Biogenesis of phagolysosomes: the 'kiss and run' hypothesis

Bosutinib Stimulates Macrophage Survival, Phagocytosis, and Intracellular Killing of Bacteria

Host-acting compounds are emerging as potential alternatives to combating antibiotic resistance. Here, we show that bosutinib, an FDA-approved chemotherapeutic for treating chronic myelogenous leukemia, does not possess any antibiotic activity but enhances macrophage responses to bacterial infection. In vitro, bosutinib stimulates murine and human macrophages to kill bacteria more effectively. In a murine wound infection with vancomycin-resistant Enterococcus faecalis, a single intraperitoneal bosutinib injection or multiple topical applications on the wound reduce the bacterial load by approximately 10-fold, which is abolished by macrophage depletion. Mechanistically, bosutinib stimulates macrophage phagocytosis of bacteria by upregulating surface expression of bacterial uptake markers Dectin-1 and CD14 and promoting actin remodeling. Bosutinib also stimulates bacterial killing by elevating the intracellular levels of reactive oxygen species. Moreover, bosutinib drives NF-κB activation, which protects infected macrophages from dying. Other Src kinase inhibitors such as DMAT and tirbanibulin also upregulate expression of bacterial uptake markers in macrophages and enhance intracellular bacterial killing. Finally, cotreatment with bosutinib and mitoxantrone, another chemotherapeutic in clinical use, results in an additive effect on bacterial clearance in vitro and in vivo. These results show that bosutinib stimulates macrophage clearance of bacterial infections through multiple mechanisms and could be used to boost the host innate immunity to combat drug-resistant bacterial infections.

PF-431396 hydrate inhibition of kinase phosphorylation during adherent-invasive Escherichia coli infection inhibits intra-macrophage replication and inflammatory cytokine release

Adherent-invasive Escherichia coli (AIEC) have been implicated in the aetiology of Crohn's disease (CD). They are characterized by an ability to adhere to and invade intestinal epithelial cells, and to replicate intracellularly in macrophages resulting in inflammation. Proline-rich tyrosine kinase 2 (PYK2) has previously been identified as a risk locus for inflammatory bowel disease and a regulator of intestinal inflammation. It is overexpressed in patients with colorectal cancer, a major long-term complication of CD. Here we show that Pyk2 levels are significantly increased during AIEC infection of murine macrophages while the inhibitor PF-431396 hydrate, which blocks Pyk2 activation, significantly decreased intramacrophage AIEC numbers. Imaging flow cytometry indicated that Pyk2 inhibition blocked intramacrophage replication of AIEC with no change in the overall number of infected cells, but a significant reduction in bacterial burden per cell. This reduction in intracellular bacteria resulted in a 20-fold decrease in tumour necrosis factor α secretion by cells post-AIEC infection. These data demonstrate a key role for Pyk2 in modulating AIEC intracellular replication and associated inflammation and may provide a new avenue for future therapeutic intervention in CD.

Mapping Sphingolipid Metabolism Pathways during Phagosomal Maturation

Phagocytosis is an important physiological process, which, in higher organisms, is a means of fighting infections and clearing cellular debris. During phagocytosis, detrimental foreign particles (e.g. pathogens and apoptotic cells) are engulfed by phagocytes (e.g. macrophages), enclosed in membrane-bound vesicles called phagosomes, and transported to the lysosome for eventual detoxification. During this well-choreographed process, the nascent phagosome (also called early phagosome, EP) undergoes a series of spatiotemporally regulated changes in its protein and lipid composition and matures into a late phagosome (LP), which subsequently fuses with the lysosomal membrane to form the phagolysosome. While several elegant proteomic studies have identified the role of unique proteins during phagosomal maturation, the corresponding lipidomic studies are sparse. Recently, we reported a comparative lipidomic analysis between EPs and LPs and showed that ceramides are enriched on the LPs. Further, we found that this ceramide accumulation on LPs was orchestrated by ceramide synthase 2, inhibition of which hampers phagosomal maturation. Following up on this study, here, using biochemical assays, we first show that the increased ceramidase activity on EPs also significantly contributes to the accumulation of ceramides on LPs. Next, leveraging lipidomics, we show that de novo ceramide synthesis does not significantly contribute to the ceramide accumulation on LPs, while concomitant to increased ceramides, glucosylceramides are substantially elevated on LPs. We validate this interesting finding using biochemical assays and show that LPs indeed have heightened glucosylceramide synthase activity. Taken together, our studies provide interesting insights and possible new roles of sphingolipid metabolism during phagosomal maturation.

High Glucose Concentrations Impair the Processing and Presentation of Mycobacterium tuberculosis Antigens In Vitro

Type 2 diabetes is an established risk factor for tuberculosis, but the underlying mechanisms are largely unknown. We established an in vitro model to analyze the effect of high glucose concentrations in antigen processing and presentation in antigen-presenting cells. Human monocyte-derived macrophages (MDMs) were exposed to high (11 mM and 30 mM) and low (5.5 mM) glucose concentrations and infected with Mycobacterium tuberculosis (Mtb). Flow cytometry was used to analyze the effect of high glucose concentrations in histocompatibility complex (MHC) class II molecules (HLA-DR) and co-stimulatory molecules (CD80 and CD86), indispensable for an adequate antigenic presentation and CD4+ T cell activation. HLA-DR and CD86 were significantly decreased by high glucose concentrations compared with low glucose concentrations. Confocal microscopy was used to detect Rab 5 and Lamp-1, proteins involved in the kinetics of antigen processing as early markers, and Rab 7 and cathepsin D as late markers. We observed a delay in the dynamics of the acquisition of Rab 7 and cathepsin D in high glucose concentrations. Moreover, the kinetics of the formation M. tuberculosis peptide-MHC II complexes in MDMs was decreased under high glucose concentrations, reducing their capacity for T cell activation. These findings suggest that high glucose concentrations directly affect antigenic processing, and therefore antigenic presentation.

