Abstract
Non-alcoholic fatty liver disease (NAFLD) with its more progressive form non-alcoholic steatohepatitis (NASH) has become the most common chronic liver disease, thereby representing a great burden for patients and healthcare systems. Specific pharmacological therapies for NAFLD are still missing. Inflammation is an important driver in the pathogenesis of NASH, and the mechanisms underlying inflammation in NAFLD represent possible therapeutic targets. In NASH, various intra- and extrahepatic triggers involved in the metabolic injury typically lead to the activation of different immune cells. This includes hepatic Kupffer cells, i.e. liver-resident macrophages, which can adopt an inflammatory phenotype and activate other immune cells by releasing inflammatory cytokines. As inflammation progresses, Kupffer cells are increasingly replaced by monocyte-derived macrophages with a distinct lipid-associated and scar-associated phenotype. Many other immune cells, including neutrophils, T lymphocytes – such as auto-aggressive cytotoxic as well as regulatory T cells – and innate lymphoid cells balance the progression and regression of inflammation and subsequent fibrosis. The detailed understanding of inflammatory cell subsets and their activation pathways prompted preclinical and clinical exploration of potential targets in NAFLD/NASH. These approaches to target inflammation in NASH include inhibition of immune cell recruitment via chemokine receptors (e.g. cenicriviroc), neutralization of CD44 or galectin-3 as well as agonism to nuclear factors like peroxisome proliferator-activated receptors and farnesoid X receptor that interfere with the activation of immune cells. As some of these approaches did not demonstrate convincing efficacy as monotherapies, a rational and personalized combination of therapeutic interventions may be needed for the near future.
Introduction
Non-alcoholic fatty liver disease (NAFLD) has become the most common chronic liver disease (Cotter & Rinella 2020). Various stages are grouped under the term NAFLD (Fig. 1): the initial stages of non-alcoholic fatty liver (NAFL) can progress to non-alcoholic steatohepatitis (NASH) characterized by persistent inflammatory processes, which in turn can progress to NASH fibrosis. The long-term risk is the transition to liver cirrhosis. NAFLD is very often associated with other diseases of the metabolic syndrome and is partly described as its hepatic manifestation. Thus, in the early stages, cardiovascular events represent the main cause of death in NAFLD, whereas in the later stages, liver-associated causes of death as decompensated cirrhosis or hepatocellular carcinoma (HCC) predominate (Sanyal et al. 2021, Simon et al. 2022). NAFLD and NASH are already a major burden in health economics, and prevalence rates for NAFLD as well as HCC caused by NASH continue to rise (Allen et al. 2018, Huang et al. 2021, Gu et al. 2022). Despite the high prevalence and economic relevance, direct therapeutic options are still lacking. A necessary requirement for the successful development of therapies is a profound understanding of the underlying pathogenic mechanisms (Ratziu et al. 2022).
Spectrum of non-alcoholic fatty liver disease (NAFLD) and involved factors in pathogenesis and regression. NAFLD affects about 25% of the general population and comprises different stages, ranging from steatosis (non-alcoholic fatty liver, NAFL) to non-alcoholic steatohepatitis (NASH) to fibrosis and its life-threatening sequelae cirrhosis and hepatocellular carcinoma (HCC). Relevant mechanisms of pathogenesis at the different disease stages as well as of regression are mentioned in the boxes. HSC, hepatic stellate cells. Image created with BioRender.com.
Citation: Journal of Endocrinology 256, 1; 10.1530/JOE-22-0194
The initial basis of the pathogenesis is metabolic injury, which occurs both intra- and extrahepatically (Schuster et al. 2018). However, not all patients with NAFL develop progression (Hagstrom et al. 2017). Rather, the transition to an inflammatory stage is the key mechanism of pathogenesis of NASH (Parthasarathy et al. 2020). A multitude of different immune cells, both of the innate and the acquired immune system, are involved in this process (Huby & Gautier 2022, Peiseler et al. 2022). The proinflammatory pathways can subsequently lead to fibrosis through the activation of hepatic stellate cells (HSCs) (Carter & Friedman 2022). The aim of this review is to provide a comprehensive overview of relevant inflammatory mechanisms in NAFLD and currently researched therapeutic options targeting these mechanisms.
Pathogenesis
Metabolic injury and hepatocytes
There are several intra- as well as extrahepatic triggers for inflammation in NASH that relate to the metabolic injury to the liver (Schuster et al. 2018). While this review does not aim to provide a detailed overview of the processes involved, it is important to understand the basic mechanisms, as they represent potential targets for pharmacological intervention. Insulin resistance is nearly always present in NAFLD, and there is a strong association between insulin resistance and the development of hepatic steatosis (Khan et al. 2019, Grzych et al. 2021). The visceral adipose tissue itself is not only involved in impaired insulin metabolism but also contributes to the generation of a systemic inflammatory environment through the secretion of inflammatory cytokines such as CC-chemokine ligand (CCL) 2 (Kanda et al. 2006, Adolph et al. 2017). Alterations in the gut–liver axis represent another extrahepatic trigger for hepatic inflammation as NASH is often associated with intestinal dysbiosis (Brandl & Schnabl 2017). Intestinal dysbiosis (Boursier et al. 2016) in combination with intestinal barrier dysfunction (Rahman et al. 2016, Mouries et al. 2019) is thought to lead to increased bacterial translocation and higher secretion of inflammatory cytokines and interferons (IFNs) (Jiang et al. 2015), which in turn activate intrahepatic inflammatory pathways. For instance, more circulating endotoxins can be detected in NASH (Kitabatake et al. 2017). Recent work has even demonstrated the influence of the microbiome on the localization of immune cells within the liver (Gola et al. 2021).
