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REVIEWS
Liver: An Organ with Predominant Innate Immunity
Bin Gao,1 Won-Il Jeong,1 and Zhigang Tian2
Blood circulating from the intestines to the liver is rich in bacterial products, environmental
toxins, and food antigens. To effectively and quickly defend against potentially toxic agents
without launching harmful immune responses, the liver relies on its strong innate immune
system. This comprises enrichment of innate immune cells (such as macrophages, natural
killer, natural killer T, and ␥␦ T cells) and removal of waste molecules and immunologic
elimination of microorganisms by liver endothelial cells and Kupffer cells. In addition, the
liver also plays an important role in controlling systemic innate immunity through the
biosynthesis of numerous soluble pathogen-recognition receptors and complement components. Conclusion: The liver is an organ with predominant innate immunity, playing an
important role not only in host defenses against invading microorganisms and tumor transformation but also in liver injury and repair. Recent evidence suggests that innate immunity
is also involved in the pathogenesis of liver fibrosis, providing novel therapeutic targets to
treat such a liver disorder. (HEPATOLOGY 2008;47:729-736.)
T
he liver is the largest solid organ in the body with
dual inputs for its blood supply. It receives 80%
of its blood supply from the gut through the portal vein, which is rich in bacterial products, environment
toxins, and food antigens. The remaining 20% is from
vascularization by the hepatic artery. Seventy percent of
the cell number or 80% of the liver volume is composed of
Abbreviations: ␣1-CPI, ␣1-cysteine proteinase inhibitor; ␣2M, ␣2-macroglobulin; AAP, acute phase protein; AAT, antitrypsin; ACT, antichymotrypsin; C1-INH,
C1 inhibitor; CRP, C-reactive protein; CTC, connective tissue component; HBV,
hepatitis B virus; HCV, hepatitis C virus; HSC, hepatic stellate cell; IFN, interferon; Ig, immunoglobulin; IL, interleukin; LBP, lipopolysaccharide-binding protein; LEAP, liver-expressed antimicrobial peptide; LPS, lipopolysaccharide;
MAp19, mannan-binding lectin-associated protein 19; MASP, mannan-binding
lectin-associated serine protease; MBL, mannan-binding lectin; MHC, major histocompatibility complex; NK, natural killer; NKT, natural killer T; NOD, nucleotide-binding oligomerization domain; NS, nonstructural protein; PAMP,
pathogen-associated molecular pattern; PGLYP2, peptidoglycan-recognition protein-2; PGRP, peptidoglycan-recognition protein; PRR, pattern-recognition receptor; RAE-1, retinoic acid early inducible gene 1; SAP, serum amyloid P; SPRR,
secreted pattern-recognition molecule; TCR, T cell receptor; TGF-␤, transforming
growth factor ␤; TLR, toll-like receptor; TNF-␣, tumor necrosis factor ␣; TRAIL,
tumor necrosis factor–related apoptosis-inducing ligand.
From the 1Section on Liver Biology, Laboratory of Physiologic Studies, National
Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda,
MD; and 2Institute of Immunology, School of Life Sciences, University of Science
and Technology of China, Hefei, China.
Received June 20, 2007; accepted September 5, 2007.
Address reprint requests to: Bin Gao, M.D., Ph.D., Section on Liver Biology, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, 5625
Fishers Lane, Room 2S-33, Bethesda, MD 20892. E-mail: [email protected]; fax:
301-480-0257.
Copyright © 2007 by the American Association for the Study of Liver Diseases.
Published online in Wiley InterScience (www.interscience.wiley.com).
DOI 10.1002/hep.22034
Potential conflict of interest: Nothing to report.
Supplementary material for this article can be found on the HEPATOLOGY Web
site (http://interscience.wiley.com/jpages/0270-9139/suppmat/index.html).
hepatocytes that fulfill the metabolic and detoxifying
needs of the body. The remaining cells are made up of
nonparenchymal cells, including endothelial cells, stellate
cells, Kupffer cells, and lymphocytes. Emerging evidence
suggests that the liver is an important part of the body’s
immune response and is therefore considered an immunologic organ.1 In this review, evidence is presented demonstrating that the liver plays a key role in innate immune
defenses against pathogens, which supports the notion
that the liver is an organ with predominant innate immunity and acts as an organ barrier or a filter between the
digestive tract and the rest of the body (see details below).
