Species differences influencing the safety assessment of CD3 T-cell engagers.

Bispecific CD3 T-cell engagers (TCE) redirect endogenous T-cells to tumor cells and have demonstrated substantial clinical activity, but their mechanism of action also drives immune-mediated toxicities that are often underpredicted in nonhuman primate (NHP) studies. To investigate the biological basis of this translational gap, we performed a comparative evaluation of human and cynomolgus macaque T-cells using immunophenotyping, functional characterization with TCE, and single-cell transcriptomic profiling. Human and NHP donors showed broadly similar T-cell frequencies, subset distributions, and robust responses to polyclonal stimulation, confirming preserved intrinsic effector capacity in both species. However, across multiple CD20-, BCMA-, and GPRC5D-targeted TCE, human PBMC consistently exhibited greater cytotoxic potency and markedly elevated IFNγ production relative to NHP. Transcriptomic analyses revealed that although global TCR signaling architecture is conserved, human T-cells exhibit increased expression of some proximal signaling mediators, including LAT, PAG1, SKP1, and PI3K-family genes. These functional and molecular differences provide a mechanistic explanation for reduced pharmacologic responsiveness observed in NHP toxicology studies. While NHP studies can provide valuable insight for identifying antigen-dependent cytotoxic hazards, their limitations underscore the increasing importance of incorporating human-relevant models in nonclinical safety assessment.