Unusual intracellular trafficking of Salmonella typhimurium in human melanoma cells

Salmonella spp. are enterobacteria capable of invading and replicating in both professional and non-professional phagocytes. Here, we investigate the fate of S. typhimurium in human melanoma MelJuSo cells. The bacterium entered MelJuSo cells by a trigger mechanism and resided within a unique organelle, the Salmonella-containing vacuole (SCV). The SCV acquired early endosomal markers transiently and then underwent a series of membrane modifications. In HeLa cells, vacuole maturation is characterized by the simultaneous acquisition of the lysosomal membrane glycoproteins (Lgps) Lamp1, CD63 and vacuolar (v)-ATPase; in MelJuSo cells, however, acquisition of CD63 and v-ATPase preceded that of Lamp1. A very striking event in MelJuSo cells was the arrest of bacterial septation starting from 8 h after infection. Bacteria nevertheless continued to elongate, remained morphologically intact and viable and were eventually exocytosed. This original feature was observed in several skin-related cells including melanocytes, suggesting that it may provide the basis for an efficient host defence mechanism against Salmonella infection.

Disruption of the actin filament network affects delivery of endocytic contents marker to phagosomes with early endosome characteristics: the case of phagosomes with pathogenic mycobacteria

Phagosomes containing live virulent mycobacteria undergo fusion with early endosomes, but they are unable to mature normally. Accordingly, they do not fuse with lysosomes. Although M. avium-containing phagosomes retain fusion and intermingling characteristics of early endosomes indefinitely, fusions with early endosomes are increasingly restricted as bacteria multiply. In addition, when endocytic tracers, such as horseradish peroxidase (HRP), are added to M. avium-infected macrophages at 1 or up to 15 days after infection, an atypical time course of acquisition of the tracer by the phagosomes is observed, i.e., a 10 to 20 min lag, instead of immediate acquisition as is typical for early endosomes (and phagosomes with early endosome characteristics). These events coincide with a marked disorganization of the actin filament network in M. avium-infected macrophages. In the present study, we have therefore addressed the following question: Do actin filaments play a role in fusion and intermingling of contents between early endosomes and immature phagosomes that undergo homotypic fusion with early endosomes? We examined the time course of acquisition of subsequently internalized endocytic marker (HRP) by early endosome-like preexisting phagosomes, i.e. 2 hour-old phagosomes with either hydrophobic latex particles, virulent or avirulent M. avium, after depolymerization of the actin filament network with cytochalasin D or after repolymerization of the actin filament network with jasplakinolide, in cases where the network had been depolymerized (macrophages infected with M. avium, at 1 or up to 7 days after infection). By direct morphological observation at the electron microscope level and by a kinetic approach, we show here that depolymerization of the actin filament network with cytochalasin D delays acquisition of HRP whereas repolymerization restores immediate acquisition of the marker. We conclude that the actin filament network is involved in fusion and intermingling of endocytic contents between early endosomes and early endosome-like phagosomes, and that disruption of this network by M. avium is the cause for the atypical acquisition of content marker by phagosomes containing these pathogenic mycobacteria.

Pathogenic mycobacteria disrupt the macrophage actin filament network

Phagosomes with pathogenic mycobacteria retain fusion and intermingling characteristics of early endosomes indefinitely. The time course of acquisition of newly endocytosed tracers becomes, however, atypical (lag instead of immediate acquisition) starting from day 1 postinfection (p.i.), thereby suggesting that additional factors affect this process. Disruption of the actin filament (F-actin) network by cytochalasin D perturbs the movement of early endosomes and probably fusion events among early endosomes and phagosomes. Here we compare, by immunofluorescence microscopy, the morphology and distribution of F-actin in macrophages infected with virulent Mycobacterium avium, in uninfected macrophages, or in macrophages after phagocytosis of nonpathogenic bacteria (Mycobacterium smegmatis or Bacillus subtilis) or hydrophobic latex particles. In uninfected cells, F-actin appeared as a network of small filaments distributed throughout the cell; about 80% of the cells also displayed one or two small patches of F-actin at the cell periphery. Virulent M. avium caused a marked disorganization of the F-actin network starting from day 1 p.i. The most salient features were the formation of several large patches, the progressive disappearance of the small filaments, and the appearance of large numbers of tiny punctate structures starting from day 2 p.i. With the three other particles, the F-actin network was unmodified compared to that in uninfected cells. The atypical lag in acquisition of newly endocytosed tracers by M. avium-containing phagosomes, therefore, seems to coincide with the disorganization of the F-actin network.

Construction of chimeric phagosomes that shelter Mycobacterium avium and Coxiella burnetii (phase II) in doubly infected mouse macrophages: an ultrastructural study

Dual infection of cells may divert pathogens to intracellular compartments different from those occupied in mono-infected cells. In the present studies, mouse bone marrow in vitro-derived macrophages were first infected with virulent Mycobacterium avium, which are normally singly lodged within tight phagosomes. These phagosomes do not mature; they undergo homotypic fusion with early endosomes and do not fuse with lysosomes. Seven days later, the cultures were superinfected with phase II (non-virulent) Coxiella burnetii, organisms sheltered in lysosome- (or prelysosome)-like, multi-occupancy phagosomes. The latter can attain large size and engage in efficient homo- and heterotypic fusion with other phagosomes. Cultures were fixed for transmission electron microscopy 6, 12, 24, and 48 h later. Other M. avium-infected cultures were superinfected with amastigotes of the trypanosomatid flagellate Leishmania amazonensis, which are also sheltered in lysosome- (or prelysosome)-like multi-occupancy vacuoles, and fixed at the same time periods. Chimeric phagosomes containing both M. avium and C. burnetii, were found already at 6 h and the proportion of M. avium that colocalized with C. burnetii in the same phagosomes reached over 90% after 48 h. In such phagosomes, both organisms were ultrastructurally well preserved. In contrast, colocalization of M. avium and L. amazonensis was rarely found. Speculative scenarios that could underlie the formation of chimeric phagosomes could involve delayed maturation of C. burnetii-containing phagosomes in presence of M. avium, which would allow for fusion of C. burnetii- and M. avium-containing phagosomes; the production, by C. burnetii, of molecules that upregulate the fusion of M. avium-containing phagosomes with those that contain C. burnetii; and the secretion of factors that could favour the survival of M. avium within chimeric vacuoles.

