Composite cellular defence stratagem in the avian respiratory system: functional morphology of the free (surface) macrophages and specialized pulmonary epithelia

Abstract

Qualitative and quantitative attributes of the free respiratory macrophages (FRMs) of the lung -air sac systems of the domestic fowl (Gallus gallus variant domesticus) and the muscovy duck (Cairina moschata) were compared with those of the alveolar macrophages of the lung of the black rat (Rattus rattus). The birds had significantly fewer FRMs compared to the rat. In the birds, the FRMs were found both in the lungs and in the air sacs. Under similar experimental conditions, the most robust FRMs were those of the domestic fowl followed by those of the rat and the duck. Flux of macrophages onto the respiratory surface from the subepithelial compartment and probably also from the pulmonary vasculature was observed in the birds but not in the rat. In the duck and the domestic fowl, a phagocytic epithelium that constituted over 70% of the surface area of the blood-gas (tissue) barrier lines the atrial muscles, the atria and the infundibulae. The epithelial cells of the upper respiratory airways contain abundant lysosomes, suggesting a high lytic capacity. By inference, the various defence strategies in the avian lung may explain the dearth of FRMs on the respiratory surface. We counter-propose that rather than arising directly from paucity of FRMs, an aspect that has been over-stressed by most investigators, the purported high susceptibility of birds (particularly table birds) to respiratory ailments and afflictions may be explained by factors such as inadequate management and husbandry practices and severe genetic manipulation for fast growth and high productivity, manipulations that may have weakened cellular and immunological defences.

The avian lung-associated immune system: a review

“…of the lung but also from the air sacs as elegantly shown in domestic fowls and ducks [55]. Differences in the sampling technique as well as in the chicken strains and immune status of the birds may account for the variable results obtained within [76] and between studies.…”

“…Whereas Klika et al [37] did not find evidence for the migration of FARM from the interstitial tissue to the surface of the conducting airways other investigators reported the opposite [25,77]. Nganpiep and Maina [55] compared the number of macrophages obtained by first and subsequent lung lavages and found a clear increase in cell numbers. The authors concluded that these cells emanated through efflux from the subepithelial compartment or even from the vascular system itself.…”

“…In the mammalian lung alveolar macrophages constitute an important defence line at the gas-blood barrier [9,10] that protects the extensive surface area which is only separated by a thin, easily assailable tissue barrier [55]. Relatively little is known about the corresponding cell type in the avian lungair sac system.…”

“…In the literature these cells have been designated as "avian respiratory macrophages or phagocytes -ARP" [26,76] or "free avian respiratory macrophages -FARM" [42,77]. Alveolar macrophages can easily be collected by pulmonary lavage in mammals, however this procedure yields only few FARM in birds [25,46,55,77,81]. From these studies it was estimated that alveolar macrophages in mice or rats were 20 times more frequent compared to FARM in the lung of the much larger chicken [46,79].…”

Induction of respiratory immune responses in the chicken; implications for development of mucosal avian influenza virus vaccines

“…Adjacent to the muscle fibers that form the parabronchi, the lumen is bordered by 74.3 stained cells, and these cells are expected to act as mammalian alveolar MPh (Jeurissen et al 1994). Epithelial cells lining the atrial muscles, the atrial floor and the infundibulae have been shown to be phagocytic and may therefore play a role in the defence of the thin, extensive and highly vulnerable tissue barrier (Nganpiep and Maina 2002). It is unclear if phagocytosed material is presented to the local immune system in the BALT, or in the follicles present in the interstitium between parabronchi or if it is transported to the spleen or a combination of these option depending on the antigen.…”