macrophage function: A therapeutic strategy in the treatment of acute liver failure

Clinical Application of Basic Science

Modulation of monocyte/macrophage function: A therapeutic strategy in the treatment of acute liver failure Lucia A. Possamai1, Mark R. Thursz1, Julia A. Wendon2, Charalambos Gustav Antoniades1,2,⇑ 1

Department of Hepatology, St Mary’s Campus, Imperial College London, London, UK; 2Liver Intensive Care Unit, Institute of Liver Sciences, King’s College London, London, UK

Summary Acute liver failure (ALF) is a condition with a high mortality and morbidity for which new treatments are desperately required. We contend that although the initial event in ALF is liver cell death, the clinical syndrome of ALF and its complications including multi-organ dysfunction and sepsis, are largely generated by the immune response to liver injury. Hepatic macrophages fulfil a diversity of roles in ALF, from proinflammatory to pro-resolution. Their inherent plasticity means the same macrophages may have a variety of functions depending on the local tissue environment at different stages of disease. A better understanding of the mechanisms that regulate macrophage plasticity during ALF will be an essential step towards realising the potential of immune-modulating therapies that re-orientate macrophages to promote the desirable functions of attenuating liver injury and promoting liver repair/regenerative responses. The key dynamics: temporal (early vs. late phase), regional (hepatic vs. systemic), and activation (pro-inflammatory vs. pro-resolution) are discussed and the potential for novel ALF therapies that modulate monocyte/macrophage function are described. Ó 2014 European Association for the Study of the Liver. Published by Elsevier B.V. All rights reserved.

Acute liver failure as a model of dysregulated immune-cell driven inflammation Acute liver failure (ALF) is a clinical syndrome consisting of jaundice, encephalopathy, and coagulopathy due to overwhelming hepatocyte death. Currently, specific treatments for ALF include the use of N-acetylcysteine, liver transplantation (OLT), and best supportive care. Although OLT has markedly improved the survival in ALF, it commits patients to life-long immunosuppression and utilises the precious resource of a transplantable organ [1].

Keywords: Acute liver failure; Macrophage; Monocyte; Plasticity; Biomarkers. Received 5 February 2014; received in revised form 12 March 2014; accepted 27 March 2014 ⇑ Corresponding author. Address: Liver & Antiviral Unit, QEQM Building, St Mary’s Hospital, Praed Street, London W2 1NY, UK. Tel.: +44 2033126454; fax: +44 2077249369. E-mail address: [email protected] (C.G. Antoniades).

Immune dysregulation is central to the pathogenesis of ALF, in which massive leucocyte infiltration of the injured liver is contrasted by a depletion and dysfunction of immune cells in the circulation [2]. From a clinical perspective, it is widely accepted that the mortality in ALF is a consequence of profound activation of systemic inflammatory responses (SIRS) and its attendant complications of recurrent sepsis and extra-hepatic organ dysfunction [3,4]. In this review, we show that ALF is an innate immune-driven disorder, in which monocytes/macrophages are key determinants of the initiation, propagation, and resolution phases of acute liver injury. Uncontrolled immune activation leads to the clinical consequences of recurrent sepsis and organ failure encountered in this devastating condition (Fig. 1).

Key Points •

Macrophages are central to the pathogenesis of ALF, driving the initiation, propagation, and resolution of liver injury

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Macrophage plasticity provides an exciting platform to base therapies aimed at functional re-orientation, thereby promoting the resolution of inflammation

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Mechanistic biomarkers will have an important role in signposting the phases of immune activation in ALF, thus guiding the timing of intervention

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The relationship between beneficial local (hepatic) and deleterious systemic (circulatory) effects of key mediators is likely to be a challenge in developing immunotherapeutic strategies

Immunopathology of acute liver failure Local immunopathology of acute liver failure: The role of tissue macrophages Macrophages are tissue resident members of the mononuclear phagocytic system that play a central role in the innate immune response to infection and sterile inflammation and are additionally able to recruit adaptive immune cells. In the adult liver, macrophages may originate from either a haematopoiesis-independent, self-renewing population derived

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Please cite this article in press as: Possamai LA et al. Modulation of monocyte/macrophage function: A therapeutic strategy in the treatment of acute liver failure. J Hepatol (2014), http://dx.doi.org/10.1016/j.jhep.2014.03.031

