From steatosis to inflammation: Innate lymphocytes as hidden orchestrators in metabolic dysfunction-associated steatotic liver disease

Abstract

Metabolic dysfunction-associated steatotic liver disease (MASLD) is a progressive condition that ranges from simple hepatic steatosis to steatohepatitis, persistent inflammation, and fibrosis. Although metabolic alterations and hepatocellular injury are established as central drivers of MASLD, accumulating evidence underscores the pivotal contribution of innate and innate-like lymphocytes in modulating immune responses throughout disease progression. Among these, gamma delta T cells, natural killer cells, natural killer T cells, innate lymphoid cells, and mucosal-associated invariant T cells are highly represented in the liver and rapidly respond to metabolic stress and inflammatory stimuli. These populations promote cytokine secretion, hepatocyte injury, recruitment of additional immune subsets, and activation of hepatic stellate cells, thereby sustaining inflammation and tissue remodeling. Depending on the disease stage and the surrounding microenvironment, they may exert either protective or pathogenic roles, ultimately determining whether the process resolves or progresses toward fibrosis. This review provides an overview of their phenotypic features, effector mechanisms, and interactions within the hepatic immune microenvironment, highlighting their potential as diagnostic biomarkers and therapeutic targets in MASLD and its complications.

Key Words: Innate lymphocytes cells; Gamma delta T cells; Mucosal-associated invariant T cells; Natural killer cells; Natural killer T cell; Metabolic dysfunction-associated steatotic liver disease; Fibrosis

Core Tip: Metabolic dysfunction-associated steatotic liver disease is now recognized as a leading cause of chronic liver disease worldwide. Beyond metabolic stress and hepatocellular injury, recent evidence highlights the critical involvement of innate and innate-like lymphocytes, such as gamma delta T cells, natural killer cells, natural killer T cells, innate lymphoid cells, and mucosal-associated invariant T cells in driving inflammation, hepatocyte injury, and hepatic stellate cell activation. Depending on the microenvironment, these cells may exert protective or pathogenic roles, shaping progression toward steatohepatitis and fibrosis. Understanding their effector functions offers opportunities for novel biomarkers and therapeutic strategies in metabolic dysfunction-associated steatotic liver disease.

INTRODUCTION

Metabolic dysfunction-associated steatotic liver disease (MASLD) is characterized by excess triglyceride accumulation within hepatocytes, or steatosis, which may progress to inflammatory steatohepatitis (MASH) and ultimately to fibrosis. In Western countries, MASLD is the most common chronic liver disease and is projected to become the leading indication for liver transplantation by 2030. Although metabolic and hepatocellular mechanisms have been described in detail, immune dysregulation has also been implicated as a determinant of clinical course and severity[1,2]. Lipid overload in hepatocytes provokes lipotoxicity and oxidative stress, disrupts organelle function, and can culminate in cell death. Before apoptosis, injured hepatocytes release damage-associated molecular patterns, cytokines, chemokines, and extracellular vesicles that recruit and activate immune cells in the liver. When this signaling persists, inflammation escalates, and fibrogenic pathways are engaged, marking the transition from simple steatosis to steatohepatitis[3,4].

Multiple immune populations participate in these events. Particular attention has turned to innate and innate-like lymphocytes because they connect metabolic stress to immune activation. These populations are characterized by limited clonal diversity and the capacity to mount rapid effector responses to hepatocyte-derived stress cues, and they can detect conserved molecular signals without classical antigen presentation, which positions them as early sensors and modulators of hepatic inflammation[5].

The term “innate-like lymphocytes” denotes lymphoid populations that share features of both innate and adaptive immunity. They arise from lymphoid precursors, express semi-invariant receptors, and can execute effector programs without prior antigen priming. This hybrid profile allows them to sense stress-related ligands and to operate within the metabolically dynamic hepatic microenvironment[6,7]. Their positioning within liver tissue and their capacity to shape inflammatory tone make them highly relevant to metabolic liver disease. This review synthesizes current evidence on gamma delta T (γδT) cells, natural killer (NK) cells, natural killer T (NKT) cells, mucosal-associated invariant T (MAIT) cells, and innate lymphoid cells (ILCs) in MASLD and MASH, with emphasis on their activation pathways, interactions within the hepatic microenvironment, and overall contribution to disease progression.

