Helper T cell-dependent human B cell differentiation mediated by a mycoplasmal superantigen bridge

Blocking Superantigen-Mediated Diseases: Challenges and Future Trends

Superantigens are virulence factors secreted by microorganisms that can cause various immune diseases, such as overactivating the immune system, resulting in cytokine storms, rheumatoid arthritis, and multiple sclerosis. Some studies have demonstrated that superantigens do not require intracellular processing and instated bind as intact proteins to the antigen-binding groove of major histocompatibility complex II on antigen-presenting cells, resulting in the activation of T cells with different T-cell receptor Vβ and subsequent overstimulation. To combat superantigen-mediated diseases, researchers have employed different approaches, such as antibodies and simulated peptides. However, due to the complex nature of superantigens, these approaches have not been entirely successful in achieving optimal therapeutic outcomes. CD28 interacts with members of the B7 molecule family to activate T cells. Its mimicking peptide has been suggested as a potential candidate to block superantigens, but it can lead to reduced T-cell activity while increasing the host's infection risk. Thus, this review focuses on the use of drug delivery methods to accurately target and block superantigens, while reducing the adverse effects associated with CD28 mimic peptides. We believe that this method has the potential to provide an effective and safe therapeutic strategy for superantigen-mediated diseases.

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.

MHC-restricted Ig V region-driven T-B lymphocyte collaboration: B cell receptor ligation facilitates switch to IgG production

B cells spontaneously process their endogenous Ig and present V region peptides on their MHC class II molecules. We have here investigated whether B cells collaborate with V region-specific CD4+ T cells in vivo. By use of paired Ig L chain-transgenic and TCR-transgenic mice and cell transfer into normal hosts, we demonstrate that B cell presentation of a V(L) region peptide to CD4+ T cells results in germinal centers, plasma cells, and Ab secretion. Because the transgenic B cells have a fixed L chain but polyclonal H chains, their B cell receptor (BCR) repertoire is diverse and may bind a multitude of ligands. In a hapten-based system, BCR ligation concomitant with V region-driven T-B collaboration induced germinal center formation and an IgM --> IgG isotype switch. In the absence of BCR ligation, mainly IgM was produced. Consistent with this, prolonged V region-driven T-B collaboration resulted in high titers of IgG autoantibodies against ubiquitous self-Ags, while natural-type Abs against exotic bacteria remained IgM. Taken together, V region-driven T-B collaboration may explain induction of natural IgM Abs (absence of BCR ligation) and IgG autoantibodies (BCR ligation by autoantigen) and may be involved in the development of autoimmunity.

Current perspective on the pathogenesis of Graves' disease and ophthalmopathy

Graves' disease (GD) is a very common autoimmune disorder of the thyroid in which stimulatory antibodies bind to the thyrotropin receptor and activate glandular function, resulting in hyperthyroidism. In addition, some patients with GD develop localized manifestations including ophthalmopathy (GO) and dermopathy. Since the cloning of the receptor cDNA, significant progress has been made in understanding the structure-function relationship of the receptor, which has been discussed in a number of earlier reviews. In this paper, we have focused our discussion on studies related to the molecular mechanisms of the disease pathogenesis and the development of animal models for GD. It has become apparent that multiple factors contribute to the etiology of GD, including host genetic as well as environmental factors. Studies in experimental animals indicate that GD is a slowly progressing disease that involves activation and recruitment of thyrotropin receptor-specific T and B cells. This activation eventually results in the production of stimulatory antibodies that can cause hyperthyroidism. Similarly, significant new insights have been gained in our understanding of GO that occurs in a subset of patients with GD. As in GD, both environmental and genetic factors play important roles in the development of GO. Although a number of putative ocular autoantigens have been identified, their role in the pathogenesis of GO awaits confirmation. Extensive analyses of orbital tissues obtained from patients with GO have provided a clearer understanding of the roles of T and B cells, cytokines and chemokines, and various ocular tissues including ocular muscles and fibroblasts. Equally impressive is the progress made in understanding why connective tissues of the orbit and the skin in GO are singled out for activation and undergo extensive remodeling. Results to date indicate that fibroblasts can act as sentinel cells and initiate lymphocyte recruitment and tissue remodeling. Moreover, these fibroblasts can be readily activated by Ig in the sera of patients with GD, suggesting a central role for them in the pathogenesis. Collectively, recent studies have led to a better understanding of the pathogenesis of GD and GO and have opened up potential new avenues for developing novel treatments for GD and GO.

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.

DBA/2J (Mls-1a) B-cell differentiation in BALB.xid recipients

Studies of superantigens (SAg) have focused primarily on their impact on CD4+ T cells, largely bypassing the impact of the sequelae of this interaction upon the antigen-presenting cell (APC). Sequelae of SAg-induced CD4+ T-cell activation include the 'bathing' of the SAg-presenting cell with cytokines that promote the differentiation of the APC. In this report, the SAg-induced differentiation of Mls+ DBA/2J B cells was studied in vivo by their transplantation into B-cell-defective BALB.xid recipients. Rapid, high-level serum immunoglobulin M (IgM) production was noted shortly after transfer, disappearing by 3 weeks. Donor B cells, as evidenced after their chemical and genetic impairment and by the use of an IgM allotype-disparate donor-recipient combination, contributed to this transient IgM production. These results clarify a discrepancy in the literature regarding donor B-cell contribution to IgM production and illustrate a model system to utilize SAg to study B-lymphocyte diversity.

Superantigens and autoimmune disease: are they involved?

