Human cell-mediated rejection of porcine xenografts in an immunodeficient mouse model

NK cells promote peritoneal xenograft rejection through an IFN-?-dependent mechanism

BACKGROUND

Natural killer (NK) cells have emerged as major players in anti-viral and anti-tumour immune responses. Like cytotoxic T lymphocytes (CTL), they express perforin and are potent secretors of gamma-interferon (IFN-gamma). However, there is conflicting evidence about their role in mediating rejection of xenogeneic tissue.

METHODS

A pig-to-mouse peritoneal cell model of xenotransplantation was used to investigate the effect of NK deficiency on xenograft recovery and the possible mechanisms behind this NK-mediated graft rejection. gamma c(-/-)RAG(-/-) mice were used as a model of NK deficiency. Additionally, NK cells were depleted in RAG(-/-) mice using anti-asialo GM1. The contributions of IFN-gamma, perforin and NKT cells were studied using knock-out mice that were depleted in vivo of T cells. Mice were injected with 10(7) pig cells intraperitoneally and peritoneal fluid was assessed 5 days later for xenograft recovery and phenotypic analysis. The requirement for NK cells for xenograft rejection was also assessed using luciferase-transfected porcine cells in a renal subcapsular model of transplantation.

RESULTS

Pig cell recovery was enhanced in both gamma c(-/-)RAG(-/-) and NK-depleted RAG(-/-) mice when compared with RAG(-/-) control mice. IFN-gamma(-/-) mice depleted of T cells also demonstrated superior graft survival compared with their B6 counterparts. However, there were minimal graft survival differences between Pfp(-/-) and B6 control mice. Similarly, a deficiency in NKT cells did not improve pig xenograft recovery from the peritoneum of these mice.

CONCLUSIONS

Therefore, we conclude that NK cells, but not NKT cells, are important mediators of xenograft rejection in the peritoneal cavity, and that their role may be unmasked in the absence of T cells. The mechanism for this xenorejection appears to involve IFN-gamma but is perforin independent.

The influence of natural antibody specificity on antigen immunogenicity

The natural antibody repertoire in humans, apes and Old World primates is distinct from the repertoire of all other placental mammals, and encodes antibodies specific for the carbohydrate epitope Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal). Here, we examined whether conjugating antigens to the alphaGal epitope can augment their immunogenicity in alpha(1,3)galactosyltransferase knockout mice (GT0 mice) which, like humans, produce alphaGal-specific antibodies. Immunization of GT0 mice with BSA conjugated to alphaGal (alphaGal-BSA) led to significant production of anti-BSA IgG antibodies without the need for adjuvant. This response was dependent on the presence of alphaGal-reactive antibodies. Immunization of wild-type mice with alphaGal-BSA failed to induce an anti-BSA response. The presence of alphaGal-reactive antibodies also led to an increase in the T cell response to BSA following immunization with alphaGal-BSA when compared with mice that received BSA alone, resulting in an increased frequency of IFN-gamma- and IL-4-producing BSA-specific T cells. In addition, the ability to produce alphaGal-reactive antibodies enhanced the cytotoxic T lymphocyte anti-viral antigen response following vaccination with murine leukemia virus transformed cell lines that express alphaGal on their cell surface. Natural antibodies that bind alphaGal therefore play a key role in increasing the efficiency of priming to antigens decorated with alphaGal epitopes.

