Prospects for oral vaccination using recombinant bacteria expressing viral epitopes

Salmonella enterica serovar Typhi Ty21a expressing human papillomavirus type 16 L1 as a potential live vaccine against cervical cancer and typhoid fever

Human papillomavirus (HPV) vaccines based on L1 virus-like particles (VLPs) can prevent HPV-induced genital neoplasias, the precursors of cervical cancer. However, most cervical cancers occur in developing countries, where the implementation of expensive vaccines requiring multiple injections will be difficult. A live Salmonella-based vaccine could be a lower-cost alternative. We previously demonstrated that high HPV type 16 (HPV16)-neutralizing titers are induced after a single oral immunization of mice with attenuated Salmonella enterica serovar Typhimurium strains expressing a codon-optimized version of HPV16 L1 (L1S). To allow the testing of this type of vaccine in women, we constructed a new L1-expressing plasmid, kanL1S, and tested kanL1S recombinants of three Salmonella enterica serovar Typhi vaccine strains shown to be safe in humans, i.e., Ty21a, the actual licensed typhoid vaccine, and two highly immunogenic typhoid vaccine candidates, Ty800 and CVD908-htrA. In an intranasal mouse model of Salmonella serovar Typhi infection, Ty21a kanL1S was unique in inducing HPV16-neutralizing antibodies in serum and genital secretions, while anti-Salmonella responses were similar to those against the parental Ty21a vaccine. Electron microscopy examination of Ty21a kanL1S lysates showed that L1 assembled in capsomers and capsomer aggregates but not well-ordered VLPs. Comparison to the neutralizing antibody response induced by purified HPV16 L1 VLP immunizations in mice suggests that Ty21a kanL1S may be an effective prophylactic HPV vaccine. Ty21a has been widely used against typhoid fever in humans with a remarkable safety record. These finds encourage clinical testing of Ty21a kanL1S as a combined typhoid fever/cervical cancer vaccine with the potential for worldwide application.

Improved efficiency of a Salmonella-based vaccine against human papillomavirus type 16 virus-like particles achieved by using a codon-optimized version of L1

Cervical cancer results from cervical infection by human papillomaviruses (HPVs), especially HPV16. An effective vaccine against these HPVs is expected to have a dramatic impact on the incidence of this cancer and its precursor lesions. The leading candidate, a subunit prophylactic HPV virus-like particle (VLP) vaccine, can protect women from HPV infection. An alternative improved vaccine that avoids parenteral injection, that is efficient with a single dose, and that induces mucosal immunity might greatly facilitate vaccine implementation in different settings. In this study, we have constructed a new generation of recombinant Salmonella organisms that assemble HPV16 VLPs and induce high titers of neutralizing antibodies in mice after a single nasal or oral immunization with live bacteria. This was achieved through the expression of a HPV16 L1 capsid gene whose codon usage was optimized to fit with the most frequently used codons in Salmonella. Interestingly, the high immunogenicity of the new recombinant bacteria did not correlate with an increased expression of L1 VLPs but with a greater stability of the L1-expressing plasmid in vitro and in vivo in absence of antibiotic selection. Anti-HPV16 humoral and neutralizing responses were also observed with different Salmonella enterica serovar Typhimurium strains whose attenuating deletions have already been shown to be safe after oral vaccination of humans. Thus, our findings are a promising improvement toward a vaccine strain that could be tested in human volunteers.

Immunogenicity against human papillomavirus type 16 virus-like particles is strongly enhanced by the PhoPc phenotype in Salmonella enterica serovar Typhimurium