Moreover, additional evidence suggests that innate immunity is also involved in the pathogenesis of liver fibrosis, which will also be discussed.
Innate Immunity
Innate immunity is an important first line of defense
against infection, quickly responding to potential attacks
by pathogens. It comprises physical barriers (for example,
skin and mucous membranes), chemical barriers (urine,
vaginal secretions, and hydrochloric acid in the stomach),
humoral factors (complements and interferons [IFNs]),
phagocytic cells (neutrophils and macrophages), and lymphocytic cells (natural killer [NK] and natural killer T
[NKT] cells). Many of these barriers can kill pathogens
nonspecifically. However, recent evidence suggests that
innate immunity can also specifically detect infection
through pattern-recognition receptors (PRRs) that recognize specific structures, called pathogen-associated molecular patterns (PAMPs), that are expressed by invading
pathogens.2 The best defined PAMPs include lipopoly729
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GAO, JEONG, AND TIAN
HEPATOLOGY, February 2008
saccharide (LPS) found on gram-negative bacteria and
peptidoglycan on gram-positive bacteria. The PRRs can
be divided into 3 categories: secreted PRRs, membranebound PRRs, and phagocytic PRRs. Secreted PRRs are a
group of proteins that kill pathogens through complement activation and opsonization of microbial cells for
phagocytosis. Some secreted PRRs also have direct bactericidal effects on bound bacteria. The best examples of
secreted PRRs include complements, pentraxins, peptidoglycan-recognition proteins, and lipid transferases,
which are mainly produced by hepatocytes and secreted
into the blood stream (Table 1). Membrane-bound or
intracellular PRRs include the toll-like receptor (TLR)
family of proteins,3 the recently identified nucleotidebinding oligomerization domain (NOD)–like receptors,
and the retinoic acid-induced gene I (RIG)-like helicases.4
Phagocytic (or endocytic) PRRs, which are expressed on
the surface of macrophages, neutrophils, and dendritic
cells, can bind directly to pathogens, and this is followed
by phagocytosis into lysosomal compartments and elimination. These phagocytic PRRs include scavenger receptors, macrophage mannose receptors, and ␤-glucan
receptors.
Hepatocytes
Biosynthesis of 80% to 90% of Complement Components and Secreted PRRs of the Innate Immune
System. Hepatocytes play a key role in controlling systemic innate immunity via production of secreted PRRs
and complement components found in plasma (Table 1).
Expression of the genes encoding these proteins is con-
trolled by liver-specific transcription factors, such as hepatocyte nuclear factors, nuclear factor-1, and CCAATenhancer-binding protein, which account for their liverspecific expression. During an acute phase or systemic
inflammatory response, a variety of proinflammatory cytokines [such as interleukin-6 (IL-6), IL-1, tumor necrosis
factor ␣ (TNF-␣), and IFN-␥] can stimulate hepatocytes
to produce high levels of complements and secreted
PRRs. Alterations in the normally stable plasma levels of
these innate proteins occur in liver diseases, resulting in
increased incidence of microbial infections. Recently, an
elegant study showed that transplant patients receiving
donor livers with a genetic predisposition to lowered production of secreted PRRs had a higher risk for bacterial
infections post transplantation, providing an unequivocal
demonstration for the important role of hepatocytes in
systemic innate immunity against infection.5
Complements. The complement system consists of
more than 35 plasma or membrane proteins that interact
with one another in a cascading fashion to protect against
infections. Three different pathways have been identified
that activate the complement system. These include the
classical pathway (target-bound antibody), the lectin
pathway (microbial repetitive polysaccharide structures),
and the alternative pathway (recognition of other foreign
surface structures). After activation, the complement system generates a wide range of biologic activities such as
opsonic, inflammatory, and cytotoxic functions. The liver
(primarily hepatocytes) is a major site that biosynthesizes
complement components found in plasma (Table 1).