Growth of Mycobacterium bovis, Bacille Calmette-Guérin, within human monocytes-macrophages cultured in serum-free medium

Mycobacterium bovis BCG is an opportunistic agent that may be responsible for disseminated disease in immunocompromised individuals. Under physiological conditions, macrophages are the natural hosts and final killers of BCG. In the context of inherited or acquired immune disorders underlying disseminated BCG infections, macrophages fail to eradicate BCG or even to restrict its intracellular growth. The direct contribution of macrophages, in this setting of impaired BCG destruction, probably depends on the type of underlying immune deficiency and remains to be experimentally investigated. As an initial approach, we document here the fate of BCG within human monocytes and human monocyte-derived macrophages (MDMs) cultured in commercially available serum-free medium (M-SFM). This medium was used to avoid potential problems associated with human or animal serum-supplemented medium. We show here that both monocytes and MDMs cultured in M-SFM display the morphological features and functional activities expected for such cells. We also show that after an initial phase of intracellular destruction, BCG grow within infected monocytes-macrophages, as shown by colony forming unit (CFU) counts and Ziehl-Nielsen staining. By an electron microscopic analysis, we show that the BCG always reside within phagosomes and that 24-h postinfection many phagosomes stain for the hydrolytic enzyme acid phosphatase. Finally, we compare bacterial growth in vitro within phagocytes from healthy individuals and patients with chronic granulomatous disease (CGD), an inheritable condition associated with disseminated BCG infection in vivo. No destruction of intracellular BCG was achieved by the patients cells, revealing the essential mycobactericidal role of the respiratory burst in human phagocytes. Investigations of BCG growth within MDM cultured in M-SFM from patients with other conditions which predispose to clinical BCG infection is therefore warranted.

The ClpC ATPase of Listeria monocytogenes is a general stress protein required for virulence and promoting early bacterial escape from the phagosome of macrophages

Under stress conditions, the facultative intracellular pathogen Listeria monocytogenes produces a ClpC ATPase, which is a general stress protein encoded by clpC and belonging to the HSP-100/Clp family. A ClpC-deficient mutant was obtained by gene disruption in strain LO28, which became highly susceptible to stress conditions in vitro. Intracellular growth of this mutant was restricted within macrophages, one of the major target cells of L. monocytogenes, during the infectious process. A quantitative electron microscope study showed that, contrary to wild-type bacteria that rapidly gain access to the cytoplasm of macrophages, mutant bacteria remained confined to membrane-bound phagosomes. Only a few mutant bacteria disrupted the phagosome membrane after 4h of incubation, then polymerized actin filaments and multiplied within the cytoplasm. The ClpC ATPase, therefore, promotes early bacterial escape from the phagosome of macrophages, thus enhancing intracellular survival. The ClpC ATPase was produced in vivo during experimental infection by wild-type bacteria. The virulence of the ClpC-deficient mutant was severely attenuated in mice, with a three-log decrease in its 50% lethal dose compared with wild-type bacteria. Bacterial growth of mutant bacteria was strongly restricted in organs, presumably because of an impairment of intracellular survival in host tissues. Our results provide evidence that a general stress protein is required for the virulence of L. monocytogenes, which behaves as a virulence factor promoting intracellular survival of this pathogen.

Intracytoplasmic delivery of listeriolysin O by a vaccinal strain of Bacillus anthracis induces CD8-mediated protection against Listeria monocytogenes

The facultative intracellular pathogen Listeria monocytogenes secretes a 58-kDa hemolysin, listeriolysin O (LLO), that allows bacteria to access the cytoplasm and to multiply inside infected cells. LLO is also a protective Ag required for the development of specific immunity. We studied the capacity of a new bacterial vector, derived from an attenuated strain of Bacillus anthracis, to deliver in vivo LLO and to induce protection against L. monocytogenes infection. The hly gene encoding LLO was fused to a B. anthracis regulatory region induced in vivo and was integrated into a resident plasmid of this bacterium. This recombinant strain secreted a functional LLO in vitro and inside phagosomes of bone marrow macrophages. This LLO production enabled the conversion of the extracellular replicating B. anthracis into an intracytoplasmic bacterium. LLO+ B. anthracis thus mimicked the intracellular behavior of L. monocytogenes in macrophages. Specific protection of mice against lethal doses of L. monocytogenes was induced by immunization with LLO+ B. anthracis. The immunity was mediated by CD8+ T lymphocytes and was associated with the activation of LLO-specific MHC class I-restricted CD8+ CTL, able to recognize the immunodominant H-2d-restricted epitope 91-99 of LLO. This study, therefore, suggests that intracytoplasmic delivery of LLO by B. anthracis is sufficient to induce a MHC class I-restricted CD8-mediated protection against L. monocytogenes. The LLO+ B. anthracis recombinant strain represents a potential vector for delivering foreign Ags involved in CD8-mediated protective responses.

