Mechanisms of allograft rejection in the rat brain

Addition of allogeneic spleen cells causes rejection of intrastriatal embryonic mesencephalic allografts in the rat

To address the importance of antigen-presenting cells for the survival of intracerebral neural allografts, allogeneic spleen cells were added to the graft tissue before transplantation. Dissociated embryonic, dopamine-rich mesencephalic and adult spleen tissues were prepared from either inbred Lewis or Sprague-Dawley rats. A mixture of neural and spleen cells was sterotaxically transplanted into the right striatum of adult Sprague-Dawley rats. Controls were neural allografts without addition of allogeneic spleen cells and syngeneic neural grafts with or without the addition of syngeneic spleen cells. Six weeks after transplantation, brain sections were processed immunocytochemically for tyrosine hydroxylase, specific for grafted dopamine neurons, and a bank of markers for various components in the immune and inflammatory responses. The neural allografts which were mixed with allogeneic spleen cells were rejected. In these rats, there were high levels of expression of major histocompatibility complex class I and II antigens, intense cellular infiltration including macrophages and activated microglial cells, and a presence of cluster of differentiation 4- and 8-immunoreactive cells in the graft sites. Moreover, there were increased levels of intercellular adhesion molecule-1, tumour necrosis factor-alpha and interleukin-6 in and around the grafts which were undergoing rejection. In contrast, syngeneic neural grafts survived well regardless of whether they were mixed with syngeneic spleen cells or not, and control neural allografts also exhibited unimpaired survival. No significant difference was observed in the number of grafted dopamine neurons among these three latter groups. The levels of expression of the different markers for inflammation and rejection were generally lower in these grafts than in implants of combined allogeneic neural and spleen cells. In summary, intrastriatal neural allografts, which normally survive well in our animal model, were rejected if allogeneic spleen cells from the same donor were added to the graft tissue. The added spleen cells caused strong host immune and inflammatory responses. The study gave support to the notion that immunological privilege of the brain does not provide absolute protection to immunogenetically histoincompatible neural grafts.

Methylprednisolone prevents rejection of intrastriatal grafts of xenogeneic embryonic neural tissue in adult rats

We studied the effects of high-dose methylprednisolone on the survival of intrastriatal neural xenografts and the host responses against them. Dissociated mesencephalic tissue from inbred mouse (CBA-strain) embryos was transplanted to the intact striatum of adult Sprague-Dawley rats. The rats received either daily injections of methylprednisolone (30 mg/kg), or cyclosporin A (10 mg/kg), or no immunosuppressive treatment. Two or six weeks after transplantation, there was good survival of xenografts in both the methylprednisolone- and cyclosporin A-treated rats. In contrast, the xenografts in untreated control rats were all rejected by six weeks. There was no marked difference in the degree of expression of MHC class I and II antigens and the accumulation of activated astrocytes and microglial cells/macrophages between the three groups. However, both methylprednisolone and cyclosporin A reduced infiltration of T lymphocytes to the transplantation sites. The expression of pro-inflammatory cytokines (interferon-gamma, tumour necrosis factor-alpha, interleukin-6) in and around the grafts was lower in the methylprednisolone- and cyclosporin A-treated groups than in untreated control rats. Although high-dose methylprednisolone caused significant body weight loss, we conclude that this treatment can prevent rejection of intrastriatal grafts of xenogeneic embryonic neural tissue in the adult.

Immunomodulation with intrathymic grafts or anti-lymphocyte serum promotes long-term intraspinal allograft survival

In this study, we sought to test whether introduction of fetal cells into the adult rat thymus would promote immunotolerance to subsequent donor-type allografts in the injured spinal cord. To first evaluate intrathymic survival of fetal central nervous system (CNS) tissue, fragments of E14 Sprague-Dawley (SD) fetal spinal cord (FSCSD) were injected into the thymuses of either adult, outbred SD, or Wistar rats. Histological examination revealed well-differentiated grafts in both the SD (10 out of 13) and Wistar (7 out of 13) recipients. We next examined whether prior intrathymic exposure to FSC graft-derived alloantigens leads to enhanced survival of subsequent allografts into the injured, adult spinal cord. Wistar rats thus first received FSCSD tissue as intrathymic grafts coupled with single-dose, anti-lymphocyte serum (ALS) ablation of the circulating host T-cell population. Ten days later, FCSSD was transplanted into an aspiration lesion of each intrathymic graft recipient's spinal cord. After 60 days, 87% of two-stage graft recipients (n = 15) exhibited viable intraspinal (IS) grafts compared to 38% (3 out of 8) observed in the controls (i.e., not receiving intrathymic grafts). Another group of Wistar rats that had received ALS (only) at the time of the IS FSCSD transplant (n = 8) also had 75% graft survival rates after 60 days. These initial findings show that the intrathymic microenvironment can be a compatible ectopic site for fetal SC graft development and survival. Also, the enhanced survival of intraspinal grafts in animals with previous intrathymic implants or ALS administered at the time of grafting suggests the potential for inducing immunoprotection of some fetal neural allografts in adult recipients.

