Despite the fact that the adjuvant properties of the heat-labile enterotoxins of Escherichia coli (LT) and Vibrio cholerae (CT) have been known for more than 20 years, there are no available oral vaccines containing these molecules as adjuvants, primarily because they are both very potent enterotoxins. A number of attempts with various degrees of success have been made to reduce or eliminate the enterotoxicity of LT and CT so they can safely be used as oral adjuvants or immunogens. In this report we characterize the structural, enzymatic, enterotoxic, and adjuvant properties of a novel mutant of LT, designated LT(R192G/L211A), or dmLT. dmLT was not sensitive to trypsin activation, had reduced enzymatic activity for induction of cyclic AMP in Caco-2 cells, and exhibited no enterotoxicity in the patent mouse assay. Importantly, dmLT retained the ability to function as an oral adjuvant for a coadministered antigen (tetanus toxoid) and to elicit anti-LT antibodies. In vitro and in vivo data suggest that the reduced enterotoxicity of this molecule compared to native LT or the single mutant, LT(R192G), is a consequence of increased sensitivity to proteolysis and rapid intracellular degradation in mammalian cells. In conclusion, dmLT is a safe and powerful detoxified enterotoxin with the potential to function as a mucosal adjuvant for coadministered antigens and to elicit anti-LT antibodies without undesirable side effects.Bacterially derived enterotoxins, such as the heat-labile enterotoxin (LT) produced by enterotoxigenic Escherichia coli (ETEC) and the closely related cholera enterotoxin (CT) produced by Vibrio cholerae, promote secretory diarrhea during microbial infection. Each enterotoxin is composed of an enzymatically active A-subunit noncovalently associated with a pentameric B-subunit. Induction of intestinal fluid secretion occurs after a series of events involving both changes to toxin structure and activation of intracellular signaling pathways (reviewed in reference 14). The B-subunit mediates binding and internalization of the toxin to cells; subsequent proteolytic cleavage and disulfide bond reduction separate the A-subunit into two domains, the enzymatically active A1 subunit and a smaller A2 peptide. Transport of A1 into the cytoplasm results in ADP-ribosylation of Gs␣, followed by irreversible activation of adenylate cyclase and increases in intracellular levels of cyclic AMP (cAMP). In intestinal epithelial cells, this induction of cAMP causes a disregulation of cAMP-sensitive ion transport mechanisms, inhibiting intracellular salt absorption, increasing electrolyte transport into the gut lumen, and creating an osmotic gradient favoring intestinal water secretion (12).An enzymatically active A-subunit is required for intestinal fluid secretion but also enables LT to act as an oral adjuvant, boosting the immune response to coadministered antigens and inducing both humoral and cellular immune responses in both the systemic and mucosal compartments. However, even low doses of LT (and CT) induce side ef...
Induction of immune responses following oral immunization is frequently dependent upon the co-administration of appropriate adjuvants that can initiate and support the transition from innate to adaptive immunity. The three bacterial products with the greatest potential to function as mucosal adjuvants are the ADP-ribosylating enterotoxins (cholera toxin and the heat-labile enterotoxin of Escherichia coli), synthetic oligodeoxynucleotides containing unmethylated CpG dinucleotides (CpG ODN), and monophosphoryl lipid A (MPL). The mechanism of adjuvanticity of the ADP-ribosylating enterotoxins is the subject of considerable debate. Our own view is that adjuvanticity is an outcome and not an event. It is likely that these molecules exert their adjuvant function by interacting with a variety of cell types, including epithelial cells, dendritic cells, macrophages, and possibly B- and T-lymphocytes. The adjuvant activities of CpG and MPL are due to several different effects they have on innate and adaptive immune responses and both MPL and CpG act through MyD88-dependent and -independent pathways. This presentation will summarize the probable mechanisms of action of these diverse mucosal adjuvants and discuss potential synergy between these molecules for use in conjunction with plant-derived vaccines.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.