Characterization of the pulmonary immune response induced by a highly protective tuberculosis vaccine using latent and acute infection mouse models

Abstract

We utilized a mouse tuberculosis (TB) latency model to evaluate multiple candidate TB vaccines for their ability to prevent reactivation of a latent infection. Among the most promising vaccine regimens tested was BCG formulated in adjuvant (Adj) (dimethyl dioctadecyl-ammonium bromide (DDA) plus D-(+)-Trehalose 6,6-Dibehenate (TDB)) with rEsat-6 delivered subcutaneously (SC) followed by an intranasal (IN) administration of an adenovirus construct expressing a fusion of Esat-6 (E6) and antigen-85B (Ag85B) (AdE6-85B) before an aerosol M. tuberculosis challenge. We designated this vaccine regimen as BAA. BAA consistently prevented reactivation of 75 - 100% of immunized animals and was also highly and significantly protective against an acute aerosol infection with a consistent 2 - 3 log10 mycobacterial CFU reduction in the lungs relative to nonimmunized mice (Naive). Likewise, the BAA vaccine was significantly more protective than BCG or BCG+Adj controls. Interestingly, we found that pre-challenge frequencies of CD4+ tissue resident memory (TRM) T cells (CD69+PD-1+CXCR3+), and CD4+ populations bearing CD153 and P2X7R, which are markers for protection, were significantly elevated in the lungs of mice immunized with the BAA vaccine relative to control groups. Additionally, we found significantly higher frequencies of multifunctional CD4+ T cells from infected lungs expressing both IL-17A and TGF{beta} or IL-17A, TGF{beta} and IFN-{gamma} than in control groups. These findings suggest that vaccine regimens that establish a population of CD4+ TRM cells in the lungs prior to infection and populations of multifunctional CD4+ T cells after infection may help control an acute pulmonary infection and prevent progression to active disease. IMPORTANCETo help curtail the TB epidemic, a new vaccine should prevent progression from a latent infection to active disease. Correlates of protective immunity, however, are presently unclear, which impedes the development of an improved TB vaccine. Hence, we tested different vaccines for their ability to prevent reactivation using a mouse latency model and identified a highly efficacious formulation using this model and, also, after testing using an acute aerosol infection model. We then examined the pulmonary immune responses induced by this vaccine both before and after an aerosol challenge and identified immune markers as well as populations of multifunctional and tissue resident memory T cells that may serve as correlates of vaccine efficacy against progression to active TB disease and control of an acute infection.