Analysis of three HLA-A*3303 binding peptide anchors using an HLA-A*3303 stabilization assay

The affinity of 232 8- to 11-mer peptides carrying HLA-A*3303 anchor residues at position 2 (P2) (Ala, Ile, Leu, Val, Phe or Tyr) and the C-terminus (Arg) was analysed by a stabilization assay using RMA-S transfectants expressing HLA-A*3303 and human beta2-microglobulin. One hundred and nineteen of these peptides (51.3%) bound to HLA-A*3303, confirming that these residues are anchors for HLA-A*3303. Evaluation of P2 residues demonstrated that binding of peptides with Phe or Tyr at P2 is stronger than that of peptides with aliphatic hydrophobic residues at P2. This was confirmed by analysis of a panel of peptides mutated at P2. Analysis of the C-terminal mutant peptides showed that substitution of Lys for Arg had minimal influence on binding to HLA-A*3303. This implies that peptides carrying HLA-A*1101 anchor residues (Val, Ile, Phe or Tyr at P2 and Lys at the C-terminus) can bind to HLA-A*3303. However, such peptides showed lower binding for HLA-A*3303 than for HLA-A*1101. Thus, Arg at the C-terminus is much stronger anchor for HLA-A*3303 than Lys. The preference for Arg and Lys at the C-terminus by HLA-A*1101 and HLA-A*3303 respectively may be due to sequences of three residues (70, 97 and 114) forming the F-pocket of these HLA class I molecules. Statistical analysis of 232 peptides further showed a positive effect of negatively charged residues at P1 for peptide binding to HLA-A*3303. Thus, residues at P1, P2 and the C-terminus play an important role in peptide binding to HLA-A*3303.