The significance of the anti-class I antibody response. I. Clinical and pathologic features of anti-class I-mediated rejection

Chronic Antibody-Mediated Rejection and Plasma Cell ER Stress: Opportunities and Challenges with Calcineurin Inhibitors

Chronic alloantibody-mediated rejection (cAMR) remains a major challenge in transplant immunology, with no FDA-approved targeted therapies currently available. Despite advancements in cellular immunosuppression, effective strategies to mitigate alloantibody-mediated rejection are still lacking. This review provides a comprehensive overview of transplant rejection with a particular focus on the pathophysiology and therapeutic landscape of cAMR. We highlight the role of plasma cell-driven alloantibody production and its susceptibility to endoplasmic reticulum (ER) stress, a pathway with potential for therapeutic intervention. Special attention is given to calcineurin inhibitors (CNIs), which, beyond their well-established T-cell inhibitory effects, exhibit differential impacts on ER stress and plasma cell viability. By delineating the mechanistic differences between cyclosporine and tacrolimus in regulating ER stress responses, we propose potential therapeutic implications for optimizing cAMR management. This review underscores the need for innovative strategies targeting plasma cell biology to improve long-term transplant outcomes.

Early and Late Microvascular Inflammation Have Differing Etiological Causes and Clinical Expression

BACKGROUND

Microvascular inflammation (MVI) is an important pathological feature of antibody-mediated rejection (AMR). How posttransplant time affects its clinicopathological expression is little understood.

METHODS

This retrospective, single-center study screened 3398 kidney transplant biopsies and dichotomized 202 MVI ≥ 2 (Banff glomerulitis + peritubular capillaritis ≥ 2) samples by 9-mo median incidence time for comparison.

RESULTS

The prevalence of MVI ≥ 2 was 12.4% in transplant kidneys, which failed more frequently than propensity-matched normal controls (n = 202; P  < 0.001). Epidemiological risk factors for early MVI ≥ 2 were delayed graft function, prior AMR, and circulating donor-specific antibodies (DSAs+). Prior recipient sensitization occurred in 72.3%. Early MVI ≥ 2 was classified AMR in 65.3% and cellular rejection in 34.7%, and demonstrated excellent functional recovery and graft survival comparable to normal control kidneys. Late MVI ≥ 2 was predicted by younger (18 = 29 y) age, female recipient, living-donation, prior methylprednisolone, cyclosporine (versus tacrolimus, levels <5 ng/mL), absent antiproliferative therapy, and DSA+ using multivariable epidemiological modeling. Nonadherence caused 49.5%, with iatrogenic minimization responsible for 47.5%, usually for recipient infection. Late MVI ≥ 2 was because of AMR in 93.1%, and characterized by greater interstitial fibrosis, tubular atrophy, complement degradation split-product 4d (C4d) staining of peritubular capillaries+, endothelial C4d staining of glomerular capillaries+, transplant glomerulopathy and vasculopathy scores, DSA strength, and graft failure than early MVI ≥ 2 or normal transplant kidneys. Death-censored graft survival in 149 unique MVI ≥ 2 kidneys was independently determined by nonadherence, serum creatinine, proteinuria, DSA+, Banff C4d staining of peritubular capillaries+, and chronic interstitial fibrosis scores. MVI score and time lost significance using multivariable Cox regression.

CONCLUSIONS

The changing expression of MVI ≥ 2 over time is best explained by differences in underimmunosuppression and microvascular injury from AMR impacting allograft function and survival.

The relationship of microvascular inflammation with antibody-mediated rejection in kidney transplantation

Microvascular inflammation (MVI) is a key diagnostic feature of antibody-mediated rejection (AMR); however, recipients without donor-specific antibodies (DSA) defy etiologic classification using C4d staining of peritubular capillaries (C4dptc) and conventional DSA assignment. We evaluated MVI ≥ 2 (Banff g + ptc ≥ 2) using Banff 2019 AMR (independent of MVI ≥ 2 but including C4dptc) with unconventional endothelial C4d staining of glomerular capillaries (C4dglom) and - arterial endothelium and/or intima (C4dart) using tissue immunoperoxidase, shared-eplet and subthreshold DSA (median fluorescence intensity, [MFI] 100-499), and capillary ultrastructure from 3398 kidney transplant samples for evidence of AMR. MVI ≥ 2 (n = 202 biopsies) from 149 kidneys (12.4% prevalence) correlated with DSA+, C4dptc+, C4dglom+, Banff cg, i, t, ti scores, serum creatinine, proteinuria, and graft failure compared with 202 propensity score matched normal controls. The laboratory reported DSA- MVI ≥ 2 (MFI ≥500) occurred in 34.7%; however, subthreshold (28.6%), eplet-directed (51.4%), and/or misclassified anti-Human leukocyte antigen (HLA) DSA (12.9%) were identified in 67.1% by forensic reanalysis, with vascular C4d+ staining in 67.1%, and endothelial abnormalities in 57.1%, totaling 87.1%. Etiologic analysis attributed 62.9% to AMR (77.8% for MVI with negative reported DSA [DSA- MVI ≥2] with glomerulitis) and pure T cellular rejection in 37.1%. C4dptc-DSA- MVI ≥ 2 was unrecognized AMR in 48.0%. Functional outcomes and graft survival were comparable to normal controls. We concluded that DSA- MVI ≥ 2 frequently signified a mild "borderline" phenotype of AMR which was recognizable using novel serologic and pathological techniques.

The Effect of Genetic HLA Matching on Liver Transplantation Outcome: A Systematic Review and Meta-Analysis

Objective

We aim to investigate the effects of genetically based HLA matching on patient and graft survival, and acute and chronic rejection after liver transplantation.

Background

Liver transplantation is a common treatment for patients with end-stage liver disease. In contrast to most other solid organ transplantations, there is no conclusive evidence supporting human leukocyte antigen (HLA) matching for liver transplantations. With emerging alternatives such as transplantation of bankable (stem) cells, HLA matching becomes feasible, which may decrease the need for immunosuppressive therapy and improve transplantation outcomes.

Methods

We systematically searched the PubMed, Embase, and Cochrane databases and performed a meta-analysis investigating the effect of genetic HLA matching on liver transplantation outcomes (acute/chronic rejection, graft failure, and mortality).

Results

We included 14 studies with 2682 patients. HLA-C mismatching significantly increased the risk of acute rejection (full mismatching: risk ratio = 1.90, 95% confidence interval = 1.08 to 3.33, P = 0.03; partial mismatching: risk ratio = 1.33, 95% confidence interval = 1.07 to 1.66, P = 0.01). We did not discern any significant effect of HLA mismatching per locus on acute rejection for HLA-A, -B, -DR, and -DQ, nor on chronic rejection, graft failure, or mortality for HLA-DR, and -DQ.

Conclusions

We found evidence that genetic HLA-C matching reduces the risk of acute rejection after liver transplantation while matching for other loci does not reduce the risk of acute rejection, chronic rejection, graft failure, or mortality.

On a Long and Winding Road: Alloantibodies in Organ Transplantation

Today we know that both the humoral and the cellular arm of the immune system are engaged in severe immunological challenges. A close interaction between B and T cells can be observed in most "natural" challenges, including infections, malignancies, and autoimmune diseases. The importance and power of humoral immunity are impressively demonstrated by the current coronavirus disease 2019 pandemic. Organ transplant rejection is a normal immune response to a completely "artificial" challenge. It took a long time before the multifaceted action of different immunological forces was recognized and a unified, generally accepted opinion could be formed. Here, we address prominent paradigms and paradigm shifts in the field of transplantation immunology. We identify several instances in which the transplant community missed a timely paradigm shift because essential, available knowledge was ignored. Moreover, we discuss key findings that critically contributed to our understanding of transplant immunology but sometimes developed with delay and in a roundabout way, as was the case with antibody-mediated rejection-a main focus of this article. These include the discovery of the molecular principles of histocompatibility, the recognition of the microcirculation as a key interface of immune damage, the refinement of alloantibody detection, the description of C4d as a footmark of endothelium-bound antibody, and last but not least, the developments in biopsy-based diagnostics beyond conventional morphology, which only now give us a glimpse of the enormous complexity and pathogenetic diversity of rejection.

