Download The properties and functions of effector T cells

NAIVE AND EFFECTOR T CELLS
The precursors of T lymphocytes are originated from bone marrow-derived
multipotent stem cells. Via the blood stream these precursors migrate into the thymus, the site
of T cell development. Once T cells have completed their developmental program, they leave
the thymus and circulate between the blood and the lymph, passing through many secondary
lymphoid organs/ tissues. Mature circulating T cells that have not recognized antigens yet are
in a resting state and defined as naive T cells. The activation of naive T cells occurs in the
secondary lymphoid organs/ tissues where they interact with professional APCs (mainly with
DCs). Activated T cells differentiate into effector and memory cells, which may remain in
the lymphoid organs or migrate to non-lymphoid tissues. To perform their various functions,
the effector cells must be activated by antigen-specific signals (Slide 2).
To participate in an adaptive immune response, a naive T cell has to meet its specific
peptide antigen in complex with a major MHC molecule expressed on professional APCs. In
addition to antigen-induced signals, the proliferation and differentiation of naive T cells also
require signals provided by costimulatory molecules expressed by professional APCs (Slide
4). The antigen-specific and the costimulatory signal have to be induced in concert to induce
naive T lymphocyte activation. The antigen-specific and costimulatory signals can be
delivered simultaneously by professional APCs only. The antigen-specific and the
costimulatory signals have to be delivered by the same professional APC (Slide 5).
Costimulatory molecules are absent or expressed only at low levels on resting APCs in
normal, healthy tissues; however, their expression is induced during infections. This
mechanism ensures that T cell responses are initiated only when needed. The major
costimulatory receptor in T cells is CD28. CD28 is present on the surface of all naive T cells
and binds the co-stimulatory ligands B7.1 (CD80) and B7.2 (CD86) expressed on activated
APCs (Slide 6). Another activating member of the CD28 family is a receptor called ICOS
(inducible costimulator), which interacts with ICOS ligand (ICOS-L) expressed on B cells
and plays an important role in follicular helper T (TFH) cell development and in germinal
center reactions (Slides 6-7). CTLA-4 is also a receptor for B7 molecules, but it inhibits early
T cell responses induced on activated T cells in the lymphoid organs and has a higher affinity
than CD28 for B7 proteins (Slides 6-7). Another inhibitory receptor of the same family is
called PD-1 (programmed death 1), which inhibits effector T cell responses in peripheral
tissues (Slides 6-7).
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When a naive T cell recognizes MHC-peptide complex on an APC as antigen, several T
cell surface proteins and intracellular signaling molecules are rapidly mobilized to the site of
T cell-APC contact. This region of physical contact between the T cell and the APC is called
an immunologic synapse or a supramolecular activation cluster (SMAC). The T cell
molecules that are rapidly mobilized to the center of the synapse include the TCR complex
(the TCR, CD3, and ζ chains), CD4 or CD8 coreceptors, receptors for costimulators (such as
CD28), enzymes and adaptor molecules associated with the cytoplasmic tails of the
transmembrane receptors. Adhesion molecules located at the periphery of the synapse, where
they stabilize the binding of the T cell to the APC (Slides 8-9).
The simultaneous recognition of antigenic and costimulatory signals by naive T cells
increases the production of IL-2 cytokine, a growth, survival and differentiation factor for T
cells, and triggers the expression of high-affinity IL-2 receptors (Slides 10). High-affinity
IL-2 receptors are transiently expressed on activation of naive T cells, whereas regulatory T
cells constitutively express this form of IL-2 receptor (Slide 11). Resting naive T cells express
low-affinity IL-2 receptors (β and γ chains) and do not produce IL-2 (or do it at a low level).
Recognition of the antigen triggers expression α chain of IL-2 receptor on the surface of naive
CD4+ and CD8+ T cells. Association of the α subunit converts the IL-2 receptor to a highaffinity form (Slide 12). Recognition of costimulators induces transcription of the IL 2 gene
and stabilizes and thus increases the half-life of IL-2 mRNA (Slide 13). The binding of IL-2
to a high-affinity IL-2 receptor composed of α, β, and γ chains drives the proliferation and
differentiation of T cells to effector and memory T cells. The activated T cell will divide two
to three times daily for about a week, producing a clone of thousands of identical antigenspecific effector T cells. In the absence of infection or tissue injury, DCs do not express B7
costimulatory molecules. Antigen-specific signal without a costimulatory signal induces
anergy (unresponsiveness to a specific antigen) in the naive T cells. This mechanism prevents
activation of naive self-reactive T cells that escape negative thymic selection and enter the
circulation (Slides 14-15).
