Antibody‐Mediated Rejection: Emergence of Animal Models to Answer Clinical Questions

Decades of experiments in small animals had tipped the balance of opinion away from antibodies as a cause of transplant rejection. However, clinical experience, especially with sensitized patients, has convinced basic immunologists of the need to develop models to investigate mechanisms underlying antibody‐mediated rejection (AMR). This resurgent interest has resulted in several new rodent models to investigate antibody‐mediated mechanisms of heart and renal allograft injury, but satisfactory models of chronic AMR remain more elusive. Nevertheless, these new studies have begun to reveal many insights into the molecular and pathological sequelae of antibody binding to the allograft endothelium. In addition, complement‐independent and complement‐dependent effects of antibodies on endothelial cells have been identified in vitro. As small animal models become better defined, it is anticipated that they will be more widely used to answer further questions concerning mechanisms of antibody‐mediated tissue injury as well as to design therapeutic interventions.     

Histopathology and Immunophenotype of the Spleen During Acute Antibody‐Mediated Rejection

Splenectomy has been reported to have a beneficial effect in treating Acute antibody‐mediated rejection (ABMR). This reason for this often rapid and profound beneficial effect is not readily apparent from what is known about normal splenic immunoarchitecture. While the spleen is rich in mature B cells, it has not been noted to be a repository for direct antibody‐secreting cells. We present a case of a Native American female who received a renal transplant and developed a severe episode of ABMR. The patient was initially refractory to both plasmapheresis and IVIG. The patient underwent an emergent splenectomy with almost immediate improvement in her renal function and a rapid drop in her DR51 antibodies. Immunohistochemical stains of the spleen demonstrated abundant clusters of CD138+ plasma cells (>10% CD138 cells as opposed to 1% CD138 cells as seen in traumatic controls). Though this is a single case, these findings offer a rationale for the rapid ameliorative effect of splenectomy in cases of antibody rejection. It is possible that the spleen during times of excessive antigenic stress may rapidly turn over B cells to active antibody‐secreting cells or serve as a reservoir for these cells produced at other sites.     

Summary of FDA Antibody‐Mediated Rejection Workshop

The Food and Drug Administration (FDA) held an open public workshop in June 2010 to discuss the current state of science related to antibody‐mediated rejection (AMR) in kidney transplantation. Desensitization, acute AMR and chronic AMR (CAMR) were considered in the context of clinical trial design. Participants discussed experiences with HLA antibody detection and quantitation and the utility of monitoring donor‐specific antibodies (DSAs) to inform the management of patients with AMR. The role for animal models was discussed. Diagnostic and prognostic features of histology were presented, followed by discussion of sensitivity and specificity of various criteria. The published literature on treatment of acute AMR was summarized, which consisted of case series and limited data from controlled clinical trials. Considerations for future clinical trials were presented, including endpoints and statistical evaluations of outcome. Although many issues need further consideration, the meeting enabled an important exchange of ideas between experts in the field.     

Microarray Diagnosis of Antibody‐Mediated Rejection in Kidney Transplant Biopsies: An International Prospective Study (INTERCOM)

In a reference set of 403 kidney transplant biopsies, we recently developed a microarray‐based test that diagnoses antibody‐mediated rejection (ABMR) by assigning an ABMR score. To validate the ABMR score and assess its potential impact on practice, we performed the present prospective INTERCOM study ( clinicaltrials.gov NCT01299168) in 300 new biopsies (264 patients) from six centers: Baltimore, Barcelona, Edmonton, Hannover, Manchester and Minneapolis. We assigned ABMR scores using the classifier created in the reference set and compared it to conventional assessment as documented in the pathology reports. INTERCOM documented uncertainty in conventional assessment: In 41% of biopsies where ABMR features were noted, the recorded diagnoses did not mention ABMR. The ABMR score correlated with ABMR histologic lesions and donor‐specific antibodies, but not with T cell–mediated rejection lesions. The agreement between ABMR scores and conventional assessment was identical to that in the reference set (accuracy 85%). The ABMR score was more strongly associated with failure than conventional assessment, and when the ABMR score and conventional assessment disagreed, only the ABMR score was associated with early progression to failure. INTERCOM confirms the need to reduce uncertainty in the diagnosis of ABMR, and demonstrates the potential of the ABMR score to impact practice.     

Bortezomib for Acute Antibody‐Mediated Rejection in Liver Transplantation

Antibody‐mediated rejection (AMR) is an uncommon, but challenging type of rejection after solid organ transplantation. We review three cases of AMR in ABO‐compatible liver transplant recipients. These cases were characterized by severe acute rejection resistant to steroids and antithymocyte globulin, histologic evidence of plasma cell infiltrates, C4d positivity and high serum anti‐HLA donor‐specific antibodies. All three patients were treated with bortezomib, a proteasome inhibitor effective in depleting plasma cells. After treatment, all patients had improved or normal liver function tests, resolution of C4d deposition and significant decline in their HLA donor‐specific antibodies.     

