An SopB-mediated immune escape mechanism of Salmonella enterica can be subverted to optimize the performance of live attenuated vaccine carrier strains

Navigating the signaling landscape of Ralstonia solanacearum: a study of bacterial two-component systems

Ralstonia solanacearum, the bacterium that causes bacterial wilt, is a destructive phytopathogen that can infect over 450 different plant species. Several agriculturally significant crop plants, including eggplant, tomato, pepper, potato, and ginger, are highly susceptible to this plant disease, which has a global impact on crop quality and yield. There is currently no known preventive method that works well for bacterial wilt. Bacteria use two-component systems (TCSs) to sense their environment constantly and react appropriately. This is achieved by an extracellular sensor kinase (SK) capable of sensing a suitable signal and a cytoplasmic response regulator (RR) which gives a downstream response. Moreover, our investigation revealed that R. solanacearum GMI1000 possesses a substantial count of TCSs, specifically comprising 36 RRs and 27 SKs. While TCSs are known targets for various human pathogenic bacteria, such as Salmonella, the role of TCSs in R. solanacearum remains largely unexplored in this context. Notably, numerous inhibitors targeting TCSs have been identified, including GHL (Gyrase, Hsp, and MutL) compounds, Walk inhibitors, and anti-TCS medications like Radicicol. Consequently, the investigation into the involvement of TCSs in virulence and pathogenesis has gained traction; however, further research is imperative to ascertain whether TCSs could potentially supplant conventional anti-wilt therapies. This review delves into the prospective utilization of TCSs as an alternative anti-wilt therapy, focusing on the lethal phytopathogen R. solanacearum.

Salmonella enterica serovar Choleraesuis vector delivering SaoA antigen confers protection against Streptococcus suis serotypes 2 and 7 in mice and pigs

Streptococcus suis is one of the major pathogens that cause economic losses in the swine industry worldwide. However, current bacterins only provide limited prophylactic protection in the field. An ideal vaccine against S. suis should protect pigs against the clinical diseases caused by multiple serotypes, or at least protect against the dominant serotype in a given geographic region. A new recombinant Salmonella enterica serotype Choleraesuis vaccine vector, rSC0011, that is based on the regulated delayed attenuation system and regulated delayed antigen synthesis system, was developed recently. In this study, an improved recombinant attenuated Salmonella Choleraesuis vector, rSC0016, was developed by incorporating a sopB mutation to ensure adequate safety and maximal immunogenicity. In the spleens of mice, rSC0016 colonized less than rSC0011. rSC0016 and rSC0011 colonized similarly in Peyer's patches of mice. The recombinant vaccine rSC0016(pS-SaoA) induced stronger cellular, humoral, and mucosal immune responses in mice and swine against SaoA, a conserved surface protein that is present in many S. suis serotypes, than did rSC0011(pS-SaoA) without sopB or rSC0018(pS-SaoA), which is an avirulent, chemically attenuated vaccine strain. rSC0016(pS-SaoA) provided 100% protection against S. suis serotype 2 in mice and pigs, and full cross-protection against SS7 in pigs. This new vaccine vector provides a foundation for the development of a universal vaccine against multiple serotypes of S. suis in pigs.

Two-Component Signal Transduction Systems of Pathogenic Bacteria As Targets for Antimicrobial Therapy: An Overview

The bacterial communities in a wide range of environmental niches sense and respond to numerous external stimuli for their survival. Primarily, a source they require to follow up this communication is the two-component signal transduction system (TCS), which typically comprises a sensor Histidine kinase for receiving external input signals and a response regulator that conveys a proper change in the bacterial cell physiology. For numerous reasons, TCSs have ascended as convincing targets for antibacterial drug design. Several studies have shown that TCSs are essential for the coordinated expression of virulence factors and, in some cases, for bacterial viability and growth. It has also been reported that the expression of antibiotic resistance determinants may be regulated by some TCSs. In addition, as a mode of signal transduction, phosphorylation of histidine in bacteria differs from normal serine/threonine and tyrosine phosphorylation in higher eukaryotes. Several studies have shown the molecular mechanisms by which TCSs regulate virulence and antibiotic resistance in pathogenic bacteria. In this review, we list some of the characteristics of the bacterial TCSs and their involvement in virulence and antibiotic resistance. Furthermore, this review lists and discusses inhibitors that have been reported to target TCSs in pathogenic bacteria.

