Up-regulation of granzyme B and perforin by staphylococcal enterotoxin C2 mutant induces enhanced cytotoxicity in Hepa1–6 cells

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.

Staphylococcal Enterotoxin C2 Mutant-Induced Antitumor Immune Response Is Controlled by CDC42/MLC2-Mediated Tumor Cell Stiffness

As a biological macromolecule, the superantigen staphylococcal enterotoxin C2 (SEC2) is one of the most potent known T-cell activators, and it induces massive cytotoxic granule production. With this property, SEC2 and its mutants are widely regarded as immunomodulating agents for cancer therapy. In a previous study, we constructed an MHC-II-independent mutant of SEC2, named ST-4, which exhibits enhanced immunocyte stimulation and antitumor activity. However, tumor cells have different degrees of sensitivity to SEC2/ST-4. The mechanisms of immune resistance to SEs in cancer cells have not been investigated. Herein, we show that ST-4 could activate more powerful human lymphocyte granule-based cytotoxicity than SEC2. The results of RNA-seq and atomic force microscopy (AFM) analysis showed that, compared with SKOV3 cells, the softer ES-2 cells could escape from SEC2/ST-4-induced cytotoxic T-cell-mediated apoptosis by regulating cell softness through the CDC42/MLC2 pathway. Conversely, after enhancing the stiffness of cancer cells by a nonmuscle myosin-II-specific inhibitor, SEC2/ST-4 exhibited a significant antitumor effect against ES-2 cells by promoting perforin-dependent apoptosis and the S-phase arrest. Taken together, these data suggest that cell stiffness could be a key factor of resistance to SEs in ovarian cancer, and our findings may provide new insight for SE-based tumor immunotherapy.

In vitro study on the toxicity of nanoplastics with different charges to murine splenic lymphocytes

Nanoplastics can be ingested by organisms and penetrate biological barriers to affect multiple physiological functions. However, few studies have focused on the effects of nanoplastics on the mammalian immune system. We evaluated the effects and underlying mechanism of nanoplastics of varying particle sizes and surface charges on murine splenic lymphocytes. We found that nanoplastics penetrated into splenic lymphocytes and that nanoplastics of a diameter of 50 nm were absorbed more efficiently by the cells. The nanoplastics decreased cell viability, induce cell apoptosis, up-regulated apoptosis-related protein expression, elicited the production of reactive oxygen species, altered mitochondrial membrane potential, and impaired mitochondrial function. Positively charged nanoplastics exerted the strongest toxicity. Negatively charged and uncharged nanoplastics caused oxidative stress and mitochondrial structural damage in lymphocytes, while positively charged nanoplastics induced endogenous apoptosis directly. Moreover, nanoplastics inhibited the expression of activated T cell markers on the T cell surface, while inhibiting the differentiation of CD8⁺ T cells and the expression of helper T cell cytokines. In terms of the mechanism, a series of key signaling molecules in the pathways of T cell activation and function were markedly down-regulated after exposure to nanoplastics.

A recombinant protein containing influenza viral conserved epitopes and superantigen induces broad-spectrum protection

Influenza pandemics pose public health threats annually for lacking vaccine that provides cross-protection against novel and emerging influenza viruses. Combining conserved antigens that induce cross-protective antibody responses with epitopes that activate cross-protective T cell responses might be an attractive strategy for developing a universal vaccine. In this study, we constructed a recombinant protein named NMHC that consists of influenza viral conserved epitopes and a superantigen fragment. NMHC promoted the maturation of bone marrow-derived dendritic cells and induced CD4+ T cells to differentiate into Th1, Th2, and Th17 subtypes. Mice vaccinated with NMHC produced high levels of immunoglobulins that cross-bound to HA fragments from six influenza virus subtypes with high antibody titers. Anti-NMHC serum showed potent hemagglutinin inhibition effects to highly divergent group 1 (H1 subtype) and group 2 (H3 subtype) influenza virus strains. Furthermore, purified anti-NMHC antibodies bound to multiple HAs with high affinities. NMHC vaccination effectively protected mice from infection and lung damage when exposed to two subtypes of H1N1 influenza virus. Moreover, NMHC vaccination elicited CD4+ and CD8+ T cell responses that cleared the virus from infected tissues and prevented virus spread. In conclusion, this study provides proof of concept that NMHC vaccination triggers B and T cell immune responses against multiple influenza virus infections. Therefore, NMHC might be a candidate universal broad-spectrum vaccine for the prevention and treatment of multiple influenza viruses.

