Allografts of CNS tissue possess a blood-brain barrier. II. Angiogenesis in solid tissue and cell suspension grafts

Atrial fibrillation

Atrial fibrillation (AF) is the most common cardiac arrhythmia despite substantial efforts to understand the pathophysiology of the condition and develop improved treatments. Identifying the underlying causative mechanisms of AF in individual patients is difficult and the efficacy of current therapies is suboptimal. Consequently, the incidence of AF is steadily rising and there is a pressing need for novel therapies. Research has revealed that defects in specific molecular pathways underlie AF pathogenesis, resulting in electrical conduction disorders that drive AF. The severity of this so-called electropathology correlates with the stage of AF disease progression and determines the response to AF treatment. Therefore, unravelling the molecular mechanisms underlying electropathology is expected to fuel the development of innovative personalized diagnostic tools and mechanism-based therapies. Moreover, the co-creation of AF studies with patients to implement novel diagnostic tools and therapies is a prerequisite for successful personalized AF management. Currently, various treatment modalities targeting AF-related electropathology, including lifestyle changes, pharmaceutical and nutraceutical therapy, substrate-based ablative therapy, and neuromodulation, are available to maintain sinus rhythm and might offer a novel holistic strategy to treat AF.

Cell Transplantation to Restore Lost Auditory Nerve Function is a Realistic Clinical Opportunity

Hearing is one of our most important means of communication. Disabling hearing loss (DHL) is a long-standing, unmet problem in medicine, and in many elderly people, it leads to social isolation, depression, and even dementia. Traditionally, major efforts to cure DHL have focused on hair cells (HCs). However, the auditory nerve is also important because it transmits electrical signals generated by HCs to the brainstem. Its function is critical for the success of cochlear implants as well as for future therapies for HC regeneration. Over the past two decades, cell transplantation has emerged as a promising therapeutic option for restoring lost auditory nerve function, and two independent studies on animal models show that cell transplantation can lead to functional recovery. In this article, we consider the approaches most likely to achieve success in the clinic. We conclude that the structure and biochemical integrity of the auditory nerve is critical and that it is important to preserve the remaining neural scaffold, and in particular the glial scar, for the functional integration of donor cells. To exploit the natural, autologous cell scaffold and to minimize the deleterious effects of surgery, donor cells can be placed relatively easily on the surface of the nerve endoscopically. In this context, the selection of donor cells is a critical issue. Nevertheless, there is now a very realistic possibility for clinical application of cell transplantation for several different types of hearing loss.

Enhanced Survival of Porcine Neural Xenografts in Mice Lacking CD1d1, But No Effect of NK1.1 Depletion

Transplantation of embryonic porcine neurons may restore neurological function in patients with Parkinson's disease, if immunological rejection could be prevented. This study was performed to investigate the role of natural killer cells (NK cells) and NK1.1+ T cells (NK T cells) in the rejection of neural xenografts. A cell suspension was prepared from the ventral mesencephalon of 26 – 27-day-old pig embryos, and 2 μl was implanted in the right striata of mutant CD1d1 null (CD1.1-/-) mice, NK1.1-depleted mice, and controls. The CD1.1-/- mice are deficient in NK T cells and the antigen-presenting molecule CD1d1. Graft survival and host responses were determined immunohistochemically using markers for dopamine neurons, CD4-, CD8- cells, microglia, and macrophages. At 2 weeks, the grafts were significantly larger in CD1.1-/- mice, 0.09 ± 0.02 μl (mean ± SEM), compared with controls, 0.05 ± 0.01 μl. There was no significant difference between NK1.1-depleted mice, 0.02 ± 0.01 μl, and controls. At 5 weeks, two grafts were still present in the CD1-/- mice, whereas only scars remained in the controls and in the NK1.1-depleted mice. Immune reactions were strong at 2 weeks and less pronounced at 5 weeks in all groups. Microglial activation was lower in NK-depleted mice than in the controls at 2 weeks. In contrast to organ xenografting, NK1.1+ cells do not seem to be important mediators of the rejection of discordant cellular neural xenografts. However, our results suggest that the antigen-presenting molecule CD1d1 may be involved in the rejection process.

Kidney Cografts Enhance Fiber Outgrowth from Ventral Mesencephalic Grafts to the 6-Ohda–Lesioned Striatum, and Improve Behavioral Recovery

Recent studies have demonstrated the presence of many different neurotrophic factors in the developing and adult kidney. Due to its production of this mixture of neurotrophic factors, we wanted to investigate whether fetal kidney tissue could be beneficial for neuritic fiber growth and/or cell survival in intracranial transplants of fetal ventral mesencephalic tissue (VM). A retrograde lesion of nigral dopaminergic neurons was performed in adult Fischer 344 male rats by injecting 6-hydroxydopamine into the medial forebain. The animals were monitored for spontaneous locomotor activity in addition to apomorphine-induced rotations once a week. Four weeks following the lesion, animals were anesthetized and embryonic day 14 VM tissue from rat fetuses was implanted stereotaxically into the dorsal striatum. One group of animals received a cograft of kidney tissue from the same embryos in the same needle track. The animals were then monitored behaviorally for an additional 4 months. There was a significant improvement in both spontaneous locomotor activity (distance traveled) and apomorphine-induced rotations with both single VM grafts and VM–kidney cografts, with the VM–kidney double grafts enhancing the motor behaviors to a significantly greater degree. Tyrosine hydroxylase (TH) immunohistochemistry and image analysis revealed a significantly denser innervation of the host striatum from the VM–kidney cografts than from the single VM grafts. TH-positive neurons were also significantly larger in the cografts compared to the single VM grafts. In addition to the dense TH-immunoreactive innervation, the kidney portion of cografts contained a rich cholinergic innervation, as evidenced from antibodies against choline acetyltransferase (ChAT). The striatal cholinergic cell bodies surrounding the VM–kidney cografts were enlarged and had a slightly higher staining density for ChAT. Taken together, these data support the hypothesis that neurotrophic factors secreted from fetal kidney grafts stimulated both TH-positive neurons in the VM cografts and cholinergic neurons in the host striatum. Thus, these factors may be combined for treatment of degenerative diseases involving both dopaminergic and cholinergic neurons.

Development of an Intact Blood-Brain Barrier in Brain Tissue Transplants is Dependent on the Site of Transplantation

Transplantation of fetal septal forebrain tissue was performed to the anterior chamber of the eye, or intracranially to the rostral hippocampal formation in rats, to evaluate the impact of transplantation site on the development of an intact blood–brain barrier (BBB). The tissue was studied at 1, 2, 3, and 4 wk following transplantation by means of intravenous injection of Trypan blue, which is a vital stain not normally penetrating the BBB, as well as with an antibody specifically directed against the rat BBB, SMI71. In the intraocular septal transplants, there was a significant leakage of Trypan blue 1 wk postgrafting, associated with a few laminin-immunoreactive blood vessels that did not contain any SMI71-immunoreactivity. However, at 2 wk postgrafting, the intraocular grats exhibited an extensive plexus of thin-walled blood vessels expressing SMI71 immunoreactivity and no Trypan blue leakage. Thus, it appeared that a BBB had developed to some degree by 2 wk postgrafting in oculo. In the intracranial grafts, on the other hand, Trypan blue leakage could be seen as long as 3 wk postgrafting, and a dense plexus of blood vessels with SMI71 immunoreactivity was first seen at 4 wk postgrafting. Thus, the development of Trypan blue impermeability was delayed with 1 to 2 wk in the intracranial versus the intraocular grafts. Control experiments using psychological stress in adult rats as a means to transiently disrupt the BBB revealed that an increase in Trypan blue leakage correlated well with the disappearance of SMI71 immunoreactivity. Taken together, these studies demonstrate that the site of transplantation can influence the development of an intact BBB in neural tissue grafts.

