The relative contribution of direct and indirect antigen recognition pathways to the alloresponse and graft rejection depends upon the nature of the transplant

Basic Understanding of Liver Transplant Immunology

The liver is a specialized organ and plays an important role in our immune system. The liver constitutes parenchymal cells which are hepatocytes and cholangiocytes (60-80%) and non-parenchymal cells like liver sinusoidal endothelial cells (LSECs), hepatic satellite/Ito cells, Kupffer cells, neutrophils, mononuclear cells, T and B lymphocytes (conventional and non-conventional), natural killer cells, and natural killer T (NKT) cells. The liver mounts a rapid and strong immune response, under unfavorable conditions and acts as an immune tolerance to a variety of non-pathogenic antigens. This delicate and dynamic interaction between different kinds of immune cells in the liver maintains a balance between immune screening and immune tolerance. The liver allografts are privileged immunologically; however, allograft rejection is not uncommon and is classified as cell or antibody-mediated. Advancements in transplant immunology help in the prevention of allografts rejection by immune reactions of the host thus leading to better graft and host survival. Fewer patients may not require immunosuppression due to systemic donor-specific T-cell tolerance. The liver tolerance mechanism is poorly studied, and LSEC and unconventional lymphocytes play an important role that dampens T cell response either by inducing apoptosis of cells or inhibiting co-stimulatory pathways. Newer cell-based therapy based on Treg, dendritic cells, and mesenchymal stromal cells will probably change the future of immunosuppression. Various invasive and non-invasive biomarkers and artificial intelligence have also been investigated to predict graft survival, post-transplant complications, and immunotolerance in the future.

Characteristics of endothelial corneal transplant rejection following immunisation with SARS-CoV-2 messenger RNA vaccine

AIM

We report two cases of endothelial corneal allograft rejection following immunisation with SARS-CoV-2 messenger RNA (mRNA) vaccine BNT162b2 and describe the implications for management of transplant recipients postvaccination for COVID-19.

METHODS

A 66-year-old woman with Fuchs endothelial corneal dystrophy (FECD) and a unilateral Descemet's membrane endothelial keratoplasty (DMEK) transplant received COVID-19 mRNA vaccine BNT162b2 14 days post-transplant. Seven days later, she presented with symptoms and signs of endothelial graft rejection. An 83-year-old woman with bilateral DMEK transplants for FECD 3 and 6 years earlier developed simultaneous acute endothelial rejection in both eyes, 3 weeks post second dose of COVID-19 mRNA vaccine BNT162b2. Rejection in both cases was treated successfully with topical corticosteroids.

CONCLUSIONS

We believe this is the first report of temporal association between corneal transplant rejection following immunisation against COVID-19 and the first report of DMEK rejection following any immunisation. We hypothesise that the allogeneic response may have been initiated by the host antibody response following vaccination. Clinicians and patients should be aware of the potential of corneal graft rejection associated with vaccine administration and may wish to consider vaccination in advance of planned non-urgent keratoplasties. Patients should be counselled on the symptoms and signs that require urgent review to allow early treatment of any confirmed rejection episode.

Manipulation of Regulatory Dendritic Cells for Induction Transplantation Tolerance

Current organ transplantation therapy is life-saving but accompanied by well-recognized side effects due to post-transplantation systematic immunosuppressive treatment. Dendritic cells (DCs) are central instigators and regulators of transplantation immunity and are responsible for balancing allograft rejection and tolerance. They are derived from monocyte-macrophage DC progenitors originating in the bone marrow and are classified into different subsets based on their developmental, phenotypical, and functional criteria. Functionally, DCs instigate allograft immunity by presenting donor antigens to alloreactive T cells via direct, indirect, and semidirect recognition pathways and provide essential signaling for alloreactive T cell activation via costimulatory molecules and pro-inflammatory cytokines. Regulatory DCs (DCregs) are characterized by a relatively low expression of major histocompatibility complex, costimulatory molecules, and altered cytokine production and exert their regulatory function through T cell anergy, T cell deletion, and regulatory T cell induction. In rodent transplantation studies, DCreg-based therapy, by in situ targeting or infusion of ex vivo generated DCregs, exhibits promising potential as a natural, well-tolerated, organ-specific therapeutic strategy for promoting lasting organ-specific transplantation tolerance. Recent early-phase studies of DCregs have begun to examine the safety and efficacy of DCreg-induced allograft tolerance in living-donor renal or liver transplantations. The present review summarizes the basic characteristics, function, and translation of DCregs in transplantation tolerance induction.

Application of PLGA nano/microparticle delivery systems for immunomodulation and prevention of allotransplant rejection

INTRODUCTION

Allograft transplantation is an effective end-point therapy to replace the function of an impaired organ. The main problem associated with allotransplantation is the induction of immune responses that results in acute and chronic graft rejection. To modulate the response of the immune system, transplant recipients generally take high dose immunosuppressant drugs for life. These drugs are associated with serious side effects such as infection with opportunistic pathogens and the development of neoplasia.

AREAS COVERED

We reviewed the obstacles to successful transplantation and PLGA-based strategies to reduce immune-mediated allograft rejection.

EXPERT OPINION

Biomaterial-based approaches using micro- and nanoparticles such as poly (lactic-co-glycolic acid) (PLGA) can be used to achieve controlled release of drugs. This approach decreases the required effective dose of drugs and enables local delivery of these agents to specific tissues and cells, whilst decreasing systemic effects.

