Granuloma formation is required to contain bacillus growth and delay mortality in mice chronically infected with Mycobacterium tuberculosis

Genetic control of immune-mediated necrosis of Mycobacterium avium granulomas

Intravenous infection of C57BL/6 and C57BL/10 mice with low doses of a highly virulent strain of Mycobacterium avium (ATCC 25291) led to the development of granulomas that underwent necrosis. In contrast, neither BALB/c nor DBA/1 mice developed granuloma necrosis after such infection despite a similar course of mycobacterial proliferation. Studies with C57BL/10 mice congenic for the Hc locus revealed that an intact complement C5 gene is required for granuloma necrosis. On the other hand, genetic disruption of the interleukin-10 gene in BALB/c mice made this strain susceptible to granuloma necrosis.

The LFA-1 adhesion molecule is required for protective immunity during pulmonary Mycobacterium tuberculosis infection

Host immunity to Mycobacterium tuberculosis is mediated by T cells that recognize and activate infected macrophages to control intracellular bacterial replication. The early appearance of T cells in the lungs of infected mice correlates with greater resistance to infection. However, it is unknown whether the trafficking of T cells to the lung following infection is dependent upon the expression of certain adhesion molecules. To address this question, we infected knockout (KO) mice that have defective expression of CD11a, CD11b, CD18, CD62, CD103, or beta7. We found that the integrins CD11a and CD18 are absolutely required for host resistance following infection with aerosolized M. tuberculosis. Although Ag-specific T cells are generated following infection of CD11a KO mice, T cell priming is delayed, T cell trafficking to the lung is impaired, and fewer ESAT6-specific CD4+ T cells are found in the lungs of CD11a KO mice compared with control mice. Thus, LFA-1 (CD11a/CD18) plays an essential role in immunity to M. tuberculosis infection.

New insights into the function of granulomas in human tuberculosis

The human tuberculous granuloma provides the morphological framework for local immune processes central to the outcome of tuberculosis. This review article describes investigations on human lung granulomas aimed at better understanding the regional host response and counter-measures to Mycobacterium tuberculosis. These findings lead to a revised view of the regional immune response in human tuberculosis. Novel insights into this dynamic cross-talk form the basis of novel intervention strategies.

Granuloma formation in vitro requires B-1 cells and is modulated by Paracoccidioides brasiliensis gp43 antigen

The mechanisms that determine granuloma formation and the significance of this type of inflammatory response in the pathogenesis of fungal diseases such as paracoccidioidomycosis are far from fully understood. We developed a granuloma model in vitro using beads to evaluate the role of isolated mouse peritoneal macrophages and B-1 cells. We also investigated granuloma formation in the presence of gp43, the main antigenic component of Paracoccidioides brasiliensis, which is secreted exocellularly. To determine whether B-1 cells, macrophages, or both, participate in granuloma formation, peritoneal cells from Xid mice, which lack B-1 cells, were used. Granuloma-like structures were not formed with Xid peritoneal cells or with cells from wild type mice that had their peritoneal and pleural cavities irradiated before the cultures were established. Granulomas were observed either when total adherent peritoneal cells or when isolated B-1 cells were added to macrophage cultures. The data strongly suggest that an interaction of B-1 cells and macrophages plays an important role in granuloma-like formation in this experimental model and that the presence of gp43 strongly stimulates this response.

Recombinant interleukin-4-treated macrophages, epithelioid cell surrogates, harbor and arrest Mycobacterium avium multiplication in vitro

Our group has previously described that murine peritoneal macrophages treated in vitro for 7 days with recombinant interleukin-4 (rIL-4) acquire morphological and functional characteristics of epithelioid cells (ECs) found in granulomatous lesions. Although EC function has not been clarified so far, it has been suggested that these cells could present antigens and control multiplication of mycobacteria. These aspects have been addressed here using in vitro EC surrogates. Using immunocytochemistry and immunofluorescence methods, we have observed an increased expression of CD11b, CD54, CD86 and CD40 molecules on rIL-4-treated macrophages when compared to untreated ones. Cytokine-treated cells were less phagocytic for latex beads (P<0.03) and more pinocytic for dextran particles than untreated macrophages. T-cell lymphoproliferation assays using ovalbumin (OVA) and Mycobacterium avium as antigens showed that both cultured macrophages were equally efficient as antigen presenting cells (APCs). However, M. avium antigens were better presented in vivo by EC surrogates (P<0.01). Both macrophage cultures were similarly infected by M. avium. However, while the infection level was maintained in the cytokine-treated population, untreated macrophages showed a progressive increase in the number of bacilli/cell with time (P<0.01) and a reduction of about 65% in cell population. After 96 h of M. avium infection, untreated cells secreted higher amounts of tumor necrosis factor-alpha (P<0.005) while rIL-4-treated macrophages showed higher, although not significant, transforming growth factor-beta production. Also, EC surrogates produced less nitric oxide than control macrophages (P<0.05). Hence, EC surrogates restrain M. avium growth and act as APCs in vitro and in vivo.

