Human mesenchymal stem cells lack tumor tropism but enhance the antitumor activity of oncolytic adenoviruses in orthotopic lung and breast tumors

Taming the Trojan horse: optimizing dynamic carrier cell/oncolytic virus systems for cancer biotherapy

Live cells offer unique advantages as vehicles for systemic oncolytic virus (OV) delivery. Recent studies from our laboratory and others have shown that virus-infected cells can serve as Trojan horse vehicles to evade antiviral mechanisms encountered in the bloodstream, prevent uptake by off-target tissues and act as microscale factories to produce OV upon arrival in tumor beds. However to be employed effectively, OV-infected cells are best viewed as dynamic biological systems rather than static therapeutic agents. The time-dependent processes of infection and in vivo cell trafficking will inevitably vary depending on which particular OV is being delivered, as well as the type of carrier cells (CC) employed. Understanding these parameters with respect to each unique CC/OV combination will therefore be required in order to effectively evaluate and harness their potential in preclinical study. In the following review, we discuss how early studies of OV delivery led us to investigate the use of cell carriers in our laboratory, and the approaches we are currently undertaking to compare the dynamics of different CC/OV systems. On the basis of these studies and others it is apparent that the success of any cell-based system for OV delivery rests upon the coordinated timing of three sequential phases--(1) ex vivo loading, (2) stealth delivery and (3) virus production at the tumor site. While at the current time, the timing of these processes are coupled to the natural cycle of infection and in vivo trafficking properties innate to each cell virus system, a quantitative delineation of their dynamics will lay the foundation for engineering CC/OV biotherapeutic systems that can be clinically deployed in a highly directed and controlled manner.

Cell vehicle targeting strategies

Use of cells as therapeutic carriers has increased in the past few years and has developed as a distinct concept and delivery method. Cell-based vehicles are particularly attractive for delivery of biotherapeutic agents that are difficult to synthesize, have reduced half-lives, limited tissue penetrance or are rapidly inactivated upon direct in vivo introduction. Initial studies using cell-based approaches served to identify some of the key factors for the success of this type of therapeutic delivery. These factors include the efficiency of cell loading with a therapeutic payload, the means of cell loading and the nature of therapeutics that cells can carry. However, one important aspect of cell-based delivery yet to be fully investigated is the process of actual delivery of the cell payload in vivo. In this regard, the potential ability of cell carriers to provide site-specific or targeted delivery of therapeutics deserves special attention. The present review focuses on a variety of targeting approaches that may be utilized to improve cell-based therapeutic delivery strategies. The different aspects of targeting that can be applied to cell vehicles will be discussed, including physical methods for directing cell distribution, intrinsic cell-mediated homing mechanisms and the feasibility of engineering cells with novel targeting mechanisms. Development of cell targeting strategies will further advance cell vehicle applications, broaden the applicability of this delivery approach and potentiate therapeutic outcomes.

Cell carriers to deliver oncolytic viruses to sites of myeloma tumor growth

Multiple myeloma (MM) is a disseminated malignancy of antibody secreting plasma cells that localize primarily to the bone marrow. Several studies have illustrated the potential of utilizing oncolytic viruses (measles, vaccinia, Vesicular Stomatitis Virus and coxsackievirus A21) for the treatment of MM, but there are significant barriers that prevent the viruses from reaching sites of myeloma tumor growth after intravenous delivery. The most important barriers are failure to extravasate from tumor blood vessels, mislocalization of the viruses in liver and spleen and neutralization by antiviral antibodies. In this review, we discuss the use of various cell types as carriers to overcome these barriers, emphasizing their relative susceptibilities to virus infection and their variable trafficking properties. Mesenchymal progenitor cells, monocytes and T cells have all shown promise as virus-delivery vehicles capable of accessing sites of myeloma growth. However, a previously unexplored alternative would be to use primary myeloma cells, or even myeloma cell lines, as delivery vehicles. Advantages of this approach are the natural ability of myeloma cells to home to sites of myeloma tumor growth and their compatibility with tumor-specific viruses that cannot propagate in other carrier cell lineages. A potential difficulty associated with the use of myeloma cells for virus delivery is that they must be exposed to supralethal doses of ionizing radiation before they can be safely administered to patients. Preliminary studies are presented in which we demonstrate the feasibility of using irradiated myeloma cells as carriers to deliver oncolytic viruses to sites of myeloma tumor growth in an orthotopic human myeloma model.

