Peptide presentation to an alloreactive CTL clone is modulated through multiple mechanisms involving polymorphic and conserved residues in HLA-B27

This study addressed the mechanisms by which HLA class I polymorphism modul ates allorecognition. CTL 27S69 is an alloreactive clone raised against HLA -B*2705, with a known peptide epitope, This CTL cross-reacts with B*2702, w hich differs from B*2705 in the D77N, T80I, and L81A changes, but not with B*2701; which has D74Y, D77N, and L81A changes, To explain this differentia l recognition, B*2705 mutants mimicking subtype changes were used, The A81 mutant was not recognized, despite binding the natural epitope in vivo, sug gesting that,when bound to this mutant, this peptide adopts an inappropriat e conformation. The N77 and 180 mutations restored recognition in the N77A8 1 or I80A81 mutants. These compensatory effects explain the cross-reaction with B*2702. The Y74 and the Y74N77 mutants were weakly recognized or not r ecognized by CTL 27S69, This correlated with the absence or marginal presen ce of the peptide epitope in the Y74N77-bound pool. As with B*2701, exogeno us addition of the peptide epitope sensitized Y74 and Y74N77 targets for ly sis, indicating that failure to cross-react with B*2701 or these mutants wa s due to poor binding of the peptide in vivo and not to inappropriate prese ntation. The abrogating effect of Y74 was critically dependent upon the K70 residue, conserved among subtypes, as demonstrated with mutants at this po sition. Thus, HLA polymorphism affects allorecognition by modulating peptid e binding or the conformation of bound peptides. Compensatory mutations and indirect effects of a polymorphic residue on residues conserved play a cri tical role.