L-selectin: A Major Regulator of Leukocyte Adhesion, Migration and Signaling.

L-selectin (CD62L) is a type-I transmembrane glycoprotein and cell adhesion molecule that is expressed on most circulating leukocytes. Since its identification in 1983, L-selectin has been extensively characterized as a tethering/rolling receptor. There is now mounting evidence in the literature to suggest that L-selectin plays a role in regulating monocyte protrusion during transendothelial migration (TEM). The N-terminal calcium-dependent (C-type) lectin domain of L-selectin interacts with numerous glycans, including sialyl Lewis X (sLex) for tethering/rolling and proteoglycans for TEM. Although the signals downstream of L-selectin-dependent adhesion are poorly understood, they will invariably involve the short 17 amino acid cytoplasmic tail. In this review we will detail the expression of L-selectin in different immune cell subsets, and its influence on cell behavior. We will list some of the diverse glycans known to support L-selectin-dependent adhesion, within luminal and abluminal regions of the vessel wall. We will describe how each domain within L-selectin contributes to adhesion, migration and signal transduction. A significant focus on the L-selectin cytoplasmic tail and its proposed contribution to signaling via the ezrin-radixin-moesin (ERM) family of proteins will be outlined. Finally, we will discuss how ectodomain shedding of L-selectin during monocyte TEM is essential for the establishment of front-back cell polarity, bestowing emigrated cells the capacity to chemotax toward sites of damage.

Loss of L-selectin-guided CD8+ , but not CD4+ , cells protects against ischemia reperfusion injury in a steatotic liver.

Steatotic liver responds with increased hepatocellular injury when exposed to an ischemic-reperfusion insult. Increasing evidence supports the role of immune cells as key mediators of this injury in a normal (lean) state, but data about their role in a steatotic liver are practically nonexistent. The objective of the current study was to delineate the contribution of specific phenotypes of T cells and adhesion molecules in exacerbated cell death in steatotic liver injury. RNA sequencing was performed on isolated steatotic primary hepatocytes, and T-cell markers were assessed in hepatic lymphocytes after ischemia reperfusion injury (IRI) in high-fat diet (HFD)-fed mice. Cluster of differentiation 8 knockout (CD8-/- ) and CD4-/- mice along with CD8 and L-selectin antibody-treated mice were fed an HFD, and hepatocellular injury was assessed by histology, propidium iodide injection, and alanine aminotransferase after IRI. RNA sequencing demonstrated a strikingly differential gene profile in steatotic hepatocytes versus lean hepatocytes. After injury, the HFD liver showed increased necrosis, infiltrating CD8+ cells, alanine aminotransferase, and proinflammatory cytokines. Hepatic lymphocytes demonstrated increased CD8+ /CD62L+ (L-selectin) cells in HFD-fed mice after IRI. CD8-/- mice and CD8-depleted C57BL/6 mice demonstrated significant protection from injury, which was not seen in CD4-/- mice. L-selectin blockade also demonstrated significant hepatoprotection from IRI. L-selectin ligand MECA-79 was increased in HFD-fed mice undergoing IRI. CONCLUSION: Blockade of CD8 and L-selectin, but not CD4, ameliorated hepatocellular injury, confirming that CD8+ cells are critical drivers of injury in a steatotic liver; this represents a therapeutic target in steatotic liver injury, underlining the importance of development of therapies specific to a steatotic liver. (Hepatology 2017;66:1258-1274).

Reduced Oxidative Burst by Primed Neutrophils in the Elderly Individuals Is Associated With Increased Levels of the CD16bright/CD62Ldim Immunosuppressive Subset.

The aim of our study was to analyze polymorphonuclear neutrophil (PMN) functions in elderly individuals compared with those in healthy young participants, directly in whole blood to avoid issues with data interpretation related to cell isolation procedures. Despite the presence of increased circulating levels of proinflammatory cytokines, resting PMNs from the elderly individuals were not activated as shown by normal CD62L and CD11b expression at the PMN surface and normal constitutive reactive oxygen species (ROS) production. However, suboptimal stimulation induced modulations of CD62L and CD11b expression, which positively correlated with the interleukin-6 circulating level, suggesting a possible in vivo preactivation of old PMNs by this cytokine. In addition, PMN phagocytosis of opsonized Escherichia Coli was decreased in elderly individuals. Furthermore, upon preincubation of elderly whole-blood samples with tumor necrosis factor-α or Toll Receptor agonists, we observed a reduced PMN oxidative burst in response to formyl peptides. Elderly participants also exhibited an increased percentage of the immunosuppressive CD16bright/CD62Ldim PMN subpopulation, which was characterized by a lower phagocytic index and a reduced ROS production compared with the CD16bright/CD62Lbright subset. Thus, the reduced phagocytosis and ROS production associated with an expansion of immunosuppressive CD16bright/CD62Ldim PMN subpopulation might be involved in the increased susceptibility to bacterial and fungal infections with old age.

