1. Y ANTIGEN RECOGNITION BY T-CELLS REQUIRES
PEPTIDE ANTIGENS AND ANTIGEN PRESENTING CELLS
THAT EXPRESS MHC MOLECULES T Y
Cell surface MHC-peptide complex
T-cell response
soluble Ag
Peptide
antigen
Native
membrane Ag
Cell surface
peptides
APC
No T-cell response

2. REQUIREMENTS FOR ANTIGEN PRESENTATION
Expression of MHC molecules
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

3. THE STRUCTURE OF MHC

4. MOLECULES CONTAINING ONE OR MORE Ig DOMAIN(S)
MEMBERS OF THE IMMUNOGLOBULIN SUPERGENE FAMILY
MOLECULES CONTAINING ONE OR MORE Ig DOMAIN(S)
V or C domain related
FUNCTION
RECOGNITION
Ig, TCR, MHC-I, MHC-II
ADHESION
ICAM-1, ICAM-2, VCAM-1, NCAM
BINDING
CD4, CD8, CD28, B7, IL-1RI, PDGFR, FcRII, poly-IgR

5. There are two types of MHC molecule, MHC class I and MHC class II.
The two classes of MHC molecule have similar overall three-dimensional structures. Where they differ is in their constituent polypeptide chains. An MHC class II molecule is made of two similarly sized polypeptides that contain two extracellular domains and are anchored in the plasma membrane. In the MHC class I molecule, a larger polypeptide contains three extracellular domains and is anchored in the plasma membrane; a smaller polypeptide comprises the fourth extracellular domain and is not attached to the membrane.

6. 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 activation
MHC class I molecules are important in immune responses agains viruses and tumour cells
MHC class II plays a role in the activation of immunocytes and in the regulation of cell cell cooperation

7. Genes and proteins of MHC

8. EGY VAGY TÖBB Ig DOMÉNT TARTALMAZÓ FEHÉRJÉK V vagy C doménhez hasonló
Some members of the immunoglobulin supergene family
EGY VAGY TÖBB Ig DOMÉNT TARTALMAZÓ FEHÉRJÉK
V vagy C doménhez hasonló
FUNKCTION
RECOGNITION
Ig, TCR, MHC-I, MHC-II
ADHESION
ICAM-1, ICAM-2, VCAM-1, NCAM
BINDING
CD4, CD8, CD28, B7, IL-1RI, PDGFR, FcRII, poly-IgR

9. MHC class I and MHC class II molecules bind to different T-cell co-receptors.
Left panel: the CD8 co-receptor of a CD8 T cells binds to an MHC class I molecule on an antigenpresenting cell (APC). Right panel: the CD4 co-receptor of a CD4 T cell binds to an MHC class II molecule on an APC. TCR, T-cell receptor. Although CD4 and CD8 perform an analogous function they have different polypeptide chains and three-dimensional structures.

10. The structure of MHC proteins
An MHC class I molecule (left panels) is composed of one membrane-bound heavy (or α) chain and noncovalently bonded β2-microglobulin. The heavy chain has three extracellular domains, of which the amino-terminal α1 and α2 domains resemble each other in structure and form the peptidebinding site. An MHC class II molecule (right panels) is composed of two membrane-bound chains, an α chain (which is a different protein from MHC class I α) and a β chain. These have two extracellular domains each, the aminoterminal two (α1 and β1) resembling each other in structure and forming the peptide-binding site. The β2 domain of MHC class II molecules should not be confused with the β2-microglobulin of MHC class I molecules. The ribbon diagrams in the lower panels trace the paths of the polypeptide backbone chains.

11. THE PEPTIDE BINDING SITE OF MHC CLASS I MOLECULES

12. THE PEPTIDE BINDING SITE OF MHC CLASS II MOLECULES

13. Cleft geometry b2-M a-chain Peptide a-chain b-chain Peptide
MHC class I accommodate
peptides of 8-10 amino acids
MHC class II accommodate
peptides of >13 amino acids

14. The number of different T cell antigen receptors is estimated to be
1,000,000,000,000,000 (1015)
How can 6 invariant molecules have the capacity to
bind to 1,000,000,000,000,000 different peptides?

15. Peptides can be eluted from MHC molecules
Acid elute peptides

16. 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
A common sequence in a peptide antigen that binds to an MHC molecule is called a MOTIF R T Y Q L V N C P E I Y S F H
Amino acids common to many peptides tether the peptide to structural features of the MHC molecule
ANCHOR RESIDUES A V T Y K Q L P S A Y I K
Tethering amino acids need not be identical but must be related
Y & F are aromatic
V, L & I are hydrophobic R G Y V Q L S I F N E K L A P G Y N L
Side chains of anchor residues bind into POCKETS in the MHC molecule
Different types of MHC molecule bind peptides with different patterns of conserved amino acids

18. 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.
Venus fly trap
Floppy
Compact
Allows 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

19. 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 anchors
and different lengths of peptide
• Adopt a flexible “floppy” conformation until a peptide binds
• Fold around the peptide to increase stability of the complex

20. MECHANISM OF ANTIGEN PRESENTATION

21. THE ENDOGENOUS AND EXOGENOUS ROUTES OF ANTIGEN PRESENTATION
Endogenous peptides are presented on MHC I (vírus proteins, tumor antigens) Tc Th
Exogenous peptides (toxins, bacterium, allergen) are presented by MHC II
Exogenous Ag
Endogenous Ag

22. The MHC I receptor binds the CD8 receptor, while
MHC II binds CD4.
The CD8 co-receptor binds to the α3 domain of the MHC class I heavy chain, ensuring that MHC class I molecules present peptides only to CD8 T cells (left panel). In a complementary fashion, the CD4 co-receptor binds to the β2 domain of MHC class II molecules, ensuring that peptides bound by MHC class II stimulate only CD4 T cells (right panel).

