Evidence for anti-tubulin autoantibodies in the form of immune complexes in human sera.

Anti-tubulin antibodies were studied in normal human serum either maintained at neutral pH to measure the free antibody activity (FAA) or treated at pH 2.8 to measure the total antibody activity (TAA): FAA + the antibody activity in the form of immune complexes (ICAA). Anti-tubulin antibody activities were assessed by measurements of the capacity of serum immunoglobulins to bind pure 125I-labelled tubulin in a liquid phase radioimmune assay or to immunoprecipitate unlabelled tubulin revealed by Western blot using anti-alpha- or anti-beta-tubulin monoclonal antibodies. Acid buffer-treated serum and untreated serum at a 1:200 dilution immunoprecipitated about 35% and 4% of labelled tubulin, respectively. TAA was therefore 8- to 10-fold higher than FFA. Anti-tubulin antibody titres corresponding to TAA and FAA were about 1:20,000 and 1:500, respectively. The Western blot analysis confirmed that the acid buffer-treatment of the serum dramatically increased the capacity of serum immunoglobulins to immunoprecipitate tubulin. TAA was studied in patients with Graves' disease with elevated FAA. TAA of the sera of control subjects and patients with Graves' disease were not significantly different, so an increase of FAA was related to a decrease of ICAA. These results indicate that (a) normal human serum contains high levels of anti-tubulin antibodies in the form of immune complexes which are dissociated by an acid buffer treatment, (b) these immune complexes exist in the presence of a small excess of free anti-tubulin antibodies, (c) the equilibrium between free and immune complex-bound anti-tubulin antibodies could be altered in patients with autoimmune diseases.