Cell-Cell Interactions, Cytokines, and Chemokines in Immune Response Mechanisms Part 1

The immune response is defined by three principles: discrimination between self and nonself, specificity, and memory [see 6:I Organs and Cells of the Immune System]. This topic will discuss the manifestation of those principles through antigen processing and presentation, the T cell response to antigen, interactions between T cells and B cells, and the actions of cytokines and chemokines. The cellular and humoral mechanisms of innate immunity are described in detail elsewhere [see 6:II Innate Immunity].

Two principal arenas of the immune response are sites of pathogenic invasion and the lymph nodes that drain these sites. The immune response begins with exposure of epithelial cells, macrophages, and dendritic cells to a pathogen. In the lymph nodes, these cells (antigen-processing cells [APCs]) concentrate and process antigens and present them to T and B cells.

The critical first step of the T cell immune response to a specific antigen is the recognition and binding of processed antigen on the surface of an APC with a T cell receptor (TCR) on the surface of a helper (CD4+) T cell [see 6:I Organs and Cells of the Immune System]. This event is relayed to the helper T cell nucleus by a cascade of cytoplasmic signaling molecules. In the nucleus, activation of specific transcription factors stimulates expression of the genes that encode soluble factors—cytokines and chemokines—that mediate the immune response. This response has two aspects: humoral and cell mediated. In the humoral response, cytokines secreted by a specific form of activated T cells induce antigen-stimulated B cells to differentiate into antibody-secreting plasma cells. In the cell-mediated response, cytokines from CD4+ T cells induce CD8+ T cells to differentiate into cytolytic effectors and also can activate macrophages, another effector cell. B cells recognize nonprocessed antigens in solution or antigens that are attached to the surface of follicular dendritic cells—a cell type that is particularly adept at antigen processing, presentation, and retention. Both T cells and B cells are induced to expand their population and control the initial infection and to produce memory cells for long-term acquired immunity.


Antigen Processing and Presentation

The major histocompatibility complex

The major histocompatibility complex (MHC) is a membrane glycoprotein complex that binds antigenic peptide in the cytoplasm of an APC and transports it to the cell surface for interaction with T cells.1 An extensive polymorphism exists in the MHC gene (i.e., there are many alleles per locus); however, each person expresses only a small number of different MHC molecules. To ensure an adequate immune response against a wide range of nonself antigens, each MHC molecule must be able to bind a large number of different peptides.

Two main classes of MHC have been identified. The two classes of molecules have a similar structure: two immunoglob-ulin-like domains and a binding site for processed antigens (peptides) [see 6:V Adaptive Immunity: Histocompatibility Antigens and Immune Response Genes]. Whereas MHC class I molecules bind only smaller peptides of defined lengths (eight to 11 amino acids), MHC class II molecules bind longer peptides with no apparent restriction on peptide length. An interesting finding is that certain peptides bind only to specific al-leles of either MHC class I or MHC class II molecules. Thus, persons who lack one of those alleles would not develop an immune response to its associated peptide.

The differences in peptide binding between MHC class I and MHC class II molecules result from small structural dissimilarities within the relatively fixed framework of the peptide-bind-ing site and probably also from fundamental differences in the mechanism of peptide processing, which takes place in the en-doplasmic reticulum (ER) for MHC class I molecules and in the endosomes and lysosomes for MHC class II molecules.

The MHC Class I Pathway

Antigen processing with MHC class I molecules essentially entails three steps: (1) generation of antigenic peptides in the cytosol of the APC, (2) transport of the peptides into the ER, and (3) assembly of the peptide-MHC class I complexes. The completed complexes then migrate through the Golgi apparatus to the cell surface and insert in the plasma membrane for presentation on the extracellular surface [see Figure 1a].

The MHC Class II Pathway

Assembly of antigenic peptide-MHC class II molecule complexes in the APC requires four steps: (1) uptake of exogenous antigens by vesicles (endosomes, lysosomes, and, possibly, undefined endosomal subcompartments), (2) proteolytic degradation of proteins in the endosome, (3) assembly of MHC class II molecules in the ER and migration of these molecules through the Golgi apparatus to the endosomes, and (4) assembly of the peptide-MHC class II complexes in the late endo-some. After the peptides are bound to MHC class II molecules, the endosome containing these complexes migrates to the cell surface and inserts in the plasma membrane [see Figure 1b]. Of note is that partially unfolded antigenic proteins can bind to MHC class II molecules before undergoing proteolytic degradation. This may explain why the length of peptides bound to MHC class II molecules is highly variable.

