CH0576/RY 1

Biology of Disease CH0576

Hyperbilirubinaemia & Jaundice IIIViral Hepatitis

CH0576/RY2

Viral Hepatitis

• A range of systemic viral infections can involve the liver and lead to hepatitis e.g. EBV, CMV or Yellow Fever.

• The term ‘viral hepatitis’ is restricted to infection by a growing group of viruses which have a particular affinity for the liver.

• Currently this group of unrelated viruses includes Hepatitis A,B,C,D,and E .

CH0576/RY3

Hepatitis A

• Hepatitis A is commonly called ‘infectious’ hepatitis.

• Hep A is a benign, self limiting disease with an incubation period of around 2-6 weeks.

• HAV infection occurs around the world and is endemic in areas of poor hygiene and sanitation.

• Clinical disease tends to be mild or asymptomatic, and is rare after childhood

CH0576/RY4

Hepatitis A

• HAV is spread by the ingestion of virally contaminated water and food, and is shed in stools for about 2 to 3 weeks before and 1 week after the onset of an overt jaundice.

• Close personal contact with an infected individual accounts for the spread of this condition among certain groups:-– Schools and nurseries– Water sports enthusiasts (in contaminated water)

CH0576/RY5

Hepatitis A

• In developed countries sporadic infections are seen largely through the consumption of contaminated shellfish, which are able to concentrate the virus from sea water contaminated by sewage outfalls.

• As HAV viraemia is only transient, blood borne transmission of HAV only rarely occurs and hence donated units are not screened for the virus.

CH0576/RY6

Hepatitis A Virus• A small non-enveloped single stranded

RNA virus.• Apparently not cytotoxic to hepatocytes• At onset of symptoms, IgM antibodies

appear and serve as a marker of acute infection.

• As the IgM titre rises, faecal shedding of the virus ends.

• Lifelong immunity to re-infection by all strains of HAV is gained.

CH0576/RY7

Hepatitis B

• HBV is the cause of ‘serum hepatitis’ and can produce a range of possible clinical outcomes.

• Number of carriers of HBV, worldwide, is estimated to be around 300,000,000.

• Unlike Hepatitis A, Hepatitis B viraemia persists during a prolonged incubation period of between 4 and 26 weeks.

CH0576/RY8

Hepatitis B

• This virus can be spread by contact with blood and body fluids/body secretions such as:-– Semen– Saliva– Sweat– Tears– Breast milk– Pathologic effusions.

CH0576/RY9

Hepatitis B• There are a number of primary risk

categories for Hep B infection:-– Transfusion– Infusion of blood products– Dialysis– Needle stick injuries in health care workers– Intravenous drug abuse– Homosexual activity.

• In 1/3 of cases the source of transmission is unknown.

CH0576/RY10

Hepatitis B• In endemic regions such as Africa and

Southeast Asia, the spread from a mother to a neonate during birth is common (vertical transmission)

• Neonates infected in this fashion often become lifelong carriers of the infection.

• Hep B virus seems not to cause direct damage to the liver cells.

• The damage to the liver is a consequence of CD8+ T-cell activity against infected cells.

CH0576/RY11

Hepatitis C• Hepatitis C was previously known as ‘non-A

non-B’ viral hepatitis, due to the failure to recognise the infective agent.

• In 1989 Choo et al successfully identified the HCV genome, using molecular cloning techniques.

• At this stage there were no culture techniques which could successfully grow the virus, nor any means of visualising the agent.

• Since then, major advances in our knowledge regarding HCV have been made.

CH0576/RY12

Hepatitis C

• HCV is an enveloped virus, and is a member of the Flavivirus family.

• The envelope consists of host cell membrane material, into which viral glycoproteins are inserted.

• The genome is RNA, and consists of around 9,600 nucleotides.

• This encodes a single ‘polyprotein’ of around 3,000 amino acids length

CH0576/RY13

Hepatitis C

• The ‘polyprotein’ is cleaved into 10 structural and non-structural proteins.

