Chapter 6Chapter 6

An Introduction to VirusesAn Introduction to Viruses

General characteristicsGeneral characteristicsViruses = obligate, Viruses = obligate, intracellular parasites that intracellular parasites that contain either DNA or RNA; contain either DNA or RNA; depend on synthetic depend on synthetic machinery of host cell for machinery of host cell for replication of the infectious replication of the infectious particle called the virionparticle called the virionSize - submicroscopic; most Size - submicroscopic; most are beyond limit of are beyond limit of resolution of light resolution of light microscope (exception = microscope (exception = poxviruses; can be seen poxviruses; can be seen under light microscope with under light microscope with proper illumination); size proper illumination); size varies from 25 varies from 25 m to 250 m to 250 m (approximately 0.1 m (approximately 0.1 diameter of diameter of E. coliE. coli cell) cell)

Structural components Structural components

Nucleic acid core = Nucleic acid core = genome - may be either genome - may be either DNA or RNA but never DNA or RNA but never both; may be either single both; may be either single stranded (ss) or double stranded (ss) or double stranded (ds); used in stranded (ds); used in classification (ssDNA, classification (ssDNA, ssRNA, dsDNA, or ssRNA, dsDNA, or dsRNA); may be either dsRNA); may be either linear or circular; found in linear or circular; found in all mature virus particles all mature virus particles (virions)(virions)

Structural componentsStructural components

Protein coat = capsid - Protein coat = capsid - shell surrounding and shell surrounding and protecting nucleic acid protecting nucleic acid core; composed of core; composed of identical subunits called identical subunits called capsomeres; subunits capsomeres; subunits undergo self-assembly undergo self-assembly into geometric shapes into geometric shapes around nucleic acid around nucleic acid core; number of core; number of capsomeres is unique capsomeres is unique for each virus; contain for each virus; contain receptors that combine receptors that combine with complementary with complementary sites on host cell surface sites on host cell surface for attachment; found in for attachment; found in all mature virusesall mature viruses

Structural componentsStructural components

Envelope - lipid and protein Envelope - lipid and protein membrane surrounding virion; membrane surrounding virion; origin = host cell membrane; origin = host cell membrane; lipid of envelope make viruses lipid of envelope make viruses susceptible to lipid solvents susceptible to lipid solvents and are more heat labile than and are more heat labile than non-enveloped viruses; found non-enveloped viruses; found only in viruses that exit cell only in viruses that exit cell through cell membrane or bud through cell membrane or bud through nuclear membrane through nuclear membrane during maturation; viruses during maturation; viruses without envelope are called without envelope are called nakednakedGlycoprotein spikes - surface Glycoprotein spikes - surface projections coded for by projections coded for by viruses; manufactured in host viruses; manufactured in host cell and become incorporated cell and become incorporated into host cell membrane prior into host cell membrane prior to virion maturation; often act to virion maturation; often act as receptors for host cells; as receptors for host cells; may agglutinate rbc'smay agglutinate rbc's

Shape Shape Protein shell may Protein shell may assume either assume either icosahedral (20 icosahedral (20 equilateral triangular equilateral triangular faces = cubic faces = cubic symmetry) or helical symmetry) or helical (rod-shaped particles (rod-shaped particles composed of repeating composed of repeating protein subunits protein subunits surrounding core in a surrounding core in a helical fashion = helical fashion = helical symmetry) helical symmetry) shape; some viruses shape; some viruses (e.g. poxviruses) have (e.g. poxviruses) have a complex shapea complex shape

Classification of Viruses Classification of Viruses

Criteria for classificationCriteria for classificationNature of disease (e.g. neurotropic viruses)Nature of disease (e.g. neurotropic viruses)Organ system most frequently involved (e.g. hepatitis Organ system most frequently involved (e.g. hepatitis viruses)viruses)Physiochemical characteristicsPhysiochemical characteristicsType of nucleic acid (ssDNA, ssRNA, dsDNA, dsRNA)Type of nucleic acid (ssDNA, ssRNA, dsDNA, dsRNA)Presence or absence of envelope - ether sensitive are Presence or absence of envelope - ether sensitive are enveloped while ether resistant = nakedenveloped while ether resistant = nakedSize (small = picornaviruses to large = poxviruses)Size (small = picornaviruses to large = poxviruses)Vertebrate viruses classified into 13 families of RNA-Vertebrate viruses classified into 13 families of RNA-containing viruses and 7 families of DNA virusescontaining viruses and 7 families of DNA viruses

