Comparison of Fetal Rabbit Brain Xenografts to Three Different Strains of Athymic Nude Rats: Electrophysiological and Immunohistochemical Studies of Intraocular Grafts

Interest in the use of neural tissue transplantation for the study of CNS development and maturation and the potential use of this technique for the treatment of certain degenerative CNS disorders has led to our use of transplantation of neural tissue across species lines. Prior to extensive transplantation studies using athymic rats as recipients, we wished to evaluate the currently available strains of athymic rat for their suitability as host animals for xenografts of neural tissue. Fetal cerebellar and cerebral cortex tissue from rabbit brain of gestational age 20-25 days was dissected and transplanted to the anterior chamber of the eye of Harlan Wisconsin, Fisher 344 Jnu, or NCI-Harlan athymic nude rat strains. The brain tissue grafts were allowed to mature for 3 mo during which time the size and vascularity of each graft was monitored through the cornea of anesthetized hosts. In each group all of the transplants survived and grew to varying extents in the anterior chamber of the eye. Following the growth study in vivo extracellular recording of single neuronal activity was performed. Spontaneous neural activity was found in most transplants in all three groups with no difference in the viability or discharge rates of neurons between the groups. Illumination of the ipsilateral eye increased the firing rate of neurons in all three groups, suggesting excitatory cholinergic innervation of the grafted neurons from the host parasympathetic iris ground plexus. Antibodies directed against neurofilament protein, glial fibrillary acidic protein, synapsin, and tyrosine hydroxylase were used to characterize the transplants immunocytochemically and revealed no differences between the grafts in the three groups of recipients. All transplants contained significant numbers of glial and neuronal elements with the distribution resembling that in adult brain tissue. Some of the transplants contained a sparse innervation of tyrosine hydroxylase–positive fibers from the sympathetic plexus of the host iris. Furthermore, synapsin-immunoreactivity suggested that synaptogenesis had taken place within the grafts. Histological examination of the grafts revealed that 67% of the grafts had been infiltrated, to varying extents, by lymphocytes which led to areas of cell lysis and necrosis. All host animals had populations of T-cell receptor positive cells, most of which also expressed the T-cell surface antigens CD4 and CD8. However, no transplants were overtly rejected over the 15 wk period of study. Our investigation demonstrates that all of the athymic strains used in this study are able to mount an immune response against grafted fetal tissue, despite the absence of rejection, and that none of these strains is superior to the others with respect to suitability as a host for the long-term study of fetal CNS xenografts in oculo.

Overcoming the Histocompatibility Barrier: Transferrins as Carriers and Modulators of Immunogenic Identity

Proteic molecules were found in the bone marrow that were later identified as transferrins. When applied to transplantation of genetically incompatible bone marrow in supralethally irradiated recipient mice, the transferrins obtained from plasma of bone marrow donors promoted engraftment, permanent hemopoietic chimerism, and donor-type immune character. A combination of donor-matched transferrins and antigens was needed for induction of xenogeneic (interspecies) “tolerance” or unresponsiveness to donor antigens in chemically immunosuppressed mice treated with human transferrins and donor leucocytes. This novel and unique property of transferrins may explain the genesis and maintenance of immunogenic identity and allow a reshaping of the immune system. © 1998 Elsevier Science Inc.

Short-Term Immunosuppression Enhances Long-Term Survival of Bovine Chromaffin Cell Xenografts in Rat Cns

Xenogeneic donors, a largely untapped resource, would solve many of the problems associated with the limited availability of human donor tissue for neural transplantation. Previous work in our laboratory has revealed that xenografts of isolated bovine chromaffin cells survive transplantation into the periaqueductal gray (PAG) of immunosuppressed adult rats. Electron microscopic analysis reveals that graft sites contain healthy chromaffin cells, but do not contain host immune cells typical of graft rejection.

The aim of the current study was to assess the necessary conditions for long-term survival of bovine chromaffin cell xenografts in the central nervous system (CNS). In particular, the need for short-course vs. permanent immunosuppressive therapy with cyclosporine A (CsA) for the long-term survival of grafted bovine chromaffin cells was addressed. Grafts from animals receiving continuous CsA treatment for either 3, 6, or 12 wk contained large clumps of dopamines-β-hydroxylase (DBH) positive cells in contrast to the few surviving cells observed in nonimmunosuppressed animals. In addition, grafts from animals that had CsA treatment terminated at 3 or 6 wk contained similarly large clumps of DBH-positive cells. Furthermore, short-term immunosuppression (3 wk) appeared to enhance the long-term survival of grafted cells, since clumps of DBH staining cells could still be positively identified in the host PAG at least 1 yr after transplantation.

Complete rejection of graft tissue depends on several factors, such as blood–brain barrier integrity, the presence of major histocompatability complex (MHC) antigens in either the host or graft, and the status of the host immune system. By using a suspension of isolated bovine chromaffin cells, potential MHC antigen presenting cells, such as endothelial cells, are eliminated. In addition, CsA treatment may negate the immunologic consequences of increased blood–brain barrier permeability following surgical trauma by attenuating the host cell mediated response. In summary, long-term survival of isolated chromaffin cell xenografts in the rat CNS may be attained by a short-term course of CsA.

Immunobiology of Cellular Transplantation

The goal of cellular transplantation is to allow long-term function of the grafted cells using minimal host immunosuppression. To this end, the major strategies to implant cells and tissues are through: (i) the pretreatment of the graft to reduce tissue immunogenicity; (ii) the application of immunoisolation technologies to prevent host sensitization to implanted cells; and (iii) the induction of immunological tolerance to the donor tissues. Further, a major dilemma facing clinical tissue grafting is the shortage of donor tissue for transplantation. This problem requires the consideration of tissues from other species (xenografts) as a potential source of donor material. In light of these issues, the focus of this discussion is on the T cell-dependent response to allogeneic and xenogeneic transplants and the implications of this reactivity on the field of cellular replacement therapy.

CD86 Blockade in Genetically Modified Porcine Cells Delays Xenograft Rejection by Inhibiting T-Cell and NK-Cell Activation

Porcine xenografts transplanted into primates are rejected in spite of immunosuppression. Identification of the triggering mechanisms and the strategies to overcome them is crucial to achieve long-term graft survival. We hypothesized that porcine CD86 (pCD86) contributes to xenograft rejection by direct activation of host T cells and NK cells. Formerly, we designed the human chimeric molecule hCD152-hCD59 to block pCD86 in cis. To test the efficacy in vivo, we have utilized a pig-to-mouse xenotransplant model. First, we showed that hCD152-hCD59 expression prevents the binding of murine CD28Ig to pCD86 on porcine aortic endothelial cells (PAEC) and dramatically reduces IL-2 secretion by Con A-stimulated mouse splenocytes in coculture. Moreover, IFN-γ secretion by IL-12-stimulated mouse NK cells was averted after coculture with hCD152-hCD59 PAEC. In vivo, control PAEC implanted under the kidney capsule were rapidly rejected (2–4 weeks) in BALB/c and BALB/c SCID mice. Rejection of hCD152-hCD59 PAEC was significantly delayed in both cases. Signs of immune modulation in the hCD152-hCD59-PAEC BALB/c recipients were identified such as early hyporesponsiveness and diminished antibody response. Thus, simply modifying the donor xenogeneic cell can diminish both T cell and NK cell immune responses. We specifically demonstrate that pCD86 contributes to rejection of porcine xenografts.

Maintenance of Functional Human Cancellous Bone and Human Hematopoiesis in NOD/SCID Mice

Attempts were made to establish models to study interactions between marrow stromal cells and hematopoietic cells in vivo. The approach was to create a NOD-SCID-hu murine model of long-term human hematopoiesis by implantation of a human adult bone fragment. Nine to 12 weeks posttransplantation, human CD45+ cells were detected in the blood and the spleen of some mice. The histology of the human transplant showed that human bone fragment was viable at 9 weeks. Moreover, vessels of human origin, as assessed by immunohistochemical detection of human β2-microglobulin, were observed in the mouse tissue surrounding the transplanted human fragment.

