Flow cytometric sorting and biochemical characterization of the late endosomal rab7-containing compartment

Grancalcin (GCA) modulates Toll-like receptor 9 (TLR9) mediated signaling through its direct interaction with TLR9

Toll-like receptors (TLRs) are playing important roles in stimulating the innate immune response and intensifying adaptive immune response against invading pathogens. Appropriate regulation of TLR activation is important to maintain a balance between preventing tumor activation and inhibiting autoimmunity. Toll-like receptor 9 (TLR9) senses microbial DNA in the endosomes of plasmacytoid dendritic cells and triggers myeloid differentiation primary response gene 88 (MyD88) dependent nuclear factor kappa B (NF-κB) pathways and type I interferon (IFN) responses. However, mechanisms of how TLR9 signals are mediated and which molecules are involved in controlling TLR9 functions remain poorly understood. Here, we report that penta EF-hand protein grancalcin (GCA) interacts and binds with TLR9 in a yeast two-hybrid system and an overexpression system. Using siRNA-mediated knockdown experiments, we also revealed that GCA positively regulates type I IFN production, cytokine/chemokine production through nuclear localization of interferon regulatory factor 7 (IRF7), NF-κB activation, and mitogen-activated protein kinase (MAPK) activation in plasmacytoid dendritic cells. Our results indicate that heterodimerization of GCA and TLR9 is important for TLR9-mediated downstream signaling and might serve to fine tune processes against viral infection.

The virulence protein SopD2 regulates membrane dynamics of Salmonella-containing vacuoles

Salmonella enterica serovar Typhimurium is a Gram-negative bacterial pathogen causing gastroenteritis in humans and a systemic typhoid-like illness in mice. The capacity of Salmonella to cause diseases relies on the establishment of its intracellular replication niche, a membrane-bound compartment named the Salmonella-containing vacuole (SCV). This requires the translocation of bacterial effector proteins into the host cell by type three secretion systems. Among these effectors, SifA is required for the SCV stability, the formation of Salmonella-induced filaments (SIFs) and plays an important role in the virulence of Salmonella. Here we show that the effector SopD2 is responsible for the SCV instability that triggers the cytoplasmic release of a sifA⁻ mutant. Deletion of sopD2 also rescued intra-macrophagic replication and increased virulence of sifA⁻ mutants in mice. Membrane tubular structures that extend from the SCV are the hallmark of Salmonella-infected cells. Until now, these unique structures have not been observed in the absence of SifA. The deletion of sopD2 in a sifA⁻ mutant strain re-established membrane trafficking from the SCV and led to the formation of new membrane tubular structures, the formation of which is dependent on other Salmonella effector(s). Taken together, our data demonstrate that SopD2 inhibits the vesicular transport and the formation of tubules that extend outward from the SCV and thereby contributes to the sifA⁻ associated phenotypes. These results also highlight the antagonistic roles played by SopD2 and SifA in the membrane dynamics of the vacuole, and the complex actions of SopD2, SifA, PipB2 and other unidentified effector(s) in the biogenesis and maintenance of the Salmonella replicative niche.

Salmonella typhimurium is a bacterial pathogen that causes diseases ranging from gastroenteritis to typhoid fever. This bacterium survives inside eukaryotic cells within a membrane-bound compartment, namely the Salmonella-containing vacuole. Salmonella injects proteins, named effectors, into the infected cell. These effectors change the biology of the infected cell and collectively support Salmonella replication and virulence. The effector SifA plays a key role in the bacterial vacuole stability and in the formation of membrane tubules that extend from the vacuole. Absence of SifA leads to the disruption of the vacuolar membrane and, therefore to the release of bacteria in the cytosolic compartment. Consequently, this mutant presents significant replication and virulence defects. Here, we show that an additional Salmonella effector, SopD2, is responsible for the membrane instability of the sifA⁻ vacuole. In addition, we demonstrate that SopD2 acts as an inhibitor of vesicle transport from the vacuole and that it down-modulates the formation of tubular structures. These findings describe a role for SopD2 as an antagonist of SifA in terms of vacuolar membrane dynamics.

