Superantigens: mechanism of T-cell stimulation and role in immune responses

Resistance to tolerance induction in the diabetes-prone biobreeding rat as one manifestation of abnormal responses to superantigens

T cells taken from normal rats treated with an exogenous source of bacterial superantigen in vivo specifically failed to proliferate following re-stimulation with the same superantigen in vitro. Responsiveness was restored following the addition of an exogenous source of interleukin-2 indicating that the T cells had been made functionally tolerant and not deleted. While staphylococcal enterotoxin treatment of normal rats virtually abolished T-cell proliferation to the same enterotoxin in vitro, T cells from similarly treated diabetes-prone Biobreeding (BB-DP) rats were markedly resistant to this in vivo effect. Responses in BB-DP rats were never reduced by more than 50% even when a 4 times more effective dose of enterotoxin was employed. The resistance of BB-DP peripheral T cells to staphylococcal enterotoxin-induced tolerance could not be attributed to differences in T-cell receptor V beta chain family usage of BB-DP vs normal T cells but was associated with qualitative differences in the way in which BB-DP T cells responded to staphylococcal enterotoxins in vitro. While under optimal stimulatory conditions BB-DP T-cell proliferative responses to staphylococcal enterotoxins appeared comparable to those from non-diabetes-prone animals, under superoptimal conditions BB-DP, but not diabetes-resistant, donor T-cell proliferative responses to staphylococcal enterotoxins could be blocked in vitro with antibodies to CD4 antigens. In addition, BB-DP T-cell proliferative responses were more sensitive to suboptimal staphylococcal enterotoxin doses in vitro. We discuss ways in which abnormal BB-DP T-cell responses to superantigens in general and resistance to staphylococcal enterotoxin-mediated tolerance induction in particular may play a role in the generation of a peripheral T-cell repertoire prone to autoimmunity.

T cell receptor (TCR) repertoire in alloimmune responses

A large number of alloantigenic determinants could be generated by both the direct and indirect alloantigen presentation pathways. Hence, a heterogeneous population of T cells expressing a wide variety of receptors would be expected to respond to this diverse array of alloantigenic determinants. However, T cells expressing highly restricted T cell receptor (TCR) variable genes have been reported in a variety of alloimmune responses. A similar phenomenon has been observed in a wide variety of other immune responses, from those induced by superantigens, to very specific responses induced by a single peptide presented by a single MHC molecule. Given this scenario, the limited number of T cell clones which dominate an allograft rejection response, or for that matter an autoimmune response or a tumor specific response, could be therapeutically targeted by virtue of the selected TCR expression.

Evidence for the promotion of positive selection of thymocytes by Ah receptor agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a ligand for the arylhydrocarbon receptor (Ah receptor), abundant in the murine thymus. In the thymus immunocompetent T cells develop. Upon exposure of murine fetal thymi in organ cultures to TCDD the distribution of mature and immature thymocytes is skewed towards apparently mature, prospective cytotoxic cells of the CD4-CD8+T cell receptor+ phenotype. The normally abundant CD4+ CD8+ cells are decreased. Proliferation of the most immature thymocyte subpopulations is inhibited and maturation of thymocytes appears accelerated by TCDD. Eventually the thymocyte number is significantly decreased. Selective treatment of stroma cells showed them to be the primary target cells of TCDD action. Thymus stroma plays a pivotal role in thymocyte maturation and is indispensable for the selection of thymocytes bearing T cell receptors specific for foreign antigen in the context of self. We tested whether the effects of TCDD on thymocyte differentiation and maturation has further consequences for the selection processes by analysing (a) the repertoire of V beta genes used as a measure for negative selection and (b) the expression of CD69 and bcl-2 by thymocytes as a parameter of positive selection. Our data indicate that TCDD does not cause gross disturbance of negative selection but provide evidence for more cells auditioning for positive selection by TCDD exposure.

