T cells Flashcards
• Describe the major steps of T cell development
BONE MARROW
- hematopoietic stem cells from Bone marrow go to the thymus
THYMUS
- they are now called thymocytes-
- in the thymus they are double negative cells - since they have CD4+ and CD8+ within the cytosol and NOT expressed on the cell membrane
- they begin to form preTBeta allele for the TCR ( within the double negative form)
- during this process Beta Selection occurs - which is when RANDOM Gene rearrangements (VDJ recombination) ( DNA break and repair mechanisms occur) of the TCR B chain loci occur
- this occurs by the expression of RAG1 and RAG2 genes - which expressed recombinase enzyme which induces the recombination of the Beta chain loci -
this process now creates the functional receptor complex called preTCR ( containing the TCR beta chain , preTalpha subunit and CD3 signalling subunits )
- this promoted cell survival and entry into the cell cycle
this expression also downregulates the RAG gene expression to prevent a rearrangement of the 2nd TCR beta allele. ( this is why all T cells in the periphery express a TCR beta allele which has single specificity = allelic exclusion!)
then the cells become Double positive - since the CD4+ and CD8+ have left the cytosol and now expressed on the cell membrane along with the preTCR complex
- during this stage the signals which induce the preTalpha subunit to become rearranged are sent - due to the expression of the preTBeta subunit and the CD4+ and CD8+ on the membrane.
- during this the RAG genes are re-expressed to induce the RANDOM rearrangement of the preTalpha subunit for the TCR.
T CELLS ARE THEN SELECTED-
- The next stage is the Positive selection stage - a thymic epithelial cell carrying either an MHC class 1 or Class 2 which has a ligand attached is exposed to the double positive preT cell -
-it depends on which Co-receptor ( e.g. CD4+ or CD8+ )
the T cell has which determines the binding in this case -
- if the preTcell successfully reacts ( weak reactivity) then survival signals are sent to activate the T cell to survive by the CD3 complex ( this complex is always present - in ( when it is double negative) or on ( double positive or single positive) all T cells and is important for signal transduction.
- if the preTcell does not show any reactivity towards the MHC/ligand complex- then no survival signals will be sent and so NO survival and the cell will die !!! - the next stage is Negative selection stage - which is when a thymic epithelial cell carrying either one of the MHC classes which has a self peptide - e.g. insulin -( which was transcribed by the AIRE genes, ) binds too strongly to a TCR - in this case this would induce APOPTOSIS - since the preTcell has attacked a self peptide - which would lead to autoimmunity ! -
so if the cell successfully is able to NOT create a strong reactive signal in this case it would survive - as it is important that it recognises the MHC complex - however even more important that it does NOT cause a reactive response to it - when it is a self peptide ! - The preTcell , before it leaves the thymus will now engage one coreceptor and lose the other ( either CD4+ or CD8+ depedning on the MHC class - MHCII binds with the coreceptor CD4+ and MHCI binds with the coreceptor CD8+
PERIPHERY
- now the T cell is ready to leave the Thymus to the periphery where it will go to the lymph nodes to encounter antigens and become activated etc.
Demonstrate a good understanding of the key extracellular signals that control T cell development
- Cytokines …
What does the T cell do when it goes out into the periphery ?
it waits around until a TCR is engaged by a pathogen , the T cell will undergo clonal expansion and differentiation
this differentiation is shaped by Cytokines
What is a TCR?
TCR is a multisubunit complex - contains specific antigen binding subunits - alpha and Beta subunit , very variable produced as the products of gene rearrangements , that recognise peptide MHC complexes - these two chains are looped and connected to invarient signalling chains - CD3 complex , and these can activate many signalling pathways -e.g. cytosolic tyrosine pathways - through ITAM interaction ( with the phosphorylation of ITAM by zap70 , and this switches on a whole signalling cascade that can change the transcriptional program of the T cells ,
T cells will each have unique receptor - about 20,000 T cell receptors on T cells - all the same on an individual T cell , but different between different T cells
What family of cytokines is really important ?
Gamma chain Cytokines - e.g. IL2 and IL7
different cytokines are needed to differentiate the T cells
Place the events of antigen capture and presentation in an anatomical context
T cell activation often occurs at anatomically distant sites to an infection Dendritic cells (DC) have remarkable capacity to capture antigen, process it, migrate to lymphoid organs and present it to T cells On sensing pathogens, DC undergo a process of ‘maturation’ which transforms them from antigen capturing cells to antigen presenting cells
Summary of diagrams:
A pathogen adheres to EPITHELIUM , if a SKIN wound occurs then it allows the pathogens to penetrate the EPITHELIUM
local infection occurs which allows innate immunity to take place , where macrophages will attack the pathogen. In the case of macrophages they will use phagocytosis to further break down the antigen.
