Immunology (Lippincott Illustrated Reviews Series) 2nd Edition

Chapter 6: Molecules of Adaptive Immunity

I. OVERVIEW

The adaptive immune system uses a broad range of molecules for its activities. Some of these molecules are also used by the innate immune system (see Chapter 5). Others, including antigen-specific B-cell receptors (BCR) and T-cell receptors (TCR) of B and T lymphocytes, are unique to the adaptive immune system. Immunoglobulins are synthesized by and are present on the surfaces of B lymphocytes. Each B cell synthesizes immunoglobulins of a single specificity that bind to a specific molecular structure (epitope). The immunoglobulins on the B-cell surface serve as the BCRs. Stimulated B cells may further differentiate into plasma cells that secrete soluble forms of these immunoglobulins. The immunoglobulins recognize and bind to the same epitopes that activate the classical complement pathway. T cells express a wide variety of membrane-bound TCRs. Each T cell produces single-specificity TCRs that recognize a specific peptide epitope contained within a major histocompatibility complex (MHC) molecule. Epitope engagement of BCRs or TCRs leads to the initiation of signal transduction pathways and the expression of both soluble (cytokines and chemokines) and cell-surface (receptors and adhesion) molecules.

II. IMMUNOGLOBULINS

Immunoglobulins are synthesized by B lymphocytes (B cells) and are both synthesized and secreted by plasma cells. Plasma cells are terminally differentiated B cells. The term antibody is applied to an immunoglobulin molecule with specificity for an epitope of the molecules that make up antigens (see Chapter 2). Antibodies noncovalently bind to antigens to immobilize them, render them harmless, or “tag” the antigen for destruction and removal by other components of the immune system. In doing so, antibodies facilitate the ability of other cells and molecules in the immune system to identify and interact with antigens. Because antibodies are often in soluble form, they are important components of humoral (soluble) immune responses (see Chapter 11).

A. Basic structure

Human immunoglobulin contains four polypeptides: two identical light chains and two identical heavy chains linked by disulfide bonds (Fig. 6.1) to form a monomeric unit. Heavy and light chains are aligned such that the amino portion (NH terminus) of a single, heavy, and a single light chain form an epitope-binding site (more about this later in the chapter). Each heavy and light chain may be subdivided into homologous regions termed domainsLight chains, termed κ (kappa) or λ (lambda), are encoded on chromosomes 2 and 22, respectively. There are five types of heavy chains, all encoded on chromosome 14, termed mu (μ), delta (δ), gamma (γ), epsilon (ε), and alpha (α). The genetically different forms of light chains (κ and λ) and of heavy chains (μδγε, and α) are known as isotypes. Immunoglobulin class or subclass is determined by the heavy chain isotype.

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Figure 6.1

Immunoglobulin monomer. An immunoglobulin monomer contains two identical light (L) chains and two identical heavy (H) chains connected by disulfide bonds. Each chain contains a variable domain and one or more constant domains.

1. Light chains: An immunoglobulin monomer contains two identical κ or two identical λ light chains but never one of each. Light or L chains contain a variable (VL) domain and a constant (CL) domain (Fig. 6.2). Each domain contains about 110 amino acids and an intrachain disulfide bond. Variable regions (in both heavy and light chains) are so named for their variation in amino acid sequences between immunoglobulins synthesized by different B cells.

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Figure 6.2

Immunoglobulin domains. Light chains are of two types (κ and λ) whereas there are five types of heavy chains (αδεγμ). Immunoglobulin light and heavy changes are divisible into domains that consist of approximately 110 amino acids and contain an intrachain disulfide bond. V L, light chain variable domain; V H, heavy chain variable domain; C L, light chain constant domain; C H, heavy chain constant domain

2. Heavy chains: Heavy chains contain one variable (VH) and three or four constant (CH) domains (Fig. 6.2). Heavy (H) chain variable domains (VH) are extremely diverse, and constant domains (CH) display a relatively limited variability for members of an isotype. The δγ, and α heavy chains contain three constant domains (CH1, CH2, CH3), and μ and ε heavy chains contain a fourth constant domain (CH4), making them both longer and heavier than δγ, or α heavy chains.

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Figure 6.3

Immunoglobulin epitope-binding regions. Two identical epitope-binding regions are formed by pairing of a single VL domain with a single VH domain.