These include C1r/s, C2, C4, and Cbp of the classical
Table 1. Biosynthesis of Cs, SPRRs, and APPs of the Innate Immune System by Hepatocytes
SPRRs and APPs
Cs
SPRRs
Other APPs
Mainly Synthesized in Hepatocytes
Functions
Classical
Alternative
Lectin
Terminal
Regulators
Pentraxins
Lipid transferase
PGRPs
sCD14
C1r/s, C2, C4, C4bp
C3, B
MBL, MASP1, MASP2, MASP3, MAp19
C5, C6, C8, C9
I, H, C1-INH
CRP, SAP
LBP
PGLYP2
Soluble CD14
Antimicrobial peptide
Clotting factors
Proteinase inhibitors
Hepcidin (also LEAP)
Fibrinogen
AAT, ACT, ␣1-CPI, ␣2M
Activate C classical pathway
Activate C alternative pathway
Activate C MBL pathway
Terminal C components
Inhibit C activation
Bind microbes and subsequently activate Cs to kill microbes
Binds LPS and subsequently transfers LPS to a receptor complex
(TLR4/MD2) via a CD14-enhanced mechanism
Antibacterial protein via digestion of peptidoglycan on the bacterial wall
Stimulates or inhibits LPS signaling dependent on its concentration and
environment
Antimicrobial peptide by limiting iron availability
A central regulator of the inflammatory response
Inactivate proteases released by pathogens and dead or dying cells
The references for Table 1 are listed in the supplementary material.
Abbreviations: ␣1-CPI, ␣1-cysteine proteinase inhibitor (thiostain); ␣2M, ␣2-macroglobulin; AAP, acute phase protein; AAT, antitrypsin; ACT, antichymotrypsin; B, factor
B; C1-INH, C1 inhibitor; CRP, C-reactive protein; Cs, complements; H, factor H; I, factor I; LBP, lipopolysaccharide-binding protein; LEAP, liver-expressed antimicrobial
peptide; LPS, lipopolysaccharide; MAp19, mannan-binding lectin-associated protein 19; MASP, mannan-binding lectin-associated serine protease; MBL, mannanbinding lectin; MD2, myeloid differentiation factor-2; PGLYP2, peptidoglycan-recognition protein-2; PGRP, peptidoglycan-recognition protein; SAP, serum amyloid P;
SPRR, secreted pattern-recognition molecule; TLR, toll-like receptor.
HEPATOLOGY, Vol. 47, No. 2, 2008
GAO, JEONG, AND TIAN
pathway, C3 and factor B of the alternative pathway,
mannan-binding lectin, mannan-binding lectin-associated serine proteases 1-3, and mannan-binding lectinassociated protein 19 of the lectin pathway, and terminal
components C5, C6, C8, and C9 of the complement
system. 6,7 Although immune cells and endothelial cells
also produce these components, their contributions are
minor compared to those of hepatocytes.6 Additionally,
hepatocytes are also primarily responsible for the biosynthesis of several complement regulator proteins found in
plasma, such as factor I, factor H, and the C1 inhibitor.6
In contrast, the membrane-bound complement regulators are expressed ubiquitously in all tissues.6,7 In addition
to being an important part of innate defenses against infection, the complement system also contributes to the
pathogenesis of a variety of liver disorders, including liver
fibrosis, alcoholic liver disease, liver ischemia/reperfusion,
and liver transplantation.7-9 However, the molecular
mechanisms underlying the involvement of complements
in liver injury and repair remain obscure.
Secreted PRRs. Hepatocytes are also the major
sources for production of secreted PRRs, which have two
main functions: complement activation and microbial
cell opsonization for phagocytosis. Four major classes of
soluble PRRs have been identified according to their domain composition: collectins, pentraxins, lipid transferases, and peptidoglycan-recognition proteins. Many of
these proteins are synthesized mainly in hepatocytes and
released into the bloodstream, thereby playing an important role in innate immunity against local and systemic
microbial infection (see Table 1). In addition, the liver is
also a major source of many other acute phase proteins,
which play key roles in innate defenses against infection
and in reducing tissue damage through inactivation of
proteinases released by pathogens and dead or dying cells.
