Phagocytosis of Leishmania donovani amastigotes is Rac1 dependent and occurs in the absence of NADPH oxidase activation

Macrophages produce little superoxide during phagocytosis of Leishmania donovani amastigotes. In this study, we characterized molecular events associated with L. donovani amastigotes uptake by mouse macrophages, to further define the mechanisms by which they are internalized without triggering superoxide production. Using transient transfections, we first showed that internalization of L. donovani amastigotes is mediated by the GTPases Rac1 and Arf6, of which Rac1 is recruited and retained on parasite-containing phagosomes. Next, we showed that, whereas internalization of amastigotes induced no superoxide release, co-internalization of serum-opsonized zymozan and amastigotes resulted in superoxide production. Furthermore, in co-internalization experiments, we detected superoxide production in over 95% of phagosomes containing IgG-opsonized SRBC compared to 5% of amastigote-harboring phagosomes. These results suggest that amastigotes evade the ability of macrophages to produce superoxide during phagocytosis. Consistently, we observed that amastigotes induced barely detectable phosphorylation of the NADPH oxidase component p47phox, leading to a defective phagosomal recruitment of p67phox and p47phox. Finally, we showed that amastigotes disrupt phagosomal lipid raft integrity, potentially interfering with NADPH oxidase assembly. Collectively, our results indicate that internalization of L. donovani amastigotes is a Rac1- and Arf6-dependent process that occurs in the absence of significant NADPH oxidase activation.     

Importance of spatial activation of Cdc42 and rac small G proteins by frabin for microspike formation in MDCK cells

BACKGROUND: Frabin is an actin filament (F-actin)-binding protein that shows GDP/GTP exchange activity for Cdc42 small G protein (Cdc42). Frabin furthermore induces indirect activation of Rac small G protein (Rac) in intact cells. We have recently shown that in nonepithelial cells, frabin induces the formation of both filopodia- and lamellipodia-like processes through the activation of Cdc42 and Rac, respectively. In epithelial cells such as MDCK cells, Cdc42 and Rac regulate cell-cell adherens junctions (AJs) via the accumulation of F-actin and E-cadherin, although neither Cdc42 nor Rac induces the formation of filopodia or lamellipodia. In this study, we have examined the effects of frabin on the reorganization of the actin cytoskeleton in MDCK cells. RESULTS: Frabin induces the formation of microspikes at the basal area of the lateral membranes through the activation of Cdc42 and Rac in MDCK cells, although a dominant active mutant of Cdc42 or Rac alone, or both, did not induce the formation of microspikes. Furthermore, frabin weakly increased the accumulation of F-actin and E-cadherin at cell-cell AJs and the formation of stress fibres through the activation of Cdc42 and Rac, under conditions where the dominant active mutant of Cdc42 or Rac markedly showed these effects. The Cdc42- and Rac-induced formation of stress fibres was dependent on the activation of Rho small G protein. CONCLUSION: These results indicate that the frabin-dependent spatial activation of Cdc42 and Rac is important for the formation of microspikes.     

Proteomic analysis of Legionella-containing phagosomes isolated from Dictyostelium

Legionella pneumophila, the agent of Legionnaires’ disease, replicates intracellularly within specialized phagosomes of human macrophages and amoebae. In this study, we have developed a protocol for the isolation of Legionella-containing phagosomes from Dictyostelium discoideum. Cell fractionation, two-dimensional gel electrophoresis and MALDI-TOF MS combined with genomic data identified 157 phagosome host proteins. In addition to proteins with an evident role in phagosome maturation, we identified proteins for which a function remains to be elucidated. Possible interactions of coronin with cytosolic NADPH oxidase components and protein kinase C inhibitors which together may lead to an inhibition of phagosomal superoxide generation are discussed. Comparative proteomics of phagosomes containing highly virulent L. pneumophila Corby versus less virulent L. hackeliae revealed distinctive protein expression patterns, e.g., an abundance of RhoGDI in L. hackeliae degrading phagosomes versus little RhoGDI in L. pneumophila Corby replicative phagosomes. We present a kinetic dissection of phagosome maturation including the complex alterations of the phagosome protein composition. A reference flow chart suggests so far unrecognized consequences of infection for host cell physiology, actin degradation on phagosomes, and a putative cysteine proteinase inhibitor interference with lysosomal enzyme sorting and activation processes.     

