Quantification of fibrosis in the lungs of rats using a morphometric method

The health effects of short fiber chrysotile and amphibole asbestos

The potential toxic effects of short chrysotile and amphibole asbestos fibers with lengths <5 to ∼10 µm have been debated over the years. This stems from the large database of epidemiology, toxicology, and in-vitro studies, each of which often provides different information in understanding and differentiating the effects of short fibers. The epidemiology studies in which the cancer potency estimates were based upon relatively high exposure concentrations provide a conservative assessment that shorter fibers would have little if any effect, especially under controlled exposure or environmental conditions that may occur today. The QSAR models have shown that fiber aspect ratio and Mg content are excellent predictors of cancer potency and that short fibers/particles of amphibole would have no effect. The studies of motor vehicle mechanics and in particular workers who serviced chrysotile containing brakes with the majority of the fibers being short provides evidence that motor vehicle mechanics, including workers who were engaged in brake repair, are not at an increased risk of mesothelioma. Several inhalation toxicology studies clearly differentiated that short chrysotile and amphibole asbestos fibers did not produce a significant carcinogenic effect in the lung or pleural cavity. Because of dosing and lack of sensitivity to biosolubility, in vitro studies can be difficult to interpret; however, a number have differentiated short chrysotile and amphibole asbestos fibers from long fibers. Integral to understanding the importance of fiber length in determining possible health effects is an understanding of the biological and physiological function of the respiratory system. Short asbestos fibers, like innocuous dust, can be cleared through the tracheobronchial ciliated mucous transport, phagocytized by macrophages and cleared via the bronchial tree, and can also be removed through the lymphatic system. While the first two methods can remove them from the lung, with lymphatic transport through one-way valves, fibers are removed from the active area of the lung where the fiber-related disease has been shown to develop and can accumulate in lymphatic sumps and lymph nodes. While short asbestos fibers are present in most occupational or environmental exposures, the large body of studies strongly supports that they do not contribute to the health effects of asbestos exposure.

Final results from a 90-day quantitative inhalation toxicology study evaluating the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos: Histopathological examination, confocal microscopy and collagen quantification of the lung and pleural cavity

The final results from this multi-dose, 90-day inhalation toxicology study in the rat with life-time post-exposure observation have shown a significant fundamental difference in pathological response and tumorgenicity between brake dust generated from brake pads manufactured with chrysotile or from chrysotile alone in comparison to the amphiboles, crocidolite and amosite asbestos. The groups exposed to brake dust showed no significant pathological or tumorigenic response in the respiratory track compared to the air control group at exposure concentrations and deposited doses well above those at which humans have been exposed. Slight alveolar/interstitial macrophage accumulation of particles was noted. Wagner grades were 1-2 (1 = control group), similar to the TiO₂ particle control group. Chrysotile was not biopersistent, exhibiting in the lung a deterioration of its matrix which results in breakage into particles and short fibers which can be cleared by alveolar macrophages and which can continue to dissolve. Particle-laden macrophage accumulation was observed, leading to a very-slight interstitial inflammatory response (Wagner grade 1-3). There was no peribronchiolar inflammation, occasional very-slight interstitial fibrosis (Wagner grade 4), and no exposure-related tumorigenic response. The pathological response of crocidolite and amosite compared to the brake dust and chrysotile was clearly differentiated by the histopathology and the confocal analysis. Crocidolite and amosite induced persistent inflammation, microgranulomas, persistent fibrosis (Wagner grades 4), and a dose-related lung tumor response. Confocal microscopy quantified extensive inflammatory response and collagen development in the lung, visceral and parietal pleura as well as pleural adhesions. These results provide a clear foundation for differentiating the innocuous effects of brake dust exposure from the adverse effects following amphibole asbestos exposure.