Expression of matrix metalloproteinase-9 in pleural effusions of tuberculosis and lung cancer

BACKGROUND

Matrix metalloproteinase (MMP)-9 has been implicated in the development of pleural effusions.

OBJECTIVES

The aim of this study was to assess the expression of MMP-9 in pleural effusions of tuberculosis, lung cancer and transudates.

METHODS

Ninety-one patients (37 tuberculous pleural effusions, 42 malignant pleural effusions of lung cancer and 12 transudates) were included. Concentrations of pleural fluid MMP-9 and tissue inhibitors of matrix metalloproteinase (TIMP)-1 were determined by immunoassay. We also investigated the cellular localization of MMP-9 and TIMP-1 by reverse-transcriptase polymerase chain reaction on lymphocytes from pleural effusions and by immunohistochemical analysis of pleural tissues.

RESULTS

Pleural fluid MMP-9 levels, MMP-9/total protein and MMP-9/TIMP-1 ratios were significantly higher in tuberculous pleural effusions, whilst TIMP-1 levels were similar in the three groups. MMP-9 levels positively correlated with TIMP-1 and lactate dehydrogenase levels, and negatively with pH and glucose levels in pleural effusions. MMP-9 mRNA expression in lymphocytes tended to be higher in malignant pleural effusions of lung cancer than in the other groups without reaching statistical significance. The strongest immunoreactivity for MMP-9 was observed in epithelioid cells of tuberculous pleural tissues. Much lower levels of MMP-9 expression were found in tumor cells of pleural tissues.

CONCLUSIONS

MMP-9 is increased in tuberculous pleural effusions compared with transudates and malignant pleural effusions of lung cancer and is produced predominantly by epithelioid cells in the granulomas of tuberculous pleural tissues.

Mycobacterium tuberculosis 6kDa early secreted antigenic target stimulates activation of J774 macrophages

The influence of the 6 kDa early-secreted antigenic target (ESAT-6) of Mycobacterium tuberculosis on macrophage activation was investigated using J774 macrophage cell line. While without effect if applied alone, ESAT-6 in a dose-dependent manner enhanced nitric oxide (NO) release by IFN-gamma-stimulated J774 cells. However, it completely failed to modulate NO production in J774 cells activated with E. coli lipopolysaccharide. The effect of ESAT-6 on NO synthesis in IFN-gamma-activated J774 cells was accompanied by up-regulated expression of inducible NO synthase (iNOS), an enzyme that catalyzes NO formation. ESAT-6-mediated enhancement of macrophage NO release was markedly suppressed by pharmacological agents that selectively inhibit iNOS enzymatic activity or protein tyrosine kinase and mitogen-activated protein kinase activation. Finally, treatment of J774 macrophages with ESAT-6 also enhanced IFN-gamma-induced expression of the surface molecules B7.1, MHC-II, and ICAM-1. Collectively, these data suggest that ESAT-6 might contribute to macrophage activation in tuberculosis.

Acid and particulate-induced aspiration lung injury in mice: importance of MCP-1

A model of aspiration lung injury was developed in WT C57BL/6 mice to exploit genetically modified animals on this background, i.e., MCP-1(-/-) mice. Mice were given intratracheal hydrochloric acid (ACID, pH 1.25), small nonacidified gastric particles (SNAP), or combined acid plus small gastric particles (CASP). As reported previously in rats, lung injury in WT mice was most severe for "two-hit" aspiration from CASP (40 mg/ml particulates) based on the levels of albumin, leukocytes, TNF-alpha, IL-1beta, IL-6, MCP-1, KC, and MIP-2 in bronchoalveolar lavage (BAL) at 5, 24, and 48 h. MCP-1(-/-) mice given 40 mg/ml CASP had significantly decreased survival compared with WT mice (32% vs. 80% survival at 24 h and 0% vs. 72% survival at 48 h). MCP-1(-/-) mice also had decreased survival compared with WT mice for CASP aspirates containing reduced particulate doses of 10-20 mg/ml. MCP-1(-/-) mice given 5 mg/ml CASP had survival similar to WT mice given 40 mg/ml CASP. MCP-1(-/-) mice also had differing responses from WT mice for several inflammatory mediators in BAL (KC or IL-6 depending on the particle dose of CASP and time of injury). Histopathology of WT mice with CASP (40 mg particles/ml) showed microscopic areas of compartmentalization with prominent granuloma formation by 24 h, whereas lung tissue from MCP-1(-/-) mice had severe diffuse pneumonia without granulomas. These results indicate that MCP-1 is important for survival in murine aspiration pneumonitis and appears to act partly to protect uninjured lung regions by promoting isolation and compartmentalization of tissue with active inflammation.

