Vitamin D as an Immune Modulator in Systemic Lupus Erythematosus: A Narrative Review.

Systemic lupus erythematosus (SLE) is a multisystem autoimmune disease in which environmental factors modulate genetically determined immune dysregulation. Vitamin D has emerged as a plausible modifier of disease expression because its active metabolite signals through the vitamin D receptor on innate and adaptive immune cells and influences antigen presentation, cytokine balance, and lymphocyte differentiation. This narrative review synthesizes current evidence on vitamin D status and supplementation in SLE with attention to organ-specific domains. Observational studies consistently report high rates of hypovitaminosis D in SLE and associations with less favorable clinical profiles, including higher global and renal disease activity, adverse cardiometabolic features, greater infection vulnerability, and neuropsychiatric manifestations. Preclinical models demonstrate neuroprotective and barrier-stabilizing actions of vitamin D analogs, supporting biological plausibility. Interventional trials indicate that supplementation safely corrects deficiency and shows signals of benefit for selected outcomes (e.g., modest activity reductions or fatigue in specific contexts), although effects on interferon signatures, complement, and autoantibodies are heterogeneous and often limited. Overall, current evidence supports optimization of vitamin D status as a low-risk adjunct in comprehensive SLE care while highlighting the need for adequately powered, organ-focused randomized trials using standardized measurements and prespecified endpoints to define causality, therapeutic targets, and long-term safety.