Effect of in vitro inoculation of bovine respiratory syncytial virus on bovine pulmonary alveolar macrophage function

Abstract

SUMMARY Viruses may predispose the respiratory tract to the development of secondary bacterial pneumonia by impairing functions of alveolar macrophages. The effects of bovine respiratory syncytial virus ( brsv ) on selected functions of bovine pulmonary alveolar macrophages ( pam ) were examined in vitro. Alveolar macrophages were obtained from nonsedated cattle, using a polypropylene tube passed intranasally into the lung. The pam lavaged from the lung were allowed to adhere to glass coverslips or plastic tissue culture plates, and were exposed to brsv for 2 hours. Control and brsv -inoculated pam were compared at intervals over a 72-hour period for their abilities to phagocytize and kill Staphylococcus epidermidis , rosette with and phagocytize antibody-coated sheep rbc : ( srbc ), phagocytize latex particles, and influence lysosomal enzyme activity. Challenge exposure with brsv did not affect the ability of pam to adhere and did not affect cell viability. There were numerical differences between control and brsv -inoculated cell populations in phagocytosis and killing of S epidermidis , but these were not significant ( P > 0.05). There was < 5% difference in the abilities of control and brsv -challenged pam to phagocytize latex beads. When Fc-receptor-mediated phagocytosis of antibody-coated srbc was compared with controls, brsv -challenged pam had significantly ( P < 0.05) impaired phagocytic function, which was maximal 72 hours after brsv inoculation; the phagocytic impairment occurred in spite of normal Fc-receptor function, as determined by resetting with antibody-coated srbc . Lysosomal enzyme activity, measured as intracellular acid phosphatase, was similar between control and brsv -challenged cell populations at 2, 24, and 48 hours after brsv inoculation; at 72 hours, enzyme levels in brsv -challenged cells were nearly twice those of control cells. Replication of brsv in pam and bovine turbinate ( bt ) cells was followed for a 60-hour period. There was no evidence of extracellular virus or cell-associated virus detected from pam over the 60-hour postinoculation period. However, extracellular and cell-associated brsv was detected over a 60-hour period after brsv inoculation of bt cells. Immunofluorescence studies at 72 hours after inoculation indicated that approximately 10% of brsv -inoculated pam contained brsv antigen. In contrast, approximately 70% of brsv -inoculated bt cells were fluorescent positive by 72 hours. These results indicated that, although bovine pam are not permissive to brsv infection, in vitro challenge with this virus did impair selected pam functions.