Effects of PEG-rHuMGDF therapy on circulating levels of antiplatelet GP IIIa_49-66 antibodies. Stimulation of megakaryocytopoiesis by PEG-rHuMGDF (3 subcutaneous twice weekly doses of 25 μg/kg) increased the peripheral concentration of platelets 10-fold (○), peaking at day 12 and followed by a gradual return to baseline values. The mean concentration of circulating antiplatelet GP IIIa_49-66 antibodies exhibited a reciprocal reduction (•) during the period when the peripheral platelet counts were elevated. This transient decrease in antibody levels is attributable to depletion resulting from high-affinity binding to the greatly expanded pool of platelets in the circulation.