In over 10 years since the definition of superantigens, much has been learned about host cell-superantigen interactions. The initial simple set of rules used to define these interactions has given way to a more complex system, in which the activation of multiple cell types can occur as a consequence of superantigen-cell interactions or as a result of bystander effects based on the induction of a specific cytokine milieu. As a consequence, our ideas concerning the ways in which superantigens might be involved in disease are also expanding rapidly. This review highlights some of the many different pathways of superantigen-associated pathogenesis currently under investigation.

c-Rel is crucial for lymphocyte proliferation but dispensable for T cell effector function

The TCR signals are essential for T cell activation and proliferation, primarily through the induction of cytokine and cytokine receptors. Several transcription factor families, including NF-kappaB/Rel, have been implicated in the regulation of cytokine gene expression in T cells in response to antigen, cytokine and mitogenic stimulation. In this study, we show that the mice with a null mutation in the lymphoid-specific c-Rel gene have normal development of lymphoid tissues and T cell compartment. However, T cells derived from the c-Rel knockout mice have several functional abnormalities. The c-Rel-deficient T lymphocytes fail to respond to activation and proliferation signals mediated by the TCR and mitogens in vitro. This is attributed to an impaired production of cytokines IL-2, IL-3 and granulocyte macrophage colony stimulating factor. In addition, the induction of IL-2R alpha chain is impaired in the c-Rel(-/-) T cells. The poor expression of cytokines and IL-2R alpha chain correlates with a reduced nuclear translocation of NF-kappaB components in c-Rel(-/-) T cells. Since activation is prerequisite for differentiation into effector cells, c-Rel(-/-) T cells failed to differentiate into cytotoxic T cells or Th cells without rescuing cytokines. However, upon supplement with exogenous IL-2, the c-Rel(-/-) cytotoxic T lymphocytes are able to execute cytotoxicity and the c-Rel(-/-) Th cells are capable of providing help to normal B cells. These data suggest that c-Rel is important for inducible cytokine and cytokine receptor expression, and a key regulator of early activation and proliferation in T cells.

Sperm as infection-potentiating cofactors in HIV transmission

In this paper I will discuss the possible role of sperm as cofactors in the genital-mucosal transmission of HIV. The ideas involved arose from my laboratory's discovery that sperm bind to HLA-DR molecules expressed on somatic cells, and from our subsequent findings that lymphocytes are activated by these interactions. Sperm binding to HLA-DR mimics one of the two ligand binding characteristics of superantigens, which also bind to T-cell receptors in a V-beta-specific fashion. This property of sperm may be significant in HIV transmission because: (a) HLA-DR plays a central role in immune recognition and response; and (b) cell interactions involving HLA-DR are involved in HIV infection and disease development. After sexual contact, sperm elicit a transient leukocytic infiltration of the mammalian cervix (Thompson, L.A., Barratt, C.L., Bolton, A.E., Cooke, I.D., 1992. The leukocytic reaction of the human cervix. Am. J. Reprod. Immunol. 28, 85; Pandya, I.J., Cohen, J., 1985. The leukocytic reaction of the human uterine cervix to sperm. Fertil. Steril. 43, 417), and human cervical cells are bound and penetrated by sperm at this time (Sievers-Altermann, R., Engelbrecht, D.V., 1990. Entry of spermatozoa into the cervical mucosa and transmission of the AIDS virus. S. Afr. Med. J. 77, 319). At present, little is known about these in vivo events, but sperm-somatic cell interactions in vitro are also followed by sperm entry into the target cell cytoplasm and target cell activation. When the target cells are leukocytes, sperm interactions increase their susceptibility to HIV infection. If similar interactions occur in the cervicovaginal environment after sexual contact, they are likely to enhance the genital-mucosal transfer of HIV from semen.

c-Rel is essential for B lymphocyte survival and cell cycle progression

c-Rel is a lymphoid-specific member of the NF-kappaB/Rel family of transcriptional factors. To investigate the role of c-Rel in B lymphocyte function, we generated a c-Rel(-/-) mouse via a gene targeting approach. Although early lymphocyte development is normal in c-Rel(-/-) mice, there are significantly fewer B cells displaying a memory (IgM/IgD-) phenotype. Upon immunization, c-Rel(-/-) mice generate fewer B cells with a germinal center (PNAhi) phenotype. In vitro, c-Rel(-/-) B cells proliferate poorly upon ligation of their surface IgM or CD40 receptors or when stimulated with either lipopolysaccharide (LPS) or T cell help. Early molecular events that precede proliferation, such as increases in RNA synthesis as well as IL-2 receptor alpha chain expression, are greatly diminished in c-Rel(-/-) B cells. Furthermore, c-Rel(-/-) B cells are impaired in the ability to receive survival signals generated by anti-IgM or LPS. In contrast, CD40-mediated cell survival is normal in c-Rel(-/-) B cells, suggesting the involvement of a survival-signaling pathway that is independent of c-Rel. When c-Rel (-/-) B cells are co-stimulated with either anti-IgM and CD40 or LPS and CD40, they are rendered capable of progressing through the cell cycle. Finally, co-culture experiments suggest that the defects observed in c-Rel(-/-) B cells are intrinsic to the cell and can not be rescued through either cell-cell contact or addition of soluble factors. Thus, c-Rel is requisite for differentiation to the germinal center and memory B cells in vivo and is required for the transduction of survival and cell cycle progression signals mediated by anti-IgM and LPS in vitro. Furthermore, while c-Rel is involved in CD40-induced proliferation, it is apparently dispensable for the survival signals transduced by CD40.

