High Endothelial Venules: Lymphocyte Traffic Control and Controlled Traffic**This article was accepted for publication on 27 September 1996

In Sickness and in Health: The Immunological Roles of the Lymphatic System

The lymphatic system plays crucial roles in immunity far beyond those of simply providing conduits for leukocytes and antigens in lymph fluid. Endothelial cells within this vasculature are distinct and highly specialized to perform roles based upon their location. Afferent lymphatic capillaries have unique intercellular junctions for efficient uptake of fluid and macromolecules, while expressing chemotactic and adhesion molecules that permit selective trafficking of specific immune cell subsets. Moreover, in response to events within peripheral tissue such as inflammation or infection, soluble factors from lymphatic endothelial cells exert “remote control” to modulate leukocyte migration across high endothelial venules from the blood to lymph nodes draining the tissue. These immune hubs are highly organized and perfectly arrayed to survey antigens from peripheral tissue while optimizing encounters between antigen-presenting cells and cognate lymphocytes. Furthermore, subsets of lymphatic endothelial cells exhibit differences in gene expression relating to specific functions and locality within the lymph node, facilitating both innate and acquired immune responses through antigen presentation, lymph node remodeling and regulation of leukocyte entry and exit. This review details the immune cell subsets in afferent and efferent lymph, and explores the mechanisms by which endothelial cells of the lymphatic system regulate such trafficking, for immune surveillance and tolerance during steady-state conditions, and in response to infection, acute and chronic inflammation, and subsequent resolution.

Sex-related differences in autoimmune-induced lung lesions in MRL/MpJ-faslpr mice are mediated by the development of mediastinal fat-associated lymphoid clusters

MRL/MpJ-Fas^lpr (lpr) mice are a model for autoimmune diseases such as systemic lupus erythematosus (SLE). These diseases mainly affect women, with a 10:1 female-to-male ratio, and cause pleuropulmonary lesions. We previously revealed a correlation between mediastinal fat-associated lymphoid cluster (MFALC) development and cellular infiltration in the lungs of lpr male mice; however, we did not report on MFALCs in females. The purpose of this investigation was to reveal sex-related differences in MFALCs in lpr mice. We compared the morphological features of MFALCs and lung mononuclear cell aggregates (LMCAs) in 5-month-old male and female lpr mice. The females showed significantly elevated anti-dsDNA autoantibody titers and larger MFALCs, with a higher ratio of lymphatic vessel (LV) and high endothelial venule (HEV) areas to MFALC area, and greater numbers of T- and B-cells, macrophages, and proliferating and dendritic cells in MFALCs and LMCAs than males. Our data indicated that MFALCs were more developed and lung lesions were more severe in female than in male lpr mice, thereby suggesting a potential role for LVs and HEVs in the establishment of MFALCs and lung lesions. Further investigation in female lpr mice will be needed for treatment of human respiratory diseases and autoimmune disorders.

Exploring local immune responses to vaccines using efferent lymphatic cannulation

The early stages of the induction of a primary immune response to a vaccine can shape the overall quality of the immune memory generated and hence affect the success of the vaccine. This early interaction between a vaccine and the immune system occurs first at the site of vaccination and can be explored using afferent cannulation. Subsequently, the vaccine and adjuvant activates the local draining lymph node. These interactions can be studied in real time in vivo using efferent lymphatic duct cannulation in large animal models and are the subject of this review. Depending on how the vaccine is delivered, the draining lymph nodes of different organs can be accessed, facilitating the testing of tissue-specific vaccinations. The efferent lymphatic cannulation model provides an avenue to study the effect of both adjuvants and antigen on the local immune system, and hence opens a pathway toward developing more effective ways of inducing immunity.

Characterization of mouse mediastinal fat-associated lymphoid clusters

The association between adipose tissue and immunity has been established and fat-associated lymphoid clusters (FALCs) are considered as a source of immune cells. We discovered lymphoid clusters (LCs) in mouse mediastinal fat tissues (MFTs). In Th1-biased C57BL/6N (B6), Th2-biased DBA/2Cr (DBA) and autoimmune-prone MRL/MpJ (MRL) mice strains, LCs without a fibrous capsule and germinal center were observed in white-colored MFTs extending from the diaphragm to the heart. The number and size of the LCs were larger in 12-month-old mice than in 3-month-old mice in all of the examined strains. Moreover, B6 had an especially large number of LCs compared with DBA and MRL. The immune cells in the LCs consisted of mainly T-cells and some B-cells. The majority of T-cells were CD4+ helper T (Th) cells, rather than CD8+ cytotoxic T-cells and no obvious immune cell population difference was present among the strains. Furthermore, high endothelial venules and lymphatic vessels in the LCs were better developed in B6 mice than in the other strains. Interestingly, some CD133+ hematopoietic progenitor cells and some c-Kit+/CD127+ natural helper cells were detected in the LCs. BrdU+ proliferating cells were more abundant in the LCs of B6 mice than in the LCs of the other strains and the number of BrdU+ cells increased with age. This is the first report of LCs in mouse MFTs. We suggest that the mouse genetic background affects LC size and number. We term the LCs "mediastinal fat-associated lymphoid clusters". These clusters can be considered as niches for Th cell production.

Endothelium and its alterations in cardiovascular diseases: life style intervention

The endothelium, which forms the inner cellular lining of blood vessels and lymphatics, is a highly metabolically active organ that is involved in many physiopathological processes, including the control of vasomotor tone, barrier function, leukocyte adhesion, and trafficking and inflammation. In this review, we summarized and described the following: (i) endothelial cell function in physiological conditions and (ii) endothelial cell activation and dysfunction in the main cardiovascular diseases (such as atherosclerosis, and hypertension) and to diabetes, cigarette smoking, and aging physiological process. Finally, we presented the currently available evidence that supports the beneficial effects of physical activity and various dietary compounds on endothelial functions.

Functional and clinical relevance of chondroitin sulfate proteoglycan 4

The lack of effective conventional therapies for the treatment of advanced stage melanoma has stimulated interest in the development of novel strategies for the management of patients with malignant melanoma. Among them, immunotherapy has attracted much attention because of the potential role played by immunological events in the clinical course of melanoma. For many years, T cell-based immunotherapy has been emphasized in part because of the disappointing results of the monoclonal antibody (mAb)-based clinical trials conducted in the early 1980s and in part because of the postulated major role played by T cells in tumor growth control. More recently, mAb-based therapies have gained in popularity given their clinical and commercial success for a variety of malignant diseases. As a result, there has been increased interest in identifying and characterizing antibody-defined melanoma antigens. Among them, the chondroitin sulfate proteoglycan 4 (CSPG4), also known as high molecular weight-melanoma associated antigen (HMW-MAA) or melanoma chondroitin sulfate proteoglycan (MCSP), has attracted much attention in recent years because of the growing experimental evidence that it fulfills two requirements for immunotherapy to be therapeutically effective: (1) targeting of cancer stem cells (CSC) and (2) development of combinatorial therapies to counteract the escape mechanisms driven by the genetic instability of tumor cells. With this in mind, in this chapter, we have reviewed recent information related to the distribution of CSPG4 on various types of tumors, including CSC, its expression on pericytes in the tumor microenvironment, its recognition by T cells, its role in cell biology as well as the potential mechanisms underlying the ability of CSPG4-specific immunity to control malignant cell growth.

