Signal transduction and phagosome biogenesis in human macrophages during phagocytosis of Mycobacterium bovis BCG

Calcium Signaling Commands Phagosome Maturation Process

Phagosome-lysosome (P-L) fusion is one of the central immune-effector responses of host. It is known that phagosome maturation process is associated with numerous signaling cascades and among these, important role of calcium (Ca²⁺) signaling has been realized recently. Ca²⁺ plays key roles in actin rearrangement, activation of NADPH oxidase and protein kinase C (PKC). Involvement of Ca²⁺ in these cellular processes directs phagosomal maturation process. Some of the intracellular pathogens have acquired the strategies to modulate Ca²⁺ associated pathways to block P-L fusion process. In this review we have described the mechanism of Ca²⁺ signals that influence P-L fusion by controlling ROS, actin and PKC signaling cascades. We have also discussed the strategies implemented by the intracellular pathogens to manipulate Ca²⁺ signaling to consequently subvert P-L fusion. A detail study of factors associated in manipulating Ca²⁺ signaling may provide new insights for the development of therapeutic tools for more effective treatment options against infectious diseases.

Regulation of impaired protein kinase C signaling by chemokines in murine macrophages during visceral leishmaniasis

The protein kinase C (PKC) family regulates macrophage function involved in host defense against infection. In the case of Leishmania donovani infection, the impairment of PKC-mediated signaling is one of the crucial events for the establishment of parasite into the macrophages. Earlier reports established that C-C chemokines mediated protection against leishmaniasis via the generation of nitric oxide after 48 h. In this study, we investigated the role of MIP-1alpha and MCP-1 in the regulation of impaired PKC activity in the early hours (6 h) of infection. These chemokines restored Ca2+-dependent PKC activity and inhibited Ca2+-independent atypical PKC activity in L. donovani-infected macrophages under both in vivo and in vitro conditions. Pretreatment of macrophages with chemokines induced superoxide anion generation by activating NADPH oxidase components in infected cells. Chemokine administration in vitro induced the migration of infected macrophages and triggered the production of reactive oxygen species. In vivo treatment with chemokines significantly restricted the parasitic burden in livers as well as in spleens. Collectively, these results indicate a novel regulatory role of C-C chemokines in controlling the intracellular growth and multiplication of L. donovani, thereby demonstrating the antileishmanial properties of C-C chemokines in the disease process.

Involvement of heterotrimeric G proteins in phagocytosis and recycling from the phagosomal compartment

Phagocytosis is a receptor-mediated process by which specialized cell types engulf large extracellular particles. Phagosome maturation involves a series of intracellular membrane fusion and budding events resulting in the delivery of particles to compartments enriched in lysosomal hydrolases where they are digested. Substantial amounts of plasma membrane and many phagosomal proteins, such as receptors, rapidly recycle to the plasma membrane following phagosome formation. Despite the importance of this recycling pathway in phagosome maturation and in the retrieval of immunogenic peptides from phagosomes, the molecular machinery involved is largely unknown. To assess the participation of GTPases in phagocytosis and recycling from phagosomes we used aluminum fluoride (AIF(-)(4)), which activates the GDP-bound form of stimulatory and inhibitory trimeric G proteins. AlF(-)(4) inhibited both the uptake to and the recycling from the phagosomal compartment. Cholera toxin, which activates Galphas, and pertussis toxin, which uncouples Gi and Go from receptors, were effective inhibitors of phagocytosis. However, both toxins stimulated recycling from phagosomes. These results suggest that more than one GTP-binding protein participates either directly or indirectly not only in phagocytosis, but also in maturation and recycling from phagosomes, and thereby assign a role for heterotrimeric G proteins in controlling traffic through the phagocytic pathway.

Monitoring internalization of Histoplasma capsulatum by mammalian cell lines using a fluorometric microplate assay

We have developed a fluorometric microtiter plate assay to quantify the internalization of Histoplasma capsulatum yeasts by macrophages. The assay utilizes the fluorescent dye Calcofluor White to label the yeast cell wall and the vital dye trypan blue, which does not enter viable macrophages, to quench fluorescence of extracellular labeled yeasts. Murine RAW 264.7 cells showed more efficient internalization of strain G217B yeasts than human U937 cells. Both cell lines exhibited a dependence upon actin, and, to a lesser degree, microtubules, in G217B uptake.

Role of Protein Kinase C-α in the Control of Infection by Intracellular Pathogens in Macrophages

The protein kinase C (PKC) family regulates macrophage function involved in host defense against infection. In this study, we investigated the role of macrophage PKC-α in the uptake and subsequent fate of Leishmania donovani promastigotes and Legionella pneumophila infections. To this end, we used clones of the murine macrophage cell line RAW 264.7 overexpressing a dominant-negative (DN) mutant of PKC-α. While phagocytosis of L. donovani promastigotes was not affected by DN PKC-α overexpression, their intracellular survival was enhanced by 10- to 20-fold at 48 h postinfection. Intracellular survival of a L. donovani mutant defective in lipophosphoglycan repeating units synthesis, which normally is rapidly degraded in phagolysosomes, was enhanced by 100-fold at 48 h postinfection. However, IFN-γ-induced leishmanicidal activity was not affected by DN PKC-α overexpression. Similar to macrophages from genetically resistant C57BL/6 mice, control RAW 264.7 cells were not permissive for the intracellular replication of Legionella pneumophila. In contrast, DN PKC-α-overexpressing RAW 264.7 clones were phenotypically similar to macrophages from genetically susceptible A/J mice, as they allowed intracellular replication of L. pneumophila. Permissiveness to L. pneumophila was not the consequence of a general defect in the microbicidal capacities because killing of a temperature-sensitive mutant of Pseudomonas aeruginosa was normal in DN PKC-α-overexpressing RAW 264.7 clones. Collectively, these results support a role for PKC-α in the regulation of innate macrophage functions involved in the control of infection by intracellular parasites.