The virtual crossmatch--a screening tool for sensitized pediatric heart transplant recipients

Aspectos técnicos y clínicos de la prueba cruzada de histocompatibilidad en el trasplante de órganos sólidos

La presencia de anticuerpos dirigidos contra los antígenos leucocitarios humanos (Human Leukocyte Antigens, HLA) que se expresan en las células del donante, es uno de los factores de riesgo más importantes asociados con las complicaciones clínicas después del trasplante. La prueba cruzada es una de las pruebas de histocompatibilidad más eficaces para la detección de anticuerpos específicos contra el donante en los receptores de injertos. En los primeros métodos de la prueba cruzada, se utilizaba la citotoxicidad dependiente del complemento, que es útil para detectar dichos anticuerpos responsables del rechazo hiperagudo del injerto, pero carece de la sensibilidad adecuada. Por ello, se desarrollaron métodos de pruebas cruzadas más sensibles, entre ellas, la prueba cruzada por citometría de flujo que hoy se considera el método preferido.

En este artículo se revisa la evolución de la prueba cruzada y los factores más importantes que deben tenerse en cuenta al realizarla y al interpretar los resultados de esta prueba fundamental para la supervivencia a largo plazo del injerto.

The clinical utility and thresholds of Virtual and Halifaster Flow crossmatches in lung transplantation

INTRODUCTION

Immune sensitization, defined as the presence of alloreactive donor-specific antibodies (DSA), is associated with increased wait-times and inferior transplant outcomes. Identifying pre-transplant DSA with a physical cell-based assay is critical in defining immunological risk. However, improved solid phase antibody detection has provided the potential to forgo this physical assay. Here, we evaluated the association between DSA mean fluorescence intensity (MFI) and the recently introduced Halifaster Flow cytometry crossmatch (FXM) to determine if MFI could predict the outcome of FXM and whether a virtual crossmatch (VXM) would provide an accurate risk assessment.

METHODS

Sera from 134 waitlisted lung patients was retrospectively assessed by Halifaster FXM against lymphocytes preparations from 32 donors, resulting in 265 FXMs. HLA typing was performed to 2-field allelic level and Luminex single antigen beads (SAB) used to identify DSA. The association between FXM and Luminex MFI was calculated using ROC analysis. MFI threshold accuracy was confirmed using a separate validation cohort (174 recipient sera and 34 donors), whereby both virtual crossmatch (VXM) and FXMs were compared.

RESULTS

From the 265 FXM performed, 48 (18%) T-cell (TFXM) and 56 (21%) B-cell (BFXM) were positive. In the evaluation cohort, MFI thresholds of 2000 for HLA-A, B, DRB1 and >4000 for DQB1, were predictive of a positive FXM. The validation cohort of 233 paired FXM and VXM confirmed these MFI thresholds for both TFXM and BFXM with an accuracy of 91.4% and 89.3% respectively.

CONCLUSION

A positive VXM, defined with HLA-specific MFI thresholds predicts Halifaster FXM reactivity, and can potentially expedite organ allocation, by minimising the need for the more time-consuming flow cytometry crossmatch. This article is protected by copyright. All rights reserved.

Prevalence of Clinically Significant anti-HLA Antibodies in Renal Transplant Patients: Single-center Report from North India

Introduction:

Solid organ transplantation is the preferred therapeutic modality of treatment in patients affected by terminal organ failures. Human leukocyte antigens (HLAs) plays an important role in graft survival. In many of the cases of rejection, antibodies are directed against HLA antigens expressed on the cells of the transplanted organ. Pre-transplant compatibility testing involves the use of different methodologies for the determination of anti-HLA antibodies. Luminex single-antigen bead (SAB) assay demonstrates higher sensitivity and specificity in detecting anti-HLA antibodies. The aim of this study was to determine the prevalence of anti-HLA antibodies in pre-transplant work up recipients, planned for renal transplant at a tertiary care center in India.

Methods:

1640 patients visiting tertiary care hospital for pre-transplant compatibility testing were screened with complement-dependent micro-lymphocytotoxicity crossmatch (CDC-XM) and flow cytometric crossmatch (FC-XM). The patients positive for either or both screening tests were assayed with the Luminex SAB tests in order to establish defined antigen specificity of the alloantibodies and determining donor-specific antibody (DSA).

Results:

The two most frequent antibodies identified in each A, B, C locus of HLA class I were -A*24:03 (43.9%), A*25:01 (36.6%), B*57:01 (40.3%), B*15:12 (37.1%), C*17:01 (61.9%), C*07:01 (52.4%) and in DR, DQ DP locus in HLA class II were DRB1*09:01(40.0%), DRB1*14:04(37.6%), DQA1*04:01/DQB1*03:03 (58.4%), DQA1*05:01/DQB1*03:01 (55.1%), DPA1*02:01/DPB1*17:01 (55.0%), DPA1*02:01/DPB1*05:01 (45.0%).

Conclusion:

This study has found the prevalence and specificity of anti-HLA antibodies in north India.