Novel Flow Cytometry-Based Method for Detection of Anti-HLA Complement Activating Donor-Specific Antibodies in Renal Transplant Recipients and Its Comparison With Other Conventional Detection Methods

BACKGROUND

Presence of preformed donor specific antibodies (DSAs) detected by complement-dependent cytotoxicity (CDC-XM) is a strong contraindication for transplant. However, it has limitations including its sensitivity and its inability to distinguish between HLA-specific and other non-HLA-specific antibodies. In this study, we standardized CDC-XM by flow cytometry and determined its relevance by comparing its results with other methods of DSA detection, such as routine CDC-XM, antibody binding assay by flow cytometry (FC-XM), and Luminex-based crossmatch assays, such as Luminex crossmatch (LXM) and virtual crossmatch (VXM).

MATERIALS AND METHODS

A total of 79 serum samples were tested for DSAs by the flow cytometric complement-dependent cytotoxicity crossmatch assay (FC-CDC-XM) and then the results of FC-CDC-XM were compared with other detection methods such as CDC-XM, FC-XM, LXM, and VXM.

RESULTS

We found that the FC-CDC-XM assay is more sensitive than routine CDC-XM. Out of total 79 sera, 24 sera were detected positive (T cells positive: 1 case and B cells positive: 23) by FC-CDC-XM as compared with 3 sera using CDC-XM; these 3 sera also showed positivity by FC-CDC-XM. After FC-XM assay, 23 samples were positive by FC-XM and out of these 23 samples, 13 were also positive by FC-CDC-XM. On comparing the FC-CDC-XM results with VXM and LXM, 10 sera of 24 FC-CDC-XM positive had HLA class II antibodies detected on a Luminex platform.

CONCLUSIONS

The FC-CDC-XM is a more sensitive and specific method for detection of HLA-specific complement-fixing antibodies than CDC-XM and FC-XM. FC-CDC-XM should be used in tissue-typing laboratories after intra- and inter- laboratory validation.

Molecular diagnosis of ABMR with or without donor-specific antibody in kidney transplant biopsies: Differences in timing and intensity but similar mechanisms and outcomes

We studied the clinical, histologic, and molecular features distinguishing DSA-negative from DSA-positive molecularly defined antibody-mediated rejection (mABMR). We analyzed mABMR biopsies with available DSA assessments from the INTERCOMEX study: 148 DSA-negative versus 248 DSA-positive, compared with 864 no rejection (excluding TCMR and Mixed). DSA-positivity varied with mABMR stage: early-stage (EABMR) 56%; fully developed (FABMR) 70%; and late-stage (LABMR) 58%. DSA-negative patients with mABMR were usually sensitized, 60% being HLA antibody-positive. Compared with DSA-positive mABMR, DSA-negative mABMR was more often C4d-negative; earlier by 1.5 years (average 2.4 vs. 3.9 years); and had lower ABMR activity and earlier stage in molecular and histology features. However, the top ABMR-associated transcripts were identical in DSA-negative versus DSA-positive mABMR, for example, NK-associated (e.g., KLRD1 and GZMB) and IFNG-inducible (e.g., PLA1A). Genome-wide class comparison between DSA-negative and DSA-positive mABMR showed no significant differences in transcript expression except those related to lower intensity and earlier time of DSA-negative ABMR. Three-year graft loss in DSA-negative mABMR was the same as DSA-positive mABMR, even after adjusting for ABMR stage. Thus, compared with DSA-positive mABMR, DSA-negative mABMR is on average earlier, less active, and more often C4d-negative but has similar graft loss, and genome-wide analysis suggests that it involves the same mechanisms. SUMMARY SENTENCE: In 398 kidney transplant biopsies with molecular antibody-mediated rejection, the 150 DSA-negative cases are earlier, less intense, and mostly C4d-negative, but use identical molecular mechanisms and have the same risk of graft loss as the 248 DSA-positive cases.

The Role of Fc Gamma Receptors in Antibody-Mediated Rejection of Kidney Transplants

For the past decades, complement activation and complement-mediated destruction of allograft cells were considered to play a central role in anti-HLA antibody-mediated rejection (AMR) of kidney transplants. However, also complement-independent mechanisms are relevant in the downstream immune activation induced by donor-specific antibodies, such as Fc-gamma receptor (FcγR)-mediated direct cellular activation. This article reviews the literature regarding FcγR involvement in AMR, and the potential contribution of FcγR gene polymorphisms to the risk for antibody mediated rejection of kidney transplants. There is large heterogeneity between the studies, both in the definition of the clinical phenotypes and in the technical aspects. The study populations were generally quite small, except for two larger study cohorts, which obviates drawing firm conclusions regarding the associations between AMR and specific FcγR polymorphisms. Although FcγR are central in the pathophysiology of AMR, it remains difficult to identify genetic risk factors for AMR in the recipient’s genome, independent of clinical risk factors, independent of the donor-recipient genetic mismatch, and in the presence of powerful immunosuppressive agents. There is a need for larger, multi-center studies with standardised methods and endpoints to identify potentially relevant FcγR gene polymorphisms that represent an increased risk for AMR after kidney transplantation.

Peritubular Capillaritis in Native Kidney Biopsies

OBJECTIVES

To determine the frequency and predictors of peritubular capillaritis (PTCitis) among native kidney biopsies.

METHODS

Consecutive native kidney biopsies of 169 patients were reexamined for capturing possible PTCitis according to the Banff Classification. The relation of PTCitis with demographic and clinicopathological findings was evaluated. Logistic regression analysis was performed to determine predictors of PTCitis.

RESULTS

Peritubular capillaritis was captured in 90 (53.3%) patients, with scores of 1, 2, and 3 in 57 (33.7%), 31 (18.3%), and 2 (1.2%) patients, respectively. The highest frequency of PTCitis was observed in pauci-immune glomerulonephritis. In univariate analysis, male sex, the presence of interstitial inflammation, pauci-immune glomerulonephritis, and a higher serum creatinine level were associated with a higher risk of PTCitis, while severe interstitial fibrosis/tubular atrophy was associated with a lower risk. The presence of interstitial inflammation (odds ratio [OR], 5.94 [95% confidence interval (CI), 1.41-25.03]; P = .015), pauci-immune glomerulonephritis (OR, 3.08 [95% CI, 1.01-9.36]; P = .048), and a higher serum creatinine level (per 1 mg/dL) (OR, 1.56 [95% CI, 1.14-2.11]; P = .005) were independent predictors of PTCitis development in a multivariate regression model.

CONCLUSIONS

Peritubular capillaritis is common in native biopsies and more likely to be observed in the presence of interstitial inflammation, pauci-immune glomerulonephritis, and a higher serum creatinine level.

The Clinical and Pathological Phenotype of Antibody-Mediated Vascular Rejection Diagnosed using Arterial C4d Immunoperoxidase

Introduction

The diagnosis of antibody-mediated vascular rejection (AM-VR) should be reliable and accurate. We hypothesized that arterial C4d (C4d_art) immunoperoxidase deposition represents endothelial interaction with antibody.

Methods

From 3309 consecutive, kidney transplant biopsies from a single center, 100 vascular rejection (VR) cases were compared against rejection without arteritis (n = 540) and normal controls (n = 1108). The clinical utility of C4d_art for diagnosis and classification of AM-VR was evaluated against an independent reference test.

