Retrovirology 102: Human Immunodeficiency Virus

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Retrovirology 102 Human Immunodeficiency Virus

• Molecular Virology
• How does HIV replicate?
• Pathogenesis
• How does HIV cause AIDS?
• Clinical Stages of HIV Infection and Selected
  Pathogenesis Issues
• The Epidemic and its Impact

Original art Paul Spearman Other Figures
Coffin, Hughes, Varmus, eds., Retroviruses, 1997.
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Questions to Ponder

• What are the stages of HIV infection that can be
  defined clinically?
• What characteristics of HIV make it particularly
  hard to defeat with therapy?
• What characteristics of the virus allow it to
  evade and eventually destroy the host immune
  response?

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• HIV/SIV
• Receptor
• CD4

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ButWhy Isnt CD4 Sufficient?

• Some cells expressing CD4 do not allow
  replication block is at entry
• Some viruses replicate well in macrophages
  (M-tropic) others replicate well in T-cell lines
  (T-tropic) Both have CD4

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Evidence for HIV Entry Cofactors
Mouse cells Expressing human CD4
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Entry Cofactor Coreceptor

• Berger and colleagues discovered first HIV
  coreceptor fusin
• Now known as CXCR4
• 7-transmembrane protein, G-coupled receptor
• CD4 CXCR4 Receptor and coreceptor for T-tropic
  HIV strains

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CCR5

• Several groups identified CCR5 as the M-tropic
  HIV coreceptor. CCR5 binds ?-chemokines (MIP-1?,
  MIP-1?, RANTES)
• Subsequently individuals homozygous for deletion
  in CCR5 shown to be protected from HIV infection
  (1 of Caucasions of W. European descent)
• CCR5 is predominant coreceptor utilized during
  transmission events

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T- vs. M-tropism is determined by ENV-Coreceptor
Interaction
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HIV Entry Process

• Virus binds to CD4 through interactions between
  the 1st immunoglobulin-like domain of CD4 and the
  CD4 binding site on gp120
• Conformational changes are induced in gp120
• A second binding event occurs between the V3
  loop/bridging sheet domains of gp120 and either
  CXCR4 or CCR5
• Further conformational changes occur in gp120,
  leading to release of the fusion peptide domain
  of gp41

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Envelope Protein Structure/Organization
gp120
gp41
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Gp120/CD4/17b Crystal Structure
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Model of Entry/Fusion Process
CD4 Binding
gp41
gp120
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Tat

• An essential viral protein
• 101 amino acid protein
• Potent transactivator of HIV gene expression
• Binds to a stable stem-loop RNA structure known
  as TAR
• TAR present at 5 end of all HIV-1 RNAs
  (nucleotides 1 - 59)

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TAR RNA
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Tat Mechanism of Action

• In absence of Tat, HIV transcripts initiate, but
  fail to elongate
• Transcriptional elongation requires recruitment
  of cellular factors
• Positive Transcription Elongation Factor (pTEFb).
  pTEFb includes cyclin T, CDK9.
• pTEFb phosphorylates RNA polymerase II, rendering
  it more processive

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Proposed Model of Tat Function
RNAPII
CTD
TATAA
NF-kB
SP1
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Tat Additional Functions

• Activation of quiescent T-cells
• Induction of apoptosis
• Modulation of expression of cellular genes
• Extracellular activities chemokine-like, growth
  factor-like activities

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Multiple HIV RNA Species Produced by Splicing
Rev-dependent
gag, pol
AAA
vif
AAA
vpr
AAA
AAA
vpu, env
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Rev

• 13 Kd, RNA-binding phosphoprotein
• Contains nuclear localization, nuclear export,
  and multimerization domains
• Continuously shuttles between the nucleus and
  cytoplasm
• Binds to a specific RNA sequence termed the
  Rev-response element (RRE), within the second
  intron of HIV RNA
• Essential for cytoplasmic transport of all
  incompletely spliced HIV mRNA species

