Notions de virologie

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Notions de virologie

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

Virus hélicaux • Tube spiralé de protéines (“colimaçon”) • Rage, virus mosaïque du tabac

Virus icosahédriques • Semblent ronds au microscope • 20 faces triangulaires • Hépatite B

Virus enveloppés • Sphériques • Ces virus utilisent la membrane de l’hôte pour former une enveloppe • Des Glycoprotéines (“récepteurs”) à la surface du virus lui permettent de ne pas être reconnu • HIV, grippe

Virus enveloppés

Bactériophage • Tête hexagonale et queue en hélice • La queue sert à injecter l’ADN du virus dans la bactérie hôte • Phage T4

Bactériophage

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 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

Taille des virus

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Réplication et cycle de vie

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Une grande variété de mécanismes d’entrée • 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 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 15

Poliovirus

16

Mécanismes d’exocytose

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Génétique virale • 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

Génomes viraux

Expression génétique coordonnée

Evolution des virus • 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 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

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

Le virus influenza (la grippe)

Quels traitements? • Les antibiotiques ne fonctionnent pas • Prévention : vaccins viraux • Les cellules immunitaires produisent des 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

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 • On peut imaginer généraliser ce système avec des peptides de synthèse, des virus vivants atténués • Thérapie génique?

Interférons • Produits naturellement par des cellules immunitaires en contact avec des virus ou des cellules infectées • Empêchent la réplication virale, stoppant l’infection • La défense naturelle principale contre les virus

L’exemple du HIV

Cycle de vie du HIV-1

Organisation génétique du HIV-1

Une histoire évolutive complexe

CHARACTERISTICS • • • • • •

To see the virus electron microscope Growth need living cells/ tissues Can not growth saprophytic Only have certain enzyme for metabolism and energy Easy mutated changes antigenic property Multiplication different from bacteria

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Virus structure & Morphology The basic design of all viruses places the nucleic acid genome on the inside of a protein shell à capsid Two basic types of virions : 1. Enveloped viruses à have a nucleocapsid of nucleic acid complexed to protein 2. Naked capsid viruses à have a nucleic acid genome within a protein shell

SUNARYATI

35

Virus structure & Morphology

Schematic drawing of two basic type of virions SUNARYATI

36

Two basic shapes of virions : 1. Cylindrical 2. Spherical Some bacteriophages combine those 2 basic shapes Functions of capsid or envelope of viruses : 1. To protect the NA genome from damage during celullar passage of the virus from 2. To aid in the process of entry into the cell 3. To package enzymes essential for the early steps process

SUNARYATI

one cell to another

extra-extra

of the infection

37

Basic viral forms •

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38

The structure and relative sizes of a number of DNA

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The structure and relative sizes of a number of RNA

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40

DNA VIRUSES

ENVELOPED Double – stranded

Icosahedral

HERPES

NAKED Double – stranded Single Single--stranded

Complex

Icosahedral

POX

Icosahedral

PAPOVA

HEPADNA

ADENO

SUNARYATI

PARVO

RNA VIRUSES Single – stranded

Double – stranded

Positive – stranded (+) Naked Enveloped *PICORNA *CALICI

Negative – stranded (-) Enveloped

*TOGA *FLAVI CORONA RETRO

Naked

BUNYA ORTHOMYXO PARAMYXO RHABDO ARENA FILO

SUNARYATI * Icosahedral; all of the rest have helical symmetry

*REO

ADSORPTION Adsorption is the first step in every viral infection. Adsorption involves : - virion attachment proteins - cell surface receptor proteins

SUNARYATI

Examples of viral receptors

• For some viruses coco-receptors are involved in adsorption à HIV HIV--1 : CD4 CD4 & chemokine receptors •

Viral spikes & phage tails carry attachment proteins



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



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



A particular kind of virus is capable to infecting only a limited spectrum of cell types à its host range Differences in host range & tissue tropism due to presence or absence of the receptors SUNARYATI

44

Entry & Uncoating Enveloped Animal Viruses Some enveloped viruses enter cells by direct fusion of plasma membrane & envelope, release the nucleocapsid directly into the cytoplasm. à Paramyxoviruses (eg. measles) retroviruses (eg. HIV-1) & herpesviruses Other enveloped & naked viruses are taken in by receptor--mediated endocytosis (viropexis). receptor à orthomycovirus (eg. influenza viruses), togaviruses (eg. rubella viruses), rhabdoviruses (eg. rabies) & coronaviruses

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45

Entry by direct fusion

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Viropexis

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Viral release by Budding

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48

Late Transcription

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Human Viral Diseases • Because viruses aren’t “alive” they must be spread by an intermediate host (vector)

Chicken Pox/Shingles: • Caused by vericella-zoster herpesvirus • Spread through the air and contact • May remain dormant as a provirus and become shingles

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

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

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

Emerging Diseases • Caused by infections in animal populations – Rainforest animals-> Ebola – Hanta virus (pneumonia)-> mice – SARS-> civet cats

• Epidemic- quickly spreading infectious disease • Pandemic- disease spreads over large regions • Bubonic plague, Small pox

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 ability to cause disease

• Vector control • Drug therapy- interfere with DNA/RNA synthesis – Acyclovir- blocks DNA polymerase

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 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 SUNARYATI • Virus shedding

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.

Identification of The Virus in Cell Culture 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.

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. • 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).

Serologic Procedures • •

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. Rise in antibody titer that is 4 fold or greater in the convalescent serum sample compared to the acute sample can be used to make a diagnosis.

Detection of Viral Antigen & Nucleic Acids • •

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.

Molecular Diagnostic Methods • • • • • • •

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 intraocular 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).

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).

PREVENTION AND TREATMENT OF VIRAL INFECTIONS

1. VIRAL VACCINES

– Killed-virus vaccines – Attenuated live-virus vaccines – Future prospect : - attenuation of viruses by genetic mapping - avirulent viral vectors - purified proteins produced using cloned genes - synthetic peptides - subunit vaccines - DNA vaccines SUNARYATI

2. INTERFERONS

IFNs : Mhost-coded proteins of large cytokine family Minhibit viral replication Mproduced by intact animal or cell culture in response to viral infection or other inducers Mfirst line of defense against viral infection SUNARYATI

3. ANTIVIRAL CHEMOTHERAPY

A. Nucleoside analogs • Acyclovir & valacyclovir • Didanosine • Gancyclovir • Idoxuridine • Lamivudin (3TC)

– Ribavirin – Stavudine (d4T) – Trifluridine – Vidarabine – Zalzitabine (ddC) – Zidovudine (AZT) SUNARYATI

B. Nucleotide analogs

Cidofovir : active against CMV & HSV inhibit s viral DNA polymerase C. Nonnucleoside reverse transcriptase inhibitor Nevirapine : inhibit reverse transcriptase of HIV D. Protease inhibitors Ritonavir, Saquinavir HIV E. Other types Amantadine & rimantadine Foscarnet Methiasone SUNARYATI

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Notions de virologie

Notions de virologie Table des matières • • • • • Morphologies de virus Qu’est ce qu’un virus Cycle de vie Organisation génétique des virus Virus e...

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