Results

C4d_art occurred in 20.4% of acute, 11.0% of subclinical, and 46% of VR episodes. Semiquantitative C4d_art score significantly correlated with immunodominant donor-specific antibodies (DSAs) (rho = 0.500, P < 0.001), peritubular capillary C4d (C4d_ptc), microvascular inflammation, and Banff v scores. Banff v3 arteritis suggested AM-VR. Addition of C4d_art to Banff antibody-mediated rejection (AMR) schema increased diagnostic sensitivity for AM-VR from 57.9% to 93.0%, accuracy 74.0% to 92.0%, and specificity 95.4% to 90.2% versus Banff 2019 (using C4d_ptc). Death-censored graft failure was associated with C4d_art AM-VR criteria using Cox regression (adjusted hazard ratio [HR] 4.310, 95% CI 1.322–14.052, P = 0.015). VR was then etiologically classified into AM-VR (n = 57, including 36 mixed VR) or “pure” (TCM-VR, n = 43). AM-VR occurred within all post-transplant periods, characterized by greater total, interstitial, and microvascular inflammation, arterial and peritubular C4d, DSA levels, and graft failure rates compared with TCM-VR. Mixed VR kidneys had the greatest inflammatory burden and graft loss (P < 0.001).

Conclusion

C4d_art is a suggestive biomarker of the humoral alloresponse toward muscular arteries. Inclusion of C4d_art into the Banff schema improved its diagnostic performance for detection of AM-VR and etiologic classification of arteritis.

Banff classification from 1991 to 2019. A significant contribution to our understanding and reporting of allograft renal biopsies

The Banff schema of classification of renal allograft biopsies, first proposed at the meeting in Banff, Canada in 1991 has evolved through subsequent meetings held once in two years and is the internationally accepted scheme of classification which is consensual, current, validated and in clinical use. This review traces the evolution of the classification and our understanding of renal transplant pathology, with emphasis on alloimmune reactions. The proceedings of the meetings and the important studies which have shaped the classification are covered.

The Biology and Molecular Basis of Organ Transplant Rejection

Allograft rejection is defined as tissue injury in a transplanted allogeneic organ produced by the effector mechanisms of the adaptive alloimmune response. Effector T lymphocytes and IgG alloantibodies cause two different types of rejection that can occur either individually or simultaneously: T cell-mediated rejection (TCMR) and antibody-mediated rejection (ABMR). In TCMR, cognate effector T cells infiltrate the graft and orchestrate an interstitial inflammatory response in the kidney interstitium in which effector T cells engage antigen-presenting myeloid cells, activating the T cells, antigen-presenting cells, and macrophages. The result is intense expression of IFNG and IFNG-induced molecules, expression of effector T cell molecules and macrophage molecules and checkpoints, and deterioration of parenchymal function. The diagnostic lesions of TCMR follow, i.e. interstitial inflammation, parenchymal deterioration, and intimal arteritis. In ABMR, HLA IgG alloantibodies produced by plasma cells bind to the donor antigens on graft microcirculation, leading to complement activation, margination, and activation of NK cells and neutrophils and monocytes, and endothelial injury, sometimes with intimal arteritis. TCMR becomes infrequent after 5-10 years post-transplant, probably reflecting adaptive mechanisms such as checkpoints, but ABMR can present even decades post-transplant. Some rejection is triggered by inadequate immunosuppression and non-adherence, challenging the clinician to target effective immunosuppression even decades post-transplant.

Allorecognition and the spectrum of kidney transplant rejection

Detection of mismatched human leukocyte antigens by adaptive immune cells is considered as the main cause of transplant rejection, leading to either T-cell mediated rejection or antibody-mediated rejection. This canonical view guided the successful development of immunosuppressive therapies and shaped the diagnostic Banff classification for kidney transplant rejection that is used in clinics worldwide. However, several observations have recently emerged that question this dichotomization between T-cell mediated rejection and antibody-mediated rejection, related to heterogeneity in the serology, histology, and prognosis of the rejection phenotypes. In parallel, novel insights were obtained concerning the dynamics of donor-specific anti-human leukocyte antigen antibodies, the immunogenicity of donor-recipient non-human leukocyte antigen mismatches, and the autoreactivity against self-antigens. Moreover, the potential of innate allorecognition was uncovered, as exemplified by natural killer cell-mediated microvascular inflammation through missing self, and by the emerging evidence on monocyte-driven allorecognition. In this review, we highlight the gaps in the current classification of rejection, provide an overview of the expanding insights into the mechanisms of allorecognition, and critically appraise how these could improve our understanding and clinical approach to kidney transplant rejection. We argue that consideration of the complex interplay of various allorecognition mechanisms can foster a more integrated view of kidney transplant rejection and can lead to improved risk stratification, targeted therapies, and better outcome after kidney transplantation.

C4d Deposition after Allogeneic Renal Transplantation in Rats Is Involved in Initial Apoptotic Cell Clearance

In the context of transplantation, complement activation is associated with poor prognosis and outcome. While complement activation in antibody-mediated rejection is well-known, less is known about complement activation in acute T cell-mediated rejection (TCMR). There is increasing evidence that complement contributes to the clearance of apoptotic debris and tissue repair. In this regard, we have analysed published human kidney biopsy transcriptome data clearly showing upregulated expression of complement factors in TCMR. To clarify whether and how the complement system is activated early during acute TCMR, experimental syngeneic and allogeneic renal transplantations were performed. Using an allogeneic rat renal transplant model, we also observed upregulation of complement factors in TCMR in contrast to healthy kidneys and isograft controls. While staining for C4d was positive, staining with a C3d antibody showed no C3d deposition. FACS analysis of blood showed the absence of alloantibodies that could have explained the C4d deposition. Gene expression pathway analysis showed upregulation of pro-apoptotic factors in TCMR, and apoptotic endothelial cells were detected by ultrastructural analysis. Monocytes/macrophages were found to bind to and phagocytise these apoptotic cells. Therefore, we conclude that early C4d deposition in TCMR may be relevant to the clearance of apoptotic cells.

Donor-Specific Antibody Is Associated with Increased Expression of Rejection Transcripts in Renal Transplant Biopsies Classified as No Rejection

BACKGROUND

Donor -specific HLA antibody (DSA) is present in many kidney transplant patients whose biopsies are classified as no rejection (NR). We explored whether in some NR kidneys DSA has subtle effects not currently being recognized.

METHODS

We used microarrays to examine the relationship between standard-of-care DSA and rejection-related transcript increases in 1679 kidney transplant indication biopsies in the INTERCOMEX study (ClinicalTrials.gov NCT01299168), focusing on biopsies classified as NR by automatically assigned archetypal clustering. DSA testing results were available for 835 NR biopsies and were positive in 271 (32%).

RESULTS

DSA positivity in NR biopsies was associated with mildly increased expression of antibody-mediated rejection (ABMR)-related transcripts, particularly IFNG-inducible and NK cell transcripts. We developed a machine learning DSA probability (DSA_Prob) classifier based on transcript expression in biopsies from DSA-positive versus DSA-negative patients, assigning scores using 10-fold cross-validation. This DSA_Prob classifier was very similar to a previously described "ABMR probability" classifier trained on histologic ABMR in transcript associations and prediction of molecular or histologic ABMR. Plotting the biopsies using Uniform Manifold Approximation and Projection revealed a gradient of increasing molecular ABMR-like transcript expression in NR biopsies, associated with increased DSA (P<2 × 10^-16). In biopsies with no molecular or histologic rejection, increased DSA_Prob or ABMR probability scores were associated with increased risk of kidney failure over 3 years.

CONCLUSIONS

Many biopsies currently considered to have no molecular or histologic rejection have mild increases in expression of ABMR-related transcripts, associated with increasing frequency of DSA. Thus, mild molecular ABMR-related pathology is more common than previously realized.

