title: 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: Enterotoxigenic Escherichia coli (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.
authors.affiliation: Department of Microbiology and Immunology, Tulane University School of Medicine, New Orleans, Louisiana, USA.
authors.forename: Elizabeth B
authors.initials: EB
authors.lastname: Norton
authors.forename: Louise B
authors.initials: LB
authors.lastname: Lawson
authors.forename: Zaid
authors.initials: Z
authors.lastname: Mahdi
authors.forename: Lucy C
authors.initials: LC
authors.lastname: Freytag
authors.forename: John D
authors.initials: JD
authors.lastname: Clements
mesh_terms: D000276:Adjuvants, Immunologic / Q000008:administration & dosage* / Q000009:adverse effects; D000281:Administration, Intranasal; D000818:Animals; D001427:Bacterial Toxins / Q000008:administration & dosage* / Q000009:adverse effects; D003713:Dendritic Cells / Q000276:immunology; D004768:Enterotoxins / Q000008:administration & dosage* / Q000009:adverse effects; D029968:Escherichia coli Proteins / Q000008:administration & dosage* / Q000009:adverse effects; D007070:Immunoglobulin A / Q000276:immunology*; D007074:Immunoglobulin G / Q000276:immunology*; D051379:Mice; D008807:Mice, Inbred BALB C; D021122:Protein Subunits / Q000008:administration & dosage; D058504:Th17 Cells / Q000276:immunology*; D014612:Vaccines / Q000008:administration & dosage / Q000009:adverse effects / Q000276:immunology*
chemical_list: D000276:Adjuvants, Immunologic; D001427:Bacterial Toxins; D004768:Enterotoxins; D029968:Escherichia coli Proteins; D007070:Immunoglobulin A; D007074:Immunoglobulin G; D021122:Protein Subunits; D014612:Vaccines; C027538:heat-labile enterotoxin, E coli