Search

International audience; Within tuberculous granulomas, a subpopulation of Mycobacterium tuberculosis resides inside foamy macrophages (FM) that contain abundant cytoplasmic lipid bodies (LB) filled with triacylglycerol (TAG). Upon fusion of LB with M. tuberculosis-containing phagosomes, TAG is hydrolyzed and reprocessed by the bacteria into their own lipids, which accumulate as intracytosolic lipid inclusions (ILI). This phenomenon is driven by many mycobacterial lipases, among which LipY participates in the hydrolysis of host and bacterial TAG. However, the functional contribution of LipY's PE domain to TAG hydrolysis remains unclear. Here, enzymatic studies were performed to compare the lipolytic activities of recombinant LipY and its truncated variant lacking the N-terminal PE domain, LipY(ΔPE). Complementarily, an FM model was used where bone marrow-derived mouse macrophages were infected with M. bovis BCG strains either overexpressing LipY or LipY(ΔPE) or carrying a lipY deletion mutation prior to being exposed to TAG-rich very-low-density lipoprotein (VLDL). Results indicate that truncation of the PE domain correlates with increased TAG hydrolase activity. Quantitative electron microscopy analyses showed that (i) in the presence of lipase inhibitors, large ILI (ILI+3) were not formed because of an absence of LB due to inhibition of VLDL-TAG hydrolysis or inhibition of LB-neutral lipid hydrolysis by mycobacterial lipases, (ii) ILI+3 profiles in the strain overexpressing LipY(ΔPE) were reduced, and (iii) the number of ILI+3 profiles in the ΔlipY mutant was reduced by 50%. Overall, these results delineate the role of LipY and its PE domain in host and mycobacterial lipid consumption and show that additional mycobacterial lipases take part in these processes.

During the dormant phase of tuberculosis, Mycobacterium tuberculosis persists in lung granulomas by residing in foamy macrophages (FM) that contain abundant lipid bodies (LB) in their cytoplasm, allowing bacilli to accumulate lipids as intracytoplasmic lipid inclusions (ILI). An experimental model of FM is presented where bone marrow-derived mouse macrophages are infected with M. avium and exposed to very-low-density lipoprotein (VLDL) as a lipid source. Quantitative analysis of detailed electron microscope observations showed the following results. (i) Macrophages became foamy, and mycobacteria formed ILI, for which host triacylglycerides, rather than cholesterol, was essential. (ii) Lipid transfer occurred via mycobacterium-induced fusion between LB and phagosomes. (iii) Mycobacteria showed a thinned cell wall and became elongated but did not divide. (iv) Upon removal of VLDL, LB and ILI declined within hours, and simultaneous resumption of mycobacterial division restored the number of mycobacteria to the same level as that found in untreated control macrophages. This showed that the presence of ILI resulted in a reversible block of division without causing a change in the mycobacterial replication rate. Fluctuation between ILI either partially or fully extending throughout the mycobacterial cytoplasm was suggestive of bacterial cell cycle events. We propose that VLDL-driven FM constitute a well-defined cellular system in which to study changed metabolic states of intracellular mycobacteria that may relate to persistence and reactivation of tuberculosis.

Mycobacterium abscessusis a pathogenic, rapidly growing mycobacterium responsible for pulmonary and cutaneous infections in immunocompetent patients and in patients with Mendelian disorders, such as cystic fibrosis (CF).Mycobacterium abscessusis known to transition from a smooth (S) morphotype with cell surface-associated glycopeptidolipids (GPL) to a rough (R) morphotype lacking GPL. Herein, we show thatM. abscessusS and R variants are able to grow inside macrophages and are present in morphologically distinct phagosomes. The S forms are found mostly as single bacteria within phagosomes characterized by a tightly apposed phagosomal membrane and the presence of an electron translucent zone (ETZ) surrounding the bacilli. By contrast, infection with the R form leads to phagosomes often containing more than two bacilli, surrounded by a loose phagosomal membrane and lacking the ETZ. In contrast to the R variant, the S variant is capable of restricting intraphagosomal acidification and induces less apoptosis and autophagy. Importantly, the membrane of phagosomes enclosing the S forms showed signs of alteration, such as breaks or partial degradation. Although not frequently encountered, these events suggest that the S form is capable of provoking phagosome–cytosol communication. In conclusion,M. abscessusS exhibits traits inside macrophages that are reminiscent of slow-growing mycobacterial species.

