In the mammalian lung, respiratory macrophages provide first line of defense where they expunge and remove particulate matter and kill invading pathogens [1]. The cells are resident and numerous on the luminal surface of alveoli, a strategic location that enables them to internalize and kill pathogens before breaking the epithelia barrier with a possibility of causing local and systemic infections [2]. While substantial data on the protective roles of mammalian respiratory macrophages is well documented [3, 4], little is known about the free avian respiratory macrophages (FARM) [5]. Despite different resident tissue macrophages being phenotypically and functionally distinct [4], some investigators have extrapolated data on avian blood monocytes, splenic macrophages and peritoneal macrophages to apply to the FARM [5].

The avian lung is susceptible to respiratory inflictions owing to its anatomical features that distinguish it from the mammalian lung. The avian blood-gas respiratory tissue barrier is 56–67 % thinner than that of a mammal of equivalent body weight and the respiratory surface is 15 % greater [6]. Skin inflictions predispose the avian respiratory system to airborne infections such as air sacculitis, a condition enhanced by some air sacs diverticulae that extend beyond the coelomic cavity and lie near the skin surface [7]. Additionally, among the vertebrates, birds have the most efficient respiratory system that extracts 60 % of inhaled oxygen compared to mammalian lung that extracts 27 % of oxygen in inspired air, predisposing the avian lung air sac system to oxidative stress [8]. The vast and attenuated avian blood-gas respiratory tissue barrier not only enhances flux of gases by diffusion, but also facilitates invasion of the lungs by particulates and pathogens [9]. Without development of an apt pulmonary defense system, birds would be more vulnerable to respiratory disease conditions.

Although lavage of mammalian lung recovers numerous respiratory macrophages [10], lavage of avian lungs is associated with failure to recover FARM [13]. However, under non inflammatory conditions, some studies report recovery of few FARM through repeated lavages of the parabronchial avian lungs [11, 12]. On average, the number of respiratory macrophages harvested by lavage of mammalian lungs exceeds the number of FARM recovered by similar method in captive birds by approximately 20 times. The average number of FARM in the rock dove is 1.6 × 10⁵ [9] while that in the domestic fowl and turkey is 2.5 × 10⁵ and 1.15 × 10⁶ respectively [10, 12]. Lavage of rat and hamster lungs yielded 8.5 × 10⁶ and 4.64 × 10⁶ respiratory macrophages respectively [10, 14]. An average of 1.1 × 10⁶ FARM and 1.5 × 10⁷ respiratory macrophages were harvested by lavage of domestic duck and rabbit respectively [15]. Paucity of FARM in avian lungs has been purported to foreordain a weak first line of defense against invading particulate matter and pathogens, a condition that has been used to explain high mortality among the captive birds to respiratory disease conditions [16].

According to a review by [5], study of the avian cellular defenses processes is important for: (i) evaluation of drug delivery by aerosolization in a complex respiratory system, (ii) understanding the evolution of pulmonary cellular protection mechanisms and (iii) understanding the pathogenesis of respiratory disease conditions in birds. Microscopic techniques have been used to survey distribution of FARM in the avian lung. Numerous FARM have been localized in the atrial and infundibular subepithelium but the cells are absent in the air capillaries [17]. Morphologically, FARM are similar to mammalian alveolar macrophages exhibiting characteristic eccentric nucleus, a plasma membrane ruffled with filopodial extensions and lysosomes which appear as electron cytoplasmic vesicular bodies [15, 29].

According to [5], dearth of FARM in the avian respiratory system could imply: (a) a weak first line of defense of the avian respiratory system, (b) FARM population on the respiratory surface increases following transmigration of the cells from the avian lung subepithelium or vascular system in response to invading particulates and pathogens, (c) evolution of supplementary defenses, (d) FARM are efficiently cytotoxic to pathogens, or/and (e) FARM are quantitatively fewer but highly phagocytic. Existing data on the protective roles of FARM negate the assertion that dearth of the cells foreordains a weak first line of defense in the avian lung as outlined below.

Peroxisome proliferator activated receptors (PPAR) are transcription ligand activated nuclear receptors [30] and three isoforms: PPAR α, PPAR β and PPAR γ have been identified [31]. The PPAR γ protein is predominantly expressed in the adipose tissue [32]. Later the protein was also found to be expressed in immune cells including monocytes and macrophages [33]. Thiazolidinediones are selective synthetic PPAR γ ligands [34] and treatment of respiratory macrophages with high doses of selective synthetic PPAR γ ligands induces non phlogistic phagocytic properties in the cells with subsequent clearance of inflammatory sites in the mammalian lung [34]. Non phlogistic phagocytosis is characterized by enhanced ability to internalize particles and pathogens with attenuated production of proinflammatory cytokines by phagocytic cells [35].

Avian respiratory disorders are characterized by inflammation of the respiratory epithelium which is a complex biological response of lung and other tissues to harmful stimuli such as pathogens, damaged cells, or irritants such as reactive oxygen species [36]. Even though Inflammation is a protective attempt by an organism to remove the injurious stimuli as well as initiating the healing process for the tissue [37], prolonged inflammation has been associated with respiratory epithelial tissue destruction and pathogenesis of disease conditions such as aspergillosis in captive birds [38].

A review by [39] observes that “an understanding of the mechanisms and molecules that enhance FARM to regulate immune and inflammatory responses may permit the development of products, diets, or husbandry techniques to modulate immunity for the enhancement of the productivity of poultry”. Five specific rationales for modulating FARM function in poultry have been suggested by [39]. These are: (a) providing enhanced or sustained immune response to infectious organisms; (b) enhancement and direction of vaccination responses; (c) mitigation of immunosuppression arising from infectious diseases, dietary toxins, or stress; (d) accelerating the development and maturation of the immune system; (e) inducing tolerance to nonpathogenic environmental immunogens; and (f) mitigating the catabolic consequences of an immune response.

In a recent study that elucidated the anti-inflammatory roles of selective synthetic PPAR γ in FARM, chicken FARM were treated with high dose of troglitazone, a selective synthetic PPAR γ ligand. The study demonstrated that selective synthetic PPAR γ ligands improve the ability of freshly harvested FARM to internalize particles. The volume density of internalized particles per unit volume of a FARM was 41 and 21 % in treated and untreated FARM [29]. The study further observed that treatment of the FARM with PPAR γ ligands attenuated proinflammatory cytokine production in activated FARM [29]. These data indicate that synthetic PPAR γ ligands could be used to improve the ability of FARM to resolve inflammatory disease condition through non phlogistic clearance of inflammatory sites in the avian lung.

Paucity of FARM in the avian lung may not necessarily imply a weak first line of pulmonary defense. FARM, with exceptional bactericidal ability, transmigrate into avian lung in response to infections. An efficient aerodynamic filtration system, tight epithelial junctions that provide a physical barrier against invading pathogens and ciliated epithelium in the trachea and bronchi which removes foreign substances through an efficient mucociliary escalator system, supplement the protective roles of FARM. Available experimental data confirm FARM to have exceptionally efficient phagocytic ability. Recently, selective synthetic PPAR γ ligands have been shown to induce non phlogistic phagocytic properties in the FARM, a necessary condition for resolution of inflammatory disease conditions in the lungs. To develop effective therapeutic interventions targeting FARM in treatment and management of respiratory disease conditions in the poultry, further studies are required to fully understand the role of FARM in innate and adaptive immune responses.