The end result of TCR stimulation is the activation of the T cell to produce various cytokines (e.g., IL-2), as well as to express the high-affinity IL-2 receptor. IL-2, also known as T-cell growth factor, stimulates the T cells to multiply, resulting in “clonal proliferation” of a population of antigen-specific T cells. As they proliferate, different progeny cells of this clonal population take on one of a number of essential functions. Some of these cells remain in the secondary lymphoid organ, while others exit via the blood or efferent lymphatics and migrate to inflamed tissues where the same TCR–peptide–MHC pathways restimulate them to exert their effector functions. Figure 60–4 is an overview of the priming of naïve CD4-positive and CD8-positive T cells in a lymph node draining a site of infection.
Overview of the initial priming and differentiation of CD4-positive and CD8-positive T cells in a secondary lymphoid organ. Dendritic cells (DC) from an infected tissue take up antigen and migrate through afferent lymphatics to the T-cell zone of a draining lymph node. Endosomal antigens are presented on class II major histocompatibility complex (MHC) proteins to CD4-positive T cells (blue), and cytosolic (e.g., viral) antigens are presented on class I MHC proteins to CD8-positive T cells (green). Naïve T cells’ T-cell receptors (TCRs) recognize their cognate peptide antigens presented by the corresponding MHC proteins and become activated. As the T cells proliferate, they differentiate: CD4-positive cells become follicular helper (Tfh) cells, effector/helper T (Th) cells, or regulatory (Treg) cells, whereas CD8-positive cells become cytotoxic T cells. Depending on their function, differentiated T cells either stay in the lymph nodes or migrate through the lymphatics or bloodstream to sites of inflammation. PAMP = pathogen-associated molecular pattern.
T-cell functions can be divided into four main categories: CD4-positive cells become (1) effector/helper (Teff or Th) cells, which leave the lymphoid organ and coordinate immune responses in inflamed tissue; (2) follicular helper (Tfh) cells, which move into the B-cell follicle of the lymphoid organ and help the B cells; and (3) regulatory T (Treg) cells, which suppress inflammation. CD8-positive cells become (4) cytotoxic T cells (or cytotoxic T lymphocytes, usually abbreviated CTL), which kill virus-infected cells and tumor cells. Remember, all of these T cells require cell–cell interactions and TCR–peptide–MHC recognition, both for their initial priming and later for their effector functions. Also, after an infection is cleared, each of these T-cell types can contribute clones to the pool of memory T cells that patrol the body and respond rapidly to reinfection. Figure 60–5 is an overview of the “help” provided by the various subsets of CD4-positive effector/helper (Th-1, Th-2, and Th-17) and follicular helper T (Tfh) cells.
Subsets of CD4-positive helper T cells. After activation by a dendritic cell, in the presence of particular cytokines, a naïve CD4-positive T cell divides and differentiates into effector/helper (Th-1, Th-2 or Th-17) or follicular helper (Tfh) subsets. Each type of cell produces cytokines that “help” by activating other immune cell partners.
CD4-positive T lymphocytes perform a variety of functions that help an immune response by enhancing the functions of other cells. T effector/helper (Th) cells leave the lymph node, migrate to inflamed tissues in the body, and produce cytokines. Different infectious pathogens must be handled by the immune system in different ways. In order to provide a targeted immune defense against a specific organism, Th cells can produce various cytokines that have various effects.
However, this means that the Th cells must be programed to produce the appropriate cytokines for the appropriate organisms. This occurs through a process of further differentiation: the signals that Th cells encounter at the time of first antigen recognition commit them to become one of several specialist cell types, or Th subsets. In inflamed tissue, the Th subsets interact with APCs, and when they sense their unique antigen, they respond in a way that is defined by their subset membership.
Table 60–1 lists the major innate cytokines that influence Th subset differentiation. Analogous to a multipotent stem cell, the original naïve cell clone has the potential to become any of the subsets. But as the progeny cells divide, their transcriptional programs are reinforced through a process of epigenetic modification. The earliest cues that start the process of differentiation are only partially known, but after only a few cell divisions, clones of the original cell can be identified that have specialist capabilities, defined by their signature transcription factors and cytokines.
