Host Defenses Against Pathogens

Innate immune responses:

Occur early after infection

Are not specific for the invading pathogen

Do not induce memory

Include cytokines (esp. interferon) and natural killer cells

Adaptive immune responses

Require time to develop

Are specific for the invading pathogen

Give rise to immunological memory, making vaccination possible

Two arms: B cells (humoral immunity) and T cells (cellular immunity)

MacrophagesComplement Cascade Monocytes

Eosinophils

Neutrophils

Natural Killer Cells

cytokines

Innate Immunity

Acquired Immunity

Pathogen/Antigen

Immune System Players

Immediate Effect

Antigen Presentation

Extracellular Antigens Bacteria, Viruses

Intracellular Antigens Cells infected with

Viruses, Rickettsia, or Mycoplasma

B Cell+ +TH2 TH1CTL

Soluble Antigen, Activated B Cell

Result Humoral Immunity Cell-Mediated Immunity

Cell killing by CTL’s

Class II MHC Class I MHC

Immune System Players

Mechanisms of Innate and Acquired Immunity

Cytokines and Chemokines

The cytokines are a family of >30 signaling proteins

They play a very important role in regulating the immune system

They include the IFNs, many interleukins, TNF, among others

They are secreted by many cells and have important regulatory roles

Most are about 30 kDa in size

The chemokines are a family of >30 small proteins, 70-80 aa in length

Some are constitutive, others are induced

Some are proinflammatory

They attract leukocytes and serve to maintain, e.g., lymph nodes, to attract immune cells to sites of inflammation, and other roles

They are components of both the innate and adaptive systems

Interferons

There are two classes of interferons, which use different receptors

IFNs and use the same receptor and are called Type I IFNsThese are secreted by most cells

IFN uses a different receptorIt is secreted by T-cells and is called immune IFN

IFNs induce the transcription of many genes

They regulate the immune system

They induce the anti-viral state in which cells are resistant to viruses

They are extremely important for the control of viral infectionsIn the absence of IFN, most viral infections are

much, much more serious

Type I and II IFNs have overlapping but nonidentical effects

Characteristics of the Interferons

Type IIType I

Alternative name

IFN- IFN- IFN-

Leucocyte IFN Fibroblast IFN Immune IFN

Produced by All cells All cells T-lymphocytes

Inducing agent Viral infection or ds RNA

Viral infection or ds RNA

Antigen or mitogen

Number of species (number of genes) 22 (mouse)

1 1

Chromosomal location of gene

9 (human) 9 (human) 12 (human)4 (mouse) 4 (mouse) 10 (mouse)

Size of IFN protein 165-166aa 166aa 146aa, dimerizes

Receptors Receptor for both IFN- and INF- consists of 2 polypeptides, IFN-R1 and IFN-R2, encoded on chromosome 21(human) or 16 (mouse)

Receptor consists of 2 proteins: IFN-R1 encoded on chromosome 6 (human) or 10 (mouse) and IFN-R2 encoded on chromosome 21 (human) or 16 (mouse)

General functions Anti-viral activity Anti-viral activity Macrophage activation

MHC Class I MHC Class I MHC Class I

MHC Class II on macrophages

Number of introns None None Three

NK cell activationSome anti-viral activity

14 (human)

MHC Class II on B-cells

IgE, IgG production by B-cells

Data for this table came from Mims et al. (1993) Figure 12.9 and Fields et al. (1996) Table 3 on p. 378.

