Analysis of the mutant HLA-A*0201 heavy chain H74L: impaired TAP-dependent peptide loading.

A mutation of the HLA-A*0201 heavy chain at position 74 from histidine to leucine (H74L) resulted in a molecule with an interesting phenotype. H74L-expressing 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 tysis was due to its enhanced capability to bind exogenous peptide. Furthermore, this phenotype of improved exogenous binding and functional recognition was not peptide-specific. In contrast, the H74L molecule failed to present the HIV- HLA-A2-restricted pol peptide when expressed and processed endogenously. The inability to bind endogenous pol could be rescued by preceding the pol peptide with a signal sequence. The defect affecting endogenous presentation, therefore, appeared to be limited to the TAP-dependent pathway. Surprisingly, the H74L heavy chain was able to enter the defined MHC class I pathway and associate with beta2M, calreticulin, tapasin, and TAP. Despite the presence of the H74L heavy chain at the TAP complex, H74L was functionally inefficient at loading TAP-dependent peptides. H74L may help elucidate further steps in the process of loading TAP-dependent peptides into the class I cleft.