Autoimmunity to a 28–30 kD cell membrane DNA binding protein: occurrence in selected sera from patients with SLE and mixed connective tissue disease (MCTD)

SUMMARY Previous experiments have established the presence of a 30‐kD DNA binding protein on the surface of human leukocytes. Herein we report that selected sera from patients with systemic lupus erythematosus (SLE) and MCTD are reactive with a 28–30 kD protein on immunoblots of peripheral blood mononuclear cells (PBMC) cell membrane preparations; the reactivity is abolished by prior incubation of the blot with DNA. Antibodies ctuted from the 28–30 kD strip inhibited the binding of 3H.DNA to human PBMC. An immunomatrix of 28–30 kD reactive immunogtobulins was able to extract a 29‐kD DNA binding protein from a PBMC cell membrane preparation. Flow cytometry experiments confirmed the cell surface IgG reactivity of sera with T lymphocytes. Additional experiments indicated that cell surface IgG binding was not due to antibodies binding to cell surface DNA, DNA anti‐DNA immune complexes reacting with a DNA binding protein, anti‐histone antibodies or anti‐Sm antibodies. It is hypothesized that this autoimmune response could be one component of an idiotypic network involving anti‐DNA antibodies.