The glyceraldehyde-3-phosphate dehydrogenase and the small GTPase Rab 2 are crucial for Brucella replication

The intracellular pathogen Brucella abortus survives and replicates inside host cells within an endoplasmic reticulum (ER)-derived replicative organelle named the “Brucella-containing vacuole” (BCV). Here, we developed a subcellular fractionation method to isolate BCVs and characterize for the first time the protein composition of its replicative niche. After identification of BCV membrane proteins by 2 dimensional (2D) gel electrophoresis and mass spectrometry, we focused on two eukaryotic proteins: the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the small GTPase Rab 2 recruited to the vacuolar membrane of Brucella. These proteins were previously described to localize on vesicular and tubular clusters (VTC) and to regulate the VTC membrane traffic between the endoplasmic reticulum (ER) and the Golgi. Inhibition of either GAPDH or Rab 2 expression by small interfering RNA strongly inhibited B. abortus replication. Consistent with this result, inhibition of other partners of GAPDH and Rab 2, such as COPI and PKC ι, reduced B. abortus replication. Furthermore, blockage of Rab 2 GTPase in a GDP-locked form also inhibited B. abortus replication. Bacteria did not fuse with the ER and instead remained in lysosomal-associated membrane vacuoles. These results reveal an essential role for GAPDH and the small GTPase Rab 2 in B. abortus virulence within host cells.

A key determinant for intracellular pathogenic bacteria to ensure their virulence within host cells is their ability to bypass the endocytic pathway and to reach a safe replication niche. Brucella bacteria reach the endoplasmic reticulum (ER) to create their replicating niche called the Brucella-containing vacuole (BCV). The ER is a suitable strategic place for pathogenic Brucella. Bacteria can be hidden from host cell defences to persist within the host, and can take advantage of the membrane reservoir delivered by the ER to replicate. Interactions between BCV and the ER lead to the presence of ER proteins on the BCV membrane. Currently, no other proteins (eukaryotic or prokaryotic) have yet been associated with the BCV membrane. Here we show that non-ER related proteins are also present on the BCV membrane, in particular, the glyceraldehyde-3-phosphate dehydrogenase (GAPDH) and the small GTPase Rab 2 known to be located on secretory vesicles that traffic between the ER and the Golgi apparatus. GAPDH and the small GTPase Rab 2 are involved in Brucella replication at late post-infection. Similarly, integrity of secretory vesicle trafficking is also necessary for Brucella replication. Here, we show that recruitment of the two eukaryotic proteins GAPDH and Rab 2 on BCV membranes is necessary for the establishment of the replicative niche by sustaining interactions between the ER and secretory membrane vesicles.

DNA Internalized via Caveolae Requires Microtubule-dependent, Rab7-independent Transport to the Late Endocytic Pathway for Delivery to the Nucleus

Using cationic liposomes to mediate gene delivery by transfection has the advantages of improved safety and simplicity of use over viral gene therapy. Understanding the mechanism by which cationic liposome:DNA complexes are internalized and delivered to the nucleus should help identify which transport steps might be manipulated in order to improve transfection efficiencies. We therefore examined the endocytosis and trafficking of two cationic liposomes, DMRIE-C and Lipofectamine LTX, in CHO cells. We found that DMRIE-C-transfected DNA is internalized via caveolae, while LTX-transfected DNA is internalized by clathrin-mediated endocytosis, with both pathways converging at the late endosome or lysosome. Inhibition of microtubule-dependent transport with nocodazole revealed that DMRIE-C:DNA complexes cannot enter the cytosol directly from caveosomes. Lysosomal degradation of transfected DNA has been proposed to be a major reason for poor transfection efficiency. However, in our system dominant negatives of both Rab7 and its effector RILP inhibited late endosome to lysosome transport of DNA complexes and LDL, but did not affect DNA delivery to the nucleus. This suggests that DNA is able to escape from late endosomes without traversing lysosomes and that caveosome to late endosome transport does not require Rab7 function. Lysosomal inhibition with chloroquine likewise had no effect on transfection product titers. These data suggest that DMRIE-C and LTX transfection complexes are endocytosed by separate pathways that converge at the late endosome or lysosome, but that blocking lysosomal traffic does not improve transfection product yields, identifying late endosome/lysosome to nuclear delivery as a step for future study.

