Staphylococcal-enterotoxin-dependent cell-mediated cytotoxicity

Superantigens: The Good, the Bad, and the Ugly

Increasing evidence suggests that superantigens play a role in Immune-mediated diseases. Superantigens are potent activators of CD4* T cells, causing rapid and massive proliferation of cells and cytokine production. This characteristic of superantigens can be exploited in diseases where strong immunologic responses are required, such as in the B16F10 animal model of melanoma. Superantigen administration is able to significantly enhance Ineffective anti-tumor Immune responses, resulting in potent and long-lived protective anti-tumor immunity. However, superantigens are more well-known for the role they play in diseases. Studies using an animal model for neurologic demy-elinatlng diseases such as multiple sclerosis show that superantigens can induce severe relapses and activate auto-reactive T cells not involved in the Initial bout of disease. This may also involve epitope spreading of disease. Superantigens have also been implicated in acute diseases such as food poisoning and TSS, and in chronic diseases such as psoriasis and rheumatoid arthritis. Viral superantigens are also involved in the disease process, including superantigens derived from human Immunodeficiency virus and mouse mammary tumor virus. Finally, immunotherapies that ameliorate the role played by superantigens in disease are discussed.

Naptumomab estafenatox: targeted immunotherapy with a novel immunotoxin

Improvement of cancer therapy by introducing new concepts is still urgent even though there have been major advancements lately. Immunotherapy is well on the way to becoming an established tool in the cancer treatment armory. It seems that a combination of (1) activation of immune effector cells and selective targeting of them to tumors and (2) the inhibition of immune suppression often induced by the tumor itself are necessary to achieve the therapeutic goal. The immunotoxin naptumomab estafenatox was developed in an effort to activate and target the patient’s own T cells to their tumor, by fusing a superantigen (SAg) variant that activates T lymphocytes to the Fab moiety of a tumor-reactive monoclonal antibody. Naptumomab estafenatox targets the 5T4 tumor antigen, a 72-kDa oncofetal trophoblast protein expressed on many carcinomas, including renal cell carcinoma. The therapeutic effect is associated with activation of SAg-binding T cells. The SAg-binding T lymphocytes expand, differentiate to effector cells, and infiltrate the tumor. The therapeutic efficacy is most likely related to the dual mechanism of tumor cell killing: (1) direct lysis by cytotoxic T lymphocytes of tumor cells expressing the antigen recognized by the antibody moiety of the fusion protein and (2) secretion of cytokines eliminating antigen-negative tumor cell variants. Naptumomab estafenatox has been clinically tested in a range of solid tumors with focus on renal cell carcinoma. This review looks at the clinical experience with the new immunotoxin and its potential.

The tumor targeted superantigen ABR-217620 selectively engages TRBV7-9 and exploits TCR-pMHC affinity mimicry in mediating T cell cytotoxicity

The T lymphocytes are the most important effector cells in immunotherapy of cancer. The conceptual objective for developing the tumor targeted superantigen (TTS) ABR-217620 (naptumomab estafenatox, 5T4Fab-SEA/E-120), now in phase 3 studies for advanced renal cell cancer, was to selectively coat tumor cells with cytotoxic T lymphocytes (CTL) target structures functionally similar to natural CTL pMHC target molecules. Here we present data showing that the molecular basis for the anti-tumor activity by ABR-217620 resides in the distinct interaction between the T cell receptor β variable (TRBV) 7-9 and the engineered superantigen (Sag) SEA/E-120 in the fusion protein bound to the 5T4 antigen on tumor cells. Multimeric but not monomeric ABR-217620 selectively stains TRBV7-9 expressing T lymphocytes from human peripheral blood similar to antigen specific staining of T cells with pMHC tetramers. SEA/E-120 selectively activates TRBV7-9 expressing T lymphocytes resulting in expansion of the subset. ABR-217620 selectively triggers TRBV7-9 expressing cytotoxic T lymphocytes to kill 5T4 positive tumor cells. Furthermore, ABR-217620 activates TRBV7-9 expressing T cell line cells in the presence of cell- and bead-bound 5T4 tumor antigen. Surface plasmon resonance analysis revealed that ABR-217620 binds to 5T4 with high affinity, to TRBV7-9 with low affinity and to MHC class II with very low affinity. The T lymphocyte engagement by ABR-217620 is constituted by displaying high affinity binding to the tumor cells (K_D approximately 1 nM) and with the mimicry of natural productive immune TCR-pMHC contact using affinities of around 1 µM. This difference in kinetics between the two components of the ABR-217620 fusion protein will bias the binding towards the 5T4 target antigen, efficiently activating T-cells via SEA/E-120 only when presented by the tumor cells.

Increased T-cell stimulating activity by mutated SEC2 correlates with its improved antitumour potency

AIMS

To investigate the improved antitumour activity of SAM-3 compared with recombinant staphylococcal enterotoxins C2 (rSEC2).

METHODS AND RESULTS

Methylthiazol tetrazolium and flow cytometry assays showed that the antitumour activity of SAM-3 in vivo was improved because of enhanced T-cell stimulating potency, resulting in massive activation of T cells, particularly CD4(+) and CD8(+) T cells, and subsequent cytokine release. Quantitative real-time PCR assay showed that despite similar Vβ specificities induced by rSEC2 and SAM-3, the quantities of activated T cells bearing specific Vβin vitro were different.

CONCLUSIONS

The results strongly suggested that the increased SAM-3-T-cell receptor (TCR) binding affinity contributed to massive T-cell activation and cytokine release, substantially amplifying antitumour immune response in vivo.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provided evidence for the mechanism of SAM-3 antitumour activity improvement compared with rSEC2. Results indicated that SAM-3 could be used as a potent powerful candidate agent for tumour treatment in clinics.