Summary Mycobacterium tuberculosis thrives within macrophages by residing in phagosomes and preventing them from maturing and fusing with lysosomes. A parallel transcriptional survey of intracellular mycobacteria and their host macrophages revealed signatures of heavy metal poisoning. In particular, mycobacterial genes encoding heavy metal efflux P-type ATPases CtpC, CtpG, and CtpV, and host cell metallothioneins and zinc exporter ZnT1, were induced during infection. Consistent with this pattern of gene modulation, we observed a burst of free zinc inside macrophages, and intraphagosomal zinc accumulation within a few hours postinfection. Zinc exposure led to rapid CtpC induction, and ctpC deficiency caused zinc retention within the mycobacterial cytoplasm, leading to impaired intracellular growth of the bacilli. Thus, the use of P1-type ATPases represents a M. tuberculosis strategy to neutralize the toxic effects of zinc in macrophages. We propose that heavy metal toxicity and its counteraction might represent yet another chapter in the host-microbe arms race., Highlights ► Zinc accumulates in the M. tuberculosis (Mtb) phagosome in macrophages (Mϕ) ► Mtb P1-type ATPases, including CtpC, are induced upon exposure to zinc inside Mϕ ► CtpC enables Mtb resistance to zinc poisoning and intracellular survival in Mϕ ► P1-type zinc efflux ATPase ZntA null E. coli is highly susceptible to Mϕ killing

The appearance of drug-resistant strains of Mycobacterium tuberculosis (Mtb) poses a great challenge to the development of novel treatment programmes to combat tuberculosis. Since innovative nanotechnologies might alleviate the limitations of current therapies, we have designed a new nanoformulation for use as an anti-TB drug delivery system. It consists of incorporating mycobacterial cell wall mycolic acids (MA) as targeting ligands into a drug-encapsulating Poly dl-lactic-co-glycolic acid polymer (PLGA), via a double emulsion solvent evaporation technique. Bone marrow-derived mouse macrophages, either uninfected or infected with different mycobacterial strains (Mycobacterium avium, Mycobacterium bovis BCG or Mtb), were exposed to encapsulated isoniazid-PLGA nanoparticles (NPs) using MA as a targeting ligand. The fate of the NPs was monitored by electron microscopy. Our study showed that i) the inclusion of MA in the nanoformulations resulted in their expression on the outer surface and a significant increase in phagocytic uptake of the NPs; ii) nanoparticle-containing phagosomes were rapidly processed into phagolysosomes, whether MA had been included or not; and iii) nanoparticle-containing phagolysosomes did not fuse with non-matured mycobacterium-containing phagosomes, but fusion events with mycobacterium-containing phagolysosomes were clearly observed.

Summary The growth of pathogenic mycobacteria in phagosomes of the host cell correlates with their ability to prevent phagosome maturation. The underlying molecular mechanism remains elusive. In a previous study, we have shown that Mycobacterium avium depletes the phagosome membrane of cell surface-derived glycoconjugates (de Chastellier and Thilo, Eur. J. Cell Biol. 81, 17 – 25, 2002). We now extended these quantitative observations to the major human pathogen, Mycobacterium tuberculosis (H37Rv). At increasing times after infection of mouse bone marrow-derived macrophages, cell-surface glycoconjugates were labelled enzymatically with [ 3 H]galactose. Subsequent endocytic membrane traffic resulted in a redistribution of this label from the cell surface to endocytic membranes, including phagosomes. The steady-state distribution was measured by quantitative autoradiography at the electron microscope level. Relative to early endosomes, with which phagosomes continued to fuse and rapidly exchange membrane constituents, the phagosome membrane was depleted about 3-fold, starting during infection and in the course of 9 days thereafter. These results were in quantitative agreement with our previous observations for Mycobacterium avium . For the latter case, we now showed by cell fractionation that the depletion was selective, mainly involving glycoproteins in the 110 – 210 kDa range. Together, these results indicated that pathogenic mycobacteria induced and maintained a bulk change in phagosome membrane composition that could be of special relevance for survival of pathogenic mycobacteria within phagosomes.