The Crohn's disease-associated adherent-invasive Escherichia coli strain LF82 replicates in mature phagolysosomes within J774 macrophages

Adherent-invasive Escherichia coli (AIEC) bacteria isolated from Crohn's disease patients are able to extensively replicate within macrophages in large vacuoles. The mechanism by which AIEC bacteria survive within phagocytic cells is unknown. This report describes the maturation of AIEC LF82-containing phagosomes within J774 macrophages. LF82-containing phagosomes traffic through the endocytic pathway as shown by the sequential acquisition and loss of EEA1 and Rab7 and by accumulation of Lamp-1, Lamp-2 and cathepsin D. We demonstrated that AIEC LF82-containing phagosomes mature into active phagolysosomes where bacteria are exposed to low pH and to the degradative activity of cathepsin D. Finally, we showed that an acidic environment is necessary for replication of AIEC LF82 bacteria within J774 macrophages. Thus, evidence is provided that AIEC LF82 bacteria do not escape from the endocytic pathway but undergo normal interaction with host endomembrane organelles and replicate within acidic and cathepsin D-positive vacuolar phagolysosomes.

Intracellular trafficking of Parachlamydia acanthamoebae

Parachlamydia acanthamoebae is an obligate intracellular bacterium that naturally infects free-living amoebae. It is a potential agent of pneumonia that resists destruction by human macrophages. However, the strategy used by this Chlamydia-like organism in order to resist to macrophage destruction is unknown. We analysed the intracellular trafficking of P. acanthamoebae within monocyte-derived macrophages. Infected cells were immunolabelled for the bacteria and for various intracellular compartments by using specific antibodies. We analysed the bacteria colocalization with the different subcellular compartments by using epifluorescence and confocal microscopy. Bacterial replication took place 4-6 h post infection within acidic vacuoles. At that time, P. acanthamoebae colocalized with Lamp-1, a membrane marker of late endosomal and lysosomal compartments. A transient accumulation of EEA1 15 min post infection, and of rab7 and the mannose 6-phosphate receptor 30 min post infection confirmed that P. acanthamoebae traffic through the endocytic pathway. The acquisition of Lamp-1 was not different after infection with living and heat-inactivated bacteria. However, 24.5% and 79.5% of living and heat-inactivated P. acanthamoebae, respectively, colocalized with the vacuolar proton ATPase. Moreover, P. acanthamoebae did not colocalized with cathepsin D, a lysosomal hydrolase, suggesting that P. acanthamoebae interferes with maturation of its vacuole. Thus, P. acanthamoebae survives to destruction by human macrophages probably by controlling the vacuole biogenesis.

Macropinocytosis of Polyplexes and Recycling of Plasmid via the Clathrin-Dependent Pathway Impair the Transfection Efficiency of Human Hepatocarcinoma Cells

Knowledge of the entry mechanism and intracellular routing of polyplexes is of major importance for designing efficient gene delivery systems. We therefore investigated the internalization and trafficking of polyplexes in HepG2 cells. pDNA encoding the luciferase was complexed with histidylated polylysine (His-pLK), a polymer that requires acidic pH for pDNA endosomal release. Fluoresceinylated polyplexes (F-His-pLK or F-pDNA) were internalized by clathrin-dependent and -independent pathways. The latter most likely occurred by macropinocytosis since it was stimulated by phorbol myristate and blocked by dimethylamiloride. Intracellular routing of the plasmid was analyzed by confocal microscopy and flow cytometry. These data revealed that: (i) one part of the plasmid was present in vesicles that were not labeled with any known organelle-specific marker, (ii) the other part was in transferrin receptor-positive vesicles, and (iii) the plasmid was not transferred to late endosomes/lysosomes. Using luciferase activity as a readout for gene expression, we found that it was strongly reduced when macropinocytosis was stimulated, whereas macropinocytosis inhibitors had no effect. However, blocking clathrin-dependent internalization by chlorpromazine completely prevented gene expression. These findings demonstrate that: (i) macropinocytosis of polyplexes and (ii) plasmid recycling impair the transfection efficiency and (iii) clathrin-dependent endocytosis is the most productive route for transfection of HepG2 cells.

Quantitative analysis of phagolysosome fusion in intact cells: inhibition by mycobacterial lipoarabinomannan and rescue by an 1alpha,25-dihydroxyvitamin D3-phosphoinositide 3-kinase pathway

