Lymphocyte homing and leukocyte rolling and migration are impaired in L-selectin-deficient mice

Fever-Range Hyperthermia Enhances L-Selectin-Dependent Adhesion of Lymphocytes to Vascular Endothelium

The L-selectin leukocyte adhesion molecule plays an important role in controlling leukocyte extravasation in peripheral lymph nodes and at sites of tissue injury or infection. Although febrile responses during infection and inflammation are associated with enhanced immune activity, the contribution of fever-range temperatures to controlling lymphocyte recruitment to tissues has not been previously examined. In this report we provide evidence that direct exposure of lymphocytes to fever-range temperatures (38–41°C) in vitro for 9 to 24 h resulted in a >100% increase in L-selectin-dependent adhesion of these cells to lymph node high endothelial venules (HEV). Moreover, culture of lymphocytes under hyperthermia conditions markedly enhanced the ability of these cells to traffic in an L-selectin-dependent manner to peripheral lymph nodes, mesenteric lymph nodes, and Peyer’s patches. In contrast, febrile temperatures did not increase LFA-1 function as assessed by measuring lymphocyte adhesion to ICAM-1–3T3 transfectants. Fever-range hyperthermia further did not increase L-selectin surface density on lymphocytes or L-selectin-dependent recognition of soluble carbohydrate substrates; however, a marked increase in ultrastructural immunogold-labeling of L-selectin was observed in response to thermal stimuli. These results suggest that elevated temperatures enhance L-selectin adhesion and/or avidity through the regulation of L-selectin conformation or organization in the plasma membrane. Finally, the observed thermal effects on L-selectin adhesion were attributed to soluble factors in the conditioned medium of heat-treated cells. Taken together, these data provide new insight into the potential physiologic role of the febrile response in enhancing lymphocyte recruitment to tissues through the regulation of L-selectin adhesion.

Role of primary and secondary capture for leukocyte accumulation in vivo

Leukocyte accumulation during inflammation depends on the concerted action of selectin and integrin adhesion molecules, which promote capture, rolling, and arrest of these cells on activated endothelium. In addition to interacting with endothelial cells, leukocytes can also adhere to already adherent leukocytes through an L-selectin-dependent mechanism. Initiation of adhesion through this mechanism has been called nucleation and leads to characteristic geometric patterns (ie, clusters and strings) of adherent leukocytes in flow chambers. We have used intravital microscopy of tumor necrosis factor-alpha (TNF-alpha)-treated mouse cremaster muscles to quantitatively investigate the potential role of leukocyte-leukocyte adhesion in initiating and maintaining the leukocyte clusters that are commonly observed in inflamed venules. Our data show that in TNF-alpha-treated venules with diameters between 23 and 108 microm, leukocyte adhesion occurs in clusters that are 19 to 50 microm long and 8 to 44 microm wide. They are almost entirely made up of slow-rolling leukocytes. Of all leukocytes recruited into a cluster (100%), the majority enter the cluster rolling along the endothelium and sharply reduce their velocity in the absence (59%) or presence (15%) of other leukocytes in proximity (one cell diameter). Some of the rolling leukocytes (17%) pass through the cluster without reducing their velocity. Recruitment of leukocytes from the free flow regime into a cluster is a rare event and accounts for only 7 (1.2%) of 476 leukocytes arriving in the cluster. However, of the leukocytes captured from the free flow, 6 initiated contact with a slow-rolling leukocyte rather than making direct contact with the endothelium. Our data show that leukocyte-leukocyte interactions can occur in vivo but are not important for cluster formation. This is confirmed by the observation of normal cluster formation in L-selectin-deficient mice, in which leukocyte-leukocyte interactions under flow are abolished. We conclude that leukocyte-mediated nucleation contributes little to leukocyte recruitment during inflammation in vivo. Cluster formation appears to be dominated by areas of endothelium with a higher expression of E-selectin, because cluster formation is greatly reduced in E-selectin-deficient mice.

L-selectin expression in CD34 positive cells in chronic myeloid leukemia

L-selectin is a cell adhesion molecule, expressed on leukocytes and involved in the regulation of leukocyte traffic. This adhesion receptor is implicated in hematopoiesis by the interaction of hematopoietic stem cells and progenitors to stroma in the bone marrow microenvironment. We found that L-selectin expression on CD34++ cells from patients with chronic myelogenous leukemia (CML) is decreased or deficient, reflecting one of the features of malignant CML progenitors. In this review, we briefly describe the structure and function of L-selectin, and its role in hematopoiesis and its expression in leukemia and lymphoma. Finally, we discuss the abnormal adhesiveness of CML progenitor cells, and the role of L-selectin in this defect.

Distinct Recirculating and Non-Recirculating B-Lymphocyte Pools in the Peripheral Blood Are Defined by Coordinated Expression of CD21 and L-Selectin

The continual recirculation of lymphocytes between the blood, tissues, and lymph is essential for the coordination and dissemination of immune responses. We have compared the functional and phenotypic properties of lymphocytes isolated from blood and lymph, the two major migratory populations. Lymph-borne lymphocytes migrated readily into the lymphatic recirculation pathway, but greater than one third of all peripheral blood lymphocytes (PBLs) were excluded from the lymphatic circuit and showed an enhanced migration to the spleen. Phenotypic analysis showed that most non-recirculating PBLs were B cells. The migration competence of B cells correlated with the surface expression of CD21 and L-selectin; recirculating B cells expressed both of these molecules, whereas non-recirculating B cells lacked both antigens. These results establish that blood contains distinct pools of lymphocytes that differ in their recirculation competence. Clearly, blood sampling is not an efficient method to directly measure the status of the recirculating immune system, and implies important constraints and restrictions in the interpretation of experimental or clinical data that include phenotypic and quantitative analyses of blood lymphocytes.

Distinct Recirculating and Non-Recirculating B-Lymphocyte Pools in the Peripheral Blood Are Defined by Coordinated Expression of CD21 and L-Selectin

The continual recirculation of lymphocytes between the blood, tissues, and lymph is essential for the coordination and dissemination of immune responses. We have compared the functional and phenotypic properties of lymphocytes isolated from blood and lymph, the two major migratory populations. Lymph-borne lymphocytes migrated readily into the lymphatic recirculation pathway, but greater than one third of all peripheral blood lymphocytes (PBLs) were excluded from the lymphatic circuit and showed an enhanced migration to the spleen. Phenotypic analysis showed that most non-recirculating PBLs were B cells. The migration competence of B cells correlated with the surface expression of CD21 and L-selectin; recirculating B cells expressed both of these molecules, whereas non-recirculating B cells lacked both antigens. These results establish that blood contains distinct pools of lymphocytes that differ in their recirculation competence. Clearly, blood sampling is not an efficient method to directly measure the status of the recirculating immune system, and implies important constraints and restrictions in the interpretation of experimental or clinical data that include phenotypic and quantitative analyses of blood lymphocytes.

Induction of tolerance toward rat cardiac allografts by treatment with allochimeric class I MHC antigen and FTY720

BACKGROUND

The combination of FTY 720, a novel immunosuppressant, and allochimeric class I MHC proteins bearing donor-type amino acid (aa) epitope substitutions for host-type sequences induces tolerance of Wistar Furth (WF; RT1.Au) heart allografts in ACI (RT1.Aa) recipients.

METHODS

Allochimeric alpha(1h)l58-80-RT1.Aa proteins were produced by substituting the allogeneic nucleotide sequence encoding 10 aa residues unique to the alpha1 helical (alpha1h) region of RT1.Al Lewis (Asp58, Arg62, Glu63, Gln65, Lys66, Gly69, Asn70, Asn73, Ser77, and Asn80) for native RT1.Aa residues. The RT1.Au and the RT1.Al haplotypes share four of these aa (Arg62, Glu63, Gln65, and Gly69). A baculovirus/Spodoptera frugiperda insect cell system was used to express the alpha(1h)l58-80-RT1.Aa proteins.

