Analysis of the molecular basis of HLA-A3 recognition by cytotoxic T cells using defined mutants of the HLA-A3 molecule

The structure-function relationships in human class I HLA molecules have been examined by the analysis of two T cell-defined subtypes of HLA-A3 (A3.1 and A3.2). These subtypes differ by two amino acid residues that are located at positions 152 (GluA3.1 vs ValA3.2) and 156 (LeuA3.1 vs GlnA3.2). By the methods of site-directed mutagenesis and DNA-mediated gene transfer, mammalian cell transfectants have been produced that express only one of the above A3.2 amino acid residues at either position 152 or position 156. Previous studies using murine transfectants have shown that A3.1- and A3.2-expressing cells can be distinguished by A3.1-restricted type A influenza virus-specific CTL and A3.2-allospecific CTL and have implied that amino acid position 152 plays a key role in this specificity. To test whether these results were a function of the virus specificity, the alloantigen, or the cell type expressing the class I molecules, we have tested the recognition of human and murine cell transfectants by A3.1-restricted, A/JAP/305/57 and B/Ann Arbor-specific CTL and by A3.1- and A3.2-allospecific CTL. The results indicate that the Glu at position 152 is critical for recognition by all of the A3.1-restricted CTL populations tested and 15 of 16 of the A3.1-allospecific CTL populations tested. The A3.1 Leu at position 156 was sufficient for recognition by only one A3.1-allospecific CTL line. Substitution of the charged Glu residue for the polar Gln at position 156 of A3.2 affected recognition of some but not all A3.2-alloreactive CTL. These data demonstrate that the structural basis for epitopes that are recognized by almost all CTL that discriminate between A3.1 and A3.2 is primarily the amino acid at position 152. The implications of these data for Ag presentation and CTL recognition are discussed.