Transcript Viral Persistence

Viral Persistence
Viral Titer (Log10 scale)
Viral persistence
in vivo:
Some Examples
Virus family
Example
Host(s)
Site of
persistence
Cytocidal
in
permissive
cells
Immune
response
HIGH TITER REPLIC ATION
Arenaviridae
LCMV
Mouse
Macrophage
NO
Restricted
Hepadnaviridae
HBV
Human
Hepatocyte
NO
Restricted
LATENT INFEC TION
Herpesviridae
HSV
Human
Sensory neuron
YES
Brisk
Polyomaviridae
Papilloma
Human
Epidermal cells
YES
Brisk
SMOLDERING INFECTION
Picornaviridae
TMEV
Mouse
CNS
Glial cells
YES
Normal
Paramyxoviridae
Measles
Human
Neurons
YES
Super
normal
Lentiviridae
HIV
Human
CD4 lymphocyte
YES
Variable
Sites of persistence are usually terminally differentiated cells
Viral persistence in vivo:
Some Examples and Rules
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RNA and DNA viruses can persist
Strategies for persistence can range from “high-titer” replication to
“latency” to “smoldering” infections
Corresponding strategies for evasion of host immune responses
High titer replication:
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Noncytocidal vs rapid replenishment of target cells
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Ineffective immune clearance due to tolerance, immune complex
formation, viral variation etc.
Latency:
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Viral genome is maintained in non-replicative mode
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“Hidden” from immune surveillance
Smoldering infections:
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Continuous replication at low levels
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Effective immune clerance is prevented by antigenic variation,
infectious immune complex, transmission via intracellular bridges etc.
Cell culture models of viral
persistence
Characteristics of the carrier culture
Nonlytic virus
Lytic virus
What fraction of the cell s are i n fe cted?
~100%
<100%
Are si ngle cel l cl ones always i n fe cted?
YES
NO
Must antiviral factors be pre sen t i n the cu lture
medium to protect the cel l s?
NO
YES (1)
C an the culture be “cured” by addi ng an ti vi ral
anti body or i n te rfe ron?
NO
YES (1)
Does the carrie r cu lture resist superinfection
wi th the sam e virus ?
YES (1)
NO (2)
Determinants of viral persistence can
be mapped
Reovirus: Respiratory Enteric Orphan Virus (dsRNA genome)
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Reovirus is a lytic virus but can be
induced to cause persistent infections in
cell culture by co-infecting cultures with
a lytic wild type virus (Type 2 wt) and a
temperature sensitive variant (Type 3 ts).
The virus isolated late after persistent
infection is a reassortant carrying the
genes of the T2 wt virus except for the
S4 and S1 genes of the T3 ts variant
virus, and it appears that these two gene
segments are responsible for the
persistent phenotype
S4(s3: major outer capsid protein) and
S1(s1 viral attachment protein) from
Type 3ts reduce the efficiency of viral
entry, thus reducing likelihood of
overwhelming lytic infection
Predominant gene segments
(T ype 2wt or T ype 3ts)
Gene segments
Day 16
Day 230
L1
L2
L3
2
2/3
2
2
2
2
M1
M2
M3
2
2
2
2
2
2
S1
S2
S3
S4
3/2
2
2
3/2
3
2
2
3
Evidence for immune clearance of viral
infection: example
If immune system
can clear virus,
what accounts for
virus persistence
Log10 titer per gm
West Nile Virus: IC injection of
10 6.3 suckling mouse LD50 into adult rats
No difference in titer in first week,
implies difference is due to subsequent immune response
(which is abrogated by Cytoxan treatment)
Mechanisms of persistence and
escape from immune surveillance
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High titer persistence
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Not acutely cytocidal or
Target cells are replenished at high rate, and
Tolerance (absence of virus-specific immunity)
 Deletion of naive T-cell clones
 Exhaustion of peripheral virus-specific T-cell clones
 Absence of specific Ab response
E.g. HBV (fig. 6.4), LCMV (fig. 7.4) and HIV
Mechanisms of High Titer Persistence:
LCMV & Immune tolerance
(Fig. 7.4)
Mouse infected at high dose---> viral persistence
Viremia
Exhaustion
of LCMV-specific
CTL
Log10 titer per gm
Log10 titer per gm
CTL
Mechanisms of persistence and
escape from immune surveillance
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High titer persistence
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Nonlytic viruses
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Not acutely cytocidal or
Target cells are replenished at high rate, and
Tolerance (absence of virus-specific immunity)
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Deletion of naive T-cell clones
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Exhaustion of peripheral virus-specific T-cell clones
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Absence of specific Ab response
E.g. HBV (fig. 6.4), LCMV (fig. 7.4) and HIV
a-LCMV Abs circulate as immune complexes
LCMV persistence can be terminated by adoptive transfer of virus specific CTL
Similar for HBV ( see fig. 5.11)
“Lytic” viruses
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SIV/HIV
Constant replenishment of target cell pool
