Cross-linking of major histocompatibility complex class II molecules by staphylococcal enterotoxin A superantigen is a requirement for inflammatory cytokine gene expression

A genetically engineered therapeutic lectin inhibits human influenza A virus infection and sustains robust virus-specific CD8 T cell expansion

Seasonal influenza continues to be a global health problem. Current existing vaccines and antivirals against influenza have limited effectiveness, and typically do not stay ahead of the viral evolutionary curve. Broad-spectrum antiviral agents that are effective therapeutically and prophylactically are much needed. We have created a promising new broad-spectrum anti-influenza agent using molecular engineering of a lectin from bananas, H84T, which is well-tolerated and protective in small animal models. However, the potency and effect of H84T on human immune cells and influenza-specific immune responses are undetermined. We found that H84T efficiently inhibited influenza A virus (IAV) replication in primary human dendritic cells (DCs) isolated from blood and tonsil, preserved DC viability and allowed acquisition and presentation of viral antigen. Excitingly, H84T-treated DCs subsequently initiated effective expansion of IAV-specific CD8 T cells. Furthermore, H84T preserved the capacity of IAV-exposed DCs to present a second non-IAV antigen and induce robust antigen-specific CD8 T cell expansion. Our data support H84T as a potent antiviral in humans as it not only effectively inhibits IAV infection, but also preserves induction of robust pathogen-specific adaptive immune responses against diverse antigens, which likely is clinically beneficial.

A genetically engineered therapeutic lectin inhibits human influenza A virus infection and sustains robust virus-specific CD8 T cell expansion

Native banana lectin (BanLec) is antiviral but highly mitogenic, which limits its therapeutic value. In contrast, the genetically engineered H84T BanLec (H84T) is not mitogenic but remains effective against influenza A virus (IAV) infection in mouse models. However, the potency and effect of H84T on human immune cells and IAV-specific immune responses is undetermined. We found that H84T efficiently inhibited IAV replication in human dendritic cells (DCs) from blood and tonsils, which preserved DC viability and allowed acquisition and presentation of viral antigen. Consequently, H84T-treated DCs initiated effective expansion of IAV-specific CD8 T cells. Furthermore, H84T preserved the capacity of IAV-exposed DCs to present a second non-IAV antigen and induce robust CD8 T cell expansion. This supports H84T as a potent antiviral in humans as it effectively inhibits IAV infection without disrupting DC function, and preserves induction of antigen-specific adaptive immune responses against diverse antigens, which likely is clinically beneficial.

Keratinocytes Present Staphylococcus aureus Enterotoxins and Promote Malignant and Nonmalignant T Cell Proliferation in Cutaneous T-Cell Lymphoma

Cutaneous T-cell lymphoma is characterized by malignant T cells proliferating in a unique tumor microenvironment dominated by keratinocytes (KCs). Skin colonization and infection by Staphylococcus aureus are a common cause of morbidity and are suspected of fueling disease activity. In this study, we show that expression of HLA-DRs, high-affinity receptors for staphylococcal enterotoxins (SEs), by KCs correlates with IFN-γ expression in the tumor microenvironment. Importantly, IFN-γ induces HLA-DR, SE binding, and SE presentation by KCs to malignant T cells from patients with Sézary syndrome and malignant and nonmalignant T-cell lines derived from patients with Sézary syndrome and mycosis fungoides. Likewise, preincubation of KCs with supernatant from patient-derived SE-producing S aureus triggers proliferation in malignant T cells and cytokine release (including IL-2), when cultured with nonmalignant T cells. This is inhibited by pretreatment with engineered bacteriophage S aureus-specific endolysins. Furthermore, alteration in the HLA-DR-binding sites of SE type A and small interfering RNA-mediated knockdown of Jak3 and IL-2Rγ block induction of malignant T-cell proliferation. In conclusion, we show that upon exposure to patient-derived S aureus and SE, KCs stimulate IL-2Rγ/Jak3-dependent proliferation of malignant and nonmalignant T cells in an environment with nonmalignant T cells. These findings suggest that KCs in the tumor microenvironment play a key role in S aureus-mediated disease activity in cutaneous T-cell lymphoma.

Staphylococcus aureus induces drug resistance in cancer T cells in Sézary syndrome

ABSTRACT

Patients with Sézary syndrome (SS), a leukemic variant of cutaneous T-cell lymphoma (CTCL), are prone to Staphylococcus aureus infections and have a poor prognosis due to treatment resistance. Here, we report that S aureus and staphylococcal enterotoxins (SE) induce drug resistance in malignant T cells against therapeutics commonly used in CTCL. Supernatant from patient-derived, SE-producing S aureus and recombinant SE significantly inhibit cell death induced by histone deacetylase (HDAC) inhibitor romidepsin in primary malignant T cells from patients with SS. Bacterial killing by engineered, bacteriophage-derived, S aureus-specific endolysin (XZ.700) abrogates the effect of S aureus supernatant. Similarly, mutations in major histocompatibility complex (MHC) class II binding sites of SE type A (SEA) and anti-SEA antibody block induction of resistance. Importantly, SE also triggers resistance to other HDAC inhibitors (vorinostat and resminostat) and chemotherapeutic drugs (doxorubicin and etoposide). Multimodal single-cell sequencing indicates T-cell receptor (TCR), NF-κB, and JAK/STAT signaling pathways (previously associated with drug resistance) as putative mediators of SE-induced drug resistance. In support, inhibition of TCR-signaling and Protein kinase C (upstream of NF-κB) counteracts SE-induced rescue from drug-induced cell death. Inversely, SE cannot rescue from cell death induced by the proteasome/NF-κB inhibitor bortezomib. Inhibition of JAK/STAT only blocks rescue in patients whose malignant T-cell survival is dependent on SE-induced cytokines, suggesting 2 distinct ways SE can induce drug resistance. In conclusion, we show that S aureus enterotoxins induce drug resistance in primary malignant T cells. These findings suggest that S aureus enterotoxins cause clinical treatment resistance in patients with SS, and antibacterial measures may improve the outcome of cancer-directed therapy in patients harboring S aureus.

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.

Novel SPEA Superantigen Peptide Agonists and Peptide Agonist-TGFαL3 Conjugate. In Vitro Study of Their Growth-Inhibitory Effects for Targeted Cancer Immunotherapy

Bacterial superantigens (SAgs) are effective T-cell stimulatory molecules that lead to massive cytokine production. Superantigens crosslink between MHC class II molecules on the Antigen Presenting Cells (APC) and TCR on T-cells. This enables them to activate up to 20% of resting T cells, whilst conventional antigen presentation results in the activation of 0.001-0.0001% of the T cell population. These biological properties of superantigens make them attractive for use in immunotherapy. Previous studies have established the effectiveness of superantigens as therapeutic agents. This, however, was achieved with severe side effects due to the high lethality of the native toxins. Our study aims to produce superantigen-based peptides with minimum or no lethality for safer cancer treatment. In previous work, we designed and synthesized twenty overlapping SPEA-based peptides and successfully mapped regions in SPEA superantigen, causing a vasodilatory response. We screened 20 overlapping SPEA-based peptides designed and synthesized to cover the whole SPEA molecule for T-cell activation and tumor-killing ability. In addition, we designed and synthesized tumor-targeted superantigen-based peptides by fusion of TGFαL3 either from the N' or C' terminal of selected SPEA-based peptides with an eight-amino acid flexible linker in between. Our study identified parts of SPEA capable of stimulating human T-cells and producing different cytokines. We also demonstrated that the SPEA-based peptide conjugate binds specifically to cancer cells and can kill this cancer. Peptides induce T-cell activation, and tumor killing might pave the way for safer tumor-targeted superantigens (TTS). We proposed the combination of our new superantigen-based peptide conjugates with other immunotherapy techniques for effective and safer cancer treatment.

