Mobilization of sialidase from intracellular stores to the surface of human neutrophils and its role in stimulated adhesion responses of these cells

Desialylation of glycoconjugates on the surface of monocytes activates the extracellular signal-related kinases ERK 1/2 and results in enhanced production of specific cytokines

Modulation of the sialic acid content of cell-surface glycoproteins and glycolipids influences the functional capacity of cells of the immune system. The role of sialidase(s) and the consequent desialylation of cell surface glycoconjugates in the activation of monocytes have not been established. In this study, we show that desialylation of glycoconjugates on the surface of purified monocytes using exogenous neuraminidase (NANase) activated extracellular signal-regulated kinase 1/2 (ERK 1/2), an intermediate in intracellular signaling pathways. Elevated levels of phosphorylated ERK 1/2 were detected in desialylated monocytes after 2 h of NANase treatment, and increased amounts persisted for at least 2 additional hours. Desialylation of cell surface glycoconjugates also led to increased production of interleukin (IL)-6, macrophage inflammatory protein (MIP)-1alpha, and MIP-1beta by NANase-treated monocytes that were maintained in culture. Neither increased levels of phosphorylated ERK 1/2 nor enhanced production of cytokines were detected when NANase was heat-inactivated before use, demonstrating the specificity of NANase action. Treatment of monocytes with gram-negative bacterial lipopolysaccharide (LPS) also led to enhanced production of IL-6, MIP-1alpha, and MIP-1beta. The amount of each of these cytokines that was produced was markedly increased when monocytes were desialylated with NANase before exposure to LPS. These results suggest that changes in the sialic acid content of surface glycoconjugates influence the activation of monocytes.

Dimeric galectin-1 induces surface exposure of phosphatidylserine and phagocytic recognition of leukocytes without inducing apoptosis

We report that human galectin-1 (dGal-1), a small dimeric beta-galactoside-binding protein, induces phosphatidylserine (PS) exposure, measured by Annexin V staining, on human promyelocytic HL-60 cells, T leukemic MOLT-4 cells, and fMet-Leu-Phe-activated, but not resting, human neutrophils. This effect of dGal-1 on HL-60 and MOLT-4 cells is enhanced by pretreatment of the cells with neuraminidase, but treatment of resting neutrophils with neuraminidase does not enhance their sensitivity to dGal-1. Although the induction of staining with Annexin V is often associated with apoptosis, the dGal-1-treated HL-60 cells, MOLT-4 cells, and activated neutrophils do not undergo apoptosis, and there is no detectable DNA fragmentation. HL-60 and MOLT-4 cells treated with dGal-1 continue to grow normally. By contrast, camptothecin-treated HL-60 cells, etoposide-treated MOLT-4 cells, and anti-Fas-treated neutrophils exhibit extensive DNA fragmentation and/or cell death. Lactose inhibits the dGal-1-induced effects, indicating that dGal-1-induced signaling requires binding to cell surface beta-galactosides. The dimeric form of Gal-1 is required for signaling, because a monomeric mutant form of Gal-1, termed mGal-1, binds to cells but does not cause these effects. Importantly, dGal-1, but not mGal-1, treatment of HL-60 cells and activated human neutrophils significantly promotes their phagocytosis by activated mouse macrophages. These dGal-1-induced effects are distinguishable from apoptosis, but like apoptotic agents, prepare cells for phagocytic removal. Such effects of dGal-1 may contribute to leukocyte homeostasis.

Recruitment of murine neutrophils in vivo through endogenous sialidase activity

Upon activation with various noncytokine stimuli, polymorphonuclear leukocytes (PMNs) mobilize intracellular sialidase to the plasma membrane, where the sialidase releases sialic acid from the cell surface. This desialylation enhances PMN adherence, spreading, deformability, and motility, functions critical to diapedesis. We now have examined the role of sialidase activity in PMN adhesion to and migration across the endothelium in vivo. A polyclonal antibody prepared against Clostridium perfringens neuraminidase 1) detected surface expression of sialidase on human PMNs stimulated with IL-8 in vitro and on murine PMNs stimulated in vivo, but not on that of unstimulated cells, 2) recognized proteins in human PMN lysates and granule preparations that were not detected by preimmune antibody, 3) inhibited bacterial neuraminidase and human PMN sialidase activities in vitro, and 4) inhibited both pulmonary leukostasis in mice systemically infused with cobra venom factor and intrapulmonary transendothelial migration of PMNs into the bronchoalveolar compartment of mice intranasally challenged with interleukin-8. We conclude that the chemokine interleukin-8, like other PMN agonists, induces the translocation of sialidase to the PMN surface and that surface expression of this sialidase is a prerequisite to PMN recruitment in vivo. The ability of antibodies raised against a prokaryotic neuraminidase to recognize eukaryotic sialidase extends the concept of the neuraminidase superfamily to mammalian enzymes. Inhibition of mobilized endogenous sialidase may provide a novel strategy for limiting the inflammatory response.

