Effect of phagocytosis on membrane transport of nonelectrolytes

Mechanotransduction as a major driver of cell behaviour: mechanisms, and relevance to cell organization and future research

How do cells process environmental cues to make decisions? This simple question is still generating much experimental and theoretical work, at the border of physics, chemistry and biology, with strong implications in medicine. The purpose of mechanobiology is to understand how biochemical and physical cues are turned into signals through mechanotransduction. Here, we review recent evidence showing that (i) mechanotransduction plays a major role in triggering signalling cascades following cell-neighbourhood interaction; (ii) the cell capacity to continually generate forces, and biomolecule properties to undergo conformational changes in response to piconewton forces, provide a molecular basis for understanding mechanotransduction; and (iii) mechanotransduction shapes the guidance cues retrieved by living cells and the information flow they generate. This includes the temporal and spatial properties of intracellular signalling cascades. In conclusion, it is suggested that the described concepts may provide guidelines to define experimentally accessible parameters to describe cell structure and dynamics, as a prerequisite to take advantage of recent progress in high-throughput data gathering, computer simulation and artificial intelligence, in order to build a workable, hopefully predictive, account of cell signalling networks.

A Multiparametric and High-Throughput Assay to Quantify the Influence of Target Size on Phagocytosis

Phagocytosis by macrophages represents a fundamental process essential for both immunity and tissue homeostasis. It consists in the uptake of pathogenic or cellular targets larger than 0.5 μm. For the biggest particles, the phagocytic process involves a massive reorganization of membrane and actin cytoskeleton as well as an important intracellular deformation all in a matter of minutes. The study of the role of the size of objects in their phagocytosis has led to contradictory results in the last decades. We designed a method using confocal microscopy, automated image analysis, and databases for fast quantitative analysis of phagocytosis assays. It yields comprehensive data on the cells and targets geometric and fluorescence intensity parameters, automatically discriminates internalized from external targets, and stores the relationship between a cell and the targets it has engulfed. We used two types of targets (solid polystyrene beads and liquid lipid droplets) to investigate the influence of size on the phagocytic uptake of macrophages. The method made it possible not only to perform phagocytic assays with functionalized droplets and beads of different sizes but to use polydisperse particles to further our understanding of the role of size in phagocytosis. The use of monodisperse and polydisperse objects shows that whereas smaller monodisperse objects are internalized in greater numbers, objects of different sizes presented simultaneously are internalized without preferred size. The total surface engulfed by the cell is thus the main factor limiting the uptake of particles, regardless of their nature or size. A meta-analysis of the literature reveals that this dependence in surface is consistently conserved throughout cell types, targets' nature, or activated receptors.

Lipopolysaccharide-induced vacuoles in macrophages: Their origin is plasma membrane-derived organelles and endoplasmic reticulum, but not lysosomes

Lipopolysaccharide (LPS) is one of the potent activators of macrophages. LPS was shown to induce cell spreading and large vacuoles in the cytoplasm of a macrophage-like cell line, JY3. These vacuoles were negative for acid phosphatase histochemistry and did not take up Lucifer yellow added to the medium. Latex beads were incorporated into cytoplasmic vesicles distinct from the vacuoles. These results indicated that the vacuoles are neither phagosomes nor lysosomes.

DiIC18(3), a specific marker of endoplasmic reticulum (ER), stained the vacuoles intensely, and DiOC6(3) stained the vacuoles at a density similar to nuclear envelope, suggesting ER origin of their membrane. Glucose-6-phosphatase, however, was not detected histochemically.

Vacuoles were also stained with wheat germ agglutinin-horseradish peroxidase (WGA-HRP) or WGA-biotin, suggesting that the vacuoles originated from plasma membrane-endosome-trans Golgi network-secretory granule pathway. Golgi markers, TPPase or BODIPY-ceramide were not localized to the vacuolar membrane.

These results indicate that the vacuoles may have dual origins; ER and plasma membrane-derived organelles.

Lipid rafts in mast cell biology

Mast cells have long been recognized to have a direct and critical role in allergic and inflammatory reactions. In allergic diseases, these cells exert both local and systemic responses, including allergic rhinitis and anaphylaxis. Mast cell mediators are also related to many chronic inflammatory conditions. Besides the roles in pathological conditions, the biological functions of mast cells include roles in innate immunity, involvement in host defense mechanisms against parasites, immunomodulation of the immune system, tissue repair, and angiogenesis. Despite their growing significance in physiological and pathological conditions, much still remains to be learned about mast cell biology. This paper presents evidence that lipid rafts or raft components modulate many of the biological processes in mast cells, such as degranulation and endocytosis, play a role in mast cell development and recruitment, and contribute to the overall preservation of mast cell structure and organization.

Mast cell-specific gangliosides and FcepsilonRI follow the same endocytic pathway from lipid rafts in RBL-2H3 cells

Recent studies have shown that, in mast cells, membrane microdomains rich in cholesterol and glycosphingolipids called lipid rafts play an important role in FcepsilonRI signaling. The present study demonstrates that, in RBL-2H3 cells following stimulation, the mast cell-specific gangliosides associated with FcepsilonRI are internalized from lipid rafts along with the receptor. When the cells are labeled with iodinated antibodies against the gangliosides or against FcepsilonRI and the cell components are then fractionated on Percoll density gradients, in stimulated cells the gangliosides are internalized with the same kinetics as FcepsilonRI and at 3 hr are present in the dense lysosome fraction. Using transmission electron microscopy, with antibody against the gangliosides conjugated to horseradish peroxidase and antibody against FcepsilonRI conjugated to colloidal gold, it was possible to demonstrate that the gangliosides and FcepsilonRI are internalized in the same coated vesicles. At 5 min, the gangliosides and FcepsilonRI can be identified in early endosomes and at 3 hr are found together in acid phosphatase-positive lysosomes. This study demonstrates that the mast cell-specific gangliosides are internalized from lipid rafts in the same vesicles and traffic intracellularly with the same kinetics as FcepsilonRI. This study contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials.

