Kinetics of Interaction and Fate of Pasteurella hemolytica in Bovine Alveolar Macrophages

Publisher Website
Google Scholar
Abstract
To study the role of pulmonary alveolar macrophages (PAM) in phagocytizing P. haemolytica, an in vitro cultivation method was developed for preparing them. This procedure provided an adherent monolayer of PAM which were nonspecific esterase-positive and phagocytized latex beads. The phagocytosis and fate of P. hemolytica (biotype A, serotype 1) by PAM in suspension were studied. The kinetics of phagocytosis were determined by quantitatively measuring the uptake of 24 h [3H]thymidine-labeled bacteria by the PAM in the presence of opsonins. The uptake of P. hemolytica was enhanced in the presence of normal serum or antiserum. A total of 90% of the bacteria were phagocytized in the presence of normal adult bovine serum and up to 95% were phagocytized in the presence of an antiserum. Normal serum, but not fetal calf serum, contained heat-stable natural antibodies which readily initiated the opsonization of P. hemolytica. The heat-labile complement system was also involved in the opsonization. The fate of P. hemolytica inside the PAM was investigated by transmission electron microscopy and by the viable plate count method. Approximately 90% of the normal serum- or antiserum-opsonized P. hemolytica were phagocytized by PAM at a bacteria/PAM ratio of 20:1 and were completely degraded after 60 min of exposure. Prolonged incubation of this mixture of bacteria and PAM resulted in cytotoxic changes and destruction of PAM. At a low bacteria/PAM ratio (10:1 or less), there was phagocytosis and killing of bacteria but no cytotoxic changes on the PAM. The exact mechanism which initiated this phenomenon was not demonstrated. Perhaps toxic substance(s) released by the excess unphagocytized bacteria caused the cytotoxic changes to the PAM.