Anti-Ro and anti-La antibodies, also known as SS-A and SS-B, respectively, are commonly found in primary Sjögrens syndrome, an autoimmune disorder that affects the exocrine glands. The presence of both antibodies is found in 30–60% of Sjögrens syndrome, anti-Ro antibodies alone are found in 50–70% of Sjögrens syndrome and 30% of SLE with cutaneous involvement, and anti-La antibodies are rarely found in isolation. Anti-La antibodies are also found in SLE; however, Sjögrens syndrome is normally also present. Anti-Ro antibodies are also found less frequently in other disorders including autoimmune liver diseases, coeliac disease, autoimmune rheumatic diseases, cardiac neonatal lupus erythematosus and polymyositis. During pregnancy, anti-Ro antibodies can cross the placenta and cause heart block and neonatal lupus in babies. In Sjögren's syndrome, anti-Ro and anti-La antibodies correlate with early onset, increased disease duration, parotid gland enlargement, disease outside the glands and infiltration of glands by lymphocytes. Anti-Ro antibodies are specific to components of the Ro-RNP complex, comprising 45kDa, 52kDa, 54kDa and 60kDa proteins and RNA. The 60kDa DNA/RNA binding protein and 52kDa T-cell regulatory protein are the best characterised antigens of anti-Ro antibodies. Collectively, these proteins are part of a ribonucleoprotein (RNP) complex that associate with the human Y RNAs, hY1-hY5. The La antigen is a 48kDa transcription termination factor of RNA polymerase III, which associates with the Ro-RNP complex.
The mechanism of antibody production in Sjögrens syndrome is not fully understood, but apoptosis (programmed cell death) and molecular mimicry may play a role. The Ro and La antigens are expressed on the surface of cells undergoing apoptosis and may cause the inflammation within the salivary gland by interaction with cells of the immune system. The antibodies may also be produced through molecular mimicry, where cross reactive antibodies bind to both virus and human proteins. This may occur with one of the antigens, Ro or La, and may subsequently produce antibodies to other proteins through a process known as epitope spreading. The retroviral gag protein shows similarity to the La protein and is proposed as a possible example for molecular mimicry in Sjögrens syndrome.