Immunomodulation with Intrathymic Grafts or Anti-Lymphocyte Serum Promotes Long-Term Intraspinal Allograft Survival

In this study, we sought to test whether introduction of fetal cells into the adult rat thymus would promote immunotolerance to subsequent donor-type allografts in the injured spinal cord. To first evaluate intrathymic survival of fetal central nervous system (CNS) tissue, fragments of E14 Sprague–Dawley (SD) fetal spinal cord (FSC,SD) were injected into the thymuses of either adult, outbred SD, or Wistar rats. Histological examination revealed well-differentiated grafts in both the SD (10 out of 13) and Wistar (7 out of 13) recipients. We next examined whether prior intrathymic exposure to FSC graft-derived alloantigens leads to enhanced survival of subsequent allografts into the injured, adult spinal cord. Wistar rats thus first received FSCSD tissue as intrathymic grafts coupled with single-dose, anti-lymphocyte serum (ALS) ablation of the circulating host T-cell population. Ten days later, FCSSD was transplanted into an aspiration lesion of each intrathymic graft recipient's spinal cord. After 60 days, 87% of two-stage graft recipients (n = 15) exhibited viable intraspinal (IS) grafts compared to 38% (3 out of 8) observed in the controls (i.e., not receiving intrathymic grafts). Another group of Wistar rats that had received ALS (only) at the time of the IS FSCSD transplant (n = 8) also had 75% graft survival rates after 60 days. These initial findings show that the intrathymic microenvironment can be a compatible ectopic site for fetal SC graft development and survival. Also, the enhanced survival of intraspinal grafts in animals with previous intrathymic implants or ALS administered at the time of grafting suggests the potential for inducing immunoprotection of some fetal neural allografts in adult recipients.

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.

Consequences of cytotoxic T lymphocyte interaction with major histocompatibility complex class I-expressing neurons in vivo

Neurons have evolved strategies to evade immune surveillance that include an inability to synthesize the heavy chain of the class I major histocompatibility complex (MHC), proteins that are necessary for cytotoxic T lymphocyte (CTL) recognition of target cells. Multiple viruses have taken advantage of the lack of CTL-mediated recognition and killing of neurons by establishing persistent neuronal infections and thereby escaping attack by antiviral CTL. We have expressed a class I MHC molecule (Db) in neurons of transgenic mice using the neuron-specific enolase (NSE) promoter to determine the pathogenic consequences of CTL recognition of virally infected, MHC-expressing central nervous system (CNS) neurons. The NSE-Db transgene was expressed in H-2b founder mice, and transgene-derived messenger RNA was detected by reverse transcriptase-polymerase chain reaction in transgenic brains from several lines. Purified primary neurons from transgenic but not from nontransgenic mice adhered to coverslips coated with a conformation-dependent monoclonal antibody directed against the Dv molecule and presented viral peptide to CTL in an MHC-restricted manner, indicating that the Db molecule was expressed on transgenic neurons in a functional form. Transgenic mice infected with the neurotropic lymphocytic choriomeningitis virus (LCMV) and given anti-LCMV, MHC-restricted CTL displayed a high morbidity and mortality when compared with controls receiving MHC-mismatched CTL or expressing alternative transgenes. After CTL transfer, transgenic brains showed an increased number of CD8+ cells compared with nontransgenic controls as well as an increased rate of clearance of infectious virus from the CNS. Additionally, an increase in blood-brain barrier permeability was detected during viral clearance in NSE-Db transgenic mice and lasted several months after clearance of virus from neurons. In contrast, LCMV-infected, nontransgenic littermates and mice expressing other gene products from the NSE promoter showed no CNS disease, no increased intraparenchymal CTL, and no blood-brain barrier damage after the adoptive transfer of antiviral CTL. Our study indicates that viral infections and CTL-CNS interactions may induce blood-brain barrier disruptions and neurologic disease by a "hit-and-run" mechanism, triggering a cascade of pathogenic events that proceeds in the absence of continual viral stimulation.

Characterization of perivascular cells in astrocytic tumours and peritumoral oedematous brain