Genes Induced by Interferons

Protein Induced by

IFN- IFN- IFN-

Functions/phenotype

Indoleamine 2,3-dioxygenase+ + +++ Tryptophan degradation

56 + + +++ Trp-tRNA synthetase

GBP/g57 + + +++ Guanylate binding

+++ +++ +MxA Inhibits replication of influenza and VSV

IRF1/ISGF2 ++ ++ Transcription factor++

IRF2 ++ ++ Transcription factor

MHC Class I +++ +++ +++ Upregulation of antigen presentation

RING 12 +++ Proteosome subunit

RING 4 +++ +++ Putative TAP

2-microglobulin +++ +++ +++ MHC light chain

Induction of anti-viral state esp. anti-picornavirus

(also dsRNA)(2’-5’) (An) synthetase (2’-5’) (An) synthesis+++ +++ + ISRE

p68 Kinase (PKR) Protein KinaseInduction of anti-viral state

+++ +++ + ISRE

MHC Class II ++ Upregulation of antigen presentationnot ISRE nor GAS

Inducible Element

INF-

INF-INF-

INF

-R

1

INF

-R

1

INF

-R

2

INF

-R

2

JAK1JAK1

JAK2TYK2

GAS

Protein-protein interactions

Phosphorylation

Migration to nucleus

ISRE

INF-

NUCLEUS

ISRE Interferon Stimulated Response Element

GAS IFN-Gamma Activation Site

Interaction resulting in phosphorylation

Transcription initiation

STAT1 STAT1

Recruitment

STAT1STAT2

p48

Phosphorylation Dimerization

Signal Transduction by Interferons

Development of the antiviral state

HOST CELL

Induction by IFN

NUCLEUS

Latent RNase L Latent 2’-5’ OS Latent PKR

ACTIVATION

PKR

EIF2

PHOSPHORYLATION

2’-5’ OS

SYNTHESIS

ACTIVATION

RNase L

2’-5’A

NUCLEUS

dsRNA

EIF2

ds RNA

Phosphate group

Ribosome EIF2

Activated RNase L

Phosphorylated EIF2

Cells in the Antiviral State inhibit Viral Replication

ds RNA

Phosphate group

NO VIRAL REPLICATION

NUCLEUS

Uncoating

Viral mRNA

EIF2

TRANSLATION INITATION BLOCKED

Ribosome

RNaseL

VIRAL mRNA DEGRADED

EIF2

Activated RNase L

Phosphorylated EIF2

Effects of IFNs

IFNs induce the antiviral state in which viral RNA cannot be translated

The inducer of Type I IFNs is usually double-stranded RNA

Double-stranded RNA is also required for the activity of the induced enzymes RNase L and PKR

Thus, dsRNA plays a pivotal role in IFN action

IFN has toxic effects and tight regulation of its action is necessary

IFNs are also powerful regulators of the adaptive system

They upregulate production of MHC and of other proteins required for function of the adaptive system

They activate many types of immune cells

Functional GroupName (Abbreviation) Therapeutic targets Side effects of Therapy

Therapeutic Uses of some Cytokines

Antiviral cytokinesType I Interferon(IFN- Chronic hepatitis B, hepatitis C, herpes zoster, papilloma viruses, rhinovirus, HIV(?), warts

Fever, malaise, fatigue, muscle pain Toxic to kidney,liver,heart, bone marrow

Type II interferon IFN- Lepromatous leprosy, leishmaniansis, toxoplasmosis

As above for Type I interferons

Inflammatory cytokines

Tumor necrosis factor (TNF)

Anti-TNF in septic shock Shock with marked hypotension

Interleukin 1 (IL-1) Receptor antagonist in septic shock

???

Interleukin 6

Regulators of Lymphocyte Functions

Interleukin 2 Leprosy, local treatment of skin lesions

Vascular leak syndrome, hypotension, edema, ascites, renal failure, hepatic failure, mental changes and coma

Interleukin 4

Interleukin 5

Interleukin 7

Interleukin 9

Interleukin 10

Interleukin 12

Interleukin 13

Transforming growth factor (TGF-

Septic shock

Septic shock

Normal Biological Function

Inhibits viral replication

Inhibits viral replication, upregulates expression of class I and class II MHC, enhances activity of macrophages

Cytotoxic for tumor cells, induces cytokine secretion by inflammatory cells.Costimulates T-helper cells, promotes maturation of B-cells, enhances activity of NK cells, attracts macrophages and neutrophils

Promotes differentiation of B cells, stimulates Ab secretion by plasma cells

Induces proliferation of T-cells, B- cells, and CTLs, stimulates NK cells

Stimulates activity of B cells, and proliferation of activated B-cells, induces class switch to IgG and IgE

Stimulates activity of B cells, and proliferation of activated B-cells, induces class switch to IgA

Induces differentiation of stem cells, increases IL-2 in resting cells

Mitogenic activity

Suppresses cytokines in macrophages

Induces differentiation of T-cells into CTLs

Regulates inflammatory response in macrophages

Chemotactically attracts macrophages, limits inflammatory response, promotes wound healing

Symptoms similar to those for IL-2, especially shock and hypotension

Natural Killer Cells

NK cells are a first line of defense against viral infection

They increase in activity in the first 2-3 days after infection and then decline

They kill virus-infected cells, probably because these cells display too little class I MHC

