《免疫学》（英文版） Chapter 15 Leukocyte Migration and Inflammation

eukocyte migration chapter 15 and inflammation ANY TYPES OF LEUKOCYTES MOVE FROM ONE part of the body to another. This is espe cially true of lymphocytes, which circulate continually in the blood and lymph and, in common with other types of leukocytes, migrate into the tissues at sites of infection or tissue injury. This recirculation not only in creases the chance that lymphocytes specific for a particular antigen will encounter that antigen but also is critical to Lymphocytes Attached to the Surface of a High-Endothelial Venule development of an inflammatory response. Inflammation is a complex response to local injury or other trauma; it is characterized by redness, heat, swelling, and pain. Inflam- a Lymphocyte Recirculation mation involves various immune-system cells and numer- a Cell-Adhesion Molecules ous mediators. Assembling and regulating inflammatory responses would be impossible without the controlled Neutrophil Extravasation migration of leukocyte populations. This chapter covers the a Lymphocyte Extravasation molecules and processes that play a role in leukocyte migra- tion, various molecules that mediate inflammation, and the a Chemokines-Key Mediators of Inflammation characteristic physiologic changes that accompany inflam a Other Mediators of Inflammation matory responses a The Inflammatory Process Anti-Inflammatory Agents Lymphocyte Recirculation Lymphocytes are capable of a remarkable level of recircula on, continually moving through the blood and lymph to the various lymphoid organs(Figure 15-1). After a brief transit time of approximately 30 min in the bloodstream, and back again as often as 1-2 times per day. Since only directly to the spleen, where they reside for approximately gen, it would appear that a large number of T or B cells must 5 h Almost equal numbers(42%)of lymphocytes exit from contact antigen on a given antigen-presenting cell within a the blood into various peripheral lymph nodes, where they short time in order to generate a specific immune response reside for about 12 h. A smaller number of lymphocytes The odds of the small percentage of lymphocytes committed (10%)migrate to tertiary extralymphoid tissues by crossing to a given antigen actually making contact with that antigen between endothelial cells that line the capillaries. These tis- when it is present are elevated by the extensive recircula- sues normally have few, if any, lymphoid cells but can import tion of lymphocytes. The likelihood of such contacts is them during an inflammatory response. The most immuno- profoundly increased also by factors that regulate, organize, interface with the external environment, such as the skin presenting cells culation of lymphocytes and antigen logically active tertiary extralymphoid tissues are those that and direct the ci and various mucosal epithelia of the gastrointestinal, pul- monary, and genitourinary tracts The process of continual lymphocyte recirculation allows Cell-Adhesion molecules maximal numbers of antigenically committed lymphocytes to encounter antigen. an individual lymphocyte may make a The vascular endothelium serves as an important gate- omplete circuit from the blood to the tissues and lymph keeper, "regulating the movement of blood-borne molecules

■ Lymphocyte Recirculation ■ Cell-Adhesion Molecules ■ Neutrophil Extravasation ■ Lymphocyte Extravasation ■ Chemokines—Key Mediators of Inflammation ■ Other Mediators of Inflammation ■ The Inflammatory Process ■ Anti-Inflammatory Agents Leukocyte Migration and Inflammation M       part of the body to another. This is espe￾cially true of lymphocytes, which circulate continually in the blood and lymph and, in common with other types of leukocytes, migrate into the tissues at sites of infection or tissue injury. This recirculation not only in￾creases the chance that lymphocytes specific for a particular antigen will encounter that antigen but also is critical to development of an inflammatory response. Inflammation is a complex response to local injury or other trauma; it is characterized by redness, heat, swelling, and pain. Inflam￾mation involves various immune-system cells and numer￾ous mediators. Assembling and regulating inflammatory responses would be impossible without the controlled migration of leukocyte populations. This chapter covers the molecules and processes that play a role in leukocyte migra￾tion, various molecules that mediate inflammation, and the characteristic physiologic changes that accompany inflam￾matory responses. Lymphocyte Recirculation Lymphocytes are capable of a remarkable level of recircula￾tion, continually moving through the blood and lymph to the various lymphoid organs (Figure 15-1). After a brief transit time of approximately 30 min in the bloodstream, nearly 45% of all lymphocytes are carried from the blood directly to the spleen, where they reside for approximately 5 h. Almost equal numbers (42%) of lymphocytes exit from the blood into various peripheral lymph nodes, where they reside for about 12 h. A smaller number of lymphocytes (10%) migrate to tertiary extralymphoid tissues by crossing between endothelial cells that line the capillaries. These tis￾sues normally have few, if any, lymphoid cells but can import them during an inflammatory response. The most immuno￾logically active tertiary extralymphoid tissues are those that interface with the external environment, such as the skin and various mucosal epithelia of the gastrointestinal, pul￾monary, and genitourinary tracts. The process of continual lymphocyte recirculation allows maximal numbers of antigenically committed lymphocytes to encounter antigen. An individual lymphocyte may make a complete circuit from the blood to the tissues and lymph and back again as often as 1–2 times per day. Since only about one in 105 lymphocytes recognizes a particular anti￾gen, it would appear that a large number of T or B cells must contact antigen on a given antigen-presenting cell within a short time in order to generate a specific immune response. The odds of the small percentage of lymphocytes committed to a given antigen actually making contact with that antigen when it is present are elevated by the extensive recircula￾tion of lymphocytes. The likelihood of such contacts is profoundly increased also by factors that regulate, organize, and direct the circulation of lymphocytes and antigen￾presenting cells. Cell-Adhesion Molecules The vascular endothelium serves as an important “gate￾keeper,” regulating the movement of blood-borne molecules chapter 15 FPO Lymphocytes Attached to the Surface of a High-Endothelial Venule

Leukocyte Migration and Inflammation CHAPTER 15 339 Efferent lymph A number of endothelial and leukocyte CAMs have (52%) loned and characterized, providing new details about the extravasation process. Most of these Cams belong to four families of proteins: the selectin family, the mucin-like fam ily, the integrin family, and the immunoglobulin(lg)super- family( Figure 15-2 (45%)(42% SELECTINs The selectin family of membrane glycoproteins has a distal lectin -ike domain that enables these molecules Blood to bind to specific carbohydrate groups. Selectins interact lymphocyte pool lymph (10%) primarily with sialylated carbohydrate moieties, which are (30 min) often linked to mucin-like molecules. The selectin family includes three molecules, designated L, E, and P. Most cir- culating leukocytes express L-selectin, whereas E-selecti (10%)\ lymphocytes and P-selectin are expressed on vascular endothelial cells. Selectin molecules are responsible for the initial stickiness of leukocytes to vascular endothelium Tertiary extralymphoid Bone marrow Epithelial surface Mucosal epithelia in gut MUCINS The mucins are a group of serine- and threonine- lungs, and genitourinary tracts rich proteins that are heavily glycosylated. Their extended structure allows them to present sialylated carbohydrate recognizes sialylated carbohydrates on two mucin-like mole cules(CD34 and glyCAM-1)expressed on certain endothelial cells of lymph nodes. Another mucin-like molecule(PSGL-1) FIGURE15-1 Lymphocyte recirculation routes. The percentage of found on neutrophils interacts with E- and P-selectin ex- the lymphocyte pool that circulates to various sites and the average pressed on inflamed endothelium transit times in the major sites are indicated. Lymphocytes migrate from the blood into lymph nodes through specialized areas in post- INTEGRINS The integrins are heterodimeric proteins(consist- capillary venules called high-endothelial venules(HEVs). Although ing of an a and a B chain)that are expressed by leukocytes most lymphocytes circulate, some sites appear to contain lympho- and facilitate both adherence to the vascular endothelium and that do not. /Adapted from A. Ager, 1994, Trends Cell Biol. 4: 326] other cell-to-cell interactions. The integrins are grouped into categories according to which p subunit they contain. Differ at integrins are expressed by different populations of leuko- cytes, allowing these cells to bind to different CAMs that belong to the immunoglobulin superfamily expressed along the vascular endothelium. As described later, some integrins and leukocytes into the tissues. In order for circulating leuko- must be activated before they can bind with high affinity to cytes to enter inflamed tissue or peripheral lymphoid organs, their ligands. The importance of integrin molecules in leuko- the cells must adhere to and pass between the endothelial cyte extravasation is demonstrated by leukocyte-adhesion de- cells lining the walls of blood vessels, a process called extra- ficiency(LAD), an autosomal recessive disease described later vasation. Endothelial cells express leukocyte-specific cell- in this chapter(see the Clinical Focus). It is characterized by adhesion molecules(CAMs). Some of these membrane pro- recurrent bacterial infections and impaired healing of wounds. teins are expressed constitutively; others are expressed only in response to local concentrations of cytokines produced ICAMS Several adhesion molecules contain a variable num during an inflammatory response. Recirculating lympho- ber of immunoglobulin-like domains and thus are classified cytes, monocytes, and granulocytes bear receptors that bind in the immunoglobulin superfamily. Included in this group to CAMs on the vascular endothelium, enabling these cells to are ICAM-1, ICAM-2, ICAM-3, and VCAM, which are ex- extravasate into the tissues pressed on vascular endothelial cells and bind to various In addition to their role in leukocyte adhesion to vascular integrin molecules. An important cell-adhesion molecule endothelial cells, CAMs on leukocytes also serve to increase called MAdCAM-1 has both Ig-like domains and mucin-like the strength of the functional interactions between cells of domains. This molecule is expressed on mucosal endothe- the immune system. Various adhesion molecules have been lium and directs lymphocyte entry into mucosa. It binds to shown to contribute to the interactions between TH cells and integrins by its immunoglobulin-like domain and to selectins APCs, TH and b cells, and Ctls and target cells by its mucin-like domain

