Overcoming Chronic Rejection—Can it B?

Harnessing the B Cell Response in Kidney Transplantation - Current State and Future Directions

Despite dramatic improvement in kidney transplantation outcomes over the last decades due to advent of modern immunosuppressive agents, long-term outcomes remain poor. Antibody-mediated rejection (ABMR), a B cell driven process, accounts for the majority of chronic graft failures. There are currently no FDA-approved regimens for ABMR; however, several clinical trials are currently on-going. In this review, we present current mechanisms of B cell response in kidney transplantation, the clinical impact of sensitization and ABMR, the B cell response under current immunosuppressive regimens, and ongoing clinical trials for ABMR and desensitization treatment.

B cells in transplant tolerance and rejection: friends or foes?

Our understanding of the role of B cells in organ transplantation remains incomplete and continues to grow. The majority of research has focused on the detrimental role of antibodies that drive the development of pathogenesis of the transplanted organ. However, it has been shown that not all donor-specific antibodies are harmful and in some circumstances can even promote tolerance through the mechanism of accommodation. Furthermore, B cells can have effects on transplanted organs through their interaction with T cells, namely antigen presentation, cytokine production, and costimulation. More recently, the role and importance of Bregs was introduced to the field of transplantation. Due to this functional and ontogenetic heterogeneity, targeting B cells in transplantation may bring undesired immunologic side effects including increased rejection. Therefore, the selective control of B cells that contribute to the humoral response against donor antigens will continue to be an important and challenging area of research and potentially lead to improved long-term transplant outcomes.

Regulatory T Cells for the Induction of Transplantation Tolerance

Organ transplantation is the optimal treatment for terminal and irreversible organ failure. Achieving transplantation tolerance has long been the ultimate goal in the field of transplantation. Regulatory T cell (Treg)-based therapy is a promising novel approach for inducing donor organ-specific tolerance. Tregs play critical roles in the maintenance of immune homeostasis and self-tolerance, by promoting transplantation tolerance through a variety of mechanisms on different target cells, including anti-inflammatory cytokine production, induction of apoptosis, disruption of metabolic pathways, and mutual interaction with dendritic cells. The continued success of Treg-based therapy in the clinical setting is critically dependent on preclinical studies that support its translational potential. However, although some initial clinical trials of adoptive Treg therapy have successively demonstrated safety and efficacy for immunosuppressant minimization and transplantation tolerance induction, most Treg-based hematopoietic stem cell and solid organ clinical trials are still in their infancy. These clinical trials have not only focused on safety and efficacy but also included optimization and standardization protocols of good manufacturing practice regarding cell isolation, expansion, dosing, timing, specificity, quality control, concomitant immunosuppressants, and post-administration monitoring. We herein report a brief introduction of Tregs, including their phenotypic and functional characterization, and focus on the clinical translation of Treg-based therapeutic applications in the setting of transplantation.

Manipulation of Regulatory Dendritic Cells for Induction Transplantation Tolerance

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

Quantitative analysis of humoral immunity by flow-cytometric crossmatch using molecules of equivalent soluble fluorochromosome (FCXM-MESF)

OBJECTIVES

In this study, we examined the quantitative significance of humoral immunity by flow-cytometric crossmatch using molecules of equivalent soluble fluorochromosome (FCXM-MESF) in recipients of kidney transplantation. We stratified the recipients into four sensitization classes, from non-sensitized to strongly sensitized by the results of the FCXM-MESF assay, and compared the pathological results after transplantation by the sensitization status.

MATERIALS AND METHODS

We stratified 140 recipients into four groups according to their sensitization status, as follows; none/NDSA, defined by FCXM-MESF values of below the cut-off value (n = 79), mildly sensitized, defined by FCXM-MESF values of less than 3000 (N = 45); moderately sensitized, defined by FCXM-MESF values of between 3000 and 8000 (N = 12); strongly sensitized, defined by FCXM-MESF values exceeding 8000 (N = 4).

RESULTS

We employed tailor-made immunosuppressive regimens according to the FCXM-MESF values for the 140 recipients between 2009 and 2011. In regard to the pathological results, 4% (2/51), 3% (1/35), 20% (2/10) and 75% (3/4) of the none/Non Donor Specific Antibody (NDSA), mildly sensitized, moderately sensitized and strongly sensitized patients showed antibody mediated rejection (AMR). Thus, FCXM may be more useful for the detection of anti-non-HLA as well as for that of anti-HLA antibodies than the solid phase assay (SPA) or panel reactive antibody (PRA) assay.

