One of the remarkable properties of the normal immune system is that it can react to an enormous variety of microbes but does not react against the individual’s own (self) antigens. This unresponsiveness to self antigens, also called immunologic tolerance , is maintained despite the fact that the molecular mechanisms by which lymphocyte receptor specificities are generated are not biased to exclude receptors for self antigens. In other words, lymphocytes with the ability to recognize self antigens are constantly being generated during the normal process of lymphocyte maturation. Furthermore, many self antigens have ready access to the immune system, so unresponsiveness to these antigens cannot be maintained simply by concealing them from lymphocytes. The process by which antigen-presenting cells (APCs) display antigens to T cells does not distinguish between foreign and self proteins, so self antigens are normally seen by lymphocytes. It follows that there must exist mechanisms that prevent immune responses to self antigens. These mechanisms are responsible for one of the cardinal features of the immune system—namely, its ability to discriminate between self and nonself (usually microbial) antigens. If these mechanisms fail, the immune system may attack the individual’s own cells and tissues. Such reactions are called autoimmunity , and the diseases they cause are called autoimmune diseases. In addition to tolerating the presence of self antigens, the immune system has to coexist with many commensal microbes that live on the epithelial barriers of their human hosts, often in a state of symbiosis, and the immune system of a pregnant female has to accept the presence of a fetus that expresses antigens derived from the father. Unresponsiveness to commensal microbes and the fetus is maintained by many of the same mechanisms involved in unresponsiveness to self.
In this chapter we address the following questions:
How does the immune system maintain unresponsiveness to self antigens?
What are the factors that may contribute to the loss of self-tolerance and the development of autoimmunity?
How does the immune system maintain unresponsiveness to commensal microbes and the fetus?
This chapter begins with a discussion of the important principles and features of self-tolerance. Then we discuss the different mechanisms that maintain tolerance to self antigens, as well as commensal microbes and the fetus, and how tolerance may fail, resulting in autoimmunity.
Immunologic Tolerance: General Priniciples and Significance
Immunologic tolerance is a lack of response to antigens that is induced by exposure of lymphocytes to these antigens . When lymphocytes with receptors for a particular antigen encounter this antigen, any of several outcomes is possible. The lymphocytes may be activated to proliferate and to differentiate into effector and memory cells, leading to a productive immune response; antigens that elicit such a response are said to be immunogenic . The lymphocytes may be functionally inactivated or killed, resulting in tolerance; antigens that induce tolerance are said to be tolerogenic . In some situations, the antigen-specific lymphocytes may not react in any way; this phenomenon has been called immunologic ignorance, implying that the lymphocytes simply ignore the presence of the antigen. Normally, microbes are immunogenic and self antigens are tolerogenic.
The choice between lymphocyte activation and tolerance is determined largely by the nature of the antigen and the additional signals present when the antigen is displayed to the immune system. In fact, the same antigen may be administered in different ways to induce an immune response or tolerance. This experimental observation has been exploited to analyze what factors determine whether activation or tolerance develops as a consequence of encounter with an antigen.
The phenomenon of immunologic tolerance is important for several reasons. First, as we stated at the outset, self antigens normally induce tolerance, and failure of self-tolerance is the underlying cause of autoimmune diseases. Second, if we learn how to induce tolerance in lymphocytes specific for a particular antigen, we may be able to use this knowledge to prevent or control unwanted immune reactions. Strategies for inducing tolerance are being tested to treat allergic and autoimmune diseases and to prevent the rejection of organ transplants. The same strategies may be valuable in gene therapy to prevent immune responses against the products of newly expressed genes or vectors and even for stem cell transplantation if the stem cell donor is genetically different from the recipient.
Immunologic tolerance to different self antigens may be induced when developing lymphocytes encounter these antigens in the generative (central) lymphoid organs, a process called central tolerance, or when mature lymphocytes encounter self-antigens in peripheral (secondary) lymphoid organs or peripheral tissues, called peripheral tolerance ( Fig. 9.1 ). Central tolerance is a mechanism of tolerance only to self antigens that are present in the generative lymphoid organs—namely, the bone marrow and thymus. Tolerance to self antigens that are not present in these organs must be induced and maintained by peripheral mechanisms. We have only limited knowledge of which self antigens induce central or peripheral tolerance or are ignored by the immune system.
