Inhalation of ozone produces a decrease in superoxide anion radical production in mouse alveolar macrophages

Lung macrophages: current understanding of their roles in Ozone-induced lung diseases

Through the National Ambient Air Quality Standards (NAAQS), the Clean Air Act of the United States outlines acceptable levels of six different air pollutants considered harmful to humans and the environment. Included in this list is ozone (O₃), a highly reactive oxidant gas, respiratory health hazard, and common environmental air pollutant at ground level. The respiratory health effects due to O₃ exposure are often associated with molecular and cellular perturbations in the respiratory tract. Periodic review of NAAQS requires comprehensive scientific evaluation of the public health effects of these pollutants, which is formulated through integrated science assessment (ISA) of the most policy-relevant scientific literature. This review focuses on the protective and pathogenic effects of macrophages in the O₃-exposed respiratory tract, with emphasis on mouse model-based toxicological studies. Critical findings from 39 studies containing the words O₃, macrophage, mice, and lung within the full text were assessed. While some of these studies highlight the presence of disease-relevant pathogenic macrophages in the airspaces, others emphasize a protective role for macrophages in O₃-induced lung diseases. Moreover, a comprehensive list of currently known macrophage-specific roles in O₃-induced lung diseases is included in this review and the significant knowledge gaps that still exist in the field are outlined. In conclusion, there is a vital need in this field for additional policy-relevant scientific information, including mechanistic studies to further define the role of macrophages in response to O₃.

Macrophage phagocytosis: effects of environmental pollutants, alcohol, cigarette smoke, and other external factors

The ability of a pathogen to evade host immunity successfully, in contrast to the host's capacity to defend itself against a foreign invader, is a complex struggle, in which eradication of infection is dictated by a robust immunologic response. Often, there are external factors that can alter the outcome by tipping the scale to benefit pathogen establishment rather than resolution by the host's defense system. These external sources, such a cigarettes, alcohol, or environmental pollutants, can negatively influence the effectiveness of the immune system's response to a pathogen. The observed suppression of immune function can be attributed to dysregulated cytokine and chemokine production, the loss of migratory potential, or the inability to phagocytose pathogens by immune cells. This review will focus on the mechanisms involved during the toxin-induced suppression of phagocytosis. The accumulated data support the importance of studying the mechanisms of phagocytosis following exposure to these factors, in that this effect alone cannot only leave the host susceptible to infection but also promote alterations in many other macrophage functions necessary for pathogen clearance and restoration of homeostasis.

Nitric oxide and peroxynitrite in ozone-induced lung injury

One of the hallmarks of the inflammatory response associated with tissue injury is the accumulation of macrophages at sites of damage. These cell types release proinflammatory cytokines and cytotoxic mediators to destroy invading pathogens and initiate wound repair. However, when produced in excessive amounts, these macrophage-derived mediators may actually contribute to tissue injury. This process involves both direct damage to target tissues and amplification of the inflammatory response. One group of macrophage-derived mediators of particular interest are reactive nitrogen intermediates including nitric oxide and peroxynitrite which have been implicated in tissue injury induced by a variety oftoxicants. Our laboratory has been investigating the role of reactive nitrogen intermediates in lung injury induced by oxidants such as ozone. Inhalation of ozone causes epithelial cell damage and Type II cell hyperplasia. This is associated with an accumulation of activated macrophages in the lower lungs which we have demonstrated contribute to toxicity. To analyze the role of macrophage-derived reactive nitrogen intermediates in ozone toxicity, we used transgenic mice lacking the gene for inducible nitric oxide synthase (NOSII). Treatment of wild type control animals with ozone (0.8 ppm) for 3 hr resulted in an increase in bronchoalveolar lavage (BAL) fluid protein reaching a maximum 24-48 hr after exposure. This was correlated with increased expression of NOSII protein and mRNA by alveolar macrophages and increased production of nitric oxide as well as peroxynitrite. Ozone inhalation also resulted in the appearance of nitrotyrosine residues in the lungs, an in vivo marker of peroxynitrite-induced damage. In contrast, in NOSII knockout mice, BAL protein was not increased demonstrating that these mice were protected from ozone-induced epithelial injury. Moreover, alveolar macrophages from the transgenic mice did not produce nitric oxide or peroxynitrite even after ozone inhalation. There was also no evidence for the formation of nitrotyrosine in lung tissue. These data indicate that ozone-induced lung injury is mediated by reactive nitrogen intermediates.

