Table 11-1. Comparative Features of the Cytokines of Innate and Adaptive
Immunity
Features
Innate immunity
Adaptive immunity
Examples
TNF-α, IL-1, IL-12, IFN-γ *
IL-2, IL-4, IL-5, IFN-γ *
Major cell source
Macrophages, NK cells
T lymphocytes
Principal physiologic Mediators of innate immunity and
functions
inflammation (local and systemic)
Adaptive immunity: regulation of lymphocyte growth
and differentiation; activation of effector cells
(macrophages, eosinophils, mast cells)
Stimuli
LPS (endotoxin), bacterial
peptidoglycans, viral RNA, T cellderived cytokines (IFN-γ)
Protein antigens
Amounts produced
Local or systemic
effects
May be high; detectable in serum
Both
Generally low; usually undetectable in serum
Usually local only
Roles in disease
Systemic diseases (e.g., septic
shock)
Local tissue injury (e.g., granulomatous
inflammation)
Inhibitors of
synthesis
Corticosteroids
Cyclosporine, FK-506
*IFN-γplays important roles in innate and adaptive immunity.
Table 11-2. Signal Transduction Mechanisms of Cytokine Receptors
Signal transduction Cytokine receptors
pathway
using this pathway
Signaling mechanism
JAK/STAT pathway
Type I and type II cytokine
receptors
JAK-mediated phosphorylation and activation of
STAT transcription factors (see Box 11-2)
TNF receptor signaling by
TRAFs
TNF receptor family: TNRRII, CD40
Binding of adapter proteins, activation of
transcription factors (see Box 11-1)
TNF receptor signaling by
death domains
TNF receptor family: TNF-RI, Binding of adapter proteins, caspase activation (see
Fas
Box 11-1)
Receptor-associated
tyrosine kinases
M-CSF receptor, stem cell
factor receptor
Intrinsic tyrosine kinase activity in receptor
G protein signaling
Chemokine receptors
GTP exchange and dissociation of Gα. GTP from
Gβγ, Gα. GTP activates various cellular enzymes
page 248
page 249
Table 11-3. Cytokines of Innate Immunity
Cytokine
Size
Principal cell source
Principal cell targets and biologic effects
Tumor necrosis
factor (TNF)
17 kD; 51-kD
homotrimer
Macrophages, T cells
Endothelial cells: activation (inflammation,
coagulation)
Neutrophils: activation
Hypothalamus: fever
Liver: synthesis of acute-phase proteins
Muscle, fat: catabolism (cachexia)
Many cell types: apoptosis
Interleukin-1 (IL- 17 kD mature form; Macrophages, endothelial Endothelial cells: activation (inflammation,
1)
33-kD precursors
cells, some epithelial cells coagulation)
Hypothalamus: fever
Liver: synthesis of acute-phase proteins
Chemokines
8-12 kD
(see Table 11-4)
Macrophages, endothelial Leukocytes: chemotaxis, activation;
cells, T cells, fibroblasts,
migration into tissues
platelets
Interleukin-12
(IL-12)
Heterodimer of 35- Macrophages, dendritic
kD + 40-kD
cells
subunits
T cells: TH1 differentiation
NK cells and T cells: IFN-γ synthesis,
increased cytolytic activity
Type I IFNs (IFN- IFN-α: 15-21 kD
α, IFN-β)
IFN-β: 20-25 kD
IFN-α: macrophages
IFN-β: fibroblasts
All cells: antiviral state, increased class I
MHC expression
NK cells: activation
Interleukin-10
(IL-10)
Macrophages, T cells
(mainly TH2)
Macrophages, dendritic cells: inhibition of IL12 production and expression of
costimulators and class II MHC molecules
Homodimer of 3440 kD; 18-kD
subunits
Interleukin-6 (IL- 19-26 kD
6)
Macrophages, endothelial Liver: synthesis of acute-phase proteins B
cells, T cells
cells: proliferation of antibody-producing
cells
Interleukin-15
(IL-15)
13 kD
Macrophages, others
NK cells: proliferation T cells: proliferation
(memory CD8+ cells)
Interleukin-18
(IL-18)
17 kD
Macrophages
NK cells and T cells: IFN-γ synthesis
BOX 11-1 Signaling by the TNF Receptor (TNF-R) Family
The TNF family of proteins includes secreted cytokines and membrane proteins
that share sequence homologies and fold into homotrimeric triangular
pyramidal complexes, which bind to structurally similar cell surface receptors.
