Anticardiolipin antibodies purified from serum from patients with systemic lupus erythematosus (SLE) by cardiolipin micelles were studied for their effects on lymphocytes and neutrophils. At a concentration of 160 micrograms/ml they markedly suppressed the [3H]thymidine incorporation of mononuclear cells stimulated by phytohaemagglutinin (4.9 (SEM 1.9%) of the control) and pokeweed mitogen (26.7 (10.5%) of the control). In addition, anticardiolipin antibodies changed the cell cycle of phytohaemagglutinin stimulated lymphocytes such that the S and G2+M phases were significantly diminished (G0/G1 = 64.62%, S = 20.59%, G2+M = 14.78% in the presence of normal human IgG v G0/G1 = 86.07%, S = 10.32%, G2+M = 3.59% in the presence of anticardiolipin antibodies). The suppression of lymphocyte proliferation by anticardiolipin antibodies was shown not to be caused by an alteration of T cell subpopulations. However, the interleukin 2 receptors on the cell surface and the soluble interleukin 2 receptors in the supernatant of phytohaemagglutinin stimulated mononuclear cells were decreased in the presence of anticardiolipin antibodies. On the other hand, the phagocytic activity of neutrophils was 40% inhibited at a higher concentration of anticardiolipin antibodies (300 micrograms/ml) through suppression of C3b/C4b and Fc receptors on polymorphonuclear leucocytes. These results suggest that anticardiolipin antibodies exert inhibitory effects on both lymphocytes and phagocytes in addition to the coagulation cascade. These newly found activities of anticardiolipin antibodies were mediated by the non-specific membranotropic property of the antibodies.
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