*Purpose: Following the allocation of a transplant by the allocation system, each pediatric transplant team reviews the proposal and decides based on various criteria whether to accept or refuse this proposal. Donor/recipient HLA matching assessed at the antigenic level is a major criteria in this decision. However, recent technical improvements allow a more precise assessment of HLA matching at the epitope level and epitope load is strongly associated with donor-specific antibodies appearance, which is known to reduce graft survival. The aim of this study was to assess the potential benefit of selecting transplants based on epitope matching assessed by the epitope load compared to antigenic matching.

*Methods: We conducted a retrospective study including all patients ≤ 18 years old waitlisted in 2 pediatric kidney transplant centers in Paris between 2012 and 2017. The first analysis focused on factors associated with the acceptance of the proposal. The second analysis included patients with at least one accepted proposal and one rejected proposal because of poor HLA matching. We compared the epitope loads estimated using HLA matchmaker between accepted and rejected proposals.

*Results: We included 164 patients that received a total of 757 proposals. Among them, 64 had at least one proposal accepted and one proposal rejected for poor HLA matching. The main factor associated with rejecting a transplant was the number of antigenic HLA mismatches (p<0.0001). There was a trend towards a higher probability of acceptance for high quality donors (determined by lower serum creatinine and proteinuria). We identified 27 proposals in 19 patients that were refused despite a lower epitope load than the proposal finally accepted for this patient. This resulted in an increased time on dialysis (median 4months IQ [1.4-6.1], max 22 months). Among these 27 proposals, 11 presented more antigenic mismatches despite a lower epitope load.

*Conclusions: This is the first pediatric study reporting that high quality transplants with low epitope loads are rejected by transplant teams based on the antigenic assessment of HLA matching. Thus, assessing HLA matching at the epitope level may allow the allocation of kidneys that would otherwise be rejected and the reduction of time on dialysis in some patients. The assessment of the global effect of an epitope-based allocation system requires further studies.

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