CONCLUSION

Quantitative analysis using FCXM-MESF assay accurately reflected the clinical as well as pathological aspects, and may serve as a useful guide for the selection of appropriate anti-rejection therapy.

Contemporary Strategies and Barriers to Transplantation Tolerance

The purpose of this review is to discuss immunologic tolerance as it applies to solid organ transplantation and to identify barriers that hinder the achievement of this long-term goal. First, the definition of tolerance and an introduction of mechanisms by which tolerance exists or can be achieved will be discussed. Next, a review of contemporary attempts at achieving transplant tolerance will be described. Finally, a discussion of the humoral barriers to transplantation tolerance and potential ways to overcome these barriers will be presented.

Macrophage subpopulations and their impact on chronic allograft rejection versus graft acceptance in a mouse heart transplant model

Macrophages infiltrating the allografts are heterogeneous, consisting of proinflammatory (M1 cells) as well as antiinflammatory and fibrogenic phenotypes (M2 cells); they affect transplantation outcomes via diverse mechanisms. Here we found that macrophage polarization into M1 and M2 subsets was critically dependent on tumor necrosis factor receptor-associated factor 6 (TRAF6) and mammalian target of rapamycin (mTOR), respectively. In a heart transplant model we showed that macrophage-specific deletion of TRAF6 (LysM^Cre Traf6 ^fl/fl ) or mTOR (LysM^Cre Mtor^fl/fl ) did not affect acute allograft rejection. However, treatment of LysM^Cre Mtor^fl/fl recipients with CTLA4-Ig induced long-term allograft survival (>100 days) without histological signs of chronic rejection, whereas the similarly treated LysM^Cre Traf6 ^fl/fl recipients developed severe transplant vasculopathy (chronic rejection). The presentation of chronic rejection in CTLA4-Ig-treated LysM^Cre Traf6 ^fl/fl mice was similar to that of CTLA4-Ig-treated wild-type B6 recipients. Mechanistically, we found that the graft-infiltrating macrophages in LysM^Cre Mtor^fl/fl recipients expressed high levels of PD-L1, and that PD-L1 blockade readily induced rejection of otherwise survival grafts in the LysM^Cre Mtor^fl/fl recipients. Our findings demonstrate that targeting mTOR-dependent M2 cells is critical for preventing chronic allograft rejection, and that graft survival under such conditions is dependent on the PD-1/PD-L1 coinhibitory pathway.

Crosstalk Between T and B Cells in the Germinal Center After Transplantation

Crosstalk between B and T cells in transplantation is increasingly recognized as being important in the alloimmune response. T cell activation of B cells occurs by a 3-stage pathway, culminating with costimulation signals. We review the distinct T cell subtypes required for B-cell activation and discuss the formation of the germinal center (GC) after transplantation, with particular reference to the repopulation of the GC after depletional induction, and the subsequent effect of immunosuppressive manipulation of T cell-B cell interactions. In addition, ectopic GCs are seen in transplantation, but their role is not fully understood. Therapeutic options to target T cell-B cell interactions are of considerable interest, both as immunosuppressive tools, and to aid in the further understanding of these important alloimmune mechanisms.

Interferon Gamma and Contact-dependent Cytotoxicity Are Each Rate Limiting for Natural Killer Cell-Mediated Antibody-dependent Chronic Rejection

