Multiple cell types contribute to the atherosclerotic lesion fibrous cap by PDGFRβ and bioenergetic mechanisms

Newman et al. explore the origins of myofibroblasts in atherosclerotic fibrous caps, finding that while composed of cells from multiple origins, smooth muscle cells predominate and are required for long-term plaque stability. Stable atherosclerotic plaques are characterized by a thick, extracellular matrix-rich fibrous cap populated by protective ACTA2(+) myofibroblast (MF)-like cells, assumed to be almost exclusively derived from smooth muscle cells (SMCs). Herein, we show that in murine and human lesions, 20% to 40% of ACTA2(+) fibrous cap cells, respectively, are derived from non-SMC sources, including endothelial cells (ECs) or macrophages that have undergone an endothelial-to-mesenchymal transition (EndoMT) or a macrophage-to-mesenchymal transition (MMT). In addition, we show that SMC-specific knockout of the Pdgfrb gene, which encodes platelet-derived growth factor receptor beta (PDGFR beta), in Apoe(-/-) mice fed a Western diet for 18 weeks resulted in brachiocephalic artery lesions nearly devoid of SMCs but with no changes in lesion size, remodelling or indices of stability, including the percentage of ACTA2(+) fibrous cap cells. However, prolonged Western diet feeding of SMC Pdgfrb-knockout mice resulted in reduced indices of stability, indicating that EndoMT- and MMT-derived MFs cannot compensate indefinitely for loss of SMC-derived MFs. Using single-cell and bulk RNA-sequencing analyses of the brachiocephalic artery region and in vitro models, we provide evidence that SMC-to-MF transitions are induced by PDGF and transforming growth factor-beta and dependent on aerobic glycolysis, while EndoMT is induced by interleukin-1 beta and transforming growth factor-beta. Together, we provide evidence that the ACTA2(+) fibrous cap originates from a tapestry of cell types, which transition to an MF-like state through distinct signalling pathways that are either dependent on or associated with extensive metabolic reprogramming.