Download Viruses of Humans

RNA VIRUSES
Picornaviridae

The smallest RNA-containing viruses known.

Comprise one of the largest (230 members) and most
important families of human and agricultural
pathogens.

The family is currently divided into five genera;
Rhinoviruses, Enteroviruses, Aphthoviruses,
Cardioviruses, and Hepatoviruses (REACH).

Aphtovirus (foot-and-mouth disease virus):- Infects
cloven-hoofed (footed) animals and occasionally
humans.

Cardiovirus:- Infects rodents

Three genera; Rhinovirus, Enterovirus, and
Hepatovirus include primary human pathogens with
numerous serotypes.
Genus
Rhinoviruses
Enteroviruses
Virus
Serotypes
> 100
Polio viruses
Coxsackie viruses
A
3
B
Echoviruses
Hepatovirus
Total
23
6
28
1-22, 24
(23= echorviruses 9)
1-6
1-7,9,11-21, 24-27,29-33
Echo 8 = Echo 1
Echo 10 = Reovirus 1
Echo 28= Rhino 1A
Echo 34 = Coxsackie A 24
Echo 22,23= Parechovirus
*Parechoviruses
2
*Enteroviruses 68-71 4
Previously Echo 22 and 23
New naming system since 1967
Hepatitis A virus
Previously enteroviruses 72
1
67

Enteroviruses and Hepatovirus differ from
Rhinoviruses in:
- Stability at pH 3
- Optimum T º of growth
- Mode of transmission
- Diseases caused
Enterovirus Particles

The virion is roughly spherical, naked, and range in
diameter from 24 to 30 nm.

60 subunits make up the icosahedral capsid each of
which is composed of four polypeptide chains, VP1VP4

VP1, 2 and 3 are exposed at the virion surface,
whereas VP4 lies buried in close association with the
RNA core.

Of the four proteins, VP1 exhibits the greatest
sequence variability and VP4 the least.

VP1 is also the dominant protein, playing key roles in
surface topography, antigenicity, receptor binding, and
probably viral uncoating.

Genome is a ss RNA ( 7500 nt) of positive polarity,
polyadenylated at 3- and has a protein of 22 to 24
amino acids (VPg) at the 5- end.
Receptors

The receptors for polioviruses, Coxsackie viruses,
echoviruses, and the major serogroup of rhinoviruses
have all been mapped to human chromosome 19.

The receptors for polioviruses and human
rhinoviruses have been identified as members of the
Ig superfamily, whereas the receptor for echoviruses
has been identified as a member of the integrin family.
Virus
- Rhinovirus (major)
- Rhinovirus (minor)
- Polioviruses
- Coxsackie A
- Coxsackie B
- Echovirus
Receptor
ICAM-1
LDL-R
PVR (similar to
ICAM-1) CD155
ICAM-1
Unknown
DAF, VLA-2
Enteroviruses

Arildone (pleconaril) contains a 3-methyl- isoxazole
group that binds to the floor of the VP1 canyon and
alters its conformation to prevent the uncoating of the
virus.

Poliovirus produces a protease that degrades the
200.000 Dalton cap-binding protein of eukaryotic
ribosomes, thereby blocking the translation of cellular
mRNA.
Orthomyxoviridae
INFLUENZA VIRUS
ORTHOMYXOVIRUSES
HA - hemagglutinin
NA - neuraminidase
helical nucleocapsid (RNA plus
NP protein)
lipid bilayer membrane
polymerase complex
M1 protein
type A, B, C : NP, M1 protein
sub-types: HA or NA protein

Among the RNA viruses, influenza is very special in
that all of its RNA synthesis take place in the nucleus.

Short capped primers are generated from host cell
RNAs by an influenza virus-encoded cap-dependent
endonuclease.

Influenza virus mRNAs undergo splicing in the
nucleus.
Classification

The family contains two genera: Influenza A and B
viruses, and influenza C virus.

Genera are distinguished on the basis of antigenicity
of nucleoprotein (NP) and matrix (M) proteins.

Influenza A viruses are divided into subtypes based on
the antigenicity of HA and NA glycoproteins.
Distinguishing Characteristics

Influenza A viruses naturally infect humans, several other
mammalian species and a wide variety of avian species
whereas Influenza B and C are human pathogens.