The regulation of phagosome maturation in Dictyostelium

Macropinocytosis (fluid uptake) and phagocytosis (particle uptake) are processes that result in the formation of intracellular membrane enclosed vacuoles termed macropinosomes and phagosomes, respectively. Macropinosomes and phagosomes are modified by fission and fusion reactions with the endo-lysosomal pathway that eventually transform these vacuoles into a lysosomal environment. Many human bacterial pathogens, including species of Mycobacteria, Legionella, and Chlamydia, are thought to survive by disrupting the normal membrane trafficking events that usually result in the formation of phago-lysosomes and death of the microorganism. In addition, a number of important pathogens facilitate homotypic phagosome fusion in order to generate an intracellular environment conducive for survival. A greater understanding of the regulation of phagosomal maturation and fusion will be critical in designing new therapies to treat infections caused by intracellular pathogens. The genetically tractable phagocyte, D. discoideum, has proven extremely useful in dissecting the signaling pathways regulating macropinocytosis, phagocytosis, phagosomal maturation and phagosome–phagosome fusion. A body of knowledge has accumulated and demonstrates important roles for Rab GTPases, the cytoskeleton, phosphoinositide metabolism and pH regulation in regulating phagosome maturation. This review will summarize the current state of knowledge.     

The Mycobacterium tuberculosis SecA2 system subverts phagosome maturation to promote growth in macrophages

The ability of Mycobacterium tuberculosis to grow in macrophages is critical to the virulence of this important pathogen. One way M. tuberculosis is thought to maintain a hospitable niche in macrophages is by arresting the normal process of phagosomes maturing into acidified phagolysosomes. The process of phagosome maturation arrest by M. tuberculosis is not fully understood, and there has remained a need to firmly establish a requirement for phagosome maturation arrest for M. tuberculosis growth in macrophages. Other intracellular pathogens that control the phagosomal environment use specialized protein export systems to deliver effectors of phagosome trafficking to the host cell. In M. tuberculosis, the accessory SecA2 system is a specialized protein export system that is required for intracellular growth in macrophages. In studying the importance of the SecA2 system in macrophages, we discovered that SecA2 is required for phagosome maturation arrest. Shortly after infection, phagosomes containing a ΔsecA2 mutant of M. tuberculosis were more acidified and showed greater association with markers of late endosomes than phagosomes containing wild-type M. tuberculosis. We further showed that inhibitors of phagosome acidification rescued the intracellular growth defect of the ΔsecA2 mutant, which demonstrated that the phagosome maturation arrest defect of the ΔsecA2 mutant is responsible for the intracellular growth defect. This study demonstrates the importance of phagosome maturation arrest for M. tuberculosis growth in macrophages, and it suggests there are effectors of phagosome maturation that are exported into the host environment by the accessory SecA2 system.     

Modulation of phagolysosome biogenesis by the lipophosphoglycan of Leishmania

Promastigotes of the protozoan parasite Leishmania are inoculated into the mammalian host by an infected sandfly and are phagocytosed by macrophages. There, they differentiate into amastigotes, which replicate in phagolysosomes. A family of glycoconjugates, the phosphoglycans (PGs), plays an important role in the ability of promastigotes to survive the potentially microbicidal consequences of phagocytosis. Lipophosphoglycan (LPG), an abundant promastigote surface glycolipid, has received considerable attention over the past several years. Of interest for this review, lipophosphoglycan confers upon Leishmania donovani promastigotes the ability to inhibit phagolysosome biogenesis. This inhibition correlates with an accumulation of periphagosomal F-actin, which may potentially form a physical barrier that prevents L. donovani promastigote-harboring phagosomes from interacting with late endosomes and lysosomes. Thus, similar to several other pathogens, Leishmania promastigotes hijack the host cell's cytoskeleton early during the infection process. Here, we review this phenomenon and discuss the potential underlying mechanisms.     

The early phagosomal stage of Francisella tularensis determines optimal phagosomal escape and Francisella pathogenicity island protein expression

