Intro to virology Flashcards
Virus
genetic elements that cannot replicate independently of a living (host) cell;
however viruses posses their own genetic information.
Viruses rely on the host cell for
Energy, Metabolic Intermediates and Protein synthesis
Virus particle (Virion)
extracellular form of a virus that enables the virus to
- Exists outside host and facilitates transmission from one host cell to another
- Contains nucleic acid genome surrounded by a protein coat and, in some cases, other layers of material
To multiply viruses must
enter a host in which they can replicate
-process called infection
Viruses are
obligate intracellular parasites
- too small to be seen by a light microscope
- come in many shapes and sizes
Viruses can replicate in a way
that is destructive to the host (agents of disease)
virology
The study of viruses
Viruses can exist in either
Extracellular or Intracellular forms
Extracellular form
Virion is a microscopic particle containing nucleic acid surrounded by a protein coat and sometimes other macromolecule
_______ is metabolically ____ and cannot generate _____ or carryout ______
Virion, inert, energy, biosynthesis
virus _____ moves from the _____ cell to another ____ inside the ______
genome, host, cell, virion
Intracellular form
once in the new cell, the intracellular stage begins and the virus replicate
-New copies of the viral genome are produced and the components of the virus coat are synthesized
oAll cells, Prokaryotes and Eukaryotes contain
double-stranded DNA genomes
viruses can be either
DNA or RNA
-some use both as genomic material at different stages in their replication cycle.
Only one type of what is present in the virion of any particular type of virus?
nucleic acid
Some viral genomes are ____, but most are _____
circular, linear
Viral genomes vary almost a
thousand fold in size from
smallest to largest
circovirus
tiny
-1.8kb single-stranded genome
mega virus
1.25 Mb double-stranded genome
RNA genomes, whether single or double stranded, are typically ______ than DNA viruses
smaller
coronavirus
32kb
MS2,a bacteriophage
3.5kb
Viral genomes are
very small
-encode proteins whose functions viruses can not usurp from their hosts
virus redirects
host metabolic functions to support virus replication and the assembly of new virions
-New viral particles are released, and the process can repeat itself
Viruses can be classified on the basis of the _____ they infect as well as by
their ______
hosts, genomes
Most viruses are smaller than
prokaryotic cells
-range from 0.02
to 0.3 μm.
Viral structure
quite diverse
-varies in shape and chemical composition
Capsid (the protein coat)
surrounds the genome of a virus particle is composed of a number of protein molecules arranged in a precise and highly repetitive patterns around the nucleid acid
Capsomere:
subunit of the capsid (smallest morphological unit visible with an electron microscope)
Nucleocapsid
complete complex of nucleic acid and protein packaged in the virion.
Enveloped virus
virus that contains additional
layers around the nucleocapsid
-envelopes typically are derived from portions of the host cell membranes (phospholipids and proteins) but include some Viral glycoproteins.
-Functionally viral envelopes are used to help viruses enter host cells.
-viral envelope then fuses with the host’s membrane, allowing the capsid and viral genome to enter and infect the host
Glycoproteins
on the surface of the envelope serve to identify and bind to receptor sites on the host’s membrane
characteristic structure of the virus is determined by
of the capsomeres
of which it is constructed
Helical symmetry
rod-shaped viruses
-Length of virus determined by length of nucleic acid
-Width of virus determined by size and packaging of
protein subunits
Icosahedral symmetry
spherical viruses
-Most efficient arrangement of subunits in a closed
shell
-uses the smallest number of capsomeres to build the shell
-symmetric structure containing 20 triangular faces and 12 vertices and is roughly spherical in shape
Complex Viruses
Virions composed of several parts, each with separate shapes and symmetries
- Bacterial viruses contain complicated structures
- Icosahedral heads and helical tails
In some bacterial viruses, such as ________ of E.
coli, the tail itself has a complex structure with about___ different _____
Bacteriophage T4, 20, proteins
The complete tail is formed as a
sub-assembly
-tail is added to the DNA-containing head
In general, ______ do not carry out metabolic processes and thus a virus is metabolically ____ outside a host cell
virions, inert
True or False: some virions contain enzymes critical to infection
True
Lysozyme
Makes holes in bacterial cell walls
- allows the virus to inject its nucleic acid into the cytoplasm of the host.
