Abstract
Tolerance to bacterial components represents an essential regulatory mechanism during bacterial infection. Bacterial lipoprotein (BLP)-induced tolerance confers protection against microbial sepsis by attenuating inflammatory responses and augmenting antimicrobial activity in innate phagocytes. It has been well-documented that BLP tolerance-attenuated proinflammatory cytokine production is associated with suppressed TLR2 signalling pathway; however, the underlying mechanism(s) involved in BLP tolerance-enhanced antimicrobial activity is unclear. Here we report that BLP-tolerised macrophages exhibited accelerated phagosome maturation and enhanced bactericidal activity upon bacterial infection, with upregulated expression of membrane-trafficking regulators and lysosomal enzymes. Notably, bacterial challenge resulted in a strong activation of NF-κB pathway in BLP-tolerised macrophages. Importantly, activation of NF-κB pathway is critical for BLP tolerance-enhanced antimicrobial activity, as deactivation of NF-κB in BLP-tolerised macrophages impaired phagosome maturation and intracellular killing of the ingested bacteria. Finally, activation of NF-κB pathway in BLP-tolerised macrophages was dependent on NOD1 and NOD2 signalling, as knocking-down NOD1 and NOD2 substantially inhibited bacteria-induced activation of NF-κB and overexpression of Rab10 and Acp5, two membrane-trafficking regulators and lysosomal enzymes contributed to BLP tolerance-enhanced bactericidal activity. These results indicate that activation of NF-κB pathway is essential for BLP tolerance-augmented antimicrobial activity in innate phagocytes and depends primarily on both NOD1 and NOD2.
Highlights
Transmembrane Toll-like receptors (TLRs), in particular TLR4 and TLR2, are the best known PRRs and play a key role in the host defence against gram-negative and gram-positive bacterial infection by activation of TLR-mediated intracellular signal transduction pathways and initiation of both inflammatory and antimicrobial responses in innate phagocytes including polymorphonuclear neutrophils (PMNs) and monocytes/macrophages, which culminate in eliminating the invading microbial pathogens[9,11,12,13]
bacterial lipoprotein (BLP) tolerance-afforded protection against microbial sepsis is closely associated with BLP-induced reprogramming in innate phagocytes characterised by hyporesponsiveness in producing proinflammatory cytokines and simultaneously, an augmented antimicrobial activity[16,20,21]
We demonstrated that BLP-tolerised macrophages exhibited accelerated phagosome maturation and enhanced bactericidal activity in response to bacterial infection
Summary
Transmembrane Toll-like receptors (TLRs), in particular TLR4 and TLR2, are the best known PRRs and play a key role in the host defence against gram-negative and gram-positive bacterial infection by activation of TLR-mediated intracellular signal transduction pathways and initiation of both inflammatory and antimicrobial responses in innate phagocytes including polymorphonuclear neutrophils (PMNs) and monocytes/macrophages, which culminate in eliminating the invading microbial pathogens[9,11,12,13]. Tolerance induced by the gram-positive bacterial cell wall component bacterial lipoprotein (BLP), a TLR2 agonist, affords protection against a subsequent ‘lethal’ BLP challenge and live Staphylococcus aureus (S. aureus) and Salmonella typhimurium (S. typhimurium) infection or cecal ligation and puncture (CLP)-induced polymicrobial sepsis[20]. BLP-induced tolerance rescues TLR4-deficient mice from gram-negative S. typhimurium infection with a significant survival benefit[21]. This protection, afforded by BLP tolerance, against microbial sepsis is predominantly associated with BLP-induced reprogramming in innate phagocytes characterised by hyporesponsiveness in producing proinflammatory cytokines and simultaneously, an enhanced antimicrobial activity including upregulated phagocytic receptor expression and enhanced bacterial ingestion and killing, with accelerated bacterial clearance from the circulation and visceral organs[16,20,21]. We further show that activation of the NF-κB pathway by bacterial stimulation in BLP-tolerised macrophages appeared to be dependent on both NOD1 and NOD 2 signalling
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