Human pathogenic viruses are increasingly common vehicles for the development of novel therapeutic agents against malignant glioma (Gromeier, 2001; Gromeier and Wimmer, 2001; Gromeier et al., 2000a). The first reported incidences of viral oncolysis were due to nonintended exposure to naturally occurring viruses or after administration of live attenuated vaccine strains (DePace, 1912; reviewed in Sincovics and Horvath [1993]). New prospects for genetically manipulating viruses have opened possibilities for increasing the tumor specificity and lowering the toxicity of oncolytic viral agents (reviewed in Gromeier [2002]). These efforts have given rise to oncolytic adenoviruses (Bischoff et al., 1996), herpesviruses (Martuza et al., 1991), reoviruses (Coffey et al., 1998), vesicular stomatitis virus (Stojdl et al., 2000), and most recently, polioviruses (Gromeier et al., 2000b). The antineoplastic effects of oncolytic viruses are subject to multifaceted interactions with the host cell. The primary prerequisite for viral oncolysis is expression of cellular receptors mediating viral entry in malignant cell types. In preparation for clinical applications using oncolytic viruses, analysis of viral receptor expression in the target tissue is highly desirable in order to select tumor types and patients most likely to respond favorably to therapeutic intervention. The critical influence of virus tropism determinants on therapeutic efficacy has recently become evident with oncolytic adenoviruses and adenovirus-derived gene delivery vehicles directed to malignant tissues. Targeting of adenoviral vectors to malignant tissues is greatly limited by low or absent expression of a critical adenovirus attachment and entry molecule, the Coxsackie adenovirus receptor (CAR3; Asaoka et al., 2000; Bergelson et al., 1997; Douglas et al., 2001; Li, D., et al., 1999; Li, Y., et al., 1999; Tomko et al., 1997). Moreover, CAR expression levels in established cell lines frequently used for pre-clinical testing of these agents did not correspond to expression in tumor tissues. The failure of adenovirus-based therapeutic agents to reach and infect the intended target due to low CAR expression has inspired efforts to alter receptor specificity of adenoviruses in order to broaden its tropism (reviewed in Curiel [1999]). Cellular entry of poliovirus appears to be mediated by a single cell-surface molecule, the immunoglobulin superfamily (IGSF) member CD155 (Mendelsohn et al., 1989). Although a physiological function for CD155 is unknown, the protein binds specifically to the extracellular matrix component vitronectin (Lange et al., 2001). Transcriptional regulation of the CD155 gene is tightly controlled (Solecki et al., 1999, 2000). In accordance with the restricted tropism of poliovirus for anterior horn spinal cord motor neurons, activity of the CD155 promoter was restricted to the floor plate, notochord, and the anterior spinal cord (Gromeier et al., 2000a). Confirming the activity pattern of the CD155 promoter in the developing spinal cord, morphogenic factors active in the floor plate and notochord — the transcription factors sonic hedgehog (shh) and its downstream gli effectors—strongly activate the CD155 promoter and induce CD155 expression (Solecki et al., 2002). Both shh and gli transcription factors have been implicated in the oncogenesis of neuroectodermal tumors (Goodrich et al., 1997; Kinzler et al., 1987). Thus, the role of shh and gli transcription factors in CD155 gene regulation suggested that CD155 expression may occur ectopically in neuroectodermal malignancies. TagE4, a rodent member of the nectin family recently proposed to be the true rodent CD155 homolog (Baury et al., 2001), is ectopically upregulated in colon and mammary tumors in mice (Denis, 1998). CD155 has been reported to be overexpressed in human colorectal tumors (Masson et al., 2001). Evidence for CD155 expression in neuroectodermal tumors stems mainly from studies of neuroectodermal tumor cell lines that are susceptible to oncolytic poliovirus-based agents (Gromeier et al., 2000b). However, no systematic CD155 expression analysis has been performed on CNS tumor tissues. Determination of CD155 expression levels is notoriously difficult because of the exceedingly low expression levels, even in cell lines routinely used for the propagation of poliovirus and in confirmed sites of expression, such as human spinal cord motor neurons (Bernhardt et al., 1994). To address this problem, we have generated new polyclonal α-CD155 antibodies directed against a recombinant soluble segment of the antigen. Combining solid-phase antigen capture procedures with Western blot detection of CD155 using our new antibodies proved to be an efficient means of conducting comprehensive and reliable expression analyses in tissue samples and cell lines. Analysis of 6 cases indicated that CD155 overexpression is commonly associated with high-grade malignant glioma (HGL). CD155 expression levels in tumor tissues corresponded to those in primary tissue cultures derived from the tumors. Furthermore, CD155 expression in primary glioma explant cultures was equivalent to that found in established glioma cell lines used in preclinical evaluations of oncolytic poliovirus recombinants. In accordance with consistent expression of CD155, primary explant cultures of glioma tissue from all 6 cases displayed exquisite sensitivity to the oncolytic poliovirus recombinant PVS-RIPO. We have thus provided evidence for aberrant expression of a molecular target for oncolytic poliovirus recombinants in HGL. Our findings demonstrate that individual tumor characterization for viral cellular receptor expression can aid in the identification of targets most likely to respond to oncolytic virus therapy.