An engineered superantigen SEC2 exhibits promising antitumor activity and low toxicity

Recent studies suggested that the histidine residues at 118 and 122 play an important role for the toxicity of staphylococcal enterotoxin C subtype 2 (SEC2), and the substitutions of both histidines with alanine can severely impair the fever activity of SEC2. We hypothesized that promising SEC2 antitumor agent with low toxicity and enhanced superantigen activity can be constructed by introducing related mutations at protein functional sites of SEC2. We showed that the SEC2 mutants H122A and H118A/H122A exhibited improved superantigen activity after introducing the point mutations at Thr20 and Gly22. A resultant mutant, named as SAM-3, has considerable abilities to inhibit the growth of H22 and Hepa1-6 tumor cells in vitro and colon 26 solid tumor in vivo. Furthermore, SAM-3 also exhibits significantly reduced toxicity compared with native SEC2. The study provides a novel strategy for designing promising superantigen immunotherapeutic agent. The constructed SEC2 mutant SAM-3 can be used as a powerful candidate for cancer immunotherapy and could compensate the deficiency caused by toxicity of native SEC2 in clinic.

Enhancement of superantigen activity and antitumor response of staphylococcal enterotoxin C2 by site-directed mutagenesis

Bacterial superantigen staphylococcal enterotoxins (SEs) tremendously stimulate polyclonal T cells bearing particular TCR Vbeta domains when binding to MHC II molecules, suggesting that they could be a candidate of new antitumor agent. SEC2, an important member of superantigen family, has been used in clinical trial as an immunotherapy agent for cancer treatment in China, and obtained some encouraging effects. However, the presence of immunosuppression and endotoxic activity limits the therapeutic dosage of SEC2, and influences its antitumor effect in clinic. Therefore, the enhancement of superantigen activity and antitumor effect of SEC2 could effectively make compensation for the disadvantages mentioned above. In this study, a superantigen SEC2(T20L/G22E) mutant was generated by site-directed mutagenesis, and efficiently expressed in E. coli BL21(DE3). The results showed that SEC2(T20L/G22E) mutant exhibited a significantly enhanced superantigen activity and antitumor response, compared with native SEC2 in vitro. Further toxicity assay in vivo indicated that SEC2(T20L/G22E) mutant had no significant increase in emetic and pyrogenic activity compared with SEC2, which suggested that the mutant SEC2(T20L/G22E) could be used as a potentially powerful candidate for cancer immunotherapy, and could make compensation for the deficiency of native SEC2 in clinic.

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.

A phase II study of a 5T4 oncofoetal antigen tumour-targeted superantigen (ABR-214936) therapy in patients with advanced renal cell carcinoma

In a phase II study, 43 renal cell carcinoma patients were treated with individualised doses of ABR-214936; a fusion of a Fab recognising the antigen 5T4, and Staphylococcal enterotoxin A. Drug was given intravenously on 4 consecutive days, treatment was repeated 1 month later. Treatment was associated with moderate fever and nausea, but well tolerated. Of 40 evaluable patients, 28 had disease control at 2 months, and at 4 months, one patient showed partial response (PR) and 16 patients stable disease. Median survival, with minimum follow-up of 26 months was 19.7 months with 13 patients alive to date. Stratification by the Motzer's prognostic criteria highlights prolonged survival compared to published expectation. Patients receiving higher drug exposure had greater disease control and lived almost twice as long as expected, whereas the low-exposure patients survived as expected. Sustained interleukin-2 (IL-2) production after a repeated injection appears to be a biomarker for clinical effect, as the induced-IL-2 level on the day 2 of treatment correlated with survival. The high degree of disease control and the prolonged survival suggest that this treatment can be effective. These findings will be used in the trial design for the next generation of drug, with reduced antigenicity and toxicity.

Interplay Between Superantigens and Immunoreceptors

Superantigens (SAGs) cause a massive T-cell proliferation by simultaneously binding to major histocompatibility complex (MHC) class II on antigen-presenting cells and T-cell receptors (TCRs) on T cells. These T-cell mitogens can cause disease in host, such as food poisoning or toxic shock. The best characterized groups of SAGs are the bacterial SAGs secreted by Staphylococcus aureus and Streptococcus pyogenes. Despite a common overall three-dimensional fold of these SAGs, they have been shown to bind to MHC class II in different ways. Recently, it has also been shown that SAGs have individual preferences in their binding to the TCRs. They can interact with various regions of the variable beta-chain of TCRs and at least one SAG seems to bind to the alpha-chain of TCRs. In this review, different subclasses of SAGs are classified based upon their binding mode to MHC class II, and models of trimolecular complexes of MHC-SAG-TCR molecules are described in order to reveal and understand the complexity of SAG-mediated T-cell activation.

Staphylococcal enterotoxin H induces V alpha-specific expansion of T cells

Staphylococcal enterotoxin H (SEH) is a bacterial superantigen secreted by Staphylococcus aureus. Superantigens are presented on the MHC class II and activate large amounts of T cells by cross-linking APC and T cells. In this study, RT-PCR was used to show that SEH stimulates human T cells via the Valpha domain of TCR, in particular Valpha10 (TRAV27), while no TCR Vbeta-specific expansion was seen. This is in sharp contrast to all other studied bacterial superantigens, which are highly specific for TCR Vbeta. It was further confirmed by flow cytometry that SEH stimulation does not alter the levels of certain TCR Vbeta. In a functional assay addressing cross-reactivity, Vbeta binding superantigens were found to form one group, whereas SEH has different properties that fit well with Valpha reactivity. As SEH binds on top of MHC class II, an interaction between MHC and TCR upon SEH binding is not likely. This concludes that the specific expansion of TCR Valpha is not due to contacts between MHC and TCR, instead we suggest that SEH directly interacts with the TCR Valpha domain.