3. Antigen-binding sites: A light chain variable domain and a heavy chain variable domain together form a pocket that constitutes the antigen (epitope)-binding region of the immunoglobulin molecule. Because an immunoglobulin monomer contains two identical light chains and two identical heavy chains, the two binding sites found in each monomeric immunoglobulin are also identical (Fig. 6.3). The variability in the amino acid sequences of the VL and VH domains, together with the random pairing of light and heavy chain that occurs from one B cell to another, creates a pool of binding sites capable of recognizing a very large number of different epitopes.

4. Immunoglobulin landmarks of two proteases: Immunoglobulin molecules can be enzymatically cleaved into discrete fragments by either pepsin or papain (Fig. 6.4). Disulfide bonds join the heavy chains at or near a proline-rich hinge region, which confers flexibility on the immunoglobulin molecule.

The fragments of immunoglobulin are as follows:

Fab or antigen (epitope)-binding fragment, produced by papain cleavage of the immunoglobulin molecule, contains VH, CH1, VL, and CL. Two Fab fragments are produced by papain cleavage of an immunoglobulin monomer; each fragment has an epitope-binding site.

Fc or constant (crystallizable) fragment is produced by cleavage of the immunoglobulin molecule with papain. The Fc portion contains the CH2, CH3, and (for IgM and IgE) CH4 regions of the immunoglobulin molecule. It is responsible for many biologic activities that occur following engagement of an epitope.

Fd is the heavy chain (VH, CH1) portion of Fab.

Fd′ is a heavy chain (VH, CH1) portion of Fab. The prime (′) mark denotes extra amino acids due to a pepsin cleavage site.

F(ab)2 is a dimeric molecule produced by pepsin cleavage. An immunoglobulin monomer will produce a single F(ab′)2 fragment containing two (VH,CH1′) segments joined by disulfide bonds. An F(ab′)2contains two epitope-binding sites.

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Figure 6.4

Enzyme cleavage of immunoglobulin determines landmarks. Papain cleaves heavy chains to form two identical Fab fragments (each containing one binding site) and one Fc fragment. Pepsin cleaves heavy chains at a point that produces an F(ab′)2 fragment containing two linked binding sites and remaining heavy chain material that is degraded and eliminated.

B. Isotypes

Heavy chain isotypes (μδγα, and ε) also determine immunoglobulin isotype or class (IgM, IgD, IgG, IgA, and IgE, respectively) (Table 6.1). Normally, humans produce all five immunoglobulin isotypes. Of the two light chain isotypes, an individual B cell will produce only κ or λ chains, never both. B cells express surface-bound immunoglobulin monomers as epitope-specific receptors; B cells produce and display only one heavy chain isotype, with the exception that unstimulated B cells express both IgM and IgD. When secreted into the body fluids, soluble IgG and IgE remain monomeric, soluble IgM forms a pentamer, and soluble IgA can be found in either a monomeric or dimeric form.

IgM is found either as a cell surface-bound monomer (2μ + 2κ or 2λ) or as a secreted pentamer with 10 H and L chains linked by disulfide bonds and a J (“joining”) chain [five monomers + J, i.e., 5 × (2μ + 2κ or 2λ) + J]. Most unstimulated B cells display IgM on their cell surfaces. In general, IgM is the first immunoglobulin to be formed following antigenic stimulation. IgM is effective both at immobilizing antigen (agglutination) (see Chapter 20Fig. 20.2) and in activating the classical pathway of complement.

IgD has a monomeric structure (2δ + 2κ or 2λ) and is almost exclusively displayed on B-cell surfaces. Little is known of its function.

IgG exists as both surface and secreted monomeric (2γ + 2κ or 2λ) molecules. Four subclasses (γ1γ2γ3, and γ4) of γ heavy chains account for the four human IgG subclasses, IgG1IgG2IgG3, and IgG4. Collectively, IgG subclasses make up the greatest amount of immunoglobulin in the serum. Many IgG antibodies are effective in activating complement (see succeeding discussion), opsonizing and neutralizing microorganisms and viruses, and initiating antibody-dependent cell-mediated cytotoxicity, and they function in a wide variety of hypersensitivity functions.

IgA is present in both monomeric and dimeric forms. Monomeric IgA (2α + 2κ or 2λ) is found in the serum. The addition of a J or joining chain to two IgA monomers forms a dimer. Epithelial cells use a specialized receptor to transport the IgA dimer to mucosal surfaces. This specialized receptor becomes an accessory molecule that binds to the IgA dimers is known as secretory component (SC) [2 × (2α+ 2κ or 2λ) + J + SC]. Secretory IgA dimers are found in mucus, saliva, tears, breast milk, and gastrointestinal secretions. The SC provides increased resistance to enzymatic degradation. Two isoforms of IgA (α1 and α2) show slightly different functions. IgA1 predominates in the blood plasma and in secretions above the diaphragm. Secretory IgA2 accounts for most IgA found in the lumen of the lower portion of the gastrointestinal tract. Large amounts of IgA are synthesized and secreted daily at the mucosal surfaces of the GI tract, respiratory tracts, and other secretory epithelia. More IgA is produced daily than all the other isotypes combined.

