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

Individual Human Serum Differs in the Amount of Antibodies with Affinity for Pig Fetal Ventral Mesencephalic Cells and the Ability to Lyse These Cells by Complement Activation

Xenografting pig fetal ventral mesencephalic (pfVM) cells to repair the dopamine deficit in patients with Parkinson's disease is the focus of both experimental and clinical investigations. Although there have been marked advances in the experimental and even clinical application of these xenogeneic transplantations, questions regarding the host's xenospecific immune response remain unanswered. It has been shown that human serum is able to lyse pfVM tissue by both anti-gal-gal and non-anti-gal-gal antibodies by complement activation. The aim of this study was to investigate whether interindividual differences exist in the levels of pfVM cell-specific IgM and IgG subclass antibodies, their ability to lyse pfVM cells in vitro and the relationship between both. Pig fetal VM cells were incubated with heat-inactivated serum from 10 different individuals and binding of IgM antibodies and IgG subclass antibodies to pfVM cells was analyzed by flow cytometry. The ability to lyse pfVM cells was analyzed exposing 51Cr-labeled pfVM cells to fresh serum or isolated IgM and IgG from the same individuals and subsequent determination of released 51Cr from lysed cells. Strong differences were found between individuals in the levels of pfVM cell-specific IgM antibodies: antibody levels differed up to 40-fold. pfVM-specific IgG1 and IgG2 levels were only detectable in a few individuals. The ability to lyse pfVM cells ranged from negligible lysis up to 66.5% specific lysis. There was a strong correlation between the levels of individual pfVM-specific IgM antibodies and the ability to lyse pfVM cells in vitro. Isolated IgM, but not IgG, was able to lyse pfVM cells in the presence of complement. In conclusion, the interindividual differences in the levels of IgM with affinity for pfVM cells and their ability to lyse pfVM cells in vitro are considerable. Only few individuals possessed IgG1 and IgG2 subclass antibodies with affinity for pfVM. These findings may influence patient selection for porcine transplants and chances of graft survival in individual patients.

Porcine Fetal Ventral Mesencephalic Cells are Targets for Primed Xenoreactive Human T Cells

Xenotransplantation of porcine fetal ventral mesencephalic (pfVM) cells to overcome the dopamine shortage in the striatum of patients with Parkinson's disease seems a viable alternative to allotransplantion of human fetal donor tissue, especially because the latter is complicated by both practical and ethical issues. There is, however, little known about the xenospecific immune responses involved in such an intracerebral xenotransplantation. The aim of our study was to investigate whether 1) naive human peripheral blood mononuclear cells (PMBC) display cytotoxicity against pfVM cells of E28 pig fetuses, and 2) priming of human PBMC by xenogeneic antigen presenting cells (APC) modulates pfVM-directed cellular cytotoxicity. For this purpose fresh PMBC from nine individual donors were primed by incubation with either irradiated pfVM cells or porcine spleen cells (PSC) as APC in the presence of IL-2 for 1 week before assessing cytotoxicity in a 51Cr release assay. Also, direct NK reactivity and antibody-dependent cellular cytotoxicity (ADCC) of fresh PMBC against pfVM cells was assessed. No direct cytotoxicity of naive cells (either NK reactivity or ADCC) against pfVM cells could be determined. Only PMBC primed with PSC were capable of lysing pfVM cells. PBMC primed with pfVM cells did not show cytolytic capacity towards pfVM. Interestingly, large differences in xenospecific T-cell responses exist between individual donor PBMC. Thus, human T cells are capable of killing pfVM cells in a xenoreactive response, but only after priming by donor APC. The large interindividual differences between human donors in their xenoreactive response may influence patient selection for xenotransplantation and chances of graft survival for individual patients.

