Anti-56K: a novel, frequently occurring autoantibody specificity in connective tissue disease.

A novel frequently occurring autoantibody specificity in serum from connective tissue disease patients is described. The autoantibodies as detected by immunoblotting are directed against a 56,000 Dalton (56K) antigen, that after biochemical fractionation predominantly is found in the cytoplasmic fraction of various cell types and tissues. Attempts to localize the antigen more precisely were unsuccessful primarily because these antibodies do not produce a positive immunofluorescence pattern. The antigen in its native form is not associated with DNA or one of the common cytoplasmic or nuclear RNAs in the cell. Immunoprecipitation studies also showed that the protein is not closely associated with other proteins in a multi-component complex. Anti-56K antibodies are found in about 8% of patients with connective tissue disease, most commonly in patients with Sjögren's syndrome (14%). It is not found in patients with mixed connective tissue disease, polymyositis/dermatomyositis or scleroderma and rarely (less than 1%) in healthy control subjects. The fact that 22% of the anti-56K sera also contain La/SS-B antibodies support the idea that this antibody specificity might be characteristic for a subclass of Sjögren's syndrome patients.