Normal Immune System Function Karim Rafaat

Organs of the Immune System

Central lymphoid organs

Central (primary) lymphoid organs are the sites for generation and early maturation of lymphocytes T cells mature in the thymus (T for thymus) B cells mature in the bone marrow B for bursa of Fabricius (a lymphoid organ in birds) [After Hieronymus Fabricius (1537-1619), Italian anatomist]

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

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 *other stuff (not traffic peripheral lymphoid organs)

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

Myeloid cells

lymphoid cells

Granulocytic

Monocytic

T-cells

B-cells

Neutrophils Basophils Eosinophils

Macrophages Langerhans & Kupffer cells Dendritic cells?

Helper Cytotoxic Suppressor

Plasma cells

Dendritic cells?

Components of the Immune System

Nonspecific Humoral complement, interferon, TNF etc.

Cellular macrophages, neutrophils

Specific Humoral

antibodies

Cellular T cells; other effectors cells

Characteristics of Innate and Adaptive Immunity Innate Immunity

Adaptive Immunity

Antigen independent

Antigen dependent

No time lag

A lag period

Not antigen specific

Antigen specific

No Immunologic memory

Development of memory

Components of Innate and Adaptive Immunity Innate Immunity

Adaptive Immunity

physical barriers skin, gut Villi, lung cilia,etc

soluble factors many protein and non-protein secretions phagocytes, NK cell eosinophils, K cells

cells

non e Immunoglobulins (antibody)

T and B lymphocytes

Effector mechanisms in Innate Immunity Site Skin GI tract Lung

Component

Functions

squamous cells sweat

desquamation flushing, fatty acids

columnar cells

Peristalsis, low pH bile salts, fatty acids

tracheal cilia

mucociliary elevator surfactants

Effector mechanisms in Innate Immunity Site

Component

Functions

Nasopharynx and eye

mucus, saliva, tears

flushing, lysozyme

Blood and Lymphiod organs

Phagocytes

phagocytosis and intracellular killing

K, NK & LAK cells

direct and antibody dependent cytolysis

Effector mechanisms in Innate Immunity Site Serum and other serous fluids

Component

Functions

lactoferrin, transferrin

iron deprivation

interferons, TNF-

antiviral proteins phagocyte activation

lysozyme Fibronectin & complement

peptidoglycan hydrolysis opsonization, enhanced phagocytosis, inflammation

Phagocytes are the Most Important Cells George Bernard Shaw wrote: “There is at bottom only one genuine treatment for all diseases,…to stimulate Influenced by the work of phagocytes.theDrugs are a

Eli Metchnikoff,

delusion. …(when) the

phagocytes are stimulated; they devour the disease…”

Phagocytes: Neutrophils  phagocytosis, intracellular killing, inflammation and tissue damage  characteristic nucleus, cytoplasm  granules and CD66 membrane marker.

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 ScavengerR IgG FcR CR Toll-like R

Respiratory Burst Oxygen Dependent Myeloperoxidase Independent Reactions Glucose +NADP+ G-6-P-dehydrogenase NADPH + O2 Cytochrome b558 2O2

+

NADP +

+ 2H+

Superoxide dismutase 2O2

+ H2O2

Pentose-P + NADPH O2

H2O2 + 1O2

.OH + OH- + 1O2

Mediators of Oxygen Independent Killing in the Phago-lysosome Effector Molecule

Function

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

kills kills transformed malignant and malignant cells cells

Macrophages phagocytose & kill intracellularly identified by CD14 adherent of plastic and glass surfaces

activated by cytokines kill both intracellular organisms kill malignant and altered self targets

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 phagocytosis phagocyte attraction and activation lysis of bacteria and infected cells regulation of antibody responses clearance of immune complexes clearance of apoptic cells

Host detriment: Inflammation, anaphylaxis

Proteins of the complement system (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 activation CLASSICAL PATHWAY antibody dependent

LECTIN PATHWAY

ALTERNATIVE PATHWAY

antibody independent

Activation of C3 and generation of C5 convertase activation of C5 LYTIC ATTACK PATHWAY

