T Cell-Mediated Immunity Notes

T Cell-Mediated Immunity

Activation of T Lymphocytes by Cell-Associated Antigens

T lymphocytes play a crucial role in defending against infections caused by various microbes.
Cell-mediated immunity (CMI) is essential for defense against intracellular microbes.
Intracellular microbes find refuge inside cells, necessitating elimination through cell-mediated immune responses.
Two types of infections lead to microbes residing inside cells:
- Microbes ingested by phagocytes that survive within vesicles or escape into the cytoplasm.
- Viruses that bind to receptors on various cells and replicate in the cytoplasm.
Some microbes resist the microbicidal activities of phagocytes and survive/replicate within phagocyte vesicles or the cytosol of infected cells.
Viruses infect and replicate within cells lacking mechanisms to destroy them.
T lymphocytes also defend against microbes that don't reproduce inside cells or survive in phagocytes, such as bacteria, fungi, and helminthic parasites.
Subsets of CD4+ helper T lymphocytes contribute to both humoral immune responses (helping B cells produce antibodies) and inflammation.

Phases of T Cell Responses

Naive T lymphocytes recognize antigens in peripheral lymphoid organs, stimulating proliferation and differentiation into effector and memory cells.
Effector cells are activated by the same antigens in peripheral tissues or lymphoid organs.
T lymphocytes interact with phagocytes, infected host cells, or B lymphocytes, facilitated by the specificity for peptides displayed by MHC molecules.

Key Questions Addressed

What signals are needed to activate T lymphocytes, and what cellular receptors are used to sense and respond to these signals?
How are few naive T cells specific for any microbe converted into a large number of effector T cells that have specialized functions and the ability to eliminate diverse microbes?
What molecules are produced by T lymphocytes that mediate their communications with other cells, such as macrophages, B lymphocytes, and other leukocytes?

Induction and Effector Phases of Cell-Mediated Immunity

Induction of response: Naive CD4+ and CD8+ T cells recognize peptides from protein antigens presented by antigen-presenting cells in peripheral lymphoid organs.
T lymphocytes proliferate and differentiate into effector cells, many entering circulation.
Migration of effector T cells: Effector T cells and other leukocytes migrate through blood vessels in peripheral tissues.
- They bind to endothelial cells activated by cytokines produced in response to infection.
T cell effector functions: CD4+ T cells recruit and activate phagocytes for microbial destruction; CD8+ cytotoxic T lymphocytes (CTLs) kill infected cells.

Responses of Naive T Lymphocytes

Naive T lymphocytes' responses to cell-associated microbial antigens include sequential steps:
- Increase in antigen-specific T cells.
- Conversion of naive T cells to effector and memory cells.
Cytokine secretion occurs early, stimulating proliferation (clonal expansion).
Differentiation results in effector T cells for microbial elimination.
Effector T cells migrate to infection sites.
Some effector T cells remain in lymph nodes, eradicating infected cells or promoting antibody responses by B cells.
Progeny of T cells develop into long-lived, functionally inactive memory T cells, ready for rapid response upon re-exposure.
As effector T cells eliminate the infectious agent, stimuli that triggered T cell expansion fade, leading to the death of most antigen-specific lymphocytes, returning the system to a resting state with remaining memory cells.
The sequence is common to CD4+ and CD8+ T lymphocytes, with differences in properties and effector functions.

T Cell Migration

Naive T lymphocytes recirculate through peripheral lymphoid organs, searching for foreign protein antigens.
Microbial antigens are transported to peripheral lymphoid organs.
Antigens are processed and displayed by MHC molecules on dendritic cells, the most efficient stimulators of naive T cells.
T cell recognition of antigen leads to transient arrest on the dendritic cell and activation.
Activated cells may migrate to the inflamed tissue, the original source of the antigen.
Three stimuli are critical for T cell activation:
- Antigen recognition initiates the process.
- Costimulation maximizes the response.
- Cytokines amplify the response and direct differentiation pathways.

Antigen Recognition and Costimulation

T cell responses require multiple receptors on T cells recognizing ligands on APCs.
T cell receptor (TCR) recognizes MHC-associated peptide antigens; CD4 or CD8 coreceptors recognize MHC molecules; adhesion molecules strengthen T cell-APC binding; costimulators provide second signals.