Macrophage cell membranes were labeled with PKH26 and subsequently incubated with latex beads to generate phagosomes surrounded by a red-fluorescent membrane suitable for flow cytometry. Following cell disruption and partial purification of phagosomes, these vesicles were readily distinguished from both cell debris and free beads released from disrupted vacuoles. Flow cytometry analysis of phagosomes stained with specific mAbs and FITC-labeled secondary antibodies showed progressive acquisition of both Rab7 and LAMP-1 consistent with movement along the endocytic pathway. Alternatively, macrophages were preloaded with the lysosomal tracer FITC-dextran before membrane labeling with PKH and incubation with latex beads. Phagosome-lysosome fusion was then quantified on the basis of the colocalization of red and green signals. Using these flow cytometry-based systems, we showed that co-internalization of beads with lysates of Mycobacterium tuberculosis, but not lysates from the nonpathogenic organism Mycobacterium smegmatis, markedly decreased phagosome acquisition of Rab7 and LAMP-1 and vesicle fusion with FITC-dextran-loaded lysosomes. Inhibition of phagolysosome fusion could be attributed, at least in part, to the mycobacterial cell wall glycolipid lipoarabinomannan, and further analysis showed complete rescue of phagosome maturation when cells were pretreated with vitamin D3 before exposure to lipoarabinomannan. Moreover, the ability of vitamin D3 to reverse the phenotype of phagosomes in the presence of the glycolipid was completely abrogated by LY-294002, suggesting that vitamin D3 promotes phagolysosome fusion via a phosphoinositide 3-kinase signaling pathway. These findings establish a robust platform technology based on labeling of phagocyte cell membranes and flow cytometry capable of supporting broad-based screens to identify microbial and other bioactive compounds that influence phagosome biology.

Brucella evades macrophage killing via VirB-dependent sustained interactions with the endoplasmic reticulum

The intracellular pathogen Brucella is the causative agent of brucellosis, a worldwide zoonosis that affects mammals, including humans. Essential to Brucella virulence is its ability to survive and replicate inside host macrophages, yet the underlying mechanisms and the nature of the replicative compartment remain unclear. Here we show in a model of Brucella abortus infection of murine bone marrow-derived macrophages that a fraction of the bacteria that survive an initial macrophage killing proceed to replicate in a compartment segregated from the endocytic pathway. The maturation of the Brucella-containing vacuole involves sustained interactions and fusion with the endoplasmic reticulum (ER), which creates a replicative compartment with ER-like properties. The acquisition of ER membranes by replicating Brucella is independent of ER-Golgi COPI-dependent vesicular transport. A mutant of the VirB type IV secretion system, which is necessary for intracellular survival, was unable to sustain interactions and fuse with the ER, and was killed via eventual fusion with lysosomes. Thus, we demonstrate that live intracellular Brucella evade macrophage killing through VirB-dependent sustained interactions with the ER. Moreover, we assign an intracellular function to the VirB system, as being required for late maturation events necessary for the biogenesis of an ER-derived replicative organelle.

Salmonella typhimurium SifA effector protein requires its membrane-anchoring C-terminal hexapeptide for its biological function

SifA is a Salmonella typhimurium effector protein that is translocated across the membrane of the Salmonella-containing vacuole by the Salmonella pathogenicity island 2-encoded type III secretion system. SifA is necessary for the formation of Salmonella-induced filaments and for the maintenance of the vacuolar membrane enclosing the pathogen. We have investigated the role of the C-terminal hexapeptide of SifA as a potential site for membrane anchoring. An S. typhimurium strain carrying a deletion of the sequence encoding this hexapeptide (sifA Delta 6) was found to be attenuated for systemic virulence in mice. In mouse macrophages, sifA Delta 6 mutant bacteria displayed a reduced association with vacuolar markers, similar to that of sifA null mutant bacteria, and exhibited a dramatic replication defect. Expression of SifA in epithelial cells results in the mobilization of lysosomal glycoproteins in large vesicular structures and Sif-like tubules. This process requires the presence of the C-terminal hexapeptide domain of SifA. Ectopic expression of truncated or mutated versions of SifA affecting the C-terminal hexapeptide revealed a strong correlation between the membrane binding capability and the biological activity of the protein. Finally, the eleven C-terminal residues of SifA are shown to be sufficient to target the Aequorea green fluorescent protein to membranes. Altogether, our results indicate that membrane anchoring of SifA requires its C-terminal hexapeptide domain, which is important for the biological function of this bacterial effector.

Endoplasmic reticulum-mediated phagocytosis is a mechanism of entry into macrophages

Phagocytosis is a key aspect of our innate ability to fight infectious diseases. In this study, we have found that fusion of the endoplasmic reticulum (ER) with the macrophage plasmalemma, underneath phagocytic cups, is a source of membrane for phagosome formation in macrophages. Successive waves of ER become associated with maturing phagosomes during phagolysosome biogenesis. Thus, the ER appears to possess unexpectedly pluripotent fusion properties. ER-mediated phagocytosis is regulated in part by phosphatidylinositol 3-kinase and used for the internalization of inert particles and intracellular pathogens, regardless of their final trafficking in the host. In neutrophils, where pathogens are rapidly killed, the ER is not used as a major source of membrane for phagocytosis. We propose that intracellular pathogens have evolved to adapt and exploit ER-mediated phagocytosis to avoid destruction in host cells.