β1 Integrin as a Xenoantigen in Fetal Porcine Mesencephalic Cells Transplanted into the Rat Brain

Xenografts of fetal porcine mesencephalic cells implanted into the rat striatum are generally rejected within several weeks. The fetal donor mesencephalon predominantly consists of neurons, but also contains microglial and endothelial cells, which are more immunogenic. In the present work, we investigated the occurrence of donor endothelial cells in grafts of porcine mesencephalic cells implanted into the rat striatum. Pig endothelial cells were monitored by immunochemical methods, using a monoclonal antibody (mAb) that recognizes a peptidic epitope of the porcine β1 integrin, and isolectin IB4, for the staining of the Galα1,3Gal epitope. The analysis also involved the detection of the pig hyaluronate receptor CD44, and the cell adhesion molecule CD31. The anti-β1 integrin mAb revealed endothelial-like cells in grafts of porcine mesencephalic cells as soon as 1 week after implantation. A similar staining pattern was obtained with the IB4 lectin. Unlike aortic endothelial cells, these pig brain-derived endothelial-like cells were not recognized by the anti-CD44 antibody. They also failed to express the CD31 adhesion molecule, a fact which suggests that they remained poorly mature, even in grafts maintained during 45 days in immunosuppressed rats. Interestingly, a strong expression of β1 integrin immunoreactivity was noticed in a large proportion (80%) of the cells freshly dissociated from the fetal pig mesencephalic tissue. The immunoreactivity decreased progressively after transplantation of the cells into the rat brain. This observation suggests that dissociated neuroblasts are capable of a temporary expression of β1 integrin. This molecule is known to participate in the process of cell sorting and migration in the developing brain. Hence, its expression could be the hallmark of a rescue mechanism triggered by the disruption of the cell/matrix interactions during the dissociation of the fetal mesencephalon. This disruption might account for part of the dramatic cell death process that occurs during the manipulation of the donor tissue.

Concise Review: Innate and Adaptive Immune Recognition of Allogeneic and Xenogeneic Cell Transplants in the Central Nervous System

Over the last 30 years, numerous allogeneic and xenogeneic cell grafts have been transplanted into the central nervous system (CNS) of mice and men in an attempt to cure neurological diseases. In the early studies, human or porcine embryonic neural cells were grafted in the striatum of animals or patients in an attempt to replace lost neurons. Although the immune-privileged status of the brain as a recipient organ was widely accepted, it rapidly became evident that CNS-grafted allogeneic and xenogeneic cells could be recognized and rejected by the immune system, resulting in poor neural graft survival and limited functional recovery. Since then, the CNS transplantation field has witnessed a sharp rise in the number of studies in which allogeneic and xenogeneic neural or mesenchymal stem cells (NSCs or MSCs, respectively) are transplanted, predominantly aiming at providing trophic stimulation and promoting endogenous repair of the brain. Interestingly, in many recent NSC and MSC-based publications functional improvement was used as the principal measure to evaluate the success of cell transplantation, while the fate of transplanted cells remained largely unreported. In this review, we first attempt to understand why primary neural cell isolates were largely substituted for NSCs and MSCs in cell grafting studies. Next, we review the current knowledge on the immune mechanisms involved in the recognition and rejection of allogeneic and xenogeneic cellular grafts in the CNS. Finally, we propose strategies to reduce graft immunogenicity and to improve graft survival in order to design improved cell-based CNS therapies. Stem Cells Translational Medicine 2017;6:1434-1441.

Gal/non-Gal antigens in pig tissues and human non-Gal antibodies in the GalT-KO era

Our knowledge regarding Gal and non-Gal antigens in GalT-KO pig tissues can be summarized as α3Galactosyl-tranferase gene knock out eliminates the Galα3Galβ4GlcNAc-R antigen expression in pig tissues as well as anti-Gal antibody binding. Other Galα-terminating saccharides (e.g. iGb3 glycolipids and Galα2 determinants) may be present but have not been documented. α3Galactosyl-tranferase gene knock out slightly changes the carbohydrate antigen expression but no "new" antigens recognized by the human immune system have been found. Non-Gal antigens are both of protein and carbohydrate nature but their exact chemical structures are poorly defined. Regarding human non-Gal antibodies our knowledge is as Non-Gal antibodies exist naturally and increase in humans/non-human primate (NHP) receiving WT or GalT-KO pig grafts. Non-Gal antibodies with new antigen epitope recognition can be induced in humans/NHP after challenge by WT or GalT-KO pig grafts. Non-Gal antibodies react with both carbohydrates and proteins. Part of the protein reactivity is directed to glycoprotein carbohydrates chains. Non-Gal antibodies reacting with neuraminic acid terminated saccharides (both N-Acetyl and N-Glycoloyl variants) are present in humans/NHP. Anti-neuraminic acid antibodies are increased, as well as induced, after grafting pig organs into humans/NHP. Non-Gal antibodies does not cause hyperacute xenorejection but can be cytotoxic and cause xenoorgan damage. If humans sensitized to HLA antigens are at a higher risk of rejecting pig xenograft compared with non-sensitized individuals is not fully clarified. Clinical trials are needed to evaluate the relevance of non-Gal antigens/antibodies and for the xenofield to advance.

