Antibody-mediated renal allograft rejection: diagnosis and pathogenesis

Bortezomib in late antibody-mediated kidney transplant rejection (BORTEJECT Study): study protocol for a randomized controlled trial

Background

Despite major advances in transplant medicine, improvements in long-term kidney allograft survival have not been commensurate with those observed shortly after transplantation. The formation of donor-specific antibodies (DSA) and ongoing antibody-mediated rejection (AMR) processes may critically contribute to late graft loss. However, appropriate treatment for late AMR has not yet been defined. There is accumulating evidence that the proteasome inhibitor bortezomib may substantially affect the function and integrity of alloantibody-secreting plasma cells. The impact of this agent on the course of late AMR has not so far been systematically investigated.

Methods/design

The BORTEJECT Study is a randomized controlled trial designed to clarify the impact of intravenous bortezomib on the course of late AMR. In this single-center study (nephrological outpatient service, Medical University Vienna) we plan an initial cross-sectional DSA screening of 1,000 kidney transplant recipients (functioning graft at ≥180 days; estimated glomerular filtration rate (eGFR) >20 ml/minute/1.73 m²). DSA-positive recipients will be subjected to kidney allograft biopsy to detect morphological features consistent with AMR. Forty-four patients with biopsy-proven AMR will then be included in a double-blind placebo-controlled intervention trial (1:1 randomization stratified for eGFR and the presence of T-cell-mediated rejection). Patients in the active group will receive two cycles of bortezomib (4 × 1.3 mg/m² over 2 weeks; 3-month interval between cycles). The primary end point will be the course of eGFR over 24 months (intention-to-treat analysis). The sample size was calculated according to the assumption of a 5 ml/minute/1.73 m² difference in eGFR slope (per year) between the two groups (alpha: 0.05; power: 0.8). Secondary endpoints will be DSA levels, protein excretion, measured glomerular filtration rate, transplant and patient survival, and the development of acute and chronic morphological lesions in 24-month protocol biopsies.

Discussion

The impact of anti-humoral treatment on the course of late AMR has not yet been systematically investigated. Based on the hypothesis that proteasome inhibition improves the outcome of DSA-positive late AMR, we suggest that our trial has the potential to provide solid evidence towards the treatment of this type of rejection.

Trial registration

Clinicaltrials.gov: NCT01873157.

Transplant glomerulopathy: the interaction of HLA antibodies and endothelium

Transplant glomerulopathy (TG) is a major cause of chronic graft dysfunction without effective therapy. Although the histological definition of TG is well characterized, the pathophysiological pathways leading to TG development are still poorly understood. Electron microscopy suggests an earlier appearance of TG and suggests that endothelial cell injury is the first sign of the disease. The pathogenic role of human leukocyte antigen (HLA) antibodies in endothelial cells has been described in acute vascular and humoral rejection. However the mechanisms and pathways of endothelial cell injury by HLA antibodies remain unclear. Despite the description of different causes of the morphological lesion of TG (hepatitis, thrombotic microangiopathy), the strong link between TG and chronic antibody mediated rejection suggests a major role for HLA antibodies in TG formation. In this review, we describe the effect of classes I or II HLA-antibodies in TG and especially the implication of donor specific antibodies (DSA). We update recent studies about endothelial cells and try to explain the different signals and intracellular pathways involved in the progression of TG.

Immune responses to collagen-IV and fibronectin in renal transplant recipients with transplant glomerulopathy

Antibodies (Abs) to donor HLA (donor-specific antibodies [DSA]) have been associated with transplant glomerulopathy (TG) following kidney transplantation (KTx). Immune responses to tissue-restricted self-antigens (self-Ags) have been proposed to play a role in chronic rejection. We determined whether KTx with TG have immune responses to self-Ags, Collagen-IV (Col-IV) and fibronectin (FN). DSA were determined by solid phase assay, Abs against Col-IV and FN by enzyme-linked immunosorbent assay and CD4+ T cells secreting interferon gamma (IFN-γ), IL-17 or IL-10 by ELISPOT. Development of Abs to self-Ags following KTx increased the risk for TG with an odds ratio of 22 (p-value = 0.001). Abs to self-Ags were IgG and IgM isotypes. Pretransplant Abs to self-Ags increased the risk of TG (22% vs. 10%, p < 0.05). Abs to self-Ags were identified frequently in KTx with DSA. TG patients demonstrated increased Col-IV and FN specific CD4+ T cells secreting IFN-γ and IL-17 with reduction in IL-10. We conclude that development of Abs to self-Ags is a risk factor and having both DSA and Abs to self-Ags increases the risk for TG. The increased frequency of self-Ag-specific IFN-γ and IL-17 cells with reduction in IL-10 demonstrate tolerance breakdown to self-Ags which we propose play a role in the pathogenesis of TG.

Plasma exchange for kidney disease: what is the best evidence?

Therapeutic plasma exchange (TPE) has been used as adjunctive therapy for various kidney diseases dating back to the 1970s. In many cases, support for TPE was on mechanistic grounds given the potential to remove unwanted large molecular-weight substances such as autoantibodies, immune complexes, myeloma light chains, and cryoglobulins. More recently, growing evidence from randomized controlled trials, meta-analyses, and prospective studies has provided insights into more rational use of this therapy. This report describes the role of TPE for the 6 most common kidney indications in the 2013 Canadian Apheresis Group (CAG) registry and the evidence that underpins current recommendations and practice. These kidney indications include thrombotic microangiopathy, antiglomerular basement membrane disease, anti-neutrophil cytoplasmic antibody-associated vasculitis, cryoglobulinemia, recurrence of focal and segmental glomerulosclerosis in the kidney allograft, and kidney transplantation.