Elevated numbers of proliferating mononuclear cells in the peripheral blood of psoriatic patients correlate with disease severity

Although psoriasis is characterized by the accumulation of activated proliferating lymphoid cells in the psoriatic skin lesion, it is not known whether these cells are activated and proliferating before entry into the psoriatic plaque. The current study evaluates the number and phenotype of proliferating lymphoid cells in the blood of psoriatic patients. Proliferation of peripheral blood mononuclear cells was evaluated on cytospun preparations of these cells using autoradiographic techniques after pulsing the mononuclear cells with 3H-methyl thymidine for 2 h. The phenotypes of the labeled peripheral blood mononuclear cells were determined combining autoradiography and immunohistochemistry with monoclonal antibodies directed at CD3, CD4, CD8, CD11c, CD22, and human leukocyte antigen-DR. The data demonstrated elevated numbers of proliferating lymphoid cells in the blood of psoriatic patients compared with normal nonpsoriatic volunteers (p < 0.01). Furthermore, the number of circulating proliferating mononuclear cells increased significantly with increasing psoriasis skin disease severity (correlation coefficient 0.95; p < 0.0001). When the phenotype of the proliferating cells in the blood was examined, the numbers of T cells (CD3+, CD4+, CD8+ cells), B cells (CD22+ cells), monocytes (CD11c+ cells), and human leukocyte antigen-DR+ cells were significantly elevated compared with nonpsoriatic skin (p < 0.01) and increased with increasing disease activity (correlation coefficient range 0.48-0.74; p < 0.05). The data suggest a generalized systemic activation of T, B, and monocytic cells that results in labeling of up to 0.16% of the circulating mononuclear cells with 3H-methyl thymidine (i.e., proliferating and presumably activated) when assayed in vitro.

Reactive arthritis-associated bacteria can stimulate lymphocyte proliferation in non-exposed individuals and newborns

In reactive arthritis (ReA) a specific T cell response to the triggering bacterial antigen is present in the synovial fluid, while in paired peripheral blood T cells the response is markedly reduced. The proliferative response to ReA-associated bacteria in the peripheral blood of ReA patients was compared with that seen in the blood of healthy adults, who denied exposure to these microbes, and in the umbilical cord blood of newborns, who have clearly not been exposed to bacterial antigen. Peripheral blood mononuclear cells (PBMC) from non-exposed adults and those from umbilical cord blood proliferated to ReA-associated bacteria, whilst little response was seen in ReA PBMC. The response was MHC class II-restricted, required processing of the bacterial antigen, was seen in both CD45RO+ and CD45RA+ subsets, and was not oligoclonal. These T cell responses are similar to those previously demonstrated in non-exposed individuals to malaria, leishmania and trypanosoma antigen, and may reflect the existence of 'natural' T cell immunity to ReA-associated bacteria. The lack of such responses in ReA peripheral blood may suggest that such 'natural' responses may restrict the dissemination or progression of infection.

Analysis of T cell receptor beta chain expression by isoelectric focusing following gene amplification and in vitro translation

We describe a new approach to analysis of T cell receptor diversity based on isoelectric focusing of in vitro translation products of amplified V region genes. The method is illustrated by analysis of V beta 2 profiles in peripheral blood lymphocytes from normal donors. The primers used for V beta 2 analysis spanned the V-(D-)J junction and included the segment from amino acid residue position 53 in the variable region to residue 132 of the constant region. The isoelectric focusing patterns display approximately 13-14 bands of varying intensity. Differences in expression of V beta 2-derived peptides were detected in comparisons of the isoelectric focusing profiles from different individuals, suggesting that the method may be useful for detecting genetically determined, immune response related or disease associated differences in Tcr V region expression. The major isoelectric focusing bands have been interpreted as representing groups of V beta 2 sequences sharing J beta region and NDN region charge similarity. Quantitative differences were detected in V beta 2 profiles of CD4 and CD8 T cell subpopulations indicating there may be selection for different charge characteristics in NDNJ sequences in the two T cell subsets. The method provides a new dimension for the detection of perturbations in the T cell repertoire.

