The Banff 2022 Kidney Meeting Report: Reappraisal of microvascular inflammation and the role of biopsy-based transcript diagnostics.

The XVI-th Banff Meeting for Allograft Pathology was held at Banff, Alberta, Canada, from 19th to 23rd September 2022, as a joint meeting with the Canadian Society of Transplantation. To mark the 30th anniversary of the first Banff Classification, premeeting discussions were held on the past, present, and future of the Banff Classification. This report is a summary of the meeting highlights that were most important in terms of their effect on the Classification, including discussions around microvascular inflammation and biopsy-based transcript analysis for diagnosis. In a postmeeting survey, agreement was reached on the delineation of the following phenotypes: (1) "Probable antibody-mediated rejection (AMR)," which represents donor-specific antibodies (DSA)-positive cases with some histologic features of AMR but below current thresholds for a definitive AMR diagnosis; and (2) "Microvascular inflammation, DSA-negative and C4d-negative," a phenotype of unclear cause requiring further study, which represents cases with microvascular inflammation not explained by DSA. Although biopsy-based transcript diagnostics are considered promising and remain an integral part of the Banff Classification (limited to diagnosis of AMR), further work needs to be done to agree on the exact classifiers, thresholds, and clinical context of use.

Smoking, Alcohol Intake and Torque Teno Virus in Stable Kidney Transplant Recipients.

Torque Teno Virus (TTV) is a non-pathogenic virus that is highly prevalent among kidney transplant recipients (KTRs). Its circulating load is associated with an immunological status in KTR and is considered a promising tool for guiding immunosuppression. To allow for optimal guidance, it is important to identify other determinants of TTV load. We aimed to investigate the potential association of smoking and alcohol intake with TTV load. For this cross-sectional study, serum TTV load was measured using PCR in stable kidney transplant recipients at ≥1 year after transplantation, and smoking status and alcohol intake were assessed through questionnaires and measurements of urinary cotinine and ethyl glucuronide. A total of 666 KTRs were included (57% male). A total of 549 KTR (82%) had a detectable TTV load (3.1 ± 1.5 log10 copies/mL). In KTR with a detectable TTV load, cyclosporin and tacrolimus use were positively associated with TTV load (St. ß = 0.46, p < 0.001 and St. ß = 0.66, p < 0.001, respectively), independently of adjustment for potential confounders. Current smoking and alcohol intake of >20 g/day were negatively associated with TTV load (St. ß = -0.40, p = 0.004 and St. ß = -0.33, p = 0.009, respectively), independently of each other and of adjustment for age, sex, kidney function, time since transplantation and calcineurin inhibitor use. This strong association of smoking and alcohol intake with TTV suggests a need to account for the smoking status and alcohol intake when applying TTV guided immunosuppression in KTR.

Transcriptional and spatial profiling of the kidney allograft unravels a central role for FcyRIII+ innate immune cells in rejection.

Rejection remains the main cause of premature graft loss after kidney transplantation, despite the use of potent immunosuppression. This highlights the need to better understand the composition and the cell-to-cell interactions of the alloreactive inflammatory infiltrate. Here, we performed droplet-based single-cell RNA sequencing of 35,152 transcriptomes from 16 kidney transplant biopsies with varying phenotypes and severities of rejection and without rejection, and identified cell-type specific gene expression signatures for deconvolution of bulk tissue. A specific association was identified between recipient-derived FCGR3A+ monocytes, FCGR3A+ NK cells and the severity of intragraft inflammation. Activated FCGR3A+ monocytes overexpressed CD47 and LILR genes and increased paracrine signaling pathways promoting T cell infiltration. FCGR3A+ NK cells overexpressed FCRL3, suggesting that antibody-dependent cytotoxicity is a central mechanism of NK-cell mediated graft injury. Multiplexed immunofluorescence using 38 markers on 18 independent biopsy slides confirmed this role of FcγRIII+ NK and FcγRIII+ nonclassical monocytes in antibody-mediated rejection, with specificity to the glomerular area. These results highlight the central involvement of innate immune cells in the pathogenesis of allograft rejection and identify several potential therapeutic targets that might improve allograft longevity.

Imlifidase for Kidney Transplantation of Highly Sensitized Patients With a Positive Crossmatch: The French Consensus Guidelines.

