Chemoattractants induce a rapid and transient upregulation of monocyte alpha4 integrin affinity for vascular cell adhesion molecule 1 which mediates arrest: an early step in the process of emigration

Expression and affinity of homing-related molecules on steady-state adult and neonate human PB CD34+ cells and their SRC activity

Although the vast majority of hematopoietic progenitor cells (HPCs) reside within the bone marrow (BM), a small number of HPCs also continuously circulate in the peripheral blood (PB). The examination of the fate of blood-borne HPCs in parabiotic mice, which are surgically conjoined and share a common circulation, recently revealed that steady-state PB HPCs play a physiological role in, at least, the functional re-engraftment of unconditioned BM. To assess the possibility that human HPCs have a similar function, in this study we examined the expression level and affinity of the homing-related molecules, as well as the SCID mouse reconstituting cell (SRC) activity of human PB CD34+ cells, and compared adults with neonates. There was no remarkable difference between adults and neonates in the expression of E- and/or P-selectin ligands by PB CD34+ cells or in these cells' affinity to VCAM-1. In contrast, the expression level of CXCR4 on PB CD34+ cells was much lower in adults than in neonates. Adult cells also showed a much lower SRC activity than neonates. These results suggest that human PB HPCs may contribute to steady-state hematopoiesis in the BM of neonates to some extent, but not so much in adults.

Annexin 1 modulates monocyte-endothelial cell interaction in vitro and cell migration in vivo in the human SCID mouse transplantation model

The effect of the glucocorticoid inducible protein annexin 1 (ANXA1) on the process of monocytic cell migration was studied using transfected U937 cells expressing variable protein levels. An antisense (AS) (36.4AS; approximately 50% less ANXA1) and a sense (S) clone (15S; overexpressing the bioactive 24-kDa fragment) together with the empty plasmid CMV clone were obtained and compared with wild-type U937 cells in various models of cell migration in vitro and in vivo. 15S-transfected U937 cells displayed a reduced (50%) degree of trans-endothelial migration in response to stromal cell-derived factor-1alpha (CXC chemokine ligand 12 (CXCL12)). In addition, the inhibitory role of endogenous ANXA1 on U937 cell migration in vitro was confirmed by the potentiating effect of a neutralizing anti-ANXA1 serum. Importantly, overexpression of ANXA1 in clone 15S inhibited the extent of cell migration into rheumatoid synovial grafts transplanted into SCID mice. ANXA1 inhibitory effects were not due to modifications in adhesion molecule or CXCL12 receptor (CXCR4) expression as shown by the similar amounts of surface molecules found in transfected and wild-type U937 cells. Likewise, an equal chemotactic response to CXCL12 in vitro excluded an intrinsic defect in cell motility in clones 15S and 36.4AS. These data strongly support the notion that ANXA1 critically interferes with a leukocyte endothelial step essential for U937 cell, and possibly monocyte, transmigration both in vitro and in vivo.

Mechanisms contributing to the activity of integrins on leukocytes

Understanding how the integrins on leukocytes operate is important because these receptors control the activity of leukocytes in all phases of their lives. Thus integrins control leukocyte development and maturation in bone marrow, the circulation of naive cells in secondary lymphoid tissue, e.g. the lymph nodes, and leukocyte responses to inflammatory signals emanating from injured tissues. Using as an example LFA-1, which is expressed by all leukocytes, we outline how the activity of this integrin is modified to meet the challenges posed by these leukocyte activities. Briefly, we discuss three means by which LFA-1 is adapted to bind more efficiently to its chief ligand, ICAM-1. LFA-1 can undergo changes in conformation leading to increased affinity, can be clustered on the membrane and, finally, when activated can move into the lipid raft compartment of the membrane. The study of humans with the beta2 deficiency syndrome termed leukocyte adhesion deficiency (LAD)-1 and analysis of LFA-1 null mice has given further insight into integrin activation mechanisms and the in vivo roles of LFA-1 and other leukocyte integrins.

Rapid leukocyte integrin activation by chemokines

Chemokines control selective targeting of circulating leukocytes to the microvasculature by triggering inside-out signal transduction pathways leading to integrin-dependent adhesion. Integrin activation by chemokines is very rapid, is downmodulated within minutes and appears to involve both enhanced heterodimer lateral mobility on the plasma membrane, facilitating encounters with dispersed ligand, as well as induction of a high-affinity state. These two modalities of integrin activation by chemokines involve distinct signaling pathways in the cell, yet complement each other functionally, allowing binding of rolling cells under conditions of low as well as high ligand density. Recent data show that chemokines generate both pro- and anti-adhesive intracellular signaling events, whose equilibrium is likely to be relevant to the kinetics of adhesion and de-adhesion, and to cell movement during diapedesis and chemotaxis. Importantly, chemokines utilize different signaling mechanisms to modulate the activity of distinct integrin subtypes. These recent advances suggest that chemokines may regulate adhesive responses of immune cells based not only on patterns of chemokine receptor expression, but also on variable signaling pathways that can modulate the pro-adhesive responses of leukocytes as a function of their differentiated state, and of the local microenvironment.