Incidence and impact of allele-specific anti-HLA antibodies and high-resolution HLA genotyping on assessing immunologic compatibility

The ability to identify specific HLA molecules against which a patient has alloantibodies has revolutionized assessment of immunologic compatibility. Anti-HLA antibodies are typically evaluated as reactive against well-defined serologic antigen groups. Thus, donor HLA genotyping is aimed at defining HLA at the serologic split-antigen level to avoid incompatible antigen-antibody combinations. However, anti-HLA antibodies can have reactivities not accurately described by well-defined serologic antigens. While existence of these antibodies is acknowledged, their precise impact on clinical practice is not clear. We performed a single-center review of 2 years of pre-and post-transplant anti-HLA antibody testing data combined with high-resolution HLA genotyping data for living and deceased organ donors to evaluate the clinical impact of anti-HLA antibodies with reactivities outside of commonly defined serologic antigen groups. We find approximately 15% of patients awaiting transplantation have alloantibodies with differential reactivity for HLA proteins encoded by specific alleles within a serologic antigen group. Allele-specific antibodies are associated with positive cellular crossmatches not accurately predicted by standard donor HLA genotyping and can manifest as post-transplant donor-specific antibodies. Our data highlights the importance of evaluating anti-HLA antibodies at the allele-level and provides evidence supporting utility for high-resolution HLA genotyping in solid organ transplantation.

Out with the old, in with the new: Virtual versus physical crossmatching in the modern era

The virtual crossmatch (VXM) is gaining acceptance as an alternative approach to assess donor:recipient compatibility prior to transplantation. In contrast to a physical crossmatch, the virtual crossmatch does not require viable donor cells but rather relies on complete HLA typing of the donor and current antibody assessment of the recipient. Thus, the VXM can be performed in minutes which allows for faster transplant decisions thereby increasing the likelihood that organs can be shipped across significant distances yet safely transplanted. Here, we present a brief review of the past 50 years of histocompatibility testing; from the original complement-dependent cytotoxicity crossmatch in 1969 to the new era of molecular HLA typing, solid-phase antibody testing and virtual crossmatching. These advancements have shaped a paradigm shift in our approach to transplantation. That is, foregoing a prospective physical crossmatch in favor of a VXM. In this review, we undertake an in-depth analysis of the pros- and cons- of physical and virtual crossmatching.Finally, we provide objective data on the selected use of the VXM which demonstrate the value of a VXM in lieu of the traditional physical crossmatch for safe and efficient organ transplantation.

Management of the sensitized pediatric heart transplant patient

Despite advancements in transplant immunosuppression and techniques for managing critically ill patients awaiting heart transplantation, children who are immunologically sensitized to human leukocyte antigen remain at increased risk for morbidity and mortality, both while awaiting and after heart transplant. In this review we will discuss the epidemiology of sensitization, review the immunologic basis and methods of human leukocyte antigen antibody detection, describe outcomes for sensitized pediatric transplant candidates, and consider both pre- and post-transplant management options for sensitized patients.

Development of data-driven models for the flow cytometric crossmatch

HLA laboratories use virtual crossmatching (VXM) to predict recipient and donor compatibility using HLA antibody data and donor HLA type. Increasingly, transplant centers are utilizing VXM as the final compatibility determination prior to transplant. However, the VXM interpretation is based on HLA experience of individual transplant centers. This study developed data-driven algorithms that predicted flow cytometric crossmatch (FCXM) outcomes using HLA antibody mean fluorescent intensity (MFI) data and donor HLA typing without the need for human interpretation.Two algorithms were evaluated; an MFI Optimal-Threshold model and a Least-Squares-Fitting model. The Optimal-Threshold model correctly determined between 81.5% and 85.5% of T or B-cell responses. A class I antibody MFI threshold of 4670 was optimal for predicting T-cell response while an antibody MFI threshold of 6180 was optimal for predicting B-cell responses. HLA class I antibodies had a 1.47-fold greater influence on FCXM outcomes than class II antibodies. HLA-B antibodies influenced T and B-cell responses more than HLA-A or -C (-B > -A > -C). The Least-Squares-Fitting model increased accuracy to 94.1% and 88.8% for T and B-cell responses, respectively. The algorithms described here provide enhanced FCXM prediction and novel insights into the influence of specific HLA antibodies on the crossmatch outcome.

HLA in transplantation

The human major histocompatibility complex is a family of genes that encodes HLAs, which have a crucial role in defence against foreign pathogens and immune surveillance of tumours. In the context of transplantation, HLA molecules are polymorphic antigens that comprise an immunodominant alloreactive trigger for the immune response, resulting in rejection. Remarkable advances in knowledge and technology in the field of immunogenetics have considerably enhanced the safety of transplantation. However, access to transplantation among individuals who have become sensitized as a result of previous exposure to alloantigens is reduced proportional to the breadth of their sensitization. New approaches for crossing the HLA barrier in transplantation using plasmapheresis, intravenous immunoglobulin and kidney paired donation have been made possible by the relative ease with which even low levels of anti-HLA antibodies can now be detected and tracked. The development of novel protocols for the induction of tolerance and new approaches to immunomodulation was also facilitated by advances in HLA technology. Here, we review the progress made in understanding HLAs that has enabled organ transplantation to become a life-saving endeavour that is accessible even for sensitized patients. We also discuss novel approaches to desensitization, immunomodulation and tolerance induction that have the potential to further improve transplantation access and outcomes.

