B-cell immunity in the context of T-cell tolerance after combined kidney and bone marrow transplantation in humans

Clinical outcomes of children receiving ABO-incompatible versus ABO-compatible heart transplantation: a multicentre cohort study

BACKGROUND

ABO-incompatible heart transplantation increases donor availability in young children and is evolving into standard of care in children younger than 2 years. Previous smaller studies suggest similar outcomes to ABO-compatible heart transplantation, but persisting alterations of the immune system in ABO-incompatible recipients might increase the risk of some infections or benefit the graft owing to reduced HLA reactivity. We aimed to assess long-term outcomes in young children after they received ABO-incompatible or ABO-compatible heart transplantation.

METHODS

In this multicentre, prospective cohort study, we analysed data from the Pediatric Heart Transplant Society registry to compare children who received ABO-incompatible or ABO-compatible heart transplantation before age 2 years between Jan 1, 1999, and June 30, 2018. Given significantly different clinical demographics between the two groups, we also matched each ABO-incompatible recipient to two ABO-compatible recipients using propensity score matching. We assessed patient and graft survival, coronary allograft vasculopathy, malignancy, acute rejection (any episode resulting in augmentation of immunosuppression), and infections (requiring intravenous antibiotic or antiviral therapy or life-threatening infections treated with oral therapy).

FINDINGS

We included 2206 children who received a heart transplant before age 2 years, with 11 332·6 patient-years of cumulative observation time. Children who received an ABO-incompatible transplant (n=364) were younger and a larger proportion had congenital heart disease and ventilator and mechanical circulatory support than the ABO-compatible recipients (n=1842). After matching, only differences in blood group (more O in ABO-incompatible and more AB in ABO-compatible groups) and use of polyclonal induction therapy with anti-thymocyte globulins persisted. The two matched groups had similar post-transplantation graft survival (p=0·74), freedom from coronary allograft vasculopathy (p=0·75), and malignancy (p=0·51). ABO-incompatible recipients showed longer freedom from rejection (p=0·0021) in the overall cohort, but not after matching (p=0·48). Severe infections (p=0·0007), bacterial infections (p=0·0005), and infections with polysaccharide encapsulated bacteria (p=0·0005) that share immunological properties with blood group antigens occurred less frequently after ABO-incompatible heart transplantation.

INTERPRETATION

ABO-incompatible heart transplantation for children younger than 2 years is a clinically safe approach, with similar survival and incidences of rejection, coronary allograft vasculopathy, and malignancy to ABO-compatible recipients, despite higher-risk pre-transplant profiles. ABO-incompatible transplantation was associated with less bacterial infection, particularly encapsulated bacteria, suggesting that the acquired immunological changes accompanying ABO tolerance might benefit rather than jeopardise transplanted children.

FUNDING

Pediatric Heart Transplant Society.

Combining Treg therapy with mixed chimerism: Getting the best of both worlds

Deliberate establishment of donor-specific immunologic tolerance is considered to be the "Holy Grail" in transplantation medicine, but clinical tolerance protocols for routine organ transplantation are still an unmet need. Mixed hematopoietic chimerism is an attractive tolerance strategy with considerable potential. Recent pilot trials provide proof-of-principle that mixed chimerism can induce tolerance in renal transplant recipients. Routine clinical translation, however, is impeded by the side effects of the cytotoxic recipient conditioning necessary for the transient engraftment of HLA-mismatched BM. In murine studies recently published in The American Journal of Transplantation, we demonstrated that the therapeutic application of polyclonal recipient regulatory T cells (Tregs) leads to engraftment of practicable doses of fully allogeneic BM and to donor-specific tolerance without any cytotoxic conditioning, thereby eliminating a major impediment for the clinical translation of the mixed chimerism strategy in the experimental setting. The background and the implications of these findings are discussed.

Immune monitoring in renal transplantation: The search for biomarkers

It is now widely accepted that in order to improve long-term graft function and survival, a more personalized immunosuppressive treatment of transplant patients according to the individual anti-donor immune response status is needed. This applies to the identification of potentially "high-risk" patients likely to develop acute rejection episodes or display an accelerated decline of graft function, patients who might need immunosuppression intensification, and operationally tolerant patients suitable for immunosuppression minimization or weaning off. Such a patient stratification would benefit from biomarkers, which enable categorization into low and high risk or, ideally, identification of operational tolerant patients. Here, we report on recent developments regarding identification and performance analysis of noninvasive biomarkers such as mRNA and miRNA expression profiles, chemokines, or changes in immune cell subsets in either blood or urine of renal transplant patients. We will also discuss which future steps are needed to accelerate their clinical implementation.

Dynamics of B Cell Recovery In Kidney/Bone Marrow Transplant Recipients

BACKGROUND

Previous studies identified B cell gene signatures and predominance of specific B cell subsets as a marker of operational tolerance after kidney transplantation. These findings suggested a role for B cells in the establishment or maintenance of tolerance. Here we analyzed B cell recovery in 4 subjects, 3 of whom achieved tolerance after combined kidney/bone marrow transplantation.

