Innate lymphoid cells (ILCs) are a recently identified group of heterogeneous innate immune cells. These newly identified ILCs are distinguished from B and T cells by their thymus-independent development and lack antigen receptors encoded by rearranged genes (1). ILCs are divided into three subsets: group 1 ILCs (comprising ILC1s and conventional NK cells); group 2 ILCs (comprising ILC2s); and group 3 ILCs [comprising ILC3s and lymphoid tissue inducer (LTi) cells], based on their ability to produce type 1, type 2, and Th17 cell-associated cytokines (2). There is a striking similarity between different T helper cells and ILC subsets in terms of phenotypes and functions (3). The conventional NK (cNK) cells are also called cytotoxic ILCs because of their powerful ability to kill target cells directly (4). The other subgroups of ILCs (ILC1s, ILC2s, and ILC3s) are termed as helper-like ILCs, based on their ability to improve body defense by the secretion of cytokines (5).

Innate lymphoid cells are found at primary entry sites of pathogens, such as in mucosal surfaces of the lung and gastrointestinal system, the skin, and the liver (5). They play a vital role in tissue homeostasis, remodeling, repair of damaged tissue, and lymphoid tissue formation (6). The liver, as an important immune-tolerant organ, is known for its predominantly innate immunity and maintaining tolerance to harmless antigens (7–9). However, uncontrolled activation and proliferation of ILCs can lead to damaging inflammation (10–13). Recent studies have highlighted the potential involvement of ILC subsets in regulating liver diseases (14–16). For example, ILC1s exhibit pro-inflammatory roles in the pathogenesis of chronic hepatitis B (14). A group of IL-33-dependent hepatic ILC2s was required and sufficient for hepatic fibrosis via an IL-13-dependent mechanism (17). ILC3s play a protective role in murine acute hepatitis (18). Although accumulating data support potential roles for helper-like ILCs in regulating liver diseases, the molecular mechanisms deserve further investigation. In this review, we summarize the phenotypic characteristics of ILCs, their particular roles and mechanisms in immune-mediated liver diseases, and potential therapeutic interventions for liver diseases.

Group 1 ILCs were defined based on their ability to produce interferon-γ (IFN-γ) and a dependency on the T-box transcription factor T-bet for their function and development, similar to Th1 cells (19, 20). Despite the similarities between ILC1 and cNK cells, they differ in several important respects. For example, ILC1s exhibit limited cytotoxicity compared with cNK cells, by expressing high levels of TNF-related apoptosis-inducing ligand (TRAIL) and IFN-γ, but low levels of granzyme B (GmB), and perforin in response to IL-12 (21). ILC1s depend developmentally on transcription factor T-bet, but not on eomesodermin (Eomes), and they do not express, or express low levels of Eomes (22, 23). By contrast, cNK cells express both T-bet and Eomes, and develop in a strictly Eomes-dependent manner, but only partially require T-bet, at least for terminal NK cell maturation (1, 22, 24). However, emerging data indicate that ILC1s have overlapping, but different, phenotypes and functions in different tissues (19). Both intestinal and hepatic ILC1s are CD49a⁺CD49b⁻Eomes⁻ and produce high amounts of IFN-γ in mice. Hepatic ILC1s exhibit stronger cytotoxic potential by expressing higher levels of GmB, perforin, CD107a, TRAIL, and FasL compared with intestinal ILC1s (23). ILC1-like cells found in the salivary gland are similar to the hepatic ILC1s in expressing CD49a and TRAIL, but are different from hepatic ILC1s in that the majority of these cells also express Eomes and CD49b, and they produce very low levels of IFN-γ (25). ILC1-like cells found in mouse breast and prostate tumors exert a similar phenotype to salivary gland ILC1s and express CD49a and CD103 (26). The tissue environments may modulate the phenotypes and function of ILC1s; however, the potential mechanism(s) that results in these differences remains unclear (1). ILC1s and cNK cells have an important role in infectious diseases. They are required for the control of T. gondii infection, as confirmed in T-bet-deficient mice (19). T-bet-dependent ILC1s are essential in host defense against Clostridium difficile or Salmonella enterica infections (27, 28). Hepatic ILC1s and mucosal ILC1s are involved in tumor surveillance (1).

ILC2s are defined by their ability to produce type 2 cytokines: IL-4, IL-5, IL-9, and IL-13, with GATA-binding protein 3 (GATA3) as its signature transcription factor. ILC2s develop in bone marrow and arise from a common lymphoid progenitor (CLP). ILC2s present mainly in non-lymphoid tissues, including the brain, heart, lung, kidney, skin, intestine, and uterus, while a few ILC2s were also reported in lymphoid tissues, such as spleen and liver (29, 30). ILC2s do not express lineage markers, but express MHC II molecules, c-Kit, Sca-1, IL-33R, and IL-7Rα (31–33). They maintain tissue homeostasis, relying on the expression of IL-7Rα in response to IL-7 (31). The expression of transcription factors GATA3 and RORα allows ILC2s to produce type 2 cytokines (34, 35). ILC2s express both IL-25 and IL-33 receptors, and are responsive to IL-25 and IL-33. Similar to CD4⁺T cells, as a major regulator in type II cytokine-dependent diseases (e.g., food allergies, atopic dermatitis, sinusitis, and asthma) (36), ILC2s are able to drive type 2 inflammation (37, 38) and provide protective immunity against helminths (39).