Antibodies to kidney endothelial cells contribute to a "leaky" glomerular barrier in patients with chronic kidney diseases

Anti-endothelial cell antibodies (AECA) have been reported to cause endothelial dysfunction, but their clinical importance for tissue-specific endothelial cells is not clear. We hypothesized that AECA reactive with human kidney endothelial cells (HKEC) may cause renal endothelial dysfunction in patients with chronic kidney diseases. We report that a higher fraction (56%) of end-stage renal disease (ESRD) patients than healthy controls (5%) have AECA reactive against kidney endothelial cells (P <0.001). The presence of antibodies was associated with female gender (P < 0.001), systolic hypertension (P < 0.01), and elevated TNF-α (P < 0.05). These antibodies markedly decrease expression of both adherens and tight junction proteins VE-cadherin, claudin-1, and zonula occludens-1 and provoked a rapid increase in cytosolic free Ca(2+) and rearrangement of actin filaments in HKEC compared with controls. This was followed by an enhancement in protein flux and phosphorylation of VE-cadherin, events associated with augmented endothelial cell permeability. Additionally, kidney biopsies from ESRD patients with AECA but not controls demonstrated a marked decrease in adherens and tight junctions in glomerular endothelium, confirming our in vitro data. In summary, our data demonstrate a causal link between AECA and their capacity to induce alterations in glomerular vascular permeability.

Cell-based therapies for Parkinson's disease

Parkinson's disease (PD) is the second most common neurodegenerative disease worldwide, classically characterized by a triad of motor features: bradykinesia, rigidity and resting tremor. Neurodegeneration in PD critically involves the dopaminergic neurons of the substantia nigra pars compacta, which results in a severe reduction in dopamine levels in the dorsal striatum. However, the disease also exhibits extensive non-nigral pathology and as many non-motor as motor features. Nevertheless, owing to the relatively circumscribed nature of the nigrostriatal lesion in PD, dopaminergic cell transplantation has emerged as a potentially reparative therapy for the disease. Sources for such cells are varied and include the developing ventral mesencephalon, several autologous somatic cell types, embryonic stem cells and induced pluripotent stem cells. In this article, we review the origins of dopaminergic transplantation for PD and the emergent hunt for a suitable long-term source of transplantable dopaminergic neurons.

New therapeutic approaches to Parkinson's disease including neural transplants

Parkinson's disease (PD) is a common neurodegenerative disorder of the brain and typically presents with a disorder of movement. The core pathological event underlying the condition is the loss of the dopaminergic nigrostriatal pathway with the formation of alpha-synuclein positive Lewy bodies. As a result, drugs that target the degenerating dopaminergic network within the brain work well at least in the early stages of the disease. Unfortunately, with time these therapies fail and produce their own unique side-effect profile, and this, coupled with the more diffuse pathological and clinical findings in advancing disease, has led to a search for more effective therapies. In this review, the authors will briefly discuss the emerging new drug therapies in PD before concentrating on a more detailed discussion on the state of cell therapies to cure PD.

Immune problems in central nervous system cell therapy

Transplantation of cells and tissues to the mammalian brain and CNS has revived the interest in the immunological status of brain and its response to grafted tissue. The previously held view that the brain was an absolute "immunologically privileged site" allowing indefinite survival without rejection of grafts of cells has proven to be wrong. Thus, the brain should be regarded as a site where immune responses can occur, albeit in a modified form, and under certain circumstances these are as vigorous as those seen in other peripheral sites. Clinical cell transplant trials have now been performed in Parkinson's disease, Huntington's disease, demyelinating diseases, retinal disorders, stroke, epilepsy, and even deafness, and normally are designed as cell replacement strategies, although implantation of genetically modified cells for supplementation of growth factors has also been tried. In addition, some disorders of the CNS for which cell therapies are being considered have an immunological basis, such as multiple sclerosis, which further complicates the situation. Embryonic neural tissue allografted into the CNS of animals and patients with neurodegenerative conditions survives, makes and receives synapses, and ameliorates behavioral deficits. The use of aborted human tissue is logistically and ethically complicated, which has lead to the search for alternative sources of cells, including xenogeneic tissue, genetically modified cells, and stem cells, all of which can and will induce some level of immune reaction. We review some of the immunological factors involved in transplantation of cells to CNS.

