Inborn errors of immunity reveal molecular requirements for generation and maintenance of human CD4+ IL-9–expressing cells

CD4+ T cells play essential roles in adaptive immunity. Distinct CD4+ T-cell subsets—TH1, TH2, TH17, TH22, T follicular helper, and regulatory T cells—have been identified, and their contributions to host defense and immune regulation are increasingly well defined. IL-9–producing TH9 cells were first described in 2008 and appear to play both protective and pathogenic roles in human immunity. However, key requirements for generating human TH9 cells remain incompletely defined. We sought to define signaling pathways that regulate IL-9 production by human CD4+ T cells. Human naive and memory CD4+ T cells were cultured under different conditions, and the molecular mechanisms regulating IL-9 induction were determined by assessing the ability of CD4+ T cells from a broad range of patients (n = 92) with pathogenic variants in key immune genes (n = 21) to differentiate into IL-9+ cells. We identified 2 culture conditions that yielded IL-9–expressing cells from naive CD4+ T cells and amplified IL-9 production by in vivo–generated memory CD4+ T cells: TGF-β plus IL-4 (ie, TH9 polarizing condition), and the combination of IL-21, IL-23, IL-6, IL-1β, and TGF-β (ie, TH17 polarizing condition). Combining these conditions had a synergistic effect in generating IL-9+CD4+ T cells. IL-9 induction required STAT3-activating cytokines as well as intact signaling via the T-cell receptor and STAT5. Importantly, IL-9 induction was restrained by IFN-γ/STAT1 and IL-10. Our findings revealed critical molecules involved in inducing/restraining IL-9 production by human CD4+ T cells, thereby identifying pathways that could be targeted to modulate IL-9 in health and disease.