Migration of genetically labeled glioma cells after implantation into murine brain

Induction of prostaglandin E2 synthesis and microsomal prostaglandin E synthase–1 expression in murine microglia by glioma-derived soluble factors

Object

Microglia are one of the members of monocyte/macrophage lineage in the central nervous system (CNS) and exist as ramified microglia in a normal resting state, but they are activated by various stimuli, such as tumors. Activated microglia induce immune responses in the CNS, but the precise functions of microglia in glioma microenvironments are not clear. It has been reported that glioma cells produce prostaglandin (PG)E2, which promotes the growth of tumor cells and possesses immunosuppressive activity. The authors previously reported that PGE2 production by peritoneal macrophages was enhanced by glioma-derived soluble factors, which induce an immunosuppressive state. In this study, they investigated PGE2 production by microglia treated with glioma cells and assessed the role of microglia in glioma microenvironments in the mouse.

Methods

Microglia and peritoneal macrophages were cultured in vitro with or without lipopolysaccharide, and tumor necrosis factor (TNF) and PGE2 in the culture supernatant were measured using L929 bioassay and enzyme immunoassay. The expression of mRNA was measured using reverse transcriptase polymerase chain reaction, and the protein expression was assayed with Western blotting. In some experiments glioma cells and conditioned glioma medium were added to the microglia cultures.

Results

Glioma cells studied in this report did not produce a significant amount of PGE2. However, the coculture of microglia with glioma cells or conditioned glioma medium led to the production of a large amount of PGE2. The enhancement of PGE2 production by microglia was more significant than that by peritoneal macrophages. The expression of cyclooxygenase (COX)–2 and particularly the expression of microsomal PGE synthase (mPGES)–1 (a terminal enzyme of the arachidonate cascade) in microglia were enhanced by conditioned glioma medium. The enhancement of mPGES-1 expression in microglia was more significant than that in peritoneal macrophages. The production of TNF was suppressed when culturing microglia with conditioned glioma medium, but this suppression was abrogated by the addition of a COX inhibitor (NS-398) and a PGE2 receptor (EP4) antagonist. Furthermore, TNF production was not suppressed in microglia from mPGES-1–deficient mice.

Conclusions

These results indicate that PGE2 production by microglia is enhanced by conditioned glioma medium, which induces an immunosuppressive state in the CNS. Therefore, the manipulation of microglia, from the standpoint of PGE2, provides investigators with an important strategy to induce an effective antiglioma immune response.

Induction of macrophagic prostaglandin E2 synthesis by glioma cells

Object

It has been reported that glioma cells produce prostaglandin (PG)E2, which promotes the growth of tumor cells and possesses immunosuppressive activity, and that cyclooxygenase (COX) inhibitors impede tumor growth and infiltration. Macrophages in tumor-bearing hosts are activated to produce PGE2, which induces an immunosuppressive state. Note, however, that the precise mechanism by which PGE2 induces an immunosuppressive state is still unclear. In this study, the authors investigated the mechanism of PGE2 production in glioma-bearing hosts.

Methods

The human and murine glioma cells that were studied did not produce a significant amount of PGE2. However, the coculture of human peripheral blood mononuclear cells or murine peritoneal macrophages with glioma cells or conditioned glioma medium led to the production of a large amount of PGE2. In contrast, production of tumor necrosis factor and interleukin (IL)-12p70 by macrophages and cytotoxic T lymphocyte induction were suppressed by culturing with conditioned glioma medium; this suppression was abrogated by the addition of the COX inhibitor indomethacin. The macrophagic expression of COX-2, and particularly the expression of microsomal PGE synthase (mPGES)–1, a terminal enzyme of the arachidonate cascade, was enhanced by the glioma-derived soluble factors. Furthermore, IL-12p70 production was not clearly suppressed in macrophages from mPGES-1–deficient mice. The glioma-derived soluble factors were sensitive to treatment with heat and papain.

Conclusions

These results indicated that PGE2 production by macrophages is enhanced by glioma-derived soluble factors, which induce an immunosuppressive state in glioma-bearing hosts. Therefore, the inhibition of PGE2 synthesis, targeting COX-2 and mPGES-1, is an effective treatment for the induction of antiglioma immune responses.

