Species differences in the expression of major histocompatibility complex class II antigens on coronary artery endothelium: implications for cell-mediated xenoreactivity

Animal Models for Heart Transplantation Focusing on the Pathological Conditions

Cardiac transplant recipients face many complications due to transplant rejection. Scientists must conduct animal experiments to study disease onset mechanisms and develop countermeasures. Therefore, many animal models have been developed for research topics including immunopathology of graft rejection, immunosuppressive therapies, anastomotic techniques, and graft preservation techniques. Small experimental animals include rodents, rabbits, and guinea pigs. They have a high metabolic rate, high reproductive rate, small size for easy handling, and low cost. Additionally, they have genetically modified strains for pathological mechanisms research; however, there is a lacuna, as these research results rarely translate directly to clinical applications. Large animals, including canines, pigs, and non-human primates, have anatomical structures and physiological states that are similar to those of humans; therefore, they are often used to validate the results obtained from small animal studies and directly speculate on the feasibility of applying these results in clinical practice. Before 2023, PubMed Central^® at the United States National Institute of Health's National Library of Medicine was used for literature searches on the animal models for heart transplantation focusing on the pathological conditions. Unpublished reports and abstracts from conferences were excluded from this review article. We discussed the applications of small- and large-animal models in heart transplantation-related studies. This review article aimed to provide researchers with a complete understanding of animal models for heart transplantation by focusing on the pathological conditions created by each model.

Non-human Primate Regulatory T Cells and Their Assessment as Cellular Therapeutics in Preclinical Transplantation Models

Non-human primates (NHP) are an important resource for addressing key issues regarding the immunobiology of regulatory T cells (Treg), their in vivo manipulation and the translation of adoptive Treg therapy to clinical application. In addition to their phenotypic and functional characterization, particularly in cynomolgus and rhesus macaques, NHP Treg have been isolated and expanded successfully ex vivo. Their numbers can be enhanced in vivo by administration of IL-2 and other cytokines. Both polyclonal and donor antigen (Ag) alloreactive NHP Treg have been expanded ex vivo and their potential to improve long-term outcomes in organ transplantation assessed following their adoptive transfer in combination with various cytoreductive, immunosuppressive and "Treg permissive" agents. In addition, important insights have been gained into the in vivo fate/biodistribution, functional stability, replicative capacity and longevity of adoptively-transferred Treg in monkeys. We discuss current knowledge of NHP Treg immunobiology, methods for their in vivo expansion and functional validation, and results obtained testing their safety and efficacy in organ and pancreatic islet transplantation models. We compare and contrast results obtained in NHP and mice and also consider prospects for future, clinically relevant studies in NHP aimed at improved understanding of Treg biology, and innovative approaches to promote and evaluate their therapeutic potential.

Immunocompetent cancer-on-chip models to assess immuno-oncology therapy

The advances in cancer immunotherapy come with several obstacles, limiting its widespread use and benefits so far only to a small subset of patients. One of the underlying challenges remains to be the lack of representative nonclinical models that translate to human immunity and are able to predict clinical efficacy and safety outcomes. In recent years, immunocompetent Cancer-on-Chip models emerge as an alternative human-based platform that enables the integration and manipulation of complex tumor microenvironment. In this review, we discuss novel opportunities offered by Cancer-on-Chip models to advance (mechanistic) immuno-oncology research, ranging from design flexibility to multimodal analysis approaches. We then exemplify their (potential) applications for the research and development of adoptive cell therapy, immune checkpoint therapy, cytokine therapy, oncolytic virus, and cancer vaccines.

Advanced in vitro Research Models to Study the Role of Endothelial Cells in Solid Organ Transplantation

The endothelium plays a key role in acute and chronic rejection of solid organ transplants. During both processes the endothelium is damaged often with major consequences for organ function. Also, endothelial cells (EC) have antigen-presenting properties and can in this manner initiate and enhance alloreactive immune responses. For decades, knowledge about these roles of EC have been obtained by studying both in vitro and in vivo models. These experimental models poorly imitate the immune response in patients and might explain why the discovery and development of agents that control EC responses is hampered. In recent years, various innovative human 3D in vitro models mimicking in vivo organ structure and function have been developed. These models will extend the knowledge about the diverse roles of EC in allograft rejection and will hopefully lead to discoveries of new targets that are involved in the interactions between the donor organ EC and the recipient's immune system. Moreover, these models can be used to gain a better insight in the mode of action of the currently prescribed immunosuppression and will enhance the development of novel therapeutics aiming to reduce allograft rejection and prolong graft survival.

Unexpected enhancement of FVIII immunogenicity by endothelial expression in lentivirus-transduced and transgenic mice

Factor VIII (FVIII) replacement therapy for hemophilia A is complicated by development of inhibitory antibodies (inhibitors) in ∼30% of patients. Because endothelial cells (ECs) are the primary physiologic expression site, we probed the therapeutic potential of genetically restoring FVIII expression selectively in ECs in hemophilia A mice (FVIIInull). Expression of FVIII was driven by the Tie2 promoter in the context of lentivirus (LV)-mediated in situ transduction (T2F8LV) or embryonic stem cell-mediated transgenesis (T2F8Tg). Both endothelial expression approaches were associated with a strikingly robust immune response. Following in situ T2F8LV-mediated EC transduction, all FVIIInull mice developed inhibitors but had no detectable plasma FVIII. In the transgenic approach, the T2F8Tg mice had normalized plasma FVIII levels, but showed strong sensitivity to developing an FVIII immune response upon FVIII immunization. A single injection of FVIII with incomplete Freund adjuvant led to high titers of inhibitors and reduction of plasma FVIII to undetectable levels. Because ECs are putative major histocompatibility complex class II (MHCII)-expressing nonhematopoietic, "semiprofessional" antigen-presenting cells (APCs), we asked whether they might directly influence the FVIII immune responses. Imaging and flow cytometric studies confirmed that both murine and human ECs express MHCII and efficiently bind and take up FVIII protein in vitro. Moreover, microvascular ECs preconditioned ex vivo with inflammatory cytokines could functionally present exogenously taken-up FVIII to previously primed CD4+/CXCR5+ T follicular helper (Tfh) cells to drive FVIII-specific proliferation. Our results show an unanticipated immunogenicity of EC-expressed FVIII and suggest a context-dependent role for ECs in the regulation of inhibitors as auxiliary APCs for Tfh cells.

Xenotransplantation tolerance: applications for recent advances in modified swine

PURPOSE OF REVIEW

The aim of this study was to review the recent progress in xenotransplantation achieved through genetic engineering and discuss the potential of tolerance induction to overcome remaining barriers to extended xenograft survival.

RECENT FINDINGS

The success of life-saving allotransplantation has created a demand for organ transplantation that cannot be met by the supply of human organs. Xenotransplantation is one possible solution that would allow for a nearly unlimited supply of organs. Recent genetic engineering of swine has decreased the reactivity of preformed antibodies to some, but not all, potential human recipients. Experiments using genetically modified swine organs have now resulted in survival of life-supporting kidneys for over a year. However, the grafts show evidence of antibody-mediated rejection on histology, suggesting additional measures will be required for further extension of graft survival. Tolerance induction through mixed chimerism or thymic transplantation across xenogeneic barriers would be well suited for patients with a positive crossmatch to genetically modified swine or relatively negative crossmatches to genetically modified swine, respectively.

SUMMARY

This review highlights the current understanding of the immunologic processes in xenotransplantation and describes the development and application of strategies designed to overcome them from the genetic modification of the source animal to the induction of tolerance to xenografts.

