Tracking therapeutic T cells in vivo.

Engineered cell therapies have shown remarkable promise in treating malignancies and autoimmune diseases. As of 2025, there are seven FDA-approved chimeric antigen receptor (CAR) T therapies, each treating different cancers. Despite rapid progress in developing new therapies, there is a lack of understanding about the in vivo behavior of adoptively transferred cells. Noninvasive methods that monitor CAR T cell dynamics and persistence in vivo are critical to both assess individual patients' responses to therapy in real time and to guiding improvements to engineered cell therapies. Imaging approaches that evaluate persistence, proliferation, functionality, and distribution of T cells will aid in optimizing therapeutic development and adjustment of treatment strategies in the clinic. Molecular imaging can noninvasively track labeled cells on a whole-body level and enable long-term monitoring of adoptively transferred cells in a manner compatible with the proliferation and persistence of therapeutic cells and may someday be able to bypass the need for paired biopsies. In this chapter, we highlight examples of cell tracking and labeling strategies for engineered T cell therapies.