Regulation of Adaptive Immunity

Key Points

• T cells must be made tolerant to self-antigens. This may be accomplished through central tolerance whereby self-reactive T cells are killed. Alternatively, it may be achieved through peripheral tolerance whereby these T cells are “turned off” by inappropriate signaling.

• B cells are much harder to tolerize than T cells. They are generally regulated by peripheral mechanisms and by the absence of T cell help.

• Antigens stimulate immune responses, although very low or very high doses of antigen may cause tolerance.

• Antibodies tend to regulate antibody production through negative feedback mechanisms. This can prevent the successful vaccination of newborn animals as a result of maternal immunity.

• Immune responses may also be controlled by the activities of regulatory T cells (Treg cells). These Treg cells secrete cytokines such as IL-10 and TGF-β.

• Another T cell subset, Th17 cells, regulates inflammation by secreting a cytokine called interleukin-17 (IL-17).

• The immune system and the central nervous system are closely interconnected and influence each other.

The adaptive immune system is a sophisticated defense system. It can recognize and respond to foreign invaders and can learn from the experience so that the body responds faster and more effectively when exposed to the invader a second time. There is, however, a risk associated with this—the risk of collateral damage. One of the reasons why the adaptive immune system is so complex is that much effort must be put into ensuring that it will only attack foreign or abnormal tissues and will ignore normal healthy tissues. As might be anticipated, many different mechanisms minimize the chances of developing autoimmunity. In addition, immune responses are regulated to ensure that they are appropriate with respect to both quality and quantity (Figure 20-1).

FIGURE 20-1 Some of the bad things that can happen if the immune system is not carefully regulated.

Since both T and B cells generate antigen-binding receptors at random, it is clear that the initial production of self-reactive cells cannot be prevented. An animal cannot control the amino acid sequences and hence the binding specificity of these receptors. As a result, when first generated, as many as 50% of the T cell antigen receptors (TCRs) and B cell antigen receptors (BCRs) may bind self-antigens. If autoimmunity is to be avoided, lymphocytes with these inappropriate receptors must either be destroyed or turned off.

Tolerance

Tolerance is the name given to the situation in which the immune system will not respond to a specific antigen. Tolerance is primarily directed against self-antigens from normal tissues. In 1948 two Australian immunologists, Burnet and Fenner, recognized this need for self-tolerance and suggested that immature lymphocytes would become tolerant to an antigen if they first met it early in fetal life.

Support for this suggestion came from observations on chimeric calves. In 1945 Owen noted that when cows are carrying twin calves, blood vessels in the two placentas commonly fuse. As a result, the blood of the twins intermingles freely, and bone marrow stem cells from one animal colonize the other. Each calf is born with a mixture of blood cells, some of its own and some originating from its twin. In dizygotic (nonidentical) twins, this is called a chimera. These “foreign” blood cells persist indefinitely because each chimeric calf is fully tolerant to the presence of its twin’s cells (Figure 20-2). Burnet and Fenner suggested that this could only happen because each calf was exposed to the foreign cells early in fetal life during a period when lymphocytes become tolerant upon encountering antigens. Cells from an unrelated calf would be rejected normally if administered after birth.

FIGURE 20-2 Fusion of the placentas of dizygotic twin calves results in the development of calf chimeras. Hematopoietic stem cells from each animal colonize the bone marrow of the other. Each chimera is tolerant to its twin’s cells and will accept a skin graft from its twin despite the genetic differences.

Subsequent studies have shown that self-tolerance is of two types, central and peripheral. In central tolerance, immature self-reactive lymphocytes within the thymus, bursa, or bone marrow either die or alter their receptor specificity. In peripheral tolerance, mature lymphocytes that encounter self-antigens either die, are turned off, or are suppressed by Treg cells. By reconstituting lethally irradiated mice with T or B cells derived from normal or tolerant donors, tolerance can be shown to occur in both cell populations. However, their susceptibility to peripheral tolerance differs. T cells can be made tolerant rapidly and easily within 24 hours and remain in that state for more than 100 days (Figure 20-3). In contrast, B cells develop tolerance in about 10 days and return to normal within 50 days.