Two-step grafting significantly enhances the survival of foetal dopaminergic transplants and induces graft-derived vascularisation in a 6-OHDA model of Parkinson's disease

Following transplantation of foetal primary dopamine (DA)-rich tissue for neurorestaurative treatment of Parkinson's disease (PD), only 5-10% of the functionally relevant DAergic cells survive both in experimental models and in clinical studies. The current work tested how a two-step grafting protocol could have a positive impact on graft survival. DAergic tissue is divided in two portions and grafted in two separate sessions into the same target area within a defined time interval. We hypothesized that the first graft creates a "DAergic" microenvironment or "nest" similar to the perinatal substantia nigra that stimulates and protects the second graft. 6-OHDA-lesioned rats were sequentially transplanted with wild-type (GFP-, first graft) and transgenic (GFP+, second graft) DAergic cells in time interims of 2, 5 or 9days. Each group was further divided into two sub-groups receiving either 200k (low cell number groups: 2dL, 5dL, 9dL) or 400k cells (high cell number groups: 2dH, 5dH, 9dH) as first graft. During the second transplantation, all groups received the same amount of 200k GFP+ cells. Controls received either low or high cell numbers in one single session (standard protocol). Drug-induced rotations, at 2 and 6weeks after grafting, showed significant improvement compared to the baseline lesion levels without significant differences between the groups. Rats were sacrificed 8weeks after transplantation for post-mortem histological assessment. Both two-step groups with the time interval of 2days (2dL and 2dH) showed a significantly higher survival of DAergic cells compared to their respective standard control group (2dL, +137%; 2dH, +47%). Interposing longer intervals of 5 or 9days resulted in the loss of statistical significance, neutralising the beneficial two-step grafting effect. Furthermore, the transplants in the 2dL and 2dH groups had higher graft volume and DA-fibre-density values compared to all other two-step groups. They also showed intense growth of GFP+ vessels - completely absent in control grafts - in regions where the two grafts overlap, indicating second-graft derived angiogenesis. In summary, the study shows that two-step grafting with a 2days time interval significantly increases DAergic cell survival compared to the standard protocol. Furthermore, our results demonstrate, for the first time, a donor-derived neoangiogenesis, leading to a new understanding of graft survival and development in the field of cell-replacement therapies for neurodegenerative diseases.

Single-cell suspension methodology favors survival and vascularization of fetal striatal grafts in the YAC128 mouse model of Huntington's disease

Cell replacement therapies have yielded variable and short-lived benefits in Huntington's disease (HD) patients. This suboptimal outcome is likely due to the fact that graft survival is compromised long term because grafts are subjected to a host's microglial inflammatory response, to a lack of adequate trophic support, and possibly to cortical excitotoxicity. However, graft demise may also relate to more straightforward issues such as cell preparation methodology (solid grafts vs. cell suspension). Indeed, we recently reported that solid grafts are poorly revascularized in HD patients transplanted 9 and 12 years previously. To evaluate whether methodological issues relating to cell preparation may have an impact on graft viability, we implanted green fluorescent protein (GFP(+)) single-cell suspensions of fetal striatal neuronal cells into the striatum of YAC128 HD mice. Postmortem evaluation yielded comparable graft survival in YAC128 mice and their wild-type littermates (noncarrier) at 1 and 3 months posttransplantation. Additionally, the degrees of graft revascularization in the YAC128 and noncarrier mice were similar, with both capillaries and large-caliber vessels observable within the grafted tissue. Furthermore, GFP(+) cells interacted well with host blood vessels, indicating integration of the donor cells within the recipient brain. These observations, combined with our recent report of poor revascularization of solid grafts in the HD-transplanted patients, suggest that the success of cell transplantation can be improved by optimizing methodological aspects relating to cell preparation.

Improving the survival of human CNS precursor-derived neurons after transplantation

We have examined the effects of predifferentiation and energy substrate deprivation on long-term expanded human neural precursor cells (HNPCs). The pre-differentiation of HNPC cultures produced large numbers of neurons (>60%) and mature glial cells capable of generating glycogen stores that protected the neuronal population from experimental metabolic stress. When predifferentiated HNPCs were transplanted into intact adult rat hippocampus, fewer cells survived compared to undifferentiated HNPC transplants. This cell death was completely attenuated, however, when predifferentiated HNPC cultures were pretreated to boost glial energy stores and resulted in greatly increased neuronal survival in vivo. The transplanted cells primarily engrafted within the granular layer of the dentate gyrus, where a large proportion of the predifferentiated HNPCs co-expressed neuronal markers whereas most HNPCs outside of the neuronal layer did not, indicating that the predifferentiated cells remained capable of responding to local cues in the adult brain. Undifferentiated HNPCs migrated more widely in the brain after grafting than did the predifferentiated cells, which generally remained within the hippocampus.

Angiogenic and neurotrophic effects of vascular endothelial growth factor (VEGF165): studies of grafted and cultured embryonic ventral mesencephalic cells

The present series of experiments investigated the effects of vascular endothelial growth factor (VEGF165) on adult rat striatal cerebrovasculature and embryonic dopamine (DA) neuron allografts in a rat model of Parkinson's disease (PD). We examined VEGF165's ability to (1) alter the vascular network of the adult rat striatum, (2) influence the vascular growth of solid embryonic day 14 (E14) ventral mesencephalic (VM) grafts when placed into a VEGF-pretreated host striatum, (3) alter the function and survival of E14 VM grafts when transplanted into an adult DA-deleted striatum, and (4) influence cell survival and neurite growth in cultures of E14 VM cells. We demonstrate here that a single bolus injection of VEGF165 into the adult rat striatum significantly increases the amount of vasculature in the vicinity of the injection site in a delayed and transient manner when compared to saline controls. Transplanting solid E14 VM grafts into the VEGF165-pretreated striatum resulted in a homogeneous distribution of small blood vessels throughout the graft, a pattern that closely resembles mature adult vasculature. In contrast, grafts in the control condition contained a patchy distribution of heavily dilated vessels. Behavioral measurements indicate that VEGF pretreatment of the intrastriatal graft site accelerates recovery of amphetamine-induced rotational asymmetry in unilateral 6-OHDA lesioned rats. Unexpectedly, however, VEGF pretreatments failed to increase survival of tyrosine hydroxylase-immunoreactive (THir) neurons in the grafts. In contrast to this finding in vivo, adding VEGF165 to glial-reduced E14 rat VM cultures produced a fourfold increase in THir cell survival and a doubling in the length of THir neurites. We conclude that with the proper method of delivery, VEGF165 may prove to be one of several strategies necessary to significantly improve the survival and function of fetal VM tissue grafts.

Microvascular plasticity in aging

Understanding the bases of aging-related cognitive decline remains a central challenge in neurobiology. Quantitative studies reveal little change in the number of neurons or synapses in most of the brain but their ongoing replacement is reduced, resulting in a significant loss of neuronal plasticity with senescence. Aging also may alter neuronal function and plasticity in ways that are not evident from anatomical studies of neurons and their connections. Since the nervous system is dependent upon a consistent blood supply, any aging-related changes in the microvasculature could affect neuronal function. Several studies suggest that, as the nervous system ages, there is a rarefaction of the microvasculature in some regions of the brain, as well as changes in the structure of the remaining vessels. These changes contribute to a decline in cerebral blood flow (CBF) that reduces metabolic support for neural signaling, particularly when levels of neuronal activity are high. In addition to direct effects on the microvasculature, aging reduces microvascular plasticity and the ability of the vessels to respond appropriately to changes in metabolic demand. This loss of microvascular plasticity has significance beyond metabolic support for neuronal signaling, since neurogenesis in the adult brain is regulated coordinately with capillary growth.