Multi-Omics Studies towards Novel Modulators of Influenza A Virus-Host Interaction

Human influenza A viruses (IAVs) cause global pandemics and epidemics. These viruses evolve rapidly, making current treatment options ineffective. To identify novel modulators of IAV–host interactions, we re-analyzed our recent transcriptomics, metabolomics, proteomics, phosphoproteomics, and genomics/virtual ligand screening data. We identified 713 potential modulators targeting 199 cellular and two viral proteins. Anti-influenza activity for 48 of them has been reported previously, whereas the antiviral efficacy of the 665 remains unknown. Studying anti-influenza efficacy and immuno/neuro-modulating properties of these compounds and their combinations as well as potential viral and host resistance to them may lead to the discovery of novel modulators of IAV–host interactions, which might be more effective than the currently available anti-influenza therapeutics.

Dual 4- and 5-phosphatase activities regulate SopB-dependent phosphoinositide dynamics to promote bacterial entry

Salmonella are able to invade non-phagocytic cells such as intestinal epithelial cells by modulating the host actin cytoskeleton to produce membrane ruffles. Two type III effector proteins SopB and SopE play key roles to this modulation. SopE is a known guanine nucleotide exchange factor (GEF) capable of activating Rac1 and CDC42. SopB is a phosphatidylinositol 4-phosphatase and 5-phosphatase promoting membrane ruffles and invasion of Salmonella through undefined mechanisms. Previous studies have demonstrated that the 4-phosphatase activity of SopB is required for PtdIns-3-phosphate (PtdIns(3)P) accumulation and SopB-mediated invasion. We show here that both the 4-phosphatase as well as the 5-phosphatase activities of SopB are essential in ruffle formation and subsequent invasion. We found that the 5-phosphatase activity of SopB is likely responsible for generating PtdIns-3,4-bisphosphate (PtdIns(3,4)P(2)) and subsequent recruitment of sorting nexin 9 (SNX9), an actin modulating protein. Intriguingly, the 4-phosphatase activity is responsible for the dephosphorylation of PtdIns(3,4)P(2) into PtdIns(3)P. Alone, neither activity is sufficient for ruffling but when acting in conjunction with one another, the 4-phosphatase and 5-phosphatase activities led to SNX9-mediated ruffling and Salmonella invasion. This work reveals the unique ability of bacterial effector protein SopB to utilize both its 4- and 5-phosphatase activities to regulate phosphoinositide dynamics to promote bacterial entry.

The evolution of bacterial social life: From the ivory tower to the front lines of public health

Drug resistant bacteria are a huge and growing threat to public health. A solution exists in theory, but had not yet been put to a practical test. In this paper, it is put to a test, and performs successfully. As predicted by theory, using a drug that targets bacteria's shared secreted ‘public goods’ molecules instead of cell components, did not drive the bacterial evolution of drug resistance, and therefore retained its effectiveness. This result holds great promise for better drugs and vaccines against many infectious diseases, and also for better cancer therapies.

Drug-resistant bacteria are a huge and growing threat to public health. A solution exists in theory, but had not yet been put to a practical test. The accompanying paper by Ross-Gillespie et al., the theory is put to a test and performs successfully. As predicted, using a drug that targets bacteria's shared secreted ‘public goods’ molecules instead of cell components did not drive the bacterial evolution of drug resistance, and therefore retained its effectiveness. This result holds great promise for better drugs and vaccines against many infectious diseases, and also for better cancer therapies.

Vaccines: from empirical development to rational design

Infectious diseases are responsible for an overwhelming number of deaths worldwide and their clinical management is often hampered by the emergence of multi-drug-resistant strains. Therefore, prevention through vaccination currently represents the best course of action to combat them. However, immune escape and evasion by pathogens often render vaccine development difficult. Furthermore, most currently available vaccines were empirically designed. In this review, we discuss why rational design of vaccines is not only desirable but also necessary. We introduce recent developments towards specifically tailored antigens, adjuvants, and delivery systems, and discuss the methodological gaps and lack of knowledge still hampering true rational vaccine design. Finally, we address the potential and limitations of different strategies and technologies for advancing vaccine development.

