Immunogenicity and protective efficacy of a fusion protein vaccine consisting of antigen Ag85B and HspX against Mycobacterium tuberculosis infection in mice

Subunit vaccines have the potential advantage to boost Mycobacterium bovis Bacillus Calmette‐Guérin (BCG)‐primed immunity in adults. However, most candidates are antigens highly expressed in replicating bacilli but not in dormant or persisting bacilli, which exist during Mycobacterium tuberculosis infection. We constructed M. tuberculosis fusion protein Ag85B‐Mpt64_190–198‐HspX (AMH) and Ag85B‐Mpt64_190–198‐Mtb8.4 (AMM), which consist of Ag85B, the 190–198 peptide of Mpt64, HspX (Rv2031c) and Mtb8.4 (Rv1174c), respectively. AMH and/or AMM were mixed with adjuvants composed of dimethyl‐dioctyldecyl ammonium bromide and BCG polysaccharide nucleic acid (DDA‐BCG PSN) to construct subunit vaccines. Mice were immunized thrice with Ag85B, AMH and AMM vaccines and the immunogenicity of the fusion protein vaccines was determined. Then, mice were primed with BCG and boosted twice with Ag85B, AMH, AMM and AMM + AMH vaccines, respectively, followed by challenging with M. tuberculosis virulent strain H37Rv, and the immune responses and protective effects were measured. It was found that mice immunized with AMH vaccine generated high levels of antigen‐specific cell‐mediated responses. Compared with the group injected only with BCG, the mice boosted with AMM, AMH and AMM + AMH produced higher levels of Ag85B‐specific IgG1 and IgG2a and IFN‐γ‐secreting T cells upon Ag85B and Mycobacterium tuberculosis purified protein derivative (PPD) stimulation. It is interesting that only mice boosted with AMM + AMH had significantly lower bacterial count in the lungs than those receiving BCG, whereas mice boosted with AMH or AMM did not. The results suggest that AMH consisting of HspX, the antigen highly expressed in dormant bacilli, could be combined with antigens from replicating bacilli to enhance BCG primed immunity so as to provide better protection against both growing and non‐growing bacteria that occur during the infection process.