Myeloid related proteins activate Toll-like receptor 4 in human acute coronary syndromes.

Introduction: We previously reported increased expression of TLR4 on monocytes in thrombi from patients with acute coronary syndromes (ACS). In mice, myeloid related protein (MRP) 8 and MRP14, cytoplasmic proteins of neutrophils and monocytes, activate Toll-like receptor (TLR) 4 during sepsis. In human ACS, we investigated now whether the pro-inflammatory action of MRPs occurs through TLR4 in monocytes derived from thrombi.

Methods: Coronary thrombi and peripheral blood of 27 ACS patients were analyzed. CD14(+) monocytes were isolated and incubated with TLR2 ligand PM3SKA, TLR4 ligand lipopolysaccharide (LPS), MRP8, MRP14, or MRP8/14 heterocomplex. Anti-TLR4 antibodies (HTA125) were used to block TLR4 and polymyxin B (PMB) was employed to inhibit endotoxins. Before and after stimulation, the release of TNFα was measured by ELISA and the expression of TLR4 on CD14(+) monocytes was determined by flow cytometry. Further, selected pathways of downstream signaling were analyzed.

Results: MRP8 and MRP8/14 increased release of TNFα in cultures of CD14(+) monocytes, more in cells derived from thrombi compared with matched peripheral blood cells (p<0.001). LPS, MRP8, and MRP8/14, but much less PM3SKA and MRP14 alone, stimulated TNFα release, which can be inhibited by HTA125. MRP8/14 enhanced TLR4 expression on monocytes from thrombi (p<0.001), but not on monocytes from peripheral blood of the same patients.

Conclusion: In ACS, MRP8 and MRP8/14 complex are specific ligands of TLR4, which induce the release of TNFα and probably other pro-inflammatory agents from monocytes. This specific MRP8/14-dependent pathway with striking similarities to sepsis increasing expression of TLR4 in thrombi appears to be involved in the pathogenesis of coronary occlusion and may represent a novel therapeutic target in ACS.