Analysis of human monocyte chemoattractant binding by flow cytometry

Selective induction of phospholipase D1 in pathogen-activated human monocytes

Phospholipase D (PLD) activation is part of the complex signalling cascade induced during phagocyte activation. Two PLD isoforms have been cloned, but their role in phagocyte functions is still poorly defined. We report that resting fresh circulating human monocytes expressed PLD1. PLD1 protein expression was rapidly down-regulated during cell culture. Lipopolysaccharide and pathogen-derived agonists (Candida albicans, arabinoside-terminated lipoarabinomannan and Gram-positive bacteria, but not mannose-capped lipoarabinomannan or double-stranded RNA) strongly induced PLD1 expression at both the mRNA and protein levels. Pro-inflammatory cytokines [interleukin (IL)-1beta and tumour necrosis factor alpha] had only a weak effect, whereas immune cytokines (IL-6 and interferon gamma), anti-inflammatory cytokines (IL-13 and IL-10) and chemoattractants (fMet-Leu-Phe and macrophage chemoattractant protein 1) were inactive. None of the agonists tested induced significant changes in the basal expression of PLD2 mRNA. Consistent with PLD1 up-regulation was the observation that PLD enzymic activity was higher in monocytes treated with active-pathogen-derived agonists than in control cells, when stimulated with PMA or with chemotactic agonists (fMet-Leu-Phe and C5a). Thus PLD2 seems to be a constitutive enzyme in circulating monocytes. Conversely, PLD1 is an inducible protein, rapidly regulated during culture conditions and selectively induced during cell activation. Therefore PLD1 might have a relevant role in immune responses against pathogens and in chronic inflammation.

Heterogeneity in F-actin polymerization of cord blood polymorphonuclear leukocytes stimulated by N-formyl-methionyl-leucyl-phenylalanine

BACKGROUND

To elucidate the mechanism responsible for defects of polymorphonuclear leukocyte (PMNL) chemotaxis of neonates, we determined actin polymerization of NBD (7-nitrobenz-2-oxa-diazol)-phallacidin-stained PMNL following stimulation with either N-formyl-methionyl-leucyl-phenylalanine (fMLP) or phorbol myristate acetate (PMA) in cord blood and adult controls.

METHODS

We measured F-actin content in PMNL stained with NBD-phallacidin using flow cytometry.

RESULTS

Relative F-actin polymerization, that is, a ratio of stimulated F-actin to basal F-actin, was significantly decreased in cord blood PMNL when compared with that of adult PMNL. Distribution of fMLP-stimulated F-actin showed a bimodal pattern, while adult PMNL disclosed a single pattern. Following stimulation with PMA, however, F-actin levels were equal in both cord and adult PMNL. A fluorescein isothiocyanate-conjugated fMLP receptor assay showed no significant difference in binding capacity of fMLP receptors between adult and cord PMNL.

CONCLUSION

These results indicate that a deficiency of PMNL chemotaxis in neonates may be due, in part, to decreased relative F-actin polymerization, which may be caused by functional heterogeneity in cord blood PMNL.

Granulocyte colony-stimulating factor promotes functional maturation of O-2 generating system during differentiation of HL-60 cells to neutrophil-like cells

