2688 PART 11 Immune-Mediated, Inflammatory, and Rheumatologic Disorders

In general, cytokines exert their effects by influencing gene activation that results in cellular activation, growth, differentiation, functional cell-surface molecule expression, and cellular effector function.

In this regard, cytokines can have dramatic effects on the regulation

of immune responses and the pathogenesis of a variety of diseases.

Indeed, T cells have been categorized on the basis of the pattern of

cytokines that they secrete, which results in either humoral immune

response (TH2) or cell-mediated immune response (TH1). A third type

of T helper cell is the TH17 cell that contributes to host defense against

extracellular bacteria and fungi, particularly at mucosalsites(Fig. 349-2).

Cytokine receptors can be grouped into five general families based on

similarities in their extracellular amino acid sequences and conserved

structural domains. The immunoglobulin (Ig) superfamily represents a

large number of cell-surface and secreted proteins. The IL-1 receptors

(type 1, type 2) are examples of cytokine receptors with extracellular

Ig domains.

The hallmark of the hematopoietic growth factor (type 1) receptor

family is that the extracellular regions of each receptor contain two

conserved motifs. One motif, located at the N terminus, is rich in

cysteine residues. The other motif is located at the C terminus proximal

to the transmembrane region and comprises five amino acid residues,

tryptophan-serine-X-tryptophan-serine (WSXWS). This family can be

grouped on the basis of the number of receptor subunits they have and

on the utilization of shared subunits. A number of cytokine receptors,

i.e., IL-6, IL-11, IL-12, and leukemia inhibitory factor, are paired with

gp130. There is also a common 150-kDa subunit shared by IL-3, IL-5,

and granulocyte-macrophage colony-stimulating factor (GM-CSF)

receptors. The gamma chain (γc

) of the IL-2 receptor is common to the

IL-2, IL-4, IL-7, IL-9, and IL-15 receptors. Thus, the specific cytokine

receptor is responsible for ligand-specific binding, whereas the subunits such as gp130, the 150-kDa subunit, and γc are important in signal

transduction. The γc gene is on the X chromosome, and mutations in

the γc protein result in the X-linked form of severe combined immune

deficiency syndrome (X-SCID) (Chap. 351).

The members of the interferon (type II) receptor family include the

receptors for IFN-γ and -β, which share a similar 210-amino-acid

binding domain with conserved cysteine pairs at both the amino and

carboxy termini. The members of the TNF (type III) receptor family

share a common binding domain composed of repeated cysteine-rich

regions. Members of this family include the p55 and p75 receptors for

TNF (TNF-R1 and TNF-R2, respectively); CD40 antigen, which is an

important B-cell surface marker involved in immunoglobulin isotype

switching; fas/Apo-1, whose triggering induces apoptosis; CD27 and

CD30, which are found on activated T cells and B cells; and nerve

growth factor receptor.

The common motif for the seven transmembrane helix family was

originally found in receptors linked to GTP-binding proteins. This

family includes receptors for chemokines (Table 349-7), β-adrenergic

receptors, and retinal rhodopsin. It is important to note that two members of the chemokine receptor family, CXC chemokine receptor type

4 (CXCR4) and β chemokine receptor type 5 (CCR5), have been found

to serve as the two major co-receptors for binding and entry of HIV-1

into CD4-expressing host cells (Chap. 202).

Significant advances have been made in defining the signaling

pathways through which cytokines exert their intracellular effects.

The Janus family of protein tyrosine kinases (JAK) is a critical element involved in signaling via the hematopoietin receptors. Four JAK

kinases, JAK1, JAK2, JAK3, and Tyk2, preferentially bind different

cytokine receptor subunits. Cytokine binding to its receptor brings

the cytokine receptor subunits into apposition and allows a pair

of JAKs to transphosphorylate and activate one another. The JAKs

then phosphorylate the receptor on the tyrosine residues and allow

Inhibitory receptor

Activating receptor

No activating

ligands

Activating

ligands

Activating

ligands

No activating

ligands

No HLA

class I

No HLA

class I

HLA

class I

HLA

class I

Target NK

NK

NK

NK

Target

Target

Target

No response

No response

NK attacks

target cells

Outcome

determined by

balance of signals

A

B

C

D

FIGURE 349-4 Encounters between natural killer (NK) cells: Potential targets

and possible outcomes. The amount of activating and inhibitory receptors on the

NK cells and the amount of ligands on the target cell, as well as the qualitative

differences in the signals transduced, determine the extent of the NK response.

A. When target cells have no HLA class I or activating ligands, NK cells cannot

kill target cells. B. When target cells bear self-HLA, NK cells cannot kill targets.

C. When target cells are pathogen-infected and have downregulated HLA and

express activating ligands, NK cells kill target cells. D. When NK cells encounter

targets with both self-HLA and activating receptors, then the level of target killing

is determined by the balance of inhibitory and activating signals to the NK cell.

HLA, human leukocyte antigen. (Republished with permission of Annual Review of

Immunology, from NK Cell Recognition, L Lanier 23:225,2005: permission conveyed

through Copyright Clearance Center, Inc.)

Mannose-binding

lectin

activation pathway

MBL-MASP1-MASP2

Microbes with terminal

mannose groups

Classic

activation

pathway

Bacteria, fungi, virus,

or tumor cells

C3 (H2O)

Alternative

activation

pathway

C1q-C1r-C1s

Antigen/antibody

immune complex

C4

C4

C2

C3

C3b

C5

C6

C7

C8

poly-C9

C2

P

D

B

Terminal

pathway

Immune complex

modification

Clearance of

apoptotic cells

Anaphylatoxin

Anaphylatoxin

Lysis

Opsonin

Lymphocyte

activation

Membrane perturbation

FIGURE 349-5 The four pathways and the effector mechanisms of the complement

system. Dashed arrows indicate the functions of pathway components. (Reproduced

with permission from BJ Morley, MJ Walport: The Complement Facts Books.

London, Academic Press, 2000.)

