DNA vaccination combined with immune checkpoint inhibition eradicates tumors, inducing life-long immunity against breast cancer in mice

The employment of cancer vaccines as stand-alone or combined therapies has not yet reached clinically relevant endpoints in large clinical trials in the vast majority of patients, and there is a clear need for novel ideas and qualitatively new vaccine design approaches. In this study, we used a novel Variable Epitope Library (VEL) vaccine strategy, which incorporates thousands to millions of mutated variant epitopes within a combinatorial library, to target extreme variability and intratumoral heterogeneity of tumor antigens. A single intrasplenic vaccination with a VEL DNA vaccine encoding the amino-terminal region of mouse survivin, carrying eight mutated amino acid positions, induced significant tumor growth inhibition and suppression of lung metastasis in an aggressive and highly metastatic 4T1 triple-negative breast cancer (TNBC) preclinical model. Combining this vaccine with an immune checkpoint inhibitor (ICI) αCTLA-4 resulted in the elimination of established tumors, tumor-free survival of up to 412 days and life-long sterile immunity against tumor rechallenge in 77 % of mice. A significant increase in the number of CD3⁺ CD8⁺ Ly6C⁺ effector T cells in the lungs and spleens of vaccinated mice and the presence of central memory (T_CM) and effector memory (T_EM) T cells at different time points was documented. Likewise, the reduction of numbers of CD11b⁺ Ly6C^int Ly6G⁺ granulocytic myeloid-derived suppressor cells (G-MDSC) in vaccinated mice was observed. These data suggest that VEL immunogens are feasible candidates for inclusion/testing in clinical trials targeting multiple cancer types due to their universal nature.