T-cell and antibody response characterisation of a new recombinant pre-S1, pre-S2 and SHBs antigen-containing hepatitis B vaccine; demonstration of superior anti-SHBs antibody induction in responder mice.

The incidence of non-responders to hepatitis B (HB) virus SHBs antigen (Ag) vaccines has prompted the development of pre-S containing vaccines. The aim of this study was to characterise the murine immune response to a novel recombinant particle (Hepagene) (Medeva plc) containing pre-S1, pre-S2 and SHBsAg components. Hepagene induced potent in vitro spleen T-cell proliferative responses in both BALB/c (maximum stimulation index (SI) = 38) and SWR/J (maximum SI = 43) strains of mouse, following immunisation. High concentrations of interferon-gamma and low concentrations of interleukin-10 were detected in the media of spleen cells stimulated with Hepagene. The anti-Hepagene antibody response was higher in SWR/J mice and alhydrogel adjuvant significantly improved the titres. Anti-pre-S1 antibody was detected in both strains of mouse, whereas antipre-S2 antibody was only detected in SWR/J mice. IgG subclass analysis of the anti-Hepagene response revealed a Th2-type response in BALB/c mice and a mixed Th1/Th2 response in SWR/J mice. Hepagene induced higher anti-SHBs antibody responses than Engerix-B (11097 and 1276 IU/ml, respectively) in BALB/c mice. Hepagene therefore, stimulates strong cellular and humoral immune responses in murine models. The high anti-SHBs antibody response suggests that Hepagene is an improved hepatitis B virus vaccine.

Characterization of candidate live oral Salmonella typhi vaccine strains harboring defined mutations in aroA, aroC, and htrA.

The properties of two candidate Salmonella typhi-based live oral typhoid vaccine strains, BRD691 (S. typhi Ty2 harboring mutations in aroA and aroC) and BRD1116 (S. typhi Ty2 harboring mutations in aroA, aroC, and htrA), were compared in a number of in vitro and in vivo assays. BRD1116 exhibited an increased susceptibility to oxidative stress compared with BRD691, but both strains were equally resistant to heat shock. Both strains showed a similar ability to invade Caco-2 and HT-29 epithelial cells and U937 macrophage-like cells, but BRD1116 was less efficient at surviving in epithelial cells than BRD691. BRD1116 and BRD691 were equally susceptible to intracellular killing within U937 cells. Similar findings were demonstrated in vivo, with BRD1116 being less able to survive and translocate to secondary sites of infection when inoculated into the lumen of human intestinal xenografts in SCID mice. However, translocation of BRD1116 to spleens and livers in SCID mice occurred as efficiently as that of BRD691 when inoculated intraperitonally. The ability of BRD1116 to increase the secretion of interleukin-8 following infection of HT-29 epithelial cells was comparable to that of BRD691. Therefore, loss of the HtrA protease in S. typhi does not seem to alter its ability to invade epithelial cells or macrophages or to induce proinflammatory cytokines such as IL-8 but significantly reduces intracellular survival in human intestinal epithelial cells in vitro and in vivo.

Characterization of the T- and B-cell immune response to a new recombinant pre-S1, pre-S2 and SHBs antigen containing hepatitis B vaccine (Hepagene); evidence for superior anti-SHBs antibody induction in responder mice.

