Development of a novel in-water vaccination protocol for DNA adenine methylase deficient Salmonella enterica serovar Typhimurium vaccine in adult sheep

Essential role of Salmonella Enteritidis DNA adenine methylase in modulating inflammasome activation

Background

Salmonella Enteritidis (SE) is one of the major foodborne zoonotic pathogens of worldwide importance which can induce activation of NLRC4 and NLRP3 inflammasomes during infection. Given that the inflammasomes play an essential role in resisting bacterial infection, Salmonella has evolved various strategies to regulate activation of the inflammasome, most of which largely remain unclear.

Results

A transposon mutant library in SE strain C50336 was screened for the identification of the potential factors that regulate inflammasome activation. We found that T3SS-associated genes invC, prgH, and spaN were required for inflammasome activation in vitro. Interestingly, C50336 strains with deletion or overexpression of Dam were both defective in activation of caspase-1, secretion of IL-1β and phosphorylation of c-Jun N-terminal kinase (Jnk). Transcriptome sequencing (RNA-seq) results showed that most of the differentially expressed genes and enriched KEGG pathways between the C50336-VS-C50336Δdam and C50336-VS-C50336::dam groups overlapped, which includes multiple signaling pathways related to the inflammasome. C50336Δdam and C50336::dam were both found to be defective in suppressing the expression of several anti-inflammasome factors. Moreover, overexpression of Dam in macrophages by lentiviral infection could specifically enhance the activation of NLRP3 inflammasome independently via promoting the Jnk pathway.

Conclusions

These data indicated that Dam was essential for modulating inflammasome activation during SE infection, there were complex and dynamic interplays between Dam and the inflammasome under different conditions. New insights were provided about the battle between SE and host innate immunological mechanisms.

Bacterial DNA Methylation and Methylomes

Formation of C5-methylcytosine, N4-methylcytosine, and N6-methyladenine in bacterial genomes is postreplicative and involves transfer of a methyl group from S-adenosyl-methionine to a base embedded in a specific DNA sequence context. Most bacterial DNA methyltransferases belong to restriction-modification systems; in addition, "solitary" or "orphan" DNA methyltransferases are frequently found in the genomes of bacteria and phage. Base methylation can affect the interaction of DNA-binding proteins with their cognate sites, either by a direct effect (e.g., steric hindrance) or by changes in DNA topology. In both Alphaproteobacteria and Gammaproteobacteria, the roles of DNA base methylation are especially well known for N6-methyladenine, including control of chromosome replication, nucleoid segregation, postreplicative correction of DNA mismatches, cell cycle-coupled transcription, formation of bacterial cell lineages, and regulation of bacterial virulence. Technical procedures that permit genome-wide analysis of DNA methylation are nowadays expanding our knowledge of the extent, evolution, and physiological significance of bacterial DNA methylation.

Contaminated water delivery as a simple and effective method of experimental Salmonella infection

AIM

In most infectious disease models, it is assumed that gavage needle infection is the most reliable means of pathogen delivery to the GI tract. However, this methodology can cause esophageal tearing and induces stress in experimental animals, both of which have the potential to impact early infection and the subsequent immune response.

MATERIALS & METHODS

C57BL/6 mice were orally infected with virulent Salmonella Typhimurium SL1344 either by intragastric gavage preceded by sodium bicarbonate, or by contamination of drinking water.

RESULTS

We demonstrate that water contamination delivery of Salmonella is equivalent to gavage inoculation in providing a consistent model of infection. Furthermore, exposure of mice to contaminated drinking water for as little as 4 h allowed maximal mucosal and systemic infection, suggesting an abbreviated window exists for natural intestinal entry.

CONCLUSION

Together, these data question the need for gavage delivery for infection with oral pathogens.