Liver
Expression of Membrane-Bound PRRs of the Innate
Immune System. The liver not only is the major source
731
of secreted PRRs but also expresses membrane-bound
PRRs, such as TLRs. TLRs are a family of proteins that
recognize PAMPs expressed by microorganisms, but not
by eukaryotes. TLRs can also be activated by endogenous
signals such as uric acid activation of the NALP3/ASC/
Caspase 1 (NALP3/ASC: pyrin domain-containing protein 3/apoptosis-associated speck-like protein containing
a caspase activation and recruitment domain) and apoptotic mammalian DNA activation of TLR9.3 There are
13 different TLRs identified so far. Each of them recognizes specific PAMPS and activates specific signaling
pathways and antimicrobial responses.3 Liver cells express
a variety of TLRs,10 which have been shown to participate
in liver injury and repair, and contribute to the pathogenesis of a variety of liver diseases.11,12 However, the role of
TLRs on liver cells in host defenses against invading
pathogens is less clear. The TLR4 protein has been detected on all types of liver cells and is likely involved in the
uptake and clearance of endotoxins, production of proinflammatory and anti-inflammatory cytokines, and generation of reactive oxidative stress.11,12 Additionally,
hepatocytes express messenger RNAs for all other TLRs,10
the functions of which on hepatocytes remain to be determined. Expression of functional TLR2 has been reported in Kupffer cells, stellate cells, and sinusoidal
endothelial cells, and activation of TLR2 leads to production of proinflammatory cytokines.11,12
Recently, several other cytoplasmic PRRs have been identified, including NOD-like receptors and the RIG-like helicases.4 Among them, RIG-1 serves as a pathogen receptor to
regulate cellular permissiveness to hepatitis C virus (HCV)
replication13; however, HCV nonstructural protein 3/4A
(NS3/4A) blunts RIG-1/mitochondrial antiviral signaling
protein (MAVS) signaling, leading to persistent infection.14,15 Additional details about the effects of HCV infection on PRRs are described in Table 2. In addition,
activation of several TLRs has been shown to inhibit hepatitis B virus (HBV) and HCV infection, providing novel
strategies to treat hepatitis viral infection.16,17
Table 2. Effects of HCV Infection on PRRs
HCV
Effects of HCV on PRRs
HCV infection
HCV proteins
Core
NS3
NS4A
NS5B
HCV infection increases expression of TLRs 2, 6, 7, 8, 9, and 10 and MD-2 messenger RNA levels in both monocytes and T cells
and increases expression of TLR4 in T lymphocytes and TLR5, CD14, and MyD88 expression in monocytes.
Core protein activates TLR2, involvement of TLR1/6.
NS3 activates TLR2, involvement of TLR1/6.
NS3/4A inhibits TLR3 signaling via cleavage of the TLR3 adaptor protein TRIF.
NS3/4A blunts RIG-I/MAVS activation.
NS5B activates TLR3 signaling
References for Table 2 are listed in the supplementary material.
Abbreviations: HCV, hepatitis C virus; NS, nonstructural protein; PRR, pattern-recognition receptor; TLR, toll-like receptor; MAVS, mitochondrial antiviral signaling
protein; MyD88, myeloid differentiation protein-88; MD-2, myeloid differentiation factor-2; TRIF, TIR domain-containing adapter-inducing interferon; RIG-I, retinoic
acid-induced gene I.
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GAO, JEONG, AND TIAN
HEPATOLOGY, February 2008
Table 3. Expression of PRRs on Liver Sinusoidal Endothelial Cells and Kupffer Cells
Liver Endothelial Cells*
Receptors (R)
R for CTC
Hyaluronan R
Collagen R
Fibronectin R
Scavenger R
Mannose R
Fc-␥ R
Functions
Eliminates major matrix polysaccharides and proteoglycans such as hyaluronan and chondroitin sulfate
Eliminates collagen ␣ chains of several types of collagen
Involvement of cell attachment but not for endocytosis
Removal of physiological and foreign waste macromolecules, including LPS, intracellular
macromolecules, modified serum proteins, and microbial proteins
Clearance of a large number of molecules with mannosyl residues
Eliminate IgG-antigen complexes
Kupffer Cells†
Receptors (R)
R for Cs
C5aR
C3R, CR1, CR3, CR4, CRIg
Scavenger R: SR-AI, SR-AII
Mannose R
Fc-␥ R
Functions
Stimulates Kupffer cells to produce prostanoid and proinflammatory cytokines
Play a key role in Kupffer cell clearance of C3-opsonized immune complexes, IgM-opsonized E and
␤-glucans, and therapeutic ␤-glucan polysaccharides
Endocytosis of Ac LDL and Mal-BSA; phagocytosis of gram-negative and gram-positive bacteria
through recognition of LPS and LTA, respectively; phagocytosis of apoptotic cells and red blood
cell–derived vesicles
Clearance of a large number of molecules with mannosyl residues
Eliminate IgG-antigen complexes
References for Table 3 are listed in the supplementary material.
Abbreviations: Cs, complements; CTC, connective tissue component; Ig, immunoglobulin; LPS, lipopolysaccharide; PRR, pattern-recognition receptor; R, receptors.