Gene therapy for the central nervous system in the lysosomal storage disorders

Although great promise has been made in the field of gene therapy, a number of difficulties must be solved before successful human studies can be completed. These issues involve safety, immunological reactions to the vectors and their transgene products, persistent transgene expression, and ability to repeat administrations of the vector safely. A major hurdle that must be overcome is the ubiquitous delivery of the transgene throughout the nervous system. Significant gene delivery to the CNS of murine models of LSD has been accomplished, but we await the successful treatment of the nervous system in a larger mammalian model of LSD. As yet there is no perfect vector that can solve all of these problems. It is likely that vector technology will evolve into hybrid vectors also using synthetic components that will increase safety and efficacy of recombinant vectors. The treatment of the CNS remains complicated, but progress is being made in this area. Clinical trials already planned will give us increasing information as to the ideal gene therapy for the CNS.

Generation of stable retrovirus packaging cell lines after transduction with herpes simplex virus hybrid amplicon vectors

BACKGROUND

A number of properties have relegated the use of Moloney murine leukemia virus (Mo-MLV)-based retrovirus vectors primarily to ex vivo protocols. Direct implantation of retrovirus producer cells can bypass some of the limitations, and in situ vector production may result in a large number of gene transfer events. However, the fibroblast nature of most retrovirus packaging cells does not provide for an effective distribution of vector producing foci in vivo, especially in the brain. Effective development of new retrovirus producer cells with enhanced biologic properties may require the testing of a large number of different cell types, and a quick and efficient method to generate them is needed.

METHODS

Moloney murine leukemia virus (Mo-MLV) gag-pol and env genes and retrovirus vector sequences carrying lacZ were cloned into different minimal HSV/AAV hybrid amplicons. Helper virus-free amplicon vectors were used to co-infect glioma cells in culture. Titers and stability of retrovirus vector production were assessed.

RESULTS

Simultaneous infection of two glioma lines, Gli-36 (human) and J3T (dog), with both types of amplicon vectors, generated stable packaging populations that produced retrovirus titers of 0.5-1.2 x 10(5) and 3.1-7.1 x 10(3) tu/ml, respectively. Alternatively, when cells were first infected with retrovirus vectors followed by infection with HyRMOVAmpho amplicon vector, stable retrovirus packaging populations were obtained from Gli-36 and J3T cells producing retrovirus titers comparable to those obtained with a traditional retrovirus packaging cell line, Psi CRIPlacZ.

CONCLUSIONS

This amplicon vector system should facilitate generation of new types of retrovirus producer cells. Conversion of cells with migratory or tumor/tissue homing properties could result in expansion of the spatial distribution or targeting capacity, respectively, of gene delivery by retrovirus vectors in vivo.

Eradication of murine brain tumors by direct inoculation of concentrated high titer-recombinant retrovirus harboring the herpes simplex virus thymidine kinase gene

Implantation of retrovirus-producing cells within a tumor has been demonstrated to eliminate malignant brain tumors effectively in animal models. In our previous study, the implantation of high-titer retrovirus-producing fibroblasts into tumors resulted in highly efficient transduction in vivo. The transduced glioma cells migrated far from the implantation site, potentiating the induction of the remarkable bystander effect. It is also possible, however, that the implantation of murine fibroblast-derived virus-producing cells may induce an immune response in patients. In this study, we prepared retroviruses carrying the herpes simplex virus thymidine kinase (HTK) gene with titers of 1.4--2.5 x 10(11) colony-forming units (c.f.u.)/ml, and stereotactically inoculated only 3 microl of the HTK-bearing retroviruses into the brain tumors of mice. Following repetitive ganciclovir (GCV) intraperitoneal injection, effective killing of glioma cells in the mouse brain was observed. The transduction efficiency was nearly as high as that observed for the implantation of high-titer retrovirus-producing fibroblasts. Eighty percent of brain tumor-bearing mice were completely cured by our treatment protocol using concentrated HTK-harboring retroviruses. Our results suggest that repeated inoculations of high-titer retroviruses carrying the HTK gene followed by GCV treatment may be a promising strategy for the clinical treatment of malignant gliomas.

Targeting angiogenesis inhibits tumor infiltration and expression of the pro-invasive protein SPARC

