Induction of protection against oral infection with cytotoxin-producing Escherichia coli O157:H7 in mice by shiga-like toxin-liposome conjugate

Modeling Native EHEC Outer Membrane Vesicles by Creating Synthetic Surrogates

Enterohemorrhagic Escherichia coli (EHEC) is a zoonotic pathogen responsible for life-threating diseases such as hemolytic uremic syndrome. While its major virulence factor, the Shiga toxin (Stx), is known to exert its cytotoxic effect on various endothelial and epithelial cells when in its free, soluble form, Stx was also recently found to be associated with EHEC outer membrane vesicles (OMVs). However, depending on the strain background, other toxins can also be associated with native OMVs (nOMVs), and nOMVs are also made up of immunomodulatory agents such as lipopolysaccharides and flagellin. Thus, it is difficult to determine to which extent a single virulence factor in nOMVs, such as Stx, contributes to the molecular pathogenesis of EHEC. To reduce this complexity, we successfully developed a protocol for the preparation of synthetic OMVs (sOMVs) with a defined lipid composition resembling the E. coli outer membrane and loaded with specific proteins, i.e., bovine serum albumin (BSA) as a proxy for functional Stx2a. Using BSA for parameter evaluation, we found that (1) functional sOMVs can be prepared at room temperature instead of potentially detrimental higher temperatures (e.g., 45 °C), (2) a 1:10 ratio of protein to lipid, i.e., 100 µg protein with 1 mg of lipid mixture, yields homogenously sized sOMVs, and (3) long-term storage for up to one year at 4 °C is possible without losing structural integrity. Accordingly, we reproducibly generated Stx2a-loaded sOMVs with an average diameter of 132.4 ± 9.6 nm that preserve Stx2a’s injuring activity, as determined by cytotoxicity assays with Vero cells. Overall, we successfully created sOMVs and loaded them with an EHEC toxin, which opens the door for future studies on the degree of virulence associated with individual toxins from EHEC and other bacterial pathogens.

Enriched bovine IgG fraction prevents infections with Enterohaemorrhagic Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, and Mycobacterium avium

A bovine IgG-enriched whey fraction contains antibodies against various bacterial antigens. We investigated the protective effects of a bovine whey fraction preparation against infections with Enterohaemorrhagic Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, and Mycobacterium avium in mouse models. After infection with these pathogens, the IgG-enriched fraction or skim milk was given ad libitum at a 5% solution instead of water. The mice given the IgG-enriched fraction were significantly resistant to orally challenged EHEC O157:H7 (LD50: 4.0 × 105 CFU/mouse) infections compared with the mice given skim milk (LD50: <1.5 × 102 CFU/mouse). The mice given the IgG-enriched fraction were also significantly resistant to orally challenged S. Enteritidis (LD50: 5.0 × 106 CFU/mouse) infections compared with the mice given skim milk (LD50: <2.5 × 101 CFU/mouse). When the mice were nasally infected with M. avium, the numbers of the bacteria in lungs of mice given the IgG-enriched fraction were significantly lower than those given skim milk 2 and 3 weeks after infection. These results strongly indicate that oral administration of the bovine IgG-enriched whey fraction protects mice against food-borne infection and also that it partially protects mice against respiratory tract infection.

Nano-multilamellar lipid vesicles (NMVs) enhance protective antibody responses against Shiga toxin (Stx2a) produced by enterohemorrhagic Escherichia coli strains (EHEC)

Microlipid vesicles (MLV) have a broad spectrum of applications for the delivery of molecules, ranging from chemical compounds to proteins, in both in vitro and in vivo conditions. In the present study, we developed a new set of nanosize multilayer lipid vesicles (NMVs) containing a unique combination of lipids. The NMVs enable the adsorption of histidine-tagged proteins at the vesicle surface and were demonstrated to be suitable for the in vivo delivery of antigens. The NMVs contained a combination of neutral (DOPC) and anionic (DPPG) lipids in the inner membrane and an external layer composed of DOPC, cholesterol, and a nickel-containing lipid (DGS-NTA [Ni]). NMVs combined with a recombinant form of the B subunit of the Shiga toxin (rStx2B) produced by certain enterohemorragic Escherichia coli (EHEC) strains enhanced the immunogenicity of the antigen after parenteral administration to mice. Mice immunized with rStx2B-loaded NMVs elicited serum antibodies capable of neutralizing the toxic activities of the native toxin; this result was demonstrated both in vitro and in vivo. Taken together, these results demonstrated that the proposed NMVs represent an alternative for the delivery of antigens, including recombinant proteins, generated in different expression systems.

