Staphylococcal enterotoxin-dependent cell-mediated cytotoxicity

Targeting glioblastoma multiforme using a novel fusion protein comprising interleukin-13 and staphylococcal enterotoxin B in vitro

Targeting cell surface receptors with immunotoxins provides a novel, unique and highly potent treatment against cancers. A high expression of interleukin-13 (IL13) receptor α2 (IL13Rα2) has been reported in different types of cancers including glioblastoma multiforme (GBM). In this paper, to target IL13Rα2 on GBM cells, a fusion protein was generated comprising human IL13 and staphylococcal enterotoxin B (SEB), termed IL13-linker-SEB. The fusion protein was cloned into pET28a(+) and expressed in Escherichia coli strain BL21 (DE3); U251 (IL13Rα2-positive) and T98G (IL13Rα2-negative) GBM cell lines were employed and the functional activity of IL13-linker-SEB was evaluated by cell ELISA, cytotoxicity (MTT and LDH), apoptosis (flow cytometry and caspase-3 activity), adhesion, scratch and RT-PCR tests. SEB and chemotherapeutic drugs were employed to be compared to IL13-linker-SEB function. The IL13-linker-SEB exhibited higher binding affinity and cytotoxicity compared to SEB on U251 cells, although both recombinant proteins had shown similar behavior regarding T98G cells. Furthermore, the highest induction of apoptosis was observed in U251 cells treated with IL13-linker-SEB which was confirmed by Bax/Bcl-2 ratio. The expression of MMP2, MMP9 and VEGFR2 in U251 cells experienced a significant reduction after treatment with IL13-linker-SEB compared to SEB and T98G treated cells. The data showed that IL13-linker-SEB can be considered as a novel potential agent for GBM treatment; however, further research is needed to investigate the efficacy.

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

Staphylococcal enterotoxin C2 (SEC2) is a classical superantigen (SAg), which can tremendously activate T lymphocytes at very low dosage, thus exerting its powerful antitumor activity. As an intravenous protein drug and a bacterial toxin, SEC2 has some limitations including poor patient compliance and toxic side effects. In this research, we devoted our attention to studying the antitumor activity and toxicity of SEC2 as a potential oral administration protein drug. We proved that His-tagged SEC2 (SEC2-His) could undergo facilitated transcytosis on human colon adenocarcinoma (Caco-2) cells and SEC2-His was detected in the blood of rats after oral administration. Furthermore, oral SEC2-His caused massive cytokine release and immune cell enrichment around tumor tissue, leading to inhibition of tumor growth in vivo. Meanwhile, although SEC2-His was dosed up to 32 mg/kg in mice, no significant toxicity was observed. These data showed that SEC2 can cross the intestinal epithelium in an immunologically integral form, maintaining antitumor activity but with reduced systemic toxicity. Therefore, these results may have implications for developing SEC2 as an oral administration protein drug.

Homeostatic regulation of CD8+ T cells by perforin

To prevent uncontrolled expansion, the massive proliferation of T cells during an acute immune response has to be followed by controlled deletion. Here we show that similar to Fas, perforin is not only an important effector molecule of cytotoxic T lymphocytes (CTL) but also involved in down-regulating peripheral T cells. Mice deficient for both the CTL effector molecule perforin and the apoptosis-inducing Fas ligand spontaneously develop infiltration of highly activated CD8(+) T cells in kidney and liver and die between 5 and 12 weeks of age. Injection of staphylococcal enterotoxin B (SEB) into perforin-deficient mice results in dramatically increased selective expansion and prolonged persistence of CD8(+), but not CD4(+), SEB-reactive T cells. Also, secondary immunization of TCR transgenic perforin-deficient mice with the lymphocytic choriomeningitis virus glycoprotein-derived epitope peptide leads to an increased proliferation of transgenic CD8(+) T cells, that is not explained by failure to deplete professional antigen-presenting cells. These results point to a novel mechanism of T cell homeostasis in which the acquisition of perforin-dependent cytotoxic activity regulates the expansion and persistence of CD8(+) effector T cells in vivo.