Natural killer (NK) cells are key components of the innate immune system. In murine cardiac transplant models, donor-specific antibodies (DSA), in concert with NK cells, are sufficient to inflict chronic allograft vasculopathy independently of T and B cells. In this study, we aimed to determine the effector mechanism(s) required by NK cells to trigger chronic allograft vasculopathy during antibody-mediated rejection. Specifically, we tested the relative contribution of the proinflammatory cytokine interferon gamma (IFN-γ) versus the contact-dependent cytotoxic mediators of perforin and the CD95/CD95L (Fas/Fas ligand [FasL]) pathway for triggering these lesions. C3H/HeJ cardiac allografts were transplanted into immune-deficient C57BL/6 rag-/- γc-/- recipients, who also received monoclonal anti-major histocompatibility complex (MHC) class I DSA. The combination of DSA and wild-type NK cell transfer triggered aggressive chronic allograft vasculopathy. However, transfer of IFN-γ-deficient NK cells or host IFN-γ neutralization led to amelioration of these lesions. Use of either perforin-deficient NK cells or CD95 (Fas)-deficient donors alone did not alter development of vasculopathy, but simultaneous disruption of NK cell-derived perforin and allograft Fas expression resulted in prevention of these abnormalities. Therefore, both NK cell IFN-γ production and contact-dependent cytotoxic activity are rate-limiting effector pathways that contribute to this form of antibody-induced chronic allograft vasculopathy.

Transient blockade of delta-like Notch ligands prevents allograft rejection mediated by cellular and humoral mechanisms in a mouse model of heart transplantation

Rejection remains a major clinical challenge limiting allograft survival after solid organ transplantation. Both cellular and humoral immunity contribute to this complication, with increased recognition of Ab-mediated damage during acute and chronic rejection. Using a mouse model of MHC-mismatched heart transplantation, we report markedly protective effects of Notch inhibition, dampening both T cell and Ab-driven rejection. T cell-specific pan-Notch blockade prolonged heart allograft survival and decreased IFN-γ and IL-4 production by alloreactive T cells, especially when combined with depletion of recipient CD8(+) T cells. These effects were associated with decreased infiltration by conventional T cells and an increased proportion of regulatory T cells in the graft. Transient administration of neutralizing Abs specific for delta-like (Dll)1/4 Notch ligands in the peritransplant period led to prolonged acceptance of allogeneic hearts, with superior outcome over Notch inhibition only in T cells. Systemic Dll1/4 inhibition decreased T cell cytokines and graft infiltration, germinal center B cell and plasmablast numbers, as well as production of donor-specific alloantibodies and complement deposition in the transplanted hearts. Dll1 or Dll4 inhibition alone provided partial protection. Thus, pathogenic signals delivered by Dll1/4 Notch ligands early after transplantation promote organ rejection through several complementary mechanisms. Transient interruption of these signals represents an attractive new therapeutic strategy to enhance long-term allograft survival.

Update on Bronchiolitis Obliterans Syndrome in Lung Transplantation

Lung transplantation has become an important therapeutic option for patients with end-stage organ dysfunction; however, its clinical usefulness has been limited by the relatively early onset of chronic allograft dysfunction and progressive clinical decline. Obliterative bronchiolitis is characterized histologically by luminal fibrosis of the respiratory bronchioles and clinically by bronchiolitis obliterans syndrome (BOS) which is defined by a measured decline in lung function based on forced expiratory volume (FEV₁). Since its earliest description, a number of risk factors have been associated with the development of BOS, including acute rejection, lymphocytic bronchiolitis, primary graft dysfunction, infection, donor specific antibodies, and gastroesophageal reflux disease. However, despite this broadened understanding, the pathogenesis underlying BOS remains poorly understood and once begun, there are relatively few treatment options to battle the progressive deterioration in lung function.

Regulatory B cells: an exciting target for future therapeutics in transplantation

Transplantation is the preferred treatment for most end-stage solid organ diseases. Despite potent immunosuppressive agents, chronic rejection remains a real problem in transplantation. For many years, the predominant immunological focus of research into transplant rejection has been T cells. The pillar of immunotherapy in clinical practice is T cell-directed, which efficiently prevents acute T cell-mediated allograft rejection. However, the root of late allograft failure is chronic rejection and the humoral arm of the immune response now emerges as an important factor in transplantation. Thus, the potential effects of Abs and B cell infiltrate on transplants have cast B cells as major actors in late graft rejection. Consequently, a number of recent drugs target either B cells or plasma cells. However, immunotherapies, such as the anti-CD20 B cell-depleting antibody, can generate deleterious effects on the transplant, likely due to the deletion of beneficial population. The positive contribution of regulatory B (Breg) cells or B10 cells has been reported in the case of transplantation, mainly in mice models and highlights the primordial role that some populations of B cells can play in graft tolerance. Yet, this regulatory aspect remains poorly characterized in clinical transplantation. Thus, total B cell depletion treatments should be avoided and novel approaches should be considered that manipulate the different B cell subsets. This article provides an overview of the current knowledge on the link between Breg cells and grafts, and reports a number of data advising Breg cells as a new target for future therapeutic approaches.