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IgE is present in relatively low serum concentration; most is adsorbed on the surfaces of mast cells and eosinophils. Its basic structural formula is (2ε + 2κ or 2λ). Mast cells and basophils have isotype-specific receptors (FcεRI) for the Fc portion of free IgE molecules. Cross-linking of IgE on mast cell surfaces by antigen triggers the release of histamine and other inflammatory mediators, leading to immediate hypersensitivity (allergic) responses.

III. CLASSICAL PATHWAY OF COMPLEMENT ACTIVATION

Interaction of antibody with antigen initiates the classical pathway of complement activation. (Fig. 6.5). This biochemical cascade of enzymes and protein fragments facilitates destruction of microbes by the membrane attack complex (MAC), by increased opsonization through C3b binding of microbial surfaces and by the production of anaphylotoxins C3a, C5a, and C4a. The cascade begins with the activation of component C1.

A. Activation of C1

Binding of IgM or IgG antibody to antigen causes a conformational change in the Fc region of the immunoglobulin molecule. This conformational change enables binding of the first component of the classic pathway, C1q. Each head of C1q may bind to a CH2 domain (within the Fc portion) of an antibody molecule. Upon binding to antibody, C1q undergoes a conformational change that leads to the sequential binding and activation of the serine proteases C1r and C1s. The C1qrs complex has enzymatic activity for both C4 and C2, indicated by a horizontal bar as either C1qrs or abbreviated as C1s.

B. Production of C3 convertase

Activation of C1qrs leads to the rapid cleavage and activation of components C4, C2, and C3. In fact, both the classical and mannan-binding lectin (MBL) pathways of complement activation are identical in the cleavage and activation of C4, C2, and C3 (Fig. 6.6).

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Figure 6.5

Classical pathway of complement activation. The classical pathway is initiated by the binding of antibody (usually IgG or IgM) to an antigen and then to the C1 complement component. The pathway produces a C3 convertase (responsible for the cleavage of C3 into its component parts: C3a, C3b, etc.) and a C5 convertase (that can lead to MAC formation).

C. Production of C5 convertase

The binding of C4b2b to C3b leads to the formation of the C4b2b3b complex. This complex, a C5 convertase initiates the construction of the membrane attack complex on the microbial surface (see Figs. 5.85.9, and 6.5). Thus, as in the case of the alternative (see Fig. 5.7) and MBL pathways (see Fig. 5.10), production of C5 convertase by the classical pathway leads to the development and insertion of a structure that is capable of damaging the cell surfaces.

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Figure 6.6

Activation of complement component C1. Activation of C1 involves serial binding and activation of its three subunits (C1q, C1r, C1s).

IV. MAJOR HISTOCOMPATIBILITY MOLECULES

The major histocompatibility complex (MHC), also called the human leukocyte antigen (HLA) complex, is a segment of chromosome 6 containing several genes that are critical to immune function (Fig. 6.7). These include genes encoding various enzymes and structural molecules needed for the activation and function of B and T cells. The encoded molecules fall into three groups or classes known as MHC (or HLA) class I, II, and III molecules. MHC class III molecules include complement components C4, Bf, and C2. MHC class I and II molecules serve entirely different functions.

A. MHC class I molecules

Codominantly expressed 45-kDa MHC class I molecules, in association with β2 microglobulin (β2m, 12 kDa), are found on the surfaces of all nucleated cells. Three genetic loci, HLA-A, -B, and -C, are highly polymorphic, with more than 100 alleles at each locus (see Fig. 6.7). Altogether, up to six different class I molecules (if heterozygous at all three loci) can be displayed simultaneously on each cell.

MHC class I molecules fold to form a cleft between the α1 and α2 domains that noncovalently binds an eight- to nine-amino-acid peptide (Fig. 6.8). Because of slight structural variations in the binding cleft (or binding groove) among the different allelic forms, different peptides may preferentially fit into clefts of some MHC class I molecules better than others. Additional (“nonclassical” or class Ib) class I molecules (e.g., those encoded by the HLA-E-F-G-H loci) show limited variability and tissue distribution and may function to present carbohydrate and peptide fragments (see Fig. 13.14).

