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Type 2 innate lymphoid cells (ILC2s) are implicated in lung diseases such as idiopathic pulmonary fibrosis, but targeting these cells is a challenge. New data show that neuropilin-1 drives the ILC2 phenotype and is specific to lung-resident ILC2s in mice.

Innate lymphoid cells (ILCs) are important for tissue homeostasis and for the initiation of immune responses. Based on their transcriptional regulation and cytokine profiles, ILCs can be categorized into five subsets with defined phenotypes and functional profiles, but they also have the ability to adapt to local environmental cues by changing these profiles. This plasticity raises the question of the extent to which the cytokine production profiles of ILCs are pre-programmed or are a reflection of the tissue microenvironment. Here, we review recent advances in research on ILCs, with a focus on the plasticity of these cells. We highlight the ability of ILCs to communicate with the surrounding microenvironment and discuss the possible consequences of ILC plasticity for our understanding of the biological roles of these cells. Finally, we discuss how we might use this knowledge of ILC plasticity to develop or improve options for the treatment of inflammatory diseases.Innate lymphoid cell (ILC) subsets with defined phenotypes can adapt to local environmental cues through transdifferentiation. Studies of such plasticity have improved our understanding of the biological roles of ILCs and offer promise for new strategies to treat inflammatory diseases and cancer.

A family of innate lymphoid cells exist that is capable of rapid cytokine production. Spits and Di Santo review the developmental relationships and physiologic function of this expanding family. Research has identified what can be considered a family of innate lymphoid cells (ILCs) that includes not only natural killer (NK) cells and lymphoid tissue–inducer (LTi) cells but also cells that produce interleukin 5 (IL-5), IL-13, IL-17 and/or IL-22. These ILC subsets are developmentally related, requiring expression of the transcriptional repressor Id2 and cytokine signals through the common γ-chain of the IL-2 receptor. The functional differentiation of ILC subsets is orchestrated by distinct transcription factors. Analogous to helper T cell subsets, these evolutionarily conserved yet distinct ILCs seem to have important roles in protective immunity, and their dysregulation can promote immune pathology.

With respect to their ontogeny, emerging data support a lymphoid origin of plasmacytoid dendritic cells (pDCs), which seem to share a developmental trajectory with B cells. By contrast, conventional dendritic cells (DCs) have a myeloid origin in the common dendritic cell progenitor. Furthermore, pDCs do not seem in most cases to share the classical functional properties of conventional DCs, such as presentation of native antigen and migration via the lymphatics. Therefore, we propose here that pDCs should not be classified as dendritic cells and suggest instead that they should be assigned to a subcategory of innate lymphocytes. We emphasize their potent interferon-producing capacity as a defining feature of these cells.Emerging data suggest a lymphoid origin of plasmacytoid dendritic cells (pDCs), which, in most cases, do not share the classical functional properties of myeloid dendritic cells. This Comment proposes that pDCs should be assigned to a subcategory of innate lymphocytes and should be referred to as interferon-producing cells.

Innate lymphoid cells (ILCs) are crucial for the immune surveillance at mucosal sites. ILCs coordinate early eradication of pathogens and contribute to tissue healing and remodeling, features that are dysfunctional in patients with cystic fibrosis (CF). The mechanisms by which ILCs contribute to CF-immunopathology are ill-defined. Here, we show that group 2 ILCs (ILC2s) transdifferentiated into IL-17-secreting cells in the presence of the epithelial-derived cytokines IL-1β, IL-23 and TGF-β. This conversion is abrogated by IL-4 or vitamin D3. IL-17 producing ILC2s induce IL-8 secretion by epithelial cells and their presence in nasal polyps of CF patients is associated with neutrophilia. Our data suggest that ILC2s undergo transdifferentiation in CF nasal polyps in response to local cytokines, which are induced by infectious agents. Innate lymphoid cells (ILCs) play critical immunological roles including immune surveillance at mucosal sites. Here the authors show that during nasal inflammation pathogen-induced cytokine production guides the differentiation of ILCs.

