Notions de virologie

(1)

Notions de virologie

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Table des matières

• Morphologies de virus

• Qu’est ce qu’un virus

• Cycle de vie

• Organisation génétique des virus

• Virus et santé : l’exemple du HIV

(3)

Virus hélicaux

• Tube spiralé de protéines (“colimaçon”)

• Rage, virus mosaïque du tabac

(4)

Virus icosahédriques

• Semblent ronds au microscope

• 20 faces triangulaires

• Hépatite B

(5)

Virus enveloppés

• Sphériques

• Ces virus utilisent la membrane de l’hôte pour former une enveloppe

• Des Glycoprotéines (“récepteurs”) à la

• Des Glycoprotéines (“récepteurs”) à la surface du virus lui permettent de ne pas être reconnu

• HIV, grippe

(6)

Virus enveloppés

(7)

Bactériophage

• Tête hexagonale et queue en hélice

• La queue sert à injecter l’ADN du virus

• La queue sert à injecter l’ADN du virus dans la bactérie hôte

• Phage T4

(8)

Bactériophage

(9)

Caractéristiques

• Petite taille: 20-300 nm de diamètre

• Entièrement dépendant d’une cellule hôte pour sa survie

• Certains virus codent néanmoins pour une ADN

polymérase ou une ARN polymérase, mais ne peuvent polymérase ou une ARN polymérase, mais ne peuvent jamais se multiplier seuls

• Récepteurs de surfaces pour la reconnaissance et l’adhésion à la surface hôte

• Vecteurs très important de transfert horizontal entre espèces

(10)

Taille des virus

(11)

SUNARYATI

(12)

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(13)

Réplication et cycle de vie

13/10

(14)

(15)

•• Certains virus entrent également dans la cellule par ce

moyen, mais ne peuvent pas fusionner leur membrane avec celle de l’endosome

•Poliovirus: l’endosome est acidifié et expose les virions sur

Une grande variété de mécanismes d’entrée

15

•Poliovirus: l’endosome est acidifié et expose les virions sur la membrane, d’où ils vont ensuite être relachés dans le cytoplasme

•Reovirus: transfert du contenu de l’endosome au

lysosome, où les protéases vont détruire la capside virale et laisser l’ADN s’échapper

(16)

Poliovirus

16

(17)

Mécanismes d’exocytose

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(18)

• Le matériel génétique existe sous forme d’ADN ou d’ARN, simple brin ou double brin

• Il peut être circulaire ou linéaire

• Le génome peut etre divisé en plusieurs chromosomes, . e.g. le virus influenza et les rotavirus

Génétique virale

(19)

Génomes viraux

(20)

Expression génétique coordonnée

(21)

• Le nombre de générations est très grand, ainsi que la population, le nombre potentiel de mutations est

donc très important, même sur des temps courts

• L’évolution précise dépend beaucoup de la structure

Evolution des virus

• L’évolution précise dépend beaucoup de la structure du génome (ADN ou ARN, segmenté ou non)

• L’un des principaux mécanismes d’évolution virale répond à une forte pression de sélection pour

échapper au système immunitaire de l’hôte: on parlera de shift antigénique

(22)

Mutations et recombinaison

• Des mutations ont lieu naturellement lors de la réplication du virus par la cellule hôte (cf les virus qui ont leur propre ADN polymérase « à erreurs »)

• Les mutations sont plus fréquentes chez les virus à ARN

• Des mécanismes de recombinaison peuvent avoir lieu dans la population virale au sein

d’un hôte, ou entre les segments d’un génome

(23)

Le virus influenza (la grippe)

(24)

Quels traitements?

• Les antibiotiques ne fonctionnent pas

• Prévention : vaccins viraux

• Les cellules immunitaires produisent des interférons (un type de cytokine) pour se interférons (un type de cytokine) pour se défendre contre les virus

• Thérapies antivirales à base d’analogues de

nucléosides ou de nucléotides, d’inhibiteurs

de la reverse-transcriptase ou des protéases

(25)

Vaccins viraux

• L’idée est d’injecter à la personne un virus

modifié, qui n’est pas pathogène, mais dont la présentation des antigènes va activer le

système immunitaire système immunitaire

• On peut imaginer généraliser ce système avec des peptides de synthèse, des virus vivants

atténués

• Thérapie génique?

