The purpose of this investigation was to establish the site(s) of Stx2 accumulation in the intoxicated host, and whether administration of highly protective antibodies impacts this outcome. In addition, the site of accumulation of Stx2/antibody complexes in patients with HUS is of particular concern to nephrologists, should the site be the kidneys leading to further renal dysfunction. The results of the present study suggest that HuMAbs 5C12 and 5H8 facilitated hepatic clearance of Stx2 as maximal levels of radioactivity in these groups were present in the liver at 3 h after 125I-Stx2 administration. The liver was also a major organ of radioactivity accumulation in these 2 groups at day 1 and day 2 sampling times, largely sparing the kidney. In contrast, kidneys were the major target organ of 125I-Stx2 accumulation in the PBS group at all 3 sampling times, indicating that rapid hepatic elimination of Stx2 did not occur in this group. Other studies have previously shown that soluble immune complexes are cleared primarily by the liver through uptake into Kupffer cells [26–28].
The results of the present study show that accumulation of Stx2 in the kidneys is significantly blocked by the HuMAbs 5C12 and 5H8 as radioactivity accumulated in the kidneys of these two groups at much lower levels than the kidneys of the PBS group. As all mice in the 5C12 and 5H8 groups did not show any clinical signs of Stx2-intoxication and survived, the renal presence of Stx2 in these mice appear to have had minimal clinical impact. In contrast, all mice in the PBS group succumbed to Stx2intoxication, and toxin presence in the kidneys of this group may have contributed to their deaths. Stx2 in the kidneys of mice is toxic and causes apoptosis of medullary and cortical tubular cells, and renal failure due to the loss of functioning collecting ducts . These results suggest that Stx2/5C12 or Stx2/5H8 complexes are less likely to accumulate in the kidneys in patients with HUS, and less likely to exacerbate the condition further.
Gb3 expression on neuronal cells in mouse central nervous system and its binding with Stx2 has recently been shown , which also explains earlier observations of Stx2 localization in the spinal cord . In the present study, significantly more toxin was measured in the cerebrum but not cerebellum of the PBS and 5C12 groups than in the 5H8 group at the 3 h sampling time. This suggests that toxin can bind to the cerebrum but not the cerebellum, and that 5H8 but not 5C12 can block this binding. Significant differences in toxin accumulation in cerebrum or cerebellum did not occur on day 1 or day 2 in any of the groups. Note however, that in the present study, cerebral toxin accumulation in PBS and 5C12 groups was much less than the accumulation of toxin in the kidney, a well known target of Stx2. The spinal cord was not included in the present study.
The stomach was one of the major organs of radioactivity accumulation (especially at 3 h sampling time) in all treatment groups. The stomach has previously been shown to accumulate radioactivity in mice injected with radiolabeled Stx2 . Presumably, sequestration of the released radioiodide in the stomach may have occurred due to the dehalogenation effect .
Rutjes et al.  reported that the highest concentrations of Stx2 localized in the nasal turbinates followed by the lungs and kidneys. Stx2-specific antibodies were not used in that study . The lungs, but not the nasal turbinates were included in the present study, and with regards to toxin concentration, they were the fourth highest in the PBS and 5H8 groups, and the sixth highest in 5C12 group at 3 h sampling time (Figure 3, Additional file 1: Table S1). As the previous  and the present studies used radioiodinated Stx2 and the IV route of toxin administration, the discrepancy between the two investigations may be related to sampling time after toxin administration (1 h vs. 3 h in our study), and/or the strain of mouse used (Balb/c vs. C57BL/6 in our study). Nevertheless, the kidney was a major target organ for Stx2 in both studies . Stx2 is known to have a propensity for lymphoid cells [32–34] as is shown by binding of the toxin within MLN in the present study.