Preformed Donor HLA‐DP‐Specific Antibodies Mediate Acute and Chronic Antibody‐Mediated Rejection Following Renal Transplantation

Donor‐specific HLA alloantibodies may cause acute and chronic antibody‐mediated rejection (AMR) and significantly compromise allograft survival. The clinical relevance of antibodies directed against some HLA class II antigens, particularly HLA‐DP, is less clear with conflicting reports on their pathogenicity. We report two patients with high levels of pretransplant donor‐specific HLA‐DP antibodies who subsequently developed recurrent acute AMR and graft failure. In both cases, there were no other donor‐specific HLA alloantibodies, suggesting that the HLA‐DP‐specific antibodies may be directly pathogenic.     

Identifying Subphenotypes of Antibody‐Mediated Rejection in Kidney Transplants

The key lesions in antibody‐mediated kidney transplant rejection (ABMR) are microcirculation inflammation (peritubular capillaritis and/or glomerulitis lesions, abbreviated “pg”) and glomerular double contours (cg lesions). We used these features to explore subphenotypes in 164 indication biopsies with ABMR‐related diagnoses: 137 ABMR (109 pure and 28 mixed with T cell–mediated rejection [TCMR]) and 27 transplant glomerulopathy (TG), identified from prospective multicenter studies. The lesions indicated three ABMR subphenotypes: pgABMR, cgABMR, and pgcgABMR. Principal component analysis confirmed these subphenotypes and showed that TG can be reclassified as pgcgABMR (n = 17) or cgABMR (n = 10). ABMR‐related biopsies included 45 pgABMR, 90 pgcgABMR, and 25 cgABMR, with four unclassifiable. Dominating all time intervals was the subphenotype pgcgABMR. The pgABMR subphenotype presented earliest (median <2 years), frequently mixed with TCMR, and was most associated with nonadherence. The cgABMR subphenotype presented late (median 9 years). Subphenotypes differed in their molecular changes, with pgABMR having the most histologic–molecular discrepancies (i.e. potential errors). Donor‐specific antibody (DSA) was not identified in 29% of pgcgABMR and 46% of cgABMR, but failure rates and molecular findings were similar to cases where DSA was known to be positive. Thus, ABMR presents distinct subphenotypes, early pg‐dominant, late cg‐dominant, and combined pgcg phenotype, differing in time, molecular features, accompanying TCMR, HLA antibody, and probability of nonadherence.     

Progressive Multifocal Leukoencephalopathy in a Heart Transplant Recipient Following Rituximab Therapy for Antibody‐Mediated Rejection

We report the case of a male heart transplant recipient who developed acute antibody‐mediated rejection and was treated with 5 weeks of a rituximab‐containing regimen. Two months later he presented with progressive motor and cognitive impairments and was diagnosed with progressive multifocal leukoencephalopathy (PML). He was treated with reduction of his immunosuppressive medications, mirtazapine, IVIG and plasmapheresis. He died within weeks. We reviewed the current literature on PML and its association with immunosuppression, highlighting its impact in the setting of solid organ transplantation and considering the potential effect of newer biologic drugs on the incidence of this devastating disease in the transplant population.     

Banff 2011 Meeting Report: New Concepts in Antibody‐Mediated Rejection

The 11th Banff meeting was held in Paris, France, from June 5 to 10, 2011, with a focus on refining diagnostic criteria for antibody‐mediated rejection (ABMR). The major outcome was the acknowledgment of C4d‐negative ABMR in kidney transplants. Diagnostic criteria for ABMR have also been revisited in other types of transplants. It was recognized that ABMR is associated with heterogeneous phenotypes even within the same type of transplant. This highlights the necessity of further refining the respective diagnostic criteria, and is of particular significance for the design of randomized clinical trials. A reliable phenotyping will allow for definition of robust end‐points. To address this unmet need and to allow for an evidence‐based refinement of the Banff classification, Banff Working Groups presented multicenter data regarding the reproducibility of features relevant to the diagnosis of ABMR. However, the consensus was that more data are necessary and further Banff Working Group activities were initiated. A new Banff working group was created to define diagnostic criteria for ABMR in kidneys independent of C4d. Results are expected to be presented at the 12th Banff meeting to be held in 2013 in Brazil. No change to the Banff classification occurred in 2011.     

Anti‐huCD20 Antibody Therapy for Antibody‐Mediated Rejection of Renal Allografts in a Mouse Model

We have reported that B6.CCR5^?/? mice reject renal allografts with high serum donor‐specific antibody (DSA) titers and marked C4d deposition in grafts, features consistent with antibody‐mediated rejection (AMR). B6.huCD20/CCR5^?/? mice, where human CD20 expression is restricted to B cells, rejected A/J renal allografts by day 26 posttransplant with DSA first detected in serum on day 5 posttransplant and increased thereafter. Recipient treatment with anti‐huCD20 mAb prior to the transplant and weekly up to 7 weeks posttransplant promoted long‐term allograft survival (>100 days) with low DSA titers. To investigate the effect of B cell depletion at the time serum DSA was first detected, recipients were treated with anti‐huCD20 mAb on days 5, 8, and 12 posttransplant. This regimen significantly reduced DSA titers and graft inflammation on day 15 posttransplant and prolonged allograft survival >60 days. However, DSA returned to the titers observed in control treated recipients by day 30 posttransplant and histological analyses on day 60 posttransplant indicated severe interstitial fibrosis. These results indicate that anti‐huCD20 mAb had the greatest effect as a prophylactic treatment and that the distinct kinetics of DSA responses accounts for acute renal allograft failure versus the development of fibrosis.