Pre-exposure to Aerosolized Polyvalent Bacterial Lysates Protects Against Bleomycin-Induced Pulmonary Fibrosis in Mice

Idiopathic pulmonary fibrosis (IPF) is a chronic fibrosing interstitial pneumonia of unknown cause. No therapeutic modalities can reverse or stop its ever-deteriorating course. The stimulation of lung innate immunity using bacterial lysates was found to protect against lethal pulmonary infection. Hence, this study aimed to explore whether the immune-stimulating enhancement by pretreatment with bacterial lysates led to protection against bleomycin-induced pulmonary fibrosis. C57BL/6 mice were randomly divided into 4 groups with 20 mice in each group. The mice were exposed to an aerosolized mixture of polyvalent bacterial lysates (PVBL) or phosphate-buffered saline (PBS) three times on separate days. Twenty-four hours after the last exposure, the lungs were intratracheally infused with bleomycin (BLM) or normal saline (NS). The pulmonary morphology, Ashcroft’s scale of pulmonary fibrosis, and levels of pro-inflammatory cytokines such as interferon (IFN)-γ and interleukin (IL)-4 were evaluated 14 days after the intratracheal infusion. The exposure to PVBL did not induce any discernible structural abnormalities in the lungs, while the IFN-γ/IL-4 ratio increased. BLM-induced pulmonary fibrosis was associated with an overwhelming downregulation of IFN-γ and IL-4 expression. Pre-exposure to PVBL protected against BLM-induced pulmonary fibrosis, which was demonstrated by a greater reduction of Ashcroft’s fibrotic score and a greater decrease in the hydroxyproline level in the lungs. Although the PVBL pre-exposure did not restore the BLM-induced downregulation of IL-4 and IFN-γ levels, the IFN-γ/IL-4 ratio was still maintained greater than the animals with BLM infusion. BLM-induced murine pulmonary fibrosis is associated with downregulation of IFN-γ and IL-4 levels. Pre-exposure to the aerosolized PVBL protects against BLM-induced pulmonary fibrosis.