Mycobacterium abscessus-Induced Granuloma Formation Is Strictly Dependent on TNF Signaling and Neutrophil Trafficking

Abstract

Mycobacterium abscessus is considered the most common respiratory pathogen among the rapidly growing non-tuberculous mycobacteria. Infections with M. abscessus are increasingly found in patients with chronic lung diseases, especially cystic fibrosis, and are often refractory to antibiotic therapy. M. abscessus has two morphotypes with distinct effects on host cells and biological responses. The smooth (S) variant is recognized as the initial airway colonizer while the rough (R) is known to be a potent inflammatory inducer associated with invasive disease, but the underlying immunopathological mechanisms of the infection remain unsolved. We conducted a comparative stepwise dissection of the inflammatory response in S and R pathogenesis by monitoring infected transparent zebrafish embryos. Loss of TNFR1 function resulted in increased mortality with both variants, and was associated with unrestricted intramacrophage bacterial growth and decreased bactericidal activity. The use of transgenic zebrafish lines harboring fluorescent macrophages and neutrophils revealed that neutrophils, like macrophages, interact with M. abscessus at the initial infection sites. Impaired TNF signaling disrupted the IL8-dependent neutrophil mobilization, and the defect in neutrophil trafficking led to the formation of aberrant granulomas, extensive mycobacterial cording, unrestricted bacterial growth and subsequent larval death. Our findings emphasize the central role of neutrophils for the establishment and maintenance of the protective M. abscessus granulomas. These results also suggest that the TNF/IL8 inflammatory axis is necessary for protective immunity against M. abscessus and may be of clinical relevance to explain why immunosuppressive TNF therapy leads to the exacerbation of M. abscessus infections.

The Diverse Cellular and Animal Models to Decipher the Physiopathological Traits of Mycobacterium abscessus Infection

“…Overall, work using the zebrafish model confirmed the crucial role of MAB_4780 for Mabs cording to successfully establish acute and lethal infections. In addition, in studies evaluating a loss-of-function coupled with fluorescent reporter zebrafish lines and high resolution imaging, the contribution of TNF-mediated signaling to protective immunity against Mabs has been confirmed (Bernut et al, 2016). Furthermore, identifying the crucial role of TNF in activating macrophage bactericidal activity leading to restriction of intracellular Mabs growth and IL8-mediated neutrophil recruitment for development and maintenance of protective granulomas was demonstrated (Figure 1).…”

“…TNF-α plays a central role in the immunity to Mabs by activating the macrophage bactericidal response and, through IL-8 production, in neutrophil chemotaxis to the site of infection or to form protective granulomas. Adapted from Bernut et al (2016). …”

Humoral and Cellular Immune Response of European Seabass Dicentrarchus labrax Vaccinated with Heat‐Killed Mycobacterium marinum (iipA::kan Mutant)

“…The primary role of granulomas is preventing a widespread expansion of mycobacteria in the extracellular milieu (Bernut et al. ). The initial granulomatous structures observed after vaccination were present in the control groups throughout the 6‐month experimental period, suggesting that granulomas were unable to resolve (e.g., through phagolysosomal degradation) the physical presence of the killed mycobacteria during this period.…”

“…However, each species can be typified by a specific lipid/glycolipid signature, and subtle variations in lipid composition, structure, and size can affect granulomatogenesis (Bernut et al. ). Kinetics and functions of granulomas are thus species‐specific, and conclusions drawn from one mycobacterial species cannot always be extrapolated to others (Bernut et al.…”

“…Bernut et al. () demonstrated the crucial role of TNF signaling in a continuum of effects in Zebrafish embryos. Tumor necrosis factor modulated the engagement of neutrophils to the infection site and their subsequent recruitment to granulomas, which are essential for control of the early and later stages of Mycobacterium abscessus infection, respectively.…”