*Purpose: Epitope matching has been shown to better predict allograft survival and development of de novo donor specific antibodies. In previous study, the number of epitope matching was shown to have additional risk in the low level of HLA mismatch (HLA mismatch less than 3). We tried to assess whether its clinical usefulness is still available in Korean kidney transplantation setting, where a living donor kidney transplantation prevails.

*Methods: Patients included in Korean Organ Transplantation Registry (KOTRY) were used. Kidney transplant recipients who have been transplanted from 2014 to 2017 were enrolled. HLA 4 digits genotype were imputed by matching the serotype combination (2 digits) with the alleged 4 digits haplotype distribution in Korean population, which were publicly available data in the bone marrow donor program. DQ genotyping were imputed by using linkage-disequlibrium association. Primary outcome measurement was acute rejection within 1 year, which was defined as a mixture of any kind of biopsy proven rejection and clinical rejection.

*Results: Among 3,422 donor-recipient pairs, 4 digits haplotype of 2,806 (82.0%) pairs were successfully imputed. Mean number of mismatched class I and class II eplet were 10.5 + 6.7 (Range; 0 – 35) and 23.7 + 17.2 (0 – 85), respectively. Mean eplet mismatch numbers were increased according to the number of HLA antigen mismatch (Beta 4.503, 95% C.I. 4.391 – 4.615, p<0.001). Eplet mismatch numbers were correlated with HLA antigen mismatch numbers. (Spearman’s rho 0.760, p<0.001) Eplet mismatch was associated with acute rejection in the unadjusted models. (Class I 1.021, 95% C.I. 1.005 - 1.037, p=0.011; DR 1.019, 95% C.I. 1.006 - 1.031, p=0.003; DQ 1.011 (95% C.I. 1.001 - 1.021, p=0.034) When controlled with clinical covariates and HLA mismatch numbers, eplet mismatch numbers did not show additional predictability to acute rejection. (AUC comparison; eplet model vs. HLA serotype model, 0.706 (95% C.I 0.648 - 0.763) vs 0.704 (95% C.I. 0.557 - 0.761), p=0.819) However, eplet mismatches over 60 was shown to be significant risk predictors in the low HLA serotype mismatches (1 or 2 mismatches) in a non-linear model.

*Conclusions: Eplet mismatch numbers predicted acute rejection in similar ways as HLA antigen mismatches. Additional predictability of eplet mismatches to acute rejection was shown to be significant in low degree HLA mismatch (1 or 2 mismatches), which verifies a finding of previous study.

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