Hepatitis B virus
The History of Virus Discovery
In the first half of XX century it has become evident that at least two viral agents are responsible for so-called “infectious jaundice”. As the result, F.O. MacCallum and D. J. Bauer proposed in 1947 the designation of hepatitis B for “serum-transmitted” hepatitis and hepatitis A for the disease transmitted via fecal-oral route.
In 1967 B.S. Blumberg discovered a specific antigen in blood of an Australian aborigine that was later associated with acute hepatitis B and led to the development of specific tests for identification of HBV infection.
Finally, viral etiology of hepatitis B was firmly established by D.S. Dane in 1968-1970 by electron microscopy with detection of HBV virions (referred to as Dane particles). Dane particles were shown to react with antiserum to Australian antigen. The next study found this antigen to be the surface component of HBV virion, thereby termed as hepatitis B surface antigen (or HBsAg).
Classification of Virus
Hepatitis B virus (HBV) pertains to Hepadnaviridae family, genus Orthohepadnavirus.
This genus also harbors other viral species that infect animals (e.g., bats, woodchucks, squirrels, etc.).
Hepatitis B virus causes acute and chronic viral hepatitis, often progressing to permanent carrier states, liver cirrhosis and hepatocellular carcinoma.
HBV is a 42 nm spherical particle with icosahedral symmetry, enveloped with external protein shell.
HBV genome contains unique circular partly double-stranded DNA. Minus strand of DNA is almost a complete circle and carries overlapping genes that encode both structural proteins (pre-S, surface, and core proteins) and replicative proteins (polymerase with reverse transcriptase function and X-protein). The plus strand of DNA is shorter and variable in length. The gap in DNA must be completed at beginning of replication cycle.
Partially different 8 genotypes of HBV are known to date (A to H).
HBV contains 4 major antigens – HBs, HBc, HBe and HBx.
Viral envelope is predominantly composed of HBsAg and some lipids. External shell surrounds a 27-nm inner nucleocapsid core that contains HbcAg.
HbsAg particles exist in several forms – S (small), M (middle) and L (large). The latter two variants are composed of HbsAg and pre-S2 protein component or pre-S1 component.
It is suggested that the pre-S proteins play an important role in the attachment of HBV to hepatocytes. In addition, pre-S2 attaches to polymerized human serum albumin.
Core HBcAg is processed intracellularly to produce HBeAg, which appears in bloodstream.
HBx protein is a small transcriptional transactivator, regulating viral transcription. It is suspected to participate in emergence of HBV-induced primary hepatocellular carcinoma.
Also HBV genome encodes large polymerase (P) protein that develops DNA polymerase, reverse transcriptase, and RNase activities.
HBV virion is a stable particle. The dried virus remains viable for at least 1 week. HBV can be stored at -20°C for over 20 years; it is resistant to repeated freezing and thawing.
HBV virion (but not HBsAg) is sensitive to high temperatures being inactivated at 100°C in 1 minute. Incubation at 60°C prolongs viral survival to 10 hours.
HBsAg remains stable at pH 2.4 for up to 6 hours, but HBV infectivity is lost.
HBV is relatively sensitive to a number of disinfectants such as halides, glutaraldehyde and formaldehyde, 95% ethanol, phenol. For instance, sodium hypochlorite destroys HBV antigenicity within several minutes.
HBV Replication Cycle
HBV replication begins with binding of the virus to hepatocyte membrane with subsequent penetration and shedding of external shell.
Virus core is transported to the nucleus, where the relaxed circular DNA is converted to a covalently closed circular DNA (cccDNA), which acts as the template for all viral RNA synthesis.
DNA transcription results in pregenome RNA synthesis. Pregenomic RNA serves as a template for reverse transcription resulting in a negative strand DNA copy. The polymerase starts to synthesize positive DNA strand, but the process is not completed. Sometimes DNA becomes capable of integrating with cellular genome, but it usually doesn’t occur during the normal course of replication.
Viral RNAs are translated on ribosomes yielding viral structural proteins.
Viral cores become encapsidated with newly synthesized HbcAg, acquire HBsAg-containing envelopes in cytoplasm, and may exit the cell. Alternatively, cores may be reimported into the nucleus and initiate another round of replication in the same cell.
Pathogenesis, Clinical Findings and Immunity of Hepatitis B
HBV infection is grossly spread worldwide. According to WHO data, about 240 million people are chronically infected with hepatitis B. The estimated number of annual death cases due to hepatitis B complications (namely, cirrhosis and liver cancer) exceeds 680,000.
