Th2 Cells

Th2 cells are induced by parasitic worm infections and promote IgE-, mast cell- and eosinophilmediated destruction of these parasites (Fig. 6.8). The

signature cytokines of Th2 cells—IL-4, IL-5, and

IL-13—function cooperatively in eradicating worm

infections. Helminths are too large to be phagocytosed, so mechanisms other than macrophage activation are needed for their destruction. When Th2 and

related Tfh cells encounter the antigens of helminths,

the T cells secrete their cytokines. IL-4 produced by

Tfh cells stimulates the production of IgE antibodies,

which coat the helminths and thus help in their clearance. Eosinophils use their Fc receptors to bind to the

IgE and are activated by IL-5 produced by the Th2 cells,

Macrophage

APC

Naive

T cell

Bacteria

Th1

cells

Classical

macrophage

activation

(enhanced

microbial killing)

IFN-?

Fig. 6.5 Functions of Th1 cells. Th1 cells produce the cytokine interferon-? (IFN-?), which activates macrophages to kill

phagocytosed microbes (classical pathway of macrophage

activation). In mice, IFN-? stimulates the production of IgG

antibodies, but this has not been established in humans. APC,

Antigen-presenting cell.

126 CHAPTER 6 Effector Mechanisms of T Cell–Mediated Immunity

as well as by signals from these IgE-specific Fc receptors. Activated eosinophils release their granule contents, which are toxic to the parasites. IL-13 stimulates

mucus secretion and intestinal peristalsis, increasing

the expulsion of parasites from the intestines. IgE also

binds to mast cells and is responsible for their activation, leading to the secretion of chemical mediators

that stimulate inflammation and proteases that destroy

toxins.

Th2 cytokines inhibit classical macrophage activation and stimulate the alternative pathway of macrophage activation (Fig. 6.9). IL-4 and IL-13 shut down

the activation of inflammatory macrophages, thus terminating these potentially damaging reactions. These

cytokines also can activate macrophages to secrete

growth factors that act on fibroblasts to increase collagen synthesis and induce fibrosis. This type of macrophage response is called alternative macrophage

Activation of

macrophage

Activation of

effector cell

Responses of activated macrophages

CD40L CD40

CD40

CD4+ effector

T cell (Th1 cell)

Macrophage

with ingested

bacteria

IFN-?

receptor

ROS,

NO

Macrophage response Role in cell-mediated immunity

Increased expression of B7 costimulators,

MHC molecules

Secretion of cytokines (TNF, IL-1, IL-12)

and chemokines

Production of reactive oxygen species,

nitric oxide, increased lysosomal enzymes

Killing of microbes in phagolysosomes

(effector function of macrophages)

TNF, IL-1, chemokines: leukocyte

recruitment (inflammation)

IL-12: Th1 differentiation, IFN-? production

Increased T cell activation (amplification

of T cell response)

Increased expression

of MHC and

costimulators

(B7 molecules)

Secretion

of cytokines

(TNF, IL-1, IL-12,

chemokines)

Killing of

phagocytosed

bacteria

A

B

IFN-?

Fig. 6.6 Activation of macrophages by Th1 lymphocytes. Effector T lymphocytes of the Th1 subset recognize the antigens of ingested microbes on macrophages. In response to this recognition, the T lymphocytes express CD40L, which engages CD40 on the macrophages, and the T cells secrete interferon-? (IFN-?),

which binds to IFN-? receptors on the macrophages. This combination of signals activates the macrophages

to produce microbicidal substances that kill the ingested microbes. Activated macrophages also secrete

tumor necrosis factor (TNF), interleukin-1 (IL-1), and chemokines, which induce inflammation, and IL-12,

which promotes Th1 responses. These macrophages also express more major histocompatibility complex

(MHC) molecules and costimulators, which further enhance T cell responses. A, Illustration shows a CD4+ T

cell recognizing class II MHC–associated peptides and activating the macrophage. B, The figure summarizes

macrophage responses and their roles in cell-mediated immunity.

CHAPTER 6 Effector Mechanisms of T Cell–Mediated Immunity 127

activation, to distinguish it from classical activation,

which enhances microbicidal functions. Alternative

macrophage activation mediated by Th2 cytokines may

play a role in tissue repair following injury and may contribute to fibrosis in a variety of disease states.