and leukocytes into the tissues. In order for circulating leuko￾cytes to enter inflamed tissue or peripheral lymphoid organs, the cells must adhere to and pass between the endothelial cells lining the walls of blood vessels, a process called extra￾vasation. Endothelial cells express leukocyte-specific cell￾adhesion molecules (CAMs). Some of these membrane pro￾teins are expressed constitutively; others are expressed only in response to local concentrations of cytokines produced during an inflammatory response. Recirculating lympho￾cytes, monocytes, and granulocytes bear receptors that bind to CAMs on the vascular endothelium, enabling these cells to extravasate into the tissues. In addition to their role in leukocyte adhesion to vascular endothelial cells, CAMs on leukocytes also serve to increase the strength of the functional interactions between cells of the immune system. Various adhesion molecules have been shown to contribute to the interactions between TH cells and APCs, TH and B cells, and CTLs and target cells. A number of endothelial and leukocyte CAMs have been cloned and characterized, providing new details about the extravasation process. Most of these CAMs belong to four families of proteins: the selectin family, the mucin-like fam￾ily, the integrin family, and the immunoglobulin (Ig) super￾family (Figure 15-2). SELECTINS The selectin family of membrane glycoproteins has a distal lectin-like domain that enables these molecules to bind to specific carbohydrate groups. Selectins interact primarily with sialylated carbohydrate moieties, which are often linked to mucin-like molecules. The selectin family includes three molecules, designated L, E, and P. Most cir￾culating leukocytes express L-selectin, whereas E-selectin and P-selectin are expressed on vascular endothelial cells. Selectin molecules are responsible for the initial stickiness of leukocytes to vascular endothelium. MUCINS The mucins are a group of serine- and threonine￾rich proteins that are heavily glycosylated. Their extended structure allows them to present sialylated carbohydrate ligands to selectins. For example, L-selectin on leukocytes recognizes sialylated carbohydrates on two mucin-like mole￾cules (CD34 and GlyCAM-1) expressed on certain endothelial cells of lymph nodes. Another mucin-like molecule (PSGL-1) found on neutrophils interacts with E- and P-selectin ex￾pressed on inflamed endothelium. INTEGRINS The integrins are heterodimeric proteins (consist￾ing of an and a chain) that are expressed by leukocytes and facilitate both adherence to the vascular endothelium and other cell-to-cell interactions. The integrins are grouped into categories according to which subunit they contain. Differ￾ent integrins are expressed by different populations of leuko￾cytes, allowing these cells to bind to different CAMs that belong to the immunoglobulin superfamily expressed along the vascular endothelium. As described later, some integrins must be activated before they can bind with high affinity to their ligands. The importance of integrin molecules in leuko￾cyte extravasation is demonstrated by leukocyte-adhesion de￾ficiency (LAD), an autosomal recessive disease described later in this chapter (see the Clinical Focus). It is characterized by recurrent bacterial infections and impaired healing of wounds. ICAMS Several adhesion molecules contain a variable num￾ber of immunoglobulin-like domains and thus are classified in the immunoglobulin superfamily. Included in this group are ICAM-1, ICAM-2, ICAM-3, and VCAM, which are ex￾pressed on vascular endothelial cells and bind to various integrin molecules. An important cell-adhesion molecule called MAdCAM-1 has both Ig-like domains and mucin-like domains. This molecule is expressed on mucosal endothe￾lium and directs lymphocyte entry into mucosa. It binds to integrins by its immunoglobulin-like domain and to selectins by its mucin-like domain. Leukocyte Migration and Inflammation CHAPTER 15 339 Spleen (5 h) Bone marrow Epithelial surface Peritoneum Activated lymphocytes Nonrecirculating cells Afferent lymph Naive lymphocytes (45%) (42%) Efferent lymph (52%) Blood lymphocyte pool (30 min) Lymph nodes (12 h) (?) (10%) (10%) Tertiary extralymphoid tissue: Mucosal epithelia in gut, lungs, and genitourinary tracts Liver Brain Skin FIGURE 15-1 Lymphocyte recirculation routes. The percentage of the lymphocyte pool that circulates to various sites and the average transit times in the major sites are indicated. Lymphocytes migrate from the blood into lymph nodes through specialized areas in post￾capillary venules called high-endothelial venules (HEVs). Although most lymphocytes circulate, some sites appear to contain lympho￾cytes that do not. [Adapted from A. Ager, 1994, Trends Cell Biol. 4:326.]

40 PaRt II Immune Effector Mechanisms (a) General structure of CAM families (b)Selected CAMs belonging to each family Mucin-like cams Mucin-like CAMs: GlyCAM-1 L-selectin E-selectin MAdCAM-1 CHO side Ig-superfamily CAMs: ICAM-1-2-3 LPAM-2) p7①LPAM1) LFA-2(CD2) a6B1 (VLA-6 LFA-3(CD58) MAdCAM-1 aMB2(Mac-1) cXB2(CR4,p150/95) Lectin domain Fibrinonectin-type FIGURE 15-2 Schematic diagrams depicting the general structures of the four families of cell-adhesion molecules(a) and a list of repre- sentative molecules in each family(b). The lectin domain in selectins nteracts primarily with carbohydrate(CHO) moieties on mucin-like molecules. Both component chains in integrin molecules contribute to the binding site, which interacts with an lg domain in CAMs belonging to the Ig superfamily. MAdCAM-1 contains both mucin-like and ig-like Selectins Ig- superfamily Cams domains and can bind to both selectins and integrins Neutrophil Extravasation like cell-adhesion molecules on the neutrophil membrane or with a sialylated lactosaminoglycan called sialyl Lewis(Figure As an inflammatory response develops, various cytokines 15-3b ). This interaction tethers the neutrophil briefly to the and other inflammatory mediators act upon the local blood endothelial cell, but the shear force of the circulating blood vessels, inducing increased expression of endothelial CAMs. soon detaches the neutrophil Selectin molecules on another The vascular endothelium is then said to be activated, or endothelial cell again tether the neutrophil; this process is inflamed. Neutrophils are generally the first cell type to bind repeated so that the neutrophil tumbles end-over-end along to inflamed endothelium and extravasate into the tissues. To the endothelium, a type of binding called rolling accomplish this, neutrophils must recognize the inflamed As the neutrophil rolls, it is activated by various chemoat endothelium and adhere strongly enough so that they are not tractants; these are either permanent features of the endo- swept away by the flowing blood. The bound neutrophils thelial cell surface or secreted locally by cells involved in the must then penetrate the endothelial layer and migrate into inflammatory response. Among the chemoattractants are the underlying tissue Monocytes and eosinophils extravasate members of a large family of chemoattractive cytokines called by a similar process, but the steps have been best established chemokines. Two chemokines involved in the activation for the neutrophil, so we focus on neutrophils here process are interleukin 8(IL-8)and macrophage inflamma- The process of neutrophil extravasation can be divided into tory protein (MIP-1B). However, not all chemoattractants tractant stimulus,(3)arrest and adhesion, and(4)transendo. belong to the chemokine group. Other chemoattractants are attach loosely to the endothelium by a low-affinity selectin- duced by the breakdown of bacterial proteins during ago. thelial migration(Figure 15-3a) In the first step, neutrophils ucts C5a, C3a, and C5b67 and various N-formyl peptides 四时含 carbohydrate interaction. During an inflammatory response, tion. Binding of these chemoattractar cytokines and other mediators act upon the local endothe- neutrophil membrane nal mediated ptor. This signal induces n the integrin molecules in the neu-

