Potential of tumour cells for delivering oncolytic viruses - Nature

Gene Therapy (2008) 15, 704–710 & 2008 Nature Publishing Group All rights reserved 0969-7128/08 $30.00 www.nature.com/gt

REVIEW

Potential of tumour cells for delivering oncolytic viruses Z Raykov and J Rommelaere Infection and Cancer Program, Abt. F010 and INSERM U701, German Cancer Research Centre, Heidelberg, Germany

Autologous or allogenic tumour cells have long been used in the fight against cancer as vaccines to awaken the patient’s immune system. On the other hand, oncolytic viruses have emerged in recent years as powerful therapeutic tools for selectively killing tumour cells. Yet despite recent improvements in virus production, administration and targeting, the latter strategy remains limited by poor access of oncolytic viruses to primary and metastatic tumour cells. The present

review focuses on how to overcome these limitations on oncolytic virus delivery, at least in part, through the use of tumour-derived or in vitro transformed carrier cells. On the basis of existing evidence, novel strategies are proposed for using such cell vehicles, alone or in combination, both as virus factories and as anticancer vaccines. Gene Therapy (2008) 15, 704–710; doi:10.1038/gt.2008.34; published online 20 March 2008

Keywords: oncolytic virus; carrier cell; vehicle; metastases; cancer vaccine

Rationale for using transformed cells to deliver oncolytic viruses The use of cells as oncolytic virus (OV) delivery vehicles offers a number of potential advantages.1,2 One lies in the tissue-homing capacity of certain cancer cells.3,4 This specificity can combine with the intrinsic cell tropism of the virus and contribute to restricting virus infection to the target neoplastic lesions, thereby minimizing deleterious side effects. Second, carrier cells can be engineered to express a set of transgenes exceeding the cloning capacity of the viral vector considered and can be used to reinforce the genuine activity of the OV in various ways. Third, carrier cells can act as local virus factories allowing high amplification of the inoculated virus.5 Finally, OVs harboured by carrier cells remain at least partly protected from inactivation by the host until released at their destination.6 In the present review, emphasis will be placed on tumour cell carriers, while the use of mesenchymal, endothelial or bone marrow progenitor cells with different OV systems will be addressed elsewhere in this issue. If carrier cells are to be used for OV delivery, tumourderived or in vitro transformed cells would seem to be optimal candidates for efficient production of progeny virus bursts in the vicinity of tumours. Besides being permissive for OV multiplication, these cells should survive infection (and other treatments used to ensure their innocuousness) long enough to arrest in the target Correspondence: Dr Z Raykov, Infection and Cancer Program, Abt. F010 and INSERM U701, Deutsches Krebsforschungszentrum, German Cancer Research Centre, Nuenheimer Feld 242, Postfach 101949, Baden Wuertenberg, Heidelberg D-69009, Germany. E-mail: [email protected] Received 13 February 2008; accepted 15 February 2008; published online 20 March 2008

neoplastic tissues and sustain virus production for a sufficient time. The choice of an appropriate carrier/OV system is dictated by the nature of the target malignancy. Whether the tumour is local or disseminated is particularly relevant. Metastatic spreading of primary tumours is indeed one of the central problems of cancer in terms of both long-term patient survival and therapeutic protocol design. Targeting scattered metastases is a tougher therapeutic challenge than treating primary cancers. Metastasis-derived carrier cells are promising in this case as well, since metastatic cells combine distinct tissue-homing properties with enhanced resistance to shear stress and apoptosis upon extravasation.7 The safety concerns raised by the use of tumour-derived or transformed cells as vehicles are discussed below. Identifying optimal carrier cells for a particular cancer application is not a straightforward process. This is discussed below in relation to the pros and cons of various strategies. Some approaches are exemplified by recent data obtained in our laboratory with autonomous parvoviruses, which are naturally endowed with oncosuppressive properties.8 Some members of the genus parvovirus (PV), harboured by rodents, are currently under consideration for cancer virotherapy applications because of the following outstanding features: they fail to transform host cells, infection in humans is asymptomatic, and these viruses propagate preferentially in (oncotropism) and kill (oncolysis) neoplastically transformed cells.9 Parvovirus H-1PV exerts its lytic effect on (human) tumour cells from about 48 h post-infection, a time lag compatible with the use of these cells as PV carriers, giving the cells enough time to settle in (metastatic) neoplastic lesions before the onset of virus release. The oncoselectivity of OV implies that cells derived from tumours or generated by in vitro transformation should provide the most efficient producer cells for OV delivery. Figure 1 depicts various delivery strategies

