T-cell responses to β2GPI-treated anionic substances present in circulation. PBMCs from APS4 were cultured with or without various antigen preparations in medium supplemented with autologous APS plasma. Antigens used included β2GPI alone, as well as PS, LDL, oxLDL, resting platelets, and activated platelets, which were treated either with or without β2GPI. Thrombin, which was used to activate platelets, in combination with β2GPI served as a control. T-cell proliferative response was measured by 3H-thymidine incorporation. Results are shown as mean (column) and standard deviation (error bar) of duplicate measurements. Analogous results were obtained in samples from all 5 patients with APS.