IL-10 down-regulates costimulatory molecules on Mycobacterium tuberculosis-pulsed macrophages and impairs the lytic activity of CD4 and CD8 CTL in tuberculosis patients

Activation of T cells requires both TCR-specific ligation and costimulation through accessory molecules during T cell priming. IFNgamma is a key cytokine responsible for macrophage activation during Mycobacterium tuberculosis (Mtb) infection while IL-10 is associated with suppression of cell mediated immunity in intracellular infection. In this paper we evaluated the role of IFNgamma and IL-10 on the function of cytotoxic T cells (CTL) and on the modulation of costimulatory molecules in healthy controls and patients with active tuberculosis (TB). gamma-irradiated-Mtb (i-Mtb) induced IL-10 production from CD14(+) cells from TB patients. Moreover, CD3(+) T cells of patients with advanced disease also produced IL-10 after i-Mtb stimulation. In healthy donors, IL-10 decreased the lytic activity of CD4(+) and CD8(+) T cells whereas it increased gammadelta-mediated cytotoxicity. Furthermore, we found that the presence of IL-10 induced a loss of the alternative processing pathways of antigen presentation along with a down-regulation of the expression of costimulatory molecule expression on monocytes and macrophages from healthy individuals. Conversely, neutralization of endogenous IL-10 or addition of IFNgamma to either effector or target cells from TB patients induced a strong lytic activity mediated by CD8(+) CTL together with an up-regulation of CD54 and CD86 expression on target cells. Moreover, we observed that macrophages from TB patients could use alternative pathways for i-Mtb presentation. Taken together, our results demonstrate that the presence of IL-10 during Mtb infection might contribute to mycobacteria persistence inside host macrophages through a mechanism that involved inhibition of MHC-restricted cytotoxicity against infected macrophages.

An in vitro dual model of mycobacterial granulomas to investigate the molecular interactions between mycobacteria and human host cells

In the majority of individuals infected with Mycobacterium tuberculosis, the bacilli cause a long-term asymptomatic infection called latent tuberculosis, a state during which the bacilli reside within granulomas. Latently infected individuals have around 10% risk of progression to clinical disease at a later stage. Determining the state of the mycobacteria and the host cells during this latent phase, i.e. within the granulomas, would greatly improve our understanding of the physiopathology of tuberculosis, and thus enable the development of new therapeutic means to treat the one-third of the world's population who are latently infected. We have developed an in vitro model of human mycobacterial granulomas, enabling the cellular and molecular analysis of the very first steps in the host granulomatous response to either mycobacterial compounds or live mycobacterial species. In vitro mycobacterial granulomas mimic natural granulomas very well, with the progressive recruitment of macrophages around live bacilli or mycobacterial antigen-coated beads, their differentiation into multinucleated giant cells and epithelioid cells, and the final recruitment of a ring of activated lymphocytes. Besides morphological similarities, in vitro granulomas also functionally resemble natural ones, with the development of intense cellular co-operation and intracellular mycobactericidal activities.

High incidence of anergy in inflammatory bowel disease patients limits the usefulness of PPD screening before infliximab therapy

BACKGROUND & AIMS

Reports of tuberculosis (TB) in patients administered infliximab prompted the Food and Drug Administration to recommend that all patients being considered for this therapy be evaluated for the risk for latent TB infection by means of a tuberculin skin test (TST). The aim of this study is to evaluate the utility of a TST as an adequate screen for TB exposure in patients with inflammatory bowel disease (IBD).

METHODS

Eighty-two consecutive patients with IBD (Crohn's disease, 70 patients; ulcerative colitis, 4 patients; indeterminate colitis, 8 patients) seen at Cedars-Sinai Medical Center IBD Center (Los Angeles, CA) being treated with or considered for infliximab therapy underwent a standard intradermal purified protein derivative (PPD) TST before or between infusions of infliximab. One or more control antigens (Candida, tetanus, and/or mumps) were concurrently placed on 69 of these patients. Skin tests were read for induration at 48-72 hours after placement, and results were recorded.

RESULTS

None of 82 patients had a positive PPD TST result. Overall, 71% of patients (49 of 69 patients) with controls placed failed to react to any antigen. Eighty-three percent of patients (40 of 48 patients) who were administered corticosteroids and/or immunosuppressive medications, not including infliximab, for at least 1 month were anergic compared with 43% of patients (9 of 21 patients; P < 0.002) who were not administered those medications.

CONCLUSIONS

Given the high prevalence of anergy, a negative TST result in patients with IBD administered infliximab is an unreliable indicator for TB exposure. Evaluation for TB risks should include not only a TST, but also a detailed history of travel, TB exposures, and such symptoms as chronic cough and weight loss, and a chest radiograph should be considered.

