The PstS1 antigen is highly immunogenic, principally when combined with CFP10 during both latent and active TB infection. In the present study, a selected pstS1 gene fragment was cloned, fused with CFP10, and expressed in Escherichia coli. The product M ycobacterium tuberculosis, a highly successful parasite, is the cause of tuberculosis (TB). A major health care concern, the disease results in approximately 1.4 million annual deaths (990,000 among HIV-negative individuals), infecting roughly one-third of the world's population. Among the infected, 5% to 10% will develop active disease in their lifetimes and 90% will harbor the latent form. According to a recent mathematical projection of TB eradication, the treatment of latent TB infection (LTBI) and active TB is urgently required in order to lower and ultimately prevent the further spread of the disease at its present rate (1, 2).Two principal approaches based on the adaptive immune responses elicited by M. tuberculosis infection are currently used to identify TBI (3): the in vivo tuberculin skin test (TST) and the ex vivo gamma interferon (IFN-␥) release assay (IGRA).Developed in 1908, TST became the standard means of assessing the presence of TB infection. Via TST, prior TB exposure is measured by a type 4 delayed-type hypersensitivity reaction when a purified protein derivative (PPD) of M. tuberculosis is injected intradermally. Although TST has some biological limitations such as the occurrence of anergies in one-third of active TB cases, crossreactivity with M. bovis bacillus Calmette-Guérin (BCG) and non-TB mycobacteria does not, according to some authors, result in major sensibility differences in IGRAs (4).The IGRA was recently developed using antigens that are expressed in M. tuberculosis but not in BCG or M. bovis strains as stimuli. These antigens measure the production of IFN-␥ in peripheral blood mononuclear cells (3,5). According to a recent World Health Organization (WHO) report (1), the two currently commercially available IGRAs have yet to generate sufficient data or enough high-quality evidence regarding their performance in the low-and middle-income countries that typically have a high TB and/or HIV burden and where the coverage of BCG vaccination may cause some interference (5, 6).Both IGRAs are based on Mycobacterium tuberculosis-specific antigens, namely, early secretory antigenic target 6 (ESAT-6) and culture filtrate protein 10 (CFP10) (5). These antigens must be validated in different scenarios, but other antigen combinations also need to be evaluated to improve IGRA coverage.The 38-kDa M. tuberculosis antigen, also known as phosphatespecific transporter-1 (PstS-1), coded by a pstS1 gene that composes one of the 3 putative pst operons, is a lipoprotein phosphate transport receptor on the cell surface. These operons probably constitute a subtle biochemical adaptation of M. tuberculosis, enabling it to grow and survive under different phosphate-limiting conditions during its infectious cycle. The PstS-1 protein is ac-
