Pathogenesis of lymphoid lesions in murine experimental listeriosis

Low dose aerosol infection of mice with virulent type A Francisella tularensis induces severe thymus atrophy and CD4+CD8+ thymocyte depletion

Francisella tularensis is a gram-negative facultative intracellular bacterium and the causative agent of tularemia. Two subspecies (type A and B strains) of the pathogen exist, the former being much more virulent than the latter for humans and other higher mammals. In this study, we examined the effect of virulent strains of F. tularensis infection on the thymus and thymocytes and the potential mechanisms involved. Low-dose aerosol exposure of C57BL/6 mice with type A, but not type B, F. tularensis caused severe reduction in thymus weight and destruction of thymocytes, particularly CD4+CD8+ thymocytes, by day 4 after infection. The depletion of thymocytes was accompanied by a significant increase in circulating cortisone levels and could be partially prevented by adrenalectomy. Moreover, thymus atrophy and thymocyte depletion following infection were abolished in mice deficient in tumor necrosis factor receptors 1 and 2, but not in FasL-deficient mice. The severe destruction of the thymus and selective depletion of immature thymocytes during type A F. tularensis infection may represent a key pathogenic mechanism in tularemia and could hinder the development of an effective primary immune response against this highly virulent pathogen.

Suppurative gastritis in BALB/c mice infected with Listeria monocytogenes via the intragastric route

Suppurative gastritis was demonstrated in BALB/c mice 3 days after intragastric inoculation with 10(9) organisms of Listeria monocytogenes strain ATCC19113 (serotype 3). Also tested were four other strains of mice (C3H, C57BL/6, FVB and ICR) and three other strains of L. monocytogenes (HPB 3 [serotype 4b], HPB 410 [serotype 1/2a] and HPB 503 [serotype 1/2b]). After inoculation with ATCC19113 the numbers of bacteria found in the stomach wall were greater in C57BL/6 and ICR mice than in C3H and FVB mice; moreover, the gastritis produced in BALB/c and C57BL/6 mice was more severe than that produced in the other mouse strains. The gastritis produced in BALB/c mice with L. monocytogenes HPB 3, HPB 410 and HPB 503 was much more severe than that produced by ATCC19113. The inflammatory response occurred in the lamina muscularis and mucosa of the fundus. Massive necrosis of the gastric epithelium was observed, and there was oedema in a large part of the mucosal layer of the fundus. In addition, the submucosal layer was apparently expanded due to oedema, and in the cardia, the mucosal layer had become thin and flattened. Immunohistochemically, a polyclonal antibody against Listeria spp. produced labelling in areas of the gastric mucosa in which there was an inflammatory response and gastric epithelial necrosis.

Invasion of the central nervous system by intracellular bacteria

Infection of the central nervous system (CNS) is a severe and frequently fatal event during the course of many diseases caused by microbes with predominantly intracellular life cycles. Examples of these include the facultative intracellular bacteria Listeria monocytogenes, Mycobacterium tuberculosis, and Brucella and Salmonella spp. and obligate intracellular microbes of the Rickettsiaceae family and Tropheryma whipplei. Unfortunately, the mechanisms used by intracellular bacterial pathogens to enter the CNS are less well known than those used by bacterial pathogens with an extracellular life cycle. The goal of this review is to elaborate on the means by which intracellular bacterial pathogens establish infection within the CNS. This review encompasses the clinical and pathological findings that pertain to the CNS infection in humans and includes experimental data from animal models that illuminate how these microbes enter the CNS. Recent experimental data showing that L. monocytogenes can invade the CNS by more than one mechanism make it a useful model for discussing the various routes for neuroinvasion used by intracellular bacterial pathogens.

Host resistance of CD18 knockout mice against systemic infection with Listeria monocytogenes

