Infection by Pseudomonas aeruginosa, and bacteria in general, frequently promotes acidification of the local microenvironment, and this is reinforced by pulmonary exertion and exacerbation. However, the consequence of an acidic environment on the host inflammatory response to P. aeruginosa infection is poorly understood. Here we report that the pivotal cellular and host proinflammatory interleukin-1␤ (IL-1␤) response, which enables host clearance of the infection but can produce collateral inflammatory damage, is increased in response to P. aeruginosa infection within an acidic environment. Synergistic mechanisms that promote increased IL-1␤ release in response to P. aeruginosa infection in an acidic environment are increased pro-IL-1␤ induction and increased caspase-1 activity, the latter being dependent upon a functional type III secretion system of the bacteria and the NLRC4 inflammasome of the host. Using an in vivo peritonitis model, we have validated that the IL-1␤ inflammatory response is increased in mice in response to P. aeruginosa infection within an acidic microenvironment. These data reveal novel insights into the regulation and exacerbation of inflammatory responses to P. aeruginosa.

Acidic microenvironments commonly occur at sites of inflammation and bacterial infections. In the context of a Pseudomonas aeruginosa infection, we previously demonstrated that acidosis enhances the cellular proinflammatory interleukin (IL)-1β response in vitro. However, how pH alterations affect in vivo IL-1β responses and subsequent IL-1-driven inflammation during infection with P. aeruginosa is unclear. Here, we report that acidosis enhances in vivo IL-1β production and downstream IL-1 receptor-dependent responses during infection with P. aeruginosa in models of acute pneumonia and peritonitis. Importantly, we demonstrate that infection with P. aeruginosa within an acidic environment leads to enhanced production of a subset of proinflammatory cytokines, including chemokine (C-X-C) motif ligand 1, IL-6, and chemokine (C-C motif) ligand 2, and increased neutrophil recruitment. Furthermore, with the use of IL-1 receptor type 1-deficient mice, we identify the contribution of the IL-1 signaling pathway to the acidosis-enhanced inflammatory response and pathology. These data provide insights into the potential benefit of pH regulation during bacterial infections to control disease progression and immunopathology.

Bacterial infection can lead to acidosis of the local microenvironment, which is believed to exacerbate disease pathogenesis; however, the mechanisms by which changes in pH alter disease progression are poorly understood. We test the hypothesis that acidosis enhances respiratory epithelial cell death in response to infection with Our findings support the idea that acidosis in the context of infection results in increased epithelial cell cytotoxicity due to ExoU intoxication. Importantly, enforced maintenance of neutral pH during infection demonstrates that cytotoxicity is dependent on the acidosis. Investigation of the underlying mechanisms revealed that host cell cytotoxicity correlated with increased bacterial survival during an acidic infection that was due to reduced bactericidal activity of host-derived antimicrobial peptides. These findings extend previous reports that the activities of antimicrobial peptides are pH-dependent and provide novel insights into the consequences of acidosis on infection-derived pathology. Therefore, this report provides the first evidence that physiological levels of acidosis increase the susceptibility of epithelial cells to acute infection and demonstrates the benefit of maintaining pH homeostasis during a bacterial infection.

MCJ (DNAJC15) is a mitochondrial protein that regulates the mitochondrial metabolic status of macrophages and their response to inflammatory stimuli. CpG island methylation in cancer cellsMCJ (Methylation-Controlled J protein), also known as DNAJC15, is a small protein (147 aa) that contains a highly conserved 70 aa J domain at the C-terminus, an unusual transmembrane domain, and an N-terminal region with no homology to any other known protein [1][2][3] . The MCJ gene originated as a gene-duplication from the related gene DnaJC19, already present in flies 2 . MCJ is located in the inner mitochondrial membrane where it interacts with Complex I of the electron transport chain (ETC), interfering with the formation of supercomplexes composed of complexes I, III and IV 4,5 . MCJ is the first described endogenous negative regulator of Complex I that has also been associated with the TIM23 translocase and the import of pre-proteins to the mitochondria 3 . Silencing MCJ expression does not affect cell survival or proliferation 5 . However, loss of MCJ results in augmented mitochondrial membrane potential, increased oxidative respiration and mitochondrial ATP 5 . Although MCJ deficiency has no harmful effects under physiological conditions, increased mitochondrial metabolism in the absence of MCJ in vivo prevents the pathological accumulation of lipids in the liver during starvation or high cholesterol diet, and the development of liver steatosis 5 . MCJ is thus a modulator of mitochondrial