Species-specific regulation of innate immunity by vitamin D signaling

While many global mechanisms of innate immune responses to pathogen threat are conserved over a vast range of species, the details of those responses and their regulation appear to be highly species-specific. An array of studies over recent years has revealed that hormonal vitamin D is an important regulator of innate immunity. In humans, the hormone-bound VDR directly induces the transcription of genes encoding antimicrobial peptides (AMPs), pattern recognition receptors and key cytokines implicated in innate immune responses. We find that the vitamin D response elements (VDREs) in a number of these human genes are highly conserved in a range of primates, but not present in rodent genes. Consistent with this, VDR target genes encoding AMPs human beta-defensin 2 (HBD2) and cathelicidin (CAMP) and the pattern recognition receptor NOD2 are induced by 1,25(OH)₂D in human cells of epithelial or myeloid origin but not similarly regulated in mouse cells. In addition, while conditioned media from human epithelial cells treated with 1,25(OH)₂D produced antimicrobial activity against E. coli and the lung pathogen Pseudomonas aeruginosa, no such activity was detected in conditioned media from comparable 1,25(OH)₂D-treated mouse epithelial cells. Given that other work has provided evidence that 1,25(OH)₂D does control innate immune responses in mouse models of disease, we discuss the species-specific similarities and differences in 1,25(OH)₂D-regulated innate immunity.