Superantigen-reactive T cells that display an anergic phenotype in vitro appear functional in vivo

Apoptosis-mediated inhibition of human T-cell acute lymphoblastic leukemia upon treatment with Staphylococus Aureus enterotoxin-superantigen

Patients with relapsed T cell acute lymphoblastic leukemia (T-ALL) have limited therapeutic options and poor prognosis. The finding of efficient strategies against this refractory neoplasm is a medical priority. Superantigens (SAgs) are viral and bacterial proteins that bind to major histocompatibility complex class II molecules as unprocessed proteins and subsequently interact with a high number of T cells expressing particular T cell receptor Vβ chains. Although on mature T cells, SAgs usually trigger massive cell proliferation producing deleterious effects on the organism, in contrast, on immature T cells, they may trigger their death by apoptosis. On this basis, it was hypothesized that SAgs could also induce apoptosis in neoplastic T cells that are usually immature cells that probably conserve their particular Vβ chains. In this work, we investigated the effect of the SAg Staphylococcus aureus enterotoxin E (SEE) (that specifically interacts with cells that express Vβ8 chain), on human Jurkat T- leukemia line, that expresses Vβ8 in its T receptor and it is a model of the highly aggressive recurrent T-ALL. Our results demonstrated that SEE could induce apoptosis in Jurkat cells in vitro. The induction of apoptosis was specific, correlated to the down regulation of surface Vβ8 TCR expression and was triggered, at least in part, through the Fas/FasL extrinsic pathway. The apoptotic effect induced by SEE on Jurkat cells was therapeutically relevant. In effect, upon transplantation of Jurkat cells in the highly immunodeficient NSG mice, SEE treatment reduced dramatically tumor growth, decreased the infiltration of neoplastic cells in the bloodstream, spleen and lymph nodes and, most importantly, increased significantly the survival of mice. Taken together, these results raise the possibility that this strategy can be, in the future, a useful option for the treatment of recurrent T-ALL.

Streptococcal Pyrogenic Exotoxin A-Stimulated Monocytes Mediate Regulatory T-Cell Accumulation through PD-L1 and Kynurenine

Bacterial superantigens (SAgs) are exotoxins that promote a fulminant activation of the immune system. The subsequent intense release of inflammatory cytokines often results in hypotension, shock, and organ failure with high mortality rates. In the current paradigm, the direct and simultaneous binding of SAgs with T-cell receptor (TCR)-bearing Vβ regions and conserved structures on major histocompatibility complex class II (MHC class II) on antigen-presenting cells (APCs) induces the activation of both cell types. However, by crosslinking MHC class II molecules, APCs can be activated by SAgs independently of T lymphocytes. Recently, we showed that streptococcal pyrogenic exotoxin A (SPEA) of Streptococcus pyogenes stimulates an immunogenic APC phenotype with upregulated costimulatory molecules and inflammatory cytokines. Additionally, we revealed that SPEA triggers immunosuppressive programs in monocytes that facilitate the accumulation of regulatory T cells (Tregs) in in vitro monocyte/CD4⁺ T-cell cocultures. Immunosuppressive factors include anti-inflammatory interleukin 10 (IL-10), co-inhibitory surface molecule programmed cell death 1 ligand 1 (PD-L1), and the inhibitory indoleamine 2,3-dioxygenase (IDO)/kynurenine effector system. In the present study, we investigated the underlying mechanism of SPEA-stimulated monocyte-mediated accumulation of Tregs. Blood-derived monocytes from healthy donors were stimulated with SPEA for 48 h (SPEA-monocytes). For the evaluation of SPEA-monocyte-mediated modulation of CD4⁺ T lymphocytes, SPEA was removed from the culture through extensive washing of cells before adding allogeneic CD3/CD28-activated T cells. Results: In coculture with allogeneic CD4⁺ T cells, SPEA-monocytes mediate apoptosis of CD4⁺Foxp3⁻ lymphocytes and accumulation of CD4⁺Foxp3⁺ Tregs. PD-L1 and kynurenine are critically involved in the mediated cell death because blocking both factors diminished apoptosis and decreased the proportion of the CD25⁺/Foxp3⁺ Treg subpopulation significantly. Upregulation of PD-L1 and kynurenine as well as SPEA-monocyte-mediated effects on T cells depend on inflammatory IL-1β. Our study shows that monocytes activated by SPEA mediate apoptosis of CD4⁺Foxp3⁻ T effector cells through PD-L1 and kynurenine. CD4⁺Foxp3⁺ T cells are resistant to apoptosis and accumulate in SPEA-monocyte/CD4⁺ T-cell coculture.

