Mixed chimerism induced without lethal conditioning prevents T cell- and anti-Gal alpha 1,3Gal-mediated graft rejection

Physiological basis for xenotransplantation from genetically modified pigs to humans

The collective efforts of scientists over multiple decades have led to advancements in molecular and cellular biology-based technologies including genetic engineering and animal cloning that are now being harnessed to enhance the suitability of pig organs for xenotransplantation into humans. Using organs sourced from pigs with multiple gene deletions and human transgene insertions, investigators have overcome formidable immunological and physiological barriers in pig-to-nonhuman primate (NHP) xenotransplantation and achieved prolonged pig xenograft survival. These studies informed the design of Revivicor's (Revivicor Inc, Blacksburg, VA) genetically engineered pigs with 10 genetic modifications (10 GE) (including the inactivation of 4 endogenous porcine genes and insertion of 6 human transgenes), whose hearts and kidneys have now been studied in preclinical human xenotransplantation models with brain-dead recipients. Additionally, the first two clinical cases of pig-to-human heart xenotransplantation were recently performed with hearts from this 10 GE pig at the University of Maryland. Although this review focuses on xenotransplantation of hearts and kidneys, multiple organs, tissues, and cell types from genetically engineered pigs will provide much-needed therapeutic interventions in the future.

Xenotransplantation: Current Challenges and Emerging Solutions

To address the ongoing shortage of organs available for replacement, xenotransplantation of hearts, corneas, skin, and kidneys has been attempted. However, a major obstacle facing xenotransplants is rejection due to a cycle of immune reactions to the graft. Both adaptive and innate immune systems contribute to this cycle, in which natural killer cells, macrophages, and T-cells play a significant role. While advancements in the field of genetic editing can circumvent some of these obstacles, biomarkers to identify and predict xenograft rejection remain to be standardized. Several T-cell markers, such as CD3, CD4, and CD8, are useful in both the diagnosis and prediction of xenograft rejection. Furthermore, an increase in the levels of various circulating DNA markers and microRNAs is also predictive of xenograft rejection. In this review, we summarize recent findings on the advancements in xenotransplantation, with a focus on pig-to-human, the role of immunity in xenograft rejection, and its biomarkers.

Progress in islet xenotransplantation: Immunologic barriers, advances in gene editing, and tolerance induction strategies for xenogeneic islets in pig-to-primate transplantation

Islet transplantation has emerged as a curative therapy for diabetes in select patients but remains rare due to shortage of suitable donor pancreases. Islet transplantation using porcine islets has long been proposed as a solution to this organ shortage. There have already been several small clinical trials using porcine islets in humans, but results have been mixed and further trials limited by calls for more rigorous pre-clinical data. Recent progress in heart and kidney xenograft transplant, including three studies of pig-to-human xenograft transplant, have recaptured popular imagination and renewed interest in clinical islet xenotransplantation. This review outlines immunologic barriers to islet transplantation, summarizes current strategies to overcome these barriers with a particular focus on approaches to induce tolerance, and describes an innovative strategy for treatment of diabetic nephropathy with composite islet-kidney transplantation.

Progress in xenotransplantation: overcoming immune barriers

A major limitation of organ allotransplantation is the insufficient supply of donor organs. Consequently, thousands of patients die every year while waiting for a transplant. Progress in xenotransplantation that has permitted pig organ graft survivals of years in non-human primates has led to renewed excitement about the potential of this approach to alleviate the organ shortage. In 2022, the first pig-to-human heart transplant was performed on a compassionate use basis, and xenotransplantation experiments using pig kidneys in deceased human recipients provided encouraging data. Many advances in xenotransplantation have resulted from improvements in the ability to genetically modify pigs using CRISPR-Cas9 and other methodologies. Gene editing has the capacity to generate pig organs that more closely resemble those of humans and are hence more physiologically compatible and less prone to rejection. Despite such modifications, immune responses to xenografts remain powerful and multi-faceted, involving innate immune components that do not attack allografts. Thus, the induction of innate and adaptive immune tolerance to prevent rejection while preserving the capacity of the immune system to protect the recipient and the graft from infection is desirable to enable clinical xenotransplantation.

Progress in Xenotransplantation: Immunologic Barriers, Advances in Gene Editing, and Successful Tolerance Induction Strategies in Pig-To-Primate Transplantation

Organ transplantation is the most effective treatment for end stage organ failure, but there are not enough organs to meet burgeoning demand. One potential solution to this organ shortage is xenotransplantation using pig tissues. Decades of progress in xenotransplantation, accelerated by the development of rapid genome editing tools, particularly the advent of CRISPR-Cas9 gene editing technologies, have enabled remarkable advances in kidney and heart xenotransplantation in pig-to-nonhuman primates. These breakthroughs in large animal preclinical models laid the foundation for three recent pig-to-human transplants by three different groups: two kidney xenografts in brain dead recipients deemed ineligible for transplant, and one heart xenograft in the first clinical grade study of pig-to-human transplantation. However, despite tremendous progress, recent data including the first clinical case suggest that gene-modification alone will not overcome all xenogeneic immunologic barriers, and thus an active and innovative immunologic strategy is required for successful xenotransplantation. This review highlights xenogeneic immunologic barriers, advances in gene editing, and tolerance-inducing strategies in pig-to-human xenotransplantation.

Chimerism-Based Tolerance to Kidney Allografts in Humans: Novel Insights and Future Perspectives

Chronic rejection and immunosuppression-related toxicity severely affect long-term outcomes of kidney transplantation. The induction of transplantation tolerance – the lack of destructive immune responses to a transplanted organ in the absence of immunosuppression – could potentially overcome these limitations. Immune tolerance to kidney allografts from living donors has been successfully achieved in humans through clinical protocols based on chimerism induction with hematopoietic cell transplantation after non-myeloablative conditioning. Notably, two of these protocols have led to immune tolerance in a significant fraction of HLA-mismatched donor-recipient combinations, which represent the large majority of cases in clinical practice. Studies in mice and large animals have been critical in dissecting tolerance mechanisms and in selecting the most promising approaches for human translation. However, there are several key differences in tolerance induction between these models and humans, including the rate of success and stability of donor chimerism, as well as the relative contribution of different mechanisms in inducing donor-specific unresponsiveness. Kidney allograft tolerance achieved through durable full-donor chimerism may be due to central deletion of graft-reactive donor T cells, even though mechanistic data from patient series are lacking. On the other hand, immune tolerance attained with transient mixed chimerism-based protocols initially relies on Treg-mediated suppression, followed by peripheral deletion of donor-reactive recipient T-cell clones under antigenic pressure from the graft. These conclusions were supported by data deriving from novel high-throughput T-cell receptor sequencing approaches that allowed tracking of alloreactive repertoires over time. In this review, we summarize the most important mechanistic studies on tolerance induction with combined kidney-bone marrow transplantation in humans, discussing open issues that still need to be addressed and focusing on techniques developed in recent years to efficiently monitor the alloresponse in tolerance trials. These cutting-edge methods will be instrumental for the development of immune tolerance protocols with improved efficacy and to identify patients amenable to safe immunosuppression withdrawal.