METHODS

Peripheral B cell subsets were examined longitudinally by flow cytometry. Immunoglobulin heavy chain repertoire analysis was performed using next-generation sequencing. Lastly, the patients' serum reactivity to HLA was assessed by Luminex.

RESULTS

B cell counts recovered approximately 1 year posttransplant except for 1 subject who experienced delayed reconstitution. This subject resumed immunosuppression for acute rejection at 10 months posttransplant and underwent preemptive retransplantation at 3 years for chronic rejection. B cell recovery was accompanied by a high frequency of CD20 + CD24CD38 transitional B cells and a diversified clonal repertoire. However, all 4 subjects showed prevalence of CD20 + CD27+ memory B cells around 6 months posttransplant when B cell counts were still low and the clonal B cell repertoire very limited. The predominance of memory B cells was also associated with high levels of somatically mutated immunoglobulin heavy chain variable sequences and transient serum reactivity to HLA.

CONCLUSIONS

Our observations reveal the presence of memory B cells early posttransplant that likely escaped the preparative regimen at a time consistent with the establishment of tolerance. Further studies are warranted to characterize the functional properties of these persisting memory cells and evaluate their potential contribution to tolerance induction.

Novel Therapeutics Identification for Fibrosis in Renal Allograft Using Integrative Informatics Approach

Chronic allograft damage, defined by interstitial fibrosis and tubular atrophy (IF/TA), is a leading cause of allograft failure. Few effective therapeutic options are available to prevent the progression of IF/TA. We applied a meta-analysis approach on IF/TA molecular datasets in Gene Expression Omnibus to identify a robust 85-gene signature, which was used for computational drug repurposing analysis. Among the top ranked compounds predicted to be therapeutic for IF/TA were azathioprine, a drug to prevent acute rejection in renal transplantation, and kaempferol and esculetin, two drugs not previously described to have efficacy for IF/TA. We experimentally validated the anti-fibrosis effects of kaempferol and esculetin using renal tubular cells in vitro and in vivo in a mouse Unilateral Ureteric Obstruction (UUO) model. Kaempferol significantly attenuated TGF-β1-mediated profibrotic pathways in vitro and in vivo, while esculetin significantly inhibited Wnt/β-catenin pathway in vitro and in vivo. Histology confirmed significantly abrogated fibrosis by kaempferol and esculetin in vivo. We developed an integrative computational framework to identify kaempferol and esculetin as putatively novel therapies for IF/TA and provided experimental evidence for their therapeutic activities in vitro and in vivo using preclinical models. The findings suggest that both drugs might serve as therapeutic options for IF/TA.

Advances in diagnostics for transplant rejection

INTRODUCTION

Identification of allograft injury, including acute clinical and subclinical injury, is vital in increasing the longevity of the transplanted organ. Acute rejection, which occurs as a result of a variety of immune and non-immune factors including the infiltration of immune cells and antibodies to the donor specific epitopes, poses a significant risk to the organ. Recent years have marked an increase in the discovery of new genomic, transcriptomic, and proteomic biomarkers in molecular diagnostics, which offer better potential for personalized management of the transplanted organ by providing earlier detection of rejection episodes. Areas covered: This review was compiled from key word searches of full-text publications relevant to the field. Expert commentary: Many of the recent advancements in the molecular diagnostics of allograft injury show much promise, but before they can be fully realized further validation in larger sample sets must be conducted. Additionally, for better informed therapeutic decisions, more work must be completed to differentiate between different causes of injury. Moreover, the diagnostics field is looking at methodologies that allow for multiplexing, the ability to identify multiple targets simultaneously, in order to provide more robust biomarkers and better understanding.

Advances in pharmacotherapy to treat kidney transplant rejection

INTRODUCTION

Current immunosuppressive combination therapy provides excellent prevention of T-cell-mediated rejection following renal transplantation; however, antibody-mediated rejection remains of high concern and accounts for a large number of long-term allograft losses. The recent development of protocol biopsies resulted in the definition of subclinical rejection (SCR), showing histologic evidence for rejection but unremarkable clinical course.

AREAS COVERED

This review describes the current knowledge and evidence of pharmacotherapy to treat kidney allograft rejections and covers SCR treatment options. Each substance is analyzed with regard to its classical indication and further discussed for the treatment of other forms of rejection.

EXPERT OPINION

Despite a lack of randomized trials, early acute T-cell-mediated rejection can be treated effectively in most cases without graft loss. The necessity to treat SCR is currently unclear. Due to a lack of effective therapies, new treatment approaches for antibody-mediated rejection are an urgent medical need to improve long-term outcomes. Future research should aim to better define pathophysiology and histology, stratify risk, and develop rational treatment strategies from randomized controlled trials, in order to establish the value of novel therapies in the arsenal of rejection pharmacotherapy. However, the effective prevention of rejection with minimal side effects still remains the goal in immunosuppression.

