Vitamin D regulates MerTK-dependent phagocytosis in human myeloid cells

Vitamin D deficiency is a major environmental risk factor for the development of multiple sclerosis (MS). The major circulating metabolite of vitamin D (25OHD) is converted to the active form (calcitriol) by the hydroxylase enzyme CYP27B1. In MS lesions the tyrosine kinase MerTK expressed by microglia and macrophages regulates phagocytosis of myelin debris and apoptotic cells that can accumulate and inhibit tissue repair and remyelination. We show that calcitriol downregulates MerTK mRNA and protein expression in primary adult human microglia and monocyte-derived macrophages, thereby inhibiting myelin phagocytosis and apoptotic cell clearance. Proinflammatory myeloid cells express high levels of CYP27B1 compared to homeostatic (TGFb-treated) myeloid cells. Only proinflammatory cells in the presence of TNF-a generate calcitriol from 25OHD, resulting in repression of MerTK expression and function. The selective production of calcitriol in proinflammatory myeloid cells leading to downregulation of MerTK-mediated phagocytosis has the potential to reduce the risk for auto-antigen presentation while retaining the phagocytic ability of homeostatic myeloid cells, thereby contributing to inflammation reduction and enhanced tissue repair.