Complement regulatory proteins are expressed at low levels in embryonic human, wild type and transgenic porcine neural tissue

Allotransplantation of human foetal neural tissue for neurodegenerative disorders has been shown to provide clinical benefit but is limited by a number of issues including donor supply. The use of porcine foetal tissue as an alternative source of cells is being investigated but xenotransplants survive poorly as a result of immunological rejection, which may involve complement. In this study we investigated the expression of the membrane-bound complement regulatory proteins--decay accelerating factor (DAF), membrane co-factor protein (MCP) and CD59 in embryonic neural tissue. Cells were derived from human foetuses, wild-type porcine foetuses and porcine foetuses transgenic for human complement regulatory proteins and analysed using flow cytometry and immunocytochemistry. Functional assessment of human complement regulatory protein expression in transgenic porcine tissue was assessed by C3b deposition and cell survival on exposure to human complement. Human and wild-type porcine foetal neural tissue expressed moderate levels of MCP and CD59 but low or no levels of DAF. Neural tissue from porcine foetuses transgenic for human MCP (E174) expressed the transgene but failed to significantly inhibit human C3b deposition compared with non-transgenic tissue. In contrast, foetal neural tissue from two different human DAF transgenic pig lines (A74 and E71) known to express high levels of human DAF on endothelial cells, failed to express significant levels of human DAF in foetal neural tissue. Complement regulatory proteins such as MCP and CD59 are expressed in the human and wild-type embryonic brain but in contrast, DAF is expressed at very low levels. Pigs transgenic for human DAF express very low levels of human DAF on embryonic neural tissue. In pigs transgenic for human MCP, the transgene is expressed at similar levels to that in human embryonic neural tissue but at an insufficient level to prevent activation of the complement cascade. Thus alternative approaches to reducing complement activation by xenografted neural foetal tissue will be required if this process proves to be important in the rejection process.

Activated porcine embryonic brain endothelial cells induce a proliferative human T-lymphocyte response

Transplantation of allogeneic embryonic neural tissue is a potential treatment for patients with Parkinson's and Huntington's diseases. The supply of human donor tissue is limited, and alternatives such as the use of animal (e.g., porcine) donor tissue are currently being evaluated. Before porcine grafts can be used clinically, strategies to prevent neural xenograft rejection must be developed. Knowledge on how human T lymphocytes recognize porcine embryonic neural tissue would facilitate the development of such strategies. To investigate the ability of porcine embryonic brain microvascular endothelial cells (PBMEC) to stimulate human T-cell proliferation, PBMEC were immuno-magnetically isolated and cocultured with purified human CD4 or CD8 single-positive T cells. PBMEC had a cobblestone-like growth pattern and expressed the endothelial cell markers CD31 and CD106. PBMEC stimulated with the supernatant of phytohemagglutinin-activated porcine peripheral blood mononuclear cells or porcine IFN-gamma, but not nonstimulated PBMEC, induced proliferation of both CD8 and CD4 T cells as assessed by [3H]thymidine incorporation. Flow cytometric analyses showed that the degree of CD8 and CD4 T cell proliferation correlated with the expression levels of class I and II major histocompatibility complex (MHC) antigens, respectively. PBMEC expressed a CTLA-4/Fc-reactive molecule, most likely CD86, suggesting that these cells are able to deliver a costimulatory signal to the T cells. Human TNF-alpha, but not human IFN-gamma, induced class I, but not class II, MHC expression on PBMEC. Within a neural graft or the regional lymph nodes, PBMEC might stimulate human T cells via the direct pathway, and should therefore be removed from the donor tissue prior to transplantation.