Introduction to the Immune System

2689CHAPTER 349

signaling molecules to bind to the receptor, whereby the signaling molecules become phosphorylated. Signaling molecules bind the receptor

because they have domains (SH2, or src homology 2 domains) that can

bind phosphorylated tyrosine residues. There are a number of these

important signaling molecules that bind the receptor, such as the adapter molecule SHC, which can couple the receptor to the activation of

the mitogen-activated protein kinase pathway. In addition, an important class of substrate of the JAKs is the signal transducers and activators of transcription (STAT) family of transcription factors. STATs have

SH2 domains that enable them to bind to phosphorylated receptors,

where they are then phosphorylated by the JAKs. It appears that different STATs have specificity for different receptor subunits. The STATs

then dissociate from the receptor and translocate to the nucleus, bind

to DNA motifs that they recognize, and regulate gene expression. The

STATs preferentially bind DNA motifs that are slightly different from

one another and thereby control transcription of specific genes. The

importance of this pathway is particularly relevant to lymphoid development. Mutations of JAK3 itself also result in a disorder identical to

X-SCID; however, because JAK3 is found on chromosome 19 and not

on the X chromosome, JAK3 deficiency

occurs in boys and girls (Chap. 351).

■ THE ADAPTIVE IMMUNE

SYSTEM

Adaptive immunity is characterized by

antigen-specific responses to a foreign

antigen or pathogen. A key feature of

adaptive immunity is that following

the initial contact with antigen (immunologic priming), subsequent antigen

exposure leads to more rapid and vigorous immune responses (immunologic

memory). The adaptive immune system

consists of dual limbs of cellular and

humoral immunity. The principal effectors of cellular immunity are T lymphocytes, whereas the principal effectors of

humoral immunity are B lymphocytes.

Both B and T lymphocytes derive from

a common stem cell (Fig. 349-6).

The proportion and distribution of

immunocompetent cells in various tissues reflect cell traffic, homing patterns,

and functional capabilities. Bone marrow is the major site of maturation of

B cells, monocytes-macrophages, DCs,

and granulocytes and contains pluripotent stem cells that, under the influence

of various colony-stimulating factors,

can give rise to all hematopoietic cell

types. T-cell precursors also arise from

hematopoietic stem cells and home to

the thymus for maturation. Mature T

lymphocytes, B lymphocytes, monocytes, and DCs enter the circulation and

home to peripheral lymphoid organs

(lymph nodes, spleen) and mucosal

surface-associated lymphoid tissue

(gut, genitourinary, and respiratory

tracts) as well as the skin and mucous

membranes and await activation by foreign antigen.

T Cells The pool of effector T cells

is established in the thymus early in life

and is maintained throughout life both

by new T-cell production in the thymus

and by antigen-driven expansion of

virgin peripheral T cells into “memory”

T cells that reside in peripheral lymphoid organs. The thymus exports

~2% of the total number of thymocytes per day throughout life, with

the total number of daily thymic emigrants decreasing by ~3% per

year during the first four decades of life.

Mature T lymphocytes constitute 70–80% of normal peripheral

blood lymphocytes (only 2% of the total-body lymphocytes are

contained in peripheral blood), 90% of thoracic duct lymphocytes,

30–40% of lymph node cells, and 20–30% of spleen lymphoid cells.

In lymph nodes, T cells occupy deep paracortical areas around B-cell

germinal centers, and in the spleen, they are located in periarteriolar

areas of white pulp (Chap. 66). T cells are the primary effectors of cellmediated immunity, with subsets of T cells maturing into CD8+ cytotoxic T cells capable of lysis of virus-infected or foreign cells (shortlived effector T cells) and CD4+ T cells capable of T-cell help for

CD8+ T-cell and B-cell development. Two populations of long-lived

memory T cells are triggered by infections: effector memory and central memory T cells. Effector memory T cells reside in nonlymphoid

organs and respond rapidly to repeated pathogenic infections with

cytokine production and cytotoxic functions to kill virus-infected

CD34+

Hematopoietic

α,β Germline

Hematopoietic

stem cell

CD34+

CD7

CD2

CD3

α- Germline

β- VDJ Rearranged

Thymus medulla

and peripheral

T-cell pools

CD7

CD2

CD3, TCRαβ

CD8

CD7

CD2

CD3, TCRαβ

CD4

CD7

CD2

CD3, TCRγδ

CD8

CD7

CD2

cCD3, TCRαβ

CD1

CD4, CD8

α-VJ Rearranged

β- VDJ Rearranged

Early

pro-B cell

Late

pro-B cell

Large

pre-B cell

Small

pre-B cell

Immature

B cell

Mature

B cell

D-J

rearranging

Germline

Absent

CD34

CD10

CD38

VDJ

rearranging

Germline

Absent

CD10

CD19

CD38

CD40

VDJ

rearranged

Germline

µ H-chain at

surface as

part of the pre-B

receptor (orange)

containing

surrogate light

chain (SLC).

Receptor is

mainly intracellular

CD19

CD20

CD38

VDJ

rearranged

VJ

rearranging

µ H-chain in

cytoplasm

CD19

CD20

CD38

VDJ

rearranged

IgM expressed

on cell surface

CD19

CD20

VJ

rearranged

VDJ

rearranged

IgD and IgM

made from

alternatively

spliced

H-chain

transcripts

CD19

CD20

CD21

VJ

rearranged

Heavy-chain

genes

Light-chain

genes

Surface Ig

Surface

marker

proteins

Pro-T Pro-T Pro-T Immature T Mature T

Mature T

Mature T

CD34+

CD7lo+ or -

α,β Germline

IgD

IgM

FIGURE 349-6 Development stages of T and B cells. Elements of the developing T- and B-cell receptor for antigen

are shown schematically. The classification into the various stages of B-cell development is primarily defined by

rearrangement of the immunoglobulin (Ig) heavy (H) and light (L) chain genes and by the absence or presence of

specific surface markers. The classification of stages of T-cell development is primarily defined by cell-surface marker

protein expression (sCD3, surface CD3 expression; cCD3, cytoplasmic CD3 expression; TCR, T-cell receptor). For B-cell

development, the pre-B-cell receptor is shown as a blue-orange B-cell receptor. (From Janeway’s Immunobioogy,

9th ed by Kenneth Murphy and Casey Weaver. Copyright © 2017 by Garland Science, Taylor & Francis Group, LLC. Used

by permission of W. W. Norton & Company, Inc.)

