Informing selection of nanomaterial concentrations for ToxCast in vitro testing based on occupational exposure potential

Mimicking exposures to acute and lifetime concentrations of inhaled silver nanoparticles by two different in vitro approaches

In the emerging market of nano-sized products, silver nanoparticles (Ag NPs) are widely used due to their antimicrobial properties. Human interaction with Ag NPs can occur through the lung, skin, gastrointestinal tract, and bloodstream. However, the inhalation of Ag NP aerosols is a primary concern. To study the possible effects of inhaled Ag NPs, an in vitro triple cell co-culture model of the human alveolar/airway barrier (A549 epithelial cells, human peripheral blood monocyte derived dendritic and macrophage cells) together with an air-liquid interface cell exposure (ALICE) system was used in order to reflect a real-life exposure scenario. Cells were exposed at the air-liquid interface (ALI) to 0.03, 0.3, and 3 µg Ag/cm(2) of Ag NPs (diameter 100 nm; coated with polyvinylpyrrolidone: PVP). Ag NPs were found to be highly aggregated within ALI exposed cells with no impairment of cell morphology. Furthermore, a significant increase in release of cytotoxic (LDH), oxidative stress (SOD-1, HMOX-1) or pro-inflammatory markers (TNF-α, IL-8) was absent. As a comparison, cells were exposed to Ag NPs in submerged conditions to 10, 20, and 30 µg Ag/mL. The deposited dose per surface area was estimated by using a dosimetry model (ISDD) to directly compare submerged vs ALI exposure concentrations after 4 and 24 h. Unlike ALI exposures, the two highest concentrations under submerged conditions promoted a cytotoxic and pro-inflammatory response after 24 h. Interestingly, when cell cultures were co-incubated with lipopolysaccharide (LPS), no synergistic inflammatory effects were observed. By using two different exposure scenarios it has been shown that the ALI as well as the suspension conditions for the lower concentrations after 4 h, reflecting real-life concentrations of an acute 24 h exposure, did not induce any adverse effects in a complex 3D model mimicking the human alveolar/airway barrier. However, the highest concentrations used in the ALI setup, as well as all concentrations under submerged conditions after 24 h, reflecting more of a chronic lifetime exposure concentration, showed cytotoxic as well as pro-inflammatory effects. In conclusion, more studies need to address long-term and chronic Ag NP exposure effects.

The role of p53 in lung macrophages following exposure to a panel of manufactured nanomaterials

Manufactured nanomaterials (MNMs) have the potential to improve everyday life as they can be utilised in numerous medical applications and day-to-day consumer products. However, this increased use has led to concerns about the potential environmental and human health impacts. The protein p53 is a key transcription factor implicated in cellular defence and reparative responses to various stress factors. Additionally, p53 has been implicated in cellular responses following exposure to some MNMs. Here, the role of the MNM mediated p53 induction and activation and its downstream effects following exposure to five well-characterised materials [namely two types of TiO2, two carbon black (CB), and one single-walled carbon nanotube (SWCNT)] were investigated. MNM internalisation, cellular viability, p53 protein induction and activation, oxidative stress, inflammation and apoptosis were measured in murine cell line and primary pulmonary macrophage models. It was observed that p53 was implicated in the biological responses to MNMs, with oxidative stress associated with p53 activation (only following exposure to the SWCNT). We demonstrate that p53 acted as an antioxidant and anti-inflammatory in macrophage responses to SWCNT and CB NMs. However, p53 was neither involved in MNM-induced cellular toxicity, nor in the apoptosis induced by these MNMs. Moreover, the physicochemical characteristics of MNMs seemed to influence their biological effects-SWCNT the materials with the largest surface area and a fibrous shape were the most cytotoxic in this study and were capable of the induction and activation of p53.

