Autoantibodies from mice exposed to Libby amphibole asbestos bind SSA/Ro52-enriched apoptotic blebs of murine macrophages

Amphibole asbestos as an environmental trigger for systemic autoimmune diseases

A growing body of evidence supports an association between systemic autoimmune disease and exposure to amphibole asbestos, a form of asbestos typically with straight, stiff, needle-like fibers that are easily inhaled. While the bulk of this evidence comes from the population exposed occupationally and environmentally to Libby Amphibole (LA) due to the mining of contaminated vermiculite in Montana, studies from Italy and Australia are broadening the evidence to other sites of amphibole exposures. What these investigations have done, that most historical studies have not, is to evaluate amphibole asbestos separately from chrysotile, the most common commercial asbestos in the United States. Here we review the current and historical evidence summarizing amphibole asbestos exposure as a risk factor for autoimmune disease. In both mice and humans, amphibole asbestos, but not chrysotile, drives production of both antinuclear autoantibodies (ANA) associated with lupus-like pathologies and pathogenic autoantibodies against mesothelial cells that appear to contribute to a severe and progressive pleural fibrosis. A growing public health concern has emerged with revelations that a) unregulated asbestos minerals can be just as pathogenic as commercial (regulated) asbestos, and b) bedrock and soil occurrences of asbestos are far more widespread than previously thought. While occupational exposures may be decreasing, environmental exposures are on the rise for many reasons, including those due to the creation of windborne asbestos-containing dusts from urban development and climate change, making this topic an urgent challenge for public and heath provider education, health screening and environmental regulations.

Advances in biological functions and applications of apoptotic vesicles

BACKGROUND

Apoptotic vesicles are extracellular vesicles generated by apoptotic cells that were previously regarded as containing waste or harmful substances but are now thought to play an important role in signal transduction and homeostasis regulation.

METHODS

In the present review, we reviewed many articles published over the past decades on the subtypes and formation of apoptotic vesicles and the existing applications of these vesicles.

RESULTS

Apoptotic bodies were once regarded as vesicles released by apoptotic cells, however, apoptotic vesicles are now regarded to include apoptotic bodies, apoptotic microvesicles and apoptotic exosomes, which exhibit variation in terms of biogenesis, sizes and properties. Applications of apoptotic vesicles were first reported long ago, but such reports have been rarer than those of other extracellular vesicles. At present, apoptotic vesicles have been utilized mainly in four aspects, including in direct therapeutic applications, in their engineering as carriers, in their construction as vaccines and in their utilization in diagnosis.

CONCLUSION

Building on a deeper understanding of their composition and characteristics, some studies have utilized apoptotic vesicles to treat diseases in more novel ways. However, their limitations for clinical translation, such as heterogeneity, have also emerged. In general, apoptotic vesicles have great application potential, but there are still many barriers to overcome in their investigation. Video Abstract.

B-1 cells in immunotoxicology: Mechanisms underlying their response to chemicals and particles

Since their discovery nearly 40 years ago, B-1 cells have continued to challenge the boundaries between innate and adaptive immunity, as well as myeloid and lymphoid functions. This B-cell subset ensures early immunity in neonates before the development of conventional B (B-2) cells and respond to immune injuries throughout life. B-1 cells are multifaceted and serve as natural- and induced-antibody-producing cells, phagocytic cells, antigen-presenting cells, and anti-/pro-inflammatory cytokine-releasing cells. This review retraces the origin of B-1 cells and their different roles in homeostatic and infectious conditions before focusing on pollutants comprising contact-sensitivity-inducing chemicals, endocrine disruptors, aryl hydrocarbon receptor (AHR) ligands, and reactive particles.

Exposure to silicates and systemic autoimmune-related outcomes in rodents: a systematic review

BACKGROUND

Autoimmunity can result from the interplay between genetic background and effects of environmental and/or occupational exposure to hazardous materials. Several compounds, including silica dust, have been linked with systemic autoimmunity and systemic autoimmune diseases, based on epidemiological evidence. For asbestos, a strong link with systemic autoimmune diseases does not yet exist, however, several studies have documented features of autoimmunity following asbestos exposure. Even so, human studies are limited in their ability to identify and examine isolated exposures, making it difficult to demonstrate causation or to assess pathogenic mechanisms. Therefore, this systematic review examines the existing animal evidence regarding autoimmunity and exposure to silicates (silica and asbestos).

