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

Allergy-A New Role for T Cell Superantigens of Staphylococcus aureus?

Staphylococcus aureus superantigens (SAgs) are among the most potent T cell mitogens known. They stimulate large fractions of T cells by cross-linking their T cell receptor with major histocompatibility complex class-II molecules on antigen presenting cells, resulting in T cell proliferation and massive cytokine release. To date, 26 different SAgs have been described in the species S. aureus; they comprise the toxic shock syndrome toxin (TSST-1), as well as 25 staphylococcal enterotoxins (SEs) or enterotoxin-like proteins (SEls). SAgs can cause staphylococcal food poisoning and toxic shock syndrome and contribute to the clinical symptoms of staphylococcal infection. In addition, there is growing evidence that SAgs are involved in allergic diseases. This review provides an overview on recent epidemiological data on the involvement of S. aureus SAgs and anti-SAg-IgE in allergy, demonstrating that being sensitized to SEs—in contrast to inhalant allergens—is associated with a severe disease course in patients with chronic airway inflammation. The mechanisms by which SAgs trigger or amplify allergic immune responses, however, are not yet fully understood. Here, we discuss known and hypothetical pathways by which SAgs can drive an atopic disease.

Bacterial Superantigens Expand and Activate, Rather than Delete or Incapacitate, Preexisting Antigen-Specific Memory CD8+ T Cells

Superantigens (SAgs) released by common Gram-positive bacterial pathogens have been reported to delete, anergize, or activate mouse T cells. However, little is known about their effects on preexisting memory CD8+ T cell (TCD8) pools. Furthermore, whether SAgs manipulate human memory TCD8 responses to cognate antigens is unknown. We used a human peripheral blood mononuclear cell culture system and a nontransgenic mouse model in which the impact of stimulation by two fundamentally distinct SAgs, staphylococcal enterotoxin B and Mycoplasma arthritidis mitogen, on influenza virus- and/or cytomegalovirus-specific memory TCD8 could be monitored. Bacterial SAgs surprisingly expanded antiviral memory TCD8 generated naturally through infection or artificially through vaccination. Mechanistically, this was a T cell-intrinsic and T cell receptor β-chain variable-dependent phenomenon. Importantly, SAg-expanded TCD8 displayed an effector memory phenotype and were capable of producing interferon-γ and destroying target cells ex vivo or in vivo. These findings have clear implications for antimicrobial defense and rational vaccine design.

Mycoplasmal lipid-associated membrane proteins and Mycoplasma arthritidis mitogen recognition by serum antibodies from patients with rheumatoid arthritis

Mycoplasmal lipid-associated membrane proteins (LAMPs) and Mycoplasma arthritidis mitogen (MAM superantigen) are potent stimulators of the immune system. The objective of this work was to detect antibodies to MAM and LAMPs of Mycoplasma hominis and M. fermentans in the sera of patients affected by rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) to identify mycoplasmal products that can be involved in the etiopathogenesis of these autoimmune diseases. Serum samples from female RA and SLE patients and controls, recombinant MAM, and LAMPs of M. hominis PG21 and M. fermentans PG18 were used in Western blot assays. A similar frequency of sera from patients and controls reactive to MAM was detected. A larger number of M. hominis and M. fermentans LAMPs were recognized by sera from RA patients than controls, but no differences were detected between sera from SLE patients and controls. Among the LAMPs recognized by IgG antibodies from RA patients, proteins of molecular masses in a range of <49 and ≥20 KDa (M. hominis) and <102 and ≥58 KDa (M. fermentans) were the most reactive. These preliminary results demonstrate the strong reactivity of antibodies of RA patients with some M. hominis and M. fermentans LAMPs. These LAMPs could be investigated as mycoplasmal antigens that can take part in the induction or amplification of human autoimmune responses.

Association of Mycoplasma arthritidis mitogen with lethal toxicity but not with arthritis in mice

Mycoplasma arthritidis induces an acute to chronic arthritis in rodents. Arthritis induced in mice histologically resembles human rheumatoid arthritis and can be associated with lethal toxicity following systemic injection. The M. arthritidis mitogen (MAM) superantigen has long been implicated as having a role in pathogenesis, but its significance with respect to toxicity and arthritogenicity in mycoplasma-induced disease is unclear. To study the pathogenic significance of MAM, M. arthritidis mutants that overproduced or failed to produce MAM were developed. MAM overproduction and knockout mutants were more and less mitogenic, respectively, than the wild-type strain. The degree of mitogenic activity correlated with lethal toxicity in DBA/2J mice. In contrast, histopathological studies detected no correlation between MAM production and the severity of arthritis induced in DBA/2J and CBA/J mice.