The surface glycoproteins of influenza A virus exhibit much
greater amino acid sequence variability than their counterparts
in influenza B virus. Influenza C has a single multifunctional
glycoprotein.

Influenza A and B viruses contain 8 RNA segments, whereas
influenza C contains 7 segments
Virion Structure

Influenza A and B are morphologically indistinguishable but
there are morphological features that distinguish influenza A
and B viruses from influenza C virus.

The RNPs consist of four protein species and the RNA
genome which occurs in eight separate segments containing
10 genes.

The segments are complexed with nucleoprotein to from a
nucleocapsid with helical symmetry.

Genomic segments range from 890 to 2340 bases.
Replication

Unlike replication of other RNA viruses, replication of
orthomyxovirus depends on the presence of active
host cell DNA synthesis.

Replication in the nucleus is necessary because the
virus lacks capping and methylating enzymes activities.

The virus scavenges cap sequences from the nascent
mRNA generated in the nucleus and attaches it to its
own mRNA.
Genome Organization








RNA segment 1 codes for PB2
RNA segment 2 for codes PB1
RNA segment 3 for codes PA
RNA segment 4 for codes HA
RNA segment 5 for codes NP
RNA segment 6 for codes NA
RNA segment 7 for codes M1 and M2
RNA segment 8 for codes NS1, and NS2
Virion Proteins







PB2, PB1 (Basic), PA (acidic)
NP
HA (16 subtypes)
HEF (Influenza C)
NA (9 subtypes)
M1 and M2
NS1 and NS2
Genetics

RNA segment Reassortment (Antigenic shifts)

RNA Mutations (antigenic drifts)

RNA Recombination

Nomenclature
- A/Swine / lowa/15/30/H1N1
- A/ Bangkok/1/79/H3N2
Paramyxoviridae

Enveloped viruses with a negative single
stranded nonsegmented RNA genome.

They have special relationships with
orthomyxoviruses and rhabdoviruses.

They encode and package their own RNA
transcriptase.

They range in size from 150 – 350 nm
Classification

The Paramyxovirinae
Paramyxovirus: Parainfluenza virus types
1 and 3.
Rubulavirus:
mumps virus,
parainfluenza virus types
2, 4a and 4b.
Morbillivirus: measles virus.

The pneumovirinae
Pneumovirus: Respiratory Syncytial
Virus (RSV)

Nucleoprotein (NP)

Phosphoprotein (P
Protein)

large (L) protein

The matrix (M) protein

Envelope Glycoproteins
- Attachment protein
(HN,H, G)
- Fusion Protein (F)

Other proteins
- SH, C, V, W, I, D, NS1 and
NS2.
Parainfluenza Virus





ssRNA virus
enveloped, pleomorphic
morphology
5 serotypes: 1, 2, 3, 4a
and 4b
No common group
antigen
Closely related to
Mumps virus
Parainfluenza Viruses

Important respiratory tract pathogens of infants and
children causing 30-40% of such infections.

They are second only to RSV as a cause of serious
respiratory tract disease in infants and children (HPIV
1-3).

Pleomorphic, 150-200 nm in diameter, enveloped with
HN and F envelope glycproteins.
Respiratory Syncytial Virus (RSV)

RSV is the most important cause of viral lower
respiratory tract disease in infants and children
worlwide.

RSV infection is an important agent of disease in
immunosuppressed adults and the elderly.

Ranges in diameter from 150-300 nm

Research on RSV has been impeded because
- It grows poorly in tissue culture and most
exprerimental animals
- it does not shut off host macromolecular
synthesis
- The virion is unstable.

RSV survives on surfaces for up to 6 hours and on
gloves for less than 2 hours.

The virus loses viability with freeze-thaw cycles, in
acidic conditions and with treatment by disinfectants.

It encodes a larger number of mRNAs than do the
paramyxoviruses (10 compared with 6 or 7)

Additional genes are; SH, M2, NS1, and NS2

Although six proteins appear to correspond (N, P, M,
G/H/HN; F and L) only F and L exhibit
unambiguous sequence relatedness between the two
subfamilies.

Variation in the G glycoprotein (RSV-A and B)

RSV utilizes ICAM-1 as its receptor.
Mumps Virus

“to mump” means to grimace or grin.