Neutrophil Extravasation As an inflammatory response develops, various cytokines and other inflammatory mediators act upon the local blood vessels, inducing increased expression of endothelial CAMs. The vascular endothelium is then said to be activated, or inflamed. Neutrophils are generally the first cell type to bind to inflamed endothelium and extravasate into the tissues. To accomplish this, neutrophils must recognize the inflamed endothelium and adhere strongly enough so that they are not swept away by the flowing blood. The bound neutrophils must then penetrate the endothelial layer and migrate into the underlying tissue. Monocytes and eosinophils extravasate by a similar process, but the steps have been best established for the neutrophil, so we focus on neutrophils here. The process of neutrophil extravasation can be divided into four sequential steps: (1) rolling, (2) activation by chemoat￾tractant stimulus, (3) arrest and adhesion, and (4) transendo￾thelial migration (Figure 15-3a). In the first step, neutrophils attach loosely to the endothelium by a low-affinity selectin￾carbohydrate interaction. During an inflammatory response, cytokines and other mediators act upon the local endothe￾lium, inducing expression of adhesion molecules of the selec￾tin family. These E- and P-selectin molecules bind to mucin￾like cell-adhesion molecules on the neutrophil membrane or with a sialylated lactosaminoglycan called sialyl Lewisx (Figure 15-3b). This interaction tethers the neutrophil briefly to the endothelial cell, but the shear force of the circulating blood soon detaches the neutrophil. Selectin molecules on another endothelial cell again tether the neutrophil; this process is repeated so that the neutrophil tumbles end-over-end along the endothelium, a type of binding called rolling. As the neutrophil rolls, it is activated by various chemoat￾tractants; these are either permanent features of the endo￾thelial cell surface or secreted locally by cells involved in the inflammatory response. Among the chemoattractants are members of a large family of chemoattractive cytokines called chemokines. Two chemokines involved in the activation process are interleukin 8 (IL-8) and macrophage inflamma￾tory protein (MIP-1). However, not all chemoattractants belong to the chemokine group. Other chemoattractants are platelet-activating factor (PAF), the complement split prod￾ucts C5a, C3a, and C5b67 and various N-formyl peptides pro￾duced by the breakdown of bacterial proteins during an infec￾tion. Binding of these chemoattractants to receptors on the neutrophil membrane triggers an activating signal mediated by G proteins associated with the receptor. This signal induces a conformational change in the integrin molecules in the neu- 340 PART III Immune Effector Mechanisms S S S S S S S S S S Ig domains Lectin domain Mucin-like CAMs Integrins α β CHO side chains Selectins Ig-superfamily CAMs (a) General structure of CAM families Fibrinonectin-type domains (b) Selected CAMs belonging to each family Mucin-like CAMs: GlyCAM-1 CD34 PSGL-1 MAdCAM-1 Selectins: L-selectin P-selectin E-selectin Ig-superfamily CAMs: ICAM-1, -2, -3 VCAM-1 LFA-2 (CD2) LFA-3 (CD58) MAdCAM-1 Integrins: α4β1 (VLA-4, LPAM-2) α4β7 (LPAM-1) α6β1 (VLA-6) αLβ2 (LFA-1) αMβ2 (Mac-1) αXβ2 (CR4, p150/95) FIGURE 15-2 Schematic diagrams depicting the general structures of the four families of cell-adhesion molecules (a) and a list of repre￾sentative molecules in each family (b). The lectin domain in selectins interacts primarily with carbohydrate (CHO) moieties on mucin-like molecules. Both component chains in integrin molecules contribute to the binding site, which interacts with an Ig domain in CAMs belonging to the Ig superfamily. MAdCAM-1 contains both mucin-like and Ig-like domains and can bind to both selectins and integrins.

Leukocyte Migration and Inflammation CHAPTER 15 341 Activation Arrest/ Transendothelial FIGURE 15-3(a)The four sequential adhesion migration but overlapping steps in neutrophil ex- travasation.(b)Cell-adhesion molecules and chemokines involved in the first three Endothelium ③雪 like CAMs. a chemokine such as IL-8 ther binds to a G-protein-linked receptor on the neutrophil, triggering an activating sig- nal. This signal induces a conformational change in the integrin molecules, enabling em to adhere firmly to Ig-superfamily utrop molecules on the endothelium Chemokine or iter Mucin-Ii E-selectin O Chen 8) Step Step trophil membrane, increasing their affinity for the Ig-super- sation of lymphocytes involves interactions among a number family adhesion molecules on the endothelium. Subsequent of cell-adhesion molecules (Table 15-1). The overall process interaction between integrins and Ig-superfamily CAMs stabi- is similar to what happens during neutrophil extravasation lizes adhesion of the neutrophil to the endothelial cell, enabl- and comprises the same four stages of contact and rolling, ing the cell to adhere firmly to the endothelial cell activation, arrest and adhesion, and, finally, transendothelial Subsequently, the neutrophil migrates through the vessel migr wall into the tissues. The steps in transendothelial migration and how it is directed are still largely unknown; they may be High-Endothelial Venules Are Sites mediated by further activation by chemoattractants and sub- sequent integrin-Ig-superfamily interactions or by a separate of Lymphocyte Extravasation migration stimulus Some regions of vascular endothelium in postcapillary varlous lymphoid organs are composed of special ized cells with a plump, cuboidal ("high") shape; such re Lymphocyte Extravasation gions are called high-endothelial venules, or HEVs(Figure 5-4a, b). Their cells contrast sharply in appearance with the Various subsets of lymphocytes exhibit directed extravasa- flattened endothelial cells that line the rest of the capillary tion at inflammatory sites and secondary lymphoid organs. Each of the secondary lymphoid organ epton The recirculation of lymphocytes thus is carefully controlled of the spleen, contains HEVs. When frozen sections of lymph to ensure that appropriate populations of B and T cells are nodes, Peyers patches, or tonsils are incubated with lympho- recruited into different tissues. As with neutrophils, extrava- cytes and washed to remove unbound cells, over 85%of the Gotowww.whfreeman.com/immunology(animation

