Infection of cultured rat enterocytes by Ileal symbiont intracellularis depends on host cell function and actin polymerisation

Evaluation of the role of clathrin and bacterial viability in the endocytosis of Lawsonia intracellularis

Lawsonia intracellularis is an obligate intracellular bacterium and causative agent of proliferative enteropathy. The pathogenesis of L. intracellularis is not completely understood, including the endocytic mechanisms to access the host cell cytoplasm. In this study, we evaluated the mechanisms involved in endocytosis of L. intracellularis in vitro using intestinal porcine epithelial cells (IPEC-J2). Confocal microscopy was used to co-localize L. intracellularis and clathrin. Clathrin gene knockdown was then applied to verify whether L. intracellularis endocytosis is clathrin-dependent. Finally, internalization of viable and non-viable (bacteria were inactivated by heat) L. intracellularis organisms were assessed to study the role of the host cell during bacterial endocytosis. L. intracellularis organisms were observed co-localized with clathrin by confocal microscopy but the amount of L. intracellularis internalized in cells, with and without clathrin knockdown, did not differ statistically. The internalization of non-viable L. intracellularis showed a decrease in the internalization in cells with less clathrin synthesis (P<0.05). The present study is the first to elucidate the involvement of clathrin in the endocytosis of L. intracellularis. Clathrin-mediated endocytosis was shown to be an important, but not required, process for L. intracellularis internalization in porcine intestinal epithelial cells. Independence of bacterial viability for host cell internalization was also confirmed.

Survival of Lawsonia intracellularis in porcine peripheral blood monocyte-derived macrophages

Lawsonia intracellularis, an obligately intracellular enteric bacterium, infects intestinal epithelial cells, but may also be found within macrophages in the intestinal lamina propria of affected pigs. Macrophages play an important role in host defense against infectious agents, but the role of this cell in L. intracellularis infection is not well understood. The aim of this study was to evaluate the permissibility of macrophages to L. intracellularis infection in vitro. Pure culture of L. intracellularis was added to swine peripheral blood monocyte-derived macrophages. Viability of intracytoplasmic L. intracellularis was evaluated at different time points by transmission electron microscopy (TEM). Potential replication of L. intracellularis in macrophages was also evaluated by qPCR. By TEM, phagocytosis L. intracellularis within of phagolysosomes were observed 1-hour post-infection (hpi) and bacterial structures in binary fission at 48 hpi. The number of intracellular bacteria was determined at 1, 4, 24, 48, and 72 hpi by qPCR in infected macrophages and compared to the number of intracellular bacteria from culture in McCoy cells. In both cell lines, the amount of L. intracellularis was decreased at 4 hpiand increased at 24 hpi. The number of intracellular bacteria continued to increase in McCoy cells over time. This is the first study showing interaction, survival and propagation of L. intracellularis in macrophages. These findings are critical to establish an experimental model for future studies of the pathogenesis of porcine proliferative enteropathy and the potential persistence of L. intracellularis in macrophages during chronic infections.

Recent Advances in Understanding the Pathogenesis of Lawsonia intracellularis Infections

Proliferative enteropathy is an infectious disease caused by an obligate intracellular bacterium, Lawsonia intracellularis, and characterized by thickening of the intestinal epithelium due to enterocyte proliferation. The disease is endemic in swine herds and has been occasionally reported in various other species. Furthermore, outbreaks among foals began to be reported on breeding farms worldwide within the past 5 years. Cell proliferation is directly associated with bacterial infection and replication in the intestinal epithelium. As a result, mild to severe diarrhea is the major clinical sign described in infected animals. The dynamics of L. intracellularis infection in vitro and in vivo have been well characterized, but little is known about the genetic basis for the pathogenesis or ecology of this organism. The present review focuses on the recent advances regarding the pathogenesis and host-pathogen interaction of L. intracellularis infections.