Components of the Classical Pathway

C3 C1 complex

C4

Classical Pathway Generation of C3-convertase

Classical Pathway Generation of C3-convertase

C4b2a is C3 convertase C4b

Classical Pathway Generation of C5-convertase

C4b2a3b is C5 convertase; it leads into the Membrane Attack Pathway

C4b

C3b

Components of mannose-binding lectin pathway

MBL

MASP1

Mannose-binding lectin pathway

C4b2a is C3 convertase; it will lead to the generation of C5 convertase MASP1

MBL

Components of the alternative pathway

C3

Spontaneous C3 activation Generation of C3 convertase

C3 i

b

C3b

C3iBb complex has a very short half life

C3-activation the amplification loop If spontaneously-generated C3b is not degraded

C3b

b

C3 b

C3-activation the amplification loop

C3 b

C3b

b

C3b

C3-activation the amplification loop

C3 b

b

C3b

C3b

C3b

C3-activation the amplification loop

C3b

C3b

C3b

C3b

C3-activation the amplification loop

C3b

C3b

C3b

Control of spontaneous C3 activation via DAF

DAF prevents the binding of factor B to C3b

C3b

CR1 Autologous cell membrane

Control of spontaneous C3 activation via CR1

C3b CR1 Autologous cell membrane

C3b iC3b CR1

C3b stabilization and C5 activation C3b finds an activator (protector) membrane This is stable C5 convertase of the alternative pathway C3b

b

C3 b

C3b regulation on self and activator surfaces

C3b

C5-convertase of the two pathways C5-convertase of the Classical and lectin Pathways

C4b

C3b

C5-convertase of the Alternative Pathway

C3b

C3b

Lytic pathway

Generation of C5 convertase leads to the activation of the

Lytic pathway

Components of the lytic pathway C7

C6

C 9

Lytic pathway C5-activation

b C4b

C3b

Lytic pathway assembly of the lytic complex

C6 C7

b

Lytic pathway:

insertion of lytic complex into cell membrane

C6 C7 CC C C C9 9 9 9C 9C C C9 9 9 9

b

Biological effects of C5a

Opsonization and phagocytosis

Biological properties of C-activation products Product C2b (prokinin) C3a (anaphylatoxin)

Biological Effects edema mast cell degranulation; enhanced vascular permeability; anaphylaxis

Regulation C1-INH carboxypeptidase- B (C3-INA)

Biological properties of C-activation products Product

Biological Effects

Regulation

C3b (opsonin)

opsonization; phagocyte activation

factors H & I

C4a (anaphylatoxin)

as C3, but less potent

(C3-INA)

C4b (opsonin)

opsonization; phagocytosis

C4-BP, factor I

Biological properties of C-activation products Product

Biological Effects

Regulation

C5a (chemotactic factor)

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

carboxypeptidase-C (C3-INA)

C5b67

chemotaxis, attaches to other membranes

protein-S

Antigens

Factors Influencing Immunogenicity

Contribution of the Immunogen • Foreignness • Size • Chemical Composition – – – –

Primary Structure Secondary Structure Tertiary Structure Quarternary Structure

Sequence determinants Conformational determinants

Factors Influencing Immunogenicity

Contribution of the Immunogen • Foreigness • Size • Chemical Composition • Physical Form • Degradability – Ag processing by Ag Presenting Cells (APC)

Factors Influencing Immunogenicity

Contribution of the Biological System • Genetics – Species – Individual • Responders vs Non-responders

• Age

Factors Influencing Immunogenicity

Method 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 Antigens T-independent • Polysaccharides • Properties – Polymeric structure – Polyclonal B cell activation – Resistance to degradation

• Examples – Pneumococcal polysaccharide, lipopolysaccharide – Flagella

Types of Antigens T-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 Determinants Recognized 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

Biological Foreign bit receptor Consequence of Interaction Microbial cell wall Complement Opsonization; components Complement activation MannoseMannose-binding Opsonization; containing protein Complement carbohydrates activation Polyanions Scavenger receptors Phagocytosis Lipoproteins of Gram + bacteria Yeast cell wall components

TLR-2 (Toll-like receptor 2)

Macrophage activation; Secretion of inflammatory cytokines

Foreign bit Double stranded RNA

receptor TLR-3

LPS TLR-4 (lipopolysaccharid e of Gram – bacteria Flagellin (bacterial TLR-5 flagella)

Biological Consequence of Interaction Production of interferon (antiviral) Macrophage activation; Secretion of inflammatory cytokines 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 • Ag binding – Can result in protection

