T Cell-Mediated Immunity Flashcards
Activation of dendritic cells
- Activated peripheral DCs: Lose adhesive markers and up-regulate CCR7 expression (lymphatic endothelium)
- Mature as they migrate: Increase expression of MHC/HLA, CD80 (B7) and LFA-1
- Travel to regional secondary lymphoid tissue,
- Home by chemokines
- Present Ag to mature, naïve T cells circulating through lymphoid tissues.
- Secret cytokines to promote T cell subset differentiation
DC cytokines
IL-12 : Th1
IL-23 : Th17
IL-10 : Tregs
CD4+ vs CD8+
mature naive t cells
T Cytotoxic Cells
TCR/CD3 signaling complex including z
CD8+
MHC/HLA Class 1
CD28+
LFA-1
CCR7
L- selectin
mature naive T Helper Cytokine Secreting Cells
T Helper Cytokine Secreting Cells
TCR/CD3 signaling complex including z
CD4+
MHC/HLA Class 1
CD28+
LFA-1
CCR7
L- selectin
Naïve t cell migration in the peripheral lymphoid organs
Naïve Lymphocyte trafficking
•Naïve T cells enter the lymph nodes across the high endothelium venules (HEV) in the cortex
- Slow down – L-selection, CCR7
- Stable arrest - LFA
- T cells sample the antigen presented by APCs.
- T cells that do not encounter specific Ag leave the node through the lymphatics and travel down the chain to the next lymph node
- T cells that encounter Ag proliferate and differentiate into effector cells:
- CD4+ T cells increase 100X to 1000X
- CD8+ T cells increase 100,000X
immunological synapse
All the same molecular players for CD8 positive T cells with the exception of HLA Class I instead of Class II for CD8 binding.
CTLA-4 : Cytotoxic T lymphocyte associated protein - 4
PD-1 : Programmed cell death protein 1
Formation of the immune synapse initiates the intracellular signaling leading to complete T cell activation.
Occurs through activation of immunoreceptor tyrosine-based activation motifs (ITAMs).
Kinase associated with co-receptors CD4 and CD8, phosphorylates and activates the tyrosine kinase ZAP-70 associated with the ζ chain
Activation of ZAP-70 on the ζ chain is necessary for all downstream signaling
T Cell Activation: First Signal
FIRST SIGNAL is binding of MHC/peptide complex displayed by DC to TCR on T cell
Upon binding of TCR to MHC/HLA peptide complex:
T Cell
- CD40L expression increased
- CD 40 constitutive
Dendritic Cell
- CD28 constitutive
- B7 (CD80) expression increased
- Cytokine secretion increased
other celluar changes in t cell activation
- TCR/HLA antigen recognition changes integrin conformation on T cells from low affinity to high affinity.
- Integrin advidity Increases upon antigen recognition by T cells : LFA-1
- Allows for clustering of adhesion molecules and firm adhesion for immune synapse.
Why Two Pronged Approach : Clonal Selection
First signal : Recognition of antigenic epitope by TCR
Second signals: Maintains the specificity of the response to the specific epitope
This results in a large number of antigen-specific effector cells from rare antigen-specific naïve T cell.
Induction of anergy
T cells recognizing antigen without binding of co-stimulatory ligands or cytokine support will not become activated.
These cells become unresponsive to additional stimulus
- anergic
- tolerant
Promoting Proliferation:
Self-Stimulation by IL-2
- IL-2 is an autocrine survival signal for newly activated T cells. It is not produced until activation.
- IL-2Rbgc is constitutively expressed on mature, naïve T cells as a low affinity receptor minimizing proliferation
- Upon activation, upregulation of additional high affinity chain in IL-2R, the a chain (CD25)
-IL-2R abgc
- Binding of IL-2 to high affinity IL-2R promotes T-cell proliferation and differentiation
- IL-2a chain = CD25
Clinical application
Inhibition of IL-2 used to prevent the rejection of organ transplant by suppressing the activation and differentiation of naïve T cells and all immune responses that require activated
Activation of T Cells Triggers a Cascade of Protein Production
Substantial proliferation of activated clone to expand population of Ag specific lymphocytes
Activated CD4+ Th cells start expressing CTLA-4 and PD-1 for immune regulation.
