including some producing

anti-X antibody

Antigen X

Hybridomas

producing

monoclonal

anti-X antibody

Isolate spleen

cells from mouse

immunized

with antigen X

Fuse spleen cells

and immortal

myeloma cell line

Only fused cells

(hybridomas)

grow

Culture in

selection medium

Screen supernatants of each clone for

anti-X antibody and expand positive clones

Isolate clones derived from single hybridoma cells

Fusion

Fig. 4.5 Generation of hybridomas and monoclonal antibodies. In this procedure, spleen cells from a

mouse that has been immunized with a known antigen are fused with an enzyme-deficient myeloma cell line

that does not secrete its own immunoglobulins. The fused cells are then placed in a selection medium that

permits the survival of only immortalized hybrids; the normal B cells provide the enzyme that the myeloma

lacks, and unfused B cells cannot survive indefinitely. These hybrid cells are then grown as single-cell clones

and tested for the secretion of antibody of the desired specificity. The clone producing this antibody is

expanded and becomes a source of the monoclonal antibody.

82 CHAPTER 4 Antigen Recognition in the Adaptive Immune System

nonpolymorphic portions of MHC molecules. The functions of these TCR-associated proteins and coreceptors

are discussed in Chapter 5.

Antigen recognition by B and T lymphocyte receptors differs in important ways (Fig. 4.9). Antibodies can

bind many different types of chemical structures, often

with high affinities, which is why antibodies can bind

to and neutralize many different microbes and toxins

that may be present at low concentrations in the circulation or in the lumens of mucosal organs. TCRs only

recognize peptide-MHC complexes and bind these

with relatively low affinity, which may be why the binding of T cells to APCs has to be strengthened by additional cell surface adhesion molecules (see Chapter 5).

The three-dimensional structure of the TCR is similar

to that of the Fab region of an Ig molecule. In contrast

to membrane antibodies, in which only the heavy chain

is membrane-anchored, both TCR chains are anchored

in the plasma membrane. TCRs are not produced in a

secreted form and do not undergo isotype switching or

affinity maturation during the life of a T cell.

About 5% to 10% of T cells in the body express

receptors composed of gamma (?) and delta (d) chains.

CD20 Depletion of B cells Rheumatoid arthritis, multiple sclerosis, other

autoimmune diseases; B cell lymphoma

IL-6 receptor Blocking inflammation Rheumatoid arthritis

IgE Blocking IgE function Allergy-related asthma

TNF Blocking inflammation Rheumatoid arthritis, Crohn disease,

psoriasis

CD52 Depletion of lymphocytes Chronic lymphocytic leukemia

Glycoprotein IIb/IIIa Inhibition of platelet

aggregation

Cardiovascular disease

EGFR Growth inhibition of

epithelial tumors

Colorectal, lung, and head and neck cancers

CTLA-4 Activation of T cells Melanoma

PD-1 Activation of effector T cells Many tumors

VEGF Blocking

tumor angiogenesis

Breast cancer, colon cancer, age-related

macular degeneration

PD-L1 Activation of effector T cells Many tumors

HER2/Neu Inhibition of EGF signaling;

depletion of tumor cells

Breast cancer

Inflammatory (immunological) diseases

Target Effect Diseases

Cancer

Target Effect Diseases

Other diseases

Target Effect Diseases

Fig. 4.6 Selected monoclonal antibodies in clinical use. The figure lists some of the monoclonal antibodies that are approved for the treatment of various types of diseases.

CHAPTER 4 Antigen Recognition in the Adaptive Immune System 83

These receptors are structurally similar to the aß TCR

but have very different specificities. The ?d TCR may

recognize a variety of protein and nonprotein antigens,

usually not displayed by classical MHC molecules.

T cells expressing ?d TCRs are abundant in epithelia. This

observation suggests that ?d T cells recognize microbes

usually encountered at epithelial surfaces, but neither the

specificity nor the function of these T cells is well established. Another subpopulation of T cells, comprising less

than 5% of all T cells, express aß TCRs and surface molecules found on natural killer cells, and are therefore called

natural killer T cells (NK-T cells). NK-T cells express aß

TCRs with limited diversity, and they recognize lipid

antigens displayed by nonpolymorphic class I MHC–like

molecules called CD1. A third subset of T cells called

mucosal associated invariant T (MAIT) cells also express

aß TCRs with limited diversity, some of which are specific for bacterially derived vitamin B metabolites bound

to an MHC-like protein called MR1. MAIT cells account

for only about 5% of blood T cells, but up to 20%–40% of

human liver T cells. The physiologic functions of NK-T

cells and MAIT cells also are not well understood.

DEVELOPMENT OF B AND

T LYMPHOCYTES

Now that we have discussed the structure of antigen

receptors of B and T lymphocytes and how these receptors recognize antigens, the next question is how the

enormous diversity of these receptors is generated. As

the clonal selection hypothesis predicted, there are many

clones of lymphocytes with distinct specificities, perhaps as many as 107–109, and these clones arise before

an encounter with antigen. There are not enough genes

in the human genome for every possible receptor to be

encoded by a different gene. In fact, the immune system

S S

S

Transmembrane S

region

N N

Ca

Cß

Vß Va

Ca

Va

Cß

Vß

ß chain a chain

Disulfide bond

Ig domain

Carbohydrate

group

C C

S S

S

S

S

S

S

S

S

S

Fig. 4.7 Structure of the T cell antigen receptor (TCR). The schematic diagram of the aß TCR (left) shows

the domains of a TCR specific for a peptide-MHC complex. The antigen-binding portion of the TCR is formed

by the V domains of the a and ß chains. N and C refer to the amino-terminal and carboxy-terminal ends of the

polypeptides. The ribbon diagram (right) shows the structure of the extracellular portion of a TCR as revealed

by x-ray crystallography. Ig, Immunoglobulin; MHC, major histocompatibility complex. (From Bjorkman PJ:

MHC restriction in three dimensions: a view of T cell receptor/ligand interactions, Cell 89:167–170, 1997.

Copyright Cell Press; with permission.)

84 CHAPTER 4 Antigen Recognition in the Adaptive Immune System

has developed mechanisms for generating extremely

diverse antigen receptors from a limited number of

inherited genes, and the generation of diverse receptors

is intimately linked to the process of B and T lymphocyte maturation.

The process of lymphocyte maturation first generates a very large number of cells, each with a different antigen receptor, and then preserves the cells with

useful receptors. The generation of millions of receptors is a molecular process that cannot be influenced by

what the receptors recognize, because recognition can

only occur after receptor generation and expression.

Once these antigen receptors are expressed on developing lymphocytes, selection processes come into play

that promote the survival of cells with receptors that

can recognize antigens, such as microbial antigens, and

eliminate cells that cannot recognize any antigens or

recognize self antigens well enough to pose danger of

causing autoimmune disease. We discuss each of these

events next.