The secret lives of the pathogenic mycobacteria

Pathogenic mycobacteria, including the causative agents of tuberculosis and leprosy, are responsible for considerable morbidity and mortality worldwide. A hallmark of these pathogens is their tendency to establish chronic infections that produce similar pathologies in a variety of hosts. During infection, mycobacteria reside in macrophages and induce the formation of granulomas, organized immune complexes of differentiated macrophages, lymphocytes, and other cells. This review summarizes our understanding of Mycobacterium-host cell interactions, the bacterial-granuloma interface, and mechanisms of bacterial virulence and persistence. In addition, we highlight current controversies and unanswered questions in these areas.

Role of narK2X and narGHJI in hypoxic upregulation of nitrate reduction by Mycobacterium tuberculosis

Mycobacterium tuberculosis is one of the strongest reducers of nitrate in the genus Mycobacterium: Under microaerobic conditions, whole cells exhibit upregulation of activity, producing approximately eightfold more nitrite than those of aerobic cultures of the same age. Assays of cell extracts from aerobic cultures and hypoxic cultures yielded comparable nitrate reductase activities. Mycobacterium bovis produced only low levels of nitrite, and this activity was not induced by hypoxia. M. tuberculosis has two sets of genes, narGHJI and narX of the narK2X operon, that exhibit some degree of homology to prokaryotic dissimilatory nitrate reductases. Each of these were knocked out by insertional inactivation. The narG mutant showed no nitrate reductase activity in whole culture or in cell-free assays, while the narX mutant showed wild-type levels in both assays. A knockout of the putative nitrite transporter narK2 gene produced a strain that had aerobic levels of nitrate reductase activity but failed to show hypoxic upregulation. Insertion of the M. tuberculosis narGHJI into a nitrate reductase Escherichia coli mutant allowed anaerobic growth in the presence of nitrate. Under aerobic and hypoxic conditions, transcription of narGHJI was constitutive, while the narK2X operon was induced under hypoxia, as measured with a lacZ reporter system and by quantitative real-time reverse PCR. This indicates that nitrate reductase activity in M. tuberculosis is due to the narGHJI locus with no detectable contribution from narX and that the hypoxic upregulation of activity is associated with the induction of the nitrate and nitrite transport gene narK2.

Intracellular adhesion molecule 1 plays a key role in acquired immunity to salmonellosis

This study investigated the role of intracellular adhesion molecule 1 (ICAM-1) during Salmonella enterica serovar Typhimurium infection of mice. We show that ICAM-1 is expressed in and around granulomas on day 4 of infection in wild-type mice. However, when naive ICAM-1(-/-) mice were challenged with a sublethal dose of serovar Typhimurium, there were no detectable differences in systemic bacterial burden over the first 9 days of infection compared to wild-type control mice. When mice were immunized with the S. enterica serovar Typhimurium vaccine strain SL2361 and then challenged with the virulent S. enterica serovar Typhimurium strain C5, 100% of the ICAM-1(-/-) mice succumbed to infection, compared to 30% of wild-type mice. T-cell responses, as measured by activation via interleukin-2 production, as well as antibody responses were comparable in the ICAM-1(-/-) and wild-type mice. Following challenge, counts in organs were significantly higher in the ICAM-1(-/-) mice, and histological examination of organs showed pathological differences. Strain SL3261-immunized wild-type mice had cellular infiltrate and normal granuloma formation in the liver and spleen on days 5 and 10 after challenge with strain C5. ICAM-1(-/-) mice had a similar infiltrate on day 5, whereas on day 10 the infiltrate was more widespread and there were fewer macrophages associated with the granulomas. High circulating levels of tumor necrosis factor alpha and gamma interferon, as well as a high burden of strain C5 in the blood, accompanied the differences in histopathology. In this study we show that ICAM-1 plays a critical role during rechallenge of immunized mice with virulent S. enterica serovar Typhimurium.

The mouse as a useful model of tuberculosis

Like other animal models of tuberculosis, the mouse has provided a large amount of information that can be applied to understanding the disease process in infected humans. The model is particularly useful in providing information about the immune response, given the huge database of reagents now available, including antibodies to lymphocyte markers and the growing number of available gene disrupted mice, and the model is validated by the fact that multiple mechanisms discovered in the mouse such as the TH1 pathway and the Toll-like receptor system are similarly important in humans. The model also has its limitations, particularly in terms of the immunopathologic response, in which similar elements occur but are expressed somewhat differently.

