Oral delivery of antigens in live bacterial vectors

Oral colonization and immune responses to Streptococcus gordonii: Potential use as a vector to induce antibodies against respiratory pathogens

PURPOSE OF REVIEW

Mucosal immunization should be an excellent method of preventing respiratory infections because the local immunoglobulin A antibodies can neutralize the invading pathogens at the site of entry. Because Streptococcus gordonii, a normal inhabitant of the human oral cavity, can naturally elicit a mucosal immune response, it has been a prime candidate for investigations as a live oral vaccine vector for immunization against respiratory infections.

RECENT FINDINGS

Antigens from a number of respiratory bacteria, such as Bordetella pertussis, and one virus have been expressed extracellularly or on the cell surface of S. gordonii. The antigens expressed were single or multiple proteins from one or more pathogens. The recombinant S. gordonii expressing surface-localized heterologous antigens could colonize and persist in the oral cavity of mice and rats. Oral colonization induced a mucosal immunoglobulin A response and, in some instances, also a systemic immunoglobulin G response to the heterologous antigens. When given parenterally, the heterologous antigens generated a systemic immunoglobulin G response. These findings indicate that antigens expressed by S. gordonii are immunogenic. A new approach to the use of S. gordonii as a vaccine vector is to modulate immune responses by co-expressing cytokines with the antigen.

SUMMARY

The ability to express antigens from respiratory pathogens and induce immune responses during oral colonization suggests that S. gordonii may be developed into a live vector for oral immunization against respiratory infections. The major challenge ahead is to find ways to achieve a high level of immune response following oral colonization.

Flagellin expressed by live Salmonella vaccine strains induces distinct antibody responses following delivery via systemic or mucosal immunization routes

Salmonella flagellin, expressed as flagella in live attenuated vaccine strains, elicits distinct systemic (IgG) and secreted (IgA) antibody responses in mice following delivery via mucosal (nasal/oral) or parenteral (intraperitoneal (i.p.)) immunization routes. Reduced flagellin-specific antibodies were detected either systemically or locally following delivery of flagellated derivatives of aroA Salmonella enterica serovar Dublin SL1438 via the nasal route, the most effective mucosal site for activation of immune responses in mice. In contrast, flagellin represents the most potent Salmonella antigen for the generation of specific serum antibody (IgG) responses following i.p. inoculations. The distinct immunogenic properties of Salmonella flagellin could not be ascribed to deficient colonization, reduced invasive ability or loss of the flagellin expression by the flagellated vaccine strains.

Phase I clinical trials of aroA aroD and aroA aroD htrA attenuated S. typhi vaccines; effect of formulation on safety and immunogenicity

PBCC211, an aroA aroD derivative of S. typhi strain CDC10-80, was tested in phase I trials as a single dose typhoid fever vaccine. Three different vaccine preparations, reconstituted lyophilized bacteria, freshly grown bacteria or lyophilized bacteria reconstituted from sachets, were orally administered to a total of 86 adult volunteers. An aroA aroD htrA strain, PBCC222, was also tested in 38 volunteers. Formulation impacted on the determination of a safe and immunogenic dose; reconstituted lyophilized cultures required higher doses than the broth cultures to stimulate seroconversion. In general, doses which seroconverted the majority of group members produced undesirable symptoms regardless of attenuation or formulation. The inability to separate the presence of symptoms from achieving significant immunogenicity in these aroA aroD or aroA aroD htrA strains precludes their use as single dose typhoid vaccines in the formulations tested. Multiple doses of these strains at a lower, safe level may be effective as vectors for foreign antigens.

Expression and immunogenicity of an 18-residue epitope of HIV1 gp41 inserted in the flagellar protein of a Salmonella live vaccine

A synthetic oligonucleotide specifying residues 735-752 of the product of the env gene of HIV1 IIIB was inserted by blunt-end ligation at restriction sites in the hypervariable, antigenically determinant region IV of two flagellin genes. Its integration, in frame and correct orientation, into gene fliC(d) in plasmid pLS408 allowed production of functional flagella when the plasmid was placed in a flagellin-negative aroA live-vaccine Salmonella dublin strain, SL5928. Bacteria thus made motile were immobilized and agglutinated by anti-(735-752 peptide) serum; expression was also shown by immunoelectron-microscopy and by Western blot of whole-cell lysates. Enzyme immunoassay (EIA) of sera of mice given three doses by intraperitoneal injection of the live-vaccine strain producing chimeric flagellin, or of concentrated flagella from it, showed production of antibody with affinity for the peptide, and in some sera, also for r-gp160. Pooled serum from mice given five i.p. doses of the live vaccine strain expressing the gp41 epitope at the surface of its flagellar filaments had higher titres of anti-peptide and anti-r-gp160 antibody and weak neutralizing activity on the IIIB isolate (90% neutralization at 1/100). The sera of nine mice given two doses of the live vaccine by the oral route had either no or only very low titres of antibody to flagellar antigen d; they were therefore not tested for anti-peptide activity.

