Although a fraction of human blood memory CD4+ T cells expresses chemokine (C-X-C motif) receptor 5 (CXCR5), their relationship to T follicular helper (Tfh) cells is not well-established. Here we show that human blood CXCR5+ CD4+ T cells share functional properties with Tfh cells, and appear to represent their circulating memory compartment. Blood CXCR5+ CD4+ T cells comprised three subsets; T helper 1 (Th1), Th2 and Th17 cells. Th2 and Th17 cells within CXCR5+, but not within CXCR5−, compartment efficiently induced naïve B cells to produce immunoglobulins via interleukin-21 (IL-21). In contrast, Th1 cells from both CXCR5+ and CXCR5− compartments lacked the capacity to help B cells. Patients with juvenile dermatomyositis, a systemic autoimmune disease, displayed a profound skewing of blood CXCR5+ Th subsets towards Th2 and Th17 cells. Importantly, the skewing of subsets correlated with disease activity and frequency of blood plasmablasts. Collectively, our study suggests that an altered balance of Tfh subsets contributes to human autoimmunity.

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Summary T follicular helper (Tfh) cells help development of antibody responses via Interleukin-21 (IL-21). Here we show that activated human dendritic cells (DCs) induced naïve CD4+ T cells to become IL-21-producing Tfh-like cells through IL-12. CD4+ T cells primed with IL-12 induced B cells to produce immunoglobulins in a fashion dependent on IL-21 and inducible costimulator (ICOS), thus sharing fundamental characteristics with Tfh cells. The induction of Tfh-like cells by activated DCs was inhibited by neutralizing IL-12. IL-12 induced two different IL-21-producers: IL-21+IFN-γ+T-bet+ Th1 cells and IL-21+IFN-γ-T-bet- non-Th1 cells, in a manner dependent on signal transducer and activator of transcription (STAT)4. IL-12 also regulated IL-21 secretion by memory CD4+ T cells. Thus, IL-12 produced by activated DCs regulates antibody responses via developing IL-21-producing Tfh-like cells, and inducing IL-21 secretion from memory CD4+ T cells. These data suggest that the developmental pathway of Tfh cells differs between mice and humans, which have considerable implications for vaccine development.

Understanding the developmental mechanisms of T follicular helper (TFH) cells in humans is a highly relevant topic to clinic. However, factors that drive human CD4+ helper T (TH) cell differentiation program towards TFH cells remain largely undefined. Here we show that TGF-β provides critical additional signals for the transcription factors STAT3 and STAT4 to promote the initial TFH differentiation programs in humans. This mechanism does not appear to be shared with mouse TH cells. The developing human Bcl-6+ TFH cells also expressed RORγt, a transcription factor typically expressed by TH17 cells. Our study documents a mechanism by which TFH and TH17 cells co-emerge in inflammatory environments in humans, as often observed in many human autoimmune diseases.

Antibody responses represent a key immune protection mechanism. T follicular helper (Tfh) cells are the major CD4(+) T-cell subset that provides help to B cells to generate an antibody response. Tfh cells together with B cells form germinal centers (GCs), the site where high-affinity B cells are selected and differentiate into either memory B cells or long-lived plasma cells. We show here that interleukin-12 receptor β1 (IL-12Rβ1)-mediated signaling is important for in vivo Tfh response in humans. Although not prone to B cell-deficient-associated infections, subjects lacking functional IL-12Rβ1, a receptor for IL-12 and IL-23, displayed substantially less circulating memory Tfh and memory B cells than control subjects. GC formation in lymph nodes was also impaired in IL-12Rβ1-deficient subjects. Consistently, the avidity of tetanus toxoid-specific serum antibodies was substantially lower in these subjects than in age-matched controls. Tfh cells in tonsils from control individuals displayed the active form of signal transducer and activator of transcription 4 (STAT4), demonstrating that IL-12 is also acting on Tfh cells in GCs. Thus, our study shows that the IL-12-STAT4 axis is associated with the development and the functions of Tfh cells in vivo in humans.