Conditional deletion ofCcl2in smooth muscle cells does not reduce early atherosclerosis in mice

Abstract

AbstractBackground and aimsC-C motif chemokine ligand 2 (CCL2) is a pro-inflammatory chemokine important for monocyte recruitment to the arterial wall and atherosclerotic plaques. Global knockout ofCcl2reduces plaque formation and macrophage content in mice, but the importance of different plaque cell types in mediating this effect has not been resolved. Smooth muscle cells (SMCs) can adopt a potentially pro-inflammatory function with expression of CCL2. The present study aimed to test the hypothesis that SMC-secreted CCL2 is involved in early atherogenesis in mice.MethodsSMC-restricted Cre recombinase was activated at 6 weeks of age in mice with homozygous floxed or wildtypeCcl2alleles. Separate experiments in mice lacking the Cre recombinase transgene were conducted to control for genetic background effects. Hypercholesterolemia and atherosclerosis were induced by a tail vein injection of recombinant adeno-associated virus (rAAV) encoding proprotein convertase subtilisin /kexin type 9 (PCSK9) and a high-fat diet for 12 weeks.ResultsUnexpectedly, mice with SMC-specificCcl2deletion developed higher levels of plasma cholesterol and larger atherosclerotic plaques with more macrophages compared with wild-type littermates. When total cholesterol levels were incorporated into the statistical analysis, none of the effects on plaque development between groups remained significant. Importantly, changes in plasma cholesterol and atherosclerosis remained in mice lacking Cre recombinase indicating that they were not caused by SMC-specific CCL2 deletion but by effects of the floxed allele or passenger genes.ConclusionsSMC-specific deficiency ofCcl2does not significantly affect plaque development in hypercholesterolemic mice.Bullet pointsSMCs express CCL2 in human plaques and upon inflammatory activation of a murine SMC line in vitro.Deletion of the Ccl2 gene in SMCs using Cre recombinase does not influence the size or composition of atherosclerotic plaque in mice.Only by conducting a control study in mice without Cre recombinase was an initially observed difference in atherosclerosis concluded to be a genetic background effect.