Influence of B cell receptor ligation and TCR affinity on T-B collaboration in vitro

Co-culture of purified T and B cells obtained from cytochrome c-specific TCR- and hen egg lysozyme (HEL)-specific Ig-transgenic mice was used to examine the role of B cell receptor (BCR) ligation and TCR affinity on the efficiency of T-B cell collaboration. The results showed that BCR ligation of naive B cells with HEL was not required for effective presentation of high-affinity antigen to T cells, although it did enhance activation and division of both T and B cells. Anergic B cells were also effective at presentation of high-affinity antigen and proliferated more than naive B cells in response to T cell help, due to prior exposure to antigen in vivo. Despite the fact that induction of CD86 on anergic B cells following BCR ligation was suboptimal, these cells supported T cell activation and survival in culture as efficiently as naive B cells exposed to HEL. In contrast, when the low-affinity antigen mls-3a served as the T cell stimulus, BCR ligation was essential to elicit a detectable T cell response. Thus the in vitro model demonstrates that co-stimulation is not an absolute requirement for effective antigen presentation and delivery of T cell help to B cells. Rather, the cooperative effects of BCR ligation and TCR affinity determine the relative requirement for co-stimulation.