Cutting Edge: Marginal Zone Macrophages Regulate Antigen Transport by B Cells to the Follicle in the Spleen via CD21.

Marginal zone macrophages (MZM) are strategically located in the spleen, lining the marginal sinus where they sense inflammation and capture Ag from the circulation. One of the receptors expressed by MZM is scavenger receptor macrophage receptor with collagenous structure (MARCO), which has affinity for modified self-antigens. In this article, we show that engagement of MARCO on murine macrophages induces extracellular ATP and loss of CD21 and CD62L on marginal zone B cells. Engagement of MARCO also leads to reduction of Ag transport by marginal zone B cells and affects the subsequent immune response. This study highlights a novel function for MZM in regulating Ag transport and activation, and we suggest that MARCO-dependent ATP release regulates this through shedding of CD21 and CD62L. Because systemic lupus erythematosus patients were shown to acquire autoantibodies against MARCO, this highlights a mechanism that could affect a patient's ability to combat infections.

Human neutrophil antigen-3a antibodies induce neutrophil stiffening and conformational activation of CD11b without shedding of L-selectin.

BACKGROUND: HNA-3a antibodies induce severe transfusion-related acute lung injury (TRALI) in which neutrophils play a major role. As neutrophil passage through the pulmonary microvasculature is a critical step in the pathogenesis of TRALI, we investigated the impact of HNA-3a antibodies on two important factors that could impair granulocyte passage through lung capillaries: the elasticity of neutrophils and the expression and activation of adhesion molecules. STUDY DESIGN AND METHODS: The impact of HNA-3a antibodies on the elasticity of neutrophils was investigated using atomic force microscopy (AFM). Neutrophils were settled on poly-2-hydroxyethyl-methacrylate-coated glass slides before treatment with anti-HNA-3a plasma samples, control plasma, or control plasma containing formyl-methionyl-leucyl-phenylalanine (fMLP). Elasticity measurements were carried out in a temperature-controlled perfusion chamber using an atomic force microscopy (AFM) device. The impact of HNA-3a antibodies on the surface expression of total CD11b, activation of CD11b, and L-selectin (CD62L) shedding was investigated by flow cytometry. The functional impact of HNA-3a antibodies on neutrophil adhesion was assessed using fibrinogen-coated plates. RESULTS: HNA-3a antibodies induced stiffening of neutrophils (+24%-40%; p < 0.05) to a similar extent as fMLP. This effect was blocked by treatment of neutrophils with cytochalasin D. While total surface expression of CD11b and L-selectin on neutrophils was largely unaffected, HNA-3a antibodies induced alloantigen-specific activation of CD11b (+72%-107%; p < 0.05) and increased adhesion of neutrophils to fibrinogen. CONCLUSION: Accumulation of neutrophils in the pulmonary microvasculature during severe TRALI is likely mediated by increased rigidity and CD11b-mediated adhesion of neutrophils leading to retention of neutrophils.

L-selectin controls trafficking of chronic lymphocytic leukemia cells in lymph node high endothelial venules in vivo.