Mice with targeted mutations of CD18, the common beta2 subunit of CD11/CD18 integrins, have leukocytosis, impaired transendothelial neutrophil emigration, and reduced host defense to Streptococcus pneumoniae, a gram-positive extracellular bacterium. Previous studies using blocking monoclonal antibodies suggested roles for CD18 and CD11b in hepatic neutrophil recruitment and host innate response to Listeria monocytogenes, a gram-positive intracellular bacterium. We induced systemic listeriosis in CD18 knockout (CD18-ko) and wild-type (WT) mice by tail vein injection with Listeria. By 14 days postinjection (dpi), 8 of 10 WT mice died, compared with 2 of 10 CD18-ko mice (P < 0.01). Quantitative organ culture showed that numbers of Listeria organisms in livers and spleens were similar in both groups at 20 min postinfection. By 3, 5, and 7 dpi, however, numbers of Listeria organisms were significantly lower in livers and spleens of CD18-ko mice than in WT mice. Histopathology showed that following Listeria infection, CD18-ko mice had milder inflammatory and necrotizing lesions in both spleens and livers than did WT mice. Cytokine assays indicated that baseline interleukin-1beta and granulocyte colony-stimulating factor (G-CSF) levels were higher in CD18-ko mice than in WT mice and that CD18-ko splenocytes produced higher levels of interleukin-1beta and G-CSF than WT splenocytes under the same amount of Listeria stimulation. These findings show that CD18 is not an absolute requirement for antilisterial innate immunity or hepatic neutrophil recruitment. We propose that the absence of CD18 in the mice results in the priming of innate immunity, as evidenced by elevated cytokine expression, and neutrophilic leukocytosis, which augments antilisterial defense.

Metastatic Listeria monocytogenes infection of the peritoneum in mice with cyclosporine a-induced peritonitis

Inoculation of mice with Listeria monocytogenes intragastrically or by parenteral routes has not been reported to cause peritonitis. In this study, however, severe listerial peritonitis was induced in mice infected subcutaneously and treated intraperitoneally with cyclosporin A (Cs A) in an oil carrier. In both uninfected and listeria-infected mice, intraperitoneal administration of Cs A consistently produced overexpression of P-selectin in the peritoneal microvasculature and pyogranulomatous inflammation of the peritoneum, suggesting that Cs A causes endothelial damage. We suggest that in listeria-infected mice the non-specific irritant peritonitis induced by the intraperitoneal administration of Cs A results in transfer of listeria-infected phagocytes from the liver and spleen to the peritoneal microvasculature, producing metastatic infection.

Dendritic cells are early cellular targets of Listeria monocytogenes after intestinal delivery and are involved in bacterial spread in the host

We studied the sequence of cellular events leading to the dissemination of Listeria monocytogenes from the gut to draining mesenteric lymph nodes (MLNs) by confocal microscopy of immunostained tissue sections from a rat ligated ileal loop system. OX-62-positive cells beneath the epithelial lining of Peyer's patches (PPs) were the first Listeria targets identified after intestinal inoculation. These cells had other features typical of dendritic cells (DCs): they were large, pleiomorphic and major histocompatibility complex class II(hi). Listeria were detected by microscopy in draining MLNs as early as 6 h after inoculation. Some 80-90% of bacteria were located in the deep paracortical regions, and 100% of the bacteria were present in OX-62-positive cells. Most infected cells contained more than five bacteria each, suggesting that they had arrived already loaded with bacteria. At later stages, the bacteria in these areas were mostly present in ED1-positive mononuclear phagocytes. These cells were also infected by an actA mutant defective in cell-to-cell spreading. This suggests that Listeria are transported by DCs from PPs to the deep paracortical regions of draining MLNs and are then transmitted to other cell populations by mechanisms independent of ActA. Another pathway of dissemination to MLNs was identified, probably involving free Listeria and leading to the infection of ED3-positive mononuclear phagocytes in the subcapsular sinus and adjacent paracortical areas. This study provides evidence that DCs are major cellular targets of L. monocytogenes in PPs and that DCs may be involved in the early dissemination of this pathogen. DCs were not sites of active bacterial replication, making these cells ideal vectors of infection.

Systemic immunity and mucosal immunity are induced against human immunodeficiency virus Gag protein in mice by a new hyperattenuated strain of Listeria monocytogenes

Vaccines designed to control chronic infections by intracellular agents such as human immunodeficiency virus type 1 (HIV-1) require the induction of cell-mediated immune responses to rid the host of pathogen-infected cells. Listeria monocytogenes has characteristics that make it an attractive vaccine vector for use against such infections. Here we show that parenteral immunization with a new highly attenuated strain of this organism provided complete protection against systemic and mucosal challenges with a recombinant vaccinia virus expressing HIV-1 gag. Immunization also generated a strong, long-term memory cytotoxic-T-lymphocyte (CTL) response in spleen, mesenteric lymph nodes, and Peyer's patches directed against the gag protein. Oral immunization with this attenuated strain also produced complete, long-lasting protection against the recombinant virus but only against mucosal virus challenge. Curiously, oral immunization was associated with a transient CTL response in the three lymphoid tissues examined.

