Accommodation of grafts: implications for health and disease

Role of complement in humoral immunity

PURPOSE OF REVIEW

Antibody-mediated rejection (AMR) after solid organ transplantation remains an unsolved problem and leads to poor early and late patient outcomes. The complement system is a well recognized pathogenic mediator of AMR. Herein, we review the known molecular mechanisms of disease and results from ongoing clinical testing of complement inhibitors after solid organ transplant.

RECENT FINDINGS

Activation and regulation of the complement cascade is critical not only for the terminal effector function of donor-specific antibodies, but also for the regulation of T and B cell subsets to generate the antidonor humoral response. Donor-specific antibodies (DSA) have heterogenous features, as are their interactions with the complement system. Clinical testing of complement inhibitors in transplant patients have shown good safety profiles but mixed efficacy to date.

SUMMARY

The complement cascade is a critical mediator of AMR and clinical trials have shown early promising results. With the steady emergence of novel complement inhibitors and our greater understanding of the molecular mechanisms linking complement and AMR, there is greater optimism now for new prognostic and therapeutic tools to deploy in transplant patients with AMR.

Accommodation in allogeneic and xenogeneic organ transplantation: Prevalence, impact, and implications for monitoring and for therapeutics

Accommodation refers to acquired resistance of organs or tissues to immune or inflammatory reactions that might otherwise cause severe injury or rejection. As first observed in ABO-incompatible kidney transplants and heterotopic cardiac xenografts, accommodation was identified when organ transplants continued to function despite the presence of anti-graft antibodies and/or other reactants in the blood of recipients. Recent evidence suggests many and perhaps most organ transplants have accommodation, as most recipients mount B cell responses specific for the graft. Wide interest in the impact of graft-specific antibodies on the outcomes of transplants prompts questions about which mechanisms confer protection against such antibodies, how accommodation might be detected and whether and how rejection could be superimposed on accommodation. Xenotransplantation offers a unique opportunity to address these questions because immune responses to xenografts are easily detected and the pathogenic impact of immune responses is so severe. Xenotransplantation also provides a compelling need to apply these and other insights to decrease the intensity and toxicity of immunosuppression that otherwise could limit clinical application.

TNFRSF13B in B cell responses to organ transplantation

Antibodies directed against organ transplants are thought to pose the most vexing hurdle to enduring function and survival of the transplants, particularly organ xenotransplants, and accordingly basic and clinical investigation has focused on elucidating the specificity and pathogenicity of graft-specific antibodies. While much has been learned about these matters, far less is known about the B cells producing graft-specific antibodies and why these antibodies appear to injure some grafts but not others. With the goal of addressing those questions, we have investigated the properties of tumor necrosis factor receptor super family-13B (TNFRSF13B), which regulates various aspects of B cell responses. A full understanding of the functions of TNFRSF13B however is hindered by extreme polymorphism and by diversity of interactions of the protein. Nevertheless, TNFRSF13B variants have been found to exert distinct impact on natural and elicited antibody responses and host defense and mutations of TNFRSF13B have been found to influence the propensity for development of antibody-mediated rejection of organ transplants. Because B cell responses potentially limit application of xenotransplantation, understanding how TNFRSF13B diversity and TNFRSF13B variants govern immunity in xenotransplantation could inspire development of novel therapeutics that could in turn accelerate clinical implementation of xenotransplantation.

Endothelial Dysfunction after Hematopoietic Stem Cell Transplantation: A Review Based on Physiopathology

Endothelial dysfunction (ED) is frequently encountered in transplant medicine. ED is an argument of high complexity, and its understanding requires a wide spectrum of knowledge based on many fields of basic sciences such as molecular biology, immunology, and pathology. After hematopoietic stem cell transplantation (HSCT), ED participates in the pathogenesis of various complications such as sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD), graft-versus-host disease (GVHD), transplant-associated thrombotic microangiopathy (TA-TMA), idiopathic pneumonia syndrome (IPS), capillary leak syndrome (CLS), and engraftment syndrome (ES). In the first part of the present manuscript, we briefly review some biological aspects of factors involved in ED: adhesion molecules, cytokines, Toll-like receptors, complement, angiopoietin-1, angiopoietin-2, thrombomodulin, high-mobility group B-1 protein, nitric oxide, glycocalyx, coagulation cascade. In the second part, we review the abnormalities of these factors found in the ED complications associated with HSCT. In the third part, a review of agents used in the treatment of ED after HSCT is presented.