Oncolytic virotherapy: molecular targets in tumor-selective replication and carrier cell-mediated delivery of oncolytic viruses

Tremendous advances have been made in developing oncolytic viruses (OVs) in the last few years. By taking advantage of current knowledge in cancer biology and virology, specific OVs have been genetically engineered to target specific molecules or signal transduction pathways in cancer cells in order to achieve efficient and selective replication. The viral infection and amplification eventually induce cancer cells into cell death pathways and elicit host antitumor immune responses to further help eliminate cancer cells. Specifically targeted molecules or signaling pathways (such as RB/E2F/p16, p53, IFN, PKR, EGFR, Ras, Wnt, anti-apoptosis or hypoxia) in cancer cells or tumor microenvironment have been studied and dissected with a variety of OVs such as adenovirus, herpes simplex virus, poxvirus, vesicular stomatitis virus, measles virus, Newcastle disease virus, influenza virus and reovirus, setting the molecular basis for further improvements in the near future. Another exciting new area of research has been the harnessing of naturally tumor-homing cells as carrier cells (or cellular vehicles) to deliver OVs to tumors. The trafficking of these tumor-homing cells (stem cells, immune cells and cancer cells), which support proliferation of the viruses, is mediated by specific chemokines and cell adhesion molecules and we are just beginning to understand the roles of these molecules. Finally, we will highlight some avenues deserving further study in order to achieve the ultimate goals of utilizing various OVs for effective cancer treatment.

Cancer, stem cells, and oncolytic viruses

Cells with stem cell-like attributes, such as self-renewal and pluripotency, have been isolated from hematological malignancies and from several solid tumor types. Tumor-initiating cells, also referred to as cancer stem cells, are thought to be responsible for the initiation and growth of tumors. Like their normal counterparts, putative cancer stem cells show remarkable resistance to radiation and chemotherapy. Their capacity for surviving apparently curative treatment can result in tumor relapse. Novel approaches that target tumor-initiating cells in addition to differentiated malignant cells, which constitute the bulk of the tumor, are required for improved survival of patients with metastatic tumors. Oncolytic viruses enter cells through infection and may therefore be resistant to defense mechanisms exhibited by cancer stem cells. Oncolytic adenoviruses can be engineered to attack tumor stem cells, recognized by linage-specific cell surface markers, dysfunctional stem cell-signaling pathways, or upregulated oncogenic genes. Normal stem cells may possess innate resistance to adenoviruses, as most humans have sustained numerous infections with various wild-type serotypes. This review focuses on current literature in support of cancer stem cells and discusses the possibility of using oncolytic virotherapy for killing these tumor-initiating cells.

Oncolytic adenoviruses kill breast cancer initiating CD44+CD24-/low cells

Cancer stem cells have been indicated in the initiation of tumors and are even found to be responsible for relapses after apparently curative therapies have been undertaken. In breast cancer, they may reside in the CD44(+)CD24(-/low) population. The use of oncolytic adenoviruses presents an attractive anti-tumor approach for eradication of these cells because their entry occurs through infection and they are, therefore, not susceptible to those mechanisms that commonly render stem cells resistant to many drugs. We isolated CD44(+)CD24(-/low) cells from patient pleural effusions and confirmed stem cell-like features including oct4 and sox2 expression and Hoechst 33342 exclusion. CD44(+)CD24(-/low) cells, including the Hoechst excluding subpopulation, could be effectively killed by oncolytic adenoviruses Ad5/3-Delta24 and Ad5.pk7-Delta24. In mice, CD44(+)CD24(-/low) cells formed orthotopic breast tumors but virus infection prevented tumor formation. Ad5/3-Delta24 and Ad5.pk7-Delta24 were effective against advanced orthotopic CD44(+)CD24(-/low)-derived tumors. In summary, Ad5/3-Delta24 and Ad5.pk7-Delta24 can kill CD44(+)CD24(-/low), and also committed breast cancer cells, making them promising agents for treatment of breast cancer.