Transcript Viral Replication - BMC Dentists 2011

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Basic Virology: Introduction
• Viruses are not cells
• They are not capable of independent replication
• They synthesize neither their own energy nor their
own proteins
• They are too small to be seen in light microscope.
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Basic Virology: Introduction
Comparison of Viruses and Cells
Property
Type of nucleic acid
Proteins
Viruses
Cells
DNA or RNA but not both
DNA and RNA
Few
Many
Lipoprotein membrane Envelopeviruses present
Cell membrane present in all
in some
cells
Ribosomes
Absent
Present
Mitochondria
Absent
Present in eukaryotic cells
None or few
Many
No
Yes
Enzymes
Multiplication by binary
fission or mitosis
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Virus Size & Structure
• Viruses range in size from (~20 nm to ~300 nm) .
• Most viruses appear as spheres or rods in the
electron microscope.
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Virus Size & Structure
Capsid
• All viruses have protein coat called capsid that
covers genome.
• Capsid is composed of repeating subunits called
capsomers
virus symmetric appearance used
for classification.
• Some viruses, capsid is covered with lipoprotein
envelope.
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Virus Size & Structure
Nucleocapsid
• It composed of nucleic acid genome & capsid
proteins.
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Virus Size & Structure
Nucleocapsid
• Viral nucleocapsids have :
-Spherical (icosahedral) symmetry (enveloped or naked)
-Helical symmetry (enveloped).
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Viral Nucleic Acids
• Viruses contain either DNA or RNA, but not both.
• These DNA & RNA genomes can be either singlestranded or double-stranded.
• Some RNA viruses (e.g. influenza
rotavirus), have segmented genome.
virus
&
• All viruses have one copy of their genome (except
retroviruses, which have two copies).
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Viral Proteins
Viral surface proteins
• It mediate attachment to host cell receptors
(determines host & organ specificity of virus).
• Surface proteins are targets of antibody which
"neutralizes" (inhibits) viral replication.
• Some viruses produce antigenic variants of their
surface proteins
evade host defenses.
Internal proteins :
They are DNA or RNA polymerases.
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Viral Proteins
Matrix protein
It mediates interaction between viral nucleocapsid
proteins & envelope proteins.
• Antibody against one antigenic variant (serotype) will not neutralize different serotype.
• Some viruses have one serotype; others have multiple serotypes.
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Viral Envelope
• Envelope consists of membrane that contains lipid
derived from host cell & proteins encoded by virus.
• Envelope is acquired as virus exits from cell in
process called budding.
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Viral Envelope
• Enveloped viruses :
-Easily inactivated
-transmitted by direct contact
via blood & body fluids.
• Naked viruses
- Survive
longer
in
environment
- Transmitted
by
indirect
means e.g. fecal-oral route
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Viral Growth Curve
•One virion infects cell & hundreds of progeny
virions are produced within hours.
•This is remarkable amplification & explains rapid
spread of virus from cell to cell.
The eclipse period
It is time when no virus particles are detected within
infected cell & occurs soon after cell is infected.
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Viral Replication
The steps in viral replication are as follows:
1. Attachment
2. Penetration and uncoating.
3. Transcription & Translation
4. Assembly
5. Release
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Viral Replication
1-Attachment
• Virus particles can only infect cells possessing
surface “receptors” specific to particular virus
species.
• Virus attached to cell with:
-Capsid (Naked viruses)
-Envelope proteins
(enveloped viruses)
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Viral Replication
2-Penetration
• Viruses penetrate into cell by means of pinocytosis
(viropexis).
• In enveloped viruses, envelope fuse with cell
membrane, releasing virus into cytoplasm.
Adsorption of such an enveloped virus to two cells at the same time may result in
cell fusion
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Viral Replication
3- Uncoating
• It involves release of nucleic acid from capsid
• It is activated by cellular enzymes & contribution
from cell membranes (except smallpox virus)
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Viral Replication
4- Replication of nucleic acid
DNA viruses
• All
DNA viruses
genome
is
double-stranded
(except parvoviruses, single-stranded)
• They use host cell RNA polymerase to synthesize
viral mRNA.
• All DNA viruses replicate in nucleus (except
poxviruses in cytoplasm).
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Viral Replication
4- Replication of nucleic acid
RNA viruses
•Genome of all RNA viruses is single-stranded
(Except reoviruses, e.g., rotavirus double-stranded).
• All RNA viruses replicate in cytoplasm, except
retroviruses, influenza virus, & hepatitis D virus,
which require intranuclear step in their replication.
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Viral Replication
•Viral proteins:
• Early proteins are typically enzymes used in
synthesis
of
viral
components,
whereas
late
proteins are typically structural proteins of progeny
viruses.
