The effects of microbiota-derived short-chain fatty acids on T lymphocytes: From autoimmune diseases to cancer.

Short-chain fatty acids (SCFAs), acetate, propionate, and butyrate, are the microbial metabolites that have significant functions in host immune modulation, especially T lymphocyte function. Implication by recent evidence indicates SCFAs regulate T-cell growth, differentiation, metabolism, effector function, and apoptosis through histone deacetylase (HDAC) inhibition, G-protein-coupled receptor (GPCR) signaling, and metabolic reprogramming processes. Butyrate, for example, enhances regulatory T cell (Treg) and Interleukin 10 (IL-10)-producing T helper 1 (Th1) cell differentiation as well as context-dependent regulation on T helper 17 (Th17) cell development. SCFAs also impact cytotoxic CD8+ T cells through augmented production of IFN-γ and memory formation, which enhances antiviral and antitumor immunity. SCFAs reprogram T-cell metabolism through enhanced acetyl-CoA, mechanistic target of rapamycin (mTOR) signaling, and fatty acid oxidation (FAO), thus promoting the unique metabolic requirements of effector and memory T-cell subsets. In addition, SCFAs induce apoptosis of activated T cells through the Fas upregulation by inhibiting HDAC1. SCFA dysregulation plays a role in disease and autoimmune disorders like type 1 diabetes and rheumatoid arthritis, whereas therapeutic supplementation reduces inflammation and immune tolerance. SCFAs also amplify the antitumor effect of immune checkpoint inhibitors (eg, anti-programmed cell death protein 1 (anti-PD-1)) in cancer by driving CD8+ T-cell activation, infiltration, and Interferon gamma (IFNγ) production, partially through the transcriptional regulator Inhibitor of DNA binding 2 (ID2). Significantly, tissue- and disease-specific differential expression and functional implication of SCFA receptors (eg, GPR43, GPR41, GPR109A) emphasize the complexity of SCFA-mediated signaling. In conclusion, the current review emphasizes the multifunctional role of microbiota-derived SCFAs in T lymphocyte biology and their therapeutic potential in cancer, infection, and autoimmune diseases.