Lecture 30 – Hepatitis B
Reading Assignments: (1) Text Chapters 42. (2) Gilligan, P.H., M.L. Smiley, and D.S. Shapiro. 1997. Cases 31, 32. In: Cases in Medical Microbiology and Infectious Diseases (2nd ed.), American Society for Microbiology, Washington, D.C (3) “Bloodborne and Transplant-Associated Infections”, pp. 167-175, In McClane, B.A. and T.A. Mietzner. 1999. Microbial Pathogenesis: A Principles-Oriented Approach, Fence Creek Publishing, Madison, CT. (4) White, D.O., and F.J. Fenner. 1994. “Persistent Infections”, pp.156-158., In Medical Virology (4th ed.), Academic Press, New York.
1. HEPATITIS B VIRUS
A. Unique Features of Hepadnaviruses
B. Virion Structure
Infectious Virion – called the Dane particle – 42 nm in diameter
a. Structural Proteins:
1. Outer envelope glycoprotein -the Hepatitis B surface antigen (HBsAg).
(Includes 3 glycoproteins- L,M, and S)
a. HbsAg particles are produced and released into the serum in large
amounts.
(1) Two types: spheres and rods.
(2) 1000-fold greater number than virions. May reach
concentrations of 1g/l (1012particles/ml)
(3) Why produced in excess???
2. Icosahedral Core – Hepatitis B core antigen (HbcAg)
b. Nonstructural Proteins
1. Hepatitis B e antigen (HbeAg) a soluble non-structural protein associated with
the core; a minor component of the virion.
c. Genome – Partially double stranded circular DNA
1. Full length – strand (complementary to all mRNA's)
2. Variable length + strand – 50-80% of – stand
d. Integration of DNA into the host genome
Integration of the viral genome into the host genome is not part of the replication
cycle (unlike retroviruses). However, integration of DNA fragments into the
hepatocyte genome occurs frequently during infection. Since only fragments of
DNA are integrated, a complete pregenomic RNA cannot be expressed, however
subgenomic mRNAs and viral proteins are expressed from the integrated DNA.
There is evidence that integration of the HBV DNA causes insertional
mutagenesis leading to deregulation of growth control genes and ultimately to
hepatocellular carcinoma (HCC).
***See Text. p. 398 for additional explanations about how chronic Hepatitis
B infections may induce HCC***
C. Replication Cycle
THE BIG PICTURE: partial dsDNA ---- cccDNA ---- RNA intermediate ---- partial dsDNA
Hepadnaviruses are the only viruses that produce genome DNA by reverse
transcription with mRNA as the template.
THE KEY ENZYMES: (1) Unknown viral or cellular enzymes??? (2) Host cell DNA
dep. RNA polymerase (RNA Polymerase II), (3) Viral RNA dep. DNA polymerase
(Reverse Transcriptase- acts as 3 enzymes in one – a reverse transcriptase, and
RNAse H, and a DNA polymerase)
1. Attachment to hepatocytes via HBsAg glycoproteins. (Host cell receptors are not
known at this time)
2. Endocytosis?
3. Fusion to release the core?
4. Experimental evidence suggests that the core travels to the nuclear pore and
releases the partial dsDNA genome into the nucleus.
5. The partial DNA strand of the genome is synthesized (using host cell or viral DNA
polymerase??) and a complete double-stranded DNA covalently closed circular
molecule (cccDNA) is formed in the nucleus. This negative strand of this cccDNA
will serve as the template for all viral transcripts.
6. The cccDNA is transcribed into mRNA by host cell RNA polymerase II. Four
mRNA's are formed, including an mRNA called the pregenome that is larger than the
genome itself (3500bp). The pregenome functions both as a message (encoding
the HBc and HBe antigens, the reverse transcriptase, and a protein primer for DNA
replication) and as a template for replication of the genome . The smaller mRNA's
code for surface antigen proteins.
7. The messenger are translated into structural proteins (envelope and core proteins)
and non-structural proteins (enzymes) in the cytoplasm.
8. In the cytoplasm, the core proteins assemble around the pregenome (the 3500bp
mRNA) incorporating some of the newly synthesized reverse transcriptase enzyme.
9. Within the core, the negative sense DNA strand is synthesized by reverse
transcriptase. The RNA template is degraded (by the RNase H activity of the
reverse transcriptase) and synthesis of the complementary positive DNA strand is
begun but not completed (also catalyzed by reverse transcriptase acting as a DNA
polymerase). This leaves a gap in the positive sense DNA strand.
