Genetic control of immune-mediated necrosis of Mycobacterium avium granulomas

Ag85-focused T-cell immune response controls Mycobacterium avium chronic infection

CD4⁺ T cells are essential players for the control of mycobacterial infections. Several mycobacterial antigens have been identified for eliciting a relevant CD4⁺ T cell mediated-immune response, and numerous studies explored this issue in the context of Mycobacterium tuberculosis infection. Antigen 85 (Ag85), a highly conserved protein across Mycobacterium species, is secreted at the early phase of M. tuberculosis infection leading to the proliferation of Ag85-specific CD4⁺ T cells. However, in the context of Mycobacterium avium infection, little is known about the expression of this antigen and the elicited immune response. In the current work, we investigated if a T cell receptor (TCR) repertoire mostly, but not exclusively, directed at Ag85 is sufficient to mount a protective immune response against M. avium. We show that P25 mice, whose majority of T cells express a transgenic TCR specific for Ag85, control M. avium infection at the same level as wild type (WT) mice up to 20 weeks post-infection (wpi). During M. avium infection, Ag85 antigen is easily detected in the liver of 20 wpi mice by immunohistochemistry. In spite of the propensity of P25 CD4⁺ T cells to produce higher amounts of interferon-gamma (IFNγ) upon ex vivo stimulation, no differences in serum IFNγ levels are detected in P25 compared to WT mice, nor enhanced immunopathology is detected in P25 mice. These results indicate that a T cell response dominated by Ag85-specific T cells is appropriate to control M. avium infection with no signs of immunopathology.

Lack of the Transcription Factor Hypoxia-Inducible Factor 1α (HIF-1α) in Macrophages Accelerates the Necrosis of Mycobacterium avium-Induced Granulomas

The establishment of mycobacterial infection is characterized by the formation of granulomas, which are well-organized aggregates of immune cells, namely, infected macrophages. The granuloma's main function is to constrain and prevent dissemination of the mycobacteria while focusing the immune response to a limited area. In some cases these lesions can grow progressively into large granulomas which can undergo central necrosis, thereby leading to their caseation. Macrophages are the most abundant cells present in the granuloma and are known to adapt under hypoxic conditions in order to avoid cell death. Our laboratory has developed a granuloma necrosis model that mimics the human pathology of Mycobacterium tuberculosis, using C57BL/6 mice infected intravenously with a low dose of a highly virulent strain of Mycobacterium avium. In this work, a mouse strain deleted of the hypoxia inducible factor 1α (HIF-1α) under the Cre-lox system regulated by the lysozyme M gene promoter was used to determine the relevance of HIF-1α in the caseation of granulomas. The genetic ablation of HIF-1α in the myeloid lineage causes the earlier emergence of granuloma necrosis and clearly induces an impairment of the resistance against M. avium infection coincident with the emergence of necrosis. The data provide evidence that granulomas become hypoxic before undergoing necrosis through the analysis of vascularization and quantification of HIF-1α in a necrotizing mouse model. Our results show that interfering with macrophage adaptation to hypoxia, such as through HIF-1α inactivation, accelerates granuloma necrosis.

Virulence and immune response induced by Mycobacterium avium complex strains in a model of progressive pulmonary tuberculosis and subcutaneous infection in BALB/c mice

