Dynamically visualizing profibrotic maladaptive repair after acute kidney injury by fibroblast activation protein imaging

A major challenge in prevention and early treatment of organ fibrosis is the lack of valuable tools to assess the evolving profibrotic maladaptive repair after injury in vivo in a non-invasive way. Here, using acute kidney injury (AKI) as an example, we tested the utility of fibroblast activation protein (FAP) imaging for dynamic assessment of maladaptive repair after injury. The temporospatial pattern of kidney FAP expression after injury was first characterized. Single-cell RNA sequencing and immunostaining analysis of patient biopsies were combined to show that FAP was specifically upregulated in kidney fibroblasts after AKI and was associated with fibroblast activation and chronic kidney disease (CKD) progression. This was corroborated in AKI mouse models, where a sustained and exaggerated kidney FAP upregulation was coupled to persistent fibroblast activation and a fibrotic outcome, linking kidney FAP level to post-insult maladaptive repair. Furthermore, using positron emission tomography (PET)/CT scanning with FAP-inhibitor tracers ([¹⁸F]FAPI-42, [¹⁸F]FAPT) targeting FAP, we demonstrated the feasibility of non-invasively tracking of maladaptive repair evolution toward kidney fibrosis. Importantly, a sustained increase in kidney [¹⁸F]FAPT (less hepatobiliary metabolized than [¹⁸F]FAPI-42) uptake reflected persistent kidney upregulation of FAP and characterized maladaptive repair after AKI. Kidney [¹⁸F]FAPT uptake at hour 2-day 7 correlated with kidney fibrosis 14 days after AKI. Similar changes in [¹⁸F]FAPI-42 PET/CT imaging were observed in patients with AKI and CKD progression. Thus, persistent kidney FAP upregulation after AKI was associated with maladaptive repair and a fibrotic outcome. Hence, FAP-specific PET/CT imaging enables dynamic visualization of maladaptive repair after AKI and prediction of kidney fibrosis within a clinically actionable window.