Xenotransplantation: The Challenge to Current Psychosocial Attitudes

Because of the limited availability of transplantable human organs, xenotransplantation, the use of animal organs as an alternative source, has received considerable attention in recent years. Xenotransplantation would provide an unlimited supply of organs, and these organs would be available whenever required. Although the pig is considered the best source for organs, significant immunologic barriers currently prohibit the implementation of a clinical trial of organ transplantation. However, as medical research gains more insight into the mechanisms underlying rejection of pig organs in primates, therapeutic xenotransplantation is becoming more feasible. Clinical trials of porcine cell transplants are currently underway. Although xenotransplantation will minimize the waiting period for an organ and obviate the feelings of guilt or indebtedness commonly experienced by recipients of human organs, several psychosocial issues may hinder the reintegration of patients into society. For example, concerns that infectious pathogens could be transferred to recipients of pig organs will necessitate life-long monitoring and perhaps even temporary isolation of patients. The possible risk of the spread of a xenozoonosis from the patient to other members of the community may inspire public controversy and even fear, which may have an adverse impact on the patient's emotional state. Additionally, some patients may be psychologically disturbed by the need to incorporate pig organs into their body. This article addresses these and other psychosocial issues that may be associated with clinical xenotransplantation.

In vivo expansion of co-transplanted T cells impacts on tumor re-initiating activity of human acute myeloid leukemia in NSG mice

Human cells from acute myeloid leukemia (AML) patients are frequently transplanted into immune-compromised mouse strains to provide an in vivo environment for studies on the biology of the disease. Since frequencies of leukemia re-initiating cells are low and a unique cell surface phenotype that includes all tumor re-initiating activity remains unknown, the underlying mechanisms leading to limitations in the xenotransplantation assay need to be understood and overcome to obtain robust engraftment of AML-containing samples. We report here that in the NSG xenotransplantation assay, the large majority of mononucleated cells from patients with AML fail to establish a reproducible myeloid engraftment despite high donor chimerism. Instead, donor-derived cells mainly consist of polyclonal disease-unrelated expanded co-transplanted human T lymphocytes that induce xenogeneic graft versus host disease and mask the engraftment of human AML in mice. Engraftment of mainly myeloid cell types can be enforced by the prevention of T cell expansion through the depletion of lymphocytes from the graft prior transplantation.

Mechanisms of xenogeneic baboon platelet aggregation and phagocytosis by porcine liver sinusoidal endothelial cells

Background

Baboons receiving xenogeneic livers from wild type and transgenic pigs survive less than 10 days. One of the major issues is the early development of profound thrombocytopenia that results in fatal hemorrhage. Histological examination of xenotransplanted livers has shown baboon platelet activation, phagocytosis and sequestration within the sinusoids. In order to study the mechanisms of platelet consumption in liver xenotransplantation, we have developed an in vitro system to examine the interaction between pig endothelial cells with baboon platelets and to thereby identify molecular mechanisms and therapies.

Methods

Fresh pig hepatocytes, liver sinusoidal and aortic endothelial cells were isolated by collagenase digestion of livers and processing of aortae from GTKO and Gal+ MGH-miniature swine. These primary cell cultures were then tested for the differential ability to induce baboon or pig platelet aggregation. Phagocytosis was evaluated by direct observation of CFSE labeled-platelets, which are incubated with endothelial cells under confocal light microscopy. Aurintricarboxylic acid (GpIb antagonist blocking interactions with von Willebrand factor/vWF), eptifibatide (Gp IIb/IIIa antagonist), and anti-Mac-1 Ab (anti-α_Mβ₂ integrin Ab) were tested for the ability to inhibit phagocytosis.

Results

None of the pig cells induced aggregation or phagocytosis of porcine platelets. However, pig hepatocytes, liver sinusoidal and aortic endothelial cells (GTKO and Gal+) all induced moderate aggregation of baboon platelets. Importantly, pig liver sinusoidal endothelial cells efficiently phagocytosed baboon platelets, while pig aortic endothelial cells and hepatocytes had minimal effects on platelet numbers. Anti-MAC-1 Ab, aurintricarboxylic acid or eptifibatide, significantly decreased baboon platelet phagocytosis by pig liver endothelial cells (P<0.01).

Conclusions

Although pig hepatocytes and aortic endothelial cells directly caused aggregation of baboon platelets, only pig liver endothelial cells efficiently phagocytosed baboon platelets. Blocking vWF and integrin adhesion pathways prevented both aggregation and phagocytosis.

A humanized mouse model to study human immune response in xenotransplantation

BACKGROUND

A major barrier to the clinical application of xenotransplantation as a treatment option for patients is T cell-mediated rejection. Studies based on experimental rodent models of xenograft tolerance or rejection in vivo have provided useful information about the role of T cell immune response in xenotransplantation. However not all observations seen in rodents faithfully recapitulate the human situation. This study aimed to establish a humanized mouse model of xenotransplantation, which mimics xenograft rejection in the context of the human immune system.

METHODS

NOD-SCID IL2rgamma-/- mice were transplanted with neonatal porcine islet cell clusters (NICC) followed by reconstitution of human peripheral blood mononuclear cells (PBMC). Human leukocyte engraftment and islet xenograft rejection were confirmed by flow cytometric and histological analyses.

RESULTS

In the absence of human PBMC, porcine NICC transplanted into NOD-SCID IL2rgamma-/- mice revealed excellent graft integrity and endocrine function. Human PBMC demonstrated a high level of engraftment in NOD-SCID IL2rgamma-/- mice. Reconstitution of NICC recipient NOD-SCID IL2rgamma-/- mice with human PBMC led to the rapid destruction of NICC xenografts in a PBMC number-dependent manner.

CONCLUSIONS

Human PBMC-reconstituted NOD-SCID IL2rgamma-/- mice provide an ideal model to study human immune responses in xenotransplantation. Studies based on this humanized mouse model will provide insight for improving the outcomes of clinical xenotransplantation.

Immunobiology of liver xenotransplantation

Pigs are currently the preferred species for future organ xenotransplantation. With advances in the development of genetically modified pigs, clinical xenotransplantation is becoming closer to reality. In preclinical studies (pig-to-nonhuman primate), the xenotransplantation of livers from pigs transgenic for human CD55 or from α1,3-galactosyltransferase gene-knockout pigs+/- transgenic for human CD46, is associated with survival of approximately 7-9 days. Although hepatic function, including coagulation, has proved to be satisfactory, the immediate development of thrombocytopenia is very limiting for pig liver xenotransplantation even as a 'bridge' to allotransplantation. Current studies are directed to understand the immunobiology of platelet activation, aggregation and phagocytosis, in particular the interaction between platelets and liver sinusoidal endothelial cells, hepatocytes and Kupffer cells, toward identifying interventions that may enable clinical application.

Xenografted islet cell clusters from INSLEA29Y transgenic pigs rescue diabetes and prevent immune rejection in humanized mice

Islet transplantation is a potential treatment for type 1 diabetes, but the shortage of donor organs limits its routine application. As potential donor animals, we generated transgenic pigs expressing LEA29Y, a high-affinity variant of the T-cell costimulation inhibitor CTLA-4Ig, under the control of the porcine insulin gene promoter. Neonatal islet cell clusters (ICCs) from INSLEA29Y transgenic (LEA-tg) pigs and wild-type controls were transplanted into streptozotocin-induced hyperglycemic NOD-scid IL2Rγ(null) mice. Cloned LEA-tg pigs are healthy and exhibit a strong β-cell-specific transgene expression. LEA-tg ICCs displayed the same potential to normalize glucose homeostasis as wild-type ICCs after transplantation. After adoptive transfer of human peripheral blood mononuclear cells, transplanted LEA-tg ICCs were completely protected from rejection, whereas reoccurrence of hyperglycemia was observed in 80% of mice transplanted with wild-type ICCs. In the current study, we provide the first proof-of-principle report on transgenic pigs with β-cell-specific expression of LEA29Y and their successful application as donors in a xenotransplantation model. This approach may represent a major step toward the development of a novel strategy for pig-to-human islet transplantation without side effects of systemic immunosuppression.

