T-Cell Mediated Immunity

Regulation and termination of immune responses

Lecture Objectives

Students should know:

What molecules of T cells are involved in T cell activation?

Priming and effector phases of T cell responseAntigen presenting cellsImmature versus mature dendritic cellsCo-stimulation signal/CD28/AnergyWhat happens in T cells after TCR activation?Th1 cells/Cell-mediated immunityGranulomaTh2 cells/ Humoral ImmunityRegulatory (suppressor) T cellsFunctions of Th1/2 cell cytokines Naïve versus memory T cells?Memory and effector T cells?

T cells:

Development in thymus

Migration out of thymus into blood(Naïve T cells)

Priming phase: Antigen-dependent differentiation in lymphoid tissues(to become memory and effector T cells)

Effector phase: Migration to sites of infection and effector function

Human:

Fetal development

Birth

School

Job

Figure 6-1

Figure 6-2

Green: MHC class IIRed: Lysosomal protein

Naïve T cells enter secondary lymphoid tissues and encounter antigens presented on dendritic cell MHC

Infection at skin: Dendritic cells pick up antigen, undergo maturation, and migrate to 2o LT.

Microorganisms in the blood:Trapped in spleen.

Infection at mucosal surface:Collected in tonsils, Peyer’s patch, appendix and Bronchial-associated lymphoid tissues .

In lymph nodes, naïve T cells migrate from blood to LN through specialized blood vessels called HEV (high endothelial venules).

Figure 6-5

Naive and some memory T cells express L-selectin to adhere on HEV in lymph nodes.

Each leukocyte subset expresses unique sets of adhesion molecules and chemokine receptors.

Endothelial cells in different tissues express different counter-receptors.

Multi-step homing process into LT

Naïve T cells express L-selectin and a chemokine receptor CCR7.Endothelial cells express L-selectin receptors (GlyCAM-1 and CD34) and CCR7-activating chemokines SLC and ELC.

Rolling (L-selectin) chemokinefirm adhesiondiapedesis

Integrins

Fluorescently labeled T cells migrating into Peyer’s patches

Kim lab

Interactions between CD4 and MHC II-expressing antigen presenting cells:

(T cell left : APC right)

(i) LFA-1 : ICAM-1 (adhesion)(ii) CD2 : LFA-3 (adhesion)(iii) CD4 (Lck) : MHC II (co-activation with TCR)(iv) TCR/CD3 : MHC II/peptide (activation of T cell)(v) CD28 : B7-1/B7-2 (co-activation with TCR)

(h) End result : activation of transcription factors.

CORE

Interactions between CD8 and MHC I-expressing antigen presenting cells:

(T cell left : APC right)

(i) LFA-1 : ICAM-1 (adhesion)

(ii) CD2 : LFA-3 (adhesion)

(iii) CD8 (Lck) : MHC I (co-activation with TCR)

(iv) TCR/CD3 : MHC I / peptide (activation of T cell through PIP2/DAG, IP3 pathway)

(v) CD28 : B7-1/B7-2 (co-activation with TCR)

CORE

Interaction of T cells and APCInteraction between TCR and Ag/MHC alone is not strong enough to sustain the contact between T cells and APCIntegrins and their receptors on T and APC strengthen the interaction so that TCR and CD28 receptors on T cells can receive prolonged and stable signals.Signals through the integrins on T cells also enhance T cell activation.

TCR-Ag/MHC

TCR-Ag/MHC interaction signals for LFA-1 to bind more firmly to ICAM-1

This is to ensure stable and prolonged interaction between T cells and APC.

Co-stimulatory receptor: CD28 (or CTLA4):B7

For full activation, T cells require signals from both TCR and CD28.

TCR activation alone is not sufficient to activate T cells.

CD28 and CTLA4 are T cell receptors for B7 molecules on APC.

CD28 is expressed by all T cells

CTLA4 is expressed by activated T cells.

CTLA4:B7 interaction negatively regulate T cell activation, limiting T cell activation and proliferation.

CD28: AcceleratorCTLA4: Brake

Figure 6-10

Professional antigen presenting cells

They are dendritic cells, B cells and macrophages

They express high levels of MHC I and II molecules

They express co-stimulatory molecules (e.g. B7)

They are differently distributed in lymphoid tissues (LT).

Dendritic cells are particularly important for initiation of T cell activation in lymphoid tissues.

