Virus 2013

PowerPoint Presentation - Introduction to viruses

Introduction To Viruses1These two lectures will review some features of viruses from the basic virology to the development of immunity to virus infections.Sub microscopic entity consisting of a single nucleic acid surrounded by a protein coat and capable of replication only within the living cells of bacteria, animals or plants. Definition of a Virus2What is a virusSub microscopic entities consisting of a single nucleic acid surrounded by a protein coat and capable of replication only within the living cells of bacteria, animals or plants.[1]The key features of this definition are as follows: Single type of nucleic acid either DNA or RNA but not both Replication of the virus only with a living cell they are obligate intracellular parasites.These characteristics are typical for ALL viruses whether they infect bacteria, plants or animals. [1] Adapted from Collins English Dictionary

Definition of a VirusObligateIntracellularParasite3What is a virusSub microscopic entities consisting of a single nucleic acid surrounded by a protein coat and capable of replication only within the living cells of bacteria, animals or plants.[1]The key features of this definition are as follows: Single type of nucleic acid either DNA or RNA but not both Replication of the virus only with a living cell they are obligate intracellular parasites.These characteristics are typical for ALL viruses whether they infect bacteria, plants or animals. [1] Adapted from Collins English Dictionary

Virion StructureNucleic AcidSpike ProjectionsProteinCapsidLipid EnvelopeVirionAssociatedPolymerase4 This overhead shows the standard features found in some (but not all) viruses. A virus particle is essentially a piece of nucleic acid surrounded by a protein coat. The protein coat (i.e. the capsid) is a delivery system for transferring the virus genome from one cell to another. The protein serves to: Provide protection to the nucleic acid against the environment - e.g. nucleases etc. Function in receptor recognition - targeting a virus to a susceptible host and cell type. Surrounding this coat there may be a lipid envelope - this envelope is derived from one of the cell membranes and is not determined by the virus. There may be some modification to the lipid composition induced during virus maturation. Inserted into the lipid envelope there are usually virus proteins which are present as spike projections - these are normally glycoproteins.Due to restrictions on the coding size of many virus genomes the capsid of the virion is made up of repeating subunits, which coat the virus genomic nucleic acid. The redundancy also allows for the fact that if there is an inactivation of part of the capsid the virus does not completely lose its infectivity For example the poliovirus RNA (7kb) can specific at most 250,000 Daltons of protein altogether (some must be used for replication) but the poliovirus virion capsid weighs 6 x 106 Daltons.

Genomic Nucleic AcidViruses only possess a single type of genomic nucleic acid either DNA or RNA but not both. This nucleic acid can be in a variety of physicla forms that can be used as a valuable classification feature.Virion MorphologySimple StructureRepetitive StructureHigh Level of Redundancy5 The virions that you will see have the following common features:Simple structure The overall structure is not in general complex although they do perform complex functions (protection of the genome and entry of the virus to the cell)Repeating structure They are generally made up a very few proteins (the simple plant viruses may just have one protein in the virus capsid) OR proteins which are structurally very similar.High Level of RedundancyThere is a high degree of redundancy in the virion, which allows for the partial inactivation of some parts of the virion without actually destroying the virion completely. The general exceptions to these comments are the poxviruses.

Virus Morphology

Helical

Icosahedral61.1. Virion Types The majority of viruses fall within one of two basic structures: Helical virions Icosahedral virions The features of each of these are described in the following.1.1.1. Helical Virions Common form of structure in which the capsomeres wrap around the nucleic acid to produce a helix. In plant viruses this helix may be naked whereas in the case of viruses infecting animals all viruses have an envelope surrounding the capsid structure. The diameter of the helical capsid is determined by the characteristics of the capsomeres and the length of the nucleic acid molecule determines the length of the helix.1.1.2. Icosahedral Virions The only closed shell that can be made with repeating capsomeres is an icosahedron. The simplest icosahedron is a regular solid with 12 vertices and 20 triangular faces - to make this shell there must be sixty identical protomers (i.e. 3 per face)[2]. Larger viruses have a more complex virion. Each triangular face of the icosahedron is divided into six half-triangles. The corners of the inscribed faces are solid lines; those of the basic faces are dashed lines. Monomers are arranged in pentons around the fivefold axis and in hexons around the threefold axis. In the case of poliovirus each of the sixty subunits is made up of three monomers VP1, VP2 and VP3. In higher order capsids still the proteins located at the pentons and hexons are distinct proteins - for example in adenovirus the hexons contain three protomers rather than six as in the basic icosahedron. Icosahedral particles may exist as either naked or enveloped virions.

