A Memory B Cell Crossmatch Assay for Quantification of Donor-Specific Memory B Cells in the Peripheral Blood of HLA-Immunized Individuals

Tracking Circulating HLA-Specific IgG-Producing Memory B Cells with the B-Cell ImmunoSpot Assay

Donor-specific antibodies (DSA) against human leukocyte antigen (HLA) molecules are a major risk factor for rejection of transplanted organs (in antibody-mediated rejection [ABMR]), particularly in patients who have prior sensitization or receive insufficient immunosuppression through minimization or noncompliance. These DSA are measured routinely in the serum of patients prior to transplantation mainly using bead-based technologies or cell-based assays. However, the absence of detectable serum DSA does not always reflect the absence of sensitization or histologically defined ABMR, and so it has been proposed that the detection and measurement of memory B cells capable of secreting antibodies against donor HLA antigens could be carried out using B-cell ImmunoSpot, to better inform the degree of immune sensitization of transplant patients prior to as well as after transplantation. Such an assay is described here.

BSHI and BTS UK guideline on the detection of alloantibodies in solid organ (and islet) transplantation

Solid organ transplantation represents the best (and in many cases only) treatment option for patients with end-stage organ failure. The effectiveness and functioning life of these transplants has improved each decade due to surgical and clinical advances, and accurate histocompatibility assessment. Patient exposure to alloantigen from another individual is a common occurrence and takes place through pregnancies, blood transfusions or previous transplantation. Such exposure to alloantigen's can lead to the formation of circulating alloreactive antibodies which can be deleterious to solid organ transplant outcome. The purpose of these guidelines is to update to the previous BSHI/BTS guidelines 2016 on the relevance, assessment, and management of alloantibodies within solid organ transplantation.

Alloreactive memory B cell detection by flow cytometric cross match using polyclonally activated memory B cell culture supernatants

In addition to alloantibodies, alloreactive memory B cell (mBC) evaluation has a potential for immunological risk assessment during transplantation processes. For the alloreactive mBCs evaluation currently, direct Flow Cytometric (FC) analysis using the HLA tetramer staining is an option. Evaluation of alloantibodies produced by the polyclonally stimulated alloreactive mBCs in in vitro culture system seems to be another useful approach, but this needs further downstream applications. In this study, we investigated the usefulness of the Flow Cytometric Cross Match (FCXM-supernatant) in which in vitro polyclonally activated mBCs culture supernatants and potential donor's lymphocytes being used for the mBC detection. FCXM-supernatant assays were performed between culture supernatants of polyclonally activated mBCs obtained from 4 allosensitized multiparous women and 14 renal transplant patients, and their non-alloimmunized spouses' or donors' lymphocytes, and vice versa. HLA typing was performed by SSP method. Anti-HLA antibodies produced by in vitro activated alloreactive mBCs were also evaluated by the Luminex assays. The success of in vitro polyclonal activation of mBCs was evaluated by a total IgG ELISA test and antibody secreting cell analyses by FC. Donor specific alloreactive mBCs were detected by FCXM-supernatant in 45% of the 18 allosensitized cases. Detection rate was 85% (6 out of 7) in the strongly allosensitized cases. No alloreactive mBCs was detected in control cases without allosensitization. FCXM-supernatant negative results of the allosensitized cases were related to low level of allosensitization and insufficient polyclonal stimulation evaluated by total IgG antibody tests of the supernatants. We herein report a practical methodology for alloreactive mBC detection as a donor specific manner using the FCXM-supernatant assay so that this would easily be transformed into a routine test performed in tissue typing laboratories.

Adaptive immune cell responses as therapeutic targets in antibody-mediated organ rejection

Humoral alloimmunity of organ transplant recipient to donor can lead to antibody-mediated rejection (ABMR), causing thousands of organ transplants to fail each year worldwide. However, the mechanisms of adaptive immune cell responses at the basis of humoral alloimmunity have not been entirely understood. In this review, we discuss how recent investigations have uncovered the key contributions of T follicular helper (TFH) and B cells and their coordinated actions in driving donor-specific antibody generation and immune progression towards ABMR. We show how recognition of the role of TFH-B cell interactions may allow the elaboration of improved clinical strategies for immune monitoring and the identification of novel therapeutic targets to tackle ABMR that will ultimately improve organ transplant survival.