Allogeneic versus xenogeneic immune reaction to bioengineered skin grafts

There are conflicting reports on the survival and immune reaction to allografts and xenografts of cultured skin substitutes (CSS). In this study, we investigated the allogeneic and xenogeneic responses to CSS of human keratinocytes and genetically engineered CSS expressing keratinocyte growth factor (KGF) that forms a hyperproliferative epidermis. CSS (control and KGF modified) and neonatal human foreskins were evaluated by immunohistochemistry for the expression of MHC class I and II. To study allograft rejection, grafts were transplanted to human peripheral blood mononuclear cell (huPBMC)-reconstituted SCID mice. To study xenograft rejection, grafts were transplanted to immunocompetent mice. Graft survival and immune reaction were assessed visually and microscopically. After transplantation, control CSS formed a normal differentiated epidermis, whereas KGF CSS formed a hyperproliferative epidermis. Control and KGF CSS expressed class I similar to neonatal foreskin, but did not express class II. In the allograft model, rejection of neonatal foreskins was between 5 and 9 days. In contrast, neither control nor KGF CSS was rejected by huPBMC-SCID mice. Histology showed dense mononuclear cell infiltration in human foreskins, with few, if any, mononuclear cells in control or KGF CSS. In contrast to the allogeneic reaction, CSS (control and KGF) were rejected in the xenograft model, but rejection was delayed (9-21 days) compared with neonatal skin (5-8 days). Humanized SCID mice rejected allografts of human neonatal foreskins, but did not reject control CSS or KGF CSS, even though the KGF CSS formed a hyperproliferative epidermis. Rejection of control and KGF CSS by immunocompetent mice in a xenograft model was comparable and their survival was significantly prolonged compared with neonatal skin. These results demonstrate that control CSS and hyperproliferative KGF CSS are less immunogenic than normal human skin and that sustained hyperproliferation of the epidermis does not accelerate rejection.

Porcine cell microchimerism but lack of productive porcine endogenous retrovirus (PERV) infection in naive and humanized SCID-beige mice treated with porcine peripheral blood mononuclear cells

Pigs are considered a suitable source of cells and organs for xenotransplantation. All known strains of pigs contain porcine endogenous retrovirus (PERV) and PERV released by porcine cells may infect human cells in vitro and severe-combined immunodeficient (SCID) mice in vivo. Humanized SCID (hu-SCID) mice develop immune response to porcine antigens. Here we investigated PERV transmission in humanized SCID-beige mice using porcine peripheral blood mononuclear cells (PBMC) as the donor tissue (and the source of PERV). Mice were infused in the peritoneal cavity with 1.5-3.0 x 10(7) unfractionated human PBMC. Unfractionated porcine PBMC (1.5-3.0 x 10(7) cell/mouse) were infused to the mice simultaneously with human PBMC or 3 weeks after human PBMC infusion. The treated mice were monitored for weight and skin changes, donor cell chimerism, anti-pig antibodies and PERV transmission. All humanized mice tested 5-12 weeks after human PBMC transplantation were macrochimeric (up to 40% of cells in blood) for human cells, where 99% of the human cells were T-lymphocytes. Although human B lymphocytes were very rare in the blood of humanized mice at that point, the mice were positive for human anti-pig natural antibodies. The control SCID-beige mice or mice treated with porcine PBMC alone were negative for anti-porcine antibodies. Approximately 70% of the humanized mice treated with porcine PBMC were also microchimeric for porcine cells. Although some tissue samples of these mice were positive for PERV DNA in the absence of porcine DNA indicating PERV infection, the infection was non-productive as PERV transcripts were not detectable in those tissues. PERV infection of human and mouse cells in vitro by co-culturing with porcine PBMC was also non-productive. Humanized SCID-beige mice suffered weight loss and occasional minor skin changes due to graft vs. host disease caused by human PBMC but none of the mice showed observable effect attributable to the apparent PERV infection alone.

Direct Antigen Presentation by a Xenograft Induces Immunity Independently of Secondary Lymphoid Organs

The location of immune activation is controversial during acute allograft rejection and unknown in xenotransplantation. To determine where immune activation to a xenograft occurs, we examined whether splenectomized alymphoplastic mice that possess no secondary lymphoid organs can reject porcine skin xenografts. Our results show that these mice rejected their xenografts, in a T cell-dependent fashion, at the same tempo as wild-type recipients, demonstrating that xenograft rejection is not critically dependent on secondary lymphoid organs. Furthermore, we provide evidence that immune activation in the bone marrow did not take place during xenograft rejection. Importantly, immunity to xenoantigens was only induced after xenotransplantation and not by immunization with porcine spleen cells, as xenografted mutant mice developed an effector response, whereas mutant mice immunized by porcine spleen cells via i.p. injection failed to do so. Moreover, we provide evidence that antixenograft immunity occurred via direct and indirect Ag presentation, as recipient T cells could be stimulated by either donor spleen cells or recipient APCs. Thus, our data provide evidence that direct and indirect Ag presentation by a xenograft induces immunity in the absence of secondary lymphoid organs. These results have important implications for developing relevant xenotransplantation protocols.