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.

Plasmacytoid Dendritic Cells Die by the CD8 T Cell-Dependent Perforin Pathway during Acute Nonviral Inflammation

Regulation of the inflammatory response involves the control of dendritic cell survival. To our knowledge, nothing is known about the survival of plasmacytoid dendritic cells (pDC) in such situation. pDC are specialized in type I IFN (IFN-I) secretion to control viral infections, and IFN-I also negatively regulate pDC survival during the course of viral infections. In this study, we asked about pDC behavior in the setting of virus-free inflammation. We report that pDC survival was profoundly reduced during different nonviral inflammatory situations in the mouse, through a mechanism independent of IFN-I and TLR signaling. Indeed, we demonstrated that during inflammation, CD8(+) T cells induced pDC apoptosis through the perforin pathway. The data suggest, therefore, that pDC have to be turned down during ongoing acute inflammation to not initiate autoimmunity. Manipulating CD8(+) T cell response may therefore represent a new therapeutic opportunity for the treatment of pDC-associated autoimmune diseases, such as lupus or psoriasis.

Staphylococcal Superantigens Spark Host-Mediated Danger Signals

Staphylococcal enterotoxin B (SEB) of Staphylococcus aureus, and related superantigenic toxins produced by myriad microbes, are potent stimulators of the immune system causing a variety of human diseases from transient food poisoning to lethal toxic shock. These protein toxins bind directly to specific Vβ regions of T-cell receptors (TCR) and major histocompatibility complex (MHC) class II on antigen-presenting cells, resulting in hyperactivation of T lymphocytes and monocytes/macrophages. Activated host cells produce excessive amounts of proinflammatory cytokines and chemokines, especially tumor necrosis factor α, interleukin 1 (IL-1), IL-2, interferon γ (IFNγ), and macrophage chemoattractant protein 1 causing clinical symptoms of fever, hypotension, and shock. Because of superantigen-induced T cells skewed toward TH1 helper cells, and the induction of proinflammatory cytokines, superantigens can exacerbate autoimmune diseases. Upon TCR/MHC ligation, pathways induced by superantigens include the mitogen-activated protein kinase cascades and cytokine receptor signaling, resulting in activation of NFκB and the phosphoinositide 3-kinase/mammalian target of rapamycin pathways. Various mouse models exist to study SEB-induced shock including those with potentiating agents, transgenic mice and an “SEB-only” model. However, therapeutics to treat toxic shock remain elusive as host response genes central to pathogenesis of superantigens have only been identified recently. Gene profiling of a murine model for SEB-induced shock reveals novel molecules upregulated in multiple organs not previously associated with SEB-induced responses. The pivotal genes include intracellular DNA/RNA sensors, apoptosis/DNA damage-related molecules, immunoproteasome components, as well as antiviral and IFN-stimulated genes. The host-wide induction of these, and other, antimicrobial defense genes provide evidence that SEB elicits danger signals resulting in multi-organ damage and toxic shock. Ultimately, these discoveries might lead to novel therapeutics for various superantigen-based diseases.

T-cell activation or tolerization: the Yin and Yang of bacterial superantigens

Bacterial superantigens (SAg) are exotoxins from pathogens which interact with innate and adaptive immune cells. The paradox that SAgs cause activation and inactivation/anergy of T-cells was soon recognized. The structural and molecular events following SAg binding to antigen presenting cells (APCs) followed by crosslinking of T-cell receptors were characterized in detail. Activation, cytokine burst and T-cell anergy have been described in vitro and in vivo. Later it became clear that SAg-induced T-cell anergy is in part caused by SAg-dependent activation of T-regulatory cells (Tregs). Although the main focus of analyses was laid on T-cells, it was also shown that SAg binding to MHC class II molecules on APCs induces a signal, which leads to activation and secretion of pro-inflammatory cytokines. Accordingly APCs are mandatory for T-cell activation. So far it is not known, whether APCs play a role during SAg-triggered activation of Tregs. We therefore tested whether in SAg (Streptococcal pyrogenic exotoxin A) -treated APCs an anti-inflammatory program is triggered in addition. We show here that not only the anti-inflammatory cytokine IL-10 and the co-inhibitory surface molecule PD-L1 (CD274) but also inhibitory effector systems like indoleamine 2,3-dioxygenase (IDO) or intracellular negative feedback loops (suppressor of cytokine signaling molecules, SOCS) are induced by SAgs. Moreover, cyclosporine A completely prevented induction of this program. We therefore propose that APCs triggered by SAgs play a key role in T-cell activation as well as inactivation and induction of Treg cells.

Non-classical MHC I-E negatively regulates macrophage activation and Th17 cell development in NOD mice

Transgenic expression of I-E molecules prevents diabetes in NOD mice. So far, the precise role of these non-classical MHC II molecules remains elusive. Here, we showed that transgenic expression of I-E^k alpha 16 molecule in NOD mice selectively reduced Th17 cells in the thymus and pancreatic draining lymph nodes. The reduction in Th17 cells was associated with both attenuated IL-6 production and decreased activation of macrophages. Mechanistically, transgenic expression of the I-E molecule diminished expression of intracellular classical MHC II molecule and led to impaired TLR4-mediated signaling. In contrast to classical MHC II molecule, this non-classical MHC II molecule negatively regulates the inflammatory responses of macrophages. Altogether, our study reveals a novel regulatory role of I-E molecules in modulating inflammatory immune responses.

Superantigenicity analysis of staphylococcal enterotoxins SElK and SElQ in a mouse model

Staphylococcal enterotoxins (SEs) are superantigenic toxins secreted byStaphylococcus aureusthat is involved in causing food poisoning and human diseases.