2690 PART 11 Immune-Mediated, Inflammatory, and Rheumatologic Disorders

cells. Central memory T cells home to lymphoid organs where they

replenish long- and short-lived and effector memory T cells as

needed.

In general, CD4+ T cells are the primary regulatory cells of T and B

lymphocyte and monocyte function by the production of cytokines and

by direct cell contact (Fig. 349-2). In addition, T cells regulate erythroid

cell maturation in bone marrow and, through cell contact (CD40 ligand), have an important role in activation of B cells and induction of Ig

isotype switching. Considerable evidence now exists that colonization

of the gut by commensal bacteria (the gut microbiome) is responsible

for expansion of the peripheral CD4+ T-cell compartment in normal

children and adults.

Human T cells express cell-surface proteins that mark stages of intrathymic T-cell maturation oridentify specific functionalsubpopulations of

mature T cells. Many of these molecules mediate or participate in important T-cell functions (Table 349-1, Fig. 349-6, Chap. 350).

The earliest identifiable T-cell precursors in bone marrow are CD34+

pro-T cells (i.e., cells in which TCR genes are neither rearranged nor

expressed). In the thymus, CD34+ T-cell precursors begin cytoplasmic

(c) synthesis of components of the CD3 complex of TCR-associated

molecules (Fig. 349-6). Within T-cell precursors, TCR for antigen gene

rearrangement yields two T-cell lineages, expressing either TCR-αβ

chains or TCR-γδ chains. T cells expressing the TCR-αβ chains constitute the majority of peripheral T cells in blood, lymph node, and spleen

and terminally differentiate into either CD4+ or CD8+ cells. Cells

expressing TCR-γδ chains circulate as a minor population in blood;

their functions, although not fully understood, have been postulated

to be those of immune surveillance at epithelial surfaces and cellular

defenses against mycobacterial organisms and other intracellular bacteria through recognition of bacterial lipids.

In the thymus, the recognition of self-peptides on thymic epithelial

cells, thymic macrophages, and DCs plays an important role in shaping

T-cell repertoire. As immature cortical thymocytes begin to express

surface TCR for antigen, thymocytes with TCRs capable of interacting

with self-peptides in the context of self-MHC antigens with low affinity

are activated and survive (positive selection). Thymocytes with TCRs

that are incapable of binding to self-MHC antigens or bind with high

affinity die of attrition (no selection) or by apoptosis (negative selection). Thymocytes that are positively selected undergo maturation into

CD4 or CD8 single positive T cells, and then migrate to the thymus

medulla where they interact with self-peptide–self-MHC molecules,

where they can again undergo selection. The purpose of negative and

positive thymocyte selection is to eliminate potential pathogenic autoreactive T cells, and at the same time, select a repertoire of mature T

cells capable of recognizing foreign antigens.

Mature TCRab thymocytesthat are positively selected are functional

MHC class II–restricted CD4+ T cells (Fig. 349-2), or they are CD8+

T cells destined to become CD8+ MHC class I–restricted cytotoxic T

cells. MHC class I or class II restriction means that T cells recognize

antigen peptide fragments only when they are presented in the antigenrecognition site of a class I or class II MHC molecule, respectively.

After thymocyte maturation and selection, CD4 and CD8 thymocytes

leave the thymus and migrate to the peripheral immune system. The

thymus can continue to be a contributor to the peripheral immune system well into adult life, both normally and when the peripheral T-cell

pool is damaged, such as occurs in AIDS and cancer chemotherapy.

MOLECULAR BASIS OF T-CELL RECOGNITION OF ANTIGEN The TCR

for antigen is a complex of molecules consisting of an antigenbinding heterodimer of either αβ or γδ chains noncovalently linked

with five CD3 subunits (γ, δ, ε, ζ, and η) (Fig. 349-7). The CD3 ζ chains

are either disulfide-linked homodimers (CD3-ζ2

) or disulfide-linked

heterodimers composed of one ζ chain and one η chain. TCR-αβ or

TCR-γδ molecules must be associated with CD3 molecules to be

PtdIns (4,5)P3

Lipid raft

APC

CD3 TCR

β α

InsP3

DAG

PKC RASGRP

Activation of downstream

effectors such as NFkB, AP1,

and NFAT to induce specific

gene transcription leading to

cell proliferation and differentiation

Release of Ca2+

Translocation

of NFAT to the nucleus

ZAP70

Integrin activation

MAPK activation

Cytoskeletal

reorganization

RAS

SOS

GRB2

LAT

PLCγ

GADS

HPK1

NCK

ADAP

ITK

LCK

VAV1

LFA-1

CD28 B7-1

CD2 LFA-3

ICAM-1

FIGURE 349-7 Signaling through the T-cell receptor. Activation signals are mediated via immunoreceptor tyrosine-based activation (ITAM) sequences in LAT and CD3

chains (blue bars) that bind to enzymes and transduce activation signals to the nucleus via the indicated intracellular activation pathways. Ligation of the T-cell receptor

(TCR) by MHC complexed with antigen results in sequential activation of LCK and γ-chain-associated protein kinase of 70 kDa (ZAP70). ZAP70 phosphorylates several

downstream targets, including LAT (linker for activation of T cells) and SLP76 (SCR homology 2 [SH2] domain-containing leukocyte protein of 76 kDa). SLP76 is recruited

to membrane-bound LAT through its constitutive interaction with GADS (GRB2-related adaptor protein). Together, SLP76 and LAT nucleate a multimolecular signaling

complex, which induces a host of downstream responses, including calcium flux, mitogen-activated protein kinase (MAPK) activation, integrin activation, and cytoskeletal

reorganization. APC, antigen-presenting cell; NFAT, nuclear factor of activated T cells. (Reproduced with permission from GA Koretzky, F Abtahian, MA Silverman. SLP76

and SLP65: complex regulation of signalling in lymphocytes and beyond. Nat Rev Immunol 6:67, 2006.)

