Concentrations of NO and nitrite were determined using an NO analyzer (NOATM280; Sievers Instruments, Inc., Boulder, CO). NO concentrations were determined via analysis of nitrite accumulation in the culture supernatants of co-cultured d7EB cells and CD11bϩ cells or CD11bϩ cells alone. Media aliquots were collected at the indicated times, and were injected directly into the analyzing cell containing the reducing solution (saturated sodium iodide in 50% acetic acid). The analysis was conducted at room temperature. Authentic NaNO 2 solutions of known concentrations were used as standards. For determination of NO production from human d7EB cells, cultures were washed twice with fresh serum-free media. Medium containing 1 g/ml of LPS was replenished, and cells were undisturbed for the duration of the experiment. Media samples were collected at selected times, and NO production was assessed as nitrite accumulated in the media.