T-cell development and activation Flashcards
What is needed in terms of MHC recognition for a T cell to respond?
- self MHC
2. correct antigen for TCR
Describe the structure of a TCR
- heterdimer of an alpha and beta chain linked by disulfide bond
- no significant intracellular portion
- each chain consists of a variable region and a constain region and the variable region is what binds to specific antigen
- *unlike BCR, it remains surface structure and isn’t secreted as antibody
The antigen binding region of the TCR arises from ______________.
- rearranging gene segments
- VJC for alpha, VDJC for beta chains
Which chain of a TCR gets rearranged first?
-the beta chain, then the alpha chain
Similarities and differences between the TCR and BCR
- both have 3 CDR loci in variable region that bind antigen
- BCR sees soluble antigens; TCR sees 1-3 residues of a peptide and polymorphic MHC
- TCRs have lower affinity for antigen and do not increase in affinity during immune response like BCRs
- TCR has accessory molecules (CD8, CD4) involved in binding
T cell precursors originate in the _______, then develop in the _______.
- bone marrow
- thymus
gamma delta (yd) T cells
- do not express a-B receptors, but instead express similar protein dimer yd
- distinct T cell lineage
- often respond to lipid, not peptide antigens
- majority T cells in some tissues like epithelium
T/F: A T-cell can express aB r and yd receptor.
-false: either have one receptor type or the other; and also only have 1 receptor specificity, but that clonal receptor may respond to a fairly wide array of antigens
Natural killer T cells (NKT)
- small population of cells that have both surface markers typically found on T cells and surface markers characteristic of NK cells
- most are CD4+
- unlike NK cells which are tons of receptors, these do not
invariant NKT cells
- subpopulation of NKT cells that express the same (non-polymorphic) a and B TCR chains
- restricted by CD1 (not classical MHC) and respond to lipid antigens
- critical for protection against autoimmunity and cancer
Developmental progression of T cell precursors as they enter and mature in thymus
- enter as double negatives and CD3 (TCR) negative
- rearrange B chain for TCR to form pre T cell receptor; if this can transduce, pre T cell survives, proliferates and expresses full receptor
- now express CD8 and CD4 (double positive)
- undergo selection to weakly bind self MHC and down regulate either CD4 or CD8 and exported to periphery as mature T cells
What percentage of double positive, immature T cells in the thymus die?
-95%
Where do developing T cells undergo V(D)J recombination?
- in the thymus
- remember immature thymocytes entering thymus are negative for CD4, CD8, and TCR (CD3)
What occurs in “death by neglect” and negative selection?
- DBN: TCRs do not bind self MHC of either class; aka failure of positive selection
- Negative selection: killing of T cells which have; TCRs that bind self MHC too strongly
**developing thymocytes know if they express a forbidden receptor ( too high or too low affinity) through signals mediated by the antigen receptor
Where do positive and negative selection of T cells occur?
- in the cortex of the thymus
- single positives are found moreso in the medulla
Within the thymus, selection of immature T cells is occuring. What cells are expressing the MHC that is being used to select the T cells?
-thymic epithelial cells
3 possible outcomes of T cell selection
- lack of positive selection; death by neglect: Double positive thymocyte doesn’t recognize peptide-MHC complex on thymic epithelial cell
- positive selection: low affinity recognition of peptide-MHC complex on thymic epithelial cell and survives and converts to single positive
- negative selection: if the TCR on double positive thymocyte recognizes self MHC on thymic APC with high affinity and undergoes apoptosis
What receptor is downregulated if a immature, double positive T-cell recognizes MHC class I or class II?
- if recognize class I: down regulate CD4
- if recognize class II: down regulate CD8
What surface proteins are expressed by most cells in the thymus?
- CD4 and CD8
- mostly double positives
What are the surface markers of the most immature T cells in the thymus?
- double negatives!!
- remember, they enter thymus with TCR, CD4 and CD8
What would one expect if there was a defect that prevented a T cell from recognizing class I MHC molecules?
-only would form CD4+ cells because could only see MHC class II
Do TCRs act in isolation?
- no; found in association with a series of proteins called CD3 chains
- while TCR a and B chains have all the necessary components for antigen and MHC recognition, the CD3 chains are responsible for initiating signal transductions to inform cell that antigen has been bound
What role do CD3s play in T cell signal transduction? What part of their structure is responsible for this?
- ITAMs become phosphorylated and act as docking sites for other proteins for the cascade to continue
- increase transcriptions factors NFAT and NFkB and AP-1 due to new gene transcription
- *when TCR binds antigen, a number of biochemical second messenger cascades are stimulated
- *coordinated activation of these pathways results in transcription of genes important for T cell activation
What are 2 downstream effectors of T cell activation that are often targets of immunosuppressor drugs?
- NFAT and NFkB
- but can be found in other cells, so drugs against these have other effects as well
The a and B chains of TCRs encode the ________ components of the TCR, while the CD3 encodes the ________ components.
