Abstract
Chronic infections with human hepatitis viruses continue to be a major health burden worldwide. Despite the availability of an effective prophylactic vaccine against the hepatitis B virus (HBV) and of antiviral agents efficiently suppressing HBV replication, more than 250 million people are currently chronically infected with this hepatotropic DNA virus, and resolution of chronic hepatitis B (CHB) is rarely achieved. Moreover, coinfection with the hepatitis D virus (HDV), a human RNA satellite virus requiring the envelope proteins of HBV for productive viral spreading, substantially aggravates the disease course of CHB. The molecular mechanisms by which these viruses interact with each other and with the intrinsic innate responses of the hepatocytes are not fully understood. While HBV appears to avoid innate immune recognition, HDV elicits a strong enhancement of innate responses. Notwithstanding, such induction does not hamper HDV replication but contributes to liver inflammation and pathogenesis. Intriguingly, HDV appears to influence the ability of T cells to recognize infected hepatocytes by boosting antigen presentation. This review focuses on current knowledge regarding how these viruses can shape and counteract the intrinsic innate responses of the hepatocytes, thus affecting the immune system and pathogenesis. Understanding the distinct strategies of persistence that HBV and HDV have evolved is central for advancing the development of curative therapies.
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Introduction
Hepatitis B virus (HBV) remains the major etiological agent of chronic viral hepatitis worldwide. Persistent HBV infection is characterized by various degrees of liver inflammation, which bears the risk, over decades, to develop liver cirrhosis and hepatocellular carcinoma (HCC) [1]. The current death toll is around 880.000 deaths a year [2]. Alarmingly, the number of deaths associated with chronic viral infections is even increasing [3]. Nucleos(t)ide analogs (NUCs) are safe and well-tolerated approved antiviral agents. Because of their ability to suppress HBV replication efficiently and their high barrier to resistance, they have become the gold standard for chronic hepatitis B (CHB) treatment [4]. However, loss of circulating viral antigens (HBeAg, HBsAg) and seroconversion remain rare under NUC treatment, committing most patients to long-term antiviral therapy. Although NUCs have been shown to prevent disease progression in most patients and to reduce the risk of developing HCC in non-cirrhotic patients, NUC discontinuation is often bound to the relapse of viral activity. This is mainly due to the fact that NUCs do not target the HBV genome template that is formed from incoming virions in the nuclei of infected hepatocytes [5]. In contrast, pegylated interferon alpha (peg-IFNα) is the only approved finite treatment for chronic HBV infection, despite its limited capacity to induce seroconversion and association with side effects [6]. Thus, undetectable HBV DNA levels in the serum and HBsAg loss, which are considered endpoints for a functional cure, are rarely achieved with existing treatments [4]. Therefore, the current efforts to advance treatment options for CHB aim to increase off-treatment response rates or functional cure rates [7].
The molecular mechanisms determining either effective HBV recognition and clearance, or persistence and pathogenesis are not fully elucidated. Acute HBV infection is known to resolve spontaneously in approximately 95% of immunocompetent adults, leading to the development of long-lasting immunity [8]. In contrast, 90% of children infected before 1 year of age develop a chronic HBV infection [9]. In general, the resolution of HBV infection requires an effective viral recognition and concerted induction of innate and adaptive immune responses. Both animal and clinical studies demonstrated that in acute self-limited HBV infection, both CD8+ T cell and CD4+ T cell responses to HBV proteins are strong and polyclonal [10], whereas in chronically infected individuals, immune responses appear weak and narrowly focused [8, 11]. The inability to effectively suppress HBV infection therefore results in the persistence of high quantities of viral antigen over the years. Such chronic presentation of viral antigens progressively suppresses virus-specific T cell immunity, which appears particularly compromised in old CHB patients [12, 13]
Despite the clear role of adaptive immune responses to resolve HBV infection, both unique replication characteristics of this hepatotropic virus and its ability to avoid or even affect intrinsic innate responses appear to be key elements in determining the failure of effective HBV recognition as it is observed in the course of CHB.
The disease course of CHB is substantially aggravated by co- or super-infection with the hepatitis delta virus (HDV), which is the only known satellite virus infecting humans. According to the World Health Organization (WHO), at least 12 million individuals are HBV/HDV co-infected worldwide, although recent metanalyses indicated these numbers may be substantially higher [14]. HDV infection causes the most severe form of chronic viral hepatitis, since it is associated with more rapid progression to cirrhosis, liver decompensation, HCC, and death [15]. As a defective RNA viroid, HDV needs the expression of HBV envelope proteins for the productive release of HDV particles and propagation among human hepatocytes. Because HDV shares the same envelope proteins of HBV, HDV infection can be prevented by hepatitis B immunization in HBV-negative individuals. However, treatment options for patients with chronic HDV (CHD) are limited. Off-label use of pegylated interferon alpha (peg-IFNα) shows limited efficacy, is curative in a minority of patients, and is associated with frequent and sometimes severe side effects [16]. The development of novel in vitro and in vivo systems for HBV and HDV infection has opened new venues for the preclinical assessment of new therapeutic options [17, 18]. As a result, Myrcludex B/bulevirtide (BLV), a synthetic peptide-blocking HBV and HDV cell entry, has now reached the clinic and was conditionally approved for the treatment of HBV/HDV-co-infected patients in Europe and Russia in 2020. In clinical trials, Bulevirtide has shown excellent safety and strong effectiveness in lowering HDV RNA loads [19] both alone and combined with peg-IFNα [20]. Further, promising treatment options include peg-IFN-lambda [21, 22], lonafarnib, which is a farnesyl transferase inhibitor, and nucleic acid polymers [23, 24]. Despite these encouraging therapeutic progresses, the molecular mechanisms responsible for the more severe disease progression observed in CHD compared to chronic HBV mono-infection are not yet fully understood. The strict host and tissue tropism of these viruses have hindered in-depth understanding of the host mechanisms involved in HBV and HDV sensing and the strategies adopted by HBV to escape recognition. Elucidating the interplay between HBV and HDV in infected cells, as well as the strategies used by both viruses to evade and modulate immune responses, remains a key effort for the development of curative therapies.
HBV replication and persistence strategies
The hepatitis B virus is a small blood-borne enveloped DNA virus that can cause both acute and chronic infection by targeting the hepatocytes, which are the only cells susceptible to infection. Typical of HBV is not only its high tissue and species specificity, but also a unique genomic organization and replication mechanism, which involves the formation of an over-length RNA intermediate and the utilization of a reverse transcriptase [25]. The infectious viral particle consists of a spherical lipid envelope containing a single small circular partially double-stranded DNA (rcDNA) molecule of about 3200 nucleotides, which is covalently linked to the viral polymerase and packaged within a nucleocapsid formed by the core protein (HBcAg) [25]. The viral membrane is formed by host-derived lipids and three HBV envelope proteins that are named, according to their size, preS1 (or large), preS2 (or middle), and S (or small). The three HBV envelope proteins share the C-terminal extremity, which corresponds to the S domain of the small protein, while the middle and large proteins display N-terminal extensions of 55 (preS2) and, genotype-dependent, 107 or 118 amino acids (preS1), respectively [26]. The same C-terminal domain bears the region coding for the surface antigen (HBsAg). A crucial step in the entry process of HBV into the human hepatocytes is the high-affinity binding of the N-terminal domain of the preS1 protein to the hepatocyte-specific receptor, the Na+-taurocholate cotransporting polypeptide (NTCP) [27, 28]. Notably, the entry of both HBV and HDV is efficiently blocked by a small myristoylated lipopeptide derived from the preS1 domain of the large envelope protein [18, 29,30,31]. To establish a productive HBV infection, the viral genome needs to be conveyed to the hepatocyte nucleus (Fig. 1), where the key molecule of viral persistence, the covalently closed circular DNA (cccDNA) minichromosome, is built from the rcDNA with the support of distinct, not yet fully characterized, cellular enzymes. The formation of the cccDNA is therefore a multi-step process requiring the participation of the cellular DNA repair machinery and its association with histone and non-histone proteins (for a review, see [32]). The cccDNA serves as a template for the production of all viral RNAs that are transcribed from four largely overlapping main open reading frames. Distinct promoters and enhancer regions regulate the transcription of both subgenomic RNAs, like those responsible for the production of the envelope proteins and an over-length pregenomic RNA (pgRNA). The transcription of the pgRNA is not only indispensable for viral replication, but is also responsible for the translation of the polymerase and of the core proteins. Through the reverse transcription of the pgRNA within the nucleocapsids, newly formed rcDNA-containing nucleocapsids are enveloped and secreted as progeny viruses via the endosomal secretory pathway [33].
The smallest subgenomic RNA produces the regulatory X protein (HBx), which was reported to interfere with several cellular pathways and transcription factors and to be recruited onto the cccDNA [32, 34]. Moreover, a major function of HBx that has emerged in the last years regards its ability to hinder the host’s attempts to silence cccDNA transcription [35,36,37]. By binding to the damaged DNA binding protein 1 (DDB1) [38], HBx triggers the degradation of the structural maintenance of chromosomes 5/6 complex (SMC5/6), a multi-functional DNA-binding complex involved in chromosome dynamics and stability [39]. Thus, HBx expression appears fundamental to counteract the SMC5/6 host restriction factor and maintain active transcription of the cccDNA minichromosome. Therapeutic strategies aiming at abrogating HBx production, such as siRNA technologies, were recently shown to enable reappearance of the SMC5/6 complex and silencing of the cccDNA minichromosome in HBV-infected chimeric mice [40].