A deficiency in NK cells results in more serious viral infections

Complement

The complement system consists of >20 blood proteins

It is activated by a proteolytic cascade

It is a component of both the innate and adaptive immune systems

It can interact with antibody to kill viruses or infected cells

It can also kill pathogens in the absence of antibody

One group of effector molecules inserts into membranes to kill cells or viruses

Other effectors control pathogens in other ways

Complement is a potentially destructive system and its activity must be carefully regulated

Apoptosis

Apoptosis is a cell suicide pathway in which mitochrondria cease to function, DNA is degraded, and the cell fragments into small pieces

Apoptosis is non-inflammatory

Many events can trigger apoptosis, including stress of viral infection, withdrawal of growth factors, or a deregulated cell cycle

CTLs kill target cells by inducing apoptosis

Proteases called caspases are key players in the apoptotic pathway

By undergoing apoptosis, an infected cell prevents further production of virus

Mechanisms of Apoptosis

p53

Mdm2p53

??

Bcl-2

Bax

Effector caspases

Procaspase

Killling via Receptor Killing due to External Stimuli Killing by CTL via the Granzyme B Pathway

Perforin Channel

Granzyme

Procaspase

CTL

Uv irradiationUnscheduled DNA synthesis

Hypoxia

E2FViral infection

Caspase cascade Caspase cascadeCaspase cascade

Apoptosis

DE

D

DD

DD

DD

Cytoplasmic death domains

DE

DD

ED D

EDDeath effector

domains

Activation cleavages

FADD

Ligand

Receptor Fas

Procaspase

FASL

Induction

Inhibition

The Adaptive Immune System--CTLs

The cellular arm of the adaptive immune system consists of CTLs (cytotoxic T lymphocytes) that kill infected cells

Most CTLs express CD8 and respond to antigen presented by Class I MHC (major histocompatibility complex) molecules

Activation of CTLs requires exposure to cognate antigen and a second signal, usually supplied by T-helper cells

Upon activation, CTLs express IFN- and other proteins and begin to divide; they are programmed to undergo apoptosis once the cognate antigen is withdrawn

CTLs recognize the antigen-MHC I complex by means of a T-cell receptor that they express on their surface

Upon activation, memory T-cells are formed that persist and that are programmed to respond rapidly upon renewed exposure to antigen

T-Helper Cells

TH cells express CD4 and recognize antigen presented by Class II MHC

Whereas MHC Class I is expressed by most cells in the body, Class II is expressed primarily by T-cells, B-cells, and other cells of the immune system

TH cells secrete cytokines that help CTLs or B-cells to become activated

A spectrum of TH cells exists that secrete different assortments of cytokines and that preferentially help CTLs or B-cells

The effector cells of the adaptive immune system require at least two different inputs to become activated, thus subjecting this potentially harmful system to greater control

Class I MHC Class II MHC

S S

S

S

S

S

1 2

32m

11

2

1

2S

S

S

S

S S

Cell cytoplasm

Plasma membrane

A.

2 microglobulin

3 domain

2 domain1 domain

Peptide-bindingcleft

B.

S

S

COOHCOOH

S

S

S

S

S

S

or chain or chain

Variable regions

Constant regions

NH2NH2

S S

TM

Cell cytoplasm

Plasma membrane

Structure of the T-cell receptor (TCR)

S

S

S-S

S

S

S

S

S

S

Cytotoxic CD8+ T cell

S

S

S

S

Antigenic peptide

Almost any host cell

MHC Class I

TCR

S S

chain

chain

S

S

SS

CD8 dimer

Interaction between a cell expressing MHC Class I and a CD8+ T cell

S

S

S

S

S

S

S

S

CD4+ Helper T cell

chain

chain

S S

S

S

S

S

Antigen-presenting cell

Antigenic peptide

MHC Class II

TCR

S-S

S

S

S

S

S

S

S

S

S

S

CD4

Interaction between an Antigen-presenting Cell expressing MHC Class II and a CD4+ T cell.

