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Journal of Virology, April 2009, p. 2796-2802, Vol. 83, No. 7
0022-538X/09/$08.00+0 doi:10.1128/JVI.00996-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.
Macrophages in Hepatitis B and Hepatitis C Virus Infections
Mathis Heydtmann*
Liver Research Laboratories, Institute for Biomedical Research, Birmingham University, Birmingham B15 2TT, United Kingdom

INTRODUCTION
Hepatitis B virus (HBV) and hepatitis C virus (HCV) are major
health burdens worldwide, with over 300 and 170 million people,
respectively, infected. HBV, a DNA virus, and HCV, an RNA virus,
are both hepatotropic, and both lead to hepatitis in many patients,
with potentially fatal complications, including hepatocellular
carcinoma. A high proportion of HCV- and HBV-infected patients
develop chronic infections characterized by absent, weak, or
narrowly focused T-cell responses (
70). It is likely that early
immune avoidance mechanisms contribute to the disturbed T-cell
responses in combination with various other strategies reviewed
elsewhere (
7,
28,
31,
36,
70,
82). The two viruses differ considerably
in their interactions with the host immune system, but all current
treatment protocols aimed at clearing either virus include alpha
interferon (IFN-

). This implies that the innate immune system
is of pivotal importance in the development and maintenance
of chronic infection versus viral clearance.
Critical components of the innate immune response are liver macrophages. Here we highlight their key roles in both the favorable and adverse responses to HBV and HCV infections.

MACROPHAGES IN THE LIVER
Macrophages are phagocytic mononuclear cells of the innate immune
response which also prepare and maintain adaptive responses.
Monocytes in peripheral blood differentiate into macrophages
after migrating into tissues, where gene expression changes
are driven by the extracellular matrix, chemokine milieu, and
T cells (
13,
47,
56). Kupffer cells (KCs), resident liver macrophages,
are long lived and abundant, representing 15 to 20% of the total
liver cell population (
42,
68). Resting KCs, one of the first
types of immune cells to be exposed to materials absorbed in
the gut, contribute to the generally tolerogenic environment
in the liver (
3,
41,
68,
77,
86), including suppression of T-cell
activation (
51). Nevertheless, during immune responses, KCs,
like circulating monocytes drawn into the liver, can be activated
by various stimuli (
17,
19,
22,
26,
34,
47,
57,
59,
64,
73,
77,
83) (Table
1). Low shear stress, fenestrations of sinusoidal
cells, and a large contact area between blood and parenchymal
cells facilitate extravasation and recruitment of immune cells
to the liver (
12,
43).
Macrophages display heterogeneous phenotypes with distinct functional
capacities that vary according to tissue microenvironment and
external stimuli. They monitor their environment and mediate
phagocytosis via a plethora of plasma membrane receptors (
90).
Pattern recognition receptors, including Toll-like receptors
(TLRs) (
26,
90), recognize pathogen-associated molecular patterns
and subsequently mediate both phagocytosis and signaling, leading
to an altered macrophage phenotype (
17,
22,
26,
34,
59,
64,
73,
83) (Fig.
1). Typically, innate immune activation is particle
induced, antigen nonspecific, and T-cell independent (
26,
47),
resulting in macrophages that secrete reactive oxygen species
(ROS), nitric oxide, type I IFNs (IFN-

and IFN-β), and
other cytokines and chemokines. Additionally, innately activated
macrophages often upregulate expression of adaptive response
genes, such as the gene encoding antigen presentation major
histocompatibility complex class II (MHC-II) molecules (
42),
and promote activation of primed T cells (
26). In contrast,
adaptive macrophage activation is modulated by direct interaction
with T cells. Classical adaptive activation occurs in the presence
of IFN-

and results in macrophages with enhanced cytotoxic activity
(
19), whereas alternative adaptive activation, which is promoted
by the type 2 cytokines interleukin-4 (IL-4) and IL-13, results
in macrophages with activities optimized for combating parasitic
and extracellular pathogens (
26,
55). Other activation phenotypes,
including that for IL-10-mediated activation, have been described
(Table
1), but IL-10 has also been associated with deactivation
of macrophages, which is necessary to limit the duration and
intensity of the immune responses that downregulate MHC-II and
the expression of other cytokines (
26,
47).