With this brief background, we proceed to a discussion of the mechanisms of immunologic tolerance and how the failure of each mechanism may result in autoimmunity. Tolerance in T cells, particularly CD4 + helper T lymphocytes, is discussed first because many of the mechanisms of self-tolerance were defined by studies of these cells. In addition, CD4 + helper T cells orchestrate virtually all immune responses to protein antigens, so tolerance in these cells may be enough to prevent both cell-mediated and humoral immune responses against self proteins. Conversely, failure of tolerance in helper T cells may result in autoimmunity manifested by T cell–mediated attack against tissue self antigens or by the production of autoantibodies against self proteins.
Central T Lymphocyte Tolerance
The principal mechanisms of central tolerance in T cells are death of immature T cells and the generation of CD4 + regulatory T cells ( Fig. 9.2 ). The lymphocytes that develop in the thymus consist of cells with receptors capable of recognizing many antigens, both self and foreign. If a lymphocyte that has not completed its maturation interacts strongly with a self antigen, displayed as a peptide bound to a self major histocompatibility complex (MHC) molecule, that lymphocyte receives signals that trigger apoptosis. Thus, the self-reactive cell dies before it can become functionally competent. This process, called negative selection (see Chapter 4 ), is a major mechanism of central tolerance. The process of negative selection affects self-reactive CD4 + T cells and CD8 + T cells, which recognize self peptides displayed by class II MHC and class I MHC molecules, respectively. Why immature lymphocytes die upon receiving strong T cell receptor (TCR) signals in the thymus, while mature lymphocytes that get strong TCR signals in the periphery are activated, is not fully understood.
Some immature CD4 + T cells that recognize self antigens in the thymus with high affinity do not die but develop into regulatory T cells and enter peripheral tissues (see Fig. 9.2 ). The functions of regulatory T cells are described later in the chapter. What determines whether a thymic CD4 + T cell that recognizes a self antigen will die or become a regulatory T cell is also not established.
Immature lymphocytes may interact strongly with an antigen if the antigen is present at high concentrations in the thymus and if the lymphocytes express receptors that recognize the antigen with high affinity. Antigens that induce negative selection may include proteins that are abundant throughout the body, such as plasma proteins and common cellular proteins.
Surprisingly, many self proteins that are normally present only in certain peripheral tissues, called tissue-restricted antigens, are also expressed in some of the epithelial cells of the thymus. A protein called AIRE (autoimmune regulator) is responsible for the thymic expression of these peripheral tissue antigens. Mutations in the AIRE gene are the cause of a rare disorder called autoimmune polyendocrine syndrome. In this disorder, several tissue antigens are not expressed in the thymus because of a lack of functional AIRE protein, so immature T cells specific for these antigens are not eliminated and do not develop into regulatory cells. These cells mature into functionally competent T cells that enter the peripheral immune system and are capable of reacting harmfully against the tissue-restricted antigens, which are expressed normally in the appropriate peripheral tissues even in the absence of AIRE. Therefore, T cells specific for these antigens emerge from the thymus, encounter the antigens in the peripheral tissues, and attack the tissues and cause disease. It is not clear why endocrine organs are the most frequent targets of this autoimmune attack. Although this rare syndrome illustrates the importance of negative selection in the thymus for maintaining self-tolerance, it is not known if defects in negative selection contribute to common autoimmune diseases.
Central tolerance is imperfect, and some self-reactive lymphocytes mature and are present in healthy individuals. As discussed next, peripheral mechanisms may prevent the activation of these lymphocytes.
Peripheral T Lymphocyte Tolerance
Peripheral tolerance is induced when mature T cells recognize self antigens in peripheral tissues, leading to functional inactivation (anergy) or death, or when the self-reactive lymphocytes are suppressed by regulatory T cells ( Fig. 9.3 ). Each of these mechanisms of peripheral T cell tolerance is described in this section. Peripheral tolerance is clearly important for preventing T cell responses to self antigens that are not present in the thymus, and it also may provide backup mechanisms for preventing autoimmunity in situations where central tolerance to antigens that are expressed in the thymus is incomplete.