The virion is 120 – 200 nm in diameter

Contains in addition to the six major proteins; V
(viral) protein and S (soluble) protein.

One serotype.

MEASLES (RUBEOLA)
Measles is a relatively new disease of humans.

Probably it has evolved from an animal morbillivirus
(rinderpest).

It is related to canine distemper virus.

Abu- Becr Al- Razi of 10th century is credited with
distinguishing smallpox from measles.

He referred to measles as “hasbah eruption” in Arabic and
regarded it as a modification of smallpox.

It is highly infectious and almost always produces
clinical disease in those infected.

Virion is similar to other members of the
paramyxoviridae but it lacks the neuraminidase.

Membrane cofactor protein (MCP) or CD46 is the
receptor for the virus.

Measles virus is a stable monotypic virus with some
degree of variability (strains).
Human Metapneumovirus
In 2001, van den Hoogen and colleagues reported
that they had isolated a paramyxovirus from 28
young children in the Netherlands identified as a new
member of the metapneumovirus genus by:
- Virological data
- Sequence homology
- Gene constellation
Previously, avian pneumovirus was the sole member
of this recently assigned genus, hence the provisional
name for the newly discovered virus: human
metapneumovirus.
hMPV Features

Negative stranded RNA virus
 Paramyxoviridae family
 Related to avian pneumovirus
and turkey rhinotracheitis
virus
 Causative agent of respiratory
tract disease in humans
 Most children are seropositive
by the age of 5 years
 2 genetic clusters of hMPV
that may represent different
serotypes
Rubella virus

Rubella virus is a member of the togaviridae but
unlike most other togaviruses, rubella virus has no
known invertebrate host, and the only known natural
reservoir for rubella virus is man.

Rubella virus is a spherical, icosahedral, enveloped
particle that measures 60-70 nm in diameter.

It has a +ss RNA genome of about 10.000 nt that is
encased by multiple copies of the capsid protein (C).
Two glycoproteins, E1 and E2, are embedded in the
envelope
Rhabdoviridae

A large number of member viruses that are
serologically unrelated.

Rabies belongs to the genus lyssa virus (rabies in
Greek means mad or frenzy).

It is bullet shaped, enveloped and has a diameter
of 75X180 nm.
Rabies Virus
G, M, L, N, and NS Proteins

The genome is helical and is associated with N
protein.

Virions bud from the endoplasmic reticulum

Replication of rhabdoviruses is followed by cell
death except for rabies virus which is nonlytic
causing no discernable damage
CORONAVIRUSES
HISTORY AND CLASSIFICATION
•
Avian Infectious Bronchitis (IBV)
(Schalk and Hawn, 1931).
•
Recovery of virus in the Laboratory (Beaudette and
Hudson 1937).
•
•
Discovery of human coronaviruses
(Tyrrell and Bynoe, 1965).
Distinctive morphology.
Genome Structure


Human Coronaviruses
Genus Coronavirus
Species HCoV-229E
HCoV-OC43
SARS- CoV
HCoV-NL63
HCoV-HKU1
HCoV-EMC
• Responsible for about 10-20% of
common colds
 re-infection is common
 infections year-round, most
prevalent in fall and spring
 incubation period about 2 to 5 days
VIRION STRUCTURE

There is considerable diversity in both the lengths and
nucleotide sequences of the S1 glycoproteins of
different coronaviruses and even of different strains
of a single coronavirus.

This diversity in S1 probably results from mutation
and recombination between coronaviruses and strong
positive selection in vivo.
Functions of Coronavirus Proteins
Membrane (M) glycoprotein

May determine budding site on intracellular
membranes

Essential for envelope formation

May interact with viral nucleocapsid

May induce alpha interferon
Functions of Coronavirus Proteins
Spike (S) glycoprotein







Binds to specific host cell receptor glycoprotein
May induce fusion of viral envelope with cell
membrane
Induces cell fusion
Binds immunoglobulin at Fc receptor site
Binds to 9-O-acetylated neuraminic acid
Induces neutralizing antibody
Elicits cell-mediated immunity
Coronavirus Stability
 Stable
 In
body fluids (e.g. urine and faeces) for up to 4
days.
 For  21 days at cold temperature (4 and -80ºC).
 At a pH of 6.
 Inactivated
rapidly:
At a mild alkaline pH.
 By disinfectants
 By heating to 56 ºC

REPLICATION OF CORONAVIRUSES

Primary translation.

