Isolation of the phagocytic compartment from macrophages using a paramagnetic, particulate ligand

Phagosome-Bacteria Interactions from the Bottom Up

When attempting to propagate infections, bacterial pathogens encounter phagocytes that encase them in vacuoles called phagosomes. Within phagosomes, bacteria are bombarded with a plethora of stresses that often lead to their demise. However, pathogens have evolved numerous strategies to counter those host defenses and facilitate survival. Given the importance of phagosome-bacteria interactions to infection outcomes, they represent a collection of targets that are of interest for next-generation antibacterials. To facilitate such therapies, different approaches can be employed to increase understanding of phagosome-bacteria interactions, and these can be classified broadly as top down (starting from intact systems and breaking down the importance of different parts) or bottom up (developing a knowledge base on simplified systems and progressively increasing complexity). Here we review knowledge of phagosomal compositions and bacterial survival tactics useful for bottom-up approaches, which are particularly relevant for the application of reaction engineering to quantify and predict the time evolution of biochemical species in these death-dealing vacuoles. Further, we highlight how understanding in this area can be built up through the combination of immunology, microbiology, and engineering.

A guide to measuring phagosomal dynamics

Phagocytosis is an essential mechanism for immunity and homeostasis, performed by a subset of cells known as phagocytes. Upon target engulfment, de novo formation of specialized compartments termed phagosomes takes place. Phagosomes then undergo a series of fusion and fission events as they interact with the endolysosomal system and other organelles, in a dynamic process known as phagosome maturation. Because phagocytes play a key role in tissue patrolling and immune surveillance, phagosome maturation is associated with signaling pathways that link phagocytosis to antigen presentation and the development of adaptive immune responses. In addition, and depending on the nature of the cargo, phagosome integrity may be compromised, triggering additional cellular mechanisms including inflammation and autophagy. Upon completion of maturation, phagosomes enter a recently described phase: phagosome resolution, where catabolites from degraded cargo are metabolized, phagosomes are resorbed, and vesicles of phagosomal origin are recycled. Finally, phagocytes return to homeostasis and become ready for a new round of phagocytosis. Altogether, phagosome maturation and resolution encompass a series of dynamic events and organelle crosstalk that can be measured by biochemical, imaging, photoluminescence, cytometric, and immune‐based assays that will be described in this guide.

Regulation of phagosome functions by post-translational modifications: a new paradigm

Phagosomes are highly dynamic organelles formed by the uptake of particles through phagocytic innate immune cells such as macrophages. Their key roles in microbe elimination and antigen presentation make them essential for innate and adaptive immunity. However, phagosomes are also important for tissue homeostasis as even in healthy individuals billions of dead cells are phagocytosed each day. In this short review, we highlight how the use of latex beads as inert baits for phagocytosis and subsequent analysis by proteomics has changed our understanding of the phagosome. We further discuss recent data on post-translational modifications such as phosphorylation and ubiquitylation that regulate phagosome functions and demonstrate that the phagosome is not only a 'degradative organelle' but also serves as a subcellular signalling platform.

The Neuropeptides of Ocular Immune Privilege, α-MSH and NPY, Suppress Phagosome Maturation in Macrophages

The ocular microenvironment has evolutionarily adapted several mechanisms of immunosuppression to minimize the induction of inflammation. Neuropeptides produced by the retinal pigment epithelial cells regulate macrophage activity. Two neuropeptides, α-melanocyte–stimulating hormone (α -MSH) and neuropeptide Y (NPY), are constitutively expressed by the retinal pigment epithelial cells. Together these two neuropeptides induce anti-inflammatory cytokine production in endotoxin-stimulated macrophages and suppress phagocytosis of unopsonized bioparticles. These neuropeptides do not suppress the phagocytosis of opsonized bioparticles; however, they do suppress phagolysosome activation or formation. In this report, we studied the possibility that α-MSH with NPY suppress phagosome maturation within macrophages using opsonized OVA-coated magnetic beads to isolate and analyze the phagosomes. The magnetic bead–containing intercellular vesicles were isolated and assayed for Rab5, Rab7, LAMP1, Ia^d, and OVA. The macrophages cotreated with α-MSH and NPY were suppressed in Rab7 recruitment to the phagosome with suppression in LAMP1 expression but not in Ia^d expression. The results demonstrated that the α-MSH/NPY cotreatment suppressed phagosome maturation. In addition, the a-MSH/NPY–cotreated macrophages were suppressed in their ability to Ag stimulate CD4⁺ T cell proliferation. These results imply a potential mechanism of ocular immune privilege to divert Ag processing to prevent autoreactive effector T cells from binding their target cognate Ag within the ocular microenvironment.