MMTV superantigens coerce an unconventional topology between the TCR and MHC class II

Mouse mammary tumor virus superantigens (vSAGs) are notorious for defying structural characterization, and a consensus has yet to be reached regarding their ability to bridge the TCR to MHC class II (MHCII). In this study, we determined the topology of the T cell signaling complex by examining the respective relation of vSAG7 with the MHCII molecule, MHCII-associated peptide, and TCR. We used covalently linked peptide/MHCII complexes to demonstrate that vSAG presentation is tolerant to variation in the protruding side chains of the peptide, but can be sensitive to the nature of the protruding N-terminal extension. An original approach in which vSAG was covalently linked to either MHCII chain confirmed that vSAG binds outside the peptide binding groove. Also, whereas the C-terminal vSAG segment binds to the MHCII α-chain in a conformation-sensitive manner, the membrane-proximal N-terminal domain binds the β-chain. Because both moieties of the mature vSAG remain noncovalently associated after processing, our results suggest that vSAG crosslinks MHCII molecules. Comparing different T cell hybridomas, we identified key residues on the MHCII α-chain that are differentially recognized by the CDR3β when engaged by vSAG. Finally, we show that the highly conserved tyrosine residue found in the vSAg TGXY motif is required for T cell activation. Our results reveal a novel SAG/MHCII/TCR architecture in which vSAGs coerce a near-canonical docking between MHCII and TCR that allows eschewing of traditional CDR3 binding with the associated peptide in favor of MHCII α-chain binding. Our findings highlight the plasticity of the TCR CDRs.

Coexpression of TLR2 or TLR4 with HLA-DR potentiates the superantigenic activities of Mycoplasma arthritidis-derived mitogen

Mycoplasma arthritidis-derived mitogen (MAM) is a member of the superantigen family that structurally differs from other members while still capable of initiating cognate APC/T cell interaction. In addition to the critical role of MHC class II molecules, it has been suggested that TLR2 and TLR4 may cooperate with MHC class II during MAM-induced responses. In this study, we investigated the direct involvement of TLR2 and TLR4 in MAM binding and presentation to T cells. Our results showed that MAM fails to bind to TLR2- and TLR4-transfected cells. However, coexpression of TLR2 or TLR4 with HLA-DR significantly increases MAM binding and the subsequent T cell activation compared with cells expressing HLA-DR alone. The upregulated MAM binding and activity in HLA-DR/TLR-transfected cells is abrogated by an anti-HLA-DR Ab. Interestingly, we also found that MAM complexed with soluble HLA-DR is capable of binding to both TLR2 and TLR4. The enhancing effect of TLR2 or TLR4 on MAM-induced T cell proliferation was not due to TLR ligand contamination in the MAM preparation. Taken together, these results strongly suggest that binding of MAM to HLA-DR leads to a conformational change in MAM structure allowing its interaction with TLR2 and TLR4 and a better recognition by T cells.

HLA-DO increases bacterial superantigen binding to human MHC molecules by inhibiting dissociation of class II-associated invariant chain peptides

HLA-DO (H2-O in mice) is an intracellular non-classical MHC class II molecule (MHCII). It forms a stable complex with HLA-DM (H2-M in mice) and shapes the MHC class II-associated peptide repertoire. Here, we tested the impact of HLA-DO and H2-O on the binding of superantigens (SAgs), which has been shown previously to be sensitive to the structural nature of the class II-bound peptides. We found that the binding of staphylococcal enterotoxin (SE) A and B, as well as toxic shock syndrome toxin 1 (TSST-1), was similar on the HLA-DO(+) human B cell lines 721.45 and its HLA-DO(-) counterpart. However, overexpressing HLA-DO in MHC class II(+) HeLa cells (HeLa-CIITA-DO) improved binding of SEA and TSST-1. Accordingly, knocking down HLA-DO expression using specific siRNAs decreased SEA and TSST-1 binding. We tested directly the impact of the class II-associated invariant chain peptide (CLIP), which dissociation from MHC class II molecules is inhibited by overexpressed HLA-DO. Loading of synthetic CLIP on HLA-DR(+) cells increased SEA and TSST-1 binding. Accordingly, knocking down HLA-DM had a similar effect. In mice, H2-O deficiency had no impact on SAgs binding to isolated splenocytes. Altogether, our results demonstrate that the sensitivity of SAgs to the MHCII-associated peptide has physiological basis and that the effect of HLA-DO on SEA and TSST-1 is mediated through the inhibition of CLIP release.

Targeting surface nucleolin with multivalent HB-19 and related Nucant pseudopeptides results in distinct inhibitory mechanisms depending on the malignant tumor cell type

BACKGROUND

Nucleolin expressed at the cell surface is a binding protein for a variety of ligands implicated in tumorigenesis and angiogenesis. By using a specific antagonist that binds the C-terminal RGG domain of nucleolin, the HB-19 pseudopeptide, we recently reported that targeting surface nucleolin with HB-19 suppresses progression of established human breast tumor cells in the athymic nude mice, and delays development of spontaneous melanoma in the RET transgenic mice.

METHODS

By the capacity of HB-19 to bind stably surface nucleolin, we purified and identified nucleolin partners at the cell surface. HB-19 and related multivalent Nucant pseudopeptides, that present pentavalently or hexavalently the tripeptide Lysψ(CH2N)-Pro-Arg, were then used to show that targeting surface nucleolin results in distinct inhibitory mechanisms on breast, prostate, colon carcinoma and leukemia cells.

RESULTS

Surface nucleolin exists in a 500-kDa protein complex including several other proteins, which we identified by microsequencing as two Wnt related proteins, Ku86 autoantigen, signal recognition particle subunits SRP68/72, the receptor for complement component gC1q-R, and ribosomal proteins S4/S6. Interestingly, some of the surface-nucleolin associated proteins are implicated in cell signaling, tumor cell adhesion, migration, invasion, cell death, autoimmunity, and bacterial infections. Surface nucleolin in the 500-kDa complex is highly stable. Surface nucleolin antagonists, HB-19 and related multivalent Nucant pseudopeptides, exert distinct inhibitory mechanisms depending on the malignant tumor cell type. For example, in epithelial tumor cells they inhibit cell adhesion or spreading and induce reversion of the malignant phenotype (BMC cancer 2010, 10:325) while in leukemia cells they trigger a rapid cell death associated with DNA fragmentation. The fact that these pseudopeptides do not cause cell death in epithelial tumor cells indicates that cell death in leukemia cells is triggered by a specific signaling mechanism, rather than nonspecific cellular injury.

CONCLUSIONS

Our results suggest that targeting surface nucleolin could change the organization of the 500-kDa complex to interfere with the proper functioning of surface nucleolin and the associated proteins, and thus lead to distinct inhibitory mechanisms. Consequently, HB-19 and related Nucant pseudopeptides provide novel therapeutic opportunities in treatment of a wide variety of cancers and related malignancies.

