Novel virulence properties of the Salmonella typhimurium virulence-associated plasmid: immune suppression and stimulation of splenomegaly

Mice infected subcutaneously with wild-type Salmonella typhimurium, SR11, developed a significant splenomegaly when compared with mice infected with an equal number of a plasmid-cured strain. Further, the bacterial load in the spleen at 14 days after infection, measured as colony-forming units per gram tissue, was significantly higher in mice infected with the parent strain than in mice infected with the plasmid-cured strain. These data confirm the previously reported plasmid-associated ability of Salmonella to multiply within the spleen. In addition, lymph node cells (LNC) from mice infected with the parent strain had a significantly reduced ability to proliferate in response to concanavalin A, a T-cell mitogen, and to heat-killed S. typhimurium cells when compared with LNC isolated from mice infected with the plasmid-cured strain. Finally, reintroduction of a functional Tn5-tagged 90-kb plasmid into a plasmid-free strain restored its capacity to cause a marked splenomegaly and to suppress lymph node cell proliferation in BALB/c mice. These data demonstrate that the 90-kb plasmid of highly virulent S. typhimurium strains mediates several novel pathogenic properties in infected mice: (1) enhancement of the ability of Salmonella to multiply within the spleen; (2) stimulation of a splenic inflammatory response as displayed by marked splenomegaly; and (3) a general suppression of lymphocyte responsiveness to both T-cell mitogens and specific Salmonella antigens.     

Experimental Infection of Mice with Yersinia enterocolitica Serotype O9 by Oral and Parenteral Routes: Spreading and Enterotropism of Virulent Yersiniae

An isogenic pair of Yersinia enterocolitica serotype O9 strains, with and without virulence plasmid, was used to study the plasmid role in the infection of BALB/c mice by oral, intraperitoneal, and intravenous routes. The plasmid-bearing strain, but not its plasmid-less derivative, caused enteric infection after challenge by all three routes. The virulence plasmid did not influence the peritoneal clearance of yersiniae, but only the plasmid-bearing yersiniae were able to move from the peritoneal cavity to the bloodstream, and thus they spread to spleen and liver. Moreover, plasmid-bearing yersiniae were able to move from the liver to the gallbladder, and they shed in bile into the intestine. Western blot analysis of antibody responses to chromosomally encoded outer membrane proteins revealed similar patterns with sera from mice challenged with each one of the two strains by intraperitoneal route. In contrast, only the plasmid-bearing strain elicited an antibody response to these antigens in mice challenged by oral route. Although mice experimentally infected with plasmid-bearing O9 yersiniae developed an enteric infection, irrespective of the inoculation route, differences between the first steps in infection by oral and parenteral routes may be important, especially when the infection model is used as an approach to study the yersinia-host interactions. Received: 31 August 1998 / Accepted: 13 November 1998     

Effect of constitutively expressed phoP gene on the localization of Salmonella typhimurium within Mac-1 positive phagocytes

Intracellular localization of the wild-type (Spv+), the phoP-constitutively expressed strain (PhoPc), and the spv-deleted strain (Spv-) of Salmonella typhimurium was examined by the use of confocal laser scanning microscopy analysis of immunostained sections of mouse spleens after oral or subcutaneous inoculation. Only 40% of salmonellae of both the PhoPc and the Spv- strains were detected intracellularly within Mac-1 positive cells at day five after oral or day four after subcutaneous inoculation. In contrast, over 85% of salmonellae of the Spv+ strain were detected inside Mac-1 positive cells. In both inoculation trials, the splenic colony-forming unit values for the PhoPc and Spv- strains were significantly lower than the corresponding value for the Spv+ strain. These findings suggest that the constitutively expressed phoP gene of S. typhimurium attenuated virulence by limiting intracellular proliferation within mouse spleen phagocytes, and that the lack of spv genes had the same effect.     