RESULTS

The addition of a 3-day oral gavage of 0.05 mg/kg/day FTY720 to a single portal vein injection of 10 microg alpha(1h)l58-80-RT1.Aa protein induced permanent acceptance of WF heart allografts in 16 of 26 ACI recipients (>100 days); the alpha(1h)l58-80-RT1.Aa protein alone only modestly prolonged WF heart survival (13.8+/-0.8 days). The same tolerogenic protocol did not prolong the survival of third-party Brown Norway (RT1.An) heart allografts (14.3+/-2.5 days) compared with FTY720 alone (14.0+/-2.3 days; NS). Tolerant ACI recipients bearing primary WF heart allografts for more than 100 days accepted second WF hearts, but promptly rejected third-party Brown Norway heart grafts (9.3+/-1.5 days). The tolerant state was transferred to irradiated ACI rats (400 rad) with either purified T cells (4-10 x 10[7]) or serum (1-2 ml) from tolerant hosts, and was not broken by daily intraperitoneal injections of interleukin-2 (1000 U/day; 7 days).

CONCLUSIONS

The combination of allochimeric protein with FTY720 induces transplantation tolerance, a state that may be associated with the appearance of donor-specific regulatory factors.

Regulation of hematopoiesis by microvascular endothelium

The bone marrow microenvironment is a complex three dimensional structure where hematopoietic stem cells proliferate, mature, migrate into the sinusoidal space, and enter the circulation in an exquisitely regulated fashion. Stromal cells within the BM microenvironment provide a suitable environment for self-renewal, proliferation and differentiation of hematopoietic stem cells. Within the hematopoietic microenvironment, whether it is embryonic yolk sac, fetal liver, or adult bone marrow, microvascular endothelium not only acts as a gatekeeper controlling the trafficking and homing of hematopoietic progenitors, but also provides cellular contact and secretes cytokines that allows for the preservation of the steady state hematopoiesis. Recently, homogenous monolayers of bone marrow endothelial cells (BMEC) have been isolated and cultivated in tissue culture. Long-term coculture studies have shown that BMEC monolayers are unique type of endothelium and can support long-term proliferation of hematopoietic progenitor cells particularly megakaryocytic and myeloid progenitor cells by constitutive elaboration of lineage-specific cytokines such as G-CSF, GM-CSF, M-CSF, Kit-ligand, IL6, FLK-2 ligand, and leukemia inhibitory factor. Direct cellular contact between hematopoietic progenitor cells and BMEC monolayers through specific adhesion molecules including beta1, beta2 integrins and selectins play a critical role in trafficking and possibly proliferation of hematopoietic stem cells. Dysfunction of microvascular endothelial cells within the hematopoietic microenvironment may result in stem cell disorders and progression to aplastic anemias, and contribute to graft failure during bone marrow transplantation. Further studies on the role of microvascular endothelium in the regulation of hematopoietic stem cell homing and proliferation may enhance our understanding of the pathophysiology of stem cell and leukemic disorders.

L-selectin activates JNK via src-like tyrosine kinases and the small G-protein Rac

Selectin and alpha 4 beta 7-integrins have been shown to mediate transient leucocyte interactions with endothelial cells which is a crucial step in the initial immune response to pathogens. We have previously shown that stimulation of T lymphocytes via L-selectin results in activation of a signalling cascade from the L-selectin molecule via the tyrosine kinase p56lck and tyrosine phosphorylation of L-selectin to the stimulation of p21Ras and Rac proteins. In the present study we demonstrate that stimulation of Jurkat T lymphocytes via L-selectin results in an activation of Jun N-terminal kinase (JNK) but not of p38-K. L-selectin-initiated activation of JNK is mediated by src-like tyrosine kinases and the small G-protein Rac 1/2, since genetic or pharmacological inhibition of p56lck or Rac proteins prevent the stimulation of JNK by L-selectin. Thus, the data point to a novel signalling cascade from L-selectin via src-like tyrosine kinases and Rac proteins to JNK.

Endothelial cell adhesion molecule expression in gene-targeted mice

Gene-targeted mice are now routinely employed as tools for defining the contribution of different leukocyte and endothelial cell adhesion molecules to the leukocyte recruitment and tissue injury associated with acute and chronic inflammation. The objective of this study was to determine whether gene-targeted mice that are deficient in CD11/CD18, intracellular adhesion molecule-1 (ICAM-1), or P-selectin exhibit an altered constitutive or induced expression of the endothelial cell adhesion molecules E- and P-selectin. The gene-targeted mice were all developed in the 129Sv mouse strain and backcrossed into C57B1/6J mice. The number of backcrosses ranged between 8 (P-selectin) and 10 (CD18 and ICAM-1) generations. The dual-radiolabeled monoclonal antibody technique was used to quantify E- and P-selectin expression in different vascular beds. In the unstimulated state, E-selectin expression was significantly elevated (relative to wild-type mice) in the stomach, large intestine, and brain of mutants deficient in ICAM-1. In general, constitutive expression of P-selectin did not differ between wild-type, ICAM-1-deficient, and CD11/CD18-deficient mutants. In CD11/CD18-deficient mice, tumor necrosis factor-alpha (TNF-alpha) administration elicited a more profound upregulation of P-selectin in several vascular beds, compared with wild-type and ICAM-1-deficient mice. E-selectin expression in brain of TNF-alpha-stimulated, ICAM-1-deficient, and P-selectin-deficient mice was attenuated compared with wild-type mice. These findings indicate that chronic deficiency of some of the adhesion glycoproteins that mediate leukocyte recruitment alters basal and induced surface expression of other adhesion molecules on endothelial cells.

Leukocyte adhesion deficiency type 1 (LAD-1)/variant. A novel immunodeficiency syndrome characterized by dysfunctional beta2 integrins

Leukocyte adhesion deficiency (LAD) is characterized by the inability of leukocytes, in particular neutrophilic granulocytes, to emigrate from the bloodstream towards sites of inflammation. Infectious foci are nonpurulent and may eventually become necrotic because of abnormal wound healing. LAD-1 is characterized by the absence of the beta2 integrins (CD11/CD18) on leukocytes. When expression is completely absent, patients often die within the first year. However, low levels of beta2 expression may result in a milder clinical picture of recurrent infection, which offers a better prognosis. In this paper, we describe the in vivo and in vitro findings on a patient with clinical features of a mild LAD-1 disorder, i.e., suffering from bacterial infections without apparent pus formation in the presence of a striking granulocytosis, showing no delayed-type hypersensitivity reaction upon skin testing, no specific antibody generation, but normal in vitro T cell proliferation responses after immunization. Expression levels of CD11/CD18 proteins were completely normal, but leukocyte activation did not result in CD11/ CD18 activation and high-avidity ligand-binding. In vitro chemotaxis and endothelial transmigration of the neutrophils as well as leukocyte aggregation responses were almost absent. On the other hand, beta1 and beta3 integrin-mediated adhesion functions were completely normal. During follow-up, a bleeding tendency related to decreased beta3 activation became clinically apparent, different from previously described cellular adhesion molecule variants. Therefore, this is the first well-documented case of a clinical combined immunodeficiency syndrome that results from nonfunctional CD11/CD18 molecules, and thus designated LAD-1/ variant.

