Hydrodynamic Shear Shows Distinct Roles for LFA-1 and Mac-1 in Neutrophil Adhesion to Intercellular Adhesion Molecule-1

Trail Formation Alleviates Excessive Adhesion and Maintains Efficient Neutrophil Migration

Migrating neutrophils are found to leave behind subcellular trails in vivo, but the underlying mechanisms remain unclear. Here, an in vitro cell migration test plus an in vivo observation was applied to monitor neutrophil migration on intercellular cell adhesion molecule-1 (ICAM-1) presenting surfaces. Results indicated that migrating neutrophils left behind long-lasting, chemokine-containing trails. Trail formation tended to alleviate excessive cell adhesion enhanced by the trans-binding antibody and maintain efficient cell migration, which was associated with differential instantaneous edge velocity between the cell front and rear. CD11a and CD11b worked differently in inducing trail formation with polarized distributions on the cell body and uropod. Trail release at the cell rear was attributed to membrane ripping, in which β2-integrin was disrupted from the cell membrane through myosin-mediated rear contraction and integrin-cytoskeleton dissociation, potentiating a specialized strategy of integrin loss and cell deadhesion to maintain efficient migration. Moreover, neutrophil trails left on the substrate served as immune forerunners to recruit dendritic cells. These results provided an insight in elucidating the mechanisms of neutrophil trail formation and deciphering the roles of trail formation in efficient neutrophil migration.

Intravascular Crawling of Patrolling Monocytes: A Lèvy-Like Motility for Unique Search Functions?

Patrolling monocytes (PMo) are the organism’s preeminent intravascular guardians by their continuous search of damaged endothelial cells and harmful microparticles for their removal and to restore homeostasis. This surveillance is accomplished by PMo crawling on the apical side of the endothelium through regulated interactions of integrins and chemokine receptors with their endothelial ligands. We propose that the search mode governs the intravascular motility of PMo in vivo in a similar way to T cells looking for antigen in tissues. Signs of damage to the luminal side of the endothelium (local death, oxidized LDL, amyloid deposits, tumor cells, pathogens, abnormal red cells, etc.) will change the diffusive random towards a Lèvy-like crawling enhancing their recognition and clearance by PMo damage receptors as the integrin αMβ2 and CD36. This new perspective can help identify new actors to promote unique PMo intravascular actions aimed at maintaining endothelial fitness and combating harmful microparticles involved in diseases as lung metastasis, Alzheimer’s angiopathy, vaso-occlusive disorders, and sepsis.

The Activation and Regulation of β2 Integrins in Phagocytes and Phagocytosis

Phagocytes, which include neutrophils, monocytes, macrophages, and dendritic cells, protect the body by removing foreign particles, bacteria, and dead or dying cells. Phagocytic integrins are greatly involved in the recognition of and adhesion to specific antigens on cells and pathogens during phagocytosis as well as the recruitment of immune cells. β2 integrins, including αLβ2, αMβ2, αXβ2, and αDβ2, are the major integrins presented on the phagocyte surface. The activation of β2 integrins is essential to the recruitment and phagocytic function of these phagocytes and is critical for the regulation of inflammation and immune defense. However, aberrant activation of β2 integrins aggravates auto-immune diseases, such as psoriasis, arthritis, and multiple sclerosis, and facilitates tumor metastasis, making them double-edged swords as candidates for therapeutic intervention. Therefore, precise regulation of phagocyte activities by targeting β2 integrins should promote their host defense functions with minimal side effects on other cells. Here, we reviewed advances in the regulatory mechanisms underlying β2 integrin inside-out signaling, as well as the roles of β2 integrin activation in phagocyte functions.

Analysis of ICAM1 gene polymorphism in Slovak multiple sclerosis patients

Infiltration of immune cells into CNS is one of the essential events in multiple sclerosis (MS) development. Adhesion molecules like the intercellular adhesion molecule 1 (ICAM-1) play critical role in this process. Therefore, the ICAM1 gene containing two important single-nucleotide polymorphisms (SNPs) belongs to candidate loci with possible involvement in MS susceptibility and/or severity. The objective of our case-control study was to analyze the association of two functional ICAM1 polymorphisms rs1799969 (or G241R) and rs5498 (or K469E) with susceptibility to MS and evaluate their influence on the age at disease onset, severity, neurological disability and progression rate. Two hundred forty-eight MS subjects (mean 39.2 years) and 208 age-matched controls (mean 35.6 years) were involved in the study. Genotyping of ICAM1 rs1799969 and rs5498 SNPs was performed by PCR-RFLP. Presence of the rs3135388 polymorphism tagging the major MS risk allele HLA-DRB1*15:01 allele was determined as well. Our analysis revealed no statistically significant association of ICAM1 polymorphisms with risk of MS development in the Slovak population. Stratification of study cohorts by gender, age at onset and presence of the HLA-DRB1*15:01 risk allele showed only moderate changes. Correlation of clinical findings as age at onset, Kurtzke Expanded Disability Status Scale, Multiple Sclerosis Severity Score and progression index with ICAM1 genotypes in MS patients revealed no significant association; however, patients with earlier onset of MS showed slightly higher frequencies of the homozygous G allele at rs5498 in comparison to other genotypes (P = 0.04), suggesting that GG carriers tend to induce MS at an earlier age.

IMM-H004, a coumarin derivative, attenuated brain ischemia/reperfusion injuries and subsequent inflammation in spontaneously hypertensive rats through inhibition of VCAM-1

We studied the effect of IMM-H004 in treating brain I/R injury in spontaneously hypertensive rats and showed that IMM-H004 could efficiently ameliorate neurological defects and infarct volume in a time and dose dependent manner.

Role of fluid shear stress in regulating VWF structure, function and related blood disorders

Von Willebrand factor (VWF) is the largest glycoprotein in blood. It plays a crucial role in primary hemostasis via its binding interaction with platelet and endothelial cell surface receptors, other blood proteins and extra-cellular matrix components. This protein is found as a series of repeat units that are disulfide bonded to form multimeric structures. Once in blood, the protein multimer distribution is dynamically regulated by fluid shear stress which has two opposing effects: it promotes the aggregation or self-association of multiple VWF units, and it simultaneously reduces multimer size by facilitating the force-dependent cleavage of the protein by various proteases, most notably ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type repeats, motif 1 type 13). In addition to these effects, fluid shear also controls the solution and substrate-immobilized structure of VWF, the nature of contact between blood platelets and substrates, and the biomechanics of the GpIbα–VWF bond. These features together regulate different physiological and pathological processes including normal hemostasis, arterial and venous thrombosis, von Willebrand disease, thrombotic thrombocytopenic purpura and acquired von Willebrand syndrome. This article discusses current knowledge of VWF structure–function relationships with emphasis on the effects of hydrodynamic shear, including rapid methods to estimate the nature and magnitude of these forces in selected conditions. It shows that observations made by many investigators using solution and substrate-based shearing devices can be reconciled upon considering the physical size of VWF and the applied mechanical force in these different geometries.

In Vitro Evaluation of the Link Between Cell Activation State and Its Rheological Impact on the Microscale Flow of Neutrophil Suspensions

Activated neutrophils have been reported to affect peripheral resistance, for example, by plugging capillaries or adhering to the microvasculature. In vivo and ex vivo data indicate that activated neutrophils circulating in the blood also influence peripheral resistance. We used viscometry and microvascular mimics for in vitro corroboration. The rheological impact of differentiated neutrophil-like HL-60 promyelocytes (dHL60s) or human neutrophil suspensions stimulated with 10 nM fMet-Leu-Phe (fMLP) was quantified using a cone-plate rheometer (450 s(-1) shear rate). To evaluate their impact on microscale flow resistance, we used 10-μm Isopore® membranes to model capillaries as well as single 200 × 50 μm microchannels and networks of twenty 20 × 50 μm microfluidic channels to mimic noncapillary microvasculature. Stimulation of dHL60 and neutrophil populations significantly altered their flow behavior as evidenced by their impact on suspension viscosity. Notably, hematocrit abrogated the impact of leukocyte activation on blood cell suspension viscosity. In micropore filters, activated cell suspensions enhanced flow resistance. This effect was further enhanced by the presence of erythrocytes. The resistance of our noncapillary microvascular mimics to flow of activated neutrophil suspensions was significantly increased only with hematocrit. Notably, it was elevated to a higher extent within the micronetwork chambers compared to the single-channel chambers. Collectively, our findings provide supportive evidence that activated neutrophils passing through the microcirculation may alter hemodynamic resistance due to their altered rheology in the noncapillary microvasculature. This effect is another way neutrophil activation due to chronic inflammation may, at least in part, contribute to the elevated hemodynamic resistance associated with cardiovascular diseases (e.g., hypertension and hypercholesterolemia).