Deficiency in inducible nitric oxide synthase protects mice from ozone-induced lung inflammation and tissue injury

Inhalation of ozone causes Type I epithelial cell necrosis and Type II cell hyperplasia and proliferation. This is associated with an accumulation of activated macrophages in the lower lung, which we have demonstrated contribute to tissue injury. Nitric oxide (NO) is a highly reactive cytotoxic macrophage-derived mediator that has been implicated in lung damage. In the present studies we used knockout mice with a targeted disruption of the gene for inducible nitric oxide synthase (NOSII) to analyze the role of NO in ozone-induced lung inflammation and tissue injury. Treatment of wild-type control mice with ozone (0.8 ppm) for 3 h resulted in a time-dependent increase in protein and cells in bronchoalveolar lavage fluid, which reached a maximum 24-48 h after exposure. Alveolar macrophages isolated from animals treated with ozone were found to produce increased amounts of NO, as well as peroxynitrite. This was correlated with induction of NOSII protein and nitrotyrosine staining of lung macrophages in tissue sections and in culture. Production of superoxide anion and prostaglandin (PG)E2 by alveolar macrophages was also increased after ozone inhalation. In contrast, alveolar macrophages from NOSII knockout mice did not produce reactive nitrogen intermediates even after ozone inhalation. Moreover, production of PGE2 was at control levels. NOSII knockout mice were also protected from ozone-induced inflammation and tissue injury, as measured by bronchoalveolar lavage protein and cell number. There was also no evidence of peroxynitrite-mediated lung damage in these animals. Taken together, these data demonstrate that NO, produced via NOSII, and potentially, its reactive oxidative product peroxynitrite, play a critical role in ozone-induced release of inflammatory mediators and in tissue injury.

Effects of ozone upon macrophage-interferon interactions

Lung cell populations may be directly exposed to environmental airbone toxicants such as ozone (O3). Since pulmonary macrophages (M phi) play a pivotal role in host pulmonary immunocompetence, their function in this regard may be compromised by pollutant exposure thereby giving rise to an increased incidence of pulmonary disease. The current in vitro study was designed to provide some insight into possible mechanisms by which O3 induces decreased host pulmonary resistance against microbial pathogens. Specifically, this study investigated the impact of an acute O3 exposure upon the ability of a cultured mouse M phi cell line (WEHI-3) to interact with, and respond to, the major M phi-activating cytokine, interferon-gamma (IFN gamma). The results of this study indicate that WEHI-3 exposure to 1 ppm O3 for 4 h reduced both the binding of, and responsivity to, IFN gamma. Among the functional parameters affected by this inability to properly bind/respond to IFN gamma were: reactive oxygen intermediate production, phagocytic activity, and cellular calcium ion elevation; IFN gamma-enhanced expression of surface histocompatibility antigens was unaffected by O3 exposure. The reduced activity of any one of these critical M phi functions could provide a basis for previously-documented increases in microbial pathogen survival in the lungs, and overall compromise of host health following O3 exposure.

Assessment of toxicologic interactions resulting from acute inhalation exposure to sulfuric acid and ozone mixtures

Studies examining effects of air pollutants often use single compounds, while "real world" exposures are to more than one chemical. Thus, it is necessary to assess responses following inhalation of chemical mixtures. Rabbits were exposed for 3 hr to sulfuric acid aerosol at 0, 50, 75, or 125 micrograms/m3 in conjunction with ozone at 0, 0.1, 0.3, or 0.6 ppm, following which broncho-pulmonary lavage was performed. Various pulmonary response endpoints related to general cytotoxicity and macrophage function were examined. In addition, a goal of the study was to define an improved approach to the analysis of data sets involving binary pollutant mixtures. Results were evaluated using analysis of variance with multiple linear contrasts to determine the significance of any effect in the pollutant-exposed groups compared to sham control animals and to assess the type, and extent, of any toxicological interaction between acid and ozone. Interaction was considered to occur when the effects of combined exposure were either significantly greater or less than additive. Pollutant exposures had no effect on lavage fluid levels of lactate dehydrogenase, prostaglandins E2 and F2 alpha, nor on the numbers, viability, or types of immune cells recovered by lavage. Phagocytic activity of macrophages was depressed at the two highest acid levels and at all levels of ozone. Exposure to all mixtures showed significant antagonism. Superoxide production by stimulated macrophages was depressed by acid exposure at the two highest concentrations, while ozone alone had no effect. Significant antagonistic interaction was observed following exposure to mixtures of 75 or 125 micrograms/m3 acid with 0.1 or 0.3 ppm ozone. The activity of tumor necrosis factor elicited from stimulated macrophages was depressed by acid at 75 and 125 micrograms/m3 while ozone had no effect. Exposure to mixtures of 125 micrograms/m3 acid with 0.3 or 0.6 ppm ozone resulted in synergistic interaction. This study provided additional evidence for antagonism between two common air pollutants and demonstrated that the type of interaction between sulfuric acid and ozone depended upon the endpoint but that the magnitude of any interaction was not always related to the exposure concentrations of the constituent pollutants.