Individual genetic variations directly effect polarization of cytokine responses to superantigens associated with streptococcal sepsis: implications for customized patient care

Host immunogenetic variations strongly influence the severity of group A streptococcus sepsis by modulating responses to streptococcal superantigens (Strep-SAgs). Although HLA-II-DR15/DQ6 alleles strongly protect against severe sepsis, HLA-II-DR14/DR7/DQ5 alleles significantly increase the risk for toxic shock syndrome. We found that, regardless of individual variations in TCR-Vβ repertoires, the presentation of Strep-SAgs by the protective HLA-II-DR15/DQ6 alleles significantly attenuated proliferative responses to Strep-SAgs, whereas their presentation by the high-risk alleles augmented it. Importantly, HLA-II variations differentially polarized cytokine responses to Strep-SAgs: the presentation of Strep-SAgs by HLA-II-DR15/DQ6 alleles elicited significantly higher ratios of anti-inflammatory cytokines (e.g., IL-10) to proinflammatory cytokines (e.g., IFN-γ) than did their presentation by the high-risk HLA-II alleles. Adding exogenous rIL-10 significantly attenuated responses to Strep-SAgs presented by the high-risk HLA-II alleles but did not completely block the response; instead, it reduced it to a level comparable to that seen when these superantigens were presented by the protective HLA-II alleles. Furthermore, adding neutralizing anti-IL-10 Abs augmented Strep-SAg responses in the presence of protective HLA-II alleles to the same level as (but no higher than) that seen when the superantigens were presented by the high-risk alleles. Our findings provide a molecular basis for the role of HLA-II allelic variations in modulating streptococcal sepsis outcomes and suggest the presence of an internal control mechanism that maintains superantigen responses within a defined range, which helps to eradicate the infection while attenuating pathological inflammatory responses that can inflict more harm than the infection itself.

Construction and characterization of a novel fusion protein MG7-scFv/SEB against gastric cancer

Antibody-targeted superantigen has been developed into a new strategy to treat many malignant tumors. In this study, for specific targeting to gastric cancer cell, superantigen SEB (Staphylococcal Enterotoxin B) was genetically fused to the single-chain variable fragment of gastric carcinoma-associated antibody MG7(MG7-scFv) that recognizes the MG7 antigen frequently expressed in gastric cancer cell. The recombinant MG7-scFv/SEB fusion proteins are expressed in E. coli as inclusion bodies, and the purified MG7-scFv/SEB retains high binding affinity with gastric cancer cell SGC-7901 (positive MG7 antigen expression). When incubated with effector cell-peripheral blood mononuclear cells (PBMCs), MG7-scFv/SEB could effectively inhibit the proliferation and induce apoptosis of SGC-7901. After being treated with MG7-scFv/SEB, PBMCs remarkably increased the production of Th1 cytokines (IFN-gamma, IL-2), and slightly increased the production of Th2 cytokines (IL-4, IL-10) in vitro. It was observed that gastric-tumor-bearing rats administrated with MG7-scFv/SEB showed more inflammatory cell infiltration, more significant tumor inhibition, and longer survival time than those of rats treated with SEB or NS (Normal Saline). The data indicated that MG7-scFv/SEB fusion protein could specifically target gastric cancer cell, enhance the activity of T cells and induce tumor cell apoptosis to exert the antitumor effect on gastric cancer.

Rac/ROS-related protein kinase C and phosphatidylinositol-3-kinase signaling are involved in a negative regulating cascade in B cell activation by antibody-mediated cross-linking of MHC class II molecules

In addition to their essential role in antigen presentation, MHC class II molecules have been widely described as receptors associated with signal transduction involved in regulating B cell function. However, their precise function and mechanism in signal transduction are not yet fully elucidated. Our previous studies demonstrated that cross-linking of MHC class II molecules led to the inhibition of resting B cell activation in which various signal molecules were involved. Especially, Rac-associated ROS-dependent MAP kinases, including ERK1/2 and p38, are involved in MHC class II-associated negative signal transduction in the phorbol 12, 13-dibutyrate (PDBU)-treated, but not LPS-treated, resting B cell line, 38B9. In this study, we further illustrated that PKC regulates downstream signal molecules, including MAP kinases and NF-kappaB in PDBU-stimulated resting B cells, together with Rac and ROS. In addition, we found that phosphatidylinositol 3-kinase (PI3K)-dependent activation of ERK/p38 MAP kinases was associated with the signaling procedure in PDBU-induced B cell activation. Collectively, Rac/ROS-related PKC and PI3K signaling are involved in a negative regulation cascade through the cross-linking of MHC class II molecules by anti-MHC class II antibodies in resting B cells.

Staphylococcal enterotoxin A induces small clusters of HLA-DR1 on B cells

The superantigen SEA causes non-specific hyperactivation of T and B cells at low concentrations. Studies of mutants or soluble proteins suggest SEA is bivalent for its ligand, MHC class II. However, the interaction between these molecules on intact cells is unknown. On primary mouse B cells expressing the MHC class II allele HLA-DR1, measurements of Förster Resonance Energy Transfer between HLA-DR1 molecules on SEA-treated cells indicated specific clustering, not observed in untreated or monovalent superantigen treated cells. Tomographic visualization and electron microscopy of immunogold-labeled SEA-treated B cells revealed small clusters of surface HLA-DR1 (≤4 gold labels). These results present direct visual evidence of SEA-mediated clustering of MHC class II molecules on treated antigen presenting cells, and provide a new structural approach to addressing problems of this nature.

Construction and characterization of a novel superantigen fusion protein: bFGF/SEB

BACKGROUND

As one of the bacterial superantigens, Staphylococcal enterotoxins (SEs) are potent activators of T cells, especially for those expressing T cell receptor V(beta) chains, and can induce the production of cytokines such as IFN-gamma, TNF-alpha, IL-1, IL-2, IL-6, IL-12, etc. Thus, SEs could be used in tumor-targeting therapy when cooperated with the vectors that can specifically recognize the tumor cells.

MATERIALS AND METHODS

The coding sequences of Staphylococcal enterotxin B (SEB) was amplified and fused with human basic fibroblast growth factor (bFGF). Recombinant protein SEB and fusion protein bFGF/SEB were expressed and purified. The biological activity was detected, including splenocytes proliferation, cytokine production, and cytotoxicity in tumor cells in vitro. In addition, the binding of bFGF/SEB with tumor cells and the tumor cell apoptosis were also tested by immunofluorescent technique.

RESULTS

The fusion protein bFGF/SEB had similar biological activities compared with natural SEA and recombinant SEB, including tumor-inhibition ratio.

CONCLUSION

The recombinant bFGF/SEB-fusion protein was shown to retain the superantigenic activity of SEB, and might be a novel promising immunotherapeutic agent for the treatment of some carcinomas.

Recruitment and in situ renewal regulate rapid accumulation of CD11c+ cells in the lung following intranasal superantigen challenge

BACKGROUND

Staphylococcusaureus, a primary source of bacterial superantigen, is known to colonize the human respiratory tract and has been implicated in airway inflammation. The potential pathological effect of staphylococcal enterotoxins on the respiratory tract necessitates a detailed understanding of how they regulate innate immune cells, particularly CD11c-expressing dendritic cells (DCs).

METHODS

C57BL/6 mice were challenged intranasally with staphylococcal enterotoxin A (SEA) and at indicated time points lung tissue was perfused, digested and analyzed for CD11c+ expressing cells.

RESULTS

The pulmonary CD11c+ cells can be divided into two major populations based on their MHC II expression. One day following intranasal SEA challenge, there was rapid accumulation of CD11c+ cells expressing medium to high levels of MHC II. The peak accumulation of CD11c+ MHC II- population was observed 2 days after SEA challenge; however, careful examination of this cell population revealed that they were heterogeneous, being comprised of cells bearing CD3, CD19, NK1.1 and F4/80 along with varying levels of CD11c. Nevertheless, there was a 2-fold increase of CD11c+ MHC II- (CD3- CD19- NK1.1- F4/80-) cells in the lungs.