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.

In vivo staphylococcal superantigen-driven polyclonal Ig responses in mice: dependence upon CD4(+) cells and human MHC class II

Staphylococcal enterotoxin (SE) B and seven other staphylococcal superantigens (SAg), despite promoting vigorous Ig production in human peripheral blood mononuclear cell cultures, are exceedingly poor at eliciting Ig responses in cultures of spleen cells from C57BL/10J (B10) or C3H/HeJ mice. In contrast, SEB elicits Ig responses in cultures of spleen cells from human MHC class II-transgenic mice. Whereas i.p. administration of SEB (0.2-20 microg) to non-transgenic B10 mice elicits very weak in vivo Ig responses, identical treatment of CD4(+) cell-intact (but not CD4(+) cell-depleted) human MHC class II-transgenic mice elicits dramatic increases in both splenic Ig-secreting cells and serum Ig levels. Over a 2-week period, the SEB-induced in vivo Ig responses peak and then plateau or fall in association with a preferential increase in splenic CD8(+) cells. Nevertheless, in vivo depletion of CD8(+) cells has no sustained effect on SEB-driven Ig responses. Taken together, these observations demonstrate that the effects of SAg on in vivo humoral immune responses are highly CD4(+) cell dependent, are substantially CD8(+) cell independent and can be successfully investigated using human MHC class II-transgenic mice. This model system may be useful in investigating the polyclonally activating effects of microbial products (prototypic environmental insults) on the development of systemic autoimmunity.

Prostaglandin E2 is variably induced by bacterial superantigens in bovine mononuclear cells and has a regulatory role for the T cell proliferative response

Signal transduction in antigen presenting cells via MHC class II molecules induces production of prostaglandin E2 (PGE2) known to possess immunoregulatory potential. Since Staphylococcus aureus superantigens (SAgs) utilize MHC class II molecules as primary ligands, we wanted to know whether PGE2 is induced after in vitro SAg stimulation of bovine blood mononuclear cells (boMNC), and whether this arachidonic acid metabolite modulates the preferential SAg-induced proliferative response of bovine CD8+ T cells. SEB as well as SEA induced maximal amounts of PGE2 on day 2 of culture (1-2.5 x 10(-8) mol/l per 2 x 10(5) boMNC). PGE2 production could be inhibited completely by indomethacin (10(-5) mol/l) causing enhanced proliferation of boCD4+ T cells (174%) as well as of boCD8+ T cells (122%) between day 4 and 6 of the in vitro culture, however, only in a subset of the tested animals. Notably, the striking preference of proliferation of boCD8+ over boCD4+ T cells following SAg stimulation remained largely unchanged after inhibition of endogenous PGE2 synthesis or after addition of exogenous PGE2. Higher concentrations of exogenously added PGE2 (> or = 10(-8) mol/l) inhibited the proliferation reaction, mainly due to an increased death rate of both CD4+ and CD8+ blasts. In contrast, lower PGE2 concentrations between 10(-8)-10(-9) mol/l even slightly enhanced the proliferation of both T cell subsets, depending on the individual cell donor. Summing up: These data show that SAgs, indeed, can induce PGE2 production in boMNC which can enhance or reduce the proliferative response of bovine CD4+ and CD8+ T cells.

Modulation of cytokine profiles by the Mycoplasma superantigen Mycoplasma arthritidis mitogen parallels susceptibility to arthritis induced by M. arthritidis

Mycoplasma arthritidis mitogen (MAM) is a potent superantigen secreted by M. arthritidis, an agent of murine arthritis. Here we compare the abilities of MAM to induce a panel of cytokines in vitro and in vivo in BALB/c and C3H/HeJ mouse strains that differ in susceptibility to mycoplasmal arthritis. Splenocytes from both mouse strains produced high levels of all cytokines by 24 h following in vitro exposure to MAM. No differences in cytokine profiles were seen irrespective of the MAM dose. However, there were striking differences in cytokine profiles present in supernatants of splenocytes that had been collected from mice after intravenous (i.v. ) injection of MAM and subsequently rechallenged with MAM in vitro. Splenocytes collected 24 and 72 h after i.v. injection of MAM and challenged in vitro with MAM showed the most marked divergence in the secreted cytokines. Type 1 cytokines were markedly elevated in C3H/HeJ cell supernatants, whereas they were depressed or remained low in BALB/c cell supernatants. In contrast, the levels of type 2 cytokines were all greatly increased in BALB/c cell cultures but were decreased or remained low in C3H/HeJ supernatants. Interleukin-12 mRNA and protein was also markedly elevated in C3H/HeJ mice, as were the levels of immunoglobulin G2a. The data indicate a major skewing in cytokine profiles to a type 1 inflammatory response in C3H/HeJ mice but to a protective type 2 response in BALB/c mice. These cytokine changes appear to be associated with the severe arthritis in C3H/HeJ mice following injection of M. arthritidis in comparison to the mild disease seen in injected BALB/c mice.

