The high efficacy of therapeutic cancer vaccines in preclinical studies has yet to be fully achieved in clinical trials. Tumor immune suppression is a critical factor that hampers the desired antitumor effect. Here, we analyzed the combined effect of a cancer vaccine and the receptor tyrosine kinase inhibitor sunitinib. Sunitinib was administered intraperitoneally, alone or in combination with intramuscular immunization using a viral vector based cancer vaccine composed of Semliki Forest virus replicon particles and encoding the oncoproteins E6 and E7 (SFVeE6,7) of human papilloma virus (HPV). We first demonstrated that treatment of tumor-bearing mice with sunitinib alone dose-dependently depleted myeloid-derived suppressor cells (MDSCs) in the tumor, spleen and in circulation. Concomitantly, the number of CD8+ T cells increased 2–fold and, on the basis of CD69 expression, their activation status was greatly enhanced. The intrinsic immunosuppressive activity of residual MDSCs after sunitinib treatment was not changed in a dose-dependent fashion. We next combined sunitinib treatment with SFVeE6,7 immunization. This combined treatment resulted in a 1.5- and 3-fold increase of E7-specific cytotoxic T lymphocytes (CTLs) present within the circulation and tumor, respectively, as compared to immunization only. The ratio of E7-specific CTLs to MDSCs in blood thereby increased 10- to 20-fold and in tumors up to 12.5-fold. As a result, the combined treatment strongly enhanced the antitumor effect of the cancer vaccine. This study demonstrates that sunitinib creates a favorable microenvironment depleted of MDSCs and acts synergistically with a cancer vaccine resulting in enhanced levels of active tumor-antigen specific CTLs, thus changing the balance in favor of antitumor immunity.

Therapeutic cancer vaccines show promise in preclinical studies, yet their clinical efficacy is limited. Increased recruitment of immune cells into tumors and suppression of the immune suppressive tumor environment are critical components toward effective cancer immunotherapies. Here, we report how local low-dose irradiation, alone or with a therapeutic immunization based on Semliki Forest virus (SFV) against human papillomavirus (HPV)-related cancer, influences these immune mechanisms. We first demonstrated that immunization with SFVeE6,7 or SFVeOVA, replicon particles expressing either HPV16 E6/E7 or ovalbumin, resulted in an antigen-specific migration of CD81 T cells into HPV-and OVA-specific tumors. Local low-dose tumor irradiation alone resulted in a 2-fold increase of intratumoral CD81 T cells. When 14 Gy irradiation was combined with immunization, intratumoral numbers of CD81 T cells increased 10-fold and the number of CD81 T cells specific for the E7-epitope increased more than 20-fold. Irradiation alone however also increased the number of intratumoral myeloid-derived suppressor cells (MDSCs) 3.5-fold. Importantly, this number did not further increase when combined with immunization. As a result, the ratio of antigen-specific CD81 T cells and MDSCs in tumors increased up to 85-fold compared to the control. We furthermore demonstrated that following irradiation CCR2 and CCL2, CXCR6 and CCL16, chemokines and ligands involved in tumor homing of immune cells, were significantly up regulated. This study demonstrates that local low-dose tumor irradiation influences the intratumoral immune population induced by SFVeE6,7 immunization by a strong increase in the ratio of antitumoral to immune suppressive cells, thus changing the intratumoral immune balance in favor of antitumor activity.An essential requirement for cancer immunotherapy is activation of antigen-specific T cells and their homing into tumors. Although antitumoral immunization studies in the clinic report the presence of vaccine-induced antigen-specific T cells in periphery or at the injection site, so far the therapeutic antitumor responses are limited. [1][2][3][4] The most obvious reasons for the lack of clinical responses can be insufficient tumor infiltration of antigen-specific immune effector cells and the immunosuppressive environment of the tumor. [5][6][7][8][9] Strategies aiming at promoting T-cell influx into the tumor or at reverting the immune suppression within the tumor microenvironment are currently the focus of numerous preclinical studies. In a recent review, we addressed various antitumoral therapeutic strategies currently under investigation.