The Applicability of Mouse Models to the Study of Human Disease

The laboratory mouse Mus musculus has long been used as a model organism to test hypotheses and treatments related to understanding the mechanisms of disease in humans; however, for these experiments to be relevant, it is important to know the complex ways in which mice are similar to humans and, crucially, the ways in which they differ. In this chapter, an in-depth analysis of these similarities and differences is provided to allow researchers to use mouse models of human disease and primary cells derived from these animal models under the most appropriate and meaningful conditions.

Although there are considerable differences between mice and humans, particularly regarding genetics, physiology, and immunology, a more thorough understanding of these differences and their effects on the function of the whole organism will provide deeper insights into relevant disease mechanisms and potential drug targets for further clinical investigation. Using specific examples of mouse models of human lung disease, i.e., asthma, chronic obstructive pulmonary disease, and pulmonary fibrosis, this chapter explores the most salient features of mouse models of human disease and provides a full assessment of the advantages and limitations of these models, focusing on the relevance of disease induction and their ability to replicate critical features of human disease pathophysiology and response to treatment. The chapter concludes with a discussion on the future of using mice in medical research with regard to ethical and technological considerations.

Humanized mouse models of immunological diseases and precision medicine

With the increase in knowledge resulting from the sequencing of the human genome, the genetic basis for the underlying differences in individuals, their diseases, and how they respond to therapies is starting to be understood. This has formed the foundation for the era of precision medicine in many human diseases that is beginning to be implemented in the clinic, particularly in cancer. However, preclinical testing of therapeutic approaches based on individual biology will need to be validated in animal models prior to translation into patients. Although animal models, particularly murine models, have provided significant information on the basic biology underlying immune responses in various diseases and the response to therapy, murine and human immune systems differ markedly. These fundamental differences may be the underlying reason why many of the positive therapeutic responses observed in mice have not translated directly into the clinic. There is a critical need for preclinical animal models in which human immune responses can be investigated. For this, many investigators are using humanized mice, i.e., immunodeficient mice engrafted with functional human cells, tissues, and immune systems. We will briefly review the history of humanized mice, the remaining limitations, approaches to overcome them and how humanized mouse models are being used as a preclinical bridge in precision medicine for evaluation of human therapies prior to their implementation in the clinic.

Induction of Tolerance Towards Solid Organ Allografts Using Hematopoietic Cell Transplantation in Large Animal Models

Background

The application of hematopoietic cell transplantation for induction of immune tolerance has been limited by toxicities associated with conditioning regimens and to graft-versus-host disease (GVHD). Decades of animal studies have culminated into sufficient control of these two problems, making immune tolerance a viable alternative to life-long application of immunosuppressive drugs to prevent allograft rejection.

Methods

Studies in mice have paved the way for the application of HCT with limited toxicity in large animal models. Resultant studies in the pig, dog, and ultimately the nonhuman primate have led to appropriate methods for achieving nonmyeloablative irradiation protocols, dose, and timing of post-grafting immunosuppressive drugs, monoclonal antibody therapy, and biologicals for costimulatory molecule blockade. The genetics field has been extensively evaluated in appreciation of the ultimate need to obtain organs from MHC-mismatched unrelated donors.

Results

Nonmyeloablative conditioning regimens have been shown to be successful in inducing immune tolerance across all three animal models. Postgrafting immunosuppression is also important in assuring sustained donor hematopoiesis for tolerance. Donor chimerism need not be permanent to establish stable engraftment of donor organs, thereby essentially eliminating the risk of GVHD. Using nonmyeloablative HCT with monoclonal antibody immunosuppression, the kidney has been successfully transplanted in MHC-mismatched nonhuman primates.

Conclusions

Nonmyeloablative HCT for the establishment of temporary mixed chimerism has led to the establishment of stable tolerance against solid organ allografts in large animal models. The kidney, considered a tolerogenic organ, has been successfully transplanted in the clinic. Other organs such as heart, lung, and vascularized composite allografts (face and hands), remain distant possibilities. Further study in large animal models will be required to improve tolerance against these organs before success can be attained in the clinic.

Transplant Tolerance: Current Insights and Strategies for Long-Term Survival of Xenografts

Xenotransplantation is an attractive solution to the problem of allograft shortage. However, transplants across discordant species barriers are subject to vigorous immunologic and pathobiologic hurdles, some of which might be overcome with the induction of immunologic tolerance. Several strategies have been designed to induce tolerance to a xenograft at both the central (including induction of mixed chimerism and thymic transplantation) and peripheral (including adoptive transfer of regulatory cells and blocking T cell costimulation) levels. Currently, xenograft tolerance has been well-established in rodent models, but these protocols have not yet achieved similar success in nonhuman primates. This review will discuss the major barriers that impede the establishment of immunological tolerance across xenogeneic barriers and the potential solution to these challenges, and provide a perspective on the future of the development of novel tolerance-inducing strategies.

T Lymphocyte-Endothelial Interactions: Emerging Understanding of Trafficking and Antigen-Specific Immunity

Antigen-specific immunity requires regulated trafficking of T cells in and out of diverse tissues in order to orchestrate lymphocyte development, immune surveillance, responses, and memory. The endothelium serves as a unique barrier, as well as a sentinel, between the blood and the tissues, and as such it plays an essential locally tuned role in regulating T cell migration and information exchange. While it is well established that chemoattractants and adhesion molecules are major determinants of T cell trafficking, emerging studies have now enumerated a large number of molecular players as well as a range of discrete cellular remodeling activities (e.g., transmigratory cups and invadosome-like protrusions) that participate in directed migration and pathfinding by T cells. In addition to providing trafficking cues, intimate cell-cell interaction between lymphocytes and endothelial cells provide instruction to T cells that influence their activation and differentiation states. Perhaps the most intriguing and underappreciated of these "sentinel" roles is the ability of the endothelium to act as a non-hematopoietic "semiprofessional" antigen-presenting cell. Close contacts between circulating T cells and antigen-presenting endothelium may play unique non-redundant roles in shaping adaptive immune responses within the periphery. A better understanding of the mechanisms directing T cell trafficking and the antigen-presenting role of the endothelium may not only increase our knowledge of the adaptive immune response but also empower the utility of emerging immunomodulatory therapeutics.

Xenotransplantation: immunological hurdles and progress toward tolerance

The discrepancy between organ need and organ availability represents one of the major limitations in the field of transplantation. One possible solution to this problem is xenotransplantation. Research in this field has identified several obstacles that have so far prevented the successful development of clinical xenotransplantation protocols. The main immunologic barriers include strong T-cell and B-cell responses to solid organ and cellular xenografts. In addition, components of the innate immune system can mediate xenograft rejection. Here, we review these immunologic and physiologic barriers and describe some of the strategies that we and others have developed to overcome them. We also describe the development of two strategies to induce tolerance across the xenogeneic barrier, namely thymus transplantation and mixed chimerism, from their inception in rodent models through their current progress in preclinical large animal models. We believe that the addition of further beneficial transgenes to Gal knockout swine, combined with new therapies such as Treg administration, will allow for successful clinical application of xenotransplantation.