Intracytoplasmic delivery of listeriolysin O by a vaccinal strain of Bacillus anthracis induces CD8-mediated protection against Listeria monocytogenes

The facultative intracellular pathogen Listeria monocytogenes secretes a 58-kDa hemolysin, listeriolysin O (LLO), that allows bacteria to access the cytoplasm and to multiply inside infected cells. LLO is also a protective Ag required for the development of specific immunity. We studied the capacity of a new bacterial vector, derived from an attenuated strain of Bacillus anthracis, to deliver in vivo LLO and to induce protection against L. monocytogenes infection. The hly gene encoding LLO was fused to a B. anthracis regulatory region induced in vivo and was integrated into a resident plasmid of this bacterium. This recombinant strain secreted a functional LLO in vitro and inside phagosomes of bone marrow macrophages. This LLO production enabled the conversion of the extracellular replicating B. anthracis into an intracytoplasmic bacterium. LLO+ B. anthracis thus mimicked the intracellular behavior of L. monocytogenes in macrophages. Specific protection of mice against lethal doses of L. monocytogenes was induced by immunization with LLO+ B. anthracis. The immunity was mediated by CD8+ T lymphocytes and was associated with the activation of LLO-specific MHC class I-restricted CD8+ CTL, able to recognize the immunodominant H-2d-restricted epitope 91-99 of LLO. This study, therefore, suggests that intracytoplasmic delivery of LLO by B. anthracis is sufficient to induce a MHC class I-restricted CD8-mediated protection against L. monocytogenes. The LLO+ B. anthracis recombinant strain represents a potential vector for delivering foreign Ags involved in CD8-mediated protective responses.

Phagosome maturation and fusion with lysosomes in relation to surface property and size of the phagocytic particle

Phagosomes with pathogenic mycobacteria, or with hydrophobic polystyrene beads of 1 micron in size, do not mature, but remain fusogenic towards early endosomes and do not fuse with lysosomes (de Chastellier et al. Eur. J. Cell Biol. 68, 167-182 (1995)). Both types of phagocytic particles display a close apposition to the phagosome membrane. We have postulated that due to the absence of tubule formation of the phagosomal membrane, efficient recycling of hypothetical fusion-mediating factors is impaired thus keeping the phagosome fusogenic to early endosomes and unable to fuse with lysosomes. To test this hypothesis, we now analyzed phagosome maturation for particles which were expected to display a less close particle-membrane apposition, in addition to confirming our previous results for non-maturing phagosomes as a direct comparison. In contrast to hydrophobic latex beads as before, we now used beads with a more hydrophilic surface, being carboxylated, with and without additional coating by protein (horseradish peroxidase, HRP; or bovine serum albumin, BSA). In addition, we used hydrophobic beads of smaller sizes (0.5, 0.3, 0.1 micron), in order to determine the limiting size at which the particle no longer determined the size and the fate of the phagosome. As predicted, all the above particles displayed a less tight interaction with the phagosome membrane. Tubule formation was observed to a similar extent as for early endosomes. Morphological evidence showed that phagosomes rapidly lost their ability to fuse with early endosomes, after which they could be seen fusing with lysosomes labeled with gold-conjugated BSA. Functional evidence for the formation of phagolysosomes was based on the kinetic observation that subsequently endocytosed contents marker (HRP) was acquired by phagosomes only after a lag of about 5 min as is typical for lysosomes. The present observations could be explained in terms of a model which suggests that mycobacteria can prevent phagosome maturation and, therefore, fusion with lysosomes, by a tight interaction with constituents of the phagosomal membrane. Furthermore, these results show that it is important to choose artificial phagocytic particles according to the appropriate surface properties when using them as a model system to study phagosome processing.

The phagosomal environment protects virulent Mycobacterium avium from killing and destruction by clarithromycin

Murine bone marrow-derived macrophages (Mphis) infected with virulent strains of Mycobacterium avium (TMC 724 and a human clinical isolate) or with an avirulent opaque variant that spontaneously dissociates from the virulent human clinical isolate were subjected to a prolonged and continuous treatment with clarithromycin added at the MIC. The efficiency of this antibiotic in terms of inhibition of bacterial growth and bacterial degradation was evaluated during a 21-day treatment period. Growth was assessed by determination of CFU of intracellular bacteria and by a quantitative ultrastructural analysis which allowed us also to determine the extent of bacterial degradation. A similar treatment was applied to the same strains growing in liquid medium. Our data show that in liquid medium, clarithromycin caused a 90% decrease in CFU within 7 days of treatment. When applied to Mphis infected with virulent M. avium, clarithromycin immediately arrested bacterial growth but was unable to fully kill and degrade intracellularly growing virulent bacteria. After 21 days of treatment, 25% of intracellular bacteria were still morphologically intact. These bacteria resumed growth upon removal of the antibiotic, with a normal replication rate. These bacteria had not become more resistant to the drug, since the MIC was unchanged as compared to the one determined for the initial stock used to infect Mphis. Our data therefore suggest that the intraphagosomal environment protects bacteria from degradation. We propose that the inability of the drug to completely destroy bacteria is the result of a limited accessibility of the drug due to prevention of fusions between the immature phagosomes in which virulent bacteria reside and lysosomes in which clarithromycin accumulates. In accord with our proposal, we show that the avirulent opaque variant, which does not prevent phagosome-lysosome fusions (unpublished data), is finally destroyed by clarithromycin even within the phagosomal environment.