Molecular Mechanisms of Borrelia burgdorferi Phagocytosis and Intracellular Processing by Human Macrophages

Lyme disease is the most common vector-borne illness in North America and Europe. Its causative agents are spirochetes of the Borrelia burgdorferi sensu latu complex. Infection with borreliae can manifest in different tissues, most commonly in the skin and joints, but in severe cases also in the nervous systems and the heart. The immune response of the host is a crucial factor for preventing the development or progression of Lyme disease. Macrophages are part of the innate immune system and thus one of the first cells to encounter infecting borreliae. As professional phagocytes, they are capable of recognition, uptake, intracellular processing and final elimination of borreliae. This sequence of events involves the initial capture and internalization by actin-rich cellular protrusions, filopodia and coiling pseudopods. Uptake into phagosomes is followed by compaction of the elongated spirochetes and degradation in mature phagolysosomes. In this review, we discuss the current knowledge about the processes and molecular mechanisms involved in recognition, capturing, uptake and intracellular processing of Borrelia by human macrophages. Moreover, we highlight interactions between macrophages and other cells of the immune system during these processes and point out open questions in the intracellular processing of borreliae, which include potential escape strategies of Borrelia.

Better Together: Current Insights Into Phagosome-Lysosome Fusion

Following phagocytosis, the nascent phagosome undergoes maturation to become a phagolysosome with an acidic, hydrolytic, and often oxidative lumen that can efficiently kill and digest engulfed microbes, cells, and debris. The fusion of phagosomes with lysosomes is a principal driver of phagosomal maturation and is targeted by several adapted intracellular pathogens. Impairment of this process has significant consequences for microbial infection, tissue inflammation, the onset of adaptive immunity, and disease. Given the importance of phagosome-lysosome fusion to phagocyte function and the many virulence factors that target it, it is unsurprising that multiple molecular pathways have evolved to mediate this essential process. While the full range of these pathways has yet to be fully characterized, several pathways involving proteins such as members of the Rab GTPases, tethering factors and SNAREs have been identified. Here, we summarize the current state of knowledge to clarify the ambiguities in the field and construct a more comprehensive phagolysosome formation model. Lastly, we discuss how other cellular pathways help support phagolysosome biogenesis and, consequently, phagocyte function.

Lysophosphatidylcholine Enhances Bactericidal Activity by Promoting Phagosome Maturation via the Activation of the NF-κB Pathway during Salmonella Infection in Mouse Macrophages

Salmonella enterica serovar Typhimurium (S. Typhimurium) is a facultative intracellular pathogen that causes salmonellosis and mortality worldwide. S. Typhimurium infects macrophages and survives within phagosomes by avoiding the phagosome-lysosome fusion system. Phagosomes sequentially acquire different Rab GTPases during maturation and eventually fuse with acidic lysosomes. Lysophosphatidylcholine (LPC) is a bioactive lipid that is associated with the generation of chemoattractants and reactive oxygen species (ROS). In our previous study, LPC controlled the intracellular growth of Mycobacterium tuberculosis by promoting phagosome maturation. In this study, to verify whether LPC enhances phagosome maturation and regulates the intracellular growth of S. Typhimurium, macrophages were infected with S. Typhimurium. LPC decreased the intracellular bacterial burden, but it did not induce cytotoxicity in S. Typhimuriuminfected cells. In addition, combined administration of LPC and antibiotic significantly reduced the bacterial burden in the spleen and the liver. The ratios of the colocalization of intracellular S. Typhimurium with phagosome maturation markers, such as early endosome antigen 1 (EEA1) and lysosome-associated membrane protein 1 (LAMP-1), were significantly increased in LPC-treated cells. The expression level of cleaved cathepsin D was rapidly increased in LPCtreated cells during S. Typhimurium infection. Treatment with LPC enhanced ROS production, but it did not affect nitric oxide production in S. Typhimurium-infected cells. LPC also rapidly triggered the phosphorylation of IκBα during S. Typhimurium infection. These results suggest that LPC can improve phagosome maturation via ROS-induced activation of NF-κB pathway and thus may be developed as a therapeutic agent to control S. Typhimurium growth.

Formation and Maturation of the Phagosome: A Key Mechanism in Innate Immunity against Intracellular Bacterial Infection

Phagocytosis is an essential mechanism in innate immune defense, and in maintaining homeostasis to eliminate apoptotic cells or microbes, such as Mycobacterium tuberculosis, Salmonella enterica, Streptococcus pyogenes and Legionella pneumophila. After internalizing microbial pathogens via phagocytosis, phagosomes undergo a series of ‘maturation’ steps, to form an increasingly acidified compartment and subsequently fuse with the lysosome to develop into phagolysosomes and effectively eliminate the invading pathogens. Through this mechanism, phagocytes, including macrophages, neutrophils and dendritic cells, are involved in the processing of microbial pathogens and antigen presentation to T cells to initiate adaptive immune responses. Therefore, phagocytosis plays a role in the bridge between innate and adaptive immunity. However, intracellular bacteria have evolved diverse strategies to survive and replicate within hosts. In this review, we describe the sequential stages in the phagocytosis process. We also discuss the immune evasion strategies used by pathogens to regulate phagosome maturation during intracellular bacterial infection, and indicate that these might be used for the development of potential therapeutic strategies for infectious diseases.

The macrophage microtubule network acts as a key cellular controller of the intracellular fate of Leishmania infantum

The parasitophorous vacuoles (PVs) that insulate Leishmania spp. in host macrophages are vacuolar compartments wherein promastigote forms differentiate into amastigote that are the replicative form of the parasite and are also more resistant to host responses. We revisited the biogenesis of tight-fitting PVs that insulate L. infantum in promastigote-infected macrophage-like RAW 264.7 cells by time-dependent confocal laser multidimensional imaging analysis. Pharmacological disassembly of the cellular microtubule network and silencing of the dynein gene led to an impaired interaction of L. infantum-containing phagosomes with late endosomes and lysosomes, resulting in the tight-fitting parasite-containing phagosomes never transforming into mature PVs. Analysis of the shape of the L. infantum parasite within PVs, showed that factors that impair promastigote-amastigote differentiation can also result in PVs whose maturation is arrested. These findings highlight the importance of the MT-dependent interaction of L. infantum-containing phagosomes with the host macrophage endolysosomal pathway to secure the intracellular fate of the parasite.