Polarized granzyme release is required for antigen-driven transendothelial migration of human effector memory CD4 T cells

Human effector memory CD4 T cells may transmigrate across endothelial cell (EC) monolayers either in response to inflammatory chemokines or in response to TCR recognition of Ag presented on the surface of the EC. The kinetics, morphologic manifestations, and molecular requirements of chemokine- and TCR-driven transendothelial migration (TEM) differ significantly. In this study, we report that, whereas the microtubule organizing center (MTOC) and cytosolic granules follow the nucleus across the endothelium in a uropod during chemokine-driven TEM, MTOC reorientation to the contact region between the T cell and the EC, accompanied by dynein-driven transport of granzyme-containing granules to and exocytosis at the contact region, are early events in TCR-driven, but not chemokine-driven TEM. Inhibitors of either granule function or granzyme proteolytic activity can arrest TCR-driven TEM, implying a requirement for granule discharge in the process. In the final stages of TCR-driven TEM, the MTOC precedes, rather than follows, the nucleus across the endothelium. Thus, TCR-driven TEM of effector memory CD4 T cells appears to be a novel process that more closely resembles immune synapse formation than it does conventional chemotaxis.

Immunological challenges and therapies in xenotransplantation

Xenotransplantation, or the transplantation of cells, tissues, or organs between different species, was proposed a long time ago as a possible solution to the worldwide shortage of human organs and tissues for transplantation. In this setting, the pig is currently seen as the most likely candidate species. In the last decade, progress in this field has been remarkable and includes a better insight into the immunological mechanisms underlying the rejection process. Several immunological hurdles nonetheless remain, such as the strong antibody-mediated and innate or adaptive cellular immune responses linked to coagulation derangements, precluding indefinite xenograft survival. This article reviews our current understanding of the immunological mechanisms involved in xenograft rejection and the potential strategies that may enable xenotransplantation to become a clinical reality in the not-too-distant future.

Human dominant-negative class II transactivator transgenic pigs - effect on the human anti-pig T-cell immune response and immune status

Swine leucocyte antigen (SLA) class II molecules on porcine (p) cells play a crucial role in xenotransplantation as activators of recipient human CD4(+) T cells. A human dominant-negative mutant class II transactivator (CIITA-DN) transgene under a CAG promoter with an endothelium-specific Tie2 enhancer was constructed. CIITA-DN transgenic pigs were produced by nuclear transfer/embryo transfer. CIITA-DN pig cells were evaluated for expression of SLA class II with/without activation, and the human CD4(+) T-cell response to cells from CIITA-DN and wild-type (WT) pigs was compared. Lymphocyte subset numbers and T-cell function in CIITA-DN pigs were compared with those in WT pigs. The expression of SLA class II on antigen-presenting cells from CIITA-DN pigs was significantly reduced (40-50% reduction compared with WT; P < 0·01), and was completely suppressed on aortic endothelial cells (AECs) even after activation (100% suppression; P < 0·01). The human CD4(+) T-cell response to CIITA-DN pAECs was significantly weaker than to WT pAECs (60-80% suppression; P < 0·01). Although there was a significantly lower frequency of CD4(+) cells in the PBMCs from CIITA-DN (20%) than from WT (30%) pigs (P < 0·01), T-cell proliferation was similar, suggesting no significant immunological compromise. Organs and cells from CIITA-DN pigs should be partially protected from the human cellular immune response.

Experimental glomerular endothelial injury in vivo

The microvascular endothelium of the kidney glomerulus is injured in Shiga-like toxigenic bacterial infection, genetic or acquired loss of complement regulatory protein function, and allo-immune responses of solid-organ or bone marrow transplantation. Existing models of diseases with glomerular endothelial cell (EC) injury, collectively grouped as thrombotic microangiopathies, are problematic, impeding investigation of the mechanisms of microvascular defense and repair. To develop a model of glomerular endothelial injury in the mouse, we conjugated the M. oreades lectin to the cytotoxin, saporin, (LS) to selectively injure the glomerular endothelium. Injury of the microvasculature was evaluated by light, immunofluorescence, and electron microscopy, and by quantitative RT-PCR of cell-type specific transcripts. Renal function was evaluated by quantitation of serum creatinine. The toxin conjugate induced apoptosis of microvascular ECs in vitro, and subtle histologic features of thrombotic microangiopathy in vivo that were enhanced by co-injection of 50 μg/kg LPS. Among LS/LPS-treated animals, loss of glomerular EC staining correlated with decreased expression of EC-specific transcripts, and impaired kidney function. Selective injury of the glomerular microvasculature with LS toxin conjugate and LPS elicits histologic features of thrombotic microangiopathy and acute kidney failure.

Primate models in organ transplantation

Large animal models have long served as the proving grounds for advances in transplantation, bridging the gap between inbred mouse experimentation and human clinical trials. Although a variety of species have been and continue to be used, the emergence of highly targeted biologic- and antibody-based therapies has required models to have a high degree of homology with humans. Thus, the nonhuman primate has become the model of choice in many settings. This article will provide an overview of nonhuman primate models of transplantation. Issues of primate genetics and care will be introduced, and a brief overview of technical aspects for various transplant models will be discussed. Finally, several prominent immunosuppressive and tolerance strategies used in primates will be reviewed.

Potential of T-regulatory cells to protect xenografts

PURPOSE OF REVIEW

Immunological barriers still preclude clinical xenotransplantation. The protective role of CD4(+)CD25(+)Foxp3(+) T-regulatory cells (Treg) in allotransplantation is well described and, therefore, could represent a promising therapeutical tool for xenotransplantation. This review addresses the latest findings on Treg in xenotransplantation research.

RECENT FINDINGS

In vivo, costimulation blockade-based strategies including anti-CD154 monoclonal antibodies (mAbs) in combination with rapamycin or anti-LFA-1 mAb prolonged both concordant and discordant islets xenografts survival in a Treg-dependent manner. In vitro, IL-10 secretion was shown to be critical for the suppression of xenogeneic responses mediated by Treg. Moreover, transgenic expression of inducible costimulator-immunoglobulin or PD-L1 on porcine endothelial cells inhibited human T-cell proliferation in vitro and was associated with the induction of Treg and IL-10 secretion. CXCR3 mediated the recruitment of Treg to pig endothelium. Finally, the recruitment of human Treg was enhanced by the immobilization of human CCL17 on pig endothelium.

SUMMARY

There is increasing evidence for the potential of CD4(+)CD25(+)Foxp3(+) Treg to protect xenografts. Induction of Treg in recipients and/or recruitment of human Treg to pig endothelium may represent novel strategies to prevent cell-mediated rejection in pig-to-human xenotransplantation.

Role of the retinal vascular endothelial cell in ocular disease

Retinal endothelial cells line the arborizing microvasculature that supplies and drains the neural retina. The anatomical and physiological characteristics of these endothelial cells are consistent with nutritional requirements and protection of a tissue critical to vision. On the one hand, the endothelium must ensure the supply of oxygen and other nutrients to the metabolically active retina, and allow access to circulating cells that maintain the vasculature or survey the retina for the presence of potential pathogens. On the other hand, the endothelium contributes to the blood-retinal barrier that protects the retina by excluding circulating molecular toxins, microorganisms, and pro-inflammatory leukocytes. Features required to fulfill these functions may also predispose to disease processes, such as retinal vascular leakage and neovascularization, and trafficking of microbes and inflammatory cells. Thus, the retinal endothelial cell is a key participant in retinal ischemic vasculopathies that include diabetic retinopathy and retinopathy of prematurity, and retinal inflammation or infection, as occurs in posterior uveitis. Using gene expression and proteomic profiling, it has been possible to explore the molecular phenotype of the human retinal endothelial cell and contribute to understanding of the pathogenesis of these diseases. In addition to providing support for the involvement of well-characterized endothelial molecules, profiling has the power to identify new players in retinal pathologies. Findings may have implications for the design of new biological therapies. Additional progress in this field is anticipated as other technologies, including epigenetic profiling methods, whole transcriptome shotgun sequencing, and metabolomics, are used to study the human retinal endothelial cell.