Tumour cell carriers of oncolytic viruses Z Raykov and J Rommelaere

705 1b

2

1a

O2

FACS

3

7

4

6

5 Figure 1 Scheme of strategies and procedures used to generate tumour-derived or in vitro transformed cell carriers for delivery of oncolytic viruses (OVs). (1) Isolation of tumour cells (a) or in vitro transformation of normal cells (b) derived from either a primary tumour, from distant metastases, from draining lymph nodes or from other tissues. (2) Use of organ-specific matrices, hypoxic conditions, fluorescence-activated cell sorting (FACS) sorting or transwell migration assays, to select cell variants endowed with an enhanced potential for homing to tumours/ metastases. (3) Cell engineering so as to express additional homing receptors, immunomodulatory cytokines or safeguard elimination systems. (4) Selection of the most potent OV. (5) Possible optimization of the OV for production by the vehicle/target cell. (6) Systemic administration of infected and inactivated carrier cells to the donor or to another cancer patient. (7) Homing of carrier cells to target neoplastic tissues. As discussed in the main text, this application could be preceded by first-line treatment with free virus or with other cell-based therapeutics.

exploiting such cells. Since both autologous and allogenic cells are envisaged for such applications, both options are discussed below in terms of OV carrier potential.

Potential of autologous tumour cells for OV delivery Clinical and experimental data clearly show that blood-borne metastases from many malignancies exhibit distinct patterns of tissue distribution.3 Although mechanical or anatomical factors contribute to this distribution by leading tumour cell emboli to be trapped in the first capillary bed encountered, there is evidence supporting Paget’s ‘seed and soil’ hypothesis, according to which metastasizing cells (seed) recognize and parasitize certain organs (soil) through specific interactions with endothelial cells or the extracellular matrix.4 Furthermore, emerging evidence suggests that some primary tumour cells release soluble factors that induce a specific population of non-malignant haematopoietic cells to mobilize and settle in distant organ tissues, thereby establishing a ‘pre-metastatic niche’ that lays a foundation for incoming circulating cancer cells.10 This raises the fascinating possibility that pre-selected autologous patient-derived tumour cells isolated from the patient’s primary tumour might be used to target organs suspected of harbouring metastases, and to deliver antineoplastic products, including OVs, locally. The major advantages to be expected from autologous tumour cell (ATC) carriers can be summarized as follows: (1) As isolated ATCs are prone to adhere to

their histological counterparts, they should be especially suitable as local ‘OV factories’ disseminating OVs to non-resectable or locally disseminated tumours through cell-to-cell spreading.11 Furthermore, it is possible to select metastasis-prone populations from the primary tumour or its draining lymph nodes or from established metastases with the help of tissue-specific endothelial matrices, hypoxic growth conditions or distinct cell markers such as CD44.12–15 Carriers generated ex tempore from these ATCs should be endowed with an enhanced capacity to pick up the trail of neoplastic dissemination and to transport their OV load to the right location after systemic application. This was recently demonstrated by our finding that rat hepatoma cells can transfer H-1PV to lung metastases originating from these cells and can initiate oncolysis within these metastases.5 (2) The immune compatibility of ATCs with the patient minimizes the risk of rejection, thereby extending the time available for OV release or transgene expression. (3) Irradiated ATCs constitute genuine therapeutic anticancer vaccines, whose action can be enhanced by the carried OVs acting as adjuvants. OV-dependent potentiation of ATC-mediated anti-tumour immunization is best exemplified by clinical successes in melanoma treatment using NDV-modified autologous cell vaccines.16 Compared with commonly used subcutaneous ATC immunization, systemic administration of OV-loaded ATCs presumably adds the advantages of ‘in situ’ immunization at sites of metastasis, resulting from local oncolysis-mediated release of tumour-associated antigens from the infected vaccine.17 Furthermore, virus infection of ATCs may potentiate the immune system by generating viral danger signals, by inducing cells to Gene Therapy