Th2 cells are involved in allergic reactions to

environmental antigens. The antigens that elicit

such reactions are called allergens. They induce Th2

responses in genetically susceptible individuals, and

repeat exposure to the allergens triggers mast cell and

eosinophil activation. Allergies are the most common

type of immune disorder; we will return to these diseases in Chapter 11 when we discuss hypersensitivity

reactions. Antagonists of IL-5 are approved for the

treatment of asthma, and an antibody against the IL-4

receptor is approved for the allergic disease atopic dermatitis.

The relative activation of Th1 and Th2 cells in

response to an infectious microbe may determine the

outcome of the infection (Fig. 6.10). For example, the

protozoan parasite Leishmania major lives inside the

phagocytic vesicles of macrophages, and its elimination

requires the activation of the macrophages by L. major–

specific Th1 cells. Most inbred strains of mice make an

effective Th1 response to the parasite and are thus able to

eradicate the infection. However, in some inbred mouse

+

+

+

+

+

Dendritic cell

Dendritic cell

Dendritic cell

NK cell

Mast cells,

eosinophils

CD4+ T cell

IFN-?

A

B

C

Extracellular

fungi,

bacteria

Th1

cell

Th2

cell

Th17

cell

Intracellular

microbes

(mycobacteria)

Antigenactivated

T cell

Antigenactivated

T cell

Antigenactivated

T cell

IL-12

IL-4

STAT4

T-bet

STAT6

GATA-3

ROR?t

STAT3

IL-1

IL-6

IL-23

TGF-ß

Helminths

STAT1

Fig. 6.7 Development of Th1, Th2, and Th17 effector cells. Dendritic cells and other immune cells that

respond to different types of microbes secrete cytokines that induce the development of antigen-activated

CD4+ T cells into Th1 (A), Th2 (B), and Th17 (C) subsets. The transcription factors that are involved in T cell

differentiation are indicated in boxes in the antigen-activated T cells. IFN, Interferon-?; IL, interleukin; TGF-ß,

transforming growth factor ß; NK, natural killer.

128 CHAPTER 6 Effector Mechanisms of T Cell–Mediated Immunity

strains, the response to L. major is dominated by Th2

cells, and these mice succumb to the infection. Mycobacterium leprae, the bacterium that causes leprosy, is a

pathogen for humans that also lives inside macrophages

and may be eliminated by cell-mediated immune mechanisms. Some people infected with M. leprae are unable

to eradicate the infection, which, if left untreated, will

progress to a destructive form of the disease, called

lepromatous leprosy. By contrast, in other patients, the

bacteria induce strong cell-mediated immune responses

with activated T cells and macrophages around the

infection site and few surviving microbes; this form of

less injurious infection is called tuberculoid leprosy.

The tuberculoid form is associated with the activation

of M. leprae–specific Th1 cells, whereas the destructive

lepromatous form is associated with a defect in Th1 cell

activation and sometimes a strong Th2 response. The

same principle—that the T cell cytokine response to an

infectious pathogen is an important determinant of the

outcome of the infection—may be true for other infectious diseases.

Development of Th2 Cells

Differentiation of naive CD4+ T cells to Th2 cells is

stimulated by IL-4, which may be produced by mast

cells, other tissue cells, and T cells themselves at sites

Eosinophil

activation

Helminth

Eosinophil

IgE IgG4 (human),

IgG1 (mouse)

Antibody

production

Alternative

macrophage activation

(enhanced fibrosis/

tissue repair)

Th2 cells

Mast cell

degranulation Intestinal mucus

secretion and

peristalsis

IL-4,

IL-13

B cell

Helminths or

protein antigens

APC

Naive CD4+

T cell

Proliferation and

differentiation

Macrophage

IL-4

IL-5

IL-4,

IL-13

Tfh

cell

Fig. 6.8 Functions of Th2 cells. Th2 cells produce the cytokines interleukin-4 (IL-4), IL-5, and IL-13. IL-4

(and IL-13) act on B cells to stimulate production mainly of IgE antibodies, which bind to mast cells. Help for

antibody production may be provided by Tfh cells that produce Th2 cytokines and reside in lymphoid organs

and not by classical Th2 cells. IL-5 activates eosinophils, a response that is important in the destruction of

helminths. APC, Antigen-presenting cell; Ig, immunoglobulin; IL, interleukin.

