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

Abstract

eThe 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 1␣ (HIF-1␣) under the Cre-lox system regulated by the lysozyme M gene promoter was used to determine the relevance of HIF-1␣ in the caseation of granulomas. The genetic ablation of HIF-1␣ 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-1␣ in a necrotizing mouse model. Our results show that interfering with macrophage adaptation to hypoxia, such as through HIF-1␣ inactivation, accelerates granuloma necrosis.T he main pathological feature of human tuberculosis is the induction by Mycobacterium tuberculosis of granulomas which undergo central necrosis (caseous necrosis) (1). The mechanisms underlying the latter phenomenon are not clear, but it is generally accepted that caseation plays an essential role in the pathogenesis and dissemination of the infection. Caseous necrosis and cavitation during infection by M. tuberculosis is more prominent in rabbits and guinea pigs than in mice (2). Nevertheless, human-like pathology, as central granuloma necrosis and associated hypoxia, can be obtained in M. tuberculosis-infected I/StSnEgYCit, C3HeB/ FeJ, or IL-13-overexpressing C57BL/6 mouse strains (3-5). Confluent necrotic granulomas also are observed in mice deficient in gamma interferon (IFN-␥) or the type 1 tumor necrosis factor (TNF) receptor (2, 6, 7). However, the fact that immunodeficient human hosts, namely, AIDS patients, tend to present forms of tuberculosis with granulomas lacking necrotic features suggests that necrosis depends on the immune response (8).We have shown that mice infected with a small inoculum of M. avium ATCC 25291 establish a chronic and progressive infection characterized by the formation of discrete granulomas, which gradually increase in size and undergo central necrosis within a few months (9). This requires CD4 ϩ T cells as well as an intact interleukin-12 (IL-12)/IFN-␥ cytokine axis (9) but is independent...

Interleukin-17 limits hypoxia-inducible factor 1α and development of hypoxic granulomas during tuberculosis

“…That MDSCs themselves have been implicated as IL-17 producers during Mtb infection (37) suggests that IL-17 made by MDSCs may be a necessary feedback loop to limit excessive HIF1α production in MDSCs. Interestingly, in a systemic Mycobacterium avium model of infection, mice deficient in HIF1α expression in myeloid cells demonstrated increased liver necrosis (39). A recent study also showed that in a high-dose model of Mtb Erdman infection, mice deficient in HIF1α in myeloid cells demonstrated increased susceptibility to infection (40).…”

HIF-1α Is an Essential Mediator of IFN-γ–Dependent Immunity to Mycobacterium tuberculosis

“…First, it was demonstrated that pharmacological stabilization of HIF-1α during infection of zebrafish embryos with M. marinum leads to a reduced bacterial burden at early timepoints (19). Second, HIF-1α was found to play a role in a mouse model of granuloma caseation in livers of mice infected with M. avium (20). This granuloma formation depends upon hypoxia but is independent of effectors of IFN-γ based immunity, including NO.…”

“…Exogenously increasing the levels of active HIF-1α using pharmacological or genetic tools in zebrafish embryos enhances bactericidal activity against Mycobacterium marinum (19). Mice lacking HIF-1α in the myeloid lineage exhibit more rapid progression of hypoxic granulomatous lesions in the liver following intravenous infection with Mycobacterium avium (20). These data suggest the intriguing possibility that HIF-1α may be an important mediator of resistance to M. tuberculosis infection.…”

Hypoxia inducible factor 1α: A critical factor for the immune response to pathogens and Leishmania

“…Following this study, several other groups reported the importance of HIF-1α in pathogen clearance by myeloid cells during bacterial infections. For instance, myeloid conditional HIF-1α knockout mice fail to clear Mycobacterium infection and develop earlier emergence of necrosis in the granuloma [102]. HIF-1α is also required for effector neutrophil activity in keratitis and is fundamental for effective bacterial killing and antimicrobial peptide production during Pseudomonas aeruginosa infection [103].…”