On phagosome individuality and membrane signalling networks

Cells such as macrophages take up pathogens into specialized membrane organelles (phagosomes) that fuse with other organelles, including lysosomes, in a process termed maturation. The fully matured phagolysosome is a low-pH, hydrolase-rich killing device that some pathogens can bypass. One might expect that phagosomes containing a given type of particle that entered cells simultaneously via the same receptor would behave the same, at least in a single cell. Surprisingly, however, recent data show that phagosomes formed via the same receptors can find themselves in different chemical states even within the same macrophage. Here, I argue that each phagosome is an individual entity whose behaviour depends on a finite number of stable equilibrium states in its membrane signalling networks.

Phagocytosis: latex leads the way

Phagocytosis is the process that cells have evolved to internalise large particles such as mineral debris, which they store, or apoptotic cells and pathogens, which they have the capacity to kill and degrade. However, several important pathogens can suppress these killing functions and survive and multiply within phagosomes, causing disease. Recent advances in phagosome biology have been made possible largely by a model system that uses inert latex beads. The ability to purify latex bead-containing phagosomes has opened the door to allow comprehensive biochemical analyses and functional assays to study the molecular mechanisms governing phagosome function. These approaches have led to unique insights directly relevant for the understanding of the biology of intracellular pathogens and the ways by which they subvert their hosts.

High resolution mapping of mast cell membranes reveals primary and secondary domains of Fc(epsilon)RI and LAT

In mast cells, cross-linking the high-affinity IgE receptor (Fc(epsilon)RI) initiates the Lyn-mediated phosphorylation of receptor ITAMs, forming phospho-ITAM binding sites for Syk. Previous immunogold labeling of membrane sheets showed that resting Fc(epsilon)RI colocalize loosely with Lyn, whereas cross-linked Fc(epsilon)RI redistribute into specialized domains (osmiophilic patches) that exclude Lyn, accumulate Syk, and are often bordered by coated pits. Here, the distribution of Fc(epsilon)RI beta is mapped relative to linker for activation of T cells (LAT), Grb2-binding protein 2 (Gab2), two PLCgamma isoforms, and the p85 subunit of phosphatidylinositol 3-kinase (PI3-kinase), all implicated in the remodeling of membrane inositol phospholipids. Before activation, PLCgamma1 and Gab2 are not strongly membrane associated, LAT occurs in small membrane clusters separate from receptor, and PLCgamma2, that coprecipitates with LAT, occurs in clusters and along cytoskeletal cables. After activation, PLCgamma2, Gab2, and a portion of p85 colocalize with Fc(epsilon)RI beta in osmiophilic patches. LAT clusters enlarge within 30 s of receptor activation, forming elongated complexes that can intersect osmiophilic patches without mixing. PLCgamma1 and another portion of p85 associate preferentially with activated LAT. Supporting multiple distributions of PI3-kinase, Fc(epsilon)RI cross-linking increases PI3-kinase activity in anti-LAT, anti-Fc(epsilon)RIbeta, and anti-Gab2 immune complexes. We propose that activated mast cells propagate signals from primary domains organized around Fc(epsilon)RIbeta and from secondary domains, including one organized around LAT.

Observing FcepsilonRI signaling from the inside of the mast cell membrane

We have determined the membrane topography of the high-affinity IgE receptor, FcstraightepsilonRI, and its associated tyrosine kinases, Lyn and Syk, by immunogold labeling and transmission electron microscopic (TEM) analysis of membrane sheets prepared from RBL-2H3 mast cells. The method of Sanan and Anderson (Sanan, D.A., and R.G.W. Anderson. 1991. J. Histochem. Cytochem. 39:1017-1024) was modified to generate membrane sheets from the dorsal surface of RBL-2H3 cells. Signaling molecules were localized on the cytoplasmic face of these native membranes by immunogold labeling and high-resolution TEM analysis. In unstimulated cells, the majority of gold particles marking both FcepsilonRI and Lyn are distributed as small clusters (2-9 gold particles) that do not associate with clathrin-coated membrane. Approximately 25% of FcepsilonRI clusters contain Lyn. In contrast, there is essentially no FcepsilonRI-Syk colocalization in resting cells. 2 min after FcepsilonRI cross-linking, approximately 10% of Lyn colocalizes with small and medium-sized FcepsilonRI clusters (up to 20 gold particles), whereas approximately 16% of Lyn is found in distinctive strings and clusters at the periphery of large receptor clusters (20-100 gold particles) that form on characteristically osmiophilic membrane patches. While Lyn is excluded, Syk is dramatically recruited into these larger aggregates. The clathrin-coated pits that internalize cross-linked receptors bud from membrane adjacent to the Syk-containing receptor complexes. The sequential association of FcstraightepsilonRI with Lyn, Syk, and coated pits in topographically distinct membrane domains implicates membrane segregation in the regulation of FcstraightepsilonRI signaling.

Impact of monocyte colony-stimulating factor upon beta-very low density lipoprotein (beta-VLDL) cholesterol metabolism in tetradecanoyl phorbol acetate-derived THP-1 cells

The effect of monocyte colony stimulating factor (M-CSF) on the beta-very low density lipoprotein (beta-VLDL) metabolism in THP-1 cells (human leukemia cell line) was studied. THP-1 cells treated with M-CSF decreased Latex Bead phagocytosis, but the cells incubated with 12-tetradecanoyl-phorbol-13-acetate (TPA) enhanced phagocytosis 2.5-fold. Binding activity of 125I-M-CSF to THP-1 cells was higher than that in THP-1 cells elicited with TPA. THP-1 cells incubated with M-CSF before TPA treatment were designated MT macrophages, and those incubated with M-CSF after TPA treatment were called TM macrophages. When these cells were incubated with beta-VLDL, the cholesterol ester content in MT macrophages was less than in TM macrophages. The uptake of [3H]cholesterol oleate-beta-VLDL in MT macrophages was the same as in TM macrophages. The released radioactivity from [3H]cholesterol oleate-beta-VLDL loaded MT macrophages was higher than that from TM macrophages. Acid cholesterol esterase activity and ACAT activity were the same in both types of macrophages. Neutral cholesterol esterase activity was higher in MT than in TM macrophages. These results suggested that beta-VLDL-induced cholesterol ester deposition in THP-1 cells-derived macrophages was suppressed by M-CSF, when M-CSF acted at the stage of monocytes (THP-1 cells), and that the reduction of cholesterol ester might be due to enhanced release of cholesterol from the cells with high neutral cholesterol esterase activity.