Intrahepatically, metabolic injury primarily affects hepatocytes. In addition to inflammatory stimuli from the circulation, glucose and lipid metabolism within the hepatocytes become dysbalanced (Schuster et al. 2018). In particular, nuclear receptors such as peroxisome proliferator-activated receptors (PPARs) and farnesoid X receptor (FXR) play a crucial role in this process (Puengel et al. 2022). Accumulation of free fatty acids in hepatocytes increases the presence of lipotoxic intermediate metabolites such as palmitates, stearates, and ceramides (Wei et al. 2006, Allard et al. 2008, Garcia-Jaramillo et al. 2019). These lipotoxic metabolites as well as cholesterol and fructose can induce endoplasmic reticulum (ER) stress and mitochondrial dysfunction (Han et al. 2008, Min et al. 2012, Kim et al. 2018, Federico et al. 2021). ER stress causes an increased production of reactive oxygen species (ROS) (Zhang et al. 2014). Mitochondrial dysfunction impairs the primary metabolism of free fatty acids by β-oxidation, which can then accumulate further and are alternatively oxidated in peroxisomes and cytochromes generating even more ROS (Ramanathan et al. 2022). In the course, there is an increase in oxidative stress, loss of ATP, and subsequent decrease in mitochondrial integrity, which ultimately leads to cell death of hepatocytes (Machado & Diehl 2016). Accumulation of cholesterol may additionally disrupt membrane fluidity (Horn et al. 2022) and directly activate the NLR family pyrin domain containing 3 (NLRP3) inflammasome (Ioannou et al. 2017). Collectively, these processes ultimately lead to apoptosis or necrosis of affected hepatocytes via membrane disruption and consequent release of proteases into the cytoplasm (Feldstein et al. 2004, Hirsova et al. 2016, Liu et al. 2016). One of the most important molecular mediators of hepatocyte apoptosis in the context of NASH is represented by apoptosis signal-regulating kinase (ASK)-1, which is part of the mitogen-activated protein kinase (MAP3K) family. After activation by ROS or ER stress, ASK1 induces a downstream upregulation of JNK and p38 (Xiang et al. 2016). ASK1 has been shown to be strongly activated in NASH. Increased cell death ultimately leads to sterile inflammation due to the release of chemokines and extracellular vesicles. Apoptosis of hepatocytes also induces activation of HSC (Watanabe et al. 2007) and therefore links metabolic injury to fibrosis.
Even before undergoing apoptosis, hepatocytes suffering from lipotoxicity can also release proinflammatory extracellular vesicles (Kakazu et al. 2016), including exosomes and microvesicles, which can contain chemokines, inflammatory mediators as well as pro-fibrogenic microRNA. These vesicles influence crosstalk and can further amplify inflammation and fibrosis due to the activation of myeloid cells and HSC (Srinivas et al. 2020). This local signaling from stressed and apoptotic cells, together with the systemic proinflammatory changes, provide a strong stimulus for the activation and recruitment of various immune cells in the liver (Fig. 2).
Overview of immune cells involved in the pathogenesis of NASH. Intra- and extrahepatic triggers can lead to the activation of different immune cells in NASH, which includes various cell types of the innate and also the adaptive immune system. Kupffer cells, as the most frequent resident immune cells in the liver, adopt an inflammatory phenotype and activate other immune cells by releasing inflammatory cytokines. As inflammation progresses, Kupffer cells are replaced by monocyte-derived macrophages. Also, other immune cells as neutrophils and Type 1 T helper cells contribute to the inflammatory environment in NASH. However, also anti-inflammatory effects of immune cells in NASH are described for special subsets for natural killer cells and possibly for regulatory T cells. In addition, the effects of different cell types can differ between different stages of diseases. Thus, this figure can only provide a simplified overview of some of the relevant mechanisms. CCL, CC-chemokine ligand; CCR, CC chemokine receptor; CTLA-4, cytotoxic T-lymphocyte-associated protein 4; CXCL, chemokine (C-X-C motif) ligand; DC, dendritic cell; ECM, extracellular matrix; HSC, hepatic stellate cells; IL, interleukin, IL; INF, interferon; KC, Kupffer cells; mitDNA, mitochondrial DNA; MPO, myeloperoxidase; NASH, non-alcoholic steatohepatitis; NET, neutrophil extracellular trap; NK, natural killer cells; NKT, natural killer T cells; TGF, transforming growth factor; Th cells, T helper cells; TNF, tumor necrosis factor; Tregs, regulatory T cells. Image created with BioRender.com.
Citation: Journal of Endocrinology 256, 1; 10.1530/JOE-22-0194
Kupffer cells/macrophages
Kupffer cells (KCs) are the tissue-resident macrophages of the liver. As they are localized at the luminal site of the liver sinusoidal endothelial cells, they represent the liver’s first line of defense against potential damage. It is therefore reasonable that they also play an important role in NASH (Krenkel & Tacke 2017), especially in the early stages.
KCs register metabolic injury via damage-associated molecular patterns (DAMPs), pathogen-associated molecular patterns (PAMPs), and directly via toxic metabolites (Chatterjee et al. 2012, Wen et al. 2021). For example, mitochondrial DNA and vesicles with tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) can activate macrophages (Garcia-Martinez et al. 2016). Accumulation of cholesterol can also directly induce activation of KC via the NLRP3 inflammasome (Ioannou et al. 2017). Upon activation, KCs adopt a proinflammatory phenotype and in turn secrete a large number of different cytokines that activate a variety of other immune cells and, in part, further amplify metabolic injury in hepatocytes in a positive feedback loop (Canbay et al. 2003, Reid et al. 2016, Pan et al. 2018). This inflammatory phenotype includes increased release of IL-1β, TNFα, CCL2 and 5, TRAIL, and Fas ligand. These have different effects, for example, IL-1β enhances the accumulation of triglycerides in hepatocytes via PPARα and recruits other immune cells (Garcia-Martinez et al. 2016). In recent years, however, it has become increasingly apparent that these proinflammatory effects cannot be found in all KC subsets: In mouse NAFLD models, the inflammatory, disease-aggravating phenotype applies primarily to the so-called KC2 (CD206hiESAM+) which are more frequent in NASH whereas anti-inflammatory KC1 (CD206loESAM−) are reduced (Bleriot et al. 2021). Additionally, KC2 are known to have impaired self-renewal (Tran et al. 2020). In human NASH samples, a subset of CD68+, MARCO−, and TIMD-4− macrophages was described that resembles the KC2 (MacParland et al. 2018, Guilliams et al. 2022).
In the progression of the disease, proinflammatory cytokines drive the recruitment of monocyte-derived macrophages (MoMF) from the circulation, which gradually replace the KC (Barreby et al. 2022). The chemokines CCL1, 2, and 5 play an important role in this process (Baeck et al. 2012, Miura et al. 2012). In addition to recruitment, these chemokines can also induce a polarization of the MoMF toward an inflammatory phenotype (Ruytinx et al. 2018). Therefore, inhibition of these chemokines, especially CCL2 and 5, or of the CC chemokine receptors (CCR) 2/5 appeared as a possible therapeutic approach and was intensely studied (Krenkel et al. 2018). In addition to chemokines, CD44 also plays an important role, as it is upregulated in NASH and supports macrophage recruitment and activation (Patouraux et al. 2017). This represents another potential therapeutic approach, but unlike CCR2/5 inhibitors, which have already been investigated in a phase III trial, these approaches are much less advanced (Kodama et al. 2015).
