Advanced Immunology Flashcards
Proteasome
Splices peptides into readable fragments for antigen presentation.
TAP
Transports short peptides produced by the proteasome into the rough E.R lumen.
B1i, B2i and B5i
Sections of the proteasome that are found within the immunoproteasome form. They replace the B1,B2 and B5 cells within the constitutive proteasome.
Tapasin
Keeps empty MHC I molecules in the endoplasmic reticulum until they receive their peptide.
Promotes assembly of MHC I with peptides.
What is crucial for tapasin function?
Formation of disulphide-linked conjugates of tapasin with ERp57.
TLR role
Recognise pathogen-associated molecular patterns from exogenous sources or cell surfaces which induces dimerisation and thus, signalling occurs.
What does TLR signalling lead to?
Expression of Pro-inflammatory Cytokines
What do TLRs located on the cell surface of an APC detect?
Extracellular pathogens
What do TLRs located within the endosome membrane recognise?
Pathogenic/ foreign molecules digested and broken down by endosomic enzymes.
What are Mannose receptors and Scavenger receptors?
Pattern recognition receptors, they recognise PAMPs
Summarise the endocytic pathway of MHC class 2 presentation
- Antigen is taken up from the extracellular space and into endocytic vesicles.
- Acidification of the vesicles activates protease to degrade antigen into peptide fragments.
- Vesicles containing peptides fuse with vesicles containing MHC Class II molecules.
What is the method of MHC Class 2 presentation known as?
Endocytic
What is the method of MHC Class I presentation known as?
Cytosolic
How does mouse herpes virus protein target newly synthesized MHC class I molecules?
By adding ubiquitin to the cytoplasmic tail of MHC I. This directs the new MHC from the E.R to the proteasome in the cytosol for complete destruction.
DRiPs
Defective Ribosomal products
- Includes unusable products translated from the mRNA which appear in the cytoplasm and can be both self and pathogen derived proteins.
How are DRiPs disposed of?
They are recognised and tagged by ubiquitin for rapid destruction by the proteasome.
ERAAP
Endoplasmic reticulum aminopeptidase. Trims the amino-terminus of peptides if they are too long so they fit better into the MHC Class I groove.
What is ERAAP expression upregulated by?
The presence of IFN-y.
What must a MHC Class I molecule associate with first, prior to the peptide?
Beta2-microglobulin
Which MHC is presented on professional antigen-presenting cells?
Class 2
Which MHC is presented on all nucleated cells?
Class 1
What sort of protein is degraded the most by the proteasome?
DRiPs
What is the protein loading complex comprised of?
Calreticulum, ERp57, Tapasin, TAP and MHC I
How is the Protein loading complex formed?
Calnexin binds to B2M.
MHC I is released from Calnexin
MHC I then binds to the chaperone proteins (Calreticulum, ERp57)
This then binds to TAP via Tapasin.
What is PA28
Doorkeeper/activator of the proteasome.
Must bind to both ends of the proteasome for it to activate.
What are the chaperone proteins that assist within the protein loading complex?
Calreticulum, Tapasin and ERp57
Summarise the Cytosolic pathway of antigen presentation
Proteasome processes proteins into peptides
Peptides are transported by TAP1 and TAP2 to the E.R
Peptides associate with MHC I heavy chain (after calnexin has bound B2M) Tapasin assists this
Tapasin then allows MHC/Peptide complex to leave E.R
This is then transported to the surface for CD8 T cell inspection
At which pH does the invariant chain break down to form CLIP?
5
What does HLA-DM do?
Aids MHC folding but also checks for weakly bound or unstably bound antigenic peptides attached to MHC II.
How does HLA-DM prevent poorly bound peptides?
It will bind to the peptide/MHC II complex and remove the peptide.
What does the removal of poorly bound peptides allow?
A method of peptide editing. Makes presentation more accurate as peptides can be displayed for days upon APCs
What increases expression levels of HLA-DM?
IFN-y
What makes up the invariant chain/Class II MHC complex?
3 x MHC II molecules
3 x Invariant chains
What does the invariant chain leave in the groove of MHC II?
CLIP, a small fragment.
What is contained in the cytoplasmic tail of the invariant chain and what does it do?
Sorting signals that direct the transport of MHC II from the E.R to the trans-Golgi then towards the acidified late endosome.
What does Milatuzumab do?
Bind to CD74 and is used to treat some cancers which express this protein.
Where is MARCH-1 found?
In recycling endosomes
What does MARCH-1 do?
Ubiquitinates MHC molecules in immature dendritic cells, targeting them for degradation when there is no foriegn antigen.
What switches off the production of MARCH-1?
Crosslinking/dimerisation of TLR when a foriegn antigenic fragment is sensed.
Why is MARCH-1 turned off in the presence of an antigen?
Cell is dealing with pathogens so would be counter-productive to destroy the MHC molecules. Allows MHC to transport to the cell surface.
Where is high variation seen in MHC I molecules?
Alpha 1 and alpha 2 regions
Where is high variation seen in MHC II molecules?
Beta 1 region
How fast is the kinetics for MHC/Peptide binding?
Slow, stability allows accumulation and persistence of MHC/foriegn peptide complexes on the cell surface of the APC to allow recognition by T cells.
How fast is the kinetics for TcR/MHC binding?
Fast, disassociation rate allows several triggering events of many TcRs in seconds rather than hours or days for allowing rapid T cell activation.
What is signal 1 involved in the activation of naive T cells?
- Binding of MHC II to TCR mediated by CD4
What is signal 2 involved in the activation of naive T cells?
- Costimulation, B7 of APC binds to CD28. This must happen for continued survival of the cell.
