This is a medical students' lecture. It is among the immunology lectures. it is important for a medical student to understand immunology to some extent.

1. ANTIGEN PROCESSING
PRESENTATION AND
RECOGNITION
BY
DR SB ZAILANI
CONSULTANT CLINICAL MICROBIOLOGIST
8/15/2022
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2. OUTLINE
 Introduction
Antigens/Antigen presenting cells
Major Histocompatibility Complex(MHC)
T-lymphocytes
 Antigen Processing and Presentation
Exogenous/Endogenous antigen/ pathways
 Antigen Recognition
T-cell& B-cell recognition/ Activation
 Summary /Conclusion
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3. INTRODUCTION
 Antigens are simple or complex foreign
substances, may be organic, inorganic or
biological agents, enhance/ provoke/ elicit
immune response when they get into the
body
 May be complete/incomplete
 Types of antigens-
A)Exogenous antigens
B)Endogenous antigen
C)Superantigens
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4. SUPERANTIGENS
 Proteins produced by pathogens
 Not processed by antigen presenting cells
 Intact protein binds to variable region of β
chain on TCR of T cells and to MHC class
II on antigen presenting cells (APC)
 Large numbers of activated T cells release
cytokines having pathological effects

5. CONT’D
 Antigen processing refers to the ability of
APCs to break down a protein antigen into
peptides and to associate these peptides
with MHC molecules
 Antigen presentation is the process of
displaying peptide antigens associated with
MHC molecules to a T cell, which it
recognises resulting in appropriate immune
response
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6. ANTIGEN PRESENTING CELLS (APCS)
 Three main APC
 Dendritic cells (DCs)
 Macrophages
 B cells
• Others/functions
 Vascular EC (activation of Ag-specific Tcell at site of
Ag exposure)
 Epithelial and Mesenchymal cell (not known)
 Thymic epithelial cell (T-cell maturation)
 Keratinocytes (express cytokines & MHCII)
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7. ANTIGEN PRESENTING CELLS - DENDRITIC
CELLS
 Found in skin (Langerhans cells)
 Ingest antigens by pinocytosis
 Pick up antigen in peripheral tissues and
migrate to lymph nodes and spleen
 Found in T cell area of lymph nodes and spleen
 Most effective APC for activation of “naïve” T
cells
 Can present internalized antigen in association
with either class I or class II MHC (cross
presentation)
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9. ANTIGEN PRESENTING CELLS - MACROPHAGES
 Ingest antigens by phagocytosis
 Not as effective as DCs in activating “naïve”
T cells
 Effective in activating memory T cells
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10. ANTIGEN PRESENTING CELLS - B CELLS
 Bind antigen by surface Ig
 Ingest antigens by endocytosis
 Not as effective as DCs in activating “naïve”
T cells
 Effective in activating memory T cells
 Very effective APCs when antigen
concentration are low
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MAJOR HISTOCOMPATIBILITY COMPLEX (MHC)
Tight cluster of genes
Function in:
1) Intercellular recognition
2) Self/nonself recognition
3) Development of humoral and cell-mediated respons
4) Antigen recognition by TH and TC cells
5) Determine response of individual to infectious
diseases and development of autoimmunity
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Location and function of MHC Regions
MHC “Complex collection of genes arranged within a long
stretch of DNA on chromosome 6 in humans”
Referred to as HLA complex in humans
Genes organised into regions that code three classes of
molecules: Class I, II and III.
Class I encode glycoproteins expressed on surface of nearly all
nucleated cells.
Class I present antigens of altered self-cell necessary for
activation of TC cells
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Class II present antigens on surface of APC (macrophage
dendritic cells and B cells)
They present processed antigenic peptides to the TH cells
Class III genes encode different product which functions
in
the immune system (e.g. TNF α &ß, complement
system
components C2, C4 & factor B and two hydroxylase
enzymes )
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14. More on the Major Histocompatibility Complex
Genetics and Function
MHC class
I
MHC class II