Mixed xenogeneic porcine chimerism tolerizes human anti-pig natural antibody-producing cells in a humanized mouse model

BACKGROUND

A major obstacle to the success of organ transplantation from pigs to humans, necessitated by the shortage of human organs, is robust humoral immune rejection by pig-reactive human antibodies. Mixed xenogeneic hematopoietic chimerism induces xenoreactive B cell tolerance in rodents, but whether mixed pig/human chimerism could induce tolerance of human B cells to pig xenoantigens is unknown.

METHODS

We investigated this question using a humanized mouse model in which durable mixed (pig-human) xenogeneic chimerism can be established.

RESULTS

Human natural anti-pig cytotoxic antibodies, predominantly IgM, are detectable in non-chimeric humanized mouse serum, and pig-reactive antibodies were reduced in mixed chimeric versus non-chimeric humanized mice. This difference required persistent mixed chimerism and was not due to the adsorption of antibodies on pig cells in vivo. Furthermore, human B cells from spleens of mixed chimeric mice produced lower levels of anti-pig antibodies when stimulated in vitro compared with those from non-chimeric mice.

CONCLUSIONS

Our findings demonstrate that mixed chimerism reduces human natural antibodies to pig xenoantigens, providing the first in vivo evidence of human B cell tolerance induction by mixed xenogeneic chimerism and supporting further evaluation of this approach for inducing human B cell tolerance to xenografts.

TLR-MyD88 signaling blockades inhibit refractory B-1b cell immune responses to transplant-related glycan antigens

Refractory B cell responses to T cell-independent (TI) carbohydrate antigens (Ags) are critical drivers of rejection reactions to ABO-incompatible allogeneic grafts and xenogeneic grafts from other species. To explore the biological significance of crosstalk between Toll-like receptors (TLRs) and B cell receptors (BCRs) in the TI B cell immunity, we here used MyD88-, TRIF-, and α-galactosyltransferase-deficient mice to study B cell phenotypes and functional properties during TI transplant-related glycan Ag exposure. BCR stimulation alone induced differentiation into CD5^high (B-1a) cells, which were highly sensitive to a calcineurin inhibitor (CNI), while co-stimulation of TLRs and BCRs induced differentiation into CD5^dim (B-1b) cells in MyD88-dependent and CNI-resistant manner. MyD88-dependent TLR stimulation in B-1b cells enhanced downstream factors in the BCR-calcineurin pathway, including a nuclear factor of activated T cells, cytoplasmic 1 (NFATc1). TLR inhibitor together with CNI abrogated refractory B-1b cell immune responses against the ABO-blood group Ags, while blocking both BCRs and TLR-MyD88 by using Bruton's tyrosine kinase inhibitor and histone deacetylase inhibitor abrogated refractory B-1b cell immune responses against Gal-glycan Ags. Thus, this study provides a rationale for a novel therapeutic approach to overcome refractory transplant-related anti-glycan Ab production by blocking both BCR and TLR-MyD88 signals.

Intra-bone bone marrow transplantation from hCD47 transgenic pigs to baboons prolongs chimerism to >60 days and promotes increased porcine lung transplant survival

BACKGROUND

We have recently demonstrated that human-CD47 (hCD47) expressed on endothelial cells of porcine lung xenografts extended median graft survival from 3.5 days to 8.7 days in baboons. Intra-bone bone marrow transplantation (IBBMTx) in a pig-to-baboon model was previously shown to markedly prolong the duration of macrochimerism up to 21 days from 1 to 4 days by intravenous BMTx. We now examined whether the use of hCD47 transgenic (Tg) BM further prolonged the duration of chimerism following IBBMTx. We then tested if lung xenograft survival was prolonged following IBBMTx.

METHODS

Baboons received GalTKO-hCD47/hCD55Tg (n = 5) or -hCD55Tg (n = 1) or -hCD46/HLA-E Tg (n = 1) pig IBBMTx. Macrochimerism, anti-pig T cells and antibody responses were assessed. Animals received lung xenografts from either hCD47+ or hCD47- porcine lungs 1-3 months later.

RESULTS

All baboons that received hCD47Tg porcine IBBM maintained durable macrochimerism >30 days, and two maintained chimerism for >8 weeks. Notably, anti-pig antibody levels decreased over time and anti-pig cellular unresponsiveness developed following IBBMTx. Lungs from hCD47Tg IBBMTx matched pigs were transplanted at day 33 or day 49 after IBBMTx. These animals showed extended survival up to 13 and 14 days, while animals that received lungs from hCD47 negative pigs displayed no prolonged survival (1-4 days).

CONCLUSION

This is the first report demonstrating durable macrochimerism beyond 8 weeks, as well as evidence for B cell tolerance in large animal xenotransplantation. Using hCD47Tg pigs as both IBBMTx and lung donors prolongs lung xenograft survival. However, additional strategies are required to control the acute loss of lung xenografts.

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.

IXA Honorary Member Lecture, 2017: The long and winding road to tolerance

The last 15 years or so have seen exciting progress in xenotransplantation, with porcine organ grafts surviving months or even years in non-human primates. These advances reflect the application of new scientific knowledge, improved immunosuppressive agents, and genetic engineering. The field has recently enjoyed a renaissance of interest and hope, largely due to the exponential increase in our capacity to genetically engineer porcine source animals. However, immune responses to xenografts are very powerful and widespread clinical application of xenotransplantation will depend on the ability to suppress these immune responses while preserving the capacity to protect both the recipient and the graft from infectious microorganisms. Our work over the last three decades has aimed to engineer the immune system of the recipient in a manner that achieves specific tolerance to the xenogeneic donor while preserving otherwise normal immune function. Important proofs of principle have been obtained, first in rodents, and later in human immune systems in "humanized mice" and finally in non-human primates, demonstrating the capacity and potential synergy of mixed xenogeneic chimerism and xenogeneic thymic transplantation in tolerizing multiple arms of the immune system. Considering the fact that clinical tolerance has recently been achieved for allografts and the even greater importance of avoiding excessive immunosuppression for xenografts, it is my belief that it is both possible and imperative that we likewise achieve xenograft tolerance. I expect this to be accomplished through the availability of targeted approaches to recipient immune conditioning, understanding of immunological mechanisms of tolerance, advanced knowledge of physiological incompatibilities, and the availability of inbred miniature swine with optimized use of genetic engineering.