Study rationale, design, and pretransplantation alloantibody status: A first report of Clinical Trials in Organ Transplantation in Children-04 (CTOTC-04) in pediatric heart transplantation

Anti-HLA donor-specific antibodies are associated with worse outcomes after organ transplantation. Among sensitized pediatric heart candidates, requirement for negative donor-specific cytotoxicity crossmatch increases wait times and mortality. However, transplantation with positive crossmatch may increase posttransplantation morbidity and mortality. We address this clinical challenge in a prospective, multicenter, observational cohort study of children listed for heart transplantation (Clinical Trials in Organ Transplantation in Children-04 [CTOTC-04]). Outcomes were compared among sensitized recipients who underwent transplantation with positive crossmatch, nonsensitized recipients, and sensitized recipients without positive crossmatch. Positive crossmatch recipients received antibody removal and augmented immunosuppression, while other recipients received standard immunosuppression with corticosteroid avoidance. This first CTOTC-04 report summarizes study rationale and design and relates pretransplantation sensitization status using solid-phase technology. Risk factors for sensitization were explored. Of 317 screened patients, 290 were enrolled and 240 underwent transplantation. Core laboratory evaluation demonstrated that more than half of patients were anti-HLA sensitized. Greater than 80% of sensitized patients had class I (with or without class II) HLA antibodies, and one-third of sensitized patients had at least 1 HLA antibody with median fluorescence intensity of ≥8000. Logistic regression models demonstrated male sex, weight, congenital heart disease history, prior allograft, and ventricular assist device are independent risk factors for sensitization.

Five decades of pediatric heart transplantation: challenges overcome, challenges remaining

PURPOSE OF REVIEW

Pediatric heart transplants continue to be the therapy of choice for children with end stage heart failure. The interplay of limited donor supply, improvement in ventricular assist device (VAD) technology and utilization, and a focus on optimizing long-term outcomes make it critically important for practitioners to be aware of an evolving diagnostic and therapeutic arsenal.

RECENT FINDINGS

Data suitable to define best practices for pediatric heart transplantation consist of an amalgam of small single center series, registry reviews and judicious inference from adult studies. Large-scale prospective pediatric studies are essentially nonexistent; the pediatric heart transplant study group continues to be highly productive while new collaboratives are emerging.

SUMMARY

Outcomes for pediatric transplants continue to improve. Technology and innovation continue to drive shifts in management. Improvements in VAD support along with refinement of solid-phase assays require clinicians to develop a deeper understanding of pre and post transplant management of donor-specific antibodies and antibody-mediated rejection. Expertise in retransplantation and the care of adults with congenital heart disease will be critical in the future.

HLA Population Genetics in Solid Organ Transplantation

HLAs are fundamental to the adaptive immune response and play critical roles in the cellular and humoral response in solid organ transplantation. The genes encoding HLA proteins are the most polymorphic within the human genome, with thousands of different allelic variants known within the population. Application of the principles of population genetics to the HLA genes has resulted in the development of a numeric metric, the calculated panel-reactive antibody (CPRA) that predicts the likelihood of a positive crossmatch as a function of a transplant candidate's unacceptable HLA antigens. The CPRA is an indispensible measure of access to transplantation for sensitized candidates and is used as the official measure of sensitization for allocation of points in the US Kidney Allocation System and Eurotransplant. Here, we review HLA population genetics and detail the mathematical basis of the CPRA. An understanding of these principles by transplant clinicians will lay the foundation for continued innovation in the care of sensitized patients.

CPRA for allocation of kidneys in the US: More candidates ≥98% CPRA, lower positive crossmatch rates and improved transplant rates for sensitized patients

In 2009 calculated panel reactive antibody (CPRA) replaced PRA as the metric for HLA sensitization in the US kidney allocation system. During the next four years, registrants with at least one unacceptable antigen increased (34-40%) and registrants with ≥98% PRA/CPRA increased from 7% to 9% of the waitlist. These changes were accompanied by a reduction in kidney offers refused for positive crossmatch: 14,137 (1.7%) in 2009 and 3,310 in 2013 (0.4%). Registrants with ≥98% PRA/CPRA had highest rates of refusal but also showed substantial improvement (20% in 2009 vs 8% in 2013). For registrants with ≥98% PRA/CPRA, 45% of accepted offers in 2009 were not transplanted into the intended recipient compared to 11% in 2013. Transplant rates remained low for these patients (∼50/1000 active patient-years), but rates improved for patients with 80-97% PRA/CPRA (223/1000 active patient-years in 2009 vs 354/1000 in 2013). In 2013, 40% regraft candidates had CPRA ≥98% compared to 4% of primary graft candidates. More females than males were ≥98% CPRA (14% vs 7%) and more females had CPRA above 0 (50% vs 28%). In the CPRA era, listing of unacceptable antigens increased, positive crossmatches were diminished and transplant rates for sensitized patients improved.

Analysis of Panel Reactive Antibodies in Renal Transplant Recipients Detected by Luminex: A Single-Center Experience

OBJECTIVES

The role of panel reactive antibody has gained universal acceptance in solid-organ transplant. This parameter is used to gauge the level of sensitization of prospective solid-organ recipients. More than one-third of patients on wait lists for kidney transplant are sensitized. Most have previously formed donor-specific and non-donor-specific serum antibodies and/or positive crossmatch by complement-dependent cytotoxicity and/or flow cytometry. We present the rate of positivity at our institution for human leukocyte antigen antibodies and describe the condensation of antibodies in human leukocyte antigens for renal pretransplant recipients.