15. Cells MHC Class I MHC Class II
T cells +++ -
B cells +++ +++
Macrophages +++ +++
Dendritic cells +++ +++
Thymic Epithelia + +++
Neutrophils +++ -
Hepatocytes + -
Kidney + -
Muscle +/- -
Red blood cells - -
MHC EXPRESSION
APCs
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16. MAJOR HISTOCOMPATIBILITY COMPLEX (MHC)
GENE PRODUCTS
 Class I
 Antigen is usually endogenous (e.g. viral
proteins).
 CD8+ Cytotoxic T Lymphocytes (CTLs) recognize
antigen in association with class I MHC gene
product on APC.
 Class II Molecules
 Antigen is usually extracellular.
 CD4+ Helper T Lymphocytes recognize antigen
in association with class II MHC gene product on
APC.
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17. 15/08/2022
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T CELL SUBSET
 CD4 T cells bind to
MHC II on APC
 Helper T cells are
CD4
 Helper1 secrete
IL 2/yIFN important
in delayed
hypersensitivity
 Helper 2 cells
secrete IL4/IL5
important in IgE
antibody antigen
reaction
 CD8 T cells bind to
MHC I on APC
 Cytotoxic reactions
 Graft versus host
 Tumour immunity

18. ANTIGEN PROCESSING AND PRESENTATION
 CMI reactions and antibody responses are
directed against different determinants of the
antigen
 Thus requires degradation into peptide
fragments, processing by way of classI or
classII MHC molecules, before presentation
by APCs
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19. POINTS CONCERNING ANTIGEN PROCESSING
AND PRESENTATION
 Location of pathogen dictates type of response
 Virus in cytosol → MHC class I pathway → Tc
response
 Extracellular pathogen → MHC class II pathway
→Th2 response → antibody production
 Intracellular pathogen in vesicles → MHC class II
pathway → Th1 response → cytokine production
(activation of infected cell)

20. TWO ANTIGEN PROCESSING PATHWAYS
 Endogenous antigens in cytosol presented
on class I MHC molecules to CD8+ T cells.
 Exogenous antigens in endosomes
presented on Class II MHC molecules to
CD4+ T cells
 ?The 3rd pathways Superantigens
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21. CELLULAR COMPARTMENTS
EXOGENOUS ANTIGENS VS ENDOGENOUS ANTIGENS
 Endogenous proteins
in cytosol or in
secretory vesicles.
 Exogenous proteins
come in through
endosomes.
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22. Peptides are made from proteins in the cytosol,
transported into the ER, loaded into MHC class I
and transported to the surface of the cell
peptide
MHC class I

23. Cellular Compartments
Exogenous Antigens vs Endogenous Antigens
 Endogenous proteins
in cytosol.
 Exogenous proteins
come in through
endosomes.
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24. • Peptides bound to MHC Class II molecules are derived
from engulfed pathogens (also self proteins and
internalized TM proteins)
• Acidification of endocytic vesicles activates proteases
that degrade proteins into fragments.
• These peptide fragments are loaded onto MHC class II
molecules
How peptides are generated
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25. EXOGENOUS PATHWAY
INVARIANT CHAIN
 Invariant chain (Ii) binds to Class II MHC
molecules in ER to prevent endogenous
peptide binding.
 Ii is cleaved to leave a peptide fragment
(CLIP) in the binding groove.
CLIP (CLass II associate Invariant chain Peptide).
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26. Class II-
associated
invariant-chain
peptide (CLIP)
The invariant chain
• Ii binds to Class II MHC molecules in
the ER to prevent peptide binding.
• Signals in the cytoplasmic tail of Ii lead
to proper sorting of MHC class II

