IVIg inhibits reticuloendothelial system function and ameliorates murine passive‐immune thrombocytopenia independent of anti‐idiotype reactivity

Abstract

Summary. Although the mechanism of action of intravenous immunoglobulin (IVIg) in treating antibody-dependent thrombocytopenia remains unclear, most studies have suggested that IVIg blocks the function of Fc receptors in the reticuloendothelial system (RES) and/or the protective effect may be due to the presence of variable region-reactive (anti-idiotype) antibodies within IVIg. We evaluated the effect of IVIg on platelet counts in a murine model of passively induced immune thrombocytopenia (PIT). Although IVIg was unable to neutralize the binding of two platelet-specific monoclonal antibodies to their target antigens either in vivo or in vitro, it was able to prevent PIT as well as ameliorate pre-established PIT mediated by these antibodies. IVIg adsorbed against the antibody used to induce thrombocytopenia or endogenous murine immunoglobulin also protected against PIT, indicating that antibodies with anti-idiotype activity present in IVIg are not necessary for its effective treatment of PIT. IVIg significantly blocked the ability of the RES to clear antibody-sensitized red blood cells. F(ab H ) 2 fragments of IVIg, which are unable to block the RES but retain the idiotypic regions, were ineffective at protecting mice from PIT. Our data suggest that IVIg exerts its rapid effect by inhibiting RES function and that anti±idiotype interactions are not required.

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

“…9,25 Perhaps related to the latter points, human IgG was used in the mouse model, which may have introduced extraneous xenogenic effects such as cross-reactivity with murine red blood cells). However, other studies have successfully used human IVIg in different animal models of ITP with no apparent xenogeneic effects, [10][11][12]24,25 and, in our hands, human IVIg at least does not bind with murine red blood cells (J.W.S., unpublished data, December 1999). Thus, taken together, our data suggest that IVIg therapy is efficacious for antibodymediated thrombocytopenia but not the CD8 ϩ T cell-mediated form of the disease.…”

“…In addition, the level of thrombocytopenia in the recipient mice does not necessarily account for the increased bleeding because it is known that mice rendered thrombocytopenic by, for example, passive transfer of antiplatelet antibodies, do not bleed excessively. [9][10][11][12]47 The excessive bleeding in our model suggests that additional immune factors are involved. For example, the IgG anti-CD61 antibody response in immune CD61 KO mice was extremely strong (titers Ͼ 1:12 000) and contains antibodies with multiple CD61 epitope specificities 24,25 that not only induce FcR-mediated phagocytosis but also significantly inhibit platelet function.…”

“…8 First-line treatments for patients with chronic ITP include steroids and intravenous ␥-globulins (IVIgs), and previous studies have shown that IVIg can also protect against passive ITP in mice. [9][10][11][12] At least 70% of patients with chronic ITP have identifiable serum autoantibodies that are composed primarily of IgG1 and IgG3 isotypes and generally target platelet glycoproteins (GPs) IIb/IIIa and/or GPIbIX. [3][4][5]13,14 However, some patients with ITP have no detectible antiplatelet autoantibodies yet are thrombocytopenic, and it has been shown that, in these patients, cell-mediated immune mechanisms such as CD8 ϩ T-cell cytotoxicity may lead to the thrombocytopenia.…”

“…[9][10][11][12] Although these models have been instrumental in understanding the pathophysiology of secondary ITP and the mechanisms of action of treatments such as IVIg, they are not ideal for chronic ITP pathophysiology where both T-cell and B-cell autoimmune attacks are focused on platelet-specific antigens. Recently, however, a platelet-specific immune model of neonatal alloimmune thrombocytopenia was developed with the use of immune CD61 knockout (KO) mice 24 that were pregnant with wild-type (WT) CD61 ϩ fetuses.…”

Monoclonal versus polyclonal anti-D in the treatment of ITP

“…In the decade that followed, a murine model of passive ITP became available [9] and monoclonal antibodies to a variety of different cellular and soluble antigens were examined to determine if in fact monoclonal antibodies could potentially increase platelet counts in this new model of ITP [10][11][12]. Although antibodies to the murine equivalent of RhD are not available, there are a number of monoclonal antibodies to other murine red cell antigens.…”