I. Larsen

Prevention of Bleeding After Islet Transplantation: Lessons Learned from a Multivariate Analysis of 132 Cases at a Single Institution

Islet transplantation is being offered increasingly for selected patients with unstable type 1 diabetes. Percutaneous transhepatic portal access avoids a need for surgery, but is associated with potential risk of bleeding. Between 1999 and 2005, we performed 132 percutaneous transhepatic islet transplants in 67 patients. We encountered bleeding in 18/132 cases (13.6%). In univariate analysis, the risk of bleeding in the absence of effective track ablation was associated with an increasing number of procedures (2nd and 3rd procedures with an odds ratio (OR) of 9.5 and 20.9, respectively), platelets count <150 000 (OR 4.4), elevated portal pressure (OR 1.1 per mm Hg rise), heparin dose ≥45 U/kg (OR 9.8) and pre‐transplant aspirin (81 mg per day) (OR 2.6, p = 0.05). A multivariate analysis further confirmed the cumulative transplant procedure number (p < 0.001) and heparin dose ≥45 U/kg (p = 0.02) as independent risk factors for bleeding. Effective mechanical sealing of the intrahepatic portal catheter tract with thrombostatic coils and tissue fibrin glue completely prevented bleeding in all subsequent procedures (n = 26, p = 0.02). We conclude that bleeding after percutaneous islet implantation is an avoidable complication provided the intraparenchymal liver tract is sealed effectively.

ABO-incompatible heart transplantation in early childhood: An international multicenter study of clinical experiences and limits

BACKGROUND Intentional blood group (BG)-incompatible (ABOi) heart transplantation in childhood is emerging in many centers. Safety limits remain undetermined. In this multicenter study we have compiled experience on clinical and immunologic boundaries. METHODS Data from six centers in Europe and North America on ABOi transplantation were collected in a standardized survey. RESULTS Fifty-eight ABOi transplants were performed in 57 patients. Median age at transplant was 6.8 months (0.03 to 90 months); post-transplant follow-up was 37.7 months (0.46 to 117 months), accumulating 188 patient-years. Forty-seven percent of the patients received pretransplant mechanical circulatory support. Donors were either blood group A (n = 25), B (n = 18) or AB (n = 15). The median peak antibody titer to the donor BG pretransplant was 1:8 (0 to 1:64) for anti-A and 1:4 (0 to 1:32) for anti-B. Titers against the donor BG were lower post- than pretransplant in B recipients (p = 0.02), whereas third-party antibodies in BG O recipients developed normally post-transplant. Induction immunosuppression included anti-thymocyte globulin (61%), basiliximab (32%) or none (7%). All patients received calcineurin inhibitors, including 62% with mycophenolate mofetil, 10% with azathioprine, 2% with everolimus and 24% with steroids. There were 4 episodes of cellular rejection (Grade≥2R) and 7 antibody-mediated rejections. Five patients underwent antibody removal post-transplant. One patient developed severe graft vasculopathy. Freedom from death or retransplantation was 100%/96%/69% at 1/5/10 years. No graft loss was attributed to BG antibodies. CONCLUSIONS Successful ABOi heart transplantation can be performed at an older age and with higher isohemagglutinin titers than initially assumed and using similar immunosuppressive regimens as for ABO-compatible transplants. Rejection and graft vasculopathy are rare. Persistently low titers of antibodies to the donor BG post-transplant suggest elements of tolerance and/or accommodation.

Absence of Donor-Specific Anti-HLA Antibodies after ABO-Incompatible Heart Transplantation in Infancy – Altered Immunity or Age?

Specific B‐cell tolerance toward donor blood group antigens develops in infants after ABO‐incompatible heart transplantation, whereas their immune response toward protein antigens such as HLA has not been investigated. We assessed de novo HLA‐antibodies in 122 patients after pediatric thoracic transplantation (28 ABO‐incompatible) and 36 controls. Median age at transplantation was 1.7 years (1 day to 17.8 year) and samples were collected at median 3.48 years after transplantation. Antibodies were detected against HLA‐class I in 21 patients (17.2%), class II in 18 (14.8%) and against both classes in 10 (8.2%). Using single‐antigen beads, donor‐specific antibodies (DSAs) were identified in six patients (all class II, one additional class I). Patients with DSAs were significantly older at time of transplantation. In patients who had undergone pretransplant cardiac surgeries, class II antibodies were more frequent, although use of homografts or mechanical heart support had no influence. DSAs were absent in ABO‐incompatible recipients and class II antibodies were significantly less frequent than in children with ABO‐compatible transplants. This difference was present also when comparing only children transplanted below 2 years of age. Therefore, tolerance toward the donor blood group appears to be associated with an altered response to HLA beyond age‐related effects.

Discarded Human Thymus Is a Novel Source of Stable and Long-Lived Therapeutic Regulatory T Cells.

