Interactions of pathogenic and commensal strains of Mannheimia haemolytica with differentiated bovine airway epithelial cells grown at an air-liquid interface

Mannheimia haemolytica serotype A2 is a common commensal species present in the nasopharynx of healthy cattle. However, prior to the onset of bovine pneumonic pasteurellosis, there is sudden increase in M. haemolytica serotype A1 within the upper respiratory tract. The events during this selective proliferation of serotype A1 strains are poorly characterised. In this investigation, a differentiated bovine airway epithelial cell culture was used to study the interactions of A1 and A2 bovine isolates with the respiratory epithelium. This model reproduced the key defences of the airway epithelium, including tight junctions and mucociliary clearance. Although initial adherence of the serotype A1 strains was low, by 12 hours post-infection the bacteria was able to traverse the tight junctions to form foci of infection below the apical surface. The size, density and number of these foci increased with time, as did the cytopathic effects observed in the bovine bronchial epithelial cells. Penetration of M. haemolytica A1 into the sub-apical epithelium was shown to be through transcytosis but not paracytosis. Commensal A2 bovine isolates however were not capable of colonising the model to a high degree, and did not penetrate the epithelium following initial adherence at the apical surface. This difference in their ability to colonise the respiratory epithelium may account for the sudden proliferation of serotype A1 in the onset of pneumonia pasteurellosis. The pathogenesis observed was replicated by virulent A2 ovine isolates; however colonisation was 10-fold lower in comparison to bovine A1 strains. This investigation provides new insight into the interactions of M. haemolytica with bovine airway epithelial cells which are occurring in vivo during pneumonia pasteurellosis.