Tolerance in xenotransplantation

PURPOSE OF REVIEW

This review describes recent progress in tolerance-inducing strategies across xenogeneic immunological barriers as well as the potential benefit of a tolerance strategy for islets and kidney xenotransplantation.

RECENT FINDINGS

Using advanced gene editing technologies, xenotransplantation from multitransgenic alpha-1,3-galactosyltransferase knockout pigs has demonstrated marked prolongation of renal xenograft survival, ranging from days to greater than several months for life-supporting kidneys, and more than 2 years in a heterotopic nonlife-supporting cardiac xenograft model. Continuous administration of multiple immunosuppressive drugs has been required and attempts to taper immunosuppression have been unsuccessful. It appears likely that low levels of T cell dependent antibodies and activation of innate responses are responsible for xenograft loss. Mixed chimerism and thymic transplantation approaches have achieved xenogeneic tolerance in pig-to-mouse models and both have recently been extended to pig-to-baboon models. Encouraging results have been reported, including persistence of macrochimerism, prolonged pig skin graft survival, donor-specific unresponsiveness in vitro and detection of recent T cell emigrants in vivo.

SUMMARY

Although tolerance induction in vivo has not yet been achieved in pig-to-baboon models, recent results are encouraging that this goal will be attainable through genetic engineering of porcine donors.

PD-L1/PD-L2-expressing B-1 cells inhibit alloreactive T cells in mice

B cells constitute a complex system of antigen-presenting cells (APCs) and exist as distinct subsets that differ in their lineage affiliation, surface molecule expression, and biological function, thus potentially regulating the immune response. In this study, we investigated the immune-regulatory roles of murine B cell subsets as regulatory APCs targeting alloreactive T cells. Either splenic B cells, peritoneal cavity (PerC) B cells, or non-B cells from Balb/c mice were intravenously injected into B6 mice. Serum levels of anti-Balb/c antibodies in the recipients of PerC B cells were significantly lower than those in the recipients of splenic B cells and PerC non-B cells, as determined over a 4-week period after the injection. Mixed-lymphocyte reaction (MLR) assays using splenocytes from the B6 mice at 2 weeks after the injection revealed the significantly reduced anti-Balb/c T cell-responses in the recipients of PerC B cells, as compared to those in the recipients of splenic B cells or untreated control mice. Since PerC B cells contained MHC class II⁺ CD80⁺ CD86⁺ PD-L1⁺ PD-L2⁺ cells among the CD5⁺ B-1a cell subset, PerC B cells from Balb/c mice were pre-incubated with anti-PD-L1/PD-L2 mAbs prior to injection. This treatment abrogated their immune-regulatory effects on anti-Balb/c T cells in the MLR assays. In addition, the inoculation with Balb/c PerC B cells significantly prolonged the survival of subsequently grafted Balb/c hearts in B6 mouse recipients, whereas that with SPL B cells did not. These findings indicate that the PerC B cells, including PD-L1/PD-L2 B-1a cells, may suppress T cells responding to allostimulation, and thus may be optimal for donor lymphocyte injection.

Transplantation tolerance: don't forget about the B cells

Establishing a state of transplantation tolerance that leads to indefinite graft survival without the need for lifelong immunosuppression has been achieved successfully in limited numbers of transplant recipients in the clinic. These successes led to studies aimed at identifying potential biomarkers that diagnose allograft tolerance and identify the patients most amenable to drug minimization, and implicated an enriched B cell signature of tolerance. The emergence of a specialized subset of regulatory B cell (B_regs ), that possess immune-modulatory function in inflammation and autoimmune disease, raised the possibility that B_regs play critical roles in the promotion of transplantation tolerance and that B_regs are the underlying explanation for the B cell signature of tolerance. However, B cells are best known to play a key role in humoral immunity, and excessive production of donor specific antibodies has clear deleterious effects in transplantation. Thus, for tolerance to be persistent, alloantibody responses must also be curtailed, either through the suppression of T cell help or the induction of B cell-intrinsic dysfunction. Recent findings indicate a unique subset of follicular regulatory T cells (Tfr) that can suppress B cell function and induce epigenetic modifications that result in sustained defects in B cell differentiation and function. In this review, we summarize studies in animals and humans that suggest roles for B_regs and dysfunctional B cells in transplantation tolerance, and discuss how these insights may provide a roadmap for new approaches to diagnose, and new therapies to induce allograft tolerance.

Impact of Mixed Xenogeneic Porcine Hematopoietic Chimerism on Human NK Cell Recognition in a Humanized Mouse Model

Mixed chimerism is a promising approach to inducing allograft and xenograft tolerance. Mixed allogeneic and xenogeneic chimerism in mouse models induced specific tolerance and global hyporesponsiveness, respectively, of host mouse natural killer (NK) cells. In this study, we investigated whether pig/human mixed chimerism could tolerize human NK cells in a humanized mouse model. Our results showed no impact of induced human NK cell reconstitution on porcine chimerism. NK cells from most pig/human mixed chimeric mice showed either specifically decreased cytotoxicity to pig cells or global hyporesponsiveness in an in vitro cytotoxicity assay. Mixed xenogeneic chimerism did not hamper the maturation of human NK cells but was associated with an alteration in NK cell subset distribution and interferon gamma (IFN-γ) production in the bone marrow. In summary, we demonstrate that mixed xenogeneic chimerism induces human NK cell hyporesponsiveness to pig cells. Our results support the use of this approach to inducing xenogeneic tolerance in the clinical setting. However, additional approaches are required to improve the efficacy of tolerance induction while ensuring adequate NK cell functions.