Pathogenic mycobacteria prevent maturation of the phagosomes in which they reside inside macrophages and this is thought to be a major strategy allowing them to survive and multiply within macrophages. The molecular basis for this inhibition is only now beginning to emerge with the molecular characterization of the phagosome membrane enclosing these pathogens. We have used here several electron microscopy approaches in combination with counts of bacterial viability to analyse how expression of Nramp1 at the phagosomal membrane may influence survival of Mycobacterium avium and affect its ability to modulate the fusogenic properties of the phagosome in which it resides. The experiments were carried out in bone marrow-derived macrophages from wild-type 129sv (Nramp1(G169)) mice and from isogenic 129sv carrying a null mutation at Nramp1 (Nramp(1-/-)) following infection with a virulent strain of M. avium. We show here that Nramp1 expression has a bacteriostatic effect and that abrogation of Nramp1 restores the bacteria's capacity to replicate within macrophages. The combined analyses of the acquisition of endocytic contents markers delivered to early endosomes and/or lysosomes either prior to or after phagocytic uptake showed that in Nramp1-positive macrophages, M. avium was unable to prevent phagosome maturation and fusion with lysosomes but that in Nramp1-negative macrophages this capacity was restored. Several hypotheses are proposed to explain how Nramp1 could affect survival of M. avium. We also propose how the present observations could relate to the model according to which mycobacteria can prevent phagosome maturation by establishing a tight interaction with constituents of the phagosome membrane. Furthermore, these results show the importance of the choice of macrophages used as a model to study intracellular survival strategies of pathogens.

Pathogens have evolved a wide variety of strategies to circumvent the host microbicidal activities and to use the cellular machinery to their own advantage. This chapter is devoted to electron microscopy, which is the approach of choice for optimal resolution and precision. A wide variety of cytochemical and immunoelectron microscopy methods can be used to characterize pathogens, analyze the intracellular compartment in which they reside, and localize bacterial virulence factors or cell components involved in their survival. Some of the major morphological methods, recent and less recent, of special interest to host-pathogen interplay are reviewed in this chapter. Phagosomes that retain intermingling characteristics of early endosomes are considered to be immature; those that become mature lose their ability to fuse with early endosomes and fuse with lysosomes to become phagolysosomes. Endocytosis and phagocytosis both involve an extensive transfer of membrane in both directions between the cell surface and intracellular membrane compartments. Given the large amounts of membrane required during uptake of particles and also during replication of endoparasites within their phagocytic vacuole, one of the questions that have been raised recently was whether other cell compartments, and more especially the endoplasmic reticulum (ER), could be an additional source of membrane for forming and/or dividing phagosomes.

Mycobacterium tuberculosis is an intracellular pathogen. Within macrophages, M. tuberculosis thrives in a specialized membrane-bound vacuole, the phagosome, whose pH is slightly acidic, and where access to nutrients is limited. Understanding how the bacillus extracts and incorporates nutrients from its host may help develop novel strategies to combat tuberculosis. Here we show that M. tuberculosis employs the asparagine transporter AnsP2 and the secreted asparaginase AnsA to assimilate nitrogen and resist acid stress through asparagine hydrolysis and ammonia release. While the role of AnsP2 is partially spared by yet to be identified transporter(s), that of AnsA is crucial in both phagosome acidification arrest and intracellular replication, as an M. tuberculosis mutant lacking this asparaginase is ultimately attenuated in macrophages and in mice. Our study provides yet another example of the intimate link between physiology and virulence in the tubercle bacillus, and identifies a novel pathway to be targeted for therapeutic purposes., Author Summary Tuberculosis (TB) is still responsible for nearly 1.3 million deaths annually. There is an urgent need to identify novel drug targets in the tubercle bacillus, Mycobacterium tuberculosis, in order to develop novel therapeutics. To proliferate inside its human host, and ensure its spreading, M. tuberculosis must adapt its nutritional requirements and metabolism to the molecular environment it encounters during infection. Elucidating the origin, nature, and acquisition mechanisms of the nutrients required by M. tuberculosis inside its host may help identify targets for novel antimicrobials. In this study we asked how the TB bacillus acquires nitrogen, a vital constituent of all living organisms, from host tissues. We show the amino acid asparagine to be an important source of nitrogen for the bacillus, and we identify two bacterial proteins, AnsP2 and AnsA, that allow the pathogen to capture and ‘digest’ asparagine, respectively. In addition, we report that asparagine ‘digestion’ allows the pathogen to resist the host immune defense and to survive inside host cells and tissues. This study paves the way for future research into M. tuberculosis nitrogen metabolism, and for the development of alternative therapeutic strategies to impair nitrogen acquisition by the bacillus and treat patients with TB.