Transcription of viral RNA.

Replication of viral RNA.

Processing and intracellular transport of viral proteins
(S glycoprotein).

Assembly and release of virions.
Coronavirus Genetics




Variation is due to Mutation and
Recombination
Mutation
High frequency (several point mutations during each
round of replication).
Analysis has shown extensive sequence variability in S
and N genes especially due to deletion mutations.

The most striking example of the biological
importance of deletion mutations is the
emergence of porcine respiratory coronavirus
(PRCV) from transmissible gastroenteritis virus
(TGEV) which causes epizootic enteric infection
of pigs.

In the early 1980s, PRCV emerged in Europe as a
new virus that causes widespread, devastating
epizootics of respiratory disease in pigs.

Recombination
•
High frequency (up to 25%). Mechanism is by
discontinuous transcription and polymerase jumping
(Copy-choice). Example is acquisition of HE
glycoprotein from influenza C

The capacity of coronaviruses both to recombine and
to mutate suggests that diversity will also be a feature
of human coronaviruses and that changes in
pathogenicity may
occur over time (Kenneth
McIntosh, 1996).
Propagation and Assay in Cell Culture

In tissue culture, coronaviruses have a latent period of
about 5 to 7 hours.

Infectivity of virions is fairly stable at pH 6.0, but
rapidly inactivated at mildly alkaline pH.

Coronaviruses can cause either cytocidal or persistent
infections of cells in vitro and in vivo, depending on the
virus strain and the host cell.

None of the human coronaviruses, except HCoV-EMC,
grows well in cell culture without extensive adaptation by
passage.

They have been propagated in human embryonic tracheal
organ culture, in primary or secondary human embryonic
kidney cell lines, in many diploid human fibroblast cell lines,
and in few heteroploid lines.

The most sensitive cell line for isolation of virus from clinical
specimens appears to be the diploid intestinal cell line MA –
177.

The highest titers of both 229E and OC43 are obtained by
growth in human rhabdomyosarcoma cells.
Reoviridae

Respiratory Enteric Orphan viruses (Albert Sabin,
1959)
 Non enveloped with double-layered protein capsid,
containing 10-12 segments of the double-stranded
RNA genomes (double : double).
 Stable over wide PH and temperature ranges and in
air-borne aerosols.
 Human
Pathogens
- Orthoreoviruses
- Rotaviruses
- Orbiviruses
- Coltiviruses

Rotaviruses cause human infantile
gastroenteritis.

They account for approximately 50% of all cases
of diarrhea in children requiring hospitalization
because of dehydration.

In underdeveloped countries, rotaviruses may be
responsible for causing as many as 1 million
deaths each year from uncontrolled viral
diarrhea.
Rotavirus Particle

Proteolytic cleavage of the outer capsid activates the
virus and produces an intermediate/infectious subviral
particle (ISVP).

Rotaviruses resemble enveloped viruses and they
acquire an envelope and loose it during replication.

Reassortment of gene segments can occur and thus
create hybrid viruses.
Gene segment
Protein location
VP1
VP2
VP3
(inner capsid)
(inner capsid)
(inner capsid)
VP4
(outer capsid
spike at vertice)
NS53
VP6
Function
Polymerase
Transcriptase
mRNA capping
Activation by protease to VP5 and
VP8 in ISVP, HA and VAP
RNA binding
(inner capsid)
Groups (A-E) and
Subgroups (I,II)
Major structural protein binds to NS28 at
ER and promote outer capsid assembly
NS34
NS35
VP7
NS28
NS26
(Outer capsid)
Serotypes ( 1-7)
Type-specific antigen major, outer capsid
component
Promote inner capsid binding to ER,
transient envelopment of outer capsid
 Rotaviruses
are found in many different
mammals and birds.
 Rotavirus
is stable at room temperature and
to treatment with detergents, pH extremes
of 3.5 to 10 or even repeated freezing and
thawing.
 Infectivity
is enhanced by proteolytic
enzymes such as trypsin.