Immunomodulating effects of ozone on macrophage functions important for tumor surveillance and host defense

Ozone (O3) is a toxic gaseous pollutant that has been implicated in laboratory studies as a potential lung carcinogen or cocarcinogen in mice. To begin to assess the role of altered macrophage (M phi) responses as a possible mechanism by which O3 may influence carcinogenesis, we examined the effects of repeated in vivo O3 exposure on pulmonary M phi functional and biochemical activities deemed important in tumor surveillance, and host defense in general. Rabbits were exposed by inhalation to 1 ppm O3 for 3 d (2 h/d) and the lungs were lavaged immediately (t0) and 24 h (t24) after exposure. Results demonstrate that O3 reduced M phi viability and increased the number of neutrophils collected immediately after exposure. Effects of O3 on M phi movement were as follows: random migration was depressed immediately after the final exposure and chemotactic migration increased after 24 h. M phi-mediated cytotoxicity toward xenogeneic tumor cells in vitro was significantly depressed, compared to control, immediately and 24 h after O3 exposure. Release of cytotoxic factors deemed important for mediating tumor cell destruction was also assessed. Spontaneous and stimulated production of tumor necrosis factor, as measured by cytotoxicity toward LM cells (a clone of L-929 mouse fibroblasts), was unaffected by exposure to O3. Zymosan-stimulated production of superoxide anion radical (.O2-) was depressed at t0 and increased at t24; however, no significant effects on H2O2 production by resting or zymosan-stimulated M phi were observed at either time interval. Inhaled toxicants such as O3, which can compromise M phi functions important in tumor surveillance, could potentially alter host susceptibility to pulmonary cancer. Results of this study have important implications for human health, and demonstrate the need for further studies examining the carcinogenic/cocarcinogenic potential of O3.

Modulation of human alveolar macrophage properties by ozone exposure in vitro

We have investigated changes in human alveolar macrophage (HAM) function after exposure in vitro to ozone (O3) (0.1-1.0 ppm for 2-4 hr). The functions studied reflect concern that O3 is detrimental to host defense mechanisms in the bronchoalveolar spaces. Exposure of HAM to O3 caused a concentration-dependent increase in release of prostaglandin E2 (PGE2), an important modulator of inflammation, phagocytosis, and oxidative burst. Although phagocytosis of particulate immune complexes was decreased by O3, we found no change in the quantity of Fc receptors and complement receptors on the HAM surface. Superoxide (O2-) production in response to phorbol ester was reduced after exposure of HAM to O3 while the basal O2- release in response to plastic adherence was not affected. Growth inhibition of the opportunistic yeast Cryptococcus neoformans by HAM was not affected by O3 exposure. The production of inflammatory mediators and immune modulators such as tumor necrosis factor-alpha, interleukin 1, and interleukin 6 were not induced by exposure to O3. However, compared to controls, O3- exposed HAM produced significantly lower levels of these cytokines when stimulated with bacterial lipopolysaccharide (LPS). Two-dimensional gel electrophoretic analysis of proteins made by HAM following in vitro exposure to O3 identified 11 proteins whose rate of synthesis was significantly altered. Thus, these studies show that exposure to O3 alters the functional competence of HAM. While there is a minimal effect on protein expression or synthesis, the responses of HAM to particulate immune complexes, to bacterial LPS, and to PMA are impaired. The release of arachidonic acid and PGE2 suggest that the effect of O3 is primarily targeted to the HAM cell membrane. These changes may ultimately result in increased susceptibility to inhaled infectious agents in the O3-exposed individual.

