Shiga Toxin (Stx) Type 1a Reduces the Oral Toxicity of Stx Type 2a

(See the editorial commentary by Steiner on pages 1214–5.) Shiga toxin (Stx)–producing Escherichia coli (STEC) is a gram-negative enteric pathogen responsible for serious foodborne disease. A single serotype, O157:H7, is the most frequently reported cause of illness due to STEC in the United States [1]. The Centers for Disease Control and Prevention estimates that O157:H7 strains account for approximately 63 000 of the 175 000 estimated STEC cases each year. Cattle and other ruminants are the natural reservoir for STEC [2–4]. Infection with STEC most frequently occurs after ingestion of contaminated food, but other sources include contaminated swimming or well water or contact with cattle or infected individuals [5, 6]. The primary disease manifestation associated with STEC infection is hemorrhagic colitis. Approximately 10%–20% of individuals will progress to hemolytic uremic syndrome (HUS), a serious sequela characterized by thrombocytopenia, hemolytic anemia, and renal failure [7]. Children aged <5 years have the highest incidence of HUS, and STEC-mediated HUS is the leading cause of pediatric acute renal failure [8, 9]. STEC can encode either Stx1a and/or Stx2a, which are highly similar but antigenically distinct AB5 toxins with identical modes of action [10]. Upon binding the functional receptor, Gb3 [11], Stx is endocytosed and undergoes retrograde transport to the cytoplasm, where it acts as an N-glycosidase that inactivates ribosomes, which, in turn, stops protein synthesis and causes cell death [12]. Treatment of STEC-infected individuals is limited to supportive therapy, as antibiotics can increase the likelihood of developing HUS [7]. Epidemiological studies indicate that an individual is more likely to develop HUS if infected with an O157:H7 strain expressing Stx2a only than with strains expressing Stx1a only or both Stx1a and Stx2a [13–17]. It is not clear why the pathogenicity of a strain that produces only Stx2a is greater than that of a strain that expresses both Stx1a and Stx2a. However, we recently showed that the oral 50% lethal dose (LD50) of Stx2a in mice is 2.8 µg, whereas Stx1a-intoxicated mice failed to exhibit morbidity or mortality even at a dose of 157 µg/mouse [18]. We hypothesized, therefore, that Stx1a might have a reduced capacity to cross the intestinal barrier as compared to Stx2a. Further, we speculated that Stx1a may even interfere with the toxicity of Stx2a when both toxins are present, which, if true, might explain the reduced pathogenicity of a strain that produces both toxins. Since Stx intoxication models recapitulate at least some of the kidney pathology associated with STEC infection [18, 19], we used a murine oral intoxication model in this study to evaluate the effect of Stx1a and Stx2a cointoxication as compared to Stx2a intoxication alone. We found, to our surprise, that similar levels of Stx1a and Stx2a reach the kidney after oral intoxication. We then demonstrated that Stx1a reduced the morbidity and mortality associated with Stx2a when gavaged at the same time. These results may explain the epidemiological observation that STEC infection with Stx2a-positive strains are more severe than Stx1a/Stx2a-positive strains.