Alloimmune injury and rejection of human skin grafts on human peripheral blood lymphocyte-reconstituted non-obese diabetic severe combined immunodeficient beta2-microglobulin-null mice

Small animal models with the capacity to support engraftment of a functional human immune system are needed to facilitate studies of human alloimmunity. In the present investigation, non-obese diabetic (NOD) severe combined immunodeficient (scid) beta2-microglobulin-null (B2mnull) mice engrafted with human peripheral blood lymphocytes (hu-PBL-NOD-scid B2mnull mice) were used as in vivo models for studying human skin allograft rejection. Hu-PBL-NOD-scid B2mnull mice were established by injection of human spleen cells or PBLs and transplanted with full-thickness allogeneic human skin. Human cell engraftment was enhanced by injection of anti-mouse CD122 antibody. The respective contributions of human CD4+ and CD8+ cells in allograft rejection were determined using depleting antibodies. Human skin grafts on unmanipulated NOD-scid B2mnull mice uniformly survived but on chimeric hu-PBL-NOD-scid B2mnull mice exhibited severe immune-mediated injury that often progressed to complete rejection. The alloaggressive hu-PBLs did not require prior in vitro sensitization to elicit targeted effector cell activity. Extensive mononuclear cell infiltration directed towards human-origin endothelium was associated with thrombosis and fibrin necrosis. No evidence of graft-versus-host disease was detected. Either CD4+ or CD8+ T cells may mediate injury and alloimmune rejection of human skin grafts on hu-PBL-NOD-scid B2mnull mice. It is proposed that Hu-PBL-NOD-scid B2mnull mice engrafted with human skin will provide a useful model for analysis of interventions designed to modulate human allograft rejection.

Immunoglobulin heavy chain transgenic mice expressing Galalpha(1,3)Gal-reactive antibodies

BACKGROUND

Natural antibodies that bind the carbohydrate antigen Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal) mediate rigorous rejection of porcine xenografts and represent a major immunological hurdle to successful discordant xenotransplantation. However, little is known about how production of antibodies specific for alphaGal is regulated.

METHODS

Transgenic mice expressing an IgM heavy chain isolated from a B-cell hybridoma that produces antibodies specific for alphaGal were constructed. These mice were bred to mutant mice that lack the alphaGal epitope (GT0 mice) or wild-type (GT+) mice to generate animals in which the transgene is expressed in the presence or absence of alphaGal as a "self"-antigen. Development of transgene-expressing B cells and production of alphaGal-specific serum antibodies were then analyzed in transgenic mice on GT0 and GT+ backgrounds.

RESULTS

B cells expressing the transgenic heavy chain and transgene-encoded serum antibodies specific for alphaGal were readily detected in mice on the GT0 background. Most alphaGal-reactive antibodies in GT0 mice used the transgene rather than endogenous Ig heavy chains. In contrast, transgene-encoded serum antibodies specific for alphaGal were not detected in GT+ mice. In transgenic mice on the GT+ background, B cells expressing the transgene underwent deletion as a result of encountering alphaGal during their development, indicating that expression of alphaGal as part of self-mediated efficient negative selection of B cells expressing transgene-encoded alphaGal-specific antibodies.

CONCLUSIONS

The development of transgenic mice expressing a B cell receptor specific for alphaGal provides a novel system to study developmental regulation of B cells making carbohydrate-specific antibodies. In addition, these mice may be useful for examining methods to prevent production of alphaGal-reactive antibodies.

