Abstract 1947: Chymase Inhibition Reduces the Incidence of Intraplaque Hemorrhage and Induces a Stable Atherosclerotic Plaque Phenotype in ApoE Deficient Mice
Mast cells are abundantly present in perivascular tissue during atherosclerotic plaque progression and were previously demonstrated to contribute significantly to plaque destabilization. In this study, we aimed to investigate to what extent inhibition of chymase, one of the mast cell proteases, could enhance plaque stability. We demonstrated earlier that the specific chymase inhibitor RO501 (1 μM) was able to quench mast cell activation in vitro, as illustrated by a decreased β-hexosaminidase (−41% and −80%, respectively; P<0.05) as well as chymase activity in the releasate of activated MC/9 and peritoneal mast cells (−71% and −65%, respectively; P<0.05). Next, we assessed whether chymase inhibition was also effective in vivo. Atherosclerotic carotid artery lesions were induced in ApoE−/− mice by perivascular collar placement and mast cells were activated by DNP challenge systemically during lesion development. RO501 (50 mg/kg/day) was administered as diet supplement, leading to serum concentrations of ~2 μM. While plaque size after 6 weeks of treatment did not differ, collagen content of the lesions was 2-fold enhanced in mice treated with RO501 compared to controls (1.4 ± 0.5% and 0.7 ± 0.2%, respectively). This was accompanied by a significant decrease in necrotic core size of the plaques (controls: 52 ± 3% versus RO501: 41 ± 4%, P<0.05). To determine the effects of chymase inhibition after acute mast cell activation in advanced plaques, perivascular mast cells were focally activated in the adventitia of advanced lesions in ApoE−/− mice, which were treated with RO501 as described above. At three days after focal mast cell challenge, the incidence of intraplaque hemorrhage (IPH) was inhibited from 23% in control mice to 4.5% in RO501 treated mice, while also the plaque erythrocyte area was reduced by >90% from 1.2 ± 0.6*103 to 0.1 ± 0.08*103 μm2 (P<0.05). Also, we observed a reduction in apoptotic cells (RO501: 0.68 ± 0.20% vs. 1.01 ± 0.36% for IPH negative controls and 1.23 ± 0.42% for IPH+ plaques). In conclusion, our data suggest that chymase inhibition at least partly prevents the detrimental effects of perivascular mast cells on plaque stability, identifying chymase inhibition as a new therapeutic approach in the prevention of acute coronary syndromes.