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Figure S1. Anti-P antibodies affinity purified from the serum of SLE patients without NP symptoms induce calcium influx and apoptosis in cortical primary neurons. Anti-P (+) sera from two patients that did not manifest NP symptoms were subjected to affinity chromatography with P₁₁, and the isolated α-hP₁₁ antibodies were tested in experiments similar to those depicted in Figs. 8 and 9. For simplicity, we illustrate the results of only one of the sera, but the other serum showed similar results. Graphs show cells displaying different degrees of responsiveness. These α-hP₁₁ antibodies induced a sustained increase in calcium levels (A), which returned to basal levels after washing the antibody (white bar) and were abrogated by preincubating the antibodies with the P peptide (black bar; B), thus demonstrating reversibility and specificity. (C) Dose-dependent responses to α-hP₁₁ of nonpsychiatric patients are similar to those elicited by patients with lupus psychosis (Fig. 8). Horizontal bars represent incubation time periods. (D) Indirect immunofluorescence against activated caspase-3 revealed apoptosis in neuronal cells incubated for 48 h with 0.5 μg/ml of α-hP₁₁ antibodies but not with 100 μg/ml of control IgG. Bar, 100 μm.