Attempted depletion of passenger leukocytes by irradiation in pigs

Allograft/xenograft rejection is associated with “passenger leukocyte” migration from the organ into recipient lymph nodes. In Study 1, we attempted to deplete leukocytes from potential kidney “donor” pigs, using two regimens of total body irradiation. A dose of 700 cGy was administered, followed by either 800 cGy (“low-dose”) or 1,300 cGy (“high dose”) with the kidneys shielded. Neither regimen was entirely successful in depleting all leukocytes, although remaining T and 8 cell numbers were negligible. Study 2 was aimed at providing an indication of whether near-complete depletion of leukocytes had any major impact on kidney allograft survival. In non-immunosuppressed recipient pigs, survival of a kidney from a donor that received high-dose irradiation was compared with that of a kidney taken from a non-irradiated donor. Kidney graft survival was 9 and 7 days, respectively, suggesting that depletion had little impact on graft survival. The lack of effect may have been related to (i) inadequate depletion of passenger leukocytes, thus not preventing a direct T cell response, (ii) the presence of dead or dying leukocytes (antigens), thus not preventing an indirect T cell response, or (iii) constitutive expression of MHC class II and B7 molecules on the porcine vascular endothelium, activating recipient T cells.

Antibody-mediated rejection: emergence of animal models to answer clinical questions

Decades of experiments in small animals had tipped the balance of opinion away from antibodies as a cause of transplant rejection. However, clinical experience, especially with sensitized patients, has convinced basic immunologists of the need to develop models to investigate mechanisms underlying antibody-mediated rejection (AMR). This resurgent interest has resulted in several new rodent models to investigate antibody-mediated mechanisms of heart and renal allograft injury, but satisfactory models of chronic AMR remain more elusive. Nevertheless, these new studies have begun to reveal many insights into the molecular and pathological sequelae of antibody binding to the allograft endothelium. In addition, complement-independent and complement-dependent effects of antibodies on endothelial cells have been identified in vitro. As small animal models become better defined, it is anticipated that they will be more widely used to answer further questions concerning mechanisms of antibody-mediated tissue injury as well as to design therapeutic interventions.

Leucocyte adhesion under haemodynamic flow conditions

Vascular endothelial cells (EC) line the luminal side of all blood vessels and act as a selective barrier between blood and tissue. EC are constantly exposed to biochemical and biomechanical stimuli from the blood and underlying tissue. Fluid shear stress acts in parallel to the vessel wall, resulting from friction of blood against EC. Despite the importance of flow on normal EC function, much of the information regarding EC function and dysfunction has been derived from cells harvested, grown and studied in static culture. In order to study the effects of shear stress on EC function, a number of in vitro models have been developed. This chapter provides methodology for use of a system which enables recirculation of leucocytes and cell culture medium over the endothelium for a period of several minutes to days and enables investigation of the effects of prolonged leucocyte co-culture on both the endothelial and leucocyte populations.

Low immunogenicity of endothelial derivatives from rat embryonic stem cell-like cells

Embryonic stem cells (ESC) are suggested to be immune-privileged, but they carry the risk of uncontrolled expansion and malignancy. Upon differentiation they lose their tumor-forming capacity, but they become immunogenic by the expression of a normal set of MHC molecules. This immunogenicity might trigger rejection after application in regenerative therapies. In this study MHC expression of and immune responses to endothelial derivatives of rat embryonic stem cell-like cells (RESC) under inflammatory conditions were determined in comparison to primary rat aortic endothelial cells (ECs). Cellular as well as humoral allo-recognition was analyzed in vitro. In addition, immune reactions in vivo were assessed by allo-antibody production and determination of interferon-gamma (IFNgamma)-secreting allo-reactive T cells. RESC derivatives expressed low but significant levels of MHC class I, and no MHC class II. In response to IFNgamma MHC class I expression was enhanced, while class II transactivator induction failed completely in these cells; MHC class II expression remained consistently absent. Functionally, the RESC derivatives showed a reduced allo-stimulatory capacity, protection against humoral allo-recognition in vitro and a slightly diminished susceptibility to cytotoxic T cell lysis. Furthermore, in vivo experiments demonstrated that these cells do not trigger host immune reactions, characterized by no allo-antibody production and no induction of allo-reactive memory T cells. Our results show that endothelial derivatives of RESC have a distinctive reduced immunogenic potency even under inflammatory conditions.

In vivo observations of cell trafficking in allotransplanted vascularized skin flaps and conventional skin grafts

The problem of allogeneic skin rejection is a major limitation to more widespread application of clinical composite tissue allotransplantation (CTA). Previous research examining skin rejection has mainly studied rejection of conventional skin grafts (CSG) using standard histological techniques. The aim of this study was to objectively assess if there were differences in the immune response to CSG and primarily vascularized skin in composite tissue allotransplants (SCTT) using in vivo techniques in order to gain new insights in to the immune response to skin allotransplants. CSG and SCTT were transplanted from standard Lewis (LEW) ad Wistar Furth (WF) to recipient transgenic green fluorescent Lewis rats (LEW-GFP). In vivo confocal microscopy was used to observe cell trafficking within skin of the transplants. In addition, immunohistochemical staining was performed on skin biopsies to reveal possible expression of class II major histocompatibility antigens. A difference was observed in the immune response to SCTT compared to CSG. SCTT had a greater density cellular infiltrate than CSG (p<0.03) that was focused more at the center of the transplant (p<0.05) than at the edges, likely due to the immediate vascularization of the skin. Recipient dendritic cells were only observed in rejecting SCTT, not CSG. Furthermore, dermal endothelial class II MHC expression was only observed in allogeneic SCTT. The immune response in both SCTT and CSG was focused on targets in the dermis, with infiltrating cells clustering around hair follicles (CSG and SCTT; p<0.01) and blood vessels (SCTT; p<0.01) in allogeneic transplants. This study suggests that there are significant differences between rejection of SCTT and CSG that may limit the relevance of much of the historical data on skin graft rejection when applied to composite tissue allotransplantation. Furthermore, the use of novel in vivo techniques identified characteristics of the immune response to allograft skin not previously described, which may be useful in directing future approaches to overcoming allograft skin rejection.

Antigen presentation by human microvascular endothelial cells triggers ICAM-1-dependent transendothelial protrusion by, and fractalkine-dependent transendothelial migration of, effector memory CD4+ T cells

TCR engagement on adherent human effector memory CD4(+) T cells by TNF-treated HUVECs under flow induces formation of a transendothelial protrusion (TEP) by the T cell but fails to induce transendothelial migration (TEM). In contrast, TCR engagement of the same T cell populations by TNF-treated human dermal microvascular cells (HDMEC) not only induces TEP formation, but triggers TEM at or near the interendothelial cell junctions via a process in which TEP formation appears to be the first step. Transduction of adhesion molecules in unactivated HDMEC and use of blocking Abs as conducted with TNF-activated HDMEC indicate that ICAM-1 plays a nonredundant role in TCR-driven TEP formation and TEM, and that TCR-driven TEM is also dependent upon fractalkine. TEP formation, dependence on ICAM-1, and dependence on fractalkine distinguish TCR-induced TEM from IP-10-induced TEM. These in vitro observations suggest that presentation of Ag by human microvascular endothelial cells to circulating CD4(+) effector memory T cells may function to initiate recall responses in peripheral tissues.