Infected hepatocytes also secrete the non-structural pre-core protein, the so-called E antigen (HBeAg), as well as high amounts of subviral particles (SVPs), which are mainly composed of envelope proteins (HBsAg) but lack the capsid and the viral genome. High levels of circulating viral antigens are thought to contribute to establishing HBV persistence and a state of immune tolerance [41]. Of note, HBV DNA sequences are also found integrated into the host genome in the liver of infected individuals [42, 43]. Although HBV DNA sequences are often truncated and highly rearranged, they can contribute to the production of viral proteins, in particular, HBsAg. Moreover, additional virological markers like hepatitis B core-related antigens (HBcrAg) and enveloped particles containing HBV RNAs or empty capsids are also secreted from HBV replicating cells into the bloodstream [33]. Their relationship with cccDNA transcription, their clinical potential as surrogate biomarkers [44], and their role in the HBV life cycle, immunomodulation, and pathogenesis still need to be elucidated.
The persistence of the HBV minichromosome in the liver of CHB patients is considered the main virological reason for the rebound of HBV commonly determined in serum after cessation of NUC therapy. In an environment where liver inflammation is controlled and hepatocyte turnover remains low, the intrahepatic cccDNA pool appears stable [45]. However, knowledge of the half-life of individual cccDNA molecules is still limited, with reports estimating half-lives spanning from months to years [46,47,48]. To note, an accurate definition of the longevity of individual cccDNA molecules remains technically challenging. Adding complexity, the occurrence of new infection events is thought to play a key role in determining maintenance and renewal of intrahepatic cccDNA loads even under NUC therapy, due to the inability of polymerase inhibitors to achieve complete suppression of HBV replication [46].
It is conceivable that the kinetics of cccDNA decay are substantially affected by various biochemical and immunological conditions. Without a doubt, immune cells have the ability to recognize and destroy the cccDNA together with the infected hepatocytes, events that are expected to lower the intrahepatic cccDNA levels. Furthermore, the immune-mediated elimination of infected cells is bound to promote compensatory hepatocyte proliferation. Cell division has been shown not only to dilute the existing cccDNA molecules among dividing cells, but also to facilitate their destabilization and loss, thus leading to a substantial reduction of the intrahepatic cccDNA pool [49].
Studies with HBV-related viruses have shown that a pool of cccDNA molecules can be established not only from rcDNA molecules infecting the cells via the NTCP receptor entry pathway “external route,” but also through an “internal route” redirecting newly synthesized rcDNAs into the nucleus instead of promoting cell egress [49, 50]. However, the efficacy of such an “internal route” in HBV-infected human hepatocytes has been recently questioned, since both in vitro [43] and in vivo [31, 49] studies provide accumulating evidence that amplification and replenishment of the HBV cccDNA pool are mainly supported by new infection events. Understanding cccDNA biology and whether the transport of newly synthesized rcDNA molecules into the nucleus of human hepatocytes are key events determining maintenance of the cccDNA pool remain mandatory to assist the design of future therapeutic interventions. In this regard, the development of therapeutic approaches able to lower cccDNA amounts, to trigger its silencing, and to guard the cells from new infection events represent the main goals for achieving a functional cure. Together with strategies promoting restoration of the HBV-specific antiviral immune responses, a complete HBV cure could even be envisioned [5, 7, 24].
HDV replication and persistence strategies
The viral genome of HDV is a circular, single-stranded, negative-sense (−) RNA of approximately 1680 nucleotides. In the nucleus of infected human hepatocytes, the viral genome appears as a rod-like structure with broad intramolecular base pairing. This leads to the accumulation of three distinct RNA forms: the genomic RNA (−); the antigenomic RNA (+), which is an exact complement of the genomic RNA; and a smaller linear mRNA (+) encoding two isoforms of the only viral protein, the hepatitis delta antigen (HDAg). The small (S-) HDAg (24 kDa, 195 amino acids) is important for virus replication, while the large (L) variant (27 kDa, 214 amino acids), which is generated by an RNA editing event induced by the cellular enzyme adenosine deaminase acting on RNA (ADAR), is essential for virus assembly [51]. Abrogation of the stop-codon within the HDV antigenome enables the extension of the mRNA open reading frame and translation of L-HDAg, which harbors a nuclear export signal. The host RNA polymerases drive HDV replication using the antigenome as a template and are also responsible for the transcription of the HDV mRNA (Fig. 1). Using a double rolling-circle amplification process, HDV genomes and antigenomes are first generated as oligomers that get self-cleaved into RNA monomers through their intrinsic ribozyme activity. Newly formed HDV-RNA is associated with L-HDAg and S-HDAg to generate a ribonucleoprotein (RNP) complex. This complex is enveloped through budding into an ER-derived lipid bilayer carrying the three HBV envelope proteins to generate new virions [52].
The balance between genomic and antigenomic RNA appears crucial in guaranteeing persistence of HDV replication and is highly regulated by the two forms of HDAg, as well as through epigenetic modifications [53, 54]. Post-translational modifications are also known to play a key role in HDV replication and morphogenesis. Notably, farnesylation of the L-HDAg is essential for enabling the interaction of the HDV RNP complex with the HBsAg in the cytoplasm, thus favoring the assembly of HDV virions [55].
HBV plays an essential role as a helper virus for HDV transmission. However, HDV infection was shown to persist in patients for years also in the presence of very low levels of HBV infection [56, 57]. Intriguingly, HDAg-positive cells have been detected after liver transplantation for up to one and half years in the absence of HBV replication [58, 59], and infection studies in human liver chimeric mice revealed that HDV can infect and persist in vivo for at least 6 weeks in the absence of HBV [60]. Moreover, these in vivo experiments demonstrated that HDV mono-infection could be converted into a productive infection by super-infection with HBV [60], thus highlighting the endurance capacity of HDV in quiescent human hepatocytes. Of note, recent in vitro and in vivo studies revealed that HDV may even be rescued by alternative enveloped viruses, such as flaviviruses, hepaciviruses, and vesiculoviruses [61]. Particularly intriguing was the observation that HDV could be propagated by HCV in the liver of co-infected humanized mice, thus providing experimental evidence that HBV envelope proteins are not strictly required for HDV cell egress. However, clinical analysis of chronically infected HCV patient cohorts failed to document cases of HDV/HCV co-infection without HBV, thus indicating that HDV propagation mediated by HCV infection may rarely occur in a real clinical setting [62,63,64]. Since the interaction between the farnesylated L-HDAg and the HBsAg is weak, the evolutionary advantage of using HBV as a helper virus to exit the hepatocytes is not fully elucidated. Yet, it is conceivable that not only the tolerogenic liver environment and the peculiar low immune recognition profile of HBV (see below), but also the high production of HBV envelope proteins and of SVPs, which may serve as immunological decoy also for HDV [65], represent key benefits for HDV/HBV co-replication.
Keeping in mind the requirement of HBV envelope proteins for HDV cell-to-cell propagation, it could be hypothesized that immune-mediated cell turnover in HBV/HDV chronically infected livers could not only accelerate cccDNA loss but also affect HDV persistence. However, in vitro and in vivo experiments revealed that cell division promoted the clonal expansion of HDV-positive cell clusters, thus enabling HDV to propagate and replicate among dividing human hepatocytes even in the absence of HBV [66]. These findings highlight the strong persistence capacities of this unique RNA virus in the liver of HBV/HDV chronically infected patients.
HBV and HDV recognition by intrinsic innate mechanisms of the hepatocytes
The ability of the innate immunity to recognize intracellular pathogens, like viruses, is central to initiate the first line of defense and to coordinate immune responses adequate to achieve the control of the infection. A large range of pathogens is sensed through germline-encoded pattern recognition receptors (PRRs) that are present either on the cell surface or within multiple intracellular compartments. These PRRs include membrane-bound Toll-like receptors (TLRs); cytosolic DNA sensors, such as members of the AIM2 family; and numerous cytosolic RNA sensors, like the RIG I like receptors (RLRs) with their main players: the retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated gene 5 (MDA5). These receptors are specialized to recognize unusual structures like viral proteins or nucleic acids [67]. Their activation initiates the recruitment of distinct sets of adaptor molecules, such as Myd88 (myeloid differentiation primary response gene 88), MAVS (mitochondrial antiviral-signaling protein), STING (stimulator of interferon genes), IFI16, and TRIF (TIR-domain-containing adapter-inducing interferon-β), which trigger the main signaling pathways of NF-kB and interferon regulatory factors (IRFs). Nuclear translocation of these factors culminates in the induction of interferon-stimulated genes (ISGs) and production of different inflammatory cytokines, interferons (type I/III IFNs), and chemokines (reviewed in [68]).
