Modeling mycoplasma infections: insights from Air-Liquid Interface cultures of primary bronchial epithelial cells

Although Mycoplasma (M.) bovis is widely recognized as an emerging bacterial pathogen associated with bovine respiratory disease, the pathophysiological processes underlying M. bovis infections remain poorly understood. Another mycoplasma, Mycoplasma (M.) bovirhinis is frequently found in association with M. bovis and is suspected to contribute to the overall clinical signs, despite being considered a commensal or opportunistic organism. The present study aims to compare the interactions of M. bovis and M. bovirhinis with different pulmonary cell types. Both the embryonic bovine lung (EBL) immortalized cell line, which is easy to grow and manipulate, and primary lung epithelial cells cultured at an air-liquid interface (ALI), which better mimic in vivo tissues, were used. We confirmed the ability of M. bovis to invade epithelial cells, including the EBL cell line - as previously demonstrated- as well as type II alveolar cells and ciliated cells in ALI-cultured primary lung cells. In contrast, under our experimental conditions, M. bovirhinis efficiently invaded EBL cells but did not invade alveolar or ciliated cells in ALI-cultured primary lung cells. Both mycoplasmas induced a strong inflammatory response of epithelial cells within our ALI model, which lacks professional immune cells. However, differences were observed in the dynamics of expression of genes encoding receptors involved in pathogen detection, chemokines, cytokines and their receptors as well as antimicrobial defense effectors. The distinct patterns of cell tropism and invasion capacity observed between the two species correspond well with their differential pathogenicity in vivo and may be attributable to differences in their cell-adhesion properties.