Towards prospective identification of respiratory sensitizers: effects of piperazine, chloramine-T, and toluene-diisocyanate in an air–liquid interface model comprising human bronchial epithelial cells

Exposure to respiratory sensitizers (RSs) is the leading cause of occupational asthma. Although the prospective identification of RSs is important, there currently exists no OECD-approved test guideline for this endpoint. The adverse outcome pathway for respiratory sensitization consists of key event (KE) 1: binding of the respiratory sensitizer to a protein, KE2: activation of lung epithelial cells, KE3: activation of dendritic cells, and KE4: T-cell response. Here, we focused on KE2 by investigating whether measuring this KE could contribute to prospectively identify respiratory sensitizers. To mimic real-life exposure, cells were exposed via the air. We used an air-liquid interface model comprising the human bronchial epithelial cell line Calu-3. Exposure to the RS piperazine (3, 18, and 100 mg/m³) dose-dependently increased IL-6 production at dose levels that did not affect the other parameters tested (barrier integrity, cell metabolism, cytotoxicity, and IL-8 production). IL-6 has been linked to asthma in humans. Exposure to the RS chloramine-T (30, 300, and 3000 ng/cm²) showed only minor effects on the parameters tested. Exposure to the RS 2,4-toluene diisocyanate (10, 33, and 100 mg/m³) at the highest dose level clearly affected all parameters tested. Disrupted barrier function has been linked to asthma. In conclusion, this study may possibly suggest that different respiratory sensitizers may differentially impact KE2. Further research is needed to elucidate this.