Recombinant Salmonella strains have been widely used to deliver heterologous antigens and induce immune responses in vaccinated animals and humans. It remains to be established, however, how these bacteria mount an immune response; this has prevented the rational design of vaccines. Here we report for the first time that a particular genetic program, PhoPc, is necessary for recombinant Salmonella strains to induce an antibody response to a heterologous antigen, the human papillomaviruses type 16 (HPV16) virus-like particle (VLP). The PhoPc phenotype results from a point mutation in phoQ, the gene encoding the sensor component of a two-component regulatory system (PhoP-PhoQ) that controls the expression of a number of virulence factors in Salmonellae. To demonstrate that immunogenicity of the viral antigen expressed by the bacterial vector was dependent on the PhoPc phenotype, we have expressed the phoQ mutant gene (phoQ24) in two differently attenuated Salmonella enterica serovar Typhimurium strains. Our data show extrachromosomal phoQ24 to be dominant over the chromosomal copy of the phoQ gene, conferring the PhoPc phenotype on the recipient strains. In addition, activation of PhoPQ-regulated genes by the plasmid-encoded PhoQ24 did not alter bacterial survival and conferred immunogenicity to the HPV16 VLP expressed in the two S. enterica serovar Typhimurium backgrounds, inducing the production of HPV-specific antibodies in mice. This strongly suggests that at least one of the PhoP-regulated genes is necessary for mounting an efficient antibody response to HPV16 VLP. This finding sets the stage for further development of a Salmonella-based vaccine against HPV infection and cervical cancer.

Nasal vaccination with attenuated Salmonella typhimurium strains expressing the Hepatitis B nucleocapsid: dose response analysis

Nasal vaccination of mice with recombinant attenuated strains of Salmonella typhimurium is more efficient at inducing antibody responses than oral vaccination. However, mortality was observed when high doses [10(9) colony forming unit (CFU)], otherwise safe by the oral route, were administered. This observation was counterbalanced by the fact that nasal vaccination was still highly efficient with lower doses (10(6) CFU), which are inefficient by the oral route and this, without any incidents of mortality. Here, we further analyse in mice the effect of nasal vaccination with differently attenuated S. typhimurium strains expressing the Hepatitis B nucleocapsid (HBc). Surprisingly, as few as 100 CFU were sufficient to induce a maximal HBc specific antibody response, but only if the bacteria were inhaled. Furthermore, we observed no correlation between the inoculum dose and the number of surviving bacteria in cervical lymph nodes and spleen. Examination of lung sections revealed strong inflammation and bronchopneumonia 24 h after nasal vaccination with 10(8) CFU, while only minor signs of inflammation were detected transiently when 10(3) CFU or phosphate buffered saline (PBS) were administered. Our data suggest that the safety issue of nasal vaccination with low doses of the Salmonella vaccine strains should be addressed in humans, as it might be an efficient alternative to oral vaccination.

The nature of the attenuation of Salmonella typhimurium strains expressing human papillomavirus type 16 virus-like particles determines the systemic and mucosal antibody responses in nasally immunized mice

We have recently shown by using a recombinant Salmonella typhimurium PhoPc strain in mice the feasibility of using a Salmonella-based vaccine to prevent infection by the genital human papillomavirus type 16 (HPV16). Here, we compare the HPV16-specific antibody responses elicited by nasal immunization with recombinant S. typhimurium strains harboring attenuations that, in contrast to PhoPc, are suitable for human use. For this purpose, chi4989 (Deltacya Deltacrp) and chi4990 [Deltacya Delta(crp-cdt)] were constructed in the ATCC 14028 genetic background, and comparison was made with the isogenic PhoPc and PhoP- strains. Although the levels of expression of HPV16 virus-like particle (VLP) were similar in all strains, only PhoPc HPV16 induced sustained specific antibody responses after nasal immunization, while all strains induced high antibody responses with a single nasal immunization when an unrelated viral hepatitis B core antigen was expressed. The level of the specific antibody responses induced did not correlate with the number of recombinant bacteria surviving in various organs 2 weeks after immunization. Our data suggest that the immunogenicity of attenuated Salmonella vaccine strains does not correlate with either the number of persisting bacteria after immunization or the levels of in vitro expression of the antigen carried. Rather, the PhoPc phenotype appears to provide the unique ability in Salmonella to induce immune responses against HPV16 VLPs.