trophil membrane, increasing their affinity for the Ig-super￾family adhesion molecules on the endothelium. Subsequent interaction between integrins and Ig-superfamily CAMs stabi￾lizes adhesion of the neutrophil to the endothelial cell, enabl￾ing the cell to adhere firmly to the endothelial cell. Subsequently, the neutrophil migrates through the vessel wall into the tissues. The steps in transendothelial migration and how it is directed are still largely unknown; they may be mediated by further activation by chemoattractants and sub￾sequent integrin–Ig-superfamily interactions or by a separate migration stimulus. Lymphocyte Extravasation Various subsets of lymphocytes exhibit directed extravasa￾tion at inflammatory sites and secondary lymphoid organs. The recirculation of lymphocytes thus is carefully controlled to ensure that appropriate populations of B and T cells are recruited into different tissues. As with neutrophils, extrava￾sation of lymphocytes involves interactions among a number of cell-adhesion molecules (Table 15-1). The overall process is similar to what happens during neutrophil extravasation and comprises the same four stages of contact and rolling, activation, arrest and adhesion, and, finally, transendothelial migration. High-Endothelial Venules Are Sites of Lymphocyte Extravasation Some regions of vascular endothelium in postcapillary venules of various lymphoid organs are composed of special￾ized cells with a plump, cuboidal (“high”) shape; such re￾gions are called high-endothelial venules, or HEVs (Figure 15-4a, b). Their cells contrast sharply in appearance with the flattened endothelial cells that line the rest of the capillary. Each of the secondary lymphoid organs, with the exception of the spleen, contains HEVs. When frozen sections of lymph nodes, Peyer’s patches, or tonsils are incubated with lympho￾cytes and washed to remove unbound cells, over 85% of the Leukocyte Migration and Inflammation CHAPTER 15 341 Endothelium (a) Rolling Activation Arrest/ adhesion Transendothelial migration 1 2 3 4 (b) Step 2 Step 3 Step 1 Neutrophil Integrin Ig-superfamily E-selectin CAM Chemokine or chemoattractant receptor Mucin-like CAM Chemokine (IL-8) SS SS SS SS FIGURE 15-3 (a) The four sequential but overlapping steps in neutrophil ex￾travasation. (b) Cell-adhesion molecules and chemokines involved in the first three steps of neutrophil extravasation. Initial rolling is mediated by binding of E-selectin molecules on the vascular endothelium to sialylated carbohydrate moieties on mucin￾like CAMs. A chemokine such as IL-8 then binds to a G-protein–linked receptor on the neutrophil, triggering an activating sig￾nal. This signal induces a conformational change in the integrin molecules, enabling them to adhere firmly to Ig-superfamily molecules on the endothelium. Go to www.whfreeman.com/immunology Animation Leukocyte Extravasation

2 part I Immune Effector mechanisms TABLE 15-1 Some interactions between cell-adhesion molecules implicated in leukocyte extravasation" Step involving Receptor on cells Expressio endothelium iteraction Main function CLA or ESL-1 Effector t cells E-selectin Tethering/rolling Homing to skin and migration into inflamed tissue All leukocytes GlyCAM-1 Tethering/rolling ymphocyte recirculation CD34 via HEVs to peripheral lymph MAdCAM-1 nodes and migration into inflamed tertiary sites LFA-1(aLB2) Leukocyte CAM1,2,3 Adhesion/arrest General role in lymphocyte extravasation via HEVs and leukocyte migration into LPAM-1(a4B7) Effector t cells MAdCAM-1 Rolling/adhesion Homing of T cells to gut via VCAM-1 mucosal HEV; migration into inflamed tissue Mac-1 (aMB2) Monocytes VCAM-1 Monocyte migration into inflamed tissue PSGL-1 Neutrophils E and Tethering/rolling Neutrophil migration into P-selectin VLA-4(a4B1) Neutrophils VCAM-1 Rolling/adhesion General role in leukocyte T cells MAdCAM-1 migration into inflamed tissue VLA-6(a6B1) T cells Homing of progenitor T cells to thymus: possible role in T-cell homing to nonmucosal sit and leukocyte CAMs belong to four groups of proteins as shown in Figure 15-2. In general, molecules in the integrin family bind to lg- superfamily CAMs, and molecules in the selectin family bind to mucin-like CAMs. Members of the selectin and mucin-like families can be expressed on both leukocytes and ndothelial cells, whereas integrins are expressed only on leukocytes, and Ig.super CAMs are expressed only on endothelium See Figures 15-3a and 15-7for an illustration of steps in the extravasation process bound cells are found adhering to HEVs, even though HEVs like family(GlyCAM-1 and CD34), and the immunoglobulin account for only 1%-2% of the total area of the frozen sec- superfamily (ICAM-1, ICAM-2, ICAM-3, VCAM-1, and tion(Figure 15-4c) MAdCAM-1). Some of these adhesion molecules are distrib It has been estimated that as many as 1.4 X 10" lympho- uted in a tissue-specific manner. These tissue-specific adhe every second through HEVs into a single sion molecules have been called vascular addressins(VAs) cytes extravasate development and maintenance of HEVs in because they serve to direct the extravasation of different lymphoid organs is influenced by cytokines produced in re- populations of recirculating lymphocytes to particular lym- sponse to antigen capture. For example, HEVs fail to develop phoid organs in animals raised in a germ-free environment. The role of ntigenic activation of lymphocytes in the maintenance of Lymphocyte Homing Is Directed HEVs has been demonstrated by surgically blocking the af ferent lymphatic vasculature to a node, so that antigen entry al- by Receptor Profiles and Signals to the node is blocked. Within a short period of time, the The general process of lymphocyte extravasation is similar to HEVs show impaired function and eventually revert to a neutrophil extravasation. An important feature distinguish more flattened morphology ing the two processes is that different subsets of lymphocytes High-endothelial venules express a variety of cell-adhesion migrate differentially into different tissues. This process is molecules. Like other vascular endothelial cells, HEVs express called trafficking, or homing. The different trafficking pat CAMs of the selectin family (E-and P-selectin), the mucin- terns of lymphocyte subsets are mediated by unique combi

bound cells are found adhering to HEVs, even though HEVs account for only 1%–2% of the total area of the frozen sec￾tion (Figure 15-4c). It has been estimated that as many as 1.4  104 lympho￾cytes extravasate every second through HEVs into a single lymph node. The development and maintenance of HEVs in lymphoid organs is influenced by cytokines produced in re￾sponse to antigen capture. For example, HEVs fail to develop in animals raised in a germ-free environment. The role of antigenic activation of lymphocytes in the maintenance of HEVs has been demonstrated by surgically blocking the af￾ferent lymphatic vasculature to a node, so that antigen entry to the node is blocked. Within a short period of time, the HEVs show impaired function and eventually revert to a more flattened morphology. High-endothelial venules express a variety of cell-adhesion molecules. Like other vascular endothelial cells, HEVs express CAMs of the selectin family (E- and P-selectin), the mucin￾like family (GlyCAM-1 and CD34), and the immunoglobulin superfamily (ICAM-1, ICAM-2, ICAM-3, VCAM-1, and MAdCAM-1). Some of these adhesion molecules are distrib￾uted in a tissue-specific manner. These tissue-specific adhe￾sion molecules have been called vascular addressins (VAs) because they serve to direct the extravasation of different populations of recirculating lymphocytes to particular lym￾phoid organs. Lymphocyte Homing Is Directed by Receptor Profiles and Signals The general process of lymphocyte extravasation is similar to neutrophil extravasation. An important feature distinguish￾ing the two processes is that different subsets of lymphocytes migrate differentially into different tissues. This process is called trafficking, or homing. The different trafficking pat￾terns of lymphocyte subsets are mediated by unique combi- 342 PART III Immune Effector Mechanisms TABLE 15-1 Some interactions between cell-adhesion molecules implicated in leukocyte extravasation* Ligands on Step involving Receptor on cells Expression endothelium interaction† Main function CLA or ESL-1 Effector T cells E-selectin Tethering/rolling Homing to skin and migration into inflamed tissue L-selectin All leukocytes GlyCAM-1, Tethering/rolling Lymphocyte recirculation CD34, via HEVs to peripheral lymph MAdCAM-1 nodes and migration into inflamed tertiary sites LFA-1 (L2) Leukocyte ICAM-1, 2, 3 Adhesion/arrest General role in lymphocyte subsets extravasation via HEVs and leukocyte migration into inflamed tissue LPAM-1 (47) Effector T cells, MAdCAM-1, Rolling/adhesion Homing of T cells to gut via monocytes VCAM-1 mucosal HEV; migration into inflamed tissue Mac-1 (M2) Monocytes VCAM-1 — Monocyte migration into inflamed tissue PSGL-1 Neutrophils E- and Tethering/rolling Neutrophil migration into P-selectin inflamed tissue VLA-4 (41) Neutrophils, VCAM-1 Rolling/adhesion General role in leukocyte T cells, MAdCAM-1, migration into inflamed tissue monocytes fibronectin VLA-6 (61) T cells Laminin — Homing of progenitor T cells to thymus; possible role in T-cell homing to nonmucosal sites *Most endothelial and leukocyte CAMs belong to four groups of proteins as shown in Figure 15-2. In general, molecules in the integrin family bind to Ig-superfamily CAMs, and molecules in the selectin family bind to mucin-like CAMs. Members of the selectin and mucin-like families can be expressed on both leukocytes and endothelial cells, whereas integrins are expressed only on leukocytes, and Ig-superfamily CAMs are expressed only on endothelium. † See Figures 15-3a and 15-7 for an illustration of steps in the extravasation process.

