ANTIGEN RECOGNITION BY T-CELLS REQUIRESPEPTIDE ANTIGENS AND ANTIGEN PRESENTING CELLS

THAT EXPRESS MHC MOLECULES

YT

No T-cell response

soluble Ag

Native membrane Ag

Peptide antigen

Cell surface MHC-peptide complex

T-cell response

Cell surfacepeptides APC

REQUIREMENTS FOR ANTIGEN PRESENTATION

1. Expression of MHC molecules

2. ANTIGEN

a) Synthesis of viral antigens - intracellular

b) Uptake of protein antigens – extracellular

3. „Processing” of antigen

generation of peptides suitable for T-cell recognition

4. Presentation of peptides in complex with MHC molecules on the cell surface

T-cells with TCR are specialized for recognizing protein – derived fragments

THE STRUCTURE OF MHC

s s

s s

s

s

s s

ss s

s s

s

s

s s

s

s ss s

s

s

s

s

s

s

s

s

s ss s

L

H HL

s ss s

s ss s

ss

s

s

s

s

ss

s

s

s

s

s

s

ssss

gde

ss

ss

a b

s

s

s

s

ss

s

s

s

s

s

s

ss

ss

s

s

s

s

s

s

ss

s

s

s

s

IgM CD79 TCR MHCI MHCIIThy-1 ICAM-1

CD2 B-7CD28CD8CD4CD3

ss

ss

s s

Fc RII

Poly-Ig

MEMBERS OF THE IMMUNOGLOBULIN SUPERGENE FAMILY

FUNCTION

RECOGNITION

Ig, TCR, MHC-I, MHC-II

ADHESION

ICAM-1, ICAM-2, VCAM-1, NCAM

BINDING

CD4, CD8, CD28, B7, IL-1RI, PDGFR, FcRII, poly-IgR

MOLECULES CONTAINING ONE OR MORE Ig DOMAIN(S)

V or C domain related

There are two types of MHC molecule, MHC class I and MHC class II.

CELL SURFACE EXPRESSION OF MHC ON VARIOUS CELL TYPES

Szövet MHC I MHC II

T cells +++ +/-B cells +++ +++Makrophages +++ ++Dendritic cells +++ +++

Epithelial cells + +++

Neutrophyls +++ -Hepatocytes + -Kidney + -Brain + -Eritrocytes - -

Cell surface expression of MHC is influenced by activationMHC class I molecules are important in immune responses agains viruses

and tumour cellsMHC class II plays a role in the activation of immunocytes and in the

regulation of cell cell cooperation

Genes and proteins of MHC

s s

s s

s

s

s s

ss s

s s

s

s

s s

s

s ss s

s

s

s

s

s

s

s

s

s ss s

L

H HL

s ss s

s ss s

ss

s

s

s

s

ss

s

s

s

s

s

s

ssss

gde

ss

ss

a b

s

s

s

s

ss

s

s

s

s

s

s

ss

ss

s

s

s

s

s

s

ss

s

s

s

s

IgM CD79 TCR MHCI MHCIIThy-1 ICAM-1

CD2 B-7CD28CD8CD4CD3

ss

ss

s s

Fc RII

Poly-Ig

Some members of the immunoglobulin supergene family

FUNKCTION

RECOGNITION

Ig, TCR, MHC-I, MHC-II

ADHESION

ICAM-1, ICAM-2, VCAM-1, NCAM

BINDING

CD4, CD8, CD28, B7, IL-1RI, PDGFR, FcRII, poly-IgR

EGY VAGY TÖBB Ig DOMÉNT TARTALMAZÓ FEHÉRJÉK

V vagy C doménhez hasonló

MHC class I and MHC class II molecules bind to different T-cell co-receptors.

The structure of MHC proteins

THE PEPTIDE BINDING SITE OF MHC CLASS I MOLECULES

THE PEPTIDE BINDING SITE OF MHC CLASS II MOLECULES

PEPTIDE

MHC class I accommodatepeptides of 8-10 amino acids

Cleft geometry

MHC class II accommodatepeptides of >13 amino acids

-M

-chain

Peptide

-chain

-chainPeptide

How can 6 invariant molecules have the capacity tobind to 1,000,000,000,000,000 different peptides?