In vivo inflammatory stimulation induces a transient change in the binding of thrombin to rat peritoneal macrophages

The binding of 125I-labeled thrombin to rat peritoneal macrophages isolated 20 h after the ip injection of thioglycollate broth or lipopolysaccharide decreased to 20% of the value found in resident macrophages due to a decrease in the number of receptors. The binding returned to normal values within a week after the injection. The decline parallelled more or less the Vmax for the 5'-nucleotidase activity. This decrease in the binding of thrombin could not be explained by an immigration of monocytes into the peritoneal cavity, since the binding of 125I-labeled alpha 2-macroglobulin-trypsin complex increased 4.5-fold in the same cell population due to an increase in the number of receptors, and blood monocytes do not bind alpha 2-macroglobulin-trypsin complex. The increase in the binding of alpha 2-macroglobulin-protease complex parallelled an increase in the incorporation of glucosamine, although the latter did not increase to the same extent. Engulfment of plasma membrane after phagocytosis did not result in a decreased binding of thrombin, but preincubation at 37 degrees C with concanavalin A caused a minor reduction in the binding. There was a positive correlation between the binding of alpha 2-macroglobulin-trypsin complex and the fraction of polymorphonuclear leukocytes in the peritoneal exudate and a negative correlation between the binding of thrombin and the fraction of polymorphonuclear leukocytes in the exudate, when the inflammation was induced by a milder stimulus, sterile NaCl, indicating a common signal for the polymorphonuclear leukocyte chemotaxis and the macrophage differentiation.

Membrane sorting during phagocytosis: selective exclusion of major histocompatibility complex molecules but not complement receptor CR3 during conventional and coiling phagocytosis

We have used immunocytochemical techniques and enzyme cytochemistry to examine the distribution of plasma membrane proteins during coiling phagocytosis of Legionella pneumophila and conventional phagocytosis of Escherichia coli. Whereas class I and class II major histocompatibility complex (MHC) molecules are relatively excluded from nascent phagosomes during conventional and coiling phagocytosis, the CR3 complement receptor persists in nascent phagosomes. The staining pattern for alkaline phosphatase activity resembles that of MHC molecules, with a marked exclusion of phosphatase activity from L. pneumophila coils and nascent phagosomes. The staining pattern for 5'-nucleotidase activity, on the other hand, resembles that of CR3 with intense staining in the inner layers of L. pneumophila coils. These results demonstrate that the cell has the ability to exclude selectively certain membrane proteins from the nascent phagosome during phagocytosis, thereby producing a phagosomal membrane markedly different from the plasma membrane from which it is derived.

Sulphydryl reactivity of the HLA-B27 epitope: accessibility of the free cysteine studied by flow cytometry

HLA-B27 has an unpaired cysteine on or near its serologically defined spondylitis associated epitope, and it has been argued that its sulphydryl side chain may be chemically reactive. In a previous study it was shown that chemical treatment of HLA-B27 cells with the sulphydryl binding agent p-chloromercuriphenylsulphonic acid (pCMPSA) specifically reduced binding of antibodies to HLA-B27 by up to 80%, as measured in a cellular enzyme linked immunosorbent assay (CELISA). The effect of sulphydryl blockade on intact B27 cells was investigated using flow cytometry. Compared with the CELISA, inhibition required higher concentrations of pCMPSA, and the degree of inhibition produced by a greater than or equal to 30 microM solution of pCMPSA as measured by flow cytometry (median 28.9%) was significantly lower than that measured by CELISA (median 73.6%; p = 1.6 x 10(-6)). Analysis of unfixed, cell surface HLA-B27 by flow cytometry suggests that on most B27 molecules the unpaired sulphydryl site is not available. On the basis of this evidence for modification after translation, a new 'altered self' hypothesis is proposed for the part which HLA-B27 plays in inflammatory disease.

Modulation of human T cell functions by surface sulphydryl groups: differential effects on IL-2 production and responsiveness

An impermeable thiol blocker has been used to investigate the role of sulphydryl (SH) groups in the production of and responsiveness to IL-2 by normal human T lymphocytes. Surface SH blockade of mononuclear cells prior to incubation with mitogen (phytohaemagglutinin, concanavalin A, CD3 MoAb) had no effect on production of IL-2 but markedly impaired cellular responsiveness to exogenous IL-2. Studies using MoAbs indicated that this effect was accompanied by decreased expression of both the CD25 and p75 subunits of the IL-2 receptor. Blocking surface SH groups did not affect binding of IL-2 to p75 on unstimulated mononuclear cells, but inhibited binding to high-affinity receptors on a T lymphoma cell line. The data are consistent with the hypothesis that sulphydryl groups on the IL-2 receptor are required for its function and may be involved in the interaction of the CD25 and p75 subunits leading to generation of the high-affinity binding site. The surface thiol identified on the IL-2 receptor may be a candidate for oxidation on cells from patients with chronic inflammatory diseases such as rheumatoid arthritis and thus contribute to the aberrant function of T cells in these patients.