Imlifidase recently received early access authorization for highly sensitized adult kidney transplant candidates with a positive crossmatch against an ABO-compatible deceased donor. These French consensus guidelines have been generated by an expert working group, in order to homogenize patient selection, associated treatments and follow-up. This initiative is part of an international effort to analyze properly the benefits and tolerance of this new costly treatment in real-life. Eligible patients must meet the following screening criteria: cPRA ≥ 98%, ≤ 65-year of age, ≥ 3 years on the waiting list, and a low risk of biopsy-related complications. The final decision to use Imlifidase will be based on the two following criteria. First, the results of a virtual crossmatch on recent serum, which shall show a MFI for the immunodominant donor-specific antibodies (DSA) > 6,000 but the value of which does not exceed 5,000 after 1:10 dilution. Second, the post-Imlifidase complement-dependent cytotoxicity crossmatch must be negative. Patients treated with Imlifidase will receive an immunosuppressive regimen based on steroids, rATG, high dose IVIg, rituximab, tacrolimus and mycophenolic acid. Frequent post-transplant testing for DSA and systematic surveillance kidney biopsies are highly recommended to monitor post-transplant DSA rebound and subclinical rejection.

Clinical and Prognostic Factors in Patients with IgG4-Related Kidney Disease.

BACKGROUND: IgG4-related kidney disease is a major manifestation of IgG4-related disease, a systemic fibroinflammatory disorder. However, the clinical and prognostic kidney-related factors in patients with IgG4-related kidney disease are insufficiently defined. METHODS: We conducted an observational cohort study using data from 35 sites in two European countries. Clinical, biologic, imaging, and histopathologic data; treatment modalities; and outcomes were collected from medical records. Logistic regression was performed to identify the possible factors related to an eGFR ≤30 ml/min per 1.73 m 2 at the last follow-up. Cox proportional hazards model was performed to assess the factors associated with the risk of relapse. RESULTS: We studied 101 adult patients with IgG4-related disease with a median follow-up of 24 (11-58) months. Of these, 87 (86%) patients were male, and the median age was 68 (57-76) years. Eighty-three (82%) patients had IgG4-related kidney disease confirmed by kidney biopsy, with all biopsies showing tubulointerstitial involvement and 16 showing glomerular lesions. Ninety (89%) patients were treated with corticosteroids, and 18 (18%) patients received rituximab as first-line therapy. At the last follow-up, the eGFR was below 30 ml/min per 1.73 m 2 in 32% of patients; 34 (34%) patients experienced a relapse, while 12 (13%) patients had died. By Cox survival analysis, the number of organs involved (hazard ratio [HR], 1.26; 95% confidence interval [CI], 1.01 to 1.55) and low C3 and C4 concentrations (HR, 2.31; 95% CI, 1.10 to 4.85) were independently associated with a higher risk of relapse, whereas first-line therapy with rituximab was protective (HR, 0.22; 95% CI, 0.06 to 0.78). At their last follow-up, 19 (19%) patients had an eGFR ≤30 ml/min per 1.73 m 2 . Age (odd ratio [OR], 1.11; 95% CI, 1.03 to 1.20), peak serum creatinine (OR, 2.74; 95% CI, 1.71 to 5.47), and serum IgG4 level ≥5 g/L (OR, 4.46; 95% CI, 1.23 to 19.40) were independently predictive for severe CKD. CONCLUSIONS: IgG4-related kidney disease predominantly affected middle-aged men and manifested as tubulointerstitial nephritis with potential glomerular involvement. Complement consumption and the number of organs involved were associated with a higher relapse rate, whereas first-line therapy with rituximab was associated with lower relapse rate. Patients with high serum IgG4 concentrations (≥5 g/L) had more severe kidney disease.

mTOR Activation Underlies Enhanced B Cell Proliferation and Autoimmunity in PrkcdG510S/G510S Mice.

Autosomal recessive PRKCD deficiency has previously been associated with the development of systemic lupus erythematosus in human patients, but the mechanisms underlying autoimmunity remain poorly understood. We introduced the Prkcd G510S mutation that we previously associated to a Mendelian cause of systemic lupus erythematosus in the mouse genome, using CRISPR-Cas9 gene editing. PrkcdG510S/G510S mice recapitulated the human phenotype and had reduced lifespan. We demonstrate that this phenotype is linked to a B cell-autonomous role of Prkcd. A detailed analysis of B cell activation in PrkcdG510S/G510S mice shows an upregulation of the PI3K/mTOR pathway after the engagement of the BCR in these cells, leading to lymphoproliferation. Treatment of mice with rapamycin, an mTORC1 inhibitor, significantly improves autoimmune symptoms, demonstrating in vivo the deleterious effect of mTOR pathway activation in PrkcdG510S/G510S mice. Additional defects in PrkcdG510S/G510S mice include a decrease in peripheral mature NK cells that might contribute to the known susceptibility to viral infections of patients with PRKCD mutations.

