Patent Publication Number: US-2021163562-A1

Title: Novel IL-21 Prodrugs and Methods of Use Thereof

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from U.S. Provisional Application No. 62/703,383, filed Jul. 25, 2018, the contents of which are incorporated herein by reference in their entirety. 
    
    
     SEQUENCE LISTING 
     The instant application contains a Sequence Listing that has been submitted electronically in ASCII format. The Sequence Listing is hereby incorporated by reference in its entirety. The ASCII copy, created on Jul. 22, 2019, is named 025471_WO003_SL.txt and is 189,683 bytes in size. 
     FIELD OF THE INVENTION 
     The present application relates to novel IL-21 prodrugs that are activatable at the site of a tumor. Further included in the present disclosure are methods of using the novel prodrugs. 
     BACKGROUND OF THE INVENTION 
     Interleukin-21 (IL-21) is produced by activated CD4 +  T cells, T-follicular helper cells, and natural killer T (NKT) cells (Spolski and Leonard,  Ann Rev Immunol.  (2008) 26:57008). IL-21 has been shown to exert pleiotropic effects on the proliferation, differentiation, and cytotoxicity of various classes of lymphoid cells. More recently, IL-21 has been further shown to play a crucial role in the differentiation of CD4 +  T cells into T-helper 17 (TH 17 ) cells, a subset of T cells associated with development of inflammatory conditions and autoimmune diseases (Korn et al.,  Nature  (2007) 448(7152):484-87; Nurieva et al.,  Nature  (2007) 448(7152):480-83). 
     The receptor complex of IL-21 is composed of the private chain IL-21Rα and the common chain γC (or Rγ); the common chain is shared by five other cytokines: IL-2, IL-4, IL-7, IL-9, and IL-15 (Spolski and Leonard, supra). Human IL-21 binds to IL-21Rα with a very high affinity (K D ˜70 pM; Zhang et al.,  Biochem Biophys Res Commun.  (2003) 300(2):291-6), while binding to IL-21Rγ with a relatively low affinity (K D ˜160 μM). 
     Recombinant IL-21 has been tested in several clinical trials for the treatment of solid tumors (Zarkavelis et al.,  Transl Cancer Res.  (2017) 6(Suppl 2):S328-30). In one of the studies, the maximum tolerated dose was established at 200 μg/kg (Schmidt et al.,  Clin Cancer Res.  (2010) 16(21):5312-19). Thus, as with other cytokine therapies, systemic toxicity may severely limit the therapeutic dosage of IL-21. In addition, IL-21 may encounter “PK sinks” in vivo because it binds to its receptor IL-21Rα with a very high affinity (Zhang et al., supra). Consequently, it may be difficult for IL-21 to achieve optimal pharmacokinetics (PK) and exposure in patients. Analogs of IL-21 have been disclosed in U.S. Pat. No. 8,211,420 and Kan et al.,  J Biol Chem.  (2010) 285(16):12223-31. However, some of the analogs have selectively reduced γC binding affinity and are IL-21 antagonists. 
     There remains a need to develop IL-21-based cancer therapeutics that are more tumor site-selective and have improved PK and efficacy, while causing fewer severe side effects. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides a IL-21 prodrug comprising a cytokine moiety, a masking moiety, and a carrier moiety, wherein the cytokine moiety is an IL-21 agonist polypeptide, wherein the masking moiety binds to the cytokine moiety and inhibits a biological activity of the cytokine moiety (e.g., prevents the cytokine moiety from binding to its receptor on a target cell, or reduces one or more biological activities of the cytokine moiety, e.g., stimulation of interferon-γ secretion by immune cells such as T cells and NK cells), and wherein the cytokine moiety is fused to the carrier moiety and the masking moiety is fused to the cytokine moiety or to the carrier moiety through a cleavable peptide linker. In some embodiments, the masking moiety comprises an IL-21 receptor alpha (IL-21Rα) extracellular domain (ECD) or functional analog thereof. 
     In some embodiments, the cytokine moiety is a wildtype human IL-21 or a mutein thereof, for example, the human IL-21 agonist polypeptide such as one comprising SEQ ID NO: 1 or an amino acid sequence that is at least 90% (e.g., at least 95, 96, 97, 98, or 99 percent) identical to SEQ ID NO: 1. In some embodiments, the human IL-21 agonist polypeptide comprises one or more mutations at position(s) selected from D18, Q19, E109, and K117 (numbering according to SEQ ID NO: 1). In particular embodiments, the human IL-21 agonist polypeptide comprises an amino acid sequence selected from SEQ ID NOs: 1, 2, 3, 4, and 5. 
     In some embodiments, the masking moiety of the present prodrug comprises an ECD of human IL-21Rα or a functional analog thereof. In further embodiments, the masking moiety comprises (i) two copies of the ECD of human IL-21Rα or a functional analog thereof fused together through a peptide linker, or (ii) the ECD human IL-2Rβ or a functional analog thereof fused to an ECD of human IL-21Rγ or a functional analog thereof through a peptide linker. In some embodiments, the ECD of human IL-21Rγ or a functional analog thereof comprises SEQ ID NO: 7 or an amino acid sequence that is at least 90% (e.g., at least 95, 96, 97, 98, or 99 percent) identical to SEQ ID NO: 7. In particular embodiments, the ECD of human IL-21Rα or a functional analog thereof comprises SEQ ID NO: 6 or an amino acid sequence that is at least 90% (e.g., at least 95, 96, 97, 98, or 99 percent) identical to SEQ ID NO: 6. 
     In some embodiments, the prodrugs further comprise a second masking moiety that binds to the second cytokine moiety and inhibits a biological activity of the second cytokine moiety, wherein the second masking moiety is fused to the second cytokine moiety or to the carrier moiety through a cleavable peptide linker. In particular embodiments, the second masking moiety is selected from an ECD of IL-2 receptor beta subunit or a functional analog thereof, an ECD of an IL-7 receptor or a functional analog thereof, and an ECD of an IL-9 receptor or a functional analog thereof. 
     In some embodiments of the present prodrugs, the cytokine moieties are fused to the carrier moieties through a noncleavable peptide linker, such as one selected from SEQ ID NOs: 29-33. 
     In some embodiments of the present prodrugs, the cleavable peptide linker linking the masking moiety directly or indirectly (e.g., through the cytokine moiety) to the carrier moiety comprises a substrate sequence of urokinase-type plasminogen activator (uPA), matrix metallopeptidase (MMP) 2, or MMP9. In further embodiments, the cleavable peptide linker comprises substrate sequences of (i) both uPA and MMP2, (ii) both uPA and MMP9, or (iii) uPA, MMP2 and MMP9. In particular embodiments, the cleavable peptide linker comprises an amino acid sequence selected from SEQ ID NOs: 11-26. In certain embodiments, the cleavable peptide linker is cleavable by one or more proteases located at a tumor site or its surrounding environment, and the cleavage leads to activation of the prodrug at the tumor site or surrounding environment. 
     In some embodiments of the present prodrugs, the carrier moiety is a PEG molecule, a fatty acid chain, an albumin (e.g., a human serum albumin) or a fragment thereof, an antibody Fc domain, or an antibody or an antigen-binding fragment thereof. In particular embodiments, the carrier moiety is an IgG1 antibody Fc domain or an antibody that comprises mutations L234A and L235A (“LALA”) (EU numbering) or an IgG4 Fc domain that comprises mutations S228P/L234A/L235A (PAA). Other mutations which lead to the reduced Fc functionality such as the ones described by Tam et al.,  Antibodies  (2017) 6(12):1-34 can also be introduced when the Fc domain or the Fc of an antibody is used as the carrier moiety. 
     In particular embodiments, the carrier moiety is an antibody Fc domain or an antibody comprising knobs-into-holes mutations, and wherein the cytokine moieties and the masking moieties are fused to different polypeptide chains of the antibody Fc domain or to the different heavy chains of the antibody. In some embodiments, the cytokine moiety and the masking moiety are fused to the C-termini of the two different polypeptide chains of the Fc domain or to the C-termini of the two different heavy chains of the antibody. In other embodiments, the cytokine moiety and the masking moiety are fused to the N-termini of the two different polypeptide chains of the Fc domain or to the N-termini of the two different heavy chains of the antibody. In certain embodiments, the knobs-into-holes mutations comprise a T366Y “knob” mutation on a polypeptide chain of the Fc domain or a heavy chain of the antibody, and a Y407T “hole” mutation in the other polypeptide of the Fc domain or the other heavy chain of the antibody (EU numbering). In certain embodiments, the knobs-into-holes mutations comprise Y349C and/or T366W mutations in the CH3 domain of the “knob chain” and E356C, T366S, L368A, and/or Y407V mutations in the CH3 domain of the “hole chain” (EU numbering). 
     In particular embodiments, the prodrug comprises two polypeptide chains whose amino acid sequences respectively comprise SEQ ID NOs: 36 and 38; SEQ ID NOs: 37 and 38; SEQ ID NOs: 39 and 41; SEQ ID NOs: 40 and 41; SEQ ID NOs: 42 and 44; SEQ ID NOs: 43 and 44; SEQ ID NOs: 45 and 47; or SEQ ID NOs: 46 and 47. 
     In some embodiments, the carrier moiety is an antibody or an antigen-binding fragment thereof that specifically binds to one or more antigens selected from guanyl cyclase C (GCC), carbohydrate antigen 19-9 (CA19-9), glycoprotein A33 (gpA33), mucin 1 (MUC1), carcinoembryonic antigen (CEA), insulin-like growth factor 1 receptor (IGF1-R), human epidermal growth factor receptor 2 (HER2), human epidermal growth factor receptor 3 (HER3), delta-like protein 3 (DLL3), delta-like protein 4 (DLL4), epidermal growth factor receptor (EGFR), glypican-3 (GPC3), c-MET, vascular endothelial growth factor receptor 1 (VEGFR1), vascular endothelial growth factor receptor 2 (VEGFR2), Nectin-4, Liv-1, glycoprotein NMB (GPNMB), prostate specific membrane antigen (PSMA), Trop-2, carbonic anhydrase IX (CA9), endothelin B receptor (ETBR), six transmembrane epithelial antigen of the prostate 1 (STEAP1), folate receptor alpha (FR-α), SLIT and NTRK-like protein 6 (SLITRK6), carbonic anhydrase VI (CA6), ectonucleotide pyrophosphatase/phosphodiesterase family member 3 (ENPP3), mesothelin, trophoblast glycoprotein (TPBG), CD19, CD20, CD22, CD33, CD40, CD56, CD66e, CD70, CD74, CD79b, CD98, CD123, CD138, CD352, CD47, signal-regulatory protein alpha (SIRPα), PD1, Claudin 18.2, Claudin 6, 5T4, BCMA, PD-L1, PD-1, fibroblast activation protein alpha (FAPalpha), the melanoma-associated chondroitin sulfate proteoglycan (MCSP), and epithelial cellular adhesion molecule (EPCAM). In specific embodiments, the carrier moiety is an antibody or fragment thereof which binds to FAPalpha or 5T4. 
     In particular embodiments, the carrier moiety is an antibody comprising two heavy chain polypeptides whose amino acid sequences respectively comprise SEQ ID NO: 48 and SEQ ID No: 49; and wherein said antibody comprises a light chain polypeptide chain with an amino acid sequence as shown in SEQ ID NO: 50 or 51. 
     In some embodiments, the prodrug further comprises two or three ectodomains of a TNF ligand family member or fragments thereof or two or three ectodomains of 4-1BB ligand or fragments thereof. 
     In some embodiments, the prodrug further comprises a second effector polypeptide selected from the following: (i) a human IL-2 agonist polypeptide comprising SEQ ID NO: 8 or an amino acid sequence that is at least 90% (e.g., at least 95, 96, 97, 98, or 99 percent) identical to SEQ ID NO: 8, or (ii) a human IL-7 agonist polypeptide, or (iii) a human IL-9 agonist polypeptide, or (iv) a human IL-15 agonist polypeptide comprising SEQ ID NO: 9 or an amino acid sequence that is at least 90% (e.g., at least 95, 96, 97, 98, or 99 percent) identical to SEQ ID NO: 9, or (v) a human IL-15 agonist polypeptide and a IL-15 receptor alpha sushi domain, or (vi) a CCL19 polypeptide comprising an amino acid sequence that is at least 90% (e.g., at least 95, 96, 97, 98, or 99 percent) identical to SEQ ID NO: 27. 
     In other embodiments, the prodrug further comprises a second effector polypeptide such as a second cytokine agonist polypeptide and optionally a second masking moiety that binds this second effector polypeptide. An example of such a prodrug comprises two polypeptide chains whose amino acid sequences respectively comprise SEQ ID NO: 42 and SEQ ID NO: 44. Another example is where the prodrug comprises two polypeptide chains whose amino acid sequences respectively comprise SEQ ID NO: 43 and SEQ ID NO: 44. Another example is where the prodrug comprises two polypeptide chains whose amino acid sequences respectively comprise SEQ ID NO: 45 and SEQ ID NO: 47. Yet another example is where the prodrug comprises two polypeptide chains whose amino acid sequences respectively comprise SEQ ID NO: 46 and SEQ ID NO: 47. 
     In another aspect, the present disclosure provides polynucleotides encoding the present prodrugs, expression vectors comprising the polynucleotides, and host cells (e.g., mammalian host cells such as CHO, NS0 cells, and 293T cells) comprising the expression vectors. The present disclosure also provides methods for making the present prodrugs, comprising culturing the mammalian host cells under conditions that allow expression of the prodrugs and isolating the prodrugs. 
     The present disclosure also provides a method of treating a cancer or an infectious disease or stimulating the immune system in a patient (e.g., human patient) in need thereof, comprising administering to the patient a therapeutically effective amount of the IL-21 prodrug, or the pharmaceutical composition of the present disclosure. The patient may have, for example, a viral infection (e.g., HIV, HBV, HCV, or HPV infection), or a cancer selected from the group consisting of breast cancer, lung cancer, pancreatic cancer, esophageal cancer, medullary thyroid cancer, ovarian cancer, uterine cancer, prostate cancer, testicular cancer, colorectal cancer, and stomach cancer. Also provided herein are an IL-21 prodrug for use in treating a cancer or an infectious disease or stimulating the immune system in the present method; use of an Il-21 prodrug for the manufacture of a medicament for treating a cancer or an infectious disease or stimulating the immune system in the present method; and articles of manufacture (e.g., kits) comprising one or more dosing units of the present Il-21 prodrug. 
     Other features, objectives, and advantages of the invention are apparent in the detailed description that follows. It should be understood, however, that the detailed description, while indicating embodiments and aspects of the invention, is given by way of illustration only, not limitation. Various changes and modification within the scope of the invention will become apparent to those skilled in the art from the detailed description. 
    
