Tuberculosis, a major health problem in developing countries, has reemerged in recent years in many industrialized countries. The increased susceptibility of immunocompromised individuals to tuberculosis, and many experimental studies indicate that T cell-mediated immunity plays an important role in resistance. The lymphokine interferon gamma (IFN-gamma) is thought to be a principal mediator of macrophage activation and resistance to intracellular pathogens. Mice have been developed which fail to produce IFN-gamma (gko), because of a targeted disruption of the gene for IFN-gamma. Upon infection with Mycobacterium tuberculosis, although they develop granulomas, gko mice fail to produce reactive nitrogen intermediates and are unable to restrict the growth of the bacilli. In contrast to control mice, gko mice exhibit heightened tissue necrosis and succumb to a rapid and fatal course of tuberculosis that could be delayed, but not prevented, by treatment with exogenous recombinant IFN-gamma.

Understanding the immunological mechanisms of protection and pathogenesis in tuberculosis remains problematic. We have examined the extent to which tumor necrosis factor-alpha (TNF alpha) contributes to this disease using murine models in which the action of TNF alpha is inhibited. TNF alpha was neutralized in vivo by monoclonal antibody; in addition, a mouse strain with a disruption in the gene for the 55 kDa TNF receptor was used. The data from both models established that TNF alpha and the 55 kDa TNF receptor are essential for protection against tuberculosis in mice, and for reactive nitrogen production by macrophages early in infection. Granulomas were formed in equal numbers in control and experimental mice, but necrosis was observed only in mice deficient in TNF alpha or TNF receptor. TNF alpha and the 55 kDa TNF receptor are necessary conditions for protection against murine M. tuberculosis infection, but are not solely responsible for the tissue damage observed.

Immunological tests provide evidence of latent tuberculosis in one third of the global population, more than two billion individuals. Latent tuberculosis is defined by the absence of clinical symptoms but carries a risk of subsequent progression to clinical disease, particularly in the context of co-infection with HIV. Here we discuss the biology of latent tuberculosis as part of a broad spectrum of responses that occur following infection with Mycobacterium tuberculosis, resulting in formation of a range of physiologically distinct granulomatous lesions that provide environments with differential ability to support or suppress persistence of viable bacteria. We go on to show how this model can be used to inform a rational programme to discover drugs that will be effective in the eradication of M. tuberculosis infection.

Macrophages in granulomas are both anti-mycobacterial effector and host cell for Mycobacterium tuberculosis(M.tb), yet basic aspects of macrophage diversity and function within the complex structures of granulomas remain poorly understood. To address this, we examined myeloid cell phenotypes and expression of enzymes correlated with host defense in macaque and human granulomas. Macaque granulomas had upregulated inducible and endothelial nitric oxide synthase (iNOS and eNOS) and arginase (Arg1 and Arg2) expression and enzyme activity compared to non-granulomatous tissue. Immunohistochemical analysis indicated macrophages adjacent to uninvolved normal tissue were more likely to express CD163, while epithelioid macrophages in regions where bacteria reside strongly expressed CD11c, CD68 and HAM56. Calprotectin-positive neutrophils were abundant in regions adjacent to caseum. iNOS, eNOS, Arg1 and Arg2 proteins were identified in macrophages and localized similarly in granulomas across species, with greater eNOS expression and ratio of iNOS:Arg1 expression in epithelioid macrophages, as compared to cells in the lymphocyte cuff. iNOS, Arg1 and Arg2 expression in neutrophils was also identified. The combination of phenotypic and functional markers support that macrophages with anti-inflammatory phenotypes localized to outer regions of granulomas while the inner regions were more likely to contain macrophages with pro-inflammatory, presumably bactericidal, phenotypes. Together these data support the concept that granulomas have organized microenvironments that balance anti-microbial anti-inflammatory responses to limit pathology in the lungs.

Lung granulomas are the pathologic hallmark of tuberculosis (TB). T cells are a major cellular component of TB lung granulomas and are known to play an important role in containment of Mycobacterium tuberculosis (Mtb) infection. We used cynomolgus macaques, a non-human primate model that recapitulates human TB with clinically active disease, latent infection or early infection, to understand functional characteristics and dynamics of T cells in individual granulomas. We sought to correlate T cell cytokine response and bacterial burden of each granuloma, as well as granuloma and systemic responses in individual animals. Our results support that each granuloma within an individual host is independent with respect to total cell numbers, proportion of T cells, pattern of cytokine response, and bacterial burden. The spectrum of these components overlaps greatly amongst animals with different clinical status, indicating that a diversity of granulomas exists within an individual host. On average only about 8% of T cells from granulomas respond with cytokine production after stimulation with Mtb specific antigens, and few “multi-functional” T cells were observed. However, granulomas were found to be “multi-functional” with respect to the combinations of functional T cells that were identified among lesions from individual animals. Although the responses generally overlapped, sterile granulomas had modestly higher frequencies of T cells making IL-17, TNF and any of T-1 (IFN-γ, IL-2, or TNF) and/or T-17 (IL-17) cytokines than non-sterile granulomas. An inverse correlation was observed between bacterial burden with TNF and T-1/T-17 responses in individual granulomas, and a combinatorial analysis of pair-wise cytokine responses indicated that granulomas with T cells producing both pro- and anti-inflammatory cytokines (e.g. IL-10 and IL-17) were associated with clearance of Mtb. Preliminary evaluation suggests that systemic responses in the blood do not accurately reflect local T cell responses within granulomas.

SummaryThe granulomatous reaction is the hallmark of the host response to infection with Mycobacterium tuberculosis . Despite its apparent importance to host defence against the tubercle bacillus, the granulomatous response remains to be completely defined. The present study used histological, immunohistochemical and flow-cytometric analyses to characterize pulmonic granulomatous tissues of tuberculous mice and humans. The kinetics of recruitment of neutrophils, macrophages, dendritic cells, and T and B lymphocytes into the lungs of mice infected aerogenically with the virulent Erdman strain of M. tuberculosis was evaluated in detail in both the acute and persistent phase of infection. A hypoxia-sensing compound based on the 2-nitroimidazole structure (EF5), together with immunohistochemical studies targeting endothelial cells were used to examine the relative oxygen tension in tuberculous granulomatous tissues in mice. The results have provided evidence that: (i) the granulomatous tissues are a highly organized structure whose formation is regulated by orderly recruitment of specific immune cells exhibiting distinct spatial relationship with one another; (ii) the granulomatous reaction, at least in the mouse, may represent an exaggerated response to the tubercle bacillus that can play a role in the development of immunopathology; (iii) B lymphoid aggregates are a prominent feature in both murine and human granulomatous tissues, although the immune cells that are most prominently associated with these clusters vary among the two species; (iv) murine tuberculous granulomatous tissues are relatively aerobic, suggesting that mouse models of persistent tuberculosis may not be suitable for the study of any hypoxic response of M. tuberculosis .

R us se ll -L od ri gue, C hr is to ph er Monjure,

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