Flare-up reaction on murine contact hypersensitivity. III. Effect of staphylococcal enterotoxin B

Staphylococcal enterotoxin B (SEB), a bacterial superantigen, is known as an immunomodulator because it activates an extremely large number of T-cells, and induces the production of large amounts of cytokines. In this study, we examined the effects of SEB on the contact hypersensitivity reaction (CHR). BALB/c mice were first sensitized through haptens applied to the back, and CHR was then induced through challenge to the left ear using the same haptens. SEB was administered intravenously 4 weeks later, causing a flare-up, peaking at 24 h post-administration, in the left ear that had previously exhibited CHR. This flare-up reaction was hapten non-specific, and was inhibited by anti-mouse tumor necrosis factor (TNF)-alpha antibodies. The flare-up was also suppressed by the oral administration of cyclosporin A prior to the administration of SEB. These results suggest that SEB induces a flare-up of CHR via the production of TNF-alpha.

Echinococcus granulosus: assays for hydatid immunoregulatory factors using established lymphoid cell lines

Mitosis, mitochondrial metabolic rate and proliferation were measured in established lymphoid cell lines exposed to chromatographic fractions of equine Echinococcus granulosus hydatid fluid. In several cell lines, one or more of the three parameters were modified by the exposure. As an assay for potential immunoregulatory activity, the method was simple and repeatable. The following novel observations were made: (1) Mitotic reaction was found among lines of T-cell, B-cell and macrophage origin; (2) mitosis was accompanied by proliferation in the B-cell lines, B9 and A20, and in the macrophage lines, HL-60 and P388d. With mitotically responsive T-cells, proliferation was slight in CTLL-2 and absent in D10, implying cell-cycle modification; (3) mitotic responsiveness tended to occur in cell lines with mature characteristics; (4) among cytokine-dependent cell lines, hydatid fluid FPLC fraction 1 mimicked IL-1 and several fractions mimicked IL-2 and IL-6 in the maintenance of mitosis; and (5) there was significant statistical interaction between the influences of mammalian cytokines and hydatid fluid fractions, implying that the propensity of antigenically unprimed lymphoid cells to be regulated by E. granulosus is conditioned by cytokine activity.

Experimental acute hematogenous osteomyelitis in mice. II. Influence of Staphylococcus aureus infection on T-cell immunity

A murine model of acute hematogenous osteomyelitis was used to study the immune response following Staphylococcus aureus infection and to examine the hypothesis that the bacteria may modify T-cell responses due to the production of bacterial enterotoxins with mitogenic or superantigenic activity. Lymph-node T cell-receptor expression was assessed with use of flow cytometry and reverse transcription-polymerase chain reaction techniques, and increased apoptosis (programmed cell death) in T-cell subsets was monitored. The expression and levels of circulating cytokines and T-cell cytokines within tissues surrounding the damaged area of the proximal tibia were also investigated. Analysis of T-cell receptors in experimental osteomyelitis revealed two distinct patterns of T-cell evolution during the disease. Certain T-cell subsets (Vbeta2, Vbeta3, Vbeta9, and Vbeta10) were activated and expanded during the first 24 hours after infection; they reached maximum levels 6 days after infection, followed by a return to pre-infection levels. In contrast, other T-cell subsets (Vbeta11, Vbeta12, Vbeta13, Vbeta14, and Vbeta16) contracted during the first 24 hours after infection, followed by expansion to a maximum level 9 days after infection. Activation and proliferation of T-cell subsets (notably Vbeta14 T cells) was followed by apoptosis, suggesting that staphylococcal bone infection caused superantigenic-like effects on the mouse immune system. Analysis of cytokine responses in local tissue revealed that the T-cell cytokines interleukin-2 and interferon-gamma showed a late and relatively short activation pattern compared with the inflammatory cytokines interleukin-1, interleukin-6, and tumor necrosis factor-alpha. The results suggest that Staphylococcus aureus bone infection may undermine the antibacterial immune response through downregulation of T-cell immunity and immune-cytokine production, which could increase the severity of the systemic infection and local osseous destruction that occur with acute hematogenous osteomyelitis.

Understanding the mechanism of action of bacterial superantigens from a decade of research

In the face of the unique diversity and plasticity of the immune system pathogenic organisms have developed multiple mechanisms in adaptation to their hosts, including the expression of a particular class of molecules called superantigens. Bacterial superantigens are the most potent stimulators of T cells. The functional consequences of the expression of superantigens by bacteria can be extended not only to T lymphocytes, but also to B lymphocytes and to cells of the myeloid compartment, including antigen-presenting cells and phagocytes. The biological effects of bacterial superantigens as well as their molecular aspects have now been studied for a decade. Although there is still a long way to go to clearly understand the role these molecules play in the establishment of disease, recently acquired knowledge of their biochemistry now offers unique experimental opportunities in defining the molecular rules of T-cell activation. Here, we present some of the most recent functional and molecular aspects of the interaction of bacterial superantigens with MHC class II molecules and the T-cell receptor.

Synovial fibroblasts as target cells for staphylococcal enterotoxin-induced T-cell cytotoxicity

Rheumatoid arthritis (RA) is a chronic autoimmune disease of unknown aetiology. Recently, superantigens have been implied in the pathogenesis of RA. Superantigens activate a large fraction of T cells leading to the production of cytokines and proliferation. In addition, superantigens direct cellular cytotoxicity towards major histocompatibility complex (MHC) class II-expressing cells. There is now increasing evidence that cytotoxic T cells may be involved in the pathogenesis of RA. In the inflamed synovia class II-positive synovial fibroblasts (SFC) are found. In the present study it was tested whether MHC class II-positive SFC serve as target cells for superantigen-induced cellular cytotoxicity. SFC were stimulated with interferon-gamma to express class II antigens, then they were cultivated in the presence of CD4-positive T cells with or without staphylococcal enterotoxins (SE). Cytotoxicity of T cells was measured as release of lactate dehydrogenase from SFC. Specific cytotoxicity was only found in the presence of class II-positive SFC depending on the dose of SE. Maximum lysis was seen after 20 hr. T-cell cytotoxicity was inhibited by antibodies to MHC class II antigens. The data suggest that class II-positive SFC not only function as accessory cells for SE-mediated T-cell proliferation and interleukin-2 production but may also be the targets of superantigen-mediated cellular cytotoxicity.