    
     
       BRIEF DESCRIPTIONS OF THE DRAWINGS 
         FIG. 1  illustrates a heterodimeric IL-21 prodrug wherein the carrier is a Fc domain. The two chains of the Fc domain contain knobs-into-holes mutations. 
         FIG. 2  illustrates a tetrameric IL-21 prodrug wherein the carrier is an antibody containing knobs-into-holes mutations in the Fc domain. 
         FIG. 3A  illustrates a heterodimeric IL-21 prodrug comprising (i) an IL-21 polypeptide and its corresponding mask, and (ii) an IL-2 mutein and its corresponding mask. 
         FIG. 3B  illustrates an IL-21 prodrug comprising (i) an IL-21 polypeptide and its corresponding mask, and (ii) an IL-15 polypeptide, an IL-15Rα sushi domain and the corresponding mask. 
         FIG. 4  illustrates an IL-21 prodrug with two 4-1BBL ectodomains. 
         FIGS. 5A-C  show the size exclusion chromatography (SEC) HPLC analysis of the IL-21 Prodrug A after purification. 
         FIG. 6  shows the SDS-PAGE analysis of the IL-21 prodrugs prior to and after activation by the protease matrix metalloproteinase-2 (MMP2). Prodrug A comprises a wild type IL-21 polypeptide, while Prodrug B comprises an IL-21 mutein with the mutations Q19K and E109R. 
         FIGS. 7A and 7B  show the results of a cell-based biological activity assay of IL-21 prodrugs prior to and after activation by protease MMP2.  FIG. 7A  shows the results of the IL-21 Prodrug A, which comprises wild type IL-21.  FIG. 7B  shows the results of the IL-21 Prodrug B, which comprises an IL-21 mutein. 
         FIG. 8  shows the results of a cell-based biological activity assay of PD-1-IL-21 Prodrugs prior to and after activation by protease MMP2. 
         FIG. 9  shows the results of a PD-1 reporter assay, which assesses the ability of the anti-PD-1 antibody of the fusion molecules to block PD-L1-mediated PD-1 signaling. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As used herein and in the appended claims, the singular forms “a,” “or,” and “the” include plural referents unless the context clearly dictates otherwise. 
     Reference to “about” a value or parameter herein includes (and describes) variations that are directed to that value or parameter per se. For example, description referring to “about X” includes description of “X.” Additionally, use of “about” preceding any series of numbers includes “about” each of the recited numbers in that series. For example, description referring to “about X, Y, or Z” is intended to describe “about X, about Y, or about Z.” 
     The term “antigen-binding moiety” refers to a polypeptide or a set of interacting polypeptides that specifically bind to an antigen, and includes, but is not limited to, an antibody (e.g., a monoclonal antibody, a polyclonal antibody, a multi-specific antibody, a dual-specific or bispecific antibody, an anti-idiotypic antibody, or a bifunctional hybrid antibody) or an antigen-binding fragment thereof (e.g., a Fab, a Fab′, a F(ab′) 2 , a Fv, a disulfide linked Fv, a scFv, a single domain antibody (dAb), or a diabody, a single chain antibody, and an Fc-containing polypeptide such as an immunoadhesin). In some embodiments, the antibody may be of any heavy chain isotype (e.g., IgG, IgA, IgM, IgE, or IgD) or subtype (e.g., IgG 1 , IgG 2 , IgG 3 , or IgG 4 ). In some embodiments, the antibody may be of any light chain isotype (e.g., kappa or lambda). The antibody may be human, non-human (e.g., from mouse, rat, rabbit, goat, or another non-human animal), chimeric (e.g., with a non-human variable region and a human constant region), or humanized (e.g., with non-human CDRs and human framework and constant regions). In some embodiments, the antibody is a derivatized antibody. 
     The term “cytokine agonist polypeptide” refers to a wildtype cytokine, or an analog thereof. An analog of a wildtype cytokine has the same biological specificity (e.g., binding to the same receptor(s) and activating the same target cells) as the wildtype cytokine, although the activity level of the analog may be different from that of the wildtype cytokine. The analog may be, for example, a mutein (i.e., mutated polypeptide) of the wildtype cytokine, and may comprise at least one, at least two, at least three, at least four, at least five, at least six, at least seven, at least eight, at least nine, or at least ten mutations relative to the wildtype cytokine. 
     The term “cytokine antagonist” or “cytokine mask” refers to a moiety (e.g., a polypeptide) that binds to a cytokine and thereby inhibits the cytokine from binding to its receptor on the surface of a target cell and/or exerting its biological functions while being bound by the antagonist or mask. Examples of a cytokine antagonist or mask include, without limitations, a polypeptide derived from an extracellular domain of the cytokine&#39;s natural receptor that makes contact with the cytokine. 
     The term “effective amount” or “therapeutically effective amount” refers to an amount of a compound or composition sufficient to treat a specified disorder, condition, or disease (e.g., ameliorate, palliate, lessen, and/or delay one or more of its symptoms). In reference to a disease such as cancer, an effective amount may be an amount sufficient to delay cancer development or progression (e.g., decrease tumor growth rate, and/or delay or prevent tumor angiogenesis, metastasis, or infiltration of cancer cells into peripheral organs), reduce the number of epithelioid cells, cause cancer regression (e.g., shrink or eradicate a tumor), and/or prevent or delay cancer occurrence or recurrence. An effective amount can be administered in one or more doses. 
     The term “functional analog” refers to a molecule that has the same biological specificity (e.g., binding to the same ligand) and/or activity (e.g., activating or inhibiting a target cell) as a reference molecule. 
     The term “fused” or “fusion” in reference to two polypeptide sequences refers to the joining of the two polypeptide sequences through a backbone peptide bond. Two polypeptides may be fused directly or through a peptide linker that is one or more amino acids long. A fusion polypeptide may be made by recombinant technology from a coding sequence containing the respective coding sequences for the two fusion partners, with or without a coding sequence for a peptide linker in between. In some embodiments, fusion encompasses chemical conjugation. 
     The term “pharmaceutically acceptable excipient” when used to refer to an ingredient in a composition means that the excipient is suitable for administration to a treatment subject, including a human subject, without undue deleterious side effects to the subject and without affecting the biological activity of the active pharmaceutical ingredient (API). 
     The term “prodrug” refers to a therapeutic molecule that is inactive until having been activated in vivo. 
     The term “subject” refers to a mammal and includes, but is not limited to, a human, a pet (e.g., a canine or a feline), a farm animal (e.g., cattle or horse), a rodent, or a primate. 
     As used herein, “treatment” or “treating” is an approach for obtaining beneficial or desired clinical results. Beneficial or desired clinical results include, but are not limited to, one or more of the following: alleviating one or more symptoms resulting from a disease, diminishing the extent of a disease, ameliorating a disease state, stabilizing a disease (e.g., preventing or delaying the worsening or progression of the disease), preventing or delaying the spread (e.g., metastasis) of a disease, preventing or delaying the recurrence of a disease, providing partial or total remission of a disease, decreasing the dose of one or more other medications required to treat a disease, increasing the patient&#39;s quality of life, and/or prolonging survival. The methods of the present disclosure contemplate any one or more of these aspects of treatment. 
     It is to be understood that one, some, or all of the properties of the various embodiments described herein may be combined to form other embodiments of the present invention. The section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described thereunder. 
     IL-21 Prodrugs 
     The present disclosure provides IL-21 prodrugs that are metabolized in vivo to become active IL-21 therapeutics. The IL-21 prodrugs have fewer side effects, better in vivo PK profiles (e.g., longer half-life) and better target specificity, and are more efficacious as compared to prior IL-21 therapeutics. The present IL-21 prodrugs comprise an IL-21 agonist polypeptide (cytokine moiety) linked to a carrier moiety and masked (bound) by an IL-21 antagonist (masking moiety or cytokine mask). The IL-21 antagonist, which may be, for example, an extracellular domain of IL-21Rα, is linked to the cytokine moiety or to the carrier moiety through a cleavable linker (e.g., a cleavable peptide linker). The mask inhibits the cytokine moiety&#39;s biological functions while the mask is binding to it. The prodrugs may be activated at a target site (e.g., at a tumor site or the surrounding environment) in the patient by cleavage of the linker and the consequent release of the cytokine mask from the prodrug, exposing the previously masked cytokine moiety and allowing the cytokine moiety to bind to its receptor on a target cell and exert its biological functions on the target cell. In some embodiments, the carriers for the IL-21 prodrugs are antigen-binding moieties, such as antibodies, that bind an antigen at the target site. 
     In some embodiments, the present IL-21 prodrugs are metabolized to become active at a target site in the body targeted by the carrier. In further embodiments, the carrier in the prodrug is an antibody targeting a tumor antigen such that the IL-21 prodrug is delivered to a tumor site in a patient and is metabolized locally (e.g., inside or in the vicinity of the tumor microenvironment) through cleavage of the linker linking the cytokine mask to the carrier or the cytokine moiety, making the cytokine moiety available to interact with its receptor on a target cell and stimulating the target immune cells locally. 
     A. Cytokine Moieties of the Prodrugs 
     An IL-21 prodrug may comprise an IL-21 agonist polypeptide (cytokine moiety), a carrier (carrier moiety), and an IL-21 antagonist (masking moiety), wherein the IL-21 agonist polypeptide is fused to the carrier directly or through a linker (e.g., cleavable or noncleavable peptide linker), and the IL-21 antagonist is linked to the IL-21 agonist polypeptide or to the carrier through a cleavable peptide linker. In the present IL-21 prodrugs, the IL-21 agonist polypeptide may be a wildtype IL-21 polypeptide such as a wildtype human IL-21 (e.g., SEQ ID NO: 1), or an IL-21 mutein such as one derived from a human IL-21, e.g., one with an amino acid sequence selected from SEQ ID NOs: 2-5. The IL-21 mutein may have significantly reduced affinity for IL-21Rα or IL-21RαRγ, as compared to wild type IL-21. In some embodiments, the IL-21 mutein has a binding affinity for the high-affinity IL-2Rα that is 5 times, 10 times, 20 times, 50 times, 100 times, 300 times, 500 times, 1,000 times, or 10,000 times lower compared to wild type IL-21. Unless otherwise indicated, all residue numbers in IL-21 and IL-21 muteins described herein are in accordance with the numbering in SEQ ID NO: 1. 
     B. Masking Moieties of the IL-21 Prodrugs 
     The IL-21 antagonist, i.e., the masking moiety, in the present prodrug may comprise a peptide or an antibody or antibody fragment that binds to the cytokine moiety in the prodrug, masking the cytokine moiety and inhibiting its biological functions. 
     By way of example, IL-21 antagonists may comprise peptides and antibodies that bind IL-21 and interfere with the binding of the IL-21 to its receptors, leading to the reduced biological activities of the IL-21 moiety while masked. In some embodiments, the IL-21 antagonist comprises an IL-21Rα or IL-21Rγ extracellular domain or its functional analog such as one derived from human IL-21Rα or human IL-21Rγ (e.g., SEQ ID NO: 6 or 7). In some embodiments, the IL-21 antagonist comprises a peptide identified from the screening of a peptide library. In some embodiments, the IL-21 antagonist comprises an antibody or fragment thereof that blocks the binding of IL-21 or IL-21 muteins to an IL-21 receptor. 
     In some embodiments, the IL-21 antagonist in the present prodrug is an IL-21Rα extracellular domain (ECD). The present inventors have surprisingly found that, when the IL-21Rα ECD was cleaved by a protease, the cell-based activity of the IL-21 agonist polypeptide was nearly fully recovered, despite the high binding affinity of the IL-21 agonist polypeptide for the IL-21Rα ECD. 
     C. Carrier Moieties of the Prodrugs 
     The carrier moieties of the present IL-21 prodrugs may be an antigen-binding moiety, or a moiety that is not an antigen-binding moiety. The carrier moiety may improve the PK profiles such as serum half-life of the cytokine agonist polypeptide, and may also target the cytokine agonist polypeptide to a target site in the body, such as a tumor site. 
     1. Antigen-Binding Carrier Moieties 
     The carrier moiety may be an antibody or an antigen-binding fragment thereof, or an immunoadhesin. In some embodiments, the antigen-binding moiety is a full-length antibody with two heavy chains and two light chains, a Fab fragment, a Fab′ fragment, a F(ab′) 2  fragment, a Fv fragment, a disulfide linked Fv fragment, a single domain antibody, a nanobody, or a single-chain variable fragment (scFv). In some embodiments, the antigen-binding moiety is a bispecific antigen-binding moiety and can bind to two different antigens or two different epitopes on the same antigen. The antigen-binding moiety may provide additional and potentially synergetic therapeutic efficacy to the cytokine agonist polypeptide. 
     The IL-21 agonist polypeptide and its mask may be fused to the N-terminus or C-terminus of the light chain(s) and/or heavy chain(s) of the antigen-binding moiety. By way of example, the IL-21 agonist polypeptide and its mask may be fused to the antibody heavy chain or an antigen-binding fragment thereof or to the antibody light chain or an antigen-binding fragment thereof. In some embodiments, one terminus of the IL-21 agonist polypeptide is fused to the C-terminus of one or both of the heavy chains of an antibody, and the IL-21 mask is fused to the other terminus of the IL-21 agonist polypeptide through a cleavable peptide linker. In some embodiments, the IL-21 agonist polypeptide is fused to the C-terminus of one of the heavy chains of an antibody, and the IL-21 mask is fused to the C-terminus of the other heavy chain of the antibody through a cleavable peptide linker, wherein the two heavy chains contain mutations that allow the specific pairing of the two different heavy chains. 
     Strategies of forming heterodimers are well known (see, e.g., Spies et al.,  Mol Imm.  (2015) 67(2)(A):95-106). For example, the two heavy chain polypeptides in the prodrug may form stable heterodimers through “knobs-into-holes” mutations. “Knobs-into-holes” mutations are made to promote the formation of the heterodimers of the antibody heavy chains and are commonly used to make bispecific antibodies (see, e.g., U.S. Pat. No. 8,642,745). For example, the Fc domain of the antibody may comprise a T366W mutation in the CH3 domain of the “knob chain” and T366S, L368A, and/or Y407V mutations in the CH3 domain of the “hole chain.” An additional interchain disulfide bridge between the CH3 domains can also be used, e.g., by introducing a Y349C mutation into the CH3 domain of the “knobs chain” and an E356C or S354C mutation into the CH3 domain of the “hole chain” (see, e.g., Merchant et al.,  Nature Biotech  (1998) 16:677-81). In other embodiments, the antibody moiety may comprise Y349C and/or T366W mutations in one of the two CH3 domains, and E356C, T366S, L368A, and/or Y407V mutations in the other CH3 domain. In certain embodiments, the antibody moiety may comprise Y349C and/or T366W mutations in one of the two CH3 domains, and S354C (or E356C), T366S, L368A, and/or Y407V mutations in the other CH3 domain, with the additional Y349C mutation in one CH3 domain and the additional E356C or S354C mutation in the other CH3 domain, forming an interchain disulfide bridge (numbering always according to EU index of Kabat; Kabat et al., “Sequences of Proteins of Immunological Interest,” 5th ed., Public Health Service, National Institutes of Health, Bethesda, Md. (1991)). Other knobs-into-holes technologies, such as those described in EP1870459A1, can be used alternatively or additionally. Thus, another example of knobs-into-holes mutations for an antibody moiety is having R409D/K370E mutations in the CH3 domain of the “knob chain” and D399K/E357K mutations in the CH3 domain of the “hole chain” (EU numbering). 
     In some embodiments, the antibody moiety in the prodrug comprises L234A and L235A (“LALA”) mutations in its Fc domain. The LALA mutations eliminate complement binding and fixation as well as Fcγ dependent ADCC (see, e.g., Hezareh et al.  J. Virol.  (2001) 75(24):12161-8). In further embodiments, the LALA mutations are present in the antibody moiety in addition to the knobs-into-holes mutations. 
     In some embodiments, the antibody moiety comprises the M252Y/S254T/T256E (“YTE”) mutations in the Fc domain. The YTE mutations allow the simultaneous modulation of serum half-life, tissue distribution and activity of IgG 1  (see Dall&#39;Acqua et al.,  J Biol Chem.  (2006) 281:23514-24; and Robbie et al.,  Antimicrob Agents Chemother.  (2013) 57(12):6147-53). In further embodiments, the YTE mutations are present in the antibody moiety in addition to the knobs-into-holes mutations. In particular embodiments, the antibody moiety has YTE, LALA and knobs-into-holes mutations or any combination thereof. 
     The antigen-binding moiety may bind to an antigen on the surface of a cell, such as an immune cell, for example, T cells, NK cells, and macrophages, or bind to a cytokine. For example, the antigen-binding moiety may bind to PD-1, LAG-3, TIM-3, TIGIT, CTLA-4, or TGF-beta and may be an antibody. The antibody may have the ability to activate the immune cell and enhance its anti-cancer activity. 
     The antigen-binding moiety may bind to an antigen on the surface of a tumor cell. For example, the antigen-binding moiety may bind to FAP alpha, 5T4, Trop-2, PD-L1, HER-2, EGFR, Claudin 18.2, DLL-3, GCP3, or carcinoembryonic antigen (CEA), and may be an antibody. The antibody may or may not have ADCC activity. The antibody may also be further conjugated to a cytotoxic drug. 
     In some embodiments, the antigen-binding moiety binds to guanyl cyclase C (GCC), carbohydrate antigen 19-9 (CA19-9), glycoprotein A33 (gpA33), mucin 1 (MUC1), insulin-like growth factor 1 receptor (IGF1-R), human epidermal growth factor receptor 2 (HER2), human epidermal growth factor receptor 3 (HER3), delta-like protein 3 (DLL3), delta-like protein 4 (DLL4), epidermal growth factor receptor (EGFR), glypican-3 (GPC3), c-MET, vascular endothelial growth factor receptor 1 (VEGFR1), vascular endothelial growth factor receptor 2 (VEGFR2), Nectin-4, Liv-1, glycoprotein NMB (GPNMB), prostates-specific membrane antigen (PSMA), Trop-2, carbonic anhydrase IX (CA9), endothelin B receptor (ETBR), six transmembrane epithelial antigen of the prostate 1 (STEAP1), folate receptor alpha (FR-α), SLIT and NTRK-like protein 6 (SLITRK6), carbonic anhydrase VI (CA6), ectonucleotide pyrophosphatase/phosphodiesterase family member 3 (ENPP3), mesothelin, trophoblast glycoprotein (TPBG), CD19, CD20, CD22, CD33, CD40, CD56, CD66e, CD70, CD74, CD79b, CD98, CD123, CD138, CD352, CD47, signal-regulatory protein alpha (SIRPα), Claudin 18.2, Claudin 6, BCMA, or EPCAM. In some embodiments, the antigen-binding moiety binds to an epidermal growth factor (EGF)-like domain of DLL3. In some embodiments, the antigen-binding moiety binds to a Delta/Serrate/Lag2 (DSL)-like domain of DLL3. In some embodiments, the antigen-binding moiety binds to an epitope located after the 374th amino acid of GPC3. In some embodiments, the antigen-binding moiety binds to a heparin sulfate glycan of GPC3. In some embodiments, the antigen-binding moiety binds to Claudin 18.2 and does not bind to Claudin 18.1. In some embodiments, the antigen-binding moiety binds to Claudin 18.1 with at least 10 times weaker binding affinity than to Claudin 18.2. 
     Exemplary antigen-binding moieties include trastuzumab, rituximab, brentuximab, cetuximab, panitumumab, GC33 (or a humanized version thereof), anti-EGFR antibody mAb806 (or a humanized version thereof), anti-dPNAG antibody F598, and antigen-binding fragments thereof. In some embodiments, the antigen-binding moiety has at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to trastuzumab, rituximab, brentuximab, cetuximab, or panitumumab, GC33 (or a humanized version thereof), anti-EGFR antibody mAb806 (or a humanized version thereof), anti-dPNAG antibody F598, or a fragment thereof. In some embodiments, the antigen-binding moiety has an antibody heavy chain with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to the antibody heavy chain of trastuzumab, rituximab, brentuximab, cetuximab, panitumumab, GC33 (or a humanized version thereof), anti-EGFR antibody mAb806 (or a humanized version thereof), anti-dPNAG antibody F598, or a fragment thereof. In some embodiments, the antigen-binding moiety has an antibody light chain with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to the antibody light chain of trastuzumab, rituximab, brentuximab, cetuximab, panitumumab, GC33 (or a humanized version thereof), anti-EGFR antibody mAb806 (or a humanized version thereof), anti-dPNAG antibody F598, or a fragment thereof. The antigen-binding moiety is fused to an IL-2 agonist polypeptide. In some embodiments, the antigen-binding moiety comprises the six complementarity-determining regions (CDRs) of trastuzumab, rituximab, brentuximab, cetuximab, panitumumab, GC33, anti-EGFR antibody mAb806, or anti-dPNAG antibody F598. 
     A number of CDR delineations are known in the art and are encompassed herein. A person of skill in the art can readily determine a CDR for a given delineation based on the sequence of the heavy or light chain variable region. The “Kabat” CDRs are based on sequence variability and are the most commonly used (Kabat et al.,  Sequences of Proteins of Immunological Interest,  5th Ed. Public Health Service, National Institutes of Health, Bethesda, Md. (1991)). “Chothia” CDRs refer to the location of the structural loops (Chothia &amp; Lesk,  J. Mol. Biol.  (1987) 196:901-917). The “AbM” CDRs represent a compromise between the Kabat CDRs and Chothia structural loops, and are used by Oxford Molecular&#39;s AbM antibody modeling software. The “Contact” CDRs are based on an analysis of the available complex crystal structures. The residues from each of these CDRs are noted below in Table 1, in reference to common antibody numbering schemes. Unless otherwise specified herein, amino acid numbers in antibodies refer to the Kabat numbering scheme as described in Kabat et al., supra, including when CDR delineations are made in reference to Kabat, Chothia, AbM, or Contact schemes. Using this numbering system, the actual linear amino acid sequence may contain fewer or additional amino acids corresponding to a shortening of, or insertion into, a framework region (FR) or CDR of the variable domain. For example, a heavy chain variable domain may include a single amino acid insert (residue 52a according to Kabat) after residue 52 of H2 and inserted residues (e.g., residues 82a, 82b, and 82c, etc. according to Kabat) after heavy chain FR residue 82. The Kabat numbering of residues may be determined for a given antibody by alignment at regions of homology of the sequence of the antibody with a “standard” Kabat numbered sequence. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 CDR Delineations According to Various Schemes 
               
