W0080

Crystallographic Studies of Two HIV RT Peptide/Class I MHC Complexes. Timothy J. Kirksey, Rebecca R. Pogue Caley, Jeffrey A. Frelinger and Edward J. Collins, Dept. of Microbiology and Immunology/UNC Chapel Hill, Chapel Hill, NC 27599

We present the 2.5 Å crystal structure of the complex of MHC class I molecule HLA-A2 (A2) with a peptide variant of HIV-reverse transcriptase peptide 309-317 (RT). The RT peptide variant YLKEPVHGV (I1Y) is of immunological significance in that it is capable of stimulating a higher cytotoxic T lymphocyte (CTL) mediated immunological response in peripheral blood lymphocytes (PBLs) from HIV-seropositive donors against the wild-type RT peptide than stimulation by wild-type RT peptide itself. Thus, I1Y provides an "improved" epitope for the design of peptide-based vaccines. We are finishing the final stages of refinement on a second A2 complex with another P1 variant peptide, I1F. Both I1F/A2 and I1Y/A2 complexes show increased cell surface stability compared to the wild-type RT/A2 complex. However, the CTL stimulation observed with the I1Y peptide is not observed with the I1F peptide. Additionally, the I1F peptide is recognized like wild-type RT peptide in two wild-type RT specific CTL lines, whereas I1Y recognition is reduced in one of the cell lines. The two peptide/A2 complexes provide somewhat of a structural/immunological anomaly, in that P1 side chains of peptides bound to A2 are buried in the binding cleft and should not be recognized by the T cell receptor (TcR). Our I1Y/A2 structure shows that the hydroxyl group of the P1 tyrosine forms a hydrogen bond to the P4 glutamate side chain, thus changing the conformation in the middle of the peptide. We believe it is this feature of the complex which accounts for the increased immunogenicity of the I1Y peptide over wild-type RT or I1F peptides.