Lecture 5 - T Cells, Peptide and the MHC Flashcards
Properties of MHCI
- expressed in all nucleated cells
- antigen source is cytosol
- bind to CD8+ T cells
- kills infected cells
Properties of MHCII
- Expressed in professional antigen presenting cells (dendritic cells, macrophages, B cells)
- Antigen source is external endocytic vesicles
- MHCII binds to CD4+ T helper cells
- results in the activation of T helper cells (help B cells make antibody and help macrophage injest bacteria
How are peptides and the consequential formation of MHCI peptide complex formed?
1) only 70% proteins made functional, defective ribosomal product and old proteins (processes inside cells let them know when protein need to be replaced)
2) old/defective products are fed into proteasome (made up of many different proeins, between entry and exits sites space 8 amino acids long) which breaks down into peptides and releases products
3) a TAP protein channel transports peptides from cytosol to an MHCI molecule in the ER
4) MHCI released from TAP through the ER into the golgi apparatus
5) MHCI containing peptide exported to cell membrane
What percentage of the peptides in an MHCI molecule are self?
99% self
Why are there many different proteins involved in the regulation of the production of MHCI/peptide complexes?
No point putting MHC complexes on cell if don’t contain a peptide, functionally useless
What is the DETAILED process of formation of the MHCI/peptide complex?
1) partly folded MHC class I alpha chain binds to calnexin until B-microblobulin binds
2) MHC i alpha B-microglobulin complex released from calnexin, then binds a complex of chaperone proteins (calretculin, Erp57) and TAP via tapasin
3) Cytosolic proteins and defective ribosomal products are degraded to peptide fragments by the proteasome. TAP delivers peptides to the ER
4) a peptide bindes to MHCI and completes its folding. MHCI released from TAP complex and exported to cell membrane
How do the ICP47 (human simplex virus) and the US6 (Human cytomegalovirus) proteins allow viruses to evade the immune system?
Viral protein ICP47 and US6 block the function of the TAP molecule consequently stopping viral peptides from entering the ER from cytosol and being presented by MHCI
What are four ways different viruses may use to prevent MHC class I presentation?
1) blocking peptide entry to ER
2) retention of MHCI in ER
3) Degredation of MHC class I
4) binding MHCI on cell surface
What is the process of generation of MHCII molecules?
1) MHCII assembled in the ER, invariant chain binds to MHCII molecules to prevent peptide loading
2) Packaged into a vesicle where invarient chain is degraded leaving single peptide CLIP in the MHC II
3) Antigen is taken up from the extracellular space into the intracellular vesicles
4) In early endosomes of neutral pH endosomal proteases are inactive
5) Acidification of vesicles activates proteases to degrade antigen into peptide fragments
6) Vesicles containing peptides fuse with vesicles containing MHCII molecules, CLIP single fragment removed by HLA-DM protein, peptide binds
7) MHCII exported to cell surface and presented
By what processes can antigens be taken up into a cell?
1) phagocytosis (dendritic cells and macrophages)
2) antigen binding to antibody (b cells)
Why don’t peptides load up with peptide in the ER?
Invariant chain
What did ZINKERNAGEL and DOHERTY discover?
in order for TCR to recognise peptide, has to recognise peptide as foreighn and the corresponding correct ‘self’ MHC molecule - MHC restriction
What experiment was done to show that T cells are only activated by antigenic peptide in the presence of the CORRECT self MHC?
Already known that:
-mice infected with LCMV virus make an immune response, and secondary infection would lead to a stronger immune response. If LCMV killer CD8+ T cells bind to normal cell won’t kill, if binds to LCMV infected cell killed
Experiment:
1) Took nude mice with no T cells as no thymus, three mice with K/k class I MHC one with D and on with B
2) Infected mice with LCMV virus
3) Cytotoxic T cell assay to see whether MHC class H-2K kill virally infected cells
- nude mice could not kill any cells, no T cells
- H-2K mouse won’t kill uninfected, just infected cells
- H-2D and H-2B mouse couldn’t kill infect or non infected
-different strain of mouse with H-2K killed infected
Where does T cell education occur?
In thymic epithelial cells (primary lympoid tissue)
What is the life history of T cells?
1) T cell progenitors begin in the bone marrow and precursors migrate to the thymus where T cell rearrangement occurs
2) T cells undergo education in the thymus
3) Mature educated T cells migrate to the lymph nodes and spleen where they can be activated by MHC/peptide complexes
4) T helper cells help B cells and macrophages
5) Cytotoxic T cells kill targets
What is the process of T cell education?
!) T cell precursors enter thymus with germ line configuration (don’t express TCR, CD4 or CD8)
2) Rearrangement begins with B chain T cell receptor, leads to cytoplasmic TCR beta in the cell
3) Must be tested for competent rearrangement firstly on the first allele, then on the second BY Pre-TCRb binds to b chain, cell starts to express CD4 and CD8 molecules, if cannot express, cell dies
4) Rearrangement of chain, if useless protein dies
5) PROCESS OF POSITIVE SELECTION - If cells can interact with MHCI or MHCII survive..
- If interact with MHCI cell commits to being a CD8 cytotoxic T cell
- If interact with MHCII commit to T helper cell
What happens to T cell education in the absence of MHC?
Shown in gene knock out mice:
- delete MHCI molecule no CD8, no cytotoxic T cells
- delete MHCII no CD4 T helper cells
- delete both no CD8+ or CD4+
How does the T cell know it’s interacting with an MHC molecule?
Through concept of ‘signal strength’
- if have no interaction, no selection occurs (90%)
- if have weak interaction (1%) indicated can interact but not very well, positive selection occurs
- if have strong interaction (8%) indicated self reactive T cell, negative selection and removal of ‘self’ MHC molecules
How has negative selection in T cell education been shown experimentally?
1) In WT mice every T cell has a different TCR rearrangement. In monoclonal TCR transgenic mice all T cells have same TCR and recognise same peptide
2) In WT mice see in thymus that cells are dying all the time resulting in few scattered apoptotic cells
3) In monoclonal TCR transgenic mice when given cognate peptide induces simultaenous widespread apoptosis as all cells undergo negative selection - die
Why do T cells have to undergo negative and positive selection?
- actual number of responding T cells to any one antigen is very low
- thereofre requirement of T cells with best chancce of responding must get rid of unwanted unecessary cells - through positive selection results in T cells with greatest chance of responding to foreign antigen
- T cells cannot respond to slef peptide MHC moelcules - negataive selection
What is the basic process of T cell development
1) TCR rearrangement, test for successful protein, if unsuccessful cells die
2) If successful T cells will express both CD4+ and CD8+, T cell education
- if no signal cell die
- if weak signal, positive selection to self MHC and cells survive, becoming either cytotoxic T cell (CD8) or T helper cell (CD4) depending on interaction
- if strong signal get negative selection as auto-reactive to self MHC and die
Why does education T cells lead to transplant rejection?
Foreign organ contains different MHC molecules which leads to peptide independent binding, alloreactivity which then induces transplant rejeciton