Identification, genetic analysis and DNA sequence of a 7.8-kb virulence region of the Salmonella typhimurium virulence plasmid

The 90-kilobase (kb) virulence plasmid of Salmonella typhimurium is responsible for invasion from the intestines to mesenteric lymph nodes and spleens of orally inoculated mice. We used Tn5 and aminoglycoside phosphotransferase (aph) gene insertion mutagenesis and deletion mutagenesis of a previously identified 14-kb virulence region to reduce this virulence region to 7.8kb. The 7.8-kb virulence region subcloned into a low copy-number vector conferred a wild-type level of splenic infection to virulence plasmid-cured S. typhimurium and conferred essentially a wild-type oral LD50. Insertion mutagenesis identified five loci essential for virulence, and DNA sequence analysis of the virulence region identified six open reading frames. Expected protein products were identified from four of the six genes, with three of the proteins identified as doublet bands in Escherichia coli minicells. Three of the five mutated genes were able to be complemented by clones containing only the corresponding wild-type gene. Only one of the five deduced amino acid sequences, that of the positive regulatory element, SpvR, possessed significant homology to other proteins. The codon usage for the virulence genes showed no codon bias, which is consistent with the low levels of expression observed for the corresponding proteins. Consensus promoters for several different sigma factors were identified upstream of several of the genes, whereas only consensus Rho-dependent termination sequences were observed between certain of the genes. The operon structure of this virulence region therefore appears to be complex. The construction of the cloned 7.8-kb virulence region and the determination of the DNA sequence will aid in the further genetic analysis of the five plasmid-encoded virulence genes of S. typhimurium.     

Characterisation and in vivo behaviour of a Salmonella typhimurium aroA strain expressing Escherichia coli K1 polysaccharide

Abstract Plasmid pKT274 encoding a determinant for the Escherichia coli K1 polysaccharide was introduced into the Salmonella typhimurium aro A vaccine strain SL3261 and cells harbouring the plasmid were shown to express K1 polysaccharide at their cell surface. SL3261 (pKT274) could be detected in the livers and spleens of BALB/c mice infected by the intravenous route and viable organisms persisted for several weeks. SL3261 (pKT274) was cleared from the livers more rapidly and from the spleens more slowly than SL3261. Unlike mice infected with SL3261 those infected with SL3261 (pKT274) did not exhibit gross splenomegaly during the first three weeks after infection. Mice vaccinated with viable SL3261 (pKT274) were protected against challenge with virulent S. typhimurium but failed to produce detectable levels of humoral anti-K1 polysaccharide antibodies.     

表达乙型脑炎病毒E蛋白重组减毒沙门菌活载体疫苗株的构建

目的:构建表达乙型脑炎病毒(JEV)E蛋白的口服重组减毒鼠伤寒沙门菌活载体疫苗株。方法:克隆JEV E基因,将其插入表达载体pYA3341中,构建重组质粒pYA3341-E,将重组质粒电转入鼠伤寒沙门菌疫苗株X4550(缺失asd、cya、crp基因),获得重组疫苗菌株X4550(pYA3341-E);鉴定重组菌E蛋白的表达,测定重组菌的稳定性、生长曲线、安全性,以及小鼠的免疫试验和血清中和试验。结果:酶切鉴定和序列测定证实重组质粒构建成功;SDS-PAGE检测有目的蛋白条带;Western印迹证实表达的E蛋白能与猪抗JEV阳性血清特异性结合;重组菌株在体外营养选择压力下,可稳定地携带重组质粒传代繁殖,在体内可较稳定地定居于肠系膜淋巴结和脾脏;小鼠口服试验证实重组菌无毒性作用,安全可靠;小鼠口服重组菌免疫,ELISA检测产生了抗JEV抗体;中和试验表明产生的抗体具有中和活性。结论:构建了能稳定表达JEV E蛋白的口服减毒鼠伤寒沙门菌疫苗株X4550(pYA3341-E),为研究乙型脑炎口服基因工程疫苗奠定基础。     

Dietary Bifidobacterium lactis (HN019) enhances resistance to oral Salmonella typhimurium infection in mice

