Biology of Disease CH0576

1. Biology of Disease CH0576 Hyperbilirubinaemia & Jaundice III Viral Hepatitis CH0576/RY

2. 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/RY

3. 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/RY

4. 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/RY

5. 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/RY

6. 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/RY

7. 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/RY

8. 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/RY

9. 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/RY

10. 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/RY

11. 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/RY

12. 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/RY

13. 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/RY

14. 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/RY

15. 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/RY

16. 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/RY

17. 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/RY

18. 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/RY

19. 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/RY

20. Clinical Course of HCV • HCV RNA • Serum ALT (alanine aminotransferase) • 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. CH0576/RY

21. Proposed Life Cycle of HCV CH0576/RY

22. 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/RY

23. 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/RY

24. 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/RY

25. 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/RY

26. 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/RY

27. 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/RY

28. 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/RY

29. 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/RY

30. 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/RY

31. 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/RY

32. 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/RY

33. 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/RY

34. 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/RY

35. 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. CH0576/RY