• Throughout the world 6 major HCV genotypes have been described and >100 subtypes.

• A major hindrance to development of an effective vaccine.

CH0576/RY14

Hepatitis C

• To further complicate matters, within a given infected individual there can be identified several different, but related, HCV sequences.

• These are referred to as ‘quasi-species’.• These a re a reflection of the high

replication rate of the virus and indicates a lack of effective proof-reading of the RNA-dependent RNA polymerase.

CH0576/RY15

Hepatitis C

• The virus predominantly replicates in the cytoplasm of hepatocytes.

• HCV genome sequences have been isolated from dendritic cells and also from B cells.

• One of the proposed receptors for the virus is CD81, a component of the B-cell co-receptor.

• HCV transmission is primarily through exposure to infected blood.

CH0576/RY16

Hepatitis C• It is thought that HCV is responsible for 90-

95% of transfusion associated hepatitis.• In 1992 there was the development and

introduction of a screening test for blood donations.

• This has greatly reduced the risk from transmission by transfusion or transplantation.

• Infections still occur through intravenous drug abuse, sex with infected partners and through needle stick injuries in health care workers.

CH0576/RY17

Hepatitis C

• Vertical transmission (mother to foetus) has also been documented.

• Despite the screening of blood donations the number of new infections in the U.S is thought to be around 40,000 per annum.

• HCV has been estimated to have caused persistent infection and chronic hepatitis in >180,000,000 individuals globally.

CH0576/RY18

Hepatitis C

• Incubation period ranges from 2-26 weeks.• Persistent infection and chronic hepatitis

are classical hallmarks of HCV infection in >50% of cases.

• Cirrhosis may be present at the time of diagnosis or may develop over the following 5 to 10 years.

• There is an appreciable risk of the future development of hepatocellular carcinoma.

CH0576/RY19

Hepatitis C• A significant number of individuals

remain asymptomatic and completely unaware of infection until liver fibrosis, cirrhosis and/or hepatocellular carcinoma develop.

• This may be decades after the acute sub-clinical infection with the virus.

• Treatments including the use of IFN and ribavirin in combination have been developed BUT <50% of the patients with chronic HCV infection respond.

CH0576/RY20

Clinical Course of HCV

• HCV can leave little serological evidence following spontaneous recovery.

• Abs can completely disappear 10-20 years after infection.

• The typical picture in chronically evolving acute hepatitis is seen in fig.b.

• HCV RNA Serum ALT (alanine aminotransferase)

CH0576/RY21

Proposed Life Cycle of HCV

CH0576/RY22

Hepatitis C & Lipids• From the proposed life cycle of HCV it is

evident that lipid metabolism plays a role in the disease pathogenesis.

• HCV is associated with LDL and VLDLs in the plasma of infected individuals.

• The LDL receptor is a candidate for the viral receptor/co-receptor on host cells.

• In chronic Hep C intracellular lipids accumulate within infected hepatocytes ~ steatosis.

CH0576/RY23

Hepatitis C & Lipids• The HCV core protein seems to play a

role in steatosis:-– Mice transgenic for the core protein develop

steatosis and hepatocellular carcinoma.– Proposed mechanism being an increased

lipid peroxidation and development of mitochondrial damage, leading to generation of reactive oxygen species and damage to DNA within the host cell.

CH0576/RY24

Hepatitis C ~ Immune Evasion

• HCV employs a varied range of evasion strategies to produce persistent infection.

• These strategies are generally divided into three categories:-– Escape from T-Cell recognition,– Resistance to immune attack,– Counterattack i.e. active methods of fending

off the immune effector responses.

CH0576/RY25

HCV ~ Escape• HCV has a very high replication rate

with 1010-1012 virions produced daily.• This replication rate is thought to outrun

the immune response.• Maximal viral titres are reached several

weeks before humoral/CMI responses are detectable.

• CD8+ cells have been described as ‘stunned’ and are unable to secrete IFN- when viral load is high.

