Adenosine deaminase (ADA) is an enzyme involved in purine metabolism and has a major role in the development and function of lymphoid cells. Congenital deficiency of ADA results in severe immunodeficiency. Patients with congenital ADA deficiency treated with polyethylene glycol-conjugated bovine ADA develop antibodies to ADA. This leads us to investigate the role of anti-ADA antibodies in patients with systemic rheumatic diseases. Commercially available ADA was used in ELISA and immunoblots for detection of anti-ADA antibodies. Four out of 100 patients examined were positive for anti-ADA antibodies. Two of them had peripheral blood lymphopenia but the antibody levels did not appear to correlate with the lymphocyte counts. Immunoblotting revealed that the antibodies recognized a 40 kDa peptide of ADA, corresponding to ADA1, the major component of ADA. Affinity-purified antibodies were used to locate the distribution of ADA on Hep-2 cells and lymphocytes by indirect immunofluorescence. Anti-ADA antibodies gave a distinct nuclear speckled pattern on acetone-fixed cells. With viable cell immunofluorescence, anti-ADA antibodies also stained the cell surface of HEp-2 cells and lymphocytes, indicating surface expression of ADA. The anti-ADA antibodies failed to gain access into the cytoplasm or nuclei when added to the cultures of HEp-2 cells. In summary, this is the first report of detection of anti-ADA1 autoantibody which is a new type of ANA with discrete, speckled nuclear staining, but which may not be associated with lymphopenia.
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