Lymphoproliferation in primary sjögren's syndrome. evidence of selective expansion of a b cell subset characterized by the expression of cross‐reactive idiotypes

Objective. To evaluate the possibility that lymphoproliferation in primary Sjögren's syndrome (SS) arises within a subset of B cells. Methods. A panel of monoclonal antibodies (MAb) specific for rheumatoid factor (RF)–associated cross‐reactive idiotypes (CRI) and anti‐V K and anti‐V H subgroup antibodies were used to define the clonality of B lymphocytes undergoing neoplastic transformation in 5 patients with primary SS. Anti‐CRI antibodies were also used to study longitudinal variations in serum paraprotein levels and in vitro regulation of IgM and IgM‐RF production by peripheral blood lymphocytes. The levels of CRI, IgM, and IgM‐RF were quantitated in serum and culture supernatants by enzyme‐linked immunosorbent assay. Heavy and light chain isotypes and V H subgroups of the paraproteins were determined by immunoelectrophoresis, immunofixation, and Western blotting. Results. Paraproteins from all patients expressed an epitope associated with V K IIIb sub‐subgroup of light chains. Three of the paraproteins were cryoglobulins with RF activity, all of which expressed the V K IIIb‐associated CRI (detected by MAb 17–109) and the V H I‐associated CRI (detected by MAb G6 and G8). None of the paraproteins expressed the V H III‐associated CRI (detected by MAb B6 and D12). The CRI were consistently expressed over a period of 5–6 years. The anti‐CRI and anti‐subgroup antibodies substantially inhibited spontaneous production of IgM‐RF and IgM by peripheral blood B lymphocytes from 3 of the SS patients. Conclusion. These results suggest that lymphoproliferation in primary SS is highly selective, and that the anti‐CRI antibodies can be used as an aid to early diagnosis as well as for monitoring and modulating the lymphoproliferative process in primary SS.