Identification and characterization of major proteins carrying ABO blood group antigens in the human kidney

Detecting and exploring kidney-derived extracellular vesicles in plasma

BACKGROUND

Extracellular vesicles (EVs) have received considerable attention as ideal biomarkers for kidney diseases. Most reports have focused on urinary EVs, that are mainly derived from the cells in the urinary tract. However, the detection and the application of kidney-derived EVs in plasma remains uncertain.

METHODS

We examined the kidney-derived small EVs (sEVs) in plasma that were supposedly released from renal mesangial and glomerular endothelial cells, using clinical samples from healthy controls and patients with kidney transplants. Plasma from healthy controls underwent ultracentrifugation, followed by on-bead flow cytometry, targeting α8 integrin, an antigen-specific to mesangial cells. To confirm the presence of kidney-derived sEVs in peripheral blood, plasma from ABO-incompatible kidney transplant recipients was ultracentrifuged, followed by western blotting for donor blood type antigens.

RESULTS

Immunohistochemistry and immunoelectron microscopy confirmed α8 integrin expression in kidney mesangial cells and their sEVs. The CD9-α8 integrin double-positive sEVs were successfully detected using on-bead flow cytometry. Western blot analysis further revealed transplanted kidney-derived sEVs containing blood type B antigens in non-blood type B recipients, who had received kidneys from blood type B donors. Notably, a patient experiencing graft kidney loss exhibited diminished signals of sEVs containing donor blood type antigens.

CONCLUSION

Our findings demonstrate the potential usefulness of kidney-derived sEVs in plasma in future research for kidney diseases.

Long-term CMV monitoring and chronic rejection in renal transplant recipients

Introduction

Cytomegalovirus (CMV) is well established to be an independent risk factor for graft loss after kidney transplantation (KTx). Monitoring for CMV in the chronic phase is not defined in the current guideline. The effects of CMV infection, including asymptomatic CMV viremia, in the chronic phase are unclear.

Methods

We performed a single-center retrospective study to investigate incidence of CMV infection in the chronic phase, defined as more than 1 year after KTx. We included 205 patients who received KTx between April 2004 and December 2017. The CMV pp65 antigenemia assays to detect CMV viremia were continuously performed every 1-3 months.

Results

The median duration of the follow-up was 80.6 (13.1-172.1) months. Asymptomatic CMV infection and CMV disease were observed in 30.7% and 2.9% in the chronic phase, respectively. We found that 10-20% of patients had CMV infections in each year after KTx which did not change over 10 years. The history of CMV infection in the early phase (within 1 year after KTx) and chronic rejection were significantly associated with CMV viremia in the chronic phase. CMV viremia in the chronic phase was significantly associated with graft loss.

Discussion

This is the first study to examine the incidence of CMV viremia for 10 years post KTx. Preventing latent CMV infection may decrease chronic rejection and graft loss after KTx.

Acquisition of Antibody Against Cytomegalovirus After Kidney Transplantation in Seronegative Recipients

BACKGROUND

Cytomegalovirus (CMV) infection is one of the most important infectious diseases affecting recipients of kidney transplantation (KTx). However, the timing of seroconversion for CMV infection in seronegative recipients remains unclear. We evaluated CMV infections in CMV-seronegative recipients and the time to acquire antibodies against CMV.

METHODS

We conducted a retrospective study of 228 recipients who underwent KTx between March 1988 and February 2018 at the Niigata University Hospital. The anti-CMV IgG antibody profile before and after transplantation was analyzed. Oral acyclovir or valganciclovir was used as prophylactic therapy for at least 6 months after transplantation. Cytomegalovirus infection was defined as CMV viremia detected using the CMV pp65 antigenemia assay.

RESULTS

In this study, 50 cases (21.9%) were CMV-seronegative recipients. Over a median follow-up period of 126.7 months, 68% (34/50) of CMV-seronegative recipients experienced CMV viremia or overt disease symptoms as the first episode of CMV infection. The median duration from transplant to CMV infection was 69.0 days (range, 22-7426). All the recipients who experienced CMV infections acquired seroconversion. The median duration from KTx to seroconversion was 7.2 months (range, 2.8-252.3). Almost all CMV-seronegative recipients could acquire anti-CMV IgG antibodies within 2.5 years. In seronegative recipients whose donors were seronegative, no CMV viremia was found, and none developed anti-CMV IgG antibodies.