The genus Mycobacterium comprises more than 150 species, including important pathogens for humans which cause major public health problems. The vast majority of efforts to understand the genus have been addressed in studies with Mycobacterium tuberculosis. The biological differentiation between M. tuberculosis and nontuberculous mycobacteria (NTM) is important because there are distinctions in the sources of infection, treatments, and the course of disease. Likewise, the importance of studying NTM is not only due to its clinical significance but also due to the mechanisms by which some species are pathogenic while others are not. Mycobacterium avium complex (MAC) is the most important group of NTM opportunistic pathogens, since it is the second largest medical complex in the genus after the M. tuberculosis complex. Here, we evaluated the virulence and immune response of M. avium subsp. avium and Mycobacterium colombiense, using experimental models of progressive pulmonary tuberculosis and subcutaneous infection in BALB/c mice. Mice infected intratracheally with a high dose of MAC strains showed high expression of tumor necrosis factor alpha (TNF-α) and inducible nitric oxide synthase with rapid bacillus elimination and numerous granulomas, but without lung consolidation during late infection in coexistence with high expression of anti-inflammatory cytokines. In contrast, subcutaneous infection showed high production of the proinflammatory cytokines TNF-α and gamma interferon with relatively low production of anti-inflammatory cytokines such as interleukin-10 (IL-10) or IL-4, which efficiently eliminate the bacilli but maintain extensive inflammation and fibrosis. Thus, MAC infection evokes different immune and inflammatory responses depending on the MAC species and affected tissue.

Difference in virulence of Mycobacterium avium isolates sharing indistinguishable DNA fingerprint determined in murine model of lung infection

BACKGROUND

Opportunistic Mycobacterium avium typically causes disease in immunocompromised patients and in some groups of apparently healthy individuals. The high virulence of some bacterial lineages increases the disease risk. High-resolution molecular genotyping studies of M. avium clinical isolates demonstrated that some genotype patterns were more prevalent than others, suggesting that close genetic relatedness of these successful isolates sharing a similar genotype could determine similar biological properties associated with high virulence.

METHODS AND FINDINGS

In this study, we aimed to compare the virulence and pathogenic properties of two epidemiologically unrelated M. avium isolates sharing an indistinguishable DNA fingerprint in a well-characterized model of pulmonary infection in mice, resistant or susceptible to mycobacteria. The mice, C57BL/6 wild- type or IFN-gamma gene disrupted (GKO), respectively, were intratracheally infected with two isolates, H27 (human blood isolate) and P104 (pig lymph node isolate), and the lungs were examined for bacterial loads, histopathology and cytokine gene expression. The obtained data demonstrated significant differences in the virulence properties of these strains. Although the H27 strain grew significantly faster than P104 in the early stage of infection, this bacterium induced protective immunity that started to reduce bacterial numbers in the wild-type mice, whereas the P104 strain established a chronic infection. In the GKO mice, both strains were capable of causing a chronic infection, associated with higher bacterial burdens and severe lung pathology, in a similar manner.

CONCLUSIONS/SIGNIFICANCE

The results demonstrated that the studied isolates differed in the pathogenic properties although were indistinguishable by actually widely used genotyping techniques demonstrating that the genotype similarity does not predict similarity in virulence of M. avium isolates.

Interleukin-10 and immunity against prokaryotic and eukaryotic intracellular pathogens

The generation of an effective immune response against an infection while also limiting tissue damage requires a delicate balance between pro- and anti-inflammatory responses. Interleukin-10 (IL-10) has potent immunosuppressive effects and is essential for regulation of immune responses. However, the immunosuppressive properties of IL-10 can also be exploited by pathogens to facilitate their own survival. In this minireview, we discuss the role of IL-10 in modulating intracellular bacterial, fungal, and parasitic infections. Using information from several different infection models, we bring together and highlight some common pathways for IL-10 regulation and function that cannot be fully appreciated by studies of a single pathogen.

The death-promoting molecule tumour necrosis factor-related apoptosis inducing ligand (TRAIL) is not required for the development of peripheral lymphopenia or granuloma necrosis during infection with virulent Mycobacterium avium

