P129 Missing the forest for the trees: Using shared epitope analysis tools to understand crossmatch results

Abstract

Aim A positive crossmatch with no identifiable donor specific antibody (DSA) is often considered not to be a contraindication for transplant. However, accurate HLA antibody identification is critical for assessing the risk of transplant in the presence of a positive crossmatch. In this case study, we present a 45 year-old female who had a strong positive flow cytometric crossmatch (FCXM) but no obvious donor specific antibody (DSA) detected. The patient had multiple pregnancies with no autoimmune disease. Our goal was to investigate the cause of this patient’s positive crossmatch with no apparent DSA to assess the risk of transplanting this patient. Methods Single antigen bead (SAB), LABScreen PRA (LSPRA), and FlowPRA Screen (FlowPRA) assays were used to identify HLA antibodies. Epitope analysis was performed using the Epitope Registry and EpVix. Flow cytometry crossmatches were performed using the Rapid Optimized FCXM protocol. Initial DSA results were based on the SAB assay using the laboratory’s established positive/negative MFI cutoff ( ⩾ 1000). Results The results of the FCXM were T-cell negative (MCS = 0) and B-cell positive (MCS = 208). The donor’s class II HLA typing was DR11,-; DQ5,-; DR52. Although the DR8, 11, 12, 13, 14, 17, and 18 beads were at the top of SAB panel, all had MFI of less than 1000 MFI (DR11 MFI = 500). The LSPRA assay showed some weak reactivity; however, the class II FlowPRA assay was strong positive. Therefore, we hypothesized that antibody against a shared epitope was likely to be the cause of the strong positive FlowPRA and observed positive FCXM. Epitope analysis using the EpVix program demonstrated that the 96HK epitope is shared among DR8, 11, 12, 13, 14, 17, and 18, thus supporting our shared epitope hypothesis. To determine if patient’s antibodies may result in positive crossmatch with other donors sharing the same epitope, we performed surrogate FCXMs. The results confirmed that the patient had positive B-cell crossmatches with donors who expressed DR8, 11, 12, 13, 14, 17, or 18 antigens. Conclusions Utilizing multiple assay platforms can help elucidate antibody specificities. Performing epitope analysis is useful to confirm shared epitope reactivity. In the future, listing unacceptable epitopes may be a better method to eliminate positive crossmatch than listing unacceptable antigens.