Genetically engineered superantigens as tolerable antitumor agents

Superantigens (SAg) are a family of bacterial and viral proteins with strong immunostimulatory properties. SAg bound to major histocompatibility complex (MHC) class II molecules activate a high frequency of T cells and represent the most potent known activators of T cells to date. To explore the use of SAg for T cell-based tumor therapy we have created a tumor-reactive SAg by engineering a fusion protein composed of a tumor-reactive mAb (C215Fab) and the bacterial SAg staphylococcal enterotoxin A (SEA). A point mutation D227A was introduced at the major MHC class II binding site in SEA to reduce systemic toxicity. Treatment of tumor bearing mice with the Fab-SEA D227A fusion protein resulted in profound antitumor effects with a markedly reduced toxicity as compared with the wild-type Fab-SEA fusion protein. The reduced toxicity was probably due to a weak distribution of the SEA D227A fusion protein in tissues with a high MHC class II expression and low systemic cytokine levels as exhibited in mice and rabbits. The data presented demonstrate the efficacy of immunoconjugates containing a mutated SAg in directing a T cell attack against tumor cells with minimal systemic immune activation.

T-cell stimulation and cytokine release induced by staphylococcal enterotoxin A (SEA) and the SEAD227A mutant

Previous work demonstrated that human cytotoxic T cells activated by superantigens can lyse major histocompatibility complex (MHC) class II-positive target cells as well as MHC class II-negative tumour cells coated with conjugates of monoclonal antibodies and superantigens. In order to decrease MHC class II affinity, and therefore unwanted binding of the superantigen staphylococcal enterotoxin A (SEA) to MHC class II molecules, a point mutation was introduced into the SEA gene. This mutation (SEAD227A) resulted in an approximately 3-log reduction of affinity to human leucocyte antigen (HLA)-DR, but cytotoxicity mediated by this mutant superantigen towards antibody-labelled tumour cells is as efficient as cytotoxicity mediated by the native superantigen. We therefore compared the T-cell activating potency of native and mutated SEA. Our data show that SEAD227A is 4- to 5-log less effective than native SEA when activation of resting T cells is assayed in terms of blast formation, expression of cell surface activation markers and cytokine release. Furthermore, presenting either SEA or SEAD227A to MHC class II-negative mononuclear cells by MHC class II-negative tumour cells did not result in significant blast formation of T cells, up-regulation of CD25 or cytokine release. This suggests that lysis of MHC class II-negative tumour cells is efficiently induced by monoclonal antibody targeted superantigen, while activation of resting T cells requires additional co-stimulatory signals.

Echinococcosis

Hydatids, the intermediate stages, or metacestodes, of the tapeworm genus, Echinococcus, present a major immunological problem; they survive, grow and metastasize in immunized hosts which are protected against reinfection and possess effector mechanisms capable of killing the parasite. Explanations for this state of concomitant immunity have been made from investigations of avoidance strategies, genetics and quantitative hydatid growth. The latter study suggests that the host-parasite relationship is sustained as a dynamic equilibrium between parasite growth and acquired immunity, the balance being subject to mutual regulation and including the possibility of spontaneous rejection of the parasite. Two immunoregulatory, or cytokine-like, factors have been detected in hydatids of Echinococcus spp. One appears to be a mediator of the previously reported mitogenic effects of hydatids. Recent evidence has linked these effects to generation of T-suppressor populations. The second factor interferes with the interaction of macrophages and T-cells, mimics the effect of metacestode infection in impairing the accessory action of macrophages in lymphoproliferative responses, and is suppressive for rosette-forming cell responses against third-party antigens. It is suggested that these factors form part of a primary homeostatic mechanism regulating hydatid growth and immunity.

Primary cytotoxicity of SEB-activated human PBMC and separated CD8+ and CD4+ T lymphocytes elicited by two different stimulation protocols

The primary cytotoxicity of Staphylococcal enterotoxin B (SEB)-stimulated PBMC and separated CD8+ and CD4+ T cells against Burkitt's lymphoma target cells has been characterized by applying two stimulation protocols. In the bulk protocol, the PBMC were stimulated with 100 ng/mL SEB for 3 days before separation to CD4+ and CD8+ T subsets. In the direct protocol, the separated CD4+ and CD8+ T cells were stimulated with 100 ng SEB preabsorbed to mitomycin C (MMC)-treated APC. Comparison of the results of the two different protocols revealed the following differences: (i) PBMC in the direct protocol provided greater cytolytic activity than in the bulk protocol; and (ii) the CD4+ T cells acquired cytotoxicity only in the direct protocol. Unexpectedly, the superantigen-dependent cellular cytotoxicity (SDCC) of SEB-stimulated cells was not dominant compared with the basal cytotoxicity. The classical NK target, K-562 was also sensitive to SEB-augmented cytotoxicity. The parallel stimulation with IL-2 and SEB caused a similar extent of cytotoxicity enhancement against both types of target cells. However, the cyclosporin A (CSA) inhibited only the SEB-induced cytotoxicity. The results suggest that SEB-induced PBMC acquire mainly a LAK-like cytotoxicity, as a consequence of newly produced lymphokines. This observation might propose a different approach in pathological studies.

