Ultrastructural and cytochemical evaluation of sepsis-induced changes in the rat pulmonary intravascular mononuclear phagocytes

Pulmonary intravascular macrophages and lung health: what are we missing?

Pulmonary intravascular macrophages (PIMs) are constitutively found in species such as cattle, horse, pig, sheep, goat, cats, and whales and can be induced in species such as rats, which normally lack them. It is believed that human lung lacks PIMs, but there are previous suggestions of their induction in patients suffering from liver dysfunction. Recent data show induction of PIMs in bile-duct ligated rats and humans suffering from hepato-pulmonary syndrome. Because constitutive and induced PIMs are pro-inflammatory in response to endotoxins and bacteria, there is a need to study their biology in inflammatory lung diseases such as sepsis, asthma, chronic obstructive pulmonary diseases, or hepato-pulmonary syndrome. We provide a review of PIM biology to make an argument for increased emphasis and better focus on the study of human PIMs to better understand their potential role in the pathophysiology and mechanisms of pulmonary diseases.

Role of pulmonary intravascular macrophages in endotoxin-induced lung inflammation and mortality in a rat model

Background

Bile-duct ligated (BDL) rats recruit pulmonary intravascular macrophages (PIMs) and are highly susceptible to endotoxin-induced mortality. The mechanisms of this enhanced susceptibility and mortality in BDL rats, which are used as a model of hepato-pulmonary syndrome, remain unknown. We tested a hypothesis that recruited PIMs promote endotoxin-induced mortality in a rat model.

Methods

Rats were subjected to BDL to induce PIM recruitment followed by treatment with gadolinium chloride (GC) to deplete PIMs. Normal and BDL rats were treated intravenously with E. coli lipopolysaccharide (LPS) with or without GC pre-treatment followed by collection and analyses of lungs for histopathology, electron microscopy and cytokine quantification.

Results

BDL rats recruited PIMs without any change in the expression of IL-1β, TNF-α and IL-10. GC caused reduction in PIMs at 48 hours post-treatment (P < 0.05). BDL rats treated intravenously with E. coli LPS died within 3 hours of the challenge while the normal LPS-treated rats were euthanized at 6 hours after the LPS treatment. GC treatment of rats 6 hours or 48 hours before LPS challenge resulted in 80% (1/5) and 100% (0/5) survival, respectively, at 6 hours post-LPS treatment. Lungs from BDL+LPS rats showed large areas of perivascular hemorrhages compared to those pre-treated with GC. Concentrations of IL-1β, TNF-α and IL-10 were increased in lungs of BDL+LPS rats compared to BDL rats treated with GC 48 hours but not 6 hours before LPS (P < 0.05).

Conclusion

We conclude that PIMs increase susceptibility for LPS-induced lung injury and mortality in this model, which is blocked by a reduction in their numbers or their inactivation.

Lung inflammation following a single exposure to swine barn air

Background

Exposure to swine barn air is an occupational hazard. Barn workers following an eight-hour work shift develop many signs of lung dysfunction including lung inflammation. However, the in situ cellular and molecular mechanisms responsible for lung dysfunction induced following exposure to the barn air remain largely unknown. Specifically, the recruitment and role of pulmonary intravascular monocytes/macrophages (PIMMs), which increase host susceptibility for acute lung inflammation, remain unknown in barn air induced lung inflammation. We hypothesized that barn exposure induces recruitment of PIMMs and increases susceptibility for acute lung inflammation with a secondary challenge.

Methods

Sprague-Dawley rats were exposed either to the barn or ambient air for eight hours and were euthanized at various time intervals to collect blood, broncho-alveolar lavage fluid (BALF) and lung tissue. Subsequently, following an eight hour barn or ambient air exposure, rats were challenged either with Escherichia coli (E. coli) lipopolysaccharide (LPS) or saline and euthanized 6 hours post-LPS or saline treatment. We used ANOVA (P < 0.05 means significant) to compare group differences.

Results

An eight-hour exposure to barn air induced acute lung inflammation with recruitment of granulocytes and PIMMs. Granulocyte and PIMM numbers peaked at one and 48 hour post-exposure, respectively.

Secondary challenge with E. coli LPS at 48 hour following barn exposure resulted in intense lung inflammation, greater numbers of granulocytes, increased number of cells positive for TNF-α and decreased amounts of TGF-β2 in lung tissues. We also localized TNF-α, IL-1β and TGF-β2 in PIMMs.

Conclusion

A single exposure to barn air induces lung inflammation with recruitment of PIMMs and granulocytes. Recruited PIMMs may be linked to more robust lung inflammation in barn-exposed rats exposed to LPS. These data may have implications of workers exposed to the barn air who may encounter secondary microbial challenge.

