Vitamin D hormone (1,25-dihydroxyvitamin D) is involved in innate immunity and induces host defense peptides in epithelial cells, suggesting its involvement in mucosal defense against infections. Chlamydia trachomatis is a major cause of bacterial sexually transmitted disease worldwide. We tested the hypothesis that the vitamin D endocrine system would attenuate chlamydial infection. Vitamin D receptor knock-out mice (VDR(-/-)) and wild-type mice (VDR(+/+)) were infected with 10(3) inclusion forming units of Chlamydia muridarum and cervical epithelial cells (HeLa cells) were infected with C. muridarum at multiplicity of infection 5:1 in the presence and absence of 1,25-dihydroxyvitamin D3. VDR(-/-) mice exhibited significantly higher bacterial loading than wild-type VDR(+/+) mice (P<0.01) and cleared the chlamydial infection in 39 days, compared with 18 days for VDR(+/+) mice. Monocytes and neutrophils were more numerous in the uterus and oviduct of VDR(-/-) mice than in VDR(+/+) mice (P<0.05) at d 45 after infection. Pre-treatment of HeLa cells with 10nM or 100nM 1,25-dihydroxyvitamin D3 decreased the infectivity of C. muridarum (P<0.001). Several differentially expressed protein spots were detected by proteomic analysis of chlamydial-infected HeLa cells pre-treated with 1,25-dihydroxyvitamin D3. Leukocyte elastase inhibitor (LEI), an anti-inflammatory protein, was up-regulated. Expression of LEI in the ovary and oviduct of infected VDR(+/+) mice was greater than that of infected VDR(-/-) mice. We conclude that the vitamin D endocrine system reduces the risk for prolonged chlamydial infections through regulation of several proteins and that LEI is involved in its anti-inflammatory activity.