Anti-β₂GP1 antibodies induce cell surface E-selectin expression and the release of endothelial cell microparticles. Endothelial cells were activated by incubation with 100 nM β₂GP1 and 600 nM affinity-purified rabbit (A-B) or human (C-D) anti-β₂GPI antibodies for 8 hours. Human IgG anti-β₂GPI antibodies used in this experiment were affinity purified from a patient with a history or recurrent deep vein thrombosis who had an anti-β₂GPI antibody level of 60 SGU and a lupus anticoagulant. Results obtained using anti-β₂GPI IgG from this patient are representative of 2 additional patients that were also studied. Endothelial cell surface E-selectin (A,C) was measured as described in “Materials and methods.” Endothelial microparticles were measured in conditioned medium using flow cytometry after staining with anti-CD144 antibodies (B,D). For consistency, data are presented as the percent increase in E-selectin expression or microparticle release caused by control or anti-β₂GPI IgG relative to that by cells incubated in medium alone. All values were determined in quadruplicate, and all assays repeated at least 3 times. Differences between control and experimental conditions were assessed using the Student 2-tailed t test. Ctrl, control; Hum, human; Rab, rabbit.