T cells are able to recognize only peptide antigens presented by MHC molecules;
therefore, effector T cells act on other host cells, not on the pathogen itself. The cells on
which effector T cells act are referred to as their target cells. The processes of cell-mediated
effector mechanisms consist of the development of effector T cells from naive cells in
peripheral lymphoid organs, migration of these effector T cells and other leukocytes to the
sites of infection, and either cytokine-mediated activation of leukocytes to destroy microbes
or direct killing of infected target cells.
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The properties and functions of effector T cells
After completing their differentiation program, effector T cells detach themselves from
the professional APCs that nursed their differentiation. CD8+ cytotoxic T cells and CD4+
helper T cells (TH1, TH2, and TH17) leave the lymphoid tissues, enter the blood circulation
and migrate into the sites of infection or inflammation in peripheral (non-lymphoid) tissues.
All effector T cell functions are initiated by recognition of a peptide antigen presented by
MHC-I or MHC-II molecules on the surface of target cell. Interacting effector cell and target
cell form a conjugate pair connected by a synapse, through which the T cell delivers effector
molecules that profoundly affect the target cell. CD8+ effector cells induce their target to die,
whereas CD4+ TH1, TH2, and TH17 cells help their target fight against the pathogens. TFH cells
remain in the secondary lymphoid organs and promote activation of B cells and are involved
in germinal center reactions. Effector T cells are the progeny of naive T cells, which were
activated by a highly selective process. Requirements for activation of the effector T cells are
less demanding than those applied to naive T cells. Once a T cell has differentiated into an
effector cell, encounter with its specific antigen results in activation without the need for costimulation. According to this phenomenon, CD8+ cytotoxic T cells are able to kill any virusinfected cell, whether or not the infected cell can express co-stimulatory molecules. It is also
important for the effector function of CD4+ T cells, because they are able to activate B cells
and macrophages that have taken up antigen even if these cells are not expressing costimulatory molecules.
Migration of effector T lymphocytes to sites of infection
After their differentiation in secondary lymphoid organs or tissues, effector T cells
migrate to the sites of infection in peripheral tissues to perform their activity. The
differentiation of effector T cells from naive T cells involves changes in expression of the
chemokine receptors and adhesion molecules that determine the migratory behavior of these
cells. These changes allow effector T cells to leave the secondary lymphoid tissue via the
lymph, travel to the blood stream, and then leave the blood at the inflamed tissue. Through
these alterations of the T cell surface, effector T cells are excluded from returning to
secondary lymphoid tissues and ordered to enter the infected tissues where their services are
needed. On entering an infected site, an effector T cell forms transient contacts with the tissue
cells, searching for a target cell that is presenting its specific antigen. Without specific
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engagement of the T-cell receptor, an effector T cell interacts with a target cell for a short
period of time. When T-cell receptor recognizes MHC-peptide complex, conformational
changes are induced in some adhesion molecules that ensures a long-lived interaction between
the T cell and the target cell. The migration of effector T cells from the circulation to
peripheral sites of infection is largely independent of antigen, ensuring that the maximum
possible number of previously activated T cells have the opportunity to locate pathogens and
to eradicate the infection. Some memory T cells also migrate into peripheral non-lymphoid
tissues regardless of antigen specificity. Once in the tissues, the T cells encounter microbial
antigens presented by macrophages and other APCs. Antigen-specific effector and memory T
cells that recognize the antigen are preferentially retained in the peripheral tissue, where the
antigen is present. T cells migrated into the site of inflammation, but not specific for the
antigen may die in the tissue or return through lymphatic vessels to the circulation.
