Human Immunodeficiency Viruses

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Chapter 17

• Human Immunodeficiency Viruses

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Introduction

• 2 types HIV-1 and HIV-2 each arose from SIV
• SIV doesnt harm its natural primate host
• HIV damages the immune system susceptible to
  bacterial, viral, fungal and protozoan infections
  AIDS
• HIV-1 is more common than HIV-2
• HIV-1 is largely responsible for AIDS
• HIV-2 is restricted to West Africa
• 5 million new cases/year and 3 million deaths
• 4th leading cause of mortality world-wide
• lead to much research money for antivirals and
  effective vaccine

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HIV Virion

• General characteristics of retroviruses but
  capsid is coned shaped, usually 1 capsid per
  virion but can have 2 or more

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Proteins of HIV Virion

• NC protein is typical highly basic and having
  zinc fingers
• TM and SU proteins are gp41 and gp120,
  respectively
• heavily glycosylated and number represents size
• C of gp41 is inside virion and bound to MA
  protein
• spikes seen on surface are gp41-gp120 trimers and
  there are 14/virion
• gp38 and gp130 of HIV-2 are not related but all
  others have some relatedness
• Also contain Nef, Vpr, Vif, p1, p2, p6 and p6
• Can find MHC II associated with envelope and
  cyclophilin A associated with the capsid

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HIV Genome

• 9.3 kb in length
• Encodes auxiliary genes in addition to gag, env
  and pol making it a complex retrovirus
• Auxiliary genes control viral gene expression,
  transport viral components in cell and modify
  hosts immune response, some have multiple roles
• Uses all 3 reading frames and extensive
  overlapping
• vpu in frame 2 overlaps env in frame 3
• tat and rev are split form when splicing occurs
• HSV-2 no vpu but vpx which is related

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Attachment

• Uses CD4 as cell receptor on several cell types
  such as macrophages and T-helper cells
• Main target is the CD4 T cell
• Attachment uses a site in gp120 to recognize the
  outer domain of CD4
• Must also bind a co-receptor on the host surface
• 7 transmembrane domains and are chemokine
  receptors

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Chemokine Receptors

• Bind chemokine during immune response control
  leukocyte trafficking and T-cell differentiation
• 1 of 2 classes
• C-C is the CCR receptor
• C-X-C is the CXCR receptor
• CCR5 and CXCR4 are HIV-1 co-receptors
• HIV that uses CCR5 are R5 strains and CXCR4 are
  X4 strains, R5X4 strain can use either
  co-receptor
• strain R5 cannot infect naïve T cells but all 3
  can infect memory T-cells

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Chemokine Receptors (continued)

• Some people with repeated exposure to HIV have no
  infection found to have 32 nt deletion in the
  CCR5 gene
• homozygous for deletion are extremely resistant
  to virus
• heterozygous have increased resistance
• mainly Europeans with deletion

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Entry

• gp120 interacts with receptor and co-receptor
  causing a dramatic change in gp41
• gp41 fuses with cell membrane and releases
  contents to cytoplasm as a reverse transcription
  complex (MA, Vpr, RT and IN proteins and viral
  genome)

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RT and Transport to the Nucleus

• Reverse transcription complex associates with
  microtubule primed by tRNAlys-3
• Provirus is usually dsDNA but HIV and other
  lentiviruses have short 3 strand sequence called
  the central DNA flap
• 3 end is polyuridine tract (PPT) also central
  PPT that acts as 2nd initiation site of () DNA
  synthesis
• synthesis of () DNA initiated at 3 PPT stops
  when reach the () DNA synthesis at central PPT
  short overlapping ssDNA important in early stages
  of DNA

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Pre-Integration Complex

• After RT is done, the pre-integration complex
  which also has host proteins moves along the
  microtubule to nucleus
• Can enter nucleus while other retroviruses need
  mitosis probably thru a nuclear pore
• cell is resting T cell or macrophage
• Nuclear localization signal on MA Vpr and IN
  proteins
• Pre-integration in resting memory T-cells
  latent infection
• provide reservoir for infection even with
  antiviral therapy
• Provirus integration prelude to productive
  infection

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Early Phase of Gene Expression

• Cellular TF such as NF?B, AP-1 and Sp-1 bind
  promoter and enhancer in U3 region upstream of
  LTR
• Transcription is terminated downstream of LTR
• PolyA tail is in R region and transcripts are
  polyA at R-U5 junction
• Many genome length transcripts are spliced and 3
  size classes of viral transcripts using a
  Northern blot

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Transcripts

• Largest RNAs are genome length (9.3 kb)
• Other 2 classes are number of mRNAs that were
  spliced
• 4.5 kb single splice transcript
• 2.0 kb multiply spliced transcripts
• Number of splice donor sites/acceptor sites gt30
  mRNA species
• Early in infection RNA translated into Nef, Tat
  and Rev proteins

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Nef Negative Regulatory Protein

• Used to believe inhibited HIV infection but now
  know this protein stimulates replication
• Alters endosome trafficking pathway, reduce
  expression at cell surface of CD4, MHC I and II
  proteins

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Tat and Rev In the Nucleus
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Tat Protein Role