[1] Naked viruses with helical virions (e.g. plant viruses) are more tightly packed.[2] The smallest and simplest virions made up of 60 identical subunits are that of satellite tobacco mosaic virus. Possess a short RNA (~1600 bases) with just one gene that encoding the capsid protein. This virus can only replicate in cells already infected by tobacco necrosis virus. This virus is referred to as a satellite virus.

1.1.1. Complex virionsNot all viruses fall within these two simple categories and two examples will be given of so-called complex virions. Virus Replication1Virus attachmentand entry12Uncoating of virion23Migration ofgenome nucleicacid to nucleus34Transcription5Genome replication456Translation of virusmRNAs67Virion assembly78Release of newvirus particles87There are many variations on the virus replication and this diagram illustrates some of the basic features of the cycle.1. Attachment and entry: viruses recognise specific structures on the cell surface (referred to as virus receptors), which target the virus to specific cell types and tissue. This is one of the primary determinants for which tissues are infected by a particular virus. The receptor is a normal component of the cell, which the virus has hijacked for the infection process.2. Uncoating: The virion breaks open and releases the virus genome nucleic acid into the host cell cytoplasm. Further replication may take place in the cytoplasm or the nucleic acid may migrate to the cells nucleus.3. Transcription: Virus mRNA is produced using either cellular enzymes or virus-coded enzymes.4. Genome replication: This stage can take place in either the cytoplasm or nucleus of the infected cell. Depending on the size of the virus genome the enzymes involved in genome replication may be encoded by either the virus itself or the host cell.5. Translation: This stage uses the host cell machinery - ribosomes and enzymes etc. Various proteins are synthesised - structural - only in virion - and non-structural - detected only in the virus-infected cell.6. Virion Assembly: The newly formed virus proteins and genomic nucleic acid assemble to produce the new virus particles.7. Virion release: Various strategies are available for the release of the progeny virus from the infected cell depending on the particular virus group. The virus may bud through the cell membrane at which time it picks up the envelope surrounding the virus particle OR the virus may simply cause lysis of the cell resulting in cell death and the release of progeny virus particles.

Cytopathic Effect (cpe)

AdenovirusHerpes virus8Cell death Cytopathic effect This is the end result of many lytic virus infections in which the cell is killed following virus infection. This end result of virus infection is the cause of cpe found in cell culture systems infected with lytic viruses. The form of virus-induced cpe can take many forms ranging from the lysis of the cell to a fusion event with the formation of syncytia.Persistent infection The outcome of some virus infections is not cell death but the development of a persistent (or chronic) virus infection. This differs from the transformation of cells (described below) since in many cases the cells appear similar (or identical) to the uninfected parental cell line. The cells may continue to grow in culture and release infectious virus. These infections are characterised by the virus normally being lytic but under specific situations (e.g. host cell type) the virus establishes a chronic or persistent infection.Latent Infection The capacity of herpes virus to establish a latent infection is essentially another form of persistent infection. In this case the virus is not actually replicating but lying dormant in the host cellTransformation A transformed cell in vitro or a tumour developing in vivo is essentially a cell-type, which shows no control over its cell division with unregulated growth.Transmission of VirusesRespiratory transmissionInfluenza A virusFaecal-oral transmissionEnterovirusBlood-borne transmissionHepatitis B virusSexual TransmissionHIVAnimal or insect vectorsRabies virus9 As with many infections viruses can be transmitted between susceptible individuals by a variety of means. The details provided related mainly to viruses infecting humans. Many animal viruses do not remain infectious for very long outside the host. Respiratory: Influenza A virus (and rhinovirus). Transmission in the form of aerosols during coughing and sneezing. The viruses are fairly sensitive to drying and their transmission is highest when individuals are in close contact. Faecal-oral: Enteroviruses (e.g. poliovirus) A lot of viruses are excreted in faeces following high levels of replication in the gut. Blood borne: Hepatitis B (and HIV). Transferred through contaminated blood products or via shared needles with drug abuse. Sexual transmission: (HIV)Animal/insect vector: Rabies. In many instances the virus infection is a specific pathogen of the animal and is not normally transmitted to humans by any other means. Virus Tissue TropismTargeting of the virus to specific tissue and cell typesReceptor RecognitionCD4+ cells infected by HIVCD155 acts as the receptor for poliovirus10An Importatnt statge in virus pathogenesis is the targetting of the virus for a specific tissue or cell type. This is achieved at the initial stage of virus replication when the virus recognises a receptor on the cell surface.

