Lymphocyte arrest requires instantaneous induction of an extended LFA-1 conformation mediated by endothelium-bound chemokines

It is widely believed that rolling lymphocytes require successive chemokine-induced signaling for lymphocyte function-associated antigen 1 (LFA-1) to achieve a threshold avidity that will mediate lymphocyte arrest. Using an in vivo model of lymphocyte arrest, we show here that LFA-1-mediated arrest of lymphocytes rolling on high endothelial venules bearing LFA-1 ligands and chemokines was abrupt. In vitro flow chamber models showed that endothelium-presented but not soluble chemokines triggered instantaneous extension of bent LFA-1 in the absence of LFA-1 ligand engagement. To support lymphocyte adhesion, this extended LFA-1 conformation required immediate activation by its ligand, intercellular adhesion molecule 1. These data show that chemokine-triggered lymphocyte adhesiveness involves a previously unrecognized extension step that primes LFA-1 for ligand binding and firm adhesion.

LAD-III, a leukocyte adhesion deficiency syndrome associated with defective Rap1 activation and impaired stabilization of integrin bonds

Recently, we reported a rare leukocyte adhesion deficiency (LAD) associated with severe defects in integrin activation by chemokine signals, despite normal ligand binding of leukocyte integrins.1 We now report that the small GTPase, Rap1, a key regulator of inside-out integrin activation is abnormally regulated in LAD Epstein-Barr virus (EBV) lymphocyte cells. Both constitutive and chemokine-triggered activation of Rap1 were abolished in LAD lymphocytes despite normal chemokine signaling. Nevertheless, Rap1 expression and activation by phorbol esters were intact, ruling out an LAD defect in Rap1 guanosine triphosphate (GTP) loading. The very late antigen 4 (VLA-4) integrin abnormally tethered LAD EBV lymphocytes to its ligand vascular cell adhesion molecule 1 (VCAM-1) under shear flow due to impaired generation of high-avidity contacts despite normal ligand binding and intact avidity to surface-bound anti-VLA-4 monoclonal antibody (mAb). Thus, a defect in constitutive Rap1 activation results in an inability of ligand-occupied integrins to generate high-avidity binding to ligand under shear flow. This is a first report of an inherited Rap1 activation defect associated with a pathologic disorder in leukocyte integrin function, we herein term it “LAD-III.” (Blood. 2004;103:1033-1036)

A novel genetic leukocyte adhesion deficiency in subsecond triggering of integrin avidity by endothelial chemokines results in impaired leukocyte arrest on vascular endothelium under shear flow

Leukocyte arrest on vascular endothelium under disruptive shear flow is a multistep process that requires in situ integrin activation on the leukocyte surface by endothelium-displayed chemoattractants, primarily chemokines. A genetic deficiency of leukocyte adhesion to endothelium associated with defective beta2 integrin expression or function (LAD-1) has been described. We now report a novel severe genetic disorder in this multistep process associated with functional defects in multiple leukocyte integrins, reflected in recurrent infections, profound leukocytosis, and a bleeding tendency. This syndrome is associated with an impaired ability of neutrophil and lymphocyte beta1 and beta2 integrins to generate high avidity to their endothelial ligands and arrest cells on vascular endothelium in response to endothelial chemoattractant signals. Patient leukocytes roll normally on endothelial selectins, express intact integrins and G protein-coupled chemokine receptors (GPCR), spread on integrin ligands, and migrate normally along a chemotactic gradient. Activation of beta2 integrins in response to GPCR signals and intrinsic soluble ligand binding properties of the very late activation antigen-4 (VLA-4) integrin are also retained in patient leukocytes. Nevertheless, all integrins fail to generate firm adhesion to immobilized ligands in response to in situ GPCR-mediated activation by chemokines or chemoattractants, a result of a primary defect in integrin rearrangement at ligand-bearing contacts. This syndrome is the first example of a human integrin-activation deficiency associated with defective GPCR stimulation of integrin avidity at subsecond contacts, a key step in leukocyte arrest on vascular endothelium under shear flow.

Chemokine induction of integrin adhesiveness on rolling and arrested leukocytes local signaling events or global stepwise activation?