- Lyses bacterial cell releasing the new virions
Nucleic acid polymerases
posses an RNA-dependent
DNA polymerase (reverse transcriptase)
-transcribes the viral RNA to form a DNA intermediate
-retrovirus
Neuraminidases (surface proteins)
Enzymes that cleave glycosidic bonds
-Allows liberation of viruses from cell
Viruses replicate only in
certain types of cells or in whole organisms
Which viruses are the easiest to grow in the laboratory?
Bacterial
-for the study of bacterial viruses, pure cultures are used either in liquid or agar media
What viruses can be cultivated in tissue or cell cultures?
Animal viruses (and some plant viruses) -Plant viruses typically are most difficult because study often requires growth of whole plant
How are cell cultures obtained?
by aseptically removing pieces of tissue and dissociating the
cells by enzymatic treatment:
Primary Cell Cultures (PCC)
come directly from the animal and are not sub-cultured
- younger the source animal, the longer the cells will survive in culture
- Typically consist of a mixture of cell types (e.g. muscle and epithelial cells
- Such cells usually do not divide more than a few times, it supports growth of a wide variety of virus
Continuous Cells Lines (CCL):
cells that will reproduce for an extended number of generations
- early continuous cell lines used malignant cells because of their capacity for rapid growth
- immortal cell lines grow in the laboratory without aging,divide rapidly and repeatedly, and have simpler nutritional needs.
Virus Infectious Unit
the smallest unit that causes a detectable effect when
added to a susceptible host.
Titer:
number of infectious units per volume of fluid.
zone of lysis
clear area on the layer of growing host cells
- assumed that each plaque originated from the replication of a single virion
- When a virion initiates an infection on a layer of host cells growing on a flat surface
Plaque assay
analogous to the bacterial colon
-one way to measure virus infectivity
Plaques
clear zones that develop on lawns of host cells
- Lawn can be bacterial or tissue culture
- Each plaque results from infection by a single virus particle
Quantification of Bacterial Virus by Plaque Assay
Using the Agar Overlay Technique
dilution of a suspension containing the virus is mixed in a small amount of melted agar
with the sensitive host bacteria
-mixture is poured on the surface of an agar plate of the
appropriate medium
-host bacteria, which has been spread uniformly throughout the top
agar layer, begin to grow, and after overnight incubation form a lawn of confluent growth
-Virion-infected cells are lysed, forming plaques in the lawn
-size of the plaque depends on the virus, the host and conditions of culture.
Plaque Assay can be seen with
optical microscope or visually (pouring off the overlay medium and adding a crystal violet solution for 15 minutes until it has colored the cytoplasm, gently removing the excess with water will show uncolored the location of the dead cell
Focus Forming Assay (FFA)
variation of the plaque assay, but instead of relying on cell lysis in order to detect plaque formation, the FFA employs immunostaining techniques
-particularly useful for quantifying classes of viruses that do not lyse the cell membranes
Efficiency of plating is used in
quantitative virology
The number of PFU (Plaque-Forming Units) is almost always _____ than direct counts by
electron microscopy
lower
Intact Animal Methods
Some viruses do not show recognizable changes in cell cultures yet cause death or
disease in whole animals, in such cases quantification can be done only by titration in
infected animals
-General procedure is to carry out serial dilutions of the Virus (10-fold dil)
-Animals are infected with viral dilution, and after a suitable incubation period
General Features of Virus Replication
for a virus to replicate, it must induce a living host cell
to synthesize all the essential components needed to make more virions
-components must then be assembled into new virions that are released from the cell.
Viral Attachment and Penetration
are the first steps in the viral life cycle. We will also
mention the mechanisms by which some bacteria react to penetration by bacteriophage DNA.