Detecting donor-specific antibodies: the importance of sorting the wheat from the chaff

Human leukocyte antigen (HLA) compatibility is very important for successful transplantation of solid organs. In this paper, we focused on the humoral arm of immunity in the clinical setting of organ transplantation: how HLA antibodies develop, how they can be detected, and what they can do to injure organ transplants. Specifically, we explore the technical perspectives of detecting donor-specific antibodies (DSA) in HLA laboratories, and use real-life clinical cases to explain the principles. Currently there are many tools in our HLA antibody detection toolbox: conventional cytotoxicity cross match, flow cross match, and solid phase assays using beads conjugated with single or multiple HLA antigens. Single antigen bead (SAB) assay is the most sensitive tool available for detecting HLA antibodies and assessing the immunological risk for organ transplant. However, there are intrinsic limitations to solid-phase assays and they are prone to both false negativity and importantly, false positivity. Denatured antigens on single antigen beads might be the most prominent source of false positive reactivity, and may have been underestimated by many HLA experts. No single assay is perfect and therefore multiple methods, including the less sensitive assays, should be employed to determine the clinical relevance of detected HLA antibodies. Thoughtful process, including knowledge of HLA systems, cross reactivity, epitopes, and the patient's clinical history should be employed to correctly interpret data. The clinical team should work closely with HLA laboratories to ensure accurate interpretation of information and optimal management of patients before and after organ transplantation.

Accommodation and related conditions in vascularized composite allografts

PURPOSE OF REVIEW

The outcome of vascularized composite allografts (VCA) often appear unrelated to the presence of donor-specific antibodies (DSA) in blood of the recipient or deposition of complement in the graft. The attenuation of injury and the absence of rejection in other types of grafts despite manifest donor-specific immunity have been explained by accommodation (acquired resistance to immune-mediated injury), adaptation (loss of graft antigen) and/or enhancement (antibody-mediated antigen blockade). Whether and how accommodation, adaptation and/or enhancement impact on the outcome of VCA is unknown. Here we consider how recent observations concerning accommodation in organ transplants might advance understanding and resolve uncertainties about the clinical course of VCA.

RECENT FINDINGS

Investigation of the mechanisms through which kidney allografts avert antibody-mediated injury and rejection provide insights potentially applicable to VCA. Interaction of DSA can facilitate replacement of donor by recipient endothelial cells, modulate or decrease synthesis of antigen, mobilize antigen that in turn blocks further immune recognition and limit the amount of bound antibody, allowing accommodation to ensue. These processes also can explain the apparent dissociation between the presence and levels of DSA in blood, deposition of C4d in grafts and antibody-mediated rejection. Over time the processes might also explain the inception of chronic graft changes.

SUMMARY

The disrupted tissue in VCA and potential for repopulation by endothelial cells of the recipient establish conditions that potentially decrease susceptibility to acute antibody-mediated rejection. These conditions include clonal suppression of donor-specific B cells, and adaptation, enhancement and accommodation. This setting also potentially highlights heretofore unrecognized interactions between these 'protective' processes.

Effects of Neutralization by Soluble ABH Antigens Produced by Transplanted Kidneys From ABO-Incompatible Secretor Donors

BACKGROUND

Grafts survive despite blood group antigens on the transplant being continuously exposed to antibodies in the blood of recipients in ABO-incompatible kidney transplantation (ABOi KT), owing to the mechanism of accommodation. We analyzed the immunodynamics of soluble ABH antigens in allografts from secretor donors and the influence of such immunodynamics on accommodation and subsequent graft survival in ABOi KT.

METHODS

The genotype of a known human β-galactoside α-1,2-fucosyltransferase gene (FUT2), which determines soluble ABH antigen secretor status, was established in 32 donors for ABOi KT at the Severance Hospital, from June 2010 to July 2015. Clinical outcomes of recipients, such as anti-A/B antibody titer change, renal function, and graft survival, were evaluated.

