P018 : IDENTIFICATION OF UNACCEPTABLE HLA ANTIGENS WHEN PRESENT IN TRANSPLANT PATIENT SERA AT AN UNDILUTED OR NEAT TITER BUT NOT PRESENT AT A LOW TITER (⩽1:16)

Abstract

Characteristics identifying clinically deleterious HLA antibodies (Ab) may include specificity, mean fluorescence intensity (MFI) and C1q reactivity. These factors are also used when identifying unacceptable HLA antigens (Ags). However, if unacceptable HLA Ab specificities are present in undiluted sera but not present when sera are diluted 1:16 then these Ab specificities may be ignored when deciding on which unacceptable Ab specificities to report. This reduction in unacceptable Ags would allow for easier matching of highly sensitized patients (with high numbers of HLA Ab specificities) to an appropriate donor. Thirty-nine cI/cII high PRA sera were evaluated for the presence of HLA Abs and their Ag specificities at neat and at a 1:16 titer. The number of Ab specificities were compared at the two dilutions. One Lambda, Inc. reagents were used per the manufacturers instructions. The HLA cI/cII PRAs were 77 ± 25% and 84 ± 21% respectively for the 39 sera. The total number of HLA cI/cII Ags identified in undiluted sera were 46 ± 22 and 34 ± 22 respectively per patient. In addition, there were 7 ± 8 cI and 5 ± 4 cII falsely identified HLA Ags respectively per patient. When sera were titrated to a 1:16 dilution 71 ± 25% of cI and 51 ± 28% of cII unacceptable Ags were eliminated. The total number of cI Ags eliminated were 21 ± 10 and 15 ± 9 cII Ags. When false positive Ags were included in the evaluation the frequency for Ag elimination at a 1:16 dilution for cI became 85 ± 14% and 66 ± 24% for cII respectively. Fifty-four patients with donor specific antibody (DSA) titers of ⩽1:16 were identified. 81% (44/54) had both negative cytotoxic (AHG) and flow cytometric (FCXM) donor specific crossmatches whereas 10 patients (19%, 10/54) displayed a negative AHG but a positive FCXM. When transplanted, the FCXM negative patients had a two year graft survival of 91% vs. 60% for the FCXM positive patients ( p These data suggest that titration studies of high PRA, multiple Ab specificity presenting sera, can be used when identifying unacceptable Ags. Moreover, while the presence of DSA in low concentration (⩽1:16) may falsely suggest a positive virtual crossmatch it may not result in a real time positive crossmatch. HLA Ab specificities that are eliminated at a low titer (1:16) may allow for easier matching of highly sensitized patients to donors for transplant.