The number of different T cell antigen receptors is estimated to be

1,000,000,000,000,000 (1015)

Peptides can be eluted from MHC molecules

Acid elute peptides

Eluted peptides from MHC molecules have different sequences but contain motifs

Peptides bound to a particular type of MHC class I molecule have conserved patterns of amino acids

P E IYS F H I

A V TYK Q T L

P S AYS I K I

R T RYT Q L VN C

Tethering amino acids need not be identical but must be related

Y & F are aromaticV, L & I are hydrophobic

Side chains of anchor residues bind into POCKETS in the MHC molecule

S I I FN E K L

A P G YN P A L

R G Y YV Q Q L

Different types of MHC molecule bind peptides with different patterns of conserved amino acids

A common sequence in a peptide antigen that binds to an MHC molecule is called a MOTIF

Amino acids common to many peptides tether the peptide to structural features

of the MHC moleculeANCHOR RESIDUES

MHCI

P1P2P3P4P5P6P7P8P9Octapeptide

Decapeptide

anchoringamino acids

NH2 COOH

MHCII

P1

NH2

NH2

COOH

COOH

-1 1011-2 P4 P6 P9

anchoringamino acids

A flexible binding site?

A binding site that is flexible at an early, intracellular stage of maturation Formed by folding the MHC molecules around the peptide.

Floppy CompactAllows a single type of MHC molecule to • bind many different peptides• bind peptides with high affinity• form stable complexes at the cell surface• Export only molecules that have captured a peptide to the cell surface

Venus fly trap

MHC molecules bind peptides according to the following principals

• Use a small number of anchor residues to tether the peptide- this allows different sequences between anchorsand different lengths of peptide

• Adopt a flexible “floppy” conformation until a peptide binds

• Fold around the peptide to increase stability of the complex

MECHANISM OF ANTIGEN PRESENTATION

Endogenous peptides are presented on MHC I (vírus proteins, tumor

antigens)

Tc

Endogenous Ag

THE ENDOGENOUS AND EXOGENOUS ROUTES OF ANTIGEN PRESENTATION

Exogenous Ag

Th

Exogenous peptides (toxins, bacterium, allergen) are presented by MHC II

The MHC I receptor binds the CD8 receptor, while MHC II binds CD4.

1

3

2

2m 2

1

2

1

Allelic polymorphism is concentrated inthe peptide antigen binding site

Polymorphism in the MHC affects peptide antigen binding

Allelic variants may differ by 20 amino acids

Class II(HLA-DR)

Class I

Cytosol-derived peptides are presented by MHCI receptors

Degradation of endogenous proteins in the (immun)proteosomes

TAP: Transporter associatedWith antigen processing

Multiple proteins help Ag presentation of MHCI

Trimming of antigenic peptides by ERAP

Presentation of extracellular (Exogen) peptidesbemutatása (MHCII prezentáció)

Invariant chain protects the binding site of MHCII until it reaches the appropriate

compartment

INVARIÁNS LÁNC (Ii)

1. Chaperon – konformáció

2. Peptidkötőhely gátlása

3. Szállító/visszatartó molekula

DMA/DMB

1. A peptidet befogadó konformáció fenntartása

2. A CLIP és az exogén fehérjékből származó peptidek lecserélése

The biological function of MHC proteins

AZ MHC FUNCTIONS

– Presentation of peptides– self/altered self/foeign peptides– Continous expression of self-peptidesto monitor cellular health

– Determination of immunological self• Self MHC + self peptide – individual MHC pluss és saját peptid • Allogeneic response to fotreign MHC (transplantation) • Self MHC– autolgous foreign MHC allogeneic activation. Az The

ratio of alloreactive T-cells is very high: 1-10%

– A differentiation and selection of developing thymocytes (in the thymus)

– promotion of T-limphocyte survival in the pripheryweek” tonic” signals induced by MHC / TCR interactions

provide survival signals

– Inhibitory ligands for NK cells, maintainance of host cell integrity.

T-sejt

TCR

APC

MHC

T-sejt T-sejt

TCR TCR

APC

MHC

APC

MHC

AZ MHC restriction

TCR/ MHC + peptid complex recognized

A single TCR recognize a single MHC-peptid komplex

The same peptide presented on a different MHC is not recognized.

The same MHC molecule with a different peptide is not recognized by a given TCR (other TCRs may recognize)

MICE MICE YY

Virus AVirus A T -T - CELLS CELLS

TT

TTTT

TTTT TT

VVíírus Brus B+ + YY sejteksejtek

TT

VVíírus Arus A+ + YY sejteksejtek

TT

MICE MICE XX

VVíírus Arus A+ + X X cellscells

TT

Virus AVirus A+ + X X cellscells

TT

TT

Experiment of Peter DOHERTY & Rolph ZINKERNAGEL 1976

Cells infected with a virus are only killed if the infected cell and virus-specific T cells are from the same animal or strain. (The MHC needs to be recognized by the

CTL cells ------ MHC restriction ).