ELISpot assay and prediction of organ transplant rejection

The ELISpot assay is a sensitive technique applied to assess cytokine-producing memory/effector T cells and human leukocyte antigens (HLA)-specific IgG-producing B cells. Besides the fact that the method is laborious and is difficult to standardise between laboratories, it may provide valuable information on the immune response of recipients before and after organ transplantation. In this article, we briefly review the recent literature and discuss the clinical significance of the ELISpot assay in predicting the risk and incidence of allograft rejection and survival.

Positive flow cytometry crossmatch with discrepant antibody testing results following COVID-19 vaccination

The impact of COVID-19 vaccination on the alloimmunity of transplant candidates is unknown. We report a case of positive B cell flow cytometry crossmatch in a patient waiting for second kidney transplantation, 37 days after receiving the COVID-19 vaccine. The preliminary crossmatch, using sample collected before COVID-19 vaccination, was negative. The antibodies to mismatched donor HLA-DR7 were detected only with multi-antigen beads but not with single-antigen beads, excluding possible prozone effects in solid-phase antibody assays. The crossmatches were positive with HLA-DR7-positive surrogates (n = 2) while negative with HLA-DR7-negative surrogates (n = 3), which confirms the HLA-DR7 alloreactivity. The antigen configurations on B lymphocytes are similar to that on the multi-antigen beads while distinct from the single-antigen beads. HLA-DR7 was the repeating mismatched antigen with the failing first kidney allograft. The newly emerged antibody to HLA-DR7 probably is the consequence of bystander activation of memory response by the COVID-19 vaccination. This case highlights the importance of verifying allo-sensitization history and utilizing multiple assays, including cell-based crossmatch and solid-phase assays with multi-antigens. COVID-19 immunization may deserve special attention when assessing the immunological risk before and after organ transplantation.

Outcome of Husband-to-Wife Kidney Transplantation With Mutual Children: Single Center Experience Using T Cell-Depleting Induction and Review of the Literature

Few data on husband-to-wife transplantations with mutual children (H2W) exist in the current era. We investigated the outcome of H2W transplantations (n = 25) treated with T cell-depleting induction compared to women with prior pregnancies also receiving their first HLA-mismatched kidney transplant, but from a different donor source: (i) other living donor (n = 52) and (ii) deceased donor (n = 120). Seventy-four percent of the women had ≥2 pregnancies; median follow-up time was 5 years. Death-censored allograft survival was significantly lower in the H2W group compared to the other two groups (p = 0.03). Three of four graft losses in the H2W group were due to rejection. 5-year patient survival in the H2W group was high and similar compared to the other living donor group (100 vs. 98%; p = 0.28). The incidence of (sub)clinical antibody-mediated rejection was higher in the H2W group (36 vs. 20 vs. 18%) (p = 0.10). The frequency of infections was similar among the three groups. No immunological parameter was predictive for rejection or graft loss in H2W transplantations. In conclusion, H2W transplantation is a valuable option, but associated with a higher risk for allograft loss due to rejection despite T cell-depleting induction. Further research is required for better risk prediction on an individual patient level.

