Cytoplasmic detection of a novel protein containing a nuclear localization sequence by human autoantibodies

A great diversity of antibodies directed to cell proteins has been described in sera of patients with autoimmune diseases. Most of these sera recognize nuclear components, but some others are directed against cytoplasmic autoantigens. Some of the antibodies directed to cytoplasmic autoantigens are well characterized, such as anti-mitochrondial, anti-ribosomal, anti-microsomal and anti-Golgi complex autoantibodies, but the target of many others remains unknown. In the last 5 years we have selected 32 sera with a characteristic speckled cytoplasmic pattern in indirect immunofluorescence (IIF) assay among a total of more than 31,000 sera from patients with any kind of autoimmune manifestation who attend our Connective Tissue Disease Clinic. Using a human cDNA expression library, we have identified a new autoantibody specificity named RCD-8 in five of these sera, directed to one cytoplasmic autoantigen. Affinity-purified antibodies eluted from a positive clone reproduced the same IIF cytoplasmic staining pattern as native serum and reacted with one single band of 160 kD on an immunoblot of HeLa cell extract. The sequence was found homologous to an autoantigen recently reported named Ge-1, and contains a nuclear localization sequence (NLS), an active protein domain made by a contiguous stretch of amino acids which allows the selective entry of the protein into the nucleus. The five patients whose sera exhibited this new autoantibody specificity displayed different autoimmune pathological profiles.