Chemokines in tissue-specific and microenvironment-specific lymphocyte homing

Chemokines in tissue-specific and microenvironment-specific lymphocyte homing James J Campbell* and Eugene C Butcher† This review describes recent breakthroughs in our understanding of the roles played by chemokines in lymphocyte trafficking. These include the first demonstration that chemokines control lymphocyte/vascular recognition by shear-resistant rapid adhesion; the first example of specialized tissue-specific homing mediated by chemokines; and the implication that chemokines may control microenvironmental segregation within lymphoid organs. Addresses *Department of Pathology, Harvard University School of Medicine, Children's Hospital, 300 Longwood Avenue, Boston, MA 02115, USA; e-mail: [email protected] *† Laboratory of Immunology and Vascular Biology, Department of Pathology, Stanford University School of Medicine, Lane Building, L-235, Stanford, CA 94305-5324, USA; and the Center for Molecular Biology and Medicine, Veterans Affairs Medical Center, Building 101, Room C4-101, Mail Code 154B, 3801 Miranda Avenue, Palo Alto, CA 94304, USA † e-mail: [email protected] Current Opinion in Immunology 2000, 12:336–341 0952-7915/00/$ — see front matter © 2000 Elsevier Science Ltd. All rights reserved. Abbreviation HEV high endothelial venule

Introduction Chemokines were first recognized as a family of small protein molecules, induced by inflammation and capable of attracting inflammatory leukocytes (such as monocytes, activated T cells and neutrophils) [1]. More recent evidence, however, implicates them as key players coordinating lymphocyte traffic throughout the body during routine immunosurveillance and orchestrating movements through complex microenvironments during lymphocyte development and differentiation. The end result of these intricate homing processes is to promote proper cell positioning and cell–cell interactions, helping towards a regulated immune response. Here we focus on recent studies that illustrate the participation of chemokines in tissue-selective lymphocyte homing, a process central to regional immune responses; we also briefly discuss some emerging insights concerning chemokine receptor expression in the thymus.

Chemokines in lymphocyte/endothelial-cell recognition Chemokines and other ligands of Gαi-coupled receptors have been long known for their ability to mediate leukocyte chemotaxis and diapedesis. However, chemokines have a separate but equally important role in the complex process by which cells traveling at high speed under the shear forces of the circulation ultimately enter a tissue. When blood lymphocytes initiate the homing process by

tethering and rolling on endothelium of high endothelial venules (HEVs) [2], they do not come to a stop unless their cell-surface β2 integrins (e.g. LFA-1) are triggered into their high-affinity conformation by signaling through a Gαi-coupled receptor [3–5]. Chemokines have recently been shown capable of mediating this process on peripheral blood lymphocytes and lymph node cells [6••,7]. Some chemokines — such as SDF-1α, SLC (Exodus2/6Ckine/TCA4) and MIP-3β (ELC/Exodus-3) — are capable of triggering adhesion of most peripheral blood lymphocyte subpopulations. Other chemokines trigger adhesion of only particular specialized subsets. For example, MIP-3α (LARC, Exodus) triggers adhesion of a memory CD4+ T subset but has no effect on naive T cells [6••]; this finding raises the interesting possibility that triggering of adhesion under shear may serve as a ‘gatekeeper’ control point, allowing entry of only particular lymphoid subsets into a given tissue. Several examples in the following sections support this hypothesis. It is worth noting that adhesion-triggering apparently requires occupancy of a threshold number of chemokine receptor molecules per cell that is much higher than that necessary for mediating chemotaxis [8]; thus detectable receptor expression may not ipso facto imply adhesion-triggering ability [9••].

CCR7 as a homing receptor for secondary lymphoid organs CCR7 (EBI-1/BLR-2) was first implicated as a potential mediator of adhesion triggering for cells homing through HEVs when it was discovered to be the receptor for SLC [10••,11], a chemokine expressed at the mRNA level within HEVs of lymph nodes and Peyer’s patches [12••]. SLC triggers integrin-dependent adhesion of most peripheral blood lymphocytes under shear in vitro [6••,7] and is displayed to passing lymphocytes at the endothelial surface [13••,14••]. CCR7 and rapid adhesion of T cells