Induced T cell cytokine production is enhanced by engineered nanoparticles

Engineered nanoparticles are widely used in commercial products, and yet due to the paucity of safety information, there are concerns surrounding potential adverse health effects, especially from inhaled nanoparticles and their putative contribution to allergic airway disease. The objective of this study was to investigate whether size or surface chemistry of engineered nanoparticles can influence the immune enhancing properties of these agents on antigen-specific T cell responses. Ovalbumin (OVA)-derived peptides were presented to T cells by either spleen-derived endogenous antigen presenting cells or a mouse dendritic cell (DC) line, DC2.4. In all models, interferon (IFN)-γ and interleukin (IL)-2 production by CD8(+) or CD4(+) T cells in response to peptide OVA257-264 or OVA323-339, respectively, was measured by flow cytometry. To address the study objective, silica nanoparticles (SNPs) were modified with alkyne-terminated surfaces and appended with polyethylene glycol chains via "click" chemistry. These modified SNPs were resistant to agglomerate in in vitro culture media, suggesting that their modulation of T cell responses is the result of true nanoscale-mediated effects. Under conditions of suboptimal T-cell activation, modified SNPs (up to 10 µg/ml) enhanced the proportion of CD8(+), but not CD4(+), T cells producing IFN-γ and IL-2. Various functional groups (-COOH, -NH2 and -OH) on modified SNPs enhanced IFN-γ and IL-2 production to different levels, with -COOH SNPs being the most effective. Furthermore, 51 nm -COOH SNPs exhibited a greater enhancing effect on the CD8(+) T cell response than other sized particles. Collectively, our results show that modified SNPs can enhance antigen-specific CD8(+) T cell responses, suggesting that certain modified SNPs exhibit potential adjuvant-like properties.

Cellular interactions and biological responses to titanium dioxide nanoparticles in HepG2 and BEAS-2B cells: role of cell culture media

We showed previously that exposure of human lung cells (BEAS-2B) to TiO2 nanoparticles (nano-TiO2 ) produced micronuclei (MN) only when the final concentration of protein in the cell-culture medium was at least 1%. Nanoparticles localize in the liver; thus, we exposed human liver cells (HepG2) to nano-TiO2 and found the same requirement for MN induction. Nano-TiO2 also formed small agglomerates in medium containing as little as 1% protein and caused cellular interaction as measured by side scatter by flow cytometry and DNA damage (comet assay) in HepG2 cells. Nano-TiO2 also increased the activity of the inflammatory factor NFkB but not of AP1 in a reporter-gene HepG2 cell line. Suspension of nano-TiO2 in medium containing 0.1% protein was sufficient for induction of MN by the nanoparticles in either BEAS-2B or HepG2 cells as long the final concentration of protein in the cell-culture medium was at least 1%.

An integrated approach for the in vitro dosimetry of engineered nanomaterials

BACKGROUND

There is a great need for screening tools capable of rapidly assessing nanomaterial toxicity. One impediment to the development of reliable in vitro screening methods is the need for accurate measures of cellular dose. We present here a methodology that enables accurate determination of delivered to cell dose metrics. This methodology includes (1) standardization of engineered nanomaterial (ENM) suspension preparation; (2) measurement of ENM characteristics controlling delivery to cells in culture; and (3) calculation of delivered dose as a function of exposure time using the ISDD model. The approach is validated against experimentally measured doses, and simplified analytical expressions for the delivered dose (Relevant In Vitro Dose (RID)f function) are derived for 20 ENMs. These functions can be used by nanotoxicologists to accurately calculate the total mass (RIDM), surface area (RIDSA), or particle number (RIDN) delivered to cells as a function of exposure time.

RESULTS

The proposed methodology was used to derive the effective density, agglomerate diameter and RID functions for 17 industrially-relevant metal and metal oxide ENMs, two carbonaceous nanoparticles, and non-agglomerating gold nanospheres, for two well plate configurations (96 and 384 well plates). For agglomerating ENMs, the measured effective density was on average 60% below the material density. We report great variability in delivered dose metrics, with some materials depositing within 24 hours while others require over 100 hours for delivery to cells. A neutron-activated tracer particle system was employed to validate the proposed in vitro dosimetry methodology for a number of ENMs (measured delivered to cell dose within 9% of estimated).

CONCLUSIONS

Our findings confirm and extend experimental and computational evidence that agglomerate characteristics affect the dose delivered to cells. Therefore measurement of these characteristics is critical for effective use of in vitro systems for nanotoxicology. The mixed experimental/computational approach to cellular dosimetry proposed and validated here can be used by nanotoxicologists to accurately calculate the delivered to cell dose metrics for various ENMs and in vitro conditions as a function of exposure time. The RID functions and characterization data for widely used ENMs presented here can together be used by experimentalists to design and interpret toxicity studies.