Rat nigral xenografts survive in the brain of MHC class II-, but not class I-deficient mice

We have examined the role of the indirect pathway of antigen recognition and T cells in neural xenografts rejection by using major histocompatibility complex (MHC) class II-deficient mice as xenograft recipients. Dissociated embryonic ventral mesencephalic tissue from Sprague-Dawley rats was stereotaxically injected as a cell suspension into the striatum of MHC class II-deficient adult mice as well as MHC class I-deficient and wild-type mice as controls. All of the MHC class II-deficient mice had surviving grafts in the striatum 4 weeks post-grafting. In contrast, only a few of the MHC class I-deficient mice exhibited very small grafts and none of the wild-type mice had any surviving grafts. The mean number of surviving transplanted dopamine neurons in the MHC class II-deficient group was significantly larger than that observed in the other two groups. Moderate levels of MHC class I antigen expression were seen in the transplantation sites of some animals in the MHC class II-deficient group. No helper or cytotoxic T cells were observed infiltrating into the graft sites of this group. However, there were markedly increased levels of expression of MHC class I and class II antigens, and a number of T cells infiltrating in the graft sites in both the MHC class I-deficient and wild-type groups. These results show that rat embryonic nigral tissue can survive transplantation in the brain of the MHC class II-deficient mice for at least 4 weeks without any overt signs of rejection, suggesting that the indirect pathway of foreign antigen recognition mediated by host MHC class II molecules and helper T cells plays an important role in the rejection responses to intracerebral xenografts.

Cellular replacement therapy for Parkinson's disease--where we are today?

The concept of replacing lost dopamine neurons in Parkinson's disease using mesencephalic brain cells from fetal cadavers has been supported by over 20 years of research in animals and over a decade of clinical studies. The ambitious goal of these studies was no less than a molecular and cellular "cure" for Parkinson's disease, other neurodegenerative diseases, and spinal cord injury. Much research has been done in rodents, and a few studies have been done in nonhuman primate models. Early uncontrolled clinical reports were enthusiastic, but the outcome of the first randomized, double blind, controlled study challenged the idea that dopamine replacement cells can cure Parkinson's disease, although there were some significant positive findings. Were the earlier animal studies and clinical reports wrong? Should we give up on the goal? Some aspects of the trial design and implantation methods may have led to lack of effects and to some side effects such as dyskinesias. But a detailed review of clinical neural transplants published to date still suggests that neural transplantation variably reverses some aspects of Parkinson's disease, although differing methods make exact comparisons difficult. While the randomized clinical studies have been in progress, new methods have shown promise for increasing transplant survival and distribution, reconstructing the circuits to provide dopamine to the appropriate targets and with normal regulation. Selected promising new strategies are reviewed that block apoptosis induced by tissue dissection, promote vascularization of grafts, reduce oxidant stress, provide key growth factors, and counteract adverse effects of increased age. New sources of replacement cells and stem cells may provide additional advantages for the future. Full recovery from parkinsonism appears not only to be possible, but a reliable cell replacement treatment may finally be near.

Proliferating cellular nuclear antigen expression as a marker of perivascular macrophages in simian immunodeficiency virus encephalitis

Brain perivascular macrophages are a major target of simian immunodeficiency virus (SIV) infection in rhesus macaques and HIV infection in humans. Perivascular macrophages are distinct from parenchymal microglia in their location, morphology, expression of myeloid markers, and turnover in the CNS. In contrast to parenchymal microglia, perivascular macrophages are continuously repopulated by blood monocytes, which undergo maturation to macrophages on entering the central nervous system (CNS). We studied differences in monocyte/macrophages in vivo that might account for preferential infection of perivascular macrophages by SIV. In situ hybridization for SIV and proliferating cellular nuclear antigen (PCNA) immunohistochemistry demonstrated that SIV-infected and PCNA-positive cells were predominantly found in perivascular cuffs of viremic animals and in histopathological lesions that characterize SIV encephalitis (SIVE) in animals with AIDS. Multilabel techniques including double-label immunohistochemistry and combined in situ hybridization and immunofluorescence confocal microscopy revealed numerous infected perivascular macrophages that were PCNA-positive. Outside the CNS, SIV-infected, PCNA-expressing macrophage subpopulations were found in the small intestine and lung of animals with AIDS. While PCNA is used as a marker of cell proliferation it is also strongly expressed in non-dividing cells undergoing DNA synthesis and repair. Therefore, more specific markers for cell proliferation including Ki-67, topoisomerase IIalpha, and bromodeoxyuridine (BrdU) incorporation were used which indicated that PCNA-positive cells within SIVE lesions were not proliferating. These observations are consistent with perivascular macrophages as terminally differentiated, non-dividing cells and underscores biological differences that could potentially define mechanisms of preferential, productive infection of perivascular macrophages in the rhesus macaque model of neuroAIDS. These studies suggest that within CNS and non-CNS tissues there exist subpopulations of macrophages that are SIV-infected and express PCNA.

Evidence for target-specific outgrowth from subpopulations of grafted human dopamine neurons

Clinical and experimental grafting in Parkinson's disease has shown the need for enhanced survival of dopamine neurons to obtain improved functional recovery. In addition, it has been suggested that a limited number of surviving dopamine neurons project to the dopamine-denervated host striatum. The aim of this study was to investigate if subpopulations of ventral mesencephalic dopamine neurons project to their normal targets, i.e., dorsal vs. ventral striatum. Following implantation of human ventral mesencepahlic tissue into the lateral ventricle of dopamine-depleted rats, human-derived dopamine reinnervation was achieved both in dorsal and ventral striatum. Treatment with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) resulted in a degeneration of tyrosine hydroxylase (TH)-immunoreactive nerve fibers in dorsal striatum but not in ventral areas in some animals, while MPTP was without effect in other animals. TH-immunoreactive neurons were small and appeared shrunken in animals carrying grafts affected by the MPTP treatment. In conclusion, grafted dopamine neurons projected nerve fibers into areas that they normally innervate. Thus, when searching for factors that may enhance survival of grafted dopamine neurons it is important to study which subpopulation(s) of ventral mesencephalic dopamine neurons is affected, such that a proper reinnervation may be achieved.

Solid human embryonic spinal cord xenografts in acute and chronic spinal cord cavities: a morphological and functional study

While therapeutic spinal cord grafting procedures are of interest in the chronic spinal cord injury stage, previous experimental grafting studies, including human spinal cord tissue, have mainly focused on the acute stage. Therefore, solid human embryonic spinal cord grafts were implanted in acute or chronic spinal cord aspiration cavities of immunodeficient rats to compare the morphological and locomotor outcome to that of lesion alone cases. Locomotor function was assessed using the Basso, Beattie, and Bresnahan open-field locomotor rating scale up to 6 months, while the morphological evaluation of graft survival, growth, and integration was performed at 6 weeks or 6 months after implantation. Graft survival was 94% in both lesion models, while graft growth was enhanced in the chronic compared to the acute cavity group. Human specific Thy-1 and neurofilament immunoreactive fibers were observed up to 7 mm into host white matter, while aminergic fibers were observed up to 1 mm into the grafts. Abundant calcitonin gene-related peptide immunoreactive fibers in the grafts in the absence both of immunoreactive cell bodies and colocalized human-specific neurofilament immunoreactivity, suggested host fiber ingrowth. At 6 months, the grafted cases presented less central canal deformation and lower glial fibrillary acidic protein immunoreactivity at the host cavity border compared to that of the nongrafted cases. The strong compensatory regain of locomotor function after unilateral spinal cord lesions was not affected by the human spinal cord grafts. In conclusion, solid human embryonic spinal cord tissue transplanted to a cavity in the adult injured spinal cord results in beneficial morphological effects in both the acute and chronic spinal cord lesion.

