Phagosomal Markers Research Articles

Clathrin‐mediated phagosome resolution regenerates lysosomes and maintains degradative capacity of macrophages after phagocytosis

Phagocytosis is the selective internalization of large particles and is critical for immunity, development and tissue homeostasis. The conversion of phagosomes into microbicidal compartments is a highly regulated process, which has been studied in great detail. Conversely, the fate of phagolysosomes, after their cargo is digested, is poorly understood. By utilizing fluorescent bacteria and labelling phagosomal markers, we followed mature phagolysosomes to their resolution. Despite previous assumptions, phagolysosomes are not exocytosed, but instead undergo a continuous dismembering process resulting in the shedding of myriads of vesicles containing degraded cargo. These phagosome‐derived vesicles resemble lysosomes, based on the acquisition of lysosomal markers, acidification and fusogenicity with newly formed phagosomes. Combining pharmacological and genetic strategies, we found that phagosomal fragmentation is dependent on cargo degradation, clathrin and the integrity of the actin and microtubule cytoskeleton. In addition, we provide evidence that phagosome‐derived vesicles are required to recycle lysosomal components allowing for the formation of new phagosomes and consequently maintaining the capacity of macrophages to ingest targets. Thus, we propose that phagolysosome resolution recycles cellular resources for the continuous generation of new phagosomes and the prevention of this phase will critically impair the immune and housekeeping functions of the phagocytes.Support or Funding InformationCanadian Institutes of Health Research Canada Research Chairs Program Ryerson University Natural Sciences and Engineering Council of CanadaClathrin mediates phagosome resolution. A. Macrophages engulfed mRFP1‐expressing E. coli. At 1 h post‐phagocytosis, phagosomes are rod‐shaped, like E. coli and mRFP1 is mostly associated with phagosomes. After 4 h of phagosome maturation, phagosomes shrunk and mRFP1 is found in small vesicular compartments. Clathrin inhibitors, pitstop and ikarugamycin, impair phagosome fragmentation. B. Quantification of phagosome‐derived vesicles, where b is significantly different from other conditions.Figure 1

Histophilus somni Survives in Bovine Macrophages by Interfering with Phagosome-Lysosome Fusion but Requires IbpA for Optimal Serum Resistance.

Histophilus somni is capable of intracellular survival within professional phagocytic cells, but the mechanism of survival is not understood. The Fic motif within the direct repeat (DR1)/DR2 domains of the IbpA fibrillary network protein of H. somni is cytotoxic to epithelial and phagocytic cells, which may interfere with the bactericidal activity of these cells. To determine the contribution of IbpA and Fic to resistance to host defenses, H. somni strains and mutants that lacked all or a region of ibpA (including the DR1/DR2 regions) were tested for survival in bovine monocytic cells and for serum susceptibility. An H. somni mutant lacking IbpA, but not the DR1/DR2 region within ibpA, was more susceptible to killing by antiserum than the parent, indicating that the entire protein was associated with serum resistance. H. somni strains expressing IbpA replicated in bovine monocytes for at least 72 h and were toxic for these cells. Virulent strain 2336 mutants lacking the entire ibpA gene or both DR1 and DR2 were not toxic to the monocytes but still survived within the monocytes for at least 72 h. Monitoring of intracellular trafficking of H. somni with monoclonal antibodies to phagosomal markers indicated that the early phagosomal marker early endosome antigen 1 colocalized with all isolates tested, but only strains that could survive intracellularly did not colocalize with the late lysosomal marker lysosome-associated membrane protein 2 and prevented the acidification of phagosomes. These results indicated that virulent isolates of H. somni were capable of surviving within phagocytic cells through interference in phagosome-lysosome maturation. Therefore, H. somni may be considered a permissive intracellular pathogen.

Mycobacterium bovis BCG Interferes with miR-3619-5p Control of Cathepsin S in the Process of Autophagy.

Main survival mechanism of pathogenic mycobacteria is to escape inimical phagolysosomal environment inside the macrophages. Many efforts have been made to unravel the molecular mechanisms behind this process. However, little is known about the involvement of microRNAs (miRNAs) in the regulation of phagolysosomal biosynthesis and maturation. Based on a bottom up approach, we searched for miRNAs that were involved in phagolysosomal processing events in the course of mycobacterial infection of macrophages. After infecting THP-1 derived macrophages with viable and heat killed Mycobacterium bovis BCG (BCG), early time points were identified after co-localization studies of the phagosomal marker protein LAMP1 and BCG. Differences in LAMP1 localization on the phagosomes of both groups were observed at 30 min and 4 h. After in silico based pre-selection of miRNAs, expression analysis at the identified time points revealed down-regulation of three miRNAs: miR-3619-5p, miR-637, and miR-324-3p. Consequently, most likely targets were predicted that were supposed to be mutually regulated by these three studied miRNAs. The lysosomal cysteine protease Cathepsin S (CTSS) and Rab11 family-interacting protein 4 (RAB11FIP4) were up-regulated and were considered to be connected to lysosomal trafficking and autophagy. Interaction studies verified the regulation of CTSS by miR-3619-5p. Down-regulation of CTSS by ectopic miR-3619-5p as well as its specific knockdown by siRNA affected the process of autophagy in THP-1 derived macrophages.