High wall shear stress and spatial gradients in vascular pathology: a review

Cardiovascular pathologies such as intracranial aneurysms (IAs) and atherosclerosis preferentially localize to bifurcations and curvatures where hemodynamics are complex. While extensive knowledge about low wall shear stress (WSS) has been generated in the past, due to its strong relevance to atherogenesis, high WSS (typically >3 Pa) has emerged as a key regulator of vascular biology and pathology as well, receiving renewed interests. As reviewed here, chronic high WSS not only stimulates adaptive outward remodeling, but also contributes to saccular IA formation (at bifurcation apices or outer curves) and atherosclerotic plaque destabilization (in stenosed vessels). Recent advances in understanding IA pathogenesis have shed new light on the role of high WSS in pathological vascular remodeling. In complex geometries, high WSS can couple with significant spatial WSS gradient (WSSG). A combination of high WSS and positive WSSG has been shown to trigger aneurysm initiation. Since endothelial cells (ECs) are sensors of WSS, we have begun to elucidate EC responses to high WSS alone and in combination with WSSG. Understanding such responses will provide insight into not only aneurysm formation, but also plaque destabilization and other vascular pathologies and potentially lead to improved strategies for disease management and novel targets for pharmacological intervention.

The impact of diabetes on the pathogenesis of sepsis

Diabetes is associated with an increased susceptibility to infection and sepsis. Conflicting data exist on whether the mortality of patients with sepsis is influenced by the presence of diabetes, fuelling the ongoing debate on the benefit of tight glucose regulation in patients with sepsis. The main reason for which diabetes predisposes to infection appears to be abnormalities of the host response, particularly in neutrophil chemotaxis, adhesion and intracellular killing, defects that have been attributed to the effect of hyperglycaemia. There is also evidence for defects in humoral immunity, and this may play a larger role than previously recognised. We review the literature on the immune response in diabetes and its potential contribution to the pathogenesis of sepsis. In addition, the effect of diabetes treatment on the immune response is discussed, with specific reference to insulin, metformin, sulphonylureas and thiazolidinediones.

Determining beta2-integrin and intercellular adhesion molecule 1 binding kinetics in tumor cell adhesion to leukocytes and endothelial cells by a gas-driven micropipette assay

Interactions between polymorphonuclear neutrophils (PMNs) and tumor cells have been reported to facilitate the adhesion and subsequent extravasation of tumor cells through the endothelium under blood flow, both of which are mediated by binding β(2)-integrin to intercellular adhesion molecule 1 (ICAM-1). Here the adhesions between human WM9 metastatic melanoma cells, PMNs, and human pulmonary microvascular endothelial cells (HPMECs) were quantified by a gas-driven micropipette aspiration technique (GDMAT). Our data indicated that the cellular binding affinity of PMN-WM9 pair was 3.9-fold higher than that of the PMN-HPMEC pair. However, the effective binding affinities per molecular pair were comparable between the two cell pairs no matter whether WM9 cells or HPMECs were quiescent or cytokine-activated, indicating that the stronger adhesion between PMN-WM9 pair is mainly attributed to the high expression of ICAM-1 on WM9 cells. These results proposed an alternative mechanism, where WM9 melanoma cells adhere first with PMNs near vessel-wall regions and then bind to endothelial cells via PMNs under blood flow. In contrast, the adhesions between human MDA-MB-231 metastatic breast carcinoma cells and PMNs showed a comparable cellular binding affinity to PMN-HPMEC pair because the ICAM-1 expressions on MDA-MB-231 cells and HPMECs are similar. Furthermore, differences were observed in the intrinsic forward and reverse rates of the β(2)-integrin-ICAM-1 bond between PMN-TC and PMN-EC pairs. This GDMAT assay enables us to quantify the binding kinetics of cell adhesion molecules physiologically expressed on nucleated cells. The findings also further the understanding of leukocyte-facilitated tumor cell adhesion from the viewpoint of molecular binding kinetics.

Sequential binding of αVβ3 and ICAM-1 determines fibrin-mediated melanoma capture and stable adhesion to CD11b/CD18 on neutrophils

Fibrin (Fn) deposition defines several type 1 immune responses, including delayed-type hypersensitivity and autoimmunity in which polymorphonuclear leukocytes (PMNs) are involved. Fn monomer and fibrinogen are multivalent ligands for a variety of cell receptors during cell adhesion. These cell receptors provide critical linkage among thrombosis, inflammation, and cancer metastasis under venous flow conditions. However, the mechanisms of Fn-mediated interactions among immune cells and circulating tumor cells remain elusive. By using a cone-plate viscometer shear assay and dual-color flow cytometry, we demonstrated that soluble fibrinogen and Fn had different abilities to enhance heterotypic aggregation between PMNs and Lu1205 melanoma cells in a shear flow, regulated by thrombin levels. In addition, the involvement of integrin α(v)β(3), ICAM-1, and CD11b/CD18 (Mac-1) in fibrin(ogen)-mediated melanoma-PMN aggregations was explored. Kinetic studies provided evidence that ICAM-1 mediated initial capture of melanoma cells by PMNs, whereas α(v)β(3) played a role in sustained adhesion of the two cell types at a shear rate of 62.5 s(-1). Quantitative analysis of the melanoma-PMN interactions conducted by a parallel-plate flow chamber assay further revealed that at a shear rate of 20 s(-1), α(v)β(3) had enough contact time to form bonds with Mac-1 via Fn, which could not otherwise occur at a shear rate higher than 62.5 s(-1). Our studies have captured a novel finding that leukocytes could be recruited to tumor cells via thrombin-mediated Fn formation within a tumor microenvironment, and α(v)β(3) and ICAM-1 may participate in multistep fibrin(ogen)-mediated melanoma cell adhesion within the circulation.

Kinetics of LFA-1 mediated adhesion of human neutrophils to ICAM-1--role of E-selectin signaling post-activation

LFA-1 and Mac-1 are the two integrins involved in the arrest and firm adhesion of neutrophils. LFA-1 plays a role in the early stage of cell arrest while Mac-1 stabilizes firm adhesion. Here, we further elucidated the kinetics of LFA-1 activation and its role in mediating neutrophil adhesion to ICAM-1 in the presence of E-selectin interaction. We confirm that LFA-1 activation to high affinity is transient in nature, decaying back to low affinity within 1 min after chemotactic stimulation. However, we show for the first time that this downshift in LFA-1 affinity does not return back to the low affinity state when E-selectin interaction is present and active, but rather E-selectin signals an intermediate LFA-1 conformation through PI3-Kinase that maintains an intermediate level of neutrophil firm adhesion. We further show that this E-selectin signaling is capable of returning LFA-1 to the intermediate affinity conformation outside the 1-min window previously reported for LFA-1 functionality. While our work confirms a role for PI3-Kinase in neutrophil firm adhesion, we show that PI3-Kinase may not be important in the initial transition from rolling to firm arrest (i.e. LFA-1 shift from low to high affinity conformation occurring within seconds of chemotactic stimulation).

