9

One of the remarkable properties of the normal immune

system is that it can react to an enormous variety of

microbes but does not react against the individual’s own

(self) antigens. This unresponsiveness to self antigens,

also called immunologic tolerance, is maintained

despite the fact that the molecular mechanisms by which

lymphocyte receptor specificities are generated are not

biased to exclude receptors for self antigens. In other

words, lymphocytes with the ability to recognize self

antigens are constantly being generated during the

normal process of lymphocyte maturation. Furthermore,

many self antigens have ready access to the immune

system, so unresponsiveness to these antigens cannot

be maintained simply by concealing them from lymphocytes. The process by which antigen-presenting cells

(APCs) display antigens to T cells does not distinguish

between foreign and self proteins, so self antigens are

normally seen by lymphocytes. It follows that there must

exist mechanisms that prevent immune responses to self

antigens. These mechanisms are responsible for one of

the cardinal features of the immune system—namely,

its ability to discriminate between self and nonself (usually microbial) antigens. If these mechanisms fail, the

immune system may attack the individual’s own cells

and tissues. Such reactions are called autoimmunity,

and the diseases they cause are called autoimmune

diseases. In addition to tolerating the presence of self

antigens, the immune system has to coexist with many

commensal microbes that live on the epithelial barriers of their human hosts, often in a state of symbiosis,

and the immune system of a pregnant female has to

accept the presence of a fetus that expresses antigens

Immunologic Tolerance

and Autoimmunity

CHAPTER OUTLINE

Immunologic Tolerance: General Priniciples and

Significance, 178

Central T Lymphocyte Tolerance, 178

Peripheral T Lymphocyte Tolerance, 180

Anergy, 181

Regulation of T Cell Responses by Inhibitory

Receptors, 182

Immune Suppression by Regulatory T Cells, 184

Deletion: Apoptosis of Mature Lymphocytes, 185

B Lymphocyte Tolerance, 187

Central B Cell Tolerance, 187

Peripheral B Cell Tolerance, 188

Tolerance to Commensal Microbes and Fetal

Antigens, 188

Tolerance to Commensal Microbes in the Intestines

and Skin, 188

Tolerance to Fetal Antigens, 189

Autoimmunity, 189

Pathogenesis, 189

Genetic Factors, 189

Role of Infections and Other Environmental

Influences, 191

Summary, 194

178 CHAPTER 9 Immunologic Tolerance and Autoimmunity

derived from the father. Unresponsiveness to commensal microbes and the fetus is maintained by many of the

same mechanisms involved in unresponsiveness to self.

In this chapter we address the following questions:

• How does the immune system maintain unresponsiveness to self antigens?

• What are the factors that may contribute to the loss

of self-tolerance and the development of autoimmunity?

• How does the immune system maintain unresponsiveness to commensal microbes and the fetus?

This chapter begins with a discussion of the important principles and features of self-tolerance. Then we

discuss the different mechanisms that maintain tolerance to self antigens, as well as commensal microbes

and the fetus, and how tolerance may fail, resulting in

autoimmunity.

IMMUNOLOGIC TOLERANCE: GENERAL

PRINICIPLES AND SIGNIFICANCE

Immunologic tolerance is a lack of response to antigens that is induced by exposure of lymphocytes

to these antigens. When lymphocytes with receptors

for a particular antigen encounter this antigen, any of

several outcomes is possible. The lymphocytes may be

activated to proliferate and to differentiate into effector and memory cells, leading to a productive immune

response; antigens that elicit such a response are said to

be immunogenic. The lymphocytes may be functionally

inactivated or killed, resulting in tolerance; antigens that

induce tolerance are said to be tolerogenic. In some situations, the antigen-specific lymphocytes may not react

in any way; this phenomenon has been called immunologic ignorance, implying that the lymphocytes simply

ignore the presence of the antigen. Normally, microbes

are immunogenic and self antigens are tolerogenic.

The choice between lymphocyte activation and tolerance is determined largely by the nature of the antigen

and the additional signals present when the antigen is

displayed to the immune system. In fact, the same antigen may be administered in different ways to induce

an immune response or tolerance. This experimental

observation has been exploited to analyze what factors

determine whether activation or tolerance develops as a

consequence of encounter with an antigen.

