Patent Publication Number: US-2023151117-A1

Title: Frb antibodies

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 63/007,990, filed Apr. 10, 2020, which is incorporated by reference herein in its entirety. 
    
    
     STATEMENT REGARDING SEQUENCE LISTING 
     The Sequence Listing associated with this application is provided in text format in lieu of a paper copy and is hereby incorporated by reference into the specification. The name of the text file containing the Sequence Listing is BLUE-129_PC_ST25.txt. The text file is 24 KB, created on Apr. 9, 2021, and is being submitted electronically via EFS-Web, concurrent with the filing of the specification. 
     BACKGROUND 
     Technical Field 
     The present invention relates to FK506-binding protein (FKBP) rapamycin binding (FRB) polypeptide antibodies or antigen binding fragments thereof. 
     Description of the Related Art 
     Adoptive cellular therapy is emerging as a powerful paradigm for delivering complex biological signals to treat cancer. Immune effector cells can be modified to express receptors and redirected to target cancer cells. However, compositions and methods for detecting and/or monitoring these therapeutic cells are not routinely available. 
     BRIEF SUMMARY 
     The invention generally provides improved antibodies and antigen binding fragments thereof that bind to FK506-binding protein (FKBP) rapamycin binding (FRB) polypeptides. In particular embodiments, the antibodies and antigen binding fragments bind an FRB domain present in a dimerizing agent regulated immunoreceptor complex (DARIC) or other chimeric protein. 
     In particular embodiments, an anti-FKBP Rapamycin binding (FRB) antibody or antigen binding fragment thereof that binds an FRB polypeptide comprises a light chain variable region comprising the light chain CDRs (CDRL1-CDRL3) set forth in any one of SEQ ID NOs: 1-3, 11-13, 21-23, 31-33, and 41-43 and a heavy chain variable region comprising the heavy chain CDRs (CDRH1-CDRH3) set forth in any one of SEQ ID NOs: 4-6, 14-16, 24-26, 34-36, and 44-46. 
     In some embodiments, the antibody or antigen binding fragment thereof comprises a light chain variable region comprising the light chain CDRs (CDRL1-CDRL3) set forth in SEQ ID NOs: 1-3 and the heavy chain variable region comprising the heavy chain CDRs (CDRH1-CDRH3) set forth in SEQ ID NOs: 4-6. 
     In particular embodiments, the antibody or antigen binding fragment thereof comprises a light chain variable region comprising the light chain CDRs (CDRL1-CDRL3) set forth in SEQ ID NOs: 11-13 and the heavy chain variable region comprising the heavy chain CDRs (CDRH1-CDRH3) set forth in SEQ ID NOs: 14-16. 
     In certain embodiments, the antibody or antigen binding fragment thereof comprises a light chain variable region comprising the light chain CDRs (CDRL1-CDRL3) set forth in SEQ ID NOs: 21-23 and the heavy chain variable region comprising the heavy chain CDRs (CDRH1-CDRH3) set forth in SEQ ID NOs: 24-26. 
     In some embodiments, the antibody or antigen binding fragment thereof comprises a light chain variable region comprising the light chain CDRs (CDRL1-CDRL3) set forth in SEQ ID NOs: 31-33 and the heavy chain variable region comprising the heavy chain CDRs (CDRH1-CDRH3) set forth in SEQ ID NOs: 34-36. 
     In particular embodiments, the antibody or antigen binding fragment thereof comprises a light chain variable region comprising the light chain CDRs (CDRL1-CDRL3) set forth in SEQ ID NOs: 41-43 and the heavy chain variable region comprising the heavy chain CDRs (CDRH1-CDRH3) set forth in SEQ ID NOs: 44-46. 
     In some embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in any one of SEQ ID NOs: 7, 9, 17, 19, 27, 29, 37, 39, 47, and 49 and a variable heavy chain sequence as set forth in any one of SEQ ID NOs: 8, 10, 18, 20, 28, 30, 38, 40, 48, and 50. 
     In certain embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 7 and a variable heavy chain sequence as set forth in SEQ ID NO: 8. 
     In certain embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 9 and a variable heavy chain sequence as set forth in SEQ ID NO: 10. 
     In particular embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 17 and a variable heavy chain sequence as set forth in SEQ ID NO: 18. 
     In particular embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 19 and a variable heavy chain sequence as set forth in SEQ ID NO: 20. 
     In some embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 27 and a variable heavy chain sequence as set forth in SEQ ID NO: 28. 
     In some embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 29 and a variable heavy chain sequence as set forth in SEQ ID NO: 30. 
     In particular embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 37 and a variable heavy chain sequence as set forth in SEQ ID NO: 38. 
     In some embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 39 and a variable heavy chain sequence as set forth in SEQ ID NO: 40. 
     In particular embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 47 and a variable heavy chain sequence as set forth in SEQ ID NO: 48. 
     In particular embodiments, the antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 49 and a variable heavy chain sequence as set forth in SEQ ID NO: 50. 
     In some embodiments, the preceding claims, wherein the antibody or antigen binding fragment comprises a variable heavy chain sequence, a polypeptide linker, and a variable light chain sequence. 
     In particular embodiments, the preceding claims, wherein the antibody or antigen binding fragment comprises a variable light chain sequence, a polypeptide linker, and a variable heavy chain sequence. 
     In some embodiments, the preceding claims, wherein the antibody or antigen binding fragment is a monoclonal antibody or antigen binding fragment. 
     In particular embodiments, the preceding claims, wherein the antibody or antigen binding fragment is a single chain variable fragment (scFv). 
     In particular embodiments, a conjugate comprises the anti-FRB antibody or antigen binding fragment thereof of any of the preceding claims and a detectable label. 
     In certain embodiments, the detectable label is selected from the group consisting of: a hapten, a fluorescent dye, a fluorescent protein, a chromophore, a metal ion, a gold particle, a silver particle, a magnetic particle, a polypeptide, an enzyme, a luminescent compound, or an oligonucleotide. 
     In particular embodiments, the detectable label is a fluorescent dye selected from the group consisting of: Oregon Green®, Pacific Blue™, Pacific Orange™, Pacific Green™, Cascade Blue™, Cascade Yellow™, Lucifer Yellow™, Marina Blue™, and Texas Red® (TxRed). 
     In certain embodiments, the detectable label is an AlexaFluor® (AF) dye selected from the group consisting of: AF350, AF405, AF488, AF500, AF514, AF532, AF546, AF555, AF568, AF594, AF610, AF633, AF635, AF647, AF680, AF700, AF710, AF750, AF790, and AF800. 
     In some embodiments, the detectable label is a QDot® selected from the group consisting of: Qdot®525, Qdot®565, Qdot®585, Qdot®605, Qdot®655, Qdot®705, and Qdot®800. 
     In particular embodiments, the detectable label is a DyLight™ Dye (DL) selected from the group consisting of: DL549, DL649, DL680, and DL800. 
     In certain embodiments, the detectable label is a hapten selected from the group consisting of: fluorescein or a derivative thereof, fluorescein isothiocyanate, carboxyfluorescein, dichlorotriazinylamine fluorescein, digoxigenin, dinitrophenol (DNP), trinitrophenol (TNP), and biotin. 
     In particular embodiments, the detectable label is a Cy Dye selected from the group consisting of: Cy2, Cy3, Cy3.5, Cy5, Cy5.5, Cy7, and Cy 7.5. 
     In some embodiments, the detectable label is a fluorescent molecule selected from the group consisting of: Phycoerythrin (PE, R-Phycoerythrin (RPE)), B-Phycoerythrin (BPE), Peridinin Chlorophyll (PerCP), Allophycocyanin (APC), and C-Phycocyanin. 
     In particular embodiments, the detectable label is a fluorescent dye selected from the group consisting of: Atto 390, Atto 425, Atto 465, Atto 488, Atto 495, Atto 514Atto 520, Atto 532, Atto 550, Atto 565, Atto 590, Atto 594, Atto 610, Atto 620, Atto 633, Atto 647, Atto 655, Atto 665, Atto 680, Atto 700, Atto 725, Atto 740, Super Bright™ 436, Super Bright™ 600, Super Bright™ 645, Super Bright™ 702, Super Bright™ 780, Brilliant™ Violet 421, Brilliant™ Violet 480, Brilliant™ Violet 510, Brilliant™ Violet 605, Brilliant Violet™ 650, Brilliant Violet™ 711, Brilliant Violet™ 786, Brilliant™ Ultraviolet 395 (BUV395), Brilliant™ Ultraviolet 496 (BUV496), Brilliant™ Ultraviolet 563 (BUV563), Brilliant™ Ultraviolet 661 (BUV661), Brilliant™ Ultraviolet 737 (BUV737), Brilliant™ Ultraviolet 805 (BUV805), Brilliant™ Blue 515 (BB515), Brilliant™ Blue 700 (BB700) and IR Dye 680, IR Dye 680LT, IR Dye 700, IR Dye 700DX, IR Dye 800, IR Dye 800RS, and IR Dye 800CW. 
     In certain embodiments, the detectable label is a tandem fluorescent dye selected from the group consisting of: RPE-Cy5, RPE-Cy5.5, RPE-Cy7, RPE-CF594, RPE-AlexaFluor® tandem conjugates; RPE-Alexa610, RPE-TxRed, APC-H7, APC-R700, APC-Alexa600, APC-Alexa610, APC-Alexa750, APC-Cy5, APC-Cy5.5, and APC-Cy7. 
     In certain embodiments, the detectable label is a fluorescent protein selected from the group consisting of: GFP, eGFP, BFP, CFP, YFP, DsRed, DsRed2, mRFP, mBanana, mOrange, dTomato, tdTomato, mTangerine, mStrawberry, mCherry, mPlum, and mRaspberry. 
     In particular embodiments, the detectable label is an enzyme selected from the group consisting of: alkaline phosphatase, horseradish peroxidase, luciferase, and θ-galactosidase. 
     In certain embodiments, the detectable label comprises a radionuclide selected from the group consisting of: carbon (14C), chromium (51Cr), cobalt (57Co), fluorine (18F), gadolinium (153Gd, 159Gd), germanium (68Ge), holmium (166Ho), indium (115In, 113In, 112In, mIn), iodine (125I, 123I, 121I), lanthanium (140La), lutetium (177Lu), manganese (54Mn), molybdenum (99 Mo), palladium (103 Pd), phosphorous (32P), praseodymium (142 Pr), promethium (149Pm), rhenium (186Re, 188Re), rhodium (105Rh), rutheroium (97Ru), samarium (153Sm), scandium (47Sc), selenium (75Se), (85Sr), sulphur (35S), technetium (99Tc), thallium (201Ti), tin (113Sn, 117Sn), tritium (3H), xenon (133Xe), ytterbium (169Yb, 175Yb), and yttrium (90Y). 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment binds an FRB polypeptide sequence set forth in SEQ ID NO: 51. 
     In particular embodiments, a polynucleotide encodes the anti-FRB antibody or antigen binding fragment thereof contemplated herein. 
     In particular embodiments, a composition comprises an anti-FRB antibody or antigen binding fragment, a conjugate, or a polynucleotide contemplated herein. 
     In particular embodiments, a hybridoma comprises polynucleotides encoding anti-FRB antibody or antigen binding fragment thereof contemplated herein. 
     In particular embodiments, a hybridoma comprising an antibody contemplated herein is provided. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         FIG.  1    shows a schematic of an antibody production process. 
         FIGS.  2 A- 2 E  show antibody binding data for anti-FRB antibody clones against cells expressing a protein comprising an extracellular FRB domain. 
     
