ROLE OF HLA–C EPITOPES IN PREDICTION OF A NEGATIVE FLOW CROSSMATCH WITH HLAMATCHMAKER

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TITLE: ROLE OF HLA–C EPITOPES IN PREDICTION OF A NEGATIVE FLOW CROSSMATCH WITH HLAMATCHMAKER

R. Vorhaben,¹ S. Hassen,¹ J. Tsai,¹ B. Kauper,¹ B. Lavingia,¹ P. Stastny.¹

¹Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX

HLAMatchmaker is a computer–assisted amino acid epitope matching algorithm used to identify compatible donors for highly sensitized patients. HLAMatchmaker uses both intra– and inter–locus epitope matching and degrees of epitope immunogenicity to determine donor compatibility. We have previously reported that a highly immunogenic epitope mismatch (iemm) of <5 showed a significant correlation with retrospective negative flow cytometry crossmatches (FCXM) (n = 43, PRA > 50%). In a second prospective data set of FCXM (n = 75, PRA > 79%) a single highly immunogenic epitope mismatch within the HLA–A or –B loci was associated with a significant risk for a positive flow crossmatch. 18/18 crossmatches which had 1–2 iemm were flow positive and a cutoff of <1 iemm to predict a negative crossmatch gave a x2 = 19.75. To investigate the role of C–locus epitope mismatching, molecular SSP typing was performed. With the addition of HLA–C results, the average number of epitope mismatches increased from 3.1 to 6.9 in pairs that were well matched for the A– and B–locus epitopes (A, B locus iemm >2). There were 3 cases in which a zero A and B epitope mismatched pair had a positive crossmatch and all were mismatched for HLA–C. To further evaluate the role of C–locus epitope antibodies in highly sensitized patients we performed FCXM with mouse fibroblast cell lines transfected with HLA–C antigens. The sera were first absorbed with the untransfected mouse fibroblast cell line Lneo, to remove heterophile antibodies. Preliminary results showed that sera from highly sensitized recipients were reactive with the transfected C–locus antigens. HLA–C should be considered when using the matching algorithm, as a single highly immunogenic epitope mismatch appears to increase the probability of a positive FXCM.