miR-9-5p regulates immunometabolic and epigenetic pathways in β-glucan-trained immunity via IDH3α.

Trained immunity, induced by β-glucan in monocytes, is mediated by activating metabolic pathways that result in epigenetic rewiring of cellular functional programs; however, molecular mechanisms underlying these changes remain unclear. Here, we report a key immunometabolic and epigenetic pathway mediated by the miR-9-5p-isocitrate dehydrogenase 3α (IDH3α) axis in trained immunity. We found that β-glucan-trained miR-9-5p-/- monocytes showed decreased IL-1β, IL-6, and TNF-α production after LPS stimulation. Trained miR-9-5p-/- mice produced decreased levels of proinflammatory cytokines upon rechallenge in vivo and had worse protection against Candida albicans infection. miR-9-5p targeted IDH3α and reduced α-ketoglutarate (α-KG) levels to stabilize HIF-1α, which promoted glycolysis. Accumulating succinate and fumarate via miR-9-5p action integrated immunometabolic circuits to induce histone modifications by inhibiting KDM5 demethylases. β-Glucan-trained monocytes exhibited low IDH3α levels, and IDH3α overexpression blocked the induction of trained immunity by monocytes. Monocytes with IDH3α variants from autosomal recessive retinitis pigmentosa patients showed a trained immunity phenotype at immunometabolic and epigenetic levels. These findings suggest that miR-9-5p and IDH3α act as critical metabolic and epigenetic switches in trained immunity.