New approaches consisting of 'multistage'
vaccines against (TB) are emerging that combine early antigenic
proteins with latency-associated
antigens. In this study, HspX was tested for its potential to elicit both short- and long-term protective immune responses. HspX is a logical component in
vaccine strategies targeting protective immune responses against primary
infection, as well as against reactivation of
latent infection, because as previously shown, it is produced during latency, and as our studies show, it elicits protection within 30 days of
infection. Recent studies have shown that the current TB
vaccine, bacilli Calmette-Guerin (BCG), does not induce strong
interferon-γ T-cell responses to latency-associated
antigens like HspX, which may be in part why BCG fails to protect against reactivation disease. We therefore tested HspX
protein alone as a prophylactic
vaccine and as a boost to BCG vaccination, and found that HspX purified from M.
tuberculosis cell lysates protected mice against
aerosol challenge and improved the protective efficacy of BCG when used as a booster
vaccine. Native HspX was highly immunogenic and protective, in a dose-dependent manner, in both short- and
long-term infection models. Based on these promising findings, HspX was produced as a
recombinant protein in E. coli, as this would enable facile purification; however, recombinant HspX (rHspX) alone consistently failed to protect against
aerosol challenge. Incubation of rHspX with mycobacterial cell lysate and re-purification following incubation restored the capacity of the
protein to confer protection. These data suggest the possibility that the native form may chaperone an immunogenic and protective
antigen that is mycobacteria-specific.