ABSTRACT
Legionella pneumophila is an intracellular pathogen that modulates the biogenesis of its phagosome to evade endocytic vesicle traffic. The Legionella-containing phagosome (LCP) does not acquire any endocytic markers and is remodeled by the endoplasmic reticulum during early stages. Here we show that intracellular replication of L. pneumophila is inhibited in gamma interferon (IFN-γ)-activated, bone marrow-derived mouse macrophages and IFN-γ-activated, human monocyte-derived macrophages in a dose-dependent manner. This inhibition of intracellular replication is associated with the maturation of the LCP into a phagolysosome, as documented by the acquisition of LAMP-2, cathepsin D, and lysosomal tracer Texas Red ovalbumin, and with the failure of the LCP to be remodeled by the rough endoplasmic reticulum. We conclude that IFN-γ-activated macrophages override the ability of L. pneumophila to evade endocytic fusion and that the LCP is processed through the “default” endosomal-lysosomal degradation pathway.
Legionella pneumophila is an environmental organism that is ubiquitous in the aquatic environment, and its invasion and intracellular replication within protozoa are essential for its ecology and infectivity to humans (32, 33, 44, 48). This organism is the causative agent of Legionnaires' disease, which is manifested as pneumonia as a result of intracellular replication within phagocytic cells, primarily alveolar macrophages (25, 36). Upon entry into macrophages, L. pneumophila resides in a phagosome that is excluded from endocytic traffic (22, 23, 38). During early stages of infection, the Legionella-containing phagosome (LCP) excludes all endocytic and lysosomal markers such as LAMP-1, LAMP-2, and the lysosomal acid protease cathepsin D (11, 38, 40, 47). In addition to evading endocytic traffic, the LCP is remodeled by the rough endoplasmic reticulum (RER), and vesicles and mitochondria are recruited around the LCP within minutes of infection (26, 45, 46). Modulation of the biogenesis of the LCP, its evasion of endocytic traffic, and its remodeling by the RER are all controlled by the Dot/Icm type IV export machinery (26, 40, 46, 47). The Dot/Icm system is also essential for activation of caspase-3 during early stages of the infection, which is associated with cleavage of the Rab5 early endosome effector rabaptin-5 (15, 16, 50). During late stages of intracellular replication, the LCP membrane becomes disrupted, followed by bacterial escape into the cytoplasm where the bacteria continue to replicate (31). This transient replication in the cytoplasm is followed by lysis of the plasma membrane and bacterial egress from the host cell (2, 5, 28-30).
Interferon gamma (IFN-γ) has been demonstrated to activate host cells to control infection by L. pneumophila and other intracellular pathogens (4, 8, 9, 24, 27, 34, 35, 42). In vivo experiments have shown that L. pneumophila causes severe infection in IFN-γ-deficient mice. Treatment of L. pneumophila-infected mice with IFN-γ results in increased clearance of the bacteria from the lungs of the mice (41). The mechanisms by which IFN-γ-activated macrophages block intracellular replication of L. pneumophila are not well understood. These mechanisms may be partially mediated by limiting the availability of intracellular iron and the production of nitric oxide and reactive oxygen intermediates (20, 21). Whether activated macrophages alter trafficking of L. pneumophila and target it through a different endocytic route has never been examined. Interestingly, IFN-γ-activated macrophages prevent the escape of Listeria monocytogenes from the phagosome and lead to the killing of the organism (13). In addition, IFN-γ-activated macrophages override the ability of Mycobacterium avium to block acidification of the phagosome, leading to phagosome acidification and inhibition of intracellular replication (39). Interestingly, recent studies have shown that while Francisella tularensis modulates biogenesis of its phagosome and subsequently escapes into the cytoplasm of quiescent macrophages, the bacterial phagosome fuses to lysosomes within IFN-γ-activated macrophages (38a). Therefore, we tested the hypothesis that IFN-γ-activated macrophages may interfere with the ability of L. pneumophila to modulate the biogenesis of its phagosome.
