IFN-alpha secretion by activated pDCs. pDCs were separated from healthy donors (HD), untreated patients (MS: No Rx), and IFN-beta-treated RR MS patients (MS: IFN-beta). pDCs were activated in vitro with TLR9 agonist for 16 hours. IFN-alpha was measured in culture supernatants by ELISA, as described in Patients and Methods. IFN-alpha production was elevated in untreated patients (mean ± SEM = 8,479 ± 1,898 pg/ml, n = 11) as compared to healthy donors (3,659 ± 891 pg/ml, n = 17), p = 0.0164. MS patients treated with IFN-beta had decreased IFN-alpha production (2,826 ± 1,350 pg/ml, n = 12) as compared to untreated RR MS patients, p = 0.027. No detectable IFN-alpha was seen in control pDC cultures without TLR9 agonist. ELISA = enzyme-linked immunosorbent assay; SEM = standard error of the mean.