Structure-function relationships with IVIg

Besides being used for replacement therapy in patients with antibody deficiency, intravenous immunoglobulin (IVIg) is used in conditions such as Idiopathic Thrombocytopenia Purpura (ITP), Kawasaki syndrome and Guillain-Barre. In applications other than replacement therapy, the mechanisms of action are largely uncertain, and proposed mechanisms are for example, effects due to scavenging of complement activation products, blockade of Fc receptors, effects of IgG dimers and effects of specific antibodies (for example: cytokine neutralization).

We investigated properties such as stability of the IgG dimers present in IVIg under different physical conditions by among others size-exclusion chromatography and sodium dodecyl sulfate (SDS) electrophoresis. A substantial fraction of dimers dissociate rapidly under conditions mimicking those in patients after administering IVIg, but part of the dimers remain stable. Formation of dimers and larger aggregates may result in part from slightly denatured IgG. We found that aggregation of IgG may be counteracted by addition of fragments of IgG.

Treatment of conditions such as ITP require high doses of IVIg. It is reported that only the fraction of IgG molecules containing sialic acid is responsible for its anti-inflammatory action. These findings are based mainly on an arthritis mouse model. In cooperation with the Division of Plasma Products, IVIg was enriched for sialic acid (SA). The SA-enriched and depleted IVIg were tested in a mouse model for ITP.

Monomeric precursors for aggregation of IgG are also difficult to detect. Usually, hydrophobic fluorescent probes such as 1-anilino-8-naphthalenesulfonate are used that may detect exposed hydrophobic surfaces as a result of partial unfolding. We extended this approach by detecting binding of such probes using isothermal titration calorimetry. We also developed a method of detecting small quantities of alternatively-folded IgG using radio-labeled IgG fragments.

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Last edited on: 22 April 2013