The potential impact of CD4+ T cell activation and enhanced Th1/Th2 cytokine ratio on HIV-1 secretion in the lungs of individuals with advanced AIDS and active pulmonary infection

Bronchoalveolar lavage fluid (BALF) provides a source of mucosal CD4(+) T cells. We investigated the physiological properties of T lymphocytes from BALF and blood and their role on the dynamic of HIV-1 replication among AIDS patients with active lung infections. Pulmonary CD4(+) T cells consist mainly of effector memory cells (CD45RO(+) and CCR7(-)) with increased expression of activation markers (HLA-DR(+) and CD69(+)) when compared to the blood counterpart. We observed a high frequency of BALF cells capable of secreting HIV-1-Ags suggesting that the local lung environment may support favorable conditions for CD4(+) T lymphocytes harboring HIV-1 DNA to initiate the viral cycle. Nevertheless, the high number of IFN-γ-producing cells and the predominance of Th1 immune response in the lung could limit the secretion of HIV-1 RNA. In conclusion, the capacity of activated CD4(+) T cells to produce HIV-1 is driven by both the level and quality of cellular activation in the lung.

Role of CD44 in CTL-induced acute liver injury in hepatitis B virus transgenic mice

BACKGROUND

There are many uncertain points regarding leukocyte movement in the liver, especially interaction between liver sinus endothelial cells (LSECs) and cytotoxic T lymphocytes (CTLs). We examined the role of CD44 in these interactions using the hepatitis model.

METHODS

CTLs were administered to hepatitis B virus transgenic mice (HBVTg) mice and HBVTg x CD44 knockout (KO) mice, and alanine aminotransferase activity (ALT), number of intrahepatic leukocytes, cytokines, and chemokine mRNA level were examined. To determine the number and distribution of CTLs in the liver, 5,6-carboxyfluoroscein succinimidyl ester (CFSE)-labeled CTLs was administered to HBVTg mice with or without CD44.

RESULTS

Serum ALT activity increased after 12 h, although it had declined to 4 h in the CD44KO x HBVTg mice after CTL injection. Similarly, the levels of tumor necrosis factor (TNF)-alpha, interferon (IFN)-gamma, and macrophage inflammatory protein (MIP)-2 mRNAs were reduced in 4 h, although the levels were increased after 12 h in the CD44KO x HBVTg mice. The number of apoptotic hepatocytes increased intentionally at 24 h in the CD44KO x HBVTg livers, and this was thought to result from the lower activity of initial nuclear factor kappa B (NF-kappaB). Although the number of CTLs was lower at 4 h in the CD44KO x HBVTg livers, the difference of intercellular adhesion molecule (ICAM)-1 and CD86 expression on LSECs was not detected.

CONCLUSIONS

CD44 exerts and important effect on LSECs for CTL migration into the hepatocytes. However, because the CD44-deficient state exacerbated hepatic injury, attention is necessary for hepatitis treatment as CD44 target therapy.

CD73-generated adenosine restricts lymphocyte migration into draining lymph nodes

After an inflammatory stimulus, lymphocyte migration into draining lymph nodes increases dramatically to facilitate the encounter of naive T cells with Ag-loaded dendritic cells. In this study, we show that CD73 (ecto-5'-nucleotidase) plays an important role in regulating this process. CD73 produces adenosine from AMP and is expressed on high endothelial venules (HEV) and subsets of lymphocytes. Cd73(-/-) mice have normal sized lymphoid organs in the steady state, but approximately 1.5-fold larger draining lymph nodes and 2.5-fold increased rates of L-selectin-dependent lymphocyte migration from the blood through HEV compared with wild-type mice 24 h after LPS administration. Migration rates of cd73(+/+) and cd73(-/-) lymphocytes into lymph nodes of wild-type mice are equal, suggesting that it is CD73 on HEV that regulates lymphocyte migration into draining lymph nodes. The A(2B) receptor is a likely target of CD73-generated adenosine, because it is the only adenosine receptor expressed on the HEV-like cell line KOP2.16 and it is up-regulated by TNF-alpha. Furthermore, increased lymphocyte migration into draining lymph nodes of cd73(-/-) mice is largely normalized by pretreatment with the selective A(2B) receptor agonist BAY 60-6583. Adenosine receptor signaling to restrict lymphocyte migration across HEV may be an important mechanism to control the magnitude of an inflammatory response.

Fibrocytes: a new insight into kidney fibrosis

Fibrocytes are supposed to be a circulating connective tissue cell progenitor, which consists of a novel population of peripheral blood cells. This distinct population of blood-borne cells shares markers of leukocytes as well as mesenchymal cells. Accumulating evidence indicates that fibrosis is characteristic of progressive chronic kidney diseases of any etiologies, resulting in kidney failure. We have uncovered that CCR7-positive fibrocytes migrate into the kidney in response to secondary lymphoid tissue chemokine (SLC/CCL21) and contribute to kidney fibrosis induced by unilateral ureteral obstruction in mice. In addition, the blockade of CCL21/CCR7 signaling by anti-CCL21 antibodies reduced kidney fibrosis, which was confirmed by a decrease in fibrosis in CCR7-null mice with concomitant reduction in macrophage recruitment along with reduced renal transcripts of monocyte chemoattractant protein-1 (MCP-1/CCL2). These findings suggest that fibrocytes dependent on CCL21/CCR7 signaling pathways contribute to the pathogenesis of kidney fibrosis, thereby providing that regulating fibrocytes may provide a novel therapeutic benefit for kidney fibrosis.

The importance of regional lymph nodes for mucosal tolerance

The immune system is organized as a number of distinct lymphoid organs interconnected by recirculating lymphocytes. These organs, such as lymph nodes, spleen, and gut-associated Peyer's patches, are compartmentalized, providing separate niches for T and B cells. In addition, regional compartmentalization of lymphoid organs themselves exists, leading to the distinction between the mucosal and the systemic immune systems. This distinction not only reflects the anatomical localization but also is based on functional differences, with predominant tolerance induction via mucosal routes and immunity seen after systemic antigen exposure. These differences are associated with regional differences in the lymphoid organs and with environmental conditions of the tissues in which the immune system functions. Recirculation patterns of lymphocytes differ between mucosal and systemic lymphoid organs, and more insight into the mechanisms that imprint this behavior has been generated recently. Differences in dendritic cells have been observed between mucosal and systemic sites, and knowledge on how local factors contribute to the immune system is emerging. From our studies on mucosal tolerance in mouse models, it has become evident that regional lymph nodes draining the mucosa are important sites to direct immune responses. Here, we discuss the way regional lymph nodes contribute to the direction of immune responses and what is known about the local factors and cell behavior that form the basis for these differences.