Trial watch: Naked and vectored DNA-based anticancer vaccines

One type of anticancer vaccine relies on the administration of DNA constructs encoding one or multiple tumor-associated antigens (TAAs). The ultimate objective of these preparations, which can be naked or vectored by non-pathogenic viruses, bacteria or yeast cells, is to drive the synthesis of TAAs in the context of an immunostimulatory milieu, resulting in the (re-)elicitation of a tumor-targeting immune response. In spite of encouraging preclinical results, the clinical efficacy of DNA-based vaccines employed as standalone immunotherapeutic interventions in cancer patients appears to be limited. Thus, efforts are currently being devoted to the development of combinatorial regimens that allow DNA-based anticancer vaccines to elicit clinically relevant immune responses. Here, we discuss recent advances in the preclinical and clinical development of this therapeutic paradigm.

Immunization with a DNA adenine methylase over-producing Yersinia pseudotuberculosis vaccine confers robust cross-protection against heterologous pathogenic serotypes

Yersinia pseudotuberculosis is a foodborne pathogen that can cause serious human illness. Although the source and route of transmission often remain obscure, livestock have been implicated in some cases. The diversity of yersiniae present on farms and their widespread distribution in animal and environmental reservoirs necessitates the use of broad prophylactic strategies that are efficacious against many serotypes simultaneously. Herein, immunization of mice with a modified, live attenuated Y. pseudotuberculosis vaccine that overproduces the DNA adenine methylase (Dam(OP)) conferred robust protection against virulent challenge (150-fold LD50) with homologous and heterologous serotypes that have been associated with human disease (O:1, O:1a, O:3). Further, the dam gene was shown to be essential for cell viability in all (7 of 7) Y. pseudotuberculosis strains tested. Direct selection for the inheritance of dam mutant alleles in Y. pseudotuberculosis resulted in dam strain variants that contained compensatory (second-site suppressor) mutations in genes encoding methyl-directed mismatch repair proteins (mutHLS) that are involved in suppression of the non-viable cell phenotype in all (19/19) strains tested. Such dam mutH variants exhibited a significant increase in virulence and spontaneous mutation frequency relative to that of a Dam(OP) vaccine strain. These studies indicate that Y. pseudotuberculosis Dam(OP) strains conferred potent cross-protective efficacy as well as decreased virulence and spontaneous mutation frequency relative to those that lack Dam, which have compensatory mutations in mutHLS loci. These data suggest that development of yersiniae livestock vaccines based on Dam overproduction is a viable mitigation strategy to reduce these potential foodborne contaminants.

Salmonella Typhimurium lacking the Znuabc transporter is attenuated and immunogenic in pigs

Meat contamination by Salmonella spp. is emerging as a major cause of human enteric infections in industrialized countries. The attempts to reduce human cases of salmonellosis encompass pre- and post-harvest interventions. In this context, vaccination of pigs may represent an effective instrument in eliminating/reducing Salmonella burden through the food chain. We have previously demonstrated that Salmonella Typhimurium lacking the ZnuABC transporter (S. Typhimurium ΔznuABC) is a promising candidate live vaccine in different mouse models of Salmonella Typhimurium infection. In this study, we confirmed in pigs the attenuation of S. Typhimurium ΔznuABC. Moreover, we evaluated the safety and immunogenicity of S. Typhimurium ΔznuABC administered to pigs by the oral route. We monitored clinical conditions of animals and we conducted a microbiological culture and a quantification of the humoral and cellular immune response, respectively, on fecal and blood samples of pigs. After vaccination with attenuated S. Typhimurium ΔznuABC, pigs showed a modest degree of hyperthermia. In addition, fecal shedding of S. Typhimurium ΔznuABC could not be detected 28 days after the inoculum. Furthermore, vaccination with S. Typhimurium ΔznuABC elicited a distinct production of anti-Salmonella antibodies and IFN-γ. Taken together, these results suggest that S. Typhimurium ΔznuABC is attenuated and immunogenic in pigs. Although the vaccine dosages do not guarantee complete safety there is ample margin to set up better conditions of use, suggesting that S. Typhimurium ΔznuABC could be a promising attenuated strain to be used as live mucosal vaccine for oral delivery.