MicroRNA regulation of glycoprotein B5R in oncolytic vaccinia virus reduces viral pathogenicity without impairing its antitumor efficacy.

Vaccinia virus, once widely used for smallpox vaccine, has recently been engineered and used as an oncolytic virus for cancer virotherapy. Their replication has been restricted to tumors by disrupting viral genes and complementing them with products that are found specifically in tumor cells. Here, we show that microRNA (miRNA) regulation also enables tumor-specific viral replication by altering the expression of a targeted viral gene. Since the deletion of viral glycoprotein B5R not only decreases viral pathogenicity but also impairs the oncolytic activity of vaccinia virus, we used miRNA-based gene regulation to suppress B5R expression through let-7a, a miRNA that is downregulated in many tumors. The expression of B5R and the replication of miRNA-regulated vaccinia virus (MRVV) with target sequences complementary to let-7a in the 3'-untranslated region (UTR) of the B5R gene depended on the endogenous expression level of let-7a in the infected cells. Intratumoral administration of MRVV in mice with human cancer xenografts that expressed low levels of let-7a resulted in tumor-specific viral replication and significant tumor regression without side effects, which were observed in the control virus. These results demonstrate that miRNA-based gene regulation is a potentially novel and versatile platform for engineering vaccinia viruses for cancer virotherapy.