virus 2

• Viral Pathogenesis

  The study of how viruses cause disease in their hosts
• Virus Life Cycle

  1. Attachment

  2. Entry

  3. Uncoating

  4. Viral Gene Transcription

  5. Genome Replication

  6. Translation

  7. Assembly

  8. Release
• Pathogenicity

  The ability an organism has to cause disease
• Pathogenicity

  • Composed of two factors: Infectivity (ability to infect and colonize a host) and Virulence (ability to cause host cell damage)
• Infectivity

  Measured as infectious dose (Number of microbes necessary to initiate infection and cause disease)
• Virus Attachment

  Highly specific process involving complementary receptors on the surface of susceptible host cells
• Virus Entry

  Occurs by either cell membrane fusion (non-endocytotic pathways) or receptor-mediated endocytosis (endocytotic pathways)
• Entry of Herpes Simplex Virus

  1. Initial binding gB or gC to heparin sulphate

  2. Attachment of gD to HveA, Nectin 1 & 2

  3. Fusion of the viral envelope with cell membrane
• Entry of Human Immunodeficiency Virus

  1. Binding of HIV gp120 to CD4+ T cells

  2. Enables binding of gp120 to CCR5 or CXCR4

  3. Fusion of gp41 with cell membrane

  4. Nucleocapsids are targeted to nucleus
• Entry of Influenza Virus

  1. Binding of haemagglutinin (HA) to sialic acid receptor

  2. Internalisation in clathrin coated pit

  3. Movement into endocytotic vacuole which fuse with lysosomes

  4. Low pH triggers conformational change in HA trimer

  5. Exposes fusion domain which allows fusion of viral membrane and endosome membrane

  6. Release of nucleocapsids into cytoplasm
• Uncoating

  Release of viral nucleic acid from viral capsid
• Virus Replication

  DNA viruses use host machinery in the nucleus (except poxviruses) to make more dsDNA, while RNA viruses replicate in the cytoplasm (except influenza and retroviruses)
• Virus Assembly

  Translation of viral proteins in the cytoplasm, assembly of virus capsids from newly synthesised components, and encapsidation of the viral nucleic acid
• Virus Release

  Enveloped viruses by budding from the plasma membrane, non-enveloped viruses released by cell lysis
• Steps in the Virus Replication Cycle

  • Attachment
  • Entry
  • Uncoating
  • Viral Gene Transcription
  • Genome Replication
  • Translation
  • Assembly
  • Virus Release
• Viruses Discussed for Entry Mechanisms

  • Influenza
  • HIV
  • Herpes Simplex Virus
• Virus Tropism

  Specificity of a virus for a particular host, tissue or cell
• Factors Affecting Virus Tropism

  • Susceptibility - Appropriate cell surface receptors for entry
  • Permissiveness - Able to support replication of the virus
  • Cellular proteases can activate fusion
  • Temperature of replication
  • pH lability of viruses
  • Anatomical barriers
• Experimental Study of Virus Spread in an Infected Host

  1. Inoculate mice in the foot pad with mousepox

  2. At daily intervals look for virus in inoculated foot pad, lymph nodes, spleen, skin, and blood
• Pathogenesis is important for scientific interest, control of virus diseases, diagnosis, and understanding sources of infection/transmission routes
• For virus infection to occur, the cell must be: 

  • Susceptible

  • Appropriate cell surface receptors for entry (susceptibility) 

  • Permissive

   − Able to support replication of the virus

   − May need particular cellular proteins to complete infection

   − May need to be in a particular cell type

   E.g. Canine parvovirus needs rapidly dividing cells
• Cellular protease can activate fusion

   Some enveloped virus require proteolytic cleavage of envelope glycoprotein for activation of fusion domain 

  − Infectious virus is only found in cell types that contain proteases that cleave the glycoprotein

   − E.g. influenza haemagglutinin (HA) spike protein

   • must be cleaved into HA1 and HA2 by cellular proteases
• Protease cleavage by digestive enzymes 

  • Reoviruses are activated into infectious virions by cleavage with digestive enzymes 

  • Cleavage of VP4 spike to VP8 and VP5 

  • Conformational change permitting virus to bind to M cells in the gut
• Temperature of replication

   • Most human viruses replicate at 37°C 

  • Upper respiratory tract has a lower temp – about 33°C 

  • Rhinoviruses replicate efficiently at 33°C but poorly at 37°C

   • This limits their ability to spread beyond the upper respiratory tract
• pH Lability of viruses 

  • Gastrointestinal tract presents a harsh environment

   −Acid pH of stomach

   −Alkaline pH of intestine

   −Destructive effects of pancreatic enzymes 

  • Not many viruses can survive this

   −Most respiratory viruses are inactivated if swallowed (exception is adenovirus

  ) −Rotaviruses/ caliciviruses can survive (unenveloped viruses)
• Anatomical Barriers

  • Ability of virus to breach barriers such as blood brain barrier will limit their distribution

   • Poliovirus/ West Nile virus 

  • Sometimes (but not always) spread to CNS
• Respiratory Tract

   Defenses 

  • Specialised ciliated epithelium & mucus: MUCOCILIARY ESCALATOR (in Upper Respiratory Tract (URT) and bronchi)

   − Filters out large particles (particles <5uM can enter terminal airways and alveolar)

   • Sneezing & coughing • Innate immunological defences (e.g. alveolar macrophages, complement, cytokines, natural killer cells) 
• Spread via Bloodstream (Viraemia):

  • Some viruses can spread to distant sites in the body.
  • Virus reaches blood via the lymphatic system.
  • Primary viraemia (clinically silent) increases virus levels, allowing infection of distant organs.
  • Secondary viraemia results from virus replication in other organs, leading to high concentrations of virus in circulation.
  • Allows entry and exit routes from the host to differ, usually with a longer incubation period and more severe pathology.
  • Greater involvement of adaptive immune responses and IgG antibodies.
• Gastrointestinal (GI) Tract:

  • Defenses include low pH in the stomach, bile, proteolytic enzymes, mucous, and pH changes in the duodenum.
  • Entry via ingestion (oral route).
  • Examples: Rotavirus, Norovirus.
• Examples of viruses entering via respiratory route 

  • Influenza 

  • Foot and Mouth Disease Virus 

  • Rhinovirus (common cold)
• How do viruses enter the body & initiate infections?

   • Skin • Respiratory tract 

  • Alimentary tract (GI tract

  ) • Urogenital tract 

  • Eye Viruses attach to cells at these locations by attaching to receptor molecules on certain cells