Staphylococcal superantigens in colonization and disease

Superantigens (SAgs) are a family of potent immunostimulatory exotoxins known to be produced by only a few bacterial pathogens, including Staphylococcus aureus. More than 20 distinct SAgs have been characterized from different S. aureus strains and at least 80% of clinical strains harbor at least one SAg gene, although most strains encode many. SAgs have been classically associated with food poisoning and toxic shock syndrome (TSS), for which these toxins are the causative agent. TSS is a potentially fatal disease whereby SAg-mediated activation of T cells results in overproduction of cytokines and results in systemic inflammation and shock. Numerous studies have also shown a possible role for SAgs in other diseases such as Kawasaki disease (KD), atopic dermatitis (AD), and chronic rhinosinusitis (CRS). There is also now a rich understanding of the mechanisms of action of SAgs, as well as their structures and function. However, we have yet to discover what purpose SAgs play in the life cycle of S. aureus, and why such a wide array of these toxins exists. This review will focus on recent developments within the SAg field in terms of the molecular biology of these toxins and their role in both colonization and disease.

Enhanced anti-tumor immunity by superantigen-pulsed dendritic cells

Staphylococcal enterotoxins A (SEA) and B (SEB) are classical models of superantigens (SAg), which induce potent T-cell-stimulating activity by forming complexes with MHC class II molecules on antigen-presenting cells. This large-scale activation of T-cells is accompanied by increased production of cytokines such as interferon-γ (IFN-γ). Additionally, as we previously reported, IFN-γ-producing CD8(+) T cells act as "helper cells," supporting the ability of dendritic cells to produce interleukin-12 (IL-12)p70. Here, we show that DC pulsed with SAg promote the enhancement of anti-tumor immunity. Murine bone marrow-derived dendritic cells (DC) were pulsed with OVA(257-264) (SIINFEKL), which is an H-2Kb target epitope of EG7 [ovalbumin (OVA)-expressing EL4] cell lines, in the presence of SEA and SEB and were subcutaneously injected into naïve C57BL/6 mice. SAg plus OVA(257-264)-pulsed DC vaccine strongly enhanced peptide-specific CD8(+) T cells exhibiting OVA(257-264)-specific cytotoxic activity and IFN-γ production, leading to the induction of protective immunity against EG7 tumors. Furthermore, cyclophosphamide (CY) added to SAg plus tumor-antigens (OVA(257-264), tumor lysate, or TRP-2) pulsed DC immunization markedly enhanced tumor-specific T-cell expansion and had a significant therapeutic effect against various tumors (EG7, 2LL, and B16). Superantigens are potential candidates for enhancing tumor immunity in DC vaccines.