The effects of N-ethylmaleimide on extracellularly and intracellularly generated chemiluminescence in neutrophils indicate that the rate of deactivation of NADPH-oxidase is higher when the oxidase system is localized on the plasma membrane than when it is localized on the phagosomal membrane

Sustained generation of reactive oxygen metabolites following respiratory burst activation in neutrophils is a result of continued replenishment of a pool of active NADPH-oxidase. The sulphydryl-modifying reagent N-ethylmaleimide (NEM) has been shown to be without effect on the turnover of activated NADPH-oxidase but to inhibit the replenishment of active oxidase molecules (Akard et al., 1988). NEM was thus used to determine the rate of deactivation of extracellularly and intracellularly generated chemiluminescence in human neutrophils. We have shown that deactivation is more rapid when activation leads to a release of oxygen metabolites (extracellular chemiluminescence) than when the metabolites are generated intracellularly. The results indicate that the rate of deactivation of NADPH-oxidase is higher when the oxidase system is localized on the plasma membrane than when it is localized on the phagosomal membrane.

The opsonizing ligand on Salmonella typhimurium influences incorporation of specific, but not azurophil, granule constituents into neutrophil phagosomes

Phagosomes were purified from human neutrophils ingesting Salmonella typhimurium opsonized with adsorbed normal human serum or with rabbit IgG. Constituents within the phagosome were endogenously labeled by supplying the cells with 125INa during phagocytosis. Lactoferrin and vitamin B12 binding protein (TC1 and TC3), markers for specific granules, were present in the phagosomes from neutrophils ingesting S. typhimurium opsonized with IgG but were 3.5- to 5-fold less prominent in phagosomes from cells phagocytosing Salmonella bearing C3 fragments only. In contrast, iodinated azurophilic granule components, most prominently defensins, were the major constituents in phagosomes prepared under both opsonization conditions. Furthermore, labeled complement (CR1 and CR3) and immunoglobulin (Fc gamma RIII) receptors were incorporated in the phagosome regardless of the ligand mediating phagocytosis. These results suggest that the ligand-receptor interactions mediating phagocytosis influence incorporation of neutrophil-specific granule contents into phagosomes.

Uptake of inert particles by dog alveolar macrophages in vitro--a comparison of monolayer and suspension techniques

This study was conducted to determine if significant differences in phagocytosis occurred in pulmonary alveolar macrophages in suspension or monolayer cultures. Dog alveolar macrophages were incubated with different numbers of fluorescent polystyrene latex microspheres, both in suspension and in monolayer cultures. The particle uptake by cells was linear for 10-100 min and the fraction of cells that contained particles was similar for both incubation techniques. Differences between carboxylated and non-carboxylated microspheres were statistically not significant. The overall particle numbers associated with cells were 2-5 times higher for cells incubated in suspension compared to adherent cells. Many more cells were found with low particle numbers at all incubation times using the monolayer technique compared to the suspension technique. When cells were treated with xylene for 1 h after different times of incubation in vitro, a higher decrease in particle numbers in cells was found in suspension cultures than in monolayer cultures. This finding suggests higher particle binding when cells were incubated in suspension, but particle numbers after xylene treatment were still higher in cells incubated in suspension than in monolayers. A comparison with the uptake of similar particles by dog alveolar macrophages in vivo suggests that the suspension cultures were more similar to the events in the dog lung, but uptake rates were about a factor of two lower with suspended cells than those seen in vivo.

Cytochemical localization of plasma membrane enzyme markers during interiorization of tachyzoites of Toxoplasma gondii by macrophages

The enzyme activity of Mg++-ATPase, Na+-K+-ATPase, 5'-nucleotidase and NAD(P)H-oxidase was cytochemically detected at the ultrastructural level in mouse peritoneal macrophages infected with untreated and with specific antibody-coated Toxoplasma gondii tachyzoites. The Mg++-ATPase and 5'-nucleotidase were distributed throughout the macrophages' plasma membrane but were not observed in the membrane lining endocytic vacuoles containing ingested parasites; however, Na+-K+-ATPase activity was detected in the macrophages' plasma membrane as well as in the parasitophorous vacuoles that contained untreated or specific antibody-coated parasites. Reaction product, indicative of NAD(P)H-oxidase, was detected in the parasitophorous vacuoles that contained only specific antibody-coated parasites.

Association of glycolytic enzymes with the cytoplasmic side of the plasma membrane of glioma cells

A latex phagocytosis technique was used to prepare relatively pure plasma membranes with inside-out orientation. This method was adapted through a number of modifications in order to evaluate the association of glycolytic enzymes with the cytoplasmic side of the plasma membrane of C6 glial cells. As phosphorylation is strictly coupled with transport in these cells, glycolytic enzymes, especially hexokinase, could metabolize glucose in close vicinity to its transporter. Of the enzymes tested, hexokinase is present in considerable quantities on these membranes (nearly 40% of homogenate specific activity), followed by D-glyceraldehyde-3-phosphate dehydrogenase (10%), pyruvate kinase (8%), and 3-phosphoglycerate kinase (1%). Except for hexokinase, the enzyme pattern presented here is different from that published for other membrane preparations.

Biophysical aspects of microsphere engulfment by human neutrophils

A quantitative investigation into the mechanism of neutrophil phagocytosis of opsonized microspheres possessing well defined dimensions was undertaken. Three aspects were documented: membrane conservation, cell adhesion to the spheres, and active cell cytoplasmic projection around the microspheres. The physical act of internalizing a particle by a cell involves a reduction in its plasma membrane area and an increase in its volume. As a consequence, a cell can internalize only a finite number of particles. A store of membrane area exists on cytoplasmic granules and may be recruited during phagocytosis. Previous measurements of neutrophil membrane area and volume served as a basis for estimates of the maximum number of internalized microspheres. A comparison with experimental prediction based on membrane conservation and degranulation agrees within 10% for a range of microsphere diameters, from 0.5 to 8 microns. This suggests that the limitation for additional particle uptake in the population of engorged neutrophils is the lack of excess plasma membrane area. In a random population of neutrophils, there was a sub-group, approximately 40%, which could no longer phagocytose before depleting their membrane stores. Several aspects of the engulfment process were investigated to elucidate the cause of this phagocytosis deficiency. It could be shown by single cell observation that these cases were associated with a lack of pseudopod projection, although adhesion was still evident between the cell membrane and the microspheres.