Antibodies in transplantation: the effects of HLA and non-HLA antibody binding and mechanisms of injury

Until recently, allograft rejection was thought to be mediated primarily by alloreactive T cells. Consequently, immunosuppressive approaches focused on inhibition of T cell activation. While short-term graft survival has significantly improved and rejection rates have dropped, acute rejection has not been eliminated and chronic rejection remains the major threat to long-term graft survival. Increased attention to humoral immunity in experimental systems and in the clinic has revealed that donor specific antibodies (DSA) can mediate and promote acute and chronic rejection. Herein, we detail the effects of alloantibody, particularly HLA antibody, binding to graft vascular and other cells, and briefly summarize the experimental methods used to assess such outcomes.

Capillary dilation and rarefaction are correlated with intracapillary inflammation in antibody-mediated rejection

Antibody-mediated rejection (ABMR) remains one of the major causes of graft loss after renal transplantation. It is dominated by endothelial damage in microcirculation. Clarifying the mechanism of microcirculating damage is obviously a key step to understand the pathogenesis of ABMR. Here we characterized capillary variation in ABMR and its possible mechanisms. Compared with T cell-mediated rejection and stable grafts, there was a significant dilation and rarefaction in peritubular capillaries (PTCs) of the ABMR group; Image-Pro Plus revealed a significantly larger intra-PTC area. Interestingly, the dilation of PTCs was strongly correlated with the intra-PTC cell counting. Moreover, peritubular capillary inflammation is correlated with in situ T-bet expression, and there was a good correlation between the intra-PTC expression of T-bet and the PTC diameter. HIF-1α up-regulation could be observed in ABMR but it was not necessary for capillary dilation. In general, ABMR is characterized with early capillary dilation and rarefaction; our data confirmed that the dilation is strongly correlated with intracapillary inflammation, which in turn is correlated with in situ T-bet expression. T-bet plays an important role in the development of microcirculating injury, and thus it is a potential target for the treatment of ABMR.

Key role for CD4 T cells during mixed antibody-mediated rejection of renal allografts

We utilized mouse models to elucidate the immunologic mechanisms of functional graft loss during mixed antibody-mediated rejection of renal allografts (mixed AMR), in which humoral and cellular responses to the graft occur concomitantly. Although the majority of T cells in the graft at the time of rejection were CD8 T cells with only a minor population of CD4 T cells, depletion of CD4 but not CD8 cells prevented acute graft loss during mixed AMR. CD4 depletion eliminated antidonor alloantibodies and conferred protection from destruction of renal allografts. ELISPOT revealed that CD4 T effectors responded to donor alloantigens by both the direct and indirect pathways of allorecognition. In transfer studies, CD4 T effectors primed to donor alloantigens were highly effective at promoting acute graft dysfunction, and exhibited the attributes of effector T cells. Laser capture microdissection and confirmatory immunostaining studies revealed that CD4 T cells infiltrating the graft produced effector molecules with graft destructive potential. Bioluminescent imaging confirmed that CD4 T effectors traffic to the graft site in immune replete hosts. These data document that host CD4 T cells can promote acute dysfunction of renal allografts by directly mediating graft injury in addition to facilitating antidonor alloantibody responses.

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.

Flow cytometry and solid organ transplantation: a perfect match

In the field of transplantation, flow cytometry serves a well-established role in pre-transplant crossmatching and monitoring immune reconstitution following hematopoietic stem cell transplantation. The capabilities of flow cytometers have continuously expanded and this combined with more detailed knowledge of the constituents of the immune system, their function and interaction and newly developed reagents to study these parameters have led to additional utility of flow cytometry-based analyses, particularly in the post-transplant setting. This review discusses the impact of flow cytometry on managing alloantigen reactions, monitoring opportunistic infections and graft rejection and gauging immunosuppression in the context of solid organ transplantation.

Postanastomotic transplant renal artery stenosis: association with de novo class II donor-specific antibodies

In this study, we analyze the outcomes of transplant renal artery stenosis (TRAS), determine the different anatomical positions of TRAS, and establish cardiovascular and immunological risk factors associated with its development. One hundred thirty-seven of 999 (13.7%) patients had TRAS diagnosed by angiography; 119/137 (86.9%) were treated with angioplasty, of which 113/137 (82.5%) were stented. Allograft survival in the TRAS+ intervention, TRAS+ nonintervention and TRAS- groups was 80.4%, 71.3% and 83.1%, respectively. There was no difference in allograft survival between the TRAS+ intervention and TRAS- groups, p = 0.12; there was a difference in allograft survival between the TRAS- and TRAS+ nonintervention groups, p < 0.001, and between the TRAS+ intervention and TRAS+ nonintervention groups, p = 0.037. TRAS developed at the anastomosis, within a bend/kink or distally. Anastomotic TRAS developed in living donor recipients; postanastomotic TRAS (TRAS-P) developed in diabetic and older patients who received grafts from deceased, older donors. Compared with the TRAS- group, patients with TRAS-P were more likely to have had rejection with arteritis, odds ratio (OR): 4.83 (1.47-15.87), p = 0.0095, and capillaritis, OR: 3.03 (1.10-8.36), p = 0.033. Patients with TRAS-P were more likely to have developed de novo class II DSA compared with TRAS- patients hazard ratio: 4.41 (2.0-9.73), p < 0.001. TRAS is a heterogeneous condition with TRAS-P having both alloimmune and traditional cardiovascular risk factors.

Long-term follow-up of non-HLA and anti-HLA antibodies: incidence and importance in renal transplantation

BACKGROUND

Detection of antibody-mediated injury is becoming increasingly important in post-transplant patient care. The role of donor-specific anti-human leukocyte antigen (HLA) antibodies in kidney transplant damage is known, whereas the significance of non-HLA antibodies remains an unresolved concern. The aim of the study was to determine the presence and influence on renal function of non-HLA and anti-HLA antibodies in stable patients at 5 years after kidney transplantation.

