Salmonella typhimurium induces apoptosis in human monocyte-derived macrophages

Programmed cell death in aortic aneurysm and dissection: A potential therapeutic target

Rupture of aortic aneurysm and dissection (AAD) remains a leading cause of death. Progressive smooth muscle cell (SMC) loss is a crucial feature of AAD that contributes to aortic dysfunction and degeneration, leading to aortic aneurysm, dissection, and, ultimately, rupture. Understanding the molecular mechanisms of SMC loss and identifying pathways that promote SMC death in AAD are critical for developing an effective pharmacologic therapy to prevent aortic destruction and disease progression. Cell death is controlled by programmed cell death pathways, including apoptosis, necroptosis, pyroptosis, and ferroptosis. Although these pathways share common stimuli and triggers, each type of programmed cell death has unique features and activation pathways. A growing body of evidence supports a critical role for programmed cell death in the pathogenesis of AAD, and inhibitors of various types of programmed cell death represent a promising therapeutic strategy. This review discusses the different types of programmed cell death pathways and their features, induction, contributions to AAD development, and therapeutic potential. We also highlight the clinical significance of programmed cell death for further studies.

Small alarmones (p)ppGpp regulate virulence associated traits and pathogenesis of Salmonella enterica serovar Typhi

How Salmonella enterica serovar Typhi (S. Typhi), an important human pathogen, survives the stressful microenvironments inside the gastrointestinal tract and within macrophages remains poorly understood. We report here that S. Typhi has a bonafide stringent response (SR) system, which is mediated by (p)ppGpp and regulates multiple virulence-associated traits and the pathogenicity of the S. Typhi Ty2 strain. In an iron overload mouse model of S. Typhi infection, the (p)ppGpp⁰ (Ty2ΔRelAΔSpoT) strain showed minimal systemic spread and no mortality, as opposed to 100% death of the mice challenged with the isogenic wild-type strain. Ty2ΔRelAΔSpoT had markedly elongated morphology with incomplete septa formation and demonstrated severely attenuated motility and chemotaxis due to the loss of flagella. Absence of the Vi-polysaccharide capsule rendered the mutant strain highly susceptible to complement-mediated lysis. The phenotypes of Ty2ΔRelAΔSpoT was contributed by transcriptional repression of several genes, including fliC, tviA, and ftsZ, as found by reverse transcriptase quantitative polymerase chain reaction and gene complementation studies. Finally, Ty2ΔRelAΔSpoT had markedly reduced invasion into intestinal epithelial cells and significantly attenuated survival within macrophages. To the best of our knowledge, this was the first study that addressed SR in S. Typhi and showed that (p)ppGpp was essential for optimal pathogenic fitness of the organism.

Differential Immune Phenotypes in Human Monocytes Induced by Non-Host-Adapted Salmonella enterica Serovar Choleraesuis and Host-Adapted S. Typhimurium

We studied the effects of two Salmonella enterica serovar Typhimurium (host-adapted) strains (14028 and 4/74) and three S Choleraesuis (non-host-adapted) strains (A50, A45, and B195) in human monocytes between 2 and 24 h postinfection (p.i.) to investigate whether differences in immune response may explain the much higher prevalence of sepsis in individuals infected with S Choleraesuis. Both serovars significantly increased the production of cytokines associated with acute sepsis (tumor necrosis factor alpha [TNF-α], interleukin β [IL-β], and IL-6), but temporal differences occurred between these serovars and between different S Choleraesuis strains. Generally, all S Choleraesuis strains induced significantly higher production of inflammatory cytokines than S Typhimurium strains (P < 0.01 to 0.05). All S Choleraesuis strains very significantly increased IL-10 production by monocytes at 6 and 24 h p.i. in comparison to S Typhimurium strains (P < 0.01). In addition, ∼80% of monocytes were viable at 24 h p.i. with S Choleraesuis A50, compared to only ∼40% following S Typhimurium infection. Using S Typhimurium 14028 and S Choleraesuis A50 as examples of these two serovars, we also showed differential expression of genes within the Janus tyrosine kinase (JAK) and signal transducer and activator of transcription (STAT) (JAK/STAT) pathway via quantitative PCR (qPCR) microarray analysis. High serum IL-10 concentration and monocyte survival have been reported as markers of the development of human sepsis. We therefore conclude that high production of IL-10 by monocytes may, in part, explain the greater propensity for S Choleraesuis to induce human sepsis and that this may be greater in strains such as A50, which induces both high IL-10 production and monocyte survival.