The recruited MoMF are derived from Ly6Chi monocytes in mice, which can differentiate to different subsets. On the one hand, they can differentiate to so-called monocyte-derived KCs which are defined in mice as F4/80+, CLEC4F+, TIM4+/−, CRlg+/− cells that are proinflammatory macrophages releasing cytokines and ROS (Tran et al. 2020). But recruited MoMF can also develop to ‘lipid-associated’ and/or ‘scar-associated macrophages’, which are characterized by the expression of triggering receptor expressed on myeloid cells 2 (TREM2), CD9, and osteopontin in human NASH and mouse NASH models (Krenkel et al. 2020, Remmerie et al. 2020, Seidman et al. 2020). Although these TREM2+ scar-associated macrophages are characteristic to the fibrotic niche in human cirrhosis as well (Ramachandran et al. 2019) and were therefore thought to have a profibrotic role, recent studies showed TREM2 may not act fibrogenic, but seems to be functionally involved in limiting inflammation and fibrosis (Hou et al. 2021, Hendrikx et al. 2022). The biological function of the different MoMF subsets is not yet fully understood.
During the resolution of inflammation, macrophages can also adopt a restorative phenotype (Wan et al. 2014). In mouse models, they can transform into Ly6Clo restorative macrophages, which express regenerative growth factors and anti-inflammatory cytokines (Ramachandran et al. 2012). Influencing the switch of phenotype represents another potential therapeutic approach to target inflammation in NASH.
Neutrophils
In addition to KCs, neutrophils are activated by DAMPs, PAMPs, cytokines, and apoptotic bodies and are recruited to the injured liver (Marra & Tacke 2014, Cai et al. 2019). Thus, increased infiltration of neutrophils in the liver is observed in biopsies of NASH patients. CXCL1 and IL-8, among others, play an important role in their recruitment (Bertola et al. 2010). Activated neutrophils produce proinflammatory mediators and secrete typical peptides from their granules, including neutrophil elastase, myeloperoxidase (MPO) and lipocalin-2, the amount of which is also increased in NASH (Bertola et al. 2010, Hwang et al. 2021). MPO enhances hepatocyte injury and macrophage activation (Rensen et al. 2012, Pulli et al. 2015). However, in addition to these proinflammatory mechanisms, there is also some evidence of anti-inflammatory effects of neutrophils. For instance, microRNA (miR)-233, which is mainly found in neutrophils, is increasingly expressed in human NASH livers and probably represents a protective mechanism (Pirola et al. 2015, He et al. 2019). Via effects on lipid metabolism and macrophage polarization, miR-233 contributes to the inhibition of disease progression. Its deletion results in increased NASH in animal models.
Natural killer cells
Natural killer (NK) cells participate in the inflammatory pathogenesis of NASH (MartÃnez-Chantar et al. 2021). After their activation, they are able to produce IFNγ and thus induce an inflammatory polarization of macrophages (Wensveen et al. 2015). Although IFNγ also appears to have an antifibrotic effect, this effect has been shown to be diminished in patients with insulin resistance and NASH (Amer et al. 2018). Nonetheless, NK cells may also have antifibrotic activities by inducing de-activation and cell death of HSCs (Wallace et al. 2022).
Dendritic cells
Myeloid dendritic cells (DCs), also called classical DCs (cDCs), represent an important link between the innate and the adaptive immune system. While a protective effect against hepatic inflammation was initially postulated in mouse models (Henning et al. 2013, Heier et al. 2017), proinflammatory effects for different subsets of DC became apparent over time. In particular, a subset of XCR1+ subset of cDC type 1 was described in mice and humans that seems to favor the development and progression of NASH (Deczkowska et al. 2021). A possible therapeutic approach to interfere with proinflammatory cDC represents the inhibition of fractalkine receptor CX3CR1, which is involved in the recruitment of cDC. Treatment with an antagonist showed reduced infiltration with cDC and improved hepatic inflammation in mice (Sutti et al. 2019). Overall, however, the contribution of DCs in NASH is not yet fully understood.
T and B lymphocytes
In addition to the described effects of the innate immune system, more and more influences of the adaptive immune system in NASH are being described (Sutti & Albano 2020). Their interactions and influences are less well understood than those of the innate immune system, but it is well-known that infiltration of lymphocytes is a histological feature of NASH in patients (Sutti et al. 2014, Wolf et al. 2014). In this context, the degree of infiltration correlates with parenchymal damage and inflammation. Animal models with knockouts leading to an absence of mature B and T cells show less liver damage and inflammation (Wolf et al. 2014). This principally highlights the relevant role of lymphocytes in the pathogenesis of NASH.
The increased secretion of IFNγ in NASH causes T helper cells (Th cells) to polarize more toward Th1 cells in mice (Sutti et al. 2014). These Th1 cells in turn produce additional proinflammatory cytokines, including IFNγ, IL-2, TNFα, and lymphotoxin-a, and modulate the stimulation of macrophages to an inflammatory phenotype. Th17 cells have also been described to be more abundant in NASH, which has been linked to increased production of IL-17 although the specific mechanisms remained unclear (Gomes et al. 2016, Rau et al. 2016). Recently a distinct Th17 subset was described in animal models and patient samples that contributes to the pathogenesis of NASH. These inflammatory hepatic CXCR3+Th17 cells induce inflammation via production of IL-17, IFNγ as well as TNFα (Moreno-Fernandez et al. 2021). For Th22 cells, the evidence is rather vague, possibly showing a more antagonistic effect to Th17 in mice models and thus overall a more protective function (Rolla et al. 2016).
Cytotoxic CD8+ T cells are recruited more frequently in NASH. However, different behaviors of certain subsets are observed here as well (Wolf et al. 2014). While certain CD8+ memory cells (CD69+ CD103− CD8+ tissue resident memory T cells) may play a role in disease regression in mice and humans (Koda et al. 2021), at the same time a particularly aggressive subset (CD8+ CXCR6+ FOXO1low PD1high cells) has been described in NASH mouse models and confirmed in human samples showing a strong auto-aggressive behavior toward hepatocytes in the presence of specific lipotoxic metabolites such as acetate (Dudek et al. 2021).
The role of other T cells in the pathogenesis of NASH is insufficiently understood. While regulatory T cells (Tregs) are expected to have a more protective effect on NASH due to their inhibitory effects on CD4+ and CD8+ cells in the liver (Crispe 2014), there is conflicting data on whether their numbers increase or decrease in NASH in patients and mice (Rau et al. 2016, Wang et al. 2021). Some of these discrepancies may be related to the analysis of different mouse models and stages of the disease (Sutti & Albano 2020).
