Background— Atherosclerosis is an inflammatory disease in which monocytes and macrophages have been suggested to play an essential role. The underlying signaling mechanisms are unknown thus far. We hypothesized that the human isoform of Toll-like receptor (hTLR)-4 is involved in monocyte activation of patients with accelerated forms of atherosclerosis.

Methods and Results— Expression of hTLR4 on circulating monocytes from 30 controls, 20 patients with stable angina (SA), 40 patients with unstable angina (UA), and 28 patients with acute myocardial infarction (AMI) was compared with the use of flow-cytometry and reverse transcription–polymerase chain reaction. Regulation of interleukin (IL)-12 and B7-1 as downstream events of TLR4 activation was analyzed after lipopolysaccharide stimulation of monocytes. TLR4-transfected Chinese hamster ovary (CHO) cells were used to identify potential hTLR4 ligands in the serum of patients with UA or AMI. Circulating hTLR4⁺/CD14⁺ monocytes were ≈2.5-fold increased above controls and patients with SA in the UA and AMI groups (P<0.0001). This was paralleled by enhanced transcript levels of TLR4 and Myd88 in patients with UA and AMI (P<0.0001) and increased expression of IL-12 (UA 35.5±7.8, AMI 31.8±7.7 versus SA 2.2±0.5, controls 2.1±0.3 pg/mL; P<0.0002) and B7-1 (UA 27.3±14.4, AMI 22.6±11.1 versus SA 3.4±2.5, controls 2.4±2.3%; P<0.0001). Compared with serum from patients with UA and AMI, challenging TLR4-transfected CHO cells with serum from SA patients yielded only a weak response (P<0.0001). Coincubation with anti–heat shock protein 60 inhibited CHO cell activation.

Conclusions— UA and AMI are associated with enhanced expression and signaling events downstream of hTLR4 in circulating monocytes. These observations suggest hTLR4 activation as a signaling mechanism in immune-mediated progression of atherosclerosis.