Abstract B34: Immune response after combining radiotherapy and gene electrotransfer of plasmid DNA encoding shRNA against melanoma cell adhesion molecule in B16F10 melanoma and TS/A carcinoma
Authors: Bosnjak Masa , Brezar Simona Kranjc , Savarin Monika , Jesenko Tanja , Mrak Valter , Sersa Gregor , Cemazar Maja 
Published: March 2020

Radiotherapy is one of the most frequent treatment modalities of cancer. To improve its efficacy, it can be combined with adjuvant treatments. One of the possibilities is gene electrotransfer (GET) of plasmid encoding shRNA against melanoma cell adhesion molecule (MCAM; pMCAM). GET of pMCAM has dual action: vascular targeted effect mediated by silencing of MCAM and immunologic effect mediated by presence of foreign plasmid DNA in the cytosol, which activates cytosolic DNA sensors. The aim of our study was to determine the effects of GET of pMCAM combination with radiotherapy in two murine tumors, which differ in in immunologic status: B16F10 melanoma (immunologically “hot” tumor) and TS/A carcinoma (“cold” tumor) and to elucidate the underlying mechanisms. B16F10 melanoma, growing in C57Bl/6 mice, and TS/A carcinoma, growing in BALB/c mice, were treated by triple GET (repeated every 2 days) and single dose of irradiation (IR) 15 Gy performed 1 day after the first GET. Tumor growth delay and tumor cures were determined. Proliferation, apoptosis, necrosis, vascularization, hypoxia, and infiltration of immune cells were evaluated histologically. Silencing of MCAM potentiated the effect of IR in both tumor models. However, melanoma was significantly more responsive, even to GET of pMCAM without irradiation, than carcinoma. Combined therapy resulted in 81% complete responses (CR) in melanoma and 27% CR in carcinoma, while GET of pMCAM without irradiation resulted in 36% CR and 0% CR, respectively. Moreover, after secondary challenge of mice with CR, there was 59% CR in melanoma and 0% CR in carcinoma model. GET of pMCAM combined with IR reduced the number of blood vessels, induced hypoxia, apoptosis, necrosis, and reduced tumor cell proliferation in both tumor models to similar extent. However, the significant increase of infiltrating immune cells was observed only in melanoma model. Moreover, expression of IL-12 and TNF-γ in melanoma was elevated after combining GET with IR. Immune response can participate in overall antitumor response of combined treatment, but only in immunologically “hot” tumors, while in immunologically “cold” tumor the enhancement of IR can be ascribed only to the action of the expression of the therapeutic molecule. Thus, the properties of tumor microenvironment are crucial for sufficient immune system contribution to overall antitumor effect.

Citation Format: Masa Bosnjak, Simona Kranjc Brezar, Monika Savarin, Tanja Jesenko, Valter Mrak, Gregor Sersa, Maja Cemazar. Immune response after combining radiotherapy and gene electrotransfer of plasmid DNA encoding shRNA against melanoma cell adhesion molecule in B16F10 melanoma and TS/A carcinoma [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2019 Nov 17-20; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(3 Suppl):Abstract nr B34.