Virtual crossmatching for deceased donor transplantation: one size does not fit all

Virtual crossmatch for deceased donor kidney transplantation in the United States: A survey of histocompatibility lab directors and transplant surgeons

Virtual crossmatch (VXM) is used as an alternative to or in conjunction with a cell-based physical crossmatch (PXM) for assessing HLA (human leukocyte antigen) compatibility prior to deceased donor kidney transplantation (DDKT). Data on practice patterns and perceptions regarding VXM use in the US are limited. We performed a survey of US HLA directors and transplant surgeons regarding HLA testing and crossmatch strategies. 53 (56 %) HLA directors and 68 surgeons (representing ∼ 23 % of US transplant centers) completed the survey. Both groups agreed that VXM could reduce cold ischemia time (CIT), costs and improve allocation efficiency. VXM use increased following the 2021 kidney allocation change. Reducing CIT was the primary reason for favoring VXM over PXM. Preference for VXM reduced as candidates' panel reactive antibodies increased. Regulations, program policies and limitations of HLA technology were cited as important reasons for preferring PXM over VXM. Surgeons reported similar perceptions, but findings are limited by the low response rate. Finally, half the labs reported lacking specific protocols for VXM use. In conclusion, improved HLA technology and protocols along with changes to institutional procedures and policy regulations are needed for safer expansion of VXM in DDKT.

Early Effect of the Circular Model of Kidney Allocation in the United States

BACKGROUND

In March 2021, the United States implemented a new kidney allocation system (KAS250) for deceased donor kidney transplantation (DDKT), which eliminated the donation service area-based allocation and replaced it with a system on the basis of distance from donor hospital to transplant center within/outside a radius of 250 nautical miles. The effect of this policy on kidney discards and logistics is unknown.

METHODS

We examined discards, donor-recipient characteristics, cold ischemia time (CIT), and delayed graft function (DGF) during the first 9 months of KAS250 compared with a pre-KAS250 cohort from the preceding 2 years. Changes in discards and CIT after the onset of COVID-19 and the implementation of KAS250 were evaluated using an interrupted time-series model. Changes in allocation practices (biopsy, machine perfusion, and virtual cross-match) were also evaluated.

RESULTS

Post-KAS250 saw a two-fold increase in kidneys imported from nonlocal organ procurement organizations (OPO) and a higher proportion of recipients with calculated panel reactive antibody (cPRA) 81%-98% (12% versus 8%; P <0.001) and those with >5 years of pretransplant dialysis (35% versus 33%; P <0.001). CIT increased (mean 2 hours), including among local OPO kidneys. DGF was similar on adjusted analysis. Discards after KAS250 did not immediately change, but we observed a statistically significant increase over time that was independent of donor quality. Machine perfusion use decreased, whereas reliance on virtual cross-match increased, which was associated with shorter CIT.

CONCLUSIONS

Early trends after KAS250 show an increase in transplant access to patients with cPRA>80% and those with longer dialysis duration, but this was accompanied by an increase in CIT and a suggestion of worsening kidney discards.

Virtual crossmatching and epitope analysis in kidney transplantation: What the physician involved in kidney transplantation should know?

Solid-phase single antigen bead (SAB) assay for detection of anti-human leukocyte antigen (HLA) antibodies and high-resolution HLA typing have enabled tremendous progress in virtual crossmatch (VXM) technology in recent years. However, misinterpretation of the SAB assay may result in detrimental consequences after kidney transplantation. Meanwhile, epitope analysis could be an effective method to estimate immunizing eplets, which may provide ancillary information for better understanding of the SAB assay. To perform epitope analysis appropriately, it is necessary to understand the basic principles related to histocompatibility testing and the characteristics of the SAB assay. Therefore, knowledge of the properties and limitations of the SAB assay is critical. In this review, we aim to describe the fundamental concepts regarding immunobiological assessment, including HLA, anti-HLA antibodies, and SAB assay, and explain epitope analysis using examples.

