Hepatitis B Virus (HBV)
The smallest DNA virus infecting humans: 3.2 kb partially double-stranded relaxed circular DNA (rcDNA) with four overlapping ORFs (S, C, P, X) encoding more protein per nucleotide than virtually any other known virus. Infection is exclusively hepatotropic — HBsAg preS1 binds heparan sulfate proteoglycans for initial attachment, then engages NTCP (sodium-taurocholate cotransporting polypeptide; SLC10A1), a bile acid transporter expressed almost exclusively on hepatocytes, for high-affinity entry. In the nucleus, the viral polymerase and host DNA repair enzymes convert rcDNA to covalently closed circular DNA (cccDNA) — a ~3.2 kb episomal minichromosome associated with histones. cccDNA drives transcription of all viral RNAs including the pregenomic RNA (pgRNA) template for reverse transcription. Current nucleos(t)ide analogues (tenofovir, entecavir) suppress HBV DNA replication but do not eliminate cccDNA — the fundamental barrier to functional cure. HBx protein is an essential transcriptional co-activator that also drives oncogenic signalling (Src/Ras/ERK, PI3K/Akt), p53 inhibition, and epigenetic reprogramming, making HBV the most common cause of hepatocellular carcinoma globally.
Classification & Structure
| Genome | 3.2 kb partially double-stranded relaxed circular DNA (rcDNA); complete (−) strand + incomplete (+) strand held in circular form by cohesive overlapping ends; 4 overlapping ORFs (S, C, P, X) — remarkable information density. Replicates through an RNA intermediate (pgRNA) via reverse transcriptase, uniquely among DNA viruses. |
| Family / Genus | Hepadnaviridae / Orthohepadnavirus. 10 genotypes (A–J) with geographic distributions: genotype C (East Asia) and F (Alaska/Latin America) associated with higher HCC risk. |
| Envelope | Yes — host-derived lipid bilayer + HBsAg (Surface antigen); 3 forms: Large HBsAg (preS1+preS2+S; NTCP-binding), Medium (preS2+S), Small (S only). HBsAg subviral particles circulate at 10,000× molar excess over complete virions. |
| Size / particles | Dane particle (complete virion): 42 nm. HBsAg spherical subviral particles: 22 nm (up to 10¹³ particles/mL — acts as enormous antibody decoy). HBsAg filamentous particles: 22 nm × variable length. |
| Genotypes | A–J (10 genotypes); clinical importance: genotype C/F → higher HCC risk; genotype A → best IFN response; genotype B → more HBeAg seroconversion in young adults |
| Key proteins | Large HBsAg (LHBsAg; preS1 domain; NTCP-binding; entry); HBcAg (capsid; major immune target); HBeAg (secreted tolerogen); HBV Polymerase (RT + RNase H + terminal protein; drug target); HBx (transcriptional transactivator; oncogenic) |
Infection Mechanism
1 · Attachment and NTCP receptor binding
HBV entry is a two-step process. Step 1 (low affinity): The LHBsAg preS1 domain binds heparan sulfate proteoglycans (HSPGs) on the hepatocyte surface, concentrating virions near the cell. Step 2 (high affinity): The preS1 domain (aa 2–48 of the myristoylated N-terminal peptide) binds NTCP (SLC10A1) — the basolateral sodium-dependent bile acid transporter expressed almost exclusively on hepatocytes. NTCP expression explains HBV’s strict hepatotropism. Bulevirtide (Hepcludex), a synthetic preS1 lipopeptide competitive inhibitor of NTCP, is the first approved HBV/HDV entry inhibitor (EMA-approved for HDV co-infection).
2 · Nuclear import and cccDNA formation — the persistence mechanism
After clathrin-mediated endocytosis and cytoplasmic uncoating, the partially double-stranded rcDNA is transported into the nucleus via importin-α/β. In the nucleus, the viral polymerase completes the (+) strand and cellular DNA repair enzymes (including PCNA, RFC, Lig1) convert rcDNA to covalently closed circular DNA (cccDNA) — an episomal ~3.2 kb minichromosome wrapped around histones H3/H4 and viral core protein. cccDNA is the persistent transcriptional template for all viral RNAs. Critically, cccDNA is not targeted by tenofovir or entecavir (which act downstream on reverse transcriptase). This is why treatment suppresses but cannot eliminate HBV: cccDNA persists in non-dividing hepatocytes for decades.
