EBI2 augments Tfh cell fate by promoting interaction with IL2-quenching dendritic cells

T follicular helper (Tfh) cells are a CD4 T cell subset that is important for supporting plasma cell and germinal center (GC) responses(1,2). The initial induction of Tfh cell properties occurs within the first few days following activation by antigen recognition on dendritic cells (DCs), though how DCs promote this cell-fate decision is not fully understood(1,2). Moreover, although Tfh cells are uniquely defined by expression of the follicle-homing receptor CXCR5(1,2), the guidance receptor promoting the earlier localization of activated T cells at the B cell follicle–T zone interface has been unclear(3–5). Here we show that the G-protein coupled receptor EBI2 (GPR183) and its ligand 7α,25-dihydroxycholesterol (7α,25-OHC) mediate positioning of activated CD4 T cells at the follicle–T zone interface. In this location they interact with activated DCs and are exposed to Tfh cell-promoting ICOS ligand. IL2 is a cytokine that has multiple influences on T cell fate, including negative regulation of Tfh cell differentiation(6–10). We demonstrate that activated DCs in the outer T zone further augment Tfh cell differentiation by producing membrane and soluble forms of CD25, the IL2 receptor α chain, and quenching T cell-derived IL2. Mice lacking EBI2 in T cells or CD25 in DCs have reduced Tfh cells and mount defective T cell-dependent plasma cell and GC responses. These findings demonstrate that distinct niches within the lymphoid organ T zone support distinct cell fate decisions, and they establish a function for DC-derived CD25 in controlling IL2 availability and T cell differentiation.