Chronic Active Antibody-Mediated Rejection Is Associated With the Upregulation of Interstitial But Not Glomerular Transcripts

Molecular assessment of renal allografts has already been suggested in antibody-mediated rejection (ABMR), but little is known about the gene transcript patterns in particular renal compartments. We used laser capture microdissection coupled with quantitative RT-PCR to distinguish the transcript patterns in the glomeruli and tubulointerstitium of kidney allografts in sensitized retransplant recipients at high risk of ABMR. The expressions of 13 genes were quantified in biopsies with acute active ABMR, chronic active ABMR, acute tubular necrosis (ATN), and normal findings. The transcripts were either compartment specific (TGFB1 in the glomeruli and HAVCR1 and IGHG1 in the tubulointerstitium), ABMR specific (GNLY), or follow-up specific (CXCL10 and CX3CR1). The transcriptional profiles of early acute ABMR shared similarities with ATN. The transcripts of CXCL10 and TGFB1 increased in the glomeruli in both acute ABMR and chronic active ABMR. Chronic active ABMR was associated with the upregulation of most genes (SH2D1B, CX3CR1, IGHG1, MS4A1, C5, CD46, and TGFB1) in the tubulointerstitium. In this study, we show distinct gene expression patterns in specific renal compartments reflecting cellular infiltration observed by conventional histology. In comparison with active ABMR, chronic active ABMR is associated with increased transcripts of tubulointerstitial origin.

Lung Transplantation and the Era of the Sensitized Patient

Long term outcomes in lung transplant are limited by the development of chronic lung allograft dysfunction (CLAD). Within the past several decades, antibody-mediated rejection (AMR) has been recognized as a risk factor for CLAD. The presence of HLA antibodies in lung transplant candidates, "sensitized patients" may predispose patients to AMR, CLAD, and higher mortality after transplant. This review will discuss issues surrounding the sensitized patient, including mechanisms of sensitization, implications within lung transplant, and management strategies.

Incidence, risk factors, treatment, and consequences of antibody-mediated kidney transplant rejection: A systematic review

BACKGROUND

Antibody-mediated rejection (AMR) is a leading cause of kidney allograft failure, but its incidence, risk factors, and outcomes are not well understood.

METHODS

We searched Ovid MEDLINE, Cochrane, EMBASE, and Scopus from January 2000 to January 2020 to identify published cohorts of ≥500 incident adult or 75 pediatric kidney transplant recipients followed for ≥1 year post-transplant.

RESULTS

At least two reviewers screened 5061 articles and abstracts; 28 met inclusion criteria. Incidence of acute AMR was 1.1%-21.5%; most studies reported 3%-12% incidence, usually within the first year post-transplant. Few studies reported chronic AMR incidence, from 7.5%-20.1% up to 10 years. Almost all patients with acute or chronic AMR received corticosteroids and intravenous immunoglobulin; most received plasmapheresis, and approximately half with rituximab. Most studies examining death-censored graft failure identified AMR as an independent risk factor. Few reported refractory AMR rates or outcomes, and none examined costs. Most studies were single-center and varied greatly in design.

CONCLUSIONS

Cohort studies of kidney transplant recipients demonstrate that AMR is common and associated with increased risk of death-censored graft failure, but studies vary widely regarding populations, definitions, and reported incidence. Gaps remain in our understanding of refractory AMR, its costs, and resulting quality of life.

Histologic Antibody-Mediated Kidney Allograft Rejection in the Absence of Donor Specific HLA Antibodies

Histologic antibody-mediated rejection (hAMR) is defined as a kidney allograft biopsy satisfying the first 2 Banff criteria for diagnosing antibody-mediated rejection (AMR): tissue injury and evidence of current/recent antibody interaction with the endothelium. In approximately one-half of such cases, circulating HLA donor specific antibodies (DSA) are not detectable by current methodology at the time of biopsy. Some studies indicated a better prognosis for HLA-DSA-negative cases of hAMR compared to those with detectable HLA-DSA, whereas others found equally poor survival compared to hAMR-negative cases. We reviewed the literature regarding the pathophysiology of HLA-DSA-negative hAMR. We find 3 nonmutually exclusive possibilities: 1) HLA-DSA are involved, but just not detected; 2) non-HLA DSA (allo- or autoantibodies) are pathogenically involved; and/or 3) antibody-independent NK cell activation is mediating the process through "missing self" or other activating mechanisms. These possibilities are discussed in detail. Recommendations regarding the approach to such patients are made. Clearly, more research is necessary regarding the measurement of non-HLA antibodies, recipient/donor NK cell genotyping, and the use of antibody reduction therapy or other immunosuppression in any subset of patients with HLA-DSA-negative hAMR.

NK cells in antibody-mediated rejection - Key effector cells in microvascular graft damage

Antibody-mediated rejection (ABMR) stands as the major limitation to long-term transplant outcome. The immunologic understanding of ABMR continues to progress and has identified natural killer (NK) cells as key effector cells promoting and coordinating the immune attack on the graft microvascular endothelium. This review discusses the current concepts outlining the different ways that allow for NK cell recognition of graft endothelial cells which includes antibody-dependent as well as independent processes.

Reducing Donor-specific Antibody During Acute Rejection Diminishes Long-term Renal Allograft Loss: Comparison of Early and Late Rejection

BACKGROUND

Reduction in donor-specific antibody (DSA) has been associated with improved renal allograft survival after antibody-mediated rejection (AMR). These observations have not been separately analyzed for early and late AMR and mixed acute rejection (MAR). The purpose of this study was to evaluate long-term responses to proteasome inhibitor-based therapy for 4 rejection phenotypes and to determine factors that predict allograft survival.

METHODS

Retrospective cohort study evaluating renal transplant recipients with first AMR episodes treated with proteasome inhibitor-based therapy from January 2005 to July 2015.

RESULTS

A total of 108 patients were included in the analysis. Immunodominant DSA reduction at 14 days differed significantly (early AMR 79.6%, early MAR 54.7%, late AMR 23.4%, late MAR 21.1%, P < 0.001). Death-censored graft survival (DCGS) differed at 3 years postrejection (early AMR 88.3% versus early MAR 77.8% versus late AMR 56.7% versus late MAR 54.9%, P = 0.02). Multivariate analysis revealed that immunodominant DSA reduction > 50% at 14 days was associated with improved DCGS (odds ratio, 0.12, 95% CI, 0.02-0.52, P = 0.01).

CONCLUSIONS

In summary, significant differences exist across rejection phenotypes with respect to histological and DSA responses. The data suggest that DSA reduction may be associated with improved DCGS in both early and late AMR.

Enhancing the Value of Histopathological Assessment of Allograft Biopsy Monitoring

Traditional histopathological allograft biopsy evaluation provides, within hours, diagnoses, prognostic information, and mechanistic insights into disease processes. However, proponents of an array of alternative monitoring platforms, broadly classified as "invasive" or "noninvasive" depending on whether allograft tissue is needed, question the value proposition of tissue histopathology. The authors explore the pros and cons of current analytical methods relative to the value of traditional and illustrate advancements of next-generation histopathological evaluation of tissue biopsies. We describe the continuing value of traditional histopathological tissue assessment and "next-generation pathology (NGP)," broadly defined as staining/labeling techniques coupled with digital imaging and automated image analysis. Noninvasive imaging and fluid (blood and urine) analyses promote low-risk, global organ assessment, and "molecular" data output, respectively; invasive alternatives promote objective, "mechanistic" insights by creating gene lists with variably increased/decreased expression compared with steady state/baseline. Proponents of alternative approaches contrast their preferred methods with traditional histopathology and: (1) fail to cite the main value of traditional and NGP-retention of spatial and inferred temporal context available for innumerable objective analyses and (2) belie an unfamiliarity with the impact of advances in imaging and software-guided analytics on emerging histopathology practices. Illustrative NGP examples demonstrate the value of multidimensional data that preserve tissue-based spatial and temporal contexts. We outline a path forward for clinical NGP implementation where "software-assisted sign-out" will enable pathologists to conduct objective analyses that can be incorporated into their final reports and improve patient care.