Hepagene is a novel recombinant particle consisting of the pre-S1, pre-S2 and small surface (SHBs) antigens (Ag) of the hepatitis B virus (HBV) and is adjuvanted with alhydrogel in the final formulation. It has been primarily developed to enhance anti-SHBs antibody titres in inadequate responders, to conventional SHBsAg vaccines. Since non-compliance is also a problem with existing HBV vaccine schedules, the ability to accelerate current immunization regimens to provide more rapid protection has also been an important objective. Here we describe the T- and B-cell responses to Hepagene in two strains of responder mouse (BALB/c and SWR/J). Hepagene induced high in vitro spleen T-cell proliferative responses in both strains (max. Stimulation Index = 43), following intraperitoneal immunization. High concentrations of interferon-gamma (max. = 5000 pg/mL) were detected in the media of spleen cells cultured with non-adjuvanted Hepagene particles. SWR/J mice showed the highest serum antibody (Ab) titres to non-adjuvanted Hepagene. The presence of alhydrogel adjuvant in the vaccine formulation significantly improved the titres. Anti-pre-S1 Ab was detected in both strains of mouse but anti-pre-S2 Ab was only detected in the SWR/J strain. In BALB/c mice, the anti-Hepagene (non-adjuvanted) IgG1 Ab subclass was predominant but in SWR/J mice IgG1, IgG2a and IgG2b subclasses were of a similar magnitude. In BALB/c mice, Hepagene induced higher anti-SHBs Ab responses than Engerix-B (11097 IU/mL and 1276 IU/mL, respectively), following two doses of vaccine (10 micrograms/mouse). The vaccine therefore, induces strong cellular and humoral immune responses and these data suggest that Hepagene is an improved hepatitis B vaccine.

Regulation of host immune responses by modification of Salmonella virulence genes.

Modifying bacterial virulence genes to probe the nature of host immunity is mostly unexplored. Here we investigate whether host immune responses can be regulated by modification of bacterial virulence genes. In mice, attenuated Salmonella mutant strains with clinical relevance elicited differential host immune responses. Oral administration of a mutant strain with a PhoP-null phenotype promoted potent innate immune responses of macrophages that were sufficient for host defense. In contrast, administration of an Aro- mutant strain elicited stronger specific antibody and T-helper (Th)-cell responses, wherein Th1-type cells were required for clearance. Thus, genetic manipulation of bacteria may be used to broadly alter immune mechanisms that regulate attenuation within the host and to tailor host immunity to specific bacterial pathogens.

Immune responses in calves immunised orally or subcutaneously with a live Salmonella typhimurium aro vaccine.

Salmonella aro vaccines are able to confer solid protection against homologous virulent challenge in several animal species. Calves were protected against virulent S. typhimurium challenge following administration of a single oral dose of live BRD562 vaccine. Immune responses elicited by the S. typhimurium aro vaccine strain BRD562 were studied following administration to calves by either the oral or subcutaneous route. Serum antibodies to Salmonella polypeptides, following oral or subcutaneous vaccination, were detected by immunoblotting and the route of inoculation found to affect both the antibody isotype and the antigens detected. Oral, but not subcutaneous, immunisation induced bovine serum IgA antibodies against Salmonella antigens of 30 kDa and 65 kDa and bovine IgG2 antibodies against a 35 kDa antigen. Subcutaneous vaccination triggered responses against antigens of 52 kDa, 54 kDa and 57 kDa which were not detected by immune plasma of animals immunised orally. Antibody responses to LPS were poor in animals inoculated by either route. Subcutaneous vaccination elicited T-cell responses against Salmonella antigens as measured by in vitro peripheral blood cell thymidine incorporation. These studies show that the S. typhimurium vaccine strain BRD562 is capable of inducing both humoral and cellular immune responses. Further studies are necessary to identify the nature of the antigens responsible for protection. Oral or subcutaneous inoculation of BRD562(pTETnir15) failed to induce serum antibodies against the fragment C of tetanus toxin (TetC) but was effective in mice. Oral vaccination with this recombinant vaccine induced mucosal IgA against TetC. This is the first time that Salmonella recombinant vaccines have been shown to successfully elicit antibodies against a guest antigen in cattle after one single oral inoculation.

Safety of live oral Salmonella typhi vaccine strains with deletions in htrA and aroC aroD and immune response in humans.

