B-1a cells scavenge NETs to attenuate sepsis.

B-1a cells, a regulatory subset of B lymphocytes, produce natural IgM and interleukin-10. Neutrophil extracellular traps (NETs) play a crucial role in pathogen defense, but their excessive formation during sepsis can cause further inflammation and tissue damage. In sepsis, extracellular cold-inducible RNA-binding protein (eCIRP), a damage-associated molecular pattern, is released to induce NET formation. We hypothesize that B-1a cells clear NETs to prevent sepsis-induced injury. Sepsis in mice was induced by injecting 1 × 107 and 5 × 107 colony-forming units of Escherichia coli intraperitoneally. After 4 and 20 h, we assessed the number of B-1a cells in the peritoneal cavity using flow cytometry. Our results showed that the number of peritoneal B-1a cells was significantly decreased in E. coli sepsis mice. Importantly, replenishing B-1a cells via intraperitoneal injection in sepsis mice significantly decreased NETs in peritoneal neutrophils. We also observed a decrease in serum inflammation and injury markers and a significant increase in the overall survival rate in B-1a cell-treated septic mice. To understand the mechanism, we cocultured bone marrow-derived neutrophils with peritoneal B-1a cells in a contact or noncontact condition using an insert and stimulated them with eCIRP. After 4 h, we found that eCIRP significantly increased NET formation in bone marrow-derived neutrophils. Interestingly, we observed that B-1a cells inhibited NETs by 67% in a contact-dependent manner. Surprisingly, when B-1a cells were cultured in inserts, there was no significant decrease in NET formation, suggesting that direct cell-to-cell contact is crucial for this inhibitory effect. We further determined that B-1a cells promoted NET phagocytosis, and this was mediated through natural IgM, as blocking the IgM receptor attenuated the engulfment of NETs by B-1a cells. Finally, we identified that following their engulfment, NETs were localized into the lysosomal compartment for lysis. Thus, our study suggests that B-1a cells decrease NET content in eCIRP-treated neutrophils and E. coli sepsis mice.