Phagocytic processing of the macrophage endoparasite, Mycobacterium avium, in comparison to phagosomes which contain Bacillus subtilis or latex beads

The intraphagosomal survival strategy of pathogenic mycobacteria was studied in bone marrow-derived mouse macrophages. These bacteria survive inside phagosomes by interfering in an unknown manner with phagosome processing which normally would lead to digestion of the phagocytic particle in phagolysosomes. Here, phagosome processing was compared for different phagocytic particles: live Mycobacterium avium, degradable Bacillus subtilis, or indigestible latex beads. We show detailed electron microscopic morphological observations which characterize various phases of interaction between endocytic organelles and phagosomes. We measured fusion of phagosomes with early endosomes or with lysosomes by using newly internalized endocytic contents (horseradish peroxidase, HRP) and membrane marker (plasma membrane glycoconjugates labeled with [3H]galactose via exoglycosylation). Morphometric analysis of these observations showed that the nature of the phagocytic particle affects phagosome processing: As long as particles remain undigested, maturation of phagosomes is prevented and they remain fusogenic towards early endosomes; concurrent to particle digestion, phagosome processing proceeds towards transfer of phagocytic contents to phagolysosomes which display kinetic and compositional characteristics of lysosomes. As an intact phagocytic particle, M. avium remains in non-matured phagosomes which fuse with early endosomes, but not with lysosomes. Fusion with early endosomes is reduced, thereby indicating the stage where this endoparasite exerts its effect.

Fusion between large phagocytic vesicles: targeting of yeast and other particulates to phagolysosomes that shelter the bacterium Coxiella burnetii or the protozoan Leishmania amazonensis in Chinese hamster ovary cells

This report examines the fusion of phagocytic vesicles with the large phagolysosome-like vacuoles induced in Chinese hamster ovary cells by the bacterium Coxiella burnetti or the Protozoan flagellate Leishmania amazonensis. Infection by these organisms is compatible with cell survival and multiplication. Fusion was inferred from the transfer of microscopically identifiable particles from donor to target vesicles. Donor vesicles contained heat-killed yeast, zymosan, beta-glucan or latex beads taken up by the host cells. Yeast and zymosan were also coated with Concanavalin A to increase their uptake by the cells (Goldman, R., Exp. Cell Res. 104, 325–334, 1977). Particle localization, routinely ascertained by phase-contrast microscopy, was confirmed by confocal laser fluorescence and by transmission electron microscopy. Coxiella vacuoles admitted all the particles tested and transfer took place whether the particles were given to the cells prior to or after infection. Transfer of uncoated or Concanavalin-A-coated yeast or zymosan was dependent on the number of particles ingested and on the incubation period (between 2 and 24 hours). Furthermore, the transfer step was quite efficient, since over 85% of the particles ingested entered Coxiella vacuoles at all particle to cell ratios examined. The fraction of uncoated or Concanavalin-A-coated yeast or zymosan transferred to Leishmania vacuoles was consistently lower and diminished at higher particle loads. In addition, only rarely did latex beads enter these vacuoles. The models proposed may be useful for the delineation of biochemical and molecular mechanisms involved in the fusion of large phagocytic vesicles and the modulation of the latter by cellular and pathogen-derived signals.

Fate of Listeria monocytogenes in murine macrophages: evidence for simultaneous killing and survival of intracellular bacteria

The intracellular survival of the ubiquitous pathogen Listeria monocytogenes was studied in primary cultures of bone marrow-derived mouse macrophages. Bacteria were able to grow rapidly in these cells, with an apparent multiplication rate of about 40 min. Electron microscopy demonstrated that intracellular bacterial replication was the consequence of simultaneous intracellular killing and replication of bacteria in the same cells. Within the first hour following phagocytosis, most bacteria were destroyed in the phagosomal compartment to which they were confined. This was due to early transfer of hydrolytic enzymes to phagosomes, undoubtedly via phagosome-lysosome (P-L) fusion, as demonstrated by a quantitative analysis after staining for a lysosomal marker, acid phosphatase. One hour after infection, about 14% of the bacteria were free in the cytoplasm, in which they multiplied and induced actin polymerization and spreading to adjacent macrophages, as in epithelial cells. By using the 3-(2,4-dinitroanilino)-3'-amino-N-methyldipropylamine staining procedure, direct evidence is presented that all phagosomes were acidified immediately after phagocytosis, thus indicating that intraphagosomal bacteria were exposed to an acidic environment that might favor vacuolar lysis by listeriolysin O. Intracellular growth in macrophages, therefore, appears to be the result of a competition between the expression of the hydrolytic activity of these cells following P-L fusion and the capacity of L. monocytogenes to escape from the acidified phagosomal compartment before P-L fusion has occurred. The finding that concomitant intracellular killing and survival of L. monocytogenes occurs in the same macrophages might explain the high immunogenicity observed in vivo with live bacteria, as opposed to killed bacteria.

