Influence of preexisting pulmonary emphysema on susceptibility of rats to inhaled diesel exhaust

STP Position Paper

The Scientific and Regulatory Policy Committee of the Society of Toxicologic Pathology (STP) appointed a working group to address risk assessment for increases in alveolar macrophages following inhalation of pharmaceutical materials. This position paper provides recommendations for inhalation study–specific terminology and interpretation based on literature and information from marketed inhaled drugs. Based on a weight-of-the-evidence approach, and with appropriate consideration of the physical and pharmacological characteristics of the compound, uncomplicated increases in the size or number of alveolar macrophages in nonclinical species are interpreted as nonadverse.

Susceptibility of the aging lung to environmental injury

With an ever-increasing number of elderly individuals in the world, a better understanding of the issues associated with aging and the environment is needed. The respiratory system is one of the primary interfaces between the body and the external environment. An expanding number of studies suggest that the aging pulmonary system (>65 years) is at increased risk for adverse health effects from environmental insult, such as by air pollutants, infection, and climate change. However, the mechanism(s) for increased susceptibility in this subpopulation are not well understood. In this review, we provide a limited but comprehensive overview of how the lung ages, examples of environmental exposures associated with injury to the aging lung, and potential mechanisms underlying the increased vulnerability of the aging lung to injury from environmental factors.

Carcinogenicity studies of diesel engine exhausts in laboratory animals: a review of past studies and a discussion of future research needs

Diesel engines play a vital role in world economy, especially in transportation. Exhaust from traditional diesel engines using high-sulfur fuel contains high concentrations of respirable carbonaceous particles with absorbed organic compounds. Recognition that some of these compounds are mutagenic has raised concern for the cancer-causing potential of diesel exhaust exposure. Extensive research addressing this issue has been conducted during the last three decades. This critical review is offered to facilitate an updated assessment of the carcinogenicity of diesel exhaust and to provide a rationale for future animal research of new diesel technology. Life-span bioassays in rats, mice, and Syrian hamsters demonstrated that chronic inhalation of high concentrations of diesel exhaust caused lung tumors in rats but not in mice or Syrian hamsters. In 1989, the International Agency for Research on Cancer (IARC) characterized the rat findings as "sufficient evidence of animal carcinogenicity," and, with "limited" evidence from epidemiological studies, classified diesel exhaust Category 2A, a "probable human carcinogen." Subsequent research has shown that similar chronic high concentration exposure to particulate matter generally considered innocuous (such as carbon black and titanium dioxide) also caused lung tumors in rats. Thus, in 2002, the U.S. Environmental Protection Agency (EPA) concluded that the findings in the rats should not be used to characterize the cancer hazard or quantify the cancer risk of diesel exhaust. Concurrent with the conduct of the health effects studies, progressively more stringent standards have been promulgated for diesel exhaust particles and NOx. Engine manufacturers have responded with new technology diesel (improved engines, fuel injection, fuels, lubricants, and exhaust treatments) to meet the standards. This review concludes with an outline of research to evaluate the health effects of the new technology, research that is consistent with recommendations included in the U.S. EPA 2002 health assessment document. When this research has been completed, it will be appropriate for IARC to evaluate the potential cancer hazard of the new technology diesel.

Pulmonary inflammatory response to inhaled ultrafine particles is modified by age, ozone exposure, and bacterial toxin

Epidemiological studies demonstrate associations between increasing levels of ambient particles and morbidity in the elderly with cardiopulmonary disease. Such findings have been challenged partly because particles may not act alone to cause these effects. We hypothesized that carbonaceous ambient ultrafine particles and ozone can act together to induce greater oxidative stress and inflammation in the lung than when administered alone and that these effects would be amplified in the compromised, aging lung. Two models of a compromised lung were used: endotoxin priming and old-age emphysema (TSK mice). Young (10 wk) and old (22 mo) male F344 rats and male TSK mice (14-17 mo) were exposed to ultrafine carbon particles (count median diameter 25 nm, 110 micrograms/m3) and to ozone (1 ppm) alone and in combination for 6 h. Inhalation of low-dose endotoxin (70 and 7.5 units estimated alveolar deposited dose in rats and mice, respectively) was used to model respiratory-tract infection. Cellular and biochemical lavage parameters and oxidant release from lung lavage cells were assessed 24 h after exposure. Inflammatory cell influx into the alveolar space was observed for both species and age groups: The combination of inhaled ultrafine carbon and ozone after endotoxin priming resulted in the greatest increase in lavage-fluid neutrophils. In general, the unstimulated and stimulated release of reactive oxygen species (ROS) from lavage inflammatory cells correlated well with the neutrophil response. There were significant effects of carbon particles as well as a consistent interaction between carbon and ozone as determined by analysis of variance (ANOVA). However, this interaction was in the opposite direction in young rats versus old rats and old TSK mice: Carbon and ozone interacted such that ROS activity was depressed in young rats, whereas it was enhanced in old rats and old TSK mice, indicating age-dependent functional differences in elicited pulmonary inflammatory cells. These results demonstrate that ultrafine carbonaceous particles inhaled for short periods of time can induce significant pulmonary inflammation and oxidative stress that are modified by age, copollutants, and a compromised respiratory tract.