B-cell chronic lymphocytic leukemia (CLL) is the most common leukemia in adults. Lymph nodes (LNs) are sites of malignant proliferation and LN enlargement is associated with poor prognosis in the clinics. The LN microenvironment is believed to favor disease progression by promoting CLL cell growth and drug resistance. A better understanding of the mechanisms regulating trafficking of CLL cells to LNs is thus urgently needed. Here, we studied the first step of CLL cell migration to LNs, their interaction with high endothelial venules (HEVs), specialized blood vessels for lymphocyte extravasation in lymphoid organs. We observed that the density of HEV blood vessels was increased in CLL LNs and that CD20(+) CLL cells accumulated within HEV pockets, suggesting intense trafficking. We used intravital imaging to visualize the behavior of human CLL cells within the mouse LN microcirculation, and discovered that CLL cells bind to HEVs in vivo via a multistep adhesion cascade, which involves rolling, sticking, and crawling of the leukemic cells on the endothelium. Functional analyses revealed that the lymphocyte homing receptor L-selectin (CD62L) is the key factor controlling the binding of CLL cells to HEV walls in vivo. Interestingly, L-selectin expression was decreased on CLL cells from patients treated with idelalisib, a phosphoinositide-3-kinase δ inhibitor recently approved for CLL therapy. Interference with L-selectin-mediated trafficking in HEVs could represent a novel strategy to block dissemination of CLL cells to LNs and increase the efficacy of conventional therapy.

Lung-derived factors mediate breast cancer cell migration through CD44 receptor-ligand interactions in a novel ex vivo system for analysis of organ-specific soluble proteins.

Breast cancer preferentially metastasizes to lung, lymph node, liver, bone, and brain. However, it is unclear whether properties of cancer cells, properties of organ microenvironments, or a combination of both is responsible for this observed organ tropism. We hypothesized that breast cancer cells exhibit distinctive migration/growth patterns in organ microenvironments that mirror common clinical sites of breast cancer metastasis and that receptor-ligand interactions between breast cancer cells and soluble organ-derived factors mediate this behavior. Using an ex vivo model system composed of organ-conditioned media (CM), human breast cancer cells (MDA-MB-231,MDA-MB-468, SUM149, and SUM159) displayed cell line-specific and organ-specific patterns of migration/proliferation that corresponded to their in vivo metastatic behavior. Notably, exposure to lung-CM increased migration of all cell lines and increased proliferation in two of four lines (P < .05). Several cluster of differentiation (CD) 44 ligands including osteopontin (OPN) and L-selectin (SELL) were identified in lung-CM by protein arrays. Immunodepletion of SELL decreased migration of MDA-MB-231 cells, whereas depletion of OPN decreased both migration and proliferation. Pretreatment of cells with a CD44-blocking antibody abrogated migration effects (P < .05). "Stemlike" breast cancer cells with high aldehyde dehydrogenase and CD44 (ALDH(hi)CD44(+)) responded in a distinct chemotactic manner toward organ-CM, preferentially migrating toward lung-CM through CD44 receptor-ligand interactions (P < .05). In contrast, organ-specific changes in migration were not observed for ALDH(low)CD44(-) cells. Our data suggest that interactions between CD44(+) breast cancer cells and soluble factors present in the lung microenvironment may play an important role in determining organotropic metastatic behavior.

Reply to Shin and Bayry on "An age-related decline of CD62L and vaccine response: a role of microRNA 92a?".

Aging of the human body affects the immune system by a decline in the ability to raise a response to challenges such as microbial infections or vaccinations. In the very elderly, the decline in such functions appears to relate to a reduced expression of certain co-stimulatory molecules expressed by T lymphocytes. More recently, attention has been drawn to the adhesion molecule CD62L, where differences in expression and function of this molecule between younger and older individuals are suspected to be a part of immunosenescence in the elderly.

An age-related decline of CD62L and vaccine response: a role of microRNA 92a?

Aging process can affect T cell and antibody response to vaccination and an age-related decline in the expression of CD62L on CD8(+) T-lymphocyte is one of the important factors that contribute. A recent report demonstrated that percentage of CD3(+)CD8(+)CD62L(+) cells and CD8(+) T-lymphocyte microRNA-92a levels significantly decline with the age and were positively correlated. These results suggested that the age-related attrition of human naïve T cells could be connected to a reduced microRNA-92a in T-lymphocytes and downregulation of the microRNA-92a level might indicate exhaustion of naïve T-cells due to alteration of the immunologic condition with aging. Further studies are necessary to evaluate whether targeting microRNA-92a as microRNA mimics could be one of the therapeutic strategies in improving vaccine response in elderly.

Roles of T cell-associated L-selectin and ß7 integrins during induction and regulation of chronic colitis.

