V-L1 Virology Intro- 2013

8/12/2019 V-L1 Virology Intro- 2013

http://slidepdf.com/reader/full/v-l1-virology-intro-2013 1/26

Block 4 Infection and Immunity

Medical Virology

Behavioral

Environmental

Pharmacological

Immuno-

modulatory

Surgical

Innateimmunity

Adaptive

immunity

VIRUSES

Bacteria

Fungi

Protozoa

Animalia

Infectious

Agent

Host

Defenses

Health

Interventions



Learning Objectives for Introduction to Viruses

Students will be able to

1. define a virus.

2. define how the terms “virion” and “virus” differ.

3. identify the components of a virion.

4. list the families of viruses that infect humans.

5. describe how virus families are organized.

6. list, in order, the steps in the viral replication cycle.

7. list the kinds of infections viruses cause.



Viruses are obligate intracellular parasites

They replicate only inside living cells because they use

the cellular synthetic machinery for transcription,

translation and replication to cause the synthesis of

specialized elements called virions that serve to transfer

the viral genome, which can be either RNA or DNA, toother living cells.

Virus

• Infectious form of virus: virion

•Intracellular form of virus represented by its genome

[non-infectious form of virus]

[2 distinct stages or forms of a virus]

What is a virus?



Virion1. Genome: DNA or RNA

2. Capsid (Protein) cubic (icosahedral)

helical

complex

4. Envelope: lipid bilayer (host) with glycoprotein (virus)

external layer surrounding nucleocapsid

Virions are either enveloped or non-enveloped [naked]

Enveloped viruses are sensitive (inactivated) to

organic solvents, detergents

3. Matrix (Protein)—not in all viruseslayer between capsid and envelope



Genome

Human genome is composed of linear, double-stranded DNA with about

three billion (3x109) base pairs. It is segmented and diploid.

Viral genomes can be:

• RNA or DNA

• double stranded or single stranded

• linear or circular

• continuous (non-segmented) or segmented

• haploid or diploid

1 amino acid:1 codon (3 nucleic acid bases)

Av. protein 50,000 Daltons or ~500 amino acids

Av. Gene is 1,500 nucleic acid bases (1.5 kb)

Smallest virus ~1.5 kb (2-3 genes)

Largest viruses ~1 Mb (>1,000 genes)

Smallest bacteria "free living” ~600 kb (600 genes)

Human (~20,000 genes)



Icosahedral

Helical

C a p s

i d s

Naked Enveloped



INFLUENZA A (lab strains)

INFLUENZA C

FILAMENTOUS INFLUENZA A

(most clinical strains are filamentous)



DNA Viruses

Parvoviridae

Polyomaviridae

Adenoviridae

Herpesviridae

Poxviridae

RNA Viruses

Picornaviridae

Positive RNA

Caliciviridae

Hepeviridae

FlaviviridaeTogaviridaeCoronaviridae

Negative RNA

Orthomyxoviridae

Double-Stranded RNA

segmented

Reoviridae

Use Reverse Transcription "make DNA from RNA"

Hepadnaviridae Retroviridae

Mononegavirales

Papillomaviridae

RhabdoviridaeParamyxoviridae

BunyaviridaeArenaviridae

FiloviridaeBornaviridae

Segmented

Naked virions above red line,

Enveloped virions below red line



Learning Tips for Virology

1. DNA viruses have icosahedral symmetry, except Poxviruses

that have complex symmetry.

2. Parvo-, polyoma- papilloma- and adenoviruses are naked,

herpes- and poxviruses are enveloped.

3. Positive stranded RNA viruses have icosahedral symmetry,

except coronaviruses which have helical symmetry.

4. The picorna-, hepe- and caliciviruses are naked, the flavi-, toga-and coronaviruses are enveloped.

5. All negative stranded RNA viruses are enveloped and have

helical symmetry.

6. Reoviruses have multiple capsid shells (naked).

7. Hepadna- and Retroviruses encode a reverse transcriptase,

both are enveloped and have icosahedral symmetry.

Retroviruses have an RNA genome and make DNA on the way

in (first step after uncoating). Hepadnaviruses have a DNA

genome and make DNA on the way out (last step during

assembly).



Process by which virus is taken up into cells - Two general types

•occurs at cell surface ("pH independent”)

•occurs inside-after receptor-mediated endocytosis ("pH dependent”)

at least 5 pathways for endocytosis

•clathrin-dependent pathway

•macropinocytosis

•caveolar pathway

•clathrin- and caveolin-independent pathway•phagocytosis

Actin cytoskeleton involved in all endocytotic pathways

2. Penetration



Entry [portal of entry]

In some cases it is difficult to separate penetration from uncoating,

both occur simultaneously.

3. Uncoating

Uncoating is the separation of the genome or internal nucleocapsids fromthe outer structural components of virions

"Eclipse period" results from uncoating. After uncoating, the virus is no longer

a virion, i.e., it loses its infectivity, because the particle no longer contains its VAP.