Only these cells express B7

Different features of the three types of APC

Macrophages are good at presenting extracellular bacteriaDCs present bacterial and viral antigensB cells can efficiently present soluble antigens

Figure 6-12

Bacterial products such as LPS and cell wall products, and cytokines such as TNF-a differentiate immature DCs to mature DCs. Mature DCs express CCR7 and migrate from sites of infection to LT.Immature DCs are better in phagocytosis but lack co-stimulatory activity.Mature DCs express higher levels of MHCs and B7. They are better in antigen presentation and T cell activation but lack the phagocytic ability.

DC maturation

Figure 6-16

T cell activation process

CD4 for adhesion to MHC II and signal transduction (lck)Phosphorylation cascade occurs at early stage of signal tranduction

Figure 6-17 part 1 of 2

Intracellular events during T cell activation

Memorize the process

Major events involved in T-cell activationEvent Example

Cell-cell interaction T cell-APCCTL-target cell

Receptor-ligand binding TCR-antigen/MHC

Transmembrane signal transduction Activation of Lck

Generation of second messengers 1,4,5-IP3 and DG

Second-messenger effects Ca2+ mobilizationProtein kinase C activation

Biochemical pathways Phosphatidylinositol pathwayRas pathway

Cellular events Secretion of cytolytic granules

Early gene activation c-Myc, c-Fos

Intermediate gene activation Lymphokines, lymphokine receptors, nutrient receptors

Late gene activation Genes involved in cell proliferationIL-2, IL-2R, VLA-2 etc

A consequence of T cell activation:Enhanced IL-2 production and induction of IL-

2Rexpressionfor T cell proliferation

Immuno-suppressive drugs such as cyclosporin A and FK506 block NFAT activation induced by TCR activation, thus blocking production of IL-2.

Therefore these drugs inhibit T cell proliferation and activation.

Function of IL-2

Figure 6-23

Activated T cell express memory/effector type molecules

TCR activation without CD28/B7 co-stimulation leads to T cell anergy (unresponsiveness)

B7 is expressed only by activated professional antigen presenting cellsTherefore, recognition of Ag presented by immature APC (which don’t express B7) can induce T cell anergy.

c. Functional subsets of CD4 cells based on cytokine profile.

(1) T naive cell = Low levels of IL-2, 4, 5, 6, 10, 13, TNF-alpha, IFN-γ (these may not be detectable; all activated T cells produce IL-2)

(2) TH1 = IFN-γ, IL-2(Type 1 cytokines promote cell mediated immunity)

(3) TH2 = IL- 2, 3, 4, 5, 9, 13 (Type 2 cytokines promote humoral immunity)

(4) TH17 = IL-17, 22 (promote cutaneous inflammation)

(5) Subset control: TH1 polarization is promoted by IL-12 and IFN-γ IL-4 &10 inhibit.

TH2 polarization is promoted by IL-4; IFN-γ inhibits

TH17 polarization is promoted by IL-1 and IL-23; IL-4 and IFN-γ inhibit

CORE

T cell differentiation to Th1 or Th2

IgE response for defense against parasites or allergic responses is mediated by Th2 cells

Cell-mediated Humoral/Ig response

Regulatory T cells: suppress the function of Th1/2 cells

T cells can differentiate to Th1, Th2 or other T cell subsets.Many people believe that only Th2 cells can induce humoral immune response, while Th1 cells induce only CMI. But this is an overly simplified text book style view.Actually Th1 and Th2 cells are required for different aspects of humoral immune response.

TCR activation by APC

Thymus

Th1

Th2

IL-4

IFN

-

Tnp(Nonpolarized T cells)

Anti-microbial

Anti-parasitic

Th1 and Th2 cells produce different cytokines

Th0 (IL4+IFN-+)

Effector T cell subsets

Tregs

TGF1IL10etc

FoxP3Target

Activation signal

No activation

IL4 makes Th2, while IL12 makes Th1 cells

Cytokine secretion and biological activities of TH1 and

TH2 Subsets

Type 1Type 1 Type 2Type 2

IFN-IFN-IL-4IL-4IL-5IL-5IL-13IL-13

Cell-mediatedCell-mediatedImmune Immune responseresponse(intracellular(intracellularOrganisms)Organisms)

Some humoral Some humoral responses that responses that promote promote opsonizationopsonization

Anti parasitic, Anti parasitic, and IgE and IgE responsesresponses

General humoral General humoral responseresponse

T cellT cell

Figure 6-34

Very useful information !