Delivery of class I and class II MHC-restricted T-cell epitopes of listeriolysin of Listeria monocytogenes by attenuated Salmonella

Using a Salmonella vaccine-Listeria infection model of intracellular infection, we studied the capacity of an attenuated strain of Salmonella carrying T-cell epitopes of listeriolysin (LLO) of L. monocytogenes to elicit epitope-specific T-cell responses. Class II (LLO 215-226) or class I (LLO 91-99) MHC-restricted T-cell epitopes of LLO were inserted within a central, hypervariable domain of the flagellin protein of an attenuated delta aroA Salmonella dublin strain. T cells from Listeria-immunized mice were activated by lysates or heat-killed preparations of Salmonella construct expressing the LLO 215-226 epitope, indicating that LLO 215-226 is processed and presented to T cells when offered to antigen-presenting cells as part of a flagellin-epitope fusion protein. The chimeric flagellin genes were integrated into the chromosome of the flagellin-negative S. dublin strain to obtain stable expression of the epitopes. Immunization with the living, chromosomally integrated Salmonella construct carrying LLO 215-226 epitope as part of the flagellin protein generated T cells reactive with the corresponding LLO peptide, indicating that this chimera can stimulate a class-specific immune response in vitro. The effect of flanking residues on the processing and presentation of MHC class I LLO 91-99 epitope was studied using Salmonella vaccine strains that express chimeric flagellins containing one of three LLO 91-99 inserts: 91-99 (normal flagellin amino acids as flanking residues); KK91-99KK (Lys-Lys flanking residues); and AAA91-99AAA (Ala-Ala-Ala flanking residues).(ABSTRACT TRUNCATED AT 250 WORDS)

Immune responses to epitopes inserted in Salmonella flagellin

Plasmid pLS408 includes gene fliC(d) specifying Salmonella flagellin of antigenic type d with an in vitro deletion of a 48 base-pair EcoRV fragment in its central hypervariable antigenically-determinant region IV. Oligonucleotides specifying peptide epitopes of antigens of unrelated pathogens inserted, in correct orientation, at the unique EcoRV site of pLS408 specify chimeric flagellins and, in many instances, cause production of functional flagella when the plasmid is placed in a flagellin-deficient delta aroA live-vaccine strain of Salmonella dublin. The foreign epitope is then exposed at the surface of the flagellar filaments, as shown by the immobilizing effect of anti-epitope antibody and by immunogold electron-microscopy. The live-vaccine strain with a foreign epitope at the surface of its flagella when administered to mice by injection nearly always causes production of antibody with affinity for the foreign epitope and, sometimes, also for the source protein. Repeated injection of the live vaccine with an epitope of Streptococcus pyogenes type 5 M protein as insert caused production of opsonizing antibody and conferred partial protection against Streptococcus challenge. Injection of semi-purified chimeric flagella or flagellin, alone or with adjuvant, likewise causes antibody production, in one instance sufficient to give partial protection against influenza A virus challenge. Plasmid pLS408 with some inserts does not confer motility, either because the filaments produced are non-functional or because flagellin is made but not assembled or because little or no flagellin is produced. The features of a sequence which as insert determine production or non-production of functional flagella are not known. The effect of insertion of known T-cell epitopes and cellular immune responses to epitope inserts in flagellin are as yet little explored.

Molecular analyses of the Salmonella g. . . flagellar antigen complex

Salmonella flagellar filaments are polymers of a highly antigenic protein, termed flagellin. Eight main subfactors have been identified in the Salmonella phase-1 g. . . series flagellar antigen. To determine the molecular basis for expression of the epitopes by which the g. . . family subfactors are distinguished, 10 members of this series were selected and their fliC (the structural gene for phase-1 flagellin) genes were sequenced. Comparative analyses of the inferred primary structures of these flagellins did not allow the identification of linear epitopes responsible for the antigen subfactors. This suggests that conformational aspects are involved in determining the antigenic specificity in these cases. A phylogenetic analysis of the flagellin sequences showed that members of the g. . . series do not form a single coherent unit.