Antibodies in the sera of patients with systemic lupus erythematosus that block the binding of monoclonal anti-Ia to Ia-positive targets also inhibit the autologous mixed lymphocyte response

In this report, we examined the effect of SLE sera on the autologous MLR and found that sera from active SLE patients contain an IgG antibody that blocks the autologous MLR. Sera from 12 patients with active SLE caused 84 ± 5% inhibition of the autologous MLR vs 19 ± 11% inhibition caused by sera from 12 patients with inactive disease (mean ± SE, p < 0.001). When three SLE sera were separated into IgG and MLIgM fractions using affinity chromatography, the MLR inhibitory activity was contained in the IgG fraction (whole sera, 93 ± 8% inhibition; IgG fraction, 79 ± 16% inhibition; IgM fraction, 9 ± 18% inhibition), whereas anti-Ia blocking activity was present in both fractions. Inhibitory activity of SLE sera on the autologous MLR was highly correlated with ability of such sera to block the binding of monoclonal anti-Ia antibody to Ia-positive target cells (n = 24, r = 0.84, p < 0.001). Absorption of SLE sera with Ia-positive but not Ia-negative cell lines eliminated both the inhibition of the autologous MLR (91 ± 10% inhibition by whole sera; 11 ± 18% inhibition by sera absorbed with Ia(+) cell line; 73 ± 23% inhibition absorbed with Ia(-) cell line) and the blocking activity of monoclonal anti-Ia binding to Ia-positive target cells. We conclude that both IgG and IgM antibodies present in the sera of patients with active SLE block the binding of monoclonal anti-Ia to Ia-positive target cells and that IgG antibody blocks the autologous MLR and may be responsible for the depressed autolgous MLR seen in patients with this disease.