Autoantibodies occurring in two different rheumatic diseases react with the same nuclear ribonucleoprotein particle

Area of Science:

• Immunology
• Molecular Biology
• Biochemistry

Background:

• Patients with systemic lupus erythematosus and mixed connective tissue disease produce antibodies against nuclear antigens Sm and nuclear ribonucleoprotein (nRNP).
• A serologic partial identity exists between Sm and nRNP antigens, but its molecular basis was unclear.

Purpose of the Study:

• To isolate and characterize the nRNP particle containing both nRNP and Sm antigens.
• To elucidate the molecular basis for the partial identity between Sm and nRNP antigens.

Main Methods:

• Sucrose density gradient centrifugation of rat liver nuclear extract.
• Anti-nRNP affinity chromatography.
• Protein analysis via SDS-PAGE and immunoautoradiography.

Main Results:

• Isolation of a P30--P13--RNA particle containing both nRNP and Sm antigens.
• P13 identified as the Sm antigen, reacting independently of RNA.
• P30 identified as the nRNP antigen, reacting with anti-nRNP sera.
• Partial identity explained by shared reactivity to the P30--P13--RNA particle.

Conclusions:

• The P30--P13--RNA particle is the molecular basis for the partial identity between Sm and nRNP antigens.
• Anti-Sm antibodies bind to P13, while anti-nRNP antibodies bind to P30.
• This study provides critical insights into the serologic relationships of autoantigens in autoimmune diseases.