Fig. 2.
The conversion of 25 OHD2 to 1,25-(OH)2 D3 can be done directly in some cells of the immune system such as macrophages. The macrophage, like kidney cells, contains the enzyme 1-alpha hydroxylase that is capable of transforming Vitamin D into its active form, calcitriol. Exposure of the macrophage to some pathogens induces the production of CP27B (step 1) (which allows 25 OHD to enter the mitochondria for transformation into its active form 1,25-(OH)2 D3), (step 2) as well as vitamin D receptor (VDR) (step 3), which by binding to 1,25-(OH)2 D3 increases the production of cathelicidin. (step 4). The antimicrobial activity of vitamin D appears to be primarily dependent on the induction of cathelicidins, which perform numerous functions that enhance both innate and adaptive immunity; help improve the digestion process within the phagolysosome through a non-oxygen dependent mechanism (step 5),46 that can promote pathogen clearance by inducing apoptosis of infected epithelial cells, and induce paracrine responses in monocytes and T and B lymphocytes, among others.45 Vitamin D also has direct effects on other cells, for example, epithelial cells, eosinophils, mast cells as well as T and B lymphocytes. (step 6)47