    
    
     BRIEF DESCRIPTION OF THE SEQUENCE IDENTIFIERS 
     SEQ ID NOs: 1-3, 11-13, 21-23, 31-33, and 41-43 set forth exemplary amino acid sequences of light chain CDR1-3 sequences. 
     SEQ ID NOs: 4-6, 14-16, 24-26, 34-36, and 44-46 set forth exemplary amino acid sequences of heavy chain CDR1-3 sequences. 
     SEQ ID NO: 7, 9, 17, 19, 27, 29, 37, 39, 47, and 49 sets forth exemplary variable domain light chains. 
     SEQ ID NO: 8, 10, 18, 20, 28, 30, 38, 40, 48, and 50 sets forth an exemplary variable domain heavy chain. 
     SEQ ID NO: 51 sets forth an FRB amino acid sequence. 
     SEQ ID NOs: 52-61 set forth the amino acid sequences of various linkers. 
     DETAILED DESCRIPTION 
     A. Overview 
     The invention generally relates to improved anti-FRB antibodies and antigen binding fragments thereof, compositions, and diagnostic uses of the same. 
     Techniques for recombinant (i.e., engineered) DNA, peptide and oligonucleotide synthesis, immunoassays, tissue culture, transformation (e.g., electroporation, lipofection), enzymatic reactions, purification and related techniques and procedures may be generally performed as described in various general and more specific references in microbiology, molecular biology, biochemistry, molecular genetics, cell biology, virology and immunology as cited and discussed throughout the present specification. See, e.g., Sambrook et al.,  Molecular Cloning: A Laboratory Manual,  3d ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.;  Current Protocols in Molecular Biology  (John Wiley and Sons, updated July 2008);  Short Protocols in Molecular Biology: A Compendium of Methods from Current Protocols in Molecular Biology , Greene Pub. Associates and Wiley-Interscience; Glover,  DNA Cloning: A Practical Approach , vol. I &amp; II (IRL Press, Oxford Univ. Press USA, 1985);  Current Protocols in Immunology  (Edited by: John E. Coligan, Ada M. Kruisbeek, David H. Margulies, Ethan M. Shevach, Warren Strober 2001 John Wiley &amp; Sons, NY, N.Y.);  Real - Time PCR: Current Technology and Applications , Edited by Julie Logan, Kirstin Edwards and Nick Saunders, 2009, Caister Academic Press, Norfolk, UK; Anand,  Techniques for the Analysis of Complex Genomes , (Academic Press, New York, 1992); Guthrie and Fink,  Guide to Yeast Genetics and Molecular Biology  (Academic Press, New York, 1991) ; Oligonucleotide Synthesis  (N. Gait, Ed., 1984);  Nucleic Acid The Hybridization  (B. Hames &amp; S. Higgins, Eds., 1985);  Transcription and Translation  (B. Hames &amp; S. Higgins, Eds., 1984);  Animal Cell Culture  (R. Freshney, Ed., 1986); Perbal,  A Practical Guide to Molecular Cloning  (1984);  Next - Generation Genome Sequencing  (Janitz, 2008 Wiley-VCH);  PCR Protocols  ( Methods in Molecular Biology ) (Park, Ed., 3rd Edition, 2010 Humana Press);  Immobilized Cells And Enzymes  (IRL Press, 1986); the treatise,  Methods In Enzymology  (Academic Press, Inc., N.Y.);  Gene Transfer Vectors For Mammalian Cells  (J. H. Miller and M. P. Calos eds., 1987, Cold Spring Harbor Laboratory); Harlow and Lane, Antibodies, (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y., 1998);  Immunochemical Methods In Cell And Molecular Biology  (Mayer and Walker, eds., Academic Press, London, 1987);  Handbook Of Experimental Immunology , Volumes I-IV (D. M. Weir and C C Blackwell, eds., 1986); Roitt,  Essential Immunology,  6th Edition, (Blackwell Scientific Publications, Oxford, 1988);  Current Protocols in Immunology  (Q. E. Coligan, A. M. Kruisbeek, D. H. Margulies, E. M. Shevach and W. Strober, eds., 1991);  Annual Review of Immunology ; as well as monographs in joumals such as Advances in Immunology. 
     B. Definitions 
     Prior to setting forth this disclosure in more detail, it may be helpful to an understanding thereof to provide definitions of certain terms to be used herein. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of particular embodiments, preferred embodiments of compositions, methods and materials are described herein. For the purposes of the present disclosure, the following terms are defined below. 
     The articles “a,” “an,” and “the” are used herein to refer to one or to more than one (i.e., to at least one, or to one or more) of the grammatical object of the article. By way of example, “an element” means one element or one or more elements. 
     The use of the alternative (e.g., “or”) should be understood to mean either one, both, or any combination thereof of the alternatives. 
     The term “and/or” should be understood to mean either one, or both of the alternatives. 
     Throughout this specification, unless the context requires otherwise, the words “comprise”, “comprises” and “comprising” will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements. By “consisting of” is meant including, and limited to, whatever follows the phrase “consisting of” Thus, the phrase “consisting of” indicates that the listed elements are required or mandatory, and that no other elements may be present. By “consisting essentially of” is meant including any elements listed after the phrase, and limited to other elements that do not interfere with or contribute to the activity or action specified in the disclosure for the listed elements. Thus, the phrase “consisting essentially of” indicates that the listed elements are required or mandatory, but that no other elements are present that materially affect the activity or action of the listed elements. 
     Reference throughout this specification to “one embodiment,” “an embodiment,” “a particular embodiment,” “a related embodiment,” “a certain embodiment,” “an additional embodiment,” or “a further embodiment” or combinations thereof means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the foregoing phrases in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. It is also understood that the positive recitation of a feature in one embodiment, serves as a basis for excluding the feature in a particular embodiment. 
     A “dimerizing agent regulated immunoreceptor complex” or “DARIC” refers to a multi-chain chimeric receptor comprising a signaling component that comprises a multimerization domain polypeptide or variant thereof, e.g., FRB, a transmembrane domain, and one or more T cell intracellular signaling domains and a binding components that comprises a binding domain, a multimerization domain polypeptide or variant thereof, e.g., FKBP12, a transmembrane domain, and one or more intracellular signaling domains. 
     Additional definitions are set forth throughout this disclosure. 
     C. Antibodies 
     In particular embodiments, an antibody or antigen binding fragment thereof is provided. 
     The term “antibody” refers to a binding agent that is a polypeptide comprising at least a light chain or heavy chain immunoglobulin variable region or fragment thereof which specifically recognizes and binds an epitope of an antigen, such as a peptide, lipid, polysaccharide, or nucleic acid containing an antigenic determinant, such as those recognized by an immune cell. 
     An “isolated antibody or antigen binding fragment thereof” is one which has been identified and separated and/or recovered from a component of its natural environment. 
     The terms “specific binding affinity” or “specifically binds” or “specifically bound” or “specific binding” or “specifically targets” as used herein, describe binding of an anti-FRB antibody or antigen binding fragment thereof to an FRB polypeptide at greater binding affinity than background binding. An antibody or antigen binding fragment thereof “specifically binds” to an FRB polypeptide if it binds to or associates with an FRB polypeptide with an affinity or K a  (i.e., an equilibrium association constant of a particular binding interaction with units of 1/M) of, for example, greater than or equal to about 10 5 M −1 . In certain embodiments, an antibody or antigen binding fragment thereof binds to a target with a K a  greater than or equal to about 10 6  M −1 , 10 7  M −1 , 10 8  M −1 , 10 9  M −1 , 10 10  M − , 1011 M − , 10 12  M − , or 10 13  M −1 . “High affinity” antibody or antigen binding fragment thereof have a K a  of at least 10 7  M −1 , at least 10 8  M −1 , at least 10 9  M −1 , at least 10 10  M −1 , at least 10 11 M −1 , at least 10 12  M −1 , at least 10 13  M −1 , or greater. 
     Alternatively, affinity may be defined as an equilibrium dissociation constant (K a ) of a particular binding interaction with units of M (e.g., 10 −5  M to 10 −13  M, or less). Affinities of antibody or antigen binding fragment thereof contemplated herein can be readily determined using conventional techniques, e.g., by competitive ELISA (enzyme-linked immunosorbent assay), or by binding association, or displacement assays using labeled ligands, or using a surface-plasmon resonance device such as the Biacore T100, which is available from Biacore, Inc., Piscataway, N.J., or optical biosensor technology such as the EPIC system or EnSpire that are available from Corning and Perkin Elmer respectively (see also, e.g., Scatchard et al. (1949) Ann. N.Y. Acad. Sci. 51:660; and U.S. Pat. Nos. 5,283,173; 5,468,614, or the equivalent). 
     In one embodiment, the affinity of specific binding is about 2 times greater than background binding, about 5 times greater than background binding, about 10 times greater than background binding, about 20 times greater than background binding, about 50 times greater than background binding, about 100 times greater than background binding, or about 1000 times greater than background binding or more. 
     An “antigen (Ag)” refers to a compound, composition, or substance that can stimulate the production of antibodies or a T cell response in an animal, including compositions (such as one that includes a cancer-specific protein) that are injected or absorbed into an animal. An antigen reacts with the products of specific humoral or cellular immunity, including those induced by heterologous antigens, such as the disclosed antigens. In particular embodiments, the target antigen is an epitope of an FRB polypeptide. 
     An “epitope” or “antigenic determinant” refers to the region of an antigen to which a binding agent binds. Epitopes can be formed both from contiguous amino acids or noncontiguous amino acids juxtaposed by tertiary folding of a protein. Epitopes formed from contiguous amino acids are typically retained on exposure to denaturing solvents whereas epitopes formed by tertiary folding are typically lost on treatment with denaturing solvents. An epitope typically includes at least 3, and more usually, at least 5, about 9, or about 8-10 amino acids in a unique spatial conformation. 
