Patent Publication Number: US-2019175728-A1

Title: Nucleic acid-antigen peptide conjugate

Description:
This application claims the priority of PCT/JP2017/022283, filed Jun. 16, 2017, and JP 2016-120261, filed Jun. 16, 2016, from which the PCT application claims priority, the disclosures of which are incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a nucleic acid-antigen peptide conjugate. More specifically, the present invention relates to a nucleic acid-antigen peptide conjugate which is capable of forming a complex with a polysaccharide having a β-1,3-glucan skeleton such as schizophyllan, and which can efficiently present the antigen peptide in an antigen-presenting cell. 
     BACKGROUND ART 
     The basic principle of prevention of infectious diseases and cancer by vaccines is based on artificial pseudo-infection to induce acquired immunity, and to induce cellular immunity and antibody production against a specific pathogen. In cell-mediated immunity, first, an antigen-presenting cell phagocytoses an antigen protein and digests the antigen protein with an intracellular protease to generate an antigen peptide. The antigen peptide binds to MHC, and the antigen peptide is exposed on the membrane surface of the antigen-presenting cell. Then, information of the foreign substance is presented to T cells. Subsequently, the T cells are activated by recognizing the antigen peptide bound to MHC by the T cell receptor on the membrane surface. The activated T cells specifically damage cancer cells or virus-infected cells or induce the production of various cytokines, thereby activating immunity. 
     There are two types of MHC molecules, class I and class II, which bind peptides at different locations in the cells. Peptides generated in the cytoplasm migrate to the endoplasmic reticulum, bind to MHC class I molecules, and migrate to the cell surface. Then, the peptides presented as antigen peptides are recognized by CD8T cells. Further, proteins taken up by endocytosis are digested with lysosome, the resulting proteins are bound to MHC type II molecules in the endoplasmic reticulum and migrated to the cell surface. Then, the proteins presented as antigen peptides are recognized by CD4T cells. 
     On the other hand, there is a conventionally known method of forming a triple helix complex composed of one molecule of nucleic acid and two molecules of β-1,3 glucan in which β-1,3 glucan (e.g., schizophyllan) which forms a triple helix structure in nature is dissolved in an aprotic nonpolar organic solvent or an alkaline solution of 0.1 N or more to cause disassociation of the triple helix into a single strand, a single-stranded nucleic acid is added to the resulting mixture, and the solvent is replaced with water or the pH of the alkaline solution is adjusted to neutral. By complexation of the nucleic acid and β-1,3 glucan in the above manner, the nucleic acid may be delivered into the cell while suppressing hydrolysis of the nucleic acid molecule by nucleases, and while suppressing undesirable interactions between the nucleic acid molecule and proteins in the body such as non-specific binding of nucleic acid molecules with proteins in the blood plasma. 
     Therefore, various attempts have been made to utilize β-1,3 glucan in the vaccine therapy in which antigen peptides are administered, from the viewpoints of improving the delivery efficiency of antigen peptides to antigen-presenting cells and improving immunostimulatory activity. For example, N. Shimada, et al., “Synthesis and in Vitro Characterization of Antigen-Conjugated Polysaccharide as CpG DNA Carrier,” Bioconjugate Chem., 17, 1136-1140 (2006) (“Shimada”) discloses that, in the case of using a ternary complex of β-1,3 glucan/antigen peptide/CpG DNA obtained by bonding β-1,3 glucan to an antigen peptide and further complexing CpG DNA with the β-1,3 glucan, the antigen peptide and CpG DNA functioning as an adjuvant can be simultaneously and efficiently delivered to the antigen-presenting cell. Further, WO 2015/118789 discloses that a peptide/polynucleotide conjugate in which an antigen peptide is bonded, via a covalent bond, to a polynucleotide or a polynucleotide derivative is complexed with β-1,3 glucan to form a complex, whereby the productivity is excellent and the complex has high immunostimulatory activity. 
     Problems to be Solved by the Invention 
     It has been clarified that Dectin-1 is expressed in antigen-presenting cells such as macrophages and dendritic cells, and further that a nucleic acid-polysaccharide complex containing β-1,3 glucan (such as schizophyllan) has the property to be specifically incorporated into Dectin-1 expressing cells. Therefore, as in Shimada and WO 2015/118789, a complex containing β-1,3 glucan and antigen peptide has excellent delivery efficiency to antigen-presenting cells, and is greatly expected to be used in the vaccine therapy. 
     On the other hand, when a heterologous molecule such as polysaccharide or nucleic acid is bonded to an antigen peptide, the antigen peptide cannot be bonded to MHC after being incorporated into the antigen-presenting cell, whereby the antigen peptide is not presented on the membrane surface of the antigen-presenting cell or the efficiency of presenting the antigen peptide may be reduced. In Shimada and WO 2015/118789, there has been no adequate study as to whether the antigen peptide can be efficiently presented in the antigen-presenting cell when the antigen peptide is bonded to the heterologous molecule in any bonding mode. Indeed, the present inventors have confirmed that the technique of Shimada requires a relatively large number of peptides to induce the response of cytotoxic T cells (CTL) and cannot efficiently induce antigen-presenting cells (see Examples 5 and 6 to be described later). 
     An object of the present invention is to provide a nucleic acid-antigen peptide conjugate which is capable of forming a complex with a polysaccharide having a β-1,3-glucan skeleton such as schizophyllan, and which can efficiently present the antigen peptide in an antigen-presenting cell. 
     SUMMARY OF THE INVENTION 
     It has been found that a conjugate, where an N-terminal cysteine residue of an antigen peptide that has a cysteine residue at the N-terminal and has eight or more amino acid residues is disulfide-bonded to a linker added to polydeoxyadenine, can efficiently present the antigen peptide in an antigen-presenting cell. In addition, such a conjugate is capable of forming a complex with a polysaccharide having a β-1,3-glucan skeleton such as schizophyllan, since polydeoxyadenine exists as a single strand. The present invention has been completed by further repeated studies based on such knowledge. 
     That is, the present invention provides the following aspects: 
     1. A nucleic acid-peptide conjugate, where an N-terminal cysteine residue of an antigen peptide that has a cysteine residue at the N-terminal and has eight or more amino acid residues, is disulfide-bonded to a linker added to polydeoxyadenine. 
     2. The nucleic acid-peptide conjugate according to 1, where a C-terminal amino acid residue of the antigen peptide is a hydrophobic amino acid residue. 
     3. The nucleic acid-peptide conjugate according to 1 or 2, where the antigen peptide is a peptide presented by an MHC class I molecule. 
     4. The nucleic acid-peptide conjugate according to any one of 1 to 3, where at least a part of a phosphodiester bond of the polydeoxyadenine is phosphorothioated (S-modified). 
     5. A complex comprising: the nucleic acid-peptide conjugate according to any one of 1 to 4; and a polysaccharide having a β-1,3-glucan skeleton. 
     6. A vaccine formulation comprising: the nucleic acid-peptide conjugate according to any one of 1 to 4; or the complex according to 5. 
     7. An immunization method comprising administering the nucleic acid-peptide conjugate according to any one of 1 to 4 or the complex according to 5, to a subject who needs to be given immunity or needs to enhance immunity. 
     8. Use of the nucleic acid-peptide conjugate according to any one of 1 to 4, or the complex according to 5, for the vaccine formulation. 
     Advantages of the Invention 
     A N-terminal cysteine residue of an antigen peptide that has a cysteine residue at the N-terminal and has eight or more amino acid residues is disulfide-bonded to polydeoxyadenine via a linker, whereby the conjugate can efficiently present the antigen peptide in the antigen-presenting cell. 
     Further, in the conjugate of the present invention, a polysaccharide having a β-1,3-glucan skeleton such as schizophyllan can be complexed with a moiety of polydeoxyadenine. By complexation of the conjugate of the present invention and the polysaccharide, the conjugate can be efficiently delivered to a Dictin-1 expressing antigen-presenting cell (e.g., macrophages, dendritic cells). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows the result obtained by confirming a purified dA40(S)-peptide conjugate by electrophoresis in Example 1. 
         FIG. 2  shows the result obtained by confirming the reactivity of aminopeptidase to various dA40(S)-peptide conjugates in Example 2. 
         FIG. 3  shows the result obtained by confirming the antigen presenting ability of various dA40(S)-peptide conjugates in Example 3. 
         FIGS. 4A and 4B  show the result obtained by confirming the antigen presenting abilities of SPG/dA40(S)-OVA 257-264 , dA40(S)-OVA 257-264 , and OVA 257-264 -click-dA40(S)/SPG in Example 5. 
         FIG. 5  shows the result obtained by performing an in vivo CTL assay using SPG/dA40(S)-OVA 257-264  in Example 6. 
     
