PhD Candidate, University of Sydney
The role of inflammatory caspases in macrophage defence against mycobacterial infection
Tuberculosis is currently the worlds deadliest disease. Macrophages are the first cells to fight tuberculosis when it enters the body. Unfortunately, many macrophages instead of killing the tuberculosis, become trojan horses, housing the bacteria and allowing it to spread. One reason for this is a breakdown in macrophage self-surveillance and defence capabilities. While inflammatory caspases within macrophages are known to play powerful anti-tuberculosis roles in acute infection, their roles over time, how they are regulated by host and bacteria alike, and more particularly their role in human, as opposed to mouse studies, remains unknown. My research focuses upon the metabolism and cell death mechanisms mediated by human inflammatory caspases, through a use of knockout human macrophage cell lines, coupled with metabolic investigations and my own novel cell death assay. My research casts the magnifying glass over the intricate host innate immunity:pathogen relationship. By investigating the role of human inflammatory caspases, major sources of anti-tuberculosis weaponry, scientists are able to gain insight into why macrophages fail to defend the body against tuberculosis invasion. As knowledge is power, knowing how our macrophage weaponry is broken, is our best starting point for learning how to fix it. Tuberculosis is a human enemy that has been winning too long. My research is the next step in crippling this enemy
Abstract: Humans encode two inflammatory caspases that detect cytoplasmic LPS, caspase-4 and caspase-5. When activated, these trigger pyroptotic cell death and caspase-1-dependent IL-1β production; however the mechanism underlying this process is not yet confirmed. We now show that a specific NLRP3 inhibitor, MCC950, prevents caspase-4/5-dependent IL-1β production elicited by transfected LPS. Given that both caspase-4 and caspase-5 can detect cytoplasmic LPS, it is possible that these proteins exhibit some degree of redundancy. Therefore, we generated human monocytic cell lines in which caspase-4 and caspase-5 were genetically deleted either individually or together. We found that the deletion of caspase-4 suppressed cell death and IL-1β production following transfection of LPS into the cytoplasm, or in response to infection with Salmonella typhimurium. Although deletion of caspase-5 did not confer protection against transfected LPS, cell death and IL-1β production were reduced after infection with Salmonella. Furthermore, double deletion of caspase-4 and caspase-5 had a synergistic effect in the context of Salmonella infection. Our results identify the NLRP3 inflammasome as the specific platform for IL-1β maturation, downstream of cytoplasmic LPS detection by caspase-4/5. We also show that both caspase-4 and caspase-5 are functionally important for appropriate responses to intracellular Gram-negative bacteria.
Pub.: 16 Jul '15, Pinned: 29 Aug '17
Abstract: Inflammatory responses mediated by macrophages are part of the innate immune system, whose role is to protect against invading pathogens. Lipopolysaccharide (LPS) found in the outer membrane of Gram-negative bacteria stimulate an inflammatory response by macrophages. During the inflammatory response, extracellular LPS are recognized by Toll-like receptor 4 (TLR4), one of the pattern recognition receptors (PRRs) that activates inflammatory signaling pathways and leads to the production of inflammatory mediators. The innate immune response is also triggered by intracellular inflammasomes, and inflammasome activation induces pyroptosis and the secretion of pro-inflammatory cytokines such as interleukin-1beta (IL-1β) and IL-18 by macrophages. Cysteine-aspartic protease (caspase)-11 and the human orthologues caspase-4/caspase-5 were recently identified as components of the 'non-canonical inflammasome' that senses intracellular LPS derived from Gram-negative bacteria during macrophage-mediated inflammatory responses. Direct recognition of intracellular LPS facilitates the rapid oligomerization of caspase-11/4/5, which results in pyroptosis and the secretion of IL-1β and IL-18. LPS are released into the cytoplasm from Gram-negative bacterium-containing vacuoles by small interferon (IFN)-inducible guanylate-binding proteins encoded on chromosome 3 (GBP(chr3) )-mediated lysis of the vacuoles. In vivo studies have clearly shown that caspase-11(-/-) mice are more resistant to endotoxic septic shock by excessive LPS challenge. Given the evidence, activation of caspase-11 non-canonical inflammasomes by intracellular LPS is distinct from canonical inflammasome activation and provides a new paradigm in macrophage-mediated inflammatory responses. This article is protected by copyright. All rights reserved.
Pub.: 12 Jul '17, Pinned: 29 Aug '17
Abstract: Recent advances in immunometabolism link metabolic changes in stimulated macrophages to production of IL-1β, a crucial cytokine in the innate immune response to Mycobacterium tuberculosis. To investigate this pathway in the host response to M. tuberculosis, we performed metabolic and functional studies on human alveolar macrophages, human monocyte-derived macrophages, and murine bone marrow-derived macrophages following infection with the bacillus in vitro. M. tuberculosis infection induced a shift from oxidative phosphorylation to aerobic glycolysis in macrophages. Inhibition of this shift resulted in decreased levels of proinflammatory IL-1β and decreased transcription of PTGS2, increased levels of anti-inflammatory IL-10, and increased intracellular bacillary survival. Blockade or absence of IL-1R negated the impact of aerobic glycolysis on intracellular bacillary survival, demonstrating that infection-induced glycolysis limits M. tuberculosis survival in macrophages through induction of IL-1β. Drugs that manipulate host metabolism may be exploited as adjuvants for future therapeutic and vaccination strategies.
Pub.: 14 Feb '16, Pinned: 29 Aug '17
Abstract: The tuberculous granuloma is an elaborately organized structure and one of the main histological hallmarks of tuberculosis. Macrophages, which are important immunologic effector and antigen-presenting cells, are the main cell type found in the tuberculous granuloma and have high plasticity. Macrophage polarization during bacterial infection has been elucidated in numerous recent studies; however, macrophage polarization during tuberculous granuloma formation and development has rarely been reported. It remains to be clarified whether differences in the activation status of macrophages affect granuloma formation. In this study, the variation in macrophage polarization during the formation and development of tuberculous granulomas was investigated in both sections of lung tissues from tuberculosis patients and an in vitro tuberculous granuloma model. The roles of macrophage polarization in this process were also investigated. Mycobacterium tuberculosis (M. tuberculosis) infection was found to induce monocyte-derived macrophage polarization. In the in vitro tuberculous granuloma model, macrophage transformation from M1 to M2 was observed over time following M. tuberculosis infection. M2 macrophages were found to predominate in both necrotic and non-necrotic granulomas from tuberculosis patients, while both M1 and M2 polarized macrophages were found in the non-granulomatous lung tissues. Furthermore, it was found that M1 macrophages promote granuloma formation and macrophage bactericidal activity in vitro, while M2 macrophages inhibit these effects. The findings of this study provide insights into the mechanism by which M. tuberculosis circumvents the host immune system as well as a theoretical foundation for the development of novel tuberculosis therapies based on reprogramming macrophage polarization.
Pub.: 20 Jun '15, Pinned: 29 Aug '17