Effect of phagocytosis on receptor distribution and endocytic activity in macrophages

Phagocytosis requires the internalization of a significant fraction of the plasma membrane and results in the intracellular deposition of large particles. We evaluated the effect of phagocytosis on the cellular distribution of recycling receptors and uptake of ligand to determine whether phagocytosis affects receptor behavior. Phagocytosis of zymosan, latex particles, or IgG-coated red blood cells by rabbit alveolar macrophages did not decrease the number of cell surface receptors for transferrin, alpha 2-macroglobulin X protease complexes, maleylated proteins, or mannosylated proteins. The number of surface receptors for transferrin was also unaltered in J774 cells, a macrophage-like cell line. In both cell types extensive phagocytosis did not affect the rate of receptor-mediated endocytosis or the distribution of receptors between the endosome and the cell surface. However, fluid phase pinocytosis was reduced by phagocytosis. The major reduction appeared to be not in the rate of internalization but rather in the delivery of fluid to the lysosome. These results demonstrate that internalization of a significant amount of the plasma membrane during phagocytosis does not diminish the number of receptors on the cell surface and has no effect on receptor-mediated ligand uptake.

Specific inhibition of macrophage antibody-dependent endocytosis by p-chloromercuribenzenesulphonic acid: identification of sensitive membrane proteins

Murine macrophage Fc receptor function has been studied with the membrane-impermeable sulphydryl-blocking reagent p-chloromercuribenzenesulphonic acid (PCMBSA). Antibody-dependent endocytosis of fluorescein-labelled immune complexes was studied with video intensification microscopy. PCMBSA was found to inhibit the endocytosis of immune complexes at 10 nM. Control experiments indicate that the inhibition is due to an effect upon the cell, not the immune complex. Furthermore, specificity is suggested by the fact that complement-mediated and latex bead phagocytosis were not affected. 203Hg-PCMBSA labelled three macrophage proteins of molecular weight (MW) 25,000, 35,000 and 50,000. A 25,000 MW PCMBSA-binding protein has been found that is specifically immunoprecipitated by an anti-Fc receptor antibody. These studies suggest that perturbation of a cell surface sulphydryl group(s) of one of the three major PCMBSA binding membrane proteins, possibly an Fc receptor-associated protein, blocks a molecular signal required for antibody-dependent endocytosis.

An improved method to quantitate phagocytosis with mineral oil particles which avoid cell flotation

We have shown that actively phagocytosing human polymorphonuclear leukocytes (PMN) float on top of the incubation medium when Oil Red O containing paraffin oil particles (density 0.8740) are used as the phagocytosable material. This implies that the quantitation of phagocytosis based on the recovery of Oil Red O in phagocytosing cells pelleted after centrifugation would be underestimated. We therefore prepared particles of progressively increasing density by mixing paraffin oil (density 0.8740) with silicon oil (density 1.0802). Cell flotation also occurred with paraffin oil-silicon oil particles and could be avoided only when the density of the particles used had reached 0.9952 g/cm3. Paraffin oil-silicon oil particles of a density sufficient to dissolve the dye Oil Red O were therefore used to quantitate both the initial rate of phagocytosis and the phagocytic capacity of human PMN. With this assay the initial rate of phagocytosis was found to be 400 micrograms paraffin oil-silicon oil/min/5 X 10(6) PMN, which is about 20 times higher than that reported for the same cell type using paraffin oil particles. The calculated maximum phagocytic capacity was 2.5 mg paraffin oil-silicon oil/5 X 10(6) PMN. The uptake of paraffin oil-silicon oil particles was sensitive to inhibitors of phagocytosis, such as N-ethylmaleimide, papaverine and cytochalasin B, in a dose dependent manner. The assay also permitted the detection of increased phagocytosis such as occurs in myeloperoxidase deficient PMN.

The carbohydrates associated with the surfaces of hamster alveolar macrophages: a study using fluorescein-conjugated lectins

Alveolar macrophages were isolated from the lungs of hamsters by lavage and centrifugation. The harvesting procedure yielded about 5.2 X 10(6) cells per animal and the majority of these cells were viable as assessed by a Trypan blue exclusion test. Monolayers of macrophages were labelled with a series of FITC-conjugated lectins. The results of this study suggest that the surface coating of the alveolar macrophages contains the following carbohydrate groups: N-acetylgalactosamine, N-acetylglucosamine, D-glucose, D-mannose, L-fucose and sialic acid.

Redistribution of Na-K-ATPase in the dystrophic rat retinal pigment epithelium

Our previous studies have shown that a breakdown in tight junctions in the dystrophic retinal pigment epithelium (RPE) of Royal College of Surgeons' rats is accompanied by changes in intramembrane structure which suggest a redistribution of intramembrane particles. We have now investigated, using the p-nitrophenyl phosphate technique, the possibility that a specific membrane protein, Na-K-ATPase, is redistributed as tight junctions break down in the dystrophic RPE. In the normal RPE, Na-K-ATPase activity is restricted to the apical membrane. Junctional membranes and membranes around phagosomes are free of enzyme activity, suggesting a segregation of the transport enzyme from the junctional and phagocytic membrane. In the dystrophic RPE, prior to changes in tight junctions, enzyme activity is restricted to the apical membrane. During the initial stages of junctional breakdown, junctional membranes and membranes around cytoplasmic inclusions are also labelled. As the breakdown progresses, Na-K-ATPase activity is often present laterally and basolaterally and is sometimes absent apically. Enzyme activity is seen basally only where RPE cells have detached from Bruch's membrane and are superimposed over each other. These changes suggest that Na-K-ATPase redistributes during junctional breakdown, but that attachments between the RPE and Bruch's membrane may restrict the redistribution. The apparent reduction of enzyme activity apically suggests that active transport across the dystrophic RPE may be reduced as the tight junctions break down.