CONCLUSIONS

In the clinical practice of CMV-seronegative recipients, we should consider that physicians must closely monitor the occurrence of CMV infection up until 2.5 years after KTx.

Optimal dose of rituximab in ABO-incompatible kidney transplantation in patients with low anti-A/B antibody titers: A single-center retrospective cohort study

BACKGROUND

The clinical outcomes of ABO-incompatible (ABOi) kidney transplantation have improved with the introduction of desensitization therapy with rituximab. However, rituximab prevents not only antibody-mediated rejection (AMR) but also increases the risk of adverse events, such as infection. For ABOi kidney transplantation in patients with low anti-A/B antibody titers, we previously used a rituximab-free desensitization protocol and then initiated a single dose of 100 mg rituximab in 2016. We retrospectively compared the outcomes of ABOi kidney transplantation in patients with low anti-A/B antibody titers before and after the introduction of rituximab.

METHODS

ABOi kidney transplantations (n = 142) in patients with low anti-A/B antibody titers between 2007 and 2021 were included. Patients were divided into two groups (with and without rituximab) for desensitization. The primary outcomes were the incidence of acute AMR and infection.

RESULTS

Sixty-six patients were desensitized without rituximab (rituximab-free group), and 76 were pretreated with 100 mg rituximab (rituximab group) before transplantation. The incidence of acute AMR was significantly lower in the rituximab group than in the rituximab-free group (.0% [0/76] vs. 7.6% [5/66], respectively; p = .047). Post-transplantation anti-A/B antibody titers were also lower in the rituximab group than in the rituximab-free group. There was no significant difference in the incidence of adverse events, including infections, between the two groups.

CONCLUSION

In ABOi kidney transplantation patients with low anti-A/B antibody titers, the desensitization protocol with a single dose of 100 mg rituximab was effective in preventing acute AMR without increasing the risk of other adverse events.

Hyporesponsiveness against donor's ABO antigens of renal grafts after ABO-incompatible kidney transplantation

BACKGROUND

ABO antigens expressed on the red blood cells (RBCs) are not identical to those expressed on the renal endothelial cells. The isohemagglutinin assay employing the RBCs is the gold standard for evaluating anti-ABO antibody (Ab) levels. However, it remains unclear whether the anti-ABO Abs detected by the isohemagglutinin assay after ABO-incompatible (ABOi) kidney transplantations (KTx) that are not associated with antibody-mediated rejection can bind to renal graft endothelial cells.

METHODS

Ninety plasma samples were collected from patients with stable graft function after ABO-compatible (ABOc) or ABOi KTx. Anti-ABO Ab titers were examined by both the isohemagglutinin assay and the CD31-ABO microarray, which was developed as a mimic of the ABO antigens expressed on the renal endothelial cells.

RESULTS

The antibody titers detected by the isohemagglutinin assay and the CD31-ABO microarray after the ABOc KTx relatively correlated with each other. However, the CD31-ABO microarray results showed low antibody levels against donor blood group antigens after ABOi KTx and did not correlate with the isohemagglutinin assay. In contrast, the antibody levels against non-donor blood group antigens after ABOi KTx were comparable to those after the ABOc KTx. Fourteen patients received graft biopsies, and no antibody-mediated rejection was observed in ABOi KTx recipients, except for two patients who had anti-donor-HLA Abs.

CONCLUSION

The present study suggested that the anti-ABO Abs detected by the isohemagglutinin assay after ABOi KTx with stable graft function were hyporeactive to the ABO antigen of graft renal endothelial cells.