Experimental Sepsis: Characteristics of Activated Macrophages and Apoptotic Cells in the Rat Spleen

Sepsis, being characterized by massive translocation of bacteria into tissues, induces the suppression of the function of both leukocytes and macrophages. The aim of the study was to count activated macrophages (AMs) and apoptotic (Ao) cells in the rat spleen during the period of experimental sepsis and to clarify the associations of these parameters with each other and with leukocyte migration and bacterial translocation into different organs. The Wistar rats were intraperitoneally inoculated with Escherichia coli (E. coli) and were sacrificed after 2, 6, 24, 48, and 120 h. Bacteria and leukocytes in tissues were specifically stained. AMs were identified by immunohistological staining and Ao cells by the TUNEL assay. The high counts of E. coli at 6 h were strongly associated with a low level of the total counts of leukocytes, accompanied by the high translocation of microbes into tissues. In the spleen, lymphocytes, macrophages, and neutrophils with pyknotic nuclei were identified. The count of AMs was highest at 24 h after the inoculation with E. coli; at the same time the Ao cell count began to rise and achieved the highest level 24 h later. Our investigation indicates that the molecular peculiarities of macrophages and their responses to the inflammation process are tissue-specific. In the spleen the activation process involving hematopoietic cells and macrophages was remarkable at the late stage of sepsis, characterized by a high count of Ao cells.

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 microg/kg body weight; n=5). A second group of 10 rats was infected intraperitoneally with E. coli (2x10(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-beta2 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-beta2 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-beta2.

Protective effect of β-glucan on lung injury after cecal ligation and puncture in rats

OBJECTIVE

Understanding the biological mediators involved in the complex inflammatory response of sepsis and acute lung injury offers the possibility of future investigations targeting treatment based on these mediators. This study investigated whether macrophage activator beta-glucan has a protective effect on acute lung injury in an experimental model of sepsis.

DESIGN AND SETTING

Experimental study in an experimental research center.

MATERIALS

30 rats randomized into three groups (sham, sepsis, and beta-glucan).

INTERVENTIONS

Cecal ligation and puncture were performed in the beta-glucan and sepsis groups. The beta-glucan group was given a single intraperitoneal dose of beta-glucan (4 mg/kg) following cecal ligation.

MEASUREMENTS AND RESULTS

Rats treated with beta-glucan had fewer circulating neutrophils, more blood monocytes, and higher serum interleukin 6 levels than septic animals. The percentages of neutrophils and lymphocytes from the bronchoalveolar lavage fluid and the myeloperoxidase activity measured in the lung tissue were lower in the beta-glucan group than in the sepsis group. Less alveolar hemorrhage and neutrophil infiltration were observed in lungs from animals in the beta-glucan group in the septic groups.

CONCLUSIONS

In this rat model of intra-abdominal sepsis beta-glucan treatment partially protected against secondary lung injury, decreased lung hemorrhages, and lung neutrophilia. These results suggest that beta-glucan protects against sepsis-associated lung damage.

Depletion of pulmonary intravascular macrophages inhibits acute lung inflammation

Pulmonary intravascular macrophages (PIMs) are present in ruminants and horses. These species are highly sensitive to acute lung inflammation compared with non-PIM-containing species such as rats and humans. There is evidence that rats and humans may also recruit PIMs under certain conditions. We investigated precise contributions of PIMs to acute lung inflammation in a calf model. First, PIMs were recognized with a combination of in vivo phagocytic tracer Monastral blue and postembedding immunohistology with anti-CD68 monoclonal antibody. Second, gadolinium chloride depleted PIMs within 48 h of treatment (P < 0.05). Finally, PIMs contain TNF-alpha, and their depletion reduces cells positive for IL-8 (P < 0.05) and TNF-alpha (P < 0.05) and histopathological signs of acute lung inflammation in calves infected with Mannheimia hemolytica. The majority of IL-8-positive inflammatory cells in lung septa of infected calves were platelets. Platelets from normal cattle contained preformed IL-8 that was released upon in vitro exposure to thrombin (P < 0.05). These novel data show that PIMs, as the source of TNF-alpha, promote recruitment of inflammatory cells including IL-8-containing platelets to stimulate acute inflammation and pathology in lungs. These data may also be relevant to humans due to our ability to recruit PIMs.

