Monoclonal anti-D is not effective in the treatment of chronic autoimmune thrombocytopenic purpura

Monoclonal versus polyclonal anti-D in the treatment of ITP

Immune thrombocytopenia (ITP) is an autoimmune bleeding disorder caused by low numbers of platelets generally due to the production of anti-platelet antibodies. One effective treatment for ITP patients who express the RhD antigen on their red blood cells has been the use of blood donor-derived pooled polyclonal anti-D. Although anti-D has served us well, it needs to be replaced with a recombinant product. While the mechanism of action of anti-D in ITP remains highly speculative, this has not thwarted attempts to replace anti-D with a monoclonal product. Although a single attempt at a monoclonal antibody was not successful in the 1990s for the treatment of ITP, more recent efforts in mouse models of ITP and ITP patients now show that monoclonal antibodies can be successful in ITP. These studies also finally help substantiate the concept that it is unlikely that contaminants in the original donor-derived preparations mediate the major ameliorative activity of anti-D in ITP.

Intravenous immunoglobulin therapy: how does IgG modulate the immune system?

Intravenous immunoglobulin (IVIG) preparations comprise pooled IgG antibodies from the serum of thousands of donors and were initially used as an IgG replacement therapy in immunocompromised patients. Since the discovery, more than 30 years ago, that IVIG therapy can ameliorate immune thrombocytopenia, the use of IVIG preparations has been extended to a wide range of autoimmune and inflammatory diseases. Despite the broad efficacy of IVIG therapy, its modes of action remain unclear. In this Review, we cover the recent insights into the molecular and cellular pathways that are involved in IVIG-mediated immunosuppression, with a particular focus on IVIG as a therapy for IgG-dependent autoimmune diseases.

Therapeutic intravenous immunoglobulins

Intravenous immunoglobulins (IVIg) are concentrated formulations of human IgG prepared by industrial fractionation of large pools of individual plasma donations. IVIg were developed 20 years ago for the prophylaxis support of immunodeficient patients. However, IVIg have been increasingly used since 10 years, in the treatment of many autoimmune and inflammatory diseases raising the possibility of product shortages and ever increasing costs in the near future. Surprisingly, the immunomodulatory mechanisms of action of IVIg are unclear because of the diversity and often contradictory Fc, F(ab')(2), and non-IgG-related mechanisms that have been proposed from clinical observations and from results obtained in various in vitro and in vivo experimental models. These concepts are reviewed here and we discuss in more details three areas of active research, namely the mechanisms of IVIg action in Idiopathic Thrombocytopenic Purpura (ITP), the effects of IVIg on activated B lymphocytes and the possible involvement of autoantibodies of IgG isotype (auto-IgG) in the immunomodulatory effects of IVIg. The elucidation of the mechanisms of action of IVIg is crucial for a more rationalized clinical use of IVIg and for developing substitutes for some of the immunomodulatory indications in order to ensure long-term availability of plasma-derived IVIg for immunodeficient patients.

Monoclonal IgG can ameliorate immune thrombocytopenia in a murine model of ITP: an alternative to IVIG

Intravenous immunoglobulin (IVIG) is used to treat immune thrombocytopenia resulting from a variety of autoimmune and nonautoimmune diseases such as idiopathic thrombocytopenic purpura (ITP), heparin-induced thrombocytopenia, and posttransfusion purpura. IVIG is a limited resource and although considered safe, may nevertheless carry some risk of transferring disease. Its high cost makes monoclonal antibodies, capable of mimicking the clinical effects of IVIG, highly desirable. We show here, using a murine model of ITP, that selected monoclonal antibodies can protect against thrombocytopenia. SCID mice were pretreated with 1 of 21 monoclonal antibodies before induction of thrombocytopenia by antiplatelet antibody. Four antibodies reacted with the CD24 antigen on erythrocytes. Two antibodies were of the IgM class, and although one IgM antibody caused a minimal degree of anemia (P <.05), neither antibody ameliorated immune thrombocytopenia. One of 2 anti-CD24 antibodies of the IgG class ameliorated immune thrombocytopenia and blocked reticuloendothelial system function at the same doses that protected against thrombocytopenia. Some antibodies reactive with other circulating cell types also protected against immune-mediated thrombocytopenia while no antibody without a distinct target antigen in the mice was protective. Protective monoclonal antibodies significantly prevented thrombocytopenia at down to a 1000-fold lower dose (200 microg/kg) as compared with standard IVIG treatment (2 g/kg). It is concluded that monoclonal IgG with specificity for a circulating cellular target antigen may provide an alternative therapeutic approach to treating immune thrombocytopenia.

Enhanced efficacy of anti-D immunoglobulin for treating ITP is not explained by higher immunoglobulin polymer content

Several reports have suggested that low dose anti-D immunoglobulin is superior to high dose immunoglobulin for treatment of idiopathic thrombocytopenia purpura (ITP). However, some findings suggest that it is not the anti-D activity per se that is responsible for efficacy for treatment of ITP with anti-D immunoglobulin. Amongst alternative explanations for the mechanism of action is a relatively higher immunoglobulin polymer content of anti-D compared to other immunoglobulin products, which is more efficient in causing reticuloendothelial Fc receptor blockade. In order to investigate this we have evaluated the polymeric IgG content of anti-D and other immunoglobulin products. Different products showed considerable variation in immunoglobulin polymer content. There was no clear correlation between aggregate or dimer content and product type and anti-D as a class of product did not contain higher amounts of either dimer or aggregate compared to other products. Some manufacturers' products increased in polymer content on storage, but others did not show this effect. Therefore, higher immunoglobulin dimer and/or aggregate content cannot explain the increased efficacy of anti-D immunoglobulin for treatment of ITP. The role of polymeric Ig in efficacy for treatment of ITP is unclear.