IgG anti-T cell autoantibodies are common in SLE serum, react preferentially with activated lymphocytes, and exert early-phase inhibitory effects on antigen-induced T cell proliferation. Little is known about the target molecules in this system, however, because the low titer and low avidity of the most interesting antibodies limit their utility in conventional immunoprecipitation analyses. Therefore, Western blotting was used to demonstrate binding of IgG in anti-T cell antibody-positive SLE sera to four surface membrane molecules shared by peripheral T cells and HSB-2 cells. Molecules of Mr 90,000 and 55,000 were particularly reactive: each target was stained by IgG anti-lymphocyte antibodies in 11 patient sera (approximately 85%) in the panel. Targets of Mr 37,000 and 105,000 were encountered less frequently (six of 13 and one of 13 patients, respectively). It is unlikely that alloantibodies contributed to the staining patterns observed because reactivity with the four targets was consistently present when cell preparations from multiple unrelated donors were examined. The target molecules were localized to the plasma membrane by whole cell absorption/elution experiments, by the failure of chromatin (DNA/histone) to absorb antibodies to these antigens, and through the use of purified membranes as substrate for Western blotting. With the possible exception of the 105,000 Mr molecule, which is a major target in the IgM anti-T cell antibody system, evidence for the existence of neoantigens as a basis for increased reactivity of SLE IgG with activated T cells was not obtained. The identity of the IgG antibody-reactive molecules with respect to known T cell antigens was not determined, although evidence against the existence of antibodies to Tac (IL 2 receptor) and the transferrin receptor was obtained in monoclonal antibody pre-clearing experiments. Nonetheless, the observation that a limited number of major IgG autoantibody target antigens on activated peripheral T cells are shared by HSB-2 cells, a primitive T cell line expressing few of the differentiation antigens characteristic of mature T cells, should provide a basis for more definitive characterization of antigens in this system in the future.

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