Secondary lymphoid tissue chemokine (SLC/CCL21)/CCR7 signaling regulates fibrocytes in renal fibrosis

Fibrocytes are a distinct population of bloodborne cells that share markers of leukocytes as well as mesenchymal cells. We hypothesized that CCR7-positive fibrocytes migrate into the kidney in response to secondary lymphoid tissue chemokine (SLC/CCL21) and contribute to renal fibrosis. To investigate this hypothesis, renal fibrosis was induced by unilateral ureteral obstruction in mice. A considerable number of fibrocytes dual-positive for CD45 and type I collagen (ColI) or CD34 and ColI infiltrated the interstitium, reaching a peak on day 7. Most fibrocytes were positive for CCR7, and CCL21/CCR7 blockade reduced the number of infiltrating fibrocytes. CCL21 and MECA79 dual-positive vessels were also detected in the interstitium. The blockade of CCL21/CCR7 signaling by anti-CCL21 antibodies reduced renal fibrosis, which was confirmed by a decrease in fibrosis in CCR7-null mice with concomitant reduction in renal transcripts of pro alpha1 chain of ColI and TGF-beta1. The number of F4/80-positive macrophages decreased along with renal transcripts of monocyte chemoattractant protein 1 (MCP-1/CCL2) after the blockade of CCL21/CCR7 signaling. These findings suggest that CCR7-positive fibrocytes infiltrate the kidney via CCL21-positive vessels, thereby contributing to the pathogenesis of renal fibrosis. Thus, the CCL21/CCR7 signaling of fibrocytes may provide therapeutic targets for combating renal fibrosis.

New insights into the cell biology of the marginal zone of the spleen

In the marginal zone of the spleen the bloodstream passes through an open system of reticular cells and fibers in which various myeloid and lymphoid cells are located. Macrophages in this region are well equipped to recognize pathogens and filter the blood by virtue of unique combinations of pattern recognition receptors. They interact with a specific set of B cells that can be found only in the marginal zone and that are able to react rapidly to bacterial antigens in particular. This combination of strategically located cells is an important factor in our defense against blood-borne pathogens. New data on the development of the marginal zone itself and the marginal zone B cells are reviewed and discussed in light of the function of the spleen in host defense.

Lymphotoxin-beta receptor signaling is required for the homeostatic control of HEV differentiation and function

The lymphotoxin axis is important for the maintenance of several specialized lymphoid microenvironments in secondary lymphoid tissue. Lymphoid-tissue architecture is highly plastic and requires continual homeostatic signaling to maintain its basal functional state. The cellularity of lymph nodes in adult mice was reduced by systemic blockade of lymphotoxin-beta receptor (LTbeta R) signaling with a soluble decoy receptor both in resting and reactive settings. This reduction in cellularity resulted from greatly impaired lymphocyte entry into lymph nodes due to decreased levels of peripheral lymph node addressing (PNAd) and MAdCAM on high endothelial venules (HEV). LTbeta R signaling was required to maintain normal levels of RNA expression of MAdCAM, and also of PNAd by regulating the expression of key enzymes and scaffold proteins required for its assembly. Thus, the homeostatic maintenance of functional HEV status in adult mice relies largely on LTbeta R signaling.

Ultrastructural anatomy of CALT follicles in the rabbit reveals characteristics of M-cells, germinal centres and high endothelial venules

Conjunctiva-associated lymphoid tissue (CALT) is a part of the eye-associated lymphoid tissue (EALT) at the ocular surface. Its lymphoid follicles are usually characterized by using light microscopy, but its ultrastructure remains largely unknown. In this study, flat whole-mount conjunctival tissues (n = 42) from 21 young adult rabbits were investigated native in reflected light, and further stained and cleared (n = 6), in paraffin histology sections (n = 6), scanning electron microscopy (SEM, n = 4) and transmission electron microscopy (TEM, n = 4). Secondary lymphoid follicles accumulated into a dense group nasally towards the lacrimal punctum of the lower lid. High endothelial venules (HEV) with typical ultrastructure occurred in the parafollicular zone. The bright germinal centre (GC) contained lymphoblasts, follicular dendritic cells, apoptotic cells and tingible body macrophages. The follicle-associated epithelium (FAE) was devoid of goblet cells and contained groups of lymphoid cells. TEM showed these cells to be located in cytoplasmic pockets of superficial electron-lucent cells with a thin cytoplasmic luminal lining that contained a fine filament meshwork and numerous endocytotic vesicles. These M-cells were sitting between and on top of the ordinary dense epithelial cells that were located basally and formed pillar-like structures. In stereoscopic SEM, the surface cells were very large, had a polygonal outline and covered cavernous spaces. The rabbit has a CALT with typical follicular morphology, including HEV for regulated lymphocyte migration and epithelial cells with ultrastructural characteristics of M-cells that allow antigen transport as indicated by the GC-reaction. The arrangement of these M-cells on top of and between epithelial pillar cells may reflect a special structural requirement of the multilayered CALT FAE.

Association of CXCL13 and CCL21 expression with the progressive organization of lymphoid-like structures in Sjögren's syndrome

OBJECTIVE

Ectopic lymphoneogenesis can occur in the salivary glands of Sjögren's syndrome (SS) patients and is associated with local antigen-driven B cell responses, autoantibody formation, and potential lymphomatous transformation. CXCL13 and CCL21 have been identified in salivary glands, but their role in ectopic lymphoneogenesis in SS remains unclear. This study aimed to evaluate the microanatomic association between CXCL13 and CCL21 expression and the acquisition of lymphoid features in periductal foci.

METHODS

Salivary glands from 37 SS patients and 9 chronic sialadenitis patients were analyzed by immunohistochemistry for T cell/B cell segregation, CD21+ follicular dendritic cell networks, and peripheral lymph node addressin (PNAd)-positive high endothelial venules (HEVs) in relationship to the size of the aggregates and the expression of CXCL13 and CCL21 within infiltrating cells, epithelium, and endothelium.

RESULTS

Grade 1 aggregates (10-50 lymphocytes) demonstrated predominance of nonorganized CD3+ cells, while grade 2 (>50 lymphocytes) and grade 3 (>50 with germinal centers) showed a progressive increase in CD20+ B cells and T cell/B cell segregation. This higher degree of lymphoid organization was significantly related to an increased expression of CXCL13 within infiltrating cells and PNAd+ HEV-associated CCL21-producing cells. Conversely, no association between lymphoid organization and lymphoid chemokine expression by epithelial cells was observed.

