The relative importance of T cell subsets in immunity and immunopathology of airborne Mycobacterium tuberculosis infection in mice

Induction of TNF in human alveolar macrophages as a potential evasion mechanism of virulent Mycobacterium tuberculosis

The ability of macrophages to release cytokines is crucial to the host response to intracellular infection. In particular, macrophage-derived TNF plays an important role in the host response to infection with the intracellular pathogen Mycobacterium tuberculosis. In mice, TNF is indispensable for the formation of tuberculous granulomas, which serve to demarcate the virulent bacterium. TNF is also implicated in many of the immunopathological features of tuberculosis. To investigate the role of TNF in the local immune response, we infected human alveolar macrophages with virulent and attenuated mycobacteria. Infection with virulent strains induced the secretion of significantly higher levels of bioactive TNF than attenuated strains correlating with their ability to multiply intracellularly. Treatment of infected macrophages with neutralizing anti-TNF Abs reduced the growth rate of intracellular bacteria, whereas bacterial replication was augmented by addition of exogenous TNF. Infected and uninfected macrophages contributed to cytokine production as determined by double-staining of M. tuberculosis and intracellular TNF. The induction of TNF by human alveolar macrophages at the site of infection permits the multiplication of intracellular bacteria and may therefore present an evasion mechanism of human pathogens.

IRF and tuberculosis

Tuberculosis is the most prevalent infectious disease and causes more deaths than any other, yet only 5%-10% of people infected by the causative agent, Mycobacterium tuberculosis, will develop the disease. Thus, natural resistance among humans is the norm. Fundamental immune responses to M. tuberculosis are being elucidated, including induction of interferon regulatory factor-1 (IRF-1). Moreover, IRF-1 has been found necessary for normal resistance to infection by mycobacteria in mice. Roles for IRF-1 in a plethora of immune system functions have been described. This review considers molecular responses to infection by M. tuberculosis that might account for induction of IRF-1 and highlights putative connections between immunomodulatory functions of IRF-1 and immune responses relevant to infection by M. tuberculosis. However, the complexity inherent in pleiotropy and redundancy limits the ability to draw firm conclusions. In many cases, it remains to be demonstrated that a particular function of IRF-1 is the basis for a known response to infection. For example, although IRF-1 is required for a Th1 cell-mediated, adaptive immune response in some circumstances, it is not known if the Th1 response to infection by M. tuberculosis requires IRF-1. Conversely, some known contributions by IRF-1 to fundamental aspects of the immune system are not yet proven relevant in the host response to infection. For example, it is not known if control of T cell subset development by IRF-1 is significant for host defense against M. tuberculosis. Functions of other IRF that overlap with or are distinct from the functions of IRF-1 also could be important for the immune response to M. tuberculosis.

Alphabeta T cell receptor-positive cells and interferon-gamma, but not inducible nitric oxide synthase, are critical for granuloma necrosis in a mouse model of mycobacteria-induced pulmonary immunopathology

The immunological basis of tuberculin-induced necrosis, known for more than a century as "Koch's phenomenon," remains poorly understood. Aerosol infection in mice with the highly virulent Mycobacterium avium strain TMC724 causes progressive pulmonary pathology strongly resembling caseating necrosis in human patients with tuberculosis. To identify the cellular and molecular mediators causing this pathology, we infected C57BL/6 mice and mice selectively deficient in recombinase activating gene (RAG)-1, alphabeta T cell receptor (TCR), gammadelta TCR, CD4, CD8, beta2-microglobulin, interferon (IFN)-gamma, interleukin (IL)-10, IL-12p35, IL-12p35/p40, or iNOS with M. avium by aerosol and compared bacterial multiplication, histopathology, and respiratory physiology in these mice. The bacterial load in the lung was similarly high in all mouse groups. Pulmonary compliance, as a surrogate marker for granulomatous infiltrations in the lung, deteriorated to a similar extent in all groups of mice, except in alphabeta TCR-knockout (KO) and IL-12-KO mice in which compliance was higher, and in IFN-gamma and inducible nitric oxide synthase-KO mice in which compliance was reduced faster. Progressive caseation of pulmonary granulomas never occurred in alphabeta TCR-KO, IL-12-KO, and IFN-gamma-KO mice and was reduced in CD4-KO mice. In summary, alphabeta TCR(+) cells and IFN-gamma are essential for the development of mycobacteria-induced pulmonary caseous necrosis. In contrast, high mycobacterial load and extensive granulomatous infiltration per se are not sufficient to cause caseation, nor is granuloma necrosis linked to the induction of nitric oxide.