Figure 8.4

SS

S-S

SS

SS

SSNH

2N

H2

Germline -chain DNA

Rearranged -chain DNA

Protein product heterodimer

VD

JC

V J C

5’ V1 Vn V1 Vn JJ Jn C

J Jn C5’ V1 V2 V 3’

3’

D D2V1 Vn J C C V14

Rearranged -chain DNA

Germline -chain DNA

D1D2 J J C

5’ J

3’5’ D2C C V14J

V1 VD

J

3’

D-J joining

V-DJ joining

V-J Joining

Transcription mRNA splicing Translation

Transcription mRNA splicing Translation

Comparison of Diversity in Human Immunoglobulin and T-Cell Receptor Genes

Mechanism of Diversity

Immunogobulins T-Cell Receptors T-Cell Receptors

chain chain chain chain chain chain

Multiple germ-line gene segments

V 65 40 30 ~70 52 12 >4

D 27 0 0 0 2 0 3

J 6 5 4 61 13 3

Combinatorial Joining

Combinatorial V-J-D combinations

65 X 27 X 6=

1.0 X 104

D segments read in 3 framesrarely --- --- --- often --- ---

Joints with N and P nucleotides

2 (1) 2 1

Total Diversity

Heavy Chains Light Chains

40 X 5 = 30 X 4 =

1.0 X 104 X 320 = 3.2 X 106V gene pairs

Junctional Diversity ~3 X 107

~ 1014

70 X 61 = 52 X 2 X 13 =

1.3 X 103

430 X 1.2 X 103 = 5.8X 106

~2 X 1011

12 X 5 = 4 X 3 X 3 =

?? ??

60 X 36 = 2.1 X 103

???

???~ 1018

~ 430 60 36200 120

5

Viral protein synthesized in

the cell

NUCLEUSa

b

c

d

e

Proteasome

ER

TAP

Acidic vesicle

A

B

C

D

E

(Invariant chain

peptide)

or

External antigen or pathogen

Infected cell

MHC Class I MHC Class II

Antigen Processing by Class I and Class II MHC

Adaptive Immunity--B cells

The humoral immune response is carried out by B cells

B cells express anchored antibody on their surface

Upon exposure of the cell to an antigen recognized by the antibody, the cell can divide and produce plasma cells that secrete antibody

Activation of the B cell requires a second signal supplied by TH cells

After activation, memory cells are formed that persist and are capable of more rapid activation upon exposure to cognate antigen

Secreted antibodies are of 5 different kinds which have different functions in the immune response

CL domain

strands

Disulfide bond

VL domain

N-terminus

C-terminus

HV regions

Loops

The Immunoglobulin Fold

Heavy Chains

Constant

Variable

Hinge

Hypervariable (CDR’s)

Light Chains

Constant

Variable

Hypervariable (CDR’s)

S

S

S

S

S

S

S

S

S SS SS S

S

S

S

S

S

S

Variable regionsAntigen binding domain

Effector function domain

VL

VH

CH1

H

CH2

CH3

S SS S

S

SS

S

S

S

S

S

CL

S

S

CDR1

CDR2

Heavy chain variable region

Light chain variable region

J H

DH

CDR1

J L

S

S

CDR2

CD

R3

S

S

CD

R3

VH or VL

Structure of an Immunoglobulin G Molecule

S SS S

VL

CLVH

C1

C2

C3

IgG IgD

S S

VL

CLVH

C3

C1

C2

S S

S S

S S

VL

CLVH

C3

C1

C2

C4

IgE

S S

S S

VH

VL

C1

CL

C3

IgA dimer

S S

S S

S S

S S

C2

ss

ss

IgM pentamer

VL

CLVH

C2

C1

C3

C4

SS

SS

SSSS

s ss s

Structures of the Different Classes of Secreted Immunoglobulins

Types of Secreted Antibodies

IgM is the earliest antibody secreted by a plasma cell. It is a sign of recent infection.

IgG is long lived and circulates in the blood for years. Many cells in the body are thus exposed to it. It is also transferred to the fetus of a pregnant woman and is responsible for maternal immunity.

IgA is secreted on mucosal surfaces and if effective against viruses that replicate in the respiratory tract or the intestinal tract

IgE is most effective against large parasites and is responsible for the symptoms of hay fever when it reacts against pollen grains, mites, dust particles, or other large objects

IgD is only expressed together with IgM. Its precise role in immunity is not clearly understood.