CONCLUSION

The acquisition of lymphoid features by inflammatory foci in SS is critically associated with the enlargement of the inflammatory foci and with the expression of CXCL13 and CCL21 within the infiltrate, but is not associated with their expression by epithelial cells. These data strongly support an active participation of CXCL13 and CCL21 in regulating the progressive organization and maintenance of periductal foci.

Adhesion molecule and homing receptor expression on blood and milk polymorphonuclear leukocytes during the periparturient period of dairy cattle

Adhesion molecule and homing receptor expression on blood and milk polymorphonuclear leukocytes (PMN) from periparturient dairy cattle was studied. Both percentages and the mean fluorescence intensity (MFI) of PMN expressing CD11a, CD44, CD62L, and LPAM-1 (alpha4 beta7) were evaluated at seven time points during the twenty-one day period post calving. CD11a and CD62L were expressed on 94-100% of PMN in both blood and milk and there were no significant differences in these percentages at any time point. LPAM-1 was expressed on 3-10% of the PMN in the blood and 13-45% in the milk and the percentage of cells expressing LPAM-1 in milk was significantly (P<0.05) greater than in blood at 0, 4, 10, 14, 18 and 21 days after calving. CD44 was expressed on 11-39% of the PMN in blood and 33-69% in the milk and the percentage of cells expressing CD44 in milk was significantly (P<0.05) greater than in blood at all time points. The MFI of CD11a on milk PMN was consistently higher than that of blood PMN throughout the study period and significantly (P<0.05) higher at days 4, 10 and 18 after calving.

Naive, Effector, and Memory T Lymphocytes Efficiently Scan Dendritic Cells In Vivo: Contact Frequency in T Cell Zones of Secondary Lymphoid Organs Does Not Depend on LFA-1 Expression and Facilitates Survival of Effector T Cells

Contact between T cells and dendritic cells (DCs) is required for their subsequent interaction leading to the induction of adaptive immune responses. Quantitative data regarding the contact frequencies of T cell subsets in different lymphoid organs and species are lacking. Therefore, naive, effector, and memory CD4 T cells were injected into rats in absence of the cognate Ag, and 0.5-96 h later, spleen, lymph nodes, and Peyer's patches were removed. Cryosections were analyzed for contact between donor T cells and endogenous DCs in the T cell zone, and donor cell proliferation. More than 60% of injected naive CD4 T cells were in contact with endogenous DCs at all time points and in all organs analyzed. Surprisingly, we were unable to detect any differences between naive, effector, and memory CD4 T cells despite different expression levels of surface molecules. In addition, contact frequency was similar for T cells in lymphoid organs of rats, mice, and humans; it was unaffected by the absence of LFA-1 (CD11a/CD18), and sustained effector T cells in an activated state. Thus, the architecture of the T cell zone rather than expression patterns of surface molecules determines the contact efficiency between T cells and DCs in vivo.

Lymphocyte subsets and adhesion molecule expression in milk and blood of periparturient dairy cattle

Fifteen Holstein dairy cattle were monitored for lymphocyte subsets and expression of adhesion molecules on cells in milk and blood at parturition and at intervals up to 21 days post-partum. Using flow cytometry, we determined percentages of T cells (CD4+, CD8+, gammadelta) and B cells from milk and blood of these cows. We also measured expression of adhesion molecules (CD62L, LFA-1, LPAM-1, and CD44) on lymphocytes in milk and blood. Significantly higher percentages of CD8+ cells were found in milk than in blood at all time points while significantly higher percentages of B cells were found in blood than in milk at all time points. There were minimal to no significant differences in percentages of CD4+ or gammadelta+ cells between milk and blood. Expression of adhesion molecules was consistently higher on all subsets of milk lymphocytes compared with blood lymphocytes. These differences were most pronounced and statistically significant at calving and in the first week following calving. CD62L, LPAM-1 and CD44 were expressed on a significantly higher percentage of lymphocytes in milk at calving than in milk at subsequent sampling times, while LFA-1 expression on lymphocytes in milk was significantly lower at calving than at subsequent times.

MAdCAM-1 expressing sacral lymph node in the lymphotoxin beta-deficient mouse provides a site for immune generation following vaginal herpes simplex virus-2 infection

The members of the lymphotoxin (LT) family of molecules play a critical role in lymphoid organogenesis. Whereas LT alpha-deficient mice lack all lymph nodes and Peyer's patches, mice deficient in LT beta retain mesenteric lymph nodes and cervical lymph nodes, suggesting that an LT beta-independent pathway exists for the generation of mucosal lymph nodes. In this study, we describe the presence of a lymph node in LT beta-deficient mice responsible for draining the genital mucosa. In the majority of LT beta-deficient mice, a lymph node was found near the iliac artery, slightly misplaced from the site of the sacral lymph node in wild-type mice. The sacral lymph node of the LT beta-deficient mice, as well as that of the wild-type mice, expressed the mucosal addressin cell adhesion molecule-1 similar to the mesenteric lymph node. Following intravaginal infection with HSV type 2, activated dendritic cells capable of stimulating a Th1 response were found in this sacral lymph node. Furthermore, normal HSV-2-specific IgG responses were generated in the LT beta-deficient mice following intravaginal HSV-2 infection even in the absence of the spleen. Therefore, an LT beta-independent pathway exists for the development of a lymph node associated with the genital mucosa, and such a lymph node serves to generate potent immune responses against viral challenge.

Lymph node fibroblastic reticular cells construct the stromal reticulum via contact with lymphocytes

The sophisticated microarchitecture of the lymph node, which is largely supported by a reticular network of fibroblastic reticular cells (FRCs) and extracellular matrix, is essential for immune function. How FRCs form the elaborate network and remodel it in response to lymphocyte activation is not understood. In this work, we established ERTR7⁺gp38⁺VCAM-1⁺ FRC lines and examined the production of the ER-TR7 antigen. Multiple chemokines produced by FRCs induced T cell and dendritic cell chemotaxis and adhesion to the FRC surface. FRCs can secrete the ER-TR7 antigen as an extracellular matrix component to make a reticular meshwork in response to contact with lymphocytes. The formation of the meshwork is induced by stimulation with tumor necrosis factor-α or lymphotoxin-α in combination with agonistic antibody to lymphotoxin-β receptor in a nuclear factor-κB (RelA)–dependent manner. These findings suggest that signals from lymphocytes induce FRCs to form the network that supports the movement and interactions of immune effectors within the lymph node.