γδT CELLS

γδT cells are a type of cluster differentiation 3 (CD3+) T lymphocytes that originate from the double-negative stage during their development (they do not express CD4 or CD8) and express a T cell receptor gamma-delta. However, recent studies have demonstrated the existence of γδT cells that can be CD4+, CD8+, or CD56+ (derived from double-positive cells, presenting a phenotype similar to NKT cells)[8]. Depending on the cytokine production profile, γδT cells can be classified into different functional subsets that require specific transcription factors. For example, T-box transcription factor TBX21 and eomesodermin are crucial for the differentiation of γδT1 cells that produce interferon-gamma (IFN-γ), while retinoic acid-related orphan receptor (RORγT) and Runx1 are essential for the development of γδT17 cells that produce interleukin-17 (IL-17)[9].

These are the first T cells to emerge during prenatal development, but their population decreases dramatically after birth, representing only 5% of the total T cell population[8]. Generally, γδT cells are located in tissues with mucosal surfaces, such as the skin and intestine. They are considered a bridge between innate and adaptive immunity, as T cell receptor gamma-delta recognizes a wide variety of conserved structures on pathogens and self-cells (including non-peptide metabolites) without the restriction of antigen presentation by major histocompatibility complex (MHC). This ability enables them to mount an immune response faster than alpha beta T cells[10].

Following antigen recognition, these cells can perform several activities, including cytotoxicity against target cells, the production of IL-17 and IFN-γ, and the activation of other immune cells[11]. Their cytotoxic activity is similar to that of conventional T cells (via death receptor-mediated apoptosis or release of cytolytic granules). In contrast, the activation of other immune cells that mediate inflammatory processes is based on their ability to produce immunomodulatory cytokines, such as the aforementioned IFN-γ and IL-17, as well as IL-4, IL-5, IL-10, IL-13, transforming growth factor beta (TGF-β), and granulocyte-macrophage colony-stimulating factor[12]. γδT cells play a significant role in inflammatory responses, particularly in liver disease. They have been implicated in the pathophysiology of MASH, with human and mouse studies showing a link between MASH and type 3 inflammation, characterized by IL-17A production. γδT cells secrete this cytokine in response to T cell receptor engagement, promoting liver fibrosis[13].

MASLD induces both metabolic and inflammatory stress in hepatocytes. In this context, the NK group 2 member D (NKG2D) ligands (MHC class I polypeptide-related sequence A/B) are upregulated in regions most affected by steatosis. NKG2D is involved in the cytotoxicity and cytokine production of NK cells and innate-like T cells. When both NKG2D and the TCR are stimulated, γδT cells significantly increase IL-17A production, becoming the dominant source of this cytokine in the liver. The frequency of IL-17A-expressing cells positively correlates with the degree of steatosis and liver inflammation, increasing during the early stages of MASLD but decreasing in later stages[14].

The IL-17 exacerbates the intracellular lipid accumulation induced by free fatty acids through the inhibition of insulin-signaling pathways in hepatocytes. This combination of IL-17 and free fatty acids also increases the production of IL-6 in these cells[15]. IL-17A promotes chemokine production by liver cells, leading to the induction of chemokines such as C-X-C motif ligand 1 (CXCL1), CXCL2, and, to a lesser extent, C-C motif ligand 2, which are potent recruiters of myeloid proinflammatory cells. In murine models, it has been demonstrated that IL-17 produced by γδT cells plays a crucial role in the recruitment of neutrophils, macrophages, and T cells to the liver[14,16,17]. Activated neutrophils and macrophages produce high amounts of reactive oxygen species through the nicotinamide adenine dinucleotide phosphate oxidase cascade. Thus, IL-17, by regulating hepatic immune infiltration/inflammation, also modulates hepatic nicotinamide adenine dinucleotide phosphate oxidase-dependent reactive oxygen species production, exacerbating the hepatocellular damage in this context[18].

In NKG2D-deficient (Klrk1-/-) mice, signaling pathways related to the IL-17A receptor and some chemokines (CXCL1, CXCL2, and C-C motif ligand 2) are severely impaired. Additionally, in TCRδ-/- mice fed with a special steatosis-steatohepatitis diet (SSD), liver fibrosis and alpha-smooth muscle actin (α-SMA) expression are significantly reduced, although steatosis remains unaffected. These results suggest that NKG2D-dependent induction of IL-17A expression by γδT cells plays a key role in liver fibrosis within the context of MASLD[14]. In contrast with the above, a carbon tetrachloride (CCl₄)-induced murine model of liver fibrosis revealed that, in chronic liver inflammation, hepatic γδT cells express NK cell p46-related protein (NKp46), which contributes to the direct cytotoxicity against activated hepatic stellate cells (HSCs), mainly by γδT1 cells. This mechanism enhances the killing of these cells through tumor necrosis factor alpha (TNF-α)-related apoptosis-inducing ligand (TRAIL) and fas ligand (FasL)-mediated HSC killing, thereby protecting against liver fibrosis[19].