Effector functions of CD8+ cytotoxic T lymphocytes
All viruses and some bacteria replicate in the cytoplasm of infected cells. Once inside a
cell, these pathogens are not accessible to antibodies and other soluble proteins of the immune
system. Such infections can be eliminated only by the destruction of the infected cells. In
adaptive immune responses, activity of CD8+ cytotoxic T cells is the principal mechanism for
killing these infected cells. CD8+ cytotoxic T cells are also important mediators of tumor
immunity and the rejection of organ transplants. The development of a CD8+ cytotoxic T cell
response to infection proceeds through similar steps as those described for CD4+ T cell
responses, including antigen-mediated activation of naive CD8+ T cells in secondary
lymphoid organs, clonal expansion, differentiation, and migration of differentiated effector
CD8+ cytotoxic T cells into the infected tissues (Slide 17). Activation of naive CD8+ T cells is
mediated by antigen-specific and costimulatory signals, and autocrine growth factors,
primarily IL-2. Activated naive CD8+ T cells express high-affinity IL-2 receptor, as well as
produce IL-2 and also preferentially respond to it, ensuring that the antigen-specific T cells
are the ones that proliferate the most. Proliferation of T cells results in an increase in the size
of the antigen-specific clones, known as clonal expansion. Before antigen exposure, the
frequency of naive T cells specific for any antigen is 1 in 105 to 106 lymphocytes. After
microbial exposure, the frequency of CD8+ T cells specific for that microbe may increase to
as many as 1 in 3 CD8+ T cells. It means that CD8+ T lymphocytes may expand >50,000-fold
within a week after an acute viral infection. During this massive antigen-specific clonal
expansion, bystander T cells not specific for the virus do not proliferate. CD8+ T lymphocytes
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expand much more than do CD4+ T cells. However, it is estimated that more than 90% of the
antigen-specific T cells that arise by clonal expansion die by apoptosis as the antigen is
cleared (Slides 18-19).
Naive CD8+ T cells recognize antigens presented on the MHC-I molecules of mature
DCs. For some viral infections, the interaction of a virus-specific naive CD8+ T cell with an
infected DC that presents the MHC-I-peptide complex is sufficient for activation (Slide 20).
For other cases, such as latent viral infections, organ transplants and tumors, the full
activation of naive CD8+ T cells and their differentiation into effector and memory cells may
require the participation of CD4+ TH cells. These TH cells are activated by antigen presented
on MHC-II molecules and by B7 costimulators expressed on DCs. TH cells may promote
CD8+ T cell activation by two mechanisms (Slide 21). Helper T cells may secrete IL-2 and
other cytokines that stimulate the differentiation of CD8+ T cells. Activated TH cells express
CD40 ligand (CD40L), which binds to CD40 on antigen-loaded DCs. This interaction induces
the expression of costimulatory molecules on DCs; therefore, enhances their ability to
stimulate cytotoxic T cell differentiation (Slide 21).
The antigen presentation to CD8+ T cells by MHC-I molecules requires that protein
antigens have to be degraded in the cytosol of infected cells and then peptides have to be
transported in the endoplasmic reticulum. This requirement raises the problem that the
antigens recognized by CD8+ T cells may be proteins of viruses that do not infect DCs, or
they may be tumor antigens that are derived from a variety of cell types. The process of crosspresentation is the immune system’s solution for this problem. In this process, specialized
DCs engulf debris of infected cells, tumor cells, or proteins expressed by these cells, and
transfer the protein antigens into the cytosol. These exogenous proteins are going to enter the
MHC-I antigen presentation pathway for recognition by CD8+ T cells (Slides 22-23).
The elimination of infected cells without the destruction of healthy tissues requires the
killing mechanisms of CD8+ T cells to be accurately targeted. Cytotoxic T lymphocytes kill
target cells that express the same MHC-I-associated antigen that triggered the proliferation
and differentiation of naive CD8+ T cells from which they are derived and do not kill adjacent
uninfected cells that do not express this antigen. The killing is highly specific because an
“immunological synapse” is formed at the site of contact of CD8+ cytotoxic T cell and
antigen-expressing target cell, and the molecules that perform the killing are secreted into the
synapse and cannot diffuse to other nearby cells. Costimulatory signals and cytokines
provided by professional APCs, which are required for the activation of naive CD8+ T cells,
are not necessary for triggering the effector function of CD8+ cytotoxic T cells. Therefore,
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once a CD8+ T cell specific for an antigen has differentiated into fully functional CD8+
cytotoxic T cell, it can kill any nucleated cell that displays that antigen (Slide 24).
Killing of target cells by CD8+ cytotoxic T cells
Cytotoxic T cells kill their target cells by inducing them to undergo apoptosis. Cells
dying by apoptosis are not lysed or disintegrated, unlike cells undergoing necrosis. This
prevents the release of intact pathogens from dead cells and thus infection of healthy tissues.