METHODS

PubMed and EMBASE were systematically searched for peer-reviewed studies examining systemic autoimmune disease-related outcomes after silicate exposure in rodents. Literature databases were searched up to September 2021 for studies written in English and where the full text was available. Search strings were established based on a PECO (Population, Exposure, Comparator, Outcome) format. After title, abstract, and full-text screening, thirty-four studies were identified for further analysis. Quality assessment through ToxR tool and qualitative analysis of the results was performed.

RESULTS

Although there was significant heterogeneity in the included studies in terms of exposure protocol and genetic background of the rodent models used, it was noted that both genetic background and exposure to silicates [(crystalline) silica and asbestos] are highly relevant to the development of (sub-) clinical systemic autoimmune disease.

CONCLUSION

Parallels were observed between the findings from the animal (this review) and human (epidemiological) studies, arguing that experimental animal models are valuable tools for examining exacerbation or development of autoimmune disease after silicate exposure. However, genetic background and synergism between exposures should be considered in future studies.

Comparative health effects in mice of Libby amphibole asbestos and a fibrous amphibole from Arizona

This project developed from studies demonstrating that Libby Amphibole Asbestos (LAA) causes a non-typical set of health outcomes not generally reported for asbestos, including systemic autoimmunity and an unusual and devastating lamellar pleural thickening that progresses to severe pulmonary dysfunction and death. Further, mineral fiber mixtures with some similarities to LAA have recently been discovered in southern Nevada and northwestern Arizona, where the material exists in extensive recreational areas and is present in yards, roads, parking lots and school yards. The objective was to compare the health outcomes in mice exposed to either LAA or the fibrous amphiboles collected in Arizona at the Lake Mead National Recreational Area at very low doses to represent environmental exposures. In this study, the fibrous amphibole asbestos sample from Arizona (AzA) is composed of winchite (69%), actinolite (22%), and non-amphibole minerals (9%) and has a mean aspect ratio of 16.7±0.9. Fibrous amphibole asbestos from Libby (LAA) is composed of winchite (70%), richterite (9%), tremolite (5%), and non-amphibole minerals (16%) with a mean aspect ratio of 8.4±0.7. C57BL/6 mice were exposed by oropharyngeal aspiration to fiber suspensions at a very low dose of 3μg/mouse. After seven months, both LAA- and AzA-exposed mice had indices of chronic immune dysfunction related to a T_H17 cytokine profile, with B cell activation, autoantibody production and proteinuria, suggesting kidney involvement. In addition, both exposures led to significant lung and pleural fibrosis. These data suggest that there is risk of pulmonary disease and autoimmune outcomes with environmental exposure to amphibole asbestos, and that this is not limited to Libby, Montana.

Silica, Silicosis, and Autoimmunity

Inhalation of dust containing crystalline silica is associated with a number of acute and chronic diseases including systemic autoimmune diseases. Evidence for the link with autoimmune disease comes from epidemiological studies linking occupational exposure to crystalline silica dust with the systemic autoimmune diseases systemic lupus erythematosus, systemic sclerosis, and rheumatoid arthritis. Although little is known regarding the mechanism by which silica exposure leads to systemic autoimmune disease, there is a voluminous literature on silica exposure and silicosis that may help identify immune processes that precede development of autoimmunity. The pathophysiology of silicosis consists of deposition of silica particles in the alveoli of the lung. Ingestion of these particles by macrophages initiates an inflammatory response, which stimulates fibroblasts to proliferate and produce collagen. Silica particles are encased by collagen leading to fibrosis and the nodular lesions characteristic of the disease. The steps in the development of silicosis, including acute and chronic inflammation and fibrosis, have different molecular and cellular requirements, suggesting that silica-induced inflammation and fibrosis may be mechanistically separate. Significantly, it is unclear whether silica-induced inflammation and fibrosis contribute similarly to the development of autoimmunity. Nonetheless, the findings from human and animal model studies are consistent with an autoimmune pathogenesis that begins with activation of the innate immune system leading to proinflammatory cytokine production, pulmonary inflammation leading to activation of adaptive immunity, breaking of tolerance, and autoantibodies and tissue damage. The variable frequency of these immunological features following silica exposure suggests substantial genetic involvement and gene/environment interaction in silica-induced autoimmunity. However, numerous questions remain unanswered.

