Legionella pneumophila (green), the bacterium responsible for severe acute lung disease inside eukaryotic cells. Mitochondrial network in red, nucleus in blue. © Institut Pasteur.

Scientists at the Institut Pasteur, CNRS and Inserm, together with a team from Switzerland*, have shown that the bacterial pathogen Legionella pneumophila (the causative agent of Legionnaires’ disease or legionellosis) has developed a specific strategy to target the host cell mitochondria, the organelles in charge of cellular bioenergetics. By changing the shape of these host organelles, L. pneumophila impairs mitochondrial respiration leading to metabolic changes in the host cell that are instrumental for the pathogens replication in human cells. This work provides precious information on how a pathogen manipulates the cellular metabolism to replicate intracellularly, and proposes a new concept of protection of host cells from Legionella-induced mitochondrial changes in order to fight infection. This research is published online on August 31, at the Cell Host & Microbe website.

Intracellular pathogens adopt various strategies to circumvent the defences of the host cell and to proliferate intracellularly. One specific strategy is to target host organelles like the mitochondria. Mitochondria are well-defined cytoplasmic organelles, which take part in a variety of cellular metabolic functions. Mitochondria are important as they supply the energy to the cell, thus they are also referred to as the ‘power house’ of the cell. Some bacteria, including Legionella pneumophila, are able to alter mitochondrial functions to the pathogens advantage.

L. pneumophila is a bacterial pathogen that causes Legionellosis – a disease characterized by an acute pulmonary infection, which is often fatal when not treated promptly. In France, between 1200 and 1500 cases are identified each year, with mortality rates ranging from 5 to 15%.

Researchers from the Institut Pasteur, CNRS and Inserm, in collaboration with a team from Switzerland, have discovered a previously unknown mechanism by which L. pneumophila targets mitochondria to modulate mitochondrial dynamics and thereby impairs mitochondrial respiration which in turn leads to a change in the cellular metabolism. These metabolic changes in the host cell favour bacterial replication. Thus the rewiring of cellular bioenergetics to create a replication permissive niche in host cells is a core virulence strategy of L. pneumophila.

The researchers have identified the following mechanism: L. pneumophila establishes transient, highly dynamic contacts with host mitochondria and secretes an enzyme called MitF that modifies the shape of the mitochondria by inducing DNM1L (a host protein that is necessary for fragmenting mitochondria) depended mitochondrial fragmentation. Surprisingly, L. pneumophila induced mitochondrial fragmentation is independent of cell death and ultimately impairs mitochondrial respiration, whereas cellular glycolysis is increased. Thus the bacterial-induced changes in mitochondrial dynamics promote a Warburg-like phenotype (which is characteristic of cancerous cells) in the infected cell that favours bacterial replication.

Researchers also brought the proof of concept that protecting host cells from Legionella-induced mitochondrial changes may help to fight infection. Indeed, pre-treating of human cells with a compound that inhibits changes in mitochondrial morphology allows protecting the host cell from Legionella-induced changes of mitochondria and restricts bacterial infection of human cells.

As Carmen Buchrieser, head of the Biology of the intracellular bacteria research unit at the Institut Pasteur and researcher at CNRS, explains: “this is an important discovery as our results showcase a key strategy used by L. pneumophilia for intracellular replication. By targeting mitochondria, the bacterium ensures that the host cell will be permissive to its replication. It is therefore essential that researchers also focus their studies on metabolic changes caused by pathogenic bacteria, in order to develop new therapeutic strategies against legionellosis and other diseases linked to intracellular bacteria.”

This work sheds new light on how a pathogen shapes host metabolic responses during infection of human cells and shows that metabolic changes in the host cell are instrumental for the pathogens replication in human cells and thus to cause disease. It also proposes a new concept, which is to treat infections by inhibiting pathogen induced metabolic changes.

* The researchers and scientific teams involved are: Carmen Buchrieser, head of the Biology of the intracellular bacteria unit at the Institut Pasteur and associated to CNRS, Hubert Hilbi, of the University of Zürich Switzerland, Priscille Brodin, at the Institut Pasteur Lille and associated to CNRS and INSERM, and Jean-Christophe Olivo-Marin, head of the Bioimage analyses unit at the Institut Pasteur.

