Direct and Indirect Induction by 1,25-Dihydroxyvitamin D₃ of the NOD2/CARD15-Defensin β2 Innate Immune Pathway Defective in Crohn Disease*

Abstract

Vitamin D signaling through its nuclear vitamin D receptor has emerged as a key regulator of innate immunity in humans. Here we show that hormonal vitamin D, 1,25-dihydroxyvitamin D₃, robustly stimulates expression of pattern recognition receptor NOD2/CARD15/IBD1 gene and protein in primary human monocytic and epithelial cells. The vitamin D receptor signals through distal enhancers in the NOD2 gene, whose function was validated by chromatin immunoprecipitation and chromatin conformation capture assays. A key downstream signaling consequence of NOD2 activation by agonist muramyl dipeptide is stimulation of NF-κB transcription factor function, which induces expression of the gene encoding antimicrobial peptide defensin β2 (DEFB2/HBD2). Pretreatment with 1,25-dihydroxyvitamin D₃ synergistically induced NF-κB function and expression of genes encoding DEFB2/HBD2 and antimicrobial peptide cathelicidin in the presence of muramyl dipeptide. Importantly, this synergistic response was also seen in macrophages from a donor wild type for NOD2 but was absent in macrophages from patients with Crohn disease homozygous for non-functional NOD2 variants. These studies provide strong molecular links between vitamin D deficiency and the genetics of Crohn disease, a chronic incurable inflammatory bowel condition, as Crohn's pathogenesis is associated with attenuated NOD2 or DEFB2/HBD2 function.