Structure and function of the J chain

Complex formation between secretory component and human immunoglobulins related to their content of J chain

The J-chain content of 3 IgM and 24 IgA preparations was quantitated by an immunochemical technique after reduction with 20mM dithiothreitol. The amounts released ranged from undetectable (less than 0.1 mg) to 7.3 mg per 100 mg of Ig. Most of the J-chain-deficient proteins were monomeric, but four polymeric IgA preparations were found to contain only 0.2-0.8 mg of J chain per 100 mg. The SC-binding capacity of these polymers, expressed as percentage of the amount added (2.5 mu g SC/100 mug Ig), was 6%-12% compared with 69%-82% for IgA and IgM polymers that contained more than 4.0 mg of J chain per 100 mg. Some monomeric IgA preparations showed a slight SC-binding capacity, which was explained by the presence of contaminating J-chain-positive polymers. Bound J chain therefore seems to be a structural prerequisite for a specific noncovalent complexing of Ig polymers with SC.

Quantitative estimation of secretory immunoglobulin A by radial immunodiffusion technique

A single radial diffusion system in agar gel, which can be used for the quantitative measurement of secretory immunoglobulin A in solutions and in biological fluids, is discussed. Concentrations of secretory immunoglobulin A as low as 0.095 mg/ml were measured successfully using a 2 mul sample volume. Conditions that achieve maximum sensitivity and precision, such as gel concentration, incubation time, and temperature, are described.

Blocking effect of J chain and J-chain antibody on the binding of secretory component to human IgA and IgM

Isolated released J chain showed only a small affinity for free secretory component (SC), as indicated by a marginal but reproducible blocking effect on the binding of SC to Ig polymers. The SC-binding site was completely blocked by J-chain antibody in those Ig polymers where the bound J chains were accessible to the antibody. Along with the established masking effect of SC on the antigenicity of J chains present in secretory IgA, these results are compatible with the idea that the conformation of Ig-associated J chains contributes to the SC-binding site of Ig polymers.

J chain in malignant human IgG immunocytes

J chain was demonstrated directly by immunofluorescence in the cytoplasm of malignant IgG-producing human tumor cells. This was in contrast to IgG immunocytes accumulating in foci of chronic inflammation, which generally were devoid of J chain.