Primary human hepatocytes (PHHs) express a broad range of PRRs [69] and have the ability to sense various pathogens, as shown by studies with the hepatitis C virus (HCV). In this regard, both experimental studies [70,71,72,73] and analyses of liver specimens from HCV-infected patients provided clear evidence that HCV induces the upregulation of ISGs and a strong interferon response [74,75,76]. The enhancement of various human ISGs and chemokines, like TGFβ1 and IP10, was also demonstrated in the absence of the adaptive immune system, in HCV-infected human hepatocytes within the liver of immunodeficient uPA chimeric mice [77]. Of note, HCV is an RNA virus whose replication cycle exclusively takes place in the cytoplasm of infected cells. It is therefore plausible that PRRs, like RIG-I, MAVS, and TLR3, can sense cytosolic HCV RNA despite the ability of HCV proteins to attenuate the IFN response by counteracting components of the innate immune signaling [68]. Nevertheless, type I and III interferons are produced during HCV infection, and interferon-based treatments suppress HCV replication both in vitro [78] and in vivo [77, 79], even though sustained ISG expression has been associated with weaker responses to IFN-based treatment [80].
In contrast to HCV, after entering the hepatocytes, the HBV DNA is transported to the cell nucleus" instead of "into" - since the genome may get released from the nucleocapsid at the level of the nuclear membrane. Above all, the mimicry ability of the HBV DNA genome to reside as a minichromosome in hepatocyte nuclei appears to be a key replication strategy of this virus to avoid innate immune recognition. Transcribed HBV RNAs are generated from the cccDNA by cellular enzymes and resemble host messenger RNAs with their 5′ cap and 3′poly(A) tail, thus offering poor opportunities to ignite recognition. Only the HBV pgRNA contains an unusual hairpin loop as packaging signal [81] and was shown to induce cytosolic PRRs (RIG-I) in certain experimental conditions [82]. Nevertheless, the pgRNA becomes rapidly encapsidated by the core proteins in the cytoplasm, thus protecting the HBV replicative intermediate from PRR recognition. Such ability of HBV to escape a strong induction of the so-called antiviral state led researchers to stamp HBV as a “stealth” virus [73].
Both studies in chimpanzees [83] and in patients with acute infection [84] showed that HBV neither triggers the induction of type I/III interferons nor clearly enhances ISGs, indicating that either the sensory pathways are unable to recognize HBV or that HBV can actively block these pathways. Likewise, in vitro studies with HBV-infected human hepatocytes failed to detect upregulation of ISGs [72], and liver specimens from patients with CHB were reported to express ISG levels similar to those obtained from control individuals [85]. Nevertheless, this study showed that innate responses could be activated upon ex vivo incubation of fresh liver biopsies with TLR3 agonists.
Human liver chimeric mice are based on the repopulation of the mouse liver with primary human hepatocytes. Because these mice lack NK cells and functional adaptive immune responses, the model offers the opportunity to dissect interactions occurring between human hepatotropic viruses and intrinsic innate responses of the human hepatocytes in vivo. We and others employed these systems to investigate the capacity of human hepatocytes to sense different hepatitis viruses in vivo [21, 86], as well as the antiviral effects of therapeutic cytokines like interferons [22, 40, 87, 88] and of HBV-specific immune cells [89,90,91]. These studies revealed that HBV induces a much weaker and barely detectable enhancement of innate immunity genes [86, 92] in comparison with infections with HCV [77] or HDV [86].
As an RNA virus, HDV is expected to be recognized by various PRRs, and in particular RNA sensors like RLRs, MDA5 and to activate downstream signaling proteins (i.e., MAVS) and transcription factors, which can translocate into the cell nucleus and initiate the transcription of IFN and ISG genes. HBV/HDV co-infection in humanized mice was shown to provoke a sustained induction of the antiviral state of the human hepatocytes by promoting the enhancement of classical human ISGs, genes involved in antigen presentation (see below), and the induction of inflammatory and pro-fibrogenic cytokines (i.e., IP10, IFN-β, TGF-β), both at the transcriptional and protein levels [86]. In this regard, TNFα was recently shown to play a key role in HDV-mediated liver inflammation [93]. A similar HDV-mediated induction of ISGs was also observed in vitro [94] and in other mouse models [95,96,97]. The increased amounts of ISGs and cytokine levels may be a key determinant of liver inflammation in HDV infection, thus explaining the more severe course of disease observed in CHD patients. The enhancement and nuclear translocation of STAT in human hepatocytes also indicated that HDV triggered the JAK/STAT signaling cascade in HBV/HDV co-infected livers. Notably and in line with previous in vitro studies [98], nuclear accumulation of STAT proteins appeared most pronounced in cells displaying lower HDAg levels, suggesting that interference mechanisms may be active in the presence of high HDV protein levels [86]. Microarray analyses also reported the activation of a broad range of ISGs and production of IFN-β and IFN-λ in hepatoma cell lines (HepG2-NTCP cells) and PHH cultures infected with HDV [99]. Of note, HDV-mediated induction of IFN appeared strongly reduced upon depletion of MDA5, thus indicating that this innate cellular component can act as a key sensor of HDV RNA recognition [96, 100]. However, it remains unknown whether MDA5 can be also transported to the nucleus, which is the replication site of HDV. Thus, the ability of this virus to replicate in a “protected” cellular compartment may be seen as a countermeasure to avoid recognition from PRRs or other cellular restriction mechanisms as it has been recently described for HBV (i.e., SMC5/6 complex).
It is noteworthy that different levels of HBV and HDV infection can coexist among human cells within the same liver even in immune-deficient liver chimeric mice. As shown in Fig. 2, human hepatocytes expressing almost exclusively high levels of HDAg can be observed near cells expressing both viruses, as well as cells positive only for HBV markers. Moreover, the development of HBV viremia and the increase of intrahepatic cccDNA loads appeared substantially delayed in HBV/HDV co-infected mice in comparison with HBV mono-infected animals, suggesting that the stronger antiviral state exerted by HDV during viral spreading could interfere with HBV replication and establishment of new infection events [18, 101]. Such moderate HDV-mediated suppression of HBV activity could occur also in the absence of the adaptive immune system, as it is the case in chimeric mice, thus supporting the notion that intrinsic innate mechanisms elicited within the hepatocytes and/or direct virus/virus interferences account for a certain dampening of HBV productivity. Although the exact molecular mechanisms by which HDV can suppress HBV remain to be elucidated, these observations are in agreement with studies reporting lower levels of HBV infection in HBV/HDV co-infected patients [56].
Single-cell analyses based on in situ RNA hybridization showed that the activation of innate genes is clearly detectable not only in HBV/HDV co-infected livers [86, 96, 99], but also within the human hepatocytes [21]. While some studies reported induction of ISGs and IFN also in the setting of HDV mono-infection [95,96,97], a clear induction of the same intrinsic immune responses could not be detected in the setting of HDV mono-infection in immune-deficient humanized mice [21]. The mechanisms underlying such differences are unclear. The low amounts of infected cells achievable in vivo in the setting of HDV mono-infection may in part impact the type and amplitude of intrahepatic HDV sensing. Of note, extracellular vesicles released from HDV mono-infected cells have been recently shown to bear the ability to induce pro-inflammatory cytokines in macrophages and peripheral blood mononuclear cells [102], thus indicating that different mechanisms may participate in intercellular HDV sensing. Moreover, one could speculate that the co-existence of both viruses enabling ongoing HDV spreading among hepatocytes may play a determinant role in promoting the enhancement of most ISGs, as well as the production of pro-inflammatory cytokines. If this is true, abrogation of new infection events would be expected to lower liver inflammation in CHD infection—and provide at least in part a rationale for the amelioration of most biochemical parameters as it has been observed in the clinical trials investigating the efficacy of the HBV/HDV entry inhibitor bulevirtide [103].
Altogether, the abovementioned studies provide strong evidence that the presence of HBV in human hepatocytes co-infected with either HCV or HDV [72, 86] did not prevent the enhancement of intrinsic innate responses promoted by these RNA viruses. Therefore, such studies support the hypothesis that HBV remains nearly invisible to PRRs within the infected hepatocytes. While the ability of HBV to escape innate recognition in infected cells encounters large consensus in the scientific community, it remains unclear whether HBV can actively suppress or hinder the induction of innate mechanisms in infected cells at least in certain conditions. In this respect, downregulation of some innate genes was observed in liver samples from CHB patients [104], while other studies reported the ability of the precore/HBeAg to target and hinder the induction of the TLR signaling pathways, thus supporting the notion that HBV proteins may actively contribute to the evasion of the innate responses [105]. Moreover, the HBx protein and the HBV polymerase were reported to block the induction of innate immunity genes in vitro [106, 107]. However, the use of different experimental models and viral protein expression levels may in part explain the controversial data reported. Yet, persistent production of HBsAg and HBeAg have shown to exert immune-modulating functions contributing to viral persistence [8, 105, 108,109,110] and to the B cell exhaustion detected in chronically HBV-infected individuals [111,112,113,114,115].