23. Allelic polymorphism is concentrated in
the peptide antigen binding site
Class I
Class II
(HLA-DR) 1 3 2
2m 2 1 2 1
Polymorphism in the MHC affects peptide antigen binding
Allelic variants may differ by 20 amino acids

24. Cytosol-derived peptides are presented by MHCI receptors

25. Degradation of endogenous proteins in the
(immun)proteosomes
TAP: Transporter associated
With antigen processing
In all cells, proteasomes degrade cellular proteins that are poorly folded, damaged, or unwanted. When a cell becomes infected, pathogen-derived proteins in the cytosol are also degraded by the proteasome. Peptides are transported from the cytosol into the lumen of the endoplasmic reticulum by the protein called transporter associated with antigen processing (TAP), which is embedded in the endoplasmic reticulum membrane.

26. Multiple proteins help Ag presentation of MHCI
MHC class I heavy chains assemble in the endoplasmic reticulum with the membrane-bound protein calnexin. When this complex binds β2-microglobulin (β2m) the partly folded MHC class I molecule is released from calnexin and then associates with the TAP, tapasin, calreticulin, ERp57, and protein disulfide isomerase (PDI) to form the peptide-loading complex. The MHC class I molecule is retained in the endoplasmic reticulum until it binds a peptide, which completes the folding of the molecule. The peptide:MHC class I molecule is then released from the other proteins and leaves the endoplasmic reticulum for transport to the cell surface.

27. Trimming of antigenic peptides by ERAP
The endoplasmic reticulum aminopeptidase (ERAP) binds to MHC class I molecules in which the amino terminus of an overlong peptide hangs out of the binding site. It removes the accessible amino acid residues to leave a peptide of 8-10 amino acids with an improved fit to the heterodimer of the class I heavy chain and β2-microglobulin. It is not known whether ERAP acts on the class I molecule alone, as illustrated here, or when it is part of the peptide-loading complex, or both.

28. Presentation of extracellular (Exogen) peptides
bemutatása (MHCII prezentáció)
Processing of antigens for presentation by MHC class II or MHC class I molecules occurs in different cellular compartments.
The left half of the figure shows the fate of peptides derived from extracellular antigens and pathogens. Extracellular material is taken up by endocytosis and phagocytosis into the vesicular system of the cell, in this case a macrophage. Proteases in these vesicles break down proteins to produce peptides that are bound by MHC class II molecules, which have been transported to the vesicles via the endoplasmic reticulum (ER) and the Golgi apparatus. The peptide:MHC class II complex is transported to the cell surface in outgoing vesicles. The right half of the figure shows the fate of peptides generated in the cytosol as a result of infection with viruses or intracytosolic bacteria. Proteins from such pathogens are broken down in the cytosol by the proteasome to peptides, which enter the ER. There the peptides are bound by MHC class I molecules. The peptide:MHC class I complex is transported to the cell surface via the Golgi apparatus.

29. Invariant chain protects the binding site of
MHCII until it reaches the appropriate
compartment
The invariant chain prevents peptides from binding to an MHC class II molecule until it reaches the site of extracellular protein breakdown.
In the endoplasmic reticulum (ER), MHC class II α and β chains are assembled with an invariant chain that fills the peptide-binding groove; this complex is transported to the acidified vesicles of the endocytic system. The invariant chain is broken down, leaving a small fragment called class II-associated invariant-chain peptide (CLIP) attached in the peptide-binding site. The vesicle membrane protein HLA-DM catalyzes the release of the CLIP fragment and its replacement by a peptide derived from endocytosed antigen that has been degraded within the acidic interior of the vesicles.
INVARIÁNS LÁNC (Ii)
Chaperon – konformáció
Peptidkötőhely gátlása
Szállító/visszatartó molekula
DMA/DMB
1. A peptidet befogadó konformáció fenntartása
2. A CLIP és az exogén fehérjékből származó peptidek lecserélése

30. The biological function of MHC proteins

31. 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 priphery
week” tonic” signals induced by MHC / TCR interactions provide survival signals
Inhibitory ligands for NK cells, maintainance of host cell integrity.

