normal immune system function

Normal Immune System Function

Karim Rafaat

Organs of the Immune System

Central (primary) lymphoid organs are the sites for generation and early maturation of lymphocytes

T cells mature in the thymus

Central lymphoid

organs

B for bursa of Fabricius (a lymphoid organ in birds)[After Hieronymus Fabricius (1537-1619), Italian anatomist]

(T for thymus)

B cells mature in the bone marrow

Lymphoid organs contain lymphocytes and non-lymphoid cells such as macrophages and dendritic cells (and epithelial cells)

Lymphoid organs are important for the generation and maturation of lymphocytes, the initiation of immune responses and the perpetuation of immune responses

*other stuff (not peripheral lymphoid organs)

Peripheral lymphoid organs

1. trap antigens

2. are the sites for initiation of most immune response

3. provide signals for recirculation of lymphocytes

4. Antigen delivery to regions of increased traffic

Flow of lymph

Gut associated lymphoid tissue (GALT)(tonsils, adenoids, Peyer’s patches, appendix)

• Immature B and T cells mature in the central lymphoid organs • Then, they circulate in the blood and through the peripheral

lymphoid organs. As long as they have not encountered the specific antigen that binds their antigen receptors (BCR or TCR), the circulating lymphocytes are mature naïve lymphocytes

When they encounter antigen (bind antigen in their antigen receptor), they

1. Stick in the lymph nodes (or other peripheral lymphoid organ)(they stop circulating, i.e., altered trafficking)

2. Proliferate (divide)

3. Differentiate

Innate and Specific immunity;

Cells of the Immune System

Cells of the Immune System

BasophilsNeutrophils

Eosinophils

Granulocytic

Langerhans &Macrophages

Kupffer cellsDendritic cells?

Monocytic

CytotoxicHelper

Suppressor

T-cells

Plasmacells

B-cells Dendriticcells?

lymphoid cellsMyeloid cells

Components of the Immune System

Humoral Cellular Humoral Cellular

SpecificNonspecific

complement, interferon, TNF etc.

macrophages, neutrophils

T cells; other effectors cellsantibodies

Innate Immunity Adaptive Immunity

Characteristics of Innate and Adaptive Immunity

No Immunologicmemory

Antigen independent

No time lag

Not antigen specific

Antigen dependent

A lag period

Antigen specific

Developmentof memory

Innate Immunity Adaptive Immunity

Components of Innate and Adaptive Immunity

skin, gut Villi, lung cilia,etc

many protein andnon-protein secretions

phagocytes, NK cell eosinophils, K cells

physical barriers

soluble factors

cells

none

Immunoglobulins(antibody)

T and B lymphocytes

Site Component Functions

Effector mechanisms in Innate Immunity

Peristalsis, low pHbile salts, fatty acids

columnar cellsGI tract

Skin squamous cellssweat

desquamationflushing, fatty acids

tracheal ciliaLung mucociliary elevatorsurfactants

Site Component Functions

Effector mechanisms in Innate Immunity

Nasopharynxand eye

mucus, saliva, tears flushing, lysozyme

PhagocytesBlood andLymphiodorgans

phagocytosis and intracellular killing

K, NK & LAK cells

direct and antibody dependent cytolysis

Site Component Functions

Effector mechanisms in Innate Immunity

Serum and other serous

fluids

lactoferrin, transferrin

iron deprivation

interferons, TNF-

antiviral proteins phagocyte activation

lysozyme peptidoglycan hydrolysis

Fibronectin & complement

opsonization, enhanced phagocytosis, inflammation

Phagocytes are the Most Important Cells

Influenced bythe work of

Eli Metchnikoff,

George Bernard Shaw wrote:

“There is at bottom only one genuine treatment for all diseases,…to stimulate the phagocytes. Drugs are a delusion. …(when) the phagocytes are stimulated; they devour the disease…”

Phagocytes:Macrophages

phagocytosis, intracellular and extra-cellular killing, tissue repair, antigen presentation for specific immune response

characteristic nucleus and CD14 membrane marker.