The importance of SLC in T cell homing to secondary lymphoid organs is supported by the relative absence of T cells in the lymph nodes and Peyer’s patches of plt mice [15], a spontaneous mutant strain deficient in the SLC gene expressed in high endothelium [16•,17]. In vivo staining with monoclonal antibody to SLC confirms that (unlike normal mice) SLC protein is not present on HEVs of plt mice [13••,14••]. Indeed, normal T cells injected into plt mice (and observed by intravital fluorescent microscopy) initially interact with HEVs by rolling at normal rates, but adhesion is not triggered and T cells invariably roll along the HEVs without leaving the bloodstream [13••,14••]. Further, normal T cells roll but fail to arrest on normal HEVs in vivo when CCR7 is desensitized by exposure to high doses of CCR7 ligand. The behavior of T cells in both

Figure 1 Site of interaction

Cell type

Interactions * ~4000 microns/sec midline velocity Seconds

Minutes

~40 microns/sec

L-selectin/CLA/α4β7 SLC/TARK/TECK G-protein CCR4/CCR7/CCR9 LFA-1 (low affinity) LFA-1 (high affinity) ICAM-1

Blood * * ***

HEV

(a) Initiation of contact through microvillous receptors

C (b) Rolling

(c) Activation through G-proteinlinked receptors

(d) Activationdependent arrest (reversible over minutes)

(e) Diapedesis

~10 minutes

(f) Movement among microenvironments within the organ

Naive T cells

L-selectin–PNAd

CCR7–SLC

LFA-1–ICAM-1

CCR7–MIP-3β?

CXCR5–BLC CCR7–MIP-3β

Naive B cells

L-selectin–PNAd

?–?

LFA-1–ICAM-1

CCR7–MIP-3β?

CXCR5–BLC

Inflamed skin

Skin-memory T cells

CLA–E-selectin α4β1–VCAM-1?

CCR4–TARC

LFA-1–ICAM-1 α4β1–VCAM-1?

CCR?–CTACK

CCR?–CTACK

Small intestinal lamina propria

Gut-memory T cells

α4β7–MAdCAM-1

CCR9–TECK?

LFA-1–ICAM-1 α4β7–MAdCAM-1?

CCR9–TECK?

CCR9–TECK?

Peripheral lymph node

Current Opinion in Immunology

Proposed roles for adhesion molecules and chemokine–receptor pairs in tissue-specific homing. The speed and timescale of lymphocytes traversing HEVs from the blood are shown, as are the stages (a–f) of this process. Chemokine–ligand interactions implicated for various cell populations in different tissues are indicated.

cases is similar to that of pertussis-toxin-treated T cells interacting with normal HEVs [4,5]. Additionally, T cells homing in vivo in normal mice adhere almost exclusively to HEV sites presenting SLC protein [13••]. Interestingly, when SLC is injected subcutaneously into plt mice, afferent lymph apparently carries it into the draining node, where it is ultimately presented on the luminal surface of HEVs. In such reconstituted plt mice, the ability of T cells to adhere to HEVs and enter the organ is restored [14••]. Another CCR7 ligand, MIP-3β, is produced by the cells within the T zones of plt mice, albeit at lower levels than normal mice. However, MIP-3β is apparently not transported to the HEVs in sufficient quantities for presentation to passing T cells (or at all). Thus, although SLC and MIP-3β both act through CCR7, SLC may have a unique ability to be presented under conditions that allow rapid adhesion of T cells to HEVs. These findings may reflect selectivity in the mechanisms of chemokine delivery from lymph to HEVs (via the reticular network [18] and/or in vascular transport and presentation at the endothelial cell lumen, as described in [19]). Clearly, however, the CCR7–SLC interaction is essential for triggering of T lymphocyte adhesion to HEVs in lymphoid organs, which in turn is necessary for entry into the organs (see Figure 1).