Clinical lung xenotransplantation--what donor genetic modifications may be necessary?

Barriers to successful lung xenotransplantation appear to be even greater than for other organs. This difficulty may be related to several macro anatomic factors, such as the uniquely fragile lung parenchyma and associated blood supply that results in heightened vulnerability of graft function to segmental or lobar airway flooding caused by loss of vascular integrity (also applicable to allotransplants). There are also micro-anatomic considerations, such as the presence of large numbers of resident inflammatory cells, such as pulmonary intravascular macrophages and natural killer (NK) T cells, and the high levels of von Willebrand factor (vWF) associated with the microvasculature. We have considered what developments would be necessary to allow successful clinical lung xenotransplantation. We suggest this will only be achieved by multiple genetic modifications of the organ-source pig, in particular to render the vasculature resistant to thrombosis. The major problems that require to be overcome are multiple and include (i) the innate immune response (antibody, complement, donor pulmonary and recipient macrophages, monocytes, neutrophils, and NK cells), (ii) the adaptive immune response (T and B cells), (iii) coagulation dysregulation, and (iv) an inflammatory response (e.g., TNF-α, IL-6, HMGB1, C-reactive protein). We propose that the genetic manipulation required to provide normal thromboregulation alone may include the introduction of genes for human thrombomodulin/endothelial protein C-receptor, and/or tissue factor pathway inhibitor, and/or CD39/CD73; the problem of pig vWF may also need to be addressed. It would appear that exploration of every available therapeutic path will be required if lung xenotransplantation is to be successful. To initiate a clinical trial of lung xenotransplantation, even as a bridge to allotransplantation (with a realistic possibility of survival long enough for a human lung allograft to be obtained), significant advances and much experimental work will be required. Nevertheless, with the steadily increasing developments in techniques of genetic engineering of pigs, we are optimistic that the goal of successful clinical lung xenotransplantation can be achieved within the foreseeable future. The optimistic view would be that if experimental pig lung xenotransplantation could be successfully managed, it is likely that clinical application of this and all other forms of xenotransplantation would become more feasible.

Cardiac xenotransplantation: progress and challenges

PURPOSE OF REVIEW

Cardiac xenotransplantation (CXTx) remains a promising approach to alleviate the chronic shortage of donor hearts. This review summarizes recent results of heterotopic and orthotopic CXTx, highlights the role of non-Gal antibody in xenograft rejection, and discusses challenges to clinical orthotopic CXTx.

RECENT FINDINGS

Pigs mutated in the α 1,3 galactosyltransferase gene (GTKO pigs) are devoid of the galactose α1,3 galactose (αGal) carbohydrate antigen. This situation effectively eliminates any role for anti-Gal antibody in GTKO cardiac xenograft rejection. Survival of heterotopic GTKO cardiac xenografts in nonhuman primates continues to increase. GTKO graft rejection commonly involves vascular antibody deposition and variable complement deposition. Non-Gal antibody responses to porcine antigens associated with inflammation, complement, and hemostatic regulation and to new carbohydrate antigens have been identified. Their contribution to rejection remains under investigation. Orthotopic CXTx is limited by early perioperative cardiac xenograft dysfunction (PCXD). However, hearts affected by PCXD recover full cardiac function and orthotopic survival up to 2 months without rejection has been reported.

SUMMARY

CXTx remains a promising technology for treating end-stage cardiac failure. Genetic modification of the donor and refinement of immunosuppressive regimens have extended heterotopic cardiac xenograft survival from minutes to in excess of 8 months.

Innate cellular immunity and xenotransplantation

PURPOSE OF REVIEW

This review assesses the recent progress in xenograft rejection by innate immune responses, with a focus on innate cellular xenoreactivity.

RECENT FINDINGS

Current literature was reviewed for new insights into the role of innate cellular immunity in xenograft rejection. Increasing evidence confirms that vigorous innate immune cell activation is accounted for by a combination of xenoantigen recognition by activating receptors, and incompatibility in inhibitory receptor-ligand interactions. Although both innate humoral and cellular xenoimmune responses are predominantly elicited by preformed and induced xenoreactive antibodies in nonhuman primates following porcine xenotransplantation, innate immune cells can also be activated by xenografts in the absence of antibodies. The latter antibody-independent response will likely persist in recipients even when adaptive xenoimmune responses are suppressed. In addition to xenograft rejection by recipient innate immune cells, phagocytic cells within liver xenografts are also deleterious to recipients by causing thrombocytopenia.

SUMMARY

Strategies of overcoming innate immune responses are required for successful clinical xenotransplantation. In addition to developing better immunosuppressive and tolerance induction protocols, endeavors towards further genetic modifications of porcine source animals are ultimately important for successful clinical xenotransplantation.

Current progress in xenogeneic tolerance

PURPOSE OF REVIEW

The present review updates the current status of research regarding the immunologic responses of the recipient following xenotransplantation. Additionally, we present the recent progress with attempts to induce xenogeneic tolerance induction.

RECENT FINDINGS

There continues to be great interest in xenotransplantation. Recently, descriptions of the mechanisms responsible for attempted T-cell xenogeneic tolerance in both large and small animal models have improved xenogeneic graft survivals. Additionally, the cellular signaling mechanisms, such as those involving CD39, CD44, and CD47, are proving to be highly important. Using the mixed chimerism approach to tolerance in xenogeneic model may be encouraging, especially given the recent clarification of the role for macrophage-induced phagocytosis of xenogeneic donor cells.

SUMMARY

Induction of tolerance to xenogeneic antigens has been accomplished only in small animals; however, graft survivals in large animal models continue to improve. Further clarification of both the adaptive and innate immune responses to xenogeneic antigens is required for success to continue.

A crucial role of IL-17 and IFN-γ during acute rejection of peripheral nerve xenotransplantation in mice

Nerve injuries causing segmental loss require nerve grafting. However, autografts and allografts have limitations for clinical use. Peripheral nerve xenotransplantation has become an area of great interest in clinical surgery research as an alternative graft strategy. However, xenotransplant rejection is severe with cellular immunity, and Th1 cells play an important role in the process. To better understand the process of rejection, we used peripheral nerve xenografts from rats to mice and found that mononuclear cells expressing IFN-γ and IL-17 infiltrated around the grafts, and IFN-γ and IL-17 producing CD4+ and CD8+ T cells increased during the process of acute rejection. The changes of IL-4 level had no significant difference between xenotransplanted group and sham control group. The rejection of xenograft was significantly prevented after the treatment of IL-17 and IFN-γ neutralizing antibodies. These data suggest that Th17 cells contribute to the acute rejection process of peripheral nerve xenotransplant in addition to Th1 cells.

