Persistent allopeptide reactivity and epitope spreading in chronic rejection of organ allografts

Animal models of chronic allograft injury: contributions and limitations to understanding the mechanism of long-term graft dysfunction

Advances in immunosuppression have reduced the incidence of acute graft loss after transplantation, but long-term allograft survival is still hindered by the development of chronic allograft injury, a multifactorial process that involves both immunologic and nonimmunologic components. Because these components become defined in the clinical setting, development of animal models enables exploration into underlying mechanisms leading to long-term graft dysfunction. This review presents animal models that have enabled investigation into chronic allograft injury and discusses pivotal models currently being used. The mechanisms uncovered by these models will ultimately lead to development of new therapeutic options to prevent long-term graft dysfunction.

Pre- and posttransplantation allosensitization in heart allograft recipients: major impact of de novo alloantibody production on allograft survival

The involvement of humoral response in allograft rejection has been suggested by both immunologic and histochemistry studies. In the present study, we explored the role of alloantibodies in a large cohort of heart allograft recipients followed for 15 years. Sequential samples of sera were obtained from 950 recipients of heart allografts before and after transplantation at the time when protocol endomyocardial biopsies were performed. The presence of anti-human leukocyte antigen (HLA) antibodies was investigated using complement mediated cytotoxicity and solid phase assay (SPA). Our data reveal an inverse correlation between the development of alloantibodies after transplantation and heart allograft survival. The 15-year graft survival was highest in patients who never developed alloantibodies (70%) or who displayed them only before transplantation (71%); graft survival in recipients who showed antibodies both before and after transplantation (56%), or only after transplantation (47%), was lower. The deleterious effect of antibodies on graft survival started 8 years after transplantation, suggesting that the production of de novo antibodies may have been triggered by some later event. We found that patients with de novo antibodies appearing more than 1 year after transplantation had the poorest survival. Furthermore, the development of de novo antibodies was preceded in 76% of these patients by cellular rejection grade 3 or higher, according to the International Society for Heart Transplantation (ISHT) grading criteria. Development of antibody-mediated rejection (AMR) had a significant negative impact on graft survival (16% in AMR(+) vs 63% in AMR(-) patients, p = 0.0008). Of the 23 patients with AMR, 21 displayed cytotoxic donor-specific antibodies (DSA) at the time of diagnosis, and in 18 of these cases SPA showed that they were directed against the donors' HLA. The data demonstrate that the detection of alloantibodies permits a better definition of AMR in heart allograft recipients. Identification of patients at risk for developing AMR is of great importance for early treatment of rejection episodes.

Non-MHC antigenic targets of the humoral immune response in transplantation

There is a growing body of data supporting a role for non-HLA antibodies in acute and chronic rejection of solid organ transplants. While many of these non-HLA antigens remain poorly defined, the principal antigenic targets are expressed on cells of the allograft including endothelium and epithelium. These non-HLA antigens are classified as either alloantigens, such as the major histocompatibility complex class I chain-related gene A (MICA) or MICB, or tissue-specific autoantigens such as vimentin, cardiac myosin (CM), collagen V (Col V), agrin, and angiotensin II receptor type I (AT1). Herein we provide an overview of the non-MHC antigenic targets that have been implicated in graft rejection and discuss the interplay between alloimmunity and autoreactivity in graft rejection.

Humoral autoimmunity and transplant vasculopathy: when allo is not enough

For several decades, allograft rejection was believed to be mediated almost exclusively by cellular immune responses, but it is now realized that humoral responses also play a major role. Although directed typically against donor human leukocyte antigen, it is becoming increasingly evident that the antibody response can also target autoantigens that are shared between donor and recipient and that this autoantibody may contribute to graft rejection. Many aspects of transplant-induced humoral autoimmunity remain poorly understood and key questions persist; not least what triggers the response and how autoantibody causes graft damage. Here, we collate results from recent clinical and experimental studies in transplantation and autoimmune diseases to propose answers to these questions.

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.

Mechanism of cellular rejection in transplantation

The explosion of new discoveries in the field of immunology has provided new insights into mechanisms that promote an immune response directed against a transplanted organ. Central to the allograft response are T lymphocytes. This review summarizes the current literature on allorecognition, costimulation, memory T cells, T cell migration, and their role in both acute and chronic graft destruction. An in depth understanding of the cellular mechanisms that result in both acute and chronic allograft rejection will provide new strategies and targeted therapeutics capable of inducing long-lasting, allograft-specific tolerance.

Successful reduction of immunosuppression in older renal transplant recipients who exhibit donor-specific regulation

BACKGROUND

We hypothesized that T-regulatory cells specific for donor alloantigens would protect a renal transplant during partial withdrawal of immunosuppression.

METHODS

To test this hypothesis, 32 renal transplant recipients aged 55 years and older with excellent renal function were tested for donor-specific regulation (DSR) by trans-vivo delayed type hypersensitivity assay at the time of enrollment (T=0) and 6 months later (T=6). Twenty-two patients had prednisone withdrawn during a 3-month period, whereas 10 controls were maintained on triple therapy (prednisone, cyclosporine, and mycophenolate).

RESULTS

Of 22 patients in the steroid withdrawal group, 10 were DSR+ and 12 were DSR- at the time of enrollment (T=0). None of the DSR+ patients experienced acute rejection, nor did any have donor-specific human leukocyte antigen (HLA) antibody during or after withdrawal. Of 12 DSR- patients, three developed acute rejection, which were reversed with bolus steroid treatment, and four were donor-specific antibody+ at T=0 or T=6. Two years later, 80% (8 of 10) of DSR+ patients in the withdrawal group remain steroid free while maintaining excellent renal function, as compared with only 58% (7 of 12) DSR- patients. Patient survival at 4 years was similar for DSR+ (9 of 10) and DSR- (11 of 12) patients in the withdrawal group. Patients maintained on triple therapy remained rejection free during the 4-year follow-up regardless of initial DSR status, with patient survival rate of 70% (7 of 10).