The phenomenon of immunologic tolerance is

important for several reasons. First, as we stated at the

outset, self antigens normally induce tolerance, and

failure of self-tolerance is the underlying cause of autoimmune diseases. Second, if we learn how to induce

tolerance in lymphocytes specific for a particular antigen, we may be able to use this knowledge to prevent

or control unwanted immune reactions. Strategies for

inducing tolerance are being tested to treat allergic and

autoimmune diseases and to prevent the rejection of

organ transplants. The same strategies may be valuable

in gene therapy to prevent immune responses against

the products of newly expressed genes or vectors and

even for stem cell transplantation if the stem cell donor

is genetically different from the recipient.

Immunologic tolerance to different self antigens

may be induced when developing lymphocytes

encounter these antigens in the generative (central)

lymphoid organs, a process called central tolerance, or when mature lymphocytes encounter selfantigens in peripheral (secondary) lymphoid organs

or peripheral tissues, called peripheral tolerance (Fig.

9.1). Central tolerance is a mechanism of tolerance only

to self antigens that are present in the generative lymphoid organs—namely, the bone marrow and thymus.

Tolerance to self antigens that are not present in these

organs must be induced and maintained by peripheral

mechanisms. We have only limited knowledge of which

self antigens induce central or peripheral tolerance or

are ignored by the immune system.

With this brief background, we proceed to a discussion of the mechanisms of immunologic tolerance and

how the failure of each mechanism may result in autoimmunity. Tolerance in T cells, particularly CD4+ helper

T lymphocytes, is discussed first because many of the

mechanisms of self-tolerance were defined by studies of

these cells. In addition, CD4+ helper T cells orchestrate

virtually all immune responses to protein antigens, so

tolerance in these cells may be enough to prevent both

cell-mediated and humoral immune responses against

self proteins. Conversely, failure of tolerance in helper T

cells may result in autoimmunity manifested by T cell–

mediated attack against tissue self antigens or by the

production of autoantibodies against self proteins.

CENTRAL T LYMPHOCYTE TOLERANCE

The principal mechanisms of central tolerance in T

cells are death of immature T cells and the generation

of CD4+ regulatory T cells (Fig. 9.2). The lymphocytes

CHAPTER 9 Immunologic Tolerance and Autoimmunity 179

that develop in the thymus consist of cells with receptors capable of recognizing many antigens, both self and

foreign. If a lymphocyte that has not completed its maturation interacts strongly with a self antigen, displayed

as a peptide bound to a self major histocompatibility

complex (MHC) molecule, that lymphocyte receives

signals that trigger apoptosis. Thus, the self-reactive cell

dies before it can become functionally competent. This

process, called negative selection (see Chapter 4), is a

major mechanism of central tolerance. The process of

negative selection affects self-reactive CD4+ T cells and

CD8+ T cells, which recognize self peptides displayed by

class II MHC and class I MHC molecules, respectively.

Why immature lymphocytes die upon receiving strong

T cell receptor (TCR) signals in the thymus, while

mature lymphocytes that get strong TCR signals in the

periphery are activated, is not fully understood.

Some immature CD4+ T cells that recognize self

antigens in the thymus with high affinity do not die

but develop into regulatory T cells and enter peripheral

Peripheral tolerance:

Peripheral tissues

Central tolerance:

Generative lymphoid organs

(thymus. bone marrow)

Lymphoid

precursor

Immature

lymphocytes

Mature

lymphocytes

Development

of regulatory

T lymphocytes

(CD4+ T cells

only)

Apoptosis

(deletion)

Apoptosis

(deletion)

Anergy Suppression

Change in

receptors

(receptor

editing;

B cells only)

Recognition of self antigen

Recognition of self antigen

Regulatory

T cell

Fig. 9.1 Central and peripheral tolerance to self antigens. Central tolerance: Immature lymphocytes specific for self antigens may encounter these antigens in the generative (central) lymphoid organs and are

deleted; B lymphocytes may change their specificity (receptor editing); and some T lymphocytes develop

into regulatory T cells. Some self-reactive lymphocytes may complete their maturation and enter peripheral

tissues. Peripheral tolerance: mature self-reactive lymphocytes may be inactivated or deleted by encounter

with self antigens in peripheral tissues or suppressed by regulatory T cells.

180 CHAPTER 9 Immunologic Tolerance and Autoimmunity

tissues (see Fig. 9.2). The functions of regulatory T cells

are described later in the chapter. What determines

whether a thymic CD4+ T cell that recognizes a self

antigen will die or become a regulatory T cell is also not

established.

Immature lymphocytes may interact strongly with an

antigen if the antigen is present at high concentrations

in the thymus and if the lymphocytes express receptors

that recognize the antigen with high affinity. Antigens

that induce negative selection may include proteins that

are abundant throughout the body, such as plasma proteins and common cellular proteins.

