Immune responses to the Ro and La autoantigens

Abstract

Sjögren's syndrome (SS) and systemic lupus erythematosus (SLE) are rheumatic diseases characterized by autoantibody production directed to the Ro/SS-A and La/SS-B autoantigens. The Ro/SS-A is a complex of one of four small RNAs denoted hY RNAs, and three proteins; Ro 60kD, Ro 52kD and La. The La protein also binds to immature polymerase III transcripts and certain viral RNAs. This thesis has focussed on the possible role of anti-Ro and anti-La autoantibodies in the autoimmune exocrinopathy of SS, and to some extent the systemic disease SLE. We have also investigated a possible route for how Sindbis virus RNA might bind the Ro proteins and incorporate them into a mature virus partice.

Whether autoantibodies to Ro and La are directly pathogenic is not known, but several studies have indicated so. To address this question, we performed studies on longitudinally collected patient sera and correlated levels of anti-Ro and anti-La with clinical disease activity, studied local production of Ro and La antibodies in the inflamed target organ as well as cloned and characterized Ro antibody-producing cells from peripheral blood of a SS patient. Cytokine production in peripheral blood was also investigated in SS patients.

In longitudinally collected SS and SLE patient sera one group of sera with stable autoantibody levels was identified and one group which had variable autoantibody levels. The group with stable levels had stable disease activity, while the group with autoantibody variations had variable disease activity. From this we concluded that there is an association between levels of Ro and La autoantibodies and clinical disease activity. Local production of anti-Ro and anti-La of IgG, IgM and IgA, was demonstrated in inflamed salivary glands of SS patients, using biotinylated purified recombinant antigens for detection. The local production was highly correlated to the presence of antiRo and anti-La in serum. The presence of Ro and La antibodies in inflamed organs further implies their importance in pathogenesis of the disease.

To further characterize Ro antibodies, peripheral anti-Ro 52kD cells from a SS patient were cloned. Immunoglobulin variable genes were sequenced and epitopes recognized by the antibodies mapped. Two clones were identified, both of IgM type, which recognized epitopes within a previously identified immunodominant region of the protein. The immunoglobulin genes showed little evidence of somatic mutation, displaying few mutations derivating from germline. Cytokine expression in peripheral blood of SS patients was investigated by ELISPOT, and we identified high numbers of IL-6 and IL-10 producing cells. The possibility that the Ro 60kD binds Sindbis virus RNA was investigated using immunoprecipitation-RT-PCR on purified virus paticles. Anti-Ro 60 MoAbs precipitated viral RNA, suggesting Ro 60 binding to Sindbis RNA, and Ro 60 antibodies detected a 60kD band in immunoblotting of viral extract indicating the presence of Ro 60 in virus particles.

In conclusion, the autoantibody response to Ro/SS-A and La/SS-B appears to be regulated in common, antigen-driven and involved in the pathogenesis of SS.