Activated CD8+ T cells start expressing PD-1 for immune regulation
Trapping and Activation of Naïve T cells
- Within two days of an antigen appearing in a lymph node, it has been bound by its naïve antigen specific T cell.
- Five days after the arrival of the antigen, activated effector cells emigrate from the lymph node into the periphery
- Transient expression of CD69
-Binds to S1PR and impairs migration
Naïve T cells entering the lymph node encounter many antigen-presenting dendritic cells. Most T cells will not recognize their specific antigen and re-circulate. T cells that recognize their specific antigen are activated by the dendritic cells resulting in effector T cells. The circulation of lymphocytes allows for all antigen-specific, naïve T cell in the periphery to be ‘trapped’ by antigen within a node after 2 days. By five days after the arrival of the antigen, activated effector cells emigrate from the lymph node to the periphery.
Role of CD69 in sequestering
- S1P in high quantity in the blood and lymphatics.
- Naïve T cells express receptor for S1P (S1PR) that directs their migration through circulation.
- Newly activated T cells will transiently express CD69.
- CD69 binds to S1pR on T cell surface and sequesters it
- Leaves the T cell unable to respond to S1P migration signals.
- Ensures activated T cell has time to provide “help” to other lymphocytes
CD4+ T helper cells differentiation
- All start at CD4+ Th0
- Differentiation of each subset is induced by the types of microbes it is best able to combat.
- Antigen drives the response
- Contributing cytokine microenvironment during activation.
•Each subset of T helper cells produces cytokines that support its own development and suppress other subset development.
CD4+ T help for CD8+ T cell Activation
- Cytokine support of activation and proliferation of CTLs
- Induction of cytotoxic protein synthesis
- Cross presentation is often necessary for CD8+ T cell activation
Cross-Presentation of antigen by Dendritic cells
CD4+ T regulatory cells
- Tregs / Th3
- Influenced by IL-2, IL-10 and TGF-β
- Constitutively express CTLA-4 and CD25 (a chain)
- Novel transcription factor FOXp3
- Secrete IL-10 and TGF-β
- Peripheral tolerance
Gamma Delta T cells
- TCR composed of g/d chains instead of a/b
- Less than 5% of Ts
- Found in higher numbers at epithelial boundaries especially the gut mucosa.
- Ag restricted
- Limited diversity of peptides recognized
- Can recognize non-protein Ag
- Not restricted to MHC/HLA presentation.
effector t cell: t cytotoxic cells
T Cytotoxic Cells
TCR/CD3 signaling complex including z
CD8+
MHC/HLA Class 1
CD28+
CD25+
FasL
PD-1
LFA-1/VLA-4
CXCR3
E- and P- selectin ligand
effector t cells: t helper cells
T Helper Cells
TCR/CD3 signaling complex including z
CD4+
MHC/HLA Class 1
CD28+
CD25+
CTLA-4
PD-1
LFA-1/VLA-4
CXCR3
E- and P- selectin ligand
The Major Difference Between
Effector T cells and Resting Naïve T cells is. . .
An effector T cell is able to respond to specific antigen WITHOUT need for co-stimulation via B7 (CD80)-CD28 interaction!
Cell mediated immunity
- Can transfer immunity to intracellular microbes to non-immune individuals by transferring “immune” T cells.
- Intracellular microbes are killed by either CD8+ CTLs or activated macrophages or “cells”.
- Immune serum (antibodies) does not protect against intracellular microbes.
Effector t cell migration site of infection
Effector Ts enter the peripheral tissues by interacting with cytokines, chemokines and adhesion molecules on the endothelium at the site of infection.
The adhesion molecules and chemokines for effector Ts are different than the molecules on the HEVs naïve T cells interact with.