Introduction to the Immune System

2691CHAPTER 349

inserted into the T-cell surface membrane, TCR-α being paired with

TCR-β and TCR-γ being paired with TCR-δ. Molecules of the CD3

complex mediate transduction of T-cell activation signals via TCRs,

whereas TCR-α and -β or -γ and -δ molecules combine to form the

TCR antigen-binding site.

The α, β, γ, and δ TCR for antigen molecules have amino acid

sequence homology and structural similarities to immunoglobulin

heavy and light chains and are members of the immunoglobulin gene

superfamily of molecules. The genes encoding TCR molecules are

encoded as clusters of gene segments that rearrange during T-cell maturation. This creates an efficient and compact mechanism for housing

the diversity requirements of antigen receptor molecules. The TCR-α

chain is on chromosome 14 and consists of a series of V (variable), J

(joining), and C (constant) regions. The TCR-β chain is on chromosome 7 and consists of multiple V, D (diversity), J, and C TCR-β loci.

The TCR-γ chain is on chromosome 7, and the TCR-δ chain is in the

middle of the TCR-α locus on chromosome 14. Thus, molecules of

the TCR for antigen have constant (framework) and variable regions,

and the gene segments encoding the α, β, γ, and δ chains of these

molecules are recombined and selected in the thymus, culminating in

synthesis of the completed molecule. In both T- and B-cell precursors

(see below), DNA rearrangements of antigen receptor genes involve the

same enzymes, recombinase activating gene RAG1 and RAG2, both

DNA-dependent protein kinases.

TCR diversity is created by the different V, D, and J segments that are

possible for each receptor chain by the many permutations of V, D, and J

segment combinations, by “N-region diversification” due to the addition

of nucleotides at the junction of rearranged gene segments, and by the

pairing of individual chains to form a TCR dimer. As T cells mature in the

thymus, the repertoire of antigen-reactive T cells is modified by selection

processes that eliminate many autoreactive T cells, enhance the proliferation of cells that function appropriately with self-MHC molecules and

antigen, and allow T cells with nonproductive TCR rearrangementsto die.

TCR-αβ cells do not recognize native protein or carbohydrate antigens. Instead, T cells recognize only short (~9–13 amino acids) peptide

fragments derived from protein antigens taken up or produced in APCs.

Foreign antigens may be taken up by endocytosisinto acidified intracellular

vesicles or by phagocytosis and degraded into small peptides that associate

with MHC class II molecules (exogenous antigen-presentation pathway).

Other foreign antigens arise endogenously in the cytosol (such as from

replicating viruses) and are broken down into small peptides that associate

with MHC class I molecules (endogenous antigen-presenting pathway).

Thus, APCs proteolytically degrade foreign proteins and display peptide

fragments embedded in the MHC class I or II antigen-recognition site

on the MHC molecule surface, where foreign peptide fragments are

available to bind to TCR-αβ or TCR-γδ chains of reactive T cells. CD4

molecules act as adhesives and, by direct binding to MHC class II (DR,

DQ, or DP) molecules, stabilize the interaction of TCR with peptide

antigen (Fig. 349-7). Similarly, CD8 molecules also act as adhesives to

stabilize the TCR-antigen interaction by direct CD8 molecule binding

to MHC class I (A, B, or C) molecules.

Antigens that arise in the cytosol and are processed via the endogenous antigen-presentation pathway are cleaved into small peptides by

a complex of proteases called the proteasome. From the proteasome,

antigen peptide fragments are transported from the cytosol into the

lumen of the endoplasmic reticulum by a heterodimeric complex

termed transporters associated with antigen processing or TAP proteins.

There, MHC class I molecules in the endoplasmic reticulum membrane physically associate with processed cytosolic peptides. Following

peptide association with class I molecules, peptide–class I complexes

are exported to the Golgi apparatus, and then to the cell surface, for

recognition by CD8+ T cells.

Antigens taken up from the extracellular space via endocytosis

into intracellular acidified vesicles are degraded by vesicle proteases

into peptide fragments. Intracellular vesicles containing MHC class II

molecules fuse with peptide-containing vesicles, thus allowing peptide

fragments to physically bind to MHC class II molecules. Peptide–MHC

class II complexes are then transported to the cell surface for recognition by CD4+ T cells.

Whereas it is generally agreed that the TCR-αβ receptor recognizes

peptide antigens in the context of MHC class I or class II molecules,

lipids in the cell wall of intracellular bacteria such as M. tuberculosis

can also be presented to a wide variety of T cells, including subsets of

TCR-γδ T cells, and a subset of CD8+ TCR-αβ T cells. Importantly,

bacterial lipid antigens are not presented in the context of MHC class I

or II molecules, but rather are presented in the context of MHC-related

CD1 molecules. Some γδ T cells that recognize lipid antigens via CD1

molecules have very restricted TCR usage, do not need antigen priming to respond to bacterial lipids, and may be a form of innate rather

than acquired immunity to intracellular bacteria.

Just as foreign antigens are degraded and their peptide fragments

presented in the context of MHC class I or class II molecules on APCs,

endogenous self-proteins also are degraded, and self-peptide fragments

are presented to T cells in the context of MHC class I or class II molecules on APCs. In peripheral lymphoid organs, there are T cells that are

capable of recognizing self-protein fragments but normally are anergic

or tolerant, i.e., nonresponsive to self-antigenic stimulation, due to lack

of self-antigen upregulating APC co-stimulatory molecules such as B7-1

(CD80) and B7-2 (CD86) (see below and Chap. 350).