CONCLUSIONS

DSR before steroid withdrawal may identify a subset of transplant patients who could benefit from reduction of immunosuppression without elevated risk of rejection or deteriorating renal function.

Alloantibodies and outcomes of deceased donor kidney allografts

Analysis of the anti-HLA antibody status of 100 recipients of kidneys from deceased donors demonstrated that presensitization and the development of alloantibodies after transplantation are associated with the development of antibody mediated as well as cellular rejection. This finding indicates that the humoral arm of the immune response is also involved in cell-mediated rejection and/or that there may be a continuum between these two forms of rejection. Most episodes of rejection were successfully reversed in our population, as shown by the overall 3-year actuarial survival of 98% in nonsensitized and 91% in sensitized recipients, emphasizing the importance of comprehensive antibody studies.

In vitro assays of allosensitization

Current approaches to the management of immunosuppression are largely empiric and reactive rather than proactive due to our inability to predict accurately how the recipient immune system will respond to a given organ allograft. The validation of simple, reliable, non-invasive assays exploring allogeneic anti donor responsiveness or donor specific non-responsiveness are of interest for several reasons: (i) it would allow for early and non-invasive detection of acute or chronic allograft rejection such that intervention could be initiated before effector mechanisms and organ destruction occur, (ii) it would allow for individual immunosuppressive drug therapy thereby avoiding the unwanted consequences of over immunosuppression, and (iii) the identification of the immunological phenotype related to operational tolerance could allow for the complete cessation of immunosuppressants. This review will summarize in vitro assays of T cell reactivity that reflect allo-antigen-specific responses.

Analysis of indirect pathway CD4+ T cells in a patient with metastable tolerance to a kidney allograft: possible relevance to superior graft survival of HLA class II closely matched renal allografts

MHC class I mismatched, but class II matched kidney transplants are tolerogenic in large animal models. CD4(+) T regulatory cells specific for HLA-B1501-derived peptide p37-MA (DSDAASPRMAPRAPWIEQ) developed in a long term (>12 years) tolerant patient who received an HLA-B1501 mismatched, HLA class II closely matched renal allograft. We hypothesized that class II matching favored T regulatory cell development by allowing allopeptide presentation on either recipient (DR4/DQ7(+)) or donor (DR4/DQ8(+)) antigen presenting cells (APC). Indirect pathway CD4(+) T cell clones were generated from recipient PBMC by sorting antigen-stimulated, proliferating cells. Most clones responded to p37-MA-pulsed B1501(-) autologous, but not to DQ8(+)B1501(+) donor B-lymphoblastoid cell lines (B-LCL). However, some clones responded to both; in fact, one responded even more strongly to donor B-LCL than to p37-MA-pulsed autologous B-LCL. P37-MA contained a DQ8-binding motif and induced strong TH1 responses from DQ8 but not DQ6 transgenic mice. Microchimerism was found to be enriched in the dendritic cells (DC) cultured from adherent PBMC. This indicates donor APC could possibly present p37-MA peptide directly to p37-MA specific T cells. These data support the concept that, when the donor is MHC class II closely matched, a "hybrid" form of allorecognition (direct/indirect) occurs. This may favor the generation of a beneficial form of indirect pathway alloreactivity, i.e. allopeptide-specific CD4(+) T regulatory cells, in the context of long term DC microchimerism.

Immunosuppression for lung transplantation

As a result of advances in surgical techniques, immunosuppressive therapy, and postoperative management, lung transplantation has become an established therapeutic option for individuals with a variety of end-stage lung diseases. The current 1-year actuarial survival rate following lung transplantation is approaching 80%. However, the 5- year actuarial survival rate has remained virtually unchanged at approximately 50% over the last 15 years due to the processes of acute and chronic lung allograft rejection (1). Clinicians still rely on a vast array of immunosuppressive agents to suppress the process of graft rejection, but find themselves limited by an inescapable therapeutic paradox. Insufficient immunosuppression results in graft loss due to rejection, while excess immunosuppression results in increased morbidity and mortality from opportunistic infections and malignancies. Indeed, graft rejection, infection, and malignancy are the three principal causes of mortality for the lung transplant recipient. One should also keep in mind that graft loss in a lung transplant recipient is usually a fatal event, since there is no practical means of long-term mechanical support, and since the prospects of re-transplantation are low, given the shortage of acceptable donor grafts. This chapter reviews the current state of immunosuppressive therapy for lung transplantation and suggests alternative paradigms for the management of future lung transplant recipients.

Differential effects of donor-specific alloantibody

Alloantigen exposure typically provokes an adaptive immune response that can foster rejection of transplanted organs, and these responses present the most formidable biological barrier to kidney transplantation. Although most cellular alloimmune responses can be therapeutically controlled with T-cell-specific immunosuppressants, humoral alloimmune responses remain relatively untamed. Importantly, humoral immunity, typically manifesting as allospecific antibody production, is increasingly recognized for its variable appearance after kidney transplantation. Indeed, the appearance of alloantibody can herald the onset of rapid and destructive antibody-mediated rejection or have no demonstrable acute effects. The factors determining the end result of alloantibody formation remain poorly understood. This review will discuss the breadth of alloantibody responses seen in clinical kidney transplantation and provide an overview of potential factors explaining the phenotypic variability associated with humoral alloimmunity. We propose several avenues ripe for future investigation including the influence of innate immune components and the potential influence of heterologous immune responses in determining the ultimate clinical import of an alloantibody response.