Pretransplant BMI Should Be <25 in Japanese Kidney Transplant Recipients: A Single-Center Experience

BACKGROUND

The aim of this study was to determine the appropriate body mass index (BMI) in Japanese kidney transplant (KTx) recipients. We analyzed the effects of pre- and post-transplant (Tx) obesity on graft and patient survival, perioperative complications, post-transplant diabetes mellitus (PTDM), and cardiovascular disease (CVD) in Japanese KTx recipients.

METHODS

This retrospective study included 269 recipients who underwent KTx from 2008 through 2020 at Niigata University Hospital. Obesity was defined as a body mass index (BMI) ≥25 kg/m². We examined the association between pre- and post-Tx obesity and graft survival, patient survival, the incidence of PTDM and CVD, and perioperative surgical complications.

RESULTS

The graft survival rate was lower in the pre-Tx BMI ≥25 kg/m² group, although there was no significant difference in patient survival. There was no difference in graft and patient survival between the post-Tx BMI ≥25 kg/m² group and the <25 kg/m² group. A pre-Tx BMI ≥25 kg/m² was an independent risk factor for biopsy-proven allograft rejection. New-onset DM after transplantation was significantly more common in the BMI ≥25 kg/m² group than in the BMI <25 kg/m² group (36% vs 13%; P = .002). The incidence of CVD was significantly higher in the post-Tx BMI ≥30 kg/m² group than in the BMI <30 kg/m² group (50% vs 11%; P = .023). There were no differences in surgical operating time, intraoperative blood loss, or perioperative complications between the obese and non-obese groups.

CONCLUSION

Pre-Tx BMI ≥25 kg/m² may be a risk factor for allograft rejection and graft loss. Post-Tx BMI should be <25 kg/m² to reduce the risk for PTDM.

Protection of transplants against antibody-mediated injuries: from xenotransplantation to allogeneic transplantation, mechanisms and therapeutic insights

Long-term allograft survival in allotransplantation, especially in kidney and heart transplantation, is mainly limited by the occurrence of antibody-mediated rejection due to anti-Human Leukocyte Antigen antibodies. These types of rejection are difficult to handle and chronic endothelial damages are often irreversible. In the settings of ABO-incompatible transplantation and xenotransplantation, the presence of antibodies targeting graft antigens is not always associated with rejection. This resistance to antibodies toxicity seems to associate changes in endothelial cells phenotype and modification of the immune response. We describe here these mechanisms with a special focus on endothelial cells resistance to antibodies. Endothelial protection against anti-HLA antibodies has been described in vitro and in animal models, but do not seem to be a common feature in immunized allograft recipients. Complement regulation and anti-apoptotic molecules expression appear to be common features in all these settings. Lastly, pharmacological interventions that may promote endothelial cell protection against donor specific antibodies will be described.

Outcome of ABO-incompatible kidney transplantation using a modified desensitization protocol without plasmapheresis

OBJECTIVES

Several controversies regarding desensitization strategies for successful ABO-incompatible (ABOi) kidney transplantation still exist. This study aimed to investigate whether pretransplant anti-A/B antibody removal is mandatory in an ABOi kidney transplant recipient with low baseline isoagglutinin titers.

METHODS

We adopted a modified desensitization protocol with two doses of rituximab (RTX, 100 mg/body) without pretransplant antibody removal for ABOi kidney transplant recipients with a titer of ≤1:64 (group A; n = 35) and investigated the feasibility of this protocol by comparing it with the clinical outcomes of patients undergoing standard pretransplant plasmapheresis (group B; n = 21).

RESULTS

There was no significant difference in the rate of antibody-mediated rejection within the first month after transplantation between the two groups (11.4% in group A vs. 2% in group B, p = 0.6019). Moreover, no differences were observed in the short- and long-term graft outcomes between the groups. However, two major critical acute antibody-mediated events occurred in group A; one patient lost the graft due to hyperacute rejection, and the other patient developed thrombotic microangiopathy after surgery. Risk factors predicting these perioperative complications were not identified.

CONCLUSIONS

We conclude that not only B-cell depletion using RTX but also pretransplant antibody removal is still recommended even for patients with low isoagglutinin titers. In addition, a new diagnostic tool is needed for accurate risk stratification.