     Antibodies include, in particular embodiments, antigen binding fragments thereof, such as a Fab′ fragment, a F(ab′)2 fragment, a bispecific Fab dimer (Fab2), a trispecific Fab trimer (Fab3), an Fv, an single chain Fv protein (“scFv”), a bis-scFv, (scFv) 2 , a minibody, a diabody, a triabody, a tetrabody, a disulfide stabilized Fv protein (“dsFv”), and a single-domain antibody (sdAb, a camelid VHH, Nanobody) and portions of full length antibodies responsible for antigen binding. The term also includes genetically engineered forms such as chimeric antibodies (for example, humanized murine antibodies), heteroconjugate antibodies (such as, bispecific antibodies) and antigen binding fragments thereof, in particular embodiments. See also, Pierce Catalog and Handbook, 1994-1995 (Pierce Chemical Co., Rockford, Ill.); Kuby, J., Immunology, 3 rd  Ed., W. H. Freeman &amp; Co., New York, 1997. 
     As would be understood by the skilled person and as described elsewhere herein, a complete antibody comprises two heavy chains and two light chains. Each heavy chain consists of a variable region and a first, second, and third constant region, while each light chain consists of a variable region and a constant region. Mammalian heavy chains are classified as α, δ, ε, γ, and μ. Mammalian light chains are classified as λ or κ. Immunoglobulins comprising the α, δ, ε, γ, and μ heavy chains are classified as immunoglobulin (Ig)A, IgD, IgE, IgG, and IgM. The complete antibody forms a “Y” shape. The stem of the Y consists of the second and third constant regions (and for IgE and IgM, the fourth constant region) of two heavy chains bound together and disulfide bonds (inter-chain) are formed in the hinge. Heavy chains γ, α and δ have a constant region composed of three tandem (in a line) Ig domains, and a hinge region for added flexibility; heavy chains μ and ε have a constant region composed of four immunoglobulin domains. The second and third constant regions are referred to as “CH2 domain” and “CH3 domain”, respectively. Each arm of the Y includes the variable region and first constant region of a single heavy chain bound to the variable and constant regions of a single light chain. The variable regions of the light and heavy chains are responsible for antigen binding. 
     Light and heavy chain variable regions contain a “framework” region interrupted by three hypervariable regions, also called “complementarity-determining regions” or “CDRs.” The CDRs can be defined or identified by conventional methods, such as by sequence according to Kabat et al. (Wu, T T and Kabat, E. A., J Exp Med. 132(2):211-50, (1970); Borden, P. and Kabat E. A., PNAS, 84: 2440-2443 (1987); (see, Kabat et al., Sequences of Proteins of Immunological Interest, U.S. Department of Health and Human Services, 1991, which is hereby incorporated by reference), or by structure according to Chothia et al. (Chothia, C. and Lesk, A. M.,  J Mol. Biol.,  196(4): 901-917 (1987), Chothia, C. et al,  Nature,  342: 877-883 (1989)). 
     Illustrative examples of rules for predicting light chain CDRs include: CDR-L1 starts at about residue 24, is preceded by a Cys, is about 10-17 residues, and is followed by a Trp (typically Trp-Tyr-Gln, but also, Trp-Leu-Gln, Trp-Phe-Gln, Trp-Tyr-Leu); CDR-L2 starts about 16 residues after the end of CDR-L1, is generally preceded by Ile-Tyr, but also, Val-Tyr, Ile-Lys, Ile-Phe, and is 7 residues; and CDR-L3 starts about 33 residues after the end of CDR-L2, is preceded by a Cys, is 7-11 residues, and is followed by Phe-Gly-XXX-Gly (SEQ ID NO:62) (XXX is any amino acid). 
     Illustrative examples of rules for predicting heavy chain CDRs include: CDR-H1 starts at about residue 26, is preceded by Cys-XXX-XXX-XXX (SEQ ID NO:63), is 10-12 residues and is followed by a Trp (typically Trp-Val, but also, Trp-Ile, Trp-Ala); CDR-H2 starts about 15 residues after the end of CDR-H1, is generally preceded by Leu-Glu-Trp-Ile-Gly (SEQ ID NO:64), or a number of variations, is 16-19 residues, and is followed by Lys/Arg-Leu/Ile/Val/Phe/Thr/Ala-Thr/Ser/Ile/Ala; and CDR-H3 starts about 33 residues after the end of CDR-H2, is preceded by Cys-XXX-XXX (typically Cys-Ala-Arg), is 3 to 25 residues, and is followed by Trp-Gly-XXX-Gly (SEQ ID NO:65). 
     In one embodiment, light chain CDRs and the heavy chain CDRs are determined according to the Kabat method 
     In one embodiment, light chain CDRs and the heavy chain CDR2 and CDR3 are determined according to the Kabat method, and heavy chain CDR1 is determined according to the AbM method, which is a comprise between the Kabat and Clothia methods, see e.g., Whitelegg N &amp; Rees A R,  Protein Eng.  2000 December; 13(12):819-24 and  Methods Mol Biol.  2004; 248:51-91. Programs for predicting CDRs are publicly available, e.g., AbYsis (www.bioinf.org.uk/abysis/). 
     The sequences of the framework regions of different light or heavy chains are relatively conserved within a species, such as humans. The framework region of an antibody, that is the combined framework regions of the constituent light and heavy chains, serves to position and align the CDRs in three-dimensional space. The CDRs are primarily responsible for binding to an epitope of an antigen. The CDRs of each chain are typically referred to as CDR1, CDR2, and CDR3, numbered sequentially starting from the N-terminus, and are also typically identified by the chain in which the particular CDR is located. Thus, the CDRs located in the variable domain of the heavy chain of the antibody are referred to as CDRH1, CDRH2, and CDRH3, whereas the CDRs located in the variable domain of the light chain of the antibody are referred to as CDRL1, CDRL2, and CDRL3. Antibodies with different specificities (i.e., different combining sites for different antigens) have different CDRs. Although it is the CDRs that vary from antibody to antibody, only a limited number of amino acid positions within the CDRs are directly involved in antigen binding. These positions within the CDRs are called specificity determining residues (SDRs). 
     Illustrative examples of light chain CDRs include the CDR sequences set forth in SEQ ID NOs: 1-3, 11-13, 21-23, 31-33, and 41-43. Illustrative examples of heavy chain CDRs include the CDR sequences set forth in SEQ ID NOs: 4-6, 14-16, 24-26, 34-36, and 44-46. 
     References to “V L ” or “VL” refer to the variable region of an immunoglobulin light chain, including that of an antibody, Fv, scFv, dsFv, Fab, or other antibody fragment as disclosed herein. Illustrative examples of light chain variable regions include the light chain variable region sequences set forth in SEQ ID NO: 7, 9, 17, 19, 27, 29, 37, 39, 47, and 49. 
     References to “V H ” or “VH” refer to the variable region of an immunoglobulin heavy chain, including that of an antibody, Fv, scFv, dsFv, Fab, or other antibody fragment as disclosed herein. Illustrative examples of heavy chain variable regions include the heavy chain variable region sequences set forth in SEQ ID NO: 8, 10, 18, 20, 28, 30, 38, 40, 48, and 50. 
     A “monoclonal antibody” is an antibody produced by a single clone of B lymphocytes or by a cell into which the light and heavy chain genes of a single antibody have been transfected. Monoclonal antibodies are produced by methods known to those of skill in the art, for instance by making hybrid antibody-forming cells from a fusion of myeloma cells with immune spleen cells. Monoclonal antibodies include humanized monoclonal antibodies. In particular embodiments, the antibody is a rabbit monoclonal antibody or antigen binding fragment thereof. 
     In preferred embodiments, the antibody is a rabbit antibody (such as a rabbit monoclonal antibody) or fragment thereof that specifically binds to an FRB polypeptide. 
     In one embodiment, an antibody is a “humanized” antibody. A humanized antibody is an immunoglobulin including a human framework region and one or more CDRs from a non-human (for example a mouse, rat, or synthetic) immunoglobulin. The non-human immunoglobulin providing the CDRs is termed a “donor,” and the human immunoglobulin providing the framework is termed an “acceptor.” In one embodiment, all the CDRs are from the donor immunoglobulin in a humanized immunoglobulin. Constant regions need not be present, but if they are, they must be substantially identical to human immunoglobulin constant regions, i.e., at least about 85-90%, such as about 95% or more identical. Hence, all parts of a humanized immunoglobulin, except possibly the CDRs, are substantially identical to corresponding parts of natural human immunoglobulin sequences. Humanized or other monoclonal antibodies can have additional conservative amino acid substitutions, which have substantially no effect on antigen binding or other immunoglobulin functions. Humanized antibodies can be constructed by means of genetic engineering (see for example, U.S. Pat. No. 5,585,089). 
     Papain digestion of antibodies produces two identical antigen-binding fragments, called “Fab” fragments, each with a single antigen-binding site, and a residual “Fc” fragment, whose name reflects its ability to crystallize readily. Pepsin treatment yields an F(ab′)2 fragment that has two antigen-combining sites and is still capable of cross-linking antigen. 
     “Fv” is the minimum antibody fragment which contains a complete antigen-binding site. In one embodiment, a two-chain Fv species consists of a dimer of one heavy- and one light-chain variable domain in tight, non-covalent association. In a single-chain Fv (scFv) species, one heavy- and one light-chain variable domain can be covalently linked by a flexible peptide linker such that the light and heavy chains can associate in a “dimeric” structure analogous to that in a two-chain Fv species. It is in this configuration that the three hypervariable regions (HVRs) of each variable domain interact to define an antigen-binding site on the surface of the VH-VL dimer. Collectively, the six HVRs confer antigen-binding specificity to the antibody. However, even a single variable domain (or half of an Fv comprising only three HVRs specific for an antigen) has the ability to recognize and bind antigen, although at a lower affinity than the entire binding site. 
     The Fab fragment contains the heavy- and light-chain variable domains and also contains the constant domain of the light chain and the first constant domain (CH1) of the heavy chain. Fab′ fragments differ from Fab fragments by the addition of a few residues at the carboxy terminus of the heavy chain CH1 domain including one or more cysteines from the antibody hinge region. Fab′-SH is the designation herein for Fab′ in which the cysteine residue(s) of the constant domains bear a free thiol group. F(ab′)2 antibody fragments originally were produced as pairs of Fab′ fragments which have hinge cysteines between them. Other chemical couplings of antibody fragments are also known. 