Distribution of MHC class I and of MHC class II molecules in macrophages infected with Leishmania amazonensis

Macrophages, being apparently the only cells that in vivo allow the growth of the intracellular pathogen Leishmania, are likely candidates to present antigens to Leishmania-specific CD4+ and CD8+ T lymphocytes, known to be involved in the resolution or in the development of lesions induced by these parasites, and recognizing processed antigens bound to MHC class I and MHC class II molecules, respectively. In the present study, we analysed by confocal microscopy and by immunoelectron microscopy the subcellular distribution of both MHC class I and class II molecules in mouse (Balb/c and C57BL/6 strains) bone marrow-derived macrophages infected for 12 to 48 hours with Leishmania amazonensis amastigotes and activated with gamma interferon to determine the intracellular sites where Leishmania antigens and MHC molecules meet and can possibly interact. Double labelings with anti-MHC molecule antibodies and with either propidium iodide or an anti-amastigote antibody allowed localization of MHC molecules with regard to the endocytic compartments housing Leishmania amastigotes, organelles known as the parasitophorous vacuoles (PV) and which most likely contain the highest concentration of parasite antigens in the host cell. Both uninfected and infected macrophages from Balb/c mice expressed the MHC class I molecules H-2Kd and H-2Dd on their cell surface but no significant amount of these molecules could be detected in the PV, which indicates that, if infected macrophages play a role in the induction of Leishmania-specific CD8+ T lymphocytes, PV are probably not loading compartments for MHC class I molecules. In contrast, MHC class II molecules were found to be associated with the PV membranes as shown previously with microscopic techniques at lower resolution (Antoine et al. Infect. Immun. 59, 764–775, 1991). In addition, we show here that, 48 hours after infection of Balb/c macrophages, in about 90% of PV containing MHC class II molecules, the latter were mainly or solely localized at the attachment zone of amastigotes to PV membranes. This peculiar distribution, especially well demonstrated using confocal microscopy, was confirmed by subcellular fluorescence cytometry of infected macrophages stained for the MHC class II molecules. The following data agree with the idea that PV-associated MHC class II molecules establish specific interactions with plasma membrane components of amastigotes. First, the polarized localization of class II appeared specific to these molecules, since the distribution of the lysosomal glycoproteins Igp110 and Igp120, of the macrosialin (a macrophage-specific marker of endocytic compartments) and of the GTP-binding protein rab7p, shown here as being PV membrane components, was homogeneous.(ABSTRACT TRUNCATED AT 400 WORDS)

Implication of phagosome-lysosome fusion in restriction of Mycobacterium avium growth in bone marrow macrophages from genetically resistant mice

The ability of the host to resist infection to a variety of intracellular pathogens, including mycobacteria, is strongly dependent upon the expression of the Bcg gene. Mouse strains which express the resistance phenotype (Bcgr) restrict bacterial growth, whereas susceptible strains (Bcgs) allow bacterial growth. Expression of the Bcg allele is known to influence the priming of host macrophages (M phi s) for bactericidal function. In the present work, bone marrow-derived M phi s from congenic BALB/c (Bcgs) and C.D2 (BALB/c.Bcgr) mice were infected with the virulent strain Mycobacterium avium TMC 724 to define the mechanism involved in growth restriction of M. avium. By combining CFU measurements and ultrastructural analyses, we show that growth of this bacterium is restricted in marrow M phi s from resistant mice. Using acid phosphatase as a lysosomal marker, we provide evidence that the hydrolytic activity of M phi s, as measured by the capacity of lysosomes to fuse with and transfer active hydrolytic enzymes to phagosomes in which M. avium resides, is an expression of the Bcg gene and that this phenomenon is a key antibacterial activity responsible for growth restriction of M. avium: (i) the percentage of phagosome-lysosome fusions was twice as high in Bcgr M phi s as in Bcgs M phi s, and (ii) the percentage of intact viable bacteria residing in acid phosphatase-negative phagosomes was twice as low in Bcgr M phi s as in the Bcgs counterparts. These differences are not due to a lower activity of the enzyme in Bcgr M phi s. The mechanism by which the Bcg gene exerts control over the phagolysosomal fusion is discussed.

Transfer of zymosan (yeast cell walls) to the parasitophorous vacuoles of macrophages infected with Leishmania amazonensis

Leishmania are flagellated protozoan parasites which, in their amastigote stages, survive and multiply within phagolysosome-like parasitophorous vacuoles (PV) of mammalian macrophages (MO). This study develops an earlier ultrastructural, incidental observation that zymosan particles (Z) were transferred to the PV of macrophages infected with Leishmania amazonensis. In the present report, a pulse-chase light microscopic assay was used to delineate several features of the Z transfer. The assay reflects both the movement of internalized particles to a position adjoining a PV, and their delivery to the vacuoles. Transfer was selective, in the sense that Z, beta-glucan or heat-killed yeast particles were transferred, whereas latex beads, aldehyde-fixed, or immunoglobulin G-coated erythrocytes were not. This selectivity may be related to the high density of carbohydrate ligands displayed on the surface of yeast-derived particles, to ligand resistance to lysosomal degradation or to signals encoded in the cytosolic tails of the receptors involved in particle recognition. A few Z particles could be found within PV after 1 h of incubation with infected MO, but chase periods of several hours at 34 degrees C were required for particle transfer to the PV in a substantial proportion of the MO. Ammonium chloride, chloroquine, or monensin, compounds which increase the pH in acidified compartments, substantially enhanced the transfer of Z particles. Finally, transfer was inhibited by cytochalasin D, but was unaffected by the antitubulin nocodazole. Although it is not yet known if particle transfer occurs by fusion of donor vesicles with PV or by interiorization of the former into the latter, the model described should be useful in the study of the interactions between large phagocytic vesicles and the modulation of those interactions by cellular, parasitic, and environmental signals.