Perivascular cells (PVCs) form an immunophenotypically defined population that plays an important scavenging role in the perivascular fluid drainage pathways in the rat brain; such cells may also act as antigen-presenting cells. The present study tests the hypotheses that (a) PVCs in human brain are distinct from microglia and haematogenous macrophages, and (b) PVCs within astrocytic tumours and peritumoral oedematous brain tissue react in a similar way to PVCs in the rat brain. Paraffin sections of formalin-fixed tissue from 10 astrocytomas, 10 anaplastic astrocytomas, 10 glioblastoma multiforme, peritumoral oedematous brain and from normal human brain were examined immunocytochemically using antibodies HLA-DR beta-chain for MHC class II antigen, PGM1 and MAC 387 directed against macrophage components, MT1 for T lymphocytes and GFAP for astrocytes. No PVCs, microglia or macrophages were labelled by these techniques in paraffin sections of normal brain. Microglia, macrophages recently derived from haematogenous monocytes and PVCs were labelled by immunocytochemistry in all tumours but were more numerous in glioblastomas than in astrocytomas or anaplastic astrocytomas. Perivascular cells were distinguished by their perivascular position, their expression of MHC class II antigen and were labelled by PGM1 antibody but not by MAC 387 antibody. Microglia and monocyte/macrophages, remote from blood vessels, on the other hand, were strongly labelled by MAC 387, moderately by PGM1 and showed weak expression of MHC class II antigen. A similar pattern of staining was seen in peritumoral oedematous tissue. These findings suggest that PVCs form a defined population of resident cells in the human brain and that they are distinct from microglia, monocytes and macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)

Immune reactions following systemic immunization prior or subsequent to intrastriatal transplantation of allogeneic mesencephalic tissue in adult rats

We have previously found that dissociated mesencephalic tissue, which differs from the host at both major histocompatibility complex and non-major histocompatibility complex gene loci, can survive stereotaxic transplantation to the striatum of adult rats. We have now studied the outcome of intrastriatal neural allografts in rats that were systemically immunized by an orthotopic skin allograft either prior or subsequent to intracerebral implantation surgery. Dissociated mesencephalic tissue from Lewis rat embryos was stereotaxically injected into the dopamine-depleted striatum of hemi-parkinsonian Sprague-Dawley rats. One group was immunized by an orthotopic allogeneic skin graft of the same genetic origin as the neural graft, six weeks before the neural transplantation (the pre-immunized group). Another group was post-immunized by an orthotopic skin allograft, six weeks after the neural transplantation (the post-immunized group). A control group of rats was not challenged by a skin allograft. Marked behavioural recovery was observed in six of seven rats in the control group, in six of eight rats in the post-immunized group, and in none of the pre-immunized rats. Tyrosine hydroxylase-immunopositive cells were found in rats from the two behaviourally compensated groups, but not in the pre-immunized group. The immune responses were evaluated by OX-18 (monoclonal antibody against major histocompatibility complex class I antigen), OX-6 (major histocompatibility complex class II antigen), OX-42 (microglia and macrophages), glial fibrillary acidic protein (astrocytes), OX-8 (cytotoxic T-lymphocytes) and W3/25 (helper T-lymphocytes) immunocytochemistry. All the neural allografts in the pre-immunized group were rejected, leaving scars only. There were more intense immune responses to the allografts in the post-immunized group than the control group, in terms of immunocytochemically higher expression of major histocompatibility complex class I and II antigens and more intense cellular reactions consisting of macrophages, activated microglia and astrocytes, in addition to CD8- and CD4-positive lymphocytes. In summary, the results show the following: (i) systemic pre-immunization leads to complete rejection of intrastriatal neural allografts, implying that the status of the host immune system before transplantation determines the outcome for intrastriatal neural allografts; (ii) established intrastriatal neural allografts can survive for at least six weeks after systemic immunization, in spite of increased host immune responses in and around the allografts; (iii) there are no marked immune reactions against intrastriatal neural allografts 13 weeks after implantation in rats which have not been systemically immunized by a skin allograft; (iv) pre-immunized rats may provide a very useful animal model to investigate the role of inflammatory lymphokines in immune rejection and to test alternative immunosuppressive drugs.

Sprouting of dopaminergic fibers from spared mesencephalic dopamine neurons in the unilateral partial lesioned rat

A unilateral partially lesioned rat model of Parkinson's disease was developed following selective lesioning of the dopamine neurons of the substantia nigra pars compacta by stereotactic injection of the neurotoxin 6-hydroxydopamine. In this animal model the dopamine neurons of the ventral tegmental area and medial substantia nigra are spared. The neuronal loss in such partial lesioned models mimics more closely that seen in human mid-stage parkinsonism. Cografts of adrenal medullary cells and sciatic nerve to the partially lesioned striatum induced a sprouting response in grafted animals that was confirmed by immunocytochemical staining with antibodies to tyrosine hydroxylase (TH) and by quantification of the high affinity dopamine uptake complex using [3H]GBR 12935 binding. Enhanced TH fiber immunostaining was evident even in the presence of poor cograft survival. The origin of the TH-like immunostained fibers in the striatum was determined using Lucifer yellow retrograde axonal transport. Following discrete tracer injections into the striatum adjacent to a cograft, neurons in the medial substantia nigra and ventral tegmental area (areas A9 and A10, respectively) were labeled with Lucifer yellow. These labelled neurons displayed a morphology characteristic of dopamine neurons and, in double-labelling experiments, also immunostained for TH. These results support the utility of unilateral partially lesioned rat models of Parkinson's disease for studies investigating a host sprouting or upregulation response and confirm that the immunostained striatal fibers originate from spared dopamine neurons in the ventromedial midbrain.

