Highly sensitized patients on the kidney transplant waiting list are significantly disadvantaged because of their diverse human leukocyte antigen (HLA) antibody profiles resulting in longer waiting times for an immunologically compatible donor. In attempts to alleviate this disadvantage, United Network of Organ Sharing adopted a new kidney allocation system (KAS) in 2014. The new KAS system requires sharing of kidneys regionally and nationally for highly sensitized candidates. The new KAS has significantly increased transplant rates for these highly sensitized individuals.1 However, it is estimated that sharing kidneys regionally and nationally results in reallocation of up to 20% of organ offers. The majority of the reallocation cases are related to a positive physical crossmatch (PXM) interpreted as immunologic incompatibility. This results in reallocation to a nonintended candidate on the local or regional list and adds significant costs and cold ischemia time to the organ.
Performing a prospective PXM (either cytotoxic or flow cytometric) adds several additional hours to the allocation process, resulting in longer cold ischemia time. In addition, performing a PXM requires shipping of donor cells, resulting in further delays. Furthermore, it is estimated that up to 20% of positive PXM is due to false-positive results that inappropriately exclude patients from receiving the intended organ and thus further prolonging the waiting time for highly sensitized candidates.2
One alternative to the PXM is the virtual crossmatch (VXM).3-6 The VXM is a process of identifying unacceptable antigens based on the HLA antibody profile of the transplant candidate. This is usually done using solid-phase single antigen bead (SAB) technology. SAB testing for HLA antibodies is a highly sensitive and reliable technique with relatively low false-positive and false-negative rates. SAB testing has significantly improved the process of HLA antibody testing and risk stratifying patients for posttransplant antibody-related injury.7 VXM can be performed before the organ is allocated and therefore can prevent the need for reallocation due to an unexpected positive PXM related to a previously undetected HLA antibody or a false-positive reaction. Because the VXM does not require donor cells, it shortens the process by several hours and can prevent shipping many kidneys that will require reallocation. However, it is important to note that the VXM requires significant HLA laboratory expertise, good knowledge of the patient’s HLA antibody history, and close communication between the HLA laboratory and the transplant team.
In this issue of Transplantation, Roll et al8 describe the value of using VXM to facilitate allocation of kidneys into highly sensitized candidates awaiting kidney transplantation. Of 254 kidneys imported during the study period, 215 (84.6%) were transplanted solely based on the VXM result. Of note, 118 of 215 imported kidneys (54.9%) accepted based on the VXM result were transplanted into the most highly sensitized candidates with a calculated panel reactive antibody of 99%–100%. Their results showed that 93% of the kidneys were transplanted into the intended recipient, and only 4 kidneys (1.6%) were reallocated because of HLA incompatibility because of a positive PXM and detection of new unacceptable donor-specific antibody (DSA) in the admission sample. It is also remarkable to note that the center only excluded 8 kidney offers based on positive VXM results.
Roll et al8 suggests that this strategy of using VXM to accept kidneys for highly sensitized patients on the waiting list is safe and effective, resulting in a very low reallocation rate and posttransplant clinical complications. Although some of these patients had a positive retrospective PXM, there were no cases of “hyperacute rejection” as well as very low rates of antibody-mediated rejection (n = 1) and acute cellular rejection (n = 1); however, the follow-up was relatively short.
The lack of a control group and the retrospective single-center study design limits the generalizability of this study. This study will need to be confirmed by other studies including significant cohorts of highly sensitized patients. Studies comparing transplant centers using VXM compared with PXM are needed, using organ procurement organization and center-specific data to confirm the good outcomes reported here.
It is important to note the stringent criteria used in this study specifying when a prospective PXM was required. These are: (1) presence of multiple weak DSA (median fluorescent intensity [MFI] <2000, their cutoff value for unacceptable antigens), (2) presence of strong allele-specific DSA (MFI ≥5000), (3) lack of an up-to-date quarterly HLA antibody test available to perform the VXM, (4) occurrence of a possible sensitizing event between the most recent HLA antibody test and the organ offer, or (5) presence of only a single serum specimen tested for HLA antibodies when an HLA-mismatched organ was offered. It is also noteworthy that Bw4 and Bw6 were sensibly considered as unacceptable antigens if the patient displayed antibodies against these epitopes at any MFI value, further enhancing the safety of the VXM protocol. The reason for this is that these “public” epitopes are shared by multiple antigens, and therefore the complementary Bw4 or Bw6 antibody is spread over a large number of beads on the SAB assay. Consequently, there is less antibody binding to a single bead, significantly underevaluating its strength.9
There are no widely accepted protocols or guidelines for the utilization of VXM in kidney transplantation, particularly for highly sensitized candidates. The transplant community will need to develop a national VXM strategy with standardized HLA testing and organ acceptance criteria. Using different MFI cutoffs will affect the positive and negative predictive values and have a significant influence on the rate of late reallocation.
Roll et al8 should be congratulated for this excellent study, which clearly demonstrates a safe and efficient process for the utilization of the VXM to better serve our highly sensitized patients. If the goal is to improve the efficiency of transplanting highly sensitized candidates on the kidney transplant waiting list, Roll et al8 have demonstrated that their strategy is highly effective.
Further research is needed in this field to answer several important questions including: (1) what is the optimal MFI cutoffs for unacceptable antigens? (2) does this threshold need to vary for different HLA loci? (3) how do transplant outcomes compare to centers using PXM? (4) is there any impact on late outcomes such as the occurrence of chronic antibody-mediated rejection? and (5) what would be the effect of including other loci such as HLA-DPB1 in the VXM protocol?10
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2. Paramesh AS, Neidlinger N, Salvatore M, et al. OPO strategies to prevent unintended use of kidneys exported for high PRA (>98% cpra) recipients. Am J Transplant. 2017; 17:2139–2143
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4. Taylor CJ, Kosmoliaptsis V, Sharples LD, et al. Ten-year experience of selective omission of the pretransplant crossmatch test in deceased donor kidney transplantation. Transplantation. 2010; 89:185–193
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7. Amico P, Hirt-Minkowski P, Hönger G, et al. Risk stratification by the virtual crossmatch: a prospective study in 233 renal transplantations. Transpl Int. 2011; 24:560–569
8. Roll G, Webber A, Gae D, et al. A virtual crossmatch based strategy facilitates sharing of deceased donor kidneys for highly sensitized recipients. Transplantation. 2019
9. Konvalinka A, Tinckam K. Utility of HLA antibody testing in kidney transplantation. J Am Soc Nephrol. 2015; 26:1489–1502
10. Tambur AR, Haarberg KM, Friedewald JJ, et al. Unintended consequences of the new national kidney allocation policy in the united states. Am J Transplant. 2015; 15:2465–2469