Abstract
Mycobacterium tuberculosis (Mtb), is estimated to infect a quarter of the world’s population and causes over one million deaths annually. Tuberculosis (TB) is a contagious airborne disease that primarily affects the lungs and persists as the leading cause of death by a single pathogen worldwide. Bacille Calmette-Guérin (BCG) was initially developed in 1921 and is a live attenuated vaccine containing a weakened strain of Mycobacterium bovis given via the intradermal route. At over 100 years old and our only current licensed vaccine against TB, BCG has highly variable efficacy ranging from 0-80%. BCG is known to induce trained innate immunity, whereby cells of the innate immune system, such as macrophages and neutrophils, increase their ability to respond to a secondary infection through epigenetic remodeling. Upon inhalation, Mtb traverses the respiratory tract, passing the airway epithelium, and is deposited into the distal lung where it is faced with the alveolar epithelium. The role of the lung epithelium – a key innate immune barrier – in the protective response to Mtb infection is unknown, and it is also unknown if the lung epithelium can be “trained” in response to mucosal BCG vaccination.
This study aimed to characterise lung epithelial cell subsets and immune activation following intranasal BCG vaccination using a murine model. It was hypothesised that changes would be observed in lung epithelial cells at 1 week and sustained changes at 3-weeks post BCG- vaccination compared to controls. A multi-parameter spectral flow cytometry panel was designed to detect and characterise epithelial cells, and methodology for preparing single cell suspensions of epithelial cells from the lungs of vaccinated C57BL/6 mice was established. At 1 and 3-weeks post-vaccination, epithelial populations were identified and their ability to produce cytokines quantified. Only modest, non-significant differences in epithelial cell numbers and cytokine production were detectable between BCG and PBS controls. However, analysis of a previously acquired spatial transcriptomics data set, from intranasally BCG-vaccinated C57BL/6 mice that had been challenged with virulent Mtb, identified significant changes in lung epithelial cell markers and differential gene expression of cytokines, chemokines, antimicrobial peptides, and stress response genes in BCG-vaccinated Mtb challenged mice compared to unvaccinated Mtb challenged controls.
These findings suggest that modest epithelial changes occur following BCG-vaccination. If and how these changes influence local immune responses during subsequent Mtb infection remains to be determined. Further study into epithelial-immune crosstalk and epigenetic remodelling may uncover new targets to enhance protective immunity in the lung.