Recognition of MHC-Associated Peptides

TCR and CD4/CD8 coreceptor recognize peptide antigens and MHC molecules on APCs, providing the first signal for T cell activation.
The TCR on CD4+ and CD8+ T cells consists of α and β chains that participate in antigen recognition.
TCR of a T cell specific for a foreign peptide recognizes the displayed peptide and simultaneously recognizes residues of the MHC molecule.
Mature MHC-restricted T cells express either CD4 or CD8, which function with the TCR to bind MHC molecules.
CD4 or CD8 recognizes class II or class I MHC molecule, respectively, at a site separate from the peptide-binding cleft.
Protein antigens ingested by APCs are processed into peptides displayed by class II MHC molecules, while cytosolic protein antigens are processed into peptides displayed by class I MHC molecules.
CD4+ T cells recognize antigens ingested from extracellular microbes, while CD8+ T cells recognize peptides from cytosolic or nuclear antigens.
The specificity of CD4 and CD8 ensures that different classes of T cells respond to different types of microbes.
The TCR and its coreceptor must be engaged simultaneously to initiate the T cell response, and multiple TCRs likely need to be triggered for T cell activation to occur.
Biochemical signals that lead to T cell activation are triggered by proteins linked to the TCR (CD3 proteins and ζ chain) and by the CD4 or CD8 coreceptor.
The TCR αβ heterodimer recognizes antigens but cannot transmit biochemical signals. It is associated with transmembrane signaling molecules (CD3 and ζ chain).
The TCR, CD3, and ζ chain make up the TCR complex.
Signaling functions of TCRs are the same in all clones (CD3 and ζ proteins are invariant).
T cells can be activated by molecules that bind to the TCRs of many or all clones of T cells, regardless of the peptide-MHC specificity of the TCR. These are polyclonal activators.
Examples of polyclonal activators: antibodies specific for the TCR or associated CD3 proteins, polymeric carbohydrate-binding proteins (phytohemagglutinin, PHA), and microbial proteins (superantigens).
Polyclonal activators are used experimentally and clinically.
Microbial superantigens may cause systemic inflammatory disease by inducing excessive cytokine release from many T cells.

Role of Adhesion Molecules in T Cell Responses

Adhesion molecules on T cells recognize their ligands on APCs and stabilize the binding of the T cells to the APCs.
Most TCRs bind the peptide-MHC complexes with low affinity.
The binding of T cells to APCs must be stabilized for a sufficiently long period that the necessary signaling threshold is achieved.
This stabilization function is performed by adhesion molecules on the T cells that bind to ligands expressed on APCs.
The major T cell integrin involved in binding to APCs is leukocyte function–associated antigen 1 (LFA-1), whose ligand on APCs is called intercellular adhesion molecule 1 (ICAM-1).
On resting naive T cells, the LFA-1 integrin is in a low-affinity state.
Antigen recognition by a T cell increases the affinity of that cell’s LFA-1, providing a positive feedback loop.
Integrin-mediated adhesion is critical for the ability of T cells to bind to APCs displaying microbial antigens.

Role of Costimulation in T Cell Activation

Full activation of T cells depends on:
- Recognition of costimulators on APCs in addition to antigen.
- Costimulators provide stimuli to T cells that function together with stimulation by antigen.
The best-defined costimulators for T cells are two related proteins called B7-1 (CD80) and B7-2 (CD86), expressed on APCs.
B7 proteins are recognized by a receptor called CD28, which is expressed on virtually all T cells.
Binding of CD28 on T cells to B7 on the APCs generates signals in the T cells that work together with signals generated by TCR recognition of antigen presented by MHC proteins on the same APCs.
CD28-mediated signaling is essential for the responses of naive T cells; in the absence of CD28-B7 interactions, antigen recognition by the TCR is insufficient for T cell activation.
The requirement for costimulation ensures that naive T lymphocytes are activated fully by microbial antigens, and not by harmless foreign substances (or by self antigens), because microbes stimulate the expression of B7 costimulators on APCs.
Another set of molecules that participate in T cell responses are CD40 ligand (CD40L, or CD154) on activated T cells and CD40 on APCs.
CD40L expressed on an antigen-stimulated T cell binds to CD40 on APCs and activates the APCs to express more B7 costimulators and to secrete cytokines (e.g., IL-12) that enhance T cell differentiation.
CD40L-CD40 interaction promotes T cell activation by making APCs better at stimulating T cells.
Adjuvants function mainly by inducing the expression of costimulators on APCs and by stimulating the APCs to secrete cytokines that activate T cells. Most adjuvants are products of microbes or substances that mimic microbes.
Agents that block B7:CD28 are used in the treatment of rheumatoid arthritis, other inflammatory diseases, and graft rejection, and antibodies to block CD40:CD40L interactions are being tested in inflammatory diseases and in transplant recipients to reduce or prevent graft rejection.