SLC and CCR7 not necessary for rapid adhesion of B cells

Although secondary lymphoid organs of plt mice contain greatly reduced numbers of T cells, B cells are present in relatively normal quantities [15]. This suggests that B cells are not as dependent as T cells on SLC for entry through HEVs. Indeed, when normal B cells are injected into plt mice, they adhere to HEVs at relatively normal rates and in a pertussis-toxin-inhibitable manner [13••]. Moreover, although B cells are able to migrate well to CCR7 ligands in vitro, desensitization of CCR7 does not inhibit rapid adhesion of B cells to HEVs [13••]. Furthermore B cells in normal mice adhere to regions of the HEVs devoid of SLC protein, where co-injected T cells are unable to adhere [13••]. Interestingly, the SLC– HEVs to which B cells stick are in proximity to B cell zones of the lymphoid organs. These HEV regions are upstream of the SLC+ sites, so most B cells adhere before they have the opportunity to interact with SLC+ vessels. Thus, the rapid adhesion-triggering step in lymphoid HEVs is apparently different for B cells and T cells and may initiate the segregation of cells into B- and T-zones. Chemokines responsible for rapid adhesion of B cells to HEVs remain to be identified. In contrast to the selective deficiency of T cell homing (compared with B cell homing) in SLC-deficient plt mice, targeted mutant mice deficient in the SLC and MIP-3β receptor (CCR7) display

a paucity of both T and B cells in the secondary lymphoid organs [20•]. Since CCR7 is not necessary for B cell adhesion-triggering, it may be important for allowing B cells to enter the organ via diapedesis after adherence is established (perhaps via MIP-3β in this step). Resolution of this issue awaits performance of real-time in vivo homing of CCR7– cells in HEVs of normal mice (see Figure 1). CCR7–CD4+ T cells from peripheral blood

A peripheral blood CD4+ lymphocyte subset, termed ‘effector memory’, has been identified and displays rapid cytokine production upon TCR stimulation. Although most peripheral blood lymphocytes express CCR7 and L-selectin — molecules allowing their trafficking through lymphoid tissues — this small subset is enriched in cells lacking CCR7 [21]. Effector memory cells may be unable to home to lymph nodes or Peyer’s patches (due to their lack of CCR7) but may be targeted preferentially to nonlymphoid tissues and sites of inflammation [21,22]. However it is clear that the vast majority of skin-homing and intestinal-homing memory cells (identified by expression of tissue-specific adhesion molecules, vide infra) — including many capable of rapid cytokine expression — are quite responsive to CCR7 ligands [9••], so that ‘effector’ CD4+ cells are probably heterogeneous in their ability to recirculate through lymphoid organs.

MAdCAM-1 by endothelial cells allows intestinal lymphocytes to home into intestinal tissue via α4β7 integrin). Thus, the high expression of CCR4 on skin-homing (but not guthoming) lymphocytes and the presence of TARC on skin (but not intestinal) endothelium presents a compelling picture of the TARC–CCR4 interaction as an important decision point in cutaneous homing compared with intestinal homing. The role of CCR4 in homing to other systemic (nonintestinal) sites remains to be determined. TARC is probably most important in the rapid adhesion step, because of its presentation by endothelial cells. Indeed, TARC triggers rapid adhesion (to ICAM-1) of CLA+ T cells rolling on E-selectin in vitro (E-selectin is expressed by endothelial cells within inflamed cutaneous tissues) [9••]. In contrast to TARC, a novel chemokine called CTACK [27••] (or ALP [28]) — which also preferentially attracts the skin-homing subset of peripheral T cells [27••] — is expressed preferentially by keratinocytes within the skin. Thus, TARC may work sequentially with CTACK: TARC inducing adhesion of passing cutaneous T cells under shear and CTACK subsequently attracting the adherent cells into the tissue. Confirmation of this hypothesis awaits testing in animal models. CCR9, TECK and homing to intestinal sites

Homing of memory lymphocytes to nonlymphoid tissues Although trafficking of naive T lymphocytes is largely restricted to secondary lymphoid organs, memory T cells are found at low levels in every tissue of the body and are preferentially recruited to extralymphoid (‘tertiary’ lymphoid) sites of inflammation. Moreover, memory cells display striking tissue selectivity of homing (reviewed in [23•,24]). The best-studied lymphocyte populations targeted to specific nonlymphoid tissues are the cutaneous or skin-homing population — defined by its expression of CLA — and the intestinal or gut-homing population, which expresses high levels of α4β7 integrin. Not only do these populations home preferentially to these tissues but also immunity to cutaneous and intestinal antigens, respectively, resides within them [25,26]. Recent studies now suggest that chemokines can play essential roles in tissue specific targeting as well (see Figure 1). CCR4, TARC and CTACK in homing to cutaneous sites compared with intestinal sites