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Figure 6.7

Genetic and protein organization of MHC class I, II, and III. Located on chromosome 6, HLA (human leukocyte antigen) genes are arranged as shown. They are grouped into Class I, Class II, and Class III based on structural and functional characteristics.

B. MHC class II molecules

MHC class II molecules are normally only expressed on the surfaces of dendritic-, macrophage-, and B-cell surfaces; on some activated T cells; and on some specialized epithelial cells in the thymus and intestine. Codominantly expressed as noncovalent heterodimers, a 32- to 38-kDa α chain and a 29- to 32-kDa β chain form a binding groove (α1 and β1 domains) that can accommodate peptides of 18- to 20-amino-acid length (see Fig. 6.8). Encoded within the HLA-DP, -DQ, and -DR regions (see Fig. 6.7) are both α and β loci (DPα, DPβ, DQα, DQβ, etc.). After synthesis, MHC class II α and β chains combine only with others encoded within the same region (e.g., DPα associates only with DPβ but never with DQβ or DRβ). However, within each of these regions, α chains can combine either with β chains encoded on the same chromosome (cis) or on the other member of the chromosome pair (trans). Termed cis-trans complementation, this allows individuals that are heterozygous at one or more of the class II loci to produce a greater variety of class II dimers than would be possible if they were homozygous. The range of different MHC class I and II molecules expressed can affect the overall immune capacity of an individual.

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Figure 6.8

MHC class I molecules (HLA-A, HLA-B, or HLA-C) together with β2 microglobulin (β2m) each form a closed peptide cleft that can bind peptides of eight to nine amino acids in length. MHC class II molecules are heterodimers (DPβ + DPα, DQβ + DQα, or DRβ + DRα) that form open-ended peptide binding clefts, which bind peptide of 18–20 amino acids in length.

V. T-CELL RECEPTORS

The antigen-specific T-cell receptor (TCR) is an αβ or a γδ heterodimer polypeptide pair. (Note: Despite the similarity in terminology, αβ TCR loci/molecules and MHC class II αβ loci/molecules are genetically and molecularly distinct.) Each polypeptide of the TCR contains variable and constant domains that are genetically and molecularly distinct from immunoglobulins. The choice of whether to express an αβ or γδ heterodimer is made early in T-cell development, and clonal descendants retain the same type of TCR.

A. Basic structure

The TCR is bound to the membrane of the T cell. The short cytoplasmic tails of αβ or γδ polypeptide chains lack signaling sequences or immunoreceptor tyrosine activation motifs (ITAMs) to initiate activation signals to the nucleus (see Chapter 8). These signals are provided by the CD3 complex molecules (CD3δ, CD3γ, CD3ε, and CD247 [z chain]) that noncovalently associate with the TCR. Unlike antibodies, TCRs cannot bind soluble epitopes. They bind only to fragments of larger molecules that fit within the binding grooves of MHC class I or class II molecules as peptide-MHC (pMHC) complexes. Interaction of the TCR with pMHC is stabilized by the associated interaction of CD4 or CD8 with constant domains of MHC class II or MHC class I molecules, respectively (Fig. 6.9).

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Figure 6.9

CD4+ and CD8+ T cells only interact with peptides bound to MHC class II or class I molecules.

B. Variable and constant regions

Each polypeptide chain of the TCR pair contains a variable (Vα or Vβ, Vγ or Vδ) and a constant (Cα or Cβ, Cγ or Cδ) region domain. Together, variable regions of α and β (or γ and δ) chains form hypervariable or complementarity-determining regions (CDRs) that interact with pMHC. Similar to immunoglobulins, each T cell expresses a unique TCR. Unlike immunoglobulins, T cells must “see” pMHC and do not recognize soluble peptides.

VI. MOLECULES OF CELLULAR INTERACTION

Many adaptive and innate immune responses require leukocyte-to-leukocyte interaction. These interactions take place by direct cell-to-cell contact or by the sending and receiving of signals via soluble molecules. Leukocytes respond to these signals by up- or down-regulating their functions, by migrating to specific anatomic sites, or by making life-or-death decisions about the fate of a cell within the body.

