Evolving Role of Vitamin D in Immune-Mediated Disease and Its Implications in Autoimmune Hepatitis.

Vitamin D has immunomodulatory, anti-inflammatory, antioxidant, and anti-fibrotic actions that may impact on the occurrence and outcome of immune-mediated disease. The goals of this review are to describe the nature of these expanded roles, examine the implications of vitamin D deficiency in autoimmune hepatitis, and identify opportunities for future investigation. Abstracts were identified in PubMed by multiple search terms. Full-length articles were selected for review, and secondary and tertiary bibliographies were developed. Vitamin D receptors are expressed on the principal cell populations involved in the innate and adaptive immune responses. Macrophages and dendritic cells can produce 1,25-dihydroxyvitamin D within the microenvironment. This active form of vitamin D can inhibit immune cell proliferation, promote an anti-inflammatory cytokine profile, expand regulatory T cells, enhance glucocorticoid actions, increase glutathione production, and inhibit hepatic stellate cells. Vitamin D deficiency has been commonly present in patients with immune-mediated liver and non-liver diseases, and it has been associated with histological severity, advanced hepatic fibrosis, and non-response to conventional glucocorticoid therapy in autoimmune hepatitis. Vitamin D analogues with high potency, low calcemic effects, and independence from hepatic hydroxylation are possible interventions. In conclusion, vitamin D has properties that could ameliorate immune-mediated disease, and vitamin D deficiency has been a common finding in immune-mediated liver and non-liver diseases, including autoimmune hepatitis. Loss of vitamin D-dependent homeostatic mechanisms may promote disease progression. Vitamin D analogues that are independent of hepatic hydroxylation constitute an investigational opportunity to supplement current management of autoimmune hepatitis.