(26)

Interférons

• Produits naturellement par des cellules

immunitaires en contact avec des virus ou des cellules infectées

• Empêchent la réplication virale, stoppant

• Empêchent la réplication virale, stoppant l’infection

• La défense naturelle principale contre les virus

(27)

L’exemple du HIV

(28)

Cycle de vie du HIV-1

(29)

Organisation génétique du HIV-1

(30)

Une histoire évolutive complexe

(31)

(32)

(33)

CHARACTERISTICS CHARACTERISTICS

• To see the virus electron microscope

• Growth need living cells/ tissues

• Can not growth saprophytic

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• Can not growth saprophytic

• Only have certain enzyme for metabolism and energy

• Easy mutated changes antigenic property

• Multiplication different from bacteria

(34)

SUNARYATI

(35)

Virus structure & Morphology Virus structure & Morphology

The basic design of all viruses places the nucleic The basic design of all viruses places the nucleic acid genome on the inside of a protein shell

acid genome on the inside of a protein shell à

à capsid capsid

Two basic types of virions : Two basic types of virions :

1. Enveloped viruses 1. Enveloped viruses

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1. Enveloped viruses 1. Enveloped viruses

à

à have a nucleocapsid of nucleic acidhave a nucleocapsid of nucleic acid complexed to protein

complexed to protein 2. Naked capsid viruses 2. Naked capsid viruses

à

à have a nucleic acid genome within ahave a nucleic acid genome within a protein shell

protein shell

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(36)

Virus structure & Morphology Virus structure & Morphology

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Schematic drawing of two basic type of virions

(37)

Two basic shapes of virions : Two basic shapes of virions : 1. Cylindrical

1. Cylindrical 2. Spherical 2. Spherical

Some bacteriophages combine those 2 basic shapes Some bacteriophages combine those 2 basic shapes Functions of capsid or envelope of viruses :

Functions of capsid or envelope of viruses :

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Functions of capsid or envelope of viruses : Functions of capsid or envelope of viruses :

1. To protect the NA genome from damage during

1. To protect the NA genome from damage during extraextra-- celullar passage of the virus from

celullar passage of the virus from one cell to anotherone cell to another 2. To aid in the process of entry into the cell

2. To aid in the process of entry into the cell

3. To package enzymes essential for the early steps

3. To package enzymes essential for the early steps of the infection of the infection process

process

37

(38)

Basic viral Basic viral

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•

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Basic viral Basic viral forms

forms

(39)

The structure The structure

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The structure The structure and relative and relative sizes of a sizes of a number of number of DNA

DNA

(40)

The structure The structure and relative and relative

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40 and relative and relative sizes of a sizes of a number of number of RNA

RNA

(41)

DNA VIRUSES DNA VIRUSES

ENVELOPED

ENVELOPED NAKEDNAKED

Double

Double –– strandedstranded Double Double –– stranded stranded SingleSingle--strandedstranded

Icosahedral Complex Icosahedral Icosahedral

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Icosahedral Complex Icosahedral Icosahedral

HERPES POX PAPOVA PARVO

HEPADNA ADENO

(42)

RNA VIRUSES RNA VIRUSES

Single – stranded Double – stranded

Positive – stranded (+) Negative – stranded (-)

Naked Enveloped Enveloped Naked

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*PICORNA *TOGA BUNYA *REO

*CALICI *FLAVI ORTHOMYXO

CORONA PARAMYXO

RETRO RHABDO

ARENA FILO

* Icosahedral; all of the rest have helical symmetry

(43)

ADSORPTION ADSORPTION

Adsorption is the first step in every viral infection.

Adsorption involves :

- virion attachment proteins - cell surface receptor proteins

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Examples of viral receptors

(44)

•• For some viruses coFor some viruses co--receptors are involved in adsorption receptors are involved in adsorption à

à HIVHIV--1 1 : CD: CD4 4 & chemokine receptors& chemokine receptors

•• Viral spikes & phage tails carry attachment proteins Viral spikes & phage tails carry attachment proteins

•• In some case, a region of the capsid protein serveIn some case, a region of the capsid protein serve

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•• In some case, a region of the capsid protein serveIn some case, a region of the capsid protein serve the function of attachment

the function of attachment

•• Adsorption is enhanced by presence of multiple Adsorption is enhanced by presence of multiple attachment & receptor proteins.

attachment & receptor proteins.