The role of T cell help in the production of antibodies specific for Gal alpha 1-3Gal

The majority of xenoreactive natural Abs in humans recognize the carbohydrate Ag present on pig tissue, Galalpha1-3Galbeta1-4GlcNAc-R (alphaGal), synthesized by the enzyme UDP galactose:beta-D-galactosyl-1,4-N-acetyl-D-glucosaminide alpha(1-3)galactosyltransferase or alphaGT. Using alphaGT knockout mice (GT(0) mice), which like humans produce serum Abs that bind alphaGal, we examined the role of T cells in production of Abs specific for alphaGal. GT(0) mice were crossed with TCR-beta knockout mice (TCR-beta(0)) to generate double-knockout mice (GT(0)/TCR-beta(0)). While GT(0)/TCR-beta+ mice exhibited an age-dependent increase in the serum titer of natural Abs specific for alphaGal, a similar increase was not observed in GT(0)/TCR-beta(0) mice, and the titer of alphaGal-specific Abs in double knockouts was significantly lower than in age-matched GT(0)/TCR-beta+ mice. Immunization with pig cells resulted in a significant increase in the serum titer of alphaGal-specific Abs in GT(0)/TCR-beta+ mice, but had no effect on the level of alphaGal-specific serum Abs in GT(0)/TCR-beta(0) mice. Treatment of GT(0)/TCR-beta+ mice with anti-CD40L Abs before immunization with pig cells prevented sensitization to alphaGal. Our data suggest that the majority of alphaGal-specific Abs are T cell dependent and that production of alphaGal-specific Abs after sensitization can be prevented by blocking costimulatory pathways.

Patterns of engraftment in different strains of immunodeficient mice reconstituted with human peripheral blood lymphocytes

BACKGROUND

Models of immunodeficient mice reconstituted with a competent human immune system would represent an invaluable tool for the study of transplantation immunobiology allergy, autoimmunity, and infectious diseases. Severe combined immune deficiency (scid) mice can be successfully reconstituted with human peripheral blood lymphocytes (PBLs), but rates and levels of engraftment are poor. New strains of mice with diverse immunodeficiencies have been recently characterized or developed, which might prove to be advantageous for in vivo studies of human immune reactivity.

METHODS

We have compared rates and patterns of human PBL engraftment in four available immunodeficient murine strains; scid-beige, nonobese diabetic (NOD)-scid, NOD-scid-beta2 m- and rag-. T- and B-lymphocyte engraftment, phenotype of engrafted cells, and occurrence of graft-versus-host disease (GVHD) were studied and compared.

RESULTS

Successful engraftment of human PBL was readily obtained in the majority of scid-beige, NOD-scid, and NOD-scid-beta2 m- with a single i.p. administration of human PBLs, whereas it was seldom achieved in rag- animals. Human Ig levels were accordingly remarkably low in rag- recipients but, interestingly also in NOD-scid-beta2 m- mice. Engraftment was readily observed not only in peripheral blood but also in spleen and bone marrow of successfully reconstituted animals. Phenotypic analysis of engrafted human cells showed preserved CD4/CD8 ratios and a clear skewing toward an activated phenotype. GVHD was invariably observed in successfully reconstituted animals.

CONCLUSIONS

Our data indicate that a high rate of reconstitution with human lymphocytes can be achieved in scid-beige, NOD-scid, and NOD-scid-beta2 m- mice. Human Ig are produced at high levels, except in NOD-scid-beta2 m-, including xenoreactive natural antibodies. Scid-beige and NOD-scid appear therefore better suited than NOD-scid-beta2 m- or rag- for analysis of human immunoreactivity in vivo. An important caveat is the invariable occurrence of GVHD that precludes long-term studies in this model system.

Human T cells infiltrate and injure pig coronary artery grafts with activated but not quiescent endothelium in immunodeficient mouse hosts

BACKGROUND

We have previously demonstrated that human artery grafts transplanted to immunodeficient mice are infiltrated and injured by unsensitized allogeneic human T cells. We extended our investigations to human anti-porcine xenoresponses in this model.

METHODS

Pig coronary artery segments were interposed into the infrarenal aorta of severe combined immunodeficiency/beige mice. After 7 days, certain recipients were reconstituted with human leukocytes and/or treated with proinflammatory cytokines. The grafts were harvested after 1-70 days and examined by histology, immunohistochemistry, and morphometry.

