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Making Sense of Sensitization

Evan Kransdorf, MD, PhD

Jignesh Patel, MD, PhD
Cedars-Sinai Heart Institute
Los Angeles, CA, USA

Sensitization, the presence of circulating antibodies against human leukocyte antigens (HLA), affects approximately 25% of candidates on the heart transplant waiting list [1]. To reduce the risk of post-transplant complications including most importantly hyperacute rejection, antigens to which the candidate has strong antibodies must be excluded. This leads to a smaller potential donor pool, which in turn leads to an increase in waiting time and consequently a higher risk of adverse outcomes on the waiting list [1]. In this commentary, we address three important issues in the area of sensitization: 1) How do we measure the degree to which a candidate is sensitized? 2) What threshold should we use to define a candidate as sensitized? and 3) What should we do about sensitization?

How do we measure sensitization?
The de facto measure of sensitization has traditionally been the panel-reactive antibody (PRA) value. This value was originally derived using an assay that consisted of a panel of third-party T (for class I HLA antigens) or B (for class II HLA antigens) lymphocytes and the candidate's serum; the "PRA value" was equal to the number of cytotoxic reactions divided by the size of the "panel" x 100. Although there was no consensus about what PRA value defined a candidate as sensitized, most programs used a value of 10% [2]. This cell-based assay was subsequently replaced by an assay that measures antibodies against class I and class II HLA antigens using flow cytometry.

The next innovation in solid-phase methods was the use of Luminex technology that allowed a semi-quantitative assessment of the HLA specificities in a candidate's serum. For these methods, a new measure of sensitization had to be used as there was no longer any "panel." Zachary and Braun proposed a method that uses the gene frequencies of the excluded HLA [3] to predict the likelihood of a positive crossmatch. This method has been implemented by the United Network for Organ Sharing (UNOS), and the resulting value is known as the calculated panel-reactive antibody (CPRA). CPRA has been the official metric of sensitization for kidney transplant candidates since 2009 [4]. CPRA is exclusively a function of the HLA that are selected to be excluded.

CPRA as a metric of sensitization has both strengths and weaknesses. The major strength is that it summarizes the percentage of the potential donor population with HLA corresponding with the candidate's HLA antibodies for both the class I and class II specificities as a single numeric value, which is therefore easy to interpret. In contrast, PRA reported values separately for class I and class II HLA antigens. Furthermore, CPRA can be further refined for HLA alleles, which are in the process of being added as potential excludes by UNOS.

However, CPRA has several important limitations. First and foremost, CPRA as implemented by UNOS does not currently include the frequencies for HLA-DQA1, -DPA1 and -DPB1 and thus is an incomplete metric of sensitization. Efforts are underway to correct this, and an expanded CPRA will likely be available within the next year or two. Inclusion of these loci is likely to result in higher CPRA values for candidates with these antigens [5]. Next, for very highly sensitized candidates, CPRA is not sufficiently granular to describe the level of sensitization [6], and the addition of decimal places is helpful. This also facilitates conversion of CPRA to its converse, the Likelihood of a Compatible Donor (LCD). LCD is equal to 1 - CPRA. LCD is a very useful metric for extremely sensitized candidates. For example, candidate "A" with a CPRA of 99% has a LCD of 1% or 1 in 100 donors, whereas candidate "B" with CPRA of 99.9% has a LCD of 0.1% or 1 in 1,000 donors. Thus, although these two candidates differ by only 0.9% CPRA, candidate "A" is 10-fold more likely to find a compatible donor. Furthermore, this concept of fold-change in compatible donors may be a promising way to look at the effect of desensitization [7].

What threshold should we use to define a candidate as sensitized?
It is important to reiterate that PRA and CPRA values are distinct. PRA is determined using beads that indicate which class I and class II HLA specificities are present in the candidate's serum, whereas CPRA is based on the HLA specificities above a certain mean fluorescence intensity (MFI) threshold that are present in the candidate's serum. The MFI threshold varies by HLA laboratory [8] and generally aims to correspond with the antibody strength likely to correlate with cytotoxicity; however, determining this value can be difficult and the concept of an MFI threshold is itself controversial [9]. Nevertheless, given that PRA is no longer the preferred metric of sensitization, the "over 10%" definition of a sensitized patient needs to reassessed.

