Alpha-2 domain polymorphism and HLA class I peptide loading

Diversity within the class I HLA antigen binding groove is positioned to mo derate the presentation of peptide ligands. Polymorphism is widely disperse d about the peptide binding groove, and unravelling the functional signific ance of a given polymorphism requires comparative analysis of peptides pres ented by class I subtypes differing at the position(s) in question Previous studies have demonstrated that not all class I polymorphisms act equally, and to determine the impact of substitutions specifically located in the al pha(2) domain, peptides purified from B*1501, B*1512, B*1510, and B*1518 we re examined by pooled Edman sequencing and comparative mass spectrometric a nalysis. Molecule B*1512 differs from B*1501 at residues 166 (Glu to Asp) a nd 167 (Trp to Gly) of the alpha(2) domain. The pooled motif and ion mass l igand maps for B*1512 tightly matched those of B*1501, demonstrating that t he 166/167 polymorphism between B*1501 and B*1512 has little impact upon li gand presentation. Although the 166/167 polymorphism minimally affects pept ide binding preferences, this polymorphism makes B*1512 and B*1501 quite di stinct by serology, We then compared the B70 molecules B*1510 and B*1518. T he two are almost indistinguishable by serology and differ only by an alpha (2) polymorphism at 116. Comparative peptide mapping shows that a Tyr to Se r polymorphism at 116 drastically changes the ligands bound by B*1510 and B *1518; no overlaps could be found. Polymorphisms in alpha(2) therefore vary from subtle to extreme in the manner by which they moderate ligand present ation, and serologic crossreac- tivity did not reflect the ligands presente d by these B15 subtypes.