Group 3 ILCs contain NCR⁺ILC3s (NKp46⁺ILC3s in mice or NKp44⁺ILC3s in humans), NCR⁻ILC3s, and classical LTi cells (40). One of the most prominent features of group 3 ILCs is their production of Th17-associated cytokines IL-17, IL-22, and IFNγ, and RORγt as their signature transcription factor (10, 41). All of these ILC subsets develop in bone marrow, differentiate from CLP, and require IL-7 for their development (42). LTi cells contribute to the formation of lymph nodes and Peyer’s patches (43). NKp46⁺ILC3s produce IL-22 in response to IL-23, but do not produce IL-17 in mice, while NCR⁻ILC3s and LTi ILC3s can produce both IL-17 and IL-22 (44). Takatori et al. identified CD4⁺CD3⁻LTi-like cells expressing the IL-23 receptor, the aryl hydrocarbon receptor, and CCR6 (44). These LTi-like cells play important roles in the formation of secondary lymphoid tissues and host defense by secreting Th17-associated cytokines (44). A notable difference between LTi cells and ILC3s is that the differentiation of ILC3s, as well as ILC1s and ILC2s, depends on a transcription factor termed promyelocytic leukemia zinc finger (PLZF); however, LTi and cNK cells are independent of PLZF (45). LTis and ILC3s were reported to be major mediators in the pathogenesis of inflammatory bowel diseases (IBDs) (2, 46).

ILC1s, along with cNK cells, ILC2s, and ILC3s, arise from an early innate lymphoid precursor, which is generated from CLP (47). However, it is reported that the development of ILC1s is distinct from other ILC subsets, according to a study of fate-mapping mice (5). It is now becoming clear that developmental plasticity exists between ILC1s and other ILC subsets. A proportion of ILC3s acquire the ability to produce IFNγ by upregulating T-bet in response to IL-23 and IL-12, but completely lose RORγt expression. This process gives rise to ILC3-derived ILC1s (also called ex-RORγt⁺ILC3s) (48). ILC2s can also transdifferentiate toward ILC1s in the presence of IL-12 (49). Interestingly, this shift is reversible: both ILC2s- and ILC3s-derived ILC1s can revert to ILC2s and ILC3s under the influence of IL-4 and IL-23, respectively (48, 50).

The characteristics of the development, phenotype, and function of ILC subsets are summarized in Figure 1.

The liver is the largest solid organ in the body, with two blood supplies: the portal venous system and the hepatic arterial system. It is a unique anatomical and immunological site where blood containing various antigens or microbial products circulate through a network of sinusoids and are scanned by antigen-presenting cells (APCs) and lymphocytes (51–54). The small diameter of the sinusoids and the lower systemic venous pressure result in a relatively slow flow, which lengthens the contact between APCs and lymphocytes and facilitates the clearance of harmful substances by liver-resident cells (55). The strong innate immune system in the liver distinguishes harmless from harmful molecules (56–60). The liver immune system is characterized by particularly rich in innate immune cells (e.g., Kupffer, NK, and NKT cells) (8, 60, 61). Interestingly, ILCs are detected abundantly in the liver, with the dominant ILC subsets being cNK cells and ILC1s (nearly 95% of all ILCs) (62). Although ILC2s and ILC3s in liver are quite rare (about 5% of all ILCs), the roles of these different ILCs subpopulation in liver diseases have also attracted increasing attention (17, 18).

Recent research has made significant progress in determining the function of ILCs in liver diseases. Notably, different ILCs subpopulations might exhibit different roles in the same or different liver diseases. Even the same ILC subsets might play opposite roles in different liver diseases. These seemingly paradoxical effects might depend on the inflammation state and tissue microenvironment of different diseases. The protective or pathogenic roles of ILCs in liver diseases are summarized in Figure 2. Nevertheless, their strategic location at mucosal barriers or in liver enables ILCs to play essential roles in the maintenance of immune homeostasis by balancing destructive immunity and protective responses. It is clear that excessive activation of ILCs could lead to chronic pathologies (e.g., inflammation, liver injury, or fibrosis). Further research is needed to clarify the precise protective or pathogenic mechanisms of different ILCs in liver diseases. However, the existing findings have suggested that ILCs and their related effector molecules can be used as targets for diagnostic and therapeutic strategies. Targeting the activator or effector molecules of hepatic ILCs might be an effective treatment for liver diseases. For example, thymic stromal lymphopoietin, IL-25, and IL-33 are important activators of ILC2s, and simultaneous disruption of the three factors successfully alleviated IL-13-dependent liver inflammation and fibrosis during chronic Schistosoma mansoni infection (99). In view of the roles of ILC1s in maintaining tolerance to Ad or HCV infection in a NKG2A-IFNγ-DCs axis-dependent manner, it is suggested that blocking NKG2A on ILC1s in parallel with the use of a hepatic-resident DCs-targeted vaccine might constitute a novel strategy to induce sustainable CD8⁺T cell responses against hepatic pathogens (77). In addition, therapeutic adoptive transfer of hepatoprotective ILCs (e.g., IL-15 + IL-23-expanded NKp46⁺ILC3) or exogenously providing their effector molecules (e.g., protective IL-22) may also offer a promising strategy to prevent liver injury, excessive inflammation, or promote liver regeneration. Further studies are needed to improve our understanding of the distinct roles of each ILC subpopulation in immune-mediated liver diseases and to identify valid ILC-based targets for therapeutic intervention.

All authors listed have made substantial, direct, and intellectual contribution to the work and approved it for publication.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.