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

Abstract

Background: 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 defence 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.

Objective: Define signalling pathways that regulate IL-9 production by human CD4⁺ T cells.

Methods: 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.

Results: We identified two culture conditions that yielded IL-9-expressing cells from naïve CD4⁺ T cells, and amplified IL-9 production by in vivo-generated memory CD4⁺ T cells: TGFβ plus IL-4 (i.e. Th9 polarising condition), and the combination of IL-21, IL-23, IL-6, IL-1β and TGFβ (i.e. Th17 polarising 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 signalling via the TCR and STAT5. Importantly, IL-9 induction was restrained by IFNγ/STAT1 and IL-10.

Conclusions: 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.

Keywords: IL-9 expression; Th9 cells; cytokines; human CD4(+) T cell differentiation; inborn errors of immunity.