CD28 costimulatory bispecific antibodies (TAA×CD28) provide a targeted means of augmenting T cell activation within tumors, yet their efficacy can be limited by restricted T cell diversity and an immunosuppressive tumor microenvironment. We show that pairing TAA×CD28 bispecifics with cytotoxic therapies—including conventional chemotherapy or antibody–drug conjugates (ADCs)—substantially enhances antitumor immunity in genetically humanized murine models.
An EGFR×CD28 bispecific synergized with 5‑fluorouracil (5‑FU) to expand naïve‑like and early effector CD8⁺ T cells, increase TCR clonal diversity, and reprogram tumor‑resident myeloid cells toward immunostimulatory states that support T cell activity. These complementary effects translated into significantly improved tumor control.
A novel HER2×CD28 bispecific combined with trastuzumab deruxtecan (T‑Dxd) achieved complete, CD8⁺ T cell–dependent tumor clearance and durable memory responses. Importantly, the ADC combination uniquely increased the abundance and activation state of intratumoral dendritic cells, marked by elevated CD80/CD86 expression. This DC activation likely reflects enhanced antigen release by ADC‑mediated tumor cell killing and contributes to improved recruitment, priming, and maintenance of a broader CD8⁺ T cell repertoire under CD28 costimulation.
Across both models, cytotoxic therapies complemented CD28 signaling by broadening T cell clonotypes, enhancing immune infiltration, and reshaping the tumor microenvironment to be more permissive to productive antitumor immunity. These findings identify chemotherapy and ADCs as rational partners for CD28 costimulatory bispecific antibodies and support advancing these combination strategies for solid tumors with limited immune responsiveness.