27. EXOGENOUS PATHWAY
HLA-DM
 HLA-DM (H-2M in mice) is a Class II like
MHC molecule that binds to and stabilizes
empty Class II molecules.
 HLA-DM helps in the release of CLIP
fragment so that antigenic peptide can bind.
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28. EXOGENOUS PATHWAY
CLASS II MHC PEPTIDE LOADING
• Class II MHC loading takes place in
endosomes where acidic pH is required
for exchange of peptides.
• Invariant Chain is degraded and CLIP is
exchanged with foreign peptide.
• Chloroquine can be used to raise
vesicular pH and block loading of Class
II MHC.
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29. CD4+ TH ACTIVATED BY EXOGENOUS ANTIGENS
 Foreign antigens/extracellular pathogens
need to be taken up by APCs to get noticed
by immune system and activate Th cells.
 Thus, APCs serve as sentinels or extra
barrier to get immune system activated.
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30. CLASS I MHC PATHWAY
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32. PRESENTATION OF SUPERANTIGENS
 Superantigens
 Intact protein binds to
MHC class II
molecules and Vβ
region of TCR
 All T cells that use a
particular Vβ in their
TCR will be activated
 Activation of large
numbers of T cells
results in cytokine
release and
subsequent pathology

33. B Cell
T Cell APC
Antigen Recognition by B and T lymphocytes
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34. CELLULAR COOPERATION AND
ANTIGEN RECOGNITION
APC Extracellular
Antigen
CD4+ Helper
T
Lymphocyte
Class II
MHC-
associate
d
antigen
+
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35. B CELL ANTIGEN RECOGNITION
 Cell surface immunoglobulin receptor or B-cell
receptor (IgM and IgD)
 Antigen contact initiates B-cell activation, clonal
expansion, maturation to plasma cell
 Antigen receptor is identical to immunoglobulin that
will ultimately be produced
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36. CELLULAR COOPERATION
TH B
antigen
Antibody secretion
by plasma cells
Plasma Cells
Antigen presenting cell
Antigen presentation to T
and B cells by APC
T cells elaborate
cytokines to facilitate B
cell proliferation and
maturation
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37. ANTIGEN RECOGNITION
 MHC restriction – a self–imposed regulatory
mechanism of immune response
- Tc recognise antigen in association with MHC- I
- Th recognise antigens on macrophage & B cell in
association with MHC-II
 The phenomenon of recognition of MHC by
appropriate CD membrane component of Tcell- MHC
RESTRICTION
-Hence ,immune responses to self antigens do not
occur because the antigens are not linked to the
MHC class II molecule in the macrophage membrane
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38. T-CELL ACTIVATION
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Adhesion;T-cell LFA1 &ICAM 1&3 on
APC
Antigen specific activation- CD2(T-
cell) with CD58(APC), MHCI/II(APC)
with CD4/8(T-cell)
Costimulation-B7 on APC & CD28
on Tcell
Cytokine release signal – IL-1, IL-6
TNF-α,IL-12 &IL-15 by APC & IL-2,IL-
4,TNF-ß,INF and GM-CSF by Tcell

39. Key steps for T cell activation

40. CO-STIMULATION AND T CELL
ACTIVATION
 B7 expressed by APCs plays an
important role in stimulation of T cells
 Interacts with CD28 on T cells to deliver
second signal
 Ligation of TCR with MHC/antigen in the
absence of costimulation can lead to
clonal anergy

41. THE BOTTOM LINE
 In order to initiate an immune
response antigen must be
recognized.
 Antigen recognition depends on
detection of antigen by special
receptors.
 Antigen recognition depends on
cellular cooperation.
 Cellular cooperation is controlled
by recognition of MHC-encoded
receptors.
 Antigen “drives” the process
resulting in “effector” cells and
“memory” cells.
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42. Inside of a cell, pathogens and their products are found in either the
cytosol or in vesicles
Mostly
viruses
but some
bacteria
Mostly
bacteria,
common
bacterial
products
(less likely
to be
toxins and
viruses),
self junk
Bacteria,
bacterial
products,
viruses
(anything
that is
antigenic
)
after recognition by an appropriate T
cell
Dendritic cell
Summary

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