Alloreactivity and allorecognition of syngeneic and allogeneic mitochondria

Previously, we have demonstrated that the transplantation of autologous mitochondria is cardioprotective. No immune or autoimmune response was detectable following the single injection of autologous mitochondria. To expand the therapeutic potential and safety of mitochondrial transplantation, we now investigate the immune response to single and serial injections of syngeneic and allogeneic mitochondria delivered by intraperitoneal injection. Our results demonstrate that there is no direct or indirect, acute or chronic alloreactivity, allorecognition or damage-associated molecular pattern molecules (DAMPs) reaction to single or serial injections of either syngeneic or allogeneic mitochondria.

New Insights into Graft-Versus-Host Disease and Graft Rejection

Allogeneic transplantation of foreign organs or tissues has lifesaving potential, but can lead to serious complications. After solid organ transplantation, immune-mediated rejection mandates the use of prolonged global immunosuppression and limits the life span of transplanted allografts. After bone marrow transplantation, donor-derived immune cells can trigger life-threatening graft-versus-host disease. T cells are central mediators of alloimmune complications and the target of most existing therapeutic interventions. We review recent progress in identifying multiple cell types in addition to T cells and new molecular pathways that regulate pathogenic alloreactivity. Key discoveries include the cellular subsets that function as potential sources of alloantigens, the cross talk of innate lymphoid cells with damaged epithelia and with the recipient microbiome, the impact of the alarmin interleukin-33 on alloreactivity, and the role of Notch ligands expressed by fibroblastic stromal cells in alloimmunity. While refining our understanding of transplantation immunobiology, these findings identify new therapeutic targets and new areas of investigation.

Antigen Presentation in Transplantation

Transplantation of solid organs between genetically distinct individuals leads, in the absence of immunosuppression, to T cell-dependent transplant rejection. Activation of graft-reactive T cells relies on the presentation of transplant-derived antigens (intact donor MHC molecules or processed peptides on host MHC molecules) by mature dendritic cells (DCs). This review focuses on novel insights regarding the steps for maturation and differentiation of DCs that are necessary for productive presentation of transplant antigens to host T cells. These steps include the licensing of DCs by the microbiota, their activation and maturation following recognition of allogeneic non-self, and their capture of donor cell exosomes to amplify the presentation of transplant antigens.

Persistence of Indirect but Not Direct T Cell Xenoresponses in Baboon Recipients of Pig Cell and Organ Transplants

We investigated the contributions of direct and indirect T cell antigen recognition pathways to the immune response to porcine antigens in naïve baboons and baboon recipients of pig xenografts. In naïve baboons, in vitro culture of peripheral blood T cells with intact pig cells (direct xenorecognition pathway) or pig cell sonicates and baboon antigen-presenting cells (indirect xenorecognition pathway) induced the activation and expansion of xenoreactive T cells producing proinflammatory cytokines, interleukin-2 and interferon-γ. Primary indirect xenoresponses were mediated by preexisting memory T cells, whose presence is not typically observed in primary alloresponses. Next, baboons were conditioned with a nonmyeloablative regimen before short-term immunosuppression and transplantation of xenogeneic peripheral blood progenitor cells and a kidney, heart, or pancreatic islets from a miniature swine. All transplants were rejected acutely within 30 days after their placement. Posttransplantation, we observed an inhibition of the direct xenoresponse but a significant expansion of indirectly activated proinflammatory T cells. These results suggest that additional treatment to suppress indirect T cell immunity in primates may be required to achieve tolerance of pig xenografts through hematopoietic chimerism.

Vascularized composite allograft-specific characteristics of immune responses

Vascularized composite allograft (VCA) transplantation, or reconstructive transplantation, has revolutionized the treatment of complex tissue and functional defects. Despite arriving during an age in which the immunology of solid organ transplant rejection has been investigated in much detail, these transplants have offered new perspectives from which to explore the immunobiology of transplantation. VCAs have a number of unique molecular, cellular, and architectural features which alter the character and intensity of the rejection response. While much is yet to be clarified, an understanding of these distinct mechanisms affords new possibilities for the control of immune responses in an effort to improve outcomes after VCA transplantation.

The road to transplant tolerance is paved with good dendritic cells

After transplantation, recipient T cells can recognize donor antigens either by interacting with MHC class II on donor bone marrow-derived cells (direct allorecognition), or by recognizing allogeneic peptides bound to self-MHC class II molecules on recipient antigen presenting cells (indirect allorecognition). The activation of pro-inflammatory T cells via either of these pathways leads to allograft rejection, so the suppression of both of these pathways is needed to achieve transplantation tolerance. A study in this issue of the European Journal of Immunology [Eur. J. Immunol. 2013. 43: 734-746] shows that allogeneic dendritic cells (DCs) modified to either lack expression of CD80/86 or over-express indoleamine 2,3-dioxygenase (IDO) are able to inhibit direct and/or indirect alloresponses in vitro and in vivo in mice. Notably, both allorecognition pathways were suppressed by the coexpression of self- and allo-MHC molecules on semi-allogeneic DCs. This Commentary discusses the challenges and potential of using genetically-modified DCs to suppress alloreactivity in the context of transplant tolerance.