HBV core particles allow the insertion and surface exposure of the entire potentially protective region of Puumala hantavirus nucleocapsid protein

Core particles of the hepatitis B virus (HBV) potentiate the immune response against foreign epitopes presented on their surface. Potential insertion sites in the monomeric subunit of the HBV core protein were previously identified at the N- and C-terminus and in the immunodominant c/e1 region. In a C-terminally truncated core protein these sites were used to introduce the entire 120 amino acid (aa)-long potentially immunoprotective region of the hantavirus (serotype Puumala) nucleocapsid protein. The N- and C-terminal fusion products were unable to form core-like particles in detectable amounts. However, a suppressable stop codon located between the HBV core and the C-terminally fused hantavirus sequence restored the ability to form particles ('mosaic particles'); in contrast to the C-terminal fusion product the mosaic construct allowed the formation of particles built up by the core protein itself and the HBV core-Puumala nucleocapsid-readthrough protein. The mosaic particles exposed the 120 aa region of the PUU nucleocapsid protein on their surface as demonstrated by ELISA and immuno electron microscopy applying different monoclonal antibodies. Insertion of the hantaviral sequence into the c/e1 region not only allowed the formation of chimeric particles, but again the surface accessibility of the sequence. HBV core antigenicity itself was, however, reduced in the particles carrying insertions in the c/e1 region, probably due to a masking effect of the 120 aa long insert.

A new membrane-bound OprI lipoprotein expression vector. High production of heterologous fusion proteins in gram (-) bacteria and the implications for oral vaccination

We have previously described the development of cloning vectors for the production of OprI-based outer membrane fusion proteins in E. coli (Cornelis et al., 1996) and now describe the construction of a new vector, containing a lacI(q) gene, resulting in tight repression of the promotor and allowing its use in other Gram (-) bacteria. The new pVUB3 expression vector encodes a truncated but active LacI(q)(341) repressor which binds to the single operator in the vector. A high repression of the trc promotor was observed, resulting in a very low basal leakage of expression and very high production levels of OprI or derivatives after IPTG induction in E. coli. Bacterial viability was not affected under uninduced conditions, but the number of viable cell counts decreased after production of large amounts of the outer membrane-bound OprI lipoprotein and its derivatives, both in E. coli and Salmonella typhimurium. This highly repressible system allows us to extend the use of OprI vectors in other Gram (-) bacteria, resulting in the production of outer membrane-bound lipid-modified molecules, opening the possibility for its application in the design of potential live Salmonella-based subunit vaccines.

A live recombinant avirulent oral Salmonella vaccine expressing pneumococcal surface protein A induces protective responses against Streptococcus pneumoniae

A live oral recombinant Salmonella vaccine strain expressing pneumococcal surface protein A (PspA) was developed. The strain was attenuated with Deltacya Deltacrp mutations. Stable expression of PspA was achieved by the use of the balanced-lethal vector-host system, which employs an asd deletion in the host chromosome to impose an obligate requirement for diaminopimelic acid. The chromosomal Deltaasd mutation was complemented by a plasmid vector possessing the asd+ gene. A portion of the pspA gene from Streptococcus pneumoniae Rx1 was cloned onto a multicopy Asd+ vector. After oral immunization, the recombinant Salmonella-PspA vaccine strain colonized the Peyer's patches, spleens, and livers of BALB/cByJ and CBA/N mice and stimulated humoral and mucosal antibody responses. Oral immunization of outbred New Zealand White rabbits with the recombinant Salmonella strain induced significant anti-PspA immunoglobulin G titers in serum and vaginal secretions. Polyclonal sera from orally immunized mice detected PspA on the S. pneumoniae cell surface as revealed by immunofluorescence. Oral immunization of BALB/cJ mice with the PspA-producing Salmonella strain elicited antibody to PspA and resistance to challenge by the mouse-virulent human clinical isolate S. pneumoniae WU2. Immune sera from orally immunized mice conferred passive protection against otherwise lethal intraperitoneal or intravascular challenge with strain WU2.