We have recently used data from the UNOS database to show that CPRA is a continuous predictor of outcomes on the heart transplant waiting list [1]. At each successively higher level of sensitization, the number of candidates that were removed from the list or died increased, and the number of candidates who were transplanted decreased. As such, we would posit that the presence of any HLA antibodies defines a candidate as sensitized.

What should we do about sensitization?
Given that sensitized heart transplant candidates experience prolonged waiting times which in turn puts them at increased risk of decompensation before a transplant becomes available, several strategies can be utilized to improve outcomes for this population. First, careful consideration should be paid to selection of antigens to exclude. In addition to the Luminex single-antigen assay, our center utilizes dilution, C1q and complement-dependent cytotoxicity studies [10]. Next, desensitization with a variety of pharmacologic regimens can be implemented [11]. Desensitization has modest effects on CPRA, but as discussed above, any change in CPRA will result in a larger potential donor pool and thus increase access to transplantation.

Sensitization represents an important health disparity; most highly sensitized heart transplant candidates are women (67% of candidates with CPRA 80% to 100% are women, as compared to 20% of candidates with CPRA 0% to 20% [1]). Canada has implemented the "4S" (Category 4 for sensitized patients) system, which prioritizes organ allocation to sensitized patients based on their CPRA [12]. The 4S system was recently shown to effectively increase the size of the donor pool and improve transplant access for these very highly sensitized candidates. An allocation benefit for sensitization has existed in kidney transplantation in the United States since 1997. In 2014, the kidney allocation system was updated to provide an allocation benefit to sensitized candidates according to a sliding scale. A recent analysis has shown that this system has resulted in a significant increase in access to transplantation for extremely sensitized candidates [13].

In summary, sensitization is a common and important risk factor for adverse outcomes in heart transplant candidates and recipients. CPRA is the current best metric of sensitization and is undergoing updates, such as the ability to add decimal places, the use of LCD, calculation of CPRA based on HLA alleles and the addition of the HLA-DQA1, -DPA1 and -DPB1 loci. Further research is urgently needed to help improve access to transplant and to mitigate the immunology risk of these candidates. ■

Disclosure Statement: The authors have no conflicts of interest to disclose.


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  2. Betkowski AS, Graff R, Chen JJ, Hauptman PJ Panel-reactive antibody screening practices prior to heart transplantation. J Heart Lung Transplant 2002;21(6):644-50.
  3. Zachary AA, Braun WE Calculation of a predictive value for transplantation. Transplantation 1985;39(3):316-8.
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  6. Kransdorf EP, Pando MJ Calculated panel reactive antibody with decimals: A refined metric of access to transplantation for highly sensitized candidates. Hum Immunol 2017;78(3):252-6.
  7. Tremblay S, Shields A, Alloway R, et al. A prospective iterative trial of carfizomib-based desensitization trial: initial comparative observations. Am J Transplant 2017;17(S3):243-4.
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  9. Sullivan HC, Liwski RS, Bray RA, Gebel HM. The road to HLA antibody evaluation: do not rely on MFI. Am J Transplant 2017;17(6):1455-61.
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  11. Chih S, Patel J Desensitization strategies in adult heart transplantation-Will persistence pay off? J Heart Lung Transplant 2016:1-10.
  12. Clarke B, Ducharme A, Giannetti N, et al. Multicenter evaluation of a national organ sharing policy for highly sensitized patients listed for heart transplantation in Canada. J Heart Lung Transplant 2017;36(5):491-8.
  13. Stewart DE, Kucheryavaya AY, Klassen DK, Turgeon NA, Formica RN, Aeder MI. Changes in deceased donor kidney transplantation one year after KAS implementation. Am J Transplant 2016;16(6):1834-47.

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