TABLE 60–1Innate Cytokines that Influence Th Cell Subset Differentiation ||Download (.pdf) TABLE 60–1 Innate Cytokines that Influence Th Cell Subset Differentiation
|Cytokine ||Cell Source ||Key Biologic Effects |
|IL-1 ||Macrophages, dendritic cells ||Th-17 differentiation |
|IL-4 ||Mast cells, basophils, T cells ||Th-2 differentiation |
|IL-6 ||Macrophages, endothelial cells, T cells ||Th-17 differentiation and activation |
|IL-12 ||Macrophages, Dendritic cells ||Th-1 differentiation; in NK cells and CD8+ T cells, increases cytotoxic activity and IFN-γ production |
|IL-23 ||Macrophages, Dendritic cells ||Th-17 differentiation and activation |
Most of our understanding of Th cell subsets comes from studies in which Th cell clones can be transferred among genetically identical mice. Despite exposures to new infectious and inflammatory stimuli, these Th cells and their progeny continue to have the same signature cytokines of their original subset. For example, all effector/helper Th cell subsets express the gene PRDM1 (encoding the protein BLIMP-1) and have the ability to make copious IL-2, migrate from the secondary lymphoid organ to the site of infection, and express further cytokines upon re-stimulation of their TCR.
Th-1 cells are primarily responsible for “classical” activation of macrophages, leading to enhanced phagocytosis, phagolysosome free radical production, and granuloma formation (Figure 60–6 and Table 60–2). Th-1 cells arise after antigens arrive in the T-cell zone of the secondary lymphoid tissue. DCs present peptide fragments complexed with class II MHC proteins to nearby naïve CD4-positive T cells. Activation and clonal proliferation of antigen-specific Th cells occur as a result of TCR stimulation, co-stimulatory signals, and IL-2 production from the T cells.
In certain infections, the DCs produce IL-12 at the time of Th cell activation, leading to the differentiation of these CD4-positive cells into Th-1 cells. Th-1 cells express the signature transcription factor TBX21 and produce the cytokine gamma interferon (IFN-γ). The activated Th-1 cell clones then move out of the lymphoid tissue, into the arterial circulation, and into the inflamed tissue by means of inflammation-induced extravasation (see Figure 58–4). There, they come in contact with macrophages, and after recognizing the same peptide presented with class II MHC by these macrophages, the Th-1 cells make more IFN-γ as well as tumor necrosis factor (TNF). These cytokines activate the macrophages to be more effective killers of phagocytized intracellular organisms and help the macrophages form large granulomas to wall off microbes that are hard to kill.
Th-1 cells and macrophages play a role in host defense against many bacteria, fungi, and viruses, as well as against tumors, but individuals with deficiencies in IL-12 or IFN-γ are particularly susceptible to mycobacterial infections, such as tuberculosis. In addition to their role in controlling phagocytized pathogens, overactive Th-1 cells are associated with autoimmune and inflammatory diseases, including Crohn’s disease, psoriasis, and rheumatoid arthritis.
The Th-17 cell subset is closely related to the Th-1 cell subset but is generated by high levels of IL-1, IL-6, and IL-23 at the time of initial activation by DCs (see Figure 60–6 and Table 60–2). Th-17 cells express the signature transcription factors RORC and STAT3, which are reinforced by autocrine signaling from the cytokine IL-21. Th-17 cells also produce the cytokines IL-17 (the source of their name), which stimulates phagocytes and mucosal epithelial cells to increase the production of IL-1, IL-6, and neutrophil-attracting chemokines. Th-17 cells also make IL-22, which stimulates mucosal epithelial cells to increase the production of antimicrobial defensins and tight junction proteins. Together, the cytokines from Th-17 cells and the neutrophils they recruit defend the barrier tissues against bacterial and fungal infections.
Patients with genetic mutations causing IL-17 deficiency have particular susceptibility to mucocutaneous infections from the yeast Candida albicans. In addition, loss of Th-17 cells in HIV disease is associated with chronic translocation of small numbers of bacteria from the intestinal lumen across the bowel wall and into the portal circulation. Like Th-1 cells, overactive Th-17 cells are associated with autoimmune and inflammatory diseases.
The Th-2 cell subset is most commonly associated with infection by certain helminth worms, such as Schistosoma and Strongyloides, which have a tissue-invasive stage of their life cycle (Figure 60–7 and Table 60–2). The signature Th-2 transcription factor is GATA3, and the signature Th-2 cytokines are IL-4 and IL-13, two cytokines that share the same receptor and therefore have similar effects. These cytokines increase the production of mucus by goblet cells at barrier surfaces, cause smooth muscle cells to be hypercontractile, and cause “alternative” activation of macrophages, leading to collagen deposition often seen in wound healing. IL-4 also signals in autocrine fashion to reinforce the Th-2 transcriptional program (analogous to IL-21 for Th-17 cells). Th-2 cells also make IL-5, which is the specific factor that recruits and maintains eosinophils, and IL-9, which activates mast cells.