- Slow down: E and P selectin ligand, CXCR3
- Stable arrest: LFA-1 & VLA-4
Not every effector T cell that enters the area will be specific for that antigen. Why?
Overview of T helper subsets
Th1: IFN-gamma
Role : Macrophage activation
Defense : Intracellular pathogens
Pathology : Chronic inflammation, autoimmunity
Th2 : IL-4, IL-5, IL-13
Role: Eosinophil, mast cell and alternative macrophage activation
Defense : Helminths
Pathology: Allergy
Th17: IL-17, IL-22
Role: Neutrophil activation
Defense: Extracellular bacteria & fungi
Pathology: Autoimmunity, inflammation
Treg: IL-10, TGF-b
Role: Peripheral tolerance
Defense: Regulation of T cell responses
Pathology: Autoimmunity
Tfh: IL-21, IL-4
Role: support of B cells in germinal center
Defense: Extracellular pathogens
Pathology : Antibody mediated autoimmunity?
Migration of CD4+ T helper cells
- Effector Th cells leave the lymphatics and re-enter circulation.
- They circulate until they are exposed to inflammatory molecules that allow them to migrate into the peripheral tissues at the sight of infection.
CD4+ T helper 1 overview
Th1
Proliferate in response to IL-12 and IFN-γ secreted by Macrophages, NK cells and DCs
Unique transcription factor: T-bet
Secrete : IFN-γ and TNF-a
Role : Classical macrophage activation
Defense : Intracellular pathogens
Pathology : Chronic inflammation, autoimmunity
Don’t forget – these cells are also secreting IL-2 upon activation as well. It’s an autocrine survival signal.
T helper 1 cytokine activities
IFN-γ
- Increases microbicidal activity by macrophages against intracellular microbes
- Classical activation
- Stimulates B cells to class switch to IgG during activation (opsonization)
- Stimulates class II HLA antigen presentation and B7 (CD80) expression
- Inhibits Th2 and Th17 production
Don’t forget – these cells are also secreting IL-2 upon activation as well. It’s an autocrine survival signal.
Th1 phagocyte activation
- Effector T helpers leave the nodes and migrate to the area of infection.
- Following chemokines and other markers of inflammation and migrate into the tissues.
- They “sample” antigen presented to them by macrophages.
-Looking for the same epitope they are effector clone for.
•Immune synapse between T helper cells the macrophage fully activates the macrophage to become a better killer.
Immune synapse ligand interactions and binding the same as for the synapse between DC and T cell. The macrophage has become the APC. Remember – macrophages and B cells can present antigen to effector (activated) and memory T cells.
CD40, B7, Class II HLA/MHC/peptide complex/ICAM – macrophage
CD40L, CD28, CD3 TCR complex/LFA-1 – T cell
CD4+ T helper 2 OVerview
Th2
Proliferates in response to IL-4 secreted by mast cells, basophils and Th2 cells in immediate surroundings
Unique transcription factor: GATA-3
Secretes : IL-4, IL-5, IL-13
Role: Eosinophil, mast cell and alternative macrophage activation
Defense : Helminths
Pathology: Allergy
T helper 2 Cytokine activities
- IL-4, IL-5 and IL-13
- Stimulates B cells to class switch to IgE during activation
- FcR for IgE on cell membranes and cross-linking of bound IgE leads to physiological responses
- Increased production of mucus in epithelial cells (gut and airway, IL-13)
- Increased eosinophil migration and activation (IL-5)
- Support alternative macrophage development.
- Inhibits Th1 development and effector responses
T helper 2 Immunity
Functions as protection against helminths (large worms
- Mast cell activation
- Mucus production
- Peristalsis
- IgA production
- Eosinophil activation
Alternative Macrophage activation
Th1 versus Th2 activation of Macrophages
An example of the dichotomous development of Th1 versus Th2 immune responses by suppression of other subset. Directly influences immune responses to antigens and can impact pathological processes.
