Autoimmune sera recognize a 100 kD nuclear protein antigen (sp-100).

Autoimmune sera from patients suffering from undifferentiated connective tissue diseases (UCTD), Sjögren's syndrome (SS), primary biliary cirrhosis (PBC) and other disorders were found to contain antibodies that produce a distinctive nuclear spot pattern with HEp-2 cells in immunofluorescence studies. These spots which vary in size and number, are spread over the whole nucleus with the exception of the nucleoli. This pattern is easily distinguishable from the staining patterns of anti-centromere, anti-RNP, anti-nucleolar and anti-Scl-70 antibodies. In cells of non-human origin this pattern is discerned only at high serum concentrations. Immunoblotting experiments with a soluble protein fraction from HeLa nuclei revealed that the antigenic target common to all sera is a polypeptide of 100 kD with a pI value of about 5.2. The correlation between immunofluorescence and immunoblotting data was confirmed by affinity-purification of sp-100 specific autoantibodies followed by immunofluorescence experiments.