Recent investigations into pig antigen and anti-pig antibody expression

Genetic engineering of donor pigs to eliminate expression of the dominant xenogeneic antigen galactose α1,3 galactose (Gal) has created a sea change in the immunobiology of xenograft rejection. Antibody mediated xenograft rejection of GGTA-1 α-galactosyltransferase (GTKO) deficient organs is now directed to a combination of non-Gal pig protein and carbohydrate antigens. Glycan analysis of GTKO tissues identified no new neo-antigens but detected high levels of N-acetylneuraminic acid (Neu5Gc) modified glycoproteins and glycolipids. Humans produce anti-Neu5Gc antibody and in very limited clinical studies sometimes show an induced anti-Neu5Gc antibody response after challenge with pig tissue. The pathogenicity of anti-Neu5Gc antibody in xenotransplantation is not clear however as non-human transplant models, critical for modelling anti-Gal immunity, do not produce anti-Neu5Gc antibody. Antibody induced after xenotransplantation in non-human primates is directed to an array of pig endothelial cells proteins and to a glycan produced by the pig B4GALNT2 gene. We anticipate that immune suppression will significantly affect the T-cell dependent and independent specificity of an induced antibody response and that donor pigs deficient in synthesis of multiple xenogeneic glycans will be important to future studies.

Progress towards inducing tolerance of pig-to-primate xenografts

Xenotransplantation remains the best near-term solution to the shortage of transplantable organs that currently limits the field of transplantation. However, because the immune response to xenografts is considerably stronger than it is to allografts, the amount of non-specific immunosuppression required to avoid xenograft rejection may limit clinical applicability. For this reason, we consider it likely that the success of clinical xenotransplantation will depend on finding ways of safely inducing tolerance across xenogeneic barriers rather than relying entirely on non-specific immunosuppressive agents. In this laboratory, two approaches are being studied for the induction of pig-to-primate tolerance: a) the simultaneous transplantation of vascularized thymus and solid organs; and b) mixed hematopoietic chimerism. A summary of the development of these two approaches and their current status is the subject of this review.

The Role of Liver Sinusoidal Endothelial Cells in Induction of Carbohydrate Reactive B Cells Tolerance Through the Programmed Death 1/Programmed Death Ligand 1 Pathway

BACKGROUND

A spontaneous tolerance of B cells responding to blood group antigens frequently develops in ABO-incompatible pediatric liver transplantation (LT). Liver sinusoidal endothelial cells (LSECs), which exclusively express blood group antigens in the liver, possess a capacity to induce alloantigen-specific tolerance. In this study, we elucidated the role of LSECs in the tolerance induction of blood group antigen-reactive B cells after ABO-incompatible LT using mice that lack galactose-α(1,3)galactose (Gal) epitopes resembling blood group carbohydrate antigens.

METHODS

Using adoptive transfer of LSECs from wild type (WT) C57BL/6J mice to congenic α1,3-galactosyltransferase gene knockout (GalT) mice, we established orthotropic GalT → GalT LSEC chimera mice. Anti-Gal Ab (antibody) production was evaluated after immunization of GalT → GalT LSEC chimera mice with Gal rabbit RBC.

RESULTS

Adoptive transfer of LSECs isolated from WT GalT mice via the portal vein resulted in persistent engraftment of Gal LSECs in congenic GalT mouse livers. Only when GalT mice were splenectomized before LSEC inoculation, the GalT → GalT LSEC chimera lost the ability to produce anti-Gal Abs. The administration of blocking monoclonal Abs (mAbs) against programmed death ligand 1 to the splenectomized GalT → GalT LSEC chimera resulted in the recovery of anti-Gal Ab production.

CONCLUSIONS

These findings suggest that LSECs take a part in tolerization of immature but not mature B cells specifically for Gal. Furthermore, the programmed death 1/programmed death ligand 1 pathway likely plays a crucial role in the mechanisms underlying spontaneous tolerization of B cells responding to ABO-blood group antigens in LT.

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.

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.

Desensitization for solid organ and hematopoietic stem cell transplantation

Desensitization protocols are being used worldwide to enable kidney transplantation across immunologic barriers, i.e. antibody to donor HLA or ABO antigens, which were once thought to be absolute contraindications to transplantation. Desensitization protocols are also being applied to permit transplantation of HLA mismatched hematopoietic stem cells to patients with antibody to donor HLA, to enhance the opportunity for transplantation of non-renal organs, and to treat antibody-mediated rejection. Although desensitization for organ transplantation carries an increased risk of antibody-mediated rejection, ultimately these transplants extend and enhance the quality of life for solid organ recipients, and desensitization that permits transplantation of hematopoietic stem cells is life saving for patients with limited donor options. Complex patient factors and variability in treatment protocols have made it difficult to identify, precisely, the mechanisms underlying the downregulation of donor-specific antibodies. The mechanisms underlying desensitization may differ among the various protocols in use, although there are likely to be some common features. However, it is likely that desensitization achieves a sort of immune detente by first reducing the immunologic barrier and then by creating an environment in which an autoregulatory process restricts the immune response to the allograft.

Blockade of invariant TCR-CD1d interaction specifically inhibits antibody production against blood group A carbohydrates

Previously, we detected B cells expressing receptors for blood group A carbohydrates in the CD11b(+)CD5(+) B-1a subpopulation in mice, similar to that in blood group O or B in humans. In the present study, we demonstrate that CD1d-restricted natural killer T (NKT) cells are required to produce anti-A antibodies (Abs), probably through collaboration with B-1a cells. After immunization of wild-type (WT) mice with human blood group A red blood cells (A-RBCs), interleukin (IL)-5 exclusively and transiently increased and the anti-A Abs were elevated in sera. However, these reactions were not observed in CD1d(-/-) mice, which lack NKT cells. Administration of anti-mouse CD1d blocking monoclonal Abs (mAb) prior to immunization abolished IL-5 production by NKT cells and anti-A Ab production in WT mice. Administration of anti-IL-5 neutralizing mAb also diminished anti-A Ab production in WT mice, suggesting that IL-5 secreted from NKT cells critically regulates anti-A Ab production by B-1a cells. In nonobese diabetic/severe combined immunodeficient (NOD/SCID/γc(null)) mice, into which peripheral blood mononuclear cells from type O human volunteers were engrafted, administration of anti-human CD1d mAb prior to A-RBC immunization completely inhibited anti-A Ab production. Thus, anti-CD1d treatment might constitute a novel approach that could help in evading Ab-mediated rejection in ABO-incompatible transplant recipients.