Dendritic cell modification as a route to inhibiting corneal graft rejection by the indirect pathway of allorecognition

Dendritic cell (DC) modification is a potential strategy to induce clinical transplantation tolerance. We compared two DC modification strategies to inhibit allogeneic T-cell proliferation. In the first strategy, murine DCs were transduced with a lentiviral vector expressing CTLA4-KDEL, a fusion protein that prevents surface CD80/86 expression by retaining the co-stimulatory molecules within the ER. In the second approach, DCs were transduced to express the tryptophan-catabolising enzyme IDO. CTLA4-KDEL-expressing DCs induced anergy in alloreactive T cells and generated both CD4⁺CD25⁺ and CD4⁺CD25⁻ Treg cells (with direct and indirect donor allospecificity and capacity for linked suppression) both in vitro and in vivo. In contrast, T-cell unresponsiveness induced by IDO⁺ DCs lacked donor specificity. In the absence of any immunosuppressive treatment, i.v. administration of CTLA4-KDEL-expressing DCs resulted in long-term survival of corneal allografts only when the DCs were capable of indirect presentation of alloantigen. This study demonstrates the therapeutic potential of CTLA4-KDEL-expressing DCs in tolerance induction.

Innate immunity and resistance to tolerogenesis in allotransplantation

The development of immunosuppressive drugs to control adaptive immune responses has led to the success of transplantation as a therapy for end-stage organ failure. However, these agents are largely ineffective in suppressing components of the innate immune system. This distinction has gained in clinical significance as mounting evidence now indicates that innate immune responses play important roles in the acute and chronic rejection of whole organ allografts. For instance, whereas clinical interest in natural killer (NK) cells was once largely confined to the field of bone marrow transplantation, recent findings suggest that these cells can also participate in the acute rejection of cardiac allografts and prevent tolerance induction. Stimulation of Toll-like receptors (TLRs), another important component of innate immunity, by endogenous ligands released in response to ischemia/reperfusion is now known to cause an inflammatory milieu favorable to graft rejection and abrogation of tolerance. Emerging data suggest that activation of complement is linked to acute rejection and interferes with tolerance. In summary, the conventional wisdom that the innate immune system is of little importance in whole organ transplantation is no longer tenable. The addition of strategies that target TLRs, NK cells, complement, and other components of the innate immune system will be necessary to eventually achieve long-term tolerance to human allograft recipients.

Phenotype, distribution and alloreactive properties of memory T cells from cynomolgus monkeys

The high frequency of memory T cells present in primates is thought to represent a major barrier to tolerance induction in transplantation. Therefore, it is crucial to characterize these memory T cells and determine their functional properties. High numbers of memory T cells were detected in peripheral blood and all lymphoid tissues except lymph nodes, which were essentially the site of naïve T cells. The majority of CD8(+) memory T cells were effector memory cells located in the blood and bone marrow while most CD4(+) memory T cells were central memory cells present in the spleen. Next, memory T cells from over 100 monkeys were tested for their response to alloantigens by ELISPOT. Memory alloreactivity mediated via direct but not indirect allorecognition was detected in all animals. The frequency of allospecific memory T cells varied dramatically depending upon the nature of the responder/stimulator monkey combination tested. MHC gene matching was generally associated with a low-memory alloreactivity. Nevertheless, low anamnestic alloresponses were also found in a significant number of fully MHC-mismatched monkey combinations. These results show that selected donor/recipient combinations displaying a low memory alloresponsiveness can be found. These combinations may be more favorable for transplant tolerance induction.

Targeting MHC class I monomers to dendritic cells inhibits the indirect pathway of allorecognition and the production of IgG alloantibodies leading to long-term allograft survival

T cell depletion strategies are an efficient therapy for the treatment of acute rejections and are an essential part of tolerance induction protocols in various animal models; however, they are usually nonselective and cause wholesale T cell depletion leaving the individual in a severely immunocompromised state. So far it has been difficult to selectively delete alloreactive T cells because the majority of protocols either delete all T cells, subsets of T cells, or subpopulations of T cells expressing certain activation markers, ignoring the Ag specificity of the TCR. We have developed a model in which we were able to selectively deplete alloreactive T cells with an indirect specificity by targeting intact MHC molecules to quiescent dendritic cells using 33D1 as the targeting Ab. This strategy enabled us to inhibit the indirect alloresponse against MHC-mismatched skin grafts and hence the generation of IgG alloantibodies, which depends on indirectly activated T cells. In combination with the temporary abrogation of the direct alloresponse, we were able to induce indefinite skin graft survival. Importantly, the targeting strategy had no detrimental effect on CD4(+)CD25(+)FoxP3(+) T cells, which could potentially be used as an adjunctive cellular therapy. Transplantation tolerance depends on the right balance between depletion and regulation. For the former this approach may be a useful tool in the development of future tolerance induction protocols in non-sensitized patients.