A. Light ChainGermline chain DNA

B. Heavy Chain

Rearranged -chain DNA

Light Chain () mRNA

Light Chain () protein

Heavy Chain () protein

GermlineH-chain DNA

Rearranged H-chain DNA

Heavy Chain () mRNA

5’ V1 V23 Vn

V-J Joining

CJ5’ V1 V

CJ

J

3’

3’

Transcription, RNA splicing, polyadenylation

5’ 3’Cap (An)

V J C

Translation

Vk Ck

VH C1 C2 C3

5’ 3’JHDH1 DH7 DH13V1 Vn

JHDH1 DH6V1 V180 DJ5’ 3’

D-J joining

Vn

JHV1 V179 DJ5’ 3’V

V-DJ joining

Transcription, RNA splicing, polyadenylation

5’ 3’Cap (An)

DJV C

C C C3 C1 C2b C2a C C

C4

S S

IgM

S S

5Translation

L

L

L

L

L

L

L

Formation of the Human Immunoglobulin Heavy and Light Chains

Comparison of Diversity in Human Immunoglobulin and T-Cell Receptor Genes

Mechanism of Diversity

Immunogobulins T-Cell Receptors T-Cell Receptors

chain chain chain chain chain chain

Multiple germ-line gene segments

V 65 40 30 ~70 52 12 >4

D 27 0 0 0 2 0 3

J 6 5 4 61 13 3

Combinatorial Joining

Combinatorial V-J-D combinations

65 X 27 X 6=

1.0 X 104

D segments read in 3 framesrarely --- --- --- often --- ---

Joints with N and P nucleotides

2 (1) 2 1

Total Diversity

Heavy Chains Light Chains

40 X 5 = 30 X 4 =

1.0 X 104 X 320 = 3.2 X 106V gene pairs

Junctional Diversity ~3 X 107

~ 1014

70 X 61 = 52 X 2 X 13 =

1.3 X 103

430 X 1.2 X 103 = 5.8X 106

~2 X 1011

12 X 5 = 4 X 3 X 3 =

?? ??

60 X 36 = 2.1 X 103

???

???~ 1018

~ 430 60 36200 120

5

Primary Response

Secondary Response

Total

IgGIgM

IgG

IgMIgM

Total100

10

1

0.1

Anti

body c

oncentr

ati

on in s

eru

m

unit

s/m

l

Time after Immunization

1° Antigen 2° antigen

Time Course of Primary and Secondary Antibody Responses

DJ3’

V

Recombination between S and S1

C C C3 C1 C2b C2a C C

S S3 S1 S2b S2a S S

DJV C1 C2b C2a C C

S2b S2a S S

IgG1 mRNA

Transcription, splicing, polyadenylation

Recombination between S1 and S

JHDJV C C

STranscription, splicing,

polyadenylation

IgE mRNA5’ 3’

5’ 3’Cap An

5’

5’ 3’

DJV C1

Cap AnDJV C

S S

S S

IgG heavy chains

Translation

IgE heavy chainsS

S

SS

SS

H-Chain DNA

Class-switched H-chain DNA

Translation

Immunoglobulin Class Switching To Produce Heavy Chains For IgG And IgE

Developmental pathway

Secretion

Activation

Inhibition

IFN-LT-

MCP-1

IL-18

IFN-IL-6

IL-15

IL-1 TNF- CD4+ T-cellCD8+ T-cell

CytotoxicT Lymphocyte

(CTL)

IFN-

Th1 Th2

Cell-mediated response to viral infection

Humoral response to viral infection

IFN-IL-2LT-

IL-4IL-5IL-10IL-13

B-cell

Plasma Cell

IgM

IgA

IgE

IgG

Endothelial cell

Natural Killer cell

(NK)

IL-12

Macrophage dendritic cell

Cytokine Networks Important for Innate and Acquired Antiviral Immune Responses

Control of the Immune System and Autoimmunity

T-cells are negatively selected during development if they recognize self

Activation of B-cells or T-cells requires two signals, exposure

to the cognate antigen and cytokine stimulation from TH cells

After activation the cells die off when the cognate antigen is no longer present in sufficiently high concentrations

Failure of these control mechanisms can result in autoimmunity, which can lead to very serious illness

An inflammatory response induced by infection is important for optimal signaling and activation

Vaccines

The existence of memory in the adaptive immune system makes it possible to immunize people by vaccination

Vaccines may be attenuated viruses that infect but do not cause disease, inactivated viruses that cannot infect but which expose the person to the viral antigens, or subunit vaccines that contain only a subset of viral proteins

Attenuated vaccines usually are the most effective, but it can be difficult to balance sufficient attenuation so as not to cause disease in any individual with the necessity for sufficiently vigorous replication to induce immunity

Inactivated or subunit vaccines require that large amounts of protein be injected and it may be difficult to obtain an inflammatory response required for a vigorous response without overdoing it

Vaccines (con)