A. Cytokines

Cytokines are low-molecular-weight soluble protein messengers that are involved in all aspects of the innate and adaptive immune response, including cellular growth and differentiation, inflammation, and repair. Originally called lymphokines and monokines to reflect lymphocytic or monocytic origin, we now recognize that these substances are produced by a wide variety of leukocytes and nonleukocytes. A large number of cytokines have been identified, although the roles of many of them are not yet fully understood (Table 6.2). Many cytokines are crucial in regulating lymphocyte development and in determining the types of immune responses evoked by specific responses.

B. Chemokines

Low-molecular-weight cytokines known as chemokines (chemoattractant cytokines) stimulate leukocyte movement. Leukocytes are guided by chemokine concentration gradients to the site of an infection or inflammation (a process called homing). They are divided into four types based on the presence of certain structural motifs involving the numbers and intervals between cystine residues: C, CC, CXC, and CX3C (Table 6.3).

C. Adhesion molecules

Often, leukocytes must interact directly to contact other cells under somewhat adverse conditions such as during rapid fluid flow within the circulatory system or under weak ligand-receptor binding. Adhesion molecules provide stable cell-to-cell contact necessary for both innate and adaptive immune responses as well as for many other intercellular activities. Although a seemingly simple activity, the ability of cells to examine the surface of other cells and to establish stable contact with them is vital. For cells to communicate and for cell-surface receptors and ligands to interact, the cells must be able to establish and maintain relatively prolonged surface-to-surface contact.

Types of adhesion molecules include integrins, selectins, and addressins.

1. Integrins are found on the surfaces of many types of leukocytes. Integrins are heterodimers consisting of various combinations of α and β chains (e.g., α5β1 on monocytes and macrophages). They interact with other molecules that are based on the Ig superfamily motif (found on a wide variety of cells and has the generalized intrachain disulfide bond domain, e.g., Fig. 6.2) and with extracellular matrix. Their main function is to strengthen contact between leukocytes and many types of cells (e.g., vascular endothelium) so that more extensive interactions can then take place. Individual integrins and their activities are discussed in upcoming chapters in the more detailed descriptions of various immune responses.

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2. Selectins and addressins are limited in their tissue distribution and are designed to identify particular tissues and to facilitate the interactions of particular cell combinations. For example, newly differentiated lymphocytes need to migrate to lymph nodes to undergo their next stage of development. This migration is accomplished by interactions between selectin molecules found on the lymphocytes (e.g., CD62L, also known as L-selectin) and addressin molecules (e.g., GlyCAM-1) located on the high vascular endothelium of blood vessels passing through lymph nodes. Other selectins and addressins assist in the movement of lymphocytes and other cells to the gut, epithelium, and sites of tissue inflammation. Individual selectins and adhesions and their activities are discussed in upcoming chapters in the more detailed descriptions of various immune responses.

D. Cluster of differentiation molecules

Cluster of differentiation (CD) molecules populate the surfaces of many cell types and often serve as indicators of the functional capacities of leukocytes and other cells. Well over 350 CD molecules have been identified. More are added every month. Fortunately, for a basic understanding of the underlying mechanisms of the adaptive immune response, you need to know only a few of these. Among those that you will frequently encounter are the following:

CD3 complex contains several molecules associated with the TCR. It is composed of six polypeptides (2 CD3ε + 1 CD3γ + 1 CD3δ + 1 CD247 ξ-ξ homodimer). Its functions are to support the TCR and to transduce transmembrane signaling when the TCR is engaged.

CD4 is a single-chain member of the immunoglobulin supergene family and is expressed on the surfaces of approximately two-thirds of mature T cells. CD4 molecules recognize a nonpeptide-binding portion of MHC class II molecules. As a result, CD4+ T cells, also known as helper T (Th) cells, are “restricted” to the recognition of pMHC class II complexes.

CD8 is a two-chain cell-surface molecule expressed as a homo-dimer (αα) or heterodimer (αβ) by about one-third of mature T cells. CD8 molecules recognize the nonpeptide-binding portion of MHC class I molecules. CD8+ T cells, “restricted” to the recognition of pMHC I complexes, are also known as cytotoxic T (Tc) and suppressor T (Ts) cells.

E. Signal transduction molecules

Leukocytes use their cell-surface receptors to sense their extracellular environment. Binding of certain ligands causes a conformational change in the receptor or its accessory molecules. This change is then communicated inside the cell via the receptor’s cytoplasmic tail (the part that is inside the cell), initiating a signal transduction cascade within the cell. Such cascades usually involve the binding of one or more specific intracellular signal transduction proteins. Receptor engagement often initiates a series of chemical signals that regulate gene transcription in the nucleus and alteration of cellular activity. Two signaling pathways that use phosphorylation of tyrosine residues are described. Phosphorylated tyrosines are short-lived, and the appearance of a phosphotyrosine is a very potent intracellar signal.

