Analysis of the mutant HLA-A*0201 heavy chain H74L: Impaired TAP-dependentpeptide loading

A mutation of the HLA-A*0201 heavy chain at position 74 from histidine to l eucine (H74L) resulted in a molecule with an interesting phenotype. H74L-ex pressing targets were recognized by peptide-specific HLA-A*0201,restricted cytotoxic T lymphocytes at lower peptide concentrations than wild type HLA- A*0201. H74L's improved ability to sensitize cells for lysis was due to its enhanced capability to bind exogenous peptide. Furthermore, this phenotype of improved exogenous binding and functional recognition was not peptide-s pecific. In contrast, the H74L molecule failed to present the HIV-1 HLA-A2- restricted pol peptide when expressed and processed endogenously. The inabi lity to bind endogenous pol could be rescued by preceding the pol peptide w ith a signal sequence. The defect affecting endogenous presentation, theref ore, appeared to be limited to the TAP-dependent pathway. Surprisingly, the H74L heavy chain was able to enter che defined MHC class I pathway and ass ociate with beta 2M, calreticulin, tapasin, and TAP. Despite the presence o f the H74L heavy chain at the TAP complex, H74L was functionally inefficien t at loading TAP-dependent peptides. H74L may help elucidate further steps in the process of loading TAP-dependent peptides into the class I cleft. (C ) American Society for Histocompatibility and Immunogenetics, 1999. Publish ed by Elsevier Science Inc.