    
    
     DETAILED DESCRIPTION 
     The conjugate of the present invention is characterized in that the conjugate, where an N-terminal cysteine residue of an antigen peptide that has a cysteine residue at the N-terminal and has eight or more amino acid residues, is disulfide-bonded to a linker added to polydeoxyadenine. Hereinafter, the conjugate of the present invention will be described in detail. 
     Antigen Peptide 
     The number of amino acid residues constituting the antigen peptide used in the present invention is 8 or more and is not limited as long as the antigen peptide is an antigen peptide having antigenicity (i.e., an antigen peptide which is recognized as a foreign substance by the body&#39;s immune system, thereby causing an immune response). The number is usually from 8 to 20, preferably from 8 to 12, and more preferably from 8 to 9. 
     The amino acid sequence of the antigen peptide used in the present invention is not particularly limited as long as the N-terminal of the amino acid sequence is a cysteine residue and the amino acid sequence can induce an immune response. Examples of the antigen peptide include antigen peptides having a partial amino acid sequence capable of acting as an epitope, among proteins as allergens (such as food allergies), and proteins derived from pathogens (such as bacteria and viruses) and tumor cells. Preferable examples of the antigen peptide include antigen peptides presented by MHC type I molecules. More specific examples of the antigen peptide used in the present invention include antigen peptides containing the amino acid sequences of the antigen peptides shown in Table 1. In the case of using the antigen peptides containing the amino acid sequences of the antigen peptides shown in Table 1, cysteine may be added to the N-terminal of each of the amino acid sequences of the antigen peptides shown in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1  
               
               
                   
                   
               
               
                   
                 Antigen peptide 
                 Sequence 
               
               
                   
                   
               
             
            
               
                   
                 OVA 
                 SIINFEKL 
               
               
                   
                   
               
               
                   
                 Melanocyte gp100 for mouse 
                 EGSRNQDWL 
               
               
                   
                   
               
               
                   