Effects of the Tumor-Leukocyte Microenvironment on Melanoma-Neutrophil Adhesion to the Endothelium in a Shear Flow

The primary cause of cancer mortality is not attributed to primary tumor formation, but rather to the growth of metastases at distant organ sites. Tumor cell adhesion to blood vessel endothelium (EC) and subsequent transendothelial migration within the circulation are critical components of the metastasis cascade. Previous studies have shown polymorphonuclear neutrophils (PMNs) may facilitate melanoma cell adhesion to the EC and subsequent extravasation under flow conditions. The melanoma cell-PMN interactions are found to be mediated by the binding between intercellular adhesion molecule-1 (ICAM-1) on melanoma cells and β(2) integrin on PMNs and by endogenously secreted interleukin 8 (IL-8) within the tumor-leukocyte microenvironment. In this study, the effects of fluid convection on the IL-8-mediated activation of PMNs and the binding kinetics between PMNs and melanoma cells were investigated. Results indicate that the shear rate dependence of PMN-melanoma cell adhesion and melanoma cell extravasation is due, at least partly, to the convection of tumor-secreted proinflammatory cytokine IL-8.

Immune complexes formed following the binding of anti-platelet factor 4 (CXCL4) antibodies to CXCL4 stimulate human neutrophil activation and cell adhesion

We tested the possibility that immune complexes formed following platelet factor 4 (PF4/CXCL4) binding to anti-PF4 antibody can stimulate neutrophil activation, similar to previous reports with platelets. Monoclonal Abs against PF4 and IgG from a heparin-induced thrombocytopenia (HIT) patient were applied. We observed that although PF4 or anti-PF4 antibody alone did not alter neutrophil function, costimulation with both reagents resulted in approximately 3-fold increase in cell surface Mac-1 expression, enhanced cell adhesion via L-selectin and CD18 integrins, and degranulation of secondary and tertiary granules. The level of Mac-1 up-regulation peaked at an intermediate PF4 dose, suggesting that functional response varies with antigen-antibody stoichiometry. PF4 binding to neutrophils was blocked by chondroitinase ABC. Cell activation was inhibited by both chondroitinase ABC and anti-CD32/FcgammaRII blocking mAb, IV.3. Confocal microscopy demonstrated that immune complexes colocalize with CD32a. Studies with HIT IgG demonstrated that neutrophils could be activated in the absence of exogenous heparin. These data, together, show that leukocyte surface chondroitin sulfates promote neutrophil activation by enhancing immune-complex binding to CD32a. Studies with recombinant PF4 suggest a role for arginine 49 in stabilizing PF4-chondroitin binding. Neutrophils activated via this mechanism may contribute to thrombosis and inflammation in patients mounting an immune response to PF4-heparin.

Hydrodynamic shear rate regulates melanoma-leukocyte aggregation, melanoma adhesion to the endothelium, and subsequent extravasation

Adhesion to and subsequent extravasation through the endothelial lining of blood vessels is critical for tumor cells to establish metastases. Recent studies have indicated that polymorphonuclear neutrophils (PMNs) may enhance melanoma adhesion to the endothelium (EC) and subsequent extravasation under dynamic flow conditions. However, little is known about hydrodynamics involved in the tumor microenvironment within the microcirculation. In this study, effects of hydrodynamic flow on regulating melanoma cell adhesion to the EC have been investigated. Results indicate that under flow conditions, interactions between melanoma cells and the EC are distinctly different from PMN-EC interactions. Without expressions of surface integrins or sialylated molecules, most melanoma cells that express a high-level of intercellular adhesion molecule (ICAM-1) are not able to effectively adhere to the inflamed EC by themselves. Binding of melanoma cells and PMNs through ICAM-1 on melanoma cells and beta(2) integrins on PMNs has been shown to enhance melanoma cell arrest on the EC. Although PMN tethering on the EC is regulated by both the shear rate and shear stress, melanoma cell adhesion to the EC and subsequent extravasation via tethering PMN on the EC is predominantly regulated by shear rate, which partly is due to the shear-rate-dependent PMN-melanoma aggregation in shear flow. These findings provide a rationale and mechanistic basis for understanding of leukocyte-tumor cell interactions under flow conditions during tumor cell extravasation and metastasis.

Differential regulation of neutrophil CD18 integrin function by di- and tri-valent cations: manganese vs. gadolinium

Affinity regulation of integrin function plays an important role during both leukocyte-endothelial and leukocyte-leukocyte interactions. We compared the roles of Mn(2+) (Manganese) and Gd(3+) (Gadolinium) in regulating leukocyte CD18-integrin function. We observed that: (i) Both cations prolonged neutrophil homotypic aggregation following chemoattractant IL-8 stimulation, with Gd(3+) being effective at doses two orders of magnitude (10 microM range) lower that Mn(2+). (ii) While both Gd(3+) and Mn(2+) mediate homotypic cell aggregation via L: -selectin and CD18 integrins, their effects on the integrin subunits, LFA-1 (CD11a/CD18) and Mac-1 (CD11b/CD18), was different. Gd(3+) altered both LFA-1 and Mac-1 function, while the dominant effect of Mn(2+) was on Mac-1. This effect of Gd(3+) on LFA-1 function was confirmed in cell-free studies that measured the binding of recombinant ICAM-1 to LFA-1 immobilized on beads. (iii) Both ions augmented the binding of 327C, an antibody that recognizes active CD18 on human neutrophils, both in the presence and absence of exogenous IL-8. The effects of Mn(2+) was more pronounced since it caused 3-4-fold increase in mAb 327C binding to neutrophils compared to Gd(3+) which increased antibody binding by only approximately 80%. 327C binding was partially reduced by Ca(2+). Further, 327C binding induced by Mn(2+) did not correlate tightly with cell adhesion function. (iv) In studies that monitored intracellular Ca(2+) ([Ca(2+)](i)), the addition of Mn(2+) but not Gd(3+) to neutrophils altered [Ca(2+)](i) levels. Overall, while both Gd(3+) and Mn(2+) stabilize high affinity CD18 mediated cell adhesion, Gd(3+) affects integrin conformation while Mn(2+) may also trigger other effects.

Integrin modulation and signaling in leukocyte adhesion and migration

The movement of leukocytes from the blood into peripheral tissues plays a key role in immunity as well as chronic inflammatory and autoimmune diseases. The shear force of blood flow presents special challenges to leukocytes as they establish adhesion on the vascular endothelium and migrate into the underlying tissues. Integrins are a family of cell adhesion and signaling molecules, whose function can be regulated to meet these challenges. The affinity of integrins for their vascular ligands can be stimulated in subseconds by chemoattractant signaling. This aids in inducing leukocyte adhesion under flow conditions. Further, linkage of these integrins to the actin cytoskeleton also helps to establish adhesion to the endothelium under flow conditions. In the case of alpha4beta1 integrins, this linkage of the integrin to the cytoskeleton is mediated in part by the binding of paxillin to the alpha4 integrin subunit and the subsequent binding of paxillin to the cytoskeleton molecule talin. The movement of leukocytes along the vascular endothelium and in between endothelial cells requires the temporal and spatial regulation of small guanosine triphosphatases, such as Rac1. We describe mechanisms through which alpha4beta1 integrin signaling regulates appropriate Rac activation to drive leukocyte migration.

Multi-cell agent-based simulation of the microvasculature to study the dynamics of circulating inflammatory cell trafficking

Leukocyte trafficking through the microcirculation and into tissues is central in angiogenesis, inflammation, and the immune response. Although the literature is rich with mechanistic detail describing molecular mediators of these processes, integration of signaling events and cell behaviors within a unified spatial and temporal framework at the multi-cell tissue-level is needed to achieve a fuller understanding. We have developed a novel computational framework that combines agent-based modeling (ABM) with a network flow analysis to study monocyte homing. A microvascular network architecture derived from mouse muscle was incorporated into the ABM. Each individual cell was represented by an individual agent in the simulation. The network flow model calculates hemodynamic parameters (blood flow rates, fluid shear stress, and hydrostatic pressures) throughout the simulated microvascular network. These are incorporated into the ABM to affect monocyte transit through the network and chemokine/cytokine concentrations. In turn, simulated monocytes respond to their local mechanical and biochemical environments and make behavioral decisions based on a rule set derived from independent literature. Simulated cell behaviors give rise to emergent leukocyte rolling, adhesion, and extravasation. Molecular knockout simulations were performed to validate the model, and predictions of monocyte adhesion, rolling, and extravasation show good agreement with the independently published corresponding mouse studies.