Leukocyte Migration and Inflammation CHAPTER 15 343 Basement across FIGURE 15-4(a)Schematic cross-sectional diagram of a lymph node postcapillary venule with high endothelium. Lymphocytes are shown in various stages of attachment to the HEV and in migration across the wall into the cortex of the node. (b)Scanning electron mi 学 crograph showing numerous lymphocytes bound to the surface of a high-endothelial venule (c)Micrograph of frozen sections of lym- phoid tissue. Some 85% of the lymphocytes(darkly stained)are 代个 bound to HEVs(in cross section), which comprise only 1%-2% of the total area of the tissue section. /Part(a) adapted from A O Anderson and N. D. Anderson, 1981, in Cellular Functions in Immi lity and Inflammation, J.. Oppenheim et al.(eds ) Elsevier, North Holland: part(b)from S. D. Rosen and L. M. Stoolman, 1987, Vertebrate Lectins, Van Nostrand Reinhold: part (c) from S. D. Rosen nations of adhesion molecules and chemokines; receptors lymph nodes, Peyer's patches, tonsils, and spleen) that direct the circulation of various populations of lympho- these microenvironments, dendritic cells capture cytes to particular lymphoid and inflammatory tissues are and present it to the naive lymphocyte, resulting in its activa called homing receptors. Researchers have identified a num- tion. Naive cells do not exhibit a preference for a particular ber of lymphocyte and endothelial cell-adhesion molecules type of secondary lymphoid tissue but instead circulate hat participate in the interaction of lymphocytes with HEVs indiscriminately to secondary lymphoid tissue throughout and with endothelium at tertiary sites or sites of inflamma- the body by recognizing adhesion molecules on HEVs tion(see Table 15-1). As is described later, in the section The initial attachment of naive lymphocytes to HEVs is chemokines, these molecules play a major role in determin generally mediated by the binding of the homing receptor ing the heterogeneity of lymphocyte circulation patterns L-selectin to adhesion molecules such as GlyCAM-1 and CD34 on HEVs(Figure 15-5a). The trafficking pattern of Naive Lymphocytes Recirculate naive cells is designed to keep these cells constantly recircu to Secondary Lymphoid Tissue lating through secondary lymphoid tissue, whose primary function is to trap blood-borne or tissue-borne antigen. A naive lymphocyte is not able to mount an immune re- Once naive lymphocytes encounter antigen trapped sponse until it has been activated to become an effector cell. secondary lymphoid tissue, they become activated and en- Activation of a naive cell occurs in specialized microenviron- large into lymphoblasts. Activation takes about 48 h, and ments within secondary lymphoid tissue(e.g, peripheral during this time the blast cells are retained in the paracortical

nations of adhesion molecules and chemokines; receptors that direct the circulation of various populations of lympho￾cytes to particular lymphoid and inflammatory tissues are called homing receptors.Researchers have identified a num￾ber of lymphocyte and endothelial cell-adhesion molecules that participate in the interaction of lymphocytes with HEVs and with endothelium at tertiary sites or sites of inflamma￾tion (see Table 15-1). As is described later, in the section on chemokines, these molecules play a major role in determin￾ing the heterogeneity of lymphocyte circulation patterns. Naive Lymphocytes Recirculate to Secondary Lymphoid Tissue A naive lymphocyte is not able to mount an immune re￾sponse until it has been activated to become an effector cell. Activation of a naive cell occurs in specialized microenviron￾ments within secondary lymphoid tissue (e.g., peripheral lymph nodes, Peyer’s patches, tonsils, and spleen). Within these microenvironments, dendritic cells capture antigen and present it to the naive lymphocyte, resulting in its activa￾tion. Naive cells do not exhibit a preference for a particular type of secondary lymphoid tissue but instead circulate indiscriminately to secondary lymphoid tissue throughout the body by recognizing adhesion molecules on HEVs. The initial attachment of naive lymphocytes to HEVs is generally mediated by the binding of the homing receptor L-selectin to adhesion molecules such as GlyCAM-1 and CD34 on HEVs (Figure 15-5a). The trafficking pattern of naive cells is designed to keep these cells constantly recircu￾lating through secondary lymphoid tissue, whose primary function is to trap blood-borne or tissue-borne antigen. Once naive lymphocytes encounter antigen trapped in a secondary lymphoid tissue, they become activated and en￾large into lymphoblasts. Activation takes about 48 h, and during this time the blast cells are retained in the paracortical Leukocyte Migration and Inflammation CHAPTER 15 343 Lymphocytes passing across the wall Basement membrane High endothelium (a) (b) (c) FIGURE 15-4 (a) Schematic cross-sectional diagram of a lymph￾node postcapillary venule with high endothelium. Lymphocytes are shown in various stages of attachment to the HEV and in migration across the wall into the cortex of the node. (b) Scanning electron mi￾crograph showing numerous lymphocytes bound to the surface of a high-endothelial venule. (c) Micrograph of frozen sections of lym￾phoid tissue. Some 85% of the lymphocytes (darkly stained) are bound to HEVs (in cross section), which comprise only 1%–2% of the total area of the tissue section. [Part (a) adapted from A. O. Anderson and N. D. Anderson, 1981, in Cellular Functions in Immu￾nity and Inflammation, J. J. Oppenheim et al. (eds.), Elsevier, North￾Holland; part (b) from S. D. Rosen and L. M. Stoolman, 1987, Vertebrate Lectins, Van Nostrand Reinhold; part (c) from S. D. Rosen, 1989, Curr. Opin. Cell Biol. 1:913.]

PART Immune Effector mechanisms Naive t cell Mucosal-homing Skin- homing effector t cell effector t cel CLA L-selectin L-selectin ICAM-1( CD34 GlyCAM-1 d LPAM-1 E-selectin ICAM-1 MAdCAM-1 Intestinal lamina propria Skin dermal venule endothelium Teri FIGURE Examples of homing receptors and vascular addres- pressed on HEV cells. (b, c)Various subsets of effector T cells express sins involved in selective trafficking of naive and effector T cells.(a)Naive high levels of particular homing receptors that allow them to home T cells tend to home to secondary lymphoid tissues through their HEv to endothelium in particular tertiary extralymphoid tissues. The initial regions. The initial interaction involves the homing receptor L-selectin interactions in homing of effector T cells to mucosal and skin sites are and mucin -like cell-adhesion molecules such as CD34 or GlyCAM-1 ex- illustrated. region of the secondary lymphoid tissue. During this phase, home to these sites. Inflamed endothelium expresses a num- called the shut-down phase, the antigen-specific lympho- ber of adhesion molecules, including E-and P-selectin and cytes cannot be detected in the circulation(Figure 15-6). the Ig-superfamily molecules VCAM-1 and ICAM-1, that Rapid proliferation and differentiation of naive cells occurs bind to the receptors expressed at high levels on memory and during the shut-down phase. The effector and memory cells effector cells that are generated by this process then leave the lymphoid tis- sue and begin to recirculate Effector and Memory Lymphocytes Adopt Different Trafficking Patterns hut-down phase The trafficking patterns of effector and memory lympho- 2 cytes differ from those of naive lymphocytes Effector cells g tend to home to regions of infection by recognizing inflamed fg vascular endothelium and chemoattractant molecules that o e are generated during the inflammatory response. Memory lymphocytes, on the other hand, home selectively to the type e of tissue in which they first encountered antigen. Presumably this ensures that a particular memory cell will return to the issue where it is most likely to reencounter a subsequent threat by the antigen it recognizes Effector and memory cells express increased levels of cer- Days following antigen exposure tain cell-adhesion molecules. such as lfa-1 that interact with ligands present on tertiary extralymphoid tissue(such FIGURE 15-6 T-cell activation in the paracortical region of a lymph as skin and mucosal epithelia)and at sites of inflammation, node results in the brief loss of lymphocyte recirculation. During this allowing effector and memory cells to enter these sites. Naive shut-down phase, antigen-specific T cells cannot be detected leaving cells lack corresponding cell-adhesion molecules and do not the node in the efferent lymph