CD8+ suppressor and cytotoxic T cells recognize the same human leukocyte antigen-A2 restricted cytomegalovirus peptide

We explored the possibility that antigen-specific human CD8(+) T cells, which display cytotoxic or suppressor function, can recognize the same peptide epitope. Using the human leukocyte antigen-A0201 restricted immunodominant cytomegalovirus epitope pp65-NLVPMVATV for pulsing either mature/immunogenic or ILT3(high)ILT4(high) tolerogenic dendritic cells (DC), we generated cytotoxic and suppressor CD8(+) T-cell lines, respectively. Our data indicate that modulating the functional state of DC is crucial to the development of tolerogenic or immunogeneic peptide-specific vaccines.

De novo autoimmunity after organ transplantation: targets and possible pathways

The development of autoantibodies to different tissue-specific antigens in patients without existing history of autoimmune diseases suggests that autoimmunity may develop de novo after organ transplantation. In addition to allo-specific immune responses, tissue-specific autoimmunity also appears contribute to the host anti-graft response, and thus may affect long-term graft function. As graft failure caused by chronic rejection is a major challenge in clinical transplantation, understanding de novo autoreactivity after transplantation has important ramifications. This review investigates this emerging concept by discussing target antigens and possible pathways.

Indirect allorecognition: not simple but effective

Donor dendritic cells are potent but short-lived stimulators of early transplant rejection, and recipient antigen presenting cells presenting donor major histocompatibility complex peptides sustain immunoreactivity and contribute to chronic rejection. We briefly consider how CD4 T cells that recognize allopeptide can provide help for effector and regulatory responses and highlight the implications for promoting graft survival.

Monitoring of indirect allorecognition: wishful thinking or solid data?

Monitoring of T cells involved in the alloimmune response after transplantation requires the availability of reliable in vitro assays for the detection of T cells with both direct and indirect allospecificity. While generally accepted assays exist to measure helper and cytotoxic T cells involved in direct allorecognition, consensus about an assay for monitoring indirect T-cell allorecognition in clinical transplantation is lacking. Many studies claim a relationship between the reactivity of T cells with indirect allospecificity and graft rejection, but different protocols are used and essential controls are often lacking. In this review, the disadvantages and pitfalls of the current approaches are discussed, in some cases supported by the results of our own in vitro experiments. We conclude that an international workshop is necessary to establish and validate a uniform, robust and reliable assay for the monitoring of transplant recipients and to study the actual role of indirect allorecognition in acute and chronic rejection.

Circulating alloreactive T cells correlate with graft function in longstanding renal transplant recipients

Monitoring for alloreactive memory T cells after organ transplantation may allow individualization of immunosuppression. Two pathways of T cell allorecognition have been implicated in chronic graft dysfunction: Direct (recipient T cells respond to donor peptides presented by donor antigen-presenting cells) and indirect (donor peptides are processed and presented by recipient antigen-presenting cells). Previous studies have assessed these alloresponses only during the first 2 yr after kidney transplantation,so this study correlated the presence of circulating donor-reactive memory/effector T cells, primed by both pathways, in 34 longstanding living-donor renal transplant recipients using the highly sensitive IFN-gamma Elispot assay. Remarkably, 59% of patients had directly primed donor-reactive T cells, and their presence correlated directly with serum creatinine (P = 0.001) and inversely with estimated GFR (P = 0.042). Multivariate analysis revealed that hyporesponsiveness of direct, donor-specific T cells was the only variable that significantly correlated with graft function and that antidonor indirect alloreactivity was the only variable that significantly correlated with proteinuria. Interestingly, when both allorecognition pathways were considered together, patients with undetectable direct alloreactivity had better longterm graft function, independent of allosensitization by the indirect pathway. In conclusion, circulating donor-specific alloreactive T cells primed by both pathways are detectable long after transplantation and are associated with graft injury. Assessment of alloreactive memory/effector T cells might be helpful to tailor individual immunosuppression regimens for transplant recipients in the future.

Pretransplant donor-specific and non-specific immune parameters associated with early acute rejection

BACKGROUND

New immunosuppression protocols have resulted in decreased rates of biopsy-proven acute rejection; however, it is unclear whether recipients without biopsy-proven acute rejection are still at risk for immune complication and chronic allograft dysfunction. The aim of our studies was to determine whether pretransplant immune parameters were associated with posttransplant early acute rejection, unstable creatinine courses, and poor graft outcome.

METHODS

Immune parameters, including human leukocyte antigen (HLA) mismatch, HLA-specific antibodies, global CD4+ cellular response as measured by intracellular adenosine triphosphate (iATP) synthesis, and IFN-gamma precursor frequencies to donor or third-party cells as measured by ELISPOT were determined for a total of 126 kidney recipients treated with a protocol, including rapid discontinuation of prednisone.

RESULTS

The donor specific pretransplant parameters of HLA class I mismatches (P=0.04) and total HLA mismatches (P=0.04) with the donor as well as the pretransplant HLA-donor specific antibodies (P=0.002) were associated with biopsy-proven acute rejection. Higher pretransplant iATP levels, a donor nonspecific parameter, were found associated with biopsy proven acute rejection (P=0.04). Pretransplant iATP levels were significantly greater for recipients with early unstable creatinine levels (P=0.01). Recipients with a pretransplant iATP value greater than 375 ng/ml were 3.67 times more likely to experience acute rejection (P=0.03).

CONCLUSIONS

Pretransplant assessment of donor specific and nonspecific immune parameters may identify recipients who can benefit from closer clinical and immunological surveillance to allow for tailored immunsuppression and selective intervention aimed at optimizing both short and long-term graft outcome.

