Human myeloma IgG half-molecules. Structural and antigenic analyses,.

Area of Science:

• Immunology
• Molecular Biology
• Protein Chemistry

Background:

• Myeloma proteins are abnormal immunoglobulins produced by cancerous plasma cells.
• Human IgG myeloma proteins typically exist as four-chain (7S) structures.
• Half-molecule myeloma proteins are rare and their structural basis is not fully understood.

Purpose of the Study:

• To characterize the structure and antigenic properties of a human IgG myeloma protein that forms half-molecules.
• To investigate the molecular basis for the formation of half-molecules and the associated functional deficiencies.

Main Methods:

• Analysis of myeloma protein structure using polyacrylamide gel electrophoresis and analytical ultracentrifugation.
• Biochemical characterization of heavy chain structure, including amino acid composition and disulfide bond analysis.
• Immunological assays to assess Fc fragment antigenicity and binding interactions.

Main Results:

• The myeloma protein predominantly existed as two-chain (4.3S) half-molecules, with a small amount of 7S protein.
• The heavy chains of the half-molecule had a reduced molecular weight, suggesting a deletion.
• The Fc fragment was antigenically deficient and lacked normal noncovalent interactions, leading to instability.
• Genetic markers were absent on the third homology region of the gamma chain.

Conclusions:

• The myeloma protein likely possesses a deletion in the gamma heavy chain, probably in the third homology region.
• This deletion is responsible for the lack of Fc fragment noncovalent interactions and the deficiency in antigenic determinants.
• The findings provide insights into the structure-function relationships of human IgG and the pathogenesis of myeloma.