The cellular repair of the brain in Parkinson's disease—past, present and future

Damage to the central nervous system was once considered irreparable. However, there is now growing optimism that neural transplant therapies may one day enable complete circuit reconstruction and thus functional benefit for patients with neurodegenerative conditions such as Parkinson's disease (PD), and perhaps even those with more widespread damage such as stroke patients. Indeed, since the late 1980s hundreds of patients with Parkinson's disease have received allografts of dopamine-rich embryonic human neural tissue. The grafted tissue has been shown to survive and ameliorate many of the symptoms of the disease, both in the clinical setting and in animal models of the disease. However, practical problems associated with tissue procurement and storage, and ethical concerns over using aborted human fetal tissue have fuelled a search for alternative sources of suitable material for grafting. In particular, stem cells and xenogeneic embryonic dopamine-rich neural tissue are being explored, both of which bring their own practical and ethical dilemmas. Here we review the progress made in neural transplantation, both in the laboratory and in the clinic with particular attention to the development of stem cell and xenogeneic tissue based therapy.

Immune parameters relevant to neural xenograft survival in the primate brain

The lack of supply and access to human tissue has prompted the development of xenotransplantation as a potential clinical modality for neural cell transplantation. The goal of the present study was to achieve a better understanding of the immune factors involved in neural xenograft rejection in primates. Initially, we quantified complement mediated cell lysis of porcine fetal neurons by primate serum and demonstrated that anti-C5 antibody treatment inhibited cell death. We then developed an immunosuppression protocol that included in vivo anti-C5 monoclonal antibody treatment, triple drug therapy (cyclosporine, methylprednisolone, azathioprine) and donor tissue derived from CD59 or H-transferase transgenic pigs and applied it to pig-to-primate neural cell transplant models. Pre-formed alphaGal, induced alphaGal and primate anti-mouse antibody (PAMA) titers were monitored to assess the immune response. Four primates were transplanted. The three CD59 neural cell recipients showed an induced anti-alphaGal response, whereas the H-transferase neural cell recipient exhibited consistently low anti-alphaGal titers. Two of these recipients contained surviving grafts as detected by immunohistochemistry using selected neural markers. Graft survival correlated with high dose cyclosporine treatment, complete complement blockade and the absence of an induced PAMA response to the murine anti-C5 monoclonal antibodies.

Identification of the nonclassical HLA molecules, mica, as targets for humoral immunity associated with irreversible rejection of kidney allografts

BACKGROUND

A substantial portion of kidney allografted patients experience early acute rejection episodes and even irreversible rejections in the early posttransplantation period. The presence of HLA alloantibodies before grafting is associated with early immunological complications, but in many patients rejections and graft loss occur even in the absence of such antibodies.

METHODS

In this study, 748 serum samples taken before and at various time points after kidney transplantation from 139 patients were investigated for the presence, frequency, and specificity of kidney microvascular endothelial cell (KMEC)-reactive antibodies using major histocompatability class (MHC) I-related chain A (MICA) transfected cells and flow cytometry, antibody blocking experiments, and Western blotting. The ability of MICA-specific antibodies to fix complement and to induce a prothrombotic phenotype in KMECs was investigated.

RESULTS

A polymorphic, 62 kDa nonclassical HLA class I molecule is identified as a new target molecule for reactivity in sera from patients with irreversible rejections. Specific blocking and transfection experiments verified the target molecule as MICA. A significant correlation was established for pre- or posttransplantation MICA humoral immunity and graft loss (P<0.001). MICA-specific antibody titers increased in the posttransplantation period and were present before any signs of clinical rejection. MICA antibody-containing patient sera induced a prothrombotic phenotype in KMECs.

CONCLUSION

The increasing polymorphism detected at the MIC loci combined with the results of this study suggest that typing for the MIC loci and crossmatching for the detection of anti-MIC antibodies before transplantation should be used routinely. A better recipient-donor selection based on a negative crossmatch for both anti-donor HLA and MICA antibodies will decrease early graft rejections and losses.

Alpha Gal is widely expressed in embryonic porcine stem cells and neural tissue

Fetal porcine neural xenografts are an alternative to human fetal tissue for cell based treatments of a number of neurodegenerative conditions but are currently limited by host immunological rejection. The expression of a major immunological epitope, Galalpha1-3Galbeta1-4GlcNAcbeta-R (alphaGal) was determined on stem cells and primary cells derived from E26 porcine fetal brains. alphaGal was detected on the majority of neural stem cells and cells from primary cell suspensions. The expression of this epitope paralleled the binding of human IgG and IgM to the cells, a binding that was significantly reduced with anti-alphaGal depleted human serum. This study demonstrates that alphaGal expression is extensive in embryonic porcine neural cells and will be of relevance to any clinical trials using this tissue.

