Rheumatoid factor isotypes in serum and salivary fluid of patients with primary Sjögren's syndrome.

Salivary fluid and serum levels of immunoglobulin and rheumatoid factor (RF) isotypes were measured in patients with primary Sjögren's syndrome (SS) as well as in patients suspected of the syndrome in whom the diagnosis could be excluded (NON-SS) in order to investigate the site of RF production in primary SS and to determine the diagnostic value of determination of salivary fluid levels of RF. Increased salivary fluid levels of IgA-RF were found in 46% of 39 primary SS patients, 14% of 42 NON-SS patients, and in 2% of 41 healthy controls. Salivary fluid levels of other RF isotypes were not increased in the patient groups. The frequency of elevated titers of IgA-, IgG-, and IgM-RF in serum of primary SS patients was 51, 7, and 59%, respectively. The levels of IgA, IgG, and IgM in salivary fluid of primary SS patients were increased in 36, 31, and 23%, respectively. Comparison of RF levels in samples of serum and saliva from primary SS patients after correction for immunoglobulin levels indicated local production of IgA-RF in salivary glands. Significant positive correlations were found between IgA-RF levels in serum and saliva but not between the levels of immunoglobulin isotypes in the two compartments. The results of this study show that (1) increased levels of IgA-, but not IgG- and IgM-RF occur in saliva of primary SS patients; (2) IgA-RF is locally produced in the salivary glands; (3) the stimuli for IgA-RF production in the systemic and secretory compartment may be similar; and (4) measurement of both IgA- and IgM-RF in serum may be used as adjunctive diagnostic procedure for primary SS.