Pathophysiology of airflow limitation in chronic obstructive pulmonary disease

The airflow limitation that defines chronic obstructive pulmonary disease (COPD) is the result of a prolonged time constant for lung emptying, caused by increased resistance of the small conducting airways and increased compliance of the lung as a result of emphysematous destruction. These lesions are associated with a chronic innate and adaptive inflammatory immune response of the host to a lifetime exposure to inhaled toxic gases and particles. Processes contributing to obstruction in the small conducting airways include disruption of the epithelial barrier, interference with mucociliary clearance apparatus that results in accumulation of inflammatory mucous exudates in the small airway lumen, infiltration of the airway walls by inflammatory cells, and deposition of connective tissue in the airway wall. This remodelling and repair thickens the airway walls, reduces lumen calibre, and restricts the normal increase in calibre produced by lung inflation. Emphysematous lung destruction is associated with an infiltration of the same type of inflammatory cells found in the airways. The centrilobular pattern of emphysematous destruction is most closely associated with cigarette smoking, and although it is initially focused on respiratory bronchioles, separate lesions coalesce to destroy large volumes of lung tissue. The panacinar pattern of emphysema is characterised by a more even involvement of the acinus and is associated with alpha1 antitrypsin deficiency. The technology needed to diagnose and quantitate the individual small airway and emphysema phenotypes present in people with COPD is being developed, and should prove helpful in the assessment of therapeutic interventions designed to modify the progress of either phenotype.

Expression of adhesion molecules on milk and blood lymphocytes from periparturient dairy cattle with Johne’s disease

Twelve dairy cows infected with Mycobacterium avium subsp. paratuberculosis were monitored for lymphocyte subsets and expression of adhesion molecules on cells in blood and milk at parturition and at intervals up to 21 days post-partum. Using fluorescent antibody labeling of cells and analysis by flow cytometry, we determined percentages of T cell subsets (CD4+, CD8+, gammadelta+) and expression of adhesion molecules (CD62L, LFA-1, LPAM-1, and CD44) on cells from blood and milk of these cows. Significantly higher percentages of CD8+ cells were found in milk than in blood at all time points; there were no significant differences in percentages of CD4+ or gammadelta+ cells. CD62L, LFA-1, and LPAM-1 were expressed on a significantly higher percentage of all T cell subsets in milk than in blood at various times after parturition. No differences were seen in expression of CD44. Increased percentages of T lymphocytes expressing adhesion molecules in milk compared to blood suggest that a migratory population of cells is being selectively recruited to the mammary gland from the circulation.

Differential gene expression profile of human tonsil high endothelial cells: implications for lymphocyte trafficking

Lymphocyte recirculation is dependent on the interactions of adhesion and signaling molecules expressed on lymphocytes and their partners on high endothelial cells (HEC). Many of the events in this process have yet to be molecularly characterized. To identify novel HEC-specific proteins with potential function in the recruitment cascade, we sequenced a normalized human tonsil HEC cDNA library (generated from an inflamed tonsil) from which lymphocyte and human umbilical vein endothelial cell cDNAs had been subtracted. One-thousand forty-nine sequences were analyzed. All but three mapped to known cDNAs or genomic DNAs. The two most abundant transcripts encoded alpha2-macroglobulin and hevin. The next-abundant transcripts encoded several other protease inhibitors, making this protein class the most prominent in HEC. Several endothelial-specific transcripts were also identified, including those encoding E-selectin, vascular cell adhesion molecule-1, vascular endothelial-junctional adhesion molecule, and platelet-endothelial cell adhesion molecule-1. The library contains a great diversity of transcripts, and studies of the encoded proteins will provide further insight into the complex biology of these specialized endothelial cells.

Effects of fluorescent and nonfluorescent tracing methods on lymphocyte migration in vivo

BACKGROUND

The use of fluorescent dyes to monitor in vivo cellular migration and proliferation has greatly expanded, but little is known about their potential influence on cell migration.

METHODS

Adoptive transfer studies of lymphocytes labeled with various dyes were performed, and their in vivo homing was compared with that of coinjected unlabeled control cells. In addition, in vitro migration and binding studies were performed to analyze the various steps of transmigration separately.

RESULTS

These data showed that the intracellular fluorescent dyes calcein acetoxymethyl ester, 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester, 5-chloromethylfluorescein diacetate, 5-(and-6)-carboxyfluorescein diacetate, succinimidyl ester, and fluorescein isothiocyanate affect in vivo homing of especially B lymphocytes to lymphoid organs, without any direct effect on in vitro chemotactic or adhesive activity. The only label that did not affect migration was the extracellular and nonfluorescent molecule biotin, provided that the labeling was performed at room temperature. Interestingly, by using the highly versatile congenic Ly5.1-Ly5.2 system, we also demonstrated intrinsic differences in lymphocyte migration based on allelic differences.

CONCLUSIONS

Our data showed that fluorescent labeling of lymphocytes has a severe effect on their homing capacity in vivo. Labeling of cells with biotin appeared to be a good alternative for this purpose; however, if direct fluorescence is required, the negative effects on cell migration should be considered.

Changes in the lymph node microenvironment induced by oncostatin M

Oncostatin M (OM) transforms the lymph node (LN) into a "super lymphoid organ" with 2 striking features: massive thymus-independent T-cell development and major expansion of the memory T-cell pool. We report that T-cell development in the LckOM LN is regulated by a cyclooxygenase-2 (COX-2)-dependent neoangiogenesis involving high endothelial venules (HEVs). That LN HEVs are particularlyrich in OM-receptor beta-chain provides aplausible explanation for the fact that extrathymic T-cell development in LckOM mice is limited to the LN. Moreover, we found that increased production of the CCL20 chemokine by LN stromal cells was instrumental in the expansion of the memory phenotype CD4 T-cell pool in LckOM mice. The generality of the latter finding was demonstrated by the fact that CCL20/CCR6 interactions increase the basal proliferation rate of CD62L(lo) CD4 T cells irrespective of their thymic (in non-OM-transgenic mice) or extrathymic (in LckOM mice) origin. To our knowledge, CCL20 is the first molecule found to increase the proliferation of memory phenotype CD4 T cells. These findings identify potential targets for the creation of thymic substitutes (LN HEVs) and for expansion of the CD4 memory T-cell compartment (CCL20).

Ribosomal immunotherapy for recurrent respiratory tract infections in children

Recurrent respiratory tract infections are common in children. They reflect the immaturity of the immune system in its encounter with environmental antigens. Little or no specific protective immune response has yet been established. These infections represent an important public health problem in terms of both treatment (anti-inflammatory or antibacterial drugs for children) and economy. Immunotherapy has been proposed as a means of preventing these recurrent infections by providing children with small doses of inactive bacterial antigens liable to trigger specific and protective immune responses. Among such drugs, ribosomal preparations (to which this review is limited) appear to be not only well tolerated, but also ideally targeted to induce mucosal responses. One preparation of ribosomal mucosal vaccine is commercially available in several countries. Numerous clinical trials in the world have confirmed the positive role of this mucosal ribosomal bacterial vaccine in significantly reducing the number of infections, courses of antibacterials, and absenteeism. In vitro and ex vivo investigations have confirmed that such vaccines indeed trigger protective specific immune responses.