• poliovirus & retroviruses, translate their mRNA into
precursor polyproteins, which must be cleaved by
proteases to produce functional proteins.
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Viral Replication
Assembly and release:
• All enveloped viruses acquire their envelope by
budding through the external cell membrane as
they exit the cell,
• herpesviruses, acquire their envelope by budding
through nuclear membrane.
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Viral Replication
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Viral Replication
Lysogeny
• It
is the process by which viral DNA becomes
integrated into host cell DNA, replication stops, and
no progeny virus is made.
• Later, if DNA is damaged by, for example, UV light,
viral DNA is excised from the host cell DNA and
progeny viruses are made.
• The integrated viral DNA is called a prophage.
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Viral Replication
Lysogeny
Bacterial cells carrying a prophage can acquire new
traits, such as the ability to produce exotoxins such
as diphtheria toxin.
Transduction is the process by which viruses carry
genes from one cell to another.
Lysogenic conversion is the term used to indicate
that the cell has acquired a new trait as a result of
the integrated prophage.
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Viral Replication
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Genetics
• Mutations in the viral genome can produce
antigenic variants and drug-resistant variants.
• Mutations can also produce attenuated (weakened)
variants that cannot cause disease but retain their
antigenicity and are useful in vaccines.
• Temperature-sensitive mutants can replicate at a
low (permissive) temperature but not at a high
(restrictive) temperature.
• Temperature-sensitive mutants of influenza virus
are used in vaccines against this disease.
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Genetics
Reassortment (exchange) of segments of the
genome RNA of influenza virus is important in the
pathogenesis of the worldwide epidemics caused by
this virus.
•Complementation occurs when one virus produces
a protein that can be used by another virus.
•A medically important example is hepatitis D virus
that uses the surface antigen of hepatitis B virus as
its outer coat protein.
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Genetics
•Phenotypic mixing occurs when two different
viruses infect the same cell and progeny viruses
contain proteins of both parental viruses.
• This can endow the progeny viruses with the ability
to infect cells of species that ordinarily parental
virus could not.
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Pathogenesis
The Infected Patient
• Viral infection in the person typically has four
stages:
- incubation period,
-prodromal period
- specific-illness period,
- recovery period.
• The main portals of entry are the respiratory,
gastrointestinal, and genital tracts, but through the
skin, across the placenta, and via blood are
important as well.
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Pathogenesis
The Infected Patient
• Transmission from mother to offspring is called
vertical transmission; all other modes of
transmission, e.g., fecal–oral, respiratory aerosol,
and insect bite, are horizontal transmission.
• Transmission can be from human to human or from
animal to human.
• Most serious viral infection are systemic, i.e., the
virus travels from the portal of entry via the blood
to various organs.
• some are localized to portal of entry as common
cold & involves only upper respiratory tract.
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Pathogenesis
Pathogenesis
•The symptoms of viral diseases are usually caused
by death of the infected cells and a consequent loss
of function.
• For example, poliovirus kills neurons, resulting in
paralysis.
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Pathogenesis
Immunopathogenesis
• It is the process by which the symptoms of viral
diseases are caused by the immune system rather
than by the killing of cells directly by the virus.
• One type of immunopathogenesis is the killing of
virus-infected cell by the attack of cytotoxic T cells
that recognize viral antigens on the cell surface
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Pathogenesis
• Damage to the liver caused by hepatitis viruses
occurs by this mechanism.
• Another
is
the
formation
of
virus–antibody
complexes that are deposited in tissues.
• Arthritis associated with parvovirus B19 or rubella
virus infection occurs by this mechanism.
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Pathogenesis
• Viruses can evade host defenses by producing
multiple antigens, thereby avoiding inactivation
by antibodies.
• and by reducing the synthesis of class I MHC
proteins, Thereby decreasing the ability of a cell
to present viral antigens and blunting the ability
of cytotoxic T cells to kill the virus-infected cells.
• Viruses produce receptors for immune mediators,
as IL-1 and TNF, thereby preventing the ability of
these mediators to activate antiviral processes
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Persistent Viral Infections
Carrier state refers to people who produce virus for
long periods of time and can serve as a source of
infection for others.
The carrier state that is associated with hepatitis C
virus infection is a medically important example.
Latent infections are those infections that are not
producing virus at the present time but can be
reactivated at a subsequent time.
The latent infections that are associated with herpes
simplex virus infection are a important example.
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Persistent Viral Infections
•Slow virus infections refer to those diseases with a
long incubation period, often measured in years.
•Some,
such
as
progressive
multifocal
leukoencephalopathy, are caused by viruses,
whereas others, such as Creutzfeldt-Jakob disease,
are caused by prions.
•The brain is often the main site of these diseases.