RESULTS

Twenty-five donors were secretors (78.1%), and seven were nonsecretors (21.9%). The frequency of anti-A/B IgG or IgM antibody titer elevation or reduction post-transplantation was not significantly related to donor secretor status. However, IgM titer was rapidly reduced in recipients transplanted from nonsecretor donors (P=0.01), which could be explained by the lack of absorption effect of soluble antigens, enhancing the binding of antibodies to antigens in the allografts. Interestingly, soluble ABH antigens did not affect rejection-free graft survival, which may be due to the nature of β-galactoside α-1,2-fucosyltransferase.

CONCLUSIONS

Soluble ABH antigens produced by transplanted kidneys from secretor donors played a role in inducing accommodation within three months of KT through neutralization; however, major graft outcomes were not affected.

Issues in solid-organ transplantation in children: translational research from bench to bedside

In this review, we identify important challenges facing physicians responsible for renal and cardiac transplantation in children based on a review of the contemporary medical literature. Regarding pediatric renal transplantation, we discuss the challenge of antibody-mediated rejection, focusing on both acute and chronic antibody-mediated rejection. We review new diagnostic approaches to antibody-mediated rejection, such as panel-reactive antibodies, donor-specific cross-matching, antibody assays, risk assessment and diagnosis of antibody-mediated rejection, the pathology of antibody-mediated rejection, the issue of ABO incompatibility in renal transplantation, new therapies for antibody-mediated rejection, inhibiting of residual antibodies, the suppression or depletion of B-cells, genetic approaches to treating acute antibody-mediated rejection, and identifying future translational research directions in kidney transplantation in children. Regarding pediatric cardiac transplantation, we discuss the mechanisms of cardiac transplant rejection, including the role of endomyocardial biopsy in detecting graft rejection and the role of biomarkers in detecting cardiac graft rejection, including biomarkers of inflammation, cardiomyocyte injury, or stress. We review cardiac allograft vasculopathy. We also address the role of genetic analyses, including genome-wide association studies, gene expression profiling using entities such as AlloMap®, and adenosine triphosphate release as a measure of immune function using the Cylex® ImmuKnow™ cell function assay. Finally, we identify future translational research directions in heart transplantation in children.

AMP-activated protein kinase as a promoting factor, but complement and thrombin as limiting factors for acquisition of cytoprotection: implications for induction of accommodation

Accommodation has been termed as a condition without graft rejection even in the presence of antidonor antibody. We previously reported an in vitro accommodation model, which demonstrated that preincubation of A/B antigen-expressing endothelial cells with anti-A/B antibody resulted in ERK inactivation followed by resistance to complement-mediated cytotoxicity through the induction of complement regulatory genes. However, under the in vivo condition, the effects of complement and coagulation system cannot be ignored. The purpose of this study is to find effective ways to navigate accommodation by exploring the relevant signal transduction. Preincubation with a low level of complement or thrombin failed to induce resistance to complement-mediated cytotoxicity. AMP-activated protein kinase (AMPK) activators such as resveratrol, AICAR and metformin protected endothelial cells against complement-mediated cytotoxicity through the increase in CD55, CD59, haem oxygenase-1 (HO-1) and ferritin heavy chain (ferritin H) genes, all of which were attenuated by AMPKα knock-down. Resveratrol counteracted the inhibitory effect of pretreated complement and thrombin on acquisition of resistance to complement-mediated cytotoxicity through AMPKα. AMPK regulation in endothelial cells could become the potential strategy to induce accommodation in clinical pro-inflammation and pro-coagulation.

