Micro Ch 43
Key Concepts
 Pathogens: hepatitis viruses A through E; antigenically and genetically unrelated to each other
 Encounter: A and E acquired by ingestion via fecal-oral route; B, C, and delta primarily bloodborne or sexually
transmitted
 Entry: despite entry, viruses go for liver cells
 Spread and Replication: Hep A & E viruses produce only transient infection followed by resolution; hep B, C, and
delta can produce lifelong persistent infections w/ongoing virus replication in liver and persistent viremia
 Damage: viral hepatitis – clinical syndrome characterized by fever and signs and symptoms of liver injury; clinical
signs variable and initially indistinguishable among different hepatitis viruses
 Diagnosis: testing for presence of specific viral proteins, specific antibodies against those proteins, or viral
nucleic acid associated w/causative agent
 Treatment: acute hepatitis not treated; chronic hep B & C can be treated w/specific antiviral drugs to prevent or
forestall permanent liver damage
 Prevention: blood products routinely screened for hep viruses; effective vaccines can prevent hepatitis A and B
Agent
Size (nm)
Nucleic Acid Composition
Virus Family
Hepatitis A
27
Linear + ss RNA
Picornaviridae
Hepatitis B
43
Nicked, circular, mostly ds
Hepadnaviridae
DNA
Hepatitis C
55
Linear + ss RNA
Flaviviridae
Hepatitis delta
Circular – ss RNA
Deltaviridae
Hepatitis E
27
Linear + ss RNA
Caliciviridae
The Liver as a Site for Virus Replication
 Hepatocytes needed for
o Production of plasma proteins (albumin and blood clotting factors)
o Glucose and lipid metabolism
o Metabolism and detoxification of xenobiotics, toxins, and drugs
o Conjugation and excretion of bilirubin (breakdown product of heme)
 Liver home to cells of immune system
o Innate components (interferons)
o Adaptive responses (antibodies and cytotoxic T cells (CTLs))
 Kupffer cells – important mediators of inflammatory response (cytokine release) that aid in clearance of virus
from liver either directly or by evoking influx of additional inflammatory cells from circulation
 Lots of NK T cells; capable of dramatic cytokine production when activated; innate immune system
 Symptoms that characterize viral hepatitis – fever, abdominal pain, nausea, vomiting, jaundice (if more
extensive), impaired mental function (more severe; due to failure to detoxify things absorbed from GI tract),
bleeding can result from diminished production of clotting factors
 Lab tests show elevation of serum bilirubin and transaminases (released from damaged hepatocytes)
Infectious Strategies Used by Hepatitis Viruses
 HAV and HEV acquired from environment by oral ingestion
o Viruses go to liver where replication takes place
o Virus shed into bile ducts and returned to intestine, where it can pass out w/feces
o Infected hosts mount effective immune response that clears virus from liver and blood and generates
lasting immunity to reinfection (transient, self-limited infection)
 HBV and HCV capable of either transient or persistent infection; initiate acute infection that may or may not
result in disease symptoms but always leads to extensive replication of virus in liver
o Immune response that follows not always effective in clearing infection
o Some infected persons clear acute infection; many don’t and develop persistent lifelong infection
o Persistent infection characterized by continued virus replication in liver and release of infectious
particles into bloodstream
 5% of HBV infections become persistent; 80% of HCV persistent
o
Virus replication in hepatocytes not directly injurious to cell; damage caused by host immune response
 Response generates CTLs directed against viral antigens on MHC class I on hepatocyte surface
 When antiviral CTLs recognize antigens, hepatocytes killed
 Cytokines released from T cells promote further inflammation and tissue injury
o In some cases, inadequate response by immune system continues to injure infected hepatocytes,
generative syndrome of sustained liver damage (chronic hepatitis) w/signs and symptoms that resemble
acute hepatitis but persist
o Asymptomatic carriers – persistently infected person who doesn’t have liver damage
o Liver responds to sustained damage w/regeneration and in some cases fibrosis; can evolve to cirrhosis
(nodules of regenerating hepatocytes accompanied by large bands of CT; usually indicates extensive loss
of functioning liver cells)
o Chronic HCV infection is leading indication for liver transplantation in U.S.
