Therapeutic Administration of Mycobacterium bovis BCG Killed by Extended Freeze-Drying Modulates Airway Inflammation in a Chronic Murine Model of Asthma

Background: We previously showed that treatment with Mycobacterium bovis BCG killed by extended freeze-drying (EFD BCG) modulates inflammation through regulatory T cells (Tregs) in an acute asthma model. In this study, we investigated the kinetics of Treg induction as well as their long-term homing in spleen and lungs correlating with reduced airway hyperresponsiveness (AHR) in a murine model of acute allergic asthma. We then evaluated the therapeutic implication of EFD BCG in a chronic asthma model. Methods: Tregs expressing Foxp3 were analyzed in various organs shortly and long-term after EFD BCG, live- and Heat Killed-(HK-) BCG treatments in an acute model of asthma. We further studied EFD BCG treatment on airway inflammation using a chronic model of asthma in mice. Results: Foxp3 expression peaked in the inguinal draining lymph-nodes (iDLNs) 2-4 days after EFD BCG treatment whereas it was long-term observed in spleen (days 7 to 90). This increase in Foxp3 expression was also found in lungs upon intranasal ovalbumin (OVA) challenge in OVA-sensitized mice. The loss of protection 4 months after EFD BCG treatment was correlated with the end of this phenomenon. Moreover, major lung inflammation hallmarks of severe asthma after multiple allergen challenges promoting chronic airway inflammation in OVA sensitized mice were reduced by EFD BCG treatment: AHR, eosinophils and neutrophils in bronchoalveolar lavage (BAL), mucus metaplasia, Th2 as well as Th17 cytokine levels in BAL and sera. EFD BCG treatment also enhances PPAR-γ expression and regulates NF-κBp65 translocation in lung extracts in this model of chronic asthma. Conclusions: EFD BCG treatment induced long-term protective effect associated to Foxp3 Tregs in the spleen and lungs in an acute model of asthma and inhibits AHR in a chronic model of asthma. EFD BCG could be a new and promising immuno-modulatory alternative treatment to corticoids in severe human asthma.