Pesticides, chemical and industrial exposures in relation to systemic lupus erythematosus

Growing evidence suggests exposure to chemicals and industrial pollutants may increase risk of systemic lupus erythematosus (SLE). Here we review research on SLE associations with occupational and industrial exposures, primarily drawing on studies in human populations and summarizing epidemiologic research published in the past decade. The association of occupational silica exposure with SLE is well established, but key questions remain, including the required dose and susceptibility factors, and SLE risk due to other silicate exposures. Research on SLE and other exposures is less well developed, though several potential associations merit further consideration because of the consistency of preliminary human findings, experimental animal research, and biologic plausibility. These include pesticides and solvents, for which experimental findings also support investigation of specific agents, including organochlorines and trichloroethylene. Experimental findings and biologic plausibility suggest research on SLE and occupational exposure to hydrocarbons (i.e. mineral oils) is warranted, especially given the widespread exposures in the population. Experimental and limited human findings support further investigation of SLE related to mercury exposure, especially in dental occupations. Research on environmental risk factors in risk-enriched cohorts (family-based) is recommended, as is further investigation of exposures in relation to intermediate markers of effect (e.g. antinuclear antibodies), clinical features (e.g. nephritis), and outcomes.

Autoimmunity and asbestos exposure

Despite a body of evidence supporting an association between asbestos exposure and autoantibodies indicative of systemic autoimmunity, such as antinuclear antibodies (ANA), a strong epidemiological link has never been made to specific autoimmune diseases. This is in contrast with another silicate dust, crystalline silica, for which there is considerable evidence linking exposure to diseases such as systemic lupus erythematosus, systemic sclerosis, and rheumatoid arthritis. Instead, the asbestos literature is heavily focused on cancer, including mesothelioma and pulmonary carcinoma. Possible contributing factors to the absence of a stronger epidemiological association between asbestos and autoimmune disease include (a) a lack of statistical power due to relatively small or diffuse exposure cohorts, (b) exposure misclassification, (c) latency of clinical disease, (d) mild or subclinical entities that remain undetected or masked by other pathologies, or (e) effects that are specific to certain fiber types, so that analyses on mixed exposures do not reach statistical significance. This review summarizes epidemiological, animal model, and in vitro data related to asbestos exposures and autoimmunity. These combined data help build toward a better understanding of the fiber-associated factors contributing to immune dysfunction that may raise the risk of autoimmunity and the possible contribution to asbestos-related pulmonary disease.

Requirements for innate immune pathways in environmentally induced autoimmunity

There is substantial evidence that environmental triggers in combination with genetic and stochastic factors play an important role in spontaneous autoimmune disease. Although the specific environmental agents and how they promote autoimmunity remain largely unknown, in part because of diverse etiologies, environmentally induced autoimmune models can provide insights into potential mechanisms. Studies of idiopathic and environmentally induced systemic autoimmunity show that they are mediated by common adaptive immune response genes. By contrast, although the innate immune system is indispensable for autoimmunity, there are clear differences in the molecular and cellular innate components that mediate specific systemic autoimmune diseases, suggesting distinct autoimmune-promoting pathways. Some of these differences may be related to the bifurcation of toll-like receptor signaling that distinguishes interferon regulatory factor 7-mediated type I interferon production from nuclear factor-κB-driven proinflammatory cytokine expression. Accordingly, idiopathic and pristane-induced systemic autoimmunity require both type I interferon and proinflammatory cytokines whereas the less aggressive mercury-induced autoimmunity, although dependent on nucleic acid-binding toll-like receptors, does not require type I interferon but needs proinflammatory cytokines. Scavenger receptors and the inflammasome may contribute to silica-induced autoimmunity. Greater understanding of the innate mechanisms responsible for idiopathic and environmentally induced autoimmunity should yield new information into the processes that instigate and drive systemic autoimmunity.

Evaluation of anti-nuclear antibodies and kidney pathology in Lewis rats following exposure to Libby amphibole asbestos