A review on strategies for decreasing E. coli O157:H7 risk in animals

Enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 is a food-borne pathogen that younger children are most prone to this microorganism. Hemolytic Uremic Syndrome (HUS) caused by EHEC, leads to the destruction of red blood cells and kidney failure. The virulence of E.coli O157:H7 is attributed to fimbriae, that facilitate colonization of bacteria within the colon and verotoxins (VT) or Shiga toxins (Stx) that are released into the blood. Although, in most cases, the infection is self-limitedin young children and aged population, it may cause HUS. Therefore, several investigations are performed in order to offer effective therapies and vaccines, which can prevent and treat the infection in appropriate time. As the pathogenesis of this infection is complicated, a multi-targeted strategy is required. Since cattle are the most important reservoir of EHEC and the root of contamination, reducing E. coli O157:H7 at the farm level should decrease the risk of human illness. Several vaccine approaches have been employed with different proper outcomes in animal models, including recombinant proteins (virulence factors such as; Stx1/2, intimin, EspA, fusion proteins of A and B Stx subunits), avirulent ghost cells of EHEC O157:H7, live attenuated bacteria expressing recombinant proteins, recombinant fimbrial proteins. In addition to protein-based vaccines, DNA vaccines have provided proper prevention in the laboratory animal model. This review paper summarizes the previous studies, current status and future perspective of different immunization strategies for eradicating Enterohemorrhagic Escherichia coli O157:H7.

Particulate delivery systems for vaccination against bioterrorism agents and emerging infectious pathogens

Bioterrorism agents that can be easily transmitted with high mortality rates and cause debilitating diseases pose major threats to national security and public health. The recent Ebola virus outbreak in West Africa and ongoing Zika virus outbreak in Brazil, now spreading throughout Latin America, are case examples of emerging infectious pathogens that have incited widespread fear and economic and social disruption on a global scale. Prophylactic vaccines would provide effective countermeasures against infectious pathogens and biological warfare agents. However, traditional approaches relying on attenuated or inactivated vaccines have been hampered by their unacceptable levels of reactogenicity and safety issues, whereas subunit antigen-based vaccines suffer from suboptimal immunogenicity and efficacy. In contrast, particulate vaccine delivery systems offer key advantages, including efficient and stable delivery of subunit antigens, co-delivery of adjuvant molecules to bolster immune responses, low reactogenicity due to the use of biocompatible biomaterials, and robust efficiency to elicit humoral and cellular immunity in systemic and mucosal tissues. Thus, vaccine nanoparticles and microparticles are promising platforms for clinical development of biodefense vaccines. In this review, we summarize the current status of research efforts to develop particulate vaccine delivery systems against bioterrorism agents and emerging infectious pathogens. WIREs Nanomed Nanobiotechnol 2017, 9:e1403. doi: 10.1002/wnan.1403 For further resources related to this article, please visit the WIREs website.

Application of surface-linked liposomal antigens to the development of vaccines that induce both humoral and cellular immunity

The first characteristic identified in surface-linked liposomal antigens was the ability to induce antigen-specific, IgE-selective unresponsiveness. These results remained consistent even when different coupling procedures were employed for antigens with liposomes or for liposomes with different lipid components. The potential usefulness of surface-linked liposomal antigens for application to vaccine development was further investigated. During this investigation, a significant difference was observed in the recognition of liposomal antigens by antigen-presenting cells between liposomes with different lipid components, and this difference correlated closely with the adjuvant activity of liposomes. In addition to this "quantitative" difference between liposomes with differential lipid components, a "qualitative" difference (i.e., a differential ability to induce cross-presentation) was observed between liposomes with different lipid components. Therefore, by utilizing the ability to induce cross-presentation, surface-linked liposomal antigens might be used to develop virus vaccines that would induce cytotoxic T lymphocyte (CTL) responses. We have successfully developed a liposome vaccine that is capable of inducing CTL responses against internal antigens of influenza viruses and thus removing virus-infected cells in the host. This CTL-based liposomal vaccine might be applicable to the development of vaccines against influenza and other viruses that frequently undergo changes in their surface antigenic molecules.