Lymphocyte Development

The development of lymphocytes from bone marrow

stem cells involves commitment of hematopoietic progenitors to the B or T cell lineage, the proliferation of

these progenitors, the rearrangement and expression

of antigen receptor genes, and selection events to preserve and expand cells that express potentially useful

antigen receptors (Fig. 4.10). These steps are common

to B and T lymphocytes, even though B lymphocytes

mature in the bone marrow and T lymphocytes mature

in the thymus. Each of the processes that occurs during

lymphocyte maturation plays a special role in the generation of the lymphocyte repertoire.

• The maturation of common lymphoid progenitors

in the bone marrow results in commitment to the B

cell or T cell lineage. This commitment is associated

with the activation of several lineage-specific transcription factors and increased accessibility of Ig and

TCR genes to the gene recombination machinery,

described later.

• Developing lymphocytes undergo proliferation at

several stages during their maturation. Proliferation

of developing lymphocytes is necessary to ensure

that an adequate number of cells will be available

to express antigen receptors and mature into functionally competent lymphocytes. Survival and proliferation of the earliest lymphocyte precursors are

stimulated mainly by growth factors that are produced by stromal cells in the bone marrow and the

thymus. In humans, IL-7 maintains and expands

the number of T lymphocyte progenitors before

they express antigen receptors. The growth factors

required for expansion of human B cell progenitors

are not defined. This proliferative expansion generates a large pool of cells in which diverse antigen

receptors may be produced. Even greater proliferation of the B and T cell lineages occurs after the

developing lymphocytes have completed their first

antigen receptor gene rearrangement and assembled

a so-called preantigen receptor (described later). This

step is a quality control checkpoint in lymphocyte

development that ensures preservation of cells with

functional receptors.

• Lymphocytes are selected at multiple steps during their

maturation to preserve useful specificities. Selection

TCR

Va

Vß

a2

a1

ß2m

a3

Peptide

MHC

(class I)

Fig. 4.8 Recognition of peptide-MHC complex by a T cell

antigen receptor. This ribbon diagram is drawn from the crystal structure of the extracellular portion of a peptide-MHC complex bound to a TCR that is specific for the peptide displayed

by the MHC molecule. The peptide can be seen attached to

the cleft at the top of the MHC molecule, and one residue of

the peptide contacts the V region of a TCR. The structure of

MHC molecules and their function as peptide display proteins

are described in Chapter 3. MHC, Major histocompatibility

complex; TCR, T cell receptor; ß2m, ß2-microglobulin. (From

Bjorkman PJ: MHC restriction in three dimensions: a view of

T cell receptor/ligand interactions, Cell 89:167–170, 1997.

Copyright Cell Press; with permission.)

CHAPTER 4 Antigen Recognition in the Adaptive Immune System 85

is based on the expression of intact antigen receptor

components and what they recognize. As discussed

later, many attempts to generate antigen receptors

fail because of errors during the gene recombination

process. Therefore, checkpoints are needed at which

only cells that can express functional components of

antigen receptors are selected to survive and proliferate. Prelymphocytes and immature lymphocytes

that fail to express antigen receptor proteins die by

apoptosis (see Fig. 4.10). The gene rearrangements in

the developing lymphocytes randomly generate antigen receptors with highly diverse specificities. Some

of these may be incapable of recognizing antigens in

the individual—for instance, if the TCR cannot recognize MHC alleles present in the individual. In order

to preserve the T cells that will be functional, immature T cells are selected to survive only if they have

some affinity for MHC molecules in the thymus. This

process, called positive selection, ensures that cells

that complete maturation will be capable of recognizing microbial peptides displayed by the same MHC

molecules on APCs (which are the only MHC molecules these cells can normally encounter). Other antigen receptors may strongly recognize certain peptides

of self proteins bound to self MHC, or strongly recognize self MHC regardless of the peptide displayed.

Another selection process is needed to eliminate

these potentially dangerous lymphocytes and prevent

the development of autoimmune responses. The elimination of strongly self-reactive B and T lymphocytes

is called negative selection.

The processes of B and T lymphocyte maturation and

selection share some important features but also differ

in many respects. We start with the central event that

is common to both lineages: the recombination and

expression of antigen receptor genes.

Antigen

binding

Changes

in constant

regions

Affinity of

antigen

binding

On-rate

and off-rate

Made up of three CDRs

in VH and three CDRs

in VL

Made up of three CDRs

in Va and three CDRs

in Vß

Heavy-chain class

switching and change

from membrane to

secretory Ig

None

Kd 10-7-10-11 M;

average affinity of lgs

increases during

immune responses to

protein antigens

Kd 10-5-10-7 M;

No change during

immune responses

Rapid on-rate,

variable off-rate

Slow on-rate,

slow off-rate

Feature Antigen-binding molecule

Immunoglobulin (Ig) T cell receptor (TCR)

CD4

TCR

Peptide

Ig

Antigen

Fig. 4.9 Features of antigen recognition by immunoglobulins and T cell antigen receptors. The important similarities and differences of Ig and TCR molecules, the antigen receptors of B and T lymphocytes,

respectively.

86 CHAPTER 4 Antigen Recognition in the Adaptive Immune System

Production of Diverse Antigen Receptors

The formation of functional genes that encode B

and T lymphocyte antigen receptors is initiated by

somatic recombination of gene segments that code for

the variable regions of the receptors, and diversity is

generated during this process.

Inherited Antigen Receptor Genes

Hematopoietic stem cells in the bone marrow and early

lymphoid progenitors contain Ig and TCR genes in their

inherited, or germline, configuration. In this configuration, Ig heavy-chain and light-chain loci and the TCR

a chain and ß chain loci each contain multiple variable

region (V) gene segments, numbering about 30 to 45,

and one or a few constant region (C) genes (Fig. 4.11).

Between the V and C gene segments are groups of

several short coding sequences called diversity (D) and

joining (J) gene segments. (All antigen receptor gene

loci contain V, J, and C gene segments, but only the Ig

heavy chain and TCR ß chain loci also contain D gene

segments.) These separated gene segments cannot code

for functional antigen receptor proteins, so they have to

be brought together as lymphocytes mature.

Somatic Recombination and Expression of Antigen

Receptor Genes

The commitment of a lymphocyte progenitor to become

a B lymphocyte is associated with the recombination of

randomly selected gene segments in the Ig heavy-chain

locus—first one D gene segment with one J segment to

form a fused DJ complex, followed by the rearrangement of a V segment to the fused DJ complex (Fig. 4.12).