Adoptive transfer of allogeneic liver sinusoidal endothelial cells specifically inhibits T-cell responses to cognate stimuli

Although it is well known that liver allografts are often accepted by recipients, leading to donor-specific tolerance of further organ transplants, the underlying mechanisms remain unclear. We had previously used an in vitro model and showed that mouse liver sinusoidal endothelial cells (LSECs) selectively suppress allospecific T-cells across major histocompatibility complex (MHC) barriers. In the present study, we established an in vivo model for evaluating the immunomodulatory effects of allogeneic LSECs on corresponding T-cells. Allogeneic BALB/cA LSECs were injected intraportally into recombination activating gene 2 γ-chain double-knockout (RAG2/gc-KO, H-2(b)) mice lacking T, B, and natural killer (NK) cells. In order to facilitate LSEC engraftment, the RAG2/gc-KO mice were injected intraperitoneally with monocrotaline 2 days before the adoptive transfer of LSECs; this impaired the host LSECs, conferring a proliferative advantage to the transplanted LSECs. After orthotopic allogeneic LSEC engraftment, the RAG2/gc-KO mice were immune reconstituted intravenously with C57BL/6 splenocytes. After immune reconstitution, mixed lymphocyte reaction (MLR) assay using splenocytes from the recipients revealed that specific inhibition of host CD4(+) and CD8(+) T-cell proliferation was greater in response to allostimulation with irradiated BALB/cA splenocytes rather than to stimulation with irradiated third party SJL/jorllco splenocytes. This inhibitory effect was attenuated by administering anti-programmed death ligand 1 (PD-L1) monoclonal antibody during immune reconstitution in the above-mentioned mice, but not in RAG2/gc-KO mice engrafted with Fas ligand (FasL)-deficient BALB/cA LSECs. Furthermore, engraftment of allogeneic BALB/cA LSECs significantly prolonged the survival of subsequently grafted cognate allogeneic BALB/cA hearts in RAG2/gc-KO mice immune reconstituted with bone marrow transplantation from C57BL/6 mice. In conclusion, murine LSECs have been proven capable of suppressing T-cells with cognate specificity for LSECs in an in vivo model. The programmed death 1/PD-L1 pathway is likely involved in these suppressive effects.

Innate and adaptive immune responses are tolerized in chimeras prepared with nonmyeloablative conditioning

BACKGROUND

Mixed chimerism is an effective approach for tolerance induction in transplantation. Strategies to achieve mixed chimerism with relatively low toxicity have significantly expanded the clinical use of chimerism.

METHODS

Allogeneic bone marrow transplants were performed between B6 (H2(b)) and BALB/c (H2(d)) mice. Recipient B6 mice were nonmyeloablatively conditioned with anti-αβ-T-cell receptor, anti-CD154, or rapamycin alone or in different combinations. A total of 15 × 10(6) BALB/c bone marrow cells were transplanted after varying doses of cGy of total body irradiation.

RESULTS

Pretreatment of recipients with anti-CD154 and rapamycin with or without T-cell lymphodepletion reduced the total body irradiation requirement to 100 cGy for establishing stable mixed chimerism. The mixed chimeras accepted donor islet allografts long term. Lymphocytes from mixed chimeras did not respond to host or donor antigens, yet were reactive to major histocompatibility complex-disparate third-party alloantigens, demonstrating functional donor-specific T-cell tolerance. No antibodies against donor and host were detected in mixed chimeras, suggesting humoral tolerance. Mixed chimeras showed no cytotoxicity to donor cells, but a similar rapid killing rate for major histocompatibility complex disparate third-party B10.BR cells compared with T-cell-deficient and wild-type controls in in vivo cytotoxicity assays, suggesting donor-specific tolerance in the innate immune cells was achieved in mixed chimeras.

CONCLUSIONS

Mixed chimeras prepared with low-intensity nonmyeloablative conditioning exhibit systemic tolerance in innate immunity and tolerance in adaptive T- and B-cell immune responses.

Experimental models of B cell tolerance in transplantation

The use of conventional immunosuppression has successfully improved short-term allograft survival, however, long-term allograft survival has remained static and is complicated by serious side effects secondary to the long-term use of immunosuppressive agents. Immunological tolerance is the ultimate goal of organ transplantation, however it is an infrequent event in humans. Accordingly, over the past several decades, there has been a push to fully understand both the cellular and molecular mechanisms that play a role in the induction and maintenance of tolerance, with recent data implicating B cells and donor specific alloantibody as a barrier to and potential mediator of allograft tolerance. The study of B cells and alloantibody in transplant tolerance has evolved over recent years from using rodent models to non-human primate models. This review will discuss the role of B cells and alloantibody as antagonists and facilitators of transplantation tolerance, and highlight the experimental models developed for elucidating the mechanisms of B cell tolerance to alloantigen.

Mechanisms of donor-specific tolerance in recipients of haploidentical combined bone marrow/kidney transplantation

We recently reported long-term organ allograft survival without ongoing immunosuppression in four of five patients receiving combined kidney and bone marrow transplantation from haploidentical donors following nonmyeloablative conditioning. In vitro assays up to 18 months revealed donor-specific unresponsiveness. We now demonstrate that T cell recovery is gradual and is characterized by memory-type cell predominance and an increased proportion of CD4⁺ CD25⁺ CD127⁻ FOXP3⁺ Treg during the lymphopenic period. Complete donor-specific unresponsiveness in proliferative and cytotoxic assays, and in limiting dilution analyses of IL-2-producing and cytotoxic cells, developed and persisted for the 3-year follow-up in all patients, and extended to donor renal tubular epithelial cells. Assays in two of four patients were consistent with a role for a suppressive tolerance mechanism at 6 months to 1 year, but later (≥ 18 months) studies on all four patients provided no evidence for a suppressive mechanism. Our studies demonstrate, for the first time, long-term, systemic donor-specific unresponsiveness in patients with HLA-mismatched allograft tolerance. While regulatory cells may play an early role, long-term tolerance appears to be maintained by a deletion or anergy mechanism.

CD47 in xenograft rejection and tolerance induction

Robust immune responses to xenografts remain a major obstacle to clinical translation of xenotransplantation, which could otherwise be a potential solution to the worldwide shortage of organ donors. The more vigorous xenograft rejection relative to allograft rejection is largely accounted for by the extensive genetic disparities between the donor and recipient. Xenografts activate host immunity not only by expressing immunogenic xenoantigens that provide the targets for immune recognition and rejection, but also by lacking ligands for the host immune inhibitory receptors. This review is focused on recent findings regarding the role of CD47, a ligand of an immune inhibitory receptor SIRPalpha, in xenograft rejection and induction of xenotolerance.