Transplantation tolerance and its outcome during infections and inflammation

Much progress has been made toward understanding the mechanistic basis of transplantation tolerance in experimental models, which implicates clonal deletion of alloreactive T and B cells, induction of cell-intrinsic hyporesponsiveness, and dominant regulatory cells mediating infectious tolerance and linked suppression. Despite encouraging success in the laboratory, achieving tolerance in the clinic remains challenging, although the basis for these challenges is beginning to be understood. Heterologous memory alloreactive T cells generated by infections prior to transplantation have been shown to be a critical barrier to tolerance induction. Furthermore, infections at the time of transplantation and tolerance induction provide a pro-inflammatory milieu that alters the stability and function of regulatory T cells as well as the activation requirements and differentiation of effector T cells. Thus, infections can result in enhanced alloreactivity, resistance to tolerance induction, and destabilization of the established tolerance state. We speculate that these experimental findings have relevance to the clinic, where infections have been associated with allograft rejection and may be a causal event precipitating the loss of grafts after long periods of stable operational tolerance. Understanding the mechanisms by which infections prevent and destabilize tolerance can lead to therapies that promote stable life-long tolerance in transplant recipients.

B-cell tolerance in transplantation: is repertoire remodeling the answer?

T lymphocytes are the primary targets of immunotherapy in clinical transplantation; however, B lymphocytes and their secreted alloantibodies are also highly detrimental to the allograft. Therefore, the achievement of sustained organ transplant survival will likely require the induction of B-lymphocyte tolerance. During development, acquisition of B-cell tolerance to self-antigens relies on clonal deletion in the early stages of B-cell compartment ontogeny. We contend that this mechanism should be recapitulated in the setting of alloantigens and organ transplantation to eliminate the alloreactive B-cell subset from the recipient. Clinically feasible targets of B-cell-directed immunotherapy, such as CD20 and B-lymphocyte stimulator (BLyS), should drive upcoming clinical trials aimed at remodeling the recipient B-cell repertoire.

Induction of tolerance through mixed chimerism

"Mixed chimerism" refers to a state in which the lymphohematopoietic system of the recipient of allogeneic hematopoietic stem cells comprises a mixture of host and donor cells. This state is usually attained through either bone marrow or mobilized peripheral blood stem cell transplantation. Although numerous treatment regimens have led to transplantation tolerance in mice, the induction of mixed chimerism is currently the only treatment modality that has been successfully extended to large animals and to the clinic. Here we describe and compare the use of mixed chimerism to establish transplantation tolerance in mice, pigs, monkeys, and in the clinic. We also attempt to correlate the mechanisms involved in achieving tolerance with the nature of the tolerance that has resulted in each case.

Regulatory B cells and tolerance in transplantation: from animal models to human

Until recently, the role of B cells in transplantation was thought to be restricted to producing antibodies that have been clearly shown to be deleterious in the long-term, but, in fact, B cells are also able to produce cytokine and to present antigen. Their role as regulatory cells in various pathological situations has also been highlighted, and their role in transplantation is beginning to emerge in animal, and also in human, models. This review summarizes the different studies in animals and humans that suggest a B-cell regulatory role in the transplant tolerance mechanisms.

T-regulatory cell treatment prevents chronic rejection of heart allografts in a murine mixed chimerism model

Background

The mixed chimerism approach induces donor-specific tolerance in both pre-clinical models and clinical pilot trials. However, chronic rejection of heart allografts and acute rejection of skin allografts were observed in some chimeric animals despite persistent hematopoietic chimerism and tolerance toward donor antigens in vitro. We tested whether additional cell therapy with regulatory T cells (Tregs) is able to induce full immunologic tolerance and prevent chronic rejection.

Methods

We recently developed a murine “Treg bone marrow (BM) transplantation (BMT) protocol” that is devoid of cytoreductive recipient pre-treatment. The protocol consists of a moderate dose of fully mismatched allogeneic donor BM under costimulation blockade, together with polyclonal recipient Tregs and rapamycin. Control groups received BMT under non-myeloablative irradiation and costimulation blockade without Treg therapy. Multilineage chimerism was followed by flow cytometry, and tolerance was assessed by donor-specific skin and heart allografts.

Results

Durable multilineage chimerism and long-term donor skin and heart allograft survival were successfully achieved with both protocols. Notably, histologic examination of heart allografts at the end of follow-up revealed that chronic rejection is prevented only in chimeras induced with the Treg protocol.

Conclusions

In a mouse model of mixed chimerism, additional Treg treatment at the time of BMT prevents chronic rejection of heart allografts. As the Treg-chimerism protocol also obviates the need for cytoreductive recipient treatment it improves both efficacy and safety over previous non-myeloablative mixed chimerism regimens. These results may significantly impact the development of protocols for tolerance induction in cardiac transplantation.

Transplantation: Autoantibodies-epiphenomena or biological clues

The potential roles for autoantibodies in renal transplantation are increasing, as illustrated by a recent report of polyreactive autoantibodies produced by B-cell clones from a kidney transplant recipient that can bind apoptotic cells and activate complement. Such autoantibodies have the potential to amplify microcirculation injury caused by alloantibody in antibody-mediated transplant rejection.

