We have previously reported the development of a strategy to target the cancer stem cell (CSC) populations in melanoma and squamous cell carcinoma using CSC lysate-pulsed dendritic cells (DCs). Using mouse models we demonstrated that DCs pulsed with CSCs enriched by virtue of their expression of the CSC marker ALDH (termed CSC-DC) significantly inhibited tumor growth. However, CSC-DC vaccine therapy alone may not be sufficient to overcome the immunosuppressive components of the tumor microenvironment. CSC-mediated suppression of T cells has been reported in a process involving the PD-L1/PD-1 axis. Immunologically targeting CSCs while simultaneously blocking PD-L1/PD-1-mendiated immune suppression may significantly enhance the outcome of current immunotherapies of cancer. To test this hypothesis, we investigated the effect of using anti-PD-L1 to block the immunosuppression during the CSC-DC vaccination to augment the therapeutic efficacy of using each regimen alone. We used 4T1 cell line, a mammary carcinoma syngeneic to BALB/c mice, and found that 4T1 tumors contain approx. 6-10% ALDEFLUOR+/ALDHhigh cells, and that these cells are enriched for tumor initiating capacity compared to bulk or ALDEFLUOR-/ALDHlow cells. Inoculating 4T1 cells into the mammary fat pad induces the development of spontaneous pulmonary metastases. ALDHhigh 4T1 CSCs expressed PD-L1. 4T1 ALDHhigh CSC-DC vaccine + anti-PD-L1 administration significantly reduced pulmonary metastases and prolonged survival compared with 4T1 ALDHhigh CSC-DC vaccine or anti-PD-L1 administration alone. Anti-PD-L1 administration increased systematic anti-4T1 CSC immunity induced by CSC-DC vaccine. This is evident by the increased production of total IgG in the serum samples collected from the 4T1-bearing animals subjected to 4T1 CSC-DC vaccination with anti-PD-L1 administration. The CSC-reactivity/specificity of the immune sera was demonstrated by their specific binding to the ALDHhigh vs. ALDHlow 4T1 cells in flow cytometry. Importantly, the immunological consequence of such binding was Ab-mediated ALDHhigh 4T1 CSC lysis via complement dependent cytotoxicity. In addition, CTLs generated from the splenocytes harvested from the hosts subjected to 4T1 CSC-DC vaccination with anti-PD-L1 administration killed ALDHhigh 4T1 CSCs significantly more than ALDHlow 4T1 cells. These results are supportive of the conclusion that administration of anti-PD-L1 as an adjuvant could significantly boost the therapeutic efficacy of the CSC-DC vaccine, and that this effect was due to both antibody and T cell-mediated anti-CSC immunity.

Citation Format: Yangyang Hu, Yang Xia, Xin Chen, Li Zhou, Yangyi Bao, Shiang Huang, Xiubao Ren, Elaine Hurt, Robert E. Hollingsworth, Alfred E. Chang, Max S. Wicha, Qiao Li. Increasing the efficacy of cancer stem cell-based vaccine in the PD-L1/PD-1 blockade. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3329.