Induction and recall of immune memory by mucosal immunization with a non‐toxic recombinant enterotoxin‐based chimeric protein

Abstract

SummaryPrevious reports have suggested that peroral delivery of antigens chemically coupled to non-toxic recombinant enterotoxin B subunits, such as the cholera toxin B subunit (CTB), induces tolerance to the antigen that may be abrogated by the toxic enzyme activity of intact enterotoxins, such as cholera toxin (CT). The aim of this study was to examine the immunogenicity of a genetically coupled protein composed of the saliva-binding region (SBR) of the Streptococcus mutans surface antigen AgI/II and the non-toxic A2 and B subunits of CT (SBR-CTA2/B) compared with that of recombinant SBR admixed with CT (SBR + CT) and SBR chemically coupled to recombinant CTB (SBR-CTB) following peroral delivery by intragastric (i.g.) immunization. The results showed that i.g. immunization with SBR-CTA2/B, like SBR + CT, induced antigen-specific serum immunoglobulin G (IgG) and salivary IgA antibodies, and sensitized splenic T cells. Comparison studies with SBR-CTB produced serum IgG but not salivary IgA titres and failed to sensitize splenic cells. Immunization with SBR-CTA2/B via the intranasal route also primed for the recall of antigen-specific memory antibody responses 6 months later. These findings show that SBR-CTA2/B is an immunogenic, not tolerogenic, chimeric protein that can induce and recall antigen-specific memory responses upon mucosal immunization.

Mucosal Immunization with a Staphylococcus aureus IsdA-Cholera Toxin A2/B Chimera Induces Antigen-Specific Th2-Type Responses in Mice

“…The presence of IsdA-specific IgA in nasal, intestinal, and vaginal fluids after intranasal immunization with IsdA-CTA 2 /B suggests that IgA blasts migrated from the nasalassociated lymphoid tissue into distal mucosal effector sites in the nasal passage and gastrointestinal and genital tracts. Induction of T lymphocyte proliferation by IsdA-CTA 2 /B is consistent with previous studies showing that CT (16,18,40,62). These reports also support our findings that CT and CT derivatives promote more of a Th2-type response.…”

“…Lastly, this report supports the hypothesis that the humoral and cellular responses induced by IsdA-CTA 2 /B are superior to those stimulated by a mixed preparation of antigen and adjuvant (IsdA plus CTA 2 /B). Thus, the structure of the IsdA-CTA 2 /B chimera is optimal for the induction of antigen-specific humoral responses and potentially for presentation on MHC molecules, which is consistent with previous reports of chimeric molecules using distinct antigens (18,36,51). CT and the closely related LTI are gold-standard mucosal adjuvants with a long history of use in animals and more recent use of nontoxic derivatives in humans.…”

“…Another important issue that will be addressed in future protection studies is the potential for the induction of immune unresponsiveness or tolerance by IsdA-CTA 2 /B. While previous studies suggest that CTA 2 /B chimeras can abrogate oral tolerance for up to 6 months, many studies have also shown that chemical conjugation or genetic fusion of antigens directly to CTB induces oral tolerance (18,52). As the induction of tolerance to staphylococcal antigens may be quite detrimental in the case of invasive disease, careful long-term studies that specifically address the induction of tolerance will be required before this vaccine could advance into humans.…”

“…Stable holotoxin-like CTA 2 /B chimeras, where the toxic A 1 domain is replaced with an antigen of interest, possess a number of advantages for use as mucosal vaccines, including the absence of the toxic domain, noncovalent association of the vaccine antigen to a functional CTB subunit, and maintenance of the endoplasmic reticulum (ER)-targeting KDEL motif contained within the CTA 2 domain (22,28). Evidence suggests that CTA 2 /B genetic fusions can activate long-term humoral responses, stimulate protection, and block the promotion of oral tolerance (18,21,23,34).…”

Contribution of a Streptococcus mutans Antigen Expressed by a Salmonella Vector Vaccine in Dendritic Cell Activation

“…Numerous studies in experimental animal models have shown that immunization with AgI/II can induce specific salivary IgA antibody responses that inhibit S. mutans colonization and protect against dental caries (25,46,50). Furthermore, in our previous studies (14) and those of others (12), it was shown that the 42-kDa salivabinding region (SBR) of AgI/II also exhibits good immunogenicity in inducing specific salivary IgA response.…”