Differentiation and Activation of T Cells Flashcards
The “bubble boy” disease
People without T and B cells because are missing the recombination activating gene that allows them to make T and B cell receptors
Susceptible to ear infections, pneumonia, oral candidiasis
T cells express membrane T cell receptor (TCR) that recognize peptides displayed by…
MHC molecules
TCR on each T cell clone is specific for…
distinct peptide/MHC molecule combination
Collection of distinct T cells clones make up the…
immune repertoire. Meaning can have over 10^16 specificities
TCR transmits signals that are not specific via…
associated invariant CD3 and z membrane proteins
In TCR signaling, coreceptors CD4 and CD8 transmit…
mandatory activating signals
Structure of TCR
Membrane bound heterodimeric protein composed of an alpha and beta chain, with variable region (able to recognize antigens) in N terminal and constant region in C terminal
- three hypervariable regions where the antigen binds
Which region of the TCR is the complementary-determining region with the greatest variability and most antigen binding?
CDR3
*may be analyzed for polymorphisms
Describe the binding site for recognition of peptide-MHC complex by TCR
- Variable domains of a chains for MHC class I
- Variable domains of a and b chains for MHC class II
- recognize 1-3 residues of the peptides (processed antigen is the one recognized, does not recognize an entire protein)
- affinity of antigen binding: 10^5-10^7 M (pretty high)
How is the enormous diversity of TCR generated?
Recombination of V, D, J gene segments; somatic recombination of gene segments accounts for the diversity
*diversity is 10^16
describe the germline organization of TCR gene loci
- TCR beta chains locus (on chromosome 7) contains Variable (48), D, J, and Constant (few) genes
- TCR alpha chains locus (chromosome 14) contains Variable (45), J, and Constant (few) genes - note no D gene
- several short diversity (D) and joining (J) gene segments between V and C genes
describe the recombination of TCR genes
- Random selection of gene segments
- mediated by a lymphocyte-specific VDJ recombinase (RAG) that brings two segments close together
Step 1: recombination of D and J segments
Step 2: recombination of V segments with fused D-J elements
Step 3: recombination of C segment with fused V-D-J segments
*same idea for alpha chain, but no D segment
In the generation of TCR diversity, ___ diversity is limited, but ___ diversity is unlimited
combinatorial; junctional
How is junctional diversity in TCR mainly responsible for the tremendous diversity?
- exonucleases, which can remove nucleotides
- terminal deoxyribonucleotidyl transferase (TdT), which can add nucleotides
*these two enzymes contribute to the tremendous diversity: 10^16
How do T cells mature and get selected? (overview)
Positive and negative selection based on functional antigen receptors
Steps in maturation of lymphocytes
- cycles of proliferation and expression of antigen receptor
- cells that do not express functional receptors die
- start in bone marrow with common lymphoid progenitor
- Pro-B/T cells proliferate in response to IL-7 made in the thymus
- Pre-B/T cells start expressing one chain of the antigen receptor. Cells that don’t express this die by apoptosis (checkpoint 1)
- Cells proliferate and now express the complete antigen receptor. Cells that don’t express this die (checkpoint 2)
- Positive and negative selection: weak antigen recognition are positively selected and become mature T/B cells. Strong antigen selection is negatively selected
*99% of T cells in thymus die
Why is positive/negative selection of T cells done?
A T cell that has strong antigen recognition may be recognizing an antigen in the body strongly, leading to autoimmunity
describe the maturation of thymocytes
- TCRb gene recombination in pro-T cells and forms pre-TCR
- pre-TCR signals promote survival and TCRa gene recombination
- immature T cells express TCR and CD4 and CD8 (double positive)
what is a double negative thymocyte?