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Host Defenses
• Host defenses against viruses fall into two major
categories:
(1) Nonspecific, of which the most important are
interferons and natural killer cells
(2) Specific, including both humoral and cellmediated immunity.
• Interferons are an early, first-line defense,
whereas humoral immunity and cell-mediated
immunity are effective only later because it takes
several days to induce the humoral and cellmediated arms of the immune response
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Nonspecific Defenses
Interferons
• Interferons inhibit virus replication by blocking
production of viral proteins, by degrading viral
mRNA.
• They induce the synthesis of a ribonuclease that
specifically cleaves viral mRNA but not cell mRNA.
• Double-stranded RNA viruses are the most potent
inducers of interferons.
• Many viruses induce interferons, and many viruses
are inhibited by interferons, i.e., neither the
induction of interferons nor its action is specific.
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Nonspecific Defenses
Interferons
•Interferons act by binding to a receptor on the cell
surface that signals the cell to synthesize the
ribonuclease and the other antiviral proteins.
•Interferons do not enter the cell and have no effect
on extracellular viruses.
•Alpha & beta interferons have a stronger antiviral
action than gamma interferon.
•Gamma interferon activates macrophages.
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Nonspecific Defenses
• Natural killer (NK) cells are lymphocytes that
destroy cells infected by many different viruses,
i.e., they are nonspecific.
• NK cells do not have an antigen receptor on their
surface.
• NK cells recognize and destroy cells that do not
display class I MHC proteins on the surface.
• They kill cells
granzymes.
by
secreting
perforins
and
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Nonspecific Defenses
• Phagocytosis by macrophages and the clearance of
mucus by the cilia of the respiratory tract are also
important defenses.
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Nonspecific Defenses
• Increased corticosteroid levels suppress various
host defenses and predispose to severe viral
infections as disseminated herpesvirus infections.
• Malnutrition
predisposes
to
severe
measles
infections in developing countries.
• The very young and the very old have more severe
viral infections.
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Specific Defenses
Active immunity to viral infection is mediated by
both antibodies and cytotoxic T cells.
It can be elicited either by exposure to the virus or
by immunization with a viral vaccine.
Passive immunity consists of antibodies preformed
in another person or animal.
The duration of active immunity is much longer than
(years) that of passive immunity (weeks to a few
months).
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Specific Defenses
Passive immunity is effective immediately, whereas
it takes active immunity 7 to 10 days in the primary
response (or 3–5 days in the secondary response) to
stimulate detectable amounts of antibody.
Herd immunity is the protection of an individual that
results from immunity in many other members of the
population (the herd) that interrupts transmission of
the virus to the individual.
Herd immunity can be achieved either by
immunization or by natural infection of a sufficiently
high percentage of the population.
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Laboratory Diagnosis
Identification in Cell Culture
• The presence of a virus in a patient's specimen
can be detected by seeing a "cytopathic effect"
(CPE) in cell culture.
• CPE is not specific, i.e., many viruses cause it.
• A specific identification of the virus usually
involves an antibody-based test
as fluorescent
antibody, complement fixation, or ELISA.
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Laboratory Diagnosis
Microscopic Identification
Inclusion bodies
•They are formed by aggregates of many virus
particles, can be seen in either the nucleus or
cytoplasm of infected cells.
•They are not specific.
•Two important examples are the nuclear inclusions
formed by certain herpesviruses and the
cytoplasmic inclusions formed by rabies virus
(Negri bodies).
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Laboratory Diagnosis
Multinucleated giant cells
• They are formed by several viruses, notably certain
herpesviruses, respiratory syncytial virus &
measles virus.
• Fluorescent antibody staining of cells obtained
from the patient or of cells infected in culture can
provide a rapid, specific diagnosis.
• Electron microscopy is not often used in clinical
diagnosis but is useful in the diagnosis of certain
viruses, as Ebola virus, that have a characteristic
appearance and are dangerous to grow in culture.
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Laboratory Diagnosis
Serologic Procedures
• The presence of IgM can be used to diagnose
current infection.
• The presence of IgG cannot be used to diagnose
current infection because the antibody may be
due to an infection in the past.
• acute and convalescent serum sample should be
analyzed.
• An antibody titer that is fourfold or greater in the
convalescent serum sample compared to the
acute sample can be used to make a diagnosis.
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Laboratory Diagnosis
Detection of Viral Antigens & Nucleic Acids
• The presence of viral proteins, such as p24 of HIV
and hepatitis B surface antigen, is commonly
used in diagnosis.
• The presence of viral DNA or RNA is increasingly
becoming the "gold standard" in viral diagnosis.
• Labeled probes are highly specific and rapidly.
• Small amounts of viral nucleic acids can be
amplified using reverse transcriptase to produce
amounts detectable by the probes e.g."viral load"
assay of HIV RNA.