Dichotomous Roles of Smooth Muscle Cell–Derived MCP1 (Monocyte Chemoattractant Protein 1) in Development of Atherosclerosis

Background: Smooth muscle cells (SMCs) in atherosclerotic plaque take on multiple nonclassical phenotypes that may affect plaque stability and, therefore, the likelihood of myocardial infarction or stroke. However, the mechanisms by which these cells affect stability are only beginning to be explored. Methods: In this study, we investigated the contribution of inflammatory MCP1 (monocyte chemoattractant protein 1) produced by both classical Myh11 (myosin heavy chain 11) + SMCs and SMCs that have transitioned through an Lgals3 (galectin 3) + state in atherosclerosis using smooth muscle lineage tracing mice that label all Myh11 + cells and a dual lineage tracing system that targets Lgals3-transitioned SMC only. Results: We show that loss of MCP1 in all Myh11 + smooth muscle results in a paradoxical increase in plaque size and macrophage content, driven by a baseline systemic monocytosis early in atherosclerosis pathogenesis. In contrast, knockout of MCP1 in Lgals3-transitioned SMCs using a complex dual lineage tracing system resulted in lesions with an increased Acta2 (actin alpha 2, smooth muscle) + fibrous cap and decreased investment of Lgals3-transitioned SMCs, consistent with increased plaque stability. Finally, using flow cytometry and single-cell RNA sequencing, we show that MCP1 produced by Lgals3-transitioned SMCs influences multiple populations of inflammatory cells in late-stage plaques. Conclusions: MCP1 produced by classical SMCs influences monocyte levels beginning early in disease and was atheroprotective, while MCP1 produced by the Lgals3-transitioned subset of SMCs exacerbated plaque pathogenesis in late-stage disease. Results are the first to determine the function of Lgals3-transitioned inflammatory SMCs in atherosclerosis and highlight the need for caution when considering therapeutic interventions involving MCP1.