Dendritic cell (DC)-based vaccines have received attention as a new therapeutic modality against cancer. However, increased STAT3 activity in the tumor microenvironment makes DCs tolerogenic and suppresses their antitumor activity. In this study, we explored the effects of a combination treatment consisting of a proteasome inhibitor, bortezomib, and an antigen specific STAT3-ablated (STAT3⁻/⁻) DC-based vaccine on the control of TC-1(P3) tumors, a p53-degraded immune resistant cancer cells. We found that E7-antigen expressing STAT3⁻/⁻ DC (E7-DC-1STAT3⁻/⁻) vaccination enhanced generation of E7-specific CD8⁺ T cells, but was not enough to control TC-1(P3) cancer cells. Therefore, we investigated whether bortezomib could create a synergistic effect with E7-DC-1STAT3⁻/⁻ vaccination. We found that apoptosis via down-regulation of STAT3 and NF-κB and up-regulation of Fas and death receptor 5 (DR5) expression in TC-1(P3) induced by bortezomib was independent of p53 status. We also observed that TC-1(P3) cells pretreated with bortezomib had markedly enhanced anti-tumor effects on E7-specific CD8⁺ T cells through a Fas/DR5-mediated mechanism. In addition, TC-1(P3) tumor-bearing mice treated with bortezomib prior to vaccination with E7-DC-1STAT3⁻/⁻ demonstrated enhanced generation of E7-specific CD8⁺ T cells and prolonged survival compared to those treated with monotherapy. These results suggest that the anti-tumor effects against a p53-degraded immune resistant variant generated by antigen-expressing STAT3-ablated mature DCs may be enhanced by bortezomib via death receptor-mediated apoptosis.
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