Superantigen-Driven, CD8+ T Cell-Mediated Down-Regulation: CD95 (Fas)-Dependent Down-Regulation of Human Ig Responses Despite CD95-Independent Killing of Activated B Cells

Staphylococcal superantigens, including staphylococcal enterotoxin B (SEB), promote vigorous T cell-dependent Ig responses at low dose (0.01 ng/ml). In contrast, more mitogenic high dose SEB (100 ng/ml) profoundly inhibits the Ig responses. To assess the contribution of CD8+ T cells to this inhibition, high dose SEB-dependent killing of activated B cells and down-regulation of Ig responses were determined. Rapid killing (4 h) of activated B cells was effected by high dose SEB-activated CD8+ T cells (CD8*), but not by high-dose SEB-activated CD4+ T cells (CD4*), and required the presence of high dose SEB during the cytotoxicity assay. This killing was abrogated by chelation of extracellular calcium or by treatment with concanamycin A but was only modestly affected by treatment with brefeldin A, suggesting a perforin-based pathway of killing. Despite their widely disparate abilities to rapidly kill activated B cells, CD8* and CD4* demonstrated similar quantitative abilities to effect high dose SEB-dependent down-regulation of Ig responses. Antagonist anti-CD95 mAb substantially reversed high dose SEB-dependent down-regulation effected by CD8* but had no appreciable effects on high dose SEB-dependent killing of activated B cells. These observations strongly suggest that the small fraction of activated B cells that secrete Ig are selectively sensitive to CD95-based killing but resistant to CD95-independent killing. This finding may help explain why clinical autoimmunity associated with increased titers of autoantibodies is a predominant feature of defects in CD95 or CD95 ligand.

Cytokine production by cryopreserved human peripheral blood mononuclear cells in response to periodontal pathogens

Freezing techniques provide a way of repeating and extending immunological assays by using frozen portions of an individual's peripheral blood mononuclear cell fraction. Earlier work shows that the lymphocytes that are stored frozen retain their ability to respond to polyclonal B-cell activators, mitogens, superantigens and bacterial extracts of oral interest. These studies extend previous findings by determining cytokine production by lymphocytes following frozen storage for up to 24 weeks. Production of interleukin (IL)-1beta, IL-2, IL-6, and tumour necrosis factor (TNF)-beta by stimulated lymphocytes after cyropreservation was not significantly different from those responses before storage, with one exception: IL-6 production was negligible after 24 weeks' frozen storage when thawed cells were cocultured with pokeweed mitogen. After stimulation with extracts from Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans, the proliferative capacity of the frozen cells was maintained as well as the production of IL-1beta, IL-2, and IL-6. TNF-beta was not produced in response to bacterial antigen stimulation. The ability of peripheral blood mononuclear cells to retain functional activity after frozen storage should permit more effective monitoring during longitudinal clinical studies and a better evaluation of changes in cytokine production in patients with advanced periodontitis both during and after treatment.

Human autoimmune neuropathies

Peripheral nerve diseases are among the most prevalent disorders of the nervous system. Because of the accessibility of the peripheral nervous system (PNS) to direct physiological and pathological study, neuropathies have traditionally played a unique role in developing our understanding of basic mechanism of nervous system injury and repair. At present they are providing new insight into the mechanisms of immune injury to the nervous system. A rapidly growing catalogue of PNS disorders are now suspected to be immune-mediated, and in the best understood of these disorders, the molecular and cellular targets of immune attack are known, and the pathophysiology follows directly from the specific immune injury. This review summarizes the immunologically relevant features of the PNS, then considers selected immune-mediated neuropathies, focusing on pathogenetic mechanisms. Finally, the PNS is providing a testing ground for new immunotherapies and approaches to protection and regeneration, including the use of trophic factors. The current status of treatment and implications for future approaches is reviewed.

Expression of the superantigen Mycoplasma arthritidis mitogen in Escherichia coli and characterization of the recombinant protein

Mycoplasma arthritidis mitogen (MAM), is a soluble protein with classical superantigenic properties and is produced by an organism that causes an acute and chronic proliferative arthritis. Unfortunately, the process of obtaining purified MAM from M. arthritidis culture supernatants is extremely time-consuming and costly, and very little material is recovered. Thus, our laboratory has expressed MAM in Escherichia coli by using a protein fusion expression system. The construction and expression of recombinant MAM (rMAM), as well as a comparison of the biological properties of rMAM to those of native MAM, are discussed. Briefly, conversion of the three UGA codons to UGG codons was required to obtain full-length expression and mitogenic activity of rMAM. Antisera to native MAM recognized both rMAM and the fusion protein. The T-cell receptor Vbeta and major histocompatibility complex class II receptor usages by rMAM and the fusion protein were identical to that of native MAM. In addition, the ability to induce suppression and form the superantigen bridge could also be demonstrated with rMAM. Importantly, dose-response experiments indicated that homogeneous native MAM and rMAM were of equal potency. Thus, MAM has been successfully expressed in E. coli, thereby creating a viable alternative to native MAM.

Staphylococcal enterotoxin B induces arthritis in female DBA/1 mice but fails to induce activation of type II collagen-reactive lymphocytes

It has been proposed that superantigens are involved in the pathogenesis of autoimmune diseases. To test the possibility of superantigens inducing arthritis in naive mice, V beta 8-reactive superantigen staphylococcal enterotoxin B (SEB) was injected into naive mice. We used female DBA/1 mice, because they were susceptible to collagen-induced arthritis (CIA), in which the pathogenic T cells were supposed to preferentially use limited V betas of T cell receptors including V beta 8. Mild monoarthritis developed in uninjected hindlimbs of mice administered with SEB in higher frequency (an average incidence of 24%) than the control phosphate-buffered saline-injected mice (4.2%). Autoimmune responses in mice administered with SEB were compared with those in mice developing CIA. However, activation of type II collagen (IIC)-reactive T cells was not detected in SEB-injected mice. Production of autoantibodies, anti-IIC antibody and rheumatoid factor was also undetected. Although exact mechanisms of pathogenesis of this arthritis remain to be known, V beta 8+ T cells were activated for a long period and the unresponsiveness of V beta 8+ T cells was not detected in this strain. From these results, we discuss the pathogenesis of arthritis induced by SEB and the possibility that superantigen may play a role in the induction of autoimmune diseases.