What is signal 3 involved in the activation of naive T cells?
- From cytokines (mostly pro-inflammatory) coming from APCs, binding to receptors on the T cell and finally the T cell differentiates.
What does IL23 do?
Differentiates T cells into T helper 1 cells which help CD8 cytotoxic T cells.
What does IL-4 do?
Differentiates T cells into T helper 2 cells which help B cells.
What does IL-6 do?
Induces T cell differentiation
What is anergy?
State of a T cell which has been completely turned off and cannot be restimulated.
Can T cells recieve signal 1 from self cells?
No, this is the cause of autoimmunity and should not happen.
What is the role of the invariant chain?
Blocks accidental MHC cytosolic binding. Prevents peptides already in the cytosol/E.R from binding by mistake.
What would happen in the TcR affinity for MHC/peptide was very high?
Wouldn’t get a lot of activation events.
What is peptide editing?
Use of HLA-DM binding to peptide-MHC II complexes allows peptides of higher affinity to bind at a higher stability.
Why is peptide editing needed?
MHC II/peptide complex must persist at surface of APC for many days = needs to be stable.
What increases the expression of HLA-DM
IFN-y
When is it useful to block costimulation?
Transplants, to prevent rejection
Autoimmunity, switching off autoimmune responses
What is a Langerhans cell?
Highly immature DC cell
Which 3 cells are able to take up antigen?
Dendritic cells, Macrophages and B cells
How do Dendritic cells take up antigen?
Macropinocytosis and Phagocytosis
How do Macrophages take up antigen?
Macropinocytosis and Phagocytosis
How do B cells take up antigen?
Antigen-specific receptor
How do DCs present viral antigens even when they’re not infected?
Via cross-presentation, allows viral peptides to enter cytosol.
What is cross-presentation?
Method of presenting viral peptides on MHC I to CD8 T cells.
What molecule assists transportation across the endosomal membrane into the cytosol for cross-presentation?
Sec61
What is Sec61?
ATP-dependent transporter/translocon
What allows Sec61 to leave the E.R to reach individual endosomes for transportation?
TRIF signalling
What initiates TRIF signalling?
TLRs
Process of TRIF mediated transport
TLRs detect viral proteins
Initiates TRIF signaling
Allows Sec61 to move from E.R to enter endosomal membrane to allow transport
What is Autophagy?
The reverse of cross-presentation.
Cytoplasmic material is cannibalised by specialized vesicles (autophagosomes) which then fuse with lysosomes that degrade proteins into peptides.
Why do cells deploy autophagy?
It is a method of getting protein aggregates/whole foreign microorganisms from within the cell but are too big to be degraded by the proteasome
What induces autophagy?
TLRs, NOD-like receptors, DAMPs, IFN-y, TNF-a
What happens once DCs mature?
Express heightened levels of B7, MHCI and MHCII and a number of adhesion molecules.
Which chemokine receptor hones DCs to lymph nodes?
CCR7
What type of cells do not always need to recieve signal 2 of costimulation to survive?
Effector B and T cells
What does transfection of tumour cells with GM-CSF gene allow?
Increased production of GM-CSF for activation of neighboring DC cells.
Allows presentation of tumour antigens upon DCs and costimulation to both Th cells and CTL precursors.
What are the MHC I classes in humans?
A,B and C - hereditary
What are the MHC II classes in humans?
DP,DQ,DR - affects immunological susceptibility
What are the MHC III classes in humans?
C4,C2 and BF –> nothing to do with histocompatibility
Autograft
From one part of the body to another.
E.g trunk to arm
Isograft
Between genetically identical individuals.
E.g monozygotic twins
Allografts
Between different members of the same species
e.g Mr Smith to Mr Jones
Xenografts
Between members of different species.
e.g monkey to man
Which types of graft are usually accepted?
Autografts and Isografts
Which grafts are accepted with immunosuppression?
Allografts
THE LAWS OF TRANSPLANTATION OF SKIN GRAFTS (INBRED MICE)
Inbred mice - skin transplant onto exactly the same mouse
ACCEPTED
THE LAWS OF TRANSPLANTATION OF SKIN GRAFTS (INBRED MICE)
Brown mouse onto grey
Skin recognised as foriegn = REJECTED
THE LAWS OF TRANSPLANTATION OF SKIN GRAFTS (INBRED MICE)
Brown mouse onto offspring AxB
ACCEPTED –> not technically foreign
THE LAWS OF TRANSPLANTATION OF SKIN GRAFTS (INBRED MICE)
AxB onto brown mouse
REJECTION –> never seen AxB antigen before
Steps of graft acceptance
- Revascularisation days 3-7
- Healing days 7-10
- Resolution days 12-14
Steps of First set rejection
- Revascularisation days 3-7
- Cellular infiltration days 7- 10
- Thrombosis and necrosis CLOT FORMATION days 10-14
Steps of second set rejection
- Cellular infiltration days 3-4
- Thrombosis and Necrosis days 5-6
What are minor HLA antigens?
Antigens that can cause some graft rejection. Minor HLA antigens can cause slow rejection of grafts even between members of MHC-identical mouse strains
How are alloantigens in grafted organs recognized?
Via direct and indirect recognition
Direct allorecognition
Recognition of donor graft antigens (peptide and foreign MHC) on the surfaces of donor APCs that have migrated to recipient LN.
Indirect allorecognition
Recognition of processed and presented donor antigenic peptides on recipient APCs for presentation within the LN.
What chemokines are required for CD8 T cell activation
IL-2 and IFN-y
What chemokines are required for B cell activation?
IL-2, IL-4 and IL-5