Approaches for the characterization of clinically relevant pre-transplant human leucocyte antigen (HLA) antibodies in solid organ transplant patients

The avoidance of antibody-mediated rejection (AMR) attributed to human leucocyte antigen (HLA) antibody incompatibility remains an essential function of clinical Histocompatibility and Immunogenetics (H&I) laboratories who are supporting solid organ transplantation. Developments in HLA antibody identification assays over the past thirty years have greatly reduced unexpected positive cellular crossmatches and improved solid organ transplant outcomes. For sensitized patients, the decision to register unacceptable HLA antigen mismatches is often heavily influenced by results from solid phase antibody assays, particularly the Luminex^® Single Antigen Bead (SAB) assays, although the clinical relevance of antibodies identified solely by these assays remains unclear. As such, the identification of non-clinically relevant antibodies may proportionally increase the number of unacceptable transplant mismatches registered, with an associated increase in waiting time for a compatible organ. We reflect on the clinical relevance of antibodies identified solely by the Luminex SAB^® assays and consider whether the application of additional assays and/or tools could further develop our ability to define the clinical relevance of antibodies identified in patient sera. Improvements in this area would assist equity of access to a compatible transplant for highly sensitized patients awaiting a solid organ transplant.

Memory B Cells in Pregnancy Sensitization

Memory B cells play an important role in immunity to pathogens as these cells are poised to rapidly differentiate into antibody-secreting cells upon antigen re-encounter. Memory B cells also develop over the course of HLA-sensitization during pregnancy and transplantation. In this review, we discuss the potential contribution of memory B cells to pregnancy sensitization as well as the impact of these cells on transplant candidacy and outcomes. We start by summarizing how B cell subsets are altered in pregnancy and discuss what is known about HLA-specific B cell responses given our current understanding of fetal antigen availability in maternal secondary lymphoid tissues. We then review the molecular mechanisms governing the generation and maintenance of memory B cells during infection - including the role of T follicular helper cells - and discuss the experimental evidence for the development of these cells during pregnancy. Finally, we discuss how memory B cells impact access to transplantation and transplant outcomes for a range of transplant recipients.

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.

Generation and reactivity analysis of human recombinant monoclonal antibodies directed against epitopes on HLA-DR

In kidney transplantation, eplet mismatches between donor and recipient have been associated with de novo donor-specific antibody development. Eplets are theoretically defined configurations of polymorphic amino acids and require experimental verification to establish whether they can be bound by alloantibodies. Human HLA-specific monoclonal antibodies (mAbs) have been instrumental for this purpose but are largely lacking for HLA class II. In this study, we isolated single HLA-DR-specific memory B cells from peripheral blood of immunized individuals (n = 3) using HLA class II tetramers to generate recombinant human HLA-DR antigen-reactive mAbs (n = 5). Comparison of the amino acid composition of the reactive HLA alleles in relation to the antibody reactivity patterns led to identification of 3 configurations, 70Q 73A, 31F 32Y 37Y, and 14K 25Q recognized, respectively, by HLA-DRB1*01:01, HLA-DRB1*04:01, and HLA-DRB1*07:01 antigen-reactive mAbs. The first 2 correspond to eplets 70QA and 31FYY and can now be considered antibody verified. The latter indicates that eplet 25Q needs to be redefined before being considered as antibody verified. Generation and reactivity analysis of human HLA-DR mAbs allowed for identification of amino acid configurations corresponding to known eplets, whereas the other patterns may be used to redefine eplets with similar, but not identical predicted amino acid composition.

Sensitization in transplantation: Assessment of risk (STAR) 2019 Working Group Meeting Report

The purpose of the STAR 2019 Working Group was to build on findings from the initial STAR report to further clarify the expectations, limitations, perceptions, and utility of alloimmune assays that are currently in use or in development for risk assessment in the setting of organ transplantation. The goal was to determine the precision and clinical feasibility/utility of such assays in evaluating both memory and primary alloimmune risks. The process included a critical review of biologically driven, state-of-the-art, clinical diagnostics literature by experts in the field and an open public forum in a face-to-face meeting to promote broader engagement of the American Society of Transplantation and American Society of Histocompatibility and Immunogenetics membership. This report summarizes the literature review and the workshop discussions. Specifically, it highlights (1) available assays to evaluate the attributes of HLA antibodies and their utility both as clinical diagnostics and as research tools to evaluate the effector mechanisms driving rejection; (2) potential assays to assess the presence of alloimmune T and B cell memory; and (3) progress in the development of HLA molecular mismatch computational scores as a potential prognostic biomarker for primary alloimmunity and its application in research trial design.