Kinetoplastid parasites of the genus Leishmania are responsible for a diverse spectrum of mammalian infectious diseases, the leishmaniases, including cutaneous, mucocutaneous, and mucosal pathologies. Infectious metacyclic promastigotes of infected female Phlebotomus sandflies are injected into the host at the site of the bite during the sandfly blood meal, after which they are internalized by host professional phagocytic neutrophils and macrophages. Leishmania infantum is an etiological agent of potentially fatal visceral pathology. This study molecularly dissects the maturation of L. infantum-containing phagosomes/parasitophorous vacuoles (PVs) in host macrophages. We reveal the requirement of vacuolar movement along macrophage microtubule tracks for the phagosome trafficking toward the endolysosomal pathway necessary for the development of the mature tight-fitting PV crucial for L. infantum survival and proliferation.

Phagocytosis: Our Current Understanding of a Universal Biological Process

Phagocytosis is a cellular process for ingesting and eliminating particles larger than 0.5 μm in diameter, including microorganisms, foreign substances, and apoptotic cells. Phagocytosis is found in many types of cells and it is, in consequence an essential process for tissue homeostasis. However, only specialized cells termed professional phagocytes accomplish phagocytosis with high efficiency. Macrophages, neutrophils, monocytes, dendritic cells, and osteoclasts are among these dedicated cells. These professional phagocytes express several phagocytic receptors that activate signaling pathways resulting in phagocytosis. The process of phagocytosis involves several phases: i) detection of the particle to be ingested, ii) activation of the internalization process, iii) formation of a specialized vacuole called phagosome, and iv) maturation of the phagosome to transform it into a phagolysosome. In this review, we present a general view of our current understanding on cells, phagocytic receptors and phases involved in phagocytosis.

Understanding the early host immune response against Mycobacterium tuberculosis

Generation of immune response is a crucial activity of host defense against any microbial attack. When facultative organism Mycobacterium tuberculosis (MTB) invades its host, various pathways are activated in the host to mount immune responses against invading pathogen for nullifying its actions. During this host-pathogen interaction, interplay of complex network of cytokines and chemokines, initiation of phagocytosis, and formation of granuloma play an important role in containing MTB infections at host side. Simultaneously, MTB also evolves a plethora of specialized mechanisms to evade the host’s killing cascades on other side, and during this bilateral cross-talk, many mycobacterial products play crucial role in survival of MTB inside the host. Hence, a better understanding of these phenomena is necessary not only for getting clear picture of pathogenesis of MTB, but also for developing effective, preventive, and therapeutic modalities against the pathogen. With some suggestions on future work, an insight into diversity of immune response of host against MTB was provided in the present review.

Hydroalcoholic extract from Origanum vulgare induces a combined anti-mycobacterial and anti-inflammatory response in innate immune cells

In nature, many plants or their extracted compounds have been found to possess anti-inflammatory features and therapeutic properties against infectious as well as non-infectious diseases, including cancer. In this study, we analysed the immunomodulatory effects on innate immune cells of hydroalcoholic extract from Origanum vulgare L. ssp. hirtum (HyE-Ov), a plant traditionally known for its anti-oxidative properties. The effects of HyE-Ov were tested on human monocyte derived dendritic cells (DC), type-1 (M1) and type-2 macrophages (M2) infected with M. bovis Bacille Calmette-Guérin (BCG), used as a model of persistent intracellular bacterium. DC, M1 and M2 treated with HyE-Ov significantly enhanced their mycobactericidal activity, which was associated with phagosomal acidification in M1 and M2 and increase of phagosomal, but not mitochondrial ROS production in M1, M2, and DC. Treatment of BCG-infected DC with HyE-Ov significantly reduced TNF-α and IL-12 production and increased TGF-β synthesis. Finally, experiments were repeated using eight different HPLC fractions of HyE-Ov. Results showed that the capability to activate anti-microbial and anti-inflammatory response is shared by different fractions, suggesting that diverse bioactive molecules are present within the hydroalcoholic extract. Altogether, these results show that HyE-Ov promotes anti-mycobacterial innate immunity and limits inflammatory response in vitro and suggest that this plant extract may be exploitable as phytocomplex or nutraceutical for novel host-directed therapeutic approaches.

The phagosome and redox control of antigen processing

In addition to debris clearance and antimicrobial function, versatile organelles known as phagosomes play an essential role in the processing of exogenous antigen in antigen presenting cells. While there has been much attention on human leukocyte antigen haplotypes in the determination of antigenic peptide repertoires, the lumenal biochemistries within phagosomes and endosomes are emerging as equally-important determinants of peptide epitope composition and immunodominance. Recently, the lumenal redox microenvironment within these degradative compartments has been shown to impact two key antigenic processing chemistries: proteolysis by lysosomal cysteine proteases and disulfide reduction of protein antigens. Through manipulation of the balance between oxidative and reductive capacities in the phagosome-principally by modulating NADPH oxidase (NOX2) and γ-interferon-inducible lysosomal thiol reductase (GILT) activities-studies have demonstrated changes to antigen processing patterns leading to modified repertoires of antigenic peptides available for presentation, and subsequently, altered disease progression in T cell-driven autoimmunity. This review focuses on the mechanisms and consequences of redox-mediated phagosomal antigen processing, and the potential downstream implications to tolerance and autoimmunity.

Defective Phagocytic Properties of HIV-Infected Macrophages: How Might They Be Implicated in the Development of Invasive Salmonella Typhimurium?

Human immunodeficiency virus type 1 (HIV-1) infects and kills T cells, profoundly damaging the host-specific immune response. The virus also integrates into memory T cells and long-lived macrophages, establishing chronic infections. HIV-1 infection impairs the functions of macrophages both in vivo and in vitro, which contributes to the development of opportunistic diseases. Non-typhoidal Salmonella enterica serovar Typhimurium has been identified as the most common cause of bacterial bloodstream infections in HIV-infected adults. In this review, we report how the functions of macrophages are impaired post HIV infection; introduce what makes invasive Salmonella Typhimurium specific for its pathogenesis; and finally, we discuss why these bacteria may be particularly adapted to the HIV-infected host.

Phagosomal transport depends strongly on phagosome size

Macrophages internalize pathogens for intracellular degradation. An important part of this process is the phagosomal transport from the cell periphery to the perinuclear region. Biochemical factors are known to influence the fate of phagosomes. Here, we show that the size of phagosomes also has a strong influence on their transport. We found that large phagosomes are transported persistently to the nucleus, whereas small phagosomes show strong bidirectional transport. We show that dynein motors play a larger role in the transport of large phagosomes, whereas actin filament-based motility plays a larger role in the transport of small phagosomes. Furthermore, we investigated the spatial distribution of dyneins and microtubules around phagosomes and hypothesize that dynein and microtubule density differences between the nucleus-facing side of phagosomes and the opposite side could explain part of the observed transport characteristics. Our findings suggest that a size-dependent cellular sorting mechanism might exist that supports macrophages in their immunological roles.