Francisella tularensis is an intracellular pathogen that can survive and replicate within macrophages. Following phagocytosis and transient interactions with the endocytic pathway, F. tularensis rapidly escapes from its original phagosome into the macrophage cytoplasm, where it eventually replicates. To examine the importance of the nascent phagosome for the Francisella intracellular cycle, we have characterized early trafficking events of the F. tularensis subsp. tularensis strain Schu S4 in a murine bone marrow-derived macrophage model. Here we show that early phagosomes containing Schu S4 transiently interact with early and late endosomes and become acidified before the onset of phagosomal disruption. Inhibition of endosomal acidification with the vacuolar ATPase inhibitor bafilomycin A1 or concanamycin A prior to infection significantly delayed but did not block phagosomal escape and cytosolic replication, indicating that maturation of the early Francisella-containing phagosome (FCP) is important for optimal phagosomal escape and subsequent intracellular growth. Further, Francisella pathogenicity island (FPI) protein expression was induced during early intracellular trafficking events. Although inhibition of endosomal acidification mimicked the early phagosomal escape defects caused by mutation of the FPI-encoded IglCD proteins, it did not inhibit the intracellular induction of FPI proteins, demonstrating that this response is independent of phagosomal pH. Altogether, these results demonstrate that early phagosomal maturation is required for optimal phagosomal escape and that the early FCP provides cues other than intravacuolar pH that determine intracellular induction of FPI proteins.     

Association of frabin with specific actin and membrane structures

BACKGROUND: Frabin is an actin filament (F-actin)-binding protein with GDP/GTP exchange activity specific for Cdc42 small G protein. Expression of frabin forms filopodia-like microspikes through the direct activation of Cdc42, and lamellipodia through indirect activation of Rac small G protein. Frabin consists of the F-actin-binding domain (FAB), the Dbl homology domain (DH), the first pleckstrin homology domain (PH1), the FYVE-finger domain (FYVE), the second PH domain (PH2) from the N-terminus in this order. Although DH and PH1 show exchange activity, FAB, in addition to DH and PH1, is required for the formation of microspikes, whereas FYVE and PH2, in addition to DH and PH1, are required for the formation of lamellipodia. RESULTS: Various truncated mutants of frabin were co-expressed with a dominant active mutant (DA) of Cdc42, Rac1DA, or full-length frabin in L fibroblasts. FAB was recruited to the Cdc42DA-formed filopodia-like microspikes. FAB and a fragment containing DH, PH1, FYVE and PH2 were recruited to the Rac1DA-formed membrane ruffles. Furthermore, each of these fragments served as a dominant negative mutant of frabin when co-expressed with full-length frabin, and inhibited the full-length frabin-formed morphological changes. CONCLUSION: These results suggest that frabin recognizes a specific actin structure(s) through FAB and a specific membrane structure(s) through FAB and the region containing DH, PH1, FYVE and PH2. It is likely that frabin associates with the specific actin and membrane structures and activates Cdc42 and Rac in the vicinity of these structures, eventually leading to morphological changes.     

Stable accumulation of p67phox at the phagosomal membrane and ROS production within the phagosome

Production of ROS by the leukocyte NADPH oxidase is essential for the destruction of pathogenic bacteria inside phagosomes. The enzyme is a complex of cytosolic and membranous subunits that need to assemble upon activation. Biochemical data suggest that the complex is renewed continuously during activity. Furthermore, it is generally assumed that complex assembly and activity occur in parallel. However, information about the oxidase assembly in individual phagosomes in live cells is scarce. We studied the dynamic behavior of the crucial cytosolic NADPH oxidase component p67(phox) during phagocytosis by videomicroscopy. p67(phox) is involved in the regulation of electron flow from NADPH to oxygen, leading to superoxide radical formation inside the phagosome. p67(phox)-citrine, expressed in myeloid PLB-985 cells, accumulated at the phagosomal membrane during phagocytosis of yeast particles. Using photobleaching techniques (FRAP, FLIP), we demonstrated that p67(phox)-citrine diffused freely in this phagosomal membrane, but the phagosomal pool of p67(phox)-citrine did not exchange with the cytosolic pool. This result suggests that once assembled in the NADPH oxidase complex, p67(phox) is stable in this complex. Furthermore, the time of the presence of p67(phox)-citrine at the phagosome increased substantially in the presence of complement in the opsonizing serum compared with decomplemented serum. PI(3)P also accumulated around phagosomes for twice as long in the presence of complement. The presence of p67(phox)-citrine was correlated with the duration of phagosomal ROS production in different opsonization conditions. These data support the critical role of p67(phox) for ROS production on the level of individual phagosomes.     

Role of small GTPases in Trypanosoma cruzi invasion in MDCK cell lines

Trypanosoma cruzi can modulate a large number of host intracellular responses during its invasion. GTPases such as RhoA, Rac1 and Cdc42 are examples of molecules that could be activated at this moment and trigger changes in the pattern of F-actin cytoskeleton leading to the formation of structures like stress fibers, lamellipodium and fillopodium, respectively. Here we investigate the role of these GTPases in the cytoskeletal rearrangement of MDCK cell transfectants expressing variants of RhoA, Rac1 and Cdc42 during T. cruzi infection. The adhesion, internalization and the survival rate were determined. Rac1 mutants showed the higher adhesion and internalization indexes but the lower survival index after 48 h of infection. Confocal laser scanning microscopy showed changes in the pattern of F-actin distribution and reorganization at the site of trypomastigote invasion. These observations suggest that these GTPases act in the signaling mechanisms that affect the F-actin cytoskeleton during T. cruzi invasion.     