Eplet-Predicted Antigens: An Attempt to Introduce Eplets into Unacceptable Antigen Determination and Calculated Panel-Reactive Antibody Calculation Facilitating Kidney Allocation

(1) Calculated panel-reactive antibody (CPRA) is a measure of sensitization based on unacceptable antigens (UAs). Determination of UAs based on single-antigen bead assays at allele or antigen levels may be inappropriate. We aimed to introduce eplets for better assessment of sensitization; (2) 900 recipients and 1427 donors were enrolled for candidate or donor pools, respectively. Eplets were from the HLA Epitope Registry. UAs were determined by anti-HLA antibodies identified using LIFECODES Single Antigen (LSA) kits. CPRA values were calculated using a simplified method of donor filtering; (3) HLA antigens containing all eplets of an HLA antigen in LSA kits (LSA antigen) were defined as eplet-predicted (EP) antigens, the reactivity of which could be predicted by that LSA antigen. High reactivity concordance was found between LSA and EP antigens. More HLA antigens were covered by EP antigens in the population than LSA antigens. CPRA values at the EP level were higher than at the allele level and lower than at the antigen level. The EP antigens facilitated UA determination for non-LSA antigens and avoided acute rejection; (4) UA determination using EP antigens can lead to more accurate assessment of sensitization, enabling a high probability of compatible organs and a low risk of adverse outcomes.

Principles of Virtual Crossmatch Testing for Kidney Transplantation

Human leukocyte antigens (HLAs) are the primary determinants of alloimmunity. A crossmatch test is a test that determines the immunologic risk of a recipient with a potential donor by ensuring that there are no transplant-relevant circulating antibodies in the recipient directed against donor antigens. Physical crossmatch (PXM) tests, such as complement-dependent cytotoxicity crossmatch (CDCXM) and flow cytometry crossmatch (FCXM), require mixing of patient serum and donor cells, are labor intensive, and are logistically challenging. Virtual crossmatch (VXM) test assesses immunologic compatibility between recipient and potential donor by analyzing the results of 2 independently done physical laboratory tests—patient anti-HLA antibody and donor HLA typing. The goal of VXM is pretransplant risk stratification—though there is no consensus on whether such risk assessment involves predicting the PXM result or the posttransplant outcome. Although the concept of VXM is not new, the advent of solid-phase assays for detecting circulating antibodies in the recipient directed against individual HLA and DNA-based methods for typing donor HLA specificities at a higher resolution makes the routine use of VXM a reality. Accordingly, VXM may be applied at different scenarios—both for sensitized and nonsensitized patients. Implementation of VXM-based approach has resulted in statistically significant reduction in cold ischemia time without an increase in hyperacute rejection episodes. Though there are considerable challenges, VXM is expected to be used more often in the future, depending on the transplant center’s tolerance of immunologic risk.

Trends and impact on cold ischemia time and clinical outcomes using virtual crossmatch for deceased donor kidney transplantation in the United States

For assessing human leukocyte antigen compatibility in deceased donor kidney transplantation, virtual crossmatch is used as an alternative to physical crossmatch and has potential to reduce cold ischemia time. The 2014 United States kidney allocation system prioritized highly sensitized candidates but led to increased shipping of kidneys. Using data from the Scientific Registry of Transplant Recipients, we evaluated changes in virtual crossmatch use with the new allocation policy and the impact of virtual crossmatch use on cold ischemia time and transplant outcomes. This was a retrospective cohort study of adult deceased donor kidney recipients in the United States (2011-2018) transplanted with either 9,632 virtual or 71,839 physical crossmatches. Before allocation change, only 9% of transplants were performed relying on a virtual crossmatch. After the 2014 allocation change, this increased by 2.4%/year so that 18% transplants in 2018 were performed with just a virtual crossmatch. There was significant variation in virtual crossmatch use among transplant regions (range 0.7-36%) and higher use was noted among large volume centers. Compared to physical crossmatches, virtual crossmatches were significantly associated with shorter cold ischemia times (mean 15.0 vs 16.5 hours) and similar death-censored graft loss and mortality (both hazard ratios HR 0.99) at a median follow-up of 2.9 years. Thus, our results show that virtual crossmatch is an attractive strategy for shortening cold ischemia time without negatively impacting transplant outcomes. Hence, strategies to optimize use and reduce practice variation may allow for maximizing benefits from virtual crossmatch.

