Myocardial TLR4 is a determinant of neutrophil infiltration after global myocardial ischemia: mediating KC and MCP-1 expression induced by extracellular HSC70.

Cardiac surgery with global myocardial ischemia-reperfusion (I/R) induces a myocardial inflammatory response that impairs cardiac recovery. Chemokines contribute to the overall myocardial inflammatory response through inducing leukocyte infiltration. Although Toll-like receptor 4 (TLR4) has an important role in postischemic myocardial injury, the relative roles of myocardial tissue and leukocyte TLR4 in leukocyte infiltration, as well as the role of TLR4 in myocardial chemokine expression, are unclear. Our recent study, in an isolated mouse heart model of global I/R, found that the 70-kDa heat shock cognate protein (HSC70) is released from cardiac cells and mediates the expression of cardiodepressant cytokines via a TLR4-dependent mechanism. In the present study, we tested the hypotheses that myocardial tissue TLR4 has a major role in mediating neutrophil infiltration and that myocardial TLR4 and extracellular HSC70 contribute to the mechanisms underlying cardiac chemokine response to global I/R. We subjected hearts isolated from TLR4-defective and TLR4-competent mice to global I/R and examined myocardial neutrophil infiltration and expression of keratinocyte-derived chemokine (KC) and monocyte chemoattractant protein-1 (MCP-1). TLR4-defective hearts exhibited reduced neutrophil infiltration regardless of the phenotypes of neutrophils perfused during reperfusion and expressed lower levels of KC and MCP-1. HSC70-specific antibody reduced myocardial expression of KC and MCP-1 after I/R. Furthermore, perfusion of HSC70 increased KC and MCP-1 expression in TLR4-competent hearts but not in TLR4-defective hearts, and HSC70 also induced the chemokine response in macrophages in a TLR4-dependent fashion. A recombinant HSC70 fragment lacking the substrate-binding domain was insufficient to induce chemokine expression in hearts and cells. This study demonstrates that myocardial tissue TLR4, rather than neutrophil TLR4, is the determinant of myocardial neutrophil infiltration after global I/R. TLR4 mediates myocardial chemokine expression, and the mechanisms involve extracellular HSC70. These results imply the HSC70-TLR4 interaction as a novel mechanism underlying the myocardial chemokine response to global I/R.