Subclinical rejection-free diagnostic after kidney transplantation using blood gene expression.

We previously established a six-gene-based blood score associated with operational tolerance in kidney transplantation which was decreased in patients developing anti-HLA donor-specific antibodies (DSA). Herein, we aimed to confirm that this score is associated with immunological events and risk of rejection. We measured this using quantitative PCR (qPCR) and NanoString methods from an independent multicenter cohort of 588 kidney transplant recipients with paired blood samples and biopsies at one year after transplantation validating its association with pre-existing and de novo DSA. From 441 patients with protocol biopsy, there was a significant decrease of the score of tolerance in 45 patients with biopsy-proven subclinical rejection (SCR), a major threat associated with pejorative allograft outcomes that prompted an SCR score refinement. This refinement used only two genes, AKR1C3 and TCL1A, and four clinical parameters (previous experience of rejection, previous transplantation, sex of recipient and tacrolimus uptake). This refined SCR score was able to identify patients unlikely to develop SCR with a C-statistic of 0.864 and a negative predictive value of 98.3%. The SCR score was validated in an external laboratory, with two methods (qPCR and NanoString), and on 447 patients from an independent and multicenter cohort. Moreover, this score allowed reclassifying patients with discrepancies between the DSA presence and the histological diagnosis of antibody mediated rejection unlike kidney function. Thus, our refined SCR score could improve detection of SCR for closer and noninvasive monitoring, allowing early treatment of SCR lesions notably for patients DSA-positive and during lowering of immunosuppressive treatment.

Data-Driven Chronic Allograft Phenotypes: A Novel and Validated Complement for Histologic Assessment of Kidney Transplant Biopsies.

BACKGROUND: No validated system currently exists to realistically characterize the chronic pathology of kidney transplants that represents the dynamic disease process and spectrum of disease severity. We sought to develop and validate a tool to describe chronicity and severity of renal allograft disease and integrate it with the evaluation of disease activity. METHODS: The training cohort included 3549 kidney transplant biopsies from an observational cohort of 937 recipients. We reweighted the chronic histologic lesions according to their time-dependent association with graft failure, and performed consensus k-means clustering analysis. Total chronicity was calculated as the sum of the weighted chronic lesion scores, scaled to the unit interval. RESULTS: We identified four chronic clusters associated with graft outcome, based on the proportion of ambiguous clustering. The two clusters with the worst survival outcome were determined by interstitial fibrosis and tubular atrophy (IFTA) and by transplant glomerulopathy. The chronic clusters partially overlapped with the existing Banff IFTA classification (adjusted Rand index, 0.35) and were distributed independently of the acute lesions. Total chronicity strongly associated with graft failure (hazard ratio [HR], 8.33; 95% confidence interval [CI], 5.94 to 10.88; P<0.001), independent of the total activity scores (HR, 5.01; 95% CI, 2.83 to 7.00; P<0.001). These results were validated on an external cohort of 4031 biopsies from 2054 kidney transplant recipients. CONCLUSIONS: The evaluation of total chronicity provides information on kidney transplant pathology that complements the estimation of disease activity from acute lesion scores. Use of the data-driven algorithm used in this study, called RejectClass, may provide a holistic and quantitative assessment of kidney transplant injury phenotypes and severity.

Inverted direct allorecognition triggers early donor-specific antibody responses after transplantation.