Susceptibility of mice deficient in the MHC class II transactivator to infection with Mycobacterium tuberculosis

Major histocompatibility complex (MHC) class II antigen presentation and subsequent CD4+ T-cell activation are critical for acquired immunity to Mycobacterium tuberculosis infection. MHC class II gene expression is primarily controlled by the master transactivator CIITA protein. Without functional CIITA protein, MHC class II expression is lost, impairing immune responses and increasing susceptibility to infection. In this study, we compared protective immune responses of CIITA-deficient mice and wild-type C57BL/6 controls with low dose aerosol M. tuberculosis infection. After aerogenic challenge, CIITA-/- mice failed to limit mycobacterial growth (2.5 and 2.0 log10 > WT lung and spleen CFUs, respectively, at day 58). Lung histopathology involved extensive necrosis, severe pneumonitis and overwhelming inflammation in the gene knockout mice. Mean survival time for CIITA-/- mice was significantly reduced (57 versus >300 days for WT). This extreme sensitivity to tuberculous infection was largely attributed to the absence of CD4+ cells. Flow cytometric studies detected virtually no CD4+ cells in CIITA-/- mouse spleens after infection versus elevated numbers in WT spleens. Failed CD4+ T-cell expansion markedly reduced interferon-gamma (IFN-gamma production in CIITA-/- mice versus WT controls. These results suggest the necessity of a functional CIITA pathway for controlling tuberculous infections and that interventions targeting CIITA expression may be useful antimycobacterial therapeutics.

The lymphotoxin beta receptor is critically involved in controlling infections with the intracellular pathogens Mycobacterium tuberculosis and Listeria monocytogenes

Containment of intracellularly viable microorganisms requires an intricate cooperation between macrophages and T cells, the most potent mediators known to date being IFN-gamma and TNF. To identify novel mechanisms involved in combating intracellular infections, experiments were performed in mice with selective defects in the lymphotoxin (LT)/LT beta R pathway. When mice deficient in LT alpha or LT beta were challenged intranasally with Mycobacterium tuberculosis, they showed a significant increase in bacterial loads in lungs and livers compared with wild-type mice, suggesting a role for LT alpha beta heterotrimers in resistance to infection. Indeed, mice deficient in the receptor for LT alpha(1)beta(2) heterotrimers (LT beta R-knockout (KO) mice) also had significantly higher numbers of M. tuberculosis in infected lungs and exhibited widespread pulmonary necrosis already by day 35 after intranasal infection. Furthermore, LT beta R-KO mice were dramatically more susceptible than wild-type mice to i.p. infection with Listeria monocytogenes. Compared with wild-type mice, LT beta R-KO mice had similar transcript levels of TNF and IFN-gamma and recruited similar numbers of CD3(+) T cells inside granulomatous lesions in M. tuberculosis-infected lungs. Flow cytometry revealed that the LT beta R is expressed on pulmonary macrophages obtained after digestion of M. tuberculosis-infected lungs. LT beta R-KO mice showed delayed expression of inducible NO synthase protein in granuloma macrophages, implicating deficient macrophage activation as the most likely cause for enhanced susceptibility of these mice to intracellular infections. Since LIGHT-KO mice proved to be equally resistant to M. tuberculosis infection as wild-type mice, these data demonstrate that signaling of LT alpha(1)beta(2) heterotrimers via the LT beta R is an essential prerequisite for containment of intracellular pathogens.

Mechanisms of cell recruitment in the immune response to Mycobacterium tuberculosis

Recent advances in understanding cell traffic, especially the roles of adhesion proteins, chemokines, and chemokine receptors, provide the opportunity for understanding mechanisms involved in the immune response to tuberculosis. This review concentrates on the roles of these molecules and the immune response in tuberculosis, based on studies of humans and mice infected with Mycobacterium tuberculosis.

Immunological basis of the development of necrotic lesions following Mycobacterium avium infection

Normal C57BL/6 mice infected with 106 colony-forming units of a highly virulent strain of Mycobacterium avium developed a progressive infection characterized by loss of T cells from the tissues and infiltration with high numbers of heavily infected macrophages. In contrast, when C57BL/6 mice were infected with 102 colony-forming units of the same strain they retained T cells and T-cell reactivity in the tissues, and granulomas evolved into large masses that, at 4 months of infection, exhibited central necrosis. The development of these necrotic lesions did not occur in nude mice, nor in mice genetically deficient in CD4, interleukin-12 (IL-12) p40, interferon-gamma (IFN-gamma) and CD40 and were reduced in mice deficient in CD54 or IL-6. They were less numerous but bigger in mice deficient in IL-10 or the inducible nitric oxide synthase, correlating with the increased resistance to mycobacterial proliferation of these strains as compared to control mice. The appearance of necrosis was not affected in mice deficient in CD8alpha, T-cell receptor delta, tumour necrosis factor receptor p55, and perforin, nor was it affected in mice over-expressing bcl2. The appearance of necrosis could be prevented by administering antibodies specific for CD4, IL-12p40, or IFN-gamma from the second month of infection when organized granulomas were already found. Our results show that the immunological mediators involved in the induction of protective immunity are also major players in the immunopathology associated with mycobacteriosis.

