Circulating autoantibodies to IgD in rheumatic diseases.

Most mature B lymphocytes possess IgD on their cell surface. Antibodies to IgD produce an adjuvant-like effect in many experimental systems. Employing sensitive class-specific radioimmunoassays, elevated concentrations of autoantibodies to IgD of the IgA, IgM, and IgG classes were detected in the sera of 55% of patients with adult onset rheumatoid arthritis (RA), of 45% with systemic lupus erythematosus (SLE), of 67% with mixed connective tissue disease syndrome (MCTD), and of only 8% of juvenile RA. Marked differences in the class of anti-IgD were noted. An elevated IgA anti-IgD was detected in 45% (p less than 0.01) of RA sera, 27% of SLE sera, and 58% (p less than 0.01) of MCTD sera. IgG anti-IgD was increased in only 26% of RA, 32% of SLE, and 8% of MCTD sera. IgM anti-IgD was elevated in only one patient. IgA anti-IgD was inhibited greater than 90% by IgD but not at all by IgG. Correlations between the concentration of immunoglobulin G and IgA anti-IgD were noted in RA (rs = 0.37, p = 0.02), SLE (rs = 0.46, p = 0.015), and MCTD (rs = 0.4). These data suggest a potential role for autoantibodies to cell surface IgD as modulators of the immune system in some patients with autoimmune disorders.