Microbiology Case 2: HIV - Skildum

Question 1

What is the classification of HIV?

• family
• genome
• envelope
• capsid

Answer 1

• Family: Retroviridae
• Genome: (+) ssRNA
• Envelope: Yes
• Capsid: Icosahedral

Question 2

What is the tropism of HIV-1?

Answer 2

• CD4+ T-cells
  
  • also CD4+ monocytes & macrophages, others

Question 3

What does the HIV-1 genome consist of?

Answer 3

two copies of positive single-stranded RNA

Question 4

What are the four main genes encoded in the small & compact genome of HIV-1?

Answer 4

• gag
  
  • 3 capsid proteins (p24)
• pol
  
  • reverse transcriptase
  • protease
  • integrase
• env
  
  • envelope glycoproteins gp120 and gp41
• LTRs
  
  • integration sites
  • bind host transcription factors NF-_KB, Sp1, TBP

Question 5

How does HIV-1 bind to host cells?

Answer 5

• chemokine coreceptors CXCR4/CCR5
  
  • help guide/bind HIV-1 envelope glycoprotein
• HIV-1 gp120 binds to CD4 receptor on CD4+ T-cell
  
  • initiates another contact between a chemokine receptor and gp41 → stimulate fusion of viral envelope with plasma membrane of host cell

Question 6

After the HIV-1 envelope fuses with the host cell membrane, what are the steps of the life cycle of HIV-1?

Answer 6

• Genome uncoats
• Reverse transcription of viral ssRNA → dsDNA
  
  • takes place in cytosol
  • two rounds of reverese transcriptase:
  • 1) RNA → ssDNA, 2) ssDNA → dsDNA
• Viral DNA migrates to the nucleus
• Viral DNA integrates into host cell DNA
  
  • via viral integrase enzyme
• Expression of viral genes to make viral proteins & viral genomes
  
  • Txn of both mRNAs that make capsid/envelope and full length RNA that will be packaged into new virus
  • NF-_KB (TF) activates/recruits RNA pol II for transcription of viral genome
• Proteolytic processing of viral proteins
  
  • cleavage of unspliced RNA done by viral protease
• Assembly of viral particles & budding from host

Question 7

What is the major cause of CD4+ T cell loss in HIV-1 infection?

Answer 7

• HIV-1 infected T-cells (provirus) express env proteins on their cell surface
  
  • binding of env to CXCR4 induces autophagy
    
    • leads to apoptosis/cell death

Question 8

What normally binds to CXCR4 on lymphocytes and directs their homing to particular tissues?

Answer 8

SDF-1

Question 9

What are the normal function of CD4+ T cells in cellular immunity? How does HIV infection perturb the immune system?

Answer 9

• Normal fxn:
  
  • release cytokines
  • help suppress or regulate immune responses
  • signal B cell antibody class switching
  • activate cytotoxic T cells
  • maximize bactericidal activity of macrophages
• Loss of CD4+ T cells in HIV:
  
  • allows opportunistic infections to take hold
  • increased severity/duration of infections

Question 10

What are the most common opportunistic infections in patients with AIDS?

Answer 10

• Viral:
  
  • Kaposi sarcoma virus (KSV, HHV-8)
  • Cytomegalovirus (CMV, HHV-4)
• Fungal:
  
  • Candida
• Protozoa:
  
  • Cryptococcus
  • Pneumocystis jirovecii

Question 11

What is the key diagnostic protein in HIV-1 infection?

Answer 11

p24

(capsid protein encoded by gag gene in HIV-1)

Question 12

How is HIV-1 infection diagnosed?

Answer 12

• ***Main Screening Test = Indirect ELISA
  
  • 9-12+ weeks out
  • detect presence of anti-p24 Ab
  • confirmed by Western blot
• Early Screening test = Direct ELISA
  
  • 3-9 weeks out
  • detect presence of p24 antigen

Question 13

What tests are used to monitor patient’s with HIV-1 infection?

Answer 13

• CD4+ T cell enumeration by flow cytometry
  
  • looks at cell surface markers
• Polymerase chain reaction
  
  • viral RNA or DNA

Question 14

Which two enzymes are required for the HIV-1 particle to infect a cell and become a provirus?

Answer 14

reverse transcriptase & integrase