Viral classification, structure and replication of DNA Viruses: Adenoviridae, Poxviridae Flashcards
What are viral particals called?
Virions
This is considered the mature, infectious virus particle
What is a capsid?
This is a protein shell that encloses and protects the viral nucleic acid.
What is the nucleocapsid?
This is the internal part of the virus.
Consists of the nucleic acid and other associated proteins.
What is the enevlope?
This is the viral membrane
Consists of a lipid bilayer, proteins, and glycoproteins
Originally belonged to the host cell
What are the characteristics of animal viruses?
Small size, 0.03 um to 0.3 um
DNA or RNA is the genetic material
Amount of genetic material varies by virus type / species
Some viruses have segmented genomes and others have a single piece of nucleic acid
Usually only have one type of genetic material, DNA or RNA. Exception is the Mimivirus.
What is the Baltimore Classification of viruses?
The Baltimore Classification was named after David Baltimore
Class I - dsDNA
Class II - ssDNA
Class III - dsDNA
Class IV - ssRNA + strand
Class V - ssRNA - strand
Class VI - RNA viruses requiring a DNA intermediate for replication
What are the characteristics of the viral capsid?
Made up of protein subunits called capsomers
It protects the viral nucleic acid
Complex: Poxvirus
Icosahedral: Solid w/ 20 triangular faces, 12 verticies, 2,3,5 fold symmetry
Helical: Helical morphology
What are the characteristics of the viral envelope?
It surrounds the nucleocapsid
Formed from modified host cell membrane
Contains host-derived phospholipid bilayer
Contains virus-derived proteins and glycoproteins for attachment and viral fusion
Matrix proteins (M proteins) are often found associated with the inner layer of the envelope. Aid in viral structure.
Fusion proteins (F proteins) are found on the envelope surface. They cause viral membranes to fuse with cellular membranes.
How many families of human viruses are there and how are they classified?
There are six DNA virus families
13 RNA virus families
These viruses are classified based on their:
Nucleic acid
Symmetry of the nucleocapsid
Presence of an envelope
Dimensions of the virion and capsid
Nucleic acid sequence similarities
How are RNA viral families classified?
Symmetry of capsid - Icoshedreal or Helical
Envelope - Naked or eneveloped
Genome structure - SS, DS, + or - sense
How are DNA viruses classified?
Shape of capsid - Icosahederal, Helical, or Complex
Envelope - Naked or enveloped
Genome structure - SS, DS, linear, + or - sense, or circular
What are the steps of viral infection of a host cell?
Attachment to host cell
Entry of the virus into the host cell
Synthesis of viral proteins and genome
Assembly and release of mature virions
What is needed for a virus to attach to a host cell?
Viral receptors. These help determine host specificity.
Viruses tend to be very host specific and can be tissue specific.
What is the receptor for HIV-1?
CD4
Present on Macrophages and T cells
CD4 normally binds to MHC Class II in order to mediate T cell responses
What are the co-receptors for HIV 1?
Cxcr4 and Ccr5
Normally, these are chemokine receptors
True or false; The mode of entry into a cell depends on the viral family.
True
How do non-enveloped viruses enter cells?
1) Rearrangement of their capsid protein
2) Virus engulfed via receptor mediated endocytosis
3) Partial break up of capsid in vacuole followed by migration into the cytoplasm for further uncoating
How do enveloped viruses enter the host cell?
1) Fusion of viral membrane and cell membrane mediated by viral F protein
2) Phagocytosis and fusion of viral membrane with membrane of phagosome. Mediated by viral F protein.
Describe the F protein.
The F protein has a hydrophobic domain that can insert into the host cell membrane
The virus and host cell are brought close together and they are ready to fuse
How does HIV-1 binding take place?
SU binds CD4
Binding sites for Cxcr4 & Ccr5 are exposed
SU binds Cxcr4 or Ccr5 and induces a conformational change
Peptide is inserted into membrane
Individuals that are homozygous for deletions in Ccr5 are resistant to HIV-1 infection
What is the eclipse period?
This is the time from when nucleic acid is uncoated from its capsid
No infectious particles are present
What is the latent period?