CONCLUSION

The mechanism of increase in the CD11c+ MHC II- immune progenitor population was mainly due to cellular division rather than migration from blood to lung. In contrast, the early and rapid accumulation of CD11c+ MHC II(hi) cells, conventionally known as DCs, in the lung on day 1 was mostly due to migration from blood. Thus this study examines the pulmonary innate immune response to a powerful immune stimulus.

The diverse functions of Src family kinases in macrophages

Macrophages are key components of the innate immune response. These cells possess a diverse repertoire of receptors that allow them to respond to a host of external stimuli including cytokines, chemokines, and pathogen-associated molecules. Signals resulting from these stimuli activate a number of macrophage functional responses such as adhesion, migration, phagocytosis, proliferation, survival, cytokine release and production of reactive oxygen and nitrogen species. The cytoplasmic tyrosine kinase Src and its family members (SFKs) have been implicated in many intracellular signaling pathways in macrophages, initiated by a diverse set of receptors ranging from integrins to Toll-like receptors. However, it has been difficult to implicate any given member of the family in any specific pathway. SFKs appear to have overlapping and complementary functions in many pathways. Perhaps the function of these enzymes is to modulate the overall intracellular signaling network in macrophages, rather than operating as exclusive signaling switches for defined pathways. In general, SFKs may function more like rheostats, influencing the amplitude of many pathways.

Mutagenesis, biochemical, and biophysical characterization of Mycoplasma arthritidis-derived mitogen

Mycoplasma arthritidis-derived mitogen (MAM) is a superantigen (SAg) that can activate large fractions of T cells bearing particular TCR Vbeta elements. Here we report the mutagenesis, biochemical and biophysical studies on the dimerization of MAM in solution. Our studies showed that although MAM mainly exists as a monomer in solution, a small percentage of MAM molecules form homodimer at high protein concentration, regardless of the presence of Zn2+. A distinct peak corresponding to a MAM homodimer was detected in the presence of EDTA, using both chemical cross-linking and analytical ultracentrifugation methods. Further mutagenesis studies revealed that single mutation of residues at the interface of the crystallographic dimer of MAM does not significantly affect the dimerization of MAM in solution. Circular dichroism (CD) analysis indicated that addition of Zn2+ does not induce conformational changes of MAM from its apo-state. Thermal denaturation experiments indicated that addition of Zn2+ to MAM solution resulted in a decrease of melting point (Tm), whereas addition of EDTA did not affect the Tm of MAM. These results imply that there is no defined Zn2+-binding site on MAM.

The superantigen staphylococcal enterotoxin A (SEA) and monoclonal antibody L243 share a common epitope but differ in their ability to induce apoptosis via MHC-II

Crosslinking of MHC class II (MHC-II) molecules by antibodies or by superantigens (SAg) induces a variety of functional responses in the antigen presenting cell. We were able to allocate K39 as the residue that is essential for binding of antibody L243 to the alpha chain of HLA-DR. K39 is also essential for binding of staphylococcal enterotoxin A (SEA). However, the functional responses of the two ligands differ considerably exemplified by the ability of L243 to induce apoptosis in monocytic cells and in B cells, whereas SEA is unable to activate the apoptosis pathway. Despite the differences in functional responses, both ligands induce cell aggregation in MonoMac-1 cells. The SEA molecule with its two different binding sites associates one MHC alpha chain with one beta chain as opposed to two alpha chains that are brought into close proximity by the two identical antigen binding sites of L243. We therefore conclude that the spatial orientation of dimerized MHC-II and their associated proteins is an important factor for the nature of the transduced signal and consequently the outcome of functional responses.

Cross-linking of MHC class II molecules interferes with phorbol 12,13-dibutyrate-induced differentiation of resting B cells by inhibiting Rac-associated ROS-dependent ERK/p38 MAP kinase pathways leading to NF-kappaB activation

In addition to their essential role in antigen presentation, major histocompatibility complex (MHC) class II molecules have been described as the receptor associated with signal transduction regulating B-cell function. In previous experiments, we found that cross-linking of MHC class II molecules with corresponding anti-MHC class II antibodies inhibited NF-kappaB-activated signaling pathways associated with the proliferation and differentiation of the LPS-stimulated primary and resting B-cell line, 38B9. We also found that exposure to the anti-MHC class II antibody reduced the production of ROS, which function as secondary signal transducers, in the phorbol 12,13-dibutyrate (PDBU)-treated (but not in the LPS-treated) resting B-cell line. In this study, we investigated the molecular mechanisms in the ROS-associated signaling pathway leading to PDBU-induced NF-kappaB activation that results in B-cell differentiation and speculated that the signaling pathway was inhibited by exposure to the anti-MHC class II antibody. We also found that this inhibition was mediated through down-regulation of the activated Rac/ROS-associated ERK/p38 MAPK signaling pathway in PDBU-treated 38B9 cells. Collectively, these findings suggest that ROS-associated molecules are involved in MHC class II-associated negative signal transduction in resting B cells.

Staphylococcus aureus enterotoxins induce IL-8 secretion by human nasal epithelial cells

Background

Staphylococcus aureus produces a set of proteins which act both as superantigens and toxins. Although their mode of action as superantigens is well understood, little is known about their effects on airway epithelial cells.

Methods

To investigate this problem, primary nasal epithelial cells derived from normal and asthmatic subjects were stimulated with staphylococcal enterotoxin A and B (SEA and SEB) and secreted (supernatants) and cell-associated (cell lysates) IL-8, TNF-α, RANTES and eotaxin were determined by specific ELISAs.

Results

Non-toxic concentrations of SEA and SEB (0.01 μg/ml and 1.0 μg/ml) induced IL-8 secretion after 24 h of culture. Pre-treatment of the cells with IFN-γ (50 IU/ml) resulted in a further increase of IL-8 secretion. In cells from healthy donors pretreated with IFN-γ, SEA at 1.0 μg/ml induced release of 1009 pg/ml IL-8 (733.0–1216 pg/ml, median (range)) while in cells from asthmatic donors the same treatment induced significantly higher IL-8 secretion – 1550 pg/ml (1168.0–2000.0 pg/ml p = 0.04). Normal cells pre-treated with IFN-γ and then cultured with SEB at 1.0 μg/ml released 904.6 pg/ml IL-8 (666.5–1169.0 pg/ml). Cells from asthmatics treated in the same way produced significantly higher amounts of IL-8 – 1665.0 pg/ml (1168.0–2000.0 pg/ml, p = 0.01). Blocking antibodies to MHC class II molecules added to cultures stimulated with SEA and SEB, reduced IL-8 secretion by about 40% in IFN-γ unstimulated cultures and 75% in IFN-γ stimulated cultures. No secretion of TNF-α, RANTES and eotaxin was noted.

Conclusion

Staphylococcal enterotoxins may have a role in the pathogenesis of asthma.

