Synergistic effects of mesenchymal stromal cells and immunotherapy in experimental brain tumors

Glioblastoma multiforme (GBM) is the most common and aggressive form of primary brain tumor. In spite of surgical resection, combined radiation therapy and chemotherapy, the mean survival is less than 15 months following diagnosis. Using the established N29 and N32 animal models of glioma, shown to have many similarities with the human tumor, we are able to study and develop therapeutic strategies against GBM. The tumor models were first characterized according to cancer-initiating capacity. It was established that both models concomitantly and homogenously express several immature and mature markers of neural origin. Furthermore, the glioma cells were highly clonogenic in vitro and showed a robust tumorigenicity in vivo. Next multipotent mesenchymal stromal cells (MSCs) were compared to neural precursor cells (NPCs) in capacity of survival and migration following intratumoral grafting, which revealed that MSCs were superior to NPCs as potential vectors in tumor therapy. It was also concluded that MSCs do not migrate towards a pre-established tumor when grafted behind it or in contact with the corpus callosum. Neither do the MSCs proliferate following grafting, further confirming that MSCs are suitable as cellular vehicles. MSCs derived from adult rat bone marrow are relatively easily isolated and cultured as well as able to be stably transduced to express a therapeutic gene/drug. When MSCs are grafted intratumorally following peripheral immunizations with IFNγ-secreting autologous irradiated tumor cells, a synergistic effect on anti-tumor immunity is noticed with a prolonged survival and an increased tumor infiltration of immune cells. Rat MSCs were modified to produce the cytokine interleukin 7 (IL-7), a growth factor for immune cells. IL-7 secreting MSCs injected intratumorally resulted in a decreased tumor area compared to control. When combined with peripheral immunization the mean area of pre-established tumors was further decreased. The effect on tumor was coupled to an increase of tumor-infiltrating T cells. In conclusion this thesis points to a synergistic effect of intratumorally located MSCs and peripheral immunotherapy with IFNγ-producing tumor cells. Hopefully this combination might be further developed into a clinically useful treatment strategy

Characterization of the subventricular zone neurogenic response to rat malignant brain tumors

The subventricular zone (SVZ) is one of the neurogenic regions of the adult brain. We characterized the neurogenic response of the SVZ to the growth of brain tumors in the rat striatum. Abundant nestin positive cells, most likely representing reactive astrocytes, were found surrounding the tumor. However, we observed no substantial migration of nestin positive cells from the SVZ toward the tumor. Tumor growth resulted in decreased numbers of bromodeoxyuridine positive and Ki-67 positive proliferating cells and a concomitant increase in doublecortin and polysialylated neural cell adhesion molecule immunoreactivity within the SVZ. Neuroblasts were observed in high numbers in the area between the SVZ and the tumor, most likely pointing to the SVZ as the principal source of these cells. Neuroblasts located between the SVZ and the tumor expressed the transcription factor Pbx, a marker for immature striatal neurons. However, no evidence of neuroblast differentiation into fully mature neurons was found. This study thus demonstrates increased neuroblast immunoreactivity within the SVZ ipsilateral to a brain tumor in the striatum. SVZ-derived neuroblasts attracted by the tumor adopt an immature striatal phenotype indicating a region specific reparative mechanism in response to a malignant tumor. (C) 2007 IBRO. Published by Elsevier Ltd. All rights reserved

Intratumoral IL-7 delivery by mesenchymal stromal cells potentiates IFNgamma-transduced tumor cell immunotherapy of experimental glioma.

The present study reports regression of pre-established experimental rat gliomas as a result of combining peripheral immunization using interferon gamma (IFNgamma) transduced autologous tumor cells with local intratumoral delivery of interleukin 7 (IL-7) by mesenchymal stromal cells. IL-7 alone significantly decreased the tumor area and this effect was enhanced with IFNgamma immunization. A higher density of intratumoral T-cells was observed in animals receiving combined therapies compared to rats receiving either cytokine alone suggesting that the therapeutic effect is dependent on a T-cell response

Rat Multipotent Mesenchymal Stromal Cells Lack Long-Distance Tropism to 3 Different Rat Glioma Models

BACKGROUND: Viral gene therapy of malignant brain tumors has been restricted by the limited vector distribution within the tumors. Multipotent mesenchymal stromal cells (MSCs) and other precursor cells have shown tropism for gliomas, and these cells are currently being explored as potential vehicles for gene delivery in glioma gene therapy. OBJECTIVE: To investigate MSC migration in detail after intratumoral and extratumoral implantation through syngeneic and orthotopic glioma models. METHODS: Adult rat bone marrow-derived MSCs were transduced to express enhanced green fluorescent protein and implanted either directly into or at a distance from rat gliomas. RESULTS: We found no evidence of long-distance MSC migration through the intact striatum toward syngeneic D74(RG2), N32, and N29 gliomas in the ipsilateral hemisphere or across the corpus callosum to gliomas located in the contralateral hemisphere. After intratumoral injection, MSCs migrated extensively, specifically within N32 gliomas. The MSCs did not proliferate within tumors, suggesting a low risk of malignant transformation of in vivo grafted cell vectors. Using a model for surgical glioma resection, we found that intratumorally grafted MSCs migrate efficiently within glioma remnants after partial surgical resection. CONCLUSION: The findings point to limitations for the use of MSCs as vectors in glioma gene therapy, although intratumoral MSC implantation provides a dense and tumor-specific vector distribution

Bone Marrow Multipotent Mesenchymal Stroma Cells Act as Pericyte-like Migratory Vehicles in Experimental Gliomas

Bone marrow–derived multipotent mesenchymal stroma cells (MSCs) have emerged as cellular vectors for gene therapy of solid cancers. We implanted enhanced green fluorescent protein–expressing rat MSCs directly into rat malignant gliomas to address their migratory capacity, phenotype, and effects on tumor neovascularization and animal survival. A single intratumoral injection of MSCs infiltrated the majority of invasive glioma extensions (72 ± 14%) and a substantial fraction of distant tumor microsatellites (32 ± 6%). MSC migration was highly specific for tumor tissue. Grafted MSCs integrated into tumor vessel walls and expressed pericyte markers α-smooth muscle actin, neuron-glia 2, and platelet-derived growth factor receptor-β but not endothelial cell markers. The pericyte marker expression profile and perivascular location of grafted MSCs indicate that these cells act as pericytes within tumors. MSC grafting did not influence tumor microvessel density or survival of tumor-bearing animals. The antiangiogenic drug Sunitinib markedly reduced the numbers of grafted MSCs migrating within tumors. We found no MSCs within gliomas following intravenous (i.v.) injections. Thus, MSCs should be administered by intratumoral implantations rather than by i.v. injections. Intratumorally grafted pericyte-like MSCs might represent a particularly well-suited vector system for delivering molecules to affect tumor angiogenesis and for targeting cancer stem cells within the perivascular niche