10. It is believed that two things now happen to the mature cores, both of which are
important for the viral life cycle:
a. Because cccDNA is subject to degradation, some cores return to a nuclear
pore releasing their genomes back into the nucleus to form more cccDNA,
thus amplifying the pool of intranuclear HBV genomes. (This pool must be
continued to ensure viral persistence.)
b. Other cores become associated with glycoproteins which have previously
been inserted into the endoplasmic reticulum and are enveloped by budding. The mature virions are transported to the cell surface in the Golgi apparatus and they leave the cell by exocytosis.
2. HEPATITIS B DISEASE
A. Epidemiology
1. Distribution - worldwide
2. Prevalence:
a. 1/3 to 1/2 of the world's population have been infected by the virus, and 10%
(350 million people) are chronically infected. Worldwide, there are 1 million
deaths/year to HBV liver disease and Hepatocellular Carcinoma (HCC)
b. Distribution in the World
1. Highest in Italy, Greece, Africa, Southeast Asia. Serocoversion 50%.
Hepatocellular Carcinoma (HCC) is endemic in these regions of the
world.
2. In the U.S., 300,000 people infected each year. 1 million carriers in
the U.S; 25% of carriers develop chronic active hepatitis.
c. Transmission –
1. Blood –seen in iv drug abusers, acupuncture, tattooing, and ear and
body piercing. (Screening donor units of blood has greatly decreased
the chance of transmission from blood transfusions).
2. Sexual transmission- multiple sex partners; close contact with
semen, vaginal secretions.
3. From mother to child during birth, or breast-feeding.
liver disease and are at high risk of developing HCC.
4. Other routes: All bodily fluids potentially infectious, even
saliva.
d. High risk groups- drug abusers, institutionalized individuals,
health care personnel, multiply-transfused patients, organ
transplant patients, hemodialysis patients and staff, highly
promiscuous persons, and newborn infants born to mothers with
HBV.
B. Clinical Syndromes- The course of hepatitis B infection may follow several paths:
CASE STUDIES: Mr. P, Text p. 393 and Clinical Case in Reading Assignment (3)
1. Acute Hepatitis – occurs 90-99% in adults an older children; 5-20% in neonates
and infants.
Acute Hepatitis B infection can have 3 possible clinical outcomes:
a. Subclinical – no symptoms 90% will totally
recover
b. Icteric disease - classic symptoms
1. Long incubation period (6-26 weeks) with an insidious
onset.
2. Prodromal phase – malaise, lethargy, anorexia, nausea,
vomiting and upper right abdominal pain.
A minority of patients develop a type of serum
sickness (Type III Hypersensitivity) due to circulating
complexes of HBsAg and anti-HBsAg antibody. These
complexes produce symptoms of rash, fever and
polyarthritis.
3. Icteric Phase –2 days-2 weeks later
4. Convalescent Phase – long and drawn out, with malaise and
fatigue lasting for weeks.
c. Fulminant Hepatitis (1%)- fatal 70-90% of the time. May be
associated with co-infection or superinfection with HDV.
2. Chronic Hepatitis (disease lasting over 6 months) ; 1-10% in adults and older
children; 80-95% in neonates and infants. Usually develops in patients with
immature immune systems or those with impaired immune systems. There is a
spectrum of possible clinical symptoms:
a. Asymptomatic carrier state
b. Chronic persistent hepatitis –nonspecific symptoms of malaise, lack of energy
and becoming easily fatigued.
c. Chronic active hepatitis – intermittent acute disease which over time can
cause causing progressive liver damage leading to ----- cirrhosis (irreversible
liver disease) -------- hepatocellular carcinoma (20 years later). HBV is the
major cause of HCC.
C. Pathogenesis and Immunity
1. HBV virus enters the host and is transported to the hepatocytes via the bloodstream; it
enters the hepatocytes.
2. The virus begins replication within 3 days after acquisition, but symptoms may not
been seen for 45 days or longer depending on the route of infection, the dose, and
the immune status of the host.
3. The virus replicates within hepatocytes with minimal CPE.
4. Cell mediated immunity and inflammation are responsible for the symptoms and
for resolving the infection!
a. Acute Hepatitis
1. The virus replicates productively in the liver (unintegrated in host cell DNA)
and the infectious virions circulate in the plasma along with an excess of
HBsAg.
2. Symptoms of hepatocyte damage and death are caused by a strong cell-
mediated immune response and the attendant inflammation.