            
           
           
               
               
               
               
               
            
               
                 CDR 
                 Kabat 
                 AbM 
                 Chothia 
                 Contact 
               
               
                   
               
               
                 VL-CDR1 
                 L24-L34 
                 L24-L34 
                 L26-L32 
                 L30-L36 
               
               
                 VL-CDR2 
                 L50-L56 
                 L50-L56 
                 L50-L52 
                 L46-L55 
               
               
                 VL-CDR3 
                 L89-L97 
                 L89-L97 
                 L91-L96 
                 L89-L96 
               
               
                 VH-CDR1  
                  H31-H35B 
                  H26-H35B 
                 H26-H32 
                  H30-H35B 
               
               
                 (Kabat nos.) 
                   
                   
                   
                   
               
               
                 VH-CDR1  
                 H31-H35 
                 H26-H35 
                 H26-H32 
                 H30-H35 
               
               
                 (Chothia nos.) 
                   
                   
                   
                   
               
               
                 VH-CDR2 
                 H50-H65 
                 H50-H58 
                 H53-H55 
                 H47-H58 
               
               
                 VH-CDR3 
                  H95-H102 
                  H95-H102 
                  H95-H101 
                  H93-H101 
               
               
                   
               
            
           
         
       
     
     In some embodiments, the CDRs are “extended CDRs,” and encompass a region that begins or terminates according to a different scheme. For example, an extended CDR can be as follows: L24-L36, L26-L34, or L26-L36 (VL-CDR1); L46-L52, L46-L56, or L50-L55 (VL-CDR2); L91-L97 (VL-CDR3); H47-H55, H47-H65, H50-H55, H53-H58, or H53-H65 (VH-CDR2); and/or H93-H102 (VH-CDR3). 
     In some embodiments, the antigen-binding moiety binds to HER2, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 52, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 53, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 52, and CDR1, CDR2, and CDR3 from SEQ ID NO: 53. 
     In some embodiments, the antigen-binding moiety binds to CD20, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 54, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 55, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 54, and CDR1, CDR2, and CDR3 from SEQ ID NO: 55. 
     In some embodiments, the antigen-binding moiety binds to CD30, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 56, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 57, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 56, and CDR1, CDR2, and CDR3 from SEQ ID NO: 57. 
     In some embodiments, the antigen-binding moiety binds to EGFR, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 58 or 60, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 59 or 61, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NOs: 58 and 59, or CDR1, CDR2, and CDR3 from SEQ ID NOs: 60 and 61. 
     In some embodiments, the antigen-binding moiety binds to c-MET, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 62, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 63, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 62, and CDR1, CDR2, and CDR3 from SEQ ID NO: 63. 
     In some embodiments, the antigen-binding moiety binds to GPC3, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 64, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 65, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 64, and CDR1, CDR2, and CDR3 from SEQ ID NO: 65. 
     In some embodiments, the antigen-binding moiety binds to Claudin 18.2, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 66, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 67, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 66, and CDR1, CDR2, and CDR3 from SEQ ID NO: 67. 
     In some embodiments, the antigen-binding moiety binds to FAP alpha, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 80 or 81, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 82, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 80 or 81, and CDR1, CDR2, and CDR3 from SEQ ID NO: 82. 
     In some embodiments, the antigen-binding moiety binds to FAP alpha, and comprises a light chain variable domain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 83, and a heavy chain variable domain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 84. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 84, and CDR1, CDR2, and CDR3 from SEQ ID NO: 84. 
     In some embodiments, the antigen-binding moiety binds to PDL1, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 89, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 90, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 89, and CDR1, CDR2, and CDR3 from SEQ ID NO: 90. 
     In some embodiments, the antigen-binding moiety binds to 5T4, and comprises a light chain variable domain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 87 or 88, and a heavy chain variable domain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 85 or 86, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 87 or 88, and CDR1, CDR2, and CDR3 from SEQ ID NO: 85 or 86. 
     In some embodiments, the antigen-binding moiety binds to Trop-2, and comprises a light chain variable region comprising a CDR1 comprising an amino acid sequence of KASQDVSIAVA (SEQ ID NO: 68), a CDR2 comprising an amino acid sequence of SASYRYT (SEQ ID NO: 69), and a CDR3 comprising an amino acid sequence of QQHYITPLT (SEQ ID NO: 70); and a heavy chain variable region comprising a CDR1 comprising an amino acid sequence of NYGMN (SEQ ID NO: 71), a CDR2 comprising an amino acid sequence of WINTYTGEPTYTDDFKG (SEQ ID NO: 72), and a CDR3 comprising an amino acid sequence of GGFGSSYWYFDV (SEQ ID NO: 73). 
     In some embodiments, the antigen-binding moiety binds to mesothelin, and comprises light chain variable region comprising a CDR1 comprising an amino acid sequence of SASSSVSYMH (SEQ ID NO: 74), a CDR2 comprising an amino acid sequence of DTSKLAS (SEQ ID NO: 75), and a CDR3 comprising an amino acid sequence of QQWSGYPLT (SEQ ID NO: 76); and a heavy chain variable region comprising a CDR1 comprising an amino acid sequence of GYTMN (SEQ ID NO:77), a CDR2 comprising an amino acid sequence of LITPYNGASSYNQKFRG (SEQ ID NO: 78), and a CDR3 comprising an amino acid sequence of GGYDGRGFDY (SEQ ID NO: 79). 
     In some embodiments, the antigen-binding moiety binds to PD-1, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 50, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 91, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 50, and CDR1, CDR2, and CDR3 from SEQ ID NO: 91. 
     In some embodiments, the antigen-binding moiety binds to PD-1, and comprises a light chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 92, or a fragment thereof, and a heavy chain having an amino acid sequence with at least 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% identity to SEQ ID NO: 93, or a fragment thereof. In some embodiments, the antigen-binding domain comprises CDR1, CDR2, and CDR3 from SEQ ID NO: 92, and CDR1, CDR2, and CDR3 from SEQ ID NO: 93. 
     In some embodiments, the antigen-binding moiety comprises one, two or three antigen-binding domains. For example, the antigen-binding moiety is bispecific and binds to two different antigens selected from the group consisting of HER2, HER3, EGFR, 5T4, FAP alpha, Trop-2, GPC3, VEGFR2, Claudin 18.2 and PD-L1. In some embodiments, said bispecific antigen-binding moiety binds to two different epitopes of HER2. 
     2. Other Carrier Moieties 
     Other non-antigen-binding carrier moieties may be used for the present prodrugs. For example, an antibody Fc domain (e.g., a human IgG 1 , IgG 2 , IgG 3 , or IgG 4  Fc), a polymer (e.g., PEG), an albumin (e.g., a human albumin) or a fragment thereof, or a nanoparticle can be used. 
     By way of example, the IL-21 agonist polypeptide and its antagonist may be fused to an antibody Fc domain, forming an Fc fusion protein. In some embodiments, the IL-21 agonist polypeptide is fused (directly or through a peptide linker) to the C-terminus or N-terminus of one of the Fc domain polypeptide chains, and the cytokine mask is fused to the corresponding C-terminus or N-terminus of the other Fc domain polypeptide chain through a cleavable peptide linker, wherein the two Fc domain polypeptide chains contain mutations that allow the specific pairing of the two different Fc chains. In some embodiments, the Fc domain comprises the holes-into-holes mutations described above. In further embodiments, the Fc domain may comprise also the YTE and/or LALA mutations described above. 
     The carrier moiety of the prodrug may comprise an albumin (e.g., human serum albumin) or a fragment thereof. In some embodiments, the carrier moiety comprises an albumin fragment (e.g., a human serum albumin fragment) that is about 10 or more, 20 or more, 30 or more 40 or more, 50 or more, 60 or more, 70 or more, 80 or more, 90 or more, 100 or more, 120 or more, 140 or more, 160 or more, 180 or more, 200 or more, 250 or more, 300 or more, 350 or more, 400 or more, 450 or more, 500 or more, or 550 or more amino acids in length. In some embodiments, the albumin fragment is between about 10 amino acids and about 584 amino acids in length (such as between about 10 and about 20, about 20 and about 40, about 40 and about 80, about 80 and about 160, about 160 and about 250, about 250 and about 350, about 350 and about 450, or about 450 and about 550 amino acids in length). In some embodiments, the albumin fragment includes the Sudlow I domain or a fragment thereof, or the Sudlow II domain or the fragment thereof. 
     D. Linker Components of the Prodrugs 
     The IL-21 agonist polypeptide may be fused to the carrier moiety with or without a peptide linker. The peptide linker may be noncleavable. In some embodiments, the peptide linker is selected from SEQ ID NOs: 29-33. In particular embodiments, the peptide linker comprise the amino acid sequence GGGGSGGGGSGGGGS (SEQ ID NO: 31). 
     The IL-21 mask may be fused to the cytokine moiety or to the carrier through a cleavable linker. The cleavable linker may contain one or more (e.g., two or three) cleavable moieties (CM). Each CM may be a substrate for an enzyme or protease selected from legumain, plasmin, TMPRSS-3/4, MMP2, MMP9, MT1-MMP, cathepsin, caspase, human neutrophil elastase, beta-secretase, uPA, and PSA. Examples of cleavable linkers include, without limitation, those comprising an amino acid sequence selected from SEQ ID NOs: 11-26. By way of example, a cleavable peptide linker is used to link the IL-21Rγ ECD to an IL-21 Rα-subunit ECD. Preferably, there is a cleavable peptide linker which links one IL-21Rα-subunit ECD to the other α-subunit ECD to form a dimer. 
     E. IL-21 Prodrugs with an Additional Effector Polypeptide 
     In specific embodiments, the IL-21 prodrugs further comprise a second effector polypeptide such as a second cytokine moiety. In such cases, the prodrugs may further comprise a second masking moiety that binds to and inhibits a biological activity of the second effector polypeptide. 
     By way of example, the IL-21 agonist polypeptide and its mask may be fused to separate Fc chains at one end of the Fc domain, while the second cytokine moiety and its mask may be fused to separate Fc chains at the other end of the Fc domain, wherein the masks are fused to the Fc chains through cleavable peptide linkers. In certain embodiments, the two Fc domain polypeptide chains contain mutations that allow the specific pairing of the two different Fc chains. 
     Examples of prodrugs comprising two effector polypeptides and two masking moieties include those comprising two polypeptide chains whose amino acid sequences respectively comprise (i) SEQ ID NOs: 42 and 44; (ii) SEQ ID NOs: 43 and 44; (iii) SEQ ID NOs: 45 and 47; or (iv) SEQ ID NOs: 46 and 47. The exemplary structure of an IL-21 prodrug that comprises an IL-2 agonist polypeptide (second effector polypeptide) and its corresponding mask is illustrated in  FIG. 3A . The exemplary structure of an IL-21 prodrug that comprises an IL-15 agonist polypeptide, the sushi domain and its corresponding mask is illustrated in  FIG. 3B . 
     In some embodiments, the IL-21 prodrugs further comprise one or more (e.g., two or three) copies of the ectodomains of the ligand of a tumor necrosis factor (TNF) superfamily member. In some embodiments, the TNF superfamily member is 4-1BB. The structure of an exemplary IL-21 prodrug further comprising two copies of a 4-1BB ligand (4-1BBL) ectodomain is illustrated in  FIG. 4 . The carrier of the IL-21 prodrug may be an antibody that binds to an antigen expressed in a tumor, for example, FAP or 5T4. 
     Specific, nonlimiting examples of IL-21 agonist polypeptides, cytokine masks, carriers, peptide linkers, and prodrugs are shown in the Sequences section below. Further, the prodrugs of the present disclosure may be made by well-known recombinant technology. For examples, one or more expression vectors comprising the coding sequences for the polypeptide chains of the prodrugs may be transfected into mammalian host cells (e.g., CHO cells), and the cells are cultured under conditions that allow the expression of the coding sequences and the assembly of the expressed polypeptides into the prodrug complex. In order for the prodrug to remain inactive, the host cells that express no or little uPA, MMP2 and/or MMP9 may be used. In some embodiments, the host cells may contain null mutations (knockout) of the genes that encode these proteases. 
     Pharmaceutical Compositions 
     Pharmaceutical compositions comprising the prodrugs and muteins (i.e., the active pharmaceutical ingredient or API) of the present disclosure may be prepared by mixing the API having the desired degree of purity with one or more optional pharmaceutically acceptable excipients (see, e.g.,  Remington&#39;s Pharmaceutical Sciences,  16th Edition., Osol, A. Ed. (1980)) in the form of lyophilized formulations or aqueous solutions. Pharmaceutically acceptable excipients (or carriers) are generally nontoxic to recipients at the dosages and concentrations employed, and include, but are not limited to: buffers containing, for example, phosphate, citrate, succinate, histidine, acetate, or another inorganic or organic acid or salt thereof; antioxidants including ascorbic acid and methionine; preservatives (such as octadecyldimethylbenzyl ammonium chloride; hexamethonium chloride; benzalkonium chloride; benzethonium chloride; phenol, butyl or benzyl alcohol; alkyl parabens such as methyl or propyl paraben; catechol; resorcinol; cyclohexanol; 3-pentanol; and m-cresol); low molecular weight (less than about 10 residues) polypeptides; proteins, such as serum albumin, gelatin, or immunoglobulins; hydrophilic polymers such as polyvinylpyrrolidone; amino acids such as glycine, glutamine, asparagine, histidine, arginine, or lysine; monosaccharides, disaccharides, and other carbohydrates including sucrose, glucose, mannose, or dextrins; chelating agents such as EDTA; sugars such as sucrose, mannitol, trehalose or sorbitol; salt-forming counter-ions such as sodium; metal complexes (e.g. Zn-protein complexes); and/or non-ionic surfactants such as polyethylene glycol (PEG). 
     Buffers are used to control the pH in a range which optimizes the therapeutic effectiveness, especially if stability is pH dependent. Buffers are preferably present at concentrations ranging from about 50 mM to about 250 mM. Suitable buffering agents for use with the present invention include both organic and inorganic acids and salts thereof, such as citrate, phosphate, succinate, tartrate, fumarate, gluconate, oxalate, lactate, and acetate. Additionally, buffers may comprise histidine and trimethylamine salts such as Tris. 
     Preservatives are added to retard microbial growth, and are typically present in a range from 0.2%-1.0% (w/v). Suitable preservatives for use with the present invention include octadecyldimethylbenzyl ammonium chloride; hexamethonium chloride; benzalkonium halides (e.g., chloride, bromide, iodide), benzethonium chloride; thimerosal, phenol, butyl or benzyl alcohol; alkyl parabens such as methyl or propyl paraben; catechol; resorcinol; cyclohexanol, 3-pentanol, and m-cresol. 
     Tonicity agents, sometimes known as “stabilizers” are present to adjust or maintain the tonicity of liquid in a composition. When used with large, charged biomolecules such as proteins and antibodies, they are often termed “stabilizers” because they can interact with the charged groups of the amino acid side chains, thereby lessening the potential for inter- and intra-molecular interactions. Tonicity agents can be present in any amount between 0.1% to 25% by weight, or more preferably between 1% to 5% by weight, taking into account the relative amounts of the other ingredients. Preferred tonicity agents include polyhydric sugar alcohols, preferably trihydric or higher sugar alcohols, such as glycerin, erythritol, arabitol, xylitol, sorbitol and mannitol. 
