Germinal center B cells present antigen obtained in vivo to T cells in vitro and stimulate mixed lymphocyte reactions.

In the present study we sought to test the ability of GC B cells to activate T cells by antigen presentation and mixed lymphocyte reactions. The first set of experiments demonstrated that GC B cells obtained three or more weeks after immunization could induce IL 2 production by T cells only by adding antigen to the cell cultures. In the next (table; see text) series, it was demonstrated that GC B cells isolated as early as 1 day and for over a week following an antigenic challenge with OVA (i.e. where they obtained antigen from the FDC-derived iccosomes in vivo), were able to stimulate high levels of IL 2 production in the absence of exogenous OVA. This response was maximal on Day 5 which corresponded precisely with the keinetics of the ultrastructural studies which document the uptake of antigen by GC B cells in vivo (see Szakal et. al, this volume). Furthermore, it was shown that the FDC-derived antigen was remarkably immunogenic when compared with exogenous antigen. The data from the MLRs demonstrated that GC B cells, which are low density cells, are potent stimulators of T cells compared to the resting, low density B cells. In addition, GC B cells even stimulated a significant syngeneic response. These results indicate that GC B cells can present antigen and interact with T cells efficiently. This is believed to be important in the germinal center reaction and in the induction and maintenance of the immune responses.