CCR4 is expressed at very high levels by CLA+ cutaneous memory T cells (and by a subset of those memory CD4+ cells lacking both cutaneous and intestinal markers [9••]) but is rare on gut-homing cells (among which the few expressors have ~10-fold lower expression than cutaneous cells). The CCR4 ligands, TARC and MDC (STCP-1), attract these memory subsets with efficiencies consistent with their levels of CCR4 expression [9••]. Further, TARC protein is present on endothelial cells of venules within inflamed and (at lower levels) normal skin whereas it is not on MAdCAM-1-expressing venules of intestinal lamina propria (expression of

The chemokine TECK attracts a subset of intestinal (α4β7hi) memory CD4+ cells but not cutaneous or other systemic memory CD4+ cells [29••]. The receptor for TECK, CCR9 (GPR-96) [29••,30–34], is expressed on a subset of intestinal but not cutaneous or other systemic memory CD4+ cells [29••]. (It is worth noting that another chemokine receptor was previously named CCR9 [35,36] but this name was withdrawn when the receptor was found incapable of transducing a signal.) TECK is expressed at the mRNA level by small intestine [37] and by intestinal epithelial cells [29••,33] but it is not yet known where the protein is presented. Thus it is unclear whether TECK may be involved in the rapid adhesion step of lymphocyte homing to gut, in the subsequent diapedesis steps, or both (see Figure 1).

Chemokines and the homing of Th1/Th2 cells An effort has been made over the past few years to associate expression of particular chemokine receptors with the cytokine secretion phenotype of particular T cell subsets (reviewed in [38•]) and such associations are apparent between cultured Th1 and Th2 cell lines generated under prescribed conditions in vitro. However it now appears that Th1- and Th2-associated chemokine receptor expression patterns from cultured/polarized cells are not necessarily maintained under physiological conditions in vivo [39]. Thus, specific physiologic chemoattractant receptors that define Th1 and Th2 cells remain elusive.

Homing within lymphoid tissues Chemokines and the organization of lymph nodes

The observation of disorganized lymphoid organs in CXCR5or CCR7-deficient animals [20•,40] strongly supports the

Figure 2 Chemokine responses and/or receptor expression at critical milestones in thymic development of CD4+ T cells associated with changes in anatomical location. The + and – signs indicate the presence (and extent) or absence of the activity or receptor expression.

Successful rearrangement of TCRβ chain

Developmental event

Survival of negative selection

Positive selection

Outer cortex

Inner cortex

Medulla

Periphery

CCR9 expression and/or TECK responsiveness

−

+

+

−

CCR7 expression and/or MIP-3β/SLC responsiveness

−

−

+

++

CCR4 expression and/or TARC/MDC responsiveness

−

−

+

−

CXCR4 expression and/or SDF-1α responsiveness

+

+

+

+

Histological location

Current Opinion in Immunology

notion that chemokines are instrumental in development and maintenance of the strictly segregated microenvironments characteristic of secondary lymphoid organs and in the movement of lymphocytes in and out of these areas. One example, a type of in-vitro-polarized T cell that normally homes to the outer PALS (periarteriolar lymphoid sheath) near the B cells zones of spleen, clusters instead within the central PALS (where stromal cells express the CCR7 ligand MIP-3β) after forced expression of CCR7 [41]. Although it is now clear that CCR7 and CXCR5 may be involved in organization of T and B cells zones, there is still much to be learned about the roles chemokines play in the maintenance of these complex organs. It is clear for example that CXCR5 expression is required for B cell follicular homing in some (but not other) organs but that CXCR5 alone is insufficient to target them to B cell follicles, even though follicles constitutively express the CXCR5 ligand BLC. The recent findings of this field have been extensively reviewed elsewhere [42••,43••]. Combinatorial control of lymphocyte chemotaxis

Chemokines probably act in an overlapping, sequential fashion to control lymphocyte navigation. Recent studies in model systems show that leukocytes can integrate chemotactic signals from competing attractant sources and that they can navigate in a step-by-step fashion through chemoattractant arrays [44]. Such multistep navigation provides an explanation for the multiplicity of chemoattractant receptors each lymphocyte uses in its migration in vivo. Moreover, since chemotactic fields must overlap significantly and in some instances more than one receptor–ligand pair may participate at any given step in the sequence, the model may explain how receptor promiscuity (with significant receptor

redundancy) and specificity are combined in physiologic lymphocyte targeting in vivo. Chemokines and thymic subsets