Clinical Application of Basic Science EXTRA-HEPATIC ORGANS

Microglial activation

Gut epithelial apoptosis

Multi-organ dysfunction

Bacterial translocation

HE

Vascular endothelial dysfunction & microcirculatory disturbance CIRCULATION

Functional monocyte deactivation

Pro-inflammatory mediators TNFα, IL-1 Chemokines

SIRS

Sepsis

Monocyte & neutrophil recruitment

Anti-inflammatory mediators IL-10, SLPI

LIVER DAMP/TLR signalling Proliferation KC Activation Hepatocyte death INITIATION

Activated M1-type macrophages

Pro-resolution or M2-type macrophages

PROPAGATION

Hepatocyte regeneration

RESOLUTION

Fig. 1. A simplified model of monocyte and macrophage function in acute liver failure. KCs detect hepatocyte death through DAMP/TLR signalling and initiate a proinflammatory response. Bone-marrow derived monocytes traffic to the liver to contribute to an expanded macrophage population which are initially pro-inflammatory. During the propagation phase, immune activation is self-perpetuating with recruitment of effectors driving further cytokine and chemokine production. The release of cytokines and vasoactive mediators into the systemic circulation provokes SIRS. Macrophage-derived mediators contribute to vascular endothelial dysfunction and microcirculatory disturbances that result in extra-hepatic organ dysfunction. Later, the presence of pro-resolution macrophages aids tissue recovery. The spill over of antiinflammatory mediators from the liver into the circulation contributes to functional monocyte deactivation and an increased susceptibility to sepsis.

from embryonic yolk sac cells (Kupffer cells) or from the recruitment of bone marrow derived circulating monocytes [5]. Macrophage plasticity: the importance of the microenvironment Macrophages have pleiotropic actions and diverse roles in inflammation. Their protoxicant responses, such as the production of cytokines and reactive oxygen species, are aimed at microbial elimination and tissue destruction. Equally, they possess anti-inflammatory properties that resolve acute inflammation and trigger tissue remodelling and repair [6]. The functional characteristics of macrophages have been broadly categorised as either ‘M1’, pro-inflammatory, or ‘M2’, anti-inflammatory, phenotypes in a classification analogous to TH1 and TH2 responses. In recent years however, this definition has evolved with the recognition that the dichotomous phenotypes really represent extremes, between which numerous macrophage activation states exist [7]. Furthermore it is understood that macrophages exhibit considerable plasticity during inflammation, being able to reversibly transition through distinct activation states in response to microenvironmental mediators [8]. The microenvironmental milieu is a critical determinant of macrophage function during tissue inflammation. In early inflammation the presence of TLR ligands such as HMGB1, DNA, and pro-inflammatory cytokines drive macrophage polarisation 2

towards a classical ‘M1-like’ activation state through the activation of NF-jB and STAT1 signalling pathways. This leads to the propagation and perpetuation of inflammation as it induces further pro-inflammatory cytokine secretion and MHCII expression. At the later stages of inflammation, macrophages undergo a functional ‘‘switch’’ towards an anti-inflammatory or pro-resolution ‘‘M2-like’’ phenotype; with an increasing number of microenvironmental mediators responsible for this functional switch being identified [9] (Fig. 2). Hepatic macrophages in acute liver failure In ALF, resident Kupffer cells are important in the initial transduction and amplification of the ‘alarm’ signal following an injurious event. Hepatocyte death leads to the release of DAMPS, which initiate the activation of innate immune and tissue destructive responses through production of NF-jB dependent pro-inflammatory mediators (e.g., TNFa, IL-1b, IL-6) and reactive nitrogen and oxygen species [10]. Within 12 h of injury in experimental models of ALF, there is a massive expansion in the number of hepatic macrophages. These may be derived from proliferation of resident Kupffer cell population and/or recruitment from the circulating pool of monocytes. Evidence from human and experimental models of acetaminophen toxicity (APAP) suggests that the CCR2-dependent influx

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Please cite this article in press as: Possamai LA et al. Modulation of monocyte/macrophage function: A therapeutic strategy in the treatment of acute liver failure. J Hepatol (2014), http://dx.doi.org/10.1016/j.jhep.2014.03.031

JOURNAL OF HEPATOLOGY Tissue environment rich in: Glucocorticoids TGFβ IL-10 IL-4/IL-13 Immune-complexes Resolvin D1 Lipid SPMs

