Effects of Lipoxin A₄ on antimicrobial actions of neutrophils in sepsis

In sepsis, hyperactivation of neutrophils can lead to tissue injury. Later, neutrophil dysregulation with reduced levels of migration, decreased apoptosis and inadequate phagocytosis may impair the host[U+05F3]s ability to clear infection. Lipoxin A₄ (LXA₄) is a pro-resolution lipid mediator which reduces neutrophil migration and inflammatory mediator expression. As neutrophil migration and activation are important in bacterial clearance, the role of LXA₄ in regulating neutrophil function for bacterial clearance is unclear. Using the cecal ligation and puncture (CLP) rat model of sepsis, LXA₄ given after 1h reduced blood bacterial load at 24h. LXA₄ treatment decreased neutrophil migration to the peritoneum but augmented blood neutrophil phagocytic ability and promoted apoptosis without affecting free radical production. In contrast, LXA₄ increased peritoneal neutrophil phagocytic ability without affecting apoptosis or free radical production suggesting that in vivo effects of LXA₄ were compartment specific. To investigate if LXA₄ acted directly on neutrophils, blood and peritoneal leukocytes were taken from CLP rats 1h after surgery and incubated ex vivo with and without LXA₄. LXA₄ (1nM) increased phagocytosis in blood neutrophils without affecting apoptosis or free radical production. Ex vivo LXA₄ had no effect on peritoneal neutrophils which suggests that LXA₄ enhanced peritoneal neutrophil phagocytic ability in vivo by an indirect mechanism. The results suggest that LXA₄ reduced neutrophil migration, but increased neutrophil bacteria clearing function without excessive free radical production. This phenotype was associated with reduced blood bacteria load.