The virulent clinical isolate of L. pneumophila strain AA100 and its isogenic dotA mutant (GL10) have been described previously (50). The bacteria were grown on buffered charcoal-yeast extract agar for 72 h. The wild-type L. pneumophila and dotA mutant strains harbored the plasmid pAM239, which encodes green fluorescent protein (37). The plates for gfp-transformed AA100 and dotA were supplemented with 5 μg/ml chloramphenicol.
Immortalized macrophage cell lines were established by infecting bone marrow-derived macrophages from A/J mice with the murine recombinant J2 retrovirus as previously described (6, 12). After a 6-day culture period in L-cell-conditioned medium, macrophages were collected by centrifugation and suspended in RPMI 1640 containing 10% heat-inactivated fetal bovine serum (Gibco BRL). Peripheral blood monocytes were isolated from adult volunteers who had no history of Legionnaires' disease, and informed consent was obtained from each volunteer as approved by the University of Louisville. The monocytes were obtained using the Ficoll-Hypaque gradient as described previously (18). The monocytes were allowed to mature to quiescent macrophages in Teflon tissue culture flasks as described previously (49).
The human monocyte-derived macrophages (hMDMs) (5 × 105 cells per well in 96-, 12-, or 24-well plates) and A/J mouse macrophages (5 × 105 cells per well in 96-well plates) were incubated for 24 h at 37°C in 5% CO2 with RPMI 1640 containing 10% fetal bovine serum and different concentrations (0.2, 1, or 2 μg/ml) of IFN-γ. Unless specified, all infections were carried out for 1 h followed by a washing off of the extracellular bacteria, and the infected cells were incubated for the indicated periods of infections specific for each experiment. A multiplicity of infection of 10 was used for intracellular growth kinetics, confocal laser scanning microscopy, and electron microscopy. For growth kinetics, 105 hMDMs and A/J mouse macrophages per well of 96-well plates were treated with IFN-γ and infected as described above. After 1, 15, 24, and 48 h of incubation, the cells were washed and either lysed immediately or incubated further. The number of L. pneumophila CFU in each well was determined on buffered charcoal-yeast extract agar as described previously (1).
Confocal microscopy to examine colocalization with endocytic markers was performed as we described previously (17). The anti-LAMP-2 (H4B4) (1:2,000) monoclonal antibody (developed by J. T. August and J. E. K. Hildreth) was obtained from the Developmental Studies Hybridoma Bank (University of Iowa). Monoclonal antibodies anti-cathepsin D (1:200) (BD Transduction) and anti-KDEL (1:200) were obtained from Stressgen. Anti-mouse secondary antibodies conjugated to Alexa fluor 594 were obtained from Molecular Probes (Oregon). To label the lysosomes, macrophages were incubated for 1.5 h with 100 μg/ml Trov (Molecular Probes), followed by a 30-min chase prior to infection with L. pneumophila. The cells were fixed, permeabilized, and stained with secondary antibody as described above. At least 100 infected cells from multiple coverslips were analyzed, and the experiments were performed at least three times. Transmission electron microscopy was performed as we described previously (31).
Inhibition of intracellular replication of L. pneumophila in IFN-γ-activated macrophages.
The role of IFN-γ in intracellular replication of the clinical isolate of L. pneumophila strain AA100 was examined in bone marrow-derived macrophages from A/J mice and in hMDMs. Macrophages were pretreated for 24 h with different concentrations of IFN-γ and infected by L. pneumophila for 1 h, washed, and either lysed immediately or lysed after an incubation period of 1, 15, 24, or 48 h. Control hMDMs and control mouse macrophages were infected with the dotA mutant (3). IFN-γ pretreatment of mouse macrophages and hMDMs inhibited intracellular replication of L. pneumophila in a dose-dependent manner (Fig. 1). In quiescent mouse and human macrophages, the number of L. pneumophila CFU increased by ∼10-fold 15 h after infection, while in activated macrophages, bacterial replication was inhibited. The maximum number of bacteria in quiescent macrophages was reached at 48 h after infection. Pretreatment with IFN-γ (1 μg/ml or higher) was essential to achieve maximal inhibition of L. pneumophila multiplication within macrophages. Addition of IFN-γ to the macrophages at the time of infection partially reduced intracellular replication (data not shown). Therefore, activation of macrophages by IFN-γ blocks intracellular replication of L. pneumophila in human and mouse macrophages, consistent with previous observations.
