Transcytosis, Antitumor Activity and Toxicity of Staphylococcal Enterotoxin C2 as an Oral Administration Protein Drug

Staphylococcus aureus Infections and Human Intestinal Microbiota

Staphylococcus aureus (S. aureus) is a common pathogen that can cause many human diseases, such as skin infection, food poisoning, endocarditis, and sepsis. These diseases can be minor infections or life-threatening, requiring complex medical management resulting in substantial healthcare costs. Meanwhile, as the critically ignored "organ," the intestinal microbiome greatly impacts physiological health, not only in gastrointestinal diseases but also in disorders beyond the gut. However, the correlation between S. aureus infection and intestinal microbial homeostasis is largely unknown. Here, we summarized the recent progress in understanding S. aureus infections and their interactions with the microbiome in the intestine. These summarizations will help us understand the mechanisms behind these infections and crosstalk and the challenges we are facing now, which could contribute to preventing S. aureus infections, effective treatment investigation, and vaccine development.

Low-Dose Staphylococcal Enterotoxin C2 Mutant Maintains Bone Homeostasis via Regulating Crosstalk between Bone Formation and Host T-Cell Effector Immunity

Studies in recent years have highlighted an elaborate crosstalk between T cells and bone cells, suggesting that T cells may be alternative therapeutic targets for the maintenance of bone homeostasis. Here, it is reported that systemic administration of low-dose staphylococcal enterotoxin C2 (SEC2) 2M-118, a form of mutant superantigen, dramatically alleviates ovariectomy (OVX)-induced bone loss via modulating T cells. Specially, SEC2 2M-118 treatment increases trabecular bone mass significantly via promoting bone formation in OVX mice. These beneficial effects are largely diminished in T-cell-deficient nude mice and can be rescued by T-cell reconstruction. Neutralizing assays determine interferon gamma (IFN-γ) as the key factor that mediates the beneficial effects of SEC2 2M-118 on bone. Mechanistic studies demonstrate that IFN-γ stimulates Janus kinase/signal transducer and activator of transcription (JAK-STAT) signaling, leading to enhanced production of nitric oxide, which further activates p38 mitogen-activated protein kinase (MAPK) and Runt-related transcription factor 2 (Runx2) signaling and promotes osteogenic differentiation. IFN-γ also directly inhibits osteoclast differentiation, but this effect is counteracted by proabsorptive factors tumor necrosis factor alpha (TNF-α) and interleukin 1 beta (IL-1β) secreted from IFN-γ-stimulated macrophages. Taken together, this work provides clues for developing innovative approaches which target T cells for the prevention and treatment of osteoporosis.

Soluble Expression of Fc-Fused T Cell Receptors Allows Yielding Novel Bispecific T Cell Engagers

The specific recognition of T cell receptors (TCR) and peptides presented by human leukocyte antigens (pHLAs) is the core step for T cell triggering to execute anti-tumor activity. However, TCR assembly and soluble expression are challenging, which precludes the broad use of TCR in tumor therapy. Herein, we used heterodimeric Fc to assist in the correct assembly of TCRs to achieve the stable and soluble expression of several TCRs in mammalian cells, and the soluble TCRs enable us to yield novel bispecific T cell engagers (TCR/aCD3) through pairing them with an anti-CD3 antibody. The NY-ESO-1/LAGE-1 targeted TCR/aCD3 (NY-TCR/aCD3) that we generated can redirect naïve T cells to specific lysis antigen-positive tumor cells, but the potency of the NY-TCR/aCD3 was disappointing. Furthermore, we found that the activation of T cells by NY-TCR/aCD3 was mild and unabiding, and the activity of NY-TCR/aCD3 could be significantly improved when we replaced naïve T cells with pre-activated T cells. Therefore, we employed the robust T cell activation ability of staphylococcal enterotoxin C2 (SEC2) to optimize the activity of NY-TCR/aCD3. Moreover, we found that the secretions of SEC2-activated T cells can promote HLA-I expression and thus increase target levels, which may further contribute to improving the activity of NY-TCR/aCD3. Our study described novel strategies for soluble TCR expression, and the optimization of the generation and potency of TCR/aCD3 provided a representative for us to fully exploit TCRs for the precision targeting of cancers.