     The term “diabodies” refers to antibody fragments with two antigen-binding sites, which fragments comprise a heavy-chain variable domain (VH) connected to a light-chain variable domain (VL) in the same polypeptide chain (VH-VL). By using a linker that is too short to allow pairing between the two domains on the same chain, the domains are forced to pair with the complementary domains of another chain and create two antigen-binding sites. Diabodies may be bivalent or bispecific. Diabodies are described more fully in, for example, EP 404,097; WO 1993/01161; Hudson et al.,  Nat. Med.  9:129-134 (2003); and Hollinger et al.,  PNAS USA  90: 6444-6448 (1993). Triabodies and tetrabodies are also described in Hudson et al.,  Nat. Med.  9:129-134 (2003). 
     “Single domain antibody” or “sdAb” or “nanobody” refers to an antibody fragment that consists of the variable region of an antibody heavy chain (VH domain) or the variable region of an antibody light chain (VL domain) (Holt, L., et al,  Trends in Biotechnology,  21(11): 484-490). 
     “Single-chain Fv” or “scFv” antibody fragments comprise the VH and VL domains of antibody, wherein these domains are present in a single polypeptide chain and in either orientation (e.g., VL-VH or VH-VL). Generally, the scFv polypeptide further comprises a polypeptide linker between the VH and VL domains which enables the scFv to form the desired structure for antigen binding. For a review of scFv, see, e.g., Pluckthün, in  The Pharmacology of Monoclonal Antibodies , vol. 113, Rosenburg and Moore eds., (Springer-Verlag, New York, 1994), pp. 269-315. 
     In preferred embodiments, the anti-FRB antigen binding fragment is an scFv. In particular embodiments, the scFv is a rabbit scFv. Single chain antibodies may be cloned form the V region genes of a hybridoma specific for a desired target. The production of such hybridomas has become routine. A technique which can be used for cloning the variable region heavy chain (VH) and variable region light chain (VL) has been described, for example, in Orlandi et al.,  PNAS,  1989; 86: 3833-3837. 
     In various embodiments, an anti-FRB antibody or antigen binding fragment thereof comprises a variable light chain sequence comprising CDRL1-CDRL3 sequences set forth in SEQ ID NOs: 1-3, 11-13, 21-23, 31-33, and 41-43, and/or a variable heavy chain sequence comprising CDRH1-CDRH3 sequences set forth in SEQ ID NOs: 4-6, 14-16, 24-26, 34-36, and 44-46. In some embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises a variable light chain sequence as set forth in SEQ ID NO: 7, 9, 17, 19, 27, 29, 37, 39, 47, and 49 and/or a variable heavy chain sequence as set forth in SEQ ID NO: 8, 10, 18, 20, 28, 30, 38, 40, 48, and 50. 
     FRB-specific binding domains provided in particular embodiments also comprise one, two, three, four, five, or six CDRs. Such CDRs may be nonhuman CDRs or altered nonhuman CDRs selected from CDRL1, CDRL2 and CDRL3 of the light chain and CDRH1, CDRH2 and CDRH3 of the heavy chain. In certain embodiments, an FRB-specific binding domain comprises (a) a light chain variable region that comprises a light chain CDRL1, a light chain CDRL2, and a light chain CDRL3, and (b) a heavy chain variable region that comprises a heavy chain CDRH1, a heavy chain CDRH2, and a heavy chain CDRH3. 
     In one embodiment, an FRB-specific binding domain comprises light chain CDR sequences set forth in SEQ ID NOs: 1-3, 11-13, 21-23, 31-33, and 41-43. In a particular embodiment, an FRB-specific binding domain comprises light chain CDR sequences with at least 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% amino acid identity to the light chain CDR sequences set forth in SEQ ID NOs: 1-3, 11-13, 21-23, 31-33, and 41-43. 
     In one embodiment, an FRB-specific binding domain comprises heavy chain CDR sequences set forth in SEQ ID NOs: 4-6, 14-16, 24-26, 34-36, and 44-46. In a particular embodiment, an FRB-specific binding domain comprises heavy chain CDR sequences with at least 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% amino acid identity to the heavy chain CDR sequences set forth in SEQ ID NOs: 4-6, 14-16, 24-26, 34-36, and 44-46. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises one or more light chain CDRs as set forth in any one of SEQ ID NOs: 1-3, 11-13, 21-23, 31-33, and 41-43 and/or one or more heavy chain CDRs as set forth in any one of SEQ ID NOs: 4-6, 14-16, 24-26, 34-36, and 44-46. In certain embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises a variable light chain sequence with at least 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% amino acid identity to the variable light chain sequence set forth in SEQ ID NO: 7, 9, 17, 19, 27, 29, 37, 39, 47, and 49 and/or a variable heavy chain sequence with at least 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99% amino acid identity to the variable heavy chain sequence set forth in SEQ ID NO: 8, 10, 18, 20, 28, 30, 38, 40, 48, and 50. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 1-3 and the heavy chain CDRs as set forth in SEQ ID NOs: 4-6 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 7 and the variable heavy chain sequence as set forth in SEQ ID NO: 8. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 1-3 and the heavy chain CDRs as set forth in SEQ ID NOs: 4-6 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 9 and the variable heavy chain sequence as set forth in SEQ ID NO: 10. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 11-13 and the heavy chain CDRs as set forth in SEQ ID NOs: 14-16 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 17 and the variable heavy chain sequence as set forth in SEQ ID NO: 18. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 11-13 and the heavy chain CDRs as set forth in SEQ ID NOs: 14-16 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 19 and the variable heavy chain sequence as set forth in SEQ ID NO: 20. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 21-23 and the heavy chain CDRs as set forth in SEQ ID NOs: 24-26 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 27 and the variable heavy chain sequence as set forth in SEQ ID NO: 28. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 21-23 and the heavy chain CDRs as set forth in SEQ ID NOs: 24-26 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 29 and the variable heavy chain sequence as set forth in SEQ ID NO: 30. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 31-33 and the heavy chain CDRs as set forth in SEQ ID NOs: 34-36 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 37 and the variable heavy chain sequence as set forth in SEQ ID NO: 38. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 31-33 and the heavy chain CDRs as set forth in SEQ ID NOs: 34-36 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 39 and the variable heavy chain sequence as set forth in SEQ ID NO: 40. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 41-43 and the heavy chain CDRs as set forth in SEQ ID NOs: 44-46 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 47 and the variable heavy chain sequence as set forth in SEQ ID NO: 48. 
     In particular embodiments, the anti-FRB antibody or antigen binding fragment thereof comprises the light chain CDRs as set forth in SEQ ID NOs: 41-43 and the heavy chain CDRs as set forth in SEQ ID NOs: 44-46 and at least 95% amino acid identity to the variable light chain sequence as set forth in SEQ ID NO: 49 and the variable heavy chain sequence as set forth in SEQ ID NO: 50. 
     D. Conjugates 
     In various embodiments, a conjugate comprising an anti-FRB antibody or antigen binding fragment thereof and a label is provided. In particular embodiments, a conjugate comprises an anti-FRB antibody or antigen binding fragment thereof, and a detectable label or a label capable of producing a detectable signal. In more particular embodiments, a conjugate comprises an anti-FRB antibody or antigen binding fragment thereof, coupled to a detectable label. In even more particular embodiments, a conjugate comprises an anti-FRB antibody or antigen binding fragment thereof, covalently bound, or chemically coupled to, a detectable label. 
     As used herein, the term “label” refers to a detectable label or a label capable of producing a detectable signal. In particular embodiments, a label comprises a radionuclides, nucleic acid, small molecule, or polypeptide. In some embodiments, labels are directly detectable. In some embodiments, labels are indirectly detectable. 
     Illustrative examples of detectable labels suitable for use in conjugates contemplated in particular embodiments include, but are not limited to: haptens, fluorescent molecules, fluorescent dyes, fluorescent proteins, chromophores, metal ions, gold particles, silver particles, magnetic particles, radionuclides, polypeptides, enzymes, luminescent compounds, or oligonucleotides. 
     Illustrative examples of molecules suitable for use as detectable labels in particular embodiments include, but are not limited to: Oregon Green®; Pacific Blue™; Pacific Orange™; Pacific Green™; Cascade Blue™; Cascade Yellow™; Lucifer Yellow™; Marina Blue™; Texas Red® (TxRed); AlexaFluor® (AF) dyes, e.g., AF350, AF405, AF488, AF500, AF514, AF532, AF546, AF555, AF568, AF594, AF610, AF633, AF635, AF647, AF680, AF700, AF710, AF750, AF790, and AF800; QDot® nanocrystals, e.g., Qdot®525, Qdot®565, Qdot®585, Qdot®605, Qdot®655, Qdot®705, and Qdot®800; DyLight™ Dyes (DL), e.g., DL549, DL649, DL680, and DL800; fluorescein or a derivative thereof, e.g., fluorescein isothiocyanate, carboxyfluorescein, and dichlorotriazinylamine fluorescein; digoxigenin; dinitrophenol (DNP); trinitrophenol (TNP); biotin; Cy dyes, e.g., Cy2, Cy3, Cy3.5, Cy5, Cy5.5, Cy7, and Cy 7.5; Phycoerythrin (PE, R-Phycoerythrin (RPE)); B-Phycoerythrin (BPE); Peridinin Chlorophyll (PerCP); Allophycocyanin (APC); C-Phycocyanin; Atto® Dyes, e.g., Atto 390, Atto 425, Atto 465, Atto 488, Atto 495, Atto 514Atto 520, Atto 532, Atto 550, Atto 565, Atto 590, Atto 594, Atto 610, Atto 620, Atto 633, Atto 647, Atto 655, Atto 665, Atto 680, Atto 700, Atto 725, and Atto 740; Super Bright™ Dyes, e.g., Super Bright™ 436, Super Bright™ 600, Super Bright™ 645, Super Bright™ 702, and Super Bright™ 780; Brilliant™ Dyes, e.g., Brilliant™ Violet 421, Brilliant™ Violet 480, Brilliant™ Violet 510, Brilliant™ Violet 605, Brilliant Violet™ 650, Brilliant Violet™ 711, Brilliant Violet™ 786, Brilliant™ Ultraviolet 395 (BUV395), Brilliant™ Ultraviolet 496 (BUV496), Brilliant™ Ultraviolet 563 (BUV563), Brilliant™ Ultraviolet 661 (BUV661), Brilliant™ Ultraviolet 737 (BUV737), Brilliant™ Ultraviolet 805 (BUV805), Brilliant™ Blue 515 (BB515), and Brilliant™ Blue 700 (BB700); and IR Dyes, e.g., IR Dye 680, IR Dye 680LT, IR Dye 700, IR Dye 700DX, IR Dye 800, IR Dye 800RS, and IR Dye 800CW. 
     Illustrative examples of tandem fluorescent dye molecules suitable for use as detectable labels include, but are not limited to: RPE-Cy5, RPE-Cy5.5, RPE-Cy7, RPE-CF594, RPE-AlexaFluor® tandem conjugates; RPE-Alexa610, RPE-TxRed, APC-H7, APC-R700, APC-Alexa600, APC-Alexa610, APC-Alexa750, APC-Cy5, APC-Cy5.5, and APC-Cy7. 