                 Melanocyte gp100 for human 
                 KVPRNQDWL 
               
               
                   
                   
               
               
                   
                 TRP2 
                 SVYDFFVWL 
               
               
                   
                   
               
               
                   
                 CT26 
                 SPSYVYHQF 
               
               
                   
                   
               
               
                   
                 Influenza HA 
                 JYSTVASSL 
               
               
                   
                   
               
               
                   
                 Influenza NP 
                 ASNENMDTM 
               
               
                   
                   
               
               
                   
                 Influenza PA 
                 SSLENFRAYV 
               
               
                   
                   
               
               
                   
                 β-galactosidase 
                 DAPIYTNV 
               
               
                   
                   
               
               
                   
                 MuLV p15E 
                 KSPWFTTL 
               
               
                   
                   
               
               
                   
                 SeV 
                 FAPGNYPAL 
               
               
                   
                   
               
               
                   
                 MCMV IEI 
                 YPHFMPTNL 
               
               
                   
                   
               
               
                   
                 LCMV gp33 
                 KAVYNFATM 
               
               
                   
                   
               
               
                   
                 LCMV NP396 
                 FQPQNGQFI 
               
               
                   
                   
               
               
                   
                 LCMV NP118 
                 RPQASGVYM 
               
               
                   
                   
               
               
                   
                 Malaria Pb9 
                 SYIPSAEKI 
               
               
                   
                   
               
               
                   
                 HIV P18-110 
                 RGPGRAFVTI 
               
               
                   
                   
               
               
                   
                 BCG MPT51 
                 GGPHAVYLL 
               
               
                   
                   
               
               
                   
                 Human CEA 
                 EAQNTTYL 
               
               
                   
                   
               
               
                   
                 P815 
                 LPYLGWLVF 
               
               
                   
                   
               
               
                   
                 HBsAg 
                 IPQSLDSWWTSL 
               
               
                   
                   
               
               
                   
                 HSV-I gB 
                 SSIEFARL 
               
               
                   
                   
               
               
                   
                 HY Uty 
                 WMHHNMDLI 
               
               
                   
                   
               
               
                   
                 EGFP 
                 HYLSTQSAL 
               
               
                   
                   
               
               
                   
                 HER2 
                 TYLPTNASL 
               
               
                   
                   
               
               
                   
                 VSV NP 
                 RGYVYQGL 
               
               
                   
                   
               
               
                   
                 Polyomavirus MT 
                 RRLGRTLLL 
               
               
                   
                   
               
            
           
         
       
