Monogenic mutations differentially affect the quantity and quality of T follicular helper cells in patients with human primary immunodeficiencies

Background: Follicular helper T (T-FH) cells underpin T cell-dependent humoral immunity and the success of most vaccines. T-FH cells also contribute to human immune disorders, such as autoimmunity, immunodeficiency, and malignancy. Understanding the molecular requirements for the generation and function of T-FH cells will provide strategies for targeting these cells to modulate their behavior in the setting of these immunologic abnormalities. Objective: We sought to determine the signaling pathways and cellular interactions required for the development and function of T-FH cells in human subjects. Methods: Human primary immunodeficiencies (PIDs) resulting from monogenic mutations provide a unique opportunity to assess the requirement for particular molecules in regulating human lymphocyte function. Circulating follicular helper T (cT(FH)) cell subsets, memory B cells, and serum immunoglobulin levels were quantified and functionally assessed in healthy control subjects, as well as in patients with PIDs resulting from mutations in STAT3, STAT1, TYK2, IL21, IL21R, IL10R, IFNGR1/2, IL12RB1, CD40LG, NEMO, ICOS, or BTK. Results: Loss-of-function (LOF) mutations in STAT3, IL10R, CD40LG, NEMO, ICOS, or BTK reduced cT(FH) cell frequencies. STAT3 and IL21/R LOF and STAT1 gain-of-function mutations skewed cT(FH) cell differentiation toward a phenotype characterized by overexpression of IFN-gamma and programmed death 1. IFN-gamma inhibited cT(FH) cell function in vitro and in vivo, as corroborated by hypergammaglobulinemia in patients with IFNGR1/2, STAT1, and IL12RB1 LOF mutations. Conclusion: Specific mutations affect the quantity and quality of cT(FH) cells, highlighting the need to assess T-FH cells in patients by using multiple criteria, including phenotype and function. Furthermore, IFN-gamma functions in vivo to restrain T-FH cell-induced B-cell differentiation. These findings shed new light on T-FH cell biology and the integrated signaling pathways required for their generation, maintenance, and effector function and explain the compromised humoral immunity seen in patients with some PIDs.