CHAPTER 6 Effector Mechanisms of T Cell–Mediated Immunity 129

of helminth infection (see Fig. 6.7B). IL-4 activates

the transcription factor Stat6 and antigen-induced

signals in combination with IL-4 induce expression

of a transcription factor GATA-3, which is required

for Th2 differentiation. Analogous to Th1 cells, these

transcription factors stimulate the expression of Th2

cytokines and proteins involved in cell migration

and thus promote Th2 responses. IL-4 produced by

Th2 cells enhances further Th2 differentiation, thus

amplifying the Th2 response.

Th17 Cells

Th17 cells develop in response to extracellular bacterial and fungal infections and induce inflammatory

reactions that destroy these organisms (Fig. 6.11). The

major cytokines produced by Th17 cells are IL-17 and

IL-22. This T cell subset was discovered during studies of

inflammatory diseases, many years after Th1 and Th2 subsets were described, and its role in host defense was established later.

The major function of Th17 cells is to stimulate the recruitment of neutrophils and, to less

extent, monocytes, thus inducing the inflammation

that accompanies many T cell–mediated adaptive

immune responses. Recall that inflammation also

is one of the principal reactions of innate immunity (see Chapter 2). Typically, when T cells stimulate inflammation, the reaction is stronger and more

prolonged than when it is elicited by innate immune

responses only. IL-17 secreted by Th17 cells stimulates the production of chemokines from other cells,

and these chemokines are responsible for leukocyte

recruitment. Th17 cells also stimulate the production

of antimicrobial substances, called defensins, that

ROS, NO,

lysosomal enzymes IL-10,

TGF-ß IL-1, IL-12,

IL-23,

chemokines

Monocyte

IL-13,

IL-4

Microbicidal

actions:

phagocytosis

and killing

of bacteria

and fungi Inflammation

Anti-inflammatory

effects, wound

repair, fibrosis

Classically activated

macrophage (M1)

Alternatively activated

macrophage (M2)

Microbial

TLR-ligands,

IFN-?

Fig. 6.9 Classical and alternative macrophage activation. Classically activated (M1) macrophages are

induced by microbial products binding to TLRs and cytokines, particularly interferon-? (IFN-?), and are microbicidal and proinflammatory. Alternatively activated (M2) macrophages are induced by interleukin-4 (IL-4) and

IL-13 (produced by certain subsets of T lymphocytes and other leukocytes) and are important in tissue repair

and fibrosis. The M1 and M2 populations may represent extreme phenotypes, and there may be other macrophage populations that express different sets of proteins. Also, in most immune responses, various mixtures

of these macrophages are likely induced. NO, Nitric oxide; ROS, reactive oxygen species; TGF-ß, transforming

growth factor ß; TLR, Toll-like receptor.

130 CHAPTER 6 Effector Mechanisms of T Cell–Mediated Immunity

function like locally produced endogenous antibiotics. IL-22 produced by Th17 cells induces epithelial

cell defensin production, helps to maintain the integrity of epithelial barriers and may promote repair of

damaged epithelia.

These reactions of Th17 cells are critical for defense

against fungal and bacterial infections, especially in

epithelial barrier tissues. These microbes can survive

outside cells but are rapidly destroyed once they are

phagocytosed, especially by neutrophils. Rare individuals who have inherited defects in Th17 responses are

prone to developing chronic mucocutaneous candidiasis and bacterial abscesses in the skin. Th17 cells are

also implicated in numerous inflammatory diseases,

and antagonists of IL-17 and of the Th17-inducing cytokine IL-23 are very effective treatments for psoriasis,

an inflammatory skin disease. An antagonist that neutralizes IL-12 and IL-23 (by binding to a protein shared

by these two-chain cytokines), and thus inhibits the

development of both Th1 and Th17 cells, is used for the

treatment of inflammatory bowel disease and psoriasis.

