Identification of antibodies cross-reactive with woodchuck immune cells and activation of virus-specific and global cytotoxic T cell responses by anti-PD-1 and anti-PD-L1 in experimental chronic hepatitis B and persistent occult hepadnaviral infection

Woodchuck (Marmota monax) infected with woodchuck hepatitis virus (WHV) is the most pathogenically compatible naturally occurring model of human hepatitis B virus (HBV) infection, chronic hepatitis B, and HBV-induced hepatocellular carcinoma. This system plays a crucial role in discovery and preclinical evaluation of anti-HBV therapies. Its utilization remains tempered by the relatively narrow range of validated immunologic and molecular tools. We evaluated commercial antibodies against immune cell phenotypic markers and T cell molecules for cross-reactivity with woodchuck antigenic equivalents. The confirmed antibodies against programed cell death protein-1 (PD-1) and its ligand (PD-L1) were examined for ex vivo ability to activate WHV-specific, global and bystander cytotoxic T cells (CTLs) in chronic hepatitis and asymptomatic infection persisting after self-resolved acute hepatitis. Examination of 65 antibodies led to identification or confirmation of 23 recognizing woodchuck T, regulatory T, B and natural killer cells, T cell-associated PD-1, PD-L1, CTLA-4 and TIM-3 molecules, CD25 and CD69 markers of T cell activation, and interferon gamma (IFNγ). Antibodies against woodchuck PD-1 and PD-L1 triggered in vitro highly individualized WHV-specific and global activation of CTLs in both chronic hepatitis and persistent occult infection. WHV-specific CTLs were more robustly augmented by anti-PD-1 than by anti-PD-L1 in chronic hepatitis, while global IFNγ-positive CTL response was significantly suppressed in chronic hepatitis compared to persistent occult infection. Anti-PD-1 and anti-PD-L1 also occasionally activated CTLs to specificities other than those tested suggesting their potency to trigger side effects. This was particularly apparent when T cells from chronic hepatitis were treated with anti-PD-L1. The current findings indicate that inhibition of the PD-1/PD-L1 pathway could reactivate virus-specific and global T cell responses in both chronic hepatitis and asymptomatic persistent infection. They suggest a mechanism of potential reactivation of clinically silent infection during anti-PD-1/PD-L1 treatment and indicate that this therapy may also subdue occult HBV infection.

Hepadnaviral Lymphotropism and Its Relevance to HBV Persistence and Pathogenesis

Since the discovery of hepatitis B virus (HBV) over five decades ago, there have been many independent studies showing presence of HBV genomes in cells of the immune system. However, the nature of HBV lymphotropism and its significance with respect to HBV biology, persistence and the pathogenesis of liver and extrahepatic disorders remains underappreciated. This is in contrast to studies of other viral pathogens in which the capability to infect immune cells is an area of active investigation. Indeed, in some viral infections, lymphotropism may be essential, and even a primary mechanism of viral persistence, and a major contributor to disease pathogenesis. Nevertheless, there are advances in understanding of HBV lymphotropism in recent years due to cumulative evidence showing that: (i) lymphoid cells are a reservoir of replicating HBV, (ii) are a site of HBV-host DNA integration and (iii) virus genomic diversification leading to pathogenic variants, and (iv) they play a role in HBV resistance to antiviral therapy and (v) likely contribute to reactivation of hepatitis B. Further support for HBV lymphotropic nature is provided by studies in a model infection with the closely related woodchuck hepatitis virus (WHV) naturally infecting susceptible marmots. This animal model faithfully reproduces many aspects of HBV biology, including its replication scheme, tissue tropism, and induction of both symptomatic and silent infections, immunological processes accompanying infection, and progressing liver disease culminating in hepatocellular carcinoma. The most robust evidence came from the ability of WHV to establish persistent infection of the immune system that may not engage the liver when small quantities of virus are experimentally administered or naturally transmitted into virus-naïve animals. Although the concept of HBV lymphotropism is not new, it remains controversial and not accepted by conventional HBV researchers. This review summarizes research advances on HBV and hepadnaviral lymphotropism including the role of immune cells infection in viral persistence and the pathogenesis of HBV-induced liver and extrahepatic diseases. Finally, we discuss the role of immune cells in HBV diagnosis and assessment of antiviral therapy efficacy.