METHODS

We evaluated the antibodies in 35 consecutive patients with stable renal function at 5 years after transplantation.

RESULTS

Pretransplant screening for donor-specific antibodies by CDC cross-matches was negative in all patients. Anti-endothelial cell antibodies (AECA), anti-angiotensin II type 1 receptor antibodies (anti-AT1R), and anti-endothelin receptor antibodies (anti-ETAR) were assayed as non-HLA antibodies. Non-HLA antibodies were observed in 12 (34%) patients, including AECA (n = 5; 14%), anti- AT1R (n = 6; 17%), anti-ETAR (n = 4; 11%), and both anti-AT1R and anti-ETAR (n = 3). Among 13 (37%) patients with anti-HLA antibodies, 7 also had both non-HLA antibodies: AECA (n = 1), anti-AT1R (n = 3), and anti-ETAR (n = 3). The antibody-negative group (n = 13) showed significantly better renal function than the antibody-positive group (non-HLA and/or anti-HLA; n = 22). Biopsy-proven acute rejection had occurred in 2 of 13 (15%) antibody-negative versus 8 of 22 (36%) antibody-positive patients. These preliminary data revealed an high prevalence of autoantibody and alloantibody production among stable patients at 5 years after kidney transplantation.

CONCLUSION

Simultaneous production of these antibodies and their association with reduced renal function suggests that active humoral immune responses are poorly controlled by immunosuppression.

Significance of isoagglutinin titer in ABO-incompatible kidney transplantation

ABO-incompatible (ABO-i) kidney transplantation (KT) has emerged for overcoming the shortage of organ donors. Although this technique initially achieved only low graft survival due to isoagglutinin, recently developed desensitization protocols have improved survival to levels that are comparable to ABO-compatible KT. However, isoagglutinin is still regarded as a major obstacle to ABO-i KT. In this study, we evaluate the impact of isoagglutinin titer on clinical outcomes as well as factors that may influence isoagglutinin titers. In total, data from 95 patients who underwent ABO-i KT were analyzed. Preoperatively, rituximab administration and plasmapheresis were performed until the titer was reduced to ≤1:4. Retrospective analysis included blood group; timing and dosage of rituximab; isoagglutinin titer; number of plasmapheresis; and clinical outcomes including graft survival and serum creatinine. Graft survival was 95.8% (n = 91) and average serum creatinine at 1- and 1.5-year post-ABOi-KT was 1.3. Three patients died of sepsis. The identified predictors of titer-rebound after transplant were short interval (<7 days) between rituximab and first plasmapheresis (P = 0.004); high initial titer (≥256) (P = 0.023); low titer-reduction rate (P < 0.001); and blood group O (P < 0.001). One patient who experienced a rebound developed antibody-mediated rejection. With low-dose (200 mg) rituximab, the change in isoagglutinin titer-rebound was not significant and the infection rate was significantly decreased (P = 0.001). In conclusion, isoagglutinin titer-rebound within the first 2 weeks after KT may be a risk factor for rejection. The factors identified as affecting titer-rebound after KT were high initial isoagglutinin titer, low titer-reduction rate, short interval, and blood group O.

Absence of C4d urinary excretion in the early post-transplant period is associated with improved long-term kidney graft survival

INTRODUCTION

C4d urinary excretion varies according to the risk of graft rejection or progression of chronic allograft nephropathy, but its influence on long-term kidney transplant (KTx) outcomes remains unclear. The aim of the study was to determine whether the initial (1-3 months post KTx) level of C4d urinary excretion may help to predict long-term kidney allograft transplantation outcomes.

MATERIALS AND METHODS

The study involved 185 patients who had undergone KTx. The urinary specimens taken from the morning urine portion were assessed by ELISA test for C4d excretion. To increase the objectivity of the assessment, all measurements were divided by urinary creatinine excretion (ng/mgCr). The study population was grouped according to the C4d excretion cut-off value into low (LC4d, 109 participants) and high (HC4d, 76 participants) C4d excretion groups. Additionally a subgroup with absence of C4d in the urine (ZC4d, 26 patients) was formed.

RESULTS

The calculated Roc curve indicated the cut off value of the urinary C4d excretion as 12.4ng/mgCr (AUC 0.77; 95%CI 0.73-0.95). The mean C4d urinary excretions in LC4d and HC4d were 1.9±3.27 and 20.6±4.6ng/mgCr, respectively, whereas after exclusion of ZC4d subgroup, the mean C4d was 14.9±6.3ng/mgCr in the remainder. Kaplan-Meier curve analysis demonstrated a slightly higher graft survival rate (GSR) in LC4d than in HC4d group (p=0.04 by log-rank). The subsequent analysis showed the highest GSR in ZC4d subgroup (p=0.0006 by log-rank).

CONCLUSION

Although lower C4d urinary excretion in the early post-transplant period seems to be associated with better long-term kidney allograft transplantation outcomes, only its absence in the urine appears to be a solid predictor of improved graft survival.

Influence of preformed donor-specific antibodies and C4d on early liver allograft function

INTRODUCTION. The impact of preformed donor-specific antibodies (DSA) is incompletely understood in liver transplantation. The incidence and impact of preformed DSA on early post liver transplant were assessed and these were correlated with compliment fragment C4d on allograft biopsy. METHODS. Pretransplant serum from 41 consecutive liver transplant recipients (brain dead donors; DBD = 27 and cardiac death donors; DCD = 14) were tested for class-specific anti-human leukocyte antigen (HLA) and compared against donor HLA types. Liver biopsies were taken during cold storage (t-1) and post-reperfusion (t0) stained with C4d and graded for preservation-reperfusion injury (PRI). RESULTS. Of the 41 recipients, 8 (20%) had anti-HLA class I/II antibodies pretransplant, 3 (7%) were confirmed preformed DSA; classes I and II (n=1) and class I only (n=2). No biopsies showed definite evidence of antibody-mediated rejection. Graft biopsies in overall showed only mild PRI with ischemic hepatocyte C4d pattern similar in both positive and negative DSA patients. One DSA-positive (33%) compared with four DSA-negative patients (10%) had significant early graft dysfunction; severe PRI causing graft loss from primary nonfunction was seen only in DSA-negative group. Allograft biopsy of preformed DSA-positive patient demonstrated only minimal PRI; however, no identifiable cause could be attributed to graft dysfunction other than preformed DSA. CONCLUSION. Preformed DSA are present in 5-10% liver transplant recipients. There is no association between anti-HLA DSA and PRI and C4d, but preformed DSA may cause early morbidity. Larger studies on the impact of DSA with optimization of C4d techniques are required.

