Overview
This lecture covers the five known hepatitis viruses (A-E), their virology, transmission routes, the pathogenesis of acute versus persistent infection, incubation periods and clinical presentation, laboratory diagnosis, disease burden, host genetic susceptibility, and prevention/treatment options including host-directed and virus-directed antivirals for HBV and HCV.
What is viral hepatitis
Hepatitis is inflammation of the liver. It can be caused by hepatitis viruses, by infection from bacteria and other parasites, by damage from alcohol or poison, or by immune cells attacking the liver (autoimmune hepatitis). At least five hepatitis viruses are known: A, B, C, D, and E. They belong to different families and are biologically unrelated, but all target hepatocytes (liver cells). Hepatitis B and Hepatitis C viruses are the most concerning because they can become chronic.
The five hepatitis viruses
- Hepatitis A virus (HAV): member of Picornaviridae, linear RNA genome, non-enveloped virion, but released from cells covered in an “envelope”; infects mostly higher primates.
- Hepatitis B virus (HBV): member of Hepadnaviridae, enveloped virion, circular DNA genome that is partially double-stranded (one strand, -, is nicked; the other, +, is incomplete); the genome is repaired to a complete circle (cccDNA, covalently closed circular DNA) in the host cell nucleus, and copies are made through reverse transcription. Multiple particle forms occur in infected cells: large spherical virions (Dane particles, containing HBV Pol, core protein HBc, and rcDNA/relaxed-circular DNA, surrounded by surface proteins S-HBs, M-HBs, L-HBs), small spherical sub-viral particles (sSVP), and filamentous sub-viral particles (fSVP). HBV infects mostly higher primates and enables hepatitis D virus dissemination.
- Hepatitis D virus (HDV): a satellite virus that needs a helper virus (HBV) to complete its life cycle. HDV virions contain just a circular 1.7 kb RNA genome with no nucleocapsid, exist only in HBV-infected cells, and acquire the HBV envelope for transmission.
- Hepatitis C virus (HCV): member of Flaviviridae, enveloped virion, linear RNA genome; infects mostly higher primates. The virion has envelope glycoproteins E2 and E1 in the outer layer, a core protein, and single-stranded RNA genome inside a hexagonal capsid.
- Hepatitis E virus (HEV): member of Hepeviridae, linear RNA genome, normally a non-enveloped (“naked”) virion, but a ‘quasi-enveloped’ virion also exists (same capsid core wrapped in an outer lipid envelope studded with HEV-ORF3 protein). Infects humans and other mammals.
Transmission
- HAV and HEV: faecal-oral route, via contaminated food and water.
- HBV, HCV, and HDV: direct contact with bodily fluids (semen, blood), blood transfusion, sexual intercourse, sharing needles/razors, and mother-to-child (perinatal) transmission.
Outcomes of infection
Viral hepatitis is usually an acute infection, but HBV and HCV infection may become chronic.
- Acute infection: the virus is cleared from the host; short incubation period (hours to weeks); a large number of progeny virus is produced; the host cell dies; clinical symptoms are present.
- Persistent infection: the virus is not cleared from the host; long incubation period (days to months); infection can persist for life and the host cell “survives”. Two sub-types:
- Latent: no progeny is produced, only the genome and a few proteins are present, no clinical symptoms.
- Chronic: progeny is produced, can be passed on to host offspring, symptoms present.
- Transformative infection: the infected cell is transformed into a cancerous cell. Some viruses encode viral oncogenes that promote uncontrolled cell division, leading to cancer; this can occur via activation of a host proto-oncogene (in humans) or insertion of an oncogene (in other animals).
Incubation periods and clinical symptoms
- HAV/HEV: average 28 days (range 15-50 days).
- HBV: average 90 days (range 60-150 days).
- HCV: average 60 days (range 15-180 days).
Clinical symptoms across all hepatitis viruses are nearly identical: nausea, vomiting, fever, abdominal pain, dark urine, and jaundice, with almost identical histopathologic liver lesions. Because clinical distinction between the viruses is difficult, laboratory diagnosis is required.
Chronic infection and liver cancer
Hepatitis B, B/D (co-infection), and C infections may progress to liver cancer (hepatocellular carcinoma, HCC), following a progression from healthy liver to chronic hepatitis to cirrhosis to HCC.