Control of Phagocytosis by Microbial Pathogens

Phagocytosis is a fundamental process of cells to capture and ingest foreign particles. Small unicellular organisms such as free-living amoeba use this process to acquire food. In pluricellular organisms, phagocytosis is a universal phenomenon that all cells are able to perform (including epithelial, endothelial, fibroblasts, etc.), but some specialized cells (such as neutrophils and macrophages) perform this very efficiently and were therefore named professional phagocytes by Rabinovitch. Cells use phagocytosis to capture and clear all particles larger than 0.5 µm, including pathogenic microorganisms and cellular debris. Phagocytosis involves a series of steps from recognition of the target particle, ingestion of it in a phagosome (phagocytic vacuole), maturation of this phagosome into a phagolysosome, to the final destruction of the ingested particle in the robust antimicrobial environment of the phagolysosome. For the most part, phagocytosis is an efficient process that eliminates invading pathogens and helps maintaining homeostasis. However, several pathogens have also evolved different strategies to prevent phagocytosis from proceeding in a normal way. These pathogens have a clear advantage to perpetuate the infection and continue their replication. Here, we present an overview of the phagocytic process with emphasis on the antimicrobial elements professional phagocytes use. We also summarize the current knowledge on the microbial strategies different pathogens use to prevent phagocytosis either at the level of ingestion, phagosome formation, and maturation, and even complete escape from phagosomes.

Intracellular fate of Ureaplasma parvum entrapped by host cellular autophagy

Genital mycoplasmas, including Ureaplasma spp., are among the smallest human pathogenic bacteria and are associated with preterm birth. Electron microscopic observation of U. parvum showed that these prokaryotes have a regular, spherical shape with a mean diameter of 146 nm. U. parvum was internalized into HeLa cells by clathrin‐mediated endocytosis and survived for at least 14 days around the perinuclear region. Intracellular U. parvum reached endosomes in HeLa cells labeled with EEA1, Rab7, and LAMP‐1 within 1 to 3 hr. After 3 hr of infection, U. parvum induced the cytosolic accumulation of galectin‐3 and was subsequently entrapped by the autophagy marker LC3. However, when using atg7^−/−MEF cells, autophagy was inadequate for the complete elimination of U. parvum in HeLa cells. U. parvum also colocalized with the recycling endosome marker Rab11. Furthermore, the exosomes purified from infected HeLa cell culture medium included U. parvum. In these purified exosomes ureaplasma lipoprotein multiple banded antigen, host cellular annexin A2, CD9, and CD63 were detected. This research has successfully shown that Ureaplasma spp. utilize the host cellular membrane compartments possibly to evade the host immune system.

Growth factor signaling to mTORC1 by amino acid-laden macropinosomes

The rapid activation of the mechanistic target of rapamycin complex-1 (mTORC1) by growth factors is increased by extracellular amino acids through yet-undefined mechanisms of amino acid transfer into endolysosomes. Because the endocytic process of macropinocytosis concentrates extracellular solutes into endolysosomes and is increased in cells stimulated by growth factors or tumor-promoting phorbol esters, we analyzed its role in amino acid-dependent activation of mTORC1. Here, we show that growth factor-dependent activation of mTORC1 by amino acids, but not glucose, requires macropinocytosis. In murine bone marrow-derived macrophages and murine embryonic fibroblasts stimulated with their cognate growth factors or with phorbol myristate acetate, activation of mTORC1 required an Akt-independent vesicular pathway of amino acid delivery into endolysosomes, mediated by the actin cytoskeleton. Macropinocytosis delivered small, fluorescent fluid-phase solutes into endolysosomes sufficiently fast to explain growth factor-mediated signaling by amino acids. Therefore, the amino acid-laden macropinosome is an essential and discrete unit of growth factor receptor signaling to mTORC1.

The small GTPase Rab7 as a central regulator of hepatocellular lipophagy

Autophagy is a central mechanism by which hepatocytes catabolize lipid droplets (LDs). Currently, the regulatory mechanisms that control this important process are poorly defined. The small guanosine triphosphatase (GTPase) Rab7 has been implicated in the late endocytic pathway and is known to associate with LDs, although its role in LD breakdown has not been tested. In this study, we demonstrate that Rab7 is indispensable for LD breakdown ("lipophagy") in hepatocytes subjected to nutrient deprivation. Importantly, Rab7 is dramatically activated in cells placed under nutrient stress; this activation is required for the trafficking of both multivesicular bodies and lysosomes to the LD surface during lipophagy, resulting in the formation of a lipophagic "synapse." Depletion of Rab7 leads to gross morphological changes of multivesicular bodies, lysosomes, and autophagosomes, consequently leading to attenuation of hepatocellular lipophagy.

CONCLUSION

These findings provide additional support for the role of autophagy in hepatocellular LD catabolism while implicating the small GTPase Rab7 as a key regulatory component of this essential process.

Macrophages phagocytose nonopsonized silica particles using a unique microtubule-dependent pathway

Cells can take up particles by both opsonized and nonopsonized pathways. Silica and latex, but not zymosan, can be taken up by the nonopsonized pathway. Uptake of silica, but not latex, is toxic to macrophages. Nonopsonized phagocytosis is characterized and found to have key differences from the complement- and antibody-opsonized pathways.

Silica inhalation leads to the development of the chronic lung disease silicosis. Macrophages are killed by uptake of nonopsonized silica particles, and this is believed to play a critical role in the etiology of silicosis. However, the mechanism of nonopsonized-particle uptake is not well understood. We compared the molecular events associated with nonopsonized- and opsonized-particle phagocytosis. Both Rac and RhoA GTPases are activated upon nonopsonized-particle exposure, whereas opsonized particles activate either Rac or RhoA. All types of particles quickly generate a PI(3,4,5)P₃ and F-actin response at the particle attachment site. After formation of a phagosome, the events related to endolysosome-to-phagosome fusion do not significantly differ between the pathways. Inhibitors of tyrosine kinases, actin polymerization, and the phosphatidylinositol cascade prevent opsonized- and nonopsonized-particle uptake similarly. Inhibition of silica particle uptake prevents silica-induced cell death. Microtubule depolymerization abolished uptake of complement-opsonized and nonopsonized particles but not Ab-opsonized particles. Of interest, regrowth of microtubules allowed uptake of new nonopsonized particles but not ones bound to cells in the absence of microtubules. Although complement-mediated uptake requires macrophages to be PMA-primed, untreated cells phagocytose nonopsonized silica and latex. Thus it appears that nonopsonized-particle uptake is accomplished by a pathway with unique characteristics.

