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

2. 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-a, IFN-g, 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

3. Cell-mediated Immunpathology: LCMV, an experimental model
Inflammatory Cytokines
CTL
Effector Cell
TCR
Cytopathic attack on
choriodal cells can damage meninges
Convulsions (?)
Inflammatory Macrophage
attraction
NOS
Nitric Oxide
Viral antigen
MHC Class I
Infected Choriodal Cells

4. Cell-mediated Immunpathology: LCMV, an experimental model
Lymphocytic Choriomeningitis Virus causes fatal choriomeningitis (inflammation of meninges) in immunocompetent mice, but not in immunosuppressed mice
Adoptive Transfer:
Syngeneic normal mice “immunized” IP with LCMV. Anti-serum or lymphocytes then adoptively transferred
Intracerebral Innoculation
e.g. via Cyclophosphamide Rx,
or irradiation

5. 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
% Cr51 release
Percent Alive
Serum GLDH
Log10 Virus Titer

6. Cerebellar involution in newborn rats
Immune mediated disease: Tissue destruction in addition to inflammation
Hepatotropic LCMV
Liver destruction
Cerebellar involution in newborn rats
Pathogenic consequences: severe ataxia
Non-Immunosuppressed
Immunosuppressed

7. CD4+ T cells can enhance immune pathology
(N.B. in the book, should be RSV should be Respiratory Syncytial Virus, NOT Rous Sarcoma Virus)
Respiratory Syncytial Virus: virus induced immunopathology
(bronchioaveolitis) is more dependent on CD4 T lymphocytes
Complement Fixation
(neg.)
IgG3>IgG1>IgG3>>>(IgG4)
Depletion is usually
accomplished by infusing
mice with antibodies
(complement-fixing)
against the specific cell-
surface molecule
Passsive transfer of antibodies (from RSV infected animals)
cannot re-produce aveolitis
Conclusion: alveolitis is mediated mainly by CD4+ effector cells

8. 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
% Maximum
% Maximum

9. HBV
All three particles contain HBsAg (also known as the major protein)
The L(large) and M(middle) are also contained in the Dane particles
The L protein when expressed alone gets trapped in ER
~45 nm “Dane”particles
(infectious)
~20 nM spheres
HBsAg (major) and middle proteins and host lipids;
No nucleic acid, non-infectious
Present in 103 to 106 fold excess over Dane particles
Can reach 1012 particles/ml in infected patients
Highly immunogenic, original source of
1st generation HBV vaccine
~20 nM in diameter filaments
(contains the L protein, but lack nucleic acids;
Non-infectious)

10. 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

11. 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, IFNg 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)

12. 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)

13. 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
Infectious Ab-Ag complex
Can be demonstrated by reduction in infectious titer using antibodies against mouse IgG Y
= anti-LCMV
= anti-mouse IgG
virus

14. 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)

15. Y Y ADE +++ - monocyte Total IgG F(ab)2
(Heterotypic
anti-Dengue Ab) Y
Enhancement is dependent on the titer of the Abs
monocyte
Fc Receptor
Enhancement is dependent on Fc portion of Abs
(Fab fragments from immune sera do not have
Enhancing ability) Y
Total IgG
F(ab)2
ADE
+++ -

16. 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

17. Y Y Y Y Enhanced attachment, free envelope spikes
available to mediate fusion
No free envelope
spikes to
mediate fusion
(Heterotypic
anti-Dengue Ab) Y Y Y Y
monocyte
Fc Receptor
(Heterotypic
anti-Dengue Ab)
monocyte
Fc Receptor

18. Y Y Y Fc ADE +++ - +++ [+anti-F(ab)2] ADE Total IgG F(ab)2
Enhancement is dependent on the titer of the Abs
Enhancement is dependent on Fc portion of Abs
(Fab fragments from immune sera do not have
Enhancing ability) Y Y
Total IgG Y
F(ab)2 Fc
ADE
+++ -
+++
[+anti-F(ab)2]
ADE

19. 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-a, IFNg 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)

20. 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

21. 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

22. Virus-Induced Immunopathology
Defensive Effects
Destructive Effects
---CTL mediated
destruction
---CTL removal of
infected cells
---ADE;
Immune Complex Disease
---Ab neutralization