MODULATION OF PHAGOCYTIC FUNCTION OF BOVINE MONONUCLEAR PHAGOCYTES BYHAEMOPHILUS-SOMNUS

The interactions between bovine mononuclear cells and Haemophilus somn us are known to be complex. To study this interaction, a flow cytometr ic assay was developed to assess the effect of H. somnus on phagocytos is of killed opsonized Staphylococcus aureus by bovine alveolar macrop hages and blood monocytes. Using this in vitro system, it was found th at log phase H. somnus significantly inhibited the phagocytosis of kil led opsonized S. aureus by bovine alveolar macrophages obtained both f rom healthy calves and from cattle experimentally infected with H. som nus. However, killed log-phase H. somnus, in vitro passaged and statio nary phase H. somnus had no effect on the phagocytic activity of these cells. In contrast to bovine alveolar macrophages, blood monocytes sh owed a significant increase in their phagocytic activity following in vitro exposure to either log or stationary phase H. somnus. Using a ly pophilic, non-toxic fluorophore PKH2 to label live H. somnus, it was p ossible to simultaneously measure the uptake of both S. aureus and H. somnus. Stationary and log phase H. somnus were taken up by macrophage s equally well, even though phagocytosis of S. aureus was inhibited by only log phase H. somnus. These results demonstrate the ability of H. somnus to modulate bovine mononuclear phagocytic function which might contribute towards the pathogenesis of bovine hemophilosis. (C) 1997 Academic Press Limited