Antitumor Response Elicited by a Superantigen- Transmembrane Sequence Fusion Protein Anchored onto Tumor Cells

Superantigens stimulate T cells bearing certain TCR β-chain variable regions when bound to MHC II molecules. We investigated whether the superantigen toxic shock syndrome toxin-1 (TSST1) could induce an antitumor immune response when anchored onto MHC II-negative tumor cells. Our approach was to facilitate association of TSST1 with cell membranes by fusing its coding region to the transmembrane region (TM) sequence of the proto-oncogene c-erb-B-2. TSST1-TM was expressed in bacteria with an N-terminal histidine tag and purified using nickel-agarose affinity chromatography. Purified TSST1-TM added to cultures of several different MHC II-negative tumor cells spontaneously associated with cell membranes, as detected by flow cytometry. Because superantigens can direct cell-mediated cytotoxicity against MHC II-positive cells, a TM fusion protein lacking the TSST1 MHC II binding domain (TSST88–194-TM) was also constructed. Tumor cells precoated with TSST1-TM or TSST88–194-TM stimulated proliferation of human peripheral blood lymphocytes in vitro whereas uncoated tumor cells did not. Mice preimmunized with TSST1-TM- or TSST88–194-TM-coated tumor cells mounted a systemic response that resulted in significant antitumor immunity as measured by regression of a parental tumor challenge. TSST1-TM and TSST88–194-TM fusion proteins represent a useful new strategy for attaching superantigens or potentially other proteins onto tumor cell surfaces without genetic manipulation.

Prevention of superantigen-induced down-regulation of T-cell mediated cytotoxic activity by IL-2 in vivo

Administration of staphylococcal enterotoxin A (SEA) to mice induces profound activation, cytokine production and cytotoxic activity of both CD4+ and CD8+ T cells, but subsequently activated cells are deleted or become anergic. This study demonstrates that administration of interleukin-2 (IL-2) can prevent sea-induced hyporesponsiveness in CD8+ cytotoxic T lymphocytes (CTL). Repeated injections with sea every fourth day resulted in severely reduced cytotoxic activity in the spleen, which correlated with a reduced number of sea-responsive T-cell receptor (TCR)-V beta 11+ CD8+ cells. Studies of purified TCR-V beta 11+ CD8+ cells showed that they possessed intact cytotoxic activity per cell compared with cells from mice given a single injection of SEA, indicating that deletion was the main mechanism for the reduced cytotoxic activity. Combined treatment with SEA and IL-2 increased the number of cytotoxic cells in the spleen after each SEA injection and prevented the down-regulation of cytotoxic activity. Increased cytotoxic activity could be related to increased number and proliferation of CD8+ IL-2R alpha + cells, suggesting that administration of IL-2 maintained IL-2 responsiveness among CD8+ cells. Studies of sorted TCR-V beta 11+ CD8+ cells demonstrated that combined treatment with SEA and IL-2 also increased cytotoxic activity per cell compared with treatment with SEA alone. Taken together, IL-2 administration in vivo augmented SEA-induced expansion of T cells as well as the cytotoxic activity per CTL, and prevented SEA-induced cell deletion.

Antibody-targeted superantigens are potent inducers of tumor-infiltrating T lymphocytes in vivo

Recruitment of antigen-specific tumor-infiltrating lymphocytes (TILs) is a major goal for immunotherapy of malignant tumours. We now describe that T-cell-activating superantigens targeted to a tumor by monoclonal antibodies induced large numbers of pseudospecific TILs and eradication of micrometastases. As a model for tumor micrometastases, syngeneic B16 melanoma cells transfected with the human colon carcinoma antigen C215 were injected intravenously into C57BL/6 mice and therapy with an anti-C215 Fab fragment-staphylococcal enterotoxin A (C215Fab-SEA) fusion protein reacting with the C215 antigen was initiated when visible lung metastases were established. More than 90% reduction of the number of lung metastases was observed when mice carrying 5-day-old established lung metastases were treated with C215Fab-SEA. The antitumor effect of C215Fab-SEA was shown to be T-cell-dependent since no therapeutic effect was seen in T-cell-deficient nude mice. Depletion of T-cell subsets by injection of monoclonal antibody demonstrated that CD8+ cells were the most prominent effector cells although some contribution from CD4+ cells was also noted. C215Fab-SEA treatment induced massive tumor infiltration of CD4+ and CD8+ T cells, while only scattered T cells were observed in untreated tumors. SEA treatment alone induced a slight general inflammatory response in the lung parenchyme, but no specific accumulation of T cells was seen in the tumor. TILs induced by C215Fab-SEA were mainly CD8+ but a substantial number of CD4+ cells were also present. Immunohistochemical analysis showed strong production of the tumoricidal cytokines tumor necrosis factor alpha and interferon gamma in the tumor. Thus, the C215Fab-SEA fusion protein targets effector T lymphocytes to established tumors in vivo and provokes a strong local antitumor immune response.