Encapsulated cell grafts to treat cellular deficiencies and dysfunction

Cell transplantation provides a therapeutic alternative to whole organ transplantation in the management of diseases arising from the absence or failure of specialized cells. Though allogenic transplantation is favorable in terms of graft acceptance, xenotransplantation can provide a potentially unlimited source of cells and can overcome shortage of human donors. Effective immunoisolation of the xenografts is critical for their long term survival and function. Encapsulation of cells in polymeric matrices, organic or inorganic, provides a physical selectively permeable barrier between the host and the graft, thereby immunoisolating the graft. Microencapsulation of cells in alginate hydrogels has been pervasive, but this approach does not provide precise control over porosity, whereas micro- and nano-fabrication technologies can provide precise and reproducible control over porosity. We highlight both encapsulation approaches in this review, with their relative advantages and challenges. We also highlight the therapeutic potential of encapsulated cells for treating a variety of diseases, detailing the xenotransplantation of pancreatic islets in diabetes therapy as well as the grafting of engineered cells that facilitate localized enzyme-prodrug therapy of pancreatic cancer.

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.

Mechanisms involved in antibody- and complement-mediated allograft rejection

Antibody-mediated rejection has become critical clinically because this form of rejection is usually unresponsive to conventional anti-rejection therapy, and therefore, it has been recognized as a major cause of allograft loss. Our group developed experimental animal models of vascularized organ transplantation to study pathogenesis of antibody- and complement-mediated endothelial cell injury leading to graft rejection. In this review, we discuss mechanisms of antibody-mediated graft rejection resulting from activation of complement by C1q- and MBL (mannose-binding lectin)-dependent pathways and interactions with a variety of effector cells, including macrophages and monocytes through Fcgamma receptors and complement receptors.

Peritransplant immunoadsorption for positive crossmatch deceased donor kidney transplantation

Various desensitization protocols were shown to enable successful living donor kidney transplantation across a positive complement-dependent cytotoxicity crossmatch (CDCXM). Positive crossmatch transplantation, however, is less well established for deceased donor transplantation. We report a cohort of 68 deceased donor renal allograft recipients who, on the basis of broad sensitization (lymphocytotoxic panel reactivity ≥40%), were subjected to a protocol of peritransplant immunoadsorption (IA). Treatment consisted of a single session of immediate pretransplant IA (protein A) followed by posttransplant IA and antilymphocyte antibody therapy. Twenty-one patients had a positive CDCXM, which could be rendered negative by pretransplant apheresis. Solid phase HLA antibody detection revealed preformed donor-specific antibodies (DSA) in all 21 CDCXM-positive and in 30 CDCXM-negative recipients. At 5 years, overall graft survival, death-censored graft survival and patient survival were 63%, 76% and 87%, respectively, without any differences between CDCXM-positive, CDCXM-negative/DSA-positive and CDCXM-negative/DSA-negative recipients. Furthermore, groups did not differ regarding rates of antibody-mediated rejection (24% vs. 30% vs. 24%, p = 0.84), cellular rejection (14% vs. 23% vs. 18%, p = 0.7) or allograft function (median 5-year serum creatinine: 1.3 vs. 1.8 vs. 1.7 mg/dL, p = 0.62). Our results suggest that peritransplant IA is an effective strategy for rapid desensitization in deceased donor transplantation.

Antibodies in transplantation

Transplantation of cells, tissues, and organs from one individual to another can incite the production of antibodies specific for foreign antigens, especially major histocompatibility antigens, in the graft. Antibodies specific for a graft provide an index of immunity and a potential trigger for injury and rejection. However, the index of immunity can sometimes miss antibody-mediated rejection and besides causing injury the antibodies against a graft can also protect a graft from injury by blocking immune recognition, called enhancement, regulating activation of complement, and inducing changes in the graft that resist damage. Reviewed here are potential limitations in the use of antibodies as an index of immunity and the ways antibodies cause and/or prevent injury.

Escaping from rejection

Since the earliest days of transplantation, immunobiologists have sought means to prevent recognition and rejection of foreign tissue. The goal of these strategies is the retention of recipient immune function while selectively avoiding graft injury. Although considerable theoretical and technical problems remain, an analogous problem and solution already exists in nature. Here, we discuss the mechanisms by which organisms preclude or control autotoxicity, and for each, consider the corollaries between prevention of autotoxicity and graft rejection. Further study of these controls, including structural and conditional tolerance and accommodation, will offer insight into new therapies for allo- and xenotransplantation.