FIGURE 20-3 The duration of tolerance in T cells and B cells. T cells are much more easily rendered tolerant than B cells. Once tolerant, they remain that way for much longer.

Tolerance will result if there are no functional T cells with receptors that can bind self-antigens (Figure 20-4). Although the body generates an enormous diversity of TCRs, far fewer receptors are actually used by mature T cells than might be anticipated. Several processes serve to limit receptor diversity. First, the mechanisms used to generate diverse TCRs inevitably result in the production of nonfunctional receptors. For example, two thirds of possible gene arrangements will be out of frame. Cells with these nonfunctional TCRs undergo apoptosis. As T cells mature within the thymus, positive selection ensures that the cells that recognize self-MHC molecules survive. At this point, however, the cells whose receptors bind too strongly to self-antigens die by apoptosis (Figure 20-5). The timing and extent of this apoptosis depend on the affinity of the TCR for a self-antigen. T cells that bind self-antigens strongly die earlier and more completely than weakly binding cells. Thus the T cells that eventually leave the thymus have been purged of dangerous, self-reactive cells.

FIGURE 20-4 The principal ways by which T cells are made tolerant.

The negative selection process is assisted by the presence of many different self-antigens in the thymus. Normally each tissue possesses its own tissue-specific antigens. Thus “skin antigens” are usually restricted to the skin, whereas “liver antigens” are restricted to the liver, and so forth. However, the epithelial cells in the thymic medulla show uniquely “promiscuous” gene expression. Thymic epithelial cells use a transcription regulator, called the autoimmune regulator (AIRE), that promotes the expression of many different proteins once thought to be restricted to other tissues. Examples include insulin, thyroglobulin, and myelin basic protein. In this way, the thymic epithelial cells ensure that self-reactive T cells encounter many normal tissue antigens and are therefore eliminated. In addition, some normal tissue antigens may be taken up by macrophages and carried to the thymus. Self-reactive T cells that respond to these antigens are also eliminated. However, this raises another question: What about self-antigens that are not expressed in, or do not enter, the thymus? For example, antigens in the eye, testis, or brain are not processed in this way, and as a result central tolerance to these antigens does not develop.

Positive and negative selection acting together ensure that cells that can bind self-MHC molecules are positively selected and those that bind the MHC molecules with very low or very high affinity are subsequently deleted. As a result, the moderate-affinity clones survive and can recognize foreign antigens. An additional factor that probably determines thymocyte survival is the dose of antigen presented to the cells. If the amount of a specific antigen is high (as one might anticipate for a self-antigen), multiple TCRs will be occupied on each thymocyte and trigger apoptosis. In contrast, if the amount of an antigen is low, this will occupy only a few TCRs, and the weak signal may cause positive selection and thymocyte proliferation.

Receptor Editing.

When the antigen receptors of a developing T cell bind to self-antigens, another strategy employed to prevent autoimmunity is receptor editing (Chapter 34). Although cell maturation stops when it leaves the thymus, the RAG genes remain active and as a result, V(D)J recombination continues. Consequently, the TCR genes continue to diversify, and changed receptors are expressed on the cell surface. This process is called receptor editing. If a cell successfully edits its receptors, its maturation can proceed. Failure to do so will result in its apoptosis. This is a potentially hazardous process since it permits the development of self-reactive T cells that have not undergone careful selection within the thymus.

Peripheral T Cell Tolerance

Clonal Anergy.

Low-affinity self-reactive T cells may survive the selection process and leave the thymus and must then be suppressed by peripheral tolerance mechanisms. One form of peripheral tolerance is clonal anergy, the prolonged, antigen-specific suppression of T cell function. Clonal anergy can be triggered by the signals received by the T cell. T cells normally require multiple signals from several sources in order to respond to antigen. If these signals are insufficient or inappropriate, T cell responses to antigen will be suppressed.