Phagocyte Response to Infection

The SOS Signals–N-formyl methionine–Clotting system

peptides–Complement products

Phagocyte response–Vascular adherence–Diapedesis–Chemotaxis–Activation–Phagocytosis and killing

Initiation of Phagocytosis

Attachment via

IgG FcR

ScavengerR

CRToll-like R

Respiratory BurstOxygen Dependent Myeloperoxidase

Independent Reactions

2O2- + H2O2

.OH + OH- + 1O2

Glucose +NADP+

G-6-P-dehydrogenasePentose-P + NADPH

NADPH + O2

Cytochrome b558

NADP++ O2-

2O2- + 2H+

Superoxide dismutase

H2O2 + 1O2

Effector Molecule Function

Mediators of Oxygen Independent Killing in the Phago-lysosome

Cationic proteins (cathepsin) Damage to microbial membranes

Lysozyme Hydrolyses mucopeptides in the cell wall

Lactoferrin Deprives pathogens of iron

Hydrolytic enzymes (proteases) Digests killed organisms

Non-specific Killer Cells

NK and LAK cells

ADCC (K) cell

Activated

macrophages

Eosinophils

They all kill foreign and altered self targets

Natural Killer (NK) cells

also known as large granular lymphocytes (LGL)

kill infected and malignant cells

are identified by the presence of CD56 & CD16 and absence of CD3

activated by IL2 and IFN-γ to become LAK cells

Lymphokine Activated Killer (LAK) cell

IL2

IFNIFN

IL2

kills malignant

cells

kills transformed

and malignant cells

Innate Immunity

• The complement system

Complement:history

Discovered in 1894 by Bordet

It represents lytic activity of fresh serum

Complement functions

Host benefit:opsonization to enhance phagocytosisphagocyte attraction and activationlysis of bacteria and infected cellsregulation of antibody responsesclearance of immune complexesclearance of apoptic cells

Host detriment:Inflammation, anaphylaxis

Proteins of the complementsystem (nomenclature)

C1(qrs), C2, C3, C4, C5, C6, C7, C8, C9

factors B, D, H and I, properdin (P)

mannose binding lectin (MBL), MBL associated serine proteases (MASP-1 MASP-2)

C1 inhibitor (C1-INH, serpin), C4-binding protein (C4-BP), decay accelerating factor (DAF),

C1 receptor (CR1), protein-S (vitronectin)

