Cell biology and immunology lessons taught by Legionella pneumophila

The Influence of Acanthamoeba-Legionella Interaction in the Virulence of Two Different Legionella Species

The genus Legionella comprises more than 60 species, and about half are associated with infection. Legionella pneumophila is the most commonly associated with these infections and by far the most studied, but L. non-pneumophila species, such as L. feeleii, L. anisa, etc., may also present clinical importance. Free-living amoebae are their preferred environmental host, where these bacteria not only survive but also succeed in multiplying, and this relationship can lead to an increase in bacterial virulence. The goal of this study was to evaluate the alterations of Legionella pathogenicity due to its interaction with Acanthamoeba. For this, the expression of protein effectors SdhA, LegK2, and SidK were evaluated in L. pneumophila and L. feeleii, before and after infecting Acanthamoeba. Additionally, the host response was evaluated by measuring the production of IL-6, IL-8, and IFN-γ in infected macrophages. Regarding the virulence factors, an increase in SdhA expression was observed after these bacteria infected Acanthamoeba, with a higher increase in the macrophage cultures infected with L. feeleii. Also, an increase in the expression of LegK2 was observed after infecting Acanthamoeba, but it was more intense in the cultures infected with L. pneumophila. With regard to SidK, it was increased in L. feeleii after infecting Acanthamoeba, however the same effect was not observed for L. pneumophila. In cytokine production, the effect on IL-6 and IL-8 was similar for both cytokines, increasing their concentration, but higher production was observed in the cultures infected with L. feeleii, even though it demonstrated slightly lower production with the inoculum obtained from Acanthamoeba. Concerning IFN-γ, induction was observed in both species but higher in the infection by L. pneumophila. Nevertheless, it is not known if this induction is enough to promote an efficient immune response against either L. pneumophila or L. feeleii. Altogether, these alterations seem to increase L. feeleii virulence after infecting Acanthamoeba. However, this increase does not seem to turn L. feeleii as virulent as L. pneumophila. More studies are necessary to understand the aspects influenced in these bacteria by their interaction with Acanthamoeba and, thus, identify targets to be used in future therapeutic approaches.

Crystal structure of lpg1832, a VirK family protein from Legionella pneumophila, reveals a novel fold for bacterial VirK proteins

VirK family [Pfam06903] consists of 14 bacterial VirK proteins of around 145 residues in length. The function of this family is unknown. Herein, using single-wavelength anomalous diffraction, we determined the crystal structure of lpg1832, a VirK family protein from Legionella pneumophila, at 2.0 Å resolution. This is the first structural determination of a VirK domain-containing protein. Lpg1832 is a type II secretion system-dependent extracellular protein that folds into a novel barrel-shaped structure. It is found to adopt a quaternary assembly comprising a homotetramer. The three-dimensional structure of lpg1832 provides the first structural information pertaining to the VirK family and allows us to possibly identify its functionally important regions.

Intracellular Growth and Morphological Characteristics of Legionella pneumophila during Invasion and Proliferation in Different Cells

Various studies have been made to attempt to study the interaction between Legionella pneumophila and the host cells. In this research, we successfully constructed a L. pneumophila mutant strain that stably expressed high levels of green fluorescent protein and used this strain to evaluate the adherence, invasion and proliferation of L. pneumophila in association with several cell lines, including seven cell lines [human macrophage-like cell lines (U937, THP-1), murine macrophage-like cell lines (J774.1A, Raw264.7), human bronchial epithelial cell lines (16HBE, Beas-2B) and human cerrical cancer cell line (HeLa)] which have been used as the host models of L. pneumophila, and two breast carcinoma cell lines (MCF-7 and MDA-MB-231). Our results showed that the two newly tested cell lines are able to support the intracellular proliferation of L. pneumophila, and there were some morphological variations during the invasion and intracellular replication of L. pneumophila in different cell lines. These results can help us find out the common and special patterns of invasion and proliferation of L. pneumophila within different hosts. This is conducive to our knowledge on the relationship and interaction between bacteria and host.

Ubiquitination independent of E1 and E2 enzymes by bacterial effectors

Signaling by ubiquitination regulates virtually every cellular process in eukaryotes. Covalent attachment of ubiquitin to a substrate is catalyzed by the E1, E2 and E3 three-enzyme cascade ¹, which links the C terminus of ubiquitin via an isopeptide bond mostly to the ε-amino group of a lysine of the substrate. Given the essential roles of ubiquitination in the regulation of the immune system, it is not surprising that the ubiquitination network is a common target for diverse infectious agents ². For example, many bacterial pathogens exploit ubiquitin signaling using virulence factors that function as E3 ligases, deubiquitinases ³ or as enzymes that directly attack ubiquitin ⁴. The bacterial pathogen Legionella pneumophila utilizes approximately 300 effectors that modulate diverse host processes to create a niche permissive for its replication in phagocytes ⁵. Here we demonstrate that members of the SidE effector family (SidEs) of L. pneumophila ubiquitinate multiple Rab small GTPases associated with the endoplasmic reticulum (ER). Moreover, we show that these proteins are capable of catalyzing ubiquitination without the need for the E1 and E2 enzymes. A putative mono ADP-ribosyltransferase (mART) motif critical for the ubiquitination activity is also essential for the role of SidEs in intracellular bacterial replication in a protozoan host. The E1/E2-independent ubiquitination catalyzed by these enzymes is energized by NAD which activates ubiquitin by the formation of ADP-ribosylated ubiquitin (ADPR-Ub). These results establish that ubiquitination can be catalyzed by a single enzyme whose activity does not require ATP.