Chapter 8 Part 2- Cell Mediated Immunity Flashcards
What changes are observed in naive T cells when they differentiate into effector T cells? (4)
- Acquire ability to synthesize certain cytokines and
molecules to help eradicate different pathogens - Reduced requirement for co-stimulation
- Changes in adhesion molecules, chemokine receptors
- Also, increased expression of CTLA-4
CTLA-4
An additional and complementary B7 receptor that is expressed when T cells are activated. CTLA4 is structurally similar to CD28 but it binds to B7 in a much stronger manner than CD28. When B7 interacts with CTLA4, it acts as a brake that inhibits both the activation and proliferation of T cells
Co-stimulatory signal
Any signal that is required for the activation of a naive lymphocyte in addition to the signal delivered via the antigen receptor (T cell with the peptide-MHC complex). The co-stimulatory signal is also necessary for the cell to survive and divide. It is delivered when CD28 on the T cell binds to the B7 molecule on the dendritic cell. In this context, B7 is a co-stimulator and CD28 acts as its co-stimulatory receptor
CD28 and B7 signaling
CD28 (on the T-cell) binds to B7 (on the dendritic cell). This allows the peptide-MHC complexes to engage both the T cell receptor and the co-receptor. The intracellular signals generated by the antigen receptor, co-receptor, and co-stimulatory receptor are necessary for the activation of the naive T-cell, as well as its proliferation and the differentiation of its progeny to become effector cells
TH1 cells
Produce immune responses against intracellular pathogens (bacteria, viruses, protozoa). IFN-γ activates macrophages and helps eradicate microbes (viral and intracellular bacterial infections) that survive/replicate within macrophages. Especially ones that are able to evade
intracellular killing mechanisms (NADPH oxidase, enzymes, etc.). Expresses CD40L. TF1 cells also promote B-cell class switching to opsonizing IgG antibodies. TH1 development is induced by IL-12 (DC) and IFN-γ (NK cells) produced during an innate response
TH2 cells
Produce immune responses against extracellular pathogens (worms, parasites). IL-4, IL-5, and IL-13 are cytokines that enhance innate effector functions against parasites. They also activate a preferential Ig class switch to IgE and activate mast cells to secrete anti-parasitic molecules. These cells are linked to allergies and asthma
Types of effector CD4 T cells (5)
- TH1
- TH2
- TH17
- TFH
- Treg
How are the types of CD4 effector T cells differentiated from each other? (4)
- Cytokines that induce their differentiation (local environment)
- Unique transcription factor
- Cytokines produced
- Role in immune response
TH1 development is induced by
IL-12 (DC) and IFN-γ (NK cells) produced during an innate response
TH2 functions
Controls infections by extracellular parasites, promotes eosinophil and mast cell responses, promotes B-cell class switching to IgE antibodies. Secretes non-inflammatory cytokines. Development is induced by IL-4, secreted by either basophils or NK cells
TH2 development is induced by
IL-4, secreted by either basophils or NK cells
TH17 development is induced by
IL-16 and TGF-β, expressed during extracellular bacterial and fungal infections
TH17 functions (8)
- Development induced by extracellular bacterial and fungal infections- release IL-17 and IL-22
- Play a key role in maintaining mucosal and epithelial barriers
- Enhances the production of neutrophils in the bone marrow
- Induces stromal cells to secrete chemokines needed to recruit neutrophils to infection (like CXCL8)
- Induces production of anti-microbial peptides by epithelial cells, by secreting beta defensins
- Promotes wound healing
- Promote B-cell switching to opsonizing IgG2 and IgG3
TFH cell functions
Follicular helper cells- they promote germinal center responses. Promote B cell class switching to the appropriate isotype, depending on the infection type. They also activate B cells to refine the antibody response- trigger naive B cell activation and differentiation into plasma cells
Treg cell functions (3)
- Regulatory helper cells, development is induced by TGF-β and the absence of IL-6
- Suppress other T cell responses to limit damage and promote healing
- Prevent autoimmunity- cytokine secretion, cell-cell contact, and cytolysis
T-cell activation
Also called T-cell priming. It is the stimulation of mature naive T cells by an antigen, which is presented to them by professional antigen-presenting cells. It causes their proliferation and differentiation into effector T cells. T-cell activation is the first stage of a primary adaptive immune response
Polarized T cell response
When the cytokines responsible for driving the differentiation of TH1 and TH2 cells are secreted, it results in positive reinforcement- causes the functional effector T cells to drive further differentiation of the same effector T cell type. This can cause rapid expansion of a population of pathogen-specific CD4 T cells, and either TH1 or TH2 cells become dominant. When this occurs, the T-cell response becomes polarized
T-bet
A transcription factor that drives naive T cells to become TH1 cells and suppresses TH2 differentiation. When the T cells are exposed to IFN-γ, T-bet is up regulated and GATA3 is down regulated
GATA-3
A transcription factor that drives cells to become TH2 cells and suppresses TH1 differentiation. When T cells are exposed to IL-4, GATA3 is up regulated and T-bet is down regulated
Which effector T cells stay in secondary lymphoid tissue?