Pathways of complement

activationCLASSICALPATHWAY

ALTERNATIVEPATHWAY

activationof C5

LYTIC ATTACKPATHWAY

antibodydependent

LECTINPATHWAY

antibodyindependent

Activation of C3 andgeneration of C5 convertase

Components of the Classical Pathway

C4C2 C3

C1 complex

Ca++

C1r C1s

C1q

Ca++

C1r C1s

C1q

C4

C4a

b

Classical Pathway Generation of C3-

convertase

Classical Pathway Generation of C3-

convertase

C4b

Mg++

C4a

Ca++

C1r C1s

C1q

C2

C2ba

C2a

C4b2a is C3 convertase

Classical Pathway Generation of C5-

convertase

C4b

Mg++

C4a

Ca++

C1r C1s

C1q

C2b

C2a

C3

C3a

b

C4b2a3b is C5 convertase; it leads into the Membrane

Attack Pathway

Components of mannose-binding lectin pathway

C4

MBL C2 MASP1

MASP2

Mannose-binding lectin pathway

C4

MBL

C4b

C4a

C4b

C2

C2b

C2a

C2a

C4b2a is C3 convertase; it will lead to the generation of

C5 convertaseMASP1

MASP2

Components of thealternative pathway

C3 B

D

P

Spontaneous C3 activation

C3

H2O

i B

D

Generation of C3 convertase

C3iBb complex has a very short half life

b C3

C3a

b

B

D

bC3b

If spontaneously-generated C3b is not degraded

C3-activationthe amplification loop

C3C3a b

C3a

B

D

BbC3b

C3 b

C3-activationthe amplification

loop

C3b

C3a

b

C3a

C3a BbC3b

C3bC3 BbB

D

bb

C3a

C3-activationthe amplification

loop

C3b

C3a

C3a BbC3b

BbBbC3b

C3a

C3-activationthe amplification

loop

C3bC3b

C3a

C3a BbC3b

BbBb

C3a

C3-activationthe amplification

loop

C3bC3b

Control of spontaneousC3 activation via DAF

C3bDAF prevents

the binding of

factor B to C3b

B

Autologous cell membrane

DA

F

CR1

Autologous cell membrane

C3b C3b Bb

H

I

iC3b

Control of spontaneousC3 activation via CR1

B b

I

iC3b DA

F

CR1DA

F

CR1

C3b stabilization andC5 activation

C3b

C3b finds an activator (protector) membrane

C3

C3a

bB

D

b

P This is stable C5 convertase of the alternative pathway

C3b regulation on self and activator surfaces

C3b

C5-convertase of the two pathways

C3b Bb C3b

C5-convertase of the Alternative Pathway

C4b C2a C3b

C5-convertase of the Classical and lectin

Pathways

Generation of C5 convertase leads to the activation of the

Lytic pathway

Lytic pathway

Components of the lytic pathway

C6

C9

C8

C7

C5

Lytic pathwayC5-activation

C3b C2 aC4b

C5 b

C5a

Lytic pathwayassembly of the lytic

complex

C5 b

C6

C7

Lytic pathway:insertion of lytic complex into cell

membrane

C5 b

C6

C7C8

C9C

9C9 C

9C9

C9 C

9 C9

C9

Biological effects of C5a

Opsonization and phagocytosis

Product Biological Effects Regulation

Biological properties of C-activation products

C2b (prokinin) edema C1-INH

C3a (anaphylatoxin)

mast cell degranulation; enhanced vascular permeability; anaphylaxis

carboxy-peptidase- B (C3-INA)

Product Biological Effects Regulation

Biological properties of C-activation products

as C3, but less potent

(C3-INA)C4a (anaphylatoxin)

opsonization; phagocytosis

C4b (opsonin)

C4-BP, factor I

C3b (opsonin)

opsonization; phagocyte activation

factors H & I

Product Biological Effects Regulation

Biological properties of C-activation products

anaphylactic as C3, but much more potent;attracts & activates PMN causes neutrophil aggregation, stimulation of oxidative metabolism and leukotriene release

C5a (chemotactic factor)

carboxy-peptidase-C (C3-INA)

C5b67 protein-Schemotaxis, attaches to other membranes

Antigens

Factors Influencing ImmunogenicityContribution of the Immunogen• Foreignness• Size

Conformational determinants

Sequence determinants

• Chemical Composition– Primary Structure– Secondary Structure– Tertiary Structure– Quarternary Structure

Factors Influencing ImmunogenicityContribution of the Immunogen• Foreigness• Size• Chemical Composition• Physical Form• Degradability

– Ag processing by Ag Presenting Cells (APC)

Factors Influencing ImmunogenicityContribution of the Biological System• Genetics

– Species– Individual

• Responders vs Non-responders• Age

Factors Influencing ImmunogenicityMethod of Administration• Dose• Route

– Subcutaneous > Intravenous > Intragastric

• Adjuvant– Substances that enhance an immune

response to an Ag

Chemical Nature of Immunogens• Proteins• Polysaccharides• Nucleic Acids• Lipids

– Some glycolipids and phosopholipids can be immunogenic for T cells and illicit a cell mediated immune response

Types of AntigensT-independent• Polysaccharides• Properties

– Polymeric structure

– Polyclonal B cell activation

– Resistance to degradation

• Examples– Pneumococcal polysaccharide,

lipopolysaccharide– Flagella

Types of AntigensT-dependent• Proteins• Structure• Examples

– Microbial proteins

– Non-self or Altered-self proteins

Antigenic Determinants Recognized by B cells and Ab• Composition

– Proteins, polysaccharides, nucleic acids

• Size– 4-8 residues

Antigenic DeterminantsRecognized by T cells• Composition

– Proteins (some lipids)– Sequence determinants

• Processed• MHC presentation (lipid presentation by

MHC-like CD1)• Size

– 8 -15 residues• Number

– Limited to those that can bind to MHC

Foreign bit receptorBiological

Consequence of Interaction

Microbial cell wall components

Complement Opsonization; Complement activation

Mannose-containing carbohydrates

Mannose-binding protein

Opsonization; Complement activation

Polyanions Scavenger receptors PhagocytosisLipoproteins of Gram + bacteriaYeast cell wall components