Biologic agents in islet transplantation

Islet transplantation is today an accepted modality for treating selected patients with frequent hypoglycemic events or severe glycemic lability. Despite tremendous progress in islet isolation, culture, and preservation, clinical use is still restricted to a limited subset, and lifelong immunosuppression is required. Issues surrounding limited islet revascularization and immune destruction remain. One of the major challenges is to prevent alloreactivity and recurrence of autoimmunity against β-cells. These two hurdles can be effectively reduced by immunosuppressive therapy combining induction and maintenance treatments. The introduction of highly potent and selective biologic agents has significantly reduced the frequency of acute rejection and has prolonged graft survival, while minimizing the complications of this therapeutic scheme. This review will address the most important biological agents used in islet transplantation. We provide a historical perspective of their introduction into clinical practice and their role in current clinical protocols, aiming at improved engraftment efficiency, increased long-term survival, and better overall results of clinical islet transplantation.

The site of allergen expression in hematopoietic cells determines the degree and quality of tolerance induced through molecular chimerism

The transplantation of allergens (e.g. Phl p 5 or Bet v 1) expressed on BM cells as membrane-anchored full-length proteins leads to permanent tolerance at the T-cell, B-cell, and effector-cell levels. Since the exposure of complete allergens bears the risk of inducing anaphylaxis, we investigated here whether expression of Phl p 5 in the cytoplasm (rather than on the cell surface) is sufficient for tolerance induction. Transplantation of BALB/c BM retrovirally transduced to express Phl p 5 in the cytoplasm led to stable and durable molecular chimerism in syngeneic recipients (∼20% chimerism at 6 months). Chimeras showed allergen-specific T-cell hyporesponsiveness. Further, Phl p 5-specific TH 1-dependent humoral responses were tolerized in several chimeras. Surprisingly, Phl p 5-specific IgE and IgG1 levels were significantly reduced but still detectable in sera of chimeric mice, indicating incomplete B-cell tolerance. No Phl p 5-specific sIgM developed in cytoplasmic chimeras, which is in marked contrast to mice transplanted with BM expressing membrane-anchored Phl p 5. Thus, the expression site of the allergen substantially influences the degree and quality of tolerance achieved with molecular chimerism in IgE-mediated allergy.

Moving beyond HLA: a review of nHLA antibodies in organ transplantation

Given the finite graft life expectancy of HLA identical organ transplants and the recognition of humoral graft injury in the absence of donor directed anti-HLA antibodies, the clinical impact of antibodies against non-HLA (nHLA) antigens in transplant injury is being increasingly recognized. The recognition of the impact of nHLA antigen discrepancies between donor and recipient on transplant outcomes is timely given the advances in rapid and lower cost sequencing methods that can soon provide complete maps of all recipient and donor HLA and nHLA mismatch data. In this review, we present a summary of recent reports evaluating the role of nHLA antibodies and their relevance to the field of organ transplantation.

Composite tissue allotransplantation and dysregulation in tissue repair and regeneration: a role for mesenchymal stem cells

Vascularized composite tissue allotransplantation is a rapidly evolving area that has brought technological advances to the forefront of plastic surgery, hand surgery, and transplant biology. Composite tissue allografts (CTAs) may have profound functional, esthetic, and psychological benefits, but carry with them the risks of life-long immunosuppression and the inadequate abilities to monitor and prevent rejection. Allografts may suffer from additional insults further weakening their overall benefits. Changes in local blood flow, lack of fully restored neurologic function, infection, inflammation with subsequent dysregulated regenerative activity, and paucity of appropriate growth factors may all be involved in reducing the potential of CTAs and therefore serve as new therapeutic targets to improve outcomes. Strategies involving minimized immunosuppression and pro-regenerative therapy may provide a greater path to optimizing long-term CTA function. One such strategy may include mesenchymal stem cells (MSCs), which can provide unique anti-inflammatory and pro-regenerative effects. Insights gained from new studies with MSCs on composite allografts, advances in tissue regeneration reported in other MSC-based clinical studies, as well as consideration of newly described capacities of MSCs, may provide new regenerative based strategies for the care of CTAs.

Long-term prevention of chronic allograft rejection by regulatory T-cell immunotherapy involves host Foxp3-expressing T cells

Administration of donor-specific regulatory T cells prevents chronic rejection of BM and skin allografts in the mouse. Injected regulatory T cells induce the emergence of host regulatory T cells with similar specificity thus ensuring persistence of tolerance.

Non-invasive biomarkers to guide management following renal transplantation: the need for a multiplatform approach

PURPOSE OF REVIEW

Balancing the level of anti-rejection therapy is the key challenge facing clinicians managing recipients of a solid organ transplant. Identification of biomarkers in non-invasive samples will allow serial monitoring which can prompt changes in therapy at an earlier stage without the need for invasive tests, and before significant damage to the graft or side effects have occurred. In this review we provide an update on the present status of such biomarker discovery in renal transplantation.

RECENT FINDINGS

Previous studies focusing on candidate biomarkers have identified a number of genes that have the potential to identify patients undergoing rejection. Advances in technology now allow screening of samples for markers which have led to identification of further genes as well as adding microRNAs and proteins to the list. Similarly, by studying patients in whom anti-rejection therapy has been withdrawn, a gene signature for operational tolerance has been described.

