Intrinsic activation of the vitamin D antimicrobial pathway by M. leprae infection is inhibited by type I IFN.

Following infection, virulent mycobacteria persist and grow within the macrophage, suggesting that the intrinsic activation of an innate antimicrobial response is subverted by the intracellular pathogen. For Mycobacterium leprae, the intracellular bacterium that causes leprosy, the addition of exogenous innate or adaptive immune ligands to the infected monocytes/macrophages was required to detect a vitamin D-dependent antimicrobial activity. We investigated whether there is an intrinsic immune response to M. leprae in macrophages that is inhibited by the pathogen. Upon infection of monocytes with M. leprae, there was no upregulation of CYP27B1 nor its enzymatic activity converting the inactive prohormone form of vitamin D (25-hydroxyvitamin D) to the bioactive form (1,25α-dihydroxyvitamin D). Given that M. leprae-induced type I interferon (IFN) inhibited monocyte activation, we blocked the type I IFN receptor (IFNAR), revealing the intrinsic capacity of monocytes to recognize M. leprae and upregulate CYP27B1. Consistent with these in vitro studies, an inverse relationship between expression of CYP27B1 vs. type I IFN downstream gene OAS1 was detected in leprosy patient lesions, leading us to study cytokine-derived macrophages (MΦ) to model cellular responses at the site of disease. Infection of IL-15-derived MΦ, similar to MΦ in lesions from the self-limited form of leprosy, with M. leprae did not inhibit induction of the vitamin D antimicrobial pathway. In contrast, infection of IL-10-derived MΦ, similar to MΦ in lesions from patients with the progressive form of leprosy, resulted in induction of type I IFN and suppression of the vitamin D directed pathway. Importantly, blockade of the type I IFN response in infected IL-10 MΦ decreased M. leprae viability. These results indicate that M. leprae evades the intrinsic capacity of human monocytes/MΦ to activate the vitamin D-mediated antimicrobial pathway via the induction of type I IFN.

MCF-7/VD(R): a new vitamin D resistant cell line.

Several in vitro and in vivo experiments have demonstrated potent cell regulatory effects of vitamin D compounds in cancer cells. Moreover, a promising phase I study with the vitamin D analogue Seocalcitol (EB 1089) in patients with advanced breast and colon cancer has already been carried out and more clinical trials evaluating the clinical effectiveness of EB 1089 in other cancer types are in progress (Mørk Hansen et al. [2000a]). However, only little is known about the mechanisms underlying the actions of vitamin D or about the possible development of drug resistance in the patients. Therefore, in an attempt to gain more insight into these aspects, we have developed the MCF-7/VD(R) cell line, a stable subclone of the human MCF-7 breast cancer cell line, which is resistant to the growth inhibitory and apoptosis inducing effects of 1alpha,25(OH)(2)D(3). Despite this characteristic, receptor studies on the VDR have clearly demonstrated that the MCF-7/VD(R) cells contain fully functional VDRs, although in a lower number than seen with the parental MCF-7 cells. The regulation of the 24-hydroxylase enzyme appeared to be intact in the MCF-7/VD(R) cells and no differences with regard to growth rate and morphological appearance between the MCF-7/VD(R) cells and the parental MCF-7 cells were observed. Interestingly, however, the sensitivity of the MCF-7/VD(R) cells to the pure anti-estrogen ICI 182,780 was found to be increased. The MCF-7/VD(R) cell line shows characteristics different from those of previously described vitamin D resistant breast cancer cell lines but also some similarities. Together such vitamin D resistant cell lines therefore serve as a useful tool for studying the exact mechanism of action of vitamin D and the development of vitamin D resistance.