©Fotolia

Teams from the virology, hematology-bone marrow transplant and biostatistics departments of the Saint-Louis Hospital, AP-HP, Université Paris Diderot and INSERM, in collaboration with scientists from the University of California-San Francisco (USA) discovered that a group of viruses in the intestine may trigger the onset of graft-versus-host disease. Researchers demonstrated the role of this intestinal “virome” (the population of viruses found in the intestine) in the onset of graft-versus-host disease by analyzing the intestinal virome of 44 patients receiving a bone marrow transplant.

These results appeared in a letter published in Nature Medicine.

Graft-versus-host disease (GVHD) affects up to 60% of patients who receive a stem cell graft (bone marrow transplant) with a high risk of death. Even though past studies have identified biomarkers associated with the severity of the disease, none of them have been used in practice to date in order to predict the onset of graft-versus-host disease.

The researchers carried out a longitudinal study analyzing the intestinal virome of 44 patients before a bone marrow transplant and six weeks after the transplant. By using high-throughput metagenomic techniques, they sequenced the DNA and RNA in the stool samples, in order to analyze and compare the flux over time in bacterial and viral populations.

The results show that GVHD is associated with significant changes in bacteria, bacteriophage (i.e. the viruses that infect bacteria) and virus populations in the stool.

In patients with intestinal GVHD, the persistent DNA viruses (Anelloviridae, Herpesviridae and Polyomaviridae) are absent from the intestinal tract in the first 2-3 weeks following the graft and then they increase in a significant manner. However, the presence of these viruses is stable as soon as the graft is performed in patients without GVHD.

In particular, the researchers detected the presence of a group of RNA viruses, picobirnavirus (PBV), in more than a third of patients after transplanting stem cells. As opposed to other viruses, the presence of PBV predicts the development of GVHD and is highly correlated to the severity biomarkers of intestinal graft-versus-host disease.

Metagenomic tools have made it possible to identify PBVs that have been little studied to date, since their extreme variability makes developing a test to detect them difficult. These viruses have been described in cases of intestinal gastroenteritis, notably in patients with AIDS, but their pathogenic role has not yet been determined. The potential utility of PBV as a predictive marker of GVHD has rekindled the interest in developing adapted to tools for clinical practice.

These results improve the state of knowledge of this complex disease, notably in terms of the role of viral infections in digestive inflammatory diseases and has opened the door to new therapeutic opportunities.

Cross section of newborn mouse’s testis (Ø = 20 µm), where we can see the seminiferous tubules (red) surrounded by macrophages (green).  Confocal micrograph.

© Noushine Mossadegh-Keller and Sébastien Mailfert / CIML

The origin, development, and characteristics of two types of testicular macrophage have been described by a CNRS team at the Centre d’Immunologie de Marseille-Luminy (CNRS / INSERM / Aix-Marseille University). To elucidate the nature of these immune cells, the researchers used a novel cell tracing method. Their findings were published on August 7, 2017, in the Journal of Experimental Medicine, and are of fundamental importance. They may help understand certain kinds of infertility in men and find new treatments for them.

From the start of life, an individual’s immune system learns to distinguish self—that is, native cells—from other, potentially pathogenic cells. But in males, as sperm only appear at puberty, they may be mistaken for foreign cells by certain elements of the immune system.  Testicular macrophages are special immune cells that rush to the defense of sperm. By releasing specific molecules, these guardians of fertility prevent other immune system agents from entering the testes.

Macrophages not only migrate to sites of infection and phagocytose pathogens, but also modulate immune system activity to ensure proper organ function and regeneration. They may arise from either embryonic progenitors or bone marrow cells in adults. Research with mice has enabled the team of Michael Sieweke from the Centre d’Immunologie de Marseille-Luminy (CNRS / INSERM / Aix-Marseille University) to describe both testicular macrophage populations in depth.

The testis is divided into two compartments. One kind of testicular macrophage is found in the interstitial spaces, where testosterone-producing Leydig cells are also located. These interstitial macrophages are of embryonic origin: they are present from the beginning of the individual’s life. The other kind is peritubular—that is, located on the surface of the seminiferous tubules that house sperm cell precursors. Each macrophage population has distinctive cellular markers.

The researchers used a new cell tracing method to follow the movement of peritubular macrophages from the bone marrow to the testes. They discovered that these macrophages only appear two weeks after the mice are born, which corresponds to the pubescent stage in human males. Surprisingly, once they have been established in the testes, macrophages of both populations remain there for the rest of their long lives. Sieweke’s team will next focus their research efforts on the relationships between macrophages, sperm, and testosterone production, which may yield innovative treatments for certain kinds of male infertility.