Yersinia pseudotuberculosis blocks the phagosomal acidification of B10.A mouse macrophages through the inhibition of vacuolar H(+)-ATPase activity.

Yersinia pseudotuberculosis survived and multiplied in the phagosomes of B10.A mouse peritoneal macrophages. As one of the possible mechanisms for the bacteria's survival in the phagosomes, we demonstrated that live Y. pseudotuberculosis inhibited the phagosomal acidification; pH within phagosomes containing the live Y. pseudotuberculosis remained at about 6.0, whereas pH within phagosomes containing the dead Y. pseudotuberculosis fell to about 4. 5. This ability to inhibit intraphagosomal acidification was also shared by mutants lacking the 42 Md virulence plasmid, indicating that it is chromosomally encoded. The phagosomes containing dead bacteria raised the pH to 6.2 after the treatment of their macrophages with an inhibitor (bafilomycin A1) specific for V-ATPase. Although the amount of V-ATPase in the A and B subunits on the phagosomes was not significantly different between the live and dead bacteria infection, the phagosomes containing live bacteria had a 10-fold smaller V-ATPase activity than those containing the dead bacteria. These results indicated that the inhibition of phagosomal acidification by Y. pseudotuberculosis infection was due to the attenuation of V-ATPase activity, and not due to the exclusion of V-ATPase subunits from the phagosome membrane as found in Mycobacterium avium.     

Involvement of Cdc42 and Rac small G proteins in invadopodia formation of RPMI7951 cells

BACKGROUND: Invadopodia are membrane protrusions into the extracellular matrix by aggressive tumour cells. These structures are associated with sites of matrix degradation and invasiveness of malignant tumour cells in an in vitro fibronectin degradation/invasion assay. The Rho family small G proteins, consisting of the Rho, Rac and Cdc42 subfamilies, are implicated in various cell functions, such as cell shape change, adhesion, and motility, through reorganization of the actin cytoskeleton. We studied the roles of the Rho family small G proteins in invadopodia formation. RESULTS: We first demonstrated that invadopodia of RPMI7951 human melanoma cells extended into the matrix substratum on a vertical view using a laser scanning confocal microscope system. We confirmed that invadopodia were rich in actin filaments (F-actin) and visualized clearly with F-actin staining on a vertical view as well as on a horizontal view. We then studied the roles of Rho, Rac, and Cdc42 in invasiveness of the same cell line. In the in vitro fibronectin degradation/invasion assay, a dominant active mutant of Cdc42 enhanced dot-like degradation, whereas a dominant active mutant of Rac enhanced diffuse-type degradation. Furthermore, frabin, a GDP/GTP exchange protein for Cdc42 with F-actin-binding activity, enhanced both dot-like and diffuse-type degradation. However, a dominant active mutant of Rho did not affect the fibronectin degradation. Moreover, inhibition of phosphatidylinositol-3 kinase (PI3K) disrupted the Rac and Cdc42-dependent actin structures and blocked the fibronectin degradation. CONCLUSION: These results suggest that Cdc42 and Rac play important roles in fibronectin degradation and invasiveness in a coordinate manner through the frabin-Cdc42/Rac-PI3K signalling pathway.     

Virulent and avirulent strains of Francisella tularensis prevent acidification and maturation of their phagosomes and escape into the cytoplasm in human macrophages

