Investigations of regulatory T cell induction by 2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin during a graft-versus-host response

Graduation date: 2010The immune toxicity of 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been studied for over 35 years, but only recently has the profound immune suppression associated with TCDD exposure been linked to induction of regulatory T cells (Tregs). The effects of TCDD are mediated through binding the aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor. Subsequent AhR-mediated effects of TCDD in T cells that induce Tregs are not yet known. To address this, studies to further characterize CD25+CD4+ T cells induced in TCDD-treated mice on day 2 of an acute graft-versus-host (GVH) response were performed by comparing them to naturally-derived Tregs. Results show that TCDD-induced Tregs are similar to natural Tregs with a lack of IL-2 production, in vitro suppressive function, and reversal of suppressive function through ligation of GITR. However, TCDD-induced Tregs are unique in that they suppress naïve T cells while proliferating, do not express Foxp3, and secrete IL-10. A highly upregulated gene transcript in TCDD-induced Tregs was IL-12Rb2. IL-12Rb2 protein was found to be increased on T cells exposed to TCDD in the presence of IL-12. This correlated with increased binding of AhR upstream of the IL-12Rb2 gene. However, transfer of IL-12Rb2 KO T cells into TCDD-treated mice did not affect induction of the Treg phenotype. A second gene found to be upregulated in the TCDD-induced Tregs was IL-10. To inhibit IL-10 expression, a phosphorodiamidate morpholino oligomer conjugated to a cell-penetrating peptide (P-PMO) was utilized. Of several cell-penetrating peptides screened for delivery into murine leukocytes, the arginine-rich (RXR)4 peptide was most effective, particularly into activated T cells. Dosing of TCDD-treated host mice with IL-10 P-PMO-(RXR)4 did not affect induction of the day 2 Treg phenotype. However on day 6, an increased frequency of donor CD69+CD4+ and CD69+CD122+CD8+ T cells was identified in TCDD-treated mice, phenotypes associated with regulatory function. The increased frequency of these populations was suppressed by IL-10 P-PMO treatment. Taken together, the results suggest TCDD induces adaptive CD4+ and CD8+ regulatory T cells during a GVH response that is partially influenced by IL-10. These studies support Treg induction as a mechanism for suppression of T cell-mediated responses by TCDD