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.

Pathogenic mycobacteria survive within macrophages by residing in phagosomes, which they prevent from maturing and fusing with lysosomes. Although several bacterial components were seen to modulate phagosome processing, the molecular regulatory mechanisms taking part in this process remain elusive. We investigated whether the phagosome maturation block (PMB) could be modulated by signaling through Ser/Thr phosphorylation. Here, we demonstrated that mycolic acid cyclopropane synthase PcaA, but not MmaA2, was phosphorylated by mycobacterial Ser/Thr kinases at Thr-168 and Thr-183 both in vitro and in mycobacteria. Phosphorylation of PcaA was associated with a significant decrease in the methyltransferase activity, in agreement with the strategic structural localization of these two phosphoacceptors. Using a BCG ΔpcaA mutant, we showed that PcaA was required for intracellular survival and prevention of phagosome maturation in human monocyte-derived macrophages. The physiological relevance of PcaA phosphorylation was further assessed by generating PcaA phosphoablative (T168A/T183A) or phosphomimetic (T168D/T183D) mutants. In contrast to the wild-type and phosphoablative pcaA alleles, introduction of the phosphomimetic pcaA allele in the ΔpcaA mutant failed to restore the parental mycolic acid profile and cording morphotype. Importantly, the PcaA phosphomimetic strain, as the ΔpcaA mutant, exhibited reduced survival in human macrophages and was unable to prevent phagosome maturation. Our results add new insight into the importance of mycolic acid cyclopropane rings in the PMB and provide the first evidence of a Ser/Thr kinase-dependent mechanism for modulating mycolic acid composition and PMB.

International audience; Pathogenic mycobacteria survive in macrophages of the host organism by residing in phagosomes which they prevent from undergoing maturation and fusion with lysosomes. Several molecular mechanisms have been associated with the phagosome maturation block. Here we show for Mycobacterium avium in mouse bone marrow-derived macrophages that the maturation block required an all-around close apposition between the mycobacterial surface and the phagosome membrane. When small (0.1 microm) latex beads were covalently attached to the mycobacterial surface to act as a spacer that interfered with a close apposition, phagosomes rapidly acquired lysosomal characteristics as indicators for maturation and fusion with lysosomes. As a result, several mycobacteria were delivered into single phagolysosomes. Detailed electron-microscope observations of phagosome morphology over a 7-day post-infection period showed a linear correlation between bead attachment and phagosome-lysosome fusion. After about 3 days post infection, conditions inside phagolysosomes caused a gradual release of beads. This allowed mycobacteria to re-establish a close apposition with the surrounding membrane and sequester themselves into individual, non-maturing phagosomes which had lost lysosomal characteristics. By rescuing themselves from phagolysosomes, mycobacteria remained fully viable and able to multiply at the normal rate. In order to unify the present observations and previously reported mechanisms for the maturation block, we discuss evidence that they may act synergistically to interfere with 'Phagosome Membrane Economics' by causing relative changes in incoming and outgoing endocytic membrane fluxes.

International audience; A major virulence factor of pathogenic mycobacteria is their ability to parasitize the host's scavenger cells and more particularly macrophages. The present overview discusses the known cellular and molecular mechanisms of intracellular survival of Mtb and other pathogenic mycobacteria within different intracellular niches, i.e. the macrophage in which they replicate and the granuloma in which they persist in a non-replicating state. After phagocytic uptake by macrophages, mycobacteria reside in phagosomes which they prevent from maturing and, as a result, from fusing with acidic and hydrolase-rich lysosomes. Two major points are highlighted: (i) the requirement for a close apposition between the phagosome membrane and the mycobacterial surface all around, and (ii) the ability for mycobacteria targeted to phagolysosomes to avoid degradation and to be rescued from this cytolytic environment to again reside in non-maturing phagosomes with a closely apposed membrane in which they can replicate. Concerning Mtb in granulomatous lesions, this review discusses the occurence of mycobacteria in lipid-rich foamy macrophages in which they persist in a non-replicating state. This overview highlights the major contribution of host cholesterol and/or fatty acids (triacylglycerol) in both prevention of phagosome maturation and persistence in granulomatous lesions.