• Transactivator of transcription protein enhance
  transcription
• nuclear localization signal takes Tat to nucleus
• binds 5 end if nascent viral transcript at the
  TAR (transactivating response) element
• cell proteins bind TAR one being kinase which
  phosphorylates components of RNA pol complex
• increases processivity along proviral template
• Tat functions as TF but is unusual as binds RNA
  not DNA
• No Tat transcripts are incomplete , early
  infection do get some Tat to increase synthesis
  of genome-length RNA

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Rev Role

• Regulator of expression of viral protein has
  nuclear localization signal
• Rev accumulates in nucleus and cause shift from
  early to late protein synthesis by binding to Rev
  response element (RRE) in virus RNA
• RRE is present in unspliced and singly spliced
  transcripts but absent in multiply spliced
  transcript
• late genes from genome-length and singly spliced
  but mRNA are not transported from nucleus until
  have multiple Rev bound

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Late Gene Expression

• Gag and Gag-Pol translated from unspliced
  transcript
• get Gag-Pol translated when ribosomal frameshift
  takes place
• 5 of time in UUUUUUA sequence (slippery
  sequence) between NC and p1 domains of Gag
• allows ribosome to shift from frame 1 to 3 also
  using downstream 2? structure
• after frameshift continues to make pol Gag-Pol
  polyprotein

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Vif, Vpr, Vpu and Env

• Translated singly spliced transcript
• Vpu and Env translated in RER from bicistronic
  mRNA
• Env is heavily glycosylated MW of 160kD
• trimers of Env form before cleavage to gp120 and
  gp41
• cleavage by Golgi protein furin (host protease)
  
• Vpu is membrane associated and required for
  efficient budding of virions from plasma membrane
• Vpu and MA region in Gag are phosphorylated

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Assembly and Exit
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Assembly

• Form RNA dimer by base pairing between
  complementary sequence in loop near 5 end of
  each RNA kissing loop complex stabilizes dimer
• Molecules of Gag and Gag-Pol form orderly
  arrangement bind viral genome and other
  proteins that become part of virion
• Basic NC domain with Zn finger bind genome at
  domain called ? - main part of packaging signal
• CA binds host protein cyclophilin A, p6 domain
  binds Vpr
• p6 is late domain responsible for budding of
  virion from host
• Vpr has Pro-Thr-Ala-Pro sequence that binds cell
  proteins to pinch off bud
• Vif some incorporated into virion and also
  keeps APOBEC 3F and G from entering virus binds
  and degrades
• vif gene mutated APOBEC 3 gets in new virus ant
  then new cell making lethal mutations
  deaminates C to U
• Gag-Pol dimers form and undergo self-cleavage
  to form cellular enzymes protease which cleaves
  Gag to mature the virion

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HIV-1 Variability

• High error rate in genome replication
• no proofreading ability in RT
• evolve into groups and subgroups
• Manifested in
• antigens
• host cell range
• resistance to drugs

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Antigen Variability

• gp120 is one of the most variable even with the
  overlap between env and vpu genes
• has 5 variable domains
• Presumably because of pressure from immune system
• Nef, not on the surface of the cell or virus but
  still highly variable

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Host Cell Range and Drug Resistance

• Transmission to new host always associated with
  R5 strains predominantly during acute and
  asymptomatic phases
• in 50 infected people evolve X4 or R5X4
  strains as AIDS develops
• Antiretroviral drugs exert evolutionary pressure
  on virus causing resistance

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Progression of HIV Infection

• After infection see huge rise in viremia
• some have illness resembling glandular fever or
  influenza
• Immune system controls and get period of
  asymptomatic infection
• without drugs usually 8-10 years
• may be shorter or longer depending on host and
  virus
• extensive viral replication 1010 HIV/day,
  persist even with immune system
• CD4 T cells and macrophages are killed by virus
  and uninfected CD4 T cells can be killed by
  apoptosis
• CD4 cells aid B cells, CTL precursors, NK cells
  and macrophages to function
• Virus not cleared because virus evolves as
  infection proceeds new antigenic variants no
  Ab or T cell recognition latent cells are
  shielded from immune system
• People who shift to X4 co-receptor likely to
  proceed to AIDS more rapidly
• Can tolerate onslaught on immune system by
  replacing cells, eventually CD4 cells decline,
  viremia level rises and AIDS develop
• infections with pathogens, disease of brain
  and/or cancers may develop, Kaposi sarcoma and
  non-Hodgkins lymphoma
• HIV-2 linger asymptomatic state and slower
  progression

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Progression
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Prevention

• Test for presence of HIV in blood, organs and
  semen by looking for HIV-specific Ab
• Prevent spread by using auto-disposable syringes
  so cant reuse syringe
• plunger breaks is try to reuse
• Transmission between mom and baby is 15-40 with
  upper end seen in breast feeding moms
• greatly reduced it take antiviral drugs before
  and after delivery
• drugs are not a cure for the patient
• Antiviral drugs given as post-exposure
  prophylaxis following needle sticks and risky
  sexual activity
• Want to create an effective vaccine to reduce
  transmission
• still no suitable vaccine available may be in
  part to rapidly evolving antigenic variants

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