B cells display an abnormal distribution and an impaired suppressive function in patients with chronic antibody-mediated rejection

In kidney transplantation, the composition of the B-cell compartment is increasingly identified as an important determinant for graft outcome. Whereas naive and transitional B cells have been associated with long-term allograft survival and operational tolerance, memory B cells have been linked to graft rejection and graft loss. Chronic antibody-mediated rejection now represents a major complication in transplantation and is a challenge in current therapeutics. Here, we show that patients with chronic antibody-mediated rejection display a unique B-cell phenotype with a reduced ratio of activated to memory B cells associated with an impaired immunosuppressive activity. The regulatory functions of the B cells depended on their maturation status. Thus, phenotypic and functional analyses of the B-cell compartment may be indicated for appropriate follow-up after transplantation and drive therapy in the establishment of transplant tolerance processes.

The diagnostic value of transcription factors T-bet/GATA3 ratio in predicting antibody-mediated rejection

Background. Previous data showed that the predominance of intraglomerular T-bet or GATA3 is correlated with different mechanisms of rejection, suggesting that the ratio of T-bet/GATA3 might be used to distinguish antibody-mediated rejection (ABMR) and T-cell-mediated rejection (TCMR). Methods. We compared the intraglomerular T-bet/GATA3 ratio in ABMR and TCMR. The intragraft expression of T-bet and GATA3 was studied via immunohistochemistry. The correlation of the diagnosis of AMR with the ratio of T-bet/GATA3 was examined. Results. Both intraglomerular T-bet- and GATA3-expressing cells were increased during acute rejection. T-bet/GATA3>1 was strongly correlated with ABMR (93.3% versus 18.2%). The incidence of positive HLA-I/II antibodies and glomerulitis is significantly higher in T-bet/GATA3>1 group (P < 0.001, 0.013, resp.). The scores of peritubular capillary inflammation and glomerulitis were also higher in T-bet/GATA3>1 group (P = 0.052, P < 0.001, resp.). Nevertheless, T-bet/GATA3>1 is also correlated with C4d-negative ABMR and resistance to steroid treatment. Compared with C4d deposition, T-bet/GATA3>1 had a slight lower (90% versus 100%) specificity but a much higher (87.5% versus 68.8%) sensitivity. Conclusion. Our data suggested that intraglomerular predominance of T-bet over GATA3 might be used as diagnosis maker of ABMR in addition to C4d, especially in C4d-negative cases.

Impact of de novo donor-specific HLA antibodies detected by Luminex solid-phase assay after transplantation in a group of 88 consecutive living-donor renal transplantations

De novo donor-specific HLA antibodies (DSA) after renal transplantation are known to be correlated with poor graft outcome and the development of acute and chronic rejection. Currently, data for the influence of de novo DSA in patient cohorts including only living-donor renal transplantations (LDRT) are limited. A consecutive cohort of 88 LDRT was tested for the occurrence of de novo DSA by utilizing the highly sensitive Luminex solid-phase assay for antibody detection. Data were analyzed for risk factors for de novo DSA development and correlated with acute rejection (AR) and graft function. Patients with de novo DSA [31 (35%)] showed a trend for inferior graft function [mean creatinine change (mg/dL/year) after the first year: 0.15 DSA (+) vs. 0.02 DSA (-) (P = 0.10)] and a higher rate of AR episodes, especially in case of de novo DSA of both class I and II [6 (55%), (P = 0.05)]. Antibody-mediated rejection (AMR) appeared in five patients and was significantly correlated with de novo DSA (P = 0.05). Monitoring for de novo DSA after LDRT may help to identify patients at risk of declining renal function. Especially patients with simultaneous presence of de novo DSA class I and class II are at a high risk to suffer AR episodes.

Experimental glomerular endothelial injury in vivo

The microvascular endothelium of the kidney glomerulus is injured in Shiga-like toxigenic bacterial infection, genetic or acquired loss of complement regulatory protein function, and allo-immune responses of solid-organ or bone marrow transplantation. Existing models of diseases with glomerular endothelial cell (EC) injury, collectively grouped as thrombotic microangiopathies, are problematic, impeding investigation of the mechanisms of microvascular defense and repair. To develop a model of glomerular endothelial injury in the mouse, we conjugated the M. oreades lectin to the cytotoxin, saporin, (LS) to selectively injure the glomerular endothelium. Injury of the microvasculature was evaluated by light, immunofluorescence, and electron microscopy, and by quantitative RT-PCR of cell-type specific transcripts. Renal function was evaluated by quantitation of serum creatinine. The toxin conjugate induced apoptosis of microvascular ECs in vitro, and subtle histologic features of thrombotic microangiopathy in vivo that were enhanced by co-injection of 50 μg/kg LPS. Among LS/LPS-treated animals, loss of glomerular EC staining correlated with decreased expression of EC-specific transcripts, and impaired kidney function. Selective injury of the glomerular microvasculature with LS toxin conjugate and LPS elicits histologic features of thrombotic microangiopathy and acute kidney failure.