Studies of proliferative responses by long-term-cryopreserved peripheral blood mononuclear cells to bacterial components associated with periodontitis

Freezing techniques provide a means for repeating and extending immunological assays with frozen aliquots of an individual's peripheral blood mononuclear cell fraction. Lymphocytes which are stored frozen for a limited time retain their ability to respond to polyclonal B-cell activators, mitogens, and antigens of dental interest. Our studies extend these previous findings by determining lymphocyte functional activity following frozen storage for up to 100 weeks. In addition, the autologous immune response was measured by spontaneous lymphocyte proliferation following 0, 1, 40, and 60 weeks of frozen storage. Peak responses for all individuals occurred at day 7 of incubation. The lymphocyte proliferative response to the superantigens toxic shock syndrome toxin-1 (TSST-1) and Staphylococcus enterotoxin A (SEA) were not changed after 100 weeks of frozen storage. Maximum responses varied among the individuals but occurred at equivalent stimulator concentrations. However, slopes generated from data obtained following 0, 4, 13, 20, 30, 50, 88, and 100 weeks of frozen storage showed no significant deviation from zero (P > 0.05) for all individuals tested. After 100 weeks of storage, the total changes in proliferative activity (counts per minute per week) were -2.1% +/- 16.8% and -5.5% +/- 17.0% for TSST-1 and SEA, respectively. The lymphocyte proliferative responses to pokeweed mitogen, concanavalin A, and sonicates of two periodontal pathogens (Porphyromonas gingivalis and Actinobacillus actinomycetemcomitans) following frozen storage were similar to those with TSST-1 and SEA. These results indicate that peripheral blood mononuclear cells stored frozen may serve as appropriate controls to monitor changes in the disease state long-term periodontal treatment.

Immunomodulatory and superantigen activities of bacteria associated with adult periodontitis

Immune dysfunctions are frequently associated with chronic inflammatory diseases. Several investigators have reported that patients with severe periodontitis show reduced or negligible levels of proliferative responses of peripheral blood and gingival lymphocytes to periodontopathic organisms. The purpose of this study was to evaluate the influences of products from Porphyromonas gingivalis (P. gingivalis) and Actinobacillus actinomycetemcomitans (A. actinomycetemcomitans) on lymphocytes obtained from periodontally diseased and non-diseased individuals in order to extend our understanding of the possible role of such bacterial components as immune modulators. Pooled cultures of either P. gingivalis or A. actinomycetemcomitans were disrupted using glass beads in a bead mill to prepare whole cell homogenates. These homogenates were then co-cultured with human peripheral blood mononuclear cells (PBMC) and known lymphocyte stimulators. Cultures were pulsed with tritiated thymidine, harvested, and radio label incorporation was determined. Responses to toxic shock syndrome toxin-1 (TSST-1) and pokeweed mitogen (PWM) were inhibited at high concentrations of bacterial homogenate. However, as the concentration was reduced, responses induced by PWM were restored while TSST-1 induced responses remained inhibited. Such results suggest that P. gingivalis and A. actinomycetemcomitans contain potent immunosuppressants with differential influences on lymphocyte population. These effects on B- and T-cells are independent of periodontal disease status and appear to exert their influence through non-toxic mechanisms. In addition, work currently underway presently indicates that obligate oral anaerobic bacteria such as P. gingivalis produce substances with some of the characteristics of superantigens.

Fas expression and apoptosis in human B cells

Mechanisms of B cell apoptosis are critical in reducing aberrant B cell proliferations such as those that arise in autoimmune disease and in B cell malignancies. The physiologic interaction of CD4+ helper T cells and B lymphocytes has been extensively studied over the past two decades. Although CD4+ T cells are considered primarily to offer positive costimulatory signals for B cell differentiation into active immunoglobulin-secreting cells, recent studies have shown that CD4+ T cells are crucial in downregulating the humoral immune response. In the course of cognate interaction between CD40 ligand (CD40L)-bearing CD4+ T cells and CD40-expressing germinal center B cells, CD40 ligation results in augmented Fas expression at the B cell surface. Like CD40L, Fas ligand is expressed on activated CD4+ Th1 cells and when bound to Fas receptor on the B cell surface, initiates an apoptotic signal in that cell. Thus, CD4+ T cells limit the growth of autologous germinal center B cells by first inducing Fas expression and then instigating a death signal via Fas ligand. In this work, we will consider these observations about CD4+ T-cell-induced, Fas-mediated B cell death in the context of other factors that affect apoptosis in B cells, normal and malignant.

Increased expression of CD40 ligand on systemic lupus erythematosus lymphocytes

The specificity of T cell help for B cell activation and differentiation is maintained by the brief expression on the T cell surface, following T cell receptor-mediated triggering, of CD40 ligand (CD40L). Interaction of T helper (Th) cell CD40L with B cell CD40 induces B cell activation, cell surface expression of activation antigens, proliferation, and initiation of immunoglobulin isotype switch. We predicted that in patients with systemic lupus erythematosus (SLE), in whom Th cell-dependent production of autoantibodies results in immune complex-mediated tissue damage, CD40L expression might be augmented, prolonged, or abnormally regulated. Baseline expression of CD40L was increased in some SLE patients studied, when compared with control subjects. While Th cells from normal subjects (n = 14) and rheumatic disease control patients (n = 9) showed maximal expression of CD40L, after in vitro activation with phorbol myristate acetate (PMA) and ionomycin, at 6 h of culture with diminished levels observed at 24 and 48 h, Th cells from SLE patients (n = 19) maintained high level cell surface expression of CD40L through 24 and 48 h of culture. The prolonged expression of CD40L was functionally significant, as 24 h-activated SLE T cells, when cocultured with target B cells, induced greater B cell surface CD80 (B7-1) expression than did 24 h-activated normal T cells. These results document impaired regulation of CD40L expression in SLE T cells and identify an important potential target for therapy in this systemic autoimmune disease.