Expression of CD11a and CD45R isoforms defines distinct subsets of CD8+ TCR alpha beta and TCR gamma delta CTL in vivo

Intraperitoneal immunization of rats with a syngeneic lymphoma and allogeneic leukocytes induced enrichment of antigen-selective TCR alpha beta+ and TCR gamma delta+ CTL. The peritoneal cavity seems to be a suitable site for enrichment of antigen-selective CTL, since the induced effector cells executed strong cytotoxicity without any requirement for in vitro reactivation. Tumor-selective CTL expressed high cell surface levels of CD45RC, allogeneic CTL expressed a variable level of CD45RC, while SAg-reactive CTL had low CD45RC expression. CTL with tumor and allogeneic selectivity as well as SAg-induced CTL all expressed the LFA-1high phenotype, suggesting that upregulation of LFA-1 is a hallmark for in vivo activated CTL. RT-PCR analyses showed that all CD8+TCR alpha beta+ CTL lost expression of CD45R exon 4 mRNA, which is compatible with the view that effector/memory T cells are CD45RA-. In contrast, TCR gamma delta+ CTL retained the CD45RA phenotype but showed a down-regulation of CD45R exon 8 mRNA. Since isoforms of the CD45 tyrosine phosphatase have been reported to differentially affect T-cell activation, the unique CD45R splice pattern observed in TCR alpha beta+ and TCR gamma delta+ CTL implies that CD45R may influence the TCR signal transduction distinctly in various effector CTL subsets.

Enhanced and prolonged efficacy of superantigen-induced cytotoxic T lymphocyte activity by interleukin-2 in vivo

The bacterial superantigen, staphylococcal enterotoxin A (SEA) activates T cells with high frequency and directs them to lyse MHC-class-II-expressing cells in superantigen-dependent cell-mediated cytotoxicity (SDCC). Treatment of mice with SEA induced strong CD8+ T-cell(CTL)-mediated SDCC, as well as abundant cytokine production from CD4+ and CD8+ T cells. However, both cytotoxicity and cytokine release were transient. In contrast, combined treatment with SEA and recombinant interleukin-2 (rIL-2) increased peak levels and maintained CTL activity. These effects were concomitant with an increased number of SEA-reactive V beta 11+ T cells. Both the CD4+ and CD8+ populations contained higher frequencies of cells expressing IL-2 receptor (IL-2R) alpha beta, which suggests that continuous IL-2R signaling preserves its high expression and subsequently prevents loss of growth factor signals necessary for expansion of T cells. Although IL-2R expression was increased among both CD4+ and CD8+ cells, only the cytotoxic function of CTL, but not cytokine production from either CD4 or CD8, was augmented. These findings demonstrate that treatment with rIL-2 potentiates superantigen-induced cytotoxicity and maintains high CTL activity. rIL-2 might therefore be useful in improving superantigen-based tumor therapy.

Bacterial pyrogenic exotoxins as superantigens

The recent discovery of the mode of interaction between a group of microbial proteins known as superantigens and the immune system has opened a wide area of investigation into the possible role of these molecules in human diseases. Superantigens produced by certain viruses and bacteria, including Mycoplasma species, are either secreted or membrane-bound proteins. A unique feature of these proteins is that they can interact simultaneously with distinct receptors on different types of cells, resulting in enhanced cell-cell interaction and triggering a series of biochemical reactions that can lead to excessive cell proliferation and the release of inflammatory cytokines. However, although superantigens share many features, they can have very different biological effects that are potentiated by host genetic and environmental factors. This review focuses on a group of secreted pyrogenic toxins that belong to the superantigen family and highlights some of their structural-functional features and their roles in diseases such as toxic shock and autoimmunity. Deciphering the biological activities of the various superantigens and understanding their role in the pathogenesis of microbial infections and their sequelae will enable us to devise means by which we can intervene with their activity and/or manipulate them to our advantage.

Monoclonal antibody-superantigen fusion proteins: tumor-specific agents for T-cell-based tumor therapy

The bacterial superantigen staphylococcal enterotoxin A (SEA) is an extremely potent activator of T lymphocytes when presented on major histocompatibility complex (MHC) class II molecules. To develop a tumor-specific superantigen for cancer therapy, we have made a recombinant fusion protein of SEA and the Fab region of the C215 monoclonal antibody specific for human colon carcinoma cells. SEA as part of a fusion protein showed a > 10-fold reduction in MHC class II binding compared to native SEA, and accordingly, the affinity of the FabC215-SEA fusion protein for the C215 tumor antigen was approximately 100-fold stronger than to MHC class II molecules. The FabC215-SEA fusion protein efficiently targeted T cells to lyse C215+ MHC class II- human colon carcinoma cells, which demonstrates functional substitution of the MHC class II-dependent presentation of SEA with tumor specificity. Treatment of mice carrying B16 melanoma cells expressing a transfected C215 antigen resulted in 85-99% inhibition of tumor growth and allowed long-term survival of animals. The therapeutic effect was dependent on antigen-specific targeting of the FabC215-SEA fusion protein, since native SEA and an antigen-irrelevant FabC242-SEA fusion protein did not influence tumor growth. The results suggest that Fab-SEA fusion proteins convey superantigenicity on tumor cells, which evokes T cells to suppress tumor growth.