SUMMARY

It has become clear that to obtain a test suitable for clinical purposes, combinations of markers are required to identify specific clinical phenotypes. Identification and validation of such marker sets in large cohorts is urgently required to allow progression to clinical trials with the ultimate goal of offering recipients personalized anti-rejection therapy regimes.

Anti-LFA-1 or rapamycin overcome costimulation blockade-resistant rejection in sensitized bone marrow recipients

While costimulation blockade-based mixed chimerism protocols work well for inducing tolerance in rodents, translation to preclinical large animal/nonhuman primate models has been less successful. One recognized cause for these difficulties is the high frequency of alloreactive memory T cells (Tmem) found in the (pre)clinical setting as opposed to laboratory mice. In the present study, we therefore developed a murine bone marrow transplantation (BMT) model employing recipients harboring polyclonal donor-reactive Tmem without concomitant humoral sensitization. This model was then used to identify strategies to overcome this additional immune barrier. We found that B6 recipients that were enriched with 3 × 10(7) T cells isolated from B6 mice that had been previously grafted with Balb/c skin, rejected Balb/c BM despite costimulation blockade with anti-CD40L and CTLA4Ig (while recipients not enriched developed chimerism). Adjunctive short-term treatment of sensitized BMT recipients with rapamycin or anti-LFA-1 mAb was demonstrated to be effective in controlling Tmem in this model, leading to long-term mixed chimerism and donor-specific tolerance. Thus, rapamycin and anti-LFA-1 mAb are effective in overcoming the potent barrier that donor-reactive Tmem pose to the induction of mixed chimerism and tolerance despite costimulation blockade.

BAFF and BAFF-R levels are associated with risk of long-term kidney graft dysfunction and development of donor-specific antibodies

There are lines of evidence that B cells may play a role in transplantation. B cell activating factor, BAFF, is a homotrimer that has been shown to play a role in B cell survival, maturation and activation. To date, little is known of the role of BAFF and its receptors in transplantation. We analyzed the level of BAFF mRNA and its soluble protein, as well as transcripts coding for its receptors, BAFF-R, TACI and BCMA, in the blood of 143 patients with stable kidney transplant function 5 years or more posttransplantation. Three endpoints were analyzed: the time to renal dysfunction, the time to appearance of anti-HLA antibodies and the time to development of donor-specific antibodies. We established threshold values for BAFF and BAFF-R and showed that (1) stable patients with high BAFF-R levels had a higher risk of developing graft dysfunction, (2) patients with lower levels of BAFF transcripts or a higher level of soluble BAFF had a significantly higher risk of developing donor-specific antibodies. These data suggest that BAFF constitutes a risk factor for renal graft dysfunction and development of donor-specific antibodies. They also suggest that agents targeting BAFF-R interactions may offer new therapeutic opportunities in transplantation.

Biologics in organ transplantation

The last two decades have witnessed a pandemic in antibody development, with over 600 entering clinical studies and a total of 28 approved by the FDA and European Union. The incorporation of biologics in transplantation has made a significant impact on allograft survival. Herein, we review the armamentarium of clinical and preclinical biologics used for organ transplantation--with the exception of belatacept--from depleting and IL-2R targeting induction agents to costimulation blockade, B-cell therapeutics, BAFF and complement inhibition, anti-adhesion, and anti-cytokine approaches. While individual agents may be insufficient for tolerance induction, they provide possibilities for reduction of steroid or calcineurin inhibitor use, alternatives to rejection episodes refractory to conventional therapies, and specialized immunosuppression for highly sensitized patients.

Hematopoietic chimerism and transplantation tolerance: a role for regulatory T cells

The immunosuppressive regimens currently used in transplantation to prevent allograft destruction by the host's immune system have deleterious side effects and fail to control chronic rejection processes. Induction of donor-specific non-responsiveness (i.e., immunological tolerance) to transplants would solve these problems and would substantially ameliorate patients' quality of life. It has been proposed that bone marrow or hematopoietic stem-cell transplantation, and resulting (mixed) hematopoietic chimerism, lead to immunological tolerance to organs of the same donor. However, a careful analysis of the literature, performed here, clearly establishes that whereas hematopoietic chimerism substantially prolongs allograft survival, it does not systematically prevent chronic rejection. Moreover, the cytotoxic conditioning regimens used to achieve long-term persistence of chimerism are associated with severe side effects that appear incompatible with a routine use in the clinic. Several laboratories recently embarked on different studies to develop alternative strategies to overcome these issues. We discuss here recent advances obtained by combining regulatory T cell infusion with bone-marrow transplantation. In experimental settings, this attractive approach allows development of genuine immunological tolerance to donor tissues using clinically relevant conditioning regimens.