Dual effects of the alloresponse by Th1 and Th2 cells on acute and chronic rejection of allotransplants

The contribution of direct and indirect alloresponses by CD4(+) Th1 and Th2 cells in acute and chronic rejection of allogeneic transplants remains unclear. In the present study, we addressed this question using a transplant model in a single MHC class I-disparate donor-recipient mouse combination. BALB/c-dm2 (dm2) mutant mice do not express MHC class I L(d) molecules and reject acutely L(d+) skin grafts from BALB/c mice. In contrast, BALB/c hearts placed in dm2 mice are permanently accepted in the absence of chronic allograft vasculopathy. In this model, CD4(+) T cells are activated following recognition of a donor MHC class I determinant, L(d) 61-80, presented by MHC Class II A(d) molecules on donor and recipient APC. Pre-transplantation of recipients with L(d) 61-80 peptide emulsified in complete Freund's adjuvant induced a Th1 response, which accelerated the rejection of skin allografts, but it had no effect on cardiac transplants. In contrast, induction of a Th2 response to the same peptide abrogated the CD8(+) cytotoxic T cells response and markedly delayed the rejection of skin allografts while it induced de novo chronic rejection of heart transplants. This shows that Th2 cells activated via indirect allorecognition can exert dual effects on acute and chronic rejection of allogeneic transplants.

The taming of the shrew? The immunology of corneal transplantation

Corneal transplantation, first reported a century ago, is the oldest and most frequent form of solid tissue transplantation. Although keratoplasty is also considered as the most successful transplant procedure, several studies indicate that the long term survival of corneal grafts is even lower than that of transplanted parenchymatous organs. Despite the immune privilege enjoyed by the cornea and anterior segment of the eye, immunologic graft rejection is a major limitation to corneal transplantation. This review gives an update on corneal immunobiology and the mechanisms of corneal graft rejection, focusing on antigen presentation, as well as on the molecular and cellular mediators of this particular immune response.

Regulation of blood vessel versus lymphatic vessel growth in the cornea

PURPOSE

In the present study, the authors developed novel models to stimulate blood vessel formation (hemangiogenesis) versus lymphatic vessel formation (lymphangiogenesis) in the cornea.

METHODS

Micropellets loaded with high-dose (80 ng) or low-dose (12.5 ng) basic fibroblast growth factor (bFGF) were placed in BALB/c corneas. Angiogenic responses were analyzed by immunohistochemistry to quantify blood neovessels (BVs) and lymphatic neovessels (LVs) to 3 weeks after implantation. Areas covered by BV and LV were calculated and expressed as a percentage of the total corneal area (percentage BV and percentage LV). Hemangiogenesis (HA) and lymphangiogenesis (LA) were also assessed after antibody blockade of VEGFR-2 or VEGFR-3

RESULTS

Although high-dose bFGF stimulation induced a more potent angiogenic response, the relative LV (RLV=percentage LV/percentage BV x 100) was nearly identical with high- and low-doses of bFGF. Delayed LA responses induced 3 weeks after implantation of high-dose bFGF resulted in a lymphatic vessel-dominant phenotype. Interestingly, the blockade of VEGFR-2 significantly suppressed BV and LV. However, the blockade of VEGFR-3 inhibited only LV (P=0.0002) without concurrent inhibition of BV (P=0.79), thereby resulting in a blood vessel-dominant phenotype

CONCLUSIONS

An HA-dominant corneal phenotype can be obtained in BALB/c mice 2 weeks after implantation of an 80-ng bFGF micropellet with VEGFR-3 blockade. Alternatively, an LA-dominant corneal phenotype can be obtained 3 weeks after implantation of an 80-ng bFGF micropellet without supplementary modulating agents. These models will be useful in evaluating the differential contribution of BV and LV to a variety of corneal abnormalities, including transplant rejection, wound healing and microbial keratitis.

In situ-targeting of dendritic cells with donor-derived apoptotic cells restrains indirect allorecognition and ameliorates allograft vasculopathy

Chronic allograft vasculopathy (CAV) is an atheromatous-like lesion that affects vessels of transplanted organs. It is a component of chronic rejection that conventional immuno-suppression fails to prevent, and is a major cause of graft loss. Indirect allo-recognition through T cells and allo-Abs are critical during CAV pathogenesis. We tested whether the indirect allo-response and its impact on CAV is down-regulated by in situ-delivery of donor Ags to recipient's dendritic cells (DCs) in lymphoid organs in a pro-tolerogenic fashion, through administration of donor splenocytes undergoing early apoptosis. Following systemic injection, donor apoptotic cells were internalized by splenic CD11c^hi CD8α⁺ and CD8⁻ DCs, but not by CD11c^int plasmacytoid DCs. Those DCs that phagocytosed apoptotic cells in vivo remained quiescent, resisted ex vivo-maturation, and presented allo-Ag for up to 3 days. Administration of donor apoptotic splenocytes, unlike cells alive, (i) promoted deletion, FoxP3 expression and IL-10 secretion, and decreased IFN-γ-release in indirect pathway CD4 T cells; and (ii) reduced cross-priming of anti-donor CD8 T cells in vivo. Targeting recipient's DCs with donor apoptotic cells reduced significantly CAV in a fully-mismatched aortic allograft model. The effect was donor specific, dependent on the physical characteristics of the apoptotic cells, and was associated to down-regulation of the indirect type-1 T cell allo-response and secretion of allo-Abs, when compared to recipients treated with donor cells alive or necrotic. Down-regulation of indirect allo-recognition through in situ-delivery of donor-Ag to recipient's quiescent DCs constitutes a promising strategy to prevent/ameliorate indirect allorecognition and CAV.

