Antigen Rv is a secretory antigen and a virulence factor

Antigen Rv0577 is a secretory antigen and a virulence factor associated with the replication of M. tuberculosis. The antigen is an agonist of Toll-like receptor 2 (TLR2) and can promote maturation of dendritic cells (Byun et al., 2012). Rv2875 encodes a cell membrane-associated antigen, MPT70, of M. tuberculosis that has strong immunogenicity (Windish et al., 2011). Rv3044 is a gene that is present in the genomes of M. tuberculosis complex, including BCG and its substrains. The gene product functions in the uptake and transport of Fe in the phagosome (Wagner et al., 2005). Rv2073c is located in the RD9 region that is unique to the genome of virulent M. tuberculosis in the genus Mycobacteria (Behr et al., 1999; Tan et al., 2015). With the exception of Rv0577, the aforementioned three genes are characteristically expressed by M. tuberculosis in hypoxic environments in vivo (Sassetti and Rubin, 2003). With the goal of developing an efficient vaccine for adult TB, these four purchase BTL105 were screened using a recombinant CFP21-MPT64-whole blood interferon-gamma (IFN-γ) release assay (rCM-WBIA) (Fu et al., 2009). The antigens were strongly recognized by T cells from LTBIs and ATBs. A multistage fusion protein subunit, CMFO (Rv2875-Rv3044-Rv2073c-Rv0577), based on these four antigens was constructed. DMT was used as the adjuvant because of its ability as a strong inducer of Th1-type immunity (Teng et al., 2015). The protective efficacy of DMT adjuvant and CMFO was compared with that of the subunits of single-stage CTT3H (CFP10-TB10.4-TB8.4-Rv3615c-HBHA) (Teng et al., 2015) and two-stage A1D4 (Rv1813-Rv2660c-Ag85B-Rv2623-HspX) (Wang et al., 2015) in immunized mice against primary infection, latent infection, and reactivation, respectively. The immunological mechanisms associated with the protection elicited by CMFO-DMT were characterized.
Materials and Methods
Results
Discussion Although BCG is used globally, the lack of effective vaccines to prevent adults against TB is a significant obstacle toward the WHO goal of global eradication of TB by 2050. The development of an efficient vaccine against adult TB is the key strategy for successful control of TB and remains a global priority. However, a rational strategy to develop an efficient vaccine for adult TB remains elusive. In the current study, the protective efficacy of DMT adjuvant subunits of multistage CMFO, two-stage A1D4, and single-stage CTT3H was compared in mice models of primary infection, latent infection, and reactivation. Our results demonstrate that only the multistage CMFO-DMT vaccine produces effective protection in BCG-primed mice to eliminate latent infection and prevent reactivation. In addition, CMFO-DMT provides protection against primary infection with M. tuberculosis that is comparable to the effect of BCG vaccine, while each multistage antigen from CMFO confers a lower level of protection against aerosol infection with M. tuberculosis when compared with that in BCG-vaccinated mice (Bertholet et al., 2008). M. tuberculosis expresses different antigens when adapting to conditions at different stages during infection, which has crucial role in escaping the pre-established immunity in the infected host (Ernst, 2012). The genome of M. tuberculosis contains 4018 open reading frames and about 450 antigens are secreted during the early acute infection (de Souza et al., 2008). Forty-eight genes controlled by DosR in the genome of M. tuberculosis were demonstrated to be expressed during hypoxia in vitro, by mimicking the microenvironment in the host during latent infection (Park et al., 2003). At least 230 genes were found to be characteristically up-regulated during hypoxia in vitro (Rustad et al., 2008). In particular, genes of M. tuberculosis persistently expressed in infected mice were also identified (Sassetti and Rubin, 2003). We selected vaccine targets from this potential antigenic reservoir, based on the high recognition frequencies of T cells from LTBIs and ATBs. CTT3H was constructed based on low molecular-weight proteins like CFP-10 and TB10.4, which are the main species secreted by M. tuberculosis and presented in the early culture supernatant in vitro (Teng et al., 2015). Currently, one-stage vaccine candidates, such as the fusion subunits Ag85B-ESAT-6 and Ag85B-TB10.4, are being evaluated in clinical trials (Kaufmann, 2014; Reither et al., 2014). Among our three subunit vaccines, CTT3H-DMT conferred the worst protection against primary infection. Our results indicate that the strategy of subunit vaccine that is based solely on the early secretory antigens of M. tuberculosis might be not very effective for the prevention of primary infection, as evidenced by the co-existence of actively growing and latent M. tuberculosis after infection. Although we previously reported that A1D4 in the adjuvant of MTO provided inferior protection against primary infection with BCG (Wang et al., 2015), DMT adjuvanted A1D4 was as efficacious in the lung as BCG and CMFO-DMT, and the results were consistent with previously reported subunit vaccines of two-stage H56 and ID93, and multistage WH121. Because the protection against primary infection induced by repeat BCG vaccination was inferior to that induced by BCG vaccination alone (Cruz et al., 2010), two- or multistage subunit vaccines might be a suitable replacement of BCG to prevent against primary infection in adults.