Download 18 AIDS

Acquired
Immunodeficiency
Syndrome
Chapter 44
HIV and AIDS
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HIV infection usually leads to
Immunosuppression
Subsequent susceptibility to opportunistic infections
Cancers
Wasting
Central nervous system degeneration
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Molecular and Biologic Features of
HIV
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HIV structure and genes
HIV is a diploid RNA virus
Genome is 9 kb
9 genes
16 proteins
Protein core
Enveloped
Long-terminal repeats (LTR) flank the genome and contain ciselements recognized by cellular transcription factors
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The cellular transcription factors are expressed only when the T cell
participates in an immune response
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Molecular and Biologic Features of
HIV
Viral life cycle
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Viral glycoproteins gp120 and gp41 protruding from the envelope bind
to CD4 and a chemokine receptor (CCR), respectively, on the target
cell surface
Only cells that express both CD4 and CCR are susceptible to HIV
infection
The viral envelope fuses with the cell's membrane and the core is
dumped into the cytoplasm
The core proteins dissociate, liberating the viral genome and several
enzymes into the cytoplasm
The viral reverse transcriptase (RT) synthesizes a double-stranded
DNA copy of the viral genome
The viral DNA is then integrated into the host cell chromosome
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Viral enzyme is integrase
Integrated viral genome is called a provirus
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Molecular and Biologic Features of
HIV
Viral Life Cycle (cont.)
It is evident that viral gene expression requires cell activation
Cellular transcription factors (trans-elements) recognize the
proviral LTRs and stimulate viral gene expression
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Cellular NFκB
Cellular Sp1
Several viral polypeptides are post-translationally cleaved into
functional proteins by the viral protease
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env polypeptide (gp160) to gp41 and gp120
gag polypeptide (core proteins) to p17, p24, p7, p6
Viral proteins and nucleic acids assemble into a core
gp120 and gp41 protrude from the cell's surface
The core travels to the surface where it buds through the
membrane, picking up its envelope containing gp120 and gp41
proteins
When the rate of budding exceeds the rate of membranesynthesis, the infected cell begins to die
Course of HIV Disease
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Early on, HIV infection is contained by the adaptive immune
response
Infected individuals often have a flu-like illness
Viral loads are high
Immune response clears the majority of infected cells and virus
Some virus is sequestered in the lymphoid tissues
Follicular dendritic cells trap HIV (but they are not infected)
Virus remains viable for more than 1 year
Many T helper cells become infected with the virus, but the virus
remains dormant because the majority of T cells are not activated
As infected Th cells engage a pathogen, the virus becomes active
and produces more viruses and killing the T cell
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Course of HIV Disease (cont.)
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This results in a hole in the T cell repertoire
Subsequent exposure to the microbe goes unchecked because all
the T cells to the microbe have been depleted
Loss of Th cells compromises the immune system's ability to
contain pathogens
Other cells can be infected without CPE and act as a reservoir of
virus
Macrophages
Dendritic cells
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Immune evasion by HIV
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High mutation rate changes immunogenicity of the virus
Antagonistic HIV peptides often emerge that induce T cell
anergy
HIV nef protein downregulates MHC class I expression
nef also downregulates CD4 expression
vif inhibits APOBEC3G, an antiretrovirus protein (induces C > U
mutations)
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Long-Term Nonprogressors
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A small percentage of infected persons fail to
develop disease
Some are HIV+ but do not progress to AIDS
It is unknown why most of these individuals
remain healthy (but infectious), but...
Some of them are heterozygous for the
CCR5Δ32 allele of chemokine receptor 5.
