Cell Signaling

Cytokines and Their Receptors

The immune systems form complex networks involving many different cell types, each sending and receiving multiple messages from many different sources. Intercellular signals are transmitted in two general ways. One method is called volume transmission. In volume transmission, a mediator molecule is released by the signaling cell and diffuses through the extracellular fluid to the receiving cell, where it binds to cell surface receptors. The second method, called network transmission, occurs when two cells come into direct contact using complementary receptors. Signals are then transmitted directly between these two receptors.

Irrespective of how the signals are transmitted, by signaling through appropriate receptors, target cells can be directed to behave in a specific manner. They may be told to divide or stop dividing; they may be stimulated to secrete their own signaling molecules or express new receptors; they may be told to commit suicide. Each cell may be exposed to many different signaling molecules at any one time. The target cell must integrate these signals and respond appropriately. In this chapter, we review the signaling molecules secreted by cells, the receptors that receive these signals, and the way in which the received signals are interpreted by the receiving cell.

The cells of the immune system can synthesize and secrete hundreds of different proteins that control the immune responses by communicating among cells. These proteins are called cytokines (Box 8-1). Cytokines differ from conventional hormones in several important respects. For example, unlike conventional hormones, which tend to affect a single target cell type, cytokines can affect many different cell types. Second, immune system cells rarely secrete a single cytokine at a time. For instance, macrophages secrete at least four interleukins (IL-1, IL-6, IL-12, IL-18) as well as tumor necrosis factor-α (TNF-α). Third, cytokines are “redundant” in their biological activities in that many different cytokines have similar effects. For example, IL-1, TNF-α, TNF-β, IL-6, high mobility group box protein-1 (HMGB1), and the chemokine CCL3 all act on the brain to cause a fever. Finally, cytokine-mediated signals are transient, and the messages delivered may vary over time as the cytokine environment changes.

Box 8-1 Properties of Cytokines

• Short-lived proteins

• Highly diverse structures and receptors

• Can act locally and/or systemically

• Pleiotropic: affect many different cells

• Redundant: exhibit biologically overlapping functions

• Carefully regulated

• Toxic in high doses

Cytokine Nomenclature

The nomenclature and classification of the cytokines is not based on any systematic relationship among these proteins. Many were originally named after their cell of origin or the bioassay used to identify them.

The interleukins, for example, are cytokines that mediate signaling between lymphocytes and other leukocytes. They are numbered sequentially in the order of their discovery. Because their definition is so broad, the interleukins are a heterogeneous mixture of proteins with little in common except their name. As of 2011, 37 different numbered interleukins have been described. As might be expected, we know a lot about some of these molecules and very little about others. Likewise, some are clearly critical to a successful immune response, whereas others appear to be much less essential.

The interferons are cytokines produced in response to virus infection or immune stimulation. Their name is derived from the fact that they interfere with viral RNA and protein synthesis and so have antiviral activity (Chapter 26). There are three types of interferon. Type I interferons are a diverse family, the most important of which are interferon-α (IFN-α) and IFN-β. There is a single type II interferon, called IFN-γ. Three type III interferons (IFN–λ) have been identified. Type I interferons are primarily antiviral with a secondary immunoregulatory role. For type II and type III interferons such as IFN-γ and IFN–λ, the reverse is the case. Many type I interferons also play an important role in the maintenance of pregnancy.

TNFs are cytokines secreted by macrophages and T cells. As their name suggests, they can kill tumor cells, although this is not their primary function. Thus TNF-α is the key mediator of acute inflammation. The TNFs belong to a large family of related cytokines, the TNF superfamily, which is involved in immune regulation and inflammation. Other important members of the TNF superfamily include CD178 (also called CD95L or Fas ligand) (Chapter 18), and CD154 (CD40 ligand) (Chapter 14).

Many cytokines are growth factors (or colony-stimulating factors) and regulate blood cell production by regulating stem cell activities. They thereby ensure that the body is supplied with sufficient cells to defend itself.

Chemokines are a family of at least 50 small cytokines that play an important role in leukocyte chemotaxis, circulation, migration, and activation, especially in inflammation. A typical example of a chemokine is CXCL8 (also known as IL-8). Chemokines are described in detail in Chapter 3.

Cytokine Functions

Cytokines are produced in response to many different stimuli. Examples of these stimuli include antigens acting through the T cell or B cell antigen receptors; antigen-antibody complexes acting through antibody receptors (FcR); pathogen-associated molecular patterns (PAMPs) such as lipopolysaccharides acting through toll-like receptors (TLRs); and other cytokines acting through cytokine receptors (Figure 8-1).

FIGURE 8-1 Three of the most important pathways that trigger cytokine release are the combination of antigens with their receptors on T and B cells, the combination of PAMPs with toll-like receptors on sentinel cells; and the combination of antibodies with Fc receptors on phagocytic cells.

Cytokines act on many different cellular targets. They may, for example, bind to receptors on the cell that produced them and thus have an autocrine effect. Alternatively, they may bind only to receptors on nearby cells; this is called a paracrine effect. Some cytokines may spread throughout the body, affecting target cells in distant locations, and thus have an endocrine effect (Figure 8-2).

