4.2 Tregs response to antiviral therapy
CHB infection is characterized by an impaired antiviral immune response.
Tregs positively associated with the levels of serum HBV DNA and HBsAg
in CHB patients. At present, antiviral drugs mainly include
nucleoside/nucleotide analogs (NAs) and pegylated interferon (PEG-IFN)
[109]. Previous studies revealed
that IL-2, IFN-γ, TNF-α, and IL-4 increased in CHB patients after NA
treatment, and the numbers of Tregs were
reduced[110,
111]. However, in contrast to this
opinion, Zhang et al. argue that Tregs (CD4 + CD25+) levels were lower in the group that responded to
lamivudine treatment than in the group that did not respond. No
significant change of Tregs was observed in the entecavir treatment
group[112], which may be due to the
difference in the specific markers selected for Tregs. Th17 cells
mediate non-specific inflammation, and the Th17/Treg imbalance is
associated with disease progression in patients with CHB
infection[113]. Liu reported that
the Th17/Treg(CD4+ CD25+CD127dim) ratio was reduced in the therapeutic
response (TR) group which responded to NAs therapy, and no differences
were founded between the TR group and
HCs[114]. In addition, the frequency
of CD25+CD4+ T cells reduced
following Peg-IFN-α-2b therapy[115].
Tregs in the progression of HBV-induced liver fibrosis
The global prevalence of HBV infection in patients with liver cirrhosis
(LC) was 42%[116]. Cirrhosis
imposes a substantial health burden on many countries, which has
increased at the global level since 1990, partly due to population
growth and ageing[4].Many inflammatory
factors were expressed during liver cirrhosis. Tregs suppresses the
activation of T and other immune cells and thus inhibits
inflammation[117]. Many studies
demonstrate a very close relationship between increased Tregs and
CD8+T-cell impairment and poor survival in
HCC[118]. But the role of Tregs in
developing liver fibrosis (LF) remains controversial.
Some studies reported that Tregs can activate HSC to promote LF. In
turn, activated HSC upregulate the levels of Tregs via the PGE2/EP2 and
EP4 pathway. And TGF-β produced by Tregs may also worsen fibrosis by
activating HSC. However, Tregs are likely to be only a minor source of
TGF-β in the liver[119,
120]. In addition, IL-10 produced by
Tregs has an anti-fibrotic effect by inhibiting the activation and
proliferation of HSC[121]. Instead,
Deng et al. reported that Tregs had an anti-fibrosis function by
improving the anti-cirrhosis activity of human amniotic mesenchymal stem
cells (hAMSC) in a mouse model[117].
The different Tregs identification, species (human/mouse), and disease
models might partially explain the different roles of Tregs in LF. As
discussed above, Tregs and Th17 cells have a common origin and belong to
CD4+ T cell subsets. They have antagonistic effects on
each other during inflammation. A study revealed that the frequencies of
CD4+CD25+ Tregs and Th17 cells
significantly increased in mouse liver fibrosis models. Elevated Tregs
are induced to antagonize the Th17 cells. And Tregs directly
downregulated the pro-fibrotic features of
HSC[86]. Lan et al. found that
circulating Tregs were reduced in LC compared with HC, Tregs and Th17
cells were negatively
correlated[113]. Addtionally, many
studies thought that Treg/Th17 ratio was negatively related to the
severity of liver fibrosis[122-124].
Currently, it was limited research on the role of Tregs in HBV-LF. In
the future, more studies are necessary to ascertain the exact function
of Tregs, especially in the liver microenvironment.
Tregs in HBV-HCC
The interaction of Tregs with adaptive immune cells in
HBV-HCC
HBV is the most common cause of HCC, with an estimated prevalence of
44%-55% of HCC cases
worldwide[125]. Persistent HBV
infection can lead to varying liver damage, eventually leading to
hepatitis, fibrosis, cirrhosis, and HCC. The mechanism underlying
HBV-HCC is not completely clear. As immunosuppressive cells, Tregs and
their associated factors, such as metabolites and secreting cytokines,
mediate the immune tolerance of the tumor microenvironment. A recent
study reported that the HBV-HCC microenvironment is more
immunosuppressive and exhausted than the non-viral-HCC
microenvironment[126]. It is
well-documented that HCC patients with a high lymphocyte infiltration
level in the tumors have a lower risk of recurrence and a better
prognosis[127]. The percentage of
Tregs in the peripheral blood and intra-hepatic is increased in HBV-HCC.
Increased Tregs may impair CD8+T cells and be
associated with cancer progression and poor survival in
HBV-HCC[128-130]. In addition, an
elevated level of Tregs is accompanied by reduced infiltration of
CD8+T cells in tumor regions compared with nontumor
regions. CD8+T cells in HCC tumor tissues can
differentiate into CTLs, which then produce cytokines (IFN-γ, TNF-α) and
cytotoxic enzymes (perforin and granulosin B) to clear cancer
cells[131]. However, the expression
of granzyme A, granzyme B, and perforin was decreased significantly in
tumor-infiltrating CD8+T
cells[128]. The exhausted
CD8+ effector T cells eventually weaken the tumor
surveillance of the adaptive immune system and lead to the immune escape
of tumor cells.