BACKGROUND: L-selectin (CD62L) and ß(7) integrins are important for trafficking of naive T cells under steady-state conditions. The objectives of this study were to dissect the requirements for T cell-associated CD62L and ß(7) integrins during initiation, progression, and regulation of chronic colitis. METHODS: Using the T-cell transfer model, we compared colitogenic potential between T cells lacking one or both of these molecules with wild-type T cells. To assess trafficking of cells to the secondary lymphoid tissue and the gut, we performed co-homing experiments. RESULTS: Adoptive transfer of wild-type, CD62L(-/-) or ß(7)(-/-) single-deficient T cells induced moderate to severe disease with slightly different kinetics. However, transfer of CD62L(-/-) ß(7)(-/-) double-deficient (DKO) T cells produced significantly attenuated gut inflammation, which correlated with fewer T cells and reduced levels of proinflammatory cytokines in the colon lamina propria. Our subsequent experiments established that lack of colitogenic potential of these cells was due to inability of DKO T cells to home to the secondary lymphoid tissue. Furthermore, homing of in vitro-generated effector DKO T cells to the inflamed intestine was significantly impaired. Lastly, DKO regulatory T cells were ineffective at suppressing colitis induced by wild-type T cells. CONCLUSIONS: We established that T cells can use either CD62L(-/-) or ß(7)(-/-) integrins to induce chronic colitis, but lack of both abrogates their colitogenic potential. Effector T cells critically rely on ß(7) integrin during their recruitment to the inflamed intestinal mucosa. Finally, regulation of intestinal inflammation by regulatory T cells requires one or both of these adhesion molecules.

Age is an important determinant in humoral and T cell responses to immunization with hepatitis B surface antigen.

The aging of the immune system, also named immunosenescence, affects vaccine responses. However, the onset of age-related immunosenescence has been uncertain, in particularly with regard to vaccine responses. Here, we show that the formation of antibodies in response to vaccination against hepatitis B virus infection was significantly reduced for donors with a mean age of 61 y compared with a group with a mean age of 33 y. Booster vaccination sero-converted the elderly donors, but only at a reduced level, while a stronger response was found for the group of young donors. Agreeing with these findings, the hepatitis B surface antigen-specific proliferative responses by donor-derived T cells were reduced for the elder donors. Interestingly, the association between expression of the adhesion molecule CD62L (L-selectin) on naïve and central memory T cells and the formation of antigen-specific antibodies differed significantly between younger and elder donors. This finding corresponds well with the observation made previously that CD62L gene ablation in animals alters the formation of antigen-specific antibodies. We suggest that a complex interplay between the expression of CD62L and its ligands is a determinant in early-age immunosenescence affecting the response to HBV vaccination.

Hydrogen sulfide reduces neutrophil recruitment in hind-limb ischemia-reperfusion injury in an L-selectin and ADAM-17-dependent manner.

BACKGROUND: Reperfusion following ischemia leads to neutrophil recruitment into injured tissue. Selectins and ß2-integrins regulate neutrophil interaction with the endothelium during neutrophil rolling and firm adhesion. Excessive neutrophil infiltration into tissue is thought to contribute to ischemia-reperfusion injury damage. Hydrogen sulfide mitigates the damage caused by ischemia-reperfusion injury. This study's objective was to determine the effect of hydrogen sulfide on neutrophil adhesion receptor expression. METHODS: Human neutrophils were either left untreated or incubated in 20 µM hydrogen sulfide and/or 50 µg/ml pharmacologic ADAM-17 inhibitor TAPI-0; activated by interleukin-8, fMLP, or TNF-α; and labeled against P-selectin glycoprotein ligand-1, leukocyte function associated antigen-1, Mac-1 α, L-selectin, and ß2-integrin epitopes CBRM1/5 or KIM127 for flow cytometry. Cohorts of three C57BL/6 mice received an intravenous dose of saline vehicle or 20 µM hydrogen sulfide with or without 50 µg/ml TAPI-0 before unilateral tourniquet-induced hind-limb ischemia for 3 hours followed by 3 hours of reperfusion. Bilateral gastrocnemius muscles were processed for histology before neutrophil infiltration quantification. RESULTS: Hydrogen sulfide treatment significantly increased L-selectin shedding from human neutrophils following activation by fMLP and interleukin-8 in an ADAM-17-dependent manner. Mice treated with hydrogen sulfide to raise bloodstream concentration by 20 µM before ischemia or reperfusion showed a significant reduction in neutrophil recruitment into skeletal muscle tissue following tourniquet-induced hind-limb ischemia-reperfusion injury. CONCLUSIONS: Hydrogen sulfide administration results in the down-regulation of L-selectin expression in activated human neutrophils. This leads to a reduction in neutrophil extravasation and tissue infiltration and may partially account for the protective effects of hydrogen sulfide seen in the setting of ischemia-reperfusion injury.

