I-A-restricted T cell antigen recognition. Analysis of the roles of A alpha and A beta using DNA-mediated gene transfer.

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Abstract
The contributions of A.alpha. and A.beta. chains, and of subregions of A.beta., to Ia-restricted recognition of antigen by Th lymphocytes were analyzed using a panel of L cells transfected with various pairs of A.alpha. b,d,ork genes and recombinant or wild-type A.beta. b,d,ork genes. The A.beta. genes included all possible exchanges of the whole NH2-terminal (.beta.1) domain or halves of the .beta.1 domain among these three allelic A.beta. genes. The Ia+ L cells derived from such transfections were used as antigen-presenting cells with a 21 member panel of responding Ia-restricted T hybridoma cells of differing nominal antigen specificity and Ia-restriction. Special care was taken to account for quantitative variation in levels of Ia expression throughout the experiments. The results of this analysis reveal that (a) only 2 of the 21 Th cells recognized Ia molecules involving either a nonparental A.alpha. or a nonparental A.beta. chain, and in both cases the degeneracy extended to only one of the two other alleles tested. This suggests that allele specific contributions from both A.alpha. and A.beta. chains are important in restricted recognition for most, if not all I-A-restricted Th cells. (b) In no case did substitution of the A.beta.2 domain from either of the alternative haplotypes lead to any functionally detectable effects, demonstrating the polymorphisms in the A.beta.1 domain can entirely account for the restriction imposed on Th cells responses by the entire A.beta. chain. (c) For 90% of the cells tested, replacement of the NH2-terminal portion of the .beta.1 domain with an allogeneic segment led to Ia molecules unable to elicit Th responses. Furthermore, of all the cells permissive of the substitution of one or other half of the .beta. 1 domain, only two permitted the substitution of sequence from both alternative haplotypes. Taken together, these data strongly suggest that antigen recognition by most, if not all, I-A-restricted Th cells involves contributions from both halves of the A.beta.1 domain. These data suggest that the role of I-A molecules in restricted Th cell recognition of antigen depends on conformational determinants unique to a particular combination of polymorphic .alpha. and .beta. chains, and that multiple such sites exist on a single Ia molecule. This conclusion suggests caution in attempting to physically interpret the result of MHC mutagenesis experiments, and a need for testing numerous T cells in several antigenic systems before general conclusions about Ia structure-function relationships can be drawn.