Development of Th17 Cells

The development of Th17 cells from naive CD4+ cells

is driven by cytokines secreted by dendritic cells (and

macrophages) in response to fungi and extracellular

bacteria (see Fig. 6.7C). Recognition of fungal glycans

and bacterial peptidoglycans and lipopeptides by innate

immune receptors on dendritic cells stimulates the

secretion of several innate proinflammatory cytokines,

including IL-1, IL-6, and IL-23. IL-6 and IL-23 activate

the transcription factor Stat3. Signals induced by these

innate inflammatory cytokines and another cytokine

called transforming growth factor ß (TGF-ß), in combination with TCR signals, induce the expression of the

transcription factor ROR?T. These transcription factors are required for Th17 differentiation. Interestingly,

TGF-ß is a powerful inhibitor of immune responses, but

Th1 cell

Th2 cell

Macrophage

activation:

cell-mediated

immunity

Naive

CD4+

T cell

Infection Response Outcome

Most mouse strains: Th1

BALB/c mice: Th2

Recovery

Disseminated

infection

Some patients: Th1 Tuberculoid leprosy

Leishmania

major

Mycobacterium

leprae Some patients: Defective

Th1 or dominant Th2

Lepromatous leprosy

(high bacterial count)

IFN-?, TNF

IL-4, IL-13

Inhibits

microbicidal

activity of

macrophages

Fig. 6.10 Balance between Th1 and Th2 cell activation determines outcome of intracellular infections. Naive CD4+ T lymphocytes may differentiate into Th1 cells, which activate phagocytes to kill ingested

microbes, and Th2 cells, which inhibit classical macrophage activation. The balance between these two subsets may influence the outcome of infections, as illustrated by Leishmania infection in mice and leprosy in

humans. IFN, Interferon; IL, interleukin; TNF, tumor necrosis factor.

CHAPTER 6 Effector Mechanisms of T Cell–Mediated Immunity 131

when present together with IL-6 or IL-1, it promotes the

development of Th17 cells.

DIFFERENTIATION AND FUNCTIONS OF

CD8+ CYTOTOXIC T LYMPHOCYTES

Phagocytes are best at killing microbes that are confined to vesicles, and microbes that directly enter the

cytosol (e.g., viruses) or escape from phagosomes into

the cytosol (e.g., some ingested bacteria) are relatively

resistant to the microbicidal mechanisms of phagocytes. Eradication of such cytosolic pathogens requires

another effector mechanism of T cell–mediated immunity: CD8+ CTLs. CTLs also serve a vital role in defense

against cancers (see Chapter 10).

CD8+ T lymphocytes activated by antigen and

other signals differentiate into CTLs that are able to

kill infected cells expressing the antigen. Naive CD8+

T cells can recognize antigens but are not capable of killing antigen-expressing cells. The differentiation of naive

CD8+ T cells into fully active CTLs is accompanied by

the synthesis of molecules involved in cell killing, giving

these effector T cells the functional capacity that is the

basis for their designation as cytotoxic. CD8+ T lymphocytes recognize class I MHC–associated peptides

on infected cells and tumor cells. The sources of class I–

associated peptides are protein antigens synthesized

in the cytosol and protein antigens of phagocytosed

microbes that escape from phagocytic vesicles into the

cytosol (see Chapter 3). In addition, some dendritic cells

may capture the antigens of infected cells and tumors,

transfer these antigens into the cytosol, and thus present the ingested antigens on class I MHC molecules, by

the process known as cross-presentation (see Fig. 3.16,

Chapter 3). The differentiation of naive CD8+ T cells into

functional CTLs and memory cells requires not only antigen recognition but also costimulation and, in some situations, help from CD4+ T cells (see Fig. 5.7, Chapter 5).

CD8+ CTLs recognize class I MHC–peptide complexes on the surface of infected cells and kill these

cells, thus eliminating the reservoir of infection. The

T cells recognize MHC-associated peptides by their

TCR and the CD8 coreceptor. These infected cells also

are called targets of CTLs, because they are destroyed

by the CTLs. The TCR and CD8, as well as other signaling proteins, cluster in the CTL membrane at the site of

contact with the target cell and are surrounded by the

leukocyte function–associated antigen 1 (LFA-1) integrin. These molecules bind their ligands on the target cell,

forming an immune synapse (see Chapter 5).

Antigen recognition by CTLs results in the activation

of signal transduction pathways that lead to the exocytosis of the contents of the CTL’s granules into the synapse

between the CTL and the target cell (Fig. 6.12). Because

all nucleated cells express class I MHC, and differentiated CTLs do not require costimulation or T cell help for

activation, the CTLs can be activated by and are able to

kill any infected cell in any tissue. CTLs kill target cells