Inhibition of posttraumatic microglial proliferation in a genetic model of macrophage colony-stimulating factor deficiency in the mouse

Activation and proliferation of microglia are common cellular hallmarks in many different pathological processes of the central nervous system. Although a number of colony-stimulating factors enhance microglial proliferation in vitro, little is known about the endogenous mitogens. In the present study we show a strong and selective inhibition of microglial proliferation in the facial nucleus of osteopetrotic (op/op) mice, with a genetic deficiency in biologically active macrophage colony-stimulating factor (MCSF). Posttraumatic activation of adjacent axotomized neurons and reactive astrocytes was not affected, emphasizing the specificity of MCSF as a microglial growth factor.

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.

Morphological assessment of grafted rat and mouse cortical neurons: a light and electron microscopic study

The morphology of cortical neurons grafted into (or near) the rat striatum was studied by means of intracellular Lucifer yellow injections in fixed slices. Rat donor syngeneic cortical tissue (from postnatal day 1 old rats; AO strain) as well as mouse donor xenogeneic cortical tissue (prenatal day 19; C3H/HE strain) were grafted as solid pieces into 8-12 week-old rats (AO strain). Recipients of mouse xenografts were immunosuppressed with a monoclonal antibody against the interleukin-2 receptor. After perfusion and sectioning of the graft-containing areas, individual slices were incubated in the DNA stain 4.6-diamidino-2-phenylindole (DAPI) to visualize the cell nuclei. Grafts could be easily identified by a surrounding rim of astrocytes which outline the border between grafted and host tissue. Grafted cortical neurons were intracellularly filled with Lucifer yellow, DAB-photoconverted, and further processed for light and electron microscopy. In general, no cortical lamination could be observed in the grafted rat and mouse cortical tissue, but neurons were loosely packed throughout the graft. Two major cell types could be identified in all grafts investigated so far. The majority resembled those described as spiny neurons (85%), which could be further classified into pyramid-like, spiny stellate-like or fusiform spiny neurons, with somata ranging between 15 and 25 microns in diameter. The remaining 15% resembled non-spiny neurons with either a multipolar basket-like or fusiform morphology. Dendrites of spiny and non-spiny neurons, which could extend to distances up to 400 microns, were never seen to cross the astrocytic border, but some main axon and axonal collaterals of spiny neurons were found to leave the graft. On the basis of light microscopic observations no difference was found between mouse and rat grafted cortical neurons. The results of this study show that grafted cortical neurons retain some of the characteristic features of neurons in the intact adult cerebral cortex, although there appears to be a greater preponderance of spiny neurons in grafted tissue. This may reflect an immaturity of the grafted tissue or a response to the striatal environment.

Cellular forms and functions of brain microglia

Consistent with the recent characterization of microglial cells as macrophages, an overall picture for the unique function of these cells in CNS tissue has developed. The microglia are derived from blood monocytes that migrate into the tissue during fetal development and subsequently remain after complete formation of the blood-brain barrier. These monocytes give rise to the ramified microglia of adult tissue through the developmental intermediate of amoeboid microglia. Ramified microglia appear uniquely adapted in contrast to other tissue macrophages based on their stability or lack of turnover and mitotic capability. The ramified cells, while usually downregulated, can convert into active macrophages termed reactive microglia; this conversion appears to occur nonspecifically in response to any injury. Further, reactive microglial cells can fuse to form giant multinucleated cells during viral infections. Each microglia cell form possesses a characteristic morphology and differing functional state with regard to macrophage activity. In their role as tissue macrophages, microglia are involved in immune responses, tissue transplantation, and AIDS dementia complex, as well as many other neurological mechanisms and diseases.

Myelin formation by mouse glia in myelin-deficient rats treated with cyclosporine

Previous attempts to generate myelin in the myelin-deficient rat spinal cord by transplanting mouse glia were not successful. In order to determine whether this result was due to graft rejection or to interspecies mismatch of cellular or molecular components at the axoglial junction, we have repeated the experiment in cyclosporine-treated rats. Our results show that in the immunosuppressed hosts, foetal glial xenografts form an abundance of myelin within the dorsal columns at or near the injection site about two weeks after the operation. In some cases, myelination extends virtually across the entire width of the dorsal columns. Ultrastructurally, the myelin sheaths are normal in all respects, including the presence of the 'radial component'. The lateral edges of the myelin lamellae form typical paranodal axoglial junctions, some displaying periodic 'transverse bands'. We infer that previous mouse to rat xenograft failures reflect host immune response rather than mismatch of heterologous junctional components. We also compared foetal, early post-natal and adult xenografts. Foetal donor cells, containing an abundance of precursors but virtually no mature oligodendrocytes, are more effective than neonatal donor cells in forming myelin, and after adult grafts, we found no myelin formation. Thus, in xenografts, as in allografts, foetal precursor cells are far more suitable than glia from mature donors in generating significant amounts of myelin.