Continued production of xenoimmune antibodies 6-8 years after clinical transplantation of fetal pig islet-like cell-clusters

We have monitored the humoral immune responses of 10 type I diabetic patients, xenotransplanted with fetal porcine islet-like cell clusters for up to 8 years after xenotransplantation. We investigated the immunoglobulin subclass distribution as well as specificity differences of xenoreactive antibodies. Hemagglutintion tests, using pig erythrocytes, showed that some patients maintained higher titers of xenoreactive IgM antibodies during the entire follow up period, compared with pretransplant levels. In microcytotoxicity tests all but one patient tested showed higher than pretransplant levels of cytotoxic antibodies against pig peripheral blood mononuclear cells (PBMC) 6-8 years after transplantation. Levels of Gal alpha 1,3Gal specific antibodies, were also high. Antibody dependent cellular cytotoxicity (ADCC) activity against a Gal alpha 1,3Gal expressing human B cell line was detected in four patients while ADCC reactivity against adult pig islet cells was detected in only two patients, 6-8 years after transplantation. Immune sera collected 30 days and 1 year after transplantation showed positive staining of adult pig islet cells in fluoromicroscopy whereas sera from later time points did not. Western blot experiments showed that some patients had IgG1 antibodies reactive against epitopes on pig cells other than Gal alpha 1,3Gal, while xenoreactive IgM and IgG2 antibodies mainly reacted with Gal alpha 1,3Gal-containing epitopes as shown by absorption experiments. These results show that patients continue to produce higher than pretransplant levels of IgM and IgG2 xenospecific antibodies against Gal alpha 1,3Gal for extended time periods following xenotransplantation. Some patients also produce xenoreactive IgG1 antibodies directed against non-Gal alpha 1,3Gal epitopes.

The in vitro activity and specificity of human endothelial cell-specific promoters in porcine cells

The chronic shortage of human organs, tissues and cells for transplantation has inspired research on the possibility of using animal donor tissue instead. Transplantation over a species barrier is associated with rejections which are difficult to control. Therefore, it is generally agreed that successful pig to human xenotransplantation requires donor pigs to be genetically modified. Vascular endothelium is the most immediate barrier between the xenogeneic donor organ and host immune and nonimmune defense systems. Thus, these cells are the prime targets for such genetic modifications. Luciferase assays were used to evaluate the activity and specificity of human endothelial-cell specific promoters in porcine aortic-, microvascular- and nonendothelial cells. The promoters for human Flk-1 (fetal liver kinase-1), Flt-1 (fms-like tyrosine kinase), ICAM-2 (intercellular adhesion molecule-2), thrombomodulin and vWf (von Willebrand factor) supported similar levels of luciferase expression in human and porcine aortic endothelial cells, with the Flk-1 promoter being the strongest followed by the thrombomodulin promoter. Relative to the activity of the CMV promoter, the human endothelial cell-specific promoters all showed less activity in porcine kidney microvascular endothelial cells than in liver or brain microvascular endothelial cells. The thrombomodulin and Flk-1 promoters exhibited similar activity in liver and kidney microvascular endothelial cells, whereas the Flk-1 promoter was stronger in aortic and brain microvascular endothelial cells. Human endothelial cell-specific promoters also showed some degree of specificity in pig, because they supported less luciferase activity in porcine nonendothelial cell lines. Based on the in vitro data and previously published in vivo data, the human Flk-1 and thrombomodulin promoters are good candidate promoters for strong endothelial cell-specific gene expression in transgenic pigs.

Xenotransplantation for brain repair: reduction of porcine donor tissue immunogenicity by treatment with anti-Gal antibodies and complement

BACKGROUND

Transplantation of embryonic neural tissue is a potential treatment for Parkinson's disease. Because human donor material is in short supply, porcine xenografts are considered a useful alternative. Current immunosuppressive therapies fail, however, to protect intracerebral neural xenografts from host CD4 T lymphocytes. To reduce the immunogenicity of porcine donor tissue, we attempted to remove microglial cells with antibodies against the alpha-galactosyl epitope (Galalpha1,3Galbeta1,4GlcNAc-R), or anti-Gal, and complement, and studied whether this pretreatment can reduce direct and indirect T-cell responses to the tissue.

