Development of Mycobacterium tuberculosis attenuated strains as live vaccine candidates for tuberculosis

Tuberculosis (TB) is an infectious disease caused mainly by the bacillus Mycobacterium tuberculosis. The World Health Organization estimates that 8.6 million new TB cases occurred in 2012, resulting in 1.3 million deaths [1]. TB is the second leading cause of death caused by an infectious disease worldwide after immunodeficiency virus (HIV). Despite the availability of an effective chemotherapy and a moderately protective Bacille-Calmette-Guérin (BCG) vaccine, TB remains a major global health problem [1]. BCG provides efficient protection against TB in newborns, but its efficacy against the establishment of latent or pulmonary TB in adults is highly variable. The variability of BCG protection in adults might be related to the absence of more than a hundred genes when compared with the Mycobacterium bovis pathogenic strain. Among the missing genes in BCG is the RD1 region which encodes potent antigens and virulence factors [2]. Thus, there is an urgent need for the emergence of new prophylactic strategies to decrease TB incidence worldwide. The development of genetic tools to manipulate mycobacteria and the completion of M. tuberculosis genome sequencing have been contributing to a better understanding of genes involved in TB virulence and pathogenesis, and consequently to the emergence of novel vaccine candidates. Two major strategies have been used to develop new vaccine candidates against TB: (i) substitution of BCG in which an improved version of BCG or a new attenuated live M. tuberculosis vaccine would have a higher efficiency than BCG and replace it as a prime vaccine; and (ii) a prime-boost strategy in which BCG continues to be given to neonates, and a new vaccine is given as a booster dose to extend the protection and efficacy [1]. Following these approaches, numerous vaccine candidates against TB are currently in preclinical and clinical trials, including recombinant BCGs, attenuated M. tuberculosis strains, recombinant viral-vectored platforms, protein/adjuvants combinations and mycobacterial extracts [1]. The strategy to develop novel vaccines based on the construction of rationally attenuated M. tuberculosis strains holds the advantage of potentially eliciting a more sustained protective immune response than viral vectored and recombinant protein candidates. The successful isolation of allelic exchange mutants of M. tuberculosis is dependent on the ability of the genetic tools to enable the efficient detection and selection of mutants among the total population of transformants [3]. Suicide and conditionally-replicating delivery plasmids that combines both selectable and counter-selectable markers, reporter gene, and a mycobacterial thermosensitive origin of replication have been widely used to demonstrate attenuation of M. tuberculosis mutant strains [3]. The first step towards the development of a new live vaccine candidate against TB is the evaluation of mutant strain attenuation followed by the investigation of its ability to generate an immune response. Live vaccine candidates have to mimic natural infection as closely as possible without causing disease, in which, a right balance between attenuation and immunogenicity has to be reached, since over attenuated bacteria may not produce in vivo some key antigens for the induction of a protective immunity [4]. An efficient strategy to rationally attenuate M. tuberculosis consists on the construction of double-deletion mutant of M. tuberculosis in the panC and panD genes, both involved in the de novo biosynthesis of pantothenate. Pantothenic acid (vitamin B5) is an essential molecule required for the synthesis of coenzyme A and acyl carrier Centro de Pesquisas em Biologia Molecular e Funcional (CPBMF), Instituto Nacional de Ciência e Tecnologia em Tuberculose (INCT-TB), PUCRS. Av. Ipiranga, 6681 TECNOPUC Prédio 92A Partenon Porto Alegre, CEP 90619-900, Brazil Villela et al. BMC Proceedings 2014, 8(Suppl 4):O5 http://www.biomedcentral.com/1753-6561/8/S4/O5