Abstract Body: Anti-inflammatory therapies such as Colchicine and Canakinumab can reduce cardiovascular disease, yet the mechanisms underlying their benefit are not completely understood. Their therapeutic benefit may be due in part to atherosclerotic lesion stabilization, because Colchicine was shown to promote fibrous cap thickening in patients, which is a major feature associated with plaque stabilization. In addition, we have shown that inflammasome inhibition in models of clonal hematopoiesis (CH) promotes fibrous cap thickening in association with fibroblast-like cell accumulation in lesions. Here we aimed to determine how fibroblasts accumulate in lesions to gain better understanding of their function. To do this we first sought to identify specific markers of plaque fibroblasts so we could target them. We conducted scRNA-Seq on cells isolated from aortas of mice modeling Tet2 and Jak2 CH and found Pdgfrα was selectively enriched in fibroblast-like cells. The generation of Pdgfrα-Cre-Zsgreen1 reporter mice revealed that Pdgfra⁺ fibroblast-like cells were present in lesions and did not express ACTA2 or MAC2. Proteomics analysis of Pdgfrα-Cre-ZsGreen1 cells from lesions revealed a protein enrichment profile similar to the fibroblast transcriptome profile identified by scRNA-Seq, suggesting Pdgfrα-Cre-ZsGreen1 cells are plaque fibroblasts. We then modeled Jak2^VF CH in Pdgfrα-Cre-ZsGreen1 mice and induced atherosclerosis with Pcsk9 AAV8 and a 16-week high-fat diet. For the last 6 weeks of feeding, mice were administered IgG or IL-1ß antibodies. Consistent with our scRNA-Seq data, IL-1ß antagonism led to a significant increase in Pdgfra⁺ fibroblast-like cells in lesions. To determine the mechanisms underlying fibroblast accumulation in lesions, we conducted ingenuity pathway analysis of scRNA-Seq data and found IL-1ß inhibition was associated with increased insulin-like growth factor 1 (IGF1) signaling. Panther proteomics analysis of Pdgfra⁺ cells similarly found increased IGF1 signaling, as well as enhanced DNA replication. Ex vivo aortic ring cultures revealed that IGF1 was capable of promoting fibroblast-like cell proliferation, and IL-1ß inhibited this IGF1 mediated proliferation. In vivo IL-1ß antibody administration led to a significant increase in Pdgfra⁺ cell proliferation, marked by Edu incorporation. Collectively, our data indicates that IL-1ß antagonism promotes Pdgfrα⁺ fibroblast proliferation in atherosclerosis, which may be dependent on IGF-1 signaling.