Estimating the effective density of engineered nanomaterials for in vitro dosimetry

The need for accurate in vitro dosimetry remains a major obstacle to the development of cost-effective toxicological screening methods for engineered nanomaterials. An important key to accurate in vitro dosimetry is the characterization of sedimentation and diffusion rates of nanoparticles suspended in culture media, which largely depend upon the effective density and diameter of formed agglomerates in suspension. Here we present a rapid and inexpensive method for accurately measuring the effective density of nano-agglomerates in suspension. This novel method is based on the volume of the pellet obtained by bench-top centrifugation of nanomaterial suspensions in a packed cell volume tube, and is validated against gold-standard analytical ultracentrifugation data. This simple and cost-effective method allows nanotoxicologists to correctly model nanoparticle transport, and thus attain accurate dosimetry in cell culture systems, which will greatly advance the development of reliable and efficient methods for toxicological testing and investigation of nano-bio interactions in vitro.

Critical experimental parameters related to the cytotoxicity of zinc oxide nanoparticles

The increasing use of zinc oxide nanoparticles (ZnO-NPs) has raised concerns about their potential hazards to human and environmental health. In this study, the characterization and cytotoxicity of two ZnO-NPs products (Z-COTE and Z-COTE HP1) were investigated. The zinc content of Z-COTE and Z-COTE HP1 was 82.5 ± 7.3 and 80.1 ± 3.5 %, respectively. Both ZnO-NP samples contained sub-cytotoxic levels of iron and copper, and silicon was detected from the surface coating of Z-COTE HP1. All samples were highly agglomerated, and the primary particles appeared as variable polyhedral structures. There was no significant difference in size distribution or average diameter of Z-COTE (53 ± 23 nm) and Z-COTE HP1 (54 ± 26 nm). A dose-dependent cytotoxicity was observed 24 h after exposure to ZnO-NPs, and monocytes were more sensitive than lung epithelial cells or lymphoblasts in both human and mouse cells. There was a significant difference in cytotoxicity between nano- and fine-forms, but only at the threshold cytotoxic dose with cellular metabolism assays. Compared to uncoated ZnO-NPs, the surface coating with triethoxycaprylylsilane marginally attenuated cellular oxidative stress and protected cellular metabolic activity. These results demonstrate the importance of model cell type, dose selection, and cytotoxicity assessment methodology to accurately evaluate the potential toxicity of various nanoparticles in vitro.

Interaction of differently functionalized fluorescent silica nanoparticles with neural stem- and tissue-type cells

Engineered amorphous silica nanoparticles (SiO2 NPs), due to simple and low cost production, are increasingly used in commercial products and produced on an industrial scale. Despite the potential benefits, there is a concern that exposure to certain types of SiO2 NPs may lead to adverse health effects. As some NPs can cross the blood--brain barrier and may, in addition, reach the central nervous system through the nasal epithelium, this study addresses the responses of different neural tissue-type cells including neural stem cells, neurons, astrocytes and microglia cells to increasing doses of 50 nm fluorescent core/shell SiO2 NPs with different [-NH2, -SH and polyvinylpyrrolidone (PVP)] surface chemistry. The SiO2 NPs are characterized using a variety of physicochemical methods. Assays of cytotoxicity and cellular metabolism indicates that SiO2 NPs cause cell death only at high particle doses, except PVP-coated SiO2 NPs which do not harm cells even at very high concentrations. All SiO2 NPs, except those coated with PVP, form large agglomerates in physiological solutions and adsorb a variety of proteins. Except PVP-NPs, all SiO2 NPs adhere strongly to cell surfaces, but internalization differs depending on neural cell type. Neural stem cells and astrocytes internalize plain SiO2, SiO2-NH2 and SiO2-SH NPs, while neurons do not take up any NPs. The data indicates that the PVP coat, by lowering the particle-biomolecular component interactions, reduces the biological effects of SiO2 NPs on the investigated neural cells.

Implementation of alternative test strategies for the safety assessment of engineered nanomaterials

