1,25-D3 synergizes with IFN-γ to increase NO, nitrite, and iNOS protein levels in human monocyte-derived macrophages (MDMs). MDMs were pretreated with IFN-γ, or 1,25-D3 (20 nM), alone or in combination, for 18 h, with or without L-NMMA (100 µM) or L-NAME (100 µM). Following pretreatment, PPD or MTB H37Ra (MOI = 1) was added to all cultures. (A) Effect of IFN-γ, 1,25-D3, or both on PPD-induced nitrite production over time (at 1, 4, and 10 days). (B) PPD-induced NO generation in human MDMs pretreated with IFN-γ, 1,25-D3, or both. The generation of NO was detected by staining with DAF-2 DA (10 µM) for 1 h and monitored by confocal microscopy. Upper panel, representative immunofluorescence. Lower panel, quantitative analysis (C) Expression of iNOS protein by MTB H37Ra-stimulated MDMs pretreated with IFN-γ, 1,25-D3, or both at 6 h. Western blot analysis was performed using rabbit anti-iNOS Ab. Results are representative of three experiments. ***p<0.001 vs. PPD-treated control.