Time Course of thePrimary Immune Response

Innate immunity

Acquired immunity

Ig Maturation

Journey of a B Cell

Contact Between the TCR and MHC/peptide:

Not All Peptides are Created Equal

The Two-Signal Theory of T-cell Activation

APC = Antigen-presenting cellsTCR = T-cell receptor for antigenDC = Dendritic cellCD80 = Co-stimulatory receptor

2 1 1 2

No responseNo response

or AnergyActivation

V

C C

V

peptide

CD3

TCRCD4

MHC II

V

C C

V

CD3

TCR

MHC I

CD8

(1) Interacts with MHC class II expressing cells (APCs)

(2) Helps B cells to synthesize antibody(3) Induces and activates macrophages(4) Secretes cytokines

(1) Interacts with MHC class I-expressing cells (all nucleated cells)

(2) Kill MHC class I-expressing target cells(3) Secretes cytokines

CD4+ T cell CD8+ T cell

peptide

Two Major Functional T Cell Subsets

LckLck

APC APC

CD4+ T Cells Activate Macrophages and B cells

ImmunityImmunity Tolerance ToleranceActivation Suppression

Regulation of the Immune Response:

a Conceptual View

ImmunityImmunity Tolerance Tolerance

Activation Suppression

Regulation of the Immune Response:

a Conceptual View

AutoimmunityAutoimmunity ImmunodeficiencyImmunodeficiency

ImmunityImmunity Tolerance ToleranceActivation

Suppression

Regulation of the Immune Response:

a Conceptual View

AutoimmunityAutoimmunity ImmunodeficiencyImmunodeficiency

Antibodies: Secreted or Transmembrane

TCR: Transmembrane

Figure 4-2DNA Rearrangement RemovesSequences Between V, D and J Segments

RNA Splicing Removes Sequences Between J and C Segments

DNA

RNA

Antigen-Independent B-Cell Development

1. DNA rearrangements establish theprimary repertoire, creating diversity

2. Allelic exclusion ensures that each clone expresses a single antibody on the surface, establishing specificity

3. Deletion of self-reactive clones establishes tolerance

Bone Marrow

THE B CELL RECEPTOR

1. Bound antigen is inter- nalized and presented to T cells.

2. Bound antigen gives signals to the B cell to proliferate and differentiate.

Signalling from the BCR

Lack of Btk causes Bruton’s X-linkedagammaglobulinemia

(blocked at pre-B stage)

Antigen-Dependent B Cell Development

In Periphery (spleen and LN)

Antigen and TH cells give B cells two signals: 1) proliferate 2) differentiate

T-cell dependent responses are refined two ways: 1) higher affinity antibodies

2) IgG/A/E (“switched”) isotypes

Two products of B cell development:1) plasma cells secrete Ig (final effector)

2) memory cells respond to IIo antigen

T Cell-B Cell Communication

1. Cell-cell signals from CD40L/CD40 andother surface molecules.

2. Secreted cytokines

(B cells signal T cells by presenting Ag inassociation with MHC II)

T cells provide 2 kinds of help to B cells:

The Germinal Center

1. Affinity maturationa. Somatic hypermutationb. Selection for high affinity clones

2. Isotype switch recombination

4. Final maturation to memory or plasma cell.

3. Peripheral tolerance

Proliferation

Ag(FDC) + T cell help

(Iterative cycles)

SURVIVAL

butT help and no Ag(eliminates low affinity clones)

orAg and no T help(eliminates self-reactive clones, giving tolerance)

DEATH

+ Somatic Hypermutation

AFFINITY MATURATION IN THE GC

Darkzone

Lightzone

1. Memory B cells

Surface Ig, usually IgGHigh affinity for antigenLong-lived, even in the absence of antigenRespond rapidly to secondary stimulation

2. Plasma Cells

Secrete copious amounts of Ig, no surface IgNon-dividingSome are short-lived, some become long-lived in the bone marrow

Secreted AntibodiesFunction in Various WaysTo EliminateForeign Invaders

Notion of immunological “SELF”Skin graft transplantation compatibility

• Graft compatibility genetically determined

Extremely polymorphic trait -many alleles

Governed by genes of major histocompatibility complex (MHC) that encode MHC molecules, the principal targets of rejection

• Rejection by adaptive immune system T cell response

Differences in MHC molecules between individuals are central to determining “SELF”

How T cells recognize antigen

The selection process (Thymic “education”) has two stages

• First stage selects clones capable of recognizing self peptide in an individual’s own MHC molecules - positive selection

• Second stage eliminates overtly self reactive clones with high affinity for self peptide:MHC- negative selection

(Self-peptides are used as a surrogate for foreign peptides since there are few non-self peptides)

***

• The specificity of self/non-self peptide binding to MHC molecules determined by pockets that only bind certain amino acid side chains

• MHC genes are extremely polymorphic and alleles encode pockets with specificities for different amino acid side chains