Sequential intrastriatal grafting of allogeneic embryonic dopamine-rich neuronal tissue in adult rats: will the second graft be rejected?

An important issue in clinical neural grafting is whether a second instriatial allograft can survive well in a patient who has received an allograft before. In this study, the survival, immunogenicity and function of intrastriatal grafts of allogeneic or syngeneic embryonic dopamine-rich tissue in rats which had previously received either an intrastriatal allo- or syn-graft or sham injections were examined. The first graft tissue was taken from inbred Lewis or Sprague-Dawley rat embryos and grafted into an intact striatum of adult Sprague-Dawley rats subjected to a unilateral 6-hydroxydopamine lesion on the contralateral side. Eight weeks after the first transplantation, either allogeneic or syngeneic tissue was grafted as dissociated tissue into the dopamine depleted striatum. The function of the second grafts was assessed by rotational asymmetry at two different time points, i.e. eight and 14 weeks after the second transplantation. There were significant reductions of rotational asymmetry in all groups over time, but no significant difference between groups. Tyrosine hydroxylase immunocytochemistry was used to assess dopamine cell survival and graft size. Statistical analysis revealed no significant differnce in the mean number of tyrosine hydroxylase immunoreactive cells or the mean volume of the second grafts placed on the right side (lesioned side) between groups. Monoclonal antibodies were used to evaluate cellular immune reactions and the major histocompatibility complex class I and class II expression in and around grafts. No major histocompatibility complex class I expression was seen in any of the graft combinations. The expression of the major histocompatibility complex class II antigens was generally higher in patches in and around the second allograft of rats which had previously received an allograft than that in and around any other type of grafts. However, the expression of the major histocompatibility complex class II antigens was low throughout the grafts and did not appear as marked perivascular infiltrates. All the major histocompatibility complex class II positive cells displayed a microglia-like morphology, supported by the parallel microglia and macrophage-specific OX-42 immunostaining. The results show that there is no marked on-going immune reactions in or around the implantation site in any group fourteen weeks after a second transplantation. It may be concluded, therefore, that sequential allografting, using stereotaxic implantation of dissociated embryonic neural tissue into the striatal parenchyma, is possible to perform without a major risk of graft rejection, provided that an atraumatic technique is used.

Sequential intracerebral transplantation of allogeneic and syngeneic fetal dopamine-rich neuronal tissue in adult rats: will the first graft be rejected?

The immune response against intracerebral grafts of allogeneic fetal dopamine-rich tissue was assessed in adult rats. Sprague-Dawley rats, now outbred, but originating from an inbred stock, were given unilateral 6-hydroxy-dopamine lesions of the mesostriatal pathway, and grafted intrastriatally with mechanically dissociated ventral mesencephalic tissue (embryonic day 13-15) obtained from an inbred Lewis strain. Graft survival was assessed by functional recovery of amphetamine-induced rotational behavior on four different occasions postsurgery, and histologically using catecholamine histofluorescence and tyrosine hydroxylase immunohistochemistry. The following groups were analysed: long-term survival of a single allogeneic graft; survival of a first allogeneic graft with a syngeneic second graft; survival of a first allograft combined with a second allogeneic graft; the survival of bilateral allogeneic grafts following a subsequent orthotopic allogeneic skin graft. Evidence for recipient immunization was obtained using an indirect fluorescent antibody detection technique, Simonsen's Spleen Index (S I) test. Viable grafts, giving rise to behavioral compensation, were present after 40 wk in rats from all groups. The "first" allograft always displayed good survival and function, even following a second intracerebral allograft. However, five of nine "second" allogeneic intracerebral grafts survived poorly. In contrast, all secondary syngeneic grafts survived well. Following the application of a subsequent orthotopic allogeneic skin graft in a subgroup of rats, there was a significantly lower survival of grafted dopamine neurons in the "first" graft.

Molecular profile of reactive astrocytes--implications for their role in neurologic disease

The central nervous system responds to diverse neurologic injuries with a vigorous activation of astrocytes. While this phenomenon is found in many different species, its function is obscure. Understanding the molecular profile characteristic of reactive astrocytes should help define their function. The purpose of this review is to provide a summary of molecules whose levels of expression differentiate activated from resting astrocytes and to use the molecular profile of reactive astrocytes as the basis for speculations on the functions of these cells. At present, reactive astrocytosis is defined primarily as an increase in the number and size of cells expressing glial fibrillary acidic protein. In vivo, this increase in glial fibrillary acidic protein-positive cells reflects predominantly phenotypic changes of resident astroglia rather than migration or proliferation of such cells. Upon activation, astrocytes upmodulate the expression of a large number of molecules. From this molecular profile it becomes apparent that reactive astrocytes may benefit the injured nervous system by participating in diverse biological processes. For example, upregulation of proteases and protease inhibitors could help remodel the extracellular matrix, regulate the concentration of different proteins in the neuropil and clear up debris from degenerating cells. Cytokines are key mediators of immunity and inflammation and could play a critical role in the regulation of the blood-central nervous system interface. Neurotrophic factors, transporter molecules and enzymes involved in the metabolism of excitotoxic amino acids or in the antioxidant pathway may help protect neurons and other brain cells by controlling neurotoxin levels and contributing to homeostasis within the central nervous system. Therefore, an impairment of astroglial performance has the potential to exacerbate neuronal dysfunction. Based on the synopsis of studies presented, a number of issues become apparent that deserve a more extensive analysis. Among them are the relative contribution of microglia and astrocytes to early wound repair, the characterization of astroglial subpopulations, the specificity of the astroglial response in different diseases as well as the analysis of reactive astrocytes with techniques that can resolve fast physiologic processes. Differences between reactive astrocytes in vivo and primary astrocytes in culture are discussed and underline the need for the development and exploitation of models that will allow the analysis of reactive astrocytes in the intact organism.

