Next-generation in vivo CAR-T expansion strategies: Unlocking durable anti-tumor responses in solid tumors

The efficacy of chimeric antigen receptor T (CAR-T) cell therapy is directly associated with the abundance of functionally active anti-tumor CAR-T cells. However, the complexity of CAR-T manufacturing limits clinical accessibility and increases the risk of disease progression during treatment preparation. To improve CAR-T production, multiple strategies have been developed to expand CAR-T cells either in vitro or in vivo, including genetic engineering, vaccine platforms, cytokine stimulation, and the application of nanoparticles, engineered bacteria, and their outer membrane vesicles (OMVs). With the advancement of in vivo CAR-T approaches, the development of biocompatible materials that enable safe and effective in vivo expansion has become increasingly important. This review systematically examines the challenges associated with in vivo CAR-T cell expansion in solid tumors, comprehensively summarizes the mechanisms, advantages, and limitations of current expansion platforms, and discusses their translational potential, providing practical insights to guide the development of in vivo CAR-T therapies.