A carbohydrate neoepitope that is up-regulated on human mononuclear leucocytes by neuraminidase treatment or by cellular activation

The expression of cell-surface antigens can delineate specific leucocyte developmental or functional stages. For example, certain membrane glycoproteins are expressed selectively on leucocyte subsets only after activation. Leucocyte activation can also induce changes in carbohydrate epitopes expressed on surface antigens. In the present studies, we report on a novel monoclonal immunoglobulin M antibody (mAb 13.22) that recognizes a unique carbohydrate epitope expressed on human leucocyte membrane proteins. Characterization of mAb 13.22 specificity by immunoblotting showed that it recognized proteins of MW approximately 95 000 and 150 000, including both CD18 and CD11b. The mAb 13.22 epitope was removed by N-glycosidase F but not by endoglycosidase H or fucosidase, demonstrating that it is an N-linked carbohydrate antigen. Interestingly, immunoblot staining was enhanced after neuraminidase treatment, suggesting that the antibody epitope might also be partially masked by sialic acid. In resting leucocytes, the mAb 13.22 antigen was expressed strongly on neutrophils, while dull staining was present on monocytes, and no lymphocyte staining was observed. In marked contrast, treatment of leucocytes with neuraminidase resulted in exposure of a mAb 13.22 neoepitope on a subset of lymphocytes (primarily T lymphocytes and natural killer cells) as well as up-regulated staining more than 18-fold on monocytes. Activation of lymphocytes in culture with phytohaemagglutinin or concanavalin A also unmasked the mAb 13.22 neoepitope on approximately 37% of the CD45RO+ lymphocytes. Furthermore, analysis of leucocytes collected from the synovial fluid of patients with rheumatoid arthritis showed that approximately 18% of the lymphocytes present expressed the mAb 13.22 neoepitope. Taken together, our results suggest that the mAb 13.22 carbohydrate neoepitope could represent a physiologically relevant marker that is up-regulated on leucocyte subsets during the inflammatory response.

Comparative enzymology, biochemistry and pathophysiology of human exo-alpha-sialidases (neuraminidases)

This review summarizes the current research on human exo-alpha-sialidase (sialidase, neuraminidase). Where appropriate, the properties of viral, bacterial, and human sialidases have been compared. Sialic acids are implicated in diverse physiological processes. Sialidases, as enzymes acting upon sialic acids, assume importance as well. Sialidases hydrolyze the terminal, non-reducing, sialic acid linkage in glycoproteins, glycolipids, gangliosides, polysaccharides, and synthetic molecules. Therefore, a variety of assays are available to measure sialidase activity. Human sialidase is present in several organs and cells. Its cellular distribution could be cytosolic, lysosomal, or in the membrane. Human sialidase occurs in a high molecular-mass complex with several other proteins, including cathepsin A and beta-galactosidase. Multi-protein complexation is important for the in vivo integrity and catalytic activity of the sialidase. However, multi-protein complexation, the occurrence of isoenzymes, diverse subcellular localization, thermal instability, and membrane association have all contributed to difficulties in purifying and characterizing human sialidases. Human sialidase isoenzymes have recently been cloned and sequenced. Even though crystal structures for the human sialidases are not available, the highly conserved regions of the sialidase from various organisms have facilitated molecular modeling of the human enzyme and raise interesting evolutionary questions. While the molecular mechanisms vary, genetic defects leading to human sialidase deficiency are closely associated with at least two well-known human diseases, namely sialidosis and galactosialidosis. No therapy is currently available for either disease. A thorough investigation of human sialidases is therefore crucial to human health.

Sialidase treatment exposes the beta1-integrin active ligand binding site on HL60 cells and increases binding to fibronectin

The migration of neutrophils from the circulation to areas of inflammation is the result of the sequential activation of multiple cellular adhesion molecules. beta1-Integrins are cell surface glycoproteins and the class of adhesion molecules responsible for binding to the extracellular matrix. The goal of this study was to determine the contribution of glycosylation, specifically the presence of sialic acid, to beta1-integrin adhesion in a neutrophil model. beta1-Integrins on differentiated HL60 cells were remodeled by treatment with the exoglycosidases, sialidase and beta-galactosidase. beta1-Integrin activity was determined by measuring adherence to the extracellular matrix protein fibronectin. The expression of beta1-integrins, beta2-integrins and activated beta1-integrins was determined by flow cytometry. Remodeling of beta1-integrins by treatment with sialidase increased adhesion by greater than 1,000%. Flow cytometric analysis of remodeled beta1-integrins demonstrated an increased expression of the activated beta1-integrin, but only minor increases in the expression of total beta1- and beta2-integrins. We postulate that glycosidase treatment increases adhesion and expression of activated beta1-integrins by exposure of the normally hidden ligand-binding site. The glycosylation of beta1-integrins on neutrophils may act to hide the ligand-binding site in unstimulated cells thereby contributing to the affinity modulation observed in neutrophil beta1-integrin function.

Characterization of cytosolic sialidase from Chinese hamster ovary cells: part I: cloning and expression of soluble sialidase in Escherichia coli