Xenotransplantation-associated infectious risk: a WHO consultation

Xenotransplantation carries the potential risk of the transmission of infection with the cells or tissues of the graft. The degree of risk is unknown in the absence of clinical trials. The clinical application of xenotransplantation has important implications for infectious disease surveillance, both at the national and international levels. Preclinical data indicate that infectious disease events associated with clinical xenotransplantation from swine, should they occur, will be rare; data in human trials are limited but have demonstrated no transmission of porcine microorganisms including porcine endogenous retrovirus. Xenotransplantation will necessitate the development of surveillance programs to detect known infectious agents and, potentially, previously unknown or unexpected pathogens. The development of surveillance and safety programs for clinical trials in xenotransplantation is guided by a "Precautionary Principle," with the deployment of appropriate screening procedures and assays for source animals and xenograft recipients even in the absence of data suggesting infectious risk. All assays require training, standardization and validation, and sharing of laboratory methods and expertise to optimize the quality of the surveillance and diagnostic testing. Investigation of suspected xenogeneic infection events (xenosis, xenozoonosis) should be performed in collaboration with an expert data safety review panel and the appropriate public health and competent authorities. It should be considered an obligation of performance of xenotransplantation trials to report outcomes, including any infectious disease transmissions, in the scientific literature. Repositories of samples from source animals and from recipients prior to, and following xenograft transplantation are essential to the investigation of possible infectious disease events. Concerns over any potential hazards associated with xenotransplantation may overshadow potential benefits. Careful microbiological screening of source animals used as xenotransplant donors may enhance the safety of transplantation beyond that of allotransplant procedures. Xenogeneic tissues may be relatively resistant to infection by some human pathogens. Moreover, xenotransplantation may be made available at the time when patients require organ replacement on a clinical basis. Insights gained in studies of the microbiology and immunology of xenotransplantation will benefit transplant recipients in the future. This document summarizes approaches to disease surveillance in individual recipients of nonhuman tissues.

Clinical xenotransplantation: the next medical revolution?

The shortage of organs and cells from deceased individuals continues to restrict allotransplantation. Pigs could provide an alternative source of tissue and cells but the immunological challenges and other barriers associated with xenotransplantation need to be overcome. Transplantation of organs from genetically modified pigs into non-human primates is now not substantially limited by hyperacute, acute antibody-mediated, or cellular rejection, but other issues have become more prominent, such as development of thrombotic microangiopathy in the graft or systemic consumptive coagulopathy in the recipient. To address these problems, pigs that express one or more human thromboregulatory or anti-inflammatory genes are being developed. The results of preclinical transplantation of pig cells--eg, islets, neuronal cells, hepatocytes, or corneas--are much more encouraging than they are for organ transplantation, with survival times greater than 1 year in all cases. Risk of transfer of an infectious microorganism to the recipient is small.

Genetically-engineered pig-to-baboon liver xenotransplantation: histopathology of xenografts and native organs

Orthotopic liver transplantation was carried out in baboons using wild-type (WT, n = 1) or genetically-engineered pigs (α1,3-galactosyltransferase gene-knockout, GTKO), n = 1; GTKO pigs transgenic for human CD46, n = 7) and a clinically-acceptable immunosuppressive regimen. Biopsies were obtained from the WT pig liver pre-Tx and at 30 min, 1, 2, 3, 4 and 5 h post-transplantation. Biopsies of genetically-engineered livers were obtained pre-Tx, 2 h after reperfusion and at necropsy (4–7 days after transplantation). Tissues were examined by light, confocal, and electron microscopy. All major native organs were also examined. The WT pig liver underwent hyperacute rejection. After genetically-engineered pig liver transplantation, hyperacute rejection did not occur. Survival was limited to 4–7 days due to repeated spontaneous bleeding in the liver and native organs (as a result of profound thrombocytopenia) which necessitated euthanasia. At 2 h, graft histology was largely normal. At necropsy, genetically-engineered pig livers showed hemorrhagic necrosis, platelet aggregation, platelet-fibrin thrombi, monocyte/macrophage margination mainly in liver sinusoids, and vascular endothelial cell hypertrophy, confirmed by confocal and electron microscopy. Immunohistochemistry showed minimal deposition of IgM, and almost absence of IgG, C3, C4d, C5b-9, and of a cellular infiltrate, suggesting that neither antibody- nor cell-mediated rejection played a major role.

Pig-to-nonhuman primates pancreatic islet xenotransplantation: an overview

The therapy of type 1 diabetes is an open challenging problem. The restoration of normoglycemia and insulin independence in immunosuppressed type 1 diabetic recipients of islet allotransplantation has shown the potential of a cell-based diabetes therapy. Even if successful, this approach poses a problem of scarce tissue supply. Xenotransplantation can be the answer to this limited donor availability and, among possible candidate tissues for xenotransplantation, porcine islets are the closest to a future clinical application. Xenotransplantation, with pigs as donors, offers the possibility of using healthy, living, and genetically modified islets from pathogen-free animals available in unlimited number of islets. Several studies in the pig-to-nonhuman primate model demonstrated the feasibility of successful preclinical islet xenotransplantation and have provided insights into the critical events and possible mechanisms of immune recognition and rejection of xenogeneic islet grafts. Particularly promising results in the achievement of prolonged insulin independence were obtained with newly developed, genetically modified pigs islets able to produce immunoregulatory products, using different implantation sites, and new immunotherapeutic strategies. Nonetheless, further efforts are needed to generate additional safety and efficacy data in nonhuman primate models to safely translate these findings into the clinic.

Molecular studies of the Oka varicella vaccine

Varicella zoster virus (VZV) is one of eight members of the Herpesviridae family for which humans are the primary host; it causes two distinct diseases, varicella (chickenpox) and zoster (shingles). Varicella results from primary infection, during which the virus establishes latency in sensory neurons, a characteristic of all members of the Alphaherpesvirinae subfamily. Zoster is caused by reactivation of latent virus, which typically occurs when cellular immunity is impaired. VZV is the first human herpesvirus for which a vaccine has been licensed. The vaccine preparation, v-Oka, is a live-attenuated virus stock produced by the classic method of tissue culture passage in animal and human cell lines. Over 90 million doses of the vaccine have been administered in countries worldwide, including the USA, where varicella morbidity and mortality has declined dramatically. Over the last decade, several laboratories have been committed to investigating the mechanism by which the Oka vaccine is attenuated. Mutations have accumulated across the genome of the vaccine during the attenuation process; however, studies of the contribution of these changes to vaccine attenuation have been hampered by the lack of a suitable animal model of VZV disease and by the heterogeneity that exists among the viral population within the vaccine preparation. Notwithstanding, a wealth of data has been generated using various laboratory methodologies. Studies of the vaccine virus in human xenografts implanted in severe combined immunodeficiency-hu mice, have enabled analyses of the replication dynamics of the vaccine in dorsal root ganglia, T lymphocytes and skin. In vitro assays have been used to investigate the effect of vaccine mutations on viral gene expression and sequence analysis of vaccine rash viruses has permitted investigations into spread of the vaccine virus in a human host. We present here a review of what has been learned thus far about the molecular and phenotypic characteristics of the Oka vaccine.

Why the histocompatibility system exists and how transplant surgeons can xenograft without rejection

The histocompatibility system is responsible for the rejection of allografts. The system exists to counter the explosive speed of viral replication by directing the defensive immune attack by cytotoxic T cells on to histocompatibility antigens on the infected cell's surface. This enables destruction of the virus factories before the cytotoxic T cells are swamped by the myriad numbers of new virions, a thousand coming from each infected cell every 10 h. The immunity system mistakes alloantigens for virus-infected host cells that need swift destruction. For transplantation, Sykes has improved Kaplan's technique by adding recipient bone marrow cells to the donor ones injected for reconstitution of the recipient after immune ablation. This technique should enable the use of xenografts from pigs.

Co-transplantation of fetal bone tissue facilitates the development and reconstitution in human B cells in humanized NOD/SCID/IL-2Rγnull (NSG) mice

BACKGROUND

In terms of the function and reconstitution efficacy of human immune cells, co-transplantation of human fetal tissues, such as thymus and liver, with CD34(+) hematopoietic stem cells (HSCs) has potential advantages in the generation of humanized mice.

OBJECTIVE AND METHODS

To examine the effects of bone tissues in the reconstitution of human immune cells, particularly in B cells, we generated a new humanized mice co-transplanted with human fetal thymus (hFT)/fetal bone (hFB) tissues and human fetal liver-derived CD34(+) cells.

