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Notions de virologie

Table des matières•Morphologies de virus•Qu'estcequ'unvirus•Cycle de vie•Organisationgénétique des virus•Organisationgénétique des virus•Virus et santé : l'exempledu 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•DesGlycoprotéines("récepteurs") à la •DesGlycoprotéines("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 •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 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

SUNARYATI

SUNARYATI

Réplication et cycle de vie13/10

••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 Unegrande variété de mécanismes d'entrée15•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

Poliovirus16

Mécanismesd'exocytoseSUNARYATI

•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 rotavirusGénétique virale

Génomes viraux

Expression génétique coordonnée

•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 Evolutiondes 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

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 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 immunitairesystème 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 •Empêchent la réplication virale, stoppant l'infection•La défense naturelle principale contre les virus

L'exempledu HIV

Cycle de vie du HIV-1

Organisationgénétique du HIV-1

Une histoire évolutive complexe

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

SUNARYATI

Virus structure & MorphologyVirus structure & MorphologyThe 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 shellacid genome on the inside of a protein shellààcapsid capsid Two basic types of virions :Two basic types of virions :1. Enveloped viruses1. Enveloped virusesSUNARYATI1. Enveloped viruses1. Enveloped virusesààhave a nucleocapsid of nucleic acidhave a nucleocapsid of nucleic acidcomplexed to proteincomplexed to protein2. Naked capsid viruses2. Naked capsid virusesààhave a nucleic acid genome within ahave a nucleic acid genome within aprotein shellprotein shell35

Virus structure & MorphologyVirus structure & MorphologySUNARYATI36Schematic drawing of two basic type of virions

Two basic shapes of virions :Two basic shapes of virions :1. Cylindrical1. Cylindrical2. Spherical2. SphericalSome bacteriophages combine those 2 basic shapesSome bacteriophages combine those 2 basic shapesFunctions of capsid or envelope of viruses :Functions of capsid or envelope of viruses :SUNARYATIFunctions 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 another2. To aid in the process of entry into the cell2. To aid in the process of entry into the cell3. To package enzymes essential for the early steps 3. To package enzymes essential for the early steps of the infection of the infection processprocess37

Basic viral Basic viral SUNARYATI•38Basic viral Basic viral formsforms

The structure The structure SUNARYATIThe structure The structure and relative and relative sizes of a sizes of a number of number of DNADNA

The structure The structure and relative and relative SUNARYATI40and relative and relative sizes of a sizes of a number of number of RNARNA

DNA VIRUSESDNA VIRUSESENVELOPEDENVELOPEDNAKEDNAKEDDouble Double --strandedstrandedDouble Double --stranded stranded SingleSingle--strandedstrandedIcosahedralComplex IcosahedralIcosahedralSUNARYATIIcosahedralComplex IcosahedralIcosahedralHERPESPOXPAPOVA PARVOHEPADNAADENO

RNA VIRUSESRNA VIRUSESSingle -strandedDouble -strandedPositive -stranded (+)Negative -stranded (-) NakedEnvelopedEnvelopedNakedSUNARYATI*PICORNA *TOGABUNYA*REO*CALICI*FLAVIORTHOMYXOCORONA PARAMYXORETRORHABDOARENAFILO* Icosahedral; all of the rest have helical symmetry

ADSORPTIONADSORPTIONAdsorption is the first step in every viral infection.Adsorption involves :-virion attachment proteins-cell surface receptor proteinsSUNARYATIExamples of viral receptors

••For some viruses coFor some viruses co--receptors are involved in adsorption receptors are involved in adsorption ààHIVHIV--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 serveSUNARYATI44••In some case, a region of the capsid protein serveIn some case, a region of the capsid protein servethe 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 onlya limited spectrum of cell types a limited spectrum of cell types ààits host rangeits host rangeDifferences in host range & tissue tropism due toDifferences in host range & tissue tropism due topresence or absence of the receptors presence or absence of the receptors

