169-P: SIGNIFICANCE OF FLOW CYTOMETRIC CROSSMATCH IN THE REALM OF VIRTUAL CROSSMATCH: A CASE OF ALTERED HLA EXPRESSION ON CELL SURFACE

Abstract

Aim Refinement and better understanding of solid phase assays (SpA) to detect anti-HLA antibodies has resulted in a good correlation between actual crossmatch (XM) and a virtual crossmatch (VxM). This has lead to a greater reliance on VxM for organ allocation and selection. We present a case of weak positive flow cytometric XM (FxM) that was predicted to be a strong positive FxM on the basis of VxM. Methods Case: 46 year old male with diabetes for second renal transplant. Patient is alloimmunized and demonstrated allele specific antibody to the donor (DSA) HLA-DQB1∗02:01-DQA1∗05:01 (DQ2.5) with mean fluorescence intensity (MFI) of 15413. In our laboratory this would be predicted to result in a strong positive FxM defined as mean channel shift (MCS) >250 (cutoff >106). B-cell FxM (B-FxM) with donor cells was positive with MCS: 173 and presence of an autoantibody. Results Investigations for this case of low positive XM with an autoantibody and DSA MFI 15413 are summarized: 1. Repeat B-FxM MCS: 171 2. B-FxM with index donor cells and serum from three other patients with DQ2.5 antibody around 15000 MFI was low positive:[ Table 1 ] 3. B-FxM using index patients serum and a surrogate donor with DQ2.5 was MCS: 288 (strong positive). 4. HLA-DQ2 expression on donor cells by flow cytometer using monoclonal (Mab) anti-DQ (FM5148) and anti-DQ2 (FD100) antibodies revealed no DQ2 expression on the donor cells.[ Table 2 ]Patient was transplanted (Tx) and monitored post-Tx with SpA. At 2 months post-Tx serum creatinine is 1.0 mg/dL and DQ2.5 MFI: 13276. Conclusions This is a case of low positive FxM in the presence of DSA MFI around 15000 due to altered HLA expression on donor cells. Only a VxM could have resulted in the patient being denied Tx indicating that actual XM is required for organ allocation and selection. Chen: Thermo Fisher Scientific: Employee. Lee: Thermo Fisher Scientific: Employee.