Superantigen recognition by HLA class II on monocytes up-regulates toll-like receptor 4 and enhances proinflammatory responses to endotoxin

The devastating systemic effects of bacterial superantigens may be explained by powerful proinflammatory synergy with lipopolysaccharide (LPS). However, the mechanism underlying this phenomenon remains unclear and has never been investigated in humans. Specifically, there is no known link between superantigen-induced immune effects and the pattern recognition of LPS at toll-like receptor 4 (TLR4). Here we show that bacterial superantigens induce rapid transcription and increased membrane expression of TLR4 in primary human monocytes by ligation of major histocompatibility complex (MHC) class II. We also demonstrate that superantigens are solely responsible for monocyte TLR4 up-regulation induced by products from Gram-positive bacteria. In parallel with enhanced TLR4 expression, priming of purified monocytes or mixed peripheral blood mononuclear cells with superantigens significantly enhanced the induction of proinflammatory cytokines by known TLR4 ligands. Staphylococcal enterotoxin A constructs containing targeted mutations were used to demonstrate a requirement for MHC class II ligation in both TLR4 up-regulation and enhanced responses to endotoxin. In contrast to results from animal models, superantigen-endotoxin interaction was not dependent on T-cell receptor ligation by superantigen or interferon gamma production. Pattern recognition of bacterial superantigens by MHC class II receptors may exacerbate the proinflammatory response of monocytes to Gram-negative infection or endotoxin by up-regulation of TLR4.

Crystal structure of Mycoplasma arthritidis mitogen complexed with HLA-DR1 reveals a novel superantigen fold and a dimerized superantigen-MHC complex

Mycoplasma arthritidis-derived mitogen (MAM) is a superantigen that can activate large fractions of T cells bearing particular TCR Vbeta elements. Here we report the crystal structure of MAM complexed with a major histocompatibility complex (MHC) antigen, HLA-DR1, loaded with haemagglutinin peptide 306-318 (HA). The structure reveals that MAM has a novel fold composed of two alpha-helical domains. This fold is entirely different from that of the pyrogenic superantigens, consisting of a beta-grasped motif and a beta barrel. In the complex, the N-terminal domain of MAM binds orthogonally to the MHC alpha1 domain and the bound HA peptide, and to a lesser extent to the MHC beta1 domain. Two MAM molecules form an asymmetric dimer and cross-link two MHC antigens to form a plausible, dimerized MAM-MHC complex. These data provide the first crystallographic evidence that superantigens can dimerize MHC molecules. Based on our structure, a model of the TCR2MAM2MHC2 complex is proposed.

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.

Identification of the antigenic epitopes in staphylococcal enterotoxins A and E and design of a superantigen for human cancer therapy

Monoclonal antibodies have a potential for cancer therapy that may be further improved by linking them to effector molecules such as superantigens. Tumor targeting of a superantigen leads to a powerful T cell attack against the tumour tissue. Encouraging results have been observed preclinically and in patients using the superantigen staphylococcal enterotoxin A, SEA. To further improve the concept, we have reduced the reactivity to antibodies against superantigens, which is found in all individuals. Using epitope mapping, antibody binding sites in SEA and SEE were found around their MHC class II binding sites. These epitopes were removed genetically and a large number of synthetic superantigens were produced in an iterative engineering procedure. Properties such as decreased binding to anti-SEA as well as higher selectivity to induce killing of tumour cells compared to MHC class II expressing cells, were sequentially improved. The lysine residues 79, 81, 83 and 84 are all part of major antigenic epitopes, Gln204, Lys74, Asp75 and Asn78 are important for optimal killing of tumour cells while Asp45 affects binding to MHC class II. The production properties were optimised by further engineering and a novel synthetic superantigen, SEA/E-120, was designed. It is recognised by approximately 15% of human anti-SEA antibodies and have more potent tumour cell killing properties than SEA. SEA/E-120 is likely to have a low toxicity due to its reduced capacity to mediate killing of MHC class II expressing cells. It is produced as a Fab fusion protein at approximately 35 mg/l in Escherichia coli.

[Major histocompatibility complex (MHC) class II: are lipid rafts the missing link?]

Aside from their crucial roles in the presentation of nominal antigen to CD4+ T cells and susceptibility to autoimmune diseases, substantial evidences suggest that MHC class II molecules act as signal transducer receptors as well. The signals transmitted affect diverse biological functions. Paradoxically, the cytoplasmic and transmembrane domains of these molecules are devoid of classic signaling motifs. The recent discovery of the presence of membrane microdomains, also called lipid rafts, that are enriched in kinases and adaptor molecules, may contribute to the elucidation of the mechanisms by which MHC class II molecules transmit their signals.

Early activation events differentiate the reactivity of two T-cell families to Staphylococcus enterotoxin A

Analysis of early activation events in two SEA responsive T-cell families demonstrated that low doses of SEA induced CD4+Vbeta22 T-cells to down-regulate their TCR and express CD69, considerably earlier than CD4+Vbeta5 T-cells. The rapid down-regulation of Vbeta22 TCR led to its proliferation, whereas even a 10-fold higher dose of toxin induced only a partial down-regulation of Vbeta5 TCR. Stimulation with SEA induced a significantly higher percentage of Vbeta22 T-cells to produce IFN-gamma compared to Vbeta5 T-cells. SEAF47A, a mutant of SEA, known to have a lower binding affinity for the MHC class II molecule, failed to activate Vbeta5 T-cells whereas Vbeta22 T-cell activation was slightly decreased. Hence, early activation events highlighted the differential requirements of T-cell families to respond to SEA.

Zinc-binding sites in the N terminus of Mycoplasma arthritidis-derived mitogen permit the dimer formation required for high affinity binding to HLA-DR and for T cell activation

Zinc-dependent superantigens can be divided into two subfamilies based on how they use zinc ions for interactions with major histocompatibility complex (MHC) class II molecules. Members of the first subfamily use zinc ions for interactions with histidine 81 on the beta-chain of MHC class II molecules, whereas members of the second subfamily use zinc ions for dimer formation. The zinc-binding motif is located in the C terminus of the molecule in both subfamilies. While our recent studies with Mycoplasma arthritidis-derived mitogen (MAM) have provided the first direct evidence demonstrating the binding to MHC class II molecules in a zinc-dependent manner, it still not known how zinc coordinates the interaction. Data presented here show that the zinc ion is mainly required to induce MAM/MAM dimer formation. Residues in the N terminus of MAM are involved in dimer formation and MHC class II binding, while histidine 14 and aspartic acid 31 of the MAM sequence are the major residues mediating MAM/MAM dimerization. Zinc-induced dimer formation is necessary for MAM binding, MHC class II-induced cell-cell adhesion, and efficient T cell activation. Together these results depict the unique mode of interaction of MAM in comparison with other superantigens.