Members of this family include TNF, lymphotoxin (TNF-β), Fas ligand, CD40
ligand, OX-40, receptor activator of NF-κB ligand (RANK-L), TNF-related
apoptosis-inducing ligand (TRAIL), and several other proteins. The cell surface
receptors for the TNF family of proteins are type I membrane proteins that can
also be grouped into a family, called the TNF-R family, on the basis of
sequence homology in their cysteine-rich extracellular ligand-binding domains.
Members of the TNF-R family include TNF-RI, TNF-RII, lymphotoxin-β receptor
(LT-βR), Fas, CD40, OX-40 ligand, RANK, and others. The binding of TNF
family members to their respective receptors initiates a wide variety of
responses. Many of these responses are proinflammatory and depend on
activation of the NF-κB and AP-1 transcription factors leading to new gene
transcription. In contrast, some TNF-R family members (TNF-RI and Fas)
deliver signals that cause apoptotic cell death. The specific responses to the
cytokines depend on the particular receptor and cell type. Many of the
molecular details of the signaling pathways engaged by TNF-R family
molecules are now known. Although the members of this receptor family share
structural features in their extracellular ligand-binding regions, they have widely
divergent cytoplasmic domain structures. Nonetheless, there are shared
features in the signaling pathways associated with these receptors. We discuss
signaling by the TNF-R family, with an emphasis on how a protypical member
of the family, TNF-RI, can induce anti-apoptotic and inflammatory responses in
some circumstances or apoptosis in others.
As is the case with many membrane receptors in the immune system, the
cytoplasmic tails of the TNF-R family members do not contain intrinsic
enzymatic activities, but they do contain structural motifs that bind to
cytoplasmic signaling molecules and promote the assembly of signaling
complexes. One of these structural motifs is called the death domain, and it
binds, by homotypic interactions, to death domains in a variety of cytoplasmic
signaling/adapter molecules. The cytoplasmic tails of both TNF-RI and Fas
contain death domains, and mutation or deletion of this region prevents TNF-RI
or Fas molecules from delivering apoptosis-inducing signals. However, death
domain interactions are also essential for the anti-apoptotic, proinflammatory
signaling pathways. The cytoplasmic signaling molecules that contain death
domains and that are involved in TNF-R signaling include TRADD (TNF
receptor-associated death domain), FADD (Fas-associated death domain), and
RIP (receptor interacting protein).
The second type of motif that plays a key role in signaling by several TNF-R
family members binds to one of a family of molecules called TNF receptorassociated factors (TRAFs) (see Figure). TNF-RII, LT-βR, and CD40 all bind
TRAF proteins. To date, six TRAFs have been identified, and they are called
TRAF-1 to TRAF-6. All TRAFs share a C-terminal region of homology called a
TRAF domain, which mediates binding to the cytoplasmic domains of TNF-R
family members, and TRAFs 2 to 6 have RING and zinc finger motifs that are
involved in signaling.
Multiple different interactions of death domain proteins and TRAFs are known
to occur in the signaling pathways associated with the different TNF-R family
members. In the case of TNF-RI, a current model of these interactions
illustrates how TNF may lead to either apoptosis or inflammatory responses
(see Figure). In this model, the death domain protein TRADD binds directly to
the TNF-RI death domain and acts as a pivotal adapter molecule leading to
one of two different signaling cascades. In one of these pathways, the death
domain protein FADD binds to TRADD, and the aspartyl-directed cysteine
protease caspase-8 binds to FADD. Caspase-8 binding initiates the activation
of a cascade of caspases that eventually causes apoptosis. In response to Fas
ligand binding, Fas mediates apoptosis by a similar pathway, but in this case,
FADD binds directly to the cytoplasmic tail of Fas (see Chapter 10, Box 10-2).