Surprisingly, many self proteins that are normally

present only in certain peripheral tissues, called tissuerestricted antigens, are also expressed in some of the

epithelial cells of the thymus. A protein called AIRE

(autoimmune regulator) is responsible for the thymic

expression of these peripheral tissue antigens. Mutations

in the AIRE gene are the cause of a rare disorder called

autoimmune polyendocrine syndrome. In this disorder,

several tissue antigens are not expressed in the thymus

because of a lack of functional AIRE protein, so immature

T cells specific for these antigens are not eliminated and

do not develop into regulatory cells. These cells mature

into functionally competent T cells that enter the peripheral immune system and are capable of reacting harmfully

against the tissue-restricted antigens, which are expressed

normally in the appropriate peripheral tissues even in

the absence of AIRE. Therefore, T cells specific for these

antigens emerge from the thymus, encounter the antigens

in the peripheral tissues, and attack the tissues and cause

disease. It is not clear why endocrine organs are the most

frequent targets of this autoimmune attack. Although

this rare syndrome illustrates the importance of negative

selection in the thymus for maintaining self-tolerance, it

is not known if defects in negative selection contribute to

common autoimmune diseases.

Central tolerance is imperfect, and some self-reactive

lymphocytes mature and are present in healthy individuals. As discussed next, peripheral mechanisms may

prevent the activation of these lymphocytes.

PERIPHERAL T LYMPHOCYTE TOLERANCE

Peripheral tolerance is induced when mature T cells

recognize self antigens in peripheral tissues, leading

to functional inactivation (anergy) or death, or when

the self-reactive lymphocytes are suppressed by regulatory T cells (Fig. 9.3). Each of these mechanisms of

peripheral T cell tolerance is described in this section.

Peripheral tolerance is clearly important for preventing

T cell responses to self antigens that are not present in

the thymus, and it also may provide backup mechanisms

for preventing autoimmunity in situations where central

tolerance to antigens that are expressed in the thymus is

incomplete.

Antigen recognition without adequate costimulation results in T cell anergy or death or makes T cells

Immature

T cells specific

for self antigen

Regulatory

T cell

Negative

selection:

deletion

Development

of regulatory

T cells

Thymus Periphery

Fig. 9.2 Central T cell tolerance. Strong recognition of self antigens by immature T cells in the thymus may

lead to death of the cells (negative selection, or deletion), or the development of regulatory T cells that enter

peripheral tissues.

CHAPTER 9 Immunologic Tolerance and Autoimmunity 181

sensitive to suppression by regulatory T cells. As noted

in previous chapters, naive T lymphocytes need at least

two signals to induce their proliferation and differentiation into effector and memory cells: Signal 1 is always

antigen, and signal 2 is provided by costimulators that

are expressed on APCs, typically as part of the innate

immune response to microbes (or to damaged host

cells) (see Chapter 5, Fig. 5.6). It is believed that dendritic cells in normal uninfected tissues and peripheral

lymphoid organs are in a resting (or immature) state, in

which they express little or no costimulators, such as B7

proteins (see Chapter 5). These dendritic cells constantly

process and display the self antigens that are present in

the tissues. T lymphocytes with receptors for the self

antigens are able to recognize the antigens and thus receive

signals from their antigen receptors (signal 1), but the T

cells do not receive strong costimulation because there

is no accompanying innate immune response. Thus, the

presence or absence of costimulation is a major factor

determining whether T cells are activated or tolerized.

Anergy

Anergy in T cells refers to long-lived functional

unresponsiveness that is induced when these cells

recognize self antigens (Fig. 9.4). Self antigens are

normally displayed with low levels of costimulators,

as discussed earlier. Antigen recognition without adequate costimulation is thought to be the basis of anergy

induction, by mechanisms that are described later.

Anergic cells survive but are incapable of responding

to the antigen.

The two best-defined mechanisms responsible for the

induction of anergy are abnormal signaling by the TCR

complex and the delivery of inhibitory signals from

receptors other than the TCR complex.

• When T cells recognize antigens without costimulation, the TCR complex may lose its ability to transmit

activating signals. In some cases, this is related to the

activation of enzymes (ubiquitin ligases) that modify

signaling proteins and target them for intracellular

destruction by proteases.