Production of Viral Nucleic Acid and Protein
once a host has been infected, new copies of
the viral genome must be made and virus specific proteins must be synthesized in order for the
virus to replicate.
Phases of Viral Replicat
1) Attachment (adsorption) of the virus to a susceptible host cell
2) Penetration (entry, injection) of the virion or its nucleic acid
3) Synthesis of virus nucleic acid and protein by cell metabolism as redirected by virus
4) Assembly of capsids (and membrane components in enveloped viruses) and packaging of viral genomes into new virions (maturation)
5) Release of mature virions from host cell
Virus replication typically characterized by a one-step
growth curve:
Latent period: eclipse + maturation
- Burst size: number of virions released, from few to few
thousands
-The duration of the virus replication cycle varies from 20-60 min (bacterial viruses) to 8-40 hs (in most animal viruses).
Following Attachment (adsorption)
infectivity of the virus particles disappears (Eclipse)
-due to the uncoating of the virus particles
-During the Latent period, viral Nucleic acid replicates and
protein synthesis occurs
Maturation occurs when
virus Nucleic acid and proteins are Assembled into mature virus particles
-virions are released, with or without cell lysis
Attachment of virion to host cell is highly specific
Requires complementary receptors on the surface of a susceptible host and its infecting virus
-Receptors on host cell carry out normal functions for cell
-Receptors include proteins, carbohydrates, glycoproteins, lipids, lipoproteins, or
complexes
The attachment of a virus to its host cell results in
changes to both virus and cell surface that facilitate penetration
Permissive cell:
host cell that allows the complete replication cycle of a virus to occur (e.g. HBV in Hepatocytes).
Bacteriophage T4
virus of E. coli
-one of the most complex penetration mechanisms
-Virions attach to bacterial cells via tail fibers that interact with polysaccharides on E. coli cell
envelope
– Tail fibers retract and tail core makes contact with E. coli cell wall
– Lysozyme-like enzyme forms small pore in peptidoglycan
– Tail sheath contracts and viral DNA passes into cytoplasm
Many eukaryotes possess
mechanisms to diminish viral infections
Both Bacteria and Archaea
possess an antiviral mechanism similar to iRNA known as CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) are a distinctive feature of the genomes of most
Bacteria (40%) and Archaea (90%)
-thought to be involved in resistance to bacteriophages
prokaryotes destroy
double-stranded viral DNA after it has been injected by using
restriction endonucleases enzymes that cleaves foreign DNA at specific sites preventing its replication, restriction
Restriction modification systems
DNA destruction system; only effective against double-stranded DNA viruses
– Restriction enzymes (restriction endonucleases) cleave DNA at specific sequences
– Modification of hosts own DNA at restriction enzyme recognition sites prevents
cleavage of own DNA
Viral mechanisms to evade bacterial restriction systems
– Chemical modification of viral DNA (glycosylation or methylation)
– Production of proteins that inhibit host cell restriction system
David Baltimore, Howard Temin, and Renato Dulbecco discoved
Retroviruses and
Reverse transcriptase
– Shared 1975 Nobel Prize for Physiology or Medicine
Baltimore Classification Scheme:
based on relationship
of viral genome to its mRNA or replication strategy
-Class I are double-stranded (ds) DNA viruses: e.g. majority of bacteriophages.
– Class II are single-stranded (ss) DNA viruses e.g. Parvoviruses B19
– Class III are dsRNA e.g. Reoviruses.
– Class IV and V are ssRNA (+ or -) e.g. SARS, SARS-CoV-2, Zika virus, HAV, HCV
(IV); Ebola virus (V)
– Class VI are retroviruses, e.g. HIV
– Class VII are dsDNA viruses that replicate through an RNA intermediate, e.g. HBV
mRNA is
complementary in base sequence to the template strand of DNA
-mRNA is of the plus (+) configuration, complement is -
Once a host has been infected
new copies of the viral genome must be made and virus-specific proteins synthesized in order for the virus to replicate
- Generation of messenger RNA (mRNA) occurs first.
