Restriction of IL-22–Producing T Cell Responses and Differential Regulation of Regulatory T Cell Compartments by Zinc Finger Transcription Factor Ikaros

Abstract

Proper immune responses are needed for controlling pathogen infection at mucosal surfaces. IL-22-producing CD4+ T cells play an important role in controlling bacterial infection in the gut; however, transcriptional regulation of these cells remains elusive. Here, we show that mice with targeted deletion of the fourth DNA-binding zinc finger of the transcription factor Ikaros had increased IL-22- but not IL-17-producing CD4+ T cells in the gut. Adoptive transfer of CD4+ T cells from these Ikaros mutant mice conferred enhanced mucosal immunity against Citrobacter rodentium infection. Despite an intact in vivo thymic-derived Treg compartment in these Ikaros mutant mice, TGF-β, a cytokine well known for induction of Tregs, failed to induce Foxp3 expression in Ikaros mutant CD4+ T cells in vitro but instead promoted IL-22. Aberrant upregulation of IL-21 in CD4+ T cells expressing mutant Ikaros was at least in part responsible for the enhanced IL-22 expression in a Stat3-dependent manner. Genetic analysis using compound mutations further demonstrated that the aryl hydrocarbon receptor (Ahr), but not RORγt, was required for aberrant IL-22 expression by Ikaros mutant CD4+ T cells, whereas forced expression of Foxp3 was sufficient to inhibit this aberrant cytokine production. Together, our data has uncovered new functions for Ikaros in maintaining mucosal immune homeostasis by restricting IL-22 production by CD4+ T cells.

Ikaros Inhibits Group 3 Innate Lymphoid Cell Development and Function by Suppressing the Aryl Hydrocarbon Receptor Pathway

“…These data are in sharp contrast to our previous data showing enhanced anti-bacterial immunity mediated by Th22 cells in Rag 1 −/− mice after adoptive transfer of Ikzf1 ΔF4/ΔF4 CD4 + T cells, in which the innate immune system is presumably intact in the Rag1 −/− recipients (Heller et al, 2014). These paradoxical effects of Ikaros in regulation of T cell responses may be due to the contribution of Ikaros in other cellular compartments that are potentially altered under the steady state or during infection.…”

“…We investigated the binding of endogenous Ahr and ARNT in primary Th17 cells differentiated in vitro from wildtype or Ikzf1 ΔF4/ΔF4 mice. Ahr protein level was decreased in Ikzf1 ΔF4/ΔF4 Th17 cells (Figure S4E), consistent with our previous study (Heller et al, 2014). However, enhanced binding between Ahr and ARNT was observed in these cells after normalization of Ahr protein expression, supporting that Ikaros may perturb Ahr-ARNT dimerization (Figure S4E).…”

“…Without infection, all three Ikaros-targeted mice showed enhanced production of IL-22 by T cells under the steady state (Figure S6, and data not shown (Heller et al, 2014) ), consistent with the role of Ikaros in suppressing Th22 cells (Heller et al, 2014). However, during C. rodentium infection, at day 9 when T cell responses were pronounced in wildtype littermate mice (Figures 5D to 5F and S7A to S7C), IL-22 and IL-17 expression by T cells were significantly reduced in Ikzf1 ΔF4/ΔF4 mice and Ikzf1 f/f Cd4 -cre mice and to a less extent in Ikzf1 f/f Rorc -cre mice (Figures 5D to 5E and S7A to S7C).…”

“…Consistent with the protective role of Th22 cells in C. rodentium infection (Basu et al, 2012), our previous data suggest that adoptive transfer of Ikzf1 ΔF4/ΔF4 T cells can more efficiently protect the host from the infection due to elevated IL-22 production (Heller et al, 2014). Given the upregulation of IL-22 + ILC3s in Ikzf1 ΔF4/ΔF4 mice, we next investigated the role of Ikaros in gut immunity during C. rodentium infection.…”

AHR Function in Lymphocytes: Emerging Concepts

“…Recent studies have reported direct binding of Ikaros, a zinc finger transcription factor, to the Ahr locus by ChIP assay, and concomitant reduced levels of Ahr transcription in Ikaros-null T cells or Ikaros mutant T cells lacking DNA-binding zinc finger 4 [35, 36], suggesting a positive role for Ikaros in Ahr transcription. However, the amount of Il22 transcripts is increased in T cells that express mutant Ikaros lacking zinc finger 4, whereas Cyp1a1 expression is not; this is surprising because both genes are canonical Ahr targets [36].…”