A Novel Method of CD31-Combined ABO Carbohydrate Antigen Microarray Predicts Acute Antibody-Mediated Rejection in ABO-Incompatible Kidney Transplantation

Isohemagglutinin assays employing red blood cells (RBCs) are the most common assays used to measure antibody titer in ABO-incompatible kidney transplantation (ABOi KTx). However, ABO antigens expressed on RBCs are not identical to those of kidney and antibody titers do not always correlate with clinical outcome. We previously reported that CD31 was the main protein linked to ABO antigens on kidney endothelial cells (KECs), which was different from those on RBCs. We developed a new method to measure antibody titer using a microarray of recombinant CD31 (rCD31) linked to ABO antigens (CD31-ABO microarray). Mass spectrometry analysis suggested that rCD31 and native CD31 purified from human kidney had similar ABO glycan. To confirm clinical use of CD31-ABO microarray, a total of 252 plasma samples including volunteers, hemodialysis patients, and transplant recipients were examined. In transplant recipients, any initial IgG or IgM antibody intensity >30,000 against the donor blood type in the CD31-ABO microarray showed higher sensitivity, specificity, positive predictive value, and negative predictive value of AABMR, compared to isohemagglutinin assays. Use of a CD31-ABO microarray to determine antibody titer specifically against ABO antigens expressed on KECs will contribute to precisely predicting AABMR or preventing over immunosuppression following ABOi KTx.

Multifaceted role of glycosylation in transfusion medicine, platelets, and red blood cells

Glycosylation is highly prevalent, and also one of the most complex and varied posttranslational modifications. This large glycan diversity results in a wide range of biological functions. Functional diversity includes protein degradation, protein clearance, cell trafficking, cell signaling, host-pathogen interactions, and immune defense, including both innate and acquired immunity. Glycan-based ABO(H) antigens are critical in providing compatible products in the setting of transfusion and organ transplantation. However, evidence also suggests that ABO expression may influence cardiovascular disease, thrombosis, and hemostasis disorders, including alterations in platelet function and von Willebrand factor blood levels. Glycans also regulate immune and hemostasis function beyond ABO(H) antigens. Mutations in glycogenes (PIGA, COSMC) lead to serious blood disorders, including Tn syndrome associated with hyperagglutination, hemolysis, and thrombocytopenia. Alterations in genes responsible for sialic acids (Sia) synthesis (GNE) and UDP-galactose (GALE) and lactosamine (LacNAc) (B4GALT1) profoundly affect circulating platelet counts. Desialylation (removal of Sia) is affected by human and pathogenic neuraminidases. This review addresses the role of glycans in transfusion medicine, hemostasis and thrombosis, and red blood cell and platelet survival.

Effect of simultaneous presence of anti-blood group A/B and -HLA antibodies on clinical outcomes in kidney transplantation across positive crossmatch: a nationwide cohort study

ABO-incompatible (ABOi) and positive crossmatch (XM) kidney transplantation (KT) have been considered immunologically challenging. The present study analyzed the clinical outcomes in XM positive KT based on ABO incompatibility. We used data from the Korea Organ Transplantation Registry, a nationwide database, and a single-center registry. A total of 263 patients with positive XM were divided into an ABO compatible (ABOc) & XM positive (ABOc/XM+, n = 176) group and an ABOi & XM positive (ABOi/XM+, n = 87) group. The overall rejection rate one year after KT was significantly higher in the ABOi/XM+ group than in the ABOc/XM+ group (P < 0.01). A total of four mortalities occurred, all in the ABOi/XM+ patients (P < 0.01). There were no differences in surgical complications or the occurrence of infection-related complications, including BK virus nephropathy. Multivariate analysis indicated that female vs. male (odds ratio (OR), 2.27; P = 0.03), DSA class I (MFI/1000) (OR, 1.10; P = 0.03), DSA class II (MFI/1000) (OR, 1.10; P < 0.01), and ABOi & XM+ status (OR, 2.38; P < 0.01) were significant risk factors for acute rejection during the year after transplantation. Overall graft survival was inferior in ABOi/XM+ patients than in ABOc/XM+ patients (P = 0.02). ABO incompatibility in XM-positive KT patients was found to be a significant risk factor for the development of rejection within one year after transplantation as well as for long-term graft survival. The anti-blood group A, B and anti-HLA antibodies may show synergistic activity.

