Identification of a secondary zinc-binding site in staphylococcal enterotoxin C2. Implications for superantigen recognition

Staphylococcal Enterotoxin C-An Update on SEC Variants, Their Structure and Properties, and Their Role in Foodborne Intoxications

Staphylococcal enterotoxins are the most common cause of foodborne intoxications (staphylococcal food poisoning) and cause a wide range of diseases. With at least six variants staphylococcal enterotoxin C (SEC) stands out as particularly diverse amongst the 25 known staphylococcal enterotoxins. Some variants present unique and even host-specific features. Here, we review the role of SEC in human and animal health with a particular focus on its role as a causative agent for foodborne intoxications. We highlight structural features unique to SEC and its variants, particularly, the emetic and superantigen activity, as well as the roles of SEC in mastitis and in dairy products. Information about the genetic organization as well as regulatory mechanisms including the accessory gene regulator and food-related stressors are provided.

Structural basis of metal hypersensitivity

Metal hypersensitivity is a common immune disorder. Human immune systems mount the allergic attacks on metal ions through skin contacts, lung inhalation and metal-containing artificial body implants. The consequences can be simple annoyances to life-threatening systemic illness. Allergic hyper-reactivities to nickel (Ni) and beryllium (Be) are the best-studied human metal hypersensitivities. Ni-contact dermatitis affects 10 % of the human population, whereas Be compounds are the culprits of chronic Be disease (CBD). αβ T cells (T cells) play a crucial role in these hypersensitivity reactions. Metal ions work as haptens and bind to the surface of major histocompatibility complex (MHC) and peptide complex. This modifies the binding surface of MHC and triggers the immune response of T cells. Metal-specific αβ T cell receptors (TCRs) are usually MHC restricted, especially MHC class II (MHCII) restricted. Numerous models have been proposed, yet the mechanisms and molecular basis of metal hypersensitivity remain elusive. Recently, we determined the crystal structures of the Ni and Be presenting human MHCII molecules, HLA-DR52c (DRA*0101, DRB3*0301) and HLA-DP2 (DPA1*0103, DPB1*0201). These structures revealed unusual features of MHCII molecules and shed light on how metal ions are recognized by T cells.

Inhibition of emetic and superantigenic activities of staphylococcal enterotoxin A by synthetic peptides

Staphylococcus aureus is a major human pathogen producing different types of toxins. Enterotoxin A (SEA) is the most common type among clinical and food-related strains. The aim of the present study was to estimate functional regions of SEA that are responsible for emetic and superantigenic activities using synthetic peptides. A series of 13 synthetic peptides corresponding to specific regions of SEA were synthesized, and the effect of these peptides on superantigenic activity of SEA including interferon γ (IFN-γ) production in mouse spleen cells, SEA-induced lethal shock in mice, spleen cell proliferation in house musk shrew, and emetic activity in shrews were assessed. Pre-treatment of spleen cells with synthetic peptides corresponding to the regions 21-40, 35-50, 81-100, or 161-180 of SEA significantly inhibited SEA-induced IFN-γ production and cell proliferation. These peptides also inhibited SEA-induced lethal shock. Interestingly, peptides corresponding to regions 21-40, 35-50 and 81-100 significantly inhibited SEA-induced emesis in house musk shrews, but region 161-180 did not. These findings indicated that regions 21-50 and 81-100 of SEA are important for both superantigenic and emetic activities of SEA molecule while region 161-180 is involved in superantigenic activity but not emetic activity of SEA. These regions could be important targets for therapeutic intervention against SEA exposure.

Biological analysis of the deletion mutants of Staphylococcal enterotoxin C2

To investigate the functional domains involved in the biological activity of staphylococcal enterotoxin (SEC2), a series of SEC2 mutants were constructed. Deletion of the last 77 amino acids at the C-terminus of SEC2 did not affect its native superantigen and fever activities, and further removal of the C-terminal residues reduced SEC2 activities significantly. On the other hand, the mutants lacking 18 or more N-terminal residues severely impaired superantigen activity. These data indicated that the functional regions for the biological activities of SEC2 were confined to N-terminal domain, further implied that the proper three-dimensional structure of SEC2 is not needed for its biological activities. Our results deliver valuable information that it is possible to design new SEC2 immunotherapeutic agents which have the superantigen activity and low molecular weight for permeability.