No extracellular virus can be detected
What are the two mechanisms of virus release from a cell?
Disintegration of the infected cell (Burst) - Esp. in case of naked nucleocapsid virus
Slow release - Virus buds off of host cell membrane and acquires membrane in this way
What does viral assembly refer to?
This is the ‘putting together’ of the virion protein shells
Can be done from individual polypeptides, a polyprotein precursor and/or chaperone-mediated
What are the laboratory methods of confirming a viral diagnosis?
Isolation and growth of the virus
Cytopathic studies on cells in culture
Electron microscopy
Detection of viral components (Proteins, enzymes, nucleic acids)
Evaluation of patients immune response (Serology)
What are the consequences of viral infection?
Depends on the virus:
Cell death and lysis
Transformation of host cell into cancerous phenotype
Fusion of membranes of adjacent cells (synctia) - Feature of cells that have been infected with virus that have F protein
Development of inclusion bodies in cells - HSV causes Cowdry type A and Rabies causes negri bodies
How would one determine the infectious units of a virus?
Live cultivation
Use serial dilution end point method to determine:
Plaque formation (Viral lysis of cells in culture)
Focal Cytopathic changes in tissue culture
Focal proliferation of cells in tissue culture
Pock formation in membranes of chicken embryonated egg
Infection in experimental animals
What are other laboratory procedures to investigate viruses?
Electron microscopy
Hemagglutinin
Immunological testing via Complement fixation or direct fluorescent antibody
Western Blot
ELISA
PCR
How would one measure antibodies produced in response to a viral infection?
Complement fixation
Hemagglutination inhibition
Latex agglutination
ELISA
Indirect fluorescent antibody
Western immunoblot
What are important immunological responses to viral infection?
Viral neutralization by antibodies
Cell-mediated immunity
Interferon production - Interferons inhibit viral multiplication and may provide temporary, localized protection for nearby cells
NK cells - Kill virally infected cells
What do cytotoxic T cells do in response to viral infection?
Virally-infected cells process endogenously produced viral antigens and present them to cytotoxic T cells
Cytotoxic T cells are activated and are then produce cytotoxic substances
How do antibodies prevent viral infection?
Antibodies bind to virus (Opsonize)
Endocytosis / phagocytosis is blocked
Uncoating is blocked via capsid stabilization and fusion interference
Virus is destroyed
What are the most important cytokines in antiviral immune responses?
Type 1 interferons (alpha and beta)
The synthesis of the Type I interferons is induced in a cell by viral infection
This induction may be induced by active and inactivated virus, by ds RNA and other compounds
Interferons are host specific but not virus specific
How do Type I interferons work?
After Type I interferons are released from a cell, they bind to other cells
A protein transcription factor is activated inside the cell that then turns on a set of genes
The products o f these genes inhibit viral protein synthesis, degrade viral mRNA, and promote apoptosis in virally infected cells
What is the pattern of viral activity during an infection?
Localized infections: Viral multiplication and cell damage localized near the site of entry
Disseminated infection: Local multiplication at the site of entry, spread through the lymphatics to blood stream and circulation, multiplication at secondary sites, secondary viremia, infection of the target organ
What does the incubation period refer to?
This is an asymptomatic period where infection is not apperant.
How does varicella zoster virus infect and replicate?
Day 0: Infection via eye conjunctiva and/or upper respiratory tract
Replication in primary lymph nodes
Day 4-6: Primary viremia; replication in liver, spleen, and other organs
Day 14: Secondary viremia; infection of skin. Appearance of a rash. Infection of sensory ganglia and establishment of latent infection.
What is an inapparant infection?
These are asymptomatic viral infections
Very common; may result from infection by attenuated virus or from effective host defense mechanisms
These are important medically because they represent an often unrecognized source for the dissemination of a virus. This may confer immunity on a host.
What is an acute infection?
The virus infects the host and shortly after, disease ensues.
After a defined but short period of time, the host recovers.
The virus may be totally eliminated from the body or it can hide within the body and will result in a latent infection.
What are the categories of persistent infections?
Latent Infections
Chronic Infections
What is a latent infection?
Viral particles are not detectable but they will reappear during recurrences of the disease.
