A new preclinical study highlighted TREM2 as a switch that can reprogram CAR macrophages to attack breast cancer more effectively. Researchers reported that engagement of TREM2-dependent pathways can steer engineered macrophages away from an immunosuppressive phenotype and toward tumoricidal activity. The work positions TREM2 as a potential “third rail” for cell therapy design—beyond the CAR construct itself—by targeting how innate immune cells integrate signals inside the tumor microenvironment. For developers, the approach could help address the long-standing problem that macrophage-based therapies can be redirected by tumor ecology. The results also suggest combination strategies might be possible, using checkpoint biology to tune trafficking, persistence, and function of macrophage effectors within solid tumors.