The principal mechanism of cytotoxic T cell-mediated killing is the release of specialized
cytotoxic granules upon recognition of antigen on the surface of a target cell. Cytotoxic
granules are modified lysosomes that contain cytotoxic effector proteins. These proteins are
stored in the cytotoxic granules in an active form, but conditions within the granules prevent
them from functioning until their release. One consequence of the activation of the CD8+
cytotoxic T cells is that the cytotoxic granules are transported along microtubules and become
concentrated in the region of the “immunological synapse”, and the granule membrane fuses
with the plasma membrane. Membrane fusion results in exocytosis of the cytotoxic granule
contents into the confined space within the synaptic ring (Slides 25-26). One of the cytotoxic
proteins of CD8+ cytotoxic T cells is known as perforin. Perforin is a membrane-perturbing
molecule homologous to the C9 complement protein. Another class of cytotoxic proteins
comprises a family of serine proteases, called granzymes. The granules also contain a
sulfated proteoglycan, serglycin, which serves to assemble a complex containing granzymes
and perforin. Complexes of granzymes, perforin, and serglycin released from cytotoxic T cells
are internalized by the target cell. Perforin is responsible for the delivery of granzymes from
the endosomes into the cytosol of the target cell. Once in the cytoplasm, the granzymes cleave
various substrates, including caspases, and initiate apoptotic death of the target cell (Slide 28).
Cytotoxic proteins are synthesized and loaded into the granules during the first encounter of a
naive CD8+ T cell with its specific antigen. Ligation of the T-cell receptor similarly induces
de novo synthesis of cytotoxic proteins in effector CD8+ T cells. Once new granules have
been made, the cytotoxic T cell is able to kill another target cell. In this manner, a single
CD8+ T cell can kill a series of target cells in succession (Slide 27).
After delivering the lethal hit, the cytotoxic T cells are released from their target cells,
which usually occurs even before the target cell goes on to die. CD8+ cytotoxic T cells
themselves are not injured during target cell killing, probably because the directed granule
exocytosis process preferentially delivers granule contents into the target cell. In addition,
cytotoxic granules contain a proteolytic enzyme called cathepsin B, which is delivered to the
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cytotoxic T cells’ surface on granule exocytosis, where it degrades errant perforin molecules
that come into the vicinity of the membrane of CD8+ T cells.
Cytotoxic T cells also use a cytotoxic granule-independent mechanism of killing that is
mediated by interactions of members of the TNF family on the CD8+ cytotoxic T cells and
target cells. Upon activation, CD8+ cytotoxic T cells express a membrane protein, called Fas
ligand (FasL), which binds to the death receptor Fas expressed on many cell types. This
interaction also results in activation of caspases and apoptosis of Fas-expressing targets. In
contrast to the killing of infected cells, this mechanism is used mainly to regulate lymphocyte
numbers. Activated lymphocytes express both Fas and FasL, and thus activated CD8+ T cells
can kill other lymphocytes via the activation of caspases leading to apoptosis in the target
lymphocyte. This mechanism is important in terminating lymphocyte proliferation after
successful elimination of the pathogen, which initiated the activation of adaptive immune
responses. When T cells are repeatedly activated, FasL is expressed on the cell surface, and it
binds to surface Fas on the same T cells. This activates caspases, which ultimately cause the
apoptotic death of the cells. Cell death that occurs as a consequence of exposure of mature T
cells to antigen is sometimes called activation-induced cell death (Slides 29-30).
In addition to direct cell killing, CD8+ T cells also contribute to immune response by
secreting cytokines. One of the released cytokines is IFN-γ, which inhibits the replication of
viruses in infected cells and increases the processing and presentation of viral antigens by
MHC-I molecules. IFN-γ also activates macrophages in the proximity of the cytotoxic T cells
(Slide 31).
Some viruses (e.g. hepatitis B and C viruses) are not cytopathic and replicate within the
host cells without killing them. However, the immune system senses and reacts against these
relatively harmless viruses. The infected cells are killed by the host CD8+ T cells (and NK
cells) and not directly by the viruses (Slide 32).