Significance of HLA class I antibody-induced antioxidant gene expression for endothelial cell protection against complement attack

It has been observed that a graft organ continues to survive and function normally even in the presence of anti-graft antibodies. However, the mechanisms behind acquirement of this condition remain unknown. Here we report that the anti-HLA ligation on endothelial cells induces PI3K/AKT activation followed by antioxidant gene induction through Nrf2-mediated antioxidant-responsive element (ARE) activation. Activation of PI3K/AKT in endothelial cells by a low concentration of anti-HLA ligation enhances protection from complement attack. A real-time quantitative PCR and flow-cytometry experiment showed that ferritin H and HO-1 mRNAs were induced in a PI3K/AKT-dependent manner, while CD55 and CD59 expression were not enhanced by anti-HLA ligation. Anti-HLA ligation on endothelial cells activates ferritin H ARE and induces Nrf2 binding on its enhancer element. Finally, overexpression of Nrf2 in endothelial cells attenuates complement-mediated cytotoxicity. These experiments suggest that induction of PI3K/AKT-dependent cytoprotective genes by Nrf2 is an important mechanism to prevent complement attack. Thus, a protocol to activate this pathway would be a potential strategy for avoidance of graft rejection in transplantation.

Lessons from cardiac transplantation in infancy

Besides correcting devastating pathophysiology, cardiac transplantation in infancy offers an incomparable model for exploring the structure and function of the immune system. Infants and young children have relatively few memory B cells and T cells. Hence, the response of the young to allotransplantation better represents a primary immune response and may be better suited to testing spontaneous tolerance. Those who undergo cardiac transplantation in infancy are also unique because they are often subjected to thymectomy and depletion of mature T cells. These subjects can have a dramatic contraction of T cell diversity, allowing the testing of how diversity contributes to function.

Effect of antibodies on endothelium

Patients developing posttransplant antibodies against HLA and non-HLA antigens expressed by the endothelium of the graft undergo more frequent episodes of rejection and have decreased long-term graft survival. Antibodies against the endothelium can alter/damage the cells of the graft through several mechanisms. Historically, antibodies were thought to elicit endothelial cell injury via complement-dependent mechanisms. New research has shown that antibodies can also contribute to the process of transplant rejection by stimulating proinflammatory and proproliferation signals. Antibody ligation leads to several functional alterations in EC including Weibel Palade body exocytosis, leukocyte recruitment, growth factor expression and cell proliferation. In contrast, under certain circumstances, antibodies may induce prosurvival signals and graft accommodation. The signaling events regulating accommodation vs. rejection appear to be influenced by the specificity and concentration of the anti-HLA antibody and the degree of molecular aggregation. Knowledge of the HLA and non-HLA antibody-mediated signaling pathways has the potential to identify new therapeutic targets to promote accommodation and prevent acute and chronic antibody-mediated rejection.

The role of antibodies in transplantation

For the past 40 years, T cells have been considered the primary threat to the survival of allografts. However, antibodies can induce severe vascular disease of organ transplants, and this disease, particularly "antibody-mediated" rejection, has become a major clinical challenge. Not only do antibodies cause rejection, the rejection caused by antibodies resists treatment by conventional drug regimens. On the other hand, antibodies can induce a condition in which grafts seemingly resist antibody-mediated injury, which is accommodation. In this communication, we discuss the role of antibodies in the diagnosis and pathogenesis of rejection and accommodation, and suggest what we considered the major gaps in knowledge and directions research into this subject might productively take.

Acute cellular rejection and the subsequent development of allograft vasculopathy after cardiac transplantation

BACKGROUND

Cardiac allograft vasculopathy (CAV) is primarily immune-mediated. We investigated the role of cellular rejection in CAV development.

METHODS

The study comprised 252 cardiac transplant recipients (mean age, 49.02 +/- 17.05 years; mean follow-up, 7.61 +/- 4.49 years). Total rejection score (TRS) based on the 2004 International Society of Heart and Lung Transplantation R grading system (0R = 0, 1R = 1, 2R = 2, 3R = 3) and any rejection score (ARS; calculated as 0R = 0, 1R = 1, 2R = 1; 3R = 1, or the number of rejections of any grade) were normalized for the total number of biopsy specimens. CAV was defined as coronary stenosis of 40% or more and/or distal pruning of secondary side branches. Thirty-two patients had undergone 3-dimensional intravascular ultrasound (IVUS) at baseline and with virtual histology (VH) IVUS at 24 months.