Adhesion molecules in haemopoiesis

In the adult mammal, haemopoiesis is restricted to the extravascular compartment of the bone marrow (BM) where primitive haemopoietic stem cells (HSC) and their clonogenic progeny develop in intimate contiguity with a heterogeneous population of stromal cells that comprise the haemopoietic micro-environment (HM). Although the importance of cellular interactions between primitive haemopoietic progenitor cells (HPC) and marrow stromal cells is well established, precise definition of the nature of many of these interactions at the molecular level is lacking and remains an objective of fundamental importance to understanding of haemopoietic regulation. Current data suggest that a wide variety of cell surface molecules representing several adhesion molecule superfamilies, including integrins, selectins, sialomucins and the immunoglobulin gene superfamily, are involved in supporting cell-cell and cell-extracellular matrix (ECM) interactions. These diverse CAM-ligand interactions, rather than simply serving to initiate and maintain contact between HPC and stromal cells and ECM components, also have an additional, more direct role in controlling the growth and development of primitive haemopoietic cells.

Analysis of expression of lymphocyte homing-related adhesion molecules in ALY mice deficient in lymph nodes and Peyer's patches

The aly, alymphoplasia, is an autosomal recessive mutation in mice of an unknown etiology, which induces total aplasia of lymph nodes and Peyer's patches. We hypothesized that the lack of lymphoid tissue may be due to abnormalities of lymphocyte traffic into these tissues. Therefore, we analyzed the expression of various adhesion molecules associated with lymphocyte homing. Among the adhesion molecules examined, all were normally expressed except the mucosal addressin MAdCAM-1. In aly/aly mice MAdCAM-1 was absent in the spleen at mRNA and protein levels, but was normally expressed in the intestinal venules. The FISH analysis and linkage analysis using microsatellite markers demonstrated that the MAdCAM-1 gene is located on chromosome 10, indicating that MAdCAM-1 is not encoded by the aly gene, which is located on chromosome 11. Our results indicate that the aberrant expression of MAdCAM-1 is not the direct cause of aly mutation but rather a secondary defect.

Vascular adhesion protein 1 (VAP-1) mediates lymphocyte subtype-specific, selectin-independent recognition of vascular endothelium in human lymph nodes

Interactions between lymphocyte surface receptors and their ligands on vascular endothelial cells regulate the exit of lymphocytes from the circulation. Distinct subsets of mononuclear cells bind to high endothelial venules (HEVs) in different lymphoid organs to a different extent, but the molecular mechanisms behind this selectivity have remained poorly characterized. Here we show that vascular adhesion protein-1 (VAP-1) mediates subtype-specific binding of CD8-positive T cells and natural killer cells to human endothelium. VAP-1-dependent, oligosaccharide-dependent peripheral lymph node (PLN) HEV adhesion under shear was independent of L-selectin, P-selectin glycoprotein ligand 1, and alpha4 integrins, the known lymphocyte receptors involved in the initial recognition of endothelial cells. PLN HEV adhesion was also critically dependent on peripheral lymph node vascular addressins (PNAds), but lymphocyte L-selectin was absolutely required for PNAd binding. Most lymphocytes relied on both PNAd and VAP-1 in HEV binding. The overlapping function of L-selectin ligands and VAP-1 in PLN introduces a new control point into the lymphocyte extravasation process. Finally, intravital microscopy revealed that VAP-1 is involved in initial interactions between human lymphocytes and endothelial cells in inflamed rabbit mesenterial venules in vivo. In conclusion, VAP-1 is a novel contact-initiating ligand that discriminates between different subpopulations of mononuclear cells and is an appealing target for selective modulation of adhesion of CD8- and CD16-positive effector cells.

The fate of activated T cells migrating through the body: rescue from apoptosis in the tissue of origin

After activation within a lymphoid tissue, T lymphocytes enter the blood, where they circulate and then re-enter many organs. However, they predominantly end up in the tissue of origin, a phenomenon so far thought to be caused by organ-specific homing. We analyzed the fate of T cells from different sources stimulated via the T cell receptor and CD28 and then injected intravenously into rats. Our results showed that preferential proliferation and reduced apoptosis, rather than preferential immigration, were responsible for the accumulation of activated T cells in the tissue of origin, explaining how immune responses can spread from site to site but still be restricted to certain regions. Manipulating the life span of such cells might be a promising approach to influencing immune responses.

In vitro experimental studies of sialyl Lewis x and sialyl Lewis a on endothelial and carcinoma cells: crucial glycans on selectin ligands

Extravasation from the blood of malignant tumour cells that form metastasis and leukocytes that go into tissues require contact between selectins and their sialyl Lewis x and sialyl Lewis a (sLe(x) and sLe(a) respectively) decorated ligands. Endothelial cells have been shown to express sLe(x) epitopes in lymph nodes and at sites of inflammation, and this is crucial for the selectin-dependent leukocyte traffic. Besides the ability to synthesize sLe(x) on sialylated N-acetyllactosamine via the action of alpha(1,3)fucosyltransferase(s), endothelial cells can also degrade sLe(x) to Lewis x through the action of alpha(2,3)sialidase(s). In addition, several epithelial tumors possess the machinery to synthesize sLe(x), which facilitates their adhesion to endothelial E- and P-selectin.

Selectin-carbohydrate interactions during inflammation and metastasis

L-, E-, and P-selectin are membrane-anchored, C-type lectins that initiate tethering and rolling of flowing leukocytes on endothelial cells, platelets, or other leukocytes during inflammation. The selectins bind to sialylated, fucosylated, or, in some cases, sulfated glycans on glycoproteins, glycolipids, or proteoglycans. However, they bind with relatively high affinity or avidity to only a few, appropriately modified glycoproteins on leukocytes or endothelial cells. One leukocyte mucin, PSGL-1, tethers flowing leukocytes to P-selectin on activated platelets or endothelial cells, and also helps tether leukocytes to L-selectin on other leukocytes. The physiologic expression of the selectins is tightly controlled to limit the inflammatory response. But dysregulated expression of the selectins may contribute to inflammatory and thrombotic disorders, and perhaps to tumor metastases.

The role of E- and P-selectin in neutrophil and monocyte migration in adjuvant-induced arthritis in the rat

The role of the endothelial adhesion molecules E- and P-selectin in leukocyte accumulation in arthritis is not known. We investigated this role in rat adjuvant arthritis by employing adhesion function-blocking monoclonal antibodies (mAb) to rat P- and E-selectin. The acute migration (2 h) of radiolabeled rat blood neutrophils and monocytes to joints and skin was determined. Anti-P-selectin mAb significantly reduced accumulation of monocytes (by 50%) and neutrophils (by 40%) in the talar joint, and of neutrophils in tail joints (by 90%). Anti-E-selectin mAb alone did not attenuate leukocyte migration, but when combined with anti-P-selectin mAb, it enhanced inhibition of neutrophil accumulation in the talar and carpal joints. In the same animals, anti-P-selectin mAb significantly inhibited neutrophil and monocyte migration to dermal inflammatory reactions induced by zymosan-activated rat serum (ZAS) containing the chemotactic factor C5ades Arg, endotoxin (LPS), interferon-gamma (IFN-gamma) and tumor necrosis factor-alpha (TNF-alpha). In contrast, anti-E-selectin mAb alone had no effect on monocyte or neutrophil accumulation in inflamed skin of arthritic animals, but again enhanced the inhibition when combined with mAb to P-selectin. The addition of anti-L-selectin mAb to anti-P- and E-selectin mAb did not further suppress monocyte or neutrophil migration to inflamed skin or joints. These results demonstrate that optimal leukocyte migration to arthritic joints and inflamed skin is P-selectin dependent, and E-selectin is not essential. However, E-selectin contributes to migration when P-selectin mechanisms are not operative. L-selectin does not play a role in E- and P-selectin-independent leukocyte migration to joints or skin inflammation in arthritic rats. However, it is likely that additional selectin-independent pathways also mediate neutrophil and monocyte migration to joint and skin inflammation.

