Fc-independent phagocytosis: implications for IVIG and other therapies in immune-mediated thrombocytopenia.

Phagocytes were first described by Dr. Metchnikoff in 1873. The roles of phagocytes in innate and adaptive immunity have been well established to date, although the molecular mechanisms involved in initiating phagocytosis (through Fc or other receptors) remain to be further explored. Phagocytes in the reticuloendothelial system, particularly macrophages, have been implicated in the clearance of senescent blood cells. The destruction of these cells may be primarily mediated via an Fc-independent pathway. Fc-independent phagocytosis may also play an important role in platelet clearance, including in autoimmune thrombocytopenia (ITP), and in clearance of platelet-rich emboli detached from sites of vascular injury. In ITP, the two major platelet auto-antigens have been located on glycoprotein (GP)IIbIIIa and the GPIb complex. It has been demonstrated that anti-GPIb antibodies, in contrast to anti-GPIIbIIIa, can induce thrombocytopenia in an Fc-independent manner. We further demonstrated in an animal model that intravenous IgG (IVIG) is unable to ameliorate thrombocytopenia caused by most anti-GPIb antibodies, despite its efficacy in anti- GPIIbIIIa-mediated thrombocytopenia. Our data was supported by subsequent retrospective studies with ITP patients by several independent groups. Most recently, we found that anti-GPIb-mediated ITP was also resistant to steroid therapy and that platelet activation and apoptosis induced by anti-GPIb antibodies may be involved in the Fc-independent platelet clearance. Therefore, identification of antibody specificity in patients, e.g. anti-GPIIbIIIa (Fc-dependent) versus anti-GPIb (Fc-independent), may be important for therapies against ITP, as well as other immune-mediated thrombocytopenias.

A murine model of severe immune thrombocytopenia is induced by antibody- and CD8+ T cell-mediated responses that are differentially sensitive to therapy.

Immune thrombocytopenia (ITP) is a bleeding disorder characterized by antibody-opsonized platelets being prematurely destroyed in the spleen, although some patients with ITP may have a cell-mediated form of thrombocytopenia. Although several animal models of ITP have been developed, few mimic primary chronic ITP nor have any shown cell-mediated platelet destruction. To create this type of model, splenocytes from CD61 knockout mice immunized against CD61(+) platelets were transferred into severe combined immunodeficient (SCID) (CD61(+)) mouse recipients, and their platelet counts and phenotypes were observed. As few as 5 x 10(4) splenocytes induced a significant thrombocytopenia and bleeding mortality (80%) in recipients within 3 weeks after transfer. Depletion of lymphocyte subsets before transfer showed that the splenocyte's ability to induce thrombocytopenia and bleeding completely depended on CD4(+) T helper cells and that both CD19(+) B cell (antibody)- and CD8(+) T cell (cell)-mediated effector mechanisms were responsible. Treatment of the SCID mouse recipients with intravenous gamma-globulins raised platelet counts and completely prevented bleeding mortality induced by antibody-mediated effector mechanisms but did not affect cell-mediated disease. This novel model not only shows both antibody- and cell-mediated ITP and bleeding but also suggests that these 2 effector mechanisms have a differential response to therapy.

Fc-independent phagocytosis: implications for intravenous IgG therapy in immune thrombocytopenia.

Phagocytes were first described 120 years ago. Although the molecular mechanisms involved in initiating phagocytosis (through Fc or other receptors) are still not fully understood, the roles of phagocytes in innate and adaptive immunity have been well studied. Phagocytes in the reticuloendothelial system, particularly macrophages, have been implicated in the clearance of senescent blood cells. The destruction of these cells may be primarily mediated through an Fc-independent pathway. Fc-independent phagocytosis may also play an important role in platelet clearance, including immune thrombocytopenia (ITP). The two major platelet antigens targeted in ITP are GPIIbIIIa and the GPIb complex. It has been demonstrated that anti-GPIbalpha antibodies, in contrast to anti-GPIIbIIIa, can induce thrombocytopenia in an Fc-independent manner. We further demonstrated in an animal model that intravenous IgG is not able to ameliorate thrombocytopenia caused by most anti-GPIbalpha antibodies, though it is effective in anti-GPIIbIIIa mediated thrombocytopenia. Our data was supported by a recent retrospective study of ITP patients. Therefore, identification of antibody specificity (e.g. anti-GPIIbIIIa (Fc-dependent) versus anti-GPIbalpha (Fc-independent)) in patients may be important for ITP therapy.

Relative efficacy of intravenous immunoglobulin G in ameliorating thrombocytopenia induced by antiplatelet GPIIbIIIa versus GPIbalpha antibodies.

Intravenous immunoglobulin G (IVIG) is used 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 GPIbalpha 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-GPIbalpha antibodies in mice. Thrombocytopenia was induced in BALB/c mice using monoclonal antibodies against either mouse GPIIbIIIa (JON1, JON2, and JON3) or GPIbalpha (p0p3, p0p4, p0p5, p0p9, and p0p11). Pretreatment with IVIG significantly ameliorated ITP in all anti-GPIIbIIIa-injected animals. Conversely, IVIG failed to prevent ITP in all anti-GPIbalpha-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-GPIbalpha antibodies may be less responsive to IVIG treatment.