Itaconate inhibits alternative activation of macrophages by targeting Janus Kinase 1

In innate immune cells, TCA cycle components are not simply inert metabolites, but are key signaling molecules that can modulate inflammatory responses. Itaconate, an immunometabolite made by the enzyme aconitase decarboxylase (encoded by the gene Irg1), is highly upregulated in classically activated M1 macrophages and regulates immune responses by dampening inflammation. The role of itaconate in activation of alternative macrophage programs is of interest, as itaconate has recently been shown to be taken up by M2 macrophages. Here, we show that expression of Irg1 is specific to M1 macrophages and opposes regulators of IL-4 mediated alternative activation. Overexpression of Irg1 in murine M2 macrophages inhibited their program. Similarly, exogenous addition of a cell-permeable derivative of itaconate inhibited M2 gene expression and blocked activation of Stat6 and AKT in IL-4 activated macrophages. Itaconate-treated M2 macrophages also showed decreased oxidative phosphorylation, altogether suggesting that itaconate constrains IL-4-mediated alternative activation of macrophages. Mechanistically, this novel role of itaconate was not through previously reported pathways Nrf2 or Complex II (SDH), but rather via inhibition of the phosphorylation of Janus Kinase 1. We also show that an M2-specific microRNA, miR-378a, targets Irg1 to prevent itaconate production in these cells. Thus, we demonstrate that itaconate antagonizes M2 macrophage activation, suggesting a mechanism by which M1 macrophages inhibit alternative macrophage polarization. This may be useful in considering the therapeutic potential of itaconate as an inhibitor of M2 macrophages in such conditions as allergic asthma and fibrosis.