     Illustrative examples of fluorescent proteins suitable for use as detectable labels include, but are not limited to: GFP, eGFP, BFP, CFP, YFP, DsRed, DsRed2, mRFP, mBanana, mOrange, dTomato, tdTomato, mTangerine, mStrawberry, mCherry, mPlum, and mRaspberry. 
     Illustrative examples of enzymes suitable for use as detectable labels include, but are not limited to: alkaline phosphatase, horseradish peroxidase, luciferase, and β-galactosidase. 
     Illustrative examples of radionuclides suitable for use as detectable labels include, but are not limited to: carbon (14C), chromium (51Cr), cobalt (57Co), fluorine (18F), gadolinium (153Gd, 159Gd), germanium (68Ge), holmium (166Ho), indium (115In, 113In, 112In, min), iodine (125I, 123I, 121I), lanthanium (140La), lutetium (177Lu), manganese (54Mn), molybdenum (99Mo), palladium (103Pd), phosphorous (32P), praseodymium (142 Pr), promethium (149Pm), rhenium (186Re, 188Re), rhodium (105Rh), rutheroium (97Ru), samarium (153Sm), scandium (47Sc), selenium (75Se), (85Sr), sulphur (35S), technetium (99Tc), thallium (201Ti), tin (113Sn, 117Sn), tritium (3H), xenon (133Xe), ytterbium (169Yb, 175Yb), and yttrium (90Y). 
     In particular embodiments, a conjugate comprises an antibody or antibody fragment that is conjugated, coupled, or linked (e.g., covalently bonded) to one or more labels. In certain embodiments, a label may be conjugated, coupled, or linked to an antibody or fragment either directly or indirectly (e.g., via a linker group). An antibody can be directly covalently bound to one or more labels when the antibody and the label each possesses a substituent capable of reacting with the other. For example, a nucleophilic group, such as an amino or sulfhydryl group, on one may be capable of reacting with a carbonyl-containing group, such as an anhydride or an acid halide, or with an alkyl group containing a good leaving group (e.g., a halide) on the other. 
     In particular embodiments, it may be desirable to couple, conjugate, or link an antibody or antibody fragment to one or more labels via a monovalent or polyvalent linker or a spacer. A linker or spacer can be used to provide sufficient distance between an antibody and a label to avoid steric hindrance or interference with antibody binding capabilities. It will be evident to those skilled in the art that a variety of bifunctional or polyfunctional reagents, both homo- and hetero-functional (such as those described in the catalog of the Pierce Chemical Co., Rockford, Ill.), may be employed as the linker group. Coupling may be effected, for example, through amino groups, carboxyl groups, sulfhydryl groups or oxidized carbohydrate residues. There are numerous references describing such methodology, e.g., U.S. Pat. No. 4,671,958, to Rodwell et al. 
     In certain embodiments, a linker has an overall chain length of about 1-100 atoms, 1-80 atoms, 1-60 atoms, 1-40 atoms, 1-30 atoms, 1-20 atoms, or 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 atoms, wherein the atoms in the chain comprise C, S, N, P, and 0. 
     Illustrative examples of linkers or linkages useful in particular embodiments of the present invention include, but are not limited to one or more of the following: —C(O)—, —NH—C(O)—, —C(O)—NH—, —C(O)—NH—(CH 2 ) 2-6 —NH—C(O)—, —NH—(CH 2 ) 2-6 —NH—C(O)—, -triazole-(CH 2 ) 2-6 —NH—C(O)—, —S—(CH 2 ) 2-6 —NH—C(O)—, —S—(CH 2 ) 0-6 —CH(CONH 2 )—(CH 2 ) 0-6 —NH—C(O)—, —S—(CH 2 ) 0-6 —CH(CONH-PEG)-(CH 2 ) 0-6 —NH—C(O)—, —S—S—(CH 2 ) 2-6 —NH—C(O)—, —S—S—(CH 2 ) 0-6 —CH(CONH 2 )—(CH 2 ) 0-6 —NH—C(O)—, —S—S—(CH 2 ) 0-6 —CH(CONH-PEG)-(CH 2 ) 0-6 —NH—C(O)—, —NH—(CH 2 ) 0-6 —CH(CONH-PEG)-(CH 2 ) 0-6 —NH—C(O)—, —NH—(CH 2 ) 0-6 —CH(CONH 2 )—(CH 2 ) 0-6 —NH—C(O)— —C═N—O—(CH 2 ) 2-6 —NH—C(O)—, —C═N—NH—(CO)—(CH 2 ) 2-6 —NH—C(O)—, -succinimide-(CH 2 ) 2-6 —NH—C(O)—, -diazodicarboxamide-(Phenyl)-J-(CH 2 ) 2-6 —NH—C(O)—, J is O, CH 2 , NH, S, NH(CO), (CO)NH, —NH—(CH 2 ) 2-6 —, (CH 2 ) 1-6 —NH—C(O)—NH—(CH 2 ) 2-6 —, —C(S)—(CH 2 ) 0-6 —, —(CH 2 ) 1-6 —C(O)—NH—(CH 2 ) 2-6 —, —(CH 2 ) 1-6 —NH—C(O)—(CH 2 ) 2-6 —, —(CH 2 ) 1-6 —O—C(O)—NH—(CH 2 ) 2-6 —, —(CH 2 ) 1-6 —NH—C(O)—O—(CH 2 ) 2-6 —, (CH 2 ) 1-6 —NH—(CH 2 ) 2-6 , (CH 2 ) 1-6 —C(O—(CH 2 ) 2-6 —, branched or unbranched —C 1 -C16-alkyl, branched or unbranched —C1-C16-alkyl where one of the carbon atoms can be optionally substituted with a heteroatom, R 2 —NH—(CH 2 ) 2-6 —NH—C(O)—, R 2 —S—(CH 2 ) 2-6 —NH—C(O)—, R 2 -triazole-(CH 2 ) 2-6 —NH—C(O)—, R 2 —NH—O—(CH 2 ) 2-6 —NH—C(O)—, R 2 ═N—NH—(CO)—(CH 2 ) 2-6 —NH—C(O)—, R 2  is one to three bifunctional or trifunctional substituted cross-linking organic radicals selected from the group consisting of alkyl, substituted alkyl, cycloalkyl, aryl, heteroaryl, polyethylene glycol (PEG) [i.e., —(CH 2 CH 2 O) 1-20 ]. 
     In particular embodiments, a conjugate comprises an anti-FRB antibody or antibody fragment covalently bound to a polypeptide-based label, e.g., a fluorescent protein or enzyme, via a polypeptide linker contemplated elsewhere herein, infra. 
     E. Polypeptides 
     Various polypeptides are contemplated herein, including, but not limited to antibodies and antigen binding fragments thereof. 
     “Polypeptide,” “polypeptide fragment,” “peptide” and “protein” are used interchangeably, unless specified to the contrary, and according to conventional meaning, i.e., as a sequence of amino acids. Polypeptides are not limited to a specific length, e.g., they may comprise a full-length protein sequence or a fragment of a full-length protein, and may include post-translational modifications of the polypeptide, for example, glycosylations, acetylations, phosphorylations and the like, as well as other modifications known in the art, both naturally occurring and non-naturally occurring. 
     An “isolated peptide” or an “isolated polypeptide” and the like, as used herein, refer to in vitro isolation and/or purification of a peptide or polypeptide molecule from a cellular environment, and from association with other components of the cell, i.e., it is not significantly associated with in vivo substances. In particular embodiments, an isolated polypeptide is a synthetic polypeptide, a semi-synthetic polypeptide, or a polypeptide obtained or derived from a recombinant source. 
     Similarly, an “isolated cell” refers to a cell that has been obtained from an in vivo tissue or organ and is substantially free of extracellular matrix. 
     Polypeptides include “polypeptide variants.” Polypeptide variants may differ from a naturally occurring polypeptide in one or more substitutions, deletions, additions and/or insertions. Such variants may be naturally occurring or may be synthetically generated, for example, by modifying one or more of the above polypeptide sequences. For example, in particular embodiments, it may be desirable to improve the binding affinity and/or other biological properties of an anti-FRB antibody or antigen binding fragment thereof by introducing one or more amino acid substitutions, deletions, additions and/or insertions. In particular embodiments, polypeptides include polypeptides having at least about 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 86%, 97%, 98%, or 99% amino acid identity to any of the reference sequences contemplated herein, typically where the variant maintains at least one biological activity of the reference sequence. In particular embodiments, the biological activity is binding affinity. 
     Polypeptides variants include biologically active “polypeptide fragments.” Illustrative examples of biologically active polypeptide fragments include binding domains and the like. As used herein, the term “biologically active fragment” or “minimal biologically active fragment” refers to a polypeptide fragment that retains at least 100%, at least 90%, at least 80%, at least 70%, at least 60%, at least 50%, at least 40%, at least 30%, at least 20%, at least 10%, or at least 5% of the naturally occurring polypeptide activity. In certain embodiments, a polypeptide fragment can comprise an amino acid chain at least 5 to about 500 amino acids long. It will be appreciated that in certain embodiments, fragments are at least 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 150, 200, 250, 300, 350, 400, or 450 amino acids long. Particularly useful polypeptide fragments include functional domains, including antigen-binding domains or fragments of antibodies. In the case of an anti-FRB antibody, useful fragments include, but are not limited to: a CDR region, a CDR3 region of the heavy or light chain; a variable region of a heavy or light chain; a portion of an antibody chain or variable region including two CDRs; and the like. 
     The polypeptide may also be fused in-frame or conjugated to a linker or other sequence for ease of synthesis, purification or identification of the polypeptide (e.g., poly-His), or to enhance binding of the polypeptide to a solid support. 
     As noted above, polypeptides may be altered in various ways including amino acid substitutions, deletions, truncations, and insertions. Methods for such manipulations are generally known in the art. For example, amino acid sequence variants of a reference polypeptide can be prepared by mutations in the DNA. Methods for mutagenesis and nucleotide sequence alterations are well known in the art. See, for example, Kunkel (1985 , Proc. Natl. Acad. Sci. USA.  82: 488-492), Kunkel et al., (1987 , Methods in Enzymol,  154: 367-382), U.S. Pat. No. 4,873,192, Watson, J. D. et al., ( Molecular Biology of the Gene , Fourth Edition, Benjamin/Cummings, Menlo Park, Calif., 1987) and the references cited therein. Guidance as to appropriate amino acid substitutions that do not affect biological activity of the protein of interest may be found in the model of Dayhoff et al., (1978)  Atlas of Protein Sequence and Structure  (Natl. Biomed. Res. Found., Washington, D.C.). 
     In certain embodiments, a polypeptide variant comprises one or more conservative substitutions. A “conservative substitution” is one in which an amino acid is substituted for another amino acid that has similar properties, such that one skilled in the art of peptide chemistry would expect the secondary structure and hydropathic nature of the polypeptide to be substantially unchanged. Modifications may be made in the structure of the polynucleotides and polypeptides contemplated in particular embodiments and still obtain a functional molecule that encodes a variant or derivative polypeptide with desirable characteristics. When it is desired to alter the amino acid sequence of a polypeptide to create an equivalent, or even an improved, variant polypeptide, one skilled in the art, for example, can change one or more of the codons of the encoding DNA sequence, e.g., according to Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Amino Acid Codons 
               