Analysis of the prevalence of systemic de novo thrombotic microangiopathy after ABO-incompatible kidney transplantation and the associated risk factors

OBJECTIVES

To analyze the prevalence of systemic de novo thrombotic microangiopathy in ABO-incompatible kidney transplantation and risk factors associated with this condition.

METHODS

A total of 201 patients who received living-donor kidney transplantation (114 patients with ABO-identical kidney transplantation and 87 patients with ABO-incompatible kidney transplantation) were retrospectively analyzed. Systemic de novo thrombotic microangiopathy was diagnosed clinically according to the presence of thrombocytopenia with microangiopathic hemolytic anemia and pathological findings of thrombotic microangiopathy. Anti-A and anti-B antibodies were purified from human plasma, and these antibodies' bindings to human kidney were investigated in vitro.

RESULTS

ABO-incompatible kidney transplantation was a significant risk factor of systemic de novo thrombotic microangiopathy (odds ratio 55.9, 95% CI 1.8-8.9, P < 0.001) after transplantation. Multivariate logistic regression analysis showed that non-use of mycophenolate mofetil, pretreatment immunoglobulin G antibody titer ≥64-fold and pretransplant immunoglobulin M antibody titer ≥16-fold were significant risk factors for systemic de novo thrombotic microangiopathy in ABO-incompatible kidney transplantation. Microvascular inflammation of 1-h post-transplant biopsy could be observed more frequently in thrombotic microangiopathy patients than in non-thrombotic microangiopathy patients. Anti-A and anti-B antibodies purified from human plasma showed a strong in vitro reaction against human kidney when the antibody titer was ≥16-fold.

CONCLUSIONS

Antibody titer should be decreased to ≤16-fold until the day of ABO-incompatible kidney transplantation by desensitization therapy including mycophenolate mofetil. The 1-h biopsy results might help to diagnose systemic de novo thrombotic microangiopathy.

Biologic roles of the ABH and Lewis histo-blood group antigens part II: thrombosis, cardiovascular disease and metabolism

The ABH and Lewis antigens were among the first of the human red blood cell polymorphisms to be identified and, in the case of the former, play a dominant role in transfusion and transplantation. But these two therapies are largely twentieth-century innovations, and the ABH and related carbohydrate antigens are not only expressed on a very wide range of human tissues, but were present in primates long before modern humans evolved. Although we have learned a great deal about the biochemistry and genetics of these structures, the biological roles that they play in human health and disease are incompletely understood. This review and its companion, which appeared in a previous issue of Vox Sanguinis, will focus on a few of the biologic and pathologic processes which appear to be affected by histo-blood group phenotype. The first of the two reviews explored the interactions of two bacteria with the ABH and Lewis glycoconjugates of their human host cells, and described the possible connections between the immune response of the human host to infection and the development of the AB-isoagglutinins. This second review will describe the relationship between ABO phenotype and thromboembolic disease, cardiovascular disease states, and general metabolism.

Efficacy of 2 Doses of Rituximab on B-Cell and Antidonor Antibody and Outcomes of ABO-Incompatible Living-Donor Pediatric Kidney Transplant

OBJECTIVES

Rituximab treatment strategies vary in ABOincompatible pediatric kidney transplant recipients. Here, we present the efficacy of 2 doses of rituximab and subsequent outcomes in ABO-incompatible pediatric kidney transplant patients.

MATERIALS AND METHODS

Our study of ABO-incompatible pediatric kidney transplant recipients included 21 who were pretreated with desensitization that included 2 doses of 100 mg rituximab (rituximab group) at 10 and 1 day pretransplant and 14 who received splenectomy without rituximab (splenectomy group). Both groups received immunosuppression. Basiliximab was administered during transplant and 4 days posttransplant. Double-filtration plasmapheresis and/or plasma exchange procedures were performed pretransplant in those with higher antidonor antibody titers. CD19-positive and CD20-positive B cells were measured sequentially in the rituximab group. Maximum titers of antidonor antibody pre- and posttransplant, patient and graft survival, biopsy-proven rejection, and complications/infections were compared between groups.

