Introduction
Mycobacterium tuberculosis (
Mtb) causes persistent infection known as tuberculosis (TB). This disease is one of the main health problems in the world due to 2 million morbidity and mortality annually (
Yuan et al., 2012). Nowadays, TB vaccination is one of the best manners to prevent and control this infectious disease (
Junqueira-Kipnis et al., 2014;
Niu et al., 2015). The only approved vaccine against TB is BCG vaccine, which is a live attenuated strain of
Mycobacterium bovis (BacilleCalmette–Guerin). BCG vaccine is able to protect newborns and children against TB, but its protection against adult pulmonary TB ranging from 0% to 80% (
Xin et al., 2013). The inefficiency of BCG vaccine is attributed to some limitations including: (i) inability to eliminate the latent TB, (ii) decreased protectivity during the time, (iii) inability to protect adults against pulmonary TB, and (iv) problems in patients with immune deficiency (
Britton and Palendira, 2003,
Andersen and Kaufmann, 2014,
Soleimanpour et al., 2015). About one third of the global population has latent TB infection. They are the major reservoir of adult TB infection (
Yuan et al., 2015). Therefore, new vaccines to control the TB disease in adults, especially in latent infection, are needed. In the same time the increasing cases of multidrug resistant (MDR) and the extensively drug resistant (TDR) TB have increased the difficulties in elimination of disease (
Xin et al., 2013). These are progresses in order to improve or replace the current vaccine (
Yuan et al., 2012). The effective vaccination strategies against TB are recombinant BCG, attenuated
M. tuberculosis, DNA vaccines and protein subunit vaccines (
Mir et al., 2009). The protein based subunit vaccines are effective vaccine candidates have potentials in long-term protection against the disease (
Niu et al., 2015). To design a good protein subunit vaccine, various immunodominant antigens of
M. tuberculosis has been tested. The present review has focused on a dormancy-related antigen of
M. tuberculosis as an ideal candidate in the development of a new vaccine against TB.
Heat-shock protein X antigen (HspX antigen)
HspX is
a-crystalline protein or a heat shock protein of
M. tuberculosis which also called Hsp16.3 (molecular weight; 16.3 kDa), and Acr (HspX encoding gene, Rv2031c). HspX is produced during the lag phase, and also slightly during the active phase, or under stress conditions such as hypoxia, nutrient scarcity, and the presence of nitric oxide (
Shi et al., 2010;
Niu et al., 2011;
Yuan et al., 2012;
O'Garra et al., 2013;
Mosavat et al., 2016). Under these conditions its expression could be reached to 25% of the total bacterial protein expression (
Geluk et al., 2007). HspX expression co-regulated with DosR regulon (48 genes) in different environmental conditions (
Aagaard et al., 2009). This protein is involved in the long term survival of bacteria in macrophages and it escape from the host innate immune system by increasing the stability of the proteins and cellular structures during the latent phase of infection (
Shi et al., 2010;
Taylor et al., 2012;
Jung et al., 2014;
Wang et al., 2015). HspX also causes slow-growing of bacteria (
Marongiu et al., 2013). The most important immune response against
M. tuberculosis infection is activation of cell-mediated immunity (Th1 and Tc) (
O'Garra et al., 2013). HspX is an important dormancy antigen which could be effectively recognized by both human CD4
+ and CD8
+ T cells and strongly induces Th1 cytokines such as IFN-
g and TNF-
a. It can induce strong cellular and humoral immune responses in latent phase of tuberculosis infection (LTBI) (
Lin et al., 2007;
Li et al., 2011;
de Sousa et al., 2012;
Soleimanpour et al., 2015).
Role of Hsp X in tuberculosis vaccines
To improve TB vaccine, many studies have performed in pre-clinical and clinical phases. They showed immunogenicity and efficacy of HspX protein for induction of strong Th1 immune responses and production of high levels of Th1 cytokines, especially in latent TB (
Shi et al., 2010;
Yuan et al., 2012;
Silva et al., 2014). Therefore, HspX is a good antigen candidate for vaccination against TB (
Jeon et al., 2011). In this article several studies performed on the immunogenicity and efficacy of HspX protein, have been reviewed.
In a study conducted by Chunwei et al.,, a recombinant BCG strain (rBCG) was constructed which expressed high levels of HspX protein. After vaccination of mice with this vaccine, a significant increase in antigen specific IFN-
g against HspX antigen was observed. Their findings suggest that HspX protein may be promising target for developing rBCG (
Shi et al., 2010).
Yuan et al., were constructed a plasmid DNA vaccine expressing a fusion protein of
M. tuberculosis antigens including Ag85B, Esat6 and HspX. They were assessed the immunogenicity of this vaccine in a mice model. Vaccination with this DNA vaccine elicited high levels of antigen specific IFN-
g. They were also observed higher levels of HspX specific T cell proliferation, as compared with vaccination with BCG (
Yuan et al., 2012).
Similar study to Yuan et al., a new recombinant BCG by overexpressing immunodominant multistage antigens of Ag85B and HspX were constructed. After vaccination of C57BL/6 mice with vaccine, immunogenicity of this new vaccine compared with the immunogenicity of BCG. The results showed that new recombinant BCG had better protection against intranasal infection of
M. tuberculosis than BCG (
Yuan et al., 2015).