Natural killer T (NKT) cells are able to recognize lipid antigens directly, providing a direct pathway for activation of the acquired immune system by metabolic injury (Marrero et al. 2018). After their activation, they secrete different cytokines (IL-4, IL-10, IFNγ, and TNF), which in turn can activate Th1, Th2, as well as Treg (Lee et al. 2015). Thus, different effects on NASH are plausible. γδ-T cells are found to be increased in NASH in animal models, and their deletion results in less liver damage and less inflammation, possibly mediated via IL-17 (Li et al. 2017).
Another proinflammatory influence is attributed to B cells, especially the B2 subset. In patients with NASH, they form part of the inflammatory infiltrates and there is also an upregulation of B-cell activating factor (BAFF) (Miyake et al. 2013) In mouse models of NAFLD and NASH with B cell-specific deletions, a decrease of disease was shown, possibly mediated by reduced Th1, which are otherwise activated by the proinflammatory stimuli of B cells (Bruzzi et al. 2018, Barrow et al. 2021).
Interactions of immune cells with stellate cells
The various immune cells of the innate and acquired immune system present a large number of interactions with each other and with hepatocytes, which may contribute to the maintenance of inflammation and metabolic injury in NASH. However, to trigger progression from NASH to NASH-fibrosis, activation of the HSC, the main fibrogenic cell type in the liver, is required. Many of the triggers and stimuli already described are involved in this process. HSCs are considered the cell type with the highest number of interactions in the liver, including with immune cells in particular (Xiong et al. 2019). The variety of stimuli for HSC in NASH includes hepatocyte apoptosis, with apoptotic bodies and pre-apoptotic vesicles with inflammatory cytokines acting as DAMPs and being recognized by HSC (Watanabe et al. 2007, Zisser et al. 2021). Activation of the NLRP3 inflammasome, as described earlier, may also occur directly in HSC (Inzaugarat et al. 2019, Gaul et al. 2021). Various inflammatory stimuli originate from immune cells: B cells activate HSC via TNFα and IL-8, CD8+ via TNFα and IFNγ, NKT cells transmit activating signals via sonic hedgehog and osteopontin, and macrophages via transforming growth factor (TGF) β, platelet-derived growth factor, TNFα, ROS, and IL-1β (Tsuchida & Friedman 2017). HSCs upon activation secrete inflammatory cytokines and recruit additional immune cells (Carter & Friedman 2022). Most relevantly, activation of quiescent HSC leads to their differentiation and proliferation into inflammatory myofibroblasts which then produce high amounts of extracellular matrix, resulting in fibrosis (Tsuchida & Friedman 2017).
Resolution of inflammation and fibrosis
Overall, the mechanisms involved in disease regression are less well understood, but immune cells play a relevant role in this process as well (Fig. 3). As already briefly mentioned earlier liver macrophages have the potential to transform into Ly6Clo restorative macrophages in mice (Wan et al. 2014). These have the potential to support the resolution of inflammation through the secretion of regenerative growth factors and anti-inflammatory cytokines. Additionally, there is also evidence that these cells lead to regression of fibrosis through apoptosis of activated HSC and degradation of the extracellular matrix. Also, neutrophils may improve inflammation through the secretion of different mediators, e.g. annexin A1, phosphatidylserine, and lactadherin (Soehnlein et al. 2017). Regression of fibrosis requires inactivation or apoptosis of HSC to prevent the further accumulation of collagen. Their apoptosis can be induced via FAS-FASL by CD8+ memory cells (Koda et al. 2021). In animal models of chronic liver disease also γδ-T cells were able to induce apoptosis of HSC (Liu et al. 2019). Further understanding of these mechanisms may yield additional therapeutic targets and therefore represents an important area of research.
Overview of mechanisms of disease regression in non-alcoholic fatty liver disease. Regression of NASH involves apoptosis or inactivation of hepatic stellate cells, a change in the phenotype of macrophages and secretion of specialized pro-resolving mediators – mainly by neutrophils. The layout of these mechanisms does not portray chronological order, rather different processes probably occur simultaneously. γδ T cell, gamma delta T cell; HSC, hepatic stellate cell. Image created with BioRender.com.
Citation: Journal of Endocrinology 256, 1; 10.1530/JOE-22-0194
Therapeutic options and directions of drug development
Currently, no specific pharmacotherapy has been approved for the treatment of NASH. Thus, current treatment strategies focus on optimizing the pharmacotherapy of comorbidities (e.g. for type 2 diabetes) and promoting weight loss, either by lifestyle changes and/or bariatric surgery. Weight loss has shown beneficial effects on NAFLD (Marchesini et al. 2016, Romero-Gomez et al. 2017, Lee et al. 2019), such that a decrease in body weight of ≥10% can lead to a resolution of NASH. However, lifestyle changes are rarely sustained in the majority of patients, and bariatric surgery is accompanied by surgical risks and is currently only available for selected patients, depending on the country and health insurance policies. More recently, pharmacotherapies demonstrated effective weight loss by targeting glucagon-like peptide (GLP-1) such as semaglutide (Wilding et al. 2021) and/or the glucose-dependent insulinotropic polypeptide (GIP) such as the dual GLP-1/GIP agonist tirzapetide (Jastreboff et al. 2022). However, their long-term effects on NASH fibrosis and liver-related outcomes remain to be demonstrated.
In general, pharmacological therapeutic approaches have been proposed that target the different steps of pathogenesis. Drugs have been developed to prevent or attenuate metabolic injury by, for example, improving insulin sensitivity, inhibiting de novo lipogenesis, or improving the metabolism of fatty acids. This approach includes agonists of the nuclear receptors PPARs and FXR, but also fibroblast growth factor (FGF) analogs and thyromimetics (i.e. synthetic analogs of thyroid hormones with liver-specific thyroid hormone actions). Other substance groups attempt to prevent hepatocyte injury, these include ASK1 inhibitor and caspase inhibitors. A third approach is to inhibit the recruitment or activation of immune cells. Of the substances that have already been well studied in regard to modulation of inflammation, CCR2/5 antagonists deserve special mention. As fibrosis is the endpoint of pathogenesis, antifibrotic therapies are also being investigated.
In order to facilitate drug development, the regulatory agencies have agreed to accept the clear histological improvement of NASH and/or regression of fibrosis in longitudinal liver biopsies as surrogate parameters that could lead to conditional drug approval, while patient-related end-points (e.g. mortality, liver-related complication, cardiovascular events) are being monitored in long-term trials (Rinella et al. 2019). For some of the abovementioned proposed ‘NASH drugs’, such interim data on surrogate endpoints are being available from clinical trials (see later).
