Therapeutic Potential Of Nscs

The recognition that NSCs propagated in culture could be reimplanted into mammalian brain, where they could reintegrate appropriately and stably express foreign genes (4,26) made this strategy an appealing alternative for CNS gene therapy and repair. Numerous subsequent studies over the past decade (27) reaffirmed that neural progenitors from many regions and developmental stages could be maintained, perpetuated, and passaged in vitro by epigenetic, and genetic methods. Examples include the transduction of genes interacting with cell cycle proteins (e.g., vmyc) and by mitogen stimulation (e.g., epidermal growth factor and/or basic fibroblast growth factor; (4,26,28-32). Some of these methods may operate through common cellular mechanisms. This speculation is supported by the observation that many progenitor cell lines behave similarly in their ability to reintegrate into the CNS, despite that they were generated by different methods, obtained from various locations, and reimplanted into various CNS regions. Some NSC lines appear sufficiently plastic to participate in normal CNS development from germinal zones of multiple regions along the neuraxis and at multiple stages of development from embryo to old age (4,6,9,3337). In addition, they appear to model the in vitro and in vivo behavior of some primary fetal and adult neural cells (38-43), suggesting that insights gleaned from these NSC lines may legitimately reflect the potential of CNS progenitor or stem cells.

The inherent biologic properties of NSCs may circumvent limitations of other techniques for treating metabolic, degenerative, or other widespread lesions in the brain. They are easy to administer (often directly into the cerebral ventricles), are readily engraftable, and circumvent the blood-brain barrier. Unlike BMT, a preconditioning regime is not required before administration (e.g., total-body irradiation). One important property of NSCs is their apparent ability to develop into integral cytoarchitectural components (47) of many regions throughout the host brain as neurons, astrocytes, oligodendrocytes, and even incompletely differentiated, but quiescent, progenitors. Therefore, they may be able to replace a range of missing or dysfunctional neural cell types. A given NSC clone can give rise to multiple cell types within the same region. This is important in the likely situation where return of function may require the reconstitution of the whole milieu of a given region, e.g., not just the neurons but also the glia, and support cells required to nurture, detoxify, and/or myelinate the neurons. They appear to respond in vivo to neurogenic signals not only when they occur appropriately during development, but even when induced at later stages by certain neurodegenerative processes, like during apoptosis (4,45). NSCs may be attracted to regions of neurodegeneration in the young, as well as in the elderly (12,46-48; Fig. 2).

NSCs also appear to accommodate to the engraftment region, perhaps obviating the necessity to obtain donor cells from many specific CNS regions or the imperative for precise targeting during reimplantation. The cells might express certain genes of interest intrinsically (e.g., many neurotrophic factors), or they can be engineered ex vivo to do so because they are readily transduced by gene transfer vectors. These gene products can be delivered to the host CNS in a direct, immediate, and stable manner (6,7,47,49). Although NSCs can migrate and integrate widely throughout the brain

Fig. 2. The injured brain interacts reciprocally with NSCs supported by scaffolds to reconstitute lost tissue—evidence from hypoxic-ischemic (HI) injury. (Modified from ref. 8.)

(I) Characterization of NSCs in vitro when seeded upon a PGA scaffold. Cells seen with scanning electron microscopy at 5 d after seeding were able to attach to, impregnate, and migrate throughout a highly porous PGA matrix (arrow). The NSCs differentiated primarily into neurons (>90%) that sent out long, complex processes that adhered to, enwrapped, and interconnected the PGA fibers.