The ability of a newly identified probiotic lactic acid bacterial strain, Bifidobacterium lactis (HN019), to confer protection against Salmonella typhimurium was investigated in BALB/c mice. Feeding mice with B. lactis conferred a significant degree of protection against single or multiple oral challenge with virulent S. typhimurium, in comparison to control mice that did not receive B. lactis. Protection included a ten-fold increase in survival rate, significantly higher post-challenge food intake and weight gain, and reduced pathogen translocation to visceral tissues (spleen and liver). Furthermore, the degree of pathogen translocation showed a significant inverse correlation with splenic lymphocyte proliferative responses to mitogens, blood and peritoneal cell phagocytic activity and intestinal mucosal anti-S. typhimurium antibody titers in infected mice; all of these immune parameters were enhanced in mice fed B. lactis. Together, these results suggest that dietary B. lactis can provide a significant degree of protection against Salmonella infection by enhancing various parameters of immune function that are relevant to the immunological control of salmonellosis. Thus dietary supplementation with B. lactis provides a unique opportunity for developing immune-enhancing probiotic dairy food products with proven health benefits.     

Importance of the host genetic background on immune responses to Helicobacter pylori infection and therapeutic vaccine efficacy

In order to investigate the role of host factors in Helicobacter pylori infection and immunity, two different strains of inbred mice, C57BL/6 and BALB/c, were infected with a standard H. pylori strain, SS1. A month later, infected mice were immunized orally with whole-cell lysates of H. pylori SS1 and cholera toxin on days 1, 3, 6, 30, and 54. Ten days after the last immunization, mice were sacrificed and the stomach was collected to assess H. pylori colonization density by quantitative culture. H. pylori SS1 colonization was significantly greater in C57BL/6 than in BALB/c (P<0.02 and P<0.003 at 2 and 13 weeks post-inoculation, respectively). Colonization in C57BL/6 persisted at equivalent levels for 13 weeks but the colonization density in BALB/c decreased significantly during this period. In contrast to the pattern of bacterial colonization, antibody responses following H. pylori SS1 infection were greater in BALB/c than in C57BL/6, suggesting that host factors may modulate the immune responses to H. pylori infection. Following therapeutic immunization, H. pylori colonization in BALB/c mice was also significantly reduced (P<0.03), while no significant differences in bacterial density were observed in C57BL/6. These observations collectively demonstrate the great importance of host factors in H. pylori infection and the development of effective immune responses.     

Th1-Th2 cytokine kinetics in the bronchoalveolar lavage fluid of mice infected with Cryptococcus neoformans of different virulences

Th1 immune response plays an important role in protection against infection with Cryptococcus neoformans in mice. We investigated the effect of virulence of C. neoformans on cytokine production in the lung of a mouse model of pulmonary cryptococcosis. BALB/c mice were inoculated intratracheally with a high or low virulence strain of C. neoformans, followed by serial measurements of Th1 and Th2 cytokine concentrations in the bronchoalveolar lavage (BAL) fluid using appropriate enzyme-linked immunosorbent assay kits. The number of colony-forming units (CFU) increased with time, and all mice infected with the highly virulent strain were dead at 28 days after inoculation. In contrast, the number of microorganisms diminished with time in the mice infected with the low virulence strain during the 4-week study. The numbers of neutrophils and lymphocytes in the BAL fluid paralleled those of CFU. High neutrophil counts were observed in the BAL fluid of mice infected with the highly virulent strain, while lymphocyte counts were increased only in the later part of the study in mice infected with the high and low virulence strains. The concentrations of Th2 cytokine, interleukin (IL)-4 were significantly higher in mice infected with the highly virulent strain at days 14 and 21 of infection, whereas the level of Th1 cytokine, interferon-gamma, was significantly higher in the latter strain at days 7 and 14. Our results suggest that strain-specific difference in the organism's ability to induce (or evade) the host immune system contributes to the outcome of infection.     

