The amount of Stat1 protein, as measured by the mfi, was increased in lymphocytes of SLE patients as compared with healthy lymphocytes (21.4 ± 14.9 [mean ± standard deviation] versus 7.04 ± 1.54, P < 0.0001, t test), and in monocytes from SLE patients as compared with healthy monocytes (25.1 ± 13.2 versus 10.4 ± 2.54, P < 0.0001). Lymphocytic and monocytic Stat1 mfi correlated both for SLE patients (Pearson r = 0.57, P < 0.005) and for healthy individuals (r = 0.75, P < 0.005). The amount of phosphorylated Stat1, as measured by pStat1 mfi, was increased in SLE as compared with healthy lymphocytes (1.63 ± 0.40 versus 1.36 ± 0.22, P < 0.02), but not significantly increased in SLE monocytes as compared with healthy monocytes (4.03 ± 1.64 versus 3.24 ± 1.21, P = not significant). Nevertheless, the pStat1 mean fluorescence intensities of lymphocytes and monocytes were highly correlated, both for SLE patients and healthy individuals (r = 0.84, P < 0.0001, and r = 0.91, P < 0.0001, respectively).
As compared with incubation in medium alone, incubation with either IFN-α or IFN-γ increased the amount of pStat1 in SLE lymphocytes (from 1.57 ± 0.29 in medium alone to 1.82 ± 0.36 [P < 0.01, paired t test] with IFN-α and to 1.85 ± 0.40 [P < 0.002] with IFN-γ). In contrast, IFN-α, but not IFN-γ, increased the pStat1 mfi in healthy lymphocytes (from 1.41 ± 0.15 in medium alone to 1.83 ± 0.48 [P < 0.002] with IFN-α, but to 1.40 ± 0.18 [P = not significant] with IFN-γ). Likewise, SLE monocytes increased their pStat1 contents upon incubation with either IFN-α or IFN-γ (from 3.74 ± 1.18 to 4.66 ± 1.50 [P < 0.05] and 7.28 ± 4.14 [P < 0.001] for IFN-α and IFN-γ, respectively), while healthy monocytes responded to IFN-α (pStat1 mfi from 3.52 ± 1.11 to 5.23 ± 2.25, P < 0.01), but not IFN-γ (pStat1 mfi 3.96 ± 5.36, P < 0.001 versus medium).