A single-dose, oral Salmonella typhi vaccine strain has been sought as a carrier or vector of cloned genes encoding protective antigens of other pathogens. Such a hybrid vaccine, administered orally, would stimulate immune responses both at the mucosal surface and in the systemic compartment and would potentially provide protection against multiple pathogens. S. typhi CVD 908 and CVD 906, which harbor deletions in aroC and aroD, were further engineered by deletion in htrA to produce strains CVD 908-htrA and CVD 906-htrA, which are unable to sustain growth and are severely impaired in their ability to survive in host tissues. These strains were fed to humans at doses of 5 x 10(7) to 5 x 10(9) CFU with buffer, and safety and immune responses were assessed. CVD 908-htrA and CVD 906-htrA were well tolerated in volunteers; mild diarrhea in 3 of 36 volunteers and mild fever in 1 volunteer were the only notable adverse responses. The vaccine strains were not detected in blood cultures and only transiently detected in stool. Serum immune responses to S. typhi lipopolysaccharide and H antigens were observed in 75 to 100% of volunteers who received 5 x 10(8) to 5 x 10(9) CFU, and cells secreting S. typhi-specific antibodies were found in all volunteers after ingestion of either strain. Sixty-three percent to 83% of volunteers developed lymphoproliferative responses to S. typhi flagellar and particulate antigens after the higher doses. These studies demonstrate the potential of CVD 908-htrA as a live vector for the delivery of heterologous genes, and a clinical trial of such a construct is planned.

Oral immunization of interleukin-4 (IL-4) knockout mice with a recombinant Salmonella strain or cholera toxin reveals that CD4+ Th2 cells producing IL-6 and IL-10 are associated with mucosal immunoglobulin A responses.

Mucosal immunoglobulin A (IgA) responses are often associated with Th2-type cells and derived cytokines, and interleukin-4 (IL-4) knockout (IL-4-/-) mice with impaired Th2 cells respond poorly to oral antigens. However, we have noted that IL-4-/- mice have normal mucosal IgA levels, which led us to query whether different oral delivery systems could elicit mucosal immunity. Two oral regimens were used: (i) a live recombinant Salmonella strain which expresses fragment C (ToxC) of tetanus toxin, and (ii) soluble tetanus toxoid (TT) with cholera toxin (CT) as an adjuvant. Oral immunization of IL-4-/- mice with recombinant Salmonella vaccine expressing ToxC induced brisk mucosal IgA and serum IgG (mainly IgG2a) anti-TT antibody responses. TT-specific CD4+ T cells from spleen or Peyer's patches produced gamma interferon, indicative of Th1 responses; however, IL-6 and IL-10 were also seen. Oral immunization of IL-4-/- mice with TT and CT induced weak mucosal IgA to TT; however, brisk IgA anti-CT-B responses and CT-B-specific CD4+ T cells producing IL-6 and IL-10 were also noted. These results show that although IL-4-dependent antibody responses are impaired, mucosal IgA responses are induced in IL-4-/- mice. These result suggest that certain cytokines, i.e., IL-6 and IL-10 from Th2-type cells, play an important compensatory role in the induction and regulation of mucosal IgA responses.

Regulation of mucosal and systemic antibody responses by T helper cell subsets, macrophages, and derived cytokines following oral immunization with live recombinant Salmonella.

We have assessed regulatory Th cell and cytokine responses in mice after oral immunization with recombinant Salmonella (BRD 847) expressing fragment C of tetanus toxoid, since little information is available to explain how these vectors induce mucosal IgA responses. A single dose of BRD 847 elicited serum IgG2a and mucosal IgA anti-tetanus toxoid Ab responses. To assess Th1-and Th2-type responses, CD4+ T cells from Peyer's patches and spleen were restimulated in vitro, and cytokine-specific ELISPOT, ELISA, and reverse transcriptase-PCR assays were used to assess cytokine patterns. CD4+ T cells produced IFN-gamma and IL-2 as well as IL-10, but not IL-4 or IL-5. Although IL-6 was elevated, further purification of cells from in vitro cultures into CD4+ Mac-1- T cells and Mac-1+ CD4- cells revealed that only the latter cell population had consistently elevated IL-6 gene expression, whereas both sorted populations exhibited increased IFN-gamma and IL-10 gene expression. Thus, orally administered recombinant Salmonella expressing fragment C of tetanus toxoid elicited dominant Ag-specific Th1-type responses together with Th2-type cells producing IL-10 in both mucosal and systemic tissues. Macrophages producing IL-6 were also evident. Our results are consistent with the suggestion that Ag-specific Th1 cells and their derived cytokines, IFN-gamma and IL-2, and Th2-derived IL-10 together with IL-6 produced by macrophages provide important signals for the development of mucosal IgA and serum IgG subclass responses in the absence of preferential expression of Th2 cytokines IL-4 and IL-5.