            
           
           
               
               
               
               
            
               
                   
                 One 
                 Three 
                   
               
               
                   
                 letter 
                 letter 
               
               
                 Amino Acids 
                 code 
                 code 
                 Codons 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Alanine 
                 A 
                 Ala 
                 GCA 
                 GCC 
                 GCG 
                 GCU 
               
            
           
           
               
               
               
               
               
            
               
                 Cysteine 
                 C 
                 Cys 
                 UGC 
                 UGU 
               
               
                 Aspartic acid 
                 D 
                 Asp 
                 GAC 
                 GAU 
               
               
                 Glutamic acid 
                 E 
                 Glu 
                 GAA 
                 GAG 
               
               
                 Phenylalanine 
                 F 
                 Phe 
                 UUC 
                 UUU 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Glycine 
                 G 
                 Gly 
                 GGA 
                 GGC 
                 GGG 
                 GGU 
               
            
           
           
               
               
               
               
               
            
               
                 Histidine 
                 H 
                 His 
                 CAC 
                 CAU 
               
            
           
           
               
               
               
               
               
               
            
               
                 Isoleucine 
                 I 
                 Iso 
                 AUA 
                 AUC 
                 AUU 
               
            
           
           
               
               
               
               
               
            
               
                 Lysine 
                 K 
                 Lys 
                 AAA 
                 AAG 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Leucine 
                 L 
                 Leu 
                 UUA 
                 UUG 
                 CUA 
                 CUC 
                 CUG 
                 CUU 
               
            
           
           
               
               
               
               
            
               
                 Methionine 
                 M 
                 Met 
                 AUG 
               
            
           
           
               
               
               
               
               
            
               
                 Asparagine 
                 N 
                 Asn 
                 AAC 
                 AAU 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Proline 
                 P 
                 Pro 
                 CCA 
                 CCC 
                 CCG 
                 CCU 
               
            
           
           
               
               
               
               
               
            
               
                 Glutamine 
                 Q 
                 Gln 
                 CAA 
                 CAG 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Arginine 
                 R 
                 Arg 
                 AGA 
                 AGG 
                 CGA 
                 CGC 
                 CGG 
                 CGU 
               
               
                 Serine 
                 S 
                 Ser 
                 AGC 
                 AGU 
                 UCA 
                 UCC 
                 UCG 
                 UCU 
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Threonine 
                 T 
                 Thr 
                 ACA 
                 ACC 
                 ACG 
                 ACU 
               
               
                 Valine 
                 V 
                 Val 
                 GUA 
                 GUC 
                 GUG 
                 GUU 
               
            
           
           
               
               
               
               
            
               
                 Tryptophan 
                 W 
                 Trp 
                 UGG 
               
            
           
           
               
               
               
               
               
            
               
                 Tyrosine 
                 Y 
                 Tyr 
                 UAC 
                 UAU 
               
               
                   
               
            
           
         
       
     
     Guidance in determining which amino acid residues can be substituted, inserted, or deleted without abolishing biological activity can be found using computer programs well known in the art, such as DNASTAR, DNA Strider, Geneious, MacVector, or Vector NTI software. Preferably, amino acid changes in the protein variants disclosed herein are conservative amino acid changes, i.e., substitutions of similarly charged or uncharged amino acids. A conservative amino acid change involves substitution of one of a family of amino acids which are related in their side chains. Naturally occurring amino acids are generally divided into four families: acidic (aspartate, glutamate), basic (lysine, arginine, histidine), non-polar (alanine, valine, leucine, isoleucine, proline, phenylalanine, methionine, tryptophan), and uncharged polar (glycine, asparagine, glutamine, cysteine, serine, threonine, tyrosine) amino acids. Phenylalanine, tryptophan, and tyrosine are sometimes classified jointly as aromatic amino acids. In a peptide or protein, suitable conservative substitutions of amino acids are known to those of skill in this art and generally can be made without altering a biological activity of a resulting molecule. Those of skill in this art recognize that, in general, single amino acid substitutions in non-essential regions of a polypeptide do not substantially alter biological activity (see, e.g., Watson et al.  Molecular Biology of the Gene,  4th Edition, 1987, The Benjamin/Cummings Pub. Co., p. 224). 
     In making such changes, the hydropathic index of amino acids may be considered. The importance of the hydropathic amino acid index in conferring interactive biologic function on a protein is generally understood in the art (Kyte and Doolittle, 1982, incorporated herein by reference). Each amino acid has been assigned a hydropathic index on the basis of its hydrophobicity and charge characteristics (Kyte and Doolittle, 1982). These values are: isoleucine (+4.5); valine (+4.2); leucine (+3.8); phenylalanine (+2.8); cysteine/cysteine (+2.5); methionine (+1.9); alanine (+1.8); glycine (−0.4); threonine (−0.7); serine (−0.8); tryptophan (−0.9); tyrosine (−1.3); proline (−1.6); histidine (−3.2); glutamate (−3.5); glutamine (−3.5); aspartate (−3.5); asparagine (−3.5); lysine (−3.9); and arginine (−4.5). 
     It is known in the art that certain amino acids may be substituted by other amino acids having a similar hydropathic index or score and still result in a protein with similar biological activity, i.e., still obtain a biological functionally equivalent protein. In making such changes, the substitution of amino acids whose hydropathic indices are within ±2 is preferred, those within ±1 are particularly preferred, and those within ±0.5 are even more particularly preferred. It is also understood in the art that the substitution of like amino acids can be made effectively on the basis of hydrophilicity. 
     As detailed in U.S. Pat. No. 4,554,101, the following hydrophilicity values have been assigned to amino acid residues: arginine (+3.0); lysine (+3.0); aspartate (+3.0±1); glutamate (+3.0±1); serine (+0.3); asparagine (+0.2); glutamine (+0.2); glycine (0); threonine (−0.4); proline (−0.5±1); alanine (−0.5); histidine (−0.5); cysteine (−1.0); methionine (−1.3); valine (−1.5); leucine (−1.8); isoleucine (−1.8); tyrosine (−2.3); phenylalanine (−2.5); tryptophan (−3.4). It is understood that an amino acid can be substituted for another having a similar hydrophilicity value and still obtain a biologically equivalent, and in particular, an immunologically equivalent protein. In such changes, the substitution of amino acids whose hydrophilicity values are within ±2 is preferred, those within ±1 are particularly preferred, and those within ±0.5 are even more particularly preferred. 
     As outlined above, amino acid substitutions may be based on the relative similarity of the amino acid side-chain substituents, for example, their hydrophobicity, hydrophilicity, charge, size, and the like. 
     Polypeptide variants further include glycosylated forms, aggregative conjugates with other molecules, and covalent conjugates with unrelated chemical moieties (e.g., pegylated molecules). Covalent variants can be prepared by linking functionalities to groups which are found in the amino acid chain or at the N- or C-terminal residue, as is known in the art. Variants also include allelic variants, species variants, and muteins. Truncations or deletions of regions which do not affect functional activity of the proteins are also variants. 
     Polypeptides contemplated in particular embodiments include fusion polypeptides. In preferred embodiments, fusion polypeptides and polynucleotides encoding fusion polypeptides are provided, e.g., light chain or heavy chain variable regions linked by a linker sequence. 
     Fusion polypeptides may optionally comprise a linker that can be used to link the one or more polypeptides or domains within a polypeptide. A peptide linker sequence may be employed to separate any two or more polypeptide components by a distance sufficient to ensure that each polypeptide folds into its appropriate secondary and tertiary structures so as to allow the polypeptide domains to exert their desired functions. Such a peptide linker sequence is incorporated into the fusion polypeptide using standard techniques in the art. Suitable peptide linker sequences may be chosen based on the following factors: (1) their ability to adopt a flexible extended conformation; (2) their inability to adopt a secondary structure that could interact with functional epitopes on the first and second polypeptides; and (3) the lack of hydrophobic or charged residues that might react with the polypeptide functional epitopes. Preferred peptide linker sequences contain Gly, Asn and Ser residues. Other near neutral amino acids, such as Thr and Ala may also be used in the linker sequence. Amino acid sequences which may be usefully employed as linkers include those disclosed in Maratea et al.,  Gene  40:39-46, 1985; Murphy et al.,  Proc. Natl. Acad. Sci. USA  83:8258-8262, 1986; U.S. Pat. Nos. 4,935,233 and 4,751,180. Linker sequences are not required when a particular fusion polypeptide segment contains non-essential N-terminal amino acid regions that can be used to separate the functional domains and prevent steric interference. Preferred linkers are typically flexible amino acid subsequences which are synthesized as part of a recombinant fusion protein. Linker polypeptides can be between 1 and 200 amino acids in length, between 1 and 100 amino acids in length, or between 1 and 50 amino acids in length, including all integer values in between. 
     Exemplary linkers include, but are not limited to the following amino acid sequences: glycine polymers (G) n ; glycine-serine polymers (G 1-5 S 1-5 ) n , where n is an integer of at least one, two, three, four, or five; glycine-alanine polymers; alanine-serine polymers; GGG; DGGGS (SEQ ID NO: 52); TGEKP (SEQ ID NO: 53) (see e.g., Liu et ed.,  PNAS  5525-5530 (1997)); GGRR (SEQ ID NO: 54) (Pomerantz et al. 1995, supra); (GGGGS) n  (SEQ ID NO: 55) (Kim et al.,  PNAS  93, 1156-1160 (1996.); EGKSSGSGSESKVD (SEQ ID NO: 56) (Chaudhary et al., 1990 , Proc. Natl. Acad. Sci. U.S.A.  87:1066-1070); KESGSVSSEQLAQFRSLD (SEQ ID NO: 57) (Bird et al., 1988 , Science  242:423-426), GGRRGGGS (SEQ ID NO: 58); LRQRDGERP (SEQ ID NO: 59); LRQKDGGGSERP (SEQ ID NO: 60); LRQKD(GGGS) 2 ERP (SEQ ID NO: 61). Alternatively, flexible linkers can be rationally designed using a computer program capable of modeling both DNA-binding sites and the peptides themselves (Desjarlais &amp; Berg,  PNAS  90:2256-2260 (1993),  PNAS  91:110911-13103 (1994) or by phage display methods. 
     Fusion polypeptides are typically linked C-terminus to N-terminus, although they can also be linked C-terminus to C-terminus, N-terminus to N-terminus, or N-terminus to C-terminus. The polypeptides of the fusion protein can be in any order or a specified order. Fusion polypeptides or fusion proteins can also include conservatively modified variants, polymorphic variants, alleles, mutants, subsequences, and interspecies homologs, so long as the desired activity of the fusion polypeptide is preserved. Fusion polypeptides may be produced by chemical synthetic methods or by chemical linkage between the two moieties or may generally be prepared using other standard techniques. Ligated DNA sequences comprising the fusion polypeptide are operably linked to suitable transcriptional or translational control elements as discussed elsewhere herein. 
     In one embodiment, a fusion partner comprises a sequence that assists in expressing the protein (an expression enhancer) at higher yields than the native recombinant protein. Other fusion partners may be selected so as to increase the solubility of the protein or to enable the protein to be targeted to desired intracellular compartments or to facilitate transport of the fusion protein through the cell membrane. 
     Fusion polypeptides may further comprise a polypeptide cleavage signal between each of the polypeptide domains described herein. In addition, a polypeptide cleavage site can be put into any linker peptide sequence. Exemplary polypeptide cleavage signals include polypeptide cleavage recognition sites such as protease cleavage sites, nuclease cleavage sites (e.g., rare restriction enzyme recognition sites, self-cleaving ribozyme recognition sites), and self-cleaving viral oligopeptides (see deFelipe and Ryan, 2004 . Traffic,  5(8); 616-26). 
     F. Polynucleotides 
     In preferred embodiments, a polynucleotide encoding an antibody or antigen binding fragment thereof contemplated herein is provided. As used herein, the terms “polynucleotide” or “nucleic acid” refers to messenger RNA (mRNA), RNA, genomic DNA (gDNA), complementary DNA (cDNA) or recombinant DNA. Polynucleotides include single and double stranded polynucleotides. In particular embodiments, polynucleotides include polynucleotides or variants having at least about 50%, 55%, 60%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 86%, 97%, 98%, or 99% sequence identity to any of the reference sequences contemplated herein. In various illustrative embodiments, polynucleotides encoding a polypeptide contemplated herein, including, but not limited to the polypeptide sequences set forth in SEQ ID NOs: 1-40. 
     In particular embodiments, polynucleotides are provided that encode at least about 5, 10, 25, 50, 100, 150, 200, 250, 300, 350, 400, 500, 1000, 1250, 1500, 1750, or 2000 or more contiguous amino acid residues of a polypeptide, as well as all intermediate lengths. It will be readily understood that “intermediate lengths,” in this context, means any length between the quoted values, such as 6, 7, 8, 9, etc., 101, 102, 103, etc.; 151, 152, 153, etc.; 201, 202, 203, etc. 
     As used herein, the terms “polynucleotide variant” and “variant” and the like refer to polynucleotides displaying substantial sequence identity with a reference polynucleotide sequence or polynucleotides that hybridize with a reference sequence under stringent conditions that are defined hereinafter. These terms include polynucleotides in which one or more nucleotides have been added or deleted or replaced with different nucleotides compared to a reference polynucleotide. In this regard, it is well understood in the art that certain alterations inclusive of mutations, additions, deletions and substitutions can be made to a reference polynucleotide whereby the altered polynucleotide retains the biological function or activity of the reference polynucleotide. 
     The recitations “sequence identity” or, for example, comprising a “sequence 50% identical to,” as used herein, refer to the extent that sequences are identical on a nucleotide-by-nucleotide basis or an amino acid-by-amino acid basis over a window of comparison. Thus, a “percentage of sequence identity” may be calculated by comparing two optimally aligned sequences over the window of comparison, determining the number of positions at which the identical nucleic acid base (e.g., A, T, C, G, I) or the identical amino acid residue (e.g., Ala, Pro, Ser, Thr, Gly, Val, Leu, Ile, Phe, Tyr, Trp, Lys, Arg, His, Asp, Glu, Asn, Gln, Cys and Met) occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison (i.e., the window size), and multiplying the result by 100 to yield the percentage of sequence identity. Included are nucleotides and polypeptides having at least about 50%, 55%, 60%, 65%, 66%, 67%, 68%, 69%, 70%, 71%, 72%, 73%, 74%, 75%, 76%, 77%, 78%, 79%, 80%, 81%, 82%, 83%, 84%, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 86%, 97%, 98%, or 99% sequence identity to any of the reference sequences described herein, typically where the polypeptide variant maintains at least one biological activity of the reference polypeptide. 
     Terms used to describe sequence relationships between two or more polynucleotides or polypeptides include “reference sequence,” “comparison window,” “sequence identity,” “percentage of sequence identity,” and “substantial identity”. A “reference sequence” is at least 12 but frequently 15 to 18 and often at least 25 monomer units, inclusive of nucleotides and amino acid residues, in length. Because two polynucleotides may each comprise (1) a sequence (i.e., only a portion of the complete polynucleotide sequence) that is similar between the two polynucleotides, and (2) a sequence that is divergent between the two polynucleotides, sequence comparisons between two (or more) polynucleotides are typically performed by comparing sequences of the two polynucleotides over a “comparison window” to identify and compare local regions of sequence similarity. A “comparison window” refers to a conceptual segment of at least 6 contiguous positions, usually about 50 to about 100, more usually about 100 to about 150 in which a sequence is compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. The comparison window may comprise additions or deletions (i.e., gaps) of about 20% or less as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. Optimal alignment of sequences for aligning a comparison window may be conducted by computerized implementations of algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package Release 7.0, Genetics Computer Group, 575 Science Drive Madison, Wis., USA) or by inspection and the best alignment (i.e., resulting in the highest percentage homology over the comparison window) generated by any of the various methods selected. Reference also may be made to the BLAST family of programs as for example disclosed by Altschul et al., 1997, Nucl. Acids Res. 25:3389. A detailed discussion of sequence analysis can be found in Unit 19.3 of Ausubel et al., Current Protocols in Molecular Biology, John Wiley &amp; Sons Inc, 1994-1998, Chapter 15. 
     As used herein, “isolated polynucleotide” refers to a polynucleotide that has been purified from the sequences which flank it in a naturally-occurring state, e.g., a DNA fragment that has been removed from the sequences that are normally adjacent to the fragment. An “isolated polynucleotide” also refers to a complementary DNA (cDNA), a recombinant DNA, or other polynucleotide that does not exist in nature and that has been made by the hand of man. 
     Terms that describe the orientation of polynucleotides include: 5′ (normally the end of the polynucleotide having a free phosphate group) and 3′ (normally the end of the polynucleotide having a free hydroxyl (OH) group). Polynucleotide sequences can be annotated in the 5′ to 3′ orientation or the 3′ to 5′ orientation. For DNA and mRNA, the 5′ to 3′ strand is designated the “sense,” “plus,” or “coding” strand because its sequence is identical to the sequence of the premessenger (premRNA) [except for uracil (U) in RNA, instead of thymine (T) in DNA]. For DNA and mRNA, the complementary 3′ to 5′ strand which is the strand transcribed by the RNA polymerase is designated as “template,” “antisense,” “minus,” or “non-coding” strand. As used herein, the term “reverse orientation” refers to a 5′ to 3′ sequence written in the 3′ to 5′ orientation or a 3′ to 5′ sequence written in the 5′ to 3′ orientation. 
     The terms “complementary” and “complementarity” refer to polynucleotides (i.e., a sequence of nucleotides) related by the base-pairing rules. For example, the complementary strand of the DNA sequence 5′ A G T C A T G 3′ is 3′ T C A G T A C 5′. The latter sequence is often written as the reverse complement with the 5′ end on the left and the 3′ end on the right, 5′ C A T G A C T 3′. A sequence that is equal to its reverse complement is said to be a palindromic sequence. Complementarity can be “partial,” in which only some of the nucleic acids&#39; bases are matched according to the base pairing rules. Or, there can be “complete” or “total” complementarity between the nucleic acids. 
     Moreover, it will be appreciated by those of ordinary skill in the art that, as a result of the degeneracy of the genetic code, there are many nucleotide sequences that encode a polypeptide, or fragment of variant thereof, as described herein. Some of these polynucleotides bear minimal homology to the nucleotide sequence of any native sequence. Nonetheless, polynucleotides that vary due to differences in codon usage are specifically contemplated in particular embodiments, for example polynucleotides that are optimized for human and/or primate codon selection. 
     Polynucleotides can be prepared, manipulated and/or expressed using any of a variety of well-established techniques known and available in the art. In order to express a desired polypeptide, a nucleotide sequence encoding the polypeptide, can be inserted into appropriate vector. 
     G. Sequence Listing 
       
