Intrinsic immune receptor triggering mechanism informs rational chimeric receptor design

The T cell receptor (TCR) couple antigen recognition to T-cell activation and immune homeostasis. As an eight-subunit assembly, TCR activation operates as a highly coordinated, multi-subunit signaling event. However, which subunit serves as the primary trigger upon ligand engagement still remains unresolved. we identify CD3ε as a triggering subunit whose intracellular domain (ICD) is released from the plasma membrane during TCR activation. Elevated membrane tension facilitates this transition from a membrane-proximal to a membrane-distal conformation. The released ICD forms condensates through phase separation with free or co-receptor-associated Lck. These condensates recruit additional CD3ε ICDs from neighboring TCRs, triggering resting receptors and promoting further recruitment of TCRs and Lck to amplify proximal signaling. The CD3ε ICD alone support robust T cell activation independently of its own ITAM. We observe a similar triggering mechanism in several activating receptors in T and nature killer (NK) cell. These findings support a unifying model in which designated subunits initiate multi-subunit immune receptor activation through phase separation with key kinases. Guided by this model, we engineered triggering molecules (TRIG) that lack canonical intracellular signaling modules. These TRIG molecules recruit endogenous immune receptors in a ligand-dependent manner and efficiently activate both T cells and NK cells. This mechanism provides a basis for designing chimeric receptors that activate immune cells through endogenous receptor signaling.