Cancer vaccines compensate for the insufficient induction of protective tumor-specific immunity of CD3 bispecific antibody therapy.

Background: CD3 bispecific antibody (CD3 bsAb) therapy has become an established treatment modality for some cancer types and exploits endogenous T cells irrespective of their specificity. However, durable clinical responses are hampered by immune escape through loss of tumor target antigen expression. Induction of long-lasting tumor-specific immunity might therefore improve therapeutic efficacy, but has not been studied in detail yet for CD3 bsAbs. Here, we examined multiple combination strategies aiming to improve survival rates in solid tumors and, simultaneously, install endogenous immunity capable of protection to tumor rechallenge.

Methods: Two syngeneic mouse tumor models were employed: The immunologically "cold" B16F10 melanoma and the immunologically "hot" MC38.TRP1 colon carcinoma model. Mice were treated with CD3xTRP1 bsAb (murine Fc-inert immunoglobulin G2a) as monotherapy, or in combination with agonistic costimulatory antibodies, Fc-active tumor-opsonizing antibodies, or tumor-(non)specific vaccines. Treatment efficacy of primary tumors and protection from rechallenge was monitored, as well as induction of tumor-specific T-cell responses.

Results: In the immunologically "cold" B16F10 model, all combination therapies improved antitumor activity compared with CD3 bsAb monotherapy and induced systemic tumor-specific T-cell responses. However, this endogenous T-cell immunity swiftly waned and failed to protect mice from subsequent tumor rechallenge, except for combination therapy with tumor-specific vaccination. These vaccines strongly improved the therapeutic efficacy of CD3 bsAb against primary tumors and led to long-term immunological protection. In the immunologically "hot" MC38.TRP1 model, CD3 bsAb combined with only the vaccine adjuvant was sufficient to generate protective T-cell immunity and, moreover, prevented tumor escape via antigen loss.

Conclusions: These results demonstrate the impact of tumor antigenicity on the induction of protective endogenous antitumor immunity during CD3 bsAb treatment and, importantly, show that the combination with tumor-specific vaccines improves therapeutic efficacy and installs long-term immunological memory in both "hot" and "cold" tumors.