Proteolytic cleavage of MHC class I by complement C1-esterases--an overlooked mechanism?

The complement C1-esterases have been shown to cleave the MHC class I molecules, which are important participants in the activation of T lymphocytes, between the alpha 2- and the alpha 3-domain of the heavy chain. The possible involvement of the C1-esterases in the regulation of peripheral self-tolerance is discussed. It is hypothesized that the C1-esterase-mediated cleavage of the MHC class I molecules either induces: a soluble fragment of the outer two domains of the MHC class I molecule, in association with beta 2-microglobulin, to bind to the T cell receptors and prevent the cells from being activated, or produces a change in the exposure of the alpha 3-domain that remain on the cell surface, acting as mediator of a 'veto signal' that prevents these cells from being activated.

Induction of lymphocytes cytotoxic to oral epithelial cells by Streptococcus mitis superantigen

The preparation of a superantigenic fraction F-2 from the culture supernatant of Streptococcus mitis 108, a fresh isolate from human tooth surfaces, was reported previously. Now, to determine the possible pathogenic role of the superantigen in oral mucosal diseases, we examined the cytotoxic effects of human peripheral blood T-cells activated with F-2 on human oral epithelial cells. T-cells activated with F-2 were cytotoxic to the human squamous carcinoma HO-1-N-1 cells derived from the oral mucosa, similar to those activated with Staphylococcus aureus enterotoxin B (SEB). This cytotoxic effect was increased in a dose-dependent manner by the addition of the respective stimulant, F-2 or SEB, to the cytotoxic assay system. F-2 endowed mainly CD8+ T-cells with cytotoxic activity. Pretreatment with human interferon gamma increased the sensitivity of the HO-1-N-1 cells to the cytotoxic effects of F-2-activated T-cells. The F-2-activated T-cells were also cytotoxic to human keratinocytes derived from gingiva. There was no correlation between the degree of cytotoxicity and the levels of tumor necrosis factor alpha in co-cultures of F-2-activated T-cells and HO-1-N-1 cells. A double-chamber plate experiment revealed no cytotoxic effects when the F-2-activated T-cells were separated from the HO-1-N-1 cells. Supernatants of the co-cultures of target and effector cells were not cytotoxic to HO-1-N-1 cells. These findings suggest that the cytotoxic effects of the F-2-activated T-cells on HO-1-N-1 cells were mediated not by soluble factors but by the direct interaction between the activated T-cells and the target cells. The cytotoxicity of the F-2-activated T-cells against HO-1-N-1 cells was markedly inhibited by monoclonal antibodies (MAbs) against CD11a and CD54, but was only slightly inhibited by MAbs against human leukocyte antigen (HLA)-DR and CD2. Thus, the interaction between lymphocyte-function-associated antigen-1 (LFA-1) and intercellular adhesion molecule-1 (ICAM-1) was crucial for the F-2-dependent T-cell-mediated cytotoxicity against oral epithelial cells, while HLA-DR and CD2 molecules are not necessarily involved in the cytotoxicity observed.

T cell-dependent antibody response to staphylococcal enterotoxin B

Treatment of mice with staphylococcal enterotoxin B (SEB) induces specific T-cell tolerance to this superantigen, characterized by partial deletion of V beta 8+ T cells in vivo and T cell anergy in vitro. In this study we examined the humoral response to SEB in BALB/c mice. Immunization of mice with SEB results in a detectable anti-SEB antibody response. Upon further treatment of mice with SEB, specific antibody levels increase significantly and the response is accelerated--characteristics of a secondary humoral response. The secondary antibody response is T cell dependent, can be transferred to T cell deficient mice with splenocytes and is composed mainly of IgM, IgG1 and IgG2b isotypes, suggesting that Th2 cells provide B cell help in this response. These data demonstrate that at the same time as inducing in vitro unresponsiveness, SEB primes SEB-specific T helper cells to provide help for B cells in a secondary antibody response.

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.

Frequency of toxic shock syndrome toxin- and enterotoxin-producing clinical isolates of Staphylococcus aureus

In 183 clinical isolates of Staphylococcus aureus, toxic shock syndrome toxin was detected in 13.7%, enterotoxin A in 20.2%, enterotoxin B in 7.7%, enterotoxin C in 5.5% and enterotoxin D in 3.3%. Seventy-three (39.9%) of the strains were found to produce one or more toxins. Multiple secretion of toxins was rare (< 1% of all strains) except for the combinations enterotoxin A with toxic shock syndrome toxin and enterotoxin A with enterotoxin D. These combinations were observed at significantly higher frequencies (4.9% and 2.2%, respectively) than would have been expected from values calculated from respective individual frequencies (0.88% and 0.06%) (p < 0.0001). In comparison with Staphylococcus aureus strains isolated from miscellaneous material, Staphylococcus aureus blood culture isolates produced enterotoxin D in significantly larger amounts and at higher frequencies (p = 0.01 in both cases). These toxins might play an important pathogenic role in Staphylococcus aureus infections.