TLR-2 (Toll-like receptor 2)

Macrophage activation; Secretion of inflammatory cytokines

Foreign bit receptorBiological

Consequence of Interaction

Double stranded RNA

TLR-3 Production of interferon (antiviral)

LPS (lipopolysaccharide of Gram – bacteria

TLR-4 Macrophage activation; Secretion of inflammatory cytokines

Flagellin (bacterial flagella)

TLR-5 Macrophage activation; Secretion of inflammatory cytokines

Immunoglobulins:Structure and Function

Immunoglobulins:Structure and Function• Definition: Glycoprotein molecules that

are produced by plasma cells in response to an immunogen and which function as antibodies

General Functions of Immunoglobulins

• Effector functions – Fixation of

complement– Binding to various

cells

(Usually require Ag binding)

• Ag binding– Can result in

protection

Immunoglobulin Structure• Heavy & Light

Chains• Disulfide

bonds– Inter-chain– Intra-chain CH1

VL

CL

VH

CH2 CH3

Hinge Region

Carbohydrate

Disulfide bond

Immunoglobulin Structure

• Variable & Constant Regions– VL & CL

– VH & CH

• Hinge RegionCH1

VL

CL

VH

CH2 CH3

Hinge Region

Carbohydrate

Disulfide bond

Immunoglobulin Fragments: Structure/Function RelationshipsAg Binding

Complement Binding Site

Placental Transfer

Binding to Fc Receptors

IgG• Structure

– Monomer (7S)

IgG1, IgG2 and IgG4 IgG3

IgG• Properties

– Major serum Ig– Major Ig in extravascular spaces– Placental transfer – Does not require Ag

binding – Fixes complement Binds to Fc receptors

Phagocytes - opsonization• K cells - ADCC

IgM

• Structure– Pentamer (19S)– Extra domain

(CH4)– J chain C4

J Chain

IgM• Properties

– 3rd highest serum Ig

– First Ig made by fetus and B cells

– Fixes complement

Fixation of C1 by IgG and IgM Abs

C1r C1s

C1q C1r C1s

C1q

No activation Activation

IgM• Properties

– 3rd highest serum Ig

– First Ig made by fetus and B cells

– Fixes complement

Tail Piece

– Agglutinating Ig– Binds to Fc

receptors– B cell surface Ig

B Cell Antigen Receptor (BcR)

Ig-Ig- Ig-Ig-

IgA• Structure

– Serum - monomer– Secretions (sIgA)

• Dimer (11S)• J chain• Secretory

component

J ChainSecretory Piece

IgA• Properties

– 2nd highest serum Ig– Major secretory Ig (Mucosal or Local

Immunity)• Tears, saliva, gastric and pulmonary

secretions – Does not fix complement (unless

aggregated)– Binds to Fc receptors on some cells

IgD

• Structure– Monomer– Tail piece

Tail Piece

IgD• Properties

– 4th highest serum Ig– B cell surface Ig– Does not bind complement

IgE• Structure

– Monomer– Extra domain

(CH4)

C4

IgE• Properties

– Least common serum Ig• Binds to basophils and mast cells (Does not

require Ag binding)– Allergic reactions– Parasitic infections (Helminths)