Alternative antigen-presenting complexes

A third class of MHC includes CD1 molecules, which are related to MHC class I molecules but which bind lipid and gly-colipid antigens for presentation to T cells. For example, CD1 molecules present glycolipids from Mycobacterium tuberculosis to a restricted group of T cells.

The pathways for formation of antigenic peptide-MHC molecule complexes on antigen-presenting cells. (a) In the MHC class I molecule pathway, endogenous proteins are broken down by proteasomes into smaller peptides. In the endoplasmic reticulum (ER), an antigenic peptide binds to the peptide binding site in an MHC class I molecule. The peptide-MHC complex then migrates through the Golgi apparatus to the cell surface. (b) In the MHC class II molecule pathway, the a and p subunits of the MHC class II molecule bind to the invariant chain (Ii) in the ER. Ii is partially degraded in an endosomal compartment. The portion of Ii that occupies the antigenic peptide binding site on the MHC class II molecule (called CLIP) is removed with the help of HLA-DM, freeing the molecule for binding processed antigen. Once antigenic peptide has bound to an MHC class II molecule, the complex migrates to the cell surface.

Figure 1 The pathways for formation of antigenic peptide-MHC molecule complexes on antigen-presenting cells. (a) In the MHC class I molecule pathway, endogenous proteins are broken down by proteasomes into smaller peptides. In the endoplasmic reticulum (ER), an antigenic peptide binds to the peptide binding site in an MHC class I molecule. The peptide-MHC complex then migrates through the Golgi apparatus to the cell surface. (b) In the MHC class II molecule pathway, the a and p subunits of the MHC class II molecule bind to the invariant chain (Ii) in the ER. Ii is partially degraded in an endosomal compartment. The portion of Ii that occupies the antigenic peptide binding site on the MHC class II molecule (called CLIP) is removed with the help of HLA-DM, freeing the molecule for binding processed antigen. Once antigenic peptide has bound to an MHC class II molecule, the complex migrates to the cell surface.

This ability to recognize nonprotein microbial antigens suggests that T cells recognize a broader range of antigens than was once thought. A subset of CD1+ T cells often reacts to self-antigens; these cells have been implicated in such autoimmune diseases as type 1 diabetes mellitus and systemic lupus erythematosus. It has been suggested that they are involved at the early innate phase of these immune responses.3 Another molecule related to MHC class I, MR1, is found on a subset of T cells that are preferentially located in the gut lamina propria and are called mucosal-associated invariant T (MAIT) cells. MAIT cells are probably involved in the host response at the site of pathogen entry and may regulate intestinal B cell activity. MAIT cells are absent in humans and mice that have B cell deficiency, which suggests that the selection or expansion of this cell population requires B cells.

Superantigens

Superantigens constitute a class of immunostimulatory proteins derived from microbial agents (e.g., viral proteins and the staphylococcal toxins that cause toxic-shock syndrome and food poisoning). Superantigens bind to MHC class II molecules outside the conventional antigen-binding site and stimulate massive T cell activation5 [see 7:XXXSepsis]. Different MHC class II alleles have distinct binding constants for superantigens; thus, superantigens can activate distinct segments of the T cell repertoire.

Professional antigen-presenting cells

Whereas MHC class I molecules are expressed on the surface of all eukaryotic cells, MHC class II molecules have a restricted tissue distribution. In fact, certain cells—including B cells, dendritic cells, macrophages, Langerhans cells, and en-dothelial cells—are termed professional APCs because they present antigenic peptide with MHC class II molecules more efficiently than other APCs.6 This efficiency is primarily attributed to their ability to process endocytosed antigens. Professional APCs also interact with T cells more efficiently because they have several cell surface markers that bind to costimulato-ry molecules on the surface of T cells (see below). The most potent APCs are dendritic cells, which present antigen only on maturation and migration to the lymph nodes. This maturation process is triggered by the uptake of antigen.

T Cell Responses to Antigen

T cell recognition of antigen proceeds in two distinct stages. The first step, which is nonspecific, is the adhesion of a T cell to an APC. The second step, which is specific, is an interaction between the TCR and a compatible antigen-MHC complex on the APC. This process highlights two fundamental properties of T cells: their broad scope of action (i.e., their ability to migrate throughout the body and adhere to many types of cells) and their great specificity for particular antigens.

