Module 3 Flashcards
(24 cards)
APCs can be? and what are they interacting with?
Macrophages or dendrites, interacting with T-lympocytes receptors (CD4 or CD8)
T-cell receptor features
Variable (top) and constant (bottom) regions
Left is alpha
Right is beta
Disulfide bond
Hinge region
Has a CDR
Gene rearrangment.
B-chain - VDJ regions recombine first (chromosome 7)
a-chain - VJ region recombine (chromosome 14)
Rearranged genes go onto transcription, splicing translation to get a TCR
-> variability in T-cell high!
TCR complex features
TCR + zeta chains + CD3
- Zeta-chains are homologous & sit inside the cell that initiate intracellular signalling.
- CD3 made of two pair molecules needed for TCR surface expression -> epsilon:delta and epsilon:gamma
- ITAM helps in the translation of the signal to environment of the cell
- Tyrosine kinases activate a cell when interaction with external environment occurs
- Antigen is recognised by TCR on MHC, cell gets activated.
Features of the gamma:delta TCR
- Delta chain from chromosome 14 within a-chain
- Alpha rearrangement produces alpha -> deletes delta = aB TCR
- Delta rearrangement produces delta -> no delete delta = gamma:delta (only 1-5% of TCR’s)
- usually found in the gut, lungs and reproductive cells
Steps of how T-cells see antigens?
- pathogen proteins in a human cell
- protein broken down into fragments
- presentation of peptide by MHC molecule (of human cell)
- TCR recognises the MHC:antigen complex
Two types of MHC classes?
MHC I - CD8, are cytotoxic T-cells. CD8 directly kills cells with infected virus
MHC II - CD4, helper cells. CD4 helps B-cells to produce Antibody, heps T-cells and CD8
Interaction of TCRs to macrophages and B-cells?
CD4 T-cell recognises bacterial antigens presented by MHC class II on a macrophage. CD4 secretes cytokines activating macrophage to increase ability to kill bacteria.
CD4 T-cell recognises bacterial antigens presented by MHC class II on a B-cell. CD4 T-cell secretes cytokines that drive differentiation of the B-cell into a plasma cell making antibodies.
CD8 and CD4 structural features
CD8 is a homodimer. Binds to the a3 domain of MHC class I. Tyrosine kinases produce stimulation in the activation of T-cell.
CD4 made of 4 domains. Binds to MHC via D1 and D2 domains. Increases cell signal after 100 folds. It binds to the B1 and B2 domains of MHC class II.
Components of TCR signalling?
- Phosphorylation comes from the APC:TCR. CD4 interacts with MHC molecule and initiates FURTHER phosphorylation
- Leads to transcription factors that generate activation of IL-2.
- Need 100 MHC:peptides to activate cell, with costimulation
- No costimulation needs 10000 MHC:peptides
Structure of MHC class I and MHC class II molecules
MHC I
- unique B2m (invariant chain)
- has a1, a2, and a3. binding cleft between a1 and a2
- a3 binds to coreceptor CD8 on Tc cells.
- the a-helices lie on a sheet of 9 parallel beta sheets
- highest variability between a1 and a2
MHC II
- has an a and B, binding is at a1 and b1 domain where t-cell receptor sees.
- has two transmembrane glycoproteins , a and B, with 2 domains each
- Peptide-binding cleft is formed by non-covalently a1 and B1 domains. open at both ends
- CD4 receptor binds to B2 chain
What are the two different cell compartments?
Endogenous compartment (MHC class I): Virus infects cell and utilize the DNA of cell in nucleus to produce its own proteins (replication) into cytosol.
Exogenous pathway (MHC class II): toxin or bacteria, they get phagocytosed by endocytic compartment lysozomes and encounters MHC II molecules that identifies that pathway & complex of MHC II. Will be presented to CD4 cells
MHC I endogenous pathway
- Proteosome induced when T-cell is activated in presence of IFN-y via addition of LMP2 and LMP7. These intracellular pathogens produce proteins in the cytosol by processing of the proteosome. Adds ubiquitin to amino group of lysine side-chain so it signals to digest the protein. Goes through proteolytic proteasome subunit and generates large number of peptides.