An essential role for gp39, the ligand for CD40, in thymic selection

The interactions between CD40 on B cells and its ligand gp39 on activated T helper cells are known to be essential for the development of thymus-dependent humoral immunity. However, CD40 is also functionally expressed on thymic epithelial cells and dendritic cells, suggesting that gp39-CD40 interactions may also play a role in thymic education, the process by which self-reactive cells are deleted from the T cell repertoire. Six systems of negative selection were studied for their reliance on gp39-CD40 interactions to mediate negative selection. In all cases, when the antigen/superantigen was endogenously expressed (in contrast to exogenously administered), negative selection was blocked by loss of gp39 function. Specifically, blockade of gp39-CD40 interactions prevented the deletion of thymocytes expressing V beta 3, V beta 11, and V beta 12, specificities normally deleted in BALB/c mice because of the endogenous expression of minor lymphocyte-stimulating determinants. Independent verification of a role of gp39 in negative selection was provided by studies in gp39-deficient mice where alterations in T cell receptor (TCR) V beta expression were also observed. Studies were also performed in the AND TCR transgenic (Tg) mice, which bear the V alpha 11, V beta 3 TCR and recognize both pigeon cytochrome c (PCC)/IEk and H-2As. Neonatal administration of anti-gp39 to AND TCR Tg mice that endogenously express H-2As or endogenously produce PCC prevented the deletion of TCR Tg T cells. In contrast, deletion mediated by high-dose PCC peptide antigen (administered exogenously) in AND TCR mice was unaltered by administration of anti-gp39. In addition, deletion by Staphylococcus enterotoxin B in conventional mice was also unaffected by anti-gp39 administration. gp39 expression was induced on thymocytes by mitogens or by antigen on TCR Tg thymocytes. Immunohistochemical analysis of B7-2 expression in the thymus indicated that, in the absence of gp39, B7-2 expression was substantially reduced. Taken together, these data suggest that gp39 may influence negative selection through the regulation of costimulatory molecule expression. Moreover, the data support the hypothesis that, for negative selection to some endogenously produced antigens, negative selection may be dependent on TCR engagement and costimulation.

Immune deficiency following thermal trauma is associated with apoptotic cell death

Thermal injury-associated specific immune deficiency occurs despite indicators of systemic activation of the lymphoid compartment. We investigated the possibility that postburn immune failure and T cell activation are casually related through activation-induced (apoptotic) cell death. The relationship between the cellular immune response and cell mortality was examined in cultures of peripheral blood mononuclear cells (PBMC) from 14 immunosuppressed patients with extensive burns (35-90% total body surface area). Impaired cellular immunity coincided with significantly reduced cell viability as ascertained by propidium iodide staining and dye reduction assays. Following stimulation with the mitogenic lectin, phytohemagglutinin (PHA), the majority of DNA in patient cultures was fragmented, suggesting the occurrence of apoptotic cell death. Even without stimulation a portion of patient cells was apoptotic as indicated by oligonucleosomal bands on agarose gel electrophoresis. Exogenous interleukin-2 or phorbol ester markedly reduced constitutive as well as PHA-induced DNA fragmentation. In situ demonstration of DNA strand breaks in freshly isolated patient PBMC, by a TdT-based labeling technique, confirmed that a larger fraction (up to 60%) of circulating lymphocytes was undergoing apoptosis on the periphery. These novel observations suggest that apoptosis may play a major role in thermal injury-related cellular immunodeficiency.

Overexpression of the T-cell receptor V beta 3 in transgenic mice increases mortality during infection by enterotoxin A-producing Staphylococcus aureus

We have previously demonstrated that staphylococcal enterotoxins contribute to arthritis and mortality during staphylococcal infection. To further explore the mechanism by which bacterial superantigens contribute to the pathogenesis of Staphylococcus aureus septicemia, T-cell receptor V beta 3 transgenic (TGV beta 3) mice and nontransgenic (non-TG) littermates were inoculated intravenously with S. aureus AB-1, which produces large amounts of staphylococcal enterotoxin A, which specifically reacts with T-cell receptor V beta 3. Within 9 days after inoculation, 85% of the TGV beta mice died, compared with 31% of their non-TG littermates (P < 0.01). The high mortality of TGV beta 3 mice was accompanied by elevated bacterial burdens in the blood, spleen, and kidneys. The in vivo kinetics of cytokine mRNA expression was studied by an in situ hybridization technique. Staphylococcal infection gave rise to increased expression of interleukin 1 beta (IL-1 beta) mRNA and sparsely expressed tumor necrosis factor alpha (TNF-alpha), IL-4, and IL-10 mRNAs in both groups. Gamma interferon mRNA expression increased on day 3 and was maintained at a detectable level in the late phase of infection in TGV beta 3 mice, in contrast to non-TG mice. Impressively, significantly higher expression of TNF-beta mRNA in TGV beta 3 mice was noted throughout the course of infection than in non-TG littermates. These findings suggest that overproduction of TNF-beta and gamma interferon, the Th1 cytokines, may play a crucial role in the pathogenesis of septicemia caused by enterotoxin-secreting staphylococci.