     Non-ionic surfactants or detergents (also known as “wetting agents”) are present to help solubilize the therapeutic agent as well as to protect the therapeutic protein against agitation-induced aggregation, which also permits the formulation to be exposed to shear surface stress without causing denaturation of the active therapeutic protein or antibody. Non-ionic surfactants are present in a range of about 0.05 mg/ml to about 1.0 mg/ml, preferably about 0.07 mg/ml to about 0.2 mg/ml. 
     Suitable non-ionic surfactants include polysorbates (20, 40, 60, 65, 80, etc.), polyoxamers (184, 188, etc.), PLURONIC® polyols, TRITON®, polyoxyethylene sorbitan monoethers (TWEEN®-20, TWEEN®-80, etc.), lauromacrogol 400, polyoxyl 40 stearate, polyoxyethylene hydrogenated castor oil 10, 50 and 60, glycerol monostearate, sucrose fatty acid ester, methyl cellulose and carboxymethyl cellulose. Anionic detergents that can be used include sodium lauryl sulfate, dioctyle sodium sulfosuccinate and dioctyl sodium sulfonate. Cationic detergents include benzalkonium chloride or benzethonium chloride. 
     The choice of pharmaceutical carrier, excipient or diluent may be selected with regard to the intended route of administration and standard pharmaceutical practice. Pharmaceutical compositions may additionally comprise any suitable binder(s), lubricant(s), suspending agent(s), coating agent(s) or solubilizing agent(s). 
     There may be different composition/formulation requirements dependent on the different delivery systems. By way of example, pharmaceutical compositions useful in the present invention may be formulated to be administered using a mini-pump or by a mucosal route, for example, as a nasal spray or aerosol for inhalation or ingestible solution, or parenterally in which the composition is formulated by an injectable form, for delivery, by, for example, an intravenous, intramuscular or subcutaneous route. 
     In some embodiments, the pharmaceutical composition of the present disclosure is a lyophilized protein formulation. In other embodiments, the pharmaceutical composition may be an aqueous liquid formulation. 
     Methods of Treatment 
     The IL-21 prodrug can be used to treat a disease, depending on the antigen bound by the antigen-binding domain. In some embodiments, the IL-21 prodrug is used to treat cancer. In some embodiments, the IL-21 prodrug is used to treat an infection. 
     In some embodiments, a method of treating a disease (such as cancer, a parasitic infection, a viral infection, or a bacterial infection) in a subject comprises administering to the subject an effective amount of an IL-21 prodrug. 
     In some embodiments, the cancer is a solid cancer. In some embodiments, the cancer is a blood cancer or a solid tumor. Exemplary cancers that may be treated include, but are not limited to, leukemia, lymphoma, kidney cancer, bladder cancer, urinary tract cancer, cervical cancer, brain cancer, head and neck cancer, skin cancer, uterine cancer, testicular cancer, esophageal cancer, liver cancer, colorectal cancer, stomach cancer, squamous cell carcinoma, prostate cancer, pancreatic cancer, lung cancer such as non-small cell lung cancer, cholangiocarcinoma, breast cancer, and ovarian cancer. 
     In some embodiments, the IL-21 prodrug is used to treat a viral infection. In some embodiments, the virus causing the viral infection is hepatitis C virus (HCV), hepatitis B virus (HBV), human immunodeficiency virus (HIV), or human papilloma virus (HPV). In some embodiments, the antigen-binding moiety binds to a viral antigen. 
     In some embodiments, the IL-21 prodrug is used to treat a bacterial infection such as sepsis. In some embodiments, the bacteria causing the bacterial infection is drug-resistant bacteria. In some embodiments, the antigen-binding moiety binds to a bacterial antigen. 
     Generally, dosages and routes of administration of the present pharmaceutical compositions are determined according to the size and conditions of the subject, according to standard pharmaceutical practice. In some embodiments, the pharmaceutical composition is administered to a subject through any route, including orally, transdermally, by inhalation, intravenously, intra-arterially, intramuscularly, direct application to a wound site, application to a surgical site, intraperitoneally, by suppository, subcutaneously, intradermally, transcutaneously, by nebulization, intrapleurally, intraventricularly, intra-articularly, intraocularly, intracranially, or intraspinally. In some embodiments, the composition is administered to a subject intravenously. 
     In some embodiments, the dosage of the pharmaceutical composition is a single dose or a repeated dose. In some embodiments, the doses are given to a subject once per day, twice per day, three times per day, or four or more times per day. In some embodiments, about 1 or more (such as about 2, 3, 4, 5, 6, or 7 or more) doses are given in a week. In some embodiments, the pharmaceutical composition is administered weekly, once every 2 weeks, once every 3 weeks, once every 4 weeks, weekly for two weeks out of 3 weeks, or weekly for 3 weeks out of 4 weeks. In some embodiments, multiple doses are given over the course of days, weeks, months, or years. In some embodiments, a course of treatment is about 1 or more doses (such as about 2, 3, 4, 5, 7, 10, 15, or 20 or more doses). 
     Unless otherwise defined herein, scientific and technical terms used in connection with the present disclosure shall have the meanings that are commonly understood by those of ordinary skill in the art. Exemplary methods and materials are described below, although methods and materials similar or equivalent to those described herein can also be used in the practice or testing of the present disclosure. In case of conflict, the present specification, including definitions, will control. Generally, nomenclature used in connection with, and techniques of, cell and tissue culture, molecular biology, immunology, microbiology, genetics, analytical chemistry, synthetic organic chemistry, medicinal and pharmaceutical chemistry, and protein and nucleic acid chemistry and hybridization described herein are those well-known and commonly used in the art. Enzymatic reactions and purification techniques are performed according to manufacturer&#39;s specifications, as commonly accomplished in the art or as described herein. Further, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular. Throughout this specification and embodiments, the words “have” and “comprise,” or variations such as “has,” “having,” “comprises,” or “comprising,” will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers. It is understood that aspects and variations of the invention described herein include “consisting” and/or “consisting essentially of” aspects and variations. All publications and other references mentioned herein are incorporated by reference in their entirety. Although a number of documents are cited herein, this citation does not constitute an admission that any of these documents forms part of the common general knowledge in the art. 
     Exemplary Embodiments 
     Further particular embodiments of the present disclosure are described as follows. These embodiments are intended to illustrate the compositions and methods described in the present disclosure and are not intended to limit the scope of the present disclosure.
     1. A Prodrug of IL-21 which comprises an IL-21 agonist polypeptide (A), a masking moiety (MM), a carrier (C), and at least one cleavable peptide linker; wherein:
       a) said IL-21 agonist polypeptide (A) has an amino acid sequence at least 95% identical to that of SEQ ID NO: 1;   b) said masking moiety (MM) is selected from IL-21 receptor extracellular domain, its analog, and a fusion protein of the IL-21 receptor extracellular domain and the extracellular domain of the gamma chain;   c) said carrier is selected from an albumin, an albumin fragment, a Fc, and an antibody; and   d) said cleavable peptide linker links the said masking moiety (MM) to the carrier or to the IL-21 agonist polypeptide.   
       2. The prodrug of embodiment 1, wherein said IL-21 agonist polypeptide (A) is human IL-21 with amino acid sequence of SEQ ID NO: 1 or its analog, wherein said analog contains one or more mutations at D18, E109, K117, and or K119; and wherein said mutations are referred to according to the numbering of the human IL-21 with amino acid sequence of SEQ ID NO: 1.   3. The prodrug of embodiment 1, wherein said IL-21 agonist polypeptide (A) has an amino acid sequence selected from SEQ ID NOs: 2-5.   4. The prodrug of embodiments 1-3, wherein IL-21 receptor extracellular domain has an amino acid sequence at least 95%, at least 96%, at least 97%, at least 98%, at least 99% or 100% identical as that of SEQ ID NO: 6; and wherein said gamma chain has an amino acid as shown in SEQ ID NO: 7.   5. The prodrug of embodiments 1-4, wherein said cleavable peptide linker has an amino acid sequence selected from SEQ ID NOs: 11-26.   6. The prodrug of embodiments 1-5, wherein said carrier (C) is an albumin or a fragment thereof.   7. The prodrug of embodiments 1-5, wherein said carrier(C) is an Fc fragment.   8. The prodrug of embodiment 7, wherein said IL-21 agonist polypeptide is fused to the C-terminal of one Fc fragment, optionally through a peptide linker, and said masking moiety (MM) is fused to the C-terminal of the second Fc fragment through a cleavable peptide linker; and wherein the Fc-fusion proteins form a heterodimer through “knob-into-hole” mutations.   9. The prodrug of embodiment 7, wherein said IL-21 agonist polypeptide is fused to the N-terminal of one Fc fragment, optionally through a peptide linker, and said masking moiety (MM) is fused to the N-terminal of the second Fc fragment through a cleavable peptide linker; and wherein above said two Fc-fusion proteins form a heterodimer through “knob-into-hole” mutations.   10. The prodrug of embodiments 1-6, wherein said carrier(C) is an antibody.   11. The prodrug of embodiment 10, wherein said IL-21 agonist polypeptide is fused to the N-terminal of the heavy chains of said antibody, optionally through a peptide linker, and said masking moiety (MM) is fused to the N-terminal of the light chain through a cleavable peptide linker.   12. The prodrug of embodiment 10, wherein said IL-21 agonist polypeptide is fused to the C-terminal of one of the heavy chains of said antibody, optionally through a peptide linker, and said masking moiety (MM) is fused to the C-terminal of the 2 nd  heavy chain through a cleavable peptide linker; and wherein above the heavy chain-fusion proteins form a heterodimer through “knob-into-hole” mutations.   13. The prodrug of embodiment 10, wherein said IL-21 agonist polypeptide is fused to the N-terminal of one of the heavy chains of said antibody, optionally through a peptide linker, and said masking moiety (MM) is fused to the N-terminal of the 2 nd  heavy chain through a cleavable peptide linker; and wherein above said two heavy chain-fusion proteins form a heterodimer through “knob-into-hole” mutations.   14. The prodrug of embodiment 10, wherein one said IL-21 agonist polypeptide is fused to each of the C-terminals of the heavy chains of said antibody, optionally through a peptide linker, and said masking moiety (MM) is fused to the C-terminal of the IL-21 agonist polypeptide through a cleavable peptide linker.   15. The prodrug of embodiments 10-14, wherein said antibody binds to one or more antigens on a cancer cell.   16. The prodrug of embodiments 10-14, wherein said antibody is an antibody against human PD-L1 or CD47.   17. The prodrug of embodiments 10-14, wherein said antibody is selected from anti-CEA antibody PR1A3, humanized PR1A3, and anti-FAP-alpha antibody sibrotuzumab (BIBH1).   18. The prodrug of embodiments 10-14, wherein said antibody binds to one or more antigen selected from Guanyl cyclase C (GCC), carbohydrate antigen 19-9 (CA19-9), glycoprotein A33 (gpA33), mucin 1 (MUC1), carcinoembryonic antigen (CEA), insulin-like growth factor 1 receptor (IGF1-R), human epidermal growth factor receptor 2 (HER2), human epidermal growth factor receptor 3 (HER3), delta-like protein 3 (DLL3), delta-like protein 4 (DLL4), epidermal growth factor receptor (EGFR), glypican-3 (GPC3), c-MET, vascular endothelial growth factor receptor 1 (VEGFR1), vascular endothelial growth factor receptor 2 (VEGFR2), Nectin-4, Liv-1, glycoprotein NMB (GPNMB), prostate-specific membrane antigen (PSMA), Trop-2, carbonic anhydrase IX (CA9), endothelin B receptor (ETBR), six transmembrane epithelial antigen of the prostate 1 (STEAP1), folate receptor alpha (FR-α), SLIT and NTRK-like protein 6 (SLITRK6), carbonic anhydrase VI (CA6), ectonucleotide pyrophosphatase/phosphodiesterase family member 3 (ENPP3), mesothelin, trophoblast glycoprotein (TPBG), CD19, CD20, CD22, CD33, CD40, CD56, CD66e, CD70, CD74, CD79b, CD98, CD123, CD138, CD352, CD47, signal-regulatory protein alpha (SIRPα), PD1, Claudin 18.2, Claudin 6, and FAP-alpha., BCMA, and EPCAM, CD3, TGF-beta, LAG3, TIM3, TIGIT, and CTLA-4.   19. The prodrug of embodiments 10-18, wherein it further comprises a CCL19 polypeptide with an amino acid sequence at least 95% identical as that of SEQ ID NO: 27.   20. The prodrug of embodiments 1-19, wherein said cleavable peptide linker is cleavable by a protease or proteases found at a tumor site or its surrounding environment.   21. Prodrug of embodiments 1-19, wherein said prodrug is activated at the site of a tumor.   22. The prodrug of embodiment 16, wherein said antibody binds to PD-L1; wherein said PD-L1 antibody is selected from atezolizumab, durvalumab, avelumab.   23. Prodrug of embodiment 10-14, wherein said antibody binds to PD-1; wherein said anti-PD-1 antibody is selected from Nivolumab and Pembrolizumab.   24. A prodrug of IL-21, wherein its IL-21 activity is activatable or increase by at least 200% at the site of a tumor or its surrounding area, comprising:
       a) An antibody, which binds to PD-L1, comprising two identical light chains and a 1 st  heavy chain and a 2 nd  heavy chain; wherein said heavy chains contain “knob-and-hole” mutations;   b) An IL-21 agonist polypeptide, wherein said IL-21 agonist polypeptide is fused to the C-terminal of the 1 st  heavy chain of said antibody, optionally through a peptide linker with amino acid sequence (GGGGS) n , wherein n=1, 2, 3, and 4 (SEQ ID NO: 97);   c) A masking moiety (MM), wherein said masking moiety comprises one or two extracellular domains of the IL-21 receptor extracellular domain; and;   d) a cleavable peptide linker which links the said masking moiety (MM) to the C-terminal of the 2 nd  heavy chain of said antibody,    wherein said IL-21 agonist polypeptide comprises an amino acid sequence selected from SEQ ID NO: 1-5; and wherein said IL-21 receptor beta subunit extracellular domain has an amino acid sequence as that of SEQ ID NO: 6.   
       25. A prodrug of IL-21, wherein its IL-21 activity is activatable or increase by at least 200% at the site of a tumor or its surrounding area, comprising:
       a) An antibody, which binds to CD47, comprising two identical light chains and a 1 st  heavy chain and a 2 nd  heavy chain; wherein said heavy chains contain “knob-and-hole” mutations;   b) An IL-21 agonist polypeptide, wherein said IL-21 agonist polypeptide is fused to the C-terminal of the 1 st  heavy chain of said antibody, optionally through a peptide linker with amino acid sequence (GGGGS) n , wherein n=1, 2, 3, and 4 (SEQ ID NO: 97);   c) A masking moiety (MM), wherein said masking moiety comprises one or two extracellular domains of the IL-21 receptor extracellular domain; and;   d) a cleavable peptide linker which links the said masking moiety (MM) to the C-terminal of the 2 nd  heavy chain of said antibody,    wherein said IL-21 agonist polypeptide comprises an amino acid sequence selected from SEQ ID NO: 1-5; and wherein said IL-21 receptor beta subunit extracellular domain has an amino acid sequence as that of SEQ ID NO: 6.   
       26. Polynucleotides encoding the albumin fusion protein, the Fc fusion protein, the light chain fusion protein, or the heavy chain fusion protein of the prodrugs of embodiments 1-25.   27. An expression vector comprising the polynucleotide of embodiment 26.   28. A host cell transfected with the vector of embodiment 27.   29. A method of producing said prodrug of embodiments 1-25 comprising culturing the host cell of embodiment 28.   30. A pharmaceutical composition comprising as active ingredient the prodrug of embodiments 1-25.   31. A method of treating breast, lung, pancreatic, esophageal, medullary thyroid, ovarian, uterine, prostatic, testicular, colon, rectal or stomach cancer or infectious disease comprising administering to a human subject with breast, lung, pancreatic, esophageal, medullary thyroid, ovarian, uterine, prostatic, testicular, colon, rectal or stomach cancer, or an infectious disease comprising administration of said pharmaceutical composition of Embodiment 30.   32. The method of embodiment 31, wherein the dosage administrated is 0.1-18 mg/kg.   33. The method of embodiment 31, wherein the dosage is selected from the group consisting of 0.1 mg/kg, 0.2 mg/kg, 0.5 mg/kg, 1 mg/kg, 2 mg/kg, 3 mg/kg, 4 mg/kg, 5 mg/kg, 6 mg/kg, 8 mg/kg, 10 mg/kg, and 15 mg/kg.   34. The method of embodiment 33, wherein the dosage is administered to the human subject once or twice a week on a schedule with a cycle selected from the group consisting of: (i) weekly; (ii) every other week; (iii) one week of therapy followed by two, three or four weeks off; (iv) two weeks of therapy followed by one, two, three or four weeks off; (v) three weeks of therapy followed by one, two, three, four or five weeks off; (vi) four weeks of therapy followed by one, two, three, four or five weeks off; (vii) five weeks of therapy followed by one, two, three, four or five weeks off; and (viii) monthly.   