706

produce immunomodulating factors or by modifying cell death patterns so as to favour cross-presentation of tumour-associated antigens. Experimental studies have shown that antigen presentation in the liver, a common target of metastasis, does not necessarily result in immunization. In particular, presentation by liver sinusoidal endothelial cells of antigens derived from apoptotic tumour cells appears to be tolerogenic for CD8+ T cells.18 Infection of ATCs might shift the immune balance in favour of vaccination. For example, the fusion of tumour cells due to expression of viral fusogenic membrane glycoproteins can reverse their suppressive effects on dendritic cells.19 In gliomas, furthermore, parvoviruses have been shown to induce a cathepsin-mediated cell death mechanism that might initiate maturation of antigen-presenting cells more effectively than physiological apoptosis of metastasizing cells.20,21 (4) Another advantage worth mentioning is the possibility of using autologous tumour biopsies and derived ATCs to guide the choice of patient-tailored treatments. From a panel of potential candidates, it should be possible to select the most efficient OV against a given tumour. It should even be possible to use ATC cultures to adapt OVs ex vivo to specific types of cancer, as discussed below. On the basis of the advantages just described, one might propose a sequential two-step strategy for treating (metastatic) cancers with OVs. First-line treatment would involve multiple systemic applications of naked virions. Then, upon development of humoural anti-viral immunity, second-line treatment would involve the administration of infected ATC carriers via the same route. Virologically, this strategy should optimize the exposure of tumours to infectious viruses by shielding OVs from neutralizing antibodies after seroconversion. Immunologically, one might expect a cooperative protective effect due to priming of a cellular immune response against the virus from infected tumour cells in the first step, followed by boosting with the infected-carrier vaccine, at the second stage. Cross-presentation of tumour antigens should redirect against metastases the power of this anti-viral secondary immune response, resulting in a double therapeutic benefit. It is worth mentioning in this respect that anti-viral T-cell immunity does not prevent carrier-delivered OVs from infecting target cells.6 The first step in this protocol is likely to have a lesser impact if the patient has developed immunity against a chosen OV prior to treatment, either naturally (against adenoviruses, for example) or as part of a regular vaccination schedule (as in the case of the measles virus). In conclusion, ATCs hold special promise, in our opinion, as vehicles for patient-tailored delivery of OVs to disseminated tumours. Yet progress must be made in preparing ATCs that follow the natural route of metastatic spread and in establishing safety protocols (discussed below).

Capacity of allogenic tumour cells to carry OVs The rationale for using allogenic tumour cells (ALTCs) lies in their availability as established cell lines that can Gene Therapy

be selected and adapted for efficient OV production, while retaining the potential to spread in a manner that mimics the metastatic pattern of a given malignancy. A practical approach would be to use standard cell lines derived either from metastases of the same type of cancer as the targeted one or from a different malignancy with a similar metastatic pattern. Typically, the ALTC-carrier approach does not aim at long-term expression and production of OVs at tumour sites, as the ALTCs should eventually be rejected because of their immune incompatibility with the patient. This makes it necessary to identify highly OV-permissive ALTCs in order to achieve maximum virus production over a limited period of time. The use of established ALTC lines as OV carriers offers various advantages. First, these cell lines can be engineered in various ways for optimal OV delivery. One strategy could be to provide ALTCs with tissuehoming or immunomodulating transgenes. For example, carriers could be modified to express chemokine receptors (for example CXCR4) causing the cells to follow chemokine gradients to the metastatic niche.22 ALTC vehicles may also represent the standard packaging cell lines for some defective vectors, and serve as local (recombinant) vector factories. Further possibilities might be to build in antisense systems rendering the ALTCs insensitive to anti-viral defence mechanisms or to improve their adhesion to endothelial matrices or their predilection for hypoxic conditions. A second advantage of allogenic cells is their immunogenicity, which is a great asset to their use as vaccines.23,24 The massive alloreaction-associated activation of CD4+ and CD8+ cells at ALTC-homing sites (in particular metastatic lesions) should create a local microenvironment favouring the additional engagement of any tumour-specific lymphocytes present at those sites. Lastly, ALTCs are more convenient than ATCs for the in vitro selection of OVs adapted for efficient multiplication in a given tumour type. We are currently tackling this approach in our laboratory, using oncolytic parvoviruses known to mutate at a frequency enabling them to escape extra- or intracellular restrictions.25,26 This raises hopes of isolating a series of parvovirus variants showing increased fitness for replication in a specific ALTC line, making it possible to create improved ‘PV vehicle’ pairs for particular malignancies (see Figure 1). Selecting appropriate cell lines is a key issue in ALTCmediated OV delivery. The choice is obviously dictated by the type of tumour to be targeted. For the therapy of lung cancer or pulmonary metastases, cells derived from solid tumours of non-haematopoietic origin are worth considering, because mechanical constraints result in their initial trapping in lung capillaries after systemic injection. On the other hand, cells derived from haematological malignancies are likely to be suitable for transferring OV to certain disseminated cancers, because they retain some of the features of their untransformed counterparts.6,27 For instance, the observation that monocytes play a role in transport of the measles virus during natural infection has led to the successful use of corresponding cell lines as carriers for this virus.27 Established cell lines derived from promonocytic leukaemias are another example. These cells appear as good candidates for the delivery of parvovirus H-1PV because they are permissive for this agent28 and,


