THE MINIMAL POLYMORPHISM OF CLASS-II E-ALPHA CHAINS IS NOT DUE TO THE FUNCTIONAL NEUTRALITY OF MUTATIONS

Given the extensive allelic amino acid sequence polymorphism present in the first domain of A(alpha), A(beta), and E(beta) chains and its profound effects on class II function, the minimal polymorphism in the mouse E(alpha) chain (and in its human homologue DR(alpha)) is paradox. Two possible explanations for the lack of polymorphism in E(alpha) are: (1) the E(alpha) chain plays such a uniquely critical structural/functional role in antigen presentation, T-cell activation, repertoire selection, and/or pairing with E(beta) or other proteins for expression that it cannot vary, and mutations are selected against; (2) the E(alpha) chain plays a less significant role than the outer domains of other major histocompatibility complex (MHC) proteins in determining the interactions with processed peptides or with T-cell receptor (TCR), so there is no selective pressure to maintain new mutations. To explore this question we compared the ability of transfectants expressing wild type (wt) E(alpha)E(beta)(d) and mutant E(alpha) wt E(beta)(d) proteins to present peptides and bacterial superantigens to T-cell hybridomas. Mutations at the E(alpha) amino acid positions 31, 52, and 65&66, to residues that represent allelic alternatives in A(alpha) chains, significantly reduced activation of peptide-specific T hybridomas, and mutations at 71 sometimes enhanced T-cell stimulation. None of the E(alpha) mutations reduced, and some enhanced, superantigen stimulation of T-cell hybridomas. These results argue against the hypothesis that E(alpha) chains are minimally polymorphic because mutations in E(alpha) are functionally neutral.