Lysosomal storage disorders and iron

Lysosomal storage disorders are a group of about 50 rare metabolic diseases that result from defects in lysosomal function. The majority is recessively inherited and caused by mutations in genes encoding lysosomal proteins as the basis for its pathobiology. The lysosome plays a pivotal role in a cell's ability to recycle and degrade unwanted material. One of its functions relates to regulating iron levels throughout the body. Iron is a double-edged sword: It is absolutely required for an organism's survival, but high levels of iron quickly lead to cell death. In addition, recent results have put the lysosome on the map of pathways leading to common neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. It is plausible that the mechanisms through which the lysosome acts in these diseases also involve iron and this would have significant implications in our understanding of the molecular etiology of these disorders.

Secreted acid phosphatase (SapM) of Mycobacterium tuberculosis is indispensable for arresting phagosomal maturation and growth of the pathogen in guinea pig tissues

Tuberculosis (TB) is responsible for nearly 1.4 million deaths globally every year and continues to remain a serious threat to human health. The problem is further complicated by the growing incidence of multidrug-resistant TB (MDR-TB) and extensively drug-resistant TB (XDR-TB), emphasizing the need for the development of new drugs against this disease. Phagosomal maturation arrest is an important strategy employed by Mycobacterium tuberculosis to evade the host immune system. Secretory acid phosphatase (SapM) of M.tuberculosis is known to dephosphorylate phosphotidylinositol 3-phosphate (PI3P) present on phagosomes. However, there have been divergent reports on the involvement of SapM in phagosomal maturation arrest in mycobacteria. This study was aimed at reascertaining the involvement of SapM in phagosomal maturation arrest in M.tuberculosis. Further, for the first time, we have also studied whether SapM is essential for the pathogenesis of M.tuberculosis. By deleting the sapM gene of M.tuberculosis, we demonstrate that MtbΔsapM is defective in the arrest of phagosomal maturation as well as for growth in human THP-1 macrophages. We further show that MtbΔsapM is severely attenuated for growth in the lungs and spleen of guinea pigs and has a significantly reduced ability to cause pathological damage in the host when compared with the parental strain. Also, the guinea pigs infected with MtbΔsapM exhibited a significantly enhanced survival when compared with M.tuberculosis infected animals. The importance of SapM in phagosomal maturation arrest as well as in the pathogenesis of M.tuberculosis establishes it as an attractive target for the development of new therapeutic molecules against tuberculosis.

Deciphering the intracellular fate of Propionibacterium acnes in macrophages

Propionibacterium acnes is a Gram-positive bacterium that colonizes various niches of the human body, particularly the sebaceous follicles of the skin. Over the last years a role of this common skin bacterium as an opportunistic pathogen has been explored. Persistence of P. acnes in host tissue has been associated with chronic inflammation and disease development, for example, in prostate pathologies. This study investigated the intracellular fate of P. acnes in macrophages after phagocytosis. In a mouse model of P. acnes-induced chronic prostatic inflammation, the bacterium could be detected in prostate-infiltrating macrophages at 2 weeks postinfection. Further studies performed in the human macrophage cell line THP-1 revealed intracellular survival and persistence of P. acnes but no intracellular replication or escape from the host cell. Confocal analyses of phagosome acidification and maturation were performed. Acidification of P. acnes-containing phagosomes was observed at 6 h postinfection but then lost again, indicative of cytosolic escape of P. acnes or intraphagosomal pH neutralization. No colocalization with the lysosomal markers LAMP1 and cathepsin D was observed, implying that the P. acnes-containing phagosome does not fuse with lysosomes. Our findings give first insights into the intracellular fate of P. acnes; its persistency is likely to be important for the development of P. acnes-associated inflammatory diseases.

A Drosophila model to image phagosome maturation

Phagocytosis involves the internalization of extracellular material by invagination of the plasma membrane to form intracellular vesicles called phagosomes, which have functions that include pathogen degradation. The degradative properties of phagosomes are thought to be conferred by sequential fusion with endosomes and lysosomes; however, this maturation process has not been studied in vivo. We employed Drosophila hemocytes, which are similar to mammalian professional macrophages, to establish a model of phagosome maturation. Adult Drosophila females, carrying transgenic Rab7-GFP endosome and Lamp1-GFP lysosome markers, were injected with E. coli DH5α and the hemocytes were collected at 15, 30, 45 and 60 minutes after infection. In wild-type females, E. coli were detected within enlarged Rab7-GFP positive phagosomes at 15 to 45 minutes after infection; and were also observed in enlarged Lamp1-GFP positive phagolysosomes at 45 minutes. Two-photon imaging of hemocytes in vivo confirmed this vesicle morphology, including enlargement of Rab7-GFP and Lamp1-GFP structures that often appeared to protrude from hemocytes. The interaction of endosomes and lysosomes with E. coli phagosomes observed in Drosophila hemocytes was consistent with that previously described for phagosome maturation in human ex vivo macrophages. We also tested our model as a tool for genetic analysis using 14-3-3ε mutants, and demonstrated altered phagosome maturation with delayed E. coli internalization, trafficking and/or degradation. These findings demonstrate that Drosophila hemocytes provide an appropriate, genetically amenable, model for analyzing phagosome maturation ex vivo and in vivo.

Isolation of pathogen-containing vacuoles

Dictyostelium discoideum cells are "professional phagocytes," as they ingest a large variety of bacteria, yeast, and inert particles. Several bacterial pathogens are able to survive intracellularly within specialized vacuoles of D. discoideum by interfering with host signaling pathways. To better understand the molecular mechanisms underlying these evolutionary conserved processes we have established a method for the isolation of pathogen-containing vacuoles (PCVs). The isolation protocol describes the infection of D. discoideum cells with the intracellular pathogen Legionella pneumophila, loading of the lysosomal compartment with colloidal iron, mechanical lysis of host cells, iodophenylnitrophenyltetrazolium (INT) heavy labeling of mitochondria, removal of nucleic acid by Benzonase treatment, separation of nuclei by low-speed centrifugation, and the magnetic removal of lysosomes. The subcellular fractionation in a discontinuous sucrose density OptiPrep gradient allows the separation of mitochondria and to prepare PCVs with high purity. The proteins isolated from PCVs have been successfully subjected to mass spectrometry and allowed to analyze pathogen-directed maturation processes of vacuoles. The method can also be applied for subsequent protein modification analyses and lipidome comparisons.