Location and time-dependent control of rejection by regulatory T cells culminates in a failure to generate memory T cells

Adaptive CD25(+)CD4(+) regulatory T cells (Treg) can be induced following exposure to alloantigen and may function alongside naturally occurring Treg to suppress allograft rejection when present in sufficient numbers. However, the location of the Treg as they function in vivo and the mechanisms used to control donor-reactive T cells remains ill-defined. In this study, we used a CD8(+) TCR transgenic model of skin allograft rejection to characterize in vivo activity of donor-reactive Treg cells during induction of transplantation tolerance. We demonstrate that, initially after skin transplantation, Treg attenuate the priming of donor-reactive naive CD8(+) T cells in the lymphoid tissue draining the graft site. However, with time, peripheral suppression is overcome despite the continued presence of Treg, resulting in the priming of donor-reactive CD8(+) T cells and graft infiltration by the resultant effector T cells and induction of a "Tc1-like" intragraft gene expression profile. These intragraft effector CD8(+) T cells are then prevented from eliciting rejection by Treg that simultaneously infiltrate the skin allografts, resulting in a failure to generate donor-reactive memory CD8(+) T cells. Overall, these data demonstrate for the first time that donor-reactive Treg can suppress allograft rejection using distinct mechanisms at different sites in vivo with the overall outcome of preventing the generation of donor-reactive memory T cells.

Thrombotic microangiopathy associated with humoral rejection of cardiac xenografts from alpha1,3-galactosyltransferase gene-knockout pigs in baboons

Heterotopic cardiac xenotransplantation from alpha1,3-galactosyltransferase gene-knockout (GalT-KO) swine to baboons was performed to characterize immunological reaction to the xenograft in the absence of anti-Gal antibody-mediated rejection. Eight baboons received heterotopic cardiac xenografts from GalT-KO porcine donors. All baboons were treated with chronic immunosuppressive therapy. Both histological and immunohistochemical studies were performed on biopsy and graftectomy samples. No hyperacute rejection was observed. Three baboons were euthanized or died 16 to 56 days after transplantation. The other five grafts ceased beating between days 59 and 179 (median, 78 days). All failing grafts exhibited thrombotic microangiopathy (TM) with platelet-rich fibrin thrombi in the microvasculature, myocardial ischemia and necrosis, and focal interstitial hemorrhage. TM developed in parallel with increases in immunoglobulin (IgM and IgG) and complement (C3, C4d, and C5b-9) deposition, as well as with subsequent increases in both TUNEL(+) endothelial cell death and procoagulant activation (increased expression of both tissue factor and von Willebrand factor and decreased expression of CD39). CD3(+) T-cell infiltration occurred in all grafts and weakly correlated with the development of TM. In conclusion, although the use of GalT-KO swine donors prevented hyperacute rejection and prolonged graft survival, slowly progressive humoral rejection--probably associated with non-Gal antibodies to the xenograft--and disordered thromboregulation represent major immunological barriers to long-term xenograft survival.

A heterotopic primate model for facial composite tissue transplantation

The purpose of this study was to develop a nonhuman primate model for heterotopic composite tissue facial transplantation in which to study the natural history of facial transplantation and evaluate immunosuppressive regimens.A composite oromandibular facial segment transplant based on the common carotid artery was evaluated. Flaps from 7 cynomolgus monkeys were transplanted to the groins of 7 recipients at the superficial femoral artery and vein. The immunosuppressive regimen consisted of thymoglobulin, rapamycin, and tacrolimus. Allograft survival ranged from 6 to 129 days. Histology performed in the long-term survivor at the time of necropsy revealed extensive inflammation and necrosis of the allograft skin; however, muscle and bone elements were viable, with minimal inflammation. This heterotopic facial transplantation model avoids the potential morbidity of mandibular resection and orthotopic facial transplantation. Our work also concurs with the work of other groups who found that the skin component is the most antigenic.

The effects of tolerance on allograft damage caused by the innate immune system

BACKGROUND

It is not known whether tolerance can be induced in a strong proinflammatory milieu or whether the induction of tolerance can prevent interferon (IFN)-gamma-associated graft injury. To address these questions, we studied the effects of rIFN-gamma infusion on porcine cardiac allograft survival.

METHODS

Recombinant interferon (rIFN)-gamma was continuously infused into the left anterior descending artery of hearts transplanted into major histocompatibility complex-inbred miniature swine treated with a 12-day course of cyclosporine A. Group 1 recipients received a nearly syngeneic heart, group 2 recipients received a class I disparate heart, and group 3 recipients were cotransplanted with a class I-disparate heart and kidney, a procedure demonstrated to induce tolerance to both grafts. A fourth group of animals were not transplanted but received intracoronary rIFN-gamma infusion into the native heart.

RESULTS

rIFN-gamma perfusion not only accelerated the acute rejection of class I-disparate hearts (mean survival time, 19+/-7.21 vs. 38+/-8.19; P=0.025) but caused near-syngeneic heart transplants, which otherwise survived indefinitely, to reject within 35 days. In contrast, rIFN-gamma perfusion had no demonstrable effects on hearts grafts in tolerant recipients or on autologous hearts.

CONCLUSIONS

These results suggest that tolerance induction can occur in the presence of IFN-gamma-mediated inflammation, and that tolerance induction can prevent the tissue injury caused by the overproduction of IFN-gamma. This suggests that the beneficial effects of tolerance may include protection from nonspecific inflammatory responses, such as those produced by ischemia-reperfusion injury and brain death.

In Vivo Suppression of Major Histocompatibility Complex Class II Expression on Porcine Vascular Endothelial Cells by an HMG-CoA Reductase Inhibitor

BACKGROUND

Vascular endothelial cells of man and pig, but not rodents, strongly express major histocompatibility complex (MHC) class II antigens in vivo, probably via the inducible promoter IV of the class II transactivator. There is abundant in vitro evidence that MHC class II positive vascular endothelial cells can activate T cells. Peripheral antigen presentation by endothelial cells is potentially important for organ-specific immunity, for allograft rejection, and possibly for immune responsiveness in general. Given the reported effects of statins on promoter IV of the class II transactivator, we evaluated in vivo expression of MHC class II antigens in pigs treated with atorvastatin calcium.

METHODS

Pigs were given 3 mg/kg/day of atorvastatin orally daily for 16 days, and then killed 24 hr after the last dose. Heart, kidney, and liver were removed for immunohistological and quantitative absorption analysis.

RESULTS

Double-labeling studies using immunofluorescence on frozen section for Factor VIII and MHC class II showed a marked suppression of MHC class II on vascular endothelial cells in all 4 treated pigs, in comparison with untreated pigs. This was confirmed using immunoperoxidase techniques on frozen sections. Quantitative absorption analysis showed up to 25-fold reduction in MHC class II expression.

CONCLUSIONS

Statins substantially suppress endothelial cell MHC class II expression in vivo. This is likely to inhibit organ-specific immune responses, and possibly also general immune responsiveness. In a transplantation setting, in addition to other regulatory effects on the recipients immune system, statins might reduce the long-term capacity of the donor organ to activate rejection mechanisms.

Effects of Mycophenolate Mofetil on Cardiac Allograft Survival and Cardiac Allograft Vasculopathy in Miniature Swine

BACKGROUND

Chronic rejection, as manifested by cardiac allograft vasculopathy, remains the leading cause of late graft failure in heart transplant recipients. Despite recent clinical trials, the efficacy of mycophenolate mofetil in preventing human cardiac allograft vasculopathy remains controversial. We investigated whether mycophenolate mofetil would prevent cardiac allograft vasculopathy and prolong cardiac allograft survival in our well-established miniature swine model of heart transplantation.