Recent Advances on Biomarkers of Early and Late Kidney Graft Dysfunction

New biomarkers of early and late graft dysfunction are needed in renal transplant to improve management of complications and prolong graft survival. A wide range of potential diagnostic and prognostic biomarkers, measured in different biological fluids (serum, plasma, urine) and in renal tissues, have been proposed for post-transplant delayed graft function (DGF), acute rejection (AR), and chronic allograft dysfunction (CAD). This review investigates old and new potential biomarkers for each of these clinical domains, seeking to underline their limits and strengths. OMICs technology has allowed identifying many candidate biomarkers, providing diagnostic and prognostic information at very early stages of pathological processes, such as AR. Donor-derived cell-free DNA (ddcfDNA) and extracellular vesicles (EVs) are further promising tools. Although most of these biomarkers still need to be validated in multiple independent cohorts and standardized, they are paving the way for substantial advances, such as the possibility of accurately predicting risk of DGF before graft is implanted, of making a “molecular” diagnosis of subclinical rejection even before histological lesions develop, or of dissecting etiology of CAD. Identification of “immunoquiescent” or even tolerant patients to guide minimization of immunosuppressive therapy is another area of active research. The parallel progress in imaging techniques, bioinformatics, and artificial intelligence (AI) is helping to fully exploit the wealth of information provided by biomarkers, leading to improved disease nosology of old entities such as transplant glomerulopathy. Prospective studies are needed to assess whether introduction of these new sets of biomarkers into clinical practice could actually reduce the need for renal biopsy, integrate traditional tools, and ultimately improve graft survival compared to current management.

Pre-existing Alloreactive T and B Cells and Their Possible Relevance for Pre-transplant Risk Estimation in Kidney Transplant Recipients

In allogeneic transplantation, genetic disparities between patient and donor may lead to cellular and humoral immune responses mediated by both naïve and memory alloreactive cells of the adaptive immune system. This review will focus on alloreactive T and B cells with emphasis on the memory compartment, their role in relation to kidney rejection, and in vitro assays to detect these alloreactive cells. Finally, the potential additional value of utilizing donor-specific memory T and B cell assays supplementary to current routine pre-transplant risk assessment of kidney transplant recipients will be discussed.

What have we learned about how to prevent and treat antibody-mediated rejection in kidney transplantation?

Antibody-mediated rejection (ABMR) in kidney transplantation is a major cause of late graft loss, and despite all efforts to date the "standard of care" remains plasmapheresis, IVIg, and steroids, which itself is based on low quality evidence. This review focuses on the risk factors leading to memory and de novo donor-specific antibody (DSA)-associated ABMR, the optimal prevention strategies for ABMR, and advances in  adjunctive and emerging therapies for ABMR. Because new agents require regulatory approval via a Phase 3 randomized control trial (RCT), an overview of progress in innovative trial design for ABMR is provided. Finally, based on the insights gained in the biology of ABMR, current knowledge gaps are identified for future research that could significantly affect our understanding of how to optimally treat ABMR.

HLA-specific memory B-cell detection in kidney transplantation: Insights and future challenges

Humoral alloimmunity mediated by anti‐human leucocyte antigen (HLA) antibodies is a major challenge in kidney transplantation and impairs the longevity of the transplanted organ. The immunological risk of an individual patient is currently mainly assessed by detection of HLA antibodies in the serum, which are produced by long‐lived bone marrow‐residing plasma cells. However, humoral alloimmunity is complex, and alloreactive memory B cells constitute an additional factor in the interplay of immune cells. These recirculating “silent” cells are responsible for the immunological recall response by differentiating into antibody‐producing cells upon antigen re‐encounter. Historically, due to the lack of appropriate and routinely applicable assays to determine the presence and HLA specificity of alloreactive memory B cells, their contribution to the humoral alloimmune response has clinically often been suspected but could not be determined. In this review, we give an overview of recent advances in techniques to detect alloreactive memory B cells and discuss their strengths and limitations. Furthermore, we summarize experiences with these techniques in alloimmunized individuals and transplant recipients, thereby emphasizing unmet needs to be addressed in future studies.