It can be noted that also HDV adopts unusual replication modalities, which enable the virus to limit contacts with the intracellular PPRs by nuclear compartmentalization of the HDV RNA-RNA replication. In addition, the formation of a circular RNA genome without accessible 5′ or 3′ ends prevents PRR recognition by RIG I [116], while other viral RNA motives (like GC rich elements) that are potentially recognized by innate defense mechanisms are shielded by the delta antigens forming tight ribonucleoprotein (RNP) complexes [117] that may remain spatially inaccessible to cellular factors such as nucleases. Although it is currently unclear whether the HDAg can contribute or counteract innate immune recognition of HDV infection, the RNP complex is considered to play a key role also in preventing over-accumulation of editing events by ADAR1 at sites of the viral RNA other than the amber/w site. Such control of editing levels appears central to control HDV RNA replication levels and the likely outcome of infection [118].
Similarly to HBV, the HDV RNP complex is further enveloped by HBsAg before budding into the endosomal compartment (for more details on RNA receptors see review [119]), thus limiting the steps where HDV can be recognized by the intrinsic immune mechanisms of the hepatocytes. Finally, HDAg was predicted to generate only few peptides that can be processed and bind to human leukocyte antigen (HLA) molecules; steps that are essential to activate HDV-specific CD8 T cells [120]. Moreover, immune evasion mutations on predicted T cell epitopes were observed in chronic HBV/HDV infected individuals [121].
Hepatocyte antigen presentation of HBV and HDV and immunogenic properties of HDV
The liver represents a particularly attractive organ for many pathogens. Being exposed to high amounts of food-derived metabolites, toxins, and bacterial products coming from the gut, the liver is characterized by unique tolerogenic properties, where immune activation is kept tightly controlled. While the liver parenchymal cells, the hepatocytes, ensure metabolic and detoxification functions, large amounts of infiltrating and resident immune cells are found in the healthy liver. Innate lymphoid cells, like the Kupffer cells, which are the liver resident macrophages, build together with liver sinusoidal endothelial cells (LSECs) and hepatic stellate cells (HSCs) the so-called non-parenchymal cell (NPC) compartment. In addition to many liver-specific NPCs, natural killer cells and various circulating monocytes, like dendritic cells and lymphocytes, including mucosa-associated invariant T (MAIT) cells, infiltrate the liver and contribute to the orchestration of the immune response [67]. Despite such unique richness of resident and circulating lymphoid and professional antigen-presenting cells, the distinct ability of infected and innate cells to sense and signal the presence of pathogens may differently impact the effectiveness of the immune responses elicited.
Antigen presentation is certainly a required checkpoint to initiate adaptive, pathogen-specific, immune responses. The hepatotropic nature of HBV and HDV and the fact that hepatocytes are in direct contact with blood flow means that naive and effector/memory T cells are in direct contact with the virus-infected parenchymal cells [122, 123]. Priming of naive T cells by antigens exclusively presented by hepatocytes is known to generate T cell tolerance [124], but the mechanisms of antigen presentation during natural infection in the presence or absence of inflammatory events can profoundly alter T cell priming [125] and the ability of effector T cells to recognize infected hepatocytes [126]. Recent work in animal models has elegantly demonstrated that priming by HBV-infected hepatocytes causes the induction of HBV-specific CD8+ T cells that do not become classical effector CD8+ T cells [125]. Moreover, this study pointed out the key role of IL-2 in promoting the expansion and proliferation of primed HBV-specific T cells. Similarly to what is observed in so-called immunotolerant patients (HBeAg+ chronic infection), this specific tolerogenic priming was shown to occur in a liver environment without inflammatory events. Indeed, HBV-specific T cells of immune tolerant CHB patients were shown to expand and become functional upon the addition of IL-2. In contrast, priming in an inflammatory liver environment appears to be mediated mainly by professional antigen-presenting cells (Kupffer cells, endothelial cells). However, such priming was shown to induce more classical effector T cells that became exhausted and could be rescued by anti-PD-1 treatment [125]. Chronic HDV infection was shown to be associated with increased levels of pro-inflammatory cytokines like IL-12 and IL-18 and to engage MAIT cells, which are a subset of innate-like T cells, causing their functional impairment and progressive depletion as the HDV-associated liver disease progresses (Dias 2019). Of note, only a modest MAIT cells decrease was observed in HBV mono-infected patients (Dias), thus suggesting that higher cytokine levels often determined in HDV infection, may contribute to liver damage and disease progression also by the activation and subsequent loss or exhaustion of different innate and adaptive immune responses.
The infection of HDV and its ability to directly activate innate immunity in hepatocytes radically changes the liver microenvironment and the ability of T cells to recognize HBV-infected hepatocytes. Recent work performed in an in vitro HDV infection system, liver biopsies of HBV/HDV co-infected patients and mice with humanized livers, have shown that HDV infection not only enhances the gene expression of HLA class I molecules, B2M, immunoproteasome, and co-stimulatory molecules genes, but it also increases the presentation of viral epitopes and, as a consequence, the efficiency of T cell recognition of infected hepatocytes [126].
We do not have data related to how HBV/HDV co-infected hepatocytes might differentially prime naive T cells. However, HDV has clear effects on the presentation of viral antigens to effector CD8+ T cells. This could explain the high incidence of CD8+ T cell escape mutations found in HDV epitopes [121, 127], but also explain the epidemiological evidence of a better HBV control, or at least frequent HBV suppression, in HBV/HDV co-infected patients than in HBV mono-infected patients. The compact nature of the HBV DNA genome poses a limit on the generation of mutated HBV viruses, and this could in part explain why dually infected HBV/HDV patients become HBsAg-negative at a higher rate than HBV-mono-infected patients [128].
The increased ability of HDV to boost the presentation of HBV antigens was shown to not be exclusive to the co-infected HBV/HDV hepatocytes but also to neighboring HBV mono-infected cells, through a mechanism likely mediated by the increased production of IFN-β and IFN-λ [126]. In this regard, infected or damaged cells and cellular events eliciting pathogen recognition result in the production of a variety of soluble factors, such as danger signals, pro-inflammatory cytokines and chemokines, and extracellular vesicles [102]. These signals may play a pivotal role in promoting bystander innate immune activation of neighboring cells.
The increased antigen presentation induced by HDV also supports the concept that therapeutic interventions designed to boost HBV-specific CD8+ T cell responses, with anti-PD1 therapy or with the use of chimeric antibody receptor engineered (CAR) or T cell receptor (TCR)-redirected T cells or TCR-like antibodies might be better suited for the treatment of HBV-HDV chronic hepatitis. The demonstration that HDV can persist intracellularly in replicating human hepatocytes despite blocking re-infection by administering the entry inhibitor bulevirtide [66] suggests that immune-mediated destruction of a substantial fraction of HDV-infected cells is required to substantially lower intrahepatic HDV infection.
Sensitivity of HBV and HDV to therapeutic cytokines
The persistence of high antigen levels is considered a major factor driving functional exhaustion of HBV-specific immune cells, and various studies indicated a poor restoration of immune cell functions in the early phases of IFN treatment [8, 129,130,131]. Moreover, the limited rates of cccDNA reduction determined in patients receiving IFN-based therapy [132, 133] do not fully explain the early kinetics of HBsAg decline. Thus, the responsiveness of the hepatocytes to IFN therapy may be central to trigger the initial reduction of viral antigens and thereby facilitate the functional reconstitution of antiviral T cell responses. An early HBsAg decline is indeed observed in patients responding to peg-IFNα therapy [131]. Thus, IFN-based therapy may be beneficial in some patients not only by acting as an immune-modulator, but also by directly lowering HBV RNA levels in infected hepatocytes. Studies in humanized mice showed that administration of conventional IFNα led only to a transient epigenetic suppression of the cccDNA [88] and that the responsiveness of HBV-infected human hepatocytes appeared in part impaired [92]. Moreover, some human ISGs appeared less efficiently induced after one single injection of peg-IFNα in HBV-infected livers compared to the enhancement induced in uninfected animals, suggesting the existence of an initial, albeit partial, impairment of the responsiveness of the HBV-infected hepatocytes to IFNα, which, however, could be breached by repeated administrations of the longer-active peg-IFNα [87]. At any rate, several weeks of treatment with peg-IFNα were sufficient to provoke a strong decrease of circulating and intrahepatic viral antigens despite the absence of immune cell responses in this system [40, 87]. Of note, recent studies showed that not only administration of peg-IFNα but also RNA interference strategies targeting all HBV transcripts could abrogate the production of all HBV proteins, including HBx, in a substantial amount of hepatocytes in vivo, leading to the reappearance of the SMC5/6 complex and cccDNA silencing [40]. Intriguingly, sustained silencing of cccDNA transcription could be maintained in a substantial fraction of infected cells by applying the entry inhibitor bulevirtide, thus by shielding the hepatocytes from new infection events [40].
Due to its narrow genome size and the lack of expression of its own viral polymerase, therapy options for HDV are still limited with peg-IFNα being the drug of common use [23] and more recently bulevirtide, which does not directly block viral activity but rather promotes the reduction of HDV loads by blocking cell entry [103]. Unfortunately, outcomes in patients treated with peg-IFNα remain unsatisfactory and the mechanisms by which IFN exerts anti-HDV effects in human hepatocytes are not yet elucidated. Previous in vitro studies mostly relied on a particular cell culture-derived strain of HDV genotype 1 [134] that resulted insensitive to IFNα in vitro [99]. Contrarily, a patient-derived HDV-1 strain was shown to respond both to peg-IFNα and to peg-IFNλ in human liver chimeric mice [22], thus providing evidence that these therapeutic cytokines can lower HDV loads in infected human hepatocytes also in a system lacking adaptive immune responses. Understanding the mode of action of interferon in HDV-infected cells, as well as the mechanisms responsible for the different responsiveness to interferon among distinct HDV isolates shall greatly assist the design of therapies aiming to accelerate HDV loss and achieve HDV cure.
