Transient%20Reversions%20%20in%20%20MHC-matched%20Hosts

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Dynamic immune responses maintain cytotoxic T
lymphocyte epitope mutations in transmitted
simian immunodeficiency virus variants Barouch
et al.,2005 Nat. Imm. 6(3) 247-252 Hane
Hiim, Rebecca Powell, Sacha Bhinder
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The HIV (Retroviral) Lifecycle
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Reverse Transcription
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CD8
CD4

• Virus-specific CD8 CTL response critical for
  control of HIV-1 replication in humans and SIV
  replication in rhesus monkeys
• Stimulating this response with a vaccine is a
  potential preventative strategy

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• Mamu-A01-positive monkeys vaccinated with
  plasmids encoding epitopes of SIV proteins known
  to be dominantly recognized by their CTLs
• initially control viral replication after
  challenge
• Long term studies show breakthroughs of viral
  replication disease progression
• Correlates with SIV escape mutations in these
  dominant epitopes

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• How genetically stable are these viral variants?
• Are these variants pathogenic?
• Is there an evolutionary interplay between viral
  fitness and immunological pressure?
• Infected naïve
• Mamu-A01-positive and
• negative rhesus monkeys with SIV variants

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Assessing SIV Variant Pathogenicity

• Challenge with SIV viral epitope variants
• Variants escape Mamu-A01-restricted CTL Gag p11C
  epitope
• Infect naïve monkeys with three SIV variants

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Assessing SIV Variant Pathogenicity

• Infected naïve monkeys with three SIV variants
• p11C mutations T182 to S, I, or A.
• Peak viral RNA ?107-108 copies/ml
• 7/9 monkeys died due to disease progression

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Broad Cellular ResponseMamu-A01 Positive
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Broad Humoral Response Mamu-A01 Positive
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Variable Gag-specific Cellular Response

• Mamu-A01 positive monkey
• Gag-specific cellular response showed
  considerable variation.
• Variable despite stable viral RNA
• Following 1º viremia resolution
• Secondary peaks of Gag-specific cellular

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Variable Gag-specific Cellular Response

• Mamu-A01 negative monkey
• Stable Gag response
• Stable Env and Pol in positive and negative

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Variable Gag-specific Cellular Response

• Mamu-A01 negative monkey
• Stable Gag response
• Stable Env and Pol in positive and negative

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Transient Reversions in MHC-matched Hosts
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Method Identification Quantification
of p11C-specific CD8 T-cells
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The MHCpeptide tetramer is made from recombinant
Mamu-A01-p11C complexes, bound to streptavidin
via biotin, labeled with phycoerythrin
T cell
Used in conjuction with FITC-labeled mAb to human
CD8? and allophycocyanin-labeled mAb to rhesus
monkey CD3
- 5 x 105 PBMCs from each monkey stained -
Analysed by FACS for triple stain
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Figure 2

• - No p11C-specific responses during acute
  infection (0-10 wks)
• Distinct but transient expansions of specific
  T-cells in 3/4 monkeys,
• Correlates with increased Gag-specific ELISPOT
  (fig 1)

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Figure 2
Wild-type
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• Data suggests that viruses containing wild-type
  p11C epitope rapidly stimulated expansion of
  specific CTLs
• gtExerted immunological pressure on virus
  to reselect SIV mutants
• escape CTL
  recognition
• p11C-specific CTL function confirmed by
  chromium-release assay (data not shown)
• Magnitude of specific CTL response at 0.3-0.5 of
  cells
• - 10 of typical response
• - likely reflects transient p11C
  antigen stimulus compared to innoculation
  with wild-type virus

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Permanent Reversions In MHC-mismatched Hosts
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Figure 3

• Percentage of viral clones with wild-type p11C
  sequences

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Results demonstrate fitness advantage of
wild-type SIV in the absence of immunological
selection pressure (MHC mismatch)
Time to reversion suggests de novo mutations
(possible wild-type species in innoculum)
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Replicative Capacity in vitro of Mutant SIV

• Assessed the replicative capacity of WT SIV and
  natural mutant SIV variants in the p11C epitope
• in vitro infection model
• SIV Gag (p27) by ELISA

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Replicative Capacity in vitro of Mutant SIV

• Similar kinetics of replication between WT and
  natural mutants
• Small mutation cost does not result in
  replication defect for natural mutants

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Replicative Capacity in vitro of Mutant SIV

• Introduction of natural SIV mutations into WT SIV
  resulted in a replicative defect in vitro

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Replicative Capacity in vitro of Mutant SIV

• Introduction of natural SIV mutations into WT SIV
  resulted in a replicative defect in vitro
• Natural viral variants display higher replication
  rate than engineered variants
• Engineered variants slower in replication

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Replicative Capacity in vitro of Mutant SIV

• Limited replicative capacity of initial CTL
  epitope mutants
• Compensatory viral mutation restore replicative
  capacity.
• Great mutant shift vs. transition to replicative
  mutant

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The Great Escape

• Nature of SIV and HIV result in generation of
  mutation
• Escape CTL detection through epitope mutation
• Selective advantage for escapees results in
  population-level dominance of epitope-mutant
  variants

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but do they last

• Immunological selection results in a fine balance
  between viral replicative potential and CTL
  avoidance
• Epitope mutants persist in MHC-matched hosts
• Reversion to WT transient in face of CTL
  expansion and pressure for mutant selection

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Expansion and Oscillation

• Maximize Fitness--Select for highest replication
  rate with avoidance of a suppressive
  anti-wild-type CTL response
• Transmission changes immunological constraints

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From SIV to HIV

• HIV partially contained following
  infectionlimited success of antiviral response
• Mutation of epitopes provides an explanation for
  viral escape from CTL generation
• Escape constrained by MHC diversity
• MHC diversity enhances CTL epitope specificity
  and viral inhibition
• However, HIV has high rate of mutation,
  replication, and adaptation

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HIV Vaccine Constraints

• Ideally, vaccine epitopes cover all epitopes for
  all population MHC genotypes
• Maximally restrict escapee generation
• Diversity of epitopes exploits MHC
  diversitycounter HIV mutability and adaptability

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