Stimuli for Activation of CD8+ T Cells

Activation of CD8+ T cells is stimulated by recognition of class I MHC–associated peptides and requires costimulation and/or helper T cells.
Responses of CD8+ T cells may differ from responses of CD4+ T lymphocytes.
One feature unique to CD8+ T cell activation is that its initiation often requires cytoplasmic antigen from one cell (e.g., virus-infected cells) to be cross-presented by dendritic cells.
Differentiation of CD8+ T cells into fully active cytotoxic T lymphocytes (CTLs), and into memory cells, may require the concomitant activation of CD4+ helper T cells.
When virus-infected cells are ingested by dendritic cells the APC may present antigens from the cytosol in complex with class I MHC molecules and from vesicles in complex with class II MHC molecules.
CD4+ T cells may produce cytokines or membrane molecules that help to activate the CD8+ T cells.
Defective CTL responses to many viruses in patients infected with the human immunodeficiency virus (HIV) can be explained by the requirement for helper T cells in CD8+ T cell responses to some viruses.

Biochemical Pathways of T Cell Activation

On recognition of antigens and costimulators, T cells express proteins that are involved in proliferation, differentiation, and effector functions of the cells.
Antigen recognition leads to new gene transcription and protein synthesis in the activated T cells.
The activation of enzymes, recruitment of adaptor proteins, and production of active transcription factors are the parts of biochemical pathways that link antigen recognition with T cell responses.
Biochemical pathways are initiated when the TCR complexes and the appropriate coreceptor are brought together by binding to MHC-peptide complexes on the surface of APCs.

Immunologic Synapse

Orderly redistribution of proteins in both the APC and T cell membranes at the point of cell-to-cell contact takes place.
The TCR complex, CD4/CD8 coreceptors, and CD28 coalesce to the center and the integrins move to form a peripheral ring.
Redistribution of signaling and adhesion molecules is thought to be responsible for optimal induction of activating signals in the T cell.
The region of contact between the APC and T cell, including the redistributed membrane proteins, is called the immunologic synapse.
Effector molecules and cytokines may be secreted through this region.
Enzymes that serve to degrade or inhibit signaling molecules are recruited to the synapse, so it may be involved in terminating lymphocyte activation.
The CD4 and CD8 coreceptors facilitate signaling through a protein tyrosine kinase called Lck that is noncovalently attached to the cytoplasmic tails of these coreceptors.
Several transmembrane signaling proteins are associated with the TCR, including the CD3 and ζ chains.
CD3 and ζ contain immunoreceptor tyrosine-based activation motifs (ITAMs), which are critical for signaling.
Lck phosphorylates tyrosine residues contained within the ITAMs of the CD3 and ζ proteins.
The phosphorylated ITAMs of the ζ chain become docking sites for a tyrosine kinase called ZAP-70 (zeta-associated protein of 70 kD), which also is phosphorylated by Lck and thereby made enzymatically active.
The active ZAP-70 then phosphorylates various adaptor proteins and enzymes, which assemble near the TCR complex and mediate additional signaling events.
Major signaling pathways linked to ζ-chain phosphorylation and ZAP-70 are the calcium-NFAT pathway, the Ras– and Rac–MAP kinase pathways, the PKCθ–NF-κB pathway, and the PI-3 kinase pathway.