The take of live virus vaccines can be interfered with by concurrent infection with another virus, which is not a problem with inactivated virus vaccines

Live virus vaccines are less stable than inactivated vaccines

Inactivated virus vaccines require large amounts of material, multiple injections, and give less solid immunity

Some candidate inactivated virus vaccines have given unbalanced responses that resulted in potentiating more serious illness upon subsequent infection by the virus rather than in immunity to the virus

Characteristics of Anti-viral Vaccines

Live Attenuated Virus

Inactivated Virus and Subunit Vaccines

Poliovirus (Sabin) Poliovirus (Salk)

Measles Influenza

Mumps Rabies

Rubella Hepatitis B

Yellow Fever Hepatitis A

Vaccinia Japanese encephalitis

Varicella-Zoster Western equine encephalitis (experimental)

Adenovirus (in military recruits)

Rotavirus*

Junin (Argentine hemorrhagic fever)

Currently Licensed Vaccines:

In addition, live attenuated vaccines for the following viruses are close to release to the public: human cytomegalovirus, hepatitis A, influenza, dengue, human parainfluenza, and Japanese encephalitis

* withdrawn

Live Attenuated Virus

Inactivated Virus and Subunit Vaccines

Characteristics of the Immune Response after Vaccination

Antibody induction (B-cells) +++ +++

CD4+ helper T-cells +++ +++

CD8+ cytotoxic T-cells +++ -

Reactivity against all viral antigens

Usually Seldom

Longevity of immunity Years/decades Months/years

Cross-reactivity among viral strains

+++ +

Risk of viral disease + -

Type of Vaccine

Many Vaccines Have Been Successful in Controlling Viruses

Smallpox has been eradicated

Poliovirus is on the verge of being eradicated

Good vaccines exits for mumps, measles, rubella, yellow fever, tick-borne encephalitis, and other viruses

However, it has not yet been possible to develop vaccines against some viruses, such as HIV and RSV

Although a good vaccine against measles exists, it has not been possible to eradicate the virus because infants in some developing countries become infected with the wild-type virus as soon as maternal immunity is lost

Kuwai t

Equator

per 100,000 Population

>100

1-10

10-100

No data

None

Cases of Measles

Worldwide Incidence of Measles as of August 1998.

Kuwait

Equator

Worldwide Immunization Coverage for Measles as of August 1998.

Vaccine Coverage for Measles

Low, <50% vaccinated

High, >80% vaccinated

Medium, 50-80% vaccinated

No data

5 10 15 20 25 30 35 40Age (Months)

EZ-HT

SW-HT

Standard

Mort

ali

ty (

per

10

00

Ch

ild

ren

at

5 M

on

ths)

200

150

100

50

0

Child Mortality after High-Titer Measles Vaccination in Senegal

Major Strategies used by Viruses to Evade the Immune System

Interference with MHC Class I Antigen Presentation

Downregulation of transcription of MHC class I molecules

Downregulation of transcription of TAP

Interference with the activity of TAP

Retention of MHC class I molecules within the cell

Degradation of MHC class I molecules

Interference with natural killer (NK) cell function

Inhibition of Apoptosis

Interference with MHC Class II Antigen Presentation

Evasive strategies of viral antigen production

Restricted gene expression; virus remains latent with minimal or no expression of viral proteins

Infection of sites not readily accessible to the immune system

Antigenic variation; antigenic epitopes mutate rapidly

Interference with antiviral cytokine function

Production of viral homologues of cellular regulators of cytokines

Neutralization of cytokine activities

Production of soluble cytokine receptors

Inhibition of the function of IFN

Rapid shutdown of host macromolecular synthesis

Inhibition by Ad12

Proteosome

TAP

MHC class I molecules

ER

Nucleus

E3-19K

E1A

TAP

Inhibition by Ad2

E1A(Inhibits transcription

of TAP mRNA)(Inhibits transcription of class I molecules)

(Keeps class I molecules in ER)

Adenoviruses Inhibit Antigen Presentation by Class I MHC Molecules

MHC Class I

Interference by HCMV

Interference by HSV

Interference by EBV

Proteosome

TAP

ER

Nucleus

US3

(Binds to 2 microglobulin)

UL18

ICP47

(Blocks transport by TAP)

US6

Acts on TAP (Blocks transport by TAP)

US2, US11(Causes MHC molecules to be degraded)

(Keeps class I molecules in ER)

EBNA-1 (Protein not processed)

Herpesviruses and Antigen Presentation by Class I MHC Molecules

Some Viruses that Alter Antigen Presentation by Class I MHC Molecules

Downregulation of class I protein at the surface of cells

Interference level Virus Family Virus Virus ProteinMechanism

Downregulation of MHC class I expression at cell surface

Adenoviridae Ad 2 E3-19K Viral protein binds to class I molecules and keeps them in the ER

Ad12 E1A Inhibits transcription of class I mRNA

Hepadnavirus HepB Ag epitopes Epitopes mutate so that they are no longer recognized by CTLs

Picornavirus FMDV ??