MACROPHAGE INVOLVEMENT IN IMMUNE RECOGNITION OF HCV AND HBV INFECTIONS
In the case of HBV infection, viral replication inside infected
hepatocytes occurs within capsids, with the viral genome hidden
from pattern recognition receptors (PRRs), preventing the initial
HBV infection from being detected by the innate immune system
(
95) (Fig.
2B). In contrast, the HCV life cycle is cytoplasmic
in replication complexes. Although these membranous webs may
shield the virus somewhat from PRRs, ubiquitously expressed
cytoplasmic PRRs that detect nucleic acids, such as RIG-1 and
MDA-5, likely recognize HCV and trigger secretion of detectable
amounts of type I IFNs by hepatocytes (
84,
85). Indeed, in studies
with chimpanzees, no hepatic transcription changes are detected
during the first weeks of HBV infection, whereas many hepatic
transcription changes relating to the type I IFN response are
seen upon HCV infection (
70). There is debate as to whether
macrophages, in addition to hepatocytes, are infected by HCV
and support replication (
4,
9,
20,
44,
69,
72,
74). If infected,
macrophages may also contribute to the early induction of type
I IFN signaling through RIG-1 and MDA-5.
HCV, unlike HBV, spreads rapidly in the liver. Therefore, it
is likely that during the early stages of HCV infection and
in the later stages of HBV infection, KCs (and other cells)
are exposed to free viral nucleic acids and proteins. KCs and
macrophages express TLR1 through TLR6 and TLR8 (
87). Thus, TLR3
and/or TLR8 (located in late endosomes and lysosomes), which
are activated by double-stranded RNA (dsRNA; TLR3) and single-stranded
RNA (TLR8), likely recognize HCV via phagocytosis, become activated,
and contribute to the induction of type I IFNs (Fig.
2). In
the case of HBV infection, exactly which TLRs detect viral infection
have not been delineated, though it is likely that phagocytosed
viral particles detected by TLR8 trigger innate activation of
macrophages. It has been suggested that HBcAg can bind macrophages
via TLR2 and activate TNF-

, IL-6, and IL-12 expression (
10),
although in an immortalized human hepatocyte cell line transgenic
for HBV it is specifically TLR3 agonists that inhibit HBV replication
in isolated KCs through an IFN-β-dependent mechanism (
97).
The role of TLRs in combating HBV infection has been generally
demonstrated by using a transgenic mouse model of HBV infection,
where stimulation of most TLRs inhibits viral replication (
33).
Several studies report that HCV can interfere with type I IFN signaling, in particular through interfering with pathways downstream of TLR3 (1, 30, 46, 62; reviewed in references 5 and 82) (Fig. 2A). Given that some type I IFN responses are observed in HCV infections, this finding suggests that the interference is not 100% but that the resultant response is too weak or too late to be effective. Nevertheless, therapeutic IFN-
can lead to the elimination of HCV in chronic infection.
Macrophages, including liver macrophages, upregulate their antigen-presenting phenotype in response to IFN-
(88), leading to several-hundred-fold-increased expression of MHC-II (19). In HCV infection, most KCs express high levels of MHC-II molecules compatible with this phenotype (8, 38). MHC-II allows antigen presentation to CD4 T cells, but macrophages are also able to cross-present viral epitopes to CD8 cells. Here, cell-associated particulate viral antigen and viral dsRNA are taken up by macrophages, degraded in phagosomes, and cross-presented via MHC-I (2). This cross-priming or cross-presentation could provide an efficient means for viral antigens within dead cells to be presented to T cells by uninfected macrophages, thereby inducing adaptive immunity or facilitating activation of primed HCV-specific T cells (26). Indeed, cross-priming of viral dsRNA seems to be essential for the priming of cytotoxic T cells (76). However, in the absence of co-stimulatory molecules, cross-presentation of antigens by macrophages is more likely to promote tolerance in naïve T cells (3), which might play a role in HBV- and/or HCV-induced hepatitis, but data on these diseases are lacking.
In summary, HCV evades by antagonizing the immune response, whereas HBV hides from detection by the early immune system, including macrophages, and both viruses lead to persistent liver infections in many cases.

