A Salmonella typhimurium htrA live vaccine expressing multiple copies of a peptide comprising amino acids 8-23 of herpes simplex virus glycoprotein D as a genetic fusion to tetanus toxin fragment C protects mice from herpes simplex virus infection

Bacterial AB toxins and host-microbe interactions

AB toxins are protein virulence factors secreted by many bacterial pathogens, contributing to the pathogenicity of the cognate bacteria. AB toxins consist of two functionally distinct components: the enzymatic "A" component for pathogenicity and the receptor-binding "B" component for toxin delivery. Consistently, unlike other virulence factors such as effectors, AB toxins do not require additional systems to deliver them to the target host cells. Target host cells are located in the infection site and/or located distantly from infected host cells. The first part of this review discusses the structural and functional features of single-peptide and multiprotein AB toxins in the context of host-microbe interactions, using several well-characterized examples. The second part of this review discusses toxin neutralization strategies, as well as applications of AB toxins relevant to developing intervention strategies against diseases.

Recombinant KRAS G12D Protein Vaccines Elicit Significant Anti-Tumor Effects in Mouse CT26 Tumor Models

Drug development targeting the most frequently mutation G12D of KRAS has great significance. As an attractive immunotherapy, cancer vaccines can overcome binding difficulties of small molecules; however, the weak immunogenicity and production difficulties of reported KRAS mutation vaccines limit their clinical application. To improve antigen-specific immune responses and Anti-Tumor effects on tumors expressing KRAS G12D mutation, we designed recombinant proteins containing KRAS peptide (amino acids 5–21) with G12D (called SP) in two forms: DTT-SP₄ and DTSP. DTT-SP₄ was constructed by fusing four copies of SP to the C-terminal of the translocation domain of diphtheria toxin (DTT), and DTSP was constructed by grafting SP onto DTT. The two vaccines in combination with aluminum hydroxide (Alum) and cytosine phosphoguanine (CpG) successfully induced conspicuous SP-specific humoral and cellular immune responses, and displayed prominent protective and therapeutic Anti-Tumor effects in mouse CT26 tumor models. Surprisingly, the DTSP-treated group displayed better Anti-Tumor effects in vivo compared with the DTT-SP₄-treated and control groups. Moreover, 87.5 and 50% of DTSP-treated mice in the preventive and therapeutic models were tumor free, respectively. Notably, in the DTSP-treated group, the interferon-γ (IFN-γ) expression of T cells in vitro and the T-helper 1 (Th1)–related cytokine expression in tumor tissues indicated that the activated Th1 immune response may be involved in Anti-Tumor activity. Furthermore, DTSP treatment remarkably altered the subpopulation of T cells in splenocytes and tumor-infiltrating lymphocytes. The percentage of effector CD8⁺ T cells increased, whereas that of immunosuppressive CD4⁺Foxp3⁺ T cells remained reduced in the DTSP group. Dramatic tumor-inhibitory effects of DTSP, which is easily prepared, make it a more attractive strategy against KRAS G12D tumors.

Attenuated Salmonella VNP20009 mutant (ΔhtrA) is a promising candidate for bacteria-mediated tumour therapy in hosts with TNFR1 deficiency

VNP20009 is a genetically modified strain of Salmonella typhimurium and has a good anticancer effect wildly used in tumour therapy on animal models. For its clinical application, an accurate bio-safety assessment on sensitive models is necessary. In this study, we use TNFR1 KO mice as a susceptive model to assess the virulence of bacterial VNP20009 and its derivative ΔhtrA. By intraperitoneal administration of Salmonella, the increased lethality was observed in TNFR1 KO mice infected with VNP20009, but not with ΔhtrA. We performed a systemically comparative analysis of their toxicity, and ΔhtrA shows a better bio-safety for TNFR1 KO mice. Since the macrophages with TNFR1 deficiency exhibit a reduced ability of bacteria clearance, ΔhtrA with lower survival ability in normal macrophages restores its viability in TNFR1 KO macrophages. Thus, ΔhtrA was further tested for its antitumour effect in TNFR1 KO mice bearing a B16F10 melanoma model. It displays a moderate antitumour effect, suggesting ΔhtrA instead of VNP20009 might be a promising candidate for bacteria-mediated tumour therapy specific to those with low immunity.

SIGNIFICANT AND IMPACT OF THE STUDY

VNP20009 is attenuated Salmonella with a good safety widely used for tumour-targeting bacterial therapies. Little is known about its toxicity in hosts with low immunity. This study is the first systemically comparative analysis of their toxicity of VNP20009 and its mutant ΔhtrA in TNFR1-KO mice. Research on toxicity of tumour-targeting Salmonella in mice with immunodeficiency can facilitate the optimization of bacterial therapies with reduced adverse effects in future clinical trials.

Mucosal immunization with an attenuated Salmonella vaccine partially protects white-tailed deer from chronic wasting disease

Prion disease is a unique category of illness, affecting both animals and humans, in which the underlying pathogenesis is related to a conformational change of a normal, self-protein called PrP(C) (C for cellular) to a pathological and infectious conformer known as PrP(Sc) (Sc for scrapie). Bovine spongiform encephalopathy (BSE), a prion disease believed to have arisen from feeding cattle with prion contaminated meat and bone meal products, crossed the species barrier to infect humans. Chronic wasting disease (CWD) infects large numbers of deer and elk, with the potential to infect humans. Currently no prionosis has an effective treatment. Previously, we have demonstrated we could prevent transmission of prions in a proportion of susceptible mice with a mucosal vaccine. In the current study, white-tailed deer were orally inoculated with attenuated Salmonella expressing PrP, while control deer were orally inoculated with vehicle attenuated Salmonella. Once a mucosal response was established, the vaccinated animals were boosted orally and locally by application of polymerized recombinant PrP onto the tonsils and rectal mucosa. The vaccinated and control animals were then challenged orally with CWD-infected brain homogenate. Three years post CWD oral challenge all control deer developed clinical CWD (median survival 602 days), while among the vaccinated there was a significant prolongation of the incubation period (median survival 909 days; p=0.012 by Weibull regression analysis) and one deer has remained CWD free both clinically and by RAMALT and tonsil biopsies. This negative vaccinate has the highest titers of IgA in saliva and systemic IgG against PrP. Western blots showed that immunoglobulins from this vaccinate react to PrP(CWD). We document the first partially successful vaccination for a prion disease in a species naturally at risk.

Identification of a novel linear epitope in tetanus toxin recognized by a protective monoclonal antibody: implications for vaccine design

Tetanus, a severe infectious disease, is caused by tetanus toxin (TT) from Clostridium tetani, which remains one of the most critical unsolved health problems despite preventive strategies. The carboxyl terminal of TT (TTC) is responsible for the binding of TT to neurons and for its toxicity and has been proven to be immunogenic and protective in various forms. It would therefore be extremely interesting to identify the epitope on TTC at a molecular level. In this study, we generated a neutralizing monoclonal antibody, 5C4, which inhibited TT binding to its receptor and was efficiently protective at 73.7 IU/mg. Moreover, 5C4 recognized a novel linear epitope on TT, namely TC((1155-1171)), which spans from Lys1155 to Val1171. In addition, TC((1155-1171)) was shown to elicit the production of a serum IgG that protected mice against a challenge with TT. These results suggested that TC((1155-1171)) and the monoclonal antibody 5C4 are good candidates for the development of epitope-based vaccines and therapeutic antibodies against tetanus.