CD40 ligand mediates inflammation independently of CD40 by interaction with Mac-1

BACKGROUND

Strong evidence supports a role for CD40 ligand (CD40L) as marker and mediator of inflammatory diseases such as atherosclerosis. Despite extensive characterization of CD40, the classic receptor of CD40L, its role in immune defense against inflammatory diseases remains uncertain. The present study aimed to characterize the contribution of CD40 signaling to atherogenesis.

METHODS AND RESULTS

Surprisingly, mice deficient in both CD40 and the low-density lipoprotein receptor did not develop smaller lesions in the aortic arch, root, and thoracoabdominal aorta compared with mice deficient only in the low-density lipoprotein receptor that consumed an atherogenic diet for 8 and 16 weeks. By flow cytometry, radioactive binding assays, and immunoprecipitation, we demonstrate that CD40L interacts with the integrin Mac-1, which results in Mac-1-dependent adhesion and migration of inflammatory cells as well as myeloperoxidase release in vitro. Furthermore, mice deficient in CD40L show significantly reduced thioglycolate-elicited invasion of inflammatory cells into the peritoneal cavity compared with mice deficient in CD40 and wild-type controls. Inhibition of Mac-1 in low-density lipoprotein receptor-deficient mice attenuates lesion development and reduces lesional macrophage accumulation.

CONCLUSIONS

These observations identify the interaction of CD40L and Mac-1 as an alternative pathway for CD40L-mediated inflammation. This novel mechanism expands understanding of inflammatory signaling during atherogenesis.

Interplay between shear stress and adhesion on neutrophil locomotion

Leukocyte locomotion over the lumen of inflamed endothelial cells is a critical step, following firm adhesion, in the inflammatory response. Once firmly adherent, the cell will spread and will either undergo diapedesis through individual vascular endothelial cells or will migrate to tight junctions before extravasating to the site of injury or infection. Little is known about the mechanisms of neutrophil spreading or locomotion, or how motility is affected by the physical environment. We performed a systematic study to investigate the effect of the type of adhesive ligand and shear stress on neutrophil motility by employing a parallel-plate flow chamber with reconstituted protein surfaces of E-selectin, E-selectin/PECAM-1, and E-selectin/ICAM-1. We find that the level and type of adhesive ligand and the shear rate are intertwined in affecting several metrics of migration, such as the migration velocity, random motility, index of migration, and the percentage of cells moving in the direction of flow. On surfaces with high levels of PECAM-1, there is a near doubling in random motility at a shear rate of 180 s(-1) compared to the motility in the absence of flow. On surfaces with ICAM-1, neutrophil random motility exhibits a weaker response to shear rate, decreasing slightly when shear rate is increased from static conditions to 180 s(-1), and is only slightly higher at 1000 s(-1) than in the absence of flow. The random motility increases with increasing surface concentrations of E-selectin and PECAM-1 under static and flow conditions. Our findings illustrate that the endothelium may regulate neutrophil migration in postcapillary venules through the presentation of various adhesion ligands at sites of inflammation.

Biomechanics of P-selectin PSGL-1 bonds: shear threshold and integrin-independent cell adhesion

Platelet-leukocyte adhesion may contribute to thrombosis and inflammation. We examined the heterotypic interaction between unactivated neutrophils and either thrombin receptor activating peptide (TRAP)-stimulated platelets or P-selectin-bearing beads (Ps-beads) in suspension. Cone-plate viscometers were used to apply controlled shear rates from 14 to 3000/s. Platelet-neutrophil and bead-neutrophil adhesion analysis was performed using both flow cytometry and high-speed videomicroscopy. We observed that although blocking antibodies against either P-selectin or P-selectin glycoprotein ligand-1 (PSGL-1) alone inhibited platelet-neutrophil adhesion by approximately 60% at 140/s, these reagents completely blocked adhesion at 3000/s. Anti-Mac-1 alone did not alter platelet-neutrophil adhesion rates at any shear rate, though in synergy with selectin antagonists it abrogated cell binding. Unstimulated neutrophils avidly bound Ps-beads and activated platelets in an integrin-independent manner, suggesting that purely selectin-dependent cell adhesion is possible. In support of this, antagonists against P-selectin or PSGL-1 caused dissociation of previously formed platelet-neutrophil and Ps-bead neutrophil aggregates under shear in a variety of experimental systems, including in assays performed with whole blood. In studies where medium viscosity and shear rate were varied, a shear threshold for P-selectin PSGL-1 binding was also noted at shear rates <100/s when Ps-beads collided with isolated neutrophils. Results are discussed in light of biophysical computations that characterize the collision between unequal-size particles in linear shear flow. Overall, our studies reveal an integrin-independent regime for cell adhesion and weak shear threshold for P-selectin PSGL-1 interactions that may be physiologically relevant.

Shear stress and shear rate differentially affect the multi-step process of leukocyte-facilitated melanoma adhesion

Previous studies have shown that neutrophils (PMNs) facilitate melanoma cell extravasation [M.J. Slattery, C. Dong, Neutrophils influence melanoma adhesion and migration under flow conditions, Intl. J. Cancer 106 (2003) 713-722] Little is known, however, about the specific interactions between PMNs, melanoma and the endothelium (EC) or the molecular mechanism involved under flow conditions. The aim of this study is to investigate a "two-step adhesion" hypothesis that involves initial PMN tethering on the EC and subsequent melanoma cells being captured by tethered PMNs. Different effects of hydrodynamic shear stress and shear rate were analyzed using a parallel-plate flow chamber. Results indicate a novel finding that PMN-facilitated melanoma cell arrest on the EC is modulated by shear rate, which is inversely-proportional to cell-cell contact time, rather than by the shear stress, which is proportional to the force exerted on formed bonds. Beta2 integrins/ICAM-1 adhesion mechanisms were examined and the results indicate LFA-1 and Mac-1 cooperate to mediate the PMN-EC-melanoma interactions under shear conditions. In addition, endogenously produced IL-8 contributes to PMN-facilitated melanoma arrest on the EC through the CXC chemokine receptors 1 and 2 (CXCR1 and CXCR2) on PMN. These results provide new evidence for the complex role of hemodynamic forces, secreted chemokines and PMN-melanoma adhesion in the recruitment of metastatic cancer cells to the EC.

Molecular mechanics and dynamics of leukocyte recruitment during inflammation

Discovery of new genes and proteins directly supporting leukocyte adhesion is waning, whereas there is heightened interest in the cell mechanics and receptor dynamics that lead from transient tethering via selectins to affinity shifts and adhesion strengthening through integrins. New optical tools enable real-time imaging of leukocyte rolling and arrest in parallel plate flow channels (PPFCs), and detection of single-molecule force spectroscopy provides an inner view of the intercellular adhesive contact region. Leukocyte recruitment during acute inflammation is triggered by ligation of G protein-coupled chemotactic receptors (GPCRs) and clustering of selectins. This, in turn, activates beta(2)-integrin (CD18), which facilitates cell capture and arrest in shear flow. This review provides a conceptual model for the molecular events supporting leukocyte recruitment.

Transport features, reaction kinetics and receptor biomechanics controlling selectin and integrin mediated cell adhesion

The distinct and overlapping roles of adhesion molecules belonging to the selectin and integrin families control the rate of leukocyte adhesion to stimulated vascular endothelial cells under hydrodynamic shear flow. Crystal structures have appeared for some of these interactions which complement molecular biology experiments, and clarify the molecular mechanism of the receptor-ligand binding interactions. Binding affinity data have also appeared using surface plasmon resonance and single-molecule biophysics experiments. These studies confirm and extend the predictions of previous experiments carried out in parallel-plate flow chambers, and cone and plate viscometers. This review discusses the current state of understanding on how molecular bond formation rates coupled with cellular and hydrodynamic features regulate leukocyte binding to endothelial cells.

