Immunology V: T-Lymphocytes

Question 1

Polarizing cytokines that transform a naive Th into a Th2:
The major polarizing cytokine is:

Answer 1

IL-4:
-The source of IL-4 is a bit of a mystery- when a dendritic cell presents antigen to a naive Th, the dendritic cell does not seem to secrete IL-4.

-IL-4 productyion increases when a particular extracellular PRRs (ex: NLRs, C-lectin receptors) are bound by their ligand

FYI: IL-25 may also aid in polarization to a Th2 phenotype

Question 2

Main determination transcription factor:

Answer 2

GATA-3
(All Th cells express GATA-3)

Question 3

Th2 effector functions:

Answer 3

Production of IgE and IgA:
-IgE enhances mast cell immune activity, and IgA is secreted across membranes to bind to pathogens that try to gain access to the mucosal lining.

Secretion of IL-4, IL-5, IL-13:
-IL-4: is potent Th2 polarizing factor (positive feedback)
-IL-5: aids eosinophil activity and migration
-IL-13 increases the production of IgE

Effector functions will be discussed more in the antibody and mucosal immunity lectures.

Question 4

Physiological and clinical relevance of Th2 cells:

Answer 4

Seem to enhance the ability of the host to fight off helminthic and other parasitic infections, especially at membranes (cutaneous or mucosal)
-Recruitment of cells (eosinophils) that are specialized to fight parasitic infections
-Antibody types that have their main effect at membrane surfaces

Are implicated as major causative factors in allergic disease
-Atopic dermatitis, allergic rhinitis
-Asthma
-Other autoimmune disorders may also be mostly Th2 mediated

Question 5

Major costimulator:

Answer 5

CD80/86 on the APC
-Binds to CD28 on the Th

FYI: CD80/86 used to be known as B7

Question 6

Costimulators are not usually expressed at high levels on APCs:

Answer 6

HLA-2 and costimulator expression increase only when the APC recognizes a DAMP or a PAMP.

Question 7

Interleukin-2 (IL-2) is a major ______ & _______ growth factor.

Answer 7

T-cell & B-cell

-Proliferation and differentiation of activated naive T cells

-Proliferation and differentiation of B cells

Once activated, the naive T cell begins to synthesize IL-2 and the high affinity form of the IL-2 receptor.
-IL-2 binding to the Th IL-2R acts in an autocrine fashion
-Activates the Th cell and causes it to enter the cell cycle, as well as avoid apoptosis (upregulates BcI-2-apoptosis inhibitor)

Question 8

Synthesis of both IL-2 and the high affinity IL-2R is induced after CD28-CD80/86 binding and recognition of antigen by the TCR (via HLA-2):

Answer 8

Ag recognition in absence of co-stimulation causes T-cell Anergy.
-Anergic T cells cannot produce IL-2
-Prevents anergic T cells from proliferating and differentiating into effector cells when they encounter Ag-even if presented by APC that express co-stimulatory molecules.

Question 9

How do helper T cells aid immunity?

Answer 9

CD4+ helper T-lymphocytes (Th) never kill pathogens or other “foreign-looking” entities (like cancer cells) directly instead they:
-Activate or inhibit other cells through direct contact (ex: the immunologic synapse); CD8+ cytotoxic T-cells, B-cells, machrophages
-Activate or inhibit other cells through secretion of cytokines

Th cells will differentiate after they are activated so that they “help” in a specific way-This is known as T-cell polarization.

Question 10

T-helper cells become polarized after they are activated:

Answer 10

Polarization= specialized Th phenotype => the Th secretes a “profile” of cytokines that mediate distinct effector cell functions

Question 11

Th type is determined by the environment it is found in:

Answer 11

The types of cytokines that are present in high concentrations in the immediate vicinity of the newly-activated Th-these are known as polarizing cytokines.

Question 12

Key information about the Th polarization:

Answer 12

Inducing cytokines and the transcription factors that they activate in the Th

Cytokines secreted by the Th

The microenvironment that the polarized Th “works in” and the effects of the cytokines it secretes.

Question 13

How does Th polarization work?

