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.

Seasonal influenza vaccine protects 60 to 90% of healthy young adults from influenza infection. The immunological events that lead to the induction of protective antibody responses remain poorly understood in humans. We identified the type of CD4+ T cells associated with protective antibody responses after seasonal influenza vaccinations. The administration of trivalent split-virus influenza vaccines induced a temporary increase of CD4+ T cells expressing ICOS, which peaked at day 7, as did plasmablasts. The induction of ICOS was largely restricted to CD4+ T cells co-expressing the chemokine receptors CXCR3 and CXCR5, a subpopulation of circulating memory T follicular helper cells. Up to 60% of these ICOS+CXCR3+CXCR5+CD4+ T cells were specific for influenza antigens and expressed interleukin-2 (IL-2), IL-10, IL-21, and interferon-γ upon antigen stimulation. The increase of ICOS+CXCR3+CXCR5+CD4+ T cells in blood correlated with the increase of preexisting antibody titers, but not with the induction of primary antibody responses. Consistently, purified ICOS+CXCR3+CXCR5+CD4+ T cells efficiently induced memory B cells, but not naïve B cells, to differentiate into plasma cells that produce influenza-specific antibodies ex vivo. Thus, the emergence of blood ICOS+CXCR3+CXCR5+CD4+ T cells correlates with the development of protective antibody responses generated by memory B cells upon seasonal influenza vaccination.

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Our understanding of the origin and functions of human blood CXCR5+ CD4+ T cells found in human blood has changed dramatically in the past years. These cells are currently considered to represent a circulating memory compartment of T follicular helper (Tfh)-lineage cells. Recent studies have shown that blood memory Tfh cells are composed of phenotypically and functionally distinct subsets. Here we review the current understanding of human blood memory Tfh cells and the subsets within this compartment. We present a strategy to define these subsets based on cell surface profiles. Finally, we discuss how increased understanding of the biology of blood memory Tfh cells may contribute insight into the pathogenesis of autoimmune diseases and the mode of action of vaccines.

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.