Topical superantigen exposure induces epidermal accumulation of CD8+ T cells, a mixed Th1/Th2-type dermatitis and vigorous production of IgE antibodies in the murine model of atopic dermatitis

Patients with atopic dermatitis (AD) have repeated cutaneous exposure to both environmental allergens and superantigen-producing strains of Staphylococcus aureus. We used a murine model of AD to investigate the role of staphylococcal enterotoxin B (SEB) in the modulation of allergen-induced skin inflammation. Mice were topically exposed to SEB, OVA, a combination of OVA and SEB (OVA/SEB), or PBS. Topical SEB and OVA/SEB exposure induced epidermal accumulation of CD8+ T cells and TCRVbeta8+ cells in contrast to OVA application, which induced a mainly dermal infiltration of CD4+ cells. SEB and OVA/SEB exposure elicited a mixed Th1/Th2-associated cytokine and chemokine expression profile within the skin. Restimulation of lymph node cells from OVA- and OVA/SEB-exposed mice with OVA elicited strong production of IL-13 protein, whereas substantial amounts of IFN-gamma protein were detected after SEB stimulation of cells derived from SEB- or OVA/SEB-exposed mice. Topical SEB treatment elicited vigorous production of SEB-specific IgE and IgG2a Abs and significantly increased the production of OVA-specific IgE and IgG2a Abs. The present study shows that topical exposure to SEB provokes epidermal accumulation of CD8+ T cells, a mixed Th2/Th1 type dermatitis and vigorous production of specific IgE and IgG2a Abs, which can be related to the chronic phase of atopic skin inflammation.

Transcriptional modulation of TCR, Notch and Wnt signaling pathways in SEB-anergized CD4+ T cells

Gene expression changes in CD4 + Vbeta8+ T cells energized by in vivo exposure to staphylococcal enterotoxin B (SEB) bacterial superantigen compared to CD4 + Vbeta8+ non-energic T cells were assessed using DNA microarrays containing 5184 murine complementary DNAs. Anergy in splenic T cells of SEB-immunized BALB/c mice was verified by dramatically reduced proliferative capacity and an 8 x overexpression of GRAIL mRNA in CD4 + Vbeta8+ T cells taken from mice 7 days after injection. At an Associative t-test threshold of P<0.0005, 96 genes were overexpressed or detected only in anergic T cells, while 256 genes were suppressed or not detected in anergic T cells. Six of eight differential expressions tested using real-time quantitative PCR were validated. Message for B-Raf was detected only in non-anergic cells, while expression of the TCR signaling modulator Slap (Src-like adapter protein) and the TCR zeta-chain specific phosphatase Ptpn3 was enhanced. Modulation of multiple genes suggests downregulation of Wnt/beta-catenin signaling and enhanced Notch signaling in the anergic cells. Consistent with previous reports in a non-superantigen in vivo anergy model, mRNA for CD18 and the transcription factor Satb1 (special AT-rich-binding protein 1) was increased in SEB-energized T cells. This is the first report of global transcriptional changes in CD4+ T cells made anergic by superantigen exposure.

Natural killer T cells are required for the development of a superantigen-driven T helper type 2 immune response in mice

We show, here, that one single injection or weekly injections of staphylococcal enterotoxin B (SEB), starting in 1-day-old newborn mice, induced a powerful immune response with a T helper type 2 (Th2) pattern, as judged by the isotype and cytokine profile, with the production of large amounts of SEB-specific immunoglobulin G1 (IgG1), detectable levels of SEB-specific IgE and increased production of interleukin-4 by spleen cells. These protocols also induced an increase in the levels of total IgE in the serum. Memory of SEB was transferred to secondary recipients by using total spleen cells from primed animals. The secondary humoral response in transferred mice was diminished if spleen cells from SEB-treated mice were previously depleted of CD3+ or Vbeta8+ T cells or NK1.1+ cells. In vivo depletion of NK1.1+ cells in adult mice resulted in a marked reduction in the SEB-specific antibody response in both the primary and secondary immune responses. Additionally, purified NK1.1+ T cells were able to perform SEB-specific helper B-cell actions in vitro and in vivo. These results suggest that NK1.1+ T cells are required for the full development of humoral immunological memory, whilst making neonatal tolerance to SEB unachievable.