The ongoing pursuit of a prophylactic HSV vaccine

HSV is among the most common human pathogens in the world. It is known to cause painful, persistent skin lesions, while also being the most common cause of fatal non-epidemic encephalitis as well as the leading cause of corneal blindness. The development of prophylactic vaccines could substantially reduce global health problems associated with HSV. So far, HSV vaccine strategies have shown noticeable efficacy in early development during preclinical phases but remained unsuccessful or unproven in human trials. New understanding of how the immune system mounts a defence against HSV offers practical strategies for vaccine development. A number of promising vaccine candidates are currently awaiting clinical development or already undergoing clinical testing. Therefore, this is a suitable time to assess the progress of HSV vaccine development and consider existing challenges and future improvements needed to achieve an effective prophylactic HSV vaccine.

Immunogenicity of a bovine herpes virus I peptide expressed in tandem copies in attenuated Salmonella

A live system to release heterologous antigens using an attenuated Salmonella strain was developed. We transformed Salmonella typhimurium LVR03 (S. LVR03) with a recombinant pTECH2 vector encoding 0, 1, 2, and 4 tandem copies of an imunogenic peptide of bovine herpes virus-1 (BoHV-1) glycoprotein D (gD). The system used yielded peptides fused to the non-toxic C fragment of the tetanus toxin (TetC), which has been shown to have adjuvant properties. Inoculation of BALB/c mice with the transformed Salmonella strains gave rise to a mild self-limited infection, with primary replication of bacteria occurring in Peyer's patches, even when the bacteria was administered intranasally. Humoral and cellular immune responses directed against the BoHV-1 antigens were evaluated after oral or intranasal administration of the recombinant bacteria. The results showed that the S. LVR03-dimer vaccine induced specific humoral (IgG in serum and IgG(1) and IgA in saliva), and cellular immune responses (lymphoproliferation and lymphokine secretion), against not only the selected peptide and whole gD, but also against BoHV-1, when administered intranasally. This is the first time Salmonella has been used as an expression vector to induce immunity against BoHV-1. This work demonstrates the feasibility of using this antigen-release system and encourages future experimentation with a bovine experimental model.

Nasal immunization with attenuated Salmonella Typhimurium expressing an MrpA-TetC fusion protein significantly reduces Proteus mirabilis colonization in the mouse urinary tract

The development of effective strategies to prevent urinary tract infections (UTIs) has become an important goal in public health. Proteus mirabilis is commonly associated with complicated UTIs and expresses several virulence factors, including mannose-resistant Proteus-like (MR/P) fimbriae. Here, a fusion protein formed from MrpA, the structural protein of MR/P fimbriae, and TetC, a non-toxic but highly immunogenic fragment of tetanus toxin, to be delivered by an attenuated Salmonella Typhimurium mutant in vivo was constructed. The ability of this strain to induce an immune response and to protect mice against a urinary tract challenge with P. mirabilis was investigated. The protein was successfully expressed in S. Typhimurium. After two immunization doses, intra-nasally vaccinated mice showed a significant increase in specific serum IgG against MrpA and against Salmonella lipopolysaccharide, as well as a significant decrease in kidney and bladder colonization by P. mirabilis after challenge. However, no significant correlation was observed between antibody response and kidney or bladder colonization. MrpA fused to TetC and expressed in S. Typhimurium effectively protected mice against an experimental P. mirabilis UTI.

Diminished amyloid-beta burden in Tg2576 mice following a prophylactic oral immunization with a salmonella-based amyloid-beta derivative vaccine

Immunotherapy holds great promise for Alzheimer's disease (AD) and other conformational disorders but certain adverse reactions need to be overcome. Prior to the side effects in the first Elan/Wyeth AD vaccine trial, we proposed using amyloid-beta (Abeta) derivatives as a safer approach. The route of administration may also affect vaccine safety. To assess the feasibility of oral immunization that promotes mucosal immunity, Tg2576 AD model mice were treated prophylactically three times over 6 weeks starting at 3-5 months of age with a Salmonella vaccine expressing K6Abeta(1-30). At 22-24 months of age, cortical Abeta plaque burden and total Abeta(40/42) levels were reduced by 48-75% in the immunized mice compared to controls, which received unmodified Salmonella. Plaque clearance was not associated with increased microglial activation, which may be explained by the long treatment period. Furthermore, cerebral microhemorrhages were not increased in the treated mice in contrast to several passive Abeta antibody studies. These results further support our findings with this immunogen delivered subcutaneously and demonstrate its efficacy when given orally, which may provide added benefits for human use.

Influence of promoter, gene copy number, and preexisting immunity on humoral and cellular responses to a vectored antigen delivered by a Salmonella enterica vaccine

Attenuated Salmonella strains are currently in production as vaccines for protection of animals against salmonellosis. Such commercial strains offer the potential to deliver heterologous antigen to protect animals against other diseases. One vaccine strain, attenuated Salmonella enterica serovar Typhimurium (STM-1), was tested for the ability to deliver ovalbumin and to induce immune responses in mice. Two vaccine trials were performed testing the influence of promoter choice, the location of the encoding DNA (plasmid or chromosome), and the effect of preexisting homologous or heterologous immunity. The results demonstrated that humoral and T-cell responses were induced from either of two promoters, from either the plasmid or the chromosome, and that preexposure to the empty homologous vector, STM-1, or the heterologous vector, S. enterica serovar Enteritidis, had no detrimental effect on subsequent antigen-specific responses. In the case of homologous preexposure, responses were generally greater, and this was correlated with an increased uptake of Salmonella by macrophages in vitro after opsonization with immune sera.

An oral recombinant vaccine in dogs against Echinococcus granulosus, the causative agent of human hydatid disease: a pilot study

Dogs are the main source of human cystic echinococcosis. An oral vaccine would be an important contribution to control programs in endemic countries. We conducted two parallel experimental trials in Morocco and Tunisia of a new oral vaccine candidate against Echinococcus granulosus in 28 dogs. The vaccine was prepared using two recombinant proteins from adult worms, a tropomyosin (EgTrp) and a fibrillar protein similar to paramyosin (EgA31), cloned and expressed in a live attenuated strain of Salmonella enterica serovar typhimurium.

In each country, five dogs were vaccinated with the associated EgA31 and EgTrp; three dogs received only the vector Salmonella; and six dogs were used as different controls. The vaccinated dogs received two oral doses of the vaccine 21 d apart, and were challenged 20 d later with 75,000 living protoscoleces. The controls were challenged under the same conditions. All dogs were sacrificed 26–29 d postchallenge, before the appearance of eggs, for safety reasons.

We studied the histological responses to both the vaccine and control at the level of the duodenum, the natural localization of the cestode. Here we show a significant decrease of parasite burden in vaccinated dogs (70% to 80%) and a slower development rate in all remaining worms. The Salmonella vaccine EgA31-EgTrp demonstrated a high efficacy against E. granulosus promoting its potential role in reducing transmission to humans and animals.

In many countries in the world, livestock and humans are affected with hydatid disease, which is caused by the development, in the viscera, of the larval stage of the cestode Echinococcus granulosus. They become infected by ingesting the eggs of this parasite, which are passed in the feces of the dog—the host of the adult worm. Domestic dogs are key in the transmission to livestock and humans.

