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The C-type lectin receptor CLEC-2 signals through a pathway that is critically dependent on the tyrosine kinase Syk. We show that homozygous loss of either protein results in defects in brain vascular and lymphatic development, lung inflation, and perinatal lethality. Furthermore, we find that conditional deletion of Syk in the hematopoietic lineage, or conditional deletion of CLEC-2 or Syk in the megakaryocyte/platelet lineage, also causes defects in brain vascular and lymphatic development, although the mice are viable. In contrast, conditional deletion of Syk in other hematopoietic lineages had no effect on viability or brain vasculature and lymphatic development. We show that platelets, but not platelet releasate, modulate the migration and intercellular adhesion of lymphatic endothelial cells through a pathway that depends on CLEC-2 and Syk. These studies found that megakaryocyte/platelet expression of CLEC-2 and Syk is required for normal brain vasculature and lymphatic development and that platelet CLEC-2 and Syk directly modulate lymphatic endothelial cell behavior in vitro.

According to the prevailing paradigm, neutrophils are short-lived cells that undergo spontaneous apoptosis within 24 hours of their release from the bone marrow. However, neutrophil survival can be significantly prolonged within inflamed tissue by cytokines, inflammatory mediators, and hypoxia. During screening experiments aimed at identifying the effect of the adhesive microenvironment on neutrophil survival, we found that VCAM-1 (CD106) was able to delay both spontaneous and Fas-induced apoptosis. VCAM-1-mediated survival was as efficient as that induced by the cytokine IFN-β and provided an additive, increased delay in apoptosis when given in combination with IFN-β. VCAM-1 delivered its antiapoptotic effect through binding the integrin α9β1. The α9β1 signaling pathway shares significant features with the IFN-β survival signaling pathway, requiring PI3 kinase, NF-κB activation, as well as de novo protein synthesis, but the kinetics of NF-κB activation by VCAM-1 were slower and more sustained compared with IFN-β. This study demonstrates a novel functional role for α9β1 in neutrophil biology and suggests that adhesive signaling pathways provide an important extrinsic checkpoint for the resolution of inflammatory responses in tissues.

Vascular endothelial cells are able to sense changes in the forces acting on them and respond, for instance, by modifying expression of a range of genes. However, there is little information on how such responses are integrated to modify homeostatic functions. We hypothesized that different shear stresses experienced in different regions of the circulation might influence endothelial sensitivity to inflammatory stimuli. We cultured human endothelial cells in tubes and exposed them for varying periods to shear stresses ranging from those typically found in postcapillary venules to those in arteries. When tumor necrosis factor-α was included in the flow cultures, we found startling differential effects of shear stress on the ability of endothelial cells to induce adhesion and migration of flowing neutrophils. Compared with static cultures, endothelial cells cultured at low shear stress (0.3 Pa) captured similar numbers of neutrophils but failed to induce their transendothelial migration. After exposure of endothelial cells to high shear stress (1.0 or 2.0 Pa), capture of neutrophils was largely ablated. The modification in response was detectable after 4 hours of exposure to flow but was much greater after 24 hours. From analysis of gene expression, loss of capture or migration was attributable to reduction in tumor necrosis factor–induced expression of selectins or CXC-chemokines, respectively. Thus, conditioning of endothelial cells by different flow environments may underlie variations in susceptibility to inflammation between different tissues or parts of the vascular tree.

Neutrophils must adhere to the vessel wall, migrate, and degranulate in an ordered manner to perform their protective function. Disruption of these processes may be pathogenic. Current knowledge of the degranulation process is derived almost exclusively from studies on neutrophils in suspension, in which priming with the nonphysiological agent cytochalasin B is necessary to obtain elastase release in response to activating agents. To avoid this, we have adopted a different approach. Using a novel flow-based adhesion system, we have been able to quantify the release of elastase from the primary granules of activated neutrophils adherent to immobilized platelets or purified receptors without priming. Comparing stimuli, formyl tripeptide (fMLP), interleukin-8 (IL-8), activated complement fragment C5a, and platelet-activating factor (PAF) all induced rapid conversion to CD11b/CD18 (MAC-1) -mediated stationary adhesion when perfused over neutrophils already rolling on platelet monolayers or purified P-selectin. However, fMLP, C5a, and IL-8, but not PAF, induced release of elastase from the adherent cells in minutes. Neutrophils stimulated in suspension showed little degranulation. Treatment of neutrophils with an inhibitor of 5-lipoxygenase–activating protein (MK886) and thus synthesis of leukotrienes (LTs) or with an antagonist of the LTB4 receptor (LY223982) blocked the release of elastase. This indicated that endogenous synthesis of 5-lipoxygenase products such as LTs and autocrine activation of neutrophils was required for fMLP-driven elastase release. We hypothesize that the differential ability of PAF and fMLP to induce elastase release from surface-adherent neutrophils could arise from differential ability to generate leukotrienes, such as LTB4, and would be an appropriate mechanism for the control of elastase release during inflammation in vivo, where it is important that cytotoxic agents are not released until activated neutrophils have migrated into the extravascular tissues.