Monoclonal antibodies and superantigens: a novel therapeutic approach

We have developed a monoclonal antibody (mAb) based therapy intended for the treatment of solid tumors utilizing both main arms of the immune system by incorporating the colon carcinoma recognizing mAb C215 and the T cell activating bacterial staphylococcal enterotoxin A (SEA) in a single hybrid molecule. The recombinant tumor specific superantigen C215-SEA retained excellent antigen binding properties while the binding to MHC class II was markedly reduced and should allow targeting of a large fraction of T cells to tumors in vivo. C215-SEA mediated T cell killing of C215 expressing tumor cells irrespective of their expression of MHC class II antigens and induced levels of IFN-gamma and TNF in mononuclear cells sufficient to completely suppress the growth of colon carcinoma cells in vitro. In initial studies of anti-tumor effects, C215Fab-SEA was found to markedly inhibit the growth of colon carcinoma cells transplanted to Scid mice adoptively transferred with human mononuclear cells.

T cell killing of human colon carcinomas by monoclonal-antibody-targeted superantigens

The bacterial superantigen staphylococcal enterotoxin A (SEA) induces T cell activation as well as directing activated T cells to kill major-histocompatibility-complex-class-II-expressing tumours such as freshly prepared leukemia cells. We now report that conjugates of SEA and the colon-carcinoma-reactive mAb C215 mediate T-cell-dependent killing of freshly isolated cells obtained from surgical specimens of human colon carcinomas. Cytotoxicity was observed at nanomolar concentrations of conjugate while no or very low effects were seen with the mAb C215 or SEA alone. Tumour-infiltrating lymphocytes (TIL) did not exert any cytotoxicity against conjugate-treated tumour cells immediately after isolation. In vitro culture of TIL with interleukin-2 and SEA resulted in SEA-mAb-conjugate-dependent killing of freshly isolated tumour cells. This suggests that mAb-SEA conjugates may be of potential use to target T lymphocytes, including TIL, against colon carcinoma cells in vivo.

Superantigen-based tumor therapy: in vivo activation of cytotoxic T cells

We have recently demonstrated that the superantigen staphylococcal enterotoxin A (SEA) targets in vitro activated cytotoxic T lymphocytes against tumor cells expressing major histocompatibility complex (MHC) class II antigens. In this report we analyze the use of SEA as an immunoactivator in vivo. Treatment of mice with SEA activated a fraction of CD3+ T cells apparently as a function of their T cell receptor V beta expression. SEA induced interleukin-2 receptor expression and proliferation in both CD4+ and CD8+ T cells. This proliferative response was dose-dependent (0.1-100 micrograms/mouse), peaked during day 1 after treatment and declined to background levels within 4 days. The cytotoxic response, measured as cytotoxicity to SEA-coated MHC class II+ target cells (staphylococcal-enterotoxin-dependent cell-mediated cytotoxicity, SDCC), was maximal at a dosage of 1 microgram SEA/mouse. The SDCC was confined to the CD8+ T cell compartment, peaked 2 days after treatment and declined to background levels within 4 days. A second injection of SEA on day 5 after the first SEA treatment resulted in SDCC function with kinetics and magnitude identical to that seen after one injection. These results pave the way for the use of SEA in the treatment of MHC class II+ tumors.

Targeting of superantigens

The bacterial superantigen staphylococcal enterotoxin A (SEA) is an extremely potent activator of T lymphocytes when presented on MHC class II antigens. In order to induce T lymphocytes to reject a tumor, we substituted the specificity of SEA for MHC class II molecules with specificity for tumor cells by combining SEA with a MAb recognizing colon carcinomas. Chemical conjugates or recombinant fusion proteins of the MAb C215 and SEA retained excellent antigen binding properties whereas the binding to MHC class II was markedly reduced. The hybrid proteins directed SEA responsive T cells to tumors with specificity determined by the specificity of the MAb. Significant tumor cell killing was obtained at picomolar concentrations of the hybrid proteins and was the result of direct cell mediated by cytotoxicity as well as production of tumoricidal cytokines by T cells. Targeting of superantigens represents a novel approach to specific immunomodulation and deserves further study as a potential therapy for malignant disease.