     
     In the amino acid sequences in the table, left end represents N-terminal and right end represents C-terminal. 
     As long as the amino acid sequences of the antigen peptides shown in Table 1 are capable of inducing an immune response, the amino acid sequences may be those in which one or several amino acids are substituted, deleted, and/or inserted. In a case in which mutations such as substitution, deletion or insertion of amino acids are introduced into each of the amino acid sequences of the antigen peptides shown in Table 1, the number of the amino acid residues to which mutations are introduced may be appropriately set within a range where the immune response can be induced. The number is, for example, from 1 to 4, preferably from 1 to 3, more preferably 1 or 2, and particularly preferably 1. 
     Furthermore, in the amino acid sequences of the antigen peptides shown in Table 1, as long as the immune response can be induced, one or more amino acid residues may be added to at least one of the N-terminal and the C-terminal. In a case in which amino acid residues are added to the N-terminal and/or C-terminal of each of the amino acid sequences of the antigen peptides shown in Table 1, the number of the amino acid residues to be added may be appropriately set within a range where the immune response can be induced. The number is, for example, from 1 to 10, preferably from 1 to 5, and more preferably from 1 to 2. The antigen peptides preferably have a total length of 9 to 16 amino acid residues. Leucine, methionine, and phenylalanine are preferred as the amino acids to be added. Since the antigen peptide is recognized as an antigen from the C-terminal side, it is desirable that the amino acid residue is not added to the C-terminal side of each of the amino acid sequences of the antigen peptides shown in Table 1, except for the case where one hydrophobic amino acid residue to be described later is added. 
     As shown in Example 3 to be described later, it is confirmed that the conjugate of the present invention is taken up by the antigen-presenting cell, the C-terminal side of the antigen peptide is recognized, the C-terminal amino acid residue of the antigen peptide greatly affects the antigen presenting ability, and the antigen presentation is performed more efficiently when the C-terminal of the antigen peptide is a hydrophobic amino acid residue. Accordingly, the C-terminal of the amino acid sequence of the antigen peptide used in the present invention is preferably the hydrophobic amino acid residue. Specific examples of the hydrophobic amino acid residue include leucine, glycine, alanine, valine, isoleucine, methionine, proline, phenylalanine, and tryptophan. Among these hydrophobic amino acid residues, leucine is preferable. 
     In the case of using antigen peptides which contain the amino acid sequences of the antigen peptides shown in Table 1 and in which the C-terminal is the hydrophobic amino acid residue, regarding antigen peptides in which the C-terminal of each of the amino acid sequences of the antigen peptides shown in Table 1 is not the hydrophobic amino acid residue, one or more amino acid residues may be added so that the C-terminal is the hydrophobic amino acid residue, or one or more amino acid residues may be deleted from the C-terminal side so that the C-terminal is the hydrophobic amino acid residue. 
     The antigen peptide used in the present invention can be obtained by any known method such as hydrolysis of proteins as a source or peptide synthesis. 
     Polydeoxyadenine 
     The number of deoxyadenines constituting the polydeoxyadenine used in the present invention may be appropriately set within a range where it is possible to form a complex with a polysaccharide having a β-1,3-glucan skeleton described later. The number is, for example, from 10 to 100, preferably from 20 to 100, more preferably from 20 to 80, and still more preferably from 30 to 50. 
     Further, at least a part of the phosphodiester bond of polydeoxyadenine used in the present invention may be phosphorothioated (S-modified). Polydeoxyadenine is S-modified in this manner, thereby imparting resistance against nuclease and making the stability in vivo more excellent. S-modification of polydeoxyadenine can be carried out in accordance with a conventionally known method. 
     In the case of S-modifying polydeoxyadenine used in the present invention, the S-modification ratio is not particularly limited, and is usually 50% or more, preferably 80% or more, and more preferably 100%. In the present specification, the S-modification ratio of polydeoxyadenine indicates a ratio (%) of S-modified phosphodiester bonds to the total number of phosphodiester bonds in polydeoxyadenine. Further, the term “S-modified phosphodiester bond” refers to a bond structure in which one of the oxygen atoms of the phosphate residue in the phosphodiester bond moiety is substituted with a sulfur atom. 
     Binding of Antigen Peptide to Polydeoxyadenine 
     In the conjugate of the present invention, the thiol group of the N-terminal cysteine residue of the antigen peptide is disulfide-bonded to a linker (spacer) added to polydeoxyadenine. 
     In the conjugate of the present invention, either the 5′ end side or the 3′ end side of the polydeoxyadenine may be bonded to the N-terminal side of the antigen peptide via a linker, but the 5′ end side of the polydeoxyadenine is preferably bonded to the N-terminal side of the antigen peptide via a linker. 
     In the conjugate of the present invention, the molecular weight of the linker moiety varies depending on the structure of the linker to be used, and the molecular weight is, for example, from 100 to 5000, preferably from 100 to 2000, and more preferably from 100 to 500. 
     The linker in the conjugate of the present invention may have a structure that is covalently bonded to the 5′ end side or the 3′ end side of the polydeoxyadenine and disulfide-bonded to the thiol group of the N-terminal cysteine residue of the antigen peptide. It is possible to use a linker obtained by linking one or two or more known bifunctional compounds. 
     In the conjugate of the present invention, the linkage between the polydeoxyadenine and the antigen peptide via a linker can be carried out, for example, by bonding a linker having a functional group capable of disulfide bonding to a thiol group to the 5′ end side or the 3′ end side of the polydeoxyadenine, and forming a disulfide bond between the thiol group of the N-terminal cysteine residue of the antigen peptide and the functional group. 
     In order to bond a linker having a functional group capable of forming a disulfide bond with a thiol group to the 5′ end side or the 3′ end side of the polydeoxyadenine, for example, a bifunctional compound having a functional group capable of covalently bonding to an amino group and a functional group capable of disulfide bonding to a thiol group may be bonded to polydeoxyadenine having an amino group added to the 5′ end side or the 3′ end side. 
     The polydeoxyadenine having an amino group added to the 5′ end side or the 3′ end side is, for example, polydeoxyadenine in which any of the groups of the following Formulae (1) to (7) is substituted with a hydrogen atom constituting the 5′ end phosphate residue of polydeoxyadenine or a hydrogen atom constituting the hydroxyl group of the deoxyribonucleotide at the 3′ end of polydeoxyadenine. 
       Formula 1 
       —(CH 2 ) n1 —NH 2   (1)
 
       —(CH 2 ) n2 —NH—CO—O—(CH 1 ) n3 —NH 2   (2)
 
       —CO—(CH 2 ) n1 —NH 2   (3)
 
       —NH—(CH 2 ) n1 —NH 2   (4)
 
       —CH(CH 2 ) n4 —CH(CH 2 CH 2 O)—NH 2   (5)
 
       —CH 2 —(CH 2 CH 2 O) n4 —NH 2   (6)
 
       —CO—CH 2 —(CH 2 CH 2 O) n4 —NH 2   (7)
 