CD62L is critical for maturation and accumulation of murine hepatic NK cells in response to viral infection.

NK cells play critical roles in the first line of defense against viruses and other pathogens. However, the factors that control NK cell recruitment into local sites to exert effector functions during viral infection remain poorly understood. In this study, we found that murine NK cells in various organs could be divided into CD62L(-) and CD62L(+) subsets, the latter of which were less abundant in the liver and exhibited a relatively mature NK cell phenotype and a stronger cytotoxic function. Moreover, NK cells acquired CD62L expression after birth, and the frequency of CD62L(+) NK cells gradually increased during postnatal development. In models of polyinosinic-polycytidylic acid administration and adenovirus infection in vivo, CD62L(+) NK cell frequency and absolute numbers in the liver rapidly and markedly increased as a result of the augmented differentiation of CD62L(-) to CD62L(+) NK cells and recruitment of peripheral mature NK cells to the liver. However, blocking CD62L prior to administering viral stimuli in vivo abolished viral stimulation-induced NK cell accumulation and maturation in the liver. Collectively, these data suggest that CD62L marks a mature NK cell subset, as well as affects the magnitude of the local NK cell response to viral infection.

Recruitment of monocytes/macrophages in different tumor microenvironments.

After emigration from the bone marrow into the peripheral blood, monocytes enter tissues and differentiate into macrophages. Monocytes/macrophages have many roles in immune regulation, angiogenesis, and tumor metastasis and invasion. In addition, studies have revealed that these cells are essential to tumor progression. Recently, an accumulation of evidence has indicated that macrophages in distinct regions of tumor masses have distinct origins. For instance, classical monocytes appear to be a major source of macrophages in tumor epithelial, perivascular, and hypoxic regions. In contrast, non-classical monocytes are an important source of macrophages in the tumor perivascular region. During the past century, it has been demonstrated that several chemoattractants can regulate the recruitment of monocytes/macrophages to tumor sites. Despite the importance of monocytes/macrophages in tumor progression, there had been, until recently, no efforts to summarize receptor-ligand pairs between tumor-derived chemokines and corresponding receptors in monocytes in different microenvironments. In this review, we present a cohesive view of the distinct expression patterns of chemokine receptors in two different monocyte subsets (classical and non-classical monocytes) and describe their roles in monocyte/macrophage recruitment into distinct tumor microenvironments. This review provides insight into the behavior of monocytes/macrophages in different tumor microenvironments.

Cell adhesion molecules in human embryo implantation.

The process of human embryo implantation is mediated not only by evolutionarily conserved mechanisms, but also by a mechanism unique to humans. Evidence suggests that the cell adhesion molecules, L-selectin and trophinin, play a unique role in human embryo implantation. Here, we describe the dual roles of mucin carbohydrate ligand for L-selectin and trophinin protein and of the trophinin-associated proteins bystin and tastin. We then describe trophinin-mediated signal transduction in trophectoderm cells and endometrial epithelial cells. This review also covers cadherin and integrin in human embryo implantation.

Radiation treatment inhibits monocyte entry into the optic nerve head and prevents neuronal damage in a mouse model of glaucoma.

Glaucoma is a common ocular disorder that is a leading cause of blindness worldwide. It is characterized by the dysfunction and loss of retinal ganglion cells (RGCs). Although many studies have implicated various molecules in glaucoma, no mechanism has been shown to be responsible for the earliest detectable damage to RGCs and their axons in the optic nerve. Here, we show that the leukocyte transendothelial migration pathway is activated in the optic nerve head at the earliest stages of disease in an inherited mouse model of glaucoma. This resulted in proinflammatory monocytes entering the optic nerve prior to detectable neuronal damage. A 1-time x-ray treatment prevented monocyte entry and subsequent glaucomatous damage. A single x-ray treatment of an individual eye in young mice provided that eye with long-term protection from glaucoma but had no effect on the contralateral eye. Localized radiation treatment prevented detectable neuronal damage and dysfunction in treated eyes, despite the continued presence of other glaucomatous stresses and signaling pathways. Injection of endothelin-2, a damaging mediator produced by the monocytes, into irradiated eyes, combined with the other glaucomatous stresses, restored neural damage with a topography characteristic of glaucoma. Together, these data support a model of glaucomatous damage involving monocyte entry into the optic nerve.