MACROPHAGES DETERMINE CELLULAR AND CYTOKINE MILIEU IN HCV- AND HBV-INFECTED LIVERS
In the murine cytomegalovirus (
75) and
Listeria monocytogenes (
11,
42) models of liver disease, it has been shown that KCs
and macrophages integrate a cascade of innate inflammatory events
bridging the innate and adaptive immune responses: type I IFN-dependent
CCL2 produced by KCs recruits circulating monocytes to the liver,
where they produce CCL3 (
75). CCL3 recruits NK cells producing
high levels of IFN-

, which triggers widespread classical activation
of macrophages, inducing CXCL9 production, which in combination
with CCL3 promotes recruitment of CD4
+ T cells (
75,
99). Additionally,
based on general immunology research, immune responses are expected
to be further amplified by KCs and macrophages as follows: KC-secreted
IL-12 and IL-18 should ensure that virus-specific CD4
+ T cells
differentiate into type 1 helper (Th1) cells, which secrete
IFN-

, creating an activating feedback loop between KCs and Th1
cells and promoting infiltration of more T cells (
47) (Fig.
1); type I IFNs, released from hepatocytes (and KCs), should
promote the expansion and activity of cytotoxic CD8
+ T cells
(
84). Cell-to-cell interactions between macrophages and T cells
which are complementary to the primary T-cell receptor-MHC-II
interaction by CD40 (on macrophages) and CD40L (on T cells)
should result in bilateral signaling; the macrophages produce
more IL-2, TNF-

, and nitric oxide and increase MHC-II expression,
while the Th cells produce more IFN-

(
47,
50).
Such a cascade would be difficult to demonstrate in human HCV or HBV infection. However, in HCV-infected chimpanzees, a comparable IFN-
response is found in all animals, but resultant chemokine- and IFN-
-induced genes are correlated with viral elimination (85). This suggests that the early immune cascade that will include KCs is crucial in determining outcome. Not surprisingly, histological analysis of chronically HCV-infected livers demonstrates a dramatic increase in total as well as antigen-presenting macrophages in HCV infection (38, 53). Activated macrophages are found immunohistochemically in clusters with CD4+ T cells (8). Furthermore, on microarrays, genes of macrophage activation are very prominent in chronic HCV-infected liver (80). KC-derived chemokines (CCL3) are important in the recruitment of dendritic cells (DCs) (52, 99) and NK cells (which correlate with the outcome of HCV infection [39]). Moreover, KCs regulate DC migration into Disse's space and to lymph, which is vital for antigen presentation and the development of adaptive immune responses (63, 99).
A number of studies indicate aberrant macrophage function in chronically HCV- and HBV-infected individuals. In HBeAg-positive HBV infection, TLR2 expression in KCs is reduced (93), perhaps suppressing immune surveillance. Also, activated macrophages produce less TNF-
in vitro when exposed to lipopolysaccharide (TLR4 ligand) in the presence of HBV particles (61), and specifically, HBsAg binds to macrophages, inhibiting lipopolysaccharide-induced macrophage activities such as IL-12 production. Similarly, via interactions with gC1qR (a surface complement receptor), the HCV core protein inhibits TLR4-induced production of IL-12 by macrophages (94). Peripheral blood monocytes taken from HCV-infected patients preferentially express IL-10 upon exposure to recombinant HCV, which may contribute to reduced T-cell responses (96) and exhibit defective responses to TLR3 and TLR4 ligands (92). It is currently not clear whether these cells are functionally impaired by the virus or whether they have adapted an altered and reversible phenotype due to external stimuli (plasticity). Evidence for a direct effect is HCV core protein binding surface-expressed TLR2, which has been suggested to bias macrophage activation toward secretion of IL-10 and TNF-
(14, 16) (Fig. 2A). This cytokine combination is proposed to predispose plasmacytoid DCs (pDCs) to apoptosis and low IFN-
production when exposed to TLR9 ligands (CpG DNA). The HCV core (32, 58), NS4A, NS4B (35), and NS5A (6, 25, 37, 66) have all been shown to induce CXCL8 expression in vitro. CXCL8 is elevated in the sera of HCV-infected patients (35, 54, 67, 71) and may inhibit the antiviral activity of IFNs (35, 66) and/or exacerbate inflammation (32, 35). Furthermore, when mixed with T cells, HCV core protein-expressing macrophage cell lines are less effective at promoting proliferation and IFN-
production in T cells (45).
In summary, HBV and HCV, via an assortment of mechanisms, disturb immune responses and establish chronic infections, with macrophages as key regulators of the early immune responses being targeted by both viruses.