In mice induced to develop steatohepatitis through a methionine-choline-deficient (MCD) diet, a substantial infiltration of γδT cells is observed, with γδT cells increasing to approximately 10% of total CD3+ T cells in the liver, compared to around 3% in the control group[13]. This increase is primarily due to local proliferation rather than peripheral infiltration. Cells expressing NKG2D and RORγT proliferate faster than γδT cells lacking these markers[14]. These γδT cells exhibit higher C-C chemokine receptors (CCR) 2, CCR5, and CCR6 expression levels. They show a marked increase in IL-17A production, making them the principal producers of this cytokine in MASH. Depletion of γδT cells in Tcrδ-/- and Klrk1-/- knockout murine models results in diminished hepatic leukocytic infiltration and reduced expression of intrahepatic inflammatory mediators, such as IL-1β and IL-6-regulating genes[11,12].

γδT cells have been shown to play a role in the progression of MASLD, primarily through inflammatory and profibrotic mechanisms. Although there is evidence of a possible protective role, most studies suggest that they contribute to damage exacerbation, making them a potential target for new therapeutic approaches. The discrepancy in results may be due to intrinsic differences between the studies. For example, models with SSD, high-fat diet (HFD), or variants aim to mimic an unhealthy western-like diet, focusing on the development of MASLD/MASH hallmarks that are also present in humans (such as body weight gain, accumulation of visceral adipose tissue, hepatomegaly, and an increase in the liver enzymes aspartate aminotransferase and alanine aminotransferase)[14]. On the other hand, models like the MCD diet and CCl₄-induced fibrosis focus on the development of this pathological stage. While the MCD diet develops fibrosis in 2-4 weeks, accompanied by low insulin resistance, HFD requires around 24 weeks to show a similar presentation of steatohepatitis with insulin resistance. As for CCl₄-induced fibrosis, it can even evolve into hepatocellular carcinoma in 15-17 weeks[20,21]. The HFD-fed model fails to replicate the full features of histological changes associated with MASH, as in humans, while the MCD model cannot establish the transition from MASLD to the advanced stage (MASH). Thus, the difference between the molecular pathways involved in each model may explain why some cells react in a distinct manner. For example, in both HFD and MCD models, inflammatory genes are upregulated, but this occurs much earlier in the MCD diet. In the case of fibrosis, HFD requires 24-25 weeks to show an upregulation in fibrotic pathway genes, whereas the same model, when applied for less than 16 weeks, does exhibit this effect; meanwhile, the same changes occur within only 2 weeks of MCD administration[21].

NK CELLS

NK cells are essential components of the innate immune system that eliminate virus-infected and malignant cells without prior exposure to the corresponding antigens. They achieve this through recognition of stress-induced ligands or downregulated MHC-I on target cells. NK cells mediate cytotoxicity via the release of perforin and granzymes, which initiate apoptosis in target cells[22]. In addition, NK cells contribute to immune modulation by producing proinflammatory cytokines such as INF-γ and TNF-α, which enhance macrophage activity and support T cell responses. NK cells orchestrate key responses during infection, inflammation, and tumor surveillance by bridging innate and adaptive immunity[23]. NK cells mature primarily in the bone marrow through defined developmental stages, regulated by transcription factors such as T-box transcription factor TBX21 and eomesodermin[24]. They are broadly classified into CD56bright (immunoregulatory) and CD56dim (cytotoxic) subsets in humans. Among their surface receptors, NKp46+ cells represent a key functional subset involved in antiviral defense and tissue homeostasis, including liver immune regulation[25,26]. NK cells exhibit complex and dynamic interactions with various liver-resident and immune cells, contributing to the regulation of both early inflammation and late-stage fibrosis in MASLD. These interactions depend on the liver microenvironment throughout the progression of the disease.