(II) Implantation of NSC-PGA complexes into a region of cavity formation following extensive HI brain injury and necrosis. (A) Brain of an untransplanted (non-Tx) mouse subjected to right-HI injury with extensive infarction and cavitation of the ipsilateral right cortex, striatum, thalamus, and hippocampus (arrow). In contrast with part B, the brain of a similarly injured mouse implanted with an NSC-PGA complex (PGA+NSCs) (generated in vitro as per part I. into the infarction cavity 7 d after the induction of HI (arrow) (n = 60). At maturity (age-matched to the animal pictured in part A), the NSC-scaffold complex appears in this whole-mount to have filled the cavity (arrow) and become incorporated into the infracted cerebrum. Representative coronal sections through that region are seen at higher magnification in parts C and D, in which parenchyma appears to have filled in spaces between the dissolving black polymer fibers (white arrow in part C) and,

^ as seen in part D, even to support neovascularization by host tissues. (Blood vessel is indicated by closed black arrow in part D; open arrow in part D points to degrading black polymer fiber.) Scale bars (C and D): 100 ^m.

(III) Characterization in vivo of the neural composition of NSC-PGA complexes within the HI-injured brain. At 2 wk following transplantation of the NSC-PGA complex into the infarction cavity, donor-derived cells showed robust engraft-ment within the injured region. An intricate network of multiple long, branching NF+ (green) processes were present within the NSC-PGA complex and its parenchyma enwrapping the PGA fibers (orange autofluorescent tube-like structures under a Texas Red filter), adherent to and running along the length of the fibers (arrows), often interconnecting and bridging the fibers (arrowheads). Those NF+ processes were of both host and donor derivation. In other words, not only were donor-derived neural cells present, but host-derived cells also seemed to have entered the NSC-PGA complex, migrating and becoming adherent to the PGA matrix. In a reciprocal manner, donor-derived (lacZ+) neurons (NF+ cells) within the complex appeared to send processes along the PGA fibers out of the matrix into host parenchyma, as seen in part IV. Scale bars: 100 ^m. (Figure 2 caption is continued on the next page.)

Fig. 2. (continued) (IV) Long-distance neuronal connections extend from the transplanted NSC-PGA complexes in the HI-injured brain toward presumptive target regions in the intact contralateral hemisphere. By 6 wk following engraftment, donor-derived lacZ+ cells appeared to extend many exceedingly long, complex NF+ processes along the length of the disappearing matrix, apparently extending into host parenchyma. To confirm the suggestion that long-distance processes projected from the injured cortex into host parenchyma, a series of tract-tracing studies were performed. [G-G"] BDA-FITC was injected (G) into the contralateral intact cortex and external capsule (green arrow) at 8 wk following implantation of the NSC-PGA complex into the infarction cavity (NSC/PGA-Tx). Axonal projections (labeled green with fluorescein under an FITC filter) are visualized (via the retrograde transport of BDA), leading back to (across the interhemispheric fissure (IHF) via the corpus callosum ["cc"]), and emanating from, cells in the NSC-PGA complex within the damaged contralateral cortex and penumbra (seen at progressively higher magnification in parts G' (region indicated by arrow to part G) and G" (region indicated by an arrow and asterisk in part G). In part G", the retrogradely BDA-FITC-labeled perikaryon of a representative neuron adherent to a dissolving PGA fiber is well visualized. The fact that such cells are neurons of donor derivation is supported by their triple labeling (H-J) for lacZ (H) (Pgal), BDA-FITC (I), and the neuronal marker NF (J); arrow in (H-J), indicates the same cell in all three panels). Such neuronal clusters were never seen under control conditions, g i.e., in untransplanted cases or when the vehicle, or even an NSC suspension unsupported by scaffolds, was injected into the infarction cavity. Scale bars: (G) 500 ^m; (G") 20 ^m; (H-J) 30 ^m.

