Xiao, Xie, Cao, and Lei et al. showed that prophylactic and therapeutic i.p. or s.c. prime-boost heterologous vaccine strategies using exogenous or endogenous tumor-associated CD4+ T cell epitopes reduced tumor growth (which required CD4+ and CD8+ T cells), metastases, and post-resection recurrence, and enabled recall responses in mouse models. Vaccination induced CD4+ TILs that were TH1-polarized, promoted polyclonal expansion and effector function of tumor-specific CD8+ TILs, synergized with anti-PD-L1, and sustained antitumor effects by preventing tumor-specific CD8+ T cell re-exhaustion after stopping anti-PD-L1 treatment.
Contributed by Paula Hochman
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 (T(H)1) 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 T(H)1 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 T(H)1 cells and evaluated its antitumor efficacy and its capacity to potentiate PD-1/PD-L1 immunotherapy. METHODS: Listeria monocytogenes vector and influenza A virus (PR8 strain) vector stably expressing lymphocytic choriomeningitis virus (LCMV) glycoprotein-specific I-A(b)-restricted CD4(+) T cell epitope (GP(61-80)) or ovalbumin-specific CD4(+) T cell epitope (OVA(323-339)) 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 T(H)1 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 T(H)1 and CTL response, leading to the efficient tumor control. This strategy can also potentiate PD-1/PD-L1 immune checkpoint blockade (ICB) against cancer.