TLR2 signaling downregulates chemokine receptor CXCR2 and impairs neutrophil migration in severe polymicrobial sepsis

Background

There is a marked defect in neutrophil migration into the infectious focus during severe sepsis, which is associated with the severity of disease. We recently demonstrated that this phenomenon is a consequence of the downregulation of chemokine receptor CXCR2 on the surface of circulating neutrophils. Toll-like receptors (TLRs) are pattern-recognition receptors that are important in innate immune responses to bacterial infection. TLR activation in phagocytes produces proinflammatory cytokines and chemokines that contribute directly to elimination of infectious agents. However, a sustained inflammatory response can result in tissue damage and sepsis. Here, we address the role of TLR2 in the downregulation of CXCR2 and establishment of neutrophil migration impairment in severe sepsis.

Materials and methods

TLR2-deficient (TLR2^-/-) and C57BL/6 (WT) mice were subjected to severe polymicrobial sepsis by the cecal ligation and puncture (CLP) model, and neutrophil migration, bacteremia, CXCR2 expression and cytokines levels were evaluated.

Results

It was observed that TLR2 is critical for downregulation of CXCR2 expression on circulating neutrophils during severe sepsis, since this event was prevented in TLR2^-/- mice. In accordance, TLR2^-/- mice did not present failure of neutrophil migration into the infectious focus and, consequently, they presented lower bacteremia and did not display systemic inflammation determined by reduced levels of circulating cytokines, showing an improved survival rate. Furthermore, in vitro, TLR2 agonist (lipoteichoic acid) was able to downregulate CXCR2 expression and to markedly inhibit neutrophil chemotaxis induced by CXCR2-ligand. The downregulation of CXCR2 was associated with enhanced expression of G-protein-coupled receptor kinase-2 (GRK-2), which is known to play an important role in the desensitization and internalization of this chemokine receptor. Finally, we showed that in vitro lipoteichoic-acid-stimulated neutrophils adoptively transferred into naïve WT mice display a significantly reduced competence to migrate into the perinoneal cavity in response to thioglycolate.

Conclusion

Altogether, these findings suggest that TLR2 through GRK-2 signaling downregulates CXCR2 expression on the surface of circulating neutrophils, which is a critical determinant of impairment of neutrophil migration into the infection focus during severe sepsis.