RESUMEN
Vitamin D has effects on several body systems, from well-established role in bone metabolism to emerging effects on the immune system. Increasing evidence supports an immunomodulatory effect including inhibition of the pro-inflammatory lymphocyte subsets while enhancing their anti-inflammatory counterpart, in favour of a more tolerogenic status. Vitamin D deficiency is increasingly recognised in association with autoimmune and inflammatory diseases, also with evidence from the field of asthma where vitamin D supplementation may overcome steroid resistance. In the HSCT setting, vitamin D deficiency has been variably associated with increased complications, including graft-versus-host disease (GvHD), with a potential impact on survival outcomes. In this review we provide an overview and critical appraisal of the current literature of the role of vitamin D (and its deficiency) in relation to immunity in both allogeneic and autologous HSCT settings. We conclude that the evidence base is mixed, but a greater understanding of the role of vitamin D in relation to immune reconstitution following HSCT is warranted. Given its potential benefits, its inexpensive cost and favourable side effect profile, consideration of vitamin D levels and its supplementation could be easily incorporated into prospective studies in GvHD, including clinical trials of novel therapeutics, supportive care and biomarkers.