2025 Risk Assessment & Mixtures Specialty Section Student Award Winners: Assessment Approaches to Wildfires
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Hosted by the SOT Risk Assessment & Mixtures Specialty Sections
Evaluating the Response of Primary Human Bronchial Epithelial Cells to Polycyclic Aromatic Hydrocarbons from Wildfire Smoke in the Presence and Absence of THP-1 Macrophages
Kyle Burns, MixSS 2025 Best Abstract Winner
PhD Candidate, Environmental and Molecular Toxicology, Oregon State University
Wildfires are a growing public health crisis with wildfire occurrence, duration, and intensity increasing in recent years. In addition, wildfire emissions are chemically complex, containing polycyclic aromatic hydrocarbons (PAHs) among other pollutants, warranting further evaluation in mechanistically relevant models. In the respiratory system, induction of airway inflammation is orchestrated through signaling between airway epithelial cells and macrophages, which are two of the most abundant cell types in the respiratory tract contributing to the innate immune response. Activation of airway immune response and inflammation are critical events leading to adverse health effects and damage to the respiratory system after chemical exposure. To evaluate the combined contribution of airway epithelial-macrophage response to PAHs from wildfires, we established a co-culture model utilizing 3D organotypic primary human bronchial epithelial cells (HBEC) and THP-1 macrophages. HBEC in the presence and absence of THP-1 macrophages were exposed to mixtures of PAHs (0-600 µM) previously identified from wildfires in the Pacific Northwest (PNW) by passive air sampling. These mixtures were comprised of PAHs detected at the highest concentrations at sites in the PNW with an AQI > 100, with one mixture being based on abundance (SW12) and the other mixture only including abundant PAHs with 4 or more aromatic rings (SFM). Cultures were assessed for cytotoxicity by LDH activity, cytokine protein secretion by cytometric bead array, and apoptosis by flow cytometry. In addition, a panel of biomarkers were evaluated by qPCR for CYP450 induction and cytokine response in HBEC. Each wildfire mixture resulted in unique patterns of toxicity and inflammatory response that was specific to each mixture and dependent on the presence of macrophages. For example, SW12 but not SFM caused significant cytotoxicity (p-adj<0.05) and only in the co-culture. SW12 and SFM also resulted in distinct responses for CYP1A1, CYP1B1, and inflammatory cytokine (IL-1β, IL-6, IL-8, and TNF-α) gene expression in HBEC. Of interest, only SFM significantly (p-adj<0.05) induced both CYP1A1/1B1 transcripts in both models in a dose-dependent manner compared to SW12. Overall, it appears that signaling between epithelial and macrophage cells is necessary for a more sensitive cytotoxic response in HBEC. Furthermore, data show that unique mechanisms likely mediate toxicity by SW12 and SFM and that the macrophages have a distinct role in response to each mixture. These studies fill critical knowledge gaps regarding the role of inflammation in toxicity of inhaled PAHs from wildfires in an organotypic co-culture model.
Characterizing Individual and Mixtures-Based Chemical Contributions to Wildfire Smoke Toxicity Through In Vitro Transcriptomics Screening
Sarah Miller, RASS 2025 John Doull Award Winner
PhD Candidate, Julia E. Rager PhD Laboratory, Curriculum in Toxicology & Environmental Medicine, UNC School of Medicine, Chapel Hill
Wildfire incidence and severity have steadily increased in recent decades, leading to increases in smoke emissions that pose risks to public health. Due to the wide variety of chemical mixtures in wildfire smoke, there is a need to couple high-throughput toxicity screening approaches with computational methods to assess wildfire-relevant mixtures toxicity. This study sought to evaluate transcriptomic responses of respiratory epithelial cells to individual components, defined mixtures, and complex woodsmoke mixtures with two goals: (1) identifying potential modes of action that are shared versus distinct between wildfire components and (2) quantifying relative impacts across these various mixtures to inform human health risk estimates. The six chemicals prioritized for this study (benzo(a)pyrene, benz(a)anthracene, coniferyl aldehyde, vanillin, sodium dichromate, and copper sulfate) frequently co-occur across 10 different wildfire-relevant burn scenarios (eucalyptus, pine, pine needles, peat, and red oak burned under flaming or smoldering conditions) and were previously positively or negatively correlated with toxicity endpoints in exposed mice through mixtures modeling. Human bronchial epithelial (16HBE) cells were exposed at five different concentrations to these single chemicals, equimolar binary mixtures, an equimolar defined mixture of all six candidate chemicals, and four complex biomass mixtures. Differentially expressed genes (DEGs), gene set enrichment, benchmark concentration (BMC) modeling, transcriptomic point of departure (tPOD) derivation, and concentration addition mixtures modeling (CAM) analyses were performed on bulk RNA sequencing results to evaluate transcriptomic response. Patterns in DEGs, significantly enriched pathways, BMCs, and tPODs were shared between the complex biomass mixtures, defined mixture, sodium dichromate alone, and sodium dichromate in binary mixture with copper sulfate. Comparisons of observed versus CAM-predicted tPODs revealed that this metals binary mixture and the defined mixture also exhibited synergistic effects compared to their CAM-predicted tPODs (e.g., 0.120 µM observed versus 0.265 µM predicted for metals). Additionally, Ras/ERK and Ras/PI3K signaling were identified as top ranking enriched pathways highly shared across these exposure conditions, suggesting a potential shared mode of action. Altogether, these results suggest that metals and metal mixtures may be a particular chemical class and/or mixture of public health concern when considering the respiratory health effects of wildfire smoke exposure.