The protective effect of CD8+CD28- T suppressor cells on the acute rejection responses in rat liver transplantation

Regulatory T cells (Tr) or T-suppressor cells (Ts), which include CD4+CD25+ T cells and CD8+CD28- T cells respectively, have been shown to be essential for the induction and maintenance of immune tolerance. We have investigated the effect of CD8+CD28- Ts and CD4+CD25+ Tr on acute rejection responses in rat liver transplantation (OLT).

METHODS

CD8+CD28- Ts/CD4+CD25+ Tr were obtained from inbred and naïve rats that show spontaneous tolerance to OLT. Adoptive transfers were performed in acute rejection models of various strain combinations with survival times observed to evaluate suppressive effects. Donor-specific blood transfusion (DST) was used to induce CD8+CD28- Ts in naïve rats, which were assayed in vitro using carboxyfluorescein diacetate succinimidyl easter-labeled one-way mixed lymphocyte reactions. Secondary adoptive transfers of DST-induced CD8+CD28- Ts were also performed in an acute OLT rejection model.

RESULTS

CD8+CD28- Ts from tolerant OLT model rats possessed immunosuppressive activity in allogeneic recipients; adoptive transfers of these cells alleviated the acute rejection responses. However, CD4+CD25+ Tr derived from tolerant or naïve rats failed to do so. In vitro DST-induced CD8+CD28- Ts inhibited alloantigen T-cell responses in naïve syngeneic rats in an antigen-specific manner. Secondary adoptive transfer of DST-induced CD8+CD28-Ts further reduced acute rejection but not chronic rejection responses.

CONCLUSIONS

CD8+CD28- Ts cells protected allogeneic recipients from acute rejection in rat OLT. Furthermore, this activity was not present in CD4+CD25+Tr. DST was observed to be an effective method to generate functional CD8+CD28-Ts in naïve rats.

Pretransplant donor-specific interferon-gamma ELISPOT assay predicts acute rejection episodes in renal transplant recipients

Interferon (IFN)-gamma enzyme-linked immunosorbent spot (ELISPOT) assay is a powerful tool for measuring the frequency of alloantigen-specific T cells, reflecting cellular immunity. We correlated the pretransplant frequencies of donor-specific and third-party-specific IFN-gamma ELISPOT tests, with the posttransplant outcomes of 45 recipients of living donor renal transplantations. The mean frequency of pretransplant donor-specific ELISPOT was significantly greater among patients with acute rejection episodes (ARE) than those without ARE (18.0 [12 to 50] versus 8.8 [5 to 30.4]) spots per 200,000 peripheral blood lymphocytes (PBLs; P=.024). A cutoff level of 12 spots per 200,000 PBLs on the donor-specific ELISPOT identified an ARE-positive patient with a sensitivity of 81.8% and a specificity of 64.7%. The recipients with pretransplant donor-specific ELISPOT+showed higher serum creatinine levels and lower glomerular filtration rate (GFR) at 6 posttransplant months (P<.05). Although the pretransplant third-party-specific ELISPOT results correlated with the donor-specific ELISPOT results (r=.783; P<.001), there was no significant difference in the third-party ELISPOT results between the ARE-positive and ARE-negative recipients. In conclusion, an analysis of pretransplant donor-specific IFN-gamma ELISPOT may identify the posttransplant risk of developing ARE and displaying decreased GFR at 6 months.

Mechanisms of chronic rejection in cardiothoracic transplantation

Despite significant improvements in early post-transplantation survival rates, long-term patient and graft survival have remained poor, due in large part to the vexing problem of chronic allograft rejection. Attempts to combat this problem with intensification of immunosuppression have led to concomitant increases in the rates of fatal malignancies and infections. In cardiac transplantation, chronic rejection is manifested primarily by a disease entity known as cardiac allograft vasculopathy, an occlusive narrowing of the coronary vessels. In lung transplantation, chronic rejection is typified by obliterative bronchiolitis, an airflow limiting narrowing of the bronchioles. From an immunologic standpoint, chronic rejection is believed to be the end result of repeated immune and non-immune insults to the graft. This review examines the pathophysiology of heart and lung chronic, with emphasis on both immune and non-immune causes.

Analysis of arterial intimal hyperplasia: review and hypothesis

BACKGROUND

Despite a prodigious investment of funds, we cannot treat or prevent arteriosclerosis and restenosis, particularly its major pathology, arterial intimal hyperplasia. A cornerstone question lies behind all approaches to the disease: what causes the pathology?

HYPOTHESIS

I argue that the question itself is misplaced because it implies that intimal hyperplasia is a novel pathological phenomenon caused by new mechanisms. A simple inquiry into arterial morphology shows the opposite is true. The normal multi-layer cellular organization of the tunica intima is identical to that of diseased hyperplasia; it is the standard arterial system design in all placentals at least as large as rabbits, including humans. Formed initially as one-layer endothelium lining, this phenotype can either be maintained or differentiate into a normal multi-layer cellular lining, so striking in its resemblance to diseased hyperplasia that we have to name it "benign intimal hyperplasia". However, normal or "benign" intimal hyperplasia, although microscopically identical to pathology, is a controllable phenotype that rarely compromises blood supply. It is remarkable that each human heart has coronary arteries in which a single-layer endothelium differentiates early in life to form a multi-layer intimal hyperplasia and then continues to self-renew in a controlled manner throughout life, relatively rarely compromising the blood supply to the heart, causing complications requiring intervention only in a small fraction of the population, while all humans are carriers of benign hyperplasia. Unfortunately, this fundamental fact has not been widely appreciated in arteriosclerosis research and medical education, which continue to operate on the assumption that the normal arterial intima is always an "ideal" single-layer endothelium. As a result, the disease is perceived and studied as a new pathological event caused by new mechanisms. The discovery that normal coronary arteries are morphologically indistinguishable from deadly coronary arteriosclerosis continues to elicit surprise.

