Intravenous immunoglobulin G (IVIG) isused to treat idiopathic thrombocytopenic purpura (ITP). Although many patients benefit from IVIG, some are refractory to this therapy. ITP is characterized by platelet clearance mediated primarily by antiplatelet antibodies against GPIIbIIIa and/or the GPIb␣ complex. These 2 groups of antibodies may induce ITP through different mechanisms. We tested the hypothesis that IVIG may not be equally effective in preventing ITP caused by anti-GPIIbIIIa versus anti-GPIb␣ antibodies in mice. Thrombocytopenia was induced in BALB/c mice using monoclonal antibodies against either mouse GPIIbIIIa (JON1, JON2, and JON3) or GPIb␣ (p0p3, p0p4, p0p5, p0p9, and p0p11). Pretreatment with IVIG significantly ameliorated ITP in all anti-GPIIbIIIainjected animals. Conversely, IVIG failed to prevent ITP in all anti-GPIb␣-treated mice, except for p0p4. These results were repeated in C57BL/6 mice, and with different IVIG preparations. These data in mice suggest that patients with ITP mediated by anti-GPIb␣ antibodies may be less responsive to IVIG treatment. IntroductionIdiopathic thrombocytopenic purpura (ITP) is an autoimmune disease caused by autoantibodies generated against a patients' own platelets, leading to decreased platelet counts and a bleeding diathesis. 1-3 Platelet surface glycoproteins IIbIIIa (GPIIbIIIa, ␣IIb␤3 integrin) and Ib␣ (GPIb␣) are the 2 most common antigenic targets in ITP. [4][5][6] Approximately 70% to 80% of patients have autoantibodies to GPIIbIIIa, 20% to 40% to the GPIb complex, and a significant number of patients simultaneously have antibodies to both or to other glycoproteins (eg, GPIV and Ia/IIa). 2,6,7 Reports that anti-GPIb␣ may cause thrombocytopenia through a different mechanism (Fc-independent pathway) from anti-GPIIbIIIa (Fc-dependent pathway) 8 are intriguing, though the mechanism of Fc-independent platelet clearance is unknown. It is not clear whether pathogenesis and therapeutic responses are different for ITP induced by these 2 groups of antibodies.IVIG is used to treat several autoimmune diseases, including ITP. 9-11 However, the mechanism of action of IVIG is not fully understood. 12 Although IVIG effectively ameliorates thrombocytopenia in many patients with ITP, a significant number (15%-25%) are refractory to this therapy. 13 It is unknown whether refractory cases result from IVIG being less efficacious in ITP caused by certain antibody specificities (ie, anti-GPIb␣). To test this hypothesis, we induced thrombocytopenia in a murine model 14 using well-characterized rat monoclonal antibodies against either mouseGPIIbIIIa or GPIb␣, and tested the ability of IVIG to attenuate ITP induced by these antibodies in this model. Study design AnimalsBALB/c and C57BL/6 mice, 6 to 8 weeks of age, were purchased from Charles River Canada (Montreal, QC, Canada), and maintained in the local research vivarium. Platelet preparation and countingWhole blood (10 L) was obtained from the saphenous vein and immediately diluted 1:100 in 990 L of 1% (vol/vol) EDTA/PBS, pH 7...

Idiopathic autoimmune thrombocytopenia (ITP) is an autoimmune disease characterized by autoantibody-mediated platelet destruction. The major platelet antigens that autoantibodies are directed against in ITP are GPIIbIIIa and GPIbα. Intravenous immunoglobulin G (IVIG) is a treatment used in ITP; however, some patients are refractory to this treatment and the mechanisms of action of IVIG in this disease are not fully understood. We tested the hypothesis that IVIG may not be equally effective in preventing ITP caused by GPIIbIIIa versus GPIbα anti-platelet antibodies. Thrombocytopenia was induced in Balb/c mice using three monoclonal antibodies against either mouse platelet GPIIbIIIa (JON1, 2, −3) or against GPIbα (p0p 3, 4, −5). Mouse platelet counts were performed daily from day one (pre-treatment) until day five using a flow cytometric assay. The efficacy of IVIG in inhibiting monoclonal antibody-induced platelet destruction in mice was assessed by intraperitoneal injection of IVIG (2 g/kg) 24-hours before anti-platelet antibody injections (7 mg/mouse, i.v.) on day two. We found that pre-treatment with IVIG resulted in a significantly higher platelet count on day three in all groups of anti-GPIIbIIIa-injected animals (n=3/antibody; for JON1, P=0.002; JON2, P=0.015; JON3, P=0.002) as compared to albumin-treated controls. In contrast, IVIG failed to prevent platelet clearance and thrombocytopenia on day three in two anti-GPIbα antibody-treated groups (p0p3 and p0p5, n=3/antibody), but did inhibit platelet destruction in mice that were injected with p0p4 antibody (P=0.0003). Our results indicate that the efficacy of IVIG treatment in preventing ITP induced by these anti-platelet antibodies is affected by their antigen specificities. These data also suggest that patients with ITP mediated by GPIbα may be less responsive to IVIG treatment than those with ITP mediated by GPIIbIIIa antibodies.