Superantigens: mechanisms by which they may induce, exacerbate and control autoimmune diseases

Superantigens are polypeptide molecules produced by a broad range of infectious microorganisms which elicit excessive and toxic T-cell responses in mammalian hosts. In light of this property and the fact that autoimmune diseases are frequently the sequelae of microbial infections, it has been suggested that superantigens may be etiologic agents of autoreactive immunological responses resulting in initiation, exacerbation or relapse of autoimmune diseases. This article relates the biology of superantigens to possible mechanisms by which they may exert these activities and reviews the evidence for their roles in various human and animal models of autoimmune disease. Finally, a mechanism of active suppression by superantigen-activated CD4+ T-cells that could be exploited for therapy as well as prophylaxis of human autoimmune diseases is proposed.

The role of superantigens in human diseases: therapeutic implications for the treatment of skin diseases

Although it is well established that immune mechanisms contribute to the pathogenesis of chronic inflammatory skin diseases such as atopic dermatitis (AD) and psoriasis, the actual events that trigger the immunological pathways resulting in these skin diseases are not well understood. Colonization and infection with Staphylococcus aureus and streptococci has been reported to exacerbate AD and psoriasis. Recent studies demonstrating that bacterial toxins can act as superantigens provide mechanism(s) by which S. aureus and streptococci could mediate an inflammatory skin lesion that consists predominantly of activated T-cells and monocytes. This review will explore the diverse mechanisms by which bacterial superantigens can induce skin inflammation following systemic or local infection. These observations provide a new direction for the development of novel approaches for the treatment of skin inflammation.

Molecular biology and pathogenicity of mycoplasmas

The recent sequencing of the entire genomes of Mycoplasma genitalium and M. pneumoniae has attracted considerable attention to the molecular biology of mycoplasmas, the smallest self-replicating organisms. It appears that we are now much closer to the goal of defining, in molecular terms, the entire machinery of a self-replicating cell. Comparative genomics based on comparison of the genomic makeup of mycoplasmal genomes with those of other bacteria, has opened new ways of looking at the evolutionary history of the mycoplasmas. There is now solid genetic support for the hypothesis that mycoplasmas have evolved as a branch of gram-positive bacteria by a process of reductive evolution. During this process, the mycoplasmas lost considerable portions of their ancestors' chromosomes but retained the genes essential for life. Thus, the mycoplasmal genomes carry a high percentage of conserved genes, greatly facilitating gene annotation. The significant genome compaction that occurred in mycoplasmas was made possible by adopting a parasitic mode of life. The supply of nutrients from their hosts apparently enabled mycoplasmas to lose, during evolution, the genes for many assimilative processes. During their evolution and adaptation to a parasitic mode of life, the mycoplasmas have developed various genetic systems providing a highly plastic set of variable surface proteins to evade the host immune system. The uniqueness of the mycoplasmal systems is manifested by the presence of highly mutable modules combined with an ability to expand the antigenic repertoire by generating structural alternatives, all compressed into limited genomic sequences. In the absence of a cell wall and a periplasmic space, the majority of surface variable antigens in mycoplasmas are lipoproteins. Apart from providing specific antimycoplasmal defense, the host immune system is also involved in the development of pathogenic lesions and exacerbation of mycoplasma induced diseases. Mycoplasmas are able to stimulate as well as suppress lymphocytes in a nonspecific, polyclonal manner, both in vitro and in vivo. As well as to affecting various subsets of lymphocytes, mycoplasmas and mycoplasma-derived cell components modulate the activities of monocytes/macrophages and NK cells and trigger the production of a wide variety of up-regulating and down-regulating cytokines and chemokines. Mycoplasma-mediated secretion of proinflammatory cytokines, such as tumor necrosis factor alpha, interleukin-1 (IL-1), and IL-6, by macrophages and of up-regulating cytokines by mitogenically stimulated lymphocytes plays a major role in mycoplasma-induced immune system modulation and inflammatory responses.

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.