A potential role of neutrophil extracellular traps (NETs) in kidney acute antibody mediated rejection

BACKGROUND

The aim of this study was to evaluate neutrophil extracellular traps (NETs) in kidney transplant recipients (KTR) and their potential involvement in acute antibody-mediated rejection (AAMR).

METHODS

We studied 3 groups: KTR with AAMR (KTR-Cases, n = 14); KTR without any immunologic event (KTR-Controls, n = 14) and donors (n = 12). Spontaneous and lipopolysaccharide-induced NETosis were evaluated by immunofluorescence indirect (IFI) (NET/cells ratio). Plasmatic cH3-DNA complexes were evaluated by ELISA, (Optic Density Index - ODI). The expression of MPO and citrullinated histone 4 (cH4) was evaluated in renal biopsies.

RESULTS

We found an enhanced spontaneous NETosis in KTR regardless of whether they had rejection. The Nets/cells ratio in spontaneous NETosis was 0.203 (IQR 0.12-0.34) in Total-KTR and 0.094 (IQR 0.01-0.17) in donors, p = .011. Likewise, the ODI of cH3-DNA was 1.41 (IQR 0.94-1.72) in Total-KTR, and 0.95 (IQR 0.83-1.27) in donors, p = .019. KTR-Cases had the higher amount of NETs 1.70 (IQR 1.19-1.91). In two KTR-Cases, expression of MPO and cH4 was found in biopsies.

CONCLUSIONS

KTR show enhanced NETosis. This may indicate a permanent activation of neutrophils. Although more studies are needed, the higher amount of NETs and netting neutrophils in biopsies of KTR-Cases suggest a role of NETosis in AAMR.

The Role of Long-Lived Plasma Cells in Antibody-Mediated Rejection of Kidney Transplantation: An Update

Background

Antibody-mediated rejection (ABMR) following kidney transplant is closely associated with poor prognosis of the recipients. Long-lived plasma cells (LLPCs) produce alloantibodies as long as life time and play a crucial role in ABMR.

Summary

LLPCs generate from germinal centers and reside in survival niches in the bone marrow as well as the inflamed tissues. They are the main and long-term source of the antibodies. LLPCs mediate ABMR via the generation of preformed antibodies in sensitized patients and de novo antibodies after transplantation. They have been acknowledged as the leading causes of ABMR; however, LLPCs are insensitive to traditional immunosuppressive therapy that removes B cells. Strategies targeting LLPCs, such as antithymocyte globulin, proteasome inhibitors as well as monoclonal antibodies, are promising methods to persistently and thoroughly clear the entire PC pool.

Key Message

LLPCs play an important role in ABMR by producing alloantibodies continually, and targeting LLPCs might be a novel and effective approach against ABMR.

Specialized Roles of Human Natural Killer Cell Subsets in Kidney Transplant Rejection

Background: Human natural killer (NK) cells are key functional players in kidney transplant rejection. However, the respective contributions of the two functionally distinct human NK cell subsets (CD56bright cytokine-producing vs. CD56dim cytotoxic effector) in episodes of allograft rejection remain uncertain, with current immunohistochemical methods unable to differentiate these discrete populations. We report the outcomes of an innovative multi-color flow cytometric-based approach to unequivocally define and evaluate NK cell subsets in human kidney allograft rejection. Methods: We extracted renal lymphocytes from human kidney transplant biopsies. NK cell subsets were identified, enumerated, and phenotyped by multi-color flow cytometry. Dissociation supernatants were harvested and levels of soluble proteins were determined using a multiplex bead-based assay. Results were correlated with the histopathological patterns in biopsies-no rejection, borderline cellular rejection, T cell-mediated rejection (TCMR), and antibody-mediated rejection (AMR). Results: Absolute numbers of only CD56bright NK cells were significantly elevated in TCMR biopsies. In contrast, both CD56bright and CD56dim NK cell numbers were significantly increased in biopsies with histopathological evidence of AMR. Notably, expression of the activation marker CD69 was only significantly elevated on CD56dim NK cells in AMR biopsies compared with no rejection biopsies, indicative of a pathogenic phenotype for this cytotoxic NK cell subset. In line with this, we detected significantly elevated levels of cytotoxic effector molecules (perforin, granzyme A, and granulysin) in the dissociation supernatants of biopsies with a histopathological pattern of AMR. Conclusions: Our results indicate that human NK cell subsets are differentially recruited and activated during distinct types of rejection, suggestive of specialized functional roles.

Non anti-HLA antibodies and acute rejection: A critical viewpoint

In solid organ transplantation, the deleterious effect of antibodies directed against donor HLA antigens, whether preformed or de novo, is well established. Anti-HLA antibodies have been associated not only with the risk of antibody-mediated rejection but also with late graft dysfunction and are now considered to be the leading cause of allograft loss after renal transplantation. In addition to HLA antibodies, the possible involvement of non-HLA antibodies targeting donor endothelial cells has long been the subject of intense research. The purpose of this review is to discuss current knowledge and remaining issues related to the involvement of non-HLA antibodies in solid organ transplantation. More specifically, the clinical data underlying the hypothesis of the role of non-HLA antibodies will be discussed, as well as the different techniques for antibody detection, their clinical relevance and their antigenic targets.

Natural Killer Cells in Kidney Health and Disease

Natural killer (NK) cells are a specialized population of innate lymphocytes that have a major effector function in local immune responses. While their immunological functions in many inflammatory diseases are well established, comparatively little is still known about their roles in kidney homeostasis and disease. Our understanding of kidney NK cells is rapidly evolving, with murine studies highlighting the functional significance of NK cells in acute and chronic forms of renal disease. Recent progress has been made in translating these murine findings to human kidneys, with indications of NK cell subset-specific roles in disease progression in both native and allograft kidneys. Clearly, a better understanding of the molecular mechanisms driving NK cell activation and importantly, their downstream interactions with intrinsic renal cells and infiltrating immune cells is necessary for the development of targeted therapeutics to halt disease progression. In this review, we discuss the properties and potential functions of kidney NK cells.

Value of monitoring circulating donor-reactive memory B cells to characterize antibody-mediated rejection after kidney transplantation

Antibody-mediated rejection (ABMR) is defined by specific histopathological lesions and evidence of circulating donor-specific antibodies (DSA). Although DSA are not always detectable, monitoring donor-reactive memory B cells (mBC) could identify patients at risk of developing ABMR. Peripheral donor-reactive mBC using a novel HLA B cell ELISpot assay, serum DSA, and numbers of different B cell subsets were assessed in 175 consecutive kidney transplants undergoing either for-cause or 6- and 24-month surveillance biopsies for their association with main histological lesions of ABMR and impact on allograft outcome. In 85 incident for-cause biopsies, high frequencies of donor-reactive mBC were detected in all 16 (100%) acute ABMR/DSA+ and most chronic ABMR, with or without DSA (24/30[80%] and 21/29[72.4%], respectively). In a longitudinal cohort of 90 nonsensitized patients, a progressively higher expansion of donor-reactive mBC than de novo DSA was observed at 6 and 24 months (8.8% vs 7.7% and 15.5% vs 11.1%, respectively) and accurately identified patients with ongoing subclinical ABMR (area under the curve = 0.917 and area under the curve = 0.809, respectively). An unsupervised hierarchical cluster analysis revealed a strong association between donor-reactive mBC with main fundamental allograft lesions associated with ABMR and conferred a significant deleterious impact on graft outcome. Monitoring donor-reactive mBC may be useful to further characterize humoral rejection after kidney transplantation.