Differential Expression of Woodchuck Toll-Like Receptors 1-10 in Distinct Forms of Infection and Stages of Hepatitis in Experimental Hepatitis B Virus Infection

Woodchucks infected with woodchuck hepatitis virus (WHV) represent a highly valuable model of human hepatitis B virus (HBV) infection, chronic hepatitis (CH), and virus induced-primary liver cancer. Toll-like receptors (TLRs) are important mediators of immune responses playing pivotal roles in the pathogenesis of viral diseases; however, their expression profiles in different forms of infection and stages of hepatitis, and in healthy animals remain essentially unknown. In this study, woodchuck TLRs 1–10 exon fragments were identified and TLR genes transcription quantified in livers, primary hepatocytes, peripheral blood mononuclear cells (PBMC), and in selected organs during experimental WHV infection. Among others, liver biopsies from acute hepatitis (AH) and CH showed significantly augmented expression of the majority of TLRs when compared to healthy and woodchucks prior to AH, with resolved AH or primary occult infection. In contrast to the liver tissue, significant upregulation of TLR3, TLR4, and TLR10, but downregulation of TLR7, characterized hepatocytes derived from livers of animals with resolved AH accompanied by secondary occult infection. Hepatocytes from CH showed significantly lower expression or a trend toward suppression of several TLRs when compared to hepatocytes from healthy animals and woodchucks with other forms of infection or hepatitis, suggesting that hepatocyte innate immune response is downregulated during CH. Contrastingly, upregulated transcription of some TLRs characterized PBMC throughout CH. Our study uncovered that TLR expression significantly varies between different forms of hepadnaviral infection and whether infection is accompanied or not by hepatitis. The results showed that the profiles of TLRs’ expression in circulating lymphomononuclear cells do not mirror accurately those of livers and hepatocytes from infected animals. These findings are of importance to the understanding of immune process operating at different sites targeted by virus in the course of hepadnaviral infection and evaluation of future therapies modifying antiviral innate responses in the woodchuck model.

Asymptomatic Hepadnaviral Persistence and Its Consequences in the Woodchuck Model of Occult Hepatitis B Virus Infection

Woodchuck hepatitis virus (WHV) is molecularly and pathogenically closely related to hepatitis B virus (HBV). Both viruses display tropism towards hepatocytes and cells of the immune system and cause similar liver pathology, where acute hepatitis can progress to chronic hepatitis and to hepatocellular carcinoma (HCC). Two forms of occult hepadnaviral persistence were identified in the woodchuck-WHV model: secondary occult infection (SOI) and primary occult infection (POI). SOI occurs after resolution of a serologically apparent infection with hepatitis or after subclinical serologically evident virus exposure. POI is caused by small amounts of virus and progresses without serological infection markers, but the virus genome and its replication are detectable in the immune system and with time in the liver. SOI can be accompanied by minimal hepatitis, while the hallmark of POI is normal liver morphology. Nonetheless, HCC develops in about 20% of animals with SOI or POI within 3 to 5 years. The virus persists throughout the lifespan in both SOI and POI at serum levels rarely greater than 100 copies/mL, causes hepatitis and HCC when concentrated and administered to virus-naïve woodchucks. SOI is accompanied by virus-specific T and B cell immune responses, while only virus-specific T cells are detected in POI. SOI coincides with protection against reinfection, while POI does not and hepatitis develops after challenge with liver pathogenic doses >1000 virions. Both SOI and POI are associated with virus DNA integration into the liver and the immune system genomes. Overall, SOI and POI are two distinct forms of silent hepadnaviral persistence that share common characteristics. Here, we review findings from the woodchuck model and discuss the relevant observations made in human occult HBV infection (OBI).

Primary seronegative but molecularly evident hepadnaviral infection engages liver and induces hepatocarcinoma in the woodchuck model of hepatitis B

