Vitamin D Induces Innate Antibacterial Responses in Human Trophoblasts via an Intracrine Pathway1

Abstract

The active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)2D), is a potent inducer of the antimicrobial protein cathelicidin, CAMP (LL37). In macrophages this response is dependent on intracrine synthesis of 1,25(OH)2D from precursor 25-hydroxyvitamin D (25OHD), catalyzed by the enzyme 25-hydroxyvitamin D-1alpha-hydroxylase (CYP27B1). In view of the fact that trophoblastic cells also express abundant CYP27B1, we postulated a similar intracrine pathway for induction of CAMP in the placenta. Analysis of placenta explants, primary cultures of human trophoblast, and the 3A trophoblastic cell line treated with 1,25(OH)2D (1–100 nM) revealed dose-dependent induction of CAMP similar to that observed with primary cultures of human macrophages. Also consistent with macrophages, induction of trophoblastic CAMP was enhanced via intracrine conversion of 25OHD to 1,25(OH)2D. However, in contrast to macrophages, induction of CAMP by vitamin D in trophoblasts was not enhanced by costimulation with Toll-like receptor ligands, such as lipopolysaccharide. Despite this, exposure to vitamin D metabolites significantly enhanced antibacterial responses in trophoblastic cells: 3A cells infected with Escherichia coli showed decreased numbers of bacterial colony-forming units compared with vehicle-treated controls when treated with 25OHD (49.6% ± 10.9%) or 1,25(OH)2D (45.4% ± 9.2%), both P < 0.001. Treatment with 25OHD (1–100 nM) or 1,25(OH)2D (0.1–10 nM) also protected 3A cells against cell death following infection with E. coli (13.6%–26.9% and 22.3%–40.2% protection, respectively). These observations indicate that 1,25(OH)2D can function as an intracrine regulator of CAMP in trophoblasts, and may thus provide a novel mechanism for activation of innate immune responses in the placenta.

Vitamin D and the Regulation of Placental Inflammation

“…Coexpression of the receptor for 1,25(OH) 2 D 3 (vitamin D receptor [VDR]) in both decidua (9) and trophoblast (10) suggests that 1,25 (OH) 2 D 3 produced by the placenta may function in an autocrine or paracrine fashion. In support of this, recent studies by our group and others have shown that autocrine metabolism of 25OHD 3 to 1,25(OH) 2 D 3 promotes antibacterial and anti-inflammatory responses in maternal decidua (11) and fetal trophoblast (12,13). Such immunomodulatory actions of vitamin D are likely to be compromised under conditions of low maternal 25OHD 3 status, with potentially detrimental consequences for placental physiology.…”

“…However, it is important to recognize that vitamin D is also able to influence other aspects of immunity, notably the stimulation of antibacterial innate immune responses (18,22). In previous studies using human tissue, we have shown that induction of the antimicrobial protein cathelicidin by vitamin D is not restricted to the immune system but can also be observed with cells from both the maternal and fetal sides of the placenta (11,12). In future studies, it will be important to determine whether similar antibacterial mechanisms are also influenced by vitamin D in the placenta.…”

Vitamin D and Human Pregnancy

“…However, it is also important to recognize that novel mechanisms involving non-classical actions of vitamin D may further expand the role of vitamin D during pregnancy. In recent studies we have shown that vitamin D is a potent stimulator of innate antimicrobial responses to infection in human placental cells, whilst simultaneously inhibiting inflammation 155,156 . Given the link between infection, inflammation and risk of preterm birth 164 , it is tempting to speculate that vitamin D may have even broader beneficial effects in the general maintenance of pregnancy.…”

“…It therefore seems likely that the high levels of local 1,25(OH) 2 D production within the placenta will impact on these antimicrobials. Studies in-vitro using human placenta tissue have shown that 25OHD and 1,25(OH) 2 D induce expression of cathelicidin in both maternal decidua 155 and fetal trophoblast 156 . In the latter case, vitamin D-induced cathelicidin expression promoted intracellular killing of Escherichia coli (E. coli), underlining the potential importance of vitamin D as a key mediator of placental response to infection 156 .…”

“…Studies in-vitro using human placenta tissue have shown that 25OHD and 1,25(OH) 2 D induce expression of cathelicidin in both maternal decidua 155 and fetal trophoblast 156 . In the latter case, vitamin D-induced cathelicidin expression promoted intracellular killing of Escherichia coli (E. coli), underlining the potential importance of vitamin D as a key mediator of placental response to infection 156 . It is interesting to note studies showing that maternal vitamin Dinsufficiency is associated with increased rates of bacterial vaginosis in the first trimester of pregnancy.…”