Monitoring tolerance and rejection in organ transplant recipients

To avoid toxic side effects caused by permanent immunosuppressive treatment, research in transplantation focuses on new treatment strategies inducing tolerance or allowing drug weaning. Implementing drug minimization into clinical routine can be only safely achieved when guided by biomarkers reflecting the individual immune reactivity. We review recently described biomarkers and assays allowing identification of patients suitable for drug weaning or at risk of rejection. However, the majority of described biomarkers and assays have not been validated in prospective clinical trials. Thus, collaborative efforts are needed to design and perform prospective multicenter trials to validate the identified biomarkers across different laboratories.

Combined CD4 T-cell and antibody response to human minor histocompatibility antigen DBY after allogeneic stem-cell transplantation

BACKGROUND

Antibody responses to HY antigens in male recipients are frequent after transplantation of stem cells from female donors (Miklos et al., Blood 2005; 105: 2973; Miklos et al., Blood 2004; 103: 353). However, evidence that this B-cell immunity is accompanied by T-cell responses to the cognate antigens is scarce. Here, we examined T- and B-cell responses to DBY antigen in a male patient who received hematopoietic stem cells from a human leukocyte antigen-identical female sibling.

MATERIALS AND METHODS

We used 93 overlapping peptides representing the entire DBY protein to detect and characterize T-cell and antibody responses to DBY by enzyme-linked immunosorbent spot (ELISPOT) and enzyme-linked immunosorbent assay.

RESULTS

High frequency CD4+ T cells specific for a unique DBY peptide were detected in the patient blood. We isolated the corresponding T-cell clone and characterized the recognized epitope as an 18-mer peptide restricted by human leukocyte antigen-DRB1*0101. Upon stimulation, this clone produced cytokines with no evidence of Th1 or Th2 polarization. Remarkably, this clone also recognized the DBX homologue peptide and responded to female donor dendritic cells stimulated with poly I/C or lipopolysaccharide, indicating that the peptide was endogenously processed in these cells. High titer DBY-specific antibodies were also found in the patient serum which, in contrast to the T-cell response, did not cross-react with DBX.

CONCLUSION

We show here the development of a coordinated B and T-cell response to DBY in a recipient of sex mismatched allogeneic hematopoietic stem-cell transplantation. Our findings support a role for CD4+ T cells in the development of humoral immunity to minor histocompatibility antigens.

Experimental models of B cell tolerance in transplantation

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

Progressive histological damage in renal allografts is associated with expression of innate and adaptive immunity genes

The degree of progressive chronic histological damage is associated with long-term renal allograft survival. In order to identify promising molecular targets for timely intervention, we examined renal allograft protocol and indication biopsies from 120 low-risk pediatric and adolescent recipients by whole-genome microarray expression profiling. In data-driven analysis, we found a highly regulated pattern of adaptive and innate immune gene expression that correlated with established or ongoing histological chronic injury, and also with development of future chronic histological damage, even in histologically pristine kidneys. Hence, histologically unrecognized immunological injury at a molecular level sets the stage for the development of chronic tissue injury, while the same molecular response is accentuated during established and worsening chronic allograft damage. Irrespective of the hypothesized immune or nonimmune trigger for chronic allograft injury, a highly orchestrated regulation of innate and adaptive immune responses was found in the graft at the molecular level. This occurred months before histologic lesions appear, and quantitatively below the diagnostic threshold of classic T-cell or antibody-mediated rejection. Thus, measurement of specific immune gene expression in protocol biopsies may be warranted to predict the development of subsequent chronic injury in histologically quiescent grafts and as a means to titrate immunosuppressive therapy.

Biomarkers of tolerance: searching for the hidden phenotype

Induction of transplantation tolerance remains the ideal long-term clinical and logistic solution to the current challenges facing the management of renal allograft recipients. In this review, we describe the recent studies and advances made in identifying biomarkers of renal transplant tolerance, from study inceptions, to the lessons learned and their implications for current and future studies with the same goal. With the age of biomarker discovery entering a new dimension of high-throughput technologies, here we also review the current approaches, developments, and pitfalls faced in the subsequent statistical analysis required to identify valid biomarker candidates.

The involvement of SMILE/TMTC3 in endoplasmic reticulum stress response

Background

Thestate of operational tolerance has been detected sporadically in some renal transplanted patients that stopped immunosuppressive drugs, demonstrating that allograft tolerance might exist in humans. Several years ago, a study by Brouard et al. identified a molecular signature of several genes that were significantly differentially expressed in the blood of such patients compared with patients with other clinical situations. The aim of the present study is to analyze the role of one of these molecules over-expressed in the blood of operationally tolerant patients, SMILE or TMTC3, a protein whose function is still unknown.

Methodology/Principal Findings

We first confirmed that SMILE mRNA is differentially expressed in the blood of operationally tolerant patients with drug-free long term graft function compared to stable and rejecting patients. Using a yeast two-hybrid approach and a colocalization study by confocal microscopy we furthermore report an interaction of SMILE with PDIA3, a molecule resident in the endoplasmic reticulum (ER). In accordance with this observation, SMILE silencing in HeLa cells correlated with the modulation of several transcripts involved in proteolysis and a decrease in proteasome activity. Finally, SMILE silencing increased HeLa cell sensitivity to the proteasome inhibitor Bortezomib, a drug that induces ER stress via protein overload, and increased transcript expression of a stress response protein, XBP-1, in HeLa cells and keratinocytes.

