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GMS6121 Exam 3 with complete solutions
Examples of acute viral diseases - ANSWER-Common cold caused by Rhinovirus Diarrhea caused by Norwalk virus Examples of chronic viral diseases - ANSWER-Genital herpes caused by HSV Mononucleosis caused by EBV Examples of lethal viral diseases - ANSWER-Hemorrhagic fevers caused by Ebola and Marburg Rabies caused by Rabies virus Individual viruses exhibit tropism for particular cell types and organs. - ANSWER- Examples: HCV, HBV target liver Rotavirus, norovirus target gut Definition of a virus - ANSWER-Obligate intracellular parasite comprised of:
- Nucleic acid genome
- Capsid protein coat In addition to a nucleic acid genome and capsid protein coat, some viruses have ______________ - ANSWER-An envelope, a lipid membrane with glycoproteins provided by the host
Viruses are intracellular parasites because they lack four key features - ANSWER-1) Enzymes that produce chemical building blocks
- Enzyme systems that generate energy
- Enzymes, tRNAs, and ribosomes that enable protein synthesis
- Membrane bound organelles Virion - ANSWER-A complete, single, infectious viral particle Nucleocapsid - ANSWER-capsid and nucleic acid together Naked viruses are resistant to - ANSWER-Drying Heat Detergents Acid Enveloped viruses are sensitive to - ANSWER-Drying Heat Detergents Acid Naked viruses stability and consequences for immune control - ANSWER-Stability Remain infectious upon drying Survive on surfaces and fomites Survive acidity of GI tract Consequences for immune control Must lyse host cells in order to release mature particles Humoral/antibody response is sufficient to neutralize infection Enveloped viruses stability and consequences for immune control - ANSWER-Stability Cannot survive acidity of GI tract Must remain wet for transmission Trasmit by droplets and secretions Consequence for immune control Don't need to lyse cells in order to release mature particles Require Humoral and cellular responses to neutralize infection
- ss/dsDNA hybrid + RT Virus taxonomy - ANSWER-Order, family, subfamily, genus, species General steps of viral replication - ANSWER-1) Bind to cell receptor
- Entry and uncoating
- Early gene expression
- Replication of viral genome
- Late gene expression
- Assembly
- Exit dsDNA viral replication - ANSWER-dsDNA directly used to make mRNA > Protein dsDNA directly used to make additional copies of dsDNA ssDNA viral replication - ANSWER-ssDNA > dsDNA > mRNA > Protein dsDNA is used to make + or - ssDNA. Either one can be packaged, but not both +ssRNA viral replication - ANSWER-+ssRNA directly serves as mRNa to make proteins Rdrp is used to make -ssRNA from +ssRNA -ssRNA viral replication - ANSWER-Rdrp is used to make +ssRNA which can serve directly as mRNA or be used to generate new copies of -ssRNA dsRNA viral replication - ANSWER-Rdrp converts dsRNA to +ssRNA which serves as mRNA to make protein and to generate copies of dsRNA +ssRNA Retrovirus + RT viral replication - ANSWER-RT is used to make dsDNA from the +ssRNA. This occurs while still inside the capsid. Becomes integrated into host genome as a provirus. +mRNA and +ssRNA are generated from the dsDNA Virus grown in embryonated eggs - ANSWER-Older method Inoculation into various parts of egg: allantoic cavity, yolk sac, amniotic Flu vaccines often still made in eggs Virus grown in cell culture - ANSWER-Gold standard
Add sample onto monolayer of primary cells, wait 1-5 days and see if there is growth Recognizing virus growth in cell culture - ANSWER-1) CPE (lysis, rounding, syncytia, inclusion bodies)
- Hemadsorption
- Transformation Negri bodies - ANSWER-Example of inclusion body produced in brain when infected with rabies Hemadsorption test - ANSWER-Add virus and RBCs to cell monolayer Transformation key features - ANSWER-Immortalization Colony formation Tumor formation Loss of contact inhibition Loss of anchorage dependence Biological assays to quantify viruses - ANSWER-Plaque assay (Gold standard) Endpoint calcs. (TCID50, ID50) Focus formation assay Physical assays to quantify viruses - ANSWER-Hemagglutination Direct particle count Nucleic acid testing (PCR) Multiplicity of infection (MOI) - ANSWER-number of INFECTIOUS particles per cell Only plaque assay gives you actual number of infectious particles Direct detection of virus - ANSWER-Directly detects virus particles, proteins, nucleic acids High confidence Indirect detection of virus - ANSWER-Serology (detects antibodies)
Often measured by ELISA Sometimes measured by neutralization or hemagglutination inhibition Diagnostics methods depend on ________ - ANSWER-Suspected virus Clinical symptoms Available tests Syndromic testing - ANSWER-Use of multiplex panels Ex: BIoFire FilmArray GI Panel Cellular outcomes of viral infection - ANSWER-Cell death Abortive infection Persistent infection Transformation Cell death/CPE due to infection - ANSWER-Due to: Diversion of cell's energy Shutoff of synthesis Competition for ribosomes and TFs Results in pyrogens: Fever inducing Abortive infection - ANSWER-Virus initiates infection but does not generate infectious particles Due to: Lack of replication conditions Host antivirals Cell may be damaged, killed, or transformed, but does not release infectious progeny Persistent infection - ANSWER-No cell death or alteration
- Chronic
- Latent
- Recurrent