•• A particular kind of virus is capable to infecting onlyA particular kind of virus is capable to infecting only a limited spectrum of cell types

a limited spectrum of cell types àà its host rangeits host range Differences in host range & tissue tropism due to Differences in host range & tissue tropism due to presence or absence of the receptors

presence or absence of the receptors

(45)

Entry & Uncoating Entry & Uncoating

Enveloped Animal Viruses Enveloped Animal Viruses

Some enveloped viruses enter cells by direct fusiondirect fusion of plasma membrane & envelope, release the nucleocapsid directly into the cytoplasm.

à Paramyxoviruses (eg. measles) retroviruses (eg. HIV-1) & herpesviruses

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Other enveloped & naked viruses are taken in by receptor

receptor--mediated endocytosismediated endocytosis (viropexis).

à orthomycovirus (eg. influenza viruses),

togaviruses (eg. rubella viruses), rhabdoviruses (eg. rabies) & coronaviruses

(46)

Entry by Entry by

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Entry by Entry by direct fusion direct fusion

(47)

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Viropexis

(48)

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Viral release by Budding

(49)

Late Transcription

49/11

(50)

Human Viral Diseases

• Because viruses aren’t “alive” they must be

spread by an intermediate host (vector)

(51)

Chicken Pox/Shingles:

• Caused by vericella-zoster herpesvirus

• Spread through the air and contact

• May remain dormant as a provirus and become shingles

become shingles

(52)

Viral Hepatitis

• Inflammation of the liver caused by 5 different viruses

• A and E spread by fecal matter

• B,C and D spread by sexual contact or

blood transfusion

(53)

AIDS

• Acquired Immune Deficiency Syndrome

• Caused by the HIV virus

• Glycoproteins on the surface of the

virus bind to receptor sites of immune cells (macrophages)

• Retrovirus, Provirus

• Genetically diverse

(54)

Is there a cure?

• Azydothymidine- inhibits reverse transcriptase

• Protease inibitors- blocks synthesis of new capsids

• These drugs can only slow the

progression to full-blown AIDS

(55)

Emerging Diseases

• Caused by infections in animal populations

– Rainforest animals-> Ebola

– Hanta virus (pneumonia)-> mice

– SARS-> civet cats

(56)

• Epidemic- quickly spreading infectious disease

• Pandemic- disease spreads over large regions

• Bubonic plague, Small pox

(57)

Treatment

• Vaccine- harmless version of a virus used to build an immune response

(microevolution)

– Inactivated virus- not able to replicate

– Attenuated virus- weakened form of a virus; no – Attenuated virus- weakened form of a virus; no

ability to cause disease

• Vector control

• Drug therapy- interfere with DNA/RNA synthesis

– Acyclovir- blocks DNA polymerase

(58)

PATHOGENESIS OF VIRAL DISEASES PATHOGENESIS OF VIRAL DISEASES Viral pathogenesis :

interaction of viral and host factors

leads to disease production Virus pathogenic if : can infect and cause signs of

disease of the host

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disease of the host

Virus virulent : produce more severe disease Steps in viral pathogenesis :

• Viral entry & primary replication

• Viral spread and cell tropism

• Cell injury & clinical illness

• Recovery from infection

• Virus shedding

(59)

Laboratory Diagnosis

• Identification of the virus in cell culture;

• Microscopic identification directly in the specimen;

• Serologic procedures to detect a rise in antibody titer or the presence of Ig M antibody;

• Detection of viral antigen in blood or body fluids;

• Detection of viral nucleic acids in blood or patients cells.

(60)

The presence of a virus in a patient's specimen can be detected by cytopathic effect in cell culture but CPE is not specific. A specific identification of the virus usually involves an antibody based test as fluorescent antibody, complement fixation or ELISA.

Identification of The Virus in Cell Culture Identification of The Virus in Cell Culture

as fluorescent antibody, complement fixation or ELISA.

(61)

Microscopic identification directly in the specimen Microscopic identification directly in the specimen

Electron microscopy is the most common method used to study the morphology of viruses.