Diminished survival of mesencephalic dopamine neurons grafted into aged hosts occurs during the immediate postgrafting interval

The survival rate of dopamine (DA) neurons in mesencephalic grafts to young adult rats is poor, estimated at 5-20%, and even poorer in grafts to the aged striatum. Grafted cells die in young adult rats during the first 4 days after implantation. The present study was undertaken to determine whether the decreased survival of DA neurons in grafts to aged rats is (1) due to additional cell death during the immediate postgrafting interval or (2) due to protracted cell loss during longer postgrafting intervals. We compared survival rates of tyrosine hydroxylase-immunoreactive (THir) neurons in cell suspension grafts to young adult (3 months) and aged (24 months) male Fischer 344 rats at 4 days and 2 weeks after transplantation. At 4 days after grafting, mesencephalic grafts within the aged rat striatum contain approximately 25% of the number of THir neurons in the same mesencephalic cell suspension grafted to young adult rats. This corroborates the decreased survival of grafted DA neurons we have demonstrated previously at 10 weeks postgrafting. THir neurons in grafts to the intact striatum possessed a significantly shorter "long axis" than their counterparts on the lesioned side. No significant differences in the number of apoptotic nuclear profiles or total alkaline phosphatase staining between mesencephalic grafts to young and aged rats were detectable at 4 days postgrafting. In summary, the present study indicates that the exaggerated cell death of grafted DA neurons that occurs following implantation to the aged striatum occurs during the immediate postgrafting interval, timing identical to that documented for young adult hosts.

Survival of intrastriatal xenografts of ventral mesencephalic dopamine neurons from MHC-deficient mice to adult rats

Previous studies of neural xenografts have used immunosuppressive agents to prevent graft rejection. In the present study we have examined the survival of mouse dopamine neurons lacking either MHC class I or MHC class II molecules transplanted into rat brains and the host immune and inflammatory responses against the xenografts. Survival of neural grafts was immunocytochemically determined at 4 days, 2 weeks, and 6 weeks after transplantation by counting tyrosine hydroxylase (TH)-positive cells in the graft areas. In addition, the host immune and inflammatory responses against neural xenografts were evaluated by semiquantitatively rating MHC class I and class II antigen expression, accumulation of macrophages and activated microglia, and infiltration of CD4- and CD8-positive T-lymphocytes. For the negative controls, the mean number of TH-positive cells in rats that received wild-type mouse tissue progressively decreased at various time periods following transplantation. In contrast, intrastriatal grafting of either MHC class I or MHC class II antigen-depleted neural xenografts resulted in a prolonged survival and were comparable to cyclosporin A-treated rats that had received wild-type mouse tissue. These results indicate that genetically modified donor tissue lacking MHC molecules can be used to prevent neural xenograft rejection.

Time course of apoptotic cell death within mesencephalic cell suspension grafts: implications for improving grafted dopamine neuron survival

The vast majority ( congruent with 90%) of embryonic mesencephalic dopamine (DA) neurons die following transplantation to the striatum. Recent reports indicate that at least a subpopulation of grafted cells undergo apoptotic cell death at early times following implantation. This study examines the temporal pattern and magnitude of apoptotic cell death following the implantation of mesencephalic cell suspension grafts. Two techniques, a modified terminal deoxynucleotide-mediated nucleotide end labeling (TUNEL) technique and cresyl violet staining, are used to assess apoptotic cell death by detection of its biochemical and morphological identifiers, respectively. Male, Fischer 344 rats were examined at 1, 4, 7, and 28 days following implantation of embryonic day 14 (E14) ventral mesencephalic cells to the DA-denervated striatum. Results indicate that the overwhelming majority of apoptotic cell death occurs within the first 7 days after transplantation. However, the impact of the apoptosis that occurs over the first week following grafting only appears to limit grafted tyrosine hydroxylase-immunoreactive (THir) neuron survival during the first 4 days. No significant differences between the survival rates of THir neurons at 4 days after grafting and at 28 days after grafting were found. Therefore, it appears that the critical interval during which an estimated 90% of grafted DA neurons die is during the first 4 days postimplantation and that a major contributor to this cell death is apoptosis.

Protein synthesis inhibition in neocortical grafts evaluated by systemic amino acid uptake autoradiography

The temporal pattern of protein synthesis inhibition was examined in grafted neocortical neurons using [(3)H]valine in vivo autoradiography. Neuronal uptake levels of systemically administered (3)H-labeled amino acids which cross the blood-brain barrier (BBB) via endothelial cell neutral carriers have long been a hallmark in studies of experimental ischemic pathology; there is likely a strong correlation between persistent protein synthesis inhibition and the progression of cell damage. Because the grafting procedure involves the loss of blood flow and the subsequent reperfusion of the donor tissue there are, mechanistically, important similarities to reversible ischemia models. The effects of ischemic injury on grafted CNS neurons are not fully understood. Quantitative analysis of grain distribution in individual graft or control (adjacent host cortex) neurons indicated an initial breakdown of the amino acid barrier system, subsequent recovery, and progressive reduction of amino acid uptake by 1 year. Up to 3 weeks after surgery grafts were flooded with the [(3)H]valine tracer but individual neurons contained relatively few silver grains. After this time, the tracer was normally distributed within graft neurons but at significantly lower levels than in controls. Grain density gradually decreased over time such that 12-month grafted neurons had approximately half that compared to control and only 58% of that in 2-month grafts; the 12-month levels were comparable to those observed at early (10 days) postoperative times. Autoradiography of immunostained sections for MAP-2, SMI 311 (neurofilament marker), and neuron-specific enolase showed reduced expression of these proteins in neurons coupled with weak amino acid tracer uptake. The results further suggest that grafted neurons bear intriguing similarities to neurons placed at ischemic risk, particularly "penumbral" neurons, which are affected by reduced blood flow and are metabolically weakened. The loss of BBB properties in early grafts may also extend to the endothelial cell amino acid carrier system, and the delayed revascularization process could affect neuronal uptake mechanisms.

Blood-brain barrier formation of grafted human umbilical vein endothelial cells in athymic mouse brain

Human umbilical vein endothelial cells (HUVECs) were transplanted in athymic mouse brain and neovascularization of grafted endothelial cells was studied. HUVECs were transfected by a reporter gene pEGFPE-N1 in vitro and grafted stereotactically in unilateral striatum of adult nude mice. Histological studies in 4 weeks revealed that grafted HUVECs newly formed microvessels in brain, which were migrated and fused with host vessels. Intravenous injection of Evans blue before sacrificing animals resulted in no extravasation of dye, indicating that a blood-brain barrier (BBB) was formed by the grafted HUVECs. Immunohistochemistry demonstrated that host astrocytes extended glial feet on the grafted endothelial cells and a part of the newly formed vessels was positive with glucose transporter-1. These results indicate that endothelial cells from an ectopic origin have the potential to form a BBB after grafting in the central nervous system.