Effector functions of CD4+ helper T cells
Effector CD4+ T cells function by secreted cytokines and cell surface molecules to
activate other cells to eliminate pathogens. CD4+ T cells also participate indirectly in host
defense by helping the activation of B lymphocytes and by promoting the development of
fully functional CD8+ cytotoxic T cells. All four subsets of effector CD4+ helper T cells (TH1,
TH2, TH17 and TFH) develop from the same naïve CD4+ T cells. Cytokines produced at the site
of antigen recognition drive their differentiation into one or the other subset. These cytokines
are produced by APCs (primarily DCs and macrophages) and other immune cells (such as NK
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cells, basophils and mast cells) present in the lymphoid organ where the immune response is
initiated. Stimuli other than cytokines may also influence the pattern of helper T cell
differentiation. Differentiation of each subset is induced by the types of microbes that the
subset is best able to combat. Commitment to each subset is driven by transcription factors.
Each subset of differentiated effector cells produces cytokines that promote its own
development and may suppress the development of the other subsets.
Function of TFH cells
Follicular helper T cells represent a distinct subset of CD4+ TH cells that regulate the
development of B cell immunity. Upon exposure to an antigen, TFH cells help B cells’
activation and differentiation to antibody-producing plasma cells and long-lived memory B
cells. TFH cells are identified by elevated expression of CXCR5 chemokine receptor, ICOS
costimulatory receptor and Bcl-6 transcription factor, as well as enhanced IL-21 secretion
(Slide 38). Differentiation of TFH cells from naive CD4+ T cells requires two steps: initial
activation by antigen-presenting DCs and a subsequent activation by B cells in the T cell zone
of secondary lymphoid tissue (Slide 38). Increased expression of CXCR5 helps TFH cells’
migration into B cell follicles. The interaction of ICOS with ICOS ligand on activated B cells
promotes the differentiation of T cells into TFH cells and also found to be essential for the
germinal center reaction. The defining cytokine of TFH cells is IL-21, which is required for
germinal center development and contributes to the generation of plasma cells in the germinal
center reaction. TFH cells can further specialize through upregulation or activation of
transcription factors in response to different environmental stimuli. In addition to IL-21, these
distinct TFH cell subpopulations secrete other cytokines, including IFN-γ or IL-4, and likely
low levels of IL-17 as well, and all of these cytokines participate in isotype switching. For
example, during a helminth infection, TFH cells can express the transcription factor GATA-3
and produce IL-4. Thus, a model is proposed whereby CD4+ T cells may receive pathogenand milieu-specific signals to induce different levels of expression of TH cell subset-specific
transcription factors, enabling a customized response critical for the development of
protective humoral immunity. This model also suggests that TFH cells are not terminally
differentiated, but retain the ability to acquire characteristics of other TH cell subsets upon
subsequent challenge (Slide 39).
TFH cells play important roles in the activation and differentiation of B cells in the
germinal center reaction and are also involved in several autoimmune diseases, including
systemic lupus erythematosus (SLE) and Sjögren's syndrome.
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The remainder of the chapter focuses on TH1, TH2, and TH17 cells, which are able to
migrate into the sites of infection or inflammation in peripheral tissues. The functions of these
CD4+ effector cells in cell-mediated immunity can be divided into the following steps:

Effector TH cells are recruited from the circulation, activated by antigens displayed by
macrophages that have phagocytosed microbes. Effector TH cells use CD40L and
cytokines to activate phagocytes and chemokines to recruit more leukocytes.

Recruitment of other leukocytes. The recruitment of neutrophils, monocytes, and
eosinophils to the site of the inflammation is mediated by chemokines produced by T
cells and by other cells in response to cytokines secreted by effector T cells. Different
subsets of CD4+ effector cells recruit different types of leukocytes into the immune
reaction.

Activation of the recruited leukocytes. The mechanisms by which CD4+ T cells activate
other leukocytes involve expression of the surface protein CD40L and secretion of
cytokines. The CD40L-mediated pathway is best defined for TH1-mediated activation of
macrophages and is described in this context later. The roles of cytokines in activation
of different leukocyte subsets are also described later for each subset of effector T cells.

Amplification of the response. As in all adaptive immune responses, there are several
positive feedback loops that serve to amplify the response. For instance, cytokines
produced by T cells activate macrophages to produce other cytokines that in turn act on
the T cells and increase their responses.

Downregulation of the response. Because effector T cells are typically short-lived, they
die after performing their function. As the antigen is eliminated, the stimuli for
propagating the response are lost, and the response declines over time. Special control
mechanisms may also operate to limit effector responses. For instance, both TH1 cells
and activated macrophages produce IL-10 cytokine, which functions mainly to inhibit
further TH1 differentiation and macrophage activation. Additional inhibitory
mechanisms, such as other anti-inflammatory cytokines and receptors that turn off T
cell activation, are also involved in controlling T cell–mediated responses (Slide 40).