*Liver endothelial cells are exclusively responsible for endocytosis of soluble macromolecules and collides that are smaller than 100 nm.
†Kupffer cells are mainly responsible for phagocytosis of insoluble particles and also contribute to endocytosis of some soluble macromolecules.
Elimination of Soluble Macromolecules via Sinusoidal Endothelial Cells and Elimination of Insoluble
Waste via Kupffer Cells. The liver is the major site for
removing circulating macromolecules and microorganisms from the systemic circulation through the hepatic
reticuloendothelial system, which is composed of sinusoidal endothelial cells and Kupffer cells. The latter accounts
for 80% to 90% of the total population of fixed tissue
macrophages in the body. Sinusoidal endothelial cells are
mainly responsible for removal of soluble macromolecular and colloidal waste (smaller than 100 nm) from the
circulation by endocytosis through 5 types of endocytosis
receptors (Table 3), whereas Kupffer cells are responsible
for elimination of insoluble waste by phagocytosis
through a variety of receptors (Table 3).
One of the Richest Sources for Innate Immune
Cells: NK, NKT, and T Cell Receptor ␥␦ (TCR␥␦) T
Cells. Liver lymphocytes are abundant in NK, NKT, and
TCR␥␦ T cells (see Table 4).18,19 NK cells represent a
third class of lymphocytes distinct from B and T cells and
do not express a clonally distributed antigen receptor that
is subject to somatic diversification. Mouse liver lymphocytes contain about 10% NK cells, whereas rat and human liver lymphocytes contain about 30% to 50% NK
cells. The functions of NK cells are controlled by a balance of signals from the stimulatory and inhibitory receptors expressed on NK cells. Stimulatory receptors can be
activated by stimulatory ligands expressed on infected,
transformed, or stressed cells, whereas binding of inhibitory receptors to self class I major histocompatibility complex (MHC) molecules leads to inhibition of NK cell
function.20 Hepatic NK cells, originally termed Pit cells
in rats, are not only enriched in the liver but also naturally
activated as they show higher cytotoxicity against tumor
cells than splenic or peripheral blood NK cells in rodents21 and in humans.22 This may be due to an up-
Table 4. The Liver Lymphocytes Are Enriched in Innate Immune Cells
Liver
NK
NKT
TCR␥␦ T cells
Mice
Marker
% liver lymphocytes
Marker
% liver lymphocytes
Marker: classical
% liver lymphocytes
Marker
% liver lymphocytes
NK1.1⫹CD3⫺,
5%-10%
NK1.1⫹CD3⫹
30%-40%
CD1d
10%-30%
TCR␥␦⫹
3%-5%
DX5⫹
Rats
Humans
NK1P⫹CD3⫺
CD56⫹CD3⫺
30%-50%
CD56⫹CD3⫹
5%-10%
CD1d
⬍1%
TCR␥␦⫹
3%-5%
30%-50%
NK1P⫹CD3⫹
5%-10%
CD1d
⬍1%
TCR␥␦⫹
3%-5%
References for Table 4 are listed in the supplementary material.
Abbreviations: NK, natural killer; NKT, natural killer T; and TCR␥␦, T cell receptor ␥␦; Ac LDL, acetylated low-density lipoprotein; Mal-BSA, maleylated bovine serum
albumin; LTA, lipoteichoic acid; Fc-␥, immunoreceptor Fc gamma.
HEPATOLOGY, Vol. 47, No. 2, 2008
regulated expression of tumor necrosis factor–related
apoptosis-inducing ligand (TRAIL) or perform/granzymes
on liver NK cells compared with peripheral NK cells.
