T follicular helper cell dynamics in germinal centers.

T follicular helper (T(FH)) cells are a specialized subset of effector T cells that provide help to and thereby select high-affinity B cells in germinal centers (GCs). To examine the dynamic behavior of T(FH) cells in GCs in mice, we used two-photon microscopy in combination with a photoactivatable fluorescent reporter. Unlike GC B cells, which are clonally restricted, T(FH) cells distributed among all GCs in lymph nodes and continually emigrated into the follicle and neighboring GCs. Moreover, newly activated T(FH) cells invaded preexisting GCs, where they contributed to B cell selection and plasmablast differentiation. Our data suggest that the dynamic exchange of T(FH) cells between GCs ensures maximal diversification of T cell help and that their ability to enter ongoing GCs accommodates antigenic variation during the immune response.