EE = early endosome

AT = actin dependent stage

ECV = endocytic carrier vesicle

or vesicular EE

ME = maturing endosome

LE = late endosome

MT = microtubule

Adsorption and Entry



Infection pathway of enveloped alphaviruses

* *

* * *

* * *

*** * **

*

**

*

*

*

*

*

*

**

**

****

**

**

**

** *

* *

*

**

*

*

*

*

** *

*

*

^ ^

^

^

H +

Receptor binding

Endocytosis

Nuc leocaps id rel ease

Fusion

* *

* * *

* * *

*** * **

*

**

*

*

*

*

*

*

**

**

****

**

**

**

** *

* *

*

**

*

*

*

*

** *

*

*

^ ^

^

^

H +

Receptor binding

Endocytosis

Nuc leocaps id rel ease

Fusion

Virus

replication

Ex: Togavirus E1

membrane-inserted

homotrimers (HTs)pH ~6.5-5.5

FUSION

Viral membrane

Cell membrane

Fusion mechanism: Membrane

bending through hairpin formation



Viruses aren't the only microorganism that are obligate, intracellular

parasites or that enter the cells through endocytosis!

• Chlamydia (bacteria) are obligate intracellular parasites with biphasic growth cycle.

• EB [elementary body] infectious, metabolically inert, extracellular form

• RB [r eticulate body] metabolically active form, conventional bacteria-like

endosome



4. Synthetic Phase

Synthetic phase starts after uncoating. The virus genome directs the cell tosynthesize "nonstructural" & "structural" proteins and more genomes.

• structural proteins - proteins found in virion particles

• nonstructural proteins - proteins produced in infected cells but

not found in virions

How the virus accomplishes this task depends on the "replication

strategy of the virus."



Cellular Machines used by viruses

1. DNA Synthesis [Replication]

DNA polymerase "Replicase"2. RNA synthesis [Transcription] -- mRNA synthesis

RNA polymerase "Transcriptase"3. Protein synthesis [Translation]

Ribosome, t-RNA and protein factors"Translation machinery"

DNA RNA protein reverse

transcription

121 3



• During assembly, the virions are formed by packaging the genomes

with capsid proteins; and if the virus is enveloped, wrapping the

nucleocapsid in an envelope.

• Non-enveloped viruses accumulate in the cell and are released whenthe cell dies.

• Enveloped viruses acquire their envelope by budding through a

membrane of the cell (released in same step).

5. Assembly

6. Release



"Budding" is the process by which a virion acquires an envelope

Budding occurs at

plasma membrane

or internally (atGolgi, ER, nuclear

membranes) and is

specific to each

virus



Influenza budding – EM kindly provided by David Hockley & Milan Nermut



ONE STEP GROWTH CYCLE

V

i r i o n s

( p f u o r p l a q u

e f o r m i n g u n i t s )

106

107

108

109

1010

105

104

103

time (hours)

2 4 6 8 10 12

E C L I S P E

P E R I O D

HYPERBOLIC

GROWTH

A single infected cell can make and

release 10,000 virions within 4-6 hours.

PFU is a plaque f orming unit: one virus can infect a single cell,

spread and kill surrounding cells to cause a plaque (a clear

area of dead cells) surrounded by live cells.



Chronic - infectious virus is continuously present

Latent - the virus genome is present in cells but not

infectious virions

Recurrent - a latent infection that reactivates and yieldsinfectious virus before becoming latent (non-replicating) again

2. Persistent virus infection -virus is not cleared; immune response may

be fully active.

1. Transient virus infection - virus is cleared due to immune responses.

"acute" vs “transient”: both transient and persistent infections produce

acute signs of viral infection.

3. Slow virus infection - a chronic persistent infection with a prolonged

incubation period of months to years and in which the disease symptomsprogressively worsen to death

Example:

Subacute sclerosing panencephalitis [SSPE]: slow infection of measles virus

Transient vs Persistent



Virus-specific antibodies:

block receptor binding (neutralizing antibodies)

bind virus and prevent uncoating (entry blocking antibodies, also neutralizing)

bind virus and remove coated virus particles via phagocytosis

may require complement to neutralize certain viruses

Virus-specific T cells:

kill virus infected cells when they recognize virus antigens on cell membranes

kill infected cells that present viral peptides using MHC I

Concepts: Virus-specific antibodies “neutralize” or remove virus particles from blood

or respiratory secretions, while virus-specific T cells eliminate virus infected

cells, preventing persistence of the virus and of disease.

How does the immune system "clear" virus?



How do viruses cause disease?

1. Virus kills cells and results in organ failure.

• Example: viral encephalitis. Patient dies less than 1 week post-infection.

2. Immune reaction to virus infection, e.g. cytotoxic T cells destroy tissues and causeorgan failure.

• Example: viral hepatitis. Patient dies more than 1 week after infection

3. Virus infection induces autoimmune disease.

• Example: type 1 diabetes

4. Congenital infection prevents or distorts organ development.

•

Example: congenital rubella5. Virus infection provokes inflammatory state.

• Example: post-infection (vaccination) encephalomyelitis

6. Virus infection induces an immunodeficient state.

• Example: HIV, AIDS, measles

“undifferentiated symptoms of viral infection”

• fever

• myalgia

• malaise

• anorexia

Symptoms occur after the host has responded immunologically

and virus often is no longer present.



Questions?

V-L1 Virology Intro- 2013

Documents

Transcript of V-L1 Virology Intro- 2013