Giardia reduces host cell nitric oxide (NO) production. A, Expression changes of inducible nitric oxide synthase (nos2) in differentiated Caco-2 cells in medium with (+ arginine) and without (- arginine) arginine as assessed by qPCR in technical quadruplicates. Data is expressed as fold change expression compared to the 0 h timepoint. Significant expression changes compared to 0 h are indicated by asterisks. B, Expression changes of nos2 upon host cell (HCT-8) stimulation with cytokines (TNF-α (200 ng/mL), IL-1α (200 ng/mL), IFN-γ (500 ng/mL)) and Giardia infection 40 h later. Data is expressed as fold change expression compared to the 0 h unstimulated control (squares). C, NO production of host cells (HCT-8) stimulated with cytokines 5 h after infection with Giardia trophozoites of 3 different isolates (WB, GS, P15). This experiment was repeated two times independently and lead to similar results. D, Giardia (isolate WB) infected host cells (HCT-8) were stimulated by cytokines to produce NO after 5 h of infection. Subsequently arginine (Arg) or citrulline (Citr) were added to 0.4 mM after 1 h of interaction. NO production was measured 40 h later. The described experiment was repeated two times independently and lead to similar results. Significant differences in the figure are indicated by asterisks (*for p < 0.5 and **for p < 0.01).