Relationship between serum levels of IL‐18 and IgG1 in patients with primary Sjögren's syndrome, rheumatoid arthritis and healthy controls

SUMMARY Primary Sjögren's syndrome (SS) is characterized by inflammation in salivary and lachrymal glands, with a local predominance of Th1‐like cytokines, as well as the pleiotropic cytokine interleukin (IL) 18. High serum levels of polyclonal IgG are common, with a subclass imbalance in which IgG1 is increased and IgG2 is normal or low. IL‐18 is also of pathogenetic importance in rheumatoid arthritis. In the present study we looked for any relationship between serum IL‐18 as well as transforming growth factor (TGF) β 1 versus IgA, IgM, and IgG subclass levels in SS ( n  = 16), rheumatoid arthritis (RA) ( n  = 15), and healthy controls ( n  = 15). SS was defined by the revised American‐European classification criteria. IL‐18 and TGF‐ β 1 were analyzed with enzyme immunoassays (EIA), and IgG1, IgG2 and IgG3 by single radial immunodiffusion. In the composite group of RA, SS and normal controls, IgG1 and IL‐18 were related ( R =  0·52, P  = 0·0005). No relation was found neither between IL‐18 versus IgG2, IgG3 or IgA, nor between serum TGF‐ β 1 versus any of the immunoglobulins. Since serum levels of IL‐18 are related to serum IgG1, IL‐18 may be of importance for IgG1 switch and/or release.