Calcium-NFAT Pathway

Nuclear factor of activated T cells (NFAT) is a transcription factor present in an inactive phosphorylated form in the cytoplasm of resting T cells.
NFAT activation and its nuclear translocation depend on the concentration of calcium ( $Ca^{2+}$ ) ions in the cell.
The signaling pathway is initiated by ZAP-70–mediated phosphorylation and activation of an enzyme called phospholipase Cγ (PLCγ), which catalyzes the hydrolysis of a plasma membrane inositol phospholipid called phosphatidylinositol 4,5-bisphosphate (PIP2).
One byproduct of PLCγ-mediated PIP2 breakdown, called inositol 1,4,5-triphosphate (IP3), binds to IP3 receptors on the endoplasmic reticulum (ER) membrane and stimulates release of $Ca^{2+}$ from the ER, thereby raising the cytosolic $Ca^{2+}$ concentration.
In response to the loss of calcium from intracellular stores, a plasma membrane calcium channel is opened, leading to the influx of extracellular $Ca^{2+}$ into the cell, which sustains the elevated $Ca^{2+}$ concentration for hours.
Cytoplasmic $Ca^{2+}$ binds a protein called calmodulin, and the $Ca^{2+}$ -calmodulin complex activates a phosphatase called calcineurin.
Calcineurin removes phosphates from cytoplasmic NFAT, enabling it to migrate into the nucleus, where it binds to and activates the promoters of several genes, including the genes encoding the T cell growth factor interleukin-2 (IL-2) and components of the IL-2 receptor.
A drug called cyclosporine binds to and inhibits the phosphatase activity of calcineurin and thus inhibits the NFAT-dependent production of cytokines by T cells. This agent is widely used as an immunosuppressive drug to prevent graft rejection.

Ras/Rac–MAP Kinase Pathways

The Ras/Rac–MAP kinase pathways include the guanosine triphosphate (GTP)– binding Ras and Rac proteins, several adaptor proteins, and a cascade of enzymes that eventually activate one of a family of mitogen-activated protein (MAP) kinases.
These pathways are initiated by ZAP-70–dependent phosphorylation and accumulation of adaptor proteins at the plasma membrane, leading to the recruitment of Ras or Rac, and their activation by exchange of bound guanosine diphosphate (GDP) with GTP.
Ras•GTP and Rac•GTP initiate different enzyme cascades, leading to the activation of distinct MAP kinases.
The terminal MAP kinases in these pathways, called extracellular signal–regulated kinase (ERK) and c-Jun amino-terminal (N-terminal) kinase (JNK), respectively induce the expression of a protein called c-Fos and the phosphorylation of another protein called c-Jun.
c-Fos and phosphorylated c-Jun combine to form the transcription factor activating protein 1 (AP-1), which enhances the transcription of several T cell genes.

PKCθ–NF-κB Pathway

Another major pathway involved in TCR signaling consists of activation of the θ isoform of the serine-threonine kinase called protein kinase C (PKCθ) and activation of the transcription factor nuclear factor κB (NF-κB).
PKC is activated by diacylglycerol, which, like IP3, is generated by PLC-mediated hydrolysis of membrane inositol lipids.
PKCθ acts through adaptor proteins recruited to the TCR complex to activate NF-κB.
NF-κB exists in the cytoplasm of resting T cells in an inactive form, bound to an inhibitor called IκB.
TCR-induced signals, downstream of PKCθ, activate a kinase that phosphorylates IκB and targets it for destruction. As a result, NF-κB is released and moves to the nucleus, where it promotes the transcription of several genes.

PI-3 Kinase Pathway

T cell receptor signal transduction also involves a lipid kinase called phosphatidylinositol-3 (PI-3) kinase, which phosphorylates membrane PIP2 to generate PIP3.
The phospholipid PIP3 is required for the activation of a number of crucial targets including a serine-threonine kinase called protein kinase B, or Akt, which has many roles, including stimulating expression of anti-apoptotic proteins and thus promoting survival of antigen-stimulated T cells.
The PI-3 kinase/Akt pathway is triggered not only by the TCR but also by CD28 and IL-2 receptors.
Closely linked to the Akt pathway is mTOR (mammalian target of rapamycin), a serine-threonine kinase that is involved in stimulating protein translation and in cell survival and growth. A drug that binds to and inactivates mTOR, rapamycin, is used to treat graft rejection.