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.

Size-dependent mechanism of cargo sorting during lysosome-phagosome fusion is controlled by Rab34

Phagosome maturation is an essential part of the innate and adaptive immune response. Although it is well established that several Ras-related proteins in brain (Rab) proteins become associated to phagosomes, little is known about how these phagosomal Rab proteins influence phagosome maturation. Here, we show a specific role for Rab34 and mammalian uncoordinated 13-2 (Munc13-2) in phagolysosome biogenesis and cargo delivery. Rab34 knockdown impaired the fusion of phagosomes with late endosomes/lysosomes and high levels of active Rab34 promoted this process. We demonstrate that Rab34 enhances phagosome maturation independently of Rab7 and coordinates phagolysosome biogenesis through size-selective transfer of late endosomal/lysosomal cargo into phagosomes. More importantly, we show that Rab34 mediates phagosome maturation through the recruitment of the protein Munc13-2. Finally, we report that the alternative maturation pathway controlled by Rab34 is critical for mycobacterial killing because Rab34 silencing resulted in mycobacterial survival, and Rab34 expression led to mycobacterial killing. Altogether, our studies uncover Rab34/Munc13-2 as a critical part of an alternative Rab7-independent phagosome maturation machinery and lysosome-mediated killing of mycobacteria.

The role of dendritic cells in Mycobacterium tuberculosis infection

The immune response against Mycobacterium tuberculosis is multifactorial, involving a network of innate and adaptive immune responses. Characterization of the immune response, a clear understanding of the dynamics and interplay of different arms of the immune response are critical to allow the development of better tools for combating tuberculosis. Dendritic cells (DCs) are one of the key cells in bridging innate and adaptive immune response through their significant role in capturing, processing and presenting antigens. The outcome of interaction of M. tuberculosis with DCs is not fully understood and the available reports are contradictory were some findings reported that DCs strengthen the cellular immune response against mycobacterium infection whereas others reported M. tuberculosis impairs the function of DCs were infected DCs are poor stimulators of M. tuberculosis Ag-specific CD4 T cells. Other studies showed that the outcome depends on M. tuberculosis strain type and type of receptor on DCs during recognition. In this review I shall highlight the recent findings in the outcome of interaction of Mycobacterium tuberculosis with DCs.

The protein tyrosine phosphatase SHP-1 regulates phagolysosome biogenesis

The process of phagocytosis and phagosome maturation involves the recruitment of effector proteins that participate in phagosome formation and in the acidification and/or fusion with various endocytic vesicles. In the current study, we investigated the role of the Src homology region 2 domain-containing phosphatase 1 (SHP-1) in phagolysosome biogenesis. To this end, we used immortalized bone marrow macrophages derived from SHP-1-deficient motheaten mice and their wild-type littermates. We found that SHP-1 is recruited early and remains present on phagosomes for up to 4 h postphagocytosis. Using confocal immunofluorescence microscopy and Western blot analyses on purified phagosome extracts, we observed an impaired recruitment of lysosomal-associated membrane protein 1 in SHP-1-deficient macrophages. Moreover, Western blot analyses revealed that whereas the 51-kDa procathepsin D is recruited to phagosomes, it is not processed into the 46-kDa cathepsin D in the absence of SHP-1, suggesting a defect in acidification. Using the lysosomotropic agent LysoTracker as an indicator of phagosomal pH, we obtained evidence that in the absence of SHP-1, phagosome acidification was impaired. Taken together, these results are consistent with a role for SHP-1 in the regulation of signaling or membrane fusion events involved in phagolysosome biogenesis.

Bacteria and protozoa differentially modulate the expression of Rab proteins

Phagocytic cells represent an important line of innate defense against microorganisms. Uptake of microorganisms by these cells involves the formation of a phagosome that matures by fusing with endocytic compartments, resulting in killing of the enclosed microbe. Small GTPases of the Rab family are key regulators of vesicular trafficking in the endocytic pathway. Intracellular pathogens can interfere with the function of these proteins in order to subvert host immune responses. However, it is unknown if this subversion can be achieved through the modulation of Rab gene expression. We compared the expression level of 23 distinct Rab GTPases in mouse macrophages after infection with the protozoan Plasmodium berghei, and the bacteria Escherichia coli and Salmonella enterica. We found that P. berghei induces an increase in the expression of a different set of Rab genes than E. coli and S. enterica, which behaved similarly. Strikingly, when one of the Rab proteins whose expression was increased by P. berghei, namely Rab14, was silenced, we observed a significant increase in the phagocytosis of P. berghei, whereas Rab14 overexpression led to a decrease in phagocytosis. This suggests that the parasite might induce the increase of Rab14 expression for its own advantage. Similarly, when Rab9a, whose expression was increased by E. coli and S. enterica, was silenced, we observed an increase in the phagocytosis of both bacterial species, whereas Rab9a overexpression caused a reduction in phagocytosis. This further suggests that the modulation of Rab gene expression could represent a mechanism of immune evasion. Thus, our study analyzes the modulation of Rab gene expression induced by bacteria and protozoa and suggests that this modulation could be necessary for the success of microbial infection.

Receptor-mediated phagocytosis of Leishmania: implications for intracellular survival

The extracellular promastigote stage of Leishmania spp. is transmitted to mammals by a sand fly vector. Leishmania promastigotes ligate host macrophage receptors, triggering phagocytosis and subsequent internalization, a crucial step for survival. Parasites transform intracellularly to the amastigote stage. Many studies document different receptors detecting promastigotes and amastigotes, but the relative importance of each interaction is ill-defined. Recent studies suggest that the macrophage receptors utilized during phagocytosis impact the intracellular fate of the parasite. This review summarizes the receptors implicated in Leishmania phagocytosis over the past 30 years. It then proceeds to weigh the evidence for or against their potential roles in intracellular parasite trafficking.