IFN-γ-activated A/J mouse macrophages (left panels) and hMDMs (right panels) inhibit intracellular replication of L. pneumophila. Macrophages were pretreated with the indicated concentrations of IFN-γ. The cells were infected for 1 h, followed by a wash of extracellular bacteria, and a determination was made of the number of intracellular bacteria at the indicated time points. The dotA mutant was used as a control for a replication-defective mutant. The error bars represent standard deviations of triplicate samples, and the results shown are representative of three independent experiments.
Previous studies have shown that simultaneous pretreatment of macrophages with IFN-γ and iron for 24 h prior to infection overcomes inhibition of intracellular replication of L. pneumophila (8, 35). We thought overloading the cells with iron for such an extended period of time may be nonphysiologic and may interfere with endocytic fusion events. Therefore, we examined whether a supplement of iron (8, 35) at the time of infection or 1 h or 24 h prior to infection would overcome inhibition of intracellular replication in IFN-γ-activated macrophages. Our data showed that the supplement of excess iron 24 h prior to infection abolished the effect of IFN-γ on inhibition of intracellular replication, consistent with previous observations (8, 35) (data not shown). In contrast, supplementation of IFN-γ-activated macrophages with excess iron at the time of infection or 1 h prior to infection did not rescue the inhibition of intracellular replication (data not shown). We conclude that iron supplementation of IFN-γ-activated macrophages does not reverse the ability of activated macrophages to inhibit intracellular replication of L. pneumophila.
Alteration of intracellular trafficking of L. pneumophila within IFN-γ-activated macrophages.
We examined, by confocal laser scanning microscopy, colocalization of the LCP with the late endosomal-lysosomal marker LAMP-2 and the lysosomal acid protease enzyme cathepsin D. For a positive control, we utilized the dotA mutant that is defective in the Dot/Icm export machinery, and its phagosome was trafficked through the endosomal-lysosomal degradation pathway (3, 40, 47). The hMDMs were pretreated with IFN-γ for 24 h and then infected by L. pneumophila for 1 h. The cells were fixed and processed for confocal microscopy at 1 h postinfection. The data showed that in quiescent hMDMs, LAMP-2 did not colocalize with the LCPs of the wild-type (WT) strain AA100 (Fig. 2 and data not shown). In contrast, 90% of the LCPs within IFN-γ-activated macrophages colocalized with the late endosomal-lysosomal marker LAMP-2 (Fig. 2 and data not shown). Approximately 80% of dotA mutant phagosomes acquired LAMP-2, consistent with previous observations (Fig. 2 and data not shown).
Quantitation of acquisition of late endosomal and lysosomal markers by the LCP within quiescent and IFN-γ-activated hMDMs. FK, Formalin-killed L. pneumophila. At least 100 infected cells from multiple coverslips were examined in each experiment. Results are representative of three independent experiments performed in triplicate. Some of the error bars are too small to be displayed.
While 95% of WT strain AA100 phagosomes excluded the lysosomal acid protease cathepsin D (14) in quiescent hMDMs, 80% of the WT LCPs colocalized with cathepsin D at 1 h postinfection of IFN-γ-activated hMDMs (Fig. 2 and data not shown). For the dotA mutant, used as a positive control, 78% of LCPs colocalized with cathepsin D (Fig. 2 and data not shown).