- these peptides are transported to the endoplasmic reticulum, Achieved through a complex called TAP where the peptides are moved across a membrane.
- inside the ER, MHC class I bind to the peptides. If peptide is too long between the a1 and a2 domains then ERAP will remove amino acid residues from amino terminus. Peptide is ready to be presented by MHC class I.
3a. This step will occur based on peptide affinity. if peptide does not bind tightly, Tapasin opens up the two helices of MHC molecule a1 and a2. Peptide leaves space and another peptide will come in. New peptide tests for match. If a high-affinity peptide comes along, Tapasin is removed.
Features of the peptide-binding complex
Calnexin: maintains form of MHC class I molecule before B2m is bound. (a1, a2, a3 & B2m)
Calcireticulin: helps stabilise the molecule.
Peptide delivered by TAP which binds to the class I heavy chain. Once peptide is bound, exported out through the golgi appartus to go to cell membrane for presentation
MHC II exogenous pathway
- Antigens reside outside the cell and are endocytosed through the intracellular vesicles.
- As they are in the endosomal compartments (low pH). Acidification of vesicles activates proteases to degrade Ag into smaller peptides inside phagolysosome.
- Vesicles then fuse with MHC class II vesicles from the Golgi apparatus. With peptide and MHC class II molecule in same vesicle, an invariant chain on the MHC molecule is blocking peptide attachment. Invariant chain is then cleaved leaving a CLIP fragment bound. DM molecule (chaperone molecule) facilitates release of CLIP & peptide binds.
Peptide binding model MHC class I features
- 8-10 amino acids
- 2-3 anchors peptide. Y in second position & V in ninth position. In the middle an amino acid bulges out of Ag binding-site to be visible to TCR.
- Rigid structure
- carboxy termini are conserved
- closed at both ends
Peptide binding model MHC class II features
- 13-23 amino acids
- 2-3 anchors. distributed along length of peptide
- flexible structure
- Carboxy termini are very conserved. open at both ends
Key points about MHC.
Snell, Dauset, and Benocerraf established that there is a set of genes that determine what an individual has an identity & what they can or can’t accept in terms of transplantation.
- Class I region: MHCI and MHCII
- Class III region: complement, cytokines
- Polygenic & polymorphic
Class I of HLA genes
- A, B and C encoding a-chains which present epitopes
- HLA-G expressed in placenta protects from NK killing
- HLA-E product inhibits killing NK-cells by binding to receptor NKG2A
- HLA-F expressed in the NK-cells.
Class II of HLA genes
- DR, DP and DQ encode a- and B- chains which present epitopes
- LMP, TAP, DM, DO genes help remove peptide in lysosomal compartment
Class III of HLA genes
- Complement proteins C4, C2, factor B
- cytokines -> TNF-a
Translation into MHC facts
Importance of MHC
- chromosome 6 produces a1, a2, and a3 which have high variability when combining (MHC class I)
- chromosome 6 produces 2 beta and 2 alpha and recombine (MHC class II)
- MHC is highly-polymorphic and gene expression is codominant
- There are 10^12 possible MHC haplotypes
- certain alleles more heavily represented in populations -> evolution and advantages that the system has to express alleles
- HLA-AO201 is highly expressed in >45% caucasian population.
MHC restriction of the immune response
MHC class I: Say you have an animal with a H2K MHC molecule. Take cells (Tc), they are only able to kill cells that express that MHC molecule. Not able to kill H2B, only targets cells with H2K haplotype. Need to recognise self-MHC molecules
MHC class II: Helps recognition of antigen in the CD4+ helper T-cells (not to kill), help other cells to produce an immune response.
Macrophage (APC) gets in contact with T-cell. Macrophage can only present the antigen if the MHC they carry is the same as the MHC the animal was strained. Thymus is where T-cells learn what they can recognise and they get selected to recognise only the MHC of your own.
What is Allo-reactivity?
We carry a number of T-cells that recognise foreign MHC molecules. If an individual receives a graft of another individual whom has a different MHC molecule - a number of cells are able to recognise that quickly and eliminate them.
-> why we cannot receive transplant from person carrying different MHC molecule
-> Allo-reactivity comes into place to eliminate those cells