Transcription Factors

The various transcription factors, including NFAT, AP-1, and NF-κB, stimulate transcription and subsequent production of cytokines, cytokine receptors, cell cycle inducers, and effector molecules such as CD40L.
All these signals are initiated by antigen recognition, because binding of the TCR and coreceptors to antigen (peptide-MHC complexes) is necessary to initiate signaling in T cells.
Recognition of costimulators, such as B7 molecules, by their receptor (CD28) is essential for full T cell responses.

Functional Responses of T Lymphocytes to Antigen and Costimulation

The recognition of antigen and costimulators by T cells initiates a set of responses that culminate in the expansion of the antigen-specific clones of lymphocytes and the differentiation of the naive T cells into effector cells and memory cells.
Many of the responses of T cells are mediated by cytokines that are secreted by the T cells and act on the T cells themselves and on many other cells involved in immune defenses.

Secretion of Cytokines and Expression of Cytokine Receptors

In response to antigen and costimulators, T lymphocytes, especially CD4+ T cells, rapidly secrete several different cytokines that have diverse activities.
Cytokines are a large group of proteins that function as mediators of immunity and inflammation.
In adaptive immunity, cytokines are secreted by T cells, mainly CD4+ cells.
The first cytokine to be produced by CD4+ T cells, within 1 to 2 hours after activation, is interleukin-2 (IL-2).
Activation also rapidly enhances the ability of T cells to bind and respond to IL-2, by increasing the expression of the high-affinity IL-2 receptor.
The receptor for IL-2 is a three-chain molecule.
Naive T cells express two signaling chains but do not express the chain that enables the receptor to bind IL-2 with high affinity.
Within hours after activation by antigens and costimulators, the T cells produce the third chain of the receptor, and now the complete IL-2 receptor is able to bind IL-2 strongly.
IL-2 produced by antigen-stimulated T cells preferentially binds to and acts on the same T cells (autocrine cytokine action).
The principal functions of IL-2 are to stimulate the survival and proliferation of T cells, resulting in an increase in the number of the antigen-specific T cells; IL-2 was originally called T cell growth factor.
Differentiated effector CD4+ T cells produce many other cytokines.
CD8+ T lymphocytes that recognize antigen and costimulators do not appear to secrete large amounts of IL-2, but these lymphocytes proliferate prodigiously during immune responses.
Antigen recognition and costimulation may be able to drive the proliferation of CD8+ T cells, or IL-2 may be provided by CD4+ helper T cells.

Clonal Expansion

T lymphocytes activated by antigen and costimulation begin to proliferate within 1 or 2 days, resulting in expansion of antigen-specific clones.
The magnitude of clonal expansion is remarkable, especially for CD8+ T cells.
Before infection, the frequency of CD8+ T cells specific for any one microbial protein antigen is about 1 in $10^5$ or $10^6$ lymphocytes.
At the peak of some viral infections, possibly within a week after the infection, as many as 10% to 20% of all the lymphocytes in the lymphoid organs may be specific for that virus. This means that the antigen-specific clones have increased by more than 10,000-fold, with an estimated doubling time of about 6 hours.
Even in infections with complex microbes that contain many protein antigens, a majority of the expanded clones are specific for only a few (less than five) immunodominant peptides of that microbe.
The expansion of CD4+ T cells appears to be much less than that of CD8+ cells, probably 100-fold to 1000-fold. This difference in the magnitude of clonal expansion of CD8+ T cells and CD4+ T cells may reflect differences in their functions.
Within a week or two of their activation, some of the expanded T cells have differentiated into effector and memory cells, and the majority die as the stimuli that initiated the response are eliminated.