Hepadnavirus at very low doses establishes in woodchucks asymptomatic, serologically undetectable but molecularly evident persistent infection. This primary occult infection (POI) preferentially engages the immune system and initiates virus-specific T cell response in the absence of antiviral antibody induction. The current study aimed to determine whether POI with time may culminate in serologically identifiable infection and hepatitis, and what are, if any, its pathological consequences. Juvenile woodchucks were intravenously injected with inocula containing 10 or 100 virions of woodchuck hepatitis virus (WHV) to induce POI and followed for life or up to 5.5 years thereafter. All 10 animals established molecularly detectable infection with virus DNA in serum (<100–200 copies/mL) and in circulating lymphoid cells, but serum WHV surface antigen and antibodies to WHV core antigen remained undetectable for life. By approximately 2.5–3.5 years post-infection, circulating virus transiently increased to 10³ copies/mL and virus replication became detectable in the livers, but serological markers of infection and biochemical or histological evidence of hepatitis remained undetectable. Nonetheless, typical hepatocellular carcinoma (HCC) developed in 2/10 animals. WHV DNA integration into hepatic and lymphatic system genomes was identified in 9/10 animals. Virus recovered from the liver virus-negative or virus-positive phases of POI displayed the wild-type sequence and transmitted infection to healthy woodchucks causing hepatitis and HCC. In summary, for the first time, our data demonstrate that an asymptomatic hepadnaviral persistence initiated by very small amounts of otherwise pathogenic virus, advancing in the absence of traditional serological markers of infection and hepatitis, coincides with virus DNA integration into the host's hepatic and immune system genomes, retains liver pro-oncogenic potency and is capable of transmitting liver pathogenic infection. This emphasizes the role for primary occult hepatitis B virus infection in the development of seemingly cyptogenic HCC in seronegative but virus DNA reactive patients.

Introduction of highly sensitive molecular assays for detection of hepatitis B virus (HBV) identified the existence of persistent occult HBV infection years after recovery from an episode of hepatitis B and in individuals exposed to HBV but without symptoms and classical markers of infection. Because HBV integrates into human DNA and is a potent human carcinogen, it is postulated that occult HBV infection can be a cause of hepatic cancer in many individuals in which the tumor origin remains currently unknown. A causative relation between occult HBV infection and hepatocarcinoma is highly challenging to investigate in humans since occult HBV persistence is rarely diagnosed with current clinical assays and cancer development takes 15–30 years. However, we have established excellent models of occult HBV infection in the eastern North American woodchucks which are naturally susceptible to a virus closely related to HBV and in which chronic infection advances to liver cancer. In the current study, exploring experimental primary occult infection in woodchucks, we proved that the silently progressing infection, which is not detectable by serological markers, can culminate in hepatocellular carcinoma and that the persisting virus remains infectious, and causes hepatitis and liver cancer when transmitted to virus-naïve hosts.

Virological characteristics of occult hepatitis B virus in a North American cohort of human immunodeficiency virus type 1-positive patients on dual active anti-HBV/HIV therapy

BACKGROUND

Occult hepatitis B virus infection (OBI) is defined as low-level HBV DNA presence in serum, liver and/or peripheral blood mononuclear cells (PBMC) in individuals that lack serum hepatitis B virus surface antigen (HBsAg). HIV+ patients with OBI may be at risk for HBV reactivation, and often receive dual active anti-HBV/HIV therapy, such as lamivudine (LMV).

OBJECTIVES

To determine the presence of OBI in a North American cohort of HIV-1-positive patients.

STUDY DESIGN/METHODS

45 HIV-1-positive, serum HBsAg-negative patients, reactive for antibodies to HBV core antigen (anti-HBc), were tested for HBV DNA in plasma and for HBV DNA and covalently closed circular DNA (cccDNA) in PBMC. Ten patients were re-tested after ∼5-10 years, including genotyping and clonal sequence analysis of the HBV polymerase (P) gene and overlapping HBV surface (S) gene from 8 PBMC samples.

RESULTS

Overall, 42% (19/45) tested HBV DNA positive, especially in PBMC (18/45), including 3/18 that were reactive for HBV cccDNA, compared to 17% (8/45) that were HBV DNA reactive in plasma. In 8 patients on LMV, sequence analysis in PBMC showed that all were HBV genotype C or D. Several carried HBV P region variants at residues associated with anti-HBV drug resistance and overlapping S gene region within the major HBsAg "a determinant".

CONCLUSION

OBI is common in HIV-positive, anti-HBc reactive patients on anti-HBV/HIV therapy, particularly in PBMC. HBV sequence analysis revealed that all had HBV genotype C or D and often had P/overlapping S gene variants possibly associated with dual-active anti-HIV/HBV therapy.