Nanotechnology introduces a new field that requires novel approaches and methods for hazard and risk assessment. For an appropriate scientific platform for safety assessment, nanoscale properties and functions of engineered nanomaterials (ENMs), including how the physicochemical properties of the materials relate to mechanisms of injury at the nano-bio interface, must be considered. Moreover, this rapidly advancing new field requires novel test strategies that allow multiple toxicants to be screened in robust, mechanism-based assays in which the bulk of the investigation can be carried out at the cellular and biomolecular level whilst maintaining limited animal use and is based on the contribution of toxicological pathways to the pathophysiology of disease. First, a predictive toxicological approach for the safety assessment of ENMs will be discussed against the background of a '21st-century vision' for using alternative test strategies (ATSs) to perform toxicological assessment of large numbers of untested chemicals, thereby reducing a backlog that could otherwise become a problem for nanotechnology. An ATS is defined here as an alternative to animal experiments or refinement/reduction alternative to traditional animal testing. Secondly, the approach of selecting pathways of toxicity to screen for the pulmonary hazard potential of carbon nanotubes and metal oxides will be discussed, as well as how to use these pathways to perform high-content or high-throughput testing and how the data can be used for hazard ranking, risk assessment, regulatory decision-making and 'safer-by-design' strategies. Finally, the utility and disadvantages of this predictive toxicological approach to ENM safety assessment, and how it can assist the 21st-century vision, will be addressed.

An in vivo and in vitro toxicological characterisation of realistic nanoscale CeO₂ inhalation exposures

Nanoscale CeO₂ is increasingly used for industrial and commercial applications, including catalysis, UV-shielding and as an additive in various nanocomposites. Because of its increasing potential for consumer and occupational exposures, a comprehensive toxicological characterisation of this nanomaterial is needed. Preliminary results from intratracheal instillation studies in rats point to cytotoxicity and inflammation, though these studies may not accurately use realistic nanoscale exposure profiles. By contrast, published in vitro cellular studies have reported limited toxicological outcomes for the case of nano-ceria. Here, the authors present an integrative study evaluating the toxicity of nanoscale CeO₂ both in vitro, using the A549 lung epithelial cell line, and in vivo using an intact rat model. Realistic nano-ceria exposure atmospheres were generated using the Harvard Versatile Engineered Nanomaterial Generation System (VENGES), and rats were exposed via inhalation. Finally, the use of a nanothin amorphous SiO₂ encapsulation coating as a means of mitigating CeO₂ toxicity was assessed. Results from the inhalation experiments show lung injury and inflammation with increased PMN and LDH levels in the bronchoalveolar lavage fluid of the CeO₂-exposed rats. Moreover, exposure to SiO₂-coated CeO₂ did not induce any pulmonary toxicity to the animals, representing clear evidence for the safe by design SiO₂-encapsualtion concept.

A multi-stakeholder perspective on the use of alternative test strategies for nanomaterial safety assessment

There has been a conceptual shift in toxicological studies from describing what happens to explaining how the adverse outcome occurs, thereby enabling a deeper and improved understanding of how biomolecular and mechanistic profiling can inform hazard identification and improve risk assessment. Compared to traditional toxicology methods, which have a heavy reliance on animals, new approaches to generate toxicological data are becoming available for the safety assessment of chemicals, including high-throughput and high-content screening (HTS, HCS). With the emergence of nanotechnology, the exponential increase in the total number of engineered nanomaterials (ENMs) in research, development, and commercialization requires a robust scientific approach to screen ENM safety in humans and the environment rapidly and efficiently. Spurred by the developments in chemical testing, a promising new toxicological paradigm for ENMs is to use alternative test strategies (ATS), which reduce reliance on animal testing through the use of in vitro and in silico methods such as HTS, HCS, and computational modeling. Furthermore, this allows for the comparative analysis of large numbers of ENMs simultaneously and for hazard assessment at various stages of the product development process and overall life cycle. Using carbon nanotubes as a case study, a workshop bringing together national and international leaders from government, industry, and academia was convened at the University of California, Los Angeles, to discuss the utility of ATS for decision-making analyses of ENMs. After lively discussions, a short list of generally shared viewpoints on this topic was generated, including a general view that ATS approaches for ENMs can significantly benefit chemical safety analysis.

Developmental neurotoxicity of engineered nanomaterials: identifying research needs to support human health risk assessment

Increasing use of engineered nanomaterials (ENM) in consumer products and commercial applications has helped drive a rise in research related to the environmental health and safety (EHS) of these materials. Within the cacophony of information on ENM EHS to date are data indicating that these materials may be neurotoxic in adult animals. Evidence of elevated inflammatory responses, increased oxidative stress levels, alterations in neuronal function, and changes in cell morphology in adult animals suggests that ENM exposure during development could elicit developmental neurotoxicity (DNT), especially considering the greater vulnerability of the developing brain to some toxic insults. In this review, we examine current findings related to developmental neurotoxic effects of ENM in the context of identifying research gaps for future risk assessments. The basic risk assessment paradigm is presented, with an emphasis on problem formulation and assessments of exposure, hazard, and dose response for DNT. Limited evidence suggests that in utero and postpartum exposures are possible, while fewer than 10 animal studies have evaluated DNT, with results indicating changes in synaptic plasticity, gene expression, and neurobehavior. Based on the available information, we use current testing guidelines to highlight research gaps that may inform ENM research efforts to develop data for higher throughput methods and future risk assessments for DNT. Although the available evidence is not strong enough to reach conclusions about DNT risk from ENM exposure, the data indicate that consideration of ENM developmental neurotoxic potential is warranted.