CONTRIBUTION OF MACROPHAGES TO LIVER INFLAMMATION
Characteristic pathological features of chronic HBV and HCV
infections are chronic inflammation and apoptosis of infected
and bystander hepatocytes (
70,
82). KCs and macrophages are
thought to be major contributors of bystander killing. In HCV
infection, recently recruited (MAC387
+) macrophages are increased
in the focal areas of erosions (
40,
53), and KC-derived IL-18
correlates with hepatitis and liver injury (
53). Similarly,
in HBV infection, more activated KCs expressing FasL are observed
during episodes of liver damage (
89), and activated KCs together
with oval cells have been described in areas of inflammation
and regeneration (
81). Indeed, CD68 (macrophage marker) and
CD14 (activated macrophages [
27]) are both upregulated in viral
hepatitis and correlate with liver injury (
49,
53,
91,
98).
It has been proposed that in chronic HCV infection, a combination
of viral (e.g., serum HCV core protein) and host (e.g., IFN-
and unphagocytosed endotoxins) factors cause KCs and recruited
macrophages to be continuously and inappropriately activated
(
15). Activated KCs potentially kill hepatocytes via several
mechanisms. KC-derived TNF-

is known to be injurious to hepatocytes
in various models of T-cell-dependent acute liver damage (
65),
and KC expression of FasL may also promote hepatocyte apoptosis
(
42,
65). Local production of nitric oxide and ROS by activated
macrophages may also contribute to bystander hepatocyte cell
death (
42).

MACROPHAGES INFLUENCE LIVER FIBROSIS AND CIRRHOSIS AND CANCER
Important complications of HBV and HCV infections are fibrosis
and cirrhosis and the development of hepatocellular carcinoma
(HCC). KCs produce profibrogenic factors (e.g., transforming
growth factor β and platelet-derived growth factor) (
77)
and yet also represent a major source of enzymes and factors
important for matrix breakdown and turnover (collagenase, metalloproteinases,
and IL-6) (
64,
78,
79). Whether KCs exert a pro- or antifibrogenic
effect depends on their cytokine environment (
18,
21) and on
interactions with stellate cells and hepatocytes (
42). The serum-
and glycocorticoid-regulated kinase is induced by transforming
growth factor β and is associated with fibronectin formation,
and in HBV and HCV infections, serum- and glycocorticoid-regulated
kinase is upregulated in activated KCs, suggesting a profibrotic
effect in these diseases (
24). In a transgenic mouse model of
HBV infection with hepatic HBV envelope protein expression,
KCs exhibiting high levels of superoxides are observed specifically
beside proliferating hepatocytes (
29). HCV core- and NS5A-induced
ROS and nitric oxide produced by activated KCs may also promote
DNA damage in hepatocytes and induce oncogenesis (
48). In addition,
it has recently been postulated that KC-derived IL-6 contributes
to hepatocyte proliferation and HCC development and that estrogen
inhibition of IL-6 production reduces HCC risk in females (
60).
Therefore, KCs are not only relevant in HBV- and HCV-induced
inflammation but also in the development of associated fibrosis
and HCC. One study of KC depletion leading to attenuated liver
injury (
23) suggested an adverse net effect of liver macrophages,
but the possible influence of macrophages will depend on their
phenotypes and activation status.

CONCLUDING REMARKS
Liver macrophages and KCs contribute to the tolerogenic environment
of the undiseased liver and influence every stage of virus-induced
liver disease. They recognize viral antigens and initiate the
innate immune response, of which they are a major component.
KCs determine the cellular and cytokine milieu of the virally
infected liver and thus play a major pathological role in chronic
virus-associated diseases. Future research is necessary to answer
many of the unresolved questions relating to macrophages in
HCV and HBV infections, such as the following. Are monocytes
and macrophages productively infected by HCV, and is this a
significant parameter in immune escape by HCV? Do liver macrophages
function efficiently during HCV and/or HBV disease, and can
they be a target for immune therapy to control HBV and/or HCV
disease?

ACKNOWLEDGMENTS
I acknowledge B. Rehermann, S. J. Forbes, and J. Iredale for
helpful discussions at various stages of the manuscript.

FOOTNOTES
* Mailing address: The Liver Research Laboratories, Institute for Biomedical Research, Birmingham University, Birmingham B15 2TT, United Kingdom. Phone: 44 7764533068. Fax: 44 124158700. E-mail:
m.heydtmann{at}bham.ac.uk 
Published ahead of print on 8 October 2008. 

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Journal of Virology, April 2009, p. 2796-2802, Vol. 83, No. 7
0022-538X/09/$08.00+0 doi:10.1128/JVI.00996-08
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