     Here, in Formulae (1), (3), and (4), n1 is an integer of 1 to 18, preferably an integer of 1 to 12, and more preferably an integer of 1 to 6. 
     In Formula (2), n2 is an integer of 1 to 18, preferably an integer of 1 to 12, and more preferably an integer of 1 to 6; and n3 is an integer of 1 to 18, preferably an integer of 1 to 12, and more preferably an integer of 1 to 6. 
     In Formulae (5) to (7), n4 is an integer of 1 to 20, preferably an integer of 1 to 10, and more preferably an integer of 1 to 5. 
     The groups of Formulae (1) to (7) can be added to polydeoxyadenine in accordance with a known method. 
     Further, the bifunctional compound having a functional group capable of covalently bonding to an amino group and a functional group capable of disulfide bonding to a thiol group may be appropriately selected from known bifunctional cross-linking agents and used. Specific examples thereof include succinimidyl-6-[3′(2-pyridyldithio)propionato]hexanoate (LC-SPDP), N-succinimidyl-3-(2-pyridyldithio)propionate (SPDP), N-succinimidyl-S-acetylthioacetate (SATA), N-succinimidyl-S-acetylthiopropionate (SATP), and iminothiolane. Among these bifunctional linkers, succinimidyl-6-[3′(2-pyridyldithio)propionamido]hexanoate is preferable. 
     The bond between the polydeoxyadenine to which the amino group is added and the bifunctional compound as well as the disulfide bond between the bifunctional linker and the thiol group of the N-terminal cysteine residue can be formed by a known method. 
     Complexation with Polysaccharide Having β-1,3-Glucan Skeleton 
     The conjugate of the present invention may be directly formulated as a vaccine. Desirably, the conjugate is used after forming a complex with the polysaccharide having a β-1,3-glucan skeleton. Thus, the conjugate is complexed with the polysaccharide having a β-1,3-glucan skeleton, whereby the conjugate can be efficiently delivered to the Dicten-1-expressing antigen-presenting cells (macrophages, dendritic cells, etc.). Accordingly, it is possible to obtain more excellent immunostimulatory effect. 
     β-1,3-glucan is a polysaccharide composed of glucoses linked via β13 glycosidic linkages, and there are various known polysaccharides in which the ratio of the number of glucose residues in the side chain to the number of glucose residues in the main chain (hereinafter abbreviated as “side chain ratio”) varies. The polysaccharide having a β-1,3-glucan skeleton has helix parameters resembling those of nucleic acids such as poly(C) (e.g., see Takahashi, Obata, Suzuki, Prog. Polym. Phys. Jpn. 27, p. 767; and “Conformation of Carbohydrates”, Sharwood Academic Publisher, 1998), and also has hydroxyl groups that are capable of hydrogen-bonding with nucleic acid bases. Thus, it is known that one polydeoxyadenine molecule and two polysaccharide molecules that have a β-1,3-glucan skeleton interact with one another to form a stable complex that has a triple helix structure. 
     Specific examples of the polysaccharide having a β-1,3-glucan skeleton used in the present invention include schizophyllan, curdlan, lentinan, pachyman, grifolan, and scleroglucan. Among them, schizophyllan is a preferable polysaccharide in the present invention, because it can significantly improve the introduction into the antigen-presenting cell and the resistance to enzymatic degradation. Further, in the case of using schizophyllan as the polysaccharide, the glucose residue of the side chain of schizophyllan is chemically modified, so that it is possible to introduce a functional group having an affinity for a cellular receptor (such as an amino group, an amino acid group, a peptide group or a cholesterol group). 
     The weight average molecular weight of the polysaccharide having a β-1,3-glucan skeleton is not particularly limited, and may be appropriately set according to the type of polysaccharide to be used and the chain length of polydeoxyadenine in the conjugate of the present invention. Specifically, the weight average molecular weight of the polysaccharide having a β-1,3-glucan skeleton is usually from 25,000 to 2,500,000, and preferably from 25,000 to 150,000. 
     The polysaccharide having a β-1,3-glucan skeleton can be obtained in accordance with a known method. For example, schizophyllan can be obtained in accordance with the standard method described in the literature (A. C. S. 38 (1), 253 (1997); Carbohydrate Research, 89, 121-135 (1981)). 
     Specifically, the conjugate of the present invention can be complexed with the polysaccharide having a β-1,3-glucan skeleton in the following manner. The polysaccharide having a β-1,3-glucan skeleton has a triple helix structure in nature or in water. The process of including: dissolving the polysaccharide in a polar solvent such as dimethyl sulfoxide (DMSO) or an alkaline aqueous solution such as an aqueous sodium hydroxide solution to cause disassociation of the triple helix into a single strand; adding the conjugate of the invention; and replacing the solvent with water or adjusting the pH of the alkaline aqueous solution to neutral (regenerating process) is performed, whereby two molecules of the polysaccharide having a β-1,3-glucan skeleton are bonded to the single-stranded moiety of the polydeoxyadenine linked to the conjugate of the present invention to form a triple helix complex structure (association structure). It is considered that the complex of the polydeoxyadenine and the polysaccharide is mainly formed through hydrogen bonding and hydrophobic interaction. 
     Desirably, complexation of the conjugate of the present invention and the polysaccharide having a β-1,3-glucan skeleton is carried out by mixing the conjugate with the polysaccharide at a molar ratio of the conjugate of the present invention to the polysaccharide having a β-1,3-glucan skeleton of 20:1 to 1:5, preferably 10:1 to 1:1. The molar ratio is used for the conditions of forming a complex of the conjugate of the present invention and the polysaccharide having a β-1,3-glucan skeleton, as a result of which both the conjugate and the polysaccharide efficiently interact to each other. This leads to improvement in the production efficiency of the complex of the conjugate of the present invention and the polysaccharide having a β-1,3-glucan skeleton. 
     Usage and Dosage Form 
     The conjugate of the present invention and the complex of the conjugate of the present invention and the polysaccharide having a β-1,3-glucan skeleton can efficiently present antigen peptides in the antigen-presenting cells. Thus, the conjugate and the complex can be formulated as vaccine formulations for treating and preventing infectious diseases caused by infection of pathogens (e.g., bacteria and viruses) and for tumors such as cancer. 
     The vaccine formulation can be prepared by mixing the conjugate of the present invention or the complex of the conjugate of the present invention and the polysaccharide having a β-1,3-glucan skeleton, as appropriate, using a pharmaceutically acceptable carrier or excipient, and formulating the mixture in accordance with a known formulation technique. The dosage form of the vaccine formulation is not particularly limited, and examples thereof include tablets, suppositories, capsules, syrups, microcapsules, injectable solutions, aerosols, and sprays. 
     The vaccine formulation can be orally or parenterally administered to humans or warm-blooded animals (such as mice, rats, rabbits, sheep, pigs, cattle, horses, chickens, cats, dogs, and monkeys). Examples of parenteral administration include subcutaneous injection, intradermal injection, intramuscular injection, intraperitoneal administration, intravenous drip, nasal mucous membrane spray administration, and pharyngeal region spray administration. 
     The dose of the vaccine formulation may be appropriately set according to activity, the disease to be treated, the type, body weight, sex, and age of the animal to be administered, severity of the disease, administration method, and the like. For example, in the case of administration to an adult, the daily dose may be from about 1 to about 50,000 μg, and preferably from about 10 to about 10000 pig in terms of weight of antigen peptide. 
     If necessary, the vaccine formulation may contain a CpG oligodeoxynucleotide adjuvant, a Freund&#39;s adjuvant, an aluminum hydroxide (Alum) adjuvant, and an alum adjuvant, and the like. Alternatively, the vaccine formulation may be administered together with these adjuvants. 
     EXAMPLES 
     Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. In this regard, 40-mer polydeoxyadenine having an S-modification ratio of 100% may be hereinafter referred to as “dA40(S)”. 
     Example 1: Preparation of Conjugate of Peptide and dA40(S) 
     1 mol of dA40(S) having an amino group added to the 5′ end (40-mer polydeoxyadenine in which a hydrogen atom constituting the phosphate residue at the 5′ end was substituted with the —(CH 2 ) 6 —NH—CO—O—(CH 2 ) 2 —NH 2  group and which had an S-modification ratio of 100%) and 30 mol equivalent of succinimidyl-6-[3′(2-pyridyldithio)propionamido]hexanoate (LC-SPDP) were mixed in a phosphate buffer solution (PBS), and the mixture was reacted at 40° C. for 3 hours. 
     Then, unreacted LC-SPDP was removed using an anion exchange column (NAP-5 column) to obtain dA40(S)-SPDP in which LC-SPDP was bonded to the 5′ end of dA40(S). 
     25 mol equivalent of peptides 1 to 4 (peptides 1 to 3, i.e., ovalbumin (OVA) antigen peptides) shown in Table 2 was added to 1 mol of dA40(S)-LC-SPDP, and the mixture was reacted at 40° C. for 18 hours. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2  
               
