Organ dysfunction and failure in critical illness: Role of mesenteric lymph and lymphatics

Background: Critical illness (CI), such as haemorrhagic shock and severe acute pancreatitis, is characterised by periods of systemic inflammation and may progress to multiple organ dysfunction syndrome (MODS), which is often fatal. The pathophysiology of MODS is not fully understood, but the intestine has long been suspected to be involved in the pathophysiology of systemic inflammation and MODS. Hypotheses and aims: The hypotheses of this thesis were that: CI causes failure of the gut barrier, leading to formation and transport of conditioned mesenteric lymph (CML) to the systemic circulation, thus impacting the heart and lungs (Part I); and toxic mediators in lymph, which are not yet fully characterised, may contain pancreatic proteases and messenger molecules that impact distant organs (Part II). The aims of this research were to identify the functional derangements at the organ (heart), cell (endothelial), and organelle (mitochondrial) levels and to investigate the compositional changes in CI using a translational approach, i.e., microRNAs, and by probing the protease-antiprotease milieu. Methods: A range of CI models was investigated, including acute pancreatitis (AP), severe acute pancreatitis (SAP), and haemorrhagic shock (HS). Initially, hearts from animals with CI that had either lymphatic diversion or no diversion were analysed functionally. Similarly, hearts from normal animals were perfused with CML from animals with CI. Endothelial cells were incubated with ML and serum from animals with CI and analysed for survival as well as production of further inflammatory mediators. Mitochondria from the heart and lung tissues of normal animals were incubated with physiological concentrations of ML to determine mitochondrial function. The microRNA profile found in ML from animals with AP and SAP was analysed to determine the compositional change in the CML. The protease-antiprotease activity in ML and serum from animals with CI was analysed. Based on these results, a systemic study was then conducted to investigate the role of serum proteases as prognostic markers in prediction of severity of AP. Results: Hearts from CI animals with lymphatic diversion were found to have preserved cardiac function. Infusion of CML into normal hearts caused cardiac dysfunction similar to that observed in animals without lymphatic diversion. The endothelium showed signs of apoptosis when incubated with CML and the supernatant fluids also demonstrated significantly elevated levels of inflammatory mediators. The heart and lung tissues incubated with CML demonstrated significantly deranged mitochondrial respiration along with damage to the outer membrane, thus causing mitochondrial dysfunction. It was demonstrated that ML carries some microRNAs that may be involved in progression of AP to SAP and MODS. There was significantly elevated protease activity and significantly reduced antiprotease activity in CML, which caused a significant protease-antiprotease imbalance. The systematic review revealed that PMN elastase, a leukocyte protease,…