Macrophages and T cells control distinct phases of B cell differentiation in the humoral immune response in vitro.

The differentiation of B cells in the in vitro PFC-response to red blood cell antigen proceeds through 2 phases. Antigen-reactive B cells acquire the ability to interact with helper T cells in the first phase. This phase is controlled by macrophages through a mediator that they release (Interleukin 1 ([Il-1]). B cells convert into antibody-secreting cells (PFC) in the second phase, which is controlled by helper T cells or by a mediator that they release (T cell-replacing factors [TRF]). This is demonstrated in experiments in which Il-1 increases the number of B cells capable of responding to T cell help. The majority of antigen-reactive B cells reaches that state of differentiation within 40 hr of incubation with Il-1. After this time, the response of B cells depends no longer on the presence of Il-1 but on the presence of T cells or TRF. The presented data suggest that antigen-primed helper T cells (but not unprimed T cells) induce the release of Il-1 by macrophages, thereby also influencing the early phase of B cell differentiation.