This disease remains a major economic and public health problem in affected countries. Because dogs are quickly reinfected, control programs in these locations include monthly anthelmintic deworming. These control measures, however, are burdensome for the owner, so they often fail. In contrast, vaccination can take place in control programs at different stages of the parasite life cycle. For example, currently an effective recombinant vaccine for sheep has been developed that should work indirectly to reduce infection in dogs, which tend to eat sheep offal. However, we propose that a recombinant oral vaccine given to the small number of dogs keeping the herd would decrease the number of Echinococcus granulosus adult worms and, consequently, the number of infective eggs. This measure would help reduce the contamination risk factors for humans and livestock, and would be cost-effective for the owners of the dogs.

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.

Recombinant vesicular stomatitis virus vectors expressing herpes simplex virus type 2 gD elicit robust CD4+ Th1 immune responses and are protective in mouse and guinea pig models of vaginal challenge

Recombinant vesicular stomatitis virus (rVSV) vectors offer an attractive approach for the induction of robust cellular and humoral immune responses directed against human pathogen target antigens. We evaluated rVSV vectors expressing full-length glycoprotein D (gD) from herpes simplex virus type 2 (HSV-2) in mice and guinea pigs for immunogenicity and protective efficacy against genital challenge with wild-type HSV-2. Robust Th1-polarized anti-gD immune responses were demonstrated in the murine model as measured by induction of gD-specific cytotoxic T lymphocytes and increased gamma interferon expression. The isotype makeup of the serum anti-gD immunoglobulin G (IgG) response was consistent with the presence of a Th1-CD4+ anti-gD response, characterized by a high IgG2a/IgG1 IgG subclass ratio. Functional anti-HSV-2 neutralizing serum antibody responses were readily demonstrated in both guinea pigs and mice that had been immunized with rVSV-gD vaccines. Furthermore, guinea pigs and mice were prophylactically protected from genital challenge with high doses of wild-type HSV-2. In addition, guinea pigs were highly protected against the establishment of latent infection as evidenced by low or absent HSV-2 genome copies in dorsal root ganglia after virus challenge. In summary, rVSV-gD vectors were successfully used to elicit potent anti-gD Th1-like cellular and humoral immune responses that were protective against HSV-2 disease in guinea pigs and mice.

Mucosal vaccination delays or prevents prion infection via an oral route

In recent years major outbreaks of prion disease linked to oral exposure of the prion agent have occurred in animal and human populations. These disorders are associated with a conformational change of a normal protein, PrP(C) (prion protein cellular), to a toxic and infectious form, PrP(Sc) (prion protein scrapie). None of the prionoses currently have an effective treatment. A limited number of active immunization approaches have been shown to slightly prolong the incubation period of prion infection. Active immunization in wild-type animals is hampered by auto-tolerance to PrP and potential toxicity. Here we report that mucosal vaccination with an attenuated Salmonella vaccine strain expressing the mouse PrP, is effective at overcoming tolerance to PrP and leads to a significant delay or prevention of prion disease in mice later exposed orally to the 139A scrapie strain. This mucosal vaccine induced gut anti-PrP immunoglobulin (Ig)A and systemic anti-PrP IgG. No toxicity was evident with this vaccination approach. This promising finding suggests that mucosal vaccination may be a useful method for overcoming tolerance to PrP and preventing prion infection among animal and potentially human populations at risk.

HtrA stress protein is involved in intramacrophagic replication of adherent and invasive Escherichia coli strain LF82 isolated from a patient with Crohn's disease

Adherent and invasive Escherichia coli (AIEC) bacteria isolated from Crohn's disease patients are able to greatly replicate within macrophages without escaping from the phagosome and without inducing macrophage death. In the present study, evidence is provided that in AIEC strain LF82 the htrA gene encoding the stress protein HtrA is essential for intracellular replication within J774-A1 macrophages. Deletion of the htrA gene in strain LF82 induced increased sensitivity of the isogenic mutant to oxidative stress caused by hydrogen peroxide and a reduced rate of growth in an acid and nutrient-poor medium partly reproducing the microenvironment of the phagosome. In vitro experiments using an LF82 htrA gene promoter fusion with the lacZ gene revealed a 38-fold activation of the promoter in AIEC LF82 intramacrophagic bacteria. The CpxRA two-component signaling pathway was not involved in this activation. In addition, the activation of the LF82 htrA gene promoter was not observed in the nonpathogenic E. coli K-12 intramacrophagic bacteria, indicating that the AIEC LF82 genetic background is crucial for induction of htrA gene transcription during phagocytosis.

Sm14 of Schistosoma mansoni in fusion with tetanus toxin fragment C induces immunoprotection against tetanus and schistosomiasis in mice

We have constructed vectors that permit the expression in Escherichia coli of Schistosoma mansoni fatty acid-binding protein 14 (Sm14) in fusion with the nontoxic, but highly immunogenic, tetanus toxin fragment C (TTFC). The recombinant six-His-tagged proteins were purified by nickel affinity chromatography and used in immunization and challenge assays. Animals inoculated with TTFC in fusion with or coadministered with Sm14 showed high levels of tetanus toxin antibodies, while animals inoculated with Sm14 in fusion with or coadministered with TTFC showed high levels of Sm14 antibodies. In both cases, there were no changes in the type of immune response (Th2) obtained with the fusion proteins compared to those obtained with the nonfused proteins. Mice immunized with the recombinant proteins (TTFC in fusion with or coadministered with Sm14) survived the challenge with tetanus toxin and did not show any symptoms of the disease. Control animals inoculated with either phosphate-buffered saline (PBS) or Sm14 died with severe symptoms of tetanus after 24 h. Mice immunized with the recombinant proteins (Sm14 in fusion with or coadministered with TTFC) showed a 50% reduction in worm burden when they were challenged with S. mansoni cercariae, while control animals inoculated with either PBS or TTFC were not protected. The results show that the expression of other antigens in fusion at the carboxy terminus of TTFC is feasible for the development of a multivalent recombinant vaccine.

Intranasal immunization with a colloid-formulated bacterial extract induces an acute inflammatory response in the lungs and elicits specific immune responses

Nonspecific stimulation of lung defenses by repeated oral administration of immunomodulators, such as bacterial extracts, has shown potential for the prevention of respiratory tract infections. Here, we show that intranasal (i.n.) immunization with a bacterial extract formulated as a colloid induces an acute inflammatory response in the lungs characterized by increased production of CCL and CXCL chemokines and a major influx of dendritic cells (DCs) and neutrophils, with a higher proportion of DCs showing an activated phenotype (high CD80/CD86 expression). Cytokine levels measured in bronchoalveolar-lavage samples showed a small increase in the production of tumor necrosis factor alpha and similar levels of the other cytokines measured (interleukin 10 [IL-10], IL-12, and gamma interferon [IFN-gamma]) in immunized mice compared with control mice. However, the recall response of primed animals after antigenic challenge induced increased expression of IL-12 and IFN-gamma mRNAs in lung homogenates. Overall, all these effects were not due to the lipopolysaccharide content in the bacterial extract. Furthermore, we found that three i.n. doses administered 2 to 3 weeks apart were enough to elicit long-lasting specific serum immunoglobulin G (IgG) and secretory IgA antibody responses. Assessment of IgG subclasses showed a balanced pattern of IgG1-IgG2a responses. The serum total IgE concentrations were also elevated in immunized mice 2 weeks after the third dose, but they significantly decreased soon afterwards. Our results suggest that simple formulations of bacterial extracts administered i.n. are highly immunogenic, eliciting local and systemic immune responses, and may serve as the basis for cost-effective immunotherapies for the prevention and treatment of respiratory infections.

