Healthy donor T cell receptors expand functional neoantigen recognition beyond patient vaccination.

A major challenge in advancing neoantigen-targeted therapies lies in the limited capacity of predicted neoantigens to robustly activate CD8+ T cells, emphasizing the critical need for comprehensive functional validation before patient vaccination. Here, we provide a direct comparison of neoantigen-specific T cell responses between vaccinated patients with cancer and HLA-matched healthy donors. Despite vaccination, patient-derived T cells recognized only a small fraction of predicted neoantigens, whereas healthy donor T cells consistently exhibited broader and more robust neoantigen reactivity. Furthermore, we successfully expanded neoantigen-specific T cells from allogeneic donors, revealing that their T cell receptors (TCRs) can recognize targets that the patient's own T cells fail to engage with, partly due to poor T cell fitness. Collectively, these results indicate that cancer vaccines may be insufficient to overcome intrinsic defects in patient-derived T cell responses, supporting the use of healthy donor-derived TCRs as a complementary approach.