Hongxia et al., were used a multistage fusion protein of Mtb10.4 and HspX and evaluated its immunogenicity in vivo. This protein was strongly recognized by human T cells and generated strong humoral and cell-mediated immunity. Their study showed that Mtb10.4–HspX fusion protein vaccine induced strong antigen specific immune response in both humans and mice (
Niu et al., 2011).
Trentini et al., were demonstrated significant differences between the mice immunized with HspX and non-vaccinated animals. HspX-containing vaccines were immunogenic. Mice immunized with CpG DNA and HspX showed strong humoral and cell-mediated (mainly TNF-
a and IFN-
g) responses (
Trentini et al., 2014).
In a study performed by Jeon et al.,, the immune responses against Ag85A and HspX antigens were evaluated in mice model. This recombinant protein (Ag85A and HspX of
M. tuberculosis) was used as a vaccine candidate. After 30 days from the last immunization, mice immunized with this vaccine were challenged with
M. tuberculosis. Ag85A and HspX antigens induced high levels of IFN-
g responses. At day 30 after the challenge, the protective subunit vaccine against tuberculosis was Ag85A, but HspX subunit vaccine induced significant protective effect after 90 days from the challenge with
M. tuberculosis (
Jeon, Kim et al., 2011).
Liang et al. (
2015) produced a new recombinant BCG strain which overexpressed Ag85A, Ag85B, and HspX antigens of
M. tuberculosis. They were evaluated immunogenicity of this vaccine in C57BL/6 mice. Their results showed that recombinant BCG was induced strong Th1 immune responses in the infected mice and increased the IFN-
g and IL-12 levels. In addition, this recombinant BCG was able to induce stronger immune protection against primary TB infection in C57BL/6 mice.
In the study of Geluk et al.,, levels of IFN-
g responses to HspX antigen in
M. tuberculosis-exposed individuals were significantly higher than the
M. tuberculosis-unexposed BCG vaccines. Their result revealed that vaccination with BCG cannot induce T cell responses against the latency antigens such as HspX (
Geluk et al., 2007).
Niu et al., reported a new subunit vaccine against TB. This multicomponent and multistage vaccine contained ESAT6-Ag85B-MPT64-Mtb8.4-HspX antigens along with two adjuvants,N, N’-dimethyl-N, N’-dioctadecylammonium bromide and polyriboinosinicpolyribocytidylic acid. After challenge of immunized mice with
M. tuberculosis aerosol, this formulation could induce strong immunogenicity and high protective efficacy, even higher than traditional BCG vaccine (
Niu, Peng et al., 2015).
As discussed in the study by Li et al. (
2011), HspX antigen when combined with the replicating bacilli antigens, as multistage subunit vaccine, can enhance BCG primed immunity and also provides a better protection against both growing and non-growing bacteria.
The immunogenicity profile of a recombinant
M. tuberculosis Ag85C-MPT51-HspX fusion protein has been investigated in mice and human by de Sousa et al., They showed the immunogenicity of fusion protein both in mice and humans (
de Sousa et al., 2012).
In another study by Xin et al.,, Mtb10.4-HspX could improve BCG-primed protective efficacy against
M. tuberculosis infection in mice (
Xin et al., 2013). Marongiu et al., indicated that ESAT-6 and HspX together enable BCG-treated human DCs to elicit T and NK cells immune responses. Therefore, ESAT-6 and HspX may serve as promising candidates for improving the effectiveness of the BCG vaccine as a booster vaccine (
Marongiu et al., 2013).
HspX antigen of
M. tuberculosis could protect the immunized mice against aerosol challenge. Administration of this antigen as a booster for BCG vaccine could increase the protective efficacy (
Taylor et al., 2012). The immunogenicity of a recombinant
M. tuberculosis vaccine consisting of HspX and four other multistage antigens was evaluated in C57BL/6 mice. This novel vaccine could provide significantly better immunogenicity against
M. tuberculosis infection than the control groups (
Wang, Zhang et al., 2015). A new recombinant BCG vaccine expressing Ag85C, MPT51, and HspX of
M. tuberculosis was constructed by Costa et al., Immunogenicity of this novel vaccine was tested in mice model and demonstrated that specific immune response to vaccine was induced in the lungs and spleen cells. After vaccination with this new vaccine, the bacterial load of the lung was lower than the bacterial load of the lung after vaccination with BCG (da Costa et al., 2014).
Conclusion
According to the studies reviewed in this article, the immunogenicity of HspX antigen against TB infection could be confirmed. In all studies, HspX antigen could stimulate strong immune responses against latent TB. Therefore, it may act as an appropriate candidate antigen for immunization against latent phase of infection. Also, HspX antigen was able to prevent reactivation of latent TB infection. Therefore, it could be recommended as a prophylactic vaccine. Finally, it was observed that the association of HspX antigen with immunogenic antigens of early phase and preparation of multistage subunit vaccine could induce strong immune responses against different stages of M. tuberculosis infection. Such these multistage antigens could be used as effective candidates for protection against TB.
Higher Education Press and Springer-Verlag GmbH Germany, part of Springer Nature
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