A Retrospective Study on Human Leukocyte Antigen Types and Haplotypes in a South African Population

CONTEXT.—

Human leukocyte antigen (HLA) is a polymorphic protein of the immune system with a central role in organ transplantation. Organ recipients can be sensitized against HLA from previous exposure, which increases the likelihood of antidonor immune responses and subsequently organ rejection. HLA matching represents an attractive option to improve graft function, reduce sensitization of recipients in first transplantations, and improve organ allocation.

OBJECTIVE.—

To examine the feasibility of the reintroduction of HLA matching into the criteria in the Johannesburg program, we retrospectively assessed HLA types in our donor population.

DESIGN.—

HLA types of 782 deceased and related living donors from 2015 until 2019 were recorded and analyzed to identify the most common HLA types and haplotypes. A virtual crossmatch was also done to examine the anti-HLA antibodies in the recipient population compared with the common HLA types identified in this study.

RESULTS.—

Of the most common HLA types identified, at least 1 was present in 732 (93.6%) of the renal donors assessed. The virtual crossmatch confirmed that most recipients are sensitized against most donors, and this greatly impacts the number of recipients who can receive organ transplants.

CONCLUSIONS.—

This study determined the most common HLA types and haplotypes in a South African organ donor population. This information, combined with the evidence suggesting the immunogenic potential of these common types, the high number of recipients with antibodies against common HLA types, and the ethnic distribution of the donor and recipient populations, informs the recommendation that the pretransplantation workup should not reinclude HLA matching.

The challenge of using the virtual crossmatch as a singular tool for the detection of Anti-HLA antibodies- A study from a tertiary care institute from South India

INTRODUCTION

Detection of donor specific antibodies (DSA) is critical in both solid organ and mismatched haematopoietic stem cell transplants. The single antigen bead assay (SAB) is widely used as a virtual crossmatch in these settings. However, HLA allele variation across ethnicities and differing genetic backgrounds is a well-known and acknowledged fact and representation of alleles prevalent in a population is key while using a virtual crossmatch as a sole decision making tool. Against this background, this study was performed to assess the feasibility of using the SAB as a single tool to identify DSA in our population.

MATERIALS AND METHODS

The HLA alleles identified in the study population were analysed to assess their representation on SAB panels from two different vendors.

RESULTS

The study population comprised of a total of 966 subjects for whom 6 loci high resolution HLA typing was done. A total of 241 different alleles were assigned in the population. Among the 241 alleles identified in our study population, 48.55% (n = 117) alleles were represented in the SAB A panel and 48.13% (n = 116) represented in the SAB B panel. Unrepresented alleles were 51.45% (n = 124) in panel A and 51.87% (n = 125) in panel B. All the twelve alleles were represented for 16.05% (n = 155) and 16.25% (n = 157) of study population in panel A and in panel B respectively. The remaining individuals (83.95%, (n = 811) in panel A and 83.75%, (n = 809) in panel B) had at least one allele unrepresented.

CONCLUSION

Our study addresses an important limitation in utilizing the SAB as a single tool to identify DSA, owing to non-representation of locally prevalent / unique alleles in our population. More than 50% of alleles were unrepresented in both the SAB assays we studied, which included alleles from both Class I and Class II. We recommend therefore that, until a comprehensive coverage of alleles is provided, or epitope matching becomes robust, that the SAB be combined with a physical crossmatch when mismatched alleles are not represented.