Shiga-like toxin 2 (SLT2)

Shiga-like toxin 2 (SLT2) or Shiga toxin 2 (Stx2), is an extremely pathogenic toxin produced by certain strains of Escherichia coli (e.g., enterohemorrhagic E. coli (EHEC). In its structural identity, SLT2 is classified as an AB5 toxin. Over the past few years, there has been massive research into the pharmacology, toxicity spectrum and function of SLT2 in disease – specifically in Hemolytic Uremic Syndrome (HUS).

Structure and Mechanism of Action of SLT2

SLT2 is an AB5-family toxin in which the A subunit is N-glycosidase and the B pentamer attaches to receptors on host cells. The A subunit cuts the host's 28S ribosomal RNA, halting protein production and inducing cell death. The A subunit of SLT2 is composed of 293 amino acids and contains the toxic active site. The B subunit binds specifically to the Gb3 receptor (globotriaosylceramide) which is highly present in cells such as renal microvascular endothelial cells and some intestinal epithelial cells, making SLT2 extremely harmful to these cells.

In vitro, SLT2 is less toxic in animal studies than Shiga-like toxin 1 (Stx1) but more pathogenic in cells, perhaps due to receptor specificity and cell type targeting. It is also possible that SLT2's B subunit has a different binding mode and structure than SLT1, which affects its binding to host cells. SLT2 is divided into different subtypes (e.g., Stx2a, Stx2c, and Stx2d) that differ in toxicology, immunity and outcome during infection, with Stx2a associated with severe pathologies like HUS.

Figure 1. Cartoon Representation of Stx Structure
(Source: Melton-Celsa AR, 2014)

Regulation of SLT2 Toxicity

The regulation of SLT2 toxicity is complex and influenced by various environmental factors. Research suggests that iron levels modulate SLT2 expression via the Fur protein, affecting toxicity levels. Specifically, under high iron conditions, Fur protein inhibits SLT2 expression, while low iron enhances its toxicity. Additionally, the gene encoding SLT2 is often found within inducible lysogenic bacteriophages, meaning toxin production and release are influenced by the infection stage. For instance, when E. coli infects a host, the bacteriophage DNA integrates into the host genome, and SLT2 expression is induced under appropriate environmental stimuli, leading to host toxicity.

The B subunit of SLT2, through its pentameric structure, binds to Gb3 receptors on cell surfaces—a process regulated by membrane cholesterol, lipid rafts, and other membrane components. Notably, the B subunit has specific amino acid sites across toxin subtypes, leading to differences in cellular pathways and binding efficiency. The elastase activation mechanism in Stx2d further underscores the critical role of the B subunit in SLT2's toxicity regulation.

Figure 2. Intracellular Transport of Shiga Toxin
(Source: Johannes L, 2017)

Potential Applications of SLT2 in Vaccines and Therapies

Due to SLT2's strong association with HUS and other serious diseases, vaccine development and antibody therapy have become key research areas. The nontoxic B subunit of SLT2 is a potential vaccine target, capable of inducing specific immune responses to prevent the toxin's pathogenic effects. The pentameric structure of the B subunit also serves as an important model for antibody testing and functional studies.

Current research focuses on developing subunit-specific antibodies against SLT2 to block its binding to Gb3 receptors and thus reduce toxicity. Studies have shown that specific antibodies targeting SLT2 can effectively neutralize the toxin and reduce HUS incidence in animal models. However, the structural complexity of SLT2 and the differences between subtypes require further validation to optimize antibody targeting and efficacy.

As a highly toxic bacterial toxin, SLT2 plays a complex role in renal diseases, particularly in HUS. Its unique structure and receptor-binding mechanism enable it to exert potent toxicity on host cells, with the B subunit's pentameric structure crucial for cell binding, immune evasion, and inflammation induction. SLT2 has promising applications in vaccine and antibody research, with future studies needing to further explore its toxicity regulation mechanisms to develop more effective treatments and preventive measures.

SLT ELISA Kits

SLT Antibodies

SLT Antigens