Gene Expression Alterations in Immune System Pathways in the Thymus after Exposure to Immunosuppressive Chemicals
Frawley, Rachel, White, Kimber, Jr., Brown, Ronnetta, Musgrove, Deborah, Walker, Nigel, Germolec, Don, Environmental Health Perspectives
BACKGROUND: Dysregulation of positive and negative selection, antigen presentation, or apoptosis in the thymus can lead to immunosuppression or autoimmunity. Diethylstilbestrol (DES), dexa-methasone (DEX), cyclophosphamide (CPS), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are immunosuppressive chemicals that induce similar immunotoxic effects in the thymus, however, the mechanism of toxicity is purported to be different for each compound.
OBJECTIVES: We hypothesized that genomic analysis of thymus after chemical-induced atrophy would yield transcriptional profiles that suggest pathways of toxicity associated with reduced function.
METHODS: Female B6C3F2 mice were exposed to these immunosuppressive agents and changes in gene expression and immune cell subpopulations were evaluated.
RESULTS: All four chemicals induced thymic atrophy and changes in both the relative proportion and absolute number of [CD3.sup.+], [CD4.sup.+]/[CD8.sup.-], [CD4.sup.-]/[CD8.sup.+], and [CD4.sup.+]/[CD8.sup.+] thymocytes. The most significant impact of exposure to DEX, DES, and CPS was modulation of gene expression in the T-cell receptor (TCR) complex and TCR and CD28 signaling pathways; this could represent a common mechanism of action and play a pivotal role in lineage commitment and development of T cells. Up-regulation of genes associated with the antigen presentation and dendritic cell maturation pathways was the most distinctive effect of TCDD exposure. These elements, which were also up-regulated by DEX and DES, contribute to positive and negative selection.
CONCLUSIONS: Genomic analysis revealed gene expression changes in several pathways that are commonly associated with xenobiotic-induced immune system perturbations, particularly those that contribute to the development and maturation of thymic T cells.
KEY WORDS: cyclophosphamide, dexamethasone, diethylstilbestrol, T cell, thymus, toxicogenomics, 2,3,7,8-tetrachlorodibenzo-p-dioxin. Environ Health Perspect 119:371-376 (2011). doi:10.1289/ ehp.1002358 [Online 1 November 2010]
The immune system is a complex set of cellular, chemical, and soluble protein components that interact with each other in a sequential, regulated manner to protect the body against foreign substances. Early T-lymphocyte (T-cell) progenitors originate in the bone marrow and migrate to the thymus, where they differentiate, undergo positive and negative selection, and mature, before being released into circulation. Appropriate and carefully regulated apoptosis is one of the major mechanisms for homeostasis and the selection and development of fully competent T cells (Chaplin 2010). Recent efforts have sought to use transcriptional profiling to discover genomic patterns that are predictive of toxicity and to investigate the molecular responses to chemical stressors (Auerbach et al. 2010; Boverhof et al. 2009). Identifying chemical-induced changes in gene expression would contribute to a deeper understanding of the molecular mechanisms of immunotoxicity and would allow for an assessment of the immune system within the confines of traditional toxicology studies.
Exposure to the immunosuppressive agents dexamethasone (DEX) (Marchetti et al. 2003; van Vliet et al. 1986), diethylstilbestrol (DES) (Gould et al. 2000), cyclophosphamide (CPS) (Luster et al. 1981; Wang and Cai 1999), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (Camacho et al. 2005; Dean et al. 1985) results in hypocellularity, atrophy, and apoptosis in the thymus; however, the putative chemical mechanisms of action and cellular targets differ (Luebke et al. 2006). The nitrogen mustard derivative CPS induces DNA mutation by alkylation and cross-linking (Wang and Cai 1999). DEX (Marchetti et al. 2003) and DES (Ahmed 2000), synthetic hormones that bind to the glucocorticoid and estrogen receptors, respectively, translocate to the nucleus bound to the receptors and regulate gene expression and protein synthesis. …