As pointed out in Chapter 14, binding of an antigen to a TCR is by itself insufficient to trigger T cell responses. Indeed, occupation of the TCR in the absence of co-stimulation causes tolerance. For example, protein solutions normally contain some aggregated molecules. These aggregated molecules are readily taken up and processed by dendritic cells and thus are highly immunogenic. If a solution of such a protein, such as bovine γ-globulin, is ultracentrifuged so that all the aggregates are removed, then the aggregate-free solution will induce anergy. This is due to the lack of co-stimulation from antigen-presenting cells (APCs) (Figure 20-6).

FIGURE 20-6 Peripheral tolerance through clonal anergy will develop if a TCR is stimulated by antigen in the absence of simultaneous co-stimulation through the CD28/CD80 or CD28/CD86 pathway.

Binding to the TCR by an antigen alone activates the tyrosine kinases and phospholipase C of the T cell and raises its intracellular Ca²⁺. This results in enhanced production of IκB that inhibits NF-κB and effectively prevents the cell from making cytokines, especially IL-2. Tolerant Th1 cells produce about 1% to 3% of normal IL-2 levels and much less IFN-γ and TNF-α. Once induced, their anergy can last for several weeks. Induction of the transcription factor FoxP3 also silences anti-inflammatory cytokine genes and leads to the production of Treg cells.

Triggering of T cell responses normally requires prolonged interactions with antigen-presenting cells. Tolerance induction, on the other hand, is characterized by relatively short interactive episodes. Thus a key difference between T cell activation and anergy may simply be the duration of their encounter with APCs.

Very high doses of an antigen can induce a form of clonal anergy called immune paralysis (Figure 20-7). The high doses of the antigen probably bypass APCs, reach the Th cell receptors directly, and in the absence of co-stimulation trigger anergy.

FIGURE 20-7 The ability of different doses of antigen to induce peripheral tolerance. Both very low and very high doses can induce tolerance. Moderate doses, in contrast, induce an immune response.

B Cell Tolerance

Unlike the TCR repertoire, B cell antibody diversity is generated in two phases. The first phase involves VDJ rearrangement or gene conversion in the primary lymphoid organs; the second phase involves random somatic mutation in secondary lymphoid organs. B cells therefore have several opportunities to generate receptors that can bind self-antigens. It has been estimated that 55% to 75% of early immature B cells have self-reactive receptors, so suppression of these self-reactive B cells must begin at an early stage in an animal’s development.

Immature B cells within the bone marrow can be made tolerant once they have arranged their V-region genes and are committed to express complete immunoglobulin M (IgM) molecules. When these immature cells encounter and bind antigen, the BCR transmits a signal that arrests cell development, blocks synapse formation, and triggers apoptosis. An immature B cell population can be rendered tolerant by one millionth of the dose of an antigen required to make mature B cells tolerant. Immature B cells may also undergo receptor editing as described previously. If receptor editing fails to generate a non-self-reactive B cell, it will die.

Peripheral B Cell Tolerance

Peripheral B cell tolerance is induced by multiple mechanisms, including apoptosis, clonal anergy, clonal exhaustion, and blockage of BCRs.

Because BCRs undergo random somatic mutation within germinal centers, self-reactive B cells can still develop in secondary lymphoid organs. These cells will not, however, make autoantibodies if APCs and helper T cells are absent or if Treg cells are active (Figure 20-8). This is not, however, a foolproof method of preventing self-reactivity. In the absence of T cell help, B cells may be activated by pathogen-associated molecular patterns (PAMPs) such as bacterial lipopolysaccharide (LPS), flagellins, or unmethylated CpG DNA acting through their toll-like receptors (TLRs). B cells may also be activated by either cross-reacting epitopes or a foreign carrier molecule stimulating nontolerant helper T cells (Figure 34-2).