Human and animal rotaviruses are divided into

7 serotypes on the basis of antigenicity of VP7 and
VP4

5 groups on the basis of electrophoretic mobility of
VP6 DNA

2 subgroups on the basis of antigenicity of the inner
capsid protein VP6.
Caliciviruses and Related Agents

Caliciviruses are 27-38 nm, non enveloped, icosahedral viruses with a
(+) ss RNA genome (7500 bases).

Norovirus (previously called Norwalk agent) was discovered
by EM in stool from adults during and epidemic of an acute
gastroenteritis in 1968 in Norwalk, Ohio.
Noroviruses

Noroviruses (genus Norovirus, family Caliciviridae) are a group
of related, single-stranded RNA, nonenveloped viruses that
cause acute gastroenteritis in humans.

Norovirus was recently approved as the official genus name
for the group of viruses provisionally described as “Norwalklike viruses” (NLV).

Currently, there are at least five norovirus genogroups (GI,
GII, GIII, GIV and GV), which in turn are divided into at
least 31 genetic clusters.

Astroviruses have a five or six-pointed star shape of 28-30
nm in diameter with an icosahedral symmetry.
The Retroviridae
Genome
- 2 identical molecules of ss RNA
- Gene order is invariably; gag- pro/ pol - env.
- other genes are present in some viruses.


Mode of Replication
Classification
- 6 genera, and human pathogens belong to 2 genera;
HTLV-BLV and Lentiviruses.


HTLV-1 (1981), HTLV-2 (1982)
Human Immunodeficiency Viruses (HIV)

Pneumocystis carinii and Kaposi’s sarcoma among initial
“ 4 H ” club of AIDS.

Isolation of LAV by Montagnier In April 1983.

A year later, Galo at NIH, isolated HTLV-3.

HTLV-3 and LAV showed 98-99% identity.

LAV-2 and HTLV-4 were isolated In 1986

In 1986, the ICVT renamed the viruses HIV-1 and
HIV-2.

HIV-1 and HIV-2 are lentiviruses (lenti =slow).

HIV-1 and HIV-2 share about 40% of their genome
sequence.

Similarity is remarkable between HIV-1 and SIVcmp,
and between HIV-2 and SIVsmm.

Greatest sequence variation exists in the env. gene

The genome is composed of 2 identical copies of 9.749 kb ss
RNA of positive polarity.

A tRNA molecule is positioned near the 5- end of each
strand, with 10-50 copies of reverse transcriptase. The tRNA
is used as a primer for DNA synthesis.

At each end, there are LTR sequences which contain
promoters, enhancers, and other gene sequences for binding
different cellular transcriptional factors.

Although of positive polarity, HIV genome is not infectious

cleavage
P55 →
P17(MA), P24(CA), P9(NC), and
P7 (?).

P100 → P10 (pro) P51/P66 (RT/ RNAse H),
P32(1N).

P160 → gP120 (SU) and gP 41(TM).
HIV Genome
Other Genes of HIV






tat:
rev:
vif:
vpr:
vpu:
nef:
Positive regulator of transcription
Regulator of viral expression
Affects viral infectivity
Positive regulator of transcription,
augments virion production.
Down regulates CD4
So-called negative-regulation factor. It
augments viral replication and down
regulates CD4
Antigenic Variation

The reverse transcriptase is very error prone and lacks
proof reading which contribute to HIV diversity.

The immune response of the host is unable to
completely curtail viral replication.

Virus gene products may be relatively invisible to the
immune response and the virus may be able to mask
or change its antigenic specificity.

The envelope gene displays frequent mutations.

HIV envelope glycoproteins have two unusual
features.
- They are extensively glycosylated
- They contain hypervariable regions that permit the
virus to present new antigenic configurations to the
host.

HIV can constantly vary its surface antigenic
composition which may allow it to avoid
inactivation.

Such a mechanism hinders the development of
an effective vaccine containing the surface
glycoproteins.

Major sequence differences exist between the
two HIV types; antibodies against the surface
glycoprotein of HIV-1 only partially cross react
with HV-2.
Arboviruses

Epidemiologic classification, but taxonomically
diverse.

More than 400, all of which are RNA viruses
and about 100 of them infect humans.