Local and systemic immune responses to Echinococcus granulosus in experimentally infected dogs

Local and systemic immune responses were studied in six dogs experimentally infected with the dog/sheep tapeworm Echinococcus granulosus. All dogs developed similar IgG antibody response to parasite antigens. In contrast, IgE and IgA responses differed widely. No relationship between IgA responses and parasite burden at the end of the infection were observed. Further, clear differences in the anti-parasite IgA response in serum as compared with specific IgA forming cells in mesenteric lymph nodes were observed within the same dog. An inverse association of anti-parasite IgE and parasite load seemed to be present, with the strongest IgE response in the one dog that had no worms in the intestine at the end of the experiment. No differences were observed in the numbers of intestinal mast cells and goblet cells among all infected dogs. However, the dog with no detectable parasite load had a marked reduction of detected mast cells in the submuscular and muscular layer of the mucosa. Our data give new insight into the immune response of dogs during E. granulosus infection and provide information that may be useful for the rational design of vaccines for the control of hydatid disease.

In vitro and in vivo stability of recombinant plasmids in a vaccine strain of Salmonella enterica var. Typhimurium

This study examined the ability of different plasmid vectors encoding H(C) fragment, the non-toxic binding portion of tetanus toxin, to be stably retained by Salmonella enterica var. Typhimurium (Salmonella typhimurium) vaccine strain BRD509 and, upon immunisation, to induce an antibody response against the carried antigen. The H(C) fragment expression cassette containing the transcription/translation signals, H(C) fragment open reading frame and the downstream TrpA terminator, was excised from pTETtac4 and incorporated into the plasmids pIC20H, pBR322, pACYC184 and pRSF1010. The resulting constructs were transferred into attenuated S. typhimurium, BRD509, and the level of H(C) fragment expression was examined by Western blot analysis. The relative stability of each plasmid in S. typhimurium was determined in vitro in the absence of antibiotic selection, and in vivo following immunisation. The ability of each H(C) fragment-expressing strain to induce lipopolysaccharide- and tetanus toxoid-specific antibody responses was assayed by an enzyme-linked immunosorbent assay. These studies showed that all the vaccine vector constructs, except the S. typhimurium carrying the expression vector based on pIC20H, were able to elicit a high titre immune response. The level of tetanus toxoid-specific antibody induced by S. typhimurium directly correlated with the level of in vitro and in vivo stability of the H(C) fragment expression plasmid carried by the bacterium, and not with an increased copy number of the parent plasmid vector.

Animal models paving the way for clinical trials of attenuated Salmonella enterica serovar Typhi live oral vaccines and live vectors

Attenuated Salmonella enterica serovar Typhi (S. Typhi) strains can serve as safe and effective oral vaccines to prevent typhoid fever and as live vectors to deliver foreign antigens to the immune system, either by the bacteria expressing antigens through prokaryotic expression plasmids or by delivering foreign genes carried on eukaryotic expression systems (DNA vaccines). The practical utility of such live vector vaccines relies on achieving a proper balance between minimizing the vaccine's reactogenicity and maximizing its immunogenicity. To advance to clinical trials, vaccine candidates need to be pre-clinically evaluated in relevant animal models that attempt to predict what their safety and immunogenicity profile will be when administered to humans. Since S. Typhi is a human-restricted pathogen, a major obstacle that has impeded the progress of vaccine development has been the shortcomings of the animal models available to assess vaccine candidates. In this review, we summarize the usefulness of animal models in the assessment of the degree of attenuation and immunogenicity of novel attenuated S. Typhi strains as vaccine candidates for the prevention of typhoid fever and as live vectors in humans.

A single dose of recombinant Salmonella typhimurium induces specific humoral immune responses against heterologous Eimeria tenella antigens in chicken

Salmonella typhimurium vaccine strains were used as antigen delivery system for oral immunisation of chickens against two antigens of the coccidian parasite Eimeria tenella. The cDNAs of the known E. tenella proteins, SO7 and TA4, were isolated from total RNA and subcloned into the expression vectors pQE30 and pTECH2. Subcutaneous immunisation of chickens with Escherichia coli-expressed SO7 and TA4 revealed that both proteins were immunogenic. Both cDNAs were subcloned into plasmids of the pTECH2 vector system, which allows them to be expressed as fusion proteins with the highly immunogenic fragment C of the tetanus toxin under control of the anaerobically inducible nirB promoter. Plasmids were introduced into the S. typhimurium vaccine strains SL3261, C5aroD and C5htrA. SDS-PAGE and Western blot analysis revealed expression of both fusion proteins in all strains under anaerobic culture conditions. Three-week-old white leghorn chickens were orally immunised with 10(9) CFU per animal. The stability of the recombinant bacteria was revealed by recovery of viable Salmonella containing the respective plasmids from the liver of the immunised chickens at day 3 after inoculation. Specific serum IgG antibodies against the SO7-or TA4-antigens were detectable by ELISA 2 weeks after oral immunisation and remained for at least 6 weeks, while specific IgA antibodies were restricted to the bile of the birds. All chickens produced serum IgG and IgA to S. typhimurium lipopolysaccharides. Our data show that a single oral inoculation with recombinant S. typhimurium SL3261, C5aroD and C5htrA can induce specific antibody responses to heterologous Eimeria antigens in chickens, suggesting that recombinant Salmonella are a suitable delivery system for vaccines against Eimeria infections.

Recent progress in herpes simplex virus immunobiology and vaccine research

Herpes simplex virus types 1 and 2 (HSV-1 and HSV-2) cause prevalent, chronic infections that have serious outcomes in some individuals. Neonatal herpes may occur when the infant traverses the cervix during maternal genital herpes. Genital herpes is a major risk factor for human immunodeficiency virus type 1 transmission. Considerable efforts have been made to design and test vaccines for HSV, focusing on genital infection with HSV-2. Several protein subunit vaccines based on HSV-2 envelope glycoproteins have reached advanced-phase clinical trials. These antigens were chosen because they are the targets of neutralizing-antibody responses and because they elicit cellular immunity. Encouraging results have been reported in studies of treatment of HSV-seronegative women with a vaccine consisting of truncated glycoprotein D of HSV-2 and a novel adjuvant. Because most sexual HSV transmission occurs during asymptomatic shedding, it is important to evaluate the impact of vaccination on HSV-2 infection, clinically apparent genital herpes, and HSV shedding among vaccine recipients who acquire infection. There are several other attractive formats, including subunit vaccines that target cellular immune responses, live attenuated virus strains, and mutant strains that undergo incomplete lytic replication. HSV vaccines have also been evaluated for the immunotherapy of established HSV infection.

Salmonella vaccines for use in humans: present and future perspectives

In recent years there has been significant progress in the development of attenuated Salmonella enterica serovar Typhi strains as candidate typhoid fever vaccines. In clinical trials these vaccines have been shown to be well tolerated and immunogenic. For example, the attenuated S. enterica var. Typhi strains CVD 908-htrA (aroC aroD htrA), Ty800 (phoP phoQ) and chi4073 (cya crp cdt) are all promising candidate typhoid vaccines. In addition, clinical trials have demonstrated that S. enterica var. Typhi vaccines expressing heterologous antigens, such as the tetanus toxin fragment C, can induce immunity to the expressed antigens in human volunteers. In many cases, the problems associated with expression of antigens in Salmonella have been successfully addressed and the future of Salmonella vaccine development is very promising.