T cell

TCR

Regulatory

T cell

B7 CD28

Dendritic

cell

Effector

and

memory

T cells

Functional

unresponsiveness

Apoptosis

Block in

activation

T cell response

to microbes

and vaccines

Anergy

Deletion

Suppression

A

BMechanisms of tolerance

Fig. 9.3 Peripheral T cell tolerance. A, Normal T cell responses require antigen recognition and costimulation. B, Three major mechanisms of peripheral T cell tolerance are illustrated: cell-intrinsic anergy, suppression by regulatory T cells, and deletion (apoptotic cell death). TCR, T cell receptor.

182 CHAPTER 9 Immunologic Tolerance and Autoimmunity

• On recognition of self antigens, T cells also may preferentially use one of the inhibitory receptors of the

CD28 family, cytotoxic T lymphocyte–associated

antigen 4 (CTLA-4, or CD152) or programmed cell

death protein 1 (PD-1, CD279), which were introduced in Chapter 5. Anergic T cells may express

higher levels of these inhibitory receptors, which will

inhibit responses to subsequent antigen recognition.

The functions and mechanisms of action of these

receptors are described in more detail below.

Regulation of T Cell Responses by Inhibitory

Receptors

Immune responses are influenced by a balance between

engagement of activating and inhibitory receptors.

This idea is established for B and T lymphocytes and

natural killer (NK) cells. In T cells, the main activating

receptors are the TCR complex and costimulatory receptors such as CD28 (see Chapter 5), and the best-defined

inhibitory receptors, also called coinhibitors, are CTLA-4

and PD-1. The functions and mechanisms of action of

these inhibitors are complementary (Fig. 9.5).

• CTLA-4. CTLA-4 is expressed transiently on activated CD4+ T cells and constitutively on regulatory

T cells (described later). It functions to suppress the

activation of responding T cells. CTLA-4 works by

blocking and removing B7 molecules from the surface

of APCs, thus reducing costimulation by CD28 and

preventing the activation of T cells (see Fig. 9-5A).

The choice between engagement of CTLA-4 or CD28

is determined by the affinity of these receptors for B7

and the level of B7 expression. CTLA-4 has a higher

affinity for B7 molecules than does CD28, so it binds

B7 tightly and prevents the binding of CD28. This

competition is especially effective when B7 levels are

low (as would be expected normally when APCs are

displaying self and probably tumor antigens); in these

situations, the receptor that is preferentially engaged

is the high-affinity blocking receptor CTLA-4. However, when B7 levels are high (as in infections), not all

the ligands will be occupied by CTLA-4 and some B7

will be available to bind to the low-affinity activating

receptor CD28, leading to T cell costimulation.

• PD-1. PD-1 is expressed on CD8+ and CD4+ T cells

after antigen stimulation. Its cytoplasmic tail has inhibitory signaling motifs with tyrosine residues that are

phosphorylated upon recognition of its ligands PD-L1

or PD-L2. Once phosphorylated, these tyrosines bind

a tyrosine phosphatase that inhibits kinase-dependent

activating signals from CD28 and the TCR complex

(see Fig. 9-5B). Because the expression of PD-1 on T

cells is increased upon chronic T cell activation and

Unresponsive

(anergic)

T cell

Inhibitory

receptor

APC presenting

self antigen

Recognition of

self antigen

Signaling block

Engagement of

inhibitory receptors

Naive

T cell

Fig. 9.4 T cell anergy. If a T cell recognizes antigen without strong costimulation, the T cell receptors may

lose their ability to deliver activating signals, or the T cell may engage inhibitory receptors, such as cytotoxic T

lymphocyte–associated protein 4 (CTLA-4) and programmed cell death protein 1 (PD-1), that block activation.

APC, Antigen-presenting cells.

CHAPTER 9 Immunologic Tolerance and Autoimmunity 183

Major site

of action

Secondary

lymphoid organs

Peripheral

tissues

Stage of immune

response that is

inhibited

Cell type that

is inhibited

Cellular

expression

Induction (priming)

CD4+ same as or

more than CD8+

Tregs, activated

T cells

Main signals

inhibited

Role in Treg-mediated

suppression of

immune responses

Competitive inhibitor of CD28

costimulation by binding to B7 with

high affinity and removing B7

from APCs

Yes

Effector phase

CD8+ > CD4+

Mainly activated

T cells

Signaling inhibitor of CD28 and TCR:

inhibits kinase-depending signals

by activating phosphatase

No

CTLA-4 PD-1

T cell

(activated

T cell or

Treg)

 CTLA-4 blocks and

 removes B7

B7

APC

CTLA-4

T cell

ZAP-70

ITAM

TCR

?

