Reconciling Smooth Muscle Cell Oligoclonality and Proliferative Capacity in Experimental Atherosclerosis.

Despite decades of research, the relative contribution of vascular smooth muscle cells (SMC) to the pathogenesis of atherosclerosis has remained unclear and controversial. Most of the controversy came from the lack of rigorous lineage tracing studies to unequivocally identify the cellular origins of lesion cells. Indeed, as extensively described in recent reviews, the plasticity of multiple lesion cell types (SMC, myeloid cells, and endothelial cells), including their ability to downregulate their lineage-specific markers and to express markers of alternative cell types during atherosclerotic plaque progression, makes their identification using traditional immunostaining for such markers highly inconclusive and greatly confounds attempts to understand the functional roles of these cells within lesions. With respect to SMC, tremendous progress has been made since the development of rigorous and unambiguous murine lineage tracing systems allowing conditional, inducible, definitive, and efficient labeling of medial Myh11 SMC and tracking of their fate in atherosclerosis and simultaneous SMC-specific conditional knockout (KO) of genes postulated to control SMC phenotype and overall lesion pathogenesis. Using these capabilities, our laboratory recently demonstrated that (1) >80% of SMC-derived cells within advanced lesions of atheroprone ApoE mice fed a western diet for 18 weeks lacked detectable expression of SMC markers such as Acta2 or Myh11 typically used to identify them, (2) Myh11 medial SMC can undergo multiple phenotypic transitions characterized by activation of markers of macrophages, mesenchymal stem cells, and myofibroblasts, and (3) contrary to the long-standing dogma that SMC play a beneficial role in lesion pathogenesis by contributing to the formation of a protective fibrous cap, results from our recent studies in which we performed SMC-specific conditional KO of the pluripotency factors Klf4 and Oct4 demonstrated that SMC can play either an atheroprotective or atheropromoting role depending on the nature of their phenotypic transitions. For example, Klf4-dependent transitions, including formation of SMC-derived macrophage marker foam cells, exacerbated lesion pathogenesis, whereas Oct4-dependent transitions were atheroprotective, including being critical for migration and investment of SMC into the fibrous cap. However, our studies failed to clearly resolve several critical questions. First, are SMC-derived cells that populate lesions derived from many or only a few differentiated medial SMC, a hypothesis originally proposed by Benditt and Benditt in 1973 in their monoclonal theory of atherosclerosis? Second, can a single differentiated medial SMC give rise to multiple SMC phenotypes or are individual SMC limited in their plasticity?