ANCA induces β2integrin and CXC chemokine-dependent neutrophil-endothelial cell interactions that mimic those of highly cytokine-activated endothelium

Antineutrophil cytoplasm antibodies (ANCA) activate neutrophils to undergo a series of coordinated interactions, leading to transendothelial migration, eventually culminating in vascular destruction. The molecular events underlying neutrophil recruitment in ANCA-associated vasculitis need to be defined to enable effective therapeutic manipulation. A flow-based adhesion assay was used to investigate the role of beta2 integrins (CD11a/CD18 and CD11b/CD18) and chemokine receptors [CXC chemokine receptor (CXCR)1 and CXCR2] in neutrophil migration through the endothelium. Two endothelial models were used: a highly activated model stimulated with 100 U/ml tumor necrosis factor alpha (TNF-alpha) and a minimally activated model stimulated with 2 U/ml TNF-alpha and in which ANCA was present as a secondary neutrophil stimulus. CD11a/CD18, CD11b/CD18, and CXCR2 contributed to adhesion and transendothelial migration in both models. However, when the endothelium was minimally activated with TNF-alpha, CD11b/CD18 played an important role in stabilizing adhesion induced by ANCA immunoglobulin G (IgG). Analysis of beta2 integrins and chemokine receptors demonstrated that ANCA IgG had no effect on expression levels at the neutrophil surface but enabled an active conformational change of CD11b/CD18. Similar molecular mechanisms control neutrophil adhesion and migration through highly or minimally TNF-alpha-activated endothelium. However, the direct ANCA-mediated neutrophil stimulation is needed to drive migration through minimally activated endothelium, and CD11b/CD18 is recruited for greater stability of adhesion during this process and can undergo an activatory, conformational change in response to ANCA IgG.

Avidity modulation activates adhesion under flow and requires cooperativity among adhesion receptors

An early step in activation of leukocyte adhesion is a release of integrins from cytoskeletal constraints on their diffusion, leading to rearrangement and, consequently, increased avidity. Static adhesion assays using purified ligand as a substrate have demonstrated that very low doses of cytochalasin D disconnect beta2-integrins from their cytoskeletal links, allowing rearrangement and activating adhesion. The adhesion process in blood vessels is poorly simulated by these assays, however, for two reasons: leukocyte adhesion to endothelium 1), occurs in the presence of blood flow and 2), involves the simultaneous interactions of multiple sets of adhesion molecules. We investigated the effect of cytochalasin D, at concentrations that increase integrin diffusion but do not alter leukocyte shape and surface features, on adhesion of leukocytes to endothelial cells under flow. Cytochalasin D increased the number of rolling cells, the number of firmly adherent cells, and the duration of both rolling and firm adhesion. These effects required endothelial cell expression of ICAM-1, the ligand for leukocyte beta2-integrins. The beta2-integrin-ICAM-1 interaction alone was not sufficient, however. Experiments using purified substrates demonstrated that avidity effects on activation of adhesion under flow require functional cooperativity between integrins and other adhesion receptors.

Hemodynamic forces regulate mural macrophage infiltration in experimental aortic aneurysms

Blood flow (BF) and wall shear stress (WSS) influence reactive oxygen species production and oxidative stress in abdominal aortic aneurysm (AAA) disease. To gain further insight into the mechanisms of hemodynamic influences on AAA inflammation, we examined aneurysm macrophage density, chemotaxis and survival under varying aortic flow conditions. Rat AAAs were created via porcine pancreatic elastase (PPE) infusion. In selected cohorts, AAA flow was increased via left common femoral arteriovenous fistula (AVF) creation (HF-AAA) or decreased by left common iliac ligation (LF-AAA). WSS was highest in HF-AAA (10.4 +/- 2.3 dyn/cm(2) vs. 2.4 +/- 0.4 and 0.5 +/- 0.2 for NF- and LF-AAA, respectively, P < 0.001) 7 days after PPE infusion, with reduced medial macrophage density and increased apoptosis. Adventitial macrophage density was not significantly influenced by flow. Monocyte chemoattractant protein-1 (MCP-1) and granulocyte-macrophage colony-stimulating factor (GM-CSF) gene expression correlated with observed macrophage densities in the media and adventitia. Luminal flow conditions regulate AAA inflammation in part via influences on medial macrophage density. Hemodynamic forces may modulate AAA inflammation and diameter enlargement via direct regulation of intimal macrophage adhesion, transmural migration or survival.

Intercellular adhesion molecule-1: a protective haplotype against multiple sclerosis

Intercellular adhesion molecule-1 (ICAM-1) and its receptors are adhesion molecules that play a key role in the transmigration of inflammatory cells through the blood-brain barrier, one of the earliest events in multiple sclerosis (MS), which leads to demyelination in the central nervous system. To investigate the role of genes encoding ICAM-1 and its receptors, we used a strategy of genetic linkage and association in 439 case-parent MS families of French origin, well characterized according to HLA status and severity. We demonstrate that the genes encoding ICAM-1 receptors do not influence MS susceptibility or severity. ICAM-1 had a modest, but significant effect on MS genetic susceptibility, independent of HLA and disease severity. We observed a rare, and an as yet unreported, ICAM-1 gene haplotype defined by amino acids K469 and R241 that was never transmitted to patients suggesting a protective effect against MS in our population.

Neutrophils influence melanoma adhesion and migration under flow conditions

We have studied human melanoma cell (C8161) adhesion and migration in response to stimulation by soluble collagen IV (CIV) using a modified Boyden chamber. In this modified chamber, shear flow can be introduced over the cell-substrate interface, affecting tumor cell chemotactic migration through a microporous filter. A relatively high level of intercellular adhesion molecule-1 (ICAM-1) was found on C8161 cells. In contrast, levels of beta(2)-integrins (e.g., LFA-1 and Mac-1), the molecules that would be necessary for C8161 stable adhesion to the endothelium substrate, were found to be very low on these melanoma cells. As a result, C8161 transendothelial migration under a flow condition of 4 dyn/cm(2) decreased by 70% as compared to static migration. When human neutrophils (PMNs) were present in the tumor cell suspension, C8161 migration recovered by 85% over C8161 cells alone under the 4 dyn/cm(2) flow condition. Blocking ICAM-1 on C8161 cells or Mac-1 on PMNs significantly inhibited C8161-PMN adhesion and subsequent C8161 migration through the endothelium under flow conditions. In addition, increased interleukin-8 production and Mac-1 expression by PMNs were detected when they were co-cultured with C8161 melanoma cells. These results suggest that transmigration of C8161 cells under flow conditions can be influenced by PMNs, mediated by Mac-1/ICAM-1 adhesive interactions and enhanced by altered cytokine production.

Oxidative stress in septic shock and disseminated intravascular coagulation

Oxidative stress results from an oxidant/antioxidant imbalance, an excess of oxidants and/or a depletion of antioxidants. A considerable body of recent evidence suggests that oxidant stress plays a major role in several aspects of septic shock and disseminated intravascular coagulation (DIC), and it is the subject of this review. Immunohistochemical and biochemical evidence demonstrate the significant role of reactive oxygen species (ROS) in endotoxic and hemorrhagic shock, and in endothelial injury associated with DIC syndrome. Initiation of lipid peroxidation, direct inhibition of mitochondrial respiratory chain enzymes, inactivation of glyceraldehyde-3-phosphate dehydrogenase, inhibition of membrane Na+/K+ ATP-ase activity, inactivation of membrane sodium channels, and other oxidative protein modifications contribute to the cytotoxic effect of ROS. In addition, reactive oxygen species are potent triggers of DNA strand breakage, with subsequent activation of the nuclear enzyme poly-ADP ribosyl synthetase, with eventual severe energy depletion of the cells. Pharmacological evidence suggests that the peroxynitrite-poly-ADP ribosyl synthetase pathway contributes to the cellular injury in shock and endothelial injury. Treatment with superoxide dismutase mimetics (SODms), which selectively mimic the catalytic activity of the human superoxide dismutase enzymes, have been shown to prevent in vivo shock and the cellular energetic failure associated with shock.