The alternative proinflammatory and anti-apoptotic pathway involves the
binding of TRAF-2 and RIP-1 to TRADD and results in NF-κB-and AP-1dependent gene transcription. Both TRAF-2 and RIP-1 are involved in the
activation of IκB kinases, which mediate the initial steps in NF-κB activation
(see Chapter 8, Box 8-4). Gene knockout studies suggest that TRAF-5 may be
able to substitute for TRAF-2 in NF-κB activation. TRAF-2 also activates the
mitogen-activated protein kinase (MAP kinase) cascade that leads to JNK
activation, phosphorylation of c-Jun, and formation of the AP-1 transcription
factor composed of c-Fos and c-Jun. The combination of NF-κB and AP-1
activation promotes the transcription of a variety of genes involved in
inflammation, including endothelial adhesion molecules, cytokines, and
chemokines. Furthermore, NF-κB also enhances expression of a family of
cellular inhibitors of apoptosis (cIAPs), which block the function of caspases.
Therefore, when the proinflammatory pathway is engaged, there is active
inhibition of the apoptotic pathway. It is still unclear what determines whether
TNF binding to TNFR-1 will activate the inflammatory or apoptotic signaling
pathways.
Other members of the TNF-R family use TRAF-dependent signaling pathways
that lead to NF-κB and AP-1 activation similar to the TNF-RI pathway, but in
contrast to TNF-RI, these other receptors directly bind TRAFs to their
cytoplasmic tails. TRAF-1 and TRAF-2 interact with TNF-RII, TRAF-2 and
TRAF-3 interact with CD40, and TRAF-4 interacts with LT-βR. CD40 delivers
activating signals to B cells and macrophages through TRAF binding (see
Chapters 9 and 13). TNF-RII, which lacks a death domain, can sometimes
deliver apoptotic signals, but the mechanism is not understood. Interestingly,
one of the transforming gene products of the Epstein-Barr virus encodes a selfaggregating TRAF domain-containing protein that binds TRAF molecules, and
therefore infection by the virus mimics TNF-or CD40-induced signals.
Although the IL-1 receptor is not a member of the TNF-R family, many of the
biologic effects of IL-1 are similar to the effects of TNF-α because the IL-1
signaling pathway shares biochemical intermediates with the TNF pathway.
Binding of IL-1 to the type I IL-1 receptor (IL-1R) leads to the recruitment of an
adapter protein, called MyD88, and a serine/threonine kinase called IL-1
receptor-associated kinase (IRAK). IRAK autophosphorylates and dissociates
from the complex and subsequently binds TRAF-6. The IRAK-TRAF-6 complex
activates both NF-κB and AP-1. This signaling pathway is also used by the
Toll-like receptors that are involved in innate immunity (see Chapter 12, Box
12-1). IL-18 also signals by a similar mechanism, and the cytoplasmic regions
of the receptors for IL-1 and IL-18 are homologous to each other.
BOX 11-2 Cytokine Signaling by Janus Kinases and Signal Transducers
and Activators of Transcription
One of the best understood mechanisms by which cytokines transduce signals
that elicit specific responses in target cells involves enzymes called Janus
kinases (JAKs) and transcription factors called signal transducers and
activators of transcription (STATs). The JAK/STAT signaling pathways are
used by all type I and type II cytokine receptors. Studies of these pathways
have revealed direct links between cytokine binding to receptors and
transcriptional activation of target genes.
The discovery of the JAK/STAT pathways came from biochemical and genetic
analyses of interferon (IFN) signaling. The promoter regions of genes
responsive to IFNs contain sequences that bind cellular proteins that are
phosphorylated upon IFN treatment of the cells. These proteins were shown to
activate transcription of cytokine-responsive genes, and they were therefore
called STAT proteins. Mutant cell lines were generated that were unresponsive
to IFNs. Some of these mutant cell lines were found to lack particular STAT
proteins, and introduction of the STATs by gene transfection restored cytokine
responsiveness in the cells. This established the essential roles of the STATs
in responses to the cytokines. Other mutant cell lines were found to be deficient
in one or more related tyrosine kinases, which were called Janus kinases after
the two-headed Roman god because of the presence of two kinase domains
(only one of which is active). Subsequent studies, including analyses of
knockout mice lacking the various STATs and JAKs, have shown that
JAK/STAT pathways are involved in responses to many cytokines (see Table).