Cellular oxidative responses and mycobacterial growth inhibition in aerosol and intradermal BCG-immunized guinea-pigs

Although the dissemination of tuberculosis is aerogenic, less than 10% of infected subjects develop the active disease. Local immunity plays a major role in systemic cell-mediated immunity against this disease. BCG immunization may be more effective if administered via aerosol rather than intradermally. In this study, the immune responses seen in guinea-pigs vaccinated with a BCG aerosol were compared with those seen following intradermal vaccination. At regular intervals after each vaccination, the activation of alveolar macrophage was determined by their capacity to produce superoxides, phagosome-lysosome fusion and the inhibition of in vitro BCG growth. Concurrently, BCG multiplication or growth inhibition in the target organs was also determined. This study demonstrates that the alveolar route of BCG administration activated broncho-alveolar macrophage more effectively than the intradermal route. Superoxide production correlated with in vitro and in vivo inhibition of BCG growth. The spread, by the BCG inoculum, to the draining lymph nodes and spleen was similar for both test routes of administration. However, the lung BCG counts were significantly lower following intradermal vaccination. In contrast, the activation of broncho-alveolar macrophage was higher following aerogenic, rather than intradermal, BCG immunization.

Intramacrophage growth of Mycobacterium avium during infection of mice

Growth of the virulent Mycobacterium avium strain TMC 724 in host tissues during persistent infection of mice was studied. Following intravenous infection of C57BL/6 mice, the kinetics of bacterial growth was biphasic in the spleen and liver, with a significant reduction of the multiplication rate after day 21 to 28 of infection. An electron-microscopic study of the liver and spleen of infected mice showed that the bacteria were strictly intracellular. They were observed within inflammatory macrophages populating granulomas disseminated in host tissues. The bacteria were confined to the phagosome compartment, and they were encapsulated. Phagosome-lysosome fusions were encountered, but the bacteria showed no visible signs of degradation and continued to multiply. These results are the first in vivo evidence that virulent M. avium multiplies exclusively intracellularly and that encapsulated bacteria resist the microbicidal mechanisms of macrophages inside the phagosomal compartment.

Localization of major histocompatibility complex class II molecules in phagolysosomes of murine macrophages infected with Leishmania amazonensis

Leishmania-infected macrophages are potential antigen-presenting cells for CD4+ T lymphocytes, which recognize parasite antigens bound to major histocompatibility complex class II molecules (Ia). However, the intracellular sites where Ia and antigens may interact are far from clear, since parasites grow within the modified lysosomal compartment of the host cell, whereas Ia molecules seem to be targeted to endosomes. To address this question, the expression and fate of Ia molecules were studied by immunocytochemistry in Leishmania amazonensis-infected murine macrophages stimulated with gamma interferon. In uninfected macrophages, Ia molecules were localized on the plasma membrane and in perinuclear vesicles, but they underwent a dramatic redistribution after infection, since most of the intracellular staining was then associated with the periphery of the parasitophorous vacuoles (p.v.) and quite often polarized towards amastigote-binding sites. The Ii invariant chain, which is transiently associated with Ia during their intracellular transport, although well expressed in infected macrophages, apparently did not reach the p.v. Similar findings were observed with macrophages from mice either resistant or highly susceptible to Leishmania infection. In order to determine the origin of p.v.-associated Ia, the fate of plasma membrane, endosomal, and lysosomal markers, detected with specific antibodies, was determined after infection. At 48 h after infection, p.v. was found to exhibit a membrane composition typical of mature lysosomes. Overall, these data suggest that (i) Ia located in p.v. originate from secondary lysosomes involved in the biogenesis of this compartment or circulate in several endocytic organelles, including lysosomes and (ii) p.v. could play a role in antigen processing and presentation. Alternatively, the presence of high amounts of Ia in p.v. could be due to a Leishmania-induced mechanism by means of which this organism may evade the immune response.

Endocytic membrane traffic with respect to phagosomes in macrophages infected with non-pathogenic bacteria: phagosomal membrane acquires the same composition as lysosomal membrane

A morphometric analysis was made to study membrane traffic in bone marrow-derived macrophages, containing phagosomes with partially degraded Bacillus subtilis. Cell surface glycoproteins, labeled with radioactive galactose by terminal glycosylation, provided a covalent autoradiographic membrane marker. Membrane compartments were characterized in terms of cytochemical staining for horseradish peroxidase taken up by receptor-mediated endocytosis. The area, composition, and exchange rates of endocytic membrane compartments were measured as in a previous analysis for non-infected macrophages, devoid of phagosomes. In direct comparison with this earlier study, the present data allowed an assessment of the involvement of phagosomes in the interactions between endocytic membrane compartments. The presence of phagosomes led to a 30% reduction of lysosomal membrane area. The rate at which cell surface-derived label flowed into the lysosomal membrane pool was reduced by the same fractional amount. This suggested a linear relationship between flow rate and membrane area. The initial flow rate of label into phagosomes was higher than expected, based on their membrane area being only about 60% that of lysosomes. This rate could only be measured during the early phase of the experiments when phagosomes were younger, therefore displaying a fast exchange rate, reminiscent of the endosome compartment. However, steady-state conditions, at late times, strongly suggested that phagosomes with degraded contents finally acquire membrane of lysosomal origin. First, the composition of phagosome membrane became the same as that of lysosomes, remaining unchanged as compared to non-infected cells. Second, the membrane area of phagosomes amounted to the loss of lysosomal membrane area in infected cells.

