Allogeneic carrier-specific enhancement of hapten-specific secondary B- cell responses

We have analyzed the capacity of carrier-specific T cells to enhance the immune response of hapten-specific secondary B cells which do not share genes in the H-2 complex with the T cells. For this analysis we have used the in vitro splenic focus technique which allows assessment of monoclonal responses of B cells isolated in splenic fragment cultures of irradiated reconstituted carrier primed mice. A previous report from this laboratory demonstrated that syngeny in the I region of the H-2 complex was necessary between collaborating hapten-specific primary (nonimmune) B cells and carrier-specific T cells for responses yielding IgG1 but not IgM antibody. These findings lead up to postulate that the expression of I-region gene products on the surface of primary B cells and I-region syngeny with collaborating carrier-specific T cells were essential elements in the triggering events leading to IgG1 synthesis by primary B cells. The results presented in the present report indicate that, unlike primary B cells, the majority of secondary B cells can be stimulated to produce IgG1 antibody in carrier-primed allogeneic recipients. Although the enhancement of secondary IgG1 responses is slightly greater with syngeneic T cells, the allogeneic collaborative interaction requires both carrier priming of recipient mice and stimulation with the homologous hapten-carrier complex and thus appears to be specific. These findings clearly discriminate secondary from primary B cells and indicate that the mechanism of stimulation of secondary B cells to yield IgG1-producing clones differs fundamentally from the stimulation of primary B cells in that the requisite for I-region syngeny is obviated.