Biomarkers of rejection in kidney transplantation

PURPOSE OF REVIEW

To provide an update of the literature on the use of new biomarkers of rejection in kidney transplant recipients.

RECENT FINDINGS

The kidney allograft biopsy is currently considered the gold standard for the diagnosis of rejection. However, the kidney biopsy is invasive and could be indeterminate. A significant progress has been made in discovery of new biomarkers of rejection, and some of them have been introduced recently for potential use in clinical practice including measurement of serum donor-derived cell free DNA, allo-specific CD154 + T-cytotoxic memory cells, and gene-expression 'signatures'. The literature supports that these biomarkers provide fair and reliable diagnostic accuracy and may be helpful in clinical decision-making when the kidney biopsy is contraindicated or is inconclusive.

SUMMARY

The new biomarkers provide a promising approach to detect acute rejections in a noninvasive way.

An Easy and Sensitive Method to Profile the Antibody Specificities of HLA-specific Memory B Cells

Supplemental Digital Content is available in the text.

Background.

Pretransplant immunological risk assessment is currently based on donor–specific HLA antibodies in serum. Despite being an excellent source for antibodies produced by bone marrow–residing plasma cells, serum analysis does not provide information on the memory B–cell compartment. Although B–cell culture supernatants can be used to detect memory B cell–derived HLA antibodies, low IgG concentrations can preclude detectability of HLA antibodies in luminex single–antigen bead (SAB) assays.

Methods.

Culture supernatants of polyclonally activated B cells from alloantigen exposed (n = 13) or nonexposed (n = 10) individuals were either concentrated 10–fold, or IgG was isolated by using a protein G affinity purification method to increase the IgG concentration. These processed culture supernatants, as well as paired serum samples were tested for the presence of HLA antibodies using luminex SAB analysis.

Results.

In immunized individuals, 64% were found to have HLA–specific B–cell memory in concentrated supernatants, whereas 82% showed HLA–specific B–cell memory when IgG isolated supernatants were used for HLA antibody detection. IgG–isolated supernatants showed higher mean fluorescence intensity values compared with concentrated supernatants without increased background. In some individuals, HLA–specific B–cell memory was detected in the absence of accompanying serum antibody specificities.

Conclusions.

We developed a novel, highly sensitive method to assess the HLA–specific memory B–cell compartment using luminex SAB technology. This assay allows direct comparison to the serum compartment and may therefore provide a more complete picture of the humoral alloimmune response in patients with a history of alloantigen exposure.

Recombinant human monoclonal HLA antibodies of different IgG subclasses recognising the same epitope: Excellent tools to study differential effects of donor-specific antibodies

In the field of transplantation, the humoural immune response against mismatched HLA antigens of the donor is associated with inferior graft survival, but not in every patient. Donor‐specific HLA antibodies (DSA) of different immunoglobulin G (IgG) subclasses may have differential effects on the transplanted organ. Recombinant technology allows for the generation of IgG subclasses of a human monoclonal antibody (mAb), while retaining its epitope specificity. In order to enable studies on the biological function of IgG subclass HLA antibodies, we used recombinant technology to generate recombinant human HLA mAbs from established heterohybridomas. We generated all four IgG subclasses of a human HLA class I and class II mAb and showed that the different subclasses had a comparable affinity, normal human Fc glycosylation, and retained HLA epitope specificity. For both mAbs, the IgG1 and IgG3 isotypes were capable of binding complement component 3d (C3d) and efficient in complement‐dependent cell lysis against their specific targets, while the IgG2 and IgG4 subclasses were not able to induce cytotoxicity. Considering the fact that the antibody‐binding site and properties remained unaffected, these IgG subclass HLA mAbs are excellent tools to study the function of individual IgG subclass HLA class I and class II‐specific antibodies in a controlled fashion.

