Cell-Positive, B-Cell-Negative Flow Cytometry Crossmatch: Frequency, HLA Locus Specificity, and Mechanisms Among 3073 Clinical Flow Cytometry Crossmatch Tests

Objectives: A T-cell-positive and B-cell-negative flow cytometry crossmatch result remains a conundrum since HLA class I antigens are expressed on both T and B cells. We investigated the frequency, donor HLA specificity of the antibodies, and mechanisms for these crossmatch results. Materials and Methods: We analyzed 3073 clinical flow cytometry crossmatch tests performed in an American Society of Histocompatibility and Immunogenetics-accredited histocompatibility laboratory. The sera associated with the T-cell positive and B-cell negative flow cytometry crossmatches were also tested for donor HLA immunoglobulin G antibodies using LABScreen single antigen assays. Results: Among the 3073 test results, 1963 were T-cell negative and B-cell negative, 811 were T-cell negative and B-cell positive, 274 were T-cell positive and B-cell positive, and 25 were T-cell positive and B-cell negative. The LABScreen single antigen assay detected HLA class I immunoglobulin G donor-specific antibodies in 23 of 25 sera associated with a T-cell positive and B-cell negative flow cytometry crossmatch result, and donor-specific antibodies directed at not only HLA-Cw but also at HLA-A or HLA-B were observed. In addition, we identified that the B-cell channel shift threshold used to classify a B-cell flow cytometry crossmatch was a potential contributor to a T-cell-positive and B-cell-negative flow cytometry crossmatch result. Conclusions: Our analysis of 3073 flow cytometry crossmatches, in addition to demonstrating that HLA antibodies directed at the HLA-A, -B, or -Cw locus were associated with a T-cell-positive and B-cell-negative result, identified mechanisms for the surprising T-cell-positive and B-cell-negative flow cytometry crossmatch result.