having retained the capacity of normal monocytes for transendothelial migration, they can transport viruses through the endothelial barrier.29 Interestingly, the sensitivity of monocytic leukaemia cells to H-1PV replication can be temporarily suppressed by treating them with the differentiation inducer PMA (unpublished observation). This opens the prospect of loading these cells with virus in PMA-containing media (to prevent the onset of virus multiplication and cytopathic effects) prior to their systemic injection. Upon injection, the cells should undergo diapedesis into metastasis-affected tissues30 where, in the absence of the PMA block, they should revert to virus producers. Other promising candidates are B-cell lymphoma lines. These appear to retain the homing capacity of normal lymphocytes and can be infected productively with OVs and notably with parvovirus H-1PV.31 Interestingly, the recent identification of markers required for metastastatic dissemination of lymphomas (for example CD44 and PSGL-1) makes it possible to select cell variants for the delivery of OVs to such lesions.32 Also of great interest are choriocarcinoma cells. Choriocarcinoma, a tumour originating from the placental trophoblast layer, is the only neoplasia known to grow and metastasize in histoincompatible hosts. The ability of normal trophoblastic cells to invade the uterine wall is very reminiscent of the behaviour of invasive metastatic cancer. The haematogenous spread of choriocarcinoma is reported to result in lung metastases in 80% of the clinical cases studied.33 Because of their fetal origin, these cells have an atypical pattern of major histocompatibility complex I and cytokine expression, which favours their implantation and persistence in maternal host tissues despite their allogenic nature.34,35 This implantation potential of choriocarcinoma cells, together with their ability to support persistent adeno-associated virus (AAV) infection, might be used to achieve long-term expression of transgenes transduced by AAV vectors.36 Choriocarcinoma cells are also interesting candidates for the sustained delivery of OVs, since the choriocarcinoma line JAR has been found to support parvovirus H-1PV replication in vitro and to release substantial amounts of progeny virus over an extended period of time, even after cell inactivation by g-irradiation.5

Use of in vitro pre-activated or transformed cells to deliver OVs An alternative strategy is to transform cells in vitro by genetic manipulation, so as to drive them into a preneoplastic state and thus to enhance their capacity for OV production. Malignant transformation is a stepwise process,37 and cells may become permissive toward OV infection before reaching the stage of tumourigenicity, thus providing safer vehicles for OV delivery. For example, both spontaneously and SV40-immortalized human keratinocytes are sensitive to parvovirus H-1PV, irrespective of their additional tumourigenic transformation with the H-ras oncogene.38 Immune and endothelial cells are prime candidates for this approach. They can be manipulated in vitro or through their responsiveness to various cytokines and growth factors and have the capacity to home to metastases and neoangiogenesis centres. Notably, pre-activation of

T cells by phytohaemagglutinin results in increased susceptibility to the measles virus, although infection remains non-productive and viral transfer is achievable only through heterofusion of the vehicle with the target cell.39 A very recent report describes a potent strategy using cytokine-induced killer cells supporting replication of vaccinia OVs and their transport into tumours upon intravenous administration.40 Especially attractive candidates would include tumour-infiltrating lymphocytes (TILs). Transformation of TILs with HTLV elements (Tax), for instance, results in their immortalization and in the acquisition of a more invasive phenotype, with upregulation of metalloproteases and extravasation through the endothelium.41 On the other hand, the endotheliotropism of certain parvoviruses raises the possibility of using activated endothelial cells to deliver these agents.42 Whether other precursor cells might be conditioned so as to transfer OVs to specific tissues remains to be explored. As stated above, it would be desirable to ensure that pre-loaded carrier cells do not produce virus until they reach their (metastatic) tumour targets. One way to delay virus production in a controlled manner is to render cell transformation conditional, so that the appearance of the transformed phenotype, with the associated permissiveness, can be induced at will. Various strategies are currently available for achieving this, such as placing the oncogene under the control of a tetracycline- or hypoxiaresponsive promoter.