Failure to express

antigen receptor;

cell death

Failure to express

preantigen receptor;

cell death Negative

selection

Positive

Pro- selection

B/T cells

Common

lymphoid

progenitor

Pre-B/T cell:

expresses

one chain of

antigen receptor

Immature

B/T cell:

expresses

complete

antigen

receptor

Weak

antigen

recognition

Mature

T/B cell

Pre-B/T antigen

receptor

expression

Antigen

receptor

expression

Positive and

negative

selection

Commitment Proliferation Proliferation

Strong

antigen

recognition

Fig. 4.10 Steps in maturation of lymphocytes. During their maturation, B and T lymphocytes go through cycles

of proliferation and expression of antigen receptor proteins by gene recombination. Cells that fail to express intact,

functional receptors die by apoptosis, because they do not receive the necessary survival signals. At the end of

the process, the cells undergo positive and negative selection. The lymphocytes shown may be B or T cells.

CHAPTER 4 Antigen Recognition in the Adaptive Immune System 87

Ig ? chain locus (chromosome 2)

Extracellular

domain

Transmembrane and

cytoplasmic domains

Ig H chain locus (chromosome 14)

TCR a chain locus (chromosome 14)

TCR ß chain locus (chromosome 7)

V1

5' 3'

L V1

(n = ~35)

Vn J C

5'

3'

V1

(n = ~45)

Vn D (n =23) J Cµ Cd C?3 C?1

Ca1 C?2 C?4 Ce Ca2

5' 3'

5' 3'

L D1 D2

(n = ~48)

Vn J1 C1 J2 C2

L V1

(n = ~45)

Vn J (n = ~50) C

L

Ig ? chain locus (chromosome 22)

5'

L V1 (n = ~30)

L Vn J1 C1 J2 C2 J3 C3 J7 C7

3'

Fig. 4.11 Germline organization of antigen receptor gene loci. In the germline, inherited antigen receptor gene

loci contain coding segments (exons, shown as colored blocks of various sizes) that are separated by segments that

are not expressed (introns, shown as gray sections). Each immunoglobulin (Ig) heavy-chain constant (C) region and

T cell receptor (TCR) C region consists of multiple exons, which are not shown, that encode the domains of the C

regions; the organization of the Cµ exons in the Ig heavy-chain locus is shown as an example. The diagrams illustrate

the antigen receptor gene loci in humans; the basic organization is the same in all species, although the precise order

and number of gene segments may vary. The numbers of V, D, and J gene segments are estimates of functional

gene segments (those that can code for proteins). The sizes of the segments and the distances between them are

not drawn to scale. D, Diversity; J, joining; L, leader sequence (a small stretch of nucleotides that encodes a peptide

that guides proteins through the endoplasmic reticulum and is cleaved from the mature proteins); V, variable.

Thus, the committed but still-developing B cell now

has a recombined VDJ exon in the heavy-chain locus.

This gene is transcribed, and in the primary RNA transcript, the VDJ exon is spliced to the C-region exons of

the µ chain, the most 5' C region, to form a complete

µ messenger RNA (mRNA). The µ mRNA is translated

to produce the µ heavy chain, which is the first Ig protein synthesized during B cell maturation.

88 CHAPTER 4 Antigen Recognition in the Adaptive Immune System

RNA processing

(splicing)

Transcription

V1D1J1

V

V1 Vn J 1-6 Cµ

Translation

AAA

V

AAA

CDR3 CDR3

D1-Dn

V1 Vn

3'

D1J1 Cµ V1 Vn

3'

D3J2 Cµ

VnD3J2

VnD3J2

Cµ

V1D1J1

V1D1J1 Cµ

Cµ Cµ

Somatic recombination

(D-J joining) in

two B cell clones

Somatic recombination

(V-DJ joining) in

two B cell clones

Recombined

 DNA in two

B cell clones

Recombined

 DNA in two

B cell clones

Ig µ chains

in two

B cell clones

Germline

DNA at

Ig H locus

Primary RNA

transcript

Messenger

RNA (mRNA)

Cµ

Cµ Cµ

VnD3J2 Cµ

5' 3'

5'

5' 3' 3'

5' 3' 5' 3'

5'

5'

Fig. 4.12 Recombination and expression of immunoglobulin (Ig) genes. The expression of an Ig heavy

chain involves two gene recombination events (D-J joining, followed by joining of a V region to the DJ complex, with deletion of intervening gene segments). The recombined gene is transcribed, and the VDJ complex

is spliced onto the C region exons of the first heavy-chain RNA (which is µ), to give rise to the µ messenger

RNA (mRNA). The mRNA is translated to produce the µ heavy-chain protein. The recombination of other

antigen receptor genes—that is, the Ig light chain and the T cell receptor (TCR) a and ß chains—follows

essentially the same sequence, except that in loci lacking D segments (Ig light chains and TCR a), a V gene

recombines directly with a J gene segment.

Essentially the same sequence of DNA recombination and RNA splicing leads to production of a light

chain in B cells, except that the light-chain loci lack D

segments, so a V region exon recombines directly with

a J segment. The rearrangement of TCR a chain and ß

chain genes in T lymphocytes is similar to that of Ig L

and H chains, respectively.

Mechanisms of V(D)J Recombination

The somatic recombination of V and J, or of V, D,

and J, gene segments is mediated by a lymphoid-specific enzyme, the VDJ recombinase, and additional

enzymes, most of which are not lymphocyte specific

and are involved in repair of double-stranded DNA

breaks introduced by the recombinase. The VDJ

CHAPTER 4 Antigen Recognition in the Adaptive Immune System 89

recombinase is composed of the recombination-activating gene 1 and 2 (RAG-1 and RAG-2) proteins. It

recognizes DNA sequences that flank all antigen receptor V, D, and J gene segments. As a result of this recognition, the recombinase brings two Ig or TCR gene

segments close together and cleaves the DNA at specific

sites. The DNA breaks are then repaired by ligases, producing a full-length recombined VJ or VDJ exon without the intervening DNA segments (see Fig. 4.12). The

VDJ recombinase is expressed only in immature B and

T lymphocytes. Although the same enzyme can mediate recombination of all Ig and TCR genes, intact Ig

heavy-chain and light-chain genes are rearranged and

expressed only in B cells, and TCR a and ß genes are

rearranged and expressed only in T cells. The lineage

specificity of receptor gene rearrangement appears to

be linked to the expression of lineage-specific transcription factors. In B cells, B lineage-specific transcription

factors “open” the Ig gene locus at the chromatin level

but not the TCR locus, whereas in developing T cells,

transcriptional regulators help open the TCR locus but

not the Ig locus. The “open” loci are the ones that are

accessible to the recombinase.

Generation of Ig and TCR Diversity

Diversity of antigen receptors is produced by the use

of different combinations of V, D, and J gene segments

in different clones of lymphocytes (called combinatorial diversity) and even more by changes in nucleotide

sequences introduced at the junctions of the recombining V, D, and J gene segments (called junctional diversity; Fig. 4.13). Combinatorial diversity is limited by the

number of available V, D, and J gene segments, but junctional diversity is almost unlimited. Junctional diversity

is produced by three mechanisms, which generate more

sequences than are present in the germline genes:

• Exonucleases may remove nucleotides from V, D, and

J gene segments at the sites of recombination.