The solid growth of high-grade glioma appears to be critically dependent on tumor angiogenesis. It remains unknown, however, whether the diffuse infiltration of glioma cells into healthy adjacent tissue is also dependent on the formation of new tumor vessels. Here, we analyze the relationship between tumor angiogenesis and tumor cell infiltration in an experimental glioma model. C6 cells were implanted into the dorsal skinfold chamber of nude mice, and tumor angiogenesis was monitored by intravital fluorescence videomicroscopy. Glioma infiltration was assessed by the extent of tumor cell invasion into the adjacent chamber tissue and by expression of SPARC, a cellular marker of glioma invasiveness. To test the hypothesis that glioma angiogenesis and glioma infiltration are codependent, we assessed tumor infiltration in both the presence and the absence of the angiogenesis inhibitor SU5416. SU5416 is a selective inhibitor of the VEGF/Flk-1 signal-transduction pathway, a critical pathway implicated in angiogenesis. Control tumors demonstrated both high angiogenic activity and tumor cell invasion accompanied by strong expression of SPARC in invading tumor cells at the tumor-host tissue border. SU5416-treated tumors demonstrated reduced vascular density and vascular surface in the tumor periphery accompanied by marked inhibition of glioma invasion and decreased SPARC expression. A direct effect of SU5416 on glioma cell motility and invasiveness was excluded by in vitro migration and invasion assays. These results suggest a crucial role for glioma-induced angiogenesis as a prerequisite for diffuse tumor invasion and a possible therapeutic role for anti-angiogenic compounds as inhibitors of both solid and diffuse infiltrative tumor growth.

Modulation of phenotype and induction of irregular vessels accompany high tumorigenic potential of clonal human glioma cells xenografted to nude-rat brain

Three phenotypically different clonal human glioma cell lines were injected stereotactically into nude-rat brains, to determine their individual growth potential and to establish an in vivo system in which different therapeutic modalities could be tested. As assessed by serial sectioning, microscopic evaluation, and computer analysis, the mean approximate tumour volume after 3-7 weeks in vivo was 0.42 mm(3) for U-343 MG, 2.6 mm(3) for U-343 MGa Cl2:6, and 50.3 mm(3) for U-343 MGa 31L. When compared with the initial injected cell volume, only U-343 MGa 31L had increased in size, U-343 MGa Cl2:6 remained approximately the same but showed a certain proliferative potential, and U-343 MG regressed. Thus, only U-343 MGa 31L cells had high tumorigenic potential, invaded and replaced brain tissue in every direction, while U-343 MGa Cl2:6 cells grew in sheet-like tumour extensions along white-matter nerve-fibre tracts, in this respect mimicking foetal astrocytes. The tumorigenic potential of the U-343 MGa 31L cell clone was associated with a variable phenotype, as observed when the in vivo and in vitro characteristics were compared. The in vivo phenotype was characterized by the loss of GFAP immunoreactivity, the gain of heterogeneously distributed cellular tenascin, fibronectin, and laminin, but absence of extracellularly deposited material, and by the formation of irregular vessels. It appears that the intrinsic capacity of glioma cells to adapt to in vivo conditions is decisive for their tumorigenicity in the brain, rather than any single phenotypic property in itself. Moreover, the 2 glioma cell clones best suited for in vitro growth were no longer tumorigenic.

In situ use of suicide genes for therapy of brain tumours

Suicide gene therapy represents a new therapeutic approach to the treatment of patients with otherwise incurable malignant brain tumours. This strategy involves the introduction of a gene that renders the tumour cell susceptible to an otherwise nontoxic prodrug. The most often used genetic prodrug activation system is the herpes simplex virus thymidine kinase/ganciclovir (HSV-tk/GCV) paradigm. An important aspect of this system is the 'bystander effect', the extension of cytotoxic effects to untransduced cells. For gene delivery, retroviral, adenoviral vectors and HSV-1 mutants have been used. Clinical studies have revealed that the HSV-tk/GCV approach is safe, but also that responses are observed only in very small brain tumours, indicating insufficient vector distribution and very low transduction efficiency with replication-deficient vector systems. To improve treatment efficacy, the use of replication-competent oncolytic vectors in combination with new or improved prodrug-suicide gene systems as a part of a multimodal approach is warranted. In the context of replication-competent vectors, suicide genes might also be used as fail-safe genes in the case of runaway infection.

Single-step conversion of cells to retrovirus vector producers with herpes simplex virus-Epstein-Barr virus hybrid amplicons

We report here on the development and characterization of a novel herpes simplex virus type 1 (HSV-1) amplicon-based vector system which takes advantage of the host range and retention properties of HSV-Epstein-Barr virus (EBV) hybrid amplicons to efficiently convert cells to retrovirus vector producer cells after single-step transduction. The retrovirus genes gag-pol and env (GPE) and retroviral vector sequences were modified to minimize sequence overlap and cloned into an HSV-EBV hybrid amplicon. Retrovirus expression cassettes were used to generate the HSV-EBV-retrovirus hybrid vectors, HERE and HERA, which code for the ecotropic and the amphotropic envelopes, respectively. Retrovirus vector sequences encoding lacZ were cloned downstream from the GPE expression unit. Transfection of 293T/17 cells with amplicon plasmids yielded retrovirus titers between 10(6) and 10(7) transducing units/ml, while infection of the same cells with amplicon vectors generated maximum titers 1 order of magnitude lower. Retrovirus titers were dependent on the extent of transduction by amplicon vectors for the same cell line, but different cell lines displayed varying capacities to produce retrovirus vectors even at the same transduction efficiencies. Infection of human and dog primary gliomas with this system resulted in the production of retrovirus vectors for more than 1 week and the long-term retention and increase in transgene activity over time in these cell populations. Although the efficiency of this system still has to be determined in vivo, many applications are foreseeable for this approach to gene delivery.