Recently terminated advanced trials
Developing effective, specific drugs for treating NASH has proven to be very challenging, as several monotherapies have not reached the desired efficacy endpoints in clinical trials. In the last few years, results of several phase IIb or III trials have been reported (Table 1). While the investigated agents showed promising results in the preceding preclinical and early clinical studies, expectations were not met for many agents. This includes compounds from the PPAR agonist group, elafibrinor (Ratziu et al. 2016) and seladelpar (Haczeyni et al. 2017), the FGF analogs aldafermin (FGF-19) and pegbelfermin (FGF-21), the apoptosis inhibitors emricasan and selonsertib, as well as cenicriviroc as a CCR2/5 inhibitor. Unfortunately, all of these failed to meet their respective primary endpoints.
Recently terminated advanced trials in NASH (selection).
| Name | Type | Phase | Result | Patients |
|---|---|---|---|---|
| Elafibranor | PPARα/δ agonist | III | No significant resolution of NASH (72 weeks) | NASH F1–3 fibrosis |
| Cenicriviroc | CCR2/5 inhibitor | III | No significant improvement in liver fibrosis (52 weeks) | NASH F2/3 fibrosis |
| Selonsertib | ASK1 inhibitor | III | No significant improvement in liver fibrosis or cirrhosis (48 weeks) | Compensated NASH cirrhosis (STELLAR-4) NASH F3 fibrosis (STELLAR-3) |
| Emricasan | Caspase inhibitor | IIb | No significant improvement in liver fibrosis (72 weeks) | NASH F1–3 fibrosis |
| Aldafermin | FGF-19 analogon | IIb | No significant improvement of fibrosis (24 weeks) | NASH F2/3 fibrosis |
| Pegbelfermin | FGF-21 analogon | IIb | No significant improvement of fibrosis (24 weeks – FALCON 1, 48 weeks – FALCON 2) | NASH cirrhosis (FALCON 2) NASH F3 fibrosis (FALCON 1) |
| Seladelpar | PPARδ agonist | II | No significant reduction in liver fat (week 12) | NASH F1-3 fibrosis |
ASK, apoptosis signal-regulating kinase; CCR, CC-chemokine receptor; FGF, fibroblast growth factor; NASH, non-alcoholic steatohepatitis; PPAR, peroxisome proliferator-activated receptor.
The PPARα/δ agonist elafibrinor failed to achieve significant resolution of NASH at 72 weeks in patients with NASH fibrosis F1–3 (Harrison et al. 2020c), and the PPARδ agonist seladelpar also was unable to achieve the primary goal of reducing hepatic fat in MRI-estimated proton density fat fraction at 12 weeks (Harrison et al. 2020b). The apoptosis inhibitors selonsertib (ASK-1 inhibitor) and emricasan (caspase inhibitor) did not achieve higher rates of improvement in fibrosis compared with placebo control at 48 and 72 weeks, respectively (Harrison et al. 2020d, Harrison et al. 2020a). Additionally, no improvement in clinical events could be achieved for emricasan in patients with decompensated NASH cirrhosis either (Garcia-Tsao et al. 2020). For both FGF analogs, aldafermin and pegbelfermin, the primary endpoint also measured fibrosis suppression, but this endpoint was not met at 24 weeks (Armstrong 2021, Harrison et al. 2022b). Cenicriviroc is a dual chemokine receptor CCR2/CCR5 inhibitor that intends to inhibit inflammation by affecting macrophage recruitment and polarization in NASH. Additionally, there are direct effects on HSC by preventing their activation through TGFβ (Kruger et al. 2018). In the phase IIb study, significant improvement in fibrosis was achieved after 1 year, but improvements were not sustained after 2 years (Friedman et al. 2018, Ratziu et al. 2020). The phase III study was terminated because no improvement in fibrosis was achieved in the interim analysis after 1 year.
Currently ongoing single-drug advanced trials
Currently, a wide variety of substances continue to be investigated in advanced clinical trials (Vuppalanchi et al. 2021). This includes other representatives of the substance groups mentioned earlier, but also different therapeutic approaches (Table 2). Phase III trials are currently ongoing for obeticholic acid (OCA), an FXR agonist; resmetiron, a so-called tyreomimetic, which is an agonist for the hepatic thyroid hormone receptor (THR) β1; aramchol, a modulator of fatty acid metabolism; lanifibranor, a pan-PPAR agonist; and semaglutide, a GLP-1 analog, which has been approved for the treatment of type 2 diabetes for several years already. In addition, there are two adaptive IIb/III studies, one for cotadutide, a dual GLP-1 and glucagon agonist, and belapectin, a galectin 3 inhibitor.
Currently ongoing advanced trials analyzing mono-drug therapies in NASH (selection).