Regulatory T cell (Treg)–based therapy is a promising approach to treat many immune‐mediated disorders such as autoimmune diseases, organ transplant rejection, and graft‐versus‐host disease (GVHD). Challenges to successful clinical implementation of adoptive Treg therapy include difficulties isolating homogeneous cell populations and developing expansion protocols that result in adequate numbers of cells that remain stable, even under inflammatory conditions. We investigated the potential of discarded human thymuses, routinely removed during pediatric cardiac surgery, to be used as a novel source of therapeutic Tregs. Here, we show that large numbers of FOXP3+ Tregs can be isolated and expanded from a single thymus. Expanded thymic Tregs had stable FOXP3 expression and long telomeres, and suppressed proliferation and cytokine production of activated allogeneic T cells in vitro. Moreover, expanded thymic Tregs delayed development of xenogeneic GVHD in vivo more effectively than expanded Tregs isolated based on CD25 expression from peripheral blood. Importantly, in contrast to expanded blood Tregs, expanded thymic Tregs remained stable under inflammatory conditions. Our results demonstrate that discarded pediatric thymuses are an excellent source of therapeutic Tregs, having the potential to overcome limitations currently hindering the use of Tregs derived from peripheral or cord blood.

Chemical Basis for Qualitative and Quantitative Differences Between ABO Blood Groups and Subgroups: Implications for Organ Transplantation.

Blood group ABH(O) carbohydrate antigens are carried by precursor structures denoted type I–IV chains, creating unique antigen epitopes that may differ in expression between circulating erythrocytes and vascular endothelial cells. Characterization of such differences is invaluable in many clinical settings including transplantation. Monoclonal antibodies were generated and epitope specificities were characterized against chemically synthesized type I–IV ABH and related glycans. Antigen expression was detected on endomyocardial biopsies (n = 50) and spleen (n = 11) by immunohistochemical staining and on erythrocytes by flow cytometry. On vascular endothelial cells of heart and spleen, only type II–based ABH antigens were expressed; type III/IV structures were not detected. Type II–based ABH were expressed on erythrocytes of all blood groups. Group A1 and A2 erythrocytes additionally expressed type III/IV precursors, whereas group B and O erythrocytes did not. Intensity of A/B antigen expression differed among group A1, A2, A1B, A2B and B erythrocytes. On group A2 erythrocytes, type III H structures were largely un‐glycosylated with the terminal “A” sugar α‐GalNAc. Together, these studies define qualitative and quantitative differences in ABH antigen expression between erythrocytes and vascular tissues. These expression profiles have important implications that must be considered in clinical settings of ABO‐incompatible transplantation when interpreting anti‐ABO antibodies measured by hemagglutination assays with reagent erythrocytes.

Chemical Basis for Qualitative and Quantitative Differences Between ABO Blood Groups and Subgroups

Blood group ABH(O) carbohydrate antigens are carried by precursor structures denoted type I–IV chains, creating unique antigen epitopes that may differ in expression between circulating erythrocytes and vascular endothelial cells. Characterization of such differences is invaluable in many clinical settings including transplantation. Monoclonal antibodies were generated and epitope specificities were characterized against chemically synthesized type I–IV ABH and related glycans. Antigen expression was detected on endomyocardial biopsies (n = 50) and spleen (n = 11) by immunohistochemical staining and on erythrocytes by flow cytometry. On vascular endothelial cells of heart and spleen, only type II–based ABH antigens were expressed; type III/IV structures were not detected. Type II–based ABH were expressed on erythrocytes of all blood groups. Group A1 and A2 erythrocytes additionally expressed type III/IV precursors, whereas group B and O erythrocytes did not. Intensity of A/B antigen expression differed among group A1, A2, A1B, A2B and B erythrocytes. On group A2 erythrocytes, type III H structures were largely un‐glycosylated with the terminal “A” sugar α‐GalNAc. Together, these studies define qualitative and quantitative differences in ABH antigen expression between erythrocytes and vascular tissues. These expression profiles have important implications that must be considered in clinical settings of ABO‐incompatible transplantation when interpreting anti‐ABO antibodies measured by hemagglutination assays with reagent erythrocytes.

Statin therapy is not associated with improved outcomes after heart transplantation in children and adolescents

BACKGROUND Although used routinely, the pleiotropic benefits of statins remain understudied in children after heart transplantation. We hypothesized that statin therapy would reduce the incidence of rejection, cardiac allograft vasculopathy (CAV) and post-transplant lymphoproliferative disease (PTLD). METHODS This study was a retrospective review of 964 pediatric (ages 5 to 18 years) heart transplant recipients in the multicenter Pediatric Heart Transplant Study registry from 2001 to 2012. Patients were excluded if they were undergoing re-transplantation, survived <1 year post-transplant, or had missing data regarding statin use. The effects of statins beyond the first year were estimated by Kaplan-Meier and Cox regression multivariable analysis for freedom from PTLD, rejection requiring treatment, any severity of CAV, and survival. RESULTS Statin use was variable among participating centers with only 30% to 35% of patients ≥10 years of age started on a statin at <1 year post-transplant. After the first year post-transplant, statin-treated children (average age at transplant 13.24 ± 3.29 years) had significantly earlier rejection (HR 1.42, 95% CI 1.11 to 1.82, p = 0.006) compared with untreated children (transplanted at 12 ± 3.64 years) after adjusting for conventional risk factors for rejection. Freedom from PTLD, CAV and overall survival up to 5 years post-transplant were not affected by statin use, although the number of events was small. CONCLUSIONS Statin therapy did not confer a survival benefit and was not associated with delayed onset of PTLD or CAV. Early (<1 year post-transplant) statin therapy was associated with increased later frequency of rejection. These findings suggest that a prospective trial evaluating statin therapy in pediatric heart transplant recipients is warranted.