New technologies in developing recombinant attenuated Salmonella vaccine vectors

Recombinant attenuated Salmonella vaccine (RASV) vectors producing recombinant gene-encoded protective antigens should have special traits. These features ensure that the vaccines survive stresses encountered in the gastrointestinal tract following oral vaccination to colonize lymphoid tissues without causing disease symptoms and to result in induction of long-lasting protective immune responses. We recently described ways to achieve these goals by using regulated delayed in vivo attenuation and regulated delayed in vivo antigen synthesis, enabling RASVs to efficiently colonize effector lymphoid tissues and to serve as factories to synthesize protective antigens that induce higher protective immune responses. We also developed some additional new strategies to increase vaccine safety and efficiency. Modification of lipid A can reduce the inflammatory responses without compromising the vaccine efficiency. Outer membrane vesicles (OMVs) from Salmonella-containing heterologous protective antigens can be used to increase vaccine efficiency. A dual-plasmid system, possessing Asd+ and DadB+ selection markers, each specifying a different protective antigen, can be used to develop multivalent live vaccines. These new technologies have been adopted to develop a novel, low-cost RASV synthesizing multiple protective pneumococcal protein antigens that could be safe for newborns/infants and induce protective immunity to diverse Streptococcus pneumoniae serotypes after oral immunization.

Mechanisms used by virulent Salmonella to impair dendritic cell function and evade adaptive immunity

Innate and adaptive immunity are inter-related by dendritic cells (DCs), which directly recognize bacteria through the binding of pathogen-associated molecular patterns (PAMPs) to specialized receptors on their surface. After capturing and degrading bacteria, DCs present their antigens as small peptides bound to MHC molecules and prime naive bacteria-specific T cells. In response to PAMP recognition DCs undergo maturation, which is a phenotypic change that increases their immunogenicity and promotes the activation of naive T cells. As a result, a specific immune response that targets bacteria-derived antigens is initiated. Therefore, the characterization of DC-bacteria interactions is important to understand the mechanisms used by virulent bacteria to avoid adaptive immunity. Furthermore, any impairment of DC function might contribute to bacterial survival and dissemination inside the host. An example of a bacterial pathogen capable of interfering with DC function is Salmonella enterica serovar Typhimurium (S. Typhimurium). Virulent strains of this bacterium are able to differentially modulate the entrance to DCs, avoid lysosomal degradation and prevent antigen presentation on MHC molecules. These features of virulent S. Typhimurium are controlled by virulence proteins, which are encoded by pathogenicity islands. Modulation of DC functions by these gene products is supported by several studies showing that pathogenesis might depend on this attribute of virulent S. Typhimurium. Here we discuss some of the recent data reported by the literature showing that several virulence proteins from Salmonella are required to modulate DC function and the activation of host adaptive immunity.

Salmonella enterica as a vaccine carrier

Salmonella enterica is an invasive, facultative intracellular gastrointestinal pathogen causing human diseases such as gastroenteritis and typhoid fever. Virulence-attenuated strains of this pathogen have interesting capacities for the generation of live vaccines. Attenuated live typhoidal and nontyphoidal Salmonella strains can be used for vaccination against Salmonella infections and to target tumor tissue. Such strains may also serve as live carriers for the development of vaccination strategies against other bacterial, viral or parasitic pathogens. Various strategies have been developed to deploy regulatory circuits and protein secretion systems for efficient expression and delivery of foreign antigens by Salmonella carrier strains. One prominent example is the use of type III secretion systems to translocate recombinant antigens into antigen presenting cells. In this review, we will describe the recent developments in strategies that utilize live attenuated Salmonella as vaccine carriers for prophylactic vaccination against infectious diseases and therapeutic vaccination against tumors. Considerations for generating safe, attenuated carrier strains, designing stable expression systems and the use of adjuvants for live carrier strategies are discussed.

Live recombinant Salmonella Typhi vaccines constructed to investigate the role of rpoS in eliciting immunity to a heterologous antigen