It is known that binding of extracellular antibodies against the major sialoglycoprotein, glycophorin A, reduced the deformability of the red blood cell membrane. This has been taken to result from new or altered interactions between the glycophorin A and the membrane skeleton. We have shown by means of the micropipette aspiration technique that antibodies against the preponderant transmembrane protein, band 3, induce similar effects. A definite but much smaller reduction in elasticity of the membrane is engendered by univalent Fab fragments of the anti-band 3 antibodies. By examining cells genetically devoid of glycophorin A or containing a variant of this constituent, truncated at the inner membrane surface, we have shown that the anti-band 3 antibodies do not act through the band 3-associated glycophorin A. We examined the effect of anti-glycophorin A antibodies on homozygous Wr(a+b-) cells, in which an amino acid replacement in band 3 annihilates the Wright b (Wrb) epitope (comprising sequence elements of glycophorin A and band 3) and thus, by implication disrupts or perturbs the band 3-glycophorin A interaction; these cells show a much smaller response to an anti-glycophorin A antibody than do normal controls. We infer that in this case anti-glycophorin A antibodies exert their rigidifying effect through the associated band 3. Another anti- glycophorin A antibody, directed against an epitope remote from the membrane surface, however, increases the rigidity of both Wr(a+b-) and normal cells. This implies that not all antibodies act in the same manner in modifying the membrane mechanical properties. The effect exerted by anti-band 3 antibodies appears not to be transmitted through the band 3-ankyrin-spectrin pathway because the rigidifying effect of the intact antibody persists at alkaline pH, at which there is evidence that the ankyrin-band 3 link is largely dissociated. The large difference between the effects of saturating concentrations of the divalent and univalent anti-band 3 antibodies implies the existence of an overriding effect on rigidity, resulting from the bifunctionality of the intact antigen. Freeze-fracture electron microscopy shows that the anti-band 3 promotes the formation of small clusters of intra-membrane proteins. Extracellular ligands may in general act by promoting strong or transient interactions between integral membrane proteins, thereby impeding local distortion of the membrane skeletal network in response to shear.

Adhesion of parasitized red blood cells (RBCs) to vascular endothelium is thought to be a key factor in the pathology of falciparum malaria. However, quantitative analyses of the intercellular forces and of the effects of flow on adhesion have been lacking. We have characterized cytoadhesion of RBCs parasitized by the strains ITO4 (which can bind to receptors ICAM-1 or CD36) and FCR3A2 (which can bind to CD36 only) using micropipette manipulation and flow chamber techniques. Target cells were unfixed or glutaraldehyde-fixed human umbilical vein endothelial cells (HUVEC, bearing ICAM-1 only) or human amelanotic melanoma cells (C32, bearing CD36 and ICAM-1). In the static, micropipette assay, 60% to 70% of parasitized cells would adhere when tested at up to three successive sites. The percentage of cells adhering and the force required for their detachment (approximately 10(- 10) N) were similar for each combination of parasite strain and adhesion target (ITO4/HUVEC, ITO4/C32, FCR3A2/C32). In the flow chamber, efficiency of initial adhesion of parasitized cells was essentially constant (at about 1%) up to a stress of 0.1 Pa, and then decreased rapidly with increasing stress. Either receptor (ICAM-1 or CD36) could immobilize flowing cells at a physiologic flow stress (0.1 Pa), but the numbers of cells adhering varied for the different combinations (ITO4/C32 greater than ITO4/HUVEC greater than FCR3A2/C32). When flow was increased in steps, adhered cells were gradually washed off but many could withstand stresses at which they would not initially adhere. The force for detachment estimated in this way was similar to the pipette value, and again, was similar for the different combinations of strains and targets. Adhesion from flow depends on the affinity between surfaces being above a critical level, and once adhesion is established, the fracture energy determines resistance to disruption of adhesion. The results show that the fracture energy is greater than the affinity (ie, that adhesion becomes stabilized after it is initially established) and that the ratio of affinity to fracture energy is different for different receptor/ligand pairs, with ICAM-1 appearing to be the more efficient immobilizing receptor. Also, static and flow-based assays of adhesion clearly differ; the affinity is less critical in the static situation, so that most parasitized cells were capable of adhering in a static assay, but fewer did so under flow. Adhesiveness varied markedly from cell to cell, both for targets and parasitized cells.(ABSTRACT TRUNCATED AT 400 WORDS).

