Antibody screening is routinely performed on patients waiting for solid organ transplants and measures HLA antibodies. In 1985 Zachary and Braun20 described a useful mathematical method for calculating a predictive value for transplantation known as CPRA. This CPRA mathematical formula gave rise to the OPTN-CPRA calculator being widely used by the UNOS to generate CPRA scores for transplant candidates that reflect the distribution of HLA antigens in the donor pool. In addition to improving the efficiency of organ allocation, the implementation of CPRA significantly improved the deceased donor transplant rates among candidates with CPRA greater than 98% in the United States25.
The Federal Law No 5 of 2016 allowing transplantation of human organs and tissues26 and the Ministerial decree No 550 in 2017 to enable confirmation of brain death resulting from complete loss of brain function26 has led to the establishment of a multi-organ transplant program in the UAE27.
The Union71 Immunology and Histocompatibility Laboratory at SKMC serves the UAE national transplant program and UAE national transplant committee with its function in waitlist management and organ allocation. It also serves the various transplant programs in the country with, HLA typing of potential donors and recipients, Luminex SA testing of potential waitlist recipients, virtual cross-matching, physical cross-matching and clinical consultations.
In 2005 our group published the frequencies of HLA-A, -B, -DR, and -DQ phenotypes in the United Arab Emirates resident population28. Since then the population demographics of UAE have changed having more nationalities immigrating to UAE.
As the transplant program in UAE is growing, it becomes evident that we need a customized CPRA calculator to ensure there is equity in organ allocation so no group of the population gets disenfranchised. At this early stage of the UAE deceased organ transplant journey, two observations have come to light, firstly there is a high polymorphism of HLA coupled with ethnographical differences between the UAE people and populations in the west which would require us to exercise due diligence in using readily available online CPRA calculators. Secondly, replacing % SA with CPRA as a sensitization measure would give a more uniform and consistent indicator of sensitization for all potential candidates across all transplant centers in the UAE29.
We developed our UAE-CPRA calculator on similar lines to the UNOS-CPRA calculator, enrolling five HLA locus unacceptable antigens (HLA-A, -B, -C, -DRB1, and -DQB1) from retrospective HLA data of 1002 healthy living donors14. The relatively small donor size is a major limitation of our calculator. Nevertheless, its intended use is to serve as a tool in the UAE where the ethnic HLA phenotypes are best represented and where western online CPRA calculators cannot be applied. We are aware that the frequency of some of the HLA antigens could be over or underestimated but the percentages of positive reactions may not be skewed by the sample size of the donor panel18. The preliminary UAE-CPRA calculator is the first version that was developed as proof of concept and it can easily be upscaled and adapted to have a more accurate representation of the countries’ donor population by introducing the HLA typing of new donors as more data becomes available. It may also be noted that Luminex SA assays allow for the identification of HLA-DQA1 and DP antibodies too, but they were not included in the CPRA calculator build as we do not usually HLA type or report these loci. However, all deceased donors are tested for HLA-A, -B, -C, -DRB1, -DRB3, 4, 5, -DQA1, -DQB1, -DPA1, and -DPB1. So in the event of strong anti HLA- DQA1 or DP antibodies being present, potential candidates may be excluded by a positive virtual crossmatch. Comprehensive HLA typing of donors is important for virtual crossmatch because sensitized patients have antibodies against the antigen products of all HLA loci. To overcome this limitation and to improve the accuracy of the UAE-CPRA calculator, the next version will include extended HLA typing to include HLA-DQA1, DPA1 and DPB1 of deceased donors, as in the current Canadian CPRA calculator{Tinckam, 2015 #29} and the NMDP-CPRA calculator. This will further improve equity, as many highly sensitized candidates also have antibodies to these loci30,31.