Antibody-mediated rejection in pediatric small bowel transplantation: Capillaritis is a major determinant of C4d positivity in intestinal transplant biopsies

The diagnostic criteria for antibody-mediated rejection (ABMR) after small bowel transplantation (SBT) are not clearly defined, although the presence of donor-specific antibodies (DSAs) has been reported to be deleterious for graft survival. We aimed to determine the incidence and prognostic value of DSAs and C4d in pediatric SBT and to identify the histopathologic features associated with C4d positivity. We studied all intestinal biopsies (IBx) obtained in the first year posttransplantation (N = 345) in a prospective cohort of 23 children. DSAs and their capacity to fix C1q were identified by using Luminex technology. Eighteen patients (78%) had DSAs, and 9 had the capacity to fix C1q. Seventy-eight IBx (22.6%) were C4d positive. The independent determinants of C4d positivity were capillaritis grades 2 and 3 (odds ratio [OR] 4.02, P = .047 and OR 5.17, P = .003, respectively), mucosal erosion/ulceration (OR 2.8, P = .019), lamina propria inflammation grades 1 and 2/3 (OR 1.95, P = .043 and OR 3.1, P = .016, respectively), and chorion edema (OR 2.16, P = .028). Complement-fixing DSAs and repeated C4d-positive IBx were associated with poor outcome (P = .021 and P = .001, respectively). Our results support that capillaritis should be considered as a feature of ABMR in SBT and identify C1q-fixing DSAs and repeated C4d positivity as potential markers of poor outcome.

Clinical and histopathologic features of antibody-mediated rejection among pediatric renal transplant recipients with preformed vs de novo donor-specific antibodies

Preformed and de novo donor specific antibodies (pDSA and dnDSA) are risk factors for ABMR. This study compares the effects of pDSA vs dnDSA in pediatric kidney transplant recipients. Sixteen pediatric patients with biopsy-proven ABMR were evaluated. Strong DSA (MFI >10 000) was recorded at transplant, rejection, and follow-up. DSAs with the highest MFI were termed iDSAs. Allograft biopsies were scored according to Banff 2013 criteria. Seven of 16 (44%) patients had pDSA at transplant; 9 (56%) developed dnDSA. Patients with pDSA developed ABMR earlier (median = 63 vs 1344 days, P = .017), while patients with dnDSA were more likely to have strong Class II iDSA (100% vs 28%, P = .009). Viral infection or non-adherence was more common in patients developing dnDSA (88.8% vs 28.6%, P < .01). Pathology in those with pDSAs demonstrated worse transplant glomerulitis (g score 1.57 ± 0.98 vs 0.56 ± 0.73, P = .031); however, those with dnDSAs exhibited higher C4d+ ABMR (P = .013). Patients developing dnDSAs showed ABMR later post-transplant with predominance of HLA-Class II iDSAs. Inadequate immunosuppression likely contributes to dnDSA formation. Patients with no DSA who have unprotocolized decreases in immunosuppression should be screened for dnDSA as it could lead to early intervention and potentially better outcomes.

Comprehensive Analysis of Transcript Changes Associated With Allograft Rejection: Combining Universal and Selective Features

We annotated the top transcripts associated with kidney transplant rejection by p-value, either universal for all rejection or selective for T cell-mediated rejection (TCMR) or antibody-mediated rejection (ABMR; ClinicalTrials.gov NCT01299168). We used eight class-comparison algorithms to interrogate microarray results from 703 biopsies, 205 with rejection. The positive comparators were all rejection, TCMR, or ABMR; the negative comparators varied from normal biopsies to all nonrejecting biopsies, including other diseases. The universal algorithm, rejection versus all nonrejection, identified transcripts mainly inducible by interferon γ. Selectivity for ABMR or TCMR required the other rejection class as well as nonrejection biopsies in the comparator to avoid selecting universal transcripts. Direct comparison of ABMR versus TCMR yielded only transcripts related to TCMR, the stronger signal. Transcripts highly associated with rejection were never completely specific for rejection: Many were increased in biopsies without rejection, reflecting sharing between rejection and injury-induced innate immunity. Union of the top 200 transcripts from universal and selective algorithms yielded 454 transcripts that permitted unsupervised analysis of biopsies in principal component analysis: PC1 was rejection, and PC2 was separation of TCMR from ABMR. Appreciating rejection-associated molecular changes requires a diverse case mix, accurate histologic classification (including C4d-negative ABMR), and both selective and universal algorithms.

Rejection of the Renal Allograft in the Absence of Demonstrable Antibody and Complement

BACKGROUND

Recent literature has stressed the prominent role of antibodies in graft loss. This study was designed to assess a growing perception that T cell-mediated rejection (TCMR) is no longer clinically relevant.

METHODS

Five hundred forty-five renal allograft recipients over a 3-year period were screened for biopsies with: (a) TCMR including borderline change (BL), (b) negative complement protein C4 degradation fragment, and (c) absence of donor-specific antibody at time of transplant, within 30 days of the biopsy, and up to 4 measurements at later time points.

RESULTS

These stringent requirements identified 28 "pure" cases of late TCMR/BL. Low-grade glomerulitis, peritubular capillaritis, or chronic transplant glomerulopathy were found in 9/28 (32%) biopsies. Serum creatinine showed complete short-term remission in 7/10 (70%) BL and 9/18 (50%) TCMR patients 1 month postbiopsy. Yet, both treated and untreated patients demonstrated further decline in graft function as assessed by serum creatinine and estimated glomerular filtration rate.

CONCLUSIONS

Late TCMR seen in 7.9% of biopsies can contribute to significant deterioration of graft function in patients in whom the dominant contribution of antibody-mediated injury has been reasonably excluded. Our data also reinforce existing literature showing that microvascular lesions do not have absolute specificity for a diagnosis of antibody-mediated rejection.

Assessing the potential of angiotensin II type 1 receptor and donor specific anti-endothelial cell antibodies to predict long-term kidney graft outcome

Endothelial cell antigens have been reported as potential targets for antibodies in the context of organ transplantation, leading to increased risk for graft failure. Serum samples from 142 consecutive living donor kidney recipients were tested for the presence of antibodies to angiotensin II - type 1 receptor (AT1R), donor endothelial cells, and donor HLA. Graft survival was monitored for five years post-transplant, and secondary outcomes, including biopsy-proven rejection, proteinuria, biopsy-proven vasculopathy, and renal function based on serum creatinine were also assessed for the first two to three years. AT1R antibody levels were positive (>17U/mL) in 11.3%, 18.8% and 8.1% of patients pre-transplant, post-transplant and at time of indication biopsy, respectively. XM-ONE assay was positive in 17.6% of patients pre-transplant (7 IgG+; 15 IgM+; 3 IgG+/IgM+). Overall, 4 patients experienced antibody-mediated rejection (AMR), 31 borderline cellular rejection (BCR), 19 cellular rejection (CR) and 3 mixed AMR and CR within the first 24months. While pre-existing and de novo donor-specific HLA antibodies were associated with graft failure and many secondary outcomes, no statistical association was found for either anti-endothelial or anti-AT1R antibodies, indicating that these tests may not be the best predictors of graft outcome in living donor renal transplantation.

Current status of pediatric renal transplant pathology

Histopathology is still an indispensable tool for the diagnosis of kidney transplant dysfunction in adult and pediatric patients. This review presents consolidated knowledge, recent developments and future prospects on the biopsy procedure, the diagnostic work-up, classification schemes, the histopathology of rejection, including antibody-mediated forms, ABO-incompatible transplants, protocol biopsies, recurrent and de novo disease, post-transplant lymphoproliferative disorder, infectious complications and drug-induced toxicity. It is acknowledged that frequently the correct diagnosis can only be reached in consensus with clinical, serological, immunogenetical, bacteriological and virological findings. This review shall enhance the understanding of the pediatric nephrologist for the thought processes of nephropathologists with the aim to facilitate teamwork between these specialist groups for the benefit of the patient.