Once engagement of mature T-cell TCR by foreign peptide occurs

in the context of self-MHC class I or class II molecules, binding of

non-antigen-specific adhesion ligand pairs such as CD54-CD11/CD18

and CD58-CD2 stabilizes MHC peptide-TCR binding, and the expression of these adhesion molecules is upregulated (Fig. 349-6). Once

antigen ligation of the TCR occurs, the T-cell membrane is partitioned

into lipid membrane microdomains, or lipid rafts, that coalesce the key

signaling molecules TCR/CD3 complex, CD28, CD2, LAT (linker for

activation of T cells), intracellular activated (dephosphorylated) src

family protein tyrosine kinases (PTKs), and the key CD3ζ-associated

protein-70 (ZAP-70) PTK (Fig. 349-7). Importantly, during T-cell activation, the CD45 molecule, with protein tyrosine phosphatase activity,

is partitioned away from the TCR complex to allow activating phosphorylation events to occur. The coalescence of signaling molecules

of activated T lymphocytes in microdomains has suggested that T cell–

APC interactions can be considered immunologic synapses, analogous

in function to neuronal synapses.

After TCR-MHC binding is stabilized, activation signals are transmitted through the cell to the nucleus and lead to the expression of

gene products important in mediating the wide diversity of T-cell

functions such as the secretion of IL-2. The TCR does not have intrinsic signaling activity but is linked to a variety of signaling pathways via

ITAMs expressed on the various CD3 chains that bind to proteins that

mediate signal transduction. Each of the pathways results in the activation of particular transcription factors that control the expression of

cytokine and cytokine receptor genes. Thus, antigen-MHC binding to

the TCR induces the activation of the src family of PTKs, Fyn and Lck

(Lck is associated with CD4 or CD8 co-stimulatory molecules); phosphorylation of CD3ζ chain; activation of the related tyrosine kinases

ZAP-70 and Syk; and downstream activation of the calcium-dependent

calcineurin pathway, the ras pathway, and the protein kinase C pathway. Each of these pathways leads to activation of specific families of

transcription factors (including NF-AT, fos and jun, and rel/NF-κB)

that form heteromultimers capable of inducing expression of IL-2, IL-2

receptor, IL-4, TNF-α, and other T-cell mediators.

In addition to the signals delivered to the T cell from the TCR complex

and CD4 and CD8, molecules on the T cell, such as CD28 and inducible

co-stimulator (ICOS), and molecules on DCs, such as B7-1 (CD80) and

B7-2 (CD86), also deliver important co-stimulatory signals that upregulate T-cell cytokine production and are essential for T-cell activation. If

signaling through CD28 or ICOS does not occur, or if CD28 is blocked,

the T cell becomes anergic rather than activated (see “Immune Tolerance

and Autoimmunity” below and Chap. 350). CTLA-4 (CD152) is similar

to CD28 in its ability to bind CD80 and CD86. Unlike CD28, CTLA-4

transmits an inhibitory signal to T cells, acting as an off switch.

T-CELL EXHAUSTION IN VIRAL INFECTIONS AND CANCER In chronic

viral infections such as HIV-1, hepatitis C virus, and hepatitis B virus

and in chronic malignancies, the persistence of antigen disrupts memory

2692 PART 11 Immune-Mediated, Inflammatory, and Rheumatologic Disorders

T-cell function, resulting in defects in memory T-cell responses. This

has been defined as T-cell exhaustion and is associated with T-cell programmed cell death protein 1 (PD-1) (CD279) expression. Exhausted

T cells have compromised proliferation and lose the ability to produce

effector molecules, like IL-2, TNF-α, and IFN-γ. PD-1 downregulates T-cell responses and is associated with T-cell exhaustion and

disease progression. For this reason, inhibition of T-cell PD-1 activity

to enhance effector T-cell function is being explored as a target for

immunotherapy in both viral infections and certain malignancies

(Chap. 350).

T-CELL SUPERANTIGENS Conventional antigens bind to MHC class

I or II molecules in the groove of the αβ heterodimer and bind to T

cells via the V regions of the TCR-α and -β chains. In contrast, superantigens bind directly to the lateral portion of the TCR-β chain and

MHC class II β chain and stimulate T cells based solely on the Vβ

gene segment used independent of the D, J, and Vα sequences present.

Superantigens are protein molecules capable of activating up to 20% of

the peripheral T-cell pool, whereas conventional antigens activate <1 in

10,000 T cells. T-cell superantigens include staphylococcal enterotoxins and other bacterial products. Superantigen stimulation of human

peripheral T cells occurs in the clinical setting of staphylococcal toxic

shock syndrome, leading to massive overproduction of T-cell cytokines

that leads to hypotension and shock (Chap. 147).

B CELLS Mature B cells constitute 5–10% of human peripheral blood

lymphocytes, 20–30% of lymph node cells, 50% of splenic lymphocytes, and ~10% of bone marrow lymphocytes. B cells express on

their surface intramembrane immunoglobulin (Ig) molecules that

function as BCRs for antigen in a complex of Ig-associated α and β

signaling molecules with properties similar to those described in T cells

(Fig. 349-8). Unlike T cells, which recognize only processed peptide

fragments of conventional antigens embedded in the notches of MHC

class I and class II antigens of APCs, B cells are capable of recognizing

and proliferating to whole unprocessed native antigens via antigen

binding to B-cell surface Ig (sIg) receptors. B cells also express surface

receptors for the Fc region of IgG molecules (CD32) as well as receptors

for activated complement components (C3d or CD21, C3b or CD35).

The primary function of B cells is to produce antibodies. B cells also

serve as APCs and are highly efficient at antigen processing. Their

antigen-presenting function is enhanced by a variety of cytokines.

Mature B cells are derived from bone marrow precursor cells that arise

continuously throughout life (Fig. 349-6).

B lymphocyte development can be separated into antigenindependent and antigen-dependent phases. Antigen-independent

B-cell development occurs in primary lymphoid organs and includes

all stages of B-cell maturation up to the sIg+ mature B cell. Antigendependent B-cell maturation is driven by the interaction of antigen

with the mature B-cell sIg, leading to memory B-cell induction, Ig

class switching, and plasma cell formation. Antigen-dependent stages

of B-cell maturation occur in secondary lymphoid organs, including

lymph node, spleen, and gut Peyer’s patches. In contrast to the T-cell

repertoire that is generated intrathymically before contact with foreign

antigen, the repertoire of B cells expressing diverse antigen-reactive

sites is modified by further alteration of Ig genes after stimulation

by antigen—a process called somatic hypermutation—that occurs in

lymph node germinal centers.