Monocyte recruitment from the vasculature involves sequential engagement of multiple receptors, culminating in transendothelial migration and extravasation. Junctional adhesion molecule-C (JAM-C) is localized at endothelial intercellular junctions and plays a role in monocyte transmigration. Here, we show that blockade of JAM-B/-C interaction reduced monocyte numbers in the extravascular compartment through increased reverse transmigration rather than by reduced transmigration. This was confirmed in vivo, showing that an anti–JAM-C antibody reduced the number of monocytes in inflammatory tissue and increased the number of monocytes with a reverse-transmigratory phenotype in the peripheral blood. All together, our results suggest a novel mechanism of reducing accumulation of monocytes at inflammation sites by disruption of JAM-C–mediated monocyte retention.

Although changes in the mechanical properties of infected red cells may contribute to the pathophysiology of malaria, such changes have not previously been described in detail. In this study, the physical properties of individual cells from both clinical and cultured samples infected with Plasmodium falciparum were tested using micropipette aspiration techniques. Cells containing ring forms took about 50% longer to enter 3 microns pipettes compared with nonparasitised cells, and there was a similar increase in the critical pressure required to induce cell entry. These abnormalities were similar in clinical and cultured samples. More mature cultured parasites (ie, trophozoites and schizonts containing pigment) caused much greater loss of deformability, with entry time and pressure increased four to sixfold. The decrease in deformability of the ring forms was attributable to a deficit in cell surface area/volume ratio (based on micropipette measurement of the surface area and volume of individual cells) and slight stiffening of the cell membrane (shear elastic modulus increased 13%, as measured by pipette aspiration of small membrane tongues). Measurement of the rate of cell shape recovery indicated that the membrane of parasitised cells was not more viscous. The main factor in the drastic loss of deformability of the trophozoites and schizonts was the presence of the large very resistant parasite itself. Otherwise, the cell surface area/volume deficit was slightly less and membrane rigidification slightly greater compared with ring forms. The above abnormalities should cause the trophozoites and schizonts to have great difficulty in traversing splenic or marrow sinuses and could contribute to microvascular occlusion and sequestration. On the other hand, the ring forms may be expected to circulate relatively unhindered.

Little data exist for the mechanical properties of individual irreversible or reversible sickle cells (ISC and RSC, respectively), nor is the process of ISC formation well understood. For oxygenated ISC and density-fractionated RSC, we have used micropipette techniques to measure cell surface area (SA) and volume (V), membrane shear elastic modulus (mu), time constant for viscoelastic shape recovery (tc), and hence to calculate membrane surface viscosity (eta = mu X tc). Volume loss associated with increasing cell density was accompanied by a proportionately smaller surface area decrease; SA/V ratio thus increased for denser cells, with ISC having the highest values. Membrane area loss by fragmentation must thus be accompanied by an accelerated decrease in cell volume. ISC had relatively rigid membranes (mu 130% above normal controls) and tc close to normal values, so that their effective membrane viscosity was more than double control. RSC had viscoelastic properties close to control, but showed wider variation between sickle cell donors and within samples. Measurements on density-separated RSC showed that, on average, mu was nearly constant, but that tc was longer for the densest cells, with their eta approaching ISC levels. A small subpopulation of RSC were found that had mu close to ISC values. Hypotonically swollen ISC (with internal hemoglobin concentration decreased to normal levels) retained their increased membrane stiffness but had markedly decreased tc, so that their eta approached normal values. The results show that elevated hemoglobin concentration influences the viscoelastic behavior of ISC and RSC, but that an irreversible change in membrane elasticity also occurs for ISC. These data suggest that ISC formation occurs via a two- stage process: (1) accelerated volume loss leading to increased cytoplasmic and effective membrane viscosity; (2) a sharp rise in membrane rigidity, presumably linked to membrane structural alteration.