Transient coexpression of nestin, GFAP, and vascular endothelial growth factor in mature reactive astroglia following neural grafting or brain wounds

The spatial and temporal immunoexpression of the intermediate filament (IF) protein nestin and its relationship to glial fibrillary acidic protein (GFAP), vascular endothelial growth factor (VEGF), and its receptor flt-1 (VEGF-R1) in reactive astroglia was examined following stab wounds or transplants of fetal CNS tissue into the adult brain. Since developmentally regulated proteins and gene transcripts can be reexpressed in reactive astroglia following certain brain injuries, we analyzed the nestin profile in these experimental paradigms in order to more fully understand the nature of the gliotic "scar." Nestin expression was transiently up-regulated in some but not all astrocytes which often had a different morphology than the typical stout, stellate GFAP (+) cells; the processes of the nestin (+) cells tended to be slender and elongated. In reactive astroglia from the mature brain, nestin expression was robust but generally localized to the wound or graft site, peaked at 7-10 days postoperative, and was absent by 28 days, whereas GFAP (+) astrocytes were far more widespread and persisted for many months. Only nestin was strongly expressed immediately adjacent to early stab wounds, whereas GFAP (+) cells were located further from the wound sites. In contrast, there was marked nestin/GFAP colocalization at the graft/host interface. Semiquantitative analysis combined with confocal microscopy revealed a unique compartmentalization of protein expression; processes from single astrocytes could be entirely nestin (+), GFAP (+), or could show coexpression. At 4, 7, and 14 days postoperative, 41, 58, and 32% of the immunoexpression, respectively, was accounted for by nestin at the graft/host interface, and it was essentially undetectable at 28 days postoperative. In situ hybridization studies showed nestin transcripts within GFAP (+) cells primarily between 4 and 10 days postoperative and absent by 28 days. Many nestin (+) astrocytes, as shown by electron microscopy, were closely related to the vasculature. Therefore we further examined the expression of vascular endothelial growth factor (VEGF), an endothelial cell mitogen associated with angiogenesis. Nestin colocalized with VEGF in some astrocytes (7%) but far more prominently with the VEGF flt-1 receptor (25%). Early astroglial activation may involve several different IF components and possibly a distinct astrocytic population that shows a rapid, transient nestin expression adjacent to injury sites. Expression of the nestin IF phenotype within affected astrocytes in the surgical vicinity may be indicative of a reversion to an immature phenotype that might be less susceptible to attendant hypoxia after injury. Since injured astrocytes are well known to express many bioactive compounds, such transient reexpression of early, developmentally regulated proteins may be a hallmark for the elaboration of growth factors such as VEGF.

Human natural antibodies cytotoxic to pig embryonic brain cells recognize novel non-Galalpha1,3Gal-based xenoantigens

Transplantation of porcine embryonic brain cells, including dopaminergic neurons, from ventral mesencephalon (VM) is considered a potential treatment for patients with Parkinson's disease. In the present study, we characterized the distribution among VM cells of the major porcine endothelial xenoantigen, the Galalpha1,3Gal epitope, and evaluated the cytotoxic effect of anti-Galalpha1,3Gal antibody-depleted and nondepleted human AB serum on VM cells. Overall levels of Galalpha1,3Gal-epitope expression was very low on the VM cell population using Bandeiraea simplicifolia IB(4) lectin staining of resuspended VM cells in flow cytometric analyses or staining of SDS-PAGE-separated, solubilized VM cell membrane proteins in Western blot analyses. Lectin-histochemical staining of sections of pig embryonal VM regions with BSA IB(4) lectin showed staining restricted to endothelial cells and microglia. In the presence of complement, both nondepleted and anti-Galalpha1,3Gal antibody-depleted AB sera were shown to be cytotoxic to VM cells as assessed in microcytotoxicity- and flow cytometry-based cytotoxicity assays. Purified IgM and IgG were both cytotoxic in the presence of complement. Three major VM cell membrane antigens of approximately 210, 105, and 50 kDa were reactive with natural IgM antibodies present in pooled human AB sera. Thus, antibody-dependent cytotoxicity may contribute to pig to human brain cell xenorejection, necessitating donor tissue modifications prior to a more widespread utilization of neural tissue xenografting.

Quinolinic acid-induced inflammation in the striatum does not impair the survival of neural allografts in the rat

It has been suggested that inflammation related to intracerebral transplantation surgery can affect the survival of intrastriatal neural allografts. To test this hypothesis, we transplanted dissociated embryonic mesencephalic tissue from one of two rat strains, Lewis (allogeneic grafts) or Sprague-Dawley (syngeneic grafts), to the striatum of Sprague-Dawley rats. The target striatum was either intact or had received a local injection of quinolinic acid 9 days earlier, in order to induce a marked inflammation. At 6 or 12 weeks after transplantation, there was no significant difference between the different groups regarding the number of surviving grafted tyrosine hydroxylase immunoreactive neurons. However, the graft volume of both the syngeneic and allogeneic implants was significantly larger in the quinolinate-lesioned than in the intact striatum. There were dramatically increased levels of expression of major histocompatibility complex class I and II antigens, marked infiltrates of macrophages, activated microglia and astrocytes, and accumulation of large numbers of CD4 and CD8 positive T-lymphocytes in the quinolinate-lesioned striatum. In contrast, these immunological markers were much less abundant around both syngeneic and allogeneic grafts placed in intact striatum. We conclude that severe inflammation caused by quinolinic acid does not lead to rejection of intrastriatal neural allografts.

A non-invasive system for delivering neural growth factors across the blood-brain barrier: a review

Intraventricular administration of nerve growth factor (NGF) in rats has been shown to reduce age-related atrophy of central cholinergic neurons and the accompanying memory impairment, as well as protect these neurons against a variety of perturbations. Since neurotrophins do not pass the blood-brain barrier (BBB) in significant amounts, a non-invasive delivery system for this group of therapeutic molecules needs to be developed. We have utilized a carrier system, consisting of NGF covalently linked to an anti-transferrin receptor antibody (OX-26), to transport biologically active NGF across the BBB. The biological activity of this carrier system was tested using in vitro bioassays and intraocular transplants; we were able to demonstrate that cholinergic markers in both developing and aged intraocular septal grafts were enhanced by intravenous delivery of the OX-26-NGF conjugate. In subsequent experiments, aged (24 months old) Fischer 344 rats received intravenous injections of the OX-26-NGF conjugate for 6 weeks, resulting in a significant improvement in spatial learning in previously impaired rats, but disrupting the learning ability of previously unimpaired rats. Neuroanatomical analyses showed that OX-26-NGF conjugate treatment resulted in a significant increase in cholinergic cell size as well as an upregulation of both low and high affinity NGF receptors in the medial septal region of rats initially impaired in spatial learning. Finally, OX-26-NGF was able to protect striatal cholinergic neurons against excitotoxicity and basal forebrain cholinergic neurons from degeneration associated with chemically-induced loss of target neurons. These results indicate the potential utility of the transferrin receptor antibody delivery system for treatment of neurodegenerative disorders with neurotrophic substances.

Regeneration of neurosecretory axons into various types of intrahypothalamic grafts is promoted by the absence of blood brain barrier: fine structural analysis

Isogenous grafts of neural lobe and optic nerve and autologous grafts of sciatic nerve were placed into contact with the intrahypothalamically transected hypothalamo-neurohypophysial tract, and their fine structural characteristics examined at various time periods thereafter. The vascular bed of neural lobe grafts is composed primarily of fenestrated capillaries, that are permeable to blood-borne HRP throughout the entire experimental period. The microvasculature of sciatic nerve grafts consists of continuous, as well as fenestrated capillaries, which are similarly permeable to HRP. Fenestrated capillaries and HRP leakage in optic nerve grafts are observed at 10 days, but only in grafts located ventrally in the hypothalamus at 30 days. Neurosecretory axon regeneration is seen only in grafts or adjacent hypothalamus where the blood-brain barrier is breached. Regenerating axons are closely associated with the specific glial cells of the respective graft. Based on these observations, we conclude that blood-borne factors are necessary to initiate and sustain regeneration of transected neurosecretory axons, and that such regeneration occurs only in the presence of glial cells.

