Replication-competent oncolytic viruses are being developed as a promising strategy for treating certain types of cancer. JX-594 is an oncolytic vaccinia virus that lacks thymidine kinase and expresses human granulocyte-macrophage colony stimulating factor (hGM-CSF). Replication of JX-594 is promoted by EGFR/Ras pathway signaling in cancer cells. In addition to having direct oncolytic effects on tumor cells and recruiting an immune response, JX-594 has been found to cause rapid reduction of tumor blood flow in preclinical models and in clinical trials. To understand the mechanism of this vascular effect, we injected mouse-adapted JX-594 vaccinia virus (mJX-594, on Western Reserve backbone and expressing hGM-CSF) intravenously into RIP-Tag2 transgenic mice and examined the pancreatic neuroendocrine tumors at 6 hours or 1, 2, or 5 days after injection. Dot-like vaccinia immunoreactivity was widespread in endothelial cells of tumor vessels at 6 hours. Scattered endothelial cells had intense staining. Vaccinia immunoreactivity in tumor blood vessels was still widespread at 1 day, but at 5 days, most tumor vessels were narrowed, some appeared fragmented, and tumor vascularity was reduced by 40%. Normalization of tumor vessels, as observed after inhibition of VEGF signaling, was not found at any time examined. Intratumoral hypoxia was evident as diffuse regions of pimonidazole staining at 6 hours. Pimonidazole staining increased from 1 to 5 days, and was especially strong in regions of vascular pruning. Extravascular vaccinia immunoreactivity was located in scattered cells at 1 day and increased with time. At 5 days, most tumors had large patches of vaccinia-positive cells. This staining was not present in the surrounding normal acinar pancreas. Apoptotic cells stained for activated caspase-3 were scattered throughout tumors at 6 hours. Most of these apoptotic cells were endothelial cells. Apoptotic tumor cells were sparse at 6 hours, but at 5 days apoptotic cells were 23-fold more numerous than at baseline and even more widespread than vaccinia immunoreactivity, and tumors tended to be smaller than corresponding controls. These findings are consistent with oncolytic virus mJX-594 causing rapid infection of tumor vascular endothelial cells in RIP-Tag2 mice. Endothelial cell infection is followed by regression of tumor blood vessels, exaggeration of intratumoral hypoxia, and oncolytic actions on tumor cells that lead to widespread tumor cell apoptosis.

Citation Format: Barbara Sennino, Brian J. Schriver, Caroline J. Breitbach, Naomi De Silva, John C. Bell, David H. Kirn, Donald M. McDonald. Widespread endothelial cell infection and tumor cell apoptosis after intravenous injection of oncolytic vaccinia virus JX-594 into RIP-Tag2 mice. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2841. doi:10.1158/1538-7445.AM2013-2841