Role of the htrA gene in Klebsiella pneumoniae virulence

We recently described the use of mini-Tn5 to generate complement-sensitive mutants derived from a complement-resistant Klebsiella pneumoniae clinical isolate deficient in the lipopolysaccharide O side chain. One mutant with a reduced capacity to survive in nonimmune human sera carried the transposon inserted in the htrA gene. We cloned and sequenced the gene and predicted from the deduced amino acid sequence that the putative HtrA homolog contains structural features similar to those of previously described HtrA proteins. To investigate the biological functions and the role of the htrA gene in the virulence of K. pneumoniae, we constructed an isogenic mutant by insertion-duplication mutagenesis. Characterization of the mutant showed that it had greater sensitivity to temperature (50 degrees C) and oxidative stress (H(2)O(2)) than the parent strain. Furthermore, the htrA mutant produced less capsule, bound more molecules of complement component C3, and was more sensitive to complement and whole-blood killing than was the parent strain. Finally, disruption of the htrA gene in a virulent K. pneumoniae strain caused a reduction of its virulence in a mice model. Our results indicate that the htrA gene plays an important role in the virulence of K. pneumoniae.

Salmonella: immune responses and vaccines

Salmonella infections are a serious medical and veterinary problem world-wide and cause concern in the food industry. Vaccination is an effective tool for the prevention of Salmonella infections. Host resistance to Salmonella relies initially on the production of inflammatory cytokines leading to the infiltration of activated inflammatory cells in the tissues. Thereafter T- and B-cell dependent specific immunity develops allowing the clearance of Salmonella microorganisms from the tissues and the establishment of long-lasting acquired immunity to re-infection. The increased resistance that develops after primary infection/ vaccination requires T-cells cytokines such as IFNgamma TNFalpha and IL12 in addition to opsonising antibody. However for reasons that are not fully understood seroconversion and/or the presence of detectable T-cell memory do not always correlate with the development of acquired resistance to infection.Whole-cell killed vaccines and subunit vaccines are used in the prevention of Salmonella infection in animals and in humans with variable results. A number of early live Salmonella vaccines derived empirically by chemical or u.v. mutagenesis proved to be immunogenic and protective and are still in use despite the need for repeated parenteral administration. Recent progress in the knowledge of the genetics of Salmonella virulence and modern recombinant DNA technology offers the possibility to introduce multiple defined attenuating and irreversible mutations into the bacterial genome. This has recently allowed the development of Salmonella strains devoid of significant side effects but still capable of inducing solid immunity after single oral administration. Live attenuated Salmonella vaccines have been used for the expression of heterologous antigens/proteins that can be successfully delivered to the immune system. Furthermore Salmonella can transfer plasmids encoding foreign antigens under the control of eukaryotic promoters (DNA vaccines) to antigen-presenting cells resulting in targeted delivery of DNA vaccines to these cells. Despite the great recent advances in the development of Salmonella vaccines a large proportion of the work has been conducted in laboratory rodents and more research in other animal species is required.

Oral transgene vaccination mediated by attenuated Salmonellae is an effective method to prevent Herpes simplex virus-2 induced disease in mice

An attenuated strain of Salmonella typhimurium has been used as a carrier for oral genetic immunization. The eukaryotic expression vector pCMV containing the gene of the glycoprotein D (gD) of the herpes simplex virus 2 was used to transform Salmonellae. The oral immunization with the transformed salmonellae elicited a strong cellular immune response in both, the mucosal and systemic compartments (spleen, ileal lymph nodes and Peyer patches). The immune response mainly consisted in a dramatic activation of IFN-gamma-secreting cells. Twenty hours following the challenge with five lethal doses of virus, mRNA for IFN-gamma was observed in vaginal tissues from mice immunized with salmonella harboring the plasmid pgD but not in tissues from mice immunized by the intramuscular route with pgD. After an intravaginal challenge all immunized mice survived without developing symptoms. Furthermore, the immunization with Salmonella resulted in a more effective control of viral shedding than intramuscular immunization. We have unequivocally demonstrated by the introduction of an intron in the green fluorescent protein that the expression of the plasmid was due to the transcription of the protein by an eukaryotic nuclear process and not as a result of expression of the protein by the bacteria. Macrophages and dendritic cells were found expressing the protein in systemic and mucosal compartments of the immune system.

Mucosal and systemic immune responses to chimeric fimbriae expressed by Salmonella enterica serovar typhimurium vaccine strains

Recombinant live oral vaccines expressing pathogen-derived antigens offer a unique set of attractive properties. Among these are the simplicity of administration, the capacity to induce mucosal and systemic immunity, and the advantage of permitting genetic manipulation for optimal antigen presentation. In this study, the benefit of having a heterologous antigen expressed on the surface of a live vector rather than intracellularly was evaluated. Accordingly, the immune response of mice immunized with a Salmonella enterica serovar Typhimurium vaccine strain expressing the Escherichia coli 987P fimbrial antigen on its surface (Fas(+)) was compared with the expression in the periplasmic compartment (Fas(-)). Orally immunized BALB/c mice showed that 987P fimbriated Salmonella serovar Typhimurium CS3263 (aroA asd) with pCS151 (fas(+) asd(+)) elicited a significantly higher level of 987P-specific systemic immunoglobulin G (IgG) and mucosal IgA than serovar Typhimurium CS3263 with pCS152 (fasD mutant, asd(+)) expressing 987P periplasmic antigen. Further studies were aimed at determining whether the 987P fimbriae expressed by serovar Typhimurium chi4550 (cya crp asd) could be used as carriers of foreign epitopes. For this, the vaccine strain was genetically engineered to express chimeric fimbriae carrying the transmissible gastroenteritis virus (TGEV) C (379-388) and A (521-531) epitopes of the spike protein inserted into the 987P major fimbrial subunit FasA. BALB/c mice administered orally serovar Typhimurium chi4550 expressing the chimeric fimbriae from the tet promoter in pCS154 (fas(+) asd(+)) produced systemic antibodies against both fimbria and the TGEV C epitope but not against the TGEV A epitope. To improve the immunogenicity of the chimeric fimbriae, the in vivo inducible nirB promoter was inserted into pCS154, upstream of the fas genes, to create pCS155. In comparison with the previously used vaccine, BALB/c mice immunized orally with serovar Typhimurium chi4550/pCS155 demonstrated significantly higher levels of serum IgG and mucosal IgA against 987P fimbria. Moreover, mucosal IgA against the TGEV C epitope was only detected with serovar Typhimurium chi4550/pCS155. The induced antibodies also recognized the epitopes in the context of the full-length TGEV spike protein. Hence, immune responses to heterologous chimeric fimbriae on Salmonella vaccine vectors can be optimized by using promoters known to be activated in vivo.

Surface-displayed viral antigens on Salmonella carrier vaccine

We have developed a recombinant live oral vaccine using the ice-nucleation protein (Inp) from Pseudomonas syringae to display viral antigens on the surface of Salmonella spp. Fusion proteins containing viral antigens were expressed in the oral vaccine strain, Salmonella typhi Ty21a. Surface localization was verified by immunoblotting and fluorescence-activated cell sorting. The immunogenicity of surface-displayed viral antigens on the recombinant live vaccine strain was assessed in mice inoculated intranasally and intraperitoneally. Inoculation resulted in significantly higher serum antibody level than those induced by viral antigens expressed intracellularly. Thus, this multivalent mucosal live vaccine may provide an effective means for inducing mucosal or systemic immune responses against multiple viral antigens.

