Comparison of mAb 1562 and 6H4 binding to resting and activated normal and PNH platelets.

Resting (R) and activated (A) platelets of healthy donors and PNH patients were double labeled with CD55 antibody and anti PrPc mAbs. Binding of mAbs 1562 and 6H4 to CD55⁺ donor (n = 6) and patient (n = 8) platelets and to CD55⁻ patient platelets was evaluated by flow cytometry. Mean fluorescence values were converted to RFI. Results are presented as box and whiskers plots of RFI, indicating the median (full line) and the 25th to 75th and the 10th to 90th percentiles of the group distribution as boxes and error bars, respectively. Outlying values are plotted as full circles. Resting and activated normal platelets expressed more PrPc than did CD55⁺ patient platelets. Resting CD55⁻platelets were negative for PrPc (note different scale), whereas activated CD55⁻ platelets expressed low levels of PrPc. This up-regulated PrPc on CD55⁻ platelets was not detected by mAb 6H4 (with one exception), indicating that the C-terminal part of the molecule was not accessible to antibody.