The effect of human β2-microglobulin on major histocompatibility complex I peptide loading and the engineering of a high affinity variant -: Implications for peptide-based vaccines

The ability to directly load cell surface major histocompatibility complex (MHC) class I molecules with peptides provides a potentially powerful approach toward the development of vaccines to generate cell-mediated immunity. We demonstrate that exogenous beta(2)-microglobulin (beta(2)m) stabilizes human cell surface MHC I molecules and facilitates their loading with exogenous peptides, Additionally, using three-dimensional crystal structures and known interaction sites between MHC I heavy chains and beta(2)m, we engineered variants of human beta(2)m (h beta(2)m) with a single serine substitution at residue 55, This alteration was predicted to promote hydrophobic interactions at the MHC I heavy chain/beta(2)m interface and displace an ordered water molecule. Compared with h beta(2)m, the serine to valine substitution at residue 55 had improved ability to bind to cell surface HLA-A1, HLA-A2, and HLA-A3 molecules, facilitate exogenous peptide loading, and promote recognition by peptide-specific T cells. The inclusion of h beta(2)m or higher affinity variants when pulsing cells with MHC-restricted peptides increases the efficiency of peptide loading 50-80-fold. Therefore, the inclusion of h beta(2)m in peptide-based vaccines may increase cell surface antigen densities above thresholds that allow recognition of peptide antigens by the immune system, particularly for cryptic, subdominant, or marginally antigenic peptides.