Answer 13

1) An infection occurs: likely viral or an intracellular bacterium

2) The infection causes the emergence of PAMPs (or DAMPs, if damage)

3) Dendritic cells phagocytose the pathogen => PRR activation in dendritic cells => DCs enter the lymphatics and migrate to a lymph node or other secondary lymphatic organ (SLO)

FYI: It is possible that certain dendritic cells migrate to the LN in response to certain stimuli: for a Th1 response it may be a dendritic cell type known as the conventional dendritic cell, type 1 (cDC1)

4) when the dendritic cell arrives at the SLO, it presents the phagocytosed antigen via its HLA-2 to a naive Th cell.
-PRR => increased expression of HLA-2 and CD 80/86 on the dendritic cell

5) If the TCR on the Th recognizes the antigen it becomes activated (no longer naive)
-IL-2 secretion => division (activated clones express the same TCR)

6) The dendritic cell secretes Th1 polarizing cytokines, and these bind to receptors on the activated Th cells.
-Major Th1 polarizing cytokines: IL-12, IL-18

7) IL-12 and IL-18 bind to their receptors on the Th cell => activation/production of the Th1 transcription factor
-Th1 transcription facter => Tbet
-This transcription factor is what polarizes the Th: it causes expression of genes that “do Th1 things”

8) Tbet causes the Th to secrete IFN-y => effective macrophage and APC activator, can also tailor some B-cell responses:
-Secretion of (more) IL-12, increased phagocytosis
-Stimulation of dendritic cells to aid cytotoxic T-cell activation (if CD8+ TCR recognizes the antigen)
-Stimulates B-cells to secrete antibody types that are effective opsonins

9) The activated Th1 cell might stay i the LN or migrate into a site of inflammation: hopefully the same place the dendritic cell came from:
-In the lymph node: continued activation of the Tc and B-cells that recognize the antigen
-At the sight of infection: local activation of macrophages that fight infection

Question 14

Macrophage activation is one of the principle effector actions of the Th1 cells.

They require 2 signals for activation:`

Answer 14

-IFN-gamma
-Cell contact => CD40-CD40L

Armed effector Th1 cells can deliver both signals

Question 15

Th cells usually first activated and polarized in secondary lymphatic tissue/organs (SLO):

Answer 15

-HLA-2-TCR + costimulation + IL2 (usually) production

-Usually APC is a dendritic cell (sometimes macrophage), and the Th cell remains in the lymph node or other SLO, dividing and becoming activated for several days
(Dendritic cells migrate to the secondary lymphatic tissue via the lymphatics

Polarization type (Th1, Th2, Th17, Tfh, Treg) usually seems to be dependent on the APC:
-Dendritic cell type may influence polarization “choice”
-What activated the dendritic cell likely influences polarization “choice”

Question 16

The cells can function as effectors in SLOs or in peripheral tissue:

Answer 16

If they migrate to the peripheral tissue, then it is likely at the same (or similar) site to where the APC migrated from in the first place.

Question 17

After a Th cell is polarized, it tends to stay as that cell type:

Answer 17

ex: it is uncommon for a Th1 cell to become a Th2 cell (though it can happen)
-Exception: Tfh cells often “repolarize” to another Th type after the antibody response they help is completed

Signals generated by a polarized Th cell tend to prevent other local Th cells from being polarized into a different type (cross-regulation)

Question 18

Th17 an innate activating Th type cell:

Answer 18

Key Th type implicated in protection from infection and many autoimmune disorders.
-Bacterial and fungal disorders of the skin and mucosal surfaces
-Rheumatoid arthritis, inflammatory bowel disease, psoriasis, MS

Question 19

“Pro-inflammatory” cytokines:

Answer 19

IL-6, and IL-23(produced by activated dendritic cells)

Question 20

“Pro-fibrotic” cytokines:

Answer 20

TGF-beta
-Enhances wound healing, sometimes reduces inflammation
-Can inhibit growth and activation of lymphocytes and macrophages (if found in isolation)

Question 21

Naive Th becomes Th17 cells:

Answer 21

In secondary lymphoid organs (SLOs), stimulated by DCs secreting IL-6, TGF-beta, IL-23

Likely some are generated outside of SLOs as well-many APCs can secrete these cytokines and the pattern of cytokines is typical of chronically inflamed tissue.