Tetanus toxin fragment C expressed in live Salmonella vaccines enhances antibody responses to its fusion partner Schistosoma haematobium glutathione S-transferase

Tetanus toxoid has been used widely as an adjuvant. The atoxic fragment C from tetanus toxin (TetC) is potently immunogenic when expressed in Salmonella vaccine strains and has been used as a fusion partner for antigens (Ag). However, there has been no formal comparison of the immunomodulatory impact of TetC on its fusion partners. In this study, we have addressed this important issue. The protective 28-kDa glutathione S-transferase (GST) from Schistosoma haematobium (Sh28GST) was expressed either as a fusion to TetC or as the full-length Sh28GST alone in a nonvirulent aroA-attenuated strain of Salmonella enterica serovar Typhimurium. The Sh28GST proteins were soluble and stably expressed in Salmonella, as evaluated by Western blotting with TetC and/or Sh28GST antisera. Mice were immunized orally with a single dose of the live recombinant Salmonella. The constructs were stable in mice but, dramatically, only the strain expressing the TetC-Sh28GST fusion elicited significant antibody (Ab) responses directed against Sh28GST as determined by enzyme-linked immunosorbent assay. An analysis of the isotype profiles showed that these mice also produced anti-Sh28GST immunoglobulin A and GST-neutralizing assays revealed high levels of neutralizing Abs in sera. These are important correlates of protection in schistosomiasis. In addition, stimulation of spleen cells from immunized mice with Sh28GST Ag showed that both strains, expressing Sh28GST alone or the TetC-Sh28GST fusion, were able to stimulate the secretion of Th1-related cytokines (gamma interferon and interleukin 2) to comparable levels. Thus, TetC has modulated the immune responses generated against its fusion partner, Sh28GST, by markedly enhancing the Ab responses elicited. These results have important implications in the rational development of live vaccines.

Prospects for control of herpes simplex virus disease through immunization

Herpes simplex viruses (HSVs) can cause a variety of infections, including genital herpes. Despite effective antiviral therapy, HSV infections remain a significant worldwide public health problem. Vaccines offer the best hope for controlling spread and limiting HSV disease. This article discusses the pathogenesis and immunobiology of mucocutaneous HSV infections, summarizes the spectrum of diseases caused by HSV, and provides a review of the field of HSV vaccine research. This article also discusses what might be realistically expected of a vaccine intended for control of genital herpes and explores the question of whether a vaccine that is effective in controlling genital HSV disease might also be effective in controlling nongenital HSV disease. The efficacy of such vaccines for the full spectrum of HSV disease will eventually determine the timing and targeting of immunization, ranging from selective immunization in preadolescence to universal childhood immunization as part of the routine childhood regimen.

Use of the stationary phase inducible promoters, spv and dps, to drive heterologous antigen expression in Salmonella vaccine strains

We have investigated the ability of the growth phase regulated promoters dps and spv, to drive expression of heterologous antigens in Salmonella vaccine strains. Reporter plasmids were constructed which directed beta-galactosidase expression from dps (pDpslacZ) or spv (pSpvlacZ) and these were introduced independently into the Salmonella typhimurium vaccine strain SL3261 (aroA(-)). beta-galactosidase expression was induced 20-fold and 100-fold when broth cultures of SL3261 (pDpslacZ) or SL3261 (pSpvlacZ) respectively, entered the stationary phase of growth. Within macrophages, beta-galactosidase expression was induced 3.5-fold with SL3261 (pDpslacZ) and 7-fold with SL3261 (pSpvlacZ). The spv and dps promoters were used to drive independent expression of the C fragment domain of tetanus toxin (TetC) from plasmids harboured in S. typhimurium SL3261. Levels of anti-TetC antibodies were significantly higher in the sera of BALB/c mice perorally inoculated with SL3261 (pSpvtetC) or SL3261 (pDpstetC) compared to unvaccinated controls. This suggests that these promoter systems may be used to drive foreign antigen expression in live oral Salmonella vaccines.

Local and systemic neutralizing antibody responses induced by intranasal immunization with the nontoxic binding domain of toxin A from Clostridium difficile

Fourteen of the 38 C-terminal repeats from Clostridium difficile toxin A (14CDTA) were cloned and expressed either with an N-terminal polyhistidine tag (14CDTA-HIS) or fused to the nontoxic binding domain from tetanus toxin (14CDTA-TETC). The recombinant proteins were successfully purified by bovine thyroglobulin affinity chromatography. Both C. difficile toxin A fusion proteins bound to known toxin A ligands present on the surface of rabbit erythrocytes. Intranasal immunization of BALB/c mice with three separate 10-microg doses of 14CDTA-HIS or -TETC generated significant levels of anti-toxin A serum antibodies compared to control animals. The coadministration of the mucosal adjuvant heat labile toxin (LT) from Escherichia coli (1 microg) significantly increased the anti-toxin A response in the serum and at the mucosal surface. Importantly, the local and systemic antibodies generated neutralized toxin A cytotoxicity. Impressive systemic and mucosal anti-toxin A responses were also seen following coadministration of 14CDTA-TETC with LTR72, an LT derivative with reduced toxicity which shows potential as a mucosal adjuvant for humans.

Attenuated deltaguaBA Salmonella typhi vaccine strain CVD 915 as a live vector utilizing prokaryotic or eukaryotic expression systems to deliver foreign antigens and elicit immune responses

Attenuated Salmonella typhi strain CVD 915, harboring a deletion in guaBA that interrupts the biosynthesis of guanine nucleotides, was evaluated as a live vector vaccine for delivering foreign antigens utilizing prokaryotic or eukaryotic expression systems. Plasmids pTETnir15 and pcDNA3tetC encoding fragment C (Frag C) of tetanus toxin under the control of prokaryotic or eukaryotic promoters, respectively, were introduced into CVD 915 and administered intranasally to mice. Purified pcDNA3tetC and Frag C were given intramuscularly. High titers of serum IgG1, IgG2a, and IgG2b antibodies against Frag C were elicited by CVD 915(pTETnir15) and CVD 915(pcDNA3tetC). These responses were significantly higher than those induced by pcDNA3tetC. Proliferative responses and IL-2 and IFN-gamma production were observed in splenocytes exposed to S. typhi antigens and Frag C. We conclude that CVD 915 is a highly efficient live vector to carry foreign genes under eukaryotic or prokaryotic control and elicit potent immune responses.

Immunogenicity of a Salmonella typhimurium aroA aroD vaccine expressing a nontoxic domain of Clostridium difficile toxin A

The C-terminal repeat domain of Clostridium difficile toxin A harbors toxin-neutralizing epitopes and is considered to be a candidate component of a vaccine against C. difficile-associated disease (CDAD). Fourteen of the 38 C-terminal toxin A repeats (14CDTA) were cloned into pTECH-1 in frame with the immunogenic fragment C of tetanus toxin (TETC) to generate plasmid p56TETC. Expression of the TETC-14CDTA fusion protein was driven from the anaerobically inducible nirB promoter within attenuated Salmonella typhimurium BRD509 (aroA aroD). The TETC-14CDTA fusion protein was purified and shown to bind to known toxin A receptors found on the surface of rabbit erythrocytes. Intranasal (i.n.) and intragastric (i.g.) immunization with 10(7) and 10(10) CFU, respectively, of BRD509(p56TETC) generated significant (P < 0.05) anti-toxin A serum responses after a single dose. Antibody titers were elevated following a boosting dose with either live vaccine or a subcutaneous injection of 0.5 microgram of purified 14CDTA protein. Importantly, serum from mice immunized with BRD509(p56TETC) neutralized toxin A cytotoxicity. Both i.n. and i.g. immunizations also generated toxin A-specific immunoglobulin A on the pulmonary and intestinal mucosa, respectively. Intranasal vaccination induced consistently higher serum and mucosal anti-toxin A antibody responses. Significant anti-tetanus toxoid serum and mucosal antibodies were also generated by both immunization routes. The availability of live attenuated Salmonella typhi for human use may allow the development of a multivalent mucosal vaccine against CDAD, tetanus, and typhoid.

