610. Elucidating the mechanism(s) for tumor specificity of attenuated measles virus

Attenuated Edmonston strain of measles virus (MV-Edm) is a potent oncolytic agent which inhibited the growth or caused complete remission of a variety of human tumor xenografts in mice. Interestingly, the virus is tumor selective, causing extensive cytopathic effects (cpe) of cell-to-cell fusion (syncytium) in tumor cells but minimal cytopathic damage or cell killing in non-transformed cells. The mechanism(s) underlying tumor specificity of MV-Edm is unknown and is the focus of this study. The oncolytic activity of MV-Edm is mediated through the CD46, a complement regulatory protein, and the measles hemagglutinin (H) and fusion (F) protein. The virus uses its H protein to attach to CD46 and uses F to mediate virus-cell entry or cell-to-cell fusion which results in syncytia formation. Hence, a differential in cpe between tumor cells and non-transformed cells could be due to a difference in receptor expression on the surfaces of these cells or due to a difference in H/F expression levels. Indeed, tumor cells are known to express higher levels of CD46 receptors compared to their normal counterparts. To define the role of CD46 receptor density in MV-Edm infection, a panel of Chinese hamster ovary (CHO) clones expressing a range of surface densities of CD46 was generated. Using these CHO transfectants, a recombinant MV-Edm expressing a green fluorescent protein (MV-eGFP) and an adenoviral vector expressing MV-H/F proteins, we established the correlation between CD46 receptor density, virus entry and cell fusion. Virus entry increased progressively with CD46 receptor density and was quite efficient even at relatively low receptor densities. In contrast, syncytium formation and cell killing was minimal at low CD46 densities but increased rapidly above a critical threshold in CD46 expression level, more typical of tumor cells. The median survival of mice bearing low, medium and high CD46 expressing CHO clones infected with oncolytic MV-Edm, and injected into the flank of nude mice confirmed that high CD46 receptor density is critical to the therapeutic potency of this oncolytic agent. This unique relationship between cytopathic effects and CD46 density, coupled with overexpression of CD46 on tumor cells makes MV-Edm an appealing targeted agent for virotherapy.