• A lymphocyte-specific enzyme called terminal

deoxyribonucleotidyl transferase (TdT) catalyzes the

random addition of nucleotides that are not part of

germline genes to the junctions between V and D

segments and D and J segments, forming so-called N

regions.

• During an intermediate stage in the process of V(D)J

recombination, the two broken strands of the DNA at

each end of the cut DNA form hairpin loops. As a first

step in the repair process, the loops are asymmetrically

cut, forming overhanging DNA sequences. These

overhangs have to be filled in with new nucleotides,

which are called P-nucleotides, introducing even more

variability at the sites of recombination.

As a result of these mechanisms, the nucleotide

sequence at the site of V(D)J recombination in antibody or TCR genes in one clone of lymphocytes differs

from the sequence at the V(D)J site of antibody or TCR

molecules made by every other clone. These junctional

sequences and the D and J segments encode the amino

acids of the CDR3 loop, mentioned earlier as the most

variable of the CDRs and the most important for antigen recognition. Thus, junctional diversity maximizes

the variability in the antigen-recognizing portions of

antibodies and TCRs. In the process of creating junctional diversity, many genes may be produced with

out-of-frame sequences that cannot code for proteins

and are therefore useless. This is the price the immune

system pays for generating tremendous diversity. The

risk of producing nonfunctional genes also is why the

process of lymphocyte maturation contains checkpoints

at which only cells with useful receptors are selected to

survive.

The uniqueness of CDR3 sequences in every lymphocyte clone can be exploited to distinguish neoplastic

and reactive proliferations of B and T lymphocytes. In

tumors arising from these cells, all the cells of the tumor

will have the same CDR3 (because they all arose from

a single B or T cell clone), but in proliferations that are

reactions to external stimuli, many CDR3 sequences

will be present. The same principle can be used to define

the magnitude of an immune response—measuring

the number of CDR3 sequences present in a population before and during a response is an indicator of the

amount of proliferative expansion of a B or T cell clone.

Maturation and Selection of B Lymphocytes

The maturation of B lymphocytes occurs mainly in the

bone marrow (Fig. 4.14). Progenitors committed to the

B cell lineage proliferate, giving rise to a large number

of precursors of B cells, called pro-B cells. Subsequent

maturation involves antigen receptor gene expression

and selection.

Early Steps in B Cell Maturation

The Ig heavy-chain locus rearranges first, and only

cells that are able to make an Ig µ heavy-chain protein are selected to survive and become pre-B cells.

90 CHAPTER 4 Antigen Recognition in the Adaptive Immune System

V1 C Vn C

D1 J1 D2 J2

Mechanism

Heavy

chain

Immunoglobulin T cell

receptor

? a ß

Number of variable (V)

gene segments

Number of diversity (D)

gene segments

Number of joining (J)

gene segments

~45

23

6

35

0

5

45

0

50

48

2

12

Combinatorial diversity:

 Number of possible

 V(D)J combinations Ig: ~3x106 TCR: ~6x106

Junctional diversity:

 Total potential

 repertoire with

 junctional diversity

Ig: ~1011 TCR: ~1016

V1 D1 J1 C

Removal of

nucleotides

Addition of nucleotides

(N-region or P-nucleotides)

C

D1 J1

V1 C

D1 J1

V1

?

30

0

4

Fig. 4.13 Mechanisms of diversity in antigen receptors. Diversity in immunoglobulins and T cell receptors

is produced by random combinations of V, D, and J gene segments, which is limited by the numbers of these

segments and by removal and addition of nucleotides at the V-J or V-D-J junctions, which is almost unlimited.

The numbers of gene segments refer to the average numbers of functional genes (which are known to be

expressed as RNA or protein) in humans. Junctional diversity maximizes the variations in the CDR3 regions

of the antigen receptor proteins, because CDR3 includes the junctions at the site of V-J and V-D-J recombination. The diversity is further enhanced by the juxtaposition of the V regions of the two types of chains in Ig

or TCRs to form the complete antigen binding sites, and thus the total diversity is theoretically the product

of the total diversity of each of the juxtaposed V regions. The estimated contributions of these mechanisms

to the total possible numbers of distinct B and T cell antigen receptors are shown. Although the upper limit on

the number of immunoglobulin (Ig) and TCR proteins that may be expressed is extremely large, each individual contains on the order of only 107–109 clones of B cells and T cells with distinct specificities and receptors;

in other words, only a fraction of the potential repertoire may actually be expressed. (Modified from Davis

MM, Bjorkman PJ: T-cell antigen receptor genes and T-cell recognition, Nature 334:395–402, 1988.)

CHAPTER 4 Antigen Recognition in the Adaptive Immune System 91

Pro-B cells cease to divide, and then any one D segment of the Ig heavy-chain locus is joined to a randomly selected J segment at the same locus. Next, a

random upstream Ig V gene segment is recombined to

the previously rearranged DJ unit in each pro-B cell.

Given that junctional nucleotides are randomly added

both when the D-J joint is made and when a V segment fuses with a DJ unit, in the majority of cells, the

number of junctional nucleotides will not add up to a

multiple of three. Because three nucleotides code for

one amino acid, only some pro-B cells will create junctions that allow a functional Ig heavy-chain protein to

be made. The cells that successfully make functional

heavy-chain gene rearrangements and synthesize the

Ig heavy-chain µ protein are called pre-B cells. Pre-B

cells are therefore defined by the presence of the Ig µ

heavy-chain protein. As cells become pre-B cells, they

express the µ protein on the cell surface in association

with two other invariant proteins called surrogate light

chains because they resemble light chains and associate with the µ heavy chain. The complex of µ chain

and surrogate light chains associates with the Iga and

Igß signaling molecules to form the pre-B cell receptor

(pre-BCR) complex.

Role of the Pre-BCR Complex in B Cell Maturation

The assembled pre-BCR serves essential functions in

the maturation of B cells:

• Signals from the pre-BCR complex promote the survival and proliferation of B lineage cells that have

made a productive rearrangement at the Ig H chain

locus. This is the first checkpoint in B cell development, and it selects and expands the pre-B cells

that express a functional µ heavy chain (which is

an essential component of the pre-BCR and BCR).

Pre-B cells that make out-of-frame (nonproductive)

rearrangements at the heavy-chain locus fail to make

the µ protein, cannot express a pre-BCR or receive

pre-BCR signals, and die by programmed cell death

(apoptosis). The pre-BCR signaling pathway includes

a downstream tyrosine kinase called Btk, which is

encoded on the X chromosome. Mutations in Btk

in boys results in the failure of pre-B cells to survive

and the subsequent absence of B cells. This disease is

called X-linked agammaglobulinemia.

• The pre-BCR complex signals to shut off recombination of Ig heavy-chain genes on the second chromosome, so each B cell can express an Ig heavy chain

from only one of the two inherited parental alleles.