Intracytoplasmic domains of MHC class II molecules are essential for lipid-raft-dependent signaling

In addition to their role in antigen presentation, major histocompatibility complex (MHC) class II molecules have been widely described as signaling proteins in diverse antigen-presenting cells (APCs) including B cells and dendritic cells. By contrast, little is known of the signaling function of MHC class II molecules expressed in solid tumors. We describe the functional organization and signaling ability of I-Ak expressed in a sarcoma, and report the recruitment of I-Ak to lipid rafts after MHC class II engagement. Lipid raft integrity was required for I-Ak-mediated reorganization of the actin cytoskeleton and translocation of protein kinase C-alpha(PKC-alpha) to the precise site of stimulation via I-Ak. Truncation of the intracytoplasmic domains of I-Ak did not perturb I-Ak recruitment to lipid rafts but abrogated PKC-alpha translocation and actin rearrangement. PKC-alpha was detected in lipid microdomains and enrichment of activated PKC-alphain lipid rafts was induced by I-Ak signaling. Ordering of the molecular events following engagement of the MHC class II molecules revealed that I-Ak recruitment to lipid rafts precedes signaling. This is consistent with the absence of a requirement for the intracytoplasmic tails for localization to lipid rafts. These data reveal that lipid-rich microdomains play a key role in MHC class II-mediated signaling in a solid tumor.

Prediction of the multimeric assembly of staphylococcal enterotoxin A with cell-surface protein receptors

Staphylococcal enterotoxin A (SEA) cross-links two class II major histocompatibility complex (MHC) molecules and forms a multimeric assembly with T-cell receptors (TcRs). The X-ray crystal structure of SEA has been solved, yet details describing molecular recognition and association remain unclear. We present a structural model for the interactions of SEA with cell-surface proteins. Molecular docking calculations predicting SEA association with the class II MHC molecule HLA-DR1 were performed by using a rigid-body docking method. Docked orientations were evaluated by a Poisson-Boltzmann model for the electrostatic free energy of binding and the hydrophobic effect calculated from molecular surface areas. We found that the best-scoring SEA conformers for the DR1alpha interface display a binding mode similar to that determined crystallographically for staphylococcal enterotoxin B bound to HLA-DR1. For the zinc-binding site of SEA, docking DR1beta yielded several orientations exhibiting tetrahedral-like coordination geometries. Combining the two interfaces, tetramers were modeled by docking an alphabeta TcR with trimolecular complexes DR1beta-SEA-DR1alpha and SEA-betaDR1alpha-SEA. Our results indicate that the complex DR1beta-SEA-DR1alpha provides a more favorable assembly for the engagement of TcRs, forming SEA molecular contacts that are in accord with reported mutagenesis studies. In contrast, the cooperative association of two SEA molecules on a single DR1 molecule sterically inhibits interactions with TcRs. We suggest that signal transduction stimulated by SEA through large-scale assembly is limited to four or five TcR-(DR1beta-SEA-DR1alpha) tetramers and requires the dimerization of class II MHC molecules, while TcR dimerization is unlikely.

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.

Lipid raft-dependent and -independent signaling through HLA-DR molecules

Lipid rafts are plasma membrane microdomains that are highly enriched in signaling molecules and that act as signal transduction platforms for many immune receptors. The involvement of these microdomains in HLA-DR-induced signaling is less well defined. We examined the constitutive presence of HLA-DR molecules in lipid rafts, their possible recruitment into these microdomains, and the role of these microdomains in HLA-DR-induced responses. We detected significant amounts of HLA-DR molecules in the lipid rafts of EBV(+) and EBV(-) B cell lines, monocytic cell lines, transfected HeLa cells, tonsillar B cells, and human monocytes. Localization of HLA-DR in these microdomains was unaffected by the deletion of the cytoplasmic domain of both the alpha and beta chains. Ligation of HLA-DR with a bivalent, but not a monovalent, ligand resulted in rapid tyrosine phosphorylation of many substrates, especially Lyn, and activation of ERK1/2 MAP kinase. However, the treatment failed to induce further recruitment of HLA-DR molecules into lipid rafts. The HLA-DR-induced signaling events were accompanied by the induction of cell-cell adhesion that could be inhibited by PTK and Lyn but not ERK1/2 inhibitors. Disruption of lipid rafts by methyl-beta-cyclodextrin (MbetaCD) resulted in the loss of membrane raft association with HLA-DR molecules, inhibition of HLA-DR-mediated protein tyrosine phosphorylation and cell-cell adhesion. MbetaCD did not affect the activation of ERK1/2, which was absent from lipid rafts. These results indicate that although all the HLA-DR-induced events studied are dependent on HLA-DR dimerization, some require the presence of HLA-DR molecules in lipid rafts, whereas others do not.

Crystal structure of a SEA variant in complex with MHC class II reveals the ability of SEA to crosslink MHC molecules

Although the biological properties of staphylococcal enterotoxin A (SEA) have been well characterized, structural insights into the interaction between SEA and major histocompatibilty complex (MHC) class II have only been obtained by modeling. Here, the crystal structure of the D227A variant of SEA in complex with human MHC class II has been determined by X-ray crystallography. SEA(D227A) exclusively binds with its N-terminal domain to the alpha chain of HLA-DR1. The ability of one SEA molecule to crosslink two MHC molecules was modeled. It shows that this SEA molecule cannot interact with the T cell receptor (TCR) while a second SEA molecule interacts with MHC. Because of its relatively low toxicity, the D227A variant of SEA is used in tumor therapy.

Staphylococcal enterotoxin H contrasts closely related enterotoxins in species reactivity

Staphylococcus aureus enterotoxin H (SEH) belongs to the staphylococcal enterotoxin (SE) family of superantigens (SAgs). SEH has structural similarities to other SE; however, its biological properties are less well characterized. SEH binds with high affinity to human major histocompatibility complex (MHC) class II and exhibits strong mitogenic activity in human T cells, although it was found to be less potent than the related SEA. Surprisingly and in sharp contrast to related SEs, SEH did not possess superantigen activity in murine T cells and T cells from three investigated rat strains. However, SEH bound to a high extent to murine MHC class II expressing cells and when presented by these cells SEH stimulated human T cells to proliferate. Thus, SEH interacts with the murine MHC class II molecule in a functional manner. Notably, SEH had an inhibitory effect on murine SEA response, demonstrating that SEH interferes with the SEA interactions with murine cells. Despite this, murine T cells did not proliferate regardless of whether SEH was presented on human or murine MHC class II expressing cells. Consequently, SEH differs in species reactivity as compared to related SEs and lacks critical properties for T-cell activation in mice. We propose that unlike other SEs, SEH does not interact with murine T cells since it is not recognized by murine T-cell receptors.

Cross-linking HLA-DR molecules on Th1 cells induces anergy in association with increased level of cyclin-dependent kinase inhibitor p27(Kip1)

HLA class II molecules play pivotal roles in antigen presentation to CD4+ T cells. We investigated signaling via HLA-DR molecules expressed on CD4+ T cells. When HLA-DR or CD3 molecules on cloned CD4+ T cells were cross-linked by solid-phase mAbs, T cells proliferated, and this resulted in anergy. Whereas cross-linking of HLA-DR and CD3 resulted in secretion of the same levels of IFN-gamma and IL-8, secretion of IL-10 induced by cross-linking of HLA-DR was less than that induced by cross-linking of CD3 on CD4+ T cells. Interestingly, expression of p27(Kip1) but not p21(Cip1) increased after stimulation by either anti-HLA-DR or anti-CD3 mAb. This was indeed the case, when T cells were rendered anergic using a soluble form of antigenic peptide. In contrast, T cells stimulated by peptide-pulsed PBMC expressed little p27(Kip1). We propose that signaling via HLA-DR molecules on CD4+ T cells at least in part contributes to the induction of T cell anergy, through the upregulated expression of the p27(Kip1). The implication of our finding is that HLA-DR molecules play a role in human T cell anergy induced by a soluble form of antigenic peptide.