Th1/Th2 Cytokine influence on disease
Th1 effector T cells are necessary to combat intracellular microbes.
A predominant Th2 response to an intracellular microbe can lead to poor disease outcomes.
T helper 17 overview
Th17 / Th17
Proliferates in response to IL-1 and IL-6 secreted from DCs during activation.
Secretes : IL-17, IL-22
Unique transcription factor: RORγt
Role: Neutrophil activation
Defense: Extracellular bacteria & fungi
Pathology: Autoimmunity, inflammation
T helper 17 cytokine activity
IL-17 : Inflammatory
Recruitment of pro-inflammatory leukocytes
- Increases pro-inflammatory cytokine and chemokine production
- Stimulates anti-microbial peptide (defensins) production
IL-22 : Protective
Promotes regenerative responses within epithelial barrier
•Stimulates anti-microbial peptide (defensins) production
T helper 17 immunity
Destruction of extracellular bacteria and fungi by inducing neutrophilic inflammation
- Important in barrier function integrity
- Role in inflammatory autoimmune diseases including multiple sclerosis, inflammatory bowel disease, and rheumatoid arthritis
- Role in integument pathology
- Role in cancers such as breast/lung?
CD4+ T helper cells effector functions in the periphery
- Effector Th cells leave the circulation and migrate into the peripheral tissues at the site(s) of inflammation.
- They perform their specific effector functions in the periphery, not in the lymphoid tissues where activated.
CD8+ Cytotoxic T Lymphocytes
- Tc / CTL / Cytotoxic T
- TCR recognizes MHC/HLA Class I plus altered-self peptides
- Effector function is killing of cells infected with intracellular pathogens or tumor-transformed cells by release of lytic enzymes or Fas/FasL induced cell death.
- Th1 effector cells enhance proliferation, differentiation and clonal expansion of activated CD8+ T cells by providing additional IL-2
What is not needed with CD8+ cells are the CD40/CD40L and CD28/B7 ligand interactions that are seen with effector Th1s and macrophages. Why? Because the CD8+ need to be able to kill any infected cell in the body, not just antigen presenting cells. CD8+ Ts and NK cells are tumor surveillance cells. NKs are able to respond non-specifically while effector CD8+ Ts respond very specifically, these CD8+ are sometimes referred to as tumor infiltrating lymphocytes (TIL). This is a very large area of clinical research for many types of cancer.
Overview of CD8+ Cytotoxic
T Lymphocyte Function
MHC I restricted cytotoxicity for non-self or altered self peptides
Involved in the killing of intracellular pathogens
•Host cell must be killed to eliminate the pathogen
Two killing mechanisms:
- Cytotoxins: preformed granules contain lytic cytotoxins
•Granular proteins
–Perforin- necessary for delivery of granzymes through endocytic vesicle membrane
–Granzymes- activate caspases, induce apoptosis
Cytotoxins are delivered directly onto the surface of the infected target cell by cell exocytosis upon recognition of altered peptide displayed within MHC/HLA Class I
- FasL and Fas (CD95) induction of apoptotic pathways in target cell.
Migration of CD8+ Effector Cells
Effector Th cells leave the lymphatics and re-enter circulation.
They circulate until they are exposed to inflammatory molecules that allow them to migrate into the peripheral tissues at the sight of infection.
CTL Killing: Cytotoxic enzymes
- Antigen recognition and immune synapse formation
- Directional release of enzymes and targeted killing of infected/altered cells
- Endocytosis of Perforin and granzymes
- Perforin “pokes holes” in endocytic vesicle membrane allowing granzymes to enter cytosol to induce apoptosis
CTL Killing: Fas/FasL induction
Effector CTLs express Fas ligand (FasL)
•binds to the death receptor Fas expressed on many cell types
Activation of caspases and apoptosis of Fas-expressing altered target cells
CTLS Are Serial Killers
There is a definite pattern in killing
- Antigen specific recognition of infected/transformed cells
- Induction of apoptosis to prevent pathogen replication and release of infectious material
- Move to the next target cell
In vitro studies suggest that effector CTL can kill hundreds of cells a day. In vivo modeling suggests many less cells a day do to limitations of extracellular matrix, trafficking, etc.