Effector prediction in host-pathogen interaction based on a Markov model of a ubiquitous EPIYA motif

Background

Effector secretion is a common strategy of pathogen in mediating host-pathogen interaction. Eight EPIYA-motif containing effectors have recently been discovered in six pathogens. Once these effectors enter host cells through type III/IV secretion systems (T3SS/T4SS), tyrosine in the EPIYA motif is phosphorylated, which triggers effectors binding other proteins to manipulate host-cell functions. The objectives of this study are to evaluate the distribution pattern of EPIYA motif in broad biological species, to predict potential effectors with EPIYA motif, and to suggest roles and biological functions of potential effectors in host-pathogen interactions.

Results

A hidden Markov model (HMM) of five amino acids was built for the EPIYA-motif based on the eight known effectors. Using this HMM to search the non-redundant protein database containing 9,216,047 sequences, we obtained 107,231 sequences with at least one EPIYA motif occurrence and 3115 sequences with multiple repeats of the EPIYA motif. Although the EPIYA motif exists among broad species, it is significantly over-represented in some particular groups of species. For those proteins containing at least four copies of EPIYA motif, most of them are from intracellular bacteria, extracellular bacteria with T3SS or T4SS or intracellular protozoan parasites. By combining the EPIYA motif and the adjacent SH2 binding motifs (KK, R4, Tarp and Tir), we built HMMs of nine amino acids and predicted many potential effectors in bacteria and protista by the HMMs. Some potential effectors for pathogens (such as Lawsonia intracellularis, Plasmodium falciparum and Leishmania major) are suggested.

Conclusions

Our study indicates that the EPIYA motif may be a ubiquitous functional site for effectors that play an important pathogenicity role in mediating host-pathogen interactions. We suggest that some intracellular protozoan parasites could secrete EPIYA-motif containing effectors through secretion systems similar to the T3SS/T4SS in bacteria. Our predicted effectors provide useful hypotheses for further studies.

Application of a pig ligated intestinal loop model for early Lawsonia intracellularis infection

Background

Porcine proliferative enteropathy in pigs is caused by the obligate, intracellular bacterium Lawsonia intracellularis. In vitro studies have shown close bacterium-cell interaction followed by cellular uptake of the bacterium within 3 h post inoculation (PI). However, knowledge of the initial in vivo interaction between porcine intestinal epithelium and the bacterium is limited. The aims of the present study were to evaluate the usefulness of a ligated small intestinal loop model to study L. intracellularis infections and to obtain information on the very early L. intracellularis-enterocyte interactions.

Methods

A ligated small intestinal loop model using three different L. intracellularis inocula was applied to 10-11-week-old pigs. The inocula were 1) wild type bacteria derived from overnight incubation of L. intracellularis bacteria from spontaneous disease, 2) crude vaccine bacteria (Enterisol^® Ileitis Vet), and 3) vaccine bacteria propagated in cell culture. The bacteria-enterocyte interaction was visualised using immunohistochemistry on specimens derived 1, 3 and 6 h PI respectively.

Results

Although at a low level, close contact between bacteria and the enterocyte brush border including intracellular uptake of bacteria in mature enterocytes was seen at 3 and 6 h PI for the vaccine and the propagated vaccine inocula. Interaction between the wild-type bacteria and villus enterocytes was scarce and only seen at 6 h PI, where a few bacteria were found in close contact with the brush border.

Conclusions

The ligated intestinal loop model was useful with respect to maintaining an intact intestinal morphology for up to 6 h. Furthermore, the study demonstrated that L. intracellularis interacts with villus enterocytes within 3 to 6 h after inoculation into intestinal loops and that the bacterium, as shown for the vaccine bacteria, propagated as well as non-propagated, was able to invade mature enterocytes. Thus, the study demonstrates the early intestinal invasion of L. intracellularis in vivo.