The indirect alloresponse impairs the induction but not maintenance of tolerance to MHC class I-disparate allografts

We studied the effects of indirect allorecognition on the induction and maintenance phases of tolerance in miniature swine cotransplanted with heart and kidney allografts. MHC class I-mismatched heart and kidney grafts were cotransplanted in recipients receiving CyA for 12 days. Recipients were unimmunized or immunized with a set of donor-derived or control third-party MHC class I peptides either 21 days prior to transplantation or over 100 days after transplantation. T-cell proliferation, delayed type hypersensitivity reaction (DTH) and antibody production were assessed. All animals injected with donor MHC class I peptides developed potent indirect alloresponses specific to the immunizing peptides. While untreated recipients developed stable tolerance, all animals preimmunized with donor allopeptides rejected kidney-heart transplants acutely. In contrast, when peptide immunization was delayed until over 100 days after kidney-heart transplantation, no effects were observed on graft function or in vitro measures of alloimmunity. Donor peptide immunization prevented tolerance when administered to recipients pre transplantation but did not abrogate tolerance when administered to long-term survivors post transplantation. This suggests that the presence of T cells activated via indirect allorecognition represent a barrier to the induction but not the maintenance of tolerance.

Transplantation immunology: what the clinician needs to know for immunotherapy

The liver is unique among transplanted organs with respect to its interaction with the host immune system. There is evidence, both anecdotal and documented, that some liver recipients who cease taking immunosuppressive drugs maintain allograft function, suggesting robust tolerance is in place. Moreover, recipients of human liver allografts require less immunosuppression than do other organ recipients, and liver transplants confer protection on other organ grafts from the same donor. Hence, the liver shows features of immune privilege. Still, the liver can display destructive immunologic processes such as rejection in approximately one quarter of patients. The understanding of the cellular and molecular mechanisms operant in tolerance vs allograft rejection is important for developing new agents to improve long-term outcome, minimize infectious complications (including recurrence of hepatotropic viruses), and deliver immunosuppression without long-term toxicity. This review describes the unique aspects of the hepatic immune response, the pathways involved in T-cell activation and alloantigen recognition, effector cells and pathways mediating liver allograft rejection, the role of regulatory T cells, and targets of current and future immunosuppressive agents.

Activation of Tim-3-Galectin-9 pathway improves survival of fully allogeneic skin grafts

T cell immunoglobulin and mucin domain (Tim)-3 is a molecule expressed on terminally differentiated murine Th1 cells but not on Th2 cells. Identification of Galectin-9 as a ligand for Tim-3 has now firmly established the Tim-3-Galectin-9 pathway as an important regulator of Th1 immunity, which results in apoptosis of Th1 cells. Here, we demonstrate that engagement of Tim-3 by mouse recombinant Galectin-9 remarkably suppresses allograft rejection and improves survival of allogeneic skin grafts. Furthermore, administration of recombinant Galectin-9 decreases Tim-3 positive cells in draining lymph node and selectively inhibits production of IFN-gamma after skin transplantation. At last, even low dose of Galectin-9 (1 microg/ml) can obviously inhibit TCR crosslinking-induced primary T cell proliferation in vitro. These observations suggest that Tim-3-Galectin-9 pathway plays an important role in the termination of productive Th1-immune response and could lead to developing novel therapies in transplant medicine.

Skin tolerance: in search of the Holy Grail

In 1943, Gibson and Medawar opened the modern era of transplantation research with a paper on the problem of skin allograft rejection. Ten years later Billingham, Brent and Medawar demonstrated that it was possible to induce selective immune acceptance of skin grafts in mice, a state of tolerance. After over six decades, however, the precise mechanism of skin allograft rejection remains still ill-defined. Furthermore, it has not been possible to achieve reliably clinical tolerance allowing the widespread application of skin allotransplantation techniques. The first successful applications of skin allotransplantation have included the hand and face. However, complications from the chronic immunosuppression regimens limit the application of these techniques. Induction of tolerance to skin (and the other tissues in the allograft) would be the most effective way to overcome all these difficulties, but this is yet to be achieved reliably, stimulating some to look for other ways to surmount the current limitations. This paper summarizes alternatives to enlarge the scope of skin allotransplantation techniques, current understanding of mechanisms of skin rejection, and the utility and limitations of animal models used to study skin rejection and tolerance induction. Finally, manipulation strategies to achieve skin tolerance are outlined.

Initial steroid bolus injection promotes vigorous CD8+ alloreactive responses toward early graft acceptance immediately after liver transplantation in humans

We have found that steroid bolus withdrawal prior to graft reperfusion increased the incidence of acute cellular rejection (ACR). This study aims to clarify how initial steroid bolus (ISB) injection at reperfusion influences the kinetics of CD8(+) alloreactive immune responses immediately after living donor liver transplantation (LDLT). A total of 49 hepatitis C virus (HCV)-infected recipients were classified into 3 groups according to hierarchical clustering by preoperative CD8(+)CD45 isoforms. The naive T cell proportion was considerably higher in Group I than in Groups II and III, whereas Group II recipients had the highest effector memory (EM) T cells and Group III the highest effector T cells. The frequency of ACR was significantly higher in recipients without ISB than in those with ISB. In particular, the ACR rates were the highest in Group II without ISB. Following ISB, the proportion of effector T cells was promptly upregulated within 6 hours after graft reperfusion, simultaneously with the upregulation of CD27(-)CD28(-) subsets, interferon-gamma (IFN-gamma), tumor necrosis factor-alpha and perforin expression, which significantly correlated with increasing interleukin (IL)-12 receptor beta 1 cells. These were then downregulated to below preoperative levels by tacrolimus (Tac) administered at 24 hours. These changes did not occur in the absence of ISB. In Group II without ISB, the downregulation of IL-12Rbeta1(+) cells was the greatest, consistent with the highest rates of ACR and mortality (60%). In conclusion, ISB must be done in place, especially in Group II with preexisting high EM T cells, to enable the development of early allograft acceptance.

