Immune Thrombocytopenia Mediated by Anti-GPIbα Antibodies May Occur Via an FcR-Independent Pathway: A Potential Explanation for Refractory Cases to IVIG Therapy.

Intravenous immunoglobulin G (IVIG) is used to treat immune thrombocytopenia (ITP). While benefit has been observed in many cases, for unknown reasons some patients are refractory to this therapy. ITP is characterized by platelet clearance mainly mediated by antibodies against platelet surface glycoproteins (GP) GPIIbIIIa and GPIbα. Approximately 70–80% patients have antibodies against GPIIbIIIa, 20–40% patients have antibodies against GPIbα complex, and some patients have antibodies to both. Phagocytotic macrophages activated via their Fc receptors (FcR) are thought to be responsible for the clearance of opsonized platelets. However, anti-GPIbα antibodies may be able to induce ITP in an FcR-independent manner. Since the prevailing view of the mechanism of action of IVIG is Fc receptor blockade, we hypothesized that IVIG may not be effective at preventing thrombocytopenia induced by anti-GPIbα antibodies. Thrombocytopenia was induced in BALB/c mice using monoclonal antibodies against mouse GPIbα (p0p3, p0p4, p0p5, p0p9, and p0p11). Pretreatment with different doses of IVIG (1, 2, and 4g/kg of mouse) failed to prevent ITP in all anti-GPIbα-treated mice, except for p0p4. These results were repeated with p0p3, p0p4, and p0p5 using 2g/kg IVIG in C57BL/6 mice, and using 2g/kg of a different IVIG preparation. It has been shown that F(ab)₂’ fragments of the p0p antibodies can induce thrombocytopenia to the same extent as intact antibodies. To confirm that these anti-GPIbα antibodies act in an FcR-independent manner, we demonstrated that p0p3, p0p4, and p0p5 are able to induce thrombocytopenia in mice lacking activating Fc receptors (FcRγ chain −/−). Interestingly, IVIG was still able to prevent thrombocytopenia induced by p0p4 in these FcRγ chain −/− mice. We demonstrated in vitro that there are no anti-idiotype antibodies against any of the anti-GPIbα antibodies used present in IVIG. Thus, the amelioration of thrombocytopenia in p0p4-treated mice is not due to a neutralization of the antibody by IVIG. In contrast to the anti-GPIbα antibodies, anti-GPIIbIIIa monoclonal antibodies (JON1, JON2, and JON3) that induce thrombocytopenia were sensitive to IVIG treatment, which is consistent with earlier studies using other monoclonal antibodies against GPIIbIIIa. Also, JON2 was unable to induce thrombocytopenia in FcRγ chain −/− mice, confirming that this anti-GPIIbIIIa antibody is FcR-dependent. Our results suggest that ITP induced by anti-GPIbα antibodies may be mediated by FcR-independent mechanisms, many of which may be refractory to IVIG therapy. Thus, IVIG may not be as effective in treating patients with predominantly anti-GPIbα antibodies.