Francisella tularensis, the agent of tularemia, is an intracellular pathogen, but little is known about the compartment in which it resides in human macrophages. We have examined the interaction of a recent virulent clinical isolate of F. tularensis subsp. tularensis and the live vaccine strain with human macrophages by immunoelectron and confocal immunofluorescence microscopy. We assessed the maturation of the F. tularensis phagosome by examining its acquisition of the lysosome-associated membrane glycoproteins (LAMPs) CD63 and LAMP1 and the acid hydrolase cathepsin D. Two to four hours after infection, vacuoles containing live F. tularensis cells acquired abundant staining for LAMPs but little or no staining for cathepsin D. However, after 4 h, the colocalization of LAMPs with live F. tularensis organisms declined dramatically. In contrast, vacuoles containing formalin-killed bacteria exhibited intense staining for all of these late endosomal/lysosomal markers at all time points examined (1 to 16 h). We examined the pH of the vacuoles 3 to 4 h after infection by quantitative immunogold staining and by fluorescence staining for lysosomotropic agents. Whereas phagosomes containing killed bacteria stained intensely for these agents, indicating a marked acidification of the phagosomes (pH 5.5), phagosomes containing live F. tularensis did not concentrate these markers and thus were not appreciably acidified (pH 6.7). An ultrastructural analysis of the F. tularensis compartment revealed that during the first 4 h after uptake, the majority of F. tularensis bacteria reside within phagosomes with identifiable membranes. The cytoplasmic side of the membranes of approximately 50% of these phagosomes was coated with densely staining fibrils of approximately 30 nm in length. In many cases, these coated phagosomal membranes appeared to bud, vesiculate, and fragment. By 8 h after infection, the majority of live F. tularensis bacteria lacked any ultrastructurally discernible membrane separating them from the host cell cytoplasm. These results indicate that F. tularensis initially enters a nonacidified phagosome with LAMPs but without cathepsin D and that the phagosomal membrane subsequently becomes morphologically disrupted, allowing the bacteria to gain direct access to the macrophagic cytoplasm. The capacity of F. tularensis to alter the maturation of its phagosome and to enter the cytoplasm is likely an important element of its capacity to parasitize macrophages and has major implications for vaccine development.     

Phagosomal processing of Mycobacterium tuberculosis antigen 85B is modulated independently of mycobacterial viability and phagosome maturation

Control of Mycobacterium tuberculosis infection requires CD4 T-cell responses and major histocompatibility complex class II (MHC-II) processing of M. tuberculosis antigens (Ags). We have previously demonstrated that macrophages process heat-killed (HK) M. tuberculosis more efficiently than live M. tuberculosis. These observations suggested that live M. tuberculosis may inhibit Ag processing by inhibiting phagosome maturation or that HK M. tuberculosis may be less resistant to Ag processing. In the present study we examined the correlation between M. tuberculosis viability and phagosome maturation and efficiency of Ag processing. Since heat treatment could render M. tuberculosis Ags more accessible to proteolysis, M. tuberculosis was additionally killed by antibiotic treatment and radiation. Processing of HK, live, radiation-killed (RadK), or rifampin-killed (RifK) M. tuberculosis in activated murine bone marrow macrophages was examined by using an I-A(b)-restricted T-cell hybridoma cell line (BB7) that recognizes an epitope derived from Ag 85B. Macrophages processed HK M. tuberculosis more rapidly and efficiently than they processed live, RadK, or RifK M. tuberculosis. Live, RadK, and RifK M. tuberculosis cells were processed with similar efficiencies for presentation to BB7 T hybridoma cells. Furthermore, phagosomes containing live or RadK M. tuberculosis expressed fewer M. tuberculosis peptide-MHC-II complexes than phagosomes containing HK M. tuberculosis expressed. Since only live M. tuberculosis was able to prevent acidification of the phagosome, our results suggest that regulation of phagosome maturation does not explain the differences in processing of different forms of M. tuberculosis. These findings suggest that the mechanisms used by M. tuberculosis to inhibit phagosomal maturation differ from the mechanisms involved in modulating phagosome Ag processing.     

Membrane retrieval in neutrophils during phagocytosis: inhibition by M protein-expressing S. pyogenes bacteria

During phagocytosis and phagosome maturation, complex membrane traffic events must be coordinated. We have observed, using fluorescent fluid-phase and membrane markers, that in the human neutrophil, internalization of nonopsonized, Gram-positive bacteria, but not of latex beads, is accompanied by a rapid and localized formation of pinosomal structures. This pinocytic response is calcium-dependent but insensitive to actin cytoskeleton disruption and wortmannin treatment. Contrary to what we observe, endosomal structures usually are considered to participate in phagosome formation by providing necessary membrane to forming phagosomes. Instead, our results show a coupling between neutrophil secretory and membrane-retrieval processes during phagosome maturation, and we suggest that the observed, localized pinocytic response is linked to the secretion of azurophilic granules toward nascent phagosomes. Accordingly, M and M-like protein-expressing Streptococcus pyogenes bacteria, which are able to survive inside neutrophil phagosomes, inhibit both the secretion of azurophilic granules to phagosomes and pinosome formation.     