When dysregulated, Th-2 cells cause allergic disease, such as atopic dermatitis, allergic asthma, and eosinophilic gastrointestinal disease. As shown in Figure 60–7, another important part of the Th2 immune response is IgE, which mast cells and eosinophils use to detect antigen. Tfh cells are likely the main source of IL-4 that helps B cells mature to become IgE-producing plasma cells.
The Th-1 immune response protects against phagocytized bacteria during infection. Free antigen circulates through the lymph into the T-cell zone of the draining lymph node, and dendritic cells (DC) that have phagocytized bacteria migrate to the T-cell zone by increasing their surface expression of the chemokine receptor CCR7. DCs process the complex protein antigens of endosomal bacteria (red stars) and present them as short peptides complexed with the class II major histocompatibility complex (MHC) proteins to CD4-positive T cells, along with co-stimulatory B7 (turquoise). During this process, if DCs produce interleukin (IL)-12, this promotes the activated antigen-specific Th cells to differentiate into Th-1 cells as they make IL-2 and undergo clonal proliferation. Alternatively, if the DCs produce IL-23, IL-1, and IL-6, this promotes the activated Th cells to differentiate into Th-17 cells. These cells make IL-2 and undergo clonal proliferation.
TABLE 60–2Important Effector Cytokines Produced by T Cells ||Download (.pdf) TABLE 60–2 Important Effector Cytokines Produced by T Cells
|Cytokine ||Key Cell Source ||Key Target Cells and Biologic Effects |
|IL-2 ||All T cells || |
T and NK cells: proliferation
Treg cells: differentiation, survival, and function
|IL-4 ||CD4+ T cells (Th-2 and Tfh), primarily in the secondary lymphoid tissue || |
B cells: isotype switching to IgE
T cells: reinforce Th-2 cell differentiation
Other cells: same effects as IL-13 below
|IL-5 ||CD4+ T cells (Th-2) ||Eosinophils: survival, maturation, and mobilization from bone marrow |
|IL-10 ||CD4+ T regulatory cells (Treg) ||Myeloid cells: suppression of co-stimulatory signals and proinflammatory cytokines |
|IL-13 ||CD4+ T cells (Th-2), primarily in the nonlymphoid tissue || |
Epithelial cells: mucus
Smooth muscle cells: hypercontractility
Macrophages: “alternative” activation and collagen deposition
|IL-17 ||CD4+ T cells (Th-17) || |
Macrophages: increased chemokine and cytokine production (e.g., IL-1, IL-6, TNF)
Epithelial cells: increased IL-1
Neutrophils: chemokines, antimicrobial peptides
|IL-21 ||CD4+ T cells (Th-17 and Tfh) || |
B cells: isotype switching to all Ig subclasses and affinity maturation
Tfh cells: reinforces Tfh cell differentiation
Th-17 cells: reinforces Th-17 differentiation
|IL-22 ||CD4+ T cells (Th-17) ||Epithelial cells: increased production of defensins and tight junction adhesion proteins |
|IFN-γ (i.e., type II interferon) ||T cells (Th-1, CD8+ cells, Tfh), NK cells (and others) || |
B cells: isotype switching to opsonizing and complement-fixing IgG subclasses; increased MHC class I and class II expression
T cells: reinforces Th-1 cell differentiation
Macrophages: “classical” activation, enhanced phagocytosis, and intracellular killing
|TNF ||T cells (Th-1, Th-17) || |
Endothelial cells: inflammatory chemokines
The Th-2 immune response reacts to migratory helminths and allergens. In this example, free antigen circulates from the inflamed tissue through the lymph into the T-cell zone of the draining lymph node. Dendritic cells (DCs) that have phagocytized bacteria also migrate to the T-cell zone by increasing their surface expression of the chemokine receptor CCR7. Some free antigens also circulate to the B-cell follicle, where they are taken up by the B cells through B-cell receptor (BCR)-mediated endocytosis. DCs and B cells process the complex protein antigens of endosomal bacteria (red stars) and present them as short peptides complexed with the class II major histocompatibility complex (MHC) proteins to CD4-positive T cells (blue). The DCs also produce co-stimulatory B7 (turquoise). A few of the naïve CD4-positive clones differentiate into T follicular helper cells, which migrate to the B-cell follicle and provide help for naïve B cells (yellow) in the form of CD40-ligand (CD40L) and interleukin (IL)-4.