Tissue environment rich in: TLR Ligands IFNγ

STAT1 NFκB

IRF5

CD206 CD163 IL-10R CXCR1

CD16 CCR2 MHCII Potential for reversible change in activation state depending on environmental cues

STAT3 STAT6 IRF4

IL-12hi IL-10lo IL-23hi IL-1β, TNFα, IL-6 NOS2, ROS and NO

IL-10hi IL-12lo IL-23lo IL-1RA, IL decoy receptor II, CCL24 & 17 YM1, Arginase 1

Activated M1-type macrophages

Pro-resolution or M2-type macrophages

Fig. 2. Microenvironmental mediators are key determinants of macrophage functional polarization. This simplified schematic shows how the dominant microenvironmental mediators can reversibly influence macrophage function. In the presence of TLR ligands and IFNc, macrophages adopt a ‘M1-like’ profile with high expression levels of pro-inflammatory cytokines, including IL-12 and IL-23. If the local microenvironment contains anti-inflammatory signals such as glucocorticoids, SPMs (e.g., resolvins, maresins), and IL-10 an ‘M2-like’ activation state is induced. These phenotypes are simplified, with numerous potential macrophage phenotypes possible, which are likely to be tissue-specific and depend on the precise tissue milieu at different stages of disease.

of circulation-derived monocytes account for the observed increase in macrophages at the early stages of liver injury [2,11]. In early experimental studies there was conflicting evidence regarding the function of these newly recruited monocytederived macrophages in ALF, some showing that depletion of monocytes/macrophages to be protective, reducing liver injury [12,13]. However, recent studies show that depletion of both resident and infiltrating macrophage populations in fact impair resolution responses of acute liver injury [11,14,15]. These inconsistencies probably relate to differential susceptibility of macrophage sub-groups to the various experimental depletion techniques and to the inherent functional plasticity of macrophages during inflammation. Experimental models suggest there is a biphasic macrophage response following sterile tissue injury, namely an initial tissue destructive, ‘M1’, response followed by a resolution/tissue repair, ‘M2’, macrophage response [16,17]. Controversy exists regarding the origin of the M2-like macrophages. Arnold et al. showed that, that early infiltrating, pro-inflammatory monocyte-derived macrophages switched their functional characteristics from proinflammatory to anti-inflammatory in response to micro-environmental cues following experimental muscle injury [16]. In a murine myocardial infarction model, ‘‘M2-like’’ macrophages were derived from recruitment of committed Ly6Clo monocyte precursors from the circulation at the later stages of injury [17]. Furthermore, recent data show that the tissue-resident macrophage population proliferates in response to tissue injury and promotes resolution responses [18]. In line with this theory, our group has recently demonstrated a marked proliferation of resident macrophages within the regenerating livers of ALF patients implicating them in the resolution of acute liver injury [2]. Systemic immunopathology of acute liver failure: Dysregulation of systemic inflammatory responses The innate inflammatory response has evolved to react rapidly to tissue injury or infection in order to promote host survival. The

systemic release of cytokines and chemokines is imperative for signalling to recruit effector cells to sites of injury. In ALF, there is massive death of hepatocytes resulting in a marked activation of the innate immune responses with ensuing production of vast quantities of inflammatory mediators. It is the ‘‘spill-over’’ of inflammatory mediators from the injured liver that cause the profound systemic disturbances and clinical manifestations of ALF. Pro-inflammatory response and SIRS The SIRS response is a recognised clinical constellation of signs including elevated temperature, white cell count, heart rate, and respiratory rate that may accompany infection or sterile inflammation [19]. For over a decade, it has been established that the presence of a SIRS response, reflected by elevations of circulating inflammatory mediators, is closely associated with the development of multi-organ dysfunction and adverse outcome in ALF [20–22]. The development of intracranial hypertension in ALF is particularly strongly associated with SIRS responses and is termed the ‘‘liver-brain proinflammatory axis’’ [23]. Here, pro-inflammatory mediators released from the injured liver cross the blood-brain barrier and, synergistically with elevated ammonia levels, activate microglial cells and result in astrocyte swelling [24]; highlighting the profound effect of the inflamed liver on extrahepatic organ function. Anti-inflammatory response and immune dysfunction In parallel to the pro-inflammatory response, a compensatory anti-inflammatory response (CARS) develops in ALF [25], which is due to a concomitant release of anti-inflammatory mediators, such as IL-10 and TGFb1, from the inflamed liver. Similar to other inflammatory pathologies, elevated circulating concentrations of anti-inflammatory mediators are detected from the very early stages of ALF but it is the persistently high levels of IL-10 and the CARS response that predicts a poor outcome [26]. Although within the inflamed liver, the production anti-inflammatory