• Binds to Fc receptor on eosinophils– Does not fix complement

Nature of Ag/Ab Reactions

• Lock and Key Concept• Non-covalent Bonds

– Hydrogen bonds– Electrostatic bonds– Van der Waal forces– Hydrophobic bonds

• Reversible

• Multiple Bonds

Source: Li, Y., Li, H., Smith-Gill, S. J., Mariuzza, R. A., Biochemistry 39, 6296, 2000

http://www.med.sc.edu:85/chime2/lyso-abfr.htm

Ab formation

Hallmarks of the Immune Response

• Self/Non-self Discrimination• Memory• Specificity

Kinetics of the Ab ResponseT-dependent Ag; 1o Response

• Lag phase• Log phase• Plateau phase• Decline phase Ag

D a y s A f t e r I m m u n i z a t i o n

A b

T i

t e r

LAG LOG DECLINEPLATEAU

Kinetics of the Ab ResponseT-dependent Ag; 2o Response

* Specificity

• Lag phase• Log phase• Plateau phase• Decline phase

1o Ag 2o Ag

D a y s A f t e r I m m u n i z a t i o n

A b

T i

t e r

Qualitative Ab Changes during 1o and 2o Responses

• Class variation– 1o - IgM– 2o - IgG, IgA or IgE 1o Ag 2o Ag

Total Ab

IgM Ab

IgG Ab

D a y s A f t e r I m m u n i z a t i o n

A b

T i

t e r

Cellular Events in 1o Response to T-dependent Ags

• Lag– Clonal selection

• Log– IgM– Class switching

• Stationary• Decline• Memory Cell

Pool

IgM

Memory Cells

IgG

1o Ag

Cellular Events in 2o Response to T-dependent Ags• Lag phase– Virgin cells– Memory cells

• Log phase– Pool size– IgG, IgA or IgE

• Stationary• Decline

– Sustained production

IgM

Memory Cells

IgG

IgG

Memory Cells

Memory Pool

Virgin B cell

Cell-Cell Interactions, T-Independent Antigens, CD5 B Cells, Cytokines

T Cell-B Cell Interactions(hapten-carrier effect)• Th cells recognize carrier, B cells

recognize hapten• Th and B cells cooperate by

interacting• Interactions are class II self-MHC

restricted

Uniqueness of B Cells• Express both immunoglobulin (Ig)

and class II MHC on cell surface• Capable of producing antibody of

same specificity as that of its surface Ig

AND• Capable of functioning as an

antigen presenting cell

Mechanism of Hapten-Carrier• Hapten recognized by Ig

receptor on B cell• Hapten-carrier endocytosed• Carrier processed and

presented on class II MHC to Th cell

• Activated Th cell produces cytokines

• Cytokines enable B cell to be activated to produce anti-hapten antibodies

CD40

Immunoglobulinreceptor

MHC II

B7 CD28

TCRT helpercell

Antigen

1. Antigen presentation toTh cell

2. B7 expressed 3. Th cell is activated

and expresses CD40 ligand,

Cytokines secreted

CD40 ligand

Cytokine

Cytokine receptor

Bcell

Bcell

Bcell

T helpercell

4. Cytokine binds to cytokine receptor,

CD40 ligand binds to CD40

Bcell

Bcell

Bcell

5. B cell activated

6. B cells proliferate, differentiate, secrete Ig

Class II MHCAPCTh cell B

cellTh cell

Bcell

Bcell

B cell takes up and presents antigen

Th cell Th cellB

cell

Th cells are primed by antigen-presenting cell

B-T cell cooperationB cells receive signals from T cells

B cells divide

Bcell

Bcell

Bcell

Bcell

Antibody formingcell

Antibody formingcell

Antibody formingcell

Bmemory

cell

B Cells In Secondary Responses

• Memory cells created during primary response

• Have high-affinity Ig receptors• Can therefore take up antigens at

much lower concentrations than other antigen presenting cells that lack Ig antigen receptors

Cytokines Non-antibody proteins acting as

mediators between cells, termed:• Monokines – mononuclear

phagocytes• Lymphokines – activated T cells,

especially helper T cells• Interleukins – abbreviated IL with

a number

Properties of Cytokines1. Produced by cells involved in both

natural and specific immunity2. Mediate and regulate immune

responses3. Secretion brief and limited

- not stored pre-formed - synthesis initiated by gene transcription - mRNA short-lived - cytokines produced as needed

Properties of Cytokines(continued)

4.Can be produced by many cell types and act on many cell types (pleiotropic)

