Immunology (Lippincott Illustrated Reviews Series) 2nd Edition

Chapter 12: Regulation of Adaptive Responses

I. OVERVIEW

What happens when the immune system goes awry? When functioning properly, the innate and adaptive immune systems recognize and attack nonself while leaving self relatively undisturbed. The innate immune system expresses a finite number of genomically “hard-wired” receptors that recognize molecules widely expressed by potentially pathogenic organisms but not by the host (self). The adaptive immune system faces a daunting task because its receptors are somatically and randomly generated. Selection mechanisms in the thymus and bone marrow eliminate overtly self-reactive T cells and B cells during development. These mechanisms, however, cannot eliminate all potentially self-reactive cells because the adaptive immune system often encounters self molecules that were not present during receptor selection within the thymus or did not appear until a later point of development (e.g., those arising during and after puberty). Autoimmunity is a condition in which the immune system perceives self as nonself. Many serious and potentially fatal diseases, such as multiple sclerosis and systemic lupus erythematosus, are caused by autoimmune reactions.

Fortunately, the adaptive immune system has evolved several mechanisms to deal with potentially self-reactive lymphocytes. Unregulated adaptive immune responses are harmful. Without immune regulation, the adaptive immune response would be in a constant state of immunologic outrage, lashing out at nonself epitopes to which we are constantly exposed (e.g., food, drink, cosmetics), many of which pose no threat, and at those vital epitopes to which we are infrequently exposed (e.g., maternal–fetal interactions).

II. TOLERANCE

Normally, the immune system’s offensive machinery is reserved for use against external threats. Positive and negative thymic selection ensures that mature T cells recognize self MHC I or II molecules (positive selection) but are not overtly self-reactive against self peptides (negative selection). Thymocytes that are unable to make these distinctions meet an apoptotic death. The efficiency of negative selection is greatly increased by the action of the AIRE (αutoimmune regulator) gene. This gene operates in the thymic cells that are responsible for negative selection of developing thymocytes, as well as in other cells and tissues. AIRE causes the thymic epithelial reticular cells (see Chapter 9) to express a large number of molecules normally associated with non–thymic cells and tissues. As a result, negative selection can induce central tolerance to a range of both thymic and non–thymic self peptides. Rare individuals lacking a normally functioning AIRE gene develop autoimmune polyendocrinopathy-candidiasis-ectodermal dystrophy (APECED), an autoimmune syndrome also known as autoimmune polyendocrinopathy syndrome type I, that results in hypoparathyroidism, adrenal cortical dysfunction, and chronic mucocutaneous candidiasis (CMC).

No system is perfect, however; not all self peptides are presented within the thymus, and some self peptides arise after the waning of thymic function. In addition, some peptides are restricted to anatomic sites that are not easily accessible to the immune system. Consequently, some potentially autoreactive T cells slip through positive and negative selection. As a result, the adaptive immune system must use additional means to avoid self-reactivity. Selective nonresponsiveness or tolerance requires that when the adaptive immune system does recognize self, it should adopt a nondestructive strategy. Several tolerance mechanisms have evolved to minimize potential harm caused by postdevelopmental selection autoreactive cells.

A. Anergy

Anergy is a state of lymphocyte nonresponsiveness. It occurs following peptide + major histocompatibility complex (pMHC) engagement (T cells) or free epitope engagement (B cells). In the absence of additional “instruction” from antigen-presenting cells (APC) in the case of T cells) or from CD4+ cells in the case of B cells, the immune system does not respond. Anergy is therefore a form of regulation imposed upon the activation of naïve T and B cells.

In Chapter 10, we saw that naïve T cells require interaction with both pMHC and a set of costimulatory second signals from an APC (usually a dendritic cell) to become activated. The importance of this two-signal activation can be understood by considering what might happen, in the absence of such regulation, with T cells that escape negative selection in the thymus. Because all nucleated cells of the body express MHC I molecules presenting self peptides, naïve CD8+ T cells specific for self pMHC class I (pMHC I) could become activated by simply recognizing, via their T cell receptor (TCR, first signal), appropriate pMHC I complexes on any normal nucleated body cell. Once so activated, they would be able to bind and kill other normal body cells. The need for second signals from APCs minimizes this risk. TCR binding of self-reactive naïve CD8+ T cells with normal non-APC body cells (that are unable to provide the appropriate second signals) causes the CD8+ T cell to become anergic rather than activated. In other words, receipt of signal 1 in the absence of simultaneous receipt of signal 2 removes the T cell from the immunologic arena. How CD4+ T cells are anergized remains unclear, primarily because their interaction is almost always with APCs.

