, 2003). The high sensitivity of double-positive cells agrees with the presently proposed role of T cell activation in mediating the toxic activity of DON. In the normal thymus, depletion of precursor T lymphocytes that respond to auto-antigens occurs at the double-positive stage as well (Starr et al.,

2003). Therefore, double-positive T cells will be much more sensitive for DON-induced T cell activation than the very early or late precursor T cells. Genes encoding proteins for cellular components as mitochondria, ribosomes, and cytoplasm/nuclei were downregulated Androgen Receptor Antagonists high throughput screening by DON. It is tempting to relate the downregulation of ribosome- and protein translation-related genes to the ribotoxic stress response. Since mitochondria- and cytoplasm/nuclei-related genes are downregulated as well, these findings are more likely correlated to the depletion of early lymphocytes that have a higher metabolization rate than the thymus epithelial and stromal cells. Likewise, the upregulation of genes related to cell adhesion and cytoskeleton is most likely due to the relative increase of the proportion of stromal cells. In relation to the toxic effect of DON, it is surprising that the expression data provide little indications for induction of apoptosis. This agrees, however, with

previously published data showing that after 12-h treatment with 12.5 mg/kg DON less than 0.5% of the CD4+ CD8+ cells have apoptotic (subdiploid) nuclei

(Islam et al., 2003). At this same time point, however, 25% of the CD4+ CD8+ cells are depleted from the thymus. Therefore, depletion of see more DON-affected cells likely precedes induction of apoptosis, meaning that there were apoptotic cells present in the thymus, but before the end of the treatment period, those cells Sitaxentan were already depleted from the thymus. This rapid depletion likely occurs via phagocytosis, which agrees with our findings of a fast invasion of leucocytes and macrophages into the DON-treated thymus. Deletion via phagocytosis is also found during negative selection in the thymus (Sun and Shi, 2001 and Elliott et al., 2009). In summary, the present findings indicate that DON induces cellular events that also occur after activation of the T cell receptor, such as release of calcium from the endoplasmatic reticulum. This T cell activation is rapidly followed by negative selection of thymocytes, particularly those at the double-positive stage. At lower exposure levels (5 and 10 mg/kg), this effect is reversible, while it is irreversible at least 24 h after exposure to 25 mg/kg. This provides a plausible explanation for the high sensitivity for DON of immune cells, above all thymocytes, compared to other cell types. The following are the supplementary materials related to this article. Supplementary Fig. 1.