MATERIALS AND METHODS

Between January 2011 and December 2012, six hundred twenty consecutive renal transplant recipients on the wait list at the Baskent University were evaluated for this retrospective study. Panel reactive antibody screening and definition tests were studied with Luminex assays for the combination of class I (A, B, C) and class II antigens (DR, DQ).

RESULTS

We found a panel reactive antibody screening positivity in 20.4% of our patients on renal transplant waiting list. Panel reactive antibody defining tests were meaningful in 12.2% of the whole list. We observed that only panel reactive antibody class I positivity was seen in 2.2%, only panel reactive antibody class II positivity was seen in 2.7%, and both panel reactive antibody class I and class II positivities were seen in 7.2% of the defining tests.

CONCLUSIONS

The estimated risk of sensitization for patients with a living donor is determined from the combined results of the crossmatch with the donor and those of the recipient's panel reactive and donor-specific antibodies. Compared with complement-dependent cytotoxicity crossmatch, Luminex assays provide greater sensitivity and specificity in detection of donor-specific antibodies.

Changes in the methodology of pre-heart transplant human leukocyte antibody assessment: an analysis of the United Network for Organ Sharing database

BACKGROUND

We sought to investigate temporal trends in the methodology of human leukocyte antibody assessment in heart transplantation.

METHODS

The United Network for Organ Sharing database was queried from June 2004 to March 2013 to obtain pre-heart transplantation human leukocyte antibody results. The % panel reactive antibody for class I and II antibodies was recorded along with the methodology of assessment. Allosensitization was defined as class I and/or II panel reactive antibody of ≥ 10%. The primary outcome measure was graft survival.

RESULTS

During the study period, 12,858 patients with available data underwent heart transplantation. The prevalence of allosensitization increased, with 16.8% in 2005-2006 sensitized at the time of transplantation compared to 23.1% in 2010-2011 (p < 0.001); this occurred in conjunction with an increase in the utilization of flow cytometry (77.2% in 2005-2006; 97.0% in 2010-2011, p < 0.001). Using multivariable analysis, a positive pre-heart transplantation panel reactive antibody by flow cytometry independently predicted graft loss.

CONCLUSIONS

There has been a recent increase in flow cytometric assessment of human leukocyte antibodies prior to heart transplantation, which may be associated with an increase in the prevalence of pre-transplant patients being characterized as allosensitized. Flow cytometry may identify patients with the highest likelihood of graft loss.

Human leukocyte antigens and alloimmunization in heart transplantation: an open debate

Considerable advances in heart transplantation outcome have been achieved through the improvement of donor-recipient selection, better organ preservation, lower rates of perioperative mortality and the use of innovative immunosuppressive protocols. Nevertheless, long-term survival is still influenced by late complications. We support the introduction of HLA matching as an additional criterion in the heart allocation. Indeed, allosensitization is an important factor affecting heart transplantation and the presence of anti-HLA antibodies causes an increased risk of antibody-mediated rejection and graft failure. On the other hand, the rate of heart-immunized patients awaiting transplantation is steadily increasing due to the limited availability of organs and an increased use of ventricular assist devices. Significant benefits may result from virtual crossmatch approach that prevents transplantation in the presence of unacceptable donor antigens. A combination of both virtual crossmatch and a tailored desensitization therapy could be a good compromise for a favorable outcome in highly sensitized patients. Here, we discuss the unresolved issue on the clinical immunology of heart transplantation.

Pediatric heart transplantation-indications and outcomes in the current era

Pediatric heart transplantation (HTx) remains an important treatment option in the care of children with end-stage heart disease, whether it is secondary to cardiomyopathy or congenital heart disease (CHD). As surgical outcomes for CHD have improved, the indications for pediatric HTx have had to be dynamic, not only for children with CHD but also for the growing population of adults with CHD. As the field of pediatric HTx has evolved, the outcomes for children undergoing HTx have improved. This is undoubtedly due to the continued research efforts of both single-center studies, as well as research collaboratives such as the International Society for Heart and Lung Transplantation (ISHLT) and the Pediatric Heart Transplant Study (PHTS) group. Research collaboratives are increasingly important in pediatric HTx as single center studies for a limited patient population may not elicit strong enough evidence for practice evolution. Similarly, complications that limit the long term graft survival may occur in a minority of patients thus pooled experience is essential. This review focuses on the indications and outcomes for pediatric HTx, with a special emphasis on studies generated by these research collaboratives.

Pediatric heart transplantation-indications and outcomes in the current era

Pediatric heart transplantation (HTx) remains an important treatment option in the care of children with end-stage heart disease, whether it is secondary to cardiomyopathy or congenital heart disease (CHD). As surgical outcomes for CHD have improved, the indications for pediatric HTx have had to be dynamic, not only for children with CHD but also for the growing population of adults with CHD. As the field of pediatric HTx has evolved, the outcomes for children undergoing HTx have improved. This is undoubtedly due to the continued research efforts of both single-center studies, as well as research collaboratives such as the International Society for Heart and Lung Transplantation (ISHLT) and the Pediatric Heart Transplant Study (PHTS) group. Research collaboratives are increasingly important in pediatric HTx as single center studies for a limited patient population may not elicit strong enough evidence for practice evolution. Similarly, complications that limit the long term graft survival may occur in a minority of patients thus pooled experience is essential. This review focuses on the indications and outcomes for pediatric HTx, with a special emphasis on studies generated by these research collaboratives.