IgM IgM

Pre-BCR

Ig H chain

gene

Ig L chain

gene

HSC Pro-B Large Pre-B Immature B Mature B

Germline D to J

rearrangement

V to DJ

rearrangement

Rearranged

VDJ

Rearranged

VDJ

Germline Germline Germline Rearranged

VJ

Rearranged

VJ

Ig protein None None pre-BCR

Small Pre-B

Rearranged

VDJ

V to J

rearrangement

Intracellular µ IgM IgM, IgD

IgD

µ

Fig. 4.14 Steps in the maturation and selection of B lymphocytes. The maturation of B lymphocytes

proceeds through sequential steps, each of which is characterized by particular changes in immunoglobulin

(Ig) gene expression and in the patterns of Ig protein expression. Pro-B cells begin to rearrange Ig heavy-chain

genes and large pre-B cells are selected to survive and proliferate if they successfully rearrange an Ig heavychain gene and assemble a pre-BCR. The pre-BCR consists of a membrane-associated Ig µ protein attached to

two other proteins called surrogate light chains because they take the place of the light chain in a complete Ig

molecule. Small pre-B cells initiate Ig light-chain gene rearrangement, immature B cells assemble a complete

membrane IgM receptor, and mature B cells coexpress IgD, with the same V regions and specificity as in the

first Ig produced. BCR, B cell receptor; HSC, hematopoietic stem cell; mRNA, messenger RNA.

92 CHAPTER 4 Antigen Recognition in the Adaptive Immune System

This process is called allelic exclusion, and it helps

ensure that each cell can only express a receptor of a

single specificity.

• Signals from the pre-BCR complex shut off expression

of the surrogate light-chain genes and open up the Ig

? light-chain locus making it available for recombination. The cells transiently stop dividing, and can

express the µ protein only in the cytoplasm (and not

on the cell surface) because they have no surrogate

light-chain proteins or regular light-chain proteins.

At this stage, these cells are called small pre-B cells.

• In small pre-B cells, V to J rearrangement of the ? lightchain gene is initiated, leading to production of the ?

protein and the assembly of cell surface IgM. The cells

at this next stage of differentiation are called immature B cells. The ? light chain is produced only if the

rearranged ? chain locus fails to express a functional

protein or if the ? chain generates a potentially harmful self-reactive receptor and has to be eliminated, by a

process called receptor editing, described later.

In immature B cells, the BCR complex delivers signals that promote survival, thus preserving cells that

express complete antigen receptors; this is the second

checkpoint during B cell maturation. Signals from the

antigen receptor also shut off production of the recombinase enzyme and further recombination at light-chain

loci. As a result, each B cell produces either one ? or

one ? light chain from one of the inherited parental

alleles. The presence of two sets of light-chain genes in

the genome simply increases the chance of completing

successful gene recombination and receptor expression.

Completion of B Cell Maturation

Further maturation occurs after the immature B cells leave

the bone marrow and enter the spleen. The final maturation step involves coexpression of IgD with IgM; this

occurs because in any given B cell, the recombined heavychain VDJ unit may be spliced either to Cµ or Cd exons in

the primary RNA transcript, giving rise to µ or d mRNA,

respectively. We know that the ability of B cells to respond

to antigens develops together with the coexpression of

IgM and IgD, but why both classes of receptor are needed

is not known. The IgM+IgD+ cell is the mature B cell, able

to respond to antigen in peripheral lymphoid tissues.

Selection of Mature B Cells

Developing B cells are positively selected based mainly

on expression of complete antigen receptors and not on

the recognition specificity of these cells. (This is fundamentally different in maturing T cells, as discussed

later.) The B cell repertoire is further shaped by negative

selection. In this process, if an immature B cell binds

an antigen in the bone marrow with high affinity, it

may re-express the VDJ recombinase enzyme, undergo

additional light-chain V-J recombination, generate a

different light chain, and thus change the specificity of

the antigen receptor, a process called receptor editing

(see Chapter 9). Some B cells that encounter antigens

in the bone marrow may die by apoptosis, also known

as deletion. The antigens that developing B cells may

recognize in the bone marrow are mostly self antigens

that are abundantly expressed throughout the body (i.e.,

are ubiquitous), such as blood proteins, and membrane

molecules common to all cells. Negative selection therefore eliminates potentially dangerous cells that can recognize and react against ubiquitous self antigens.

The process of Ig gene recombination is random

and cannot be inherently biased toward recognition of

microbes. However, the receptors produced are able to

recognize the antigens of many, varied microbes that the

immune system must defend against. The repertoire of B

lymphocytes is selected positively for expression of functional receptors and selected negatively against strong

recognition of self antigens. What is left after these

selection processes is a large collection of mature B cells,

which by chance include cells that are able to recognize

almost any microbial antigen that may be encountered.

Subsets of Mature B Cells

Most mature B cells are called follicular B cells because

they are found within lymph node and spleen follicles.

Marginal-zone B cells, which are found at the margins

of splenic follicles, develop from bone-marrow–derived

hematopoietic stem cells, as do follicular B cells. B-1 lymphocytes, a distinct population found at mucosal sites

and the peritoneal cavity, develop earlier from fetal-liver–

derived hematopoietic stem cells. The role of these B cell

subsets in humoral immunity is described in Chapter 7.

Maturation and Selection of T Lymphocytes

T cell progenitors migrate from the bone marrow to

the thymus, where the entire process of maturation

occurs (Fig. 4.15). The process of T lymphocyte maturation has some unique features, primarily related to the

specificity of different subsets of T cells for peptides displayed by different classes of MHC molecules.

CHAPTER 4 Antigen Recognition in the Adaptive Immune System 93

Early Steps in T Cell Maturation

The least developed progenitors in the thymus are called

pro-T cells or double-negative T cells because they do

not express CD4 or CD8. These cells expand in number

mainly under the influence of IL-7 produced in the thymus. TCR ß gene recombination, mediated by the VDJ

recombinase, occurs in some of these double-negative

cells. (The ?d T cells undergo similar recombination

involving TCR ? and d loci, but they belong to a distinct

lineage and are not discussed further.) If VDJ recombination is successful in one of the two inherited loci and

a TCR ß chain protein is synthesized, it is expressed on

the cell surface in association with an invariant protein

called pre-Ta, to form the pre-TCR complex of pre-T

cells. If the recombination in one of the two inherited

loci is not successful, recombination will take place on

the other locus. If that too fails and a complete TCR ß

chain is not produced in a pro-T cell, the cell dies.