ILCs and T helper cells have been shown to exert bi-directional regulation in mice. However, how crosstalk between ILCs and CD4 + T cells influences immune function in humans is unknown. Here we show that human intestinal ILCs co-localize with T cells in healthy and colorectal cancer tissue and display elevated HLA-DR expression in tumor and tumor-adjacent areas. Although mostly lacking co-stimulatory molecules ex vivo, intestinal and peripheral blood (PB) ILCs acquire antigen-presenting characteristics triggered by inflammasome-associated cytokines IL-1β and IL-18. IL-1β drives the expression of HLA-DR and co-stimulatory molecules on PB ILCs in an NF-κB-dependent manner, priming them as efficient inducers of cytomegalovirus-specific memory CD4 + T-cell responses. This effect is strongly inhibited by the anti-inflammatory cytokine TGF-β. Our results suggest that circulating and tissue-resident ILCs have the intrinsic capacity to respond to the immediate cytokine milieu and regulate local CD4 + T-cell responses, with potential implications for anti-tumor immunity and inflammation. Murine ILCs can modulate T cell responses in MHCII-dependent manner. Here the authors show that human ILCs process and present antigens and induce T-cell responses upon exposure to IL-1-family cytokines; along with the article by Lehmann et al, this work elucidates how cytokines set context specificity of ILC-T cell crosstalk by regulating ILC antigen presentation.

Here we identify a group 2 innate lymphoid cell (ILC2) subpopulation that can convert into interleukin-17 (IL-17)-producing NKp44 − ILC3-like cells. c-Kit and CCR6 define this ILC2 subpopulation that exhibits ILC3 features, including RORγt, enabling the conversion into IL-17-producing cells in response to IL-1β and IL-23. We also report a role for transforming growth factor-β in promoting the conversion of c-Kit − ILC2s into RORγt-expressing cells by inducing the upregulation of IL23R , CCR6 and KIT messenger RNA in these cells. This switch was dependent on RORγt and the downregulation of GATA-3. IL-4 was able to reverse this event, supporting a role for this cytokine in maintaining ILC2 identity. Notably, this plasticity has physiological relevance because a subset of RORγt + ILC2s express the skin-homing receptor CCR10, and the frequencies of IL-17-producing ILC3s are increased at the expense of ILC2s within the lesional skin of patients with psoriasis. Innate lymphoid cells (ILCs) can exhibit considerable plasticity. Humbles and colleagues identify a subpopulation of ILC2s in humans that can convert to proinflammatory ILC3s with implications for human skin diseases.

The innate immune system is composed of a diverse array of evolutionarily ancient haematopoietic cell types, including dendritic cells, monocytes, macrophages and granulocytes. These cell populations collaborate with each other, with the adaptive immune system and with non-haematopoietic cells to promote immunity, inflammation and tissue repair. Innate lymphoid cells are the most recently identified constituents of the innate immune system and have been the focus of intense investigation over the past five years. We summarize the studies that formally identified innate lymphoid cells and highlight their emerging roles in controlling tissue homeostasis in the context of infection, chronic inflammation, metabolic disease and cancer.

Mice have lineage-negative IL-7Rα + (innate lymphoid) cells that contribute to type 2 immunity. Spits and colleagues identify a similar CRTH2 + CD161 + population in human lungs and gut that produces IL-13 after stimulation. Innate lymphoid cells (ILCs) are emerging as a family of effectors and regulators of innate immunity and tissue remodeling. Interleukin 22 (IL-22)- and IL-17-producing ILCs, which depend on the transcription factor RORγt, express CD127 (IL-7 receptor α-chain) and the natural killer cell marker CD161. Here we describe another lineage-negative CD127 + CD161 + ILC population found in humans that expressed the chemoattractant receptor CRTH2. These cells responded in vitro to IL-2 plus IL-25 and IL-33 by producing IL-13. CRTH2 + ILCs were present in fetal and adult lung and gut. In fetal gut, these cells expressed IL-13 but not IL-17 or IL-22. There was enrichment for CRTH2 + ILCs in nasal polyps of chronic rhinosinusitis, a typical type 2 inflammatory disease. Our data identify a unique type of human ILC that provides an innate source of T helper type 2 (T H 2) cytokines.