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.