RESULTS

In the rituximab group, CD19- and CD20-positive B cells were depleted on transplant, persistently depleted at 3 months, and under 5% until 1 year posttransplant. Maximum titers of antidonor antibodies decreased significantly posttranplant in the rituximab (P < .001) but not in the splenectomy group (P = .174), with maximum titers posttransplant significantly lower than shown in the splenectomy group (P < .001). No rituximab patients had clinical rejection, but 5 splenectomy group patients had clinical T-cell-mediated rejection, with 2 also having antibody-mediated rejection. Six in the rituximab group had cytomegalovirus viremia but no cytomegalovirus disease; however, 5 splenectomy group recipients had cytomegalovirus disease and viremia. In the rituximab group, 3 had late-onset neutropenia. One child died of hypertrophic cardio myopathy with a functioning graft; all others survived with no failed grafts. All splenectomy group children survived, although 2 had deteriorated graft function.

CONCLUSIONS

Two doses of rituximab were effective in long-term B-cell depletion to suppress antidonor antibodies. The possibility of late-onset neutropenia must be considered.

Acquired Downregulation of Donor-Specific Antibody Production After ABO-Incompatible Kidney Transplantation

The mechanism of long-term B cell immunity against donor blood group antigens in recipients who undergo ABO-incompatible (ABOi) living-donor kidney transplantation (LKTx) is unknown. To address this question, we evaluated serial anti-A and anti-B antibody titers in 50 adult recipients. Donor-specific antibody titers remained low (≤1:4) in 42 recipients (84%). However, antibodies against nondonor blood group antigens were continuously produced in recipients with blood type O. We stimulated recipients' peripheral blood mononuclear cells in vitro to investigate whether B cells produced antibodies against donor blood group antigens in the absence of graft adsorption in vivo. Antibodies in cell culture supernatant were measured using specific enzyme-linked immunosorbent assays (ELISAs). Thirty-five healthy volunteers and 57 recipients who underwent ABO-compatible LKTx served as controls. Antibody production in vitro against donor blood group antigens by cells from ABOi LKTx patients was lower than in the control groups. Immunoglobulin deposits were undetectable in biopsies of grafts of eight recipients with low antibody titers (≤1:4) after ABOi LKTx. One patient with blood type A1 who received a second ABOi LKTx from a type B donor did not produce B-specific antibodies. These findings suggest diminished donor-specific antibody production function in the setting of adult ABOi LKTx.

Successful ABO-Incompatible Renal Transplantation: Blood Group A1B Donor Into A2B Recipient With Anti-A1 Isoagglutinins

OBJECTIVES

Transplantation of the blood group A2B in a recipient was successfully performed in the setting of receiving a deceased donor kidney from an "incompatible" A1B donor.

METHODS

The donor and recipient were both typed for ABO blood group, including ABO genotyping. The donor and recipient were tested for ABO, non-ABO, and human leukocyte antigen (HLA) antibodies. The donor and recipient were typed for HLA antigens, including T- and B-flow cytometry crossmatch tests.

RESULTS

The recipient's RBCs were negative with A1 lectin, and immunoglobulin G anti-A1 was demonstrated in the recipient's plasma. The donor-recipient pair was a four-antigen HLA mismatch, but final T- and B-flow cytometry crossmatch tests were compatible. The transplant procedure was uneventful; the patient experienced immediate graft function with no episodes of rejection or readmissions more than 2 years later.

CONCLUSIONS

It may be safe to transplant across the A1/A2 blood group AB mismatch barrier in the setting of low titer anti-A1 isoagglutinins without the need for pretransplant desensitization even if the antibody produced reacts with anti-human globulin.

