Vulnerabilities in Yersinia pestis caf operon are unveiled by a Salmonella vector

The Peptidoglycan-associated lipoprotein Pal contributes to the virulence of Burkholderia mallei and provides protection against lethal aerosol challenge

Burkholderia mallei

is a highly pathogenic bacterium that causes the fatal zoonosis glanders. The organism specifies multiple membrane proteins, which represent prime targets for the development of countermeasures given their location at the host-pathogen interface. We investigated one of these proteins, Pal, and discovered that it is involved in the ability of B. mallei to resist complement-mediated killing and replicate inside host cells in vitro, is expressed in vivo and induces antibodies during the course of infection, and contributes to virulence in a mouse model of aerosol infection. A mutant in the pal gene of the B. mallei wild-type strain ATCC 23344 was found to be especially attenuated, as BALB/c mice challenged with the equivalent of 5,350 LD₅₀ completely cleared infection. Based on these findings, we tested the hypothesis that a vaccine containing the Pal protein elicits protective immunity against aerosol challenge. To achieve this, the pal gene was cloned in the vaccine vector Parainfluenza Virus 5 (PIV5) and mice immunized with the virus were infected with a lethal dose of B. mallei. These experiments revealed that a single dose of PIV5 expressing Pal provided 80% survival over a period of 40 days post-challenge. In contrast, only 10% of mice vaccinated with a PIV5 control virus construct survived infection. Taken together, our data establish that the Peptidoglycan-associated lipoprotein Pal is a critical virulence determinant of B. mallei and effective target for developing a glanders vaccine.

Oral administration of a recombinant attenuated Yersinia pseudotuberculosis strain elicits protective immunity against plague

A Yersinia pseudotuberculosis PB1+ (Yptb PB1+) mutant strain combined with chromosome insertion of the caf1R-caf1A-caf1M-caf1 operon and deletions of yopJ and yopK, χ10068 [pYV-ω2 (ΔyopJ315 ΔyopK108) ΔlacZ044::caf1R-caf1M-caf1A-caf1] was constructed. Results indicated that gene insertion and deletion did not affect the growth rate of χ10068 compared to wild-type Yptb cultured at 26 °C. In addition, the F1 antigen in χ10068 was synthesized and secreted on the surface of bacteria at 37 °C (mammalian body temperature), not at ambient culture temperature (26 °C). Immunization with χ10068 primed antibody responses and specific T-cell responses to F1 and YpL (Y. pestis whole cell lysate). Oral immunization with a single dose of χ10068 provided 70% protection against a subcutaneous (s.c.) challenge with ∼ 2.6 × 10(5) LD50 of Y. pestis KIM6+ (pCD1Ap) (KIM6+Ap) and 90% protection against an intranasal (i.n.) challenge with ∼ 500 LD50 of KIM6+Ap in mice. Our results suggest that χ10068 can be used as an effective precursor to make a safe vaccine to prevent plague in humans and to eliminate plague circulation among humans and animals.

The Autotransporter BpaB Contributes to the Virulence of Burkholderia mallei in an Aerosol Model of Infection

Burkholderia mallei is a highly pathogenic bacterium that causes the zoonosis glanders. Previous studies indicated that the genome of the organism contains eight genes specifying autotransporter proteins, which are important virulence factors of Gram-negative bacteria. In the present study, we report the characterization of one of these autotransporters, BpaB. Database searches identified the bpaB gene in ten B. mallei isolates and the predicted proteins were 99-100% identical. Comparative sequence analyses indicate that the gene product is a trimeric autotransporter of 1,090 amino acids with a predicted molecular weight of 105-kDa. Consistent with this finding, we discovered that recombinant bacteria expressing bpaB produce a protein of ≥300-kDa on their surface that is reactive with a BpaB-specific monoclonal antibody. Analysis of sera from mice infected with B. mallei indicated that animals produce antibodies against BpaB during the course of disease, thus establishing production of the autotransporter in vivo. To gain insight on its role in virulence, we inactivated the bpaB gene of B. mallei strain ATCC 23344 and determined the median lethal dose of the mutant in a mouse model of aerosol infection. These experiments revealed that the bpaB mutation attenuates virulence 8-14 fold. Using a crystal violet-based assay, we also discovered that constitutive production of BpaB on the surface of B. mallei promotes biofilm formation. To our knowledge, this is the first report of a biofilm factor for this organism.

