CCR2 expression on neutrophils leads to detrimental tissue infiltration during sepsis

Chemokines display a central role in mediating the neutrophil migration to inflammatory focus. Neutrophils respond to CXC chemokines, such as CXCL8, but are usually unresponsive to CC chemokines, such as CCL2. It is known that chemokine responsiveness in neutrophils can be modulated by some inflammatory conditions, such as sepsis. Here, we investigate whether Toll-like receptors (TLRs) modulate the expression of CCR2 in neutrophils and the consequence of this modulation on sepsis onset. Purified neutrophils from septic patients or from WT and CCR2^-/- mice subjected to sepsis by cecal ligation and puncture (CLP) and neutrophils from naïve mice or healthy humans stimulated with lipoteichoic acid (LTA) or lipopolysaccharide (LPS) were assayed to CCR2 expression by FACS or immunofluorescence and the chemotaxis response to CCL2. Treatments of neutrophils from naïve mice or healthy humans with TLR agonists, LTA or LPS, induce an upregulation of the CCR2 expression, leading to CCL2 responsiveness such as chemotaxis and F-actin polymerization. CCL2 expression induced by TLR activation was blocked by NF-κB or synthesis protein inhibitors. Moreover, LTA-induced or LPS-induced CCL2 chemotaxis was not observed in TLR2^-/-or TLR4^-/- neutrophils, respectively. Interestingly, neutrophils from septic patients or septic mice presented high CCR2 expression and CCL2 responsiveness, when compared with neutrophils from healthy donors or naive mice. In vivo, we found that CCR2^-/- mice subjected to severe sepsis by CLP exhibited reduced neutrophil infiltration in the heart, lung and kidney and an enhanced survival rate when compared with WT mice subjected to severe sepsis. Finally, severity of illness of the septic patients, judged by their APACHE II score and PaO₂/FiO₂ relation, had greater correlation with CCL2 responsiveness by neutrophils (r² = 0.77, P = 0.001 and r² = 0.59, P = 0.001, respectively). Our findings demonstrated that TLR activation induced the CCR2 expression and CCL2 responsiveness in human and murine neutrophils, and this expression profile in neutrophils is involved in the detrimental infiltration of these cells in distant tissues during server sepsis. CCR2 blockage is therefore a potential strategy for human sepsis treatment.