Harnessing the unique immune biology of cytomegalovirus for cancer immunotherapy

Abstract

Abstract Pre-clinical models in non-human primates demonstrate that cytomegalovirus (CMV)-vectored vaccines are unique in their ability to elicit and indefinitely maintain high frequencies of polyfunctional effector memory T cells to heterologous pathogen antigens, including in animals that are already chronically CMV infected. By introducing defined genetic modifications into the CMV backbone it is possible to program CD8+ T cell responses that are either directed to MHC-I, MHC-II or the non-classical MHC-I molecule MHC-E. In progress clinical studies are evaluating CMV-based vaccine immunity in humans as potential vaccines against HIV. To see if the pre-clinical findings could be extended to cancer antigens we inserted known tumor associated antigens (TAA) or viral TAA into genetically modified rhesus CMV (RhCMV) and characterized the T cell response in rhesus macaques. We found T cell responses to all TAA that were comparable to pathogen antigen-specific responses in frequency, duration, phenotype, epitope density and MHC-restriction. Since many of these TAA are expressed in healthy tissue, this suggests that CMV-vectored cancer vaccines are well-suited to overcome immunological tolerance. As such, CMV-based vectors expressing TAAs could be a powerful new tool for cancer immunotherapy. We show that TAA-specific, MHC-E restricted CD8+ T cells from RhCMV/TAA-immunized RM are stimulated by TAA-expressing cancer tissues and cell lines, indicating that cancer cells can present TAA-derived peptides via MHC-E. Since MHC-E is often upregulated in cancer cells to engage the inhibitory NKG2A receptor on tumor-infiltrating T and NK cells, these results suggest that MHC-E could be used as a novel target for T cell-based immunotherapies.