Abstract Body (Do not enter title and authors here): Immune checkpoint inhibitors (ICIs) like the FDA-approved monoclonal antibody nivolumab, which blocks the interaction between the programmed death 1 (PD-1) receptor on the surface of B and T cells and its ligands PD-L1/PD-L2 on cancer cells, have revolutionized the treatment of multiple cancers, and indications for their use is expanding. Concurrent with the increased use and success of ICIs has been the emergence of ICI-associated cardiovascular disease, including a ~3-fold higher risk of atherosclerotic coronary artery disease (CAD) within 2 years of ICI initiation, independent of baseline cardiovascular risk factors. During atherogenesis, vascular smooth muscle cells (SMCs) undergo complex phenotypic modulation, where they de-differentiate and variably express markers typical of macrophages, fibroblasts, osteogenic cells, and stem cells. We have established that atherosclerosis-associated phenotypic modulation of SMCs is in part driven by increased cellular cholesterol entering the endoplasmic reticulum (ER) and activating ER stress, particularly PERK signaling, and that SMC-specific deletion of Perk reduces plaque burden by ~80% in male hypercholesterolemic mice. We also discovered that early onset atherosclerosis associated with inherited genetic variants expressed in SMCs can activate heat shock factor 1 (HSF1), which in turn increases intracellular cholesterol by augmenting its biosynthesis, involving increased HMG-CoA reductase (HMGCR) activity, thus activating PERK signaling and augmenting SMC phenotypic modulation. To determine if HSF1-HMGCR-PERK signaling in SMCs underlies ICI-induced atherosclerosis, we exposed SMCs explanted from the ascending aortas of healthy human donors to nivolumab. We observed that nivolumab activates HSF1, upregulates HMGCR, resulting in elevated levels of intracellular cholesteryl esters, increased PERK signaling and augmented SMC phenotypic modulation. Importantly, both co-treatment with the HMGCR inhibitor pravastatin and SMC-specific deletion of Perk reversed nivolumab-induced SMC phenotypic modulation. Taken together, our data identify a novel effect of nivolumab on SMCs: activation of HSF1-HMGCR-PERK signaling, promoting pro-atherogenic phenotypic modulation. These results suggest that targeting cholesterol biosynthesis or PERK signaling may offer promising therapeutic strategies to prevent or treat ICI-induced atherosclerosis without compromising anti-tumor efficacy.