The role of LFA-1 modulation in the biological effects of Mannheimia haemolytica LKT on bovine leukocytes

Bovine respiratory disease (BRD) is a multifactorial disease complex caused by stressful management practices, and interactions between viral and bacterial pathogens. Bovine herpesvirus-1 (BHV-1) has been reported to increase the susceptibility of cattle to infection with the bacterial respiratory pathogen Mannheimia (Pasteurella) haemolytica A1. Despite many attempts to better understand the pathogenesis of BRD, little progress has been made in elucidating the synergistic effects of respiratory viruses and M. haemolytica.; The purpose of my study was to identify a mechanism that might explain, at least in part, the increased susceptibility of viral infected cattle to M. haemolytica. I hypothesize in this study that one mechanism that might be involved is exposure of leukocytes to BHV-1-induced inflammatory mediators (i.e. cytokines). These cytokines, in turn could modulate the migration, and functional activation of β₂-integrins on bovine leukocytes, making the latter more susceptible to the binding and biological effects of M. haemolytica LKT.; In this study we observed that exposure of peripheral blood leukocytes to the inflammatory cytokines interleukin 1 (IL-1β), tumor necrosis alpha (TNF-α), or gamma interferon (IFN-γ) enhanced LFA-1 expression on bovine PMNs and bovine peripheral blood mononuclear leukocytes (PBMCs), and subsequently increased LKT binding and cytotoxicity to these cells. We also demonstrated that experimental BHV-1 infection of cattle increased LFA-1 expression on their peripheral blood leukocytes, and enhanced LKT binding and cytotoxicity to their bronchoalveolar and peripheral blood leukocytes ex vivo. We observed that PBMCs infected in vitro with BHV-1 exhibited enhanced LFA-1 expression, LKT binding and LKT cytotoxicity. We also observed an enhanced IL-1β and IFN-γ mRNA expression on BHV-1 infected PBMC. Furthermore, we observed that exposure of PMNs to conditioned media from BHV-1 infected PBMC increase their LFA-1 levels and enhanced LKT binding and cytotoxicity. We also observed that exposure to M. haemolytica LKT and LPS increase LFA-1 surface expression on bovine neutrophils, resulting in increased LKT binding and cytotoxicity to these cells. In summary these studies provide evidence that inflammatory cytokines might play an important role in the BHV-1/M. haemolytica synergism, and might explain, in part, the severe pneumonia that can be observed in BRD.