Herpes simplex virus vaccines

Herpes simplex virus (HSV) infections are common and produce not only a primary infection, but also latent and recurrent infections. Therefore, the goals of a HSV vaccine are different from other vaccines. In this review, the goals of both prophylactic and therapeutic HSV vaccines are discussed and related to the immunobiology of acute and recurrent HSV infections. Next, the vaccine strategies that have been and are being evaluated for control of HSV disease are described. Current approaches take advantage of many of the modern methodologies for vaccine development. Thus, as we await final evaluations of subunit HSV glycoprotein vaccines, early phase I trails are evaluating newer vaccine approaches including DNA-based vaccines and replication-impaired viruses. The definitive HSV vaccine may await increased knowledge of the optimal viral antigen(s) and routes to induce genital tract immunity.

Approaches to new vaccines

The explosive technological advances in the fields of immunology and molecular biology in the last 5 years had an enormous impact on the identification of candidate vaccines against diseases, which until a few years ago seemed uncontrollable. Increased knowledge of the immune system has helped to define the mechanisms that underlie successful immunization and is now being exploited to develop improved versions of existing vaccines and new vaccines against emerging pathogens, tumors, or autoimmune diseases. An understanding of the mechanisms of action of novel adjuvants and the development of new vector and delivery systems will have a major impact on vaccine strategies. The use of DNA encoding antigens from pathogenic viruses, bacteria, and parasites as vaccines is a new approach that is receiving considerable attention. This and other innovative approaches, including vaccine production in plants, are appraised in this review. The successful eradication of smallpox and the imminent eradication of poliomyelitis by worldwide immunization campaigns provide positive examples of how the vaccine-mediated approach can lead to disease elimination; with the advent of new vaccines and improved delivery systems, there is no scientific reason why these successes cannot be repeated.

Lipid A mutant Salmonella with suppressed virulence and TNFalpha induction retain tumor-targeting in vivo

Systemically administered tumor-targeted Salmonella has been developed as an anticancer agent, although its use could be limited by the potential induction of tumor necrosis factor alpha (TNFalpha)-mediated septic shock stimulated by lipid A. Genetic modifications of tumor-targeting Salmonella that alter lipid A and increase safety must, however, retain the useful properties of this bacteria. We report here that disruption of the Salmonella msbB gene reduces TNFalpha induction and increases the LD50 of this pathogenic bacteria by 10,000-fold. Notwithstanding this enormous difference, Salmonella retains its tumor-targeting properties, exhibiting tumor accumulation ratios in excess of 1000:1 compared with normal tissues. Administration of this bacteria to mice bearing melanoma results in tumors that are less than 6% the size of tumors in untreated controls at day 18. Thus, the antitumor activity previously demonstrated using tumor-targeting Salmonella with normal lipid A is retained. Lipid modification of tumor-specific bacterial vectors provides a means for reducing septic shock and further suggests that the antitumor activity of these bacteria may be independent of TNFalpha.

Protection against herpes B virus infection in rabbits with a recombinant vaccinia virus expressing glycoprotein D

Herpes B virus infects naturally monkeys of the macaque genus in whom it can cause recurrent oral and genital lesions. However, when the virus infects humans it causes a neurological illness with a high case fatality rate. Successful treatment is possible but this depends on diagnosis prior to the onset of respiratory arrest, and fatalities over the last 10 years have been the result of late or no diagnostic data on which to base anti-viral intervention. An effective vaccine would be an ideal way to combat the risk of herpes B virus disease in humans working with potentially infected monkeys or their tissues. A recombinant vaccinia virus expressing herpes B virus glycoprotein D (gD) was constructed and rabbits inoculated with the chimeric virus were tested for immunoglobulin responses to herpes B virus by virus neutralisation, ELISA and Western blot analyses. Anti-gD humoral responses were detected in all vaccinated animals by ELISA and Western blot but neutralising antibody was not detected prior to challenge with herpes B virus. Non-vaccinated rabbits died within 8 days of challenge while 10/11 vaccinated animals were protected against herpes B virus disease. No antibodies to herpes B virus proteins other than gD were detectable in surviving animals, suggesting minimal herpes B virus replication post challenge. Autopsies were carried out on 4/10 rabbits which had remained healthy at 31 days post challenge and the dorsal root ganglia adjacent to the inoculation site were removed. Attempts to detect herpes B virus DNA by PCR followed by hybridisation proved negative suggesting protection against latent herpes B virus infection.

Control of STDs--the role of prophylactic vaccines against herpes simplex virus

OBJECTIVES

To summarise the current status of genital herpes simplex virus (HSV) vaccine development and provide a discussion of the potential benefits and limitations of genital herpes vaccines.

METHODS

Literature review.

RESULTS

Genital herpes simplex virus infection has a complex pathogenesis that has contributed to it becoming a serious worldwide problem. In an attempt to control the problem five different types of genital herpes vaccines have been developed. These include inactivated virion derived vaccines, adjuvanted subunit vaccines, vectored vaccines, replication limited live viral vaccines, genetically attenuated live viral vaccines, and nucleic acid vaccines. While available commercially in some parts of the world, inactivated virion derived vaccines have not been proved effective. Of the others, adjuvanted subunit vaccines, replication limited live viral vaccines, and nucleic acid vaccines are currently in clinical trials and vectored vaccines and genetically attenuated live viral vaccines are in preclinical development.

CONCLUSION

With regard to HSV vaccines in general, it is reasonable to expect that the newer vaccines may protect the individual from developing symptomatic genital herpes but may not protect against asymptomatic viral infection. With widespread use HSV vaccines might help to prevent the spread of genital herpes.

Oral vaccination against tetanus: comparison of the immunogenicities of Salmonella strains expressing fragment C from the nirB and htrA promoters

We have found the in vivo-regulated nirB promoter (PnirB) to be effective for directing expression of a number of antigens in salmonella in vivo. We wished to determine if other in vivo-regulated promoters have utility for antigen expression in salmonella and to compare the effectiveness of these promoters with that of PnirB. To this end, we have devised a scheme that allows the promoter element of the PnirB-fragment C plasmid pTETnir15 to be swapped with other promoters of interest. We demonstrate the usefulness of this system by replacing PnirB with PhtrA to create plasmid pTEThtrA1. htrA is a stress response gene that is required for virulence of salmonella in mice and survival within macrophages. Expression of fragment C in Salmonella typhimurium BRD509 (aroA aroD) harboring pTEThtrA1 (strain BRD937) correlated with growth temperature in vitro. A comparison was made of the immune responses to fragment C elicited in mice immunized orally with BRD937 or BRD847 (BRD509/pTETnir15) or subcutaneously with purified fragment C plus alhydrogel. High levels of anti-fragment C antibodies that persisted for at least 12 weeks were present in all groups of mice. Vaccination with BRD937 was the most effective means of immunization: the serum immunoglobulin G (IgG), IgA, and IgM anti-fragment C titers were higher in the BRD937-immunized mice throughout the duration of the study than in mice in the other groups. The kinetics of the serum anti-fragment C responses were different in different groups. The response was most rapid in the BRD937 group, with the titers almost at peak levels at 2 weeks postimmunization. Only the mice immunized with BRD937 or BRD847 developed an intestinal IgA response to fragment C. Again, the response was superior in the BRD937 group. The peak of the intestinal response was delayed with respect to the serum response. Analysis of the IgG subtype response to fragment C revealed a dominant IgG2a response in the salmonella-immunized mice, indicating a type 1 helper T-cell response to fragment C, whereas the major subtype in the group parenterally immunized with fragment C plus alhydrogel was IgG1. The IgG1/IgG2a ratio was much higher in sera of BRD937-immunized mice than in sera of BRD847-immunized mice. At 15 to 20 weeks after immunization, the mice immunized with BRD937 or BRD847 were solidly immune to tetanus toxin and salmonella. The immune responses to fragment C seen in mice immunized with BRD937 are the strongest we have observed and indicate that the htrA promoter may be very useful for expressing foreign antigens in salmonella vaccine strains.