There are various receptors defined for viruses and two are shown here:CD4+ cells are infected with HIV these are largely T4 helper cellsCD155 is the receptor for poliovirus and the pattern of the expression of this protein folows the specifi cells infected by poliovirus under natural conditions.In vivo Disease ProcessesCell destructionVirus-induced changes to gene expressionImmunopathogenic disease11 The diseases caused by viruses are due to a number of mechanisms of which the major ones are as follows (these have analogies to the manner in which the virus interacts with cells in vitro): Cell destruction following virus infection: Essentially the virus infection leads to the death of the infected cells. This is equivalent to the cytopathic effect observed during the infection of cells in vitro. Virus-induced changes to cellular gene expression: The presence of the virus within the host cell may lead to virus-induced changes in the expression of specific cellular genes. This will be discussed further with respect to the occurrence of virus-induced tumours.Immunopathogenic disease: Some viruses may induce changes in the pattern of the immune response by the host - e.g. infection of specific cells in the immune system, altered exposure of host antigens to the immune system, disturbed expression of specific cytokines during virus infection. Acute Virus InfectionAmount of virusTimeSymptomsVirus12 The graph illustrates the typical pattern of virus replication during an acute virus infection: Following a short incubation period of a few days there a maximal virus production Visible symptoms generally appear just after this peak of virus replication. Depending on the virus the symptoms may last just a few days. The patient recovers and an immune response is generated and the virus is eliminated within 1 or 2 weeks.Acute Virus InfectionsLocalised to specific site of bodyDevelopment of viraemia with widespread infection of tissues13 The spread of virus during acute virus infection can be variable with two general patterns: Localised to specific site of body: The virus infects at a specific site of the body and does not spread beyond that site - i.e. little or no viraemia.Development of viraemia with widespread infection of tissues: The virus can infect at one site in the body but develops a viraemia with extensive spread beyond the initial entry to cause a number of diverse disease symptoms. Poliovirus

14 A human virus infection that is largely controlled by vaccination and is likely to be fully eradicated in the next few years.The infection here is associated with infantile paralysis affecting the lower limbs but in severe cases there can be paralysis of the muscles controlling respiration. PoliovirusEnterovirus.Possesses a RNA genome.Transmitted by the faecal oral route.Cause of gastrointestinal illness and poliomyelitis.

Properties of the virus15Enterovirus.Possesses a RNA genome.Transmitted by the faecal oral route.Cause of gastrointestinal illness and poliomyelitis.

Poliovirus Infection

GutVirusInfectionVirus excretionin the faecesViraemiaNon-neuronaltissuesNeuronaltissuesParalysis16 The main features of an enterovirus infection are as follows: The virus is transmitted by the faecal oral route and is ingested from contaminated food or water - the contamination of these sources is due to the excretion of virus in the faeces of an infected individual. The initial site of infection is the gut and in the majority of cases (ca 90% for poliovirus) the virus does not spread further and the infection may be inapparent. The virus can spread beyond the initial site of infection as a viraemia with virus spread via the blood and lymph. During the viraemia the virus can infect tissues beyond the initial site of entry - e.g. non-neuronal tissues (heart for CBV) as well as neuronal tissues.Enteroviruses are an important cause of aseptic meningitis and in the case of neuronal infections by poliovirus there can be severe paralysis leading to fatalities. Incidence of Poliomyelitis4030201001950196019701980Number of cases (in thousands)ABPoliovirus vaccinesA: Salk killed inactivated vaccine.B: Sabin live attenuated vaccine17The incidence of poliovirus is declining and is due for complete eradication in the next 5-10 years.This has been achieved by the use of two vaccines for poliovirus.Salk Vaccine: The first to be produced. This is a killed vaccine in which the virus is no longer able to replicate but it can still act as an antigen to stimulate an immune response in the vaccinee.Sabin vaccine: Here the virus is alive but attentiated I.e. it can replicate but does not produce disese. It can still induce an immune response. This is the most widelty used vaccine particurlt in the UK. It produces the most efficient immune response.Influenza A virusMyxovirusEnveloped virus with a segmented RNA genomeInfects a wide range of animals other than humansUndergoes extensive antigenic variationMajor cause of respiratory infections

Properties of the virus18Influenza A virus is the second acute infection to be discussed.MyxovirusEnveloped virus with a segmented RNA genomeInfects a wide range of animals other than humansUndergoes extensive antigenic variationMajor cause of respiratory infectionsInfluenza A virus InfectionSpread by respiratory routeVirus infects cells of the respiratory tractDestruction of respiratory epitheliumSecondary bacterial infectionsAltered cytokine expression leading to fevere.g interleukin-1 and interferonSpread of influenza virus

20Respiratory aerosoles can be generated from the respiratory tract by various means from speaking to sneezing.

