The CCR7 ligand elc (CCL19) is transcytosed in high endothelial venules and mediates T cell recruitment

Leukocyte extravasation: chemokine transport and presentation by the endothelium

At sites of inflammation and in normal immune surveillance, chemokines direct leukocyte migration across the endothelium. Many cell types that are extravascular can produce chemokines, and for these mediators to directly elicit leukocyte migration from the blood, they would need to reach the luminal surface of the endothelium. This article reviews the evidence that endothelial cells are active in transcytosing chemokines to their luminal surfaces, where they are presented to leukocytes. The endothelial binding sites that transport and present chemokines include glycosaminoglycans (GAGs) and possibly the Duffy antigen/receptor for chemokines (DARC). The binding residues on chemokines that interact with GAGs are discussed, as are the carbohydrate structures on GAGs that bind these cytokines. The expression of particular GAG structures by endothelial cells may lend selectivity to the type of chemokine presented in a given tissue, thereby contributing to selective leukocyte recruitment. At the luminal surface of the endothelium, chemokines are preferentially presented to blood leukocytes on the tips of microvillous processes. Similarly, certain adhesion molecules and chemokine receptors are also preferentially distributed on leukocyte and endothelial microvilli, and evidence suggests an important role for these structures in creating the necessary surface topography for leukocyte migration. Finally, the mechanisms of chemokine transcytosis and presentation by endothelial cells are incorporated into the current model of chemokine-driven leukocyte extravasation.

CD4 effector T cell subsets in the response to influenza: heterogeneity, migration, and function

The immune response of naive CD4 T cells to influenza virus is initiated in the draining lymph nodes and spleen, and only after effectors are generated do antigen-specific cells migrate to the lung which is the site of infection. The effector cells generated in secondary organs appear as multiple subsets which are a heterogeneous continuum of cells in terms of number of cell divisions, phenotype and function. The effector cells that migrate to the lung constitute the more differentiated of the total responding population, characterized by many cell divisions, loss of CD62L, down-regulation of CCR7, stable expression of CD44 and CD49d, and transient expression of CCR5 and CD25. These cells also secrete high levels of interferon gamma and reduced levels of interleukin 2 relative to those in the secondary lymphoid organs. The response declines rapidly in parallel with viral clearance, but a spectrum of resting cell subsets reflecting the pattern at the peak of response is retained, suggesting that heterogeneous effector populations may give rise to corresponding memory populations. These results reveal a complex response, not an all-or-none one, which results in multiple effector phenotypes and implies that effector cells and the memory cells derived from them can display a broad spectrum of functional potentials.

T-cell activation: a multidimensional signaling network

Naïve T cell activation requires the interactions of antigen receptors, adhesion molecules and co-stimulatory molecules. Antigen receptors and adhesion molecules are involved in spatio-temporal movement to form a stable immunological synapse. This stable junction interrupts T cell migration, and provides a platform for temporally regulated co-stimulatory receptor signaling spanning a period of days.

Travellers in many guises: the origins and destinations of dendritic cells

The migratory behaviour of dendritic cells (DC) is tightly linked to their differentiation state. Precursor DC constitutively repopulate normal tissues from the bloodstream, and are recruited in elevated numbers to sites of inflammation. Whilst maturing in response to antigenic stimulation, DC acquire the capability to enter lymph nodes via afferent lymphatic vessels, thus facilitating their presentation of antigen to naïve T cells. Peripheral blood monocytes constitute a second DC precursor population, which during an inflammatory response are recruited to the affected site where some differentiate into functional DC. The availability of separate DC precursor populations is thought to be significant for the character, amplification and perpetuation of the resultant immune response. In addition, the balance between steady-state trafficking of incompletely activated DC bearing self-antigens from the periphery, and the migration of fully mature DC from inflammatory sites into lymph nodes might have profound effects upon tolerance induction and activation of T cells, respectively.

Chemokines and dendritic cells: a crucial alliance

Dendritic cells (DC) are bone marrow-derived professional antigen-presenting cells that function as sentinels of the immune system. Their importance in immunity resides in their unique ability to prime or tolerize T lymphocytes, thereby initiating or inhibiting immune responses. They reside in all tissues and organs and upon appropriate activation, migrate to secondary lymphoid organs to present antigen to T lymphocytes in the T cell zones. Because of this central role in T cell activation, there is a great deal of interest in using DC therapeutically to deliver positive or negative signals to the immune system. The DC system is critically dependent on the ability of DC at different stages of maturation to respond to a range of soluble and cell-bound signals, including members of the chemokine gene superfamily. This review will describe the interactions between DC and the chemokine system.