Late and chronic antibody-mediated rejection: main barrier to long term graft survival

Antibody-mediated rejection (AMR) is an important cause of graft loss after organ transplantation. It is caused by anti-donor-specific antibodies especially anti-HLA antibodies. C4d had been regarded as a diagnosis marker for AMR. Although most early AMR episodes can be successfully controlled or reversed, late and chronic AMR remains the leading cause of late graft loss. The strategies which work in early AMR have limited effect on late/chronic episodes. Here, we reviewed the lines of evidence that late/chronic AMR is the leading cause of late graft loss, characteristics of late AMR, and current strategies in managing late/chronic AMR. More effort should be put on the management of late/chronic AMR to make a better long term graft survival.

T-cell co-stimulatory blockade in transplantation: two steps forward one step back!

INTRODUCTION

The concern about nephrotoxicity with calcineurin inhibitors led to the search of novel agents for immunosuppression. Based on the requirement of T-cell co-stimulatory signals to fully activated naïve T cells, it became clear that blocking these pathways could be an appealing therapeutic target. However, some unexpected findings were noticed in the recent clinical trials of belatacept, including a higher rate of rejection, which warranted further investigation with some interesting concepts emerging from the bench.

AREAS COVERED

This article aims to review the literature of the B7:CD28 co-stimulatory blockade in transplantation, including the basic immunology behind its development, clinical application and potential limitations.

EXPERT OPINION

Targeting co-stimulatory pathways were found to be much more complex than initially anticipated due to the interplay between not only various co-stimulatory pathways but also various co-inhibitory ones. In addition, co-stimulatory signals have different roles in diverse immune cell types. Therefore, targeting CD28 ligands with cytotoxic T lymphocyte antigen-4 (CTLA4)-Ig may have some deleterious effects, including the inhibition of regulatory T cells, blockade of co-inhibitory signals (CTLA4) and promotion of Th17 cells. Co-stimulatory independence of memory T cells was another unforeseen limitation. Learning how to better integrate co-stimulatory targeting with other immunosuppressive agents will be critical for the improvement of long-term graft survival.

Pretransplant sensitization against angiotensin II type 1 receptor is a risk factor for acute rejection and graft loss

The angiotensin II type 1 receptor (AT1R) is an emerging target of functional non-HLA antibodies (Ab). We examined the potential of determining the degree of presensitization against AT1R as a risk factor for graft survival and acute rejection (AR). The study included 599 kidney recipients between 1998 and 2007. Serum samples were analyzed in a blinded fashion for anti-AT1R antibodies (AT1R-Abs) using a quantitative solid-phase assay. A threshold of AT1R-Ab levels was statistically determined at 10 U based on the time to graft failure. An extended Cox model determined risk factors for occurrence of graft failure and a first AR episode. AT1R-Abs >10 U were detected in 283 patients (47.2%) before transplantation. Patients who had a level of AT1R-Abs >10 U had a 2.6-fold higher risk of graft failure from 3 years posttransplantation onwards (p = 0.0005) and a 1.9-fold higher risk of experiencing an AR episode within the first 4 months of transplantation (p = 0.0393). Antibody-mediated rejection (AMR) accounted for 1/3 of AR, whereby 71.4% of them were associated with >10 U of pretransplant AT1R-Abs. Pretransplant anti-AT1R-Abs are an independent risk factor for long-term graft loss in association with a higher risk of early AR episodes.

The specificity of acute and chronic microvascular alterations in renal allografts

The diagnosis of an antibody-mediated rejection (AMR) is made when there is evident histologic injury in the presence of detectable donor-specific alloantibodies (DSA) and diffuse peritubular capillary C4d staining (C4d-pos). In the presence of only detectable DSA or C4d-pos, the tissue injury is currently considered "presumptive" for antibody causation. In acute antibody-mediated rejection (AAMR), diagnostic morphologic features include microvascular inflammation (MVI), specifically glomerulitis and peritubular capillaritis. In the case of chronic active AMR (CAAMR), these inflammatory lesions have progressed to chronic microvascular injury, transplant glomerulopathy (TG) and peritubular capillary basement membrane multilayering (PTCBMML). Either TG or PTCBMML is sufficient morphological evidence for a diagnosis of CAAMR. Unfortunately, these lesions are not specific. MVI, TG, and PTCBMML are found in the setting of cell-mediated immunity, as well as in association with non-alloimmune mechanisms. The available treatments for AMR and CMR are different, and it is important to ascertain the dominant mechanism when approaching an individual patient. At present, no gold standard exists to establish the specific pathogenesis in the more ambiguous cases. We detail here the differential diagnosis of MVI, TG, and PTCBMML.

Polyreactive antibodies developing amidst humoral rejection of human kidney grafts bind apoptotic cells and activate complement

Antibody mediated rejection (AMR) is associated with a variety of graft-reactive antibodies following kidney transplant. To characterize these antibodies, we immortalized 107 B cell clones from a patient with AMR. In a previous study, we showed that six clones were reacting to multiple self-antigens as well as to HLA and MICA for two of them, thus displaying a pattern of polyreactivity. We show here that all six polyreactive clones also reacted to apoptotic but not viable cells. More generally we observed a nearly perfect overlap between polyreactivity and reactivity to apoptotic cells. Functionally, polyreactive antibodies can activate complement, resulting in the deposition of C3d and C4d at the surface of target cells. Testing the serum of 88 kidney transplant recipients revealed a significantly higher IgG reactivity to apoptotic cells in AMR patients than in patients with stable graft function. Moreover, total IgG purified from AMR patients had increased complement activating properties compared to IgG from non-AMR patients. Overall, our studies show the development of polyreactive antibodies cross-reactive to apoptotic cells during AMR. Further studies are now warranted to determine their contribution to the detection of C4d in graft biopsies as well as their role in the pathophysiology of AMR.