- Viral genome serves as template for viral mRNA
- In some cases essential transcriptional enzymes are contained in the virion
Positive-strand RNA virus
single-stranded RNA genome with same orientation as
its mRNA
Negative-strand RNA virus
single-stranded RNA genome with orientation complementary to its mRNA.
Retroviruses:
animal viruses responsible for causing certain types of cancers and acquired immunodeficiency syndrome (AIDS).
- ssRNA in their virions but replicate through a dsRNA intermediate (class VI)
- Require reverse transcriptase (RNA-dependent DNA polymerase)
- Class VI and VII viruses (have dsDNA in their virions but replicate through an RNA intermediate
Early proteins
– synthesized soon after infection
– necessary for replication of virus nucleic acid
– typically act catalytically
– synthesized in smaller amounts
Late proteins
- Synthesized later
- Include proteins of virus coat (capsid)
- Typically structural components
- Synthesized in larger amounts
True or False: Bacteriophages arent diverse
False
Best-studied bacteriophages infect
enteric bacteria
Phages
most contain dsDNA genomes, -the most common in nature
-Most are naked (no further layers outside the capside),
but some possess lipid envelopes
-structurally complex, containing heads, tails,
and other components
Virulent mode (lytic):
viruses lyse or kill their host cells after infection
Temperate mode (lysogenic)
viruses replicate their genomes in tandem with host genome and without killing host
T4 infection
early in infection T4 directs the synthesis of its own RNA and also begins to replicate its unique DNA.
About 1min after attachment and penetration of the host by T4 DNA
the synthesis of the host DNA and RNA ceases and transcription of specific phage genes begins
-Translation of viral mRNA begins soon after, and within 4 min of infection the phage
DNA replication has begun.
T4 genome can be divided into three parts
Early, Middle, and Late proteins:
– Early and Middle proteins: enzymes needed for DNA replication and transcription
– Late proteins: head and tail proteins and enzymes required to liberate mature phage particles
True or False: T4 does not encode its own RNA polymerase
True
Following injection of DNA
early and middle mRNA is produced that codes for nucleases, DNA polymerase, new phage specific sigma factors (a π needed only for initiation of RNA synthesis) and other proteins needed for DNA replication.
- bacterial transcription initiation factor that enables specific binding of RNA polymerase to gene promoters.
- Late mRNA codes for structural proteins of the phage virion and for T4 lysozyme, which is needed to lyse the cell and release new phage particles
Temperate viruses:
can undergo a stable genetic relationship within the host.
-can also kill cells through lytic cycle
Lysogeny:
state where most virus genes are not expressed and the virus genome (prophage) is replicated in synchrony with host chromosome without harm.
Lysogen
a bacterium containing a prophage
Under certain stressful conditions ______ viruses may revert to the _____ pathway and begin to produce _____
lysogenic, lytic, virions
The two best-characterized temperate phages are
Lambda and P1.
Bacteriophage lambda
– Infects E. coli, both virulent and the temperate pathways are possible. – The lambda genome consists of linear dsDNA – Complementary, single-stranded regions 12 nucleotides long at the 5¢ terminus of each strand – Upon penetration, DNA ends base-pair, forming the cos site, and the DNA ligates and forms double-stranded circle – When lambda is lysogenic, its DNA integrates into E. coli chromosome at the lambda attachment site (attl)
The alternatives upon infection are:
Replication and release of mature virus (lysis) -Lysogeny, often by integration of the virus DNA into the host DNA -Lysogen can be induced to produce mature virus and lyse
Integration of Lambda DNA into the host
Integration always occurs at specific attachment sites (att sites) on both the host DNA and the phage.
-Some host genes near the attachment site are given: gal operon, galactose utilization; bio operon, biotin synthesis;
moa operon, molybdenum cofactor synthesis
-site-specific enzyme (integrase) is required, and specific pairing of the complementary ends results in integration of phage DNA
Regulation of Lytic vs. Lysogenic events in lambda is controlled by
a complex genetic switch
-Key elements are two repressor proteins:
• cI protein (the lambda repressor): causes repression of lambda lytic events
• Cro repressor: controls activation of lytic events
To establish Lysogeny, what two events must happen?
