Bispecific targeting of 4-1BB and CCR8 boosts antitumor immunity via Ti-Treg depletion and CD8 activation.

Immune checkpoint therapy has transformed cancer treatment, yet efficacy and safety challenges persist. Selectively inhibiting tumor-infiltrating regulatory T cells (Ti-Tregs) while enhancing CD8 T cell function are complementary strategies in cancer immunotherapy. Here, we engineered a bispecific antibody, FRP303, targeting 4-1BB and CCR8, which are co-expressed on a highly immunosuppressive subset of Ti-Tregs. FRP303 outperformed monoclonal antibodies in CT26 and MC38 colorectal tumors and poorly immunogenic B16F10 melanoma. Treatment with FRP303 reduced Ti-Treg frequency, increased CD8 T cell infiltration, and elevated antitumor cytokines IFN-γ and TNF-α. Safety assessments showed FRP303 does not disrupt immune homeostasis in peripheral tissues or induce significant hepatotoxicity. Moreover, FRP303 demonstrated strong synergistic effects when combined with a PD-1 antibody. In summary, FRP303 mediated anti-tumor activity through a dual mechanism involving the selective depletion of Ti-Tregs and the enhancement of CD8 T cell function, offering a promising strategy for cancer immunotherapy.