Antigen processing and presentation allows T cells to recognize antigens by requiring their presentation on major histocompatibility complex (MHC) molecules on cell surfaces. There are two pathways for antigen presentation: class I MHC presents endogenous antigens from inside cells to CD8 T cells, while class II MHC presents exogenous antigens from outside cells to CD4 T cells. The endogenous pathway involves cytosolic degradation and transport of antigens to the endoplasmic reticulum for binding to class I MHC, while the exogenous pathway involves antigen phagocytosis and degradation within endosomes and lysosomes before binding to class II MHC. These pathways keep self and non-self antigens separated and allow the appropriate T cell response to be activated.

1. Antigen Processing and
Presentation
Unit I
BT6602
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2. Antigen processing and presentation
• Why is it needed?
• How does it happen?
• How are the pathways of endogenous and
exogenous antigen kept apart?
• What are the consequences?
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3. T cell receptor: antigen receptor of T cells
Cannot recognise antigen in solution
Must have the antigen processed
and presented to it
Specialised molecules present
antigens to T cells:
MHC molecules
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4. Antigen presentation cells required
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5. T-Cell receptor
Antigen
MHC
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6. MHC
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8. Antigen Processing and Presentation
• Ag recognition by T cells REQUIRES presentation
by MHC on a cell membrane (MHC restriction)
• Pathways for Ag presentation:
a) Class I MHC associated with peptides from
endogenous Ag
b) Class II MHC associated with peptides from
exogenous Ag
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9. Type of T cell activated
• MHC class I activate CD8 T cytotoxic cells
• Present endogenous (eg viral) antigens
• Target cell is killed by CD8 cell
• Any cell can become virally
infected/neoplastic
• Therefore: MHC class I is on all nucleated
cells
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10. Type of T cell activated
• MHC class II activate CD4 T helper cells
• Present exogenous (eg bacteria) antigens
• CD4 T cells upregulate all immune
functions
• MHC class II found only on cells that
sample the extracellular environment
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11. A question of language
• Cells that express MHC class I, present
endogenous ag
– Are killed by CD8+ cells
– Are called Target cells
• Cells that express MHC class II, present
exogenous ag
– Activate the CD4 T cell response
– Are called Antigen presenting cells

12. • Target cells:
– All nucleated cells express MHC class I
– Can be a target cell if virally infected/neoplastic
• Antigen presenting cells
– Limited number of cells express MHC class II
– Must sample extracellular environment
– Monocytes, MΦ, B cells

13. Distinctions between MHC I and MHC II
• Most cells (target cells) can
present Ag with MHC I to TC’s
• Nearly all nucleated cells
infected by MO/virus, or
abnormal proteins produced by
cancer cells, aging cells, or by
allogeneic cells from
transplants
• Associated with MHC I
requires replication of foreign
entity (i.e., abnormal protein
synthesis) within the target cell
• Only APC’s can present Ag with
MHC II to TH’s
• APC’s are of 2 categories:
– Professional APC’s
– Non-professional APC’s
• Associated with MHC II does not
require replication of entity with in
target cells
• Phagocytosis is important in Ag-
processing
MHC I MHC II
MHC
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14. Ag is processed through 2 separate pathways:
*MHC I interacts with peptides from cytosolic degradation
*MHC II interacts with peptides from endocytic degradation
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15. Processing Endogenous Ag:
Cytosolic pathway
• Cellular components of proteins are constantly
regulated; most have a brief half-life and are “turned
over”… the same holds true for endogenous Ag’s!
• Processing of endogenous Ag involves 3 activities:
• Peptide generation from proteolysis
• Transport to ER
• Peptide binding to MHC I
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16. Endogenous Ag processing…
Peptide generation
• Proteins targeted for lysis
combine with a small protein
 ubiquitin
• Ubiquitin-protein complex
is degraded by a proteosome
• Specific proteosomes generate
peptides which can bind to
MHC I
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17. Endogenous Ag processing…
Transport to ER
• Peptides from proteolysis bind to a
“transporter protein associated with
Ag processing” (TAP)
• TAP is a heterodimer which uses
ATP to help transport peptides (8-
10 aa’s) to lumen of ER
• Usually basic amino acids at COOH
end of peptide chain.
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18. • MHC I assembly occurs with the aid
of chaperone proteins to promote
folding (calnexin + MHC I α chain)
• Tapasin + calreticulin brings TAP/
peptide close to MHC assembly
• Allows MHC I to bind to peptides
• MHC I-Ag exits ER to Golgi to
plasma membrane
Endogenous Ag processing…
Peptide binding to MHC I
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19. Assembly and stabilization of MHC I – Ag complex
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20. Processing of Exogenous Ag’s:
Endocytic pathway
• Exogenous Ag’s are typically phagocytized/
endocytized by MØ and APC’s
– Foreign Ag is degraded with in endocytic vacuole of
endocytic pathway. The pathway includes:
– Early endosomes (pH 6-6.5)
– Late endosomes or endolysosome (pH 5-6)
– Lysomes (pH 4.5 – 5)
• Ag is degraded into 13-18 aa polypeptides which bind to
MHC II
• Eventually endocytic vacuole returns to PM  recycling
surface receptors 20

21. Processing of Exogenous Ag’s:
Endocytic pathway
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22. Processing of Exogenous Ag’s:
Manufacture of MHC II
• With in ER, α and β chains of MHC II combine with a protein – “the invariant chain” (Ii,
CD74)
• The IC binds to MHC at peptide binding cleft & then exits the ER to Golgi apparatus.
• As proteolytic activity continues, the IC is degraded to a small fragment (CLIP*)
• Another MHC II (HLA-DM (found in endosomes)) substitutes Ag for CLIP with in lysosome
• MHC II – Ag complex is transported to the PM
*CLIP = class II associated invariant chain peptide 22

23. Comparison of Ag-processing pathways
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24. Thank you
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