Internalization of Listeria monocytogenes by nonprofessional and professional phagocytes

Listeria species escape from the phagosomes of interleukin-4-deactivated human macrophages independent of listeriolysin

Listeria monocytogenes is the causative agent of infections like sepsis and meningitis, especially in immunocompromised hosts. Human macrophages are able to phagocytose and digest L. monocytogenes but IL-4 prevents human macrophages from killing the bacteria, the mechanisms of which are unknown. In the present study, we examined various listeria species and strains including wild-type and deletion mutants in human macrophages pretreated with IL-4. To analyse the IL-4-mediated deactivation process, we combined quantitative infection assays with various morphologic methods. IL-4 facilitates survival and escape of the pathogenic L. monocytogenes wild-type strain 10403S from the macrophage phagosomes. In untreated macrophages, the isogenic listeriolysin deletion mutant strain DP-L2161 was killed and did not escape from the phagolysosomes. However, after macrophage deactivation with IL-4 DP-L2161 survived and escaped from the phagosomes. This was also the case, but to a lesser extent, even for the naturally avirulent L. innocua. As detected by confocal laser-scanning fluorescence microscopy and electron microscopy, IL-4 permitted the escape of all listeria species tested, including DP-L2161 and L. innocua from the phagosomal compartment of the macrophages. We conclude that escape from the phagosome and survival of the listeria species tested in IL-4-deactivated human macrophages is independent of the virulence factor listeriolysin.

Characterization of relA and codY mutants of Listeria monocytogenes: identification of the CodY regulon and its role in virulence

Listeria monocytogenes is a Gram-positive intracellular parasite and the causative organism of human listeriosis. In this article we demonstrate that L. monocytogenes encodes a functional member of the CodY family of global regulatory proteins that is responsive to both GTP and branched chain amino acids. By transcript analyses we identified the CodY regulon in L. monocytogenes and demonstrated that it comprises genes involved in amino acid metabolism, nitrogen assimilation as well as genes involved in sugar uptake and incorporation, indicating a role for CodY in L. monocytogenes in both carbon and nitrogen assimilation. A DeltarelA mutation reduced expression of the CodY regulon in early stationary phase and introduction of a DeltacodY mutation into a DeltarelA strain restored virulence. These data indicate that the avirulence of the DeltarelA mutant can in part be explained by the continued repression of the CodY regulon. The phenotypes of DeltarelA and DeltacodY mutants were studied in J774.A1 and Caco-2 cells and the DeltarelA mutation shown to effect intracellular growth. These results provide the first direct evidence that the activity of a CodY-type protein influences pathogenesis and provides new information on the physiological adaptation of L. monocytogenes to post-exponential phase growth and virulence.

Deletion of the gene encoding p60 in Listeria monocytogenes leads to abnormal cell division and loss of actin-based motility

Protein p60 encoded by the iap gene is regarded as an essential gene product of Listeria monocytogenes. Here we report, however, the successful construction of a viable iap deletion mutant of L. monocytogenes EGD. The mutant, which produces no p60, shows abnormal septum formation and tends to form short filaments and hooked forms during logarithmic growth. These abnormal bacterial cells break into almost normal sized single bacteria in the late-stationary-growth phase. The iap mutant is strongly attenuated in a mouse model after intravenous injection, demonstrating the importance of p60 during infection, and the invasiveness of the Deltaiap mutant for 3T6 fibroblasts and Caco-2 epithelial cells is slightly reduced. Upon uptake by epithelial cells and macrophages, the iap mutant escapes from the phagosome into the cytosol with the same efficiency as the wild-type strain, and the mutant bacteria also grow intracellularly at a rate similar to that of the wild-type strain. Intracellular movement and cell-to-cell spread are drastically reduced in various cell lines, since the iap-negative bacteria fail to induce the formation of actin tails. However, the bacteria are covered with actin filaments. Most intracellular bacteria show a nonpolar and uneven distribution of ActA around the cell, in contrast to that for the wild-type strain, where ActA is concentrated at the old pole. In an iap(+) revertant strain that produces wild-type levels of p60, intracellular movement, cell-to-cell spread, and polar distribution of ActA are fully restored. In vitro analysis of ActA distribution on the filaments of the Deltaiap strain shows that the loss of bacterial septum formation leads to ActA accumulation at the presumed division sites. In the light of data presented here and elswhere, we propose to rename iap (invasion-associated protein) cwhA (cell wall hydrolase A).