• Effector functions

(Usually require Ag binding) – Fixation of complement – Binding to various cells

Immunoglobulin Structure • Heavy & Light

Disulfide bond

Chains • Disulfide bonds – Inter-chain – Intra-chain

Carbohydrate

CL VL CH2

CH1 VH

Hinge Region

CH3

Immunoglobulin Structure Disulfide bond

• Variable &

Carbohydrate

Constant Regions – VL & CL – VH & CH

• Hinge Region

CL VL CH2

CH1 VH

Hinge Region

CH3

Immunoglobulin Fragments: Structure/Function Relationships Ag Binding

Complement Binding Site Binding to Fc Receptors Placental Transfer

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

J Chain

– Pentamer (19S) – Extra domain (CH4) – J chain C:4

IgM • Properties – 3rd highest serum Ig – First Ig made by fetus and B cells – Fixes complement

Fixation of C1 by IgG and IgM Abs

No activation

Activation

IgM • Properties – 3rd highest serum Ig – First Ig made by fetus and B cells – Fixes complement – Agglutinating Ig – Binds to Fc receptors – B cell surface Ig

Tail Piece

B Cell Antigen Receptor (BcR)

Ig-$ Ig-"

Ig-" Ig-$

IgA • Structure – Serum - monomer – Secretions (sIgA) • Dimer (11S) • J chain • Secretory component

Secretory Piece

J Chain

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)

C,4

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

• Lock and Key Concept • Non-covalent Bonds – Hydrogen bonds – Electrostatic bonds – Van der Waal forces – Hydrophobic bonds

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

Ab formation

Hallmarks of the Immune Response • Self/Non-self Discrimination • Memory • Specificity

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

LAG

LOG

PLATEAU

DECLINE

Ab Titer

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

Ag

Days After Immunization

Kinetics of the Ab Response

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

Ab Titer

T-dependent Ag; 2o Response

1o Ag

2o Ag

* Specificity Days After Immunization

Qualitative Ab Changes during 1o and 2o Responses Total Ab

• Class variation

IgG Ab

IgM Ab

– 2o - IgG, IgA or IgE

Ab Titer

– 1o - IgM 1o Ag

2o Ag

Days After Immunization

Cellular Events in 1o Response to T-dependent Ags • Lag – Clonal selection

• Log – IgM – Class switching

• Stationary • Decline • Memory Cell Pool

1o Ag IgM

IgG Memory Cells

Cellular Events in T-dependent Ags

o 2

Response to

• Lag phase

– Virgin cells – Memory cells

Virgin B cell

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

• Stationary • Decline – Sustained production

IgM

IgG

Memory Pool

Memory Cells

IgG

Memory Cells

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 HaptenCarrier • 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

Antigen CD40

B

MHC II

cell

Cytokine Immunoglobulin receptor receptor

B

B

cell

cell

cell

B7 CD28

2. B7 expressed

4. Cytokine binds to cytokine receptor, CD40 ligand binds to CD40

B 5. B cell activated

6. B cells proliferate, differentiate, secrete Ig

TCR

cell

cell

1. Antigen presentation to Th cell

B

T helper

B

cell

3. Th cell is activated and expresses CD40 ligand, Cytokines secreted

CD40 ligand

T helper cell Cytokine

Th cell

Class II MHC APC

B

Th cell

cell

B cell takes up and presents antigen

Th cells are primed by antigenpresenting cell Th cell

Th cell B cell

B-T cell cooperation B cells receive signals from T cells

B

B

cell

cell

B cells divide

B

B

B

B

cell

cell

cell

cell

Antibody forming cell

Antibody forming cell

Antibody forming cell

B memory 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 Cytokines 1. Produced by cells involved in both 2. 3.

natural and specific immunity Mediate and regulate immune responses 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 affinity 9. 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

Mediators and Regulators of Natural Immunity • 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)

Tumor Necrosis Factor (TNFα) • 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 T cell B cell Monocyte Activation

IL-2 secretion

NK

Stimulation of division

T cell Stimulation of division and IFN gamma release (and other Increase in NK mediators) Cell activity

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

Th1 cell > Tc cell

NK Granulocyte

Endothelial cell

Activation

Activation

IFN gamma secretion

Macrophage

NK

Activation Many cell types

Weak anti-viral activity, Stops cell division, Stops hematopoiesis

T cell T cell activation

B cell

Increase in NK cell activity

Many cell types

Differentiation, Induction of class I Stops cell division and class II MHC

Major Histocompatibility Complex and T Cell Receptor

Differential expression of MHC antigens  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.)