The diversity of the variable regions in TCRs facilitates highly specific responses to antigens [see 6:III Adaptive Immunity: Antigens, Antibodies, and T Cell and B Cell Receptors]. The prototypical T cells, ap T cells, have a TCR made up of a and p chains, which are expressed in association with CD3. A subset of peripheral T cells, y6 T cells—so called because their TCR is made up of y and 6 chains—may recognize antigen that has not been processed.

APCs process many peptide antigens simultaneously and thus express on their cell surfaces a large number of different antigen-MHC complexes. A given ap T cell clone can recognize only a small number of these complexes. T cells screen APCs for compatible antigen-MHC complexes by adhering to the APC. Adhesion is aborted if the TCRs do not recognize specific antigen; adhesion is intensified when a TCR makes contact with a compatible antigen-MHC complex. On adhesion, TCR-CD3 complexes aggregate on the surface of the T cell and bind to the antigen-MHC complexes that have aggregated on the surface of the APC. In the absence of antigen recognition, TCR-CD3 complexes are unable to cluster and detachment occurs immediately.

Interactions between a helper T cell clone and an APC or between a cytotoxic T cell and its target cell follow the same general pattern. Initially, only a very small number of antigen-MHC complexes engage with specific TCRs in an area of cell- cell contact established by adhesion. Subsequently, TCR-CD3 complexes and MHC molecules bearing the correct antigenic peptide migrate into the contact region. This establishes a high local density of TCRs, which promotes antigen binding and T cell activation. The existence of these clustered TCRs has been demonstrated experimentally, through the use of tagged monoclonal antibodies.7 In addition, visualization of clusters of TCRs using fluorescence-tagged antigen-MHC complexes has permitted incisive studies of T cell responses to infections with Ep-stein-Barr virus or HIV.8,9

Costimulatory molecules

The recognition of antigen by the TCR and subsequent activation of the T cell is regulated by other T cell surface molecules [see Figure 2 and Table 1].10 Costimulatory molecules are involved in both adhesion and T cell signaling and play a major role in the coordination and kinetic regulation of T cell activation. Clinically, these costimulatory molecules are important because aberrant activation of the TCR can initiate disastrous immune responses, such as those seen in autoimmune diseases.

The costimulatory molecules CD28 and cytotoxic T cell-associated antigen 4 (CTLA-4) initiate a signaling pathway that is different from, and often independent of, the pathway mediated by the TCR-CD3 complex. CD28 is expressed on the surface of essentially all CD4+ and most CD8+ T cells. CD28 binds to B7 on APCs, leading to T cell activation and proliferation. Although CTLA-4 is homologous to CD28, it has opposite effects: binding of CTLA-4 to B7 delivers an inhibitory signal that leads to downregulation of T cell proliferation. Of note is that research entailing manipulation of CD28-CTLA-4 interactions with their natural ligands suggests that these costimulatory molecules have a potential role in the treatment of such diseases as arthritis, multiple sclerosis, and asthma and in protection against HIV infection.11 For example, a soluble construct of CTLA-4 (called CTLA-4-Ig) is able to inhibit T cell activation when administered early in the immune response. Several new members of the B7 and CD28-CTLA-4 families have recently been discovered, and these may also be important for regulating the responses of previously activated T cells.12

The SLAM (signaling lymphocyte activation molecule) family of receptors serves as costimulatory molecules on T cells (and other immune cells). In T cells and natural killer (NK) cells, SLAM-associated protein (SAP) regulates signaling of the SLAM family of receptors. The SAP (or SH2D1A) gene is defective or absent in patients with X-linked lymphoproliferative syndrome.

Fas (also called APO-1 and CD95) has been implicated in both positive and negative signaling events after binding with its li-gand (CD95L) on cytolytic effector cells. Fas is a member of the tumor necrosis factor receptor (TNFR) family, and binding of Fas on T cells with its ligand typically causes the programmed death of the target cell (i.e., apoptosis; see below). However, Fas can also function as a costimulatory molecule for TCR-CD3 activation. Thus, a single molecule can have different signaling outcomes at different stages of T cell development.

T Cell Signaling after Antigen Recognition

In response to antigen recognition, resting T cells undergo a complex series of events known as T cell activation.15,16 The recognition and binding of TCRs to antigen-MHC complexes and of costimulatory molecules to their appropriate ligands— all of which takes place on the cell surface—is followed by an intracellular cascade of biochemical events (i.e., signal transduction) that ultimately reaches specific genes in the nucleus. The resulting production of cytokines, chemokines, and other immunomodulatory molecules leads to cell proliferation, differentiation, and expression of unique effector functions.