     
       
         
           
               
               
               
             
               
                   
               
               
                 SEQ 
                   
                   
               
               
                 ID 
                   
                   
               
               
                 NO 
                 NAME 
                 SEQUENCE 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 1 
                 2K1 LCDR1 
                 QASESIYDYLS 
               
               
                   
               
               
                 2 
                 2K1 LCDR2 
                 RVSTLAS 
               
               
                   
               
               
                 3 
                 2K1 LCDR3 
                 QQAYQYDVINL 
               
               
                   
               
               
                 4 
                 2H1 HCDR1 
                 GIDLSHYDIN 
               
               
                   
               
               
                 5 
                 2H1 HCDR2 
                 AIYGSDNTYYARWAKG 
               
               
                   
               
               
                 6 
                 2H1 HCDR3 
                 DVPGYGAFDAFDS 
               
               
                   
               
               
                 7 
                 2K1 Light 
                 MDTRAPTQLLGLLLLWLPGARCADIVMTQTPASVEAAVGGTITIKCQ 
               
               
                   
                 Chain 
                 ASESIYDYLSWYQQKPGQPPKLLIYRVSTLASGVSSRFSASGYGTEF 
               
               
                   
                 Variable 
                 SLTISGVECADAATYYCQQAYQYDVINLFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 8 
                 2H1 Heavy 
                 METGLRWLLLVAVLKGVQCQSLEESGGRLVTPGTPLTLTCTVSGIDL 
               
               
                   
                 Chain 
                 SHYDINWIRQAPGKGLEWVGAIYGSDNTYYARWAKGRVTISKTSTTV 
               
               
                   
                 Variable 
                 ELIMSSPTTEDTATYFCARDVPGYGAFDAFDSWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 9 
                 2K1 Light 
                 DIVMTQTPASVEAAVGGTITIKCQASESIYDYLSWYQQKPGQPPKLL 
               
               
                   
                 Chain 
                 IYRVSTLASGVSSRFSASGYGTEFSLTISGVECADAATYYCQQAYQY 
               
               
                   
                 Variable 
                 DVINLFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 10 
                 2H1 Heavy 
                 QCQSLEESGGRLVTPGTPLTLTCTVSGIDLSHYDINWIRQAPGKGLE 
               
               
                   
                 Chain 
                 WVGAIYGSDNTYYARWAKGRVTISKTSTTVELIMSSPTTEDTATYFC 
               
               
                   
                 Variable 
                 ARDVPGYGAFDAFDSWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 11 
                 3K1 LCDR1 
                 QSSQSVYTNNDLA 
               
               
                   
               
               
                 12 
                 3K1 LCDR2 
                 RASRLAS 
               
               
                   
               
               
                 13 
                 3K1 LCDR3 
                 LGGYDDDADNA 
               
               
                   
               
               
                 14 
                 3H3 HCDR1 
                 GIDLNINNVA 
               
               
                   
               
               
                 15 
                 3H3 HCDR2 
                 WLSSSGSTYYAIWAKG 
               
               
                   
               
               
                 16 
                 3H3 HCDR3 
                 GPGWDTGILFNL 
               
               
                   
               
               
                 17 
                 3K1 Light 
                 MDTRAPTQLLGLLLLWLPGATFAAVLTQTPSPVSAAVGGTVTISCQS 
               
               
                   
                 Chain 
                 SQSVYTNNDLAWYQQKPGQPPKVLIYRASRLASGVPSRFSGSGSGTQ 
               
               
                   
                 Variable 
                 FSLTISGVQCDDAATYYCLGGYDDDADNAFGGGTEVWK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 18 
                 3H3 Heavy 
                 METGLRWLLLVAVLKGVQCQSVEESGGRLVTPGTPLTLTCTVSGIDL 
               