     In order that this invention may be better understood, the following examples are set forth. These examples are for purposes of illustration only and are not to be construed as limiting the scope of the invention in any manner. 
     EXAMPLES 
     Example 1 
     Transient Transfection of the IL-21 Prodrugs Using HEK293 Cells 
     Expression plasmids were co-transfected into 3×10 6  cell/ml freestyle HEK293 cells at 2.5-3 μg/ml using PEI (polyethylenimine). For Fc-based IL-21 prodrugs (A and B), the Fc-IL-21 fusion polypeptide and the Fc-masking moiety fusion polypeptide were in a 1:2 ratio. For antibody-based IL-21 prodrugs, the knob heavy chain (containing IL-21 agonist polypeptide) and hole heavy chain (containing the masking moiety) and the light chain DNA were in a 2:1:2 molar ratio. The cell cultures were harvested 6 days after transfection by centrifuging at 9,000 rpm for 45 min followed by 0.22 μM filtration. 
     Two IL-21 Prodrugs (A and B) were expressed. Prodrug A consists of two polypeptides having SEQ ID NOs: 94 and 38, respectively. Prodrug B consist of two polypeptides having SEQ ID NOs: 95 and 38, respectively. Their corresponding controls, the Fc-IL-21 fusion molecules without the masking moiety, were also expressed. The sequence ID NOs are listed in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Sequence Information of the samples. 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 SEQ ID for Fc-Masking 
                   
               
               
                 Prodrug or Control 
                 SEQ ID for Fc-IL-21 
                 Moiety Fusion 
                 SEQ ID for the Fc 
               
               
                 Name 
                 Fusion Polypeptide 
                 Polypeptide 
                 Polypeptide 
               
               
                   
               
               
                 IL-21 Prodrug A 
                 SEQ ID NO: 94 
                 SEQ ID NO: 38 
                 — 
               
               
                 Fc-IL-21 Control 1 
                 SEQ ID NO: 94 
                 — 
                 SEQ ID NO: 96 
               
               
                 IL-21 Prodrug B 
                 SEQ ID NO: 95 
                 SEQ ID NO: 38 
                 — 
               
               
                 Fc-IL-21 Control 2 
                 SEQ ID NO: 94 
                 — 
                 SEQ ID NO: 96 
               
               
                   
               
            
           
         
       
     
     Example 2 
     Purification of the Fc-Based IL-21 Prodrugs 
     The purifications of the proteins of the Fc-based IL-21 prodrugs A and B were carried out by using three chromatography steps: Protein A Affinity, Capto Adhere (Flow-through mode), and Capto SP ImpRes. Briefly, the supernatant of the transient expression cell culture was loaded onto a Protein A column, which was equilibrated with 25 mM Tris-HCl, 30 mM NaCl, pH 7.8 (buffer A) before applying the sample. The column was washed with 5-column volumes of buffer A and the bound protein was eluted with 50 mM acetic acid, pH 3.6. The pH of the eluted protein was adjusted to 5.2 using 1 M Tris-base and loaded onto a Capto Adhere column, which was equilibrated with 50 mM acetic acid, 30 mM NaCl, pH 5.2 (buffer B). The flow-through was collected and further loaded onto a buffer B equilibrated Capto SP ImpRes column. The column was washed with 5-column volumes of buffer B, and the bound protein was eluted with a 30-column volume gradient from 0% to 100% of 50 mM acetic acid, 1 M NaCl, pH 5.2 (buffer C). The eluted samples from each step were analyzed by HPLC-SEC. The fractions of the Capto SP ImpRes step with aggregation less than 10% were pooled for the further analyses. 
     Example 3 
     SEC-HPLC Analysis 
     SEC-HPLC was carried out using an Agilent 1100 Series of HPLC system with a TSKgel G3000SWXL column (7.8 mmIDX 30 cm, 5 μm particle size) ordered from Tosoh Bioscience. A sample of up to 100 μl was loaded. The column was run with a buffer containing 200 mM K 3 PO 4 , 250 mM KCl, pH 6.5. The flow rate was 0.5 ml/min. The column was run at room temperature. The protein elution was monitored both at 220 nm and 280 nm. The in-process pools of the IL-21 Prodrug A were analyzed by the SEC-HPLC.  FIG. 5A  shows the assay result for the Protein A column pool;  FIG. 5B  shows the assay result for the Capto Adhere column pool; and  FIG. 5C  shows the assay result for the Capto Sp ImpRes column pool. The data show that the Protein A column purified prodrug comprised a main peak with some aggregates ( FIG. 5A ). It had a main peak purity of about 80% as analyzed by SEC-HPLC. The aggregates were significantly reduced by the subsequent chromatography steps and the Capto SP Impres pool showed a product purity of over 98% as tested by SEC-HPLC ( FIG. 5C ). 
     Example 4 
     SDS-PAGE Analysis 
     10 μl of the culture supernatants or 10-20 μg of purified protein samples were mixed with Bolt™ LDS Sample Buffer (Novex) with or without reduce reagents. The samples were heated at 70° C. for 3 min and then loaded to a NuPAGE™ 4-12% BisTris Gel (Invitrogen). The gel was run in NuPAGE™ MOPS SDS Running buffer (Invitrogen) at 200 Volts for 40 min and then stained with Coomassie. The purified samples of Prodrugs A and B together with the ones treated with the protease MMP-2 (see below) were analyzed by the SDS-PAGE analysis, as shown in  FIG. 6 . The data show that the masking moieties of both Prodrug A and Prodrug B were completely removed by the protease digestion, and that the activated molecules migrated at the expected molecular weights. 
     Example 5 
     Proteolytic Treatment 
     The proteases, human MMP2, human MMP9, mouse MMP2 and mouse MMP9 were purchased from R&amp;D systems. The protease digestion was carried out by incubating 10 μg-50 μg of prodrugs with 1 μg of human MMP2, human MMP9, mouse MMP2 or mouse MMP9 in the HBS buffer (20 mM HEPES, 150 mM NaCl 2 , pH 7.4) containing 2 mM CaCl2 and 10 μM ZnCl 2  at 37° C. for 12 hours. The prodrugs prior to and after digestion were analyzed by SDS-PAGE ( FIG. 6 ) and the cell-based activity assay (see below). 
     Example 6 
     Cell-Based Activity Assay 
     The prodrugs prior to and the protease digestion and the control samples were tested by the cell-based activity assay. Briefly, NK92 cells were grown in the RPMI-1640 medium supplemented with L-glutamine, 10% fetal bovine serum, 10% non-essential amino acids, 10% sodium pyruvate, and 55 μM beta-mercaptoethanol. NK92 cells were non-adherent and maintained at 1×10 5  −1×10 6  cells/ml in medium with 100 ng/ml of IL-2. Generally, cells were split twice per week. For bioassays, it was best to use cells no less than 48 hours after passage. IL-21 functional activity was determined by culturing NK92 cells at 5×10 4  cells/well with serial dilutions of the samples in the presence of a constant amount of IL-2 for 2 days. Supernatants were then assayed for interferon-γ by ELISA. The results are shown in  FIGS. 7A and 7B . The data show that the bioassay activities of the prodrugs were significantly enhanced by the protease MMP2 treatment. 
     The protease-treated (or activated) prodrugs showed similar activities as those of the control Fc-IL-21 fusion molecules, even though the masking moiety, i.e., IL-21Rα ECD, was not removed from the protease-treated sample. Surprisingly, the presence of the masking moiety released by the protease digestion did not seem to interfere with the IL-21 bioassay, given that IL-21 binds to IL-21Rα with very high affinity (a K D  of ˜70 pM). 
     Example 7 
     Anti-PD-1 Antibody-IL-21 Prodrug Fusion Molecule 
     An Anti-PD-1 antibody-based IL-21 prodrug was constructed with two identical light chains (with an amino acid sequence as shown in SEQ ID NO: 50). A first heavy chain polypeptide chain (with an amino acid sequence as shown in SEQ ID NO: 48) and a second heavy chain polypeptide chain (with an amino acid sequence as shown SEQ ID NO: 49). The molecule was expressed and purified. As a control, the anti-PD-1 antibody-IL-21 fusion molecule without the mask was also expressed and purified. The cell-based activity assays for the cytokine prodrug prior to and after activation were tested using the same method as described above. The data are shown in  FIG. 8 . The results show that the IL-21 activity prior to activation was minimal. After activation, the IL-21 activity was similar as that of the IL-21 in the PD-1-IL-21 fusion molecule. 
     The activity of the anti-PD-1 antibody was also tested prior to and after the activation using the PD1/PD-L1 blockade reporter assay. The ability of anti-PD-1 antibody to block PD-L1 mediated PD1 signaling was measured using two engineered cell lines. The first was a CHO-K1 cell line (CHO-K1/TCRA/PD-L1, BPS Bioscience cat #60536) expressing both human PD-L1 and a T cell receptor activator. The second cell line (PD1/NFAT, BPS Bioscience cat #60535) was a Jurkat T cell line expressing PD-1 and an NFAT firefly luciferase reporter. The T cell receptor activator on the CHO-K1 cells would activate the Jurkat cells resulting in expression of the NFAT luciferase reporter. However, since the CHO-K1 cells also express PD-L1, signaling via PD-1 would result in inhibition of NFAT activation. Blocking the PD-L1/PD-1 interaction would restore NFAT activation and luciferase activity. 
     To carry out the assay, CHO-K1/TCRA/PD-L1 cells were seeded in 96-well flat bottom plates at 35,000 cells/well in 50 μL assay medium (RPMI-1640, 10% fetal bovine serum, non-essential amino acids, 2-mercaptoethanol, and gentamicin) in 96-well white walled, flat bottom plates. After overnight culture, the culture medium was removed and samples and standards were added at 2× concentration in 50 μL/well. Plates were incubated 20 minutes, and 40,000 PD1/NFAT cells were added to each well in 50 μL. Plates were incubated 6 hours at 37° C. Plates were cooled to room temperature for 5 minutes, and 100 μL/well luciferase reagent (Pierce Firefly Luc One-Step Glow Assay Kit, Thermo Scientific cat #16197) was added. Plates were incubated for 15 minutes, then luminescence was measured on a luminometer. 
     The assay results ( FIG. 9 ) indicate that the anti-PD-1 antibodies in the fusion molecules had retained their biological functionality. 
     Example 8 
     In Vivo Efficacy Study with a Syngeneic Tumor Model 
     Six-week old Balb/c mice (Taconic Biosciences) are injected subcutaneously with 1×10 6  CT26/18.2 cells. After 7 days, tumors are measured using digital calipers and tumor volume was calculated (V=(ab2)p/6, where b is the shorter of 2 dimensions measured). Mice are then randomized into treatment groups such that all groups have approximately the same mean tumor size (˜100 mm 3 ). Mice are then treated with placebo or test article at 0.5-5 mg/kg in 100 μl via intraperitoneal injection. Dosing was performed on days 7, 9, 11, 13, 15 and 18. Tumors are measured every 2-3 days, and mice are sacrificed when tumors reach 2000 mm 3 . 
     The above non-limiting examples are provided for illustrative purposes only in order to facilitate a more complete understanding of the disclosed subject matter. These examples should not be construed to limit any of the embodiments described in the present specification, including those pertaining to the antibodies, pharmaceutical compositions, or methods and uses for treating cancer, a neurodegenerative or an infectious disease. 
     