Disseminated infection with virulent Mycobacterium avium in C57Bl/6 (B6) mice leads to severe lymphocyte depletion in secondary lymphoid organs. In this study, we found an up-regulation of caspase-8 activity in spleen cell extracts from M. avium 25291-infected B6 mice compared to non-infected mice. The activation of this extrinsic apoptotic pathway correlated with an increase in inter-nucleosomal DNA fragmentation in CD4(+) spleen cells, as analysed by the terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) assay. These data suggest the involvement of death receptors in the induction of lymphocyte loss in the spleen, but previous work has excluded a role for tumour necrosis factor (TNF) receptors and Fas/CD95 in M. avium-induced lymphopenia. TNF-related apoptosis-inducing ligand (TRAIL) is expressed by different cell types of the immune system and induces apoptosis and killing of tumour cells while sparing normal cells. Here we used TRAIL(-/-) mice to determine if the absence of TRAIL prevented M. avium-induced immune pathology. We found that TRAIL-deficient mice still developed splenic lymphopenia during disseminated infection or granuloma necrosis during low-dose infections while exhibiting slightly increased susceptibility to M. avium 25291 when compared to B6 mice. However, in vivo proliferation of less virulent strains of M. avium was not influenced by TRAIL deficiency despite a decrease in interferon-γ production in infected B6.TRAIL(-/-) mice compared to B6 mice. Our results show that TRAIL does not play a significant role in either M. avium-induced pathology or protective immunity.

Host genetics in granuloma formation: human-like lung pathology in mice with reciprocal genetic susceptibility to M. tuberculosis and M. avium

Development of lung granulomata is a hallmark of infections caused by virulent mycobacteria, reflecting both protective host response that restricts infection spreading and inflammatory pathology. The role of host genetics in granuloma formation is not well defined. Earlier we have shown that mice of the I/St strain are extremely susceptible to Mycobacterium tuberculosis but resistant to M. avium infection, whereas B6 mice show a reversed pattern of susceptibility. Here, by directly comparing: (i) characteristics of susceptibility to two infections in vivo; (ii) architecture of lung granulomata assessed by immune staining; and (iii) expression of genes encoding regulatory factors of neutrophil influx in the lung tissue, we demonstrate that genetic susceptibility of the host largely determines the pattern of lung pathology. Necrotizing granuloma surrounded by hypoxic zones, as well as a massive neutrophil influx, develop in the lungs of M. avium-infected B6 mice and in the lungs of M. tuberculosis-infected I/St mice, but not in the lungs of corresponding genetically resistant counterparts. The mirror-type lung tissue responses to two virulent mycobacteria indicate that the level of genetic susceptibility of the host to a given mycobacterial species largely determines characteristics of pathology, and directly demonstrate the importance of host genetics in pathogenesis.

IL-10 underlies distinct susceptibility of BALB/c and C57BL/6 mice to Mycobacterium avium infection and influences efficacy of antibiotic therapy

Increased production of IL-10 has been frequently associated with augmented susceptibility to infection. However, the correlation between IL-10 activity and susceptibility to mycobacterial infection is still uncertain. Although studies using transgenic mice overexpressing IL-10 consistently showed an increased susceptibility to mycobacterial infection, experimental approaches in which IL-10 activity was reduced or abrogated originated inconclusive data. We show here that this controversy might be due to the mouse strains used in the various experimental procedures. Our results show that BALB/c mice are more susceptible than C57BL/6 to Mycobacterium avium infection. This increased susceptibility of BALB/c mice is, to a great extent, due to distinct activity of IL-10 between the two mouse strains. In accordance, reduction of IL-10 activity through the administration of anti-IL-10R mAb, or the absence of IL-10 as studied in IL-10 knockout mice, clearly decreased the susceptibility of BALB/c mice to M. avium but had a less obvious effect in C57BL/6 mice. Moreover, abrogation of IL-10 activity in infected BALB/c mice increased the efficacy of antimycobacterial therapy, whereas for the C57BL/6 mice it produced no effect. These observations show that the activity of IL-10 in response to the same mycobacterial stimulus influences not only the susceptibility to infection but also the efficacy of antimycobacterial therapy. This should now be considered in the context of human response to mycobacterial infection, particularly as a possible strategy to improve treatment against infections by mycobacteria.