The generation of antibodies against donor-specific major histocompatibility complex (MHC) antigens, a type of donor-specific antibodies (DSAs), after transplantation requires that recipient's allospecific B cells receive help from T cells. The current dogma holds that this help is exclusively provided by the recipient's CD4+ T cells that recognize complexes of recipient's MHC II molecules and peptides derived from donor-specific MHC alloantigens, a process called indirect allorecognition. Here, we demonstrated that, after allogeneic heart transplantation, CD3ε knockout recipient mice lacking T cells generate a rapid, transient wave of switched alloantibodies, predominantly directed against MHC I molecules. This is due to the presence of donor CD4+ T cells within the graft that recognize intact recipient's MHC II molecules expressed by B cell receptor-activated allospecific B cells. Indirect evidence suggests that this inverted direct pathway is also operant in patients after transplantation. Resident memory donor CD4+ T cells were observed in perfusion liquids of human renal and lung grafts and acquired B cell helper functions upon in vitro stimulation. Furthermore, T follicular helper cells, specialized in helping B cells, were abundant in mucosa-associated lymphoid tissue of lung and intestinal grafts. In the latter, more graft-derived passenger T cells correlated with the detection of donor T cells in recipient's circulation; this, in turn, was associated with an early transient anti-MHC I DSA response and worse transplantation outcomes. We conclude that this inverted direct allorecognition is a possible explanation for the early transient anti-MHC DSA responses frequently observed after lung or intestinal transplantations.

Evolution of humoral lesions on follow-up biopsy stratifies the risk for renal graft loss after antibody-mediated rejection treatment.

BACKGROUND: The standard-of-care protocol, based on plasma exchanges, high-dose intravenous immunoglobulin and optimization of maintenance immunosuppression, can slow down the evolution of antibody-mediated rejection (AMR), but with high interindividual variability. Identification of a reliable predictive tool of the response to AMR treatment is a mandatory step for personalization of the follow-up strategy and to guide second-line therapies. METHODS: Interrogation of the electronic databases of 2 French university hospitals (Lyon and Strasbourg) retrospectively identified 81 renal transplant recipients diagnosed with AMR without chronic lesions (cg score ≤1) at diagnosis and for whom a follow-up biopsy had been performed 3-6 months after initiation of therapy. RESULTS: The evolution of humoral lesions on follow-up biopsy (disappearance versus persistence versus progression) correlated with the risk for allograft loss (logrank test, P = .001). Patients with disappearance of humoral lesions had ∼80% graft survival at 10 years. The hazard ratio for graft loss in multivariate analysis was 3.91 (P = .04) and 5.15 (P = .02) for patients with persistence and progression of lesions, respectively. The non-invasive parameters classically used to follow the intensity of humoral alloimmune response (evolution of immunodominant DSA mean fluorescence intensity) and the decline of renal graft function (estimated glomerular filtration rate decrease and persistent proteinuria) showed little clinical value to predict the histological response to AMR therapy. CONCLUSION: We conclude that invasive monitoring of the evolution of humoral lesions by the mean of follow-up biopsy performed 3-6 months after the initiation of therapy is an interesting tool to predict long-term outcome after AMR treatment.

International Survey of Clinical Monitoring Practices in Pancreas and Islet Transplantation.

BACKGROUND: The long-term outcomes of both pancreas and islet allotransplantation have been compromised by difficulties in the detection of early graft dysfunction at a time when a clinical intervention can prevent further deterioration and preserve allograft function. The lack of standardized strategies for monitoring pancreas and islet allograft function prompted an international survey established by an International Pancreas and Islet Transplant Association/European Pancreas and Islet Transplant Association working group. METHODS: A global survey was administered to 24 pancreas and 18 islet programs using Redcap. The survey addressed protocolized and for-cause immunologic and metabolic monitoring strategies following pancreas and islet allotransplantation. All invited programs completed the survey. RESULTS: The survey identified that in both pancreas and islet allograft programs, protocolized clinical monitoring practices included assessing body weight, fasting glucose/C-peptide, hemoglobin A1c, and donor-specific antibody. Protocolized monitoring in islet transplant programs relied on the addition of mixed meal tolerance test, continuous glucose monitoring, and autoantibody titers. In the setting of either suspicion for rejection or serially increasing hemoglobin A1c/fasting glucose levels postpancreas transplant, Doppler ultrasound, computed tomography, autoantibody titers, and pancreas graft biopsy were identified as adjunctive strategies to protocolized monitoring studies. No additional assays were identified in the setting of serially increasing hemoglobin A1c levels postislet transplantation. CONCLUSIONS: This international survey identifies common immunologic and metabolic monitoring strategies utilized for protocol and for cause following pancreas and islet transplantation. In the absence of any formal studies to assess the efficacy of immunologic and metabolic testing to detect early allograft dysfunction, it can serve as a guidance document for developing monitoring algorithms following beta-cell replacement.

