Human CD5+ B lymphocytes produce autoantibodies that bind to self- and exogenous antigens. Extremely high percentages of CD5+ B lymphocytes are present in the fetal and newborn periods, whereas they constitute only a minority of B lymphocytes in healthy adults. Increased percentages of circulating CD5+ lymphocytes have previously been demonstrated in several autoimmune diseases, including rheumatoid arthritis, progressive systemic sclerosis, Graves' disease, and Sjögren's syndrome. We measured the percentages of B lymphocytes that expressed the CD5 determinant in 93 control subjects (age range 1 day to 59 yr, mean ± SD 22.6 ± 17.7 yr), 17 subjects with newly diagnosed insulin-dependent diabetes mellitus (IDDM; range 5'29 yr, mean ± SD 13 ± 5.9 yr), 31 high-risk islet cell antibody (ICA)-positive nondiabetic subjects (range 4–45 yr, mean ± SD 19.8 ± 14.1 yr), and 13 subjects with IDDM of >5 yr duration (range 10–43 yr, mean ± SD 24.2 ± 9.9 yr). We report that CD5+ B-lymphocyte percentages are strikingly age dependent in healthy control subjects, declining progressively from the newborn period to the middle-age years (r = −0.75, P = 0.0001). In ICA+ nondiabetic and recent-onset IDDM subjects <29 yr of age, the percentage of circulating CD5+ B lymphocytes fell within the 95% confidence intervals established for control subjects. However, the age-dependent rate of decline in the percentage of CD5+ B lymphocytes within the control range was slower in ICA+ and newly diagnosed IDDM subjects than in control subjects.

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