Hepatitis B virus quasispecies in hepatic and extrahepatic viral reservoirs in liver transplant recipients on prophylactic therapy

The characterization of hepatitis B virus (HBV) quasispecies in different compartments in liver transplant (LT) recipients may be helpful in optimizing prophylaxis regimens. The aims of this study were to evaluate liver, peripheral blood mononuclear cells (PBMC), and plasma samples for HBV and to compare the quasispecies in hepatic and extrahepatic sites in LT recipients on long-term prophylaxis. For 12 patients followed for up to 15 years post-LT, liver, plasma, and PBMC samples [all HBV DNA-negative according to conventional polymerase chain reaction (PCR) assays] were evaluated for HBV DNA by a sensitive nested PCR method [covalently closed circular DNA (cccDNA) for liver and PBMC samples] and by the sequencing and phylogenetic analysis of polymerase quasispecies. For the 10 patients on prophylaxis with no clinical recurrence (median time post-LT = 15.5 months, range = 12-96 months), liver samples were HBV DNA-reactive in 9 of 10 cases, plasma samples were HBV DNA-reactive in 3 of 10 cases, and PBMC samples were HBV DNA-reactive in 2 of 7 cases (including 1 case with HBV cccDNA in PBMCs). The sequence analysis showed that all HBV clones had a wild-type (WT) sequence in the liver and PBMCs. In 2 patients with early HBV recurrence post-LT who were treated with nucleosides only, HBV DNA was detected in serum, PBMC, and liver samples, and HBV cccDNA was found in liver samples. An HBV lamivudine-resistant variant with an M204I mutation was identified in liver (70% and 18% of the clones) and plasma samples (100% of the clones), but a WT sequence was found in 70% and 100% of the PBMC clones. In conclusion, despite prophylaxis and the absence of HBV DNA in serum according to conventional assays, HBV is detectable in the serum, liver, and PBMCs of almost all patients, and this supports the use of continued anti-HBV therapy in this group. Antiviral drug-resistant variants are more frequent in the liver versus PBMCs, but both compartments are potential sources of reinfection.

Intrahepatic expression of genes affiliated with innate and adaptive immune responses immediately after invasion and during acute infection with woodchuck hepadnavirus

The importance of effective immune responses in recovery from acute hepadnaviral hepatitis has been demonstrated. However, there is no conclusive delineation of virological and immunological events occurring in the liver immediately after hepadnavirus invasion and during the preacute phase of infection. These very early events might be of primary importance in determining the recovery or progression to chronic hepatitis and the intrinsic hepadnaviral propensity to persist. In this study, applying the woodchuck model of acute hepatitis B, the hepatic kinetics of hepadnavirus replication and activation of genes encoding cytokines, cytotoxicity effectors, and immune cell markers were quantified in sequential liver biopsies collected from 1 h postinoculation onward by sensitive real-time cDNA amplification assays. The results revealed that hepadnavirus replication is established in the liver as early as 1 hour after infection. In 3 to 6 h, significantly augmented intrahepatic transcription of gamma interferon and interleukin-12 were evident, suggesting activation of antigen-presenting cells. In 48 to 72 h, NK and NKT cells were activated and virus replication was transiently but significantly reduced, implying that this early innate response is at least partially successful in limiting virus propagation. Nonetheless, T cells were activated 4 to 5 weeks later when hepatitis became histologically evident. Collectively, our data demonstrate that virus replication is initiated and the innate response activated in the liver soon after exposure to a liver-pathogenic dose of hepadnavirus. Nevertheless, this response is unable to prompt a timely adaptive T-cell response, in contrast to infections caused by other viral pathogens.

Repeated passage of wild-type woodchuck hepatitis virus in lymphoid cells does not generate cell type-specific variants or alter virus infectivity

Woodchuck hepatitis virus (WHV), which is closely related to human hepatitis B virus, infects the liver but also invariably establishes persistent infection in the lymphatic system. Although the dose of invading virus appears to be the main factor in determining whether WHV infection is restricted to the lymphatic system or also engages the liver, the nature of WHV lymphotropism remains unclear and a role for a specific lymphotropic variant was not excluded. The availability of woodchuck lymphocyte and hepatocyte cultures susceptible to WHV infection allows investigation of this issue in vitro. We hypothesized that repeated passage of wild-type WHV in lymphoid cells should lead to enrichment of a lymphotropic virus variant, if in fact such a variant exists. For this purpose, wild-type WHV with a homogeneous sequence was used as the inoculum, while lymphoid cells from a single healthy woodchuck donor and a normal woodchuck WCM-260 hepatocyte line served as infection targets. The serial passage of the wild-type virus repeated up to 13 times for both cell types did not lead to the emergence of cell type-specific WHV variants, as revealed by sequence analysis of the virus envelope and the core and X gene sequences. Moreover, the virus passaged in both cell types remained infectious for naive woodchucks, produced infection profiles that depended upon virus dose but not on virus cellular origin, and retained its initial DNA sequence. These results imply that WHV lymphotropism is a natural propensity of the wild-type virus and is not a consequence of infection with a viral variant.