               
                   
                   
               
               
                   
                   
                 Amino acid sequence 
               
               
                   
                   
               
             
            
               
                   
                 Peptide 1 
                 SIINFEKLC (SEQ ID NO: 1) 
               
               
                   
                   
               
               
                   
                 Peptide 2 
                 CSIINFEKL (SEQ ID NO: 2) 
               
               
                   
                   
               
               
                   
                 Peptide 3 
                 CQLESIINFEKL (SEQ ID NO: 3) 
               
               
                   
                   
               
               
                   
                 Peptide 4 
                 CSIINFEKK (SEQ ID NO: 4) 
               
               
                   
                   
               
            
           
         
       
     
     Thereafter, the reaction solution was subjected to HPLC to separate the conjugate of dA40(S) and peptide. The HPLC measurement conditions were set as follows: 
     
       
         
           
               
               
             
               
                   
                   
               
             
            
               
                   
                 Solution A: acetonitrile 
               
               
                   
                 Solution B: 0.1M TEAA (pH 7.0) 
               
               
                   
                 Column: ZORBAX Eclipse Plus C18 
               
               
                   
                 Eluent 
               
            
           
           
               
               
               
               
            
               
                   
                 0 min 
                 Solution A: 10%, 
                 Solution B: 90% 
               
               
                   
                 to 25 min 
                  30%, 
                 70% 
               
               
                   
                 to 30 min 
                 100%, 
                  0% 
               
               
                   
                 to 32 min 
                 100%, 
                  0% 
               
               
                   
                 to 35 min 
                  10%, 
                 90% 
               
               
                   
                   
               
            
           
         
       