Expression of a P-selectin ligand in zona pellucida of porcine oocytes and P-selectin on acrosomal membrane of porcine sperm cells. Potential implications for their involvement in sperm-egg interactions

The selectin family of cell adhesion molecules mediates initial leukocyte adhesion to vascular endothelial cells at sites of inflammation. O-glycan structural similarities between oligosaccharides from human leukocyte P-selectin glycoprotein ligand-1 (PSGL-1) and from zona pellucida glycoproteins of porcine oocytes indicate the possible existence of a P-selectin ligand in the zona pellucida. Here, using biochemical as well as morphological approaches, we demonstrate that a P-selectin ligand is expressed in the porcine zona pellucida. In addition, a search for a specific receptor for this ligand leads to the identification of P-selectin on the acrosomal membrane of porcine sperm cells. In vitro binding of porcine acrosome-reacted sperm cells to oocytes was found to be Ca2+ dependent and inhibitable with either P-selectin, P-selectin receptor-globulin, or leukocyte adhesion blocking antibodies against P-selectin and PSGL-1. Moreover, porcine sperm cells were found to be capable of binding to human promyeloid cell line HL-60. Taken together, our findings implicate a potential role for the oocyte P-selectin ligand and the sperm P-selectin in porcine sperm-egg interactions.

L-selectin is not essential for naive CD4 cell trafficking or development of primary responses in Peyer's patches

We showed previously that L-selectin-dependent recirculation of naive CD4 cells is essential for development of primary responses in peripheral lymph nodes. Recent studies suggest that L-selectin is also required for lymphocyte entry into gut mucosal lymphoid tissues that include Peyer's patches and mesenteric lymph nodes. Here we show that anti-L-selectin antibody, MEL-14, inhibited homing of a rigorously purified, homogenous population of naive CD4 cells into both of these tissues as well as peripheral lymph nodes, directly demonstrating a role for this receptor in regulating entry into gut-associated sites. However, in intact animals, treatment with MEL-14 resulted in the loss of naive CD4 cells (CD45RBhi, CD44lo from peripheral lymph nodes but not Peyer's patches, whereas mesenteric lymph nodes were intermediate in this regard. In mice primed by parenteral immunization with keyhole limpet hemocyanin (KLH), primary CD4 responses were readily detected in both. Peyer's patches and mesenteric lymph nodes, and were not affected by exposure to MEL-14. Indeed, similar frequencies of KLH-specific CD4 cells were recovered from both of these tissues irrespective of MEL-14 treatment. The results indicate that interactions with L-selectin can be circumvented to allow entry of naive CD4 cells into Peyer's patches but not peripheral lymph nodes.

Selectins, T-cell rolling and inflammation

The selectins, a family of Ca(2+)-dependent lectins, are expressed on inflamed vascular endothelium and some leukocyte subsets, and mediate adhesive contacts between blood cells and vessel walls. These interactions are loose and reversible, operate under conditions of shear flow, and result in leukocyte rolling along the vessel wall. The structure of the selectins and their ligands makes them uniquely suited for supporting the type of bond formation and dissociation that must prevail in order for a cell to be able to roll under conditions of flow. Because rolling precedes (and appears to be essential for) the integrin-mediated firm arrest before extravasation in response to inflammatory or infectious stimuli, inhibition of selectin function has potential for anti-inflammatory therapy, but also presents some significant challenges because of the complexity of the processes involved.

Isolated P-selectin glycoprotein ligand-1 dynamic adhesion to P- and E-selectin

Leukocyte adhesion to vascular endothelium under flow involves an adhesion cascade consisting of multiple receptor pairs that may function in an overlapping fashion. P-selectin glycoprotein ligand-1 (PSGL-1) and L-selectin have been implicated in neutrophil adhesion to P- and E-selectin under flow conditions. To study, in isolation, the interaction of PSGL-1 with P- and E-selectin under flow, we developed an in vitro model in which various recombinant regions of extracellular PSGL-1 were coupled to 10-microm-diameter microspheres. In a parallel plate chamber with well defined flow conditions, live time video microscopy analyses revealed that microspheres coated with PSGL-1 attached and rolled on 4-h tumor necrosis factor-alpha-activated endothelial cell monolayers, which express high levels of E-selectin, and CHO monolayers stably expressing E- or P-selectin. Further studies using CHO-E and -P monolayers demonstrate that the first 19 amino acids of PSGL-1 are sufficient for attachment and rolling on both E- and P-selectin and suggest that a sialyl Lewis x-containing glycan at Threonine-16 is critical for this sequence of amino acids to mediate attachment to E- and P-selectin. The data also demonstrate that a sulfated, anionic polypeptide segment within the amino terminus of PSGL-1 is necessary for PSGL-1-mediated attachment to P- but not to E-selectin. In addition, the results suggest that PSGL-1 has more than one binding site for E-selectin: one site located within the first 19 amino acids of PSGL-1 and one or more sites located between amino acids 19 through 148.

Differential Effect of E-Selectin Antibodies on Neutrophil Rolling and Recruitment to Inflammatory Sites

The selectins are inducible adhesion molecules critically important for the inflammatory response. We investigate here the functional effects of three monoclonal antibodies (MoAbs) raised against murine E-selectin (9A9, 10E6, and 10E9.6) on neutrophil recruitment in vivo, leukocyte rolling and circulating leukocyte concentrations in vivo, and adhesion of myeloid cells to E-selectin transfectants and recombinant E-selectin–IgG fusion protein in vitro. MoAbs 9A9 and 10E6 map to the lectin and epidermal growth factor (EGF)-like domains of murine E-selectin, whereas 10E9.6 binds to the consensus repeat region. 10E9.6 blocked neutrophil recruitment in a model of thioglycollate-induced peritonitis in Balb/c mice by more than 90% but had no effect in C57BL/6 mice. 9A9 and 10E6 blocked neutrophil recruitment in this assay only when combined with a P-selectin antibody, 5H1. Neither 9A9 nor 10E9.6 alone blocked leukocyte rolling in tumor necrosis factor-α–treated venules of Balb/c mice, but 9A9 almost completely inhibited leukocyte rolling when combined with the function-blocking murine P-selectin MoAb, RB40.34. In contrast, 10E9.6 had no effect on leukocyte rolling in RB40.34-treated Balb/c or C57BL/6 mice. 10E9.6 did not affect adhesion of myeloid cells to E-selectin transfectants or attachment, rolling, and detachment of myeloid cells to murine E-selectin–IgG fusion protein. However, adhesion was completely blocked in the same assays by 9A9. Taken together, these results indicate that E-selectin serves a function, other than rolling, that appears to be critically important for neutrophil recruitment to inflammatory sites in Balb/c mice.

Antibody to VLA-4, but not to L-selectin, protects neuronal M2 muscarinic receptors in antigen-challenged guinea pig airways

Antigen challenge of sensitized guinea pigs decreases the function of inhibitory M2 muscarinic autoreceptors on parasympathetic nerves in the lung, potentiating vagally induced bronchoconstriction. Loss of M2 receptor function is associated with the accumulation of eosinophils around airway nerves. To determine whether recruitment of eosinophils via expression of VLA-4 and L-selectin is critical for loss of M2 receptor function, guinea pigs were pretreated with monoclonal antibodies to VLA-4 (HP1/2) or L-selectin (LAM1-116). Guinea pigs were sensitized and challenged with ovalbumin, and M2 receptor function was tested. In controls, blockade of neuronal M2 muscarinic receptors by gallamine potentiated vagally induced bronchoconstriction, while in challenged animals this effect was markedly reduced, confirming M2 receptor dysfunction. Pretreatment with HP1/2, but not with LAM1-116, protected M2 receptor function in the antigen-challenged animals. HP1/2 also inhibited the development of hyperresponsiveness, and selectively inhibited accumulation of eosinophils in the lungs as measured by lavage and histology. Thus, inhibition of eosinophil influx into the lungs protects the function of M2 muscarinic receptors, and in so doing, prevents hyperresponsiveness in antigen-challenged guinea pigs.