I/St mice hypersusceptible to Mycobacterium tuberculosis are resistant to M. avium

We previously demonstrated that mice of the I/St strain are extremely susceptible to Mycobacterium tuberculosis, as well as to the taxonomically distant intracellular bacteria Chlamydia pneumoniae and Salmonella enterica. To broaden our knowledge about the control of susceptibility to intracellular pathogens, we studied the infection caused by Mycobacterium avium virulent strain 724 in a panel of inbred mouse strains and found that I/St mice are resistant to M. avium. By comparing I/St mice with B6 mice, we demonstrated that (i) B6 mice are much more susceptible to infection caused by M. avium in terms of bacterial multiplication in the lung tissue and severity of lung pathology; (ii) in B6 mice but not in I/St mice infection leads to prolonged leukocyte infiltration of the lung tissue, development of necrotic lung granulomata, and lethality; and (iii) the unfavorable infectious course in B6 mice is accompanied by elevated production of gamma interferon, tumor necrosis factor alpha, and especially interleukin-12 in the lungs. Importantly, M. avium-resistant I/St mice carry a functional r allele of the Slc11a1 (formerly Nramp1) gene, while B6 mice have the Slc11a1(s) genotype. Segregation genetic analysis of (I/St x B6) F2 hybrids demonstrated that susceptibility or resistance to infection caused by M. avium largely depended upon the Slc11a1 genotype and that other genetic traits had a relatively weak influence. This close-to-monogenic pattern differs sharply from the host control of many other intracellular bacterial infections, for which the involvement of numerous quantitative trait loci has been ubiquitously observed.

Interferon-gamma-dependent mechanisms of mycobacteria-induced pulmonary immunopathology: the role of angiostasis and CXCR3-targeted chemokines for granuloma necrosis

The mechanisms leading to granuloma caseation, a hallmark of tuberculosis (TB) in humans, are poorly understood. Lung histopathology of C57BL/6 (WT) mice 16 weeks after aerosol infection with Mycobacterium avium strain TMC724 is uniquely characterized by centrally necrotizing granulomas, strongly resembling human TB lesions. However, IFN-gamma-deficient (GKO) and IFN-gamma-receptor-deficient (GRKO) mice did not develop granuloma necrosis following M. avium infection. Comparison of differentially expressed genes in infected WT and GKO lungs by DNA microarray and RNase protection assays revealed that the angiostatic chemokines CXCL9-11 were significantly reduced in GKO mice. In contrast, angiogenic mediators such as angiopoietin and vascular endothelial growth factor, and angiogenic chemokines such as CXCL2, CCL3, and CCL4, remained unchanged or were expressed at higher levels than in infected WT mice, suggesting impaired neovascularization of the granuloma as a possible mechanism for caseation in WT mice. Granuloma vascularization was significantly decreased in central, but not peripheral, areas of granulomas of infected WT compared to GKO mice. In contrast to GRKO mice, WT mice showed signs of severe hypoxia in cells immediately surrounding the necrotic core of granulomas as measured immunohistochemically with a reagent detecting pimonidazole adducts. To test the hypothesis that CXCR3, the common receptor for the angiostatic chemokines CXCL9-11, is involved in granuloma caseation, histomorphology was assessed in M. avium-infected mice deficient for CXCR3 (CXCR3-KO). 16 weeks after infection, these mice developed caseating granulomas similar to WT mice. We conclude that IFN-gamma causes a dysbalance between angiostatic and angiogenic mediators and a concomitant reduction in granuloma vascularization, but that CXCR3-targeted chemokines are not sufficient to induce granuloma necrosis in a mouse model of mycobacteria-induced immunopathology.

Pathogenesis of Mycobacterium avium infection: typical responses to an atypical mycobacterium?

Studying infections with Mycobacterium avium in mouse models has allowed the dissection of the antimycobacterial pathways of the mammalian host. Whereas the paradigm of cell-mediated immunity to intracellular pathogens has been confirmed, namely with regard to the pivotal roles of CD4+ T cells, macrophages, and the IL12-IFNgamma cytokine axis, atypical features have been uncovered such as the resistance to NO, the involvement of minor players in the induction of type 1 protective immunity (such as TLR2, CD40, and CD30), and the development of immunopathology during the infection with highly virulent strains such as the development of caseous necrosis of granulomas or the progressive emergence of severe lymphopenia.