P145 Further evidence of the relevance of DP epitope analysis in the virtual and actual crossmatch in solid organ transplantation. A case report

Abstract

Aim Epitope-based HLA matching has been developed as a better matching method in solid organ transplantation. Here we present a highly sensitized patient with 100% cPRA but low donor specific antibody (DSA) level at the DP locus. Virtual crossmatch (vXM) with a cadaveric donor predicted a negative result but the actual crossmatch gave an unexpected positive result in B cells maybe due to the presence of a cross-reactive epitope. The clinical relevance of antibodies directed against HLA-DP antigens is discussed in the literature with conflicting reports. Methods Single antigen bead assay (SAB) was used to detect the levels of alloantibodies in the patient serum. Flow crossmatches were performed with 3 conjugated antibodies: FITC-anti-Human IgG, CD3-RD1 and CD19-PC5. Serum-incubated cells were stained with fluorochrome conjugated antibodies prior to running on Navios instruments. Test was done using current and historical sera from the patient. Results SAB analysis showed high levels of alloantibodies in both Class I and Class II but only a DSA for DP6. In our lab, DSA are considered “Positive” for HLA-DP when MFI values are ⩾ 15,000. MFI values for current and historical sera were 7536 and 10520 respectively suggesting a negative crossmatch. FCXM results were negative in both T and B cells for the last serum but unexpected positive crossmatch only in B cells was observed in the historical serum. Retrospective analysis of previous sera showed a well-defined antibody to the DP epitope, 56E, with gradual decreasing levels of MFI. The apparent discrepancies between the virtual and actual crossmatch results can be probably due to the “peanut butter” effect of this cross-reactive epitope. The patient was transplanted based on the negative crossmatch from current serum followed by immunosuppression protocol. Conclusions Our results provide evidence to the impact of epitope analysis in the vXM and its effect in the actual crossmatch. Implementation of epitope-based HLA matching in our lab will help to better select compatible donors. Characterization and matching DP epitopes using computational methods seems to be more relevant than common matching alleles. Transplant in the presence of DSA at the epitope level needs to be closely monitored to determine their true clinical relevance in solid organ transplantation.