| Name | Type | Phase | Status | Primary endpoint | Patients |
|---|---|---|---|---|---|
| Obetichol acid | FXR agonist | III | Active, not recruiting | Improvement in fibrosis by at least one stage (18 months) or achieving NASH resolution (only REGENERATE) |
NASH F2/3 fibrosis (REGENERATE) Compensated cirrhosis (REVERSE) |
| Tropifexor | FXR agonist | II | Recruiting | Improvement in fibrosis by at least one stage or achieving NASH resolution (48 weeks) |
NASH F2/3 fibrosis (ELIVATE) |
| EDP-305 | FXR agonist | IIb | Recruiting | Improvement in fibrosis by at least one stage or achieving NASH resolution (72 weeks) |
NASH F2/3 fibrosis |
| Resmetirom | THR-β1 agonist | III | Recruiting | Achieving NASH resolution (52 weeks) | NASH F2/3 fibrosis (MAESTRO-NASH) |
| VK2809 | THR-β1 agonist | IIb | Recruiting | Achieving NASH resolution (52 weeks) – only secondary endpoint | NASH F1–3 fibrosis |
| ASC41 | THR-β1 agonist | Seamless IIa/IIb | Not yet recruiting | Improvement of NASH (52 weeks) | NASH (NAS≥4) |
| Aramchol | Inhibition lipidogenesis | III | Recruiting | Improvement in fibrosis by at least one stage or achieving NASH resolution (72 weeks) |
NASH F2/3 fibrosis |
| TVB-2640 | Inhibitor fatty acid synthase | IIb | Recruiting | Improvement or resolution of NASH or (52 weeks) | NASH F2/3 fibrosis |
| Lanifibranor | Pan-PPAR agonist | III | Recruiting | Improvement in fibrosis by at least one stage or achieving NASH resolution (72 weeks) |
NASH F2/3 fibrosis |
| Saroglitazar | PPARα/δ agonist | IIb | Recruiting | Achieving NASH resolution (72 weeks) | NASH F2/3 fibrosis |
| Semaglutide | GLP-1 receptor agonist | III | Recruiting | Improvement in fibrosis by at least one stage or achieving NASH resolution (72 weeks) |
NASH F2/3 fibrosis |
| Tirzepatide | Dual GLP-1 and GIP agonist | II | Recruiting | Achieving NASH resolution (52 weeks) | NASH F2/3 fibrosis |
| BI 456906 | Dual GLP-1 and glucagon agonist | II | Recruiting | Improvement of NASH (48 weeks) | NASH F1–3 fibrosis |
| Licogliflozin | SGLT1/2-inhibitor | II | Recruiting | Improvement in fibrosis by at least one stage or achieving NASH resolution (48 weeks) |
NASH F2/3 fibrosis |
| Cotadutide | Dual GLP-1 and glucagon agonist | Adaptive IIb/III | Not yet recruiting | Improvement in fibrosis by at least one stage or achieving NASH resolution (48 weeks) |
NASH F2/3 fibrosis |
| Belapectin | Galectin 3 inhibitor | Adaptive IIb/III | Recruiting | Development of new esophageal varices (18 months) | NASH cirrhosis without varices |
| Efruxifermin | FGF-21 analogon | IIb | Active, not recruiting (Harmony) Recruiting (Symmetry) |
Improvement in fibrosis by at least one stage (24 weeks, Harmony; 36 weeks Symmetry) |
NASH cirrhosis (Harmony) NASH F2/3 fibrosis (Symmetry) |
| MK-3655 | Anti-FGFR1/KLB agonist | IIb | Recruiting | Achieving NASH resolution (52 weeks) | NASH |
| BFKB8488A | Anti-FGFR1/KLB agonist | II | Active, not recruiting | Achieving NASH resolution (52 weeks) | NASH F2/3 fibrosis |
| NNC0194-0499 | FGF agonist | II | Recruiting | Improvement in fibrosis by at least one stage (52 weeks) |
NASH F2–4 fibrosis |
| AXA1125 | Amino acids | II | Recruiting | Improvement of NASH (48 weeks) | NASH fibrosis |
| Namodenoson | A3AR agonist | IIb | Recruiting | Improvement of NASH (36 weeks) | NASH |
| Icosabutate | Fatty acid | IIb | Active, not recruiting | Resolution of NASH (52 weeks) | NASH F1–3 fibrosis |
| Rencofilstat | Cyclophilin inhibitor | IIb | Not yet recruiting | Improvement in fibrosis by at least one stage or achieving NASH resolution (12 months) |
NASH F2/3 fibrosis |
| Norursodeoxycholic | Modified bile acid | IIb | Recruiting | Achieving NASH resolution (72 weeks) | NASH fibrosis |
| ION224 | Antisense inhibiting DGAT2 | II | Recruiting | Improvement of NASH (49 weeks) | NASH |
Phase IIa studies are not included.
A3AR, A3 adenosine receptor; DGAT, diglyceride acyltransferase; FGF, fibroblast growth factor; FXR, farnesoid X receptor; GIP, glucose-dependent insulinotropic polypeptide; GLP-1, glucagon-like peptide-1; NASH, non-alcoholic steatohepatitis; PPAR, peroxisome proliferator-activated receptor; SGLT, sodium-dependent glucose cotransporters; THR, thyroid hormone receptor.
OCA is a selective agonist of FXR, which is a nuclear receptor expressed mainly in the liver and intestine. OCA influences hepatic glucose metabolism, affecting glycogenolysis and gluconeogenesis, as well as the oxidation of fatty acids. Indirectly, this leads to decreased inflammation and subsequently reduces fibrosis. In addition to metabolic effects, inhibition of inflammatory cytokines in macrophages has also been observed, at least in animal studies (Yao et al. 2014). Furthermore, an enhancement of inflammatory polarization of macrophages could be shown in vitro and in vivo (McMahan et al. 2013). The IIb study was terminated ahead of schedule upon significant histologic improvement (Neuschwander-Tetri et al. 2015). In the REGENERATE phase III study in patients with F2/3 fibrosis, the interim analysis at 18 months demonstrated a significantly more frequent improvement in fibrosis by at least 1 stage as well as a trend toward improvement in NASH with the reduction in ballooning as well as inflammation (Younossi et al. 2019). With these interim results, an application for conditional approval was submitted to the FDA. However, this has not yet been approved, and further data have been requested. In addition, the REVERSE Phase III trial is currently ongoing to evaluate OCA in compensated NASH cirrhosis. However, the use of OCA is limited by dose-dependent pruritus, which affects approximately 50% of patients at high dosages. While several second-generation FXR agonists have been developed with the aim to overcome this problem, among others cilofexor and tropifexor, these continue to cause pruritus in some cases (Lucas et al. 2020, Patel et al. 2020).
The thyromimetics studied in NASH, such as resmetirom, act via THR-β1, which is expressed almost exclusively hepatically. Via its stimulation, mitochondrial activity is modulated and β-oxidation is increased, thus improving overall hepatic lipid metabolism. In the phase IIb study of resmetirom, this resulted in a decrease of steatosis by MRI, an improvement of lipid parameters in serum and histologically at least partially in an improvement of NASH (Harrison et al. 2019). Recently, interim results of the phase III study MAESTRO-NAFLD-1 were presented. In cohorts of patients without cirrhosis as well as with compensated cirrhosis, a radiological reduction over liver fat could be reached. Additionally, the positive effects on lipid profiles of the preceding study were confirmed (Harrison et al. 2022a). The interim results of the MAESTRO-NASH study, which included histological assessment after 52 weeks of therapy, are expected to be presented in the near future, and an additional phase 3 study investing the effect of resmetirom on progression to cirrhosis and hepatic decompensation was announced. In addition to resmetirom, two other members of the substance group (VK2809 and ASC41) are currently being investigated, but only in phase II yet. Thyromimetics are orally available and show very good safety and tolerability so far (Wirth et al. 2022).
Aramchol is composed of a synthetic bile acid and a fatty acid conjugate and inhibits the hepatic stearoyl-CoA desaturase 1, which is the limiting enzyme of monounsaturated fatty acid biosynthesis. Thus, aramchol inhibits the hepatic fatty acid synthesis and subsequently increases β-oxidation. This lowers the oxidative stress of hepatocytes. In the proof-of-concept study as well as in phase IIb, a decrease in hepatic fat was achieved radiologically, and in phase IIb, there was a trend toward histological improvement of NASH, although not significant (Safadi et al. 2014, Ratziu et al. 2021).