Fig. 2. (continued) (V) Adverse secondary events that typically follow injury (e.g., monocyte infiltration and astroglial scar formation) are minimized by and within the NSC-PGA complex. (A-D) Photomicrographs of H&E-stained sections prepared to visualize the degree of monocyte infiltration in relation to the NSC-PGA complex and the injured cortex 3 wk following implantation into the infarction cavity. Monocytes are classically recognized under H&E as very small cells with small round nuclei and scanty cytoplasm (e.g., inset in part D, arrowhead). Although some localized monocyte infiltration was present immediately surrounding a blood vessel (BV in part C, arrow) that grew into the NSC-PGA complex from the host parenchyma, there was little or no monocyte infiltration either in the center of the NSC-PGA complex (B) or at the interface between the NSC-PGA complex and host cortical penumbra (A). This is in stark contrast to the excessive monocyte infiltration seen in an untransplanted infarct of equal duration, age, and extent (D), the typical histopathologic picture otherwise seen following HI brain injury (see inset, a higher magnification of the region indicated by the asterisk in part D; a typical monocyte is indicated by the arrowhead). Whereas neural cells (nuclei of which are seen in A-C) adhere exuberantly to the many polymer fibers (P in parts A-C), monocyte infiltration was minimal, compared to that in part D. (E,F) Astroglial scarring (another pathological condition confounding recovery from ischemic CNS injury) is also much constrained and diminished following implantation of the NSC-PGA complex. While GFAP+ cells (astrocytes) were among ^ the cell types into which NSCs differentiated when in contact with the PGA fibers, there was minimal astroglial presence either of donor or host origin away from the fibers (*). (E) GFAP immunostaining that is recognized by a fluorescein-conjugated secondary antibody (green) is observed. Note little scarring in the regions indicated by the asterisk. Under a Texas red filter (F) (merged with the fluorescein filter image), the tube-like PGA fibers (arrowhead in both panels) become evident (as autofluorescent orange), and most of the donor-derived astrocytes (arrows) (yellow because of their dual lacZ and GFAP immunoreactivity) are seen to be associated with these fibers, again leaving most regions of the infarct (*) astroglial scar-free. (Arrows in parts E and F point to the same cells.) Far from creating a barrier to the migration of host- or donor-origin cells, or to the ingrowth/outgrowth of axons of host- or donor-origin neurons (as per parts III and IV), NSC-derived astrocytes may have helped provide a facilitating bridge. Scale bars: (A) 10 ^m; (C,D) and (E,F) 20 ^m.

particularly well when implanted into germinal zones, allowing reconstitution of enzyme or cellular deficiencies in a global manner (6,7,47), this extensive migratory ability is present even in the parenchyma of the diseased adult and aged brain (6,7,50). Despite their extensive plasticity, NSCs never give rise to cell types inappropriate to the brain, such as muscle, bone, teeth, or yield neoplasms.

These attributes of NSCs may provide multiple strategies to treat CNS dysfunction. As proof of principle, they were first tested experimentally in mouse models of genetically based neurodegeneration. Their ability to mediate gene therapy was affirmed in a model of the neurogenetic lysoso-mal storage disease (LSD)—mucopolysaccharidosis type VII (MPS VII; 6). Mice homozygous for a frameshift mutation in the P-glucuronidase gene are devoid of the secreted enzyme P-glucuronidase (GUSB). The enzymatic deficiency results in lysosomal accumulation of undegraded glycosami-noglycans in the brain and other tissues, causing a fatal progressive degenerative disorder. Treatments for MPS VII and most other LSDs are designed to provide a source of normal enzymes for uptake by diseased cells—a process termed cross-correction (51). The goal of ex vivo gene therapy is to engineer donor cells to express the normal GUSB protein for export to other host cells. The engraftment and integration of GUSB overexpressing NSCs throughout the newborn MPS VII mutant brain succeeded in providing a sustained, lifelong, widespread source of cross-correcting enzyme in a manner not previously achieved (6).