Conclusions
Elucidation of the different mechanisms that infected hepatocytes use to unveil the presence of these human hepatotropic viruses to uninfected bystander cells and to different types of resident and circulating immune cells is central to understand key mechanisms determining the resolution of HBV and HDV infection versus persistence. Whereas experimental infection systems and patient analyses support the notion that HBV avoids innate immune recognition, co-infection with HDV appears to cause profound changes in the infected liver. The clear enhancement of various ISGs, the higher production of chemokines and inflammatory cytokines, as well as the increased antigen presentation capabilities determined in HBV/HDV infection may act however as a double sword, boosting the ability of immune cells to recognize infected cells on the one side, but also augmenting liver inflammation and thus accelerating pathogenesis. Through the development of various in vitro and in vivo infection models, as well as of sophisticated technologies enabling the dissection of events occurring at the single-cell level, the role of distinct HBV and HDV proteins in modulating the antiviral responses in infected hepatocytes is gaining recognition, also highlighting the importance of viral activity in counteracting the first line of host defenses. Understanding the interplay between viral proteins and the innate responses remains central for developing curative treatment strategies against HBV and HDV.
References
Revill PA, Chisari FV, Block JM, Dandri M, Gehring AJ, Guo H, Hu J, Kramvis A, Lampertico P, Janssen HLA, Levrero M, Li W, Liang TJ, Lim SG, Lu F, Penicaud MC, Tavis JE, Thimme R, Zoulim F, Arbuthnot P, Boonstra A, Chang KM, Chen PJ, Glebe D, Guidotti LG, Fellay J, Ferrari C, Jansen L, Lau DTY, Lok AS, Maini MK, Mason W, Matthews G, Paraskevis D, Petersen J, Rehermann B, Shin EC, Thompson A, van Bömmel F, Wang FS, Watashi K, Yang HC, Yuan Z, Yuen MF, Block T, Miller V, Protzer U, Bréchot C, Locarnini S, Peters MG, Schinazi RF (2019) A global scientific strategy to cure hepatitis B. Lancet Gastroenterol Hepatol 4(7):545–558
who. (2018) Hepatitis B key facts.
Thomas DL (2019) Global elimination of chronic hepatitis. N Engl J Med 380(21):2041–2050
Cornberg M., et al., (2019) Guidance for design and endpoints of clinical trials in chronic hepatitis B - report from the 2019 EASL-AASLD HBV Treatment Endpoints Conference. J Hepatol.
Dandri M, Petersen J (2020) cccDNA maintenance in chronic hepatitis B - targeting the matrix of viral replication. Infect Drug Resist 13:3873–3886
Terrault NA, Lok ASF, McMahon BJ, Chang KM, Hwang JP, Jonas MM, Brown RS Jr, Bzowej NH, Wong JB (2018) Update on prevention, diagnosis, and treatment of chronic hepatitis B: AASLD 2018 hepatitis B guidance. Hepatology 67(4):1560–1599
Fanning GC, Zoulim F, Hou J, Bertoletti A (2019) Therapeutic strategies for hepatitis B virus infection: towards a cure. Nat Rev Drug Discov 18(11):827–844
Bertoletti A, Ferrari C (2012) Innate and adaptive immune responses in chronic hepatitis B virus infections: towards restoration of immune control of viral infection. Gut 61(12):1754–1764
Paganelli M, Stephenne X, Sokal EM (2012) Chronic hepatitis B in children and adolescents. J Hepatol 57(4):885–896
Guidotti LG, Chisari FV (2001) Noncytolytic control of viral infections by the innate and adaptive immune response. Annu Rev Immunol 19:65–91
Bertoletti A, Ferrari C (2016) Adaptive immunity in HBV infection. J Hepatol 64(1 Suppl):S71–S83
Kennedy PT et al (2012) Preserved T-cell function in children and young adults with immune-tolerant chronic hepatitis B. Gastroenterology 143(3):637–645
Le Bert N et al (2020) Effects of hepatitis B surface antigen on virus-specific and global T cells in patients with chronic hepatitis B virus infection. Gastroenterology 159(2):652–664
Rizzetto M, Hamid S, Negro F (2021) The changing scenario of hepatitis D. J Hepatol 74:1200–1211
Fattovich G, Giustina G, Christensen E, Pantalena M, Zagni I, Realdi G, Schalm SW (2000) Influence of hepatitis delta virus infection on morbidity and mortality in compensated cirrhosis type B. The European Concerted Action on Viral Hepatitis (Eurohep). Gut 46(3):420–426
Trepo C, Chan HL, Lok A (2014) Hepatitis B virus infection. Lancet 384(9959):2053–2063
Allweiss L, L M, Volz T, Bornscheuer T, Lohse AW, Petersen J, Dandri M (2012) Pegylated-interferon-alpha alone or in combination with entecavir restores ISG responsiveness and reduces intrahepatic viral loads and antigenemia in hepatitis B virus infected humanized mice. J Hepatol 56(Supplement 2):S18–S19
Lutgehetmann M et al (2012) Humanized chimeric uPA mouse model for the study of hepatitis B and D virus interactions and preclinical drug evaluation. Hepatology 55(3):685–694
Loglio A, Ferenci P, Uceda Renteria SC, Tham CYL, van Bömmel F, Borghi M, Holzmann H, Perbellini R, Trombetta E, Giovanelli S, Greco L, Porretti L, Prati D, Ceriotti F, Lunghi G, Bertoletti A, Lampertico P (2019) Excellent safety and effectiveness of high-dose myrcludex-B monotherapy administered for 48weeks in HDV-related compensated cirrhosis: a case report of 3 patients. J Hepatol 71(4):834–839
Allweiss, L., et al., (2019) PS-155-HBV entry inhibition after interferon alpha treatment hinders HBV rebound in hepatocytes that became negative for all HBV markers during interferon treatment. Journal of Hepatology ( The International Liver Congress 2019 Abstract Book).
Giersch K, Hermanussen L, Volz T, Kah J, Allweiss L, Casey J, Sureau C, Dandri M, Lütgehetmann M (2021) Murine hepatocytes do not support persistence of hepatitis D virus mono-infection in vivo. Liver Int 41(2):410–419
Giersch K, Homs M, Volz T, Helbig M, Allweiss L, Lohse AW, Petersen J, Buti M, Pollicino T, Sureau C, Dandri M, Lütgehetmann M (2017) Both interferon alpha and lambda can reduce all intrahepatic HDV infection markers in HBV/HDV infected humanized mice. Sci Rep 7(1):3757
Petersen J, Thompson AJ, Levrero M (2016) Aiming for cure in HBV and HDV infection. J Hepatol 65(4):835–848
Urban S, Bertoletti A (2018) Editorial overview: antiviral strategies: virological and immunological basis for HBV cure. Curr Opin Virol 30:iv–vi
Nassal M (2015) HBV cccDNA: viral persistence reservoir and key obstacle for a cure of chronic hepatitis B. Gut 64(12):1972–1984
Urban S, Bartenschlager R, Kubitz R, Zoulim F (2014) Strategies to inhibit entry of HBV and HDV into hepatocytes. Gastroenterology 147(1):48–64
Li W, Urban S (2016) Entry of hepatitis B and hepatitis D virus into hepatocytes: basic insights and clinical implications. J Hepatol 64(1 Suppl):S32–S40
Yan, H., et al., (2012) Sodium taurocholate cotransporting polypeptide is a functional receptor for human hepatitis B and D virus. elife. 3.
Lempp FA, Wiedtke E, Qu B, Roques P, Chemin I, Vondran FWR, le Grand R, Grimm D, Urban S (2017) Sodium taurocholate cotransporting polypeptide is the limiting host factor of hepatitis B virus infection in macaque and pig hepatocytes. Hepatology 66(3):703–716
Petersen J, Dandri M, Mier W, Lütgehetmann M, Volz T, von Weizsäcker F, Haberkorn U, Fischer L, Pollok JM, Erbes B, Seitz S, Urban S (2008) Prevention of hepatitis B virus infection in vivo by entry inhibitors derived from the large envelope protein. Nat Biotechnol 26(3):335–341
Volz T, Allweiss L, ḾBarek MB, Warlich M, Lohse AW, Pollok JM, Alexandrov A, Urban S, Petersen J, Lütgehetmann M, Dandri M (2013) The entry inhibitor Myrcludex-B efficiently blocks intrahepatic virus spreading in humanized mice previously infected with hepatitis B virus. J Hepatol 58(5):861–867
Dandri M (2020) Epigenetic modulation in chronic hepatitis B virus infection. Semin Immunopathol 42(2):173–185
Hu J, Liu K, (2017) Complete and incomplete hepatitis B virus particles: formation, function, and application. Viruses. 9(3).
