Immunoregulation of Liver Regeneration by the Aryl Hydrocarbon Receptor: Role of Lymphocytes and Interferon-Gamma

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent environmental contaminant that elicits toxicity by activating the aryl hydrocarbon receptor. The toxic effects associated with TCDD exposure include immunotoxicity and dysregulated cell cycle control, although the mechanisms are poorly understood. A previous report indicates that exposure to TCDD suppresses hepatocyte proliferation in a mouse model of liver regeneration induced by 70% partial hepatectomy (PH). Based on reports that liver regeneration is negatively regulated by interferon (IFN)-γ produced by activated natural killer (NK) cells, along with the well-established immunotoxic effects of TCDD in other model systems, we hypothesized that TCDD treatment attenuates liver regeneration by enhancing NK cell activation and IFN-γ production in the regenerating liver. We also considered the effects of TCDD on natural killer (NK) T and T cells, which are capable of producing IFN-γ as well. Mice were treated with TCDD (20 μg/kg) one day prior to surgical PH. Hepatocyte proliferation in the remnant liver was measured based on the incorporation of a thymidine analog, bromodeoxyuridine. Lymphocytes were collected from the spleen and remnant liver and analyzed by flow cytometry. IFN-γ was measured by intracellular staining followed by flow cytometry. Consistent with other reports, we found that TCDD treatment suppressed hepatocyte proliferation in the regenerating liver by 50-80%. However, contrary to our hypothesis, exposure to TCDD had no effect on the number of lymphocytes in the spleen or liver after PH, nor did it increase IFN-γ+ production by lymphocytes in the regenerating liver. Moreover, exposure to TCDD did not increase the number of NK and NKT cells expressing CD69, an early activation marker. To determine the relevance of NK cells to the TCDD-mediated suppression of liver regeneration, mice were treated with an α-asialo-GM-1 antibody to deplete NK cells prior to TCDD administration and PH. Depletion of NK cells did not restore hepatocyte proliferation in TCDD-treated mice, indicating that NK cells are not required for the attenuation of liver regeneration by TCDD. Hence, exposure to TCDD suppresses in vivo hepatocyte proliferation by a mechanism that does not involve NK cells or enhanced production of IFN-γ