Drosophila melanogaster as an organism model for studying cystic fibrosis and its major associated microbial infections.

Cystic fibrosis (CF) is a human genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator gene that encodes a chloride channel. The most severe clinical manifestation is associated with chronic pulmonary infections by pathogenic and opportunistic microbes. Drosophila melanogaster has become the invertebrate model of choice for modeling microbial infections and studying the induced innate immune response. Here, we review its contribution to the understanding of infections with six major pathogens associated with CF (Staphylococcus aureus, Pseudomonas aeruginosa, Burkholderia cepacia, Mycobacterium abscessus, Streptococcus pneumoniae, and Aspergillus fumigatus) together with the perspectives opened by the recent availability of two CF models in this model organism.

Mycobacterium abscessus Opsonization Allows an Escape from the Defensin Bactericidal Action in Drosophila.

Mycobacterium abscessus, an intracellular nontuberculous mycobacterium, is considered the most pathogenic species among the group of rapidly growing mycobacteria. The resistance of M. abscessus to the host innate response contributes to its pathogenicity in addition to several virulence factors. We have recently shown in Drosophila that antimicrobial peptides (AMPs), whose production is induced by M. abscessus, are unable to control mycobacterial infection. This could be due to their inability to kill mycobacteria and/or the hidden location of the pathogen in phagocytic cells. Here, we demonstrate that the rapid internalization of M. abscessus by Drosophila macrophages allows it to escape the AMP-mediated humoral response. By depleting phagocytes in AMP-deficient flies, we found that several AMPs were required for the control of extracellular M. abscessus. This was confirmed in the Tep4 opsonin-deficient flies, which we show can better control M. abscessus growth and have increased survival through overproduction of some AMPs, including Defensin. Furthermore, Defensin alone was sufficient to kill extracellular M. abscessus both in vitro and in vivo and control its infection. Collectively, our data support that Tep4-mediated opsonization of M. abscessus allows its escape and resistance toward the Defensin bactericidal action in Drosophila. IMPORTANCE Mycobacterium abscessus, an opportunistic pathogen in cystic fibrosis patients, is the most pathogenic species among the fast-growing mycobacteria. How M. abscessus resists the host innate response before establishing an infection remains unclear. Using Drosophila, we have recently demonstrated that M. abscessus resists the host innate response by surviving the cytotoxic lysis of the infected phagocytes and the induced antimicrobial peptides (AMPs), including Defensin. In this work, we demonstrate that M. abscessus resists the latter response by being rapidly internalized by Drosophila phagocytes. Indeed, by combining in vivo and in vitro approaches, we show that Defensin is able to control extracellular M. abscessus infection through a direct bactericidal action. In conclusion, we report that M. abscessus escapes the host AMP-mediated humoral response by taking advantage of its internalization by the phagocytes.

Mycobacterium abscessus resists the innate cellular response by surviving cell lysis of infected phagocytes.

Mycobacterium abscessus is the most pathogenic species among the predominantly saprophytic fast-growing mycobacteria. This opportunistic human pathogen causes severe infections that are difficult to eradicate. Its ability to survive within the host was described mainly with the rough (R) form of M. abscessus, which is lethal in several animal models. This R form is not present at the very beginning of the disease but appears during the progression and the exacerbation of the mycobacterial infection, by transition from a smooth (S) form. However, we do not know how the S form of M. abscessus colonizes and infects the host to then multiply and cause the disease. In this work, we were able to show the hypersensitivity of fruit flies, Drosophila melanogaster, to intrathoracic infections by the S and R forms of M. abscessus. This allowed us to unravel how the S form resists the innate immune response developed by the fly, both the antimicrobial peptides- and cellular-dependent immune responses. We demonstrate that intracellular M. abscessus was not killed within the infected phagocytic cells, by resisting lysis and caspase-dependent apoptotic cell death of Drosophila infected phagocytes. In mice, in a similar manner, intra-macrophage M. abscessus was not killed when M. abscessus-infected macrophages were lysed by autologous natural killer cells. These results demonstrate the propensity of the S form of M. abscessus to resist the host's innate responses to colonize and multiply within the host.