707

Safety issues related to the administration of tumour or transformed cells Any strategy involving the administration of tumour or transformed cells to cancer patients in order to deliver viral agents raises safety issues linked to the residual oncogenic risk of these cells. This makes it necessary to test a range of safety mechanisms in pre-clinical models before envisaging the use of transformed carriers in clinical trials. Like conventional tumour cell vaccines, infected carrier cells can be inactivated by irradiation prior to their injection into patients.16 This treatment, together with the oncolytic properties of the harboured viruses, should deprive carrier cells of any tumourigenic potential. Yet the transformed carrier cells subjected to this double safety procedure must remain alive for a while, so as to produce OVs at the target site. Postinfection g-irradiation appears to be a treatment of choice, as it suppresses the growth of H-1PV carrier cells while preserving parvovirus replication and prolonging the production and release of progeny virions.5,11 In certain cases (for example AAV), ionizing radiation even enhances cell permissiveness for viral DNA replication.43 Additional safeguards can be implemented against the uncontrolled multiplication of transformed carrier cells. For instance, the cell vehicle can be equipped with cassettes whose expression eventually leads to senescence (through production of antisense RNAs directed against human telomerase reverse transcriptase, for instance). Another strategy is to arm carrier cells with conditional suicide genes like herpes simplex virus thymidine kinase, or with genes encoding caspases that can be activated with non-toxic, lipid-permeable chemical inducers of dimerization.44 Or Gene Therapy


708

carrier cells might be selected to be highly sensitive to a certain drug (or made so through expression of nucleoside transporters, for instance) and eradicated at the desired point in time by administration of that drug. Besides these approaches in which cells are engineered so as to be doomed to an early or drug-dependent death, other treatments can be used, should the need arise, to eliminate certain carriers specifically. Examples of such treatments include Tax-specific cytotoxic T lymphocytes against HTLV-transformed cells or chemotherapeutics to which choriocarcinoma cells are especially sensitive.45,46

Efficacy of OV protection by tumour cells against humoural immunity The pre-existence or rapid induction of neutralizing antibodies constitutes a major obstacle to systemic application of OVs. Such antibodies can completely prevent virus transfer into targeted tumours. One of the main problems that prompted the development of carrier-cell technologies was the need to shield therapeutic viruses against this physiological humoural immune reaction. Different in vitro and in vivo systems have been used to assess the capacity of carrier cells to protect OVs from the antibody response. Most in vitro assays rely on measuring cytotoxicity or on estimating syncytia formation upon co-cultivation of infected carriers with susceptible target cells in the presence and absence of virus-specific antibodies.5,27 Successful protection is exemplified by the results of a recent experiment carried out in our laboratory, where we tested the effect of an H-1PV-specific rat antiserum (diluted 1:200) on the ability of H-1PV to infect sensitive transformed cells. The parvovirus was delivered at the same total dose in two different ways, as naked virions and by producer carrier cells (effector-to-target cell ratio, 1:10). In the former case, the neutralizing serum completely abolished infection, but in the latter viral cytotoxicity was unaffected by the antiserum (unpublished observations). Yet one should be cautious in extrapolating such results, however encouraging, to the in vivo situation. In a hepatoma model, for instance, i.v. injection of various H-1PV-infected carriers resulted in virus transfer and suppression of lung metastases in naive rats but not in animals having high titres of neutralizing antibodies.47 In other OV/tumour systems, virus delivered by carrier cells was found to avoid seroneutralization partially or totally in actively or passively immunized animals.6,39 Thus, carrier cell-mediated delivery has a potential to shield input virions against neutralizing antibodies in vivo, but the level of protection appears to depend on the OV/carrier cell pair, the target tumour, the therapeutic protocol (notably treatment repetition, antibody titres). Accordingly, the impact of these parameters should be assessed in immunocompetent animal models where the OV induces a potent neutralizing response.5,6 Furthermore, the use of carrier cells might be combined with other means aimed at taming anti-viral reactions. For example, applying carriers together with plasmapheresis might be a more physiological means of taming anti-viral immune reactions than drug-induced immunosuppression. Gene Therapy

