DIFFERENT EFFECTS OF SUBSTITUTIONS AT RESIDUES 224 AND 228 OF MHC CLASS-I ON THE RECOGNITION OF CD8

Previous studies indicated that weak xenoresponse to HLA class I by mo use T cells is due to the inefficient interaction of mouse CD8 with th e alpha3 domain of HLA class I. The present study using chimeric H-2K( b) molecules with recombinant alpha3 domain between H-2K(b) and HLA-B7 as well as single amino acid mutants of H-2K(b) demonstrated that eac h substitution at residues 224 and 228 affects recognition of CD8-depe ndent mouse CTL clones. On the other hand, reactivity of IL-2-producin g H-2K(b)-specific T cell hybridoma transfected with mouse CD8alpha wa s abrogated by substitution at residue 224 but not by that at residue 228. This indicates that the substitution at residue 228 affects recog nition of CD8-dependent CTL but does not critically affect binding of CD8 to MHC class I molecules, although residue 224 abrogates binding o f CD8. The model structure of the alpha3 domain of H-2K(b) suggests th at the substitution at residue 224 induces conformational change of CD 8 binding loop, whereas minimum structure change by the substitution a t residue 228 is expected. It is therefore speculated that minimum str ucture change of CD8 binding loop by substitution at residue 228 may i nfluence binding affinity of CD8, which abrogates recognition of CD8-d ependent CTL but not IL-2 production of the CD8-dependent T cell hybri doma.