The cDNA of Chinese hamster ovary (CHO) cell cytosolic sialidase was amplified by RT-PCR and cloned into the pGEX-2T plasmid vector encoding for glutathione S-transferase (GST). Screening revealed transformed Escherichia coli clones with the constructed plasmid encoding the CHO cell sialidase sequence. After isopropyl-beta-D-thiogalactopyranoside (IPTG) induction, SDS-PAGE of the total protein extracts revealed a new protein of about 70 kDa, correlating with the molecular weight of a fusion protein composed of the GST (26 kDa) and the cloned cytosolic CHO cell sialidase (43 kDa). A soluble fusion protein was purified from sonified E. coli homogenates by one-step affinity chromatography on Glutathione Sepharose 4B, which showed sialidase activity towards 4-methyl-umbelliferyl-alpha-D-N-acetylneuraminic acid (MUF-Neu5Ac) substrate. Induction of cells with 0.1, 0.5, and 1.0 mM IPTG revealed highest total protein amounts after induction with 1.0 mM IPTG, but highest specific activity for affinity chromatography purified eluates from cultures induced with 0.1 mM IPTG. Therefore, large scale production was performed by inducing cells during exponential growth in a 25 L bioreactor for 3 h with 0.1 mM IPTG after chilling the cell suspension to 25 degrees C. The amount of 26.46 mg of 40-fold purified GST-sialidase with a specific activity of 0.999 U/mg protein was obtained from crude protein extracts by one-step affinity chromatography. 2-Deoxy-2,3-dehydro-N-acetylneuraminic acid (Neu5Ac2en) and Neu5Ac were competitive inhibitors for the sialidase, the former being the more effective one using MUF-Neu5Ac as the substrate. The cytosolic sialidase is capable of desialylating a wide spectrum of different types of gangliosides using a thin-layer chromatography overlay kinetic assay without detergents. This is the subject of the accompanying paper (Müthing, J.; Burg, M. Carbohydr. Res. 2001, 330, 347-356).

Characterization of cytosolic sialidase from Chinese hamster ovary cells: part II. Substrate specificity for gangliosides

Cytosolic Chinese hamster ovary (CHO) cell sialidase has been cloned as a soluble glutathione S-transferase (GST)-sialidase fusion protein with an apparent molecular weight of 69 kD in Escherichia coli. The enzyme has then been produced in mg quantities at 25-L bioreactor scale and purified by one-step affinity chromatography on glutathione sepharose (Burg, M.; Müthing, J. Carbohydr. Res. 2001, 330, 335-346). The cloned sialidase was probed for desialylation of a wide spectrum of different types of gangliosides using a thin-layer chromatography (TLC) overlay kinetic assay. Different gangliosides were separated on silica gel precoated TLC plates, incubated with increasing concentrations of sialidase (50 degreesU/mL up to 1.6 mU/mL) without detergents, and desialylated gangliosides were detected with specific anti-asialoganglioside antibodies. The enzyme exhibited almost identical hydrolysis activity in degradation of GM3(Neu5Ac) and GM3(Neu5Gc). A slightly enhanced activity, compared with reference Vibrio cholerae sialidase, was detected towards terminally alpha(2-3)-sialylated neolacto-series gangliosides IV3-alpha-Neu5Ac-nLc4Cer and VI3-alpha-Neu5Ac-nLc6Cer. The ganglio-series gangliosides G(D1a), G(D1b), and G(T1b), the preferential substrates of V. cholerae sialidase for generating cleavage-resistant G(M1), were less suitable targets for the CHO cell sialidase. The increasing evidence on colocalization of gangliosides and sialidase in the cytosol strongly suggests the involvement of the cytosolic sialidase in ganglioside metabolism on intracellular level by yet unknown mechanisms.

Glycocalyx modulation is a physiological means of regulating cell adhesion

Here we present experimental evidence that phagocytic cells use modulation of specific components of their glycocalyx to regulate their binding capacity. Particles coated with antibodies specific for the CD32 medium affinity IgG receptor were driven along human monocytic THP-1 cells (expressing CD32) in a flow chamber operated at low shear rate. Surprisingly, only minimal adhesion was observed. However, when cells were activated by exposure to fibronectin-coated surfaces and/or soluble γ interferon, adhesion efficiency was dramatically increased, whereas the apparent glycocalyx thickness displayed 20% decrease, and the surface density of CD43/leukosialin carbohydrate epitopes displayed 30–40% decrease on activated cells. The existence of a causal link between adhesion increase and glycocalyx alteration was strongly supported by the finding that (i) both phenomena displayed similar kinetics, (ii) an inverse relationship between THP-1 cell binding capacity and glycocalyx density was demonstrated at the individual cell level, and (iii) adhesion enhancement could not be ascribed to an increased binding site density or improved functional capacity of activated cells. Additional experiments revealed that cell-to-particle adhesion resulted in delayed (i.e. more than a few minutes) egress of CD43/leukosialin from contact areas. Since the time scale of particle attachment was less than a second, surface mobility should not affect the potential of CD43 to impair the initial step of adhesion. Finally, studies performed with fluorescent lectins suggested that THP-1 cell activation and increased adhesive potential were related to a decrease of O-glysosylation rather than N-glycosylation of surface glycoproteins.

Decrease in cell surface sialic acid in etoposide-treated Jurkat cells and the role of cell surface sialidase

The present study investigated the mechanism underlying alterations of cell surface sugar chains of Jurkat cells by inducing apoptosis with etoposide, an inhibitor of topoisomerase II. Within 3 h of etoposide treatment, flowcytometric analysis revealed a decrease in Maackia amurensis agglutinin recognized alpha2,3-linked sialic acid moieties and an increase in Ricinus communis agglutinin recognized galactose. The results suggested that asialo-sugar chains on glycoconjugates were rapidly induced on the etoposide-treated cell surface. To clarify the desialylation mechanism, we studied alpha2,3-sialyltransferase mRNA expression and the activity of sialidase on the cell surface during etoposide-induced apoptosis. The expression of hST3Gal III and hST3Gal IV mRNAs were down-regulated and sialidase activity on the cell surface increased threefold within 2 h of etoposide treatment. Moreover, the decrease in alpha2,3-linked sialic acid levels was significantly suppressed in the presence of 2,3-dehydro-2-deoxy-N-acetylneuraminic acid, an inhibitor of sialidase. These results suggested that activation or exposure of sialidase on the cell surface was induced by etoposide treatment and was the main cause of the decrease in sialic acids.