Effect of selenium deficiency on the development of central nervous system lesions in murine listeriosis

The effect of selenium (Se) deficiency, produced by feeding a Se-deficient diet, on the development of central nervous system (CNS) lesions was studied in mice infected with Listeria monocytogenes, administered in drinking water for 1 or 7 days in a daily dose of 10(9)organisms, or for 7 days in a daily dose of 10(7). Se-deficient mice differed from Se-normal controls in developing CNS lesions significantly more frequently. Moreover, regardless of Se status, mice receiving repeated doses of 10(9)organisms differed from those receiving a single 10(9)dose in showing CNS lesions at least twice as often. The majority of animals with CNS lesions showed an inflammatory pattern of rhombencephalitis (17/24), while only two of 24 showed choroiditis-ventriculitis-meningitis; five of 24 animals showed both inflammatory patterns. Listeria monocytogenes antigen was identified within the areas of inflammation by an immunoperoxidase technique. Neuritis of the trigeminal nerve was present in eight animals. The relative lack of pathological changes in the liver and spleen validates this murine model for the study of CNS listeriosis.

The hamster model of intraperitoneal Burkholderia mallei (glanders)

Thirty-one female Syrian hamsters (Mesocricetus auratus) were inoculated intraperitoneally with a lethal dose of Burkholderia mallei (Budapest strain). Hamsters were killed postinoculation on days 0 through 6. Lesions were first noted in the spleens on postinoculation day 1, and in mediastinal and mesenteric lymph nodes, mediastinum, liver, and bone marrow on day 2. Lesions were present in the lung and submandibular lymph nodes on day 3, and in the brain on day 5. The characteristic histopathologic change was necrotizing pyogranulomatous inflammation, often with hemorrhage. Lesions indicative of impaired vascular perfusion, such as ischemia and infarction, were evident at the later time points. Pathologic changes generally increased in severity and distribution with time, and almost all tissues were ultimately affected. Our findings suggest that intraperitoneal bacteria were rapidly transported to mediastinal lymph nodes by transdiaphragmatic lymphatics and ultimately seeded other tissues hematogenously. The results of the study indicate that the Syrian hamster is a useful small animal model for glanders.

Infection of rabbit Peyer's patches by Shigella flexneri: effect of adhesive or invasive bacterial phenotypes on follicle-associated epithelium

In order to invade the colonic mucosa, the bacterial pathogen Shigella flexneri must find a site of entry. Experiments with the rabbit ligated intestinal loop model described here confirm that M cells of the follicle-associated epithelium (FAE) that covers lymphoid structures of the Peyer's patches represent a major site of entry for invasive microorganisms. In addition, in an isogenic Shigella background, expression of an adhesive phenotype, or of an invasive phenotype, is required for bacteria to efficiently colonize the FAE. A nonadhesive, noninvasive mutant barely interacted with FAE. Adhesive and invasive strains induced dramatic but different alterations on FAE. Invasive strain M90T caused major inflammation-mediated tissue destruction after 8 h of infection. Adhesive strain BS15 caused limited inflammation, but major architectural changes, characterized by an increase in the size of M cells that became stretched over large pockets containing an increased number of mononuclear cells, were observed. M cells progressively occupied large surface areas of the FAE at the expense of enterocytes. This contributed to enterocytes losing contact with the lumen. These experiments demonstrate that various remodeling patterns may occur in Peyer's patches in response to bacterial pathogens, depending on the virulence phenotype expressed by the pathogenic strain.

Tuberculosis in the brushtail possum (Trichosurus vulpecula) after intratracheal inoculation with a low dose of Mycobacterium bovis

Six possums were each inoculated with approximately 125 colony-forming units of Mycobacterium bovis via a cannula inserted per os into the trachea. Five other possums were sham inoculated and housed separately. At postmortem examination 55 to 57 days after inoculation, all six infected possums showed extensive macroscopical lesions of tuberculosis in the lungs and bronchial lymph nodes and some also had lesions in the liver, kidney, spleen and hepatic lymph nodes. Mycobacterium bovis was isolated from all of these possums. No evidence of M. bovis infection was detected in the five control animals. In the M. bovis-infected possums, microscopical examination of organs and lymph nodes demonstrated a much wider distribution of lesions than did macroscopical examination. The location of early lesions indicated that the paracortical region of the lymph nodes, the marginal zone and periphery of lymphoid follicles in the spleen, and the cortex of the kidney were predilection sites for lesions resulting from haematogenous spread of infection. This method for reproducing bovine tuberculosis in the possum was more suitable for experimental studies than previously published methods.

