The effect of parainfluenza virus type 3 on the phagocytic cell response of the ovine lung to Pasteurella haemolytica

Abstract

Groups of Caesarian-derived, colostrum-deprived lambs were inoculated by the intratracheal route with Pasteurella haemolytica 4 or 6 days after the inoculation of parainfluenza virus type 3 (PI 3). Some were killed immediately (0 h) and others 24 h later. Control groups were inoculated with PI 3 alone, P. haemolytica alone or media alone. Pulmonary phagocytic cells, P. haemolytica and PI 3 were recovered by pulmonary lavage. The phagocytes were separated into alveolar macrophage (AM) and neutrophil fractions by density gradient centrifugation and examined biochemically and microbiologically. Twenty-four hours after the inoculation of P. haemolytica bacterial proliferation to greater than 0 h levels had occurred in four of six animals inoculated with P. haemolytica alone, two of eight inoculated with P. haemolytica 4 days after PI 3 and all of eight inoculated with P. haemolytica 6 days after PI 3. Mean bacterial numbers in animals inoculated with P. haemolytica 6 days after PI 3 and killed at 24 h (10 9.1±1.9) were significantly higher than they were in the other two groups killed at this time (PI 3 4 days, P. haemolytica 24 h, mean = 10 5.3±1.7; P. haemolytica alone 24 h, mean = 10 4.5±2.9). Pneumonic lesions were also more severe in the first group. This defect in pulmonary clearance and increase in the severity of pneumonia in animals inoculated with P. haemolytica 6 days after PI 3 coincided with a 1000-fold decrease in virus titres in the lung between Day 6 and Day 7 after virus inoculation and the first detectable evidence of the host's immune response. The virus infection resulted in a significant increase in the number of AM that could be recovered from the lung and an increase in the number of AM with cytoplasmic vacuolation. However, there was no difference in the total number of AM or the number of vacuolated AM between animals that controlled the P. haemolytica infection and those in which proliferation of P. haemolytica occurred. The inoculation of P. haemolytica resulted in a 100-fold increase in the number of neutrophils in the lavage fluid, but there were no differences between virus-infected and uninfected animals, nor was there a difference between animals that controlled the P. haemolytica infection and those that did not. Levels of acid phosphatase, β-glucuronidase and total protein in lavage fluid were increased in animals inoculated with P. haemolytica 6 days after PI 3 and killed at 0 and 24 h; levels of lactate dehydrogenase were also increased in these animals. Therefore, the increased amounts of lysosomal enzymes probably reflected increased cell damage rather than changes in phagocytic activity. When lysates of purified neutrophils and AM were examined there were no differences between groups in the levels of acid phosphatase, β-glucuronidase and total protein. However, the bactericidal activity of neutrophil lysates was significantly reduced in animals inoculated with P. haemolytica 6 days after PI 3, the group where the major defect in clearance was observed.