The progression of human tuberculosis (TB) to active disease and transmission involves the development of a caseous granuloma that cavitates and releases infectious Mycobacterium tuberculosis bacilli. In the current study, we exploited genome-wide microarray analysis to determine that genes for lipid sequestration and metabolism were highly expressed in caseous TB granulomas. Immunohistological analysis of these granulomas confirmed the disproportionate abundance of the proteins involved in lipid metabolism in cells surrounding the caseum; namely, adipophilin, acyl-CoA synthetase long-chain family member 1 and saposin C. Biochemical analysis of the lipid species within the caseum identified cholesterol, cholesteryl esters, triacylglycerols and lactosylceramide, which implicated low-density lipoprotein-derived lipids as the most likely source. M. tuberculosis infection in vitro induced lipid droplet formation in murine and human macrophages. Furthermore, the M. tuberculosis cell wall lipid, trehalose dimycolate, induced a strong granulomatous response in mice, which was accompanied by foam cell formation. These results provide molecular and biochemical evidence that the development of the human TB granuloma to caseation correlates with pathogen-mediated dysregulation of host lipid metabolism.

Protective immunity against pulmonary tuberculosis (TB) is characterized by the formation in the lungs of granulomas consisting of macrophages and activated T cells producing tumor necrosis factor alpha and gamma interferon, both required for the activation of the phagocytes. In 90% of immunocompetent humans, this response controls the infection. To understand why immunity fails in the other 10%, we studied the lungs of six patients who underwent surgery for incurable TB. Histologic examination of different lung lesions revealed heterogeneous morphology and distribution of acid-fast bacilli; only at the surface of cavities, i.e., in granulomas with a patent connection to the airways, were there numerous bacilli. The mutation profile of the isolates suggested that a single founder strain of Mycobacterium tuberculosis may undergo genetic changes during treatment, leading to acquisition of additional drug resistance independently in discrete physical locales. Additional drug resistance was preferentially observed at the cavity surface. Cytokine gene expression revealed that failure to control the bacilli was not associated with a generalized suppression of cellular immunity, since cytokine mRNA was up regulated in all lesions tested. Rather, a selective absence of CD4 ؉ and CD8 ؉ T cells was noted at the luminal surface of the cavity, preventing direct T-cell-macrophage interactions at this site, probably allowing luminal phagocytes to remain permissive for bacillary growth. In contrast, in the perinecrotic zone of the granulomas, the two cell types colocalized and bacillary numbers were substantially lower, suggesting that in this microenvironment an efficient bacteriostatic or bactericidal phagocyte population was generated.

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Pathogenetic processes that facilitate the entry, replication, and persistence of Mycobacterium tuberculosis (MTB) in the mammalian host likely include the regulated expression of specific sets of genes at different stages of infection. Identification of genes that are differentially expressed in vivo would provide insights into host-pathogen interactions in tuberculosis (TB); this approach might be particularly valuable for the study of human TB, where experimental opportunities are limited. In this study, the levels of selected MTB mRNAs were quantified in vitro in axenic culture, in vivo in the lungs of mice, and in lung specimens obtained from TB patients with active disease. We report the differential expression of MTB mRNAs associated with iron limitation, alternative carbon metabolism, and cellular hypoxia, conditions that are thought to exist within the granulomatous lesions of TB, in the lungs of wild-type C57BL͞6 mice as compared with bacteria grown in vitro. Analysis of the same set of mRNAs in lung specimens obtained from TB patients revealed differences in MTB gene expression in humans as compared with mice.T he clinical course of tuberculosis (TB) comprises a sequence of different stages (1). Airborne bacilli that implant in the lung alveoli are phagocytosed by alveolar macrophages (2). Intracellular replication of Mycobacterium tuberculosis (MTB) results in a primary lesion, followed by lymphohematogenous dissemination and secondary lesions in the lungs and other organs. In most individuals, disease progression is arrested at this stage by the acquired immune response, and a clinically latent state ensues. Postprimary disease arises from the subsequent reactivation of dormant tuberculous foci (3, 4). For the nearly 2 billion individuals worldwide who have been infected with MTB, the lifetime risk of developing TB is Ϸ10% (5). In the absence of effective treatment, TB case fatality rates can exceed 50%, as evidenced by longitudinal clinical studies in the preantimicrobial era (6).Despite the global importance of TB as a leading cause of morbidity and mortality, little is known about the host-pathogen interactions at each stage of infection in humans. This information has been difficult to obtain because medical ethics demand that anti-TB therapy be initiated immediately on diagnosis. The uninterrupted course of disease can be studied in TB patients who are unresponsive to therapy, typically due to multidrugresistant MTB. In such cases, palliative resection of diseased tissue is sometimes undertaken (7). Resected human tissues, which would otherwise be discarded, can be used to analyze host-pathogen interactions in situ (8, 9).Adaptation of MTB to environmental changes in the course of infection is likely mediated by differential gene expression. An important role for transcriptional regulation in the life cycle of MTB is suggested by genes encoding 13 RNA polymerase -factors and Ͼ100 putative regulatory proteins in the genome (10). Although the cellular processes controlled by these transcription f...