Entry & UncoatingEntry & UncoatingEnveloped Animal VirusesEnveloped Animal VirusesSome enveloped viruses enter cells by direct fusiondirect fusionof plasma membrane & envelope, release the nucleocapsid directly into the cytoplasm.àParamyxoviruses (eg. measles) retroviruses(eg. HIV-1) & herpesvirusesSUNARYATI45Other enveloped & naked viruses are taken in byreceptorreceptor--mediated endocytosismediated endocytosis(viropexis).àorthomycovirus (eg. influenza viruses),togaviruses (eg. rubella viruses), rhabdoviruses(eg. rabies) & coronaviruses

Entry by Entry by SUNARYATIEntry by Entry by direct fusiondirect fusion

SUNARYATIViropexisViropexis

SUNARYATI48Viral release by Budding

Late Transcription49/11

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 shinglesbecome 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 •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 •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 -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 DISEASESPATHOGENESIS OF VIRAL DISEASESViral pathogenesis :interaction of viral and host factorsleads to disease productionViruspathogenicif : can infect and cause signs of disease of the hostSUNARYATIdisease of the hostVirus 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

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 antigen in blood or body fluids;•Detection of viral nucleic acids in blood or patients cells.

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.

Microscopic identification directly in the specimenMicroscopic identification directly in the specimenElectron microscopyis the most common method used to study the morphologyof viruses.•Inclusion bodies,formed by aggregates of many virus particles, can be seen in either the nucleus or cytoplasm of infected cells.•Multinucleated giantcells are formed by several viruses e.g. herpes, respiratory syncytical virus and measles virus.respiratory syncytical virus and measles virus.•Fluorescent antibodystaining of cells obtained from the patients or of the cells infected in culture can provide a rapid specific diagnosis.•Electron microscopyis 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).

•The presence of Ig Mantibody can be used to diagnose current infection.•The presence of Ig Gantibody 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 AcidsDetection of Viral Antigen & Nucleic AcidsSerologic ProceduresSerologic 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.

•The goal is in the detection of non culturableagents such as human papilloma virus, human parvovirus,•Detecting viruses difficult to culture,including enteric adenovirus, some coxsackie viruses,•Detecting viruses that are dangerousto 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 volumeof specimen is available (forensic samples or intra-Molecular Diagnostic MethodsMolecular Diagnostic Methods•Important when a tiny volumeof 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).

RecombinationRecombination•Processof intermolecular exchange,of chromosomescombining genetic informationfrom different sources, typically two genomesof a givenspecies.•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 INFECTIONSPREVENTION AND TREATMENT OF VIRAL INFECTIONS1.VIRAL VACCINES-Killed-virus vaccines-Attenuated live-virus vaccines-Future prospect : -attenuation of viruses by genetic SUNARYATI-attenuation of viruses by genetic mapping-avirulent viral vectors-purified proteins produced using cloned genes-synthetic peptides-subunit vaccines-DNA vaccines

2. INTERFERONS2. INTERFERONSIFNs : Mhost-coded proteins of large cytokine familySUNARYATIMinhibit viral replicationMproduced by intact animal or cell culture in response to viral infection or other inducersMfirst line of defense against viral infection

3. ANTIVIRAL CHEMOTHERAPY3. ANTIVIRAL CHEMOTHERAPYA.A.Nucleoside analogsNucleoside analogs•Acyclovir & valacyclovir-Ribavirin-Stavudine (d4T)SUNARYATIvalacyclovir•Didanosine•Gancyclovir•Idoxuridine•Lamivudin (3TC)-Stavudine (d4T)-Trifluridine-Vidarabine-Zalzitabine (ddC)-Zidovudine (AZT)

B. Nucleotide analogsCidofovir : active against CMV & HSVinhibit s viral DNA polymeraseC. Nonnucleoside reverse transcriptase inhibitorC. Nonnucleoside reverse transcriptase inhibitorNevirapine : inhibit reverse transcriptase of HIV Nevirapine : inhibit reverse transcriptase of HIV D. Protease inhibitorsSUNARYATID. Protease inhibitorsRitonavir, SaquinavirHIVE. Other types Amantadine & rimantadineFoscarnetMethiasone

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