At the onset of MASLD, metabolic stress in hepatocytes leads to the upregulation of NKG2D ligands, such as MHC class I polypeptide-related sequence A/B, which are recognized by tissue-resident NK cells and γδ T cells, triggering NKG2D-mediated activation and the release of cytotoxic mediators, including TNF-α and TRAIL, that promote hepatocyte apoptosis. In experimental models using an SSD, this early activation also induces IL-17A production, which further licenses hepatocytes to produce chemokines that recruit inflammatory monocytes and macrophages, amplifying liver inflammation and contributing to the progression toward fibrosis[14]. During the initial stages, liver-resident CD56bright cells, enriched within hepatic sinusoids[27,28], predominate and act mainly as immunoregulatory cells. Through INF-γ and TNF-α secretion, they shape monocyte and macrophage recruitment, limit excessive hepatocyte injury, and help maintain local immune balance. As inflammation progresses, peripheral CD56dim NK cells, characterized by higher cytotoxic potential and abundant perforin and granzyme expression, are recruited to the liver, where they amplify hepatocyte apoptosis and perpetuate tissue damage[29].

In early stages of liver fibrosis, resident hepatic NK cells exert a protective antifibrotic role by inducing apoptosis of activated HSCs through NKG2D-retinoic acid early inducible 1 interactions and the release of cytotoxic mediators such as IFN-γ, perforin, granzyme B, TRAIL, and FasL, thereby limiting extracellular matrix accumulation and preventing collagen deposition[30]. Thus, antifibrotic activity is mainly mediated by CD56dim NK cells, which possess strong cytolytic capacity against activated HSC, whereas CD56bright NK cells contribute indirectly by modulating macrophage polarization and cytokine-driven regulation of pathways, including TGF-β/small mother against decapentaplegic, IL-6/signal transducer and activator of transcription 3, and nuclear factor kappa B signaling cascades that control HSC activation and extracellular matrix deposition[31,32]. In advanced stages of MASLD and cirrhosis, resident hepatic NK cells progressively lose their cytotoxic function due to downregulation of effector genes such as FasL, IFN-γ, granzyme B, and TNF, leading to impaired apoptosis of HSCs and uncontrolled extracellular matrix accumulation that drives fibrosis. In contrast, conventional NK cells acquire a proinflammatory phenotype characterized by increased expression of chemokines such as C-C motif chemokine ligand 3 (CCL3), CCL4, and XCL1, the activation marker CD69, and receptors like IL-12 receptor subunit beta-2 and IL-18 receptor accessory protein, which contribute to monocyte recruitment and perpetuation of chronic hepatic inflammation[32]. Thus, as MASLD progresses, a functional shift occurs from immunoregulatory CD56bright NK cells dominating early inflammation and fibrosis control to predominance of exhausted or proinflammatory CD56dim NK cells in advance of disease.

In obesity-driven MASLD, hepatic NK cell function is notably affected by the liver microenvironment, particularly via the presence of TGF-β. Murine obesity models reveal that TGF-β reprograms NK cells into a less cytotoxic phenotype, reducing their ability to eliminate activated HSCs[33]. This dysfunction is exacerbated by additional inflammatory pathways, including IL-15-driven activation of NK cells, which exacerbates liver inflammation and contributes to disease progression[34]. NK cells further enhance liver damage by producing IFN-γ and TRAIL, amplifying hepatocellular apoptosis and inflammation[35]. TRAIL, together with IFN-γ, plays a crucial role in the ability of NK cells to eliminate activated HSC, thereby limiting fibrosis; however, this antifibrotic mechanism becomes impaired as NK cells lose their cytotoxic potential[36].

The antifibrotic role of NK cells, particularly those expressing NKp46+, is demonstrated through modulation of hepatic macrophage polarization. Experimental models using MCD diets demonstrate that NKp46+ cells produce IFN-γ, which regulates the polarization of macrophages to a proinflammatory M1 phenotype, thereby preventing fibrosis[37]. Nonetheless, persistent NK-derived INF-γ also promotes sustained inflammation, eventually favoring the chronic inflammation characteristic of advanced MASLD[36]. The duality of NK cell functions highlights their potential as therapeutic targets in MASLD. Strategies to modulate NKG2D signaling to prevent hepatocyte apoptosis or restore the cytotoxic function of resident NK cells could offer significant clinical benefits, addressing both inflammation and fibrosis in different stages of MASLD[14,32].