- antigen binding
- signal transduction
What is the result of a naive T cells being stimulated by the TCR alone?
- anergy or unresponsiveness
- productive naive T cell activation requires stimulation of TCR by antigen/MHC and “accessory” or stimulatory receptors
How many signals are required to activate a naive T cell?
2;
signal 1: is due to engagement of TCR by MHC plus antigen
signal 2: co-stimulation
What are the effects on a naive T cell if it receives a co-stimulatory signal alone vs. a signal 1 alone?
- only signal 2: nothing
- only signal 1: anergy; prolonged T cell inactivation
What is the major co-stimulatory R, which cells is present on? How about the ligand?
- major co-stimulatory receptor on T cells in CD 28
- ligand is B7 which is present in low level on non-activated APCs but high levels on activated APCs
What happens to a naive T cells that receives MHC/peptide from nonactivated APC?
-anergy; nonactivated APCs lack B7 so no signal 2 can be given
What is the role of CD40: CD40L in T cells activation?
- activated T cells express CD40L, which binds to CD40 on APC
- this induces APCs to express more B7 and secrete T cell activating cytokines
- enhances T cell proliferation and differentiation
In addition to co-receptors that stimulate T cell function, T cells can express co-receptors that inhibit their function. Give an example of this.
- CTLA4
- binds B7 more avidly than CD28
- NOT expressed on naive cells and instead arises once cells are activated as a means to control T cell responses
2 ways CTLA4 terminates T cell activation
- prevents CD28 signaling
2. itself delivers inhibitory signals
T cell stimulation and activation takes a long time, how do T cells and APCs stay in contact for this period of time?
- TCR stimulation leads to increased adhesion between T cells and APCs
- integrins on T cell surface increase their avidity
After a T cell hasreceived 2 signals, what does it produce and what is this critical for?
- Interleukin-2 (IL-2) and make more components for IL-2R that increase its affinity (alpha chain)
- critical for their proliferation
What 2 forms does IL-2 come in and which cells have each?
- low-affinity receptor that consists of beta and gamma chain
- high-affinity receptor that contains a third chain, alpha
- Naive T cells only express low affinity R and are resistant to growth-promoting effects of IL-2
- Activated T cells produce IL-2 and express alpha chain to R
IL-2 R (cytokine receptor) is also important in initiating biochemical signals during T cell activation to elicit proliferation. What signalling system does it use?
- Jak-STAT
- Jak p-lated TFs (STATs) resulting in new gene transcription and T cell proliferation
Rearrange the following changes in T cell activation to the order that they occur: DNA synthesis IL-2 IL-2 R CD40L
- CD40L
- IL-2
- IL-2R
- DNA synthesis (prep for proliferation)
For an intracellular pathogen, would you want to receive an immunized animals serum or T cells?
- T cells
- intracellular pathogen would not be killed by antibodies; T cells would give immunity
CD4+ cells function primarily by ____________, while CD8+ cells function by _______________.
- stimulating other cells through cytokines
- killing host cells that are infected by pathogens
4 effector functions of T cells
- produce cytokines (CD4+ and CD8+): recruit and activate cells of innate immune system; stimulate proliferation and increase effector functions of other T and B cells
- Promote antibody secretion and antibody class switching by B cells (CD4+ helper T cells)
- kill infected targets expressing viral specific antigens (CD8+ killer cells)
- act as negative regulators of immune responses (regulatory T cells which are also CD4+)
Describe the different adhesion molecules that naive and activated T cells express
- naive T cells express L-selectin; its ligand is expressed at high levels on high endothelial venules causing naive T cells to arrest in the LN, giving them opportunity to survey the APCs for their antigen
- if productive TCR signal is delivered, L-selectin is shed, and E and P-selectins are expressed. Ligands for these integrins are found on the tissue endothelium fascilitating arrest of activated T cells at site of infection
Cytotoxic T Lymphocytes (CTLs)
- subset of CD8+ T cells
- recognize peptides that arise from cytoplasmic proteins and are processed via proteasome and are embedded in MHC class I
- effector CTLs are activated by antigen/MHC recognition and NO longer need co-stimulation bc they are mature
- signals delivered by the TCR lead to release of granules towards the target cells which die by APOPTOSIS
What 2 things to CTLs granules contain?
- perforin: forms pores in target cell membranes
2. granzymes: activate caspases which cleave target cell proteins; apoptotsis
How do granzymes enter target cell?
-through pores formed by perforin
Do bystander cells in presence of CTLs also die? How is specific aim at target achieved?
- no; they do not die if not expressing the stimulating antigen
- more specific than complement
- polarization of intracellular cytotoxic granules towards stimulatory target cell
T/F: CTLs only kill one cell and then die
- false
- they can kill many cells by inducing apoptosis
How (very generally) do CTLs kill their target cells?
-apoptosis
How does a naive T cell become an effector CTL?
-receive TCR plus costimulatory signals in LN
What happens if a naive T cell recognizes antigen on epithelial cell without costimulatory signal? but then encounters a DC with same self antigen?