B cells, too, require a second signal following B-cell receptor (BCR) engagement. If they fail to receive a second signal, they become unresponsive to the combined restimulation by both first and second signals. Anergized cells are not killed but remain in circulation and cannot, under normal circumstances, be reactivated.

B. The role of CD152 (CTLA-4) in anergy

T cells constitutively express CD28 that engages CD80 (B7.1) or CD86 (B7.2) costimulatory molecules displayed by APCs (see Chapter 10). TCR engagement of the appropriate pMHC (first signal) + CD28:CD80/86 (second signal) stimulates the T cell to produce IL-2, express IL-2 receptor (IL-2R), and enter into the cell cycle. After activation of the T cell, CD152 (cytotoxic T-lymphocyte-associated antigen 4, or CTLA-4), which is normally sequestered within the Golgi apparatus of naïve T cells, moves to the outer cell membrane, where it binds with CD80/86 with an avidity that is 100-fold greater than that of CD28. CD152 engagement inhibits IL-2 mRNA expression by the T cell and its progression through the cell cycle. This mechanism ensures that activated T cells do not continue to act once they are no longer needed. If the stimulus remains, the anergic T cells are replaced by newly activated ones. If the antigenic stimulus has been removed, the response ends. The reactive T cells disappear, with the exception of quiescent memory cells (Fig. 12.1).

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Figure 12.1

CD152 (CTLA-4) in T-cell regulation. The expression of CD152 by T cells begins only after they have been activated. CD152 competes with CD28 for binding to CD80/CD86 and does so with a greater affinity than CD28. Binding of CD152 by CD80/86 provides a signal for anergy that inactivates the T cell, providing a means for imposing a finite period of activity on each activated T cell.

C. Regulatory T cells

Regulatory T cells may also maintain tolerance. Characteristically, they inhibit the activity of autoreactive lymphocytes. The molecular basis for their regulatory action is still unclear, but they appear to fall into one of two categories: T regulatory (Treg) and T suppressor (Ts) cells. Treg cells express both CD4 and CD25 molecules and are thought to be important inhibitors of immune-mediated inflammatory diseases such as inflammatory bowel disease. Ts cells are CD8+ and inhibit the activation and proliferation of CD4+ T cells including Th1 cells. Both Treg and Ts may inhibit specific antibody production by B cells. One additional mechanism for minimizing unwanted reactivity may be apparent in the cytokines produced by Th1 and Th2 cells. Th1-produced IFN-γ inhibits the maturation of Th0 cells into Th2 cells, and Th2-produced IL-4 inhibits the differentiation of cells along the Th2 pathway.

1. CD4+ Treg cells: These cells express both CD4 and CD25 (the α chain of the IL-2 receptor, IL-2R). These CD4+CD25+ T cells, estimated to constitute 5% to 10% of peripheral CD4+ T cells, have been identified in various tissues and have been implicated in the prevention of some autoimmune responses and some responses against nonself as well (Fig. 12.2). They are present in the absence of intentional immunization and are therefore sometimes called natural Tregs. Although their activation requires TCR engagement, the inhibitory effects of Tregs appear to be nonspecific and seem to inhibit the activation of CD4+CD25+ T cells specific for various epitopes. A subset of CD4+CD25+ T cells also express the RO isoform of CD45 (CD45RO), glucocorticoid-induced tumor necrosis factor receptor (GITC), and CD152 (CTLA-4) molecules on their surface and the Foxp3 transcription factor within their nuclei. These cells are believed to be the actual suppressive subpopulation.

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Figure 12.2

CD4+25+ Treg cells. Thymus-derived CD4+CD25+ Treg. The ability of Th1 or Th2 cells to mutually inhibit one another’s activity through cytokine signals provides a model system for studying regulatory arrangements among lymphocyte subpopulations.