The role of Th1 and Th2 cells for mucosal IgA responses.

We have used cytokine-knockout mice to help determine the precise requirements for CD4+ Th cell regulation of IgA responses. In these studies, we have used two different oral delivery systems to induce mucosal and systemic antibody responses to the vaccine TT. In normal mice, oral administration of TT with CT as adjuvant induces Th2 cells and cytokines, which give rise to mucosal IgA and serum IgG1, IgA, and IgE responses. On the other hand, oral immunization with rSalmonella expressing Tox C results in Th1-type responses as well as Th2 cell-derived IL-10 and macrophage-derived IL-6, which correlate with mucosal IgA and serum IgG2a antibody responses. Two major conclusions can be drawn from our studies with these two regimens in normal, IFN-gamma-/-, and IL-4-/- mice. First, oral administration of rSalmonella, which elicits classical Th1-type responses also induces significant mucosal IgA responses when given to mice with defective Th1- (IFN-gamma-/-) or Th2- (IL-4-/-) cytokine pathways. Interestingly, we detect Th2-type cells producing IL-10 and macrophage-secreting IL-6 in both normal and cytokine-deficient mice, and we postulate that these two cytokines are of most importance for murine IgA responses. Second, oral administration of TT plus CT as adjuvant induces classical Th2-type responses in both normal and IFN-gamma-/- mice. Further, lack of IL-4 results in failure to induce mucosal IgA responses. Thus, the IL-4 pathway is necessary for the CT adjuvant effect for mucosal IgA responses after oral immunization with a protein vaccine.

Mucosal immunity: regulation by helper T cells and a novel method for detection.

The mechanisms which regulate mucosal IgA responses to orally administered protein vaccines are not yet fully elucidated. We have used two delivery systems, soluble tetanus toxoid (TT) with the mucosal adjuvant cholera toxin (CT) and recombinant Salmonella expressing Tox C, a fragment of TT, to assess the nature of CD4+ T helper (Th) cells and derived cytokines which support mucosal IgA responses in both normal and cytokine knockout (interferon gamma knockout; IFN-gamma-/- and IL-4-/-) mice. Our results provide important new information regarding Th cell and cytokine regulation of mucosal IgA responses. Whereas TT coadministered with CT induces predominant TT-specific Th2-type responses, rSalmonella delivery of Tox C induced dominant Th1-type responses along with synthesis of the Th2-cytokine IL-10. Both vaccine regimen elicited high levels of mucosal S-IgA and IL-6 production by macrophages. Further oral immunization of IFN-gamma-/- and IL-4-/- mice with rSalmonella Tox C also induced macrophage-derived IL-6 and Th2-derived IL-10 as well as S-IgA responses, suggesting that IFN-gamma from Th1-type cells as well as traditional Th2 cells producing IL-4 and IL-5 are not essential for mucosal IgA responses. Rather, induction of second level Th2 cells producing IL-10 together with high levels of IL-6 from other cell sources may be sufficient for mucosal IgA responses in the absence of traditional Th2 cells. These studies were facilitated by the development of a sensitive new luminometry assay which allowed detection of cytokines and cell surface molecules which are below the levels of detection by current solid phase assays.

Expression of fragment C of tetanus toxin fused to a carboxyl-terminal fragment of diphtheria toxin in Salmonella typhi CVD 908 vaccine strain.