Mechanisms of persistence and
escape from immune surveillance
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Latency (e.g HSV, VZV, EBV, CMV)
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Virus enters and replicates in permissive cells at portal of
entry, after immune induction, virus appears cleared but
actually becomes latent in another cell type
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Genome may be maintained chromosomally (integrated) or
episomally
If genome is in terminally differentiated cells (e.g. neurons for
HSV), no need to replicate genome, but signals required for reactivation (e.g. fever, sunburn, trigeminal nerve insult)
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Axoplasmic spread towards periphery,
conducts virus to skin-->”cold sores”
Herpes Simplex Virus
Retrograde transport of virions from exposure site
to dorsal root ganglion
Remains latent
Activation results in anterograde transport to
epithelial surfaces via peripheral sensory nerves,
replication in epithelium results in vesicles
Mechanisms of persistence and
escape from immune surveillance
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Latency (e.g HSV, VZV, EBV, CMV)
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Virus enters and replicates in permissive cells at portal of
entry, after immune induction, virus appears cleared but
actually becomeslatent in another cell type
Genome may be maintained chromosomally (integrated) or
episomally
If genome is in terminally differentiated cells (e.g. neurons for
HSV), no need to replicate genome, but signals required for reactivation (e.g. fever, sunburn, trigeminal nerve insult)
Latently infected cells express little if any viral proteins,
permitting escape from immune surveillance
Mechanisms of persistence and
escape from immune surveillance
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Smoldering Infections
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Infectious virus is produced, but at minimal levels
Virus continues to spread, may produce
progressive chronic disease
Detectable immune response, sometimes immune
response may even be hypernormal (due to
chronic viral antigenic challenge)
Paradox: why does virus continue to spread in the
presence of a potentially effective immune
response
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
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Brain
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Blood brain barrier limits trafficking
of lymphocytes thru the brain
Neurons express little or no MHC
Class I (resulting in little
presentation of viral antigens and
ineffective CTL response)
Experimental evidence:
allogeneic/xenogeneic grafts
survive better in the brain than in
the skin or other sites
Kidney
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LCMV is cleared more slowly from
kidney than any other tissues
??inability of lymphocytes to cross
subendothelial basement
membrane
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Intracelluar Bridges
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Cell to cell spread of virus without
exposure to immune effector
mechanisms (e.g. Abs)
Measles in SSPE: neuron to
neuron spread in the presence of
high titers of neutralizing
antibodies
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About 1:100,000 primary measles
infection results in SSPE
Virus implicated in SSPE is
maturation defective--either
mutations in matrix protein or
envelope glycoprotein, thus,
selects for efficient cell to cell
spread
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Intracelluar Bridges
Suppression of MHC Class I
Expression
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Virus infected cells rendered
less sensitive to CTL attack
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Adenovirus E1A
SIV/HIV nef
Viremia
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Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Intracelluar Bridges
Suppression of MHC Class I
Expression
Infectious Immune Complexes
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Ab-coated virus remains infectious
Ab-virus complex may be
internalized by Fc receptors on
macrophages-- Ab dissociates
from virus in vacuoles, permitting
infection of macrophages
LCMV, Aleutian Disease Virus can
form infectious immune complexes
and macrophages are major host
cell
Tre atmen t of se ra
From mice persisten tly
i nfected wi th LC MV
LCMV titer
(log10 LD50 per 0.02 m l)
An ti-mouse imm u noglobu l i n
<1.0
Controls
Normal rabbi t se rum
An ti-mouse albu m i n
3.7
3.5
Addition of anti-mouse IgG
remove infectivity
Virus
a-LCMV
Y
Y
Y Ya-mouse IgG
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Intracelluar Bridges
Suppression of MHC Class I
Expression
Infectious Immune Complexes
Impaired CTL function
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E (Effector Cell)
CTL
MHC Class I
T (Target Cell)
Ag specific CTL may be deficient
in effector molecules
HIV specific CTL identified by
tetramer staining are deficient in
perforin content
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HIV antigen
Y Y
Y YY Staining using
a-perforin Abs
What would be your controls?