Defective monocyte accessory function due to surface sulphydryl (SH) oxidation in rheumatoid arthritis

Low serum sulphydryl (SH) levels are a feature of active rheumatoid arthritis (RA). We have investigated whether a similar blockade of membrane SH groups on mononuclear cells modifies the function of these cells in this disease. Using pokeweed mitogen stimulated IgG synthesis as the assay system, we have found that the accessory cell function of peripheral blood monocytes is totally dependent on free SH groups on the cell surface. Monocytes from patients with active RA display poor accessory cell function when compared with healthy monocytes or with cells from patients treated with D-penicillamine. The poor function of the rheumatoid accessory cells may be corrected in vitro by 2-mercaptoethanol (2-ME). Addition of 2-ME to the culture system also enhances IgG synthesis by rheumatoid mononuclear cells to normal levels. We suggest that surface SH-dependent mechanisms are responsible, at least in part, for the depressed mononuclear cell functions of rheumatoid cells in vitro and may explain some effects of D-penicillamine therapy in rheumatoid patients.

Cytochemical study of liposome and lipid vesicle phagocytosis

Electron microscopy cytochemistry has been used to study the cytoplasmic location of liposomes and lipid vesicles following specific antibody-dependent phagocytosis. The vesicle compositions were 94-99 mol% 'fluid' lipid (egg phosphatidylcholine or dimyristoylphosphatidylcholine at 37 degrees C or 'solid' lipid (dipalmitoylphosphatidylcholine at 37 degrees C). In some cases, 4 mol% phosphatidylserine was included in the vesicle membrane so as to vary the surface charge density. These vesicles undergo specific antibody-dependent phagocytosis by RAW264 macrophages when the lipid membranes contain 1-2 mol% dinitrophenyl lipid hapten in the presence of rabbit anti-dinitrophenyl IgG antibody. Internalized lipid vesicles can be visualized with the electron microscope when ferritin is trapped in the internal aqueous compartments prior to internalization. The lipid vesicles were demonstrated to be internal to the macrophage plasma membranes by selectively staining the plasma membranes with Ruthenium red. The cytoplasmic location of vesicles and liposomes was studied by electron microscopic staining for activities of the following enzymes: (1) acid phosphatase; (2) inorganic trimetaphosphatase; (3) adenosine triphosphatase; and (4) glucose-6-phosphatase. The first two enzymatic activities were found in association with ferritin-containing vesicles after antibody-dependent phagocytosis, showing the formation of vesicle-containing phagolysosomes. Adenosine triphosphatase and glucose-6-phosphatase were primary not associated with the vesicles, suggesting a minimal association of vesicles with plasma membrane, Golgi, endoplasmic reticulum and perinuclear cisternae. Phagosome-lysosome fusion did not appear to depend on the type of target lipid vesicle or liposome, on the 'fluidity' of the target membrane, or the presence of phosphatidylserine in the target membrane.

Redistribution of membrane 5'-nucleotidase in rabbit peritoneal polymorphonuclear leucocytes during phagocytosis

Using a quantitative phagocytic model involving oil droplet internalisation rabbit polymorphonuclear leucocytes display selective segregation of membrane constituents during the phagocytic event. A resting cell surface membrane and fractions representing vesicle membranes and uninvolved surface membranes from the active cells have been purified by density gradient sedimentation and free flow electrophoresis. The specific activity of 5'-nucleotidase, a major neutrophil surface membrane glycoprotein, was 3-fold higher in the uninvolved membrane of phagocytosing cells than in the resting cell membrane. The activity in the vesicle membranes was substantially depleted. In contrast Lens culinaris receptors showed no redistribution during phagocytosis: the two surface domains showing essentially the same enrichment with respect to homogenate as the resting cell surface membrane.

C3b acceptors on macrophages: inhibition of Fc gamma-receptor-mediated phagocytosis by acceptor-bound C3b

The binding of nascent human C3b (i.e. the fragment of C3 just after trypsin cleavage) to mouse peritoneal macrophages was demonstrated by immune adherence. Acceptor-bound C3b could be detected longer than 24 h on the cell membrane. The rosette formation and phagocytosis of SRBC coated with anti-SRBC rat IgG was inhibited by preincubation of the cells with C3 and trypsin (15 min, 37 degrees C). However, the phagocytosis of opsonized yeast particles was not influenced by acceptor-bound C3b, proving that C3b-C3b acceptor interaction did not alter the function of C3b-receptors. Acceptor-bound C3b on the macrophages failed to mediate phagocytosis of human 0,Rh+ red cells having C3b-receptors.

Changes in the lateral ordering of the macrophage plasma membrane during Fc receptor mediated phagocytosis

The macrophage plasma membrane was labelled with an intercalated 5-doxyl stearic acid spin probe, and structural changes induced by IgG-coated erythrocytes (EA) were followed with particular emphasis on the possible role of lipid reordering in the sequential events of phagocytosis. We present three lines of experimental evidence to show that these structural changes were induced by the lateral aggregation of cell surface Fc receptors. Cytochalasin B, an inhibitor of microfilament function, blocked this membrane reordering; if it was added after EA binding induced membrane reordering had already been detected for 15 min, a rapid reversal process was observed resulting in a reversible restoration of the initial order parameter value. We suggest that these structural changes indicate lipid-lipid lateral phase separation, in line with morphological findings.