     
     The separated solution was exchanged with distilled water using an Amicon Ultra Ultracel-3K filter and the resulting solution was freeze-dried to obtain a dA40(S)-peptide conjugate. Purification of the resulting dA40(S)-peptide conjugate was confirmed by acrylamide gel electrophoresis. 
     In the case of using peptide 2, the conjugate was subjected to electrophoresis, the stained conjugate was stained with SYBR Gold, and a fluorescent image thereof was obtained. The result is shown in  FIG. 1 . In  FIG. 1 , the left lane shows the result of dA40(S) alone, the middle lane shows the result of the product after reacting the peptide with dA40(S), and the right lane shows the result of the purified dA40(S)-peptide conjugate. From  FIG. 1 , it is understood that, in the lane (middle) showing the result of the product after reacting the peptide with dA40(S), the band of dA40(S) moves to the polymer side, and the peptide is bound to dA40(S). However, unreacted dA40(S) was observed, but only the band of the conjugate was observed after HPLC separation. This result confirmed that the dA40(S)-peptide conjugate was synthesized and purified. 
     Example 2: Cleavage of dA40(S)-Peptide Conjugate by Endoplasmic Reticulum Aminopeptidase 
     Each of the dA40(S)-peptide conjugates (1.5 μM) and murine endoplasmic reticulum aminopeptidase (0.5 μg/ml) (R &amp; D Systems catalog Number 2500-ZN) were added to a 25 mM Tris buffer (pH 8.0) and the mixture was incubated at 37° C. for 18 hours. After completion of the reaction, polyacrylamide gel electrophoresis was carried out to observe the state of release from the dA40(S)-peptide conjugate. 
       FIG. 2  shows the results of polyacrylamide gel electrophoresis. In  FIG. 2 , pep(C)-dA40(S) represents a dA40(S)-peptide conjugate obtained by using peptide 1, dA40(S)-(C)pep represents a dA40(S)-peptide conjugate obtained by using peptide 2, dA40(S)-(C)pep long represents a dA40(S)-peptide conjugate obtained by using peptide 3, and dA40(S)-(C)pep long miss (L→K) represents a dA40(S)-peptide conjugate obtained by using peptide 4. In the conjugate in which dA40(S) was bonded, via a disulfide bond, to the C-terminal cysteine residue of peptide 1 having a cysteine residue at the C-terminal, no change was observed in the electrophoretic distance even when subjected to enzyme treatment. In other words, it was found that the dA40(S)-peptide conjugate in this orientation was hardly recognized by the enzyme. Conversely, in the conjugate in which dA40(S) was bonded, via a disulfide bond, to the N-terminal cysteine residue of peptide 2 having a cysteine residue at the N-terminal, a band of dA40(S) was slightly observed after the enzyme treatment. Further, in peptide 3 in which the number of amino acids was increased as compared to peptide 2, the peptide was almost completely released and a band of dA40(S) was observed. These facts show that dA40(S) is bonded, via a disulfide bond, to the N-terminal cysteine residue of the antigen peptide, whereby the antigen peptide and dA40(S) can be cleaved by aminopeptidase. Further, the dA40(S)-peptide conjugate obtained by using peptide 4 in which the C-terminal of peptide 2 was substituted with leucine from lysine was not recognized by the enzyme. Aminopeptidase is one of the enzymes that trim antigen peptides to present the antigen peptides to MHC. Accordingly, it is important for antigen presentation using the dA40(S)-peptide conjugate that even in the case of the peptide in which polydeoxyadenine is bonded to the N-terminal cysteine, the C-terminal side is composed of the hydrophobic amino acid residue. This fact was confirmed. 
     Example 3: Evaluation of Antigen Presenting Ability Using dA40(S)-Peptide Conjugate 
     3 ml of thioglycollate medium was administered to the peritoneal cavity of C57BL/6J mice 3 days later, macrophages were recovered from the peritoneal cavity. The resulting macrophages were seeded in 48 wells (1.5×10 5  cells/well), and further each of the dA40(S)-peptide conjugates was added to the wells at a concentration of 10 μM. Cells were fixed with 4% paraformaldehyde in a CO 2  incubator for 18 hours, the fixed cells were stained with Anti-Mouse OVA257-264 (SIINFEKL) peptide bound to H-2Kb PE, and the resulting cells were subjected to fluorescence microscopic observation. 
       FIG. 3  shows the results of fluorescence microscopic observation. In  FIG. 3 , pep(C)-dA40(S) represents a dA40(S)-peptide conjugate obtained by using peptide 1, dA40(S)-(C)pep represents a dA40(S)-peptide conjugate obtained by using peptide 2, dA40(S)-(C)pep long represents a dA40(S)-peptide conjugate obtained by using peptide 3, and dA40(S)-(C)pep long miss (L→K) represents a dA40(S)-peptide conjugate obtained by using peptide 4. Very weak fluorescence was observed in the dA40(S)-peptide conjugate obtained by using peptide 1. Further, very strong fluorescence was observed at about the same degree in the dA40(S)-peptide conjugate obtained by using peptides 2 and 3. Further, antigen presentation hardly occurred in the dA40(S)-peptide conjugate obtained by using peptide 4. These results indicate that the dA40(S)-antigen peptide conjugate is introduced into the antigen-presenting cells, the C-terminal side of the dA40(S)-antigen peptide conjugate is recognized, and it is important for the antigen presentation that the C-terminal is the hydrophobic amino acid residue. 
     Example 4: Preparation of Complex of dA40(S)-Peptide Conjugate and Schizophyllan 
     Ovalbumin (OVA) antigen peptide (OVA 257-264 ; amino acid sequence: SIINFEKL) was used to prepare a dA40(S)-OVA 257-264  conjugate (dA40(S)-OVA 257-264 ) in the manner described in Example 1. Schizophyllan (SPG) was dissolved in a 0.25 N aqueous sodium hydroxide solution for 2 to 5 days to obtain an SPG solution in which the triple helix structure was dissociated. Next, the SPG solution in which the triple helix structure was dissociated, the dA40(S)-OVA 257-264  conjugate, and a phosphate buffer solution (330 mM NaH 2 PO 4 , pH 4.7) were mixed, and the mixture was allowed to stand at 4° C. overnight. In the mixture, the concentration of dA40(S)-OVA 257-264  was 60 μM, and the ratio of [dA40(S)-OVA 257-264 ]/[SPG] was 4. In this manner, a complex (SPG/dA40(S)-OVA 257-264 ) of dA40(S)-OVA 257-264  and SPG was prepared. 
     Reference Example 1: Preparation of Complex of CpG-dA40(S) and SPG 
     CpG-dA40(S) in which dA40(S) was ligated to the 3′ end of CpG (base sequence: ATCGACTCTCGAGCGTTCTC) was provided. CpG-dA40(S) was used to form a complex (CpG-dA40(S)/SPG) of CpG-dA40(S) and SPG, in the same manner as in Example 4. 
     Reference Example 2: Preparation of Conjugate of dA40(S) and OVA 257-264  Shown in WO 2015/084374 and Preparation of Complex of the Conjugate and SPG 
     Ovalbumin (OVA) antigen peptide (OVA 257-264 ; amino acid sequence: SIINFEKL) (OVA (N3) described below) having an azido group introduced into the C-terminal was reacted with dA40(S) (dA40(S) described below) (alkyne) having alkyne introduced into the 5′ end according to the method described in Example 1 of WO 2015/084374 to cause a cycloaddition reaction between the azide group and the alkyne, thereby preparing a conjugate (OVA 257-264 -click-dA40(S)) of dA40(S) and OVA 257-264 . 
     