HLA and MICA allosensitization patterns among patients supported by ventricular assist devices

BACKGROUND

Ventricular assist devices (VADs) are increasingly being used as a bridge to transplantation and have been implicated as a risk factor for allosensitization to human leukocyte antigens (HLA). We investigate the association between VAD and allosensitization to human leukocyte antigens (HLA) and major-histocompatibility-complex (MHC) class I-related Chain A (MICA) antigens.

METHODS

We considered all patients who received a VAD at our institution between 2000 and 2009; 89 of them had pre-VAD and post-VAD (≤6 months after implant) HLA antibody screening. A control group of non-VAD heart transplant candidates was constructed with at least 2 pre-transplant panel-reactive antibody (PRA) tests within 8 months. Two controls were randomly selected/VAD patient matched for year (n = 178). Patients and controls with available sera from these time-points were tested by Luminex/flow PRA single-antigen beads and by MICA antibody Luminex single-antigen beads. Medical records were reviewed for comparison of pre-transplant immunologic risk factors and post-transplant outcomes between the 2 groups.

RESULTS

Compared with controls, VAD patients had greater Class I differences between peak and initial PRA (18% vs. 0%, p < 0.0001) and higher peak PRA (24% vs. 6%, p < 0.0001). The differences between the 2 groups in Class II were less pronounced than in Class I. Of patients who had single-antigen testing, VAD implantation was significantly associated with development of new HLA antibody specificities (Class I and/or Class II) post-VAD with an increase in calculated PRA (cPRA) post-VAD compared with controls (16% vs. 0%, p < 0.0001). This risk was still present after adjusting for age, gender, pre-VAD PRA, transfusion and duration of follow-up in a multivariate analysis (p < 0.0001 and 0.02, respectively). There were no differences in development of MICA antibodies between the 2 groups (14% in both). There was no significant difference in the incidence of pre-transplant positive T-cell crossmatch, pre-transplant donor-specific HLA antibodies, rejection episodes or graft survival between the 2 groups.

CONCLUSION

Our results suggest that VAD is associated with significant HLA allosensitization independent of common risk factors.

Lung transplantation

Lung transplantation may be the only intervention that can prolong survival and improve quality of life for those individuals with advanced lung disease who are acceptable candidates for the procedure. However, these candidates may be extremely ill and require ventilator and/or circulatory support as a bridge to transplantation, and lung transplantation recipients are at risk of numerous post-transplant complications that include surgical complications, primary graft dysfunction, acute rejection, opportunistic infection, and chronic lung allograft dysfunction (CLAD), which may be caused by chronic rejection. Many advances in pre- and post-transplant management have led to improved outcomes over the past decade. These include the creation of sound guidelines for candidate selection, improved surgical techniques, advances in donor lung preservation, an improving ability to suppress and treat allograft rejection, the development of prophylaxis protocols to decrease the incidence of opportunistic infection, more effective therapies for treating infectious complications, and the development of novel therapies to treat and manage CLAD. A major obstacle to prolonged survival beyond the early post-operative time period is the development of bronchiolitis obliterans syndrome (BOS), which is the most common form of CLAD. This manuscript discusses recent and evolving advances in the field of lung transplantation.

Successful isolated intestinal transplantation in sensitized recipients with the use of virtual crossmatching

We evaluated virtual crossmatching (VXM) for organ allocation and immunologic risk reduction in sensitized isolated intestinal transplantation recipients. All isolated intestine transplants performed at our institution from 2008 to 2011 were included in this study. Allograft allocation in sensitized recipients was based on the results of a VXM, in which the donor-specific antibody (DSA) was prospectively evaluated with the use of single-antigen assays. A total of 42 isolated intestine transplants (13 pediatric and 29 adult) were performed during this time period, with a median follow-up of 20 months (6-40 months). A sensitized (PRA ≥ 20%) group (n = 15) was compared to a control (PRA < 20%) group (n = 27) to evaluate the efficacy of VXM. With the use of VXM, 80% (12/15) of the sensitized patients were transplanted with a negative or weakly positive flow-cytometry crossmatch and 86.7% (13/15) with zero or only low-titer (≤ 1:16) DSA. Outcomes were comparable between sensitized and control recipients, including 1-year freedom from rejection (53.3% and 66.7% respectively, p = 0.367), 1-year patient survival (73.3% and 88.9% respectively, p = 0.197) and 1-year graft survival (66.7% and 85.2% respectively, p = 0.167). In conclusion, a VXM strategy to optimize organ allocation enables sensitized patients to successfully undergo isolated intestinal transplantation with acceptable short-term outcomes.