Complement-binding anti-HLA antibodies and kidney-allograft survival

BACKGROUND

Anti-HLA antibodies hamper successful transplantation, and activation of the complement cascade is involved in antibody-mediated rejection. We investigated whether the complement-binding capacity of anti-HLA antibodies plays a role in kidney-allograft failure.

METHODS

We enrolled patients who received kidney allografts at two transplantation centers in Paris between January 1, 2005, and January 1, 2011, in a population-based study. Patients were screened for the presence of circulating donor-specific anti-HLA antibodies and their complement-binding capacity. Graft injury phenotype and the time to kidney-allograft loss were assessed.

RESULTS

The primary analysis included 1016 patients. Patients with complement-binding donor-specific anti-HLA antibodies after transplantation had the lowest 5-year rate of graft survival (54%), as compared with patients with non-complement-binding donor-specific anti-HLA antibodies (93%) and patients without donor-specific anti-HLA antibodies (94%) (P<0.001 for both comparisons). The presence of complement-binding donor-specific anti-HLA antibodies after transplantation was associated with a risk of graft loss that was more than quadrupled (hazard ratio, 4.78; 95% confidence interval [CI], 2.69 to 8.49) when adjusted for clinical, functional, histologic, and immunologic factors. These antibodies were also associated with an increased rate of antibody-mediated rejection, a more severe graft injury phenotype with more extensive microvascular inflammation, and increased deposition of complement fraction C4d within graft capillaries. Adding complement-binding donor-specific anti-HLA antibodies to a traditional risk model improved the stratification of patients at risk for graft failure (continuous net reclassification improvement, 0.75; 95% CI, 0.54 to 0.97).

CONCLUSIONS

Assessment of the complement-binding capacity of donor-specific anti-HLA antibodies appears to be useful in identifying patients at high risk for kidney-allograft loss.

Renal arteriolar C4d deposition: a novel characteristic of hematopoietic stem cell transplantation-associated thrombotic microangiopathy

BACKGROUND

The mechanism of kidney injury in hematopoietic stem cell transplantation (HSCT)-associated thrombotic microangiopathy (TA-TMA) is not completely understood. Renal C4d staining is a marker of classic complement activation and endothelial injury and has been described in preliminary reports of HSCT recipients with TA-TMA. Our objective was to evaluate complement in the pathogenesis of small vessel injury in children receiving HSCT. We hypothesized that kidney tissue from children with TA-TMA would more frequently show C4d deposition compared with HSCT recipients without histologic TA-TMA.

METHODS

We reviewed kidney specimens (biopsy or autopsy) from children who had undergone HSCT at a single center. Using histologic criteria alone, subjects were divided into TA-TMA (n = 8) and non-TA-TMA (control) groups (n = 12). C4d staining was performed by immunohistochemistry and evaluated on arterioles, peritubular capillaries, glomeruli, and tubular basement membranes.

RESULTS

Diffuse or focal renal arteriolar C4d staining was more common in subjects with histologic TA-TMA (75%) compared with controls (8%). Rare peritubular capillary C4d staining was present in 50% of TA-TMA samples and was absent in controls. Glomerular C4d staining was seen at a similar frequency in cases and controls, whereas tubular basement membrane staining was less frequently observed and only in subjects with TA-TMA.

CONCLUSIONS

Arteriolar C4d deposition may be a pathologic marker of TA-TMA, implicating localized complement fixation in HSCT recipients with kidney disease secondary to small vessel injury. Further studies to better characterize the preferential arteriolar C4d staining may identify a renal compartment of injury, possibly explaining the dramatic hypertension seen in TA-TMA.

The clinical and genomic significance of donor-specific antibody-positive/C4d-negative and donor-specific antibody-negative/C4d-negative transplant glomerulopathy

BACKGROUND

This study investigated the mechanisms involved in development of donor-specific antibody (DSA) and/or C4d-negative transplant glomerulopathy (TGP) by allograft gene expression profiles using microarrays.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This cohort study was conducted in kidney transplant recipients. Patients were eligible for inclusion if they required a clinically indicated biopsy at any time point after their transplant. They were then classified according to their histopathology findings and DSA and C4d results. Eighteen chronic antibody-mediated rejection (CAMR), 14 DSA+/C4d- TGP, 25 DSA-/C4d- TGP, and 47 nonspecific interstitial fibrosis/tubular atrophy (IFTA) biopsy specimens were identified. In a subset of patients from the study population, biopsy specimens in each group and normal transplant kidney specimens were analyzed with Affymetrix Human Gene 1.0 ST Arrays.

RESULTS

The mean sum score of glomerulitis and peritubular capillaritis increased from 0.28±0.78 in IFTA specimens to 0.75±0.85 in DSA-/C4d- TGP specimens, 1.71±1.49 in DSA+/C4d-/TGP specimens, and 2.11±1.74 in CAMR specimens (P<0.001). During a median follow-up time of 2 (interquartile range, 1.4-2.8) years after biopsy, graft loss was highest in CAMR specimens (27.8%) compared to IFTA specimens (8.5%), DSA+/C4d- TGP specimens (14.3%), and DSA-/C4d- TGP specimens (16%) (P=0.01). With use of microarrays, comparison of the gene expression profiles of DSA-/C4d- TGP specimens with glomerulitis + peritubular capillaritis scores > 0 to normal and IFTA biopsy specimens revealed higher expression of quantitative cytotoxic T cell-associated transcripts (QCAT). However, both CAMR and DSA+/C4d- TGP specimens had higher expression of not only QCAT but also IFN-γ and rejection-induced, constitutive macrophage-associated, natural killer cell-associated, and DSA-selective transcripts. Endothelial cell-associated transcript expression was upregulated only in CAMR biopsy specimens.