Systemic interleukin 12 displays anti-tumour activity in the mouse central nervous system

In various systemic cancers, interleukin 12 (IL-12) induces anti-tumour immunity mediated by T lymphocytes and natural killer cells. To determine whether IL-12 has anti-tumour activity against malignant gliomas in the central nervous system (CNS), which is considered to be an immunologically privileged site, we treated mice with meningeal gliomatosis by intraperitoneal (i.p.) or intrathecal (i.t.) administration of recombinant murine IL-12. Although untreated mice revealed symptoms, such as body weight loss or paraplegia as a result of the meningeal gliomatosis within 8 days after tumour inoculation, 80% of the mice treated with IL-12 at 0.5 microg i.p. were cured. Many lymphocytes, mostly CD4+ and CD8+ cells, infiltrated to the tumours of IL-12-treated mice. The numbers of these cells increased in the cervical lymph nodes, into which the cerebrospinal fluid drains, and there they secreted a considerable amount of interferon-gamma. Mice cured by IL-12 rejected subcutaneous or i.t. rechallenge with their original glioma cells, but the same mice were not able to reject other syngeneic tumour cells. These results indicate that the immune system recognizes malignant glioma cells in the subarachnoid space of the CNS and that systemic IL-12 may produce effective anti-tumour activity and long-lasting tumour-specific immunity.

Targeted killing of migrating glioma cells by injection of HTK-modified glioma cells

The "bystander effect" describes the killing of nearby unmodified cells and herpes simplex thymidine kinase (HTK)-transduced cells by ganciclovir (GCV) treatment. This effect is thought to be produced by contact between these cells. In this study, we showed that injected glioma cells migrated rapidly to a place distant from the injection point whereas injected virus-producing fibroblast cells did not migrate in a murine brain model. Moreover, the initially injected glioma cells and glioma cells injected at a later time mix very well, even at a place distant from the injection point. This suggested that glioma cells migrating after injection could be targeted by HTK-modified glioma cells introduced in a second injection and be killed together by GCV treatment. Therefore, we injected HTK-modified glioma cells 3 days after injection of wild glioma cells to investigate whether wild-type glioma cells that migrated to a place distant from the injection point could also be killed by GCV treatment. Tumor growth was suppressed after the GCV treatment. Suppression of tumor growth of wild glioma cells is not solely mediated by the immune response, which may be triggered by the killing of HTK-modified glioma cells with GCV, because inoculation of HTK-modified glioma to the contralateral side followed by GCV treatment did not cure the initial wild glioma. Moreover, the migration of the second inoculum of glioma cells is necessary for effective killing, because early administration of GCV resulted in insufficient killing.

Glioma invasion in the central nervous system

Invading glioma cells seem to follow distinct anatomic structures within the central nervous system. Tumor cell dissemination may occur along structures, such as the basement membranes of blood vessels or the glial limitans externa, that contain extracellular matrix (ECM) proteins. Frequently, invasive glioma cells are also found to migrate along myelinated fiber tracts of white matter. This behavior is most likely a consequence of using constitutive extracellular ligands expressed along the pathways of preferred dissemination. The extracellular space in anatomic structures, such as blood vessel basement membranes or between myelinated axons, is profoundly different, thus suggesting that glioma cells may be able to use a multiplicity of matrix ligands, possibly activating separate mechanisms for invasion. In addition, enzymatic modification of the extracellular space or deposition of ECM by the tumor cells may also create a more permissive environment for tumor spread into the adjacent brain. Tumor cell invasion is defined as translocation of neoplastic cells through host cellular and ECM barriers. This process has been studied in other cancers, in which a cascade of events has been described that involves receptor-mediated matrix adhesion, degradation of matrix by tumor-secreted metalloproteinases, and, subsequently, active cell locomotion into the newly created space. Although some of these mechanisms may play an important role in glioma invasion, there are some significant differences that are mainly the result of the profoundly different composition of the extracellular environment within the brain. This review focuses on the composition of central nervous system ECM and the recent evidence for the use by glioma cells of multiple invasion mechanisms in response to this unique environment.