Mutants of Listeria monocytogenes defective in In vitro invasion and cell-to-cell spreading still invade and proliferate in hepatocytes of neutropenic mice

An Immunomodulatory Transcriptional Signature Associated With Persistent Listeria Infection in Hepatocytes

Listeria monocytogenes causes severe foodborne illness in pregnant women and immunocompromised individuals. After the intestinal phase of infection, the liver plays a central role in the clearance of this pathogen through its important functions in immunity. However, recent evidence suggests that during long-term infection of hepatocytes, a subpopulation of Listeria may escape eradication by entering a persistence phase in intracellular vacuoles. Here, we examine whether this long-term infection alters hepatocyte defense pathways, which may be instrumental for bacterial persistence. We first optimized cell models of persistent infection in human hepatocyte cell lines HepG2 and Huh7 and primary mouse hepatocytes (PMH). In these cells, Listeria efficiently entered the persistence phase after three days of infection, while inducing a potent interferon response, of type I in PMH and type III in HepG2, while Huh7 remained unresponsive. RNA-sequencing analysis identified a common signature of long-term Listeria infection characterized by the overexpression of a set of genes involved in antiviral immunity and the under-expression of many acute phase protein (APP) genes, particularly involved in the complement and coagulation systems. Infection also altered the expression of cholesterol metabolism-associated genes in HepG2 and Huh7 cells. The decrease in APP transcripts was correlated with lower protein abundance in the secretome of infected cells, as shown by proteomics, and also occurred in the presence of APP inducers (IL-6 or IL-1β). Collectively, these results reveal that long-term infection with Listeria profoundly deregulates the innate immune functions of hepatocytes, which could generate an environment favorable to the establishment of persistent infection.

Listeria monocytogenes exploits efferocytosis to promote cell-to-cell spread

Efferocytosis, the process by which dying or dead cells are removed by phagocytosis, has an important role in development, tissue homeostasis and innate immunity. Efferocytosis is mediated, in part, by receptors that bind to exofacial phosphatidylserine (PS) on cells or cellular debris after loss of plasma membrane asymmetry. Here we show that a bacterial pathogen, Listeria monocytogenes, can exploit efferocytosis to promote cell-to-cell spread during infection. These bacteria can escape the phagosome in host cells by using the pore-forming toxin listeriolysin O (LLO) and two phospholipase C enzymes. Expression of the cell surface protein ActA allows L. monocytogenes to activate host actin regulatory factors and undergo actin-based motility in the cytosol, eventually leading to formation of actin-rich protrusions at the cell surface. Here we show that protrusion formation is associated with plasma membrane damage due to LLO's pore-forming activity. LLO also promotes the release of bacteria-containing protrusions from the host cell, generating membrane-derived vesicles with exofacial PS. The PS-binding receptor TIM-4 (encoded by the Timd4 gene) contributes to efficient cell-to-cell spread by L. monocytogenes in macrophages in vitro and growth of these bacteria is impaired in Timd4(-/-) mice. Thus, L. monocytogenes promotes its dissemination in a host by exploiting efferocytosis. Our results indicate that PS-targeted therapeutics may be useful in the fight against infections by L. monocytogenes and other bacteria that use similar strategies of cell-to-cell spread during infection.

Animal models of listeriosis: a comparative review of the current state of the art and lessons learned

Listeriosis is a leading cause of hospitalization and death due to foodborne illness in the industrialized world. Animal models have played fundamental roles in elucidating the pathophysiology and immunology of listeriosis, and will almost certainly continue to be integral components of the research on listeriosis. Data derived from animal studies helped for example characterize the importance of cell-mediated immunity in controlling infection, allowed evaluation of chemotherapeutic treatments for listeriosis, and contributed to quantitative assessments of the public health risk associated with L. monocytogenes contaminated food commodities. Nonetheless, a number of pivotal questions remain unresolved, including dose-response relationships, which represent essential components of risk assessments. Newly emerging data about species-specific differences have recently raised concern about the validity of most traditional animal models of listeriosis. However, considerable uncertainty about the best choice of animal model remains. Here we review the available data on traditional and potential new animal models to summarize currently recognized strengths and limitations of each model. This knowledge is instrumental for devising future studies and for interpreting current data. We deliberately chose a historical, comparative and cross-disciplinary approach, striving to reveal clues that may help predict the ultimate value of each animal model in spite of incomplete data.

Identification of the insulin-like growth factor II receptor as a novel receptor for binding and invasion by Listeria monocytogenes

The gram-positive bacterium Listeria monocytogenes causes a life-threatening disease known as listeriosis. The mechanism by which L. monocytogenes invades mammalian cells is not fully understood, but the processes involved may provide targets to prevent and treat listeriosis. Here, for the first time, we have identified the insulin-like growth factor II receptor (IGFIIR; also known as the cation-independent mannose 6-phosphate receptor (CI)M6PR or CD222) as a novel receptor for binding and invasion of Listeria species. Random peptide phage display was employed to select a peptide sequence by panning with immobilized L. monocytogenes cells; this peptide sequence corresponds to a sequence within the mannose 6-phosphate binding site of the IGFIIR. All Listeria spp. specifically bound the labeled peptide but not a control peptide, which was demonstrated using fluorescence spectrophotometry and fluorescence-activated cell sorting. Further evidence for binding of the receptor by L. monocytogenes and L. innocua was provided by affinity purification of the bovine IGFIIR from fetal calf serum by use of magnetic beads coated with cell preparations of Listeria spp. as affinity matrices. Adherence to and invasion of mammalian cells by L. monocytogenes was significantly inhibited by both the synthetic peptide and mannose 6-phosphate but not by appropriate controls. These observations indicate a role for the IGFIIR in the adherence and invasion of L. monocytogenes of mammalian cells, perhaps in combination with known mechanisms. Ligation of IGFIIR by L. monocytogenes may be a novel mechanism that contributes to the regulation of infectivity, possibly in combination with other mechanisms.

