Regulatory ILCs Suppress ILC1- and ILC3-Induced Innate Intestinal Inflammation
Innate lymphoid cells (ILCs) are a recently described population of lymphocytes that play a critical role in innate immune responses and tissue remodeling. Multiple studies suggest that ILCs are particularly important for maintaining epithelial barriers and regulating immune responses at mucosal surfaces such as the lungs and intestines. Similar to T cells, ILCs consist of several subtypes that are characterized by the expression of different transcription factors and cytokines, and these correspond with the transcription factors and cytokines expressed by either CD8+ T cells or the Th1, Th2, or Th17 subsets of T helper cells. Group 1 ILCs consist of natural killer (NK) cells and ILC1 cells, which both express the transcription factor T-bet and produce IFN-gamma. NK cells have been suggested to be the innate counterpart of cytotoxic CD8+ T cells, while ILC1s are the innate equivalent of Th1 cells. Group 2 ILCs (ILC2s) are the innate counterpart of Th2 cells as both cell types express the transcription factor GATA-3 and produce IL-4, IL-5, IL-9, IL-13, and Amphiregulin, while group 3 ILCs (ILC3s), although heterogeneous, all express the Th17-related transcription factor, ROR gamma t, and secrete IL-17, IFN-gamma, IL-22, or some combination of these cytokines. Significantly, a regulatory subset of ILCs with anti-inflammatory functions mirroring that of regulatory T cells (Tregs) was never identified prior to a report published in the September 21st issue of Cell.
In this report, Wang, S. et al. describe a new regulatory subset of ILCs (ILCregs) that is present in the intestines of both mice and humans. Using IL-10-GFP reporter mice, the authors identified these cells as a subpopulation of Lin-CD45+CD127/IL-7 R alpha+ ILCs that constitutively express IL-10. Based on the expression of additional cell surface and intracellular molecules, this IL-10-producing subset was determined to be different than the previously described ILC subsets. Notably, these cells also lacked expression of both CD4 and FoxP3, two markers commonly used to identify Tregs. ILCregs were found to arise from a common helper-like innate lymphoid precursor (CHILP) but not a PLZF+ common ILC precursor (ILCP), which is a precursor cell downstream of the CHILP that gives rise to the ILC1, ILC2, and ILC3 subsets, indicating that ILCregs are a distinct lineage. Like the other ILC subsets, ILCregs were shown to require the transcriptional regulator, Id2 for their development, but additionally the authors showed that the development and/or maintenance of ILCregs is dependent on Id3, a transcriptional regulator that is not required for the development of other ILC subsets. Similar to Tregs, ILCregs expressed high levels of TGF-beta RI, TGF-beta RII, IL-2 R beta, and IL-2 R gamma, and IL-2 and autocrine TGF-beta 1 were determined to be required for the expansion and/or maintenance of ILCregs during intestinal inflammation.
To look at the function of ILCregs in the intestine, the authors treated Rag1-/- mice with inflammatory stimuli and found that the number of ILCregs gradually increased following treatment with dextran sodium sulfate (DSS), S. typhimurium, or C. rodentium, and peaked at 8 days post-stimulation. Furthermore, they showed that adoptive transfer of ILCregs into Rag1-/-IL-10-/-mice prevented the pathogenesis of innate colitis induced by dextran sodium sulfate (DSS) treatment, and improved the overall survival rate of DSS-treated Rag1-/-IL-10-/-mice. Using in vitro experiments, ILCregs isolated from the lamina propria of the small intestine were demonstrated to inhibit the activation and cytokine secretion of ILC1 and ILC3 cells. Although ILCregs produced high levels of both IL-10 and TGF-beta 1 in response to inflammatory stimulation, IL-10 but not TGF-beta 1, was shown to be required to inhibit the secretion of IFN-gamma or IFN-gamma and IL-17A by ILC1 or ILC3 cells, respectively.
To confirm the inhibitory function of ILCregs in vivo, activated ILC1 or ILC3 cells were transferred either alone or with ILCregs from DSS-treated mice into Rag1-/-IL-2R gamma-/- mice, which lacked T cells, B cells, NK cells, and ILCs. While transfer of activated ILC1 or ILC3 cells alone induced innate colitis, co-transfer of ILCregs protected Rag1-/-IL-2R gamma-/- mice from innate colitis and suppressed the secretion of inflammatory cytokines by ILC1 and ILC3 cells. Secretion of IL-10 by ILCregs was found to be critical for this effect as neutralization of IL-10 blocked the inhibitory effect of ILCregs. Significantly, the authors also showed that depletion of ILCregs following co-transfer with ILC1 and ILC3 cells into Rag1-/-IL-2R gamma-/- mice and subsequent DSS challenge caused an increase in cytokine secretion by ILC1 and ILC3 cells and severe innate intestinal inflammation, while depletion of Tregs in a similar experiment did not have an effect on the activities of ILC1 or ILC3 cells or innate intestinal inflammation.
This report by Wang, S. et al. is the first to show that a regulatory subset of IL-10-producing ILCs exists and that these cells can suppress the activities of ILC1 and ILC3 cells in innate intestinal inflammation. Whether ILCregs are involved in regulating innate inflammation in other tissues or disease states still awaits further investigation.