The A Subunit of Escherichia coli Heat-Labile Enterotoxin Functions as a Mucosal Adjuvant and Promotes IgG2a, IgA, and Th17 Responses to Vaccine Antigens

Abstract

<jats:title>ABSTRACT</jats:title><jats:p>Enterotoxigenic<jats:named-content xmlns:xlink="http://www.w3.org/1999/xlink" content-type="genus-species" xlink:type="simple">Escherichia coli</jats:named-content>(ETEC) produces both heat-labile (LT) and heat-stable (ST) enterotoxins and is a major cause of diarrhea in infants in developing countries and in travelers to those regions. In addition to inducing fluid secretion, LT is a powerful mucosal adjuvant capable of promoting immune responses to coadministered antigens. In this study, we examined purified A subunit to further understand the toxicity and adjuvanticity of LT. Purified A subunit was enzymatically active but sensitive to proteolytic degradation and unable to bind gangliosides, and even in the presence of admixed B subunit, it displayed low cyclic AMP (cAMP) induction and no enterotoxicity. Thus, the AB5 structure plays a key role in protecting the A subunit from proteolytic degradation and in delivering the enzymatic signals required for secretion. In contrast, the A subunit alone was capable of activating dendritic cells and enhanced immune responses to multiple antigens following intranasal immunization; therefore, unlike toxicity, LT adjuvanticity is not dependent on the AB5 holotoxin structure or the presence of the B subunit. However, immune responses were maximal when signals were received from both subunits either in an AB5 structure or with A and B admixed. Furthermore, the quality of the immune response (i.e., IgG1/IgG2 balance and mucosal IgA and IL-17 secretion) was determined by the presence of an A subunit, revealing for the first time induction of Th17 responses with the A subunit alone. These results have important implications for understanding ETEC pathogenesis, unraveling immunologic responses induced by LT-based adjuvants, and developing new mucosal vaccines.</jats:p>

Characterization of a Novel Fusion Protein from IpaB and IpaD of Shigella spp. and Its Potential as a Pan-Shigella Vaccine

“…In particular, the frequencies of IFN-␥-secreting cells and IL-17 secretion levels were higher in cells obtained from mice immunized with the DB fusion. Even if dmLT has the capacity of eliciting IL-17 responses by itself (36), the presence of the adjuvant in both formulations indicates the possibility that the fusion could be recognized by the immune system in a different manner than the individual proteins. Most importantly, this demonstrates that the DB fusion has a unique advantage in the generation of cell-mediated immunity, which can be important for control of Shigella.…”

A Double Mutant Heat-Labile Toxin from Escherichia coli, LT(R192G/L211A), Is an Effective Mucosal Adjuvant for Vaccination against Helicobacter pylori Infection

“…The question still remains how dmLT with its minimal ADPribosylating activity (11,38) can be as potent as CT in inducing mucosal immune responses. In an attempt to elucidate the adjuvant mechanism of dmLT, Norton et al (11) reported that the mutations in the dmLT molecule that prevent the proteolytic cleavage of LTA subunit into A1 and A2 subunits also led to rapid degradation of the A subunit in the cytosol of intestinal epithelial cells (11).…”

Simultaneous Exposure to Escherichia coli Heat-Labile and Heat-Stable Enterotoxins Increases Fluid Secretion and Alters Cyclic Nucleotide and Cytokine Production by Intestinal Epithelial Cells

“…The A subunit is made up of two components, A1 and A2. The A1 subunit (21 kDa), the enzymatically active portion of the toxin, is noncovalently linked to the B pentamer via the A2 peptide (7 kDa) (10)(11)(12).…”