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Please cite this article in press as: Possamai LA et al. Modulation of monocyte/macrophage function: A therapeutic strategy in the treatment of acute liver failure. J Hepatol (2014), http://dx.doi.org/10.1016/j.jhep.2014.03.031

Clinical Application of Basic Science mediators serve to dampen pro-inflammatory responses and limit the extent of tissue injury, their release into the systemic circulation is the cause for the predisposition to infection and adverse patient outcome [2]. Our group has shown that ALF patients exhibit features of immuneparesis, typified by monocytes that are hyporesponsive to microbial challenge [27,28]. This functional impairment in circulating monocytes renders them less able to respond to secondary infectious insults and may account for the high rates and poor outcomes from sepsis encountered in ALF patients. Monocyte dysfunction and immuneparesis have been attributed to the action of anti-inflammatory cytokines, in particular IL-10. However numerous other mediators released in the injured liver can enter the systemic circulation and modulate the function of circulating immune cells. One such example is secretory leucocyte protease inhibitor (SLPI); a serine protease with immunomodulatory properties is produced within the injured liver during ALF and spills-over into the systemic circulation. This molecule is known to inhibit NF-jB signalling, neutralise neutrophil elastase, and exert tissue-preserving effects and thus is likely to be protective within the hepatic microenvironment. However, ex vivo analyses show that SLPI is capable of inducing immune deficits in monocytes that contribute to their functional impairment [27]. This example illustrates one of the major challenges in developing immune therapy for ALF in that potential molecular targets may have differential local vs. systemic effects.

Use of biomarkers in determining optimal time points for interventional strategies in clinical practice

Acute liver injury/inflammation

The paradigm of acute liver failure as a discrete injurious event followed either by deterioration or a period of regeneration is

Macrophage activation Neopterin sCD163 Acetylated HMGB1

Initiation In the early, ‘initiation’ stages of acetaminophen-induced ALF markers of cell death predominate, with total HMGB1 and total and caspase-cleaved cytokeratin-18 showing significant elevations that sharply decline between day 1 and 3 of admission [29,30]. microRNA 122 has also been shown to be a specific marker of hepatocyte death that appears transiently in the very early stages of liver injury [31]. Propagation Macrophage activation markers are also seen early in the course of liver injury, but these tend to persist at high levels in serum for at least 3 d, reflecting persisting stimulation of macrophages after the majority of hepatocyte death has ceased. This phase can be considered the ‘propagation phase’, when immune stimulation is self-perpetuating and un-coupled from the initial injurious stimulus. Serum neopterin and the soluble form of the macrophage scavenger receptor CD163 (sCD163) have been proposed as sur-

Regenerative response αFP Phosphate

Suppressed immunity Monocyte deactivation Anti-inflammatory cytokines

Cell death-necrosis and apoptosis Total HMGB1 Total and caspase-cleaved CK18 microRNA 122

INITIATION

perhaps most applicable to acetaminophen poisoning. In other aetiologies the liver insult, by viral or autoimmune mechanisms for example, there is on-going hepatocyte injury and attempted resolution and regeneration will occur concomitantly. However as acetaminophen hepatotoxicity is the commonest and beststudied cause of ALF in the USA and UK a picture of the different phases of injury and inflammation in this condition is emerging through the measurement of serum biomarkers (Fig. 3). These biomarkers have potential clinical utility, not only in determining prognosis in ALF, but in defining the optimal timing of appropriate immune interventions.

PROPAGATION

RESOLUTION

Fig. 3. A schematic representation of mechanistic biomarkers in acute liver failure. Biomarkers may help to define the stage of hepatic injury in acute liver failure and guide the timing of potential immune therapies. The initiation stage is characterised by the presence of biomarkers of hepatocyte death, which peak early and decline. Markers of macrophage activation are also elevated early in the course of ALF, but tend to persist. Late markers of regeneration and immunosuppression characterise the regenerative phase. This model is simplified and the dynamic changes of these biomarkers are likely to be most instructive (i.e., falling cell death markers, with rising macrophage activation markers).