We hypothesized that the immunogenicity of live Salmonella enterica serovar Typhi vaccines expressing heterologous antigens depends, at least in part, on its rpoS status. As part of our project to develop a recombinant attenuated S. Typhi vaccine (RASTyV) to prevent pneumococcal diseases in infants and children, we constructed three RASTyV strains synthesizing the Streptococcus pneumoniae surface protein PspA to test this hypothesis. Each vector strain carried ten engineered mutations designed to optimize safety and immunogenicity. Two S. Typhi vector strains (chi9639 and chi9640) were derived from the rpoS mutant strain Ty2 and one (chi9633) from the RpoS(+) strain ISP1820. In chi9640, the nonfunctional rpoS gene was replaced with the functional rpoS gene from ISP1820. Plasmid pYA4088, encoding a secreted form of PspA, was moved into the three vector strains. The resulting RASTyV strains were evaluated for safety in vitro and for immunogenicity in mice. All three RASTyV strains were similar to the live attenuated typhoid vaccine Ty21a in their ability to survive in human blood and human monocytes. They were more sensitive to complement and were less able to survive and persist in sewage and surface water than their wild-type counterparts. Adult mice intranasally immunized with any of the RASTyV strains developed immune responses against PspA and Salmonella antigens. The RpoS(+) vaccines induced a balanced Th1/Th2 immune response while the RpoS(-) strain chi9639(pYA4088) induced a strong Th2 immune response. Immunization with any RASTyV provided protection against S. pneumoniae challenge; the RpoS(+) strain chi9640(pYA4088) provided significantly greater protection than the ISP1820 derivative, chi9633(pYA4088). In the pre-clinical setting, these strains exhibited a desirable balance between safety and immunogenicity and are currently being evaluated in a Phase 1 clinical trial to determine which of the three RASTyVs has the optimal safety and immunogenicity profile in human hosts.

Immune responses to recombinant pneumococcal PsaA antigen delivered by a live attenuated Salmonella vaccine

Streptococcus pneumoniae is a leading cause of morbidity and mortality among children worldwide and particularly in developing countries. In this study, we evaluated PsaA, a conserved antigen important for S. pneumoniae adhesion to and invasion into nasopharynx epithelia, for its ability to induce protective immunity against S. pneumoniae challenge when delivered by recombinant attenuated Salmonella vaccine (RASVs) strains. RASVs were engineered to synthesize PsaA peptides of various lengths. Vaccination with an RASV synthesizing full-length PsaA induced high titers of anti-PsaA antibodies in both systemic (IgG in serum) and mucosal (IgA in vaginal washes, nasal washes, and lung homogenates) sites. BALB/c (haplotype H2(d)) or C57BL/6 (haplotype H2(b)) mice vaccinated either orally or intranasally exhibited a significant reduction in colonization of nasopharyngeal tissues after intranasal challenge with S. pneumoniae strains compared to controls, although protection was not observed with all challenge strains. None of the vaccine constructs provided protection against intraperitoneal challenge with S. pneumoniae strain WU2 (serotype 3). Immunization with RASVs synthesizing truncated PsaA generated lower titers of IgA and IgG and did not provide significant protection. Our results showed that RASVs synthesizing full-length PsaA can provide protection against nasal colonization by some S. pneumoniae strains. PsaA may be a useful addition to a multivalent vaccine, providing protection against pneumonia, otitis media, and other diseases caused by S. pneumoniae.

Immunogenicity of a live recombinant Salmonella enterica serovar typhimurium vaccine expressing pspA in neonates and infant mice born from naive and immunized mothers

We are developing a Salmonella vectored vaccine to prevent infant pneumonia and other diseases caused by Streptococcus pneumoniae. One prerequisite for achieving this goal is to construct and evaluate new recombinant attenuated Salmonella vaccine (RASV) strains suitable for use in neonates and infants. Salmonella enterica serovar Typhimurium strain chi9558(pYA4088) specifies delivery of the pneumococcal protective antigen PspA and can protect adult mice from challenge with S. pneumoniae. This strain is completely safe for oral delivery to day-old and infant mice. Here we assess the colonizing ability, immunogenicity, and protective efficacy of chi9558(pYA4088) in neonatal mice. Colonization was assessed in mice 0, 2, 4, or 7 days of age after oral inoculation. In the presence of maternal antibodies, the colonization of lymphoid tissues was delayed, but the immune responses were enhanced in mice born to immunized mothers. Both oral and intranasal routes were used to assess immunogenicity. All orally or intranasally immunized neonatal and infant mice born to either immunized or naïve mothers developed PspA-specific mucosal and systemic immune responses. Mice born to immunized mothers produced higher titers of PspA-specific antibodies in the blood and mucosa and greater numbers of PspA-specific interleukin-4 (IL-4)-secreting cells than mice born to naïve mothers. More importantly, mice born to immune mothers showed a significant increase in protection against S. pneumoniae challenge. These results suggest that strain chi9558(pYA4088) can circumvent some of the limitations of the immature immune system in neonatal and infant mice, generating enhanced protective immune responses in the presence of maternal antibodies.