Biological and Epidemiological Consequences of MTBC Diversity

Tuberculosis is caused by different groups of bacteria belonging to the Mycobacterium tuberculosis complex (MTBC). The combined action of human factors, environmental conditions and bacterial virulence determine the extent and form of human disease. MTBC virulence is a composite of different clinical phenotypes such as transmission rate and disease severity among others. Clinical phenotypes are also influenced by cellular and immunological phenotypes. MTBC phenotypes are determined by the genotype, therefore finding genotypes responsible for clinical phenotypes would allow discovering MTBC virulence factors. Different MTBC strains display different cellular and clinical phenotypes. Strains from Lineage 5 and Lineage 6 are metabolically different, grow slower, and are less virulent. Also, at least certain groups of Lineage 2 and Lineage 4 strains are more virulent in terms of disease severity and human-to-human transmission. Because phenotypic differences are ultimately caused by genotypic differences, different genomic loci have been related to various cellular and clinical phenotypes. However, defining the impact of specific bacterial genomic loci on virulence when other bacterial determinants, human and environmental factors are also impacting the phenotype would contribute to a better knowledge of tuberculosis virulence and ultimately benefit tuberculosis control.

Vasoactive intestinal peptide (VIP) differentially affects inflammatory immune responses in human monocytes infected with viable Salmonella or stimulated with LPS

We compared the effect of VIP on human blood monocytes infected with Salmonella typhimurium 4/74 or stimulated with LPS. VIP (10(-7)M) increased monocyte viability by 24% and 9% when cultured for 24h with 4/74 or Salmonella LPS (100ng/ml), respectively. Significantly increased (P<0.05) numbers of 4/74 were also recovered from monocytes co-cultured with VIP after 6h post-infection (pi) and this remained high after 24h pi. Both 4/74 and LPS increased (P<0.05) the concentration of TNF-α, IL-1β and IL-6 measured in monocyte supernatants. However, LPS induced this effect more rapidly while, with the exception of IL-6, 4/74 induced higher concentrations (P<0.05). VIP significantly decreased (P<0.05) TNF-α and IL-1β production by 4/74-infected monocytes after 6 pi, but only after 24h in LPS-cultured monocytes. This trend was reversed for IL-6 production. However, TNF-α and IL-1β production by 4/74-infected monocytes, cultured with VIP, still remained higher (P<0.05) than concentrations measured in supernatants cultured only with LPS. VIP also increased (P<0.05) production of anti-inflammatory IL-10 in both 4/74 and LPS cultures after 24h. We also show a differential effect of VIP on the expression of TNFα and IL-6 receptors, since VIP was only able to decreased expression in LPS-stimulated monocytes but not in 4/74-infected monocytes. In conclusion, we show a differential effect of VIP on human monocytes infected with virulent Salmonella or stimulated with LPS. Our study suggests that the use of VIP in bacteraemia and/or sepsis may be limited to an adjunctive therapy to antibiotic treatment.

Macrophage cell death upon intracellular bacterial infection

Macrophage-pathogen interaction is a complex process and the outcome of this tag-of-war for both sides is to live or die. Without attempting to be comprehensive, this review will discuss the complexity and significance of the interaction outcomes between macrophages and some facultative intracellular bacterial pathogens as exemplified by Francisella, Salmonella, Shigella and Yersinia. Upon bacterial infection, macrophages can die by a variety of ways, such as apoptosis, autophagic cell death, necrosis, necroptosis, oncosis, pyronecrosis, pyroptosis etc, which is the focus of this review.

A Salmonella enterica serovar Typhi plasmid induces rapid and massive apoptosis in infected macrophages

pR(ST98) is a chimeric plasmid isolated from Salmonella enterica serovar Typhi (S. typhi) that mediates the functions of drug resistance and virulence. Previously, we reported that Salmonella plasmid virulence (spv) genes were present in S. typhi. In our current study, we investigated whether plasmid pR(ST98) exhibits significant cytotoxicity in macrophages. pR(ST98) was transferred into the avirulent Salmonella enterica serovar Typhimurium (S. typhimurium) strain RIA to create the transconjugant pR(ST98)/RIA. The standard S. typhimurium virulent strain SR-11, which carries a 100-kb virulence plasmid, was used as a positive control. The bacterial strains were incubated with a murine macrophage-like cell line (J774A.1) in vitro. Apoptosis of J774A.1 cells was examined by electron microscopy and flow cytometry after annexin-V/propidium iodide labeling, and the survival of Salmonella strains in J774A.1 cells was determined. Results showed that macrophages infected with strain pR(ST98)/RIA displayed greater levels of apoptosis than those infected with RIA and that pR(ST98 )may increase bacterial survival in macrophages. Further studies showed that the pR(ST98)-induced death of macrophages was associated with the loss of mitochondrial membrane potential and that pR(ST98 )may activate caspase-9 and then caspase-3. The research data indicate that the virulence of bacteria that contain the pR(ST98) plasmid is enhanced; the presence of this plasmid increases the survival of the bacterial pathogen and acts through the mitochondrial pathway to mediate macrophage apoptosis.

