Antigen processing and presentation and role of major histocompatibility complex molecules- Dr Somshekhar Hogtapur, PhD Scholor, Dept Of Microbiology, Vety college Bangaluru, India

1. Innate Immunity
 Is that enough?
……………..not enough

2. Triggering Acquired Immunity
 Innate Immune Defenses:
Invaders with low virulence are rapidly
eliminated
Inflammation-not foolproof &
uncomfortable and damaging
To DEFEND EFFECTIVELY ……
Acquired Immune System

3. Antigen Presentation & Role of
Major Histocompatibility
Complex
Course-VPP 703
Course Teacher-Dr Suguna Rao Prof
Presentation By-Dr Somshekhar

4. Triggering Acquired Immunity….
 Foreign Material-captured, processed,
and presented
Antigen Processing & Presenting Cells-
◦ Dendritic cells
◦ Macrophages
 B cells (Minor role in Primary Response)

5. Antigen Processing
g
Attracted by alarmins(damaged
cells)

6. T cells do not recognise native antigens
B B
B B B BB
B B
Y Y Y Y
Y YYY Y
Y
Y
Y
Y
Y
Y
Cross-linking of surface
Proliferation and antibody
membrane Ig
production
T T No proliferation
No cytokine release
Y Y

7. Antigens must be processed in order
to be recognised by T cells
T
Y
Cell surface peptides of Ag
presented by cells that
express MHC antigens
Soluble
Soluble
native Ag
peptides
of Ag Cell surface
Cell surface
native Ag peptides
of Ag
ANTIGEN
PROCESSING
No T cell No T cell No T cell No T cell T cell
response response response response
response

8. The site of pathogen replication or mechanism of antigen
uptake determines the antigen processing pathway used
EXTRACELLULAR OR
ENDOSOMAL REPLICATION
Vesicular Compartment
Y Contiguous with extracellular fluid
Exogenous processing
(Streptococcal, Mycobacterial antigens)
INTRACELLULAR REPLICATION
Cytosolic compartment
Endogenous processing
(Viral antigens)
Distinct mechanisms of antigen generation are used to raise
T cells suited to the elimination of endogenous or exogenous pathogens

9. Antigens generated by endogenous and exogenous
antigen processing activate different effector functions
EXOGENOUS
PATHOGENS
Y ENDOGENOUS
PATHOGENS
Eliminated by: Eliminated by:
Antibodies and phagocyte Killing of infected cells by CTL that
activation by T helper cells that use antigens generated by
use antigens generated by ENDOGENOUS PROCESSING
EXOGENOUS PROCESSING

10. 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

11.  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)

12. Structures of MHC Class I & II

13. Antigen Processing
 Exogenous Antigen-Endocytic or
exogenous processing pathway
Class II MHC molecules bind peptides
and present to CD4+ T cells
 Endogenous Antigen-Cytosolic or
endogenous processing pathway
Class I MHC molecules bind peptides
and present to CD8+ T cells

14. Uptake of exogenous antigens
Membrane Ig
receptor mediated
uptake
Phagocytosis
Y
Complement receptor
mediated phagocytosis
Pinocytosis
Y Fc receptor mediated phagocytosis
Uptake mechanisms direct antigen into intracellular vesicles
for exogenous antigen processing

15. Exogenous pathway
Cell surface
Protein antigens
Uptake In endosome
Endosomes
Increase
in acidity
To lysosomes
Cathepsin B, D and L proteases are activated by the decrease in pH
Proteases produce ~24 amino acid long peptides from antigens
Drugs that raise the pH of endosomes inhibit antigen processing

16. MHC class II maturation and invariant chain
In the endoplasmic reticulum
Need to prevent newly Invariant chain stabilises MHC class II by
synthesised, unfolded self non- covalently binding to the immature
proteins from binding to MHC class II molecule and forming a
immature MHC nonomeric complex

17. Class II associated invariant chain peptide (CLIP)
Cell surface
Endosomes
Uptake
( inv)3 complexes Cathepsin L degrades MHC Class II
directed towards Invariant chain containing vesicles
endosomes by CLIP blocks groove in MHC fuse with antigen
invariant chain molecule containing vesicles

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

19. HLA-DM catalyses the removal of CLIP
HLA-DM
Replaces 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
HLA-DR HLA-DM role in regulating DM
Sequence in cytoplasmic tail
retains HLA-DM in
MIIC compartment endosomes

20. Surface expression of MHC class II-
peptide complexes
Exported to the cell surface (t1/2 = 50hr)
Sent to lysosomes for degradation
MIIC compartment sorts peptide-MHC complexes for surface expression or
lysosomal degradation

21. Peptide antigens produced in the cytoplasm are
physically separated from newly formed MHC class I
ENDOPLASMIC RETICULUM
Newly synthesised
MHC class I molecules
Peptides need
access to the ER in
CYTOSOL order to be loaded onto
MHC class I molecules

22. Transporters associated with
antigen processing (TAP1 & 2)
Hydrophobic
transmembrane
Lumen of ER domain
Peptide
ER membrane
Cytosol Peptide
Peptide
Peptide antigens ATP-binding cassette
from proteasome (ABC) domain
Transporter has preference for >8 amino acid peptides
with hydrophobic C termini.

23. Maturation and loading of MHC class I
Peptide
Peptide
Peptide
Endoplasmic reticulum
Calnexin binds B2-M Tapasin, calreticulin, TAP Cytoplasmic peptides
to nascent binds and 1 & 2 form a complex with are loaded onto the
class I chain stabilises the floppy MHC MHC molecule and the
until 2-M binds floppy structure becomes
MHC compact

24. Fate of MHC class I
Exported to the cell surface
Sent to lysosomes for degradation

25. Evasion of immunity by interference with endogenous
antigen processing
Peptide
Peptide
Endoplasmic reticulum
Sent to lysosomes
for degradation
HSV protein blocks transport
of viral peptides into ER

26. Evasion of immunity by interference with
endogenous antigen processing
Normally exported to the cell surface
Adenoviral
protein
retains MHC
class I in the ER
Sent to lysosomes for degradation

27. 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

28. Summary
• 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

30. Any
Queries????????
???????????
Hope Not…………

31. THANK
YOU........