Cooperation Exists Between CD4+ and CD8+ T Cell Subsets
- Immunity to intracellular bacteria.
- Phagocytozed microbe is processed and presented with MCH/HLA Class II by macrophage to effector Th cells.
- Phagocytosed microbe escapes phagolysosome.
- Microbe in cytosol.
- Cytosolic microbe is processed and presented with MCH/HLA Class I by macrophage to effector CTLs cells.
- Need both populations of effector cells for the eradication of some intracellular pathogens.
Cooperation between NK Cell and CTL Killing
Both kill tumor cells and virus-infected cells
Both kill by granzymes, perforin or Fas/FasL induction of apoptosis
CD8+ killing activated by altered peptide MHC/HLA Class I
NK killing activated by decreased display of MCH/HLA Class I (inhibitory receptor).
Why is two different recognition systems necessary for transformed/infected cells?
Some intracellular pathogens and cancers downregulate expression of MHC/HLA Class I
Antibody-Dependent
Cell-Mediated Cytotoxicity (ADCC)
- NK cells
- Macrophages
- Neutrophils
- Eosinophils
- Targeted recognition through IgG or IgE binding.
- Killing by lytic enzymes, TNF and Perforin/ granzyme by specific cell types
basic premise – antibody binds to target, target killed by cells binding to Fc region of antibody.
Migration of CD8+ Effector Cells
Effector CD8+ CTLs leave the lymphatics and re-enter circulation.
They perform their specific effector functions in the periphery, not in the lymphoid tissues where activated.
Contraction of Immune Responses
- Number of specific clones decrease as antigenic stimulus is removed
- The majority of effector T lymphocytes will undergo apoptosis.
- A small percentage of clones will become memory cells
-Express increased levels of anti-apoptotic protein, Bcl-2
CD4+ T Regulatory Cells
- Tregs/ Th3
- Constitutively express CTLA-4 & CD25
- FOXp3 transcription factor
- CTLA-4 binds to B7 co-stimulatory ligand on APC and competitively blocks APC signaling.
-Binds more avidly than CD28
Regulatory Receptors for contraction of response
•CTLA-4 inducible on activated Th cells
- Competitive binding to B7/CD80 for downregulation of presentation
- Binds with higher affinity than CD28
•PD-1 inducible on Ts, Bs and myeloid cells after activation
-Contains ITIM signaling domain
ITIM: Immunoreceptor tyrosine-based inhibition motifs
Effector T cell exhaustion
- Regulatory ligand expression can also lead to inability of T effectors respond effectively to chronic infections.
- Effector cells become “exhausted” as they express increasing amounts of PD-1 and CTLA-4 on cell membrane.
Induction of anergy
T cells recognizing antigen without binding of co-stimulatory ligands or cytokine support will not become activated.
These cells become unresponsive to additional stimulus
- anergic
- tolerant
Memory T cells
- Activated T cell clones differentiate into effector cells and memory cells
- Most become effector T cell clones which are short-lived
- A small population will become memory cells specific for their antigenic epitope
- Memory CD4+ and CD8+ T cells require reactivation to regain their effector function
- Respond more rapidly than naïve
- Can respond in peripheral tissues
- Require IL-7 and IL-15 for survival
Evasion Mechanisms
- Pathogens have adapted to escape our immune processes.
- Viruses and intracellular pathogens can downregulate the cell mediated immune responses by several different mechanisms.
- Some are overlapping mechanisms, others are pathogen specific.
- Most are related to antigen processing or cytokine driven.
Do not memorize table for specific pathogens. Understand the basic premise of how pathogens evade cellular response mechanisms. You will be coming back to the individual pathogenic mechanisms in MOD and systems based pathology content.
Lymphocyte activation
