Strain and sex differences in chloromethylisothiazolinone and methylisothiazolinone-induced lung injury in mice

Mouse strain and sex variability may provide a better understanding of the isothiazolinone-associated respiratory toxicity profile; however, this remains poorly understood. In this study, we investigated and compared the respiratory effects of repeated exposure to a mixture of chloromethylisothiazolinone and methylisothiazolinone (CMIT/MIT) in two commonly used mouse strains, BALB/c and C57BL/6, including both males and females. CMIT/MIT-induced lung injury was analyzed after six times intratracheal instillation of CMIT/MIT using differential cell counts and cytokine measurements in bronchoalveolar lavage fluid (BALF), histological analysis, and gene expression profiling of lung tissue. In both female and male C57BL/6 mice, CMIT/MIT exposure led to increased infiltration of inflammatory cells, particularly eosinophils, and elevated levels of Type 2 helper T cell (Th2)-associated cytokines in BALF. Histopathological findings revealed granulomatous inflammation, mucinous cell hyperplasia, eosinophilic cell infiltration, and lung fibrosis in these mice. Female BALB/c mice exhibited similar but less severe pathological changes. In contrast, male BALB/c mice showed a predominance of macrophages and neutrophils, with no notable histopathological alterations. Gene expression analysis revealed upregulation of genes associated with inflammatory and fibrotic lung injury and Th2 signaling in female and male C57BL/6 mice and female BALB/c mice, but not in male BALB/c mice. Collectively, these findings indicate that C57BL/6 mice are more susceptible to CMIT/MIT-induced lung injury than BALB/c mice, which is closely associated with the genetic characteristics of increased eosinophil and Th2-mediated responses.