A rapid intraventricular injection technique was devised for the diffuse engraftment of the NSCs. Injecting the progenitors into the cerebral ventricles presumably allowed them to gain access to most of the subventricular germinal zone (SVZ), as well as to networks of cerebral vasculature, along the surface of which they would also migrate. This approach worked equally well in the fetus, where donor NSCs gained access to the ventricular germinal zone (47), migrating into the parenchyma within 24-48 h. This engraft-ment technique, exploiting many of the inherent properties of NSCs, permitted missing gene products to be delivered without disturbing other neurobiological processes and was a potential strategy for gene therapy of a class of neurogenetic diseases that had not been adequately treated thus far (Fig. 3A). Although MPS VII may be regarded as "uncommon," the broad category of diseases that it models (neurogenetic conditions) afflicts as many as 1 in 1500 children and serves as a model for many adult neurodegenerative processes of genetic origin. (Alzheimer's disease could broadly fall into this category.) Therapy instituted early in life might arrest disease progression and prevent irreversible CNS alterations. Even in the adult brain, there are routes of relatively extensive migration followed by both endogenous and transplanted NSCs (52,53). If injected into the cerebral ventricles of normal adult mice, NSCs (including those expressing transgenes) will integrate into the SVZ and migrate long distances, e.g., to the olfactory bulb, where they differentiate into interneurons, and occasionally into subcortical parenchyma, where they become glia (9,34,35,54,55). These migratory paths are still relatively restricted and stereotyped, compared to that seen in the fetal or newborn brain. However, in the degenerating, abnormal, or injured adult brain (as discussed below), migration by foreign gene-expressing NSCs can be extensive and directed specifically to regions of pathology—a phenomenon observed with stroke, head injury, dopaminergic dysfunction, brain tumors, and amyloid plaques.

The therapeutic paradigm described above can be extended to other untreatable neurodegenerative diseases that are characterized by an absence of gene products and/or the accumulation of toxic metabolites. In almost all cases, NSCs, because they are normal cells, constitutively express normal amounts of the particular enzyme in question. The extent to which this amount needs to be augmented may vary from model to model and enzyme to enzyme. Reassuringly, in most inherited metabolic diseases, the amount of enzyme required to restore normal metabolism and forestall CNS disease may be quite small. It is significant to note that while the histograms in Fig. 1B illustrate the widespread distribution of a lysosomal enzyme, they could similarly reflect the NSC-mediated distribution of other diffusible (e.g., synthetic enzymes, neurotrophins, viral vectors; 56,57) and nondiffus-ible (e.g., myelin, extracellular matrix) factors, as well as the distribution of "replacement" neural cells (see the following section). For example, neural progenitors and stem cells have been used for the local expression of NT-3 within the rat spinal cord, nerve growth factor and brain-derived neu-rotrophic factor within the septum, and tyrosine hydroxylase, Bcl-2, and glial cell-derived neurotrophic factor to the striatum (58-65). These earlier studies helped to advance the idea that NSCs, as a prototype for stem cells from any solid organ, might aid in reconstructing both the molecules, along with the cells of a maldeveloped or damaged organ. A further complexity, however, is the recognition that the same NSC may not be able to be engineered to express certain neurotrophic agents simultaneously, because they may be processed antagonistically within the cell and/or within the environment. Therefore, a greater knowledge of the NSC processing of certain molecules is a prerequisite (66).

-,

Fig. 3. NSCs possess an inherent mechanism for rescuing dysfunctional neurons: evidence from the effects of NSCs in the restoration of mesencephalic dopaminergic function. (Modified from ref. 12.)