RESULTS

Pig artery grafts from untreated mice had no evidence of injury, leukocytic infiltrate, or endothelial cell activation up to 70 days postoperatively, despite deposition of murine complement. Host reconstitution with human peripheral blood mononuclear cells resulted in a discrete population of circulating T cells that did not infiltrate or injure the grafts up to 28 days after adoptive transfer. Administration of porcine interferon-gamma for up to 28 days sustained the expression of graft vascular cell adhesion molecule-1 and major histocompatibility complex antigens, but did not initiate recruitment of human leukocytes. In contrast, treatment with human tumor necrosis factor for 7 days induced the de novo expression of porcine E-selectin by graft endothelial cells and elicited human T cell infiltration and human peripheral blood mononuclear cell-dependent vascular injury.

CONCLUSIONS

The human peripheral blood mononuclear cell-severe combined immunodeficiency/beige mouse model identifies a significant difference between human T cell allogeneic and xenogeneic responses in vivo. Xenografts with quiescent endothelium are not infiltrated or injured by T cells under the same conditions in which allografts are rejected. Activation of pig coronary artery endothelial cells by human tumor necrosis factor, but not porcine interferon-gamma, elicits cellular xenoresponses.

Characterization of human killer cell reactivity against porcine target cells: differential modulation by cytokines

The cytotoxic cell response to porcine cells by human lymphocytes, and the modulation of cytolytic cellular activity by human cytokines were investigated. Human peripheral blood mononuclear cells (PBMC) and purified lymphocyte subsets were co-cultured with fresh irradiated porcine stimulator cells and examined for the development of lytic activity and for their proliferative response. Porcine target cells included a new cell line, MS-PBMC-J2 (designated J2; SLA-DR+MHC class I+CD2+CD3 CD8+CDI6+CD45+), aortic and microvascular endothelial cells. Initial results showed that natural killer (NK) cells were fivefold more efficient in killing porcine target cells compared with T cells. IL-12 augmented the killing of porcine target cells by human NK cells beyond that induced by stimulation with cells alone. In contrast, IL-2 and IL-15 often induced substantial human NK cell mediated killing of porcine target cells, including endothelial cells in the case of IL-2 where such targets were examined, even in the absence of stimulator cells. Finally, neither IL-18 nor IL-8 had any effect beyond background on NK cell mediated killing of porcine target cells. These findings show that cytokines that would be produced in a xenograft setting clearly modulate the ability of human cytolytic cells to kill porcine targets. In addition, fresh unstimulated human NK cells lysed J2 and porcine aortic endothelial cells, but not porcine microvascular endothelial cells, suggesting the possibility of rapid attack of xenografts by NK cells, and differential susceptibility of endothelial cells from different vascular structures to this attack.

Induction of B-cell tolerance by retroviral gene therapy

The primary immunologic barrier to overcome before clinical xenotransplantation can be successful is rejection mediated by preformed natural antibodies in the host, directed toward a single carbohydrate epitope Galα1-3Galβ1-4GlcNAc-R (αGal) present on porcine tissue, encoded for by the enzyme glucosyltransferase UDP galactose:β-D-galactosyl-1,4-N-acetyl-D-glucosaminide α(1-3)galactosyltransferase (EC 2.4.1.151) or simply αGT. Although we have shown previously that a gene therapy approach could be used to prevent production of natural antibodies specific for αGal, the ability to induce and maintain tolerance after rigorous antigen challenge would be required if similar approaches are to be used clinically. Here, we demonstrate in αGT knockout mice (GT0 mice), which, like humans, contain in their serum antibodies that bind αGal, that the efficient transduction and expression of a retrovirally transduced αGT gene in bone marrow–derived cells induces stable long-term tolerance to the αGal epitope. GT0 mice reconstituted with αGT-transduced bone marrow cells were unable to produce antibodies that bind αGal after extensive immunization with pig cells. Furthermore, using ELISPOT assays, we were unable to detect the presence of B cells that produce αGal reactive antibodies after immunization, suggesting that such B cells were eliminated from the immunologic repertoire after gene therapy. Interestingly, after tolerance to αGal is induced by gene therapy, the antiporcine non-αGal humoral response changes from a predominantly IgM to an IgG response. This suggests that once the natural antibody barrier is eliminated by the induction of tolerance, the antipig response changes to a typical T-cell–dependent response involving isotype switching. Thus, gene therapy approaches may be used to overcome immunologic responses leading to xenograft rejection, and similar gene therapy approaches could be used to overcome autoimmunity.