NKT CELLS

NKT cells constitute a subset of immune cells present in different peripheral organs. They were named NKT because they express both the TCR and the NK cell receptor (NK1.1). NKT cells express a semi-invariant TCR that recognizes glycolipids from both self and microbial origins, presented by the MHC class I-like molecule CD1d[38,39]. NKT cells are subclassified into invariant or type I NKT (iNKT) cells and type II NKT cells, depending on their TCR specificity. iNKT cells present an invariant TCR Vα24-Jα18α chain paired with TCR Vβ11, which mainly responds to α-galactosylceramide glycopeptide. On the other hand, type II NKT cells use different TCR rearrangements to recognize a diverse repertoire of lipids[40].

Upon activation via TCR, NKT cells exert their immunoregulatory functions via the production of large amounts of chemokines: For example, they produce helper T cell (Th) 1/Th2 cytokines such as IFN-γ, TNF-α, IL-2, IL-4, IL-13, and IL-5, which regulate the functions of dendritic cells, macrophages, B cells, T cells, and NK cells. Due to their innate-like phenotype, NKT cells respond rapidly to stimuli and directly or indirectly regulate other immune cells. They have been reported to play an essential role in various diseases. In the liver, they represent 30%-50% of total lymphocytes[38,39].

In the absence of NKT cells (as shown in BALB/c CD1d-/- murine model, which lacks both type I and type II NKT cells), a HFD produce weight gain and adipose-tissue expansion, higher levels of IL-6 in the liver and adipose tissue and a higher infiltration of macrophages in the liver due to the presence of monocyte chemotactic protein-1 and macrophage inflammatory protein-1[38]. This same model also reveals a significant increase in liver peroxisome proliferator-activated receptor gamma coactivator-1, a transcriptional coactivator crucial in the induction of peroxisome proliferator-activated receptor-α expression, and enhanced mitochondrial β-oxidation, resulting in increased lipogenesis[38].

Mice lacking iNKT (Jα18-/-) show significantly less liver damage than normal mice. Proteins and genes involved in fibrosis, such as α-SMA, tissue inhibitor of metalloproteinase 1 (TIMP-1), collagen type I alpha 1 (COL1A1), and connective tissue growth factor, are significantly reduced in the liver of mice lacking iNKT following a choline-deficient, l-amino acid-defined (CDAA) diet. This model also shows a significant reduction in hepatic infiltration of neutrophils and CD8+ T cells, as well as in the expression levels of genes producing proinflammatory proteins such as TNF-α, IL-6, CXCL-2, nucleotide-binding domain, leucine-rich pyrin containing 3, and IL-1β[41]. In humans, iNKT cells from MASH patients secrete significantly more IFNγ and IL-17A than both MASLD patients and healthy controls, the latter having a population of iNKT cells that constitutively secrete more IL-4 than IFNγ[41].

In a CDAA diet murine model, IL-17+- and IL-22+-iNKT cells (NKT17) as well as IL-10+-iNKT cells (NKT10) increased significantly during the progression of steatosis, reaching a peak at 3 months. However, there was no significant increase in IL-17+ and IL-22+ conventional T cells. Interestingly, IFNγ+-, IL-4+-, and IL-13+-iNKT cells (NKT1/NKT2) increased significantly much later during steatosis/fibrosis. These findings indicate a shift in the cytokine secretion profile of iNKT that correlates with the progression from steatosis to fibrosis[41].

On the other hand, other studies show no apparent role of NKT cells in MASH. No significant reduction in fibrosis levels was observed in a CD1d-/- murine model placed on an SSD[14]. The difference in the results obtained may be due to the stage of fibrosis. A murine model with a fast-food diet and MCD diet demonstrated that CD1d deficiency has almost no effect on hepatic fibrogenesis at the early stage of fibrosis. However, it plays a role in the progression of liver fibrosis at both the molecular and morphological levels. At both stages, intracellular IFN-γ production from NKT rises. Nevertheless, at early-stage fibrosis, there is no change in the expression level of genes associated with HSC activation and fibrosis (COL1A1, TIMP-1, α-SMA, TGF-β1). However, at the progressive fibrosis level, COL1A1 and TIMP-1 mRNA expression are elevated, while α-SMA and TGF-β1 remain unchanged[42]. The role of NKT cells in different stages of steatosis and fibrosis suggests that they are important participants in the cellular and molecular events that occur during the transition from MASLD to MASH, making them potential targets for the diagnosis and treatment of these conditions, as well as other liver-related diseases.