Long fibre growth by axons of embryonic mouse hippocampal neurons microtransplanted into the adult rat fimbria

We have described a method for the microtransplantation of a suspension of a few thousand cells from mid to late embryonic mouse hippocampi into the fimbria of immunosuppressed adult rat hosts. There was close graft-to-host contact, across a non-scarred interface. The transplanted cells included CA3 type pyramids, and were enclosed within the host myelinated fibre tract, whose glial framework was largely undisturbed. Immunohistochemistry of two species-specific markers (M6 and Thy-1.2) showed that the donor mouse neurons grew fine (< 0.5 micron diameter) axons which extended singly or in fascicles through the rat host fimbria for a maximum distance of at least 10 mm. The donor axons were intimately integrated among and closely aligned to the host tract axons and to the interfascicular glial rows of the host tract. The axons travelled (i) laterally through the ipsilateral fimbria, (ii) medially across the midline in the ventral hippocampal commissure to reach the contralateral fimbria and alveus, and (iii) rostro-medially to the septum. On approaching the terminal fields appropriate to hippocampal CA3 pyramidal cell axons, the transplant axons gave rise to fine preterminal branches which were continuous with a reticular or amorphous immunoreactivity in the stratum oriens and stratum pyramidale of the ipsilateral hippocampus, and in the lateral and triangular septal nuclei. The donor axons extended along the host fimbria at a rate of approximately 1 mm per day, reaching their terminal field destinations by approximately 1-2 weeks. At 7 weeks the projections were maintained, but with little further extension. These observations indicate that the microenvironment of myelinated adult fibre tracts is permissive for an abundant and rapid growth of axons from transplanted embryonic cell suspensions. These axons can leave host tracts to invade appropriate terminal fields.

Localization of TNF alpha and IL-1 alpha immunoreactivities in striatal neurons after surgical injury to the hippocampus

Since the inflammatory process develops after transplantation to the brain, we sought to determine the presence of cytokines following a surgical trauma to the brain of an adult mouse. We report the early and marked presence of tumor necrosis factor-alpha and interleukin-1 alpha in neuronal somata of the striatum following a surgical injury to the hippocampus. The expression of cytokines later extends to neuronal cells of the hippocampus, thalamus, cerebral cortex, brain stem, and cerebellum and to glial cells of the corpus callosum. By contrast, these cytokines are not expressed by neuronal cells following injury to other regions, such as the striatum, cerebellum, and cortex. This study suggests a possible role for certain neurons in the brain's early reaction to a penetrating injury.

A monoclonal antibody to the interleukin-2 receptor enhances the survival of neural allografts: a time-course study

A time-course study of the survival and immunological characteristics of rat neural allografts was undertaken in animals treated with a murine monoclonal antibody to the alpha-chain (p55) of the rat interleukin-2 receptor. This antibody, NDS 63, was administered for ten days following grafting beginning on the day of operation. Inbred rat strains differing at both major and minor histocompatibility loci were selected as donor and host. Furthermore, the recipient strain displayed a high responder major histocompatibility complex haplotype. All grafts were placed in the lateral ventricle. Comparison was drawn between NDS 63-treated recipients and two groups of controls; an untreated group and a second group treated with the monoclonal antibody NDS 66, directed at a second epitope on the alpha-chain of the interleukin-2 receptor, which has been shown to be ineffective in competing with interleukin-2 for binding. Immunocytochemical analysis of the transplants was performed at several time-points up to 150 days following grafting. Grafts of NDS 63-treated recipients exhibited 100% survival with minimal induction of major histocompatibility complex antigens (both class I and class II) and negligible leukocyte infiltration at all time-points studied. In contrast grafts from both groups of controls showed evidence of a chronic immune response with most grafts undergoing rejection as shown by markedly elevated major histocompatibility complex antigen expression accompanied by specific immune cell infiltration. This was a protracted process with several grafts undergoing complete rejection by 60 days and a majority, but not all, by 150 days after transplantation. It is concluded that NDS 63, a monoclonal antibody to the interleukin-2 receptor, may diminish the immune response to transplanted allogeneic neural tissue and thereby enhance its prospects for long-term survival.

