Impact of Virtual Cross Match on Waiting Times for Heart Transplantation

Utilizing principal component analysis in the identification of clinically relevant changes in patient HLA single antigen bead solid phase testing patterns

BACKGROUND

HLA antibody testing is essential for successful solid-organ allocation, patient monitoring post-transplant, and risk assessment for both solid-organ and hematopoietic transplant patients. Luminex solid-phase testing is the most common method for identifying HLA antibody specificities, making it one of the most complex immunoassays as each panel contains over 90 specificities for both HLA class I and HLA class II with most of the analysis being performed manually in the vendor-provided software. Principal component analysis (PCA), used in machine learning, is a feature extraction method often utilized to assess data with many variables.

METHODS & FINDINGS

In our study, solid organ transplant patients who exhibited HLA donor-specific antibodies (DSAs) were used to characterize the utility of PCA-derived analysis when compared to a control group of post-transplant and pre-transplant patients. ROC analysis was utilized to determine a potential threshold for the PCA-derived analysis that would indicate a significant change in a patient's single antigen bead pattern. To evaluate if the algorithm could identify differences in patterns on HLA class I and HLA class II single antigen bead results using the optimized threshold, HLA antibody test results were analyzed using PCA-derived analysis and compared to the clinical results for each patient sample. The PCA-derived algorithm had a sensitivity of 100% (95% CI, 73.54%-100%), a specificity of 75% (95% CI, 56.30%-92.54%), with a PPV of 65% (95% CI, 52.50%-83.90%) and an NPV of 100%, in identifying new reactivity that differed from the patients historic HLA antibody pattern. Additionally, PCA-derived analysis was utilized to assess the potential over-reactivity of single antigen beads for both HLA class I and HLA class II antibody panels. This assessment of antibody results identified several beads in both the HLA class I and HLA class II antibody panel which exhibit over reactivity from 2018 to the present time.

CONCLUSIONS

PCA-derived analysis would be ideal to help automatically identify patient samples that have an HLA antibody pattern of reactivity consistent with their history and those which exhibit changes in their antibody patterns which could include donor-specific antibodies, de novo HLA antibodies, and assay interference. A similar method could also be applied to evaluate the over-reactivity of beads in the HLA solid phase assays which would be beneficial for lot comparisons and instructive for transplant centers to better understand which beads are more prone to exhibiting over-reactivity and impact patient care.

Evaluation of a new complement-dependent lymphocytotoxicity cross match method using an automated cell counter, the NucleoCounter® NC-3000™

Complement-dependent lymphocytotoxicity cross match (CDC-XM) is the ultimate test of donor/recipient compatibility prior to organ transplantation. This test is based on cell viability, evaluated under fluorescence microscopy by an operator after proper staining. The determination of the positivity threshold may vary depending on the operator. We developed a new method in which the final step of determining cell viability is automated using the NC-3000™ (Chemometec®), an image cytometer able to precisely determine the percentage of dead/live cells in a suspension. After T and B donor cells isolation by negative selection, complement-dependent lysis was performed in macrovolumes in a PCR plate. Then, cell viability was measured by the NC-3000™. The sensitivity and routine CDC-XM results of this new method were compared to those of CDC-XM reference method using Terasaki plates. The sensitivity of CDC-XM expressed in the ASHI scoring system of this method was similar to the reference method results for a dilution range of the positive controls. Similarly, the results of the new method were comparable in a clinical situation to those obtained with the reference method after a study of 10 cross-matches, of which 5 cross-matches with DSA were positive and five cross-matches without DSA were negative. Moreover, ASHI scores were similar to those obtained using the reference method, and the mortality percentage was reproducible (CV < 15%). The assessment of cell viability by the NC-3000™ is easy to perform and highly reproducible but requires CDC-XM to be performed by the macrovolume method. The determination of a precise percentage of viability/mortality by the automation excludes operator variability and allows a better understanding of results close to the decision threshold.

Intermediate-term outcomes of heart transplantation for cardiac amyloidosis in the current era

BACKGROUND

Cardiac amyloidosis (CA) has been historically noted with poor outcomes after heart transplant (HTx). However, strict patient selection, appropriate multi-organ transplant, and aggressive post-transplant therapy can result in favorable outcomes. We present the experience in the largest single-center cohort of CA patients post-HTx in the recent era.

METHODS

Between January 2010 and December 2018, 51 CA patients underwent HTx-13 light-chain amyloidosis (AL) and 38 transthyretin amyloidosis (ATTR), 49 were included. Endpoints included 3-year survival, freedom from cardiac allograft vasculopathy (CAV), and freedom from non-fatal major adverse cardiac events (NF-MACE).

