The production of high-affinity antibodies to T-dependent antigens requires the interaction of B cells and T helper cells expressing receptors specific for the same antigen. Although several mechanisms have been elucidated that regulate B-cell trafficking within lymphoid organs, less is known about molecular cues that guide the small subpopulation of CD4+ follicular T helper cells to B-cell follicles. Using adoptive transfer of transgenic T cells in mice, we demonstrate that antigen-induced activation leads to a finely tuned positioning of T cells either to the T-cell area or the B-cell follicle. We show that expression of CXCR5 is indispensable for T cells to enter B-cell follicles, whereas expression of CCR7 provides a counteracting signal to retain activated T cells in the T-cell area. Although only few T cells transiently migrate from the T-cell area to the B-cell follicle of peripheral lymph nodes following antigenic challenge, this step is essential to provide the help B cells require to produce antibodies efficiently. Thus, we demonstrate that the balanced expression of CCR7 and CXCR5 determines the positioning and proper function of follicular T helper cells.
