The study shows that people with Crohn's disease who have a gut infection (MAP) have lower levels of the immune peptide LL‑37 and the active form of vitamin D (calcitriol). Giving calcitriol (or its precursor calcifediol) boosts LL‑37 production, cuts down the bacteria inside immune cells, and reduces inflammation. This suggests that raising active vitamin D could help the body’s natural defenses, but direct calcitriol use needs medical oversight.

Vitamin D is a key regulator in calcium and phosphorus metabolism which are essential for maintaining bone health. Recent reports also showed a role for vitamin D in immune regulation which may be linked to vitamin D deficiency in autoimmune disorders including inflammatory diseases and Crohn's disease (CD). This study examines the role of vitamin D deficiency in the regulation of Cathelicidin Antimicrobial Peptide (<i>CAMP</i>) in CD-like macrophages. The latter includes macrophages infected with <i>Mycobacterium avium</i> subsp. <i>paratuberculosis</i> (MAP) isolated from CD patient. Initially, we measured cathelicidin and calcitriol in <i>ex vivo</i> plasma samples from CD patients with or without MAP infection (<i>N=40</i> per group). We also measured the expression and production of <i>CAMP</i>/LL-37, TNF-&#x3b1;, IL-1&#x3b2;, IL-10, cellular oxidative stress markers, and bacterial viability following treatment of MAP-infected macrophages with four different forms of vitamin D (D2, D3, calcifediol, and calcitriol). From these studies, we determined that LL-37 and calcitriol were significantly lower in CD samples from MAP-positive patients [155.55 &#xb1; 49.77 ng/mL and 51.48 &#xb1; 31.04 pg/mL, respectively] compared to MAP-negative patients [193.01 &#xb1; 78.95 ng/mL and 272.36 &#xb1; 94.77 pg/mL, respectively]. Moreover, calcitriol and calcifediol upregulated <i>CAMP</i> expression by nearly 5-fold and 3-fold, respectively. However, following MAP infection, only calcitriol increased <i>CAMP</i> by 3-folds. Both calcitriol and LL-37 reduced intracellular MAP viability by ~3 folds and inhibited TNF-&#x3b1; and IL-1&#x3b2; expression and production in these cells. Treating co-culture of Caco-2 monolayers and MAP-infected macrophages with LL-37 or calcitriol have shown a reduction in <i>NOX-1</i> expression and DHE signal, in addition to a higher NADPH/NADPt ratio. Notably, calcitriol's anti-inflammatory effects were lost upon <i>CAMP</i> knockdown by CAMP-siRNA transfection. Altogether, the data indicate that MAP infection and burden is significant in CD by disrupting the conversion of calcifediol to calcitriol and downregulation of <i>CAMP</i> expression leading to vitamin D deficiency.