Question 22

Th17 effector cells produce IL-22 & IL-17 when they are activated and polarized:

Answer 22

They both induce the release of IL-6, IL-1, TNF-alpha, GM-CSF

Cause the release of chemokines that recruit neutrophils and macrophages

Cause secretion of anti-microbial proteins from cells in inflamed tissue (especially barrier tissues like skin or mucosa)

Question 23

Th17 do their main jobs where?

Answer 23

Outside of the SLOs

Question 24

Transcription factor that polarizes a Th17 cell after it is exposed to IL-6, TGF-beta, and IL-23:

Answer 24

ROR-gammat:
-Presence of this TF tends to prevent the Th17 cell from secreting non Th-17 cytokines

Question 25

In many ways, the activity of Th1 and Th17 cells overlap:

Answer 25

-Activation of macrophages and increased phagocytosis (recruitment of neutrophils and macrophages into inflamed tissue)

-Less likely to enhance antibody secretion: Th1 cells only encourage one particular antibody subtype. Th17 cells do not stimulate B-lymphocytes to produce antibodies.

-Some Th cells may secrete cytokines that overlap between Th1 and Th17 subtypes

-Therefore, immunologists often talk about Th1 & Th17 responses as a general “Type 1 response”

Question 26

______________________ will influence B-cells to make particular types of antibodies that accomplish particular tasks, but cannot aid other important B-cell activities.

Answer 26

ex: Th1 cells cause B-cells to form antibodies that are good at opsonizing targets

-They dont help B-cells divide very well, nor can they aid “antibody refinement” in germinal centers of SLOs.

Follicular Th cells (Tfh) are the main Th cell that aids full B-cell development int he SLOs

Question 27

What do the Tfh cells do?

Answer 27

-Stay in the SLOs for the duration of their activation

-Remain associated with the B-cells and induce the formation of germinal centers in the SLOs

Question 28

2 rounds of antibodies that we experience in infection that our immune system has trouble clearing:

Answer 28

Round 1: lower affinity antibodies, many epitopes

Round 2: B-cells “reshuffle” their antibodies and the ones that are very high affinity are selected to reproduce in germinal centers

Known at affinity maturation and is crucial.

Without Tfh, it does not happen.

Question 29

2 major steps if Tfh polarization:

Answer 29

1) Naive Th interacts with dendritic cells in SLOs and becomes the Tfh effector cell.
-(signal 1) TCR-HLA2 & (signal 2) CD28-CD80/86 interaction w/ limited Li-2 production (LI-2 inhibits the Tfh development.

2) Tfh migrates to the B-cell region of the SLO and interacts with B-cells that present antigen via their HLA-2
-Tfh stays in the follicle and “prefers” the B-cell

Question 30

Where does the naive Th cell and dendritic cell interact?

Answer 30

Occurs in main T-cell/DC zone of the SLO

Typical TCR-HLA2 interaction w/ the DC expressing costimulators CD80/86 that bind to CD28 on the Th

Polarizing cytokines in humans: IL-12(secreted by DCs)
IL-6 polarizing cytokine in mice.

New Tfh produces little IL-2

IL-12 and weak IL-2 signal => expression of the main lineage-determining transcription factor for Tfh-BcI6

Question 31

The new Tfh expresses a chemokine that alows it to migrate to the :

Answer 31

B-cell zone of an SLO (fyi: CXC5)

Tfh interacts witht he B-cell via TCR-HLA2 & two other new costimulator interactions:
-iCOSL (iCOS ligand) o B-cells interacts with iCOS on Tfh
-CD40L (CD40 ligand) on teh Tfh interacts with CD40 on B-cells (stimulates the B-cell to increase antibody production)

Question 32

After the second Th-APC interaction, the Tfh will secrete typical Tfh effectors cytokines.

Major Tfh cytokine:

Answer 32

IL-21

IL-21 and CD40-CD40L are key for full B-cell maturation and development of the germinal centers where most antibodies are produced.

Most B-cells will die prematurely without these.