METHODS

Hearts disparate at the major histocompatibility complex class I locus were heterotopically transplanted into miniature swine recipients treated with a 12-day course of mycophenolate mofetil (n = 3) or cyclosporine A (n = 3). Allograft survival, acute rejection, and chronic rejection were monitored in the two groups.

RESULTS

Hearts transplanted with 12 days of cyclosporine were rejected between 46 and 61 days, whereas two of the three hearts transplanted with mycophenolate mofetil remained beating beyond 120 days (p = 0.02). At necropsy, there was a 4.9% mean prevalence of cardiac allograft vasculopathy in the mycophenolate mofetil group as compared with 16.6% in the cyclosporine group (p = 0.03). Cardiac allograft rejection and vasculopathy in the cyclosporine-treated group was associated with prominent myocardial interferon-gamma gene expression, a finding absent in two thirds of the mycophenolate mofetil-treated swine. Moreover, the mycophenolate mofetil-treated swine failed to develop IgM or IgG alloantibodies.

CONCLUSIONS

A short course of mycophenolate mofetil resulted in a longer allograft survival than a similar course of cyclosporine. Moreover, mycophenolate mofetil reduced the prevalence of cardiac allograft vasculopathy as compared with cyclosporine-treated controls. The salutary effect of mycophenolate mofetil may be related to its ability to decrease interferon-gamma expression in the myocardium and prevent the generation of alloantibodies.

CD26 mediates dissociation of Tollip and IRAK-1 from caveolin-1 and induces upregulation of CD86 on antigen-presenting cells

CD26 is a T-cell costimulatory molecule with dipeptidyl peptidase IV enzyme activity in its extracellular region. We have previously reported that the addition of recombinant soluble CD26 resulted in enhanced proliferation of human T lymphocytes induced by the recall antigen tetanus toxoid (TT) via upregulation of CD86 on monocytes and that caveolin-1 was a binding protein of CD26, and the CD26-caveolin-1 interaction resulted in caveolin-1 phosphorylation (p-cav-1) as well as TT-mediated T-cell proliferation. However, the mechanism involved in this immune enhancement has not yet been elucidated. In the present work, we perform experiments to identify the molecular mechanisms by which p-cav-1 leads directly to the upregulation of CD86. Through proteomic analysis, we identify Tollip (Toll-interacting protein) and IRAK-1 (interleukin-1 receptor-associated serine/threonine kinase 1) as caveolin-1-interacting proteins in monocytes. We also demonstrate that following stimulation by exogenous CD26, Tollip and IRAK-1 dissociate from caveolin-1, and IRAK-1 is then phosphorylated in the cytosol, leading to the upregulation of CD86 via activation of NF-kappaB. Binding of CD26 to caveolin-1 therefore regulates signaling pathways in antigen-presenting cells to induce antigen-specific T-cell proliferation.

Advances in strategies for inducing central tolerance in organ allograft recipients

Encouraging results in large animal models and from the clinic have been reported recently suggesting that the deliberate induction of transplantation tolerance using central deletional protocols may be closer to becoming a reality. The induction of central tolerance would be especially applicable to pediatric organ transplant recipients. In this review, we discuss three promising protocols of central tolerance induction and why they are relevant to pediatric organ transplantation.

Early T cell response to allografts occurring prior to alloantigen priming up-regulates innate-mediated inflammation and graft necrosis

The early inflammatory response within organ allografts is initiated by ischemia/reperfusion (I/R) and promotes subsequent alloantigen-primed T cell recruitment into and rejection of the graft. Polymorphonuclear leukocyte (PMN)-mediated tissue damage is a primary component of the early inflammation in allograft rejection. We sought to compare and elucidate the mechanism of early PMN infiltration into cardiac isografts and allografts. Despite identical production of PMN attractant chemokines, PMN infiltration following reperfusion into syngeneic and allogeneic grafts was not equivalent. PMN infiltration into isografts peaked at 9 to 12 hours post-transplant and quickly resolved. In contrast, PMN infiltration into allografts continued to elevated levels, peaking at 24 hours post-reperfusion. This amplified PMN infiltration into allografts did not resolve until 72 hours post-reperfusion and was accompanied by marked parenchymal necrosis. This early innate inflammatory response was regulated by IFN-gamma-producing CD8+ T cells present in the recipient before detectable alloantigen T cell priming. Co-culture with CD62L(low) CD8+ T cells, but not CD62L(high) CD8+ or CD62L(low) CD4+ T cells, harvested from naïve animals induced allogeneic endothelial cells to express IFN-gamma-dependent chemokines. These data demonstrate CD8+ T cell-mediated attack on the vascular endothelium of allografts within hours following organ reperfusion that amplifies innate immune-mediated intra-graft inflammation and necrosis.

T lymphocyte-endothelial cell interactions

Human vascular endothelial cells (EC) basally display class I and II MHC-peptide complexes on their surface and come in regular contact with circulating T cells. We propose that EC present microbial antigens to memory T cells as a mechanism of immune surveillance. Activated T cells, in turn, provide both soluble and contact-dependent signals to modulate normal EC functions, including formation and remodeling of blood vessels, regulation of blood flow, regulation of blood fluidity, maintenance of permselectivity, recruitment of inflammatory leukocytes, and antigen presentation leading to activation of T cells. T cell interactions with vascular EC are thus bidirectional and link the immune and circulatory systems.

Of mice and not men: differences between mouse and human immunology

Mice are the experimental tool of choice for the majority of immunologists and the study of their immune responses has yielded tremendous insight into the workings of the human immune system. However, as 65 million years of evolution might suggest, there are significant differences. Here we outline known discrepancies in both innate and adaptive immunity, including: balance of leukocyte subsets, defensins, Toll receptors, inducible NO synthase, the NK inhibitory receptor families Ly49 and KIR, FcR, Ig subsets, the B cell (BLNK, Btk, and lambda5) and T cell (ZAP70 and common gamma-chain) signaling pathway components, Thy-1, gammadelta T cells, cytokines and cytokine receptors, Th1/Th2 differentiation, costimulatory molecule expression and function, Ag-presenting function of endothelial cells, and chemokine and chemokine receptor expression. We also provide examples, such as multiple sclerosis and delayed-type hypersensitivity, where complex multicomponent processes differ. Such differences should be taken into account when using mice as preclinical models of human disease.

Constitutive expression of major histocompatibility complex class II antigens in pulmonary epithelium and endothelium varies among different species

We have observed high constitutive levels of class II antigen expression on porcine and human coronary endothelium, but not on the endothelium of rats and mice. This study examines whether a similar interspecies difference exists in the expression of class II molecules on pulmonary epithelium and endothelium. Lung tissues from naïve human, porcine, and rodent sources were stained with the monoclonal antibody ISCR3 and examined by light microscopy. Immunoperoxidase staining of class II molecules was observed on human and porcine pulmonary epithelium and endothelium, but was absent in rats and mice. By using an antibody with cross-species reactivity, we demonstrated that naïve swine pulmonary epithelium and endothelium, unlike those of rodent species, express basal levels of class II antigens in a manner similar to that observed in human lung tissue. These interspecies differences may explain experimental differences observed between murine and large-animal constructs.

Histomorphometric comparison of cardiac allograft vasculopathy in miniature swine

BACKGROUND

Whether the pathologic characteristics of vascular lesions manifested in recipients with cardiac allograft vasculopathy (CAV) differ with the severity of the histocompatibility barrier crossed at transplantation or with the type or amount of immunosuppression used to prolong graft survival is unclear. We used miniature swine to determine whether a wide variance in heart transplantation protocols, both in histoincompatibility and immunosuppression, affects the histomorphometry of CAV.

