Development of an enterotoxigenic Escherichia coli vaccine based on the heat-stable toxin

Abstract

Infection with enterotoxigenic Escherichia coli (ETEC) is an important cause of diarrhea-related illness and death among children under 5 years of age in low- and middle-income countries (LMIC). Recent studies have found that it is the ETEC subtypes that produce the heat-stable enterotoxin (ST), irrespective of whether they also secrete the heat-labile enterotoxin (LT), which contribute most importantly to the disease burden in children from LMIC. Therefore, adding an ST toxoid would importantly complement ongoing ETEC vaccine development efforts. The ST's potent toxicity, its structural similarity to the endogenous peptides guanylin and uroguanylin, and its poor immunogenicity have all complicated the advancement of ST-based vaccine development. Recent remarkable progress, however, including the unprecedented screening for optimal ST mutants, mapping of cross-reacting ST epitopes and improved ST-carrier coupling strategies (bioconjugation and genetic fusion), enables the rational design of safe, immunogenic, and well-defined ST-based vaccine candidates.

Immunizations with Enterotoxigenic Escherichia coli Heat-Stable Toxin Conjugates Engender Toxin-Neutralizing Antibodies in Mice That Also Cross-React with Guanylin and Uroguanylin

“…This implies that a multivalent ETEC vaccine will be required. There are several ongoing efforts to develop subunit vaccine components based on colonization factors and other conserved ETEC antigens, which could be combined with the highly immunogenic but nontoxic B subunit of LT (LTB) and an ST vaccine component (7).…”

“…ETEC exerts its toxicity by secreting the heat-stable toxin (ST) and/or the heat-labile toxin (LT) into the small intestinal lumen (6). There are two ST variants found in ETEC strains infecting humans: the 19-amino-acid human ST (STh) and the 18-amino-acid porcine ST (STp), named after the species from which the ETEC strains were initially isolated (7,8). STh and STp share 14 amino acids and have 3 disulfide bonds that define and stabilize their structure (7,9).…”

“…There are two ST variants found in ETEC strains infecting humans: the 19-amino-acid human ST (STh) and the 18-amino-acid porcine ST (STp), named after the species from which the ETEC strains were initially isolated (7,8). STh and STp share 14 amino acids and have 3 disulfide bonds that define and stabilize their structure (7,9). Importantly, ETEC strains that produce ST, with or without LT, have recently been identified as among the most important causes of moderate and severe diarrhea in children (10).…”

“…Moreover, ETEC strains that produce STh are more closely associated with childhood diarrhea than those that produce STp (8,11,12). This implies that the STs in general, and STh specifically, are attractive targets for ETEC vaccine development, and ST-based vaccine antigens could be added to other ETEC vaccine candidate formulations to create vaccines with broad coverage (7).…”

“…Three key challenges must be overcome to develop an ST-based vaccine: ST must be made immunogenic, it must be made nontoxic by mutation and still be able to elicit antibodies that neutralize native ST, and immunization with ST-based vaccines should not elicit antibodies that cross-react with the endogenous peptides (7). The nonimmunogenic property of ST has been thoroughly studied, and ST has previously been made immunogenic by coupling it to protein carriers by chemical conjugation or genetic fusion (7). Important progress has also been made on identifying ST mutants that have reduced or abolished toxicity but that still have the ability to elicit neutralizing antibodies (ST toxoids) when coupled to a carrier (28)(29)(30).…”