EBV EBNA-1 EBNA-1 protein with Gly-Ala repeat is not processed by proteasome

Alteration of antigen processing

Adenoviridae Ad12 E1A Inhibits transcription of TAP1 and TAP2

Herpesviridae HSV ICP47 Binds to TAP and prevents transport of antigenic peptide to MHC class I

HCMV US6 Blocks transport by TAP

Ad12 = adenovirus 12; HCMV = human cytomegalovirus; FMDV = foot and mouth disease virus; HIV = human immunodeficiency virus; HSV = herpes simplex virus; EBV = Epstein-Barr virus; HepB = hepatitis B virus.

Herpesviridae HCMV US2, US11 Gene products lead to degradation of class I molecules

HCMV UL18 Binds to 2 microglobulin

Lentivirus HIV Tat Inhibits transcription of MHC class I mRNANef Downregulates surface expression of class I proteins

Alteration of spectrum of antigens presented

DE

D

DD

DD

DD

Cytoplasmic death domains

DE

DD

ED D

EDDeath effector

domains

Activation cleavages

FADD

Ligand

Receptor Fas or TNFR

Procaspase

Apoptosis

Perforin Channel

Granzyme B

Procaspase

Cytotoxic T-cell

Caspase Cascade

crmA

crmA

crmA

T2

SPI-1

Killling via Receptor Killing by CTL via the granzyme B pathway

cowpox

Inhibiting factor

Virus

myxoma

vaccinia

MC 159 molluscum contagiosum

SPI-1

SPI-1

FasL or TNF-

MC159

T2

How Poxviruses Inhibit Apoptosis

Inhibition of Apoptosis by Adenoviruses

Virus Family Viral Protein Mode of Interference

Vaccinia SPI-2 crmA homologue, inhibits activation of caspasesPoxviridae Cowpox crmA Serpin homologue, inhibits proteolytic activation of caspases

Myxoma M11L, T2 T2 is a homologue of TNFR, and inhibits interaction of TNA- with TNFR; M11L has a novel function

African swine fever virusLMW5-HL Homologue of Bcl-2

HHV 8 KS bcl-2 Homologue of Bcl-2

Inhibits p53 activity; has some sequence similarity to Bcl-2

Gammaherpesvirinae viral FLIPs Inhibits signalling from death domains to caspases

Adenoviridae E1B-55K Binds to and inactivates p53E3-14.7K

Adenovirus

E1B 19K Functional homologue of Bcl-2; interacts with Bax, Bi, and Bak

Polymaviridae SV40 Large T antigen Binds to and inactivates p53

Baculoviridae AcMNPV p35 Forms a complex with caspases; inhibits caspase-mediated cell death

Hepadnavirus HepB pX Binds to p53

Herpesviridae herpes simplex g34.5 gene Prevents shutoff of protein synthesis in neuroblastoma cellsherpes saimiri ORF 16 product Homologue of Bcl-2

Epstein-Barr (latent) LMP1 Upregulates transcription of Bcl-2 and A20 mRNAs; inhibits p53-mediated apoptosis

(lytic)

HCMV IE-1, IE-2 Downregulates transcription of p53 mRNA

HPVs E6 Binds to p53 and targets it for ubiquitin-mediated proteolysis

Interacts with caspase-8Blocks caspase 8 activation by destruction of Fas

E4 orf 6 Binds to and inactivates p53

Virus

Asfarviridae

Molluscum contagiosumM159, 160 Has death domains like FADD, inhibits FADD activation of caspase 8

