Spatial FAP Expression as Detected by ⁶⁸ Ga-FAPI-46 Identifies Myofibroblasts Beyond the Infarct Scar After...

Myocardial infarction (MI) triggers complex cellular responses essential for tissue repair and remodeling, including myofibroblast activation. Fibroblast activation protein alpha (FAP) identifies activated myofibroblasts post-MI, however its spatial distribution relative to the scar and area at risk (AAR) is unclear. Non-invasive FAP-imaging with PET radiotracer ⁶⁸ Ga-FAPI-46 shows uptake beyond the infarct scar. We therefore aimed to characterize FAP expression in the AAR using a myocardial ischemia-reperfusion (MI/R) model in mice.

We induced MI/R in male C57BL/6N mice. The AAR was identified by in vivo lectin staining, and expression of FAP, CD68, and hypoxic tissues were measured using immunohistochemistry. Spatial FAP was further interrogated by ⁶⁸ Ga-FAPI-46 in mice by autoradiography and humans by PET. Additionally, human cardiac tissues from acute MI patients were examined for fibroblasts and inflammatory cells by expression of FAP, CD13, and α-smooth muscle actin.

FAP expression peaked three days post-MI/R predominantly within the AAR (p < 0.05 vs. d0). Consistent between murine models and human tissues, FAP⁺ myofibroblasts accumulated within the infarct scar and borderzone, occasionally extending into non-ischemic myocardium. CD68⁺ macrophages peaked similarly at three days post-MI/R (p < 0.05 vs. d0). FAP expression weakly correlated with CD68 but not with extent of ischemic or hypoxic territory post-MI/R. FAP imaging in mice and humans revealed aligned non-uniform ⁶⁸ Ga-FAPI-46 uptake extending from the infarct scar into surviving myocardium after MI.

Our findings demonstrate a distinct FAP expression pattern post-MI/R. The alignment of ex vivo ⁶⁸ Ga-FAPI-46 signal with myofibroblasts in the AAR supports its identification of a unique substrate in myocardial injury complementing other non-invasive imaging measurements of perfusion, viability and fibrosis.