A role for microglia in the maintenance of photoreceptors in retinal transplants lacking pigment epithelium

Studies on intact retina have pointed to a necessary role for retinal pigment epithelium in the maintenance of photoreceptor outer segments and for regeneration of visual pigment. However, it has been shown that when embryonic retinae are separated from the pigment epithelium and transplanted into the brain of neonatal rats, the transplanted photoreceptors develop outer segments and the retina responds to light in the apparent absence of pigment epithelial cells. We confirm that there are no retinal pigment epithelium cells associated with transplanted retinae in the present series of experiments and show that a row of cells, composed predominantly of microglia of host origin, border the graft. These cells can be seen to contain engulfed outer segments when they are apposed to the outer retina, suggesting that the microglia have assumed, at the least, the phagocytic function normally associated with retinal pigment epithelium. Microglial cells and their processes are also found within the transplant, but these cells are typically devoid of phagosomes, indicating an absence of phagocytic activity. The close physical association of these resting microglia with the transplant may facilitate their role in antigen presentation under specific conditions of immune provocation.

Fetal cell grafts into resection and contusion/compression injuries of the rat and cat spinal cord

This article reviews recent findings concerning the feasibility, basic neurobiology, and potential functional benefits of fetal CNS tissue grafts into acute and chronic lesions of the adult spinal cord. In the rat, neuro-anatomical observations suggest that transplants into resection cavities establish neuritic projections that could functionally reunite separated rostral and caudal segments of the host spinal cord. Furthermore, some complementary electrophysiological evidence has been obtained for synaptic connectivity between host and graft neurons. In these studies, extracellular single-unit activity was evoked in fetal spinal cord (FSC) transplants by stimulating host dorsal roots that had been juxtaposed to donor tissue at the time of transplantation. In other investigations, we examined whether grafts could also establish axonal projections to appropriate areas of gray matter in the chronically injured spinal cord. For this purpose, fetal serotoninergic (5-HT) neurons were injected caudal to complete spinal cord transections that had been made 1-3 months earlier. Immunocytochemistry revealed that these cells projected their axons into gray matter regions normally innervated by bulbospinal 5-HT neurons. To investigate transplantation in a more clinically relevant lesion model, a third group of experiments involved injection of dissociated cell suspensions into acute [less than 24 h postinjury (p.i.)]), subchronic (7-10 days p.i), and chronic (greater than or equal to one month, p.i.) contusion lesions. Such grafts routinely filled areas that otherwise would have been regions of cavitation extending rostral-caudal distances of approximately 7 mm. FSC transplants in such injuries also appeared to influence some aspects of motoneuron excitability and hindlimb locomotion. More recent studies of the cat spinal cord have extended these findings in the rat by showing long-term survival (greater than 2 years) of fetal CNS allografts in recipients with either subtotal transection or compression lesions. Preliminary studies of connectivity have also shown host-graft projection patterns similar to those seen in the rat. Behavioral analyses are currently underway to examine the effects of fetal grafts in cats with chronic postcompression lesions. These observations in the rat and cat are discussed in the general context of basic biological and clinical issues relevant to the long-term objective of promoting functional improvement in the damaged spinal cord.

Survival and distribution of transplanted human IMR-32 neuroblastoma cells

The visualisation of transplanted cell lines is essential to determine both their viability and possible functional properties. Fluorescent latex microspheres were used to label cultured human neuroblastoma IMR-32 cells prior to transplantation. IMR-32 cells were first rendered amitotic by treatment with mitomycin C and bromodeoxyuridine and subsequently incubated with fluorescent microspheres for 3 days. Cell suspensions were prepared from these cultures and transplanted into the cortex and hippocampus of male Sprague-Dawley rats bearing ibotenate lesions of forebrain cholinergic projections. The animals were perfused at 4, 8 and 12 weeks post-transplantation and tissue was prepared for electron and light microscopy. IMR-32 cells containing fluorescent microspheres were clearly visualised in cryostat sections at all time points. Greater survival was seen in the hippocampus, with evidence of migration of cells from the site of implantation. Macrophages were seen at the electron and light microscope level, and were distinct from the discrete fluorescent labelled IMR-32 cells. Ultrastructurally, transplanted IMR-32 cells resembled cells in vitro, with microspheres clearly distinguished within the cytoplasm. Fluorescent microspheres provide a simple and direct labelling technique suitable for long-term transplant experiments using characterised cell lines.

The immune response to intracerebral neural grafts

Neural transplantation offers a potential therapeutic approach to a variety of neurological disorders, most notably those of a degenerative nature. However, the degree of immunological privilege (i.e. isolation from an immune response) in the brain, which is not absolute, may be a significant impediment to the survival of histoincompatible grafts. The nature of this privilege, together with the specific immune events leading to neural graft rejection, are discussed. As a consequence of this immune-mediated rejection, immunosuppression in some form might be necessary to guarantee long-term graft survival. Various strategies are being explored to suppress the immune response to neural grafts, not only for future use in clinical therapies, but also to bring intracerebral allo- and xenotransplantation to the attention of the general neurobiologist.

