Induction of Thymus Atrophy and Disruption of Thymocyte Development by Fipronil through Dysregulation of IL-7-Associated Genes

The susceptibility of the immune system to immunotoxic chemicals is evident, particularly in the thymus, a vital primary immune organ prone to atrophy due to exposure to toxicants. Fipronil (FPN), a widely used insecticide, is of concern due to its potential neurotoxicity, hepatotoxicity, and immunotoxicity. Our previous study showed that FPN disturbed the antigen-specific T-cell functionality in vivo. As T-cell lineage commitment and thymopoiesis are closely interconnected with the normal function of the T-cell-mediated immune responses, this study aims to further examine the toxic effects of FPN on thymocyte development. In this study, 4-week-old BALB/c mice received seven doses of FPN (1, 5, 10 mg/kg) by gavage. Thymus size, medulla/cortex ratio, total thymocyte counts, double-positive thymocyte population, and IL-7-positive cells decreased dose-dependently. IL-7 aids the differentiation of early T-cell precursors into mature T cells, and several essential genes contribute to the maturation of T cells in the thymus. Foxn1 ensures that the thymic microenvironment is suitable for the maturation of T-cell precursors. Lyl1 is involved in specifying lymphoid cells and maintaining T-cell development in the thymus. The c-Kit/SCF collaboration fosters a supportive thymic milieu to promote the formation of functional T cells. The expression of IL-7, IL-7R, c-Kit, SCF, Foxn1, and Lyl1 genes in the thymus was significantly diminished in FPN-treated groups with the concordance with the reduction of IL-7 signaling proteins (IL-7, IL-7R, c-KIT, SCF, LYL1, FOXO3A, and GABPA), suggesting that the dysregulation of T-cell lineage-related genes may contribute to the thymic atrophy induced by FPN. In addition, FPN disturbed the functionality of thymocytes with an increase of IL-4 and IFN-γ production and a decrease of IL-2 secretion after T-cell mitogen stimulation ex vivo. Collectively, FPN significantly deregulated genes related to T-cell progenitor differentiation, survival, and expansion, potentially leading to impaired thymopoiesis.