Salmonella typhimurium strains carrying independent mutations display similar virulence phenotypes yet are controlled by distinct host defense mechanisms

The outcome of Salmonella infection in the mammalian host favors whoever succeeds best in disturbing the equilibrium between coordinate expression of bacterial (virulence) genes and host defense mechanisms. Intracellular persistence in host cells is critical for pathogenesis and disease, because Salmonella typhimurium strains defective in this property are avirulent. We examined whether similar host defense mechanisms are required for growth control of two S. typhimurium mutant strains. Salmonella pathogenicity island 2 (SPI2) and virulence plasmid-cured Salmonella mutants display similar virulence phenotypes in immunocompetent mice, yet their gene loci participate in independent virulence strategies. We determined the role of TNF-alpha and IFN-gamma as well as different T cell populations in infection with these Salmonella strains. After systemic infection, IFN-gamma was essential for growth restriction of plasmid-cured S. typhimurium, while SPI2 mutant infections were controlled in the absence of IFN-gamma. TNFRp55-deficiency restored systemic virulence to both Salmonella mutants. After oral inoculation, control of plasmid-cured bacteria substantially relied on both IFN-gamma and TNF-alpha signaling while control of SPI2 mutants did not. However, for both mutants, ultimate clearance of bacteria from infected mice depended on alphabeta T cells.     

In vitro and in vivo stability of plasmids in attenuated Salmonella enterica serovar typhimurium used as a carrier of DNA vaccine is associated with its replication origin.

The ability of live attenuated Salmonella enterica serovar typhimurium (S. typhimurium) as a carrier of DNA vaccine was evaluated using model plasmid encoding beta-galactosidase (beta-Gal) and BALB/c mice. We constructed pBRCMVbeta, beta-Gal expression apparatus having a replication origin from low copy pBR322. Comparison of the plasmid stability showed that pBRCMVbeta remained stable in Salmonella even after oral administration, while pUC-based pCMVbeta tended to be lost quickly. However, titers for beta-Gal specific IgG in sera did not significantly increase in mice orally administered S. typhimurium harboring pBRCMVbeta. These data suggest that the stability of plasmid in S. typhimurium is associated with its replication origin. Further studies are required to scientifically establish this methodology.     

Strain typing with IS200 fingerprints in Salmonella abortusovis.

A collection of Salmonella abortusovis isolates was examined for the presence of insertion element IS200. All proved to contain three or four copies of the element. One IS200 hybridization band of approximately 9 kb was found in all isolates, indicating that all S. abortusovis strains carry an IS200 element in similar or identical locations; this band can be potentially useful for serovar identification. S. abortusovis collection isolates from distinct geographic areas were highly polymorphic, suggesting that IS200 fingerprints might provide information on the geographic origin of S. abortusovis strains. Isolates obtained from the same geographic area (the island of Sardinia, Italy) were less polymorphic: all shared three constant IS200 hybridization bands, indicating that they derive from a single ancestor. Most strains analyzed contained an additional copy of IS200 in the variable region of the virulence plasmid. Certain Sardinian flocks proved to be infected by only one S. abortusovis strain, while others harbored two strains. Strain typing with IS200 fingerprints proved to be more reliable than plasmid analysis, because the latter yielded a high degree of polymorphism, even among isolates from the same flock.     

Recovery from chronic rotavirus infection in mice with severe combined immunodeficiency: virus clearance mediated by adoptive transfer of immune CD8+ T lymphocytes.

Severe combined immunodeficient (SCID) mice lack both functional T and B cells. These mice develop chronic rotavirus infection following an oral inoculation with the epizootic diarrhea of infant mice (EDIM) rotavirus. Reconstitution of rotavirus-infected SCID mice with T lymphocytes from immunocompetent mice allows an evaluation of a role of T-cell-mediated immunity in clearing chronic rotavirus infection. Complete rotavirus clearance was demonstrated in C.B-17/scid mice 7 to 9 days after the transfer of immune CD8+ splenic T lymphocytes from histocompatible BALB/c mice previously immunized intraperitoneally with the EDIM-w strain of murine rotavirus. The virus clearance mediated by T-cell transfer was restricted to H-2d-bearing T cells and occurred in the absence of rotavirus-specific antibody as determined by enzyme-linked immunosorbent assay, neutralization, immunohistochemistry, and radioimmunoprecipitation. Temporary clearance of rotavirus was observed after the transfer of immune CD8+ T cells isolated from the intestinal mucosa (intraepithelial lymphocytes [IELs]) or the spleens of BALB/c mice previously infected with EDIM by the oral route. Chronic virus shedding was transiently eliminated 7 to 11 days after spleen cell transfer and 11 to 12 days after IEL transfer. However, recurrence of rotavirus infection was detected 1 to 8 days later in all but one SCID recipient receiving cells from orally immunized donors. The viral clearance was mediated by IELs that were both Thy1+ and CD8+. These data demonstrated that the clearance of chronic rotavirus infection in SCID mice can be mediated by immune CD8+ T lymphocytes and that this clearance can occur in the absence of virus-specific antibodies.     