they do not express CD4 or CD8: called Pro-T cells
IL-7
Proliferation of pre-B & pre-T cells
Causes Pro-T cell to express Pre-TCR (Pre-T cells)
Describe the maturation and selection of T cells
- weak MHC + peptide recognition leads to positive selection of mature CD4/CD8 T cells (only one of the CDs will be expressed now based on which class of MHC is recognized)
- strong MHC + peptide recognition or no recognition leads to death (negative selection/death by neglect)
- selected T cells are single positive
Describe Goldilocks principle of T cell selection
The appropriate amount of antigen recognition in the thymus allows T cells to be selected and migrate to the periphery: strong recognition and no recognition lead to negative selection/death by neglect, but weak recognition leads to positive selection where the cells can then go to the periphery
How are T cells activated? (overview)
Antigen recognition and costimulation results in clonal expansion (2 signals)
describe the induction and effector phases of cell-mediated immunity
- naive T cells recognize antigens in the lymph node as presented by dendritic cells
- proliferation and differentiation
- migration of effector T cells to antigen sites
- reactivation of effector T cells in tissues (effector T cells encounter antigens in peripheral tissues and are activated; checks to make sure correct antigen is being attacked)
- effector T cells carry out their functions in tissues: leukocyte activation (inflammation), phagocytosis and killing of microbes, CTL killing of infected cell
Steps in activation of T cells
- antigen recognition
- cytokine secretion and cytokine receptor expression: IL-2 is crucial for T-cell survival and proliferation
- proliferation and differentiation into different types of T cells
- effector functions of CD4+ and CD8+
effector functions of CD4+
activation of macrophages, B cells, and other cells
effector functions of CD8+
killing of infected “target cells”; macrophage activation
what is the role of costimulation in T cell activation?
- antigen recognition by T cells (signal 1) without costimulation induce unresponsiveness
- engagement of B7 on APC by CD28 on T cells provides 2 signals and induces proliferation
*need both signals for T cell proliferation and differentiation to occur
CD28
receptor on T cells that interacts with B7 co-stimulatory molecules to promote T-cell activation
IL-2
T cell growth factor
inhibitory receptors
- critical for limiting/terminating immune responses
- induced in activated T cells
- homologous to CD28
- CTLA-4 (B7-2 ligand) and PD-1 are the inhibitory receptors
activation of CD8 T cells
Concomitant activation of CD4 T cells provides help (CD4+ helper T cells produce molecules that stimulate CTL differentiation)
describe the proteins produced by antigen-stimulated T cells
Proteins expressed by T cells upon stimulation induce proliferation and effector functions
- c-Fos first expressed: transcription factor
- IL-2: proliferation
- CD69: adhesion molecule and activation marker
- IL-2alpha: high affinity receptor
- CD40 ligand: receptor for antigen presenting cells, important for mature cells and interactions between T and B cells
After 3 days cells start dividing. All of these proteins must be expressed first before proliferation and effector function can occur
IL2/IL2R and T cell proliferation
- naive T cells express low affinity IL2Rbg
- upon activation, T cells produce IL2 and express IL2Ra
- IL2Rabg is high affinity and binds IL2 and induces T cell proliferation
- T cell activation by antigen + costimulator
- secretion of IL-2
- expression of IL-2alpha chain; formation of high-affinity IL-2RaBg complex
- IL-2 induced T cell proliferation
describe the expansion and contraction of T cells
- Antigen-specific T cell clones expand in response to antigen to provide a large pool of effector cells to fight infection. Reaches a plateau that correlates with elimination of infection (no longer needed), and will contract to become memory cells
- CD4 100-1000 fold
- CD8 10,000 fold
central memory cells reside in ___ while effector memory cells reside in ___
lymphoid organs; mucosal and peripheral tissues
How are effector function and migration of T cells determined? (overview)
Differentiation and modulation of adhesion molecules/chemokine receptors - allows them to stay in the LN to allow them to get the right signal from APCs and after to migrate to where they need to go
Describe the development of effector CD4 T cells
- naive T cells proliferate and differentiate
- most effector T cells leave LN except for Tfh which function is to help B cells (Tfh cells remain in lymphoid organ, migrate into follicles to help B cells to produce high-affinity antibodies)
- effector T cells and antibodies that go into the circulation go to sites of infection of eliminate microbes
describe the effector function of CD4 helper T cells
- activate phagocytes via IFNg production and CD40 (on macrophage)-CD40-ligand (on T cell) for killing of phagocytosed microbes
- activate B cell to produce Ig via cytokine production and CD40-CD40L: secretion of antibodies with enhanced abilities to neutralize and eliminate antigens
describe the migration of naive and effector T cells
a combination of adhesion molecules and chemokine receptors allows T cell movement
adhesion molecules and chemokine receptors for naive T cell migration
L-selectin, LFA-1, and CCR7-CCL19/21
adhesion molecules and chemokine receptors for effector/memory T cells
E/P-selectin ligand, LFA-1/VLA-4 and CXCR3-CXCL10
T cell pathway through the circulation/lymph node
T cells come in through the artery and get into the LN through the high endothelial venule in the LN. Here they then interact with dendritic cells, but need adhesion molecules to allow them to stay in these venules so that they have the ability to interact.
