The RNA from Pseudomonas aeruginosa impairs neutrophil responses favoring bacterial survival

Epithelial and endothelial cells are essential in the modulation of innate immune responses in the lung, including the arrival of neutrophils (PMN), which are crucial cells for the antibacterial host defense. These cells are exposed to prokaryotic RNA (pRNA) during bacterial infections and pRNA has been shown to promote or attenuate the inflammatory response on different immune cells. Pseudomonas aeruginosa (PAE) can cause severe pneumonia and has several immune-evading mechanisms. The aim of this study was to determine the effects of the RNA from PAE (RNAPAE) on lung epithelial, endothelial cells and PMN, and its impact on bacterial elimination. For this purpose, we purified total RNAPAE, and used it as a stimulus to evaluate different functions on Calu-6, HMEC-1 and isolated human PMN. We found that RNAPAE neither induced a pro-inflammatory response on Calu-6 or HMEC-1, as measured by ICAM-1 surface expression, or IL-6 and IL-8 secretion. Also, RNAPAE failed to activate PMN, as measured by forward-scatter (FSC) increase, CD11b surface expression, chemotaxis and IL-8 secretion. Pre-stimulation with RNAPAE diminished CD11b surface expression, chemotaxis and microbicidal activity when PMN were challenged with live bacteria. Moreover, we found that phagocytosis was affected in the presence of RNAPAE. Fragments of short RNA (<200 bp) were responsible for the PMN microbicidal attenuation during bacterial elimination. In conclusion, our results indicated that short fragments of RNAPAE diminished the immune response on PMN even in the presence of live bacteria. AUTHOR SUMMARYPseudomonas aeruginosa (PAE) pneumonia constitutes a major problem for human health. Therapies are frequently inefficient due to immune evasion mechanisms of PAE. Therefore, it is imperative to understand the relationship between PAE (or its components) with the immune system to improve therapeutic strategies. Since some bacterial RNA are immunosuppressive, our hypothesis was that the RNA from PAE (RNAPAE) might negatively modulate the immune response in a lung infection. We investigated the effects of the RNAPAE on lung epithelial, and microvascular endothelial cells, central cells that respond to PAE early during infection, and on neutrophils (PMN), the first immune cell that arrives at the site of infection. We found that RNAPAE failed to induce any response on pulmonary epithelium, endothelium, or PMN. Moreover, RNAPAE-treated PMN showed reduced migration, activation, and bactericidal response against live bacteria. Exploring deeper into this phenomenon, we found that increased bacterial survival was due to a lower phagocytic capacity of RNAPAE-treated PMN. Our results indicate that RNAPAE may act as another evasion strategy to favor PAEs survival in a pulmonary infection. Understanding the mechanisms by which PAE reduces the response of cells that participate in pulmonary immunity is crucial for planning interventions that may benefit infected patients.