Thesis Type:

Abstract:

   Polymorphonuclear neutrophils (PN) play a key role in host defense mechanisms against microbial infections. This innate function is dependent on two major PN responses ; a massive production of superoxide anion (02-°) by the NADPH oxidase system (a phenomenon termed respiratory burst) leading to the formation of hydrogen peroxide (H2O2), and the release of granular enzymes (degranulation) among which Myeloperoxidase (MPO) which uses H2O2 to kill bacteria. In some pathologies such as decompensated alcoholic cirrhosis, patients exhibit an increased susceptibility for mostly lethal infections suggesting deficiencies of defensive responses of PN.
   In this study, we compared the MPO release from PN of these cirrhotic patients and healthy volunteers in response to stimulation by the bacterial peptide fMet-Leu-Phe(fMLP), and the activation state of major signaling effectors potentially involved(AKT, p44/42-MAP kinases (ERK1/2) et p38-MAP-kinases). In addition, the, respective contribution of the three signaling effectors in MPO degranulation was explored using healthy PN and pharmacological antagonists. Our results show that fMLP-induced myeloperoxidase release was strongly impaired in patients’ neutrophils whereas the intracellular myeloperoxidase stock was unaltered. The fMLP-induced activation of signaling effectors measured by their phosphorylated forms was also strongly deficient despite a normal expression of the three effectors and the fMLP receptor, fPR. However, based on the pharmacological inhibition of the signaling effectors in healthy neutrophils, AKT and p38-MAPK, but not ERK1/2, up-regulated fMLP-induced myeloperoxidase exocytosis. Interestingly, patients’ neutrophils also exhibited a defective bactericidal capacity that was reversed ex-vivo by the Toll-like receptor 7/8agonist CL097. This agent potentiated both the degree and duration of the AKT/p38-
MAPK signaling axis induced by fMLP, thus enhancing myeloperoxidase release.

   In conclusion, this study provides first evidences that neutrophils from patients with decompensated alcoholic cirrhosis exhibit a deficient AKT/p38-MAPK signaling, myeloperoxidase release and bactericidal activity. These deficiencies which may increase patients’ susceptibility to bacterial infections, can be reversed via TLR7/8 activation, and thus raises therapeutical perspectives to boost host-defense mechanisms of immuno-depressed patients.
   Key words: Neutrophil, Exocytosis, Myeloperoxidase, Hepatitis, Phagocytes, Signaling, Reactive oxygen species.

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