The alarmin IL-33 drives a ST2+ Treg-mediated anti-inflammatory immune response during immune-mediated hepatitis

Introduction:

In patients with acute and chronic liver disease, serum levels of the alarmin IL-33 are elevated, indicating an immunomodulatory function of IL-33 in hepatic inflammation. During Concanavalin A (ConA)-induced immune-mediated hepatitis, IL-33 is released by necrotic hepatocytes and induces immune responses by signaling through the IL-33 receptor ST2. We have shown previously that IL-33 pre-treatment prevented development of immune-mediated hepatitis, suggesting an immunosuppressive role of IL-33 in liver disease. Since regulatory T cells (Tregs) expressing ST2 respond to IL-33, we aimed at investigating the IL-33-driven ST2+ Treg response in the inflamed liver.

Methods:

To induce immune-mediated hepatitis, C57BL/6 and FIR x tiger mice received i.v. ConA and were analyzed 24 hours later. To address the immunosuppressive effect of IL-33 on disease pathology, mice were treated with recombinant murine IL-33 on three consecutive days before ConA challenge and analyzed 24 hours later. The phenotype of hepatic Tregs was determined by flow cytometry.

Results:

In homeostasis, we showed that the frequency of ST2+ Foxp3+ Tregs was strongly elevated in the liver compared to the spleen. Hepatic ST2+ Tregs expressed higher levels of Foxp3 than ST2- Tregs and the frequencies of ST2+ Tregs expressing ICOS, KLRG1, CD103, TIGIT, and GITR were also increased, indicating an activated phenotype of this Treg subset in the steady-state. Moreover, hepatic ST2+ Tregs were characterized by expression of the transcription factor GATA3, the regeneration-associated epidermal growth factor amphiregulin, and strong production of the anti-inflammatory cytokine IL-10.

In immune-mediated hepatitis, we demonstrated an enhanced frequency of hepatic ST2+ Tregs, which up-regulated expression of CD25, ICOS, CD103, TIGIT, GITR, CTLA-4 as well as PD-L1, and maintained high IL-10 expression. We further showed that ST2-/- mice developed more severe immune-mediated hepatitis, despite an enhanced frequency of hepatic Foxp3+ Tregs, underlining the importance of the IL-33/ST2 pathway for hepatic immune regulation. Furthermore, IL-33 pre-treatment before induction of hepatitis highly elevated the frequency of activated hepatic ST2+ Tregs in comparison to ConA-treated mice, which was associated with reduced liver inflammation, tissue damage, and necrosis.

Conclusion:

The immune regulatory function of IL-33 in immune-mediated hepatitis might be driven by expansion and recruitment of a highly activated Treg population expressing ST2.