Molecular characterization of the surface of apoptotic neutrophils: implications for functional downregulation and recognition by phagocytes

We have used a panel of monoclonal antibodies and lectins to examine the profile of surface molecule expression on human neutrophils that have undergone spontaneous apoptosis during in vitro culture. Neutrophil apoptosis was found to be accompanied by down-regulation of the immunoglobulin superfamily members PECAM-1 (CD31), ICAM-3 (CD50), CD66acde, and CD66b and the integrin-associated proteins CD63 and urokinase plasminogen activator receptor (CD87) that may alter the potential for adhesive interactions. Cellular interactions may be further influenced by the reduction of the expression of surface carbohydrate moieties, including sialic acid. Reduced expression of FcgammaRII (CD32), complement receptor type 1 (CD35) and receptors for pro-inflammatory mediators C5a (CD88) and TNFalpha (CD120b) associated with apoptosis might limit neutrophil responsiveness to stimuli that trigger degranulation responses. Although many of the receptors we have examined are expressed at reduced levels on apoptotic neutrophils, we found that there was differential loss of certain receptors (e.g. CD16, CD15 and CD120b) and increased expression of aminopeptidase-N (CD13). Together with our previous data showing that expression of certain molecules e.g. LFA-3 (CD58) is not altered during neutrophil apoptosis, these data are suggestive of specific changes in receptor mobilisation and shedding associated with apoptosis. Although reduced expression of CD63 (azurophilic granules) and CR1 (specific granules) indicates that granule mobilisation does not accompany apoptosis, a monoclonal antibody (BOB78), that recognises a 90 kDa antigen localised in intracellular granules, defines a subpopulation of apoptotic neutrophils that exhibit nuclear degradation yet retain intact plasma membranes. BOB78 positive neutrophils were found to bind biotinylated thrombospondin, suggesting that this mAb defines surface molecular changes associated with exposure of thrombospondin binding moieties.

Periodic formation of nascent lamellae is driven by changes in the stable F-actin pool of polymorphonuclear neutrophils after stimulation with chemotactic peptide and cross-linking of CD18 or CD61

Cell motility and changes in cell shape are largely powered by actin polymerization and depolymerization. Eight to ten second periodic changes in human polymorphonuclear neutrophil (PMN) shape were detected by video-image analysis of PMN crawling on a surface and by right angle light scattering (RALS) in suspended PMN. However, sustained RALS oscillations in suspended PMN requires pre-treatment with an inhibitor of phosphatidylinositol 3-kinase or an activator of protein kinase C. Here, we show that cross-linking of the beta(2) (CD18) or beta(3) (CD61), but not beta(1) (CD 29) integrins in the presence of a low dose of formyl-Methionyl-Leucyl-Phenylalanine (fMLP) enables similar 8-s periodic RALS oscillations in suspended PMN in response to stimulation with two consecutive doses of chemoattractants. This effect did not appear to be due to increased surface expression of CD18 or CD61. RALS oscillations occurred in phase with 8-s oscillations in the stable F-actin pool and peaks in F-actin correlated with predominance of cells exhibiting a nascent lamella. Thus, simulation of surface attachment by CD18 and CD61 cross-linking after exposure to fMLP in suspended cells supports shape oscillations that are the result of actin-driven cyclic extension/retraction of nascent lamellae at the same frequency as the shape changes previously observed in crawling PMN.

Cryptic sialic acid binding lectins on human blood leukocytes can be unmasked by sialidase treatment or cellular activation

We recently reported that the sialic acid-specific binding sites of CD22 molecules on B cells are masked by endogenous ligands, and can be unmasked by sialidase treatment or cellular activation. Here, we show that many other human blood leukocyte types have endogenous sialic acid binding sites that can be unmasked by sialidase treatment. Truncation of sialic acid side chains on the soluble probes used for detection abolishes all binding, indicating the specificity of the interaction for the details of sialic acid structure. There is limited overlap between alpha2-6- and alpha2-3-sialic acid-specific binding sites, which are unmasked on monocytes, natural killer cells, a minority of mature T cells, neutrophils, and some cultured human leukemic cell lines. Activation with phorbol ester and calcium ionophore causes spontaneous exposure of some of the binding sites, occurring over a period of minutes on neutrophils and several hours on monocytes and U937 leukemia cells. Activation is accompanied by some evidence for desialylation of cell surface molecules. Thus, many human blood cells have specific binding sites for sialic acids, masked by endogenous sialylated ligands. Cellular activation can unmask these sites, possibly by the action of an endogenous sialidase. The nearly universal masking of such sites in unactivated blood cells could explain why many of these sialic acid-binding lectins have not been previously discovered. Similar considerations may apply to sialic acid binding lectins of other cell types and tissues.