&spleen

12.Cell mediated immune (CMI) responses

a. Function:(1) Control intracellular pathogens, tumors (2) Mediate transplant rejection(3) Mediate Type IV hypersensitivity(4) Contribute to granuloma formation(5) Contribute to chronic inflammation

Th1 cells regulate CMI

CORE

12. CMIb. Predominant cells and mediators

(1) TH1 lymphocytes (activated by Ag, IL-12 and IFN release IL-2 and IFN- induce TC, NK, and macrophage activation

(2) CD8+ TC1 and TC2 (activated by Ag/MHC I [endogenous pathway] and TH1 cytokines) have cytokine profile similar to TH1 and TH2, respectively; both are cytotoxic via TNFβ, perforin, granzymes, and Fas-induced apoptosis

(3) NK cells (see above for activation, etc.) release IFN-which activates macrophages and stimulates TH1 activity; also release GM-CSF that stimulates PMNs; lyse targets predominantly via granule components (e.g. perforin)

(4) Macrophages: next page

CORE

Monocytes/macrophages [mononuclear phagocytes] (activated by IFN and TLR ligands [e.g. LPS] or other cytokines [e.g. GM-CSF, TNF) (a) Tissue types: Kupffer, alveolar, etc. (b) Surface markers: C3b receptor (CR-1), LFA-1, Fc receptor, MHC II

(c) Macrophages play a central role in CMI and inflammation: (i) Antigen presenting cell, important in killing bacteria and lysing tumor cells

(ii) effector/mediator of delayed type hypersensitivity (DTH, see Type IV hypersensitivity)(iii) release IL-12 & induces TH1 response(iv) release IL-10 & suppress immune responses(v) release IL-1, 6, TNF and , INF and (vi) release enzymes, coagulation factors, complement components, superoxide (etc.), leukotrienes, and prostaglandins

12 b (4) Macrophages

CORE

Mediators of Cell Mediated ImmunityTh1 cells: DCs present Ag to naïve T cells and activate them. IL-12 produced from dendritic cells induce polarization of T cells into Th1. Produce IFN-g which activates CD8 T, NK and macrophages

CD8 T cells: also called T cytotoxic T cells. Kill virus-infected or tumor cells by TNF-b, perforin, granzymes and FAS ligand.

NK cells: IL-2, IL-12 and IFN-g activate NK cells to make them more efficient killer cells.

Macrophages: They are also activated by Th1 cytokine IFN-. Activated macrophages are more efficient in killing of engulfed or intracellular pathogens. They produce IL-12, IL-10, IL-1, IL-6, TNF- and IFN-, and release inflammatory mediators (coagulation factors, complement components, superoxide, leukotrienes and prostaglandins).

CD8 T cells (also called cytotoxic T cells, Tc or CTL)

• Cytolytic activity of CTL is promoted by IL-2, IL-12 & IFN

• Activated CTLs lyse targets with TNF, degranulation (perforin, granzymes), and/or Fas-induced apoptosis.

Cytotoxic CD8 T cells detect infected cells through TCR/Ag recognition and kill them

CD8+ T cells need the CD28 signal from APC at priming stage to become effector CD8+ cells,

But effector CD8+ cells don’t need the CD28 signal (at the effector stage) to kill target cells (APOPTOTIC cell death).

TCR activation polarization of CD8+ T cell cytoskeleton release lytic granules to kill target cells

Cytotoxins of CD8+ cells:

GranzymeGranulolysinPerforins

FAS-ligand: activates FAS to induce apoptosis

12 c.Role of endothelium Inflamed enothelial cells express P- and E-selectin and recruit

effector cells that mediate CMI (i.e. phagocytosis and cell killing)

d. Detection of cell killing activity by CD8 T cells:Cytolytic assays (51Cr release assay)

51CrCD8 51CrCD8

released

Target cells

51Cr

1. Labeling 2. Co-culture 3. Lysis & release of 51Cr

CORE

Figure 6-33

Th1 cells activate macrophages, making them highly microbicidal

GranulomaAggregated infected macrophages surrounded by T cells

A sign of chronic microbial infection

Pathogens persist in macrophages for long time

Insufficient macrophage activation by Th1 cells

Typical in M. tuberculosis infection

It is a defense mechanism that limits the spread of pathogens

A compromised situation between pathogens and the immune system

IFN- promotes antigen presentation, Th1 cell development, production of opsonizing Ab and macrophage activation

Functions of Th2 cells

Figure 6-36

Th2 cells activate B cells through cytokines and CD40L for IgG and IgE response

IgE, IgG4

Th1/2 choice made by the immune system has profound effect on the outcome of some diseases

(e.g. Leprosy)

Tuberculoid Leprosy

Infection by Mycobacterium leprae.

Th1 cells produce IFN-Activation of Macrophages

Destruction of intracellular bacteria.

Milder damage.

Patients survive.

Lepromatous Leprosy

Infection by Mycobacterium leprae.

Th2 cells cannot activateMacrophages.

