Cytokine Gene Expression during Ontogeny in Murine Thymus on Activation of the Aryl Hydrocarbon Receptor by 2,3,7,8-Tetrachlorodibenzo-p-dioxin

Abstract

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) binds and activates the aryl hydrocarbon receptor (Ah-R), an endogenous transcription factor that is expressed in the thymus. TCDD exposure leads, among other effects, to thymus atrophy and immunosuppression. We previously analyzed the interference of TCDD with differentiation processes in fetal thymus organ cultures and found that in the presence of TCDD, the proliferation rate of immature (CD4- CD8- and CD4- CD8+ HSA+) thymocytes is inhibited, whereas the maturation along the CD4/CD8 path is accelerated. Moreover, the differentiation of thymocytes is skewed by TCDD at < or = 40% (compared with approximately 15% without TCDD) of the CD8 single-positive subset of future cytotoxic T cells, and apparently more cells audition for and pass positive selection. The fetal murine thymus expresses functional Ah-R mRNA, as shown by reverse transcription-polymerase chain reaction and TCDD-inducible CYP1A1 and CYP1B1 expression. Because the differentiation of thymocytes is to a considerable extent controlled by cytokines and many cytokine genes are potential targets of the Ah-R due to Ah-R-binding elements (xenobiotic response elements) in their promoters, we analyzed the cytokine expression in fetal thymus organ culture exposed to TCDD. Fetal thymi were cultured from gestation day 15 for < or = 8 days, thus covering ex vivo the period after population of the thymus anlage until birth. We show with semiquantitative reverse transcription-polymerase chain reaction that more interleukin (IL)-1beta, IL-2, IL-6, tumor growth factor (TGF)-beta3, and tumor necrosis factor-alpha are produced in TCDD-exposed thymi, whereas other cytokines (e.g., TGF-beta1, PAI-2, or IL-4) are only slightly up- and down-modulated during the culture period or not modulated at all (e.g., IL-1beta, IL-7, interferon-gamma, and TGF-beta2).

The MurineIL-2Promoter Contains Distal Regulatory Elements Responsive to the Ah Receptor, a Member of the Evolutionarily Conserved bHLH-PAS Transcription Factor Family

“…As shown in Fig. 4B, TCDD increased IL-2 mRNA in total thymocytes, confirming previously published results (15). The overall increase was attributable to the immature CD4 Ϫ CD8 Ϫ cells and future cytotoxic CD4 Ϫ CD8 ϩ cells, both of which responded to TCDD exposure with a strong (ϳ3-and 5-fold) increase of IL-2 mRNA.…”

“…IL-2 promoter activity is regulated by a number of elements, i.e., Oct-1, NFAT, NF-B, and AP-1, which cooperate and thus confer the lineage specificity and external regulation of IL-2. Others and we had shown that IL-2 expression could be induced by TCDD, i.e., the AHR (15,31,32). Extending these findings, our analysis of IL-2 induction in subpopulations of thymocytes showed that TCDD can induce IL-2 mRNA in…”

“…Semiquantitative RT-PCR assays were performed as previously described (15). Total RNA (1 g) was used for cDNA synthesis with mouse mammary tumor virus (MMTV) reverse transcriptase.…”

“…Expression of several cytokine mRNAs was shown to be inducible by TCDD (15,16). IL-2, which is comparatively strongly inducible, is produced by T cells and has central functions for growth and differentiation of T cells, B cells, NK cells, and macrophages (17).…”

Workshop to identify critical windows of exposure for children's health: immune and respiratory systems work group summary.

“…(13). Additionally, in vitro studies using fetal mouse thymus cultures have suggested that TCDD exposure can disrupt the pattern of thymocyte maturation (14).…”