Behavioural endocrine immune-conditioned response is induced by taste and superantigen pairing

Administration of bacterial superantigen, such as staphylococcal enterotoxin B (SEB), induces in vivo stimulation of T cell proliferation and cytokine production such as interleukin-2 (IL-2). It has been previously reported that SEB administration induces fever, c-Fos expression in the brain, and hypothalamus-pituitary-adrenal axis activation, demonstrating that the brain is able to sense and respond to SEB. Previously it had been shown that immune functions can be behaviourally conditioned pairing a novel gustatory stimulus together with an immunomodulatory drug or an antigen. We designed an experimental protocol using Dark Agouti rats in which saccharin taste, as conditioned stimulus, was paired with an i.p. injection of SEB (2 mg/kg), as unconditioned stimulus. Six days later, when conditioned animals were re-exposed to the conditioned stimulus they displayed strong conditioned taste avoidance to the saccharin. More importantly, re-exposure to the conditioned stimulus significantly increased IL-2, interferon-gamma and corticosterone plasma levels, in comparison with conditioned animals which had not been re-exposed to saccharin taste. These results demonstrate a behavioural-immune-endocrine conditioned response using a superantigen as unconditioned stimulus. In addition, they illustrate the brain abilities to mimic the unconditioned effects of a superantigen by yet unknown mechanisms.

In vitro and in vivo T cell oligoclonality following chronic stimulation with staphylococcal superantigens

Microbial superantigens (SAg), including SEB and TSST-1, polyclonally activate T cells belonging to specific TCR BV families. A pathogenic role for SAg in various human diseases has been suggested, but enthusiasm for this view has been tempered by the T cell oligoclonality in these disorders. To assess whether T cell oligoclonality can emerge following protracted SAg stimulation, human PBMC were stimulated with SEB, TSST-1, or anti-CD3 mAb and maintained in culture with exogenous IL-2. Oligoclonality was appreciated by day 14 among CD4(+) and CD8(+) T cells. In addition, mice transgenic for human DR2 and DQ8 were injected weekly with SEB, and splenic CD4(+) and CD8(+) T cells were analyzed for oligoclonality. In mice that received one or three such injections, little-to-no oligoclonality was detected. In contrast, considerable oligoclonality was detected in mice that received eight weekly SEB injections. Many of these T cell oligoclones were identical to "spontaneously" arising oligoclones detected in SEB-naive mice. Thus, T cell oligoclonality can emerge following chronic SAg stimulation. In hosts who have lost tolerance to self Ag, chronic exposure to SAg may preferentially promote expansion of autoreactive T cells and facilitate development of clinical disease.

Apoptosis of superantigen-activated T cells induced by mycophenolate mofetil treatment

BACKGROUND

Mycophenolate mofetil (MMF), an ester prodrug of mycophenolic acid (MPA), is a potent immunosuppressive agent used in clinical organ transplantation. MPA preferentially inhibits the type II isoform of inosine monophosphate dehydrogenase, depletes GTP, suppresses transfer of mannose and fucose to glycoproteins, and prevents lymphocyte proliferation in vivo. Whether MMF can also delete activated T cells in vivo by triggering an apoptotic signal was addressed in this study. To this end we analyzed the activity of MMF in mice injected with the bacterial superantigen staphylococcal enterotoxin B (SEB). Superantigens bind to MHC class II molecules without requirement for processing, and activate subsets of CD4+ and CD8+ T cells whose T cell receptor beta chains express Vbeta family-specific homologous sequences. This model that shares several features with direct allorecognition has the unique advantage of allowing a precise monitoring of activated T cells.

METHODS

BALB/c mice treated with MMF (100 mg/kg/ day) or vehicle were injected with SEB. Serum cytokines, CD4+ and CD8+ Vbeta8+ cells were monitored in blood and lymphoid tissues, and apoptosis was determined by externalization of membrane phosphatidyl serine, double strand DNA breaks, and expression of B220 antigen by Vbeta8+ cells.