FIGURE 8-2 The distinction among autocrine, paracrine, and endocrine effects. Cytokines differ from hormones in that most of their effects are autocrine or paracrine, whereas hormones usually act on distant cells in an endocrine fashion.

When cytokines bind to receptors on target cells, they affect cell behavior. They may induce the target cell to divide or differentiate, or they may stimulate the production of new proteins. Alternatively they may inhibit these effects—preventing division, differentiation, or new protein synthesis. Most cytokines act on many different target cell types, perhaps inducing different responses in each one, a feature that is called pleiotropy. Conversely, many different cytokines may act on a single target, a feature known as redundancy. For example, IL-3, IL-4, IL-5, and IL-6 all affect B cell function. Some cytokines work best when paired with other cytokines in a process called synergy. For example, the combination of IL-4 and IL-5 stimulates B cells to make immunoglobulin E (IgE) and hence triggers an allergic response. Synergy can also occur in sequence when, for example, one cytokine induces the target cell to express the receptor for another cytokine. Finally, some cytokines have opposing effects and may antagonize the effects of others. The best example of this is the mutual antagonism of IL-4 and IFN-γ.

Cytokine Structure

Cytokines are complex proteins with many diverse structures. They are classified based on these structures (Table 8-1). Thus the largest family, the group I cytokines (or hematopoietins), consist of four α helices bundled together. This family includes many different interleukins as well as growth hormone and leptin. Within the group I cytokines are two major subfamilies, the interferon subfamily and the IL-10 subfamily. Group II cytokines consist of long-chain β-sheet structures. They include the TNFs, the IL-1 family, and transforming growth factor-β (TGF-β). Group III cytokines are small proteins with both α helices and β sheets. These include the chemokines and related molecules. Group IV cytokines use mixtures of domains with mixtures of structural motifs and include the IL-12 family. Many cytokines, such as the IL-17 family, IL-14, and IL-16, are structurally unique proteins and do not belong to any of these major structural families.

Table 8-1

Molecular Classification of Selected Cytokines

STRUCTURAL FAMILIES	STRUCTURE	EXAMPLES
Group 1	Four α helix bundle	IL-2, -3, -4, -5, -6, -7, -9, -11, -13, -15, -21, -23, -30; GM-CSF, erythropoietin, G-CSF, prolactin, leptin
	IFN subfamily	IFN-α/β, IFN-γ, IFN–λ
	IL-10 subfamily	IL-10, -19, -20, -22, -24, -26
Group 2	β Sheets	TNFs; TGF-β; IL-1α, -1β, -18
Group 3	α Helices and β sheets	Chemokines
Group 4	Mixed motifs	IL-12
Ungrouped	Unique structures	IL-17A-F, -14, -16

Patterns may also be seen in the biological activities of these cytokines. Thus group I cytokines tend to be involved in immune regulation or stem cell regulation. Group II cytokines are mainly involved in the growth and regulation of cells, cell death, and inflammation. Group III cytokines are involved in inflammation. The activities of the group IV cytokines depend on their subcomponents. For example, IL-12 is formed by a combination of a group I structure with a stem cell receptor, but it acts like a group I cytokine.

Cytokine Receptors

Cytokines act through cell surface receptors. These receptors consist of at least two functional units, one for ligand binding and one for signal transduction (Figure 8-3). These units may or may not be on the same protein chain. Cytokine receptors can also be classified into classes based on their structure.

FIGURE 8-3 The structure of a cytokine receptor—in this case, the IL-2 receptor complex. The complete trimer serves as a high-affinity receptor, whereas the β–γ dimer serves as a low-affinity receptor. Note that different receptor components each serve different functions.

One class of receptor includes the channel-linked receptors, which act as transmitter-gated ion channels. Thus the receptor itself is a channel, and binding of its ligand opens that channel, allowing ions to pass through it. Channel-linked receptors are found in inflammatory and immune cells, but their roles are unclear. They do not serve as cytokine receptors.

A second class of receptor consists of proteins that also act as tyrosine kinases (Figure 8-4). These are typically growth factor and cytokine receptors. In these cases, binding of the ligand to two adjacent receptors forms an active dimer. The ligand-binding site, the membrane-spanning region, and the tyrosine kinase are usually separate domains of a single protein. Thus when the ligand binds to the extracellular domain, the receptor chains dimerize so that the two tyrosine kinases are brought together and activate each other. These kinases phosphorylate tyrosine residues on other proteins or even the receptor itself (autophosphorylation). Since many of these other proteins are also tyrosine kinases, phosphorylation also converts them to an active state. In this way a cascade of expanding phosphorylations develops within the cell (Figure 8-5). Phosphorylation triggers changes in cellular activities (Box 8-2). Many cytokines and other immunological signals operate through this type of receptor (especially through tyrosine kinases of the src family).

Box 8-2 Protein Phosphorylation

Central to most signaling is protein phosphorylation by receptor kinases. Phosphorylation is a form of reversible modification of proteins. All signal transduction systems involve the use of a high-energy phosphate-rich compound such as adenosine triphosphate (ATP) to modify a protein and send a signal to a cell. Cell growth, cell division, and other critical processes are all regulated by protein phosphorylation. Protein kinases enzymatically phosphorylate the amino acids serine, threonine, and tyrosine.

Protein+ATP⟶Protein kinaseprotein-P+ADP