1. JAK–STAT pathway: Many extracellular stimuli activate a JAK (an acronym that stands for “Janus kinase” or sometimes “just another kinase”)–STAT (signal transducers and activators of transcription) signal transduction pathway. Ligand (e.g., cytokines, growth factors)-binding induces receptor polypeptides to dimerize and bind cytosolic (Fig. 6.10). Activated JAKs are tyrosine kinases that phosphorylate tyrosine residues within the intracellular portion of the receptor chains. The phosphorylated tyrosine residues provide docking sites for SRC homology 2 (SH2) domains of inactive, cytosolic STAT molecules. Receptor-bound STAT molecules are tyrosine phosphorylated by the receptor-associated JAKs, allowing the STATs to disassociate from the cytoplasmic tail and dimerize with another tyrosine phosphorylated STAT. The STAT dimer translocates to the nucleus, where it binds to specific DNA response element(s) to regulate tyrosine phosphorylated gene transcription.

2. Ras–MAP kinase pathway: This pathway is named for Ras, guanosine triphosphate (GTP)-binding protein and MAP or mitogen-activated protein. Following ligand-receptor binding, receptor dimerization promotes the phosphorylation of intracellular tyrosine kinase domains on the cytoplasmic tail of a catalytic receptor with intrinsic tyrosine kinase activity or allows activation of receptor-associated tyrosine kinases such as JAKs (Fig. 6.11). The receptor’s phosphotyrosine provides a “docking” or binding site for a specific intracellular SH2-containing (e.g., SHC and Grb2) adaptor. Upon docking, SHC activates its SH3 domain and binds the SOS protein. SOS is a guanine-nucleotide exchange factor (GEF) for Ras, a monomeric plasma membrane protein. The SHC-SOS-Ras complex exchanges GDP for GTP on Ras. Ras-GTP promotes binding of the Raf serine protein kinase (also known as MAPKKK, which stands for mitogen-activated protein kinase kinase kinase). Raf initiates a sequential phosphorylation cascade involving MAPKK (also known as MEK, which translocates to the nucleus) and MAPK (also known as ERK or extracellular-signal regulated kinases). The cascade terminates with a phosphorylation of a transcription factor (such as ELK) that binds to DNA to promote transcription of specific genes.

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Figure 6.10

A stylized JAK–STAT signal transduction pathway. Ligand (e.g., a cytokine) engagement induces receptor dimerization and the binding and activation of cytosolic JAKs (Janus kinases). Phosphorylation of STATs induces their dimerization, translocation to the nucleus, and binding to specific DNA response elements.

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Figure 6.11

A stylized Ras-MAPK signal transduction pathway. Ligand engagement induces receptor dimerization and intrinsic tyrosine kinase activity by the cytoplasmic tail of the receptor and its phosphorylation. A sequential phosphorylation cascade, involving several intermediates, results in the phosphorylation of MAPK and its movement into the nucleus, terminating in the binding of transcription elements to specific DNA response elements.

Chapter Summary

• The immunoglobulin monomer contains two identical light and two identical heavy polypeptide chains linked by disulfide bonds. Light chains contain one variable and one constant domainHeavy chains contain one variable and three or four constant domains. The combination of one light and one heavy chain variable domains form an epitope-binding site.

• Normal individuals express five immunoglobulin classes or isotypesIgM, the heaviest, is present as either a cell-surface-bound monomer or as a secreted pentamer. IgD, a monomer, is almost exclusively displayed on B-cell surfaces. Human IgG is a monomer present in four subclasses: IgG1IgG2IgG3, and IgG4. Monomeric IgA is present in the serum and in its dimeric form is found in association with mucosal surfaces and secretions. IgE is present in relatively low serum concentration; most is adsorbed on the surfaces of mast cells, basophils, and eosinophils.

• Binding of IgM or IgG antibody to antigen causes a conformational change in the Fc region of the immunoglobulin molecule, initiating the classical pathway of complement activation.

• The major histocompatibility complex (MHC), also called the human leukocyte antigen (HLA) complex, is a segment of chromosome 6 containing several genes critical to immune function. Codominantly expressed MHC class I molecules, in association with β2 microglobulin (β2m), are found on the surfaces of all nucleated cells. MHC class II molecules are normally expressed only on dendritic-, macrophage-, and B-cell surfaces, on some activated T cells, and on some specialized epithelial cells in the thymus and intestine.