RESULTS

MMF treatment decreased tumor necrosis factor alpha, interferon gamma, and interleukin-10 secretion induced by SEB. It did not modify other early activation events (blast transformation, CD69 and CD25 expression) but completely inhibited SEB-induced expansion of Vbeta8+ cells by inducing apoptosis of SEB-reactive T cells. A similar effect was observed in CD95-ligand-deficient mice. Repeated SEB injections associated with MMF resulted in a marked decrease of CD8+ Vbeta8+ T cells. SEB-induced increase of Vbeta8+ thymocytes was not prevented by MMF treatment.

CONCLUSION

Results obtained in this in vivo model suggest that MMF treatment may induce deletion of activated peripheral T cells and decrease early cytokine responses.

Inhibition of thymidine synthesis by folate analogues induces a Fas-Fas ligand-independent deletion of superantigen-reactive peripheral T cells

Methotrexate (MTX), a folate antagonist with multiple enzymatic targets, is used in the treatment of malignancies as well as in autoimmune and chronic inflammatory diseases, and ZD1694 (tomudex), a water-soluble quinazoline specific inhibitor of thymidylate synthase (TS), is used in the treatment of adenocarcinomas. In this study, we investigated the effects of these folate analogues on superantigen (SAg)-reactive peripheral T cells in vivo. In BALB/c mice, staphylococcal enterotoxin B (SEB)-induced cytokine secretion, IL-2R (CD25) expression and early deletion of a fraction of SEB-reactive V(beta)8(+) T cells were not impaired by either MTX (7 mg/kg/day) or tomudex (5 mg/kg/day). However, both MTX and tomudex prevented V(beta)8-selective T cell expansion and accelerated their peripheral elimination. Administration of thymidine (500 mg/kg/12 h) completely abrogated this effect, indicating that inhibition of TS but not that of other folate-dependent enzymes was the main mechanism involved. Furthermore, a marked increase of apoptotic cells restricted to the V(beta)8(+) T cell subset indicated that proliferation inhibition was associated with apoptosis. In contrast with peripheral V(beta)8(+) T cell deletion, MTX and tomudex did not prevent the increase of V(beta)8(+) thymocytes triggered by SEB. Experiments in C57BL/6-lpr/lpr mice further demonstrated that deletion of V(beta)8(+) T cells induced by folate analogues was independent of Fas-Fas ligand interaction. Our results provide evidence that folate analogues may selectively delete dividing peripheral T cells through TS inhibition, but do not interfere with other events triggered by SAg.

Cutting edge: cell autonomous rather than environmental factors control bacterial superantigen-induced T cell anergy in vivo

Anergic T cells display a marked decrease in their ability to produce IL-2 and to proliferate in the presence of an appropriate antigenic signal. Two nonmutually exclusive classes of models have been proposed to explain the persistence of T cell anergy in vivo. While some reports indicate that anergic T cells have intrinsic defects in signaling pathways or transcriptional activities, other studies suggest that anergy is maintained by environmental "suppressor" factors such as cytokines or Abs. To distinguish between these conflicting hypotheses, we employed the well-characterized bacterial superantigen model system to evaluate in vivo the ability of a trace population of adoptively transferred naive or anergized T cells to proliferate in a naive vs anergic environment upon subsequent challenge. Our data clearly demonstrate that bacterial superantigen-induced T cell anergy is cell autonomous and independent of environmental factors.

Anti-Vbeta8 antibodies induce and maintain staphylococcal enterotoxin B-triggered Vbeta8+ T cell anergy