Antigen recognition without adequate costimulation results in T cell anergy or death or makes T cells sensitive to suppression by regulatory T cells. As noted in previous chapters, naive T lymphocytes need at least two signals to induce their proliferation and differentiation into effector and memory cells: Signal 1 is always antigen, and signal 2 is provided by costimulators that are expressed on APCs, typically as part of the innate immune response to microbes (or to damaged host cells) (see Chapter 5 , Fig. 5.6 ). It is believed that dendritic cells in normal uninfected tissues and peripheral lymphoid organs are in a resting (or immature) state, in which they express little or no costimulators, such as B7 proteins (see Chapter 5 ). These dendritic cells constantly process and display the self antigens that are present in the tissues. T lymphocytes with receptors for the self antigens are able to recognize the antigens and thus receive signals from their antigen receptors (signal 1), but the T cells do not receive strong costimulation because there is no accompanying innate immune response. Thus, the presence or absence of costimulation is a major factor determining whether T cells are activated or tolerized.
Anergy in T cells refers to long-lived functional unresponsiveness that is induced when these cells recognize self antigens ( Fig. 9.4 ). Self antigens are normally displayed with low levels of costimulators, as discussed earlier. Antigen recognition without adequate costimulation is thought to be the basis of anergy induction, by mechanisms that are described later. Anergic cells survive but are incapable of responding to the antigen.
The two best-defined mechanisms responsible for the induction of anergy are abnormal signaling by the TCR complex and the delivery of inhibitory signals from receptors other than the TCR complex.
When T cells recognize antigens without costimulation, the TCR complex may lose its ability to transmit activating signals. In some cases, this is related to the activation of enzymes (ubiquitin ligases) that modify signaling proteins and target them for intracellular destruction by proteases.
On recognition of self antigens, T cells also may preferentially use one of the inhibitory receptors of the CD28 family, cytotoxic T lymphocyte–associated antigen 4 (CTLA-4, or CD152) or programmed cell death protein 1 (PD-1, CD279), which were introduced in Chapter 5 . Anergic T cells may express higher levels of these inhibitory receptors, which will inhibit responses to subsequent antigen recognition. The functions and mechanisms of action of these receptors are described in more detail below.
Regulation of T Cell Responses by Inhibitory Receptors
Immune responses are influenced by a balance between engagement of activating and inhibitory receptors . This idea is established for B and T lymphocytes and natural killer (NK) cells. In T cells, the main activating receptors are the TCR complex and costimulatory receptors such as CD28 (see Chapter 5 ), and the best-defined inhibitory receptors, also called coinhibitors, are CTLA-4 and PD-1. The functions and mechanisms of action of these inhibitors are complementary ( Fig. 9.5 ).
CTLA-4. CTLA-4 is expressed transiently on activated CD4 + T cells and constitutively on regulatory T cells (described later). It functions to suppress the activation of responding T cells. CTLA-4 works by blocking and removing B7 molecules from the surface of APCs, thus reducing costimulation by CD28 and preventing the activation of T cells (see Fig. 9-5A ). The choice between engagement of CTLA-4 or CD28 is determined by the affinity of these receptors for B7 and the level of B7 expression. CTLA-4 has a higher affinity for B7 molecules than does CD28, so it binds B7 tightly and prevents the binding of CD28. This competition is especially effective when B7 levels are low (as would be expected normally when APCs are displaying self and probably tumor antigens); in these situations, the receptor that is preferentially engaged is the high-affinity blocking receptor CTLA-4. However, when B7 levels are high (as in infections), not all the ligands will be occupied by CTLA-4 and some B7 will be available to bind to the low-affinity activating receptor CD28, leading to T cell costimulation.
PD-1 . PD-1 is expressed on CD8 + and CD4 + T cells after antigen stimulation. Its cytoplasmic tail has inhibitory signaling motifs with tyrosine residues that are phosphorylated upon recognition of its ligands PD-L1 or PD-L2. Once phosphorylated, these tyrosines bind a tyrosine phosphatase that inhibits kinase-dependent activating signals from CD28 and the TCR complex (see Fig. 9-5B ). Because the expression of PD-1 on T cells is increased upon chronic T cell activation and expression of the ligands is increased by cytokines produced during prolonged inflammation, this pathway is most active in situations of chronic or repeated antigenic stimulation. This may happen in responses to chronic infections, tumors, and self antigens, when PD-1–expressing T cells encounter the ligand on infected cells, tumor cells, or APCs.