Mycobacterium massiliense Induces Macrophage Extracellular Traps with Facilitating Bacterial Growth

Abstract

Human neutrophils have been known to release neutrophil extracellular traps (NETs), antimicrobial DNA structures capable of capturing and killing microbes. Recently, a similar phenomenon has been reported in macrophages infected with various pathogens. However, a role for macrophages extracellular traps (METs) in host defense responses against Mycobacterium massiliense (M. mass) has yet to be described. In this study, we show that M. mass, a rapid growing mycobacterium (RGM), also induces the release of METs from PMA-differentiated THP-1 cells. Intriguingly, this process is not dependent on NADPH oxidase activity, which regulates NET formation. Instead, M. mass-induced MET formation partially depends on calcium influx and requires phagocytosis of high bacterial load. The METs consist of a DNA backbone embedded with microbicidal proteins such as histone, MPO and elastase. Released METs entrap M. mass and prevent their dissemination, but do not have bactericidal activity. Instead, they result in enhanced bacterial growth. In this regard, METs were considered to provide interaction of M. mass with cells and an environment for bacterial aggregation, which may facilitate mycobacterial survival and growth. In conclusion, our results demonstrate METs as an innate defense response against M. mass infection, and suggest that extracellular traps play a multifaceted role in the interplay between host and bacteria.

Macrophage Extracellular Traps: A Scoping Review

“…Shen et al [40] found similar results with AAPVtreated mouse peritoneal macrophages infected with S. aureus. In contrast, Je et al [26] noted AAPV-treated THP-1 macrophage-like cells infected with M. massili ense were unaffected with regard to MET release despite elastase being identified within the MET structures. Muñoz-Caro et al [23] inhibited MET release by bovine monocytes infected with Besnoitia besnotii trachyzoites using the MPO inhibitor, 4-aminobenzoic acid hydrazide (ABAH).…”

“…Additional verification that proposed structures are ETs can be obtained by staining for known ET components such as histones, elastase, or MPO. Elastase, often considered a neutrophil-specific marker, has been identified in the METs of human peripheral-blood monocytes and in THP-1 macrophage-like cells [22,26]. Similarly, MPO has been identified in the METs of diverse macrophage populations including human glomerular macrophages, human peripheral-blood monocytes, THP-1 macrophage-like cells, murine J774A.1 macrophage-like cells, bovine monocytes, and caprine monocytes [22,23,[26][27][28][29] (Table 1).…”

“…In order to separate killing that results from MET-dependent and MET-independent mechanisms (mainly phagocytosis), some studies have used the actin cytoskeletal inhibitor, cytochalasin D, to inhibit macrophage phagocytosis [24,28]. There have been mixed results using this technique as some groups have noted decreased MET release in cells pretreated with cytochalasin D [25,26]. To date, there are reports of potential MET microbicidal activity against pathogens including Staphylococcus aureus, S. agalactiae, E. coli, and C. albicans [28,[38][39][40].…”

“…As with neutrophils, there are reports of alternative pathways of METosis [26,28]. Je et al [26] suggested that calcium influx into PMA-differentiated THP-1 macrophage-like cells may influence MET production by means of an NADPH oxidase-independent mechanism.…”

“…Je et al [26] suggested that calcium influx into PMA-differentiated THP-1 macrophage-like cells may influence MET production by means of an NADPH oxidase-independent mechanism. Treatment of THP-1 cells with 20 μM 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid or 1 μM ethylene glycol tetraacetic acid to chelate calcium diminished, but did not eliminate, MET release.…”