Characterization of a 180 kDa molecule apparently reactive with recombinant L-selectin

In the present study we identified a 180 kDa molecule (p180) in rat lymph nodes (LN) apparently reactive with silkworm derived recombinant L-selectin (LEC-IgG) in a Ca(2+)-dependent manner. Analysis of amino acid sequence revealed that p180 has a strong homology to the macrophage mannose receptor (MMR), which was corroborated by the observation that p180 reacted with polyclonal anti-alveolar MMR antibody and mannosyl-BSA-agarose. In agreement with this notion, the binding of p180 to the silkworm LEC-IgG was inhibited by alpha-methyl-D-mannoside. However, in sharp contrast to its reactivity against the silkworm LEC-IgG, p180 failed to bind LEC-IgG produced by COS-7 cells, suggesting that p180 reacted with the silkworm LEC-IgG through the recognition of oligomannose-type oligosaccharides expressed on the silkworm products and that the lectin activity of L-selectin was not involved in the interaction. These results, together with the immunohistochemical studies showing that p180 was absent from the majority of high endothelial venules (HEV) but present in medullary macrophages, led us to conclude that p180 obtained from LN lysates by the use of the silkworm LEC-IgG is not a physiological ligand for L-selectin, warning against the use of recombinant proteins expressed in the baculovirus/ silkworm expression system for the detection of carbohydrate ligands.

Intervention in endotoxin shock by sulfatide (I3SO3-GalCer) with a concomitant reduction in tumor necrosis factor alpha production

Accumulating evidence indicates that tumor necrosis factor alpha (TNF-alpha) is a principal mediator of endotoxin shock. We previously reported that the action as well as the production of TNF requires the adhesion of leukocytes to the endothelium through integrin beta2 and intercellular adhesion molecule 1. In order to elucidate the roles of the initial interaction of the leukocytes with the endothelium through the selectins, we have examined the effects of a ligand for L- and P-selectins, sulfatide, on endotoxin shock in mice. Consistent with previous reports, a single injection of a high dose of endotoxin caused acute lethality, marked hypotension, leukopenia, and elevation in serum TNF-alpha levels. Pretreatment with sulfatide prevented acute lethality and hypotension, but not leukopenia, with a concomitant reduction in the increase in serum TNF-alpha levels. Moreover, pretreatment with sulfatide inhibited lipopolysaccharide (LPS)-induced TNF-alpha production by a human monocytic cell line, THP-1, in a dose-dependent manner. These results suggest either that selectin is critically involved in conferring the responsiveness of leukocytes to LPS or that sulfatide interferes with the intracellular signaling pathway which leads to TNF-alpha gene activation.

HECA-452+ T cells migrate through superficial vascular plexus but not through deep vascular plexus endothelium

The skin is nourished by two interconnected vascular systems, the superficial vascular plexus coursing just beneath the epidermis and the deep vascular plexus located above the subcutaneous tissue. Skin inflammatory T cells in diseases, such as psoriasis or dermatitis, strikingly aim for the superficial vascular plexus without involving the deep vascular plexus, and the infiltrating T cells bear a distinct phenotype expressing the cutaneous lymphocyte-associated antigen, which is recognized by mAb HECA-452. We wanted to know whether HECA-452+ lymphocytes indeed are able to distinguish between superficial and deep vascular plexus homing sites. Employing the hu-SCID mouse model grafted with human skin and human T cells, as described previously, we developed a new skin-grafting strategy providing superficial and deep vascular plexus skin specimens placed separately onto the same mouse. Fourteen days after allogeneic human T cell grafting, both human skin sites were densely infiltrated by human T cells, but only T cells within the superficial vascular plexus, but not within the deep vascular plexus, expressed the cutaneous lymphocyte-associated antigen. IL-2 and IFN-gamma expression and allogeneic vessel destruction were present within both superficial and deep vascular plexus skin. This model provides direct evidence that expression of a specific homing receptor is indeed able to direct lymphocyte traffic, not only to a distinct organ but also to a distinct vascular bed within one organ.

L-selectin regulates actin polymerisation via activation of the small G-protein Rac2

L-selectin mediated adhesion to endothelial cells is a crucial step in the immune response to pathogens (1, 2) and in lymphocyte homing (3, 4). Selectin molecules mediate leukocyte rolling on endothelial cells, the initial step of adhesion (5, 6). We have previously shown that stimulation of Jurkat T-lymphocytes via L-selectin results in activation of the p21Ras pathway and synthesis of reactive oxygen intermediates (7). Here, we show that cellular stimulation via L-selectin induces a change of cytoskeleton organisation demonstrated by a tenfold increase of actin filament polymerisation. This actin polymerisation is mediated by a Ras and Rac2 regulated pathway, since inhibition of Ras by transient transfection of transdominant inhibitory N17Ras or suppression of Rac2 protein expression by antisense oligonucleotides prevents L-selectin triggered actin polymerisation. Our results point to a signaling cascade from L-selectin via Ras and Rac2 to actin filaments, which might be important for leukocyte adhesion.

Monocyte adhesion to activated aortic endothelium: role of L-selectin and heparan sulfate proteoglycans

This study examines the role of L-selectin in monocyte adhesion to arterial endothelium, a key pathogenic event of atherosclerosis. Using a nonstatic (rotation) adhesion assay, we observed that monocyte binding to bovine aortic endothelium at 4 degrees C increased four to nine times upon endothelium activation with tumor necrosis factor (TNF)-alpha. mAb-blocking experiments demonstrated that L-selectin mediates a major part (64 +/- 18%) of monocyte attachment. Videomicroscopy experiments performed under flow indicated that monocytes abruptly halted on 8-h TNF-alpha-activated aortic endothelium, approximately 80% of monocyte attachment being mediated by L-selectin. Flow cytometric studies with a L-selectin/IgM heavy chain chimeric protein showed calcium-dependent L-selectin binding to cytokine-activated and, unexpectedly, unactivated aortic cells. Soluble L-selectin binding was completely inhibited by anti-L-selectin mAb or by aortic cell exposure to trypsin. Experiments with cycloheximide, chlorate, or neuraminidase showed that protein synthesis and sulfate groups, but not sialic acid residues, were essential for L-selectin counterreceptor function. Moreover, heparin lyases partially inhibited soluble L-selectin binding to cytokine-activated aortic cells, whereas a stronger inhibition was seen with unstimulated endothelial cells, suggesting that cytokine activation could induce the expression of additional ligand(s) for L-selectin, distinct from heparan sulfate proteoglycans. Under flow, endothelial cell treatment with heparinase inhibited by approximately 80% monocyte attachment to TNF-alpha-activated aortic endothelium, indicating a major role for heparan sulfate proteoglycans in monocyte-endothelial interactions. Thus, L-selectin mediates monocyte attachment to activated aortic endothelium, and heparan sulfate proteoglycans serve as arterial ligands for monocyte L-selectin.