CD95 (Fas)-Based, Superantigen-Dependent, CD4+ T Cell-Mediated Down-Regulation of Human In Vitro Immunoglobulin Responses

Naturally occurring microbial superantigens (SAg) have been implicated in several human idiopathic disorders, and a compelling argument for the role of SAg in autoantibody-associated disorders, such as systemic lupus erythematosus, has been proposed. To test the effects of SAg on human in vitro Ig responses, CD4+ T cell + B cell cultures were stimulated with graded doses of staphylococcal enterotoxin B (SEB). Ig-secreting cell (IgSC) responses were very weak in CD4+ T cell + B cell cultures stimulated with SEB at the optimal mitogenic concentration (high dose SEB; 100 ng/ml) but were strong in parallel cultures stimulated with low dose SEB (0.01 ng/ml). High dose SEB actually enhanced B cell differentiation in the presence of CD4+ T cell soluble helper factors as long as the B cells were prevented from physically contacting the CD4+ T cells. However, when cell-cell contact between CD4+ T cells and B cells was permitted, high dose, but not low dose, SEB promoted increased CD4+ T cell-mediated B cell apoptosis with resulting decreases in viable CD20+ B cells and IgSC. High dose, but not low dose, SEB triggered increased levels of soluble CD95 ligand, and down-regulation of IgSC responses and incremental apoptosis of activated B cells were prevented by antagonist anti-CD95 mAb. This strongly suggests that CD4+ T cell-mediated CD95-based killing of activated B cells plays a major role in controlling SEB-driven IgSC responses. Defects in SAg-based down-regulation may contribute to autoimmune disorders such as SLE.

Mycoplasma superantigen is a CDR3-dependent ligand for the T cell antigen receptor

Superantigens are defined as proteins that activate a large number of T cells through interaction with the Vbeta region of the T cell antigen receptor (TCR). Here we demonstrate that the superantigen produced by Mycoplasma arthritidis (MAM), unlike six bacterial superantigens tested, interacts not only with the Vbeta region but also with the CDR3 (third complementarity-determining region) of TCR-beta. Although MAM shares typical features with other superantigens, direct interaction with CDR3-beta is a feature of nominal peptide antigens situated in the antigen groove of major histocompatibility complex (MHC) molecules rather than superantigens. During peptide recognition, Vbeta and Valpha domains of the TCR form contacts with MHC and the complex is stabilized by CDR3-peptide interactions. Similarly, recognition of MAM is Vbeta-dependent and is apparently stabilized by direct contacts with the CDR3-beta region. Thus, MAM represents a new type of ligand for TCR, distinct from both conventional peptide antigens and other known superantigens.

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.

Allelic polymorphisms at the H-2A and HLA-DQ loci influence the response of murine lymphocytes to the Mycoplasma arthritidis superantigen MAM

Mycoplasma arthritidis, an agent of rodent arthritis, produces a potent superantigen (SAg), MAM. Previous work established that MAM is presented to T cells by murine H-2E or the homologous human HLA-DR molecules and that lymphocytes lacking a functional H-2E molecule fail to respond to MAM. Recently, more potent and purified preparations of MAM of known protein content have become available. This enabled us to more effectively compare the response of MAM with that of other SAgs by using lymphocytes from mice whose cells express different H-2A and HLA-DQ molecules. Here we demonstrate that cells from some H-2E-negative mouse strains respond to higher concentrations of MAM. By use of inbred, congenic, and recombinant mice, we show that these differences are, in fact, exercised at the level of the major histocompatibility complex (MHC) and that allelic polymorphisms at H-2A influence reactivity to MAM. In addition, polymorphisms at HLA-DQ, the human homolog of H-2A, also influence responsiveness to MAM. Cells expressing DQw6 (HLA-DQA1*0103 and DQBI*0601 chains) gave much higher responses to MAM than did cells expressing DQw8 (DQA1*0301 and DQB1*0302 chains). In fact, responses of lymphocytes expressing DQB1*0601 chains homozygously were as high as those observed for cells expressing a functional H-2E molecule. Murine lymphocytes responded less well to staphylococcal enterotoxin B (SEB) and SEA, but mouse cells expressing human MHC molecules gave much higher responses. The patterns of reactivity observed with cells expressing the various murine and human alleles differed for MAM, SEB, and SEA, suggesting that each of these SAgs interacts with different regions or residues on MHC molecules. It has been hypothesized that SAgs might play a role in susceptibility to autoimmune disease. Allelic polymorphisms at MHC loci might therefore influence susceptibility to autoimmune disease by affecting immunoreactivity to specific superantigens.