The influence of xenotransplant immunogenicity and immunosuppression on host MHC expression in the rat CNS

During the early stages following neural transplantation, host immune responses are initiated that are not normally found in the CNS including the induction of major histocompatibility antigens (MHC I and II). Previous laboratory findings have demonstrated prolonged survival of bovine chromaffin cells (BCC) in the rat CNS following transient immunosuppression with cyclosporin A (CSA) providing chromaffin cells are isolated from highly immunogenic passenger cells. To assess the influence of passenger and chromaffin cells on host MHC I and II expression, either BCC, nonchromaffin cell adrenal constituents (NCC), or adrenal medullary endothelial cells (EC) were implanted into the host. At 2 weeks postimplantation, robust BCC survival was obtained in CSA-treated animals. This correlated with low expression of MHC I at the host-graft border and the virtual absence of MHC II. Good BCC survival with reduced MHC I expression only was seen at 6 weeks postimplantation in animals transiently immunosuppressed (4 weeks). In contrast, poor survival was seen in the EC group (even with CSA treatment). In addition, marked MHC I and II expression was found in and around these grafts at 2 weeks, and was particularly intense in EC implanted animals. The results of this study suggest that nonchromaffin passenger cells in BCC preparations, most notably endothelial cells, can induce strong immune responses even in the presence of immunosuppression. Based on MHC staining, removal of these passenger cells can reduce host responses and improve long term survival of xenogeneic chromaffin cells in the CNS.

Glial and endothelial cell response to a fetal transplant of purified neurons

Astrocytes, microglia and endothelial cells display very specific phenotypic characteristics in the intact adult CNS, which appear quite versatile when grown in culture without neurons. Indirect evidence from in vitro co-culture studies and analysis of the effects of specific neuronal removal in vivo, does accordingly favour a role of neurons for the phenotypic repression of these cells in the intact brain. In order to provide more direct evidence for such neuronal influence, we attempted to induce, in the rat brain, a reversal of the post-lesional activation of astrocytes, microglia and endothelial cells by transplantation of fetal neurons purified by immunopanning. Host microglial cells which have been activated by the lesion process, penetrated the neuronal graft during the few days after the transplantation. Reactive astrocytes began to appear in the lesioned parenchyma and gathered around the transplant. Thereafter they first sent their processes in the direction of the neuronal graft, before they migrated into the graft a few days later. At this time, which was at the end of the first week post-transplantation, the host endothelial cells sprouted "streamers" of basal lamina within the graft forming small capillaries. During the second week post-transplantation, numerous astrocytes and microglial cells, both displaying a reactive hypertrophied morphology, were observed throughout the grafts. Finally, by the end of the first month, the activated cells differentiated towards a quiescent, resting morphology. At this time the grafts contained a vascular network with morphological characteristics comparable to those observed in the intact brain parenchyma. The results indicate that the interaction of activated astroglia and microglia and endothelial cells with neurons causes the cells to re-differentiate and regain phenotypic features characteristic of intact brain parenchyma, strongly suggesting that neurons play an essential role in the phenotypic restriction of glial and endothelial cells in the adult central nervous system.

Regeneration of neurosecretory axons into various types of intrahypothalamic graft is promoted by the absence of the blood-brain barrier: a neurophysin-immunohistochemical and horseradish peroxidase-histochemical study

In order to test the hypothesis that neurosecretory axon regeneration occurs only in the presence of specific vascular, perivascular, and glial microenvironments, isografts of neural lobe and optic nerve and autografts of sciatic nerve were transplanted into the hypothalamo-neurohypophysial tract at the lateral retrochiasmatic area of adult male rats. The integrity of the blood-brain barrier (BBB) to intravenously administered horseradish peroxidase (HRP), the regenerative process of neurosecretory axons, and functional recovery from lesion-induced diabetes insipidus were analyzed at 18 hr, 36 hr, 10 days, 30 days, and 80 days postsurgery. Neurophysin-positive axons invaded all grafts, as well as perivascular spaces of the adjacent hypothalamus. Wherever neurosecretory axon regeneration occurred, the BBB was breached. Reestablishment of the BBB was paralleled by a decrease in both density and staining intensity of regenerated neurophysin-positive axons. These observations illustrate that neurosecretory axon regeneration is tributary of the absence of BBB. It is speculated that blood-borne factors, provided when the BBB is breached, initiate and sustain neurosecretory axon regeneration. In addition, products of glial elements may enhance or complement the above stimulatory processes.

Quantitative analysis of vascularization and cytochrome oxidase following fetal transplantation in the contused rat spinal cord

In the normal adult central nervous system, a coupling between energy consumption and vascular density is well established. Likewise, the survival of fetal neural tissue grafts is highly dependent on the establishment of functional vascular integration with the host. However, to what degree graft vascularization and tissue metabolism influence the normal host response to traumatic injury has not been extensively studied. In the present report, embryonic day 14 fetal spinal cord suspension grafts were made into the lesion epicenter of subchronic (10 days) contusion-injured rats. Three months later, intraspinal transplants were analyzed using correlative cytochrome oxidase histochemistry and vascular morphometric analysis. The same approaches were applied to the host spinal cord and injured, non-transplanted animals in order to determine the ability of a graft to alter the level of post-injury vascularization and/or metabolism. In general, graft vascular density was increased over that measured in normal or injured gray matter. Vascular density in gray matter near the host/graft interface was markedly increased when compared to either gray matter of the same spinal level in injured non-grafted animals or normal control spinal gray matter. Vascular changes were not noted in gray matter 3 mm distal to the lesion epicenter (rostral or caudal) in all groups analyzed. Cytochrome oxidase was up-regulated at this time in the graft and gray matter at the host/graft interfaces when compared to either gray matter of the same spinal level in injured, non-grafted animals or that of uninjured controls. These data indicate that an intraspinal transplant placed into the contused adult rat spinal cord reaches a metabolic capacity that is likely to be associated with high levels of oxidative metabolism in the well-vascularized graft neuropil. In addition, transplantation chronically alters vascularization and metabolic patterns of adjacent spinal gray matter following contusion injury.

Neovascularization of rat fetal neocortical grafts transplanted into a previously prepared cavity in the cerebral cortex: a three-dimensional morphological study using the scanning electron microscope

Neovascularization within syngeneic rat fetal neocortical grafts transplanted into a previously prepared cavity in the cerebral cortex was studied 1 to 3 months after transplantation, utilizing scanning electron microscopy of vascular corrosion casts. The grafts were easily identified and the outer surface of the grafts, especially at the host-graft interface, was surrounded by large regenerated vessels of leptomeninges and connective tissue (e.g. dura). Large vessels originating from the choroid plexus also coated the grafts in animals whose lateral ventricles had been opened at the time of cavitation. These large regenerated vessels were mainly observed on the surface of the grafts, and they ramified markedly to form capillary networks in the vicinity of the host-graft interface. Occasionally several relatively large regenerated vessels were noted to extend into the grafts, and to ramify and connect with graft capillary networks having the same features as that of the host brain. Moreover, direct vascular connections between host capillaries and those within the grafts were observed. In some animals, arteries and arterioles which fed the grafts were identified in the perimeter of the grafts with their characteristic morphology. The interior microvasculature structure of the grafts was largely composed of the capillary network of graft origin, and of several relatively large penetrating vessels originating from the regenerated leptomeningeal vessels or the vessels of the choroid plexus. The present study demonstrated that the blood supply to the solid grafts transplanted into the previously prepared cavities originated primarily from the regenerated host vessels. These host vessels perfused the intrinsic graft vessels via new anastomoses which formed predominantly at the host-graft interface.