Endosomal proteolysis of internalized insulin at the C-terminal region of the B chain by cathepsin D

The endosomal compartment of hepatic parenchymal cells contains an acidic endopeptidase, endosomal acidic insulinase, which hydrolyzes internalized insulin and generates the major primary end product A(1--21)-B(1--24) insulin resulting from a major cleavage at residues Phe(B24)-Phe(B25). This study addresses the nature of the relevant endopeptidase activity in rat liver that is responsible for most receptor-mediated insulin degradation in vivo. The endosomal activity was shown to be aspartic acid protease cathepsin D (CD), based on biochemical similarities to purified CD in 1) the rate and site of substrate cleavage, 2) pH optimum, 3) sensitivity to pepstatin A, and 4) binding to pepstatin A-agarose. The identity of the protease was immunologically confirmed by removal of greater than 90% of the insulin-degrading activity associated with an endosomal lysate using polyclonal antibodies to CD. Moreover, the elution profile of the endosomal acidic insulinase activity on a gel-filtration TSK-GEL G3000 SW(XL) high performance liquid chromatography column corresponded exactly with the elution profile of the immunoreactive 45-kDa mature form of endosomal CD. Using nondenaturating immunoprecipitation and immunoblotting procedures, other endosomal aspartic acid proteases such as cathepsin E and beta-site amyloid precursor protein-cleaving enzyme (BACE) were ruled out as candidate enzymes for the endosomal degradation of internalized insulin. Immunofluorescence studies showed a largely vesicular staining pattern for internalized insulin in rat hepatocytes that colocalized partially with CD. In vivo pepstatin A treatment was without any observable effect on the insulin receptor content of endosomes but augmented the phosphotyrosine content of the endosomal insulin receptor after insulin injection. These results suggest that CD is the endosomal acidic insulinase activity which catalyzes the rate-limiting step of the in vivo cleavage at the Phe(B24)-Phe(B25) bond, generating the inactive A(1--21)-B(1--24) insulin intermediate.

Rab-interacting lysosomal protein (RILP): the Rab7 effector required for transport to lysosomes

Rab7 is a small GTPase that controls transport to endocytic degradative compartments. Here we report the identification of a novel 45 kDa protein that specifically binds Rab7GTP at its C-terminus. This protein contains a domain comprising two coiled-coil regions typical of myosin-like proteins and is found mainly in the cytosol. We named it RILP (Rab-interacting lysosomal protein) since it can be recruited efficiently on late endosomal and lysosomal membranes by Rab7GTP. RILP-C33 (a truncated form of the protein lacking the N-terminal half) strongly inhibits epidermal growth factor and low-density lipoprotein degradation, and causes dispersion of lysosomes similarly to Rab7 dominant-negative mutants. More importantly, expression of RILP reverses/prevents the effects of Rab7 dominant-negative mutants. All these data are consistent with a model in which RILP represents a downstream effector for Rab7 and both proteins act together in the regulation of late endocytic traffic.

Accumulation and aggregation of amyloid beta-protein in late endosomes of Niemann-pick type C cells

There is growing evidence suggesting that cholesterol metabolism is linked to susceptibility to Alzheimer's disease by influencing amyloid beta-protein (Abeta) metabolism. However, the precise cellular linkage sites between cholesterol and Abeta have not yet been clarified. To address this issue, we investigated Niemann-Pick type C (NPC) model cells and NPC mutant cells, which showed aberrant cholesterol trafficking. We observed a remarkable Abeta accumulation in late endosomes of both NPC model cells and mutant cells where cholesterol accumulates and a significant accumulation in the NPC mouse brain. This Abeta accumulation was independent of its constitutive secretion and production through an endocytic pathway. In addition, it is characterized by a marked predominance of Abeta42 and insolubility in SDS, suggesting the presence of aggregated Abeta in late endosomes. Most importantly, Abeta accumulation is coupled with the cholesterol levels in late endosomes. Thus, late endosomes of NPC cells are a novel pool of aggregated Abeta42 as well as cholesterol, suggesting a direct interaction between aggregated Abeta and cholesterol.