Exposure of silver-nanoparticles and silver-ions to lung cells in vitro at the air-liquid interface

Background

Due to its antibacterial properties, silver (Ag) has been used in more consumer products than any other nanomaterial so far. Despite the promising advantages posed by using Ag-nanoparticles (NPs), their interaction with mammalian systems is currently not fully understood. An exposure route via inhalation is of primary concern for humans in an occupational setting. Aim of this study was therefore to investigate the potential adverse effects of aerosolised Ag-NPs using a human epithelial airway barrier model composed of A549, monocyte derived macrophage and dendritic cells cultured in vitro at the air-liquid interface. Cell cultures were exposed to 20 nm citrate-coated Ag-NPs with a deposition of 30 and 278 ng/cm² respectively and incubated for 4 h and 24 h. To elucidate whether any effects of Ag-NPs are due to ionic effects, Ag-Nitrate (AgNO₃) solutions were aerosolised at the same molecular mass concentrations.

Results

Agglomerates of Ag-NPs were detected at 24 h post exposure in vesicular structures inside cells but the cellular integrity was not impaired upon Ag-NP exposures. Minimal cytotoxicity, by measuring the release of lactate dehydrogenase, could only be detected following a higher concentrated AgNO₃-solution. A release of pro-inflammatory markers TNF-α and IL-8 was neither observed upon Ag-NP and AgNO₃ exposures as well as was not affected when cells were pre-stimulated with lipopolysaccharide (LPS). Also, an induction of mRNA expression of TNF-α and IL-8, could only be observed for the highest AgNO₃ concentration alone or even significantly increased when pre-stimulated with LPS after 4 h. However, this effect disappeared after 24 h. Furthermore, oxidative stress markers (HMOX-1, SOD-1) were expressed after 4 h in a concentration dependent manner following AgNO₃ exposures only.

Conclusions

With an experimental setup reflecting physiological exposure conditions in the human lung more realistic, the present study indicates that Ag-NPs do not cause adverse effects and cells were only sensitive to high Ag-ion concentrations. Chronic exposure scenarios however, are needed to reveal further insight into the fate of Ag-NPs after deposition and cell interactions.

Nanomaterials toxicity and cell death modalities

In the last decade, the nanotechnology advancement has developed a plethora of novel and intriguing nanomaterial application in many sectors, including research and medicine. However, many risks have been highlighted in their use, particularly related to their unexpected toxicity in vitro and in vivo experimental models. This paper proposes an overview concerning the cell death modalities induced by the major nanomaterials.

Incorporating exposure information into the toxicological prioritization index decision support framework

The Toxicological Prioritization Index (ToxPi) decision support framework was previously developed to facilitate incorporation of diverse data to prioritize chemicals based on potential hazard. This ToxPi index was demonstrated by considering results of bioprofiling related to potential for endocrine disruption. However, exposure information is required along with hazard information to prioritize chemicals for further testing. The goal of this analysis is to demonstrate the utility of the ToxPi framework for incorporating exposure information to rank chemicals and improve understanding of key exposure surrogates. The ToxPi tool was applied to common exposure surrogates (i.e., fate parameters, manufacturing volume, and occurrence measurements) and the relationship between resulting rankings and higher-tiered exposure estimates was investigated. As information more directly relevant to human exposure potential is incorporated, relative rank of chemicals changes. Binned ToxPi results are shown to be consistent with chemical priorities based on crude measures of population-level exposure for a limited set of chemicals. However, these bins are not predictive of higher tiered estimates of exposure such as those developed for pesticide registration. Although rankings based on exposure surrogates are used in a variety of contexts, analysis of the relevance of these tools is challenging. The ToxPi framework can be used to gain insight into the factors driving these rankings and aid identification of key exposure metrics. Additional exposure data is required to build confidence in exposure-based chemical prioritization.

