IL-9 and Blimp-1 protect the transcriptional identity of group 2 innate lymphocytes in allergic asthma

Allergic asthma is driven by type 2 immune responses, including type 2 innate lymphoid cells (ILC2s). Although ILC2s are activated by the tissue alarmins interleukin (IL)-33 and IL-25, these signals do not intrinsically enforce type 2 identity and the mechanisms that maintain type 2 cytokine expression remain unclear. Here we show that allergen-induced IL-33 and IL-25 rapidly induce IL-9, which in turn upregulates the transcriptional repressor Blimp-1 in ILC2s. Blimp-1 sustains type 2 immunity by directly repressing type 1 inflammatory programs, including expression of interferon-γ and tumor necrosis factor. Deletion of Blimp-1 in ILC2s increased type 1 cytokine production and reduced IL-5 and IL-13 expression, eosinophil recruitment and mucus production in the lung. In contrast, IL-9 expression was enhanced in the absence of Blimp-1, leading to increased mast cell recruitment. Together, these findings identify Blimp-1 as a key regulator of ILC2 transcriptional fidelity that stabilizes type 2 inflammation while constraining divergent inflammatory programs during allergic responses. Poholek and colleagues show that allergic alarmin-induced IL-9 increases expression of the transcriptional repressor Blimp-1 in ILC2 cells, sustaining their cellular identity and type 2 immune responses.