Functions of TH1 cells
TH1 differentiation is driven mainly by the cytokines IL-12 and IFN-γ, which activate
the transcription factors T-bet, STAT1, and STAT4. The principal function of TH1 cells is to
activate macrophages to engulf and destroy microbes. At any site of infection, resident
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macrophages and those differentiated from blood-derived monocytes phagocytose and destroy
the pathogen, as important part of the innate immune response. However, some
microorganisms, such as mycobacteria, survive and even grow within the phagosomes of
macrophages. These pathogens maintain themselves in the usually hostile environment of the
phagocyte by inhibiting the fusion of lysosomes to the phagosomes in which they replicate, or
by preventing the acidification of the phagosomes that is required to activate lysosomal
proteases. In these infected cells, microbial antigens are processed and presented as peptides
associated with MHC-II molecules. At the same time, TH1 effector cells are generated in an
adaptive immune response in secondary lymphoid tissues. Effector TH1 cells are recruited to
the site of infection, where they recognize antigenic peptides displayed by the microbebearing macrophages, and activate these cells to kill the ingested microbes. Activation leads
to increased expression of various proteins that endows macrophages with the capacity to
perform specialized functions, such as efficient killing of engulfed microbes.
When the TH1 cells are stimulated by antigen, the cells express CD40L on their surface
and secrete IFN-γ. The actions of IFN-γ on macrophages, described later, synergize with the
actions of CD40L, and together they are potent stimuli for macrophage activation (Slide 42).
The importance of the CD40 pathway in cell-mediated immunity is illustrated by the
immunologic defects in humans with inherited mutations in CD40L (X-linked hyper-IgM
syndrome). This disorder is characterized by severe deficiencies in cell-mediated immunity to
intracellular microbes replicating within the phagosomes. TFH cells stimulate B lymphocyte
proliferation and differentiation by CD40-mediated signals and cytokines. Therefore, in the
absence of CD40L, virtually no specific antibodies are made against T-cell dependent
antigens and there are no germinal centers in the secondary lymphoid tissues resulting in
increased susceptibility to infections with pyogenic bacteria.
TH1-mediated
macrophage
activation
is
involved
in
injurious
delayed-type
hypersensitivity reactions, which is a component of many inflammatory diseases, and in
granulomatous inflammation, which is typical of tuberculosis and is also seen in some other
infectious and inflammatory disorders.
Cytokines produced by TH1 cells
IFN-γ. The signature cytokine of TH1 cells is IFN-γ, which promotes the differentiation of
CD4+ T cells to TH1 subset and inhibits the differentiation of TH2 and TH17 cells. These
actions of IFN-γ serve to amplify the TH1 responses. IFN-γ is the principal macrophageactivating cytokine and serves essential functions in immunity against intracellular microbes.
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IFN-γ is also called immune or type II IFN. Although it has some antiviral activity, it is not
the most potent antiviral cytokine, and it functions mainly as an activator of effector cells of
the immune system. In addition to CD4+ TH1 cells, IFN-γ is also produced by NK cells and
CD8+ T cells. IFN-γ activates macrophages to kill phagocytosed microbes. The combination
of intracellular signals coming from IFN-γ receptors and CD40 stimulates the expression of
several enzymes in macrophages, including phagocyte oxidase, which induces the production
of reactive oxygen species (ROS); inducible nitric oxide synthase (iNOS), and lysosomal
enzymes. These substances destroy ingested microbes in the phagolysosomes and are
responsible for the microbicidal function of activated macrophages. These microbicidal agents
may also be released into neighboring tissues, where they kill extracellular microbes and may
also cause damage to healthy tissues. However, this tissue injury is usually limited in extent
and duration, and it resolves when the infection is cleared.
The actions of IFN-γ together result in increased ingestion of microbes and the
destruction of the ingested pathogens. This pathway of macrophage activation is called
“classical” to distinguish it from “alternative” activation, described later (Slide 44).
Individuals with rare inherited inactivating mutations in the IFN-γ receptor or in molecules
required for TH1 differentiation or IFN-γ signaling are susceptible to infections with
intracellular microbes, such as mycobacteria, because of defective macrophage-mediated
killing of the microbes.
