B cells bind PF4/heparin complexes ex vivo and in vivo. (A) Heparin displaces variable amounts of PF4 leading to B-cell binding of PF4/heparin complexes in some healthy subjects. Blood from 6 healthy donors was incubated with various concentrations of UFH as indicated. Binding of PF4/heparin complexes to B cells as assessed by binding of KKO is shown. (B) Detection of PF4/heparin complex bound circulating B cells in 3 patients receiving heparin. Blood was stained for CD19 and KKO. Dot plots show variation in binding of KKO to lymphocyte-gated populations. (C) Variations in KKO binding of PF4/heparin complexes to B cells in vivo in patients receiving UFH for medical indications (MI; n = 8) or for cardiopulmonary bypass surgery (CPB; n = 8). Each symbol represents B cell binding in an individual patient. The y-axis indicates maximal percentage of KKO-bound B cells. (D) In vivo B-cell binding is heparin dependent. A patient undergoing UFH therapy for treatment of cerebral venous thrombosis was monitored for percentage of KKO-positive B cells over 48 hours of UFH therapy. Time from start of heparin therapy, UFH dose, and activated partial thromboplastin time (aPTT) are shown.