region of the secondary lymphoid tissue. During this phase, called the shut-down phase, the antigen-specific lympho￾cytes cannot be detected in the circulation (Figure 15-6). Rapid proliferation and differentiation of naive cells occurs during the shut-down phase. The effector and memory cells that are generated by this process then leave the lymphoid tis￾sue and begin to recirculate. Effector and Memory Lymphocytes Adopt Different Trafficking Patterns The trafficking patterns of effector and memory lympho￾cytes differ from those of naive lymphocytes. Effector cells tend to home to regions of infection by recognizing inflamed vascular endothelium and chemoattractant molecules that are generated during the inflammatory response. Memory lymphocytes, on the other hand, home selectively to the type of tissue in which they first encountered antigen. Presumably this ensures that a particular memory cell will return to the tissue where it is most likely to reencounter a subsequent threat by the antigen it recognizes. Effector and memory cells express increased levels of cer￾tain cell-adhesion molecules, such as LFA-1, that interact with ligands present on tertiary extralymphoid tissue (such as skin and mucosal epithelia) and at sites of inflammation, allowing effector and memory cells to enter these sites. Naive cells lack corresponding cell-adhesion molecules and do not home to these sites. Inflamed endothelium expresses a num￾ber of adhesion molecules, including E- and P-selectin and the Ig-superfamily molecules VCAM-1 and ICAM-1, that bind to the receptors expressed at high levels on memory and effector cells. 344 PART III Immune Effector Mechanisms (a) Naive T cell L-selectin L-selectin CD34 GlyCAM-1 HEV Tertiary extralymphoid tissue LFA-1 LPAM-1 ICAM-1 E-selectin CLA (b) L-selectin LFA-1 ICAM-1 (c) Mucosal-homing effector T cell Skin-homing effector T cell Intestinal lamina propria endothelium Skin dermal venule endothelium MAdCAM-1 SS SS SS SS SS SS SS SS SS SS SS SS SS FIGURE 15-5 Examples of homing receptors and vascular addres￾sins involved in selective trafficking of naive and effector T cells. (a) Naive T cells tend to home to secondary lymphoid tissues through their HEV regions. The initial interaction involves the homing receptor L-selectin and mucin-like cell-adhesion molecules such as CD34 or GlyCAM-1 ex￾pressed on HEV cells. (b, c) Various subsets of effector T cells express high levels of particular homing receptors that allow them to home to endothelium in particular tertiary extralymphoid tissues. The initial interactions in homing of effector T cells to mucosal and skin sites are illustrated. 2468 Days following antigen exposure Shut-down phase Antigen-specific T cells in efferent lymph FIGURE 15- 6 T-cell activation in the paracortical region of a lymph node results in the brief loss of lymphocyte recirculation. During this shut-down phase, antigen-specific T cells cannot be detected leaving the node in the efferent lymph

Leukocyte Migration and Inflammation CHAPTER 15 345 Unlike naive lymphocytes, subsets of the memory and stream. Figure 15-7 depicts the typical interactions in ex- effector populations exhibit tissue-selective homing behavior. travasation of naive T cells across HEVs into lymph no Such tissue specificity is imparted not by a single adhesion The first step is usually a selectin-carbohydrate interaction receptor but by different combinations of adhesion molecules. similar to that seen with neutrophil adhesion Naive lympho- For example, a mucosal homing subset of memory/effector cytes initially bind to HEVs by l-selectin, which serves as a cells has high levels of the integrins LPAM-1( Ba4B7)and homing receptor that directs the lymphocytes to particular LFA-1(aLb2), which bind to MAdCAM and various ICAMs tissues expressing a corresponding mucin-like vascular ad- on intestinal lamina propria venules(see Figure 15-5b)How- dressin such as CD34 or GlyCAM-1 Lymphocyte rolling is ever, these cells avoid direction to secondary lymphoid tissues less pronounced than that of neutrophils. Although the ini because they have low levels of the L-selectin that would facil- tial selectin-carbohydrate interaction is quite weak, the slow itate their entry into secondary lymphoid tissue. A second sub- rate of blood flow in postcapillary venules, particularly in set of memory/effector cells displays preferential homing to regions of HEVs, reduces the likelihood that the shear force the skin. This subset also expresses low levels of L-selectin but of the flowing blood will dislodge the tethered lymphocyte displays high levels of cutaneous lymphocyte antigen(CLA) In the second step, an integrin-activating stimulus is medi and LFA-1, which bind to E-selectin and ICAMs on dermal ated by chemokines that are either localized on the endothelial venules of the skin(see Figure 15-5c). Although effector and surface or secreted locally. The thick glycocalyx covering of the memory cells that express reduced levels of L-selectin do not HEVs may function to retain these soluble chemoattractant tend to home through HEVs into peripheral lymph nodes, factors on the HEvs. If, as some have proposed, HEVs secrete they can enter peripheral lymph nodes through the afferent lymphocyte-specific chemoattractants, it would explain why lymphatic vessels. neutrophils do not extravasate into lymph nodes at the HEvs even though they express L-selectin Chemokine binding to Adhesion-Molecule Interactions play G-protein-coupled receptors on the lymphocyte leads to acti- ritical roles in Extravasation vation of integrin molecules on the membrane as occurs in neutrophil extravasation. Once activated, the integrin mole The extravasation of lymphocytes into secondary lymphoid cules interact with Ig-superfamily adhesion molecules(e.g tissue or regions of inflammation is a multistep process in- ICAM-1), so the lymphocyte adheres firmly to the endothe- volving a cascade of adhesion-molecule interactions similar lium. The molecular mechanisms involved in the final step, to those involved in neutrophil emigration from the blood- transendothelial migration, are poorly understood rolling Activation Arrest/adhesion T cell L-selectin IA1(④ Transendothelial migration Chemokine ICAM-1 CD34 FIGURE 15-7 Steps in extravasation of a naive T cell through a high- cell-adhesion molecules differ. Activation of the integrin LFA-1, induced endothelial venule into a lymph node Extravasation of lymphocytes in- by chemokine binding to the lymphocyte, leads to firm adhesion fol cludes the same basic steps as neutrophil extravasation but some of the lowed by migration between the endothelial cells into the tissue