5.Can have similar actions (redundant)

Properties of Cytokines (continued)6. Can influence synthesis of other

cytokines - produce cascades

- enhance or suppress production of

other cytokines

- exert positive or negative regulatory mechanisms for immune responses

7. Influence action of other cytokines - can be antagonistic, additive, synergistic

Properties of Cytokines (continued)8. Bind to receptors with high affinity9. Cells responding to cytokine can be: - same cell (autocrine) - nearby cell (paracrine) - distant cell by circulation

(endocrine)10.Cellular responses to cytokines are

slow, require new mRNA and protein synthesis

• Tumor Necrosis Factor-alpha (TNF-α)

• Interleukin-1 (IL-1) • Chemokines (Chemotactic

cytokines)• Type I Interferons (IFN-α and IFN-

β)• Interleukin-12 (IL-12)• Interleukin-10 (IL-10)

Mediators and Regulators of Natural Immunity

Tumor Necrosis Factor (TNF-α)• Produced by activated macrophages• Most important mediator of acute

inflammation in response to microbes, especially Gram-negative bacteria (LPS)

• Mediates recruitment of neutrophils and macrophages to site of inflammation

• Acts on hypothalamus to produce fever

• Promotes production of acute phase proteins

Interleukin-1 (IL-1)• Produced by activated

macrophages• Effects similar to those of TNF-α

Chemokines• Produced by many different

leukocytes and tissue cells• Large family of >50 substances• Recruit leukocytes to sites of

infection• Play a role in lymphocyte

trafficking

Type I Interferons (IFN-α and β)

• IFN-α a family of many proteins produced by macrophages, IFN-β a single protein produced by many cells

• Both IFNs inhibit viral replication• Both increase expression of class

I MHC• Both activate NK cells

Interleukin-12 (IL-12)• Produced by activated

macrophages and dendritic cells

• Stimulates production of IFN-γ• Induces differentiation of Th

cells to become Th1 cells• Enhances cytolytic functions of

cytotoxic T cells and NK cells

Interleukin-10 (IL-10)• Produced by activated

macrophages, Th2 cells• An inhibitory cytokine• Inhibits cytokine production by

activated macrophages• Inhibits expression of class II

MHC and costimulatory molecules on macrophages

Mediators and Regulators of Specific Immunity• Interleukin-2 (IL-2)• Interleukin-4 (IL-4)• Interleukin-5 (IL-5)• Interleukin-10 (IL-10)• Interferon-gamma (IFN-γ)

Interleukin-2 (IL-2)• Produced by Th>>Tc• Main growth factor for T cells

IL-2secretion

T cell

NKIncrease in NK

Cell activity

B cell Stimulationof division

T cellStimulation

of division and IFN gamma release (and other

mediators)

Monocyte

Activation

Interleukin-4 (IL-4)• Produced by Th2 cells• Stimulates Ig class switching to

IgE isotype• Stimulates development of Th2

cells from naïve Th cells• Promotes growth of

differentiated Th2 cells

Interleukin-5 (IL-5)• Produced by Th2 cells• Promotes growth and

differentiation of eosinophils• Activates mature eosinophils• IL-4 and IL-5 can work together

Helminths opsonized with IgE can be killed by activated eosinophils

Interleukin-10 (IL-10)• Produced by activated

macrophages, Th2 cells• Inhibits production of IFN-γ by

Th1 cells needed to activate macrophages

Interferon-gamma (IFN-γ)• Produced by Th cells >> Tc and NK

cells• Numerous functions in both natural

and specific immunity

Bcell

MacrophageNK

T cell

Th1 cell > Tc cell

NK

Many cell types Many cell types

Induction of class I and class II MHC

Increase in NK

cell activity

Differentiation,Stops cell divisionT cell activation

Weak anti-viral activity,Stops cell division,

Stops hematopoiesis

Activation

Granulocyte

ActivationEndothelial cell

ActivationIFN gammasecretion

Major Histocompatibility Complex and T Cell Receptor

Class-I expressed on all nucleated cells in man, and also on erythrocytes in mice.