       
         
           
               
             
               
                   
               
               
                 SEQUENCES 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 SEQ ID NO: 1 - human IL-21 
               
               
                 QGQDRHMIRM RQLIDIVDQL KNYVNDLVPE FLPAPEDVET NCEWSAFSCF QKAQLKSANT 
               
               
                   
               
               
                 GNNERIINVS IKKLKRKPPS TNAGRRQKHR LTCPSCDSYE KKPPKEFLER FKSLLQKMIH 
               
               
                   
               
               
                 QHLSSRTHGS EDS 
               
               
                   
               
               
                 SEQ ID NO: 2 - IL-21 Mutein A 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                 GNNERIINVS IKKLKRKPPS TNAGRRQKHR LTCPSCDSYE KKPPKEFLER FKSLLQKMIH 
               
               
                   
               
               
                 QHLSSRTHGS EDS 
               
               
                   
               
               
                 SEQ ID NO: 3 - IL-21 Mutein B 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                 GNNERIINVS IKKLKRKPPS TNAGRRQKHR LTCPSCDSYE KKPPKEFLER FKSLLQKMIH 
               
               
                   
               
               
                 QHLSSRTHGS EDS 
               
               
                   
               
               
                 SEQ ID NO: 4 - IL-21 Mutein C 
               
               
                 QGQDRHMIRM RQLIDIVDQL KNYVNDLVPE FLPAPEDVET NCEWSAFSCF QKAQLKSANT 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                 QHLSSRTHGS EDS 
               
               
                   
               
               
                 SEQ ID NO: 5 - IL-21 Mutein D 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                 QHLSSRTHGS EDS 
               
               
                   
               
               
                 SEQ ID NO: 6 - IL-21 receptor ECD (source: uniprot.org/uniprot/Q9HBE5) 
               
               
                 CPDLVCYTDY LQTVICILEM WNLHPSTLTL TWQDQYEELK DEATSCSLHR SAHNATHATY 
               
               
                   
               
               
                 TCHMDVFHFM ADDIFSVNIT DQSGNYSQEC GSFLLAESIK PAPPFNVTVT FSGQYNISWR 
               
               
                   
               
               
                 SDYEDPAFYM LKGKLQYELQ YRNRGDPWAV SPRRKLISVD SRSVSLLPLE FRKDSSYELQ 
               
               
                   
               
               
                 VRAGPMPGSS YQGTWSEWSD PVIFQTQSEE LKE 
               
               
                   
               
               
                 SEQ ID NO: 7 - Human IL-21 Rγ ECD 
               
               
                 LNTTILTPNG NEDTTADFFL TTMPTDSLSV STLPLPEVQC FVFNVEYMNC TWNSSSEPQP 
               
               
                   
               
               
                 TNLTLHYWYK NSDNDKVQKC SHYLFSEEIT SGCQLQKKEI HLYQTFVVQL QDPREPRRQA 
               
               
                   
               
               
                 TQMLKLQNLV IPWAPENLTL HKLSESQLEL NWNNRFLNHC LEHLVQYRTD WDHSWTEQSV 
               
               
                   
               
               
                 DYRHKFSLPS VDGQKRYTFR VRSRFNPLCG SAQHWSEWSH PIHWGSNTSK ENPFLFALEA 
               
               
                   
               
               
                 SEQ ID NO: 8 - Human IL-2 
               
               
                 APTSSSTKKT QLQLEHLLLD LQMILNGINN YKNPKLTRML TFKFYMPKKA TELKHLQCLE 
               
               
                   
               
               
                 EELKPLEEVL NLAQSKNFHL RPRDLISNIN VIVLELKGSE TTFMCEYADE TATIVEFLNR 
               
               
                   
               
               
                 WITFCQSIIS TLT 
               
               
                   
               
               
                 SEQ ID NO: 9 - Human IL-15 
               
               
                 NWVNVISDLK KIEDLIQSMH IDATLYTESD VHPSCKVTAM KCFLLELQVI SLESGDASIH 
               
               
                   
               
               
                 DTVENLIILA NNSLSSNGNV TESGCKECEE LEEKNIKEFL QSFVHIVQMF INT 
               
               
                   
               
               
                 SEQ ID NO: 10 - Human IL-15 Rα sushi domain 
               
               
                 ITCPPPMSVE HADIWVKSYS LYSRERYICN SGFKRKAGTS SLTECVLNKA TNVAHWTTPS 
               
               
                   
               
               
                 LKCIRDPALV HQRPA 
               
               
                   
               
               
                 SEQ ID NO: 11-17 - MMP2/MMP9 Cleavable peptide linkers 
               
               
                 GPLGVR (SEQ ID NO: 11) 
               
               
                   
               
               
                 PLGMWSR (SEQ ID NO: 12) 
               
               
                   
               
               
                 PLGLWAR (SEQ ID NO: 13) 
               
               
                   
               
               
                 PQGIAGQR (SEQ ID NO: 14) 
               
               
                   
               
               
                 PLGLAG (SEQ ID NO: 15) 
               
               
                   
               
               
                 LALGPR (SEQ ID NO: 16) 
               
               
                   
               
               
                 GGPLGMLSQS (SEQ ID NO: 17) 
               
               
                   
               
               
                 SEQ ID NO: 18-26 - urokinase plasminogen activator (uPA) Cleavable 
               
               
                 peptide linkers 
               
               
                 GGGGRRGGS (SEQ ID NO: 18) 
               
               
                   
               
               
                 TGRGPSWV (SEQ ID NO: 19) 
               
               
                   
               
               
                 SARGPSRW (SEQ ID NO: 20) 
               
               
                   
               
               
                 TARGPSFK (SEQ ID NO: 21) 
               
               
                   
               
               
                 TARGPSW (SEQ ID NO: 22) 
               
               
                   
               
               
                 GGWHTGRN (SEQ ID NO: 23) 
               
               
                   
               
               
                 HTGRSGAL (SEQ ID NO: 24) 
               
               
                   
               
               
                 PLTGRSGG (SEQ ID NO: 25) 
               
               
                   
               
               
                 LTGRSGA (SEQ ID NO: 26) 
               
               
                   
               
               
                 SEQ ID NO: 27 - Human CCL19 
               
               
                 TNDAEDCCLS VTQKPIPGYI VRNFHYLLIK DGCRVPAVVF TTLRGRQLCA PPDQPWVERI 
               
               
                   
               
               
                 IQRLQRTSAK MKRRSS 
               
               
                   
               
               
                 SEQ ID NO: 28 - Human IL-7 
               
               
                 DCDIEGKDGK QYESVLMVSI DQLLDSMKEI GSNCLNNEFN FFKRHICDAN KEGMFLFRAA 
               
               
                   
               
               
                 RKLRQFLKMN STGDFDLHLL KVSEGTTILL NCTGQVKGRK PAALGEAQPT KSLEENKSLK 
               
               
                   
               
               
                 EQKKLNDLCF LKRLLQEIKT CWNKILMGTK EH 
               
               
                   
               
               
                 SEQ ID NO: 29-33 Peptide Linker 
               
               
                 GGGGS (SEQ ID NO: 29) 
               
               
                   
               
               
                 GGGGSGGGGS (SEQ ID NO: 30) 
               
               
                   
               
               
                 GGGGSGGGGSGGGGS (SEQ ID NO: 31) 
               
               
                   
               
               
                 GGGGSGGGGXGGGGSGGGGS (SEQ ID NO: 32), X = A or N 
               
               
                   
               
               
                 GGGGSGGGGXGGGGYGGGGS (SEQ ID NO: 33), X = S, A or N, and Y = A or N 
               
               
                   
               
               
                 SEQ ID NO: 34 - IgG1 Fc 
               
               
                 DKTHTCPPCP APELLGGPSV FLFPPKPKDT LMISRTPEVT CVVVDVSHED PEVKFNWYVD 
               
               
                   
               
               
                 GVEVHNAKTK PREEQYNSTY RVVSVLTVLH QDWLNGKEYK CKVSNKALPA PIEKTISKAK 
               
               
                   
               
               
                 GQPREPQVYT LPPSRDELTK NQVSLTCLVK GFYPSDIAVE WESNGQPENN YKTTPPVLDS 
               
               
                   
               
               
                 DGSFFLYSKL TVDKSRWQQG NVFSCSVMHE ALHNHYTQKS LSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 35 - IgG4 Fc 
               
               
                 AESKYGPPCP PCPAPEAAGG PSVFLFPPKP KDTLMISRTP EVTCVVVDVS QEDPEVQFNW 
               
               
                   
               
               
                 YVDGVEVHNA KTKPREEQFN STYRVVSVLT VLHQDWLNGK EYKCKVSNKG LPSSIEKTIS 
               
               
                   
               
               
                 KAKGQPREPQ VYTLPPSQEE MTKNQVSLTC LVKGFYPSDI AVEWESNGQP ENNYKTTPPV 
               
               
                   
               
               
                 LDSDGSFFLY SRLTVDKSRW QEGNVFSCSV MHEALHNHYT QKSLSLSLGK 
               
               
                   
               
               
                 SEQ ID NO: 36 - Fc with knobs mutations - IL-21 
               
               
                 DKTHTCPPCPAPEAAGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTK 
               
               
                   
               
               
                 PREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPPCRDELTK 
               
               
                   
               
               
                 NQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNVFSCSVMHE 
               
               
                   
               
               
                 ALHNHYTQKSLSLSPGA(GGGGS) n QGQDRHMIRMRQLIDIVDQLKNYVNDLVPEFLPAPEDVETNCEWSA 
               
               
                   
               
               
                 FSCFQKAQLKSANTGNNERIINVSIKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKKPPKEFLERFKSLLQ 
               
               
                   
               
               
                 KMIHQHLSSRTHGSEDS; wherein n = 0, 1, 2, 3, 4, or 5. 
               
               
                   
               
               
                 SEQ ID NO: 37 - Fc with knobs mutations - IL-21 mutein 
               
               
                 DKTHTCPPCPAPEAAGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTK 
               
               
                   
               
               
                 PREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPPCRDELTK 
               
               
                   
               
               
                 NQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNVFSCSVMHE 
               
               
                   
               
               
                 ALHNHYTQKSLSLSPGA(GGGGS) n QGQDRHMIRMRQLIDIVDKLKNYVNDLVPEFLPAPEDVETNCEWSA 
               
               
                   
               
               
                 FSCFQKAQLKSANTGNNERIINVSIKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKKPPKEFLRRFKSLLQ 
               
               
                   
               
               
                 KMIHQHLSSRTHGSEDS; wherein n = 0, 1, 2, 3, 4, or 5. 
               
               
                   
               
               
                 SEQ ID NO: 38 - Fc with holes mutations - IL-21 Receptor ECD 
               
               
                 DKTHTCPPCPAPEAAGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTK 
               
               
                   
               
               
                 PREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVCTLPPSRDELTK 
               
               
                   
               
               
                 NQVSLSCAVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLVSKLTVDKSRWQQGNVFSCSVMHE 
               
               
                   
               
               
                 ALHNHYTQKSLSLSPGAGGGGSGGGGSGPLGVRGGGGSCPDLVCYTDYLQTVICILEMWNLHPSTLTLTW 
               
               
                   
               
               
                 QDQYEELKDEATSCSLHRSAHNATHATYTCHMDVFHFMADDIFSVNITDQSGNYSQECGSFLLAESIKPA 
               
               
                   
               
               
                 PPFNVTVTFSGQYNISWRSDYEDPAFYMLKGKLQYELQYRNRGDPWAVSPRRKLISVDSRSVSLLPLEFR 
               
               
                   
               
               
                 KDSSYELQVRAGPMPGSSYQGTWSEWSDPVIFQTQSEELKE 
               
               
                   
               
               
                 SEQ ID NO: 39 - IL-21 - Fc with knobs mutations 
               
               
                 QGQDRHMIRMRQLIDIVDQLKNYVNDLVPEFLPAPEDVETNCEWSAFSCFQKAQLKSANTGNNERIINVS 
               
               
                   
               
               
                 IKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKKPPKEFLERFKSLLQKMIHQHLSSRTHGSEDS(GGGGS) n   
               
               
                   
               
               
                 DKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTK 
               
               
                   
               
               
                 PREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPPCRDELTK 
               
               
                   
               
               
                 NQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNVFSCSVMHE 
               
               
                   
               
               
                 ALHNHYTQKSLSLSPGK; wherein n = 0, 1, 2, 3, 4, or 5. 
               
               
                   
               
               
                 SEQ ID NO: 40 - IL-2 mutein - Fc with knobs mutations 
               
               
                 QGQDRHMIRMRQLIDIVDKLKNYVNDLVPEFLPAPEDVETNCEWSAFSCFQKAQLKSANTGNNERIINVS 
               
               
                   
               
               
                 IKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKKPPKEFLRRFKSLLQKMIHQHLSSRTHGSEDS(GGGGS) n   
               
               
                   
               
               
                 DKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTK 
               
               
                   
               
               
                 PREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPPCRDELTK 
               
               
                   
               
               
                 NQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNVFSCSVMHE 
               
               
                   
               
               
                 ALHNHYTQKSLSLSPGK; wherein n = 0, 1, 2, 3, 4, or 5. 
               
               
                   
               
               
                 SEQ ID NO: 41 - IL-21Rα ECD - Fc with holes mutations 
               
               
                 CPDLVCYTDY LQTVICILEM WNLHPSTLTL TWQDQYEELK DEATSCSLHR SAHNATHATY 
               
               
                   
               
               
                 TCHMDVFHFM ADDIFSVNIT DQSGNYSQEC GSFLLAESIK PAPPFNVTVT FSGQYNISWR 
               
               
                   
               
               
                 SDYEDPAFYM LKGKLQYELQ YRNRGDPWAV SPRRKLISVD SRSVSLLPLE FRKDSSYELQ 
               
               
                   
               
               
                 VRAGPMPGSS YQGTWSEWSD PVIFQTQSEE LKEGGGGSGG GGSGPLGVRG GGGSDKTHTC 
               
               
                   
               
               
                 PPCPAPELLG GPSVFLFPPK PKDTLMISRT PEVTCVVVDV SHEDPEVKFN WYVDGVEVHN 
               
               
                   
               
               
                 AKTKPREEQY NSTYRVVSVL TVLHQDWLNG KEYKCKVSNK ALPAPIEKTI SKAKGQPREP 
               
               
                   
               
               
                 QVCTLPPSRD ELTKNQVSLS CAVKGFYPSD IAVEWESNGQ PENNYKTTPP VLDSDGSFFL 
               
               
                   
               
               
                 VSKLTVDKSR WQQGNVFSCS VMHEALHNHY TQKSLSLSPG K 
               
               
                   
               
               
                 SEQ ID NO: 42 - IL-15-Sushi - Fc with knobs mutations - IL-21 
               
               
                 NWVNVISDLKKIEDLIQSMHIDATLYTESDVHPSCKVTAMKCFLLELQVISLESGDASIHDTVENLIILA 
               
               
                   
               
               
                 NNSLSSNGNVTESGCKECEELEEKNIKEFLQSFVHIVQMFINT (GGGGS) n1   
               
               
                   
               
               
                 ITCPPPMSVEHADIWVKSYSLYSRERYICNSGFKRKAGTSSLTECVLNKATNVAHWTTPSLKCIRDPALV 
               
               
                   
               
               
                 HQRPA(GGGGS) n2 DKTHTCPPCPAPEAAGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNW 
               
               
                   
               
               
                 YVDGVEVHNAKTKPREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQ 
               
               
                   
               
               
                 VYTLPPCRDELTKNQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRW 
               
               
                   
               
               
                 QQGNVFSCSVMHEALHNHYTQKSLSLSPGA(GGGGS) n3 QGQDRHMIRMRQLIDIVDQLKNYVNDLVPEFL 
               
               
                   
               
               
                 PAPEDVETNCEWSAFSCFQKAQLKSANTGNNERIINVSIKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKK 
               
               
                   
               
               
                 PPKEFLERFKSLLQKMIHQHLSSRTHGSEDS; wherein n1 = 0, 1, 2, 3, 4, or 5; n2 = 
               
               
                   
               
               
                 0, 1, 2, 3, 4, or 5; and n3 = 0, 1, 2, 3, 4, or 5. 
               
               
                   
               
               
                 SEQ ID NO: 43 - IL-2 analog - Fc with knobs mutations - IL-21 mutein 
               
               
                 APASSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTSMLTKKFAMPKKATELKHLQCLEEALKPLEEVL 
               
               
                   
               
               
                 NLTQSKNFHLRPRDLISNINVIVLELKGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT 
               
               
                   
               
               
                 (GGGGS) n1 DKTHTCPPCPAPEAAGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGV 
               
               
                   
               
               
                 EVHNAKTKPREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLP 
               
               
                   
               
               
                 PCRDELTKNQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNV 
               
               
                   
               
               
                 FSCSVMHEALHNHYTQKSLSLSPGA(GGGGS) n2 QGQDRHMIRMRQLIDIVDKLKNYVNDLVPEFLPAPED 
               
               
                   
               
               
                 VETNCEWSAFSCFQKAQLKSANTGNNERIINVSIKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKKPPKEF 
               
               
                   
               
               
                 LRRFKSLLQKMIHQHLSSRTHGSEDS; wherein n1 = 0, 1, 2, 3, 4, or 5; and n2 = 
               
               
                   
               
               
                 0, 1, 2, 3, 4, or 5. 
               