Altered antigen-presenting capacity of human monocytes after phagocytosis of bacteria

The antigen-presenting and accessory functions of monocytes were studied after phagocytosis of bacteria. Peripheral blood monocytes isolated from mononuclear cells by counterflow elutriation were incubated with suspensions of opsonized bacteria (Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, or Salmonella enteritidis) under conditions in which at least 80% of the monocytes engulfed microorganisms. Either the cells were pulsed with antigen (purified derivative of tuberculin or tetanus toxoid) and used as antigen-presenting cells for autologous T lymphocytes or the accessory function of the cells was examined in pokeweed mitogen-activated cultures of T cells. It has been found that phagocytosis of bacteria by monocytes reduces their ability to trigger antigen- and mitogen-induced proliferation. The reduced proliferative response of T lymphocytes was not due to a change of the kinetics of the response or triggering of suppressor mechanisms. Furthermore, antigen processing was not affected much after phagocytosis of bacteria since antigen-pulsed and paraformaldehyde-fixed cells containing bacteria were comparable to control cells in their antigen-presenting capacity. This phenomenon was observed after phagocytosis of both living and dead bacteria and was not correlated to the viability of monocytes, which were more affected after phagocytosis of living bacteria than of dead ones. As a result of phagocytosis of bacteria, reduced expression of CD54, CD14, and HLA-DQ, variable reduction of HLA-DP, CD58, and CD64, and reduced viability of monocytes were observed. In conclusion, phagocytosis of bacteria by monocytes affects their antigen-presenting and accessory functions presumably because of changes in the expression of molecules essential for monocyte-T-cell interactions and reduction of their viability.

Combined activation of murine lymphocytes with staphylococcal enterotoxin and interleukin-2 results in additive cytotoxic activity

This report demonstrates that in vitro activation of murine spleen cells with interleukin-2 (IL-2) or the bacterial superantigen staphylococcal enterotoxin A (SEA) results in different patterns of activation and function of cytotoxic cells. Lymphokine-activated killer activity and antibody-dependent cellular cytotoxicity (ADCC) are mainly mediated by IL-2 activated natural killer (NK) cells. SEA is the most powerful T cell mitogen known so far and retargets cytotoxic T lymphocytes (CTL) to tumors expressing major histocompatibility complex (MHC) class II in staphylococcal-enterotoxin-dependent cellular cytotoxicity (SDCC). Culture of mouse spleen cells with SEA led to expansion and activation of T cells, which demonstrated strong SDCC activity and some NK-like cytotoxicity after 5 days in culture. Cell sorting revealed that both CD8+ and CD4+ T cells mediated SDCC but the former were more effective. Phenotypic analysis showed that SEA preferentially stimulated and expanded T cells expressing T cell receptor V beta 11, in particular CD8+ T cells. Combined activation with SEA and IL-2 resulted in simultaneous induction of T and NK cell cytotoxicity. Moreover, IL-2 had additive effects on SEA-induced SDCC. Combined treatment with SEA and IL-2 might therefore be an approach to induce maximal cytotoxicity against tumors and to recruit both T and NK cells in tumor therapy.

Role of T lymphocytes in experimental Staphylococcus aureus arthritis

Using a recently developed murine model of haematogenously induced Staphylococcus aureus, the authors have characterized the phenotypes and functional properties of inflammatory cells present in the synovium of arthritic mice. The results show that large numbers of granulocytes and macrophages were observed in the inflamed synovia within 24 h of inoculation of S. aureus strain LS-1. Many of the synovial macrophage-like cells strained for cytoplasmic IL-1 alpha and TNF-alpha. Subsequently (> or = 48 h later), a prominent infiltration of T lymphocytes, predominantly of CD4+ phenotype, was observed. Some of the T lymphocytes stained for IL-2 receptor and intracytoplasmic interferon-gamma (IFN-gamma). Surprisingly, in spite of the severe inflammatory process, very few cells expressed MHC class-II molecules in the arthritic synovia. In addition, in vivo depletion of CD4+ T-cells resulted in a considerably milder course of staphylococcal arthritis. The similarities in the phenotype expression of synovial cells and central role of T-cells in S. aureus arthritis as well as in non-infectious models of arthritis, indicate that the process governing joint destruction is likely to be the same, irrespective of the initial stimulus.

Absence of peripheral clonal deletion and anergy in immune responses of T cell-reconstituted athymic mice

Superantigens induce clonal deletion of reactive T cells in the thymus and clonal deletion and anergy in the periphery of euthymic mice. In this report we have assessed the ability of Staphylococcal enterotoxin B (SEB) to induce peripheral tolerance in nude mice reconstituted with normal, syngeneic T cells. Immunization of reconstituted nude mice with SEB resulted in lethal toxic shock in a large fraction of the animals. Such lethality was never observed in the normal donor mouse strain. Analysis of lymphokine production in response to SEB showed that reconstituted nude mice produced higher levels of interleukin-2 and tumor necrosis factor-alpha, but lower levels of interleukin-4, than euthymic control mice. Furthermore, SEB was unable to promote either clonal elimination or induction of anergy in the SEB-responsive peripheral T cells, despite the fact that reconstituted nude mice did produce high levels of corticosterone upon treatment with SEB. These results imply a lack of control over immune responses to superantigen in T cell-reconstituted athymic mice.

Staphylococcal enterotoxin B promotes deletion and functional inactivation of CD4V beta 8-positive cells in the absence of CD8 T cells

The staphylococcal enterotoxins stimulate discrete subsets of T cells depending on their expression of particular V genes. Among these, staphylococcal enterotoxin B (SEB) vigorously stimulates V beta 8+ cells. This stimulation results in proliferation of both CD4+V beta 8+ and CD8+ T cells and eventually to anergy and clonal deletion in the former subset. We have examined the possible role of CD8+ T cells in the response of CD4+ cells to SEB, by in vivo CD8+ T-cell-depletion. We found no qualitative difference in the responses of untreated and CD8+ T-cell depleted mice to SEB; however, a small quantitative difference in deletion was observed. Thus it appears that on the whole the response of CD4+V beta 8+ T cells to SEB is independent of CD8+ T-cell effector function, although the latter may play a partial role.