Animal models of emphysema and their relevance to studies of particle-induced disease

Emphysema is a pulmonary disease that may be exacerbated by inhaled particles. Over the years, many animal models of emphysema have been developed that may be useful in studying the effects of inhaled particles on humans with emphysema. Models have been described in many species, and many approaches have been described for inducing emphysema. Emphysema in humans is a parenchymal component of chronic obstructive pulmonary disease and frequently coexists in a complex with disease of the airways such as bronchitis. Animal models of emphysema usually recapitulate only one or a few aspects of this complex disease. Thus, the emphysema model must be selected carefully in order to answer specific questions about the interactive effects of particles and emphysema.

Toxic responses of the lung to inhaled pollutants: benefits and limitations of lung-disease models

The widely accepted notion that certain individuals are more susceptible to air pollutants than others has been revitalized by recent epidemiology that strongly suggests that the elderly, particularly those with underlying cardiopulmonary diseases (e.g. chronic obstructive pulmonary disease (COPD), infection), and children with asthma are more susceptible to the adverse outcomes associated with ambient particulate matter (PM). Pulmonary toxicologists have adopted 'susceptibility' as an issue that can be approached experimentally and have begun to develop as well as study more relevant animal models. These models may have specific genetic traits or cardiopulmonary impairments analogous to human diseases. The goal is to identify potential susceptibility characteristics and elucidate whether responsiveness is due to impair compensation or some unique mechanisms. Several rodent models have been used with PM: pulmonary vasculitis, bronchitis, COPD, allergic asthma, infectious lung diseases, systemic hypertension, and congestive heart disease. Transgenic and knockout mice are of growing interest but have seen limited use in air pollutants studies, with primary interest being directed to specific mechanistic questions. No model should be used without careful consideration of its strengths and limitations. However, when interpreted in the context of field and epidemiology findings, they may reveal generic susceptibility attributes or useful biomarkers.

Impaired pulmonary inflammatory responses are a prominent feature of streptococcal pneumonia in mice with experimental emphysema

Little is known about why patients with chronic obstructive pulmonary disease are susceptible to bacterial infections. Using an animal model of pulmonary emphysema, we investigated the inflammatory responses to bacterial infection. After intratracheal infection with Streptococcus pneumoniae (10(3)-10(7) cfu/mouse), the control mice did not die. However, the mice with emphysema died in a dose-dependent manner. Bronchoalveolar lavage fluid, examined 24 hours after infection showed that the numbers of total cells and neutrophils, in addition to murine tumor necrosis factor-alpha and macrophage inflammatory protein-2 concentrations, were significantly less in the mice with emphysema compared with the control mice. Histopathologic findings revealed that the alveoli were filled with inflammatory cells and exudate in the control mice but not in the mice with emphysema. Seventy-two hours after infection, serum cytokine levels were significantly higher in the mice with emphysema, and significant numbers of S. pneumoniae were detected in both the whole lung tissues and the blood of mice with emphysema. These findings suggest that the inflammatory response in mice with emphysema was impaired at the site of bacterial infection despite the bacteremia, which accelerated severe systemic inflammatory responses. Accordingly, intra-alveolar but not systemic immune responses to bacterial infection were impaired in the presence of experimental emphysema.

Rodent models of susceptibility: what is their place in inhalation toxicology?