               
                   
               
               
                 SEQ ID NO: 44 - IL-2Rβ ECD - Fc with holes mutations - IL-21Rα ECD 
               
               
                 AVNGTSQFTCFYNSRANISCVWSQDGALQDTSCQVHAWPDRRRWNQTCELLPVSQASWACNLILGAPDSQ 
               
               
                   
               
               
                 KLTTVDIVTLRVLCREGVRWRVMAIQDFKPFENLRLMAPISLQVVHVETHRCNISWEISQASHYFERHLE 
               
               
                   
               
               
                 FEARTLSPGHTWEEAPLLTLKQKQEWICLETLTPDTQYEFQVRVKPLQGEFTTWSPWSQPLAFRTKPAAL 
               
               
                   
               
               
                 GKDTGGGGSGGGGSGPLGVRGGGGSDKTHTCPPCPAPEAAGGPSVFLFPPKPKDTLMISRTPEVTCVVVD 
               
               
                   
               
               
                 VSHEDPEVKFNWYVDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKT 
               
               
                   
               
               
                 ISKAKGQPREPQVCTLPPSRDELTKNQVSLSCAVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFF 
               
               
                   
               
               
                 LVSKLTVDKSRWQQGNVFSCSVMHEALHNHYTQKSLSLSPGAGGGGSGGGGSGPLGVRGGGGSCPDLVCY 
               
               
                   
               
               
                 TDY LQTVICILEM WNLHPSTLTL TWQDQYEELK DEATSCSLHR SAHNATHATY TCHMDVFHFM 
               
               
                   
               
               
                 ADDIFSVNIT DQSGNYSQEC GSFLLAESIK PAPPFNVTVT FSGQYNISWR SDYEDPAFYM 
               
               
                   
               
               
                 LKGKLQYELQ YRNRGDPWAV SPRRKLISVD SRSVSLLPLE FRKDSSYELQ VRAGPMPGSS 
               
               
                   
               
               
                 YQGTWSEWSD PVIFQTQSEE LKE 
               
               
                   
               
               
                 SEQ ID NO: 45 - IL-21 - Fc with knobs mutations - Sushi-IL-15 
               
               
                 QGQDRHMIRM RQLIDIVDQL KNYVNDLVPE FLPAPEDVET NCEWSAFSCF QKAQLKSANT 
               
               
                   
               
               
                 GNNERIINVS IKKLKRKPPS TNAGRRQKHR LTCPSCDSYE KKPPKEFLER FKSLLQKMIH 
               
               
                   
               
               
                 QHLSSRTHGSEDS(GGGGS) n DKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHED 
               
               
                   
               
               
                 PEVKFNWYVDGVEVHNAKTKPREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAK 
               
               
                   
               
               
                 GQPREPQVYTLPPCRDELTKNQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKL 
               
               
                   
               
               
                 TVDKSRWQQGNVFSCSVMHEALHNHYTQKSLSLSPGA(GGGGS) n2  ITCPPPMSVE HADIWVKSYS 
               
               
                   
               
               
                 LYSRERYICNSGFKRKAGTSSLTECVLNKATNVAHWTTPSLKCIRDPALVHQRPA(GGGGS) n3 NWVNVIS 
               
               
                   
               
               
                 DLKKIEDLIQSMHIDATLYTESDVHPSCKVTAMKCFLLELQVISLESGDASIHDTVENLIILANNSLSSN 
               
               
                   
               
               
                 GNVTESGCKECEELEEKNIKEFLQSFVHIVQMFINT; wherein n1 = 0, 1, 2, 3, 4, or 
               
               
                   
               
               
                 5; n2 = 0, 1, 2, 3, 4, or 5; and n3 = 0, 1, 2, 3, 4, or 5. 
               
               
                   
               
               
                 SEQ ID NO: 46 - IL-21 mutein - Fc with knobs mutations - IL-2 analog 
               
               
                 QGQDRHMIRM RQLIDIVDKL KNYVNDLVPE FLPAPEDVET NCEWSAFSCF QKAQLKSANT 
               
               
                   
               
               
                 GNNERIINVS IKKLKRKPPS TNAGRRQKHR LTCPSCDSYE KKPPKEFLRR FKSLLQKMIH 
               
               
                   
               
               
                 QHLSSRTHGSEDS(GGGGS) n1 DKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHE 
               
               
                   
               
               
                 DPEVKFNWYVDGVEVHNAKTKPREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKA 
               
               
                   
               
               
                 KGQPREPQVYTLPPCRDELTKNQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSK 
               
               
                   
               
               
                 LTVDKSRWQQGNVFSCSVMHEALHNHYTQKSLSLSPGA(GGGGS) n2 APASSSTKKTQLQLEHLLLDLQMI 
               
               
                   
               
               
                 LNGINNYKNPKLTSMLTKKFAMPKKATELKHLQCLEEALKPLEEVLNLTQSKNFHLRPRDLISNINVIVL 
               
               
                   
               
               
                 ELKGSETTFMCEYADETATIVEFLNRWITFSQSIISTLT; wherein n1 = 0, 1, 2, 3, 4, 
               
               
                   
               
               
                 or 5; and n2 = 0, 1, 2, 3, 4, or 5. 
               
               
                   
               
               
                 SEQ ID NO: 47 - IL-21Rα ECD - Fc with holes mutations - IL-2Rβ ECD 
               
               
                 CPDLVCYTDY LQTVICILEM WNLHPSTLTL TWQDQYEELK DEATSCSLHR SAHNATHATY 
               
               
                   
               
               
                 TCHMDVFHFM ADDIFSVNIT DQSGNYSQEC GSFLLAESIK PAPPFNVTVT FSGQYNISWR 
               
               
                   
               
               
                 SDYEDPAFYM LKGKLQYELQ YRNRGDPWAV SPRRKLISVD SRSVSLLPLE FRKDSSYELQ 
               
               
                   
               
               
                 VRAGPMPGSSYQGTWSEWSDPVIFQTQSEELKEGGGGSGGGGSGPLGVRGGGGSDKTHTCPPCPAPELLG 
               
               
                   
               
               
                 GPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTKPREEQYNSTYRVVSVL 
               
               
                   
               
               
                 TVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVCTLPPSRDELTKNQVSLSCAVKGFYPSD 
               
               
                   
               
               
                 IAVEWESNGQPENNYKTTPPVLDSDGSFFLVSKLTVDKSRWQQGNVFSCSVMHEALHNHYTQKSLSLSPG 
               
               
                   
               
               
                 AGGGGSGGGGSGPLGVRGGGGSAVNGTSQFTCFYNSRANISCVWSQDGALQDTSCQVHAWPDRRRWNQTC 
               
               
                   
               
               
                 ELLPVSQASWACNLILGAPDSQKLTTVDIVTLRVLCREGVRWRVMAIQDFKPFENLRLMAPISLQVVHVE 
               
               
                   
               
               
                 THRCNISWEISQASHYFERHLEFEARTLSPGHTWEEAPLLTLKQKQEWICLETLTPDTQYEFQVRVKPLQ 
               
               
                   
               
               
                 GEFTTWSPWSQPLAFRTKPAALGKDT 
               
               
                   
               
               
                 SEQ ID NO: 48 - PD1-HC with knob mutations - IL-21 
               
               
                 QVQLVESGGGVVQPGRSLRLDCKASGITFSNSGMHWVRQAPGKGLEWVAVIWYDGSKRYYADSVKGRFTI 
               
               
                   
               
               
                 SRDNSKNTLFLQMNSLRAEDTAVYYCATNDDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGC 
               
               
                   
               
               
                 LVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNTKVDKR 
               
               
                   
               
               
                 VESKYGPPCPPCPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSQEDPEVQFNWYVDGVEVHNA 
               
               
                   
               
               
                 KTKPREEQFNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPCQEE 
               
               
                   
               
               
                 MTKNQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFSCSV 
               
               
                   
               
               
                 MHEALHNHYTQKSLSLSLGAGGGGSGGGGSGGGGSQGQDRHMIRMRQLIDIVDQLKNYVNDLVPEFLPAP 
               
               
                   
               
               
                 EDVETNCEWSAFSCFQKAQLKSANTGNNERIINVSIKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKKPPK 
               
               
                   
               
               
                 EFLERFKSLLQKMIHQHLSSRTHGSEDS 
               
               
                   
               
               
                 SEQ ID NO: 49 - PD1-HC with hole mutations - IL-21Rα ECD 
               
               
                 QVQLVESGGGVVQPGRSLRLDCKASGITFSNSGMHWVRQAPGKGLEWVAVIWYDGSKRYYADSVKGRFTI 
               
               
                   
               
               
                 SRDNSKNTLFLQMNSLRAEDTAVYYCATNDDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGC 
               
               
                   
               
               
                 LVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNTKVDKR 
               
               
                   
               
               
                 VESKYGPPCPPCPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSQEDPEVQFNWYVDGVEVHNA 
               
               
                   
               
               
                 KTKPREEQFNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKGLPSSIEKTISKAKGQPREPQVCTLPPSQEE 
               
               
                   
               
               
                 MTKNQVSLSCAVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLVSRLTVDKSRWQEGNVFSCSV 
               
               
                   
               
               
                 MHEALHNHYTQKSLSLSLGAGGGGSGGGGSGPLGVRGGGGSCPDLVCYTDYLQTVICILEMWNLHPSTLT 
               
               
                   
               
               
                 LTWQDQYEELKDEATSCSLHRSAHNATHATYTCHMDVFHFMADDIFSVNITDQSGNYSQECGSFLLAESI 
               
               
                   
               
               
                 KPAPPFNVTVTFSGQYNISWRSDYEDPAFYMLKGKLQYELQYRNRGDPWAVSPRRKLISVDSRSVSLLPL 
               
               
                   
               
               
                 EFRKDSSYELQVRAGPMPGSSYQGTWSEWSDPVIFQTQSEELKE 
               
               
                   
               
               
                 SEQ ID NO: 50 - PD1-LC 
               
               
                 EIVLTQSPATLSLSPGERATLSCRASQSVSSYLAWYQQKPGQAPRLLIYDASNRATGIPARFSGSGSGTD 
               
               
                   
               
               
                 FTLTISSLEPEDFAVYYCQQSSNWPRTFGQGTKVEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFY 
               
               
                   
               
               
                 PREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFN 
               
               
                   
               
               
                 RGEC 
               
               
                   
               
               
                 SEQ ID NO: 51 - PD1-LC - IL-2 basal 
               
               
                 EIVLTQSPATLSLSPGERATLSCRASQSVSSYLAWYQQKPGQAPRLLIYDASNRATGIPARFSGSGSGTD 
               
               
                   
               
               
                 FTLTISSLEPEDFAVYYCQQSSNWPRTFGQGTKVEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFY 
               
               
                   
               
               
                 PREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFN 
               
               
                   
               
               
                 RGECGGGGSGGGGSGGGGSAPASSSTKKTQLQLEHLLLDLQMILNGINNYKNPKLTSMLTAKFAMPKKAT 
               
               
                   
               
               
                 ELKHLQCLEEALKPLEEVLNLAQSKNFHLRPRDLISX aa88 INVIVLELKGSETTFMCEYADETATIVEFLN 
               
               
                   
               
               
                 RWITFSX aa126 SIISTLT; wherein X aa88  is amino acid selected from N and A, 
               
               
                   
               
               
                 and X aa126  is an amino acid selected from Q, H, W or A. 
               
               
                   
               
               
                 SEQ ID NO: 52 - trastuzumab light chain 
               
               
                 DIQMTQSPSSLSASVGDRVTITCRASQDVNTAVAWYQQKPGKAPKLLIYSASFLYSGVPSRFSGSRSGTD 
               
               
                   
               
               
                 FTLTISSLQPEDFATYYCQQHYTTPPTFGQGTKVEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFY 
               
               
                   
               
               
                 PREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFN 
               
               
                   
               
               
                 RGEC 
               
               
                   
               
               
                 SEQ ID NO: 53 - trastuzumab heavy chain 
               
               
                 EVQLVESGGGLVQPGGSLRLSCAASGFNIKDTYIHWVRQAPGKGLEWVARIYPTNGYTRYADSVKGRFTI 
               
               
                   
               
               
                 SADTSKNTAYLQMNSLRAEDTAVYYCSRWGGDGFYAMDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSG 
               
               
                   
               
               
                 GTAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPS 
               
               
                   
               
               
                 NTKVDKKVEPPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFN 
               
               
                   
               
               
                 WYVDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREP 
               
               
                   
               
               
                 QVYTLPPSRDELTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSR 
               
               
                   
               
               
                 WQQGNVFSCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 54 - rituximab light chain 
               
               
                 QIVLSQSPAILSASPGEKVTMTCRASSSVSYIHWFQQKPGSSPKPWIYATSNLASGVPVRFSGSGSGTSY 
               
               
                   
               
               
                 SLTISRVEAEDAATYYCQQWTSNPPTFGGGTKLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYP 
               
               
                   
               
               
                 REAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNR 
               
               
                   
               
               
                 GEC 
               
               
                   
               
               
                 SEQ ID NO: 55 - rituximab heavy chain 
               
               
                 QVQLQQPGAELVKPGASVKMSCKASGYTFTSYNMHWVKQTPGRGLEWIGAIYPGNGDTSYNQKFKGKATL 
               
               
                   
               
               
                 TADKSSSTAYMQLSSLTSEDSAVYYCARSTYYGGDWYFNVWGAGTTVTVSAASTKGPSVFPLAPSSKSTS 
               
               
                   
               
               
                 GGTAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKP 
               
               
                   
               
               
                 SNTKVDKKAEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFN 
               
               
                   
               
               
                 WYVDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREP 
               
               
                   
               
               
                 QVYTLPPSRDELTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSR 
               
               
                   
               
               
                 WQQGNVFSCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 56 - brentuximab light chain 
               
               
                 DIVLTQSPASLAVSLGQRATISCKASQSVDFDGDSYMNWYQQKPGQPPKVLIYAASNLESGIPARFSGSG 
               
               
                   
               
               
                 SGTDFTLNIHPVEEEDAATYYCQQSNEDPWTFGGGTKLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLL 
               
               
                   
               
               
                 NNFYPREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVT 
               
               
                   
               
               
                 KSFNRGEC 
               
               
                   
               
               
                 SEQ ID NO: 57 - brentuximab heavy chain 
               
               
                 QIQLQQSGPEVVKPGASVKISCKASGYTFTDYYITWVKQKPGQGLEWIGWIYPGSGMTKYNEKFKGKATL 
               
               
                   
               
               
                 TVDTSSSTAFMQLSSLTSEDTAVYFCANYGNYWFAYWGQGTQVTVSAASTKGPSVFPLAPSSKSTSGGTA 
               
               
                   
               
               
                 ALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNTK 
               
               
                   
               
               
                 VDKKVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVD 
               
               
                   
               
               
                 GVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYT 
               
               
                   
               
               
                 LPPSRDELTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQG 
               
               
                   
               
               
                 NVFSCSVMHEALHNHYTQKSLSLSPG 
               
               
                   
               
               
                 SEQ ID NO: 58 - cetuximab light chain 
               
               
                 DILLTQSPVILSVSPGERVSFSCRASQSIGTNIHWYQQRTNGSPRLLIKYASESISGIPSRFSGSGSGTD 
               
               
                   
               
               
                 FTLSINSVESEDIADYYCQQNNNWPTTFGAGTKLELKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFY 
               
               
                   
               
               
                 PREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFN 
               
               
                   
               
               
                 RGEC 
               
               
                   
               
               
                 SEQ ID NO: 59 - cetuximab heavy chain 
               
               
                 QVQLKQSGPGLVQPSQSLSITCTVSGFSLTNYGVHWVRQSPGKGLEWLGVIWSGGNTDYNTPFTSRLSIN 
               
               
                   
               
               
                 KDNSKSQVFFKMNSLQSNDTAIYYCARALTYYDYEFAYWGQGTLVTVSAASTKGPSVFPLAPSSKSTSGG 
               
               
                   
               
               
                 TAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSN 
               
               
                   
               
               
                 TKVDKKVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWY 
               
               
                   
               
               
                 VDGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQV 
               
               
                   
               
               
                 YTLPPSRDELTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQ 
               
               
                   
               
               
                 QGNVFSCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 60 - panitumumab light chain 
               
               
                 DIQMTQSPSSLSASVGDRVTITCQASQDISNYLNWYQQKPGKAPKLLIYDASNLETGVPSRFSGSGSGTD 
               
               
                   
               
               
                 FTFTISSLQPEDIATYFCQHFDHLPLAFGGGTKVEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFY 
               
               
                   
               
               
                 PREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFN 
               
               
                   
               
               
                 RGEC 
               
               
                   
               
               
                 SEQ ID NO: 61 - panitumumab heavy chain 
               
               
                 GHIYYSGNTNYNPSLKSRLTISIDTSKTQFSLKLSSVTAADTAIYYCVRDRVTGAFDIWGQGTMVTVSSA 
               
               
                   
               
               
                 STKGPSVFPLAPCSRSTSESTAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTV 
               
               
                   
               
               
                 PSSNFGTQTYTCNVDHKPSNTKVDERKCCVECPAGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPE 
               
               
                   
               
               
                 VQFNWYVDGVEVHNAKTKPREEQFNSTFRVVSVLTVVHQDWLNGKEYKCKVSNKGLPAPIEKTISKTKGQ 
               
               
                   