A Probabilistic Approach to Histologic Diagnosis of Antibody-Mediated Rejection in Kidney Transplant Biopsies

Histologic diagnosis of antibody-mediated rejection (ABMR) in kidney transplant biopsies uses lesion score cutoffs such as 0 versus >0 rather than actual scores and requires donor-specific antibody (DSA); however, cutoffs lose information, and DSA is not always reliable. Using microarray-derived molecular ABMR scores as a histology-independent estimate of ABMR in 703 biopsies, we reassessed criteria for ABMR to determine relative importance of various lesions, the utility of equations using actual scores rather than cutoffs, and the potential for diagnosing ABMR when DSA is unknown or negative. We confirmed that the important features for ABMR diagnosis were peritubular capillaritis (ptc), glomerulitis (g), glomerular double contours, DSA and C4d staining, but we questioned some features: arterial fibrosis, vasculitis, acute tubular injury, and sum of ptc+g scores. Regression equations using lesion scores predicted molecular ABMR more accurately than score cutoffs (area under the curve 0.85-0.86 vs. 0.75). DSA positivity improved accuracy, but regression equations predicted ABMR with moderate accuracy when DSA was unknown. Some biopsies without detectable DSA had high probability of ABMR by regression, although most had HLA antibody. We concluded that regression equations using lesion scores plus DSA maximized diagnostic accuracy and can estimate probable ABMR when DSA is unknown or undetectable.

The effect of histopathologic and clinical features on allograft survival in renal transplant patients with antibody-mediated rejection

Background: Antibody-mediated rejection is a frequent cause of graft failure; however, prognostic indications of this complication have not been well defined. The aim of this study was to evaluate the association of histopathological and clinical features and to determine the effect of these findings on allograft survival in patients with AMR.

Methods: Fifty-two patients suffered from AMR (30 male; mean age 39 ± 11 years) were included in the study. Data were investigated retrospectively and graft survival was analyzed. All transplant biopsies were evaluated according to Banff 2009 classification.

Results: Of the 52 cases, 45 were transplanted from living-donors. Twenty-one patients were diagnosed in the first 3-months after transplantation. Graft survival was 65% at 12 months and 54% at 36 months. Mean serum creatinine at time of biopsy was 3.8 ± 3.6 mg/dL. Thirty-five of the 52 cases showed diffuse C4d positivity, 12 cases showed focal and 5 remained C4d negative. One of the patients died, 13 experienced graft loss and 38 survived with functioning grafts. Serum creatinine levels at time of biopsy were correlated with graft survival (p = .021: OR = 1.10: 95 % CI = 1.015–1.199). In terms of the impact of pathological findings; tubulitis (p=.007: OR = 2.62: 95 % CI = 1.301–5.276), intimal arteritis (p=.017: OR = 2.85: 95% CI = 1.205–6.744) and interstitial infiltration (p=.004: OR = 3.37: 95% CI = 1.465–7.752) were associated with graft survival.

Conclusions: Serum creatinine at time of biopsy, tubulitis, intimal arteritis and interstitial infiltration were significantly associated with graft survival. Antibody-mediated rejection is associated with reduced long-term graft survival.

The Spectrum of Renal Allograft Failure

Background

Causes of “true” late kidney allograft failure remain unclear as study selection bias and limited follow-up risk incomplete representation of the spectrum.

Methods

We evaluated all unselected graft failures from 2008–2014 (n = 171; 0–36 years post-transplantation) by contemporary classification of indication biopsies “proximate” to failure, DSA assessment, clinical and biochemical data.

Results

The spectrum of graft failure changed markedly depending on the timing of allograft failure. Failures within the first year were most commonly attributed to technical failure, acute rejection (with T-cell mediated rejection [TCMR] dominating antibody-mediated rejection [ABMR]). Failures beyond a year were increasingly dominated by ABMR and ‘interstitial fibrosis with tubular atrophy’ without rejection, infection or recurrent disease (“IFTA”). Cases of IFTA associated with inflammation in non-scarred areas (compared with no inflammation or inflammation solely within scarred regions) were more commonly associated with episodes of prior rejection, late rejection and nonadherence, pointing to an alloimmune aetiology. Nonadherence and late rejection were common in ABMR and TCMR, particularly Acute Active ABMR. Acute Active ABMR and nonadherence were associated with younger age, faster functional decline, and less hyalinosis on biopsy. Chronic and Chronic Active ABMR were more commonly associated with Class II DSA. C1q-binding DSA, detected in 33% of ABMR episodes, were associated with shorter time to graft failure. Most non-biopsied patients were DSA-negative (16/21; 76.1%). Finally, twelve losses to recurrent disease were seen (16%).

Conclusion

This data from an unselected population identifies IFTA alongside ABMR as a very important cause of true late graft failure, with nonadherence-associated TCMR as a phenomenon in some patients. It highlights clinical and immunological characteristics of ABMR subgroups, and should inform clinical practice and individualised patient care.

The prognostic values of caveolin-1 immunoreactivity in peritubular capillaries in patients with kidney transplantation

The low sensitivity of C4d immunoreactivity in peritubular capillaries (PTCs) hinders its use in the diagnosis of chronic active antibody-mediated rejection (CAAMR). C4d-negative CAAMR was defined in the 2013 Banff classification, which included the expression of endothelial-associated transcripts (ENDATs). We previously showed that the ENDAT caveolin-1 (CAV-1) is a distinct feature of CAAMR. In this study, we investigated the prognostic value of CAV-1 immunoreactivity in PTCs in kidney transplant patients. Ninety-eight kidney transplant recipients were included in this study. The prognostic value of CAV-1 immunoreactivity in PTCs was evaluated by double immunostaining for CAV-1 and pathologische Anatomie Leiden endothelium (PAL-E, a PTC marker) in the PTCs of kidney allograft biopsy samples. The patients were divided into two groups: CAV-1/PAL-E<50% and CAV-1/PAL-E≥50%. Kaplan-Meier curves showed that CAV-1/PAL-E≥50% patients had a significantly worse prognosis than that of CAV-1/PAL-E<50% patients (log-rank; P<.001). C4d staining of PTCs was not associated with the development of graft failure (log-rank; P=.345), whereas in a multivariate Cox regression analysis, CAV-1 immunoreactivity in PTCs was independently associated with graft failure (hazard ratio: 11.1; P=.0324). CAV-1 immunoreactivity in PTCs may serve as a prognostic marker for kidney allograft survival.

Mechanisms of antibody-mediated acute and chronic rejection of kidney allografts

PURPOSE OF REVIEW

Antibody-mediated rejection is responsible for up to half of acute rejection episodes in kidney transplant patients and more than half of late graft failures. Antibodies cause acute graft abnormalities that are distinct from T cell-mediated rejection and at later times posttransplant, a distinct pathologic lesion is associated with capillary basement membrane multilayering and glomerulopathy. Despite the importance of donor-reactive antibodies as the leading cause of kidney graft failure, mechanisms underlying antibody-mediated acute and chronic kidney graft injury are poorly understood. Here, we review recent insights provided from clinical studies as well as from animal models that may help to identify new targets for therapy.

RECENT FINDINGS

Studies of biopsies from kidney grafts in patients with donor-specific antibody versus those without have utilized analysis of pathologic lesions and gene expression to identify the distinct characteristics of antibody-mediated rejection. These analyses have indicated the presence of natural killer cells and their activation during antibody-mediated rejection. The impact of studies of antibody-mediated allograft injury in animal models have lagged behind these clinical studies, but have been useful in testing the activation of innate immune components within allografts in the presence of donor-specific antibodies.

SUMMARY

Most insights into processes of antibody-mediated rejection of kidney grafts have come from carefully designed clinical studies. However, several new mouse models of antibody-mediated kidney allograft rejection may replicate the abnormalities observed in clinical kidney grafts and may be useful in directly testing mechanisms that underlie acute and chronic antibody-mediated graft injury.

Pathogenesis of non-HLA antibodies in solid organ transplantation: Where do we stand?

