Figure 1.

Direct and indirect anti-tumor effects of oncolytic viruses (OVs). Normal cells resist OV infection because of viral life-cycle blockage prior to the induction of cytopathic effects. In contrast, OVs can disturb at least three cell types within the tumor. Infected cancer cells undergo an immunogenic type of death (direct oncolysis), which is sometimes (e.g., figure) but not always accompanied by virus production; this oncolysis leads to the activation of various immune cells (green), irrespective of their direct virus infection, thereby priming anti-cancer immune responses. Furthermore, the (abortive) OV infection of immune and endothelial cells inside the tumor results in cytotoxic cytokine production and anti-vascular/angiogenic effects, respectively, which both contribute to virus-mediated oncolysis in an indirect way. Uninfected tumor cells can also serve as targets for these indirect oncolytic effects.