CD4(+) T cells are required for the development of cytotoxic CD8(+) T cells during Mycobacterium tuberculosis infection

The control of acute and chronic Mycobacterium tuberculosis infection is dependent on CD4(+) T cells. In a variety of systems CD8(+) T cell effector responses are dependent on CD4(+) T cell help. The development of CD8(+) T cell-mediated immune responses in the absence of CD4(+) T cells was investigated in a murine model of acute tuberculosis. In vitro and in vivo, priming of mycobacteria-specific CD8(+) T cells was unaffected by the absence of CD4(+) T cells. Infiltration of CD8(+) T cells into infected lungs of CD4(-/-) or wild-type mice was similar. IFN-gamma production by lung CD8(+) T cells in CD4(-/-) and wild-type mice was also comparable, suggesting that emergence of IFN-gamma-producing mycobacteria-specific CD8(+) T cells in the lungs was independent of CD4(+) T cell help. In contrast, cytotoxic activity of CD8(+) T cells from lungs of M. tuberculosis-infected mice was impaired in CD4(-/-) mice. Expression of mRNA for IL-2 and IL-15, cytokines critical for the development of cytotoxic effector cells, was diminished in the lungs of M. tuberculosis-infected CD4(-/-) mice. As tuberculosis is frequently associated with HIV infection and a subsequent loss of CD4(+) T cells, understanding the interaction between CD4(+) and CD8(+) T cell subsets during the immune response to M. tuberculosis is imperative for the design of successful vaccination strategies.

The inducible nitric oxide synthase locus confers protection against aerogenic challenge of both clinical and laboratory strains of Mycobacterium tuberculosis in mice

Murine macrophages effect potent antimycobacterial function via the production of nitric oxide by the inducible isoform of the enzyme nitric oxide synthase (NOS2). The protective role of reactive nitrogen intermediates (RNI) against Mycobacterium tuberculosis infection has been well established in various murine experimental tuberculosis models using laboratory strains of the tubercle bacillus to establish infection by the intravenous route. However, important questions remain about the in vivo importance of RNI in host defense against M. tuberculosis. There is some evidence that RNI play a lesser role following aerogenic, rather than intravenous, M. tuberculosis infection of mice. Furthermore, in vitro studies have demonstrated that different strains of M. tuberculosis, including clinical isolates, vary widely in their susceptibility to the antimycobacterial effects of RNI. Thus, we sought to test rigorously the protective role of RNI against infection with recent clinical isolates of M. tuberculosis following both aerogenic and intravenous challenges. Three recently isolated and unique M. tuberculosis strains were used to infect both wild-type (wt) C57BL/6 and NOS2 gene-disrupted mice. Regardless of the route of infection, NOS2(-/-) mice were much more susceptible than wt mice to any of the clinical isolates or to either the Erdman or H37Rv laboratory strain of M. tuberculosis. Mycobacteria replicated to much higher levels in the organs of NOS2(-/-) mice than in those of wt mice. Although the clinical isolates all exhibited enhanced virulence in NOS2(-/-) mice, they displayed distinct growth rates in vivo. The present study has provided results indicating that RNI are required for the control of murine tuberculous infection caused by both laboratory and clinical strains of M. tuberculosis. This protective role of RNI is essential for the control of infection established by either intravenous or aerogenic challenge.

Immune responses to intracellular bacteria

The multifaceted dialogue between intracellular bacteria and the mammalian host continues to be an exciting issue from both the scientific and public-health viewpoint. The recent year has witnessed some particularly impressive progress in knowledge about the two major culprits affecting the health of mankind, Mycobacterium tuberculosis and Salmonella typhi - the causative agents of tuberculosis and typhoid fever.

Overexpression of IL-15 in vivo enhances protection against Mycobacterium bovis bacillus Calmette-Guérin infection via augmentation of NK and T cytotoxic 1 responses

To investigate the immunomodulating effects of IL-15 in vivo on mycobacterial infection, we used IL-15-transgenic (Tg) mice, which were recently constructed with cDNA-encoding secretable isoform of IL-15 precursor protein under the control of a MHC class I promoter. The IL-15-Tg mice exhibited resistance against infection with Mycobacterium bovis bacillus Calmette-Guérin (BCG), as assessed by bacteria growth. IFN-gamma level in serum was significantly higher in IL-15-Tg mice than in non-Tg mice after BCG infection. NK cells were remarkably increased, and Ag-specific T cytotoxic 1 response mediated by CD8+ T cells producing IFN-gamma was significantly augmented in the IL-15-Tg mice following BCG infection. Neutralization of endogenous IFN-gamma by in vivo administration of anti-IFN-gamma mAb deteriorated the clearance of the bacteria. Depletion of of NK cells or CD8+ T cells by in vivo administration of anti-asialo-GM(1) Ab or anti-CD8 mAb hampered the exclusion of bacteria. Thus, overexpression of IL-15 in vivo enhanced protection against BCG infection via augmentation of NK and T cytotoxic 1 responses.