TFH cells stay in secondary lymphoid tissue, all other cells leave to enter the circulation to seek the site of infection. TFH cells move to the B cell area of secondary lymphoid tissue and activate the B cell response to infection.
All T cell functions are activated
By TCR recognition of peptide antigens presented by MHC class 1 or class 2
T-cell synapse
The localized area of contact between a T cell receptor/co-receptor and an interacting B cell or macrophage. The TCR and co-receptor engage the peptide antigen bound to MHC on the target cell. This is where receptors and their ligands come together, signals are transduced, and cytokines are exchanged. The synapse is a region of contact and communication and is a dynamic structure
Supramolecular activation complex (SMAC)
Molecules found inside the immunological synapse. Includes an inner structure called the central SMAC (c-SMAC) and an outer structure called the peripheral SMAC (p-SMAC)
c-SMAC
Concentrates TCRs, co-receptors, co-stimulatory receptors, CD2 adhesion molecules, and signaling molecules. This localized concentration of signals is necessary for T cell activation
p-SMAC
Contains the integrin LFA-1, the cell adhesion molecule ICAM-1, and a cytoskeletal protein called talin. Together, they form a tight seal around the c-SMAC
L-selectin
Expressed by naive T cells and is responsible for the naive cells to home to lymph nodes. L-selectin is therefore necessary for T cells to enter the lymph node. However, its expression is lower on effector T cells.
VLA-4
An integrin present on the surface of T cells. It binds to mucosal cell adhesion molecules, such as VCAM-1, which is expressed on endothelial cells in infected and inflamed tissue. This interaction is important for T cells to be able to leave the blood and enter infected tissues. VLA-4 is also important for preventing effector T cells from returning to secondary lymphoid organs. Expression is higher on effector T cells
Effector cell transition to infected tissue is facilitated by
Changes in cell-surface components that occurred in their differentiation from naive to effector T cells
LFA-1
An integrin that strengthens contact between the naive T cell and the endothelium. LFA-1 is found on the T cell surface. Effector T cells express 2-4x greater amounts of LFA-1
How are effector T cells different from naive T cells?
Effector T cells express different molecules- they express 2-4x greater amounts of LFA-1. They do not have L-selection, but VLA-4 and VCAM-1 are present. They have better engagement with target cells at site of infection and reduced requirement for co-stimulation. CD4 and CD8 cells do not require co-stimulatory signals.
VCAM-1
Expressed on endothelial cells in infected and inflamed tissue. It binds the integrin VLA-4 on effector T cells, which allows effector T cells to enter sites of infection. Expression of VCAM-1 on the endothelial cells of blood vessels allows the cells to stop passing effector T cells and direct them into the infected tissue. These interactions stop the effector T cells from returning to secondary lymphoid tissue.
What must happen for a T cell to have lasting interaction with a target cell?
The T cell receptor must be engaged by binding a peptide-MHC complex. When this occurs, a conformational change is induced in LFA-1, which strengthens its hold on ICAM-1 and ensures a long-lived interaction between the effector T cell and its target cell
Co-stimulation of effector T cells
Signals from the T-cell receptor and co-receptor are sufficient to activate effector T cells, while those signals alone would induce anergy in naive T cells. Therefore, co-stimulation mediated by CD28 binding to B7 isn’t needed to activate the effector T-cell response. The relaxation in stimulation requirements allows CD8 T cells to kill virus infected cells. CD4 cells can now respond to antigens presented on any cell that expresses MHC class 2. This is in contrast to naive T cells, where they are only activated by specific antigen presented on dendritic cells.