MiceMice XXTThymechymectomytomy

No No TT cellscellsMiceMiceYY

MiceMiceXX

Tissue compatibility is encoded by the MHC genes and tissue rejection requires the presence of T cells

No rejectionNo rejection

The MHC locus MHC protein (HLA)- coding genes

3,838,986 bp224 gene

6 kromoszóma

http://webace.sanger.ac.uk/cgi-bin/ace/pic/6ace?name=MHC&class=Map&click=400-1

MHC sequencing consortium

Nature 401, 1999

The full sequence and the map of the human MHC locus HUMAN GENOME PROJECT

TNF

LMP TAP

21-hydroxilase

Class III Non polymorphic genesClass II Class I

E G F

HSP

Class II

DMA/B

MICA/B

B C A

C2/C4

DQDP DN DR

B2 B2 B2A2 A2 B3B1 B1 (B4)A1 A1 AB B1A

DO

Klasszikus MHC gének POLIMORPHIC

HLA – Human Leukocyte Antigen system

HLA –A,B, C I osztály

ALL NUCLEATED CELLLS

HLA – DR, DP, DQ Class II

ON PROFESSIONAL APC

Non- classical MHC genes

E, G, F

6 kromoszóma rövid karja MHC

15 kromoszóma 2m

STRUCTURE OF THE MHC

Class III

A24A1

A24A11

A1A2

A11A2

A11A1

A24A2

1. 2. 3. 4.

INHERITENCE OF CLASS I AND CLASS II MHC GENES

HUMAN LEUKOCYTE ANTIGEN

HLA

EVERY CELL

α1β1 α2β2

PROFESSIONAL APC

transz

A24

A24

C1

C1

B8

B8

B27

B27

C6

C6

A1

A1

b2b1a1

transz

a2

DQ1cisz

cisz

transz

DQ2

I osztály II osztály

Ko-domináns öröklésmenet

In reality allels are not inherited randomy. Allels are linked, and there must have been strong

selection favoring certain allelic variants. Non-random distribution.

15.18

28.65

13.38

4.46

0.02

5.72

18.88

8.44

9.92

1.88

4.48

24.63

2.64

1.76

0.01

CAU AFR ASI

Frequency (%)

HLA-A1

HLA- A2

HLA- A3

HLA- A28

HLA- A36

Allels

506

274

A B C

A p

oli

mo

rfiz

mu

s (a

llél

ek)

szám

a

CLASS I1652 allels

2

466

15114

2566

CLASS II688 allels

872

DR DP DQ

~6 x 1015 combinations

POLIMORPHYSM OF MHC IN HUMAN POPULATIONS

Classical MHC genes are inherited as haplotypes

B C ADP DQ DR

B C ADP DQ DR

B C ADP DQ DR

B C ADP DQ DR

X

ParentsDP-1,2DQ-3,4DR-5,6B-7,8C-9,10A-11,12

DP-9,8DQ-7,6DR-5,4B-3,2C-1,8A-9,10

DP-1,8DQ-3,6DR-5,4B-7,2C-9,8A-11,10

DP-1,9DQ-3,7DR-5,5B-7,3C-9,1A-11,9

DP-2,8DQ-4,6DR-6,4B-8,2C-10,8A-12,10

DP-2,9DQ-4,7DR-6,5B-8,3C-10,10A-12,9

B C ADP DQ DR

B C ADP DQ DR

B C ADP DQ DR

B C ADP DQ DR

B C ADP DQ DR

B C ADP DQ DR

B C ADP DQ DR

B C ADP DQ DR

Offspring

MHC MOLECULES ARE EXPRESSED WITH BOUND PEPTIDES DERIVED FROM SELF OR NON-SELF PROTEINS

B-cell, macrophage, dendritic cell

Kidney epithelial cell

Liver cell

Present intracellular environment

Present intra- and extracellular environment

Class I MHC

Peptides of restricted size, which derive from cytosolic or nuclear

proteins

Class II MHC

Overlapping peptides of various sizes, which derive from

membrane proteins

70% derives from MHC molecules

MHC Polimorphysm is maintained by the presence of pathogens

THE OUTCOME OF INFECTION IN A POPULATION WITH POLYMORPHIC MHC GENES

MHC-GenMHC-Gen

vv

vv

vv

v

vv

v

v

v

v

v v v

v

vv

v

vv

v

vv

Example: If MHC X was the only type of MHC molecule

Population threatenedwith extinction

V – virus infection

Pathogen that evades MHC

X

MHCXX

Population is protected