RESULTS

Overall 3-year survival was 81.6% (69.2% for AL and 86% for ATTR) and was comparable to survival for patients transplanted for non-amyloid restrictive cardiomyopathy (RCM) in the same period (89%, p = .46). Three-year freedom from CAV (84% vs. 89%, p = .98), NF-MACE (82% vs. 83%, p = .96), and any-treated rejection (95% vs. 89%, p = .54) were also comparable in both groups. No recurrence in amyloid was noted in endomyocardial biopsies. Six patients (46%) with AL amyloidosis underwent autologous stem cell transplant 1-year post-HTx, and two patients (8%) with variant ATTR-CA underwent combined heart-liver transplant due to cardiac cirrhosis.

CONCLUSION

In the current era, both AL and ATTR cardiac amyloidosis patients have acceptable outcomes after heart transplantation.

Clinical Outcomes of Perioperative Desensitization in Heart Transplant Recipients

Background.

Sensitization remains a barrier to heart transplantation (HT). Perioperative desensitization strategies have been described; however, a paucity of evidence exists to demonstrate efficacy and safety in HT.

Methods.

This single-center, retrospective study consisted of adults who received an HT. Perioperative desensitization was initiated if virtual crossmatch or flow-cytometry crossmatch was positive. Therapy consisted of plasmapheresis, intravenous immunoglobulin, and rabbit antithymocyte globulin. Historical controls received standard immunosuppression or induction. The primary endpoint was survival at 12 mo. Secondary endpoints included freedom from acute rejection, cardiac allograft vasculopathy (CAV), and infectious complications.

Results.

Of the 104 patients included, 48 received no induction, 46 received induction, and 10 underwent perioperative desensitization. No differences were observed in the primary endpoint at 12 mo (90.0% versus 97.9%, P = 0.25 for desensitization versus no-induction; 90.0% versus 100%, P = 0.72 for desensitization versus induction). Rates of acute rejection were lower with induction and desensitization compared with no-induction. There were no significant differences in CAV between the groups. Infectious complications were also similar among the groups (10.0% versus 16.7%, P = 0.62 for desensitization versus no-induction; 10.0% versus 30.4%, P = 0.34 for desensitization versus induction).

Conclusions.

This study suggests that a perioperative desensitization strategy triggered by positive virtual crossmatch or flow-cytometry crossmatch allows for successful transplantation of sensitized HT recipients and results in acceptable rates of survival, rejection, CAV, and infection at 12 mo.

Combined Heart and Kidney Transplantation: Clinical Experience in 100 Consecutive Patients

Background

Combined heart and kidney transplantation (HKTx) is performed in patients with severe heart failure and advanced renal insufficiency. We analyzed the long‐term survival after HKTx, the influence of age and dialysis status, the rates of cardiac rejection, and the influence of sensitization.

Methods and Results

From June 1992 to December 2016, we performed 100 HKTx procedures. We compared older (≥60 years, n=53) with younger (<60 years, n=47) recipients, and recipients on preoperative dialysis (n=49) and not on dialysis (n=51). We analyzed actuarial freedom from any cardiac rejection, acute cellular rejection, and antibody‐mediated rejection, and survival rates by sensitized status with panel‐reactive antibody levels <10%, 10% to 50%, and >50%, and compared these survival rates with those from the United Network for Organ Sharing database. There was no difference in 15‐year survival between the 2 age groups (35±12.4% and 49±17.3%, ≥60 versus <60 years; P=0.45). There was no difference in 15‐year survival between the dialysis and nondialysis groups (44±13.4% and 37±15.2%, P=0.95). Actuarial freedom from any cardiac rejection (acute cellular rejection>0 or antibody‐mediated rejection>0) was 92±2.8% and 84±3.8%, acute cellular rejection (≥2R/3A) 98±1.5% and 94±2.5%, and antibody‐mediated rejection (≥1) 96±2.1% and 93±2.6% at 30 days and 1 year after HKTx. There was no difference in the 5‐year survival among recipients by sensitization status with panel‐reactive antibody levels <10%, 10% to 50%, and >50% (82±5.9%, 83±10.8%, and 92±8.0%; P=0.55). There was no difference in 15‐year survival after HKTx between the United Network for Organ Sharing database and our center (38±3.2% and 40±10.1%, respectively; P=0.45).