Increased immunogenicity of tumor-associated antigen, mucin 1, engineered to express alpha-gal epitopes: a novel approach to immunotherapy in pancreatic cancer

Mucin 1 (MUC1), a bound mucin glycoprotein, is overexpressed and aberrantly glycosylated in >80% of human ductal pancreatic carcinoma. Evidence suggests that MUC1 can be used as a tumor marker and is a potential target for immunotherapy of pancreatic cancer. However, vaccination with MUC1 peptides fails to stimulate the immune response against cancer cells because immunity toward tumor-associated antigens (TAA), including MUC1, in cancer patients is relatively weak, and the presentation of these TAAs to the immune system is poor due to their low immunogenicity. We investigated whether vaccination with immunogenetically enhanced MUC1 (by expressing alpha-gal epitopes; Galalpha1-3Galbeta1-4GlcNAc-R) can elicit effective antibody production for MUC1 itself as well as certain TAAs derived from pancreatic cancer cells and induced tumor-specific T-cell responses. We also used alpha1,3galactosyltransferase (alpha1,3GT) knockout mice that were preimmunized with pig kidney and transplanted with B16F10 melanoma cells transfected with MUC1 expression vector. Vaccination of these mice with alpha-gal MUC1 resulted in marked inhibition of tumor growth and significant improvement of overall survival time compared with mice vaccinated with MUC1 alone (P = 0.003). Furthermore, vaccination with pancreatic cancer cells expressing alpha-gal epitopes induced immune responses against not only differentiated cancer cells but also cancer stem cells. The results suggested that vaccination using cells engineered to express alpha-gal epitopes is a novel strategy for treatment of pancreatic cancer.

The applications of bone marrow-derived stem cells to induce tolerance and chimerism in organ transplantation

Progress in understanding the cellular and molecular biology of the immune system, in the second half of the 20(th) century brings the transplantation of replacement organs and tissues in clinical reality to cure disease. Immunosuppressive agents that are part of nearly every transplantation procedure, are toxic to some extent and their chronic use predisposes the patient to the development of infection and cancer. Alternatives to immunosuppression include modulation of host immune system to reduce the immune response and the induction of a state of immunologic tolerance. Induction of hematopoietic mixed chimerism through donor bone marrow transplantation offers a promising approach for tolerance induction as a prelude to organ transplantation. Furthermore, mesenchymal stromal cells have important effects on the host immune system and possess immune modulation properties that make them attractive for potential use in organ transplantation as immunosuppressant. Both modalities might potentially provide novel therapeutic options for treatment/prevention of rejection and/or repair of organ allografts through their multifaceted properties. In this review, evidences for the tolerogenic properties and mechanisms of hematopoietic mixed chimerism as well as mesenchymal stromal cells effects on allograft surveillance are summarized.

Induction of human T-cell tolerance to pig xenoantigens via thymus transplantation in mice with an established human immune system

Thymus xenotransplantation has been shown to induce tolerance to porcine xenografts in mice and to permit survival of alpha1,3Gal-transferase knockout porcine kidney xenografts for months in nonhuman primates. We evaluated the ability of porcine thymus xenotransplantation to induce human T-cell tolerance using a humanized mouse (hu-mouse) model, where a human immune system is preestablished by implantation of fetal human thymus tissue under the kidney capsule and intravenous injection of CD34(+) hematopoietic stem/progenitor cells. Human T-cell depletion with an anti-CD2 mAb following surgical removal of human thymic grafts prevented the initial rejection of porcine thymic xenografts in hu-mice. In these hu-mice, porcine thymic grafts were capable of supporting human thymopoiesis and T-cell development, and inducing human T-cell tolerance to porcine xenoantigens. Human T cells from these mice responded strongly to third-party pig, but not to the thymic donor swine leukocyte antigen (SLA)-matched pig stimulators in a mixed lymphocyte reaction (MLR) assay. Anti-pig xenoreactive antibodies declined in these hu-mice, whereas antibody levels increased in nontolerant animals that rejected porcine thymus grafts. These data show that porcine thymic xenotransplantation can induce donor-specific tolerance in immunocompetent hu-mice, supporting this approach for tolerance induction in clinical xenotransplantation.

Hematopoietic cell transplantation for tolerance induction: animal models to clinical trials

The induction of donor-specific immune tolerance is the "holy grail" of transplantation, as it would avoid the toxicities of chronic immunosuppressive therapies while preventing acute and chronic graft rejection. A large number of approaches to tolerance induction have been described in the experimental literature, but only hematopoietic cell transplantation has shown preliminary success for intentional tolerance induction in pilot clinical trials. This review summarizes the conditions that allow progress to be made in moving strategies for tolerance induction from the bench to the bedside and discuss the mechanisms by which tolerance may be achieved through hematopoietic cell transplantation.

High antigen levels do not preclude B-cell tolerance induction to alpha1,3-Gal via mixed chimerism

BACKGROUND

Studies of bone marrow transplantation (BMT) from wild-type mice or rats to alpha1,3-galactosyltransferase (GalT) knockout mice have demonstrated that induction of mixed chimerism tolerizes not only T cells, but also natural antibody-producing B cells, even across xenogeneic barriers. Given that rodent cells express lower levels of the alphaGal epitope than the more clinically relevant porcine species, the consequences of exposure to cells expressing high levels of alphaGal on the ability to induce B-cell tolerance are unknown.

METHODS

The effects on chimerism and anti-alphaGal B-cell tolerance of an i.p. injection of 10(9) porcine RBC were evaluated in GalT knockout mice receiving wild-type allogeneic BMT after non-myeloablative conditioning with T-cell depleting monoclonal antibodies, thymic irradiation, and low-dose total body irradiation.

RESULTS

Achievement of mixed chimerism and tolerance of anti-alphaGal-producing B cells was not affected by exposure to high-density alphaGal at the time of BMT. The absence of induced anti-alphaGal or anti-pig antibody responses in conditioned control mice suggested that the B-cell xeno-response to pig is T-cell-dependent.