IFN-γ stimulates expression of several different proteins that contribute to enhanced
antigen presentation by MHC molecules and the initiation and amplification of T-cell
dependent immune responses. These proteins include MHC molecules, many proteins
involved in antigen processing, including the transporter associated with antigen processing
(TAP), components of the proteasome, HLA-DM, as well as B7 costimulators on professional
APCs (Slide 44).
Other TH1 mediators. In addition to IFN-γ, TH1 cells produce TNF-α, and various
chemokines, which contribute to the recruitment of leukocytes and enhance inflammation
(Slide 43). TH1 cells are also important sources of IL-10, which functions mainly to inhibit
DCs and macrophages and thus to suppress TH1 activation. This is an example of a negative
feedback loop in T cell responses.
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Functions of TH2 cells
TH2 differentiation is stimulated by IL-4 via activation of the transcription factor
STAT6, which, together with T-cell receptor-mediated signals, induces expression of the
transcription factor GATA-3. TH2 cells orchestrate the defense responses against parasites
that colonize the tissues and mucosal surfaces of the human body. These biological diverse
organisms include many pathogens that are too large to be phagocytosed by neutrophils and
macrophages and are more resistant to the microbicidal activities of these phagocytes than
most bacteria and viruses. Therefore, special effector mechanisms are needed for defense
against parasitic infections. The IgE-mediated mechanisms can provide protection against
parasites, which are prevalent in tropical regions. In contrast, in the developed countries
where parasitic infections are rare, IgE-mediated responses tend to be triggered by contact
with non-threatening substances in the environment.
Cytokines produced by TH2 cells
IL-4. IL-4 is the signature cytokine of the TH2 cells and functions as both an inducer of the
development and an autocrine growth factor for differentiated TH2 cells (Slide 46). IL-4
signaling promotes the Ig heavy chain class switching to IgE isotype in B cells (see function
of TFH cells). IL-13 can also contribute to class switching to the IgE isotype. IgE antibodies
play a pivotal role in defense mechanisms against parasitic infections and also in allergic
reactions. Mast cells express high affinity Fcε receptors and may be activated by parasites and
allergens that bind to IgE on the surface of the mast cells, resulting in degranulation. The
granule contents of mast cells include vasoactive amines, proteases and other degradative
enzymes. Activated mast cells secrete various cytokines such as TNF-α, IL-4, IL-5, and IL-13
and chemokines, and lipid mediators, all of which induce local inflammation that helps to
destroy the parasites. Mast cell mediators are also responsible for the vascular abnormalities
and inflammation observed in allergic reactions.
IL-4, together with IL-13, contributes to an alternative form of macrophage activation
that is distinct from the “classical” macrophage response to IFN-γ, which results in potent
microbicidal functions. Macrophages that are activated by TH2 cytokines contribute to tissue
remodeling and fibrosis at the site of chronic parasitic infections and allergic responses.
Alternatively activated macrophages may also serve to initiate tissue repair after diverse types
of injury in that may not involve pathogens or immune responses. In these situations, the
activating cytokines, such as IL-4, may be produced by eosinophils and other cell types in
tissues. Alternatively activated macrophages induce the formation of fibrous tissue by
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releasing growth factors that stimulate fibroblast proliferation (platelet-derived growth factor),
collagen synthesis (transforming growth factor-β /TGF-β/), and angiogenesis (fibroblast
growth factor) (Slide 47).
IL-13. IL-13 is structurally and functionally similar to IL-4 and also plays an essential role in
defense against parasites and in allergic diseases. IL-13 has limited sequence homology but
displays significant structural similarity to IL-4. IL-13 is produced mainly by the TH2 cells,
but basophils and eosinophils may also secrete this cytokine. The functional IL-13 receptor
can bind both IL-13 and IL-4 with high affinity. This phenomenon accounts for the fact that
most of the biologic effects of IL-13 are shared with IL-4. The IL-13 receptor is expressed on
a wide variety of cells; however, T cells do not express it. Therefore, unlike IL-4, IL-13 is not
involved in TH2 differentiation. IL-13 receptor signaling is similar to IL-4 receptor signaling,
both of them activate the STAT6 protein, which induces transcription of genes that account
for many of the actions of these cytokines. IL-13, together with IL-4, stimulates the
recruitment of leukocytes, primarily eosinophils, by promoting expression of adhesion
molecules on endothelial cells and the secretion of chemokines that bind chemokine receptors
on eosinophils (Slide 46).
