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

Preventing hyperacute rejection in a deceased donor kidney transplant recipient

A 36-year-old, highly sensitized (cPRA 99.99 %) male listed for his third kidney transplant for more than 12 years received a deceased donor offer. The virtual crossmatch (VXM) indicated that the patient had two weak donor-specific antibodies (DSA) to DR10 and DP17 against potential Donor 1, which would correlate with a negative T/B physical crossmatch (PXM). However, the PXM results were unexpectedly positive over 300 median channel shifts (MCS). Investigation into the discrepancy ruled out new sensitization and prozone effect. Repeated PXM showed results consistent with the initial findings and ruled out a sample swap of the donor cells or patient serum.Conditions such as auto-immune diseases and HIV, which could cause false positive PXM were also ruled out. Coincidentally, the patient received a second deceased donor offer (Donor 2) with identical HLA typing to Donor 1 from the same Organ Procurement Organization (OPO) on the same day. Further investigation of the ethnicity of the two donors revealed that Donor 1 was of African American (AFA) origin, and Donor 2 was Caucasian (CAU). Based on HLA disequilibrium, it is highly impossible that the two donors would share the same HLA typing, as A30-B42-DR18-DQ4 is the most common haplotype in AFA, and nearly absent in CAU populations. Repeating HLA typing of Donor 1 showed discrepant results from the initial HLA typing provided by the OPO. The updated VXM with the correct typing for Donor 1 revealed strong DSA to A1, A33, Cw8, DQ7, and DQA1*05, which corresponded with the PXM result.

Guidelines From the French-Speaking Society of Histocompatibility and Immunogenetics for Virtual Crossmatching for Deceased Donor Kidney Transplantation and the Use of Wet Crossmatch in the Decision-Making Process

The systematic use of Single Antigen Flow Beads assays and the implementation of high-resolution HLA typing for donors and kidney transplant recipients allow a precise identification of anti-HLA donor-specific antibodies. In France, the availability of detailed molecular biology HLA typing for deceased donors in the national organ allocation software enables anticipation of wet crossmatch results and estimation of the immunological risk for a recipient/donor pair. This key process, named virtual crossmatching, involves a thorough analysis of the recipient's anti-HLA sensitisation records. Its main goal is to reduce cold ischaemia time in order to extend graft survival. In this article, we present the guidelines for virtual crossmatching developed by a working group from the French-speaking Society of Histocompatibility and Immunogenetics. The guidelines address several considerations regarding HLA typing, anti-HLA antibody testing, and sensitisation event history, which are required to perform virtual crossmatching. We also propose a decision-making process, which situates prospective or retrospective wet crossmatch depending on virtual crossmatch results. The guidelines specifically emphasise the need for a strong clinical-biological agreement to standardise practices and provide a framework for omission of wet crossmatch for both non-sensitised and sensitised recipients.

From hero to zero: a single center retrospective review of the utility of routine physical crossmatching

Pretransplantation human leukocyte antigen antibody testing is necessary to assess compatibility between donor and recipient pairs. Over the past decade, the virtual crossmatch (VXM) has replaced the physical crossmatch (PXM) as the main assessment of pretransplant histocompatibility. At our center, most transplants have proceeded based solely on the VXM, followed by a retrospective PXM. In this study, we sought to determine whether VXMs alone are sufficient and if routine retrospective PXMs can be discontinued. A review of PXMs for single solid organ transplant cases between May 19, 2020, and December 9, 2022, was performed to examine the concordance between the PXM and VXM (including predictions of the PXM made in the VXM report). The relationship between PXM results and donor-specific antibody (DSA) mean fluorescence intensity values was also examined. The overall PXM prediction/PXM result concordance rate was 99.7% based on PXMs attributed to human leukocyte antigen antibody. Furthermore, DSA mean fluorescence intensity values can guide clinical decisions on whether to perform a PXM. The data confirm that, in the era of the VXM, routine PXMs performed for all transplants are not warranted. However, a PXM may still be informative in select cases where low-to-intermediate strength DSAs are present or when the virtual assessment is ambiguous.

Introduction of the donor centre virtual crossmatch in Eurotransplant

On 24 January 2023, Eurotransplant has introduced the virtual crossmatch for kidney and pancreas allocation as a better alternative for the physical Complement Dependent Cytotoxicity (CDC) crossmatches at the donor centre, which were associated with a longer cold ischaemia time and false positive reactions. For the time being, the physical CDC crossmatch at the recipient centre will remain in place as the final histocompatibility check. While Eurotransplant is certainly not the first organ allocation organisation to introduce virtual crossmatching, several novel aspects have been introduced, such as calculation of the virtual panel reactive antibody (vPRA) on 11 loci at the second-field level in addition to the serological broad and split level, electronic HLA typing data transmission using Histoimmunogenetics Markup Language (HML) file format, and the actual virtual crossmatch based on ambiguous, second-field HLA typing of the donor on all 11 loci. This short communication will focus on these novel aspects of the virtual crossmatch in Eurotransplant.