Synaptotagmin XI regulates phagocytosis and cytokine secretion in macrophages

Synaptotagmins (Syts) are a group of type I membrane proteins that regulate vesicle docking and fusion in processes such as exocytosis and phagocytosis. All Syts possess a single transmembrane domain, and two conserved tandem Ca(2+)-binding C2 domains. However, Syts IV and XI possess a conserved serine in their C2A domain that precludes these Syts from binding Ca(2+) and phospholipids, and from mediating vesicle fusion. Given the importance of vesicular trafficking in macrophages, we investigated the role of Syt XI in cytokine secretion and phagocytosis. We demonstrated that Syt XI is expressed in murine macrophages, localized in recycling endosomes, lysosomes, and recruited to phagosomes. Syt XI had a direct effect on phagocytosis and on the secretion of TNF and IL-6. Whereas small interfering RNA-mediated knockdown of Syt XI potentiated secretion of these cytokines and particle uptake, overexpression of an Syt XI construct suppressed these processes. In addition, Syt XI knockdown led to decreased recruitment of gp91(phox) and lysosomal-associated membrane protein-1 to phagosomes, suggesting attenuated microbicidal activity. Remarkably, knockdown of Syt XI ensued in enhanced bacterial survival. Our data reveal a novel role for Syt XI as a regulator of cytokine secretion, particle uptake, and macrophage microbicidal activity.

Analysis of phagosomal proteomes: from latex-bead to bacterial phagosomes

Phagosomal proteome characterization has contributed significantly to the understanding of host-pathogen interaction and the mechanism of infectious diseases caused by intracellular bacteria. The latex bead-containing phagosome has been widely used as a model system to study phagosomal proteomes at a global level. In contrast, the study of bacteria-containing phagosomes at a similar level has just begun. A number of intracellular microbial species are studied for their proteomes during the invasion of a host, providing insight into their metabolic adaptation in host cells and interaction with host-cell antimicrobial environments. In this review, we attempt to summarize the most recent advancements in the proteomic study of microbial phagosomes, especially those originating from mouse or human cells. We also briefly describe the proteomics of latex bead-containing phagosomes because they are often used as model phagosomes for study. We provide descriptions on major biological and technological components in phagosomal proteome studies. We also discuss the role of phagosomal proteome study in the broader horizon of systems biology and the technological challenges in phagosomal proteome characterization.

Use of fluorescent probes to detect lipid signaling intermediates in macrophages

To fulfill their function in host defense, professional phagocytes such as neutrophils and macrophages exhibit the ability to ingest (phagocytose), kill, and dispose of pathogenic microorganisms. Recent studies have provided strong evidence for the importance of membrane lipids such as polyphosphoinositides in these processes. In turn, reversible phosphorylation events, involving protein and lipid kinases and phosphatases, regulate signaling pathways involving metabolism of membrane lipids. Our ability to study lipid signaling events has been greatly facilitated by the development of fluorescent molecular imaging techniques. In particular, the expression of recombinant fusions of derivatives of the jellyfish-derived green fluorescent proteins (GFP) coupled to reporter molecules enables real-time monitoring of signaling events in live cells. Here, we discuss methods to monitor alterations in membrane polyphosphoinositides involved in signaling events regulating phagocytosis. To illustrate the use of this technology, we will focus on the role of protein tyrosine phosphatase MEG2 in phagocytosis and its modulation by phosphatidylinositol-3,4,5-triphosphate (PIP3). This approach enables investigators to ascertain the involvement of lipid intermediates in diverse signaling pathways.

Effects of prostaglandin E2 on the subcellular localization of Epac-1 and Rap1 proteins during Fcgamma-receptor-mediated phagocytosis in alveolar macrophages

Recent studies have demonstrated a central role for the exchange protein activated by cAMP (Epac) in the inhibition of Fcgamma-receptor-mediated phagocytosis and bacterial killing by prostaglandin E(2) (PGE(2)) in macrophages. However, the subcellular localization of Epac, and its primary target Rap1, has yet to be determined in primary macrophages. Therefore, we used immunofluorescent techniques and phagosome isolation to localize Epac-1 and Rap1 in alveolar macrophages. Epac-1 was predominantly expressed on punctate and tubular membranes throughout the cell body; on the plasma membrane; and co-localized with microtubule organizing centers (MTOCs). Rap1 was abundant on punctate membranes, less abundant on plasma membrane, and also found on MTOCs. Following PGE(2) treatment, Epac-1, but not Rap1, accumulated on the nuclear envelope and disappeared from MTOCs. By immunofluorescent microscopy, both Epac-1 and Rap1 were seen to associate with phagosomes containing IgG-opsonized beads, but this association appeared weak, as we failed to observe such interactions in phagosomes isolated from cells at various time points after bead ingestion. Strikingly, however, Epac-1, but not Rap1, appeared to accumulate on maturing phagosomes, but only after PGE(2) treatment (or treatment with a selective Epac-1 agonist). This association was confirmed in isolated phagosome preparations. The changes in Epac-1 localization were too slow to account for the inhibitory effects of PGE(2) on phagocytosis. However, the appearance of Epac-1 on late phagosomes following PGE(2) treatment might be important for suppressing H(2)O(2) production and inhibiting the killing of intraphagosomal pathogens. The absence of Rap1 on late phagosomes suggests that the effect of Epac-1 might not require Rap1.