(I) TH expression in mesencephalon and striatum of aged mice following MPTP lesioning and unilateral NSC engraft-ment into the substantia nigra/ventral tegmental area (SN/VTA). A model that emulates the slow dysfunction of aging dopaminergic neurons in the SN was generated by giving aged mice high doses of MPTP repeatedly. Scheme (top) indicates the levels of analyzed transverse sections along the rostrocaudal axis of the mouse brain. Representative coronal sections through the striatum are presented in the left column (A,C,E,G) and through the SN/VTA area in the right column (B,D,F,H). (A,B) Immunodetection of TH (black cells) shows the normal distribution of DA-producing TH+ neurons in coronal sections in the intact SN/VTA (B) and their projections to the striatum (A). (C,D) Within 1 wk, MPTP treatment caused extensive and permanent bilateral loss of TH immunoreactivity in both the mesostriatal nuclei (C) and the striatum (D), which lasted lifelong. Shown in this example, and matching the time point in (G,H), is the result of 4-wk MPTP treatment in a mock-grafted animal. (E,F) Unilateral (right side) stereotactic injection of NSCs into the nigra is associated (within 1 wk after grafting) with substantial recovery of TH synthesis within the ipsilateral DA nuclei (F) and their ipsilateral striatal projections (E). By 3 wk posttransplant, however (G,H), the asymmetric distribution of TH expression disappeared, giving ^ rise to TH immunoreactivity in the midbrain (H) and striatum (G) of both hemispheres that approached that of intact controls (A,B) and gave the appearance of mesostriatal restoration. Similar observations were made when NSCs were injected 4 wk after MPTP treatment (not shown). Bars: 2 mm (left), 1 mm (right). Note the ectopically placed TH+ cells in part H. These are analyzed in greater detail, along with the entire SN in part II. (Figure 3 caption is continued on the next page.)

Fig. 3. (continued) (II) Immunohistochemical analyses of TH, DAT, and BrdU-positive cells in MPTP-treated and grafted mouse brains. The presumption was initially that the NSCs had replaced the dysfunctional TH neurons. However, examination of the reconstituted SN with dual P-gal (green) and TH (red) ICC showed that (a,c) 90% of the TH+ cells in the SN were host-derived cells, which had been rescued and were only 10% donor-derived (d). Most NSC-derived TH+ cells were just above the SN ectopically (blocked area in part a, enlarged in part b). These photomicrographs were taken from immunostained brain sections from aged mice exposed to MPTP, transplanted 1 wk later with NSCs, and sacrificed after 3 wk. The following combinations of markers were evaluated: TH (red) with P-gal (green) (a-d); NeuN (red) with P-gal (green) (e); GFAP (red) with P-gal (green) (f); CNPase (green) with P-gal (red), as well as TH (brown) and BrdU (black) (k); GFAP (brown) with BrdU (black) (l); and CNPase (brown) with BrdU (black) (m). Anti-DAT-stained areas are revealed in green in the SN of intact (h), mock-grafted (i), and NSC-grafted (j) brains. Three different fluorescence filters specific for Alexa Fluor 488 (green), Texas Red (red), and a double filter for both types of fluorochromes (yellow) were used to visualize specific antibody binding. Parts c, d, and h-j are single-filter exposures; a,b, and e-g are double-filter exposures. Part a shows a low-power overview of the SN+VTA of both hemispheres, similar to the image in part H of Fig. 2. The majority of TH+ cells (red cells in part a) within the nigra are actually of host origin (~90%), with a much smaller proportion of donor derivation (green ^ cells) (~10%) (representative close-up of such a donor-derived TH+ cell in part d). Although a significant proportion of NSCs did differentiate into TH+ neurons, many of these resided ectopically, dorsal to the SN (boxed area in part a, enlarged in b; high-power view of donor-derived (green) cell that was also TH+ (red) in part c), where the ratio of donor-to-host cells was inverted: ~90% donor-derived vs ~10% host-derived. Note the almost complete absence of a green P-gal-specific signal in the SN/VTA whereas ectopically, many TH+ cells were double-labeled and thus NSC-derived (appearing yellow-orange in higher power under a red/green double-filter in panel b). (c-g) NSC-derived non-TH neurons (NeuN+) (e, arrow), astrocytes (GFAP+) (f), and oligodendrocytes (CNPase+) (g, arrow) were also seen, both within the mesencephalic nuclei and dorsal to them. (h-j) The green DAT-specific signal in panel j suggests that the reconstituted mesencephalic nuclei in the NSC-grafted mice (as in panel I-h were functional DA neurons, comparable to those seen in intact nuclei (h), but not in MPTP-lesioned sham-engrafted controls (i). This further suggests that the TH+ mesostriatal DA neurons affected by MPTP are indeed functionally impaired. (Note that sham-grafted animals (i) contain only punctate residual DAT staining within their dysfunctional fibers, whereas DAT staining in normal (h) and, similarly in engrafted (j) animals, was normally and robustly distributed within processes and throughout their cell bodies.) (k-m) Any proliferative BrdU+ cells after MPTP insult and/or grafting were confined to glial cells, whereas the TH+ neurons (k) were BrdU-. This finding suggested that the reappearance of TH+ host cells was not the result of neurogenesis, but rather the recovery of extant host TH+ neurons. Bars: 90 ^m (a); 20 ^m (c,d,e); 30 ^m (f); 10 ^m (g); 20 ^m (h-j); 25 ^m, (k); 10 ^m, (l); and 20 ^m (m).