Like secondary lymphoid organs, the thymus also has multiple microenvironments occupied by cells of different developmental stages (reviewed in [45]). As several chemokines are expressed in the thymus at high levels, it is hypothesized that they may play a role in compartmental organization. The best-recognized thymic migrations are, firstly, the movement of positively selected cells from the cortex to the medulla and, secondly, the movement of cells surviving negative selection out of the thymus and into the circulating pool. Maturation of thymocytes through various stages of development is associated with dramatic changes in responsiveness to chemokines [46••,47] (see Figure 2). Expression of CCR9 first appears on thymocytes after successful rearrangement of a TCRβ chain, which precipitates the conversion of CD4–CD8– (double-negative) cells into double-positive cells [32]. (induction of CCR9 expression can be mimicked in Rag-2-deficient mice by cross-linking of CD3ε [32].) Responsiveness to TECK is maintained until the cells reach the most mature medullary stage of CD4+CD8– single-positive development, just before release into the periphery [46••]. Thus association with thymic emigration would be consistent with TECK as a retention factor, not allowing cells to leave the organ until loss of TECK responsiveness. Responsiveness to the CCR7 ligands, MIP-3β and SLC, first appears at the earliest stages of positive selection and

continues to increase until release from the thymus [46••,47]. CCR7 expression first appears at the mRNA level after positive selection [46••]. This pattern of upregulation is consistent with a role in movement of positively selected thymocytes into the medulla or may simply be a step in the maturation process, as CCR7 will be needed for homing into secondary lymphoid organs after release. Responsiveness to CCR4 ligands, MDC and TARC, exists only during a brief window of CD4 T cell thymic development — between the late cortical and early medullary stages [46••]. This is consistent with a role in migration of positively selected cells from the cortex to the medulla (like the CCR7 ligands), as well as a potential role in retention of CD4+ single-positive cells until maturation (like TECK). CCR4, like CCR7, first appears at the mRNA level in the earliest stage following thymic selection [46••]. The dramatic changes in chemokine responsiveness and receptor expression at critical stages of thymic development are intriguing and suggestive of a role for chemokines in the maturation process; however direct testing of the roles of chemokines in microenvironmental movements within the thymus is required.

Conclusions Since 1998, there have been three major conceptual advances in our view of chemokines and the immune system: firstly, the first direct experimental evidence demonstrating a role for chemokines in vascular recognition at the level of integrin-dependent adhesion triggering and arrest; secondly, the first examples of a role for chemokines in specialized tissue-specific lymphocyte homing and thus in defining and segregation of regional immune responses; and, thirdly, implication of chemokines in the microenvironmental segregation of lymphocyte subsets within primary and secondary lymphoid organs. These recent findings are probably only the tip of the iceberg, as the chemokine gene family continues to grow and the roles of these chemokines continue to be elucidated. The pervasiveness of chemokines in all of these immunological events suggests that chemokines and their receptors will be fruitful targets for discovery of specific immunomodulatory drugs and treatments in the future.

References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as: • of special interest ••of outstanding interest 1.

Murphy PM: The molecular biology of leukocyte chemoattractant receptors. Annu Rev Immunol 1994, 12:593-633.

2.

Butcher EC, Picker LJ: Lymphocyte homing and homeostasis. Science 1996, 272:60-66.

3.

Bargatze RF, Jutila MA, Butcher EC: Distinct roles of L-selectin and integrins α4β7 and LFA-1 in lymphocyte homing to Peyer’s patchHEV in situ: the multistep model confirmed and refined. Immunity 1995, 3:99-108.

4.

Bargatze RF, Butcher EC: Rapid G protein-regulated activation event involved in lymphocyte binding to high endothelial venules. J Exp Med1993, 178:367-372.

5.

Warnock RA, Askari S, Butcher EC, von Andrian UH: Molecular mechanisms of lymphocyte homing to peripheral lymph nodes. J Exp Med 1998, 187:205-216.

6. ••

Campbell JJ, Hedrick J, Zlotnik A, Siani MA, Thompson DA, Butcher EC: Chemokines and the arrest of lymphocytes rolling under flow conditions. Science 1998, 279:381-384. These authors provide the first direct experimental evidence demonstrating a role for chemokines in vascular recognition at the level of integrin-dependent adhesion triggering and arrest; the authors suggest that adhesion under shear may serve as a ‘gatekeeper’ control point, allowing entry of only particular lymphoid subsets into a given tissue. 7.