Three-dimensional morphological study of microvascular regeneration in cavity wall of the rat cerebral cortex using the scanning electron microscope: implications for delayed neural grafting into brain cavities

The present study was carried out to quantify the subsequent vascular regeneration around a lesion cavity made in the rat cerebral cortex and to decide the origin of the regenerated microvessels. A quantitative study utilizing computerized image analysis after microvascular perfusion with India ink indicated approximately 25 and 160% increase of the vascular density adjacent to the cavity compared to the contralateral cortex at 4 and 21 days, respectively, after lesioning. The microvasculature around the cavity was also evaluated by scanning electron microscopy of vascular corrosion casts. Newly formed leptomeningeal vessels began to grow down toward the floor of the cavity 4 days after lesioning and nearly covered the walls of the cavity 21 days after lesioning. A neovascular network of leptomeninges and connective tissue (e.g., dura) was formed as a roof over the cavity. Numerous branches of these newly formed vessels and prominent anastomoses with the capillary network in the walls and floor of the cavity were observed. Newly formed vessels also originated from the choroid plexus in cases where the lateral ventricle had been opened at the time of lesioning. These results document the plasticity of the vascular system in the cerebral cortex after a mechanical injury. The regenerated vascular network may offer a suitable condition for survival of transplanted tissues.

Angiogenesis and the blood-brain barrier in intracerebral solid and cell suspension grafts

Solid and suspension grafts of fetal central nervous system (CNS) tissue rapidly reform an intact blood-brain barrier (BBB), whereas solid grafts of peripheral nervous system (PNS) tissue fail to establish a BBB as detected by horseradish peroxide (HRP) leakage, administrated intravenously. We examined the acute changes in the BBB after grafting of fetal CNS tissue in solid and suspension form and superior cervical ganglion (SCG) and PNS tissue in the same manner. Adult rats (n = 20) received fetal (day 14-15) forebrain grafts (either solid or cell suspension) to their rostral corpus callosum bilaterally. A second group (n = 20) received SCG solid and cell suspension grafts at the same coordinates with the same technique. The animals were killed on first, third, seventh, and tenth days after grafting. Intravenous HRP (Sigma, type VI, 75 mg/5-g rat) was given 1 hour before perfusion with mixed aldehydes. Fifty-micron coronal sections were examined for the presence and location of the graft by cresyl violet and AChE staining and Mesulam's TMB method to detect HRP leakage. HRP leakage was detected in the parenchyma in all groups on the first and the third days post-transplantation indicating a disrupted BBB. No HRP reaction was seen at days 7 and 10 in groups receiving fetal forebrain tissue whether solid or cell suspension. Solid grafts of SCG consistently demonstrated HRP leakage from the first through the tenth day. However, cell suspension of SCG established a BBB by 7 days. These results suggest that within the solid grafts of CNS and PNS tissue, the permeability of the vessels is dictated by the transplanted tissue itself. When cell suspensions of the same tissue are introduced, host CNS tissue dominates as the local environment resulting in non-leaky vasculature within the graft.

Accumulation of blood-borne horseradish peroxidase in medial portions of the mouse hippocampus

The intracerebral distribution of intravenously injected horseradish peroxidase (HRP) in young adult DDD mice was examined. HRP-tetramethylbenzidine reaction products were observed in the medial portions of the hippocampus, particularly the medial CA1 region and medial dentate gyrus. Reaction products were observed in the subfornical organ in mice decapitated 5 min after HRP injection, and then also progressively more caudally in the medial portions of the hippocampus as postinjection survival time increased. These findings suggest that blood-borne macromolecules have ready access to the medial portions of the hippocampus, particularly the medial CA1 region and medial dentate gyrus.

Allografts of CNS tissue possess a blood-brain barrier: III. Neuropathological, methodological, and immunological considerations

Development of a blood-brain barrier (BBB) within mammalian CNS grafts, placed either intracerebrally or peripherally, has been controversial. Published data from this laboratory have emphasized the presence or the absence of a BBB within solid mammalian tissue or cell suspension grafts is determined intrinsically by the graft and not by the surrounding host parenchyma (e.g., brain, kidney, testis, etc.). Nevertheless, correctly interpreting whether or not a BBB exists within brain grafts is manifested by methodologies employed to answer the question and by ensuing neuropathological and immunological consequences of intracerebral grafting. The present study addresses these issues and suggests misinterpretation for the absence of a BBB in brain grafts can be attributed to: (1) rupture of interendothelial tight junctional complexes in vessels of CNS grafts fixed by perfusion of the host; (2) damage to host vessels and BBB during the intracerebral grafting procedure; (3) graft placement in proximity to inherently permeable vessels (e.g., CNS sites lying outside the BBB) supplying the subarachnoid space/pial surface and circumventricular organs such as the median eminence, area postrema, and choroid plexus; and (4) graft rejection associated with antigen presenting cells and the host immune response. The latter is prevalent in xenogeneic grafts and exists in allogeneic grafts with donor-host mismatch in the major and/or minor histocompatibility complex. CNS grafts between non-immunosuppressed outbred donor and host rats of the same strain (e.g., Sprague Dawley or Wistar rats) can be rejected by the host; these grafts exhibit populations of immunohistochemically identifiable major histocompatibility complex class I+ and class EE+ cells (microglia, macrophages, etc.) and CD4+ T-helper and CD8+ T-cytotoxic lymphocytes. PC12 cell suspension grafts placed within the CNS of non-immunosuppressed Sprague Dawley rats are rejected similarly. Donor cells from solid CNS grafts placed intracerebrally and stained immunohistochemically for donor major histocompatibility complex (MHC) class I expression are identified within the host spleen and lymph nodes; these donor MHC expressing cells may initiate the host immune response subsequent to the cells entering the general circulation through host cerebral vessels damaged during graft placement. Rapid healing of damaged cerebral vessels is stimulated with exogenously applied basic fibroblast growth factor, which may prove helpful in reducing the potential entry of donor cells to the host circulation. These results have implication clinically for the intracerebral grafting of human fetal CNS cell suspensions.

Human fetal neocortical tissue grafted to rat brain cavities survives, leads to reciprocal nerve fiber growth, and accumulates host IgG

The human-to-rat xenograft approach offers possibilities to study aspects of primate cortex development and function without monkeys. Human fetal cortical tissue was grafted to prepared cortical cavities of immunosuppressed host rats. Fetal tissue fragments were collected after routine low-pressure vacuum aspiration abortions performed in the first trimester of gestation. Human derived neurons and human nerve fiber outgrowth were visualized by immunohistochemistry with antibodies against human neurofilament protein 70 kD (hNFP70). Ingrowth from rat host striatum or cortex into the grafts was analyzed by immunohistochemistry with antibodies against tyrosine hydroxylase. Astrocytes were evaluated by immunohistochemistry with antibodies against glial fibrillary acidic protein. The grafts grew into different sizes (1-10 mm in diameter) and contained large numbers of hNFP70-positive nerve fibers. All grafts gave rise to outgrowth of hNFP70-positive fibers into the host with partly a cortical layering; layers III and IV received a majority of the human fibers. In several cases, the graft-derived nerve fibers entered the host brain at restricted areas, while there was no crossing over of nerve fibers at the rest of the graft-host interface. Tyrosine hydroxylase-positive fibers were usually not abundant in the grafts. Interestingly, cases of massive ingrowth occurred from host striatum into the graft in a pattern suggesting "permissive sites" at the graft-host interface in the same way as outgrowth from graft to host was found. Additionally, tyrosine hydroxylase-immunoreactive fibers from host cortex were found to grow into the transplant. Glial fibrillary acidic protein immunoreactivity was increased at the interfaces between graft and host cortex or host striatum. Immunohistochemistry using antibodies against rat IgG indicated the presence of rat IgG within the grafts, and in bordering areas of host brain, possibly indicating a defective graft-host barrier. Taken together, these results show that human cortical tissue pieces grafted to cortical cavities of immunosuppressed rats survive grafting and develop, and that reciprocal nerve fiber growth between grafts and hosts occur. Human cortical neurons can grow into the rat host brain in a pattern which is partly determined by host cortical architecture.