Over the past several years, many studies have shown
that hepatic NK cells play an important role in innate
immune responses against tumors, viruses, intracellular
bacteria, and parasites. The antitumor effects of NK cells
in the liver have been well documented in a variety of
experimental liver tumor models.23 Clinical studies also
suggest that NK cells contribute to innate defenses against
primary liver tumors and liver metastases in patients. It
has been reported that hepatic NK cell numbers are
greatly elevated in patients with hepatic malignancy, accounting for up to 90% of all hepatic lymphocytes. Moreover, the reduced activity of NK cells in patients appears
to be associated with the progression of hepatocellular
carcinoma. The antitumor action of NK cells in the liver
is likely mediated via direct killing of tumor cells and
induction of tumor-specific immunity. The antiviral effect of NK cells has been well documented in animal
models infected with several viruses, particularly murine
cytomegalovirus. However, the role of NK cells in human
HBV and HCV infections is less clear because of a lack of
suitable small-animal models. Studies using transgenic
mice overexpressing HBV genome suggest that NK cells
inhibit HBV replication in vivo through production of
IFNs and direct killing of infected hepatocytes.24 In vitro
culture experiments showed that NK cells inhibit HCV
replication in human hepatoma cells via an IFN-␥– dependent mechanism. Recently, a retrospective study revealed that individuals with a genetic predisposition to
enhanced NK function had greater chances of spontaneously clearing HCV during acute infection,25 suggesting
that NK cells play an important role in early antiviral
defenses against HCV. In contrast, HCV can escape the
antiviral response of NK cells by inhibiting NK cell function, and this results in chronic HCV infection in the
majority of patients. Finally, activation of NK cells has
also been implicated in liver injury, fibrosis, and repair.26-28 Taken together, these data show that NK cells
not only play an important role in innate response against
pathogens in the liver but also contribute to the pathogenesis of liver disease.
NKT cells are a subset of lymphocytes that express
both ␣␤ TCR (T cell marker) and cell surface receptors
characteristic of NK cells (NK1.1 in C57BL/6 mice).
Among them, classical NKT cells are controlled developmentally by ␤2-microglobulin–associated nonpolymorphic CD1d. Classical NKT cells are reactive to lipid
antigen (␣-galactosylceramide) and produce both type I
and type II cytokines. Nonclassical NKT cells are CD1dindependent and produce only type I cytokines.29 NKT
GAO, JEONG, AND TIAN
733
cells have been suggested to play important roles in linking innate immunity with adoptive immunity, antiviral
defenses, antibacterial defenses, and antitumor defenses
in the liver.29,30 Mouse liver lymphocytes contain about
20% to 30% NKT cells, which are further elevated to
50% to 60% after partial hepatectomy or liver ischemia
reperfusion.26 NKT cells appear to be involved in induction of liver injury in these models as well as other models
of liver injury induced by concanavalin A, ␣-galactosylceramide, alcohol, and drugs.31
TCR␥␦ T cells represent a minority of T cells in lymphoid organs and peripheral blood, but a high percentage
of ␥␦ T cells is found in the intraepithelial lymphocytic
compartments of skin, intestine, and genitourinary. Interestingly, liver lymphocytes are also enriched in ␥␦ T
cells. In normal mouse livers, ␥␦ T cells account for 3% to
5% of total liver lymphocytes but 15% to 25% of total
liver T cells, and the liver is one of the richest sources of ␥␦
T cells in the body. The percentage of ␥␦ T cells in the
liver is significantly increased in the liver of tumor-bearing
mice. Elevation of ␥␦ T cells was also found in the livers of
patients with viral hepatitis infection, but not in patients
with nonviral hepatitis. However, the role of ␥␦ T cells in
the liver has not been paid much attention. Emerging
evidence suggests that ␥␦ T cells may play a prominent
role in innate defenses against viral and bacterial infection
and against tumor formation.32 Thus, elevated ␥␦ T cells
in the liver may also play an important role in innate
defenses against pathogens and transformed cells.
As shown in Table 4, the distribution of innate immune cells in the liver is different in mice and humans.
For instance, mouse liver lymphocytes contain about
30% to 40% NKT cells, whereas human liver lymphocytes contain about 5% to 10% NKT cells. In addition,
unlike human NKT cells, murine NKT cells express
CD28 molecules constitutively, which are important costimulatory molecules found on T cells.33 These findings
suggest that mice may be more sensitive to NKT-mediated liver injury, whereas humans may be resistant to such
injury. Indeed, treatment with ␣-galactosylceramide, an
NKT activator, was shown to induce liver injury in
mice,34 but results from a phase I clinical study revealed
that ␣-galactosylceramide injection failed to produce
signs of liver injury in humans. In contrast, it could be
speculated that NK cells play more important roles in
human liver diseases than in murine liver disease. For
example, because NK cells inhibit liver fibrosis in
mice27,28 and human liver lymphocytes contain more NK
cells, it is likely that NK cells may be more effective in
inhibiting liver fibrosis in humans than in mice.