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Please cite this article in press as: Possamai LA et al. Modulation of monocyte/macrophage function: A therapeutic strategy in the treatment of acute liver failure. J Hepatol (2014), http://dx.doi.org/10.1016/j.jhep.2014.03.031

JOURNAL OF HEPATOLOGY rogate markers of macrophage activation in ALF [32,33]. It is important to note that although sCD163 is a biomarker of macrophage activation following liver injury, it does not reflect the functional characteristics of these cells per se. Both markers remain elevated in ALF between day 1 and 3 following admission and levels correlate with severity of extra-hepatic complications and mortality. Serum levels of sCD163 were seen to diverge between day 1 and 3 in spontaneous survivors and those who died or required transplantation, with persisting macrophage activation indicative of poor prognosis [33]. Acetylated HMGB1 is secreted by monocytes and macrophages as a pro-inflammatory mediator, unlike the non-acetylated form that is more widely expressed and passively released by necrotic cells. Levels of acetylated HMGB1 are observed in ALF and correlate well with poor outcome. In both a rodent experimental model and ALF patient group acetylated HMGB1 elevations were a delayed event (15 h and 3 d post-injury in mice and humans respectively) after markers of hepatocyte death had declined [29,34]. This temporal relationship has mechanistic implications, suggesting that activated macrophages are unlikely to be involved in the initial phase of hepatocyte injury. Regeneration/resolution A number of serum markers of hepatocyte regeneration have been investigated in acute liver failure. Alpha-fetoprotein (aFP) is elevated on day 1 in liver injury, suggesting the regenerative hepatic response occurs early. A rise in aFP between day 1 and 3 is associated with a better outcome from ALF, showing the importance of early and sustained liver regeneration to survival [35,36]. Hyperphosphataemia in the presence of renal dysfunction late in the course of ALF is associated with a poor outcome as it is a marker of absent or limited hepatic regeneration [37]. In patients with significant hepatic regeneration, hyperphosphataemia does not develop despite renal dysfunction as hepatocyte proliferation consumes phosphate. Multi-organ dysfunction and systemic immune paresis are common manifestations of ALF and can be observed late in the course of ALF. Systemic immune dysfunction as evidenced by a monocyte phenotype characterised by low HLA-DR, high CD163 expression is associated with sepsis risk and poor prognosis [28].

Potential approaches to improve outcomes in acute liver failure Attenuating the severity of acute liver injury during the initiation and propagation phases Inhibition of innate immune activation At the initiation phase of ALF, immune therapy should be targeted at restricting the massive activation of the innate immune system. As the early communication of cellular distress or death from hepatocytes to KCs is through DAMP:TLR interactions and NFjB signalling, these are clear molecular targets for immune therapy. Inhibition of signalling through TLR2, TLR3, and TLR4 has been shown to ameliorate liver injury in murine models of acetaminophen hepatotoxicity [38–40]. HMGB1 inhibition has also shown some benefit in an experimental model [40]. In a rodent model of acetaminophen toxicity, knockout of the NFjB

