Engineered T cells expressing chimeric antigen receptors (CARs) have already been proven to be an effective treatment in cancer immunotherapy. For this reason, recent research approaches are investigating the use of CAR T cell therapies in non-malignant immune-mediated diseases such as infectious diseases, autoimmune or fibrotic diseases. Fibrotic diseases represent a number of different diseases characterized by the overgrowth, hardening, and/or scarring of various tissues caused by excessive deposition of extracellular matrix components. The main cellular mediators of fibrotic diseases are activated fibroblasts, which serve as the primary extracellular matrix-producing cells. While many surface antigens on activated fibroblasts have already been identified, the fibroblast activation protein (FAP) represents the most promising target. To date, the vast majority of CAR-T cells in development are based on stable genetic modification by viral vectors. However, the long-term consequences, for example, due to off-target effects, are still poorly understood. An alternative to stable genetic modification is the transient modification of cells by introducing an mRNA encoding the CAR protein into the cells. mRNA-based CAR cell therapies thus offer the possibility of a safe and pharmacokinetically controllable immunotherapy. Here, we present results for the reprogramming of T cells with CAR mRNA directed against FAP for the treatment of fibrotic diseases. The work includes the optimization of the anti-FAP CAR mRNA for prolonged CAR expression and reduced immunogenicity. In addition, novel image-based cytotoxicity assays using live cell microscopy have been developed, providing new insights into the kinetics of CAR T cell killing.