Antibodies present in normal human serum inhibit invasion of human brain microvascular endothelial cells by Listeria monocytogenes

Listeria monocytogenes causes meningitis and encephalitis in humans and crosses the blood-brain barrier by yet unknown mechanisms. The interaction of the bacteria with different types of endothelial cells was recently analyzed, and it was shown that invasion into, but not adhesion to, human brain microvascular endothelial cells (HBMEC) depends on the product of the inlB gene, the surface molecule InlB, which is a member of the internalin multigene family. In the present study we analyzed the role of the medium composition in the interaction of L. monocytogenes with HBMEC, and we show that invasion of HBMEC is strongly inhibited in the presence of adult human serum. The strong inhibitory activity, which is not present in fetal calf serum, does not inhibit uptake by macrophage-like J774 cells but does also inhibit invasion of Caco-2 epithelial cells. The inhibitory component of human serum was identified as being associated with L. monocytogenes-specific antibodies present in the human serum. Human newborn serum (cord serum) shows only a weak inhibitory activity on the invasion of HBMEC by L. monocytogenes.

InlA- but not InlB-mediated internalization of Listeria monocytogenes by non-phagocytic mammalian cells needs the support of other internalins

To determine the contribution of the previously identified internalins, InlA, InlB, InlC, InlE, InlG, and InlH, to internalization of Listeria monocytogenes by non-professional phagocytic mammalian cells, we constructed mutants with various combinations of deletions in the respective inl genes. Internalization of these mutants into the epithelial-like Caco-2 and the microvascular endothelial HBMEC cell lines were studied. Deletion of the inlGHE gene cluster, or of the single genes, led to a two to fourfold increased internalization by HBMEC and other non-phagocytic mammalian cells. Invasion into HBMEC was totally blocked in the absence of InlB, and InlB-dependent internalization did not require the presence of any of the other internalins. Internalization by Caco-2 cells was reduced to a level of about 1% in the absence of InlA and InlB, and was most efficient in the presence of InlA, InlB and InlC and in the absence of InlG, InlH and InlE. InlB and InlA, in each case in the absence of the other internalins, led (compared with the wild-type strain) to reduced internalization of about 20% and less than 10% respectively. InlA-dependent internalization (in the absence of InlB) required the additional function of InlC and InlGHE. The deletion of inlGHE enhanced the expression of InlA and InlB. The increased amount of InlA led to an increase in early association of L. monocytogenes with Caco-2 cells without enhancing its uptake in the absence of the other internalins, whereas the larger amount of InlB did not enhance early association of L. monocytogenes with HBMEC but led to an increase in internalization of L. monocytogenes. The results suggest that InlB is able to induce phagocytosis in HBMEC and (at a lower efficiency) in Caco-2 cells by itself, but InlA needs the supportive functions of the other internalins to trigger phagocytosis. None of these internalins seems to be required for cell-to-cell spread by L. monocytogenes, as shown by microinjection of Caco-2 cells with appropriate inl mutants.

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.

Regulation of virulence genes in Listeria

As in all pathogenic bacteria, virulence of the facultative intracellular Listeria species is a multifactorial trait. The expression of the bacterial genes involved in the different steps of the infectious process--invasion, intracellular multiplication and spreading--is temporally and spatially controlled, thus ensuring the presence of the respective gene products at the right moment and place. So far, one network which is involved in the regulation of listerial virulence, the PrfA regulon, has been characterized rather well. The key element of this regulon, PrfA, belongs to the Crp/Fnr family of transcriptional regulators. Its synthesis and activity are influenced by a variety of physico-chemical signals outside and inside of eukaryotic host cells. The analysis of virulence gene expression in vivo, i.e. in infected host cells, indicates that yet uncharacterized bacterial factors other than PrfA, and possibly also host factors, modulate the expression of the PrfA regulon. Essentially nothing is known about the signal transduction pathways involved in the observed differential expression of virulence genes. Fermentable carbon sources seem to have a particular role in virulence gene regulation. In addition to the PrfA regulon, the Clp stress proteins have an impact on Listeria virulence. These two regulons interact with each other by an unknown mechanism.

Interaction of Listeria monocytogenes with human brain microvascular endothelial cells: an electron microscopic study

Internalization of Listeria monocytogenes into human brain microvascular endothelial cells (HBMEC) has recently been demonstrated to be dependent upon the inlB gene. In the present scanning electron microscopic study we show that L. monocytogenes efficiently interacts with the surface of HBMEC in an inlB-independent manner which is also different from invasion. The inlB-dependent invasion of HBMEC by L. monocytogenes is accompanied by intracellular multiplication, movement, and production of bacterium-containing protrusions. These protrusions extend from the cell surface without perturbation of any adjacent cellular membrane.