Use of combination strategies to improve carrier cell-mediated OV delivery Cancer progression involves a complex interplay between tumours and the surrounding tissues. Homing of infected carrier cells might thus be improved by preconditioning the tumour environment. A fascinating possibility would be, prior to OV/carrier treatment, to begin by injecting cells known to respond to chemotactic molecules and growth factors produced by tumours (for example mesenchymal, endothelial or bone marrow progenitors).48–52 The natural or built-in chemoresistance of these cells should enhance their participation in tumour vessel formation in patients under chemotherapy.53 By administering such Trojan horses at the time of primary tumour resection, which often correlates with the release of metastatic growth from anti-angiogenic restrictions,54 it should be possible to promote their infiltration into the pre-metastatic niche.10 These frontline cells could be further armed with specific secreted or membrane-bound molecules providing an anchorage for subsequent homing of infected OV carriers or systemically applied viruses. This strategy is obviously not devoid of risk, as it should initially promote tumour feeding. Yet these conditioning cells should be equipped with a suicide system to be turned on after OV delivery, leading to destruction not only of these cells but also of neighbouring endogenous vascular cells through a bystander mechanism. Furthermore, these front-line cells could be engineered to express a whole panel of secreted immunostimulatory transgenes enhancing different steps of the cellular immune response and generating conditions favourable to initiation of an anticancer immune response by OVs. In recombinant autonomous parvoviruses where the VP genes have been replaced with a transgene, viral propagation cannot proceed after a single round of infection. The sequential use of carriers harbouring recombinant and wild-type viruses, respectively, offers the opportunity to superinfect the former cells with helper viruses acting as capsid donors and allowing a second round of infection by the recombinants.5 Besides complementing the defect of the recombinant vectors, the helper viruses might exert genuine oncolytic activity. On the other hand, some integrating viral vectors, such as AAVs, can be delivered by specific vehicles to metastatic sites so as to express a transgene for a prolonged period. At a later stage, the site can be targeted with an adenovirus known to provide helper functions, injected either systemically or through a second-line carrier to boost production and novel seeding with the initial AAV. In conclusion, the use of cell vehicles for the targeted delivery and production of OVs opens a number of new prospects to anticancer therapy. Yet strategies involving infected carrier cells need to be considered by comparison or in combination with existing therapies like chemo- or immunotherapy, so as to take advantage of the clinical experience gained through these more established therapeutic approaches.

Acknowledgements Dr Z Raykov was supported by a fellowship (IV-BUL/ 1118365 STP) from the Alexander von Humboldt


709

Foundation. We are grateful to Dr D Nettelbeck for critical reading of this manuscript. 20

References 1 Harrington K, Alvarez-Vallina L, Crittenden M, Gough M, Chong H, Diaz RM et al. Cells as vehicles for cancer gene therapy: the missing link between targeted vectors and systemic delivery? Hum Gene Ther 2002; 13: 1263–1280. 2 Power AT, Bell JC. Cell-based delivery of oncolytic viruses: a new strategic alliance for a biological strike against cancer. Mol Ther 2007; 15: 660–665. 3 Netland PA, Zetter BR. Organ-specific adhesion of metastatic tumor cells in vitro. Science 1984; 224: 1113–1115. 4 Fidler IJ. The pathogenesis of cancer metastasis: the ‘seed and soil’ hypothesis revisited. Nat Rev Cancer 2003; 3: 453–458. 5 Raykov Z, Balboni G, Aprahamian M, Rommelaere J. Carrier cell-mediated delivery of oncolytic parvoviruses for targeting metastases. Int J Cancer 2004; 109: 742–749. 6 Power AT, Wang J, Falls TJ, Paterson JM, Parato KA, Lichty BD et al. Carrier cell-based delivery of an oncolytic virus circumvents antiviral immunity. Mol Ther 2007; 15: 123–130. 7 Sahai E. Illuminating the metastatic process. Nat Rev Cancer 2007; 7: 737–749. 8 Rommelaere J, Cornelis J. Antineoplastic activity of parvoviruses. J Virol Methods 1991; 33: 233–251. 9 Russell SJ, Brandenburger A, Flemming CL, Collins MK, Rommelaere J. Transformation-dependent expression of interleukin genes delivered by a recombinant parvovirus. J Virol 1992; 66: 2821–2828. 10 Kaplan RN, Riba RD, Zacharoulis S, Bramley AH, Vincent L, Costa C et al. VEGFR1-positive haematopoietic bone marrow progenitors initiate the pre-metastatic niche. Nature 2005; 438: 820–827. 11 Coukos G, Makrigiannakis A, Kang EH, Caparelli D, Benjamin I, Kaiser LR et al. Use of carrier cells to deliver a replicationselective herpes simplex virus-1 mutant for the intraperitoneal therapy of epithelial ovarian cancer. Clin Cancer Res 1999; 5: 1523–1537. 12 Miller BE, Aslakson CJ, Miller FR. Efficient recovery of clonogenic stem cells from solid tumors and occult metastatic deposits. Invasion Metastasis 1990; 10: 101–112. 13 Bruns CJ, Harbison MT, Kuniyasu H, Eue I, Fidler IJ. In vivo selection and characterization of metastatic variants from human pancreatic adenocarcinoma by using orthotopic implantation in nude mice. Neoplasia 1999; 1: 50–62. 14 Sullivan R, Graham CH. Hypoxia-driven selection of the metastatic phenotype. Cancer Metastasis Rev 2007; 26: 319–331. 15 Kogerman P, Sy MS, Culp LA. Overexpressed human CD44s promotes lung colonization during micrometastasis of murine fibrosarcoma cells: facilitated retention in the lung vasculature. Proc Natl Acad Sci USA 1997; 94: 13233–13238. 16 Schirrmacher V. Clinical trials of antitumor vaccination with an autologous tumor cell vaccine modified by virus infection: improvement of patient survival based on improved antitumor immune memory. Cancer Immunol Immunother 2005; 54: 587–598. 17 Endo T, Toda M, Watanabe M, Iizuka Y, Kubota T, Kitajima M et al. In situ cancer vaccination with a replication-conditional HSV for the treatment of liver metastasis of colon cancer. Cancer Gene Ther 2002; 9: 142–148. 18 Berg M, Wingender G, Djandji D, Hegenbarth S, Momburg F, Hammerling G et al. Cross-presentation of antigens from apoptotic tumor cells by liver sinusoidal endothelial cells leads to tumor-specific CD8+ T cell tolerance. Eur J Immunol 2006; 36: 2960–2970. 19 Bateman AR, Harrington KJ, Kottke T, Ahmed A, Melcher AA, Gough MJ et al. Viral fusogenic membrane glycoproteins kill

