Date Published: May 31, 2019
Publisher: Public Library of Science
Author(s): Spela Kos, Alessandra Lopes, Veronique Preat, Maja Cemazar, Ursa Lampreht Tratar, Bernard Ucakar, Kevin Vanvarenberg, Gregor Sersa, Gaelle Vandermeulen, Fabrizio Mattei.
We aimed to explore whether the combination of intradermal DNA vaccination, to boost immune response against melanoma antigens, and immune checkpoint blockade, to alleviate immunosuppression, improves antitumor effectiveness in a murine B16F10 melanoma tumor model. Compared to single treatments, a combination of intradermal DNA vaccination (ovalbumin or gp100 plasmid adjuvanted with IL12 plasmid) and immune checkpoint CTLA-4/PD-1 blockade resulted in a significant delay in tumor growth and prolonged survival of treated mice. Strong activation of the immune response induced by combined treatment resulted in a significant antigen-specific immune response, with elevated production of antigen-specific IgG antibodies and increased intratumoral CD8+ infiltration. These results indicate a potential application of the combined DNA vaccination and immune checkpoint blockade, specifically, to enhance the efficacy of DNA vaccines and to overcome the resistance to immune checkpoint inhibitors in certain cancer types.
In recent years, the field of cancer immunotherapy has considerably expanded with several new treatment options . Among them, DNA vaccines hold a great promise in prevention and treatment of different types of cancer. DNA vaccines are promising for cancer immunotherapy since they induce a broad immune response  with activation of both cellular and humoral arms of the adaptive immune system . However, the clinical ability of DNA vaccines is still limited due to the poor immune response initially observed in humans. In order to increase the immunogenicity of DNA vaccines, novel improvements have been incorporated to the DNA vaccine platform, such as plasmid optimization, delivery by in vivo gene electrotransfer and use of genetically encoded immune adjuvants . Gene electrotransfer is a well-established non-viral gene delivery method that has been used to deliver naked DNA or RNA to various tissues. Among them, gene electrotransfer of DNA vaccines into the skin has raised much attention, mainly due to the extended number of dendritic cells present in skin layers . These cells are key players of the immune system able to orchestrate the activation and proliferation of T lymphocytes . Skin appears thus as an ideal target for DNA vaccine administration and cutaneous gene electrotransfer of DNA has already demonstrated to be safe and efficient delivery technique, highly applicable to the clinical setting [7–9].
The combined treatment of intradermal DNA vaccination and dual CTLA-4 and PD-1 blockade generated more robust antitumor activity compared to each treatment alone. It is now clear that different tumor types differentially respond to immunotherapy . In this study performed with an aggressive murine melanoma model, we observed at day 15 that tumors were smaller in mice treated with αCTLA-4/αPD-1 antibodies alone compared to the naïve group. However, at later time-points, tumors were growing and the survival of mice was finally not improved by this single treatment. In addition, DNA vaccination alone failed to reject B16F10-OVA tumors although immunized mice strongly responded against OVA antigen with complete killing of OVA SIINFEKL peptide-loaded target cells and high production of detected anti-OVA IgG antibodies. Compared to single treatments, combined treatments resulted in increased intratumoral CD8+ T cells, what seems to be a crucial factor for successful tumor elimination based on present experiments and literature . Indeed, the combination of pOVA DNA vaccine and immune checkpoint inhibitors induced strong activation of antigen-specific immune response. In B16F10-OVA tumor-bearing mice, this was demonstrated by the complete elimination of target cells pulsed with OVA antigen together with significantly increased OVA-specific cytotoxic CD8+ T cells in tumors and increased production of anti-OVA antibodies in serum. Similarly, combined treatment with pGP100 DNA vaccine and immune checkpoint blockade contributed to a higher percentage of CD4+ T helper cells in spleen together with increased infiltration of cytotoxic CD8+ T cells in tumors. Each treatment alone failed to induce the intratumoral infiltration of cytotoxic lymphocyte T, indicating that DNA vaccination or antibody therapies alone are insufficient to promote complete tumor cell killing. In contrast, strong activation of the immune response induced by combined treatment resulted in significant tumor growth delay and prolonged survival of the treated mice with a high level of complete responses. Half of the mice in combined group completely rejected a secondary B16F10 tumor re-challenge, indicating that the mice treated with DNA vaccine together with immune checkpoint blockade developed memory T cells and established long-term immunological memory to tumor antigens expressed in B16F10 tumors [27,28].