The role of heat shock proteins in antigen presentation by MHC class I molecules

The ability of heat shock proteins (hsps) to bind antigenic peptides is central to their ability to elicit specific cellular immunity. Immunizing mice with hsp (gp96 or hsp70) preparations isolated from cells expressing known antigens is shown to result in generation of antigen-specific cytotoxic T lymphocytes (CTLs). It is further demonstrated that antigen presenting cells can utilize hsp-peptide complexes to induce antigen-specific CTLs in vitro. Immunization with hsps derived from influenza-infected cells is shown to protect mice from subsequent challenge with a lethal dose of influenza virus. To examine the role of hsp-peptide binding in vivo, Mixed-Lymphocyte Tumor Cultures (MLTCs) were treated with the hsp70- and hsp90-binding drug deoxyspergualin (DSG). Treatment with DSG diminishes antigen-dependent recognition of antigen presenting cells by CTLs and the response is recovered by addition of cognate peptide. A combination of FACS analysis and immunoprecipitation studies suggests that cell surface expression of MHC I-peptide complexes, but not synthesis of MHC I molecules, decreases following treatment of cells with DSG. These results indicate that treatment of cells with DSG prevents MHC I molecules from reaching the cell surface by blocking transport of antigenic peptides into the endoplasmic reticulum. The ability of hsp-peptide complexes generated in vitro to serve as immunologically active complexes in vivo was also tested. These experiments show that hsp-peptide complexes, but not mixtures of hsp and peptide, generate peptide-specific CTLs and tumor rejection. This adjuvanticity is specific to hsps, as shown by the inability of the peptide-binding protein mouse serum albumin, when complexed with the same peptides, to prime mice and generate peptide-specific CTLs. The results presented here show that association of hsps with antigenic peptides is physiologically relevant and are consistent with the proposed relay line model in which hsps chaperone and transfer antigenic peptides from cytosolic hsps 70 and 90 to the endoplasmic reticulum resident hsp gp96. These, in turn, transfer peptides to MHC I molecules for presentation on the cell surface