o Prolonged hepatocyte death and regeneration may lead to liver cancer, most likely due to host
mutations that accompany DNA synthesis process
Transmission of Hepatitis Viruses
Transmission
Hepatitis A
Hepatitis B
Hepatitis C
Hepatitis D
Hepatitis E
Fecal-oral
Yes
No
no
No
Yes
Sexual
Yes (anal & oral)
Yes
Yes
Yes
Yes (anal & oral)
Vertical
No
Yes
Yes
Yes
No
Parenteral
Yes (briefly)
yes
Yes
yes
Yes (briefly)
Hepatitis A Virus
 Responsible for most cases of infectious hepatitis; non-enveloped ss, + sense virus in Picornaviridae family
 Closely related to other picornaviruses (i.e., polio)
 Replication in liver robust, and virus titers in stool enormous
 Some isolates have been adapted to cell-culture growth (facilitated research and development of vaccine)
 Spread primarily through ingestion of fecally contaminated food or water; once HAV reaches intestine, it’s
absorbed into bloodstream and reaches liver through portal system
o Virus initiates acute infection of hepatocytes; newly synthesized virus particles exported into bile ducts
and from there, eventually excreted into feces
o Virus also released into bloodstream, causing transient viremia and brief period in which HAV can be
transmitted parentally
o Replication in liver triggers substantial immune response (humoral and cell mediated)
o Symptoms coincident w/initiation of immune response (IgM antibodies specific for structural proteins)
 Symptoms more severe in adults than children
 Virus rapidly cleared, and patients left w/lasting immunity to reinfection; NO CHRONIC INFECTION
 HAV vaccine generated by inactivation of cell-cultured-derived HAV particles; safe, highly immunogenic, and
very effective
 Those exposed to HAV offered administration of serum immune globulin (pooled antibody from normal blood
donors), which has high titers of anti-HAV antibodies; if given soon after exposure, anti-HAV antibodies in serum
immune globulin can prevent HAV infection or reduce extent and severity (postexposure prophylaxis)
Hepatitis B Virus
 Hepadnavirus family – partially double-stranded circular DNA genome; replication via reverse transcription
 Marked tropism for hepatocytes and can’t be grown in cells in culture
 Smallest genome of any human virus (except hep D which isn’t traditional virus)
 Minus (nonencoding) strand nicked; polymerase molecule attached to 5’ end
 Plus strand contains short RNA oligonucleotide at 5’ end and is shortened at 3’ end (circular DNA genome has
singl-stranded gap)
 Infectious virions surrounded by envelope and contain 5 proteins
o Envelope layer contains 3 surface antigens (HBsAg-S, HBsAg-M, and HBsAg-L; S, M, and L mean small,
medium, and large) embedded in lipid bilayer; each protein translated from different initiatior met
codon on mRNA, but proteins share much of carboxy-terminal sequences
o HBsAg-S is main constituent of envelope and required for virion assembly
o






HBsAg-L contains all of HBsAg-S plus additional amino acid sequences at amino-terminus; involved in
receptor binding and essential for infection
o HBV capsid icosahedral and composed entirely of core proteins (HBcAg) that surround viral DNA and
reverse transcriptase
o Reverse transcriptase – Pol protein; copies RNA to DNA
 Has RNase H domain that digests RNA template soon after it’s copied
 Once RNA removed, enzyme copies remaining DNA strand into duplex DNA; doesn’t go to
completion in infected cell (why DNA in virion only partially double-stranded)
Blood of HBV-infected individuals contains 1000x excess of particles w/empty envelopes (HBsAg); most spherical
and composed of mainly HBsAg-S (HBsAg-L in smaller amounts)
o HBsAg measured in blood and is useful diagnostic marker of HBV infection
HBV replication cycle
o HBV travels to liver after introduction to bloodstream; attaches to surface of hepatocytes
 After uptake and uncoating, viral genome delivered to nucleus, where partially double-stranded
DNA converted into fully double-stranded, covalently closed, circular species (cccDNA)
o cccDNA serves as template for transcription of viral RNA, which is synthesized by host RNA polymerase II
o Viral RNA to be used for reverse transcription packaged into immature viral capsid w/viral Pol protein
o Viral RNA used as template for reverse transcription, resulting in formation of viral DNA
 Viral polymerase serves as primer for first-strand synthesis, which is why product DNA
covalently linked to enzyme
o DNA-laden viral cores bud through intracellular membranes to pick up envelope of HBsAg; progeny
virions released from infected cells
In U.S., Canada, and northern Europe, HBV infections common only w/high-risk groups (IV drug users, those
w/multiple sex partners); in most of Asia and sub-Saharan Africa, infection prevalent in general population
HBV transmission occurs through exchange of body fluids (blood, semen, vaginal secretions)
o Transmits vertically from mother to infant (child’s exposure to maternal blood during birth)
Clinical pattern can follow several paths
o Primary infection can
by asymptomatic or
result in liver injury of
varying severity
o Following primary
infection, immune
response either
sufficient to clear
infection (top graph)
or not (bottom graph)
o Those w/chronic
infection have little or
no freely circulating
anti-HBs IgG; viral
products may be
found in blood for decades
o Persistence of HBsAg in
blood for 6 months or more
confirms diagnosis of
chronic HBV infection