The pre-TCR complex delivers intracellular signals

once it is assembled, similar to the signals from the

pre-BCR complex in developing B cells. These signals

promote survival, proliferation, and TCR a gene recombination and inhibit VDJ recombination in the second

ß chain locus (allelic exclusion). Failure to express the a

Pre-TCR

Negative

selection

Positive

selection

Positive

selection

Strong recognition of

either class I or class II

MHC + peptide

Weak recognition of

class I MHC + peptide

Weak recognition of

class II MHC + peptide

Apoptosis

Apoptosis

Stem

cell

Double

negative

(CD4–CD8–)

Pro-T cell

Double

positive

(CD4+CD8+)

immature

T cell

Pre-T

cell

Mature

CD4+ T cell

Mature

CD8+ T cell

Failure of

positive

selection

(death by

neglect)

No recognition of

MHC + peptide

Thymocyte APC

Fig. 4.15 Steps in the maturation and selection of major histocompatibility complex (MHC)–restricted

T lymphocytes. The maturation of T lymphocytes in the thymus proceeds through sequential steps often

defined by the expression of the CD4 and CD8 coreceptors. The T cell receptor (TCR) ß chain is first expressed

at the double-negative pre-T cell stage, and the complete T cell receptor is expressed in double-positive cells.

The pre-TCR consists of the TCR ß chain associated with a protein called pre-Ta. Maturation culminates in the

development of CD4+ and CD8+ single-positive T cells. As with B cells, failure to express antigen receptors at

any stage leads to death of the cells by apoptosis. Only class II MHC is shown for negative selection, but the

same process eliminates self-reactive class I MHC–restricted CD8+ T cells.

94 CHAPTER 4 Antigen Recognition in the Adaptive Immune System

chain and the complete TCR again results in death of the

cell. The surviving cells express the complete aß TCR

and both the CD4 and CD8 coreceptors; these cells are

called double-positive T cells.

Selection of Mature T Cells

Different clones of double-positive T cells express different aß TCRs. If the TCR of a T cell recognizes an MHC

molecule in the thymus, which must be a self MHC molecule displaying a self peptide, and if the interaction is of

low or moderate affinity, this T cell is selected to survive.

T cells that do not recognize an MHC molecule in the

thymus die by apoptosis; these T cells would not be useful

because they would be incapable of seeing MHC-displayed

cell-associated antigens in that individual. This preservation of self MHC–restricted (i.e., useful) T cells is the

process of positive selection. During this process, T cells

whose TCRs recognize class I MHC–peptide complexes

preserve the expression of CD8, the coreceptor that binds

to class I MHC, and lose expression of CD4, the coreceptor specific for class II MHC molecules. Conversely, if a

T cell recognizes class II MHC–peptide complexes, this

cell maintains expression of CD4 and loses expression of

CD8. Thus, what emerges are single-positive T cells (or

single-positive thymocytes), which are either CD8+ class I

MHC restricted or CD4+ class II MHC restricted. During

positive selection, the T cells also become committed to

different functional fates: the CD8+ T cells will differentiate into CTLs on activation, and the CD4+ cells will differentiate into cytokine-producing helper T cells.

Immature, double-positive T cells whose receptors

strongly recognize MHC-peptide complexes in the thymus undergo apoptosis. This is the process of negative

selection, and it serves to eliminate T lymphocytes that

could react in a harmful way against self proteins that are

expressed in the thymus. If a T cell that recognizes a self

peptide with high avidity were allowed to mature, recognition of the same self antigen in the periphery could

lead to harmful immune responses against self tissues,

so such a T cell must be eliminated. Some immature T

cells that recognize self antigens in the thymus do not

die but develop into regulatory T cells (see Chapter 9).

Most of the proteins present in the thymus are self proteins, because foreign (microbial and tumor) antigens

are typically captured and taken to secondary lymphoid

organs. Some of these self proteins are present throughout the body, and others are proteins that are restricted

to particular tissues but are expressed in thymic epithelial cells by special mechanisms, as discussed in Chapter

9 in the context of self-tolerance.

It may seem surprising that both positive selection

and negative selection are mediated by recognition of

the same set of self MHC–self peptide complexes in

the thymus. The two factors that determine the choice

between positive and negative selection are the affinity

of the TCR and the concentration of the self antigen in

the thymus. If a TCR strongly recognizes an abundant

self antigen in the thymus, that T cell will be negatively

selected, which makes sense because strong recognition

of an abundant self antigen has the potential for causing autoimmunity. However, if a TCR recognizes a self

peptide–self MHC complex weakly, that T cell will be

positively selected because there is a reasonable chance

the T cell will recognize a foreign peptide presented by

self MHC strongly. This is the process that gives rise to

the repertoire of functional T cells.

SUMMARY

• In the adaptive immune system, the molecules

responsible for specific recognition of antigens are

antibodies and T cell antigen receptors.

• Antibodies (also called immunoglobulins) may

be produced as membrane receptors of B lymphocytes and as proteins secreted by antigen-stimulated

B cells that have differentiated into antibody-secreting plasma cells. Secreted antibodies are the effector

molecules of humoral immunity, capable of neutralizing microbes and microbial toxins and eliminating

them by activating various effector mechanisms.

• T cell receptors (TCRs) are membrane receptors and

are not secreted.

 • The core structure of antibodies consists of two identical heavy chains and two identical light chains,

forming a disulfide-linked complex. Each chain consists of a variable (V) region, which is the portion that

recognizes antigen, and a constant (C) region, which

provides structural stability and, in heavy chains,

performs the effector functions of antibodies. The

V region of one heavy chain and of one light chain

together form the antigen-binding site, and thus the

core structure has two identical antigen-binding sites.

 • T cell receptors consist of an a chain and a ß chain.

Each chain contains one V region and one C region,

and both chains participate in the recognition of

CHAPTER 4 Antigen Recognition in the Adaptive Immune System 95

antigens, which for most T cells are peptides displayed by MHC molecules.

• The V regions of immunoglobulin (Ig) and TCR molecules contain hypervariable segments, also called

complementarity-determining regions (CDRs),

which are the regions of contact with antigens.

• The genes that encode antigen receptors consist of

multiple segments separated in the germline and

brought together during maturation of lymphocytes.

In B cells, the Ig gene segments undergo recombination as the cells mature in the bone marrow, and in

T cells, the TCR gene segments undergo recombination during maturation in the thymus.

• Receptors of different specificities are generated in

part by different combinations of V, D, and J gene

segments. The process of recombination introduces

variability in the nucleotide sequences at the sites of

recombination by adding or removing nucleotides

from the junctions. The result of this introduced

variability is the development of a diverse repertoire

of lymphocytes, in which clones of cells with different antigen specificities express receptors that differ

in sequence and recognition, and most of the differences are concentrated at the regions of gene recombination.

• During their maturation, lymphocytes are selected to

survive at several checkpoints; only cells with complete functional antigen receptors are preserved and

expanded. In addition, T lymphocytes are positively

selected to recognize peptide antigens displayed

by self MHC molecules and to ensure that the recognition of the appropriate type of MHC molecule

matches the coreceptor preserved.

• Immature lymphocytes that strongly recognize self

antigens are negatively selected and prevented from

completing their maturation, thus eliminating cells

with the potential of reacting in harmful ways against

self tissues.

