Figure 1.

Induction of C4d deposition on the surface of platelets in vitro. (A) Schematic illustration of the in vitro P-C4d induction experiments. C4d-negative (C4d−) cells are incubated with plasma or Ig isolated from C4d-positive (C4d+) SLE patients, and subsequently with normal human serum (complement). After treatment, levels of C4d deposited on platelet surfaces were analyzed by flow cytometry; (B) Platelets from a healthy individual were incubated with the plasma of two P-C4d+/aPL+ SLE patients, one P-C4d−/aPL− SLE patient, three P-C4d−/aPL+ SLE patients, and one healthy control, followed by incubation with complement. Note the high levels of C4d deposited on cell surfaces after treatment with P-C4d+/aPL+ SLE plasma, but not with P-C4d−/aPL− SLE plasma, P-C4d−/aPL+ SLE plasma, or healthy control plasma. Purple histogram: isotype control; green open histogram: anti-C4d; (C) C4d deposition was induced in vitro only when platelets were treated with P-C4d+/aPL+ plasma, followed by normal human serum. C1q-depleted or heat-inactivated human serum was incapable of inducing C4d deposition; (D) Plasma of patients with primary antiphospholipid syndrome was incapable of inducing C4d deposition on platelets in vitro. P-C4d: platelet-bound C4d; aPL: antiphospholipid antibodies; P-C4d+/aPL+: P-C4d positive/aPL positive; P-C4d−/aPL−: P-C4d negetive/aPL negative; P-C4d−/aPL+: P-C4d negetive/aPL negative; SLE: systemic lupus erythematosus.