Replication Cycle Replication Cycle

Attachment & AdsorptionAttachment & Adsorptioncell surface phenomenon; adsorption involves specific cell surface phenomenon; adsorption involves specific interaction between sites on viruses and receptors on interaction between sites on viruses and receptors on cell membrane of host cell via ionic or electrostatic cell membrane of host cell via ionic or electrostatic interactions (e.g. HIV attaches via gp120 of virus to CD4 interactions (e.g. HIV attaches via gp120 of virus to CD4 receptor on T cells); tropism = susceptibility of only receptor on T cells); tropism = susceptibility of only certain cells to particular viral agent - may be related to certain cells to particular viral agent - may be related to presence or absence of receptors on host cellpresence or absence of receptors on host cell

PenetrationPenetrationusually occurs by phagocytosis (naked viruses) usually occurs by phagocytosis (naked viruses) or fusion (enveloped viruses)or fusion (enveloped viruses)

Uncoating & BiosynthesisUncoating & BiosynthesisUncoating - involves removal of protein coat and release Uncoating - involves removal of protein coat and release of viral nucleic acid; may involve host enzymes of viral nucleic acid; may involve host enzymes (proteolytic) and/or viral coded enzymes brought in with (proteolytic) and/or viral coded enzymes brought in with virus or synthesized in new hostvirus or synthesized in new host

BiosynthesisBiosynthesis Involves synthesis of viral nucleic acid, viral coat Involves synthesis of viral nucleic acid, viral coat

proteins and virus-specific enzymes - each is proteins and virus-specific enzymes - each is synthesized separatelysynthesized separately

Site of viral synthesis varies depending on type of NA; Site of viral synthesis varies depending on type of NA; generalizations include:generalizations include:

Most DNA viruses synthesize DNA in nucleus of Most DNA viruses synthesize DNA in nucleus of host cell & proteins (structural & enzymatic) in host cell & proteins (structural & enzymatic) in cytoplasmcytoplasmMost RNA viruses synthesize all viral components Most RNA viruses synthesize all viral components in cytoplasmin cytoplasm

No infectious virus particles are detected during this No infectious virus particles are detected during this period = eclipse periodperiod = eclipse period

Maturation (Assembly of virions)Maturation (Assembly of virions)

Maturation (Assembly of virions) - involves Maturation (Assembly of virions) - involves combining of synthesized NA and protein; combining of synthesized NA and protein; occurs in nucleus with most DNA viruses occurs in nucleus with most DNA viruses & cytoplasm with most RNA viruses; most & cytoplasm with most RNA viruses; most capsids self-assemble around genome; capsids self-assemble around genome; defective (incomplete) viruses occur when defective (incomplete) viruses occur when NA is not coated by protein (empty coats NA is not coated by protein (empty coats or non-viral NA placed in coat)or non-viral NA placed in coat)

Release Release Host cell disintegration (lysis) - Host cell disintegration (lysis) - occurs when cell fills with virus occurs when cell fills with virus particles; causes death of cell particles; causes death of cell and release of naked virusesand release of naked viruses

Slow release through cell Slow release through cell membrane as part of budding membrane as part of budding process - results in release of process - results in release of enveloped viruses; cell may not enveloped viruses; cell may not die for a long time & can die for a long time & can continue to shed virus particles; continue to shed virus particles; some proteins synthesized some proteins synthesized under viral control become under viral control become incorporated into CM of host incorporated into CM of host and become incorporated into and become incorporated into virion as virus buds through virion as virus buds through membrane (glycoprotein membrane (glycoprotein spikes); important in cellular spikes); important in cellular immunity (proteins are antigenic immunity (proteins are antigenic and host recognizes these and host recognizes these proteins as foreign mounting proteins as foreign mounting immune response against cells immune response against cells with these proteins in their with these proteins in their membranes)membranes)

Effects of Viruses on Host CellsEffects of Viruses on Host Cells

Effects of viruses on host Effects of viruses on host cells = cytopathic effects cells = cytopathic effects (CPE) – morphologic (CPE) – morphologic alterations that occur in alterations that occur in virus-infected cells; often virus-infected cells; often visible in light microscopevisible in light microscope