RESULTS

Humanized mice exhibited effective reconstitution of human immune cells earlier compared to control humanized mice. In terms of quantity, the number of immune cells, such as human T, B, and monocyte/macrophages was significantly increased. Furthermore, significant increase of B cell progenitors and immature/naïve B cells could be detected in the bone marrow and spleen of humanized mice.

CONCLUSION

Our results demonstrate that co-transplantation of hFB tissue may facilitate the reconstitution of human B and T cells, and therefore the humanized model may be used to develop therapeutic human antibodies for clinical use.

Controlling coagulation dysregulation in xenotransplantation

PURPOSE OF REVIEW

Deletion of the α1,3-galactosyltransferase (GalT) gene in pigs has removed a major xenoantigen but has not eliminated the problem of dysregulated coagulation and vascular injury. Rejecting GalT knockout organ xenografts almost invariably show evidence of thrombosis and platelet sequestration, and primate recipients frequently develop consumptive coagulopathy. This review examines recent findings that illuminate potential mechanisms of this current barrier to successful xenotransplantation.

RECENT FINDINGS

The coagulation response to xenotransplantation differs depending on the type of organ and quite likely the distinct vasculatures. Renal xenografts appear more likely to initiate consumptive coagulopathy than cardiac xenografts, possibly reflecting differential transcriptional responses. Liver xenografts induce rapid and profound thrombocytopenia resulting in recipient death within days due to bleeding; ex-vivo data suggest that liver endothelial cells and hepatocytes are responsible for platelet consumption by a coagulation-independent process.It has been proposed that expression of recipient tissue factor on platelets and monocytes is an important trigger of consumptive coagulopathy. Finally, pigs transgenic for human anticoagulants and antithrombotics are slowly but surely coming on line, but have not yet been rigorously tested to date.

SUMMARY

Successful control of coagulation dysregulation in xenotransplantation may require different combinatorial pharmacological and genetic strategies for different organs.

Therapeutic issues in the treatment of vascularized xenotransplants using gal-knockout donors in nonhuman primates

PURPOSE OF REVIEW

Solid organ xenotransplantation could be the future of transplantation, but improved outcomes are required in experimental models before clinical trials are justified. This review summarizes recent advances in solid organ xenotransplantation using organs from α1,3-galactosyltransferase gene-knockout (GTKO) pigs (with or without other genetic modifications) and novel therapeutic approaches.

RECENT FINDINGS

Work on the development of genetically engineered pigs has been considerable during the past few years, with many research institutes reporting the outcomes of research. Multiple gene modifications on a GTKO background have been reported, and the results of transplantation using organs from these pigs have been published. Progress, however, has been variable, and several obstacles, for example, coagulation dysregulation, have been identified. Heterotopic pig heart xenotransplantation has been associated with graft survival up to 8 months, but kidney graft survival has not improved significantly.

SUMMARY

The availability of GTKO pigs with additional genetic modifications aimed toward expression of multiple complement-regulatory proteins and/or human thromboregulatory genes, combined with novel immunosuppressive regimens, for example, the inclusion of B cell-depleting agents, should improve pig organ survival in the near future.

Surgical and nonsurgical complications of a pig to baboon heterotopic heart transplantation model

A modified immunosuppressive regimen, developed at the National Institutes of Health, has been employed in a large animal model of heterotopic cardiac xenotransplantation. Graft survival has been prolonged, but despite this, our recipients have succumbed to various surgical or nonsurgical complications. Herein, we have described different complications and management strategies. The most common complication was hypercoagulability (HC) after transplantation, causing thrombosis of both small and large vasculature, ultimately leading to graft loss. While managing this complication we discovered that there was a delicate balance between HC and consumptive coagulopathy (CC). CC encountered in some recipient baboons was not able to be reversed by stopping anticoagulation and administering multiple blood transfusions. Some complications had iatrogenic components. To monitor the animals, a solid state left ventricular telemetry probe was placed directly into the transplanted heart via the apex. Induction of hypocoagulable states by continuous heparin infusion led to uncontrollable intra-abdominal bleeding in 1 baboon from this apical site. This occurrence necessitated securing the probe more tightly with multiple purse strings and 4-quadrant pledgeted stay sutures. One instance of cardiac rupture originated from a lateral wall infarction site. Earlier studies have shown infections to be uniformly fatal in this transplant model. However, owing to the telemetry placement, infections were identified early by temperature spikes that were treated promptly with antibiotics. We had several cases of wound dehiscence due to recipients disrupting the suture line. These complications were promptly resolved by either re-approximating the wound or finding distractions for the baboon. A few of the most common problems we faced in our earlier experiments were related to the jacket, tether, and infusion pumps. It was difficult to keep the jackets on some baboons and the tether had to be modified several times before we assured long-term success. Infusion catheter replacement resulted in transplant heart venous obstruction and thrombosis from a right common femoral venous line. Homeostatic perturbations such as HC and CC and baboon-induced wound complications comprised most complications. Major bleeding and death due to telemetry implantation and infarct rupture occurred in 2 baboons. Despite the variety of complications, we achieved significant graft prolongation in this model.

Xenotransplantation of breast cancers

Three experimental systems based on mouse models are currently used to study breast cancer: transgenic mice, carcinogen-induced models, and xenografts of breast cancers. Each of these models has advantages and limitations. This chapter focuses on xenotransplantation of breast cancers and reviews the techniques used so far in establishing this model, the advantages and limitations compared to other experimental systems, and finally, the technical questions that remain to be answered.

Encapsulated cell grafts to treat cellular deficiencies and dysfunction

Cell transplantation provides a therapeutic alternative to whole organ transplantation in the management of diseases arising from the absence or failure of specialized cells. Though allogenic transplantation is favorable in terms of graft acceptance, xenotransplantation can provide a potentially unlimited source of cells and can overcome shortage of human donors. Effective immunoisolation of the xenografts is critical for their long term survival and function. Encapsulation of cells in polymeric matrices, organic or inorganic, provides a physical selectively permeable barrier between the host and the graft, thereby immunoisolating the graft. Microencapsulation of cells in alginate hydrogels has been pervasive, but this approach does not provide precise control over porosity, whereas micro- and nano-fabrication technologies can provide precise and reproducible control over porosity. We highlight both encapsulation approaches in this review, with their relative advantages and challenges. We also highlight the therapeutic potential of encapsulated cells for treating a variety of diseases, detailing the xenotransplantation of pancreatic islets in diabetes therapy as well as the grafting of engineered cells that facilitate localized enzyme-prodrug therapy of pancreatic cancer.

Inhibition of rejection in murine islet xenografts by CTLA4Ig and CD40LIg gene transfer

BACKGROUND

Costimulatory signals play a vital role in T cell activation. Blockade of costimulatory pathway by CTLA4Ig or CD40LIg have enhanced graft survival in experimental transplantation models yet mechanisms remain undetermined. We investigated the effects of CTLA4Ig and CD40LIg gene transfer on islet xenografts rejection in rats.

METHODS

Human islets were infected with recombinant adenoviruses containing CTLA4Ig and CD40LIg genes and implanted beneath the kidney capsule of diabetic rats. Levels of blood sugar, morphological changes, and survival of grafts were recorded. Expressions of CTLA4Ig, CD40LIg and insulin were detected by immunohistochemical staining and cytokines levels were quantified by enzyme-linked immunosorbent assay (ELISA).