Class-II expressed primarily on antigen presenting cells (dendritic cells, macrophages and B cells, etc.)

Differential expression of MHC antigens

Structure of Class I MHC

NH2

Alloantigenicsites

CHO

NH2

COOH

COOH

P

α1

α2

α3

β2

OH

Plasma membrane

Disulfide bridge

Papain cleavage

Cytoplasm

NH2

Structure of Class II MHC

Plasma membrane

Cytoplasm

CHO

CHO

CHO

NH2 NH2

COOH COOH

α1

α2 β2

β1

Aspects of MHC1.MHC molecules are membrane-

bound. Recognition by T cells requires cell-cell contact.

2.Peptide from cytosol associates with class I MHC and is recognized by Tc cells. Peptide from vesicles associates with class II MHC and is recognized by Th cells.

Aspects of MHC (continued)3.Although there is a high degree of polymorphism for a species, an individual has maximum of six different class I MHC products and only slightly more class II MHC products.

A peptide must associate with a given MHC of that individual, otherwise no immune response can occur. That is one level of control.

Aspects of MHC (continued)4.Mature T cells must have a T

cell receptor that recognizes the peptide associated with MHC. This is the second level of control.

5.Each MHC molecule has only one binding site. The different peptides a given MHC molecule can bind all bind to the same site, but only one at a time.

Structure of T Cell Receptor

CHO CHO

CHOCHO

Variable region “V”

Constant region “C”

Hinge “H”

Alphachain

Betachain

Disulfide bridge

Transmembrane region

Cytoplasmic tail

++ +

Structure of T Cell Receptor(continued)• Hypervariable regions in V

contribute to diversity of TCR• TCR recognizes portions of MHC

molecule and peptide bound in the groove

• Small population of T cells has a TCR comprised of γ and δ chains – γδ TCR specificity differs from αβ TCR

Antigen Processing and Presentation

What Does The B Cell Immunoglobulin (Ig) Receptor Recognize?

1.Proteins (conformational determinants, denatured or proteolyzed determinants)

2.Nucleic acids3.Polysaccharides4.Some lipids5.Small chemicals (haptens)

What Does the αβ T Cell Receptor (TCR) Recognize?1.Only fragments of proteins

(peptides) associated with MHC molecules on surface of cells

• Helper T cells (Th) recognize peptide associated with MHC class II molecules

• Cytotoxic T cells (Tc) recognize peptide associated with MHC class I molecules

Antigen Processing and Presentation• Fragmentation of protein into

peptides• Association of peptide with an

MHC molecule• Transport to cell surface for

expression• Different cellular pathways for

association of peptide with MHC class I and class II molecules

Class I MHC Pathway Viral protein is made

on cytoplasmicribosomes

Plasma membrane

Proteasome degrades protein topeptides

Peptide transporterprotein moves peptide into ER

MHC class I alpha and beta proteinsare made on the rER

Peptide associateswith MHC-I complex

Peptide with MHCgoes to Golgi body

Peptide passeswith MHC from Golgi

body to surface

Peptide is presentedby MHC-I to CD8cytotoxic T cell

Golgi body

rER

Globular viralprotein - intact

Class II MHC PathwayGlobular protein

Endocytosis

Protein is processed topeptides in endosome

or lysosome

Endosome

Lysosome

Fusion of endosomeand exocytic vesicle

Endoplasmic reticulum

Class II MHCSynthesis

3 chains: α,β and Ii

Golgibody

Exocytic vesicle fuseswith endosome

releasing Ii from αβ dimer

αβ Ii

Immunodominantpeptide binds

to class II MHC

Endosome fuses withplasma membrane

Peptide MHC-II complex is presentedto CD4 helper T cell

CD4 helper T cell

Points Concerning Antigen Processing and Presentation1. Location of pathogen • viruses in cytosol, MHC class I

pathway, Tc response

• extracellular bacteria, MHC class II pathway, Th2 response, Ab formation

• intracellular bacteria, MHC class II pathway, Th1 response

Points Concerning Antigen Processing and Presentation2.Peptides derived from both self

and non-self proteins can associate with MHC class I and class II molecules.