Inclusions = intracellular Inclusions = intracellular structures; may be structures; may be cytoplasmic or nuclear; cytoplasmic or nuclear; represent aggregates of represent aggregates of viral nucleic acid or viral nucleic acid or protein, mature virions or protein, mature virions or degenerative cell reaction degenerative cell reaction products; serve as products; serve as markers of certain viral markers of certain viral infections (e.g. Negri infections (e.g. Negri bodies = intracytoplasmic bodies = intracytoplasmic inclusions of rabies inclusions of rabies infection; Owl's eyes = infection; Owl's eyes = cytomegaloviruses)cytomegaloviruses)

Effects of Viruses on Host CellsEffects of Viruses on Host Cells

Syncytia (multinucleated giant cells) - represent fusion Syncytia (multinucleated giant cells) - represent fusion of virus-infected cells with contiguous uninfected cells; of virus-infected cells with contiguous uninfected cells; form giant cells with many nuclei; caused by virus-form giant cells with many nuclei; caused by virus-induced changes in CM; allows cell to cell spread of induced changes in CM; allows cell to cell spread of virus without coming in contact with humoral immunity virus without coming in contact with humoral immunity (e.g. respiratory syncytial viruses)(e.g. respiratory syncytial viruses)

Cell necrosis or death - rate of destruction depends on Cell necrosis or death - rate of destruction depends on degree of shut-down of host cellular functions and rate degree of shut-down of host cellular functions and rate of synthesis with accumulation of toxic viral structural of synthesis with accumulation of toxic viral structural proteins; cytolytic effects are visible microscopically and proteins; cytolytic effects are visible microscopically and when destruction is extensive, effects can be seen with when destruction is extensive, effects can be seen with naked eye forming areas referred to as plaques in tissue naked eye forming areas referred to as plaques in tissue cultures; in vivo, destruction of large numbers of cells cultures; in vivo, destruction of large numbers of cells results in impairment of tissue or organ structure & results in impairment of tissue or organ structure & functionfunction

Host Cell ResponseHost Cell Response

Non-immunological = interferon productionNon-immunological = interferon production

ImmunologicalImmunological Humoral - inhibits adsorption of virus to host Humoral - inhibits adsorption of virus to host

cell surface (blocks receptors on virus)cell surface (blocks receptors on virus) Cell mediated - acts against viral-specific Cell mediated - acts against viral-specific

proteins incorporated into host cell proteins incorporated into host cell membrane; involves destruction of infected membrane; involves destruction of infected cell by sensitized T cellscell by sensitized T cells

Laboratory Techniques with Laboratory Techniques with Viruses Viruses Viral isolation - Viral isolation -

require present of require present of living cellsliving cellsLaboratory animals - Laboratory animals - suckling animals suckling animals usually more usually more susceptible than susceptible than adult animals (e.g. adult animals (e.g. suckling mice or suckling mice or guinea pigs)guinea pigs)Embryonated eggs - Embryonated eggs - chick or duck eggs chick or duck eggs used most often; used most often; viruses grow in viruses grow in embryo or on embryo or on membranesmembranes

Laboratory Techniques with Laboratory Techniques with VirusesViruses

Tissue cultureTissue culturePrimary cell lines - prepared directly from Primary cell lines - prepared directly from tissue excised from humans or animals tissue excised from humans or animals (e.g. primary monkey kidney = PMK cells)(e.g. primary monkey kidney = PMK cells)Continuous cell lines - cells that have been Continuous cell lines - cells that have been passaged indefinitely; are usually derived passaged indefinitely; are usually derived from malignant or premalignant cells (e.g. from malignant or premalignant cells (e.g. HeLa from human cervical carcinoma and HeLa from human cervical carcinoma and HEp-2 from human laryngeal carcinomaHEp-2 from human laryngeal carcinoma

Laboratory Techniques with Laboratory Techniques with VirusesViruses

Serological methodsSerological methodsPatient's serum - Patient's serum - show rising titer of show rising titer of antibodies to specific antibodies to specific virusesvirusesTissue culture - used Tissue culture - used to identify viruses that to identify viruses that have been grown in have been grown in the laboratory; have the laboratory; have specific antisera for specific antisera for viruses; most often viruses; most often show neutralization of show neutralization of effect by antisera as effect by antisera as indication of identity of indication of identity of virusvirus