Infectious microorganisms have developed many ways to drive the host-pathogen interaction during the infection process. Among this large arsenal of virulence factor, the secretion of phosphatase by pathogen appears to be one of the most efficient tactics to interfere with the host cell response. Indeed, the reaction of phosphorylation/dephosphorylation of protein and lipids is the most common modification used by cell to control their cellular machinery.
The laboratory is therefore focusing its research on newly discovered phosphatases secreted by two distinct pathogens: (1) the bacteria Listeria monocytogenes and (2) the parasite Leishmania major.
LipA is a new virulence factor recently discovered in the intracellular Gram-positive bacteria Listeria monocytogenes, the causative agent of Listeriosis. This Food-borne disease is rare but can cause clinically serious symptoms with a high case fatality rate in immuno-compromised individuals. After ingestion of contaminated food, L. monocytogenes crosses the epithelial barrier of the intestine and disseminates in deeper tissues via the lymphatics and the blood stream. At first, spleen and liver are infected. There, the bacterium replicates and uses these organs as reservoir for further spreading in the brain and the placenta. This pathogenicity relies on an elaborated arsenal of virulence factors.
We recently identified a new virulence factor of L. monocytogenes , the secreted phosphatase LipA (Kastner et al. 2011). This enzyme exhibits unusual features such as tyrosine as well as phosphoinositol-phosphate (PIP) phosphatase activity in vitro. Strikingly, bacteria lacking LipA are severely attenuated in virulence in vivo independently of the infection route, which goes in line with its expression pattern strictly dependent on the entry of the bacteria in the blood stream.
To control the infection by L. monocytogenes, organism strongly relies on the early innate immunity, while adaptive immunity such as cytotoxic T cell is required for the final clearance. Neutrophils are critical players during the early phase of Listeria infection (14). They are among the first cells to phagocyte bacteria and kill them via the production of reactive oxygen species (ROS). This bactericidal activity is based on the NADPH oxidase activation and assembly at the neutrophil membrane. This process is tightly regulated by multiple receptors controlling through the transient synthesis of PIPs.
We discovered the in vitro specificity of the Listeria phosphatase LipA and we showed its involvement in the virulence of the bacteria. However, the cellular and molecular targets of this phosphatase still need to be discovered.
Leishmania major is the causative agent of cutaneous leishmaniasis, a worldwide prevalent parasitic disease transmitted by sand fly bite to human. In its insect vector, the parasite resides in an extracellular, highly infective, promastigote form. Once in its mammal host, the parasite enters myeloid host cells with a high prevalence for macrophages. There, they transform into intracellular amastigotes, which facilitate the spread in the host organism. However, only few Leishmania virulence factors have been characterized to date.
Our laboratory identified two putative new virulence factor produced by Leishmania parasites (named Lp1 and Lp4). These two homologous tyrosine phosphatases were shown to be partly secreted by Leishmania major via the exosome pathway. Interestingly, these phosphatases have a strong structural homology with the human phosphatase PRL-1 (Phosphatase of Regenerating Liver 1) that is involved in the regulation of numerous cellular features such as growth and cell motility. Based on this homology, we hypothesized that the Leishmania phosphatases could interfere with the PRL-1 function to modulate the host response during infection.