Adenoviral Vectored Neoantigen Vaccine Augments Hyperexpanded CD8+ T Cell Control of Tumor Challenge in Mice

Neoantigens are promising immunogens for cancer vaccines and are traditionally delivered as adjuvanted peptide vaccines. Our goal was to understand how an adenoviral vectored neoantigen vaccine would induce tumor immunity compared to a peptide neoantigen vaccine. We generated adenovirus serotype 26 (Ad26) vaccines encoding MC38-specific neoantigens and compared them to an adjuvanted peptide MC38 neoantigen vaccine. The single-shot Ad26 vaccines induced greater neoantigen specific IFN- CD8+ T cell immune responses than the two-shot adjuvanted peptide vaccine in mice, and Ad26.VP22.7Epi also provided superior protective efficacy compared to the peptide vaccine following tumor challenge. Ad26.VP22.7Epi induced a robust immunodominant CD8+ T cell response against the Adpgk neoantigen, while the peptide vaccine induced lower responses against both Adpgk and Reps1 neoantigens. Tumor infiltrating lymphocytes (TILs) from both vaccine groups were analyzed using scRNA-seq and TCR-seq. Vaccinated mice showed increased CD8+ T cell infiltration, with the peptide vaccine inducing more infiltrating CD8+ T cells than the Ad26.VP22.7Epi vaccine. However, Ad26.VP22.7Epi induced CD8+ T cells showed more upregulation of T cell maturation, activation, and Th1 pathways compared to peptide vaccine induced CD8+ T cells, suggesting improved functional T cell quality. TCR-seq of these TILs also demonstrated that Ad26.VP22.7Epi generated larger T cell hyperexpanded clones compared to the peptide vaccine. These results suggest that the Ad26.VP22.7Epi vaccine led to improved tumor control compared with the peptide vaccine due to increased T cell hyperexpansion and functional activation. Our data suggest that future cancer vaccine development strategies should focus on inducing functional hyperexpanded CD8+ T cell responses and not only maximizing tumor infiltrating CD8+ T cell numbers.