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Background The cross-talk between the host and its microbiota plays a key role in the promotion of health. The production of metabolites such as polyamines by intestinal-resident bacteria is part of this symbiosis shaping host immunity. The polyamines putrescine, spermine, and spermidine are abundant within the gastrointestinal tract and might substantially contribute to gut immunity. Objective We aimed to characterize the polyamine spermidine as a modulator of T-cell differentiation and function. Methods Naive T cells were isolated from wild-type mice or cord blood from healthy donors and submitted to polarizing cytokines, with and without spermidine treatment, to evaluate CD4+ T-cell differentiation in vitro. Moreover, mice were subjected to oral supplementation of spermidine, or its precursor l-arginine, to assess the frequency and total numbers of regulatory T (Treg) cells in vivo. Results Spermidine modulates CD4+ T-cell differentiation in vitro, preferentially committing naive T cells to a regulatory phenotype. After spermidine treatment, activated T cells lacking the autophagy gene Atg5 fail to upregulate Foxp3 to the same extent as wild-type cells. These results indicate that spermidine's polarizing effect requires an intact autophagic machinery. Furthermore, dietary supplementation with spermidine promotes homeostatic differentiation of Treg cells within the gut and reduces pathology in a model of T-cell transfer-induced colitis. Conclusion Altogether, our results highlight the beneficial effects of spermidine, or l-arginine, on gut immunity by promoting Treg cell development.

The progression through different steps of T-cell development, activation, and effector function is tightly bound to specific cellular metabolic processes. Previous studies established that T-effector cells have a metabolic bias toward aerobic glycolysis, whereas naive and regulatory T cells mainly rely on oxidative phosphorylation. More recently, the field of immunometabolism has drifted away from the notion that mitochondrial metabolism holds little importance in T-cell activation and function. Of note, T cells possess metabolic promiscuity, which allows them to adapt their nutritional requirements according to the tissue environment. Altogether, the integration of these metabolic pathways culminates in the generation of not only energy but also intermediates, which can regulate epigenetic programs, leading to changes in T-cell fate. In this review, we discuss the recent literature on how glycolysis, amino acid catabolism, and fatty acid oxidation work together with the tricarboxylic acid cycle in the mitochondrion. We also emphasize the importance of the electron transport chain for T-cell immunity. We also discuss novel findings highlighting the role of key enzymes, accessory pathways, and posttranslational protein modifications that distinctively regulate T-cell function and might represent prominent candidates for therapeutic purposes.