Exchange kinetics and composition of endocytic membranes in terms of plasma membrane constituents: a morphometric study in macrophages

Intracellular membrane traffic, during endocytosis in mouse bone marrow-derived macrophages, was studied quantitatively by morphometric and kinetic analysis. Three functionally different markers were used: Horseradish peroxidase (HRP) served as a fluid-phase (FP) marker (1000 micrograms HRP/ml in the presence of mannan) or as a receptor-mediated (RM) membrane marker (25 micrograms HRP/ml) and, third, plasma membrane (PM) glycoconjugates, enzymatically labeled with [3H]galactose at the cell surface, served as a covalent membrane marker. The cell surface was labeled with [3H]galactose, followed by either FP or by RM uptake of HRP. The kinetics of the intracellular appearance of the markers were measured as the membrane area stained by HRP-reaction product and as the number of autoradiographic grains associated with these membranes. The following compartments were distinguished: PM, coated vesicles (VI), pinosomes or endosomes (VII), secondary lysosomes (VIII), and HRP-negative vesicles (EV). Tubular structures of VII became labeled with HRP only during RM uptake. The markers flowed first into VI and VII, and after 5 min into VIII. EV became labeled with the covalent membrane marker starting from 5 min. The ratio of autoradiographic grain number to HRP-stained membrane area remained constant with time although substantially different for the various compartments, viz. 100% (VI), 50% (VII and EV) and 30% (VIII) as compared to the PM (100%). This indicated that endosomes were only partially derived from internalized PM and that secondary lysosomes contained a substantial pool of PM constituents. The observed kinetics suggested that once every 30 to 40 min the entire PM was internalized, the bulk of which was recycled after 4 min from a prelysosomal compartment(s) leaving only 12 to 20% for recycling via membranes of secondary lysosomes after a residence time of 24 to 33 min.

Evidence for inhibition of fusion of lysosomal and prelysosomal compartments with phagosomes in macrophages infected with pathogenic Mycobacterium avium

Bone marrow-derived cultured macrophages were infected with the pathogenic organism Mycobacterium avium. Immediately after infection and at 1 to 28 days later, cells either were stained for acid phosphatase activity or given horseradish peroxidase, which served as a pinocytotic marker. With the former, fusions between phagosomes and lysosomes exclusively were assessed; with the latter, those between phagosomes and both pinosomes and lysosomes were determined. As a control, similar experiments were undertaken by infecting macrophages with gamma ray-killed M. avium and the nonpathogenic live organisms Mycobacterium aurum and Bacillus subtilis. After infection with live M. avium, fusions between phagosomes and acid phosphatase-positive vesicles (lysosomes) were inhibited. The same inhibition was observed whether phagosomes contained damaged or structurally intact (presumed to be live) bacteria, except for the early time points. This inhibition was, however, partial, suggesting that some of the live bacteria are resistant to the hydrolytic enzymes of the phagolysosomal environment. Fusions between horseradish peroxidase-positive vesicles (pinosomes and lysosomes) and phagosomes depended upon the morphological state of the bacteria. Damaged bacteria did not inhibit fusions, whereas with intact bacteria, a partial inhibition which increased with time was observed. The two types of experiment suggest that viable M. avium can impair phagosome-pinosome fusions.

Fluid phase and mannose receptor-mediated uptake of horseradish peroxidase in mouse bone marrow-derived macrophages. Biochemical and ultrastructural study

A detailed study of horseradish peroxidase (HRP) uptake by in vitro cultured bone marrow-derived macrophages was undertaken. Biochemical quantitations performed over a wide range of HRP concentrations, in the absence or presence of yeast mannan, showed that these macrophages pinocytose HRP by both fluid phase and mannose receptor-mediated uptake. The relative contribution of these two types of endocytosis varied with the concentration of enzyme in the extracellular medium. A morphological study at the light and electron microscope levels conducted in parallel confirmed the biochemical data.

Exchange of material between the extracellular medium and macrophage phagosomes containing different species of bacteria including mycobacteria

Pathogenic mycobacteria survive and multiply once they have infected macrophages. The aim of the present work was to determine whether the persistence of pathogenic bacteria such as M. avium inside the host cell phagosomes had any effect on the exchanges that normally occur between the extracellular medium and the macrophage vacuolar compartment. Our results indicate that fusions between phagosomes and lysosomes or/and incoming pinosomes appear to be slowed down by the presence of mycobacteria and even partially inhibited when phagosomes contain viable pathogenic mycobacteria.

Membrane shuttle between plasma membrane, phagosomes, and pinosomes in Dictyostelium discoideum amoeboid cells

The intracellular redistribution of membrane internalized during endocytosis was studied quantitatively by a biochemical approach and by a morphometric analysis of autoradiographs in electron microscopy. Plasma membrane glycoconjugates, enzymatically labelled with radioactive galactose, were used as a membrane marker. In cells labelled at their surface either before or after the phagocytotic uptake of latex beads, subsequent endocytosis led to a redistribution of label between the plasma membrane and endosomal membranes until a steady-state was reached after about 1 h with 43% of the label on the plasma membrane. The steady-state resulted when all participating membranes carried the same surface density of label. During phagocytosis or pinocytosis the equivalent of the plasma membrane was internalized and recycled once every 20 min or 40 min, respectively. Compared to this rate a very rapid and complete mixing of membranes was observed between newly formed phagosomes and preexisting digestive vacuoles or between newly formed pinosomes and preexisting phagosomes. Due to this rapid mixing, the membranes enclosing undigestible latex beads remained fully linked to the shuttle of membrane to and from the cell surface.