Two-compartment behavior during transport of folate compounds in L1210 cell plasma membrane vesicles

The transport of [3H] 1,L 5-formyltetrahydrofolate, [3H] folic acid, and [3H]methotrexate by L1210 cell plasma membrane vesicles exhibited multicompartmental behavior. Two separate vesicular compartments (parallel relationship) of approximately equal volume were revealed during measurements of influx and efflux. Flux in one compartment was rapid, saturable, highly temperature-sensitive, and inhibited by pCMBS. Flux in the other compartment exhibited all of the characteristics of passive diffusion. These results imply that our plasma membrane vesicle preparations consist of a mixture of two functional species. Transport of folate into one of these species occurs by passive diffusion alone, whereas transport into the other kind of vesicle occurs by both passive diffusion and carrier-facilitated transport.

Phagosomal membrane lipids of LM fibroblasts

Murine fibroblasts, LM cells, were cultured in suspension or monolayer in a chemically defined medium without serum and exposed to polystyrene beads. The LM cells endocytized the beads in direct proportion to the bead/cell ratio and the bead surface area. However, equal volumes of beads irrespective of size or surface area were internalized. The lipid composition of the phagosome membrane differed significantly from the parent primary membrane in having higher contents of phosphatidylcholine, phosphatidylserine, and sterol but lower contents of sphingomyelin and lysophosphatidylcholine. When phagosomes isolated from suspension-cultured LM fibroblasts were exposed to trinitrobenzene-sulfonic acid at 4 degrees C, 55 +/- 1.6% of the phagosomal membrane phosphatidylethanolamine was trinitrophenylated. The asymmetric distribution of phosphatidylethanolamine across the phagosomal membrane was not affected by the bead/cell ratio, bead diameter, or exposure time of LM fibroblasts to the beads. When cells were reacted with trinitrobenzenesulfonic acid at 4 degrees C prior to phagocytosis, the amount of trinitrophenylphosphatidylethanolamine was greater in the isolated phagosomes than in the parent primary plasma membrane. Culturing LM fibroblasts in suspension or monolayer had no effect on the asymmetric distribution of phosphatidylethanolamine across primary plasma membrane bilayers. The data are consistent with the observation that LM fibroblasts grown either in suspension or monolayer internalize polystyrene beads at selective sites in the surface membrane.

Acrolein: a potent modulator of lung macrophage arachidonic acid metabolism

Resting rat pulmonary alveolar macrophages exposed to acrolein were stimulated to synthesize and release thromboxane B2 and prostaglandin E2 in a dose-dependent manner. Zymosan-activated pulmonary alveolar macrophages released approximately twice as much prostaglandin E2 as thromboxane B2, whereas acrolein-activated pulmonary alveolar macrophages released 4-5 times less prostaglandin E2 than thromboxane B2. In the zymosan-stimulated pulmonary alveolar macrophages, acrolein also induced a reversal in the relative amounts of prostaglandin E2 and thromboxane B2 synthesized and released into the culture medium. This reversal was achieved by a dose-dependent reduction in prostaglandin E2 synthesis. Although phagocytosis was also inhibited in a dose-dependent manner, the reduction in prostaglandin E2 appeared to be partially independent of particle ingestion since thromboxane B2 synthesis was not affected by low doses of acrolein. In fact, high doses induced a slight enhancement in thromboxane B2 synthesis. These results suggest that acrolein selectively inhibited the enzyme, prostaglandin endoperoxide E isomerase, necessary for the conversion of the endoperoxide to prostaglandin E2. Sulfhydryl reagents such as N-ethylmaleimide and 5,5'-dithiobis (2-nitrobenzoic acid) mimicked acrolein's effects, and reduced glutathione afforded protection against the effects of acrolein. These results indicated the possible involvement of acrolein's sulfhydryl reactivity in the inhibition of the isomerase enzyme. Propionaldehyde had no effect on macrophage arachidonic acid metabolism whereas crotonaldehyde mimicked the effects of acrolein. Pulmonary macrophages were unable to reverse the acrolein effects on arachidonate metabolite synthesis after 6 h in an acrolein-free environment. These data indicated the necessity of the unsaturated carbon bond for the acrolein effects on arachidonic acid metabolism and the relative irreversibility of acrolein's reaction with the macrophage.

Isolation and characterization of phagosomes containing Chlamydia psittaci from L cells

The obligate intracellular procaryote Chlamydia psittaci enters host cells by a mechanism similar to, but distinct from, conventional phagocytosis. To better understand chlamydial uptake, L-cell phagosomes containing a single chlamydial cell were isolated and studied. Two rounds of dextran rate-zonal gradient centrifugation of L cells homogenized 1 h after infection with C. psittaci yielded phagosomes relatively free of other membranous structures. In double-label experiments, the phagosomes were enriched over 40-fold for radioactivity derived from chlamydiae as compared with the initial homogenate. Several lines of evidence showed that the structures isolated on dextran gradients were chlamydial phagosomes. These structures and free chlamydiae banded at different positions on discontinuous sucrose gradients. The difference was destroyed by the nonionic detergent Nonidet P-40, which disrupts plasma membranes but has no effect on C. psittaci. Material labeled on the surface of the L-cell plasma membrane cosedimented with the phagosome fractions. Electron microscopy of these fractions revealed structures having the appearance of a chlamydial elementary body surrounded by a unit membrane. Sodium dodecyl sulfate-polyacrylamide gels of the phagosome membranes displayed 10 major protein bands, less than the total number of surface-labeled proteins in the L-cell plasma membrane. Seven of the proteins of phagosome membranes had electrophoretic mobilities corresponding to those of proteins exposed on the surface of L cells. Two of them were cleaved by both trypsin and chymotrypsin, enzymes that decrease the susceptibility of L cells to infection with C. psittaci. These proteins may therefore be involved in the attachment and ingestion of C. psittaci by L cells.