HIV induces homing of resting T lymphocytes to lymph nodes

In vivo infection with human immunodeficiency virus type 1 (HIV-1) leads to gradual depletion of CD4+ T lymphocytes from the peripheral blood and later from the lymphoid organs. The mechanism of CD4 cell depletion is not known. HIV can only replicate in dividing lymphocytes, but greater than 98% of the lymphocytes in vivo at any given time are resting and are not permissive for productive infection. We found that exposure of resting CD4+ T lymphocytes to HIV-1 transiently upregulated expression of cell surface CD62L (L-selectin), the receptor for homing to lymph nodes, with concomitant enhanced ability of these cells to bind to lymph node high endothelial venules in an ex vivo homing assay (increased approximately 12-fold, P < 0.001) and to home from the blood into lymph nodes following intravenous injection into SCID mice. This suggested the possibility that decreases in numbers of CD4+ T lymphocytes in the blood of HIV-1-infected subjects may reflect enhanced homing of abortively infected, resting lymphocytes into lymph nodes rather than direct virus replication in and killing of these cells, and may explain development of lymphadenopathy at a time when numbers of CD4+ T lymphocytes in the blood fall.

L-selectin from human, but not from mouse neutrophils binds directly to E-selectin

L-Selectin on neutrophils as well as inducible E- and P-selectin on endothelium are involved in the recruitment of neutrophils into inflamed tissue. Based on cell attachment assays, L-selectin was suggested to function as a carbohydrate presenting ligand for E- and P-selectin. However, previous affinity isolation experiments with an E-selectin-Ig fusion protein had failed to detect L-selectin among the isolated E-selectin ligands from mouse neutrophils. We show here that L-selectin from human neutrophils, in contrast to mouse neutrophils, can be affinity-isolated as a major ligand from total cell extracts using E-selectin-Ig as affinity probe. Binding of human L-selectin to E-selectin was direct, since purified L-selectin could be reprecipitated with E-selectin-Ig. Recognition of L-selectin was abolished by sialidase-treatment, required Ca2+, and was resistant to treatment with endoglycosidase F. Binding of L-selectin to a P-selectin-Ig fusion protein was not observed. In agreement with the biochemical data, the anti-L-selectin mAb DREG56 inhibited rolling of human neutrophils on immobilized E-selectin-Ig but not on P-selectin-Ig. No such inhibitory effect was seen with the anti-mouse L-selectin mAb MEL14 on mouse neutrophils. Rolling of E-selectin transfectants on purified and immobilized human L-selectin was inhibited by mAb DREG56. We conclude that L-selectin on human neutrophils is a major glycoprotein ligand among very few glycoproteins that can be isolated by an E-selectin affinity matrix. The clear difference between human and mouse L-selectin suggests that E-selectin-binding carbohydrate moieties are attached to different protein scaffolds in different species.

Threshold levels of fluid shear promote leukocyte adhesion through selectins (CD62L,P,E)

Leukocyte adhesion through L-selectin to peripheral node addressin (PNAd, also known as MECA-79 antigen), an L-selectin ligand expressed on high endothelial venules, has been shown to require a minimum level of fluid shear stress to sustain rolling interactions (Finger, E.B., K.D. Puri, R. Alon, M.B. Lawrence, V.H. von Andrian, and T.A. Springer. 1996. Nature (Lond.). 379:266-269). Here, we show that fluid shear above a threshold of 0.5 dyn/cm2 wall shear stress significantly enhances HL-60 myelocyte rolling on P- and E-selectin at site densities of 200/microm2 and below. In addition, gravitational force is sufficient to detach HL-60 cells from P- and E-selectin substrates in the absence, but not in the presence, of flow. It appears that fluid shear-induced torque is critical for the maintenance of leukocyte rolling. K562 cells transfected with P-selectin glycoprotein ligand-1, a ligand for P-selectin, showed a similar reduction in rolling on P-selectin as the wall shear stress was lowered below 0.5 dyn/cm2. Similarly, 300.19 cells transfected with L-selectin failed to roll on PNAd below this level of wall shear stress, indicating that the requirement for minimum levels of shear force is not cell type specific. Rolling of leukocytes mediated by the selectins could be reinitiated within seconds by increasing the level of wall shear stress, suggesting that fluid shear did not modulate receptor avidity. Intravital microscopy of cremaster muscle venules indicated that the leukocyte rolling flux fraction was reduced at blood centerline velocities less than 1 mm/s in a model in which rolling is mediated by L- and P-selectin. Similar observations were made in L-selectin-deficient mice in which leukocyte rolling is entirely P-selectin dependent. Leukocyte adhesion through all three selectins appears to be significantly enhanced by a threshold level of fluid shear stress.

Cryptococcal polysaccharides bind to CD18 on human neutrophils

CD18, the beta chain of the beta 2 integrin family of adhesion molecules, is associated with three different alpha chains (CD11a, -b, and -c) and is expressed on the surface of all types of leukocytes. CD18-containing molecules are up-regulated on the surface of neutrophils (polymorphonuclear cells [PMN]) in response to chemotactic agents and are implicated in mediating adhesion to an inflamed endothelium, which is a prerequisite to migration of PMN into infected tissues. In a previous study, we found that a cryptococcal culture filtrate (CneF), when injected into the bloodstream of mice to simulate the antigenemia in cryptococcosis, inhibits PMN accumulation at the site of an inflammatory stimulus. In the present study, we assessed the ability of CneF and its individual components, i.e., glucuronoxylomannan (GXM), galactoxylomannan (GalXM), and mannoprotein (MP), to interact with CD18 on human PMN. CneF labeled with 14C was shown to bind to human PMN in a dose-dependent manner. Pretreatment of PMN with anti-CD18, but not an isotype-matched control monoclonal antibody (MAb) or anti-CD11a MAb, blocked the binding of 14C-labeled CneF to PMN. In addition, CneF, GXM, and GalXM but not MP significantly blocked the binding of the anti-CD18 MAb to CD18 on the surface of unactivated and formyl methionyl leucyl phenylalanine-activated PMN as determined by indirect immunofluorescence staining and flow cytometric analysis. In the same experiments, the cryptococcal polysaccharides did not affect the binding of an anti-CD11a or anti-L-selectin MAb to the surface of PMN at 4 degrees C. The results suggest that CneF and its components GXM and GalXM bind to CD18 on human PMN. Based on our findings, we propose that CD18 is a possible molecular target of cryptococcal polysaccharides and that binding of the polysaccharides to CD18 has the potential to inhibit leukocyte infiltration into inflammatory sites.

Neutrophil margination, sequestration, and emigration in the lungs of L-selectin-deficient mice

These studies tested the hypothesis that L-selectin plays a role in neutrophil traffic in the lungs, particularly in neutrophil margination, sequestration, and emigration, using L-selectin-deficient mice. No defect in neutrophil margination within either capillaries or arterioles and venules was observed in uninflamed lungs of L-selectin-deficient mice. The initial rapid sequestration of neutrophils within the pulmonary capillaries 1 min after intravascular injection of complement fragments was not prevented. In contrast, L-selectin did contribute to the prolonged neutrophil sequestration (> or = 5 min). Interestingly, neutrophil accumulation within noncapillary microvessels required L-selectin at both 1 and 5 min after complement injection. During bacterial pneumonias, L-selectin played a role in neutrophil accumulation within noncapillary microvessels in response to either Escherichia coli or Streptococcus pneumoniae and within capillaries in response to E. coli but not S. pneumoniae. However, L-selectin was not required for emigration of neutrophils or edema in response to either organism. These studies demonstrate a role for L-selectin in the prolonged sequestration of neutrophils in response to intravascular complement fragments, in the intracapillary accumulation of neutrophils during E. coli-induced pneumonia, and in the accumulation of neutrophils within noncapillary microvessels when induced by either intravascular complement fragments or

Mechanisms of inflammation and leukocyte activation

This article reviews the current status of the knowledge of mechanisms of activating inflammatory responses. It also describes inflammatory mediators, adhesion proteins, the inflammatory process itself, and the molecular mechanisms controlling inflammatory cell activation and regulation.