21 22

23 24

25

26

27

28

29

30

31 32

33

34

35

36

37

38

solid tumor cells by nonapoptotic mechanisms that promote cross presentation of tumor antigens by dendritic cells. Cancer Res 2002; 62: 6566–6578. Di Piazza M, Mader C, Geletneky K, Herrero Y, Calle M, Weber E et al. Cytosolic activation of cathepsins mediates parvovirus H-1induced killing of cisplatin and TRAIL-resistant glioma cells. J Virol 2007; 81: 4186–4198. Schmid D, Mu¨nz C. Innate and adaptive immunity through autophagy. Immunity 2007; 27: 11–21. Sipkins DA, Wei X, Wu JW, Runnels JM, Cote D, Means TK et al. In vivo imaging of specialized bone marrow endothelial microdomains for tumour engraftment. Nature 2005; 435: 969–973. Fabre JW. The allogeneic response and tumor immunity. Nat Med 2001; 7: 649–652. Errington F, Bateman A, Kottke T, Thompson J, Harrington K, Merrick A et al. Allogeneic tumor cells expressing fusogenic membrane glycoproteins as a platform for clinical cancer immunotherapy. Clin Cancer Res 2006; 12: 1333–1341. Lopez-Bueno A, Mateu MG, Almendral JM. High mutant frequency in populations of a DNA virus allows evasion from antibody therapy in an immunodeficient host. J Virol 2003; 77: 2701–2708. Lopez-Bueno A, Villarreal LP, Almendral JM. Parvovirus variation for disease: a difference with RNA viruses? Curr Top Microbiol Immunol 2006; 299: 349–370. Iankov ID, Blechacz B, Liu C, Schmeckpeper JD, Tarara JE, Federspiel MJ et al. Infected cell carriers: a new strategy for systemic delivery of oncolytic measles viruses in cancer virotherapy. Mol Ther 2007; 15: 114–122. Rayet B, Lopez-Guerrero JA, Rommelaere J, Dinsart C. Induction of programmed cell death by parvovirus H-1 in U937 cells: connection with the tumor necrosis factor alpha signalling pathway. J Virol 1998; 72: 8893–8903. Ronald JA, Ionescu CV, Rogers KA, Sandig M. Differential regulation of transendothelial migration of THP-1 cells by ICAM-1/LFA-1 and VCAM-1/VLA-4. J Leukoc Biol 2001; 70: 601–609. Swallow CJ, Murray MP, Guillem JG. Metastatic colorectal cancer cells induce matrix metalloproteinase release by human monocytes. Clin Exp Metastasis 1996; 14: 3–11. Pals ST, de Gorter DJ, Spaargaren M. Lymphoma dissemination: the other face of lymphocyte homing. Blood 2007; 110: 3102–3111. Raes G, Ghassabeh GH, Brys L, Mpofu N, Verschueren H, Vanhecke D et al. The metastatic T-cell hybridoma antigen/ P-selectin glycoprotein ligand 1 is required for hematogenous metastasis of lymphomas. Int J Cancer 2007; 121: 2646–2652. Kumar J, Ilancheran A, Ratnam SS. Pulmonary metastases in gestational trophoblastic disease: a review of 97 cases. Br J Obstet Gynaecol 1988; 95: 70–74. Gobin SJ, Wilson L, Keijsers V, Van den Elsen PJ. Antigen processing and presentation by human trophoblast-derived cell lines. J Immunol 1997; 158: 3587–3592. Bianchi DW, Zickwolf GK, Weil GJ, Sylvester S, DeMaria MA. Male fetal progenitor cells persist in maternal blood for as long as 27 years postpartum. Proc Natl Acad Sci USA 1996; 93: 705–708. Kiehl K, Schlehofer JR, Schultz R, Zugaib M, Armbruster-Moraes E. Adeno-associated virus DNA in human gestational trophoblastic disease. Placenta 2002; 23: 410–415. Hahn WC, Counter CM, Lundberg AS, Beijersbergen RL, Brooks MW, Weinberg RA. Creation of human tumour cells with defined genetic elements. Nature 1999; 400: 464–468. Chen YQ, Tuynder MC, Cornelis JJ, Boukamp P, Fusenig NE, Rommelaere J. Sensitization of human keratinocytes to killing by parvovirus H-1 takes place during their malignant transformation but does not require them to be tumorigenic. Carcinogenesis 1989; 10: 163–167. Gene Therapy


