Abstract
Sepsis is a global health problem, with a 25-30% mortality rate. This pathological condition may lead to systemic inflammation and multiple organ failure, that often includes with liver and lung injury. In spite of medical advancements, sepsis remains a leading cause of hospital death due to the rise in infections caused by bacteria resistant to multiple antimicrobial agents. Because of its severity and widespread occurrence of sepsis, it is essential to understand the pathogenesis of sepsis to identify novel target treatments.
Endogenous Hydrogen sulfide (H2S) synthesized via cystathionine-γ-lyase (Cth) has been demonstrated to play a key role in inflammation and the injury of an organ. These studies have used either D, L-propargylglycine (PAG-a pharmacological inhibitor of Cth) or Cth deletion. Some studies have indicated that PAG also acts by inhibiting other pyridoxal phosphate (PLP)-dependent enzymes. In addition, the pleiotropic effect of other genes could also show a compensation role for the absence of Cth activity in the KO model. Therefore, this thesis investigated the role of H2S synthesized via Cth in acute sepsis induced by caecal ligation and puncture in a mouse model by using two complementary and independent approaches. These were Cth pharmacological inhibition and gene deletion in order to understand the role of H2S in the inflammatory response in liver and lung injury via adhesion molecules in the endothelial cells in sepsis. The use of the two approaches has revealed a major role for H2S in the inflammatory response. Firstly, there was an increase in myeloperoxidase (MPO) activity (indicator for neutrophil infiltration- demonstrating the severity of systemic inflammation). Secondly, the pro-inflammatory cytokines and chemokines increased and finally there was phosphorylation of ERK 1/2 and activation of NF-κB in tissue. These responses were attenuated by both pharmacological inhibition or Cth gene deletion. This study also revealed novel evidence
for the key role H2S has in the expression of adhesion molecules on the endothelial cells, resulting in liver and lung injury.
Kupffer cells, the resident tissue macrophages in the liver, are activated during sepsis, releasing cytokines (mainly TNF α), resulting into systemic inflammatory response and multiple organ injury. Hence, this thesis focused on understanding the role of Kupffer cells on the adhesion molecules in the endothelial cells of the liver and lungs. Inhibition of Kupffer cell activity by GdCl3 administration altered the expression of ICAM-1 on the endothelial cells of both the liver and lungs. However, there is no change in the expression of VCAM-1 after GdCl3 treatment. This showed that GdCl3 has differentially regulated ICAM-1 and VCAM-1.
The progress of knowledge of H2S from the translation of animal study to human study from previous studies highlighted its contribution to pro-inflammatory response. In addition, animal and human studies, H2S has been demonstrated to promote inflammation via substance P. However, all the prior human studies involved patients in the ICU who were severely ill, experiencing a range of infections with diverse clinical trajectories before admission to the ICU. Thus, this thesis investigated the role of H2S and substance P in human subjects by comparing the concentration of H2S and substance P in peripheral blood of patients admitted to general hospital wards (non-ICU) with acute infections identified by Escherichia coli or Klebsiella pneumoniae- bacteraemia with a matched healthy population. H2S and substance P concentrations were both increased acutely in the bacteraemic patients, but the time course of the response differed, with H2S returning more quickly toward baseline than substance P. In addition, the response of substance P was higher in patients with abdominal than urinary tract infections despite there being no difference in clinical severity scores. This demonstrated there is a heterogenous responses of substance P in relation the anatomical site of the infection.
The work presented in this thesis demonstrated the role of H2S as a pro-inflammatory mediator and that makes a key contribution in the expression of adhesion molecules on endothelial cells during organ injury. The role of Kupffer cells in modulating the expression of adhesion molecules was demonstrated, which requires further studies. Additionally, the clinical study confirms the pro-inflammatory role of H2S in humans, and emphasizes the potential for drug therapy inhibiting the H2S pathway to alleviate sepsis.