CONCLUSION

Two questions should inform the priorities of our research: (1) what controls switch the single cell-layer intimal phenotype into normal hyperplasia? (2) how is normal (benign) hyperplasia maintained? We would be hard-pressed to gain practical insights without scrutinizing our premises.

Allopeptide-specific CD4(+) T cells facilitate the differentiation of directly alloreactive graft-infiltrating CD8(+) T Cells

To investigate the mechanism of CD4(+) T-cell help during the activation and differentiation of directly alloreactive CD8(+) T cells, we examined the development of obliterative airways disease (OAD) following transplantation of airways into fully mismatched recipient mice deficient in CD4(+) T cells. BALB/c trachea allografts became fibrosed significantly less frequently in B6 CD4(-/-) recipients as compared to wildtype controls. Furthermore, class I-directed cytotoxicity failed to develop in the absence of CD4(+) T cells. The infiltration of graft tissue by primed L(d)-specific directly alloreactive 2C CD8(+) T cells was not found to depend on the presence of CD4(+) T cells. Nevertheless, graft-infiltrating 2C CD8(+) T cells failed to express CD69 and granzyme B when CD4(+) T-cell help was unavailable. Importantly, reconstitution of B6 CD4(-/-) recipient mice with graft peptide-specific TCR-Tg CD4(+) T cells (OT-II or TEa) capable of recognizing antigen only on recipient APC allowed for full expression of CD69 and granzyme B by the directly alloreactive CD8(+) T cells and restored the capacity of recipients to reject their allografts. These results demonstrate that indirectly alloreactive CD4(+) T cells ensure the optimal activation and differentiation of graft-infiltrating directly alloreactive CD8(+) T cells independent of donor APC recognition.

CD4+CD25+Foxp3+ indirect alloreactive T cells from renal transplant patients suppress both the direct and indirect pathways of allorecognition

Alloreactive T cells recognize donor antigens by two routes: direct and indirect pathways of allorecognition. Although the direct pathway is reported to be dominant in allograft rejection, indirect allorecognition also plays an important role. Indirect alloreactivity is also observed in renal transplant patients irrespective of rejection. Previously we showed a predominance of interleukin (IL)-10 induced by indirect allorecognition of donor human leucocyte antigen (HLA)-DR peptides, suggesting the existence of indirect alloreactive T cells displaying regulatory activity. In the present work, our objective was to characterize these regulatory T cells. We detected indirect alloproliferation of peripheral blood mononuclear cells (PBMC) from renal transplant patients, induced by donor HLA-DR peptides, dependent on IL-4 or IL-10, suggesting regulatory activity as part of the alloreactive T-cell repertoire. PBMC-derived indirect alloreactive T-cell lines were established and produced both inflammatory and regulatory cytokines. We showed that two of these T-cell lines which were able to inhibit both direct and indirect alloproliferation of another T-cell line from the same patient presented a CD4(+)CD25(+)Foxp3(+) T-cell population. These data support the idea that indirect alloreactive T cells may also have regulatory activity and may contribute to the maintenance of the human renal allograft.

FasL expression in hepatic antigen-presenting cells and phagocytosis of apoptotic T cells by FasL+ Kupffer cells are indicators of rejection activity in human liver allografts

Fas-Fas ligand (FasL) interaction and apoptosis are important in the mechanism of allograft rejection. However, the interaction between donor and recipient cells, specifically focusing on antigen-presenting cells (APCs), under various conditions is poorly understood in human liver allografts. FasL expression on APCs, its association with apoptosis, and the origin of apoptotic lymphocytes in human liver allografts were assessed by immunohistochemistry and in situ hybridization. We found increased expression of FasL on Kupffer cells (KCs) and endothelium in acute cellular rejection (n = 20) and to lesser extent in chronic rejection (n = 6) and septic cholangitis (n = 5) compared with stable grafts and normal controls. In addition, the graft specificity of infiltrating T cells was confirmed by polymerase chain reaction examination of T-cell receptor-gamma loci. T-cell apoptosis occurred at a higher rate in acute cellular rejection than in chronic rejection or septic cholangitis. The number of apoptotic bodies derived from recipient lymphocytes correlated with the severity of rejection and was reversed by treatment. FasL(+) KCs phagocytosed CD4(+) interferon-gamma(+) T cells, rather than CD4(+) interleukin-4(+) T cells, suggesting a role of KCs in regulating CD4(+) T-cell subset differentiation. In conclusion, our data suggest that FasL expression on APCs and phagocytosis of apoptotic T cells by FasL(+) KCs are indicators of rejection activity in human liver allografts.

Class I MHC allochimeric presentation of composite immunogenic and self epitopes induces tolerance to genetically diverse rat strains

Functional topography of rat class I major histocompatibility complex (MHC) molecule was studied. The alpha1-helical sequences that are shared by class I RT1.A(l) and RT1.A(u) were substituted in the RT1.A(a) molecule to produce the composite [alpha(1h)(l/u)]-RT1.A(a) MHC class I allochimeric molecule. Dominant immunogenic epitopes that induce accelerated rejection were identified within the hypervariable regions of the alpha1 domain of RT1.A(a), RT1.A(l), and RT1.A(u). Peri-transplant portal venous delivery of MHC class I allochimeric proteins, that included composite alpha1 helical immunodominant epitopes of RT1.A(u) and RT1.A(l), induced donor-specific tolerance to RT1(u) (Wistar Furth, WF) and RT1(l) Lewis, LEW) disparate cardiac allografts in ACI (RT1(a)) hosts. Allochimeric generated tolerance was characterized by absence of T cell deletion or anergy. Donor specific IgM allo-Abs was not detected, while IgG alloresponse was markedly attenuated in sera of tolerant hosts. Further, long-term allografts in allochimeric-conditioned hosts exhibited moderate B cell infiltration when compared to rejecting controls. Analysis of intragraft cytokines revealed selective upregulation of IL-10 and marked inhibition of IL-2, IFN-gamma, and IL-4. Our findings indicate the emergence of a peripherally induced tolerant state, afforded by the novel approach of soluble class I allochimeric conditioning that presents donor immunogenic epitopes in the context of recipient class I determinants.