- The production of late proteins must be prevented
2. A copy of the lambda genome must be integrated into the host chromosome.
If cI is made
it represses the synthesis of all other lambda-encoded proteins and lysogeny is established.
If Cro is made
in high amounts, lambda is committed to the lytic pathway
-Cro indirectly represses the expression of the lambda cII and cIII proteins, which are needed to maintain lysogeny, by inducing synthesis of the cI.
The first known therapeutic use of phages occurred in
1919
-d’Herelle and several hospital interns ingested a phage cocktail to check its safety, then gave it to a 12-year-old boy with severe dysentery
Phage therapy fell out of favor in the U.S. and most of Europe with
the advent of antibiotics.
-Only in regions where antibiotics were not as easily accessed — namely what is now Russia, Poland and
the Republic of Georgia — did phage therapy and commercial production continue
Benefits of phages as therapy
-may help overcome the main drawbacks to today’s antibiotics
resistant crisis.
-Phages are very specific about the bacteria they infect, so the collateral damage to other bacteria or human cells is minimal.
-Antibiotics take years to develop, whereas a phage cocktail can be identified and matched to a patient’s specific bacterial infection and
purified within a matter of days, making personalized phage therapy-ondemand
a potential reality in the future.
Risk of phages as therapy
phage therapy testing to date has largely been observational, or conducted in small, non-randomized trials
-researchers don’t have the full picture of how it works and the potential risks.
-They don’t know the extent of potential short- and long-term side effects of phage therapy. -Decades of anecdotal reports from Russia, Poland and the Republic of Georgia, as well as preclinical studies in animals,
indicate that phage therapy is likely safe for most people.
-Septic shock is the main worry considering phage therapy.
-many types of bacterial cells release endotoxins when broken up by phages, which can lead to an overwhelming immune response and organ failure (also a concern for some currently available antibiotics)
-able to transfer DNA from one bacterium to another, in a naturally and commonly occurring process(transduction)
-Phage manipulation and engineered introduction could theoretically introduce
new virulence factors or toxins to already pathogenic bacteria, or convert non-pathogenic bacteria into pathogens
-issue can be overcome by pre-selecting phages that have been carefully screened for toxins and virulence factors — an effort that can be facilitated by using ever-expanding phage libraries that
several teams are currently developing around the world.
Challenges of phages as therapy
-Health care providers need to know exactly which bacterial
strain is causing the infection.
-They must have several phages that specifically target that
strain readily available, ideally from a large phage library that
can be screened for a suitable phage cocktail that matches
the bacteria.
-most pharmaceutical companies are reluctant to dedicate resources to phage therapy development and
commercialization
-phage therapy is almost
100 years old, making it difficult to patent and generate revenue to justify the initial development costs.
-Lack of regulatory approval for phage therapy is also an issue. –Phage cocktails need to be customized for each patient’s infection and constantly adjusted as the bacteria evolve and develop resistance.
-Regulatory agencies such as the (FDA) currently lack streamlined review and
approval mechanisms to accommodate personalization and flexibility on a large scale.
-Most experts agree that phage therapy will never completely replace antibiotics.
-may be used in combination with antibiotics, or as the last line of defense for patients with infections that have not responded to any other treatments.
-alarming increase in the number of life-threatening multidrug-resistant infections in recent years, the need for investigating the potential role of phage therapy and other alternatives to antibiotics is urgent.