Pachynski RK, Wu SW, Gunn MD, Erle DJ: Secondary lymphoidtissue chemokine (SLC) stimulates integrin alpha 4 beta 7mediated adhesion of lymphocytes to mucosal addressin cell adhesion molecule-1 (MAdCAM-1) under flow. J Immunol 1998, 161:953-956.

8.

Campbell JJ, Qin S, Bacon KB, Mackay CR, Butcher EC: The biology of chemokine and classical chemoattractant receptors: differential requirements for adhesion-triggering versus chemotactic responses in lymphoid cells. J Cell Biol 1996, 134:255-266.

9. ••

Campbell JJ, Haraldsen G, Pan J, Rottman J, Qin S, Ponath P, Andrew DP, Warnke R, Ruffing N, Kassam N et al.: The chemokine receptor CCR4 in vascular recognition by cutaneous but not intestinal memory cells. Nature 1999, 400:776-780. These authors provide the first examples of a role for chemokines in specialized tissue-specific lymphocyte homing and thus in defining and segregating regional immune responses, by elucidating the association between the TARC–CCR4 interaction in cutaneous lymphocyte homing. 10. Campbell JJ, Bowman EP, Murphy K, Youngman KR, Siani MA, •• Thompson DA, Wu L, Zlotnik A, Butcher EC: 6-C-kine (SLC), a lymphocyte adhesion-triggering chemokine expressed by high endothelium, is an agonist for the MIP-3β receptor CCR7. J Cell Biol 1998, 141:1053-1059. These authors identify SLC as a ligand for CCR7 and suggest that CCR7 may be an important homing receptor for secondary lymphoid tissues. 11. Yoshida R, Nagira M, Kitaura M, Imagawa N, Imai T, Yoshie O: Secondary lymphoid-tissue chemokine is a functional ligand for the CC chemokine receptor CCR7. J Biol Chem 1998, 273:7118-7122. 12. Gunn MD, Tangemann K, Tam C, Cyster JG, Rosen SD, Williams LT: •• A chemokine expressed in lymphoid high endothelial venules promotes the adhesion and chemotaxis of naive lymphocytes. Proc Natl Acad Sci USA 1998, 95:258-263. These authors demonstrate that SLC is expressed within the endothelial cells of HEVs and suggest that SLC might provide the adhesion-triggering stimulus necessary for entry into secondary lymphoid organs. 13. Warnock RA, Campbell JJ, Dorf ME, Matsuzawa A, McEvoy LM, •• Butcher EC: The role of chemokines in the microenvironmental control of T versus B cell arrest in Peyer’s patch high endothelial venules. J Exp Med 2000, 191:77-88. These authors demonstrate that SLC protein is present on HEVs of Peyer’s patches from normal mice, that SLC is absent from Peyer’s patches of plt mice, that SLC is important for adhesion triggering of T cells but not B cells and that T cells and B cells arrest on different areas of HEVs. 14. Stein JV, Rot A, Luo Y, Narasimhaswamy M, Nakano H, Gunn MD, •• Matsuzawa A, Quackenbush EJ, Dorf ME, von Andrian UH: The CC chemokine thymus-derived chemotactic agent 4 (TCA-4, secondary lymphoid tissue chemokine, 6Ckine, Exodus-2) triggers lymphocyte function-associated antigen-1-mediated arrest of rolling T lymphocytes in peripheral lymph node high endothelial venules. J Exp Med 2000, 191:61-75. These authors demonstrate that SLC protein is present on HEVs of peripheral lymph nodes of normal mice, that SLC is absent from peripheral lymph nodes of plt mice, that SLC is important for adhesion triggering of T cells in peripheral lymph nodes and that SLC protein injected into nonlymphoid tissue can be presented by HEVs in the draining lymph node. 15. Nakano H, Tamura T, Yoshimoto T, Yagita H, Miyasaka M, Butcher E, Nariuchi H, Kakiuchi T, Matsuzawa A: Genetic defect in T lymphocytespecific homing into peripheral lymph nodes. Eur J Immunol 1997, 27:215-221. 16. Gunn MD, Kyuwa S, Tam C, Kakiuchi T, Matsuzawa A, Williams LT, • Nakano H: Mice lacking expression of secondary lymphoid organ chemokine have defects in lymphocyte homing and dendritic cell localization. J Exp Med 1999, 189:451-460. These authors demonstrate that SLC is not produced by the HEVs of plt mice.

