Flow cytometry with β₂GPI. Binding of open or closed β₂GPI to monocytes was investigated. Cells (1 × 10⁶ cells/mL) were incubated for 30 minutes with fluorescently labeled β₂GPI ± LPS or β₂GPI ± fluorescently labeled LPS. Open β₂GPI (●), but not plasma-purified closed β₂GPI (▴), showed a concentration- (A) and time-dependent (B) binding to monocytes (both n = 5). Incubation of plasma-purified closed β₂GPI (0.2μM) with fluorescently labeled LPS showed binding of β₂GPI to monocytes (; n = 3; C). Similar binding could be observed with preincubated fluorescently labeled plasma-purified β₂GPI (0.2μM) and LPS (; n = 3) (D). (E) Binding of open β₂GPI (0.3μM) to monocytes could be partially inhibited (approximately 70%) by addition of RAP. (F) Addition of increasing concentrations of RAP (0-10μM) also reduced the inhibitory effect of β₂GPI (1μM) on LPS-induced TF expression on monocytes (n = 3). Results are expressed as mean ± SEM.