Figure 1: An integrated approach towards prevention & cure of Mtb. We combine the study of host-pathogen interactions with genetic and biochemical analyses to discover novel biology and therapeutic directions.
Discovering the routes of transmission, survival and escape of Mtb from its human host
Three key phases in a pathogen’s life cycle dictate its ability to cause disease, namely, i) invasion ii) survival and propagation and iii) escape beyond the host to infect naïve individuals. To date, how Mtb crosses the mucosa and enters the human body is incompletely understood. Likewise, the full repertoire of mechanisms used by Mtb to manipulate and persist within host macrophages is unknown. Additionally, our knowledge of macrophage antimicrobial mechanisms in host defense against Mtb and other pathogens remains incomplete. Enhancing such antimicrobial mechanisms via host-directed therapies is a promising new approach to Mtb treatment. Finally, how Mtb facilitates its own transmission through cough induction has not been studied. Thus, we are addressing these areas of Mtb biology through a series of hypothesis-driven approaches. First, how does Mtb penetrate the nasopharyngeal and respiratory mucosa to cause disease? Second, what are the mechanisms Mtb uses to manipulate host processes to facilitate survival? Third, can host antimicrobial pathways be leveraged to enhance the eradication of intracellular bacteria such as Mtb? Finally, how does Mtb trigger coughing to mediate its spread?
Figure 2: Schematic of the initial entry, survival and propagation of Mtb in macrophages. Aerosolized Mtb interact with MALT (NALT, tonsils, adenoids, and BALT) where they contact and are translocated by microfold cells (purple, upper panel). Macrophages either in alveoli or in the submucosa below M-cells ingest Mtb, and Mtb secretes membrane binding virulence factors or small molecules to modulate the host immune response (lower panel).
How does Mtb cross the airway mucosa?
To address the question of how Mtb crosses the airway mucosa to establish infection, we are testing the hypothesis that a rare epithelial cell called a microfold or M-cell mediates Mtb transmission (Fig. 2).
In the oropharynx and respiratory tree, Mtb encounters mucosa associated lymphoid tissue (MALT) such as nasal associated lymphatic tissue (NALT), tonsils, and adenoids. Specialized M-cells overlying MALT transcytose antigens from the apical to the basolateral compartment. Whether Mtb leverage M-cells to initiate infection is unknown. We recently established a mouse intranasal infection model and found that cervical lymph node dissemination by Mtb occurred without contacting alveolar macrophages. Mice genetically lacking M-cells or depleted of M-cells had reduced Mtb invasion and dissemination to draining lymph nodes. M-cell depletion delayed mouse mortality in a low-dose aerosol infection, indicating a vital role for M-cells in respiratory Mtb disease.
Our ongoing experiments are aimed at identifying how Mtb adheres and traffics through M-cells, and the consequences of M-cell transcytosis for innate and adaptive immunity. We have preliminary data that a cell surface receptor on M-cells mediates transcytosis, and are characterizing this interaction with Mtb in vitro and in vivo. We are also determining how M-cell transcytosis influences the subsequent interaction of Mtb with macrophages and dendritic cells. Finally, since M-cells overlie human tonsils, we are establishing an M-cell translocation assay in human tonsillar explants to assess the role of M-cells, Mtb receptors, and transcytosis in primary human tissue. The identified mechanisms will fundamentally advance the fields of microbiology and host defense, and provide new approaches towards reducing Mtb infectivity.