Retroviruses

• Retroviruses

  A family of RNA viruses that replicate through a DNA intermediary
• Retroviruses

  • Icosahedral
  • Enveloped
  • RNA viruses (two)
  • Use reverse transcriptase
• Reverse transcriptase


  Enzyme that transcribes RNA into DNA
• The family Retroviridae is responsible for slow diseases with long incubation periods
• Three important genera of Retroviruses

  • Spumavirus group
  • Oncovirus group
  • Lentivirus group
• Spumavirus group

  • Bovine Syncytial Virus (BSV)
  • Feline Syncytial Forming Virus (FSFV)
  • Simian Foamy Virus (SFV)
• Oncovirus group

  • Bovine Leukemia Virus (BLV)
  • Feline Leukemia Virus (FLV)
  • HTLV-I (Human leukemia)
  • HTLV-II (Human leukemia)
  • Rous Sarcoma Virus (chicken leukemia)
• Lentivirus group

  • HIV-1
  • HIV-2
  • SIV (simian virus)
  • FelV (Feline virus)
  • Visna virus of sheep
• HIV infections have killed 36 million people
• 35.3 million people were living with HIV in 2012
• Sub-Saharan Africa accounts for 60% of all HIV cases
• There is no cure for HIV but effective therapy is available
• Retroviridae

  • Single stranded RNA
  • More complex than other RNA viruses
  • Extra membrane-like outer covering (an envelope)
  • Lipid bilayer
• Envelope proteins

  gp120
• Capsule protein

  p24
• Matrix protein

  p17
• HIV infection process

  1. Fusion of HIV to the host cell surface

  2. Co-receptor binding

  3. Viral RNA is formed by reverse transcription

  4. Viral DNA integrates into host DNA

  5. New viral RNA and proteins move to the cell surface

  6. Virus matures by protease releasing individual HIV proteins
• HIV is the causative agent for AIDS first described in 1981
• CD4 helper T-lymphocytes


  Cells that HIV infects
• HIV co-receptors

  • CXCR4
  • CCR5
• HIV-1 was isolated in 1983
• HIV-2 was isolated in 1986
• Both HIV-1 and HIV-2 lead to the destruction of CD4 cells
• Loss of cell-mediated immunity occurs due to HIV infection
• HIV-1 virus groups

  • M
  • N
  • O
• The predominant M group of HIV-1 contains 9 subtypes (A-K)
• HIV-2 has six subtypes (A-F)
• Inactivation methods for HIV

  • 10% household bleach
  • 50% ethanol
  • 35% isopropanol
  • 0.5% Lysol
  • 0.3% hydrogen peroxide
  • Heating serum at 56°C for 30 min
  • Boiling for 10 min
  • Autoclaving at 121°C for 20 min
• Routes of HIV transmission

  • Blood
  • Semen/vaginal secretions
  • From mother to child during perinatal period
  • Needle stick transmission
• HIV is not transmitted by insects
• Risk factors for HIV transmission

  • Unprotected anal or vaginal sex
  • Having another sexually transmitted infection
  • Sharing contaminated needles
  • Unsafe practices (e.g., transfusion, piercing)
  • Accidental needle stick injuries
• Pathogenesis of HIV/AIDS

  1. Primary infection

  2. Dissemination of virus to lymphoid organs

  3. Clinical latency

  4. Elevated HIV expression

  5. Clinical disease and death
• Typical course of HIV/AIDS is 2-15 years
• Primary infections have a 4-11 day period between mucosal infection and initial viraemia
• Viraemia is detectable for about 8-12 weeks
• Dissemination of virus to lymphoid organs is critical
• Monocytes and macrophages express CD4 antigen
• Dendritic cells can also be infected by HIV
• CD4 cell counts in uninfected individuals range from 500-1,600 cells/mm³
• AIDS diagnosis is automatic with <200 CD4 cells