Comparison of the abilities of different attenuated Salmonella typhimurium strains to elicit humoral immune responses against a heterologous antigen

We compared the abilities of different Salmonella enterica var. Typhimurium (S. typhimurium) strains harboring mutations in the genes aroA, aroAD, purA, ompR, htrA, and cya crp to present the heterologous antigen, C fragment of tetanus toxin, to the mouse immune system. Plasmid pTETtac4, encoding C fragment, was transferred into the various S. typhimurium mutants, and the levels of antigen expression were found to be equivalent. After primary oral immunization of BALB/c mice, all attenuated strains were capable of penetrating the gut epithelium and colonizing the Peyer's patches and spleens of mice. Of all strains compared, the delta purA mutant colonized and persisted in the Peyer's patches at the lowest level, whereas the delta htrA mutant colonized and persisted in the spleen at the lowest level. The level of specific antibody elicited by the different strains against either S. typhimurium lipopolysaccharide or tetanus toxoid was strain dependent and did not directly correlate to the mutants' ability to colonize the spleen. The level of immunoglobulin G1 (IgG1) and IgG2a antibody specific for tetanus toxoid was determined in mice immunized with four S. typhimurium mutants. The level of antigen-specific IgG1 and IgG2a was significantly lower in animals immunized with S. typhimurium delta purA. Antigen-specific T-cell proliferation assays indicated a degree of variability in the capacity of some strains to elicit T cells to the heterologous antigen. Cytokine profiles (gamma interferon and interleukin-5) revealed that the four S. typhimurium mutants tested induced a Th1-type immune response. Mice were challenged with a lethal dose of tetanus toxin 96 days after oral immunization. With the exception of the S. typhimurium delta purA mutant, all strains elicited a protective immune response. These data indicate that the level of total Ig specific for the carried antigen, C fragment, does not correlate with the relative invasiveness of the vector, but it is determined by the carrier mutation and the background of the S. typhimurium strain.

Expression and immunogenicity of an Echinococcus granulosus fatty acid-binding protein in live attenuated Salmonella vaccine strains

Fatty acid-binding proteins (FABPs) are candidate molecules for vaccines against several parasitic platyhelminths. A FABP from the cestode Echinococcus granulosus (EgDf1) was expressed in Salmonella vaccine strains as a C-terminal fusion to fragment C of tetanus toxin (TetC) by using expression vector pTECH. The fusion protein was equally expressed in several attenuated vaccine strains derived from bacteria with different genetic backgrounds and different attenuating mutations. Single-dose immunization experiments with the aroA Salmonella typhimurium strain SL3261 carrying the pTECH-EgDf1 construct were conducted with mice, using both the intravenous and the oral routes. Surprisingly, the antibody response to EgDf1 and the antigen-specific cytokine production in spleen cells were stronger in mice immunized orally. Furthermore, immune mouse sera strongly reacted with fixed sections of the worm's larval stage. Analysis of the isotype distribution of the specific anti-EgDf1 antibodies showed similar production of immunoglobulin G1 (IgG1) and IgG2a together with specific IgA antibodies. In addition, stimulation of spleen cells from mice immunized with the different constructs with either Salmonella lysate, TetC, or EgDf1 showed that, together with Th1-related cytokines (gamma interferon and interleukin 2 [IL-2]), significant levels of a Th2 cytokine (IL-5) were produced specifically, indicating a Th2 component to the response to the Salmonella carrier and to the recombinant antigens. Salmonellae expressing the TetC-rEgDfl fusion are currently under evaluation as potential vaccines against E. granulosus.

Salmonella typhimurium aroA, htrA, and aroD htrA mutants cause progressive infections in athymic (nu/nu) BALB/c mice

Athymic (nu/nu) BALB/c mice and their euthymic (nu/+) littermates were inoculated intravenously with live attenuated vaccine strains of Salmonella typhimurium. All strains caused progressive infections in the athymic mice but not in their euthymic littermates. Athymic mice given strain SL3261, an aroA derivative of SL1344, in doses between log 4.7 and 5.7 CFU were all severely ill and were killed by weeks 4 to 5. Athymic mice given log 4.7 CFU of a derivative of S. typhimurium C5 carrying a mutation in htrA, encoding a stress protein, were ill and were killed by week 7 in one experiment but survived to week 13 in another. Athymic mice given log 4.6 CFU of a C5 aroD htrA double mutant were ill and were killed at week 7. Athymic mice given SL3261 had high bacterial counts in the reticuloendothelial system at 4 weeks. Athymic mice given SL3261 or C5 htrA made immunoglobulin G3 (IgG3) (and to a lesser extent IgM) antibody to lipopolysaccharide (LPS), whereas euthymic mice made IgM, IgG1, IgG2a, IgG2b, and IgG3 anti-LPS antibodies. The results indicate that both aroA and htrA strains will produce slow, progressively lethal infections in athymic mice, that the htrA strain is more attenuated than the aroA strain as measured by time to death in this model, and that IgG3 anti-LPS antibody alone cannot suppress the progress of infections by very attenuated strains in athymic mice.

Vaccine efficacy of Salmonella strains expressing glycoprotein 63 with different promoters

The development of Salmonella vaccine vectors has been hindered by both the requirement for multiple doses to induce immune responses and a lack of plasmid stability. Direct comparisons of different promoter systems with the same antigen are necessary to address these important issues. We have previously described an AroA- AroD- deletion mutant of Salmonella typhimurium (GID101) which expresses the gene encoding the Leishmania major promastigote surface glycoprotein gp63 (GID101). While this construct provided significant protection against L. major challenge to highly susceptible BALB/c mice, this required at least two oral doses. We report here the use of two different inducible promoters, the nirB and osmC promoters, to improve vaccine efficacy. These constructs (termed GID105 and GID106, respectively) expressed gp63 in vitro under inducible conditions and colonized BALB/c mice after oral administration. GID105 demonstrated greater plasmid stability in vitro and in vivo than did either GID106 or GID101, which expresses gp63 constitutively. Spleen and lymph node cells from mice immunized with a single oral dose of GID105 proliferated in vitro in response to L. major and secreted gamma interferon, whereas cells from mice given the other constructs did not. Mice immunized with a single oral dose of GID1O5 or GID106 developed significantly smaller lesions upon challenge with L. major, whereas mice administered GID101 did not. Mice administered GID105 also showed considerable resistance to Leishmania donovani infection. These data provide a direct comparison of promoter systems and demonstrate that the use of inducible promoters such as the nirB promoter allows a considerable improvement over the previous vaccine construct in terms of protection against infection.