Regulation of in vitro glia-induced microvessel morphogenesis by urokinase

Plasminogen activators (PAs) regulate a variety of processes involved in tissue morphogenesis and differentiation. We used a coculture system in which microvascular endothelial cells are induced by glial cells to form capillary-like structures in order to examine the role of urokinase-type PA (uPA) during microvessel morphogenesis within the central nervous system (CNS). Endothelia-derived uPA activity decreased sevenfold within glial-endothelial cocultures when capillary-like structures were formed. Incubation of cocultures with concentrations of phorbol 12-myristate 13-acetate (0.1 and 1.0 nM) that induced endothelial uPA activity (45-210%) inhibited endothelial differentiation (25-70%). Furthermore, incubation of cocultures with proteolytically active low molecular weight uPA (5-500 IU/ml) inhibited endothelial differentiation (37-75%), whereas the amino terminal cell-binding fragment of uPA had minimal effect. Inhibition of plasminogen activation in cocultures with the serine protease/plasmin inhibitors aprotinin and soybean trypsin inhibitor increased glia-induced capillary-like structure formation (96-98%). These findings establish a paracrine/autocrine function for urokinase and its inhibitors in regulating endothelial responses to perivascular glia and provide insight into mechanisms of microvascular reactions to CNS pathology.

Vascularization of fetal neocortical grafts implanted in brain infarcts in spontaneously hypertensive rats

The vascularization of neural grafts in ischemic brain was studied in spontaneously hypertensive rats grafted with a suspension of fetal neocortical tissue into the infarcted area five to six days after ligation of the middle cerebral artery. The brain vasculature was examined by scanning electron microscopy of corrosion vascular casts and the cortical microvasculature was stereologically quantified in light microscopy three months after the occlusion. Patent anastomoses were present between the middle cerebral artery distal to occlusion and the proximal part, as well as to the anterior and posterior cerebral arteries, in both grafted and non-grafted rats. A vascular plexus covering the infarct cavities and the grafts contained leptomeningeal vessels intermingled with a thin capillary network which is not normally found on the brain surface. The graft vessels were derived from this vascular plexus. The regular pattern of arterioles and venules penetrating from the cortical surface in normal neocortex was absent in the grafts but the capillary morphology was similar in both types of tissue. The grafts had a lower capillary density than normal tissue and lacked the laminar distribution of capillaries characteristic of normal neocortex. The results demonstrate the plasticity of the vascular system where remodeling of the vascular tree after an ischemic insult provides suitable conditions for the vascularization of neocortical grafts.

Development of neural lobe-like neurovascular contact regions after intrahypothalamic transection of the hypothalamo-neurohypophysial tract

Fifteen days after bilateral transection of the hypothalamo-neurohypophysial tract at the level of the lateral retrochiasmatic area, neurovascular contact regions had developed proximal to 66% of the lesions. Contact regions developed in every case when neural lobe explants were placed into the lesions, and near approximately half of the lesions into which small pieces of sciatic or optic nerve were transplanted. Neurovascular contact regions were characterized by microvascular networks surrounded by dense neurophysin-immunoreactive plexuses. At the fine structural level, the organization of such regions resembled that of the neural lobe, with the single exception that capillaries were not fenestrated. Numerous neurosecretory axons were present, and palisades of neurosecretory axon terminals abutted perivascular basal laminae. Lamellopodia from glial cells partially ensheathed regenerating neurosecretory axons and often lay between terminals and the perivascular basal lamina. Terminals with many microvesicles and few neurosecretory granulated vesicles provided morphological evidence of hormone release.

Immunocytochemical expression of the endothelial barrier antigen (EBA) during brain angiogenesis

The antibody to the endothelial barrier antigen (anti-EBA) is localized to the luminal plasma membrane of endothelia that have a blood-brain barrier (BBB) but not to other vessels, for instance those in the circumventricular organs, which lack barrier function. We have examined EBA expression in the rat in certain tissues and in brain microvessels in models of brain angiogenesis such as development, wound healing and neural transplantation. All brain microvessels including pial ones stained for anti-EBA whereas those of the dura, median eminence and choroid plexus did not. Vessels of the iris which are characterized by tight junctions and barrier function expressed EBA strongly. Embryonic day 18 brain did not stain at all for anti-EBA although vessels were readily localized with anti-laminin. Following stab wounds to mature brain, directly injured and adjacent microvessels lacked EBA expression for a period of approximately 2 weeks which is a similar time frame of BBB breakdown. Following this period, EBA expression gradually returned to a normal pattern by 3-4 weeks. Likewise, in intraparenchymal transplants of fetal neocortex EBA expression was not observed for 2 weeks and while at later times transplant vessels expressed EBA whereas some interface vessels associated with inflammatory cells did not. Permeable choroid plexus vessels vascularizing intraventricular transplants did not stain for anti-EBA at any time period and neither did vessels in adrenal medulla transplants. The present study shows that while EBA expression is a postnatal event unlike the development of a barrier to serum protein, its expression may be lost or delayed in injured vessels or ones associated with inflammatory cells or reactive astrocytes.

Morphological and neurochemical features of cultured primary skin fibroblasts of Fischer 344 rats following striatal implantation

In order to assess the feasibility of using primary skin fibroblasts as a donor population for genetic modification and subsequent intracerebral grafting, the present study examines the structural and neurochemical characteristics of intrastriatal grafts of isogeneic primary fibroblasts over a period of 6 months. In culture, primary skin fibroblasts obtained from a female Fischer 344 rat display robust growth, but once confluent these cells exhibit contact inhibition. Following the implantation of cultured primary cells within the striatum of other adult rats from the same inbred strain, isologous grafts stain immunohistochemically for fibronectin at 1 week, and this immunostaining persists up to 6 months. Immunoreactivity for laminin is intense within the grafts from 1 to 8 weeks, but decreases by 6 months. Astrocytes within the striatum respond dramatically to the implantation of primary fibroblasts, such that immunohistochemical staining for glial fibrillary acidic protein increases markedly from 1 to 8 weeks after implantation. Although the intensity of immunostaining for glial fibrillary acidic protein diminishes among striatal astrocytes between 8 weeks and 6 months, the astrocytic border between the grafts and striatal neuropil remains intensely immunoreactive. Capillaries within the grafts stain immunohistochemically for glucose transporter (a facilitated glucose uptake carrier) as early as 3 weeks after implantation. Following intravenous infusions of peroxidase, capillaries within fibroblast grafts do not permit the extravasation of this macromolecule at 8 weeks and 6 months. Thus, capillaries formed within intracerebral grafts of primary skin fibroblasts exhibit a functional impermeable barrier to macromolecules similar to those capillaries of the host striatum. At the ultrastructural level, grafts possess numerous fibroblasts and have an extracellular matrix filled with collagen. Reactive astrocytic processes filled with intermediate filaments are found throughout the grafts. Hypertrophied astrocytes and their processes also appear to form a continuous border between the grafts and striatal neuropil. Grafts of primary fibroblasts also possess an extensive vasculature that is composed of capillaries with nonfenestrated endothelial cells; the occurrence of reactive astrocytic processes closely associated with or enveloping capillaries is variable. These results provide direct morphological and neurochemical evidence for the long-term survival of isologous fibroblasts after implantation within the rat striatum. From these data, we propose that isologous skin fibroblasts can be considered as donor candidates for successful intracerebral grafting following gene transfer.