ILCS

ILCs are a family of immune cells that lack antigen-specific receptors but mirror the functions of T helper cells through their cytokine production. They are broadly categorized into three groups: ILC1, ILC2, and ILC3. ILC1s produce IFN-γ and are primarily involved in antiviral and anti-tumor responses; ILC2s secrete IL-5 and IL-13 and play roles in allergic inflammation and tissue repair, while ILC3s, which generate IL-17 and IL-22, are important for mucosal immunity and maintaining epithelial barrier integrity[43]. Recent findings show that ILC subsets are present in the human liver, and their composition changes in chronic liver conditions such as MASLD, where increased frequencies of ILC2s and ILC3s have been associated with fibrotic progression and local inflammation[44]. Their tissue-resident nature and ability to rapidly respond to local cues suggest a potential role in shaping the liver immune microenvironment in MASLD.

ILCs significantly modulate inflammation, fibrosis, and tissue remodeling in MASLD, exhibiting context-dependent functions that depend on the disease stage and the specific ILC subset. Murine models utilizing high-fat diets have shown that ILC3 can exhibit hepatoprotective properties early in disease progression by secreting IL-22, thereby promoting hepatocyte survival, reducing lipid-induced apoptosis, and mitigating inflammation by modulating macrophage polarization and lipid metabolism[45]. However, in advanced stages of MASLD, the functional profile of ILCs shifts toward a profibrotic and proinflammatory phenotype. Studies in murine ischemia-reperfusion injury models reveal that ILC1s become more abundant in fibrotic livers and adopt a proinflammatory phenotype characterized by the increased secretion of IFN-γ and TNF-α, exacerbating hepatic inflammation and promoting hepatocellular apoptosis and extracellular matrix remodeling[46]. The hepatic inflammatory microenvironment, enriched with IL-12, IL-15, and IL-18, further supports ILC1 activation and subsequent HSC transdifferentiation into collagen-producing myofibroblasts[47,48].

Recent human studies have identified a distinct IL-13-producing ILC3-like subset, enriched in fibrotic livers, that contributes significantly to disease progression. Human liver biopsies demonstrated that the IL-13 secretion by these cells leads to direct activation of HSCs and modulation of hepatic macrophages[49]. IL-13 induces CXCL8 production in HSCs, facilitating the recruitment of proinflammatory monocytes and neutrophils. Additionally, the interaction of these cells with Kupffer cells promotes the release of TGF-β1, a key profibrotic cytokine that reinforces HSC activation and fibrosis. In the steatotic liver, elevated levels of TGF-β may also influence the plasticity of ILCs, particularly by shifting the phenotype of conventional NK cells toward ILC1-like cells, which lack cytotoxic capacity and contribute to the maintenance of a chronic inflammatory state[33]. Although this phenotypic conversion primarily involves NK cells, the resulting ILC1-like population exhibits a cytokine profile and pro-fibrogenic impact similar to those of conventional ILC1.

ILC2s also play an increasingly recognized role in the progression of MASLD. As fibrosis worsens, hepatic ILC2 populations become activated and produce higher levels of IL-13, which directly promotes the activation and differentiation of HSCs via IL-33-dependent mechanisms[50]. This IL-33-ILC2-IL-13 axis emerges as a key pathway in sustaining fibrotic responses, particularly in the later stages of liver disease. The bidirectional interaction between ILC and other hepatic immune cells, including Kupffer cells and infiltrating macrophages, further amplifies inflammation and fibrogenesis[51]. In this context, ILC3-derived IL-17A serves as a central mediator of neutrophil recruitment and tissue damage, whereas IL-22 may exert a protective or regulatory role depending on the local balance of signals[52]. Collectively, these findings highlight the multifaceted role of hepatic ILCs in the initiation and progression of MASLD. Their ability to sense local inflammatory cues and rapidly produce effector cytokines positions them as key orchestrators of liver immune homeostasis and potential therapeutic targets. Targeting specific ILC subsets or their cytokine products, such as IL-17A, IL-13, or IL-22, may offer new avenues to modulate immune responses, suppress fibrosis, and restore hepatic function across different stages of MASLD.

MAIT CELLS

MAIT cells are innate-like lymphocytes that recognize microbial-derived vitamin B2 metabolites presented by the MHC class I-related gene protein 1 molecule, allowing them to detect infected or dysregulated cells without prior antigen exposure[53]. MAIT cells mediate cytotoxicity through the release of perforin and granzymes following stimulation via cytokines or TCR, inducing damage in target cells[54]. In addition, MAIT cells can modulate immune responses by secreting proinflammatory cytokines such as IFN-γ, TNF-α, and IL-17, enhancing macrophage activity and promoting adaptive T cell responses[55,56]. By bridging innate and adaptive immunity, MAIT cells orchestrate critical immune responses in infection, inflammation, and tissue homeostasis.