A murine model of intranasal immunization to assess the immunogenicity of attenuated Salmonella typhi live vector vaccines in stimulating serum antibody responses to expressed foreign antigens

The lack of a practical small animal model to study the immunogenicity of Salmonella typhi-based live vector vaccines expressing foreign antigens has seriously impeded the vaccine development process. For some foreign antigens, stimulation of serum IgG antibody is the desired, protective immune response. We administered to mice, by orogastric or intranasal (i.n.) routes, attenuated delta aroC delta aroD S. typhi CVD 908 carrying a plasmid encoding fragment C (fragC) of tetanus toxin fused to the eukaryotic cell receptor binding domain of diphtheria toxin (fragC-bDt), and monitored serum antibody. While orogastric inoculation of three doses was not immunogenic, i.n. immunization elicited high titers of serum IgG tetanus antitoxin, generating peak ELISA geometric mean titers (GMT) of 27024 and 35658 with 10(8) and 10(9) c.f.u. dosages, respectively; 10(9) c.f.u. i.n. of an delta aroA S. typhimurium live vector stimulated a peak antitoxin GMT of 376 405. Mice immunized with the S. typhi live vector were 100% protected against challenge with 100 50% lethal doses of tetanus toxin that rapidly killed all control mice. Intranasal immunization with two doses of S. typhi expressing unfused fragment C under control of an anaerobically-activated promoter derived from nirB stimulated significantly higher titers of serum neutralizing antitoxin than fused fragC-bDt controlled by the same promoter (GMT 0.10 AU ml-1 vs 0.01 AU ml-1, P = 0.0095). Two i.n. doses of S typhi encoding fragC under control of powerful constitutive promoter 1pp led to significantly higher peak serum neutralizing antitoxin titers than the otherwise identical construct utilizing the nirB promoter (peak GMT 0.72 AU ml-1 vs 0.10 AU ml-1, P = 0.022). The i.n. route of inoculation of mice may constitute a practical breakthrough that could expedite the development of some S. typhi-based live vector vaccines by allowing, for the first time, quantitative measurement of serum antibody responses to candidate constructs following i.n. mucosal immunization.

Oral and rectal immunization of adult female volunteers with a recombinant attenuated Salmonella typhi vaccine strain

An attenuated strain of Salmonella typhi delta(cya) delta(crp-cdt) delta(asd) expressing a gene encoding a hepatitis B virus core-pre-S protein was tested in female adult volunteers for its ability to elicit a systemic and a mucosal immune response. Specifically, our purpose was to evaluate the potential of such a vaccine strain to induce specific secretory immunoglobulin A (sIgA) at genital and rectal surfaces. Oral and rectal routes of immunization were compared: oral immunization induced seroconversion against the bacterial lipopolysaccharide (LPS) in six out of seven volunteers, while after rectal immunization only one out of six volunteers seroconverted against LPS. To our disappointment, the latter volunteer was also the only one who seroconverted against the carried antigen (pre-S1), demonstrating the poor ability of this live vaccine to induce an immune response against the carried antigen. Anti-LPS sIgA was found in both the vaginal and cervical secretions of a volunteer who presented a strong seroconversion after oral immunization (16-fold increase in anti-LPS IgG). Smaller amounts of anti-LPS sIgA were found in the rectal secretions of one orally and one rectally immunized volunteer and in the saliva of three orally and one rectally immunized woman. Our data show for the first time that it is possible to induce specific sIgA in the genital and rectal tracts of women by using an S. typhi vaccine strain.

A recombinant Salmonella typhimurium vaccine induces local immunity by four different routes of immunization

Immunization of mice with an attenuated Salmonella typhimurium strain (Phopc) carrying a plasmid encoding a hybrid form of the hepatitis B virus core antigen (HBc) induced specific antibody responses against the bacterial lipopolysaccharide (LPS) and HBc. Different mucosal routes of immunization, i.e., oral, nasal, rectal, and vaginal, were compared for their ability to induce a systemic as well as a mucosal response at sites proximal or distant to the site of immunization. Anti-LPS and anti-HBc immunoglobulin A (IgA) antibodies were measured in saliva, in feces, and in genital, bronchial, and intestinal secretions. Specific antibodies in serum and secretions were observed after immunization via all routes; however, the response to LPS was independent of that against HBc. In serum, saliva, and genital and bronchial secretions, high amounts of anti-HBc IgA were obtained by the nasal route of immunization. Vaginal immunization resulted in two different responses in mice: high and low. We observed a correlation between the level of specific immune response and the estrous status of these mice at the time of immunization. Rectal immunization induced high amounts of IgA against HBc and LPS in colonorectal secretions and feces but not at distant sites. These data suggest that S. typhimurium is able to invade different mucosal tissues and induce long-lasting local IgA responses against itself and a carried antigen after a single immunization.