P

P

PD-1 inhibits signals from

the TCR complex and CD28

APC

CD28

B7-1/

B7-2

PD-1

PD-L1/

PD-L2

SHP2

P PI3K P

T cell activation

A B

C

CD28

Fig. 9.5 Mechanisms of action and properties of cytotoxic T lymphocyte–associated protein 4 (CTLA-4)

and programmed cell death protein 1 (PD-1). A, CTLA-4 is a competitive inhibitor of the B7-CD28 interaction. B, PD-1 activates a phosphatase that inhibits signals from the TCR complex and CD28. C, Some of

the major differences between these checkpoint molecules are summarized. APC, Antigen-presenting cells;

TCR, T cell receptor.

184 CHAPTER 9 Immunologic Tolerance and Autoimmunity

expression of the ligands is increased by cytokines

produced during prolonged inflammation, this pathway is most active in situations of chronic or repeated

antigenic stimulation. This may happen in responses

to chronic infections, tumors, and self antigens, when

PD-1–expressing T cells encounter the ligand on

infected cells, tumor cells, or APCs.

One of the most impressive therapeutic applications of

our understanding of these inhibitory receptors is treatment of cancer patients with antibodies that block these

receptors. Such treatment leads to enhanced antitumor

immune responses and tumor regression in a significant

fraction of the patients (see Chapter 10). This type of

therapy has been termed checkpoint blockade, because

the inhibitory receptors impose checkpoints in immune

responses, and the treatment blocks these checkpoints

(“removes the brakes” on immune responses). Predictably, patients treated with checkpoint blockade often

develop autoimmune reactions, consistent with the idea

that the inhibitory receptors are constantly functioning

to keep autoreactive T cells in check. Rare patients with

mutations in one of their two copies of the CTLA4 gene,

which reduce expression of the receptor, also develop

multiorgan inflammation (and a profound, as yet unexplained, defect in antibody production).

Several other receptors on T cells other than CTLA-4

and PD-1 have been shown to inhibit immune responses

and are currently being tested as targets of checkpoint

blockade therapy. Some of these receptors are members

of the tumor necrosis factor (TNF) receptor family or

other protein families. Their role in maintaining tolerance to self antigens is not clearly established.

Immune Suppression by Regulatory T Cells

Regulatory T cells develop in the thymus or peripheral

tissues on recognition of self antigens and suppress

the activation of potentially harmful lymphocytes

specific for these self antigens (Fig. 9.6). The majority

FOXP3

FOXP3

Recognition of

self antigen

in thymus

Effector

T cells

Recognition of antigen

in secondary

lymphoid tissues

Regulatory

T cells

Thymus Lymph node

DC Naive

T cell B cell

NK cell

Inhibition of

T cell responses

Inhibition of

other cells

FOXP3

FOXP3

Fig. 9.6 Development and function of regulatory T cells. CD4+ T cells that recognize self antigens may

differentiate into regulatory cells in the thymus or peripheral tissues, in a process that is dependent on

the transcription factor FoxP3. (The larger arrow from the thymus, compared with the one from peripheral

tissues, indicates that most of these cells probably arise in the thymus.) These regulatory cells inhibit the

activation of naive T cells and their differentiation into effector T cells by contact-dependent mechanisms or

by secreting cytokines that inhibit T cell responses. The generation and maintenance of regulatory T cells also

require interleukin-2 (not shown). DC, Dendritic cell; NK, natural killer.

CHAPTER 9 Immunologic Tolerance and Autoimmunity 185

of self-reactive regulatory T cells probably develop in

the thymus (see Fig. 9.2), but they may also arise in

peripheral lymphoid organs. Most regulatory T cells are

CD4+ and express high levels of CD25, the a chain of the

interleukin-2 (IL-2) receptor. They also express a transcription factor called FoxP3, which is required for the

development and function of the cells. Mutations of the

gene encoding FoxP3 in humans or in mice cause a systemic, multiorgan autoimmune disease, demonstrating

the importance of FoxP3+ regulatory T cells for the maintenance of self-tolerance. The human disease is known

by the acronym IPEX, for immune dysregulation, polyendocrinopathy, enteropathy, X-linked syndrome.

The survival and function of regulatory T cells

are dependent on the cytokine IL-2. This role of IL-2

accounts for the severe autoimmune disease that develops in mice in which IL-2 or IL-2 receptor genes are

deleted and in humans with homozygous mutations in

the a or ß chain of the IL-2 receptor. Recall that we

introduced IL-2 in Chapter 5 as a cytokine made by

antigen-activated T cells that stimulates proliferation

of these cells. Thus, IL-2 is an example of a cytokine

that serves two opposite roles: it promotes immune

responses by stimulating T cell proliferation, and it

inhibits immune responses by maintaining functional

regulatory T cells. Numerous clinical trials are testing

the ability of IL-2 to promote regulation and control

harmful immune reactions, such as inflammation in

autoimmune diseases and graft rejection.

