Immunity Against Mycobacterium avium Induced by DAR-901 and BCG

<b>Background:</b> The prevalence of pulmonary nontuberculous mycobacteria (NTM) is increasing in Europe and North America. Most pulmonary NTM cases are caused by <i>Mycobacterium avium</i> complex (MAC). The treatment of pulmonary MAC is suboptimal with failure rates ranging from 30% to 40% and there is a need to develop new vaccines. <b>Methods</b>: We tested the ability of two whole-cell vaccines, DAR-901 (heat-killed <i>M. obuense</i>) and BCG (live-attenuated <i>M. bovis</i>), to induce MAC cross-reactive immunity by first immunizing BALB/c mice and then performing IFN-γ ELISPOT assays after overnight stimulation of splenocytes with live MAC. To study the ability of these vaccines to protect against MAC infection, BALB/c mice were vaccinated with DAR-901 (intradermal) or BCG (subcutaneous or intranasal) and challenged with aerosolized MAC 4 weeks later. A group of mice vaccinated with BCG were also treated with clarithromycin via gavage. Lung colony-forming units (CFU) in immunized mice and unvaccinated controls were quantified 4 weeks after infection. Histopathology was used to quantify lung inflammation and flow cytometry was used to study lung immunity in BCG-vaccinated and unvaccinated mice following MAC infection. To increase the safety profile of mucosal BCG vaccination, we studied BCG with a "kill switch" (tetR BCG) in <i>scnn1b</i>-transgenic mice (i.e., mice prone to cystic fibrosis-type lung diseases). <b>Results</b>: Our results showed that (i) DAR-901 induced cross-reactive immunity to MAC to a similar level as BCG, (ii) DAR-901 and BCG protected against aerosol MAC challenge, (iii) mucosal BCG vaccination, compared to systemic BCG and DAR-901 vaccinations, provided the best protection against MAC challenge, (iv) BCG vaccination did not interfere with anti-MAC activities of clarithromycin, (v) BCG-vaccinated mice had increased inflammation and increased frequencies of activated CD4 and CD8 T cells following MAC infection, and (vi) doxycycline treatment of tetR BCG-vaccinated mice decreased lung BCG CFU without affecting MAC immunity. <b>Conclusions</b>: Both DAR-901 and BCG vaccinations induce MAC cross-reactive immunity and protect against aerosolized MAC challenges. Mucosal BCG vaccination provides the best protection and TetR BCG could enhance the safety of mucosal BCG vaccination.