During B-cell development, diversity of the antigen-binding variable

region of Ig is generated by an ordered set of Ig gene rearrangements

Fab region

Heavy chain

Light chain

BCR

SLP65

Release

of Ca2+

Activation of

downstream

effectors

Igβ

Igα PtdIns(4,S)P3

InsP3

a

b

MAPK

activation

Cytoskeletal

reorganization

RAS

PLCγ

NCK

VAV1

LYN

SOS

GRB2

DAG

RASGRP PKCβ

BTK

SYK

FIGURE 349-8 B-cell receptor (BCR) activation results in the sequential activation of protein tyrosine kinases, which results in the formation of a signaling complex and

activation of downstream pathways as shown. Whereas SLP76 is recruited to the membrane through GADS and LAT, the mechanism of SLP65 recruitment is unclear.

Studies have indicated two mechanisms: (a) direct binding by the SH2 domain of SLP65 to immunoglobulin (Ig) of the BCR complex or (b) membrane recruitment through a

leucine zipper in the amino terminus of SLP65 and an unknown binding partner. ADAP, adhesion- and degranulation-promoting adaptor protein; AP1, activator protein 1; BTK,

Bruton’s tyrosine kinase; DAG, diacylglycerol; GRB2, growth factor receptor-bound protein 2; HPK1, hematopoietic progenitor kinase 1; InsP3

, inositol-1,4,5-trisphosphate;

ITK, interleukin-2-inducible T-cell kinase; NCK, noncatalytic region of tyrosine kinase; NF-B, nuclear factor B; PKC, protein kinase C; PLC, phospholipase C; PtdIns(4,5)P2

,

phosphatidylinositol-4,5-bisphosphate; RASGRP, RAS guanyl-releasing protein; SOS, son of sevenless homologue; SYK, spleen tyrosine kinase. (Reproduced with permission

from GA Koretzky, F Abtahian, MA Silverman. SLP76 and SLP65: complex regulation of signalling in lymphocytes and beyond. Nat Rev Immunol 6:67, 2006.)

Introduction to the Immune System

2693CHAPTER 349

that are similar to the rearrangements undergone by TCR α, β, γ, and

δ genes. For the heavy chain, there is first a rearrangement of D segments to J segments, followed by a second rearrangement between a V

gene segment and the newly formed D-J sequence; the C segment is

aligned to the V-D-J complex to yield a functional Ig heavy chain gene

(V-D-J-C). During later stages, a functional κ or γ light chain gene is

generated by rearrangement of a V segment to a J segment, ultimately

yielding an intact Ig molecule composed of heavy and light chains.

The process of Ig gene rearrangement is regulated and results in

a single antibody specificity produced by each B cell, with each Ig

molecule comprising one type of heavy chain and one type of light

chain. Although each B cell contains two copies of Ig light and heavy

chain genes, only one gene of each type is productively rearranged and

expressed in each B cell, a process termed allelic exclusion.

There are ~300 Vκ genes and 5 J

κ genes, resulting in the pairing of Vκ

and J

κ genes to create >1500 different kappa light chain combinations.

There are ~70 Vλ genes and 4 J

λ genes for >280 different lambda light

chain combinations. The number of distinct light chains that can be generated is increased by somatic mutations within the V and J genes, thus

creating large numbers of possible specificities from a limited amount

of germline genetic information. As noted above, in heavy chain Ig gene

rearrangement, the VH domain is created by the joining of three types

of germline genes called VH, DH, and J

H, thus allowing for even greater

diversity in the variable region of heavy chains than of light chains.

The most immature B-cell precursors (early pro-B cells) lack cytoplasmic Ig (cIg) and sIg (Fig. 349-6). The large pre-B cell is marked by

the acquisition of the surface pre-BCR composed of μ heavy (H) chains

and a pre-B light chain, termed V pre-B. V pre-B is a surrogate light

chain receptor encoded by the non-rearranged V pre-B and the γ5 light

chain locus (the pre-BCR). Pro- and pre-B cells are driven to proliferate

and mature by signals from bone marrow stroma—in particular, IL-7.

Light chain rearrangement occurs in the small pre-B-cell stage such

that the full BCR is expressed at the immature B-cell stage. Immature B cells have rearranged Ig light chain genes and express sIgM.

As immature B cells develop into mature B cells, sIgD is expressed as

well as sIgM. At this point, B lineage development in bone marrow is

complete, and B cells exit into the peripheral circulation and migrate to

secondary lymphoid organs to encounter specific antigens.

Random rearrangements of Ig genes occasionally generate selfreactive antibodies, and mechanisms must be in place to correct these

mistakes. One such mechanism is BCR editing, whereby autoreactive

BCRs are mutated to not react with self-antigens. If receptor editing

is unsuccessful in eliminating autoreactive B cells, then autoreactive B

cells undergo negative selection in the bone marrow through induction

of apoptosis after BCR engagement of self-antigen.

After leaving the bone marrow, B cells populate peripheral B-cell

sites, such as lymph node and spleen, and await contact with foreign

antigens that react with each B cell’s clonotypic receptor. Antigendriven B-cell activation occurs through the BCR, and a process known

as somatic hypermutation takes place whereby point mutations in

rearranged H- and L-genes give rise to mutant sIg molecules, some

of which bind antigen better than the original sIg molecules. Somatic

hypermutation, therefore, is a process whereby memory B cells in

peripheral lymph organs have the best binding or the highest-affinity

antibodies. This overall process of generating the best antibodies is

called affinity maturation of antibody.

