Virus-Induced Immunopathology Cell mediated Immunopathology Antibody-mediated Immunopathology Virus-initiated Immunopathology.

Virus-Induced Immunopathology

Cell mediated Immunopathology Antibody-mediated Immunopathology Virus-initiated Immunopathology

Cell mediated immunopathology Contact between CTL (CD8+) and

target TCR recognizes cognate peptide

presented by MHC I Activation of CTL; release of

membrane granules with perforin and granzyme

Perforin: Pore formation in target cells resuting in cytolysis

Granzyme B: induction of apoptosis

Release of Inflammatory cytokines (TNF-, IFN-, ILs)

CD4+ T cells can enhance CD8-mediated immunopathology

NK Cell Non-specific of “non-self” ADCC

CTL

MHC Class I

Viral antigen

Effector Cell

Target Cell

TCR

MHC Class I

Viral antigen

Infected Choriodal Cells

Inflammatory Cytokines

Inflammatory Macrophageat

tract

ion

NOS

Nitric Oxide

Convulsions (?)

CTL

Effector Cell

TCR

Cytopathic attack on choriodal cells can damage meninges

Cell-mediated Immunpathology: LCMV, an experimental model

Cell-mediated Immunpathology: LCMV, an experimental model

Lymphocytic Choriomeningitis Virus causes fatal choriomeningitis (inflammation of meninges) in immunocompetent mice, but not in immunosuppressed mice

Intracerebral Innoculation

e.g. via Cyclophosphamide Rx,or irradiation

Adoptive Transfer:Syngeneic normal mice “immunized” IP with LCMV. Anti-serum or lymphocytes then adoptively transferred

CTL activity is required for LCMV immunopathology

A: Perforin is required for CTL effector function

B: Perforin, hence CTL function, is required for virus clearance (IV challenge)

C: Perforin, CTL activity, mediates fatal choriomeningitis (IC infection)

D: CTL is also involved in hepatopathology (liver damage) by hepatotropic LCMV

% C

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Immune mediated disease: Tissue destruction in addition to

inflammation Hepatotropic LCMV

Liver destruction Cerebellar involution in newborn rats

Pathogenic consequences: severe ataxia

Immunosuppressed Non-Immunosuppressed

CD4+ T cells can enhance immune pathology

Immunization(day 0)

Depletiondays 18, 20, 23

IntranasalRSV infection

day 21

Alveolitis(% lung area)

day 25

Formalin-inactivatedRSV(all groups)

NoneCD8CD4

CD8 + CD4

YESYESYESYES

9.6%3.9%0.5%0.3%

Respiratory Syncytial Virus: virus induced immunopathology (bronchioaveolitis) is more dependent on CD4 T lymphocytes

Passsive transfer of antibodies (from RSV infected animals) cannot re-produce aveolitisConclusion: alveolitis is mediated mainly by CD4+ effector cells

(N.B. in the book, should be RSV should be Respiratory Syncytial Virus, NOT Rous Sarcoma Virus)

Depletion is usually accomplished by infusingmice with antibodies (complement-fixing) against the specific cell-surface molecule

IgG3>IgG1>IgG3>>>(IgG4)(neg.)

Complement Fixation

Cell-mediated Immunopathology:Human disease

Hepatitis B Virus infection Clinical disease associated

with development of high-titer antibodies

Presence of high titer virus (viremia) in the absence of clinical disease (hepatitis) suggest that the disease is not caused by infection per se

Anti-HBsAg may contribute to transient acute hepatitis, but may synergize with CTL mediated clerance of virus from hepatocytes

CTL response itself can result in acute hepatitis

% M

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Ma

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~45 nm “Dane”particles(infectious)

~20 nM spheresHBsAg (major) and middle proteins and host lipids;No nucleic acid, non-infectiousPresent in 103 to 106 fold excess over Dane particlesCan reach 1012 particles/ml in infected patientsHighly immunogenic, original source of 1st generation HBV vaccine

~20 nM in diameter filaments(contains the L protein, but lack nucleic acids; Non-infectious)

HBVAll three particles contain HBsAg (also known as the major protein)The L(large) and M(middle) are also contained in the Dane particlesThe L protein when expressed alone gets trapped in ER

Cell-mediated Immunopathology:Human disease

Evidence from mice model Transgenic mice can be made to express large, middle or major

envelope proteins Wt. Mice can be vaccinated with envelope proteins and Ag-specific

CTL clones can be made directed against specific epitopes in large, middle or major envelope proteins

System allows dissection of which immune response, targeted against which epitope, that is responsible for the immunopathology

Cell-mediated Immunopathology:Human disease

Evidence from mice model Transgenic mice expressing HbsAg

Adoptive transfer of CTL specific for a HepB envelope protein results in acute hepatitis

Direct cytotoxic effects of CTLs is limited to small numbers of hepatocytes (why? If all liver cells express the HBsAg)

However, IFN secreted by CTLs can attract other WBCs (phagocytes, PMNs) leading to necroinflammatory foci

However, these cytokines are also involved in viral clearance; down regulates viral transcription

Transgenic mice expressing large Hep B envelope protein (intracellular accumulation)

Adoptive transfer of CTL specific for a HepB large envelope protein results in progessive inflammation and liver necrosis

ER accumulation of L protein may be cytotoxic resulting in activation of liver macrophages (Kupffer cells)

Antibody-mediated Immunopathology

Ab-Ag complex formation during viremic infections

Deposition of immune complex in glomerulus can initiate complement cascade and cause inflammation, scarring and eventual kidney failure