32. 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)

33. Experiment of Peter DOHERTY & Rolph ZINKERNAGEL 1976
Vírus B
+ Y sejtek T
Virus A
T - CELLS T
MICE Y
Vírus A
+ Y sejtek T
MICE X
Vírus A
+ X cells T
Virus A
+ X cells T
Tion)
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 ). M e d i a G r a p h i c s I n t e r n a t i o n a l

34. No T cells No rejection MiceY Thymectomy Mice X MiceX
Tissue compatibility is encoded by the MHC genes and tissue rejection requires the presence of T cells
MiceY
MiceX
No rejection
Mice X
Thymectomy
No T cells

35. MHC protein (HLA)- coding genes
The MHC locus
MHC protein (HLA)- coding genes

36. The full sequence and the map of the human MHC locus
HUMAN GENOME PROJECT
3,838,986 bp
224 gene
6 kromoszóma
MHC sequencing consortium
Nature 401, 1999

37. STRUCTURE OF THE MHC Non- classical MHC genes
6 kromoszóma rövid karja MHC
15 kromoszóma 2m
Non- classical MHC genes
E, G, F
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
Class III

38. Of the human MHC class I isotypes, HLA-A, HLA-B, and HLA-C are highly polymorphic. They present peptide antigens to CD8 T cells and also interact with NK-cell receptors. HLA-E and HLA-G are oligomorphic and interact with NK-cell receptors. HLA-F is intracellular and of unknown function, and occurs as a single isotype. Of the human MHC class II isotypes, HLA-DP, HLA-DQ, and HLA-DR are polymorphic and present peptide antigens to CD4 T cells, whereas HLA-DM and HLA-DO occur in only a few isotypes, are intracellular, and regulate the loading of peptides onto HLA-DP, HLA-DQ, and HLA-DR.

39. INHERITENCE OF CLASS I AND CLASS II MHC GENES HUMAN LEUKOCYTE ANTIGEN
HLA
Ko-domináns öröklésmenet
I osztály
II osztály
EVERY CELL
α1β1
α2β2
PROFESSIONAL APC

41. A polimorfizmus (allélek) száma
~6 x 1015 combinations
POLIMORPHYSM OF MHC IN HUMAN POPULATIONS
872
CLASS I
1652 allels
506
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
274
A polimorfizmus (allélek) száma A B C
CLASS II
688 allels
466
114 66 15 25 2
In reality allels are not inherited randomy. Allels are linked, and there must have been strong selection favoring certain allelic variants. Non-random distribution. a b DR DP DQ

42. Classical MHC genes are inherited as haplotypesB C A DP DQ DR
Offspring
DP-1,8
DQ-3,6
DR-5,4
B-7,2
C-9,8
A-11,10
DP-1,9
DQ-3,7
DR-5,5
B-7,3
C-9,1
A-11,9
DP-2,8
DQ-4,6
DR-6,4
B-8,2
C-10,8
A-12,10
DP-2,9
DQ-4,7
DR-6,5
B-8,3
C-10,10
A-12,9 B C A DP DQ DR X
Parents B C A DP DQ DR
DP-1,2
DQ-3,4
DR-5,6
B-7,8
C-9,10
A-11,12
DP-9,8
DQ-7,6
DR-5,4
B-3,2
C-1,8
A-9,10 B C A DP DQ DR B C A DP DQ DR

43. Kidney epithelial cell B-cell, macrophage, dendritic cell
MHC MOLECULES ARE EXPRESSED WITH BOUND PEPTIDES DERIVED FROM SELF OR NON-SELF PROTEINS
Kidney epithelial cell
B-cell, macrophage, dendritic cell
Present intra- and extracellular environment
Liver cell
Class II MHC
Overlapping peptides of various sizes, which derive from membrane proteins
70% derives from MHC molecules
Present intracellular environment
Class I MHC
Peptides of restricted size, which derive from cytosolic or nuclear proteins

44. MHC Polimorphysm is maintained by the presence of pathogens
In our example population there are four different MHC haplotypes, each represented by a different color. The frequencies of different genotypes are represented by the 20 circles in each panel, the frequencies of the four haplotypes being given underneath. First panel: the population experiences a period characterized by balancing selection arising from successive epidemic infections, after which only heterozygotes survive (second panel) and in which 30% of the population die, as indicated by circles containing an X. After recovery of the population during a period of relative calm and health (third panel) it becomes subject to directional selection by a new and particularly nasty infection. Only individuals with the blue MHC haplotype survive and 75% of the population dies (fourth panel). As a result of these selections the frequencies of the MHC haplotypes change considerably, but all four MHC haplotypes are retained within the population.
Figure 5.35 MHC heterozygosity delays the progression to AIDS in people infected with HIV-1.
When people who have been infected with HIV-1 start to make detectable antibodies to the virus they are said to have undergone seroconversion. The onset of overt symptoms of AIDS occurs years after seroconversion. The rate of progress to AIDS decreases with the extent of HLA heterozygosity, as shown here by a comparison of individuals who are heterozygous for all the highly polymorphic HLA class I and II loci (red) with those who are homozygous for one locus (yellow) or for two or three loci (blue).

45. THE OUTCOME OF INFECTION IN A POPULATION WITH POLYMORPHIC MHC GENES
Example: If MHC X was the only type of MHC molecule
MHC-Gen v
Pathogen that evades MHC X
MHC XX
Population threatened
with extinction
V – virus infection
Population is protected