3 · Reverse transcription — RNA intermediate to rcDNA
cccDNA serves as template for the ~3.5 kb pregenomic RNA (pgRNA), which is packaged with HBV polymerase into new core particles. The polymerase then acts as a reverse transcriptase: it synthesises the (−) DNA strand using pgRNA as template (degrading the pgRNA via the RNase H domain), then partially synthesises the (+) strand from the (−) strand template — producing the characteristic rcDNA molecule. This RNA-intermediate strategy is the mechanistic basis for HBV susceptibility to NRTI drugs originally developed for HIV reverse transcriptase.
4 · Immunological tolerance vs. immune-mediated damage
HBV itself is minimally cytopathic — liver damage is almost entirely immune-mediated. In perinatal infection, maternal HBeAg crosses the placenta and tolerises the foetal/neonatal immune system to HBV antigens; combined with an immature adaptive immune system, this leads to 90% chronicity. In adult infection, a robust CD4+ Th1 and CD8+ CTL response drives vigorous hepatocyte killing — the molecular basis of acute hepatitis B — but resolves infection in >95% of adults. The characteristic ALT/AST elevation directly reflects the magnitude of immune-mediated hepatocyte destruction, not viral replication per se.
5 · HBx protein — cccDNA transcription activator and oncogene
The X protein (HBx; 154 aa; no known enzymatic activity) is essential for cccDNA transcription — it interacts with SMC complex/chromatin remodelling machinery at the cccDNA minichromosome. Beyond transcriptional function, HBx activates Src/Ras/ERK, PI3K/Akt/mTOR, and NF-&kB; signalling pathways; inhibits p53-mediated apoptosis; and drives epigenetic reprogramming of host hepatocytes through histone methyltransferase recruitment. HBsAg integrations into host chromosomes (a stochastic by-product of rcDNA recombination, accumulating over decades) can directly disrupt tumour suppressor genes and provide a cis-acting transcriptional activation of oncogenes — HBx drives HCC development even in patients with suppressed viral replication.
6 · HBsAg subviral particles — antibody decoy at extraordinary scale
During active replication, HBsAg spherical and filamentous subviral particles (22 nm; no nucleocapsid) are secreted at 10,000–100,000× molar excess over infectious Dane particles — reaching up to 10¹²–10¹³ particles/mL in serum. These particles, dominated by Small HBsAg (S), absorb anti-HBs antibodies and delay immune clearance. The sheer excess of non-infectious antigen overwhelms humoral responses, explaining why anti-HBs antibodies appear only after HBsAg clearance (anti-HBs and HBsAg do not coexist in serum in natural infection). This “antigen flood” is a major immune evasion strategy for persistence.
Host Immune Response
Disease Spectrum — Chronic Hepatitis B Phases
| Phase | HBeAg / HBV DNA | ALT / Histology | Clinical significance |
|---|---|---|---|
| Acute hepatitis B | HBsAg+, HBeAg+, HBV DNA high | ALT often >1,000 IU/L; lobular inflammation, spotty necrosis | >95% adults self-resolve; fulminant hepatic failure rare (<1%) |
| HBeAg+ immune tolerant | HBeAg+; DNA >10⁷ IU/mL | Normal ALT; minimal histology | Perinatal acquisition; low immediate fibrosis risk; treatment usually not indicated; long phase in Asians |
| HBeAg+ immune active | HBeAg+; DNA high | Elevated ALT; active hepatitis, fibrosis | Progressive fibrosis risk; antiviral therapy indicated |
| HBeAg− immune inactive (low replication) | HBeAg−; DNA <2,000 IU/mL | Normal ALT; minimal histology | Favourable; monitor every 6–12 months; low fibrosis progression |
| HBeAg− immune active (preCore/BCP mutant) | HBeAg−; DNA moderate–high (>2,000 IU/mL) | Elevated ALT; active hepatitis; fibrosis progression | HBeAg-negative variant — preCore A1896 and BCP mutations abolish HBeAg; most common chronic HBV globally; treat |
| Cirrhosis (Metavir F4) | Variable | Variable ALT; bridging fibrosis/cirrhosis | Portal hypertension; oesophageal varices; ascites; SBP; HRS; hepatic encephalopathy; 15–20% annual mortality (decompensated) |