Intestinal immunization of mice with antigen conjugated to anti-MHC class II antibodies

We have explored a new technique for immunization of the intestinal tract of mice, using protein antigens bound to antibodies with specificity for murine MHC class II molecules (MHC-II). Either of two protein antigens, hen avidin (AV) or hen egg lysozyme (HEL) were covalently conjugated to anti-MHC-II antibodies and the purified conjugates were given orally (p.o.) or by direct intraduodenal (i.d.) injection into the intestinal lumen of mice. A secondary immunization p.o. with the same conjugate or with the non-conjugated antigen in the presence of cholera toxin (CTX) resulted in production of both intestinal secretory IgA and serum IgA antibody by those mice. In addition, serum IgG antibodies were produced. Conjugates with appropriate MHC-II specificity targeted the antigen because they induced more IgA and IgG antibody than conjugates with irrelevant antibody specificity or antigen alone, and because they induced antibody in mice that were genetic low responders to antigen. The results indicate the feasibility of oral subunit type vaccines with antibody targeting technology.

Mucosal immunity and strategies for novel microbial vaccines

Infectious diseases continue to be the leading cause of morbidity and mortality worldwide. Increased awareness of the fact that mucosal membranes are the most frequent portals of entry of pathogenic microorganisms has prompted studies aimed at the development of vaccination protocols and antigen delivery systems that would lead to an increased protection of mucosae. Although systemic and strictly local immunizations are of limited effectiveness in the induction of mucosal protection, ingestion or inhalation of antigens results in a generalized immune response manifested by the appearance of specific antibodies of the secretory immunoglobulin (Ig) isotype in external secretions due to the dissemination of IgA precursor cells from IgA-inductive lymphoid tissues. Furthermore, additional inductive sites strategically positioned at the opening of the respiratory and digestive tracts may also be suitable targets for induction of immune responses at desired effector sites. To prevent degradation and the increase of ingested antigens absorption, novel strategies including enclosure of antigens into biodegradable microspheres, liposomes or their expression in viral and bacterial vectors and plants are currently being considered. Forthcoming technological advances in antigen preparation and routes of delivery will undoubtedly have a profound impact on immunization practices in the future.

Protection of immunosuppressed mice against translocation of Pseudomonas aeruginosa from the gut by oral immunization with recombinant Pseudomonas aeruginosa outer membrane protein I expressing Salmonella dublin

AroA Salmonella dublin was used as recipient for a plasmid coding for the outer membrane protein I (OprI) of Pseudomonas aeruginosa. Oral immunization of Balb/c mice with recombinant S. dublin induced serum IgG and IgA antibodies against P. aeruginosa. In spleen and Peyer's patches anti-P. aeruginosa IgG- and IgA-secreting cells could be measured by the ELISPOT technique. In an oral challenge of immunosuppressed mice with P. aeruginosa the orally immunized animals had a 58-fold higher LD50 than control animals.

Hybrid hepatitis B virus core-pre-S proteins synthesized in avirulent Salmonella typhimurium and Salmonella typhi for oral vaccination