B-cell superantigens: definition and potential impact on the immune response

Superantigens have been extremely helpful tools in exploring fundamental questions in immunobiology including mechanisms of cell activation, tolerance, and autoimmunity. Until recently, attention has been focused exclusive on T-cell superantigens. However, new data suggest that there are superantigens that directly activate B cells. By definition, these agents (1) stimulate a high frequency of B cells, (2) target B cells that have restricted usage of VH or VL family genes, and (3) bind to immunoglobulins outside the sites that bind conventional antigens. A candidate B-cell superantigen that has received considerable attention in this laboratory is staphylococcal protein A. This agent is best known to the immunologist because of its ability to bind to the Fc fragment of IgG. This binding has been localized to two alpha-helical structures on each of four or five homologous regions that comprise the extracellular domain of protein A. However, it is now clear that protein A contains a second site that binds to determinants on the Fab regions of certain immunoglobulins independently of their heavy-chain isotype. In man this so-called alternative site appears to bind only to immunoglobulins that utilize heavy-chain genes of the VH3 subfamily. In the mouse this type of binding is restricted to immunoglobulins using heavy chains belonging to the S107 and J606 VH families. In this review, we examine the growing list of microbial products that dominate B-cell superantigenic properties. Using staphylococcal protein A as a model for a B-cell superantigen, we consider the potential impact of this novel class of antigens on the immune response. We focus on the ability of B-cell superantigens to influence the expression of the B-cell repertoire. In addition, we consider the hypothesis that the interaction of a B-cell superantigen with its reactive serum immunoglobulins activates the classical complement cascade and thus represents a powerful stimulant of tissue inflammation.

Direct presentation of nonpeptide prenyl pyrophosphate antigens to human gamma delta T cells

Human V gamma 2V delta 2+ T cells recognize mycobacterial nonpeptide antigens, such as isopentenyl pyrophosphate, and their synthetic analogs, such as monoethyl phosphate, through a TCR-dependent process. Here, we examine the presentation of these antigens. V gamma 2V delta 2+ T cells recognized secreted prenyl pyrophosphate antigens in the absence of other accessory cells but, under such conditions, required T cell-T cell contact. Recognition required neither the expression of classical MHC class I, MHC class II, or CD1a, CD1b, and CD1c molecules, nor MHC class I or class II peptide loading pathways. Fixed accessory cells also presented the prenyl pyrophosphate antigens to gamma delta T cells. Thus, in contrast with the presentation of conventional peptide antigens, protein antigens, and superantigens to alpha beta T cells, prenyl pyrophosphate antigens are presented to gamma delta T cells through a novel extracellular pathway that does not require antigen uptake, antigen processing, or MHC class I or class II expression. This pathway allows for the rapid recognition of bacteria by gamma delta T cells and suggests that gamma delta T cells play a role in the early response to bacterial infection.

Intracellular Leishmania amazonensis amastigotes internalize and degrade MHC class II molecules of their host cells