Functional expression of CCR2 by human fetal astrocytes

Astrocytes from different sources bind the chemokine monocyte chemoattractant factor (MCP-1), yet functional expression in these cells of CCR2, the major receptor for this ligand, has been a matter of controversy. Here we show that cultured human fetal astrocytes express CCR2 at the mRNA and protein levels, and display chemotaxis and calcium flux in response to MCP-1. Surface CCR2 protein expression and MCP-1 binding activity were observed to undergo near parallel downmodulation and recovery following MCP-1 exposure, supporting the argument that CCR2, and not another receptor, mediates MCP-1 ligation in these cells. Downmodulation was further determined to occur via receptor internalization, and to apparently proceed via both clathrin-coated vesicles and caveolae, the latter being a novel mode for the endocytosis of chemokine receptors. Insofar as MCP-1 is thought to mediate inflammatory and developmental processes within the central nervous system (CNS), such astrocyte responses to this chemokine are likely to significantly impact physiological and pathophysiological events at the blood-brain barrier and within the CNS parenchyma.

Differing activities of homeostatic chemokines CCL19, CCL21, and CXCL12 in lymphocyte and dendritic cell recruitment and lymphoid neogenesis

Despite their widespread expression, the in vivo recruitment activities of CCL19 (EBV-induced molecule 1 ligand chemokine) and CXCL12 (stromal cell-derived factor 1) have not been established. Furthermore, although CXCL13 (B lymphocyte chemoattractant) has been shown to induce lymphoid neogenesis through induction of lymphotoxin (LT)alpha1beta2, it is unclear whether other homeostatic chemokines have this property. In this work we show that ectopic expression in pancreatic islets of CCL19 leads to small infiltrates composed of lymphocytes and dendritic cells and containing high endothelial venules and stromal cells. Ectopic CXCL12 induced small infiltrates containing few T cells but enriched in dendritic cells, B cells, and plasma cells. Comparison of CCL19 transgenic mice with mice expressing CCL21 (secondary lymphoid tissue chemokine) revealed that CCL21 induced larger and more organized infiltrates. A more significant role for CCL21 is also suggested in lymphoid tissues, as CCL21 protein was found to be present in lymph nodes and spleen at much higher concentrations than CCL19. CCL19 and CCL21 but not CXCL12 induced LTalpha1beta2 expression on naive CD4 T cells, and treatment of CCL21 transgenic mice with LTbetaR-Fc antagonized development of organized lymphoid structures. LTalpha1beta2 was also induced on naive T cells by the cytokines IL-4 and IL-7. These studies establish that CCL19 and CXCL12 are sufficient to mediate cell recruitment in vivo and they indicate that LTalpha1beta2 may function downstream of CCL21, CCL19, and IL-2 family cytokines in normal and pathological lymphoid tissue development.

Chemokine requirements for B cell entry to lymph nodes and Peyer's patches

B cell entry to lymph nodes and Peyer's patches depends on chemokine receptor signaling, but the principal chemokine involved has not been defined. Here we show that the homing of CXCR4-/- B cells is suppressed in CCL19 (ELC)- and CCL21 (SLC)-deficient paucity of lymph node T cells mice, but not in wild-type mice. We also find that CXCR4 can contribute to T cell homing. Using intravital microscopy, we find that B cell adhesion to high endothelial venules (HEVs) is disrupted when CCR7 and CXCR4 are predesensitized. In Peyer's patches, B cell entry is dependent on CXCR5 in addition to CCR7/CXCR4. CXCL12 (SDF1) is displayed broadly on HEVs, whereas CXCL13 (BLC) is found selectively on Peyer's patch follicular HEVs. These findings establish the principal chemokine and chemokine receptor requirements for B cell entry to lymph nodes and Peyer's patches.