Signaling through L-selectin mediates enhanced chemotaxis of lymphocyte subsets to secondary lymphoid tissue chemokine.

L-selectin functions as an important adhesion molecule that mediates tethering and rolling of lymphocytes by binding to high endothelial venule (HEV)-expressed ligands during recirculation. Subsequent lymphocyte arrest and transmigration require activation through binding of HEV-decorated homeostatic chemokines such as secondary lymphoid tissue chemokine (SLC; CCL21) to its counterreceptor, CCR7. Importantly, L-selectin also functions as a signaling molecule. In this study, signaling induced by ligation of L-selectin using mAb or endothelial cell-expressed ligand significantly enhanced the chemotaxis of murine T cells and B cells to SLC but not to other homeostatic chemokines. Consistent with the expression levels of L-selectin in different lymphocyte subsets, L-selectin-mediated enhancement of chemotaxis to SLC was observed for all naive lymphocytes and effector/memory CD8(+) T cells, whereas only a subpopulation of effector/memory CD4(+) T cells responded. During in vivo mesenteric lymph node migration assays, the absence of L-selectin on lymphocytes significantly attenuated both their ability to migrate out of the HEV and their chemotaxis away from the vessel wall. Notably, ligation of L-selectin and/or CCR7 did not result in increased CCR7 expression levels, internalization, or re-expression. Pharmacologic inhibitor studies showed that L-selectin-mediated enhanced chemotaxis to SLC required intact intracellular kinase function. Furthermore, treatment of lymphocytes with the spleen tyrosine kinase family inhibitor piceatannol reduced their ability to migrate across the HEV in peripheral lymph nodes. Therefore, these results suggest that "cross-talk" in the signaling pathways initiated by L-selectin and CCR7 provides a novel mechanism for functional synergy between these two molecules during lymphocyte migration.

L-selectin--a dynamic regulator of leukocyte migration.

The leukocytic cell adhesion receptor L-selectin mediates the initial step of the adhesion cascade, the capture and rolling of leukocytes on endothelial cells. This event enables leukocytes to migrate out of the vasculature into surrounding tissues during inflammation and immune surveillance. Distinct domains of L-selectin contribute to proper leukocyte migration. In this review, we discuss the contributions of these domains with respect to L-selectin function: the regulation by serine phosphorylation of the cytoplasmic tail, the role of the transmembrane domain in receptor positioning on the cell surface as well as the N-glycosylation of the extracellular part and the identification of novel binding partners.

sLeX/L-selectin mediates adhesion in vitro implantation model.

The complex implantation process is initiated by the recognition and adhesion between the embryo and uterine endometrial epithelium. The expression and interactions between the adhesive molecules from both fetal and maternal sides are crucial for the successful implantation. In this study, we aimed to investigate the expression and adhesive function of sLeX on the trophoblasts and L-selectin on uterine epithelial cells mediated the adhesion at the fetal-maternal interface, and to further explore whether this adhesion system could induce endometrial apoptosis, using in vitro implantation model consisting of the human trophoblast cell line (JAR) and human uterine epithelial cell line (RL95-2). The results showed that sLeX was expressed on JAR cells by indirect immunofluorescence staining. After transfection of JAR cells with fucosyltransferase VII (FUT7) which is the key enzyme for sLeX synthesis, the expression of FUT7 and sLeX synthesis were increased, and the percent adhesion of trophoblast cells to RL95-2 cell monolayer was significantly increased (P < 0.01). L-selectin was strongly expressed but not E- and P-selectin on epithelial RL95-2 cells by RT-PCR, Western blot. Blocking L-selectin with specific antibody or heparin pretreatment in RL95-2 cells inhibited the adhesion of JAR cells to RL95-2 cell monolayer. Furthermore, regulating the expression of sLeX on JAR cells or blocking L-selectin on RL95-2 cells could activate the apoptosis of uterine epithelial cells. These results suggest the sLeX/L-selectin adhesion system at fetal-maternal interface not only mediates the adhesion of embryo to uterine epithelium, but also effectively induces the apoptosis in uterine epithelium. The study supplies a molecular basis for the elucidation of the initial recognition and adhesion during embryo implantation.