Peptidoglycan turnover during growth recovery after chloramphenicol treatment in a Dap-Lys-mutant of Bacillus megaterium

The study of diaminopimelic acid (DAP) incorporation and turnover during growth recovery in chloramphenicol-treated (CMP-treated) Bacillus megaterium cells showed that two kinds of turnover occurred. A low acid-soluble turnover appeared as soon as growth resumed in bacteria labeled before the CMP treatment and at the middle of the first generation in those labeled during the treatment. The acid-insoluble turnover appeared only at the beginning of the second generation of growth resumption in bacteria labeled before CMP addition and at the beginning of the third generation in those labeled during the CMP treatment. The acid-soluble release observed during the period of cell wall thinning is too low to account for the decrease of the wall thickness and the acid-insoluble loss appears after this period. When bacteria were transferred into partially spent medium instead of fresh culture medium the acid-insoluble release started to appear half a generation sooner. Electron microscopic observations showed that in this case, large scales detached from the cell wall. This activity of wall degradation was not observed when the partially spent medium was previously heated for 10 min at 100 degree C. The persistence of a thick wall on cell ends during the first generation does not reflect an absence of growth sites because their labeling on autoradiographs is high. Rather, it seems to be due to a low lytic activity at the poles.

Morphometric and cytochemical studies of Dictyostelium discoideum in vegetative phase. Digestive system and membrane turnover

The morphometric analysis of growing cells shows that the membranes of the digestive apparatus have a surface area equal to the cell surface area. After yeast phagocytosis, the surface area of the membrane surrounding the ingested yeast is equal to 40% of the surface area of the cell membrane. In spite of this internalization, the cell surface remains constant. Its renewal is insured by the translocation of the membrane of the digestive system, the surface area that concomitantly decreases by 40%. This means that the influx of plasma membrane is continually compensated for by the same outflow of internal membranes. During this turnover, the characteristic polysaccharide stainability (two different stains were used) of the plasma membrane is maintained after internalization, at the level of the digestive system, despite the presence of hydrolases in the digestive vacuoles. The cytochemical demonstration of acid phosphatase shows that this enzyme penetrates into phagosomes by fusion between phagosomes and vacloles of various sizes. The debris of digested yeast are released into the culture medium after 2 h. This process of defecation is accompanied by the appearance of new pinocytotic vacuoles, which indicates that the uptake of axenic medium has resumed. A model of membrane turnover is proposed to explain these observations.

Changes of the cell surface and of the digestive apparatus of Dictyostelium discoideum during the staruation period triggering aggregation

The effects of starvation on the cell morphology of Dictyostelium discoideum were studied with different cytochemical techniques, and with a morphometric method by which the surface areas of the cell membrane and of the digestive system can be determined. During the first 2 h, the cell membrane becomes very wrinkled and many phagocytic cups and filopods are formed. These changes are in accord with the 40 percent increase in the cell surface area to cytoplasmic volume ratio observed, which is mainly due to a strong decrease in the cytoplasmic volume. At this time of starvation, cells are able to ingest twice as many yeast as during growth. Afterwards, while the phagocytic ability decreases, the phagocytic cups disappear, and all the cells become bristled with many thin filopods. In spite of these morphological changes, no quantitative or topological differences have been observed concerning the polysaccharide content of the plasma membrane, whether it was stained with phosphotungstic acid, silver proteinate, or ruthenium red. During this time, the digestive vacuoles imbricate one into the other. Part of the vacuoles are degraded by this process, thus leading to an atrophy of the digestive apparatus. The digestive apparatus is progressively replaced by an autophagic system. Polysaccharide stainings and morphological observations show that the cytosegresomes seem to originate from the food vacuoles which flatten and sequester portions of cytoplasm. After 5 h of starvation, the digestive system is entirely transformed into an autophagic apparatus. The cell population appears to be homogeneous with respect to these changes. Therefore, potential precursors of prestalk and prespore cells were not observed.

Cell wall growth of Bacillus megaterium: cytoplasmic radioactivity after pulse-labeling with tritiated diaminopimelic acid

Study of the cell wall growth in Bacillus megaterium by pulse-labeling a DAP- Lys- mutant with tritiated diaminopimelic acid (DAP) had revealed the presence of intracytoplasmic radioactivity. The nature of this radioactivity was studied on one hand by autoradiographic analysis of bacteria treated in different ways and on the other hand by chromatography of the radioactive compounds extracted with boiling water. It is shown that cytoplasmic radioactivity corresponds neither to free DAP nor to DAP metabolized into lysine, but to murein precursors. Autoradiographic analysis of bacteria in which all murein precursors were removed gives exactly the same cell wall growth pattern as the one previously obtained for untreated bacteria. It can be concluded that, in B. megaterium, cell wall elongation occurs by diffuse intercalation of newly synthesized murein along the cylindrical part of the cell wall and that only cross wall formation occurs in a precise growth zone.

Study of cell wall growth in Bacillus megaterium by high-resolution autoradiography

Growth of the cell wall of Bacillus megaterium was studied by pulse-labeling the cell wall of a DAP- Lys- mutant for a very short time with tritium-labeled diaminopimelic acid. The distribution of radioactivity along the cell wall was examined by high-resolution autoradiography on isolated cell walls and thin sections of bacteria. The results indicate that cell wall elongation occurs by diffuse intercalation of newly synthesized murein into the expanding cell wall during exponential growth, as well as during germination, and that the only zone of highly localized diaminopimelic acid incorporation is found at the cross wall during its synthesis. This zone contains about 30% of the radioactivity incorporated into the cell wall. Analysis of autoradiographs of thin sections of bacteria shows that the total radioactivity incorporated per bacterium doubles during the life cycle. This doubling occurs in the cylindrical part of the cell wall but not in the polar caps. This seems to indicate that elongation of the bacterium is not constant during the life cycle but increases with the length of the cell.