T helper cell-dependent, microbial superantigen-mediated B cell activation in vivo

We have utilized a severe combined immune-deficient (SCID) mouse adoptive transfer model to explore the in vivo immunostimulatory effects of bacterial superantigens (SAg). B cell reconstituted SCID recipients were treated with the Staphylococcus aureus-derived toxic shock syndrome toxin (TSST-1) alone or in conjunction with syngeneic L3T4+ TSST-1-reactive Th cells. Over several months of study, the repetitive administration of TSST-1 resulted in a prompt, transient increase in serum IgG levels. This response required both biologically active TSST-1 and Th cells. These findings demonstrate that certain bacterial SAgs can promote Th cell-dependent B cell activation and differentiation in vivo. These studies strengthen the analogy between SAg-mediated and allospecific Th-B cell interactions responsible for the autoimmune sequelae of graft-versus-host disease.

CD40 ligation induces Apo-1/Fas expression on human B lymphocytes and facilitates apoptosis through the Apo-1/Fas pathway

The Apo-1/Fas antigen (CD95) mediates programmed cell death of lymphocytes when bound by Fas ligand or anti-Apo-1/Fas antibody. In contrast, the CD40 antigen provides a potent activation and survival signal to B lymphocytes when it is engaged by its T cell ligand (CD40L, gp39) or cross-linked by anti-CD40 antibody. In this study, we use human tonsillar B cells and the Ramos Burkitt's lymphoma B cell line, which serves as a model for human germinal center B lymphocytes, to study the effectors of Apo-1/Fas expression and apoptosis of human B cells. We found that Apo-1/Fas expression was upregulated on both malignant and normal human B lymphocytes after CD40 ligation induced by (a) cognate T helper-B cell interaction mediated by microbial superantigen (SAg); (b) contact-dependent interaction with CD40L+, but not CD40L- Jurkat mutant T cell clones; and (c) monoclonal anti-CD40, but not any of a panel of control antibodies. Enhanced B cell Fas/Apo-1 expression is functionally significant. Coculture of Ramos Burkitt's lymphoma line cells with irradiated SAg-reactive CD4+ T cells with SAg or CD40L+ Jurkat T cells results in B cell apoptosis, evidenced by reduced cell viability and DNA laddering. This process is augmented by the addition of anti-Apo-1/Fas monoclonal antibody, consistent with an acquired susceptibility to Apo-1/Fas-mediated apoptosis. These data support an immunoregulatory pathway in which seemingly contradictory signals involving the B cell proliferation/survival antigen CD40, as well as the Apo-1/Fas molecule, which mediates programmed cell death of lymphocytes, are linked in the process of human B cell activation.

Superantigens initiate cognate CD4+ T cell/B cell interactions leading to early activation and proliferation of B cells

Dimerization or even multimerization of various receptors is commonly required for signal transduction. We report here that clustering of major histocompatibility complex class II molecules in human B cells by biotinylated staphylococcal enterotoxin A (SEA) cross-linked with avidin induces an increase in the level of intracellular calcium. This response was abolished by prior treatment with protein tyrosine kinase (PTK) inhibitors, suggesting that SEA-triggered calcium mobilization in B cells is probably dependent on the activation of PTK. The implication of PTK in SEA-induced early B cell activation was then confirmed by demonstrating that cross-linked SEA induces a significant increase in the level of tyrosine phosphorylation in B cells. The requirement of biotinavidin cross-linking in SEA-induced calcium mobilization in B cells can be fulfilled by the addition CD4+ T cells, suggesting a role for CD4 molecules. Using the murine CD4- T cell hybridoma 3DT, or its derivative I1B3 transfected with human CD4 that both express SEA-specific TCR, we confirmed the CD4 requirement for B cell calcium mobilization and that both specific TCR and CD4 molecules are required in early events of B cell activation induced by SEA. The role of CD4 in SEA-induced B cell proliferation was then investigated. SEA-stimulated B cells proliferated in the presence of CD4+ T cells, whereas no response was observed in the presence of CD8+ T cells. The addition of clone I1B3 CD4+ T cells failed to fulfill the requirement of CD4+ T cells in SEA-induced B cell proliferation, indicating the possible involvement of other CD4+ T cell surface molecules in this response. This issue is currently under investigation.

B-cell superantigens: implications for selection of the human antibody repertoire

For several decades, B-cell interactions with antigens were thought to occur only through a clonal activation mechanism, in which the hypervariable regions of the immunoglobulin receptor are exclusively involved in ligand binding. However, an additional mode of interaction can occur, whereby molecules, termed B-cell superantigens, can bind human B cells bearing immunoglobulin receptors of a given variable (V)-gene family. This mechanism requires contributions from regions outside the conventional hypervariable loops and results in a B-cell response of increased magnitude. Here, Moncef Zouali reviews recent in vitro and in vivo observations on human B-cell superantigens in the context of the current consensus of B-cell development, and discusses the implications of these novel concepts with respect to pathogenesis.

The Mycoplasma arthritidis superantigen MAM: purification and identification of an active peptide

The prototypical superantigen MAM is an extracellular T-cell mitogen produced by Mycoplasma arthritidis, an organism which causes chronic proliferative arthritis of rodents. We here describe purification of MAM to homogeneity. Pure MAM exhibits all of the major properties previously described for partially purified MAM, including preference for H-2E molecules in presention to T cells, V beta T-cell receptor specificity for T-cell activation, and in vivo inhibition of T-cell functions but enhancement of B-cell activity as mediated by the superantigen bridge. Edman degradation of pure MAM gave a 54-residue partial amino-terminal sequence. The oligopeptide MAM15-31-C, synthesized according to the Edman sequence, blocked mitogenicity of MAM and supported assignment of the amino acid sequence.

MHC class II signaling in B-cell activation

The cognate interaction between T cells and antigen-presenting cells (APCs), mediated by major histocompatibility complex (MHC) class II molecules, results in the delivery of activation signals to the APC. These signals contribute to the expression of co-stimulatory activity by APCs and have important consequences for cell effector function. MHC class II molecules also serve as receptors for B-cell stimulation by microbial superantigens. In this review, Paul Scholl and Raif Geha discuss recent advances in our understanding of mechanisms of MHC class II signaling and analyse their role in human B-cell activation.