The cytokine transforming growth factor ß (TGFß) also plays a role in the generation of regulatory T

cells, perhaps by stimulating expression of the FoxP3

transcription factor. Many cell types can produce

TGF-ß, but the source of TGF-ß for inducing regulatory T cells in the thymus or peripheral tissues is not

defined.

Regulatory T cells may suppress immune responses

by several mechanisms.

• Some regulatory cells produce cytokines (e.g., IL-10,

TGF-ß) that inhibit the activation of lymphocytes,

dendritic cells, and macrophages.

• Regulatory cells express CTLA-4, which, as discussed earlier, may block or remove B7 molecules

made by APCs and make these APCs incapable of

providing costimulation via CD28 and activating T

cells.

• Regulatory T cells, by virtue of the high level of expression of the IL-2 receptor, may bind and consume this

essential T cell growth factor, thus reducing its availability for responding T cells.

The great interest in regulatory T cells has in part been

driven by the hypothesis that the underlying abnormality in some autoimmune diseases in humans is defective

regulatory T cell function or the resistance of pathogenic

T cells to regulation. There is also growing interest in cellular therapy with regulatory T cells to treat graft-versushost disease, graft rejection, and autoimmune disorders.

Deletion: Apoptosis of Mature Lymphocytes

Recognition of self antigens may trigger pathways

of apoptosis that result in elimination (deletion) of

the self-reactive lymphocytes (Fig. 9.7). There are two

likely mechanisms of death of mature T lymphocytes

induced by self antigens:

• Antigen recognition induces in T cells the production of proapoptotic proteins that cause mitochondrial proteins, such as cytochrome c, to leak out

and activate cytosolic enzymes called caspases that

induce apoptosis. In normal immune responses,

the activity of these proapoptotic proteins is counteracted by antiapoptotic proteins that are induced

by costimulation and by growth factors produced

during the responses. However, self antigens, which

are recognized without strong costimulation, do not

stimulate production of antiapoptotic proteins, and

the relative deficiency of survival signals induces

death of the cells that recognize these antigens.

• Recognition of self antigens may lead to the coexpression of death receptors and their ligands. This ligandreceptor interaction generates signals through the death

receptor that culminate in the activation of caspases and

apoptosis. The best-defined death receptor–ligand pair

involved in self-tolerance is a protein called Fas (CD95),

which is expressed on many cell types, and Fas ligand

(FasL), which is expressed mainly on activated T cells.

Evidence from genetic studies supports the role of

apoptosis in self-tolerance. Eliminating the mitochondrial pathway of apoptosis in mice results in a failure of

deletion of self-reactive T cells in the thymus and also

in peripheral tissues. Mice with mutations in the fas

and fasl genes and children with mutations in FAS all

develop autoimmune diseases with lymphocyte accumulation. Children with mutations in the genes encoding caspase-8 or -10, which are downstream of FAS

signaling, also have similar autoimmune diseases. The

human diseases, collectively called the autoimmune

186 CHAPTER 9 Immunologic Tolerance and Autoimmunity

lymphoproliferative syndrome (ALPS), are rare and

are the only known examples of defects in apoptosis

causing an autoimmune disorder.

From this discussion of the mechanisms of T cell tolerance, it should be clear that self antigens differ from

foreign microbial antigens in several ways, which contribute to the choice between tolerance induced by the

former and activation by the latter (Fig. 9.8).

• Self antigens are present in the thymus, where they

induce deletion and generate regulatory T cells; by

contrast, most microbial antigens tend to be excluded

from the thymus because they are typically captured

from their sites of entry and transported into peripheral lymphoid organs (see Chapter 3).

• Self antigens are displayed by resting (costimulatordeficient) APCs in the absence of innate immunity,

thus favoring the induction of T cell anergy or death, or

suppression by regulatory T cells. By contrast, microbes

elicit innate immune reactions, leading to the expression of costimulators and cytokines that promote T cell

proliferation and differentiation into effector cells.

• Self antigens are present throughout life and may

therefore cause prolonged or repeated TCR engagement, again promoting anergy, apoptosis, and the

development of regulatory T cells.

