Antigen Processing and Presentation

Question 1

Examples of APCs

Answer 1

DC, Macrophages, B cells\

Myeloid Lineage Cells- Dendritic Cells: Antigen uptake in peripheral sites, Antigen presentation
professional antigen presenting cell (APC)

• Essential for priming naive T cells to respond to Ag
• Macrophage and B cells= also APC

Question 2

Different functions/ characteristics of immature vs mature DC

Answer 2

Myeloid Lineage Cells

Dendritic Cells

• Antigen uptake in peripheral sites
• Antigen presentation

Question 3

Origin (in cells) of protein Ag’s presented by MHC-I and MHC-II

Answer 3

MHC I: cytoplaam

MHC II: Endocytic vesicles

Question 4

Understand process by which peptides generated and loaded onto MHC-1

Answer 4

• Class-I MHC molecule= hetero-dimeric membrane protein with alpha chain (NO beta chain), non covalently associated with beta 2 microglobulin. MHC alpha chain genes are encoded in MHC. In humans ( HLA-A, -B, -C), In mice (H-2K, H-2D, H-2L)
• Beta 2- microglobulin (Beta2m)= the invariant subunit associated with MHC-I molecules
• MHC chaperone proteins: Calnexin (ER resident chaperone protein), Calreticulin (binds initially to MHC-I and MHC-II and other proteins that contain Ig- like domains), Erp57 (or TAP associated proteins, acts in assembly of MHC-I molecules). These chaperones plus empty MHC-I form the “peptide loading complex”
• Proteasome: Large protein complex located in cytosol of cells which has proteolytic activity responsible for degrading proteins that have been marked for destruction by ubiquitination
• TAP: Transporters of Ag Processing (TAP) and Presentation. They are ATP binding proteins involved in transporting short peptides from CYTOSOL→ lumen of ER where they are → associated with MHC-I molecules

Question 5

Understand process by which peptides generated and loaded onto MHC-II

Answer 5

• Class II MHC molecules: hetero dimeric membrane proteins with non covalently associated alpha AND beta chains- both of which are encoded in MHC. In mice ( H-2 IA, and IE), in humans ( HLA-DP, DO and DR). At the cell surface MHC-I molecules present peptides ~8-20 AA long (“ empty MHC-II are unstable, they are quickly internalised and recycled)
• Invariant Chain (li): component of MHC-II molecule that stabilises molecule before it has acquired a peptide
• HLA-DM: a non classical MHC-II molecule that catalyses exchange of CLIP for another peptide. HLA-DM is found in endosomal compartment (not found at cell surface). In mice the equivalent molecule is H-2M.

Question 6

How Ag presentation influences T cell biology and immune responses

Answer 6

Review and Note the Importance to Virus Infection

• CD8T: cytosolic proteins (peptides) presented by MHC-I molecule to CD8 T cells. Eg. viruses that enter and replication in cytoplasm. ANtiviral (virus peptide specific) T cells will therefore be CD8 T cells (most viral epitopes are presented to CD8 T cells)
• CD4T: Endocytosed proteins (peptides) presented by MHC-II to CD4 T cells e. Viruses that gain entry by endocytosis. Antiviral (viral peptide specific) T cells will therefore be CD4 T cells.
• CD4T are helper cells: they produce cytokines= IL-4 (help B cell make Ab’s), TRAIL (helps CD8T cells be cytotoxic)
• CD8T cells are cytotoxic: they make TNF and IFN y (directly antiviral). They kill virus infected cells.

Question 7

MHC Polymorphism and “MHC-Restriction” is

Answer 7

1. Viral Ag mutate- Antigenic “shift” and “drift”: changes in peptide that are presented

•Protein mutations affect whether peptides are generated and whether they fit into the groove of the MHC complex

Peptide Ag Motifs ( “Anchors” and mutations)

Peptides as Ag’s

• These differences can be within the Ag- binding site of MHC molecule and thus influence:
• 1. The “binding specificity” of the MHC molecule for a given peptide” and hence also
• 2. The affinity of the TCR for given peptide/ MHC complex

2. Human HLA Class I and II expressed by different cells
3. HLA is polymorphic

• Thus a) changes (mutations/ genetic variation) in either Ag (peptide) or b) differences in HLA
• Both influence what peptide and the affinity of TCR recognition for peptide/MHC

Question 8

Important to virus infection and hence to pathogen immune evasion

Answer 8

•HLA is polymorphic= there are many different HLC molecules and also allelic differences (can be within the Ag binding site of the MHC molecule and this influence “binding specificity: of MHC molecule for a given peptide

Question:

What happens if Ag processing fails to generate a peptide that can bind to MHC?

•Thus both Ag processing and peptide mutations and MHC polymorphism affect immune recognition of pathogens

Question 9

Rules can be broken : cross presentation

Answer 9

• Cross presentation: apoptotic cells/ Apoptotic bodies are phagocytosed by APCs. This brings what was once cytosolic proteins into new APC via exogenous pathways= these proteins (once presented by MHCI and CD8 cells) are now able to be processed and presented by MHCII and CD4 cells. ALso some ingested (phagocytosed) proteins may be released from phagolysosome and enter into cytosol. They can be processed via endogenous pathway.
• Autophagy: the cytoplasm is taken into autophagosomes, processed into lysosomes and degraded- catabolism. Thus some cytosolic proteins, in phagosome can enter the exogenous pathway of Ag presenting