Attenuated Leishmania induce pro-inflammatory mediators and influence leishmanicidal activity by p38 MAPK dependent phagosome maturation in Leishmania donovani co-infected macrophages.

Promastigote form of Leishmania, an intracellular pathogen, delays phagosome maturation and resides inside macrophages. But till date limited study has been done to manipulate the phagosomal machinery of macrophages to restrict Leishmania growth. Attenuated Leishmania strain exposed RAW 264.7 cells showed a respiratory burst and enhanced production of pro-inflammatory mediators. The augmentation of pro-inflammatory activity is mostly attributed to p38 MAPK and p44/42 MAPK. In our study, these activated macrophages are found to induce phagosome maturation when infected with pathogenic Leishmania donovani. Increased co-localization of carboxyfluorescein succinimidyl ester labeled pathogenic L. donovani with Lysosome was found. Moreover, increased co-localization was observed between pathogenic L. donovani and late phagosomal markers viz. Rab7, Lysosomal Associated Membrane Protein 1, Cathepsin D, Rab9, and V-ATPase which indicate phagosome maturation. It was also observed that inhibition of V-type ATPase caused significant hindrance in attenuated Leishmania induced phagosome maturation. Finally, it was confirmed that p38 MAPK is the key player in acidification and maturation of phagosome in attenuated Leishmania strain pre-exposed macrophages. To our knowledge, this study for the first time reported an approach to induce phagosome maturation in L. donovani infected macrophages which could potentiate short-term prophylactic response in future.

Phagolysosomal Trafficking Assay.

Phagolysosomal trafficking is an important innate defense pathway that clears microbes by delivering them to lysosomes, the degradative compartment of the cell. Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis, subverts this host defense mechanism by arresting maturation of the phagosome. The ability of Mtb to arrest its delivery to the lysosome can be demonstrated by the prolonged co-localization of bacteria containing phagosomes/vacuole with early phagosomal markers [such as, Ras-related proteins in the brain 5 (Rab5) and Transferrin receptor (TfR)], and a failure to acquire late phagosomal and lysosomal markers (such as Rab7 and LAMP1) (Deretic and Fratti, 1999, Mehra et al., 2013). Here, a protocol is outlined for infection of macrophages with mycobacterial species like pathogenic Mtb, vaccine strain Mycobacterium bovis- bacillus Calmatte- Guérin (BCG) and rapidly dividing non-pathogenic Mycobacterium smegmatis (Msmeg), followed by indirect-immunofluorescence microscopy to visualize host vacuolar markers. Thereafter, automated quantification of degree of co-localization between mycobacteria and host vacuolar markers like TfR and LAMP1 is done by processing the binary images of bacteria using mathematical tools. This results in quantification of the mean fluorescence intensity (MFI) of these host markers directly around the bacteria/bacterial clusters with increased sensitivity relative to when done manually. By manipulating host or pathogen, this assay can be used to evaluate host or bacterial determinants of intracellular trafficking. The basic method can be applied to studying trafficking of other bacteria or particles like beads, although the kinetics of infection and phagosome maturation will depend upon the phagocytic cargo. The mathematical analysis tools are available in many standard imaging analysis programs. However, any adaption for similar analysis should be confirmed by the individual user with their imaging and analysis platform.

Application of Quantum Dots to the Study of Liposome Targeting in Leishmaniasis and Malaria

Nanotechnological devices for therapeutic applications are massively addressed to diseases prevalent in the developed world, particularly cancer, because of the wrong assumption (for both ethical and technical reasons) that nanomedicines are too expensive and thus they can not be applied to diseases of poverty. Here we have applied quantum dots to study at the cellular level the delivery of the contents of liposomes to erythrocytes infected by the malaria parasite Plasmodium falciparum, and to macrophages infected by the leishmaniasis causative agent Leishmania infantum. A number of works have reported on the encapsulation in liposomes of drugs against both diseases as a strategy to increase therapeutic efficacy and decrease unspecific toxicity. Liposome- carried drugs end up inside Plasmodium-infected red blood cells (pRBCs) and in the phagolysosome system of Leishmania- infected macrophages but some knowledge gaps still obscure subcellular events related to these processes. As a proof of concept, we have used confocal fluorescence microscopy to follow the fate in pRBCs and infected macrophages of quantum dots encapsulated in liposomes, and of lysosomes, leishmaniasis and malaria parasites, nuclei, and phagosomes. Our data indicate that liposomes merge their lipid bilayers with pRBC plasma membranes but are engulfed by macrophages, where they fuse with lysosomes. Lysosomes have not been observed to join with phagosomes harboring single Leishmania parasites, whereas in phagosomes where the parasite has divided there is lysosome-specific fluorescence with a concomitant disappearance of lysosomes from the cytosol. In later stages, all the lysosome-specific label is found inside phagosomes whereas the phagosomal marker cadaverine strongly stains the macrophage nucleus, suggesting that Leishmania infection induces in its later stages nuclear degeneration and, possibly, apoptosis of the host cell. These results indicate that induction of macrophage apoptosis should be explored as a possible strategy used by Leishmania to prepare its egress.