Persistent occult hepatitis B virus infection: Experimental findings and clinical implications

Hepatitis B virus (HBV) is a highly pathogenic virus that causes chronic liver diseases in millions of people globally. In addition to a symptomatic, serologically evident infection, occult persistent HBV carriage has been identified since nucleic acid amplification assays of enhanced sensitivity became introduced for detection of hepadnaviral genomes and their replicative intermediates. Current evidence indicates that occult HBV infection is a common and long-term consequence of resolution of acute hepatitis B. This form of residual infection is termed as secondary occult infection (SOI). The data from the woodchuck model of HBV infection indicate that exposure to small amounts of hepadnavirus can also cause primary occult infection (POI) where virus genome, but no serological makers of exposure to virus, are detectable, and the liver may not be involved. However, virus replicates at low levels in the lymphatic system in both these forms. We briefly summarize the current understanding of the nature and characteristics of occult hepadnaviral persistence as well as of its documented and expected pathological consequences.

Antagonistic expression of hepatitis C virus and alpha interferon in lymphoid cells during persistent occult infection

Detection of residual HCV in individuals with SVR after treatment of CHC can be significantly heightened by analyzing ex vivo mitogen-activated T and B lymphocytes and applying sensitive nucleic acid amplification assays. However, it remained unknown if synergistic activation of lymphocytes and monocytes would further augment HCV detection, if viral replication becomes universally upregulated in treated cells, and if examining sequential sera and lymphoid cells would improve detection of occult infection. Using paired sera and lymphoid cells collected 1 year apart from 17 individuals with normal liver enzymes for up to 72 months after SVR, it was found that simultaneous activation of lymphocytes and monocytes enhanced identification of silent HCV infection and revealed that in some cases monocytes were the principal immune cell type where HCV persisted. Testing of serial samples further increased detection of occult infection. Ultimately, by combining the above two approaches, all individuals with SVR were found to be silent carriers of HCV. Clonal sequencing revealed HCV variations in sera and lymphoid cells and evolution of viral genomes confirming ongoing virus replication. Surprisingly, similar to those with CHC, naive lymphoid cells from some individuals carried approximately 10(3) HCV copies/microg total RNA. HCV loads in naive lymphoid cells predetermined the outcome of ex vivo stimulation with respect to upregulation or inhibition of HCV replication. HCV RNA levels in occult infection were inversely proportional to the expression of IFNalpha and IFN-inducible MxA, but not to IFNgamma or tumour necrosis factor alpha in naive and mitogen-treated lymphoid cells.

Profound suppression of chronic hepatitis C following superinfection with hepatitis B virus

BACKGROUND

Hepatitis B virus (HBV) and hepatitis C virus (HCV) share similar transmission routes; thus, coinfection is frequent. The consequences of acute HBV infection in patients with chronic hepatitis C are unknown.

METHODS

We describe a 47-year-old male with chronic hepatitis C who acquired HBV and then spontaneously and apparently completely cleared HCV but developed chronic hepatitis B. Five serum samples collected over 14 months and lymphoid cells obtained after acquiring HBV were tested for HCV and HBV by both standard assays and ultra-sensitive polymerase chain reaction/nucleic acid hybridization (PCR/NAH) (sensitivity approximately 2 IU/ml).

RESULTS

After superinfection with HBV, HBV surface antigen-positive chronic hepatitis developed with readily detectable HBV DNA. All sera collected after acquisition of HBV, which tested negative for HCV RNA by standard laboratory assay, were positive for HCV genomes when analysed by PCR/NAH. Peripheral lymphoid cells carried HBV DNA and covalently closed circular DNA, but were negative for HCV.

CONCLUSIONS

This is the first reported case of profound suppression of chronic hepatitis C after superinfection with HBV and establishment of chronic hepatitis B. It is hypothesized that HBV infection precipitated generalized and/or virus-specific cellular immune responses that profoundly suppressed HCV replication and yet failed to inhibit progression to chronic hepatitis B.