Rates of chronic infection much
higher in those w/depressed
immune systems (although 5% of
exposed completely competent
adults chronically infected)
o
Once chronic infection established, some patients suffer from intermittent or persistent episodes of liver
injury (elevated aminotransferase)
 Serologic Assays for Hepatitis B Virus
HBsAg
Anti-HBs
Anti-HBc
Anti-HBe
HBeAg
Interpretation
neg
neg
neg
neg
neg
No prior exposure
neg
pos
neg
neg
neg
Prior vaccination
neg
pos
pos
pos
neg
Prior acute
infection,
resolved
pos
neg
pos
neg
pos
Acute or chronic
infection
pos
neg
pos
pos
neg
Later stage in
chronic infection
 After many years of chronic infection, some patients progress to cirrhosis
 Long-term chronic HBV infection predisposes to development of hepatocellular carcinoma (HCC)
 Effective vaccine now available (HBsAg particles prepared from yeast engineered to express HBsAg)
o Infants of HBsAg-positive mothers should receive both vaccine and anti-HBs IgG
 Treatment options for chronic carriers
o Interferon-α (natural antiviral cytokine) – expensive, toxic, of limited efficacy; most patients who
respond have substantial reductions in circulating viral load, though HBsAg remains positive
 If responses sustained, lasting improvement in liver function results
 For 70% of HBV-infected persons, benefit of treatment transient, and viral levels rebound
o Nucleoside analogs – selectively inhibit viral polymerase
 Adefovir, lamivudine, and tenofovir initially developed for use against HIV
 Adfovir more active against HBV than HIV
 Entecavir and telbivudine developed specifically for HBV; relatively selective for HBV; can reduce
viral load and improve liver function
 Available in oral form; more convenient and less expensive than interferon-based regimens
 When used as single agents, resistance develops slowly to drugs
Hepatitis Delta Virus
 Subviral agent incapable of disseminating w/o help from HBV
 Small single-stranded circular RNA genome; encodes single protein (HDAg), which binds to viral RNA and is
important in viral replication and assembly
o Doesn’t encode any envelope proteins; requires presence of HBV to provide envelope proteins
 Only infects those simultaneously infected w/HBV (coinfection) or who are HBV chronic carriers (superinfection)
 Associated w/increased risk of fuliminant hepatitis during acute phase of hepatitis; tends to increase hepatitis
severity during chronic phase
 Acquired by same routes as HBV
 HDV-infected persons make antibodies direct against HDAg; antibodies diagnostic of HDV infection
 No effective antiviral drugs specific for HDV, but since envelope contains HBsAg, HBV vaccination protects
against primary acquisition of both HBV and HDV
Hepatitis C Virus
 Responsible for most cases of non-A and non-B hepatitis
 Spread by exchange of body fluids; frequently associated w/IV drug use
o Can be spread by sex, but less efficiently than HBV
o HCV-infected mothers can transmit virus to newborn infants, but rate much lower than HBV
 Diagnosed by appearance of antibodies directed against viral proteins and detection of viral RNA in blood using
variation of PCR
 Enveloped, single-stranded, positive-sense RNA virus in Flaviviridae family
o 11 genotypes (genotype 1 most prevalent in U.S.)
o Clinical isolates can’t be routinely grown in cultured cells; only susceptible lab animal is chimp
o Few isolates replicate in cell culture and spread from cell to cell

HCV RNA genome translated into protein following entry into cell; not capped because uses internal ribosome
entry site (IRES) at 5’ end as ribosomal binding sequence that directs internal initiation of RNA translation
o Product of translation is single large polyprotein that has 2 protease domains capable of cleaving
polyprotein into number of functional subunits
o Once HCV polyprotein processed, viral nonstructural proteins participate in genome replication
o Viral RNA-dependent RNA polymerase copies positive-sense RNA into negative-sense RNA and copies
that into more positive-sense RNA; highly error-prone, so high mutation rate (person contains whole
population of quasi-species (related viruses to original))
 Liver diseases similar to HBV but milder, especially in early stages of infection
 Following several decades of chronic HCV infection, incidence of cirrhosis about 20%; risk of HCC substantial
 Infection associated w/extrahepatic diseases (mixed cryoglobulinemia – immune complex disorder of kidneys
and other sites)
o Antiviral antibodies complexed to virions deposited in affected tissues, triggering inflammatory
responses that result in tissue damage
 Current therapy w/interferon-α with ribavirin (antiviral agent) results in sustained virologic responses in 30-50%
of those infected w/genotype 1 strains (70-75% in genotypes 2 and 3)
o Regimen expensive and difficult to tolerate (side effects of interferon include fever, joint and muscle
aches, low blood cell counts, and CNS depression; side effects of ribavirin include anemia and it is
teratogenic)
o Emergence of drug-resistant viruses after monotherapy rapid, so multiple drug combo required
Hepatitis E Virus
 Foodborne and waterborne; nonenveloped, single-stranded, positive-sense RNA virus; causes only acute disease
 Resembles HAV in routes of transmission and associated diseases; symptoms associated w/acute HEV more
severe than HAV, particularly in pregnant women (can be life-threatening)
 Vaccine promising, but commercialization slow