Modulation of Mac-1 (CD11b/CD18)-mediated adhesion by the leukocyte-specific protein 1 is key to its role in neutrophil polarization and chemotaxis

Leukocyte-specific protein 1 (LSP1) is an intracellular filamentous-actin binding protein which modulates cell motility. The cellular process in which LSP1 functions to regulate motility is not yet identified. In this study, we show that LSP1 negatively regulates fMLP-induced polarization and chemotaxis of neutrophils through its function on adhesion via specific integrins. Using LSP1-deficient (Lsp1(-/-)) mice, we show increased neutrophil migration into mouse knee joints during zymosan-induced acute inflammation, an inflammatory model in which the number of resident synoviocytes are not affected by LSP1-deficiency. In vitro chemotaxis experiments performed by time-lapse videomicroscopy showed that purified Lsp1(-/-) bone-marrow neutrophils exhibit an increased migration rate toward a gradient of fMLP as compared with wild-type neutrophils. This difference was observed when cells migrated on fibrinogen, but not fibronectin, suggesting a role for LSP1 in modulating neutrophil adhesion by specific integrins. LSP1 is also a negative regulator of fMLP-induced adhesion to fibrinogen or ICAM-1, but not to ICAM-2, VCAM-1, or fibronectin. These results suggest that LSP1 regulates the function of Mac-1 (CD11b/CD18), which binds only to fibrinogen and ICAM-1 among the substrates we tested. fMLP-induced filamentous actin polarization is also increased in the absence of LSP1 when cells were layered on fibrinogen, but not on fibronectin. Our findings suggest that the increased neutrophil recruitment in Lsp1(-/-) mice during acute inflammation derives from the negative regulatory role of LSP1 on neutrophil adhesion, polarization, and migration via specific integrins, such as Mac-1, which mediate neutrophil responses to chemotactic stimuli.

The chemokine stromal cell-derived factor-1 alpha modulates alpha 4 beta 7 integrin-mediated lymphocyte adhesion to mucosal addressin cell adhesion molecule-1 and fibronectin

The interaction between the integrin alpha(4)beta(7) and its ligand, mucosal addressin cell adhesion molecule-1, on high endothelial venules represents a key adhesion event during lymphocyte homing to secondary lymphoid tissue. Stromal cell-derived factor-1alpha (SDF-1alpha) is a chemokine that attracts T and B lymphocytes and has been hypothesized to be involved in lymphocyte homing. In this work we show that alpha(4)beta(7)-mediated adhesion of CD4(+) T lymphocytes and the RPMI 8866 cell line to mucosal addressin cell adhesion molecule-1 was up-regulated by SDF-1alpha in both static adhesion and cell detachment under shear stress assays. Both naive and memory phenotype CD4(+) T cells were targets of SDF-1alpha-triggered increased adhesion. In addition, SDF-1alpha augmented alpha(4)beta(7)-dependent adhesion of RPMI 8866 cells to connecting segment-1 of fibronectin. While pertussis toxin totally blocked chemotaxis of CD4(+) and RPMI 8866 cells to SDF-1alpha, enhanced alpha(4)beta(7)-dependent adhesion triggered by this chemokine was partially inhibited, indicating the participation of Galpha(i)-dependent as well as Galpha(i)-independent signaling. Accordingly, we show that SDF-1alpha induced a rapid and transient association between its receptor CXCR4 and Galpha(i), whereas association of pertussis toxin-insensitive Galpha(13) with CXCR4 was slower and of a lesser extent. SDF-1alpha also activated the small GTPases RhoA and Rac1, and inhibition of RhoA activation reduced the up-regulation of alpha(4)beta(7)-mediated lymphocyte adhesion in response to SDF-1alpha, suggesting that activation of RhoA could play an important role in the enhanced adhesion. These data indicate that up-regulation by SDF-1alpha of lymphocyte adhesion mediated by alpha(4)beta(7) could contribute to lymphocyte homing to secondary lymphoid tissues.