A high endothelial venule secretory protein, mac25/angiomodulin, interacts with multiple high endothelial venule-associated molecules including chemokines

We previously reported that mac25/angiomodulin (AGM), a 30-kDa secretory protein, is abundantly expressed in high endothelial venules (HEVs), which play a crucial role in lymphocyte trafficking to the lymph nodes and Peyer's patches. We report that mac25/AGM interacts preferentially with certain molecules that are expressed in or around HEVs. In particular, mac25/AGM interacted with not only the extracellular matrix proteins and glycosaminoglycans that are expressed in most blood vessels including HEVs, but also with some chemokines that are implicated in the regulation of lymphocyte trafficking, such as the secondary lymphoid-tissue chemokine (SLC; CCL21), IFN-gamma-inducible protein 10 (IP-10; CXCL10), and RANTES (CCL5). The binding of mac25/AGM to SLC and IP-10 was dose-dependent and saturable. The binding to IP-10 could be inhibited by SLC but not by a non-mac25/AGM-binding chemokine, EBI1-ligand chemokine (ELC; CCL19). Interestingly, mac25/AGM failed to interact with 18 other chemokines, suggesting that it binds to certain chemokines preferentially. Immunohistochemical analysis indicated that mac25/AGM colocalizes at least partially with SLC and IP-10 at the basal lamina of HEVs. Upon binding with mac25/AGM, SLC and IP-10 retained all their Ca(2+)-signaling activity in vitro, suggesting that mac25/AGM can hold and present chemokines in the basal lamina of HEVs. These results imply that mac25/AGM plays a multifunctional role, serving not only as an adhesion protein to interact with glycosaminoglycans and extracellular matrix proteins but also as a molecule to present chemokines so that lymphocytes extravasating through HEVs receive further directional cues subsequent to the luminal encounter with lymphoid chemokines.

Molecular characterization of NF-HEV, a nuclear factor preferentially expressed in human high endothelial venules

Lymphocyte homing to secondary lymphoid tissue and lesions of chronic inflammation is directed by multi-step interactions between the circulating cells and the specialized endothelium of high endothelial venules (HEVs). In this study, we used the PCR-based method of suppression subtractive hybridization (SSH) to identify novel HEV genes by comparing freshly purified HEV endothelial cells (HEVECs) with nasal polyp-derived microvascular endothelial cells (PMECs). By this approach, we cloned the first nuclear factor preferentially expressed in HEVECs, designated nuclear factor from HEVs (NF-HEV). Virtual Northern and Western blot analyses showed strong expression of NF-HEV in HEVECs, compared to human umbilical vein endothelial cells (HUVECs) and PMECs. In situ hybridization and immunohistochemistry revealed that NF-HEV mRNA and protein are expressed at high levels and rather selectively by HEVECs in human tonsils, Peyers's patches, and lymph nodes. The NF-HEV protein was found to contain a bipartite nuclear localization signal, and was targeted to the nucleus when ectopically expressed in HUVECs and HeLa cells. Furthermore, endogenous NF-HEV was found in situ to be confined to the nucleus of tonsillar HEVECs. Finally, threading and molecular modeling studies suggested that the amino-terminal part of NF-HEV (aa 1-60) corresponds to a novel homeodomain-like Helix-Turn-Helix (HTH) DNA-binding domain. Similarly to the atypical homeodomain transcription factor Prox-1, which plays a critical role in the induction of the lymphatic endothelium phenotype, NF-HEV may be one of the key nuclear factors that controls the specialized HEV phenotype.

Junctional adhesion molecule-2 (JAM-2) promotes lymphocyte transendothelial migration

The molecular mechanisms underlying lymphocyte extravasation remain poorly characterized. We have recently identified junctional adhesion molecule-2 (JAM-2), and have shown that antibodies to JAM-2 stain high endothelial venules (HEVs) within lymph nodes and Peyer patches of adult mice. Here we show that mouse lymphocytes migrate in greater numbers across monolayers of endothelioma cells transfected with JAM-2. The significance of these findings to an understanding of both normal and pathologic lymphocyte extravasation prompted us to clone the human homologue of JAM-2. We herein demonstrate that an anti-JAM-2 antibody, or a soluble JAM-2 molecule, blocks the transmigration of primary human peripheral blood leukocytes across human umbilical vein endothelial cells expressing endogenous JAM-2. Furthermore, we show that JAM-2 is expressed on HEVs in human tonsil and on a subset of human leukocytes, suggesting that JAM-2 plays a central role in the regulation of transendothelial migration.

A composite picture of TcR alpha/beta(+) CD4(-)CD8(-) T Cells (alpha/beta-DNTCs) in humans with autoimmune lymphoproliferative syndrome

The discovery of an unusual T-cell subset characterized by the expression of the alpha/beta T-cell receptor without expression of either CD4 or CD8 [alpha/beta-double-negative T cells (alpha/beta-DNTCs)] provided critical insights in the evaluation of a "new" lymphoproliferative disorder known as autoimmune lymphoproliferative syndrome (ALPS). ALPS is a disorder of defective Fas-mediated lymphocyte apoptosis, manifested by accumulation of alpha/beta-DNTCs and other lymphocyte subsets, leading to lymphadenopathy and splenomegaly, autoimmunity, and an increased risk of lymphoma. The expanded population of alpha/beta-DNTCs from ALPS patients has a remarkable uniform phenotype that is for the most part similar to alpha/beta-DNTCs from mice with defective Fas (lpr) or Fas ligand (gld). This is in contrast to the minor alpha/beta-DNTC compartment in healthy individuals that contains multiple, immunophenotypically distinct subpopulations. Current data indicate that alpha/beta-DNTCs from ALPS patients are derived from cytotoxic CD8(+) T cells, chronically activated in vivo but anergic in vitro. Their anergic state may be related to persistent modifications of O-linked carbohydrates on cell surface molecules, such as CD43 and CD45, as well as to the increased presence of interleukin-10. Although largely consistent with a model of (linear) CD8(+) cytotoxic T-cell differentiation, the expression patterns of certain surface molecules, such as CD27 and CD28, are not consistent with this model. This may be the result of the perturbed homeostasis of lymphocytes in ALPS, thereby revealing pathways of differentiation and immunophenotypes, including phenotypes pertaining to cell surface glycosylation that are hidden from view in healthy individuals.

The strict regulation of lymphocyte migration to splenic white pulp does not involve common homing receptors

Although the spleen is the largest secondary lymphoid organ, little is known about the regulation of lymphocyte migration towards its different compartments of red and white pulp, in contrast to the well-studied mechanisms of lymphocyte homing to lymph nodes. Here we show that short-term trypsin treatment of lymphocytes cleaved off molecules involved in entry into lymph nodes, while homing to the splenic white pulp was unaltered. Prolonged trypsin treatment also abolished the ability of lymphocytes to enter the white pulp. Analysis of affected cell surface molecules and adoptive transfer studies in combination with blocking antibodies revealed that l-selectin, CD44, PSGL-1 and the alpha4 integrins are not required for migration to the white pulp. Although lymphocyte function-associated antigen-1 (LFA-1) is critical for entry into lymph nodes, we show here that in the absence of functional LFA-1 molecules, lymphocytes can still enter the white pulp, in spite of the high expression of intercellular adhesion molecule-1 on sinus lining cells in the marginal zone. The data indicate that adhesion molecules involved in lymphocyte homing to lymph nodes are not essential for migration towards the splenic white pulp, but that additional, trypsin-sensitive, and so far unidentified, molecules are required.