Selective Elimination and Rationalization of Cell-based Assays in Deceased Donor Kidney Transplant Crossmatching

Background

While there is increasing reliance on a negative virtual crossmatch to proceed with deceased donor kidney transplantation, a flow cytometry crossmatch (FCXM) is still usually performed after the transplant has already occurred. Our center has eliminated pretransplant physical crossmatches for most patients, and since 2018, we have eliminated the systematic performance of posttransplant FCXMs.

Methods

We studied all deceased donor kidney transplants in our program between June 1, 2018, and March 31, 2021, to evaluate the impact of eliminating retrospective FCXMs on resource utilization and graft outcomes (ie, the occurrence of antibody-mediated rejection [AMR] in the first 3-mo posttransplant).

Results

A total of 358 kidney transplants occurred during the study period, and approximately 70% of these transplants proceeded without the performance of any FCXM. Incidence rates of AMR were low (9.63 per 1000 person-months), which compared favorably with the incidence rate of AMR during the 3-y period preceding the policy (4.82 per 1000 person-months, P = 0.21).

Conclusions

Our results suggest that moving away from retrospective FCXM and relying exclusively on the virtual crossmatch is safe and efficient for kidney allocation.

Utilizing proficiency testing survey data to create advanced educational content: the virtual crossmatch challenge model

Proficiency testing (PT) surveys include data from laboratories across the world and are ideal for creating advanced educational content, beyond just consensus grading. Educational challenges provide a unique opportunity to probe common laboratory practices and risk assessment, especially in cases where there is no "analyte" tested. Human leukocyte antigen (HLA) compatibility evaluation between donor and recipient pairs has been traditionally assessed using T-cell and B-cell physical crossmatches. However, advancements in our ability to identify and characterize HLA antibodies using solid phase assays, in combination with changing deceased donor allocation schemes and improved HLA typing, have shifted the paradigm from performing physical crossmatches to the use of the virtual crossmatch (VXM). VXM is a compatibility assessment relying on the interpretation of pre-transplant HLA laboratory data and as such, it is not an "analyte". However, VXM results are used in clinical decision-making. The VXM assessment depends on patient characteristics as well as laboratory and transplant center practices but must ensure safe transplantation outcomes while maintaining equity in access to transplantation. In this manuscript, we describe the American Society for Histocompatibility and Immunogenetics (ASHI) PT Educational VXM Challenge, as a model for creating educational content using PT survey data. We discuss the different components of the VXM Challenge and highlight major findings and learning points acquired from ASHI VXM Challenges performed between 2018-2022, such as the lack of correlation between the VXM and the physical crossmatch in the presence of low level donor-specific antibodies (DSA), or when the DSA were aimed against donor alleles that are not present on the antibody panel, and in the presence of an antibody to a shared eplet. Finally, we show that the VXM Educational Challenge serves as a valuable tool to highlight the strengths and pitfalls of the VXM assessment and reveals differences in testing and result interpretation among participating HLA laboratories.

Virtual and Reality: An Analysis of the UCLA Virtual Crossmatch Exchanges

The "virtual" crossmatch (VXM) has become a critical tool to predict the compatibility between an organ donor and a potential recipient. Yet, nonstandardized laboratory practice can lead to variability in VXM interpretation. Therefore, UCLA's VXM Exchange survey was designed to understand factors that influence the variability of VXM prediction in the presence of HLA donor-specific antibody (DSA). Thirty-six donor blood samples and 72 HLA reference sera were sent to 35 participating laboratories to perform HLA antibody testing, flow crossmatch (FXM), and VXM from 2014 to 2019, consisting of 144 T/B-cell FXM pairs and 112 T/B-cell VXM pairs. In the FXM survey, 86% T-cell FXM and 84% B-cell FXM achieved >80% concordance among laboratories. In the VXM survey, 81% T-cell VXM and 80% VXM achieved >80% concordance. The concordance between FXM and VXM was 79% for T cell and 87% for B cell. The consensus between VXM and FXM was high with strong DSA. However, significant variability was observed in sera with (1) very high titer antibodies that exit prozone effect; (2) weak-to-moderate DSA, particularly in the presence of multiple weak DSAs; and (3) DSA against lowly expressed antigens. With the increasing use the VXM, standardization and continuous learning via exchange surveys will provide better understanding and quality controls for VXM to improve accuracy across all centers.

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.

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.