Treatment of transgenic murine retinoblastoma with attenuated herpes virus

Purpose: To test the effect of a recombinant herpes simplex virus (HSV) attenuated for neurovirulence on growth and morphology of retinoblastoma in vivo. Methods: At the age of 16 weeks the right eye of 17 LHβ transgenic mice, a murine model for retinoblastoma, were injected with 10⁶ pfu of an intertypic recombinant of HSV1 and HSV2, termed RE6. Mice were sacrificed 3 weeks later, eyes were enucleated and fixed in formalin. Tumor characteristics were graded on hematoxylin and eosin stained sections and the tumor area was measured by digital image analysis. The viral antigen HSV1 was identified using immunohistochemical staining with a polyclonal antibody. Viral titers were measured in 5 eyes from LHβ transgenic mice and in 5 eyes from normal mice at 1, 3, 5, 7 and 10 days after injection with 10⁶ pfu of RE6 virus. Results: Tumor area was significantly smaller in treated compared to control eyes (p=0.038). Treated tumors were less differentiated (p=0.04) and showed fewer rosettes (p=0.049) than control tumors. Other morphologic changes were similar in both groups.The RE6 virus grew less well in LHβ transgenic mice but cleared by 7 days post infection in both groups of mice. Immunohistochemistry for the HSV1 antigen revealed focal areas of positive staining within the tumor and the retinal pigment epithelium in treated but not in control eyes. Conclusions: Attenuated herpesvirus is able to significantly reduce growth of an endogenously arising tumor and induces few other pathologic changes.