The proportion of Tregs in HBV-HCC microenvironment was significantly
higher than that of non-virus-associated HCC, and the expression levels
of inhibitory receptors PD-1 and LAG-3 were higher, showing a more
potent immunosuppressive
capability[126]. Qiu et al.
demonstrated that Furin or TGFβ1 knockdown in Tregs promoted Teff cell
proliferation and enhanced the killing activity of CTLs against HCC
cells and HBV in vitro[58]. Nishida
et al. reported that CCR4+ Tregs were the principal
type of Tregs in HBV-associated HCC, which were related to sorafenib
resistance and HBV load titers. In addition, CCR4+Tregs can express more IL-10 and IL-35 compared with
CCR4- Tregs, which enhanced the ability of suppressing
CD8+ T cells[132].
CCR4+ Tregs are recruited into tumor microenvironment
by binding to CCL22 and CCL17 in
HBV-HCC[133]. HBV-encoded gene HBx
induces substantial IL8 production through activating MEK-ERK signal.
The activity of IL8 is mainly determined by binding to its receptor
CXCR1 and CXCR2. The IL8-CXCR1 axis on the hepatic vascular endothelium
promotes the growth and dissemination of HCC by recruiting Tregs in the
liver microenvironment[134]. Tregs
play a vital role in HBV-related diseases. Thus, an in‐depth
investigation of factors that regulate Tregs functions is crucial for
the better treatment of HBV‐HCC and CHB.
The interaction of Tregs with innate immune cells in HBV-HCC
Natural killer (NK) cells are an essential component of innate immunity
in the liver, the site of HBV
replication[135]. The study revealed
that CD4+CD25+ Tregs directly
suppressed NK cell-mediated hepatocytotoxicity through membrane-bound
TGF-β (mTGF-β) and OX40/OX40L interaction in HBV-associated liver
disease[136]. In a mouse model, Liu
et al. found that MicroRNA-15a/16-1 prevented Kupffer cells (KCs) from
overproducing CCL22 by inhibiting nuclear factor-κB, which activates the
transcription of CCL22. By reducing CCL22 binding to C-C chemokine
receptor type 4 on Tregs, microRNA-15a/16-1 can impair Treg
function[137]. Thus,
microRNA-15a/16-1 prevents HCC by disrupting the communication between
KCs and Tregs, which provides new ideas for the treatment of HCC. Under
the hypoxic state.A large amount of triggering receptor expressed on
myeloid cells-1 positive (TREM-1+) tumor-associated
macrophages(TAM) were enriched in tumor tissues. Wu et al. reported that
TREM-1+ TAMs abundant at advanced HCC indirectly
impaired the killing function of CD8+ T cells and
induced CD8+ T-cells apoptosis, and recruited
CCR6+ Foxp3+ Tregs into tumor
tissues by secreting a large number of chemokine CCL20 to promote the
progression of HCC[138].
The myeloid-derived suppressor cells (MDSC) is a group of regulatory
immune cell population residing in the liver. HBV can enhance the
secretion of TGF-β and IL-10 by MDSC, and then induce the formation of
Tregs. APCs that play a crucial role
in activating T cell-mediated, antigen-specific adaptive immune
responses. B7-H4 belongs to B7 family member, which can negatively
regulate T cell responses[139].
Kryczek et.al showed that Tregs can induce APCs to produce high levels
of IL-10 and, in turn, stimulate APC B7-H4 expression. Thus, APCs can
suppress T cell activation via B7-H4
induction[140]. TIGIT was expressed
on regulatory, memory and activated T cells. Poliovirus receptor (PVR,
also called CD155), which is highly expressed on DCs, bound TIGIT with
high affinity. TIGIT interacted with CD155 on DCs to enhance the
secretion of IL-10 by DCs, which impedes CD4+ T-cell
proliferation and function[141]. In
a mouse model of acute HBV infection, NK cells have been shown to be
critical for HBV clearance[142]. NK
cell dysfunction is associated with impaired CD8+ T
cell responses in CHB[143]. Ma et
al. demonstrated that HBeAg induces IL-10 production in Tregs, which
subsequently upregulates the expression of NKG2A on NK cells, leading to
NK cell exhaustion, which in turn suppresses the anti-tumor immunity of
organism and evades the monitoring of the immune system in
HCC[144,
145]. (Figure.3)
Therapies targeting Tregs in HBV or HBV-HCC
Tregs
play a critical role in maintaining peripheral tolerance, thus garnering
substantial interest as a potential therapeutic approach for preventing
autoimmunity and establishing transplantation
tolerance[146]. However, in the
context of HCC, immune suppression can facilitate immune escape of tumor
cells. Over the past years, Tregs have been implicated in the
suppression of virus-specific immune responses, thereby providing a
mechanism for the persistence of HBV. Notebly, the depletion of Tregs
during AHB has been shown to impede its transition to a chronic
state[22]. Moreover, Tregs and
associated factors play a key role in immunosuppressive tumor
microenvironment of HCC, thus highlighting the potential of targeting
Tregs to inhibit HCC progression. Immune checkpoint inhibitors (ICIs)
are designed to target immune checkpoints that dampen immune cell
activity and exhibit ant-tumor effects. Several immune checkpoints, such
as TIM-3, TIGIT, or LAG-3 are expressed in Tregs, presenting promising
avenues for future research in HCC treatment aimed at inhibiting Treg
function [147,
148]. Non-virally associated HCC
generally exhibits higher levels of IFN-γ, IL-17, Granzyme B, and TNFα,
whereas virally-associated tumors have increased PD-1 expression on T
cells, indicative of a more suppressive environment fostered by HBV
infection [126].