Functional analysis of Mycoplasma arthritidis-derived mitogen interactions with class II molecules

The ability of superantigens (SAGs) to trigger various cellular events via major histocompatibility complex (MHC) class II molecules is largely mediated by their mode of interaction. Having two MHC class II binding sites, staphylococcal enterotoxin A (SEA) is able to dimerize MHC class II molecules on the cell surface and consequently induces cytokine gene expression in human monocytes. In contrast, cross-linking with specific monoclonal antibodies or T-cell receptor is required for staphylococcal enterotoxin B (SEB) and toxic shock syndrome toxin 1 (TSST-1) to induce similar responses. In the present study, we report how Mycoplasma arthritidis-derived mitogen (MAM) may interact with MHC class II molecules to induce cytokine gene expression in human monocytes. The data presented indicate that MAM-induced cytokine gene expression in human monocytes is Zn2+ dependent. The MAM-induced response is completely abolished by pretreatment with SEA mutants that have lost their capacity to bind either the MHC class II alpha or beta chain, with wild-type SEB, or with wild-type TSST-1, suggesting that MAM induces cytokine gene expression most probably by inducing dimerization of class II molecules. In addition, it seems that SEA and MAM interact with the same or overlapping binding sites on the MHC class II beta chain and, on the other hand, that they bind to the alpha chain most probably through the regions that are involved in SEB and TSST-1 binding.

Rabies superantigen as a Vbeta T-dependent adjuvant

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

The sequence of the Mycoplasma arthritidis superantigen, MAM: identification of functional domains and comparison with microbial superantigens and plant lectin mitogens

Mycoplasma arthritidis, an agent of chronic proliferative arthritis of rodents, secretes a potent soluble superantigen, MAM, that is active for both murine and human T and B lymphocytes. We now report the complete nucleotide and amino acid sequence of MAM and show it to be distinct from other proteins and not closely related phylogenetically to other superantigens. Two functional domains on MAM are identified based on the ability of peptides encompassing these regions to inhibit lymphocyte proliferation by the intact MAM molecule. One of these domains shares short sequences or epitopes with other microbial superantigens. The second domain contains the consensus legume lectin motif-beta, which is important for T cell activation by concanavalin (Con) A. MAM and Con A peptides containing this motif are functionally cross reactive, suggesting a novel secondary pathway for T cell activation by MAM.

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.

Modulation of Mycoplasma arthritidis-derived superantigen-induced cytokine gene expression by dexamethasone and interleukin-4

Activation of human monocytes or monocytic cell lines with all known stimuli coordinately induces the gene expression of various cytokines, including tumor necrosis factor alpha (TNF-alpha), interleukin-1 beta (IL-1 beta), and the IL-1 receptor antagonist (IL-1Ra). In contrast, superantigens induce TNF-alpha and IL-1 beta but fail to affect IL-1Ra gene expression, suggesting that activation of monocytes via major histocompatibility complex class II is distinct from other signal transduction pathways. In the present study, we analyzed the regulation of the Mycoplasma arthritidis-derived superantigen (MAM)-induced IL-1 beta and TNF-alpha gene expression by studying the effects of two different anti-inflammatory agents: dexamethasone (DEX) and the T-cell-derived cytokine IL-4. Both agents contributed to the downregulation of MAM-induced IL-1 beta and TNF-alpha gene expression. They accelerated the normal decline of the gene expression of both MAM-induced cytokines by decreasing the stability of mRNAs via the induction or enhanced synthesis of one or more regulatory proteins. In addition, IL-4, but not DEX, induced a strong and rapid expression of IL-1Ra mRNA in MAM-stimulated and unstimulated THP-1 cells in a de novo protein synthesis-independent manner. The capacity of IL-4 to induce IL-1Ra gene expression reinforces its anti-inflammatory activity. This study illustrates some of the mechanisms by which MAM-induced proinflammatory monokine gene expression can be downregulated by IL-4 and DEX.

Increased frequency of V beta 17-positive T cells in patients with rheumatoid arthritis

OBJECTIVE

To identify the T lymphocytes that mediate disease in rheumatoid arthritis (RA).

METHODS

A panel of monoclonal antibodies reactive with T cell receptor (TCR) V beta gene products was used to analyze the RA T cell repertoire.

RESULTS

Of 5 TCR V beta gene products studied, only V beta 17-positive T cells were increased in peripheral blood and synovial fluid (SF) from RA patients, compared with controls (P < 0.01 and P = 0.0006, respectively). Thirty-one percent of the 49 RA SF samples and none of the 19 non-RA SF samples contained > 10% V beta 17-positive T cells. Activated (Tac-positive) T cells were enriched among V beta 17-positive synovial T cells.

CONCLUSION

The selective increase of V beta 17-positive T cells suggests a role for those T cells in the pathogenesis of RA.

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.