Abstract Body (Do not enter title and authors here): Background: Atrial fibrosis induces atrial fibrillation (AF) and increases the risk of stroke. In our previous study, we demonstrated that tumor endothelial marker 1 (TEM1/CD248), a transmembrane protein only expressed in mesenchymal cells during embryonic development, is re-expressed in atrial cardiac fibroblasts (CFs) of AF patients (pts) and TEM1 modulates CF cellular behaviors. We further explored the role of TEM1 in atrial inflammation, a key driver of atrial fibrosis. Methods and Results: Data are presented as mean ± standard error. Left atrial (LA) appendages were collected from 30 AF pts (mean age 64.0±1.6 yrs; 76.7% male) undergoing cardiac surgery. Western blot analysis and real-time quantitative polymerase chain reaction (RT-qPCR) were used to assess expression levels of TEM1 and pro-inflammatory mediators in the LA tissues. Pearson correlation analyses of western blot revealed significantly positive correlations between TEM1 expression with interleukin (IL)-6 (r=0.53), IL-1β (r=0.52), intercellular adhesion molecule 1 (ICAM1) (r=0.54), and vascular cell adhesion molecule 1 (VCAM1) (r=0.69, all p<0.01). Multivariate regression analysis confirmed these associations were independent of age, sex, hypertension, diabetes, and dyslipidemia. RT-qPCR also shows positive correlations of relative TEM1 expression level with IL-1β (r=0.99), tumor necrosis factor (TNF)-α (r=0.99), ICAM1 (r=0.76), VCAM1 (r=0.69) and the macrophage marker CD68 (r=0.68, all p<0.001). Immunohistochemical staining with F4/80 demonstrated increased macrophage infiltration in the LA tissues of AF pts compared to normal control (US Biomax). Osmotic mini-pumps delivering angiotensin II (Ang II, 1000 ng/kg/min for 4 wks) were implanted in mice to induce atrial fibrosis. Ang II significantly increased atrial macrophage infiltration in wild-type (WT) mice (n=3), which was markedly attenuated in TEM1-deficient transgenic mice (n=3) (F4/80-positive/total area [%]: 0.07±0.01 [WT+saline] vs. 1.62±0.19 [WT+Ang II] vs. 0.53 ± 0.03 [TEM1-deficient+Ang II], p<0.001). Sirius Red staining showed significantly reduced atrial fibrosis in TEM1-deficient mice compared to WT mice after Ang II infusion. Conclusions: TEM1 expression in atrial CFs promotes atrial inflammation and contributes to atrial fibrosis.