question: Fibroblast activation protein (FAP) is a marker of activated fibroblasts in various human cardiovascular diseases. FAP is of special interest because it is expressed exclusively in activated, pathogenic fibroblasts and has been proposed as a target for chimeric antigen receptor (CAR) T cell ablation therapy. It has been shown previously that fibroblasts from patients with hypertrophic and dilated cardiomyopathy express FAP, while fibroblasts from non-failing hearts did not. The aim of the project was to examine FAP expression in other human cardiac diseases by obtaining heart samples from patients who suffered diseases characterized by cardiac fibrosis so that we could determine which cardiac diseases might be amenable to FAP CAR T therapy. In addition, we sought to characterize an in vitro model of FAP expression to aid in the ability to assess the effectiveness of engineered T cells to kill FAP-expressing cells. Heart tissue from patients diagnosed with various cardiac disorders, including idiopathic giant cell myocarditis, COVID-19, and sarcoidosis were stained for FAP expression and compared to non-failing, control hearts. The results showed significant FAP expression in all cardiac specimens with interstitial fibrosis compared to non-failing hearts. Notably, samples from patients who died from COVID-19 exhibited significant fibrosis and FAP expression. This suggests that there may be a large and growing population that may be amenable to anti-fibrotic therapy. In order to develop an in vitro model of FAP expression to assay anti-FAP CAR T cell activity, we treated IMR-90 human fibroblasts with TGF-B, a known activator of fibrosis, and assayed for FAP expression. TGF-B exposure resulted in an increase in FAP expression 24-hours after treatment. Our observation of fibroblast restricted FAP expression in multiple different cardiovascular diseases supports the potential use of FAP CAR T cell therapy as an anti-fibrotic treatment in multiple heart diseases.