Only mass human vaccination with efficient hepatitis B vaccine created the barrier against the infection, thereby preventing the development of disease complications.
Nevertheless, high prevalence of HBV infection creates a serious problem in certain world regions, e.g. in sub-Saharan Africa, South and East Asia, where 5-10% of the adults is chronically infected.
Hepatitis B is anthroponotic disease. The source of infection is human carrier of sick person.
The infectious dose for HBV is low but not yet firmly established (usually less than 100 viral particles).
The main routes of HBV transmission are: parenteral after medical manipulations via blood and its products (“artificial route”), sexual intercourse, and vertical transmission from mother to child.
As the result, HBV infection is a substantial occupational hazard for healthcare workers due to the elevated risk of their exposure to infected blood and low infectious dose of the pathogen.
Incubation period of hepatitis B is long and lasts from 1 to 6 months.
When appeared in the bloodstream, HBV enters the liver, binds to membrane liver bile acid transporter protein (LBAT) by viral HBsAg and replicates within hepatocytes.
It is generally assumed that HBV is not directly cytopathic to infected hepatocytes, but cytotoxic T cells attack the infected hepatic cells. Thus, cellular autoimmune reactions promote cytolysis of hepatocytes.
Clinical findings of acute HBV infection include fever, nausea and vomiting, jaundice with dark urine and pale stools.
In some cases extrahepatic autoimmune manifestations occur resulting in skin rashes, arthralgias or arthritis. Severe (fulminant) forms of hepatitis B develop in 1-2% of patients with lethality above 60%.
The disease has a tendency to chronicity eventually observed in 5-10% of cases. The risk of chronic infection is highest (up to 90%) for infants infected during pregnancy.
Chronic HBV infection elevates the risk of hepatocellular carcinoma. Therefore, HBV is regarded as established biological carcinogen.
Despite possible autoimmune injury of hepatocytes, T cell-mediated immunity is considered to be essential for patient recovery. Elimination of infected cells and inhibition of viral replication through the release of cytokines, e.g., γ-interferon and TNF-α, is a cornerstone of viral clearance during acute HBV infection. It is generally assumed that chronic infection is related to a weak T cell response to viral antigens.
Binding of specific antibodies to viral envelope antigens also contributes to clearance of the virus.
Viral DNA, antigens and antiviral antibodies are revealed in blood and liver biopsy specimens. HBsAg can be detected also in saliva, semen, vaginal secretions, etc. The most valuable tests are ELISA for detection of HBV antigens and antibodies and PCR for viral DNA.
HBsAg indicates HBV infection – acute or chronic. HBsAg is usually detectable 2-6 weeks prior to clinical and biochemical signs of hepatitis. It persists throughout the clinical course of the disease but typically disappears by the 6th month after exposure. HBeAg correlates with active HBV replication and high serum load with hepatitis B viruses
Anti-HBs antibodies indicate past infection with immunity to HBV, or immune response to hepatitis B vaccine. High levels of IgM anti-HBc-specific antibodies are frequently determined at the onset of clinical illness.
Anti-HBe antibodies are present in serum of persons with chronic HBV infection with low titers of HBV. Viral DNA detection by PCR correlates with active viral replication.
Principles of Disease Treatment and Prophylaxis
Hepatitis B is treated with reverse transcriptase inhibitors (e.g., lamivudine) and inhibitors of viral DNA synthesis (telbivudine).
Recombinant interferon-alpha shows beneficial effect in the treatment of patients with chronic HBV infection. For passive post-exposure prophylaxis hepatitis B immune globulin is administered within 12 hours after percutaneous exposure to HBsAg-positive blood, or after sexual intercourse with HBsAg-positive person.
For active specific prophylaxis HBsAg recombinant vaccine is effectively used worldwide. HBV vaccination is an essential part of national immunization programs in many countries. Primary injection of vaccine is made during 12 h after birth; two boosters are conducted in 1 and 6 months. Full vaccination course confers specific immunity to 99% of persons. Pre-exposure prophylaxis with hepatitis В vaccine is conducted for all susceptible high-risk groups, including medical personnel. The protective immunity lasts for 10-15 years.
Non-specific prophylaxis includes the procedures that can limit the risk of infection to healthcare workers, laboratory personnel, and other susceptible individuals. All blood remnants, body fluids and contaminated materials should be treated as if they are infectious for HBV, HCV, HIV, and other parenteral viruses. The medical instruments are sterilized by autoclaving.