Leukocyte infiltration and glial reactions in xenografts of mouse brain tissue undergoing rejection in the adult rat brain. A light and electron microscopical immunocytochemical study

Neural mouse xenografts undergoing rejection in the adult recipient rat brain were characterized with regard to infiltrating host leukocytes and reactions of graft and host astro- and microglial cells. Rejection occurred within 35 days with infiltration of the grafts by in particular macrophages and T-cells as well as blood-brain barrier (BBB) leakage for IgG. In the surrounding host brain microglial cells showed increased histochemical staining for nucleoside diphosphatase (NDPase) and increased immunocytochemical expression of complement receptor type 3 (CR3), while astroglial cells displayed an increased immunoreactivity for glial fibrillary acidic protein (GFAP). Light microscopic findings of rat major histocompatibility complex (MHC) antigen class I on microglial cells, endothelial cells and leukocytes were confirmed at the ultrastructural level and extended to include a few astrocytes. Rat and mouse MHC antigen class II was only detected on leukocytes and activated microglia. We suggest that host macrophages and activated host and xenograft microglial cells act in situ as immunostimulatory cells on T-helper cells, and that increased levels of donor MHC antigen class I may further enhance the killer activity exerted by host T-cytotoxic cells.

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

Angiogenesis and patency of blood vessels were analyzed qualitatively in solid CNS and peripheral tissue syngeneic, allogeneic, and xenogeneic grafts and in individual cell suspension grafts of astrocytes, fibroblasts, PC12, and three additional tumor cell lines placed intracerebrally in adult host mice. Postgrafting survival times were 1 day through 4 weeks. The patency of graft vessels was determined in sections from immersion-fixed tissues incubated to reveal the endogenous peroxidase activity of host red cells trapped within the lumen of blood vessels. Additionally, horseradish peroxidase (HRP) was administered intravenously to live hosts; HRP labels host brain and graft vessels on the luminal surface and reveals the presence or absence of a blood-brain barrier (BBB) within the grafts. The origins of blood vessels supplying solid tissue xenografts were identified immunohistochemically with primary antibodies against host (athymic AKR mice) and donor (fetal Lewis rats) major histocompatibility complex (MHC) class I. Blood vessels supplying solid CNS grafts at 1-7 days post-transplantation were identified ultrastructurally and possessed interendothelial tight junctional complexes; however, they were not perfused with either host blood or blood-borne HRP prior to 8 days. Graft vessels at 10 days were outlined consistently by peroxidase-positive red cells in immersion-fixed material and labeled with blood-borne HRP. These vessels provided a BBB to the circulating HRP and exhibited interendothelial tight junctions. Evidence of angiogenesis within solid anterior pituitary grafts and the variety of cell suspension grafts was obtained prior to 3 days post-transplantation in immersion-fixed preparations; the vessels, with the notable exception of those supplying astrocyte cell suspensions, failed to present a BBB to blood-borne peroxidase. Endothelia in the solid pituitary allografts and the PC12 cell grafts were highly fenestrated and exhibited open interendothelial junctions; those in the tumor and fibroblast cell grafts, for the most part, appeared nonfenestrated, and many possessed open interendothelial junctional complexes. Immunostaining for host and donor MHC class I revealed that donor blood vessels predominate over host vessels in CNS xenografts and supply pituitary xenografts exclusively; in both preparations, donor vessels were not identified within the host CNS. Because cell suspension grafts were derived from endothelia-free preparations grown in culture, blood vessels supplying these grafts were necessarily of host CNS origin and manifested a morphological transformation from a BBB to a non-BBB endothelium. The data suggest that angiogenesis in solid CNS grafts placed into the adult host CNS, compared to similarly placed solid peripheral tissue/cell suspension grafts, is not rapid.(ABSTRACT TRUNCATED AT 400 WORDS)

Transplantation of postnatal rat enteric ganglia into denervated adult rat hippocampus

These experiments explore the possible value of the myenteric plexus as a source of donor cells for autografting into the central nervous system. Neurons and glia from 10-12-day postnatal rat myenteric plexus survive for at least one month after transplantation into cholinergically denervated syngeneic adult rat hippocampus. A population of donor cholinergic neurons has acetylcholinesterase-positive processes, but these appear not to innervate host tissue. Host gliosis in response to these implants seems to be less than that seen with other peripheral ganglia, and unlike Schwann cells, the enteric glia form end-feet on brain capillaries.

Microglia: immune network in the CNS

In recent years much progress has been made toward a better understanding of the nature and function of microglial cells. This review summarizes new developments and attempts to provide a perspective for future avenues to take in microglial research. Microglia are considered to play an active role in a variety of neurological diseases. Their function in forming a network of immune competent cells within the CNS is discussed.