Pathogenicity and immunogenicity of a vaccine strain of Listeria monocytogenes that relies on a suicide plasmid to supply an essential gene product

Listeria monocytogenes is a bacterial pathogen that elicits a strong cellular immune response and thus has potential use as a vaccine vector. An attenuated strain, L. monocytogenes dal dat, produced by deletion of two genes (dal and dat) used for d-alanine synthesis, induces cytotoxic T lymphocytes and protective immunity in mice following infection in the presence of d-alanine. In order to obviate the dependence of L. monocytogenes dal dat on supplemental d-alanine yet retain its attenuation and immunogenicity, we explored mechanisms to allow transient endogenous synthesis of the amino acid. Here, we report on a derivative strain, L. monocytogenes dal dat/pRRR, that expresses a dal gene and synthesizes d-alanine under highly selective conditions. We constructed the suicide plasmid pRRR carrying a dal gene surrounded by two res1 sites and a resolvase gene, tnpR, which acts at the res1 sites. The resolvase gene is regulated by a promoter activated upon exposure to host cell cytosol. L. monocytogenes dal dat/pRRR was thus able to grow in liquid culture and to infect host cells without d-alanine supplementation. However, after infection of these cells, resolvase-mediated excision of the dal gene resulted in strong down-regulation of racemase expression. As a result, this system allowed only transient growth of L. monocytogenes dal dat/pRRR in infected cells and survival in animals for only 2 to 3 days. Nevertheless, mice immunized with L. monocytogenes dal dat/pRRR generated listeriolysin O-specific effector and memory CD8(+) T cells and were protected against lethal challenge by wild-type Listeria. This vector may be an attractive vaccine candidate for the induction of protective cellular immune responses.

Failure to induce enhanced protection against tuberculosis by increasing T-cell-dependent interferon-gamma generation

We evaluated the use of recombinant human interleukin-6 (rhIL-6) and a monoclonal antibody specific for interferon-gamma (IFN-gamma) as co-adjuvants in a subunit vaccine against tuberculosis consisting of the culture filtrate proteins of Mycobacterium tuberculosis (ST-CF) emulsified in the adjuvant dimethyl-dioctadecylammonium bromide (DDA). Both the addition of rhIL-6 and the neutralization of IFN-gamma resulted in an increased T helper type 1 (Th1) response characterized by enhanced IFN-gamma production and cell proliferation. Nevertheless, this did not result in the enhancement of protection against either an intravenous or an aerosol M. tuberculosis challenge. Our data stress the need to identify further correlates of protection in addition to IFN-gamma production to screen vaccines against tuberculosis infection.

Listeria pathogenesis and molecular virulence determinants

The gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Pregnant women, neonates, the elderly, and debilitated or immunocompromised patients in general are predominantly affected, although the disease can also develop in normal individuals. Clinical manifestations of invasive listeriosis are usually severe and include abortion, sepsis, and meningoencephalitis. Listeriosis can also manifest as a febrile gastroenteritis syndrome. In addition to humans, L. monocytogenes affects many vertebrate species, including birds. Listeria ivanovii, a second pathogenic species of the genus, is specific for ruminants. Our current view of the pathophysiology of listeriosis derives largely from studies with the mouse infection model. Pathogenic listeriae enter the host primarily through the intestine. The liver is thought to be their first target organ after intestinal translocation. In the liver, listeriae actively multiply until the infection is controlled by a cell-mediated immune response. This initial, subclinical step of listeriosis is thought to be common due to the frequent presence of pathogenic L. monocytogenes in food. In normal individuals, the continual exposure to listerial antigens probably contributes to the maintenance of anti-Listeria memory T cells. However, in debilitated and immunocompromised patients, the unrestricted proliferation of listeriae in the liver may result in prolonged low-level bacteremia, leading to invasion of the preferred secondary target organs (the brain and the gravid uterus) and to overt clinical disease. L. monocytogenes and L. ivanovii are facultative intracellular parasites able to survive in macrophages and to invade a variety of normally nonphagocytic cells, such as epithelial cells, hepatocytes, and endothelial cells. In all these cell types, pathogenic listeriae go through an intracellular life cycle involving early escape from the phagocytic vacuole, rapid intracytoplasmic multiplication, bacterially induced actin-based motility, and direct spread to neighboring cells, in which they reinitiate the cycle. In this way, listeriae disseminate in host tissues sheltered from the humoral arm of the immune system. Over the last 15 years, a number of virulence factors involved in key steps of this intracellular life cycle have been identified. This review describes in detail the molecular determinants of Listeria virulence and their mechanism of action and summarizes the current knowledge on the pathophysiology of listeriosis and the cell biology and host cell responses to Listeria infection. This article provides an updated perspective of the development of our understanding of Listeria pathogenesis from the first molecular genetic analyses of virulence mechanisms reported in 1985 until the start of the genomic era of Listeria research.

Pathogenicity islands and virulence evolution in Listeria

As in other bacterial pathogens, the virulence determinants of Listeria species are clustered in genomic islands scattered along the chromosome. This review summarizes current knowledge about the structure, distribution and role in pathogenesis of Listeria virulence loci. Hypotheses about the mode of acquisition and evolution of these loci in this group of Gram-positive bacteria are presented and discussed.