Conclusion/Significance

In this study we showed that SMILE is involved in the endoplasmic reticulum stress response, by modulating proteasome activity and XBP-1 transcript expression. This function of SMILE may influence immune cell behavior in the context of transplantation, and the analysis of endoplasmic reticulum stress in transplantation may reveal new pathways of regulation in long-term graft acceptance thereby increasing our understanding of tolerance.

The significance of non-T-cell pathways in graft rejection: implications for transplant tolerance

Both innate and adaptive immune cells are actively involved in the initiation and destruction of allotransplants, and there is a true need now to look beyond T cells in the allograft response, examining various non-T-cell types in transplant models and how such cell types interact with T cells in determining the fate of an allograft. Studies in this area may lead to further improvement in transplant outcomes.

Thymus transplantation for treatment of cancer: lessons from murine models

It is well known that tumor-bearing mammals, including humans, show decreased T-cell function due to involution of the thymus. This decrease results in faster tumor growth, susceptibility to infection and reduced life expectancy. Thus, the best strategy to restore T-cell function might be to transplant the thymus from the fetus or newborn. Based on this hypothesis, this article introduces our recent findings using mice and evidence is provided that, in humans, thymus transplantation in conjunction with bone marrow transplantation could become a valuable strategy for suppressing tumor growth, thereby prolonging survival.

Emerging role of B cells in chronic allograft dysfunction

B cells have many possible mechanisms by which they can affect allograft survival, including antigen presentation, cytokine production, immune regulation, and differentiation into alloantibody-producing plasma cells. This report reviews the last mechanism, which the authors regard as most critical for the long-term survival of allografts, namely, the promotion of chronic rejection by alloantibodies. Chronic humoral rejection characteristically arises late after transplantation and causes transplant glomerulopathy, multilamination of peritubular capillary basement membranes, and C4d deposition in PTCs and glomeruli. Circulating antidonor human leukocyte antigen class II antibodies are commonly detected and may precede the development of graft injury. Prognosis is poor, especially when recognized after graft dysfunction has developed. Improved detection and treatment are critically needed for this common cause of late graft loss.

The Immune Tolerance Network at 10 years: tolerance research at the bedside

Immune tolerance-inducing therapies reprogramme immune cells to eliminate pathogenic immune responses while preserving protective immunity. The Immune Tolerance Network (ITN), sponsored by the US National Institutes of Health, was established in 1999 to evaluate new tolerance-inducing therapies and carry out mechanistic studies using a unique interactive approach in partnership with industry, academia and foundations. Ten years later, the ITN has carried out approximately 36 clinical trials and tolerance studies examining innovative tolerogenic approaches in the settings of allergy, autoimmune diseases and organ transplantation. ITN investigators have published more than 80 original research papers based on this work. This Timeline article summarizes the progress and challenges of clinical research in the ITN.

Toward a B-cell signature of tolerance?

Operational tolerance is a rare and still unexplained phenomenon in organ transplantation. Pallier et al. analyzed the peripheral B-cell compartment of 12 renal transplant patients with drug-free long-term graft function and found a peculiar blood B-cell phenotype, with an expansion of memory activated B cells and increased expression of inhibitory molecules, suggesting a role of B cells in maintaining graft tolerance.

Bringing transplantation tolerance into the clinic: lessons from the ITN and RISET for the Establishment of Tolerance consortia

PURPOSE OF REVIEW

Fundamental discoveries during the 1990s revolutionized our understanding of transplantation tolerance and our ability to create it in animal models. The Immune Tolerance Network (ITN) and Reprogramming the Immune System for the Establishment of Tolerance (RISET) consortia were created to leverage these advances and work towards the goal of achieving clinical tolerance in transplantation. This article highlights their accomplishments and challenges during the past decade.

RECENT FINDINGS

In interventional trials, renal allograft tolerance has been achieved using bone marrow transplantation with nonmyeloablative protocols to induce transient hematopoietic chimerism. Drug minimization in renal transplantation was achieved with Campath-1H induction therapy and also with cellular therapy using 'transplant acceptance inducing cells'. Successful drug withdrawal was accomplished in long-term stable pediatric liver transplant recipients. Finally, 'registry' trials of tolerant kidney recipients revealed a B-cell signature of tolerance, which will form the basis for future investigations of its use as a biomarker for drug minimization or withdrawal in selected patients.

SUMMARY

Although transplantation tolerance is not yet reliably achieved in a clinical setting, collaborative efforts, such as those of the ITN and RISET networks, are an effective means to synergize intellectual and financial resources to bring this goal closer to reality.

The (re)emergence of B cells in organ transplantation

PURPOSE OF REVIEW

To outline recent advances in our understanding of the role of B cells in transplantation.