Conclusions

HKTx is safe to perform in patients 60 years and older or younger than 60 years and with or without dialysis dependence, with excellent outcomes. The degree of panel‐reactive antibody sensitization did not appear to affect survival after HKTx.

Serum dilutions as a predictive biomarker for peri-operative desensitization: An exploratory approach to transplanting sensitized heart candidates

Antibody-mediated rejection (AMR) of cardiac allografts mediated by anti-HLA Donor Specific Antibodies (DSA) is one of the major barriers to successful transplantation for the treatment of end-stage heart failure. Therapeutic plasma exchange (TPE) is a first-line treatment for pre-transplant desensitization. However, indications for treatment regimens and treatment end-points have not been well established. In this study, we investigated how sera dilutions could guide TPE regimens for effective peri-operative desensitization and early AMR treatment. Our data show that 1:16 dilutions of EDTA-treated sera and 1.5 volume TPE reduce anti-HLA class I and class II antibody levels in the same manner and, therefore, allows to predict which antibodies would respond to peri-operative TPE. We successfully applied this approach to transplanting three highly sensitized cardiac recipients (CPRA 85-93%) with peri-operative desensitization based on a virtual crossmatch performed on 1:16 diluted serum. Furthermore, we have used sera dilutions to guide DSA treatment post-transplant. Although these findings have to be confirmed in a larger prospective study, our data suggest that serum dilutions can serve as a predictive biomarker to guide peri-operative desensitization and post-transplant immunologic management.

Human Leukocyte Antigen Sensitization in Solid Organ Transplantation: A Primer on Terminology, Testing, and Clinical Significance for the Apheresis Practitioner

The human leukocyte antigen (HLA) system is an important immunologic barrier that must be considered for successful solid organ transplantation. Formation of donor-specific HLA antibodies in solid organ transplantation is an important cause of allograft injury and may contribute to recipient morbidity and mortality. Therapeutic plasma exchange is often requested to lower HLA antibody levels prior to or after transplantation and for management of HLA antibodies in the context of organ rejection. In this review, we summarize the current terminology, laboratory testing, and clinical significance of HLA sensitization in the solid organ transplant population. Furthermore, to illustrate applications of HLA testing in clinical practice, we summarize our own lung and kidney institutional protocols for managing HLA antibodies in the peri-transplant setting.

Virtual HLA Crossmatching as a Means to Safely Expedite Transplantation of Imported Pancreata

BACKGROUND

Imported pancreata accumulate cold ischemia time (CIT), limiting utilization and worsening outcomes. Flow cytometric crossmatching (FXM) is a standard method to assess recipient and donor compatibility, but can prolong CIT. Single-antigen bead assays allow for detection of recipient donor-specific HLA antibodies, enabling prediction of compatibility through a "virtual crossmatch" (VXM). This study investigates the utility and outcomes of VXM after transplantation of imported pancreata.

METHODS

We retrospectively compared outcomes of 153 patients undergoing pancreas transplantation at our institution over a 3.5-year period.

RESULTS

Three patient groups were analyzed based on geographic source of the pancreas graft and the type of prospective crossmatch performed: (1) imported VXM-only, n = 39; (2) imported VXM + FXM, n = 12; and (3) local VXM + FXM, n = 102. There were no episodes of hyperacute rejection and 1 episode of early antibody-mediated rejection (<90 days) in the imported VXM group. Death-censored graft survival, patient survival, and rejection rates were comparable among the recipient groups. For pancreata imported from United Network of Organ Sharing regions 3 and 4, proceeding to surgery without an FXM reduced CIT by 5.1 hours (P < 0.001). The time from organ arrival at the hospital to operation start was significantly shorter in the VXM-only group compared with the VXM + FXM group (P < 0.001).

CONCLUSIONS

Virtual crossmatch helps minimize CIT without increasing rejection or adversely affecting graft survival, making it a viable method to increase pancreas graft utilization across distant organ sharing regions.

Desensitization strategies in adult heart transplantation-Will persistence pay off?

