Cellular defense of the avian respiratory system: influx and nonopsonic phagocytosis by respiratory phagocytes activated by Pasteurella multocida.

Poultry have a very limited number of resident macrophages in the normal steady-state respiratory tract. Thus, poultry must rely heavily on active migration of phagocytic cells to the lungs and air sacs in defending against respiratory pathogens. Intratracheal administration of a live, apathogenic Pasteurella multocida vaccine (Choloral; Clemson University strain) increased the number of avian respiratory phagocytes (ARP; obtained by lavage of lungs and air sacs) within 24 h by 3 orders of magnitude compared with the number of ARP obtained from mock-inoculated controls and from nonreacting chickens. Chickens yielding a high number of ARP did not show any sign of respiratory disease. Flow cytometric analysis of ARP that were exposed to 20 nonopsonized fluorescent microspheres per ARP for 30 min at 37 degrees C demonstrated a fivefold increase in the percentage of actively phagocytic cells in the ARP populations of stimulated chickens compared with the percentage of phagocytic ARP for mock-inoculated control birds. The phagocytic capacity (relative number of engulfed microspheres) of ARP from stimulated birds doubled during the same time. The flow cytometric observations were confirmed by fluorescence microscopy. These results indicate that activation by avirulent replicating agents of phagocytic cells of chicken to migrate to the respiratory tract may be a means of defending poultry against air sacculitis and pneumonia.