Cell immortalization enhances Listeria monocytogenes invasion

Recent outbreaks of human listeriosis have emphasized the importance of food in the etiology of epidemic listeriosis, suggesting that the gastrointestinal tract is the natural site of entry for Listeria monocytogenes into the organism. L. monocytogenes invasion of finite cell lines derived from the porcine ileum exhibited a 100-fold lower penetration level, without any intracellular multiplication, when compared to CaCo-2 cells, a widely used in vitro model for L. monocytogenes invasion. Same results were obtained with both pig kidney primary cells and mouse kidney finite cell lines. To demonstrate that cell immortalization enhances L. monocytogenes invasion, finite cell lines from porcine ileum and from murine kidney were immortalized by Simian virus 40 (SV40) large T oncogene. Unlike their untransformed counterparts, the immortal cells obtained were invaded by L. monocytogenes, as observed for CaCo-2 cells as well as for spontaneously immortal human (HeLa) and murine (3T3) cell lines. Extensive electron microscopy examinations of porcine epithelioid cells infected by L. monocytogenes showed numerous bacteria within the immortal cells, whereas neither intracellular bacteria nor any bacterial antigen were revealed inside finite cell lines. These data suggested that L. monocytogenes were not destroyed inside finite cell lines but only poorly entered the finite or primary cells. Speculating that L. monocytogenes invasion is under control of differentiation or proliferation of the cells, only an enterocyte subset at a defined state of differentiation or expressing particular receptors could be invaded in vivo.

Salmonella typhimurium initiates murine infection by penetrating and destroying the specialized epithelial M cells of the Peyer's patches

Salmonella species are known to initiate infection of mammalian hosts by penetrating the intestinal epithelium of the small bowel. These bacteria preferentially interact with Peyer's patches which are collections of lymphoid follicles making up the gut-associated lymphoid tissue. We infected murine ligated intestinal loops with invasive and noninvasive Salmonella typhimurium strains for 30, 60, 120, and 180 min and examined the infected tissue by transmission electron microscopy. Within 30 min, we found that invasive S. typhimurium exclusively entered M cells found within the follicle-associated epithelium (FAE) of the Peyer's patches. Initially, interactions between invasive bacteria and enterocytes adjacent to the M cells were not found. Invasion of M cells was associated with the ability of the bacteria to invade tissue culture cells. S. typhimurium mutants, which were noninvasive for tissue culture cells, could not be found in ligated loops associated with M cells or enterocytes after incubations of 30, 60, 120, or 180 min. At 60 min, internalized invasive S. typhimurium were cytotoxic for the M cells. Destruction of an M cell formed a gap in the FAE which allowed organisms to invade enterocytes adjacent to the dead cell. Later in the infection process (120 and 180 min), the presence of bacteria beneath the FAE correlated with changes in the cytoarchitecture of the lymphoid follicle. In addition, replicating Salmonella began to enter both the apical and basolateral surfaces of enterocytes adjacent to infected M cells.

Choroiditis and meningitis in experimental murine infection with Listeria monocytogenes

In a study of central nervous system involvement in experimental listeriosis 27 Swiss CD1 mice were inoculated subcutaneously with Listeria monocytogenes. Systemic infection developed, as shown by the isolation of Listeria monocytogenes and histopathological lesions in the spleen and liver. In the central nervous system a mixed inflammatory infiltration in the ventricular system, especially in the choroid plexus, and leptomeningitis were the most relevant lesions. Inflammatory lesions were associated with the presence of Listeria monocytogenes, as demonstrated by a positive anti-Listeria monocytogenes immunoperoxidase reaction within phagocytic cells. It is suggested that choroiditis and meningitis developed as a consequence of hematogenous dissemination of Listeria monocytogenes within mononuclear phagocytes and penetration of these cells into the ventricular system through the choroid plexus.

A microbiological, histopathological and immunohistological study of the intragastric inoculation of Listeria monocytogenes in mice

The course of murine infection after intragastric inoculation of L. monocytogenes was investigated by immunocytochemical, histopathological and microbiological techniques. L. monocytogenes antigen was observed in epithelial cells of intestinal mucosa overlying Peyer's patches, but not in mucosa devoid of them. This suggests that penetration of L. monocytogenes into the host organism may take place through epithelium overlying Peyer's patches. The efficiency of bacterial penetration appeared to be low, as shown by the small amounts of L. monocytogenes antigen detected and the low counts of bacteria in organs. Gross or histopathological lesions in the intestinal tract were not observed. The presence of L. monocytogenes in spleen, liver and in maxillary and mesenteric lymph nodes, confirmed that the systemic course of infection by this route of inoculation is similar to that of the parenteral routes. The results emphasize the subclinical character of murine listeriosis by the oral route.