METHODS

We compared explanted hearts from major histocompatibility complex Class I-disparate recipients who were treated for 12 days with cyclosporine (Group 1) with minor-antigen-disparate hearts transplanted into mixed chimeric recipients previously engrafted with donor hematopoietic progenitor cells (Group 2). We analyzed coronary intimal lesions using computerized morphometry, immunohistochemistry, and TUNEL assay. Myocardial cytokine-gene expression was determined using RNAse protection assays and reverse-transcriptase polymerase chain reaction.

RESULTS

The prevalence of CAV in Group 2 was significantly less than that observed in Group 1, but the severity of the lesions in both groups was similar. The vascular lesions that developed in both groups demonstrated the presence of alpha-smooth-muscle-actin-positive spindle cells expanding the intima, with few inflammatory cells. We noted an absence of proliferating cell nuclear antigen activity and TUNEL-positive cells in both groups. We observed prominent myocardial interferon-gamma gene expression only in Group 1.

CONCLUSION

Despite differences in myocardial interferon-gamma gene expression, the histology and severity of the vascular lesions in CAV did not vary significantly with different histoincompatibilities or treatment protocols. These results suggest that the origin of CAV cannot be determined by histology alone.

Hypercholesterolemia-Induced Long-Term Increase of Macrophages in the Myocardium of New Zealand White Rabbits

The effect of hypercholesterolemia on the number, immunological phenotype and oxidative stress-dependent processes of macrophages (MPhi) and dendritic cells (DC) was studied in New Zealand White rabbits. The left ventricular myocardium was immunohistochemically analyzed in group I (control), which was on standard chow, and groups II and III, which both received a 0.5% cholesterol-enriched diet for 96 days, but thereafter, only group III was fed standard chow for 4 months. In the myocardial interstitium of group I, (1) significantly less RAM-11-immunoreactive (ir) MPhi than S-100-ir DC were found; (2) both, MPhi and DC, were similar major histocompatibility complex (MHC) class II molecules LN3-, ISCR3-, and 2.06-ir; (3) all MPhi and most DC were manganese superoxide dismutase (MnSOD)-ir and homing receptor CD44-ir. In group II, only MPhi increased about 10-fold in the myocardium in parallel to the about 40-fold increase of the serum cholesterol levels. In group III, the elevated serum cholesterol levels significantly decreased (about 90%), while the MPhi still remained significantly increased (about 8-fold). The cellular immunoreactivities of MHC class II molecules, as well as MnSOD and CD44 did not change in groups II and III in comparison to group I. We suggest that mainly the MPhi, which increase within the myocardium of rabbits after elevation of serum cholesterol levels and remain significantly increased for a long time after decrease of the blood lipid levels, might initiate or aggravate eventual complications such as coronary atherosclerosis and myocardial fibrosis.

Tolerance: of mice and men

Little is known about the effect of an individual's immune history on his or her response to an allogeneic tissue transplant. An important study now reveals that individuals harboring virally-induced memory T cells that are cross reactive with donor alloantigen are resistant to conventional strategies designed to induce transplant tolerance.

T-cell mediated induction of allogeneic endothelial cell chemokine expression

BACKGROUND

The goal of the current study was to test the ability of T cells to stimulate allogeneic endothelial cells to express chemokines, particularly the T-cell recruiting factors monokine induced by interferon-gamma (Mig) and inducible protein (IP)-10.

METHODS

Lymph node cells from C57BL/6 (H-2b) recipients of C3H (H-2k) skin grafts or from naïve mice were added to monolayers of C3H-derived endothelial cell line 2F-2B. After 5 or 24 hr, the lymph node cells were removed, and RNA was prepared from the endothelial cells and tested by ribonuclease protection assay or Northern blot hybridization for endothelial cell expression of chemokines.

RESULTS

Alloantigen-primed T cells induced endothelial cell expression of regulated on activation normal T-cell expressed and secreted (RANTES), IP-10, Mig, monocyte chemotactic protein-1, macrophage inflammatory protein-1alpha, and macrophage inflammatory protein-1beta within 5 hr of coculture. In vitro chemotaxis assays demonstrated the production of T-cell chemoattractants by the endothelial cells. With the exception of low levels of monocyte chemotactic protein-1 and RANTES, culture with naïve C57BL/6 lymph node T cells did not induce endothelial cell chemokine expression. Alloantigen-primed CD4 T cells induced endothelial expression of IP-10 and RANTES but none of the other chemokines tested, whereas primed CD8 T cells induced all of the chemokines tested. Expression of IP-10 and Mig was not induced when alloantigen-primed T cells from interferon-gamma deficient recipients of C3H skin grafts were cultured with the endothelial cells. This expression was blocked by addition of intercellular adhesion molecule-1 or lymphocyte function-associated antigen-1 specific antibodies to the cultures. CONCLUSIONS These results demonstrate the ability of alloantigen-primed CD8 T cells to quickly and directly stimulate endothelial cells to express and produce chemokines, including those recruiting T cells.

Mouse vascular endothelium activates CD8+ T lymphocytes in a B7-dependent fashion

Despite several studies examining the contribution of allorecognition pathways to acute and chronic rejection of vascularized murine allografts, little data describing activation of alloreactive T cells by mouse vascular endothelium exist. We have used primary cultures of resting or IFN-gamma-activated C57BL/6 (H-2(b)) vascular endothelial cells as stimulators and CD8(+) T lymphocytes isolated from CBA/J (H-2(k)) mice as responders. Resting endothelium expressed low levels of MHC class I, which was markedly up-regulated after activation with IFN-gamma. It also expressed moderate levels of CD80 at a resting state and after activation. Both resting and activated endothelium were able to induce proliferation of unprimed CD8(+) T lymphocytes, with proliferation noted at earlier time points after coculture with activated endothelium. Activated endothelium was also able to induce proliferation of CD44(low) naive CD8(+) T lymphocytes. Activated CD8(+) T lymphocytes had the ability to produce IFN-gamma and IL-2, acquired an effector phenotype, and showed up-regulation of the antiapoptotic protein Bcl-x(L). Treatment with CTLA4-Ig led to marked reduction of T cell proliferation and a decrease in expression of Bcl-x(L). Moreover, we demonstrate that nonhemopoietic cells such as vascular endothelium induce proliferation of CD8(+) T lymphocytes in a B7-dependent fashion in vivo. These results suggest that vascular endothelium can act as an APC for CD8(+) direct allorecognition and may, therefore, play an important role in regulating immune processes of allograft rejection.

Autologous graft-versus-host disease in a porcine bone marrow transplant model

BACKGROUND

Autologous graft-versus-host disease (autoGvHD) has been reported in patients and can be induced in rodents by syngeneic bone marrow transplantation (BMT) and a brief administration of cyclosporine A (CsA). To our knowledge, there is no previous large-animal model for this phenomenon, nor is there a model in which autoGvHD occurs spontaneously after autologous bone marrow transplant (autoBMT) in the absence of CsA induction. During our studies of autoBMT in miniature swine, performed without CsA treatment, we noted the frequent occurrence of a rash consistent with autoGvHD. We hypothesized that the extent of peripheral blood contamination of the bone marrow (BM) inoculum before transplant may have correlated with the incidence of such autoGvHD.

METHODS

Retrospective analysis of the prevalence of autoGvHD in swine was carried out in all animals that had become engrafted after autoBMT in our laboratory. Subsequent prospective experiments attempted to induce autoGvHD by transplanting autologous BM enriched with autologous peripheral blood into lethally irradiated animals.