CD4 is required for the development of a protective granulomatous response to pulmonary tuberculosis

To confirm the primary role of CD4 T cells in pulmonary tuberculosis, mice with a disruption of their CD4 gene (CD4 KO) were exposed to an aerosol of Mycobacterium tuberculosis and survival, cellular responses in the lung and granuloma development followed. CD8 and NK cells from the lungs of infected CD4 KO mice expressed IFN-gamma and were recruited in numbers similar to those seen in the C57BL/6 mice; recruitment correlated with initial control of bacteria. The major defect in mice lacking CD4 was the significant reduction in total cellular recruitment into the lungs. CD4 KO mice did not generate the typical mononuclear granulomatous lesions, instead the cellular influx was macrophage in character and was localized as perivascular cuffing. Early control of M. tuberculosis growth is therefore independent of CD4+ cells but such cells are required to ensure recruitment of mononuclear cells to the lung and thus ensure long-term survival.

Nonreplicating persistence of mycobacterium tuberculosis

There is ample clinical evidence, as well as evidence from animal experiments, that Mycobacterium tuberculosis can persist in tissues for months to decades without replicating, yet with the ability to resume growth and activate disease. Our knowledge of both macrophage physiology and the nature of tuberculous lesions in man and animals suggests that hypoxia is a major factor in inducing nonreplicating persistence (NRP) of tubercle bacilli. In vitro models reinforce this conclusion and provide insights into mechanisms that make NRP possible. There is evidence from in vitro models that the strategies employed by the bacilli to permit hypoxic NRP include restriction of biosynthetic activity to conserve energy, induction of alternative energy pathways, and stabilization of essential cell components to lessen the need for repair or replacement.

Dynamic nature of host-pathogen interactions in Mycobacterium marinum granulomas

Mycobacterium marinum causes long-term subclinical granulomatous infection in immunocompetent leopard frogs (Rana pipiens). These granulomas, organized collections of activated macrophages, share many morphological features with persistent human tuberculous infection. We examined organs of frogs with chronic M. marinum infection using transmission electron microscopy in conjunction with immunohistochemistry and acid phosphatase cytochemistry to better define the bacterium-host interplay during persistent infection. Bacteria were always found within macrophage phagosomes. These phagosomes were often fused to lysosomes, in sharp contrast to those formed during in vitro infection of J774 macrophage-like cells by M. marinum. The infected macrophages in frog granulomas showed various levels of activation, as evidenced by morphological changes, including epithelioid transformation, recent phagocytic events, phagolysosomal fusion, and disintegration of bacteria. Our results demonstrate that even long-term granulomas are dynamic environments with regard to the level of host cell activation and bacterial turnover and suggest a continuum between constantly replicating bacteria and phagocytic killing that maintains relatively constant bacterial numbers despite an established immune response. Infection with a mutant bacterial strain with a reduced capacity for intracellular replication shifted the balance, leading to a greatly reduced bacterial burden and inflammatory foci that differed from typical granulomas.

Cathepsin K--a marker of macrophage differentiation?

Cathepsin K is a cysteine protease with high matrix-degrading activity. Initially, cathepsin K was described as being expressed exclusively by osteoclasts. It was suggested that cathepsin K expression is a specific feature of cells involved in bone remodelling. The aim of this study was to investigate the hypothesis that cathepsin K is expressed not only in bone-resorbing macrophages, but also more generally in specifically differentiated macrophages, such as epithelioid cells and multinucleated giant cells in soft tissues. Specimens obtained from different organs and anatomical locations of patients suffering from sarcoidosis, tuberculosis, granulomas caused by foreign materials, and sarcoid-like lesions were investigated for the expression of cathepsins B, K, and L. Immunohistochemistry and in situ hybridization showed cathepsin K in epithelioid cells and multinucleated giant cells irrespective of the pathological condition and anatomical location, but not in normal resident macrophages. By immunoelectron microscopy, cathepsin K was discovered in cytoplasmic granules of multinucleated giant cells. In contrast, cathepsin B and cathepsin L were expressed ubiquitously in CD68-positive tissue macrophages, epithelioid cells, and multinucleated giant cells. The results demonstrate that cathepsin K, but not cathepsin B or cathepsin L, differentiates specific phenotypes of macrophages independently of the anatomical site. Its enzymatic characteristics, particularly its high matrix-degrading activity, suggest that cathepsin K-positive epithelioid cells and multinucleated giant cells are characterized by an enhanced specific proteolytic capability.