• The epitope-specific T-cell receptor contains αβ or γδ heterodimer polypeptide pairs. Each polypeptide contains one variable and one constant region domain. TCRs recognize and bind peptides that lie within the binding grooves of MHC class I or class II molecules as peptide-MHC (pMHC) complexes.

• Cytokines are low-molecular-weight soluble protein messengers that are involved in all aspects of the innate and adaptive immune response, including cellular growth and differentiation, inflammation, and repair.

• Low-molecular-weight cytokines known as chemokines (chemoattractant cytokines) stimulate leukocyte movement.

• Adhesion molecules provide stable cell-to-cell contact. Integrins are found on the surfaces of a wide variety of leukocytes. Selectins and addressins are limited in their tissue distribution and designed to identify particular tissues and to facilitate the interactions of particular cell combinations.

• Cluster of differentiation molecules populate the surfaces of a wide variety of cell types and serve as indication of the functional capacities leukocytes and a number of other cells.

• Leukocytes use receptors to sense their extracellular environment. Ligand binding by a receptor leads to a signal transduction from the receptor-bound ligand to the nucleus involving phosphorylation of tyrosine residues. Two often-used tyrosine kinase signaling pathways use JAK-STAT and Ras-MAP kinase.

Study Questions

6.1. Epitope-specific receptors of T lymphocytes are found

A. as either cytosolic or membrane-bound proteins.

B. in blood plasma, lymph, and other secretory fluids.

C. on the surface of plasma cells.

D. as transmembrane polypeptides.

E. in the nuclear lipid bilayer.

The answer is D. The epitope specific receptors of T cells (TCRs) are displayed as membrane-bound molecules on their cell surfaces. TCRs are not found as soluble molecules. Epitope-specific molecules produced by plasma cells are genetically distinct from T cell receptor molecules.

6.2. Antibodies (immunoglobulins)

A. are synthesized and secreted by both B and T cells.

B. bind to several different epitopes simultaneously.

C. contain four different light chain polypeptides.

D. recognize specific epitopes together with self molecules.

E. tag antigens for destruction and removal.

The answer is E. Antibodies bind to epitopes on antigens to identify them or tag them for destruction by other elements of the immune system. They are synthesized only by B cells and plasma cells. An antibody molecule contains two (IgD, IgG, IgE, and serum IgA), four (secretory IgA), or ten (secreted IgM) identical epitope-binding sites. An antibody monomer contains two identical light chains and two identical heavy chains. Self-recognition is not required for antibody molecules.

6.3. The constant regions of the five major types of heavy chains of immunoglobulin molecules dictate the molecule’s

A. epitope.

B. Fab fragment.

C. isotype.

D. tyrosine activation motif.

E. variable domain.

The answer is C. The heavy chain constant regions determine immunoglobulin isotypes: mu (μ, IgM), delta (δ, IgD), gamma (γ, IgG), epsilon (ε, IgE), and alpha (α, IgA). Fab fragments are enzymatic cleavage products of immunoglobulin monomers. (Immunoreceptor) tyrosine activation motifs are not present on immunoglobulin molecules. Variable domains show extensive amino acid sequence variability among immunoglobulins, even within the same isotype.

6.4. When an immunoglobulin molecule is subjected to cleavage by pepsin, the product(s)

A. are individual heavy and light chains.

B. can no longer bind to antigen.

C. consist of two separated antigen-binding fragments.

D. crystallize during storage in the cold.

E. is a dimeric antigen-binding molecule.

The answer is E. Enzymatic cleavage of the immunoglobulin monomer by pepsin occurs distal to the variable domain and distal to heavy–heavy chain disulfide bonds, which remain intact, resulting in a molecule with two epitope-binding sites. Interchain disulfide bonds are unaffected by pepsin cleavage. The epitope-binding site remains intact on pepsin cleavage of the heavy chain. Papain cleavage of the immunoglobulin monomer occurs distal to the variable domain but proximal to the heavy–heavy chain disulfide bond, resulting in two separate epitope-binding Fab fragments. Pepsin enzymatically degrades the CH2 portion of the immunoglobulin molecule resulting in fragments that rarely, if ever, form crystals.

6.5. In an individual with an immediate hypersensitivity response (allergy) to dust mites, cross-linking of which of the following dust-mite-specific molecules will trigger inflammatory mediator release?

