BACKGROUNDCathepsin S and its endogenous inhibitor cystatin C are implicated in the pathogenesis of atherosclerosis, especially in the plaque destabilization and rupture leading to acute coronary syndrome. However, whether circulating cathepsin S and cystatin C also change in association with coronary plaque morphology is unknown yet.

METHODSWe recruited 98 patients with unstable angina (UA, n = 6) or stable angina (SA, n = 2) who had a segmental stenosis resulting in > 20% and < 70% diameter reduction in one major coronary artery on coronary angiography. Thirty-one healthy subjects served as controls. Intravascular ultrasound (IVUS) was used to evaluate plaque morphology. Plasma cathepsin S and cystatin C were measured as well.

RESULTSAt the culprit lesion site, plaque area ((7.85 +/- 2.83) mm(2) vs (6.53 +/- 2.92) mm(2), P = 0.027), plaque burden ((60.92 +/- 11.04)% vs (53.87 +/- 17.52)%, P = 0.025), remodeling index (0.93 +/- 0.16 vs 0.86 +/- 0.10, P = 0.004) and eccentricity index (0.74 +/- 0.17 vs 0.66 +/- 0.21, P = 0.038) were bigger in UA group than in SA group. Plasma cathepsin S and cystatin C were significantly higher in patients than in controls (P < 0.01). Plasma cathepsin S was higher in UA group ((0.411 +/- 0.121) nmol/L) than in SA group ((0.355 +/- 0.099) nmol/L, P = 0.007), so did the plasma cystatin C ((0.95 +/- 0.23) mg/L in UA group, (0.84 +/- 0.22) mg/L in SA group; P = 0.009). Plasma cathepsin S positively correlated with remodeling index (r = 0.402, P = 0.002) and eccentricity index (r = 0.441, P = 0.001), and plasma cystatin C positively correlated with plaque area (r = 0.467, P < 0.001) and plaque burden (r = 0.395, P = 0.003) in UA group but not in SA group.

CONCLUSIONSPlasma cathepsin S and cystatin C increased significantly in UA patients. In angina patients, higher plasma cathepsin S may suggest the presence of vulnerable plaque, and higher plasma cystatin C may be a clue for larger atherosclerotic coronary plaque.