3. HBV producing hepatocytes are eliminated initially by interferon and then by
a strong TC response against HBc, HBe, and HBs protein antigens. Peptide
fragments of these antigens are displayed on the surface of infected
hepatocytes (endogenous antigen presentation on MHC I). Abundant TH
cells specific for HBc and HBe are also present with fewer TH specific for
HBsAg.
4. Antibody response to HBV envelope proteins is T-cell dependent. Antibody
can:
a. complex with free virions removing them from the circulation and
preventing their attachment and uptake by susceptible cells
(neutralization). Neutralizing antibody prevents reinfection.
b. bind with excess circulating HBsAg in the serum:
1. preventing virus neutralization, or
2. forming circulating Ag-Ab complexes which can be
deposited in kidneys and arterioles causing a
serum sickness-like immune complex disease.
b. Chronic Hepatitis
1. The virus evades the immune response and establishes a persistent infection
– not fully understood how.
2. There is a weak or undetectable cell mediated response against the virus.
Few T cells are present in the liver of patients with chronic Hepatitis B - only
low levels of TC and TH specific for HBs or HBc are found. The low number of
T cells may account for the mild symptoms and continuous inflammation that
are seen with chronic disease.
3. The genome is more commonly integrated into the chromosome of the
heptocytes, and transcription is restricted to subgenomic mRNA. There is
production of HBsAg, but little or no antibody against HBsAg is detected
during the chronic carrier state. (There is deposition of antigen-antibody
complexes in kidneys and arterioles causing “immune complex disease”.
This suggests that antibody is made but that it is absorbed by circulating
HBsAg, and thus is ineffective in clearing the infection.)
4. As many as 90% of infants infected perinatally become chronic carriers.
Infants and young children have an immature cell-mediated response and
thus they have less tissue damage and milder symptoms than chronically
infected adults. Viral replication persists for long periods and this makes
them an important source of infection to others.
1. Clinical Symptoms (especially if jaundice is present) and epidemiological clues.
2. Elevated Liver Enzymes (ALT, AST), elevated bilirubin.
3. Serology – detection of antigens and antibodies; correlates with different stages of disease
(See Text p. 397):
a. HBsAg – appears during the incubation period, is present during the prodrome, and
acute disease. Falls during convalescence in most cases. Indicates infection.
b. HBeAg – appears during the incubation period; associated with HBV replication and
high titers of HBV in serum, and with infectivity of the serum.
c. Anti HBc antibodies are detected before Anti-HBe and Anti-HBs antibodies and can
be used to make the diagnosis after antigens have disappeared from the serum. The
best indicator of a recent infection with HBV is anti-IgM antibody against HBcAg.
d. “The Window” is a period when anti-HBsAg antibodies cannot be found. This does
not indicate that the antibody is not being made, but rather that it is undetectable
because it is bound to soluble HbsAg in serum.
e. Anti-HbsAg antibody indicates past infection, the presence of passive antibody if
Hepatitis B Immune globulin has been given, or an immune response from HBV
vaccine.
f. Characteristic findings in chronic hepatitis are a prolonged HBsAg and no anti-
HBsAg (IgG).
E. Treatment, Prevention and Control
1. Treatment for Chronic Carriers:
a. Alpha interferon – difficult to cure; influenza like side effects
b. Lamivudine – a nucleoside analog that inhibits the reverse transcriptase of HIV is also
effective against HBV. Drug resistance may be a problem.
2. Prevention- Vaccine, Hyperimmune Globulin, or Both
a. Vaccine – HBsAg produced in yeasts by genetic engineering (Insertion of a plasmid
containing the S gene for HBsAg into S. cerevisiae.) The first recombinant vaccine
approved for use in humans.
b. Hepatitis B immune globulin (HBIG) contains high titers of anti-HBsAg because it is
prepared from sera of patients who have recovered from Hepatitis B.
c. “Passive-Active” Immunization – short term and long term protection recommended
for:
1. Newborns whose mothers are infected with HBV
2. Medical Personnel – needle-stick injury from a patient with
HBsAg positive blood.
3. Control
a. Screening of blood and blood products for HBsAg.
b. Education of high risk groups such as i.v. drug users.
c. Vaccinations for medical personnel and other high risk groups.
d. Universal blood and body fluid precautions for medical personnel.
Gloves required for blood and body fluids; protective clothing
and eyeglasses. Careful disposal of needles and sharp
instruments.
e. Unlike most enveloped viruses, HBV is not inactivated by detergents and HBV
contaminated materials must be disinfected with 10% bleach.