BHFR1

Papillomaviridae

K13 vFLIPS, prevents activation of caspases by death receptors

E3 10.4 K/14.5 K

Like FLIPs, inhibits activation of caspasesIAP

DNA Viruses that Interfere with Apoptosis

Virus Manipulation of Cytokine Signalling

Virus Family Virus Cellular Target or HomologViral Factor Mode of Action

Herpesviridae HCMV TNF receptor UL144Chemokine receptors US28 Competitive CC-chemokine receptor,

sequesters CC-chemokines

HHV-8 Type 1 IFNs vIRF K9 Blocks transcription activation in response to IFN

Virus-encoded chemokinesvMIP-I, vMIP-II TH-2 chemoattractant, chemokine receptor antagonist

Reverses IFN-induced translation blockHSV Type 1 IFNs 134.5

vIRF-2 May modulate expression of early inflammatory genes

RNase L 2’-5’ (A) RNA analog, inhibits RNase L

Unknown function, retained intracellularly

EBV Type 1 IFNs EBNA-2 Downregulates IFN-stimulated transcriptionPKR EBER-1 Blocks PKR activityChemokine receptors BARF-1 Secreted, sequesters CSF-1IL-10 BCRF-1 IL-10 homologue, antagonizes TH-1 responses

Adenoviridae Adenovirus Type 1 IFNs E1A Blocks IFN-induced JAK/STAT pathway

PKR VA 1 RNA Blocks PKR activity

Hepadnaviridae HepB Type 1 IFNs Terminal protein Blocks IFN signalling

Flaviviridae HepC PKR E2 Inhibits PKR activation in response to type 1 IFN

Retroviridae HIV PKR TAR RNA Recruits cellular PKR inhibitor TRBP

Poxviridae See Table 8.J

Abbreviations: HCMV = human cytomegalovirus; TNF = tumor necrosis factor; HHV-8 = human herpesvirus eight (Kaposi’s); HSV = herpes simplex virus; IFN = interferon; PKR = ds RNA-dependent protein kinase; CSF-1 = colony stimulating factor; HIV = human immunodeficiency virus.

TNF- E3 proteins Various mechanisms

Adapted from Evans (1996) ; and a review by Tortorella et al (2000).

Binds ds RNA, nuclear localization inhibits activation of PKR, IFN resistance.

System Target Virus Gene Homolog Properties

Complement C4B and C3B

Vaccinia C3L C48 binding protein

4SCRs, secreted,binds and inhibits C4B and C3B, virulence factor

Variola D15L

? Vaccinia B5R Complement control proteins

4 SCRs, EEV class I membrane glycoprotein, for virus egress

? Variola B6R

Variola C3LSwinepox K3L

Interferon

PKR Vaccinia K3L eIF-2 Binds PKR, inhibits phosphorylation of eIF-2, IFN resistance

Variola E3LdsRNA Vaccinia E3L PKR

SCR = 60 amino acid sequence called: ”Short consensus sequence”; EEV = extracellular enveloped virions; PKR = dsRNA-dependent protein kinase; IFN = interferon; SERPIN = serine protease inhibitor superfamily

Vaccinia B8RVariola B8RSwinepox C61

MyxomaIFN- T7 IFN- receptor

Secreted, binds and inhibits IFN-

IL-1 ICE Cowpox crmA SERPIN Prevents proteolytic activation of IL-1, inhibits inflammatory response, inhibits apoptosis

Vaccinia B14RVariola B12R

CowpoxIL-1 Vaccinia B15R IL-1 receptor Secreted glycoprotein, binds

and inhibits IL-1

IL-8 IL-8 Swinepox ecrf3 IL-8 receptor

Binds IL-8Swinepox K2R

TNF TNF-,TNF- Myxoma Vaccinia Variola Cowpox

T2G2R, (truncated)

TNF receptor Secreted, binds and inhibits TNF-, TNF-

Pox Defense Molecules

Type 1 IFN Vaccinia B18R IFN receptor Binds to and inhibits IFN-

Crm B

CC- chemokines

Myxoma Vaccinia Variola Cowpox

p35 Chemokine receptor

Secreted, binds to CC-chemokines

Pathogens and the Immune System

Humans cannot live without an immune system but this is the result of a long process of coevolution

The humoral system may have evolved to fight bacteria and the CTL system to fight viruses, but both are important for the control of any pathogen

This coevolution required mutual adaptation

Pathogens that are too virulent for their reservoir host become attenuated (e.g., rabbit myxoma virus)

Pathogens that are effectively defeated by the immune system often evolve new tricks to get around it (e.g., counterdefenses evolved by herpes-, pox-, and adenoviruses)

Hosts that cannot control their pathogens are removed from the gene pool in favor of variants that can (e.g., rabbits and myxomavirus)