Positron emission tomography (PET) imaging of tumor-localized Salmonella expressing HSV1-TK

In order to noninvasively detect Salmonella delivery vectors within tumors, we used a genetically modified Salmonella, VNP20009, that expresses the herpes simplex thymidine kinase (HSV1-tk) reporter gene. VNP20009-TK were able to selectively localize within murine tumor models and to effectively sequester a radiolabeled nucleoside analogue, 2'-fluoro-1-beta-D-arabino-furanosyl-5-iodo-uracil (FIAU). A quantitative relationship between the level of radioactivity accumulated and the number of bacteria in tumor and different tissues was demonstrated. The in vivo accumulation of [14C]FIAU measured in tissue sample homogenates and sections were related to Salmonella number and to immunohistochemical bacterial staining, respectively. Quantitative autoradiography (QAR) revealed the relative intensity of [14C]FIAU accumulation in a tumor cross-section, demonstrating that the peripheral region of the tumor was significantly less active than internal regions. [124I]FIAU positron emission tomography (PET) and subsequent tissue radioactivity and bacterial concentration measurements were compared. A log-log relationship was found, and the PET images could identify multiple tumor sites. The ability to noninvasively detect Salmonella vectors by PET imaging has the potential to be conducted in a clinical setting, and could aid in development of these vectors by demonstrating the efficiency and duration of targeting as well as indicating the locations of tumors.

Survival of Salmonella serovar Typhimurium inside porcine monocytes is associated with complement binding and suppression of the production of reactive oxygen species

The development of the carrier state in swine after infection with Salmonella serovar Typhimurium (S. Typhimurium) has not been elucidated yet. Possibly, phagocytes like macrophages play a crucial role. It was the aim of the present study to characterize the interaction of a S. Typhimurium strain and its hilA and ssrA mutants with porcine peripheral blood monocytes (PBM). Exposure of porcine PBM to S. Typhimurium induced the production of reactive oxygen species (ROS), requiring bacterial protein synthesis. The numbers of intracellular bacteria sharply decreased over a period of 3h. Monocytes obtained from different pigs differed markedly in their ROS production and in their ability to kill the bacteria. Interestingly, high ROS production did not coincide with increased intracellular killing. Using diphenylene iodonium inhibition of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity, it was shown that bacterial killing was ROS-dependent only within 1h post inoculation, but was ROS-independent from 1h post inoculation onwards. This might be explained by the finding that metabolically active Salmonella bacteria were capable of suppressing the respiratory burst activity in a SPI-1- and SPI-2-independent manner without causing measurable cell damage. Opsonization with complement did not alter the ROS production. Nevertheless, it increased intracellular survival of the bacteria. In conclusion, survival of S. Typhimurium inside porcine PBM is promoted by suppression of respiratory burst activity and complement binding.

The Salmonella effector protein SopB protects epithelial cells from apoptosis by sustained activation of Akt

Invasion of epithelial cells by Salmonella enterica is mediated by bacterial "effector" proteins that are delivered into the host cell by a type III secretion system. Although primarily known for their roles in actin rearrangements and membrane ruffling, translocated effectors also affect host cell processes that are not directly associated with invasion. Here, we show that SopB/SigD, an effector with phosphoinositide phosphatase activity, has anti-apoptotic activity in Salmonella-infected epithelial cells. Salmonella induced the sustained activation of Akt/protein kinase B, a pro-survival kinase, in a SopB-dependent manner. Failure to activate Akt resulted in increased levels of apoptosis after infection with a sopB deletion mutant (DeltasopB). Furthermore, cells infected with wild type bacteria, but not the DeltasopB strain, were protected from camptothecin-induced cleavage of caspase-3 and subsequent apoptosis. The anti-apoptotic activity of SopB was dependent on its phosphatase activity, because a catalytically inactive mutant was unable to protect cells from the effects of camptothecin. Finally, small interfering RNA was used to demonstrate the essential role of Akt in SopB-mediated protection against apoptosis. These results provide new insights into the mechanisms of apoptosis and highlight how bacterial effectors can intercept signaling pathways to manipulate host responses.

