Enhancement of antitumor response of staphylococcal enterotoxin C2 mutant 2M-118 by promoting cell-mediated antitumor immunity

BACKGROUND

Staphylococcal enterotoxin C2 (SEC2) is used as an immunotherapeutic drug in China. However, SEC2 are limited due to its immunosuppressive and toxic effects. A SEC2 2M-118 (H118A/T20L/G22E) mutant generated by site-directed mutagenesis was studied to elucidate the underlying antitumor mechanism.

METHODS

The effects of 2M-118 on mouse fibrosarcoma (Meth-A) cells and cytokine responses were tested in vitro using a transwell assay and ELISA, respectively. 2M-118 effect on immune function in tumor-bearing mice was tested. Cytokine levels and antitumor responses were measured using ELISA and flow cytometry, respectively. TUNEL staining and immunohistochemistry were employed to detect the tumor apoptosis and CD4+ and CD8+ tumor infiltrating lymphocytes (TILs) in tumor tissue.

RESULTS

2M-118 demonstrated the growth inhibition on tumor cells, increase of cytokines production (IL-2, IFN-γ, and TNF-α) and splenocyte proliferation in vitro. 2M-118 effectively inhibited tumor development and increased lymphocytes and cytokines in a tumor-bearing mouse model. Additionally, 2M-118 regulated the tumormicroenvironment by reducing the number of myeloid-derived suppressor cells (MDSCs), increasing the number of TILs, and inducing tumorcell apoptosis.

CONCLUSION

2M-118 promotes immune function and enhances antitumor response. This indicates that 2M-118 could potentially be developed as a novel anti-tumor drug with-highefficiencyandlowtoxicity.

Bidirectional Functional Effects of Staphylococcus on Carcinogenesis

As a Gram-positive cocci existing in nature, Staphylococcus has a variety of species, such as Staphylococcus aureus and Staphylococcus epidermidis, etc. Growing evidence reveals that Staphylococcus is closely related to the occurrence and development of various cancers. On the one hand, cancer patients are more likely to suffer from bacterial infection and antibiotic-resistant strain infection compared to healthy controls. On the other hand, there exists an association between staphylococcal infection and carcinogenesis. Staphylococcus often plays a pathogenic role and evades the host immune system through surface adhesion molecules, α-hemolysin, PVL (Panton-Valentine leukocidin), SEs (staphylococcal enterotoxins), SpA (staphylococcal protein A), TSST-1 (Toxic shock syndrom toxin-1) and other factors. Staphylococcal nucleases (SNases) are extracellular nucleases that serve as genomic markers for Staphylococcus aureus. Interestingly, a human homologue of SNases, SND1 (staphylococcal nuclease and Tudor domain-containing 1), has been recognized as an oncoprotein. This review is the first to summarize the reported basic and clinical evidence on staphylococci and neoplasms. Investigations on the correlation between Staphylococcus and the occurrence, development, diagnosis and treatment of breast, skin, oral, colon and other cancers, are made from the perspectives of various virulence factors and SND1.

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.

The T cell activating properties and antitumour activity of Staphylococcal Enterotoxin-like Q