- anergy
- unresponsive
Helper T cells
- CD4+ T cells respond to peptide antigen presented in groove of MHC class II
- following delivery of signal 1 and signal 2. CD4+ cells proliferate and gain effector function
- these cells act mainly as recruiters and activators of other immune cell components via secretion of cytokines
The spectrum of cytokines produced by CD4+ T cells is regulated by __________. What do cytokines determine?
- key transcription factors
- influence what and how immune effector cells are activated, hence the type of pathogen that can be combated
List 5 cytokines made by CD4+ T cells (Helper T cells) and their principal action
1) IL-2: survival, proliferation and differentiation of effector and regulatory T cells
2) IL-4: B cell switching to IgE and IgG
3) IL-5: activation of eosinophils
4) Interferon-y (IFNy): activation of macrophages
5) TGF-B: inhibition of T cell activation; differentiation of regulatory T cells
What determines what type of effector cell a stimulated naive T cell will turn into?
-the cytokines produced by APCs that are stimulating them
What do T cells do after being activated in secondary lymphoid tissue?
-migrate to sites of infection and enter tissues to combat infection
CD4+ effector cells express _______ and produce cytokines that stimulate what 2 cell populations?
- CD40L
- macrophages and B cells who express R for CD40 and cytokines when activated
The signature cytokine of Th1 cells is _________ and what does this do?
- IFNy
- activates macrophages
Th2 cells stimulate B cells to make antibody and class switch, particularly to ________, by producing ________ and recruit eosinophils by producing ________.
- IgE
- IL-4
- IL-5
When Th17 cells are stimulated, what do they produce?
-IL-17, a potent inflammatory cytokine
What is a result of Th1 cells producing IFN-y?
- enhanced macrophage phagocytosis of microbes and increased IL-12 production which is positive feedback look to potentiate more Th1 cell development
- stimulates B cells to produce IgGs which coat bacteria leading to opsonization and destruction and to activation of complement
Th2 cells use what 3 cytokines and what are the effects of these chemicals?
- IL4: induce B cell switch to IgE stimulating mast cell degranulation; tissue fibrosis
- IL-5: recruits and stimulates eosinophils to counter helminths
- IL-13:tissue fibrosis
- *negative feedback on to APCs to diminish production of IL-12 and inhibit Th1 cell development
Th17 has cell roles in _________ immunity. What cytokines does it use and what are the effects?
- cell-mediated
- IL-17: acts on other cells to make chemoattractants for neutrophils to site of inflammation
- IL-22: improves barrier function of epithelium, most notably in intestinal tract to prevent infection by gut microbes
- *proinflammatory helper cells
Name the APC cytokines and TF induced to cause T cell differentiation into Th1, Th2, and Th17 cells.
- Th1 cells: IFNy and IL-12; T-bet
- Th2 cells: IL-4; GATA-3
- Th17 cells: TGF-B, IL-6, IL-23; RORyT
What environmental conditions are necessary to cause Regulatory T cell (Tregs) differentiation?
-stimulation of naive CD4+ T cells in presence of TGF-B but in absence of IL-6 induces TF FoxP3
What TF determined Tregs and what cytokines do these cells produce and what is the function?
- Fox3P
- produce IL-10 and TGF-B that dampen immune responses
How was it should that Tregs were important in self tolerance?
-deleting Fox3P causes a lack of Treg formation and the mouse dies of autoimmune attacks
T/F: CD4+ T cell subsets negatively regulate eachother and each balance struck changes in response to different pathogens.
-True
List the diseases due to over-activity of 4 Th cells subtypes
- Th1 overactivity may lead to inflammatory conditions
- Th2 overactivity may lead to allergy, asthma
- Th17 overactivity may lead to autoimmunity
- uncontrolled tregs may lead to diminished immune responses against tumors
Th1 cells: transcription factor, cytokines that enhance development, what they produce and do.
- T-bet TF
- development is enhanced by IL-12 and IFN-y
- produce IFN-y; stimulate phagocytic cells
Th2 cells:transcription factor, cytokines that enhance development, what they produce and do.
- GATA-3 TF
- development enhanced by IL-4
- produce IL-4 and IL-13
- stimulate class switching in B cells
Th17 cells:transcription factor, cytokines that enhance development, what they produce and do.
- RORyT TF
- development enhanced by IL-6
- produce IL-17, IL-22
- stimulate inflammation
Treg cells: transcription factor, cytokines that enhance development, what they produce and do.
- FoxP3 TF
- development enhanced by TGF-B
- produce IL-10 and TGF-B
- block development of other antigen specific T cell subsets
When pathogenic challenge is met, most of the effector T cells _________. What do a minority do?
- die by apoptosis
- remain as memory cells which respond more briskly to the next antigenic challenge
Where are memory T cells found? What is their action?
- found in circulation and in secondary lymphoid organs
- do not actively produce cytokines or exert effector function, but they do so rapidly if antigen is encountered again