Tolerance in Heart Transplantation: The Holy Grail, or an Attainable Goal?

The father of cardiac transplantation, Norman Shumway, famously predicted that tolerance was the future of the field, and always would be. Although his prediction remains true to date, significant progress has been made toward this goal, the "Holy Grail" for transplant clinicians. Current efforts are fueled by disappointing long-term outcomes associated with chronic immunosuppression, and the promise that partial or complete tolerance will impact long-term results favorably. This article provides a clinical definition of tolerance primarily based on lessons learned from animal heart allograft models. It reviews several promising strategies for inducing tolerance and detecting its presence through the use of biomarkers in peripheral blood or the graft, and outlines a possible path toward making this vision a clinical reality.

Basic Transplantation Immunology

Humans are protected from a daily onslaught of pathogenic organisms by an immune system that provides multiple layers of protection. Until solid organ transplantation became technically feasible in the early twentieth century, this constant state of surveillance for foreign cells that are associated with the immune response mostly was viewed as advantageous. Unfortunately for patients who have end-stage failure of heart, lungs, kidney, liver, and pancreas, the immune system is incapable of distinguishing between the presence of beneficial foreign tissue and harmful foreign pathogens; it mounts an effective attack against both. Improving our understanding of the factors that initiate and perpetuate the alloimmune response will result in the development of more refined and better tolerated immunosuppressive strategies.

Heterogeneous Alterations in Human Alloimmunity Associated with Immunization

BACKGROUND

The presence of alloantibodies and/or alloreactive T cells in a patient prior to a transplant can impact graft outcome. Environmental factors, including therapeutic vaccinations, may influence the strength and/or specificity of alloimmunity.

METHODS

To address this issue, we prospectively evaluated the effects of two different immunization protocols in human subjects on cellular alloimmunity using an IFNgamma ELISPOT assay and on alloantibody reactivity by flow cytometric analysis of HLA-coated beads.

RESULTS

Vaccination/immunization was associated with augmentation of cellular and/or humoral alloimmune reactivity in >50% of the test subjects. The effects were heterogeneous in that some detected increases were transient, peaking 30-60 days postimmunization, whereas others persisted for the length of the study. Antibodies reactive to the immunizing agent did not cross react with the detected alloantibodies, suggesting that the augmentation of alloimmune reactivity was most likely due to a nonspecific adjuvant effect from the vaccine.

CONCLUSIONS

Therapeutic vaccinations can alter the strength of cellular and humoral alloimmunity in humans. The results suggest that serial immune monitoring of alloreactivity might be beneficial when immunizations are administered to potential transplant recipients.

Progression of alloresponse and tissue-specific immunity during graft coronary artery disease

Chronic rejection remains the major obstacle for long-term transplant survival. Both indirect alloresponse and tissue-specific autoimmunity have been implicated in its pathogenesis. The interrelationship between these two types of host anti-graft response remains poorly understood. We have developed an immunosuppression-free mouse model of graft coronary artery disease (GCAD), in which all FVB (H-2(q)) cardiac allografts placed into minor Ag (mHC)-mismatched DBA/1 (H-2(q)) hosts survived more than 112 days, and developed GCAD. We then examined the kinetics of both anti-mHC alloresponse and host autoimmunity against heart-specific antigen, cardiac myosin (CM). At 8 days post-transplantation, recipient mice showed minimal intragraft inflammation and apoptosis, and limited expansion of allo-specific T cells. In addition, we observed early production of anti-myosin IgG1 autoantibodies, which occurred in the absence of activated CM-specific T lymphocytes. By day 56, GCAD indices, the numbers of mHC- and CM-reactive T cells, and the levels of circulating allo- and CM-specific antibodies were all significantly increased. While host alloresponse was exhausted at 112 days post-transplant, T-cell reactivity against CM persisted. Our data suggest that both allo- and tissue-specific immunity might contribute to the induction of GCAD. They indicate that continual autoimmune response against graft tissue antigens may provide for GCAD sustenance.

Targeted gene disruption of the heat shock protein 72 gene (hsp70.1) in the donor tissue is associated with a prolonged rejection-free survival in the murine skin allograft model