710 39 Ong HT, Hasegawa K, Dietz AB, Russell SJ, Peng KW. Evaluation of T cells as carriers for systemic measles virotherapy in the presence of antiviral antibodies. Gene Ther 2007; 14: 324–333. 40 Thorne SH, Negrin RS, Contag CH. Synergistic antitumor effects of immune cell-viral biotherapy. Science 2006; 311: 1780–1784. 41 Mori N, Sato H, Hayashibara T, Senba M, Hayashi T, Yamada Y et al. Human T-cell leukemia virus type I Tax transactivates the matrix metalloproteinase-9 gene: potential role in mediating adult T-cell leukemia invasiveness. Blood 2002; 99: 1341–1349. 42 Giese NA, Raykov Z, DeMartino L, Vecchi A, Sozzani S, Dinsart C et al. Suppression of metastatic hemangiosarcoma by a parvovirus MVMp vector transducing the IP-10 chemokine into immunocompetent mice. Cancer Gene Ther 2002; 9: 432–442. 43 Ferrari FK, Samulski T, Shenk T, Samulski RJ. Second-strand synthesis is a rate-limiting step for efficient transduction by recombinant adeno-associated virus vectors. J Virol 1996; 70: 3227–3234. 44 Shariat SF, Desai S, Song W, Khan T, Zhao J, Nguyen C et al. Adenovirus-mediated transfer of inducible caspases: a novel ‘death switch’ gene therapeutic approach to prostate cancer. Cancer Res 2001; 61: 2562–2571. 45 Kannagi M. Immunologic control of human T-cell leukemia virus type I and adult T-cell leukemia. Int J Hematol 2007; 86: 113–117. 46 Lurain JR, Singh DK, Schink JC. Primary treatment of metastatic high-risk gestational trophoblastic neoplasia with EMA-CO chemotherapy. J Reprod Med 2006; 51: 767–772.

Gene Therapy

47 Raykov Z, Grekova S, Galabov AS, Balboni G, Koch U, Aprahamian M et al. Combined oncolytic and vaccination activities of parvovirus H-1 in a metastatic tumor model. Oncol Rep 2007; 17: 1493–1499. 48 Spring H, Schuler T, Arnold B, Hammerling GJ, Ganss R. Chemokines direct endothelial progenitors into tumor neovessels. Proc Natl Acad Sci USA 2005; 102: 18111–18116. 49 Komarova S, Kawakami Y, Stoff-Khalili MA, Curiel DT, Pereboeva L. Mesenchymal progenitor cells as cellular vehicles for delivery of oncolytic adenoviruses. Mol Cancer Ther 2006; 5: 755–766. 50 Arafat WO, Casado E, Wang M, Alvarez RD, Siegal GP, Glorioso JC et al. Genetically modified CD34+ cells exert a cytotoxic bystander effect on human endothelial and cancer cells. Clin Cancer Res 2000; 6: 4442–4448. 51 Rancourt C, Robertson III MW, Wang M, Goldman CK, Kelly JF, Alvarez RD et al. Endothelial cell vehicles for delivery of cytotoxic genes as a gene therapy approach for carcinoma of the ovary. Clin Cancer Res 1998; 4: 265–270. 52 Lang SI, Kottke T, Thompson J, Vile RG. Unbiased selection of bone marrow derived cells as carriers for cancer gene therapy. J Gene Med 2007; 9: 927–937. 53 Zaboikin M, Srinivasakumar N, Schuening F. Gene therapy with drug resistance genes. Cancer Gene Ther 2006; 13: 335–345. 54 Chen C, Parangi S, Tolentino MJ, Folkman J. A strategy to discover circulating angiogenesis inhibitors generated by human tumors. Cancer Res 1995; 55: 4230–4233.