Homing receptors
Effector T cells acquire homing receptors that facilitate their access to specific tissues and organs. Different T cell subsets express different chemokine receptors, for example, CCR5 on TH1 cells and CCR4 on TH2 cells. This allows travel to specific tissues where chemokines are located
Cytokines
Small, soluble proteins that can alter the behavior of cells- they are produced by effector T cells to exert their effect on target cells. Target cells are defined by the presence of a specific membrane receptor for a given cytokine. Cytokines can have autocrine, paracrine, or endocrine actions. They are considered effector molecules
Pleiotropy
A given cytokine can have different biological effects on different target cells
Synergism
When the combined effects of two cytokines on target cell activity is greater than the additive effects of each cytokine individually
Cascade induction
When the action of one cytokine on a target cell induces that cell to produce one or more cytokines, which in turn may induce other target cells to produce other cytokines
How are cytokines produced?
They are produced by effector cells to exert their effect on target cells. Cytokines are only produced and secreted after the T cell engages its target cell. The target cell must express the appropriate receptors in order to exhibit the effect
Cytotoxins
A protein made and secreted by cytotoxic T cells, that participates in the destruction of target cells. Perforins, granzymes, and granulysin are examples of cytotoxins. While all effector T cells make cytokines, only effector CD8 T cells make cytotoxins
Cytokine receptor structure
These receptors have two subunits: an alpha chain, which is the ligand-binding subunit of a receptor, and a beta or gamma chain, which are signal transducing subunits. Dimeric receptors exhibit increased affinity and are capable of signaling
Families of cytokine receptors
Receptors are categorized into families based on structure. These include Type I, Type II, Ig Superfamily, TNF family, and
Chemokine receptors
Redundancy
Multiple cytokines can exert similar actions
Antagonism
There is competition for a limited number of beta subunits. Therefore, the effects of one cytokine can inhibit or offset the effects of another cytokine
Janus kinases (JAKs)
Tyrosine kinases that transduce activating signals from cytokine receptors in the JAK-STAT pathway. They are associated with the cytoplasmic tails of the receptor polypeptides. When the receptor polypeptides are dimerized by cytokine, the Janus cytokines also dimerize and phosphorylate each other, becoming active enzymes that phosphorylate STATs
Signal transducers and activators of transcription (STATs)
Proteins that are activated to become transcription factors through the Janus kinase signaling pathway from cytokine receptors. The phosphorylated STATs dimerize, facilitating their movement from the cytoplasm to the nucleus. At this point they change the target cell’s pattern of gene expression.
Cytokine signaling mechanism (5)
- Cytokine receptors consist of two chains, each having an extracellular binding domain and an intracellular domain, each of which bind to JAKs
- Cytokine binding to the receptor causes the 2 chains to form a dimer. The JAKs are brought together in the cytoplasm
- JAKs phosphorylate the cytoplasmic tails of the cytokine receptors
- STATs bind to the phosphorylated cytokine receptor chains and are phosphorylated themselves
- The phosphorylation allows the STATs to dimerize and travel to the nucleus, where they activate gene transcription
How does IFN-γ improve macrophage function?
Secreted by TH1 cells
1. Enhanced phagocytosis
2. More efficient phagosome-lysosome fusion
3. Increase microbiocidal activities (NADPH oxidase)
CD40 ligand (CD40L)
Expressed on TH1 CD4 T cells. IFN-γ binds to IFN-γ receptor on macrophages, but CD40L delivers the secondary signal when it binds its receptor on the macrophage. The combination of intracellular signals coming from CD40 and the IFN-γ receptor induces the changes in gene expression that activate the macrophage. CD40-CD40L interactions and cytokines also lead to B cell activation, proliferation and differentiation
Macrophage activation by TH1 cells (3)
- The TH1 cell and infected macrophage come together to form a cognate
- The TH1 cell activates the macrophage through IFN-γ and CD40L signaling
- Intravesicular bacteria are killed by the activated macrophage
How do TH2 cytokines enhance effector functions against parasites? (3)
These cytokines include IL-4, IL-5, and IL-13.
