Leukemia vaccine overcomes limitations of checkpoint blockade by evoking clonal T-cell responses in a murine acute myeloid leukemia model

We have developed a personalized vaccine in which patientderived leukemia cells are fused to autologous dendritic cells, evoking a polyclonal T-cell response against shared antigens and neoantigens. We postulated that the dendritic cell/acute myeloid leukemia fusion vaccine would demonstrate synergy with checkpoint blockade by expanding tumor antigen-specific lymphocytes that would provide a critical substrate for checkpoint blockade-mediated activation. Using an immunocompetent murine leukemia model, we examined the immunological response to and therapeutic efficacy of vaccination in conjunction with checkpoint blockade with respect to leukemia engraftment, disease burden, survival and the induction of tumor-specific immunity. Mice treated with checkpoint blockade alone had rapid progression of leukemia and only a modest extension of survival. Vaccination with dendritic cell/acute myeloid leukemia fusions resulted in the expansion of tumor-specific lymphocytes and disease eradication in a subset of animals, while the combination of vaccination and checkpoint blockade induced a fully protective tumor-specific immune response in all treated animals. Vaccination followed by checkpoint blockade resulted in upregulation of genes regulating activation and proliferation in memory and effector T cells. Long-term survivors exhibited increased T-cell clonal diversity and were resistant to subsequent tumor challenge. The combination of dendritic cell/acute myeloid leukemia fusion vaccine and checkpoint blockade treatment offers unique synergy, inducing durable activation of leukemia-specific immunity, protection from lethal tumor challenge and the selective expansion of tumor-reactive clones.