The ABO Histo-Blood Group and AKI in Critically Ill Patients with Trauma or Sepsis

BACKGROUND AND OBJECTIVE

ABO blood types are determined by antigen modifications on glycoproteins and glycolipids and associated with altered plasma levels of inflammatory and endothelial injury markers implicated in AKI pathogenesis. We sought to determine the association of ABO blood types with AKI risk in critically ill patients with trauma or sepsis.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We conducted two prospective cohort studies at an urban, academic, level I trauma center and tertiary referral center; 497 patients with trauma admitted to the surgical intensive care unit between 2005 and 2010 with an injury severity score >15 and 759 patients with severe sepsis admitted to the medical intensive care unit between 2008 and 2013 were followed for 6 days for the development of incident AKI. AKI was defined by Acute Kidney Injury Network creatinine and dialysis criteria.

RESULTS

Of 497 patients with trauma, 134 developed AKI (27%). In multivariable analysis, blood type A was associated with higher AKI risk relative to type O among patients of European descent (n=229; adjusted risk, 0.28 versus 0.14; risk difference, 0.14; 95% confidence interval, 0.03 to 0.24; P=0.02). Of 759 patients with sepsis, AKI developed in 326 (43%). Blood type A again conferred higher AKI risk relative to type O among patients of European descent (n=437; adjusted risk, 0.53 versus 0.40; risk difference, 0.14; 95% confidence interval, 0.04 to 0.23; P=0.01). Findings were similar when analysis was restricted to those patients who did not develop acute respiratory distress syndrome or were not transfused. We did not detect a significant association between blood type and AKI risk among individuals of African descent in either cohort.

CONCLUSIONS

Blood type A is independently associated with AKI risk in critically ill patients with trauma or severe sepsis of European descent, suggesting a role for ABO glycans in AKI susceptibility.

Strategies to overcome the ABO barrier in kidney transplantation

Kidney transplantation across the ABO blood group barrier was long considered a contraindication for transplantation, but in an effort to increase donor pools, specific regimens for ABO-incompatible (ABOi) transplantation have been developed. These regimens are now widely used as an integral part of the available treatment options. Various desensitization protocols, commonly based on transient depletion of preformed anti-A and/or anti-B antibodies and modulation of B-cell immunity, enable excellent transplant outcomes, even in the long-term. Nevertheless, the molecular mechanisms behind transplant acceptance facilitated by a short course of anti-humoral treatment are still incompletely understood. With the evolution of efficient clinical programmes, tailoring of recipient preconditioning based on individual donor-recipient blood type combinations and the levels of pretransplant anti-A/B antibodies has become possible. In the context of low antibody titres and/or donor A2 phenotype, immunomodulation and/or apheresis might be dispensable. A concern still exists, however, that ABOi kidney transplantation is associated with an increased risk of surgical and infectious complications, partly owing to the effects of extracorporeal treatment and intensified immunosuppression. Nevertheless, a continuous improvement in desensitization strategies, with the aim of minimizing the immunosuppressive burden, might pave the way to clinical outcomes that are comparable to those achieved in ABO-compatible transplantation.

Molecular deciphering of the ABO system as a basis for novel diagnostics and therapeutics in ABO incompatible transplantation

In recent years ABO incompatible kidney transplantation (KTx) has become a more or less clinical routine procedure with graft and patient survival similar to those of ABO compatible transplants. Antigen-specific immunoadsorption (IA) for anti-A and anti-B antibody removal constitutes in many centers an important part of the treatment protocol. ABO antibody titration by hemagglutination is guiding the treatment; both if the recipient can be transplanted as well as in cases of suspected rejections if antibody removal should be performed. Despite the overall success of ABO incompatible KTx, there is still room for improvements and an extension of the technology to include other solid organs. Based on an increased understanding of the structural complexity and tissue distribution of ABH antigens and the fine epitope specificity of the ABO antibody repertoire, improved IA matrices and ABO antibody diagnostics should be developed. Furthermore, understanding the molecular mechanisms behind accommodation of ABO incompatible renal allografts could make it possible to induce long-term allograft acceptance also in human leukocyte antigen (HLA) sensitized recipients and, perhaps, also make clinical xenotransplantation possible.