Differentiation of Naive T Cells into Effector Cells

Some of the progeny of antigen-stimulated, proliferating T cells differentiate into effector cells whose function is eradicate infections.
Differentiation is the result of changes in gene expression, such as the activation of genes encoding cytokines (in CD4+ T cells) or cytotoxic proteins (in CD8+ CTLs).
Differentiation begins in concert with clonal expansion, and differentiated effector cells appear within 3 or 4 days after exposure to microbes. Many of these cells leave the peripheral lymphoid organs and migrate to the site of infection.
On recognition of antigen, the effector cells respond in ways that serve to eradicate the infection.
Effector cells of the CD4+ and CD8+ populations perform different functions and are best described separately.

CD4+ Helper T Cells

Some differentiated CD4+ helper T cells migrate into lymphoid follicles in the lymphoid organs, where they stay to help B cells.
Effector cells of the CD4+ helper lineage function to activate phagocytes and B lymphocytes by expressing various surface molecules and secreting cytokines.
The most important cell surface protein involved in the effector function of CD4+ T cells is CD40 ligand, a member of a large family of proteins structurally related to the cytokine tumor necrosis factor (TNF).
The CD40L gene is transcribed in CD4+ T cells in response to antigen recognition and costimulation, and so CD40L is expressed on activated helper T cells. It binds to its receptor, CD40, which is expressed mainly on macrophages, B lymphocytes, and dendritic cells.
Engagement of CD40 activates these cells, and thus CD40L is an important participant in the activation of macrophages and B lymphocytes by helper T cells.
The interaction of CD40L on T cells with CD40 on dendritic cells stimulates the expression of costimulators on these APCs and the production of T cell– activating cytokines, thus providing a positive feedback (amplification) mechanism for APC-induced T cell activation.

Subsets of CD4+ Helper T Cells

Analysis of cytokine production by helper T cells revealed that functionally distinct subsets of CD4+ T cells exist, distinguished by the cytokines they produce.
Subsets of CD4 + Helper T Cells Distinguished by Cytokine Profiles
CD4+ helper T cells may differentiate into at least three subsets of effector cells that produce distinct sets of cytokines and perform different functions. The subsets are TH1 cells and TH2 cells, more recently, a third population has been identified and called TH17 cells. (Regulatory T cells, another subset of CD4+ T cells, suppress immune responses and are discussed in Chapter 9 in the context of immunologic tolerance.)

TH1 Subset

TH1 cells stimulate phagocyte-mediated ingestion and killing of microbes, which is a key component of cell-mediated immunity.
The most important cytokine produced by TH1 cells is interferon-γ (IFN-γ).
IFN-γ is a potent activator of macrophages, especially the ability of macrophages to kill ingested microbes (classical macrophage activation).
IFN-γ also stimulates the production of antibody isotypes that promote the phagocytosis of microbes, because these antibodies bind directly to phagocyte Fc receptors and they activate complement, generating products that bind to phagocyte complement receptors.
Because of these actions of IFN-γ, TH1 cells are critical for ingestion and killing of intracellular microbes in phagocytes.
IFN-γ also stimulates the expression of class II MHC molecules and B7 costimulators on macrophages and dendritic cells, which may serve to amplify T cell responses.

TH2 Subset

TH2 cells stimulate phagocyte-independent, eosinophil-mediated immunity, which is especially effective against helminthic parasites.
TH2 cells produce interleukin-4, which stimulates the production of IgE antibodies, and interleukin-5, which activates eosinophils. IgE activates mast cells and binds to eosinophils.
IgE coats the helminths, eosinophils bind to the IgE, eosinophils are activated to release their granule contents, and granule enzymes kill the parasites.
Cytokines produced by TH2 cells, such as IL-4 and IL-13, promote the expulsion of parasites from mucosal organs and inhibit the entry of microbes by stimulating mucus secretion.
The cytokines of TH2 cells also activate macrophages, enhancing other functions, such as synthesis of extracellular matrix proteins involved in tissue repair (alternative macrophage activation).
Cytokines produced by TH2 cells, such as IL-4, IL-10, and IL-13, inhibit the microbicidal activities of macrophages and thus suppress TH1 cell–mediated immunity.
The efficacy of cell-mediated immune responses against a microbe may be determined by a balance between the activation of TH1 and TH2 cells in response to that microbe.

