Niacin Mitigates Cyclophosphamide-Induced Immunosuppression by Maintaining Intestinal Homeostasis and Regulating the HCAR2/NLRP3 and PTGS2/PGE2 Signaling Pathways

<b>Objectives</b>: This study is intended to reveal whether the boost in immune function in immunocompromised mice from niacin supplementation is connected to the upkeep of intestinal homeostasis and the modulation of the hydroxycarboxylic acid receptor 2 (HCAR2)/NOD-like receptor protein 3 (NLRP3) and prostaglandin endoperoxide synthase 2 (PTGS2)/prostaglandin E2 (PGE2) signaling pathways. <b>Methods</b>: Balb/c mice were employed in this study as a model for immunosuppression caused by cyclophosphamide (CTX) injection. <b>Results</b>: The study showed that niacin supplementation restored spleen and liver indices, enhanced cytokine secretion, and increased Th1/Th2 cytokine levels. Niacin effectively enhanced the phagocytic index, natural killer cell (NK cell) activity, splenic lymphocyte activity and delayed-type hypersensitivity (DTH) reaction in immunocompromised mice. Histopathological examination showed that niacin intervention alleviated injury in mice ilea. Intestinal barrier tight junction proteins were expressed at much higher levels, while the serum concentrations of diamine oxidase (DAO) and fatty acid-binding protein 2 (FABP2) were markedly lowered. Furthermore, the expression of the intestinal HCAR2/NLRP3 signaling pathway and subsequent inflammatory mediators was significantly elevated after niacin administration compared with the CTX group. Niacin supplementation improved the composition of the gut microbiota, increasing the Firmicutes/Bacteroidetes (F/B) ratio. Spearman correlation analysis showed significant correlations between cytokine-related indices and several gut microbiotas. Within a network pharmacology framework including target screening, network construction and molecular docking, PTGS2 emerged as a candidate target of niacin, suggesting its role in counteracting immunosuppression. Further experimental findings showed that niacin markedly decreased the protein expression of PTGS2 and the levels of its downstream mediators PGE2, E-prostanoid receptor type 2 (EP2) and (E-prostanoid receptor type 4 (EP4) in the ileal tissue of mice treated with CTX. <b>Conclusions</b>: In conclusion, niacin supplementation alleviated CTX-induced immunosuppression by maintaining intestinal homeostasis and regulating the intestinal HCAR2/NLRP3 and PTGS2/PGE2/EP2-EP4 pathways.