Strategies are needed to enable successful heart transplantation in highly sensitized patients. Immunologic challenges from sensitization to human leukocyte antigen (HLA) reduce access to compatible donors, extend waiting times to transplant, and increase the risks of antibody-mediated rejection and cardiac allograft vasculopathy after transplant. The prime goal of desensitization is to increase access to transplantation through expansion of the donor organ pool. Existing therapies are directed at key components of the humoral immune response with newer biologically based regimens able to target plasma cells as the source of antibody production, as well as complement activation that has a central role in antibody-mediated injury. Despite the emergence of early promising results for these agents, a significant knowledge gap remains with the current data for desensitization, extrapolated mostly from non-heart solid-organ transplants and small observational studies. Notably, no approach has demonstrated significant and sustainable reductions in HLA antibody pre-transplant, and the ideal desensitization strategy remains elusive. In addition, clinical tools to evaluate the humoral response and efficacy of therapy are limited, focusing almost exclusively on HLA antibody detection. Importantly, desensitization is associated with significant costs and potential risks, and overall long-term outcomes and cost-effectiveness have not been sufficiently evaluated. Investigation is ongoing into the development of a clinically effective desensitization strategy in heart transplantation.

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.

Anti-HLA-A, -B, -DR, -DQB1 and -DQA1 antibodies reactive epitope determination with HLAMatchmaker in multipare awaiting list for heart transplant

Human leukocyte antigen (HLA) antibodies represent a significant risk factor for transplant failure. It is very important to characterize anti-HLA antibodies as epitopes rather than antigens so that this knowledge can be applied clinically. The aim of the study was to investigate the extra reactivity patterns in sensitized multipare. Here, we have used the HLAMatchmaker program, a theoretical algorithm, to explain these unexpected antibody reactivity patterns in multipare awaiting for heart transplant. The patient was sensitized during pregnancy by alleles HLA-A(*)24:02, HLA-DRB1(*)07:01, HLA-DRB4(*)01:01, DQB1(*)02:02 and DQA1(*)02:01 mismatches with development of respective antibodies. However, the patient' sera were shown an unexpected reactivity not directed toward HLA mismatches of daughters: A(∗)23:01, A(*)24:03 and B(*)15:12 for class I and DRB4(*)01:03, DRB1(*)09:02, DRB1(*)09:01, DQB1(*)03:01, DQB1(*)03:03, DQB1(*)03:02, DQB1(*)04:02, DQB1(*)04:01 and DQB1(*)02:01 for class II. By HLAMatchmaker analysis we found that these antibodies reacted with eplet shared by antigens in single allele Luminex panels. These eplets were: 62EE, 66GKH, 70KAH, 71HS, 127K, 113YH, 144KR, 150AAH, 151AHV, 163TG and 167DG for class I and 4Q, 74RRAE, 71RRA, 98KN, 120N, and 135G, 25FT, 34HE, 41ER, 47EK2, 48LF for class II. Thus, HLAMatchmaker software together with to solid phase techniques could open new horizons for a more precise characterization of the HLA-antibodies.

Thoracic organ transplantation: laboratory methods

Although great progress has been achieved in thoracic organ transplantation through the development of effective immunosuppression, there is still significant risk of rejection during the early post-transplant period, creating a need for routine monitoring for both acute antibody and cellular mediated rejection. The currently available multiplexed, microbead assays utilizing solubilized HLA antigens afford the capability of sensitive detection and identification of HLA and non-HLA specific antibodies. These assays are being used to assess the relative strength of donor specific antibodies; to permit performance of virtual crossmatches which can reduce the waiting time to transplantation; to monitor antibody levels during desensitization; and for heart transplants to monitor antibodies post-transplant. For cell mediated immune responses, the recent development of gene expression profiling has allowed noninvasive monitoring of heart transplant recipients yielding predictive values for acute cellular rejection. T cell immune monitoring in heart and lung transplant recipients has allowed individual tailoring of immunosuppression, particularly to minimize risk of infection. While the current antibody and cellular laboratory techniques have enhanced the ability to manage thoracic organ transplant recipients, future developments from improved understanding of microchimerism and graft tolerance may allow more refined allograft monitoring techniques.

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.

The future of heart transplantation

The field of heart transplantation has seen significant progress in the past 40 years. However, the breakthroughs in long-term outcome have seen stagnation in the past decade. Through advances in genomics and transcriptomics, there is hope that an era of personalized transplant therapy lies in the future. To see where heart transplantation truly fits into the long term, searching for and understanding the alternative approaches for heart failure therapy is both important and inevitable. The application of mechanical circulatory support has contributed to the largest advancement in treatment of end stage heart failure. It has already been approved for destination therapy of heart failure, and greater portability and ease of use of the device will be the future trend. Although it is still not prime time for stem cell therapy, clinical experiences have already suggested its potential therapeutic effects. And finally, whole organ engineering is on the horizon as new techniques have opened the way for this to proceed. In the end, progress on alternative therapies largely depends on our deeper understanding of the mechanisms of heart failure and how to prevent it.