L-selectin activates the Ras pathway via the tyrosine kinase p56lck

Selectins mediate rolling, the initial step of leukocyte adhesion to endothelial cells [Springer, T. A. (1995) Annu. Rev. Physiol. 57, 827-872 and Butcher, E. C. (1991) Cell 67, 1033-1036]. In this study we show that L-selectin triggering of Jurkat cells using different antibodies or glycomimetics resulted in activation of the src-tyrosine kinase p56lck; tyrosine phosphorylation of intracellular proteins, in particular mitogen-activating protein kinase and L-selectin; and association of Grb2/Sos with L-selectin. This association correlated with an activation of p21Ras, mitogen-activating protein kinase, Rac2, and a transient increase of 2-O synthesis. Stimulation of the Ras pathway by L-selectin requires functional p56lck, since p56lck-deficient Jurkat cells (JCaM1.6) do not show tyrosine phosphorylation, association of L-selectin with Grb2/Sos, and activation of Ras upon L-selectin triggering. Transfection of JCaM1.6 cells with p56lck reconstitutes the observed signaling events. Genetic inhibition of Ras or Rac2 prevented Rac2 stimulation and 2-O synthesis, respectively. The specificity and the physiological significance of the observed signaling cascade is indicated by stimulation of L-selectin-transfected P815, L-selectin-positive CEM or peripheral blood lymphocytes resulting in the same activation events as in Jurkat cells. Our results point to a signaling cascade from L-selectin via p56lck, Grb2/Sos, Ras, and Rac2 to 2-O.

The route of antigen entry determines the requirement for L-selectin during immune responses

L-selectin, an adhesion molecule constitutively expressed on leukocytes, is important for primary adhesion and extravasation of lymphocytes at specialized high endothelial venules within lymph nodes and other leukocytes at sites of inflammation. We have generated L-selectin-deficient mice by targeted disruption, and have confirmed a previously reported phenotype which includes strikingly impaired contact hypersensitivity (CHS) responses to reactive haptens (Tedder, T.F., D.A. Steeber, and P. Pizcueta. 1995. J. Exp. Med. 181:2259-2264; Xu, J.C., I.S. Grewal, G.P. Geba, and R.A. Flavell. 1996. 183:589-598.). Since the mechanism of this impairment has not been clarified, we sought to define the stage(s) at which the CHS response is affected in L-selectin-deficient mice. We show that epidermal Langerhans cells in L-selectin-deficient mice are normal in number, migrate to peripheral lymph nodes appropriately, and are functional in presenting allogeneic and haptenic antigens. Moreover, T cells, as well as neutrophil and monocyte effector populations, are fully capable of entry into the inflamed skin sites in the absence of L-selectin. Thus, antigen presentation and effector mechanisms are intact in L-selectin deficient mice. In contrast, virtually no antigen-specific T cells can be found within draining peripheral nodes after a contact challenge, suggesting that the defect resides primarily in the inability of antigen-specific T cells to home to and be activated in these nodes. Indeed, L-selectin-deficient mice mount completely normal CHS responses when alternate routes of immunization are used. These studies pinpoint the lesion in CHS to a discrete stage of the afferent limb of the response, clarify the role of L-selectin on effector populations, and illustrate the critical importance of the route of antigen entry to the successful execution of an immune response.

Herpes simplex virus induces appearance of interferon-alpha/beta-producing cells and partially interferon-alpha/beta-dependent accumulation of leukocytes in murine regional lymph nodes

As in vivo experimental system involving the local induction of interferon-alpha/beta (IFN-alpha/beta) responses was established in mice by injecting s.c. ultraviolet (UV)-inactivated herpes simplex virus (HSV) in the right ears, the left ears receiving phosphate-buffered saline (PBS) as a control. Circulating IFN-alpha/beta was present in blood as early as 6 h postinjection, and little or none was found 24 h postinjection. Identification of IFN-alpha/beta-producing cells, carried out by immunohistochemistry and in situ hybridization, demonstrated that the IFN response occurred mainly in the lymph node draining the HSV-injected ear and not in the contralateral lymph node. Occasionally, IFN-alpha/beta-producing cells were found in the spleen and in the skin. The injected HSV caused an inflammatory reaction in the skin and an almost threefold enlargement of the draining lymph node within 6 h. The latter was characterized by a general accumulation of all major lymphocyte subsets and a striking infiltration of neutrophils. Injection s.c. of neutralizing anti-IFN-alpha/beta antibodies before HSV injection reduced the increase in size of the draining lymph node by approximately 50% at 6 h, and no significant effects were seen at 24 h. The localization of cells producing IFN-alpha/beta in the lymph node and the capacity of such IFN-alpha/beta to at least partially mediate an early accumulation of cells suggest that the local IFN-alpha/beta response may have an important role in the initiation of early antiviral immune responses.

Distinct phenotype of E-selectin-deficient mice. E-selectin is required for slow leukocyte rolling in vivo

Leukocyte capture and rolling are mediated by calcium-dependent lectins expressed on most leukocytes (L-selectin) and the vascular endothelium (P- and E-selectin). To study the role of the selectins during inflammation, we have investigated leukocyte rolling in venules of tumor necrosis factor-alpha (TNF-alpha)-treated mouse cremaster muscles in wild-type mice and gene-targeted mice with homozygous deficiency for L-, P-, or E-selectin (L-/-, P-/-, or E-/-, respectively). TNF-alpha treatment induces expression of E-selectin and increases expression of P-selectin on endothelial cells. Consistent with previous reports of redundant P- and E-selectin function, a combination of monoclonal antibodies (mAbs) against P- and E-selectin (RB40.34 and 9A9, respectively) was necessary to block rolling in wild-type mice. The rolling leukocyte flux fraction (percent rolling cells) in L-/- mice was similar to that in wild-type mice, but rolling in these mice was blocked by a P-selectin mAb. The velocity of rolling leukocytes in TNF-alpha-treated wild-type, P-/-, or L-/- mice was 5 to 10 times slower (3 to 7 microns/s) than during trauma-induced rolling (20 to 50 microns/s). In contrast, leukocytes in venules of TNF-alpha-treated E-/- mice rolled significantly faster (12 to 20 microns/s): the rolling leukocyte flux fraction was more than doubled compared with wild-type, L-/-, or P-/- mice; and the number of adherent leukocytes was reduced. Addition of an E-selectin mAb, but not a P-selectin mAb, increased rolling flux fraction and rolling velocity in wild-type mice. Histological analysis revealed that 90% to 95% of all leukocytes interacting (rolling and adherent) with the venular endothelium in TNF-alpha-treated wild-type, L-/-, P-/-, and E-/- mice were granulocytes. These results identify a previously unrecognized phenotype of E-/- mice by establishing that at the site densities prevailing in vivo, E-selectin in responsible for slow (approximately 5 microns/s) granulocyte rolling. E-selectin-dependent slow rolling drastically increases the transit time of leukocytes rolling through an inflamed tissue and thus aids in targeting leukocytes activated by chemoattractants to the inflammatory microenvironment.

