BACKGROUND AND OBJECTIVES: Follicular helper T (Tfh) cells are instrumental in the development of humoral responses. They support the terminal differentiation of B cells into memory and antibody-producing cells within germinal centers. Regulatory B (Breg) cells can modulate inflammatory reactions through the control of dendritic cell maturation and function, and through the control of T cell proliferation and Th1 polarization. Our aimed was to evaluate the control of humoral responses by Breg cells. MATERIALS AND METHODS: We developed in vitro models of human Tfh cell polarisation and function. They were obtained by stimulation of purified T cells with anti-CD3 and anti-CD28 Abs in the presence of IL-12 and IL-21. Expression of Bcl-6, IL-21, ICOS, CXCR5 and PD-1, all characteristics of Tfh cells were determined by flow cytometry. Tfh functions were studied by co-cultures with non-stimulated purified B cells. Their differentiation into memory and plasma cells was appraised by flow cytometry and by ELISA to measure the production of immunolgobulins. Breg cells were obtained by stimulation of purified B cells with CpG-ODN on CD40L-transfected fibroblasts. Their effect on Tfh cell maturation and function were studied in co-culture experiments. RESULTS: in vitro Tfh cells were obtained. Thus, Bcl-6 was up-regulated, IL-21 induced and ICOS, CXCR5 and PD-1 expression increased after stimulation. Furthermore, differentiated Tfh cells fostered the maturation of IgD-CD27 + memory B cells and CD138 + plasma cells, and triggered the secretion of IgM, IgG and IgA. Added to the T cells, Breg cells restrained the expression of Tfh markers. In the co-cultures of Tfh with B cells, Breg cells inhibited the induction of IgD-CD27 + and CD138 + B cells. They also impeded the secretion of immunoglobulins. Using blocking antibodies, suppressive activities of Bregs were found to be dependent on IL-10 and TGFbeta and based on direct contact with Tfh cells involving CD40, CD80, CD86 and CD54 molecules. CONCLUSIONS: Human Breg cells can modulate the development of humoral responses through the control of Tfh cell polarization and of Tfh-dependent terminal differentiation of B cells. Whether these properties are impaired and responsible for abnormal humoral responses in autoimmune diseases needs now to be established.