Once T cells are activated, what must they do with the adhesion and chemokine molecules?
Down-regulate so they don’t stay stuck to the venule in the LN.
What do T cells upregulate to exit the LN?
S1PR1 and follow S1P gradient to exit LN
S1P
Sphingosine 1-phosphate
A chemotactic molecule used to draw out T cells
adhesion molecules and chemokine receptors for effector/memory T cells
…
T cells express membrane _______________ that recognizes peptides displayed by MHC molecules
T cell receptor (TCR)
Describe the structure of TCR
Membrane-bound heterodimeric protein composed of an alpha chain and a beta chain
-variable region (recognizes the protein associated with MHC molecule)
-contant region
Describe the recognition of peptide-MHC complex by TCR
-Binding site
-Variable domains of alpha chains for MHC class I
-Variable domains of alpha and beta chains for MHC class II
-Recognizes 1-3 residues of the peptides (processed antigen recognized)
_______ recombination of gene segments accounts for the diversity
somatic
Describe recombination of TCR gene segments. List the 3 steps
-Random selection of segments
-Mediated by a lymphocyte-specific VDJ recombinase (RAG) that brings two segments closer together
Step 1: recombination of D and J segments Step 2: recombination of V segments with fused D-J element Step 3: recombination of C segment with fused V-D-J element
How much diversity is there of TCR?
Tremendous diversity
Pre-T cells express one chain of antigen receptor, pre-TCR. What is the purpose of pre-TCR?
promotes cell survival
What do immature T cells express?
-Complete TCR that promotes cell survival
-CD4 and CD8
What is the cycle of expression of antigen receptors?
Cells that do not express functional receptors die
What happens if there is weak recognition of MCH+peptide?
Weak recognition of MHC+peptide leads to selection.
-Selected T cells are single positive
-Weak strength of recognition is called positive selection
What happens if there is strong recognition of MHC+peptide? If there is no recognition?
Both strong recognition of MHC+peptide or no recognition lead to death (strong dies b/c we don’t want out T cells to recognize self)
-Strong strength of recognition is called negative selection
-No recognition is called neglect
The appropriate amount of antigen recognition in the thymus allows for what?
Allows T cells to be selected and migrate to the periphery
Naive T cells circulate from _____ to _____ in search of their antigens
LN to LN
Migration of effector T cells to antigen sites in tissues where they get….
reactivated and carry their function
Antigen recognition (signal 1) alone induces ______
unresponsiveness
Engagement of ____ on APC by CD28 on T cells provides signal 2 and induces ________
B7; proliferation
What are inhibitory receptors important for?
Critical for limiting/terminating immune responses
Antigen-specific T cell clones expand in response to antigens, why?
To provide a large pool of effector cell to fight infection
Niave T cells proliferate and ________
differentiate
Most effector T cells leave NL expect for _____
Tfh (follicular helper T cells) whose function is to help B cells to produce high-affinity antibodies
Describe effector function of CD4 helper T cells in cell-mediated immunity and humoral immunity
Cell-mediated immunity –> Activate phagocytes via IFNgamma production and CD40-CD40L
Humoral immunity –> Activate B cells to produce Ig via cytokine production and CD40-CD40L
What allows T cell movement?
Combination of adhesion molecules and chemokine receptor allows T cell movement