ABH‐glycan microarray characterizes ABO subtype antibodies: fine specificity of immune tolerance after ABO‐incompatible transplantation

Organ transplantation from ABO blood group–incompatible (ABOi) donors requires accurate detection, effective removal and subsequent surveillance of antidonor antibodies. Because ABH antigen subtypes are expressed differently in various cells and organs, measurement of antibodies specific for the antigen subtypes in the graft is essential. Erythrocyte agglutination, the century‐old assay used clinically, does not discriminate subtype‐specific ABO antibodies and provides limited information on antibody isotypes. We designed and created an ABO‐glycan microarray and demonstrated the precise assessment of both the presence and, importantly, the absence of donor‐specific antibodies in an international study of pediatric heart transplant patients. Specific IgM, IgG, and IgA isotype antibodies to nonself ABH subtypes were detected in control participants and recipients of ABO‐compatible transplants. Conversely, in children who received ABOi transplants, antibodies specific for A subtype II and/or B subtype II antigens—the only ABH antigen subtypes expressed in heart tissue—were absent, demonstrating the fine specificity of B cell tolerance to donor/graft blood group antigens. In contrast to the hemagglutination assay, the ABO‐glycan microarray allows detailed characterization of donor‐specific antibodies necessary for effective transplant management, representing a major step forward in precise ABO antibody detection.

Development of B-cell memory in early childhood and the impact on antigen-specific tolerance after heart transplantation

BACKGROUND Young children show better survival after heart transplant compared with older individuals and can receive heart transplants safely from ABO-incompatible donors. Children develop immunologic tolerance to donor ABH antigens reflected in persistent absence of specific antibodies. We hypothesized that immature T-independent B-cell response and lack of B-cell memory play a crucial role in tolerance of ABH antigens after ABOi transplants. METHODS We determined phenotypes of splenic lymphocytes from adults and children and peripheral blood from ABO-incompatible or ABO-compatible heart transplant recipients and control subjects by flow cytometry. In vitro immune response to T-independent stimulation, erythrocytes, and ABH antigens was assessed using proliferation assays. RESULTS A predominant role for CD27(+) B cells in T-independent activation was demonstrated; these cells were significantly less frequent in infants than older subjects. Only IgM(+)CD27(+) B cells proliferated in response to non-self erythrocytes. In blood, IgM(+) and switched IgM(-) memory B cells were rare in infants, increasing to near-adult levels in children 5 years old. IgM(+)CD27(+) B cells were significantly fewer in ABO-incompatible transplant recipients than in ABO-compatible recipients. CONCLUSIONS CD27(+) cells play a key role in T-independent B-cell activation. Response to ABH antigens is mediated by IgM(+)CD27(+) B cells, and donor ABO-specific tolerance after ABO-incompatible transplantation in children is facilitated by low prevalence of these cells. The pattern of B-cell memory development is altered after ABO-incompatible transplant. Memory B cells may be quantified to assess eligibility for ABO-incompatible transplant and represent a potential therapeutic target to extend the benefits of the immature immune system to older age groups.

Failure of neonatal B-cell tolerance induction by ABO-incompatible kidney grafts in piglets.

Background ABO-incompatible (ABOi) infant heart transplantation results in B-cell tolerance to graft A/B antigens, confirming human susceptibility to acquired immunologic or “neonatal” tolerance as described originally in murine models. Starting with this clinical observation, we sought to model neonatal ABOi organ transplantation to allow mechanistic studies of tolerance. Methods Plasma anti-A/B antibodies were measured over time in piglets to establish developmental antibody kinetics. Blood group O piglets received kidney allografts from group A (AO-incompatible) or group O (AO-compatible) donors under cyclosporine immunosuppression. Anti-A antibodies were measured serially after transplantation; A/H antigen expression and allograft rejection were assessed in graft biopsies. Results Anti-A antibodies developed in naïve piglets in a kinetic pattern analogous to human infants; anti-B remained low. After transplantation, anti-A antibodies developed similarly in AO-incompatible and AO-compatible groups and were not suppressed by cyclosporine. A/H antigen expression was persistent in all graft biopsies; however, A/H antigens were not detected in vascular endothelium. Cellular and antibody-mediated rejection was absent or minimal in early and late biopsies in both groups, with one exception. Conclusions Naturally delayed isohemagglutinin production in piglets is analogous to the developmental kinetics in human infants. However, in contrast to deficient anti-A antibody production as seen long-term after “A-into-O” infant heart transplant recipients, normal anti-A antibody production after “A-into-O” piglet kidney transplantation indicates that tolerance did not develop despite graft A antigen persistence. These findings suggest that the impact on the host immune system of exposure to nonself ABH antigens during early life in human heart versus porcine kidney grafts may depend on expression in vascular endothelium.