Sphingosine Kinase-1 (SphK-1) Regulates Mycobacterium smegmatis Infection in Macrophages

Sphingosine kinase-1 is known to mediate Mycobacterium smegmatis induced inflammatory responses in macrophages, but its role in controlling infection has not been reported to date. We aimed to unravel the significance of SphK-1 in controlling M. smegmatis infection in RAW 264.7 macrophages. Our results demonstrated for the first time that selective inhibition of SphK-1 by either D, L threo dihydrosphingosine (DHS; a competitive inhibitor of Sphk-1) or Sphk-1 siRNA rendered RAW macrophages sensitive to M. smegmatis infection. This was due to the reduction in the expression of iNOs, p38, pp-38, late phagosomal marker, LAMP-2 and stabilization of the RelA (pp-65) subunit of NF-κB. This led to a reduction in the generation of NO and secretion of TNF-α in infected macrophages. Congruently, overexpression of SphK-1 conferred resistance in macrophages to infection which was due to enhancement in the generation of NO and expression of iNOs, pp38 and LAMP-2. In addition, our results also unraveled a novel regulation of p38MAPK by SphK-1 during M. smegmatis infection and generation of NO in macrophages. Enhanced NO generation and expression of iNOs in SphK-1++ infected macrophages demonstrated their M-1bright phenotype of these macrophages. These findings thus suggested a novel antimycobacterial role of SphK-1 in macrophages.

Inactivation of Macrophage Rab7 by Burkholderia cenocepacia

Strains of the Burkholderia cepacia complex can survive within macrophages by arresting the maturation of phagocytic vacuoles. The bacteria preclude fusion of the phagosome with lysosomes by a process that is poorly understood. Using murine macrophages, we investigated the stage at which maturation is arrested and analyzed the underlying mechanism. Vacuoles containing B. cenocepacia strain J2315, an isolate of the transmissible ET12 clone, recruited Rab5 and synthesized phosphatidylinositol-3-phosphate, indicating progression to the early phagosomal stage. Despite the fact that the B. cenocepacia-containing vacuoles rarely fused with lysosomes, they could nevertheless acquire the late phagosomal markers CD63 and Rab7. Fluorescence recovery after photobleaching and use of a probe that detects Rab7-guanosine triphosphate indicated that activation of Rab7 was impaired by B. cenocepacia, accounting at least in part for the inability of the vacuole to merge with lysosomes. The Rab7 defect was not due to excessive cholesterol accumulation and was confined to the infected vacuoles. Jointly, these experiments indicate that B. cenocepacia express virulence factors capable of interfering with Rab7 function and thereby with membrane traffic.

Acid sphingomyelinase is required for efficient phago-lysosomal fusion

The acid sphingomyelinase (ASMase) localizes to the lumen of endosomes, phagosomes and lysosomes as well as to the outer leaflet of the plasma membrane and hydrolyses sphingomyelin to ceramide and phosphorylcholine. Using the facultative intracellular bacterium Listeria monocytogenes, we show that maturation of phagosomes into phagolysosomes is severely impaired in macrophages genetically deficient for ASMase. Unlike in wild-type macrophages, phagosomes containing L. monocytogenes in ASMase(-/-) macrophages remained positive for the late phagosomal markers mannose-6-phosphate receptor (M6PR) and Rab7 for at least 2 h and, correspondingly, showed delayed acquisition of lysosomal markers like lysosome associated membrane protein 1 (Lamp1). The transfer of lysosomal fluid phase markers into phagosomes containing L. monocytogenes was severely impaired in ASMase(-/-) macrophages and decreased with increasing size of the cargo. Moreover, phagosomes containing L. monocytogenes from ASMase(-/-) cells acquired significantly less listeriocidal proteases cathepsin D, B and L. The results of this study suggest that ASMase is required for the proper fusion of late phagosomes with lysosomes, which is crucial for efficient transfer of lysosomal antibacterial hydrolases into phagosomes.