Follicular regulatory T (T_fr) cells from lymph node germinal centers control follicular helper T (T_fh) cell-dependent B cell activation. These scarce cells, often described and purified as CD25⁺ cells, are thought to be derived from thymic regulatory T (T_reg) cells. However, we observed that mouse T_fr cells do not respond to interleukin-2 (IL-2), unlike T_reg cells. Stringent immunophenotyping based on B cell lymphoma 6 (Bcl6), programmed cell death protein 1 (PD-1), and CXCR5 expression revealed that T_fr cells are actually CD25^-, in mice and humans. Moreover, T_fr cell characterization based only on CXCR5 and PD-1 high expression without excluding CD25⁺ cells resulted in contamination with T_reg cells. Transcriptome studies of CD4⁺CXCR5⁺PD-1⁺Bcl6⁺Foxp3⁺CD25^- T_fr cells revealed that they express the IL-1 decoy receptor IL-1R2 and the IL-1 receptor antagonist IL-1Ra, whereas T_fh cells express the IL-1R1 agonist receptor. IL-1 treatment expanded T_fh cells in vivo and activated their production of IL-4 and IL-21 in vitro. T_fr cells suppressed the IL-1-induced activation of T_fh cells as efficiently as the IL-1 receptor antagonist Anakinra. Altogether, these results reveal an IL-1 axis in the T_fh cell control of B cell responses and an IL-2/IL-1 dichotomy for T_reg cell control of effector T cells versus T_fr cell control of T_fh cells.

Guillame Churlaud

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