Structural and functional analysis of J chain-deficient IgM

Area of Science:

• Immunology
• Molecular Biology
• Protein Structure

Background:

• Two forms of polymeric mouse immunoglobulin M (IgM) exist: a moderately cytolytic pentamer containing the J chain and a highly cytolytic hexamer lacking the J chain.
• The relationship between IgM's polymeric structure, J chain content, and its complement-activating (cytolytic) activity remains incompletely understood.

Purpose of the Study:

• To investigate how polymeric structure, J chain presence, and specific mutations affect IgM's cytolytic activity and assembly.
• To elucidate the role of the J chain in IgM structure and function, particularly in complement activation.

Main Methods:

• Production of IgM in J chain-deficient and J chain-proficient mouse hybridoma cell lines.
• Analysis of IgM structure and subunit composition using SDS-PAGE.
• Assessment of complement activation by different IgM forms.
• Characterization of a mutant IgM (IgM-S337) with a C337S substitution in the mu-chain.

Main Results:

• Both hexameric and pentameric IgM were produced regardless of J chain presence.
• Hexameric IgM activated complement approximately 100-fold more efficiently than J chain-deficient pentamer, which was more active than J chain-containing pentamer.
• The J chain was found only in the mu2kappa2 subunit of the mutant IgM-S337 pentamer, suggesting specific inter-mu bonds direct J chain incorporation.
• IgM-S337 hexamers showed complex dissociation patterns, suggesting novel structural arrangements like catenated circles.

Conclusions:

• J chain-containing pentameric IgM may not efficiently activate complement, contrary to previous assumptions.
• IgM hexamers are significantly more potent complement activators than pentamers.
• The J chain's incorporation into IgM is directed by specific inter-chain interactions, and novel quaternary structures may exist in hexameric IgM.