IL-37 Expression is Upregulated in Patients with Tuberculosis and Induces Macrophages Towards an M2-like Phenotype.

Interleukin-37 (IL-37), a member of the IL-1 family, primarily functions as an anti-inflammatory cytokine, reducing inflammation and suppressing the immune response. However, the expression and role of IL-37 in tuberculosis (TB) remains unknown. We aimed to measure serum levels of IL-37 and several important cytokines in 25 patients with active TB and to analyse their association with disease activity. We found that IL-37 levels decreased in patients with TB and recovered after treatment. IL-37 levels negatively correlated with the serum concentration of IFN-γ and IL-12 but positively correlated with IL-10 and TGF-β levels. After IL-37, secretion was blocked in peripheral blood mononuclear cells from active patients with TB, IFN-γ and IL-10 production was significantly upregulated; this was not observed in healthy donors or patients after treatment. IL-37 knockdown significantly enhanced the phagocytic activity of THP1-derived macrophages towards Mycobacterium tuberculosis (M. tb). M1/M2 polarization-associated markers were detected simultaneously, and IL-37 induced a phenotypic shift in THP1-derived macrophages towards a high CD206(+) and low CD86(+) macrophage subtype. Furthermore, this phenotypic shift was accompanied by upregulated mRNA levels of arginase 1, TGF-β and IL-10, which are characteristic hallmarks of M2 macrophages. In conclusion, our results suggest that increased levels of IL-37 in patients with TB are associated with IFN-γ, IL-12, IL-10 and TGF-β levels and that IL-37 plays a pathological role in TB infection by inhibiting the production of pro-inflammatory cytokines and inducing macrophages towards an M2-like phenotype. Thus, IL-37 may be a novel research target to understand the pathogenesis of TB infection.