Replication & Transmission Of HIV

VIRAL REPLICATION:
  • First step, HIV attaches to susceptible host cell.
  • Site of attachment is the CD4 antigen found on a variety of cells
  1. Helper T cells
  2. Macrophages
  3. Monocytes
  4. B cells
  5. Microglial brain cells
  6. Intestinal cells
  7. T cells infected later on.
  • In early phase HIV infection, initial viruses are M-tropic. 
  • Their envelope glycoprotein gp120 is able to bind to CD4 molecules and chemokine receptors called CCR5 found on macrophages
  • In late phase HIV infection, most of the viruses are T-tropic, having gp120 capable of binding to CD4 and CXCR4 found on T4-lymphocytes. 
  1. The gp120 protein on virus binds specifically to CD4 receptor on host cell with high affinity.
  2. Gp41 causes fusion of the virus to the cell membrane.
  3. After fusion virus particle enters cell.
  4. Viral genome exposed by uncoating particle.
  • Reverse transcriptase produces viral DNA from RNA.
  • Becomes a provirus which integrates into host DNA.
  • Period of latency occurs.
  • During the asymptomatic latent phase of AIDS, the virus is actively proliferating, and can be found in association with Follicular dendritic cells in lymph nodes
  • Follicular dendritic cells have receptors to the Fc portion of immunoglobulins that serve to trap HIV virions coated with anti-HIV antibodies.
  • After a period of latency lasting up to 10 years viral replication is triggered and occurs at high rate.
  • Sero conversion in HIV Infection takes place in 3-4 weeks.
  • CD4 cell may be destroyed in the process, body attempts to replace lost CD4 cells, but over the course of many years body is unable to keep the count at a safe level. Destruction of large numbers of CD4 cause symptoms of HIV to appear with increased susceptibility to opportunistic infections, disease and malignancy.
METHOD OF TRANSMISSION:
  • Sexual transmission, presence of STD increases likelihood of transmission: In the heterosexual trnsmission of HIV There is greater risk of transmission from man to woman(most common50-70%)
  • Exposure to infected blood or blood products.(Risk of HIV transmission is not seen with Plasma derived hepatitis B vaccine, Cryoprecipitate)(least common> 1%)
  • Use of contaminated clotting factors by hemophiliacs0.5%.
  • Sharing contaminated needles (IV drug users> 25%).
  • Transplantation of infected tissues or organs.(But not with Skin scraping)
  • Mother to fetus,Breast milk & perinatal transmission (30-40 %)variable, dependent on viral load and mother’s CD 4 count.
  • HIV infection in child of a HIV +ve mother is 20-30%
  • Health worker gets HIV from an accidental needle prick in1% cases
  • Possibility of vertical transfer of HIV infection if prophylaxis is given is 1% and if not then 25%
Exam Question
  • During the asymptomatic latent phase of AIDS, the virus is actively proliferating, and can be found in association with Follicular dendritic cells in lymph nodes Sero conversion in HIV Infection takes place in 3-4 weeks.
  • In the heterosexual trnsmission of HIV There is greater risk of transmission from man to woman
  • Risk of HIV transmission is not seen with Plasma derived hepatitis B vaccine, Cryoprecipitate
  • Most common mode of transmission of HIV world wide is Heterosexual
  • Least common mode of transmission of HIV is blood transfusion
  • Trans-placental spread is commonly associated with HIV
  • Means of transmission of HIV infection in newborn are Transplacental,Breast feeding
  • HIV infection in child of a HIV +ve mother is 20-30%
  • The transmission of AIDS transplacentally is 30-40 %
  • Health worker gets HIV from an accidental needle prick in1% cases
  • Possibility of vertical transfer of HIV infection if no prophylaxis is given is 25%
  • Possibility of vertical transfer of HIV infection if prophylaxis is given is 1%

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