Avirulent salmonellae expressing foreign genes are attractive for use as oral vaccine carriers. To facilitate the stable expression of heterologous genes without conferring antibiotic resistance, a deletion of the asdA1 gene was introduced into Salmonella typhimurium and S. typhi delta cya delta crp mutant vaccine strains. An asd-complementing plasmid expressing hybrid hepatitis B virus nucleocapsid-pre-S (HBcAg-pre-S) particles was constructed. These hybrid HBcAg-pre-S particle genes were stably expressed in S. typhimurium and S. typhi delta cya delta crp mutant vaccine strains in this balanced, lethal host-vector combination. A single oral immunization of BALB/c mice with a recombinant S. typhimurium delta cya delta crp mutant synthesizing hybrid HBcAg-pre-S elicited potentially virus-neutralizing anti-pre-S serum immunoglobulin G antibodies. In addition, serum immunoglobulin G recognizing S. typhimurium lipopolysaccharide was induced. Distribution in tissue after oral immunization was analyzed in one plasmid-strain combination. The recombinant S. typhimurium colonized the gut-associated lymphoid tissue and the spleen and persisted for over 4 weeks, retaining the HBcAg-pre-S expression plasmid. An isogenic virulence plasmid-cured S. typhimurium delta cya delta crp strain expressing the same HBcAg-pre-S gene had reduced immunogenicity for the carried antigen after oral immunization.

Desirable immunologic characteristics for the development of an ideal vaccine

The efficacy of vaccines for prophylactic use is a function of the immune response elicited by activated lymphocytes. Based on the current understanding of these responses, their induction, and the most effective ways to obtain long-lived immunity, a novel protocol for the vaccination of children against seven childhood diseases, involving only two visits for vaccine administration, is proposed.

Delivery of recombinant gene products with microencapsulated cells in vivo

If established cultured cell lines genetically modified to secrete desired gene products could be implanted in different allogeneic recipients without immune rejection, novel gene products would be delivered more cost effectively. We tested this strategy by encapsulating mouse Ltk- cells transfected with the human growth hormone (hGH) gene in immunoprotective perm-selective alginate microcapsules. Allogeneic mice implanted with these microcapsules demonstrated hGH in their circulation (0.1-1.5 ng/ml serum) within the first 2 weeks. Control mice implanted with only the transfected cells without microcapsules did not demonstrate significant levels of circulating hGH. By about 3 weeks, antibodies against hGH developed in the microcapsule-implanted mice. The immune response was detected only against the hGH and no other secretory products from the transfected cells. The antibody titer continued to escalate for more than three months, thus demonstrating indirectly the continued delivery of the growth hormone. The persistent expression of the transgene and survival of the transfected cells were verified when the microcapsules were retrieved periodically to demonstrate that the encapsulated cells remained viable, proliferative, and productive of hGH even by 78-111 days. In conclusion, delivering gene products with genetically modified allogeneic cells in vivo has been shown feasible for prolonged periods. This technology should have potential applications in somatic gene therapy and in treatment of other somatic diseases.

A virulent Salmonella expressing hybrid hepatitis B virus core/pre-S genes for oral vaccination

This report reviews and extends data on the use of hepatitis B virus (HBV) core (HBcAg) particles as a carrier moiety for B-cell epitopes of the HBV envelope proteins. Virus-neutralizing epitopes of the HBV pre-S region were inserted at the N-terminus, the N-terminus through a precore linker sequence, the C-terminus and an internal position of HBcAg by genetic engineering in Escherichia coli. The hybrid HBc/pre-S proteins were purified and their antigenicity and immunogenicity analysed. All purified HBc/pre-S particles were particulate. Pre-S epitopes inserted at the N-terminus through a precore polylinker, the truncated C-terminus and at the internal position between HBcAg amino acids 75 and 81 were accessible on the particle surface. N-terminal fusions required the presence of the linker sequence to become surface accessible and immunogenic. Fusions to the N- and C-termini of HBcAg did not interfere with HBcAg antigenicity and immunogenicity. In contrast, insertion at the internal site abrogated recognition of HBcAg by five out of six monoclonal antibodies and diminished recognition by human polyclonal anti-HBc antibodies as well as HBcAg immunogenicity. A pre-S(2) sequence fused to the C-terminus of HBcAg was surface accessible and weakly immunogenic. Pre-S(1) sequences fused to the N-terminus through a precore linker were surface accessible and highly immunogenic. The same sequence fused to the core methionine was not surface accessible or immunogenic. Insertion of the same pre-S(1) sequence at an internal position of HBcAg resulted in the most efficient anti-pre-S(1) antibody response.(ABSTRACT TRUNCATED AT 250 WORDS)