Fluid shear- and time-dependent modulation of molecular interactions between PMNs and colon carcinomas

This study compares the effects of fluid shear on the kinetics, adhesion efficiency, stability, and molecular requirements of polymorphonuclear leukocyte (PMN) binding to two colon adenocarcinoma cell-lines, the CD54-negative/sLe(x)-bearing LS174T cells and the CD54-expressing/sLe(x)-low HCT-8 cells. The efficiency of PMN-colon carcinoma heteroaggregation decreases with increasing shear, with PMNs binding HCT-8 more efficiently than LS174T cells at low shear (50-200 s(-1)). In the low shear regime, CD11b is sufficient to mediate PMN binding to LS174T cells. In contrast, both CD11a and CD11b contribute to PMN-HCT-8 heteroaggregation, with CD54 on HCT-8 cells acting as a CD11a ligand at early time points. At high shear, only PMN-LS174T heteroaggregation occurs, which is initiated by PMN L-selectin binding to a sialylated, O-linked, protease-sensitive ligand on LS174T cells. PMN-LS174T heteroaggregation is primarily dependent on the intercellular contact duration (or shear rate), whereas PMN-HCT-8 binding is a function of both the intercellular contact duration and the applied force (or shear stress). Cumulatively, these studies suggest that fluid shear modulates the kinetics and molecular mechanisms of PMN-colon carcinoma cell aggregation.

Estimating the efficiency of cell capture and arrest in flow chambers: study of neutrophil binding via E-selectin and ICAM-1

A mathematical model was developed to quantify the efficiency of cell-substrate attachment in the parallel-plate flow chamber. The model decouples the physical features of the system that affect cell-substrate collision rates from the biological features that influence cellular adhesivity. Thus, experimental data on cell rolling and adhesion density are converted into "frequency" parameters that quantify the "efficiency" with which cells in the flow chamber progress from the free stream to rolling, and transition from rolling to firm arrest. The model was partially validated by comparing simulation results with experiments where neutrophils rolled and adhered onto substrates composed of cotransfected cells bearing E-selectin and intercellular adhesion molecule-1 (ICAM-1). Results suggest that: 1) Neutrophils contact the E-selectin substrate on average for 4-8.5s before tethering. This contact duration is insensitive to applied shear stress. 2) At 2 dyn/cm(2), approximately 28% of the collisions between the cells and substrate result in primary capture. Also, approximately 5-7% of collisions between neutrophils in the free stream and previously recruited neutrophils bound on the substrate result in secondary capture. These percentages were higher at lower shears. 3) An adherent cell may influence the flow streams in its vicinity up to a distance of 2.5 cell diameters away. 4) Our estimates of selectin on-rate in cellular systems compare favorably with data from reconstituted systems with immobilized soluble E-selectin. In magnitude, the observed on-rates occur in the order, L-selectin > P-selectin > E-selectin.

Dynamic regulation of LFA-1 activation and neutrophil arrest on intercellular adhesion molecule 1 (ICAM-1) in shear flow

Neutrophil recruitment during acute inflammation is triggered by G-protein-linked chemotactic receptors that in turn activate beta(2) integrin (CD18), deemed a critical step in facilitating cell capture and arrest under the shear force of blood flow. A conformational switch in the I domain allosteric site (IDAS) and in CD18 regulates LFA-1 affinity for endothelial ligands including intercellular adhesion molecule 1 (ICAM-1). We examined the dynamics of CD18 activation in terms of the efficiency of neutrophil capture of ICAM-1, and we correlated this with the membrane topography of 327C, an antibody that recognizes the active conformation of CD18 I-like domain. Adhesion increased in direct proportion to chemotactic stimulus rising 7-fold over a log range of interleukin-8 (IL-8). A threshold dose of approximately 75 pm IL-8, corresponding to ligation of only approximately 10-100 receptors, was sufficient to activate approximately 20,000 CD18 and a rapid boost in the capture efficiency on ICAM-1. This was accompanied by a rapid redistribution of active LFA-1, but not Mac-1, into membrane patches, a necessary component for optimum adhesion efficiency. Shear-resistant arrest on a monolayer of ICAM-1 was reversed within minutes of chemotactic stimulation correlating with a shift from high to low affinity CD18 and dispersal of patches of active CD18. Mobility of active CD18 into high avidity patches was dependent on phosphatidylinositol 3-kinase activity and not F-actin polymerization. The data reveal that the number of chemotactic receptors bound and the topography and lifetime of high affinity LFA-1 tightly regulate the efficiency of neutrophil capture on ICAM-1.

Regulation of B16F1 melanoma cell metastasis by inducible functions of the hepatic microvasculature

We have previously shown that circulating intravascular cells generally arrest by mechanical restriction in the hepatic sinusoids, causing rapid release of nitric oxide (NO) which is cytotoxic to these cells and inhibits their growth into metastatic tumours. Here, we present evidence that these NO-dependent cytotoxic mechanisms are susceptible to upregulation by lipopolysaccharide (LPS). Five x 10(5) fluorescently labelled melanoma cells were injected into the mesenteric vein of C57BL/6 mice to effect their localisation in the hepatic microvasculature. Test mice were then given 1 mg/kg LPS intraperitoneally (i.p.) to activate the microvascular cells. By electron paramagnetic resonance (EPR) spectroscopy, the expression of NO in the liver was significantly increased by 8 h in the LPS-treated mice. The non-selective NO synthase inhibitor L-NAME inhibited the induction of NO by LPS, while its inactive enantiomer D-NAME had no significant effect. Using immunohistochemistry (IHC), iNOS-positive microvascular cells were detected in the terminal portal venule (TPV) region of the liver 8 h after LPS stimulation. LPS treatment also increased the retention of melanoma cells in the liver between 8 and 24 h, especially in the TPV region. Eight hours after cell injection, local expression of VCAM-1 and ICAM-1 was detected by double-label immunohistochemistry at the sites of tumour cell arrest. Expression of these adhesion molecules was enhanced in mice treated with LPS. Using flow cytometry, 98% of the B16F1 melanoma cells expressed VLA-4, the counter receptor of VCAM-1, and approximately 1.5% expressed LFA-1, the counter receptor of ICAM-1. LPS did not significantly alter the expression of either counter receptor on melanoma cells in vitro or in vivo. By DNA end-labelling, the rates of melanoma cell apoptosis were significantly increased from 8 to 24 h in the TPV region (but not in the sinusoids) of LPS-treated mice. Fourteen days after tumour cell injection, the LPS-treated mice had a significantly smaller hepatic metastatic tumour burden than the control mice. These data suggest that LPS can inhibit the metastasis of melanoma cells in the liver by inducing the expression of NO and adhesion molecules by the hepatic endothelium. The induction of iNOS and the inducible cytotoxic effect of LPS appear to be primarily located within the TPV region of the liver acinus.

Control of leukocyte rolling velocity in TNF-alpha-induced inflammation by LFA-1 and Mac-1

Previously it was shown that beta(2)-integrins are necessary for slow leukocyte rolling in inflamed venules. In this study, mice that are deficient for either one of the beta(2)-integrins, alpha(L)beta(2) (LFA-1) or alpha(M)beta(2) (Mac-1), were used to determine which of the beta(2)-integrins are responsible for slowing rolling leukocytes. The cremaster muscles of these mice were treated with tumor necrosis factor-alpha and prepared for intravital microscopy. The average rolling velocities in venules were elevated in LFA-1(-/-) mice (11.0 +/- 0.7 microm/s) and Mac-1(-/-) mice (10.1 +/- 1.1 microm/s) compared to wild-type mice (4.8 +/- 0.3 microm/s; P <.05), but were lower than in CD18(-/-) mice (28.5 +/- 2.1 microm/s). When both LFA-1 and Mac-1 were absent or blocked, rolling velocity became dependent on shear rate and approached that of CD18(-/-) mice. In addition, leukocyte adhesion efficiency was decreased in LFA-1(-/-) mice to near CD18(-/-) levels, but decreased only slightly in Mac-1(-/-) mice. Thus, both LFA-1 and Mac-1 contribute to slowing down rolling leukocytes, although LFA-1 is more important than Mac-1 in efficiently inducing firm adhesion.

