Monoclonal process in primary Sjögren's syndrome and cross-reactive idiotype associated with rheumatoid factor

Monoclonal or oligoclonal B cell products have been described in the sera and urine of patients with primary Sjögren's syndrome (PSS). In addition, monoclonal expansion of plasma cells has been found in the exocrine glands of PSS patients with circulating monoclonal B cell products. The goal of this study was to raise an anti-idiotype to a cryoprecipitable monoclonal IgM kappa rheumatoid factor (RF) from a PSS patient. Using the F(ab')2 fragments of the rabbit IgG anti-idiotype, an idiotype-specific ELISA was developed and sera from 32 patients with PSS (13 with monoclonal IgM kappa), 33 with rheumatoid arthritis, three with rheumatoid arthritis + Sjögren's syndrome (SS), 30 with systemic lupus erythematosus, six with Waldenström's macroglobulinaemia, and 20 healthy controls were tested. The idiotype was primarily found in PSS patients with circulating monoclonal IgM kappa and more often in those who had a ratio of kappa: lambda intracytoplasmically positive plasma cells greater than 3:1 in the lymphocytic infiltrates of minor salivary glands, and systemic manifestations. The idiotype was also found in PSS and rheumatoid arthritis patients without circulating monoclonal cryoglobulins as well as in two of the six patients with Waldenstrom's macroglobulinaemia. Our results suggest that the monoclonal process observed in PSS could involve restricted idiotypic clones that are susceptible to malignant transformation.