Probiotic Bacteria-Released Extracellular Vesicles Enhance Macrophage Phagocytosis in Polymicrobial Sepsis Through Activating the FPR1/2 Pathway

Background: Sepsis-induced organ failure and high mortality are largely ascribed to the failure of bacterial clearance from the infected tissues. Recently, probiotic bacteria-released extracellular vesicles (BEVs) have been implicated as critical mediators of intercellular communication which are widely involved in the regulation inflammatory response. However, their functional role in macrophage phagocytosis during sepsis has never been explored. Methods: BEVs were collected from three different strains of probiotics including Lactobacillus plantarum WCFS1 (LP WCFS1), Lactobacillus rhamnosus Gorbach-Goldin (LGG), and Escherichia coli Nissle 1917 (EcN), or from LGG cultured under three pH conditions (pH5-acid, pH6.5-standard, pH8-akaline) through differential centrifugation, filtering, and ultracentrifugation of their culture supernatants. In vitro phagocytosis was measured in Raw264.7 and bone marrow-derived macrophages using pHrodo red E. coli BioParticles. The in vivo therapeutic effects of BEVs were tested using a feces-intraperitoneal-injection (FIP) model of polymicrobial sepsis. Results: LGG-derived EVs (BEVLGG) were the best among these three probiotics BEVs in stimulating macrophages to take up bacteria. Furthermore, BEVLGG collected from pH8 culture condition (BEVpH8) exhibited the strongest capacity of phagocytosis, compared with BEVpH5 and BEVpH6.5. Treatment of septic mice with BEVpH8 significantly prolonged animal survival, increased bacterial clearance from the blood, peritoneal lavage, and multiple organs, decreased serum levels of pro-inflammatory cytokines, alanine aminotransferase (ALT, marker of liver injury) and creatinine (Cr, marker of kidney injury), in comparison with control-treated septic mice. Mechanistically, RNA-seq and bioinformatic analysis identified that the FPR1/2 signaling was remarkably activated, along with its downstream pathways (PI3K-Akt-MARCO and NADPH-ROS) in BEVpH8-treated macrophages, compared with control cells. Accordingly, pre-addition of Boc2, a specific antagonist of FPR1/FPR2, to macrophages significantly attenuated BEVpH8-mediated phagocytosis, compared to controls. Conclusions: This study demonstrates that LGG-derived BEVs may have therapeutic effects against sepsis-induced organ injury and mortality through enhancing FPR1/2-mediated macrophage phagocytosis.