p50 gene caused a decrease in inflammatory cytokine production, though no overall effect on outcome. Strategies that target this initial signalling pathway are likely to be most effective when administered very early in disease onset, while serum markers of cell death remain high and prior to the propagation phase of liver injury. This provides a clinical challenge, as the time window in which these therapies could be effective is likely to be short. A further important consideration is that a degree of immune activation is adaptive and necessary for the resolution of tissue injury. The recruitment and expansion of a population of tissue macrophages at the site of injury is associated with effective phagocytosis and facilitation of tissue regeneration. Strategies that limit early immune activation will need to be carefully applied so as not to prevent macrophage recruitment and consequently delay hepatic regeneration. Clearly further work is required before these approaches are trialled in patients, however they do offer the promise of curtailing the initial immune activation and its deleterious downstream consequences. Promoting pro-resolution macrophage polarisation Accepting that macrophages have a role in the initiation of injury in ALF, yet are also necessary for the later resolution of inflammation and that due to their inherent plasticity it is likely to be the same cells involved in both effects, it is clear that targeting macrophages or inhibiting their recruitment could prove counterproductive. Rather, a therapeutic approach that aims to utilise key mediators to promote an early switch in macrophage function to favour a pro-resolution phenotype should be considered. In ALF, this offers the potential to accelerate the resolution of injury and expedite hepatocyte regeneration with the recovery of vital homeostatic functions. Steroids are known to promote macrophage resolution responses, however their systemic application in ALF remains controversial [41]. Using a more targeted approach, a small trial of intra-arterial steroid injection for patients with fulminant hepatic failure has shown promise, though this requires further evaluation [42]. Ideally a therapy of this type would be able to target activated hepatic macrophages without causing a concomitant immunosuppressive effect on circulating monocytes and would permit monocyte-derived macrophage recruitment to secondary sites of inflammation or infection. Recent advancements in our understanding of macrophage plasticity will hopefully lead to the development of novel therapeutic agents for use in ALF and other sterile inflammatory conditions. Limiting the systemic effects of SIRS As described above, many of the most clinically significant sequelae of ALF are due to the multi-organ effects of a ‘storm’ of inflammatory mediator release from the acutely inflamed liver. Removal of these mediators from the circulation offers the promise of reducing the collateral damage of overwhelming innate immune activation. There is limited evidence that artificial liver support devices, based on an albumin dialysis principle, are able to remove circulating cytokines [43,44], with many studies finding no significant net removal of cytokines following liver support treatment [45,46]. Clinical studies indicate that plasmapheresis may be a potential therapeutic option in ALF, reducing the magnitude of the SIRS response and concentrations of circulating inflammatory mediators [47,48].

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Please cite this article in press as: Possamai LA et al. Modulation of monocyte/macrophage function: A therapeutic strategy in the treatment of acute liver failure. J Hepatol (2014), http://dx.doi.org/10.1016/j.jhep.2014.03.031

Clinical Application of Basic Science In contrast to the non-specific effect of plasmapheresis, individual cytokines may be targeted by immune therapies. The utility of this approach is likely to be undermined by the considerable redundancy that exists in inflammatory cytokine signalling.

Strategies to reverse immunoparesis In established ALF, sepsis is the most important cause of mortality. Reversing immunoparesis and restoring immune competence to these patients offers the possibility of reducing their predisposition to infection while allowing time for liver recovery. Evidence suggests that peripheral monocyte dysfunction is driven by the spill over of anti-inflammatory mediators such as IL-10 and SLPI from the liver. A potential therapeutic option would therefore be plasmapheresis, which could remove these soluble mediators from the circulation and as far as possible ‘isolate’ the liver from the systemic circulation. Inhibition of key mediators of immuneparesis (e.g., SLPI) may impact on the hepatic regenerative process but should be evaluated as redundancy in resolution pathways in the liver may allow inhibition of a mediator that would improve systemic monocyte function without impairing liver recovery.

Conclusion ALF is characterised by profound activation of innate immune responses, with monocytes/macrophages being the main orchestrators of disease evolution. Immunomodulatory strategies targeting the function of monocyte/macrophages offer the possibility of reducing the systemic sequelae of ALF whilst promoting hepatic repair processes. To develop clinical relevant immunomodulatory strategies in ALF, we also need to focus the research on understanding the underlying mechanisms of liver injury and biomarkers that accurately reflect the distinct phases of liver injury rather than merely relying on predictive biomarkers of disease severity. At disease initiation, strategies should be considered that attenuate the initiating pro-inflammatory response and mitigate the deleterious systemic effects of pro-inflammatory cytokine secretion, without inhibiting the necessary migration of monocytes to the injured liver. We should aim to limit the propagation phase of liver injury by early re-orientation of hepatic macrophage function towards a ‘pro-resolving’ phenotype. It is hoped that an increased understanding of the complex factors that govern tissue macrophage plasticity may allow manipulation of macrophage function to shorten the duration of inflammation and expedite tissue repair. During the resolution phase of liver injury, immune modifying strategies are required that will reverse the immuneparesis and reduce susceptibility to infection whilst allowing hepatic regenerative responses to continue unhindered.

Conflict of interest The authors declared that they do not have anything to disclose regarding funding or conflict of interest with respect to this manuscript. 6

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Please cite this article in press as: Possamai LA et al. Modulation of monocyte/macrophage function: A therapeutic strategy in the treatment of acute liver failure. J Hepatol (2014), http://dx.doi.org/10.1016/j.jhep.2014.03.031