CH0576/RY26

HCV ~ Escape• Ineffective proof reading by the HCV RNA

polymerase allows the generation of quasi-species – the virus keeps one step ahead of the evolving immune response.

• Progressive quasi species generation is associated with the generation of the chronic form of hepatitis.

• Lack of quasi species generation tends to be associated with a self-limited disease course.

CH0576/RY27

HCV ~ Escape• Any mutations which alter recognition

sites for T-cells or Abs will obviously help viral escape and are associated with chronic disease.

• HCV mutations can also affect peptide binding by MHC antigens and prevent the viral peptides being presented to T-cells.

• The non-cytopathic nature of HCV also aids its escape:-– Only very small levels of exogenous antigen

available for uptake by Dendritic cells.

CH0576/RY28

HCV ~ Resistance

• During the replication of RNA viruses there is the generation of double stranded RNA (dsRNA).

• This usually leads to the activation of IFN transcription factors and causes the production of anti-viral type 1 IFN.

• Usually there is the inhibition of both cellular and viral protein synthesis as a result.

CH0576/RY29

HCV ~ Resistance• HCV appears to be highly resistant to the anti-

viral effects of ds-RNA.• Mechanisms involved:-

– A) The NS3/4A serine protease blocks the effector action of IRF3 (a transcription factor needed for the production of IFN)

– B) HCV E2 acts as a decoy receptor for PKR (a protein kinase which inhibits protein synthesis)

– C) A further HCV product, NS5A blocks PKR activation by interacting with its catalytic site.

Uninhibited cellular & viral protein synthesis

CH0576/RY30

HCV ~ Counter-attack

• Studies indicate that HCV E2 protein causes crosslinking of CD81 and inhibits the functions of NK cells.

• There is inhibition of cytotoxicity, proliferation, IFN- and TNF- secretion.

• Dendritic cells in chronic infected HCV individuals are defective in their responses to foreign antigens.

• Viral factors are proposed to be responsible for this impaired DC function.

CH0576/RY31

HCV ~ Counter-attack• Acute HCV infection seems to cause

transient dysfunction of antigen specific T-cells.

• Perforin content, lytic activity and ability of CD8+ T-cells to proliferate are significantly reduced.

• The CD8+ T-cells of chronically infected individuals do not respond to HCV peptides in in vitro function tests.

cytotoxicity, secretion of IFN-, and proliferation in response to HCV antigens.

CH0576/RY32

Hepatitis D• This hepatitis virus is unusual in that it

shows a defective replication.• HDV is absolutely dependent on HBV co-

infection for its replication.• Hepatitis D can hence occur with:-

– Exposure to serum containing HBV and HDV– Superinfection of a chronic carrier of HBV

• The possible clinical outcomes in these two scenarios are quite different.

CH0576/RY33

Hepatitis E• HEV mode of transmission is by the oral-faecal

route, with water borne transmission occurring mainly in young to middle aged adults.

• A characteristic feature of HEV infection is the high mortality rate among pregnant women infected with the virus (around 20%)

• Average incubation period is 6 weeks.• In most cases the disease is self-limiting and not

associated with chronic liver disease.

CH0576/RY34

Clinical Syndromes• A range of possible clinical syndromes

may develop after exposure to these hepatitis viruses:– Carrier state: without apparent disease– Asymptomatic infection: serological

evidence only.– Acute hepatitis: anicteric or icteric– Chronic hepatitis: with or without

progression to cirrhosis– Fulminant hepatitis: massive hepatic cell

death.

CH0576/RY35

Clinical Syndromes• Not all of the hepatitis viruses can provoke

all of the possible clinical outcomes.– HAV and HEV do not generate a carrier state

nor do they cause chronic hepatitis.– Other infectious or non-infectious agents (e.g.

drugs and toxins) can lead to almost identical syndromes.

– Hence serological studies are essential for the differential diagnosis of viral hepatitis, and the distinction between the different viral infections.