Inflammasome regulation during bacterial and fungal superinfection in cystic fibrosis - 20/03/24

Cystic fibrosis is a genetic disorder associated with mutations in the Cftr gene, resulting in the production of thick and sticky mucus that can lead to various health complications, primarily affecting the respiratory and digestive systems. Pseudomonas aeruginosa and Aspergillus fumigatus are frequently identified microorganisms in cystic fibrosis (CF) patients. Superinfection by A. fumigatus in patients already colonized by P. aeruginosa causes hypersecretion of the inflammasome-dependent cytokine IL-1β. Unfortunately, high IL-1β release contributes to pulmonary damage in CF patients and decreases lung functions. The detrimental consequences of inflammasome overactivation pose a significant threat to CF patients. Therefore, more studies are needed to fully understand the complex interactions between inflammasomes, bacterial and fungal superinfections, and inflammation in CF.

The regulation of the inflammasome is studied in vitro during the superinfection by A. fumigatus of macrophages, derived from mice bone marrow and bronchoalveolar lavage, which are infected by the bacterium P. aeruginosa. The activation of the inflammasome is assessed through the analysis of caspase-1 protein cleavage, secretion of the cytokine IL-1β, and induction of cell death (pyroptosis) in infected and control macrophages. To decipher the molecular mechanisms enabling the overactivation of the inflammasome during a bacterial-fungal superinfection, wild-type and transgenic macrophages deleted in immune response and inflammasome signaling pathways, as well as inhibitors, have been used. We also explored the implication of the CFTR protein in inflammasome activation during the superinfection by using Cftr-/- and ΔF508d/d macrophages. Mutant bacterial and aspergillus strains are also employed to characterize the bacterial and fungal patterns involved in this inflammasome overactivation.

In our study, primary infection with P. aeruginosa showed a potentiation of macrophages allowing the overactivation of the inflammasome in response to a secondary infection with A. fumigatus. Indeed, fungal superinfection of these cells potentiated by the bacterium leads to greater NLRP3 inflammasome, caspases -1 and -8 activation, and heightened secretion of IL-1β. Results show that overactivation of the inflammasome during fungal superinfection is specific to a primary infection with the bacterium P. aeruginosa. The involvement of Cftr gene mutations in inflammasome activation during superinfection appears to depend on the mutation class. Different pathogen-associated molecular patterns (PAMPs) of P. aeruginosa and A. fumigatus are involved in this mechanism, such as type IV pili, flagellin, type II and type III secretion systems of P. aeruginosa, and galactosaminogalactan of A. fumigatus.

A primary infection with P. aeruginosa results in the potentiation of macrophages, enabling the overactivation of the inflammasome and an excessive secretion of IL-1β in response to a secondary infection by A. fumigatus. This could explain the worsening of pulmonary functions in co-infected patients. Results regarding a specific inflammasome response during superinfection based on Cftr mutations need to be confirmed. Overall, studying the immune response triggered by this interplay between bacteria and fungus in cystic fibrosis is crucial and further investigations are necessary.