The TCR is specific for both peptide and MHC- A complex ligand Polymorphic

residues of MHC

The TCR repertoire differs from individual to individual

The definition of immunologic self is made by selecting the clonal T cell repertoire on self-peptides bound to the individual’s particular allelic forms of MHC molecules

Cytosolic Virusor Pathogen

Ingested Bacteria or Endocytic Pathogen

Extracellular Pathogenor Toxin

Peptide degraded in:

Peptides bind to:

Presented to:

Effect on presenting cell of T cell recognition:

Cytosol

MHC class I

CD8 T cells

Death of cell presenting the viral antigen

Endocytic vesicles

MHC class II

CD4 T cells

Provision of help to B cell for production of antibodies

Endocytic vesicles

MHC class II (or I)

CD4 T cells (or CD8)

Activation of cell to enhance pathogen killing

Challenge:

Any cell Macrophage/DC B cell

The immune system makes this distinction by loading and recognizing peptides in either class I or class II MHC

Presenting cell:

Structure of peptide-binding class I MHC domain

N

1

2

Codominant expression of MHC alleles

a/b c/d

a/d b/c a/c b/d

a=paternal haplotype

b=paternal haplotype

c=maternal haplotype

d=maternal haplotype

Polymorphic amino acids that distinguish alleles of MHC class I molecules are found primarily in pockets that determine peptide binding or on the surface that interacts with the TCR

Because the TCR recognizes both peptide and MHC molecule, T cell recognition of MHC-peptide is both MHC restricted and specific for the immunizing peptide

Peptide from virus X

T cell

APC

HLA-B7

Peptide from virus X

APC

HLA-B27

T cell

Peptide from virus Y

T cell

APC

HLA-B7

Target killed:Yes No No

In each of the 3 experiments the T cell is from a HLA-B7 person who recovered from infection by virus “X”. The APC target cell is either infected with virus X or Y and is from an individual who is either HLA-B7 or HLA-B27

CD4 Staining

CD

8 S

tain

ing

CD4-CD8-

CD4+CD8-

CD4+CD8+

CD4-CD8+

3%

5% 80%

12%

TCR repertoire selection and thymocyte differentiation into CD4+ or CD8+ T cells

• Individuals with different MHCs have different TCR repertoires

• T cells mature into CD4 or CD8 single-positive cells as a result of positive selection.

Implications of Positive/Negative Selection

MHC class II/autopeptide

TCR

CD3

CD80

CD40

MHC class II

CD40CD40

CD40L

CD28 CD80

Activated T cell

(1) induction of cytokines/chemokines (IL-8, IL-12, TNF-, MIP-1)

(2) stimulation of CD80 and CD86 expression and co-stimulatory function with activation of T cell growth

(3) augmentation of antigen-presenting function

Key molecular interactions between T cells and APCs

Naïve CD4+ T cells differentiate into Th1 and Th2 subsets

Resting CD4+ cell

“pTh”

Activated CD4+ cell

IL-2 IL-2 IFN- TNF

IL-4 IL-5 IL-6 IL-10

IFN-, IL-12

IL-4, IL-13

IL-4 IL-10(–)

IFN-(–)

Th1 Cells

Th2 Cells

Antigen + APC

IL-2 IFN- TNF

IL-4 IL-5 IL-6 IL-10

IL-4 IL-10(–)

IFN-(–)

Th1 Cells

Th2 Cells

Functions of Th1 subsets• Activate macrophages/dendritic cells

augment antigen presentation• induce delayed type hypersensitivity

(DTH) responses important in eradicating intracellular pathogens (TB, leprosy, listeria

• mediate Th1 diseases (ie; rheumatoid arthritis, multiple sclerosis and type I diabetes

• Help B cells and induce humoral immunity

• mediate allergic and immediate hypersensitivity responses

• involved in antibody mediated immune diseases like SLE and ITP

Functions of Th2 subsets

Functions of T helper subsets

MHC Class llANTIGEN

Internalization of antigen/Ig

Antigenic peptidesbind to MHC class IImolecules

B cell

BCR(SmIg)

Peptide

Antigen binds specifically to BCR (surface membrane Ig), is internalized into vesicles and cleaved into peptides which displace and bind to MHC class II molecules. The peptide/MHC complex is then transported to the surface membrane.

Antigen Processing and Presentation by B cells

CD3TCR

CD4

Fc receptor

IL-12 IL-1IL-6IL-12TNFTGF-

MHC class II

IFN-cytotoxic granules

IL-2Receptor

IL-2

MHC II

Macrophage

Activated MacrophageActivated Th1 Cell

T Cell- Macrophage Interactions

CD4 Th1 CellCD28 B7 (CD80)

CD40L

CD28

TCR

CD4

CD80

CD80

Maximum number of different types of HLA molecules expressed on the cell surface

Class I (HLA-A)Class I (HLA-B)Class I (HLA-C)Class II (HLA-DR)Class II (HLA-DQ)Class II (HLA-DP)

Total

222000

6

Nucleated cells

Antigen presenting cells

222244

16

Each of these MHC molecules selects its own T cell repertoire that only recognizes peptides presented by that particular type of MHC molecule