Interleukin-4-induced macrophage fusion is prevented by inhibitors of mannose receptor activity

A potential role for the macrophage mannose receptor in human monocyte-derived macrophage fusion was explored by testing the effects of previously described inhibitors of its activity on the formation of interleukin-4-induced foreign body giant cells in vitro Giant cell formation was prevented or reduced in the presence of alpha-man-nan and synthetic neoglycoprotein conjugates according to the following pattern of relative inhibition: mannose-bovine serum albumin (BSA) > N-acetylgucosamine-BSA congruent to glucose-BSA. Laminarin (beta-glucan) or galactose-BSA were not inhibitory. Swainsonine and castanospermine, inhibitors of glycoprotein processing that interfere with the arrival of newly synthesized mannose receptors at the cell surface, also attenuated macrophage fusion and the formation of giant cells, whereas another glycosidase inhibitor, deoxymannojirimycin, was without effect. Mannose receptors were confirmed to be specifically up-regulated by interleukin-4 in this culture system and also demonstrated to be present and concentrated at macrophage fusion interfaces. These data suggest that the macrophage mannose receptor may be an essential participant in the mechanism of interleukin-4-induced macrophage fusion and implicate a novel function for this endocytic/phagocytic receptor in mediating foreign body giant cell formation at sites of chronic inflammation.

Cell-contact mediated modulation of the sialylation of contactinhibin

Contactinhibin was found to be involved in contact-dependent inhibition of growth. The growth inhibitory activity of contactinhibin is mediated by N-linked oligosaccharides with desialylated beta-glycosidically linked, terminal galactose residues. Here we show that in sparse human fibroblasts contactinhibin was expressed in a biologically inactive, highly sialylated form both on the plasma membrane and intracellularly, while in confluent cells plasma membrane localized contactinhibin was present in a biologically active, low sialylated form. Plasma membranes were shown to contain a glycoprotein sialidase which is suggested to be engaged in the activation of contactinhibin in a cell contact-dependent manner.

Decreased sialidase activity in mononuclear leucocytes of type 1 diabetic subjects: relationship to diabetic complications and glycaemic control

Leucocyte surface sialic acid content influences surface charge, deformability, and leucocyte-endothelial interaction. Abnormal leucocyte structure and function contributes both to microvascular damage and diabetic complications. The aim of this study was to investigate altered leucocyte SA metabolism in diabetic subjects and measure lysosomal sialidase which regulates leucocyte surface sialylation. We examined 26 Type 1 (insulin-dependent) diabetic subjects with retinopathy, 26 Type 1 diabetic subjects without complications, and 38 matched normal control subjects. Sialidase was assayed in freshly prepared sonicates of pure mononuclear leucocytes (MNLs), using the fluorometric substrate 4-methyl-umbelliferyl-N-acetylneuraminic acid. In the subjects with diabetes there was a significant negative correlation between MNL sialidase activity and both HbA1c (rs = 0.37, p = 0.007) and fructosamine (rs = -0.31, p = 0.026). MNL sialidase activity was significantly decreased in diabetic subjects with clinical evidence of complications compared to control subjects. HbA1c was significantly higher (p = 0.036) in diabetic patients with complications compared to those without. The observed decrease in MNL sialidase activity related to diabetic control may be important in the pathogenesis of vascular damage. Diabetes-associated changes in sialylation of functional cell surface glycoconjugates may have important clinical consequences.

Activation of polymorphonuclear leukocytes reduces their adhesion to P-selectin and causes redistribution of ligands for P-selectin on their surfaces

In acute inflammatory responses, selectins mediate initial rolling of neutrophils (PMNs) along the endothelial surface. This is followed by tight adhesion that requires activation-dependent up-regulation of CD11/CD18 integrins on PMNs. For emigration to occur, the initial bonds that are established at the endothelial surface must be disengaged. We show that activation of PMNs results in their detachment from P-selectin, a glycoprotein expressed at the surface of inflamed endothelium that mediates initial tethering of PMNs. Loosening of the bond occurs when PMNs are activated by platelet-activating factor, which is coexpressed with P-selectin, or by other signaling molecules. The time course of reduced adhesion to P-selectin, when compared to up-regulation of CD11/CD18 integrins, suggests that "bond trading" may occur as activated PMNs transmigrate in vivo. Activation of PMNs did not alter binding of fluid-phase P-selectin, indicating that the ligand(s) for P-selectin is not shed or internalized. Using microspheres coated with P-selectin, we found that ligands for P-selectin were randomly distributed over the surfaces of rounded, unactivated PMNs. An antibody against P-selectin glycoprotein ligand-1 (PSGL-1) completely inhibited binding of P-selectin-coated beads suggesting that P-selectin glycoprotein ligand-1 is the critical binding site in this assay. In contrast to the dispersed pattern on unactivated PMNs, the ligands for P-selectin were localized on the uropods of activated, polarized cells. Pretreating PMNs with cytochalasin D before activation prevented the change in cell shape, the redistribution of binding sites for P-selectin-coated beads, and the decrease in cellular adhesiveness for P-selectin. These experiments indicate that the distribution of ligands for P-selectin is influenced by cellular activation and by cytoskeletal interactions, and that redistribution of these ligands may influence adhesive interactions. Activation of PMNs may cause loosening or disengagement of bonds between P-selectin and its ligands, facilitating transendothelial migration.