Mycoplasma arthritidis-derived superantigen induces proinflammatory monokine gene expression in the THP-1 human monocytic cell line

Soluble factors produced by Mycoplasma arthritidis play an important role in the pathology of arthritis in rodents, which closely resembles human rheumatoid arthritis. At least one of the products of these microorganisms, M. arthritidis-T cell mitogen (MAM), has biological activities in common with superantigens. These superantigens activate T cells in a V beta-restricted fashion, and this response is strictly dependent on the presence of major histocompatibility complex (MHC) class II-positive cells. In the present study, we have examined the ability of MAM to induce proinflammatory monokine (interleukin 1 beta [IL-1 beta] and tumor necrosis factor alpha [TNF-alpha]) gene expression in the THP-1 monocytic cell line. Treatment of these cells (which express a very low level of HLA-DR molecules) with gamma interferon (INF-gamma) induced HLA-DR, -DQ, and -DP molecules and enabled them to respond to MAM in a dose-dependent manner, resulting in an increase in the level of steady-state mRNA for IL-1 beta and TNF-alpha. Stimulation of the U937 monocytic cell line (MHC class II-negative even after INF-gamma treatment) with MAM did not induce either IL-1 beta or TNF-alpha transcription. Moreover, MAM adsorption on Raji (MHC class II-positive) cells resulted in the loss of its cytokine-inducing activity to induce monokine gene expression. These findings demonstrate clearly that MAM induces monokine gene expression following interaction with MHC class II molecules. Pretreatment of INF-gamma-treated THP-1 cells with the transcription inhibitor actinomycin D prevented the induction of monokine mRNA, whereas cycloheximide superinduced mRNA after stimulation with MAM. Finally, our results, obtained with protein tyrosine kinase inhibitors and antiphosphotyrosine Western blotting (immunoblotting), indicate that protein tyrosine kinase is involved in MAM-induced IL-1 beta and TNF-alpha gene expression in the THP-1 monocytic cell line. The capacity of MAM to induce proinflammatory cytokine transcription in monocytes via MHC class II molecules can be one pathway of MAM contribution to autoimmune diseases.

Human T-cell responses to Mycoplasma arthritidis-derived superantigen

When injected into mice, Mycoplasma arthritidis causes a chronic arthritis that resembles rheumatoid arthritis, histologically. The organism produces a superantigen termed Mycoplasma arthritidis mitogen or MAM, that in humans preferentially expands T cells whose antigen receptors express V beta 17. T cells with this phenotype appear to be increased in rheumatoid synovial effusions. We describe a novel approach to isolating and characterizing human MAM-reactive T-cell lines and determining their T-cell receptor (TCR) V beta usage. Lines were prepared from T cells that clustered with dendritic cells during a 2-day exposure to MAM. Cluster and noncluster fractions of T cells were then expanded by using feeder cells and a polyclonal mitogen. Most of the MAM reactivity was found in dendritic T-cell clusters, as were most of the T cells expressing TCR V beta 17. After expansion, 76% of the cluster-derived T-cell lines were MAM reactive, while no reactivity was seen in cell lines derived from the noncluster fraction. Of the MAM-reactive lines, 49% expressed V beta 17 on some or all of the cells. Cell lines from both cluster and noncluster fractions were analyzed for TCR V beta mRNA expression by PCR amplification. Other V beta genes (5.1, 7, 8, 12, and 20) were found to be expressed by lines that were MAM reactive, although these were not a major component of the cluster-derived T cells. Some non-cluster-derived lines expressed V beta s 17, 12, and 7, but these proved to be nonreactive to MAM. Therefore, dendritic cells can be used to immunoselect and characterize T cells that express superantigen-reactive TCRs.

Triggering and exacerbation of autoimmune arthritis by the Mycoplasma arthritidis superantigen MAM

OBJECTIVE

It has been postulated that superantigens might play a role in the human rheumatic diseases, by activation of self-reactive T cells or by induction of autoantibodies. The Mycoplasma arthritidis superantigen MAM, which is derived from a naturally occurring murine arthitogenic mycoplasma, uses certain V beta chains of the murine T cell receptor (TCR) that have been proposed to be involved in murine collagen-induced arthritis (CIA). The present study was designed to determine whether MAM influences the course of arthritis mediated by immunization with porcine type II collagen (PII).

METHODS

MAM or phosphate buffered saline (PBS) was injected locally or systemically into mice convalescing from CIA or mice suboptimally immunized with collagen.

RESULTS

In contrast to PBS, MAM caused an exacerbation of arthritis in mice that were recovering from CIA. MAM also triggered arthritis onset in mice that had been suboptimally immunized with PII up to 160 days previously. Injection of MAM during the onset phase of CIA also triggered and enhanced the severity of arthritis in mice given low doses of PII.

CONCLUSION

MAM can both trigger and exacerbate murine autoimmune arthritis induced by immunization with type II collagen. Since T cells bearing the same V beta TCRs as are used by MAM have been found to comprise a major portion of the activated cells in the synovial tissue of patients with rheumatoid arthritis, it is possible that superantigens similar to MAM may play a role in this human disease.

Looking for a 'superantigen' in AIDS: a possible role for Mycoplasma?

The extensive effort made to comprehend the complex immunopathology of human immunodeficiency virus (HIV) infection has resulted in research groups focusing attention on hypotheses as disparate as the possible 'superantigen' potential of HIV and the possible requirement for a Mycoplasma 'cofactor'. Utilizing the recent observation that a Mycoplasma species possesses 'superantigen' properties, this paper attempts to reconcile these seemingly discrepant observations in a model of the acquired immunodeficiency syndrome (AIDS) which builds on the potential contribution of a 'superantigen cofactor' to the ongoing process of HIV infection. A possible role for mycoplasma-induced T-cell proliferation, T-cell dysfunction, B-cell proliferation, and hyperglobulinaemia in the exacerbation of HIV infection is discussed. The relevance of a recent observation regarding protein sequence homology between the mycoplasma adhesion protein and several human class II major histocompatibility complex (MHC) proteins is also examined and incorporated into this model.