Whole cell and membrane deformability are essential for red blood cell (RBC) survival and for effective blood flow. Neonatal RBCs display several specific properties (eg, large size, high hemoglobin F) that could influence their deformation characteristics and contribute to their shortened life span. The present study was designed to compare selected rheologic properties (cellular deformability, pressure required to aspirate RBC into micropipettes, static and dynamic viscoelastic material properties) of neonatal and adult RBCs. RBC deformability, as studied by a rheoscope, was similar for neonates and adults over a shear stress range of 2.5 to 500 dyn/cm2. The pressure required to aspirate RBCs completely into 3.3-micron diameter pipettes was 129 +/- 87 dyn/cm2 for neonatal RBCs and 71 +/- 37 dyn/cm2 for adult RBCs. The aspiration pressure for neonatal and adult RBCs increased with increasing RBC volume, suggesting that the increased mean aspiration pressure for neonatal RBCs resulted from their larger volume. When RBCs with same volume and diameter were compared, the aspiration pressure tended to be smaller for neonatal RBCs than for adult cells. To characterize material properties determining RBC deformability, we measured membrane extensional (shear) and bending elastic moduli, the time constant for elastic recovery from extensional deformation and hemoglobin viscosity (ie, cytoplasmic viscosity) of neonatal and adult RBCs. Membrane surface viscosity and time constant for recovery from bending deformation were calculated. The extensional and bending moduli of neonatal RBCs were slightly smaller (10% and 16%, respectively) compared with adult cells. This suggests that the static resistance of neonatal RBC membrane to deformation and failure in response to a given force is slightly smaller. The time constant for recovery from extensional deformation of neonatal RBCs was larger by 14%, compared with adult cells. The time constant for bending deformation related to the RBC diameter and surface area was increased by 18% in the neonates. Membrane surface viscosity and hemoglobin viscosity were similar for both cell types. These results indicate that the deformability and viscoelastic properties of neonatal RBCs deviate only slightly from those of adult RBCs and that the increased aspiration pressure of neonatal RBCs is solely due to their large size. Some of the specific deformation characteristics observed in this study (increased aspiration pressure, decreased resistance to elastic deformation) may contribute to the shortened life span of neonatal RBCs.

Although the rheological behavior of sickle cell suspensions and of hemoglobin S solutions is known to be strongly dependent on oxygen tension (PO2), little data exist concerning the influence of PO2 on the viscoelasticity of individual HbSS RBC. We have used micropipette aspiration techniques to test the deformation response of both HbSS and control HbAA RBC over a wide range of PO2 at 23 degrees C. Sickled, spiculed HbSS cells were present for PO2 approximately less than 35 mm Hg; for a number of these cells, the deformation response was essentially elastic and an effective membrane rigidity (EMR) was calculated. EMR increased with decreasing PO2 and was approximately 5 to 50 times higher than the equivalent rigidity of oxygenated HbSS RBC. In addition, the rate of membrane deformation was very slow for sickled cells; the half-time for the deformation process increased as PO2 was lowered and was about two orders of magnitude longer than the equivalent time for normal RBC. Other sickled cells exhibited plastic deformation when subjected to comparable deforming forces and experienced irreversible membrane deformation and budding. At all PO2 levels tested, some HbSS RBC remained as discocytes; these cells had normal membrane elasticity and membrane viscosity. Furthermore, changes in PO2 did not affect the membrane properties of HbAA RBC. Thus, gross abnormalities in the deformation response of HbSS RBC were only detected after morphological sickling had occurred. These abnormalities most likely arose from changes in the cytoplasmic HbS viscoelasticity and, if present in vivo, would be expected to impair the flow of HbSS cells in the microcirculation.