In contrast to the abovementioned unsuccessful trials with the PPAR agonists elafibrinaor and seladelpar, whose development for NASH has been discontinued, lanifibranor is a pan-PPAR agonist with effects on PPARα/γ/δ. In addition to effects on lipid and glucose metabolism, effects on macrophages and HSC have also been described for PPAR agonists; for example, activation of PPARδ promotes anti-inflammatory polarization of macrophages and modulates their activation (Odegaard et al. 2008, Lefere et al. 2020). The phase IIb study of lanifibranor showed an improvement in fibrosis as well as NASH after 24 weeks and additionally an improvement of secondary endpoints with relatively good tolerability (Francque et al. 2021). In addition, a phase IIb study is ongoing with saroglitazar, a dual α and γ agonist, which is already approved in India for hyperlipidemia in patients with type 2 diabetes mellitus and showed favorable results in NASH so far (Gawrieh et al. 2021).
Belapectin is a galectin 3 inhibitor. Galectin 3 is a cytosolic protein that is secreted by macrophages and activated in response to tissue damage and has a profibrotic effect (Traber & Zomer 2013). Contrary to the animal studies, no improvement of fibrosis was achieved in the phase IIb study, but an improvement of hepatic venous pressure gradient was found in the subgroup of patients without pre-existing cirrhosis (Chalasani et al. 2020). Therefore, a study in patients with NASH cirrhosis without preexisting varices is currently underway to investigate the protective effect of this drug against the development of esophageal varices.
With regard to the inhibition of inflammation, namodenoson is particularly noteworthy among the more advanced studies currently ongoing. Namodenoson is an agonist of the A3 adenosine receptor, which is significantly upregulated in inflammatory or tumorous liver. Namodenoson induces an anti-inflammatory effect via deregulation of NF-κb and Wnt pathways. In the phase II study, a dose-dependent alanine aminotransferase (ALT) reduction was achieved (Safadi et al. 2021).
Other promising targets to reduce inflammation in NASH
Other therapeutic approaches that target particularly inflammation are currently less advanced but represent a very important field of research in light of the failed therapeutic approaches with a primary focus on metabolic injury.
Thus, although cenicriviroc as a monotherapy missed to achieve significant improvement in fibrosis, a combination with a second therapeutic approach focused more on inhibiting metabolic injury represents a promising concept and is currently being investigated, for example, in combination with the FXR agonist tropifexor. In addition to CCL2 and CCL5, CD44 is also involved in macrophage recruitment (Patouraux et al. 2017). Therefore, another therapeutic approach is the neutralization of CD44 by antibodies. This has been shown to improve hepatic inflammation and reduce macrophage and neutrophil infiltration in animal models (Patouraux et al. 2017). Other therapeutic approaches to target the role of macrophages in the pathogenesis of NASH include inhibition of medium-chain free fatty acid receptor G protein-coupled receptor 84, which is involved in the chemotaxis of activated macrophages and neutrophils. In animal models, inhibition reduced macrophage recruitment (Puengel et al. 2020). A first-in-human study has also tested the feasibility of autologous macrophage therapy in cirrhosis (Moroni et al. 2019). A phase II trial has been announced, but the effects remain to be seen; however, this is not a NASH-specific approach.
Recruitment of immune cells also represents the therapeutic approach for inhibitors of amine oxidase copper-containing 3 (AOC3), also known as vascular adhesion protein (VAP-1), such as BI 1467335. AOC3 has adhesive functions and plays a role in leukocyte trafficking of CD4+ (Edwards et al. 2005). A dose-dependent reduction in ALT was achieved in humans, but the development of the drug for NASH was discontinued due to concerns about the risk of drug interactions, though no data have been published on this to date (Boehringer Ingelheim 2019). A new option with a similar approach is VAP-1 neutralizing antibodies, such as BTT-1029. In animal models, they decreased the incidence of steatohepatitis and delayed the onset of fibrosis (Weston et al. 2015). A different mechanism is the influence of proinflammatory cytokines. Pentoxifylline inhibits the transcription of TNFα and possibly also the secretion of IL-6 (Matteoni et al. 1999). In two single-center studies, this was shown to improve NASH (Zein et al. 2011, Alam et al. 2017). Also, bromodomain inhibitors affect IFN signaling and reduce inflammation in mouse models (Chan et al. 2015, Middleton et al. 2018).
Besides macrophages, other immune cells also represent potential therapeutic targets. As described earlier, the various granular proteins of neutrophils activate other immune cells. In animal models, both MPO could be inhibited by AZM198 and NSE by sivelestat. This resulted in reduced steatosis, liver damage, and NASH activity (Zang et al. 2015, Piek et al. 2019). For B cells, inhibition of BAFF is already approved in systemic lupus erythematosus and in trials in other autoimmune diseases. Upregulation of BAFF is present in NASH as well and its depletion via BAFF-neutralizing monoclonal antibody Sandy-2 was able to prevent a hepatic B2 cell response to inflammatory stimuli and reduce NASH (Miyake et al. 2013, Bruzzi et al. 2018). Overall, however, in NASH the role of B cells is rather not well enough understood to allow meaningful therapeutic intervention.
With regard to T cells, α4β7-Ak such as vedolizumab represents a potential approach. In NASH, α4β7 is expressed more strongly (Rai et al. 2020). This may play a role in the recruitment of CD4+ T cells and blockade of α4β7 could reduce this. Another concept is to change the balance of CD4+ cells in NASH, which is shifted toward Th1 and Th17, in the direction of Tregs using anti-CD3 antibodies to restore immune tolerance (Ilan et al. 2018). The NLRP3 inflammasome represents a player in the pathogenesis of NASH because it is a mechanism of how metabolic changes can activate various immune cells, including KCs and macrophages, as well as HSC (Ioannou et al. 2017, Wree et al. 2018). An NLRP3 inhibitor, MCC950 leads to improvement in fibrosis in animal models (Mridha et al. 2017). Alternative activation of the immune system is also possible by lipopolysaccharides (LPS), which activate Toll-like receptor (TLR) 4. S-adenosylmethionine, a methyl group donor, prevents LPS-induced hepatic injury and secretion of proinflammatory cytokines. Deficiency can lead to NASH in a mouse model, whereas additional administration leads to histological improvement (Cano et al. 2011, Guo et al. 2021). JKB-121 is a direct antagonist of TLR-4 and thus also inhibits liver injury by LPS. The phase II study showed a surprisingly high improvement rate in the placebo group and the agent did not have significantly superior results (Diehl et al. 2018).