Production of matrix metalloproteinases in response to mycobacterial infection

Matrix metalloproteinases (MMPs) constitute a large family of enzymes with specificity for the various proteins of the extracellular matrix which are implicated in tissue remodeling processes and chronic inflammatory conditions. To investigate the role of MMPs in immunity to mycobacterial infections, we incubated murine peritoneal macrophages with viable Mycobacterium bovis BCG or Mycobacterium tuberculosis H37Rv and assayed MMP activity in the supernatants by zymography. Resting macrophages secreted only small amounts of MMP-9 (gelatinase B), but secretion increased dramatically in a dose-dependent manner in response to either BCG or M. tuberculosis in vitro. Incubation with mycobacteria also induced increased MMP-2 (gelatinase A) activity. Neutralization of tumor necrosis alpha (TNF-alpha), and to a lesser extent interleukin 18 (IL-18), substantially reduced MMP production in response to mycobacteria. Exogenous addition of TNF-alpha or IL-18 induced macrophages to express MMPs, even in the absence of bacteria. The immunoregulatory cytokines gamma interferon (IFN-gamma), IL-4, and IL-10 all suppressed BCG-induced MMP production, but through different mechanisms. IFN-gamma treatment increased macrophage secretion of TNF-alpha but still reduced their MMP activity. Conversely, IL-4 and IL-10 seemed to act by reducing the amount of TNF-alpha available to the macrophages. Finally, infection of BALB/c or severe combined immunodeficiency (SCID) mice with either BCG or M. tuberculosis induced substantial increases in MMP-9 activity in infected tissues. In conclusion, we show that mycobacterial infection induces MMP-9 activity both in vitro and in vivo and that this is regulated by TNF-alpha, IL-18, and IFN-gamma. These findings indicate a possible contribution of MMPs to tissue remodeling processes that occur in mycobacterial infections.

A role for complement C5 in organism containment and granulomatous response during murine tuberculosis

The molecular mechanisms underlying protective granuloma formation and control of bacterial growth during infection with Mycobacterium tuberculosis (MTB) are not yet completely understood. MTB-infected mice with natural deficiency in complement component C5 are unable to develop productive granulomatous responses, and are impaired in limiting organism growth within the lung. To address the molecular basis for this histologic dysfunction, congenic complement C5-sufficient (B10.D2-H2d H2-T18c Hcl/nSnJ) and complement C5-deficient strains (B10.D2-H2d H2-T18c Hco/oSnJ) congenic mice were infected with Mycobacterium tuberculosis, and cytokine and chemokine responses were examined. Twelve and 28 days after infection, lungs showed elevated messages for multiple inflammatory cytokines in both congenic strains. Interleukin (IL)-12(p40) mRNA was also induced during infection in C5-deficient mice, although levels were significantly decreased compared to C5-sufficient congenics. C5-deficient mice also demonstrated reduced KC, MIP-2, IP-10, and MCP-1 mRNA. The defect may directly involve C5-mediated effects on macrophage responses; C5-deficient bone marrow derived macrophages had significantly reduced secretion of KC, MIP-1 alpha and MIP-2 compared to C5-sufficient macrophages following in vitro infection. These findings indicate a role for C5 in mediation of chemotactic and activation events that are the basis for granulomatous responses during murine tuberculosis.

CD8- and CD95/95L-dependent mechanisms of resistance in mice with chronic pulmonary tuberculosis

The role of CD8 T lymphocytes in the immune response to Mycobacterium tuberculosis infection remains enigmatic, with persuasive reports of both cytolytic and noncytolytic (cytokine-mediated) responses to infection. To address the importance of the cytolytic mechanisms, mice with targeted disruptions for CD8 and perforin or with gene mutations in the CD95/ CD95L signaling pathway were exposed to pulmonary infection. All mice tested showed no differences in their ability to contain the growth of infection during the early phase of disease. As the chronic phase of the disease ensued, however, both CD8- and CD95/CD95L-deficient mice gradually lost their ability to limit bacterial growth. This was associated with a tendency toward pyogenic inflammation in the lung. This tendency was not seen in the perforin gene-disrupted mice. In CD8 gene-disrupted mice, the ability to generate interferon-gamma secreting T cells was unimpaired. Although these cells were capable of entering the lung they were unable to influence the increasing bacterial load in this organ.

Expression of the nitric oxide synthase 2 gene is not essential for early control of Mycobacterium tuberculosis in the murine lung

The interleukin-12 and gamma interferon (IFN-gamma) pathway of macrophage activation plays a pivotal role in controlling tuberculosis. In the murine model, the generation of supplementary nitric oxide by the induction of the nitric oxide synthase 2 (NOS2) gene product is considered the principal antimicrobial mechanism of IFN-gamma-activated macrophages. Using a low-dose aerosol-mediated infection model in the mouse, we have investigated the role of nitric oxide in controlling Mycobacterium tuberculosis in the lung. In contrast to the consequences of a systemic infection, a low dose of bacteria introduced directly into the lungs of mice lacking the NOS2 gene is controlled almost as well as in intact animals. This is in contrast to the rapid progression of disease in mice lacking IFN-gamma or a key member of the IFN signaling pathway, interferon regulatory factor 1. Thus while IFN-gamma is pivotal in early control of bacterial growth in the lung, this control does not completely depend upon the expression of the NOS2 gene. The absence of inducible nitric oxide in the lung does, however, result in increased polymorphonuclear cell involvement and eventual necrosis in the pulmonary granulomas of the infected mice lacking the NOS2 gene.

