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Autoantibody to hLSm4 and the heptameric LSm complex in anti-Sm sera
Theophany Eystathioy1, Carol L Peebles, John C Hamel
1Scripps Research Institute, La Jolla, California 92037, USA.
Objective:
To characterize the 15-kd human SmD-like autoantigen and its associated proteins previously shown to be recognized by IgM antibodies in patients with Epstein-Barr virus (EBV)-induced infectious mononucleosis.
Methods:
The full-length complementary DNA for the 15-kd protein was expressed as recombinant protein and analyzed for reactivity using biochemical analysis and immunoprecipitation (IP).
Results:
The 15-kd protein was determined to be the human like-Sm protein LSm4 (hLSm4). Rabbit antibody raised against the C-terminal polypeptide immunoprecipitated a 68-kd complex composed of LSm4 together with a group of smaller proteins ranging in size from 6.5 to 14 kd, consistent with the reported heptameric LSm complexes involved in U4/U6 duplex formation and messenger RNA (mRNA) decapping/degradation. About 80% of all anti-Sm sera from patients with systemic lupus erythematosus (SLE) recognized the hLSm4 in vitro translated product, while 6.7% (29 of 434) immunoprecipitated from cell extracts hLSm4 together with the other members of the hLSm complex. Four sera (0.92%) showed apparently exclusive reactivity to the hLSm complex in the absence of reactivity to Sm core proteins in the IP assay.
Conclusion:
These findings document that while IgM, but not IgG, autoantibodies to LSm4 were found in sera from patients with EBV infection, IgG autoantibodies to hLSm4 are detected in a large number of anti-Sm-positive sera from patients with SLE. Importantly, in a small number of anti-Sm sera the LSm complex can be recognized independently of the Sm core protein antigens. Our data introduce the concept that "Sm" autoantigens include Sm as well as LSm complexes involved in the maturation and degradation of mRNA.
Insights
Researchers identified the 15-kd human autoantigen as LSm4. Autoantibodies to LSm4 were found in Epstein-Barr virus patients and systemic lupus erythematosus patients, suggesting Sm and LSm complexes are "Sm" autoantigens.
Area of Science:
- Immunology
- Molecular Biology
- Autoimmunity
Background:
- Autoantibodies against Sm proteins are hallmarks of systemic lupus erythematosus (SLE).
- Previous studies indicated IgM autoantibodies recognize a 15-kd protein in Epstein-Barr virus (EBV) infection.
Purpose of the Study:
- To characterize the 15-kd human autoantigen and its associated proteins.
- To investigate its recognition by autoantibodies in EBV and SLE patients.
Main Methods:
- Recombinant expression of the 15-kd protein.
- Biochemical analysis and immunoprecipitation (IP) assays.
- Analysis of patient sera (EBV and SLE) for autoantibody reactivity.
Main Results:
- The 15-kd protein was identified as human like-Sm protein LSm4 (hLSm4).
- hLSm4 forms heptameric complexes involved in mRNA processing.
- IgM autoantibodies to LSm4 were detected in EBV patients; IgG autoantibodies were found in a significant proportion of SLE patients, with some showing exclusive reactivity to the LSm complex.
Conclusions:
- IgM autoantibodies to LSm4 are associated with EBV infection.
- IgG autoantibodies to hLSm4 are prevalent in anti-Sm-positive SLE patients.
- The findings expand the definition of "Sm" autoantigens to include LSm complexes involved in mRNA metabolism.