Use of immunoglobulin enriched bovine colostrum against oral challenge with enterohaemorrhagic Escherichia coli O157:H7 in mice

An immunoglobulin enriched bovine colostrum preparation, IMMULAC (New Zealand Dairy Group, Cambridge, New Zealand), contains antibodies against various bacterial antigens. In the present study, we investigated the protective effects of a commercial bovine colostrum preparation against infections with enterohaemorrhagic Escherichia coli (EHEC) O157:H7 in a murine model. Balb/c mice were given drinking water containing streptomycin for 3 days before and following oral challenge with streptomycin-resistant EHEC O157:H7 strain (O157-SM(R)). In mice pretreated with streptomycin, EHEC O157:H7 maintained stable levels of bacterial colonization in the intestines for the 3-week experimental time period. Oral administration of colostrum resulted in rapid decrease in the bacteria numbers compared with administration of skim-milk. Colostrum showed no direct in vitro bactericidal properties against either EHEC O157:H7. When sections prepared from cecum walls of streptomycin-pretreated mice were incubated in vitro with EHEC O157:H7, the colostrum significantly prevented the attachment of the organisms to the sections when compared with skim-milk. These results indicate that oral administration of bovine colostrum effectively protects mice against food-borne infections by inhibiting bacterial attachment to the intestinal mucous membrane, colonization and growth in the intestinal tract.

Selective inhibition of systemic anti-OVA IgE production in response to oral pre-treatment with OVA-liposome conjugates

BACKGROUND

We have previously reported that intraperitoneal injection with OVA-liposome conjugates induces OVA-specific and IgE-selective unresponsiveness in mice.

METHODS

In the present study, the effects of oral pre-treatment with OVA-liposome conjugates or with plain OVA solution on anti-OVA IgG antibody production were investigated in mice after subsequent immunization with alum-adsorbed OVA. Control mice received only the immunization.

RESULTS

The levels of serum anti-OVA IgG antibody in mice receiving oral administration of OVA-liposome were comparable to those in the control mice. However, in mice receiving oral administration of the same dose of plain OVA, levels of serum anti-OVA IgG antibody were significantly lower than those in control mice. Surprisingly, anti-OVA IgE antibody production was completely inhibited in mice receiving oral administration of OVA-liposome conjugates. Splenic CD4(+) T cells of mice receiving oral administration of OVA-liposome and those of control mice produced comparable levels of cytokines, while those of mice receiving oral administration of plain OVA solution produced significantly lower levels of cytokines than those in the other two groups.

CONCLUSION

Orally administered OVA-liposome did not affect anti-OVA IgG production but did inhibit anti-OVA IgE antibody production, while orally administered OVA solution inhibited production of both IgG and IgE antibodies. These results suggest that antigen-liposome conjugates can possibly be orally administered in order to control antigen-specific IgE antibody production, without affecting IgG antibody production.