In vivo differentiated cytokine-producing CD4(+) T cells express functional CCR7

Chemokines and their receptors fulfill specialized roles in inflammation and under homeostatic conditions. CCR7 and its ligands, CCL19 and CCL21, are involved in lymphocyte recirculation through secondary lymphoid organs and additionally navigate lymphocytes into distinct tissue compartments. The role of CCR7 in the migration of polarized T effector/memory cell subsets in vivo is still poorly understood. We therefore analyzed murine and human CD4(+) cytokine-producing cells developed in vivo for their chemotactic reactivity to CCR7 ligands. The responses of cells producing cytokines, such as IFN-gamma, IL-4, and IL-10, as well as of subsets defined by memory or activation markers were comparable to that of naive CD4(+) cells, with slightly lower reactivity in cells expressing IL-10 or CD69. This indicates that CCR7 ligands are able to attract naive as well as the vast majority of activated and effector/memory T cell stages. Chemotactic reactivity of these cells toward CCL21 was absent in CCR7-deficient cells, proving that effector cells do not use alternative receptors for this chemokine. Th1 cells generated from CCR7(-/-) mice failed to enter lymph nodes and Peyer's patches, but did enter a site of inflammation. These findings indicate that CD4(+) cells producing effector cytokines upon stimulation retain the capacity to recirculate through lymphoid tissues via CCR7.

Phenotypic and functional heterogeneity of EBV epitope-specific CD8+ T cells

High frequencies of EBV-specific CD8(+) T cells have been detected during acute EBV infection, yet persistent infection inevitably results. To address this issue, we characterized the phenotype and function of epitope-specific CD8(+) T cell populations from presentation with acute through latent infection. Considerable phenotypic and functional heterogeneity within, as well as between, two different epitope-specific populations was observed over time following acute infection. B7 EBV-encoded nuclear Ag (EBNA)-3A-specific CD8(+) T cells expressed only CD45RO from acute through latent EBV infection. A2 BMLF-1-specific CD8(+) T cells expressed CD45RO during acute infection and either CD45RA or CD45RO during latent EBV infection. This difference in CD45 isoform expression between the two epitope-specific populations did not translate into differences in perforin content, the ability to produce IFN-gamma, or the ability to proliferate in response to Ag in vitro. In individuals with latent EBV infection, the frequencies of A2 BMLF-1- or B7 EBNA-3A-specific CD8(+) T cells that expressed CD45RA, CD45RO, CD62 ligand, CCR7, and perforin were stable over time. However, the expression of CD62 ligand and CCR7 was significantly higher among EBNA-3A-specific CD8(+) T cells than among BMLF-1-specific CD8(+) T cells. Further work is necessary to understand how phenotypic and functional differences between EBV epitope-specific CD8(+) T cells are related to the biology of the virus and to the equilibrium between the virus and the host during persistent infection.

The chemokine receptor CCR7 and alpha4 integrin are important for migration of chronic lymphocytic leukemia cells into lymph nodes

Malignant lymphocyte migration into lymph nodes is an important aspect of chronic lymphocytic leukemia (CLL), yet little is known about the processes involved. Here we demonstrate that CLL cells migrate across vascular endothelium in response to at least 3 chemokines, namely, CCL21, CCL19, and CXCL12. Moreover, transendothelial cell migration (TEM) in response to CCL21 and CCL19 was significantly higher for the malignant B cells of patients who had clinical lymph node involvement as compared with those of patients lacking such organomegaly. Furthermore, the expression of CCR7, the receptor for both CCL21 and CCL19, correlated with clinical lymphadenopathy, and blocking of CCR7 inhibited CLL cell TEM. By using immunohistochemistry we demonstrated that CCL21 and CCL19, but not CXCL12, are located in high endothelial venules and are, therefore, in an appropriate location to induce TEM. Regarding the adhesion receptors involved in TEM, alpha4 (most likely in association with beta1) and alphaLbeta2 were shown to be important in CLL cell TEM in vitro, but only the level of alpha4 expression correlated with the presence of clinical lymphadenopathy. The present studies are the first to shed light on the factors determining CLL cell entry into nodes and define the phenotype of circulating malignant cells likely to determine the pattern of lymph node enlargement in the disease.