Membrane activity and topography of F-Met-Leu-Phe-Treated polymorphonuclear leukocytes. Acute and sustained responses to chemotactic peptide

The chemotactic peptide N-formylmethionyl-leucyl-phenylalanine (f-Met-Leu-Phe) causes a dramatic stimulation of membrane ruffling and a fluid pinocytosis in polymorphonuclear leukocytes (PMNs). These responses are maximal by 1 minute and subside within 5-10 minutes. The same immediate responses characterize cells exposed to several peptide hormones and may thus represent an essential component of target cell activation by peptides. The stimulation of the whole membrane following f-Met-Leu-Phe binding is succeeded by the development of a polarized cell shape with a posterior uropod and a broad anterior lamellipodium, both subtended by microfilaments. Membrane components and functions segregate into distinct domains on polarized PMNs. Thus, succinyl concanavalin A-receptor complexes are capped and internalized by receptor-mediated endocytosis at the uropod; the uptake by fluid pinocytosis of fluorescein-dextran is restricted to the uropod; and coated pits and coated vesicles are concentrated at the uropod. The lamellipodium excludes coated pits and lacks pinocytic activity but shows preferential binding of immunoglobulin aggregates, presumably to Fc receptors. The origin and physiologic implications of these asymmetries of membrane molecular and functional topography on polarized cells are discussed.

Influence of macrophages on microdistribution of inhaled UO2 aerosol in rat lung

Following the inhalation of an aerosol of UO2 (mass median aerodynamic diameter = 3 microns and geometric standard deviation = 1.6) the lungs of male albino rats were populated by foci containing UO2 particles. A method of neutron-induced autoradiography on Lexan plastic was used to reveal these foci in thin sections cut from the lung. For masses of UO2 in the lung that differed by more than a factor of 10 (39 to 450 micrograms) the number of foci per g of lung increased, but not in proportion to the mass of UO2 deposited. This limited increase in the number of foci was considered to result from physiological limitations on the number of alveolar macrophages available for engulfing the UO2 particles.

Macrophage phagocytosis: analysis of particle binding and internalization

An improved radioassay for analysis of phagocytosis has been used to quantitate and characterize the binding and internalization of zymosan by monolayers of rabbit pulmonary alveolar macrophages. This method distinguishes zymosan particles reversibly bound to the cell surface from those internalized. The zymosan was radiolabeled with technetium 99m(99mTc), a gamma-emitter with a 6-h half-life. Use of 99mTc as the radiolabel also permitted simultaneous determinations of pinocytosis and cellular protein content using [14C]sucrose and [3H]amino acids, respectively. All endocytic data were normalized per adherent cell based on this latter measurement. A significant fraction of the cell-associated particles was bound to the cell surface but not internalized. Failure to correct for this compartment would have resulted in overestimation of phagocytic rate and total cellular capacity. Both binding and ingestion of functionally unopsonized zymosan were found to be saturable, temperature sensitive, dependent on glycolytic energy, dependent on a trypsin-sensitive membrane component, described by a maximal rate, and limited by a finite capacity. The time courses of both processes were found to be similar. These results led us to conclude that, in our system, both binding and internalization were active processes and that the limited capacity to ingest zymosan was not explained by a concomitant reduction in binding of the particle to the cell membrane. Furthermore, it was found that phagocytosis did not change the rate of fluid-phase pinocytosis, consistent with the concept that the cell membrane is a functional mosaic as has been previously found by others for phagocytic and transport sites in this cell type.

Identification of phagocytosis-associated surface proteins of macrophages by two-dimensional gel electrophoresis

Two-dimensional PAGE (P. Z. O'Farrell, H. M. Goodman, and P. H. O'Farrell. 1977. Cell. 12:1133-1142) has been employed to assess the effects of antibody-dependent phagocytosis on the cell surface protein composition of RAW264 macrophages. Unilamellar phospholipid vesicles containing 1% dinitrophenyl-aminocaproyl-phosphatidylethanolamine (DNP-cap-PE) were used as the target particle. Macrophages were exposed to anti-DNP antibody alone, vesicles alone, or vesicles in the presence of antibody for 1 h at 37 degrees C. Cell surface proteins were then labeled by lactoperoxidase-catalyzed radioiodination at 4 degrees C. After detergent solubilization, membrane proteins were analyzed by two-dimensional gel electrophoresis. The resulting pattern of spots was compared to that of standard proteins. We have identified several surface proteins, not apparently associated with the phagocytic process, which are present either in a multichain structure or in several discretely charged forms. After phagocytosis, we have observed the appearance of two proteins of 45 and 50 kdaltons in nonreducing gels. In addition, we have noted the disappearance of a 140-kdalton protein in gels run under reducing conditions. These alterations would not be detected in the conventional one-dimensional gel electrophoresis. This evidence shows that phagocytosis leads to a modification of cell surface protein composition. Our results support the concept of specific enrichment and depletion of membrane components during antibody-dependent phagocytosis.

Membrane transport of clindamycin in alveolar macrophages

The use of antibiotics which can penetrate phagocytic cells and kill intracellular organisms is desirable in the treatment of chronic facultative bacterial infections. Recently, we reported that several antibiotics were selectively concentrated by rabbit alveolar macrophages. Clindamycin accumulation was especially marked. In the present study we evaluated the plasma membrane transport (initial uptake) of clindamycin in alveolar macrophages. The transport of clindamycin is an active process, as documented by requirements for cellular viability, elevated environmental temperature, metabolic energy, and establishment of the 40- to 50-fold cellular/extracellular gradient. Energy for membrane transport of the drug depended at least in part upon mitochondrial oxidative respiration and cell membrane Na-K pump activity. Kinetic analysis of active clindamycin transport revealed it to be saturable, with a high binding affinity (Km = 1 mM) and a high velocity of uptake (Vmax = 15.8 nmol/45 s per 10(6) cells). Clindamycin uptake was not influenced by the presence of hexose or amino acids, but was inhibited by nucleosides (adenosine, puromycin). Decreased clindamycin transport in the presence of puromycin was typical of competitive inhibition (increased Km, unchanged Vmax). Conversely, competitive inhibition of adenosine transport by clindamycin was documented. Thus, clindamycin is transported into alveolar macrophages via the nucleoside system. The potential biological consequences of this unique antibiotic transport mechanism are of interest.