They establish life-long infections of arthropods
and humans are accidentally infected.

They have strong dependence on climatic
conditions.
Families



Togaviridae: VEE, EEE, WEE
Falviviridae: Dengue, YF, West Nile fever,
JE, St. Louis Encephalitis, Kyasanur
Forest Disease, Omsk H.F.
Bunyaviridae: CE, Rift valley fever, Crimean- Congo
virus, Sand fly fever, Hantaan virus
 Arenaviridae: Junin, Machupo, Sabia, Guanarito,
 Filoviridae:
Lassa
Marburg and Ebola viruses
The Bunyviridae

A supergroup of at least 300 viruses.

Spherical, enveloped, 90-120 nm, with 3 segments of
ambisense ssRNA.

Four genera; Bunyavirus, Phlebovirus, Niarovirus and
Hantavirus.

All, except Hantaviruses, are arthropod borne.
Hantaviruses are rodent borne.
Arenaviruses

Pleomorphic, enveloped viruses of 120 nm in
diameter

2 circles of ambisense ss RNA

A sandy appearance (arenosa = sandy in Greek)
because of the ribosomes in the virion.
Filoviruses
 Filamentous,
enveloped, with 80nm in
diameter.
-
ss RNA of helical symmetry
 They
vary in length from 800 to 14.000nm.
Hepatitis Viruses

A large number of viruses can cause hepatitis (EBV,
CMV, VZV, HSV, YF, Lassa virus … etc).

There are other viruses, however, that only cause
hepatitis.

At least six viruses, A through E and a newly
discovered virus GB, are considered hepatitis viruses.

Hepatitis viruses differ greatly in their taxonomy,
structure, mode of replication and mode of
transmission as well as in the course of the disease
they cause.
Hepatitis A Virus

It is the cause of infectious hepatitis, a term that was
coined in 1912 to describe the epidemic form of the
disease.

The virus was first isolated in 1973 by Feinstone et al
using IEM.

Previously classified as enterovirus 72, HAV has
been put in a separate genus; hepatovirus
Differences Between HAV and Enteroviruses

HAV nucleotide and amino acid sequences are dissimilar to
enteroviruses.

HAV is difficult to grow in cell culture and it usually
replicates very slowly causing no CPE (nonlytic).

HAV is stable at a PH of 1

HAV has only one serotype and one neutralization site is
dominant.

Enterovirus -specific monoclonal antibody does not react
with HAV.
Virus Structure
 HAV
is a naked 27-32 nm icosahedral
virus with a ssRNA genome of positive
polarity.
 Its
genome is 7.5 kb in length,
polyadenylated at the 3- end and carries a
protein (VPg) at the 5- end.
Hepatitis A Virus
Hepatitis B Virus
Hepatitis B Virus
Hepatitis B virus is unusual among animal viruses in
that:
o Infected cells produce multiple types of virus-related
particles.
- 42 nm double-shelled particles (Dane particles)
- 20 nm spheres, usually present in 104-106 fold excess
over Dane particles.
- Smaller quantities of filaments of 20 nm diameter and
variable length.

Hepatitis B Virus
o
o
o
The genome is partially double stranded.
It replicates utilizing an RNA intermediate
and has a reverse transcriptase.
It is unusually stable for an enveloped virus.
The Dane particle is the only infectious form.
 Envelope : HBsAg.
 Core
: HBcAg.

With less than 3200 nucleotides, HBV has the smallest
genome of any human virus.

HBV genome is relaxed circular partially duplex DNA
species, whose circularity is maintained by 5- cohesive
ends.

The genome has a coding organization that is highly
compact and over half of the sequence is translated in
more than one frame.

HBV uses its genome economically by encoding
different proteins within the same region of DNA in
different reading frames.

About half of the genome’s nucleotides are used to
code simultaneously for different proteins, and all
code for at least one protein.

The regulatory signals overlap with coding sequences
and are not separate regions.

Four ORFs are present in the DNA.
- ORF P: encodes the viral polymerase and the terminal
protein found on minus strand DNA.
- ORF C: encodes the core protein (C antigen)
- ORF S/pre-S: encodes the HBsAg.
- ORF X: encodes a protein that enhances the
expression of heterologous and homologous
genes.