We next examined whether the LCP acquired the lysosomal tracer Trov, which was preloaded into the lysosomes of quiescent and IFN-γ-activated hMDMs, followed by a chase prior to infection. The data showed that in IFN-γ-activated hMDMs, 70% of the LCPs acquired Trov. In contrast, only 12% of the LCPs in quiescent hMDMs acquired Trov (Fig. 2 and data not shown). More than 70% of the phagosomes harboring paraformaldehyde-killed bacteria, used as a positive control, acquired Trov. Considering these findings, we conclude that the LCP is trafficked through the endosomal-lysosomal degradation pathway within IFN-γ-activated macrophages.
Effect of IFN-γ on remodeling the LCP into an RER-derived phagosome.
We examined whether the L. pneumophila phagosome within IFN-γ-activated hMDMs colocalized with the RER. Confocal microscopy was used to examine colocalization of the LCP with the endoplasmic reticulum (ER) marker KDEL by the use of specific antibodies. The dotA mutant was used as a negative control, since it fails to recruit the ER. Within quiescent hMDMs, 95% of the LCPs colocalized with the ER marker KDEL. In contrast, only 19% of the LCPs colocalized with the ER within IFN-γ-activated hMDMs. Similarly, less then 20% of dotA mutant phagosomes colocalized with the ER (Fig. 3 and data not shown). These results were confirmed by transmission electron microscopy (Fig. 3 and data not shown). We conclude that failure of L. pneumophila to evade endocytic fusion in IFN-γ-activated macrophages was associated with the failure to remodel its phagosome by the RER.
Failure of the LCP to be remodeled by the ER in activated macrophages. A) Quantification of the acquisition of the KDEL marker was determined by confocal microscopy at 4 h postinfection. At least 100 infected cells from multiple coverslips were examined in each experiment. Results are representative of three independent experiments performed in triplicate. Some of the error bars are too small to be displayed. B) Representative transmission electron micrographs of remodeling of the LCP by the RER in quiescent but not IFN-γ-activated hMDMs at 4 h postinfection.
There are several potential mechanisms by which IFN-γ-activated macrophages inhibit the intracellular replication of L. pneumophila, such as activation of reactive oxygen intermediates and reactive nitrogen intermediates (7, 20, 21, 43), restriction of iron availability (8-10, 19, 21), and down-regulation of the transferrin receptors (8). However, in our current studies, when excess iron is added at 1 h prior to infection of IFN-γ-activated macrophages, the effect on inhibition of intracellular replication is not reversed by the presence of excess iron. Our data show alteration in trafficking of L. pneumophila in IFN-γ-activated macrophages where the ability of the organism to modulate the biogenesis of its phagosome into an RER-derived replicative niche is completely abolished. Instead, the phagosome harboring the organism matures into a phagolysosome within IFN-γ-activated macrophages. It is possible that the targeting of L. pneumophila into the phagolysosomes is a primary effect of activation of macrophages by IFN-γ. We speculate that multiple processes are triggered within IFN-γ-activated macrophages to restrict intracellular replication of L. pneumophila, and some of these processes lead to alteration in trafficking of the organism. It is also likely that these processes may be distinct between human and mouse macrophages, particularly the generation of reactive nitrogen intermediates.
In summary, our data clearly show that the ability of L. pneumophila to evade endocytic fusion and to remodel the LCP into an ER-derived vesicle is completely inhibited in IFN-γ-activated macrophages, similar to that of the dot/icm mutants. Within IFN-γ-activated macrophages, the LCPs are processed through the “default” endosomal-lysosomal degradation pathway similar to the pathway of dot/icm mutants and inert particles as well.
ACKNOWLEDGMENTS
Y.A.K. is supported by Public Health Service awards RO1AI43965 and R21AI038410-06A1 and by the Commonwealth of Kentucky Research Challenge Trust Fund.
FOOTNOTES
- Received 1 December 2004.
- Accepted 12 January 2005.
- Copyright © 2005 American Society for Microbiology