RECENT FINDINGS

While T-cell-mediated alloimmunity has been largely controlled using immunosuppression, the role of B cells in transplantation is just beginning to be understood. Recent studies have outlined some of the important clinical issues involving antibody including early acute humoral rejection and late transplant glomerulopathy. In addition, recent studies have identified bone-marrow-derived long-lived plasma cells that appear to be a major source of donor-specific alloantibody in sensitized renal transplant recipients. New agents are being tested that deplete these cells in vitro and in vivo. Memory B cells appear to be important in early acute humoral rejection, but few basic studies have been performed. Finally, recent studies involving patients undergoing tolerogenic regimens suggest that T-cell tolerance does not always convey tolerance in naive B cells.

SUMMARY

Several B cell types have clear and specific roles in transplant recipients. Although our understanding of B cells in transplantation has improved, important gaps remain.

Molecular diagnostics in transplantation

The past few decades are characterized by an explosive evolution of genetics and molecular cell biology. Advances in chemistry and engineering have enabled increased data throughput, permitting the study of complete sets of molecules with increasing speed and accuracy using techniques such as genomics, transcriptomics, proteomics, and metabolomics. Prediction of long-term outcomes in transplantation is hampered by the absence of sufficiently robust biomarkers and a lack of adequate insight into the mechanisms of acute and chronic alloimmune injury and the adaptive mechanisms of immunological quiescence that may support transplantation tolerance. Here, we discuss some of the great opportunities that molecular diagnostic tools have to offer both basic scientists and translational researchers for bench-to-bedside clinical application in transplantation medicine, with special focus on genomics and genome-wide association studies, epigenetics (DNA methylation and histone modifications), gene expression studies and transcriptomics (including microRNA and small interfering RNA studies), proteomics and peptidomics, antibodyomics, metabolomics, chemical genomics and functional imaging with nanoparticles. We address the challenges and opportunities associated with the newer high-throughput sequencing technologies, especially in the field of bioinformatics and biostatistics, and demonstrate the importance of integrative approaches. Although this Review focuses on transplantation research and clinical transplantation, the concepts addressed are valid for all translational research.

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.

B-lymphocyte homeostasis and BLyS-directed immunotherapy in transplantation

Current strategies for immunotherapy after transplantation are primarily T-lymphocyte directed and effectively abrogate acute rejection. However, the reality of chronic allograft rejection attests to the fact that transplantation tolerance remains an elusive goal. Donor-specific antibodies are considered the primary cause of chronic rejection. When naive, alloreactive B-cells encounter alloantigen and are activated, a resilient "sensitized" state, characterized by the presence of high-affinity antibody, is established. Here, we will delineate findings that support transient B-lymphocyte depletion therapy at the time of transplantation to preempt sensitization by eliminating alloreactive specificities from the recipient B-cell pool (ie, "repertoire remodeling"). Recent advances in our understanding of B-lymphocyte homeostasis provide novel targets for immunomodulation in transplantation. Specifically, the tumor necrosis factor-related cytokine BLyS is the dominant survival factor for "tolerance-susceptible" transitional and "preimmune" mature follicular B-cells. The transitional phenotype is the intermediate through which all newly formed B-cells pass before maturing into the follicular subset, which is responsible for mounting an alloantigen-specific antibody response. Systemic BLyS levels dictate the stringency of negative selection during peripheral B-cell repertoire development. Thus, targeting BLyS will likely provide an opportunity for repertoire-directed therapy to eliminate alloreactive B-cell specificities in transplant recipients, a requirement for the achievement of humoral tolerance and prevention of chronic rejection. In this review, the fundamentals of preimmune B-cell selection, homeostasis, and activation will be described. Furthermore, new and current B-lymphocyte-directed therapy for antibody-mediated rejection and the highly sensitized state will be discussed. Overall, our objective is to propose a rational approach for induction of humoral transplantation tolerance by remodeling the primary B-cell repertoire of the allograft recipient.

Patients with drug-free long-term graft function display increased numbers of peripheral B cells with a memory and inhibitory phenotype

Several transplant patients maintain stable kidney graft function in the absence of immunosuppression. Here we compared the characteristics of their peripheral B cells to that of others who had stable graft function but were under pharmacologic immunosuppression, to patients with chronic rejection and to healthy volunteers. In drug-free long-term graft function (DF) there was a significant increase in both absolute cell number and frequency of total B cells; particularly activated, memory and early memory B cells. These increased B-cell numbers were associated with a significantly enriched transcriptional B-cell profile. Costimulatory/migratory molecules (B7-2/CD80, CD40, and CD62L) were upregulated in B cells; particularly in memory CD19(+)IgD(-)CD38(+/-)CD27(+) B cells in these patients. Their purified B cells, however, responded normally to a polyclonal stimulation and did not have cytokine polarization. This phenotype was associated with the following specific characteristics which include an inhibitory signal (decreased FcgammaRIIA/FcgammaRIIB ratio); a preventive signal of hyperactive B-cell response (an increase in BANK1, which negatively modulates CD40-mediated AKT activation); an increased number of B cells expressing CD1d and CD5; an increased BAFF-R/BAFF ratio that could explain why these patients have more peripheral B cells; and a specific autoantibody profile. Thus, our findings show that patients with DF have a particular blood B-cell phenotype that may contribute to the maintenance of long-term graft function.