CD4 T-cell Epitope-based Heterologous Prime-boost Vaccination Potentiates Anti-tumor Immunity and PD-1/PD-L1 Immunotherapy

Background: Antitumor therapeutic vaccines are generally based on antigenic epitopes presented by major histocompatibility complex (MHC-I) molecules to induce tumor-specific CD8 T cells. Paradoxically, continuous T cell receptor (TCR) stimulation from tumor-derived CD8 T-cell epitopes can drive the functional exhaustion of tumor-specific CD8 T cells. Tumor-specific type-I helper CD4 T (T1) cells play an important role in the population maintenance and cytotoxic function of exhausted tumor-specific CD8 T cells in the tumor microenvironment. Nonetheless, whether the vaccination strategy targeting MHC-II-restricted CD4 T-cell epitopes to induce tumor-specific T1 responses can confer effective antitumor immunity to restrain tumor growth is not well studied. Here, we developed a heterologous prime-boost vaccination strategy to effectively induce tumor-specific T1 cells and evaluated its antitumor efficacy and its capacity to potentiate PD-1/PD-L1 immunotherapy.

Methods: vector and influenza A virus (PR8 strain) vector stably expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein-specific I-A-restricted CD4 T cell epitope (GP) or ovalbumin-specific CD4 T cell epitope (OVA) were constructed and evaluated their efficacy against mouse models of melanoma and colorectal adenocarcinoma expressing lymphocytic choriomeningitis virus glycoprotein and ovalbumin. The impact of CD4 T cell epitope-based heterologous prime-boost vaccination was detected by flow-cytometer, single-cell RNA sequencing and single-cell TCR sequencing.

Results: CD4 T cell epitope-based heterologous prime-boost vaccination efficiently suppressed both mouse melanoma and colorectal adenocarcinoma. This vaccination primarily induced tumor-specific T1 response, which in turn enhanced the expansion, effector function and clonal breadth of tumor-specific CD8 T cells. Furthermore, this vaccination strategy synergized PD-L1 blockade mediated tumor suppression. Notably, prime-boost vaccination extended the duration of PD-L1 blockade induced antitumor effects by preventing the re-exhaustion of tumor-specific CD8 T cells.

Conclusion: CD4 T cell epitope-based heterologous prime-boost vaccination elicited potent both tumor-specific T1 and CTL response, leading to the efficient tumor control. This strategy can also potentiate PD-1/PD-L1 immune checkpoint blockade (ICB) against cancer.