There is renewed interest in inhalation toxicology regarding 'susceptibility' as associated with host variables, including genetics, age, diet, and disease. This interest derives from epidemiology that shows air pollution-related human mortality/morbidity, especially among individuals with cardiopulmonary disease. Several animal models with experimental or genetically-based cardiopulmonary diseases are now being incorporated into inhalation toxicology studies to investigate mechanisms that underlie host susceptibility. However, current models have strengths and limitations as to how they mimic the essential features of human diseases. To date, animal models of pulmonary hypertension, bronchitis, asthma, and cardiovascular disease, but not emphysema, appear to exhibit greater susceptibility to air pollution particulate matter. As in humans, host susceptibility appears to involve multiple genetic and environmental factors, and is poorly understood, but the database of information is growing rapidly. As existing models gain wider use, our understanding of the models will improve and encourage refinements/development of models that integrate both genetic and environmental factors to better mimic the human condition.

Health effects of diesel exhaust emissions

Epidemiological studies have demonstrated an association between different levels of air pollution and various health outcomes including mortality, exacerbation of asthma, chronic bronchitis, respiratory tract infections, ischaemic heart disease and stroke. Of the motor vehicle generated air pollutants, diesel exhaust particles account for a highly significant percentage of the particles emitted in many towns and cities. This review is therefore focused on the health effects of diesel exhaust, and especially the particular matter components. Acute effects of diesel exhaust exposure include irritation of the nose and eyes, lung function changes, respiratory changes, headache, fatigue and nausea. Chronic exposures are associated with cough, sputum production and lung function decrements. In addition to symptoms, exposure studies in healthy humans have documented a number of profound inflammatory changes in the airways, notably, before changes in pulmonary function can be detected. It is likely that such effects may be even more detrimental in asthmatics and other subjects with compromised pulmonary function. There are also observations supporting the hypothesis that diesel exhaust is one important factor contributing to the allergy pandemic. For example, in many experimental systems, diesel exhaust particles can be shown to act as adjuvants to allergen and hence increase the sensitization response. Much of the research on adverse effects of diesel exhaust, both in vivo and in vitro, has however been conducted in animals. Questions remain concerning the relevance of exposure levels and whether findings in such models can be extrapolated into humans. It is therefore imperative to further assess acute and chronic effects of diesel exhaust in mechanistic studies with careful consideration of exposure levels. Whenever possible and ethically justified, studies should be carried out in humans.

Animal models for the effect of age on susceptibility to inhaled particulate matter

Epidemiological findings of associations between ambient particulate matter (PM) and respiratory and cardiovascular mortality and morbidity have fostered increased laboratory research aimed at understanding the key PM components, mechanisms, and dose-response relationships responsible for the effects. Because the health impacts are largely observed in subpopulations having characteristics known or presumed to confer increased susceptibility to PM, there is a need for identifying, developing, and using animal models of these susceptibility factors. Age, during both development and senescence of the cardiorespiratory system and its defenses, is one of the PM susceptibility factors cited frequently. This review is intended as a summary of current knowledge regarding age-related differences in the structure and function of the respiratory and pulmonary vascular systems of humans and animals. Its purpose is to facilitate the selection of appropriate animal models for research on the various facets of potential age-related susceptibility of the human respiratory tract to the effects of inhaled PM. The selection of models is a difficult challenge because no single animal species adequately models the full range of human respiratory anatomy, physiology, and age-related changes. With careful selection among the many species, strains, and comparative ages, however, animals can be selected to model most, if not all, of the individual factors hypothesized to confer increased susceptibility of humans to inhaled PM. The existing information does not provide an adequate basis for selecting models to test all of the current age-related susceptibility hypotheses. However, the information summarized in this report should facilitate the investigator's review of potential models.

Effect on single-breath washout and lung function of elastase-induced emphysema in rats

Lung volumes, diffusing capacity (DLCO), quasi-static pressure-volume curves (P-V), forced expiration (FE) and He-SF6 single-breath washout (SBW) were performed in Wistar rats with emphysema induced by different doses of pancreatic elastase in saline, instilled intratracheally 6 wk prior to the tests. Emphysema was quantitatively assessed by mean linear intercept (Lm) measurements on 5-microns lung sections. Lung volume, P-V curve, and FE dependence on Lm, as well as the nonsignificant dependence of DLCO on Lm, are generally similar to results reported by others. The most interesting observation concerns the SBW: N2 slopes of the alveolar plateau, compared for identical lung volumes, did not change with the degree of emphysema. By contrast, the He-SF6 slope difference did depend significantly on the degree of emphysema. Based on the diffusion front theory, the present work suggests that in rats with elastase-induced emphysema, the phase III slope modifications relate mainly to elastic and not to structural alterations.