Extracellular nucleotides inhibit IFN-α production in response to influenza virus or CpG ODN 2216. (A) PDCs were activated with either CpG ODN 2216, or CpG ODN 2006, or R-848 or influenza virus in the absence or presence of 100 μM ATP. Supernatants were harvested after 48 hours and concentrations of IFN-α were measured by ELISA. Data are mean values (± SEM) of 5 to 10 different donors. (B) Effect of various nucleotides, each at 100 μM, on IFN-α production in response to TLR ligands. Data represent means of percentage changes (± SEM) of 5 to 10 different donors. Mean values (± SEM) in nanograms/milliliter for CpG ODN 2216, 66.9 (± 7.6); for CpG ODN 2006, 11.3 (± 3.8); and for R-848, 47.0 (± 9.1); *P < .05.