Vascular morphogenesis and differentiation after adoptive transfer of human endothelial cells to immunodeficient mice

To establish a model for adoptive transfer of endothelial cells, we transferred human umbilical vein endothelial cells (HUVECs) to immunodeficient mice (Rag 2(-/-)). HUVECs were suspended as single cells in Matrigel and injected subcutaneously in the abdominal midline. Within 10 days after injection, HUVECs expressed pseudopod-like extensions and began to accumulate in arrays. By day 20, we observed human vessels that contained erythrocytes, and on day 30 we observed perivascular cells that expressed smooth muscle actin, thus resembling mature vessels. Throughout the experimental period, HUVECs bound Ulex europaeus lectin and expressed CD31, VE-cadherin, von Willebrand factor, as well as ICAM-2. A fraction of the cells also expressed the proliferation marker Ki67. Moreover, the sialomucin CD34, which is rapidly down-regulated in cultured HUVECs, was reinduced in vivo. However, we found no reinduction of CD34 in cells cultured inside or on top of Matrigel in vitro. We also injected cells suspended in Matrigel around the catheter tip of implanted osmotic pumps. Delivery of recombinant human interferon-gamma by this route led to strong induction of MHC class II and ICAM-1 on the human vessels. In conclusion, isolated human endothelial cells can integrate with the murine vascular system to form functional capillaries and regain in vivo properties.

The heterogeneity shown by human plasma cells from tonsil, blood, and bone marrow reveals graded stages of increasing maturity, but local profiles of adhesion molecule expression

Plasma cells (PCs) are the final B-cell differentiation stage. Recent evidence reveals relevant functional differences within the PC compartment. In rodents, early PCs formed in secondary lymphoid tissues show enhanced apoptosis and short life span, whereas PCs present in a final destination organ, such as the bone marrow (BM), have reached a stable prolonged survival state. BM PCs arrive at this organ as a circulating precursor whose cellular nature remains uncertain. An initial aim of this study was to characterize this circulating cell. We hypothesized that antibody-secreting cells detectable in the human blood after immunization might be a candidate precursor. These cells were obtained from the blood of volunteers immunized 6 days earlier with tetanus toxoid (tet), and they were unambiguously identified as PCs, as demonstrated by their expression of the CD38(h) phenotype, by morphology, by immunoglobulin (Ig) intracytoplasmic staining, and by IgG-tet-secreting capacity in vitro. In addition, by using the common CD38(h) feature, human PCs from tonsil (as a possible source of early PCs), from blood from tet-immunized donors (as the putative precursors of BM PCs), and from BM (as a deposit organ) have been purified and their phenotypes compared. The results show that a variety of differentiation molecules, proteins involved in the control of apoptosis, the B-cell transcription factors, positive regulatory domain I-binding factor 1/B lymphocyte-induced maturation protein 1 and B cell-specific activating protein and, at least partially, the chemokine receptor CXCR4 were expressed by human PCs following a gradient of increasing maturity in the direction: tonsil-->blood-->BM. However, PCs from these different organs showed a local pattern of adhesion molecule expression. These observations are discussed in light of the complex physiology of the human PC compartment.

Neutrophil gelatinase B and chemokines in leukocytosis and stem cell mobilization

Leukocytosis is a physiopathological mechanism primarily to combat infections, whereas stem cell mobilization is induced for therapeutical purposes. Both processes are dependent on the balance between leukocyte and stem cell retention and mobilization. The retention is mediated by the specific architecture of the bone marrow, adhesion molecules and the production of chemokines in the bone marrow, which attract escaped immature cells to the marrow. Mobilization is the effect of the action of "peripheral" chemokines, such as interleukin-8 (IL-8 or CXCL8) and the remodeling of the matrix and basement membranes by matrix enzymes, such as gelatinase B (MMP-9). Recent studies lead to the conclusion that neutrophils, IL-8/CXCL8 and gelatinase B/MMP-9 play control roles in leukocytosis and stem cell mobilization. Neutrophils are the predominant circulating leukocyte type and IL-8/CXCL8 is the major neutrophil chemoattractant in humans. Gelatinase B and no gelatinase A is rapidly released from prestored granules after activation of neutrophils by IL-8/CXCL8. Moreover, neutrophils do not produce TIMP-1 and can chemically activate latent progelatinase B. Activated gelatinase B catalyses the aminoterminal truncation of IL-8/CXCL8 into a tenfold more potent chemokine. This implies that, when IL-8/CXCL8 appears in the circulation, the bone marrow is instructed to release neutrophils and concomitantly stem cells. These studies suggest that IL-8/CXCL8 and gelatinase B/MMP-9 are targets for the modulation of stem cell mobilization.

L-selectin in health and disease

This article reviews recent advances in the knowledge of the role of L-selectin, an adhesion molecule that is expressed on the surface of circulating leucocytes, in animal and human physiology and pathophysiology. After a brief discussion on nomenclature and structure, it progresses through the evidence for expression and regulation of L-selectin, cell collection and purification, physiological function and roles. The special role of knock out mice and monoclonal antibodies in determining a role for L-selectin in inflammatory states is described before proceeding to discuss the importance of L-selectin ligands and shed L-selectin. A second section describes a role for L-selectin in pathophysiological states in animals and man, with special reference to trauma, systemic inflammatory syndromes and sepsis. The review concludes with a summary of the potential role of anti-inflammatory medication and L-selectin blockers in the management of inflammation.

Current concepts in lymphocyte homing and recirculation

The immune system consists of a complex collection of leukocytes and dendritic cells that surveys most tissues in the body for the appearance of foreign antigens. For an efficient immune response, the interaction and co-localization of antigen-presenting cells, costimulatory helper cells and effector cells are crucial parameters. Therefore, the migration routes of antigen-presenting cells and potential antigen-specific lymphocytes merge in secondary lymphoid organs in order to increase the likelihood and speed of a lymphocyte finding its cognate antigen. Additionally, antigen-primed effector cells are directed to the tissue where they are most likely to encounter their cognate antigen. This highly organized and efficient antigen encounter is based on a continuous recirculation of antigen-specific lymphocytes between blood, peripheral tissue, and secondary lymphoid organs. Moreover, the efficacy of the immune system is further increased by the ability of different lymphocyte subsets to recirculate only through distinct tissues. The scope of this review is to outline the concept and mechanisms of lymphocyte homing and recirculation and to discuss the significance for the immune defense. Current models in leukocyte homing and recirculation and the underlying molecular functions of implicated cell adhesion molecules, chemokines, and chemokine receptors are discussed.

