Protective immunity against tuberculosis: what does it look like and how do we find it?

Abstract

Progress towards the development of an effective vaccine against tuberculosis is hampered by the lack of correlative readouts of immune protection, coupled with our limited understanding of the immune mechanisms that determine disease progression versus containment. In this article we discuss the value of microbial readouts of bacterial fitness to probe the host immune environments and determine those host cell subsets that promote or control bacterial growth. Ultimately, we feel that these bacterial reporters will prove to be key in understanding the immune mechanisms underpinning disease outcome, and that this knowledge is critical to any program developing vaccines or immune-modulatory therapeutics as a means of controlling tuberculosis.

Figure 1

A schematic illustration of the potential outcomes of infection with Mtb. In most hosts Mtb exhibits rapid expansion of the bacterial burden over the first 3–4 weeks of infection. At this point the acquired immune response has developed and controls the bacterial burden at a subclinical level but is unable to clear the infection. In vaccinated hosts this transition to control of the bacterial burden is achieved at around a log fewer bacilli. While resolution of infection is possible theoretically most TB researchers do not believe it happens. Progression from latent disease to active disease appears to occur in the face of a robust systemic immune response that is Th1 dominant. While we have early indicators of disease progression, we have no immunological markers to detect vaccine-induced protection.

Figure 2

Demonstrating the application of the hspX’::GFP reporter strain in assessing and reporting on the localized induction of iNOS at the site of infection. PBS-immunized (naïve) and mice vaccinated with heat-killed Mycobacterium tuberculosis (vac) were infected with hspX’::GFP, smyc’::mCherry Erdman M. tuberculosis reporter strain. Fluorescence induction of the hspX promoter-dependent GFP is higher at 14 days in the vaccinated animals assessed by confocal microscopy of thick tissue sections (a), that were scored subsequently by Volocity (b). (c) The thick tissue sections were probed with antibodies against murine NOS2 (magenta) demonstrating the co-localization between GFP induction and NOS2 expression at the site(s) of infection. Data shown are detailed in Sukumar et al. [40].

Figure 3

Examination of the bacterial stress reporter strain hspX’::GFP, smyc’::mCherry Erdman Mycobacterium tuberculosis at the level of different host phagocyte populations in murine lung infection model. (a) The flow cytometry gating strategy for the identification of M. tuberculosis-infected phagocyte subsets from infected mouse lung showing preliminary identification of alveolar macrophages, interstitial macrophages, and neutrophils. (b) The level of expression of NO-driven GFP under regulation of the hspX promoter identifying those phagocytes that induce the highest level of bacterial stress, and how the stress intensifies from 14 to 28 days post-infection. The levels of induction of expression of GFP indicate that the most stressful host cells appear to be neutrophils, and the least stressful, alveolar macrophages. (c) Labeling of the host cells with antibody against NOS2 demonstrates the direct correlation between expression levels of the host nitric oxide synthase, NOS2, and levels of expression of the bacterial stress response reporter hspX’::GFP (B).