Efferocytosis and extrusion of leukocytes determine the progression of early mycobacterial pathogenesis

Abstract

Macrophages and neutrophils are the first responders to invading pathogens and contribute strongly to the host defense against intracellular pathogens. The collective interplay and dynamic interactions between these leukocytes are to a large extent not understood. In the present study, we have investigated their role using a combination of confocal laser-scanning and electron microscopy in a zebrafish model for tuberculosis, a local Mycobacterium marinum infection in the tissue of the larval tail fin. Our results show that neutrophils are efficient in phagocytosis of mycobacteria and that they contribute largely to their dissemination. Macrophages appear to play a major role in efferocytosis, phagocytosis of dead cells that contain bacterial content. Phagocytic cells with large bacterial aggregates are formed that can be extruded out of the tissue after cell death. Alternatively, these excessively infected cells can undergo necrosis leading to immediate recruitment of surrounding leukocytes and subsequent phagocytosis of released bacteria. Our data show that these necrotic burst events result in progression of the infection, whereas extrusion abates the infection.

The developmental cycle ofDictyostelium discoideumensures curing of a mycobacterial infection at both cell-autonomous level and by collaborative exclusion

“…During lytic release, D. discoideum cells died and bacteria frequently remained trapped within the debris ( Fig 3B), facilitating re-uptake by bystander cells (Fig 3D). This process is important to spread the infection, and has already been described in human phagocytes (Hosseini et al, 2016;Mahamed et al, 2017). During non-lytic egress, M. marinum was released without compromising host viability ( Fig 3C).…”

Hif-1alpha induced expression of Il-1beta protects against mycobacterial infection in zebrafish

“…The initial phase of Mm infection in zebrafish is characterised by a short period of greatly increased pro-inflammatory signalling (before 1 day post-infection, dpi) where the immune system reacts to Mm infection. This is followed by a lag-phase of decreased activity which allows for granuloma formation between 2-3dpi, before cytokine levels rise again in formed larval granulomas at 4dpi (Benard et al , 2016; Hosseini et al , 2016). However, the levels of pro-inflammatory cytokines have only been previously studied at a transcriptional level in whole embryos or FACS sorted cells, rather than detecting levels in situ , over time, in an intact organism (Benard et al , 2016).…”

“…IL-1β is an important pro-inflammatory cytokine, but its processing by caspases also plays a crucial role in immune cell pyroptosis mediated by the inflammasome (Malik & Kanneganti, 2017). Recent findings in the Mm/zebrafish model indicate that neutrophils and macrophages can efficiently phagocytose bacteria and undergo rounds of cell death and re-uptake during the initial days of infection (Hosseini et al , 2016). Although here we show a role for early pro-inflammatory il-1β transcription during Mm infection, the role of Il-1β processing and inflammasome induced pyroptosis/cell death in these early Mm immune processes remain undetermined.…”

“…Greater understanding of the diverse phenotype of macrophages immediately after infection may allow therapeutic tuning to provide maximal early control of mycobacteria during infection (McClean & Tobin, 2016; Dorhoi & Kaufmann, 2015). Recent studies in optically translucent zebrafish infection models have indicated that initial interactions between Mm and macrophages and neutrophils are more complex than originally thought, with successive rounds of bacterial internalisation and leukocyte cell death leading to granuloma formation (Hosseini et al , 2016; Cambier et al , 2017; Elks et al , 2015). The immune molecular mechanisms involved in these early processes are poorly understood.…”

Mycophenolate mofetil increases susceptibility to opportunistic fungal infection independent of lymphocytes

“…The continued recruitment of uninfected macrophages is required for control of mycobacterial infection in zebrafish as depletion of uninfected macrophages (by clodronate or in the csfr1 mutant zebrafish) increases susceptibility to infection. A proposed mechanism for this requirement is the phagocytosis of extracellular mycobacteria released following infected macrophage or neutrophil cell death (Hosseini et al, 2016). This is distinct from the progression of cryptococcal infection where there is very little macrophage lysis (discussed below) and where phagocytosis of extracellular fungal cells in limited by their polysaccharide capsule, not by the number macrophages (Bojarczuk et al, 2016).…”