Staphylococcal Enterotoxin Superantigens Induce Prophylactic Antiviral Activity Against Encephalomyocarditis Virus In Vivo and In Vitro

The staphylococcal enterotoxins (SEs) are classified as superantigens due to their potent stimulation of the immune system resulting in T cell activation and prodigious cytokine production and toxicity. This study examined the ability of superantigens to induce prophylactic antiviral activity in vivo and in vitro and evaluated potential superantigen mimetic peptides. Prophylactic treatment of mice in vivo with intraperitoneal injections of SE superantigens SEA and SEB (both at 20 μg/day for 3 days) prevented encephalomyocarditis virus (EMCV)-induced lethality in 100% and 80% of mice, respectively, as compared with control saline-treated groups in which EMCV was lethal to all mice. Furthermore, SEA (2 μg/mL) and SEB (1 μg/mL) induced antiviral activity in mouse splenocytes to produce an antiviral factor since their supernatant prevented EMCV lysis of L929 cells in tissue culture. It was found that superantigens do not directly prevent EMCV infection, but rather indirectly through inducing interferon gamma (IFNγ) production in cells as the antiviral factor. Evaluation of various superantigen mimetic peptides showed that one peptide (SEA3) had superantigen-like activity by inducing IFNγ production in cells but without the cellular proliferation, as associated with superantigens. However, the induction of IFNγ activation by the SEA3 peptide was not as pronounced, and took a much higher peptide concentration, when compared with the parent superantigen. If the negative side effects of superantigens can be eliminated, their beneficial properties can be harnessed for prophylactic treatment of viral infections and other pathologies requiring a robust immune response.

Staphylococcal Enterotoxin C2 Mutant-Directed Fatty Acid and Mitochondrial Energy Metabolic Programs Regulate CD8+ T Cell Activation

CD8⁺ T cells can switch between fatty acid catabolism and mitochondrial energy metabolism to sustain expansion and their cytotoxic functions. ST-4 is a TCR-enhanced mutant derived from superantigen staphylococcal enterotoxin C2 (SEC2), which can hyperactivate CD4⁺ T cells without MHC class II molecules. However, whether ST-4/SEC2 can enhance metabolic reprogramming in CD8⁺ T cells remains poorly understood. In this study, we found that ST-4, but not SEC2, could induce proliferation of purified CD8⁺ T cell from BALB/c mice in Vβ8.2- and -8.3-specific manners. Results of gas chromatography-mass spectroscopy analysis showed that fatty acid contents in CD8⁺ T cells were increased after ST-4 stimulation. Flow cytometry and Seahorse analyses showed that ST-4 significantly promoted mitochondrial energy metabolism in CD8⁺ T cells. We also observed significantly upregulated levels of gene transcripts for fatty acid uptake and synthesis, and significantly increased protein expression levels of fatty acid and mitochondrial metabolic markers of mTOR/PPARγ/SREBP1 and p38-MAPK signaling pathways in ST-4-activated CD8⁺ T cells. However, blocking mTOR, PPARγ, SREBP1, or p38-MAPK signals with specific inhibitors could significantly relieve the enhanced fatty acid catabolism and mitochondrial capacity induced by ST-4. In addition, blocking these signals inhibited ST-4-stimulated CD8⁺ T cell proliferation and effector functions. Taken together, our findings demonstrate that ST-4 enhanced fatty acid and mitochondria metabolic reprogramming through mTOR/PPARγ/SREBP and p38-MAPK signaling pathways, which may be important regulatory mechanisms of CD8⁺ T cell activation. Understanding the effects of ST-4-induced regulatory metabolic networks on CD8⁺ T cells provide important mechanistic insights to superantigen-based tumor therapy.