This mutation results in a substantiallyreduced infectivity of cells because HIV
cannot induce a membrane fusion event
Therefore, virus does not enter the cells
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Treatment and Prevention
Multi-drug cocktail (highly active antiretroviral therapy [HAART])
Nucleotide analogs impair viral DNA synthesis
Protease inhibitors prevent viral maturation
Multi-drug resistant HIV strains are already appearing
Life expectancy
1985 - 18 months from diagnosis
2006 - 25 years from diagnosis
2012 - Diagnosis at age 25
Expect to live another 51 years
Uninfected person expected to live another 52 years
Vaccine (0/26)
Must be safe
Must be effective
Must be cheap (i.e. 3rd world)
Must not require special care and handling (e.g. refrigeration)
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Clinical Features
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AIDS is defined by:
HIV+
T helper count below 200 cells/μl (normal is 500+)
When opportunistic infections occur
Oral candidiasis and tuberculosis; 250-500 cells/μl
Cryptotsporidiosis; 200 cells/μl
<200 cells/μl
Pneumocystis carinii pneumonia
Disseminated Mycobacterium avium
Cryptococcosis
Toxoplasmosis
Visceral herpes simplex viruses
Cytomegalovirus retinitis; <50 cells/μl
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The Global AIDS Crisis
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AIDS is altering the social structure of Africa
Many African towns have lost their entire adult populations to AIDS
This has resulted in many orphans
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These children, without parental guidance and financial support, face substantial
challenges
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No education
Many die
Some are conscripted into rebel armies
African economies cannot support adequate HIV/AIDS management
Societal collapse may occur
It is the only continent where life expectancy is declining
African AIDS Orphans
Nation
Orphans (2005)
South Africa
1,200,000
Tanzania
1,100,000
Zimbabwe
1,100,000
Kenya
1,100,000
Uganda
1,000,000
Nigeria
930,000
Zambia
710,000
DR Congo
680,000
Malawi
550,000
HIV Vaccines
HIV gp120/gp41
Obstacles
to
the
Development
of
HIV
• Antigenic diversity and hypervariability of the
Vaccines
virus
• Transmission of disease by mucosal route
• Transmission of the virus by infected cells
• Resistance of wild-type virus to
seroneutralization
• Integration of the virus genome into the host
cell chromosomes
• Latency of the virus in resting memory cells
• Rapid emergence of viral escape mutants in
the host
• Down-regulation of MHC class I antigens
• No spontaneous recovery from natural
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Principal Roles of the HIV Accessory
Proteins
Vpr
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Vpu
• Down-regulates expression of CD4 molecules by targeting them to the
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Together with matrix protein, targets the viral preintegration complex to the
nucleus
Arrests dividing cells in G2 of the cell cycle
Enhances HIV replication in macrophages
proteasome, leading to their degradation in the endoplasmic reticulum
Forms ion channels in the cell membrane, thus helping to promote the release of
virions from the cell
Neutralizes APOBEC3G, an innate antiretroviral mechanism (C>U mutase)
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Vif
• Plays a role in provirus formation and stabilizes newly synthesized DNA
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intermediates
Associates with cytoskeleton intermediate filaments and helps transport incoming
virions to the nucleus
Neutralizes an APOBEC3G antiviral mechanism
Principal Roles of the HIV Accessory
Proteins
• Nef
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Associates with cellular protein kinases (PAK65, p56tck, p59Hck)
Stimulates viral DNA synthesis and enhances virus infectivity in primary T
cells and macrophages
Enhances virus replication in vivo, contributing to high viral loads and
pathogenesis
Binds CD4 molecules at the plasma membrane and mediates their rapid
endocytosis and lysosomal degradation
Down-regulates cell surface expression of major histocompatibility complex
class I antigens and of immune costimulatory molecules of CD80 and CD86,
a cytotoxic T-lymphocyte escape mechanism
Activates the expression of FasL, which induces apoptosis in bystander
cells that express Fas
Clinical Progression of HIV Disease
determinants
and epitopes of
• AAntigenic
major obstacle
in
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effective vaccines is
the
interest
elicitation of Nabs
The principal
neutralizing determinant
appears to be the
hypervariable V3 loop
of gp120
• Antibodies against one strain often are
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not cross reactive to other strains
HIV X4 vs. HIV R5 strains
Other epitopes include V1, V2 and
CD4-binding domain (competitive)