MAIT cells develop in the thymus and mature mainly in mucosal and liver tissues, undergoing maturation regulated by transcription factors such as promyelocytic leukemia zinc finger and RORγT. They are broadly categorized into CD8+ and double-negative subsets, each with distinct effector and proliferative capacities[57]. Among their surface markers, CD161+ MAIT cells represent a key functional population responsive to cytokines like IL-12, IL-18, and IL-15, enabling rapid innate-like responses during microbial and viral infections[58]. MAIT cells play important and context-dependent roles throughout the progression of MASLD. Clinically, a significant decrease in circulating MAIT cells has been observed in MASLD patients, possibly due to peripheral redistribution or cell depletion[59]. Human intervention studies show distinct effects of lifestyle changes on MAIT cell behavior. Dietary modifications specifically reduce peripheral activation of these cells, correlating with improvements in hepatic steatosis. In contrast, aerobic exercise significantly reduces the frequency of intrahepatic MAIT cells and promotes their apoptosis, correlating with a reduction in liver fibrosis severity[60].

Functional studies further highlight the responsiveness of hepatic MAIT cells to inflammatory cytokines, such as IL-12 and IL-18, which drive the production of TNF-α, IFN-γ, IL-17A, and IL-22, suggesting potential roles in both promoting inflammation and supporting tissue repair processes[61,62]. In murine models of diet-induced MASH, IL-23-driven expansion of MAIT cells does not significantly affect hepatic inflammation or fibrosis, underscoring the complexity of their biological roles[63]. In contrast, genetically modified mice with elevated MAIT cell numbers (Vα19 mice) demonstrate downregulated lipogenic gene expression in the liver and reduced serum triglyceride and non-esterified fatty acids levels. These findings imply that MAIT cells may have stage-specific functions during MASLD progression. Moreover, their involvement in early disease phases, such as hepatic lipid accumulation, remains unclear[64].

Preclinical studies using fibrosis-inducing models, such as CCl₄ or CDAA-HFD diets, as well as human liver tissue samples, clearly identify activated MAIT cells as key contributors to liver fibrosis progression. Blocking MAIT cell activation via MHC class I-related gene protein 1 inhibitory ligands, such as Ac-6-FP, effectively reduces the expression of fibrosis-related genes (COL1A1, actin alpha 2, smooth muscle, and TGFβ1) and inflammatory cytokines (TNF-α and IL-17). This approach also promotes a shift in macrophage populations toward an antifibrotic phenotype, highlighting the therapeutic potential of targeting MAIT cells to facilitate the resolution of liver fibrosis[65].

Circulating MAIT cells progressively decline during the transition from simple steatosis to MASH, reflecting changes previously observed in related metabolic diseases such as obesity and diabetes. This reduction in peripheral MAIT cell frequency often coincides with increased infiltration into inflamed tissues, implying possible shifts in cell distribution or altered functionality associated with chronic inflammation. The exact mechanisms underlying these changes and their functional significance in MASLD progression remain to be fully characterized[66]. Collectively, these findings suggest that MAIT cells serve both protective and pathogenic roles during MASLD progression, depending on the disease stage and environmental context. Modulating MAIT cell activation represents a promising therapeutic approach for managing hepatic inflammation and fibrosis in MASLD[38,67,68] (Figure 1).

