Src-family kinase control of CAR signaling: the paradoxical and multifaceted role of LCK

• Lck is differentially regulated in TCR and CAR signaling at the proximal level. • CAR architecture influences Lck recruitment and activity, affecting signaling outcomes. • Excessive Lck activity drives exhaustion; controlled Lck improves CAR T-cell function. • Pharmacologic modulation of Lck, e.g., dasatinib, tunes CAR signaling and toxicity. • Engineering TCR motifs to tune Lck activity improves CAR design and safety. Lymphocyte-specific protein tyrosine kinase (LCK) is central to early T-cell receptor (TCR) signaling and is tightly regulated by phosphorylation, protein–protein interactions, and spatial organization to control T-cell activation and fate. Chimeric antigen receptors (CARs) co-opt core elements of the TCR signaling machinery, including LCK. Nevertheless, important mechanistic differences, which remain largely unexplored, exist in how LCK is recruited, activated, and restrained during CAR signaling relative to canonical TCR signaling. Accumulating evidence indicates that CAR architecture, particularly the nature and organization of costimulatory signaling domains, profoundly influences LCK activity, signal strength, tonic signaling, and downstream differentiation outcomes. Excessive or sustained LCK activity can drive metabolic stress and exhaustion, whereas limiting or fine-tuning LCK signaling improves CAR T-cell persistence and therapeutic efficacy. Here, we review the current understanding of LCK regulation in CAR signaling and highlight recent insights into pharmacologic and structural strategies to tune LCK activity. A deeper understanding of LCK regulation is essential for improving the efficacy and safety of CAR T cells and may help in the design of next-generation CARs.