SAHA, an HDAC inhibitor, attenuates antibody-mediated allograft rejection

BACKGROUND

Antibody-mediated rejection (AMR) is gaining increasing recognition as a critical causative factor contributing to graft loss in organ transplantation. However, current therapeutic options for prevention and treatment of AMR are very limited and ineffective. The impact of epigenetic modification in B-cell function and its involvement in AMR is still yet to be explored.

METHODS

The impacts of suberoylanilide hydroxamic acid (SAHA), a histone deacetylase inhibitor, on isolated murine B-cell viability, proliferation, apoptosis, expression of surface marker, and secretion of immunoglobulin and interleukin-10 were investigated. In vivo, a murine cardiac transplant model was used to evaluate the effect of SAHA on splenic B-cell subsets and on AMR in Rag1(-/-) recipient mice after reconstitution of allostimulated B cells.

RESULTS

SAHA possesses capability to repress B-cell function. Specifically, SAHA is potent to decrease the viability of isolated B cells by inducing apoptosis. SAHA was also found capable of suppressing the expression of B-cell costimulatory molecules and, as a result, addition of SAHA into the cultures attenuated B-cell proliferation and immunoglobulin secretion. In line with these results, administration of SAHA significantly suppressed AMR in Rag1(-/-) recipient mice after reconstitution of allostimulated B cells along with enhanced cardiac allograft survival time. Mechanistic studies revealed that SAHA promotes B-cell secretion of interleukin-10.

CONCLUSIONS

Our data support that SAHA could be a promising immunosuppressive agent with potential beneficial effect on prevention and treatment of AMR.

Impact of positive flow cytometry crossmatch on outcomes of intestinal/multivisceral transplantation: role anti-IL-2 receptor antibody

BACKGROUND

Positive crossmatch may be associated with an increased risk of acute rejection (AR) and worse overall outcomes after intestinal/multivisceral (MV) transplantation. However, the evidence from published studies in this setting is sparse and contradictory. This study reports the impact of positive flow cytometry crossmatch on clinical outcomes after intestinal/MV transplantation and the use of anti-interleukin (IL)-2 receptor antibody as a maintenance immunosuppressant.

METHODS

Records of all intestinal/MV transplants from 2003 to 2010 were reviewed. Flow cytometry was used to evaluate T- and B-cell crossmatch status. Standard immunosuppression included rabbit anti-thymocyte globulin-rituximab induction with tacrolimus and steroid maintenance. From 2008 onwards (second era), monthly anti-IL-2 receptor antibody was added to the maintenance immunosuppression in patients receiving liver-excluding transplants.

RESULTS

Of 131 intestinal/MV transplants, 27 (21%) had a positive crossmatch. Positive crossmatch was not associated with an increased incidence of AR and graft loss (30% and 37% vs. 29% and 47%; P=0.94 and 0.35, respectively). This effect was maintained in liver-excluding transplants. Overall rate of AR decreased from 39% to 22% in the second era. In liver-excluding transplants, there was a significant decrease in AR from 75% to 44% with the use of anti-IL-2 receptor antibody therapy.

CONCLUSIONS

With rabbit anti-thymocyte globulin-rituximab induction, positive crossmatch status is not associated with worse outcomes after intestinal/MV transplantation. Use of anti-IL-2 receptor antibody as a part of maintenance immunosuppression may be beneficial in liver-excluding transplants.

Cell mediated rejection

Rejection is the major barrier to successful transplantation and usually results from the integration of multiple mechanisms. Activation of elements of the innate immune system, triggered as a consequence of tissue injury sustained during cell isolation or organ retrieval as well as ischemia-reperfusion, will initiate and amplify the adaptive response. For cell mediated rejection, T cells require multiple signals for activation, the minimum being two signals; antigen recognition and costimulation. The majority of B cells require help from T cells to initiate alloantibody production. Antibodies reactive to donor HLA molecules, minor histocompatibility antigens, endothelial cells, red blood cells, or autoantigens can trigger or contribute to rejection early as well as late after transplantation.