Common HLA Haplotypes in the UAE resident population, A26-B8-Cw7-DR17-DQ2 and A2-B50-Cw6-DR7-DQ2, are not prevalent in the west. It has been previously suggested that perhaps A1 in the A1-B8-Cw7-DR17-DQ2 (A*01:01-B*08:01-C*07:01-DRB1*03:01-DQB1*02:01, Ranked 1 in the USA NMDP European Caucasians) was replaced by A26 in the Emirati population32. On the other hand, A2-B7-CW7-DR15-DQ6 and A3-B7-CW7-DR15-DQ6 (A*02:01-B*07:02-C*07:02-DRB1*15:01-DQB1*06:02, Ranked 3 and A*03:01-B*07:02-C*07:02-DRB1*15:01-DQB1*06:02 Ranked 2 in USA NMDP European Caucasians; n = 1,242,890) were not observed at all in the Emirati group33. The phenotype and haplotype frequencies that were common among the Other Arabs, South Asians, and Emiratis were overall not so frequent in the Caucasian population. These Haplotype Frequencies (HF) differences have been reflected in the correlation studies performed here, wherein the UAE-CPRA calculator scores correlated poorly with the OPTN-CPRA calculator. On the other hand, western calculators showed a good correlation with each other.
Another important factor that may influence CPRA scores is the accurate determination of HLA antibodies that are relevant to transplantation. Most solid organ transplant programs list HLA to which there are complementary antibodies in the patient’s serum as unacceptable donor mismatches. The purpose of this is to facilitate organ allocation and avoid organ transplantation with increased immunological risk. The use of solid-phase antigen assays mitigates this of associated HLA being listed as unacceptable and which unnecessarily limits the patient’s access to transplantation or results in needless desensitization treatment.
The UAE-CPRA calculator includes a high AF of HLA-Cw12 and Cw15. Since false-positive Luminex Single Antigen Cw1, Cw12, and Cw15 beads are likely caused by denatured antigens, less sensitive Luminex PRA beads, having more native antigens, may need to be used for confirmation of these antibodies. Contrarily, a failure to identify relevant HLA antibodies (false negatives) may result in the transplantation of organs with which there is an unanticipated increased risk of immunological injury34. As our deceased donation program only started in 2018, our current conversion rates are low and during the study period the deceased donation was at 1.1 per million population (pmp)35. The population of UAE is very young and with a crude mortality rate of 1.51 deaths per 1000 population36.
The laboratories’ strategies towards testing and identification of HLA antibodies should include a clear knowledge of the patient’s history of sensitization, access to high-resolution HLA typing, knowledge of Epitope or CREG analysis, use of a combination of PRA and Single antigen (SA) testing when required and utilization of serum treatment protocols such as dilution, heat inactivation, addition of EDTA or treatment with DTT.
Since our laboratory currently tests all waitlisted candidates, we can deliver consistent results in both their HLA typing and HLA antibody identification for entry of unacceptable mismatches in the CPRA program.
The development of the UAE-CPRA calculator was at no extra cost and updating it as more donors are enrolled, will be considered to overcome the disadvantage of the small donor sample pool. It will be made available to the whole UAE transplant community to help improve equity and allow more rationale and optimize deceased donor allocation. The UAE-CPRA data analysis showed that about 80% of the 110 patients in the deceased organ waiting in this study were highly sensitized (CPRA > 80%). In addition, in the highly sensitized group, the UAE CPRA calculator indicated that the correlation with western CPRA calculators was poor, additionally justifying the implementation of a CPRA calculator which more closely reflects the resident population. These candidates would benefit from the allocation of additional points in the UAE organ allocation program which is currently under development.
As the UAE-CPRA calculator mainly uses HLA typing data from donors of the Indian subcontinent and the Middle East, it may serve as a model for developing an Indian or a Middle Eastern CPRA calculator. This study has several limitations among which are the small donor sample size, a single center study and unavailability of HLA-DQA1, DPA1 and DPB1 donor data in the CPRA calculation. Despite this, the development of the UAE-CPRA calculator using the resident UAE population is a more objective way of determining the sensitization status of waitlist patients.
In the era of precision medicine and the ever-evolving histocompatibility field, our future approach will consider including High-Resolution HLA typing of all solid organ recipients and donors and epitope-based HLA matching by using specialized computer programs that extend the repertoire of acceptable antigens30,37,38,39,40.