Lead induced thymic immunosuppression in Japanese quail (Coturnix japonica) via oxidative stress-based T cell receptor pathway signaling inhibition.

The human-induced lead (Pb) contamination brings about serious environmental issues around the world and it also poses severe risks to wildlife including birds. Avian thymus is one of primary lymphoid organs and it plays an important role in regulating T cell-based cellular immunity. Therefore, in this study, we tried to examine toxic effects and potential molecular mechanism of Pb on avian thymus using a biological model species-Japanese quail (Coturnix japonica). One-week-old Japanese quails were exposed to 0, 50, 500 and 1000 ppm Pb concentrations in drinking water for three weeks when thymus reached developmental climax. The results showed body weight, thymus weight and thymic size were reduced significantly by Pb exposure. Meanwhile, histopathological changes including vacuolation, cortex atrophy and nuclear debris were detected in thymic cells of Pb exposure. In addition, ultrastructural alterations such as mitochondrial damage, chromatin condensation, and nuclear destruction were found in the thymus of Pb treatments. The increase of reactive oxygen species (ROS) and Malondialdehyde (MDA) as well as the inhibition of antioxidant system indicated that Pb exposure caused oxidative damages in the thymus. Pb exposure also increased thymic cell apoptosis. Moreover, RNA-Seq analysis revealed that thymic functional pathways were disrupted by Pb exposure. Especially, Pb exposure disturbed T cell differentiation and led to T helper type 1 (Th1) /T helper type 2 (Th2) imbalance by interfering with T cell receptor signaling and cytokine signaling. This study implied that Pb caused thymic immunosuppression through causing morphological deformation, structural destruction, oxidation and molecular signaling disruption.