The effect of anti-Rho(D) and non-specific immunoglobulins on monocyte Fc receptor function: the role of high molecular weight IgG polymers and IgG subclasses

Tetramolecular immune complexes are more efficient than IVIg to prevent antibody-dependent in vitro and in vivo phagocytosis of blood cells

Intravenous immunoglobulins (IVIg) have immunomodulatory effects in vivo and are widely used in the treatment of autoimmune diseases, such as idiopathic thrombocytopenic purpura (ITP). The mechanisms by which IVIg can prevent platelet clearance in ITP patients are not fully understood but are known to require the participation of low affinity Fcgamma receptors (FcgammaRs), which interact poorly with monomeric immunoglobulin G (IgG). Given the importance of low affinity FcgammaRs in the treatment of ITP, we hypothesized that immune complexes (IC) produced in vitro could reproduce the effects of IVIg. Small-size tetramolecular IC were prepared using mouse monoclonal anti-human IgG and human Fc fragments. The effects of tetramolecular IC and IVIg on the in vitro and in vivo inhibition of phagocytosis of opsonized blood cells were compared. The results obtained showed that tetramolecular IC were at least six times more efficient than IVIg to prevent phagocytosis of opsonized red blood cells in vitro, and clearance of platelets in the thrombocytopenic mouse model.

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.

Are there options for donor-derived i.m. anti-D IgG preparations other than to prevent Rh(D) sensitization? The intravenous route

The use of i.v. anti-Rh(D) IgG for conditions other than prevention of Rh(D) sensitization is discussed. Besides highlighting the platelet response in ATP, a putative distinctive effect on the hemorrhagic threshold is suggested. Accordingly, we have included discussion of cases of aplastic anemia, myelodysplasia, heavy chemotherapy, coagulation deficiency, and senile vascular atrophy. We have also considered attempts to replace the high-dose pooled i.v. IgG (IVIG) with the much smaller amounts of IgG present in anti-Rh(D) preparations used to prevent Rh-sensitization in pregnancy. Both Fc and variable fragments of the IgG molecule may play a role, the former potentiated by manufacture-induced IgG aggregation and the latter by donor hypersensitization. A role for IgG-anti-F(ab')2 molecules cannot be ruled out.

Anti-D immunoglobulin in the treatment of idiopathic thrombocytopenic purpura

Anti-D was evaluated in 8 RhD positive patients (6 males, 2 females) aged 2-21 years (mean 10 years) with Idiopathic Thrombocytopenic Purpura (ITP). Five patients with chronic ITP and 3 patients with acute ITP were administered Anti-D in the dosage of 50 micrograms/kg intramuscularly (IM) for 3 consecutive days. One patient of chronic ITP received two courses of Anti-D. Patients were followed up for 7 to 16 months (mean 9 months). All three cases of acute ITP had a complete response and are in remission between 3 to 12 months of follow up. Two of five cases of chronic ITP had a partial response. Rise in platelet count was observed within 72-124 hours, and duration of response varied between 10 to 15 days. None of these patients had any significant side effects of anti-D immunoglobulin therapy. Intramuscular administration of Anti-D is safe, effective and low cost alternative to IVIgG in the treatment of acute ITP.