Innate immune activation and subversion of Mammalian functions by leishmania lipophosphoglycan

Leishmania promastigotes express several prominent glycoconjugates, either secreted or anchored to the parasite surface. Of these lipophosphoglycan (LPG) is the most abundant, and along with other phosphoglycan-bearing molecules, plays important roles in parasite infectivity and pathogenesis in both the sand fly and the mammalian host. Besides its contribution for parasite survival in the sand fly vector, LPG is important for modulation the host immune responses to favor the establishment of mammalian infection. This review will summarize the current knowledge regarding the role of LPG in Leishmania infectivity, focusing on the interaction of LPG and innate immune cells and in the subversion of mammalian functions by this molecule.

Inside or outside the phagosome? The controversy of the intracellular localization of Mycobacterium tuberculosis

The localization of Mycobacterium tuberculosis (Mtb) inside the macrophage has been a matter of debate in recent years. Upon inhalation, the bacterium is taken up into macrophage phagosomes, which are manipulated by the bacterium. Subsequent translocation of the bacilli into the cytosol has been observed by several groups, while others fail to observe this phenomenon. Here, we review the available literature in favour of and against this idea, and scrutinize the existing data on how human macrophages control Mtb infection, relating this to the robustness of the host cell. We conclude that both phagosomal maturation inhibition and escape from the phagosome are part of the greater infection strategy of Mtb. The balance between the host cell and the infecting bacterium is an important factor in determining the outcome of infection as well as whether phagosomal escape occurs and can be captured.

Frustrated phagocytosis on micro-patterned immune complexes to characterize lysosome movements in live macrophages

Lysosome mobilization is a key cellular process in phagocytes for bactericidal activities and trans-matrix migration. The molecular mechanisms that regulate lysosome mobilization are still poorly known. Lysosomes are hard to track as they move toward phagosomes throughout the cell volume. In order to anticipate cell regions where lysosomes are recruited to, human and RAW264.7 macrophages were seeded on surfaces that were micro-patterned with immune complexes (ICs) as 4 μm-side squares. Distances between IC patterns were adapted to optimize cell spreading in order to constrain lysosome movements mostly in two dimensions. FcΓ receptors triggered local frustrated phagocytosis, frustrated phagosomes appeared as rings of F-actin dots around the IC patterns as early as 5 min after cells made contact with the substratum. Frustrated phagosomes recruited actin-associated proteins (vinculin, paxillin, and gelsolin). The fusion of lysosomes with frustrated phagosomes was shown by the release of beta-hexosaminidase and the recruitment of Lamp1 to frustrated phagosomes. Lysosomes of RAW264.7 macrophages were labeled with cathepsin-D-mCherry to visualize their movements toward frustrated phagosomes. Lysosomes saltatory movements were markedly slowed down compared to cells layered on non-opsonized patterns. In addition, the linearity of the trajectories and the frequency and duration of contacts of lysosomes with frustrated phagosomes were measured. Our experimental set-up is the first step toward deciphering molecular mechanisms which are involved in lysosome movements in the cytoplasm (speed, directionality, and interaction with phagosomes), and opens the door to approaches such as RNA interference, pharmacological inhibition, or mutant expression.

Latex beads internalization and quantitative proteomics join forces to decipher the endosomal proteome

The proteome analysis of endocytic compartments has been constrained by the limited purity of the organelle fractions obtained by current biochemical methods. Duclos and coworkers have developed a novel method to isolate highly purified endosomal organelles based on small latex beads internalization followed by gradient centrifugation and successfully combined it with a redundant peptide counting method to compare the relative abundance of proteins in organelles. The presence of bona fide markers in their respective subcellular organelles and the identification of several new endosomal-associated proteins, attested the applicability of their combinatory approach. Future applications of this strategy may deliver a comprehensive endosomal proteome chart: from the identification of the key players to the determination of time and signaling-dependent proteome changes. As a long-term perspective, such an approach may unveil new clues to the molecular mechanisms underlining human diseases associated with endosomal biogenesis defects.

n-3 Fatty acids uniquely affect anti-microbial resistance and immune cell plasma membrane organization

It is now well established that dietary lipids are incorporated into macrophage and T-cell membrane microdomains, altering their structure and function. Within cell membranes, there are specific detergent-resistant domains in which key signal transduction proteins are localized. These regions are classified as "lipid rafts". Rafts are composed mostly of cholesterol and sphingolipids and therefore do not integrate well into the fluid phospholipid bilayers causing them to form microdomains. Upon cell activation, rafts compartmentalize signal-transducing molecules, thus providing an environment conducive to signal transduction. In this review, we discuss recent novel data describing the effects of n-3 PUFA on alterations in the activation and functions of macrophages and T-cells. We believe that the modifications in these two disparate immune cell types are linked by fundamentally similar changes in membrane lipid composition and transmembrane signaling functions. We conclude that the outcomes of n-3 PUFA-mediated immune cell alterations may be beneficial (e.g., anti-inflammatory) or detrimental (e.g., loss of microbial immunity) depending upon the cell type interrogated.

Tuberculosis: new aspects of an old disease

Tuberculosis is an ancient infectious disease that remains a threat for public health around the world. Although the etiological agent as well as tuberculosis pathogenesis is well known, the molecular mechanisms underlying the host defense to the bacilli remain elusive. In this paper we focus on the innate immunity of this disease reviewing well-established and consensual mechanisms like Mycobacterium tuberculosis interference with phagosome maturation, less consensual mechanism like nitric oxide production, and new mechanisms, such as mycobacteria translocation to the cytosol, autophagy, and apoptosis/necrosis proposed mainly during the last decade.

Particulate vaccines: on the quest for optimal delivery and immune response

Subunit vaccines offer a safer alternative to traditional organism-based vaccines, but their immunogenicity is impaired. This hurdle might be overcome by the use of micro- and nanodelivery systems carrying the antigen(s). This review discusses the rationale for the use of particulate vaccines and provides an overview of antigen-delivery vehicles currently under investigation. It further highlights the cellular uptake, antigen processing and the presentation by antigen-presenting cells because these processes are partially governed by particle characteristics and eventually determine the immunological outcome. Finally, we address the attractive concept of concomitant delivery of antigens and immunopotentiators. The condensed knowledge could be an asset for rationally designing antigen-delivery vehicles to obtain safe and efficacious vaccines.