Evaluation of the dose-response and fate in the lung and pleura of chrysotile-containing brake dust compared to TiO2, chrysotile, crocidolite or amosite asbestos in a 90-day quantitative inhalation toxicology study - Interim results Part 2: Histopathological examination, Confocal microscopy and collagen quantification of the lung and pleural cavity

The interim results from this 90-day multi-dose, inhalation toxicology study with life-time post-exposure observation has shown an important fundamental difference in persistence and pathological response in the lung between brake dust derived from brake-pads manufactured with chrysotile, TiO₂ or chrysotile alone in comparison to the amphiboles, crocidolite and amosite asbestos. In the brake dust exposure groups no significant pathological response was observed at any time. Slight macrophage accumulation of particles was noted. Wagner-scores, were from 1 to 2 (1 = air-control group) and were similar to the TiO₂ group. Chrysotile being biodegradable, shows a weakening of its matrix and breaking into short fibers & particles that can be cleared by alveolar macrophages and continued dissolution. In the chrysotile exposure groups, particle laden macrophage accumulation was noted leading to a slight interstitial inflammatory response (Wagner-score 1-3). There was no peribronchiolar inflammation and occasional very slight interstitial fibrosis. The histopathology and the confocal analyses clearly differentiate the pathological response from amphibole asbestos, crocidolite and amosite, compared to that from the brake dust and chrysotile. Both crocidolite and amosite induced persistent inflammation, microgranulomas, and fibrosis (Wagner-scores 4), which persisted through the post exposure period. The confocal microscopy of the lung and snap-frozen chestwalls quantified the extensive inflammatory response and collagen development in the lung and on the visceral and parietal surfaces. The interim results reported here, provide a clear basis for differentiating the effects from brake dust exposure from those following amphibole asbestos exposure. The subsequent results through life-time post-exposure will follow.

Use of Severity Grades to Characterize Histopathologic Changes

The severity grade is an important component of a histopathologic diagnosis in a nonclinical toxicity study that helps distinguish treatment-related effects from background findings and aids in determining adverse dose levels during hazard characterization. Severity grades should be assigned based only on the extent (i.e., amount and complexity) of the morphologic change in the examined tissue section(s) and be clearly defined in the pathology report for critical lesions impacting study interpretation. However, the level of detail provided and criteria by which severity grades are assigned can vary, which can lead to inappropriate comparisons and confusion when evaluating pathology results. To help address this issue, a Working Group of the Society of Toxicologic Pathology's Scientific and Regulatory Policy Committee was formed to provide a "points to consider" article on the assignment and application of pathology severity grades. Overall, the Working Group supports greater transparency and consistency in the reporting of grading scales and provides recommendations to improve selection of diagnoses requiring more detailed severity criteria. This information should enhance the overall understanding by toxicologic pathologists, toxicologists, and regulatory reviewers of pathology findings and thereby improve effective communication in regulatory submissions.

Mouse models of bleomycin-induced pulmonary fibrosis

Pulmonary fibrosis is a component of many interstitial lung diseases, including idiopathic pulmonary fibrosis, a chronic, progressive disease for which there is currently no effective therapy. Bleomycin has been widely used in rodents to model pulmonary fibrosis for the study of mechanisms involved in fibrogenesis and for evaluation of potential therapies. Bleomycin induces DNA strand breaks, resulting in pulmonary inflammation, injury, and subsequent interstitial fibrosis. This unit describes methods for delivering bleomycin, either directly into the lung or systemically, to create models of pulmonary fibrosis in rodents. Also described is a rapid and easy procedure for measuring lung collagen content to quantify the severity of fibrosis.

The pathological response and fate in the lung and pleura of chrysotile in combination with fine particles compared to amosite asbestos following short-term inhalation exposure: interim results

The pathological response and translocation of a commercial chrysotile product similar to that which was used through the mid-1970s in a joint compound intended for sealing the interface between adjacent wall boards was evaluated in comparison to amosite asbestos. This study was unique in that it presents a combined real-world exposure and was the first study to investigate whether there were differences between chrysotile and amosite asbestos fibers in time course, size distribution, and pathological response in the pleural cavity. Rats were exposed by inhalation 6 h/day for 5 days to either sanded joint compound consisting of both chrysotile fibers and sanded joint compound particles (CSP) or amosite asbestos. Subgroups were examined through 1-year postexposure. No pathological response was observed at any time point in the CSP-exposure group. The long chrysotile fibers (L > 20 microm) cleared rapidly (T(1/2) of 4.5 days) and were not observed in the pleural cavity. In contrast, a rapid inflammatory response occurred in the lung following exposure to amosite resulting in Wagner grade 4 interstitial fibrosis within 28 days. Long amosite fibers had a T(1/2) > 1000 days and were observed in the pleural cavity within 7 days postexposure. By 90 days the long amosite fibers were associated with a marked inflammatory response on the parietal pleural. This study provides support that CSP following inhalation would not initiate an inflammatory response in the lung, and that the chrysotile fibers present do not migrate to, or cause an inflammatory response in the pleural cavity, the site of mesothelioma formation.