A method to purify bacteria-containing phagosomes from infected macrophages

When small particles, such as microorganisms, are taken up by macrophages, they are wrapped with a portion of the host cell plasma membrane and ingested, creating a new organelle, the phagosome. This phagosome matures stepwise as newly formed endosomes do, finally forming a phagolysosome, a process that contributes to killing of ingested microbes and to the presentation of microbial antigens on the surface of the phagocyte. Some pathogenic bacteria, however, reprogramme the phagocytic cell in such a way that the phagosome will either be arrested in an early stage of maturation or will be diverted and create an unusual, novel phagosomal compartment. To study the molecular processes that underly biogenesis of bacteria-containing phagosomes, we have established a method to isolate and to biochemically analyse bacteria-containing phagosomes. This method consists of mechanical lysis of infected macrophages, production of a postnuclear supernatant followed by fractionation in a discontinuous sucrose density gradient, separation through a Ficoll cushion, and by a final concentration step. These phagosome preparations contain very little endosomal or lysosomal contamination (the organelles of most concern when studying phagosome biogenesis) and very little Golgi- and plasma membrane-derived contamination, but do contain some mitochondrial and ER contamination. This method could also be used to study bacterial factors (proteins, RNA) produced while in phagosomes.     

Phagosome maturation proceeds independently of stimulation of toll-like receptors 2 and 4

Toll-like receptors modulate many aspects of the innate immune response. Recent reports suggest that the maturation of phagosomes following particle uptake is modulated through signaling of Toll-like receptors. In the current study, the kinetics of phagosome maturation was evaluated quantitatively by ratio fluorometry to determine the lumenal pH of the phagosomes and a FRET-based technique to determine the degree of phagosome/lysosome fusion. Profiles generated for phagosomes containing experimental particles with or without the TLR ligands Pam3Cys-Ser-(Lys)4 or LPS failed to reveal a difference in maturation despite activating TLR-signaling pathways. Moreover, while macrophages defective in individual TLRs generated phagosome maturation profiles identical to wild-type macrophages, MyD88-deficient macrophages exhibited a marked depression in phagosome/lysosome fusion that appears independent of short-term TLR-mediated effects. The results demonstrate that the rate of maturation of phagosomes proceeds independently of TLR signaling pathways.     

RNA interference reveals a role for TLR2 and TLR3 in the recognition of Leishmania donovani promastigotes by interferon-gamma-primed macrophages

Leishmania donovani promastigotes evade the induction of a proinflammatory response during their invasion of naive macrophages. However, their entry into IFN-gamma-primed macrophages is accompanied by the secretion of nitric oxide (NO) and proinflammatory cytokines. In the present study, we addressed the hypothesis that priming with IFN-gamma induces the expression of a receptor that enables mouse macrophages to recognize L. donovani promastigotes. We observed that in IFN-gamma-primed macrophages, L. donovani promastigotes stimulated Interleukin-1 receptor-associated kinase-1 (IRAK-1) activity. We next showed that Toll-like receptor (TLR)3 is barely detectable in naive macrophages but is expressed in IFN-gamma-treated macrophages. Silencing of TLR3, TLR2, IRAK-1 and myeloid differentiation factor 88 (MyD88) expression by RNA interference revealed that both TLR are involved in the secretion of NO and TNF-alpha induced by L. donovani promastigotes. Using L. donovani mutants, we showed that TLR2-mediated responses are dependent on Galbeta1,4Manalpha-PO(4)-containing phosphoglycans, whereas TLR3-mediated responses are independent of these glycoconjugates. Furthermore, our data indicate a participation of TLR2 and TLR3 in the phagocytosis of L. donovani promastigotes and a role for TLR3 in the leishmanicidal activity of the IFN-gamma-primed macrophages. Collectively, our data are consistent with a model where recognition of L. donovani promastigotes depends on the macrophage activation status and requires the expression of TLR3.     

How nascent phagosomes mature to become phagolysosomes

Phagocytosis mediates the clearance of apoptotic bodies and also the elimination of microbial pathogens. The nascent phagocytic vacuole formed upon particle engulfment lacks microbicidal and degradative activity. These capabilities are acquired as the phagosome undergoes maturation; a progressive remodeling of its membrane and contents that culminates in the formation of phagolysosomes. Maturation entails orderly sequential fusion of the phagosomal vacuole with specialized endocytic and secretory compartments. Concomitantly, the phagosomal membrane undergoes both inward and outward vesiculation and tubulation followed by fission, thereby recycling components and maintaining its overall size. Here, we summarize what is known about the molecular machinery that governs this complex metamorphosis of phagosome maturation.