Outcomes in pediatric cardiac transplantation with a positive HLA cross-match

Previous studies have shown poor outcomes in pediatric heart transplant recipients with a high PRA or a positive direct donor-recipient cross-match. This study describes outcomes in patients with a positive cross-match at a large pediatric program. Pediatric heart transplant patients at a large single center between January 1993 and July 2009 were reviewed; those with cross-match data were analyzed. Cross-match data were available in 242/262 (92.4%) patients. Indications for transplant were cardiomyopathy (58%), CHD (32%), and retransplant (7%). PRA was ≥10% in 31/213 (14.6%) patients. A retrospective cross-match was positive in 17/31 (55%) patients with PRA ≥10% and 0/182 with PRA <10%. In positive cross-match patients, rejection frequency in the first year post-transplant was higher than negative cross-match patients (1.69 vs. 0.96 episodes/pt year, p = 0.014). There was no difference in rejection frequency after the first year post-transplant (0.18 vs. 0.12 episodes/pt year, p = 0.14). Overall survival was not significantly different between the groups with a median follow-up time of 4.5 yr. Heart transplantation in patients with a positive cross-match may result in good medium-term survival but a higher frequency of early rejection. Further investigation is warranted to define which patients with a positive cross-match will do poorly.

The utility of comprehensive assessment of donor specific anti-HLA antibodies in the clinical management of pediatric kidney transplant recipients

Advances in anti-HLA antibody characterization have had a major impact on kidney transplantation. Comprehensive characterization of DSA has improved protocols for allosensitized transplant candidates, increasing access to acceptable donors. Sensitive and specific solid phase antibody detection assays have given important insight into the clinical characteristics of antibody-mediated allograft injury, resulting in the development of a classification system for acute AMR. In addition, important insights into the nature of chronic antibody-mediated allograft rejection have been achieved as a result of improvements in DSA characterization. Finally, assays initially developed as tools to detect DSA in the clinical setting have been employed in innovative research protocols, allowing the investigation of new therapeutic approaches.

Outcomes of cardiac transplantation in highly sensitized pediatric patients

Despite aggressive immunosuppressive therapy, pediatric orthotopic heart transplant (OHT) candidates with elevated pre-transplant panel reactive antibody (PRA) carry an increased risk of rejection and early graft failure following transplantation. This study has aimed to more specifically evaluate the outcomes of transplant candidates stratified by PRA values. Records of pediatric patients listed for OHT between April 2004 and July 2008 were reviewed (n = 101). Survival analysis was performed comparing patients with PRA < 25 to those with PRA > 25, as well as patients with PRA < 80 and PRA > 80. Patients with PRA > 25 had decreased survival compared with those with PRA < 25 after listing (P = 0.004). There was an even greater difference in survival between patients with PRA > 80 and those with PRA < 80 (P = 0.002). Similar analyses for the patients who underwent successful transplantation showed no significant difference in post-transplant survival between patients with a pre-transplant PRA > 25 and those with PRA < 25 (P = 0.23). A difference approaching significance was noted for patients with PRA > 80 compared with PRA < 80 (P = 0.066). Patients with significantly elevated pre-transplant PRAs at the time of listing have a significantly worse outcome compared to those with moderately increased PRA values or non-sensitized patients. Further study is necessary to guide physician and family treatment decisions at the time of listing.

Antihuman leukocyte antigen–specific antibody strength determined by complement-dependent or solid-phase assays can predict positive donor-specific crossmatches

CONTEXT

The association of circulating donor-specific antibody (DSA) strength with crossmatch results is of potential interest to predict allograft outcome.

OBJECTIVES

To systematically investigate the aforementioned association and to attempt to define a cutoff value for DSA strength that can predict a positive crossmatch result.

DESIGN

We analyzed DSA strength and crossmatch results from the 2006 to 2008 proficiency testing samples of the American Society of Histocompatibility and Immunogenetics (n  =  50). We further validated our findings in candidates for potential kidney transplant (n  =  19).

RESULTS

Proficiency test samples with positive antihuman globulin T-cell crossmatch results had significantly higher DSA strength, as assessed by Luminex (Austin, Texas) mean fluorescent intensity (MFI; MFI [SD], 7860 [4770]), compared with samples with negative crossmatch results (MFI [SD], 2900 [1820]; P  =  .001). Similarly, higher Luminex values were observed in samples from candidates for transplant with positive antihuman globulin T-cell crossmatch results (MFI [SD], 7910 [2370] versus 2840 [1960]; P < .001). The MFI value of 6540 had 61% and 75% sensitivity and 92% and 94% specificity for predicting positive antihuman globulin T-cell crossmatches in proficiency test samples and in candidates for transplant, respectively.

CONCLUSIONS

The DSA strength correlates well with crossmatch results. An MFI of 6540 predicted a positive antihuman globulin T-cell crossmatch.

Humoral Rejection after Pediatric Heart Transplantation: A Case Report

Humoral rejection was observed 2 years after heart transplantation in a 10-year-old African American girl with sickle cell disease. Hemodynamic compromise developed, and the patient started treatment with extracorporeal membrane oxygenation within 24 hours of admission. With cellular rejection initially believed to be the cause, administration of thymoglobulin and high-dose steroids was initiated. Human leukocyte antigen antibody analysis revealed high titers of donor-specific class I and II antibodies. Aggressive treatment for antibody-mediated rejection was started with plasmapheresis and administration of intravenous immune globulin and ritux-imab. The patient displayed clinical signs of infection and was treated with antimicrobial, antiviral, and antifungal agents. Computed tomography of the chest suggested asperigillous infection. The patient underwent a left upper lobectomy. The patient recovered and has done well, now 4 years after having received the heart transplant. Antibody-mediated rejection should be considered early in heart transplant patients presenting with hemodynamic compromise and may respond to aggressive antibody and B cell–directed therapy. Vigilance for secondary infections, especially during treatment for rejection, is crucial.