Chemokines in rapid leukocyte adhesion triggering and migration

Leukocyte subsets are recruited from the blood to lymphoid and non-lymphoid tissues via a multi-step process that involves distinct adhesive and activation steps. Chemokines, a family of chemotactic cytokines that signal through G-protein-coupled receptors, play critical roles in regulating the leukocyte recruitment cascade. Chemokines can be transported and immobilized on the surface of vascular endothelial cells, where they activate leukocyte subsets expressing specific receptors. Activation signals induce firm adhesion of rolling leukocytes by rapidly upregulating integrin affinity and/or avidity. Chemokines can also direct migration of adherent cells across the endothelium, and control segregation of cells into specific microenvironments within tissues. The regulated expression of chemokines and their receptors is a critical determinant for homing of specialized lymphocyte subsets, and controls both tissue and inflammation-specific immune processes.

Differential in vivo persistence of two subsets of memory phenotype CD8 T cells defined by CD44 and CD122 expression levels

The existence of distinct subsets of memory CD8 T cells with different characteristics is now well established. In this work, we describe two subsets of mouse CD8 T cells with memory characteristics that coexist in primed thymectomized TCR-transgenic F5 mice and that share some properties with the human central and effector memory cells. The first subset corresponds to CD8 T cells generated following nucleoprotein 68 peptide priming which are CD44(int)CD122(-)nucleoprotein 68/H-2D(b) tetramer(+) and express high levels of CCR7 mRNA. In contrast, CD8 T cells in the second subset are CD44(high)CD122(+), are heterogeneous in terms of Ag specificity, and express low levels of CCR7 mRNA. We have studied the functional characteristics and the persistence of these two subsets in thymectomized mice. CD44(int) CD8 T cells persist like naive cells; i.e., they are slowly lost with time. However, surviving cells maintain their phenotype and memory characteristics for the entire life span of the animal. In contrast, CD44(high) CD8 T cells are persistent and accumulate in thymectomized but not euthymic mice. This is correlated with an increased in vivo proliferative and survival potential of these cells. These results show that acquisition of enhanced functional characteristics and long-term persistence by memory T cells are independent. This may have important consequences for the design of specific vaccine.

CCL27-CCR10 interactions regulate T cell-mediated skin inflammation

The skin-associated chemokine CCL27 (also called CTACK, ALP and ESkine) and its receptor CCR10 (GPR-2) mediate chemotactic responses of skin-homing T cells in vitro. Here we report that most skin-infiltrating lymphocytes in patients suffering from psoriasis, atopic or allergic-contact dermatitis express CCR10. Epidermal basal keratinocytes produced CCL27 protein that bound to extracellular matrix, mediated adhesion and was displayed on the surface of dermal endothelial cells. Tumor necrosis factor-alpha and interleukin-1beta induced CCL27 production whereas the glucocorticosteroid clobetasol propionate suppressed it. Circulating skin-homing CLA+ T cells, dermal microvascular endothelial cells and fibroblasts expressed CCR10 on their cell surface. In vivo, intracutaneous CCL27 injection attracted lymphocytes and, conversely, neutralization of CCL27-CCR10 interactions impaired lymphocyte recruitment to the skin leading to the suppression of allergen-induced skin inflammation. Together, these findings indicate that CCL27-CCR10 interactions have a pivotal role in T cell-mediated skin inflammation.

Role for CCR7 ligands in the emigration of newly generated T lymphocytes from the neonatal thymus

Most T lymphocytes are generated within the thymus. It is unclear, however, how newly generated T cells relocate out of the thymus to the circulation. The present study shows that a CC chemokine CCL19 attracts mature T cells out of the fetal thymus organ culture. Another CC chemokine CCL21, which shares CCR7 with CCL19 but has a unique C-terminal extension containing positively charged amino acids, failed to show involvement in thymic emigration. Neonatal appearance of circulating T cells was defective in CCL19-neutralized mice as well as in CCR7-deficient mice but not in CCL21-neutralized mice. In the thymus, CCL19 is predominantly localized in the medulla including endothelial venules. These results indicate a CCL19- and CCR7-dependent pathway of thymic emigration, which represents a major pathway of neonatal T cell export.