Research indicated that Tregs induce
immune suppression via the secretion of immunosuppressive cytokines such
as IL-35, IL-10, and TGF-β. IL-35 can restrain the proliferation of
HBV-specific CTL cells and IFN-γ secretion in vitro. IL-35 is associated
with HBV-related hepatic complications, including CHB, cirrhosis, and
HCC[59]. An elevated level of IL-10
is linked to HBV serum titers and the degree of liver
inflammation[149]. A recent study
showed that TGFβ1 knockdown in Tregs can inhibit HBV replication in
vitro[58]. Therefore, blocking IL-10,
IL-35, and TGFβ1 expression helps restore the impaired function of
effector T cells. A recent study reported that CCR4+Tregs were the predominant type of Tregs recruited to HBV-HCC,
associating with sorafenib resistance and HBV load titers. Targeting
intratumoral CCR4+ Tregs was shown to overcome
sorafenib resistance and sensitize tumors to immune checkpoint blockade
in mouse models of liver
cancer[132]. Herein, severe
autoimmunity could be avoided by selectively depleting intratumoral
Tregs.
Recent studies demonstrated that blocking glycolysis can promote the
generation of Tregs through the hypoxia-inducible factor 1α (HIF1α). The
concentration of amino acids can affect the differentiation and function
of Tregs. The metabolic pathways and factors influencing Tregs
differentiation and function can be considered new therapeutic
approaches during viral infection[150,
151]. It has been found that IL-15
were downregulated in CHB patients with type 2 diabetes mellitus. IL-15
can suppress Treg function and inhibit the expression of immune
checkpoint molecules in Tregs[152].
Huang et al. showed that exosomal circGSE1 stemming from HCC cells
promotes the progression of HCC by inducing Tregs expansion via
regulating the miR-324-5p/TGFBR1/Smad3
axis[153]. In the future, exosomal
circGSE1 may be used as a promising biomarker for immunotherapy of HCC.
Other findings may open new avenues in developing therapeutic strategies
to activate specific anti-HBV immunity by diminishing Tregs
autophagy[96].
In conclusion, Tregs appear to play
a pivotal role in the maintenance of HBV infection, HBV-related
cirrhosis, and HCC. AS such, it becomes imperative to explore strategies
targeting Tregs molecules and attenuating Treg activity during specific
periods of HBV infection and HCC. Novel ICIs targeting molecules such as
anti-TIM-3, anti-TIGIT, or anti-LAG-3 have demonstrated the ability to
restore the functionality of tumor-infiltrating T cells in
vitro[154] and are currently being
evaluated in early-stage clinical trials.
Conclusions
Collectively, the role of Tregs in hepatitis viral infections appears to
exhibit a dual nature. On one hand, Tregs mediate targeted suppression
of T cells in the context of HBV infection, potentially contributing to
viral persistence, while simultaneously safeguarding against excessive
liver damage. However, it remains uncertain whether these divergent
roles can be ascribed to the same Treg populations or if they involve
distinct Treg subpopulations. Moreover, the underlying mechanisms
governing these distinct mechanisms mediate these disparate functions
requires further investigation. A promising avenue for enhancing
HBV-related disease management involves the specific targeting of Tregs
and a refined understanding of the optimal time frame for such
intervention. In addition, Tregs have the ability to shape a tumor-prone
immune microenvironment for HCC formation by weakening the immune
surveillance function of the innate and adaptive immune systems.
Consequently, strategies aimed at inhibiting Treg function and promoting
an enhanced immune state within the tumor microenvironment hold
considerable promise for the treatment of HCC. Exploring the modulation
of Treg activity and its potential impact on HCC treatment merits
further research and consideration.