The mechanism involved in the maintenance of staphylococcal enterotoxin B (SEB)-induced T cell anergy is poorly understood. We demonstrated earlier that B cells play an important role in the maintenance of SEB-induced T cell anergy in vivo and in vitro. Here, we demonstrate that B cells are not essential in SEB-induced T cell activation, but are important for the maintenance of T cell memory phenotype and anergy in vivo. Studying the activated B cell repertoire, we observe that SEB treatment increases serum anti-Vbeta8 antibody titer as detected by enzyme-linked immunosorbent assay using soluble Vbeta8 chains as antigens, and by staining of a Vbeta8-expressing thymoma. These antibodies disappear gradually after immunization with SEB, whereas the capacity of the T cells to respond to SEB in vitro is restored. Anti-Vbeta8 monoclonal antibody treatment causes Vbeta8+ T cell unresponsiveness to SEB in vitro (anergy), without affecting CD4Vbeta8+ T cell frequency. Together, these results suggest a new mechanism to explain the maintenance of SEB-induced T cell anergy, which is dependent on B cells and on anti-Vbeta8 antibody that specifically interacts with Vbeta8+ T cells.

Staphylococcal enterotoxin B primes cytokine secretion and lytic activity in response to native bacterial antigens

Superantigens stimulate T-lymphocyte proliferation and cytokine production, but the effects of superantigen exposure on cell function within a complex, highly regulated immune response remain to be determined. In this study, we demonstrate that superantigen exposure significantly alters the murine host response to bacterial antigens in an in vitro coculture system. Two days after exposure to the superantigen staphylococcal enterotoxin B, splenocytes cultured with Streptococcus mutans produced significantly greater amounts of gamma interferon (IFN-gamma) and interleukin-12 than did sham-injected controls. The majority of IFN-gamma production appeared to be CD8(+) T-cell derived since depletion of this cell type dramatically reduced the levels of IFN-gamma. To study host cell damage that may occur following superantigen exposure, we analyzed cytotoxicity to "bystander" fibroblast cells cultured with splenocytes in the presence of bacterial antigens. Prior host exposure to staphylococcal enterotoxin B significantly enhanced fibroblast cytotoxicity in the presence of bacteria. Neutralization of IFN-gamma decreased the amount of cytotoxicity observed. However, a greater reduction was evident when splenocyte-bacterium cocultures were separated from the bystander cell monolayer via a permeable membrane support. Increased cytotoxicity appears to be primarily dependent upon cell-cell contact. Collectively, these data indicate that overproduction of inflammatory cytokines may alter the activity of cytotoxic immune cells. Superantigen exposure exacerbates cytokine production and lytic cell activity when immune cells encounter bacteria in vitro and comparable activities could possibly occur in vivo.

Rabies superantigen as a Vbeta T-dependent adjuvant

Recently we reported evidence that nucleocapsid (NC) of rabies virus is a Vbeta8-specific exogenous superantigen (SAg) in humans and a Vbeta6-specific SAg in BALB/c mice. NC was also found to stimulate rabies vaccination by enhancing the rabies neutralizing antibody response. In this study, we tested the hypothesis that the stimulating effect of NC and its SAg properties are linked. To do this, we studied the effect of rabies SAg on the immune response to an unrelated antigen, the influenza virus, and compared the response in two congenic strains of mice, BALB/c and BALB/D2. BALB/c mice are rabies SAg responsive, whereas BALB/D2 mice are not responsive to SAg activation by rabies NC because they lack the SAg recognition element, the Vbeta6 T cell receptor. In BALB/c mice, coinjection of rabies SAg with inactivated influenza virus resulted in a rapid and long-term increase in (a) the titres of influenza virus-specific antibodies (IgG and IgM), including protective hemagglutination-inhibiting antibodies, (b) antigen-specific proliferation and, (c) IL-2 and IL-4 secretion by lymph node lymphocytes, when compared to mice that received influenza virus only. In contrast, in BALB/D2 mice, neither antibody nor lymphocyte responses were stimulated. Moreover, during establishment of the primary response, the increase in influenza-primed T cells was mainly restricted to those bearing a Vbeta6 TCR. These data establish that rabies SAg can stimulate both T and B cell-specific responses to an unrelated antigen, depending on expression of the SAg target (Vbeta6 T lymphocytes). This is the first report linking NC adjuvant properties with its SAg mechanism.