Chronic Allograft Nephropathy

Chronic allograft nephropathy (CAN) remains the Achilles heel of renal transplantation. In spite of the significant strides achieved in one-year renal allograft survival with newer immunosuppressant strategies, the fate of long-term renal allograft survival remains unchanged. The number of renal transplant recipients returning to dialysis has doubled in the past decade. This is especially important since these patients pose a significantly increased likelihood of dying while on the waiting list for retransplantation, due to increasing disparity between donor organ availability versus demand and longer waiting time secondary to heightened immunologic sensitization from their prior transplants. In this review we analyze the latest literature in detail and discuss the definition, natural history, pathophysiology, alloantigen dependent and independent factors that play a crucial role in CAN and the potential newer therapeutic targets on the horizon. This article highlights the importance of early identification and careful management of all the potential contributing factors with particular emphasis on prevention rather than cure of CAN as the core management strategy.

Administration of the stress protein gp96 prolongs rat cardiac allograft survival, modifies rejection-associated inflammatory events, and induces a state of peripheral T-cell hyporesponsiveness

High-dose gp96 has been shown to inhibit experimental autoimmune disease by a mechanism that appears to involve immunoregulatory CD4+ T cells. This study tested the hypothesis that high-dose gp96 administration modifies allograft rejection and associated inflammatory events. Wistar cardiac allografts were transplanted into Lewis recipient rats and graft function was monitored daily by palpation. Intradermal administration of gp96 purified from Wistar rat livers (100 microg) at the time of transplantation and 3 days later significantly prolonged allograft survival (14 vs 8 days in phosphate-buffered saline [PBS]-treated recipients; P = 0.009). Rejected allografts from gp96-treated animals were significantly less enlarged than allografts from their PBS-treated counterparts (2.8 vs 4.3 g; P < 0.004). Gp96 was also effective when administered on days 1 and 8 (13 vs 7 days), but not if it was derived from recipient (Lewis) liver tissue or administered on days 0, 3, and 6. In parallel studies, CD3+ T cells from gp96-treated untransplanted animals secreted less interleukin (IL)-4, IL-10, and interferon (IFN)-gamma after in vitro polyclonal stimulation than CD3+ T cells from PBS-treated animals. Gp96 administration might therefore influence the induction of immunity to coencountered antigenic challenges and inflammatory events by inducing what appears to be a state of peripheral T-cell hyporesponsiveness.

Indirect recognition of T-cell epitopes derived from the alpha 3 and transmembrane domain of HLA-A2

Indirect allorecognition has been implicated in the mechanism of chronic rejection and alloantibody formation but precise definition of the epitopes involved has been limited. We have undertaken a detailed assessment of the antigenic properties of peptides derived from HLA-A2. Candidate epitopes were identified in vitro by assessment of MHC class II binding. The immune response to these epitopes was determined in patients awaiting a renal transplant by the assessment of PBMC activation using gamma-interferon ELISPOT. Twenty-two of fifty-five patients responded to peptides from HLA-A2 and this was associated with but not confined to those who had made antibody to HLA-A2 (14/18). Nineteen of twenty-two patients responded to peptides derived from the hypervariable alpha1 and alpha2 domains and 18/22 responded to peptides from the alpha 3 and transmembrane domain, the sequences of which show little polymorphism. In six patients, the sequence of these peptides was identical to self, that is, the response was autoimmune. The finding of indirect epitopes derived from regions of MHC class I that exhibit little polymorphism provides a novel perspective on the immune response to alloantigen and has potential implications for the development of specific therapies.

Long-Term Cell Monitoring of Kidney Recipients After an Antilymphocyte Globulin Induction With and Without Steroids

BACKGROUND

Because of several side effects, the corticosteroid usage has been minimized in kidney transplantation. The increased acute rejection episodes associated with their withdrawal may counterbalance with induction treatment using polyclonal antilymphocyte globulin (ALG). The effects of ALG on blood cell phenotype have already been the subject of several reports. However, to date, no data are available concerning the comparison of blood phenotype when ALG is given with or without steroids and no gene profiling study has been performed.

METHODS

We report here on a longitudinal blood cell analysis of a selected cohort of kidney recipients enrolled in a randomized study of steroid avoidance or withdrawal (during 6 months) during ALG induction.

RESULTS

In the two groups, ALG quickly and massively depleted all the T cells and natural killer cells, but not B cells. Interestingly, the lymphopenia-driven homeostatic proliferation of CD4 and CD8T cells strongly differed with persistent low CD4 (including CD25CD4) T-cell counts. Effector memory CD8T cells reappeared rapidly. ALG induced apoptosis-associated molecules and increased myeloid cell genes. However, few genes were found differentially expressed with a low fold ratio between the two groups during and at distance of corticotherapy.

CONCLUSION

Thus initial steroid avoidance or withdrawal associated with ALG induction has a weak influence on phenotype and transcriptional pattern of blood leukocytes. In contrast, ALG therapy induces an early and strong depletion of all T-cell subsets with contrasted long-lasting homeostatic regulation.

