Dysfunction of hepatic regulatory T cells in experimental sclerosing cholangitis is associated with IL-12 signaling

Background: Regulatory T cells (Treg) are essential for maintenance of immunological tolerance. Reduced numbers or function of Treg have been described for several autoimmune diseases including autoimmune liver diseases. Thus, we could recently describe a reduced number and possibly reduced function of regulatory T cells (Treg) in patients with primary sclerosing cholangitis (PSC). Therefore, Treg expansion might serve as a therapeutic approach.

Methods: In this study Treg were expanded by repeated injection of IL-2/IL-2Ab complex in mouse models of cholangitis (Mdr2-/-, DDC) or colitis (DSS) as control. In vitro suppressive capacity and gene expression were analyzed in isolated hepatic and splenic Treg.

Results: We observed a significant increase of Treg numbers in the liver after enrichment of the endogenous Treg population via application of IL-2/anti-IL-2-complex (PBS: 4.6% vs. IL-2-K: 27.2%, p < 0.03). Tregs from PBS treated as well as IL-2/anti-IL-2-complex treated animals were mainly localized in the inflamed portal tracts (PBS: 640 vs. IL-2-K: 3780 Foxp3+ cells [pro mm2], p < 0.03). However, although Treg expansion was associated with reduced pro-inflammatory IL-17 (PBS: 160.8 vs. IL-2-K:103.7 [pg/ml], p < 0.03) and increased anti-inflammatory IL-10 production by hepatic lymphocytes, the severity of cholangitis was not reduced. This was in contrast to the suppression of colitis by Treg enrichment (PBS-Score 7.7; IL-2-K-Score 4.9, p < 0.02), suggesting a reduced functionality of intrahepatic Treg. Indeed, hepatic Treg manifested reduced Foxp3 expression and reduced suppressive capacity compared to splenic Treg. The reduced Foxp3 expression in Treg could be linked to increased IL-12 receptor beta 2 expression and IL-12 signaling. Accordingly, IL-12 Receptor beta 2 knockout mice (IL-12rb2-/-) were able to maintain hepatic Treg functionality.

Conclusion: Hepatic Treg expanded in vivo improved the course of experimental colitis but failed to improve the course of cholangitis, which was related to the effects of hepatic IL-12 on Treg. Therefore, neutralization of IL-12 should be considered as part of treatment strategies to improve the efficacy of Treg treatment in liver diseases.