Exocytotic Secretion of Toxins from Macrophages Infected with Escherichia coli O157.

Abstract

This study examined whether macrophages are involved in the development of pathogenicity in Shiga-like toxin (SLT)-producing enterohemorrhagic Escherichia coil (EHEC) O157:H7. Macrophages were infected with the bacteria, after which the macrophage culture medium showed a clear increase in toxicity in rats in vivo as well as in rat aortic endothelial cells in vitro. The increased toxicity resulted mainly from a rapid increase in the concentrations of SLT type I (SLT-I) and type II (SLT-II) and partly from an increase in concentrations of the proinflammatory cytokines, tumor necrosis factor alpha (TNFalpha) and interleukin-1 (IL-1), in the culture medium. Most of the EHEC O157 added to the macrophage culture were quickly incorporated to form phagosomes, which then fused with lysosomes to become phagolysosomes. During this intracellular digestion process, the EHEC O157 remained alive for about 15 min, and continued synthesizing and secreting the toxins SLT-1 and SLT-II. The bacteria were then killed and digested in the phagolysosomes with significant amounts of the toxins retained. Subsequently, the contents of the phagolysosomes were exocytotically secreted from the macrophage cell membrane into the surrounding culture medium. Such a sequence of events in macrophages may occur in vivo, suggesting the active involvement of macrophages in the rapid increase in pathogenicity, such as seen in the onset of hemolytic-uremic syndrome (HUS) in patients infected with EHEC O157. The exocytotic secretion is considered to be one of the most basic cellular functions in macrophages.

Escherichia coliO157:H7 Survives within Human Macrophages: Global Gene Expression Profile and Involvement of the Shiga Toxins

“…First, EHEC survival may enhance inflammatory responses responsible for intestinal epithelial barrier disruption, which could lead to an increase of Shiga toxins entering the bloodstream. Second, as Shiga toxins are expressed and participate in bacterial survival in macrophages, the production of Shiga toxins within macrophages may be used to increase the toxin concentration in the bloodstream, as it was shown that bacteria residing in macrophages release these toxins (53). Understanding the interactions between host and pathogen is of fundamental importance in developing vaccines and other new therapeutic agents, particularly in the case of E. coli O157:H7, as no effective treatment is presently available for humans infected with this bacterium.…”

“…Binding at the follicle-associated epithelium results in the rapid contact of E. coli O157:H7 with underlying human macrophages; however, there is very little information available about the interaction between EHEC and these cells. The interaction between EHEC and murine macrophages was investigated, and it was shown that phagocytosis of EHEC by murine macrophages caused actin rearrangements surrounding the phagosome; EHEC was rapidly killed, and high levels of Shiga toxins were produced (53). However, to our knowledge, the interaction between EHEC and human macrophages has not been investigated.…”

Upregulation of Shiga Toxin Receptor CD77/Gb3 and Interleukin‐1β Expression in the Brain of EHEC Patients with Hemolytic Uremic Syndrome and Neurologic Symptoms

“…Activation of inflammatory cascades such as increased IL-1β production has previously been demonstrated in macrophages in EHEC infection (20). Thus, we investigated key components of neuroinflammation such as tumor necrosis factor α (TNF-α) and IL-1β in EHEC patients and controls.…”

Shiga toxins

“…Shiga toxin-producing bacteria and Shiga toxins can be detected by different methods, including enzyme-linked immunosorbent assays (ELISA), cytotoxicity assays, and polymerase chain reaction (PCR) [20,50,51]. Also, flow cytometry detection of Shiga toxins bound to polymorphonuclear leukocytes was reported to be a rapid, simple, and sensitive method of detecting circulating Shiga toxins and diagnosing STEC infections [52].…”

“…If the epithelial cell barrier is disrupted, then macrophages can engulf the bacteria. Importantly, even when the bacteria are taken up and killed by the macrophages, these cells secrete toxin in the surroundings [51].…”