Follicular Helper T Cells
Tfh cells differentiate from naïve CD4-positive T cells like the other activated T cells, but rather than disperse from the lymphoid tissue to other sites, they migrate into the B-cell follicles (see Figure 60–7 and Table 60–2). The positioning of Tfh cells within the lymphoid tissue is absolutely critical to their function, as it dictates which cells they will encounter. They find the follicle by downregulating the chemokine receptor CCR7 and upregulating the chemokine receptor CXCR5, which senses chemokines produced by the stromal cells of the follicle. The earliest signals that start the Tfh cell transcriptional program are unknown, but as they proliferate and migrate to the follicle, Tfh cells begin to express the transcription factor BCL6 and suppress expression of PRDM1 (the gene encoding BLIMP-1). In addition, Tfh cells produce IL-21, which, in an autocrine fashion, increases their own BCL-6 and CXCR5 levels.
Tfh cells help B cells primarily through the production of cytokines such as IL-4 and IL-21 and also by expression of CD40 ligand (CD40L) on the Tfh cell surface, which interacts with CD40 on the surface of the B cell. (For more details on how Tfh cells promote B-cell activities, see Chapter 61.) Because of their central role in B-cell function, Tfh cells are particularly important for antibody responses, including responses to vaccines. A mutation in the gene encoding CD40L causes hyper-IgM syndrome, in which B cells are unable to “class switch” from IgM to the more mature immunoglobulin isotypes, as discussed in Chapter 68. Inappropriate Tfh activation can lead to self-reactive antibodies in autoimmune disease.
Normal immune responses can become pathologic if they are unrestrained. This can cause tissue damage, either from excessive inflammation at the site of an infection or from inappropriate activation of self-reactive (i.e., autoimmune) adaptive cells. The subset of CD4-positive cells called suppressor or regulatory T cells (Tregs) is responsible for limiting immune responses and maintaining tolerance of self-antigens and harmless commensal antigens. Some Tregs are programmed to be suppressive cells in the thymus, whereas others are programmed at the time of priming in secondary lymphoid organs.
The Treg cell subset expresses the signature transcription factor FOXP3. Patients with mutations in this gene have absent Tregs, enlarged secondary lymphoid organs, and severe autoimmune disease in numerous tissues. There are multiple mechanisms by which Tregs probably suppress immunity, although the most well-described mechanisms involve inhibiting Th and CTL activation. These mechanisms are described in greater detail in Chapter 66. There is significant interest in targeting these pathways, because boosting Treg function could be beneficial for transplant rejection and autoimmune disease, and inhibiting Treg function could be beneficial for cancer immunotherapy and chronic infections.
Table 60–2 lists the major cytokines produced by the various Th cell subsets and their main target cells.
CD8-positive cytotoxic (or cytolytic) T lymphocytes (CTLs) are particularly effective at killing virus-infected cells. Mature naïve CD8-positive T cells arise in the thymus and recognize non–self-peptide antigens complexed with class I MHC proteins (see Chapter 59). Because all nucleated cells express class I MHC, all nucleated cells bearing the cognate peptide for that CTL are a potential APC. To prevent inadvertent activation of self-reactive CTLs, there is an additional requirement that, during their activation, CTLs receive IL-2 produced by nearby CD4-positive T cells also undergoing activation.
For example, if a virus (e.g., influenza virus) infects and lyses a respiratory epithelial cell, virus particles (virions) are released to be phagocytized by DCs (Figure 60–8). The DCs transport these particles into the secondary lymphoid tissues, and viral peptide antigens appear on the surface of the DC in association with MHC proteins. Viral “peptide A” (red circle) is presented with class II MHC and recognized by the TCR of a CD4-positive T cell. In addition, through the process of “cross-presentation” (see Chapter 58), viral “peptide B” (gray circle) is presented with class I MHC and recognized by the TCR of a CD8-positive T cell (cell with green nucleus).
The CD4-positive cells help the DC–CD8-positive cell interaction in two ways: (1) When activated, the “A”-specific CD4-positive T cell expresses high surface levels of CD40L, which interacts with CD40 on the surface of the DC. This signals to the DC to further increase its expression of co-stimulatory molecules, ensuring activation of the “B”-specific CD8-positive cell. (Note that this mechanism of CD40L “licensing” of DCs is similar to the CD40L that Tfh cells provide to B cells, mentioned earlier and described in Chapter 61.) (2) The “A”-specific CD4-positive T cell also secretes IL-2, which directly signals the “B”-specific CD8-positive T-cell clone to proliferate. These new CTLs are virus-specific killers, able to recognize and kill any cell that displays viral peptide “B” on its surface.
