Initial Autoimmune Response in Lupus and Sjorgren's

Evidence implicating molecular mimicry, sub-dominant epitopes, immune ignorance, and failure of the apoptotic cellular apparatus as contributing to the mechanisms of autoimmunity have been convincingly demonstrated in various experimental systems. We propose that somatic mutation in autoantigens ia another possible mechanism that initiates autoimmunity. According to our working hypothesis new protein structures are created by somatic mutation in autoantigens which have been previously unknown to the immune system. Once the efferent immune response against the new structure(s) extends to non-mutated structures of the antigen, then an autoimmune response would be established.

Serendipitously, we detected examples of somatic frame shift mutations in a mutational hot spot of the gene encoding the autoantigen La in anti-La precipitin-positive patients. Now, we have developed a sensitive Ligase Chain Reaction assay (LCR) and a Single Strand Conformational Polymorphism assas (SSCP) to screen patient tissues for somatic mutations. 20 anti-La positive patients and 50 healthy donors were analyzed by either LCR, SSCP or both. Our data indicate that somatic mutations occur in up to 50% of La positive patients, while no mutations were detectable in healthy donors (p<0.0000001). Tissue samples taken from the same patient, but at different times, show that the somatic mutation is stable and can be detected as long as 12 years later. Thus, the somatic mutation may have occured in a stem or memory cell.

The somatic mutation leads to a subsequently altered amino acid sequence with truncation of the La antigen. Immunizing animals with a synthetic putative neoepitopoe peptide resulted in an anti-peptide response and spreading to native La. Moreover, in an assay using overlapping peptides, autoantibodies directed to the neoepitope were detected in patients which cross-reacted with the respective homologous native sequence. In conclusion, somatic mutations occur in genes encoding for autoantigens of patients. Such mutations create neoepitopes that can initiate autoantibodies.

These data served already as the basis of a grant application to the OCAST's Health Research program which was granted. It also served as the basis of a full R01 application to the NIH (GM63497-01A1).