The sequence of events in the JAK/STAT signaling pathways is now well
defined (see Figure). Inactive JAK enzymes are loosely attached to the
cytoplasmic domains of type I and type II cytokine receptors. When two
receptor molecules are brought together by binding of a cytokine molecule, the
receptor-associated JAKs become active through transphosphorylation, and
they phosphorylate tyrosine residues in the cytoplasmic portions of the
clustered receptors. Some of these phosphotyrosine moieties of the receptors
are recognized by Src homology 2 (SH2) domains of monomeric cytosolic
STAT proteins, which become attached to the receptors. The STAT proteins
are then phosphorylated by the receptor-associated JAK kinases. The SH2
domain of one STAT protein is able to bind to the phosphotyrosine residues of
another STAT protein. As a result, two STAT proteins bind to each other and
dissociate from the receptor. The STAT dimers migrate to the nucleus, where
they bind to DNA sequences in the promoter regions of cytokine-responsive
genes and activate gene transcription. After each round, new STAT proteins
can bind to the cytokine receptor, become phosphorylated, dimerize, and again
One intriguing question that arises is, What determines the specificity of responses to many
different cytokines, given the limited numbers of JAKs and STATs used by the various cytokine
receptors? The likely answer is that unique amino acid sequences in the different cytokine
receptors provide the scaffolding for specifically binding, and thereby activating, different
combinations of JAKs and STATs. The SH2 domains of different STAT proteins selectively bind to
the phosphotyrosine residues and flanking regions of different cytokine receptors. This is largely
responsible for the activation of particular STATs by various cytokine receptors and therefore for the
specificity of cytokine signaling. In addition, cytokines activate signaling pathways and transcription
factors other than STATs. For instance, the IL-2 receptor β chain activates Ras-dependent MAP
kinase pathways that may be involved in gene transcription and growth stimulation. Other cytokine
receptors may similarly activate other signaling pathways in concert with the JAK/STAT pathways to
elicit biologic responses to the cytokines.
JAK/STAT Involved cytokines*
Phenotype of knockout mice
STAT1
IFN-α/β, IFN-γ
Defect in innate immunity; no reponse to IFNs
STAT2
IFN-α/β
Defective immunity to viruses
STAT3
IL-6, IL-10
Embryonic lethal
STAT4
IL-12
Defect in TH1 development, IFN-g production
STAT5a
Prolactin
Lactation defect
STAT5b
Growth hormone
Dwarfism
STAT5a and IL-2, IL-7, IL-9 (in addition
Lactation defect, dwarfism, and defective T cell
STAT5b
to above)
proliferation in response to IL-2
STAT6
IL-4
Defect in TH2 development, IL-4-dependent Ig
isotypes
JAK1
IFN-α/β, IFN-γ, cytokines using γc and
gp130 (e.g., IL-2, IL-4, IL-6)
Perinatal lethal; defective innate immunity, possible
defect in neuronal viability
JAK2
Epo, IL-3, IFN-γ
Embryonic lethal, hematopoietic failure
JAK3
Cytokines using γc chain (IL-7, IL-2, IL-4)
Defect in T cell maturation
Tyk2
IFN-α/β, IFN-γ, IL-12, IL-10, others
Defective IL-12 response of NK cells, defective
immunity to viruses
*Selected examples of involved cytokines are shown.
