Alloactivated Lyt 1 ⁺2^- T lymphoblasts bind syngeneic Ia antigens

A large proportion of T blasts activated in the mixed lymphocyte reaction (MLR) is capable of binding plasma membrane (PM) vesicles prepared from the stimulator strain^1,2. The binding of vesicles is specific, and the antigens recognized are serologically detectable H-2K, D and I region products^3,4. T blasts binding stimulator K (and D) antigens belong to the Lyt 1 ^-2⁺ subclass, whereas those binding stimulator Ia antigens are Lyt 1 ⁺1^- (ref. 5). We report here that antibodies against heavy chain V-region determinants (V_H) ⁶, and furthermore, monoclonal⁷ or conventional antibodies against responder cell Ia determinants strongly inhibit the binding of radiolabelled stimulator vesicles. Anti-responder Ia inhibition is restricted to the Lyt 1 ⁺2^- subset of T blasts and correlates with a weak expression of Ia determinants on these cells. The relevant Ia determinants are not resynthesized after trypsin treatment of the blasts. In these conditions anti-V_H, but not anti-Ia reagents inhibit stimulator vesicle binding. Trypsinized T blasts can, however, passively absorb Ia material from supernatants of syngeneic, lipopolysaccharide (LPS)-activated spleen cells, and thus regain their susceptibility to vesicle-binding inhibition with anti-Ia sera. Acquisition of syngeneic Ia is also blocked by the anti-V_H reagent. We conclude that Lyt 1 ⁺2^- MLR blasts possess binding sites for both allogeneic (stimulator) and syngeneic Ia antigens.