RESULTS

Blood glucose levels in transplant rats decreased to normal level on the 2nd day post transplantation. The mean blood glucose in the control group, CTLA4Ig transfected group, CD40LIg transfected group and CTLA4Ig + CD40LIg cotransfected group increased on days 8, 24, 21, 68, post transplantation respectively. The grafts in control group, CTLA4Ig transfected group, CD40LIg transfected group and CTLA4Ig + CD40LIg cotransfected group survived for (8 ± 1), (29 ± 4), (27 ± 3), and (74 ± 10) days, respectively. Survival in CTLA4Ig + CD40LIg cotransfected group was significantly longer. Survivals of CTLA4Ig transfected group and CD40LIg transfected group were significantly longer than control group. In control animals, serum interleukin-2 and tumor necrosis factor α concentration significantly increased within seven days post transplantation. Haematoxylin eosin staining of grafts showed live islets in situ of transplant rats without inflammatory cell infiltration. Immunohistochemical staining confirmed the expression of insulin at islets in all experimental groups.

CONCLUSIONS

Transfer of CTLA4Ig and CD40LIg genes, especially the cotransfer of both, inhibits rejection of murine islet xenografts. Downregulated expressions of Th1 cells related cytokines might be related to the beneficial effects.

Hepatic function after genetically engineered pig liver transplantation in baboons

BACKGROUND

If "bridging" to allo-transplantation (Tx) is to be achieved by a pig liver xenograft, adequate hepatic function needs to be assured.

METHODS

We have studied hepatic function in baboons after Tx of livers from alpha1,3-galactosyltransferase gene-knockout (GTKO, n=1) or GTKO pigs transgenic for CD46 (GTKO/CD46, n=5). Monitoring was by liver function tests and coagulation parameters. Pig-specific proteins in the baboon serum/plasma were identified by Western blot. In four baboons, coagulation factors were measured. The results were compared with values from healthy humans, baboons, and pigs.

RESULTS

Recipient baboons died or were euthanized after 4 to 7 days after internal bleeding associated with profound thrombocytopenia. However, parameters of liver function, including coagulation, remained in the near-normal range, except for some cholestasis. Western blot demonstrated that pig proteins (albumin, fibrinogen, haptoglobin, and plasminogen) were produced by the liver from day 1. Production of several pig coagulation factors was confirmed.

CONCLUSIONS

After the Tx of genetically engineered pig livers into baboons (1) many parameters of hepatic function, including coagulation, were normal or near normal; (2) there was evidence for production of pig proteins, including coagulation factors; and (3) these appeared to function adequately in baboons although interspecies compatibility of such proteins remains to be confirmed.

High-resolution animal PET imaging of prostate cancer xenografts with three different 64Cu-labeled antibodies against native cell-adherent PSMA

BACKGROUND

The prostate specific membrane antigen (PSMA) is expressed by virtually all prostate cancers and represents an ideal target for diagnostic and therapeutic strategies. This article compares the in vivo behavior and tumor uptake of three different radiolabeled anti-PSMA monoclonal antibodies (mAbs) and corresponding F(ab)(2) and Fab fragments thereof.

METHODS

The mAbs 3/A12, 3/F11, and 3/E7 and fragments of 3/A12 were conjugated with the chelating agent DOTA and radiolabeled with 64Cu. For the microPET imaging studies, SCID mice bearing PSMA-positive C4-2 and PSMA-negative DU 145 prostate cancer xenografts were used. Each animal received 20-30 microg radiolabeled mAb or fragment corresponding to an activity of 8-14 MBq. Imaging was performed 3, 24, and 48 hr post-injection. After the last scan, mice were sacrificed and tracer in vivo biodistribution was measured by gamma-counting.

RESULTS

Static microPET images of mice with PSMA-positive tumors revealed a high uptake of the mAbs in the C4-2 tumors at 24 and 48 hr after tracer injection and only a minimal distribution in the DU 145 tumors and other organs. In contrast, the F(ab)(2) and Fab fragments of 3/A12 were detected at a high extend in the kidney but not in the C4-2 tumors. These results were confirmed by gamma counting of dissected organs after the final imaging.

CONCLUSIONS

Due to the high and specific uptake of the 64Cu-labeled mAbs in PSMA-positive tumors, these antibodies represent excellent tools for prostate cancer imaging.

Human immune system development and rejection of human islet allografts in spontaneously diabetic NOD-Rag1null IL2rgammanull Ins2Akita mice

OBJECTIVE

To create an immunodeficient mouse model that spontaneously develops hyperglycemia to serve as a diabetic host for human islets and stem cell-derived beta-cells in the absence or presence of a functional human immune system.

RESEARCH DESIGN AND METHODS

We backcrossed the Ins2(Akita) mutation onto the NOD-Rag1(null) IL2rgamma(null) strain and determined 1) the spontaneous development of hyperglycemia, 2) the ability of human islets, mouse islets, and dissociated mouse islet cells to restore euglycemia, 3) the generation of a human immune system following engraftment of human hematopoietic stem cells, and 4) the ability of the humanized mice to reject human islet allografts.

RESULTS

We confirmed the defects in innate and adaptive immunity and the spontaneous development of hyperglycemia conferred by the IL2rgamma(null), Rag1(null), and Ins2(Akita) genes in NOD-Rag1(null) IL2rgamma(null) Ins2(Akita) (NRG-Akita) mice. Mouse and human islets restored NRG-Akita mice to normoglycemia. Insulin-positive cells in dissociated mouse islets, required to restore euglycemia in chemically diabetic NOD-scid IL2rgamma(null) and spontaneously diabetic NRG-Akita mice, were quantified following transplantation via the intrapancreatic and subrenal routes. Engraftment of human hematopoietic stem cells in newborn NRG-Akita and NRG mice resulted in equivalent human immune system development in a normoglycemic or chronically hyperglycemic environment, with >50% of engrafted NRG-Akita mice capable of rejecting human islet allografts.

CONCLUSIONS

NRG-Akita mice provide a model system for validation of the function of human islets and human adult stem cell, embryonic stem cell, or induced pluripotent stem cell-derived beta-cells in the absence or presence of an alloreactive human immune system.

Analysis of innate defences against Plasmodium falciparum in immunodeficient mice

BACKGROUND

Mice with genetic deficiencies in adaptive immunity are used for the grafting of human cells or pathogens, to study human diseases, however, the innate immune responses to xenografts in these mice has received little attention. Using the NOD/SCID Plasmodium falciparum mouse model an analysis of innate defences responsible for the substantial control of P. falciparum which remains in such mice, was performed.

METHODS

NOD/SCID mice undergoing an immunomodulatory protocol that includes, clodronate-loaded liposomes to deplete macrophages and an anti-polymorphonuclear leukocytes antibody, were grafted with human red blood cells and P. falciparum. The systematic and kinetic analysis of the remaining innate immune responses included the number and phenotype of peripheral blood leukocytes as well as inflammatory cytokines/chemokines released in periphery. The innate responses towards the murine parasite Plasmodium yoelii were used as a control.

RESULTS

Results show that 1) P. falciparum induces a strong inflammation characterized by an increase in circulating leukocytes and the release of inflammatory cytokines; 2) in contrast, the rodent parasite P. yoelii, induces a far more moderate inflammation; 3) human red blood cells and the anti-inflammatory agents employed induce low-grade inflammation; and 4) macrophages seem to bear the most critical function in controlling P. falciparum survival in those mice, whereas polymorphonuclear and NK cells have only a minor role.

CONCLUSIONS

Despite the use of an immunomodulatory treatment, immunodeficient NOD/SCID mice are still able to mount substantial innate responses that seem to be correlated with parasite clearance. Those results bring new insights on the ability of innate immunity from immunodeficient mice to control xenografts of cells of human origin and human pathogens.

Xenogenic cancer vaccines

In the current review the latest literature data on design of cancer vaccines on the basis of xenogenic tumor-associated antigens and proteins that play an important role in tumor progression have been summarized. The main benefits and disadvantages of xenogenic cancer vaccines, the results of experimental studies and clinical trials, and perspectives of use are analyzed.

