VDR and systemic lupus erythematosus: The VDR is expressed in different immune system cells, such as activated T and B lymphocytes, neutrophils, macrophages, and dendritic cells [12,56,57]; thus, its increased expression in SLE patients could be a reflection of hyperactivation of the immune system, as well as the presence of high levels of calcitriol observed in these patients since the VDR is a ligand-dependent receptor [12].