Ultrastructural and immunocytochemical study of the pulmonary intravascular macrophages of Escherichia coli lipopolysaccharide‐treated sheep

Background Pulmonary intravascular macrophages (PIMs) of sheep, cattle, goats, and horses have a novel heparin-sensitive chain of globules, called a surface coat, on their plasma membrane. The globules are arranged at a distance of 32-39 nm from the plasma membrane of PIMs. Intravascular nonbiological tracer particles complex with these globules prior to their endocytosis by the PIMs. Methods We conducted a preliminary in vivo time-course study in sheep to investigate responses of the coat globules to a single dose of Escherichia coli lipopolysaccharide (E. coli LPS). Six sheep (6-9 months of age) were used in this study, and five of them were intravenously injected with E. coli (1 μg/kg body weight) and euthanised at 3, 8, 10, 30, and 180 min (n = 1 each) after treatment. One sheep injected with saline solution served as the control. Acid phosphatase (AcPase) cytochemistry and immunocytochemistry using a polyclonal antibody were employed to localize secretory activity and E. coli LPS respectively in the PIMs. Results The surface coat of PIMs disappeared rapidly following the LPS administration. Escherichia coli LPS micelles and coat globules were colocalized as a complex in the endosomes of PIMs. At 8-10 min following the treatment, endosomal and the other membranes were disrupted, and the LPS was identified in cytoplasm and nuclear matrix of PIMs simultaneously with the development of pulmonary interstitial edema. Progression of AcPase reactivity along the nucleus-Golgi complex axis coupled with intense buildup of coated transport vesicles within 30 min of the LPS injection suggested enhanced biosynthetic activity in the PIMS. Conclusions This study provides initial data on the sensitivity of the coat globules and their possible role in the endocytosis of E. coli LPS by the PIMs. Rapid biosynthetic activation of PIMs concurrent with loss of the coat and treatment with the LPS probably results in the secretion of inflammatory substances and contributes to the enhanced susceptibility of sheep to endotoxin-induced lung pathology. Anat. Rec. 247:214-224, 1997. © 1997 Wiley-Liss, Inc.