Patterns of de novo allo B cells and antibody formation in chronic cardiac allograft rejection after alemtuzumab treatment

Even though the etiology of chronic rejection (CR) is multifactorial, donor specific antibody (DSA) is considered to have a causal effect on CR development. Currently the antibody-mediated mechanisms during CR are poorly understood due to lack of proper animal models and tools. In a clinical setting, we previously demonstrated that induction therapy by lymphocyte depletion, using alemtuzumab (anti-human CD52), is associated with an increased incidence of serum alloantibody, C4d deposition and antibody-mediated rejection in human patients. In this study, the effects of T cell depletion in the development of antibody-mediated rejection were examined using human CD52 transgenic (CD52Tg) mice treated with alemtuzumab. Fully mismatched cardiac allografts were transplanted into alemtuzumab treated CD52Tg mice and showed no acute rejection while untreated recipients acutely rejected their grafts. However, approximately half of long-term recipients showed increased degree of vasculopathy, fibrosis and perivascular C3d depositions at posttransplant day 100. The development of CR correlated with DSA and C3d deposition in the graft. Using novel tracking tools to monitor donor-specific B cells, alloreactive B cells were shown to increase in accordance with DSA detection. The current animal model could provide a means of testing strategies to understand mechanisms and developing therapeutic approaches to prevent chronic rejection.

Enhanced de novo alloantibody and antibody-mediated injury in rhesus macaques

Chronic allograft rejection is a major impediment to long-term transplant success. Humoral immune responses to alloantigens are a growing clinical problem in transplantation, with mounting evidence associating alloantibodies with the development of chronic rejection. Nearly a third of transplant recipients develop de novo antibodies, for which no established therapies are effective at preventing or eliminating, highlighting the need for a nonhuman primate model of antibody-mediated rejection. In this report, we demonstrate that depletion using anti-CD3 immunotoxin (IT) combined with maintenance immunosuppression that included tacrolimus with or without alefacept reliably prolonged renal allograft survival in rhesus monkeys. In these animals, a preferential skewing toward CD4 repopulation and proliferation was observed, particularly with the addition of alefacept. Furthermore, alefacept-treated animals demonstrated increased alloantibody production (100%) and morphologic features of antibody-mediated injury. In vitro, alefacept was found to enhance CD4 effector memory T cell proliferation. In conclusion, alefacept administration after depletion and with tacrolimus promotes a CD4+memory T cell and alloantibody response, with morphologic changes reflecting antibody-mediated allograft injury. Early and consistent de novo alloantibody production with associated histological changes makes this nonhuman primate model an attractive candidate for evaluating targeted therapeutics.

Tolerogenic therapies in transplantation

Since the concept of immunologic tolerance was discovered in the 1940s, the pursuit of tolerance induction in human transplantation has led to a rapid development of pharmacologic and biologic agents. Short-term graft survival remains an all-time high, but successful withdrawal of immunosuppression to achieve operational tolerance rarely occurs outside of liver transplantation. Collaborative efforts through the NIH sponsored Immune Tolerance Network and the European Commission sponsored Reprogramming the Immune System for Establishment of Tolerance consortia have afforded researchers opportunity to evaluate the safety and efficacy of tolerogenic strategies, investigate mechanisms of tolerance, and identify molecular and genetic markers that distinguish the tolerance phenotype. In this article, we review traditional and novel approaches to inducing tolerance for organ transplantation, with an emphasis on their translation into clinical trials.

A personal perspective: 100-year history of the humoral theory of transplantation

The humoral theory states that antibodies cause the rejection of allografts. From 1917 to 1929, extensive efforts were made to produce antibodies against tumors. It was finally realized that the antibodies were produced against the transplant antigens present on transplantable tumors, not against the tumor-specific antigens. To get around this problem, inbred mouse strains were developed, leading to identification of the transplant antigens determined by the H-2 locus of mice. The antibodies were hemagglutinating and cytotoxic antibodies. The analogous human leukocyte antigen system was established by analysis of lymphocytotoxic alloantibodies that were made by pregnant women, directed against mismatched antigens of the fetus. The human leukocyte antigen antibodies were then found to cause hyperacute rejection, acute rejection, and chronic rejection of kidneys. Antibodies appeared in almost all patients after rejection of kidneys. With Luminex single antigen bead technology, donor-specific antibodies could be identified before rise in serum creatinine and graft failure. Antibodies were shown to be predictive of subsequent graft failure in kidney, heart, and lung transplants: patients without antibodies had superior 4-year graft survival compared with those who did have antibodies. New evidence that antibodies are also associated with chronic failure has appeared for liver and islet transplants. Four studies have now shown that removal or reduction of antibodies result in higher graft survival. If removal of antibodies prevents chronic graft failure, final validation of the humoral theory can be achieved.

