Binding and antibody blocking effects of intravenous IgG preparation on peripheral blood cells

Immune thrombocytopenia and Fc receptor-mediated phagocyte function

The response to intravenous immunoglobulin treatment (IVIG) is thought, in part, to be due to blockade of Fc receptor for IgG in the mononuclear phagocyte system (MPS). We have studied this by measuring splenic clearance of heat-damaged and IgG antibody-coated red cells in immune thrombocytopenic patients receiving IVIG. Clearance of heat-damaged red blood cells (HDRBC) labelled with technetium 99MM was measured in eight patients before and after a 5-day infusion of IVIG. In six of the eight there was an enhanced clearance of HDRBC post IVIG. Clearance studies were performed in eight rhesus-D-positive patients using autologous red cells coated with anti-D and labelled with 99MTc. The time taken for the radioactivity at 3 min to fall to 50% (T1/2) was calculated. The T1/2 increased in seven of the eight patients from a mean of 78.5 min to 137.1 min, a prolongation of 74.6%. In addition, we have examined the effect of IVIG on Fc-mediated phagocytosis by neutrophils. Platelet phagocytosis by patients' neutrophils was measured using opsonized autologous platelets in nine patients. There was a marked reduction in platelet phagocytosis as measured by formazan production in all patients at day 4-5 when compared with the preinfusion value, and in half of these this reduction was still evident at further studies, 1 week after the end of the infusion period. The platelet count rose in parallel with the fall in platelet phagocytic ability of the neutrophils, reaching a peak at 7 days post infusion, and fell with the recovery of phagocytic function in weeks 2 and 3. These findings suggest that IVIG has a marked effect on Fc-mediated phagocytosis.

The effect of non-immune IgG on antigen-antibody complexation

Two preparations of human IgG, one acylated with beta-propiolactone (acylated IgG) and one treated at pH 4 with traces of pepsin (pH 4-IgG), were used to study the effect of non-immune IgG on antigen-antibody interactions in the antigen excess zone. Employing two immunological methods together with size-exclusion chromatography, we found that the formation of human albumin-rabbit anti-human albumin complexes was inhibited in the presence of human IgG. In addition, IgG seemed to promote the aggregation of already formed complexes. Thus, non-immune IgG may modulate immune complexation by direct molecular interactions. The effect was dependent on the size and composition of the immune complexes as well as on the conformation of the IgG molecules with respect to their shape, isotype, charge, and other surface properties. Some possible mechanisms for the reactions are discussed.

Intravenous immunoglobulin in the treatment of human immunodeficiency virus-related thrombocytopenia

Fourteen patients with sexually transmitted human immunodeficiency virus (HIV)-related immune thrombocytopenia were treated with intravenous gammaglobulin (IVIG). The patients were treated with a uniform program consisting of 1 g/kg of IVIG on day 1 and day 2, followed by 1 g/kg on day 15. Most patients had pretreatment bleeding symptoms, which included petechiae, spontaneous and traumatic ecchymoses, gum bleeding, and epistaxis. Median baseline platelet count was 17,000/mm3 (range 3-61,000/mm3). After the infusion of the IGIV, all patients had a resolution of their bleeding by day 8. The median maximum platelet count achieved with the IGIV was 220,000/mm3 (range 76-426,000/mm3). No patient achieved either a sustained complete or partial remission after the conclusion of the IVIG therapy. Toxicities were minimal with the majority being headache and nausea. In conclusion, patients with sexually transmitted HIV infection and immune thrombocytopenia respond favorably to IVIG. This treatment should be considered as first-line therapy for patients with HIV-related immune thrombocytopenia who require immediate but temporary increase in their platelet count, attributable to symptoms or signs of clinical bleeding or because of the need for an invasive procedure.

Cellular changes during the infusion of high dose intravenous immunoglobulin

With the ever widening group of autoimmune conditions that are beneficially affected by infusions of high dose immunoglobulin the possible mechanisms of action of such therapy appear increasingly complex. Fc mediated blockade of the mononuclear phagocyte system is an acknowledged early effect. This is, however, accompanied by a decrease of neutrophil counts which suggests that IgG binding to the neutrophil may be a mechanism of action. The decrease of neutrophil counts is transient but in immune thrombocytopenia is inversely proportional to the platelet response observed. In parallel to the effect on the neutrophil there are changes in the lymphocyte subsets with reversal of the T helper/suppressor ratio and alterations in the individual cellular constituents of each subset that correlate with the clinical response. The observed changes in B cell numbers and function suggest that T dependent and independent antibody production is effected by intravenous immunoglobulin. It is increasingly clear that in ITP at least the clinical response to IV IgG is a summation of several cellular events and their balance reflects the ultimate outcome. It may eventually be possible to use these observations to predict the likely outcome in the individual patient of this mode of therapy.