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Infection and Immunity > MHC Class I and II Antigen Processing > Flashcards

MHC Class I and II Antigen Processing Flashcards

1
Q

What are the types of APCs?

A

Dendritic cells
Macrophages
B cells

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2
Q

Where are DCs found?

A

Under most surface epithelia and in solid organs

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3
Q

What does DC migration induce?

A

Maturation

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4
Q

What attracts DCs to the lymph nodes?

A

Specialised blood vessels - high endothelial venules (HEV) secrete a chemokine CCL21 that attracts DCs
CCL21 also contributes to DC maturation

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5
Q

What is meant by histocompatibility?

A

Tissue compatibility

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6
Q

Where are MHC structures found?

A

MHC class I and II proteins found at cell surface and form a structure that holds antigenic peptides for surveillance by T cells

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7
Q

What recognises MHC I?

A

CD8+ cytotoxic T cells

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8
Q

What recognises MHC II?

A

CD4+ helper T cells

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9
Q

What is the major histocompatibility complex?

A

Large gene complex on chromosome 6 which encodes multiple proteins involved in immune response

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10
Q

What is the structure of MHC I?

A

Alpha 1, 2, 3
Beta 2 microglobulin

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11
Q

What is the structure of MHC II?

A

Beta 1, 2
Alpha 1,2

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12
Q

Describe MHC I

A

Humans = HLA-A, B, C
Found in all nucleated cells
2 polypeptides non-covalently bound

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13
Q

Describe the alpha chain of MHC I

A

45kDa
Inserted in membrane
Glycosylated
Polymorphic

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14
Q

Describe beta 2 microglobulin in MHC I

A

12kDa
Not inserted in membrane
Not glycosylated
Not polymorphic

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15
Q

Describe MHC II

A

Humans = HLA-DR, DP, DQ
Found on professional APCs
2 polypeptides non-covalently bound

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16
Q

Describe the alpha and beta chains of MHC II

A

Both 30kDa
Both inserted in membrane
Both glycosylated
Both polymorphic

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17
Q

What are the differences between the peptide binding groove of MHC I and II?

A

Class I bind short peptides 8-10 amino acids long
No length constraints on peptides bound by class II but usually 13-17 amino acids long

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18
Q

What feature of peptide presentation do MHC molecules have?

A

Most MHC molecules can present multiple peptides (but most peptides will not bind to a given MHC molecule)

19
Q

How are so many different peptides recognised by so few invariant MHC molecules?

A

2 alleles of each of MHC I genes (HLA-A, B, C)
3 alleles of each of MHC II genes
So the actual specificity for an antigen is lower than seen with B cell or T cell

20
Q

Where are polymorphisms found?

A

In the upper peptide-binding part of the MHC protein

21
Q

What is the proteasome?

A

Cytoplasmic structure that digests proteins
- Mechanism for removing poorly folded or damaged proteins
- These proteins have been tagged for destruction by addition of ubiquitin
- So produced peptides from all internal proteins

22
Q

Where does the proteasome digest proteins?

A

In the cytosol

23
Q

What can the proteasome not do?

A

Cannot distinguish between self and non-self proteins

24
Q

What happens to the short peptide fragments?

A

They are transported to the ER lumen via TAP transporters

25
Q

What are TAP transporters?

A

Transporter associated with antigen processing
Endoplasmic reticulum membrane proteins

26
Q

What do TAP transporters do?

A

Transport peptides from cytoplasm into ER lumen
Associate with newly synthesised class I molecules and helps to load peptides into their groove

27
Q

When must stable complexes be formed?

A

Before the MHC can exit the ER

28
Q

What do chaperones do?

A

Stabilise the complex and assist with refolding in the ER

29
Q

What is the first of the chaperones?

A

Calnexin

30
Q

What is the role of calnexin?

A

Keeps the binding site of MHC I open so the peptide can be loaded

31
Q

What happens while calnexin keeps the binding site open?

A

Endoplasmic reticulum aminopeptidase associated with antigen processing (ERAAP) further trims the peptide
The trimmed peptide can then bind to the cleft in MHC I so calnexin no longer needed and beta-2-microglobulin can bind

32
Q

What is MHC II restricted to?

A

Professional APCs - DCs, macrophages, B cells

33
Q

What recognises MHC II?

A

CD4+ T cells

34
Q

How does antigen presentation occur?

A

Antigen taken up from extracellular space into intracellular vesicles
In early endosomes of neutral pH, endosomal proteases are inactive
Upon acidification of vesicles, proteases are activated
These degrade the antigen into peptide fragments
The vesicles then fuse with endosomes and processed peptides are loaded into MHC II

35
Q

When do MHC II encounter their peptide antigens?

A

After they have left the ER (they must not bind the intracellular peptide fragments they encounter in the ER)
However stable complexes must be formed before MHC can exit the ER

36
Q

What is the solution for forming MHC II stable complexes?

A

Invariant chain (Ii) binds in the groove of MHC II and acts as a chaperone
This directs the MHC II complexes towards endosomal compartment of the cell

37
Q

What happens to the invariant chain once MHC II complexes exit the ER?

A

It is degraded to leave a short peptide called CLIP (class II-associated invariant-chain peptide)

38
Q

What is required to remove CLIP to allow antigenic peptides to bind?

A

A specialised protein, HLA-DM, binds to CLIP and releases it to allow other peptides to bind
MHC II then travels to the cell surface

39
Q

What cells express MHC I?

A

All nucleated cells
Meaning all virus infected or transformed cells should present foreign or new antigens on MHC I
Cytotoxic T cells will be able to respond to these, however there is the problem of cross-presentation

40
Q

How does cross-presentation arise?

A

Naive cytotoxic T cells must be primed by profession APCs, usually DCs
But DCs are known to ingest external Ags, process them and present them via MHC II
This will not allow the priming of CD8+ T cells
DCs usually have a range of specialised processes to allow processing and prestation of external Ags through MHC I, which is known as cross-presentation

41
Q

What are the 2 pathways in which cross-presented antigens can encounter MHC?

A

Cytosolic pathway
Vacuolar pathway

42
Q

What might determine the cross-presentation pathway?

A

Size of ingested particles
- smaller particles = phagosome-cytosol pathway
- larger particles = vacuolar
Large particles may also require recruitment of extra membrane from ER, bringing ER proteins with it

43
Q

What role does autophagy have in antigen presentation?

A

Allows controlled degradation and recycling of cellular components

44
Q

Where does degradation occur?

A

Lysosomes

Infection and Immunity (33 decks)

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