T- and B-cell therapy in solid organ transplantation: current evidence and future expectations.

Cell therapy has emerged as an attractive therapeutic option in organ transplantation. During the last decade, the therapeutic potency of Treg immunotherapy has been shown in various preclinical animal models and safety was demonstrated in first clinical trials. However, there are still critical open questions regarding specificity, survival, and migration to the target tissue so the best Treg population for infusion into patients is still under debate. Recent advances in CAR technology hold the promise for Treg-functional superiority. Another exciting strategy is the generation of B-cell antibody receptor (BAR) Treg/cytotoxic T cells to specifically regulate or deplete alloreactive memory B cells. Finally, B cells are also capable of immune regulation, making them promising candidates for immunomodulatory therapeutic strategies. This article summarizes available literature on cell-based innovative therapeutic approaches aiming at modulating alloimmune response for transplantation. Crucial areas of investigation that need a joined effort of the transplant community for moving the field toward successful achievement of tolerance are highlighted.

Innate (and Innate-like) Lymphoid Cells: Emerging Immune Subsets With Multiple Roles Along Transplant Life.

Transplant immunology is currently largely focused on conventional adaptive immunity, particularly T and B lymphocytes, which have long been considered as the only cells capable of allorecognition. In this vision, except for the initial phase of ischemia/reperfusion, during which the role of innate immune effectors is well established, the latter are largely considered as "passive" players, recruited secondarily to amplify graft destruction processes during rejection. Challenging this prevalent dogma, the recent progresses in basic immunology have unraveled the complexity of the innate immune system and identified different subsets of innate (and innate-like) lymphoid cells. As most of these cells are tissue-resident, they are overrepresented among passenger leukocytes. Beyond their role in ischemia/reperfusion, some of these subsets have been shown to be capable of allorecognition and/or of regulating alloreactive adaptive responses, suggesting that these emerging immune players are actively involved in most of the life phases of the grafts and their recipients. Drawing upon the inventory of the literature, this review synthesizes the current state of knowledge of the role of the different innate (and innate-like) lymphoid cell subsets during ischemia/reperfusion, allorecognition, and graft rejection. How these subsets also contribute to graft tolerance and the protection of chronically immunosuppressed patients against infectious and cancerous complications is also examined.

Immunological Monitoring in Beta Cell Replacement: Towards a Pathophysiology-Guided Implementation of Biomarkers.

PURPOSE OF REVIEW: Grafted beta cells are lost because of recurrence of T1D and/or allograft rejection, two conditions diagnosed with pancreas graft biopsy, which is invasive and impossible in case of islet transplantation. This review synthetizes the current pathophysiological knowledge and discusses the interest of available immune biomarkers. RECENT FINDINGS: Despite the central role of auto-(recurrence of T1D) and allo-(T-cell mediated rejection) immune cellular responses, the latter are not directly monitored in routine. In striking contrast, there have been undisputable progresses in monitoring of auto and alloantibodies. Except for pancreas recipients in whom anti-donor HLA antibodies can be directly responsible for antibody-mediated rejection, autoantibodies (and alloantibodies in islet recipients) have no direct pathogenic effect. However, their fluctuation offers a surrogate marker for the activation status of T cells (because antibody generation depends on T cells). This illustrates the necessity to understand the pathophysiology when interpreting a biomarker and selecting the appropriate treatment.

Data-driven Derivation and Validation of Novel Phenotypes for Acute Kidney Transplant Rejection using Semi-supervised Clustering.

BACKGROUND: Over the past decades, an international group of experts iteratively developed a consensus classification of kidney transplant rejection phenotypes, known as the Banff classification. Data-driven clustering of kidney transplant histologic data could simplify the complex and discretionary rules of the Banff classification, while improving the association with graft failure. METHODS: The data consisted of a training set of 3510 kidney-transplant biopsies from an observational cohort of 936 recipients. Independent validation of the results was performed on an external set of 3835 biopsies from 1989 patients. On the basis of acute histologic lesion scores and the presence of donor-specific HLA antibodies, stable clustering was achieved on the basis of a consensus of 400 different clustering partitions. Additional information on kidney-transplant failure was introduced with a weighted Euclidean distance. RESULTS: Based on the proportion of ambiguous clustering, six clinically meaningful cluster phenotypes were identified. There was significant overlap with the existing Banff classification (adjusted rand index, 0.48). However, the data-driven approach eliminated intermediate and mixed phenotypes and created acute rejection clusters that are each significantly associated with graft failure. Finally, a novel visualization tool presents disease phenotypes and severity in a continuous manner, as a complement to the discrete clusters. CONCLUSIONS: A semisupervised clustering approach for the identification of clinically meaningful novel phenotypes of kidney transplant rejection has been developed and validated. The approach has the potential to offer a more quantitative evaluation of rejection subtypes and severity, especially in situations in which the current histologic categorization is ambiguous.