Belloni L, Pollicino T, de Nicola F, Guerrieri F, Raffa G, Fanciulli M, Raimondo G, Levrero M (2009) Nuclear HBx binds the HBV minichromosome and modifies the epigenetic regulation of cccDNA function. Proc Natl Acad Sci U S A 106(47):19975–19979
Decorsiere A et al (2016) Hepatitis B virus X protein identifies the Smc5/6 complex as a host restriction factor. Nature 531(7594):386–389
Minor MM, et al., (2020) Hepatitis B virus HBx protein mediates the degradation of host restriction factors through the cullin 4 DDB1 E3 ubiquitin ligase complex. Cells. 9(4).
Murphy CM, Xu Y, Li F, Nio K, Reszka-Blanco N, Li X, Wu Y, Yu Y, Xiong Y, Su L (2016) Hepatitis B virus X protein promotes degradation of SMC5/6 to enhance HBV replication. Cell Rep 16(11):2846–2854
Hodgson AJ, Hyser JM, Keasler VV, Cang Y, Slagle BL (2012) Hepatitis B virus regulatory HBx protein binding to DDB1 is required but is not sufficient for maximal HBV replication. Virology 426(1):73–82
Jeppsson K, Kanno T, Shirahige K, Sjögren C (2014) The maintenance of chromosome structure: positioning and functioning of SMC complexes. Nat Rev Mol Cell Biol 15(9):601–614
Allweiss L, Giersch K, Pirosu A, Volz T, Muench RC, Beran RK, Urban S, Javanbakht H, Fletcher SP, Lütgehetmann M, Dandri M (2021, gutjnl) Therapeutic shutdown of HBV transcripts promotes reappearance of the SMC5/6 complex and silencing of the viral genome in vivo. Gut:2020–322571
Dembek C, Protzer U, Roggendorf M (2018) Overcoming immune tolerance in chronic hepatitis B by therapeutic vaccination. Curr Opin Virol 30:58–67
Tu T, et al., (2017) HBV DNA integration: molecular mechanisms and clinical implications. Viruses 9(4).
Tu T, Zhang H, Urban S, (2021). Hepatitis B virus DNA integration: in vitro models for investigating viral pathogenesis and persistence. Viruses. 13(2).
Giersch K, Allweiss L, Volz T, Dandri M, Lütgehetmann M (2017) Serum HBV pgRNA as a clinical marker for cccDNA activity. J Hepatol 66(2):460–462
Allweiss L, Dandri M(2017) The role of cccDNA in HBV maintenance. Viruses. 9(6).
Boyd A, Lacombe K, Lavocat F, Maylin S, Miailhes P, Lascoux-Combe C, Delaugerre C, Girard PM, Zoulim F (2016) Decay of ccc-DNA marks persistence of intrahepatic viral DNA synthesis under tenofovir in HIV-HBV co-infected patients. J Hepatol 65(4):683–691
Huang Q, et al., (2020) Rapid turnover of HBV cccDNA indicated by monitoring emergence and reversion of signature-mutation in treated chronic hepatitis B patients. Hepatology.
Lai CL, Wong D, Ip P, Kopaniszen M, Seto WK, Fung J, Huang FY, Lee B, Cullaro G, Chong CK, Wu R, Cheng C, Yuen J, Ngai V, Yuen MF (2017) Reduction of covalently closed circular DNA with long-term nucleos(t)ide analogue treatment in chronic hepatitis B. J Hepatol 66(2):275–281
Allweiss L, Volz T, Giersch K, Kah J, Raffa G, Petersen J, Lohse AW, Beninati C, Pollicino T, Urban S, Lütgehetmann M, Dandri M (2018) Proliferation of primary human hepatocytes and prevention of hepatitis B virus reinfection efficiently deplete nuclear cccDNA in vivo. Gut 67(3):542–552
Wu TT, Coates L, Aldrics CE, Summers J, Mason WS (1990) In hepatocytes infected with duck hepatitis B virus, the template for viral RNA synthesis is amplified by an intracellular pathway. Virology 175(1):255–261
Ryu WS, Bayer M, Taylor J (1992) Assembly of hepatitis delta virus particles. J Virol 66(4):2310–2315
Zhang, Z. and S. Urban, (2020) Interplay between hepatitis D virus and the interferon response. Viruses. 12(11).
Sureau C, Negro F (2016) The hepatitis delta virus: replication and pathogenesis. J Hepatol 64(1 Suppl):S102–S116
Tseng CH, Cheng TS, Shu CY, Jeng KS, Lai MMC (2010) Modification of small hepatitis delta virus antigen by SUMO protein. J Virol 84(2):918–927
Glenn JS et al (1992) Identification of a prenylation site in delta virus large antigen. Science 256(5061):1331–1333
Pollicino T, Raffa G, Santantonio T, Gaeta GB, Iannello G, Alibrandi A, Squadrito G, Cacciola I, Calvi C, Colucci G, Levrero M, Raimondo G (2011) Replicative and transcriptional activities of hepatitis B virus in patients coinfected with hepatitis B and hepatitis delta viruses. J Virol 85(1):432–439
Schaper M, Rodriguez-Frias F, Jardi R, Tabernero D, Homs M, Ruiz G, Quer J, Esteban R, Buti M (2010) Quantitative longitudinal evaluations of hepatitis delta virus RNA and hepatitis B virus DNA shows a dynamic, complex replicative profile in chronic hepatitis B and D. J Hepatol 52(5):658–664
Mederacke I, Filmann N, Yurdaydin C, Bremer B, Puls F, Zacher BJ, Heidrich B, Tillmann HL, Rosenau J, Bock CT, Savas B, Helfritz F, Lehner F, Strassburg CP, Klempnauer J, Wursthorn K, Lehmann U, Manns MP, Herrmann E, Wedemeyer H (2012) Rapid early HDV RNA decline in the peripheral blood but prolonged intrahepatic hepatitis delta antigen persistence after liver transplantation. J Hepatol 56(1):115–122
Samuel D, Zignego AL, Reynes M, Feray C, Arulnaden JL, David MF, Gigou M, Bismuth A, Mathieu D, Gentilini P, Benhamou JP, Brechot C, Bismuth H (1995) Long-term clinical and virological outcome after liver transplantation for cirrhosis caused by chronic delta hepatitis. Hepatology 21(2):333–339
Giersch K, Helbig M, Volz T, Allweiss L, Mancke LV, Lohse AW, Polywka S, Pollok JM, Petersen J, Taylor J, Dandri M, Lütgehetmann M (2014) Persistent hepatitis D virus mono-infection in humanized mice is efficiently converted by hepatitis B virus to a productive co-infection. J Hepatol 60(3):538–544
Perez-Vargas J, Amirache F, Boson B, Mialon C, Freitas N, Sureau C, Fusil F, Cosset FL (2019) Enveloped viruses distinct from HBV induce dissemination of hepatitis D virus in vivo. Nat Commun 10(1):2098
Cappy, P., et al., (2020) No evidence of HCV-assisted HDV propagation in a large cohort of hepatitis C positive blood donors. J Infect Dis.