Lymphocytes that synthesize IgG, IgA, and IgE are derived from

sIgM+, sIgD+ mature B cells. Ig class switching occurs in lymph node

and other peripheral lymphoid tissue germinal centers. CD40 on B cells

and CD40 ligand on T cells constitute a critical co-stimulatory receptorligand pair of immune-stimulatory molecules. Pairs of CD40+ B cells

and CD40 ligand+ T cells bind and drive B-cell Ig class switching via

T cell–produced cytokines such as IL-4 and TGF-β. IL-1, -2, -4, -5, and

-6 synergize to drive mature B cells to proliferate and differentiate into

Ig-secreting cells.

Humoral Mediators of Adaptive Immunity: Immunoglobulins

Immunoglobulins are the products of differentiated B cells and mediate

the humoral arm of the immune response. The primary functions of

antibodies are to bind specifically to antigen and bring about the inactivation or removal of the offending toxin, microbe, parasite, or other

foreign substance from the body. The structural basis of Ig molecule

function and Ig gene organization has provided insight into the role

of antibodies in normal protective immunity, pathologic immunemediated damage by immune complexes, and autoantibody formation

against host determinants.

All immunoglobulins have the basic structure of two heavy and

two light chains (Fig. 349-8). Immunoglobulin isotype (i.e., G, M,

A, D, E) is determined by the type of Ig heavy chain present. IgG

and IgA isotypes can be divided further into subclasses (G1, G2,

G3, G4, and A1, A2) based on specific antigenic determinants on

Ig heavy chains. The characteristics of human immunoglobulins are

outlined in Table 349-9. The four chains are covalently linked by

disulfide bonds. Each chain is made up of a V region and C regions

(also called domains), themselves made up of units of ~110 amino

acids. Light chains have one variable (VL

) and one constant (CL

) unit;

heavy chains have one variable unit (VH) and three or four constant

(CH) units, depending on isotype. As the name suggests, the constant,

or C, regions of Ig molecules are made up of homologous sequences

and share the same primary structure as all other Ig chains of the

TABLE 349-9 Physical, Chemical, and Biologic Properties of Human Immunoglobulins

PROPERTY IgG IgA IgM IgD IgE

Usual molecular form Monomer Monomer, dimer Pentamer, hexamer Monomer Monomer

Other chains None J chain, SC J chain None None

Subclasses G1, G2, G3, G4 A1, A2 None None None

Heavy chain allotypes Gm (=30) No A1, A2m (2) None None None

Molecular mass, kDa 150 160, 400 950, 1150 175 190

Serum level in average adult, mg/mL 9.5–12.5 1.5–2.6 0.7–1.7 0.04 0.0003

Percentage of total serum Ig 75–85 7–15 5–10 0.3 0.019

Serum half-life, days 23 6 5 3 2.5

Synthesis rate, mg/kg per day 33 65 7 0.4 0.016

Antibody valence 2 2, 4 10, 12 2 2

Classical complement activation +(G1, 2?, 3) – ++ – –

Alternate complement activation +(G4) + – + –

Binding cells via Fc Macrophages, neutrophils,

large granular lymphocytes

Lymphocytes Lymphocytes None Mast cells, basophils, B

cells

Biologic properties Placental transfer,

secondary antibody for most

antipathogen responses

Secretory

immunoglobulin

Primary antibody

responses

Marker for

mature B cells

Allergy, antiparasite

responses

Source: Reproduced with permission from L Carayannopoulos, JD Capra, in WE Paul (ed): Fundamental Immunology, 3rd ed. New York, Raven, 1993.

2694 PART 11 Immune-Mediated, Inflammatory, and Rheumatologic Disorders

same isotype and subclass. Constant regions are involved in biologic

functions of Ig molecules. The CH2 domain of IgG and the CH4 units

of IgM are involved with the binding of the C1q portion of C1 during

complement activation. The CH region at the carboxy-terminal end of

the IgG molecule, the Fc region, binds to surface Fc receptors (CD16,

CD32, CD64) of macrophages, DCs, NK cells, B cells, neutrophils,

and eosinophils. The Fc of IgA binds to FcαR (CD89), and the Fc of

IgE binds to FcεR (CD23).

Variable regions (VL and VH) constitute the antibody-binding (Fab)

region of the molecule. Within the VL and VH regions are hypervariable

regions (extreme sequence variability) that constitute the antigenbinding site unique to each Ig molecule. The idiotype is defined as the

specific region of the Fab portion of the Ig molecule to which antigen

binds. Antibodies against the idiotype portion of an antibody molecule

are called anti-idiotype antibodies. The formation of such antibodies in

vivo during a normal B-cell antibody response may generate a negative

(or “off ”) signal to B cells to terminate antibody production.

IgG constitutes ~75–85% of total serum immunoglobulin. The four

IgG subclasses are numbered in order of their level in serum, IgG1

being found in greatest amounts and IgG4 the least. IgG subclasses

have clinical relevance in their varying ability to bind macrophage

and neutrophil Fc receptors and to activate complement (Table 349-9).

Moreover, selective deficiencies of certain IgG subclasses give rise to

clinical syndromes in which the patient is inordinately susceptible to

bacterial infections. IgG antibodies are frequently the predominant

antibody made after rechallenge of the host with antigen (secondary

antibody response).

IgM antibodies normally circulate as a 950-kDa pentamer with

160-kDa bivalent monomers joined by a molecule called the J chain, a

15-kDa nonimmunoglobulin molecule that also effects polymerization

of IgA molecules. IgM is the first immunoglobulin to appear in the

immune response (primary antibody response) and is the initial type

of antibody made by neonates. Membrane IgM in the monomeric form

also functions as a major antigen receptor on the surface of mature B

cells (Table 349-9). IgM is an important component of immune complexes in autoimmune diseases. For example, IgM antibodies against

IgG molecules (rheumatoid factors) are present in high titers in rheumatoid arthritis, other collagen diseases, and some infectious diseases

(subacute bacterial endocarditis).