Bacterial and viral superantigens: roles in autoimmunity?

Superantigens are bacterial, viral, or retroviral proteins which can activate specifically a large proportion of T cells. In contrast with classical peptide antigen recognition, superantigens do not require processing to small peptides but act as complete or partially processed proteins. They can bind to major histocompatibility complex class II molecules and stimulate T cells expressing particular T cell receptor V beta chains. The other polymorphic parts of the T cell receptor, which are crucial for classical antigen recognition, are not important for this interaction. When this strategy is used a large proportion of the host immune system can be activated shortly after infection. The activated cells have a wide variety of antigen specificities. The ability to stimulate polyclonal B (IgG) as well as T cell responses raises possibilities of a role for superantigens in the induction of autoimmune diseases. Superantigens have been a great tool in the hands of immunologists in unravelling some of the basic mechanisms of tolerance and immunity.

Human B cell differentiation induced by microbial superantigens: unselected peripheral blood lymphocytes secrete polyclonal immunoglobulin in response to Mycoplasma arthritidis mitogen

Microbial superantigens (SA) activate a significant portion of the T cell repertoire based on their dual avidity for MHC class II antigens and T cell receptor (TCR) epitopes common to products of one or several TCR beta chain variable gene families. While SA that induce massive T cell proliferation and cytokine secretion have been implicated in clinical syndromes characterized by shock and generalized immunosuppression, SA activation of a more restricted T cell response may also have significant, perhaps immunostimulatory, effects on the immune system. To investigate this issue, we measured 3H-thymidine incorporation and polyclonal IgM and IgG secretion by normal human peripheral blood mononuclear cells (PBMC) cultured with a panel of microbial SA, including the Staphylococcus aureus-derived SA, SEA, SEB, SEC-1, SEC-2, SEC-3, SEE, TSST-1, and the Mycoplasma arthritidis-derived SA, MAM. The S. aureus-derived SA induce vigorous proliferation by PBMC, while optimal MAM-induced proliferation is significantly lower in magnitude. In all 12 subjects tested, mitogenic concentrations of MAM reproducibly stimulate unselected PBMC to secrete polyclonal IgM and IgG. In contrast, the S. aureus-derived SA induce Ig production only in cultures containing isolated B cell populations and either very low numbers of untreated autologous T cells, larger numbers of X-irradiated autologous T cells, or very low concentrations of the SA. No difference in the activation of helper (CD4) versus suppressor/cytotoxic (CD8) T cells by MAM and the S. aureus-derived SA was noted. Taken together, these data suggest that MAM's capacity to induce B cell differentiation correlates with its induction of a relatively weak proliferative response by unselected human T cells. MAM-like SA, when encountered in vivo, may result in a significant perturbation of the human immune system and potentially contribute to clinical syndromes characterized by immunostimulation and hypergammaglobulinemia.

Superantigens and their potential role in human disease

In the past few years, there has been a virtual explosion of information on the viral and bacterial molecules now known as superantigens. Some structures have been defined and the mechanism by which they interact with MHC class II and the V beta region of the T cell receptor is being clarified. Data are accumulating regarding the importance of virally encoded superantigens in infectivity, viral replication, and the life cycle of the virus. In the case of MMTV, evidence also suggests that superantigens encoded by a provirus may be maintained by the host to protect against future exogenous MMTV infection. Experiments in animals have also begun to elucidate the dramatic and variable effects of superantigens on responding T cells and other immune processes. Finally, the role of superantigens in certain human diseases such as toxic shock syndrome, some autoimmune diseases like Kawasaki syndrome, and perhaps some immunodeficiency disease such as that secondary to HIV infection is being addressed and mechanisms are being defined. Still, numerous important questions remain. For example, it is not clear how superantigens with such different structures, for example, SEB, TSST-1, and MMTV vSAG, can interact with MHC and a similar region of the TCR in such basically similar ways. It remains to be determined whether there are human equivalents of the endogenous murine MMTV superantigens. The functional role of bacterial superantigens also remains to be explained. Serious infection and serious consequences from toxin-producing bacteria are relatively rare events, and it is questionable whether such events are involved in the selection pressure to maintain production of a functional superantigen. Hypotheses to explain these molecules, which can differ greatly in structure, include T cell stimulation-mediated suppression of host responses or enhancement of environments for bacterial growth and replication, but substantiating data for these ideas are mostly absent. It also seems likely that only the tip of the iceberg has been uncovered in terms of the role of superantigens in human disease. Unlike toxic shock syndrome, other associations, especially with viral superantigens, may be quite subtle and defined only after considerable effort. The definition of these molecules and mechanisms of disease may result in new therapeutic strategies. Finally, it is apparent that superantigens have dramatic effects on the immune system. One wonders whether these molecules or modifications of them can be used as specific modulators of the immune system to treat disease.

Genetic and immunological mechanisms in the pathogenesis of systemic lupus erythematosus

Recent research in systemic lupus erythematosus and animal models of lupus has provided new insight into the pathogenesis of this complex autoimmune disease. Progress has been made towards understanding the genetic contributions to disease susceptibility and induction, as well as towards elucidation of the lymphocyte abnormalities involved in pathogenic autoantibody production.

Superantigens: biology, immunology, and potential role in disease

Superantigens are unique products of bacteria and viruses which, in combination with class II major histocompatibility complex molecules, are capable of stimulating a large fraction of T cells in an affected individual. This stimulation primarily involves the variable region of the T cell receptor beta chain (V beta). The discovery of superantigens and the elucidation of their immunologic properties have provided valuable tools for the investigation of the immune system in both normal and diseased animals. Most importantly, recent work suggests that superantigens play a role in a number of diverse pathological conditions, including toxic shock syndrome and autoimmune diseases such as rheumatoid arthritis.

