Comparative In Vitro Study Of Quantitative And Qualitative Characteristics Of Free Surface Respiratory Macrophages In The Duck And The Rabbit

ABSTRACT

In mammals, the surface respiratory macrophages (SM) play a critical role in protecting the respiratory system by providing first line defense through engulfing and destroying inhaled pathogens and harmful particulates. During adaptive immune responses, SM process and present antigens to T lymphocytes. Paucity of SM has been reported in the avian respiratory system. It has been reported that the pulmonary cellular defenses in birds are inadequate. In particular susceptibility to respiratory diseases in domestic birds has been associated with dearth of SM on the lung air sac system. In view of the protective roles SM perform in the respiratory system of mammals, the objective of the current study was to carry out a comparative in vitro study of the quantitative and qualitative characteristics of the avian and the mammalian SM by determining the number and the phagocytic capacity of the SM. The domestic duck was used as the avian model while the domestic rabbit was used as the mammalian model. Seven rabbits and seven ducks were subjected to respiratory lavage for recovery of SM which were quantified using hemocytometer and co-cultured with polystyrene particles in a culture medium for determination of phagocytic capacities. Morphologic, viability and mophometric studies of the SM were characterized at light and electron microscopy. Morphologically, the duck and the rabbit SM were similar. The SM had eccentric nuclei, cytoplasmic vacuoles and the plasma membrane had filopodial extensions. Quantitatively, the SM recovered by lavage of the ducks were fewer than the SM recovered from the rabbits. With a mean number of 1.5 × 107 SM recovered, the rabbits had significantly (p ≤ 0.05) more SM than the ducks whose mean number of SM recovered by lavage was 1.1 × 106 . In the rabbits, the mean number of SM decreased steadily with progressive lavages. In the ducks, there was influx of SM indicated by significant (p ≤ 0.05) increase of the mean number of SM recovered during the first three lavages. Morphometric observations revealed that the duck and the rabbit SM had equivalent diameters of 12 µm and 13 µm respectively. However, the duck SM exhibited a significantly (p ≤ 0.05) higher phagocytic capacity by engulfing substantially more particles at 20 % than the rabbit SM at 9 %. The higher in vitro phagocytic capacity and the influx of the duck SM are two important properties that can be exploited in vaccine development. Vaccine delivery by aerosolization should focus on the ability to promote influx of SM onto avian lung-air sac system without compromising the phagocytic ability of the cells. This is an area that should be exploited in prevention and management of avian diseases.