Murine islet allograft tolerance upon blockade of the B-lymphocyte stimulator, BLyS/BAFF

BACKGROUND

Immunologic rejection is a major barrier to successful long-term outcomes in clinical transplantation. The importance of B lymphocytes-and their secretory products, alloantibodies-in the pathogenesis of allograft rejection is accepted. Furthermore, it is now clear that the dominant regulator of peripheral B-cell homeostasis and tolerance is the B-lymphocyte stimulator (BLyS), also referred to as the B-cell activating factor (BAFF). Recently, a novel class of clinical immunotherapeutic agents specific for BLyS, and its family of cytokines, has emerged for the treatment of B-cell-mediated diseases. In this study, we demonstrate the potential utility of BLyS-directed immunotherapy in preventing allograft rejection using a murine islet transplantation model.

METHODS

A transient period of mature peripheral B-cell depletion was induced by means of in vivo BLyS neutralization using a murine analog of the monoclonal antibody, Benlysta. Subsequently, fully major histocompatibility complex-mismatched islets were transplanted into naïve diabetic mice followed by a short course of rapamycin.

RESULTS

After BLyS neutralization, indefinite islet allograft survival was achieved. Induction therapy with rapamycin was necessary, but not sufficient, for the achievement of this long-term graft survival. The tolerant state was associated with (1) abrogation of the donor-specific antibody response, (2) transient preponderance of immature/transitional B cells in all lymphoid organs, (3) impaired CD4 T-cell activation during the period of B-cell depletion, and (4) presence of a "regulatory" cytokine milieu.

CONCLUSIONS

In vivo BLyS neutralization effectively induces humoral tolerance and promotes long-term islet allograft survival in mice. Therefore, B-lymphocyte-directed immunotherapy targeting the homeostatic regulator, BLyS, may be effective in promoting transplantation tolerance.

Mechanisms of rejection: current perspectives

Rejection is the major barrier to successful transplantation. The immune response to an allograft is an ongoing dialogue between the innate and adaptive immune system that if left unchecked will lead to the rejection of transplanted cells, tissues, or organs. Activation of elements of the innate immune system, triggered as a consequence of tissue injury sustained during cell isolation or organ retrieval and ischemia reperfusion, will initiate and amplify the adaptive response. T cells require a minimum of two signals for activation, antigen recognition, and costimulation. The activation requirements of naive T cells are more stringent than those of memory T cells. Memory T cells are present in the majority of transplant recipients as a result of heterologous immunity. The majority of B cells require help from T cells to initiate antibody production. Antibodies reactive to donor human leukocyte antigen molecules, minor histocompatibility antigens, endothelial cells, RBCs, or autoantigens can trigger or contribute to rejection early and late after transplantation. Antibody-mediated rejection triggered by alloantibody binding and complement activation is recognized increasingly as a significant contribution to graft loss. Even though one component of the immune system may dominate and lead to rejection being described in short hand as T cell or antibody mediated, it is usually multifactorial resulting from the integration of multiple mechanisms. Identifying the molecular pathways that trigger tissue injury, signal transduction and rejection facilitates the identification of targets for the development of immunosuppressive drugs.

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.