IgAconstitutes only 7–15% of totalserum immunoglobulin but isthe

predominant class of immunoglobulin in secretions. IgA in secretions

(tears, saliva, nasal secretions, gastrointestinal tract fluid, and human

milk) is in the form of secretory IgA (sIgA), a polymer consisting of

two IgA monomers, a joining molecule, again termed the J chain, and

a glycoprotein called the secretory protein. Of the two IgA subclasses,

IgA1 is primarily found in serum, whereas IgA2 is more prevalent in

secretions. IgA fixes complement via the alternative complement pathway and has potent antiviral activity in humans by prevention of virus

binding to respiratory and gastrointestinal epithelial cells.

IgD is found in minute quantities in serum and, together with IgM,

is a major receptor for antigen on the naïve B-cell surface. IgE, which is

present in serum in very low concentrations, is the major class of immunoglobulin involved in arming mast cells and basophils by binding to

these cells via the Fc region. Antigen cross-linking of IgE molecules on

basophil and mast cell surfaces results in release of mediators of the

immediate hypersensitivity (allergic) response (Table 349-9).

■ CELLULAR INTERACTIONS IN REGULATION OF

NORMAL IMMUNE RESPONSES

The net result of activation of the humoral (B-cell) and cellular (T-cell)

arms of the adaptive immune system by foreign antigen is the elimination of antigen directly by specific effector T cells or in concert with

specific antibody.

The expression of adaptive immune cell function is the result of

a complex series of immunoregulatory events that occur in phases.

Both T and B lymphocytes mediate immune functions, and each of

these cell types, when given appropriate signals, passes through stages,

from activation and induction through proliferation, differentiation,

and ultimately effector functions. The effector function expressed may

be at the end point of a response, such as secretion of antibody by a

differentiated plasma cell, or it might serve a regulatory function that

modulates other functions, such as is seen with CD4+ and CD8+ T

lymphocytes that modulate both differentiation of B cells and activation of CD8+ cytotoxic T cells.

CD4 helper T cells can be subdivided on the basis of cytokines produced

(Fig. 349-2). Activated TH1-type helper T cells secrete IL-2, IFN-γ, IL-3,

TNF-α, GM-CSF, and TNF-β, whereas activated TH2-type helper T cells

secrete IL-3, -4, -5, -6, -10, and -13. TH1 CD4+ T cells, through elaboration of IFN-γ, have a central role in mediating intracellular killing

by a variety of pathogens. TH1 CD4+ T cells also provide T-cell help for

generation of cytotoxic T cells and some types of opsonizing antibody,

and they generally respond to antigens that lead to delayed hypersensitivity types of immune responses for many intracellular viruses and

bacteria (such as HIV-1 or M. tuberculosis). In contrast, TH2 cells have

a primary role in regulatory humoral immunity and isotype switching.

TH2 cells, through production of IL-4 and IL-10, have a regulatory role in

limiting proinflammatory responses mediated by TH1 cells (Fig. 349-2).

In addition, TH2 CD4+ T cells provide help to B cells for specific Ig

production and respond to antigens that require high antibody levels

for foreign antigen elimination (extracellular encapsulated bacteria such

as S. pneumoniae and certain parasite infections). Additional subsets

of the CD4 TH cells have been described, one of which is termed TH17,

that secrete cytokines IL-17, -22, and -26. TH17 cells have been shown

to play a role in autoimmune inflammatory disorders in addition to

defense against extracellular bacteria and fungi, particularly at mucosal

surfaces. TH9 cells are defined by their secretion of IL-9 and have been

shown to play a role in atopic disease, inflammatory bowel disease,

and antitumor immunity. Moreover, the Tfh subset of helper T cells is

crucial for providing the necessary signals to B cells in germinal centers

to undergo affinity maturation. A subset of Tfh cells called Tfh13 cells

secrete IL-4, IL-5, and IL-13 in response to allergens and mediate anaphylaxis reactions (Fig. 349-2). In summary, the type of T-cell response

generated in an immune response is determined by the microbe PAMPs

presented to the DCs, the TLRs on the DCs that become activated, the

types of DCs that are activated, and the cytokines that are produced

(Table 349-6). Commonly, myeloid DCs produce IL-12 and activate TH1

T-cell responses that result in IFN-γ and cytotoxic T-cell induction, and

plasmacytoid DCs produce IFN-α and lead to TH2 responses that result

in IL-4 production and enhanced antibody responses.

As shown in Fig. 349-2, upon activation by DCs, T-cell subsets that

produce IL-2, IL-3, IFN-γ, and/or IL-4, -5, -6, -10, and -13 are generated and exert positive and negative influences on effector T and B

cells. For B cells, trophic effects are mediated by a variety of cytokines,

particularly T cell–derived IL-3, -4, -5, and -6, that act at sequential

stages of B-cell maturation, resulting in B-cell proliferation, differentiation, and ultimately antibody secretion. For cytotoxic T cells, trophic

factors include inducer T-cell secretion of IL-2, IFN-γ, and IL-12.

Important types of immunomodulatory T cells that control immune

responses are CD4+ and CD8+ T regulatory cells. These cells express

the α chain of the IL-2 receptor (CD25), produce IL-10, and suppress

both T- and B-cell responses. T regulatory cells (Tregs) are induced

by immature DCs and play key roles in maintaining tolerance to selfantigens. Loss of Treg cells is the cause of organ-specific autoimmune disease in mice such as autoimmune thyroiditis, adrenalitis,

and oophoritis (see “Immune Tolerance and Autoimmunity” below,

Chap. 350). Tregs also play key roles in controlling the magnitude and

duration of immune responses to microbes. Normally, after the initial

immune response to a microbe has eliminated the invader, Tregs are

activated to suppress the antimicrobe response and prevent host injury.

Some microbes have adapted to induce Treg activation at the site of

infection to promote parasite infection and survival. In Leishmania

infection, the parasite induces Treg accumulation at skin infection sites

that dampens anti-Leishmania T-cell responses and prevents parasite

elimination. Although B cells recognize native antigen via B-cell surface Ig receptors, B cells require T-cell help to produce high-affinity

antibody of multiple isotypes that are the most effective in eliminating

foreign antigen. In B cell germinal centers, the CD4 T cells that promote B cell maturation and affinity maturation are termed T follicular