In their amastigote stage, Leishmania live in mammalian macrophages within parasitophorous vacuoles (PV), organelles of phagolysosomal origin that, in macrophages activated with IFN-gamma, contain major histocompatibility complex (MHC) class II molecules apparently devoid of invariant chains. We have now studied the fate of PV-associated class II molecules in mouse bone marrow-derived macrophages infected with L. amazonensis amastigotes using immunocytochemical and biochemical approaches. We have found that at least a part of these class II molecules was internalized by amastigotes and reached structures very often located in their posterior poles. This process was much more obvious if infected macrophages were incubated with protease inhibitors like antipain, chymostatin, Z-Phe-AlaCHN2 and Z-Phe-PheCHN2, or if amastigotes were pre-treated with the irreversible cysteine protease inhibitor Z-Phe-AlaCHN2 before infection, clearly indicating that amastigotes also degraded the internalized class II molecules. Study of infected macrophage cryosections by immuno-electron microscopy allowed the identification of the class II-positive structures in amastigotes as the lysosome-like organelles known as megasomes. Other PV membrane components like the prelysosomal/lysosomal glycoproteins Igp110, Igp120 and macrosialin could not be detected in megasomes of amastigotes even after treatment of macrophages with protease inhibitors, suggesting the involvement of some specific mechanism(s) for the internalization of class II molecules. Interestingly, after treatment of infected macrophages with various protease inhibitors (antipain, leupeptin, E-64, Z-Phe-AlaCHN2, Z-Phe-PheCHN2), PV membrane as well as megasomes of amastigotes become positive for invariant chains. A quantitative analysis of amastigote-associated class II molecules based on enzyme immunoassays showed that: (a) amastigotes extracted from macrophages treated with both IFN-gamma and antipain or Z-Phe-AlaCHN2 contained a much greater amount of class II than amastigotes extracted from macrophages treated with IFN-gamma alone; (b) class II molecules associated with the former were mainly intracellular and, at least some of them, were complexed with invariant chains or fragments of invariant chains; (c) amastigotes pre-incubated with Z-Phe-AlaCHN2 before infection accumulated a greater amount of intracellular class II than amastigotes pre-incubated without inhibitor, clearly indicating that the blockade of parasite cysteine proteases was sufficient to enhance the pool of these molecules within megasomes. On the whole, these data are consistent with the idea that class II molecules reaching PV are newly synthesized and still complexed with intact invariant chains or with partially degraded invariant chains.(ABSTRACT TRUNCATED AT 400 WORDS)

Immunopathogenesis of juvenile rheumatoid arthritis: role of T cells and MHC

Juvenile rheumatoid arthritis (JRA) is defined as chronic arthritis of unknown etiology appearing in patients less than 16 years of age. The disease is heterogeneous and is classified as pauciarticular, polyarticular, or systemic-onset disease. A few lines of evidence suggest that T cells are involved in the pathogenesis of the disease. T cells infiltrating the synovial membrane bear markers of activation and produce cytokines. The association of particular subtypes of JRA with certain HLA class II alleles provides strong evidence in favor of T cell involvement through an HLA-peptide-T cell receptor complex. Limited data from a few patients with JRA on T cell receptor transcripts from synovial membrane or synovial fluid cells point towards oligoclonality. This further supports the concept that T cells infiltrating the synovial membrane or extravasating into synovial fluid in patients with JRA reflect antigen-driven T cell proliferation.

A role for the orphan steroid receptor Nur77 in apoptosis accompanying antigen-induced negative selection

The transcription factor Nur77, an orphan member of the nuclear hormone receptor superfamily, is highly expressed during T cell receptor-signaled apoptosis, suggesting a possible role for Nur77 in negative selection. We examined this by generating two sets of transgenic mice. In one set of mice, a dominant-negative Nur77 mutant is constitutively expressed and the other in which wild-type Nur77 protein is constitutively expressed in developing thymocytes. We report that inhibition of endogenous Nur77 by the dominant-negative mutant perturbed T cell development and inhibited antigen-induced negative selection in F5T cell receptor transgenic mice. Constitutive expression of wild-type Nur77 protein induced apoptosis in developing thymocytes, resulting in a decreased number of thymocytes and mature T cells. Together, these data support a role for Nur77 in the downstream signaling events in antigen-induced negative selection.

In vivo effects of superantigens

Superantigens are potent immunostimulatory molecules that activate both T cells and antigen presenting cells. The consequences of superantigen exposure range from induction of T cell proliferation, massive cytokine release and systemic shock to immunosuppression and tolerance. Superantigens have been directly implicated in a number of human conditions including food poisoning and toxic shock. In addition, there is evidence to suggest that superantigens are involved in the initiation of autoimmunity, and the immune dysfunction associated with HIV infection. Because of their possible role in human disease, and their potential use in immune therapy, it is important that we more completely understand the in vivo effects of superantigens.

