Man is not a mouse: reply

It is well documented that macrophages obtained from different mammalian species exhibit wide variation in their production of NO under conventional in vitro laboratory conditions. However, the question of greater biological interest is whether macrophages produce NO in vivo. Abundant evidence supports a role for NO production by human macrophages in vivo. Macrophages obtained from patients with inflammatory conditions such as tuberculosis, malaria, or rheumatoid arthritis have consistently been shown to express inducible NO synthase (iNOS) mRNA, produce iNOS protein, and generate NO (and l-citrulline) from l-arginine in more than 250 independent published studies from North America, South America, Europe, Asia, Africa, and Oceania 1,2,3,4,5 . Infected tissues from patients with focal bacterial infections and septic shock exhibit iNOS activity in association with iNOS expression by mononuclear cells and elevated plasma concentrations of NO oxidation products 6, 7 . Similarly, although human macrophage cell lines produce limited quantities of tetrahydrobiopterin cofactor in vitro, there is no evidence that this prevents NO generation in vivo. Tetrahydrobiopterin has been detected in all human tissues from which measurements have been obtained 8 , and synthesis of tetrahydrobiopterin and iNOS is stimulated by inflammation 9, 10 . Given the weight of evidence, it is difficult to interpret the studies cited by Drs. Schneemann and Schoedon as showing anything other than the limitations of in vitro assays that use PBMC from healthy subjects. It is interesting that these authors cite goats as another example of a species whose macrophages cannot produce NO. Yet, iNOS-positive macrophages can be detected within the tissues of Listeria-infected goats despite poor iNOS expression exhibited by caprine macrophages in vitro 11 . Man may not be a mouse, but perhaps he can still be a goat.