               
                   
                 Chain 
                 NINNVAWVRQSPGEGLEYIGWLSSSGSTYYAIWAKGRFIISKTSSTT 
               
               
                   
                 Variable 
                 VDLKMPSLTTEDTATYFCARGPGWDTGILFNLWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 19 
                 3K1 Light 
                 AAVLTQTPSPVSAAVGGTVTISCQSSQSVYTNNDLAWYQQKPGQPPK 
               
               
                   
                 Chain 
                 VLIYRASRLASGVPSRFSGSGSGTQFSLTISGVQCDDAATYYCLGGY 
               
               
                   
                 Variable 
                 DDDADNAFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 20 
                 3H3 Heavy 
                 QCQSVEESGGRLVTPGTPLTLTCTVSGIDLNINNVAWVRQSPGEGLE 
               
               
                   
                 Chain 
                 YIGWLSSSGSTYYAIWAKGRFIISKTSSTTVDLKMPSLTTEDTATYF 
               
               
                   
                 Variable 
                 CARGPGWDTGILFNLWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 21 
                 5K1 LCDR1 
                 QASQSIRTALA 
               
               
                   
               
               
                 22 
                 5K1 LCDR2 
                 DTSDLAS 
               
               
                   
               
               
                 23 
                 5K1 LCDR3 
                 QSYSYSGSRNYV 
               
               
                   
               
               
                 24 
                 5H1 HCDR1 
                 GLDFSSYWLC 
               
               
                   
               
               
                 25 
                 5H1 HCDR2 
                 CIYTGSSPYIDYASWAKG 
               
               
                   
               
               
                 26 
                 5H1 HCDR3 
                 VGYGEITYFGL 
               
               
                   
               
               
                 27 
                 5K1 Light 
                 MDTRAPTQLLGLLLLWLPGARCGDVVMTQTPASVEAAVGGTVTIKCQ 
               
               
                   
                 Chain 
                 ASQSIRTALAWYQQKPGQPPKLLISDTSDLASGVPSRFSGSGSGTEF 
               
               
                   
                 Variable 
                 TLTISGVOCEDAATYYCQSYSYSGSRNYVFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 28 
                 5H1 Heavy 
                 METGLRWLLLVAVLKGVQCQEQLVESGGGLVQPEGSLTLTCKGSGLD 
               
               
                   
                 Chain 
                 FSSYWLCWVRQAPGKGLEWIACIYTGSSPYIDYASWAKGRFTISKTS 
               
               
                   
                 Variable 
                 STSVTLQMTSLTVADTATYFCARVGYGEITYFGLWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 29 
                 5K1 Light 
                 DVVMTQTPASVEAAVGGTVTIKCQASQSIRTALAWYQQKPGQPPKLL 
               
               
                   
                 Chain 
                 ISDTSDLASGVPSRFSGSGSGTEFTLTISGVQCEDAATYYCQSYSYS 
               
               
                   
                 Variable 
                 GSRNYVFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 30 
                 5H1 Heavy 
                 QCQEQLVESGGGLVQPEGSLTLTCKGSGLDFSSYWLCWVRQAPGKGL 
               
               
                   
                 Chain 
                 EWIACIYTGSSPYIDYASWAKGRFTISKTSSTSVTLQMTSLTVADTA 
               
               
                   
                 Variable 
                 TYFCARVGYGEITYFGLWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 31 
                 6K3 LCDR1 
                 QSSESVYSNYLS 
               
               
                   
               
               
                 32 
                 6K3 LCDR2 
                 DASYLAS 
               
               
                   
               
               
                 33 
                 6K3 LCDR3 
                 AGYYSGVIYA 
               
               
                   
               
               
                 34 
                 6H2 HCDR1 
                 GFSFSTSYWIY 
               
               
                   
               
               
                 35 
                 6H2 HCDR2 
                 HIHTGTDNTYYATWAEG 
               
               
                   
               
               
                 36 
                 6H2 HCDR3 
                 DLLYDNSHL 
               
               
                   
               
               
                 37 
                 6K3 Light 
                 MDTRAPTQLLGLLLLWLPGATFAQVLTQTPSPVSVAVGGTVTINCQS 
               
               
                   
                 Chain 
                 SESVYSNYLSWYQQKPGQPPKVLIYDASYLASGVPSRFKGSGSGTQF 
               
               
                   
                 Variable 
                 TLTISDMLCDDAATYYCAGYYSGVIYAFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 38 
                 6H2 Heavy 
                 METGLRWLLLVAVLKGVQCQSLEESGGDLVKPGASLTLTCTASGFSF 
               
               
                   
                 Chain 
                 STSYWIYWVRQAPGKGLEWIAHIHTGTDNTYYATWAEGRFTISKASS 
               
               
                   
                 Variable 
                 TTVTLQVTSLTAADTATYFCARDLLYDNSHLWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 39 
                 6K3 Light 
                 AQVLTQTPSPVSVAVGGTVTINCQSSESVYSNYLSWYQQKPGQPPKV 
               
               
                   
                 Chain 
                 LIYDASYLASGVPSRFKGSGSGTQFTLTISDMLCDDAATYYCAGYYS 
               
               
                   
                 Variable 
                 GVIYAFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 40 
                 6H2 Heavy 
                 QCQSLEESGGDLVKPGASLTLTCTASGFSFSTSYWIYWVRQAPGKGL 
               
               
                   
                 Chain 
                 EWIAHIHTGTDNTYYATWAEGRFTISKASSTTVTLQVTSLTAADTAT 
               
               
                   
                 Variable 
                 YFCARDLLYDNSHLWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 41 
                 9K3 LCDR1 
                 QSSQSVYVNNDLA 
               
               
                   
               
               
                 42 
                 9K3 LCDR2 
                 RASKLAS 
               
               
                   
               
               
                 43 
                 9K3 LCDR3 
                 LGGYDDDAHNA 
               
               
                   
               
               
                 44 
                 9H2 HCDR1 
                 GIDLSSNNMA 
               
               
                   
               
               
                 45 
                 9H2 HCDR2 
                 WISSSGSTYYASWAKG 
               
               
                   
               
               
                 46 
                 9H2 HCDR3 
                 GPGWDTGILFSL 
               
               
                   
               
               
                 47 
                 9K3 Light 
                 MDTRAPTQLLGLLLLWLPGATFAAVLTQTPSPVSAAVGGTVTISCQS 
               
               
                   
                 Chain 
                 SQSVYVNNDLAWYQQKPGQPPKLLIYRASKLASGVTSRFSGSGSGTQ 
               
               
                   
                 Variable 
                 FTLTISGVQCDDAATYYCLGGYDDDAHNAFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 48 
                 9H2 Heavy 
                 METGLRWLLLVAVLKGVQCQSVEESGGRLVTPGTPLTLTCTVSGIDL 
               
               
                   
                 Chain 
                 SSNNMAWVRQSPGEGLEYIGWISSSGSTYYASWAKGRFTISKASSTT 
               
               
                   
                 Variable 
                 VDLKMTSLTPEDTATYFCARGPGWDTGILFSLWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 49 
                 9K3 Light 
                 AAVLTQTPSPVSAAVGGTVTISCQSSQSVYVNNDLAWYQQKPGQPPK 
               
               
                   
                 Chain 
                 LLIYRASKLASGVTSRFSGSGSGTQFTLTISGVQCDDAATYYCLGGY 
               
               
                   
                 Variable 
                 DDDAHNAFGGGTEVVVK 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 50 
                 9H2 Heavy 
                 QCQSVEESGGRLVTPGTPLTLTCTVSGIDLSSNNMAWVRQSPGEGLE 
               
               
                   
                 Chain 
                 YIGWISSSGSTYYASWAKGRFTISKASSTTVDLKMTSLTPEDTATYF 
               
               
                   
                 Variable 
                 CARGPGWDTGILFSLWGQGTLVTVSS 
               
               
                   
                 Region 
                   
               
               
                   
               
               
                 51 
                 FRB 
                 GSILWHEMWHEGLEEASRLYFGERNVKGMFEVLEPLHAMMERGPQTL 
               
               
                   
                 polypeptide 
                 KETSFNQAYGRDLMEAQEWCRKYMKSGNVKDLLQAWDLYYHVFRRIS 
               
               
                   
                   
                 KAS 
               
               
                   
               
            
           
         
       
     
     All publications, patent applications, and issued patents cited in this specification are herein incorporated by reference as if each individual publication, patent application, or issued patent were specifically and individually indicated to be incorporated by reference. 
     Although the foregoing embodiments have been described in some detail by way of illustration and example for purposes of clarity of understanding, it will be readily apparent to one of ordinary skill in the art in light of the teachings contemplated herein that certain changes and modifications may be made thereto without departing from the spirit or scope of the appended claims. The following examples are provided by way of illustration only and not by way of limitation. Those of skill in the art will readily recognize a variety of noncritical parameters that could be changed or modified to yield essentially similar results. 
     EXAMPLES 
     Example 1 
     Anti-Rabbit Monoclonal Antibody Production 
     Recombinant FRB (SEQ ID NO: 51) was conjugated to rabbit Fc and used to immunize rabbits. Antibody sequences were isolated from primary rabbit B cells and expressed recombinantly.  FIG.  1   . 
     Briefly, recombinant anti-FRB antibodies were produced via transient transfection of EXPI 293 cells in EXPI 293 media. Each 150 mL transient transfection was performed by complexing expression plasmid with PEI Pro. Expi 293 cells were transfected at a cell density of 3×10 6  cells/mL. After 96 hours, the cell supernatant was collected and 0.22 μM filtered. The secreted recombinant protein was purified from the cell supernatant by MAb Select Prisma affinity chromatography on an AKTA Pure. After MAb Select Prisma purification, recombinant antibody quality and purity were assessed via SDS Page electrophoresis. Each antibody was &gt;95% purity after MAb Select Prisma purification. 
     Example 2 
     Anti-Frb Monoclonal Antibodies 
     Peripheral blood mononuclear cells (PBMCs) were activated using CD3 and CD28 antibodies and transduced with a lentiviral vector encoding a chimeric protein the extracellular expresses an FRB domain and cultured for T cell expansion. 
     50,000 untransduced (UTD) T cells and T cells transduced to express FRB were washed in 200 μL of PBS and then resuspended in 20 μL of PBS and 16 μg/mL, 8 μg/mL, 4 μg/mL, 1.6 μg/mL, 0.8 μg/mL, or 0.4 μg/mL of each antibody and incubated at room temp for 30 min. Cells were then diluted and washed in PBS and resuspended in 20 μL PBS+1:500 Goat Anti-Rabbit IgG (AF647) (Invitrogen A-21246). Cells were incubated for an additional 30 min. After incubation, the cells were diluted and washed in PBS and resuspended in 200 μL PBS. Cells were analyzed via the Attune Cytometer in the APC channel. Data for each antibody clone is shown in  FIGS.  2 A- 2 E . 
     In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.