Several mechanisms of negative regulation of JAK/STAT pathways have been
identified. Proteins called suppressors of cytokine signaling (SOCS) are a
family of STAT pathway inhibitors. Members of this family are identified by the
presence of an SH2 domain and a conserved 40-amino acid C-terminal region
called a SOCS box. Eight different SOCS proteins have been identified; in
addition, other SOCS box-containing proteins that do not have SH2 domains
also exist. SOCS proteins inhibit cytokine actions by binding to
phosphotyrosines in the cytoplasmic regions of cytokine receptors, or by
binding to and inhibiting the kinase activity of JAKs. The SOCS box appears to
be involved in targeting associated proteins, such as JAKs, for proteasomal
degradation. SOCS gene knockout mice have begun to provide information on
the physiologic role of this family. SOCS-1 knockout mice, for example, die at 3
weeks of age because of excessive actions of IFN-γ, and the phenotype can be
corrected by crossing the mice with IFN-γ knockout mice. Other knockout mice
studies suggest that SOCS-2 inhibits signaling by the receptors for growth
hormone and insulin-like growth factor, whereas SOCS-3 regulates IL-6
signaling. The expression of SOCS proteins is induced by the same cytokine
stimuli that activate JAK/STAT pathways, and thus they are negative feedback
inhibitors. Other inhibitors of cytokine signaling include tyrosine phosphatases,
such as SHP-1, which can dephosphorylate and therefore deactivate JAK
molecules, and members of the protein inhibitors of activated STAT (PIAS)
family, which bind phosphorylated STAT proteins and prevent their interaction
with DNA.
Table 11-4. Cytokines of Adaptive Immunity
Cytokine
Principal cell
source
Principal cell targets and biologic
effects
Interleukin-2 (IL-2) 14-17 kD
T cells
T cells: proliferation, increased cytokine
synthesis; potentiates Fas-mediated
apoptosis
NK cells: proliferation, activation
B cells: proliferation, antibody synthesis
(in vitro)
Interleukin-4 (IL-4) 18 kD
CD4+ T cells (TH2), B cells: isotype switching to IgE T cells:
mast cells
TH2 differentiation, proliferation
Macrophages: inhibition of IFN-γmediated activation
Mast cells: proliferation (in vitro)
Interleukin-5 (IL-5) 45-50 kD; homodimer of 20kD subunits
CD4+ T cells (TH2) Eosinophils: activation, increased
production
B cells: proliferation, IgA production
Interferon-γ(IFN-γ) 50 kD (glycosylated);
homodimer of 21-to 24-kD
subunits
T cells (TH1, CD8+T Macrophages: activation (increased
cells), NK cells
microbicidal functions)
B cells: isotype switching to opsonizing
and complement-fixing IgG subclassesT
cells: TH1 differentiation
Various cells: increased expression of
class I and class II MHC molecules,
increased antigen processing and
presentation to T cells
Transforming
growth factorβ(TGF-β)
Size
25 kD; homodimer of 12.5-kD T cells,
subunits
macrophages,
other cell types
T cells: inhibition of proliferation and
effector functions
B cells: inhibition of proliferation; IgA
production Macrophages: inhibition
Lymphotoxin (LT) 21-24 kD; secreted as
homotrimer or associated
with LTβ2 on the cell
membrane
T cells
Recruitment and activation of neutrophils
Lymphoid organogenesis
Interleukin-13 (IL- 15 kD
13)
CD4+ T cells (TH2) B cells: isotype switching to IgE
Epithelial cells: increased mucus
production
Macrophages: inhibition
page
Table 11-5. Hematopoietic Cytokines
Cytokine
Size
Principal cell sources
Principal cell targets
Principal cell
populations induced
Stem cell factor (c- 24 kD
Kit ligand)
Bone marrow stromal
cells
Pluripotent stem cells
All
Interleukin-7 (IL-7) 25 kD
Fibroblasts, bone marrow Immature lymphoid
stromal cells
progenitors
B and T lymphocytes
Interleukin-3 (IL-3) 20-26 kD
T cells
All
Immature progenitors
Granulocytemonocyte CSF
(GM-CSF)
18-22 kD
T cells, macrophages,
endothelial cells,
fibroblasts
Immature and committed Granulocytes and
progenitors, mature
monocytes,
macrophages
macrophage activity
Monocyte CSF (M- Dimer of 70-90 Macrophages, endothelial Committed progenitors
CSF)
kD; 40-kD
cells, bone marrow cells,
subunits
fibroblasts
Monocytes
Granulocyte CSF
(G-CSF)
Granulocytes
19 kD
Macrophages, fibroblasts, Committed progenitors
endothelial cells
Abbreviation: CSF, colony-stimulating factor.