Prostaglandin E2 suppresses bacterial killing in alveolar macrophages by inhibiting NADPH oxidase

Prostaglandin E2 (PGE2) is a potent lipid mediator that effects changes in cell functions through ligation of four distinct G protein-coupled E prostanoid (EP) receptors (EP1-EP4). PGE2 inhibits bacterial killing and reactive oxygen intermediate (ROI) production by alveolar macrophages (AMs), although little is known about the operative molecular mechanisms. The aims of this study were to evaluate the molecular mechanisms and the specific EP receptors through which PGE2 inhibits killing of Klebsiella pneumoniae by AMs. The treatment of AMs with PGE2 suppressed the killing of K. pneumoniae, and this effect was blocked by an adenylyl cyclase inhibitor and mimicked by agonists for the stimulatory G protein (G(s))-coupled EP2 and EP4 receptors. Conversely, microbicidal activity was augmented by pretreatment with the cyclooxygenase inhibitor, indomethacin, and antagonists of EP2 and EP4. Similar results were found when ROI production was examined. PGE2 inhibition of killing and ROI generation was associated with its activation of the cAMP effectors, protein kinase A and exchange protein directly activated by cAMP-1, as well as attenuation of the phosphorylation and translocation of the reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase component, p47phox, to the phagosomal membrane. We conclude that PGE2 suppresses the microbicidal activity of AMs through the G(s)-coupled EP2/EP4 receptors, with increased cAMP inhibiting the assembly and activation of p47phox.

The phagosome: compartment with a license to kill

Phagosomes are fascinating subcellular structures. After all, there are only a few compartments that are born before our very eyes and whose development we can follow in a light microscope until their contents disintegrate and are completely absorbed. Yet, some phagosomes are taken advantage of by pathogenic microorganisms, which change their fate. Research into phagosome biogenesis has flourished in recent years - the purpose of this review is to give a glimpse of where this research stands, with emphasis on the cell biology of macrophage phagosomes, on new model organisms for the study of phagosome biogenesis and on intracellular pathogens and their interference with normal phagosome function.

Within the cell: analytical techniques for subcellular analysis

This review covers recent developments in the preparation, manipulation, and analyses of subcellular environments. In particular, it highlights approaches for (1) separation and detection of individual organelles, (2) preparation of ultra-pure organelle fractions, and (3) utilization of novel labeling strategies. These approaches, based on innovative technologies such as microfluidics, immunoisolation, mass spectrometry and electrophoresis, suggest that subcellular analyses will soon become as commonplace as single cell and bulk cellular assays.

A novel model system for characterization of phagosomal maturation, acidification, and intracellular collagen degradation in fibroblasts

Intracellular collagen degradation by fibroblasts is an important but poorly understood pathway for the physiological remodeling of mature connective tissues. The objective of this study was to determine whether gingival fibroblasts that express endogenous alpha(2)beta(1) integrin, the collagen receptor, would exhibit the cellular machinery required for phagosomal maturation and collagen degradation. There was a time-dependent increase of collagen bead internalization and a time-dependent decrease of bead-associated alpha(2)beta(1) integrin after initial bead binding. beta-Actin and gelsolin associated transiently with beads (0-30 min) followed by LAMP-2 (60-240 min) and cathepsin B (30-240 min). Cytochalasin D prevented phagosome formation and also prevented the sequential fusion of early endosomes with lysosomes. Collagen bead-associated pH was progressively reduced from 7.25 to 5.4, which was contemporaneous with progressive increases in degradation of bead-associated collagen (30-120 min). Concanamycin blocked acidification of phagolysosomes and collagen degradation but not phagosome maturation. Phagosomal acidification was partly dependent on elevated intracellular calcium. These studies demonstrate that the cellular machinery required for intracellular collagen degradation in fibroblasts closely resembles the vacuolar system in macrophages.