Induction of IL-10 suppressors in lung transplant patients by CD4+25+ regulatory T cells through CTLA-4 signaling

T cell-mediated autoimmunity to collagen V (col-V), a sequestered yet immunogenic self-protein, can induce chronic lung allograft rejection in rodent models. In this study we characterized the role of CD4+ CD25+ regulatory T cells (Tregs) in regulating col-V autoimmunity in human lung transplant (LT) recipients. LT recipients revealed a high frequency of col-V-reactive, IL-10-producing CD4+ T cells (T IL-10 cells) with low IL-2-, IFN-gamma-, IL-5-, and no IL-4-producing T cells. These T(IL-10) cells were distinct from Tregs because they lacked constitutive expression of both CD25 and Foxp3. Expansion of T IL-10 cells during col-V stimulation in vitro involved CTLA-4 on Tregs, because both depleting and blocking Tregs with anti-CTLA4 F(ab')2 mAbs resulted in loss of T IL-10 cells with a concomitant increase in IFN-gamma producing Th1 cells (TIFN-gamma cells). A Transwell culture of col-V-specific T IL-10 cells with Th1 cells (those generated in absence of Tregs) from the same patient resulted in marked inhibition of IFN-gamma and proliferation of T(IFN-gamma) cells, which was reversed by neutralizing IL-10. Furthermore, the T IL-10 cells were HLA class II restricted because blocking HLA class II on APCs resulted in the loss of IL-10 production. Chronic lung allograft rejection was associated with the loss of Tregs with a concomitant decrease in T IL-10 cells and an increase in T IFN-gamma cells. We conclude that LT patients have col-V-specific T cells that can be detected in the peripheral blood. The predominant col-V-specific T cells produce IL-10 that suppresses autoreactive Th1 cells independently of direct cellular contact. Tregs are pivotal for the induction of these "suppressor" T IL-10 cells.

From current immunosuppressive strategies to clinical tolerance of allografts

In order to prevent allograft rejection, most current immunosuppressive drugs nonspecifically target T-cell activation, clonal expansion or differentiation into effector cells. Experimental models have shown that it is possible to exploit the central and peripheral mechanisms that normally maintain immune homeostasis and tolerance to self-antigens, in order to induce tolerance to alloantigens. Central tolerance results from intrathymic deletion of T cells with high avidity for thymically expressed antigens. Peripheral tolerance to nonself-molecules can be achieved by various mechanisms including deletion of activated/effector T cells, anergy induction and active regulation of effector T cells. In this article, we briefly discuss the pathways of allorecognition and their relevance to current immunosuppressive strategies and to the induction of transplantation tolerance (through haematopoietic mixed chimerism, depleting protocols, costimulatory blockade and regulatory T cells). We then review the prospect of clinical applicability of these protocols in solid organ transplantation.

CD4+CD25+ regulatory T cell therapy for the induction of donor-specific clinical transplantation tolerance

Naturally occurring CD4+CD25+ regulatory T cells (Tregs) have been shown to play a key role in the control of autoimmunity. Interestingly, they are also capable of mediating transplantation tolerance and they can have indirect allospecificity for donor antigens. An increasing body of evidence in experimental studies has indicated that adoptive transfer of in vitro expanded CD4+CD25+ Tregs with indirect antidonor allospecificity can induce long-term donor-specific transplantation tolerance. Thus, adoptive cell therapy using patient-specific CD4+CD25+ Tregs as individualised medicine to promote clinical transplantation tolerance is promising.

Role of airway epithelial injury in murine orthotopic tracheal allograft rejection

BACKGROUND

Murine tracheal transplantation is a model used to study bronchiolitis obliterans syndrome, a major cause of morbidity and mortality after lung transplantation. Unlike murine heterotopic tracheal transplants, orthotopic transplantation does not cause luminal obliteration despite major histocompatibility antigen mismatch. Repopulation of the tracheal allografts with recipient-derived epithelium confers protection against luminal obliteration. The purpose of this study was to determine whether (1) orthotopic tracheal transplantation showed signs of allograft rejection, and (2) airway epithelial cell injury promoted orthotopic tracheal allograft rejection.

METHODS

Forty isogeneic (C57BL/6 to C57BL/6) and 40 allogeneic (BALB/c to C57BL/6) orthotopic tracheal transplants were performed. Damage to airway epithelial cells was induced by Sendai viral (SdV) infection and tracheal transplantation into non-reepithelializing matrix metalloproteinase-7 knockout (MMP7-KO) recipient mice. Percent fibrosis and lamina propria to cartilage ratio were calculated with computer assistance on harvested allografts.

RESULTS

Allografts showed significantly more intramural fibrosis compared with isografts at 30, 60, and 180 days after transplant without luminal occlusion. Tracheal allografts infected with SdV showed an increase in fibrosis and lamina propria to cartilage ratio compared with noninfected controls. Allografts retrieved from MMP7-KO recipients also showed a significant increase in fibrosis and lamina propria to cartilage ratio.

CONCLUSIONS

Although orthotopic tracheal transplantation does not cause luminal obliteration, it results in increased fibrosis in allografts. Damage to the respiratory epithelium by viral infection or defective reepithelialization after transplant as seen in MMP7-KO recipient mice leads to changes consistent with chronic allograft rejection, suggesting a role for epithelial injury in bronchiolitis obliterans syndrome development.