Modulation of lipopolysaccharide binding to human granulocytes

Using flow cytometry and fluorescein-labelled lipopolysaccharide (LPS) from Salmonella minnesota R595 (FITC-ReLPS), we studied the role of membrane proteins in the recognition of LPS by human polymorphonuclear granulocytes (PMN) in the absence of serum. Treatment of PMN with trypsin, pronase E or proteinase K reduced both the binding of FITC-ReLPS to PMN at 4 degrees and the response of PMN to LPS at 37 degrees, as measured by luminol-enhanced chemiluminescence. Neuraminidase treatment enhanced both activities. Trypsin treatment of PMN after the binding of FITC-ReLPS effectively reduced fluorescence when cells were kept at 4 degrees, while further incubation of FITC-ReLPS-labelled PMN at 37 degrees rendered fluorescence insensible to trypsin. These results indicate a protein structure of the LPS binding site, association of FITC-ReLPS with the cell membrane at 4 degrees and subsequent internalization at 37 degrees. The binding of FITC-ReLPS was not inhibited by the anti-CD14 monoclonal antibody (mAb) 3C10, which recognizes a functional epitope of CD14. Furthermore, binding of FITC-ReLPS was observed to PMN obtained from a patient with paroxysmal nocturnal haemoglobinuria who lacked membrane-bound CD14. Stimulation of PMN with tumour necrosis factor (TNF) or LPS enhanced the binding of FITC-ReLPS at 4 degrees. This was not observed after activation of PMN devoid of granules (cytoplasts), indicating that the binding of LPS at the cell surface is enhanced by mobilization of LPS-binding proteins from intracellular granules. These studies provide evidence that LPS binding and activation of PMN involves protein structures at the cell surface different from CD14, and that granules constitute a pool of LPS-binding proteins that can be translocated to the cell surface upon stimulation.

Regulation of B cell:T cell interactions: potential involvement of an endogenous B cell sialidase

In light of the ability of B cells treated with neuraminidase to interact more effectively with T cells, the increased capacity of activated, but not small resting B cells, to interact with T cells could be associated with the level of sialylation on certain B cell surface molecules which influences the effectiveness of the physical interaction between B and T cells. The purpose of this study was to determine if activation of B cells altered sialylation via an endogenous sialidase which affected both the initial interaction between T and B cells and subsequent B cell-induced T cell proliferation. The competitive neuraminidase inhibitor, 2-deoxy-2,3-dehydro-N-acetylneuraminic acid (NeuAc2en), inhibited LPS-mediated enhancement of B cell conjugate formation with Ia-specific T cell clones as well as enhancement of their capacity to stimulate a mixed lymphocyte reaction. The addition of NeuAc2en during LPS stimulation did not affect the surface expression of Ia, LFA-1, ICAM-1 or mB7, suggesting that inhibition of LPS-mediated enhancement by the sialidase inhibitor was not due to changes in the level of expression of the major B cell adhesion or co-stimulatory molecules. Short term stimulation with phorbol myristate acetate (PMA) and ionomycin also enhanced the ability of resting B cells to form antigen specific T:B conjugates. However, activation of B cells with PMA and ionomycin or with LPS did not change the capacity of a sialic acid specific lectin to bind to the B cells, suggesting that activation was not associated with global changes in surface sialic acid content. B cell stimulation did not appear to increase the activity of the most prevalent B cell sialidase activity as measured in an in vitro assay system, suggesting that the major B cell sialidase may not be responsible for the alteration of B cell sialylation levels or the ability of activated B cells to interact more effectively with T cells. The possibility of intracellular compartmentalization of sialidase activity or that a minor B cell sialidase may play a role in the regulation of a B cells ability to interact with T cells are discussed.

Freeze-stable sialidase activity in human leucocytes: substrate specificity, inhibitor susceptibility, detergent requirements and subcellular localization

Human leucocytes contain a freeze-stable sialidase (neuraminidase; EC 3.2.1.18) activity in addition to the better-characterized lysosomal freeze-labile enzyme. In order to discriminate between the sialidase activities detected with the synthetic fluorimetric substrate 4-methylumbelliferyl-alpha-D-N-acetylneuraminic acid (MU-Neu5Ac), different tritiated sialoglycoconjugate substrates were prepared. Using this sensitive radioactive assay system, leucocyte sialidase activity towards glycoproteins was shown to be labile to repeated freeze-thawing, but a Triton-stimulated activity towards gangliosides was entirely freeze-stable. Assay conditions were optimized for this freeze-stable ganglioside sialidase activity. Subcellular fractionation of mononuclear leucocytes (MNLs) on Percoll-density gradients showed that this ganglioside sialidase activity was entirely associated with the plasma membrane. Study of the detergent requirements showed that MNLs also demonstrated ganglioside sialidase activity when sodium cholate was present in place of Triton. Cholate-stimulated ganglioside sialidase activity was found to be entirely freeze-stable and localized at the plasma membrane. Studies on whole homogenates of MNLs demonstrated that the Triton-stimulated and cholate-stimulated activities showed similar acidic pH optima at < or = 3.9 and were both strongly inhibited by 2-deoxy-2,3-didehydro-N-acetylneuraminic acid and Cu2+, but not by free N-acetylneuraminic acid, N-(4-nitrophenyl)oxamic acid or heparan sulphate. These results suggest that human MNLs contain, in addition to the lysosomal freeze-labile sialidase, a single sialidase activity which is freeze-stable, ganglioside-specific, plasma membrane-associated and stimulated both by Triton and by cholate.

