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

Desensitization in heart transplant recipients: Who, when, and how

The number of heart transplant candidates who have pre-formed antibodies against human leukocyte antigens (HLAs) is increasing over time. The purpose of this review is to discuss the process of antibody desensitization for heart transplant candidates. Specifically, we review the current status of antibody detection including identification, strength, and potential pathogenicity. We discuss which patients and when should they undergo desensitization therapies during heart transplant evaluation. Specific therapies including mechanical removal of antibodies, intravenous immunoglobulins, and novel immunosuppressive agents targeting antibody production will be discussed. Finally, future research strategies to develop novel desensitization therapies for heart transplant candidates will be reviewed.

Antibody-mediated rejection in heart transplantation: new developments and old uncertainties

PURPOSE OF REVIEW

Antibody-mediated rejection (AMR) currently represents one of the main problems for clinical management of heart transplant because of its diagnostic complexity and poor evidences supporting treatments.

RECENT FINDINGS

Disorder-based diagnosis is a cornerstone in defining AMR. The limitations of the current classification have been partially overcome by novel studies improving the description of the immune-pathological graft abnormalities, and by new molecular approaches allowing a better understanding of the mechanisms behind AMR and of its relationship with cellular rejection and chronic vasculopathy. In-depth characterization of donor-specific antibodies showed to provide additional prognostic information and guide for treatment. Clinical relevance of AMR is bound to appropriate detection of graft dysfunction. In addition to traditional longitudinal evaluation by echocardiogram, cardiac magnetic resonance and detection of cell-free DNA may represent novel sensitive markers for graft injury that could prompt treatment before dysfunction becomes clinically manifest.

SUMMARY

Despite improvements in the diagnostic process, therapeutic strategies made little progress in addition to the consolidation of practices supported by limited evidences. Novel complement inhibitors appear promising in changing this scenario. Nevertheless, collaborative multicenter studies are needed to develop standardized approaches tailored to the highly variable clinical and laboratory features of AMR.

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.

Value of a flow cytometry cross-match in the setting of a negative complement-dependent cytotoxicity cross-match in heart transplant recipients

Complement-dependent cytotoxicity cross-match (CDCXM) is used for evaluation of preformed HLA-specific antibodies in patients undergoing heart transplantation. Flow cytometry cross-match (FCXM) is a more sensitive assay and used with increasing frequency. To determine the clinical relevance of a positive FCXM in the context of negative CDCXM in heart transplantation, the United Network for Organ Sharing (UNOS) database was analyzed. Kaplan-Meier analysis and Cox proportional hazard modeling were used to assess graft survival for three different patient cohorts defined by cross-match results: T-cell and B-cell CDCXM+ ("CDCXM+" cohort), CDCXM- but T-cell and/or B-cell FCXM+ ("FCXM+" cohort), and T-cell/B-cell CDCXM- and FCXM- ("XM-" cohort). During the study period, 2558 patients met inclusion criteria (10.7% CDCXM+, 18.8% FCXM+, 65.5% XM-). CDCXM+ patients had significantly decreased graft survival compared to FCXM+ and XM- cohorts (P = .003 and <.001, respectively). CDCXM- and FCXM+ patients did not have decreased graft survival compared to XM- patients (P = .09). In multivariate analysis, only CDCXM+ was associated with decreased graft survival (HR 1.22, 95% CI 1.01-1.49). In conclusion, positive FCXM in the context of negative CDCXM does not confer increased risk of graft failure. Further study is needed to understand implications of CDCXM and FCXM testing in heart transplant recipients.

Significance of Anti-HLA Antibodies on Adult and Pediatric Heart Allograft Outcomes

As methods for human leukocyte antigens (HLA) antibody detection have evolved and newer solid phase assays are much more sensitive, the last 15 years has seen a renewed focus on the importance of HLA antibodies in solid organ transplant rejection. However, there is still much controversy regarding the clinical significance of antibody level as depicted by the mean fluorescence intensity of a patient’s neat serum. Emerging techniques, including those that identify antibody level and function, show promise for the detection of individuals at risk of allograft rejection, determination of the effectiveness of desensitization prior to transplant, and for monitoring treatment of rejection. Here, we review current publications regarding the relevance of donor-specific HLA antibodies (DSA) in adult and pediatric heart transplantation (HT) with graft survival, development of antibody-mediated rejection and cardiac allograft vasculopathy (CAV). The negative impact of DSA on patient and allograft survival is evident in adult and pediatric HT recipients. Many questions remain regarding the most appropriate frequency of assessment of pre- and posttransplant DSA as well as the phenotype of DSA memory vs. true de novo antibody using large multicenter adult and pediatric cohorts and state-of-the-art methodologies for DSA detection and characterization.

Desensitization Strategies Pre- and Post-Cardiac Transplantation

OPINION STATEMENT

Panel reactive antibody (PRA) testing has become standard in the evaluation of patients prior to cardiac transplant. Sensitizing events such as blood transfusions, which result in the accumulation of pre-transplant antibodies, should be avoided as clinically feasible. Desensitization therapy might be considered in sensitized patients with cPRA > 50 % although distinct cutoff PRA values for initiating therapy pre-transplant are patient and transplant program dependent. Post-cardiac transplant, quantitative antibodies should also be periodically analyzed, at intervals individualized to the patient. Donor-specific antibodies (DSA) after cardiac transplantation have been shown to be associated with worsened survival. It appears that complement fixing DSA confer the greatest risk for antibody-mediated rejection post-transplant. Desensitization strategies aim to reduce the number of clinically important antibodies prior to and after transplant, both by removal of antibodies and cessation of further production. Current desensitization regimens include pharmacologic, procedural, and surgical modalities, and must be individualized to the patient. Currently, most cardiac transplant programs tailor the post-transplant immunosuppressive regimen based on clinical factors and immunologic assays and may include the use of cytolytic induction and/or intravenous immune gammaglobulin in higher risk patients.