THE STRUCTURAL INFLUENCE OF INDIVIDUAL RESIDUES LOCATED WITHIN PEPTIDE ANTIGEN DEPENDS UPON THEIR SEQUENCE CONTEXT

Individual residues derived from antigenic peptides are believed to co ntrol certain mAb epitopes on MHC class I molecules. To further evalua te this influence we studied the antibody recognition of H-2K(b) compl exed with the antigenic peptides OVA(257-264), OVA(55-62) and their re ciprocally substituted analogs. The mAb 20.8.4, Y-3, EH144 and AF6 rea cted strongly with K-b complexed to both OVA peptides and their analog s. However, mAb 28.8.6 bound to K-b/OVA(257-264) but not K-b/OVA(55-62 ) complexes. Recognition of K-b/OVA(55-62) by mAb 28.8.6 was restored by a single OVA(55-62)-->OVA(257-264) substitution at position 1 (OVA( 55-62)P1K-->S). Recognition by mAb 5F1 was also peptide-dependent in t hat K-b complexed to either the P4 substituted OVA(257-264)-->OVA(55-6 2) peptide (OVA(257-264)P4N-->R) or the P1 substituted OVA(55-62)-->OV A(257-264) peptide (OVA(55-62)PIK-->S) was not permissive for 5F1 reco gnition even though K-b complexed to the parental peptides OVA(55-62) (P4R) and OVA(257-264) (PIS) reacted with this mAb. These data demonst rate that the same peptide residues located within defined positions c an exert a negative influence on mAb recognition in one sequence conte xt but be permissive for antibody binding in another context. These fi ndings might be explained by conformational effects on class I heavy c hain structure, the extended structure of the peptide or altered orien tation of specific peptide side chains located at the same position bu t within different sequence contexts. Therefore the sequence context o f defined amino acids within peptide antigen can strongly influence th e fine structural contributions of these residues to MHC-peptide confo rmation.