1. Increase mucus production
2. Contract muscles to help expel worms and limit tissue damage
3. Recruitment and activation of eosinophils
Activation of a naive B cell by a TFH cell (6)
- Naive B cells are recruited to the draining lymph node and recognize the antigen (like with infection or vaccination).
- Naive B cells bind specific antigen with their surface immunoglobulin- the B cell receptor
- The antigen is internalized by receptor-mediated endocytosis and processed. Peptides from the antigen are presented on the B cell’s MHC class 2 molecules. The B cell travels to interface with the T cell area
- A TFH cell specific for a peptide-MHC complex presented by the B cell forms a cognate pair with the B cell
- Cognate interaction leads to the expression of CD40L on the T cell, which interacts with CD40 on the B cell
- TFH secretes cytokines IL-4, IL-5, and IL-6
Linked recognition
The mechanism that allows B and T cells specific for different epitopes of the same antigen to cooperate. The B cell internalizes and degrades antigen bound to its B cell receptor, and presents antigenic peptides on its MHC molecules. Helper T cells recognizing these peptide antigens on a peptide-MHC 2 complex on a B cell forms a cognate pair and synapse with the B cell. This helps activate the B cell, which then produces antibodies against its specific antigen.
What characterizes Treg cells?
They are characterized by a high surface level of CD25 (the alpha chain of the IL-2 receptor) and usually make cytokines that are immunosuppressive and anti-inflammatory. This includes IL-4, IL-10, and TGF-beta. These cytokines prevent inflammatory responses to commensal microbiota in the gut. FoxP3 is a defining transcription factor. Treg cells suppress autoreactive T cells
Types of autoreactive T cell responses suppressed by Treg cells (2)
- Natural: generated in the thymus; high affinity for self antigen and MHC
- Induced: arise in the periphery
FoxP3
A transcription factor that defines Treg cells. Without it, the cell cannot function as a regulatory T cell
What happens if an individual can’t produce FoxP3?
Infants who lack functional FoxP3 have a normal number of T cells with the characteristic cell-surface phenotype of Treg cells, but these cells can’t function as regulatory cells. This is a fatal disorder that causes death in infancy, demonstrating that Treg cells are vital for maintaining a functional immune system
Treg suppression of autoreactive T cells mechanism
Suppression of an autoreactive CD4 T cell by a Treg cell depends on both T cells interacting with the same antigen-presenting cell. There are 2 ways this can occur:
1. Treg cells enter the secondary lymphoid tissue that is generating the immune response. The Treg cells interact with the dendritic cells, preventing them from activating naive T cells through co-stimulatory signals
2. Treg cells establish direct contact with effector T cells, which prevents the effector cells from responding to antigen. They can also secrete inhibitory cytokines to stop the cells from being activated
Why are cytotoxic T cells necessary?
Once inside a cell, a pathogen becomes inaccessible to antibodies and other soluble proteins. The pathogen can be eliminated through the intrinsic defense mechanisms of a specific cell. However, if the cells are overwhelmed by an intracellular infection, the cytotoxic T cells are necessary to kill the infected cells
How do CD8 T cells specifically kill infected cells?
Due to their antigen specificity, cytotoxic T cells can pick out the infected cells for selective attack and leave healthy cells alone. Secretion of the lytic granules is focused at the synapse, which is a small, localized area of attachment to the target cell. The granule membrane fuses with the T cell’s plasma membrane and discharges its contents onto the target cell surface. This ensures that the cytotoxins don’t target healthy cells.
How are cytotoxic T cells able to kill infected target cells in succession?
When a CD8 T cell recognizes a peptide-MHC class 1 complex on an infected cell, it programs the infected cell to die by apoptosis. The T cell then detaches from the first target cell, synthesizes a new set of lytic granules, and then seeks out a second target cell to attack. The cycle is repeated with an attack on a third target cell.
Apoptosis
Cells killed by cytotoxic CD8 T cells die by apoptosis- programmed cell death. The cell shrivels and shrinks while retaining its contents, leaving a neat and tidy corpse. DNA is fragmented and proteins are degraded. Apoptosis prevents pathogen replication and the release of infectious bacteria or virus particles from the infected cell. It can be triggered by internal or external signals
Necrosis
An “untidy” form of cell death in which cells undergo lysis and disintegration. This is death by injury, caused by mechanical damage or exposure to toxic chemicals
How do CD8 T cells kill a target cell by apoptosis?