Early pathogenesis in porcine proliferative enteropathy caused by Lawsonia intracellularis

The intestinal bacterium Lawsonia intracellularis, the cause of proliferative enteropathy (PE) in pigs, is believed to infect mitotically active epithelial cells of the intestinal crypts and then multiply and spread in these cells as they divide. Further spread of infection is thought to occur by shedding of bacteria from infected crypts followed by infection of new crypts. The early stages of the pathogenesis of PE, from 0 to 48 hours post-infection (hpi), have not been studied in vivo. In the present study pigs were inoculated with L. intracellularis and killed from 12 hpi to 5 days post-infection (dpi). The localization of L. intracellularis was determined immunohistochemically and by fluorescence in-situ hybridization. At 12 hpi L. intracellularis was found within epithelial cells at the tips of villi, indicating infection of a range of epithelial cells including mature differentiated enterocytes. Furthermore, early invasion of the intestinal connective tissue was observed; with the presence of single bacteria in the lamina propria 12 hpi, and with a further spread of bacteria in the lamina propria observed at 5 dpi, suggesting an active role for the lamina propria in the course of infection.

Porcine proliferative enteropathy: an important disease with questions remaining to be solved

Proliferative enteropathy caused by the intracellular bacterium Lawsonia intracellularis is an endemic disease with high herd prevalences reported worldwide. The infection has a considerable impact on pig production and herd economics and, with the development of new diagnostic techniques, L. intracellularis is being identified in an increasing number of pig herds and a wider range of species. This paper reviews current knowledge of the disease, with a focus on the epidemiology in pigs. The prevalence of infection, transmission, predisposing factors, microbial features, pathogenesis, clinical signs, diagnosis, treatment and control are discussed. The disease is mainly controlled by antibiotic treatment and vaccination at herd level. In the development of effective measures to prevent the spread of the infection, increased knowledge of the transmission and persistence of the microorganism are crucial.

Microarray analysis of differential expression of cell cycle and cell differentiation genes in cells infected with Lawsonia intracellularis

Infection of intestinal crypt epithelial cells by the obligate intracellular bacterium Lawsonia intracellularis is directly linked to marked proliferation of the infected enterocytes within 3-5days post-infection. The virulence factor for this unique host cell-proliferative response is not known, but is considered to involve altered crypt cell cycle or differentiation events. McCoy mouse fibroblast cells were infected with L. intracellularis, and then harvested for expressed mRNA at daily time points, with matching non-infected control cell cultures. Mouse DNA microarray (>44,000 transcript targets) analysis of cDNA derived from matching mRNA samples showed over 40 identifiable genes with at least 4-fold changes between days 0 and 3 after infection with L. intracellularis. These included altered transcription of typical host cell 'alarm' response genes, such as interferon-related response genes Isgf3g and Igtp, known to be associated with invading microbial agents. Altered transcription of several genes in these cells known to be active in regulation of the cell cycle or cell differentiation genes, including usp18, Hr, Elavl2 and Slfn2, were also detected. The altered transcription of several of these genes via RT-PCR analysis was confirmed. The microarray-detected altered transcription of cell cycle and cell differentiation genes is of possible interest for links to Lawsonia-related disturbances in epithelial cell differentiation within the intestinal crypt, but this would need to be confirmed in intestinal epithelial cell studies.

Proliferative enteropathy: a global enteric disease of pigs caused byLawsonia intracellularis

Proliferative enteropathy (PE; ileitis) is a common intestinal disease affecting susceptible pigs raised under various management systems around the world. Major developments in the understanding of PE and its causative agent,Lawsonia intracellularis, have occurred that have led to advances in the detection of this disease and methods to control and prevent it. Diagnostic tools that have improved overall detection and early onset of PE in pigs include various serological and molecular-based assays. Histological tests such as immunohistochemistry continue to be the gold standard in confirmingLawsonia-specific lesions in pigspost mortem. Despite extreme difficulties in isolatingL. intracellularis, innovations in the cultivation and the development of pure culture challenge models, have opened doors to better characterization of the pathogenesis of PE throughin vivoandin vitro L. intracellularis–host interactions. Advancements in molecular research such as the genetic sequencing of the entireLawsoniagenome have provided ways to identify various immunogens, metabolic pathways and methods for understanding the epidemiology of this organism. The determinations of immunological responsiveness in pigs to virulent and attenuated isolates ofL. intracellularisand identification of various immunogens have led to progress in vaccine development.