Unlike naive lymphocytes, subsets of the memory and effector populations exhibit tissue-selective homing behavior. Such tissue specificity is imparted not by a single adhesion receptor but by different combinations of adhesion molecules. For example, a mucosal homing subset of memory/effector cells has high levels of the integrins LPAM-1 (47) and LFA-1 (Lb2), which bind to MAdCAM and various ICAMs on intestinal lamina propria venules (see Figure 15-5b). How￾ever, these cells avoid direction to secondary lymphoid tissues because they have low levels of the L-selectin that would facil￾itate their entry into secondary lymphoid tissue. A second sub￾set of memory/effector cells displays preferential homing to the skin. This subset also expresses low levels of L-selectin but displays high levels of cutaneous lymphocyte antigen (CLA) and LFA-1, which bind to E-selectin and ICAMs on dermal venules of the skin (see Figure 15-5c). Although effector and memory cells that express reduced levels of L-selectin do not tend to home through HEVs into peripheral lymph nodes, they can enter peripheral lymph nodes through the afferent lymphatic vessels. Adhesion-Molecule Interactions Play Critical Roles in Extravasation The extravasation of lymphocytes into secondary lymphoid tissue or regions of inflammation is a multistep process in￾volving a cascade of adhesion-molecule interactions similar to those involved in neutrophil emigration from the blood￾stream. Figure 15-7 depicts the typical interactions in ex￾travasation of naive T cells across HEVs into lymph nodes. The first step is usually a selectin-carbohydrate interaction similar to that seen with neutrophil adhesion. Naive lympho￾cytes initially bind to HEVs by L-selectin, which serves as a homing receptor that directs the lymphocytes to particular tissues expressing a corresponding mucin-like vascular ad￾dressin such as CD34 or GlyCAM-1. Lymphocyte rolling is less pronounced than that of neutrophils. Although the ini￾tial selectin-carbohydrate interaction is quite weak, the slow rate of blood flow in postcapillary venules, particularly in regions of HEVs, reduces the likelihood that the shear force of the flowing blood will dislodge the tethered lymphocyte. In the second step, an integrin-activating stimulus is medi￾ated by chemokines that are either localized on the endothelial surface or secreted locally. The thick glycocalyx covering of the HEVs may function to retain these soluble chemoattractant factors on the HEVs. If, as some have proposed, HEVs secrete lymphocyte-specific chemoattractants, it would explain why neutrophils do not extravasate into lymph nodes at the HEVs even though they express L-selectin. Chemokine binding to G-protein–coupled receptors on the lymphocyte leads to acti￾vation of integrin molecules on the membrane, as occurs in neutrophil extravasation. Once activated, the integrin mole￾cules interact with Ig-superfamily adhesion molecules (e.g., ICAM-1), so the lymphocyte adheres firmly to the endothe￾lium. The molecular mechanisms involved in the final step, transendothelial migration, are poorly understood. Leukocyte Migration and Inflammation CHAPTER 15 345 L-selectin Naive T cell Chemokine LFA-1 ICAM-1 CD34 HEV Rolling 1 Activation 2 Arrest/adhesion 3 Transendothelial migration 4 FIGURE 15-7 Steps in extravasation of a naive T cell through a high￾endothelial venule into a lymph node. Extravasation of lymphocytes in￾cludes the same basic steps as neutrophil extravasation but some of the cell-adhesion molecules differ. Activation of the integrin LFA-1, induced by chemokine binding to the lymphocyte, leads to firm adhesion fol￾lowed by migration between the endothelial cells into the tissue.

PaRt I Immune Effector Mechanisms tion between chemokines and their receptors is of high affin Chemokines-Key Mediators ity(K>10")and high specificity. However, as Table 15-2 of Inflammation shows, most receptors bind more than one chemokine. For example, CXCR2 recognizes at least six different chemokines, Chemokines are a superfamily of small polypeptides, most of and many chemokines can bind to more than one receptor. which contain 90-130 amino acid residues. They selectively. When a receptor binds an appropriate chemokine, it acti and often specifically, control the adhesion, chemotaxis, and vates heterotrimeric large G proteins, initiating a signal activation of many types of leukocyte populations and sub- transduction process that generate such potent second populations. Consequently, they are major regulators of leu- messengers as CAMP, IP3, Ca, and activated small G pro- kocyte traffic. Some chemokines are primarily involved in inflammatory processes, others are constitutively expressed and play important homeostatic or developmental roles Housekeeping chemokines are produced in lymphoid gans and tissues or in non-lymphoid sites such as skin, where TABLE 15-2 Human chemokines and they direct normal trafficking of lymphocytes, such as deter mining the correct positioning of leukocytes newly generated Chemokine receptors Chemokines bound by receptor by hematopoiesis and arriving from bone marrow. The thy mus constitutively expresses chemokines, and normal B cell CXC SUBGROUP lymphopoiesis is also dependent on appropriate chemokine expression. Chemokine-mediated effects are not limited CXCR1 IL-8 GCP-2 the immune system. Mice that lack either the chemokine IL-8, Gro-a, Gro-B, Gro-y CXCL12 (also called SDF-1)or its receptor(see Table 15-2) NAP-2 ENA-78 show major defects in the development of the brain and the CXCR3 IP-10, Mig, I-TAC heart. Members of the chemokine family have also been CXCR4 SDF-1. PBSE shown to play regulatory roles in the development of blood CXCR5 BCA-1 vessels(angiogenesis), and wound healing CC SUBGROUP The inflammatory chemokines are typically induced in response to infection. Contact with pathogens or the action of ccR1 MIP-1, RANTES, MCP-2, MIP-5 proinflammatory cytokines, such as TNF-oc, up-regulate the CCR2 expression of inflammatory cytokines at sites of developing CCR3 Eotaxin, RANTES MCP inflammation. Chemokines cause leukocytes to move into MCP-3 MCP-4 Eotaxin-2 various tissue sites by inducing the adherence of these cells to MIP-5 the vascular endothelium After migrating into tissues, leuko- CCR4 TARC, RANTES cytes are attracted toward high localized concentrations of MIP-1a RANTES, MIP-1B chemokines resulting in the targeted recruitment of phago- Exodus-1 cytes and effector lymphocyte populations to inflammatory ccr8 1309 sites. The assembly of leukocytes at sites of infection, orches- CCR10 MCP-1, MCP-2, MCP-3, RANTES trated by chemokines, is an essential part of mounting an appropriately focused response to infection. BOTH CC AND CXC SUBGROUPS More than 50 chemokines and at least 15 chemokine re- ceptors have been described (Table 15-2).The chemokines DARC (the Duffy Binds to a number of cc four conserved cysteine residues and based on the antigen of RBCs) and cxc chemokines position of two of the four invariant cysteine residues, almost all fall into one or the other of two distinctive subgroups .This table lists most known che d in the table are C-C subgroup chemokines, in which the conserved as follows: ELC(Ebll ligand chemokine): ENA-78(epitheliaI-cell-derived cysteines are contiguous ca, B, y(growth-related oncogene a, B, 1): MCP. C-X-C subgroup chemokines, in which the conserved chemoattractant protein 1, 2, 3, or 4): Mig(monokine induced by interferon atory protein 1a, 1B, or 5) cysteines are separated by some other amino acid (X) NAP-2(netrophil-activating protein 2): RANTES (regulated upon activation, Chemokine action is mediated by receptors whose poly- normal T-cell expressed and secreted): TARC (thymus.and a peptide chain traverses the membrane seven times. There are two subgroups of receptors, CC receptors(CCRs ) which rec- SOURCE: Adapted from Nelson and Krensky, 1998, CurTOpin Immunol. gnize CC chemokines, and CXC receptors(CXCRs), which 10:265, and Baggiolini, 1998, Nature 392.565 recognize CXC chemokines. as with cytokines, the interac-