A Major Site To Induce T Cell Tolerance. Contrary
to expressing strong innate immunity, the liver is also a
734
GAO, JEONG, AND TIAN
major site of induction of T cell tolerance as evidenced by
the spontaneous acceptance of liver allografts, the persistence of some liver pathogens (HBV, HCV, and malaria),
and the induction of oral tolerance to food antigens. Studies from many laboratories suggest that a variety of cell
types, cytokines, and innate immunity components in the
liver synergistically or additively work together within the
unique environment of the liver to induce T cell tolerance, thereby resulting in hepatic tolerance.35-37
Sterile Inflammatory Response in the Liver
In addition to critical roles in host defenses against
infection, the innate immune system can also sense danger signals from damaged hepatocytes during non–infection-related liver injury, resulting in an inflammatory
response. This so-called sterile inflammation not only
contributes to liver injury but conversely may also be involved in liver repair. For example, acetaminophen hepatotoxicity and ischemia/reperfusion liver injury are
associated with sterile neutrophilic inflammation, which
contributes to liver injury,38,39 but on the other hand,
sterile neutrophilic inflammation after partial hepatectomy can promote liver regeneration by triggering a local
inflammatory response leading to Kupffer cell– dependent release of TNF-␣ and IL-6, eventually leading to
hepatocyte proliferation.40 Moreover, an accumulation of
NK and NKT cells has also been observed in several models of liver injury induced by acetaminophen, ischemia
reperfusion, and partial hepatectomy, which appears to
contribute to liver injury and impaired liver regeneration
in these models.26,31,41-43 Although it has been well documented that the innate immune system detects infection
via recognition of PAMPs expressed by pathogens, the
molecular mechanisms underlying the sterile inflammatory response in the liver have just begun to reveal themselves. It was shown recently that IL-1 is an important
mediator of sterile neutrophilic inflammation during acetaminophen-induced and ischemia/reperfusion-induced
liver injury, but it is less important in microbial stimulus–
induced neutrophilic inflammation, providing a novel
therapeutic target to treat sterile inflammation without
markedly increasing susceptibility to infection.44,45 More
extensive studies to investigate the underlying mechanisms of sterile inflammatory responses in the liver are
required that may help to identify novel therapeutic targets to treat liver disease.
Innate Immunity, Stellate Cells, and Liver
Fibrosis
Regardless of etiology, all chronic liver diseases lead to
liver fibrosis, which is characterized by hepatic stellate cell
HEPATOLOGY, February 2008
(HSC) activation and subsequent overproduction and accumulation of collagens in the liver.46,47 HSCs are generally quiescent in normal healthy livers, but during liver
injury, they become activated and differentiate into myofibroblastic cells that are characterized by a loss of vitamin
A (retinol) and enhanced collagen expression.46,47 In the
last several decades, multiple cytokines and growth factors
have been identified to control HSC activation and liver
fibrogenesis. Among them, transforming growth factor ␤
(TGF-␤) and platelet-derived growth factor are the most
important factors that promote HSC transformation and
proliferation.46,47 Recent evidence suggests that a variety
of innate immunity components also play an important
role in regulating HSC activation and liver fibrosis.
The complement system is typically activated after
liver injury. A recent study demonstrated clearly that C5
and C5aR contribute to the pathogenesis of liver fibrosis
because C5 deficiency resulted in lowered liver fibrosis,
whereas overexpression of the C5 gene resulted in increased liver fibrosis.8 Consistently, genetic analyses also
suggest that human C5 gene variants are associated with
liver fibrosis in HCV patients.8 At present, the molecular
mechanisms by which the C5 contributes to liver fibrosis
are not fully understood and require further studies.
Because a variety of TLRs are expressed on liver cells
including HSCs,10-12 TLRs likely play an important role
in the pathogenesis of liver fibrosis. Activated HSCs express TLR receptors and respond to stimulation by TLR
ligands such as LPS, lipoteichoic acid, and N-acetyl muramyl peptide, and this suggests that LPS and other TLR
ligands may be involved in hepatic fibrogenesis via the
direct targeting of HSCs.12 TLR9-deficient mice are resistant to liver fibrosis because apoptotic hepatocyte DNA
activation of HSCs requires TLR9.48 Activation of TLR3
by polyinosinic:polycytidylic acid inhibits liver fibrosis by
activating NK cell killing of activated HSCs and producing IFN-␥.27,49 Thus, TLRs could be potential therapeutic targets to treat liver fibrosis.