Evaluation of new generation Salmonella enterica serovar Typhimurium vaccines with regulated delayed attenuation to induce immune responses against PspA

Increasing the immunogenicity to delivered antigens by recombinant attenuated Salmonella vaccines (RASV) has been the subject of intensive study. With this goal in mind, we have designed and constructed a new generation of RASV that exhibit regulated delayed attenuation. These vaccine strains are phenotypically wild type at the time of immunization and become attenuated after colonization of host tissues. The vaccine strains are grown under conditions that allow expression of genes required for optimal invasion and colonization of host tissues. Once established in the host, these virulence genes are turned off, fully attenuating the vaccine strain. In this study, we compared 2 of our newly developed regulated delayed attenuation Salmonella enterica serovar Typhimurium strains chi9088 and chi9558 with the Deltacya Deltacrp Deltaasd strain chi8133, for their abilities to express and present a secreted form of the alpha-helical region of pneumococcal surface protein A (PspA) to the mouse immune system. All 3 strains induced high levels of serum antibodies specific for PspA as well as to Salmonella antigens in orally immunized mice. However, both RASVs expressing delayed attenuation elicited significantly greater anti-PspA immune responses, including serum IgG and T cell secretion of IL-4 and IFN-gamma, than other groups. Also, vaccination with delayed attenuation strains resulted in a greater degree of protection against Streptococcus pneumoniae challenge than in mice vaccinated with chi8133 (71-86% vs. 21% survival, P </= 0.006). Together, the results demonstrate that the regulated attenuation strategy results in highly immunogenic antigen delivery vectors for oral vaccination.

A sopB deletion mutation enhances the immunogenicity and protective efficacy of a heterologous antigen delivered by live attenuated Salmonella enterica vaccines

SopB is a virulence factor of Salmonella encoded by SPI-5. Salmonella sopB deletion mutants are impaired in their ability to cause local inflammatory responses and fluid secretion into the intestinal lumen and also can enhance the immunogenicity of a vectored antigen. In this study, we evaluated the effects on immunogenicity and the efficacy of a sopB deletion mutation on two Salmonella enterica serovar Typhimurium vaccine strains with different attenuating mutations expressing a highly antigenic alpha-helical region of the Streptococcus pneumoniae surface protein PspA from an Asd(+)-balanced lethal plasmid. After oral administration to mice, the two pairs of strains induced high levels of serum antibodies specific for PspA as well as to Salmonella antigens. The levels of antigen-specific serum immunoglobulin G (IgG) and mucosal IgA were higher in mice immunized with sopB mutants. Enzyme-linked immunospot assay results indicated that the spleen cells from mice immunized with a sopB mutant showed higher interleukin-4 and gamma interferon secretion levels than did the mice immunized with the isogenic sopB(+) strain. The sopB mutants also induced higher numbers of CD4(+) CD44(hi) CD62L(hi) and CD8(+) CD44(hi) CD62L(hi) central memory T cells. Eight weeks after primary oral immunization, mice were challenged with 100 50% lethal doses of virulent S. pneumoniae WU2. Immunization with either of the sopB mutant strains led to increased levels of protection compared to that with the isogenic sopB(+) parent. Together, these results demonstrate that the deletion of sopB leads to an overall enhancement of the immunogenicity and efficacy of recombinant attenuated Salmonella vaccine strains.

Rational design of Salmonella recombinant vaccines

Salmonella enterica is an important pathogen of animals and humans causing a variety of infectious diseases. The large number of cases of typhoid fever due to S. enterica serovar Typhi infections gives rise to the continuous need for improved vaccines against this life-threatening infection. However, S. enterica is also an interesting organism to act as a live attenuated carrier for the presentation of recombinant heterologous antigens. Comprehensive experimental studies have been performed and a detailed knowledge of the molecular mechanisms of important virulence factors is available. This allows the rationale design of improved Salmonella carrier strains and the development of novel strategies for the expression and presentation of recombinant antigens. Here, we review recent advances in generation of live attenuated Salmonella vaccines and discuss criteria for expression strategies of heterologous antigens by Salmonella carrier strains.