327. Retargetable Oncolytic Measles Viruses for Cancer Virotherapy

Abstract

Replication-competent attenuated Edmonston B vaccine strains of measles virus (MV-Edm) have demonstrated potent anti-tumor activity against xenograft models of human multiple myeloma, ovarian cancer, lymphoma and glioma. The virus is selectively oncolytic in tumor cells, exhibiting extensive cell to cell fusion and finally leading to cell death. However, MV-Edm is not entirely tumor-specific and can infect a variety of cell types via its native receptors, CD46 and SLAM. Receptor recognition is mediated by the viral attachment protein H, which then triggers virus-cell fusion via the fusion protein F. CD46 is a ubiquitous regulator of complement activation found on all human nucleated cells whereas SLAM (signaling lymphocyte activation molecule) is expressed on activated T and B cells, dendritic cells and macrophages. Thus, although not yet tested clinically, it is expected that oncolytic measles viruses may cause immunosupression and unwanted damage to normal tissues. We have therefore engineered the H protein of measles virus to restrict and retarget membrane fusion through various antibody-receptor interactions (See abstract “Targeted cell fusion”). However, ablation of the native receptor interactions is incompatible with virus growth in Vero cells which greatly impedes our ability to rescue fully retargeted viruses. To address this question, we have developed a pseudoreceptor system “STAR”, which allows rescue and propagation of fully retargeted viruses that no longer bind to the native measles receptors. We have generated a Vero-αHis cells expressing membrane-anchored single-chain antibody that recognizes a six histidine peptide (H6). Viruses incorporating H6 at the C-terminus of their ablated H proteins could be rescued and propagated on pseudoreceptor expressing Vero-αHis cells under conditions where interaction between H protein and native receptor CD46 was absent. Three different single chain antibodies against CD38, epidermal growth factor receptor (EGFR), or EGFR mutant vIII were displayed at the intermediate site between the ablated H protein and H6 peptides. In all cases the viruses could be easily rescued and propagated on Vero-αHis cells and had the expected host range properties (blind to CD46 and SLAM but efficiently entering cells through their respective targeted receptors). When administered intravenously to athymic mice bearing human EGFR-positive tumor xenografts, only the EGFR-targeted MV-Edm was able to home to the tumor and express its GFP reporter gene. Tumor transduction was not observed in tumors from mice receiving viruses retargeted to CD38 or EGFR-vIII. Our data provide a first convincing in vivo demonstration of measles retargeting and suggest that it will be possible to target a variety of tumors through a broad array of cell surface antigens.