Inotuzumab ozogamicin murine analog-mediated B-cell depletion reduces anti-islet allo- and autoimmune responses

B cells participate in the priming of the allo- and autoimmune responses, and their depletion can thus be advantageous for islet transplantation. Herein, we provide an extensive study of the effect of B-cell depletion in murine models of islet transplantation. Islet transplantation was performed in hyperglycemic B-cell-deficient(μMT) mice, in a purely alloimmune setting (BALB/c into hyperglycemic C57BL/6), in a purely autoimmune setting (NOD.SCID into hyperglycemic NOD), and in a mixed allo-/autoimmune setting (BALB/c into hyperglycemic NOD). Inotuzumab ozogamicin murine analog (anti-CD22 monoclonal antibody conjugated with calicheamicin [anti-CD22/cal]) efficiently depleted B cells in all three models of islet transplantation examined. Islet graft survival was significantly prolonged in B-cell-depleted mice compared with control groups in transplants of islets from BALB/c into C57BL/6 (mean survival time [MST]: 16.5 vs. 12.0 days; P = 0.004), from NOD.SCID into NOD (MST: 23.5 vs. 14.0 days; P = 0.03), and from BALB/c into NOD (MST: 12.0 vs. 5.5 days; P = 0.003). In the BALB/c into B-cell-deficient mice model, islet survival was prolonged as well (MST: μMT = 32.5 vs. WT = 14 days; P = 0.002). Pathology revealed reduced CD3(+) cell islet infiltration and confirmed the absence of B cells in treated mice. Mechanistically, effector T cells were reduced in number, concomitant with a peripheral Th2 profile skewing and ex vivo recipient hyporesponsiveness toward donor-derived antigen as well as islet autoantigens. Finally, an anti-CD22/cal and CTLA4-Ig-based combination therapy displayed remarkable prolongation of graft survival in the stringent model of islet transplantation (BALB/c into NOD). Anti-CD22/cal-mediated B-cell depletion promotes the reduction of the anti-islet immune response in various models of islet transplantation.

The role of B cells in solid organ transplantation

The role of antibodies in chronic injury to organ transplants has been suggested for many years, but recently emphasized by new data. We have observed that when immunosuppressive potency decreases either by intentional weaning of maintenance agents or due to homeostatic repopulation after immune cell depletion, the threshold of B cell activation may be lowered. In human transplant recipients the result may be donor-specific antibody, C4d+ injury, and chronic rejection. This scenario has precise parallels in a rhesus monkey renal allograft model in which T cells are depleted with CD3 immunotoxin, or in a CD52-T cell transgenic mouse model using alemtuzumab to deplete T cells. Such animal models may be useful for the testing of therapeutic strategies to prevent DSA. We agree with others who suggest that weaning of immunosuppression may place transplant recipients at risk of chronic antibody-mediated rejection, and that strategies to prevent this scenario are needed if we are to improve long-term graft and patient outcomes in transplantation. We believe that animal models will play a crucial role in defining the pathophysiology of antibody-mediated rejection and in developing effective therapies to prevent graft injury. Two such animal models are described herein.

Late outgrowth endothelial progenitor cells engineered for improved survival and maintenance of function in transplant-related injury

Chronic allograft vasculopathy (CAV) is a major cause of organ transplant failure that responds poorly to treatment. Endothelial activation, dysfunction and apoptosis contribute to CAV, whereas strategies for protecting endothelium and maximizing endothelial repair may diminish it. Late outgrowth endothelial progenitor cells (LO-EPC) can home to areas of injury and integrate into damaged vessels, implying a role in vascular repair; however, in an allograft, LO-EPC would be exposed to the hazardous microenvironment associated with transplant-related ischaemia reperfusion (I/R) injury and persistent inflammation. We evaluated the in vitro effect of I/R injury and the proinflammatory cytokine tumour necrosis factor (TNF)-α on LO-EPC phenotype and function. We show that LO-EPC are intrinsically more tolerant than mature EC to I/R injury induced apoptosis, maintaining their proliferative, migratory and network formation capacity. Under inflammatory conditions, LO-EPC were activated and released higher levels of inflammatory cytokines, upregulated adhesion molecule expression, and were more susceptible to apoptosis. Lentiviral vector-mediated overexpression of the protective gene A20 in LO-EPC maintained their angiogenic phenotype and function, and protected them against TNF-α-mediated apoptosis, reducing ICAM-1 expression and inflammatory cytokine secretion. Administration of ex vivo modified LO-EPC overexpressing A20 might effect vascular repair of damaged allografts and protect from CAV.

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.