L-selectin crosslinking induces integrin-dependent adhesion: evidence for a signaling pathway involving PTK but not PKC

L-selectin mediates the initial contact of leukocytes with the endothelium prior to extravasation. Here we demonstrate that L-selectin engagement can induce rapid and avid integrin-dependent T cell adhesion to recombinant immobilized cell adhesion molecules (CAMs) including ICAM-1, ICAM-3, and VCAM-1, as well as to the extracellular matrix protein fibronectin (FN). L-selectin-induced adhesion to these integrin ligands shares characteristics with CD3 mAb- or phorbol ester-induced adhesion in requiring metabolic energy, tyrosine kinase and ligand-stimulated Ca2+ channel activity. However, L-selectin-induced adhesion is distinct from that induced by phorbol ester or CD3 crosslinking in being relatively independent of protein kinase C (PKC) activity and actin polymerization. Consistent with the higher levels of L-selectin expression on CD45RA+ (naive) cells, L-selectin crosslinking induces a greater proportion of naive relative to memory cell binding to CAMs and FN. In contrast, exposure to phorbol ester or CD3 crosslinking is more effective in inducing CD45RO+ (memory) cell adhesion. Thus, in addition to its role in leukocyte capture and rolling on the endothelium, L-selectin may contribute to beta 1 and beta 2 integrin-dependent binding and arrest.

Vanin-1, a novel GPI-linked perivascular molecule involved in thymus homing

Migration of hematopoietic precursor cells to the thymus is shown to depend upon a novel molecule called Vanin-1 expressed by perivascular thymic stromal cells. An anti-Vanin-1 antibody blocks the binding of pro-T cells to thymic sections in vitro, the in vivo accumulation of bone marrow cells around cortical thymic vessels, and long-term thymic regeneration. Thus, it interferes with the entry, and not the differentiation, of hematopoietic precursor cells. The Vanin-1 gene codes for a GPI-anchored 70 kDa protein that shows homology only with human biotinidase. Transfection of thymic stromal cells with the Vanin-1 cDNA enhances thymocyte adhesion in vitro. These data suggest that Vanin-1 regulates late adhesion steps of thymus homing under physiological, noninflammatory conditions.

Autoimmune disease of exocrine organs in immunodeficient alymphoplasia mice: a spontaneous model for Sjögren's syndrome

Mice homozygous for an autosomal recessive mutation aly (alymphoplasia) lack both lymph nodes and Peyer's patches, and show defects in both humoral and cellular immunity. Histopathological analysis revealed chronic inflammatory changes in exocrine organs such as the salivary gland, lacrimal gland, and pancreas of the homozygotes (aly/aly), but not the heterozygotes (aly/+). In these exocrine organs, mononuclear cells consisting mainly of CD4+ T cells infiltrate periductal areas, and, in some cases, the cell infiltration extended to lobules. The inflammatory changes in exocrine organs were transferred by a T cell-enriched fraction of spleen cells from homozygous animals. These results suggest that autoimmune mechanisms mediated by self-reactive T cells may be involved in the inflammatory lesions of various exocrine organs in the homozygous mice, although these mice show immunodeficiency. Inflammatory changes were also observed in the lung of the homozygotes. Since Sjögren's syndrome is characterized by diffuse lymphocyte infiltration in the periductal areas of the lacrimal and salivary glands and is occasionally associated with pulmonary disease, aly/aly mice may serve as a unique spontaneous model of Sjögren's syndrome.

Interactions through L-selectin between leukocytes and adherent leukocytes nucleate rolling adhesions on selectins and VCAM-1 in shear flow

We demonstrate an additional step and a positive feedback loop in leukocyte accumulation on inflamed endothelium. Leukocytes in shear flow bind to adherent leukocytes through L-selectin/ligand interactions and subsequently bind downstream and roll on inflamed endothelium, purified E-selectin, P-selectin, L-selectin, VCAM-1, or peripheral node addressin. Thus adherent leukocytes nucleate formation of strings of rolling cells and synergistically enhance leukocyte accumulation. Neutrophils, monocytes, and activated T cell lines, but not peripheral blood T lymphocytes, tether to each other through L-selectin. L-selectin is not involved in direct binding to either E- or P-selectin and is not a major counterreceptor of endothelial selectins. Leukocyte-leukocyte tethers are more tolerant to high shear than direct tethers to endothelial selectins and, like other L-selectin-mediated interactions, require a shear threshold. Synergism between leukocyte-leukocyte and leukocyte-endothelial interactions introduces novel regulatory mechanisms in recruitment of leukocytes in inflammation.

P-selectin glycoprotein ligand 1 is a ligand for L-selectin on neutrophils, monocytes, and CD34+ hematopoietic progenitor cells

Selectins play a critical role in initiating leukocyte binding to vascular endothelium. In addition, in vitro experiments have shown that neutrophils use L-selectin to roll on adherent neutrophils, suggesting that they express a nonvascular L-selectin ligand. Using a L-selectin/IgM heavy chain (mu) chimeric protein as an immunocytological probe, we show here that L-selectin can bind to neutrophils, monocytes, CD34+ hematopoietic progenitors, and HL-60 and KG-1 myeloid cells. The interaction between L-selectin and leukocytes was protease sensitive and calcium dependent, and abolished by cell treatment with neuraminidase, chlorate, or O-sialoglycoprotein endopeptidase. These results revealed common features between leukocyte L-selectin ligand and the mucin-like P-selectin glycoprotein ligand 1 (PSGL-1), which mediates neutrophil rolling on P- and E-selectin. The possibility that PSGL-1 could be a ligand for L-selectin was further supported by the ability of P-selectin/mu chimera to inhibit L-selectin/mu binding to leukocytes and by the complete inhibition of both selectin interactions with myeloid cells treated with mocarhagin, a cobra venom metalloproteinase that cleaves the amino terminus of PSGL-1 at Tyr-51. Finally, the abrogation of L- and P-selectin binding to myeloid cells treated with a polyclonal antibody, raised against a peptide corresponding to the amino acid residues 42-56 of PSGL-1, indicated that L- and P-selectin interact with a domain located at the amino-terminal end of PSGL-1. The ability of the anti-PSGL-1 mAb PL-1 to inhibit L- and P-selectin binding to KG-1 cells further supported that possibility. Thus, apart from being involved in neutrophil rolling on P- and E-selectin, PSGL-1 also plays a critical role in mediating neutrophil attachment to adherent neutrophils. Interaction between L-selectin and PSGL-1 may be of major importance for increasing leukocyte recruitment at inflammatory sites.

GlyCAM-1, a physiologic ligand for L-selectin, activates beta 2 integrins on naive peripheral lymphocytes

Naive T cells are selectively recruited from the blood into peripheral lymph nodes during lymphocyte recirculation. L-selectin, a lectin-like receptor, mediates the initial attachment of lymphocytes to high endothelial venules (HEV) in lymph nodes. A subsequent step involving the activation of beta 2 integrins has been proposed to facilitate firm adhesion, but the activating signals are poorly understood. We report here that either antibody-mediated cross-linking of L-selectin on human lymphocytes or treatment of the cells with GlyCAM-1, an HEV-derived, secreted ligand for L-selectin, stimulates their binding to ICAM-1 through the beta 2 integrin pathway. Furthermore, GlyCAM-1 causes the rapid expression of a neoepitope on beta 2 integrins associated with a high-avidity state. Naive (CD45RA+), but not memory (CD45R0+) lymphocytes, respond to L-selectin cross-linking or GlyCAM-1 treatment. Thus, the complexing of L-selectin by specific ligands may provide key signals to naive lymphocytes, contributing to their selective recruitment into peripheral lymphoid organs.

To stick or not to stick: the new leukocyte homing paradigm

The immune system is formed by leukocytes. They are passively transported through the body by the vascular system, but their entrance into tissues requires a coordinated series of events, namely activation of leukocyte integrins, adhesion to the vascular endothelium, and migration. There are four steps in this process, which begin with the rolling of leukocytes along the vascular endothelium, followed by signaling which activates leukocyte integrins, thus leading to tight adhesion to the endothelium and finally transmigration. Substantial progress has been made recently in elucidating the molecular events that induce rolling and signaling, partly as a result of the study of double-knockout mice that are deficient for genes encoding two selectins.