Induction of CD4+ regulatory T cells by stimulation with Staphylococcal Enterotoxin C2 through different signaling pathways

As a member of superantigens, Staphylococcal Enterotoxin C2 (SEC2) can potently activate T cells expressing specific Vβ repertoires and has been applied in clinic for tumor immunotherapy in China for more than 20 years. However, excessive activation of T cells by over-stimulation with superantigen are always followed by eliciting regulatory T cells (Tregs) induction and functional immunosuppression, which brings uncertainties to SEC2 application in tumor immunotherapy. In this study, we found that SEC2 could induce CD4+CD25+Foxp3+ Tregs from the murine splenocytes in dose and time related manners. The induced Tregs with high expression of GITR and CTLA-4 and low expression of CD127 were TCR Vβ8.2-specific and have character of IL-10 production in a SEC2 dose-depended manner. Importantly, SEC2-induced CD4+ Tregs showed the potent capacity of suppressing proliferation of intact murine splenocytes response to SEC2. Furthermore, by using specific inhibitors or neutralizing antibody, we proved that the signaling pathways of TCR-NFAT/AP-1, IL-2-STAT5, and TGF-β-Smad3 play crucial roles in Tregs induction by SEC2. These findings will help us better understand the balance of immune stimulation and immunosuppression mediated by SEC2 and provide valuable guidance for SEC2 application in antitumor immunology.

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.

An iRGD peptide fused superantigen mutant induced tumor-targeting and T lymphocyte infiltrating in cancer immunotherapy

Solid tumors are intrinsically resistant to immunotherapy because of the major challenges including the immunosuppression and poor penetration of drugs and lymphocytes into solid tumors due to the complicated tumor microenvironment (TME). Our previous study has created a novel superantigen mutant ST-4 to efficiently active the T lymphocytes and alleviate immune suppression. In the present study, to accumulate ST-4 into the TME, we constructed a recombinant protein, ST-4-iRGD, by fusing ST-4 to a tumor-homing peptide, iRGD. We hypothesized that ST-4-iRGD could internalize into the TME through iRGD-mediated tumor targeting and tumor tissue penetrating to activate the regional immunoreaction. The results of in vitro studies showed that ST-4-iRGD achieved improved tumor targeting and cytotoxicity in mouse B16F10 melanoma cells. The iRGD-mediated tumor tissue penetration was further confirmed by imaging and immunofluorescence studies in vivo, wherein higher distribution of ST-4-iRGD was observed in the mouse 4T1 breast tumor model. Moreover, ST-4-iRGD exhibited enhanced anti-solid tumor characteristics and induced improved lymphocyte infiltration in the B16F10 and 4T1 models. In conclusion, using iRGD to facilitate better dissemination of the therapeutic agent ST-4 throughout a solid tumor mass is feasible, and ST-4-iRGD may be a potential candidate for efficient cancer immunotherapy in the future.

Enhanced interaction between SEC2 mutant and TCR Vβ induces MHC II-independent activation of T cells via PKCθ/NF-κB and IL-2R/STAT5 signaling pathways