Comparative experimental infection of Lacazia loboi in BALB/c and B10.A mice

Both hind foot pads of BALB/c and B10.A mice strains, were inoculated with a fungal suspension of Lacazia loboi obtained from a Jorge Lobo's disease patient. The suspension had 9 x 105 cells/ml and its viability index was 45%. The animals were sacrificed at different time periods varying from 24 h to 18 months after inoculation. The BALB/c mice developed an extensive granulomatous infiltrate, similar to the disease in humans, that progressively evolved. The number of fungal elements also increased as the disease progressed, and after the seventh month of inoculation, macroscopic changes of the foot pads were evident. Although the B10.A mice developed an exuberant granulomatous infiltrate, macroscopic changes were not detected. The number of fungal cells in the infected tissues increased in number, but they were lower then the numbers found in the BALB/c strain. The viability indexes were also lower for the B10.A strain. Considering the histopathological findings, the presence of macroscopic changes and the great amount of fungal cells in the infected tissues, the authors concluded that the BALB/c mice strain was more susceptible to L. loboi infection than the B10.A strain.     

Antibodies to lipopolysaccharide and outer membrane proteins of Salmonella enteritidis PT4 are not involved in protection from experimental infection

BALB/c and Schofield mice were inoculated with formalin-killed bacteria prepared from strains of Salmonella enteritidis belonging to phage type (PT) 4 and carrying a 38 MDa plasmid and expressing long-chain lipopolysaccharide, or strains without a 38 MDa plasmid or lacking the ability to express lipopolysaccharide. Vaccinated mice were challenged with viable bacteria belonging to a virulent strain of S. enteritidis (PT4). Mice surviving this viable challenge were examined for a humoral antibody response to membrane antigens of S. enteritidis (PT4) that might relate to the possession of a given virulence property. BALB/c mice immunized with any of the test antigens were found to be immune to S. enteritidis (PT4), and this immunity was protective. Serum antibodies, of the IgG class, were detected to OmpA and a minor outer membrane protein (OMP) of 31 kDa. Schofield mice also raised IgG antibodies to these outer membrane proteins; however, non-immunized mice of this strain were resistant to infection. The virulence of S. enteritidis (PT4) was also tested using mice belonging to strains B10D2 (new), Biozzi (high), Biozzi (low), C3HeJ, B10ITYR and C57/L.     

A recombinant Salmonella typhimurium vaccine strain is taken up and survives within murine Peyer's patch dendritic cells

The attenuated Salmonella typhimurium PhoP^c strain is avirulent but immunogenic via the oral route in mice and is attenuated in survival in macrophage cell lines. In this study, the fate of PhoP^c bacteria expressing green fluorescent protein was investigated in murine Peyer's patches. The survival of PhoP^c was monitored after orogastric inoculation of BALB/c mice. Bacteria persisted for several weeks in the Peyer's patches and were also recovered from the mesenteric lymph nodes and spleen. Confocal microscopy analysis identified dendritic cells as the Peyer's patch cell type that internalized PhoP^c expressing green fluorescent protein at early time points. In addition, live PhoP^c were found in Peyer's patch dendritic cells and not in B cells 3 days after orogastric inoculation. Taken together, these results provide strong evidence that PhoP^c is internalized and survives within Peyer's patch dendritic cells. As these cells are potent antigen-presenting cells, these data could explain the immunogenicity of S. typhimurium vaccine strains in vivo .