LBP17: A SCIENTIFIC MYTH: THERE IS MORE HLA CLASS I ON B CELLS THAN ON T CELLS

Aim In 1978, before class II Ags were defined, Pellegrino et al. reported that there is more HLA on B cells than T cells with equivalent beta2m levels on each cell type. Since then, the report has been widely misinterpreted to have shown there is more class I on B cells than on T cells and has led to a long standing interpretation of a T−/B+ FXM result as presence of either class II Ab or low level class I. A T+/B− FXM result has generally been interpreted as non-HLA. We studied the relative expression of CI antigen on T and B cells in 60 pts to determine validity of this dogma.” The aim of our study was to provide direct experimental evidence for HLA-class I Ag expression on T and B cells by probing class I with pan class I MoAb and class I-DSA positive serum. Methods Class I Probing: Biotinylated W6/32 MoAb in various concentrations (conc) was incubated with 0.1X106 PBMCs for 30 mins/RT. After washes, PerCP-CD3, PE-Cy7-CD19 and SA-PE were added and incubated for another 30 mins/RT. The final cells were acquired on a Canto-II and the PE intensities on T and B cells were measured. FXM: Standard T/B FXM procedure using (1) class I-DSA serum: doubling dilutions vs corresponding cells and (2) humanized W6/32 MoAb (hW6/32; InvivoGen): same conc as class I probing expt above. LMX-IgG SAB: One Lambda LabScreen. Results class I expression was evaluated on T and B cells with the W6/32 probing assay and FXM using hW6/32 MoAb and no significant difference was found (Fig. 1). FXM with class I DSA showed no significant differences between MCS on T cells and B cells (Fig. 2). In three examples (Fig. 3), T+/B− FXM results were observed in patients where only class I DSA was detected by the LMX-SAB. Conclusion Our study shows that class I antigen is evenly distributed on T and B cells. A T+/B− FXM cannot exclude the existence of class I DSA and the presence of class I Ab alone can result in T+/B−; T−/B+ or both T+/B+ FXM. Thus, a T+/B− FXM can reflect class I DSA, be potentially clinically relevant, and warrants further investigation before attributing to non-HLA antibody or no clinical risk.