SAV
HIV-specific
CTL
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Intracelluar Bridges
Suppression of MHC Class I
Expression
Infectious Immune Complexes
Impaired CTL function
Antigenic variation
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Selection for neutralization
resistance; allows for viral
persistence in the presence of Ab
response
11 X 105
pfu/ml
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Virus
Intracelluar Bridges
isolate
(day of
Suppression of MHC Class I
infection)
Expression
0 days
Infectious Immune Complexes
20 days
44 days
Impaired CTL function
62 days
83 days
Antigenic variation
155 days
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Selection for neutralization
resistance; allows for viral
persistence in the presence of Ab
response
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In vivo selection for
neutralization escape variants
 Ab escape
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EIAV
Neutraliz ation inde x (log10) of se rum colle cted on the in dicated
day afte r infe ction
Fe ver
spike
(day of
infection)
21
44
62
83
155
0 days
20 days
44 days
62 days
83 days
155 days
0
0
0
0
0
0
0
0
0
0
0
0
0.7
1.0
0
0
0
0
2.5
1.5
3.5
0
0
0
3.2
1.5
5.4
2.0
0
0
3.2
2.5
>5.4
2.0
3.5
0
--Immune sera neutralizes virus
from earlier time points but not
concurrent virus or virus thereafter
--evidence of antigenic drift, and
explains viral persistence even in the
face of Ab response
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Intracelluar Bridges
Suppression of MHC Class I
Expression
Infectious Immune Complexes
Impaired CTL function
Antigenic variation
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Selection for neutralization
resistance; allows for viral
persistence in the presence of Ab
response
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In vivo selection for
neutralization escape variants
 Ab escape
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EIAV
LCMV
Neutralizing Ab titer
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Viremia
Log10 titer/ml
Indicates humoral immunity
plays a role in viral clearance
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Intracelluar Bridges
Viral Env from Day 16
Suppression of MHC Class I
Expression
138
212
Infectious Immune Complexes
Impaired CTL function
381
772
Antigenic variation
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Selection for neutralization
resistance; allows for viral
persistence in the presence of Ab
response
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In vivo selection for
neutralization escape variants
 Ab escape
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EIAV
LCMV
HIV
X.Wei. et. al. (2003) Antibody neutralization and
escape by HIV. Nature 422:307
Mechanisms of persistence and
escape from immune surveillance:
Smoldering Infections
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Immunological privileged sites
Intracelluar Bridges
Suppression of MHC Class I
Expression
Infectious Immune Complexes
Impaired CTL function
Antigenic variation

Selection for neutralization
resistance; allows for viral
persistence in the presence of Ab
response
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In vivo selection for
neutralization escape variants
 Ab escape
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EIAV
LCMV
HIV
CTL escape
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HIV, LCMV
DNA vaccinated rhesus macaques challenged with
pathogenic SHIV 89.6P
?
20 weeks
CTL escape via
single amino acid change
in immunodominant
CTL epitope
Viral Load
CD4
T-cell
count
p11C
Tetramer
(Immunodominant)
p41A
tetramer
p68A
tetramer
weeks
Virus family
Virus
Host
Disease
ONCOGENIC VIRUSES
Retro
Diseases
associated with
persistant viral
infections:
selected
examples
MuLV
Mice
Hematopoietic, lymphoreticular neoplasms
Hepadna
HBV
Humans
Hepatocellular Carcinoma
Papilloma
HPV
Humans
Cervival Carcinoma
Herpes
EBV
Humans
Burkitt’s lymphoma
HIG HTITER PERSISTENCE
Arena
LCMV
Mice
Glomerulonephritis, vasculitis
Parvo
Aleutian disease
Mink
Glomerulonephritis, vasculitis
LATENT INFECTIONS
Herpes
HSV
CMV
EBV
VZV
Humans
Humans
Humans
Humans
Cold sores, encephalitis
Pneumonitis, retinitis, encephalitis
Mononucleosis
Herpes zoster
SMOLDERING INFECTIONS
Morbilli
Measles
CDV
Humans
Dogs
Subacute sclerosing panencephalitis
Encephalitis, demyelination
Retro
HTLV I
Humans
Tropical spastic paraparesis (HAM)
JC
Humans
Progressive multifocal leucoencephalopathy
VMV
EIAV
HIV
Sheep
Horses
Humans
Interstitial pneumonitis, demyelination
Episodic hemolytic anemia
AIDS
Polyoma
Lenti