We report the expression of fragment C of tetanus toxin (FC) fused to the eukaryotic cell binding domain (the carboxyl-terminus) of diphtheria toxin (FC-bDt fusion) in attenuated Salmonella typhi live vector vaccine strain CVD 908. The FC-bDt protein fusion was constructed using plasmid pTETnir15 which carries the gene encoding FC under control of the nirB promoter (nirBP). The open reading frame for FC was modified to incorporate an in-frame glycine-proline hinge region and a set of four restriction sites at the 3' end of the FC gene. A 482 bp DNA fragment encoding the eukaryotic cell binding domain of diphtheria toxin was then inserted at the 3' end of the modified FC gene to create an in-frame FC-bDt fusion gene. The resulting plasmid, pOG215, was able to express the FC-bDt fusion protein in both Escherichia coli DH5a and S. typhi CVD 908, as evidenced by Western immunoblots using anti-FC and anti-C-terminal diphtheria toxin monoclonal antibodies. Maximum expression of the FC-bDt fusion protein was achieved by growing CVD 908(pOG215) at the low oxidation-reduction potential of thioglycollate broth, i.e. in conditions that activate nirBP and drive transcription of the FC-bDt fusion gene. Whereas maximum expression of FC alone was also observed using thioglycollate broth, expression of bDt alone was unsuccessful using a variety of growth conditions. FC fusions constitute one strategy to "rescue" expression of proteins which are otherwise difficult to express.

Influence of preimmunization with tetanus toxoid on immune responses to tetanus toxin fragment C-guest antigen fusions in a Salmonella vaccine carrier.

We have previously described a new system for the delivery of recombinant antigens in live Salmonella vaccines as genetic fusions to the C terminus of fragment C of tetanus toxin (TetC) driven by the anaerobically inducible nirB promoter. It has been reported that preimmunization with tetanus toxoid (TT) can suppress the antibody response to peptides chemically coupled to TT (epitope-specific suppression) in both animals and humans, which could interfere with efficacy of the Salmonella-TetC delivery system. We report that preimmunization of BALB/c mice with TT in alum did not suppress the response to either of two protective antigens of Schistosoma mansoni, the full-length S. mansoni P28 glutathione S-transferase (P28) and a construct consisting of eight tandem copies of the protective peptide comprising amino acids 115 to 131 of P28. The guest antigens were expressed in the aroA Salmonella typhimurium SL3261 vaccine strain as fusions to TetC. Preimmunization with TT 10 weeks before administration of the recombinant salmonellae did not alter the antibody response to the full-length P28, whereas the response to the peptide comprising amino acids 115 to 131 was increased by preimmunization with TT, with the increase seen mainly in the immunoglobulin G1 isotype. The antitetanus response was increased by preimmunization with TT in all groups receiving salmonellae expressing TetC. The results could be important when one is considering the use of the Salmonella-TetC delivery system in populations preimmunized with TT.

Protection against Leishmania major infection in genetically susceptible BALB/c mice by gp63 delivered orally in attenuated Salmonella typhimurium (AroA- AroD-).

The gene encoding the Leishmania major (L. major) promastigote surface glycoprotein, gp63, was introduced into the Salmonella typhimurium (S. typhimurium) aroA- aroD- live oral vaccine strain BRD509 and expressed under the control of a constitutive tac promoter in plasmid pKK233-2. This construct (GID101) expressed gp63 in vitro and was used to immunize highly susceptible BALB/c mice by the oral route. The plasmid was relatively stably inherited by bacteria growing or persisting in the mesenteric lymph nodes of immunized mice. Mice immunized with GID101 developed significant resistance against a challenge infection with L. major compared to controls immunized with BRD509 alone. Spleen and lymph node cells from immunized mice developed a strong in vitro proliferative T-cell response to killed or live L. major. The activated T cells secreted interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) which was abrogated by treatment with anti-CD4 but not with anti-CD8 antibody. The cells did not produce detectable levels of interleukin-4 (IL-4). The immunized mice also produced significant amounts of leishmanial specific IgG2a antibody but did not develop delayed-type hypersensitivity (DTH) to live parasites. No IgG1 antibody was detected. These data therefore demonstrate that gp63 gene delivered orally by a vaccine strain of S. typhimurium can preferentially induce the development of Th-1 subset of CD4+ T cells and protective immunity in the highly susceptible BALB/c mice.

The development of oral vaccines against parasitic diseases utilizing live attenuated Salmonella.

Genetically defined, live attenuated Salmonella vaccines are proving useful both as oral vaccines against salmonellosis and for the development of multivalent vaccines based on the expression of heterologous antigens in such strains. Several candidate attenuated S. typhi strains are at present being evaluated as new single dose oral typhoid vaccines in human volunteers. The emergence of such a vaccine will facilitate the development of multivalent vaccines for humans. Many antigens from different infectious organisms have been expressed in attenuated Salmonella. A focus of this work has been on developing vaccines against parasitic diseases. This review will summarize the efforts that have been made in this area.

