Sensitized patients waiting for renal allografts that have preformed antibodies against donor-specific HLA antigens are at risk of hyperacute, accelerated acute antibody-mediated rejection and poor graft outcome. Panel reactive antibodies (PRAs) have been used to measure the relative degree of sensitization in renal allograft recipients. PRA levels represent the percentage of likely cross-match incompatible donors, and are determined by testing recipient sera against cells from a panel of HLA-typed donors or solubilized HLA antigens attached to a solid phase. The panels should be representative of the local pools of potential organ donors. However, the results of PRA testing can be highly variable and inconsistent depending on the panel composition and the techniques used for HLA antibody detection [1, 2].

The development of solid phase-based assays that use solubilized HLA antigens has greatly increased the ability to detect and identify HLA-specific antibodies [3-5]. In particular, the use of recombinant single antigens (SA) in the Luminex assay makes it possible to detect HLA-specific antibodies with greater sensitivity and accuracy. Calculated panel reactive antibody (CPRA) values are based on the HLA antigens that are listed as unacceptable for renal transplant candidates. The unacceptable HLA antigens can be identified by the presence of HLA antibodies in the sera of transplant recipients [2]. This assessment can predict crossmatch-positive donor kidneys (as a virtual crossmatch) and has increased the efficiency of organ allocation.

A kidney allocation process using CPRA has been established in the United Network for Organ Sharing (UNOS) and Eurotransplant allocation system. UNOS awards sensitized patients with CPRA levels ≥80 an additional point to increase their access to potentially compatible donors. Furthermore, the organ procurement network does not offer organs expressing unacceptable HLA antigens to recipients who have HLA antibodies against those particular antigens.

In contrast, the Korean Network for Organ Sharing (KONOS) does not administer the PRA or CPRA, and only uses crossmatch results to measure sensitization for the renal allocation system. This is probably due to the variability in PRA methods, a lack of organized guidelines, and the differences between the antigen composition in commercial PRA panels and that in the Korean population. Therefore, a more uniform and accountable method for measuring sensitization to HLA antigens based on Korean HLA phenotypes is needed. In this study, we developed a CPRA calculator using the HLA phenotypes of Koreans to represent the percentage of actual donors expressing unacceptable HLA antigens; then, we compared this CPRA approach with the traditional PRA approach using Luminex technology.

The use of CPRA is rapidly being adopted by transplant laboratories worldwide. The UNOS established a CPRA calculator using HLA-A, HLA-B, HLA-DR, and HLA-DQ frequencies derived from the phenotypes of more than 12,000 donors recently entered into the OPTN registry [10]. The ETRL-CPRA program uses HLA typing data from about 4,000 organ donors, including 1,000 donors from different participating countries. The ETRL-CPRA calculation includes the frequencies of HLA-A, HLA-B, HLA-C, HLA-DR, and HLA-DQ.

In clinical laboratories in Korea, the use of PRA tests with a solid phase assay is increasing. Additionally, PRA-SA tests have recently been introduced in a few laboratories. Therefore, virtual crossmatch prediction by CPRA using unacceptable antigens could be used to increase the kidney allocation efficiency for sensitized patients awaiting transplantation.

To obtain the CPRA, a list of unacceptable antigens is required instead of a PRA value. In this study, we developed a CMC calculator based on the HLA-A, HLA-B, and HLA-DR frequencies derived from the phenotypes of 1,662 Korean donors. The CPRA represents the percentage of potential donors who have 1 or more unacceptable HLA-A, HLA-B, or HLA-DR antigens. Although the pool of HLA phenotypes for the CMC-CPRA calculator was much smaller than those for the UNOS or ETRL calculators, the phenotype data from the CMC-CPRA calculator closely corresponded to that obtained in Korean populations. Although the phenotype frequencies of several antigens differed significantly, the antigens with different frequencies compared to the previous data (by 2.1-3.7%) were B35, B62, and DR15. These discrepancies might be due to differences in population sampling or the HLA typing methods that were used. Since HLA-A, HLA-B, and HLA-DR antigen frequencies differ among various populations [11, 12], antigen frequencies in the CMC-CPRA calculator differed from those in the UNOS or ETRL programs. The most striking differences were the frequencies of HLA-A1, A3, A24, and A33 at the HLA-A locus.

Traditional PRA tests consist of a panel or pooled antigens, including either HLA class I or class II antigens, and represent the %PRA. In this study, we used the higher %PRA values based on either the class I or class II panel to compare with the %CPRA values. Although the PRA values from PRA-screen and PRA-ID tests and the CPRA values obtained from PRA-SA test varied (Fig. 2), the concordance rates for these were above 80% for the detection of broadly sensitized sera (PRA or CPRA levels greater than 50% or 80%). In a previous study, concordance was generally lower in the lower PRA groups due to an underestimation of sensitization using traditional PRA-ID testing [2]. In contrast, our study included a considerable number of cases that had higher PRA values than the CPRA in the lower CPRA groups. This may be due to less agreement for weaker antibodies, and additive or synergistic effects of multiple weak antibodies in the PRA-screen and PRA-ID assays. Because weak antibodies may not correlate with a positive flow cytometry crossmatch [13, 14], and appear to have little clinical importance [15-17], further studies will be necessary to establish the best strategy for predicting strong positive crossmatches [2]. It is also possible that the decreased agreement is due to the detection of anti-C or anti-DQ antibodies by the PRA-screen and PRA-ID tests. The HLA-C, HLA-DQ, and HLA-DP antigens are currently excluded from the CMC-CPRA calculation because adequate donor typing data to estimate their frequencies were not available. Since there were several reports indicating that donor-specific anti-C, anti-DQ, or anti-DP antibodies can induce antibody-mediated rejection [18-20], these antibodies should be included in the CPRA calculators in the future. In addition, allele-specific antibodies can be detected by PRA-SA tests, which can lead to a positive crossmatch when the target antigen is present in the donor. Therefore, it may be important to account for all HLA specificities in the CPRA calculator and to test donor HLA typing at the allele level, compatible with HLA antibody detection.

Although PRA and CPRA values had more than 80% agreement for the detection of broadly sensitized sera in this study, CPRA values would provide more consistent data for reporting sensitization than PRA values. Therefore, in KONOS, CPRA can be used to standardize sensitization measurements and can be used for standardized allocation of kidneys in an effort to compensate for the biological disadvantage of broadly sensitized patients. Moreover, high pretransplant PRA values (>50% or >80%) are known to be a poor prognostic marker in living donor renal allograft. Therefore, CPRA can also be used as a more accurate perioperative immunologic marker, and the adoption of virtual crossmatching using unacceptable antigens may increase the number of successful kidney transplantations in sensitized patients [21-23].

In conclusion, the application of unacceptable antigens and CPRA based on HLA phenotype frequencies in Koreans may be useful for the accurate and consistent measurement of sensitization and thus promoting efficient kidney allocation. Additional clinical studies are needed to confirm the benefit of CPRA calculations for organ allocation and successful transplantation.