17.

Vassileva G, Soto H, Zlotnik A, Nakano H, Kakiuchi T, Hedrick JA, Lira SA: The reduced expression of 6ckine in the plt mouse results from the deletion of one of two 6ckine genes. J Exp Med 1999, 190:1183-1188.

18. Gretz JE, Kaldjian EP, Anderson AO, Shaw S: Sophisticated strategies for information encounter in the lymph node: the reticular network as a conduit of soluble information and a highway for cell traffic. J Immunol 1996, 157:495-499. 19. Middleton J, Neil S, Wintle J, Clark-Lewis I, Moore H, Lam C, Auer M, Hub E, Rot A: Transcytosis and surface presentation of IL-8 by venular endothelial cells. Cell 1997, 91:385-395. 20. Forster R, Schubel A, Breitfield D, Kremmer E, Renner-Muller I, Wolf E, • Lipp M: CCR7 coordinates the primary immune response by establishing functional microenvironments in secondary lymphoid organs. Cell 1999, 99:23-33. These authors describe the phenotype of the CCR7 ‘knockout’ mouse. 21. Sallusto F, Lenig D, Forster R, Lipp M, Lanzavecchia A: Two subsets of memory T lymphocytes with distinct homing potentials and effector functions. Nature 1999, 401:708-712.

33. Wurbel MA, Philippe JM, Nguyen C, Victorero G, Freeman T, Wooding P, Miazek A, Mattei MG, Malissen M, Jordan BR et al.: The chemokine TECK is expressed by thymic and intestinal epithelial cells and attracts double- and single-positive thymocytes expressing the TECK receptor CCR9. Eur J Immunol 2000, 30:262-271. 34. Yu CR, Peden KW, Zaitseva MB, Golding H, Farber JM: CCR9A and CCR9B: two receptors for the chemokine CCL25/TECK/Ck beta15 that differ in their sensitivities to ligand. J Immunol 2000, 164:1293-1305. 35. Choe H, Farzan M, Konkel M, Martin K, Sun Y, Marcon L, Cayabyab M, Berman M, Dorf ME, Gerard N et al.: The orphan seventransmembrane receptor APJ supports the entry of primary T-cellline-tropic and dual tropic human immunodeficiency virus type 1. J Virol 1998, 72:6113-6118. 36. Meucci O, Fatatis A, Simen AA, Bushell TJ, Gray PW, Miller RJ: Chemokines regulate hippocampal neuronal signaling and gp120 neurotoxicity. Proc Natl Acad Sci USA 1998, 95:14500-14505. 37.

22. Mackay CR: Dual personality of T memory T cells. Nature 1999, 401:659-660. 23. Butcher EC, Williams M, Youngman K, Rott L, Briskin M: Lymphocyte • trafficking and regional immunity. Adv Immunol 1999, 72:209-253. These authors review the field of tissue-specific lymphocyte homing. 24. Mackay CR: Immunological memory. Adv Immunol 1993, 53:217-265. 25. Santamaria-Babi LF, Picker LJ, Perez-Soler MT, Drzimalla K, Flohr P, Blaser K, Hauser C: Circulating allergen-reactive T cells from patients with atopic dermatitis and allergic contact dermatitis express the skin-selective homing receptor, the cutaneous lymphocyte-associated antigen. J Exp Med 1995, 181:1935-1940. 26. Williams MB, Butcher EC: Homing of naive and memory T lymphocyte subsets to Peyer’s patches, lymph nodes and spleen. J Immunol 1997, 159:1746-1752. 27. ••

Morales J, Homey B, Vicari AP, Hudak S, Oldham E, Hedrick J, Orozco R, Copeland NG, Jenkins NA, McEvoy LM, Zlotnik A: CTACK, a skin-associated chemokine that preferentially attracts skin-homing memory T cells. Proc Natl Acad Sci USA 1999, 96:14470-14475. These authors demonstrate that CTACK attracts cutaneous lymphocytes and is expressed by keratinocytes. 28. Hromas R, Broxmeyer HE, Kim C, Christopherson K, Hou YH: Isolation of ALP, a novel divergent murine CC chemokine with a unique carboxy terminal extension. Biochem Biophys Res Commun 1999, 258:737-740.