Therapeutic Uses of NSCs Against Brain Tumors

In 2000, Aboody et al. reported that transplanted exogenous murine and human NSCs were capable of "homing in" over long distances onto intracerebral xenogeneic brain tumors deposited into rodent brains (67). The authors also demonstrated the ability of NSCs to "track" tumor cells escaping from the original inoculated tumor mass and invading the normal brain. Specially tagged NSCs were found adjacent to invading tumor cells that appeared to be infiltrating normal brain along white-matter tracts, tumor-related endot-helium, and interstitial spaces. There appeared to be a particular predilection for tumor-associated endothelium. The NSCs could be introduced either intraparenchymally or into the lateral ventricles with equal homing ability. Whether introduced into the ipsilateral or contralateral cerebral ventricles, or into the ipsilateral or contralateral cerebral parenchyma, NSCs were able to migrate toward the implanted tumor and appose themselves intimately to escaping, infiltrating tumor cells, even at far distances from the main tumor mass. Indeed, NSCs injected into the tail vein demonstrated successful intracranial tumor "homing" (establishing a paradigm, since it is used by some investigators for other intracranial pathologies, e.g., models of multiple sclerosis; 68). The blood-brain barrier normally acts as an efficient barrier to the successful delivery of many therapeutic agents and dictates the limited repertoire of brain tumor chemotherapeutic agents (69,70). In this instance, the blood-brain barrier did not appear to affect the migration and homing of NSCs toward intracranial pathology, as modeled by the neoplasms.

This "gliomatropic" capability of NSCs bodes well for their use in clinical applications. NSCs could serve as tumor-directed homing devices expressing a variety of antitumor genes, including those encoding cytotoxic, antiangiogenic, antimitotic, antimigratory, immunomodulatory, prodiffer-entiating, and/or proapoptotic agents. Ultimately, the goal is for NSCs to be used in conjunction with, and to optimize, other therapeutic modalities by providing the ability to track invading cells—the bane of most gene, radiation, surgical, and pharmacological strategies—and complete eradication of the glioma cells (71). Aboody et al. tested this hypothesis with the use of cytosine deaminase (CD)-expressing murine NSCs. This enzyme converts the nontoxic precursor 5-fluorocytosine (5-FC) into the toxic compound 5-fluorouracil. Injection of 5-FC into tumor-bearing mice that had been inoculated with NSCs engineered to express CD was followed by the dramatic reduction of intracranial tumor burden. CD has a particularly impressive bystander effect. Barresi et al. replicated this phenomenon by showing in vivo regression of implanted C6 glioma cells after coinoculation of an immortalized neural progenitor cell line ST14A expressing CD, followed by administration of 5-FC (72). This work gave further credence to the utility of exploiting NSCs to effect the well-established, but hitherto unsuccessful, "prodrug/prodrug-converting enzyme" strategy to destroy intracerebral malignancies.