Diagnosis of porcine proliferative enteropathy: detection of Lawsonia intracellularis by pathological examinations, polymerase chain reaction and cell culture inoculation

A total of 21 pigs aged 7-17 weeks with clinical symptoms suggestive for Porcine Proliferative Enteropathy were examined for Lawsonia intracellularis by analysing the following parameters: (i) intestinal gross and histological lesions, (ii) presence of comma-shaped bacteria in enterocytes by Warthin-Starry and a modified Ziehl-Neelsen stain, (iii) PCR amplification of L. intracellularis DNA from intestinal mucosa by using two oligonucleotide primer pairs targeting a 255-bp DNA fragment of the 16S rDNA-gene and a 319-bp DNA fragment of the L. intracellularis chromosome. Specificity of PCR reactions was confirmed by using DNA extracted from the L. intracellularis reference strain N343 (ATCC 55672) as well as by DNA sequence comparisons of PCR amplification products with data bank entries. Intestinal gross lesion indicative for PPE were observed in 20 pigs (95.2%). For all 21 pigs, the L. intracellularis aetiology was confirmed by histological as well as bacterioscopical examinations. Specific PCR amplification products were obtained from 20 pigs (95.2%). Taking PCR positivity as the definite criterion, L. intracellularis was diagnosed in 20 pigs from 11 herds in seven Swiss cantons (Argovia, Berne, Fribourg, Grisons, Lucerne, Schwyz, Thurgovia). To grow L. intracellularisin vitro, the cell culture method of Lawson et al. (J. Clin. Microbiol. 1993: 31, 1136-1142) was adopted. Inocula prepared from heavily infected fresh and frozen ileal mucosa of 15 pigs were cultured in rat enterocytic IEC-18 cells (ATCC CRL 1589). Six cell culture passages of 10 days each were completed. The reference strain N343 was examined for cultivability, accordingly. Except for occasional specific PCR amplifications from cell cultures up to the second passage, any indications for growth of L. intracellularis in IEC-18 cells were not found.

Immunopathogenesis of experimentally induced proliferative enteropathy in pigs

To characterize the immune response associated with Lawsonia intracellularis infection, twenty-eight, 7-week-old pigs were dosed orally with a pure culture of L. intracellularis. Animals were killed 3, 7, 14, 21, 28, 35, and 42 days postinfection. Light microscopic studies were undertaken to immunophenotype the immunologic response using specific antibodies to T-cell subsets (CD3, CD4, and CD8), B cells, major histocompatibility complex class II, cadherin, and macrophages over the course of time. The results indicate that there is a direct association between the presence of L. intracellularis and reduced T-cell and B-cell numbers. For the first time, this provides evidence of the presence of an immunosuppressive mechanism operating in this disease. Furthermore, macrophage marker studies indicated that macrophages may play a more complex and significant role in the disease process than has been previously reported, with activated macrophages accumulating in infected hyperplastic crypts.

LsaA, an antigen involved in cell attachment and invasion, is expressed by Lawsonia intracellularis during infection in vitro and in vivo

Lawsonia intracellularis has been identified recently as the etiological agent of proliferative enteropathies, which are characterized by intestinal epithelial hyperplasia and associated moderate immune responses. This disease complex has been reported in a broad range of animals, prevalently in pigs, and L. intracellularis has been linked with ulcerative colitis in humans. L. intracellularis is an obligate intracellular bacterium, and the pathogenic mechanisms used to cause disease are unknown. Using in vitro-grown organisms as a source of genomic DNA, we identified a Lawsonia gene which encodes a surface antigen, LsaA (for Lawsonia surface antigen), associated with attachment to and entry into cells. The deduced amino acid sequence of this protein showed some similarity to members of a novel protein family identified in a number of other bacterial pathogens but for which roles are not fully defined. Transcription of this gene was detected by reverse transcription-PCR in L. intracellularis grown in vitro in IEC18 cells and in bacteria present in ileal tissue from infected animals. Immunohistochemistry with specific monoclonal antibody and immunoblotting with sera from infected animals demonstrated that LsaA protein is synthesized by L. intracellularis during infection. Expression of this gene during infection in vitro and in vivo suggests that this surface antigen is involved during infection, and phenotypic analysis indicated a role during L. intracellularis attachment to and entry into intestinal epithelial cells