Sialidase in rabbit blood. Characterization of sialidase purified from rabbit erythrocyte membrane

Sialidase activities of rabbit blood cells and serum were measured. The leucocyte particulate fraction showed the highest specific activity of sialidase towards mixed gangliosides and sialyllactose, and the cytosolic fraction showed for fetuin. Predominant sialidase activity in the blood was detected in erythrocyte particulate fraction when mixed gangliosides were used as substrate. The sialidase for ganglioside was solubilized from the erythrocyte ghosts by using Triton X-100. The solubilized sialidase was purified 1886-fold by sequential chromatographies on DEAE-cellulose, EAH-Sepharose 4B, Octyl-Sepharose CL-4B, Sephadex G-100, concanavalin-A--Sepharose, N-(p-aminophenyl)oxamic acid-agarose and Heparin-Sepharose CL-6B. The optimum pH of purified sialidase was 4.5 for ganglioside mixture, and this enzyme exhibited M(r) = 48,000 by gel filtration. When the purified sialidase was subjected to SDS/PAGE, a major sialidase-active protein band at M(r) = 54,000 and another fainter inactive protein band with M(r) = 115,000 were observed. The purified enzyme was active towards oligosaccharides, gangliosides, fetuin glycopeptide and 4-methylumbelliferyl alpha-D-N-acetylneuraminic acid except for glycoproteins tested. Fe2+, Fe3+ and dithiothreitol significantly inhibited the enzyme activity, while Triton X-100 activated the enzyme. Inside-out vesicles and unsealed ghosts of rabbit erythrocyte showed the sialidase activity for mixed gangliosides but not for resealed ghosts or intact erythrocytes. These results indicate that the active site of this sialidase is oriented mainly on the inside of the erythrocyte membrane and not on the outside. Treatment of rabbit erythrocyte unsealed ghosts with phosphatidylinositol-specific phospholipase C liberated no sialidase activity toward mixed gangliosides from the ghosts.

Expression and function of an IgE-binding animal lectin (epsilon BP) in mast cells

epsilon BP (IgE-binding protein) is a 31,000 M(r) protein originally identified in rat basophilic leukemia (RBL) cells. The protein is composed of two domains with the amino-terminal domain containing a highly conserved repetitive sequence and the carboxyl-terminal domain containing consensus sequences shared by other beta-galactoside-binding soluble lectins. The protein has wide tissue distribution, is found on cell surfaces and in extracellular milieu. By combined efforts from several research groups including ours a multifunctional nature of this lectin began to emerge. This review emphasizes the following characteristics of epsilon BP: (i) epsilon BP is secreted by cells such as macrophages; (ii) like many other lectins, epsilon BP functions at least bivalently; (iii) epsilon BP has specificity for distinct oligosaccharide structures that have a terminal galactose not masked by sialic acids; and (iv) in addition to binding IgE, epsilon BP binds to surfaces of various cell types via lectin-carbohydrate interaction. Importantly, epsilon BP binds to the IgE receptor on mast cells. We propose that epsilon BP can function as a modulatory protein on various cells by cross-linking critical cell surface glycoproteins. The proposed action of epsilon BP on mast cells is presented as a model.

Human neutrophil granules and secretory vesicles

The traditional classification of neutrophil granules as peroxidase-positive (azurophil, or primary) and peroxidase-negative (specific or secondary) has proven to be too simple to explain the differential exocytosis of granule proteins and incorporation of granule membrane into the plasma membrane which is an important aspect of neutrophil activation. Combined subcellular fractionation and immunoelectron microscopy has revealed heterogeneity among both peroxidase-positive and peroxidase-negative granules with regard to their content, mobilization and time of formation. Peroxidase-negative granules may be classified according to their content of lactoferrin and gelatinase: 15% of peroxidase-negative granules contain lactoferrin, but no gelatinase. 60% contain both lactoferrin and gelatinase. The term specific or secondary granule should be reserved for these two subsets. In addition, 25% of peroxidase-negative granules contain gelatinase but no lactoferrin. These should be termed gelatinase granules or tertiary granules. Gelatinase granules are formed later than specific granules and mobilized more readily. In addition, a distinct, highly mobilizable intracellular compartment, the secretory vesicle, has now been recognized as an important store of surface membrane-bound receptors. This compartment is formed in band cells and segmented cells by endocytosis. This heterogeneity among the neutrophil granules is of functional significance, and may also be reflected in the dysmaturation which is an important feature of myeloproliferative and myelodysplastic disorders.

A role for Bacteroides fragilis neuraminidase in bacterial growth in two model systems

Two Bacteroides fragilis neuraminidase-deficient mutants were used to study the role of neuraminidase activity in growth of B. fragilis in tissue culture monolayers (CHO cells) and in the in vivo rat granuloma pouch. The nanH structural gene for neuraminidase was cloned from B. fragilis TM4000 and was used to create two isogenic strains with chromosomal disruptions at the nanH gene. B. fragilis VRC404 contains an insertion flanked by disrupted copies of the nanH gene, and B. fragilis VRC426 contains a deletion of a significant portion of nanH coding sequences. The insertion mutant VRC404 is capable of reverting to nanH+. It grew as well as the wild type in CHO monolayers. However, between 48 and 72 h after infection, the bacterial population was enriched with nanH+ bacterial cells (10 to 20%). In the rat pouch 48 h after infection, more than 90% of the population sampled had become nanH+. The deletion mutant VRC426 showed a severe growth defect in the rat pouch model. In addition, VRC426 was efficiently outgrown by the wild type in competition experiments, even when the mutant was present at 10 times the number of wild-type cells at the time of infection. A common characteristic of both model systems is a drastic decrease in the free glucose concentration 16 to 24 h postinfection. We suggest that neuraminidase activity may be required for B. fragilis to grow to maximal levels in the tissue culture and rat pouch systems by making other carbon sources available after glucose levels are reduced.