       
         
         
             
             
         
       
     
     Subsequently, OVA 257-264 -click-dA40(S) was used to form a complex (OVA 257-264 -click-dA40(S)/SPG) of OVA 257-264 -click-dA40(S) and SPG, in the same manner as in Example 4. 
     Example 5: Evaluation of Antigen Presenting Ability Using SPG/dA40(S)-OVA 257-264    
     3 ml of thioglycollate medium was administered to the peritoneal cavity of C57BL/6J mice 3 days later, macrophages were recovered from the peritoneal cavity. The resulting macrophages were seeded in 48 wells (1.5×10 5  cells/well), and further SPG/dA40(S)-OVA 257-264 , dA40(S)-OVA 257-264 , or OVA 257-264 -click-dA40(S)/SPG was added at a concentration of OVA 257-264  of 5 μg/ml. Cells were fixed with 4% paraformaldehyde in a CO 2  incubator for 18 hours, the fixed cells were stained with Anti-Mouse OVA257-264 (SIINFEKL) peptide bound to H-2Kb PE, and the resulting cells were analyzed by flow cytometry. 
     The results are shown in  FIGS. 4A and 4B . The antigen (OVA 257-264 ) presentation level of dA40(S)-OVA 257-264  was the same as that of SPG/dA40(S)-OVA 257-264 . On the other hand, antigen presentation did not occur in OVA 257-264 -click-dA40(S)/SPG. This suggests that the peptide was released from the complex in the cell (i.e., the disulfide bond was cleaved in a reducing environment), whereby the subsequent antigen presentation was efficiently carried out. 
     Example 6: In Vivo CTL Assay 
     SPG/dA40(S)-OVA 257-264  (20, 100, 500 ng in the amount of OVA 257-264 ) and CpG-dA40(S)/SPG (30 μg in the amount of CpG-dA) were intradermally administered to C57BL/6J mice. 
     On day 7 of administration, spleen cells were provided according to the following procedure. Spleen cells excised from normal C57BL/6J mice were treated with 10 μg/ml of OVA 257-264  at 37° C. for 90 minutes. Thereafter, the spleen cells were washed three times with PBS, and the spleen cells were labeled with 5 μM carboxyfluorescein succinimidyl ester (CFSE); Invitrogen, Carlsbad, Calif.). Further, spleen cells not treated with OVA 257-264  were similarly labeled with CFSE (0.5 μM). The labeled cells were washed three times with PBS. The spleen cells treated with OVA 257-264  (CFSE high ) and the spleen cells not treated with OVA 257-264  (CFSE low ) were mixed so that the number of the treated spleen cells was equal to the number of the untreated spleen cells. The resulting spleen cells were administered intravenously (4.0×10 6  cells/head) into mice on Day 7 of administration of SPG/dA40(S)-OVA 257-264  and CpG-dA40(S)/SPG, and the mice were bred for 24 hours. After that, the spleen cells were removed from the mice, and the CFSE-labeled cells were counted by flow cytometry. 
     The results are shown in  FIG. 5 . In mice immunized with SPG/dA40(S)-OVA 257-264  and CpG-dA40(S)/SPG, the decrease in OVA 257-264  presenting cells (CFSE high  spleen cells) was observed depending on the dose of OVA 257-264 . OVA 257-264  presenting cells were not observed when the dose of OVA 257-264  was 100 ng/head or more. This showed that immunized mice specifically eliminated only the antigen-containing cells. It is reported that, in OVA 257-264 -click-dA40(S)/SPG, administration of at least 2 μg/head of OVA 257-264  is required to induce the CTL response (Mochizuki, S., Morishita, H., Kobiyama, K., Aoshi, T., Ishii, K. J., and Sakurai, K. (2015) Immunization with antigenic peptides complexed with beta-glucan induces potent cytotoxic T-lymphocyte activity in combination with CpG-ODNs. J. Control. Release 220, 495-502). Hence, it is revealed that SPG/dA40(S)-OVA 257-264  can induce high CTL response with a smaller peptide dose compared to OVA 257-264 -click-dA40(S)/SPG. The difference in ability of inducing CTL reaction between SPG/dA40(S)-OVA 257-264  and OVA 257-264 -click-dA40(S)/SPG was consistent with the result of Example 5 in which the antigen presenting ability was evaluated. 
     The sequence listing entitled 4230429_1.txt Sequence Listing, submitted herewith, is hereby incorporated by reference in its entirety.