The arrest of rolling leukocytes on target endothelium is predominantly mediated by integrins, which pre-exist in largely inactive states on circulating immune cells and need to be activated in situ. These adhesion receptors acquire high avidity upon encounter with endothelial-displayed chemokines or chemoattractants, which are ligands to specific G protein-coupled receptors (GPCRs) on the leukocyte surface. In order to arrest, the leukocyte must constantly integrate endothelial-based signals as it moves along the vessel wall. It is unclear whether the chemokine signal is locally transmitted at the endothelial contact zone or whether the rolling leukocyte accumulates successive chemokine signals to reach a threshold global activation. Recent in vitro and in vivo data suggest that the induction of high integrin avidity by endothelial chemokine-transduced G(i)-signals is a general mechanism that has evolved to locally enhance integrin avidity to ligand within subseconds at restricted leukocyte-endothelial contacts. In addition, a second specialized mechanism, involving stepwise signals integrated by selectin ligands on rolling cells, seems to activate integrins on the entire leukocyte surface. This GPCR-independent and much slower pathway (10(1)-10(2) seconds) is transmitted through rolling engagements of neutrophils, primarily on E-selectin. We propose that these two mechanisms are differentially used by distinct leukocyte subsets at various vascular beds, providing much larger combinatorial diversity of integrin activation on rolling leukocytes than previously predicted.

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.

Novel chemokine functions in lymphocyte migration through vascular endothelium under shear flow

The recruitment of circulating leukocytes at vascular sites in target tissue has been linked to activation of Gi-protein signaling in leukocytes by endothelial chemokines. The mechanisms by which apical and subendothelial chemokines regulate leukocyte adhesion to and migration across endothelial barriers have been elusive. We recently found that endothelial chemokines not only stimulate integrin-mediated arrest on vascular endothelial ligands but also trigger earlier very late antigen (VLA)-4 integrin-mediated capture (tethering) of lymphocytes to vascular cell adhesion molecule 1 (VCAM-1)-bearing surfaces by extremely rapid modulation of integrin clustering at adhesive contact zones. This rapid modulation of integrin avidity requires chemokine immobilization in juxtaposition with the VLA-4 ligand VCAM-1. We also observed that endothelial-bound chemokines promote massive lymphocyte transendothelial migration (TEM). It is interesting that chemokine-promoted lymphocyte TEM requires continuous exposure of lymphocytes but not of the endothelial barrier to fluid shear. It is noteworthy that lymphocyte stimulation by soluble chemokines did not promote lymphocyte TEM. Our results suggest new roles for apical endothelial chemokines both in triggering lymphocyte capture to the endothelial surface and in driving post-arrest events that promote lymphocyte transmigration across endothelial barriers under shear flow.

Subsecond induction of alpha4 integrin clustering by immobilized chemokines stimulates leukocyte tethering and rolling on endothelial vascular cell adhesion molecule 1 under flow conditions

Leukocyte recruitment to target tissue is initiated by weak rolling attachments to vessel wall ligands followed by firm integrin-dependent arrest triggered by endothelial chemokines. We show here that immobilized chemokines can augment not only arrest but also earlier integrin-mediated capture (tethering) of lymphocytes on inflamed endothelium. Furthermore, when presented in juxtaposition to vascular cell adhesion molecule 1 (VCAM-1), the endothelial ligand for the integrin very late antigen 4 (VLA-4, alpha4beta1), chemokines rapidly augment reversible lymphocyte tethering and rolling adhesions on VCAM-1. Chemokines potentiate VLA-4 tethering within <0.1 s of contact through Gi protein signaling, the fastest inside-out integrin signaling events reported to date. Although VLA-4 affinity is not altered upon chemokine signaling, subsecond VLA-4 clustering at the leukocyte-substrate contact zone results in enhanced leukocyte avidity to VCAM-1. Endothelial chemokines thus regulate all steps in adhesive cascades that control leukocyte recruitment at specific vascular beds.

Modification of PDGFalpha receptor expression or function alters the metastatic phenotype of 3LL cells

Functional PDGFalpha receptors are selectively expressed on highly lung-metastasizing clones of the 3LL Lewis lung carcinoma, but not on low-mestastatic clones. The highly metastatic clones are also growth induced in vitro by PDGF and lung conditioned medium. To investigate whether modification of PDGFalpha receptor expression or function can affect metastatic capability, we transfected cells of a low-metastatic 3LL clone with a full length PDGFalpha receptor gene and cells of a highly-metastatic clone with a truncated kinase domain PDGFalpha receptor gene. Introduction of the full length PDGFalpha receptor conferred upon low-metastatic cells the ability to grow in vitro in the presence of PDGF-AA and to colonize the lung in experimental and spontaneous metastases assays. Conversely, introduction of a truncated version of the PDGFalpha receptor into highly metastatic cells reduced their metastatic load to control levels. Accordingly, their responses to PDGF-AA, including growth stimulation and receptor autophosphorylation, were reduced. These results demonstrate that PDGFalpha receptor expression and function can control the capacity of tumor cells to generate metastases in the lung. The response of this receptor to lung-derived PDGF-like factors may define a paracrine mode of metastatic cell growth in the target organ.