Dendritic cells contain PRR for example Toll Like receptors ( to recognise PAMPS) and so will recongnise the PAMP molecules.
Dendritic cells are a link between innate and adaptive immunity. ( since macrophages will NOT migrate !)
Dendritic cells take the infection to LYMPH NODES and stimulate the adaptive immunity. mature T cells are recruited to the LYMPH NODES and recognise the antigen and create an immune response at the SITE OF INFECTION.- this immune response would require the proliferation and differentiation of the T cells into cytotoxic T cells 9 killer cells) or T helper cells- (which are helpful to activate the cytotoxic T cells , the B cells to produce antibodies and for macrophages to break down microbes.)
Since Dendritic cells are APC ( antigen presenting cells) they are able to present the antigen to be attacked by T cells or Antibodies using the MHC molecules.
Explain why dendritic cells (DC) are critical for initiating T cell activation
- Dendritic cells are known as antigen presenting cells - and are vital for initiating T cell activation because they contain MHC complexes which present the antigen to the TCR( T cell receptor ) , this is vital to activate naive T cells to proliferate and differentiate .
they deliver 3 signals
- peptide/ MHC complexes
- costimulatory molecules
- cytokines that guide T cell differentiation
once these are activated by the DC , T cells can engage other cells e.g. B cells
Explain how pathogen-derived products signal to DC and how this links to adaptive immune responses?
pathogen sensing occurs through the reception of TLRs - which sense microbial products inside and outside the dendritic cell - this initiates several changes in the DC which would then result in the adaptive immune responses
Understand that DC provide additional signals besides MHC/peptide (e.g. costimulatory molecules and cytokines) that determine T cell responses?
Dendritic cells (DC) deliver at least 3 signals to T cells
- peptide/MHC complexes
- costimulatory molecules
- cytokines that guide T cell differentiation
it is important that dendritic cells and T cells are glued together - before T cells can actually be effective ( takes days for T cell to make effector proteins in response to this…)
note: when dendritic cells are triggered to become matured when their PRR is encountered with the microbial product - they release NFKB , IRFs , MAP kinases - which are all important for promoting the response of maturation…
Outline how DC can be involved in graft or transplant rejection?
The donated organ contains dendritic cells - which if not fully histocompatible , will migrate to host lymphoid tissue and activate the host T cells.
- alloreactive- T cells have not been tolerised to these unexpected peptide /MHC pairs , these T cells once activated may go through blood to destroy the graft !
faster rejection - direct allorecognition pathway - allorecognition T cells will attack MHC varient ( allotypes) directly !
Chronic ( slower ) rejection is driven by an indirect pathway of recognition of MHC variants ( allotypes)-
- the acute rejection wanes as donar DC die off but may first stimulate the indirect pathway of allorecognition - which would involve endocytosis by the dendritic cell of the recipient
Indirect allorecognition can lead to the induction of anti- HLA antibodies against the graft -
What is cross presentation?
Cross-presentation is the presentation of exogenous antigens on class I MHC, an important exception to the rule that only endogenous antigens appear on class I MHC
Dendritic cells take up and present antigens by various pathways - what are they ?
Virus’ are presented by MHC1 and the type of naive T cell activated is CD8 T cells. Virus’ can be taken up by :
- viral infection
- cross presentation after phagocytic or macropinocytic uptake
- transfer from incoming dendritic cell to resident dendritic cell
extracellular Bacteria ( and soluble antigens and virus particles) are presented by MHC class 2 , and the naive T cell activated is CD4 T cells
these can be taken up by:
- receptor mediated endocytosis
- macropinocytosis
What drives dendritic cell maturation?
- TLR signalling drives a progrmme of dendritic cell maturation
TLRs ( are types of Pattern recognition receptors )
TLR 4 –receptor for LPS !
TLR1 and TLR2 – senses other bacterial material
TLR3 – senses nucelic acid signatures – unique to pathogens !
TLR signalling from different types of TLR drives dendritic cell maturation
what changes are seen in DC following TLR signalling ?
- increased antigen capture
- increased biosynthesis of MHC ( and increased translocation of MHC to cell surface!)
- increased expression of costimulatory molecules : CD80 and CD86 - ( these are very similar) and CD40 !
- increaed expression of CCR7 ( a chemokine receptor that guides migration of DC towards CCL21 which is expressed in lymphoid tissues )
What do T cells do?
T cells respond to bacterial, antigens or viral antigens
and mount an immune response to clear the bacteria or virus from the body