In vitro–expanded donor alloantigen–specific CD4+CD25+ regulatory T cells promote experimental transplantation tolerance

CD4+CD25+ regulatory T (Treg) cells play a critical role in the induction and maintenance of peripheral immune tolerance. In experimental transplantation models in which tolerance was induced, donor-specific Treg cells could be identified that were capable of transferring the tolerant state to naive animals. Furthermore, these cells appeared to have indirect allospecificity for donor antigens. Here we show that in vivo alloresponses can be regulated by donor alloantigen-specific Treg cells selected and expanded in vitro. Using autologous dendritic cells pulsed with an allopeptide from H2-Kb, we generated and expanded T-cell lines from purified Treg cells of CBA mice (H2k). Compared with fresh Treg cells, the cell lines maintained their characteristic phenotype, suppressive function, and homing capacities in vivo. When cotransferred with naive CD4+CD25− effector T cells after thymectomy and T-cell depletion in CBA mice that received CBK (H2k+Kb) skin grafts, the expanded Treg cells preferentially accumulated in the graft-draining lymph nodes and within the graft while preventing CBK but not third-party B10.A (H2k+Dd) skin graft rejection. In wild-type CBA, these donor-specific Treg cells significantly delayed CBK skin graft rejection without any other immunosuppression. Taken together, these data suggest that in vitro–generated tailored Treg cells could be considered a therapeutic tool to promote donor-specific transplant tolerance.

How can we measure immunologic tolerance in humans?

Prevention and treatment of allograft rejection in organ transplant recipients relies primarily on non-antigen-specific immunosuppression, with all its associated potential hazards and costs. Currently, the status of the recipient immune response is measured by monitoring pharmacologic drug levels and clinical/pathologic evaluation of graft function. Development of reliable assays that can measure accurately the status of the immune response not only would help clinicians customize the prescription of immunosuppressive drugs in individual patients but also may allow their complete withdrawal in some patients with immunologic tolerance. Furthermore, these assays would facilitate the safe evaluation of novel tolerogenic regimens. Achieving this goal has proved to be very difficult because it requires both a more in-depth understanding of complex mechanisms of tolerance and also identification of transplant patients with acquired tolerance to an allograft that can be studied. This review discusses the current understanding of tolerance mechanisms and outlines the unique and specific challenges in development of tolerance/monitoring assays in the field of transplantation. In addition, several of the most promising candidate assays are discussed in detail.

Liver biopsy interpretation for causes of late liver allograft dysfunction

Evaluation of needle biopsies and extensive clinicopathological correlation play an important role in the determination of liver allograft dysfunction occurring more than 1 year after transplantation. Interpretation of these biopsies can be quite difficult because of the high incidence of recurrent diseases that show histopathological, clinical, and serological features that overlap with each other and with rejection. Also, more than one insult can contribute to allograft injury. In an attempt to enable centers to compare and pool results, improve therapy, and better understand pathophysiological disease mechanisms, the Banff Working Group on Liver Allograft Pathology herein proposes a set of consensus criteria for the most common and problematic causes of late liver allograft dysfunction, including late-onset acute and chronic rejection, recurrent and new-onset viral and autoimmune hepatitis, biliary strictures, and recurrent primary biliary cirrhosis and primary sclerosing cholangitis. A discussion of differential diagnosis is also presented.

ILT3+ ILT4+ Tolerogenic Endothelial Cells in Transplantation

T cells can recognize allogeneic major histocompatibility complex (MHC) antigens by two distinct routes: either directly as intact molecules or indirectly as processed peptides presented by syngeneic antigen-presenting cells (APC). The graft endothelium plays an important role in rejection eliciting and serving as a target of T cells activated via the direct and/or indirect allorecognition pathway. Recent evidence demonstrates, however, that endothelial cells are also endowed with the capacity to downregulate alloreactivity inducing tolerogenic responses. Similar to professional APC (such as dendritic cells), endothelial cells express high levels of inhibitory receptors (ILT3 and ILT4 in humans and PIR-B in rodents) and low levels of costimulatory and adhesion molecules upon interaction with allospecific CD8 T suppressor cells or exposure to inhibitory cytokines. Because of the importance of endothelial cells in the activation and control of T cell reactivity, understanding of their biology is crucial for the development of new strategies for induction of transplantation tolerance and treatment of cancer, chronic infection, and autoimmunity.

T Cell Costimulation in the Development of Cardiac Allograft Vasculopathy

Cardiac allograft vasculopathy (CAV) is a form of coronary arterial stenosis and a leading cause of death in patients who survive beyond the first year after heart transplantation. Histopathologically, this lesion is concentric diffuse intimal hyperplasia of the arterial wall that is accompanied by extensive infiltration of inflammatory cells, including T cells. Many studies have explored the potential risk factors related to this arterial lesion and its pathogenesis. Continuous minor endothelial cell damage evokes inflammatory processes including T cell activation. Costimulatory molecules play crucial roles in this T cell activation. Many costimulatory pathways have been described, and some are involved in the pathogenesis of CAV, atherogenesis, and subsequent plaque formation. In this review, we summarize the present knowledge of the role of these pathways in CAV development and the possibility of manipulating these pathways as a means to treat heart allograft vascular disease and atherosclerosis.

Sensitization after Kidney Transplantation: Table 1

Kidney transplant recipients may develop de novo anti-HLA and non-HLA antibodies after transplantation. Although these antibodies may be donor-specific or non-donor-specific, their presence may increase the risk for acute and chronic rejection, thereby decreasing allograft survival. The introduction of more sensitive and specific methods to detect anti-HLA antibodies, such as Flow Specific Beads and FlowPRA, both before and after transplantation, will help to define immunologically high-risk kidney transplant recipients. Thus, posttransplantation monitoring of anti-HLA antibody production will allow the identification of kidney transplant recipients who might be at increased risk for late allograft failure. Moreover, knowledge of alloantibody status after transplantation may help to guide the appropriate use of immunomodulatory agents to downregulate anti-HLA antibody production.