Antigen recognition T cell response

Normal

response

Cell death

caused by

engagement

of death

receptors

T cell

Apoptotic proteins

inside mitochondria

Apoptotic proteins

released from

mitochondria

T cell survival,

proliferation

and differentiation

Activated

T cells

Apoptosis

Apoptosis

Expression of

death receptor

ligand

Expression of

death receptor

Cell death

caused by

deficiency

of survival

signals

APC

Inducers of

apoptosis

Inhibitors of

apoptosis

IL-2

Fig. 9.7 Mechanisms of apoptosis of T lymphocytes. T cells respond to antigen presented by normal antigen-presenting cells (APCs) by secreting interleukin-2 (IL-2), expressing antiapoptotic (prosurvival) proteins,

and undergoing proliferation and differentiation. The antiapoptotic proteins prevent the release of mediators

of apoptosis from mitochondria. Self antigen recognition by T cells without costimulation may lead to relative

deficiency of intracellular antiapoptotic proteins, and the excess of proapoptotic proteins causes cell death

by inducing release of mediators of apoptosis from mitochondria (death by the mitochondrial [intrinsic] pathway of apoptosis). Alternatively, self antigen recognition may lead to expression of death receptors and their

ligands, such as Fas and Fas ligand (FasL), on lymphocytes, and engagement of the death receptor leads to

apoptosis of the cells by the death receptor (extrinsic) pathway.

CHAPTER 9 Immunologic Tolerance and Autoimmunity 187

B LYMPHOCYTE TOLERANCE

Self polysaccharides, lipids, and nucleic acids are

T-independent antigens that are not recognized by T cells.

These antigens must induce tolerance in B lymphocytes

to prevent autoantibody production. Self proteins may

not elicit autoantibody responses because of tolerance in

helper T cells and in B cells. It is suspected that diseases

associated with autoantibody production, such as systemic lupus erythematosus (SLE), are caused by defective tolerance in both B lymphocytes and helper T cells.

Central B Cell Tolerance

When immature B lymphocytes interact strongly with

self antigens in the bone marrow, the B cells either

change their receptor specificity (receptor editing) or

are killed (deletion) (Fig. 9.9).

• Receptor editing. Immature B cells are at a stage of

maturation in the bone marrow when they have rearranged their immunoglobulin (Ig) genes, express IgM

with a heavy chain and light chain, and have shut off

the RAG genes that encode the recombinase. If these

B cells recognize self antigens in the bone marrow,

they may reexpress RAG genes, resume light-chain

gene recombination, and express a new Ig light

chain (see Chapter 4). The heavy chain gene cannot

recombine because some segments are lost during

the initial recombination. The new light chain associates with the previously expressed Ig heavy chain to

produce a new antigen receptor that may no longer

recognize the self antigen. This process of changing

receptor specificity, called receptor editing, reduces

the chance that potentially harmful self-reactive B

cells will leave the marrow. It is estimated that 25%

to 50% of mature B cells in a normal individual may

have undergone receptor editing during their maturation. (There is no evidence that developing T cells

can undergo receptor editing.)

• Deletion. If editing fails, immature B cells that

strongly recognize self antigens receive death signals

and die by apoptosis. This process of deletion is similar to negative selection of immature T lymphocytes.

As in the T cell compartment, negative selection of

B cells eliminates lymphocytes with high-affinity

receptors for abundant, and usually widely expressed,

cell membrane or soluble self antigens.

Feature of antigen Tolerogenic self antigens Immunogenic foreign antigens

Location of

antigens

Accompanying

costimulation

Duration of

antigen exposure

Presence in generative organs

(some self antigens) induces

negative selection and other

mechanisms of central tolerance

Deficiency of costimulators may

lead to T cell anergy or apoptosis,

development of Treg, or

sensitivity to suppression by Treg

Long-lived persistence

(throughout life); prolonged TCR

engagement may induce anergy

and apoptosis

Presence in blood and peripheral

tissues (most microbial antigens)

permits concentration in peripheral

lymphoid organs

Expression of costimulators,

typically seen with microbes,

promotes lymphocyte survival

and activation

Short exposure to microbial antigen

reflects effective immune response

Tissue

Microbe

Fig. 9.8 Features of protein antigens that influence the choice between T cell tolerance and activation. This figure summarizes some of the characteristics of self and foreign (e.g., microbial) protein) antigens

that determine why the self antigens induce tolerance and microbial antigens stimulate T cell–mediated

immune responses. TCR, T cell receptor; Treg, T regulatory cells.

188 CHAPTER 9 Immunologic Tolerance and Autoimmunity

• Anergy. Some self antigens, such as soluble proteins,

may be recognized in the bone marrow with low

avidity. B cells specific for these antigens survive, but

antigen receptor expression is reduced, and the cells

become functionally unresponsive (anergic).