Viral nucleic acids are detectable between recurrences.
Herpesvirus infections are the best example of this.
What are chronic infections?
The virus is always detectable and often shed.
Infected cells produce and release virus but cellular metobolism is little affected and the infected cells can grow and divide.
How might viral diseases be controlled?
Prevention of transmission
Public health surveillance
Education
Isolation of disease cases
Passive immunization
Active immunization - Live attenuated, inactivated, or subunit vaccines
How are most virus vaccines prepared?
Live tissue culture
Some genetically engineered vaccines are now being used
Viral DNA vaccines are an active area of research. DNA is engulfed by dendritic cells and protein antigen is presented to T cells. This elicits cell mediated immunity.
What are the characteristics of a live, attenuated vaccine?
Offer many years of immunity
IgA and IgG antibodies are produced
Stimulates good cell-mediated response
Vaccine rarely reverts to virulence
What are the characteristics of an i nactivated vaccine?
Immunity does not last a lifetime
IgG antibody produced
Poor cell-mediated response elicited
No reversion to virulence
What is required to eliminate a virus from a population?
No animal reservoir
Good vaccine
Few or no subclinical cases (Latent infections ensure that virus will survive in population)
One antigenic type or only a few (No antigenic switching)
How would on treat a viral disease?
Person must be symptomatic
Immune serum is best if given early in infection, before exposure or during incubation period
A limited number of drugs are available for treatment of viral diseases
What are the characteristics of Poxviridae?
Large, brick-shaped complex particles
Replication of DNA much like that for cellular DNA but uses mainly virus-coded enzymes
Propensity to infect epidermal cells
Viral DNA replicates in cytoplasm of infected cells
What are some of the characteristics of Variola (Small pox) virus?
Large DNA virus
Dumbbell shaped core
Has a complex membrane
Humans are the only known host in nature
Major health issue until immunization became wide-spread (vaccinia-live virus vaccine)
No naturally occuring cases since 1977 and declared extinct on Friday, Oct. 28, 1979
How does smallpox cause disease?
Virus is inhaled and replicates in upper respiratory tract
Internal and dermal tissues are seeded with virus after two rounds of viremia
Hallmark “pox” forms (Lesions that are seen)
Two varients of small pox: Variola major (15-40% mortality) and Variola minor (1% mortality)
What are the Level A CDC procedures for dealing with a suspected case of small pox?
If patient has suspect rash, isolate patient in negative air pressure room
Rule out chickenpox
Obtain specimen from lesion pustules
Contact state public health lab for further guidance
What is the appearance of a patient with poxvirus?
Rashes have vesicles that appear in a centrifugal pattern
All lesions are at the same stage of debvelopment
Patients appear moribund
What is the basis for the modern smallpox virus?
Vaccinia
What is Molluscum contagiosum?
This is a pox virus
Transmitted by direct contact or fomites
Virus infects the basal cell layer and replicates
Molluscum contagiosum leaves benign epidermal tumors that can occur in people given vaccinia based vaccines (esp. in people with HIV)
Lesions are smooth, firm, shiny flesh colored to pearly white hemispheric papules with umbilicated centers confined to the skin and mucous membranes
Children, people that live in tropical climates and HIV+ are most at risk
What are the characteristics of the adenoviridae?
Adenoviruses are medium sized (90-100 nm), ds DNA
49 immunologically/serologically distinct types organized into six subgenera called A through F. All of these can cause human infections. This is a frequent cause of the common cold.
They have naked icosahedral nucleocapsids
These are very stable to chemical and physical agents as well as adverse pH. This allows for prolonged survival outside of the body.
There is a fiber with a terminal knob that projects from each penton.
What are the clinical features of adenovirus infection?
Most commonly cause respiratory illness
Depending on the infecting serotype, they may also cause various other illness such as gastroenteritis, conjunctivities, cystitis, and rash.
Respiratory symptoms range from common cold to pneumonia, croup, and bronchitis.
Patients with compromised immune systems are especially at risk.
Acute respiratory disease was first recognized among military recruits during WW II. Can be caused by adenovirus infections during conditions of crowding and stress.