Summary of 2017 FDA Public Workshop: Antibody-mediated Rejection in Kidney Transplantation

Despite major advances in understanding the pathophysiology of antibody-mediated rejection (AMR); prevention, diagnosis and treatment remain unmet medical needs. It appears that early T cell-mediated rejection, de novo donor-specific antibody (dnDSA) formation and AMR result from patient or physician initiated suboptimal immunosuppression, and represent landmarks in an ongoing process rather than separate events. On April 12 and 13, 2017, the Food and Drug Administration sponsored a public workshop on AMR in kidney transplantation to discuss new advances, importance of immunosuppressive medication nonadherence in dnDSA formation, associations between AMR, cellular rejection, changes in glomerular filtration rate, and challenges of clinical trial design for the prevention and treatment of AMR. Key messages from the workshop are included in this summary. Distinction between type 1 (due to preexisting DSA) and type 2 (due to dnDSA) phenotypes of AMR needs to be considered in patient management and clinical trial design. Standardization and more widespread adoption of routine posttransplant DSA monitoring may permit timely diagnosis and understanding of the natural course of type 2 and chronic AMR. Clinical trial design, especially as related to type 2 and chronic AMR, has specific challenges, including the high prevalence of nonadherence in the population at risk, indolent nature of the process until the appearance of graft dysfunction, and the absence of accepted surrogate endpoints. Other challenges include sample size and study duration, which could be mitigated by enrichment strategies.

A multicolour HLA-specific B-cell FluoroSpot assay to functionally track circulating HLA-specific memory B cells

Emerging evidence suggests that donor-reactive memory B cells (mBC) play a key role inducing antibody-mediated rejection (ABMR) after solid organ transplantation and show a broader antigen repertoire than plasma cells thus, being potentially present even in absence of donor-specific antibodies. Therefore, the development of novel immune assays capable of quantifying circulating donor-reactive mBC in organ transplantation is highly warranted. We developed a novel HLA-specific B-cell FluoroSpot assay capable of enumerating multiple HLA-specific Antibody Secreting Cells (ASC) originated from circulating mBC after in-vitro polyclonal activation. We performed a thorough characterization of distinct selective in-vitro mBC activation methods based on either the TLR7,8 agonist R848 plus Interleukin-2 or an anti-CD40 agonist monoclonal antibody, assessed optimal activation culture conditions, cell sources, activation time-frame as well as the advantage of measuring HLA-specific IgG-ASC as compared to HLA-IgG Ab detected in supernatants of in-vitro stimulated B-cell to characterize anti-HLA alloreactivity. Notably, using fluorescently-labeled multimerized HLA molecules as detection matrix, we show the ability of this assay to precisely quantify multiple anti-HLA mBC specificities. In conclusion, evaluating circulating donor-reactive mBC using new technology may provide novel insight of the pathogenesis of humoral rejection and may help identifying transplant recipients at high risk of allograft rejection.

Novel aspects of epitope matching and practical application in kidney transplantation

This review describes the recent developments in the applicability and clinical significance of epitope matching in kidney transplantation. As incremental human leukocyte antigen (HLA) mismatches are associated with increased risk of rejection and allograft loss, HLA-matching remains one of the standard immunological triage tests to determine transplant suitability. Advancements in tissue-typing techniques have led to innovative concepts of HLA-matching at the epitope level. Epitopes are configurations of polymorphic amino acid residues that are recognized by B cells, and antibodies reactive with these epitopes lead to rejection and/or premature allograft loss. Even though there is substantial advancement in the characterization and understanding of epitopes, evidence supporting the added clinical benefit of epitope matching over traditional antigen matching and the ability to identify clinically relevant immunogenic epitopes remains poorly defined. We present an overview of the evidence surrounding the immunogenicity of mismatched donor epitopes and the applicability of HLA epitope matching in kidney transplantation. A better understanding of the immunogenicity and structural characteristics of HLA epitopes will guide clinicians to integrate epitope matching as an important parameter for donor selection in kidney transplantation.