Transient IFN-gamma synthesis in the lymph node draining a dermal site loaded with UV-irradiated herpes simplex virus type 1: an NK- and CD3-dependent process regulated by IL-12 but not by IFN-alpha/beta

Our previous studies have shown that UV-inactivated, non-replicating herpes simplex virus type 1 (UV-HSV-1) triggers early and transient synthesis of IFN-alpha/beta in the mouse regional lymph node when delivered upstream (i.e. in the ear dermis). In this study, it is demonstrated, by use of a quantitative RT-PCR readout assay, that IFN-gamma mRNA expression was rapidly and transiently upregulated in draining lymph nodes when UV-HSV-1 was delivered in the ear dermis of C57Bl/6 mice. An increased number of IFN-gamma-producing cells was also detected in the lymph node by flow cytometric analysis. Two different subsets of cells, namely DX5(+) NK cells and CD3epsilon(+) T cells, accounted for this early IFN-gamma synthesis. Prompt upregulation of IFN-alpha and IL-12p40 mRNA was also recorded. We took advantage of IFN-alpha/beta-receptor knockout and wild-type 129 mice to study a potential role of IFN-alpha/beta in the signalling pathway leading to IFN-gamma transcription/translation. IFN-gamma mRNA upregulation still occurred in IFN-alpha/beta-receptor(-/-) mice, showing that IFN-alpha/beta was dispensable. The use of IL-12-neutralizing antibodies, prior to UV-HSV-1 delivery, confirmed the major role played by IL-12 in the early/transient IFN-gamma burst.

In vivo selection of sFv from phage display libraries

The development of phage display technology has facilitated the development of many new and sometimes novel antibody based reagents for scientific research. However, present methods for selection from phage-sFv display libraries are limited to selection against purified antigens or ex vivo cells of known origin and phenotype. Existing methods therefore preclude the isolation of sFv against unknown molecules in their natural environment, where expression is complex and subject to diverse control mechanisms. Since such a complex environment is difficult to mimic in vitro, the development of an in vivo selection procedure would greatly enhance the selection from phage display antibody libraries and lead to the development of reagents against cell surface molecules in their natural environment. This would be particularly advantageous for isolation of sFv against vascular endothelium which can readily change phenotype when cultured and is believed to express molecules in a tissue specific manner and in response to different stimuli. We describe here the development of an in vivo selection procedure in the mouse and demonstrate its potential for the selection of sFv from a phage-sFv library. The target antigen for one sFv is expressed solely on the thymic endothelium, while the second, a 165-170 kDa molecule in present on both thymic endothelium and the perivascular epithelium.

The parting of the endothelium: miracle, or simply a junctional affair?

Leukocyte extravasation from the blood across the endothelium is vital for the functioning of the immune system. Our understanding of the early steps of this process has developed rapidly. However, it is still unclear how leukocytes undergo the final step, migrating through the junctions that mediate adhesion between adjacent endothelial cells, while preserving the barrier function of the endothelium. The first stage of transmigration - tethering and rolling - is mediated by interactions between selectins on the surface of leukocytes and glycosylated proteins such as GlyCAM-1 on the surface of endothelial cells. Stimulation of the leukocyte by chemokines then induces tight adhesion, which involves binding of activated leukocyte integrins to endothelial ICAM-1/VCAM-1 molecules. Passage of the leukocyte across the endothelium appears to require delocalization of certain endothelial cell molecules and proteolytic degradation of junctional complexes.

The role of chemokines in the microenvironmental control of T versus B cell arrest in Peyer's patch high endothelial venules

Chemokines have been hypothesized to contribute to the selectivity of lymphocyte trafficking not only as chemoattractants, but also by triggering integrin-dependent sticking (arrest) of circulating lymphocytes at venular sites of extravasation. We show that T cells roll on most Peyer's patch high endothelial venules (PP-HEVs), but preferentially arrest in segments displaying high levels of luminal secondary lymphoid tissue chemokine (SLC) (6Ckine, Exodus-2, thymus-derived chemotactic agent 4 [TCA-4]). This arrest is selectively inhibited by functional deletion (desensitization) of CC chemokine receptor 7 (CCR7), the receptor for SLC and for macrophage inflammatory protein (MIP)-3beta (EBV-induced molecule 1 ligand chemokine [ELC]), and does not occur in mutant DDD/1 mice that are deficient in these CCR7 ligands. In contrast, pertussis toxin-sensitive B cell sticking does not require SLC or MIP-3beta signaling, and occurs efficiently in SLC(low/-) HEV segments in wild-type mice, and in the SLC-negative HEVs of DDD/1 mice. Remarkably, sites of T and B cell firm adhesion are segregated in PPs, with HEVs supporting B cell accumulation concentrated in or near follicles, the target domain of most B cells entering PPs, whereas T cells preferentially accumulate in interfollicular HEVs. Our findings reveal a fundamental difference in signaling requirements for PP-HEV recognition by T and B cells, and describe an unexpected level of specialization of HEVs that may allow differential, segmental control of lymphocyte subset recruitment into functionally distinct lymphoid microenvironments in vivo.

Heterogeneity of endothelial cells: the specialized phenotype of human high endothelial venules characterized by suppression subtractive hybridization

High endothelial venules (HEVs) are specialized postcapillary venules, found in lymphoid organs and chronically inflamed tissues, that support high levels of lymphocyte extravasation from the blood. Molecular characterization of HEV endothelial cells (HEVECs) has been hampered by difficulties in their purification and in vitro maintenance. To overcome these limitations, we developed a strategy combining the use of freshly purified HEVECs ( approximately 98% positive for the HEV-specific marker MECA-79) and the recently described polymerase chain reaction (PCR)-based cDNA subtraction cloning procedure called suppression subtractive hybridization (SSH). Subtracted probes prepared by SSH from small amounts of total RNA were used to screen a HEVEC cDNA library. This resulted in cloning of 22 cDNAs preferentially expressed in HEVECs, which encode the promiscuous chemokine receptor DARC, mitochondrial components, and matricellular proteins. The latter included hevin, thrombospondin-1, and mac25/IGFBP-rP1, which is a secreted growth factor-binding protein previously found to accumulate specifically in tumor blood vessels. Biochemical and histochemical analysis confirmed the identification of mac25 and DARC as novel markers of the HEVECs. Ultrastructural immunolocalization revealed a noticeable association of mac25 and MECA-79 antigens with microvillous processes near the endothelial cell junctions, suggesting a role for mac25 in the control of lymphocyte emigration. This study shows that PCR-based SSH is useful for cloning of differentially expressed genes in very small samples.