Bacterium host cell interaction : effect of Haemophilus somnus on phagocytosis, nitric oxide production and chemiluminescence response of bovine mononuclear phagocytes

Abstract

The interaction between bovine mononuclear phagocytes (BMP) and Haemophilus somnus are known to be complex. Using a flow cytometric phagocytosis assay, it was found that logarithmically growing H somnus significantly inhibited the phagocytosis of opsonized S. aureus by bovine alveolar macrophages (BAM) obtained both from healthy calves and from cattle experimentally infected with H somnus. However, neither heat- nor formalin-killed, logarithmically growing neither H somnus nor in vitro passaged H somnus showed any effect on the phagocytic activity of these cells. In contrast to BAM, bovine blood monocytes (BBM), had a significant increase in their phagocytic activity following in vitro exposure to logarithmically growing H somnus. The bactericidal ability of bovine mononuclear phagocytes in interaction with H somnus was studied using two in vitro assay systems measuring nitric oxide (NO) production and chemiluminescence response. H somnus rapidly inhibited the Luminol- dependent chemiluminescence (LDCL) of BBM, and of BAM costimulated with opsonized Staphylococcus aureus. Inhibition of the LDCL response of BBM and BAM was abrogated with either opsonized or killed H somnus. In contrast to inhibition LDCL of BMP, both BBM and BAM infected with H somnus had stimulated production of NO. Using a calorimetric bactericidal assay, it was found that: (1) H somnus was able to survive within BBM in vitro and the kinetics of its survival were similar to that seen in BBM isolated from experimentally infected cattle; (2) treatment of BBM with varying concentrations of (Formula) and E. coli LPS had no effect on the survival of H somnus. Moreover, using ultrastructural studies, and 3H_ uracil incorporation into nucleic acids, it was possible to demonstrate the survival of H somnus in BMP. These results indicate that the ability of H somnus to modulate microbicidal activity of BMP would, in turn, assist the intracellular survival and immunopathogenesis of bovine haemophilosis.