3.Chemical nature of MHC groove determines which peptides it will bind.

Self MHC Restriction• T cells recognize foreign antigen

associated with self MHC • No value for individual to have T

cells that recognize foreign antigen associated with foreign MHC

• Self MHC restriction occurs in thymus

Process of Self MHC Restriction in Thymus• T cells with TCR recognizing self MHC

molecules are retained – “positive selection”

• Retained T cells with TCR recognizing self peptide associated with self MHC are eliminated – “negative selection”

• Self MHC-restricted T cells are released

Functions of Th Cells, Th1 and Th2 Cells, Macrophages and Tc Cells. Immunoregulation

Critical Role of Th Cells in Specific Immunity

• Select effector mechanisms• Induce proliferation in appropriate effectors• Enhance functional activities of effectors

APC

Thcell

B cell

Tccell NK

AgAg

Ag

NK cell

Cytokines

Granulocyte Macrophage

Cytokines

Antigen-presenting cell

Functions of Th1 and Th2 Cells

Th1cell

Th2cell

Macrophage B cell

IFNγActivates

IL-4 IL-5

IL-10

Activates

Inhibits productionInhibits proliferation

Mast cell Eosinophil

Antibodies (including IgE)

Macrophage Macrophage

Thcell

Macrophage

Cytokines Lymphokines

CytokinesAnti-microbial functions

Anti-tumor functions

Activa

te

Invading agent Antigen presentation Activated macrophage

Central Role of Macrophages in Natural and Specific Immunity• Involved in initial defense and

antigen presentation and have effector functions

Detailed Functions of MacrophagesInflammation – Fever, Production

of: IL-6, TNF-alpha, IL-1 – act as pyrogen

ImmunitySelection of lymphocytes to be activated:IL-12 results in Th1 activationIL-4 results in Th2 activationActivation of lymphocytes:Production of IL-1Processing and presentation of antigen

Reorganization of tissues,Secretion of a variety of factors:Degradative enzymes (elastase, hyaluronidase, collagenase)Fibroblast stimulation factorsStimulation of angiogenesis

Damage to tissuesHydrolases, Hydrogen peroxide productionComplement C3aTNF alpha production

Antimicrobial actionO2–dependent production of: hydrogen peroxide, superoxide, hydroxyl radical, hypochlorous acidO2-independent production of: acid hydrolases, cationic proteins, lysozyme

Anti-tumor activity produced by:Toxic factorsHydrogen peroxideComplement C3aProteases, ArginaseNitric oxideTNF alpha

Cytolytic T (Tc) Cells• Tc exiting the thymus are pre-Tc

cells, i.e. have TCR that can recognize antigen, but are not mature and cannot kill until “armed”

• To become armed requires two signals:

1. Recognition by TCR of specific antigen associated with class I MHC, and

2. Exposure to cytokines (IL-2 and IFN-γ)

Mechanism of Arming Tc Cells

Pre-Tc cell

Tc cell

T helper cell

Class IMHC

Class II MHCAPC

1. Cell expressing class I MHC presents antigen ( )

to a pre-Tc cell

IFNIL-2

2. Antigen-presenting cell presents antigen in

association with class II MHC to Th cell3. Th cell

makes cytokines

4. Pre-Tc celldifferentiates to

functional Tc cell

5. Tc recognizes antigen onclass I MHC-expressing target cell

6. Target cellis killed

Features of Tc Killing• Antigen-specific• Requires cell-cell contact• Each Tc capable of killing many

target cells

Steps in Tc KillingTc cell

1. Tc recognizes antigen ontarget cell Target cell

Tc cell2. A lethal hit is delivered by the Tc using agents such as perforin or granzyme B

Target cell

Tc cell

3. The Tc detaches from the target cellTarget cell

4. Target cell dies by apoptosis

Target cell

Regulation of Immune Responses• Magnitude determined by balance

between the extent of lymphocyte activation and tolerance induced by an antigen

• Nature determined by specificities and functional classes of lymphocytes activated

• Regulatory mechanisms may act at the recognition, activation, or effector phases of an immune response