• Inclusion bodies, formed by aggregates of many virus particles, can be seen in either the nucleus or cytoplasm of infected cells.

• Multinucleated giant cells are formed by several viruses e.g. herpes, respiratory syncytical virus and measles virus.

respiratory syncytical virus and measles virus.

• Fluorescent antibody staining of cells obtained from the patients or of the 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 virus e.g Ebola (characteristic appearance and are dangerous to grow in culture).

(62)

• The presence of Ig M antibody can be used to diagnose current infection.

• The presence of Ig G antibody cannot be used to diagnose current infection. Risein antibody titer that is 4 foldor greater in the convalescent serum sample compared to the acute sample can be used to make a diagnosis.

Detection of Viral Antigen & Nucleic Acids Detection of Viral Antigen & Nucleic Acids

Serologic Procedures Serologic Procedures

• The presence of hepatitis B surface antigen is commonly used in diagnosis.

• The presence of vial DNA or RNA is increasingly becoming the gold standard in viral diagnosis. Molecular diagnostic procedures have been available since 1970s,when researchers first began using cloned DNA probe to detect viral nucleic acid. The new molecular diagnostic methods predicted that nucleic acid tests would rapidly replace traditional virus detection methods.

(63)

• The goal is in the detection of non culturable agents such as human papilloma virus, human parvovirus,

• Detecting viruses difficult to culture, including enteric adenovirus, some coxsackie viruses,

• Detecting viruses that are dangerous to culture such as HIV,

• Detecting viruses that are present in low numbers, for example, HIV in antibody negative patients or CMV in transplanted organs.

• Important when a tiny volume of specimen is available (forensic samples or intra-

Molecular Diagnostic Methods Molecular Diagnostic Methods

• Important when a tiny volume of specimen is available (forensic samples or intra- ocular fluid specimens).

• Allow laboratory to predict antiviral drug susceptibilities and to detect infections when viable virus cannot be obtained (latent viral infection or viruses that are present in immune complexes).

• May also used to differentiate antigenically similar viruses such as adenovirus types 40 and 41 and to detect viral genotypes that are associated with human cancers (human papilloma virus).

(64)

Recombination Recombination

• Process of intermolecular exchange, of chromosomes combining genetic information from different sources, typically two genomes of a given

species.

• This kind of break/join recombination is common in DNA viruses or those RNA viruses which have a DNA phase (retroviruses).

(65)

PREVENTION AND TREATMENT OF VIRAL INFECTIONS PREVENTION AND TREATMENT OF VIRAL INFECTIONS

1. VIRAL VACCINES

– Killed-virus vaccines

– Attenuated live-virus vaccines – Future prospect :

- attenuation of viruses by genetic

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- attenuation of viruses by genetic mapping

- avirulent viral vectors

- purified proteins produced using cloned genes

- synthetic peptides - subunit vaccines - DNA vaccines

(66)

2. INTERFERONS 2. INTERFERONS

IFNs :

M host-coded proteins of large cytokine family

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M inhibit viral replication

M produced by intact animal or cell culture in response to viral infection or other

inducers

M first line of defense against viral infection

(67)

3. ANTIVIRAL CHEMOTHERAPY 3. ANTIVIRAL CHEMOTHERAPY

A. A. Nucleoside analogs Nucleoside analogs

• Acyclovir &

valacyclovir – Ribavirin

– Stavudine (d4T)

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valacyclovir

• Didanosine

• Gancyclovir

• Idoxuridine

• Lamivudin (3TC)

– Stavudine (d4T) – Trifluridine

– Vidarabine

– Zalzitabine (ddC)

– Zidovudine (AZT)

(68)

B. Nucleotide analogs

Cidofovir : active against CMV & HSV

inhibit s viral DNA polymerase C. Nonnucleoside reverse transcriptase inhibitor C. Nonnucleoside reverse transcriptase inhibitor

Nevirapine : inhibit reverse transcriptase of HIV Nevirapine : inhibit reverse transcriptase of HIV D. Protease inhibitors

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D. Protease inhibitors

Ritonavir, Saquinavir HIV E. Other types

Amantadine & rimantadine Foscarnet

Methiasone