Quantifying rat pulmonary intravascular mononuclear phagocytes

Cells of the mononuclear phagocyte system (MPS) protect the host by clearing effete and foreign particulates from the circulation. The current study was designed to identify, quantify, harvest, and provide a partial functional characterization of the systemic host-defense cell located in the pulmonary microvasculature of the rat. Critical colloid doses of test particulates (monastral blue B [MBB] or polystyrene beads) were infused intra-arterially into anesthetized rats so that phagocytically active pulmonary intravascular phagocytes could be identified. Morphologic characterization of in situ phagocytes was performed using electron microscopy. The number of active phagocytes was then determined using tissue samples processed for light microscopy. Finally, sequential perfusion of the pulmonary vasculature with buffer, chelating agent, and collagenase allowed elution and preliminary functional characterization of the pulmonary intravascular mononuclear phagocyte (PIMP). Electron microscopy demonstrated that both mononuclear phagocytes and neutrophils contributed to pulmonary sequestration of circulating particulates. Light microscopy showed that the microvasculature of each alveolus contained 0.50+/-0.19 active mononuclear phagocytes and 0.14+/-0.12 active neutrophils. A chelation/collagenase elution technique was then used to harvest the PIMP. Histologic evaluation of the postperfusion lungs indicated that 80% of the active phagocytes were removed by the technique. In total, the elution fluids contained 2.63+/-1.04 x 10(7) cells, with 1.60+/-0.78 x 10(7), 0.49+/-0.17 x 10(7), and 0.54+/-0.26 x 10(7) of those cells being mononuclear phagocytes, neutrophils, and lymphocytes, respectively. Functionally, the mononuclear phagocyte population exhibited a spectrum of phagocytic activities, with 51.5+/-19.5% of the cells being inactive, 33.9+/-13.4% exhibiting moderate phagocytic activity, and 14.6+/-9.8% demonstrating intensive phagocytic capacity. The current study provides the first quantified demonstration that mononuclear phagocytes are primarily responsible for sequestering blood-borne foreign particulates in the pulmonary circulation of the rat. Approximately 2 x 10(7) PIMP existed in the lungs of 300 gram rats. The functionally heterogeneous mononuclear phagocytes exhibited phagocytic capacities ranging from avidly phagocytic (14.6+/-9.8%) through moderately active (33.9+/-13.4%) to inactive. The lung microvasculature's large pool of inactive mononuclear phagocytes may provide a recruitable mechanism to allow significant increases in clearance of circulating particulates. A resident pool of activatable mononuclear phagocytes might explain previous clinical observations of increased particulate localization in the lung microvasculature of septic patients.

Structural responses of pulmonary intravascular macrophages in lentivirus-infected and/or recombinant ovine interferon-tau-treated lambs

Ovine lentivirus (OvLV), a retrovirus, infects and disseminates to various tissue organs via monocytes. The differentiation of infected monocytes into macrophages is a prerequisite for viral replication, and the presence of infected macrophages in tissue organs induces chronic immunopathology such as lymphoid interstitial pneumonia. The pulmonary intravascular macrophage (PIM) is a recently identified mononuclear phagocyte in domestic animal species, including sheep. Recombinant ovine interferon-tau (roIFN-tau), a type I IFN originally named as the ovine trophoblast protein, has potent antiviral activity against OvLV and human immunodeficiency virus and prevents the development of OvLV-associated lung pathology. We investigated and compared the structural features of PIMs in OvLV-infected and/or roIFN-tau-treated 1-month-old lambs using transmission electron microscopy. The PIMs' numerical counts were performed in toluidine blue-stained sections of Epoxy-embedded lung tissues. A reduction in the number of PIMs was observed with OvLV infection and/or roIFN-tau treatment of lambs as compared to the control group (P < or = 0.05). The majority of the PIMs in OvLV-infected and/or roIFN-tau-treated groups were devoid of their surface coat. The PIMs of OvLV-infected lambs exhibited signs of biosynthetic activation such as expanded rough endoplasmic reticulum, prominent Golgi complexes, and accumulation of secretory vesicles. A few PIMs contained OvLV-like structures. In roIFN-tau-treated OvLV-infected lambs, the lymphocytes had ruffled plasma membranes and were in intimate contact with the PIMs, as is observed during cytotoxic cell-mediated killing of target cells. Most of the PIMs in roIFN-tau-treated OvLV-infected lambs appeared smaller in size. Ovine lentivirus and roIFN-tau, individually or in combination, alter the integrity of the surface coat of PIMs and cause their disappearance from the lungs. Ovine lentivirus infection induces morphological changes that correlate with cytotoxic cell behavior between lymphocytes and PIMs in roIFN-tau-treated or placebo-treated lambs. The loss of PIMs, probably infected with OvLV, either through direct killing by roIFN-tau or indirectly by roIFN-tau-activated cytotoxic T lymphocytes may represent different aspects of therapeutic actions of this cytokine.