Oral vaccine models: multiple delivery systems employing tetanus toxoid.

We have not yet directly examined the Th cell responses induced by using Salmonella/BRD 847 as a vector nor have we performed these experiments following immunization with microspheres. However, production of high serum levels of antigen-specific IgG1 may be indicative of a Th2-type response, whereas high serum levels of IgG2a may reflect a Th1-type response. An important issue in using various oral delivery systems is whether the system(s) employed affects the Th cell response to the same antigen. We therefore analyzed the serum antigen-specific IgG subclasses induced in each of the model systems we studied. Table 2 presents these results. Clearly, oral administration of soluble TT with CT as an adjuvant induced an IgG1 subclass, and encapsulation of TT within microspheres had no effect on this Th2-type response. On the other hand, oral immunization with live Salmonella expressing fragment C of tetanus toxin induced a strong IgG2a subclass response indicative of a Th1-type response. It should be noted that protection against a lethal TT challenge was afforded by elevated levels of both anti-TT IgG1 and IgG2a subclasses. We intend to examine the cytokine profiles in spleen CD4+ T cells from mice immunized with microspheres or Salmonella following in vitro antigen stimulation to confirm the T helper type responses suggested by the IgG subclass data. It will also be important to examine the cytokine patterns induced in Peyer's patch CD4+ T cells following immunization of C57BL/6 with Salmonella/BRD 847. Whereas analysis of the serum IgG subclass profile indicated a strong IgG2a response and thus a systemic Th1-type pattern, this vector also induced a good mucosal IgA response. If our current hypothesis concerning S-IgA production is correct, we would expect a predominant Th2-type profile in CD4+ T cells from Peyer's patch in these mice. Such a result would emphasize the bifurcation of T-cell responses in the systemic versus the mucosal immune environments. The data obtained to data suggest that adjuvants and various vehicle delivery systems may influence the induction of distinct T helper cell subsets to a specific antigen. The unique cytokine arrays produced by these T-cell subsets influence the immune responses in terms of systemic Ig subclasses produced, cell-mediated immune responses, and the production of mucosal S-IgA antibodies. Although additional studies are necessary, the manipulation of T-cell subsets employing adjuvants, antigen packaging, or perhaps even the addition of individual cytokines to various formulations holds significant promise for optimizing immune responses to orally administered vaccines.

Role of hns in the virulence phenotype of pathogenic salmonellae.

A TnphoA-generated mutant C5060, attenuated for virulence, was derived from the mouse-virulent Salmonella typhimurium strain C5. This mutation, designated hns-112::TnphoA, harbours the transposon in the 3' end of hns, with the alkaline phosphatase open reading frame in the opposite orientation to that of hns. Bacterial strains harbouring hns-112::TnphoA were mucoid and had altered levels of DNA supercoiling, as monitored using pUC18 as a reporter plasmid. Transduction of hns-112::TnphoA into mouse virulent strains, including S. typhimurium SL1344 and Salmonella enteritidis Se795, resulted in attenuation. When an independent hns mutation, harbouring a kanamycin-resistance cassette inserted into the Kpnl site at base pair 237 of the hns gene, was introduced into S. typhimurium C5, the isolates were also attenuated. S. typhimurium C5 isolates harbouring the multicopy plasmid pGB651, which encodes the Escherichia coli hns gene, were partially attenuated in mice. Transductional analysis, using Tn10 insertions located close to the hns gene, showed that virulence could be restored in genetic crosses that eliminated the resident hns mutations. However, some hns+ transductants were still attenuated, suggesting that secondary attenuating lesions can accumulate in hns-deficient strains. These studies show that the hns locus plays a role in Salmonella virulence.

Chimeric hepatitis B core antigen particles containing B- and Th-epitopes of human papillomavirus type 16 E7 protein induce specific antibody and T-helper responses in immunised mice.