CONCLUSION

High alphaGal density on pig cells might not preclude the ability to achieve tolerance of pre-existing alphaGal-reactive human B cells via induction of mixed chimerism. This strategy has the potential to induce B-cell tolerance to non-alphaGal epitopes, against which natural antibodies have been found in the sera of healthy humans.

Achieving tolerance in pig-to-primate xenotransplantation: reality or fantasy

Because the immunologic differences between species are far greater than those within species, it is likely that the amount of immunosuppression that would be required for successful xenografting would be so much greater than that now used for allografting, that the side-effects and complications would be unacceptable. Tolerance approaches to xenotransplantation would overcome this concern. Studies in humanized mouse models have demonstrated that human T cells can be tolerized to porcine xenografts, providing important proofs of principle of the potential feasibility of pig-to-primate xenograft tolerance. The results available from studies of pig-to-primate xenotransplantation to date have demonstrated that while chronic immunosuppressive drugs have not completely avoided either T cell responses or humoral rejection, approaches directed toward tolerance induction have been encouraging with regard to avoiding immunization at both of these levels.

Application of cyclophosphamide-induced tolerance in alpha1,3-galactosyltransferase knockout mice presensitized with Gal alpha 1-3Gal beta-4-GlcNAc antigens

PURPOSE

Hyperacute rejection (HAR) mediated by the natural antibody (nAb) against Gal alpha 1-3Gal beta-4-GlcNAc (alpha Gal) is the major obstacle in xenogeneic organ transplantation. Previously, we reported the acceptance of donor heart grafts in anti-alpha Gal nAb-producing galactosyltransferase knockout (GalT KO) mice after cyclophosphamide (CP)-induced tolerance conditioning. In the present study, we applied our tolerance induction conditioning in presensitized recipient mice.

METHODS

GalT KO (alpha Gal(-/-), H-2(b/d)) recipient mice were presensitized with alpha Gal(+) rabbit red blood cells (RRBCs). Presensitized or nonsensitized recipient mice were treated with CP-induced tolerance conditioning, consisting of AKR (alpha Gal(+/+), H-2(k)) spleen cells (SC), CP, busulfan (BU), and AKR bone marrow cells (BMC). We assessed the survival of donor hearts and skin grafts and analyzed the production of anti-alpha Gal Abs by flow cytometry.

RESULTS

Donor mixed chimerism was achieved in the presensitized GalT KO mice treated with CP-induced tolerance conditioning. In parallel with the disappearance of anti-alpha Gal Abs, permanent acceptance of donor heart grafts and skin grafts was observed in presensitized and GalT KO mice treated with CP-induced tolerance conditioning.

CONCLUSIONS

Both B-cell and T-cell tolerance was achieved in the presence of a higher titer of anti-alpha Gal Abs after treatment with CP-induced tolerance conditioning.

Manipulating the immune system for anti-tumor responses and transplant tolerance via mixed hematopoietic chimerism

SUMMARY

Stem cells (SCs) with varying potentiality have the capacity to repair injured tissues. While promising animal data have been obtained, allogeneic SCs and their progeny are subject to immune-mediated rejection. Here, we review the potential of hematopoietic stem cells (HSCs) to promote immune tolerance to allogeneic and xenogeneic organs and tissues, to reverse autoimmunity, and to be used optimally to cure hematologic malignancies. We also review the mechanisms by which hematopoietic cell transplantation (HCT) can promote anti-tumor responses and establish donor-specific transplantation tolerance. We discuss the barriers to clinical translation of animal studies and describe some recent studies indicating how they can be overcome. The recent achievements of durable mixed chimerism across human leukocyte antigen barriers without graft-versus-host disease and of organ allograft tolerance through combined kidney and bone marrow transplantation suggest that the potential of this approach for use in the treatment of many human diseases may ultimately be realized.

The persistent elimination of B cells responding to blood group A carbohydrates by synthetic group A carbohydrates and B-1 cell differentiation blockade: novel concept in preventing antibody-mediated rejection in ABO-incompatible transplantation

We demonstrated a novel strategy for specific and persistent inhibition of antibody (Ab) production against blood group A or B carbohydrate determinants necessary for successful ABO-incompatible transplantation. Similar to human blood group O or B individuals, mice have naturally occurring Abs against human blood group A carbohydrates in their sera. B cells with receptors for A carbohydrates in mice belonging to the CD5+CD11b+B-1a subset have phenotypic properties similar to those of human B cells. These cells could be temporarily eliminated by injecting synthetic A carbohydrates (GalNAcα1–3, Fucα1–2Gal) conjugated to bovine serum albumin (A-BSA) and anti-BSA Abs. In mice that received the injection of A-BSA/anti-BSA Abs, the serum levels of anti-A IgM were reduced, but immunization with human A erythrocytes resulted in increased serum levels of anti-A Abs. When combined with cyclosporin A (CsA) treatment, which blocks B-1a cell differentiation, and treatment with A-BSA/anti-BSA Abs, the serum levels of anti-A Abs were persistently undetectable in the mice even after the immunization. B cells with receptors for A carbohydrates were markedly reduced in these mice. These results are consistent with the hypotheses that treatment with A-BSA/anti-BSA Abs temporarily depletes B cells responding to A determinants, and CsA treatment prevents the replenishment of these cells.

Rapidly induced, T-cell–independent xenoantibody production is mediated by marginal zone B cells and requires help from NK cells

Xenoantibody production directed at a wide variety of T lymphocyte–dependent and T lymphocyte–independent xenoantigens remains the major immunologic obstacle for successful xenotransplantation. The B lymphocyte subpopulations and their helper factors, involved in T-cell–independent xenoantibody production are only partially understood, and their identification will contribute to the clinical applicability of xenotransplantation. Here we show, using models involving T-cell–deficient athymic recipient mice, that rapidly induced, T-cell–independent xenoantibody production is mediated by marginal zone B lymphocytes and requires help from natural killer (NK) cells. This collaboration neither required NK-cell–mediated IFN-γ production, nor NK-cell–mediated cytolytic killing of xenogeneic target cells. The T-cell–independent IgM xenoantibody response could be partially suppressed by CD40L blockade.

Immune tolerance: mechanisms and application in clinical transplantation

The achievement of immune tolerance, a state of specific unresponsiveness to the donor graft, has the potential to overcome the current major limitations to progress in organ transplantation, namely late graft loss, organ shortage and the toxicities of chronic nonspecific immumnosuppressive therapy. Advances in our understanding of immunological processes, mechanisms of rejection and tolerance have led to encouraging developments in animal models, which are just beginning to be translated into clinical pilot studies. These advances are reviewed here and the appropriate timing for clinical trials is discussed.

