IUBMB Life, 54: 95–99, 2002 Copyright c  2002 IUBMB 1521-6543/02 $12.00 + .00 DOI: 10.1080/15216540290114351 Review Article Nitric Oxide and Mycobacterium leprae Pathogenicity Paolo Visca, 1,2 Giulia Fabozzi, 2 Mario Milani, 3 Martino Bolognesi, 4 and Paolo Ascenzi 1,5 1 Department of Biology, University ‘Roma Tre,’ Viale G. Marconi 446, I-00146 Rome, Italy 2 National Institute for Infectious Diseases I.R.C.C.S. ‘Lazzaro Spallanzani,’ Molecular Microbiology Unit, Via Portuense 292, I-00149 Rome, Italy 3 Institute for Pediatric Diseases I.R.C.C.S. ‘Giannina Gaslini,’ Largo G. Gaslini 5, I-16147 Genova, Italy 4 Center for Advanced Biotechnology, Department of Physics—National Institute for Physics of the Matter, University of Genova, Largo R. Benzi 10, I-16132 Genova, Italy 5 Interdepartmental Laboratory for Electron Microscopy, University ‘Roma Tre,’ Via della Vasca Navale 79, I-00146 Rome, Italy Summary Leprosy is an old, still dreaded infectious disease caused by the obligate intracellular bacterium Mycobacterium leprae. During the infectious process, M. leprae is faced with the host macrophagic environment, where the oxidative stress and NO release, combined with low pH, low pO 2 , and high pCO 2 , contribute to limit the growth of the bacilli. Comparative genomics has unraveled massive gene decay in M. leprae, linking the strictly parasitic lifestyle with the reductive genome evolution. Compared with Mycobacterium tuber- culosis and Mycobacterium bovis, the leprosy bacillus has lost most of the genes involved in the detoxification of reactive oxygen and nitrogen species. The very low reactivity of the unique truncated hemoglobin retained by M. leprae could account for the suscepti- bility of this exceptionally slow-growing microbe to NO. IUBMB Life, 54: 95–99, 2002 Keywords Mycobacterium leprae; nitric oxide; pathogenicity; reductive genome evolution; truncated hemoglobin. Mycobacterium leprae, the causative agent of leprosy, is fa- mous for being the first microorganism definitively shown to be associated with human disease. More than one century after Hansen’s discovery of the bacterial etiology of leprosy in 1873, over 600,000 cases of such dreaded infection are still reported Received 25 June 2002; accepted 23 July 2002. Address correspondence to Paolo Ascenzi, Dipartimento di Biolo- gia, Universit` a ‘Roma Tre,’ Viale G. Marconi 446, 00146 Roma, Italy. Fax: +39-06-5517-6321. E-mail: ascenzi@bio.uniroma3.it Abbreviations: INF-γ , interferon-γ ; iNOS, inducible nitric oxide synthase; Hb, hemoglobin; flavo-Hb, flavo-hemoglobin; trHb, truncated hemoglobin. yearly 〈http://www.who.int/inffs/en/fact101.html〉, most of them concentrated in developing countries. Targeted multidrug ther- apy has dramatically reduced the prevalence of leprosy, but it has raised concern due to the emergence of multidrug-resistant strains (1). Failure to cultivate M. leprae in vitro or to mimic the disease in animal models are major drawbacks in understand- ing M. leprae pathogenesis (2, 3). The inability to propagate M. leprae in laboratory media is primarily due to its complex nutritional requirements and extremely slow growth (about 2- week generation time), both features reflecting the limited cod- ing capacity of the M. leprae genome (4). Early after entry in the host, M. leprae is faced with the host macrophagic environment, where the oxidative stress and the release of reactive nitrogen species (e.g., NO), combined with low pH, low pO 2 , and high pCO 2 , contribute to limit the growth of the bacilli (5). M. leprae evades the host immune response by invading and slowly duplicating inside the phagosomes of macrophages. Then, M. leprae infects the Schwann cells of the peripheral nervous system, blocking myelin production and leading to the characteristic features of leprosy, which include skin and nerve lesions (6). Leprosy ranges from lepromatous leprosy (i.e., LL), charac- terized by high bacterial load, to tuberculoid leprosy (i.e., TT), in which the cell-mediated immunity is responsible for bacterial suppression and extensive tissue damage. Borderline tubercu- loid (i.e., BT), midborderline (i.e., BB), and borderline lepro- matous (i.e., BL) are transitional forms of the disease between the lepromatous and tuberculoid poles (6). The locally increased NO production is associated with transitional leprosy forms (7 ). Expression of the inducible NO synthase (iNOS) and NO syn- thesis are increased in macrophage-rich granulomas in leprosy 95