Translational Study: Propionate, A Short-chain Fatty Acid Product of the Gut Microbiota, Reduces Inflammatory Responses to Lipopolysaccharide-induced Injury in Human Lung Bronchoalveolar Lavage and Tissue

Abstract Rationale: The effects of gut microbiota-derived short-chain fatty acid (SCFA) products on the immunometabolism of the resting lung via the gut-lung axis are a topic of great interest currently. Using a murine lung ischemia-reperfusion (IR) model and the lipopolysaccharide (LPS)-induced injury in vivo and in vitro, we previously reported that high fiber diets and associated gut microbiome produced SCFA propionate (C3) reduced the levels of pro-inflammatory cytokines such as IL-6, IL-1β after lung IR injury. As a next step, we conducted a translational study to determine whether propionate has the ability to moderate the inflammatory response in human BAL alveolar macrophages (AM) and human lung tissue after exposure to LPS. Methods: Bronchoalveolar lavage (BAL) cells were isolated from the right upper lobe of transplant rejected human lungs. The BAL cells were then plated in a 48-well plate at 50,000 cells/well for 2 hours before the attached cells (predominantly AM) were exposed overnight to LPS and/or propionate at different concentrations. The supernatants were collected and IL-6 measured by ELISA. In addition, the same lung lobe after BAL was cut into pieces and immersed in LPS, then incubated at 37° C for 2-18h with slow shaking. The lung tissues were collected, and tissue homogenization was performed to measure the level of IL-1β mRNA expression by qPCR or IL-6 secretion by tissue ELISA. Results: We have processed human lung samples from four donors in this study. We performed ELISAs on the supernatants of alveolar macrophages isolated from 4 human lungs and qPCRs on human lung tissues of one human lung. All the supernatants of alveolar macrophages and human lung tissues showed increased level of the inflammatory cytokines IL-6 or IL-1β after stimulation with LPS. The supernatants of AMs from 3 out of 4 human lungs showed significantly lower levels of IL-6 in the samples exposed to LPS and propionate than in the samples exposed to LPS alone. The inflammatory markers measured were also lower in the human lung tissue samples exposed to LPS and propionate than in those exposed to LPS only (Figure 1). Conclusions: LPS can stimulate alveolar macrophages and human lung tissue to release inflammatory cytokines. Consistent with in vitro and in vivo studies, propionate has the ability to reduce this cytokine production and thereby potentially protect the human lungs from anticipated lung injury during surgery or modulate lung injury responses in patients at risk.