The reading frame for HBsAg was shown to have two
in-frame initiation codons which result in three
products L, M, and S.

The bulk of HBsAg is the S protein, M protein
accounts for 5-15% of it and L for 1-2%.

The S (gp 27, 24-27 kD) glycoprotein is completely
contained in the M (gp36; 33-36 kD) glycoprotein
which is contained in the L (gp42; 39-42 kD)
glycoprotein.

The coding organization of the core protein is similar,
two in-frame AUGs were found in the core ORF.

Initiation at the upstream AUG gives rise to a Crelated protein that is secreted from infected cells into
circulation (HBeAg).

Three major mRNAs are produced
1) 2100 b mRNA: HBsAg (S, M)
2) 2400 b mRNA: HBsAg (L)
3) 3500b mRNA: HBcAg, HBeAg, polymerase and a protein
primer for DNA replication and it acts as a
template for genome replication

In addition to a minor mRNA (700 b) which codes
for X protein (a transactivator of transcription and a
protein kinase).

HBsAg contains the group specific antigenic
determinant termed a and type specific determinants
termed d or y and w or r

Type specific determinants behave like mutual alleles.
Combinations result in four possible antigenic
subtypes (adw, adr, ayw, ayr).
HBV Variation

Eight genetic groups (A-H Genotypes) and a possible 9th type (I?)
and Within genotypes 24 subtypes have been described which
differ by 4-8% of the genome.

HBV Antibody Escape Mutants
Substitution of arginine for glycine at aa 145.
Immunity to vaccine does not neutralize mutant.
Failure to detect HBsAg in donated blood




HBV Precore Mutants
Detected in patients with severe chronic liver disease and who may have
failed to respond to interferon therapy (increased pathogenicity)

Polymerase Variants ( drug resistance)

Replication
 Virus
particles acquire an envelope in the
endoplasmic reticulum or proximal Golgi
 The
surface antigen is glycosylated in the
Golgi apparatus.
 Virions
are then secreted via the
constitutive pathway of vesicular transport.
Hepatitis D virus (Delta virus)

HDV is not a true virus but a defective virus or a
natural satellite of HBV

Delta antigen is present in two forms, small (short)
with 24 kd and large (long) with 27kd.

The short form which is more abundant is required
for RNA replication whereas the long form suppresses
viral RNA replication and is required for packaging of
the HDV genome by HBsAg.

HDV replicates only in HBV-infected cells and direct
pathologic changes are limited to the liver, the only
organ in which HDV has been shown to replicate.

HDV itself seems to be cytopathic and HDV antigen
(delta) may be directly cytotoxic.
Hepatitis C virus

Was discovered in 1989 (post transfusion hepatitis).

Flaviviridae, genus Hepacivirus (HCV and GBV-C).

Enveloped, 55-65 nm.

Six genotypes with several subtypes for each genotype

+ ss RNA genome
- 9.4 kb
- over 98% contains protein coding sequence.
- a single large ORF
Hepatitis C Virus
capsid
Envelope protein
c22
protease/ helicase
c 33
RNA dependent RNA polymerase
c-100
5’
3
’
core
E1
E2
hypervariable
region
NS2
NS3
NS4
NS5

The genome codes for nine proteins; 3 structural and
6 nonstructural.

The structural proteins are; core protein (P22),
E1(gp76), and E2 (gp35) which are envelope
glycoproteins.

The nonstructural proteins are; NS2, NS3, NS4A,
NS4B, NS5A, and NS5B.

HCV has circumvented the cap requirement by
evolving an Internal Ribosome Entry Site (IRES) at its
5- end.
Hepatitis E Virus

Recognized as a distinct disease in 1980.

Virion is 32-34 nm in diameter, nonenveloped, with
an icosahedral symmetry.

Although it was originally classified in the Caliciviridae
family, the virus has since been classified into the
genus Hepevirus, but was not assigned to a viral
family.

The genome is approximately 7200 bases in length, is
a polyadenylated single-strand RNA molecule that
contains three discontinuous and partially
overlapping ORFs

ORF1 encode methyltransferase, protease, helicase,
and replicase

ORF2 encode the capsid protein

ORF3 encodes a protein of undefined function.

An in vitro culture system is not yet available.