Treg cells undergo positive and negative selection within the thymus, possibly involving interaction with the epithelial cells of the thymic Hassall corpuscles. Reportedly, Tregs do not proliferate rapidly nor produce high levels of IL-2, IL-4, IL-10, or TGF-β following stimulation. Their presence has been demonstrated, in vivo and in vitro, to be associated with the suppression of several autoimmune diseases (e.g., autoimmune gastritis, chronic colitis). Recent evidence suggests that Treg cells can also inhibit responses against some infectious agents (e.g., Leishmania). The means by which Treg cells exert their effects on other lymphocytes and perhaps on APCs as well are still unclear, as are the means by which they themselves may be regulated.

2. CD8+ suppressor cells: These cells are an inhibitory subpopulation of CD8+ T cells. These suppressor T cells (Ts) do not express CD28 surface molecules (CD8+CD28). Their presence has been associated with the suppression of graft rejection and the inhibition of some autoimmune diseases (e.g., experimental autoimmune encephalomyelitis). Their mode of action is still under investigation, but there is evidence that some of their effect might occur through their influence on APCs. Their activation requires interaction with CD4+ T helper cells. They also express the Foxp3 nuclear transcription factor that is a distinct characteristic of Treg cells.

3. Th17 cells: Unlike CD4+ Treg cells and CD8+ suppressor cells, the Th17 subset of CD4 T cells do not diminish inflammatory responses. Rather, they promote inflammatory events in various tissues. They are stimulated by IL-23 and secrete IL-17, which acts on monocytic cells (including macrophages and dendritic cells) and neutrophils. IL-17 secretion attracts them to inflammatory sites and induces them to produce inflammatory cytokines (e.g., IL-1, IL-6, TGF-β, G-CSF) and chemokines (IL-8).

III. THE TH1/TH2 PARADIGM

Immune responses often represent states of balance between different sets of response mechanisms. Often, whether an immune response is increasing or decreasing depends on the particular activity being examined. Production of Th2 cytokines against a specific antigenic stimulus may increase, whereas Th1 cytokines production stimulated by that same antigen may decrease, and vice versa. In other cases, both types of responses may increase or decrease at the same time. The mutual inhibition by CD4+ Th1 and Th2 cells provides a model for analyzing the regulatory interactions of different T-cell subpopulations (Fig. 12.3). IL-4, IL-10, and TGF-β secreted by Th2 cells promote antibody-mediated responses not only by stimulating antibody production and isotype switches, but also by simultaneously diminishing the activity of cells of the Th1 pathway. Conversely, Th1 cells promote cell-mediated responses in part by the secretion of IFN-γ that stimulates macrophage activation but also stimulates the isotype switch to IgG1 and IgG3 (the primary opsonizing antibodies promoting phagocytosis). At the same time, IFN-γ inhibits Th2 cells promoting other isotype switches.

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Figure 12.3

The Th1/Th2 paradigm. The ability of Th1 or Th2 cells to mutually inhibit one another’s activity through cytokine signals provides a model system for studying regulatory arrangements among lymphocyte subpopulations.

IV. REGULATORY CYTOKINES

Much of the regulation of lymphocyte activation and subsequent activity is mediated through cytokines. For example, T-cell–derived cytokines are critical for B-cell activation and isotype switching, whereas B cells (acting as antigen presenting cells) can use cytokines to influence T-cell activation. Even within the T-cell compartment, different subsets of lymphocytes secrete cytokines that affect one another as in the case of Th1 and Th2 cells. Table 12.1 provides a listing of some of the cytokines involved in regulation of lymphocytes.

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Chapter Summary

• In selective nonresponsiveness or tolerance, when the adaptive immune system does recognize self, it adopts a nondestructive strategy.

• Elimination of self-destructive thymocytes through negative selection within the thymus is an important mechanism for establishing tolerance. The AIRE gene increases the number of self molecules expressed within the thymus, and thus facilitates the effectiveness of negative selection.

• Naïve lymphocyte unresponsiveness or anergy results from T-cell or B-cell receptor (TCR or BCR) engagement without second signal instruction from an antigen-presenting cell or CD4+ T cell, respectively.