RESULTS

Our data showed that autoGvHD frequently occurs in swine after autoBMT, with the most severe cases of the disease occurring in animals with the highest levels of peripheral blood contamination of the BM inoculum. Furthermore, mixed lymphocyte reactions (MLR) against self antigens were positive only in animals affected by autoGvHD.

CONCLUSION

These findings provide the first evidence for autoGvHD without the use of CsA in a preclinical BMT model. The role of autologous T cells needs further delineation but may help to explain the occasional occurrences of autoGvHD that have been reported in humans after autoBMT.

Modeling chronic lung allograft rejection in miniature swine

BACKGROUND

The success of lung transplantation has been limited by the perplexing problem of chronic rejection. The development of a large-animal model for the systematic study of the mechanisms underlying chronic lung rejection has been problematic. We have developed a new preclinical model of chronic lung rejection using MHC-inbred miniature swine.

METHODS

Using standard operative techniques, four orthotopic left lung allografts were performed using MHC-matched, minor-antigen-mismatched donors. Recipient animals received a 12-day course of postoperative cyclosporine. Grafts were followed with open biopsies and high-resolution computed tomography. Cellular immune responses were monitored by mixed lymphocyte reaction, cytometric analysis of graft-infiltrating lymphocytes, and skin grafting.

RESULTS

All grafts survived > or = 5 months and developed manifestations of chronic rejection, including obliterative bronchiolitis, interstitial fibrosis, and occlusive vasculopathy. A mononuclear infiltrate was also present in all grafts by the fourth posttransplant month. High-resolution computed tomography demonstrated several cardinal radiographic findings known to correlate with chronic rejection. Cytometric analysis of graft-infiltrating lymphocytes showed a predominance of CD8+ cells. The development of alloreactivity in the host was confirmed by mixed lymphocyte reaction and skin grafting.

CONCLUSIONS

We report a reproducible, whole-lung, large-animal model of chronic lung rejection. In this immunogenetically defined construct, we have observed a full spectrum of histopathologic lesions that reproduce with fidelity those lesions observed in human lung transplant recipients suffering from chronic rejection. We anticipate that this preclinical model will facilitate further study of the pathogenesis and therapy of chronic lung rejection.

A simple method for culturing mouse vascular endothelium

Vascular endothelium is an important site for a wide array of immunological processes such as inflammation, atherosclerosis and allograft rejection. Culture methods of mouse vascular endothelium would provide an important in vitro correlate to immunological murine in vivo models. We describe a simple method to culture mouse vascular endothelium from thoracic aorta. Our cultured cells express typical phenotypic (CD105, CD31, CD106), morphological and ultrastructural (intercellular junctions, Weibel-Palade bodies) markers of vascular endothelium. They also possess functional receptors for uptake and processing of acetylated low-density lipoproteins. The mouse vascular endothelium within our system expresses high levels of MHC class I and MHC class II after activation with IFN-gamma. In addition, these cells express the accessory molecules CD80 and CD54, while they lack constitutive expression of CD86 and CD40, providing them the means to function as antigen presenting cells. Alloreactive CD4(+) and CD8(+) T lymphocytes demonstrate evidence of DNA synthesis after co-culture with activated vascular endothelium indicating their commitment to proliferation. In conclusion, we describe a simple culture system to isolate and grow mouse vascular endothelium, which provides a powerful tool to study biological interactions in vitro.

Adhesive interactions between human NK cells and porcine endothelial cells

Human natural killer (NK) cells are able to adhere to xenogeneic porcine endothelial cells (EC) and evidence from in vitro studies as well as animal models suggests a potential role for NK cells in the cellular recognition and damage of porcine xenogeneic tissues. One possible explanation for the observed NK cell-mediated xenogeneic cytotoxicity against porcine EC is the molecular incompatibility between porcine major histocompatibility complex (MHC) class I molecules and MHC-specific inhibitory receptors on human NK cells. In this review we attempt to summarize the current knowledge concerning adhesive interactions between human NK cells and porcine EC under special considerations of the cross-species receptor-ligand interactions. Methodological differences in assessing adhesion between various studies are reviewed and comparisons to the syngeneic/allogeneic adhesion mechanisms are made. Finally, the therapeutic potential of blocking antibodies and transgenic HLA expression in preventing NK-cell adhesion and xenogeneic cytotoxicity is discussed.

Decreased porcine valve antigenicity with in vitro culture

BACKGROUND

Porcine valvular prostheses may stimulate inflammation after implantation, with resultant accelerated structural degeneration. We investigated the expression of porcine major histocompatibility complex (MHC) class II molecules on valve leaflets and the possibility of decreasing valve antigenicity with in vitro culture.

METHODS

Aortic and pulmonary valves were harvested from domestic pigs under sterile conditions and cultured in vitro with either porcine or baboon serum for 4 days. Valves were harvested daily and fixed in Carnoy's or formalin solution. Microtome sections of valves were examined by hematoxylin and eosin, and by immunohistochemistry for porcine MHC class II proteins and an endothelial marker, alpha-N-acetylgalactosaminyl glycoprotein (alpha-GalNac).

RESULTS

Porcine aortic and pulmonary valves constitutively express alpha-GalNac proteins and porcine MHC class II antigens. Porcine valves continue to express both alpha-GalNac and MHC class II after 48 hours of culture in porcine serum. After 48-hour culture in baboon serum, however, MHC class II antigens became undetectable on valvular leaflets, although alpha-GalNac molecules were still detected.

CONCLUSIONS

Porcine valvular endothelial cells remain viable after 2 days of in vitro culture. Porcine valves cultured with primate serum show decreased MHC class II antigenic expression. In vitro culture before glutaraldehyde fixation may decrease inflammation associated with implantation.

TOLERANCE TO MUSCULOSKELETAL ALLOGRAFTS WITH TRANSIENT LYMPHOCYTE CHIMERISM IN MINIATURE SWINE1

BACKGROUND

Although transplantation of musculoskeletal allografts in humans is technically feasible, the adverse effects of long-term immunosuppression subject the patient to high risks for correcting a non-life-threatening condition. Achieving immunologic tolerance to musculoskeletal allografts, without the need for chronic immunosuppression, could expand the clinical application of limb tissue allografting. Tolerance to musculoskeletal allografts has been accomplished previously in miniature swine in our laboratory. Although stable, mixed chimerism has been suggested as the mechanism underlying long-term tolerance in a rat limb model, the mechanism of this tolerance induction has not been established. This report explores the possible relationship between hematopoietic chimerism and tolerance to musculoskeletal allografts in swine.

METHODS

Twelve miniature swine underwent vascularized musculoskeletal allograft transplantation from histocompatibility complex (MHC) matched, minor antigen-mismatched donors. Eight animals received a 12-day coprse of cyclosporine, one of which was excluded due to subtherapeutic levels. Four recipients were not immunosuppressed. Serial biopsies to assess graft viability and flow cytometry to assess chimerism were performed. Donor and third-party skin grafts were placed on recipients with surviving allografts greater than 100 days to validate tolerance.

RESULTS

Both groups developed early peripheral chimerism, but this chimerism became undetectable by postoperative day 19 in the cyclosporine group and by day 13 in the control group. Animals receiving cyclosporine developed permanent tolerance to their allografts, whereas those not receiving cyclosporine rejected their allografts in 6-9 weeks. Animals demonstrating tolerance to their bone allografts also demonstrated prolonged donor skin graft survival.

CONCLUSIONS

Induction of tolerance to musculoskeletal allografts can be achieved in the MHC matched swine. Although hematopoietic chimerism is present in the immediate postoperative period, persistent, long-term chimerism does not seem to be necessary for maintenance of such tolerance.