Biological characterization of the zinc site coordinating histidine residues of staphylococcal enterotoxin C2

The bacterial toxin staphylococcal enterotoxin C2 (SEC2) can cause staphylococcal toxic shock syndrome and food poisoning. Although the previously determined crystal structure of SEC2 revealed that some histidine residues (His47, His118 and His122) contribute to the binding of zinc ions, little is known about their biological roles in SEC2. This prompted us to investigate the role of the zinc site coordinating histidine residues in the biological activities of SEC2. The mutants with substitutions at positions 118 and 122 all retained T-cell stimulatory activity, whereas the histidine mutants at position 47 were defective in the ability to stimulate T-cell proliferation. Further toxicity assays in vivo indicated that mutants SEC2-H118A and SEC2-H122A were defective in emetic and febrile activities. However, mutant SEC2-H47A could cause significant emetic and febrile responses in comparison with the other two histidine mutants. These findings suggested that the zinc-coordinating histidine residues play significant roles in superantigen and toxic activities of SEC2 and further implied that superantigen and febrile activities could be separable in staphylococcal enterotoxins. The results also show that it should be possible to design new SEC2 immunotherapeutic agents that have superantigen activity and low toxicity.

Staphylococcus aureus enterotoxin C2 mutants: biological activity assay in vitro

Staphylococcal enterotoxin C2 (SEC2) is one member of bacterial superantigens produced by Staphylococcus aureus. It can be attributed to its superantigenic activity to cross-link major histocompatibility complex class II molecules with T-cell receptors and activate a large number of resting T cells resulting in release of massive cytokines, which will produce significant tumor inhibition in vivo and in vitro. However, it could be not broadly applied to cure malignant tumors in clinic because of emetic activity of SEC2. The aim of this study was to inactivate emetic activity of SEC2 through site-directed mutagenesis. Cys93, Cys110 and His118 were selected as substitutional sites based on the functional sites responsible for emesis. The mutated proteins were used to determine Peripheral blood mononuclear cell proliferation activity and anti-tumor activity in vitro. Results showed that these mutated proteins efficiently stimulated T cell and exhibited the same tumor-inhibition effect as SEC2. It is possible to inactivate emetic activity of SEC2 through site-directed mutagenesis and provide satisfying agents for tumor treatment in clinic.

Mutagenesis, biochemical, and biophysical characterization of Mycoplasma arthritidis-derived mitogen

Mycoplasma arthritidis-derived mitogen (MAM) is a superantigen (SAg) that can activate large fractions of T cells bearing particular TCR Vbeta elements. Here we report the mutagenesis, biochemical and biophysical studies on the dimerization of MAM in solution. Our studies showed that although MAM mainly exists as a monomer in solution, a small percentage of MAM molecules form homodimer at high protein concentration, regardless of the presence of Zn2+. A distinct peak corresponding to a MAM homodimer was detected in the presence of EDTA, using both chemical cross-linking and analytical ultracentrifugation methods. Further mutagenesis studies revealed that single mutation of residues at the interface of the crystallographic dimer of MAM does not significantly affect the dimerization of MAM in solution. Circular dichroism (CD) analysis indicated that addition of Zn2+ does not induce conformational changes of MAM from its apo-state. Thermal denaturation experiments indicated that addition of Zn2+ to MAM solution resulted in a decrease of melting point (Tm), whereas addition of EDTA did not affect the Tm of MAM. These results imply that there is no defined Zn2+-binding site on MAM.

Crystal structure of staphylococcal enterotoxin I (SEI) in complex with a human major histocompatibility complex class II molecule

Superantigens are bacterial or viral proteins that elicit massive T cell activation through simultaneous binding to major histocompatibility complex (MHC) class II and T cell receptors. This activation results in uncontrolled release of inflammatory cytokines, causing toxic shock. A remarkable property of superantigens, which distinguishes them from T cell receptors, is their ability to interact with multiple MHC class II alleles independently of MHC-bound peptide. Previous crystallographic studies have shown that staphylococcal and streptococcal superantigens belonging to the zinc family bind to a high affinity site on the class II beta-chain. However, the basis for promiscuous MHC recognition by zinc-dependent superantigens is not obvious, because the beta-chain is polymorphic and the MHC-bound peptide forms part of the binding interface. To understand how zinc-dependent superantigens recognize MHC, we determined the crystal structure, at 2.0 A resolution, of staphylococcal enterotoxin I bound to the human class II molecule HLA-DR1 bearing a peptide from influenza hemagglutinin. Interactions between the superantigen and DR1 beta-chain are mediated by a zinc ion, and 22% of the buried surface of peptide.MHC is contributed by the peptide. Comparison of the staphylococcal enterotoxin I.peptide.DR1 structure with ones determined previously revealed that zinc-dependent superantigens achieve promiscuous binding to MHC by targeting conservatively substituted residues of the polymorphic beta-chain. Additionally, these superantigens circumvent peptide specificity by engaging MHC-bound peptides at their conformationally conserved N-terminal regions while minimizing sequence-specific interactions with peptide residues to enhance cross-reactivity.