Response to 'Species differences in macrophage NO production are important'

Schneemann and Schoedon disagree with my statement: “the demonstration of iNOS expression by macrophages and other cell types in tissues from patients with a wide variety of infectious, autoimmune and degenerative diseases has disproved the claim that iNOS does not occur in the human immune system”¹. They argue that my review¹ ignored fundamental differences between murine and nonmurine macrophages and that none of the cited articles on the expression of iNOS in humans provided information on the cell types that release NO. For several reasons I find their criticism unjustified.

First, as stated in the introduction, my review does not focus on differences in the expression and regulation of iNOS in human versus murine cells because a number of previous reviews (which are cited) had already addressed this issue in detail and discussed all relevant publications^2,3,4,5. Second, the important regulation of iNOS activity by arginase, arginine and cofactor availability is discussed on p. 910 of my review. However, this mode of regulation is by no means restricted to human macrophages, but also occurs in murine cells. The detection of iNOS mRNA or protein certainly does not prove enzyme activity, but this limitation also applies to the numerous studies with mouse tissues. Third, the studies with patients that I referred to on p. 913 (by citing original papers or reviews) did, indeed, identify the cells expressing iNOS. For example, iNOS protein and/or activity was found in bronchoalveolar macrophages of patients with pulmonary tuberculosis or in blood monocytes isolated from patients with hepatitis C after in vivo treatment with IFN-α^6,7. Fourth, induction of iNOS in human monocytes and macrophages has undoubtedly been more difficult than in rodent cells. However, much of the frustration resulted from stimulation with IFN-γ and lipopolysaccharide². As reviewed previously^4,5, alternative stimuli (such as IFN-α/β, IL-4 + anti-CD23 or chemokines) appear to be much more effective^7,8,9,10. Fifth, irrespective of the actual amount of NO produced by human monocytes and macrophages in vivo, it is important to bear in mind that small quantities of NO (below the detection limit of the Griess assay and produced by iNOS or one of the constitutive isoforms of NOS) might still exert immunologically relevant functions, including antimicrobial activity¹¹. Finally, my statement on the expression of iNOS in humans specifically referred to macrophages and other cell types. Human blood mononuclear cells, neutrophils, keratinocytes, epithelial cells, synoviocytes, chondrocytes, hepatocytes, neurons and glial cells all express iNOS mRNA, protein and/or activity in various disease states^5,12. I therefore stand by the conclusions I drew.