Induction of B-cell tolerance by retroviral gene therapy

The primary immunologic barrier to overcome before clinical xenotransplantation can be successful is rejection mediated by preformed natural antibodies in the host, directed toward a single carbohydrate epitope Galα1-3Galβ1-4GlcNAc-R (αGal) present on porcine tissue, encoded for by the enzyme glucosyltransferase UDP galactose:β-D-galactosyl-1,4-N-acetyl-D-glucosaminide α(1-3)galactosyltransferase (EC 2.4.1.151) or simply αGT. Although we have shown previously that a gene therapy approach could be used to prevent production of natural antibodies specific for αGal, the ability to induce and maintain tolerance after rigorous antigen challenge would be required if similar approaches are to be used clinically. Here, we demonstrate in αGT knockout mice (GT0 mice), which, like humans, contain in their serum antibodies that bind αGal, that the efficient transduction and expression of a retrovirally transduced αGT gene in bone marrow–derived cells induces stable long-term tolerance to the αGal epitope. GT0 mice reconstituted with αGT-transduced bone marrow cells were unable to produce antibodies that bind αGal after extensive immunization with pig cells. Furthermore, using ELISPOT assays, we were unable to detect the presence of B cells that produce αGal reactive antibodies after immunization, suggesting that such B cells were eliminated from the immunologic repertoire after gene therapy. Interestingly, after tolerance to αGal is induced by gene therapy, the antiporcine non-αGal humoral response changes from a predominantly IgM to an IgG response. This suggests that once the natural antibody barrier is eliminated by the induction of tolerance, the antipig response changes to a typical T-cell–dependent response involving isotype switching. Thus, gene therapy approaches may be used to overcome immunologic responses leading to xenograft rejection, and similar gene therapy approaches could be used to overcome autoimmunity.

Islet xenograft destruction in the hu-PBL-severe combined immunodeficient (SCID) mouse necessitates anti-CD3 preactivation of human immune cells

Introduction of the hu-PBL-SCID mouse model has yielded a potentially useful tool for research in transplantation. The aim of this study was to define the conditions necessary for a reconstituted human immune system to destroy in a consistent manner rat islet xenografts in the alloxan-diabetic hu-PBL-SCID mouse. We examined different time points of hu-PBL reconstitution, different transplantation sites of the islets and several hu-PBL reconstitution protocols. Major differences in graft destruction were observed between the different hu-PBL reconstitution protocols, irrespective of timing of hu-PBL reconstitution or site of transplantation. Although preactivation of hu-PBL did not improve the level of hu-PBL chimerism, histological and immunohistochemical analysis of the grafts revealed a severe human lymphocytic infiltration and beta cell destruction only in the grafts of mice receiving preactivated hu-PBL. This beta cell injury resulted in impaired glucose tolerance, with in some animals recurrence of hyperglycaemia, and decreased insulin and C-peptide levels after glucose stimulation. Therefore, we conclude that activation of hu-PBL prior to transfer is essential in achieving xenograft infiltration and destruction in hu-PBL-SCID mice. The need for immune manipulation suggests that interactions between hu-PBL and xenografts in this model may be hampered by incompatibilities in cross-species adhesion and/or activation signals.

