On the killing of mycobacteria by macrophages

Molecular Pathogenesis Centre, Unit of Retrovirus and Associated Infections, Faculty of Pharmacy, University of Lisbon, Av. Forcas Armadas, 1600-083 Lisbon, Portugal.
Cellular Microbiology (Impact Factor: 4.92). 03/2008; 10(2):529-48. DOI: 10.1111/j.1462-5822.2007.01067.x
Source: PubMed


Both pathogenic and non-pathogenic mycobacteria are internalized into macrophage phagosomes. Whereas the non-pathogenic types are invariably killed by all macrophages, the pathogens generally survive and grow. Here, we addressed the survival, production of nitrogen intermediates (RNI) and intracellular trafficking of the non-pathogenic Mycobacterium smegmatis, the pathogen-like, BCG and the pathogenic M. bovis in different mouse, human and bovine macrophages. The bacteriocidal effects of RNI were restricted for all bacterial species to the early stages of infection. EM analysis showed clearly that all the mycobacteria remained within phagosomes even at late times of infection. The fraction of BCG and M. bovis found in mature phagolysosomes rarely exceeded 10% of total, irrespective of whether bacteria were growing, latent or being killed, with little correlation between the extent of phagosome maturation and the degree of killing. Theoretical modelling of our data identified two different potential sets of explanations that are consistent with our results. The model we favour is one in which a small but significant fraction of BCG is killed in an early phagosome, then maturation of a small fraction of phagosomes with both live and killed bacteria, followed by extremely rapid killing and digestion of the bacteria in phago-lysosomes.

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Available from: Luisa Jordao
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    • "In addition, actin filament assembly also plays a role in the pro-inflammatory response. Several signaling lipids, cAMP, extracellular ATP and the P2X7 receptor, were shown to be involved in actin assembly and the killing/survival of pathogenic mycobacteria (Kalamidas et al., 2006; Treede et al., 2007; Jordao et al., 2008a,b; Kühnel et al., 2008; Kuehnel et al., 2009). Furthermore, some lipid effectors that regulate actin assembly also control NF-κB, a transcription factor involved in the pro-inflammatory response (Gutierrez et al., 2009). "
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    ABSTRACT: Mycobacterium tuberculosis (Mtb) is a successful intracellular pathogen that thrives in macrophages (Mφs). There is a need to better understand how Mtb alters cellular processes like phagolysosome biogenesis, a classical determinant of its pathogenesis. A central feature of this bacteria's strategy is the manipulation of Mφ actin. Here, we examined the role of microRNAs (miRNAs) as a potential mechanism in the regulation of actin-mediated events leading to phagocytosis in the context of mycobacteria infection. Given that non-virulent Mycobacterium smegmatis also controls actin filament assembly to prolong its intracellular survival inside host cells, we performed a global transcriptomic analysis to assess the modulation of miRNAs upon M. smegmatis infection of the murine Mφ cell line, J774A.1. This approach identified miR-142-3p as a key candidate to be involved in the regulation of actin dynamics required in phagocytosis. We unequivocally demonstrate that miR-142-3p targets N-Wasp, an actin-binding protein required during microbial challenge. A gain-of-function approach for miR-142-3p revealed a down-regulation of N-Wasp expression accompanied by a decrease of mycobacteria intake, while a loss-of-function approach yielded the reciprocal increase of the phagocytosis process. Equally important, we show Mtb induces the early expression of miR-142-3p and partially down-regulates N-Wasp protein levels in both the murine J774A.1 cell line and primary human Mφs. As proof of principle, the partial siRNA-mediated knock down of N-Wasp resulted in a decrease of Mtb intake by human Mφs, reflected in lower levels of colony-forming units (CFU) counts over time. We therefore propose the modulation of miRNAs as a novel strategy in mycobacterial infection to control factors involved in actin filament assembly and other early events of phagolysosome biogenesis.
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    • "In contrast, monocytes from human peripheral blood did not to release DNA when stimulated in a fashion that normally evokes NET formation in neutrophils (Fuchs et al., 2007). Whereas neutrophils are indispensible in the defense against most extracellular bacteria, the monocyte/macrophage lineage is vital in eliminating bacteria with capacity to persist inside phagocytes, for example, mycobacteria (van der Wel et al., 2007; Jordao et al., 2008). "
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    ABSTRACT: Mononuclear phagocytes, that is, monocytes and macrophages, are central in the defense against mycobacteria. Mycobacterium abscessus is an opportunistic mycobacterial species able to cause chronic pulmonary infections in patients with cystic fibrosis but also soft tissue infections in immunocompetent individuals. Pathogenic isolates of M. abscessus with rough colony morphology form cord-like aggregates. In this study, we investigated the in vitro response of human peripheral blood mononuclear cells (PBMCs) from healthy blood donors to cord-forming M. abscessus strains from cystic fibrosis patients with clinical lung infection. Microscopic examination revealed that the PBMCs produced a coarse fibrous meshwork containing DNA and histones, which surrounded the mycobacterial cords. Thus, the bacterial cord formations were entrapped by monocytes and lymphocytes aggregated onto the DNA-rich meshwork fibers. Mycobacterium abscessus strains with smooth colony morphology, which do not form cords and are readily phagocytosed, did not induce any meshwork formation in PBMCs. The chromatin meshwork may represent a defense mechanism against nondigestible invaders.
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    • "resides largely within a phagosome-like compartment of host macrophages during infection (Jordao et al., 2008). Inflammasome activation it is postulated to occur only if these effectors or products from a signalling cascade reach the cytosol and gain access to NLRs. "

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