A. histamine

B. IgA

C. IgE

D. IgG

E. mast cells

The answer is C. Cross-linking of IgE bound to the surfaces of basophils and mast cells causes cellular degranulation and release of vasoactive amine responsible for inflammation. In humans, neither IgA nor IgG is associated with allergic responses. Histamine is released from mast cells are a result of cross-linking of surface-bound IgE.

6.6. The classical pathway of complement begins with

A. activation of C1.

B. cleavage and activation of C4, C2, and C3.

C. IgA binding to a specific epitope.

D. initiation of membrane attack complex formation.

E. production of C3 convertase.

The answer is A. The classical pathway of complement begins with the recognition of antigen–antibody complexes by the first component of complement, C1q. Subsequent steps in the classical pathway involve activation of components C4, C2, C3, and the production of C3 convertase leading to the production of C5 convertase and entry into the membrane attack complex. Antigen binding by IgA does not activate the classical pathway.

6.7. The classical pathway of complement functions to

A. cleave immunoglobulins into Fc fragments.

B. facilitate destruction of microbes.

C. recognize specific epitopes on microbes.

D. regulate lymphocyte development.

E. trigger histamine release.

The answer is B. Complement functions to facilitate the lysis of microbes by recognition of microbes tagged by antibody, by the opsonization of microbes by the attachment of C3 fragments, and by the release of anaphylotoxins C3a, C5a, and C4a. Immunoglobulin molecules are not cleaved by complement. The classical pathway is activated only by antigen–antibody complexes and by itself does not recognize microbial epitopes. Complement is not involved in lymphocyte development and does not trigger the release of histamine.

6.8. In humans, MHC class II molecules are expressed by

A. all nucleated cells.

B. B cells, dendritic cells, and macrophages.

C. erythrocytes.

D. mast cells.

E. naïve T cells.

The answer is B. B cells, dendritic cells, monocytes, and macrophages constitutively express MHC class II molecules. Only a subset of nucleated cells expresses MHC class II molecules, and it does not include mast cells or naïve T cells. Erythrocytes do not express MHC class II molecules.

6.9. The basic structure of a T cell receptor consists of

A. a membrane-bound αβ or γδ heterodimer.

B. a complex of disulfide-linked heavy and light chains.

C. covalently linked CD3 and CD247 molecules.

D. peptide-MHC complexes.

E. soluble antigen-binding homodimers.

The answer is A. The T-cell receptor (TCR) is a heterodimer composed of αβ or gd polypeptide chains. Neither the αβ or gd heterodimers nor their associated molecules (CD3 and CD247) are linked by disulfide bonds. TCR recognize pMHC complexes on antigen-presenting cells. TCRs are found only on the surfaces of T cells and are not soluble.

6.10. Migration of a B lymphocyte to specific sites (such as a lymph node) is dependent in part on the use of

A. antibodies.

B. CD8.

C. CD3.

D. complement.

E. selectins.

The answer is E. Selectins are adhesion molecules that participate in the recognition that occurs between different types of cells and tissues. Antibodies do not serve as guides for such homing. CD8 and CD3 are expressed on T cells, not on B cells, and are responsible for lymphocyte homing. Complement fragments may be chemoattractants for leukocytes, but they attract those cells to the site of immune responses rather than to specific organs.

6.11. Which of the following molecules is expressed by a mature T cell that functions as a helper T cell?

A. CD4

B. CD8

C. GlyCAM-1

D. IgA

E. IgG

The answer is A. CD4+ T cells are also called T-helper cells. CD8+ T cells have cytotoxic or suppressive functions. GlyCAM-1 is an adhesion molecule found on certain vascular epithelial cells within lymph nodes. IgA and IgG are not expressed on T cells.

6.12. Following cytokine binding to a specific cell-surface receptor, a lymphocyte is stimulated to undergo signaling via the JAK-STAT pathway. In this pathway, which of the following will be induced to translocate to the cell’s nucleus to regulate transcription?

A. JAK

B. Ras

C. SH2-containing adapter proteins

D. STAT dimers

E. tyrosine kinase

The answer is D. STAT dimers translocate into the nucleus. JAKs are cytosolic tyrosine kinases that bind to the intracellular domain of the tyrosine-phosphorylated receptor and never enter the nucleus. Ras is a membrane-bound GTP binding protein that is bound by cytosolic proteins with SH2 domains that also bind to phosphotyrosine residues within the intracellular portion of the receptor. Catalytic receptors signal by stimulating tyrosine kinase, either of the receptor itself (intrinsic activity) or by associating with nonreceptor tyrosine kinases (e.g., JAK), neither of which enters the nucleus.