RESULTS

In univariate analysis, 6-month TRS (hazard ratio [HR], 1.9; 95% confidence interval [CI], 0.99-3.90, p = 0.05) and ARS (HR, 2.22; 95% CI, 1.01-4.95; p = 0.047) were associated with increased risk of CAV. In multivariate analysis, 6-month TRS (HR, 2.84; 95% CI, 1.44-6.91, p = 0.02) was significantly associated with increased risk of CAV onset. The 12- and 24-month rejection scores were not risk factors for the onset of CAV. By Kaplan-Meier analysis, 6-month TRS exceeding 0.3 was associated with a significantly shorter time to CAV onset (p = 0.018). There was direct correlation (r = 0.44, p = 0.012) between TRS at 6 months and the percentage of necrotic core demonstrated by VH-IVUS at 24 months.

CONCLUSION

Recurrent cellular rejection has a cumulative effect on the onset of CAV. The mechanism may be due to increased inflammation resulting in increased plaque burden suggesting a relationship between the immune basis of cellular rejection and CAV.

Human leukocyte antigen antibodies in chronic transplant vasculopathy-mechanisms and pathways

Transplant recipients exhibiting posttransplant antibodies are at a higher risk for acute and chronic antibody mediated rejection (AMR). The primary alloantigens recognized by antibodies in recipients with AMR are the highly polymorphic HLA class I and class II molecules expressed on the surface of the endothelial cells (ECs) of the graft. Traditionally, anti-HLA antibodies were thought to mediate graft injury through complement-dependent mechanisms. However, recent studies indicate that antibodies can also contribute to alterations in EC function through complement-independent mechanisms by transducing intracellular signals. Anti-HLA antibodies transduce signals that are both pro-inflammatory and pro-proliferative suggesting mechanistic roles in acute and chronic AMR.

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

Five patients with end-stage kidney disease received combined kidney and bone marrow transplants from HLA haploidentical donors following nonmyeloablative conditioning to induce renal allograft tolerance. Immunosuppressive therapy was successfully discontinued in four patients with subsequent follow-up of 3 to more than 6 years. This allograft acceptance was accompanied by specific T-cell unresponsiveness to donor antigens. However, two of these four patients showed evidence of de novo antibodies reactive to donor antigens between 1 and 2 years posttransplant. These humoral responses were characterized by the presence of donor HLA-specific antibodies in the serum with or without the deposition of the complement molecule C4d in the graft. Immunofluorescence staining, ELISA assays and antibody profiling using protein microarrays demonstrated the co-development of auto- and alloantibodies in these two patients. These responses were preceded by elevated serum BAFF levels and coincided with B-cell reconstitution as revealed by a high frequency of transitional B cells in the periphery. To date, these B cell responses have not been associated with evidence of humoral rejection and their clinical significance is still unclear. Overall, our findings showed the development of B-cell allo- and autoimmunity in patients with T-cell tolerance to the donor graft.

Antibody-mediated rejection without acute graft dysfunction in adult ABO-compatible heart transplantation: a case of accommodation

Humoral rejection in heart transplantation is associated with graft dysfunction, circulating anti-donor antibodies and C4d deposits in endomyocardial biopsies. Detecting C4d capillary positivity is of diagnostic and prognostic value. C4d positivity can be found in solid-organ transplants in cases of "accommodation," a form of humoral rejection without graft dysfunction. Accommodation might reflect a change in antibodies or in the antigen, or the graft acquiring a resistance to injury by antibodies and complement. We present a case of accommodation in the setting of adult ABO-compatible orthotopic heart transplantation, which was diagnosed according to the recently introduced ISHLT criteria for humoral rejection: despite this immunologic profile, the patient never showed signs of graft dysfunction. Physicians should be aware of the accommodation phenomenon so they can identify this subset of patients and assess its long-term effects on chronic rejection and outcome in transplanted patients.