Structure of Class I MHC α1 NH2 NH2 Alloantigenic sites

α2

β2

NH2

COOH

CHO

α3

Disulfide bridge

Papain cleavage

Plasma membrane OH

P

Cytoplasm COOH

Structure of Class II MHC NH2

NH2

CHO

α1

β1

CHO

α2

β2

CHO

Plasma membrane Cytoplasm COOH

COOH

Aspects of MHC 1. 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 Alpha chain

Beta chain

CHO

CHO

CHO

CHO

Variable region “V”

Constant region “C” Hinge “H”

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 acids 3. Polysaccharides 4. Some lipids 5. 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 Peptide is presented by MHC-I to CD8 cytotoxic T cell

Peptide passes with MHC from Golgi body to surface

Plasma membrane

Viral protein is made on cytoplasmic ribosomes Globular viral protein - intact

Peptide associates with MHC-I complex

rER

Proteasome degrades protein to peptides Peptide transporter protein moves peptide into ER

Golgi body

Peptide with MHC goes to Golgi body

MHC class I alpha and beta proteins are made on the rER

Class II MHC Pathway Peptide MHC-II complex is presented to CD4 helper T cell

Endosome fuses with plasma membrane Immunodominant peptide binds to class II MHC Golgi body

Globular protein

CD4 helper T cell

Endosome Fusion of endosome and exocytic vesicle

Endocytosis Lysosome

Exocytic vesicle fuses with endosome releasing Ii from αβ dimer

Protein is processed to peptides in endosome or lysosome

Class II MHC α Synthesis 3 chains: α,β and Ii β Ii Endoplasmic reticulum

Points Concerning Antigen Processing and Presentation

1. 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 Presentation 2. 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 Ag

Ag

APC

Antigen-presenting cell

Ag

Th

B cell

cell

Cytokines Tc cell

Granulocyte

Macrophage Cytokines

NK NK cell

Functions of Th1 and Th2 Cells Th1

Th2

cell

cell

Inhibits production

IL-10

IFNγ

IL-4 IL-5

Activates

Activates

Macrophage

Mast cell

B cell

Eosinophil

Antibodies (including IgE)

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

presentation and have effector functions

Invading agent

Macrophage

Antigen presentation

Macrophage

Activated macrophage

Macrophage

Th cell

Cytokines

Lymphokines

Cytokines Anti-microbial functions Anti-tumor functions

Detailed Functions of Macrophages Inflammation – Fever, Production of: IL-6, TNF-alpha, IL-1 – act as pyrogen

Immunity Selection of lymphocytes to be activated: IL-12 results in Th1 activation IL-4 results in Th2 activation Activation of lymphocytes: Production of IL-1 Processing and presentation of antigen

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

Damage to tissues Hydrolases, Hydrogen peroxide production Complement C3a TNF alpha production Antimicrobial action O2–dependent production of: hydrogen peroxide, superoxide, hydroxyl radical, hypochlorous acid O2-independent production of: acid hydrolases, cationic proteins, lysozyme Anti-tumor activity produced by: Toxic factors Hydrogen peroxide Complement C3a Proteases, Arginase Nitric oxide TNF alpha

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

• 1. 2.

i.e. have TCR that can recognize antigen, but are not mature and cannot kill until “armed” To become armed requires two signals: Recognition by TCR of specific antigen associated with class I MHC, and Exposure to cytokines (IL-2 and IFN-γ)

Mechanism of Arming Tc Cells Class I MHC

1. Cell expressing class I MHC presents antigen ( ) to a pre-Tc cell 3. Th cell makes cytokines

Pre-Tc cell

IFN IL-2

2. Antigen-presenting cell presents antigen in association with class II MHC to Th cell

T helper cell

Class II MHC APC

4. Pre-Tc cell differentiates to functional Tc cell

Tc cell 5. Tc recognizes antigen on class I MHC-expressing target cell

6. Target cell is killed

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

Steps in Tc Killing Tc cell

Tc cell

Tc cell

Target cell

Target cell

Target cell

Target cell

1. Tc recognizes antigen on target cell

2. A lethal hit is delivered by the Tc using agents such as perforin or granzyme B 3. The Tc detaches from the target cell

4. Target cell dies by apoptosis

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