Time course of HBV & HCV infection: of those with HBV/HCV infection, 75-85% progress to chronic hepatitis; of those with chronic hepatitis, over 20-25 years, 20-30% progress to cirrhosis; of those with cirrhosis, over a further 25-30 years, 2-7% progress to HCC/end-stage liver disease (ESLD)/death.
Laboratory diagnosis
- Hepatitis viruses can be detected in liver, stool, bile, and blood, using electron microscopy, immunoassays, PCR, or RT-PCR.
- Abnormal liver function tests: alanine aminotransferase (ALT), bilirubin.
- IgM ELISA is a good indicator of acute infection.
- Liver biopsy assesses tissue damage.
Burden and host genetics
Hepatitis viruses are prevalent worldwide, including in Aotearoa New Zealand. An estimated 354 million people live with chronic HBV/HCV infection. The WHO goal is a 90% reduction in new chronic infections and a 65% reduction in deaths by 2030. Between April and June 2022, cases of acute hepatitis not caused by hepatitis viruses A-E were discovered in Europe and other countries.
Genetic polymorphism (single-nucleotide polymorphisms, SNPs) in host innate immune response genes, such as human leukocyte antigens (HLAs) and interleukins (e.g. interferon-lambda), means some ethnic groups are more susceptible to HBV/HCV chronic infection due to genetic diversity in these genes.
Prevention and treatment
- HAV: vaccine available, an inactivated whole virus (inactivated using 60°C heat and formalin).
- HBV/HDV: vaccine available (virus-like particles, VLPs); treatment with interferons and one host-directed antiviral drug.
- HCV: no vaccine available; treatment with interferons and multiple virus-directed antiviral drugs.
- HEV: no vaccine available.
Virus-like particles (VLPs) are non-replicating viral structures without a genome, formed by self-assembly of viral proteins when expressed alone; they are recognised by the immune system as virus. HBV surface proteins can self-assemble into VLPs, which is the basis of the HBV vaccine.
HBV antivirals (host-directed)
- Interferons are the first innate immune responders to infection; they strengthen the host immune system to help clear the virus.
- Myrcludex B (bulevirtide, brand name Hepcludex) is a synthetic peptide that blocks virus entry into host cells; it is also effective against HDV.
HCV antivirals (virus-directed)
The HCV RNA genome is positive-sense, single-stranded (+ssRNA), meaning it functions directly as mRNA and encodes one giant protein called the polyprotein. Virus-directed antivirals target cleavage of this polyprotein, which occurs at multiple sites by host and viral proteases.
The HCV polyprotein, from N-terminus to C-terminus: C (capsid protein) - E1 - E2 (envelope glycoproteins) - p7 (viroporin; alters pH of the secretory pathway; involved in encapsidation/envelopment) - NS2 (protease; scaffold for multiple interactions) - NS3 (protease, helicase/NTPase; NS2 protease cofactor) - NS4A (NS3 protease cofactor) - NS4B - NS5A (RNA binding; switches between replication and assembly) - NS5B (RNA-dependent RNA polymerase). Cleavage sites are cut by host signal peptide peptidase (SPP), host signal peptidase (SP), the NS2/3 protease, and the NS3/4A protease.
Glecaprevir (targets NS3/4A) combined with pibrentasvir (targets NS5A) forms Maviret (glecaprevir/pibrentasvir), which blocks the NS3/4A cleavage sites and NS5A function.
Self-test
- List the five hepatitis viruses and, for each, state its viral family, genome type, and whether the virion is enveloped or non-enveloped.
- Describe the structure and replication feature that distinguishes the HBV genome from the genomes of the other hepatitis viruses.
- Explain why hepatitis D virus is described as a satellite virus, and describe how it acquires its envelope.
- Distinguish the modes of transmission of HAV/HEV from those of HBV/HCV/HDV.
- Distinguish acute, latent, and chronic persistent infection in terms of progeny production, host cell fate, and presence of clinical symptoms.
- A patient presents with nausea, vomiting, fever, abdominal pain, dark urine, and jaundice. Why can these symptoms alone not identify which hepatitis virus is responsible, and what should be done to confirm the diagnosis?
- What is the average incubation period and range for HAV/HEV, HBV, and HCV?
- Describe the stages and approximate proportions/timescales in the progression from HBV/HCV infection to hepatocellular carcinoma.