The prevalence of anti-nuclear antibodies (ANA) and self-reported systemic autoimmune diseases were increased in residents of Libby, MT, as was the incidence of ANA in Lewis rats exposed to Libby amphibole (LA) asbestos. However, rats induced to develop rheumatoid arthritis (RA) did not develop autoantibodies associated with RA, nor was RA exacerbated by LA exposure, suggesting that increased ANA expression might be related to some other autoimmune process. Libby residents self-reported increased numbers of physician-diagnosed cases of systemic lupus erythematosus (SLE). Thus, the goal of this study was to determine if the increased incidence of ANA in Lewis rats exposed to LA is related to the development of SLE-like disease. Female Lewis rats were intratracheally instilled bi-weekly for 13 weeks with total doses of 0.15, 0.5, 1.5, or 5.0 mg of LA or 0.5 or 1.5 mg of a positive control fiber, amosite. ANA incidence was significantly increased in all asbestos dose groups, although no dose response was observed. The occurrence of proteinuria was increased in LA 0.5, LA 5.0, and amosite 0.5 dose groups; however, the microscopic appearance of the kidneys was normal, no binding of autoimmune complexes to glomerular surfaces was observed, and antibodies to double-stranded DNA were not elevated. Therefore, an increased prevalence of ANA in rats exposed to asbestos does not appear to correlate with disease markers typically observed in SLE. Analysis of ANA specificity for extractable nuclear antigens (ENA) determined that 98% of ENA(+) samples were specific for the Jo-1 antigen. Autoantibodies to Jo-1 have been reported in patients with interstitial lung disease, suggesting that autoantibodies to Jo-1 may be a biomarker for asbestos-related pulmonary disease.

Ro52/SSA sensitizes cells to death receptor-induced apoptosis by down-regulating c-FLIP(L)

Ro52/SSA is an autoantigen that presents in patients with SS (Sjögren's syndrome) and SLE (systemic lupus erythematosus). It increases cell death and redistributes itself to apoptotic blebs, but its pro-apoptotic function has not been completely identified. Overexpression of Ro52/SSA promoted cell apoptosis induced by DR (death receptor) in caspase-8-dependent manner. Ro52/SSA expression down-regulated c-FLIP(L) [cellular (Fas-associated death domain)-like interleukin 1β-converting enzyme-inhibitory protein long form] expression, and Ro52/SSA siRNAs (small interfering RNAs) increased c-FLIP(L) production, indicating that Ro52/SSA plays a role in c-FLIP(L) regulation. Ro52/SSA negatively regulated c-FLIP(L) transcriptional level probably by suppressing NF-κB (nuclear factor κB) signalling. The data suggest that Ro52/SSA is involved in DR-mediated apoptosis by regulating c-FLIP(L).

Functional expression of system x(c)- is upregulated by asbestos but not crystalline silica in murine macrophages

CONTEXT

Inhalation of asbestos or silica is associated with chronic and progressive diseases, including fibrosis, cancer, and increased risk of systemic autoimmunity. Because there is a need for treatment options for these diseases, a better understanding of their mechanistic etiologies is essential. While oxidative stress in macrophages is an early consequence of these exposures, it may also serve as a signaling mechanism involved in downstream immune dysregulation. The system x(c)(-) exchange protein is induced by oxidative stress, and exchanges equimolor levels of extracellular cystine for intracellular glutamate. Cystine is subsequently reduced to cysteine, the rate-limiting precursor for glutathione synthesis.

OBJECTIVE

As the primary transporter responsible for cystine/glutamate exchange on macrophages, system x(c)- was hypothesized to be inducible in response to asbestos and silica, and to increase viability through protection from oxidative stress.

RESULTS

When challenged with amphibole asbestos, but not crystalline silica, RAW 264.7 macrophages increased expression of xCT and the rate of cystine/glutamate exchange in sodium-free conditions. This upregulation was prevented with N-acetylcysteine, implicating oxidative stress. Cystine protected the macrophages from asbestos-induced oxidative stress and cell death, supporting the hypothesis that imported cystine was used for synthesis of cellular antioxidants. System x(c)(-) inhibitors, glutamate and S-4-carboxyphenylglycine ((S)-4-CPG), significantly increased oxidative stress and cell death of asbestos-treated macrophages.

CONCLUSION

System x(c)(-) plays a critical role in survival of macrophages exposed to asbestos, but not silica. These data demonstrate a very early difference in the cellular response to these silicates that may have important downstream implications in the pathologic outcome of exposure.