Human TNF can induce nonspecific inflammatory and human immune-mediated microvascular injury of pig skin xenografts in immunodeficient mouse hosts

TNF activates endothelial cells to express cell surface molecules that are necessary to recruit a local infiltrate of leukocytes. Because the actions of this proinflammatory cytokine are not species restricted, we investigated whether human TNF can up-regulate porcine endothelial adhesion molecules to elicit human T cell infiltration and damage of pig skin xenografts in a chimeric immunodeficient mouse model. We have previously demonstrated the vigorous rejection of human skin allografts and the absence of injury to porcine skin xenografts in human PBMC-SCID/beige mice. Intradermal administration of human TNF at high doses (600 or 2000 ng) caused nonspecific inflammatory damage of pig skin grafts, whereas low concentrations of TNF (60 or 200 ng) resulted in human PBMC-dependent injury of porcine endothelial cells. There was a strong correlation among pig skin xenograft damage, human T cell infiltration, and the TNF-induced up-regulation of swine MHC class I and class II molecules, VCAM-1, and, in particular, the de novo expression of porcine E-selectin. The microvascular damage and leukocytic infiltration elicited by TNF were enhanced by porcine IFN-gamma, suggesting that xenografts may be less prone to cytokine-mediated injury due to the species-restricted effects of recipient IFN-gamma. Our results indicate that maintenance of a quiescent endothelium, which does not express E-selectin or other activation-dependent adhesion molecules, is important in preventing human anti-porcine T cell xenoresponses in vivo and that TNF signaling molecules and TNF-responsive gene products are appropriate therapeutic targets to protect against human T cell-mediated rejection of pig xenografts.

Xenotransplantation

BACKGROUND

The success of clinical transplantation has led to a large discrepancy between donor organ availability and demand; considerable pressure exists to develop an alternative source of organs. The use of animal organs for donation is a possible solution that is not yet clinically applicable.

METHODS AND RESULTS

A literature review was performed based on a Medline search to find articles on xenotransplantation. Keywords included hyperacute, acute vascular, xenograft rejection combined with concordant and discordant. Additional references cited in these articles from journals not included in Medline were obtained from the British Library. Limited information on unpublished, preliminary work has been included from sources known to the authors, based on their research work in the field. One hundred and forty-six references and four personal communications have been included in this review article.

CONCLUSION

A greater understanding of the pathogenesis of xenograft rejection is developing rapidly. Strategies to abrogate hyperacute rejection have proved successful, but control of antibody-driven acute vascular rejection has not yet been achieved. The safety and viability of xenotransplantation as a therapeutic modality are still unproven.

Human CD4+ T Cells Mediate Rejection of Porcine Xenografts

It has previously been demonstrated that xenograft rejection in rodents is dependent on CD4+ T cells. However, because of the lack of an appropriate in vivo model, little is known about the cellular basis of human T cell-mediated rejection of xenografts. In this study, we have evaluated the ability of human T cells to mediate rejection of porcine skin grafts in a novel in vivo experimental system using immunodeficient mice as recipients. Recombinase-activating gene-1-deficient mice (R−) lacking mature B and T cells were grafted with porcine skin and received human lymphocytes stimulated in vitro with irradiated porcine PBMC. Skin grafts on mice given either unseparated, activated human lymphocytes, or NK cell-depleted lymphocyte populations were rejected within 18 days after adoptive cell transfer. In contrast, skin grafts on mice given T cell-depleted human lymphocytes or saline showed no gross or histologic evidence of rejection up to 100 days after adoptive transfer. Purified CD4+ T cells were also able to mediate rejection of porcine skin grafts. These data suggest that human CD4+ T cells are sufficient to induce rejection of porcine xenografts. Thus, strategies directed toward CD4+ T cells may effectively prevent cellular rejection of porcine xenografts in humans.

T cell-mediated xenograft rejection: specific tolerance is probably required for long term xenograft survival

T cell-mediated mechanisms of xenograft rejection appear resistant to standard immunosuppression protocols used to prevent allograft rejection and, consequently, higher doses of immunosuppressive drugs are required to promote xenograft compared to allograft survival. Evidence from recent studies suggests that porcine xenografts may be especially immunogenic in humans because of a prominent and vigorous indirect xenoresponse and because of the ability of porcine endothelium to activate human T cells. This has led to an anxiety that systemic immunosuppressives, used as the mainstay of therapy for clinical xenotransplantation, may not allow the long-term survival of porcine organs transplanted into human recipients. This article will review the biology of T cell xenoresponses, present the case for the development of novel graft-specific immunosuppressive regimes in clinical xenotransplantation, and review recent experimental progress in this area.