Utilization of CD11b knockout mice to characterize the role of complement receptor 3 (CR3, CD11b/CD18) in the growth of Mycobacterium tuberculosis in macrophages

Using CD11b knockout mice as a source of macrophages (Mphi;), we show that complement receptor 3 (CR3) mediates approximately 40-50% of nonopsonic binding and 50-60% of serum-mediated binding of Mycobacterium tuberculosis to resident Mphi;. We demonstrate that opsonic binding of M. tuberculosis to Mphi; is mediated by an immunoglobulin-independent, heat-labile component of serum, in both the presence and the absence of CD11b. The survival and replication of M. tuberculosis in an in vitro Mphi; model and an in vivo mouse model of infection were not significantly affected by the absence of CD11b, indicating that CR3-mediated uptake of M. tuberculosis is not a major factor in controlling the subsequent intracellular survival of the mycobacteria. However, whether a mycobacterium will gain access to the intracellular environment, and the type of M&phi; that the bacterium enters, is significantly affected by the presence or absence of CR3.

The stringent response of Mycobacterium tuberculosis is required for long-term survival

The stringent response utilizes hyperphosphorylated guanine [(p)ppGpp] as a signaling molecule to control bacterial gene expression involved in long-term survival under starvation conditions. In gram-negative bacteria, (p)ppGpp is produced by the activity of the related RelA and SpoT proteins. Mycobacterium tuberculosis contains a single homolog of these proteins (Rel(Mtb)) and responds to nutrient starvation by producing (p)ppGpp. A rel(Mtb) knockout strain was constructed in a virulent strain of M. tuberculosis, H37Rv, by allelic replacement. The rel(Mtb) mutant displayed a significantly slower aerobic growth rate than the wild type in synthetic liquid media, whether rich or minimal. The growth rate of the wild type was equivalent to that of the mutant when citrate or phospholipid was employed as the sole carbon source. These two organisms also showed identical growth rates within a human macrophage-like cell line. These results suggest that the in vivo carbon source does not represent a stressful condition for the bacilli, since it appears to be utilized in a similar Rel(Mtb)-independent manner. In vitro growth in liquid media represents a condition that benefits from Rel(Mtb)-mediated adaptation. Long-term survival of the rel(Mtb) mutant during in vitro starvation or nutrient run out in normal media was significantly impaired compared to that in the wild type. In addition, the mutant was significantly less able to survive extended anaerobic incubation than the wild-type virulent organism. Thus, the Rel(Mtb) protein is required for long-term survival of pathogenic mycobacteria under starvation conditions.

Restraining mycobacteria: role of granulomas in mycobacterial infections

The generation of prolonged immunity to Mycobacterium tuberculosis requires not only an antigen-specific IFN-gamma-producing T cell response, including both CD4 and CD8 T cells, but also the generation of protective granulomatous lesions, whereby the close apposition of activated T cells and macrophages acts to contain bacterial growth. The importance of the granulomatous lesion in controlling this immune response and in limiting both tissue damage and bacterial dissemination has been considered a secondary event but, as the present review illustrates, is no less important in surviving mycobacterial infection than an antigen-specific T-cell response. The formation of a protective granuloma involves the orchestrated production of a host of chemokines and cytokines, the upregulation of their receptors along with upregulation of addressins, selectins and integrins to coordinate the recruitment, migration and retention of cells to and within the granuloma. In the present review, the principal components of the protective response are outlined and the role of granuloma formation and maintenance in mediating prolonged containment of mycobacteria within the lung is addressed.

A novel nonclassic beta2-microglobulin-restricted mechanism influencing early lymphocyte accumulation and subsequent resistance to tuberculosis in the lung

In this study, we compared the course of a low-dose aerosol Mycobacterium tuberculosis infection in mice bearing gene disruptions for the beta2-microglobulin molecule, the CD8 molecule, and the CD1 molecule. Over the first 50 d of infection, the CD8- and CD1-disrupted mice were no more susceptible to infection than were the control mice. In contrast, the bacterial load in beta2-microglobulin gene-disrupted mice increased rapidly and attained much higher levels than that observed in the other gene-disrupted mice and in control mice. A second major difference between the beta2-microglobulin gene-disrupted mice and the other animals was the development of lung granulomas; both the CD8- and CD1-disrupted mice developed essentially normal granulomas except for an apparent increased lymphocyte influx in the CD8-disrupted mice. The beta2-microglobulin gene-disrupted mice, on the other hand, developed granulomas virtually devoid of lymphocytes, with these cells instead localized within prominent perivascular cuffing adjacent to the lesions. These data support the hypothesis that a beta2-microglobulin-dependent, non-CD8- and non-CD1-dependent mechanism controls the early and efficient influx of protective lymphocytes into infected lesions, and that the absence of this mechanism decreases the capacity of the animal to initially deal with pulmonary tuberculosis.