Staphylococcal enterotoxin A has two cooperative binding sites on major histocompatibility complex class II

The superantigen staphylococcal enterotoxin A (SEA) binds to major histocompatibility complex (MHC) class II molecules at two sites on either side of the peptide groove. Two separate but cooperative interactions to the human class II molecule HLA-DR1 were detected. The first high affinity interaction to the DR1 beta chain is mediated by a zinc atom coordinated by H187, H225, and D227 in SEA and H81 in the polymorphic DR1 beta chain. The second low affinity site is to the DR1 alpha chain analogous to SEB binding and is mediated by residue F47 in SEA. Binding of one SEA to the DR1 beta chain enhances the binding of a second SEA molecule to the DR1 alpha chain. The zinc site is on the opposite side of the SEA molecule from residue F47 so that one SEA molecule can readily bind two class II molecules. Both binding sites on SEA are required for maximal activity. Thus, unlike, SEB, SEA requires two separate binding sites for optimal activity, which may allow it to stabilize SEA interaction with T cell receptors, as well as to activate the antigen-presenting cell by cross-linking MHC class II.

Crystal structure of the superantigen enterotoxin C2 from Staphylococcus aureus reveals a zinc-binding site

BACKGROUND

Staphylococcus aureus enterotoxin C2 (SEC2) belongs to a family of proteins, termed 'superantigens', that form complexes with class II MHC molecules enabling them to activate a substantial number of T cells. Although superantigens seem to act by a common mechanism, they vary in many of their specific interactions and biological properties. Comparison of the structure of SEC2 with those of two other superantigens--staphylococcal enterotoxin B (SEB) and toxic shock syndrome toxin-1 (TSST-1)--may provide insight into their mode of action.

RESULTS

The crystal structure of SEC2 has been determined at 2.0 A resolution. The overall topology of the molecule resembles that of SEB and TSST-1, and the regions corresponding to the MHC class II and T-cell receptor binding sites on SEB are quite similar in SEC2. A unique feature of SEC2 is the presence of a zinc ion located in a solvent-exposed region at the interface between the two domains of the molecule. The zinc ion is coordinated to Asp83, His118, His122 and Asp9* (from the neighbouring molecule in the crystal lattice). Atomic absorption spectrometry demonstrates that zinc is also bound to SEC2 in solution.

CONCLUSIONS

SEC2 appears to be capable of binding to MHC class II molecules in much the same manner as SEB. However, structure-function studies have suggested an alternative binding mode that involves a different site on the toxin. The zinc ion of SEC2 lies within this region and thus may be important for complex formation, for example by acting as a bridge between the two molecules. Other possible roles for the metal cation, including a catalytic one, are also considered.

Characterization of human T-cell responses to Yersinia enterocolitica superantigen

We reported that antigenic preparations from Yersinia enterocolitica stimulate murine T cells in a manner consistent with that of superantigens. As a consequence we examined whether Y. enterocolitica antigenic preparations stimulate human T-cell cultures. Human T cells, enriched from peripheral blood lymphocytes, were stimulated to proliferate in the presence of Y. enterocolitica cytoplasmic and membrane preparations. This activity has also been shown to be sensitive to protease treatment, indicating the presence of a protein, and when separated by ion-exchange chromatography a single peak of activity is resolved. Furthermore, this proliferation was inhibited, in a dose-dependent manner, by the presence of antibodies directed against MHC class II antigens, indicating a requirement for these molecules. When these cells were stained with a panel of V beta-specific antibodies to determine if there was an enrichment of a particular V beta-bearing T-cell subset after stimulation, results indicate a significant enrichment of T cells bearing V beta 3, V beta 12, V beta 14, and V beta 17 over controls. Taken together, these data are consistent with a Y. enterocolitica product acting as a superantigen for human T cells.