Cytotoxic T lymphocyte (CTL) activation and killing of virus-infected cells. In this example, virus-infected cells release free virus, which circulates from the inflamed tissue through the lymph into the T-cell zone of the draining lymph node. Dendritic cells (DCs) that engulf virus also migrate to the T-cell zone by increasing their surface expression of the chemokine receptor CCR7. Some free virus particles also circulate to the B-cell follicle, where they are taken up by the B cells through B-cell receptor (BCR)-mediated endocytosis. DCs and B cells process the complex viral protein antigens (red stars) and present short peptide “A” (red circle) complexed with class II major histocompatibility complex (MHC) proteins to CD4-positive T cells (blue). DCs also cross-present peptide “B” (gray) with class I MHC proteins to CD8-positive T cells (green).
The main function of CTLs is to secrete perforins and proteases into infected cells. Perforins form a channel through the membrane, which allows the cell contents to leak out. It also allows the proteases to enter the cell cytosol and degrade cellular proteins. One of these proteases, granzyme B, cleaves procaspases into their active form, initiating apoptosis. Another mechanism by which CTLs kill target cells is the Fas-Fas ligand (FasL) interaction. Fas is a protein displayed on the surface of many cells. FasL is induced on the surface of the cytotoxic cell when its TCR recognizes its cognate antigen on the surface of a target cell. When Fas and FasL interact, the caspases that initiate apoptosis in the target cell are activated. After killing the virus-infected cell, the CTL itself is not damaged and can continue to kill other cells infected with the same virus.
Like Th-1 cells, CTLs express the transcription factor TBX21 and produce the cytokine IFN-γ. CTLs are especially important as an immune defense against viruses and some intracellular bacteria, such as Listeria monocytogenes. This is because these intracellular pathogens reside within host cells and use the cell machinery to divide and spread. These pathogens spend little time outside of host cells, meaning they are not susceptible to antibody and complement, so the best way to defeat them is for CTLs to kill the host cells, allowing phagocytes to engulf the remains. (Cytotoxic T cells have no effect on free virus, only on virus-infected cells.) In some cases, the killing effect of CTLs is actually pathogenic: the severe liver damage caused by hepatitis viruses is not the result of viral cytotoxicity, but rather the result of a robust CTL response that kills virus-infected hepatocytes.
CTLs are also important in the surveillance of the body for cancer: when malignant cells accumulate somatic mutations, they begin to generate novel (non-”self”) proteins, and CTLs that recognize and are activated by DCs presenting these peptide “neoantigens” can infiltrate the tumor and kill the malignant cells expressing those proteins. Transplanted cells from an allograft can similarly be recognized as non-”self” based on the presence of different human leukocyte antigen (HLA) polymorphisms and are therefore targets of CTLs (see Chapter 62).
Memory T cells, as the name implies, endow our host defenses with the ability to respond rapidly and vigorously for many years after the initial exposure to a microbe or other foreign material. The primary immune response occurs after the initial exposure to the antigen, when the naïve T cells are first primed. The specific T-cell clones primed during the primary response proliferate to large numbers, outnumbering many of the other T-cell clones in the circulation. For example, it is estimated that during infectious mononucleosis caused by Epstein–Barr virus (EBV), 40% of all circulating CD8-positive T cells are specific for EBV lytic phase proteins. After the infection has resolved, many of the antigen-specific T cells die by apoptosis, and the remaining few persist as memory cells.
Memory cells live for many years and have the capacity to reproduce themselves over many cell generations. On subsequent exposure to the antigen, these few T-cell clones rapidly proliferate again as part of a secondary immune response, generating many more specific T cells. This secondary response to a specific antigen is stronger and faster because: (1) the starting pool of memory cells is greater than the starting pool of that clone during the primary response, so it takes less time to reexpand this population; (2) compared with naïve cells, memory cells have a lower threshold of activation, meaning smaller amounts of antigen and co-stimulation are required; and (3) activated memory cells produce greater amounts of cytokines than do naïve T cells at the time of initial priming. (See Chapter 61 for a further discussion of primary and secondary antibody responses.)