Effects of IL-8, Gro-alpha, and LTB(4) on the adhesive kinetics of LFA-1 and Mac-1 on human neutrophils

Firm adhesion of rolling neutrophils on inflamed endothelium is dependent on beta(2) (CD18)-integrins and activating stimuli. LFA-1 (CD11a/CD18) appears to be more important than Mac-1 (CD11b/CD18) in neutrophil emigration at inflammatory sites, but little is known of the relative binding characteristics of these two integrins under conditions thought to regulate firm adhesion. The present study examined the effect of chemoattractants on the kinetics of LFA-1 and Mac-1 adhesion in human neutrophils. We found that subnanomolar concentrations of interleukin-8, Gro-alpha, and leukotriene B(4) (LTB(4)) induced rapid and optimal rates of LFA-1-dependent adhesion of neutrophils to intercellular adhesion molecule (ICAM)-1-coated beads. These optimal rates of LFA-1 adhesion were transient and decayed within 1 min after chemoattractant stimulation. Mac-1 adhesion was equally rapid initially but continued to rise for >/=6 min after stimulation. A fourfold higher density of ICAM-1 on beads markedly increased the rate of binding to LFA-1 but did not change the early and narrow time window for the optimal rate of adhesion. Using well-characterized monoclonal antibodies, we showed that activation of LFA-1 and Mac-1 by Gro-alpha was completely blocked by anti-CXC chemokine receptor R2, but activation of these integrins by interleukin-8 was most effectively blocked by anti-CXC chemokine receptor R1. The topographical distribution of beads also reflected significant differences between LFA-1 and Mac-1. Beads bound to Mac-1 translocated to the cell uropod within 4 min, but beads bound to LFA-1 remained bound to the lamellipodial regions at the same time. These kinetic and topographical differences may indicate distinct functional contributions of LFA-1 and Mac-1 on neutrophils.

Nonlinear flow affects hydrodynamic forces and neutrophil adhesion rates in cone-plate viscometers

We present a theoretical and experimental analysis of the effects of nonlinear flow in a cone-plate viscometer. The analysis predicts that flow in the viscometer is a function of two parameters, the Reynolds number and the cone angle. Nonlinear flow occurs at high shear rates and causes spatial variations in wall shear stress, collision frequency, interparticle forces and attachment times within the viscometer. We examined the effect of these features on cellular adhesion kinetics. Based on recent data (Taylor, A. D., S. Neelamegham, J. D. Hellums, et al. 1996. Biophys. J. 71:3488-3500), we modeled neutrophil homotypic aggregation as a process that is integrin-limited at low shear and selectin-limited at high shear. Our calculations suggest that selectin and integrin on-rates lie in the order of 10(-2)-10(-4)/s. They also indicate that secondary flow causes positional variations in adhesion efficiency in the viscometer, and that the overall efficiency is dependent not only on the shear rate, but also the sample volume and the cone angle. Experiments performed with isolated neutrophils confirmed these predictions. In these experiments, enhancing secondary flow by increasing the sample volume from 100 to 1000 microl at 1500/s for a 2 degrees cone caused up to an approximately 45% drop in adhesion efficiency. Our results suggest that secondary flow may significantly influence cellular aggregation, platelet activation, and endothelial cell mechanotransduction measurements made in the viscometer over the range of conditions applied in typical biological studies.

The use of lymphocyte function-associated antigen (LFA)-1-deficient mice to determine the role of LFA-1, Mac-1, and alpha4 integrin in the inflammatory response of neutrophils

After injury or infection, neutrophils rapidly migrate from the circulation into tissues by means of an orderly progression of adhesion receptor engagements. Neutrophils have been previously considered to use selectins exclusively to roll on vessels before an adhesion step mediated by the beta2 integrins, lymphocyte function-associated antigen (LFA)-1, and Mac-1. Here we use LFA-1(-/-) mice, function blocking monoclonal antibodies, and intravital microscopy to investigate the roles of LFA-1, Mac-1, and alpha4 integrins in neutrophil recruitment in vivo. For the first time, we show that LFA-1 makes a contribution to neutrophil rolling by stabilizing the transient attachment or tethering phase of rolling. In contrast, Mac-1 does not appear to be important for either rolling or firm adhesion, but instead contributes to emigration from the vessel. Blocking Mac-1 in the presence of LFA-1 significantly reduces emigration, suggesting cooperation between these two integrins. Low levels of alpha4beta1 integrin can be detected on neutrophils from LFA-1(+/+) and (-/-) mice. These cells make use of alpha4beta1 during the rolling phase, particularly in the absence of LFA-1. Thus LFA-1 and alpha4beta1, together with the selectins, are involved in the rolling phase of neutrophil recruitment, and, in turn, affect the later stages of the transmigration event.

Platelet/polymorphonuclear leukocyte adhesion: a new role for SRC kinases in Mac-1 adhesive function triggered by P-selectin

Adhesion of polymorphonuclear leukocytes (PMNLs) to activated platelets requires a P-selectin-triggered, tyrosine kinase-dependent adhesiveness of Mac-1 and is accompanied by tyrosine phosphorylation of a 110-kd protein (P-110) in PMNLs. Inhibitors of SRC tyrosine kinases were found to inhibit PMNL adhesion to activated platelets or to P-selectin expressing Chinese hamster ovary (CHO-P) cells and the tyrosine phosphorylation of P-110. Adhesion of PMNLs to activated platelets or to CHO-P cells stimulated activity of LYN and HCK. Monoclonal antibody blockade of P-selectin or beta2-integrins reduced the activation of both kinases. In PMNLs either adherent to platelets or aggregated by P-selectin-IgG chimera, Mac-1 was rapidly redistributed to the Triton X-100-insoluble cytoskeletal fraction, and large clusters of Mac-1 colocalized with patches of F-actin at the sites of cell-cell contact. In PMNLs stimulated by P-selectin-IgG chimera, SRC kinase inhibition impaired Mac-1 clustering, F-actin accumulation, and CD18 redistribution to the cytoskeleton. Disruption of the actin filament network by cytochalasin D prevented PMNL-platelet adhesion and P-selectin-induced PMNL aggregation and impaired the clustering of Mac-1. In agreement with the requirement for the beta2-integrin in the functional up-regulation of LYN and HCK, integrin blockade by monoclonal antibodies resulted in a complete inhibition of P-selectin-induced Mac-1 clustering and F-actin accumulation. Taken together, the results indicate that, after an initial P-selectin-triggered beta2-integrin interaction with the ligand, SRC kinases are activated and allow the remodeling of cytoskeleton-integrin linkages and integrin clustering that finally strengthen cell-cell adhesion. This model highlights a new role for SRC kinases in a regulatory loop by which the Mac-1 promotes its own adhesive function.

Correlating the kinetics of cytokine-induced E-selectin adhesion and expression on endothelial cells

Many human diseases are mediated through the immune system. In chronic inflammatory disorders, the processes ordinarily involved in tissue healing become destructive. Endothelial cells normally recruit leukocytes to inflamed tissue using cytokine-induced adhesion receptors on the surfaces of interacting cells. Leukocyte capture depends on specialized characteristics of these receptors, particularly the binding kinetics. This study is designed to clarify the relationship between cytokine-induced changes in cell properties and binding kinetics. Here, we measure the kinetics of expression and monoclonal antibody binding for E-selectin in interleukin-1alpha-stimulated microvascular endothelium in vitro and incorporate the data into kinetic models. Quantitative flow cytometry is used to determine molecular density (expression), and micropipette assays are used to find the probability of adhesion (function). Within five hours of interleukin-1alpha stimulation, E-selectin density increases from 0 to 742 sites/microm(2), and antibody-E-selectin adhesion probability increases from a baseline of 6.3% to 64%. A kinetic model is applied to find an apparent association rate constant, k(f), of 3.7 x 10(-14) cm(2)/sec for antibody-E-selectin binding. Although the model successfully predicts experimental results, the rate constant is undervalued for a diffusion-limited process, suggesting that functional adhesion may be modified through cytokine-induced changes in microtopology and receptor localization.