Reprogramming the phagocytic pathway--intracellular pathogens and their vacuoles (review)

Phagocytic immune cells (particularly macrophages and neutrophils) take up and digest particles that have invaded our bodies. In doing so, they represent a very early line of defence against a microbial attack. During uptake, the particles are wrapped by a portion of the phagocyte's plasma membrane, and a new endocytic compartment, the phagosome, is formed. The typical fate of a phagosome is its fusion with lysosomes to yield a phagolysosome in which the particle is digested. Recent data show that some 'intracellular microorganisms' that can cause severe illnesses (tuberculosis, leprosy, legionnaire's disease and others) manage to reprogramme the host phagocytes not to deliver them to the lysosomal compartment. This probably results in increased survival of the pathogens. The analysis of the composition of such 'novel' compartments and research on the molecular mechanisms underlying the microbial interference with host cell functions are likely to yield important insights into: (1) which endocytic/phagocytic compartments phagocytes employ to handle ingested material in general; (2) how some pathogenic microorganisms can reprogramme the phagocytic pathway; and possibly (3) how infections caused by these microorganisms can be treated more effectively. Here, some studies are presented analysing which compartments intracellular pathogens inhabit and how microbes might be able to reprogramme their host cells.

Immuno-isolation of highly purified peroxisomes using magnetic beads and continuous immunomagnetic sorting

Immuno-isolation is a powerful technique for the isolation of cells as well as subcellular organelle populations based on their antigenic properties. We have established a method for immuno-isolation of peroxisomes (PO) from both rat liver and the human hepatoblastoma cell line HepG2 using magnetic beads as solid support. A polyclonal antibody raised against the cytoplasmic C-terminal 10 amino acids of the rat 70 kDa peroxisomal membrane protein was covalently bound to magnetic beads (Dynabeads M-450). The coated beads were incubated with a light mitochondrial fraction and the organelle-bead complexes formed were separated by magnetic sorting in a free-flow system without pelleting the complexes during the isolation procedure. Scanning electron microscopy revealed decoration of beads with particles measuring 150-400 nm in diameter. The particles were identified as PO by catalase cytochemistry and biochemically by marker enzyme analysis, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) as well as immunoblotting for specific detection of peroxisomal matrix, core and membrane proteins. The functional significance of PO in man is emphasized by the existence of inherited diseases such as the Zellweger syndrome in which intact PO are lacking, but peroxisomal remnants called "ghosts" are observed instead. Peroxisomal disorders are usually studied using skin fibroblast cell lines derived from afflicted patients and immuno-magnetic separation may prove particularly useful for the investigation of such cultured cells and for further elucidation of the pathogenesis of fatal peroxisomal disorders.

Regulation of phagosomal acidification. Differential targeting of Na+/H+ exchangers, Na+/K+-ATPases, and vacuolar-type H+-atpases

Vacuolar-type (V) ATPases are thought to be the main determinant of phagosomal acidification. In phagosomes containing mycobacteria, which ostensibly impair the delivery of V-ATPases to the phagosomal membrane, the pH would be expected to be near neutral. This prediction was tested by microfluorescence ratio imaging using macrophages from mice susceptible to mycobacterial infection. Although less acidic than their counterparts containing dead bacteria, phagosomes containing live Mycobacteria bovis were nearly 1 pH unit more acidic than the cytosol, suggesting the existence of alternate H+ transport mechanisms. We therefore investigated whether Na+/H+ exchange (NHE) contributes to phagosomal acidification. Immunoblotting, reverse transcriptase-polymerase chain reaction, and pharmacological studies indicated that NHE1 is the predominant isoform of the exchanger in macrophages. Fractionation revealed that NHE1 is incorporated into the phagosomal membrane, and measurements of pH indicated that it is functional in this location. Nevertheless, acidification of the lumen of phagosomes containing either latex beads or live M. bovis was insensitive to (3-methylsulfonyl-4-piperidinobenzoyl)-guanidine methanesulfonate, a potent inhibitor of NHE1. This may have been due to the absence of an appropriate lumen to cytosol Na+ gradient, because the phagosomal membrane was found to be devoid of Na+/K+ pumps. Unexpectedly, the acidification of M. bovis phagosomes was fully reversed by specific inhibitors of the vacuolar H+-ATPase, suggesting that ATPases are present only transiently or in reduced quantities in the phagosomal membrane. Alternatively, acid equivalents accumulated in endosomes by V-ATPases may be delivered to the mycobacterial phagosome by carrier vesicles devoid of ATPases.

Coronin involved in phagocytosis: dynamics of particle-induced relocalization visualized by a green fluorescent protein Tag

Coronin is a protein involved in cell locomotion and cytokinesis of Dictyostelium discoideum. Here we show that coronin is strongly enriched in phagocytic cups formed in response to particle attachment. A fusion of coronin with green fluorescent protein (GFP) accumulates in the cups within less than 1 min upon attachment of a particle and is gradually released from the phagosome within 1 min after engulfment is completed. Phagocytic cup formation competes with leading edge formation and can be interrupted at any stage. When the cup regresses, coronin dissociates from the site of accumulation. TRITC-labeled yeast cells have been used to assay phagocytosis quantitatively in wild-type and coronin-null cells. In the mutant, the rate of uptake is reduced to about one third, which shows that coronin contributes to the efficiency of phagocytosis to about the same extent as it improves the speed of cell locomotion.