Gut ischemia/reperfusion (I/R) injury is a common clinical problem associated with significant mortality and morbidities that result from systemic inflammation and remote organ dysfunction, typically acute lung injury. The mechanisms underlying the dissemination of gut-derived harmful mediators into the circulation are poorly understood. The objective of our study was to determine the role of mesenteric lymphatic circulation in the systemic and pulmonary inflammatory response to gut I/R. Using a murine intestinal I/R model, we evaluated whether and how blocking mesenteric lymph flow affects the inflammatory response in local tissues (gut) and remote organs (lungs). We further explored the mechanisms of post-I/R lymph-induced systemic inflammation by examining neutrophil activity and interaction with endothelial cells in vitro. Mice subjected to intestinal I/R displayed a significant inflammatory response in local tissues, evidenced by neutrophil infiltration into mucosal areas, as well as lung inflammation, evidenced by increased myeloperoxidase levels, neutrophil infiltration, and elevated microvascular permeability in the lungs. Mesenteric lymph duct ligation (MLDL) had no effect on gut injury per se, but effectively attenuated lung injury following gut I/R. Cell experiments showed that lymph fluid from post-I/R animals, but not pre-I/R, increased neutrophil surface CD11b expression and their ability to migrate across vascular endothelial monolayers. Moreover, post-I/R lymph upregulated neutrophil expression of pro-inflammatory cytokines and chemokines, which was mediated by a mechanism involving nuclear factor (NF)-κB signaling. Consistently, gut I/R activated NF-κB in lung neutrophils, which was alleviated by MLDL. In conclusion, all these data indicate that mesenteric lymph circulation contributes to neutrophil activation and lung inflammation following gut I/R injury partly through activating NF-κB.

Intestinal ischemia/reperfusion (I/R) can induce systemic inflammation and remote organ injury, where cytokine production and bacteria translocation are traditionally thought to be the major underlying mechanisms. The objective of our study was to determine whether the lymphatic circulation participates in this process. We focused on mesenteric lymph duct, as it drains gut interstitial fluid, proteins, absorbed lipids, and immune cells into the blood circulation through subclavian vein; the lungs are the first organ directly exposed to lymph contents. In a mouse model of intestinal I/R, we found that I/R‐induced lung inflammation, as evidenced by neutrophil infiltration and myeloperoxidase production, was inhibited by mesenteric lymph duct ligation. We then compared the direct effects of lymph collected from pre‐I/R vs. post‐I/R mesenteric lymph duct on isolated neutrophils from normal animals. Flow cytometric analysis revealed that neutrophils treated with post‐I/R lymph exhibited increased surface expression of CD11b. Additionally, upregulated mRNA expression of neutrophil specific chemokines (e.g. Cxcl1, Cxcl2, and Cxcl5) and pro‐inflammatory markers (e.g. Il1b, Il6, and Tnfa) was detected in post‐I/R lymph‐treated neutrophils. These effects were not seen in neutrophils treated with pre‐I/R lymph. Furthermore, because intestinal I/R injury is known to be associated with gut barrier leakage and bacteria translocation into the blood, we thought to determine whether bacteria translocation occurred to the lymphatic circulation during I/R. However, PCR experiments confirmed that all lymph samples tested, regardless of I/R status, were free of bacterial 16s rRNA, excluding the direct involvement of gut bacteria in I/R lymph‐induced neutrophil activation. In conclusion, our study suggests that mesenteric lymph derived from I/R gut contributes to systemic and lung inflammation by activating neutrophils. Support or Funding Information The work is supported by the American Heart Association 15SDG22930009 (to YM) and by the National Institutes of Health HL070752 (to SYY).