TH17 Subset

TH17 cells induce inflammation, which functions to destroy extracellular bacteria and fungi and may contribute to several inflammatory diseases.
TH17 cells secrete cytokines that recruit leukocytes (mainly neutrophils but also monocytes) to sites of antigen recognition.
TH17 cells are important in defense against extracellular bacterial and fungal infections.
Mutations in genes involved in the development and functions of these cells result in increased susceptibility to bacterial and fungal infections, particularly mucocutaneous candidiasis.

Development of TH1, TH2, and TH17 Subsets

The generation of subsets is regulated by stimuli naive CD4+ T cells receive when they encounter microbial antigens.
Each subset is induced best in response to the types of microbes that subset is designed to combat.
Signals for the differentiation of naive CD4+ T lymphocytes into distinct subsets of effector cells are cytokines produced by APCs and other cells at the time of antigen stimulation.
Each effector T cell subset produces cytokines that amplify itself and inhibit the other subsets.
Differentiation of subsets is associated with the activation of transcription factors that stimulate production of various cytokines. Examples of these transcription factors are shown in Figure 5–18. The most important transcription factors for the three subsets are T-bet, GATA-3, and RORγT for TH1, TH2, and TH17, respectively. These work in concert with transcription factors called signal transducers and activators of transcription (STATs) induced by cytokines.

TH1 Differentiation

Differentiation of CD4+ T cells to the TH1 subset is driven by a combination of the cytokines IL-12 and IFN-γ.
In response to many bacteria (especially intracellular bacteria) and viruses, dendritic cells and macrophages produce IL-12, and NK cells produce IFN-γ.
When naive T cells recognize the antigens of these microbes, the T cells are exposed to IL-12 and IFN-γ. These two cytokines activate transcription factors that promote the differentiation of the T cells to the TH1 subset.
TH1 cells produce IFN-γ, which not only activates macrophages to kill the microbes but also promotes more TH1 development and inhibits the development of TH2 and TH17 cells.

TH2 Differentiation

The development of TH2 cells is stimulated by the cytokine interleukin-4 .
If an infectious microbe does not elicit IL-12 production by APCs, as may occur with helminths, the T cells themselves produce IL-4.
Helminths may activate cells of the mast cell and eosinophil lineages to secrete IL-4.
In antigen-stimulated T cells, IL-4 activates transcription factors that promote differentiation to the TH2 subset, leading to more IL-4 production and thus providing further amplification of the TH2 response.

TH17 Differentiation

The development and maintenance of TH17 cells require inflammatory cytokines
*IL-6, IL-1, and IL-23 are produced in response to fungi and some bacteria.

Differentiation of CD8+ T Cells into CTLs

CD8+ T lymphocytes activated by antigen and costimulators differentiate into cytotoxic T lymphocytes that are able to kill infected cells expressing the antigen.
CTLs kill infected cells by secreting proteins that insert into the membranes of the infected cells and facilitate the entry of enzymes that induce apoptosis of the infected cells.

Development of Memory T Lymphocytes

A fraction of antigen-activated T lymphocytes differentiates into long-lived memory cells. Memory cells survive even after the infection is eradicated and antigen is no longer present.
Memory T cells can be found in lymphoid organs, in various peripheral tissues, especially mucosa and skin, and in the circulation. Memory T cells require signals delivered by certain cytokines including IL-7, in order to stay alive.
Memory T cells can be distinguished from naive and effector cells by several criteria.
A subset of memory T cells, called central memory cells, populate lymphoid organs and are responsible for rapid clonal expansion after re-exposure to antigen. Another subset, called effector memory cells, localize in mucosal and other peripheral tissues and mediate rapid effector functions on reintroduction of antigen to these sites.

Decline of the Immune Response

During the response, the survival and proliferation of T cells are maintained by antigen, costimulatory signals from CD28, and cytokines such as IL-2.
Once an infection is cleared and the stimuli for lymphocyte activation disappear, many of the cells that had proliferated in response to antigen are deprived of these survival signals. As a result, these cells die by apoptosis (programmed cell death).
The response subsides within 1 or 2 weeks after the infection is eradicated, and the only sign that a T cell–mediated immune response had occurred is the pool of surviving memory lymphocytes.