29. Zabel BA, Agace WW, Campbell JJ, Heath H, Parent D, Roberts AI, •• Ebert EC, Kassam N, Qin S, Zovko M et al.: Human G protein-coupled receptor GPR-96/CC chemokine receptor 9 is selectively expressed on intestinal homing T lymphocytes, mucosal lymphocytes, and thymocytes and is required for thymus-expressed chemokinemediated chemotaxis. J Exp Med 1999, 190:1241-1255. These authors demonstrate that the TECK attracts a subset of only the guthoming lymphocyte subset from peripheral blood and that the TECK receptor CCR9 is expressed on this subset. 30. Zaballos A, Gutierrez J, Varona R, Ardavin C, Marquez G: Cutting edge: identification of the orphan chemokine receptor GPR-96 as CCR9, the receptor for the chemokine TECK. J Immunol 1999, 162:5671-5675. 31. Youn BS, Kim CH, Smith FO, Broxmeyer HE: TECK, an efficacious chemoattractant for human thymocytes, uses GPR-96/CCR9 as a specific receptor. Blood 1999, 94:2533-2536. 32. Norment AM, Bogatzki LY, Gantner BN, Bevan MJ: Murine CCR9, a chemokine receptor for thymus-expressed chemokine that is upregulated following pre-TCR signaling. J Immunol 2000, 164:639-648.

Vicari AP, Figueroa DJ, Hedrick JA, Foster JS, Singh KP, Menon S, Copeland NG, Gilbert DJ, Jenkins NJ, Bacon KB, Zlotnik A: TECK: a novel CC chemokine specifically expressed by thymic dendritic cells and potentially involved in T cell development. Immunity 1997, 7:291-301.

38. Sallusto F, Lanzavecchia A, Mackay CR: Chemokines and • chemokine receptors in T-cell priming and Th1/Th2-mediated responses. Immunol Today 1998, 19:568-574. This paper reviews the attempts to associate Th1 and Th2 cells with expression of specific chemokine receptors. 39. Teraki Y, Picker LJ: Independent regulation of cutaneous lymphocyte-associated antigen expression and cytokine synthesis phenotype during human CD4+ memory cell differentiation. J Immunol 1997, 159:6018-6029. 40. Forster R, Mattis AE, Kremmer E, Wolf E, Brem G, Lipp M: A putative chemokine receptor, BLR1, directs B cell migration to defined lymphoid organs and specific anatomic compartments of the spleen. Cell 1996, 87:1037-1047. 41. Randolph DA, Huang G, Carruthers CLJ, Bromley LE, Chaplin DD: The role of CCR7 in Th1 and Th2 cell localization and delivery of B cell help in vivo. Science 1999, 286:2159-2162. 42. Cyster JG: Chemokines and cell migration in secondary lymphoid •• organs. Science 1999, 286:2098-2102. This paper extensively reviews recent advances in our understanding of chemokine-mediated organization of secondary lymphoid tissues. 43. Melchers F, Rolink AG, Schaniel C: The role of chemokines in •• regulating cell migration during humoral immune responses. Cell 1999, 99:351-354. This paper extensively reviews recent advances in our understanding of chemokine-mediated organization of secondary lymphoid tissues. 44. Foxman EF, Kunkel EJ, Butcher EC: Integrating conflicting chemotactic signals: the role of memory in leukocyte navigation. J Cell Biol 1999, 147:577-588. 45. Sprent J: T Lymphocytes and the thymus. In Fundamental Immunology. Edited by Paul WE. New York: Raven Press; 1993:75-109. 46. Campbell JJ, Pan J, Butcher EC: Cutting edge: developmental •• switches in chemokine responses during T cell maturation. J Immunol 1999, 163:2353-2357. These authors describe the dramatic changes in chemokine responsiveness and receptor expression associated with important milestones in thymic development. 47.

Kim CH, Pelus LM, White JR, Broxmeyer HE: Differential chemotactic behavior of developing T cells in response to thymic chemokines. Blood 1998, 91:4434-4443.