Structural basis for abrogated binding between staphylococcal enterotoxin A superantigen vaccine and MHC-IIalpha

Staphylococcal enterotoxins (SEs) are superantigenic protein toxins responsible for a number of life-threatening diseases. The X-ray structure of a staphylococcal enterotoxin A (SEA) triple-mutant (L48R, D70R, and Y92A) vaccine reveals a cascade of structural rearrangements located in three loop regions essential for binding the alpha subunit of major histocompatibility complex class II (MHC-II) molecules. A comparison of hypothetical model complexes between SEA and the SEA triple mutant with MHC-II HLA-DR1 clearly shows disruption of key ionic and hydrophobic interactions necessary for forming the complex. Extensive dislocation of the disulfide loop in particular interferes with MHC-IIalpha binding. The triple-mutant structure provides new insights into the loss of superantigenicity and toxicity of an engineered superantigen and provides a basis for further design of enterotoxin vaccines.

Involvement of zinc in the binding of Mycoplasma arthritidis-derived mitogen to the proximity of the HLA-DR binding groove regardless of histidine 81 of the beta chain

Although our recent studies have provided the first evidence demonstrating the direct binding of Mycoplasma arthritidis-derived mitogen (MAM) to MHC class II molecules, it is not yet established how MAM interacts with these molecules. Herein, we demonstrate that MAM binds preferentially and with high affinity to HLA-DR molecules in a zinc-dependent manner. MAM's affinity (25 nM) for HLA-DR molecules is comparable to that of staphylococcal superantigens, and is slightly higher than that for murine MHC class II molecules expressed on the A20 B cell line (111 nM). The amino acid residues located between 14 - 31 and 76 - 90 of the MAM N-terminus play a critical role in MAM / HLA-DR interactions. Histidine at position 81 of the HLA-DR beta-chain, known to be critical for binding of zinc-coordinated superantigens, is not necessary for MAM / HLA-DR interactions. The HLA-DR residues involved in MAM binding are located in the proximal binding groove of the HLA-DR molecule, where the nature of the peptide of the binding groove plays an important role in MAM / HLA-DR interaction. This is the first detailed characterization of MAM's interactions with MHC class II molecules showing a mode of interaction with HLA-DR distinct from that of other superantigens.

Superantigen bacterial toxins: state of the art

Superantigens (SAgs) are viral and bacterial proteins exhibiting a highly potent polyclonal lymphocyte-proliferating activity for CD4(+), CD8(+) and sometimes gammadelta(+) T cells of human and (or) various animal species. Unlike conventional antigens, SAgs bind as unprocessed proteins to invariant regions of major histocompatibility complex (MHC) class II molecules on the surface of antigen-presenting cells (APCs) and to particular motifs of the variable region of the beta chain (Vbeta) of T-cell receptor (TcR) outside the antigen-binding groove. As a consequence, SAgs stimulate at nano-to picogram concentrations up to 10 to 30% of host T-cell repertoire while only one in 10(5)-10(6) T cells (0.01-0.0001%) are activated upon conventional antigenic peptide binding to TcR. SAg activation of an unusually high percentage of T lymphocytes initiates massive release of pro-inflammatory and other cytokines which play a pivotal role in the pathogenesis of the diseases provoked by SAg-producing microorganisms. We briefly describe in this review the molecular and biological properties of the bacterial superantigen toxins and mitogens identified in the past decade.

Immunological and biochemical characterization of streptococcal pyrogenic exotoxins I and J (SPE-I and SPE-J) from Streptococcus pyogenes

Recently, we described the identification of novel streptococcal superantigens (SAgs) by mining the Streptococcus pyogenes M1 genome database at Oklahoma University. Here, we report the cloning, expression, and functional analysis of streptococcal pyrogenic exotoxin (SPE)-J and another novel SAg (SPE-I). SPE-I is most closely related to SPE-H and staphylococcal enterotoxin I, whereas SPE-J is most closely related to SPE-C. Recombinant forms of SPE-I and SPE-J were mitogenic for PBL, both reaching half maximum responses at 0.1 pg/ml. Evidence from binding studies and cell aggregation assays using a human B-lymphoblastoid cell line (LG-2) suggests that both toxins exclusively bind to the polymorphic MHC class II beta-chain in a zinc-dependent mode but not to the generic MHC class II alpha-chain. The results from analysis by light scattering indicate that SPE-J exists as a dimer in solution above concentrations of 4.0 mg/ml. Moreover, SPE-J induced a rapid homotypic aggregation of LG-2 cells, suggesting that this toxin might cross-link MHC class II molecules on the cell surface by building tetramers of the type HLA-DRbeta-SPE-J-SPE-J-HLA-DRbeta. SPE-I preferably stimulates T cells bearing the Vbeta18.1 TCR, which is not targeted by any other known SAG: SPE-J almost exclusively stimulates Vbeta2.1 T cells, a Vbeta that is targeted by several other streptococcal SAgs, suggesting a specific role for this T cell subpopulation in immune defense. Despite a primary sequence diversity of 51%, SPE-J is functionally indistinguishable from SPE-C and might play a role in streptococcal disease, which has previously been addressed to SPE-C.

Monocytes are differentially activated through HLA-DR, -DQ, and -DP molecules via mitogen-activated protein kinases

When HLA-DR, -DQ, and -DP were cross-linked by solid-phase mAbs, monocytes produced monokines and only anti-DR markedly activated mitogen-activated protein (MAP) kinase extracellular signal-related kinase, whereas anti-DR, anti-DQ, and anti-DP all activated MAP kinase p38. Activation of extracellular signal-related kinase was not inhibited by neutralizing Ab to TNF-alpha. Anti-DR and DR-restricted T cells stimulated monocytes to produce relatively higher levels of proinflammatory monokines, such as IL-1beta, whereas anti-DQ/DP and DQ-/DP-restricted T cells stimulated higher levels of anti-inflammatory monokine IL-10. IL-10 production was abrogated by the p38 inhibitor SB203580, but rather enhanced by the MAP/extracellular signal-related kinase kinase-I-specific inhibitor PD98059, whereas IL-1beta was only partially abrogated by SB203580 and PD98059. Furthermore, DR-restricted T cells established from PBMC, which are reactive with mite Ags, purified protein derivative, and random 19-mer peptides, exhibited a higher IFN-gamma:IL-4 ratio than did DQ- or DP-restricted T cells. These results indicate that HLA-DR, -DQ, and -DP molecules transmit distinct signals to monocytes via MAP kinases and lead to distinct monokine activation patterns, which may affect T cell responses in vivo. Thus, the need for generation of a multigene family of class II MHC seems apparent.