Question 33

Other cytokines that can influence antibody type of B-cell development secreted Tfh:

Answer 33

IL-4
TNF
IL-2
Can provide IL-2 stimulation to promote B-cell division

Question 34

Step 1 of Tfh:

Answer 34

Question 35

Step 2 of Tfh:

Answer 35

Question 36

Follicular helper T-cells differ from other polarized Th cells:

Answer 36

They stay in the SLO, interacting continually and closely with B-cells.
-Without Tfh, we do not produce germinal centers in our SLOs and our antibody effectiveness is poor.

Question 37

Tfh undergo 2 separate rounds of interactions with APCs:

Answer 37

1st round: polarizes the naive Th so it becomes a Tfh, the APC is a dendritic cell.

2nd round: Provides specific help to the B-cells (Also APCs)
-Only helps the B-cells that have had their receptors (antibodies) bound to antigen.

Question 38

When the B-cell/antibody response to the infection is concluded, Tfh cells often gain the ability to switch types later:

Answer 38

same cell could become a Th1 cell later, under the right circumstances.

Question 39

Cell-mediated or antibody response?

Answer 39

Question 40

Th1 or Th2?

Answer 40

Question 41

The development of regulatory T-cells is unique in that their effector activities ____________ effector activities of other immune cells.

Answer 41

-Macrophages and other APCs
-Th cells

Question 42

T regulatory cells arise in environments where APCs present an antigen in a cytokine environment that is predominantly _______________________

Answer 42

Anti-inflammatory (no inflammatory cytokines)
-Lots of TGF-beta
-Very little or no IL-6, IL-23, IL-12, IL-1…etc…

Question 43

T regulatory cells:
Interpreted as an environment where “self” is being presented, but there is no sign of “foreign invasion”:

Answer 43

Animals or humans that cannot produce effective Tregs usually develop severe autoimmune disease.

Question 44

T regulatory cell Inducing and polarizing cytokine:

Answer 44

TGF-beta
(and no inflammatory cytokines)

Question 45

Transcription factor that causes a Th to become a Treg:

Answer 45

FoxP3

Question 46

Effector functions of the Treg:

Answer 46

Secretion of anti-inflammatory cytokines: TGF-beta, IL-10
(no other anti-inflammatory cytokines, not a complete list)

Down regulation of CD80/86 signaling by binding to Treg CTLA-4

Soaking up & stealing IL-2 from other effector T-cells.

Question 47

When a naive Th cell makes a decision to become polarized, the key event is expression of the determining transcription factor (TF):

Answer 47

ex: FoxP3 in a Treg, Tbet in a Th1

Question 48

The determining transcription factor prevents:

Answer 48

Production of other determining TFs
-ex) Tbet prevents the expression of GATA-3, FoxP3 prevents the expression of RORgammat

Production of cytokines other than those that are typical of that Th type.

Question 49

Cytokines from one Th can prevent the proliferation or survival of other Th types:

Answer 49

ex) IFN-gamma can negatively regulate Th2 cell types

Process known as cross-regulation

Question 50

Know and understand this overview:

Answer 50

Question 51

Cytotoxic T cells that express abnormal intracellular antigens on HLA-1 (virally-infected cells or malignant cells):

Answer 51

Mechanism of killing is similar to NK cells: granzyme/perforin and Fas/FasL interactions witht he infected or abnormal cell.

Question 52

Cytotoxic T-cells are activated by Th1 cells in lymphatic tissues and are dependent on two signals:

Answer 52

An activating signal from a dendritic cell that has been licensed to present antigen.
-Often these dendritic cells engage in cross presentation (ex: extracellular antigens displayed on HLA-1, intracellular antigens displayed on HLA-2
-Dendritic cells activate naive CD8+ T-cells via HLA-1

An activating signal from a Th1 cell in the near vicinity of the cytotoxic T-cell and its activating dendritic cell.

Question 53

Activation of a naive CD8+ cytotoxic T-cell:

Answer 53

Question 54

Once a CD8+ T-cell has differentiated into an armed effector cell, encounter with its specific Ag results in immune attack without the need for costimulation:

Answer 54

No CD80/86 and CD28 interaction necessary

HLA-1 and TCR interaction is still needed, since HLA-1 is necessary to present Ag to CD8+ cytotoxic T-cells