Neutrophil tethering on E-selectin activates beta 2 integrin binding to ICAM-1 through a mitogen-activated protein kinase signal transduction pathway

On inflamed endothelium selectins support neutrophil capture and rolling that leads to firm adhesion through the activation and binding of beta 2 integrin. The primary mechanism of cell activation involves ligation of chemotactic agonists presented on the endothelium. We have pursued a second mechanism involving signal transduction through binding of selectins while neutrophils tether in shear flow. We assessed whether neutrophil rolling on E-selectin led to cell activation and arrest via beta 2integrins. Neutrophils were introduced into a parallel plate flow chamber having as a substrate an L cell monolayer coexpressing E-selectin and ICAM-1 (E/I). At shears >/=0.1 dyne/cm2, neutrophils rolled on the E/I. A step increase to 4.0 dynes/cm2 revealed that approximately 60% of the interacting cells remained firmly adherent, as compared with approximately 10% on L cells expressing E-selectin or ICAM-1 alone. Cell arrest was dependent on application of shear and activation of Mac-1 and LFA-1 to bind ICAM-1. Firm adhesion was inhibited by blocking E-selectin, L-selectin, or PSGL-1 with Abs and by inhibitors to the mitogen-activated protein kinases. A chimeric soluble E-selectin-IgG molecule specifically bound sialylated ligands on neutrophils and activated adhesion that was also inhibited by blocking the mitogen-activated protein kinases. We conclude that neutrophils rolling on E-selectin undergo signal transduction leading to activation of cell arrest through beta 2 integrins binding to ICAM-1.

Shear and time-dependent changes in Mac-1, LFA-1, and ICAM-3 binding regulate neutrophil homotypic adhesion

We examined the relative contributions of LFA-1, Mac-1, and ICAM-3 to homotypic neutrophil adhesion over the time course of formyl peptide stimulation at shear rates ranging from 100 to 800 s-1. Isolated human neutrophils were sheared in a cone-plate viscometer and the kinetics of aggregate formation was measured by flow cytometry. The efficiency of cell adhesion was computed by fitting the aggregate formation rates with a model based on two-body collision theory. Neutrophil homotypic adhesion kinetics varied with shear rate and was most efficient at 800 s-1, where approximately 40% of the collisions resulted in adhesion. A panel of blocking Abs to LFA-1, Mac-1, and ICAM-3 was added to assess the relative contributions of these molecules. We report that 1) LFA-1 binds ICAM-3 as its primary ligand supporting homotypic adhesion, although the possibility of other ligands was also detected. 2) Mac-1 binding to an unidentified ligand supports homotypic adhesion with an efficiency comparable to LFA-1 at low shear rates of approximately 100 s-1. Above 300 s-1, however, Mac-1 and not LFA-1 were the predominant molecules supporting cell adhesion. This is in contrast to neutrophil adhesion to ICAM-1-transfected cells, where LFA-1 binds with a higher avidity than Mac-1 to ICAM-1. 3) Following stimulation, the capacity of LFA-1 to support aggregate formation decreases with time at a rate approximately 3-fold faster than that of Mac-1. The results suggest that the relative contributions of beta2 integrins and ICAM-3 to neutrophil adhesion is regulated by the magnitude of fluid shear and time of stimulus over a range of blood flow conditions typical of the venular microcirculation.

Endothelial apoptosis: could it have a role in the pathogenesis and treatment of disease?

Endothelial apoptosis can be found in a number of diseases. This review summarizes the current knowledge about the causes and consequences of endothelial apoptosis, and analyzes its possible role in the pathogenesis and treatment of several diseases. Novel forms of therapy based on the proposed pathophysiologic mechanisms are discussed.

Neutrophils augment the release of TNFalpha from LPS-stimulated macrophages via hydrogen peroxide

We examined the effect of polymorphonuclear cells on the release of tumor necrosis factor (TNFalpha) in endotoxin-treated macrophages. Human peripheral blood neutrophils were co-cultured with mouse peritoneal macrophages stimulated with lipopolysaccharide (LPS). In a dose-dependent manner, FMLP (n-formyl-methionyl-leucyl-phenylalanine) augmented the release of TNFalpha by LPS-stimulated macrophages in the presence, but not in the absence, of neutrophils. The stimulating effect of neutrophils on macrophages was reversed by catalase, suggesting that the release of hydrogen peroxide from neutrophils was responsible for augmenting macrophage TNFalpha. Moreover, the direct addition of hydrogen peroxide to macrophages resulted in an increased secretion of TNFalpha. In addition, insertion of a porous membrane between the neutrophils and macrophages cancelled the effect, indicating that adherence of neutrophils may be necessary for augmentation of TNFalpha release. In summary, the data suggest that hydrogen peroxide released from stimulated neutrophils may act as an activator of macrophage function by increasing their release of TNFalpha.

Sequential binding of CD11a/CD18 and CD11b/CD18 defines neutrophil capture and stable adhesion to intercellular adhesion molecule–1

The relative contributions of CD11a/CD18 and CD11b/CD18 to the dynamics and strength of neutrophil adhesion to intercellular adhesion molecule (ICAM)-1–transfected cells were examined over the time course of chemotactic stimulation. Suspensions of neutrophils and transfectants were sheared in a cone-plate viscometer, and formation of heterotypic aggregates was measured by 2-color flow cytometry. The 2-body collision theory was used to compute adhesion efficiency, defined as the proportion of collisions between neutrophils and target cells that resulted in capture. ICAM-1 surface density and shear rate both regulated adhesion efficiency. Target cells expressing approximately 1000 ICAM-1 sites/μm2 (Ilow) were captured with an efficiency of 0.15 at 100 s−1, which decreased to zero at 300 s−1. At 8-fold higher ICAM-1 expression (Ihigh) corresponding to levels measured on interleukin-1–stimulated endothelium, efficiency was 0.3 at 100 s−1 and remained above background to 900 s−1. Shear alone was sufficient for CD11a/CD18-mediated adhesion to ICAM-1, and stimulation with formyl-methionyl-leucyl-phenylalanine boosted capture efficiency through CD11a/CD18 by 4-fold. In comparison, CD11b/CD18 supported one third of this efficiency, but was necessary for aggregate stability over several minutes of shear and at shear stresses exceeding 5 dyne/cm2. Hydrodynamics influenced capture efficiency predominantly through the collisional contact duration, predicted to be approximately 9 milliseconds for successful capture of Ilow and 4 milliseconds for Ihigh. The implication is that an increase in ICAM-1 from resting levels to those on inflamed endothelium effectively increases the permissible shear in which capture through β2-integrins may occur. Neutrophil adhesion to ICAM-1 appears to be a cooperative and sequential process of CD11a-dependent capture followed by CD11b-mediated stabilization.

The use of flow cytometry to measure neutrophil function

Neutrophils are important professional phagocytic cells that provide the host with a first line of defense against acute bacterial and fungal diseases and recurrent, severe or unusual infections are associated with inherited defects of neutrophil function. Furthermore, abundant evidence links inappropriate neutrophil-mediated tissue damage to the pathogenesis of conditions such as acute respiratory distress syndrome, septicemia with multiorgan failure, ischemia-reperfusion injury and rheumatoid arthritis. Flow cytometry has been increasingly used to evaluate the functional capabilities of neutrophils. In this review, we discuss the use of flow cytometry to assess neutrophil functional responses including calcium mobilization, F-actin assembly, adhesion, aggregation, degranulation, phagocytosis and reactive oxygen species (ROS) production. The use of flow cytometry to identify neutrophil priming is also discussed. The advantage of flow cytometry is that the majority of neutrophil functions can be measured using a small volume of whole blood that reduces artifactual changes in function caused by purification procedures. The advent of numerous new fluorochromes and multiparametric analysis allows the simultaneous measurement of several neutrophil functions in the same population of cells. Flow cytometric analysis provides a rapid screen for abnormalities of neutrophil function and reflects more accurately their behavior in vivo.