• T-cell surface CD152 (CTLA-4) binds more avidly than CD28 with CD80 (B7.1) and/or CD86 (B7.2) to inhibit interleukin-2 (IL-2) production, IL-2 receptor (IL-2R) expression, and entry into cell cycle by CD4+ T cells to promote a state of anergy.

• T regulatory (Treg) cells express both CD4 and CD25 (IL-2 receptor α chain) molecules. Tregs have been implicated in the prevention of autoimmune responses (e.g., inflammatory bowel disease) and in the prevention of some nonself responses.

• Treg cells undergo positive and negative selection within the thymus, possibly involving interaction with the epithelial cells of the thymic Hassall corpuscles.

• Some subpopulations of CD8+ T cells also appear able to suppress immune responses. These suppressor T cells (Ts) are negative for the CD28 surface molecule (CD8+CD28). Their presence has been associated with the suppression of graft rejection and the inhibition of some autoimmune diseases (e.g., multiple sclerosis, systemic lupus erythematosus).

• Th17 cells promote inflammatory responses. They are triggered by IL-23 to differentiate and secrete IL-17.

Study Questions

12.1. A state of T-lymphocyte nonresponsiveness that occurs following peptide + major histocompatibility complex (pMHC) engagement is known as

A. allergy.

B. apoptosis.

C. anergy.

D. autoimmunity.

E. hypersensitivity.

The correct answer is C. Anergy is a state of nonreactivity that occurs when a lymphocyte receives a stimulus through its TCR or BCR in the absence of the additional appropriate signals provides by antigen-presenting cells or T cells. Allergy involves the degranulation of mast cells following binding of antigen to IgE molecules already affixed to the mast cell surfaces. Apoptosis is the programmed death of a cell through degradation of its nucleic acids. Autoimmunity is the active response of the immune system against self epitopes. Hypersensitivity is a response mediated by activated lymphocytes or their products. Allergy is one form of hypersensitivity.

12.2. Which of the following cells have been implicated in the prevention of autoimmune responses (e.g., inflammatory bowel disease) and in the prevention of some nonself responses?

A. Antigen-presenting cells

B. Anergized T cells

C. CD4+CD25+ Treg cells

D. Follicular dendritic cells

E. Naïve T cells

The correct answer is C. CD4+CD25+ Treg cells inhibit various responses against self epitopes as well as some responses against epitopes associated with infectious agents and tumors. Antigen-presenting cells do not have this capacity. Anergized cells are inactive. Follicular dendritic cells are involved in the display of antigen to B cells and T cells in the lymph node follicles. Naïve T cells require activation before they can begin to carry out any of their effector functions.

12.3. Which of the following cells require interaction with both pMHC and a set of costimulatory second signals from an antigen-presenting cell (usually a dendritic cell) to become activated?

A. Anergized T cells

B. B cells

C. Mast cells

D. Naïve T cells

E. Natural killer cells

The correct answer is D. Dendritic cells are the usual participants in the activation of naïve cells. Anergized T cells remain refractory to subsequent engagement of pMHC and remain quiescent. B cells do not require binding of pMHC for activation. Mast cells become activated and degranulated via the binding of antigen to IgE molecules already affixed to the mast cell surfaces. Natural killer cells do not have receptors for binding pMHC.

12.4. The Foxp3 nuclear transcription factor is expressed within

A. B cells.

B. CD4+/CD8+ (double positive) thymocytes.

C. CD8+ cytotoxic cells.

D. CD4+CD25+ T regulatory cells.

E. Th2 cells.

The correct answer is D. Expression of the Foxp3 nuclear transcription factor is a distinctive feature of CD4+CD25+ Treg cells. Foxp3 is not expressed by any of the other cell types indicated.

12.5. In activated T cells, CD152 (CTLA4)

A. becomes sequestered within the Golgi.

B. binds to the appropriate surface pMHC.

C. induces progression through the cell cycle.

D. stimulates transcription of IL-2 mRNA.

E. begins to move to the membrane and bind CD80/86.

The correct answer is E. Following activation of a T cell, CD152 begins to move from the Golgi apparatus out onto the cell surface, where it competes with CD28 for binding of CD80/CD86 on antigen-presenting cells. It does not remain sequestered in the Golgi, nor does it bind to pMHC. Its binding induces an inhibition of IL-2 mRNA and the progression of the T cell through the cell cycle.