Abstract
Mycobacterium tuberculosis (Mtb), the causative agent of the primarily pulmonary disease tuberculosis (TB), is the world’s leading single cause infectious killer. Live, attenuated bacille Calmette-Guérin (BCG) remains the only approved TB vaccine a century after its initial development in 1921. While BCG protects children and neonates against both pulmonary and disseminated TB and has greatly reduced the global burden of child and infant mortality, its protection in adulthood is limited (~50-60%). Long term follow-up of vaccinated individuals revealed reduced efficacy of BCG against TB in males compared to females.
Sex is known to influence the efficacy of several vaccines, yet the mechanisms that shape sex-driven changes in the immune response to BCG vaccination remain unclear. BCG vaccination induces trained innate immunity that enhances effector functions of innate cells and primes them for future encounters with pathogens. Given that alveolar macrophages (AMs) are the primary cellular host for intracellular Mtb, this project aimed to determine whether there were sex-differences in the lung macrophage response to the BCG vaccine. It was hypothesised that that the phagocytic function and anti-mycobacterial efficacy of AMs in the lung after BCG vaccination would be decreased in male mice compared to female mice. To test this hypothesis, C57BL/6 mice of both sexes were intranasally (IN) vaccinated with BCG, and were either challenged in vivo to model early infection using intranasal fluorescent BCG; or lung phagocytic cells were isolated and challenged in vitro with fluorescent, auxotrophic Mtb. Prior to in vitro challenge with Mtb, AM populations were enriched using magnetic beads. The uptake of fluorescent mycobacteria by phagocytic cells in vitro or in vivo was measured using flow cytometry, and mycobacterial killing in vitro was measured by plating cell culture lysates on agar and counting Mtb colony-forming units.
To determine whether there were any detectable sex-related differences in the blood of BCG-vaccinated mice after Lineage 2 or Lineage 4 Mtb challenge, a previously acquired RNA-seq data set was interrogated. This data set had been established from the peripheral blood of intravenously (IV) BCG-vaccinated male and female mice that had been challenged with Mtb, with naïve, Mtb challenged mice serving as controls.
Contrary to the hypothesis, no significant differences were observed in the phagocytic response or anti-mycobacterial activity between AMs from male and female BCG-vaccinated mice. RNA-seq analysis further revealed minimal sex-dependent gene expression. Taken together, these findings suggest that AMs and changes to gene expression in peripheral blood are unlikely to underpin the sex-dependent outcomes of BCG vaccination and TB infection.