- CD8 T cells deposit the contents of their granules on the target cell surface. Perforin, granulysin, and serglycin cooperate to form pores in the target cell membrane and center the cell
- The 5 granzymes initiate a cascade of proteolytic cleavage that leads to the presence of nucleases in the target cell nucleus
- The nucleases degrade the cell’s DNA by cleaving between nucleosomes to create DNA fragments (around 200 base pairs)
- The cell shrinks through shedding of membrane-enclosed vesicles and by the general degradation of the cell’s internal components
- Phagocytes recognize changes to the plasma membrane of the dying cell, and engulfs and destroys the cells. This includes the actual pathogen in addition to the infected cell
What ensures focused delivery of cytotoxins to a target cell?
Engagement of the antigen receptor on the T cell causes the cell to become polarized. The cortical actin cytoskeleton at the site of contact reorganizes, which allows the microtubule-organizing center, Golgi apparatus, and the lytic granules to align toward the target cell
Mechanism of cell granule release (5)
- Cytotoxic T cells contain necessary organelles such as the Golgi apparatus, MOTC, and cytotoxic granules. When cytotoxic T cells interact with their targets, the organelles are re-distributed in the cytoplasm so the cell can directly deliver its cytotoxic functions
- The adhesion molecule LFA-1 mediates the contact with the target cell, through binding to ICAM-1
- If the T cell receptor finds a specific antigen on the target cell, it is activated
- The cytoskeleton of the T cell re-arranges, the organelles move to face the target cell
- This directs granule release and new protein synthesis in the interface between the T cell and its target
How is apoptosis a normal part of development and physiology?
- Resorption of a tadpole’s tail as it morphs into an adult frog
- Removal of tissue (“webbing” between the fingers and toes of a human fetus
- Regulation of the cell number in tissues (homeostasis)
- Development of B and T cells via negative selection
- Clearance of virally-infected cells and tumor cells by CD8 T cells and NK cells
Morphological changes associated with apoptosis (6)
- Normal cells shrink
- Chromatin condenses, then collapses into crescent shapes
- Membrane begins to bulge and then bleb
- Cell breaks up into apoptotic bodies
- Apoptotic cells are rapidly ingested by phagocytic cells (e.g., macrophages)
Pathways of apoptosis (2)
- Mitochondrial (intrinsic) pathway- involves cell injury through growth factor withdrawal, DNA damage by radiation, toxins, or free radicals, or protein misfolding
- Death receptor (extrinsic) pathway- involves receptor-ligand interactions with Fas and the TNF receptor
Perforin
A caspase- forms a pore in the target-cell plasma membrane during apoptosis, allowing granzymes to diffuse into the target cell
Granzymes
Cysteine-dependent aspartate specific proteases that induce caspase-dependent apoptosis. They exist in the cytosol as inactive pro-caspases. When activated, caspases cleave other caspases into a proteolytic cascade
Caspases 8-10
Initiator caspases that start the proteolytic cascade during apoptosis
Caspases 3 and 7
Effector caspases that coordinate the “demolition” phase of apoptosis
T cell killing mechanism (7)
- Degradation of an intracellular pathogen allows their peptides to be displayed on the cell surface on MHC class 1 molecules
- CD8 T cells recognize the MHC-peptide complexes, are activated, and kill the infected cells in a sequential manner
- The killing process is initiated when the T cell receptor and CD8 both bind to the peptide-MHC complex, producing signals that activate the T cell
- T cells contain vesicles called cytotoxic granules that contain perforin, granzymes, and other caspases
- When the T cell is activated, the contents of the vesicles are released and delivered directly to the surface of the target cell. Perforin facilitates the delivery of granzymes into the cytosol
- At this point, the target cell is destined for death, and the T cell can move on to another target cell
- Granzymes target cellular proteins that regulate apoptosis
How do granzymes work?
Granzymes target cellular proteins that regulate apoptosis. It cleaves BID, and the new form of BID causes cytochrome C to be released from the mitochondria into the cytosol. Granzyme B activates pro-caspase 3, which then cleaves ICAD. ICAD is activated and migrates into the nucleus, where it degrades the DNA, ensuring the death of the cell