Titanium dioxide nanoparticles activate the ATM-Chk2 DNA damage response in human dermal fibroblasts

The use of nanoparticles in consumer products increases their prevalence in the environment and the potential risk to human health. Although recent studies have shown in vivo and in vitro toxicity of titanium dioxide nanoparticles (nano-TiO2), a more detailed view of the underlying mechanisms of this response needs to be established. Here, the effects of nano-TiO2 on the DNA damage response and DNA replication dynamics were investigated in human dermal fibroblasts. Specifically, the relationship between nano-TiO2 and the DNA damage response pathways regulated by ATM/Chk2 and ATR/Chk1 was examined. The results show increased phosphorylation of H2AX, ATM, and Chk2 after exposure. In addition, nano-TiO2 inhibited the overall rate of DNA synthesis and frequency of replicon initiation events in DNA-combed fibres. Taken together, these results demonstrate that exposure to nano-TiO2 activates the ATM/Chk2 DNA damage response pathway.

When enough is enough

The European Commission should be regulating nanosilver, not asking for yet another report on its impact on health and the environment.

Pulmonary surfactant coating of multi-walled carbon nanotubes (MWCNTs) influences their oxidative and pro-inflammatory potential in vitro

Background

Increasing concern has been expressed regarding the potential adverse health effects that may be associated with human exposure to inhaled multi-walled carbon nanotubes (MWCNTs). Thus it is imperative that an understanding as to the underlying mechanisms and the identification of the key factors involved in adverse effects are gained. In the alveoli, MWCNTs first interact with the pulmonary surfactant. At this interface, proteins and lipids of the pulmonary surfactant bind to MWCNTs, affecting their surface characteristics. Aim of the present study was to investigate if the pre-coating of MWCNTs with pulmonary surfactant has an influence on potential adverse effects, upon both (i) human monocyte derived macrophages (MDM) monocultures, and (ii) a sophisticated in vitro model of the human epithelial airway barrier. Both in vitro systems were exposed to MWCNTs either pre-coated with a porcine pulmonary surfactant (Curosurf) or not. The effect of MWCNTs surface charge was also investigated in terms of amino (−NH₂) and carboxyl (−COOH) surface modifications.

Results

Pre-coating of MWCNTs with Curosurf affects their oxidative potential by increasing the reactive oxygen species levels and decreasing intracellular glutathione depletion in MDM as well as decreases the release of Tumour necrosis factor alpha (TNF-α). In addition, an induction of apoptosis was observed after exposure to Curosurf pre-coated MWCNTs. In triple cell-co cultures the release of Interleukin-8 (IL-8) was increased after exposure to Curosurf pre-coated MWCNTs. Effects of the MWCNTs functionalizations were minor in both MDM and triple cell co-cultures.

Conclusions

The present study clearly indicates that the pre-coating of MWCNTs with pulmonary surfactant more than the functionalization of the tubes is a key factor in determining their ability to cause oxidative stress, cytokine/chemokine release and apoptosis. Thus the coating of nano-objects with pulmonary surfactant should be considered for future lung in vitro risk assessment studies.

Aggregating data for computational toxicology applications: The U.S. Environmental Protection Agency (EPA) Aggregated Computational Toxicology Resource (ACToR) System

Computational toxicology combines data from high-throughput test methods, chemical structure analyses and other biological domains (e.g., genes, proteins, cells, tissues) with the goals of predicting and understanding the underlying mechanistic causes of chemical toxicity and for predicting toxicity of new chemicals and products. A key feature of such approaches is their reliance on knowledge extracted from large collections of data and data sets in computable formats. The U.S. Environmental Protection Agency (EPA) has developed a large data resource called ACToR (Aggregated Computational Toxicology Resource) to support these data-intensive efforts. ACToR comprises four main repositories: core ACToR (chemical identifiers and structures, and summary data on hazard, exposure, use, and other domains), ToxRefDB (Toxicity Reference Database, a compilation of detailed in vivo toxicity data from guideline studies), ExpoCastDB (detailed human exposure data from observational studies of selected chemicals), and ToxCastDB (data from high-throughput screening programs, including links to underlying biological information related to genes and pathways). The EPA DSSTox (Distributed Structure-Searchable Toxicity) program provides expert-reviewed chemical structures and associated information for these and other high-interest public inventories. Overall, the ACToR system contains information on about 400,000 chemicals from 1100 different sources. The entire system is built using open source tools and is freely available to download. This review describes the organization of the data repository and provides selected examples of use cases.