Contact of lymphocytes with Helicobacter pylori augments natural killer cell activity and induces production of gamma interferon

We studied the capacity of glutaraldehyde-fixed Helicobacter pylori to stimulate natural killer (NK) cell activity. Bacteria were incubated overnight with peripheral blood lymphocytes enriched for large granular lymphocytes (LGL), the mediators of non-major histocompatibility complex-restricted cellular cytotoxicity. Then, the cytolytic activity of LGL was tested against various tumor target cells. We observed that efficient cytolytic activity was generated against resistant and nonresistant tumor target cell lines. Nine local clinical isolates of H. pylori and the reference strain NCTC 11637 were tested, and they all were equally effective in inducing NK cell activity. However, flagellin antigen, glycine extract, urease, and lipopolysaccharide prepared from H. pylori NCTC 11637 all failed to induce significant NK cell activity. The supernatants which were collected after coincubation of bacteria with LGL contained a factor(s) which could activate resting LGL into efficient cytolytic activity. The supernatants were also analyzed for interferon (IFN) activity. We observed that high titers of IFN were produced and that IFN activity was neutralized with anti-gamma interferon (IFN-gamma) antiserum, but not with anti-IFN-alpha antiserum. Thus, contact of lymphocytes with H. pylori leads to efficient stimulation of NK cell activity and the production of IFN-gamma.

Lectins and superantigens: membrane interactions of these compounds with T lymphocytes affect immune responses

1. Lectins and superantigens belong to two different families of macromolecules which are able to interact with cells of the immune system. 2. The principal mechanisms by which they modulate immune responses are presented in this review. 3. Possible similarities shared by these proteins and their common mechanisms of action upon immunocytes will be presented along with a brief discussion regarding the role of these molecules in physiological immune responses and human diseases.

CD8 is critically involved in lymphocyte activation by a T. brucei brucei-released molecule

T. brucei brucei released a lymphocyte triggering factor (TLTF), which triggered purified CD8+, but not CD4+, T cells to interferon gamma (IFN-gamma) mRNA expression and secretion and to [3H]thymidine incorporation. TLTF also induced mRNA for transforming growth factor beta, but not for interleukin-4. The action of this TLTF on mononuclear cell (MNC) cultures was blocked by anti-CD8 antibodies and by soluble CD8. MNCs from a mutant mouse strain lacking CD8 expression were not triggered by TLTF. IFN-gamma provides a growth stimulus for T. brucei brucei, and infected CD8- mice had much lower parasitemia and survived longer than CD8+ mice. The host-parasite interaction in experimental African trypanosomiasis thus involves parasite release of TLTF, which by binding to CD8 triggers CD8+ cells to produce the parasite growth-promoting cytokine IFN-gamma.

Inhibition of allostimulated HLA-DQ and DP-specific T cells by staphylococcal enterotoxin A

Bacterial superantigens have two immunologically important features. They bind MHC class II molecules and stimulate T cells bearing certain V beta TCR phenotypes. Superantigens such as SEA, SEB, and TSST bind to each of the three HLA class II isotypes (DR, DQ, and DP). Allotypic variation seems to play an important role in superantigen binding to class II molecules, but the functional implications of these differences remain largely unknown. In the present investigation, we studied the effects of SEA, SEB, and TSST on allostimulation of HLA-DR-, DQ-, and DP-allospecific T-cell clones. To avoid direct stimulation of T-cell responses by the superantigens, SEA and/or SEB nonresponsive T-cell clones were selected. We show that SEA strongly inhibited DQ- and DP-specific T-cell responses. In contrast, SEB and TSST had only weak inhibitory effects. DR-specific T-cell responses were unaffected or only weakly inhibited by the superantigens tested. The inhibition appeared not to be due to induction of cytotoxicity or suppression of either T cells or EBV-LCLs by SEA. In conclusion, the bacterial superantigen SEA can block alloantigen-specific stimulation of T clones in vitro. These results suggest that SEA binds to certain MHC class II molecules in a way that prevents MHC-TCR interactions.

The viral superantigen Mls-1a induces interferon-gamma secretion by specifically primed CD8+ cells but fails to trigger cytotoxicity

Superantigens can be operationally defined by their ability to stimulate CD4+ and CD8+ T cells via the T cell receptor beta chain variable domain (TcR V beta). We show here that effector functions of CD8+ T cells specific for superantigens differ depending upon the nature of the superantigen involved. Hence, activated CD8+ T cells bearing TcR V beta specific for the superantigen Mls-1a [encoded in the open reading frame of the 3' long terminal repeat of endogenous mouse mammary tumor virus (MMTV)] are unable to lyse Mls-1a-bearing target cells despite the fact that they release interferon-gamma (IFN-gamma) upon Mls-1a stimulation. In contrast CD8+ T cells specific for the exogenous superantigen staphylococcal enterotoxin B (SEB) readily mediate both lysis and IFN-gamma secretion when exposed to SEB-bearing target cells. This dissociation between lysis and IFN-gamma production by Mls-1a-specific CD8+ T cells is independent of the initial stimulus used for activation and appears not to be simply explained by a low Mls-1a determinant density. We suggest that this phenomenon reflects differing TcR affinity thresholds for lymphokine secretion and cytolysis. Such differences may be exploited by retroviruses such as MMTV in order to escape immunosurveillance.