Albumin inhibits neutrophil spreading and hydrogen peroxide release by blocking the shedding of CD43 (sialophorin, leukosialin)

Spreading of neutrophils on protein-coated surfaces is a pivotal event in their ability to respond to soluble, physiologic agonists by releasing large amounts of hydrolases and oxidants. Using neutrophils plated on serum-, fibrinogen- or fibronectin-coated surfaces, we investigated the effect of human serum albumin (HSA) on spreading-dependent neutrophil responses. HSA suppressed the respiratory burst of neutrophils in response to tumor necrosis factor-alpha (TNF), complement component C5a or formylated peptide, but not phorbol myristate acetate. HSA was suppressive only if added before the onset of the respiratory burst, and suppression was reversed when HSA was removed. Likewise, HSA selectively and reversibly inhibited TNF-induced cell spreading and the associated fall in cAMP. However, HSA did not hinder TNF-induced cell adherence to the same protein-coated surfaces. We investigated cell surface sialoproteins as modulators of cell spreading and as targets for the anti-spreading action of HSA. Oxidation of the cell surface with periodate followed by reduction with 3H-borohydride and immunoblotting with specific mAbs helped identify the predominant sialoprotein on human neutrophils as CD43 (sialophorin, leukosialin). Treatment of neutrophils with C. perfringens sialidase desialylated CD43, markedly enhanced the ability of the cells to respond to TNF by spreading and undergoing a respiratory burst, and antagonized the ability of HSA to inhibit these responses. TNF-treated, adherent neutrophils shed CD43, and this was blocked by HSA, but not by ovalbumin. Exogenous neutrophil elastase removed CD43 from the neutrophil surface. HSA blocked the actions of both sialidase and elastase on CD43. In contrast, ovalbumin did not block the action of sialidase on CD43, and HSA did not inhibit the ability of sialidase to hydrolyze a synthetic substrate. These results suggested that HSA might bind CD43. In fact, the extracellular portion of CD43 bound to HSA-Sepharose, but not to ovalbumin- or glycylglycine-Sepharose. Finally, two mAbs recognizing different epitopes on CD43 mimicked HSA's inhibitory effects on neutrophil function. Thus, HSA can dissociate attachment of neutrophils from spreading. This dissociation may help neutrophils migrate along a chemotactic gradient, while decreasing their release of oxidants. CD43, a long, rigid molecule with a markedly negative charge, antagonizes neutrophil spreading. HSA appears to inhibit spreading-dependent neutrophil functions by binding to CD43 and interfering with the ability of neutrophils to shed it.

Detection of antineutrophil autoantibodies by flow cytometry: use of unfixed neutrophils as antigenic targets

Antineutrophil antibodies may be found in the sera of patients with chronic neutropenia as well as in the sera of a variety of patients with neutropenia and associated autoimmune or infectious disorders. We evaluated an immunofluorescent flow cytometric technique for the measurement of antineutrophil antibodies in serum. Sera from patients with suspected immune neutropenia were studied and compared with a group of sera from normal healthy individuals, as well as with sera from patients with rheumatoid arthritis and systemic lupus erythematosus. Of 159 patients with suspected immune neutropenia and a variety of associated clinical disorders, 59 (37%) were found to have evidence for enhanced binding of IgG to normal target neutrophils, interpreted as positive for antineutrophil antibodies. Whereas 0/37 non-neutropenic patients with typical RA had positive results, 51/244 (21%) of sera from nonneutropenic patients with SLE or other collagen vascular disorders showed enhanced IgG binding to neutrophils. Living neutrophils were used to study the effects of cellular activation, and increased antibody binding was observed with certain sera that contained IgG directed against activation-dependent antigens. We found that, under controlled conditions, flow cytometry can be reliably used to detect antineutrophil autoantibodies, with unfixed, living neutrophils as antigenic targets.

Human neutrophils release their major membrane sialoprotein, leukosialin (CD43), during cell activation

Leukosialin (CD43) is a sialic acid-rich molecule with a relative molecular mass (M(r)) of 140,000 highly represented on polymorphonuclear neutrophils (PMN) and on most leukocytes. One of its functions may be to prevent nonspecific cell-to-cell interactions through negative charge repulsions. As tested by immunofluorescence, neutrophil CD43 membrane expression was shown to decrease by up to 80% upon cell activation by phorbol myristate acetate (10 ng/ml) or by N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP; 10(-6) M) in the presence of cytochalasin B. The kinetic of this decrease paralleled that of CD11b up-regulation. FMLP alone, tumor necrosis factor (TNF-alpha), lipopolysaccharide and granulocyte macrophage colony-stimulating factor had moderate or insignificant effects, while inducing striking CD11b up-regulation. Cell priming with TNF-alpha followed by FMLP stimulation resulted in up to 40% decrease of CD43 expression. Anti-CD43 mAb immunoprecipitated three fragments of M(r) 130,000, 49,000 and 34,000 from the cell-free supernatant of activated neutrophils, suggesting that CD43 is released from the membrane by proteolysis. Indeed, the decrease in CD43 expression was inhibited by phenylmethanesulfonylfluoride (PMSF). Homotypic aggregation of activated PMN was also inhibited by PMSF and could result, at least in part, from the shedding of CD43. The shedding of such a strongly anionic and major membrane protein should drastically modify PMN surface charge and may allow previously hindered interactions by exposing new adhesion molecules.