Normal livers and injured livers are enriched in innate
immune cells, which have a significant impact on hepatic
fibrogenesis. Among them, Kupffer cells and NK cells
have been shown to play an important role in regulating
liver fibrosis, whereas other innate immune cells such as
mast cells, neutrophils, and NKT cells seem to have less
effect on experimental liver fibrosis.27,46,47 The role of
Kupffer cells in liver injury and fibrosis has been extensively investigated and well documented. It is generally
believed that Kupffer cells can promote stellate cell activation via the production of cytokines/growth factors
(such as TGF-␤) and regulate the production of metalloproteinases and their inhibitors. A recent study suggests
that macrophages play a distinct and opposing role in liver
HEPATOLOGY, Vol. 47, No. 2, 2008
fibrosis: promoting extracellular matrix accumulation
during ongoing liver injury but enhancing matrix degradation during recovery.50 In contrast, NK cells seem to
have only an inhibitory effect on liver fibrogenesis via
multiple mechanisms.27,28 First, NK cells directly kill activated HSCs but not quiescent HSCs. This is because
activated HSCs express increased levels of NK cell–activating ligand retinoic acid early inducible gene 1 (RAE-1)
and TRAIL receptors but express decreased levels of NK
cell–inhibitory ligand MHC-1.27,28,51 Second, NK cells
inhibit liver fibrosis via production of IFN-␥, which induces HSC cell cycle arrest and apoptosis in a signal transducer and activator of transcription-1– dependent manner.49
Moreover, clinical data show that NK cells from HCV
patients were able to kill human HSCs and that their
activity was negatively correlated with liver fibrosis in
HCV patients, suggesting that activation of NK cells may
have a beneficial effect by inhibiting liver fibrosis in patients.28,52
Analogous to activation of the innate immune system
by cellular apoptosis, HSCs also respond to hepatocyte
apoptosis and subsequently become activated. It has been
reported that hepatocyte-specific disruption of Bcl-xL
leads to continuous hepatocyte apoptosis and subsequent
HSC activation and liver fibrosis,53 providing in vivo evidence that hepatocyte apoptosis activates HSCs. However, the molecular mechanisms by which apoptotic
hepatocytes activate HSCs have just begun to be unveiled.
Watanabe et al.48 demonstrated recently that apoptotic
hepatocyte DNA acts as an important mediator of HSC
differentiation via a TLR9-dependent mechanism. Further studies will be required to identify other signals involved in apoptotic hepatocyte DNA activation of HSCs
and determine if these signals are shared between innate
immune cells and HSCs.
In addition, emerging evidence suggests that activation
of HSCs may lead to activation of innate immunity. First,
activated HSCs synthesize and secrete a variety of growth
factors, cytokines, and chemokines, which can promote
leukocyte chemotaxis and adherence and influence leukocyte activation.54 Second, activated HSCs could participate in the innate immune response via expression of
TLR4.12 Lastly, activated HSCs express NK cell–activating ligand RAE1, which has been shown to modulate
activated macrophages and NK cells via activation of
NKG2D receptors.27,55
Concluding Remarks
In summary, the liver is an organ with strong innate
immunity contributing to the antiviral, antibacterial, and
antitumor defenses within the liver. In addition, innate
immunity also plays an important role in regulating liver
GAO, JEONG, AND TIAN
735
injury, fibrosis, and regeneration, which represents novel
therapeutic targets with which to treat chronic liver diseases. For example, activation of NK cells could be a new
strategy to treat liver fibrosis,56 which will likely have
more beneficial effects than numerous target-directed
drugs that have been proposed or used experimentally in
clinical trials to treat liver fibrosis.46 This is because not
only can activation of NK cells kill specifically activated
HSCs, thereby ameliorating liver fibrosis, but also they
have beneficial effects on inhibiting viral hepatitis infection and liver tumor formation.23-25 Indeed, IFN-␣, one
of the strongest NK cell activators, has been shown to
inhibit liver fibrosis, hepatitis virus infection, and the progression of liver cancer in HCV patients. Treatment with
IFN-␥, another NK cell activator, has also been shown to
have antifibrotic effects in animal models and some HBV
and HCV patients. In the future, it would be very interesting to investigate whether other NK cell activators
(such as IL-2, IL-15, and IL-12) have beneficial effects on
ameliorating liver fibrosis in animal models and human
patients.
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