It is known that the expression level of the heat shock protein (HSP) is elevated in allograft tissues, but the specific role of the HSP in the acute rejection has not been elucidated. This study aims to determine how and when the HSP72 molecule works immunologically in the process of acute allograft rejection from a skin graft model in which HSP72 (hsp70.1 gene) knock out (KO) mice were adopted either as a donor or as a recipient. In experiment I, tail skin was grafted from either the HSP72 KO C57BL/6 mice or wild-type C57BL/6 mice--as the allograft control--onto the trunk of B10BR mice. The grafts were observed for any signs of rejection until the 14th day after the graft. The survival of the grafted skin from the two donor groups was analyzed by a log-rank method. The grafted skin was observed for the degree of rejection using light microscopy. In addition, the degree of apoptosis was assessed using a terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). A mixed lymphocyte reaction (MLR) was observed with HSP72 KO C57BL/6 splenocytes as the stimulator and B10BR mice lymphocytes as the responder cells. The cytokine levels, such as interleukin-2 (IL-2) and interferon-gamma, were measured from an MLR supernatant using enzyme-linked immunosorbent assay (ELISA). In experiment II, the tail skin was grafted in the opposite direction from the B10BR mice to the HSP72 KO or wild-type C57BL/6 mice, and the grafts were observed for any signs of rejection, as defined in experiment I. The absence of HSP72 expression was observed in the HSP72 KO mice lymphocytes after heat stress (42 degrees C) using Western blot analysis. In experiment I, the survival of the skin grafts from the HSP72 KO C57BL/6 mice was 12+/-1.3 days (median+/-S.E.), which was significantly longer than that from the allograft control donor (9+/-0.6 days; p=0.03). This coincided with the microscopic finding of the graft tissues. The MLR response of the CD4+ lymphocytes stimulated by HSP72 KO C57BL/6 splenocytes was lower, and the interferon-gamma concentration from the MLR supernatant was also lower. On Day 9, 3.7+/-1.1(mean+/-S.D.) TUNEL-positive cells per unit high-power field (HPF) were observed from the HSP72 KO C57BL/6 skin, which was significantly lower than that from the control skin (8.7+/-2.1 cells/HPF; p=0.045). In contrast, in experiment II, there was no difference in the survival of the skin graft either from the B10BR to HSP72 KO C57BL/6 mice or from the B10BR to the wild-type C57BL/6 mice. In summary, the survival of the skin grafts from the HSP72 KO C57BL/6 mice was prolonged, and the degree of rejection was lower than that from the allograft control. This means that the presence of HSP72 in the donor tissue is related to the up-regulation of acute rejection from the recipient immune system. The HSP72 KO mice as the donor were shown to have decreased level of antigen presentation to the recipient CD4+ lymphocytes. Therefore, the HSP72 molecule from the donor cells is believed to be related to the induction of the recipient immune reaction via alloantigen presentation.

Chronic heart allograft rejection in rats demonstrates a dynamic interplay between IFN-γ and IL-10 producing T cells

The relative roles of different types of T cells and their modes of alloantigen recognition and cytokine production in chronic rejection have been controversial. This may be due to the use of models involving various immune interventions. We recently reported a rat heart allograft model (PVG.1U-to-PVG.R8) that consistently develops chronic rejection without any immune manipulations. Using this model, we investigated the frequency of indirectly activated alloreactive CD4+ T cells and directly/indirectly induced CD8+ T cells and characterized their cytokine profiles at various times posttransplantation. In vitro quantitative-proliferative and intracellular cytokine assays were performed using recipient lymphocytes as responders against donor and third party stimulator cells. Intragraft transcript levels for IFN-gamma were measured using competitive RT-PCR. We observed a steady increase in the frequency of donor reactive CD4+ T cells, peaking on day 18-22 posttransplantation, followed by a decline to background levels on day 60. These cells produced high levels of IL-10 that steadily increased throughout the 100-day experimental period. In contrast, the frequency of donor reactive CD8+ T cells was similar to that of naïve cells and remained unchanged throughout the study. CD8+ T cells produced high amounts of IFN-gamma only on days 39-45 posttransplantation when chronic rejection was apparent. There was a sharp increase in intragraft IFN-gamma transcripts on days 8-12 that peaked on days 39-45 followed by a steady decrease thereafter. Our results demonstrate that chronic rejection in this model involves a dynamic interplay between alloreactive CD8+IFN-gamma+ and CD4+IL-10+ T cells, both of which may have differential effects on the pathogenesis of the disease.

Heart, but Not Skin, Allografts from Donors Lacking Flt3 Ligand Exhibit Markedly Prolonged Survival Time

Fms-like tyrosine kinase 3 ligand (Flt3L) administration leads to dramatic increases in dendritic cells (DC) in lymphoid and nonlymphoid tissues. Conversely, mice lacking Flt3L (Flt3L(-)/(-)) show severe reductions in both myeloid (CD11c(+)CD8alpha(-)) and lymphoid-related DC (CD11c(+)CD8alpha(+)) in the thymus and secondary lymphoid organs. In this study marked reductions in CD11c(+) interstitial cardiac DC and in dermal, but not epidermal, DC (Langerhans cells) were also observed. CD11c(+) cells that migrated from Flt3L(-/-) skin explants expressed lower surface MHC class II and costimulatory molecules and naive T cell allostimulatory activity than migratory wild-type (wt) C57BL/6 (B6) CD11c(+) cells. We examined the survival of Flt3L(-)/(-) heart or tail skin grafts (H2(b)) in allogeneic wt (BALB/c; H2(d)) recipients. The outcome of transplantation of BALB/c organs into Flt3L(-)/(-) recipients was also determined. Flt3L(-)/(-) mice rejected BALB/c heart or skin grafts with similar kinetics as B6 wt recipients. Trafficking of donor DC into host spleens or draining lymph nodes was markedly reduced after transplantation of Flt3L(-)/(-) heart, but not skin grafts, respectively. Compared with wt hearts, survival of Flt3L(-)/(-) hearts was markedly prolonged in BALB/c recipients (median survival time, 37 and 15 days, respectively; p < 0.001). Skin graft survival was unaffected. Rejection of Flt3L(-/-) hearts was precipitated by infusion of wt donor DC at the time of transplant. Thus, severe depletion of interstitial heart DC resulting from targeted gene disruption prolongs, but does not indefinitely extend, heart survival. Acute rejection of wt grafts in Flt3L(-/-) recipients reflects presumably an intact role of the direct pathway of allorecognition.

