Function of hepatic type 2 innate lymphoid cells is regulated by liver inflammation-induced cytokines and effector CD4 T cells

Question:

Type 2 innate lymphoid cells (ILC2 s) are activated by IL-33 and mediate immune responses by expression of the cytokines IL-5 and IL-13. During Concanavalin (Con)A-induced immune-mediated hepatitis, effector CD4 T cells and hepatic ILC2 s drive liver inflammation. However, less is known about mechanisms involved in the regulation of ILC2 function during liver inflammation. We aim at investigating the phenotype of hepatic ILC2 s and its modulation and plasticity triggered by liver inflammation-induced cytokines. We also study the mutual influence of immune responses mediated by hepatic ILC2 s and effector CD4 T cells.

Methods:

C57BL/6 mice were treated with IL-33 on four consecutive days. Hepatic ILC2 s were isolated by FACS and cultured in the presence of IL-33, IFNγ, IL-12 or IL-1β. OVA-specific CD4 T cells or Th1 cells were cocultured with hepatic ILC2 s in the presence of OVA. The phenotype of ILC2 s and CD4 T cells was analyzed by flow cytometry.

Results:

We showed a small population of liver-resident ILC2 s that expanded during IL-33 treatment. In healthy liver, a low frequency of ILC2 s expressed KLRG1 and CD25. IL-33 increased expression of both and induced IL-4, IL-5, IL-13, TNFα and IL-6 expression by hepatic ILC2 s. In immune-mediated hepatitis, IL-33, IFNγ, IL-12 and IL-1β expression was induced in the liver. Stimulation of hepatic ILC2 s with IL-33 in vitro upregulated expression of IL-5, IL-13 and IL-6. Expression of all cytokines was suppressed by IFNγ. Interestingly, this inhibitory effect was overcome by IL-33. IL-12 and IL-1β did not induce differentiation into IFNγ-expressing ILC1 s, termed functional plasticity. Moreover, hepatic ILC2 s expressed molecules linked with APC function including MHCII, CD80 and CD86. Antigen-specific stimulation of CD4 T cells by hepatic ILC2 s induced activation and proliferation of both cell populations. We found induction of IL-2, IL-5 and IL-13 expression in ILC2-stimulated CD4 T cells and upregulation of ILC2-derived cytokine response that were inhibited in the presence of an anti-IL-2 antibody. Th1 cells were reactivated by hepatic ILC2 s and inhibited ILC2 expansion and cytokine expression. The inhibitory effect of Th1 cells on hepatic ILC2 s was partially reversed in the presence of an anti-IFNγ antibody.

Conclusion:

Hepatic ILC2 function is regulated by the cytokines IL-33 and IFNγ. The inflammation-induced phenotype remains stable as hepatic ILC2 s did not exhibit functional plasticity. Hepatic ILC2 s function as APCs and induce antigen-specific activation of CD4 T cells and Th1 cells. While ILC2-activated CD4 T cells enhanced hepatic ILC2 cytokine response Th1 cells inhibited ILC2 function.