Preexisting donor-specific HLA antibodies predict outcome in kidney transplantation

The clinical importance of preexisting HLA antibodies at the time of transplantation, identified by contemporary techniques, is not well understood. We conducted an observational study analyzing the association between preexisting donor-specific HLA antibodies (HLA-DSA) and incidence of acute antibody-mediated rejection (AMR) and survival of patients and grafts among 402 consecutive deceased-donor kidney transplant recipients. We detected HLA-DSA using Luminex single-antigen assays on the peak reactive and current sera. All patients had a negative lymphocytotoxic cross-match test on the day of transplantation. We found that 8-year graft survival was significantly worse (61%) among patients with preexisting HLA-DSA compared with both sensitized patients without HLA-DSA (93%) and nonsensitized patients (84%). Peak HLA-DSA Luminex mean fluorescence intensity (MFI) predicted AMR better than current HLA-DSA MFI (P = 0.028). As MFI of the highest ranked HLA-DSA detected on peak serum increased, graft survival decreased and the relative risk for AMR increased: Patients with MFI >6000 had >100-fold higher risk for AMR than patients with MFI <465 (relative risk 113; 95% confidence interval 31 to 414). The presence of HLA-DSA did not associate with patient survival. In conclusion, the risk for both AMR and graft loss directly correlates with peak HLA-DSA strength. Quantification of HLA antibodies allows stratification of immunologic risk, which should help guide selection of acceptable grafts for sensitized patients.

Utility of the virtual crossmatch in solid organ transplantation

PURPOSE OF REVIEW

Solid-phase assays covered with single HLA molecules - such as single-antigen flow-beads (SAFBs) - allow determining the presence of donor-specific HLA antibodies (HLA-DSAs) 'virtually' by comparison of the HLA-antibody specificities of the recipient with the HLA typing of the donor. In this review, prospects and current limitation of the virtual crossmatch are discussed.

RECENT FINDINGS

Several prospective and retrospective studies indicate that a negative virtual crossmatch is associated with a very low risk of early rejection and good long-term allograft survival. By contrast, a positive virtual crossmatch is associated with a significant risk for early rejection and decreased allograft survival. However, these studies revealed that not all HLA-DSAs detected by SAFB have a detrimental clinical impact.

SUMMARY

The virtual crossmatch has emerged as a very useful tool for pretransplant risk assessment and organ allocation. Further advances of the virtual crossmatch approach will require improvements on the technical part of SAFB analysis and a better understanding and definition of pathogenic factors of HLA-DSA. Together with an extended HLA typing of the donor, this scenario will provide us the full benefits of applications based on virtual crossmatching.

Utility of virtual crossmatch in sensitized patients awaiting heart transplantation

BACKGROUND

Organ transplant candidates with serum antibodies directed against human leukocyte antigens (HLA) face longer waiting times and higher mortality while awaiting transplantation. This study examined the accuracy of virtual crossmatch, in which recipient HLA-specific antibodies, identified by solid-phase assays, are compared to the prospective donor HLA-type in heart transplantation.

METHODS

We examined the accuracy of virtual crossmatch in predicting immune compatibility of donors and recipients in heart transplantation and clinical outcomes in immunologically sensitized heart transplant recipients in whom virtual crossmatch was used in allograft allocation.

RESULTS

Based on analysis of 257 T-cell antihuman immunoglobulin complement-dependent cytotoxic (AHG-CDC) crossmatch tests, the positive predictive value of virtual crossmatch (the likelihood of an incompatible virtual crossmatch resulting in an incompatible T-cell CDC-AHG crossmatch) was 79%, and the negative predictive value of virtual crossmatch (the likelihood of a compatible virtual crossmatch resulting in a compatible T-cell CDC-AHG crossmatch) was 92%. When used in a cohort of 28 sensitized patients awaiting heart transplantation, 14 received allografts based on a compatible virtual crossmatch alone from donors in geographically distant locations. Compared with the other 14 sensitized patients who underwent transplant after a compatible prospective serologic crossmatch, the rejection rates and survival were similar.

CONCLUSION

Our findings are evidence of the accuracy of virtual crossmatch and its utility in augmenting the opportunities for transplantation of sensitized patients.

Perception versus reality?: Virtual crossmatch--how to overcome some of the technical and logistic limitations

The goal of this work was to evaluate concordance between (a) actual flow cytometric crossmatch (FCXM) that is performed by the OPO laboratory servicing our transplant center and (b) virtual XM (vXM) prediction based on antibody identification by solid-phase methods performed in our laboratory. A total of 1586 FCXM, performed between June 2007 and September 2008, between all potential deceased donors in our region and sera from patients awaiting kidney or kidney-pancreas transplant, listed at Northwestern Memorial Hospital were evaluated. A key finding of this analysis was the understanding that a thorough vXM cannot be performed in some donor/recipient pairs due to the lack of certain antibody profile data specific to the donor in question. Obtaining more in depth and stringent information regarding antibody specificities, we demonstrate an excellent sensitivity and specificity of the vXM assays- 86.1% and 96.8%, respectively, with a positive likelihood ratio and negative likelihood ratios of 26.9 and 0.14, respectively. The vXM can serve as an outstanding tool to predict HLA compatibility between donor and recipient, with the caveat that the presence/absence of all antibodies against the potential donor and their strength have been thoroughly investigated.