Redox modulation protects islets from transplant-related injury

OBJECTIVE

Because of reduced antioxidant defenses, beta-cells are especially vulnerable to free radical and inflammatory damage. Commonly used antirejection drugs are excellent at inhibiting the adaptive immune response; however, most are harmful to islets and do not protect well from reactive oxygen species and inflammation resulting from islet isolation and ischemia-reperfusion injury. The aim of this study was to determine whether redox modulation, using the catalytic antioxidant (CA), FBC-007, can improve in vivo islet function post-transplant.

RESEARCH DESIGN AND METHODS

The abilities of redox modulation to preserve islet function were analyzed using three models of ischemia-reperfusion injury: 1) streptozotocin (STZ) treatment of human islets, 2) STZ-induced murine model of diabetes, and 3) models of syngeneic, allogeneic, and xenogeneic transplantation.

RESULTS

Incubating human islets with catalytic antioxidant during STZ treatment protects from STZ-induced islet damage, and systemic delivery of catalytic antioxidant ablates STZ-induced diabetes in mice. Islets treated with catalytic antioxidant before syngeneic, suboptimal syngeneic, or xenogeneic transplant exhibited superior function compared with untreated controls. Diabetic murine recipients of catalytic antioxidant-treated allogeneic islets exhibited improved glycemic control post-transplant and demonstrated a delay in allograft rejection. Treating recipients systemically with catalytic antioxidant further extended the delay in allograft rejection.

CONCLUSIONS

Pretreating donor islets with catalytic antioxidant protects from antigen-independent ischemia-reperfusion injury in multiple transplant settings. Treating systemically with catalytic antioxidant protects islets from antigen-independent ischemia-reperfusion injury and hinders the antigen-dependent alloimmune response. These results suggest that the addition of a redox modulation strategy would be a beneficial clinical approach for islet preservation in syngeneic, allogeneic, and xenogeneic transplantation.

Short-term administrations of a combination of anti-LFA-1 and anti-CD154 monoclonal antibodies induce tolerance to neonatal porcine islet xenografts in mice

OBJECTIVE

The objective of this study was to determine whether tolerance to neonatal porcine islet (NPI) xenografts could be achieved by short-term administrations of anti-LFA-1 and anti-CD154 monoclonal antibodies (mAbs).

RESEARCH DESIGN AND METHODS

Diabetic B6 mice received NPI transplants and short-term injections of combined anti-LFA-1 and anti-CD154 mAbs. Mice with long-term islet graft function were treated with depleting anti-CD25 mAb or re-transplanted with a second-party NPI. At the end of the study, grafts from mice with long-term islet function were examined. Their spleen cells were characterized and used for in vitro proliferation and adoptive transfer studies.

RESULTS

All mAb-treated NPI recipients maintained normoglycemia for >100 days post-transplantation. Only 5 of 50 mice rejected their grafts before 300 days post-transplantation. Intact islets, foxp3(+) immune cells, as well as interleukin (IL)-10 and transforming growth factor (TGF)-beta regulatory cytokine transcripts were detected in the NPI xenografts from tolerant mice. A higher percentage of CD4(+) T-cell population from these mice expressed regulatory markers, suggesting that tolerance to NPI xenografts may be mediated by T regulatory cells. This was confirmed when tolerant mice treated with depleting anti-CD25 mAb became diabetic. Lymphocytes from tolerant mice inhibited the proliferation of lymphocytes from B6 mice immunized with porcine cells and they displayed limited proliferation when adoptively transferred. All protected B6 mice transplanted with a second-party NPI xenograft maintained long-term normoglycemia even after removal of the first NPI graft-bearing kidney.

CONCLUSIONS

These results demonstrate that tolerance to NPI xenografts can be achieved by transient administrations of combined anti-LFA-1 and anti-CD154 mAb therapy.

Removal of alpha-Gal epitopes from porcine aortic valve and pericardium using recombinant human alpha galactosidase A

It has been reported that the immune response due to alpha-Gal epitopes is an important factor in tissue valve failure. The elimination of the interaction between the natural anti-Gal antibodies and alpha-gal epitopes on the xenografts is a prerequisite to the success of xenografts in humans. Previously, we reported that the green coffee bean alpha-galactosidase could remove all alpha-Gal epitopes from cell surface of porcine aortic valve and pericardial tissue, but it has limitations on cost effectiveness. In this study we wanted to know whether the recently produced recombinant human alpha-galactosidase A has the same effective enzymatic activity as green coffee bean alpha-galactosidase in removing alpha-Gal epitopes from the same tissues. After treating fresh porcine aortic valve and pericardial tissue with recombinant alpha-galactosidase A, each sample was stained with Griffonia simplicifolia type I isolectin B4 indirect immunoperoxidase avidin-biotin technique. We then examined whether the alpha-Gal epitopes were reduced or abolished in each consecutive concentration of recombinant alpha-galactosidase A by comparing the degree of the Griffonia simplicifolia isolectin B4 staining. As a result, the recombinant alpha-galactosidase A could remove cell surface alpha-Gals on porcine aortic valve and pericardial tissue as effectively as green coffee bean alpha-galactosidase.

Effect of Graves' intrathyroidal lymphocytes. Effect of Graves' disease intrathyroidal lymphocytes (ITL) on xenotransplants of human thyroid tissue in athymic nude mice

It has been suggested that ITL are of importance in the pathogenesis of human autoimmune thyroid disease. Aim of our study was to investigate function and morphology of xenotransplanted human thyroid tissue in nude mice following systemic application of lymphocyte preparations from patients with Graves' disease (GD) and non-toxic nodular goiter (NTG). ITL obtained from 13 patients with GD and peripheral blood lymphocytes (PBL) from 12 patients with NTG were injected into nude mice bearing 8 weeks old xenografts of normal human thyroid tissue. ITL- and PBL-subsets were analyzed by flow cytometer (FACScan) before engraftment. Two days after injection the thyroid transplants were examined histologically (HE) as well as immunohistologically by staining with: CD3, CD31, CD45R, HLA class II, ICAM-1, VCAM-1, IgG, IgM and Ki67. Flow cytometry showed a significant higher number of the lymphocyte-subsets CD3, DR+ and CD4 in GD-ITL as compared to the subsets in NTG-ITL. After injection of GD-ITL to the nude mice also a significant higher number of CD3+ human lymphocytes was found in the transplants. GD-lymphocytes stimulated thyroid tissue function significantly more pronounced than NTG-lymphocytes (histomorphological evaluation). In addition, a significant increase of HLA-class II, ICAM-1-, VCAM-1-, CD45-expression and number of Ig presenting plasma cells were observed only after injection of GD-ITL. Our data demonstrate, for the first time in vivo (nude mouse model), that GD-lymphocytes of both peripheral and intrathyroidal origin selectively migrate into human thyroid transplants ("homing"). They survive there for at least two days and induce significant functional as well as histological changes, like expression of gene products and IgG synthesis. These results suggest an important role of ITL in the pathogenetic mechanisms and clinical course of GD.

Pancreatic islet xenografts at two different transplantation sites (renal subcapsular versus intraportal): comparison of graft survival and morphology

Our primary objective in this study was to determine the effect of two different transplantation sites (renal capsule vs. portal vein) on islet xenograft survival and graft morphology. 59 chemically induced diabetic C57BL/6J mice were transplanted either intraportally (n = 30) or under the left renal capsule (n = 29) receiving 300-350 either freshly isolated or culture pretreated (37 degrees C or 22 degrees C) Lewis rat islets without any immunosuppressive therapy. Histology was performed by immunohistochemical staining to examine the morphologic pattern after rejection or after post-transplant normoglycemia for more than 120 days. Life table analysis revealed a significant (p < 0.001) prolongation of xenograft survival using the renal capsule as transplantation site. 75% graft rejection occurred 56 days after transplantation when the renal capsule was used, compared to 19.5 days intraportal. The intriguing finding was that graft morphology was different depending on the transplantation site. After transplantation under the renal capsule we observed predominantly a more periinsular infiltration with focal aggregates of mononuclear cells at the periphery of the graft. This pattern was more consistent with a non-destructive type of insulitis. In contrast, we found direct infiltration of the transplanted islets following intraportal transplantation reflecting a more destructive type of insulitis. In summary, we could demonstrate a significant prolongation of islet xenograft survival by using the renal capsule as transplantation site in contrast to intraportal transplantation. The morphological pattern possibly indicates two different mechanisms of rejection depending on the transplantation site.