The virtue of the prodrug approach is twofold. First, the NSC has to kill only a small percentage of tumor cells to have a large impact on other tumor cells as a result of the bystander effect. Like an exploding hand grenade, dying tumor cells send out toxic factors or signals that are inimical to many surrounding tumor cells. CD's action on 5-FC creates one the greatest bystander effects of all the prodrug-converting enzymes. Second, although no adverse effects or contributions to tumor growth from NSCs have ever been detected in these models, if such an unlikely situation arises, the CD within the NSC would cause it to self-eliminate if it were to become mitotic, providing a built-in safety mechanism.

Extending the use of NSCs to deliver cytolytic genes, and based on the report by Lynch et al. that NSCs could be engraftable, mobile intracranial viral-packaging lines (73), Herrlinger et al. showed that murine NSCs, engineered to release replication-conditional herpes simplex virus (HSV) thymidine kinase (TK), were efficient in the destruction of an intracranial tumor mass, as well as isolated, escaping tumor microdeposits (74). In preliminary studies, William Weiss and colleagues have validated the gliomatropism of murine NSCs in a transgenic oligodendroglioma-bearing mouse model previously described. This double transgenic mouse is the product of mating a transgenic mouse in which a mutant epidermal growth factor receptor is transcribed from the S100 promoter (S100 P-v-erbB) with an INK4aNull mouse, yielding progeny that succumb to tumors by 6 mo of age. Investigators showed that NSCs homed in on spontaneously developing tumors. With the use of CD-expressing NSCs (clone C17.2) in pilot studies, host survival improved. Because the NSCs seemed to "find" even small "subclinical" tumors, whose presence was unsuspected by the investigators' prehistological examination, NSCs may also be used in tumor diagnosis if armed with tags that can be imaged in the living state (see below). It also warrants mentioning that transgenic mice that spontaneously develop brain tumors appear to provide models for studying tumor biology and therapies, including the glioma-specific migratory potential of NSCs, which far more faithfully emulate the true clinical situation in humans and are therefore superior to older models that depend on the artificial implantation of glioma cell lines.

Immunomodulation through the judicious use of cytokines has been shown to be extremely effective in the destruction of experimental brain tumors via either direct cytotoxicity or the activation of immune-mediated antitumor effects (75-78). Benedetti and colleagues showed that intra-tumoral injection of interleukin 4 (IL-4)-expressing murine NSCs effected radiographic tumor regression and prolongation of host survival (79). In addition, the authors detected the presence of NSCs several weeks postinjection in the recipient animals. This evidence implied persistence of implanted NSCs and possibly persistence of antitumor effects in vivo. The authors also described an inherent tumor inhibitory effect exhibited by the injected stem cells. This phenomenon was first observed many years ago using murine NSC clone C17.2. Gliastatin, a membrane-associated factor isolated from these NSCs, was capable of converting C6 glioma cells to cells resembling phenotypically normal astrocytes simply via cell contact (80).

Exploiting the specific homing capability of NSCs to intracranial tumors, Ehtesham et al. transfected murine neural progenitors ex vivo with the IL-12 gene using an adenoviral vector (81). Stable expression of the IL-12 protein was demonstrated in vitro and in vivo. Implantation of IL-12 expressing NSCs into tumor-bearing syngeneic mice effected tumor destruction and improved host survival. In addition, the authors showed enhanced tumor infiltration by T lymphocytes as a result of IL-2 expression in close proximity to the tumor mass. In a separate study, the authors transfected similarly derived murine neural progenitors with the human tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) gene using a replication-deficient adenoviral vector (82). TRAIL belongs to the tumor necrosis factor superfamily of proapoptotic proteins, which has been previously shown to induce apoptosis in experimental tumor models (83). Introduction of TRAIL-expressing neural stem/progenitor cells into nude mice bearing gliablastoma multiforme xenografts was followed by the eradication of tumor via induction of apoptotic cell death.

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How To Reduce Acne Scarring

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