Pulmonary intravascular macrophages in the rabbit

Pulmonary intravascular macrophages (PIMs) promote lung inflammation and are found in ruminants, horses, pigs, cats, and dolphins, but not in primates, rats and mice. Rabbits are used to study mechanisms of lung inflammation in humans, but disagreement exists whether rabbits have PIMs. This study examined rabbits for PIMs, and their influence on endotoxin-induced lung inflammation. Rabbits were treated with gadolinium chloride (10 mg/kg intravenous: Group GC, n=6) to produce apoptosis in PIMs, or with saline (Group SAL, n=6). Rabbits were euthanized 48 hours later. Light microscopic examination of epoxy-embedded rabbit lung sections revealed mononuclear phagocytes in alveolar septa. Transmission electron microscopy confirmed PIMs with lysosomes and close attachment to capillary endothelium. Light microscopic immuno-cytochemistry using rabbit anti-macrophage antibody (RAM-11) showed staining of septal and alveolar macrophages. There was no difference in number of RAM-11 positive septal cells between SAL and GC rabbits (P=0.2). Rabbits were administered intravenous E.coli 0127:B8 endotoxin (100 μg/kg) 48 hours after GC (GC-LPS; n=5) or SAL treatment (SAL-LPS; n=6), and euthanized 24 hours later. Rabbits in both LPS treated groups were hypocalcaemic and exhibited compensated metabolic acidosis compared to SAL rabbits. Four rabbits died in the SAL-LPS group within 24 hours of the endotoxin treatment and were replaced. None died in the GC-LPS group (Chi-square comparison for survival P=0.063). Greater numbers of septal heterophils were found in groups SAL-LPS and GC-LPS compared to SAL and GC. TNFα protein in serum, and IL-1β and IL-6 mRNA in lung tissues were increased in SAL-LPS compared to SAL and GC rabbits. Lung tissues from SAL-LPS rabbits but not in GC-LPS showed moderate inflammation, but lung wet/dry ratios were not different. Lung tissue TNFα, IL-1β and IL-6 mRNA, myeloperoxidase activity, and serum TNFα were reduced in GC-LPS animals compared to SAL-LPS. Immuno-electron microscopy revealed TNFα in PIMs in normal and LPS-treated rabbits. Lung and liver tissue TNFα, IL-8 and MCP-1 protein concentrations were not different between groups. GC did not appear to reduce liver inflammation. These data show that rabbits have low numbers of PIMs. GC treatment induced apoptosis in PIMs and reduced endotoxin-induced lung inflammation and mortality