Date Published: October 12, 2018
Publisher: Public Library of Science
Author(s): Anthony M. Cadena, Forrest F. Hopkins, Pauline Maiello, Allison F. Carey, Eileen A. Wong, Constance J. Martin, Hannah P. Gideon, Robert M. DiFazio, Peter Andersen, Philana Ling Lin, Sarah M. Fortune, JoAnne L. Flynn, Padmini Salgame.
For many pathogens, including most targets of effective vaccines, infection elicits an immune response that confers significant protection against reinfection. There has been significant debate as to whether natural Mycobacterium tuberculosis (Mtb) infection confers protection against reinfection. Here we experimentally assessed the protection conferred by concurrent Mtb infection in macaques, a robust experimental model of human tuberculosis (TB), using a combination of serial imaging and Mtb challenge strains differentiated by DNA identifiers. Strikingly, ongoing Mtb infection provided complete protection against establishment of secondary infection in over half of the macaques and allowed near sterilizing bacterial control for those in which a secondary infection was established. By contrast, boosted BCG vaccination reduced granuloma inflammation but had no impact on early granuloma bacterial burden. These findings are evidence of highly effective concomitant mycobacterial immunity in the lung, which may inform TB vaccine design and development.
There has been significant debate as to whether natural Mtb infection confers protection against reinfection. Epidemiologic studies from the pre-antibiotic era suggest that primary Mycobacterium tuberculosis (Mtb) infection provides up to 80% protection against TB disease due to secondary exposure, although assessing protection against actual reinfection is not possible. However, up to ~20% of patients who complete drug treatment develop TB again, in part due to reinfection[2–6]. Recent studies also suggest that mixed infections (two or more Mtb isolates) are detectable in 2–18% of individuals with active TB[4, 7, 8], although some of these studies are in HIV+ individuals. In mice, ongoing or treated Mtb infections only reduce bacterial burdens in the lung by ~10-fold, roughly equivalent to BCG vaccination[9, 10]. These data are often used to inform the field’s understanding of the extent of protection that a primed immune response can provide against Mtb infection, which is a critical question for vaccine development. However, these conclusions are confounded by uncertainties regarding host immune status and prior exposure in the human studies, outcome measures of disease rather than establishment of a secondary infection, and the relevance of the small animal models to human infection.
In this study, we present evidence for robust concomitant immunity in the cynomolgus macaque model of TB, using a combination of PET CT imaging and molecular analysis of DNA-tagged strains of M. tuberculosis in a macaque model. We found complete protection against productive secondary infection in five macaques with ongoing infection (Figs 1D and 2A), almost no productive dissemination to lymph nodes (Fig 3D), and a ~10,000-fold decrease in live Mtb in library B (reinfection) granulomas compared to age-matched granulomas in naïve animals (Figs 2B, 2C and 3C). One reinfected monkey had no trace of library B DNA despite receiving a secondary challenge dose of 10 CFU. The number of new granulomas that were seen by PET CT and attributed to library B by Q-tag analysis at 4 weeks post-reinfection was significantly lower in macaques with ongoing primary (library A) infection, compared to the number of library B granulomas at 4 weeks post-infection in naïve (i.e. no primary infection) macaques. The new library B granulomas that were established in the reinfected animals were often sterile or had very low bacterial burden, in stark contrast to 4 week granulomas in naïve macaques, where the bacterial burden is generally quite high. Using CEQ analysis, we determined that the library B Mtb bacilli did not grow to the same extent in reinfected macaques as in naïve macaques, and that bacterial killing was increased. Thus, reduced new granuloma formation, reduced growth and increased killing resulted in a 10,000 fold decrease in live Mtb library B bacilli in reinfected macaques, compared to the bacterial burden of library B Mtb in naïve macaques at the same time point.