Mesothelial cell and anti-nuclear autoantibodies associated with pleural abnormalities in an asbestos exposed population of Libby MT

Despite data linking amphibole asbestos exposure with production of autoantibodies, the role of autoantibodies in subsequent disease is unknown. Residents of Libby, Montana have experienced significant exposure to amphibole asbestos due to the mining of asbestos-contaminated vermiculite near the community over several decades. This population predominantly exhibits pleural disease, and an autoimmune-like disorder that has yet to be well defined. This study sought to determine whether autoantibodies from asbestos-exposed subjects were associated with pleural lesions. Serum samples of subjects from Libby were evaluated for anti-nuclear antibodies (ANA) and mesothelial cell autoantibodies (MCAA) using cell based ELISA. The presence of radiographic abnormalities detected during the time frame of serum collection was determined from screening records. In accord with previous studies, 61.3% (76/124) of the Libby samples were ANA positive, a frequency much higher than expected for a healthy population. The odds of having pleural or interstitial abnormalities in Libby was nearly 3.55 times greater for individuals that tested positive for ANA compared with individuals negative for ANA (p=0.004). MCAA were also detected at a strikingly high frequency (18.5%; 23/124) in samples from Libby. Individuals with MCAA had 4.9 times the risk of having pleural abnormalities compared to MCAA-negative subjects (p=0.044). In conclusion, ANA and MCAA were elevated in a study population that was known to have chronic exposure to asbestos, and these autoantibodies were associated with pleural abnormalities, the predominant finding in the asbestos-exposed population of Libby. Additional research is needed to determine the role these autoantibodies may play in pulmonary disease.

Effects of Libby amphibole asbestos exposure on two models of arthritis in the Lewis rat

Epidemiological data suggest that occupational exposure to the amphibole-containing vermiculite in Libby, MT, was associated with increased risk for developing autoimmune diseases and had an odds ratio of 3.23 for developing rheumatoid arthritis (RA). The collagen-induced arthritis (CIA) and the peptidoglycan-polysaccharide (PG-PS) models of RA were employed to determine whether exposure to Libby amphibole (LA) induced a more rapid onset, increased expression, or prolonged course of RA. Female Lewis rats were intratracheally instilled with total doses of 0.15, 0.5, 1.5, or 5 mg LA or 0.5 or 1.5 mg amosite asbestos, and arthritis was induced with either the PG-PS or CIA model. Neither LA nor amosite exposure affected the disease course in the CIA model, or the production of rheumatoid factor (RF) or anti-cyclic citrullinated peptide (CCP) antibodies. LA exposure reduced swelling in the PG-PS model and decreased anti-PG-PS and total immunoglobulin M (IgM) antibody titers. Both amosite and LA exposure increased the number of rats with circulating anti-nuclear antibodies (ANA), the majority of which presented a speckled staining pattern. However, this ANA enhancement was not dose responsive. These results failed to show a positive correlation between LA exposure and RA disease in two animal models, although upregulated ANA suggest an altered immunological profile consistent with other systemic autoimmune diseases.

Long-term response of rats to single intratracheal exposure of Libby amphibole or amosite

In former mine workers and residents of Libby, Montana, exposure to amphibole-contaminated vermiculite has been associated with increased incidences of asbestosis and mesothelioma. In this study, long-term effects of Libby amphibole (LA) exposure were investigated relative to the well-characterized amosite asbestos in a rat model. Rat-respirable fractions of LA and amosite (aerodynamic diameter≤2.5 μm) were prepared by water elutriation. Male F344 rats were exposed to a single dose of either saline, amosite (0.65 mg/rat), or LA (0.65 or 6.5 mg/rat) by intratracheal (IT) instillation. One year after exposure, asbestos-exposed rats displayed chronic pulmonary inflammation and fibrosis. Two years postexposure, lung inflammation and fibrosis progressed in a time- and dose-dependent manner in LA-exposed rats, although the severity of inflammation and fibrosis was smaller in magnitude than in animals exposed to amosite. In contrast, gene expression of the fibrosis markers Col 1A2 and Col 3A1 was significantly greater in LA-exposed compared to amosite-exposed rats. There was no apparent evidence of preneoplastic changes in any of the asbestos-exposed groups. However, all asbestos-exposed rats demonstrated a significant increase in the expression of epidermal growth factor receptor (EGFR) 2 yr after instillation. In addition, only LA-exposed rats showed significant elevation in mesothelin (Msln) and Wilms' tumor gene (WT1) expression, suggesting possible induction of tumor pathways. These results demonstrate that a single IT exposure to LA is sufficient to induce significant fibrogenic, but not carcinogenic, effects up to 2 yr after exposure that differ both in quality and magnitude from those elicited by amosite administration at the same mass dose in F344 rats. Data showed that LA was on a mass basis less potent than amosite.

