Phagocytic Entry of <i>Legionella pneumophila</i> into Macrophages through Phosphatidylinositol 3,4,5-Trisphosphate-Independent Pathway

Abstract

Legionella pneumophila, a causative agent of Legionnaire's disease, is an intracellular pathogen. It intervenes in the signal transduction of macrophages by secreting effector molecules through the Icm/Dot type IV secretion system (T4SS). There is a connection between signaling cascades that regulate phagocytosis and the production of reactive oxygen species (ROS). Class I phosphatidylinositol 3-kinase (PI3-K) and its product phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P 3 ) play key roles in the reorganization of cytoskeleton (phagocytosis) and activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (ROS production). We investigated the production of PI(3,4,5)P 3 and recruitment of class I PI3-K and Rac1 during phagocytosis of L. pneumophila by macrophages. Transient recruitment of class I PI3-K as well as PI(3,4,5)P 3 production was observed around a phagocytosed T4SS mutant LELA3118 or avirulent mutant 25D in an early stage of infection. In contrast, class I PI3-K was recruited while accumulation of PI(3,4,5)P 3 was not observed around wild type JR32. Immunoglobulin G (IgG)-opsonized live JR32, which would activate class I PI3-K through the Fcγ receptor pathway, did not induce PI(3,4,5)P 3 production. Regardless of whether wild type or mutants were used, transient Rac1 accumulation was observed around bacteria. These results indicate that the phagocytosis of wild type L. pneumophila occurs via a special mechanism in which PI(3,4,5)P 3 production is absent. This suggests that L. pneumophila may inhibit the production of PI(3,4,5)P 3 , but not the recruitment of class I PI3-K and Rac1, in a T4SS-dependent manner. L. pneumophila may start the modulation of host signaling cascade immediately after contact with host cells to evade the ROS-dependent bactericidal system while completing entry into macrophages.

Live-Cell Imaging of Phosphoinositide Dynamics and Membrane Architecture during Legionella Infection

“…The uptake of wild-type L. pneumophila by nonpermissive murine J774A.1 macrophages or D. discoideum was partially inhibited by wortmannin or LY294002 (54, 71). In contrast, we and other groups previously found that the uptake of L. pneumophila by replication-permissive human U937 macrophages or D. discoideum occurs in a wortmannin-insensitive and PI3K-independent manner, while the uptake of an Icm/Dot mutant was strongly reduced by PI3K inhibitors (35, 72, 73). Perhaps, the conflicting results on the role of PI3Ks during L. pneumophila uptake reflect the different (permissive or nonpermissive) phagocytes and bacterial strains used.…”

Amoebal Endosymbiont Neochlamydia Genome Sequence Illuminates the Bacterial Role in the Defense of the Host Amoebae against Legionella pneumophila

“…Human isolate L. pneumophila Philadelphia I (JR32), equipped with a complete dot/icm gene set encoding a type IV secretion system, which is required for intracellular amoebal growth [48], was kindly provided by Dr. Masaki Miyake of the University of Shizuoka, Japan. L. pneumophila was cultured on BCYE agar (OXOID, Hampshire, UK) at 37°C for 2 days.…”

Knockdown of the Rhipicephalus microplus Cytochrome c Oxidase Subunit III Gene Is Associated with a Failure of Anaplasma marginale Transmission

“…These genes include one isoform of glutathione S-transferase (GST) (GenBank accession number KF784792), cytochrome c oxidase sub III (COXIII) (GenBank accession number KF784795), dynein (DYN) (GenBank accession number KF784791), synaptobrevin (SYN) (GenBank accession number KF784794) and phosphatidylinositol-3,4,5-triphosphate 3-phosphatase (PHOS) (GenBank accession number KF784793). GST is important in the detoxification of both endogenous and xenobiotic compounds and protection against oxidative stress [36]–[38]; COXIII is related to mitochondrial metabolism, including ATP production and reactive oxygen species (ROS); DYN is a motor protein involved in the conversion of chemical energy present in molecules of ATP into mechanical energy along microtubules and is involved in intracellular trafficking pathways [39]; SYN is a v-SNARE protein that participates in the exocytosis of proteins [40]–[42]; and phosphatidylinositol-3,4,5-trisphosphate 3-phosphatase plays an important role in the reorganization of the cytoskeleton, NADPH production and ROS production [43].…”