Host adaptation of pigeon isolates of Salmonella enterica subsp. enterica serovar Typhimurium variant Copenhagen phage type 99 is associated with enhanced macrophage cytotoxicity

Phage type 99 of Salmonella enterica subsp. enterica serovar Typhimurium variant Copenhagen strains isolated from pigeons were examined for the presence of genotypic and phenotypic characteristics. The pulsed-field gel electrophoresis patterns obtained with XbaI and BlnI from 38 pigeon strains were compared with those obtained from 89 porcine, poultry, and human strains of variant Copenhagen. Identical patterns with XbaI and four closely related patterns with BlnI were obtained with the pigeon strains, whereas 16 XbaI patterns were found with the other strains. The XbaI patterns of the pigeon strains showed a low genetic similarity to the patterns of the porcine, poultry, and human strains and invariably showed a low-molecular-weight band that was absent in the majority of the other strains. The virulence genes shdA, spvR, pefA, sopE, and spvB were uniformly present in six pigeon isolates representing the genetic diversity found with BlnI. These six pigeon-derived strains were highly cytotoxic for pigeon macrophages compared to three porcine strains. After experimental infection of pigeons with a pigeon strain, clinical symptoms, fecal shedding, and colonization of internal organs were more pronounced than those after infection with a porcine strain. These data suggest that the phage type 99 strains used in this study are highly adapted to pigeons and should be classified as a host-restricted lineage of the serovar Typhimurium.

Detection of hilA gene sequences in serovars of Salmonella enterica subspecies enterica

hilA gene promoter, component of the Salmonella Pathogenicity Island 1, has been found in Salmonella serovar Typhimurium, being important for the regulation of type III secretion apparatus genes. We detected hilA gene sequences in Salmonella serovars Typhi, Enteritidis, Choleraesuis, Paratyphi A and B, and Pullorum, by polymerase chain reaction (PCR) and hybridization techniques. The primers to carry out PCR were designed according to hilA sequence. A low stringency hybridization with the probe pVV441 (hilA open-reading-frame plasmid) was carried out. To find hilA gene sequences in other Salmonella sp. suggest that these serovars could have similar sequences of this kind of virulence genes.

Characterization of a novel intracellularly activated gene from Salmonella enterica serovar typhi

A Salmonella enterica serovar Typhi gene that is selectively up-regulated upon bacterial invasion of eukaryotic cells was characterized. The open reading frame encodes a 298-amino-acid hydrophobic polypeptide (30.8 kDa), which is predicted to be an integral membrane protein with nine membrane-spanning domains. The protein is closely related (87 to 94% reliability) to different transport and permease systems. Gene expression under laboratory conditions was relatively weak; however, sevenfold induction was observed in a high-osmolarity medium (300 mM NaCl). The growth pattern in a laboratory medium of a serovar Typhi strain Ty2 derivative containing a 735-bp in-frame deletion in this gene, named gaiA (for gene activated intracellularly), was not affected. In contrast, the mutant was partially impaired in intracellular survival in murine peritoneal macrophages, as well as in human monocyte-derived macrophages. However, in the case of human macrophages, this survival defect was modest and evident only at late infection times (24 h). Despite the distinct intracellular survival kinetics displayed in macrophages of different species, the gaiA null mutant was significantly affected in its potential to trigger apoptosis in both murine and human macrophages. Provision of the gaiA gene in trans resulted in complementation of these phenotypes. Interestingly, the absence of a functional gaiA gene caused a marked attenuation in the mouse mucin model, as shown by the increase (3 orders of magnitude) in the 50% lethal dose of the mutant strain over that of the parental strain Ty2 (P Collapse

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MESH Headings

• Amino Acid Sequence
• Animals
• Apoptosis
• Bacterial Proteins/genetics
• Bacterial Proteins/physiology
• Base Sequence
• DNA, Bacterial/genetics
• Gene Expression Regulation, Bacterial
• Genes, Bacterial
• Humans
• Macrophages/microbiology
• Macrophages/pathology
• Membrane Proteins/genetics
• Membrane Proteins/physiology
• Mice
• Molecular Sequence Data
• Mutation
• Salmonella Infections/etiology
• Salmonella Infections/microbiology
• Salmonella Infections/pathology
• Salmonella typhi/genetics
• Salmonella typhi/pathogenicity
• Virulence/genetics
• Virulence/physiology

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Affiliation(s)

Holger Basso Division of Microbiology, GBF-German Research Centre for Biotechnology, D-38124 Braunschweig, Germany
Faiza Rharbaoui
Lothar H Staendner
Eva Medina
Francisco García-Del Portillo
Carlos A Guzmán

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