VIRTUAL CROSSMATCHING: IT'S NOT AS SIMPLE AS IT SEEMS.

Aim Use of the virtual crossmatch (XM) has increased the efficiency of organ allocation and reduced the number of organs declined based on unexpected positive XMs (Cecka JM., et al. Am J Transplant. 2011). Despite these benefits, a cell based XM still provides a final safeguard before transplantation. False negative XM predictions may arise when reactivity is due to both antigen specific antibodies and antibodies to crossreactive epitopes. Methods HLA-specific antibodies were assessed using class I or class II phenotype panels and IgG or C1Q single antigen bead assays. (Lifecodes Immucor; Labscreen, One Lambda,Inc). One wash CDC B cell XMs were performed. Results Luminex IgG tests yielded reactivity with DR53 phenotypes ranging from 3,000-10,000 MFI with DR53 heterozygotes and >10,000 MFI with homozygotes. Reactivity with DR10 phenotypes were 2,000-3,000 MFI. Single antigen beads yielded reactions with DR53 of 20,000 MFI with undiluted serum and 17,000 MFI with serum diluted 1:8. Reactions with DR10 were 15,000 and 2,000 MFI with undiluted and 1:8 diluted serum, respectively. MFI values given above would predict negative XMs with DR10+/DR53- cells and positive XMs with DR10-/DR53+ cells. The C1q assay yielded MFI values of 28,000 with DR53 and 300 with DR10. These C1Q results substantiate the prediction of a negative CDC XM result with a DR10 cell. CDC XM tests yielded the following results: + with a titer of 2 with a DR10+/DR53- cell, + with a titer of 32 with a DR10-/DR53 homozygote cell, and negative with DR53 heterozygotes and with cells bearing other antigens crossreactive with DR10 (DR1 and DR15/51). Conclusions We propose that the CDC+ XM with the DR10 cell was due to contributions from the DR10 antibodies and the much stronger antibodies to DR53 that shares one or more epitopes with DR10. These data show that accurate prediction of crossmatches must take into account the possible contributions of antibodies positive for antigens crossreactive with the target antigen.