The effects of granulocyte colony-stimulating factor (G-CSF) on development of O-2 generating system during differentiation of HL-60 cells to neutrophil-like cells have been studied. G-CSF enhanced O-2 generating ability of HL-60 cells whose differentiation had been initiated by dimethylsulfoxide (DMSO) or retinoic acid (RA). The O-2 generations by the differentiated HL-60 cells in response to opsonized zymosan (OZ), formyl-Met-Leu-Phe (fMLP), and IgG-coated zymosan were increased two- to fourfold as a result of incubation of the cells undergoing the differentiation with G-CSF. The potentiation by G-CSF occurred in a dose-dependent manner with the maximum effect at about 10 ng/ml G-CSF. The effect of G-CSF could not be fully explained by up-regulation of the receptor expression on the HL-60 cells, because the number of C3bi receptors was not altered by G-CSF, whereas the expression of fMLP receptor was enhanced by G-CSF. On the other hand, the O2 generation of the differentiated cells activated by phorbol 12-myristate 13-acetate was not affected by the G-CSF treatment, suggesting that the biochemical events in the cells after PKC activation might not be enhanced by G-CSF. Assuming that the signaling pathways linking OZ or fMLP receptor might be enhanced by G-CSF, alteration in the cellular sn-1, 2-diacylglycerols (DAG) level upon stimulation with OZ or fMLP was compared between the G-CSF-treated and nontreated cells. Whereas DAG level was not increased by the stimulation in the cells treated with DMSO alone, a significant increase in DAG level upon the stimulation was observed in the cells treated with G-CSF and DMSO. These results suggest that G-CSF would enhance the organization of a receptor-linked DAG generating system in the differentiating cells, leading the cells to generate more O2.

Selective inhibition of neutrophil function by a peptide derived from lipocortin 1 N-terminus

A multi-faceted approach was used to investigate the effect of an anti-inflammatory peptide derived from human lipocortin 1 N-terminus region (amino acid 2-26; termed human Ac2-26) on human neutrophil activation in vitro. When incubated with purified human neutrophils. human Ac2-26 produced a concentration-dependent inhibition of elastase release stimulated by formyl-Met-Leu-Phe (fMLP), platelet-activating factor, or leukotriene B4, with an approximate EC50 of 33 microM (100 micrograms/ml). At this concentration, human Ac2-26 also inhibited (77%) the release of [3H]-arachidonic acid from neutrophils stimulated with fMLP. The peptide, however, did not inhibit the up-regulation of the beta 2-integrin CD11b and the concomitant shedding of L-selectin from neutrophil plasma membrane induced by fMLP. In adhesion experiments, human Ac2-26 inhibited neutrophil adhesion to endothelial monolayers when this was stimulated with fMLP, but not when this followed endothelial cell activation with histamine or platelet-activating factor. Again, the effect of the peptide was concentration-dependent, and an approximate EC50 of 33 microM was calculated. When a preparation of 125I-labeled human Ac2-26 was incubated with the neutrophils, the peptide was internalised in an energy-dependent fashion. All together, these observations lead us to propose a model in which this peptide derived from the N-terminus of human lipocortin 1 alters a common cellular mechanism producing a selective inhibition of neutrophil activation.

Anti-inflammatory lipocortin-derived peptides

Peptide Ac2-26, drawn from the sequence of human lipocortin 1, inhibited the release of elastase activity from cytoplasmic granules of human neutrophils, and neutrophil adhesion to monolayers of endothelial cells, in a concentration-dependent manner (approximate IC50 of 100 micrograms/ml, 33 microM). The effect of peptide Ac2-26 was not restricted to a specific neutrophil activator, being effective against formyl-Met-Leu-Phe (FMLP), leukotriene B4 (LTB4) and platelet-activating factor (PAF). Peptide Ac2-26 did not alter FMLP binding to its receptor. These in vitro observations complement in vivo data obtained with this peptide and may enable a better understanding of its pharmacology and, perhaps, that of of lipocortin 1 too.

Formylmethionyl-leucylphenylalanine and the SOS operon in Escherichia coli: a model of host-bacterial interactions

To determine the biological significance of the existence of highly specific receptors for the bacterial chemotactic peptide formylmethionyl-leucylphenylalanine (fMet-Leu-Phe) on neutrophil leucocytes, we investigated the role of this peptide in bacterial metabolism. The UmuD protein of the Escherichia coli SOS operon was identified as having an N-terminal fMet-Leu-Phe sequence and a recombinant E. coli with the umuD gene on plasmid pSB13 was shown to be an over-producer of both UmuD and fMet-Leu-Phe. Activation of SOS genes in conventional wild-type E. coli (K12) by u.v. light or hydrogen peroxide increased fMet-Leu-Phe production up to 4-fold. A RecA- strain, incapable of SOS activation, was a low basal producer of fMet-Leu-Phe and showed no increased production with u.v. light or oxidant stress. We propose that host phagocytes respond to fMet-Leu-Phe and closely related peptides because they are generated by bacteria under oxidant stress. Increased fMet-Leu-Phe production may signal to the host a change in the organism's biological status from commensal to pathogen because of the invasion into tissues exposing bacteria to high pO2 levels and oxidant stress.