IL-5. IL-5 is produced by TH2 cells and by activated mast cells. The principal actions of IL-5
are to activate mature eosinophils and to stimulate their growth and differentiation.
Eosinophils express Fc receptors specific for IgE and some IgG antibodies and are thereby
able to bind to parasites that are opsonized by these antibodies. IL-5 promotes activation of
the eosinophils and these cells release their granule contents, including major basic protein
and major cationic protein, which are capable of destroying even the tough integuments of
helminthes (Slide 46).
Cytokines produced by TH2 cells are involved in blocking entry and promoting
expulsion of microbes from mucosal surfaces. For instance, IL-13 stimulates mucus
production from airway and gut epithelial cells, and IL-4 along with IL-13 may stimulate
peristalsis in the gastrointestinal tract. IL-5 stimulates the production of IgA antibody that
plays a critical role in mucosal immunity. Thus, TH2 cells play an important role in host
defense at the barriers with the external environment, which is sometimes called barrier
immunity.
Functions of TH17 cells
The development of TH17 cells is stimulated by TGF-β and inflammatory cytokines
(mainly IL-6 and IL-1), and dependent on the transcription factors RORγt and STAT3. The
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TH17 subset is primarily involved in recruiting leukocytes and inducing inflammation.
Activated TH17 cells secrete cytokines that recruit neutrophils to sites of infection.
Neutrophils represent a major defense mechanism against extracellular bacteria and fungi,
therefore TH17 cells are involved in stimulating immune responses against these pathogens.
TH17 cells are also important in the pathogenesis of several inflammatory diseases, such as
psoriasis, inflammatory bowel disease, rheumatoid arthritis, and multiple sclerosis. TH1 and
TH17 cells may both be present in the lesions in these diseases, and their relative contribution
to the development of the disorders is an area of intense investigation.
Cytokines produced by TH17 cells
IL-17. The IL-17 cytokine family includes six structurally related proteins, of which IL-17A
and IL-17F (also known as IL-25) are the most similar molecules, and the immunologic
activities seem to be mediated primarily by IL-17A. IL-17A and IL-17F are produced mainly
by TH17 cells, whereas the other members of the family are produced by various cell types.
IL-17 receptors are expressed ubiquitously on a wide range of cells such as fibroblasts,
epithelial cells and keratinocytes. IL-17 induces these cells to secrete chemokines and
cytokines (such as TNF-α) that recruit neutrophils and, to a lesser extent, monocytes to the
site of T cell activation. It also enhances neutrophil generation by increasing the production of
the granulocyte colony-stimulating factor (G-CSF) and the expression of its receptors. IL-17
also stimulates the production of antimicrobial substances, including defensins from
numerous cell types (Slide 49).
IL-22. IL-22 is a member of the IL-10 cytokine family. It is produced in epithelial tissues,
especially of the skin and gastrointestinal tract, and serves to maintain epithelial integrity, by
promoting the barrier function and by stimulating repair reactions. IL-22 along with IL-17
induces expression of antimicrobial peptides, such as defensins, by the keratinocytes of the
epidermis (Slide 49).
In summary, T cell-mediated immunity provides excellent examples of the
specialization of adaptive immunity. The adaptive immune response to microbes that infect
and replicate in the cytoplasm of various cell types, including non-phagocytic cells, is
mediated by CD8+ cytotoxic T cells, which kill infected target cells eliminating the reservoirs
of infection. Cytotoxic T cell-mediated killing is also a mechanism for elimination of
microbes that are taken up by phagocytes but escape from phagosomes into the cytosol, where
they are not susceptible to the microbicidal activities of phagocytes.
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CD4+ effector T cells link specific recognition of microbes with activation of B
lymphocytes to produce different antibody isotypes with high affinity and the recruitment and
activation of other leukocytes that destroy the microbes.

TFH cells are present in lymphoid follicles and cooperate with B cells to promote
antibody production and to help their differentiation to long-lived memory B cells.

The adaptive immune response to microbes that are phagocytosed and live within the
phagosomes of macrophages is mediated by TH1 cells. The function of TH1 cells is to
enhance the microbicidal actions of macrophages and thus to eliminate the infection.

The response to parasites is mediated by TH2 cells, which activate eosinophils and mast
cells to eliminate these pathogens.

The response to extracellular microbes, including many fungi and bacteria, is mediated
by TH17 cells. These cells recruit neutrophils (and some monocytes), which ingest and
destroy the microbes.
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