CC chemokine receptor (CCR)4 and the CCR10 ligand cutaneous T cell-attracting chemokine (CTACK) in lymphocyte trafficking to inflamed skin

The chemokine thymus and activation-regulated chemokine (TARC; CCL17) is displayed by cutaneous (but not intestinal) venules, and is thought to trigger vascular arrest of circulating skin homing memory T cells, which uniformly express the TARC receptor CC chemokine receptor (CCR)4. Cutaneous T cell-attracting chemokine (CTACK; CCL27), expressed by skin keratinocytes, also attracts cutaneous memory T cells, and is hypothesized to assist in lymphocyte recruitment to skin as well. Here we show that chronic cutaneous inflammation induces CD4 T cells expressing E-selectin binding activity (a marker of skin homing memory cells) in draining lymph node, and that these E-selectin ligand+ T cells migrate efficiently to TARC and to CTACK. In 24 h in vivo homing assays, stimulated lymph node T cells from wild-type mice or, surprisingly, from CCR4-deficient donors migrate efficiently to inflamed skin; and an inhibitory anti-CTACK antibody has no effect on wild-type lymphocyte recruitment. However, inhibition with anti-CTACK monoclonal antibody abrogates skin recruitment of CCR4-deficient T cells. We conclude that CTACK and CCR4 can both support homing of T cells to skin, and that either one or the other is required for lymphocyte recruitment in cutaneous delayed type hypersensitivity.

Ectopic expression of the B cell-attracting chemokine BCA-1 (CXCL13) on endothelial cells and within lymphoid follicles contributes to the establishment of germinal center-like structures in Sjögren's syndrome

OBJECTIVE

To test the hypothesis that the formation of ectopic germinal center (GC)-like structures in Sjögren's syndrome (SS) is associated with the ectopic expression of the constitutive lymphoid tissue-homing chemokines B cell-attracting chemokine 1 (BCA-1; or, CXCL13) and stromal cell-derived factor 1 (SDF-1; or, CXCL12).

METHODS

Immunohistochemical and immunofluorescence analysis was used to determine the expression of the constitutive chemokines BCA-1 (CXCL13) and SDF-1 (CXCL12) in salivary glands from 5 SS patients and 3 non-SS patients. In addition, the expression of their respective receptors (CXCR5 and CXCR4) was examined on infiltrating lymphocytes. Human tonsil was used as a positive control for secondary lymphoid tissue.

RESULTS

BCA-1 (CXCL13) was expressed within lymphoid aggregates in SS, which shared many structural features with GCs in tonsil. BCA-1 (CXCL13) was completely absent in control biopsy samples from patients who did not have SS. High levels of BCA-1 (CXCL13) were also found on endothelial cells in salivary glands from SS patients. Diseased SS tissue was infiltrated by CXCR5-expressing B cells which organized into GC-like clusters. In complete contrast, SDF-1 (CXCL12), a constitutive chemokine involved in leukocyte retention within lymphoid tissue, was expressed by epithelial cells in both diseased and control samples. The chemokine receptor for SDF-1, CXCR4, was expressed on T cells that accumulated in a periductal distribution in diseased tissue.

CONCLUSION

The ectopic expression of BCA-1 (CXCL13) on endothelial cells and within GC-like structures, together with the strong expression of SDF-1 (CXCL12) on ductal epithelial cells, is a unique feature of inflamed glands in SS. By creating a local microenvironment supportive of focal B cell aggregation and differentiation, with structural features that are remarkably similar to GCs, BCA-1 (CXCL13) and SDF-1 (CXCL12) may contribute to the excessive production of high-affinity, class-switched autoantibodies and to the high incidence of B cell lymphomas classically associated with SS.

The T cell chemokine receptor CCR7 is internalized on stimulation with ELC, but not with SLC

ELC and SLC are potent agonists for CCR7, a receptor of up-most importance for the regulation of the homing and traffic of lymphocytes into and within secondary lymphoid tissues. We have studied the effects of both chemokines on receptor re-distribution in T lymphocytes and other CCR7-bearing cells by flow cytometry and by assessing receptor mediated functions. In this paper we show that ELC and SLC differ fundamentally in the ability to induce the internalization of their receptor. ELC induced a rapid time- and concentration-dependent internalization of CCR7 and markedly decreased the ability of CCR7-bearing cells to respond to a second stimulation. No receptor internalization, by contrast, was observed on stimulation with SLC. Receptors that were internalized on stimulation with ELC were re-expressed when the cells were washed. Re-expression of receptors and consequent re-activation of the cells was prevented in the presence of ELC, but was not affected in the presence of SLC. These findings could explain how T lymphocytes that enter lymphoid tissues in response to SLC produced by high-endothelial venules can subsequently migrate in response to SLC and ELC expressed within the T cell areas.