Minor pocket B influences peptide binding, peptide presentation and alloantigenicity of H-2K^b

Microsequence analysis of peptides eluted from the murine class I H-2K^b molecule together with the three-dimensional structure of the molecule co-crystallized with a homogenous population of peptides suggests that pocket B is a minor pocket that does not play a major role in peptide presentation. This is in contrast to most other class I molecules in which pocket B plays a central role in selecting and presenting antigenic peptides. To investigate the role of pocket B in antigen presentation by the K^b molecule, we analyzed site-directed mutants of position 45 in pocket B for their effect on both allo- and peptide-specific recognition. We made an identical set of mutations in K^bm8 at residue 45 in order to evaluate their influence in the context of a more open pocket B which results from the bm8 substitution at amino acid 24 (E → S). We demonstrated that this minor pocket did play a significant role in the antigenicity of both molecules and that this role was more readily apparent in the context of the more open pocket B of K^bm8. In addition, we found that some substitutions of residue 45 in the K^bms molecule restored recognition by some alloreactive and peptide specific anti-K^b T cell clones which are normally restricted to K^b, indicating that multiple configurations of amino acids in a pocket could result in similar binding and presentation capabilities.