Normal serum and IgG are potent inhibitors of immune complex-stimulated IFN-α production. A, Equal amounts of NPSLE+ CSF and normal serum (both diluted 1/10) were incubated with NE and PBMC and IFN-α quantified as in Fig. 1. B, Normal CSF or serum (1%, 2.5%, or 5% (v/v)) was added to diluted NPSLE serum (1/2000), incubated with NE and PBMCs, and tested for IFN-α production in the bioassay after 24 h. Results are expressed as percentage inhibition of IFN-α production relative to cells that did not receive normal serum or CSF. C, Purified IgG (350 μg/ml) was added to 1/10 diluted normal or four different NPSLE+ patients' CSF, and IFN-α was quantified as above. The results in A and C are representative of two to three independent experiments, and in B, inhibition of IFN-α is expressed as the mean percentage ± SEM of three to six independent experiments. n.d. indicates not detected. D, Serum from normal individuals (N), four patients with hypogammaglobulinemia (HypoGam, associated with either X-linked agammaglobulinemia or common variable immunodeficiency), or three normal sera depleted of IgG (IgG dep) were incubated at 2.5% or 5% (v/v) with stimulatory NPSLE+ serum (1/2000), U937 freeze-thawed Ag (1% (v/v)), and primed PBMC. Results are shown as the mean percentage ± SEM of four independent experiments. **, p < 0.01; ***, p < 0.001; Mann-Whitney U test.