Most of the biochemical events that occur immediately after engagement of the TCR with the antigen-MHC complex have been defined [see Figure 3]. Transduction of signals starts with the CD3 elements of the TCR-CD3 complex and proceeds to the nucleus via different pathways. One pathway involves the calcium-dependent enzyme calcineurin; other coupled pathways involve the enzymes Ras and Rac and serine-threonine kinase. These pathways converge on the activation of transcription factors that control the expression of genes mediating T cell effector function (e.g., the gene for the cytokine interleu-kin-2 [IL-2]) [see Figure 3]. Thus, the exquisitely T cell clone-specific TCR connects, via a number of intermediate molecules, with universal signal transduction pathways.

Drugs are being developed to interfere with nodal points of these signal transduction pathways. However, cyclosporine, which interferes with the calcineurin pathway, remains the most successful agent of this type.

B Cell Responses to Antigen

When a B cell receptor (BCR) binds soluble antigen, one of two events takes place: apoptosis or proliferation and further maturation of B cells. The signals for these processes are generated intracellularly by Iga and Ig|. These proteins are associated with BCRs in the way that CD3 proteins are associated with TCRs. Iga and Ig| recruit signal transduction molecules in a manner similar to that for activating T cells and involving some of the same proteins.

Relatively little is known about the signaling that takes place in B cells when they have presented an antigen to a helper T cell that recognizes that antigen. T cell-B cell interaction can also lead to either apoptosis or proliferation of B cells. With T cell help, however, proliferation results in the generation of several different classes of B cells.

(a) Two signals are necessary for activation of an antigen-specific T cell by an antigen-presenting cell (APC). Signal 1 is initiated by the interaction between the antigen bound to a class II major histocom-patibility complex (MHC) and the T cell receptor (TCR) and its corecep-tor (in this example, CD4). The costimulatory molecules B7-1 and B7-2 are transiently expressed on the surface of so-called professional APCs and are presumed to be inducible by signaling from the antigen-MHC complex. Thus, signal 2 is initiated by the binding of CD28 on the T cell to B7-1 or B7-2. CTLA-4, which is homologous to CD28, is upreg-ulated after T cell activation. (b) Activation of B cells can occur with either helper T cells or soluble antigen. Binding of soluble antigen to the B cell receptor, a cell surface immunoglobulin associated with Iga and Ig|, can lead to B cell proliferation or apoptosis. Alternatively, B cells can process antigen for presentation to TCRs on T cells. Binding of antigen-MHC complex to the TCR induces expression of the CD40 ligand (CD154) on the T cell surface, which in turn induces expression of CD40 on the B cell surface. This results in B cell proliferation and is indispensable for immunoglobulin class switching and probably for somatic mutation. CD154 signaling also plays an important role in the maturation of dendritic cells. (ICAM-1—intercellular adhesion mole-cule-1; LFA—leukocyte-function-associated antigen)

Figure 2 (a) Two signals are necessary for activation of an antigen-specific T cell by an antigen-presenting cell (APC). Signal 1 is initiated by the interaction between the antigen bound to a class II major histocom-patibility complex (MHC) and the T cell receptor (TCR) and its corecep-tor (in this example, CD4). The costimulatory molecules B7-1 and B7-2 are transiently expressed on the surface of so-called professional APCs and are presumed to be inducible by signaling from the antigen-MHC complex. Thus, signal 2 is initiated by the binding of CD28 on the T cell to B7-1 or B7-2. CTLA-4, which is homologous to CD28, is upreg-ulated after T cell activation. (b) Activation of B cells can occur with either helper T cells or soluble antigen. Binding of soluble antigen to the B cell receptor, a cell surface immunoglobulin associated with Iga and Ig|, can lead to B cell proliferation or apoptosis. Alternatively, B cells can process antigen for presentation to TCRs on T cells. Binding of antigen-MHC complex to the TCR induces expression of the CD40 ligand (CD154) on the T cell surface, which in turn induces expression of CD40 on the B cell surface. This results in B cell proliferation and is indispensable for immunoglobulin class switching and probably for somatic mutation. CD154 signaling also plays an important role in the maturation of dendritic cells. (ICAM-1—intercellular adhesion mole-cule-1; LFA—leukocyte-function-associated antigen)

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