SEC2, a major histocompatibility complex class II (MHC II)-dependent T-cell mitogen, binds MHC II and T-cell receptor (TCR) Vβs to induce effective co-stimulating signals for clonal T-cell expansion. We previously characterized a SEC2 mutant with increased recognition of TCR Vβs, ST-4, which could intensify NF-κB signaling transduction, leading to IL-2 production and T-cell activation. In this study, we found that in contrast to SEC2, ST-4 could induce murine CD4⁺ T-cell proliferation in a Vβ8.2- and Vβ8.3-specific manner in the absence of MHC II⁺ antigen-presenting cells (APCs). Furthermore, although IL-2 secretion in response to either SEC2 or ST-4 stimulation was accompanied by up-regulation of protein kinase Cθ (PKCθ), inhibitor of κB (IκB), α and β IκB kinase (IKKα/β), IκBα, and NF-κB in mouse splenocytes, only ST-4 could activate CD4⁺ T cells in the absence of MHC II⁺ APCs through the PKCθ/NF-κB signaling pathway. The PKCθ inhibitor AEB071 significantly suppressed SEC2/ST-4-induced T-cell proliferation, CD69 and CD25 expression, and IL-2 secretion with or without MHC II⁺ APCs. Further, SEC2/ST-4-induced changes in PKCθ/NF-κB signaling were significantly relieved by AEB071 in a dose-dependent manner. Using Lck siRNA, we found that Lck controlled SEC2/ST-4-induced phosphorylation of PKCθ. We also demonstrated that the IL-2R/STAT5 pathway is essential for SEC2/ST-4-induced T-cell activation. Collectively, our data demonstrate that an enhanced ST-4-TCR interaction can compensate for lack of MHC II and stimulate MHC II-free CD4⁺ T-cell proliferation via PKCθ/NF-κB and IL-2R/STAT5 signaling pathways. Compared with SEC2, intensified PKCθ/NF-κB and IL-2R/STAT5 signals induced by ST-4 lead to enhanced T-cell activation. The results of this study will facilitate better understanding of TCR-based immunotherapies for cancer.

Mutant p53 tunes the NRF2-dependent antioxidant response to support survival of cancer cells

NRF2 (NFE2L2) is one of the main regulators of the antioxidant response of the cell. Here we show that in cancer cells NRF2 targets are selectively upregulated or repressed through a mutant p53-dependent mechanism. Mechanistically, mutant p53 interacts with NRF2, increases its nuclear presence and resides with NRF2 on selected ARE containing gene promoters activating the transcription of a specific set of genes while leading to the transcriptional repression of others. We show that thioredoxin (TXN) is a mutant p53-activated NRF2 target with pro-survival and pro-migratory functions in breast cancer cells under oxidative stress, while heme oxygenase 1 (HMOX1) is a mutant p53-repressed target displaying opposite effects. A gene signature of NRF2 targets activated by mutant p53 shows a significant association with bad overall prognosis and with mutant p53 status in breast cancer patients. Concomitant inhibition of thioredoxin system with Auranofin and of mutant p53 with APR-246 synergizes in killing cancer cells expressing p53 gain-of-function mutants.

Discovery of a novel Nrf2 inhibitor that induces apoptosis of human acute myeloid leukemia cells

Nuclear factor-erythroid 2-related factor 2 (Nrf2) is persistently activated in many human tumors including acute myeloid leukemia (AML). Therefore, inhibition of Nrf2 activity may be a promising target in leukemia therapy. Here, we used an antioxidant response element-luciferase reporter system to identify a novel pyrazolyl hydroxamic acid derivative, 1-(4-(tert-Butyl)benzyl)-3-(4-chlorophenyl)-N-hydroxy-1H pyrazole-5-carboxamide (4f), that inhibited Nrf2 activity. 4f had a profound growth-inhibitory effect on three AML cell lines, THP-1, HL-60 and U937, and a similar anti-growth effect in a chick embryo model. Moreover, flow cytometry of AML cells revealed increased apoptosis with 4f (10 μM) treatment for 48 h. The protein levels of cleaved caspase-3 and cleaved poly (ADP-ribose) polymerase were enhanced in all three AML cell types. Furthermore, Nrf2 protein level was downregulated by 4f. Upregulation of Nrf2 by tert-butylhydroquinone (tBHQ) or Nrf2 overexpression could ameliorate 4f-induced growth inhibition and apoptosis. Treatment with 4f reduced both B-cell lymphoma-2 (Bcl-2) expression and Bcl-2/Bcl-2–associated X protein (Bax) ratio, which indicated that 4f induced apoptosis, at least in part, via mitochondrial-dependent signaling. Therefore, as an Nrf2 inhibitor, the pyrazolyl hydroxamic acid derivative 4f may be a promising agent in AML therapy.