REVIEW QUESTIONS

1. What are the functionally distinct domains (regions)

of antibody and TCR molecules? What features of the

amino acid sequences in these regions are important

for their functions?

2. What are the differences in the types of antigens recognized by antibodies and TCRs?

3. What mechanisms contribute to the diversity of antibody and TCR molecules? Which of these mechanisms contributes the most to the diversity?

4. What are some of the checkpoints during lymphocyte maturation that ensure survival of the useful

cells?

5. What is the phenomenon of negative selection, and

what is its importance?

Answers to and discussion of the Review Questions are

available at Student Consult.

96

Activation of T Lymphocytes

5

T Cell–Mediated Immunity

T lymphocytes perform multiple functions in defending

against infections by various kinds of microbes. A major

role for T lymphocytes is in cell-mediated immunity,

which provides defense against infections by microbes that

live and reproduce inside host cells. In all viral and some

bacterial, fungal, and protozoan infections, microbes may

find a haven inside cells, from where they must be eliminated by cell-mediated immune responses (Fig. 5.1).

• Many microbes are ingested by phagocytes as part

of the early defense mechanisms of innate immunity

and are killed by microbicidal mechanisms that are

largely limited to phagocytic vesicles (to protect the

cells themselves from damage by these mechanisms).

However, some of these microbes have evolved to

resist the microbicidal activities of phagocytes and

are able to survive, and even replicate, in the vesicles

of phagocytes. In such infections, T cells stimulate the

ability of macrophages to kill the ingested microbes.

• Some extracellular microbes, such as bacteria and

fungi, are readily destroyed if they are phagocytosed,

especially by neutrophils. Other extracellular pathogens, such as helminthic parasites, are destroyed by

special types of leukocytes (eosinophils). In these

infections, T cells provide defense by recruiting the

leukocytes that destroy the microbes.

• Some microbes, notably viruses, are able to infect and

replicate inside a wide variety of cells, and parts of

the life cycles of the viruses take place in the cytosol

and nucleus. These infected cells often do not possess

intrinsic mechanisms for destroying the microbes,

especially outside vesicles. Even some phagocytosed

microbes within macrophages can escape into the

cytosol and evade the microbicidal mechanisms of

the vesicular compartment. T cells kill the infected

cells, thus eliminating the reservoir of infection.

Other populations of T cells help B cells to produce

antibodies as part of humoral immune responses (see

Chapter 7). Although our emphasis in this chapter is

on defense against infections, the principal physiologic

function of the immune system, some T cells, especially

CHAPTER OUTLINE

Phases of T Cell Responses, 97

Antigen Recognition and Costimulation, 100

Recognition of Peptide-MHC Complexes, 100

Role of Adhesion Molecules in T Cell Responses, 102

Role of Costimulation in T Cell Activation, 103

Inhibitory Receptors of T Cells, 105

Stimuli for Activation of CD8+ T Cells, 105

Biochemical Pathways of T Cell Activation, 106

Functional Responses of T Lymphocytes to Antigen

and Costimulation, 109

Secretion of Cytokines and Expression of Cytokine

Receptors, 109

Clonal Expansion, 110

Differentiation of Naive T Cells into Effector Cells, 111

Development of Memory T Lymphocytes, 112

Migration of T Lymphocytes in

Cell-Mediated Immune Reactions, 113

Decline of the Immune Response, 116

Summary, 117

CHAPTER 5 T Cell–Mediated Immunity 97

CD8+ T cells, also destroy cancerous cells. This role of T

cells is discussed in Chapter 10.

Most of the functions of T lymphocytes—activation of phagocytes, killing of infected and tumor cells,

and help for B cells—require that the T lymphocytes

interact with other cells, which may be phagocytes,

infected host cells, or B lymphocytes. Furthermore, the

initiation of T cell responses requires that naive T cells

recognize antigens displayed by dendritic cells, which

capture antigens and concentrate them in lymphoid

organs. Thus, T lymphocytes work by communicating

with other cells. Recall that the specificity of T cells for

peptides displayed by major histocompatibility complex

(MHC) molecules ensures that the T cells can see and

respond only to antigens associated with other host cells

(see Chapters 3 and 4). This chapter discusses the way

in which T lymphocytes are activated by recognition of

cell-associated antigens and other stimuli. We address

the following questions:

• What signals are needed to activate T lymphocytes,

and what cellular receptors are used to sense and

respond to these signals?

• How are the few naive T cells specific for any microbe

converted into the large number of effector T cells

that have specialized functions and the ability to

eliminate diverse microbes?

• What molecules are produced by T lymphocytes that

mediate their communications with other cells, such

as macrophages, B lymphocytes, and other leukocytes?

After describing here how T cells recognize and

respond to the antigens of cell-associated microbes, in

Chapter 6, we discuss how these T cells function to eliminate the microbes.

PHASES OF T CELL RESPONSES

Naive T lymphocytes recognize antigens in the peripheral (secondary) lymphoid organs, which initiates

proliferation of the T cells and their differentiation

into effector and memory cells, and the effector cells

perform their functions when they are activated by the

same antigens in any infected tissue (Fig. 5.2). Naive T

cells express antigen receptors and coreceptors that function in recognizing cells harboring microbes, but naive

Intracellular microbes Examples

Intracellular bacteria:

 Mycobacteria

 Listeria monocytogenes

 Legionella pneumophila

Fungi:

 Cryptococcus neoformans

Protozoa:

 Leishmania

 Trypanosoma cruzi

Viruses:

 All

Rickettsiae:

 All

Protozoa:

 Plasmodium falciparum

 Cryptosporidium parvum

Phagocytosed microbes

that survive within

phagolysosomes

Nonphagocytic cell (e.g., epithelial cell)

Microbes that escape

from phagolysosomes

into cytoplasm

Virus

Microbes that infect

nonphagocytic cells

Cellular

receptor

for virus

A

B

Phagocyte

Fig. 5.1 Types of intracellular microbes combated by T cell–mediated immunity. A, Microbes may be

ingested by phagocytes and may survive within vesicles (phagolysosomes) or escape into the cytosol, where

they are not susceptible to the microbicidal mechanisms of the phagocytes. B, Viruses may infect many cell

types, including nonphagocytic cells, and replicate in the nucleus and cytosol of the infected cells. Rickettsiae

and some protozoa are obligate intracellular parasites that reside in nonphagocytic cells.

