Circular dichroism of immune complexes, IgG and Fab gamma with unique antigenic determinants from rheumatoid serum

Purification of antibody fragments via interaction with detergent micellar aggregates

The research described in this report seeks to present proof-of-concept for a novel and robust platform for purification of antibody fragments and to define and optimize the controlling parameters. Purification of antigen-binding F(ab')₂ fragments is achieved in the absence of chromatographic media or specific ligands, rather by using clusters of non-ionic detergent (e.g. Tween-60, Brij-O20) micelles chelated via Fe²⁺ ions and the hydrophobic chelator, bathophenanthroline (batho). These aggregates, quantitatively capture the F(ab')₂ fragment in the absence or presence of E. coli lysate and allow extraction of only the F(ab')₂ domain at pH 3.8 without concomitant aggregate dissolution or coextraction of bacterial impurities. Process yields range from 70 to 87% by densitometry. Recovered F(ab')₂ fragments are monomeric (by dynamic light scattering), preserve their secondary structure (by circular dichroism) and are as pure as those obtained via Protein A chromatography (from a mixture of F(ab')₂ and Fc fragments). The effect of process parameters on Ab binding and Ab extraction (e.g. temperature, pH, ionic strength, incubation time, composition of extraction buffer) are reported, using a monoclonal antibody (mAb) and polyclonal human IgG's as test samples.

Variations in oligosaccharide-protein interactions in immunoglobulin G determine the site-specific glycosylation profiles and modulate the dynamic motion of the Fc oligosaccharides

Glycoproteins, such as immunoglobulin G (IgG), consist of an ensemble of glycosylated variants, or glycoforms, which have different oligosaccharides attached to a common peptide. Alterations in the normal glycoform populations of IgG are associated with certain disease states, notably rheumatoid arthritis and its remission during pregnancy. In this paper, we show that two sets of IgG Fc glycoforms have quite different physical properties. The first set has 1,6 arm terminal galactose residues which interact with the protein, resulting in glycan binding to the protein surface, in agreement with the crystal structure. In contrast, the second set of glycoforms which lack galactose does not bind to the protein surface. Recently developed HPLC techniques combined with enzymatic digestion and mass spectrometry have been used to assign the glycan structures on IgG, Fab, and Fc. Comparison of Fab with Fc shows that glycosylation is site-specific. Two major glycan structures are present on Fab (fucosylated digalacto-bianntenary with and without bisect) and three on Fc (fucosylated agalacto-, 1,6 arm monogalacto-, and digalacto-bianntenary). In comparison to Fab, Fc glycans contain (i) lower levels of bisecting GlcNAc, (ii) lower levels of galactose, (iii) higher than expected levels of 1,6 arm galactose relative to 1,3 arm, and (iv) no 1,6 arm sialylation. We interpret these differences to indicate a role for both the protein quaternary structure and specific protein-glycan interactions in determining the glycoform populations. NMR relaxation measurements have been used to probe the mobility of the glycans in the Fc. By comparing two samples with different glycoform populations, we conclude that this mobility is dependent on the primary sequence of the glycan. Glycans carrying a galactose residue on the 1,6 arm have relaxation properties very similar to those of the peptide backbone and thus do not have independent motion. Glycans lacking galactose have relaxation rates 30 times slower than that of the peptide and thus a higher degree of mobility. These agalactosyl glycans do not interact with the protein, resulting in exposure of previously covered regions of the peptide surface and making the glycan more accessible. This implies that at the early stages of glycan processing the Fc glycans are mobile and only partially protected by the protein quaternary structure. Immobilization of the glycans occurs as a consequence of addition of galactose to the 1,6 arm and results in increased protection.

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.

IgG with a deviant conformation in serum and synovial fluid from rheumatoid arthritis patients

Specific rabbit antisera were prepared against an IgG with a special conformation (IgG spec.) previously detected in some sera from patients with rheumatoid arthritis. The antibodies had no affinity to normal human IgG and were not anti-idiotypic to human rheumatoid factor. The affinity of IgG spec. to the antibodies could not be explained by an antiglobulin activity to rabbit IgG. The amount of protein with affinity to immobilized specific IgG F(ab')2 of the antibodies was determined in serum and synovial fluid from patients with various joint diseases. A relationship between the content of IgG spec. and the diagnosis of seropositive rheumatoid arthritis was found on analysis of serum samples. IgG spec. also occurred in synovial fluid from some individuals with seropositive rheumatoid arthritis. Differences in the serum content of IgG spec. could not be explained by differences in the normal IgG content. Circular dichroism analysis of isolated IgG spec. showed that in the region(s) close to tyrosine residue(s) this polyclonal protein had similarities to heat-aggregated IgG.