Presence of pulmonary intravascular macrophages in the equine lung: Some structuro‐functional properties

Abstract

The pulmonary intravascular macrophages (PIMs) have been described in several species of animals. This study demonstrates for the first time that the equine lung has PIMs as resident phagocytes in its microvasculature. Their salient features such as globular surface coat, structures of the endocytic pathway, and related cell organelles closely resemble those of the calf, goat, and sheep. The exquisite organization of the coat globules in the form of a linear chain was structurally similar to the lipolytic lipase and the heparin-sensitive globular coat from PIMs of calf, goat, and sheep. Monastral blue (MB) when employed as a tracer to assess the phagocytic properties of equine PIMs induced similar modification of the globules of the coat into lipid droplets, reminiscent of neutral lipids. Lipids droplets (modified coat globules) were delivered into acid phosphatase-positive endosomes and lysosomes. Concurrently, the unaltered globules of the coat, probably internalized via fluid-phase constitutive pinocytoses, followed a different endocytic pathway. Large-scale platelet uptake by the PIMs was observed with thrombocytopenia in MB-treated ponies. The possible significance of hypothetical LDL-coat and the endocytic organelles as equivalents of synthetic apparatus of vasoactive lipids in the PIMs of horse needs to be assessed in future studies.

Depletion of pulmonary intravascular macrophages partially inhibits lipopolysaccharide-induced lung inflammation in horses

“…Pulmonary intravascular macrophages (PIMs) are relatively newly identified mononuclear phagocytes that occur in domestic animal species such as cattle, horses, sheep, pigs and cats [3,4,16,32,33]. The species that contain PIMs show marked pulmonary hypertension following infusion of tracer particles and are highly susceptible to acute lung diseases [19,[28][29][30].…”

“…We have reported that horses contain PIMs, which are activated following phagocytosis of vascular tracers and exposure to halothane [4,5,25]. Longworth et al reported physiological evidence that endotoxin-induced pulmonary arterial hypertension is neutralized following treatment with a detergent; an effect they speculated was due to the blockade of PIMs [17].…”

In vivo interaction of pulmonary intravascular macrophages with activated platelets in microvessels of equine lung after multiple exposures to halothane, isoflurane, and thiamylal: A comparative ultrastructural and cytochemical study

“…In this respect, they resemble the PIMs of sheep, goat, and cattle. Detailed ultrastructural and cytochemical features of equine PIMs are described elsewhere, especially the unique morphology of the globular surface coat and its relationship with the cell membrane and its surface structures (Atwal et al, 1992Longworth et al, 1994;Longworth, 1997). The surface coat consists of linearly arranged globular units, which are discretely separated from the outer leaflet of the cell membrane by a consistent 35-39 nm wide translucent space.…”

“…Globules of the coat, after fixation with tannic acid and paraformaldehyde-glutaraldehyde fixatives, are highly electrondense and reveal a characteristic periodicity, created by the intervening translucent space between the individual globules of the surface coat. The globular units of the coat vary in size from 40 to 120 nm in diameter, which is somewhat bigger than that of the surface coat of sheep, goat, and cattle (Atwal et al, 1992. The coat is not a static extracellular structure, but main-tains a dynamic relationship with the cell membrane, across which constitutive and receptor-mediated endocytosis of the coat globules into the vacuolar system takes place.…”

“…face coat of the cell (Atwal et al, 1992). Once the surface coat is washed away by physiological detergents, there is a drastic reduction in the pulmonary clearance of exogenous material perfused into the venous channels (Longworth et al, 1996).…”