Pulmonary intravascular monocytes/macrophages in a rat model of sepsis

Sepsis induces recruitment of neutrophils and monocytes/macrophages in the lung and enhances host susceptibility to a secondary bacterial challenge. The phenotype and functions of recruited pulmonary intravascular monocytes/macrophages (PIMMs) in sepsis remain largely unknown. Therefore, we characterized PIMM recruitment and functions in a rat model of E. coli-induced sepsis. Male Sprague-Dawley rats were injected intraperitoneally with saline (n = 10) and 48 hr after the saline treatment treated intravenously with either saline (n = 5) or E. coli lipopolysachharide (LPS; 1.5 mu g/kg body weight; n = 5). A second group of 10 rats was infected intraperitoneally with E. coli (2 X 10(7) CFU/100 g) followed by intravenous injection of either saline (n = 5) or LPS (n = 5) 48 hr after the first treatment. Rats were euthanized at 6 hr after LPS treatment. Immunocytochemistry showed more PIMMs stained with ED-1 antibody, which specifically reacts with rat monocytes/macrophages, in rats infected with E. coli compared with the controls (P < 0.05). LPS treatment of E. coli-infected rats increased the numbers of PIMMs (P < 0.05) and induced more inflammation compared to other groups. Immuno-electron microscopy localized TNF-alpha, IL-10, and TGF-beta 2 in recruited PIMMs in rats challenged with both E. coli and LPS. ELISA on lung homogenates showed higher concentrations of TNF-alpha, IL-10, and TGF-beta 2 in rats treated with both E. coli and LPS compared with those treated with only LPS or E. coli (P < 0.05). We conclude that ED-1-positive PIMMs are recruited in this model of sepsis and contain TNF-alpha, IL-10, and TGF-beta 2.