Overview of Animal Viruses
Unlike in prokaryotes, the entire virion enters the animal cell
-Eukaryotic cells contain a nucleus, the site of replication for many animal viruses
- Animal viruses contain all known modes of viral genome replication
– Many more kinds of enveloped animal viruses than enveloped bacterial viruses exist
– As animal viruses leave host cell, they can remove part of host cells lipid bilayer for their
envelope
Persistent infections:
release of virions from host cell does not result
in cell lysis
• Infected cell remains alive and continues to produce virus
Latent infections
delay between infection by the virus and lytic events. The symptoms (the result of lysed cells) reappear sporadically as the virus emerges from latency
Transformation:
conversion of normal cell into a tumor cell
Cell fusion:
some enveloped viruses promotes fusion between multiple animal cells, creating giant cells with several nuclei. Cell fusion allows viruses to avoid exposure to the immune system
Orthomyxovirus: e.g. Flu virus pathogenesis
Influenza, or the flu, is an acute contagious infectious disease of birds and mammals caused by RNA virus family known as
Orthomyxoviridae
-Epidemics occur seasonable with low fatality, more deadly pandemics occurs several times each century
-All influenzas are categorized based on the type of proteins they have on their surface; the Hemagglutinin (HA) and Neuraminidase (NA).
The three genera of Influenza virus that differ by
their antigenic properties:
- Influenza A: infects humans and other mammals, including birds (pandemics)
- Influenza B: infects humans and seals, slow mutating and hence less virulent.
- Influenza C: infects humans and pigs.
Influenza virus three main reasons for death
- Co-infection with another germ, usually a bacteria such as strep.
- Aggravation of pre-existing conditions such as: heart disease and asthma
- ‘Cytokine storm’ characterized by an overwhelming immune system response to infection, typical symptoms: fever, muscle ache
Why do newer viruses tend to kill younger people?
older population may have been exposed to a distant relative of the virus in the past.
How many people did H1N1 flu virus kill in 2009?
282 children
-may have infected 61 million people
Flu vs cold
-both respiratory illnesses but caused by different viruses
-flu virus is a Orthomyxoviruses, which belongs to a family of RNA viruses
-common cold can be caused by Rhinovirus a type of picornavirus and/or Coronavirus), Adenovirus , Human Respiratory Syncytial virus etc.
-flu is worse than the common cold, and symptoms are more common and intense
-
True or False: Colds are usually milder than the flu
True
Hepatitis
inflammation of the liver -can be caused by: oGenetic diseases o Medications o Autoimmune diseases. o Alcohol o Hepatitis virus
six known Hepatitis virus
-Hepatitis type A virus (Picornaviridae)
– Hepatitis type B virus (Hepadnaviridae)
– Hepatitis type C virus (Flaviviridae)
– Hepatitis type D virus (viroid, unclassified)
– Hepatitis type E virus (unclassified)
– Hepatitis type G virus (Flaviviridae)
of people with HIV in the U.S., about how many are coinfected with HCV? HBV?
about 25% are coinfected with HCV and about 10% are coinfected with HBV.
HIV/HCV coinfection more than ______ the risk for liver disease, liver failure, and liver-related death
triples
Hepatitis A virus (HAV)
– Picornavirus
– Positive ssRNA genome, 7.5kb
– One serotype
– Nonenveloped
o Features (HAV):
• Infectious hepatitis
• Children, young adults
• Fecal-oral transmission
• Global distribution:
– Risk of infection is very low in N. America, Europe, Australia
– Asymptomatic seroconversion (the majority).
– Uncommon: Acute hepatitis and fulminant hepatitis (less common than HBV).
– Diagnosis of acute infection is by detection of IgM antibodies against HAV.
• Poor sanitation greatest risk factor.
• It can be prevented with a safe and effective vaccine recommended for all children at
one year of age and for adults who may be at risk
Hepatitis B Virus HBV
– Serum hepatitis
-Hepadnavirus
– dsDNA virus, 3.2 kb
– Enveloped
– Predominant spike protein is hepatitis B surface antigen (HBsAg)
• Recombinant HBsAg is formulated in vaccine
– 7 polypeptides
– Unusual genome replication:
• DNA is copied into RNA transcript
• Some copies of the RNA transcript are reverse transcribed into ssDNA
• The ssDNA is transcribed into dsDNA
– The HBV is spread primarily from blood, semen, or other body fluids of an infected person enters the body of an uninfected person
–There is an effective hepatitis B vaccine available for all age groups
Hepatitis C virus HCV
-Flavivirus
-Positive ssRNA genome, 9.4 kb
-transmission primarily through blood products
– Could not be propagated in vitro until very recently.
