Phenotypical and functional characterization of alveolar macrophage subpopulations in the lungs of NO2-exposed rats.

BACKGROUND: Alveolar macrophages (AM) are known to play an important role in the regulation of inflammatory reactions in the lung, e.g. during the development of chronic lung diseases. Exposure of rats to NO2 has recently been shown to induce a shift in the activation type of AM that is characterized by reduced TNF-alpha and increased IL-10 production. So far it is unclear, whether a functional shift in the already present AM population or the occurrence of a new, phenotypically different AM population is responsible for these observations. METHODS: AM from rat and mice were analyzed by flow cytometry for surface marker expression and in vivo staining with PKH26 was applied to characterize newly recruited macrophages. Following magnetic bead separation, AM subpopulations were further analyzed for cytokine, inducible NO synthase (iNOS) and matrix metalloproteinase (MMP) mRNA expression using quantitative RT-PCR. Following in vitro stimulation, cytokines were quantitated in the culture supernatants by ELISA. RESULTS: In untreated rats the majority of AM showed a low expression of the surface antigen ED7 (CD11b) and a high ED9 (CD172) expression (ED7-/ED9high). In contrast, NO2 exposure induced the occurrence of a subpopulation characterized by the marker combination ED7+/ED9low. Comparable changes were observed in mice and by in vivo labeling of resident AM using the dye PKH26 we could demonstrate that CD11b positive cells mainly comprise newly recruited AM. Subsequent functional analyses of separated AM subpopulations of the rat revealed that ED7+ cells showed an increased expression and production of the antiinflammatory cytokine IL-10 whereas TNF-alpha production was lower compared to ED7- AM. However, iNOS and IL-12 expression were also increased in the ED7+ subpopulation. In addition, these cells showed a significantly higher mRNA expression for the matrix metalloproteinases MMP-7, -8, -9, and -12. CONCLUSION: NO2 exposure induces the infiltration of an AM subpopulation that, on the one hand may exert antiinflammatory functions by the production of high amounts of IL-10 but on the other hand may contribute to the pathology of NO2-induced lung damage by selective expression of certain matrix metalloproteinases.

Impaired superoxide radical production by bronchoalveolar lavage cells from NO(2)-exposed rats.

Production of superoxide radicals is a central property of professional phagocytes used to combat invading microorganisms. Even though the number of macrophages and neutrophils is often increased in the lungs of patients with chronic lung diseases, these patients frequently suffer from bacterially induced exacerbations. To understand the underlying mechanisms, we investigated the production of superoxide radicals by bronchoalveolar lavage (BAL) cells in a rat NO(2) exposure model (10 ppm NO(2) for 1, 3, or 20 days). We showed that cells from NO(2)-exposed animals display a significantly impaired superoxide radical release after zymosan stimulation. The use of specific inhibitors (antimycin or diphenyleneiodonium [DPI]) revealed that the major enzyme systems, NADPH oxidase and complex III of the respiratory chain, are affected. In addition, we investigated gene expression and enzyme activities of antioxidant enzymes. mRNA expression was significantly enhanced for glutathione peroxidase (GPx)-3 and CuZn-superoxide dismutase (SOD) in BAL cells from animals exposed 3 and 20 days, and GPx and SOD enzyme activities were increased in BAL cells from rats exposed 20 days. In conclusion, concomitant occurrence of reduced production and increased scavenging of superoxide radicals resulted in the drastically impaired release of these radicals from BAL cells of NO(2)-exposed rats.

Shift toward an alternatively activated macrophage response in lungs of NO2-exposed rats.

Inflammatory mechanisms are thought to play an important role in the pathogenesis of acute and chronic obstructive pulmonary diseases. In a rat inhalation model using continuous exposure to 10 ppm nitrogen dioxide for 1, 3, and 20 d, we investigated the inflammatory response with particular focus on the activation state of alveolar macrophages. Whereas the number of inflammatory cells and total protein concentration were increased in the bronchoalveolar lavage (BAL), the amount of the proinflammatory cytokine tumor necrosis factor-alpha was markedly reduced with increasing exposure time. In contrast, interleukin (IL)-10 and IL-6 were found at elevated levels and intracellular amounts of suppressor of cytokine signaling-3 protein increased in BAL cells. Upon in vitro lipopolysaccharide stimulation, BAL cells revealed reduced capability to produce the proinflammatory mediators tumor necrosis factor-alpha, IL-1 beta, and nitric oxide, but showed markedly increased transcription and protein release for IL-10. In addition, elevated levels of IL-6, scavenger receptor B, and suppressor of cytokine signaling-3 mRNA were detected in BAL cells from exposed animals. Analyses of highly purified alveolar macrophages indicated that changes in the activation state of these cells were responsible for the observed effects. In conclusion, a priming toward development of the alternatively activated macrophage phenotype occurred in the lungs of rats following nitrogen dioxide inhalation.

IL-24 is expressed by rat and human macrophages.

Recently, a number of interleukin-10 (IL-10) homologues, among them IL-24 formerly known as melanocyte differentiation factor-7 (mda-7), has been described. Since IL-10 is released by macrophages and plays an important role in the resolution of inflammatory processes, we hypothesized that IL-24 might also be expressed in cells of the monocyte/macrophage lineage. We analyzed IL-24 expression on the mRNA and protein level in stimulated rat and human macrophages. In rat alveolar macrophages and NR8383 cells, IL-24 mRNA induction was observed following stimulation with LPS and IL-4 whereas TNF-alpha failed. Intracellular IL-24 protein was detected in unstimulated and IL-4 stimulated NR8383 cells. Also human blood monocytes showed a strong up-regulation of IL-24 mRNA following preparation which was enhanced by LPS and lowered by IL-10. Furthermore, infection of human monocytes with influenza A virus A/PR/8 caused an induction of IL-24 mRNA expression. In conclusion, our data show that IL-24 expression is induced in stimulated and infected rat and human macrophages, however, more insights into the functions of IL-24 are necessary to define its physiological relevance.