Indirect Minor Histocompatibility Antigen Presentation by Allograft Recipient Cells in the Draining Lymph Node Leads to the Activation and Clonal Expansion of CD4+T Cells That Cause Obliterative Airways Disease

Ag recognition by OVA-reactive OT-II (I-Ab restricted) and DO11.10 (I-Ad restricted) TCR-Tg CD4+ T cells after heterotopic transplantation of OVA transgene-expressing tracheal grafts was examined as a model of minor histocompatibility Ag (mHAg)-induced chronic allograft rejection. In response to airway allotransplantation with grafts expressing the OVA transgene, these TCR-Tg CD4+ T cells expressed the activation markers CD69 and CD44, demonstrated evidence of blastogenesis, underwent multiple rounds of cell division leading to their clonal expansion in the draining lymph node, and proceeded to differentiate to a effector/memory T cell phenotype based on a reduction in the expression of CD45RB. These mHAg-specific TCR-Tg CD4+ T cells responded equally well to fully MHC-mismatched tracheas and to class II-deficient allografts, demonstrating that donor mHAg recognition by recipient CD4+ T cells does not rely on Ag presentation by donor-derived APC. The activation of mHAg-specific TCR-Tg CD4+ T cells after their adoptive transfer into recipient mice given MHC-matched, but mHAg-disparate, airway allografts was associated with their movement into the allograft and the near uniform destruction of the transplanted airway tissue secondary to the development of obliterative airways disease. These results demonstrate that an activation of mHAg-reactive CD4+ T cells in the draining lymph node by recipient APC that indirectly express graft mHAg-derived peptide/class II MHC complexes precedes responder T cell proliferation and differentiation, and leads to the eventual migration of these alloreactive T cells to the transplanted airway tissue and the promotion of chronic graft rejection.

Tolerance to noninherited maternal MHC antigens in mice

The phenomenon of tolerance to noninherited maternal Ags (NIMA) is poorly understood. To analyze the NIMA effect C57BL/6 (H-2(b/b)) males were mated with B6D2F(1) (H-2(b/d)) females, whereby 50% of the offspring are H-2(b/b) mice that have been exposed to maternal H-2(d) alloantigens. Controls were H-2(b/b) offspring of C57BL/6 mothers, either inbred C57BL/6 mice or F(1) backcross mice from breedings with H-2(b/d) fathers. We found that 57% of the H-2(b/b) offspring of semiallogeneic (H-2(b/d)) mothers accepted fully allogeneic DBA/2 (H-2(d/d)) heart grafts for >180 days, while similar transplants were all rejected by day 11 in controls (p < 0.0004). Foster nursing studies showed that both oral and in utero exposure to NIMA are required for this tolerogenic effect. An effect of NIMA was also found to extend the survival of skin grafts from a semiallogeneic donor (p < 0.02). Pretransplant analysis of splenocytes showed a 40-90% reduction of IL-2-, IL-5-, and IFN-gamma-producing T cells responding to H-2(d)-expressing APC in NIMA(d)-exposed vs control mice. Injection of pregnant BALB/c-dm2 (H-2L(d)-negative) female mice i.v. with H-2L(d)(61-80) peptide profoundly suppressed the offspring's indirect pathway alloreactive CD4(+) T cell response to H-2L(d). These results suggest that the natural exposure of the fetus and newborn to maternal cells and/or soluble MHC Ags suppresses NIMA-allospecific T cells of the offspring, predisposing to organ transplant tolerance in adult mice.

Dendritic cells: regulators of alloimmunity and opportunities for tolerance induction

Dendritic cells (DCs) are uniquely well-equipped antigen-presenting cells (APCs) regarded classically as sentinels of the immune response, which induce and regulate T-cell reactivity. They play critical roles in central tolerance and in the maintenance of peripheral tolerance in the normal steady state. Following cell or organ transplantation, DCs present antigen to T cells via the direct or indirect pathways of allorecognition. These functions of DCs set in train the rejection response, but they also serve as potential targets for suppression of alloimmune reactivity and promotion of tolerance induction. Much evidence from various model systems now indicates that DCs can induce specific T-cell tolerance. Although underlying mechanisms have not been fully elucidated, the capacity to induce T-regulatory cells may be an important property of tolerogenic or regulatory DCs. Efforts to generate "designer" DCs with tolerogenic properties in the laboratory using specific cytokines, immunologic or pharmacologic reagents, or genetic engineering approaches have already met with some success. Alternatively, targeting of DCs in vivo (e.g. by infusion of apoptotic allogeneic cells) to take advantage of their inherent tolerogenicity has also demonstrated exciting potential. The remarkable heterogeneity and plasticity of these important APCs present additional challenges to optimizing DC-based therapies that may lead to improved tolerance-enhancing strategies in the clinic.