| Hepatocellular carcinoma (HCC) | Any | Often elevated AFP + liver mass on US/CT/MRI | 20–30% of cirrhotics over lifetime; can arise in non-cirrhotics; 6-monthly US surveillance standard |
Treatment & Prevention
| Tenofovir (TDF / TAF) | Tenofovir disoproxil fumarate (TDF) and tenofovir alafenamide (TAF) are first-line NRTIs. TAF achieves equivalent HBV DNA suppression at 10× lower plasma tenofovir concentrations — less renal and bone toxicity. Both are potent HBV polymerase RT inhibitors; high barrier to resistance (no resistance mutations reported for TDF/TAF after 10+ years). Used indefinitely in most chronic HBV patients requiring treatment. |
| Entecavir (ETV) | Cyclopentyl guanosine analogue; potent HBV polymerase inhibitor; high barrier to resistance in treatment-naïve patients; first-line option alongside tenofovir. Reduced efficacy if lamivudine resistance present (YMDD mutations). Equivalent to TDF in long-term outcomes. |
| Pegylated interferon-α (Peg-IFN) | Immunomodulatory + antiviral; finite 48-week course; achieves HBsAg loss (“functional cure”) in ~3–7% of treated HBeAg+ patients (highest in genotype A/B); limited by significant side effects (flu-like syndrome, cytopenias, neuropsychiatric). Only option with defined treatment duration; off-treatment response possible. |
| Bulevirtide (Hepcludex) | EMA-approved (2020) for chronic HDV co-infection. Synthetic preS1 myristoylated lipopeptide that competitively inhibits LHBsAg binding to NTCP — first-in-class HBV/HDV entry inhibitor. Under investigation for HBV monoinfection in combination with other agents. |
| Functional cure strategies | HBsAg loss (± anti-HBs seroconversion) — the current practical treatment endpoint — achieved in ~1% of NA-treated patients/year. Novel pipeline: capsid assembly modulators (CAMs; block pgRNA encapsidation); siRNA/antisense oligonucleotides targeting HBsAg mRNA; cccDNA-targeting strategies (CRISPR, SMAD inhibitors); toll-like receptor agonists (TLR7/8 — innate immune reactivation); therapeutic vaccines. Multiple Phase II/III trials ongoing as of 2026. |
| HBV vaccine | 3-dose recombinant HBsAg series (0, 1, 6 months); anti-HBs ≥10 mIU/mL = protective. Neonates born to HBsAg+ mothers receive vaccine + HBIG within 12 hours of birth → reduces perinatal transmission by ~95%. Taiwan universal infant vaccination programme (from 1984) achieved 90% reduction in childhood HCC after two decades (Chang et al., NEJM 1997) — the first vaccine proven to prevent human cancer. Universal infant vaccination now WHO policy globally. |
Connections
References
- Blumberg BS, Alter HJ, Visnich S. A “new” antigen in leukemia sera. JAMA. 1965;191(7):541–6. PubMed 14325610
- Yan H, Zhong G, Xu G, et al. Sodium taurocholate cotransporting polypeptide is a functional receptor for human hepatitis B and D virus. eLife. 2012;1:e00049. doi:10.7554/eLife.00049 · PubMed 23150796
- Chang MH, Chen CJ, Lai MS, et al. Universal hepatitis B vaccination in Taiwan and the incidence of hepatocellular carcinoma in children. N Engl J Med. 1997;336(26):1855–9. PubMed 9187068
- Lok AS. Hepatitis B. N Engl J Med. 2002;346(22):1682–3. (See also: Lok AS, McMahon BJ. Chronic hepatitis B. Hepatology. 2007;45(2):507–39.) doi:10.1002/hep.21513 · PubMed 17256718
- Marcellin P, Gane E, Buti M, et al. Regression of cirrhosis during treatment with tenofovir disoproxil fumarate for chronic hepatitis B: a 5-year open-label follow-up study. Lancet. 2013;381(9865):468–75. doi:10.1016/S0140-6736(12)61425-1 · PubMed 23234725
- Terrault NA, Lok ASF, McMahon BJ, et al. Update on prevention, diagnosis, and treatment of chronic hepatitis B: AASLD 2018 hepatitis B guidance. Hepatology. 2018;67(4):1560–99. doi:10.1002/hep.29803 · PubMed 29405329
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This entry covers HBV biology, NTCP entry, cccDNA persistence, HBx oncogenesis, chronic hepatitis B phases, and approved/pipeline therapies. Planned expansions: detailed cccDNA chromatin structure, HDV co-infection biology, and HBV cure pipeline modelling. Every entry follows the same schema: structured frontmatter, peer-reviewed citations, and cross-atlas links.