CONCLUSIONS

These results suggested that DSA+/C4d- TGP biopsy specimens may be classified as CAMR. In contrast, DSA-/C4d- TGP specimens showed increased cytotoxic T cell-associated transcripts, suggesting T cell activation as a mechanism of injury.

Reversing endogenous alloreactive B cell GC responses with anti-CD154 or CTLA-4Ig

Alloantibodies mediate acute antibody-mediated rejection as well as chronic allograft rejection in clinical transplantation. To better understand the cellular dynamics driving antibody production, we focused on the activation and differentiation of alloreactive B cells in the draining lymph nodes and spleen following sensitization to allogeneic cells or hearts. We used a modified staining approach with a single MHC Class I tetramer (K(d)) bound to two different fluorochromes to discriminate between the Class I-binding and fluorochrome-streptavidin-binding B cells with a high degree of specificity and binding efficiency. By Day 7-8 postsensitization, there was a 1.5- to 3.2-fold increase in the total numbers of K(d) -binding B cells. Within this K(d) -binding B cell population, approximately half were IgD(low) , MHC Class II(high) and CD86(+), 30-45% expressed a germinal center (Fas(+) GL7(+)) phenotype and 3-12% were IRF4(hi) plasma cells. Remarkably, blockade with anti-CD40 or CTLA-4Ig, starting on Day 7 postimmunization for 1 or 4 weeks, completely dissolved established GCs and halted further development of the alloantibody response. Thus MHC Class I tetramers can specifically track the in vivo fate of endogenous, Class I-specific B cells and was used to demonstrate the ability of delayed treatment with anti-CD154 or CTLA-4Ig to halt established allo-B cell responses.

The importance of C4d in biopsies of kidney transplant recipients

Antibody-mediated rejection (AMR) is highly detrimental to the prolonged survival of transplanted kidneys. C4d has been regarded as a footprint of AMR tissue damage, and the introduction of C4d staining in daily clinical practice aroused an ever-increasing interest in the role of antibody-mediated mechanisms in allograft rejection. Despite the general acceptance of the usefulness of C4d in the identification of acute AMR, the data for C4d staining in chronic AMR is variable. The presence of C4d in the majority of the biopsies with features of chronic antibody-mediated rejection is reported, but this rejection without C4d staining is observed as well, suggesting that C4d is specific but not sensitive. Further studies on AMR with positive C4d staining in biopsy specimens are really important, as well as the study of novel routine markers that may participate in the pathogenesis of this process.

Renal transplantation across ABO barrier

In India, patients without a compatible blood group donor are usually excluded from renal transplantation. For young patients, it is a difficult therapeutic choice to stay on long-term dialysis. We describe the case of a 19-year-old male patient who had blood group O +ve and had no compatible donor in the family. His mother was B +ve and was willing to donate. The patient had an initial anti-B antibody titer of 1:512 and underwent antibody depletion with plasmapheresis (11 sessions) and intravenous immunoglobulin (IVIG) 100 mg/kg after every plasmapheresis. He also received rituximab 500 mg for 3 days prior to transplant and was induced with basiliximab. At the time of transplant, his anti-B titers were <1:8. Post-operatively, he required four sessions of plasmapheresis and IVIG as his titers rebounded to 1:64. The titers then spontaneously subsided to <1:16 and have stayed at the same level for 6 months post-transplant. The patient continues to have normal renal function with a creatinine of 1.4 mg/dl% and has had no episodes of rejection.

Comprehensive assessment and standardization of solid phase multiplex-bead arrays for the detection of antibodies to HLA

Solid phase multiplex-bead arrays for the detection and characterization of HLA antibodies provide increased sensitivity and specificity compared to conventional lymphocyte-based assays. Assay variability due to inconsistencies in commercial kits and differences in standard operating procedures (SOP) hamper comparison of results between laboratories. The Clinical Trials in Organ Transplantation Antibody Core Laboratories investigated sources of assay variation and determined if reproducibility improved through utilization of SOP, common reagents and normalization algorithms. Ten commercial kits from two manufacturers were assessed in each of seven laboratories using 20 HLA reference sera. Implementation of a standardized (vs. a nonstandardized) operating procedure greatly reduced MFI variation from 62% to 25%. Although laboratory agreements exceeded 90% (R(2) ), small systematic differences were observed suggesting center specific factors still contribute to variation. MFI varied according to manufacturer, kit, bead type and lot. ROC analyses showed excellent consistency in antibody assignments between manufacturers (AUC > 0.9) and suggested optimal cutoffs from 1000 to 1500 MFI. Global normalization further reduced MFI variation to levels near 20%. Standardization and normalization of solid phase HLA antibody tests will enable comparison of data across laboratories for clinical trials and diagnostic testing.

Regulatory T cells in the immunodiagnosis and outcome of kidney allograft rejection

Acute rejection (AR) is responsible for up to 12% of graft loss with the highest risk generally occurring during the first six months after transplantation. AR may be broadly classified into humoral as well as cellular rejection. Cellular rejection develops when donor alloantigens, presented by antigen-presenting cells (APCs) through class I or class II HLA molecules, activate the immune response against the allograft, resulting in activation of naive T cells that differentiate into subsets including cytotoxic CD8⁺ and helper CD4⁺ T cells type 1 (TH1) and TH2 cells or into cytoprotective immunoregulatory T cells (Tregs). The immune reaction directed against a renal allograft has been suggested to be characterized by two major components: a destructive one, mediated by CD4⁺ helper and CD8⁺ cytotoxic T cells, and a protective response, mediated by Tregs. The balance between these two opposite immune responses can significantly affect the graft survival. Many studies have been performed in order to define the role of Tregs either in the immunodiagnosis of transplant rejection or as predictor of the clinical outcome. However, information available from the literature shows a contradictory picture that deserves further investigation.