While previous clinical trials to prevent apoptosis of hepatocytes did not succeed as described earlier, new agents are being investigated to address these pathways. Currently, there are several potential drugs that inhibit ASK-1, which demonstrated a reduced progress of NASH or a reduction in hepatic steatosis, inflammation, fibrosis, and also metabolic effects in preclinical studies. This includes agonists of CFLAR, TNF alpha-induced protein 3, glutathione S-transferase Mu 2, dual-specificity phosphatase 8, and melanoma differentiation-associated protein 5 (Wang et al. 2017, Zhang et al. 2018, Ye et al. 2019, Lan et al. 2022, Zhang et al. 2022). However, studies in humans are not reported to date. Furthermore, more specific targeting of caspase-dependent apoptosis pathways was suggested. PXL770 is an activator of AMP-activated protein kinase that inhibits caspase-6-induced cell death. After promising results in preclinical studies, a phase IIa study demonstrated reduced hepatic fat in MRI, but the results did not reach significance (Cusi et al. 2021, Gluais-Dagorn et al. 2022). Targeting the same pathway, a phase II study investing CC-90001, a JNK inhibitor, was conducted in patients with NASH fibrosis. However, their results have not been presented so far. As described earlier, resolution of hepatic inflammation and regression of fibrosis are intensely studied to possibly identify new therapeutic targets that amplify or induce these pathways. In animal models, different pro-resolving mediators deriving from polyunsaturated fatty acids had a positive impact: Resolvin induced a switch to an anti-inflammatory phenotype in macrophages and reduced inflammatory adipokines (Rius et al. 2014). Maresin-1 was also shown to influence macrophage phenotype to pro-repair and additionally protected hepatocytes from lipotoxicity and ER stress (Rius et al. 2017, Han et al. 2019). Also, augmenting TREM2 could be a beneficial therapeutic approach, based on preclinical data (Hendrikx et al. 2022).
Discussion
NASH is a disease with a highly complex pathogenesis. Despite the increasing understanding of the pathogenesis, which consists of metabolic injury, inflammation, and fibrosis, effective pharmacological therapies are still lacking. As described in this review, in reflection on the complex pathogenesis of NASH, there is a variety of therapeutic approaches aiming at improving inflammation. In addition, of course, many other substances are being developed to inhibit metabolic injury or to induce antifibrotic effects. In order to identify promising candidates among these and to design future clinical trials in an optimized way, it seems important to understand possible reasons for the failure of previous trials.
Potential reasons for failure of clinical trials
The pathogenic pathways include a multitude of different immune and non-immune cells as well as local and systemic mechanisms. One possible reason for the lack of clear effects in the studies to date could therefore be that numerous pathways have already been activated during the course of the disease that inhibition of a single pathway is no longer sufficient, especially if signals continue to arrive from upstream. In addition, the therapeutic substances may have to target the right time point in order to be effective, as it has been shown that different immune cells can exhibit different effects at different stages of disease (Sutti & Albano 2020). Overall, NASH is a heterogeneous disease entity in which genetic polymorphisms, gender, comorbidities, and other factors contribute in addition to the extent of metabolic dysfunction. To date, however, no feasible classification is available that would properly account for these manifold dimensions of NAFLD heterogeneity (Arrese et al. 2021).
Potential ways to overcome current limitations in NASH drug development
From these different explanations for the failure of previous studies, different approaches emerge to improve the likelihood of success of future studies. Instead of monotherapy, combination therapy seems to be a reasonable approach (Dufour et al. 2020). Several moderate effects could act together and possibly even synergistically if different pathways are targeted simultaneously. This may also circumvent compensatory mechanisms or cross-reactivity that could occur if only one receptor or ligand is inhibited (Ratziu & Friedman 2020). Currently, a combination of therapies with distinctly different mechanisms seems most promising, for example, to interfere with upstream stimulation of metabolic injury while simultaneously reducing inflammation or activating antifibrotic pathways. Accordingly, several phase II trials of combination therapies, e.g., FXR-agonist plus CCR2/5 inhibitor (tropifexor and cenicriviroc) or FXR-agonist plus antidiabetic agents (cilofexor (FXR-agonist) with semaglutide (GLP-1 receptor agonist) and firsocostat (acetyl-CoA carboxylase inhibitor) and tropifexor (FXR-agonist) with licogliflozin (sodium-dependent glucose cotransporters 1/2-inhibitor)) are already ongoing. Interestingly, first trials combine drugs from different companies (Alkhouri et al. 2022), which is an important step forward to finding the optimal combinatorial approach to NASH.
Another approach might be to choose the therapeutic targets even more selectively. For example, it has been shown that subsets of immune cells can have very different functions in NASH, as it has been described, for example, for KC (Bleriot et al. 2021). Therefore, a neutralization or inhibition of whole cell types seems rather unsuitable. An even better understanding of the pathogenesis could help to develop even more targeted therapies. In the long term, personalized therapies could achieve optimal treatment outcomes. For this purpose, genomic, phenomic, and transcriptomic data could be integrated (Xiong et al. 2019, Ramachandran et al. 2020). Biopsy analysis also may allow for more accurate assessments through the implementation of new technologies (Taylor-Weiner et al. 2021). In addition, the identification of new biomarkers may allow better stratification of patients (Sadeh et al. 2021).
Conclusions
The pathogenesis of NASH is complex and includes, in addition to the initial metabolic injury, an inflammatory process involving a multitude of immune cells. Influencing inflammation by inhibiting proinflammatory pathways or by stimulating anti-inflammatory pathways seems to be a promising therapeutic approach. However, further questions remain regarding the role of individual immune cells or specific subsets. A better understanding of these may pave the way to a specific therapy for NASH.
Declaration of interest
FT’s lab was funded by Gilead, Allergan, Bristol-Myers Squibb and Inventiva.
Funding
This work was funded by the German Research Foundation (DFG SFB/TRR 296 and CRC1382, Project-ID 403224013) and the German Ministry of Education and Research (BMBF DEEP-HCC consortium). FT’s lab received research grants from Gilead, Allergan, Bristol-Myers Squibb and Inventiva.
Acknowledgements
Dr Wiering is participant in the BIH Charité Junior Clinician Scientist Program funded by the Charité – Universitätsmedizin Berlin, and the Berlin Institute of Health at Charité (BIH). Figures were created using BioRender.com.
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