The use of hepatitis B core antigen (HBcAg) as an immunogenic delivery vehicle for foreign epitopes has been reported. Here we report the insertion of DNA sequences encoding immunodominant linear B-epitopes and a "universal" T-helper epitope of human papillomavirus (HPV) type 16 E7 transforming protein into the full-length HBcAg gene cloned into an inducible bacterial expression plasmid (pPN1.0). The resulting chimeric proteins assembled into particles which were highly immunogenic. Mice immunised with particles containing one or two of the three E7 B-epitopes under consideration produced strong epitope-specific antibody responses with both IgG and IgG2a components which recognised eukaryotic E7. T-proliferative responses were elicited to the E7 T-epitope as well as HBcAg T-epitope(s). Lymph node cells from immunised mice produced IL-2 and IL-4 when specifically recalled in vitro, indicating stimulation of both Th1 and Th2 helper cell compartments. Since HBcAg particles can be administered in adjuvant acceptable for human application and can elicit mucosal responses after nasal or oral immunisation, these results have implications for vaccine design in HPV 16-associated anogenital cancer.

Use of the nirB promoter to direct the stable expression of heterologous antigens in Salmonella oral vaccine strains: development of a single-dose oral tetanus vaccine.

Plasmid pTETnir15, which directs the expression of the non-toxic immunogenic fragment C of tetanus toxin from the anaerobically inducible nirB promoter, was introduced into the Salmonella typhimurium aroA aroD live oral vaccine strain BRD509. The resulting strain, designated BRD847, was used to vaccinate orally BALB/c mice and was tested for plasmid stability and its ability to protect against a lethal tetanus toxin challenge. pTETnir15 was stably inherited by bacteria growing or persisting in the tissues of immunized mice whereas another BRD509 derivative, designated BRD753, harboring plasmid pTET85 which directs fragment C expression from the tac promoter, was highly unstable. Mice immunized with a single oral dose of BRD847 developed high levels of circulating anti-fragment C antibodies and were solidly protected against tetanus toxin challenge. Mice immunized with a single oral dose of BRD753 developed no detectable anti-fragment C antibodies. After boosting, antibodies were detected, but the mice were only partially protected against tetanus toxin challenge. Thus the use of an in vivo inducible promoter such as nirB may be a generally applicable approach to obtaining the stable in vivo expression of heterologous antigens in Salmonella vaccine strains.

Impaired resistance to infection does not increase the virulence of Salmonella htrA live vaccines for mice.

We have described a new class of live attenuated salmonella vaccines harbouring lesions in htrA, a stress protein gene previously. The virulence and invasiveness of Salmonella htrA mutants was investigated in three models of increased susceptibility to Salmonella infection. These included BALB/c mice, either given sublethal whole body irradiation (350 R) or administered rabbit anti-TNF alpha antiserum, and (CBA/NfemaleXBALB/cmale)F1 male mice which express the xid sex-linked B cell defect of CBA/N mice and are more susceptible to salmonellae than female littermates. Salmonella typhimurium htrA mutants derived from virulent strains, C5046 (C5 htrA::TnphoA) and BRD726 (SL1344 delta htrA) were not more invasive in immunosuppressed mice than in normal controls in the three mouse models of defective immunity. The results indicate that susceptibility to S. typhimurium htrA vaccines derived from virulent parents is not enhanced by conditions of impaired resistance to infection.

Evaluation of Salmonella typhimurium strains harbouring defined mutations in htrA and aroA in the murine salmonellosis model.

Derivatives of the mouse-virulent Salmonella typhimurium strain SL1344 were constructed harbouring defined mutations in htrA, aroA or htrA aroA combined. When administered orally or intravenously to BALB/c mice, all the mutants were found to be highly attenuated. All mutants were able to confer significant protection against lethal challenge with SL1344 after a single oral dose of live organisms. SL1344 htrA mutants persisted in livers and spleens at a lower level than SL1344 aroA mutants after intravenous administration. SL1344 htrA aroA mutants persisted at an even lower level and were cleared from the livers and spleens of mice within 21 days of intravenous administration. Thus htrA and htrA aroA mutants can be considered as potential oral vaccines against salmonellosis.