Chemokines—Key Mediators of Inflammation Chemokines are a superfamily of small polypeptides, most of which contain 90–130 amino acid residues. They selectively, and often specifically, control the adhesion, chemotaxis, and activation of many types of leukocyte populations and sub￾populations. Consequently, they are major regulators of leu￾kocyte traffic. Some chemokines are primarily involved in inflammatory processes, others are constitutively expressed and play important homeostatic or developmental roles. “Housekeeping” chemokines are produced in lymphoid or￾gans and tissues or in non-lymphoid sites such as skin, where they direct normal trafficking of lymphocytes, such as deter￾mining the correct positioning of leukocytes newly generated by hematopoiesis and arriving from bone marrow. The thy￾mus constitutively expresses chemokines, and normal B cell lymphopoiesis is also dependent on appropriate chemokine expression. Chemokine-mediated effects are not limited to the immune system. Mice that lack either the chemokine CXCL12 (also called SDF-1) or its receptor (see Table 15-2) show major defects in the development of the brain and the heart. Members of the chemokine family have also been shown to play regulatory roles in the development of blood vessels (angiogenesis), and wound healing. The inflammatory chemokines are typically induced in response to infection. Contact with pathogens or the action of proinflammatory cytokines, such as TNF-, up-regulate the expression of inflammatory cytokines at sites of developing inflammation. Chemokines cause leukocytes to move into various tissue sites by inducing the adherence of these cells to the vascular endothelium. After migrating into tissues, leuko￾cytes are attracted toward high localized concentrations of chemokines resulting in the targeted recruitment of phago￾cytes and effector lymphocyte populations to inflammatory sites. The assembly of leukocytes at sites of infection, orches￾trated by chemokines, is an essential part of mounting an appropriately focused response to infection. More than 50 chemokines and at least 15 chemokine re￾ceptors have been described (Table 15-2).The chemokines possess four conserved cysteine residues and based on the position of two of the four invariant cysteine residues, almost all fall into one or the other of two distinctive subgroups: ■ C-C subgroup chemokines, in which the conserved cysteines are contiguous; ■ C-X-C subgroup chemokines, in which the conserved cysteines are separated by some other amino acid (X). Chemokine action is mediated by receptors whose poly￾peptide chain traverses the membrane seven times. There are two subgroups of receptors, CC receptors (CCRs), which rec￾ognize CC chemokines, and CXC receptors (CXCRs), which recognize CXC chemokines. As with cytokines, the interac￾tion between chemokines and their receptors is of high affin￾ity (Ka > 109 ) and high specificity. However, as Table 15-2 shows, most receptors bind more than one chemokine. For example, CXCR2 recognizes at least six different chemokines, and many chemokines can bind to more than one receptor. When a receptor binds an appropriate chemokine, it acti￾vates heterotrimeric large G proteins, initiating a signal￾transduction process that generate such potent second messengers as cAMP, IP3, Ca2+, and activated small G pro- 346 PART III Immune Effector Mechanisms TABLE 15-2 Human chemokines and their receptors* Chemokine receptors Chemokines bound by receptor CXC SUBGROUP CXCR1 IL-8, GCP-2 CXCR2 IL-8, Gro-, Gro-, Gro-, NAP-2, ENA-78 CXCR3 IP-10, Mig, I-TAC CXCR4 SDF-1, PBSF CXCR5 BCA-1 CC SUBGROUP CCR1 MIP-1, RANTES, MCP-2, MIP-5 CCR2 MCP-1, MCP-2, MCP-3 CCR3 Eotaxin, RANTES, MCP-2, MCP-3, MCP-4, Eotaxin-2, MIP-5 CCR4 TARC, RANTES CCR5 MIP-1 RANTES, MIP-1 CCR6 Exodus-1 CCR7 ELC CCR8 1-309 CCR10 MCP-1, MCP-2, MCP-3, RANTES BOTH CC AND CXC SUBGROUPS DARC (the Duffy Binds to a number of CC antigen of RBCs) and CXC chemokines *This table lists most known chemokine receptors but not all chemokines. The full names for a number of the chemokines abbreviated in the table are as follows: ELC (Ebl1 ligand chemokine); ENA-78 (epithelial-cell-derived neutrophil-activating protein); GCP-2 (granulocyte chemotactic protein 2); Gro-, ,  (growth-related oncogene , , ); MCP-1, 2, 3, or 4 (monocyte chemoattractant protein 1, 2, 3, or 4); Mig (monokine induced by interferon ); MIP-1, 1, or 5 (macrophage inflammatory protein 1, 1, or 5); NAP-2 (netrophil-activating protein 2); RANTES (regulated upon activation, normal T-cell expresssed and secreted); TARC (thymus- and activation￾regulated chemokine.) SOURCE: Adapted from Nelson and Krensky, 1998, Curr. Opin. Immunol. 10:265, and Baggiolini, 1998, Nature 392:565.

Leukocyte Migration and Inflammation CHAPTER 15 347 Chemokine receptor FIGURE Chemokines signal through re Ca2+ ceptors coupled with heterotrimeric large G pro- channels teins. Binding of a chemokine to its receptor activates many signal-transduction pathways, re- sulting in a variety of modifications in the physic Adenylyl protein ogy of the target cell. If the signal-transduction PLCB pathway is not known or incompletely worked out, dashed lines and question marks are used DAG here to represent probable pathways. /Adapted from premack et al 1996. Nature Medicine PKC 2:174 CAMP Actin polymerizat Adhesion Cytoskeletal Differentiation Chemotaxis teins(Figure 15-8). Dramatic changes are effected by the Chemokine-Receptor Profiles Mediate chemokine-initiated activation of these signal transduction Leukocyte Activity pathways. Within seconds, the addition of an appropriate chemokine to leukocytes causes abrupt and extensive changes Among major populations of human leukocytes, neutrophils in shape, the promotion of greater adhesiveness to endothe- express CXCR1, 2, and-4: eosinophils have CCRl and CCR3 lial walls by activation of leukocyte integrins, and the gener-(Figure 15-9). While resting naive t cells display few types of ation of microbicidal oxygen radicals in phagocytes. These chemokine receptors, some activated T cells have CCrl, -2 ignal-transduction pathways promote other changes such 3, and-5, CXCR3 and-4, and possibly others. Clearly, a the release of granular contents, proteases in neutrophils and cell can respond to a chemokine only if it possesses a receptor macrophages, histamine from basophils, and cytotoxic pro- that recognizes it. Consequently, differences in the expression teins from eosinophils of chemokine receptors by leukocytes coupled with the N CCRI NU CXRI IL-2 Activation HNCCR2 / CXR2 NUL CCR3 nU CXR3 NL CcR4 U CXRA Activated Tlymph URE 15-9 Patterns of expression of some principal chemokine est variety of chemokine receptors has been observed on activated tors on different classes of human leukocytes. So far the great- T lymphocytes. /Adapted from M. Baggiolin, 1998, Nature 392: 565

teins (Figure 15-8). Dramatic changes are effected by the chemokine-initiated activation of these signal transduction pathways. Within seconds, the addition of an appropriate chemokine to leukocytes causes abrupt and extensive changes in shape, the promotion of greater adhesiveness to endothe￾lial walls by activation of leukocyte integrins, and the gener￾ation of microbicidal oxygen radicals in phagocytes. These signal-transduction pathways promote other changes such as the release of granular contents, proteases in neutrophils and macrophages, histamine from basophils, and cytotoxic pro￾teins from eosinophils. Chemokine-Receptor Profiles Mediate Leukocyte Activity Among major populations of human leukocytes, neutrophils express CXCR1, -2, and -4; eosinophils have CCR1 and CCR3 (Figure 15-9). While resting naive T cells display few types of chemokine receptors, some activated T cells have CCR1, -2, -3, and -5, CXCR3 and -4, and possibly others. Clearly, a cell can respond to a chemokine only if it possesses a receptor that recognizes it. Consequently, differences in the expression of chemokine receptors by leukocytes coupled with the Leukocyte Migration and Inflammation CHAPTER 15 347 α β γ Differentiation, proliferation Cytoskeletal rearrangement Adhesion Chemotaxis ? Ras Ca2+ channels Chemokine receptor ? ? cAMP Adenylyl cyclase G protein PLCβ2 IP3 DAG Ca2+ PKC Actin polymerization Neutrophil Basophil Activation T lymphocyte Resting Activated IL-2 CCR1 Eosinophil Monocyte CCR2 CCR3 CCR4 CXR1 CXR2 CXR3 CXR4 FIGURE 15-8 Chemokines signal through re￾ceptors coupled with heterotrimeric large G pro￾teins. Binding of a chemokine to its receptor activates many signal-transduction pathways, re￾sulting in a variety of modifications in the physiol￾ogy of the target cell. If the signal-transduction pathway is not known or incompletely worked out, dashed lines and question marks are used here to represent probable pathways. [Adapted from Premack et al., 1996, Nature Medicine 2:1174.] FIGURE 15-9 Patterns of expression of some principal chemokine receptors on different classes of human leukocytes. So far the great￾est variety of chemokine receptors has been observed on activated T lymphocytes. [Adapted from M. Baggiolini, 1998, Nature 392:565.]