– Mother to baby transmission only occurs in about 3% cases (≠HBV)
– Most acute infections are subclinical, most people with HCV do not know they are infected
– most will develop chronic hepatitis, which is associated with cirrhosis and even
liver cancer.
– People born from 1945-1965 (baby boomers) are 5 times more likely to have hepatitis C.
CDC recommends testing for these individuals .
o Transmission of Hep C in the 60’s through the 80’s was highest before screening of blood products in 1992
o Medical equippments or procedures before universal precautions and infectious control procedures were adopted
– About 25,000 people die from HCV infection each year in U. S.
– There is NOT a vaccine to prevent against hepatitis C
Hepatitis D virus HDV
-delta virus
-can only occur in patients already infected with HBV (superinfection) or at the same time co-infection (to provide HBsAg)
– Substantially contributes to Hepatitis B pathogenesis
– Minus-strand RNA, 1.7 kb (small genome)
-Not a virus, but a Defective Virus (satellite or incomplete virus)
-Contaminated blood and blood products.
Hepatitis E virus HEV
-enteric or epidemic virus, -predominantly spread by fecal-oral route, especially
through contaminated water (similar to A)
– Plus-strand RNA, 7.6 kb
– Oral-fecal transmission
– High fatality rate in pregnant women (20%)
Hepatitis G virus HGV
-resembles HCV
-Flavivivirus, Plus-strand RNA, 10 kb
– Transmission through blood products
– Typically induce a chronic hepatitis infection
– Detected by RT-PCR or other RNA method.
Hepatitis Virus Infections in Humans
Targets the liver
-Cause focal necrosis, leading to larger areas of necrosis
– Jaundice arising from excess of the pigment bilirubin and
typically caused by obstruction of the bile duct, by liver disease, or by excessive breakdown of red blood cells.
– If recovery occurs, liver function often returns to normal
• Substantial damage cannot be reversed
– HBV and HCV have been associated with hepatocellular carcinomas
– HBV can cause rash, arthritis, vasculitis and glomerulonephritis
– Fatality Rates
• Hep A: <0.5% (increases after age 40)
• Hep B: 1-2% (chronic in 5-10% of infections)
• Hep C: 0.5-1% (chronic in 70-90% of infections)
Hepatitis diagnoses
– Hep A • Virus detectable in blood, stool, bile, liver (biopsy) • IgM serology (ELISA) – Hep B • IgM, IgG serology • PCR – Hep C • Serology is not useful for discriminating acute or chronic infection • Real-time PCR is assay of choice (viral load: HCV-RNA) – Hep D • ELISA to HD antigen – Hep E • IgM antibodies against HEV.
Hepatitis A Virus-Host Immune Reactions
– Globally, childhood infections are common
– In developed countries Hep A is uncommon
• A large susceptible adult population
• Childhood vaccination is now routine
– Infection results in life-long immunity
Hepatitis B Virus-Host Immune Reactions
– Health care workers at higher risk
• Vaccination is routine
– The immune response to HBV is responsible for both the
viral clearance during acute infection and for disease
pathogenesis.
Hepatitis C Virus-Host Immune Reactions
– Strong and persistence CD8+ and CD4+ T-cell responses are critical in HCV clearance
– There is NO Hepatitis C vaccine available yet.
– No good animal model is available
– Only recently has the virus been propagated in cell culture
– Treatment
• Type I interferon
• Ribavirin (viral nucleoside inhibitor)
Retroviruses
envelope positive-strand ribonucleid acid (RNA) viruses
-roughly spherical in shape, with a diameter of 80 to 120 nm
-envelope contains viral glycoproteins and is acquired by budding from the plasma membrane.
-envelope surrounds a capsid that contains 2 identical copies of RNA
-Contain 10 to 50 copies of the enzyme reverse transcriptase, and replicate through a DNA
intermediate.
-DNA copy of the viral genome is then integrated into the host chromosome to become a cellular gene.