Peripheral B Cell Tolerance

Mature B lymphocytes that encounter self antigens

in peripheral lymphoid tissues become incapable

of responding to that antigen (Fig. 9.10). According

to one hypothesis, if B cells recognize a protein antigen but do not receive T cell help (because helper T

cells have been eliminated or are tolerant), the B cells

become anergic because of a block in signaling from

the antigen receptor. Anergic B cells may leave lymphoid follicles and are subsequently excluded from

the follicles. These excluded B cells may die because

they do not receive necessary survival stimuli. B cells

that recognize self antigens in the periphery may also

undergo apoptosis, or inhibitory receptors on the B

cells may be engaged, thus preventing activation. As

mentioned earlier, regulatory T cells may also contribute to B cell tolerance.

TOLERANCE TO COMMENSAL MICROBES

AND FETAL ANTIGENS

Before concluding our discussion of the mechanisms of

immunologic tolerance, it is useful to consider two other

types of antigens that are not self but are produced by

cells or tissues that have to be tolerated by the immune

system. These are products of commensal microbes that

live in symbiosis with humans and paternally derived

antigens in the fetus. Coexistence with these antigens is

dependent on many of the same mechanisms that are

used to maintain peripheral tolerance to self antigens.

Tolerance to Commensal Microbes in the

Intestines and Skin

The microbiome of healthy humans consists of approximately 1014 bacteria and viruses (which is estimated

to be almost 10 times the number of nucleated human

cells, prompting microbiologists to point out that we are

only 10% human and 90% microbial!). These microbes

reside in the intestinal and respiratory tracts and on the

skin, where they serve many essential functions. For

instance, in the gut, the normal bacteria aid in digestion and absorption of foods and prevent overgrowth of

potentially harmful organisms. Mature lymphocytes in

these tissues are capable of recognizing the organisms

but do not react against them, so the microbes are not

eliminated, and harmful inflammation is not triggered.

Self

antigen

Inhibitory

receptors

Anergy

Regulation

by inhibitory

receptors

Functional

inactivation

Apoptosis

Deletion

Fig. 9.10 Peripheral tolerance in B lymphocytes. A mature

B cell that recognizes a self-antigen without T cell help is functionally inactivated and becomes incapable of responding to

that antigen (anergy), or it dies by apoptosis (deletion), or its

activation is suppressed by engagement of inhibitory receptors.

Self antigen

recognition

Self

antigen

Receptor editing:

expression of

new Ig V region

Self-reactive

B cell

Non-self reactive Deletion

B cell

Anergic

B cell

Apoptosis

Reduced receptor

expression, signaling

Fig. 9.9 Central tolerance in immature B lymphocytes. An

immature B cell that recognizes self antigen in the bone marrow changes its antigen receptor (receptor editing), dies by

apoptosis (negative selection, or deletion), or reduces antigen

receptor expression and becomes functionally unresponsive.

Ig, Immunoglobulin.

CHAPTER 9 Immunologic Tolerance and Autoimmunity 189

In the gut, several mechanisms account for the inability

of the healthy immune system to react against commensal microbes. These mechanisms include an abundance

of IL-10–producing regulatory T cells, and an unusual

property of intestinal dendritic cells such that signaling

from some Toll-like receptors leads to inhibition rather

than activation. In addition, many commensal bacteria

are physically separated from the intestinal immune system by the epithelium. The mechanisms that maintain

tolerance to commensal bacteria in the skin are not as

well defined.

Tolerance to Fetal Antigens

The evolution of placentation in eutherian mammals

allowed the fetus to mature before birth but created the

problem that paternal antigens expressed in the fetus,

which are foreign to the mother, have to be tolerated by

the immune system of the pregnant mother. One mechanism of this tolerance is the generation of peripheral

FoxP3+ regulatory T cells specific for these paternal antigens. In fact, during mammalian evolution, placentation

is strongly correlated with the ability to generate stable

peripheral regulatory T cells. It is unclear whether women

who suffer recurrent pregnancy losses have a defect in

the generation or maintenance of these regulatory T cells.

Other mechanisms of fetal tolerance include exclusion of

inflammatory cells from the pregnant uterus, poor antigen presentation in the placenta, and an inability to generate harmful Th1 responses in the healthy pregnant uterus.

Now that we have described the principal mechanisms of immunologic tolerance, we consider the

consequences of the failure of self-tolerance—namely,

the development of autoimmunity.

AUTOIMMUNITY