Innate ImmunityIs that enough?

……………..not enough

Triggering Acquired ImmunityInnate Immune Defenses:

Invaders with low virulence are rapidly eliminatedInflammation-not foolproof & uncomfortable and damaging

To DEFEND EFFECTIVELY ……Acquired Immune System

Antigen Presentation & Role of Major Histocompatibility Complex

Course-VPP 703Course Teacher-Dr Suguna Rao Prof

Presentation By-Dr Somshekhar

Triggering Acquired Immunity….

Foreign Material-captured, processed, and presented

Antigen Processing & Presenting Cells-◦Dendritic cells◦Macrophages B cells (Minor role in Primary

Response)

Antigen Processing

g

Attracted by alarmins(damaged cells)

T cells do not recognise native antigens

YY

BYY Y Y YY

Y

BY

T

Y

T

Proliferation and antibody production

No proliferationNo cytokine release

Cross-linking of surface membrane Ig

Y

B

Y

B Y

B

Y

B

Y

B Y

B

Y

B

Cell surfacepeptidesof Ag

Antigens must be processed in orderto be recognised by T cells

YT

T cellresponse

No T cellresponse

No T cellresponse

No T cellresponse

No T cellresponse

Solublenative Ag

Cell surfacenative Ag

Soluble peptidesof Ag

Cell surface peptides of Ag presented by cells that express MHC antigens

ANTIGENPROCESSING

YThe site of pathogen replication or mechanism of antigen uptake determines the antigen processing pathway used

Y

Cytosolic compartmentEndogenous processing(Viral antigens)

Vesicular CompartmentContiguous with extracellular fluidExogenous processing(Streptococcal, Mycobacterial antigens)

Distinct mechanisms of antigen generation are used to raiseT cells suited to the elimination of endogenous or exogenous pathogens

INTRACELLULAR REPLICATION

EXTRACELLULAR ORENDOSOMAL REPLICATION

Y

Eliminated by:Killing of infected cells by CTL that use antigens generated by ENDOGENOUS PROCESSING

YEliminated by:Antibodies and phagocyteactivation by T helper cells that use antigens generated byEXOGENOUS PROCESSING

Antigens generated by endogenous and exogenous antigen processing activate different effector functions

ENDOGENOUS PATHOGENS

EXOGENOUSPATHOGENS

What is MHC??????(Gorer & George Snell)

Every mammalian species pocesses a tightly linked cluster of genes-Major Histocompability Complex (MHC), whose products play role in intracellular recognition and in discrimination between self and nonself.T cells recognize antigen only when it is combined with an self MHC molecule (MHC restricted)

w

Most cells can present antigen with class I MHC molecules to CD8+ Tc cells-Target cells

Cells that display peptides associated with class II MHC molecules to CD4+ TH cells-Antigen presenting cells (APCs)

Structures of MHC Class I & II

Antigen ProcessingExogenous Antigen-Endocytic or

exogenous processing pathwayClass II MHC molecules bind peptides and present to CD4+ T cells

Endogenous Antigen-Cytosolic or endogenous processing pathwayClass I MHC molecules bind peptides and present to CD8+ T cells

Y Y

Pinocytosis

Phagocytosis

Membrane Igreceptor mediateduptake

Y

Uptake of exogenous antigens

Complement receptormediated phagocytosis Y

Fc receptor mediated phagocytosis

Uptake mechanisms direct antigen into intracellular vesiclesfor exogenous antigen processing

Proteases produce ~24 amino acid long peptides from antigensDrugs that raise the pH of endosomes inhibit antigen processing

Endosomes

Exogenous pathway

Increasein acidity

Cell surface

To lysosomes

UptakeProtein antigensIn endosome

Cathepsin B, D and L proteases are activated by the decrease in pH

Need to prevent newly synthesised, unfolded self proteins from binding to immature MHC

Invariant chain stabilises MHC class II by non- covalently binding to the immature MHC class II molecule and forming a nonomeric complex

In the endoplasmic reticulum

MHC class II maturation and invariant chain

Endosomes

Cell surface

Uptake

Class II associated invariant chain peptide (CLIP)

(inv)3 complexesdirected towardsendosomes byinvariant chain

Cathepsin L degrades Invariant chainCLIP blocks groove in MHC molecule

MHC Class IIcontaining vesiclesfuse with antigencontaining vesicles

Removal of CLIP

?

How can the peptide stably bind to a floppy binding site?Competition between large number of peptides

HLA-DM catalyses the removal of CLIP

MIIC compartment

HLA-DMReplaces CLIP with a peptide antigen using a catalytic mechanism (i.e. efficient at sub-stoichiometric levels)

Discovered using mutant cell lines that failed to present antigen

HLA-DO may also play a role in regulating DM

Sequence in cytoplasmic tail retains HLA-DM in endosomes

HLA-DMHLA-DR

MIIC compartment sorts peptide-MHC complexes for surface expression orlysosomal degradation

Surface expression of MHC class II-peptide complexes

Exported to the cell surface (t1/2 = 50hr)

Sent to lysosomes for degradation

ENDOPLASMIC RETICULUM

CYTOSOL

Peptide antigens produced in the cytoplasm are physically separated from newly formed MHC class I

Newly synthesisedMHC class I molecules

Peptides needaccess to the ER in

order to be loaded onto MHC class I molecules

ER membrane

Lumen of ER

Cytosol

Transporters associated withantigen processing (TAP1 & 2)

Transporter has preference for >8 amino acid peptideswith hydrophobic C termini.

TAP-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

Peptide

ER membrane

Lumen of ER

Cytosol

TAP-1 TAP-2

Peptide

ATP-binding cassette(ABC) domain

Hydrophobictransmembranedomain

Peptide antigensfrom proteasome

Endoplasmic reticulum

Calnexin bindsto nascentclass I chainuntil 2-M binds

TAP-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

B2-M binds and stabilises floppy MHC

Tapasin, calreticulin, TAP 1 & 2 form a complex with the floppy MHC

Cytoplasmic peptides are loaded onto the MHC molecule and the structure becomes compact

Maturation and loading of MHC class I

Fate of MHC class I

Sent to lysosomes for degradation

Exported to the cell surface

Endoplasmic reticulum

TAP-1 TAP-2

Peptide

TAP-1 TAP-2

PeptideTA

P-1 TAP-2

Peptide

TAP-1 TAP-2

Peptide

TAP-1 TAP-2

HSV protein blocks transportof viral peptides into ER

Sent to lysosomesfor degradation

Evasion of immunity by interference with endogenous antigen processing

Sent to lysosomes for degradation

Normally exported to the cell surface

Adenoviralproteinretains MHCclass I in the ER

Evasion of immunity by interference withendogenous antigen processing

Presentation of NON PEPTIDE antigens

T cells that express the γδ TCR that react with glycolipid antigens derived from bacteria such as Mycobacterium tuberculosis

These non protein antigens are presented by members of the CD1 family of non classical class I molecules

Genes encoding CD1 are located not within MHC

• T and B cells recognise antigen differently

• Antigen must be catabolised before T cells can recognise it

• Antigen processing generates antigenic peptides

• Exogenous antigen processing takes place in lysosomes

• Endogenous processing is non-lysosomal

• The mechanism of antigen processing depends upon the compartment in which the pathogen replicates

• Endogenous and exogenous antigen processing both involve uptake, degradation, complex formation and presentation

• Exogenous antigen processing uses invariant chain and HLA-DM

• Endogenous antigen processing uses proteasomes and peptide transporters in antigen processing

• Pathogens can evade immunity by disrupting antigen processing

Summary

Any Queries???????????????????

Hope Not…………

THANK YOU........