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.