Large complexes get trapped at the basement membrane

Mesangial cells enlarge into the subepithelial (mesangial) space in an attempt to remove accumulating immune complexes

Long term: glomerular capillary gets constricted foot processes of podocytes (glomerular endothelial cells) get fused; basement membrane gets leaky but filtering is blocked

Result: Impaired glomerular function, kidney failure (no urine production)

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Antibody-mediated Immunopathology

Ab-Ag complex formation during viremic infections

Deposition of immune complex in glomerulus can initiate complement cascade and cause inflammation, scarring and

eventual kidney failure

YY

YY

YY

Y

Y= anti-LCMV

= anti-mouse IgG

virus

Infectious Ab-Ag complex Can be demonstrated by

reduction in infectious titer using antibodies against

mouse IgG

Treatment of seraFrom mice persistentlyinfected with LCMV

LCMV titer(log10 LD50 per 0.02 ml)

Anti-mouse immunoglobulin

ControlsNormal rabbit serumAnti-mouse albumin

<1.0

3.73.5

Antibody-Mediated Immunopathology: Human Disease

Dengue hemorrhagic fever Dengue virus, flavivirus transmittted by mosquitoes 4 serotypes (1-4) Disease is usually self-limiting but small percentage

develop hemorrhagic fever and shock DHF/DSS occurs most commonly in

children previously infected with a different serotype infants with primary infection but with maternal anti-dengue

antibodies against another serotype Abs against one serotype can enhance infection by second

serotype (Antibody-Dependent Enhancement)

monocyte

Fc Receptor

(Heterotypic anti-Dengue Ab)

YY

Y

Y

Enhancement is dependent on Fc portion of Abs(Fab fragments from immune sera do not have Enhancing ability)

Y YTotal IgG F(ab)2

ADE +++ -

Enhancement is dependent on the titer of the Abs

Enhancement is dependent on the titer of the Abs

At high concentrations of virus-specific Ab (low Ab dilution), percent occupancy of antibody binding sites is sufficiently high to inhibit critical steps in the viral life cycle

As antibody becomes more dilute, its inhibitory activity becomes attentuated, at some point below the neutralization end-point, antibody binding at subneutralizaing concentrations (High Ab dilution) enhances viral infectivity

Neutralization end-point

monocyte

Fc Receptor

(Heterotypic anti-Dengue Ab)

Y

Y

Y

(Heterotypic anti-Dengue Ab)

YY

Y

Y

YYY

Y

Y

Y Y

monocyte

Fc Receptor

No free envelope spikes to mediate fusion

Enhanced attachment, free envelope spikes available to mediate fusion

Enhancement is dependent on Fc portion of Abs(Fab fragments from immune sera do not have Enhancing ability)

Y YTotal IgG F(ab)2

ADE +++ -

Enhancement is dependent on the titer of the Abs

[+anti-F(ab)2]+++ADE

YYY Fc

How does ADE in secondary Dengue virus infection lead to DHF/DSS?

Increased entry/replication in target cells (Monocytes/Macrophages) results in secretion of inflamatory cytokines (TNF-, IFN and IL-2) that also have vasoactive properties

In 2º Dengue infection, pre-existing Ag-specific CD8 and CD4 T cells are activated, and also secrete similar vasoactive cytokines

Resulting “cytokine” storm can capillary fragility (associated with hemorrhage) and permeability (associated with shock--loss of intravascular osmotic pressure)

Anti-viral antibodies: the good, the bad, and the useless

LCMV immunopathogenesis: Virus infected meningeal cells become target for CTLs--

resulting in choriomeningitis CTLs are made due to hematogenous spread to spleen

and other lymphoid tissues Too high a level of viral replication in lymphoid tissue

can lead to immune exhaustion: activation induced apoptosis of Ag-specific CTLs

A: Slowly replicating Armstrong strain Ab slows hematogenous spread of LCMV, and

prevents robust CTL response; protects animals against CTL mediated choriomeningitis

B: Intermediate replicating WE strain Ab did not slow spread of LCMV sufficiently to

affect disease outcome C: High replicating strain

Usually causes immune exhaustion (“high-dose paralysis”); Ab slows down virus enough for robust CTL response, which results in fatal choriomeningitis

Virus-initiated autoimmunity:Molecular mimicry

Immune response against a viral antigen cross-reacts with a host protein

Cross-reactivity doesn’t necessarily result in autoimmune disease

Experimental Allergic Encephalitis (EAE)

Myelin Basic Protein immunization results in EAE (demyelination); “encephalitogenic” epitope confined to 10 amino-acid stretch in MBP

HBV polymerase contains similar 10 aa stretch; immunization with this peptide can cause encephalitis in rabbits

P R O TE I N S E Q U E NCE

I M MUN O L OGI CAL

CR OSS

RE A CTI V IT Y

DE MO NS T RAT E D

P ol i ov i r us V P 2

Ace t y l c h o lin e r ece pt or

STTKESRGTT

TVIKESRGTK

YE S

P ap illo m a v ir us E2

Insu lin rece pt or

SLHLESLKDS

VYGLESLKDL

YE S

Ra bi esv ir u s g lyco p ro te in

Insu lin rece pt or

TKESLVIIS

NKESLVISE

YE S

H IV p 24

Ig G co n sta n t reg io n

GVETTTPS

GVETTTPS

YE S

Mea sles v ir u s P3

Cor tico tro p h in

LECIRALK

LECIRACK

N O

Virus-Induced Immunopathology

---CTL removal of infected cells

---CTL mediated destruction

---Ab neutralization---ADE; Immune Complex Disease

Defensive Effects Destructive Effects