IFN-γ- and TNF-Independent Vitamin D-Inducible Human Suppression of Mycobacteria: The Role of Cathelicidin LL-37

Abstract

Vitamin D deficiency is associated with susceptibility to tuberculosis, and its biologically active metabolite, 1α,25 dihydroxyvitamin D₃ (1α,25(OH)₂D₃), has pleiotropic immune effects. The mechanisms by which 1α,25(OH)₂D₃ protects against tuberculosis are incompletely understood. 1α,25(OH)₂D₃ reduced the growth of mycobacteria in infected human PBMC cultures in a dose-dependent fashion. Coculture with agonists or antagonists of the membrane or nuclear vitamin D receptors indicated that these effects were primarily mediated by the nuclear vitamin D receptors. 1α,25(OH)₂D₃ reduced transcription and secretion of protective IFN-γ, IL-12p40, and TNF in infected PBMC and macrophages, indicating that 1α,25(OH)₂D₃ does not mediate protection via these cytokines. Although NOS2A was up-regulated by 1α,25(OH)₂D₃, inhibition of NO formation marginally affected the suppressive effect of 1α,25(OH)₂D₃ on bacillus Calmette Guérin in infected cells. By contrast, 1α,25(OH)₂D₃ strongly up-regulated the cathelicidin hCAP-18 gene, and some hCAP-18 polypeptide colocalized with CD14 in 1α,25(OH)₂D₃ stimulated PBMC, although no detectable LL-37 peptide was found in supernatants from similar 1α,25(OH)₂D₃-stimulated PBMC cultures. A total of 200 μg/ml of the active peptide LL-37, in turn, reduced the growth of Mycobacterium tuberculosis in culture by 75.7%. These findings suggest that vitamin D contributes to protection against TB by “nonclassical” mechanisms that include the induction of antimicrobial peptides.