Electron microscopy (EM) is the only technique that can combine sensitive protein-detection methods with detailed information on the substructure of cellular compartments. The purpose of this chapter is to describe some of the methods at the EM level that can be used to analyze the spatial organization of cell organelles with respect to phagosomes or vacuoles in which pathogens are sequestered, characterize the compartment in which pathogens are harbored, ie immature phagosomes, phagolysosomes, autophagosomes, and ER-derived vacuoles, to cite a few, and decipher the molecular mechanisms involved in survival of pathogens within infected host cells.

Legionella survives intracellularly by preventing fusion with lysosomes, due to phagosome escape from the endocytic pathway at an early stage of phagosome maturation, and by creating a replicative organelle that acquires endoplasmic reticulum (ER) characteristics through sustained interactions and fusion with the ER. Intracellular replication of Legionella pneumophila in mouse macrophages is controlled by the Lgn1 locus. Functional complementation in vivo has identified the Birc1e/Naip5 gene as being responsible for the Lgn1 effect. To understand the function and temporal site of action of Birc1e/Naip5 in susceptibility to L. pneumophila, we examined the biogenesis of Legionella-containing vacuoles (LCVs) formed in permissive A/J macrophages and in their Birc1e/Naip5 transgenic non-permissive counterpart. Birc1e/Naip5 effects on acquisition of lysosomal and ER markers were evident within 1-2 h following infection. A significantly higher proportion of LCVs formed in Birc1e/Naip5 transgenic macrophages had acquired the lysosomal markers cathepsin D and Lamp1 by 2 h post infection, whereas a significantly higher proportion of LCVs formed in permissive macrophages were positively stained for the ER markers BAP31 and calnexin, 6 h post infection. Likewise, studies by electron microscopy showed acquisition of lysosomal contents (horseradish peroxidase), within the first hour following phagocytic uptake, by LCVs formed in Birc1e/Naip5 transgenic macrophages and delivery of the ER marker glucose 6-phosphatase (G6Pase) only to the lumen of LCVs formed in A/J macrophages. Finally, a larger proportion of LCVs formed in A/J macrophages were studded with ribosomes 24 h post infection, compared with LCVs formed in Birc1e/Naip5 transgenic macrophages. These results suggest that sensing of L. pneumophila products by Birc1e/Naip5 in macrophages occurs rapidly following phagocytosis, a process that antagonizes the ability of L. pneumophila to remodel its phagosome into a specialized vacuole with ER characteristics.

SummaryPhagosomes were traditionally thought to originate from an invagination and scission of the plasma membrane to form a distinct intracellular vacuole. An alternative model implicating the endoplasmic reticulum (ER) as a major component of nascent and maturing phagosomes was recently proposed (Gagnon et al., 2002). To reconcile these seemingly disparate hypotheses, we used a combination of biochemical, fluorescence imaging, and electron microscopy techniques to quantitatively and dynamically assess the contribution of the plasmalemma and of the ER to phagosome formation and maturation. We could not verify even a transient physical continuity between the ER and the plasma membrane, nor were we able to detect a significant contribution of the ER to forming or maturing phagosomes in either macrophages or dendritic cells. Instead, our data indicate that the plasma membrane is the main constituent of nascent and newly formed phagosomes, which are progressively remodeled by fusion with endosomal and eventually lysosomal compartments as phagosomes mature into acidic, degradative organelles.

As part of their strategy for intracellular survival, mycobacteria prevent maturation of the phagosomes in which they reside inside macrophages. The molecular basis for this inhibition is only now beginning to emerge, by way of the molecular characterisation of the phagosome membrane when it encloses virulent mycobacteria. Our own work has shown that at 15 days after the phagocytic uptake of Mycobacterium avium by mouse bone marrow-derived macrophages, the phagosome membrane is depleted about 4-fold for cell surface-derived membrane glycoconjugates, labelled by exogalactosylation, in comparison to the membrane of early endosomes with which it continues to interact. Here we asked whether this depletion occurred at early or late stages after infection. We found that only about half of the depletion had occurred at about 5 hours after the beginning of phagocytic uptake, with the remainder becoming established thereafter, with a half-time of about 2.5 days. Phagosomes became depleted in relation to early endosomes with which they continued to exchange membrane constituents. Early endosomes themselves became gradually depleted by about 30% during the 15-day post-infection period. In contrast, late endosomes/lysosomes remained unchanged, with a concentration of surface-derived glycoconjugates between that of early endosomes and of phagosomes at day 15 post infection. In view of the slowness of the post-infection change of phagosome membrane composition, we proposed that this change did not play a role in preventing maturation immediately after phagosome formation, but rather correlated with the process of maintaining the phagosomes in an immature state.

