Vitamin D3 (the active form 1,25‑dihydroxyvitamin D3) makes immune cells produce the antimicrobial peptide LL‑37, and this peptide then turns on the cell's recycling system called autophagy, which helps destroy hidden bacteria like TB. The study shows the chain: vitamin D3 → LL‑37 → autophagy genes → better bacterial killing, all in lab‑grown human immune cells.

Autophagy and vitamin D3-mediated innate immunity have been shown to confer protection against infection with intracellular Mycobacterium tuberculosis. Here, we show that these two antimycobacterial defenses are physiologically linked via a regulatory function of human cathelicidin (hCAP-18/LL-37), a member of the cathelicidin family of antimicrobial proteins. We show that 1,25-dihydroxyvitamin D3 (1,25D3), the active form of vitamin D, induced autophagy in human monocytes via cathelicidin, which activated transcription of the autophagy-related genes Beclin-1 and Atg5. 1,25D3 also induced the colocalization of mycobacterial phagosomes with autophagosomes in human macrophages in a cathelicidin-dependent manner. Furthermore, the antimycobacterial activity in human macrophages mediated by physiological levels of 1,25D3 required autophagy and cathelicidin. These results indicate that human cathelicidin, a protein that has direct antimicrobial activity, also serves as a mediator of vitamin D3-induced autophagy.