Antibody-mediated rejection (ABMR) is associated with poor transplant outcome. Pathogenic alloantibodies are usually directed against human leukocyte antigens (HLAs). Histological findings suggestive of ABMR usually demonstrate an anti-HLA donor-specific antibody (DSA)-mediated injury, while a small subset of patients develop acute dysfunction with histological lesions suggestive of ABMR in the absence of anti-HLA DSAs. Although this non-HLA ABMR is not well recognized by current diagnostic classifications, it is associated with graft dysfunction and allograft loss. These clinical descriptions suggest a pathogenic role for non-HLA anti-endothelial cell antibodies. Diverse antigenic targets have been described during the last decade. This review discusses recent findings in the field and addresses the clinical relevance of anti-endothelial cell antibodies (AECAs).

Pretransplant angiotensin II type 1-receptor antibodies are a risk factor for earlier detection of de novo HLA donor-specific antibodies

BACKGROUND

Angiotensin II type 1 receptor antibodies (AT1Rabs) have been associated with significantly reduced graft survival. Earlier graft loss has been observed in patients who had pretransplant AT1Rabs and posttransplant donor-specific antibodies (DSA).

METHODS

The main goal of this retrospective cohort study was to examine the association between AT1Rabs and the time period to detection of de novo human leukocyte antigen (HLA-DSA) posttransplantation in living donor kidney transplant recipients (KTR). The analysis included 141 KTRs. Pretransplant frozen serum samples were tested for AT1Rabs by ELISA and HLA-DSA by SAB (Luminex) at both the pre- and post-KT time points.

RESULTS

The median AT1Rab level was 9.13 U (interquartile range 5.22-14.33). After a mean follow-up period of 3.55 years, 48 patients were found to harbour de novo HLA-DSAs. The presence of AT1Rabs [hazard ratio (HR) 1.009, 95% confidence interval (CI) 1.002-1.01, P = 0.010], male-to-male transplantation (HR 2.57, 95% CI 1.42-4.67, P = 0.002) and antecedent borderline changes or acute cellular rejection (ACR) (HR 2.47, 95% CI 1.29-4.75, P = 0.006) were significantly associated with de novo DSA detection. A dose-dependent association between AT1Rab levels (<10 U, 10.1-16.9 U, 17-29.9 U and >30 U) and de novo DSA detection was observed (log-rank P = 0.0031). After multivariate analysis of AT1Rab levels (continuous variable), AT1Rabs >30 U, male-to-male transplantation, donor age, higher class I percentage of Panel Reactive Antibody and antecedent borderline changes or ACR remained as independent significant risk factors for the detection of de novo DSAs.

CONCLUSIONS

The findings suggest that higher levels of pretransplant circulating antibodies against AT1R (>30 U) in kidney graft recipients constitute an independent risk factor for earlier de novo HLA-DSA detection during the posttransplant period.

The Revised (2013) Banff Classification for Antibody-Mediated Rejection of Renal Allografts: Update, Difficulties, and Future Considerations

The Banff 2013 classification (Banff 2013) for antibody-mediated rejection (ABMR) in renal allografts represents the first major revision of the original Banff classification for ABMR that was published in 2003. The main impetus for this revision was the need to include C4d-negative ABMR, although this revised classification contains a number of additional features based on findings reported from 2007 to 2013. Since its publication, several studies have examined the validity of different aspects of Banff 2013 and compared it to earlier (2003, 2007) versions of the Banff ABMR classification. Recent evidence, albeit limited, indicates that Banff 2013 represents an improvement over the previous versions, enhancing our ability to accurately diagnose cases of acute/active and chronic active ABMR on renal allograft biopsy. Molecular studies appear to justify the threshold value of glomerulitis plus peritubular capillaritis score ≥2 required by Banff 2013 for the diagnosis of C4d-negative ABMR; however, other aspects of the classification, including its overall interobserver reproducibility, the clinical significance of the category of C4d staining without evidence of rejection, and whether surrogate markers might potentially substitute for the requirement for the presence of donor-specific antibodies, require additional investigation.

Natural killer cells play a critical role in mediating inflammation and graft failure during antibody-mediated rejection of kidney allografts

While the incidence of antibody-mediated kidney graft rejection has increased, the key cellular and molecular participants underlying this graft injury remain unclear. Rejection of kidney allografts in mice lacking the chemokine receptor CCR5 is dependent on production of donor-specific antibody. Here we determine if cells expressing cytotoxic function contributed to antibody-mediated kidney allograft rejection in these recipients. Wild-type C57BL/6, B6.CCR5(-/-), and B6.CD8(-/-)/CCR5(-/-) mice were transplanted with complete MHC-mismatched A/J kidney grafts, and intragraft inflammatory components were followed to rejection. B6.CCR5(-/-) and B6.CD8(-/-)/CCR5(-/-) recipients rejected kidney allografts by day 35, whereas 65% of allografts in wild-type recipients survived past day 80 post-transplant. Rejected allografts in wild-type C57BL/6, B6.CCR5(-/-), and B6.CD8(-/-)/CCR5(-/-) recipients expressed high levels of VCAM-1 and MMP7 mRNA that was associated with high serum titers of donor-specific antibody. High levels of perforin and granzyme B mRNA expression peaked on day 6 post-transplant in allografts in all recipients, but were absent in isografts. Depletion of natural killer cells in B6.CD8(-/-)/CCR5(-/-) recipients reduced this expression to background levels and promoted the long-term survival of 40% of the kidney allografts. Thus, natural killer cells have a role in increased inflammation during antibody-mediated kidney allograft injury and in rejection of the grafts.

Analysis of the Humoral Immune Response to Human Leukocyte Antigens in Turkish Renal Transplant Candidates and Relationship Between Autoimmune Disorders

BACKGROUND

Pre-transplant sensitization to human leukocyte antigens (HLA) is a risk factor for graft failure. The purpose of the current single-center study was to evaluate humoral immune response to HLA antigens and the possible relationship between anti-HLA antibody titer and autoimmune disorders in renal transplant candidates.

METHODS

A total of 435 renal transplant candidates were analyzed; 50 sensitized patients were enrolled in this study. The HLA typing was performed by use of enzyme-linked immunoassay combined with Luminex technology. The patients were questioned for clinical evidence of hypothyroidism and systemic lupus erythematosus (SLE) and investigated for anti-nuclear antibody (ANA), anti-double-stranded DNA (anti-dsDNA), anti-thyroglobulin (anti-TG), anti-thyroid peroxidase (anti-TPO) antibodies, and thyroid function tests.

RESULTS

Among 50 patients with positive panel reactive antibody, 24 (48%) were positive for class I and negative for class II, 12 (24%) were negative for class I and positive for class II, and 14 (28%) were positive for both classes I and II. The specificities of anti-HLA antibodies-A23, A68, A69, B27, B49, DR6, and DR8-were the most frequent. ANA and anti-dsDNA antibodies were not correlated with either clinical symptoms of SLE or anti-HLA antibody titer of renal transplant candidates. Similarly, anti-TG and anti-TPO antibodies were not correlated with clinical hypothyroidism or anti-HLA antibody titer.

CONCLUSIONS

Our data have demonstrated the profile of anti-HLA antibodies in patients who were on the renal transplant waiting list in Turkey. The most frequent specificities of anti-HLA antibodies were A23, A68, A69, B27, B49, DR6, and DR8. There was no association between anti-HLA antibody titer and clinical and laboratory evidence of SLE and hypothyroidism.

Clinical relevance of circulating antibodies and B lymphocyte markers in allograft rejection

The main challenge in solid organ transplantation remains to tackle antibody-mediated rejection. Our understanding of the antibody-mediated response and the capacity to detect it has improved in the last decade. However, the sensitivity and specificity of the current clinical tools to monitor B cell activation are perfectible. New strategies, including the refinement in the characterization of HLA and non-HLA antibodies, as well as a better understanding of the circulating B cell phenotype will hopefully help to non-invasively identify patients at risk or undergoing antibody-mediated allograft damage. The current review discusses the current knowledge of the B cell biomarkers in solid organ transplantation, with a focus on circulating antibodies and peripheral B cells.