Meeting report: mode(s) of action of asbestos and related mineral fibers

BACKGROUND

Although asbestos in general is well known to cause a range of neoplastic and non-neoplastic human health effects, not all asbestos fiber types have the same disease-causing potential, and the mode of action (MOA) of specific types of asbestos and related fibers for various health outcomes are not well understood.

OBJECTIVES

A workshop was held to discuss the state of the science of the MOA for asbestos-related disease. The objective was to review the range of asbestos-induced health effects (including those at sites remote to the respiratory tract). We sought to identify existing knowledge gaps and define what research is needed to address these gaps and advance asbestos research.

DISCUSSION

Discussions centered on areas of uncertainty in the field, including the ways asbestos is defined and characterized, the role of different fiber characteristics (e.g., length and mineralogy) in disease, and the impact of low-dose exposures on human health. Studying the dosimetry and mode of action of multiple fiber types would enhance our understanding of asbestos-related disease. To better elucidate the MOA of specific asbestos fibers, the risk assessor requires data as to specific characteristics of asbestos in determining fiber toxicity (e.g., surface area, mineral type), which may inform efforts to assess and control exposures and prevent adverse human health outcomes for the diverse range of fiber types. Specific research aims were defined for these topics and for overarching issues to be addressed, including the use of standardized terminology, test materials, and better experimental models to aid in data extrapolation to humans.

CONCLUSION

To resolve these and other issues, participants agreed that diverse scientific disciplines must coordinate to better understand the MOA leading to the various asbestos-related disease end points.

Nonpulmonary outcomes of asbestos exposure

The adverse pulmonary effects of asbestos are well accepted in scientific circles. However, the extrapulmonary consequences of asbestos exposure are not as clearly defined. In this review the potential for asbestos to produce diseases of the peritoneum, immune, gastrointestinal (GIT), and reproductive systems are explored as evidenced in published, peer-reviewed literature. Several hundred epidemiological, in vivo, and in vitro publications analyzing the extrapulmonary effects of asbestos were used as sources to arrive at the conclusions and to establish areas needing further study. In order to be considered, each study had to monitor extrapulmonary outcomes following exposure to asbestos. The literature supports a strong association between asbestos exposure and peritoneal neoplasms. Correlations between asbestos exposure and immune-related disease are less conclusive; nevertheless, it was concluded from the combined autoimmune studies that there is a possibility for a higher-than-expected risk of systemic autoimmune disease among asbestos-exposed populations. In general, the GIT effects of asbestos exposure appear to be minimal, with the most likely outcome being development of stomach cancer. However, IARC recently concluded the evidence to support asbestos-induced stomach cancer to be "limited." The strongest evidence for reproductive disease due to asbestos is in regard to ovarian cancer. Unfortunately, effects on fertility and the developing fetus are under-studied. The possibility of other asbestos-induced health effects does exist. These include brain-related tumors, blood disorders due to the mutagenic and hemolytic properties of asbestos, and peritoneal fibrosis. It is clear from the literature that the adverse properties of asbestos are not confined to the pulmonary system.

Separation and characterization of respirable amphibole fibers from Libby, Montana

The vermiculite mine in Libby, Montana, was in operation for over 70 yr and was contaminated with asbestos-like amphibole fibers. The mining, processing, and shipping of this vermiculite led to significant fiber inhalation exposure throughout the community, and residents of Libby have developed numerous pulmonary diseases such as lung cancer and mesothelioma. The present study describes the separation of Libby 6-mix into respirable and nonrespirable size fractions by means of aqueous elutriation. The elutriator, designed to separate fibers with aerodynamic diameters smaller than 2.5 microm (respirable) from larger fibers, used an upward flow rate of 3.4 x 10(- 4) cm s(-1). The resultant respirable fraction constituted only 13% of the raw Libby 6-mix mass, and less than 2% of the fibers in the elutriated fraction had aerodynamic diameters exceeding 2.5 microm. Surface area of the elutriated fibers was 5.3 m(- 2) g(-1), compared to 0.53 m(-2) g(-1) for the raw fibers. There were no detectable differences in chemical composition between the larger and smaller fibers. Such harvesting of respirable fractions will allow toxicological studies to be conducted within a controlled laboratory setting, utilizing fiber sizes that may more accurately simulate historical exposure of Libby residents' lungs. Importantly, this work describes a method that allows the use of material enriched in more uniform respirable material than raw Libby 6-mix, making comparisons with other known fiber preparations more valid on a mass basis.