METHODS

Brain tissue from 27-day-old pig embryos was dissociated and treated with human anti-Gal and rabbit complement. The microglial content was analyzed by flow cytometry. [3H]thymidine incorporation in cocultures of the brain cells and purified human CD4 T cells was used to determine direct T-cell responses. Indirect T-cell responses were studied by grafting pretreated and control-pretreated (no anti-Gal) nigral tissue into the lesioned striatum of immunocompetent rats with 6-hydroxydopamine-induced hemiparkinsonism. Amphetamine-induced circling behavior was used to measure graft function.

RESULTS

Anti-Gal and complement reduced the microglial content to 11-24% of control and abolished the ability of the brain cells to induce human CD4 T-cell proliferation. Pretreated nigral tissue reduced hemiparkinsonism by more than 50% in five of eight rats at some point during the 10-week follow-up. Rats receiving control-pretreated nigral tissue did not display this degree of improvement.

CONCLUSIONS

Pretreatment with anti-Gal and complement can reduce the immunogenicity of porcine neural tissue, and might, therefore, be a valuable alternative or supplement to immunosuppression in neural xenotransplantation.

Xenotransplantation for CNS repair: immunological barriers and strategies to overcome them

Neural transplantation holds promise for focal CNS repair. Owing to the shortage of human donor material, which is derived from aborted embryos, and ethical concerns over its use, animal donor tissue is now considered an appropriate alternative. In the USA, individuals suffering from Parkinson's disease, Huntington's disease, focal epilepsy or stroke have already received neural grafts from pig embryos. However, in animal models, neural tissue transplanted between species is usually promptly rejected, even when implanted in the brain. Some of the immunological mechanisms that underlie neural xenograft rejection have recently been elucidated, but others remain to be determined and controlled before individuals with neurological disorders can benefit from xenotransplantation.

A role for complement in the rejection of porcine ventral mesencephalic xenografts in a rat model of Parkinson's disease

Vascularized whole organ discordant xenografts placed in the periphery are rejected by a rapid "hyperacute" process that involves preformed antibody binding to the xeno-antigens on the donor endothelial cells with complement activation. In the CNS, xenografts are classically thought to be rejected more slowly by a T-cell-dependent process. We now report that xenografts of embryonic porcine ventral mesencephalic tissue in the 6-hydroxydopamine-lesioned, nonimmunosuppressed rat induce both a humoral and a cell-mediated response. Over the first 10 d after implantation, the xenografts matured with identifiable TH neurons and pig-specific neurofilament fibers extending along host white matter tracts. During this period of time, IgM and complement binding were observed within the graft, as well as a CD8 cellular infiltrate, leading to rejection of the transplant over the next 25 d. These intracerebral xenografts were not associated with an early systemic antibody response. A role for complement in this rejection process was further investigated using cobra venom factor (CVF), which systemically depleted the rats of complement for 7 d. CVF treatment, when given in the period immediately before and after grafting, delayed but did not prevent the cellular immune response induced by the graft, demonstrating that xenografted neural tissue can activate the humoral arm of the rejection process, in particular the complement cascade. This suggests that interventions targeting this aspect of the immune rejection process may be of great importance for the future development of xenotransplantation for neurodegenerative conditions.

A comparison of fetal and adult porcine islets with regard to Gal alpha (1,3)Gal expression and the role of human immunoglobulins and complement in islet cell cytotoxicity

BACKGROUND

It is still debated whether fetal or adult porcine islets should be the preferred choice for future clinical islet xenotransplantation. Each type of islet preparation has advantages and disadvantages compared with the other. Here we present a direct comparison between fetal and adult porcine islets with regard to Gal alpha(1,3)Gal expression, immunoglobulin and complement binding, and cytotoxicity after exposure to fresh human serum.