Carbohydrate residues downstream of the terminal Galalpha(1,3)Gal epitope modulate the specificity of xenoreactive antibodies

Carbohydrates are involved in many immunological responses including the rejection of incompatible blood, tissues and organs. Carbohydrate antigens with Galalpha(1,3)Gal epitopes are recognized by natural antibodies in humans and pose a major barrier for pig-to-human xenotransplantation. Genetically modified pigs have been established that have no functional alpha1,3-galactosyltransferase (alpha1,3GT), which transfers alphaGal to N-acetyllactosamine (LacNAc) type oligosaccharides. However, a low level of Galalpha(1,3)Gal is still expressed in alpha1,3GT knockout animals in the form of a lipid, isoglobotrihexosylceramide (iGb3), which is produced by iGb3 synthase on lactose (Lac) type core structures. Here, we define the reactivity of a series of monoclonal antibodies (mAb) generated in alpha1,3GT-/- mice immunized with rabbit red blood cells (RbRBC), as a rich source of lipid-linked antigens. Interestingly, one mAb (15.101) binds weakly to synthetic and cell surface-expressed Galalpha(1,3)Gal on LacNAc, but strongly to versions of the antigen on Lac cores, including iGb3. Three-dimensional models suggest that the terminal alpha-linked Gal binds tightly into the antibody-binding cavity. Furthermore, antibody interactions were predicted with the second and third monosaccharide units. Collectively, our findings suggest that although the terminal carbohydrate residues confer most of the binding affinity, the fine specificity is determined by subsequent residues in the oligosaccharide.

Xenotransplantation: current status and a perspective on the future

Xenotransplantation using pigs as the transplant source has the potential to resolve the severe shortage of human organ donors. Although the development of relatively non-toxic immunosuppressive or tolerance-inducing regimens will be required to justify clinical trials using pig organs, recent advances in our understanding of the biology of xenograft rejection and zoonotic infections, and the generation of alpha1,3-galactosyltransferase-deficient pigs have moved this approach closer to clinical application. This Review highlights the major obstacles impeding the translation of xenotransplantation into clinical therapies and the potential solutions, providing a perspective on the future of clinical xenotransplantation.

Peripheral deletion of mature alloreactive B cells induced by costimulation blockade

Alloreactive B cells can contribute to graft rejection. Anti-CD154 treatment together with donor-specific transfusion (DST) results in the long-term survival of MHC-mismatched mouse heart grafts and inhibition of alloantibody production. To characterize the mechanism of B cell tolerance induced by the anti-CD154 and DST, we used 3-83Igi mice, on BALB/c (H-2K(d)) background, that express a B cell receptor that reacts with MHC class I antigens H-2K(b). Transplanting C57BL/6 (H-2K(b)) hearts into 3-83Igi mice, followed by tolerance induction, resulted in the peripheral deletion of mature but not immature 3-83 B cells. The sustained deletion of mature alloreactive B cells required the presence of the allograft and can be explained by the absence of T cell help.

Recent Progress in Tolerance Induction through Mixed Chimerism

Organ transplant recipients require life-long treatment with immunosuppressive drugs. Currently available immunosuppression is associated with substantial morbidity and mortality, and is ineffective in inhibiting chronic rejection and graft loss. Therefore, a permanent state of donor-specific tolerance remains a primary goal for transplantation research. The induction of mixed hematopoietic chimerism is an attractive concept in this regard. Hematopoietic chimerism modulates the immunologic repertoire by extending the mechanisms of self-tolerance to donor-specific allotolerance. Despite recent progress in developing nontoxic bone marrow transplantation protocols for rodents, translation to large animals has remained difficult. Here, we outline the concept of tolerance via mixed chimerism, and review recent progress and remaining challenges in bringing this approach to the clinical setting.

American society of transplantation symposium on B cells in transplantation: harnessing humoral immunity from rodent models to clinical practice

There is growing awareness that B cells and alloantibodies are important mediators of both acute and chronic allograft injury. Unfortunately, few therapies are clinically available to mitigate the function of B cells or the effects of established alloantibody. As a result, many sensitized people await transplantation without a suitable donor, and several rejection syndromes are emerging that appear to involve B cells either as antibody producers or as antigen-presenting cells. In recognition of this unmet need in transplantation, the American Society of Transplantation organized a Symposium on B cells in Organ Transplantation to foster interest in this topic amongst basic researchers attending the annual meeting of the American Association of Immunologists. This manuscript will give an overview of the presentations from this symposium including the current risks of allosensitization, adaptive accommodation, approaches toward B-cell tolerance for allo- and xenoantigens and clinical application of these concepts in ABO incompatible neonatal cardiac transplantation.

Induction of endotoxin tolerance inhibits alloimmune responses

It was recently reported that the induction of endotoxin tolerance (ET), which is defined as a reduced response to a lipopolysaccharide (LPS) challenge following the first LPS encounter, inhibits major histocompatibility complex (MHC)-restricted antigen presentation. This raises the question whether alloimmune responses can be inhibited by inducing ET in transplant donors. C57BL/6 mice were treated with a low dose of LPS prior to a challenge with a high dose of LPS to induce ET. Hearts from endotoxin-tolerized C57BL/6 mice were transplanted to BALB/c mice. The survival of the endotoxin-tolerized heart allografts was significantly prolonged. By using irradiated splenocytes from C57BL/6 mice and allogeneic splenocytes from BALB/c mice, a mixed lymphocyte reaction (MLR) assay was performed. The MLR assay used CFSE, and revealed that the splenocytes from the endotoxin-tolerized mice failed to induce the proliferation of allogeneic CD4(+) and CD8(+) T cells. Cytokine analyses of the supernatant of the MLR culture using endotoxin-tolerized stimulators revealed a distinct shift in the Th 1/Th 2 balance toward the Th 2-type response. The induction of ET increased the proportion of myeloid-related dendritic cells (DCs) expressing molecules necessary for antigen presentation, which favor the development of a Th 2 response; however, it reduced the proportion of lymphoid-related DCs expressing those molecules, which favor the development of the Th 1 response. Although the relevance of these findings with regard to the prolonged survival of the endotoxin-tolerized heart allografts remains to be elucidated, this is the first study to demonstrate that the induction of ET in donor animals inhibits alloimmune responses.