-first viruses shown to cause cancer: Rous sarcoma virus (RSV) and AIDS (HIV)
three subfamilies of human retroviruses are the
-Oncovirinae: HTLV-1, HTLV-2, HTLV-5
-Lentivirinae: HIV-1, HIV-2
-Spumavirinae: which so far has not been associated with
human disease.
Retroviruses are classified by
the disease they cause, tissue tropism and host range, virion
morphology and genetic complexity.
Oncoviruses include
the only retrovirus that can immortalize or transform target cells
-These viruses
are also categorized by the morphology of their core and capsid as type A, B, C, or D as seen on TEM.
Lentiviruses
slow viruses associated with neurologic and immunosuppressive diseases
Spumaviruses,
represented by a foamy virus, cause a distinct cytopathologic effect, which do not seem to cause clinical disease.
Retroviruses have a unique genome that consist of two identical ssRNA molecules of the plus (+) orientation
- gag: encode structural proteins
- pol: encode reverse transcriptase and integrase
- env: encode envelope proteins
overall process of Replication of a Retrovirus can be summarized as
- Entrance into the cell by fusion with the cytoplasmic membrane at site of specific
receptors. - Removal of virion envelope at the membrane, but the genome and virus-specific enzymes remain in the virus core.
- Reverse transcription of one of the two identical
genomic RNA molecules into a ssDNA converted later on to a linear dsDNA molecule, which enter the nucleus. - Integration of retroviral DNA into host genome
- Transcription of retroviral DNA, leading to the formation of viral mRNAs and viral genomic RNA.
- Assembly and packaging of genomic RNA into nucleocapsids in the cytoplasmic.
- Budding of enveloped virions at the cytoplasmic membrane and release from cell
What are “infectious agents” that resemble viruses BUT whose properties differ from this definition, and are thus NOT considered viruses?
- Defective Viruses
- Viroids
- Prions
Defective viruses
viruses that are parasitic on other viruses
- Require other virus (Helper virus) to provide some function
- Hep D virus, delta virus can only occur in patients already infected with HBV, it uses the coat of HBV
- Some rely on intact virus of the same type
Satellite viruses:
defective viruses for which no intact version exists; rely on
unrelated viruses as helpers
-found in both animals and plants
Adeno-Associated Virus (AAV)
non-envelope satellite virus of humans that depends on adenovirus as a helper.
-causes little or no damage to the host
-used as a eukaryotic
cloning vector in gene therapy to carry replacement genes to specific host tissues without causing disease itself.
Viroids
infectious RNA molecules that lack a protein coat (capsid)
- Smallest known pathogens (246–399 bp)
– Cause a number of important plant diseases
– Small, circular, ssRNA molecules
– Do not encode proteins; completely dependent on host-encoded enzymes for its replication.
– NO viroids are known that infect animals or prokaryotes.
Prions:
infectious proteins whose extracellular form contains NO nucleic acids
-Known to cause disease in animals (transmissible spongiform encephalopathies or TSE) such as: scrapie in sheep and bovine spongiform encephalopathy (BSE or “mad cow disease”)
- in humans prions cause: Creutzfeldt-Jakob disease, Kuru and some genetic abnormalities such as: Gerstmann-Straussler-Scheinker syndrome and fatal familial insomnia.
-Host cell contains gene (PrnP) that encodes native form of prion protein that is found in
healthy animals, primarily in neurons, known as PrPc (Prion Protein cellular), a cytoplasmic membrane glycoprotein.
-The pathogenic form of the prion protein is designated PrPSc which is identical in
amino acid sequence but it has a different conformation.
-Prion misfolding results in neurological symptoms of disease (e.g., resistance to
proteases, insolubility, and aggregation)
Infectious prion disease
pathogenic form of prion protein (PrPSc) is transmitted between animals or humans.
Sporadic prion disease
random misfolding of a normal, healthy prion protein in an
uninfected individual.
Inherited prion disease
mutation in prion gene yields a protein that changes more often into disease-causing form