Species differences in impairment and recovery of alveolar macrophage functions following single and repeated ozone exposures

Effects of single (0.4 ppm for 3, 6, or 12 hr) and repeated (0.4 ppm, 12 hr/day for 3 or 7 days) in vivo ozone exposures on rat and mouse alveolar macrophage functions and cell number were investigated. Single ozone exposure of rats resulted in a small (approximately 15%) decrease in Fc-receptor-mediated phagocytosis and phorbol ester-induced superoxide production by the alveolar macrophages and was followed by recovery above control levels within 12 hr of exposure. Repeated exposures of rats for up to 7 days did not alter alveolar macrophage functions, with the exception of the effects of 3 days of exposure on superoxide production (71 +/- 9% as compared with the controls). In mice, significant changes in alveolar macrophage functions were not observed until 12 hr of exposure (at that timepoint phagocytosis was 74 +/- 2%). Repeated ozone exposures of mice did not cause a further decrease in phagocytosis (at Day 7, 74 +/- 14%). Both after 3 and 7 days of repeated ozone exposure of mice, superoxide production by the alveolar macrophages was inhibited approximately 50%. In rats and mice, repeated ozone exposures led to an increase in the number of alveolar macrophages. In mice, this increase appeared at a later time point (at Day 7 vs Day 3) and was less pronounced (at Day 7, 139 +/- 9% vs 179 +/- 17%) as compared with rats. In summary, our data show that rat and mouse alveolar macrophages have different susceptibilities to both single and repeated in vivo ozone exposures.

Effects of ozone on cellular ATP levels in rat and mouse alveolar macrophages

Hydrogen peroxide (H2O2) is thought to be a major intermediate in the toxicity of ozone. In a previous study we demonstrated that ATP depletion may play an important role in the H2O2-induced inhibition of the phagocytic functions of alveolar macrophages. Ozone exposure can adversely affect the phagocytic capacities of alveolar macrophages. In the present study we investigated whether a decrease in the cellular ATP concentration may underly the effects of ozone on alveolar macrophages. Neither following single (6 and 12 h) exposure nor repeated (12 h/day for 3 and 7 days) exposures of mice or rats to 0.4 ppm ozone, were decreased levels of ATP found in the alveolar macrophages. In contrast, repeated exposures of mice for 7 days to ozone led to a significant increase (1.5-fold) in the ATP content of the alveolar macrophages. In vitro ozone exposure of rat and mouse alveolar macrophages also did not lead to a decrease in the cellular ATP concentration. These results showed that ATP depletion does not play a role in the toxicity mechanism of ozone for alveolar macrophages.

Exposure of humans to ambient levels of ozone for 6.6 hours causes cellular and biochemical changes in the lung

An acute (2 h) exposure of humans to 0.4 ppm ozone initiates biochemical changes in the lung that result in the production of components mediating inflammation and acute lung damage as well as components having the potential to lead to long-term effects such as fibrosis. However, many people are exposed to lower levels of ozone than this, but for periods of several hours. Therefore, it is important to determine if a prolonged exposure to low levels of ozone is also capable of causing cellular and biochemical changes in the lung. Nonsmoking males were randomly exposed to filtered air and either 0.10 ppm ozone or 0.08 ppm ozone for 6.6 h with moderate exercise (40 liters/min). Bronchoalveolar lavage (BAL) was performed 18 h after each exposure, and cells and fluid were analyzed. The BAL fluid of volunteers exposed to 0.10 ppm ozone had significant increases in neutrophils (PMNs), protein, prostaglandin E2 (PGE2), fibronectin, interleukin-6 (IL-6), and lactate dehydrogenase (LDH) compared with BAL fluid from the same volunteers exposed to filtered air. In addition, there was a decrease in the ability of alveolar macrophages to phagocytize yeast via the complement receptor. Exposure to 0.08 ppm ozone resulted in significant increases in PMNs, PGE2, LDH, IL-6, alpha 1-antitrypsin, and decreased phagocytosis via the complement receptor. However, BAL fluid protein and fibronectin were no longer significantly elevated. We conclude that exposure of humans to as low a level as 0.08 ppm for 6.6 h is sufficient to initiate an inflammatory reaction in the lung.

The generation of reactive oxygen-derived species by phagocytes

The generation of reactive oxygen-derived species is one main constituent of the microbicidal activity of professional phagocytes. This process is known as the respiratory or the oxidative burst. It is initiated by a cyanide- and azide-insensitive increase in O2-consumption and the concomitant generation of superoxide radicals catalyzed by a membrane-localized NADPH oxidase which is triggered by an appropriate stimulation of the cells. The generated O2 is converted to hydrogen peroxide, hydroxyl radicals and other reactive products of oxygen which, if released extracellularly (for example in connection with frustrated phagocytosis), are potentially harmful to the tissue. The oxidative burst is not necessarily dependent on phagocytosis, nor is it necessarily associated with degranulation. Therefore, the process constitutes an important independent variable of phagocyte activity, and researches aiming to characterize various forms of airway inflammation may derive valuable information from an examination of the oxidative burst.