               
               
                 PREPQVYTLPPSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPMLDSDGSFFLYSKLTV 
               
               
                   
               
               
                 DKSRWQQGNVFSCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 62 - anti-c-MET antibody light chain 
               
               
                 DIVMTQAAPSVPVTPGESVSISCRSSKSLLHSNGNTYLYWFLQRPGQSPQVLIYRMSNLASGVPDRFSGS 
               
               
                   
               
               
                 GSGTAFTLRIRRVEAEDVGVYYCMQNLEYPFTFGGGTKLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCL 
               
               
                   
               
               
                 LNNFYPREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPV 
               
               
                   
               
               
                 TKSFNRGEC 
               
               
                   
               
               
                 SEQ ID NO: 63 - anti-c-MET antibody heavy chain 
               
               
                 QVQLQQSGPELVKSGASVKMSCKASGNTLKDDHVHWVKQRPGQGLEWIGWIYPGGGRTRYNEKFKGKTTL 
               
               
                   
               
               
                 TADKPSSTVNMLLSSLTSEDSAIYFCTNLVFDVWGAGTTVTVSSASTKGPSVFPLAPSSKSTSGGTAALG 
               
               
                   
               
               
                 CLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNTKVDK 
               
               
                   
               
               
                 KVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVE 
               
               
                   
               
               
                 VHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPP 
               
               
                   
               
               
                 SREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNVF 
               
               
                   
               
               
                 SCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 64 - anti-GPC3 antibody light chain 
               
               
                 DVVMTQSPLSLPVTPGEPASISCRSSQSLVHSNANTYLHWYLQKPGQSPQLLIYKVSNRFSGVPDRFSGS 
               
               
                   
               
               
                 GSGTDFTLKISRVEAEDVGVYYCSQNTHVPPTFGQGTKLEIKRTVAAPSVFIFPPSDEQLKSGTASVVCL 
               
               
                   
               
               
                 LNNFYPREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPV 
               
               
                   
               
               
                 TKSFNRGEC 
               
               
                   
               
               
                 SEQ ID NO: 65 - anti-GPC3 antibody heavy chain 
               
               
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTDYEMHWVRQAPGQGLEWMGALDPKTGDTAYSQKFKGRVTL 
               
               
                   
               
               
                 TADKSTSTAYMELSSLTSEDTAVYYCTRFYSYTYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTAAL 
               
               
                   
               
               
                 GCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNTKVD 
               
               
                   
               
               
                 KKVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGV 
               
               
                   
               
               
                 EVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLP 
               
               
                   
               
               
                 PSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNV 
               
               
                   
               
               
                 FSCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 66 - anti-Claudin 18.2 antibody light chain 
               
               
                 DIVMTQSPSSLTVTAGEKVTMSCKSSQSLLNSGNQKNYLTWYQQKPGQPPKLLIYWASTRESGVPDRFTG 
               
               
                   
               
               
                 SGSGTDFTLTISSVQAEDLAVYYCQNDYSYPFTFGSGTKLEIKRTVAAPSVFIFPPSDEQLKSGTASVVC 
               
               
                   
               
               
                 LLNNFYPREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSP 
               
               
                   
               
               
                 VTKSFNRGEC 
               
               
                   
               
               
                 SEQ ID NO: 67 - anti-Claudin 18.2 antibody heavy chain 
               
               
                 QVQLQQPGAELVRPGASVKLSCKASGYTFTSYWINWVKQRPGQGLEWIGNIYPSDSYTNYNQKFKDKATL 
               
               
                   
               
               
                 TVDKSSSTAYMQLSSPTSEDSAVYYCTRSWRGNSFDYWGQGTTLTVSSASTKGPSVFPLAPSSKSTSGGT 
               
               
                   
               
               
                 AALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT 
               
               
                   
               
               
                 KVDKKVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYV 
               
               
                   
               
               
                 DGVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVY 
               
               
                   
               
               
                 TLPPSRDELTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQ 
               
               
                   
               
               
                 GNVFSCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 68 - anti-Trop-2 antibody light chain CDR1 
               
               
                 KASQDVSIAVA 
               
               
                   
               
               
                 SEQ ID NO: 69 - anti-Trop-2 antibody light chain CDR2 
               
               
                 SASYRYT 
               
               
                   
               
               
                 SEQ ID NO: 70 - anti-Trop-2 antibody light chain CDR3 
               
               
                 QQHYITPLT 
               
               
                   
               
               
                 SEQ ID NO: 71 - anti-Trop-2 antibody heavy chain CDR1 
               
               
                 NYGMN 
               
               
                   
               
               
                 SEQ ID NO: 72 - anti-Trop-2 antibody heavy chain CDR2 
               
               
                 WINTYTGEPTYTDDFKG 
               
               
                   
               
               
                 SEQ ID NO: 73 - anti-Trop-2 antibody heavy chain CDR3 
               
               
                 GGFGSSYWYFDV 
               
               
                   
               
               
                 SEQ ID NO: 74 - anti-mesothelin antibody light chain CDR1 
               
               
                 SASSSVSYMH 
               
               
                   
               
               
                 SEQ ID NO: 75 - anti-mesothelin antibody light chain CDR2 
               
               
                 DTSKLAS 
               
               
                   
               
               
                 SEQ ID NO: 76 - anti-mesothelin antibody light chain CDR3 
               
               
                 QQWSGYPLT 
               
               
                   
               
               
                 SEQ ID NO: 77 - anti-mesothelin antibody heavy chain CDR1 
               
               
                 GYTMN 
               
               
                   
               
               
                 SEQ ID NO: 78 - anti-mesothelin antibody heavy chain CDR2 
               
               
                 LITPYNGASSYNQKFRG 
               
               
                   
               
               
                 SEQ ID NO: 79 - anti-mesothelin antibody heavy chain CDR3 
               
               
                 GGYDGRGFDY 
               
               
                   
               
               
                 SEQ ID NO: 80 - Anti-FAP version 1 LC 
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                 TLTISSLEPEDFAVYYCQQWSFNPPTFGQGTKVEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYP 
               
               
                   
               
               
                 REAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNR 
               
               
                   
               
               
                 GEC 
               
               
                   
               
               
                 SEQ ID NO: 81 -Anti-FAP LC version 2 
               
               
                 QIVLTQSPATLSLSPGERATLSCSASSGVNFMHWYQQKPGQAPKRLIFDTSKLASGVPARFSGSGSGTDY 
               
               
                   
               
               
                 TLTISSLEPEDFAVYYCQQWSFNPPTFGQGTKVEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLLNNFYP 
               
               
                   
               
               
                 REAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVTKSFNR 
               
               
                   
               
               
                 GEC 
               
               
                   
               
               
                 SEQ ID NO: 82 - Anti-FAP VH 
               
               
                 QVQLVQSGAEVKKPGASVKVSCKASGYTFTNNGINWLRQAPGQGLEWMGEIYPRSTNTLYAQKFQGRVTI 
               
               
                   
               
               
                 TADRSSNTAYMELSSLRSEDTAVYFCARTLTAPFAFWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGTA 
               
               
                   
               
               
                 ALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNTK 
               
               
                   
               
               
                 VDKKVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVD 
               
               
                   
               
               
                 GVEVHNAKTKPREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYT 
               
               
                   
               
               
                 LPPSRDELTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQG 
               
               
                   
               
               
                 NVFSCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 83 - Humanized Light Chain variable domain of FAPalpha 
               
               
                 antibody BIBH1 
               
               
                 DIVMTQSPDS LAVSLGERAT INCKSSQSLL YSRNQKNYLA WYQQKPGQPP KLLIFWASTR 
               
               
                   
               
               
                 ESGVPDRFSG SGFGTDFTLT ISSLQAEDVA VYYCQQYFSY PLTFGQGTKV EIK 
               
               
                   
               
               
                 SEQ ID NO: 84 - Humanized Heavy Chain variable domain FAPalpha 
               
               
                 antibody BIBH1 
               
               
                 QVQLVQSGAE VKKPGASVK VSCKTSRYTFT EYTIHWVRQA PGQRLEWIGG INPNNGIPNY 
               
               
                   
               
               
                 NQKFKGRVTI TVDTSASTAY MELSSLRSED TAVYYCARRR IAYGYDEGHA MDYWGQGTLV TVSS 
               
               
                   
               
               
                 SEQ ID NO: 85 - Humanized H8 anti-5T4 version 1 VH 
               
               
                 QVQLVQSGAEVKKPGASVKVSCKASGYSFTGYYMHWVKQSPGQGLEWIGRINPNNGV 
               
               
                   
               
               
                 TLYNQKFKDRVTMTRDTSISTAYMELSRLRSDDTAVYYCARSTMITNYVMDYWGQGT 
               
               
                   
               
               
                 LWTVSS 
               
               
                   
               
               
                 SEQ ID NO: 86 - Humanized H8 anti-5T4 VH version 2 
               
               
                 QVQLVQSGAEVKKPGASVKVSCKASGYSFTGYYMHWVRQAPGQGLEWMGRINPNNGVTLYNQKFKDRVTM 
               
               
                   
               
               
                 TRDTSISTAYMELSRLRSDDTAVYYCARSTMITNYVMDYWGQGTLVTVSS 
               
               
                   
               
               
                 SEQ ID NO:87 - Humanized H8 anti-5T4 version 1 VL 
               
               
                 DIVMTQSPDSLAVSLGERATINCKASQSVSNDVAWYQQKPGQSPKLLISYTSSRYAG 
               
               
                   
               
               
                 VPDRFSGSGSGTDFTLTISSLQAEDVAVYFCQQDYNSPPTFGGGTKLEIK 
               
               
                   
               
               
                 SEQ ID NO: 88 - Humanized H8 anti-5T4 VL version 2 
               
               
                 DIVMTQSPDSLAVSLGERATINCKASQSVSNDVAWYQQKPGQPPKLLIYYTSSRYAG 
               
               
                   
               
               
                 VPDRFSGSGSGTDFTLTISSLQAEDVAVYYCQQDYNSPPTFGGGTKLEIK 
               
               
                   
               
               
                 SEQ ID NO: 89 - Anti-PDL1 atezolizumab LC 
               
               
                 DIQMTQSPSSLSASVGDRVTITCRASQDVSTAVAWYQQKPGKAPKLLIYSASFLYSGVPS 
               
               
                   
               
               
                 RFSGSGSGTDFTLTISSLQPEDFATYYCQQYLYHPATFGQGTKVEIKRTVAAPSVFIFPP 
               
               
                   
               
               
                 SDEQLKSGTASVVCLLNNFYPREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHK 
               
               
                   
               
               
                 VYACEVTHQGLSSPVTKSFNRGEC 
               
               
                   
               
               
                 SEQ ID NO: 90 - Anti-PDL1 atezolizumab HC 
               
               
                 EVQLVESGGGLVQPGGSLRLSCAASGFTFSDSWIHWVRQAPGKGLEWVAWISPYGGSTYYADSVKGRFTI 
               
               
                   
               
               
                 SADTSKNTAYLQMNSLRAEDTAVYYCARRHWPGGFDYWGQGTLVTVSSASTKGPSVFPLAPSSKSTSGGT 
               
               
                   
               
               
                 AALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTQTYICNVNHKPSNT 
               
               
                   
               
               
                 KVDKKVEPKSCDKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYV 
               
               
                   
               
               
                 DGVEVHNAKTKPREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVY 
               
               
                   
               
               
                 TLPPSREEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQ 
               
               
                   
               
               
                 GNVFSCSVMHEALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 91 - Anti-PD-1 Nivolumab HC 
               
               
                 QVQLVESGGGVVQPGRSLRLDCKASGITFSNSGMHWVRQAPGKGLEWVAVIWYDGSKRYYADSVKGRFTI 
               
               
                   
               
               
                 SRDNSKNTLFLQMNSLRAEDTAVYYCATNDDYWGQGTLVTVSSASTKGPSVFPLAPCSRSTSESTAALGC 
               
               
                   
               
               
                 LVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPSNTKVDKR 
               
               
                   
               
               
                 VESKYGPPCPPCPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSQEDPEVQFNWYVDGVEVHNA 
               
               
                   
               
               
                 KTKPREEQFNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKGLPSSIEKTISKAKGQPREPQVYTLPPSQEE 
               
               
                   
               
               
                 MTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEGNVFSCSV 
               
               
                   
               
               
                 MHEALHNHYTQKSLSLSLGK 
               
               
                   
               
               
                 SEQ ID NO: 92 - Anti-PD-1 Pembrolizumab LC 
               
               
                 EIVLTQSPATLSLSPGERATLSCRASKGVSTSGYSYLHWYQQKPGQAPRLLIYLASYLESGVPARFSGSG 
               
               
                   
               
               
                 SGTDFTLTISSLEPEDFAVYYCQHSRDLPLTFGGGTKVEIKRTVAAPSVFIFPPSDEQLKSGTASVVCLL 
               
               
                   
               
               
                 NNFYPREAKVQWKVDNALQSGNSQESVTEQDSKDSTYSLSSTLTLSKADYEKHKVYACEVTHQGLSSPVT 
               
               
                   
               
               
                 KSFNRGEC 
               
               
                   
               
               
                 SEQ ID NO: 93 - Anti-PD-1 Pembrolizumab HC 
               
               
                 QVQLVQSGVEVKKPGASVKVSCKASGYTFTNYYMYWVRQAPGQGLEWMGGINPSNGGTNFNEKFKNRVTL 
               
               
                   
               
               
                 TTDSSTTTAYMELKSLQFDDTAVYYCARRDYRFDMGFDYWGQGTTVTVSSASTKGPSVFPLAPCSRSTSE 
               
               
                   
               
               
                 STAALGCLVKDYFPEPVTVSWNSGALTSGVHTFPAVLQSSGLYSLSSVVTVPSSSLGTKTYTCNVDHKPS 
               
               
                   
               
               
                 NTKVDKRVESKYGPPCPPCPAPEFLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSQEDPEVQFNWYVD 
               
               
                   
               
               
                 GVEVHNAKTKPREEQFNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKGLPSSIEKTISKAKGQPREPQVYT 
               
               
                   
               
               
                 LPPSQEEMTKNQVSLTCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSRLTVDKSRWQEG 
               
               
                   
               
               
                 NVFSCSVMHEALHNHYTQKSLSLSLGK 
               
               
                   
               
               
                 SEQ ID NO: 94 - Fc with knobs mutations - IL-21 
               
               
                 DKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTK 
               
               
                   
               
               
                 PREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPPCRDELTK 
               
               
                   
               
               
                 NQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKSRWQQGNVFSCSVMHE 
               
               
                   
               
               
                 ALHNHYTQKSLSLSPGAGGGGSGGGGSGGGGSQGQDRHMIRMRQLIDIVDQLKNYVNDLVPEFLPAPEDV 
               
               
                   
               
               
                 ETNCEWSAFSCFQKAQLKSANTGNNERIINVSIKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKKPPKEFL 
               
               
                   
               
               
                 ERFKSLLQKMIHQHLSSRTHGSEDS 
               
               
                   
               
               
                 SEQ ID NO: 95 - Fc with knob mutations - IL-21 mutein Q19K/E109R 
               
               
                 DKTHTCPPCPAPELLGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTK 
               
               
                   
               
               
                 PREEQYASTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVYTLPPCRDELTK 
               
               
                   
               
               
                 NQVSLWCLVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGSFFLYSKLTVDKERWQQGNVFSCSVMHE 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                 ETNCEWSAFSCFQKAQLKSANTGNNERIINVSIKKLKRKPPSTNAGRRQKHRLTCPSCDSYEKKPPKEFL 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 SEQ ID NO: 96 - IgG1 Fc with holes mutations 
               
               
                 DKTHTCPPCPAPEAAGGPSVFLFPPKPKDTLMISRTPEVTCVVVDVSHEDPEVKFNWYVDGVEVHNAKTK 
               
               
                   
               
               
                 PREEQYNSTYRVVSVLTVLHQDWLNGKEYKCKVSNKALPAPIEKTISKAKGQPREPQVCTLPPSRDELTK 
               
               
                   
               
               
                 NQVSLSCAVKGFYPSDIAVEWESNGQPENNYKTTPPVLDSDGEFFLVSKLTVDKSRWQQGNVFSCSVMHE 
               
               
                   
               
               
                 ALHNHYTQKSLSLSPGK 
               
               
                   
               
               
                 SEQ ID NO: 97 - Peptide linker 
               
               
                 (GGGGS) n , wherein n = 1, 2, 3, and 4