Conidial Dihydroxynaphthalene Melanin of the Human Pathogenic Fungus Aspergillus fumigatus Interferes with the Host Endocytosis Pathway

Aspergillus fumigatus is the most important air-borne fungal pathogen of humans. The interaction of the pathogen with the host's immune system represents a key process to understand pathogenicity. For elimination of invading microorganisms, they need to be efficiently phagocytosed and located in acidified phagolysosomes. However, as shown previously, A. fumigatus is able to manipulate the formation of functional phagolysosomes. Here, we demonstrate that in contrast to pigmentless pksP mutant conidia of A. fumigatus, the gray-green wild-type conidia inhibit the acidification of phagolysosomes of alveolar macrophages, monocyte-derived macrophages, and human neutrophil granulocytes. Therefore, this inhibition is independent of the cell type and applies to the major immune effector cells required for defense against A. fumigatus. Studies with melanin ghosts indicate that the inhibitory effect of wild-type conidia is due to their dihydroxynaphthalene (DHN)-melanin covering the conidia, whereas the hydrophobin RodA rodlet layer plays no role in this process. This is also supported by the observation that pksP conidia still exhibit the RodA hydrophobin layer, as shown by scanning electron microscopy. Mutants defective in different steps of the DHN-melanin biosynthesis showed stronger inhibition than pksP mutant conidia but lower inhibition than wild-type conidia. Moreover, A. fumigatus and A. flavus led to a stronger inhibition of phagolysosomal acidification than A. nidulans and A. terreus. These data indicate that a certain type of DHN-melanin that is different in the various Aspergillus species, is required for maximal inhibition of phagolysosomal acidification. Finally, we identified the vacuolar ATPase (vATPase) as potential target for A. fumigatus based on the finding that addition of bafilomycin which inhibits vATPase, led to complete inhibition of the acidification whereas the fusion of phagosomes containing wild-type conidia and lysosomes was not affected.

Role of the (Mn)superoxide dismutase of Enterococcus faecalis in the in vitro interaction with microglia

Enterococcus faecalis is a significant human pathogen worldwide and is responsible for severe nosocomial and community-acquired infections. Although enterococcal meningitis is rare, mortality is considerable, reaching 21 %. Nevertheless, the pathogenetic mechanisms of this infection remain poorly understood, even though the ability of E. faecalis to avoid or survive phagocytic attack in vivo may be very important during the infection process. We previously showed that the manganese-cofactored superoxide dismutase (MnSOD) SodA of E. faecalis was implicated in oxidative stress responses and, interestingly, in the survival within mouse peritoneal macrophages using an in vivo-in vitro infection model. In the present study, we investigated the role of MnSOD in the interaction of E. faecalis with microglia, the brain-resident macrophages. By using an in vitro infection model, murine microglial cells were challenged in parallel with the wild-type strain JH2-2 and its isogenic sodA deletion mutant. While both strains were phagocytosed by microglia efficiently and to a similar extent, the ΔsodA mutant was found to be significantly more susceptible to microglial killing than JH2-2, as assessed by the antimicrobial protection assay. In addition, a significantly higher percentage of acidic ΔsodA-containing phagosomes was found and these also underwent enhanced maturation as determined by the expression of endolysosomal markers. In conclusion, these results show that the MnSOD of E. faecalis contributes to survival of the bacterium in microglial cells by influencing their antimicrobial activity, and this could even be important for intracellular killing in neutrophils and thus for E. faecalis pathogenesis.

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.

Ezrin promotes actin assembly at the phagosome membrane and regulates phago-lysosomal fusion

Phagosome maturation is defined as the process by which phagosomes fuse sequentially with endosomes and lysosomes to acquire an acidic pH and hydrolases that degrade ingested particles. While the essential role of actin cytoskeleton remodeling during particle internalization is well established, its role during the later stages of phagosome maturation remains largely unknown. We have previously shown that purified mature phagosomes assemble F-actin at their membrane, and that the ezrin-radixin-moesin (ERM) proteins ezrin and moesin participate in this process. Moreover, we provided evidence that actin assembly on purified phagosomes stimulates their fusion with late endocytic compartments in vitro. In this study, we further investigated the role of ezrin in phagosome maturation. We engineered a structurally open form of ezrin and demonstrated that ezrin binds directly to the actin assembly promoting factor N-WASP (Neural Wiskott-Aldrich Syndrome Protein) by its FERM domain. Using a cell-free system, we found that ezrin stimulates F-actin assembly on purified phagosomes by recruiting the N-WASP-Arp2/3 machinery. Accordingly, we showed that the down-regulation of ezrin activity in macrophages by a dominant-negative approach caused reduced F-actin accumulation on maturing phagosomes. Furthermore, using fluorescence and electron microscopy, we found that ezrin is required for the efficient fusion between phagosomes and lysosomes. Live-cell imaging analysis supported the notion that ezrin is necessary for the fusogenic process itself, promoting the transfer of the lysosome content into the phagosomal lumen.

Kinesin 5B is necessary for delivery of membrane and receptors during FcγR-mediated phagocytosis

FcγR-mediated phagocytosis is a cellular event that is evolutionary conserved to digest IgG-opsonized pathogens. Pseudopod formation during phagocytosis is a limiting step in managing the uptake of particles, and in this paper, we show that the conventional kinesin is involved in both receptor and membrane delivery to the phagocytic cup. Expression of a mutant kinesin isoform (GFP dominant negative mutant of kinesin H chain [EGFP-Kif5B-DN]) in RAW264.7 cells significantly reduced binding of IgG-sheep RBCs when macrophages were faced with multiple encounters with opsonized particles. Scanning electron microscopy analysis of EGFP-Kif5B-DN-expressing cells challenged with two rounds of IgG-sheep RBCs showed sparse, extremely thin pseudopods. We saw disrupted Rab11 trafficking to the phagocytic cup in EGFP-Kif5B-DN-transfected cells. Our particle overload assays also implicated phagosome membrane recycling in pseudopod formation. We observed reduced phagosome fission and trafficking in mutant kinesin-expressing cells, as well as reduced cell surface expression of FcγRs and Mac-1 receptors. In conclusion, anterograde trafficking via kinesin is essential for both receptor recycling from the phagosome and delivery of Rab11-containing membrane stores to effect broad and functional pseudopods during FcγR-mediated phagocytosis.