Mechanisms of endothelial dysfunction, injury, and death

Vascular endothelial cells normally perform several key homeostatic functions such as keeping blood fluid, regulating blood flow, regulating macromolecule and fluid exchange with the tissues, preventing leukocyte activation, and aiding in immune surveillance for pathogens. Injury or cell death impairs or prevents conduct of these activities, resulting in dysfunction. Most endothelial cell death is apoptotic, involving activation of caspases, but nonapoptotic death responses also have been described. Stimuli that can cause endothelial injury or death include environmental stresses such as oxidative stress, endoplasmic reticulum stress, metabolic stress, and genotoxic stress, as well as pathways of injury mediated by the innate and adaptive immune systems. Pathways of immune-mediated death include those activated by death receptors as well as those activated by cytolytic granules and reactive oxygen species. The biochemical pathways activated by these injurious stimuli are described herein and will serve as a basis for future development of endothelial protective therapies.

Clinical relevance of pretransplant human leukocyte antigen donor-specific antibodies in renal patients waiting for a transplant: a risk factor

The use of highly sensitive solid-phase antibody detection assays, including x-MAP multiple bead-based technology (Luminex), has greatly enhanced our ability to accurately detect and define very low levels of HLA antibodies. These developments have led to patients having increasing lists of antibody specificities (which may not be clinically relevant), resulting in a new "technological barrier" to transplantation in sensitized patients. Alloantibodies play a major role in all types of solid organ rejection; the presence of low-titer donor-specific antigen (DSA) identified pretransplant is associated with an increased risk of antibody-mediated rejection (AMR). However, these low-titer antibodies do not represent an absolute contraindication to transplant. Improvement in the diagnosis and treatment of AMR will allow sensitized patients with DSA to be successfully transplanted in the short term, but extended follow-up is required to ensure acceptable long-term graft survival in this group. These factors must be integrated into the decision algorithms for immunosuppressive treatment in patients at immunologic risk.

Noncognate function of B cells in transplantation

How B cells affect the outcome of transplants is a question of enduring interest. Initial efforts to answer that question suggested, wrongly, that B cells have no impact on transplantation. Now, however, B cells are known to influence not only the outcome of vascularized grafts through the production of anti-donor antibodies but also the competence of cellular immunity through a number of physiologic functions. In this study, we explain why the importance of B cells was overlooked in the past and consider the range of noncognate functions of B cells that may determine the outcome of transplants.

Posttransplant HLA alloreactivity in stable kidney transplant recipients-incidences and impact on long-term allograft outcomes

Humoral alloreactivity is well established to predict adverse allograft outcomes. However, in some recipients, alloantibodies may also occur in the absence of graft dysfunction. We evaluated if and how often complement- and noncomplement-fixing alloantibodies are detectable in stable recipients and whether, in this context, they affect long-term outcomes. Sera obtained from 164 kidney transplant recipients at 2, 6 and 12 months were evaluated by FlowPRA screening and single-antigen testing for detection of IgG- or C4d-fixing HLA panel reactivity and donor-specific antibodies (DSA). Applying stringent criteria, we selected 34 patients with an uneventful 1-year course (no graft dysfunction or rejection) and excellent graft function at 12 months [estimated glomerular filtration rate (eGFR) >or=60 mL/min and proteinuria <or=0.5 g/24 h]. Nine (27%) and 5 (15%) of these recipients tested positive by [IgG] and [C4d]FlowPRA screening, respectively. In five cases, DSA were identified. Frequencies of positive test results and DSA binding intensities were not significantly lower than those documented for patients who did not fulfill the above criteria. In recipients with an excellent 1-year course, FlowPRA reactivity was not associated with lower eGFR or increased protein excretion during 68-month median follow-up. Our results suggest cautious interpretation of antibody monitoring in patients with normal-functioning grafts.

