Differential effects of T-lymphocyte-derived soluble factor on virgin and primed B lymphocytes in vitro

Cell-free supernatants from mixed leukocyte cultures derived from histoincompatible mouse strains markedly enhanced the in vitro immune response to SRBC by splenocytes from allogeneic mice. The supernatant factor or factors from allogeneic spleen cell cultures appeared to preferentially stimulate antigen-sensitized B lymphocytes, especially those involved in secondary IgG antibody formation. Furthermore, as shown in the present study, the enhancing supernatant factor or factors had no effect on the true primary in vitro immune response to Vibrio cholerae antigen. Normal spleen cells cultured in vitro without cholera vaccine failed to develop antibody-forming cells to this bacterial antigen, despite the presence of the enhancing factor. In contrast, the true secondary immune response to vibrios was markedly enhanced when allogeneic culture supernatants were added at the time of secondary immunization of cholera-primed splenocytes in vitro. Enhancement occurred both for 7S IgG and 19S IgM vibriolytic plaque-forming cells. It appears likely that T lymphocytes present among the allogeneic splenocytes interacting in vitro to histoincompatible antigens release a factor or factors that primarily affect antigen-primed B lymphocytes but also may influence other cells such as macrophages that are important in the immune response to particulate antigens.