ANTIGEN PROCESSING AND PRESENTATION

V.Reju

The conversion of native proteins to peptides which can combine

with MHC molecules

ANTIGEN PROCESSING

The course of formation and display of peptide-MHC complexes on the surface of APCs and the course of

peptide MHC complexes recognition by T cells

ANTIGEN PRESENTATION

A group of cells that play important role in immune response which can uptake ,process antigens and

present peptide-MHC complexes to T cells

ANTIGEN PRESENTING CELLS

Capacity to uptake proteins

Ability to process proteins to peptides

Capacity to present peptides depends on type of MHC molecules expressed

Must express co-stimulatory signals for T cell stimulation

GENERAL PROPERTIES OF APC

• Dendritic cells

• Macrophages

• B cell

PROFESSIONAL APCS

• Fibroblast

• Thymus epithelial cells

• Thyroid epithelial cells

• Pancreatic beta cells

• vascular endothelial cells

NON- PROFESSIONAL APCS

First found by Steinman in 1973

Cultured in vitro in 1993 by Inaba

Generated from bone marrow in presence of cytokine granulocyte-macrophage colony stimulating factor

Can stimulate naive T cell

DENDRITIC CELLS

WHY DENDRITIC CELLS ARE CONSIDERED AS MOST

EFFECTIVE APC??????

First identified APC

Differentiated from monocyte in blood

Cannot stimulate naïve T cell

Capture antigens by phagocytosis ,pinocytosis and receptor mediated phagocytosis

MACROPHAGE ( MΦ)

Originates in bone marrow

Mature B cell co-express IgM and IgD on their cell surface

B CELLS

CD 4+ and CD 8+ cells recognize antigen only in presence of MHC

SELF MHC RESTRICTION OF T CELL

• MHC I + peptides » CD 8+ T Cell

• MHC II + peptides » CD 4+ T Cell

SELF-MHC I RESTRICTION

SELF-MHC II RESTRICTION

TWO PATHWAYS BASED ON ANTIGEN

ANTIGEN PRESENTATION BY CLASS I MHC

MHC CLASS Iα1α2

α3

β2

P

* α1, α2, α3 are the domains of α chain,45kDa* β2 microglobulin ,12kDa,small soluble domain* P→Peptide-binding site* T→Hydrophobic Transmembrane segment* ER.M→Endoplasmic reticulum membrane* C→Hydrophilic Cytoplasmic tail

T

C

ER.M

Encoded by HLA-A,B,C genes

Consist of carboxyl-terminal short domain, transmembrane hydrophobic region,extracellular domain,aminoterminal signal peptide

α-chain consists of α1, α2, α3 domain

α1 and α2 forms a groove for the peptide-binding site

α-chain

Interacts non-covalently with α3 domain

Encoded by highly conserved gene located on a different chromosome

Required for the expression of Class I molecules on cell membrane

β2-microglobulin

1.Processing of endogenous Ag

2.Transporting of antigen peptide into ER

3.Peptide loading of class I MHC molecules

4.Presenting to CD8+T cells

ANTIGEN PRESENTATION BY MHC I

Endogenous antigens are degraded by Proteasome(26S)

Consist of 20S proteasome(a catalytic core) and 19S particle(a regulatory component)

Degradation of protein requires ubiquitin molecule

Cytokine(IFNγ) enhance the production of LMP2, LMP7 and MECL-1(LMP10) and replace β1, β5, β2 respectively to form IMMUNOPROTEASOME

1.PROCESSING OF ENDOGENOUS ANTIGEN

20S

19S

αα

β

β

UBIQUITINATION OF PROTEINUb

E1

E1Ub

ATP

ADP + Pi

E2

Ub E1

Ub U

b

Ub

Ub

Ub

E3

Ub-UbiquitinE1-Ubiquitin-activating enzymeE2-Ubiquitin-conjugating enzymeE3-Ubiquitin ligase

Polyubiquitinated proteinProteasome Peptides

TAP( Transporter associated with antigen processing)

2.TRANSPORTING OF ANTIGEN PEPTIDE INTO ER

Consisting of TAP1 and TAP2 ATP dependent transporter Selective transporting (8-11aa)

TAP1 TAP2

TRANSPORTERS ASSOCIATED WITH ANTIGEN PROCESSING (TAP1 & TAP2)

ER membrane

Lumen of ER

Cytosol

TAP-1 TAP-2

Peptide

ATP-binding cassette(ABC) domain

Hydrophobictransmembranedomain

Peptide antigensfrom proteasome

3.PEPTIDE LOADING OF CLASS I MHC MOLECULES

Endoplasmic reticulum

TAP-1 TAP-2

Peptide

Calnexin bindsto nascent

class I chainUntil 2-M binds

2-M binds and

stabilises floppy MHC

Tapasin, calreticulin,ERP57, TAP 1 & 2 form a complex with

the floppy MHC

Cytoplasmic peptides are loaded onto the

MHC molecule and the structure becomes

compactβ

Calnexin

Nascent class I chain β2-M

Class I MHCERP57

Tapasin

Calreticulin

Antigen peptide-class I MHC molecules presented on cell

membrane by exocytosis

4.PRESENTING TO CD8+T CELLS

α1α2

α3

P

T

C

β2

C.M

Tc Cell

TCR

CD8

ANTIGEN PRESENTATION BY CLASS II MHC

1. Capture of exogenous antigen

2. Processing of antigen

3. Synthesis and transportation of class II molecule

4. Association of processed peptide with MHC II

5. Presenting to CD4+ T cells

EXOGENOUS OR ENDOCYTIC PATHWAY

Endocytosis

Phagocytosis

Pinocytosis

Receptor mediated endocytosis

Form Endosome

CAPTURE OF EXOGENOUS ANTIGEN

Takes place in endosome(MIIC/CIIV) and lysosome

Protease – Cathepsin cleaves peptides from Protein

PROCESSING OF ANTIGEN

Cell surface

Endosome

Peptides in endosome

MHC II MOLECULE

α1 β1

α2 β2

MHC II is synthesized in ER

Folding and assembly is aided by ER resident chaperon- calnexin

INVARIANT CHAIN ( Ii )

Ii is a trimer ,each binding to newly formed MHC II

Transport to GA through ER vesicle

Fusion with MIIC containing HLA-DM ,cathepsin S

ASSOCIATION OF PROCESSED PEPTIDE WITH MHC II

Invariant chain degradation leaving

CLIP by proteolytic enzyme Cathepsin S

Replacement of peptide in place of

CLIP by HLA -DM

Formed peptide-MHC molecule is presented on surface of APC

TCR binds with α1,β1 of MHC II and peptide

PRESENTING TO CD4 T CELLS

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