Migration of effector

T cells and other

leukocytes to

site of antigen

Inflammation,

phagocytosis and

killing of microbes

CD4+

T cells

Naive

T cell

CD8+

T cells

CD4+

effector

T cells

CD8+

T cells

(CTLs)

Differentiated

effector T cells

enter circulation

Effector T cells

encounter antigens

in peripheral tissues

Cells with intracellular microbes

Activation of

effector T cells

CTL killing of

infected cell

Induction of

response

T cell effector

functions

Antigen

recognition in

lymphoid organs

T cell

expansion and

differentiation

Cytokines

Fig. 5.2 Induction and effector phases of cell-mediated immunity. Induction of response: Naive CD4+ T

cells and CD8+ T cells recognize peptides that are derived from protein antigens and presented by dendritic

cells (DCs) in peripheral lymphoid organs. The T lymphocytes are stimulated to proliferate and differentiate

into effector cells, many of which enter the circulation. Some of the activated CD4+ T cells remain in the

lymph node, migrate into follicles, and help B cells to produce antibodies (shown in Fig. 5.13). Migration of

effector T cells and other leukocytes to site of antigen: effector T cells and other leukocytes migrate through

blood vessels in peripheral tissues by binding to endothelial cells that have been activated by cytokines produced in response to infection in these tissues. T cell effector functions: CD4+ T cells recruit and activate

phagocytes to destroy microbes, and CD8+ cytotoxic T lymphocytes (CTLs) kill infected cells.

CHAPTER 5 T Cell–Mediated Immunity 99

cells are incapable of performing the effector functions

required for eliminating the microbes. Differentiated

effector cells are capable of performing these functions,

which they do at any site of infection. In this chapter,

we focus on the initial responses of naive T cells to antigens. The development of effector T lymphocytes and

their functions in cell-mediated immunity are described

in Chapter 6 and the roles of helper T cells in antibody

responses in Chapter 7.

The responses of naive T lymphocytes to cellassociated microbial antigens consist of a series of

sequential steps that result in an increase in the number of antigen-specific T cells and the conversion of

naive T cells to effector and memory cells (Fig. 5.3).

• One of the earliest responses is the secretion of

cytokines required for growth and differentiation

and increased expression of receptors for various

cytokines. The cytokine interleukin-2 (IL-2), which

is produced by antigen-activated T cells, stimulates proliferation of these cells, resulting in a rapid

increase in the number of antigen-specific lymphocytes, a process called clonal expansion.

• The activated lymphocytes differentiate, resulting in

the conversion of naive T cells into a population of

effector T cells, which function to eliminate microbes.

• Many of the effector T cells leave the lymphoid

organs, enter the circulation, and migrate to any site

of infection, where they can eradicate the infection.

Some activated T cells may remain in the lymph

node, where they provide signals to B cells that promote antibody responses against the microbes.

• Some of the progeny of the T cells that have proliferated

in response to antigen develop into memory T cells,

which are long-lived, circulate or reside in tissues for

years, and are ready to respond rapidly to subsequent

exposure to the same microbe.

• As effector T cells eliminate the infectious agent, the

stimuli that triggered T cell expansion and differentiation also are eliminated. As a result, most of the cells

in the greatly expanded clones of antigen-specific

effector lymphocytes die, returning the system to a

resting state, with only memory cells remaining from

the immune response.

This sequence of events is common to both CD4+

and CD8+ T lymphocytes, although there are important

differences in the properties and effector functions of

CD4+ and CD8+ cells, as discussed in Chapter 6.

Naive and effector T cells have different patterns of

circulation and migration through tissues, which are

critical for their different roles in immune responses.

Lymphoid organs Peripheral tissues

Cytokine Proliferation Differentiation

secretion

and cytokine

receptor

expression

Antigen

recognition

Effector functions

APC Naive

T cell

(CD4+

or CD8+) Cytokines

(e.g., IL-2)

IL-2R

CD4+: Activation

of macrophages,

B cells, other

cells

CD8+: Killing

of infected

“target cells”;

macrophage

activation

Effector T cell

(CD4+ helper

or CD8+ CTL)

Memory T cell

(CD4+ or CD8+)

Fig. 5.3 Steps in the activation of T lymphocytes. Naive T cells recognize major histocompatibility complex

(MHC)-associated peptide antigens displayed on antigen-presenting cells and other signals (not shown). The

T cells respond by producing interleukin-2 (IL-2) and expressing receptors for IL-2, leading to an autocrine

pathway of cell proliferation. The result is expansion of the clone of T cells that are specific for the antigen.

Some of the progeny differentiate into effector cells, which serve various functions in cell-mediated immunity, and memory cells, which survive for long periods. Other changes associated with activation, such as

the expression of various surface molecules, are not shown. APC, Antigen-presenting cell; CTL, cytotoxic

T lymphocyte; IL-2R, interleukin-2 receptor.

100 CHAPTER 5 T Cell–Mediated Immunity

As discussed in previous chapters, naive T lymphocytes

constantly recirculate through peripheral lymphoid

organs searching for foreign protein antigens. The antigens of microbes are transported from the portals of

entry of the microbes to the same regions of peripheral

lymphoid organs through which naive T cells recirculate. In these organs, the antigens are processed and

displayed by MHC molecules on dendritic cells, the

antigen-presenting cells (APCs) that are the most efficient stimulators of naive T cells (see Chapter 3). When

a T cell recognizes antigen, it is transiently arrested on

the dendritic cell and it initiates an activation program.

Activation results in proliferation and differentiation,

and then the cells may leave the lymphoid organ and

migrate preferentially to the inflamed tissue, the original source of the antigen. The control of this directed

migration is discussed later in this chapter.

With this overview, we proceed to a description of

the stimuli required for T cell activation and regulation.

We then describe the biochemical signals that are generated by antigen recognition and the biologic responses

of the lymphocytes.

ANTIGEN RECOGNITION AND

COSTIMULATION

The initiation of T cell responses requires multiple

receptors on the T cells recognizing their specific

ligands on APCs (Fig. 5.4).

• The T cell receptor (TCR) recognizes MHC-associated

peptide antigens.

• CD4 or CD8 coreceptors on the T cells bind to MHC

molecules on the APC and work with the TCR complex to deliver activating signals.

• Adhesion molecules strengthen the binding of T cells

to APCs.

• Molecules called costimulators, which are expressed

on APCs after encounter with microbes, bind to

costimulatory receptors on the naive T cells, thus

promoting responses to infectious pathogens.

• Cytokines amplify the T cell response and direct it

along various differentiation pathways.

The roles of these molecules in T cell responses to

antigens are described next. Cytokines are discussed

mainly in Chapter 6.

Recognition of Peptide-MHC Complexes

The TCR for antigen and the CD4 or CD8 coreceptor

together recognize complexes of peptide antigens and

MHC molecules on APCs, and this recognition provides the initiating, or first, signal for T cell activation

(Fig. 5.5). The TCRs expressed on all CD4+ and CD8+ T

cells consist of an a chain and a ß chain, both of which

participate in antigen recognition (see Fig. 4.7). (A small

subset of T cells expresses TCRs composed of ? and d

chains, which do not recognize MHC-associated peptide antigens.) The TCR of a T cell specific for a foreign

(e.g., microbial) peptide recognizes the displayed peptide and simultaneously recognizes residues of the MHC

molecule located around the peptide-binding cleft. Every

mature MHC-restricted T cell expresses either CD4 or

CD8, both of which are called coreceptors because they