The collaborative phenotype of secondary B cells is determined by T lymphocytes during in vivo immunization.

Previous studies have demonstrated that the B cells in immune and nonimmune mice manifest different major histocompatibility complex (MHC) collaborative phenotypes with antigen-specific T cells. Immune, or secondary B cells require syngeneic-like MHC recognition by collaborating T cells, and in its absence fail to be stimulated. Primary B cells manifest a much less stringent requisite for MHC recognition by T cells, and under conditions in which secondary B cells fail to be stimulated, primary B cells are stimulated to secrete IgM antibody. Experiments were conducted to determine whether the acquisition of the secondary B cells' MHC collaborative phenotype was dependent on the presence of T cells during in vivo immunization. B cell populations from T dependently and T independently immunized conventional BALB/c and athymic BALB/c nu/nu mice were compared in their ability to collaborate with allogeneic T cells. Although antigen alone promotes the differentiation of several secondary B cell characteristics, including an increase in the frequency of antigen-specific B cells and a preference for IgG1 antibody synthesis in vitro, the acquisition of the secondary B cells' MHC collaborative phenotype was dependent on the presence of T cells during in vivo immunization. B cell populations from T dependently and T independently immunized conventional BALB/c and athymic BALB/c nu/nu mice were compared in their ability to collaborate with allogeneic T cells. Although antigen alone promotes the differentiation of several secondary B cell characteristics, including an increase in the frequency of antigen-specific B cells and a preference of IgG1 antibody synthesis in vitro, the acquisition of the secondary B cells' MHC collaborative phenotype was found to be dependent on the presence of T cells during in vivo immunization. Thus, the restriction imposed on T cell-B-cell-collaborative interactions in secondary humoral immune responses appears to be the result of T dependent antigen-driven events.