Unchecked proliferation of intracellular bacteria.

Gross destruction of tissues.

Fatal.

Figure 6-21

Th17 cellsAnother effector T cell subset

Induced from naïve CD4+ or CD8+ T cells

Induced by IL-1, IL-6, or IL-23

Produce IL-17 and IL-22;IL-17 induces expression of G-CSF and chemokines for inflammation with neutrophils; IL-22 induces anti-bacterial proteins.

Important for anti-bacterial and anti-fungal infcetion; and chronic autoimmune inflammation in skin, joints and CNS

IL-2, IL-4 and IFN- inhibit the generation of Th17 cells

e. Regulatory T cells (also called Tregs or FoxP3+ cells)

• Specialized T cells that suppress immune cells such as T cells (naïve, Th1 and Th2), B cells, dendritic cells, macrophages etc.

• FOXP3+ CD4+ CD25+T cells constitute a major subset of regulatory cells. • Express CD25 and CTLA4• Produce TGF-β, and/or IL-10 to suppress target cells.• Prevent autoimmune diseases

• Regulatory T cell deficiency due to mutations of FOXP3 causes immune dysregulation, polyendocrinopathy, enteropathy, X linked syndrome (called IPEX or XLAAD) autoimmune diseases in multiple organs

• FOXP3 is the master transcription factor for regulatory T cells

• Commonly called Tregs.

Regulatory T cells

Effector T cells

InfectionCancer

Hyper-responses orAutoimmune diseases

Balanced for health

Too much Too much

TGF-1IL-10

IL-4/5/13IFN-

13. Regulation and termination of immune responses

a. Regulation by the nature of the antigen (e.g. polysaccharidesIgM)

b. Regulation by individual history (and genetic background), antigen dose and route of administration(e.g. oral immunization tolerance)

c. Regulation by antigen presenting cell (1) Co-stimulatory signals (e.g. B7 ligands for CD28 or CTLA4;

Activated T cells express CTLA4 for downregulation of T cell activation)(2) Suppressive cytokines (e.g. IL-10 and TGF-beta)

d. Modulation by antibody Immune complexes, receptor cross-linking, and activation and suppressive Fc

receptors (e.g. Fc receptors of NK cells and B cells)

e. T cells (regulatory T cells and other cells)(1) Th1/2 and Tc1/2 subsets and cytokine control (e.g. IL4 versus IFN-g)(2) soluble forms of TCRs (?) and cytokine receptors(3) CD4+CD25+ regulatory T cells suppress immune responses(4) FAS/FASL-mediated lymphocyte apoptosis

(activated lymphocytes express FAS, susceptible to cell death)

CORE

13. Regulation and termination of immune responses

f. Neuroendocrine controls(1) hormonal controls (e.g. stress-induced immunosuppression

by glucocorticoids and catecholamines.

g. Tolerance(1) Central tolerance (positive and negative selection)(2) Peripheral tolerance (clonal deletion, apoptosis and anergy)

CORE

Brain

Hypothalamus

Pituitary

Adrenal gland

Immune cells (through specific receptors)(results: suppression of immune cells)

Testes/ovary

CortisolCorticosterone

testosterone, estradiol, progesterone

Corticotropin

LH/FSH

Neuroendocrine controls Negative regulation of the immune system by stress hormones and sex hormones

Stress pathway

Reproductive pathway

CNS and Liver

Activation of the immune system

TNF-/IL-1/IL6

fever, sleepiness, fatigue, loss of appetite decreased libido

Regulation of the CNS by inflammatory cytokines

StressCortisol

Infection

CNS regulates the immune system (IS): The central nervous system affects the immune system through the neuroendocrine humoral outflow via the pituitary, and through direct neuronal influences via the sympathetic, parasympathetic (cholinergic) and peptidergic/sensory innervation of peripheral tissues. Thus, circulating hormones or locally released neurotransmitters and neuropeptides regulate major immune functions such as antigen presentation, secretion of cytokines, chemokines and antibodies, selection of T helper (Th)1 or Th2 responses, lymphocyte activity, proliferation and traffic.

IS regulates CNS: Certain cytokines such as interleukin (IL)-1, IL-6 and tumor necrosis factor (TNF)-a , released during an immune response activate the central components of the stress system, alter neurotransmitter networks activity and, thus induce fever, sleepiness, fatigue, loss of appetite and decreased libido.

IS affects the liver: Cytokines activate the hepatic synthesis of acute phase proteins - changes referred to as 'sickness behavior' and 'acute-phase response', respectively.

Adapted from http://www.endotext.org/adrenal/adrenal28/adrenalframe28.htmBy Ilia J. Elenkov