Lawsonia intracellularis: getting inside the pathogenesis of proliferative enteropathy

Although proliferative enteropathy (PE) has been recognised for several decades, Lawsonia intracellularis, the aetiological agent, was identified formally in only 1995. This organism is both highly fastidious and obligately intracellular bacterium, characteristics which have inevitably restricted investigations in all aspects of its biology. Despite these limitations, advances have been made in characterising and understanding L. intracellularis-host interaction both in vivo and in vitro. Based upon evidence provided by mainly pathological and histological investigations conducted to date, we review salient features of our current understanding of processes involved throughout the course of infection by this unique pathogen.

Gamma interferon influences intestinal epithelial hyperplasia caused by Lawsonia intracellularis infection in mice

Lawsonia intracellularis is a recently identified bacterial pathogen which causes disease in a broad range of animals. Invasion of intestinal epithelial cells and the resultant hyperplasia of infected cells are central processes in disease pathogenesis. In this study, we aimed to establish whether immunocompetent mice were susceptible to infection and whether gamma interferon (IFN-gamma) contributed to the pathogenesis of infection. Wild-type 129-Sv-Ev mice (129 mice) and IFN-gamma receptor knockout mice based on the 129 background (IFN-gammaR(-)) were challenged orally with approximately 5.5 x 10(7) L. intracellularis cells. Both 129 and IFN-gammaR(-) mice became infected, although the extent of infection (as determined by the proportion of infected crypts) was substantially lower in 129 mice than in IFN-gammaR(-) mice. Despite these differences, infected crypts showed characteristics typical of proliferative enteropathies of other animals, i.e., intracellular colonization of epithelial cells by L. intracellularis with resultant epithelial hyperplasia. Infection in 129 mice was cleared between days 21 and 28 postchallenge, whereas infection in IFN-gammaR(-) mice was evident in 100% of animals from day 21 onward. Additionally, in IFN-gammaR(-) mice the infection was so extensive that fatalities resulted. IFN-gamma therefore plays a significant role in limiting intracellular infection and increased cellular proliferation associated with L. intracellularis. L. intracellularis infection is generally associated with modest cellular infiltration; therefore, further comparative examinations will be necessary to determine pathogenicity factors and define the role of IFN-gamma in controlling this infection.

In-vitro interactions of Lawsonia intracellularis with cultured enterocytes

Strains of the obligately intracellular bacterium Lawsonia intracellularis, the etiologic agent of porcine proliferative enteropathy, were co-cultured in rat enterocyte cell cultures (IEC-18) and examined ultrastructurally. No regular surface arrays typical of surface or S-layers were visible on any bacterial strain, with or without Triton-X-100 detergent treatment. In separate experiments, there was no difference in the ability of L. intracellularis to attach and enter enterocytes with or without the presence of added bovine plasma fibronectin, or the peptide Arg-Gly-Ser. Interestingly, there was an increase in the invasiveness of L. intracellularis in the presence of the peptide Arg-Gly-Asp (RGD), in a dose-related manner. A reduction was observed in the ability of L. intracellularis to invade enterocytes in the presence of monovalent fragments of IgG monoclonal antibodies to an outer surface component of L. intracellularis. This neutralization showed an antibody concentration-dependent titration effect and was not apparent with co-cultures incorporating control antibodies. The exact nature of ligand and cell receptor interactions for L. intracellularis remain to be determined.

