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

Om Parbhakar , Tanya Duke , Hugh Townsend , Baljit Singh

Horses are unique in their extreme sensitivity to endotoxin-induced cardio-pulmonary shock and mortality. The mechanisms behind increased sensitivity of the horse to endotoxin remain unknown. Pulmonary intravascular macrophages (PIMs) are pro-inflammatory cells occurring in horses. Because the functions of equine PIMs in endotoxemia remain unknown, we studied the role played by equine PIMs in endotoxin-induced pulmonary pathophysiology. We achieved this by using a recently developed protocol to deplete PIMs in order to compare lipopolysaccharide (LPS)-induced pulmonary responses in horses with or without PIMs. Horses treated with gadolinium chloride (GC; 10 mg/kg intravenous) to deplete PIMs or endotoxin-free saline (n = 4) were injected with Escherichia coli LPS (E. coli LPS; 50 ng/kg intravenously) 48 h after GC or saline. Control horses (n = 5) received two injections of endotoxin-free saline at 48 h intervals. All the horses were euthanized 2 h after LPS or saline challenge. Immunohistology for the PIMs showed their reduced numbers in GC-treated horses. The LPS treatment of normal and GC-treated horses increased diastolic and systolic pulmonary arterial pressures at 30 min compared to the saline-treated horses (P < 0.05). However, horses pre-treated with GC did not have an LPS-induced increase in mean pulmonary arterial pressure compared to the LPS-treated horses (P < 0.05). Light and electron microscopic immunocytochemistry detected extensive labeling for LPS in PIMs of LPS-treated horses. Both the LPS-treated groups had more alveolar septal cells positive for TNF-$\alpha$ and IL-1$\beta$ compared to control horses, which did not receive LPS (P < 0.05). However, GC-treated horses challenged with the LPS showed less IL-1$\beta$-positive cells (P < 0.05). Immuno-electron microscopy localized TNF-$\alpha$ and IL-1$\beta$ in PIMs. These new data show that PIMs endocytose LPS and contain TNF-$\alpha$ and IL-1$\beta$ and their depletion partially inhibits LPS-induced pulmonary inflammatory responses.

Mots clés: PIMs; TNF-$\alpha$; IL-1$\beta$; gadolinium chloride; light and electron microscopic immunocytochemistry

Domaines: Biologie moléculaire; Génétique animale; Biologie cellulaire; Interactions cellulaires [q-bio.CB]; Microbiologie et Parasitologie; Immunologie; Neurosciences [q-bio.NC]; Santé publique et épidémiologie; Biologie animale

Om Parbhakar, Tanya Duke, Hugh Townsend, Baljit Singh. Depletion of pulmonary intravascular macrophages partially inhibits lipopolysaccharide-induced lung inflammation in horses. Veterinary Research, 2005, 36 (4), pp.557-569. ⟨10.1051/vetres:2005016⟩. ⟨hal-00902983⟩