Clinical significance of anti-HLA antibodies detected by Luminex: enhancing the interpretation of CDC-BXM and important post-transplantation monitoring tools

B-cell crossmatch (BXM) was originally introduced to increase the sensitivity to detect anti-HLA antibodies of conventional CDC crossmatch in renal transplantation. Newer techniques such as Luminex((R)) have greater sensitivity in detecting anti-HLA antibodies but have not been directly evaluated versus BXM. We discuss our experience with Luminex testing and the significance of donor-specific antibodies (DSA) defined by Luminex in three populations, as compared with the CDC crossmatch. In the general transplant population, Luminex-defined DSA were found in only one third of positive CDC-BXM and were associated with graft rejection. Luminex testing enhanced the interpretation of CDC-BXM and identified patients with clinically relevant BXM. In the highly sensitized transplant population, Luminex-defined DSA were found in two thirds of positive BXM and were a better predictor of graft rejection. Therefore, Luminex assays rather than CDC-BXM should be used to facilitate kidney allocation in highly sensitized patients. In the post-transplantation population, Luminex antibody monitoring for DSA was shown to be important, as it defined low-level de novo DSA that were associated with development of transplant glomerulopathy and a significant predictor of graft loss in those patients. Thus Luminex testing facilitated the interpretation of CDC-BXM and provided a useful predictive tool for the detection of clinically significant DSA in post-transplantation antibody monitoring.

Organ procurement and transplantation network/united network for organ sharing histocompatibility committee collaborative study to evaluate prediction of crossmatch results in highly sensitized patients

BACKGROUND

The requirement for a prospective crossmatch limits some organ allocation to local areas. The delay necessitated by the crossmatch restricts the distance across which offers can be made without unduly increasing the ischemia time. A collaborative study involving 14 transplant centers was undertaken by the Organ Procurement and Transplantation Network/United Network for Organ Sharing (OPTN/UNOS) Histocompatibility Committee to evaluate the accuracy with which the detection of unacceptable human leukocyte antigen (HLA) antigens by most advanced solid phase immunoassays can predict crossmatch results. In addition, using actual patients' unacceptable HLA antigens, the number of compatible donors that would have been available from the OPTN deceased kidney donors during 2002 to 2004 were investigated.

METHODS

Panel reactive antibodies were performed by conventional or solid phase assays, and crossmatches were performed by cytotoxicity or flow cytometry. Analyses were stratified for T and B cell and by method of identifying unacceptable HLA antigens and crossmatch techniques.

RESULTS

Combination of solid phase immunoassays and flow cytometry crossmatches resulted in a higher prediction rates of positive T cell (86.1%-93.5%) and B-cell crossmatches (91%-97.8%). Prediction of negative crossmatches based on different combination of panel reactive antibodies and crossmatch techniques varied from 14.3% to 57.1%. Furthermore, numerous potential compatible donors were identified for each patient, regardless of their ethnicity, in the OPTN database, when predicted incompatible ones were excluded.

CONCLUSIONS

The above results showed that with the advent of solid phase immunoassays, HLA antibodies can now be accurately detected resulting in prediction of crossmatch outcome. This should facilitate organ allocation and prevents shipment of organs to distant incompatible recipients.

Tailoring antibody testing and how to use it in the calculated panel reactive antibody era: the Northwestern University experience

BACKGROUND

Patients with human leukocyte antigen antibodies constitute a significantly disadvantaged population among those awaiting renal transplantation. We speculated that more understanding of the patients' antibody makeup would allow a more "immunologic" evaluation of crossmatch data, facilitate the use of virtual crossmatch (XM), and lead to more transplantability of these patients.

METHODS

We retrospectively compared the transplantability and transplant outcome of two consecutive patient populations transplanted in our center. Group I (n=374) was evaluated using solid-phase base testing for determination of percentage panel reactive antibody ("PRA screen") with limited antibody identification testing. Group II (n=333) was tested in a more comprehensive manner with major emphasis on antibody identification, antibody strength assignment, and the use of pronase for crossmatch.

RESULTS

Given this approach, 49% (166/333) of the transplanted patients in group II were sensitized compared with 40% (150/374) of the recipients in group I; P=0.012. Transplant outcome at 1-year posttransplant was similar in both groups.

CONCLUSIONS

We conclude that comprehensive evaluation of human leukocyte antigen sensitization and application of immunologic in analyzing compatibility between donor and recipient can increase the transplantability of sensitized patients while maintaining similar outcome. Our approach is in line with United Network for Organ Sharing new guidelines for calculated panel reactive antibody and virtual XM analysis. We hope this report will prompt additional transplant programs to consider how they will use the new United Network for Organ Sharing algorithms.