(actually more)

V. Cytokines you need to know

IL-2 (big family e.g. IL-7 & IL-15)IL-4 (small family inc. IL-13)IL-6 (large family inc. G-CSF)IL-10 (growing family)IL-12 (small family inc. IL-23)IFN- IFN- (large family)

IL-1IL-18

LT-TNF-CD40LFasL

TGF- (very large family)

Chemokines (see Fig. 11.6)InflammatoryNon-inflammatory

See Figs. 11.1 (p244), 11.2 (p245), 11.3 (p248) Tables 11-3 (p249), 11.4 (p264) in Abbas

Innate Adaptive

√√

√√

√√ √

√√ √√

√√ √

√√ √√

√√

√√√√

√

√ √√

√√√

√√√√

√√

√√ √√

√√ √√

√√ √√

Type I & IICytokine Receptors

(Hematopoietin R.)

Toll (TLR) /IL-1Receptors

TNF RelatedReceptors

TGF-Receptors

ChemokineReceptors

V. Cytokines you need to know

IL-2 (big family e.g. IL-7 & IL-15)IL-4 (small family inc. IL-13)IL-6 (large family inc. G-CSF)IL-10 (growing family)IL-12 (small family inc. IL-23)IFN- IFN- (large family)

IL-1IL-18

LT-TNF-CD40LFasL

TGF- (very large family)

Chemokines (see Fig. 11.6)InflammatoryNon-inflammatory

See Figs. 11.1 (p244), 11.2 (p245), 11.3 (p248) Tables 11-3 (p249), 11.4 (p264) in Abbas

Innate Adaptive

√√

√√

√√ √

√√ √√

√√ √

√√ √√

√√

√√√√

√

√ √√

√√√

√√√√

√√

√√ √√

√√ √√

√√ √√

Type I & IICytokine Receptors

(Hematopoietin R.)

Toll (TLR) /IL-1Receptors

TNF RelatedReceptors

TGF-Receptors

ChemokineReceptors

QuickTime™ and aGIF decompressor

are needed to see this picture.

IL-2 activates T-cells in an autocrine manner

QuickTime™ and aGIF decompressor

are needed to see this picture.

VIII. Chemokines

• Important chemoattractantsImportant chemoattractants

• Regulate steady state and inflammatory Regulate steady state and inflammatory leukocyte trafficleukocyte traffic

• Signal through G-protein coupled receptorsSignal through G-protein coupled receptors

• Therefore good drug targets for big PharmaTherefore good drug targets for big Pharma

• Two chemokine receptors serve as co-recpetors Two chemokine receptors serve as co-recpetors for HIV infection (CXCR4 and CCR5)for HIV infection (CXCR4 and CCR5)

Question:

How do viruses that don’t infect “professional APCs” such as dendriticcells elicit a primary immune response?After all, virally-infected cells don’t normally traffic to 2˚ lymphoid organs

PVR expressed onnon-hematopoietic

cells.

Infection withPoliovirus

Endocytosis ofvirus, nuclear entry,

synthesis of viralproteins in cytosol.

Presentation ofviral peptides on

MHC Class I to CD8+cytotoxic T-cells

Proliferation ofcytotoxic T-cells

(CTLs)

Perforin/granzyme-mediated cell death

“Classic” view of CTL responseagainst virus-infected cells

Cross-priming of exogenousantigens by dendritic cells

PVR expressed onnon-hematopoietic

cells.

Infection withPoliovirus

Cytopathicchanges; recognition

and phagocytosisby dendritic cell

Phagosome-to-cytosolprotein export;

ubiquitin-mediatedproteolysis of viral proteins;Presentation of peptide via

MHC Class I

•Perforin/granzyme-mediated cell deathof DC; proliferation

of CD8+ CTL;Killing of virus-infectedepithelial cells by CTL

Why do NK Cells Fail to

Recognize Healthy Cells?

1. For cytotoxic CD8 T-cells, ligation of the TCR by MHC I/peptide + co-stimulation results in release of granzymes and perforin and/or FasL, leading to apoptosisof the target cells.

2. Viruses evade host defense, in part, by down-regulating MHC Class I. Uninfected dendritic cells circumvent this by “cross-priming”: phagocytosis of virus-infected cell and presentation of “exogenous” viral antigens on MHC Class I.

3. The innate immune system has a rapid onset and recognizes molecular patterns in a non-clonal fashion.

4. NK cells lack TCRs, but instead express both activating and inhibitory (e.g., KIRs) receptors at their surfaces. The relative expression and ligation of these receptors determines the outcome (i.e., killing or not) of the NK effector response.

5. Innate immune B-cells (e.g., B-1 cells and marginal zone B cells) recognizecarbohydrate antigens, secrete IgM, and are not long-lived.

6. Innate immune T-cells ( T-cells, and NK T cells) recognize non-peptide antigens innon-classical MHC-like molecules. They mediate cytotoxicity & rapid cytokine secretion.

Summary