Chemin I, Pujol FH, Scholtès C, Loureiro CL, Amirache F, Levrero M, Zoulim F, Pérez-Vargas J, Cosset FL (2021) Preliminary evidence for hepatitis delta virus exposure in patients who are apparently not infected with hepatitis B virus. Hepatology 73(2):861–864
Pfluger LS et al (2021) Hepatitis delta virus propagation enabled by hepatitis C virus-scientifically intriguing, but is it relevant to clinical practice? J Viral Hepat 28(1):213–216
Rydell GE, Prakash K, Norder H, Lindh M (2017) Hepatitis B surface antigen on subviral particles reduces the neutralizing effect of anti-HBs antibodies on hepatitis B viral particles in vitro. Virology 509:67–70
Giersch K, Bhadra OD, Volz T, Allweiss L, Riecken K, Fehse B, Lohse AW, Petersen J, Sureau C, Urban S, Dandri M, Lütgehetmann M (2019) Hepatitis delta virus persists during liver regeneration and is amplified through cell division both in vitro and in vivo. Gut 68(1):150–157
Robinson MW, Harmon C, O’Farrelly C (2016) Liver immunology and its role in inflammation and homeostasis. Cell Mol Immunol 13(3):267–276
Shin EC, Sung PS, Park SH (2016) Immune responses and immunopathology in acute and chronic viral hepatitis. Nat Rev Immunol 16(8):509–523
Faure-Dupuy S, Vegna S, Aillot L, Dimier L, Esser K, Broxtermann M, Bonnin M, Bendriss-Vermare N, Rivoire M, Passot G, Lesurtel M, Mabrut JY, Ducerf C, Salvetti A, Protzer U, Zoulim F, Durantel D, Lucifora J (2018) Characterization of pattern recognition receptor expression and functionality in liver primary cells and derived cell lines. J Innate Immun 10(4):339–348
Bigger CB, Brasky KM, Lanford RE (2001) DNA microarray analysis of chimpanzee liver during acute resolving hepatitis C virus infection. J Virol 75(15):7059–7066
Marukian S, Andrus L, Sheahan TP, Jones CT, Charles ED, Ploss A, Rice CM, Dustin LB (2011) Hepatitis C virus induces interferon-lambda and interferon-stimulated genes in primary liver cultures. Hepatology 54(6):1913–1923
Mutz P, Metz P, Lempp FA, Bender S, Qu B, Schöneweis K, Seitz S, Tu T, Restuccia A, Frankish J, Dächert C, Schusser B, Koschny R, Polychronidis G, Schemmer P, Hoffmann K, Baumert TF, Binder M, Urban S, Bartenschlager R (2018) HBV bypasses the innate immune response and does not protect HCV from antiviral activity of interferon. Gastroenterology 154(6):1791–1804 e22
Wieland SF, Chisari FV (2005) Stealth and cunning: hepatitis B and hepatitis C viruses. J Virol 79(15):9369–9380
Heim MH, Thimme R (2014) Innate and adaptive immune responses in HCV infections. J Hepatol 61(1 Suppl):S14–S25
Pfeffer LM, Madey MA, Riely CA, Fleckenstein JF (2009) The induction of type I interferon production in hepatitis C-infected patients. J Interf Cytokine Res 29(5):299–306
Thomas E, Gonzalez VD, Li Q, Modi AA, Chen W, Noureddin M, Rotman Y, Liang TJ (2012) HCV infection induces a unique hepatic innate immune response associated with robust production of type III interferons. Gastroenterology 142(4):978–988
Kah J, Volz T, Lütgehetmann M, Groth A, Lohse AW, Tiegs G, Sass G, Dandri M (2017) Haem oxygenase-1 polymorphisms can affect HCV replication and treatment responses with different efficacy in humanized mice. Liver Int 37(8):1128–1137
Castet V et al (2002) Alpha interferon inhibits hepatitis C virus replication in primary human hepatocytes infected in vitro. J Virol 76(16):8189–8199
Zeuzem S, Feinman SV, Rasenack J, Heathcote EJ, Lai MY, Gane E, O'Grady J, Reichen J, Diago M, Lin A, Hoffman J, Brunda MJ (2000) Peginterferon alfa-2a in patients with chronic hepatitis C. N Engl J Med 343(23):1666–1672
Asselah T, Bieche I, Sabbagh A, Bedossa P, Moreau R, Valla D, Vidaud M, Marcellin P (2009) Gene expression and hepatitis C virus infection. Gut 58(6):846–858
Bartenschlager R, Schaller H (1992) Hepadnaviral assembly is initiated by polymerase binding to the encapsidation signal in the viral RNA genome. EMBO J 11(9):3413–3420
Sato S, Li K, Kameyama T, Hayashi T, Ishida Y, Murakami S, Watanabe T, Iijima S, Sakurai Y, Watashi K, Tsutsumi S, Sato Y, Akita H, Wakita T, Rice CM, Harashima H, Kohara M, Tanaka Y, Takaoka A (2015) The RNA sensor RIG-I dually functions as an innate sensor and direct antiviral factor for hepatitis B virus. Immunity 42(1):123–132
Wieland S, Thimme R, Purcell RH, Chisari FV (2004) Genomic analysis of the host response to hepatitis B virus infection. Proc Natl Acad Sci U S A 101(17):6669–6674
Dunn C, Peppa D, Khanna P, Nebbia G, Jones M, Brendish N, Lascar RM, Brown D, Gilson RJ, Tedder RJ, Dusheiko GM, Jacobs M, Klenerman P, Maini MK (2009) Temporal analysis of early immune responses in patients with acute hepatitis B virus infection. Gastroenterology 137(4):1289–1300
Suslov A, Boldanova T, Wang X, Wieland S, Heim MH (2018) Hepatitis B virus does not interfere with innate immune responses in the human liver. Gastroenterology 154(6):1778–1790
Giersch K, Allweiss L, Volz T, Helbig M, Bierwolf J, Lohse AW, Pollok JM, Petersen J, Dandri M, Lütgehetmann M (2015) Hepatitis delta co-infection in humanized mice leads to pronounced induction of innate immune responses in comparison to HBV mono-infection. J Hepatol 63:346–353
Allweiss L, Volz T, Lütgehetmann M, Giersch K, Bornscheuer T, Lohse AW, Petersen J, Ma H, Klumpp K, Fletcher SP, Dandri M (2014) Immune cell responses are not required to induce substantial hepatitis B virus antigen decline during pegylated interferon-alpha administration. J Hepatol 60(3):500–507
Belloni L, Allweiss L, Guerrieri F, Pediconi N, Volz T, Pollicino T, Petersen J, Raimondo G, Dandri M, Levrero M (2012) IFN-alpha inhibits HBV transcription and replication in cell culture and in humanized mice by targeting the epigenetic regulation of the nuclear cccDNA minichromosome. J Clin Invest 122(2):529–537
Kah J, Koh S, Volz T, Ceccarello E, Allweiss L, Lütgehetmann M, Bertoletti A, Dandri M (2017) Lymphocytes transiently expressing virus-specific T cell receptors reduce hepatitis B virus infection. J Clin Invest 127(8):3177–3188
Ko C, Chakraborty A, Chou WM, Hasreiter J, Wettengel JM, Stadler D, Bester R, Asen T, Zhang K, Wisskirchen K, McKeating JA, Ryu WS, Protzer U (2018) Hepatitis B virus genome recycling and de novo secondary infection events maintain stable cccDNA levels. J Hepatol 69(6):1231–1241
Koh S, Kah J, Tham CYL, Yang N, Ceccarello E, Chia A, Chen M, Khakpoor A, Pavesi A, Tan AT, Dandri M, Bertoletti A (2018) Nonlytic lymphocytes engineered to express virus-specific T-cell receptors limit HBV infection by activating APOBEC3. Gastroenterology 155(1):180–193 e6
Lutgehetmann M et al (2011) Hepatitis B virus limits response of human hepatocytes to interferon-alpha in chimeric mice. Gastroenterology 140(7):2074–2083 2083 e1-2
Usai C, Maestro S, Camps G, Olague C, Suárez-Amaran L, Vales A, Aragon T, Hommel M, Aldabe R, Gonzalez-Aseguinolaza G (2020) TNF-alpha inhibition ameliorates HDV-induced liver damage in a mouse model of acute severe infection. JHEP Rep 2(3):100098
Alfaiate D, Lucifora J, Abeywickrama-Samarakoon N, Michelet M, Testoni B, Cortay JC, Sureau C, Zoulim F, Dény P, Durantel D (2016) HDV RNA replication is associated with HBV repression and interferon-stimulated genes induction in super-infected hepatocytes. Antivir Res 136:19–31
He W, Ren B, Mao F, Jing Z, Li Y, Liu Y, Peng B, Yan H, Qi Y, Sun Y, Guo JT, Sui J, Wang F, Li W (2015) Hepatitis D virus infection of mice expressing human sodium taurocholate co-transporting polypeptide. PLoS Pathog 11(4):e1004840
Suarez-Amaran L et al (2017) A new HDV mouse model identifies mitochondrial antiviral signaling protein (MAVS) as a key player in IFN-beta induction. J Hepatol 67(4):669–679
Winer, B.Y., et al., (2018) Preclinical assessment of antiviral combination therapy in a genetically humanized mouse model for hepatitis delta virus infection. Sci Transl Med. 10(447).
Pugnale P, Pazienza V, Guilloux K, Negro F (2009) Hepatitis delta virus inhibits alpha interferon signaling. Hepatology 49(2):398–406
Zhang Z, Filzmayer C, Ni Y, Sültmann H, Mutz P, Hiet MS, Vondran FWR, Bartenschlager R, Urban S (2018) Hepatitis D virus replication is sensed by MDA5 and induces IFN-beta/lambda responses in hepatocytes. J Hepatol 69(1):25–35
Zhang Z, Urban S (2021) New insights into HDV persistence: the role of interferon response and implications for upcoming novel therapies. J Hepatol 74(3):686–699
Giersch K, Allweiss L, Volz T, Helbig M, Bierwolf J, Lohse AW, Pollok JM, Petersen J, Dandri M, Lütgehetmann M (2015) Hepatitis delta co-infection in humanized mice leads to pronounced induction of innate immune responses in comparison to HBV mono-infection. J Hepatol 63(2):346–353
Jung S, Jacobs KFK, Shein M, Schütz AK, Mohr F, Stadler H, Stadler D, Lucko AM, Altstetter SM, Wilsch F, Deng L, Protzer U (2020) Efficient and reproducible depletion of hepatitis B virus from plasma derived extracellular vesicles. J Extracell Vesicles 10(2):e12040
Wedemeyer H et al (2020) AS072 - 48 weeks of high dose (10 mg) bulevirtide as monotherapy or with peginterferon alfa-2a in patients with chronic HBV/HDV co-infection. J Hepatol 73:S52–S53
Lebosse F et al (2017) Intrahepatic innate immune response pathways are downregulated in untreated chronic hepatitis B. J Hepatol 66(5):897–909
Lang T, Lo C, Skinner N, Locarnini S, Visvanathan K, Mansell A (2011) The hepatitis B e antigen (HBeAg) targets and suppresses activation of the toll-like receptor signaling pathway. J Hepatol 55(4):762–769
Geng M et al (2015) Molecular mechanism of hepatitis B virus X protein function in hepatocarcinogenesis. World J Gastroenterol 21(38):10732–10738
Liu Y, Li J, Chen J, Li Y, Wang W, du X, Song W, Zhang W, Lin L, Yuan Z (2015) Hepatitis B virus polymerase disrupts K63-linked ubiquitination of STING to block innate cytosolic DNA-sensing pathways. J Virol 89(4):2287–2300
Li N, Li Q, Qian Z, Zhang Y, Chen M, Shi G (2009) Impaired TLR3/IFN-beta signaling in monocyte-derived dendritic cells from patients with acute-on-chronic hepatitis B liver failure: relevance to the severity of liver damage. Biochem Biophys Res Commun 390(3):630–635
Op den Brouw, M, L., et al., Hepatitis B virus surface antigen impairs myeloid dendritic cell function: a possible immune escape mechanism of hepatitis B virus. Immunology, 2009. 126(2): p. 280-289.