Renal and urinary levels of endothelial protein C receptor correlate with acute renal allograft rejection

The Endothelial Protein C Receptor (EPCR) is expressed on leukocytes, on endothelium of large blood vessels and to a lesser extent on capillaries. Membrane bound EPCR plays an important role in the activation of protein C which has anticoagulant, anti-inflammatory and cytoprotective effects. After cleavage by a protease EPCR is also found as a soluble protein. Acute rejection of kidney allografts can be divided in T-cell-mediated rejection (TCMR) and antibody-mediated (ABMR) rejection. The latter is characterized by strong activation of coagulation. Currently no reliable non-invasive biomarkers are available to monitor rejection.

Renal biopsies were available from 81 renal transplant patients (33 without rejection, 26 TCMR and 22 ABMR), we had access to mRNA material, matched plasma and urine samples for a portion of this cohort. Renal EPCR expression was assessed by RT-PCR and immunostaining. Plasma and urine sEPCR levels were measured by ELISA.

ABMR patients showed higher levels of EPCR mRNA than TCMR patients. EPCR expression on glomeruli was significantly elevated in ABMR patients than in TCMR or control patients. In the peritubular capillaries EPCR expression was higher in ABMR patients than in control patients. EPCR expression was higher in tubules and arteries of rejection patients than in control patients. Plasma sEPCR levels did not differ. Urine sEPCR levels were more elevated in the ABMR group than in patients with TCMR or without rejection. ROC analysis demonstrated that urinary sEPCR is appropriate to discriminate between ABMR patients and TCMR or control patients. We conclude that urinary sEPCR could be a novel non-invasive biomarker of antibody mediated rejection in renal transplantation.

Banff initiative for quality assurance in transplantation (BIFQUIT): reproducibility of C4d immunohistochemistry in kidney allografts

Detection of C4d is crucial for diagnosing antibody-mediated-rejection. We conducted a multicenter trial to assess the reproducibility for C4d immunohistochemistry on paraffin tissue. Unstained slides from a tissue microarray (TMA) comprising 44 kidney allograft specimens representing a full analytical spectrum for C4d were distributed to 73 institutions. Participants stained TMA slides using local protocols and evaluated their slides following the Banff C4d schema. Local staining details and evaluation scores were collected online. Stained slides were returned for centralized panel re-evaluation. Kappa statistics were used to determine reproducibility. Poor interinstitutional reproducibility was observed (kappa 0.17), which was equally due to limitations in interobserver (kappa 0.44) and interlaboratory reproducibility (kappa 0.46). Depending on the cut-off, reproducibility could be improved by omitting C4d grading and only considering ± calls. Heat-induced epitope recovery (pH 6-7, 20-30 min, citrate buffer) with polyclonal antibody incubation (<1:80, >40 min) appeared as best practice. The BIFQUIT trial results indicated that C4d staining on paraffin sections varies considerably between laboratories. Refinement of the current Banff C4d scoring schema and harmonization of tissue processing and staining protocols is necessary to achieve acceptable reproducibility.

Antibody-mediated rejection in the liver allograft

PURPOSE OF REVIEW

There is increasing evidence to suggest that antibody-mediated mechanisms play a role in the pathogenesis of liver allograft rejection. This article will review the pathology of antibody-mediated rejection (AMR) focusing on recent studies which have improved our understanding of the clinicopathological features and diagnostic approaches.

RECENT FINDINGS

Recent studies have investigated the patterns of immunohistochemical staining for C4d as a tissue marker of AMR in posttransplant biopsies, and have correlated these findings with other histopathological changes and with the presence of donor-specific antibodies (DSAs). These studies have highlighted the diagnostic applications and limitations of C4d immunostaining. They have also emphasized the importance of using strict criteria for defining 'pure' AMR in the liver allograft - that is, graft dysfunction associated with compatible histological findings (typically resembling biliary obstruction), the presence of DSAs and diffusely positive staining for C4d.

SUMMARY

Pure AMR is relatively uncommon in ABO-compatible grafts - it should be diagnosed on the basis of strict criteria and requires treatment with antibody-depleting immunosuppression. C4d immunostaining in isolation has limited diagnostic value. However, the presence of diffuse C4d immunostaining (involving endothelium or stroma in >50% of portal tracts or sinusoids) suggests a significant component of antibody-mediated graft damage. In a person with suggestive histological features, this finding should prompt testing for DSAs. Even in the absence of typical histological features of AMR, the combined presence of DSAs and diffuse C4d positivity is associated with more frequent or severe acute and chronic rejection, which may also warrant treatment with antibody-depleting immunosuppression.

Sequential analysis of donor-specific antibodies and pathological findings in acute antibody-mediated rejection in a rat renal transplantation model

Alloantibodies contribute significantly to renal transplant rejection by activation of complement and various cytokines with a variety of effector cells, and are a major cause of allograft loss. Although there is clinical evidence of antibody- and complement-mediated injury in renal transplantation, the mechanism of antibody-mediated rejection remains largely unknown. In order to understand the sequential production of antibodies and complement components, we presensitized recipient rats by skin transplantation. Anti-donor-specific IgG levels reached a maximum 2 weeks following presensitization after which the rats underwent renal transplantation from the same donor strain. We then evaluated sequential pathological findings based on the Banff classification and several factors related to graft rejection. In this presensitized model, peritubular capillaries were already dilated and stained for C4d. Neutrophil and mononuclear cell infiltration in these capillaries was detected beginning 2 h after transplantation. Donor-specific antibody IgG levels decreased rapidly and anti-IgG antibody stained glomerular and peritubular capillaries in the grafts beginning 2 h after transplantation. Additionally, several cytokines and complement components showed marked changes in the presensitized group. Thus, in the donor-specific presensitized recipient, alloantibodies and complement were activated immediately after transplant. C4d deposition in peritubular capillaries appears to be a key factor for the diagnosis of antibody-associated rejection.