Stimulation of cutaneous T-cell lymphoma cells with superantigenic staphylococcal toxins

Stimulation of T cells by superantigenic bacterial toxins is selective for cells bearing particular B chain variable (VB) gene segments of T-cell receptor (TCR). In humans, staphylococcal exfoliating toxin (ExT) and toxic shock syndrome toxin-1 (TSST-1) are known to stimulate VB 2-bearing T cells and staphylococcal enterotoxin B (SEB) does not activate VB 2-bearing T cells. We examined the proliferative response of cutaneous T-cell lymphoma (CTCL) cells to ExT, TSST-1, and SEB. Leukemic VB 2.1-bearing CTCL cells were reactive to ExT and TSST-1, but not SEB. In addition, two leukemia CTCL-VB 2- cell samples (one of which was VB 8) showed no substantial response to ExT. Thus, it was shown that Sezary cells proliferate in response to bacterial superantigens in a manner that is restricted by their VB usage. The addition of interleukin-1 (IL-1) in combination with ExT enhanced the stimulative response of VB 2.1-bearing CTCL cells that were pre-cultured with ExT for 7 d, suggesting that IL-1 can be a co-factor for the stimulation. The present study indicates that the superantigen reaction occurs with CTCL cells and implies a possible involvement of bacterial toxins in the pathogenesis of CTCL.

Characterization of human T cells reactive with the Mycoplasma arthritidis-derived superantigen (MAM): generation of a monoclonal antibody against V beta 17, the T cell receptor gene product expressed by a large fraction of MAM-reactive human T cells

While all known microbial superantigens are mitogenic for human peripheral blood lymphocytes (PBL), the functional response induced by Mycoplasma arthritidis-derived superantigen (MAM) is unique in that MAM stimulation of PBL consistently results in T cell-dependent B cell activation characterized by polyclonal IgM and IgG production. These immunostimulatory effects of MAM on the humoral arm of the human immune system warranted a more precise characterization of MAM-reactive human T cells. Using an uncloned MAM reactive human T cell line as immunogen, we have generated a monoclonal antibody (mAb) (termed C1) specific for the T cell receptor V beta gene expressed by the major fraction of MAM-reactive human T cells, V beta 17. In addition, a V beta 17- MAM-reactive T cell population exists, assessed by MAM, induced T cell proliferation and cytotoxic T cell activity. mAb C1 will be useful in characterizing the functional properties of V beta 17+ T cells and their potential role in autoimmune disease.

The Mycoplasma arthritidis T-cell mitogen, MAM: a model superantigen

The superantigens are receiving a great deal of attention as a new group of potent immunomodulatory molecules. They are produced by diverse microbial agents including staphylococci, streptococci and mycoplasmas and are also encoded by murine tumor viruses (the Mls antigens). Superantigens activate T cells by a unique pathway which can lead to modification of the T-cell repertoire and induction of autoimmunity. Here, Barry Cole and Curtis Atkin review their observations on the Mycoplasma arthritidis superantigen, MAM, and discuss how MAM might contribute to the acute and chronic inflammatory disease mediated by this organism.

Trypanosoma cruzi attachment to lymphocyte muscarinic cholinergic and beta adrenergic receptors modulates intracellular signal transduction

Plasma membrane vesicles of Trypanosoma cruzi (PMVs) formed saturation binding isotherms with naive murine T lymphocytes. Parasite membrane attachment to the muscarinic cholinergic receptors of Lyt 2.2+T cells (suppressor cells) resulted in the synthesis of cGMP, attenuation of cAMP levels and in the secretion of prostaglandin E2, an immunoregulator effector substance. These T suppressor cell signals were blunted by atropine and by monospecific antibody against T. cruzi surface epitopes. The interaction of T. cruzi PMVs with the beta adrenergic receptors of Lyt L3T4+T cells (helper cells) resulted in the synthesis of cAMP and in the attenuation of cGMP levels. T helper cells did not secrete prostaglandin E2 when T. cruzi PMVs were added to this system. These T helper cell signals were blunted by propranolol and by monospecific antibody against T. cruzi surface epitopes. The interaction of T. cruzi with T lymphocytes may result, therefore, in the down-regulation of the immune response induced by prostaglandin E2 T suppressor cell secretion and by cAMP inhibition of proliferation of T helper cells.

T-cell-mediated activation of B cells

Antibody responses to protein antigens are dependent on helper T lymphocytes, which provide necessary growth and differentiation, inducing stimuli to antigen-specific B cells. Recent investigations have focused on the mechanisms of T-B interactions, and the nature of the T-cell-derived signals that are needed for activation and expansion of B cells.

A potential role for microbial superantigens in the pathogenesis of systemic autoimmune disease

We have attempted herein to demonstrate how microbial superantigens could promote an abnormal form of "cognate" T helper-B cell interaction, analogous to that which may occur during GVH disease, leading to B cell activation and systemic autoimmunity. In vitro studies performed at our laboratory and others have demonstrated that resting human B cells bind microbial superantigens and present them to superantigen-reactive autologous T helper cells, resulting in T cell activation and polyclonal IgM and IgG production by the superantigen-bearing B cells. In vitro studies of microbial superantigen-mediated murine T helper-B cell interactions demonstrate preferential help for B cells that have encountered specific antigen. Both in humans and in mice, the cellular interactions involved and the B cell responses induced are highly analogous to those mediated by allospecific T helper-B cell interaction. Finally, the results of studies carried out on T cell-deficient (nude) mice suggest that microbial superantigens may trigger similar T helper cell-dependent polyclonal IgM and IgG responses in vivo. These mice will be studied over time and tested for the development of autoantibodies characteristic of SLE and of autoimmune organ system damage, the occurrence of which are predicted by our model.