Toll-like receptor 2 induces chemokine receptor CXCR2 downregulation and neutrophil migration impairment in severe sepsis

There is a marked defect in neutrophil migration into the infectious focus during severe sepsis, which is associated with the severity of disease. Recently, we demonstrated that this phenomenon is a consequence of downregulation of the chemokine receptor CXCR2 on the surface of circulating neutrophils. Toll-like receptors are pattern-recognition receptors that are important in innate immune responses to bacterial infection. Toll-like receptor activation in phagocytes produces proinflammatory cytokines and chemokines that contribute directly to elimination of infectious agents. A sustained inflammatory response, however, can result in tissue damage and sepsis. Here, we address the role of Toll-like receptor 2 (TLR2) in the downregulation of CXCR2 and the establishment of neutrophil migration impairment in severe sepsis. TLR2-deficient (TLR2^-/-) and C57BL/6 (WT) mice were subjected to severe polymicrobial sepsis by the cecal ligation and puncture model, and neutrophil migration, bacteremia, CXCR2 expression and cytokine levels were evaluated. 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 improve of survival rate. Furthermore, in vitro, TLR2 agonist (lipoteichoic acid) was able to downregulate CXCR2 expression and markedly to inhibit neutrophil chemotaxis induced by CXCR2 ligand. The downregulation of CXCR2 was associated with enhanced expression of G-protein-coupled receptor kinases-2 (GRK-2), which is known to play an important role in 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 peritoneal cavity in response to thioglycolate. Altogether, these findings suggest that TLR2, through GRK2 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.