Escherichia coliHeat-Labile Enterotoxin B Subunit Is a More Potent Mucosal Adjuvant than Its Closely Related Homologue, the B Subunit of Cholera Toxin

Abstract

Although cholera toxin (Ctx) and Escherichia coli heat-labile enterotoxin (Etx) are known to be potent mucosal adjuvants, it remains controversial whether the adjuvanticity of the holotoxins extends to their nontoxic, receptor-binding B subunits. Here, we have systematically evaluated the comparative adjuvant properties of highly purified recombinant EtxB and CtxB. EtxB was found to be a more potent adjuvant than CtxB, stimulating responses to hen egg lysozyme when the two were coadministered to mice intranasally, as assessed by enhanced serum and secretory antibody titers as well as by stimulation of lymphocyte proliferation in spleen and draining lymph nodes. These results indicate that, although structurally very similar, EtxB and CtxB have strikingly different immunostimulatory properties and should not be considered equivalent as prospective vaccine adjuvants.Oral or nasal mucosal administration of protein antigen is thought to favor a state of immunological unresponsiveness by a process of peripheral tolerance, in order to avoid activation of detrimental immune responses to innocuous dietary and airborne environmental antigens (11,23). Cholera toxin (Ctx) and heat-labile enterotoxin (Etx) from enterotoxinogenic strains of Escherichia coli, however, are strongly immunogenic by both parenteral and mucosal routes and elicit strong antitoxin antibody responses when administered either orally or nasally (reviewed in reference 21). Ctx and Etx are also recognized as two of the most potent mucosal adjuvants yet identified, able to greatly enhance antibody responses to coadministered antigens (for recent reviews, see references 17 and 25). However, their toxicity is likely to preclude usage in human vaccines.The toxins are hetero-oligomeric complexes each composed of an enzymatic A subunit and five identical B subunits (9). The A subunit is responsible for toxicity, catalyzing ADP ribosylation of G s␣ , increasing cyclic AMP (cAMP) levels, and producing chloride efflux and fluid loss. The B subunits are arranged in a pentameric ring and contain five receptor-binding pockets for high-avidity association with cellular membranes containing GM 1 ganglioside. The nontoxic B subunits, CtxB and EtxB, are also potent immunogens, avoiding tolerance induction when administered mucosally and generating strong secretory and systemic antibody responses (reviewed in references 17, 21, and 25).While early studies suggested that the adjuvant activities of Ctx and Etx were completely dependent on the ability of the catalytic A subunit to raise intracellular cAMP levels and that nontoxic derivatives were devoid of adjuvanticity, it is now well established that this is not the case. Dickinson and Clements (2) constructed a single point mutation in EtxA which led to reduced ADP ribosylation activity, cAMP elevation, and cellular toxicity, though adjuvanticity was retained (5). Other completely nontoxic point mutants in both CtxA (8,(26)(27)(28) and EtxA (4, 6) are now being extensively characterized for their utility in promoting imm...

Immune Responses to Novel Pneumococcal Proteins Pneumolysin, PspA, PsaA, and CbpA in Adenoidal B Cells from Children

“…Differences between individuals may be responsible in part. Mucosal adjuvants, such as cholera toxin, E. coli heat-labile toxin, or their detoxified subunits, could improve or modify antigen-specific mucosal IgA memory responses (16,33,50). Further studies of larger numbers of individuals with more detailed clinical and immunological assessment of prior exposure to pneumococci and more extensive and detailed modeling of immune responses in in vitro culture with purified recombinant proteins will further improve our understanding of the potential of these proteins as mucosal vaccines in humans.…”

A mucosally targeted subunit vaccine candidate eliciting HIV-1 transcytosis-blocking Abs

“…To allow the efficient presentation of the P1 peptide to the mucosaassociated lymphoid tissue, while maintaining the structural integrity of the P1 peptide, we created a translational fusion between the C terminus of CTB and the peptide. The epithelialbinding subunits of bacterial enterotoxins such as CTB (41,42) and the homologous B subunit of the heat labile toxin of E. coli (31,34,43) are known to be potent mucosal immunogens, which can efficiently present fused antigenic determinants to the immune system. Based on the ability of CTB-P1, but not CTB, to bind GalCer in vitro (data not shown), and the aggregation of CTB-P1 oligomers into larger oligomeric structures (Figs.…”

Subacute Effects of Maize-Expressed Vaccine Protein, Escherichia coli Heat-Labile Enterotoxin Subunit B (LTB), on the Springtail, Folsomia candida, and the Earthworm, Eisenia fetida

“…Because of its strong immunogenicity without adverse effects on human health, LTB has been widely studied as an oral vaccine against LT-induced diarrhea disease (5). In addition to its effectiveness as a vaccine, LTB is also known as a potent mucosal adjuvant, which enhances the mucosal immunogenicity of co-administered vaccines (6). For the establishment of safe, effective, and reasonable-cost application of LTB to human and veterinary health treatments, plant-based LTB production has been investigated in several plants, such as potatoes, maize, and tobacco (7).…”