Blood group antigen-targeting peptide suppresses anti-blood group antibody binding to antigen in renal glomerular capillaries after ABO-incompatible blood reperfusion

BACKGROUND

Antibody-mediated rejection after ABO-incompatible kidney transplantation (ABO-I KTx) is a major barrier to transplantation success. The advent of immunosuppressive therapy has markedly improved graft survival in ABO-I KTx. However, compared with normal KTx, clinical conditions during ABO-I KTx are difficult to control because of overimmunosuppression. To reduce the need for immunosuppression, we aimed to develop a novel blood group antigen-neutralizing therapy.

METHODS

We screened for an ABO blood group antigen-targeting peptide (BATP) by screening of T7 phage-displayed peptide library. After screening, hemagglutination inhibition assays, enzyme-linked immunosorbent assay, and cytotoxicity assay were used to analyze the blood group antigen-blocking effect and toxicity of BATP. We also tested the inhibitory effects on anti-blood group antibody binding in normal human kidney tissues blocked with BATP and excised kidneys perfused ex vivo with BATP.

RESULTS

We identified six peptide sequences that efficiently suppressed hemagglutination of red blood cells by anti-ABO blood group antibodies and binding of these antibodies to ABO histo-blood group antigens in kidney tissues. Surprisingly, ex vivo perfusion of BATP in kidneys excised from renal cell carcinoma patients caused significant suppression of anti-blood group antibody binding to antigen and IgG and IgM deposition in renal glomerular capillaries after ABO-I blood reperfusion.

CONCLUSIONS

These data indicate that A/B blood group antigens on red blood cells and in kidney tissues may be neutralized by BATP. This approach may enable the development of a novel blood group antigen-neutralizing therapy to overcome the challenges of ABO-I KTx.

ABO-incompatible kidney transplantation

Owing to the shortage of deceased donors in Japan, since 1989, we have performed ABO-incompatible kidney transplantation (ABO-IKTx) to expand the indication for living donor kidney transplantation. During the past two decades, about 2000 ABO-IKTxs were performed. Since 2001 the success rate for these kidney transplants has reached 96% for 1-year, 91% for 5-year and 83% for 9-year graft survival, similar to outcomes of ABO-compatible kidney transplantation (ABO-CKTx). This dramatic improvement in results means that ABO-IKTx has become accepted as a therapeutic alternative for end-stage renal failure. Today ABO-IKTx accounts for approximately 30% of all living donor kidney transplantations performed in Japan. We have been making a lot of efforts to elucidate the mechanism of acute antibody-mediated rejection in ABOI-KTx in order to overcome the ABO barrier and to improve the outcome. From careful and precise clinical observations, proteomic analysis of ABO histo-blood group antigens in graft endothelial cells and deep insight into immunology and biology, we have reached the hypothesis that the structural difference of ABO histo-blood group antigens and de novo corresponding antibody production would be the key and keyhole of the development of acute AMR in ABOI-KTx. Preoperative desensitization therapy would be the best solution for the suppression of acute AMR and graft loss, which is now widespread and improves the outcome.

Effect of antibodies on endothelium

Patients developing posttransplant antibodies against HLA and non-HLA antigens expressed by the endothelium of the graft undergo more frequent episodes of rejection and have decreased long-term graft survival. Antibodies against the endothelium can alter/damage the cells of the graft through several mechanisms. Historically, antibodies were thought to elicit endothelial cell injury via complement-dependent mechanisms. New research has shown that antibodies can also contribute to the process of transplant rejection by stimulating proinflammatory and proproliferation signals. Antibody ligation leads to several functional alterations in EC including Weibel Palade body exocytosis, leukocyte recruitment, growth factor expression and cell proliferation. In contrast, under certain circumstances, antibodies may induce prosurvival signals and graft accommodation. The signaling events regulating accommodation vs. rejection appear to be influenced by the specificity and concentration of the anti-HLA antibody and the degree of molecular aggregation. Knowledge of the HLA and non-HLA antibody-mediated signaling pathways has the potential to identify new therapeutic targets to promote accommodation and prevent acute and chronic antibody-mediated rejection.