Figure 1 Immune mechanisms underlying the progression from steatosis to metabolic dysfunction-associated steatohepatitis and fibrosis. During early metabolic dysfunction-associated steatotic liver disease, metabolic stress and lipid accumulation injure hepatocytes and upregulate stress ligands (major histocompatibility complex class I chain-related proteins A/B) and damage-associated molecular patterns, which activate gamma delta T (γδT) cells and natural killer (NK) cells through NK group 2 member D. γδT cells and innate lymphoid cell (ILC)-3 release interleukin (IL)-17A, licensing liver cells to produce C-X-C motif ligand 1 (CXCL1) and CXCL2 that recruit neutrophils and C-C motif chemokine ligand 2 that recruit monocytes. NK and γδT cells promote hepatocyte apoptosis via tumor necrosis factor alpha (TNF-α)-related apoptosis-inducing ligand, Fas ligand, and TNF-α, whereas IL-22 supports hepatocyte survival. In metabolic dysfunction-associated steatohepatitis, sustained inflammation is dominated by interferon-gamma, IL-13, and IL-17A drives monocyte recruitment and hepatic stellate cell (HSC) activation. The IL-33-ILC2-IL-13 pathway, IL-13-producing ILC3-like cells that induce CXCL8, mucosal-associated invariant T cell outputs via major histocompatibility complex-related molecule 1 (TNF-α, IL-17A, interferon-gamma), and invariant NKT cell cytokines further amplify HSC activation. Persistent immune activation polarizes macrophages toward pro-fibrotic phenotypes and increases HSC expression of collagen type I α1 chain, α-smooth muscle actin, and transforming growth factor beta. The schematic integrates crosstalk between lymphocyte subsets (γδT, NK, resident NK, invariant NKT, mucosal-associated invariant T, and ILCs) and hepatic cells (hepatocytes, macrophages, and HSCs) across metabolic dysfunction-associated steatotic liver disease stages. MASH: Metabolic dysfunction-associated steatohepatitis; NKG2D: Natural killer group 2 member D; MICA/B: Major histocompatibility complex class I chain-related proteins A/B; IL: Interleukin; TRAIL: Tumor necrosis factor alpha-related apoptosis-inducing ligand; TNF: Tumor necrosis factor; CXCL: C-X-C motif ligand; IFNγ: Interferon gamma; CCL: C-C motif chemokine ligand; HSC: Hepatic stellate cell; NEFA: Non-esterified fatty acids; FasL: Fas ligand; MR1: Major histocompatibility complex-related molecule 1; COL1A1: Collagen type I α1 chain; ACTA2: Α-smooth muscle actin; TGFβ: Transforming growth factor beta; γδT: Gamma delta T; NKT: Natural killer T cell; rNK: Resident natural killer.

LIMITATIONS AND FUTURE DIRECTIONS

Although important progress has been made in understanding the contribution of innate and innate-like lymphocytes to MASLD, several aspects of this field remain incomplete. Most experimental data derive from murine models of diet-induced steatosis or fibrosis, which do not fully reproduce the metabolic and immunological complexity of human disease. This limitation makes it difficult to translate results and define how the functions of NK, γδ T cells, NKT, ILC, and MAIT cells change across the different stages of liver injury. Experimental variability across studies, including differences in dietary regimens, disease stages, and analytical endpoints, has also led to inconsistent interpretations of whether these cell populations act mainly as drivers or regulators of inflammation and fibrogenesis.

Methodological limitations also affect current knowledge. Many investigators rely on bulk tissue analyses or flow cytometry panels with limited phenotypic depth, which reduces the ability to identify rare or intermediate subsets of innate-like lymphocytes. The lack of longitudinal human studies makes it difficult to establish temporal relationships between immune alterations and the histological progression from steatosis to steatohepatitis to fibrosis. In vitro models, while useful for mechanistic exploration, rarely recapitulate the spatial and metabolic complexity of the liver, where immune, parenchymal, and stromal interactions define disease outcomes.

Future research should prioritize high-dimensional single-cell and spatial transcriptomic approaches to resolve the transcriptional and metabolic programs of hepatic innate lymphocytes at different stages of MASLD. Integrations of these datasets with metabolomic and lipidomic analyses will help clarify how metabolic stress shapes their effector and regulatory functions. The development of standardized animal models that better reflect human metabolic dysfunction, along with the validation of these findings in well-characterized patient cohorts, will be equally important. Such efforts are essential for defining the temporal sequence of immune alterations and evaluating the therapeutic potential of targeting cytokine and stress-sensing pathways, including those mediated by IL-17A, IL-13, IL-22, and NKG2D, during the progression of MASLD.

CONCLUSION

Innate and innate-like lymphocytes are immune cells present in liver tissue that can rapidly respond to mediators, such as damage-associated molecular patterns and cytokines, released by hepatic cells at different stages of MASLD. Several studies highlight the changes in their phenotype and activity during this disease, suggesting their participation from the onset of MASLD to the development and progression of MASH. Nevertheless, they have also shown, in some cases, a protective role. The variation in results is often due to different stages of the disease. However, it makes clear that further investigation is necessary to elucidate the role of these cell types in liver disease. A better comprehension of the activity and interactions between these cells and the liver environment under these circumstances will offer new perspectives for the diagnosis and treatment of MASLD and related diseases.