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RRE
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Model of Rev Function
CYTOPLASM
NUP98 NUP214
NUCLEUS
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Transcription-dependent Nuclear Localization of
Rev Stauber et al., Virology 213439 (1995)
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Modes of HIV Transmission
Doctor-patient or patient-doctor transmission (no
needlestick)
Casual contact at home or at school
Insect vectors
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Encounter HIV/AIDS Timeline
Acute retroviral syndrome
Mild symptoms
AIDS and Death
1
2
3
Months
Years
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Case 1 RobertAcute Retroviral Syndrome

• A 23 y/o WM presents to your office with fever
  and cervical lymphadenopathy. He has a macular
  rash on his upper trunk and face. He complains of
  mild headache and nausea.
• Upon review of his medical and social history,
  you note that he is gay. Three weeks ago he had
  unprotected sexual intercourse with a new
  partner. The patient has been HIV-negative on
  previous testing, but is concerned because he
  does not know the HIV status of his recent
  contact.

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Acute Antiretroviral Syndrome

• Occurs during a critical period of viral
  replication / initial host immune response
• Symptoms present in 40-90 of individuals
• Characterized by fever, fatigue, rash,
  myalgia/arthralgia, pharyngitis, lymphadenopathy,
  nausea, night sweats
• Typically clinical symptoms begin 21-28 days
  post-infection
• Very high viral load / p24 antigenemia
• May represent a unique opportunity to intervene
  and preserve immunologic function if recognized

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Acute Infection with HIV
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CD8 CTL Responses Correlate with Decline of
Viremia
Peak viremia
Setpoint
Exposure
1
2
3
Months
Years
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Case 2 LucyChronic HIV Infection

• You meet Lucy, a 36 y/o former IVDU who now is a
  cafeteria worker. She has known she was HIV for
  the past five years, but declined to seek
  therapy. She presents now to see if she should be
  receiving antiretroviral therapy.
• Lucy has no complaints and has a normal physical
  examination. Her CD4 count is 600, and her viral
  load is 35,000/mm3.
• What can you tell her about her current state of
  health? Is she feeling well because the virus is
  latent?

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Encounter Chronic Infection
Acute retroviral syndrome
Mild symptoms
AIDS and Death
1
2
3
Months
Years
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Encounter Point 2 Chronic Infection / Clinical
Latency

• Clinically asymptomatic median duration of 9
  years
• CD4 T-cells declining inverted CD4/CD8 ratio
• Viral load relatively constant
• NOT virologic latency. Viral replication and
  turnover of infected cells is high and rapid
  during this entire period
• This is the stage during which most patients are
  detected and considered for antiretroviral therapy

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A Snapshot of Viral Dynamics
Viral turnover 1010 virions produced/destroyed
per day
Productively infected CD4 T-lymphocytes turn
over rapidly (1-2 days)
Lymphoid tissue unabated replication Throughout
disease course
Latent viral reservoirs are present that may be
inaccessible to immune response and
antiretroviral therapy
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Decay Curve for Plasma Viremia On HAART Therapy
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Viral Reservoirs
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Why Do CD4 T-cell Numbers/Function Decline?

• Multiple Factors Likely
• Direct infection/killing of CD4 cells
• Immune hyperactivation state leads to increased
  apoptosis of activated T-cells
• Thymic function impaired, leading to decreased
  ability to replenish naïve lymphocytes
• Development of SI (CXCR4-tropic) virus correlates
  with rapid CD4 depletion, which may reflect
  ability to infect naïve T-cells and thymocytes
  through CXCR4

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Contributors to CD4 Decline
Direct Killing
Immune Activation Apoptosis
IL-1, TNF
Poor production of Naïve T-cells
?
Overall explanation unsatisfying requires More
research
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Loss of CD4 T-Cell Function is Central to
HIV-Induced Immune Dysfunction