Impaired recruitment of the small GTPase rab7 correlates with the inhibition of phagosome maturation by Leishmania donovani promastigotes

We have shown recently that one of the survival strategies used by Leishmania donovani promastigotes during the establishment of infection in macrophages consists in inhibiting phagosome-endosome fusion. This inhibition requires the expression of lipophosphoglycan (LPG), the predominant surface glycoconjugate of promastigotes, as parasites expressing truncated forms of LPG reside in phagosomes that fuse extensively with endocytic organelles. In the present study, we developed a single-organelle fluorescence analysis approach to study and analyse the intracellular trafficking of 'fusogenic' and 'low-fusogenic' phagosomes induced by an LPG repeating unit-defective mutant (Ipg2 KO) or by wild-type L. donovani promastigotes respectively. The results obtained indicate that phagosomes containing mutant parasites fuse extensively with endocytic organelles and transform into phagolysosomes by losing the early endosome markers EEA1 and transferrin receptor, and acquiring the late endocytic and lysosomal markers rab7 and LAMP1. In contrast, a majority of 'low-fusogenic' phagosomes containing wild-type L. donovani promastigotes do not acquire rab7, wheres they acquire LAMP1 with slower kinetics. These results suggest that L. donovani parasites use LPG to restrict phagosome-endosome fusion at the onset of infection in order to prevent phagosome maturation. This is likely to permit the transformation of hydrolase-sensitive promastigotes into hydrolase-resistant amastigotes within a hospitable vacuole not displaying the harsh environment of phagolysosomes.

Biogenesis of Salmonella typhimurium-containing vacuoles in epithelial cells involves interactions with the early endocytic pathway

In epithelial cells, the intracellular pathogen Salmonella typhimurium resides and replicates within a unique cytoplasmic organelle, the Salmonella-containing vacuole (SCV). In vitro studies have shown that the SCV is a dynamic organelle that selectively acquires lysosomal glycoproteins (Igps) without fusing directly with lyosomes. Here, we have investigated early events in SCV biogenesis using immunofluorescence microscopy and epitope-specific flow cytometry. We show that proteins specific to the early endocytic pathway, EEA1 and transferrin receptor (TR), are present on early SCVs. The association of these proteins with SCVs is transient, and both proteins are undetectable at later time points when Igp and vATPase are acquired. Analysis of the fraction of SCVs containing both TR and lamp-1 showed that TR is lost from SCVs as the Igp is acquired, and that these processes occur progressively and not as the result of a single fusion/fission event. These experiments reveal a novel mechanism of SCV biogenesis, involving previously undetected initial interactions with the early endocytic pathway followed by the sequential delivery of Igp. The pathway does not involve interactions with the late endosome/prelysosome and is distinct from traditional phagocytic and endocytic pathways. Our study indicates that intracellular S. typhimurium occupies a unique niche, branching away from the traditional endocytic pathway between the early and late endosomal compartments.

Brucella abortus transits through the autophagic pathway and replicates in the endoplasmic reticulum of nonprofessional phagocytes

Brucella abortus is an intracellular pathogen that replicates within a membrane-bounded compartment. In this study, we have examined the intracellular pathway of the virulent B. abortus strain 2308 (S2308) and the attenuated strain 19 (S19) in HeLa cells. At 10 min after inoculation, both bacterial strains are transiently detected in phagosomes characterized by the presence of early endosomal markers such as the early endosomal antigen 1. At approximately 1 h postinoculation, bacteria are located within a compartment positive for the lysosome-associated membrane proteins (LAMPs) and the endoplasmic reticulum (ER) marker sec61beta but negative for the mannose 6-phosphate receptors and cathepsin D. Interestingly, this compartment is also positive for the autophagosomal marker monodansylcadaverin, suggesting that S2308 and S19 are located in autophagic vacuoles. At 24 h after inoculation, attenuated S19 is degraded in lysosomes, while virulent S2308 multiplies within a LAMP- and cathepsin D-negative but sec61beta- and protein disulfide isomerase-positive compartment. Furthermore, treatment of infected cells with the pore-forming toxin aerolysin from Aeromonas hydrophila causes vacuolation of the bacterial replication compartment. These results are compatible with the hypothesis that pathogenic B. abortus exploits the autophagic machinery of HeLa cells to establish an intracellular niche favorable for its replication within the ER.