During a sneeze, millions of tiny droplets of water and mucus are expelled at about 200 miles per hour (100 metres per second). The droplets initially are about 10-100 micrometres diameter, but they dry rapidly to droplet nuclei of 1-4 micrometres, containing virus particles or bacteria. This is a major means of transmission of several diseases of humans.

Respiratory Tract

21There are various means by which the host is protected from infection by influenza virus.The droplets containg the virus may be filtered by fines hairs and cilia in the nasal cavity.Muco-cilliary cells lining the trachea can trap virus particles and sweep the virus to the back of the throat from where it is swallowed and excreted via the intestinal tract.Alveolar macrophages can engulf the virus if it enteres as far as the lower reaches of the lung and alveolar sac.

1988198919901991199219931994199519961997199819990100200300400500600Rate per 100 000 populationYearEpidemic activityHigher than expectedseasonal activityNormal seasonal activityBaseline activityWeekly consultation rates for influenza and influenza-like illness: WeeklyReturns Service of the Royal College of General Practitioners, 1988 to 1999CDR Weekly Report: 5th November 199922Virus epidemiologyInfleunza A virus shows regular outbreaks during the winger months. These are eitherEopidenics: widespread outbreaks within the coutryPandemics: Worldwide outbreaks of the virus affecting large numbers of people.

Point mutation of HA and NAgenesANTIGENIC DRIFTANTIGENIC SHIFT

Genetic ReassortmentHuman H3N2

Avian H3N8Human H2N2Generation of Novel Influenza A VirusesViruses and Human TumoursEpstein Barr VirusBurkitts LymphomaHuman papillomavirusBenign wartsCervical CarcinomaHuman T-cell Leukaemia Virus (HTLV-1)LeukaemiaHepatitis C virusLiver carcinoma24Epstein Barr VirusBurkitts LymphomaHuman papillomavirusBenign wartsCervical CarcinomaHuman T-cell Lymphoma Virus (HTLV-1)LekaemiaHepatitis C virusLiver carcinomaVirus-induced tumoursVirusInfectionUninfectedCell?[]Uncontrolled cellgrowth and tumourformationVirus-induced transformation

Normal cellsTransformed cells26Virus trasnformed cells show a complete deregulation of their normal growth. They tend to pile up over each other and do not stop growing as is typical of normal uninfected/non-trasnformed cells.This change in the phenotype is due to the virus influencing the gene reulation of the cell. Many of the cellular genes affected that lead to the trasnformed pheotype are associated with gowth regulation and signal recognittion.Virus-Induced TumoursVirus infects cell.Virus nucleic acid, as DNA, integrates into cellular genome.Virus causes changes in cellular gene expression.Uncontrolled cell multiplication and tumour formation.27 The following points should be noted for virus-induced tumours: Virus infects cell: The infection process of an uninfected cell by the virus follows the standard pathway. Virus nucleic acid, as DNA, integrates into cellular genome: In the majority of virus tumours the virus nucleic acid integrates and physically joins with the cellular genome DNA. This contrasts with the HSV latent infection where the virus nucleic acid remains independent from the cellular genome.For retroviruses, which have RNA as their genomic nucleic acid, a DNA copy of the virus genome RNA is made.The virus may remain integrated with the cell for an extended period without any overt sign of infection. Virus causes changes in cellular gene expression: Because the virus nucleic acid is physically incorporated into the cell genome it can influence gene expression either through the introduction of new genes (already present in the virus genome) or by activating cell genes in an uncontrolled manner.Uncontrolled cell multiplication and tumour formation: The end result of the altered gene expression is the deregulation of cell growth with the formation of a tumour. Treatment and Prevention of Virus InfectionsAntiviralsVaccines and immunisation28AntiviralsVaccines and immunisationAntiviral TargetsAttachment/EntryNucleic acid replicationVirus protein processingVirus maturation29Attachment/EntryPicornavirusesNucleic acid replicationHuman immunoideficiency virus (AZT)Herpes simplex virus (Acyclovir)Virus protein processingHIV (Protease inhbitors)Virus maturationInfluenza A virus (Neuraminidase blockers)

Problems of antivrualsDificuly in finding a virus specific site against which to direct the antivrialAs with the use of antiiotics resistant mutant scan be readily generated that are resistant to antiirals this is particuarly a problem with those against HIV where the drug has to be used for prolonged periods of time.Problems with AntiviralsIdentification of virus-specific target.Generation of resistant variants.ThanksTake Care Your self..