• Long-term Non-progressors (LTNPs) control
  viremia in absence of therapy
• LTNPs demonstrate strong HIV-specific CD4 T-cell
  responses
• Generally have strong HIV-specific CD8 CTL
  responses also require CD4 response for
  persistence
• In contrast, most progressing HIV individuals
  have weak or undetectable CD4 T-cell responses
• Lack of effective CD4 responses suggested to be
  the basis for the losing battle fought by the
  immune system of most HIV individuals

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Case 3 DonaldAIDS/End-stage HIV Infection

• A 23 y/o complains of severe dyspnea and fever.
• He is employed by a local florist. He is gay, and
  8 years ago was quite sexually promiscuous. He
  has refused any HIV testing in the past.
• He appears extremely anxious, is tachypneic, and
  has fine diffuse crackles on lung examination.

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From www.netmedicine.com
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Pneumocystis carinii
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Encounter AIDS
Acute retroviral syndrome
Mild symptoms
AIDS and Death
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2
3
Months
Years
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AIDS

• HIV-infected, CD4 lt 200 cells/mm3 or
  HIV-infected with one of 23 AIDS-defining
  conditions
• Among these Pneumocystis carinii pneumonia,
  Toxoplasmosis, Kaposis sarcoma, esophageal
  candidiasis, cryptococcal meningitis,
  HIV-associated wasting syndrome, HIV-associated
  dementia, disseminated M. avium
• Advanced HIV infection CD4lt50. Median survival
  of 12-18 months prior to HAART therapy

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HIV Wasting
Mandell, Atlas of Infectious Diseases, Volume X1
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CMV Retinitis
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AIDS-defining illnesses summary

• Opportunistic infections infections with
  organisms of low virulence, not normally
  pathogenic in immunocompetent host
• Malignancies
• KS Associated with specific risk groups exposed
  to Kaposis sarcoma-associated herpesvirus
• Lymphomas, carcinomas Probably related to loss
  of immune surveillance rather than HIV-specific
  factors
• At time of onset of AIDS, diffuse and severe
  abnormalities in immune function exist

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Immune System End-stage AIDS

• Lymphoid architecture destroyed
• Severe deficiency of CD4 T cells
• Inability to mount cellular or humoral immune
  responses to pathogens
• Often complicated by cachexia
• Viremia increases dramatically

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HIV Genetic Diversity
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HIV Diversity Multiple Levels

• Globally Cluster into subtypes or clades
• Within an individual Quasispecies develop
  variation continues throughout infection
• Very significant for drug resistance development
  and vaccine strategies
• Reasons
• 1) RNA genome with poor proofreading of
  polymerase (1-2 errors per replication cycle)
• 2) Recombination
• 3) Rapid, high-level replication

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The HIV Epidemic

• 90 of HIV cases occur in parts of the world with
  no access to antiretroviral therapy
• Numbers of individuals dying from AIDS is
  staggering
• A vaccine for HIV is an urgent global need

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About 14 000 new HIV infections a day in 2001

• More than 95 are in developing countries
• 2000 are in children under 15 years of age
• About 12 000 are in persons aged 15 to 49 years,
  of whom
• almost 50 are women
• about 50 are 1524 year olds

UNAIDS Data
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Estimated number of adults and childrennewly
infected with HIV during 2001
Eastern Europe Central Asia 250 000
Western Europe 30 000
North America 45 000
East Asia Pacific 270 000
North Africa Middle East 80 000
South South-East Asia 800 000
Caribbean 60 000
Sub-Saharan Africa 3.4 million
Latin America 130 000
Australia New Zealand 500
Total 5 million
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Adults and children estimated to be living with
HIV/AIDS as of end 2001
Eastern Europe Central Asia 1 million
Western Europe 560 000
North America 940 000
East Asia Pacific 1 million
North Africa Middle East 440 000
South South-East Asia 6.1 million
Caribbean 420 000
Sub-Saharan Africa 28.1 million
Latin America 1.4 million
Australia New Zealand 15 000
Total 40 million
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