METHOD

Islet single cell suspensions were prepared from adult and fetal islets by trypsin digestion. Fluorescein isothiocyanate-conjugated Bandeiraea simplicifolia isolectin B4 (BS-IB4) and affinity-purified chicken anti-Gal alpha(1,3)Gal antibody was used to detect Gal alpha(1,3)Gal expression. Immunoglobulin and complement binding to the islet cells and cytotoxicity for islet cells was compared after incubation with fresh and heat-inactivated human sera and with an immune serum from a diabetic patient who received a fetal porcine islet transplant. Furthermore, two pools of human AB sera were depleted of porcine endothelial cell cytotoxic human anti-Gal alpha(1,3)Gal antibodies by absorption and were used to analyze the effect of Gal alpha(1,3)Gal antibody removal on islet cell cytotoxicity.

RESULTS

Fetal islet cells readily bound both BS-IB4 and the chicken anti-Gal alpha(1,3)Gal antibody. None of 10 adult porcine islet preparations were stained by BS-IB4. In comparison, IgY anti-Gal Ab binding was detected in two of eight adult islet isolations, whereas the other six preparations showed marginal/no binding. After incubation of fetal islet cells with fresh human serum, C3c binding was strongly positive and IgM binding variable, with occasional binding of IgG and no detectable binding of IgA. Adult islet cells were also strongly positive for C3c but did not bind detectable amounts of IgM, IgG, or IgA. Immune sera from a patient who had received fetal porcine islets showed the presence of induced antibodies that bound to fetal islet cells and to porcine peripheral blood lymphocytes, whereas binding to adult islet cells was barely detectable. Fresh human sera showed a high and similar level of complement-mediated lytic activity for both adult islet cells (78+/-22%) and fetal islet cells (75+/-16%). Cytotoxicity for fetal islet cells and peripheral blood lymphocytes was significantly reduced when the corresponding sera were depleted of anti-Gal antibodies before use (P=0.002 and P=0.003, respectively). In contrast, no difference in cytotoxicity for adult islet cells was detected when anti-Gal-depleted human sera were used.

CONCLUSION

Gal alpha(1,3)Gal expression is occasionally detectable on adult porcine islet cells, but not as readily and at a lower level, compared with fetal islet cells. Thus, as porcine fetal islets mature to adult islets, the expression of the Gal alpha(1,3)Gal epitope gradually diminishes. Consequently, cytotoxic anti-Gal alpha(1,3)Gal antibodies in human serum play an important role in the lysis of fetal but not adult porcine islet cells.

Neural tissue xenotransplantation: what is needed prior to clinical trials in Parkinson's disease? Neural Tissue Xenografting Project

Embryonic allografted human tissue in patients with Parkinson's disease has been shown to survive and ameliorate many of the symptoms of this disease. Despite this success, the practical problems of using this tissue coupled to the ethical restrictions of using aborted human fetal tissue have lead to an exploration for alternative sources of suitable material for grafting, including xenogeneic embryonic dopaminergic-rich neural tissue. Nevertheless, xenografted neural tissue itself generates a number of practical, ethical, safety, and immunological issues that have to be addressed prior to any clinical xenotransplant program. In this article we review these critical issues and set out the criteria that we consider need to be met in the development of our clinical xenotransplantation research programs. We advocate that these, or similar, criteria should be adopted and made explicit by other centers contemplating similar clinical trials.

Discordant xenografts: different outcome after mouse and rat neural tissue transplantation to guinea-pigs

Embryonic neural tissue obtained from other species has been considered as a donor tissue source in repair strategies for human neurodegenerative disorders. The neuro- and immunobiology of distantly related species combinations, discordant xenografts, need to be characterised. For this purpose, a small animal model would be an important research tool. Adult guinea-pigs, and adult rats as controls, received intrastriatal grafts of either mouse or rat embryonic ventral mesencephalic tissue. The survival rates and types of host immune response were assessed at 2 weeks after grafting using stereological techniques and semi-quantitative evaluations. In the mouse-to-guinea-pig group, all transplants were rejected and no tyrosine hydroxylase-immuno reactive (TH-IR) cells remained. In the rat-to-guinea-pig group, there was good survival of TH-IR cells (5050 SEM+/-1550), similar to that in the rat-to-rat group (4900 SEM+/-1540). In the mouse-to-rat group, half of the animals had no surviving TH-IR cells (520 SEM+/-230 for the whole group). These species combinations offer inexpensive, efficient, and suitable conditions to study important survival factors for discordant xenogeneic neural tissue transplants. The factors responsible for the divergent graft outcomes between the two combinations might provide clues on how to manipulate xenogeneic tissue to increase survival rates in the future.