Characteristics of T- and NK-cell Lymphomas After Renal Transplantation: A French National Multicentric Cohort Study.

BACKGROUND: Posttransplant lymphoproliferative disorders (PTLDs) encompass a spectrum of heterogeneous entities. Because the vast majority of cases PTLD arise from B cells, available data on PTLD of T or NK phenotype (T/NK-cell PTLD) are scarce, which limits the quality of the management of these patients. METHODS: All adult cases of PTLD diagnosed in France were prospectively recorded in the national registry between 1998 and 2007. Crosschecking the registry data with 2 other independent national databases identified 58 cases of T/NK-cell PTLD. This cohort was then compared with (i) the 395 cases of B-cell PTLD from the registry, and of (ii) a cohort of 148 T/NK-cell lymphomas diagnosed in nontransplanted patients. RESULTS: T/NK-cell PTLD occurred significantly later after transplantation and had a worse overall survival than B-cell PTLD. Two subtypes of T/NK-cell PTLD were distinguished: (i) cutaneous (28%) and (ii) systemic (72%), the latter being associated with a worse prognosis. Compared with T/NK-cell lymphomas of nontransplanted patients, overall survival of systemic T/NK-cell PTLD was worse (hazard ratio: 2.64 [1.76-3.94]; P < 0.00001). CONCLUSIONS: This difference, which persisted after adjustment on tumoral mass, histological subtype, and extension of the disease at diagnosis could be explained by the fact that transplanted patients were less intensively treated and responded less to chemotherapy.

Revisiting the changes in the Banff classification for antibody-mediated rejection after kidney transplantation.

The Banff classification for antibody-mediated rejection (ABMR) has undergone important changes, mainly by inclusion of C4d-negative ABMR in Banff'13 and elimination of suspicious ABMR (sABMR) with the use of C4d as surrogate for HLA-DSA in Banff'17. We aimed to evaluate the numerical and prognostic repercussions of these changes in a single-center cohort study of 949 single kidney transplantations, comprising 3662 biopsies that were classified according to the different versions of the Banff classification. Overall, the number of ABMR and sABMR cases increased from Banff'01 to Banff'13. In Banff'17, 248 of 292 sABMR biopsies were reclassified to No ABMR, and 44 of 292 to ABMR. However, reclassified sABMR biopsies had worse and better outcome than No ABMR and ABMR, which was mainly driven by the presence of microvascular inflammation and absence of HLA-DSA, respectively. Consequently, the discriminative performance for allograft failure was lowest in Banff'17, and highest in Banff'13. Our data suggest that the clinical and histological heterogeneity of ABMR is inadequately represented in a binary classification system. This study provides a framework to evaluate the updates of the Banff classification and assess the impact of proposed changes on the number of cases and risk stratification. Two alternative classifications introducing an intermediate category are explored.

Diagnostic performance of kSORT, a blood-based mRNA assay for noninvasive detection of rejection after kidney transplantation: A retrospective multicenter cohort study.

The Kidney Solid Organ Response Test (kSORT) blood gene expression assay was developed to noninvasively detect acute rejection (AR) after kidney transplantation. Its performance in a setting with natural disease prevalence has not been evaluated. A retrospective, multicenter cohort study was conducted across all single kidney transplant recipients, transplanted between 2011 and 2015, with samples within the first year after transplantation available in existing biobanks. The primary objective was to determine the diagnostic performance of the kSORT assay to detect AR (T cell-mediated and/or antibody-mediated rejection) as compared to a concomitant renal biopsy. AR was reported on the concomitant biopsy in 188 of 1763 (10.7%) blood samples and any rejection (including borderline changes) in 614 of 1763 (34.8%) blood samples. In 320 of 1763 samples (18.2%) the kSORT risk category was indeterminate. The kSORT assay had no diagnostic value for AR (area under the curve [AUC] 0.51, 95% confidence interval [CI] 0.50-0.56; P = .46) overall, or when considering indication biopsies (N = 487) and protocol-specified biopsies (N = 1276) separately (AUC of 0.53, 95% CI 0.50-0.59, P = .44 and 0.55, 95% CI 0.50-0.61, P = .09, respectively). This large retrospective study utilizing samples obtained under real-world clinical conditions, was unable to validate the kSORT assay for detection of AR in the first year after transplantation.