Chronic infection - ANSWER-Constant production of virus at low levels Long incubation period Ex: HBV, HCV Latent infection - ANSWER-No production of viral progeny, but genome is maintained Low-level reactivation Ex: Herpesviruses Recurrent infection - ANSWER-Reactivation of a latent infection due to stress or other factors Ex: HSV Transformation - ANSWER-Virus alters cell Results in abnormal cell growth, lack of contact inhibition, loss of polarization, immortalization Viral mechanisms of transformation - ANSWER-Viral oncogene Integration into host chromosomes and disrupt regulation genes Cycle of chronic damage (HCV) Pantropic - ANSWER-Virus can replicate in many tissue types Determinants of virus tropism - ANSWER-Accessibility of permissive cell Presence of cognizant surface receptor Presence of intracellular host factors required* Absence of suppressive antivirals* Disseminated infection - ANSWER-Spread beyond primary infection site
This is routine for some viruses: HSV, Rabies This is less common for some viruses: Polio Rare due to basement membrane barrier Most common method of neural spread Neural spread via hemotagenously - ANSWER-Phagocyte enters CNS with viral particles Cytokines can increase permeability of basement membrane Virus evasion of innate immunity - ANSWER-Block IFN production and signaling Block IFN-induced antivirals Block other cytokines Suppress cell responses Virus evasion of adaptive immunity - ANSWER-Replicate in sites not ready accessible to immune cells Interfere with antigen presentation on MHC Alter MHC trafficking Spread cell to cell Evolution Immunopathology - ANSWER-Disease state resulting from host own immune reponse Viral induction of flu like symptoms Over stimulation of innate immune systm Antibody-dependent enhancement (Dengue) Generation of immune complexes
Viral induction of flu like symptoms - ANSWER-Cytokines induced during infections can act as pyrogens which act on hypothalamus to produce fever Vasoconstriction reduces heat loss > Chills Shivering to produce heat When fever stops, vasodilation and sweating cools the body Over stimulation of the innate immune response - ANSWER-Induction of cytokines can have damaging effects: Immune suppression Cytokine storm > tissue damage Ex: Ebola and dengue hemorrhagic fevers Antibody dependent enhancement of infection (ADE) - ANSWER-Occurs during second infection with different dengue serotype Patients who have already been infected with dengue once produce antibodies to another serotype. The antibodies bind the virus but doesn't neutralize it, it actually enhances infection of cells with Fc receptors Generation of immune complexes - ANSWER-In chronic infections Complexes are deposited in kidney Results in damage and sometimes malignancies Laboratory strains vs natural strains - ANSWER-Laboratory strains are selected Natural strains are a mix of genomes, more diverse, quasispecies Genotype - ANSWER-genetic makeup Phenotype - ANSWER-measurable manifestation of genotype in a given assay system One genotype can have many phenotypes - ANSWER-Sabin poliovirus vaccine
Used to: Sequence virus genome Engineer specific mutations Analyze function of a gene Seeks to find phenotypes that may derive from a genotype Picornaviruses - ANSWER-Small, naked, +ssRNA Uses IRES instead of 5' cap 5"-S/NS-3' Includes polio, HAV, FMDV, Rhinoviruses Picornavirus genome organization - ANSWER-1 ORF, 1 polyprotein Picornavirus capsid proteins - ANSWER-VP1-VP Picornavirus virion structure - ANSWER-Icosahedral capsid of 60 subunits Spherical particles Resistant to low pH: enteric Replicate in respiratory tract too Shell formed by VP1-VP VP4 is internal Picornavirus attachment - ANSWER-Wide variety of receptors: Ig-like, adhesion proteins, integrins Many share receptors
- CD55 is receptor for coxsackie A and B, echovirus, and enterovirus Some only need one receptor, some need co-receptor Picornavirus receptor binding site - ANSWER-Canyons or surface loops Picornavirus pH-dependent release from capsid - ANSWER-Acidification in endosome
For acid-labile viruses FMDV and rhinoviruses Picornavirus pore formation release from capsid - ANSWER-For pH resistant viruses Receptor-induced conformational changes results in pore formation Pocket factor: stabilizes virion Poliovirus Picornavirus polyprotein cleavage - ANSWER-By viral proteases 3C, 3CD, 2A Often occurs as the polyprotein is being produced Picornavirus RdRp/3D - ANSWER-Very efficient, but error prone Synthesizes both + and - strands Template dependent Primer dependent VPg primer for replication in picornavirus - ANSWER-Is uridylylated by RdRp/3D and is used to prime + and - strand synthesis Template for uridine addition is cre hairpin in NTR Poliovirus pathogenesis (picornavirus) - ANSWER-Replication in extraneural tissues thought to be required for CNS invasion Spread to CNS by axonal transport Once in the brain, it kills neurons Enterovirus pathogenesis (picornavirus) - ANSWER-Enters oropharynx and intestine Spreads to lymphoid tissues then to bloodstream where it can enter many tissues: skin, muscle, brain, spinal cord, meninges poliomyelitis - ANSWER-Mostly asymptomatic 25% Abortive poliomyelitis (fever, headache, sore throat) 2% Nonparalytic poliomyelitis (headache and neck stiffness) <1% Spinal paralytic poliomyelitis (flaccid limb paralysis)
5' Cap Many ORFs, many proteins Norovirus nonstructural proteins - ANSWER-p48/2AB: disrupts cellular protein trafficking, localizes to golgi membranes and causes disassembly, scaffolding role NTPase/2C p22/3A: inhibits host protein secretion, golgi disassembly VPg: covalently linked to 5' end of genomic and subgenomic mRNAs, translation initiation, protein primer 3CLpro: protease RdRp/3D