Standardized quantification of pulmonary fibrosis in histological samples

The Ashcroft scale for the evaluation of bleomycin-induced lung fibrosis is the analysis of stained histological samples by visual assessment. Based on the knowledge that this procedure is not standardized in animals and results are highly variable, we hypothesized that modification of this method may improve quantification of lung fibrosis in small animals. To prove our hypothesis, we evaluated pulmonary fibrosis in Lewis rats induced by a single intratracheal injection of 0.3 mg/kg body weight bleomycin (n = 13) compared with the same amount of saline in a control group (n = 4). We modified the Ashcroft scale by precisely defining the assignment of grades from 0 to 8 for the increasing extent of fibrosis in lung histological samples. Thirty-two observers were randomly assigned to evaluate 108 photographs of slides using either the Ashcroft scale or the modified scale. Consistent with our hypothesis, there was a significant reduction in the variability of standard deviations with the modified scale compared with the Ashcroft scale (mean of variability 0.25 versus 0.62, P < 0.0001). Applying the kappa index, the Ashcroft scale showed only a fair to moderate agreement (0.23-0.59) between the observers and a low intra-observer agreement (0.51-0.74) in contrast to the modified scale, which demonstrated a moderate to good agreement between the observers (0.65-0.93, P < 0.0001) and a high intra-observer agreement (0.87-0.91, P < 0.05). To test the modified scale in vivo, we compared both scales with the results of computed tomography (CT) of the lungs obtained from the same mice. In agreement, the modified scale demonstrated a better correlation to CT scans (R = 0.58) compared with the Ashcroft scale (R = 0.33). In summary, quantification of lung fibrosis in histological lung sections using the modified scale is reliable and reproducible.

The toxicological response of Brazilian chrysotile asbestos: a multidose subchronic 90-day inhalation toxicology study with 92-day recovery to assess cellular and pathological response

Inhalation toxicology studies with chrysotile asbestos have in the past been performed at exceedingly high doses without consideration of fiber number or dimensions. As such, the exposures have exceeded lung overload levels, making quantitative assessment of these studies difficult if not impossible. To assess the cellular and pathological response in the rat lung to a well-characterized aerosol of chrysotile asbestos, a 90-day subchronic inhalation toxicology study was performed using a commercial Brazilian chrysotile (CA 300). The protocol was based on that established by the European Commission for the evaluation of synthetic vitreous fibers. The study was also designed to assess the potential for reversibility of any such changes and to permit association of responses with fiber dose in the lung and the influence of fiber length. Wistar male rats were randomly assigned to an air control group and to 2 CA 300 exposure groups at mean fiber aerosol concentrations of 76 fibers L > 20 microm/cm3 (3413 total fibers/cm3; 536 WHO fibers/cm3) or 207 fibers L > 20 microm/cm3 (8941 total fibers/cm3; 1429 WHO fibers/cm3). The animals were exposed using a flow-past, nose-only exposure system for 5 days/wk, 6 h/day, during 13 consecutive weeks (65 exposures), followed by a subsequent nonexposure period lasting for 92 days. Animals were sacrificed after cessation of exposure and after 50 and 92 days of nonexposure recovery. At each sacrifice, subgroups of rats were assessed for the determination of the lung burden; histopathological examination; cell proliferation response; bronchoalveolar lavage with the determination of inflammatory cells; clinical biochemistry; and for analysis by confocal microscopy. Through 90 days of exposure and 92 days of recovery, chrysotile at a mean exposure of 76 fibers L > 20 microm/cm3 (3413 total fibers/cm3) resulted in no fibrosis (Wagner score 1.8 to 2.6) at any time point. The long chrysotile fibers were observed to break apart into small particles and smaller fibers. In vitro modeling has indicated that these particles are essentially amorphous silica. At an exposure concentration of 207 fibers L > 20 microm/cm3 (8941 total fibers/cm3) slight fibrosis was observed. In comparison with other studies, chrysotile produced less inflammatory response than the biosoluble synthetic vitreous fiber CMS. As predicted by the recent biopersistence studies on chrysotile, this study clearly shows that at that at an exposure concentration 5000 times greater than the U.S. threshold limit value of 0.1 f(WHO)/cm3, chrysotile produces no significant pathological response.