T cell-independent regulation of IgE antibody production induced by surface-linked liposomal antigen

Control of IgE Ab production is important for the prevention of IgE-related diseases. However, in contrast to the existing information on the induction of IgE production, little is known about the regulation of the production of this isotype, with the exception of the well-documented mechanism involving T cell subsets and their cytokine products. In this study, we demonstrate an alternative approach to interfere with the production of IgE, independent of the activity of T cells, which was discovered during the course of an investigation intended to clarify the mechanism of IgE-selective unresponsiveness induced by surface-coupled liposomal Ags. Immunization of mice with OVA-liposome conjugates induced IgE-selective unresponsiveness without apparent Th1 polarization. Neither IL-12, IL-10, nor CD8(+) T cells participated in the regulation. Furthermore, CD4(+) T cells of mice immunized with OVA-liposome were capable of inducing Ag-specific IgE synthesis in athymic nude mice immunized with alum-adsorbed OVA. In contrast, immunization of the recipient mice with OVA-liposome did not induce anti-OVA IgE production, even when CD4(+) T cells of mice immunized with alum-adsorbed OVA were transferred. In the secondary immune response, OVA-liposome enhanced anti-OVA IgG Ab production, but it did not enhance ongoing IgE production, suggesting that the IgE-selective unresponsiveness induced by the liposomal Ag involved direct effects on IgE, but not IgG switching in vivo. These results suggest the existence of an alternative mechanism not involving T cells in the regulation of IgE synthesis.

Protection of monkeys against Shiga toxin induced by Shiga toxin-liposome conjugates

BACKGROUND

We previously reported that the purified Shiga toxins (Stx) Stx1 and Stx2, when coupled with liposomes, induced substantial production of anti-Stx1 and anti-Stx2 IgG antibody, respectively, in mice. The levels of anti-Stx antibody in the sera of mice immune to Stx-liposome correlated well with the protection against subsequent challenge with Stx. Furthermore, mice immunized with a mixture of Stx1-liposome and Stx2-liposome were successfully protected against oral infection with cytotoxin-producing Escherichia coli O157:H7.

METHODS

In this study, the induction of protection against Stx2 by Stx2-liposomes was evaluated in monkeys.

RESULTS

Stx2-liposomes induced a substantial amount of anti-Stx2 IgG antibodies as well as Stx2 neutralizing antibodies in monkeys. Test monkeys were successfully protected against challenge with lethal doses of Stx2. Moreover, these monkeys showed no apparent symptoms, while nonimmunized control monkeys died within 4 days with hemorrhagic gastroenteritis and renal disorder. In addition, as shown by other cases involving antigen-liposome conjugates, Stx2-liposome did not induce anti-Stx2 IgE antibody production, though it stimulated the production of a substantial amount of anti-Stx2 IgG antibodies.

CONCLUSION

These results suggest that Stx-liposome conjugates may serve as candidate vaccines to induce protection against death caused by cytotoxin-producing E. coli infection.

Antigen-specific, IgE-selective unresponsiveness induced by antigen-liposome conjugates. Comparison of four different conjugation methods for the coupling of antigen to liposome

BACKGROUND

We have previously reported that ovalbumin (OVA) coupled with liposome via glutaraldehyde (GA) induced OVA-specific- and IgE-selective unresponsiveness in mice.

METHODS

In this study, OVA-liposome conjugates were made using four different coupling protocols: via GA, N-(6-maleimidocaproyloxy) succinimide (EMCS), disuccinimidyl suberate (DSS) and N-succimidyl-3(2-pyridyldithio)propionate (SPDP) and the induction of antigen-specific IgG and IgE antibody production was investigated for each. In addition, antigen-specific cytokine production by spleen cells of mice immunized either with OVA-liposome or with OVA adsorbed with aluminum hydroxide was investigated.

RESULTS

OVA-liposome conjugates coupled via GA or DSS did not induce anti-OVA IgE antibody production but induced substantial anti-OVA IgG antibody production. On the other hand, the induction of anti-OVA IgE unresponsiveness by OVA-liposome conjugates coupled via EMCS or SPDP was incomplete. The amount of interleukin 4 (IL-4) produced by spleen cells stimulated in vitro with OVA correlated well with anti-OVA IgE antibody production in donor mice. However, the production of no other cytokine, i.e., IL-2, IL-5, IL-10 or interferon-gamma, was correlated with in vivo IgE antibody production.

CONCLUSION

OVA-liposome coupled via GA or DSS induced complete suppression of anti-OVA IgE production. The results in this study further suggest that the regulation of IgE antibody production does not necessarily correlate with so-called Th1 cytokine production.