Immune Response to Small Intestinal Submucosa (Surgisis) Implant in Humans: Preliminary Observations

Surgisis IHM is an acellular biomaterial derived from porcine small intestinal submucosa (SIS) that induces site-specific remodeling in the organ or tissue into which it is placed. Previous animal studies have shown that the graft recipient mounts a helper T type 2-restricted immune response to the SIS xenograft without signs of rejection. The aims of this study were to evaluate the immune response to the SIS implant in a small series of humans and to examine the long-term clinical acceptance of the xenograft in these patients. Five consecutive male patients (mean age 56 years, range 34-68) who underwent inguinal hernioplasty with Surgisis IHM were assessed at 2 weeks, 6 weeks, and 6 months after implant for SIS-specific, alpha-1,3-galactose (alpha-gal) epitope and type I collagen specific antibodies. All five patients were also clinically assessed up to 2 years for signs of clinical rejection, hernia recurrence, and other complications. All 5 patients implanted with Surgisis IHM produced antibodies specific for SIS and alpha -gal with a peak between 2 and 6 weeks after implantation. By 6 months, all patients showed decreasing levels of anti-SIS antibodies. Two patients developed a transient, mild local seroma that resolved spontaneously. None of the patients showed any clinical signs of rejection, wound infection, hernia recurrence, or other complications in the follow-up out to 2 years. Thus, this study showed that in a small series of patients the SIS elicits an antibody response without clinical rejection of the xenograft and minimal postoperative complications.

Pig embryonic pancreatic tissue as a source for transplantation in diabetes: transient treatment with anti-LFA1, anti-CD48, and FTY720 enables long-term graft maintenance in mice with only mild ongoing immunosuppression

OBJECTIVE

Defining an optimal costimulatory blockade-based immune suppression protocol enabling engraftment and functional development of E42 pig embryonic pancreatic tissue in mice.

RESEARCH DESIGN AND METHODS

Considering that anti-CD40L was found to be thrombotic in humans, we sought to test alternative costimulatory blockade agents already in clinical use, including CTLA4-Ig, anti-LFA1, and anti-CD48. These agents were tested in conjunction with T-cell debulking by anti-CD4 and anti-CD8 antibodies or with conventional immunosuppressive drugs. Engraftment and functional development of E42 pig pancreatic tissue was monitored by immunohistology and by measuring pig insulin blood levels.

RESULTS

Fetal pig pancreatic tissue harvested at E42, or even as early as at E28, was fiercely rejected in C57BL/6 mice and in Lewis rats. A novel immune suppression comprising anti-LFA1, anti-CD48, and FTY720 afforded optimal growth and functional development. Cessation of treatment with anti-LFA1 and anti-CD48 at 3 months posttransplant did not lead to graft rejection, and graft maintenance could be achieved for >8 months with twice-weekly low-dose FTY720 treatment. These grafts exhibited normal morphology and were functional, as revealed by the high pig insulin blood levels in the transplanted mice and by the ability of the recipients to resist alloxan induced diabetes.

CONCLUSIONS

This novel protocol, comprising agents that simulate those approved for clinical use, offer an attractive approach for embryonic xenogeneic transplantation. Further studies in nonhuman primates are warranted.

Antibody-mediated xenograft injury: mechanisms and protective strategies

The use of porcine organs for clinical transplantation is a promising potential solution to the shortage of human organs. Preformed anti-pig antibody is the primary cause of hyperacute rejection, while elicited antibody can contribute to subsequent "delayed" xenograft rejection. This article will review recent progress to overcome antibody mediated xenograft rejection, through modification of the host immunity and use of genetically engineered pig organs.

Adenovirus-mediated gene expression of the human c-FLIP(L) gene protects pig islets against human CD8(+) cytotoxic T lymphocyte-mediated cytotoxicity

Cell-mediated immunity, especially of human CD8+ cytotoxic T lymphocytes (CTLs) is believed to have an important role in the long-term survival of pig islet xenografts. Protection against human CD8+ CTL cytotoxicity may reduce the direct damage to pig islets and enable long-term xenograft survival in pig-to-human islet xenotransplantation. We have previously reported that c-FLIP(S/L) genes, which are potent inhibitors of death receptor-mediated proapoptotic signals through binding competition with caspase-8 for recruitment to the Fas-associated via death domain (FADD), markedly suppress human CD8+ CTL-mediated xenocytotoxicity. In addition, the cytoprotective effects of c-FLIP(L) seem to be significantly stronger than those of c-FLIP(S). Accordingly, in the present study, expression of c-FLIP(L) was induced in intact pig islets by adenoviral transduction. Consequently, the cytoprotective capacity of the transgene in pig islets was examined in in vitro and in vivo exposure to human CD8+ CTLs. Cells from untransduced islets or mock islets were sensitive to CD8+ CTL-mediated lysis (59.3% +/- 15.9% and 64.0% +/- 8.9% cytotoxicity, respectively). In contrast, cells from pig islets transduced with the c-FLIP(L) gene were markedly protected from lysis (30.5% +/- 3.5%). Furthermore, prolonged xenograft survival was elicited from pig islets transduced with this molecule as assessed using an islet transplant model using the rat kidney capsule. Thus, these data indicate that intact pig islets can be transduced to express c-FLIP(L) with adenovirus. Pig islets expressing c-FLIP(L) are significantly resistant to human CTL killing and further exhibit beneficial effects to prolong xenograft survival.

HLA-E/human beta2-microglobulin transgenic pigs: protection against xenogeneic human anti-pig natural killer cell cytotoxicity

BACKGROUND

Natural killer (NK) cells participate in pig-to-primate xenograft rejection both by antibody-dependent and -independent mechanisms. A majority of human NK cells express the inhibitory receptor CD94/NKG2A, which binds specifically to human leukocyte antigen (HLA)-E, a trimeric complex consisting of the HLA-E heavy chain, beta2-microglobulin (beta2m), and a peptide derived from the leader sequence of some major histocompatibility complex class I molecules.

METHODS

To use this mechanism for protection of pig tissues against human NK cell-mediated cytotoxicity, we generated transgenic pigs by pronuclear microinjection of genomic fragments of HLA-E with an HLA-B7 signal sequence and of human beta2-microglobulin (hubeta2m) into zygotes.

RESULTS

Three transgenic founder pigs were generated. Northern blot analysis of RNA from peripheral blood mononuclear cells revealed the presence of the expected transcript sizes for both transgenes in two of the three founders. The founder with the highest expression and his offspring were characterized in detail. Fluorescence-activated cell sorting (FACS) and Western blot analyses demonstrated consistent expression of HLA-E and hubeta2m in peripheral blood mononuclear cells. Immunohistochemistry revealed the presence of HLA-E and hubeta2m on endothelial cells of many organs, including heart and kidney. In vitro studies showed that lymphoblasts and endothelial cells derived from HLA-E/hubeta2m transgenic pigs are effectively protected against human NK cell-mediated cytotoxicity, depending on the level of CD94/NKG2A expression on the NK cells. Further, HLA-E/hubeta2m expression on porcine endothelial cells inhibited the secretion of interferon (IFN)-gamma by co-cultured human NK cells.

CONCLUSIONS

This novel approach against cell-mediated xenogeneic responses has important implications for the generation of multitransgenic pigs as organ donors for clinical xenotransplantation.