The development of inbred strains of mutant mice has proven useful in ascribing specific gene functions to particular genetic loci within the regions and subregions of the H-2 complex. The B6.C-H-2bm12 (bm12) strain is of particular interest in that, compared to parental C57Bl/6Kh (B6) mice, it bears a presumptive single gene mutation altering the Ab beta chain encoded by the I-A subregion. Our data show that bm12 mice have gained the ability to respond to poly(Glu50Tyr50)(GT) and have lost the ability to make plaque-forming cell or delayed-type hypersensitivity responses to the closely related copolymer, poly(Glu60Ala30Tyr10)(GAT), although retaining the ability to mount a GAT-specific T cell proliferative response. This is in sharp contrast to the parental B6 strain, which is a GT nonresponder and a GAT responder. Thus, this study is the first to report the establishment of responder status as a consequence of mutation. Possible mechanisms accounting for the gain/loss of GT/GAT responsiveness in the context of a two-step helper T cell model are discussed.

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