P167 A case of positive virtual crossmatch and negative crossmatch: Role of fetal calf serum

Case A 69-year-old male with chronic progressive renal failure was evaluated for living donor kidney transplantation. Two prior donors were turned down due to donor specific anti-HLA antibodies (DSA) resulting in a positive flow crossmatch (FXM). Laboratory Testing Low to medium resolution HLA typing was performed on the donor-recipient pair by reverse SSO method. Recipient serum was screened for anti-HLA antibodies by single antigen beads (SAB, LabTYPE, One Lambda) and FXM was performed using donor lymphocytes. Results Donor is 4/6 mismatch with the recipient at HLA-A,B and DR (Donor DR 4,11 and recipient DR 11,13). SAB demonstrated antibodies to HLA-DR4 (DSA) and DR16 with MFI >2000 which is predicted to result in positive FXM at our center. However, FXM was T and B cell negative. Investigations To understand the clinical significance of this DSA, additional investigations were performed. Donor typed as DRB1∗04:04. Surrogate crossmatches with two additional DRB1∗04:04 donors using the same recipient sera were negative. To rule out the possibility of rare donor null allele, crossmatch with the donor cells and surrogate sera with high DRB1∗04:04 MFI, resulted in a positive crossmatch. As the HLA antigens on the beads are derived from cell lines that have been cultured in fetal calf serum (FCS), one could speculate that this reactivity maybe due to binding of naturally occurring heterophile antibodies to beads. To test this hypothesis serum was tested after pre-treating the beads with FCS. Indeed testing recipient sera after FCS treatment resulted in removal of the ‘spurious’ anti-HLA reactivity (figure). Download high-res image (416KB) Download full-size image Conclusions With increasing use of VXM to screen donors, this case adds another exception when positive VXM does not result in positive FXM due to spurious bead reactivity. Such false positive VXM may result in declining a donor for transplantation. Pre-treating the beads with FCS provides target antigens that block this spurious reactivity. We have observed this reactivity specifically to DR4-DR16 beads only.