Intact anti-LPS IgY is found in the blood after intragastric administration in mice

Severe burn injury and cirrhosis often cause the translocation of bacterial endotoxins into blood, leading to systemic damage and even death. Our previous studies have shown that anti‐lipopolysaccharide egg yolk antibody (anti‐LPS IgY) can neutralize bacterial endotoxins in vitro and in vivo effectively, thereby reducing endotoxin damage. Whether anti‐LPS IgY can be absorbed into the blood through the intestinal barrier and neutralize endotoxins in circulation remains unclear. In this study, we used in vivo small animal imaging techniques, protein purification, molecular biology, and mass spectrometry to show that intragastrically administered anti‐LPS IgY is detected in the blood of mice as an intact molecule and has the capacity to bind to LPS. Immunohistochemical analysis confirmed that anti‐LPS IgY is associated with the intestinal mucosa of mice. However, the route of absorption of this large protein molecule was not determined. This study suggests that anti‐LPS IgY can be absorbed into the circulation, with the same molecular mass as purified anti‐LPS IgY as a macromolecular protein, suggesting a new strategy for the prevention of damage caused by endotoxins.

Basis of Virulence in Enterotoxin-Mediated Staphylococcal Food Poisoning

The Staphylococcus aureus enterotoxins are a superfamily of secreted virulence factors that share structural and functional similarities and possess potent superantigenic activity causing disruptions in adaptive immunity. The enterotoxins can be separated into two groups; the classical (SEA-SEE) and the newer (SEG-SElY and counting) enterotoxin groups. Many members from both these groups contribute to the pathogenesis of several serious human diseases, including toxic shock syndrome, pneumonia, and sepsis-related infections. Additionally, many members demonstrate emetic activity and are frequently responsible for food poisoning outbreaks. Due to their robust tolerance to denaturing, the enterotoxins retain activity in food contaminated previously with S. aureus. The genes encoding the enterotoxins are found mostly on a variety of different mobile genetic elements. Therefore, the presence of enterotoxins can vary widely among different S. aureus isolates. Additionally, the enterotoxins are regulated by multiple, and often overlapping, regulatory pathways, which are influenced by environmental factors. In this review, we also will focus on the newer enterotoxins (SEG-SElY), which matter for the role of S. aureus as an enteropathogen, and summarize our current knowledge on their prevalence in recent food poisoning outbreaks. Finally, we will review the current literature regarding the key elements that govern the complex regulation of enterotoxins, the molecular mechanisms underlying their enterotoxigenic, superantigenic, and immunomodulatory functions, and discuss how these activities may collectively contribute to the overall manifestation of staphylococcal food poisoning.

Recombinant adenovirus of SEA and CD80 genes driven by MMRE and mouse TERT promoter induce effective antitumor immune responses against different types of tumor cells in vitro and in vivo

Staphylococcus enterotoxin A (SEA) is a powerful immunostimulant and can stimulate T cells bearing certain T-cell receptor β-chain variable regions when bound to major histocompatibility complex II molecules. SEA is widely used in research of antitumor therapy. The low affinity T-cell receptor (TCR) interaction with SEA in the absence of MHC class II antigens is sufficient for the induction of cytotoxicity but requires additional CD28/B7 signaling to result in proliferation of resting T cells. In this study, we constructed recombinant adenovirus (named as Ad-MMRE-mTERT-BIS) carrying membrane-expressing SEA (named as SEAtm) and CD80 driven by Myc-Max response elements (MMRE) and mouse telomerase reverse transcriptase (mTERT) promoter to reduce toxicity and to improve safety and efficiency. We demonstrated that Ad-MMRE-mTERT-BIS could make SEAtm and CD80 to co-express highly on the surface of Hepa1-6 and B16 cells, at low level on the surface of CT26 cells, but not in NIH3T3. Hepa1-6 and B16 cells infected by the recombinant adenovirus induced proliferation of CD4+ and CD8+ T cells and increased cytokine [interleukin (IL)-2, tumor necrosis factor (TNF)-α, interferon (IFN)-γ] production in vitro. Intratumoral injection of Ad-MMRE-mTERT-BIS in hepatoma and melanoma mouse models induced tumor-specific cytotoxic T cells in the spleen. Moreover, hepatoma and melanoma xenografts were suppressed by treatment with Ad-MMRE-mTERT-BIS and the survival time of treated mice was prolonged. These findings suggest that recombinant adenovirus of SEA and CD80 genes driven by mTERT promoter could induce effective antitumor immune responses against different kinds of tumor cells in vitro and in vivo.