Citrobacter rodentium is a gastrointestinal infection that requires early IL-22 from group 3 innate lymphoid cells (ILC3) for resistance. The role of vitamin D in the clearance of C. rodentium infection was tested in vitamin D sufficient (D+) and vitamin D deficient (D-) wildtype (WT) and Cyp27B1 (Cyp) KO mice (unable to produce the high affinity vitamin D ligand 1,25(OH)2D, 1,25D). Feeding Cyp KO mice D- diets reduced vitamin D levels and prevented synthesis of 1,25D. D- (WT and Cyp KO) mice had fewer ILC3 cells and less IL-22 than D+ mice. D- Cyp KO mice developed a severe infection that resulted in the lethality of the mice by d14 post-infection. T and B cell deficient D- Rag KO mice also developed a severe and lethal infection with C. rodentium compared to D+ Rag KO mice. D- WT mice survived the infection but took significantly longer to clear the C. rodentium infection than D+ WT or D+ Cyp KO mice. Treating infected D- Cyp KO mice with IL-22 protected the mice from lethality. Treating the D- WT mice with 1,25D reconstituted the ILC3 cells in the colon and protected the mice from C. rodentium. IL-22 treatment of D- WT mice eliminated the need for vitamin D to clear the C. rodentium infection. Vitamin D is required for early IL-22 production from ILC3 cells and protection from enteric infection with C. rodentium.

Diet is an important regulator of the gastrointestinal microbiota. Vitamin A and vitamin D deficiencies result in less diverse, dysbiotic microbial communities and increased susceptibility to infection or injury of the gastrointestinal tract. The vitamin A and vitamin D receptors are nuclear receptors expressed by the host, but not the microbiota. Vitamin A and vitamin D mediated regulation of the intestinal epithelium and mucosal immune cells underlies the effects of these nutrients on the microbiota. Vitamin A and vitamin D regulate the expression of tight junction proteins on intestinal epithelial cells that are critical for barrier function in the gut. Other shared functions of vitamin A and vitamin D include the support of innate lymphoid cells that produce IL-22, suppression of IFN-γ and IL-17 by T cells, and induction of regulatory T cells in the mucosal tissues. There are some unique functions of vitamin A and D; for example, vitamin A induces gut homing receptors on T cells, while vitamin D suppresses gut homing receptors on T cells. Together, vitamin A and vitamin D mediated regulation of the intestinal epithelium and mucosal immune system shape the microbial communities in the gut to maintain homeostasis.

Inflammatory bowel disease is a chronic disorder characterized by inflammation of the gastrointestinal tract and an immune mediated attack against the commensal microbiota. Vitamin D is an essential vitamin which not only promotes calcium and phosphate absorption but regulates immune function. The active form of vitamin D (1,25(OH) 2 D) has been shown to suppress symptoms of inflammatory bowel disease by inhibiting T cell responses. Host protection from gastrointestinal infection depends on T cells. Paradoxically, vitamin D deficiency increases susceptibility to inflammatory bowel disease and gastrointestinal infection. Here we will review the roles of vitamin D in immune cells using a kinetic model of the vitamin D-mediated effects on infection to explain the sometimes, paradoxical effects of vitamin D on gastrointestinal immunity.

The active form of vitamin D (1,25(OH) 2 D) suppresses experimental models of inflammatory bowel disease in part by regulating the microbiota. In this study, the role of vitamin D in the regulation of microbe induced RORγt/FoxP3+ T regulatory (reg) cells in the colon was determined. Vitamin D sufficient (D+) mice had significantly higher frequencies of FoxP3+ and RORγt/FoxP3+ T reg cells in the colon compared to vitamin D deficient (D–) mice. The higher frequency of RORγt/FoxP3+ T reg cells in D+ colon correlated with higher numbers of bacteria from the Clostridium XIVa and Bacteroides in D+ compared to D– cecum. D– mice with fewer RORγt/FoxP3+ T reg cells were significantly more susceptible to colitis than D+ mice. Transfer of the cecal bacteria from D+ or D– mice to germfree recipients phenocopied the higher numbers of RORγt/FoxP3+ cells and reduced susceptibility to colitis in D+ vs. D– recipient mice. 1,25(OH) 2 D treatment of the D– mice beginning at 3 weeks of age did not completely recover RORγt/FoxP3+ T reg cells or the Bacteriodes, Bacteriodes thetaiotaomicron , and Clostridi um XIVa numbers to D+ values. Early vitamin D status shapes the microbiota to optimize the population of colonic RORγt/FoxP3+ T reg cells important for resistance to colitis.

Vitamin D supplementation is linked to improved outcomes from respiratory virus infection, and the COVID-19 pandemic renewed interest in understanding the potential role of vitamin D in protecting the lung from viral infections. Therefore, we evaluated the role of vitamin D using animal models of pandemic H1N1 influenza and severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) infection. In mice, dietary-induced vitamin D deficiency resulted in lung inflammation that was present prior to infection. Vitamin D sufficient (D+) and deficient (D−) wildtype (WT) and D+ and D− Cyp27B1 (Cyp) knockout (KO, cannot produce 1,25(OH)2D) mice were infected with pandemic H1N1. D− WT, D+ Cyp KO, and D− Cyp KO mice all exhibited significantly reduced survival compared to D+ WT mice. Importantly, survival was not the result of reduced viral replication, as influenza M gene expression in the lungs was similar for all animals. Based on these findings, additional experiments were performed using the mouse and hamster models of SARS-CoV-2 infection. In these studies, high dose vitamin D supplementation reduced lung inflammation in mice but not hamsters. A trend to faster weight recovery was observed in 1,25(OH)2D treated mice that survived SARS-CoV-2 infection. There was no effect of vitamin D on SARS-CoV-2 N gene expression in the lung of either mice or hamsters. Therefore, vitamin D deficiency enhanced disease severity, while vitamin D sufficiency/supplementation reduced inflammation following infections with H1N1 influenza and SARS-CoV-2.