Missing Self-Induced Activation of NK Cells Combines with Non-Complement-Fixing Donor-Specific Antibodies to Accelerate Kidney Transplant Loss in Chronic Antibody-Mediated Rejection.

BACKGROUND: Binding of donor-specific antibodies (DSAs) to kidney allograft endothelial cells that does not activate the classic complement cascade can trigger the recruitment of innate immune effectors, including NK cells. Activated NK cells contribute to microvascular inflammation leading to chronic antibody-mediated rejection (AMR). Recipient NK cells can also trigger antibody-independent microvascular inflammation by sensing the absence of self HLA class I molecules ("missing self") on allograft endothelial cells. This translational study investigated whether the condition of missing self amplifies DSA-dependent NK cell activation to worsen chronic AMR. METHODS AND RESULTS: Among 1682 kidney transplant recipients who underwent an allograft biopsy at Lyon University Hospital between 2004 and 2017, 135 fulfilled the diagnostic criteria for AMR and were enrolled in the study. Patients with complement-fixing DSAs identified by a positive C3d binding assay (n=73, 54%) had a higher risk of transplant failure (P=0.002). Among the remaining patients with complement-independent chronic AMR (n=62, 46%), those in whom missing self was identified through donor and recipient genotyping exhibited worse allograft survival (P=0.02). In multivariable analysis, only proteinuria (HR: 7.24; P=0.01) and the presence of missing self (HR: 3.57; P=0.04) were independent predictors for transplant failure following diagnosis of chronic AMR. Cocultures of human NK cells and endothelial cells confirmed that addition of missing self to DSA-induced NK cell activation increased endothelial damage. CONCLUSIONS: The assessment of missing self at the time of diagnosis of chronic AMR identifies patients at higher risk for kidney transplant failure.

Transcriptional Changes in Kidney Allografts with Histology of Antibody-Mediated Rejection without Anti-HLA Donor-Specific Antibodies.

BACKGROUND: Circulating donor-specific anti-HLA antibodies (HLA-DSAs) are often absent in serum of kidney allograft recipients whose biopsy specimens demonstrate histology of antibody-mediated rejection (ABMR). It is unclear whether cases involving ABMR histology without detectable HLA-DSAs represent a distinct clinical and molecular phenotype. METHODS: In this multicenter cohort study, we integrated allograft microarray analysis with extensive clinical and histologic phenotyping from 224 kidney transplant recipients between 2011 and 2017. We used the term ABMR histology for biopsy specimens that fulfill the first two Banff 2017 criteria for ABMR, irrespective of HLA-DSA status. RESULTS: Of 224 biopsy specimens, 56 had ABMR histology; 26 of these (46.4%) lacked detectable serum HLA-DSAs. Biopsy specimens with ABMR histology showed overexpression of transcripts mostly related to IFNγ-induced pathways and activation of natural killer cells and endothelial cells. HLA-DSA-positive and HLA-DSA-negative biopsy specimens with ABMR histology displayed similar upregulation of pathways and enrichment of infiltrating leukocytes. Transcriptional heterogeneity observed in biopsy specimens with ABMR histology was not associated with HLA-DSA status but was caused by concomitant T cell-mediated rejection. Compared with cases lacking ABMR histology, those with ABMR histology and HLA-DSA had higher allograft failure risk (hazard ratio [HR], 7.24; 95% confidence interval [95% CI], 3.04 to 17.20) than cases without HLA-DSA (HR, 2.33; 95% CI, 0.85 to 6.33), despite the absence of transcriptional differences. CONCLUSIONS: ABMR histology corresponds to a robust intragraft transcriptional signature, irrespective of HLA-DSA status. Outcome after ABMR histology is not solely determined by the histomolecular presentation but is predicted by the underlying etiologic factor. It is important to consider this heterogeneity in further research and in treatment decisions for patients with ABMR histology.