- What is IgM ELISA used to indicate in the laboratory diagnosis of viral hepatitis?
- Explain, in terms of host genetics, why some ethnic groups are more susceptible to chronic HBV/HCV infection, giving two examples of the genes involved.
- What is a virus-like particle (VLP), and how is this concept used in the HBV vaccine?
- Describe the mechanisms of action of the two HBV host-directed antivirals mentioned in the lecture.
- Explain why the HCV genome can function directly as mRNA, and describe how this relates to the polyprotein and the strategy behind HCV virus-directed antivirals.
- Describe how glecaprevir and pibrentasvir act together to block the HCV polyprotein.
Answers
Reveal answers
- HAV: Picornaviridae, linear RNA genome, non-enveloped. HBV: Hepadnaviridae, circular DNA genome, enveloped. HCV: Flaviviridae, linear RNA genome, enveloped. HDV: unassigned family (satellite virus), circular 1.7 kb RNA genome, acquires HBV envelope. HEV: Hepeviridae, linear RNA genome, non-enveloped (a quasi-enveloped form also exists).
- HBV has a circular DNA genome that is partially double-stranded: one strand (-) is nicked and the other (+) is incomplete. This is repaired to a complete circle (cccDNA) in the host cell nucleus, and copies are then made through reverse transcription, unlike the other hepatitis viruses’ RNA genomes.
- HDV is a satellite virus because it needs a helper virus (HBV) to complete its life cycle: its virions contain just a circular RNA genome with no nucleocapsid and exist only in HBV-infected cells. It acquires the HBV envelope (surface proteins) to become a transmissible, enveloped particle.
- HAV and HEV spread by the faecal-oral route via contaminated food and water. HBV, HCV, and HDV spread via direct contact with bodily fluids (semen, blood), blood transfusion, sexual intercourse, sharing needles/razors, and mother-to-child (perinatal) transmission.
- Acute infection: virus is cleared, large numbers of progeny are produced, the host cell dies, and clinical symptoms are present. Latent (persistent) infection: virus is not cleared, no progeny is produced (only genome and a few proteins are present), host cell survives, no clinical symptoms. Chronic (persistent) infection: virus is not cleared, progeny is produced and can be passed to host offspring, host cell survives, and symptoms are present.
- All hepatitis viruses produce almost identical clinical symptoms and almost identical histopathologic liver lesions, so clinical presentation alone cannot distinguish them; laboratory diagnosis (e.g. detection in liver/stool/bile/blood by electron microscopy, immunoassay, PCR/RT-PCR, liver function tests, IgM ELISA, or liver biopsy) is required to confirm the cause.
- HAV/HEV: average 28 days (range 15-50 days). HBV: average 90 days (range 60-150 days). HCV: average 60 days (range 15-180 days).
- Of those infected with HBV/HCV, 75-85% progress to chronic hepatitis; of those, over 20-25 years 20-30% progress to cirrhosis; of those, over a further 25-30 years, 2-7% progress to hepatocellular carcinoma/end-stage liver disease/death.
- IgM ELISA is a good indicator of an acute infection.
- Genetic polymorphism (SNPs) in host innate immune response genes causes some ethnic groups to be more or less susceptible to developing chronic HBV/HCV infection depending on their genetic diversity in these genes; examples given are human leukocyte antigens (HLAs) and interleukins such as interferon-lambda.
- A VLP is a non-replicating viral structure without a genome, formed by self-assembly of viral proteins when expressed alone, and recognised by the immune system as virus. HBV surface proteins self-assemble into VLPs, and these VLPs form the basis of the HBV vaccine.
- Interferons are first innate immune responders that strengthen the host immune system to help clear the virus. Myrcludex B (bulevirtide/Hepcludex) is a synthetic peptide that blocks virus entry into host cells, and is also effective against HDV.
- The HCV genome is positive-sense, single-stranded RNA (+ssRNA), which means it can function directly as mRNA; it encodes one giant polyprotein that must be cleaved at multiple sites by host and viral proteases to produce the individual functional viral proteins. Virus-directed antivirals work by targeting this polyprotein cleavage process.
- Glecaprevir targets the NS3/4A cleavage site (blocking the NS3/4A protease) and pibrentasvir targets NS5A function; together, as the combination drug Maviret, they block both the NS3/4A cleavage sites and NS5A, disrupting HCV replication/assembly.