Aspirin-triggered lipoxin enhances macrophage phagocytosis of bacteria while inhibiting inflammatory cytokine production

The macrophage plays a major role in the induction and resolution phases of inflammation; however, how lipid mediator-derived signals may modulate macrophage function in the resolution of inflammation driven by microbes (e.g., in inflammatory bowel disease) is not well understood. We examined the effects of aspirin-triggered lipoxin (ATL), a stable analog of lipoxin A(4), on the antimicrobial responses of human peripheral blood mononuclear cell-derived macrophages and the monocytic THP-1 cell line. Additionally, we assessed the expression and localization of the lipoxin receptor, formyl peptide receptor 2 (FPR2), in colonic mucosal biopsies from patients with Crohn's disease to determine whether the capacity for lipoxin signaling is altered in inflammatory bowel disease. We found that THP-1 cells treated with ATL (100 nM) displayed increased phagocytosis of inert fluorescent beads and Escherichia coli in a scavenger receptor- and PI3K-dependent, opsonization-independent manner. This ATL-induced increase in phagocytosis was also observed in primary human macrophages, where it was associated with an inhibition of E. coli-induced IL-1 beta and IL-8 production. Finally, we found that FPR2 gene expression was increased approximately sixfold in the colon of patients with Crohn's disease, a finding reproduced in vitro by the treatment of THP-1 cells with interferon-gamma or lipopolysaccharide. These results suggest that lipoxin signaling is upregulated in inflammatory environments, and, in addition to their known role in tissue resolution following injury, lipoxins can enhance macrophage clearance of invading microbes.