Presentation of superantigen by human T cell clones: A model of T-T cell interaction

Superantigens (SAg) interact with T lymphocytes bearing particular V beta sequences as part of their T cell receptor (TcR). The interaction, however, requires the presence of major histocompatibility complex (MHC) class II molecules on antigen-presenting cell (APC). In peculiar circumstances, MHC class II+ T cell clones (TCC) have been shown to present peptides and selected antigens interacting with antigen-specific TCC in the absence of APC. In this report we studied the capacity of SAg to mediate a T-T cell interaction, investigating the TCC ability to present a panel of staphylococcal enteroxins (SE) independently of the presence of added APC. Upon exposure to a broad range of SE concentrations, MHC class II+ TCC showed an intense proliferative response even in the absence of professional APC. Diverse SE optimally stimulated responder TCC at different concentrations. The proliferation was inhibited by anti-DR monoclonal antibodies, both in the presence and in the absence of APC. The SE activation of TCC in the absence of APC induced the same series of phenotypic variations as that observed following the TCC stimulation with APC. Irradiated TCC efficiently presented membrane-bound SE to responder TCC as well as professional APC. These results show that a single cell of a given clone effectively presents the SE to other cells of the same clone, and provide evidence that SAg can efficiently mediate T-T cell interaction. In addition, the possibility also exists that one cell of the clone can actually undergo an auto-stimulation via SAg-mediated interactions between its own TcR and MHC class II molecule. It has recently been suggested that the V beta-selective depletion of T cells observed in acquired immunodeficiency syndrome (AIDS) patients might be a consequence of the interaction between a human immunodeficiency virus (HIV)-encoded SAg and T cells expressing a SAg complementary V beta. We suggest that the hypothesized HIV-encoded SAg might mediate T-T cell interactions that could play a relevant role in the V beta-selective depletion of T lymphocytes observed in HIV-infected patients.

Kinetics of serum levels of interleukin-6 in Staphylococcus aureus septicemia

The kinetics of Il-6 serum concentrations were analyzed in 17 patients with a culture verified Staphylococcus aureus septicemia. The analyses were performed using an antigen capture immunoassay. All patients had elevated IL-6 serum concentrations on admission. In most cases a subsequent rapid decrease to low levels was registered within 10 days. The IL-6 serum concentrations reflected the clinical course. In sera sampled on admission a relationship was found between IL-6 serum concentrations and levels of C-reactive protein.

Monoclonal antibody-targeted superantigens: a different class of anti-tumor agents

The bacterial superantigen staphylococcal enterotoxin (SE) A (SEA) directs cytotoxic T lymphocytes (CTLs) expressing particular sequences of the T-cell receptor (TCR) beta chain to lyse tumor cells expressing major histocompatibility complex (MHC) class II molecules, which serve as receptors for SEs. We now report that chemical conjugates of SEA and the colon carcinoma-reactive monoclonal antibodies (mAbs) C215 or C242 mediate T cell-dependent destruction of colon carcinoma cells lacking MHC class II molecules. SEA was covalently linked to the mAbs C215 and C242 via a PEG-based hydrophilic spacer. The C215-SEA conjugate targeted CD4+ as well as CD8+ CTLs to lyse a panel of colon carcinoma cells lacking MHC class II molecules. T-cell recognition of mAb-SEA conjugates was SEA specific, since SEB-selective T-cell lines with potent cytotoxic activity towards Raji cells coated with SEB did not respond to the C215-SEA conjugate. Unconjugated SEA did not induce T-cell lysis of MHC class II- colon carcinoma cells but efficiently directed CTLs against MHC class II+ Raji cells and certain interferon-treated MHC class II+ colon carcinoma cells. These results suggest that SEA-mAb conjugates retain the SEA-related selectivity for certain TCR beta-chain variable region (V beta) sequences but, in contrast to unconjugated SEA, mediate the TCR interaction in a MHC class II-independent manner. The cytotoxic activity mediated by C215-SEA and C242-SEA conjugates was blocked by excess of C215 mAb and C242 mAb, respectively, showing that the specificity in the targeting of mAb-SEA conjugates is defined by the antigen reactivity of the mAb. These results demonstrate that bacterial superantigens may be successfully conjugated to mAb with preserved T cell-activating capacity. The circumvention of MHC class II binding of SEs by conjugation to mAb suggests that such conjugates may find general application as antitumor agents, taking advantage of the extreme T cell-activating potency of superantigens.

Evidence for quantitative and qualitative differences in functional activation of Mls-reactive T cell clones and hybridomas by antigen or TcR/CD3 antibodies

In this study, we demonstrated that some V beta 6+, CD4+, Mls-1a-specific T cell clones had cytolytic activity when stimulated with anti-T cell receptor(TcR)/CD3 monoclonal antibodies (mAb), but not with targets expressing Mls-1a, although they produced lymphokines (interleukin 2 and interferon-gamma) in response to both types of stimuli. To examine the possibility that lack of cytolysis resulted from expression of the Mls-1a antigen on merely a fraction of splenic B blasts, we (a) used the B cell lymphoma LBB.3.4.16 and (b) measured esterase secretion which is generally concurrent with cytotoxic T lymphocyte (CTL) activity. The B cell lymphoma maximally stimulated the T cell clone for interferon-gamma production when responding and stimulating cells were incubated at a 1:1 ratio, but it was never killed by the Mls-1a-specific T cell clone unless TcR/CD3-specific mAb were added. Furthermore, a fivefold excess of the Mls-1a B cell lymphoma did not induce any secretion of esterase, which was observed only in the presence of the TcR/CD3-specific mAb. Comparison of the reactivity of two Mls-1a-specific T cell hybridomas expressing the same TcR at similar surface density, revealed both quantitative and qualitative differences between CD3-specific mAb and Mls stimulation of the hybridomas. A small quantitative difference in the sensitivity of hybridoma FJ22.5 to stimulation with V beta 6 or CD3-specific mAb resulted in a marked decrease in efficiency of stimulation by Mls-1a for interleukin 2 production and to inability to detect growth inhibition by Mls-expressing cells. A qualitative difference was observed when analyses of inositol phosphate production were performed under optimal conditions of stimulation of the highly responsive T cell hybridoma (FJ8.1): only stimulation with CD3-specific mAb, but not Mls-expressing cells, could induce detectable inositol phosphate production. Lack of cytolysis of Mls-1a class II-expressing B cells may have evolutionary significance in view of the recent mapping of Mls to mouse mammary tumor virus genes.