LcrV delivered via type III secretion system of live attenuated Yersinia pseudotuberculosis enhances immunogenicity against pneumonic plague

Here, we constructed a Yersinia pseudotuberculosis mutant strain with arabinose-dependent regulated and delayed shutoff of crp expression (araC P(BAD) crp) and replacement of the msbB gene with the Escherichia coli msbB gene to attenuate it. Then, we inserted the asd mutation into this construction to form χ10057 [Δasd-206 ΔmsbB868::P(msbB) msbB(EC) ΔP(crp21)::TT araC P(BAD) crp] for use with a balanced-lethal Asd-positive (Asd(+)) plasmid to facilitate antigen synthesis. A hybrid protein composed of YopE (amino acids [aa]1 to 138) fused with full-length LcrV (YopE(Nt138)-LcrV) was synthesized in χ10057 harboring an Asd(+) plasmid (pYA5199, yopE(Nt138)-lcrV) and could be secreted through a type III secretion system (T3SS) in vitro and in vivo. Animal studies indicated that mice orally immunized with χ10057(pYA5199) developed titers of IgG response to whole-cell lysates of Y. pestis (YpL) and subunit LcrV similar to those seen with χ10057(pYA3332) (χ10057 plus an empty plasmid). However, only immunization of mice with χ10057(pYA5199) resulted in a significant secretory IgA response to LcrV. χ10057(pYA5199) induced a higher level of protection (80% survival) against intranasal (i.n.) challenge with ~240 median lethal doses (LD50) (2.4 × 10(4) CFU) of Y. pestis KIM6+(pCD1Ap) than χ10057(pYA3332) (40% survival). Splenocytes from mice vaccinated with χ10057(pYA5199) produced significant levels of gamma interferon (IFN-γ), tumor necrosis factor alpha (TNF-α), and interleukin-17 (IL-17) after restimulation with LcrV and YpL antigens. Our results suggest that it is possible to use an attenuated Y. pseudotuberculosis strain delivering the LcrV antigen via the T3SS as a potential vaccine candidate against pneumonic plague.

The delicate balance in genetically engineering live vaccines

Contemporary vaccine development relies less on empirical methods of vaccine construction, and now employs a powerful array of precise engineering strategies to construct immunogenic live vaccines. In this review, we will survey various engineering techniques used to create attenuated vaccines, with an emphasis on recent advances and insights. We will further explore the adaptation of attenuated strains to create multivalent vaccine platforms for immunization against multiple unrelated pathogens. These carrier vaccines are engineered to deliver sufficient levels of protective antigens to appropriate lymphoid inductive sites to elicit both carrier-specific and foreign antigen-specific immunity. Although many of these technologies were originally developed for use in Salmonella vaccines, application of the essential logic of these approaches will be extended to development of other enteric vaccines where possible. A central theme driving our discussion will stress that the ultimate success of an engineered vaccine rests on achieving the proper balance between attenuation and immunogenicity. Achieving this balance will avoid over-activation of inflammatory responses, which results in unacceptable reactogenicity, but will retain sufficient metabolic fitness to enable the live vaccine to reach deep tissue inductive sites and trigger protective immunity. The breadth of examples presented herein will clearly demonstrate that genetic engineering offers the potential for rapidly propelling vaccine development forward into novel applications and therapies which will significantly expand the role of vaccines in public health.

Attenuating gene expression (AGE) for vaccine development

Live attenuated vaccines are adept in stimulating protective immunity. Methods for generating such vaccines have largely adopted strategies used with Salmonella enterica. Yet, when similar strategies were tested in other gram-negative bacteria, the virulence factors or genes responsible to incapacitate Salmonella often failed in providing the desired outcome. Consequently, conventional live vaccines rely on prior knowledge of the pathogen's virulence factors to successfully attenuate them. This can be problematic since such bacterial pathogens normally harbor thousands of genes. To circumvent this problem, we found that overexpression of bacterial appendages, e.g., fimbriae, capsule, and flagella, could successfully attenuate wild-type (wt) Salmonella enterica serovar Typhimurium. Further analysis revealed these attenuated Salmonella strains conferred protection against wt S. Typhimurium challenge as effectively as genetically defined Salmonella vaccines. We refer to this strategy as attenuating gene expression (AGE), a simple efficient approach in attenuating bacterial pathogens, greatly facilitating the construction of live vaccines.

Flagella overexpression attenuates Salmonella pathogenesis

Flagella are cell surface appendages involved in a number of bacterial behaviors, such as motility, biofilm formation, and chemotaxis. Despite these important functions, flagella can pose a liability to a bacterium when serving as potent immunogens resulting in the stimulation of the innate and adaptive immune systems. Previous work showing appendage overexpression, referred to as attenuating gene expression (AGE), was found to enfeeble wild-type Salmonella. Thus, this approach was adapted to discern whether flagella overexpression could induce similar attenuation. To test its feasibility, flagellar filament subunit FliC and flagellar regulon master regulator FlhDC were overexpressed in Salmonella enterica serovar Typhimurium wild-type strain H71. The results show that the expression of either FliC or FlhDC alone, and co-expression of the two, significantly attenuates Salmonella. The flagellated bacilli were unable to replicate within macrophages and thus were not lethal to mice. In-depth investigation suggests that flagellum-mediated AGE was due to the disruptive effects of flagella on the bacterial membrane, resulting in heightened susceptibilities to hydrogen peroxide and bile. Furthermore, flagellum-attenuated Salmonella elicited elevated immune responses to Salmonella presumably via FliC's adjuvant effect and conferred robust protection against wild-type Salmonella challenge.