B cell activation in vivo by nonantigen-specific interaction with T cells. Frequency of immune responses increased by immunization with two antigens.

The number of direct (gammaM) hemolytic plaque responses of irradiated mice, repopulated with relatively small and limiting numbers of bone marrow and thymus cells, was increased by the simultaneous immunization with two antigen complexes instead of one. Anti-sheep responses were augmented by the following antigen combinations: SRBC + HRBC, SRBC + BRBC, and SRBC + CRBC. Limiting either thymocytes or bone marrow cells indicated that the antigen mixtures acted at the level of T cells increasing severalfold the number of triggered antigen-reactive cells. It was concluded that one of the antigens could have influenced the triggering of antigen-reactive cells specific for the other by promoting synergistic or derepressive T-T cell interactions. Moreover, bone marrow precursor cells could have been activated by the thymic inducers specific for the test antigens as well as by those specific for the second of the priming antigens.