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Purpose of reviewStudy of organ crosstalk in critical illness has uncovered complex biological communication between different organ systems, but the role of microbiota in organ crosstalk has received limited attention. We highlight the emerging understanding of the gut-lung axis, and how the largest biomass of the human body in the gut may affect lung physiology in critical illness.Recent findingsDisruption of healthy gut microbial communities and replacement by disease-promoting pathogens (pathobiome) generates a maladaptive transmitter of messages from the gut to the lungs, connected via the portal venous and the mesenteric lymphatic systems. Gut barrier impairment allows for microbial translocation (living organisms or cellular fragments) to the lungs. Host-microbiota interactions in the gut mucosa can also impact lung physiology through microbial metabolite secretion or host-derived messengers (hormones, cytokines or immune cells). Clinical examples like the prevention of ventilator-associated pneumonia by selective decontamination of the digestive tract show that the gut-lung axis can be manipulated therapeutically.SummaryA growing body of evidence supports the pathophysiological relevance of the gut-lung axis, yet we are only at the brink of understanding the therapeutic and prognostic relevance of the gut microbiome, metabolites and host-microbe interactions in critical illness.

Justin Poon,1 Michael Campos,2 Robert F Foronjy,1 Sridesh Nath,1 Gayatri Gupta,1 Christopher Railwah,1 Abdoulaye J Dabo,1 Nathalie Baumlin,3 Matthias Salathe,3 Patrick Geraghty11Division of Pulmonary & Critical Care Medicine, Department of Medicine, State University of New York Downstate Medical Center, Brooklyn, NY, USA; 2Division of Pulmonary, Critical Care, and Sleep Medicine, University of Miami, Miami, FL, USA; 3Department of Internal Medicine, University of Kansas Medical Center, Kansas City, KS, USABackground: Viral infections are considered a major driving factor of chronic obstructive pulmonary disease (COPD) exacerbations and thus contribute to disease morbidity and mortality. Respiratory syncytial virus (RSV) is a frequently detected pathogen in the respiratory tract of COPD patients during an exacerbation. We previously demonstrated in a murine model that leukemia inhibitory factor (LIF) expression was increased in the lungs during RSV infection. Subduing LIF signaling in this model enhanced lung injury and airway hypersensitivity. In this study, we investigated lung LIF levels in COPD patient samples to determine the impact of disease status and cigarette smoke exposure on LIF expression.Materials and methods: Bronchoalveolar lavage fluid (BALF) was obtained from healthy never smokers, smokers, and COPD patients, by written informed consent. Human bronchial epithelial (HBE) cells were isolated from healthy never smokers and COPD patients, grown at the air–liquid interface and infected with RSV or stimulated with polyinosinic:polycytidylic acid (poly (i:c)). Mice were exposed to cigarette smoke daily for 6 months and were subsequently infected with RSV. LIF expression was profiled in all samples.Results: In human BALF, LIF protein was significantly reduced in both smokers and COPD patients compared to healthy never smokers. HBE cells isolated from COPD patients produced less LIF compared to never smokers during RSV infection or poly (i:c) stimulation. Animals exposed to cigarette smoke had reduced lung levels of LIF and its corresponding receptor, LIFR. Smoke-exposed animals had reduced LIF expression during RSV infection. Two possible factors for reduced LIF levels were increased LIF mRNA instability in COPD epithelia and proteolytic degradation of LIF protein by serine proteases.Conclusions: Cigarette smoke is an important modulator for LIF expression in the lungs. Loss of LIF expression in COPD could contribute to a higher degree of lung injury during virus-associated exacerbations.Keywords: chronic obstructive pulmonary disease, leukemia inhibitory factor, respiratory syncytial virus, cigarette smoke