Development of persistent intestinal infection and excretion of Lawsonia intracellularis by piglets

Challenge experiments using Lawsonia intracellularis as oral inocula have established its aetiological role in porcine proliferative enteropathy. Thirty piglets, in four groups, were weaned at 21 days of age and inoculated orally at 24 days. Six piglets were challenged with 1.0 x 10(8) L intracellularis strain 916/91 (NCTC 12657) passaged 12 times in vitro, six with 5.0 x 10(8) of the same strain, seven with 3.0 x 10(8) L intracellularis strain LR 189/5/83, passaged nine times, and 11 controls were dosed with sucrose-potassium glutamate buffer. An immunofluorescence assay for L intracellularis was applied to faecal smears and a polymerase chain reaction (PCR) incorporating L intracellularis-specific primers was applied to extracts of bacterial DNA derived from the faeces samples. Up to five pigs in each challenge group excreted detectable L intracellularis in faeces, in samples taken between two and 10 weeks after challenge. Some of the pigs had up to 7 x 10(8) L intracellularis g-1 faeces. The average weight gains of the higher dose challenge groups were moderately below those of the control pigs between three and nine weeks after challenge; diarrhoea was also observed in six pigs, two to four weeks after challenge. Numerous L intracellularis were detected in the intestines of all the pigs challenged with strain LR 189/5/83 and two of the pigs challenged with 916/91, but not in other tissues.

Developed and resolving lesions in porcine proliferative enteropathy: possible pathogenetic mechanisms

Proliferative enteropathy, caused by Lawsonia intracellularis, offers the opportunity to examine bacterial mechanisms that influence epithelial cell proliferation. Ultrastructural features of developed and resolving lesions included the presence of enlarged intestinal crypts containing undifferentiated immature epithelial cells and an absence of goblet cells. Numerous intracytoplasmic bacteria, identified as L. intracellularis, were consistently present within affected cells. In recovering intestinal tissue, additional features were (1) the common presence of pale, swollen, protruding epithelial cells, (2) shrunken, degenerate epithelial cells, (3) apoptotic bodies in both epithelial cells and macrophages, (4) the reappearance of normal goblet cells, and (5) reduced numbers of L. intracellularis within lesions. Bacteria were released from cells via cytoplasmic and cellular protrusions into the intestinal lumen. It is speculated that the presence of the intracytoplasmic bacterium, L. intracellularis, may disrupt normal processes of cell growth, differentiation or apoptosis in the intestinal epithelium.

Entry of the bacterium ileal symbiont intracellularis into cultured enterocytes and its subsequent release

Separate suspensions of two strains of ileal symbiont (IS) intracellularis, an obligate intracellular bacterium and the causative agent of porcine proliferative enteropathy, were added to 40 or 80 per cent confluent monolayers of established cultures of rat (IEC-18) or pig enterocytes (IPEC-J2). Peak numbers of intracellular organisms were detected within the enterocytes six days later, but no cytopathic effects were evident. After an initial close association with the cell membrane of the enterocytes, single bacteria were internalised after three hours within membranes-bound vacuoles. The formation of an electron-dense projection between cell membranes and external bacteria was only evident if the bacterial suspensions were centrifuged on to the monolayers. The release of internalised bacteria into the cytoplasm, with the breakdown and loss of membrane-bound vacuoles, was also evident three hours after infection. Internalised bacteria were associated with, but not observed within, coated membrane pits. Mitochondria were closely associated with internalised vacuoles and with released bacteria. Two to six days after infection, multiplication of the bacteria free in the cytoplasm was frequently observed. In infected cells six days after the inoculation of monolayers, groups of bacteria were found within large, balloon-like, cytoplasmic protrusions, and the subsequent release of bacteria from the monolayer provided a means of bacterial exit from the cells. Many events in the in vitro culture model closely resembled events observed at the cellular level in animals infected with IS intracellularis and the model provides a useful basis for investigating the pathogenetic mechanisms of this bacterium.