Wu J, Meng Z, Jiang M, Pei R, Trippler M, Broering R, Bucchi A, Sowa JP, Dittmer U, Yang D, Roggendorf M, Gerken G, Lu M, Schlaak JF (2009) Hepatitis B virus suppresses toll-like receptor-mediated innate immune responses in murine parenchymal and nonparenchymal liver cells. Hepatology 49(4):1132–1140
Boni C, Fisicaro P, Valdatta C, Amadei B, di Vincenzo P, Giuberti T, Laccabue D, Zerbini A, Cavalli A, Missale G, Bertoletti A, Ferrari C (2007) Characterization of hepatitis B virus (HBV)-specific T-cell dysfunction in chronic HBV infection. J Virol 81(8):4215–4225
Burton AR, Pallett LJ, McCoy LE, Suveizdyte K, Amin OE, Swadling L, Alberts E, Davidson BR, Kennedy PTF, Gill US, Mauri C, Blair PA, Pelletier N, Maini MK (2018) Circulating and intrahepatic antiviral B cells are defective in hepatitis B. J Clin Invest 128(10):4588–4603
Das A, Hoare M, Davies N, Lopes AR, Dunn C, Kennedy PTF, Alexander G, Finney H, Lawson A, Plunkett FJ, Bertoletti A, Akbar AN, Maini MK (2008) Functional skewing of the global CD8 T cell population in chronic hepatitis B virus infection. J Exp Med 205(9):2111–2124
Le Bert N et al (2020) Comparative characterization of B cells specific for HBV nucleocapsid and envelope proteins in patients with chronic hepatitis B. J Hepatol 72(1):34–44
Salimzadeh L, le Bert N, Dutertre CA, Gill US, Newell EW, Frey C, Hung M, Novikov N, Fletcher S, Kennedy PTF, Bertoletti A (2018) PD-1 blockade partially recovers dysfunctional virus-specific B cells in chronic hepatitis B infection. J Clin Invest 128(10):4573–4587
Wesselhoeft RA, Kowalski PS, Parker-Hale FC, Huang Y, Bisaria N, Anderson DG (2019) RNA circularization diminishes immunogenicity and can extend translation duration in vivo. Mol Cell 74(3):508–520 e4
Griffin BL, Chasovskikh S, Dritschilo A, Casey JL (2014) Hepatitis delta antigen requires a flexible quasi-double-stranded RNA structure to bind and condense hepatitis delta virus RNA in a ribonucleoprotein complex. J Virol 88(13):7402–7411
Casey JL (2012) Control of ADAR1 editing of hepatitis delta virus RNAs. Curr Top Microbiol Immunol 353:123–143
Girardi E, Pfeffer S, Baumert TF, Majzoub K (2021) Roadblocks and fast tracks: how RNA binding proteins affect the viral RNA journey in the cell. Semin Cell Dev Biol 111:86–100
Karimzadeh H, et al., (2018) Amino acid substitutions within HLA-B*27-restricted T cell epitopes prevent recognition by hepatitis delta virus-specific CD8(+) T cells. J Virol. 92(13).
Karimzadeh H, Kiraithe MM, Oberhardt V, Salimi Alizei E, Bockmann J, Schulze zur Wiesch J, Budeus B, Hoffmann D, Wedemeyer H, Cornberg M, Krawczyk A, Rashidi-Alavijeh J, Rodríguez-Frías F, Casillas R, Buti M, Smedile A, Alavian SM, Heinold A, Emmerich F, Panning M, Gostick E, Price DA, Timm J, Hofmann M, Raziorrouh B, Thimme R, Protzer U, Roggendorf M, Neumann-Haefelin C (2019) Mutations in hepatitis D virus allow it to escape detection by CD8(+) T cells and evolve at the population level. Gastroenterology 156(6):1820–1833
Guidotti LG, Inverso D, Sironi L, di Lucia P, Fioravanti J, Ganzer L, Fiocchi A, Vacca M, Aiolfi R, Sammicheli S, Mainetti M, Cataudella T, Raimondi A, Gonzalez-Aseguinolaza G, Protzer U, Ruggeri ZM, Chisari FV, Isogawa M, Sitia G, Iannacone M (2015) Immunosurveillance of the liver by intravascular effector CD8(+) T cells. Cell 161(3):486–500
Warren A, le Couteur DG, Fraser R, Bowen DG, McCaughan GW, Bertolino P (2006) T lymphocytes interact with hepatocytes through fenestrations in murine liver sinusoidal endothelial cells. Hepatology 44(5):1182–1190
Wong YC, Tay SS, McCaughan G, Bowen DG, Bertolino P (2015) Immune outcomes in the liver: is CD8 T cell fate determined by the environment? J Hepatol 63(4):1005–1014
Benechet AP et al (2019) Dynamics and genomic landscape of CD8(+) T cells undergoing hepatic priming. Nature 574(7777):200–205
Tham CYL, Kah J, Tan AT, Volz T, Chia A, Giersch K, Ladiges Y, Loglio A, Borghi M, Sureau C, Lampertico P, Lütgehetmann M, Dandri M, Bertoletti A (2020) Hepatitis delta virus acts as an immunogenic adjuvant in hepatitis B virus-infected hepatocytes. Cell Rep Med 1(4):100060
Kefalakes H, Koh C, Sidney J, Amanakis G, Sette A, Heller T, Rehermann B (2019) Hepatitis D virus-specific CD8(+) T cells have a memory-like phenotype associated with viral immune escape in patients with chronic hepatitis D virus infection. Gastroenterology 156(6):1805–1819 e9
Niro GA, Gravinese E, Martini E, Garrubba M, Facciorusso D, Conoscitore P, di Giorgio G, Rizzetto M, Andriulli A (2001) Clearance of hepatitis B surface antigen in chronic carriers of hepatitis delta antibodies. Liver 21(4):254–259
Micco L, Peppa D, Loggi E, Schurich A, Jefferson L, Cursaro C, Panno AM, Bernardi M, Brander C, Bihl F, Andreone P, Maini MK (2013) Differential boosting of innate and adaptive antiviral responses during pegylated-interferon-alpha therapy of chronic hepatitis B. J Hepatol 58(2):225–233
Penna A, Laccabue D, Libri I, Giuberti T, Schivazappa S, Alfieri A, Mori C, Canetti D, Lampertico P, Viganò M, Colombo M, Loggi E, Missale G, Ferrari C (2012) Peginterferon-alpha does not improve early peripheral blood HBV-specific T-cell responses in HBeAg-negative chronic hepatitis. J Hepatol 56(6):1239–1246
Thimme R, Dandri M (2013) Dissecting the divergent effects of interferon-alpha on immune cells: time to rethink combination therapy in chronic hepatitis B? J Hepatol 58(2):205–209
Lutgehetmann M, Volzt T, Quaas A, Zankel M, Fischer C, Dandri M, Petersen J (2008) Sequential combination therapy leads to biochemical and histological improvement despite low ongoing intrahepatic hepatitis B virus replication. Antivir Ther 13(1):57–66
Wursthorn K, Lutgehetmann M, Dandri M, Volz T, Buggisch P, Zollner B, Longerich T, Schirmacher P, Metzler F, Zankel M, Fischer C, Currie G, Brosgart C, Petersen J (2006) Peginterferon alpha-2b plus adefovir induce strong cccDNA decline and HBsAg reduction in patients with chronic hepatitis B. Hepatology 44(3):675–684
Kuo MY, Chao M, Taylor J (1989) Initiation of replication of the human hepatitis delta virus genome from cloned DNA: role of delta antigen. J Virol 63(5):1945–1950
Acknowledgements
We thank Katja Giersch for providing the immunofluorescence staining and for her critical reading of the manuscript and Annika Volmari and Tassilo Volz for the graphical design and editing support. I apologize to numerous investigators whose original works could not be cited due to space limitations.
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Open Access funding enabled and organized by Projekt DEAL. MD and ML received funding from the German Research Foundation (Collaborative Research Centre SFB-841: A5 & A8) and from the German Center for Infection Research (DZIF; TTU Hepatitis 05.816; 05.817; 05.820; 05.822).
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Dandri, M., Bertoletti, A. & Lütgehetmann, M. Innate immunity in hepatitis B and D virus infection: consequences for viral persistence, inflammation, and T cell recognition. Semin Immunopathol 43, 535–548 (2021). https://doi.org/10.1007/s00281-021-00864-x
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DOI: https://doi.org/10.1007/s00281-021-00864-x