(48) and Zairians living in Belgium (35) compared to control individuals with less pigmented skin, presumably due to diminished synthesis. Tuberculosis, likewise, is common among individuals with heavily pigmented skin that relocate from equatorial regions to higher latitudes, in part due to deficiencies in vitamin D synthesis within the skin (65). In addition, patients with untreated tuberculosis often have lower concentrations of 25(OH)D 3 in plasma than do healthy subjects, and tuberculosis tends to occur during the winter when exposure to sunlight is reduced and production of cholecalciferol within the skin is diminished (16). Evidence for a clear correlation between vitamin D deficiency and susceptibility to tuberculosis, however, remains controversial. In vitro studies, however, provide more compelling evidence linking vitamin D status to susceptibility to tuberculosis.Addition of 1,25(OH) 2 D 3 to monocyte-macrophage cultures infected with Mycobacterium tuberculosis suppresses bacterial growth and viability (17,53,54). The mechanism of this suppression is mediated, at least partially, by nitric oxide (NO) (53). Induction of inducible NO synthase of macrophages and subsequent generation of reactive nitrogen intermediates (RNI) toxic to mycobacteria is a potent mechanism of killing (14,15,21,22,34). Cytokines (e.g., tumor necrosis factor alpha and gamma interferon [IFN-␥]) from antigen-specific T cells and/or from macrophages stimulated directly with mycobacterial antigens is responsible for RNI-mediated antimycobacterial defense (24,60). Production of RNI is crucial for controlling acute as well as latent infections in the mouse model of virulent M. tuberculosis infection (14,15,25,34). The role of RNI in mycobacterial killing within human macrophages is less clear. Alveolar macrophages of tuberculosis patients express high levels of inducible NO synthase, suggesting a role for RNI in disease pathogenesis and/or host defense (42). Nevertheless, recent evidence suggests that human but not mouse macro-* Corresponding author. Mailing address: United States Department