Chemokines in rapid leukocyte adhesion triggering and migration

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

From rolling to arrest on blood vessels: leukocyte tap dancing on endothelial integrin ligands and chemokines at sub-second contacts

In order to extravasate the bloodstream at specific sites of inflammation or antigen presentation, circulating leukocytes must rapidly translate specific adhesive and stimulatory signals into firm adhesion. Leukocyte arrest is nearly exclusively mediated by integrin receptors. Recent in vitro and in vivo evidence suggests that specialized integrins support reversible tethers that slow down selectin-initiated rolling of leukocytes prior to their arrest. In situ activation of integrin avidity by ligand and chemokine signaling can take place within fractions of seconds, resulting either in augmented reversible adhesions or immediate arrest on the vascular endothelium. The ability of leukocyte integrins to rapidly respond to these in situ avidity modulators appears to depend on preformed affinity and clustering states, which are internally regulated by cytoskeletal constraints on integrin conformation and mobility. We discuss potential regulatory mechanisms by which a given set of chemokine receptors and integrins may interact to rapidly generate high avidity, shear-resistant integrin-mediated leukocyte arrest on vascular endothelium.

Molecular mechanisms involved in lymphocyte recruitment in inflamed brain microvessels: critical roles for P-selectin glycoprotein ligand-1 and heterotrimeric G(i)-linked receptors

Lymphocyte recruitment into the brain is a critical event in the pathogenesis of multiple sclerosis and experimental autoimmune encephalomyelitis. We developed a novel intravital microscopy model to directly analyze through the skull the interactions between lymphocytes and the endothelium in cerebral venules of mice. No adhesive interactions were observed between lymphocytes and the nonactivated endothelium in the cerebral microcirculation. When brain venules were activated by pretreating mice with TNF-alpha or LPS, proteolipid protein 139-151 autoreactive T lymphocytes rolled and arrested; notably, only a few peripheral lymph node cells rolled and firmly adhered. Abs anti-P-selectin glycoprotein ligand-1 and anti-E- and P-selectin blocked tethering and rolling of autoreactive lymphocytes, suggesting that P-selectin glycoprotein ligand-1/endothelial selectins are critical in the recruitment of lymphocytes in inflamed brain venules. E- and P-selectin were expressed on cerebral vessels upon in vivo activation and had a patchy distribution during the preclinical phase of active and passive experimental autoimmune encephalomyelitis. LFA-1/ICAM-1 and alpha(4) integrins/VCAM-1 supported rolling, but were not relevant to rolling velocity. Firm arrest was mainly mediated by LFA-1 and ICAM-1. Pretreatment of autoreactive lymphocytes with pertussis toxin blocked integrin-dependent arrest, implicating a requirement for G(i) protein-dependent signaling in vessels from nonlymphoid districts. In conclusion, our data unveils the molecular mechanisms controlling the recruitment of autoreactive lymphocytes in inflamed cerebral vessels and suggest new insights into the pathogenesis of autoimmune inflammatory diseases of the CNS.

Alpha 4 integrin signaling activates phosphatidylinositol 3-kinase and stimulates T cell adhesion to intercellular adhesion molecule-1 to a similar extent as CD3, but induces a distinct rearrangement of the actin cytoskeleton

Dynamic regulation of beta(2) integrin-dependent adhesion is critical for a wide array of T cell functions. We previously showed that binding of high-affinity alpha(4)beta(1) integrins to VCAM-1 strengthens alpha(L)beta(2) integrin-mediated adhesion to ICAM-1. In this study, we compared beta(2) integrin-mediated adhesion of T cells to ICAM-1 under two different functional contexts: alpha(4) integrin signaling during emigration from blood into tissues and CD3 signaling during adhesion to APCs and target cells. Cross-linking either alpha(4) integrin or CD3 on Jurkat T cells induced adhesion to ICAM-1 of comparable strength. Adhesion was dependent on phosphatidylinositol (PI) 3-kinase but not p44/42 mitogen-activated protein kinase (extracellular regulated kinase 1/2), because it was inhibited by wortmannin and LY294002 but not U0126. These data suggest that PI 3-kinase is a ubiquitous regulator of beta(2) integrin-mediated adhesion. A distinct morphological change consisting of Jurkat cell spreading and extension of filopodia was induced by alpha(4) integrin signaling. In contrast, CD3 induced radial rings of cortical actin polymerization. Inhibitors of PI 3-kinase and extracellular regulated kinase 1/2 did not affect alpha(4) integrin-induced rearrangement of the actin cytoskeleton, but treatment with ionomycin, a Ca(2+) ionophore, modulated cell morphology by reducing filopodia and promoting lamellipodia formation. Qualitatively similar morphological and adhesive changes to those observed with Jurkat cells were observed following alpha(4) integrin or CD3 stimulation of human peripheral blood T cells.