Foxp3⁺ T(reg) cells in humoral immunity.

T(reg) cells are essential for the maintenance of immune homeostasis and prevention of autoimmunity. In humoral immune responses, loss of T(reg) cell function causes increased levels of serum autoantibodies, hyper-IgE, spontaneous generation of germinal centres, and enhanced numbers of specialised T follicular helper cells (T(fh) cells) controlled by the lineage-defining transcription factor BCL-6 (B-cell lymphoma 6). Recent studies have demonstrated that a subset of T(reg) cells [T follicular regulatory (T(freg)) cells] are able to co-opt the follicular T-cell program by gaining expression of BCL-6 and travelling to the follicle where they have an important role in the control of expansion of T(fh) cells and the germinal centre reaction. However, the mechanisms by which they exert this control are still under investigation. In this review, we discuss the effects of T(reg) cells on humoral immunity and the mechanisms by which they exert their regulatory function.