Chemotaxis by a CNS macrophage, the microglia

Microglia demonstrate many characteristics similar to those seen in monocytes and tissue-specific macrophages, including phagocytosis, production of oxygen radicals, and growth factors and expression of MHC antigens. We have examined the ability of microglia, cultured from the cerebral cortices of neonatal rats, to demonstrate another important functional characteristic of monocytic-derived cells, that is, chemotaxis. Our results show that cultured rat microglia demonstrate chemotaxis to complement dependent chemoattractants such as recombinant C5a, zymosan activated serum, and to rat serum as well as to transforming growth factor-beta, a chemoattractant produced by platelets. Microglia fail to migrate to bacterial dependent chemoattractants such as the N-formyl peptides. The failure to respond is not dependent on maturational state of the microglia. Treatment with DMSO or casein, agents known to induce morphological and functional changes in cultured microglia reminescent of a "resting" and an "activated" macrophage, respectively, do not alter the response to fMet-Leu-Phe. In addition, the chemotactic response to serum in DMSO or casein-treated cells is the same as the response seen in untreated day 10 cultured microglia or untreated age-matched controls. The ability of microglia to migrate in response to inflammatory mediators suggests that these cells can move to sites of injury, thereby enabling them to participate in an inflammatory response.

Luminol-amplified chemiluminescence activity in human monocytes: a comparison with the activity induced in granulocytes

The characteristics of the luminol-amplified chemiluminescence (CL) induced in mononuclear phagocytes interacting with PMA, FMLP or ionomycin were determined. Azide reduced the CL activity by more than 80%, while superoxide dismutase and catalase had minor effect on the monocyte CL response. The sensitivity of the monocyte CL response, to the addition of extra peroxidase differed depending on the stimulus used. Furthermore, no direct correlation was obtained between the CL response and superoxide anion or hydrogen peroxide production. In comparison with the response in granulocytes, minor quantitative differences were observed. The mechanism for the light-generating reaction, seems to be the same in both cell types.

Time dependence of transmembrane potential changes and intracellular calcium flux in stimulated human monocytes

An important characteristic of the functional differentiation of the blood monocyte is the development of its capacity to recognize and respond to stimuli. This ability is mediated to a large extent by specific receptor glycoproteins located on the cell surface. Stimulation of mononuclear phagocytes via these receptors results in a rapid rise in intracellular Ca++ concentration, accompanied or followed by a change in membrane potential, generation of oxidative products, degranulation, and effector functions such as phagocytosis, aggregation, or locomotion. While the development of these characteristics is difficult to characterize in vivo, several investigators have demonstrated in vitro changes in these cells that correlate with the development of effector function. To examine the mechanisms of specific membrane-stimulus interactions of monocytes as they differentiate into macrophage-like cells, we studied the responses of human monocytes and of monocytes incubated in serum-containing medium for up to 96 hr to the chemotactic peptide formyl-methionyl-leucyl-phenylalanine (fMLP). Freshly isolated monocytes exhibited little change in transmembrane potential following stimulation with an optimal concentration of peptide and underwent a significant increase only after 48 hr in culture. While constant resting intracellular Ca++ concentrations were maintained during the culture period, intracellular Ca++ levels following fMLP stimulation increased with with incubation in serum, for up to 96 hr. In contrast, fMLP-induced respiratory burst activity increased from 0 to 24 hr in culture; it remained elevated at 48 hr but declined again by 96 hr. Incubation of the cells for 24 hr increased their random (unstimulated) motility in modified Boyden chambers but did not alter the cells' directed (chemotactic) response to fMLP in comparison to the response of freshly isolated monocytes. Peptide binding to the cells did not increase during the incubation period, indicating that an increase in receptor number or in affinity for fMLP was not responsible for the enhanced responsiveness to fMLP as incubation time increased. These studies indicate that incubation of monocytes in serum-containing medium leads to a complex, altered series of responses to fMLP that correlate with the differentiation of the original monocytes in vitro and may relate to the in vivo differentiation of monocytes to macrophages.