Prevention of chronic renal allograft rejection by soluble CD83

BACKGROUND

Recombinant human soluble CD83 had previously exhibited significant immunosuppressive properties that involved interference with dendritic cell maturation in both mouse and humans, inhibition of autoimmunity in mice, and induction of antigen-specific mouse cardiac allograft tolerance when used in combination with other immunosuppressive drugs. Our current research focus turned to examining the effects of peritransplant soluble CD83 (sCD83) administration on prevention of chronic renal allograft rejection.

METHODS

Fisher344-to-Lewis orthotopic rat renal transplants were performed with sequential recipient killing on postoperative days (PODs) 2, 14, and 140 to examine both the acute and chronic effects of peritransplant sCD83 treatment in rat recipients.

RESULTS

Recipients treated with sCD83 exhibited a marked decrease in IgM and IgG deposition in the graft and antidonor antibody levels in the circulation, as early as POD14 and persisting until POD140. sCD83 treatment also reduced the infiltration of T cells and monocytes into the graft tissue and inhibited intragraft expression of MyD88 and inflammatory cytokine levels during the observation period. sCD83-treated grafts demonstrated normal histology beyond POD140, including dramatic reductions in tubular atrophy and interstitial fibrosis compared with untreated recipients.

CONCLUSION

We have demonstrated that peritransplant treatment with recombinant sCD83 attenuates both innate and adaptive immune responses and leads to prevention of chronic rejection in a rat renal transplant model. Because sCD83 is of human origin, the therapeutic approach used in our rodent transplant model holds significant promise for clinical transplantation.

Acquisition of humoral transplantation tolerance upon de novo emergence of B lymphocytes

A major obstacle to transplantation tolerance is humoral immunity. In this paper, we demonstrate that the intrinsic developmental propensity of the B lymphocyte compartment for acquisition of self-tolerance can be harnessed to induce humoral unresponsiveness to transplanted alloantigens. In the current study, when transitional B cells developed in the presence of donor lymphoid cells, the mature B lymphocyte compartment failed to mount a donor-specific alloantibody response to an organ transplant--despite unrestrained acute T cell-mediated allograft rejection. Specifically, we generated an experimental system wherein a B6 strain B cell compartment developed de novo in the presence of F1 (B6xBALB/c) lymphoid cells and in a T cell-deficient setting. Following establishment of a steady-state B cell compartment, these B6 mice were transplanted with heterotopic cardiac allografts from allogeneic BALB/c donors. The mice were then inoculated with purified syngeneic B6 T cells. As expected, all cardiac allografts were acutely rejected. However, the B lymphocyte compartment of these mice was completely inert in its capacity to form a BALB/c-specific alloantibody response. Using an alloantigen-specific Ig transgenic system, we demonstrated that this profound degree of humoral tolerance was caused by clonal deletion of alloreactive specificities from the primary B cell repertoire. Thus, de novo B cell compartment development at the time of transplantation is of critical importance in recipient repertoire "remodeling" to a humoral tolerant state.

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.

Preemptive CD20+ B cell depletion attenuates cardiac allograft vasculopathy in cyclosporine-treated monkeys

Chronic rejection currently limits the long-term efficacy of clinical transplantation. Although B cells have recently been shown to play a pivotal role in the induction of alloimmunity and are being targeted in other transplant contexts, the efficacy of preemptive B cell depletion to modulate alloimmunity or attenuate cardiac allograft vasculopathy (CAV) (classic chronic rejection lesions found in transplanted hearts) in a translational model has not previously been described. We report here that the CD20-specific antibody (alphaCD20) rituximab depleted CD20+ B cells in peripheral blood, secondary lymphoid organs, and the graft in cynomolgus monkey recipients of heterotopic cardiac allografts. Furthermore, CD20+ B cell depletion therapy combined with the calcineurin inhibitor cyclosporine A (CsA) prolonged median primary graft survival relative to treatment with alphaCD20 or CsA alone. In animals treated with both alphaCD20 and CsA that achieved efficient B cell depletion, alloantibody production was substantially inhibited and the CAV severity score was markedly reduced. We conclude therefore that efficient preemptive depletion of CD20+ B cells is effective in a preclinical model to modulate pathogenic alloimmunity and to attenuate chronic rejection when used in conjunction with a conventional clinical immunosuppressant. This study suggests that use of this treatment combination may improve the efficacy of transplantation in the clinic.