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• gp41 has a three
epitopes
• Antibodies to each inhibit gp41mediated fusion
PDB ID 2B4C
Antigenic determinants and epitopes of
• T cell epitopes areinterest
not restricted to virus
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surface antigens
Any HIV polypeptide can contain T cell
epitopes
An effective antiviral immune response
requires CD8+ CTLs
• CTL responses are sustained by CD4 Th responses
• Unfortunately, this CD4+ T cell activation is precisely what HIV requires for
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virus replication
Thus, an effective Th cell response leads to the depletion of those Th cells
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CTL escape mutants (peptide epitopes)
always arise during HIV infection
• Since T cells cannot undergo affinity maturation, like B cells, these escape
mutants evade the CTL response
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formidable challenge
• Most classical vaccines provide protection from
Candidate
HIV
vaccines
The development of an HIV vaccine is a
disease but do not actually prevent infection
• They allow a limited but controlled replication of the pathogen at the portal of
entry
• This raises the question of whether an HIV
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vaccine, if unable to prevent infection, could
prevent development of disease
The difficulties of HIV vaccine testing include
Testing must be done in humans
Since controlled experiments cannot be
done, must rely on retrospective analysis
• Test population must be a high-risk group
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Whole
inactivated
and virus-like
• Not currently
beingvaccines
considered
• Regulatory andparticles
safety concerns
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(alloantigens, xenoantigens)
All HIV strains are propagated in
transformed human cell lines
• Transformed human cells cannot be used for virus propagation for
vaccines
• Formalin damages HIV epitopes, such that
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they fail to elicit strong Nabs
Virus-like particles
Canarypox virus with all HIV proteins
except RT
Produced in primary cell culture
Elicit strong Nab response when
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• All workLive,
has attenuated
been conducted
with
strains
of
vaccines
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simian immunodeficiency virus (SIV) in
monkeys
SIVmac
Directed nef deletion mutant
Administration results in low-grade infection
without clearance
Provided protection against challenge with
1,000 ID50 of virulent SIV
However:
• Some adult monkeys developed AIDS from vaccine
• Fatal to neonatal monkeys
Additional deletions of LTR and vif are
currently being tested
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Live recombinant vaccines
Potential vectors for HIV vaccine
Viruses
Vaccinia virus
Canarypox, Fowlpox
Adenovirus
Alphaviruses (VEEV,
SFV)
Flaviviruses (YF17D)
Rhabdoviruses (VSV)
Orthomyxoviruses
Paramyxoviruses
Picornaviruses
Bacteria
Bacillus sp.
Salmonella
Lactobacillus
Streptococcus
Listeria
• Subunit vaccines are composed of
polypeptides from the virus that are antigenic
Subunit vaccines
and neutralizing
Typically produced by recombinant DNA
technology
• E. coli or other bacteria
• Yeast
• Baculovirus
• DNA vaccines
Host cells for production must be able to
perform post-translational modifications
Soluble envelope glycoproteins
Exclude transmembrane domains
Coexpress gp120 and gp41
Easy to include antigens from multiple HIV
strains
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• Genetically engineered DNA molecules with
DNA
Vaccines
nontransforming
eukaryotic
cis-elements
• Typically plasmids
• Easy to produce in large quantities
• Extremely heat stable
• Requires multiple boosters
• Often require delivery by “gene gun”
• Microscopic beads coated with DNA
• Injected into the skin/muscle with high
pressure
• Presumably, the beads carry the DNA into
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cells, which then activate gene expression
from the plasmid
This produces antigen that is processed by
Human Clinical Trials
• More than 20 have been completed; all have
failed
• Many others are ongoing
• Trials to date (completed and ongoing)
• Subunit vaccines - 35 trials
• Viral vector vaccines - 35
• Bacterial vector (Salmonella typhi) - 3
• DNA vaccine - 27
Human Clinical Trials
• VaxGen gp120 phase II trial (VAX004
candidate)
• 5,095 volunteers
• Immunized with recombinant soluble gp120
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from MN and GNA8 strains
• Injections at 0, 1, 6, 12, 18, 24 and 30 months
• Assessment at 36 months
Objectives
• Does vaccine protect against HIV infection?
• What differences between vaccinated and placebo infections?
Results
• Vaccine did not protect (6.7% vaccinated vs. 7.0 % placebo)
• No differences between between groups in regard to infection responses