Anti-HLA donor-specific antibodies detected in positive B-cell crossmatches by Luminex predict late graft loss

The significance of B-cell crossmatching in kidney transplantation is controversial. Recipients (n = 471) transplanted in a single centre from 1987 to 2005 with complete T- and B-cell crossmatch records were studied. Sera from 83 patients transplanted across a positive B-cell crossmatch, with concomitant negative T-cell crossmatch (T-B+) on either current and/or peak sera were studied using Luminex to determine presence of donor-specific antibodies (DSA). Clinical outcomes of T-B+ patients were compared with 386 T-B- patients. T-B+ predicted vascular (p = 0.01), but not cellular (p = 0.82) or glomerular (p = 0.14) rejection. IgG HLA DSA were found in 33% (n = 27) of the T-B+ patients and were associated with higher risk of any (p = 0.047), vascular (p = 0.01) or glomerular (p < 0.001) rejection at 6 months. Of 27 patients with DSA, 18/21 (86%) were the complement-fixing IgG(1) and/or IgG(3) subclass antibodies. DSA imposed a statistically significant higher risk of graft loss 5 years posttransplant (1.8 [1.0-3.3], p = 0.045). This study showed that only one-third of positive B-cell crossmatch (BXM) was caused by DSA and was associated with late graft loss. Thus, using BXM to preclude kidney transplantation may potentially disadvantage >60% of patients in whom BXM is not indicative of the presence of DSA.

Accommodation and antibodies

Accommodation refers to the condition in which an organ transplant functions normally by acquiring resistance to immune-mediated injury (especially), despite the presence of anti-transplant antibodies in the recipient. This status is associated with several modifications in the recipient as well as in the graft, such as previous depletion of anti-graft antibodies and their slow return once the graft is placed; expression of several protective genes in the graft; a Th2 immune response in the recipient; and inhibition of the membrane attack complex of complement.

Accommodation in organ transplantation

PURPOSE OF REVIEW

We review recent insights into the mechanisms and prevalence of accommodation. Accommodation refers to an acquired resistance of an organ graft to humoral injury and rejection.

RECENT FINDINGS

Accommodation has been postulated to reflect changes in antibodies, control of complement and/or acquired resistance to injury by antibodies, complement or other factors. We discuss the importance of these mechanisms, highlighting new conclusions.

SUMMARY

Accommodation may be a common, perhaps the most common, outcome of organ transplantation and, in some systems, a predictable outcome of organ xenotransplantation. Further understanding of how accommodation is induced and by what mechanisms it is manifest and maintained could have a profound impact on transplantation in general and perhaps on other fields.

The anti-non-gal xenoantibody response to xenoantigens on gal knockout pig cells is encoded by a restricted number of germline progenitors

Antibodies directed at non-gal xenoantigens are responsible for acute humoral xenograft rejection when gal knockout (GalTKO) pig organs are transplanted into nonhuman primates. We generated IgM and IgG gene libraries using peripheral blood lymphocytes of rhesus monkeys initiating active xenoantibody responses after immunization with GalTKO pig endothelial cells and used these libraries to identify IgV(H) genes that encode antibody responses to non-gal pig xenoantigens. Immunoglobulin genes derived from the IGHV3-21 germline progenitor encode xenoantibodies directed at non-gal xenoantigens. Transduction of GalTKO cells with lentiviral vectors expressing the porcine alpha1,3 galactosyltransferase gene responsible for gal carbohydrate expression results in a higher level of binding of 'anti-non-gal' xenoantibodies to transduced GalTKO cells expressing the gal carbohydrate, suggesting that anti-non-gal xenoantibodies cross react with carbohydrate xenoantigens. The galactosyltransferase two gene encoding isoglobotriaosylceramide synthase (iGb3 synthase) is not expressed in GalTKO pig cells. Our results demonstrate that anti-non-gal xenoantibodies in primates are encoded by IgV(H) genes that are restricted to IGHV3-21 and bind to an epitope that is structurally related to but distinct from the Gal carbohydrate.

New insights into the functions of B cells

Organ transplants between genetically different individuals elicit powerful immune responses that invariably cause rejection in the absence of immune suppression. Among the immune responses elicited by organ allografts, B-cell responses causing antibody-mediated rejection are one of the most vexing. However, recent advances in the field indicate that B cells and antibodies' contribution to immunity extends well beyond the traditional functions ascribed to antibodies. Here we review "non-humoral" functions of B cells and the implications of these functions to transplantation.