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.

Publisher Summary Most studies on the intracellular behavior of mycobacteria are based on observations using host cells from mice or humans that are infected with a variety of mycobacterial species. Mouse peritoneal macrophages or human monocytes or monocyte-derived macrophages have generally been used in the case of Mycobacterium tuberculosis or Bacillus Calmette–Guerin (BCG). The use of cell lines for long term studies on the survival of mycobaeteria is, however, hindered by the cell growth rate, which is more rapid than that of most pathogenic mycobacteria. Artificial reduction of the cell growth rate should be avoided as it may give rise to misleading conclusions to the extent that cells may be affected in fundamental functions, including endocytosis and microbicidal activity. This chapter discusses the endocytic pathway, phagosomes, and mycobacteria. In many naturally occurring instances, mycobacteria may be phagocytosed in an opsonized state or as bacterial clumps, and this may lead to their delivery into phagolysosomes (see above). The specific interactions of mycobacteria with early endosome-derived membrane constituents may provide a rescue mechanism if mycobacteria can temporarily survive in phagolysosomes.

Macrophages represent major effector cells in the defense against a wide variety of microorganisms. However, several pathogens, such as mycobacteria (d’Arcy Hart 1982), Legionella pneumophila (Horwitz 1983),Toxoplasma gondii (Jones and Hirsch 1972), resist to the microbicidal activity of macrophages by blocking phagosome-lysosome fusions. Among the non tuberculous mycobacteria, M. avium emerges as a major pathogen in AIDS patients (Young et al. 1986). This organism survives within in vitro grown bone marrow-derived macrophages, remaining confined to the phagosome compartment in which it multiplies (Ryter et al. 1984). It resists to the hydrolytic activity of macrophages by inhibiting phagosome-lysosome (P-L) fusions (Frehel et al. 1986a) and thanks to its capsule that seems to reduce the diffusion of lysosomal enzymes (Frehel et al. 1986b).

Tuberculosis (TB) is characterized by a tight interplay between Mycobacterium tuberculosis and host cells within granulomas. These cellular aggregates restrict bacterial spreading, but do not kill all the bacilli, which can persist for years. In-depth investigation of M. tuberculosis interactions with granuloma-specific cell populations are needed to gain insight into mycobacterial persistence, and to better understand the physiopathology of the disease. We have analyzed the formation of foamy macrophages (FMs), a granuloma-specific cell population characterized by its high lipid content, and studied their interaction with the tubercle bacillus. Within our in vitro human granuloma model, M. tuberculosis long chain fatty acids, namely oxygenated mycolic acids (MA), triggered the differentiation of human monocyte-derived macrophages into FMs. In these cells, mycobacteria no longer replicated and switched to a dormant non-replicative state. Electron microscopy observation of M. tuberculosis–infected FMs showed that the mycobacteria-containing phagosomes migrate towards host cell lipid bodies (LB), a process which culminates with the engulfment of the bacillus into the lipid droplets and with the accumulation of lipids within the microbe. Altogether, our results suggest that oxygenated mycolic acids from M. tuberculosis play a crucial role in the differentiation of macrophages into FMs. These cells might constitute a reservoir used by the tubercle bacillus for long-term persistence within its human host, and could provide a relevant model for the screening of new antimicrobials against non-replicating persistent mycobacteria., Author Summary Mycobacterium tuberculosis, the causative agent of tuberculosis, is responsible for dramatic health problems globally. It is estimated that this pathogen infects one-third of the human population and causes three million deaths annually. Most individuals remain asymptomatic for several years before developing an active disease. In such individuals, the bacilli are not cleared but rather persist in a dormant state. Major goals of TB research are to (i) understand how the bacilli remain alive for years within infected individuals, and (ii) find how to prevent their reactivation and hence clinical disease. During dormancy, most of the bacilli are confined to granulomas that consist of well-defined aggregates of different host immune cells. Granulomas prevent spreading of bacilli. In this study, we analyzed the role of a particular cell population found within granulomas, the “foamy macrophages”. These cells are filled with droplets of lipids, a well-known nutrient for persistent bacilli. We found that within these cells, the bacilli do not replicate, but remain alive and seem to internalize host lipids. The foamy macrophages might thus constitute a reservoir for persisting bacilli within their human host, and could provide a relevant model for screening of new antimicrobials against non-replicating persistent mycobacteria.