Tuberculosis (TB) caused by Mycobacterium tuberculosis (Mtb) is a global disease that kills about two million people each year. Leprosy is caused by Mycobacterium leprae, and primarily affects the skin and peripheral nervous system. About 10-20% of leprosy patients suffer from reactions associated with acute inflammation that can lead to rapid and severe nerve damage. Most individuals infected with Mtb or M leprae do not develop clinical disease, which indicates that human hosts have effective defence mechanisms. In macrophages activated by cytokines such as IFN-γ, inducible nitric oxide synthase (iNOS) catalyses the production of nitric oxide (NO) from L-arginine. In an inflammatory environment, NO reacts with the superoxide radical (O₂^-) to yield peroxynitrite, an unstable metabolite that can rapidly nitrosylate tyrosine residues on proteins to form the stabile end product nitrotyrosine (NT). Many studies using experimental models have indicated that NO is important in host response to M leprae and Mtb, but this is controversial in human disease. Thus, our aim was to investigate the presence and the role of NO in the human mycobacterial infectious diseases TB and leprosy.

Levels of the NO metabolites nitrite and nitrate were initially increased in urine from patients with reactional leprosy but were normalised by treatment with prednisolone, and this was associated with clinical improvement. Immunohistochemistry revealed local production of NO in skin biopsies from patients with borderline leprosy and reversal reactions, which was detected as reactivity to iNOS and NT in macrophage-rich granulomas. Ultrastructural studies showed NT-positive aggregations of neurofilaments in dermal nerves from leprosy patients. Patients with active tuberculosis had increased urinary levels of NO metabolites, which were normalised after anti-TB treatment. Household contacts of patients with tuberculosis had increased levels of NO metabolites in plasma and serum. Immunohistochemical examination of biopsies from patients with TB indicated local, iN OS-mediated generation ofNO in macrophage-rich granulomas. In an experimental model of TB, local production of NO in the lungs was substantial in the acute phase of infection, and immunoelectron microscopy detected NT in phagosomes containing Mtb and on the surface of the bacteria. In an in vitro model, NO and peroxynitrite killed Mtb H37Ra and induced upregulation of several bacterial proteins. Peroxynitrite also mediated tyrosine nitration of albumin associated with the surface of Mtb. In a randomised, double-blind trial in Ethiopia, arginine supplementation in patients receiving conventional chemotherapy increased sputum conversion and reduced the prevalence of cough in HIV -negative, smear-positive patients with active TB.

In conclusion, these results demonstrate that iNOS-mediated production of NO occurs in human tuberculosis and leprosy; NO and peroxynitrite can kill Mtb and modify protein expression in the bacteria; and arginine leads to clinical improvement in TB patients.