Staphylococcal enterotoxins (SEs), as typical superantigens, exhibit promising antitumour activity in the clinic, but their unavoidable side effects related to fever and emesis seriously limit their application for the treatment of malignant tumours. Fortunately, the identification of Staphylococcal enterotoxin-like toxins (SEls), which possess amino acid sequences similar to those of classical SEs but exhibit no or low emetic activity, has provided a set of potential immunomodulatory candidates for cancer therapy. The aim of this study was to examine the effect of SElQ on lymphocyte activation and to further demonstrate its antitumour activity both in vitro and in vivo. High-purity SElQ was successfully harvested, and in vitro results confirmed that SElQ can significantly activate mouse- and human-derived lymphocytes in a dose-dependent manner, particularly CD4⁺ and CD8⁺ T cells, which showed significant increases in both percentage and absolute number. Further examination revealed that in addition to the originally recognized TCR Vβ5 and 21, TCR Vβ14, 17 and 18 were activated in SElQ-induced human PBMCs. Moreover, the expression of IL-2 and IFN-γ was significantly upregulated in vitro and in vivo after SElQ treatment. Based on the findings that SElQ induces lymphocyte activation and cytokine release, we then confirmed its antitumour activity both in vitro and in vivo. The data showed that treatment with a low concentration of SElQ (30 µg/mouse) could inhibit the growth of tumours by approximately 30% and no significant toxicity was observed. Taken together, our results demonstrated that SElQ can significantly induce T cell activation and cytokine release and further elicit substantial antitumour activity and thus provide support for the potential application of SElQ in cancer immunotherapy.

Staphylococcal enterotoxin C2 as an adjuvant for rabies vaccine induces specific immune responses in mice

Rabies vaccine administration is the most effective method to prevent the occurrence of rabies disease. However, administration of rabies vaccine without adjuvant always shows low efficiency. As a member of superantigen, staphylococcal enterotoxin C2 (SEC2) non-specifically activates T-cells at extremely low concentration. It enlightens us that SEC2 may be used as an adjuvant. We carried out the experiment that the mice received twice immunization with rabies vaccine in the presence or absence of SEC2 at 1-week interval. Serum and splenocytes from immunized mice were collected to measure the level of rabies-specific-IgG and the cell that secretes IFN-γ or IL-4. The promotion of antigen-specific splenocytes proliferation was also detected. Besides, a challenge test was performed to evaluate the protective efficiency of SEC2. It was shown that mice immunized with vaccine combined with SEC2 generated more specific anti-rabies-antibodies. The results for production of IFN-γ and IL-4, as well as the proliferation of splenocytes from immunized mice indicated SEC2 promoted the specific immune responses induced by rabies vaccine. Moreover, immunization of mice with vaccine combined with SEC2 provided efficient protection against the lethal rabies exposure. Taken together, our findings indicated that SEC2 can be served as an adjuvant for rabies vaccines.

Differentiation of entC1 from entC2/entC3 with a single primer pair using simple and rapid SYBR Green-based RT-PCR melt curve analysis

In spite of their involvement in foodborne illness, the epidemiological relevance of staphylococcal enterotoxin C (SEC) subtypes is poorly documented may be due to high sequence similarity. Among subtypes, SEC1, SEC2, and SEC3 exhibit more than 97 % homology because of which specific detection tools are seldom available to identify and differentiate them. In this study, a SYBR Green-based RT-PCR followed by melt curve analysis was developed for differentiation of entC1 from entC2/entC3 using a single primer pair. Nucleotide sequences of all three subtypes were analyzed using Clustal Omega program and the region with significant sequence variation/heterogeneity (where utmost SNPs were closely located and accessible for RT-PCR) was selected for amplification by designing a single primer pair that could amplify all three subtypes. In spite of same amplicon size, entC1 showed distinct melt peak at 76 °C. However, due to high similarity between entC2 and entC3, the developed format was deficient to discriminate between them and both showed melt peak at 82 °C. Reliability of developed RT-PCR was evaluated using various naturally contaminated samples and 91 food and clinical Staphylococcus aureus isolates where satisfactory results were obtained in comparison with commercial immunoassay kit and conventional PCRs using validated primers. To the best of our knowledge, this is the first method being reported to differentiate entC1 from entC2/entC3 using single primer pair which is unachievable by conventional PCR due to same amplicon size. As benefits, the method is sensitive, rapid, and inexpensive with no requirement of fluorescent probes, multiple primers, and post-PCR procedures. Thus, the assay might find its utility as a detection tool in epidemiological survey of foodborne outbreaks for simultaneous identification and differentiation of entC1 from entC2/entC3.