A series of six ligands for the human formyl peptide receptor: tetrapeptides with high chemotactic potency and efficacy

We recently isolated, from culture fluids of Staphylococcus aureus, a chemotactic peptide that comprised equimolar quantities of methionine, leucine, phenylalanine, and isoleucine. It interacted with the formylmethionyl peptide receptor of human leukocytes and had considerably higher potency and efficacy than the widely studied tripeptide agonist fMet-Leu-Phe. On the assumption that the attractant was a formylmethionyl tetrapeptide, we synthesized the six possible sequences and tested the products for chemotactic potency and efficacy, as well as their capacity to inhibit binding of fluorescein isothiocyanate-labeled fMet-Leu-Phe-Lys to human monocytes. The concentrations required for inhibition of fluorescein-labeled fMet-Leu-Phe-Lys binding by the six peptides covered three orders of magnitude. Chemotactic potency (concentration that caused 50% of the maximum chemotactic response) ranged from 3.1 X 10(-11) M to 6.4 X 10(-10) M; efficacy (percentage of monocytes migrating at optimal attractant concentration) ranged from 41% to 66%. When the six synthetic tetrapeptides were ranked for chemotactic efficacy, they paired according to the position of phenylalanine. The average percentage migration was 66% for the two peptides with phenylalanine in position 3, 51% for phenylalanine in position 4, and 41% for phenylalanine in position 2. Since the published value for the percentage of human monocytes with detectable formyl peptide receptors is 60%, it is apparent that the two tetrapeptides with phenylalanine in position 3 (fMet-Ile-Phe-Leu and fMet-Leu-Phe-Ile) are full chemotactic agonists, which are capable of inducing migration of all the receptor-bearing cells. This is in contrast to the tripeptide fMet-Leu-Phe, which induces migration of only 50% of monocytes with receptors (efficacy of 33%). Since the chemotactic efficacy of the six tetrapeptides covers a wide range, the series may be useful to investigate signals that lead to directed movement after occupancy of receptors by chemoattractants.

Population kinetic study of guinea pig monocytes and their subsets during acute inflammation

Guinea pig peripheral blood monocytes exist in four subsets which differ in cytochemistry, function, rates of production, and circulatory time during steady state. These subsets are designated small, intermediate, large, and very large vacuolated monocytes. To address the question as to whether all subsets participate equally in the acute inflammatory response, population kinetics were performed on monocyte subsets isolated by counterflow centrifugation elutriation following a single intravenous pulse of tritiated thymidine given concurrently with an intraperitoneal injection of phytohemagglutinin as phylogistic agent. Inflammation reduced the cell cycle times of the precursors for all monocyte subsets, increasing their production. However, inflammation increased the number of precursors only for large monocytes. In addition, a reserve of exclusively large monocytes existed which appeared in the circulation within 3 hr of inflammation induction. The subsequent loss of large monocytes from the circulation exceeded their production in contrast to all other monocyte fractions. Over 92% of all monocytes entering the acute inflammatory site were large monocytes despite the fact that they constituted only 58% of monocytes under steady state conditions. Small, intermediate, and very large vacuolated monocytes were only minor participants in the acute inflammatory response. These results indicate heterogeneity in the monocyte subset response to acute inflammation.