Group 2 innate lymphoid cells (ILC2s) represent a subset of newly discovered immune cells that are involved in immune reactions against microbial pathogens, as well as during host allergic reactions and tissue repair.  ILC2s differentiate from bone marrow and thymic progenitors, previously known for developing B and T lymphocytes. We have recently shown that the basic helix-loop-helix transcription factors collectively called E proteins powerfully suppress differentiation into ILC2s while promoting B and T cell development, but the basis of this suppression is not well understood.  Here we investigated the underlying molecular mechanisms of this process using approaches involving inducible gain and loss of E protein function in ILC2s and their precursors, respectively.  Examination of RNA sequencing and ATAC sequencing data obtained at different time points reveals the modulation of a set of genes which are likely direct targets of E proteins.  For example, genes encoding transcriptional repressors, Cbfa2t3 and Jdp2 are activated by gain of E protein function at an early time point.  This may contribute to a widespread down-regulation of chromatin accessibility of ILC2 signature genes at a later time point.  As a result, the large number of genes repressed by gain of E protein function leads to the down-regulation of a transcriptional network known to be important for ILC2 differentiation. Together, these findings shed light on how E protein transcription factors control the lymphoid versus innate lymphoid cell fate.

Summary The thymus has a high capacity to support the differentiation of ILCs, especially when E protein transcription factors are ablated. Whether it contributes to the homeostasis of ILC pools in tissues is not clear. Single-cell RNA sequencing analysis shows a substantial amount of ILC precursors in wild type but not athymic nude blood. The precursors express CD3 intracellularly (ic) but not on the surface. The abundance of Lin − CD127 + CD62L + icCD3ε + precursors varies with age, peaking at 2–3 months. These cells can differentiate into various ILC subsets on OP9-DL1 stroma in vitro . In the lung, small intestine, and epidermis, icCD3ε + cells differentiate into diverse ILC subsets in different tissue environments in steady state. Helminth infection promotes their differentiation toward functional ILC2s. Thus, the thymus appears to play a role in replenishing ILC pools in different peripheral tissues. Because thymic activity is age-dependent, this finding may help explain age-related differences in immune responses.

Background Acute graft-versus-host disease (aGVHD) is one of the most common causes of morbidity for patients undergoing allogeneic stem cell transplantation. There is preliminary evidence that activated Group 2 innate lymphoid cells (ILC2s) from wild type (WT) mice reduces the lethality of aGVHD and is effective in treating lower gastrointestinal (GI) tract manifestations of aGVHD. This raises the prospect that ILC2s may be used for cell-based therapy of aGVHD but vigorous investigation is necessary to assess their impacts on different aspects of aGVHD. Genetically engineered mice which either express Id1 protein (Id1tg/tg), an inhibitor of E protein transcription factors or have E protein genes knocked out (dKO) in the thymus produce massive numbers of ILC2s, thus allowing extensive evaluation of ILC2s. We investigated whether these ILC2s have protective effects in aGVHD as WT ILC2s do using an established mouse model of aGVHD. Results bone marrow transplant was performed by irradiating BALB/c strain of recipient mice and transplanting with bone marrow and T cells from the MHC-disparate C57BL/6 strain. We isolated ILC2s from Id1tg/tg and dKO mice and co-transplanted them to study their effects. Our results confirm that activated ILC2s have a protective role in aGVHD, but the effects varied depending on the origin of ILC2s. Co-transplantation of ILC2s from Id1tg/tg mice were beneficial in aGVHD and are especially helpful in ameliorating the skin manifestations of aGVHD. However, ILC2s from dKO mice were less effective at the protection and behaved differently depending on if the cells were isolated from dKO mice were pre-treated with IL-25 in vivo. Conclusion These findings support the notion that thymus-derived ILC2s from Id1tg/tg mice are protective against aGVHD, with a significant improvement of skin lesions and they behave differently from dKO mice in the setting of aGVHD.

T cells develop in the thymus from lymphoid primed multipotent progenitors or common lymphoid progenitors into αβ and γδ subsets. The basic helix-loop-helix transcription factors, E proteins, play pivotal roles at multiple stages from T cell commitment to maturation. Inhibitors of E proteins, Id2 and Id3, also regulate T cell development while promoting ILC differentiation. Recent findings suggest that the thymus can also produce innate lymphoid cells (ILCs). In this review, we present current findings that suggest the balance between E and Id proteins is likely to be critical for controlling the bifurcation of T cell and ILC fates at early stages of T cell development.