Staphylococcal enterotoxin is a bovine T cell superantigen

Different isoforms of soluble staphylococcal enterotoxins (SE): SEB, SEC-1 and SEC-2, were shown to stimulate bovine T cell proliferation, expression of cytokine messages, and IgG production. Intact metabolic function of APC was not required since peripheral blood mononuclear cells (PBMC), UV-irradiated prior to or following incubation with SE, were both capable of presenting SE, while PBMC treated with MAbs against MHC II lost the ability to stimulate T cell proliferation. SE caused approximately two fold increase of CD4+, and CD8+ T cells, but not MHC II+ APC or B cells. This model system suggests that SE transduces not only T cell activation signal, but also a non-proliferative signal for primary B cells to produce polyclonal IgG. We hypothesize that enterotoxin virulence may be in part due to its effect on activating the immune system.

Selective killing of T cells by immunotoxins directed at distinct V beta epitopes of the T cell receptor

The potency and specificity of anti-T cell receptor (TcR)-directed immunotoxins were studied in two T cell leukemia lines, HPB-ALL and Jurkat, and in primary T cells. Immunoconjugates were synthesized using anti-CD3, or distinct anti-V beta antibodies cross-linked to CRM9, a binding site-mutant of diphtheria toxin. All TcR-expressing cells display the CD3 complex on the plasma membrane. HPB-ALL cells express the V beta 5 gene product in the beta subunit of the TcR, while Jurkat cells express V beta 8. V beta expression in primary T cells isolated from buffy coats is heterogeneous. Primary T cell populations expressing specific V beta epitopes in the TcR were generated by plating CD3+ T cells on V beta-specific antibody-coated flasks or by positive immunomagnetic selection. Immunotoxins directed against the invariant CD3 epsilon epitope target and kill all T cells. Immunoconjugates targeted at distinct anti-V beta epitopes are specific for cells that express the corresponding gene product in the TcR. The results demonstrate the ability of anti-TcR-based immunotoxins selectively to kill T cells with defined V beta epitopes. These reagents may be clinically useful in disorders mediated by autoreactive T cell populations exhibiting V beta restriction and in the treatment of clonal TcR-expressing lymphomas.

T cell repertoire and clonal deletion of Mtv superantigen-reactive T cells in mice lacking CD4 and CD8 molecules

CD4-CD8- double-negative T cells constitute a lymphocyte subpopulation within the thymus and peripheral lymphatic organs that express a unique T cell receptor (TCR) repertoire and do not undergo negative selection. To test whether these cells develop as a distinct lineage or due to altered selection in the absence of CD4 and CD8 expression, we analyzed the TCR repertoire in mice lacking both CD4 and CD8 accessory molecules after homologous recombination (CD40/0CD80/0). We show that mature T cells of CD40/0CD80/0 mice express an unbiased diverse TCR V beta repertoire comparable to wild type mice. In addition, clonal deletion of mouse mammary tumor virus superantigen-reactive T cells did occur in CD40/0CD80/0 mice. These data show that the intrinsic lack of CD4 and CD8 expression has no effect on the mature TCR repertoire and that clonal deletion of superantigen-reactive cells is independent of CD4 and CD8 co-receptors.

Expression of V beta gene segments by Sezary cells

The T-cell receptor V beta repertoire expressed by Sezary cells was determined in a series of 16 patients whose samples have been shown to contain a majority of tumor cells. By using anti-V beta monoclonal antibodies, polymerase chain reaction analysis of expressed V beta, and, in selected cases, nucleotide sequencing, we have shown that the expressed V beta segments belong to five V beta families (V beta 5, V beta 6, V beta 8, V beta 13, and V beta 18), which contain a large fraction of the T-cell receptor V beta repertoire and do not share significant similarities in complementary determining region 4. V beta segments from these five families were also found to be strongly expressed by CD4 + CD7- peripheral blood cells obtained by fluorescence-activated cell sorting from two healthy donors. The diversity of the V beta repertoire expressed by Sezary cells appears to be similar to that expressed by circulating non-neoplastic T cells. These data do not support the hypothesis that a common superantigen is involved in the initiation of this form of cutaneous T-cell lymphoma.

Bacterial pyrogenic exotoxins as superantigens

The recent discovery of the mode of interaction between a group of microbial proteins known as superantigens and the immune system has opened a wide area of investigation into the possible role of these molecules in human diseases. Superantigens produced by certain viruses and bacteria, including Mycoplasma species, are either secreted or membrane-bound proteins. A unique feature of these proteins is that they can interact simultaneously with distinct receptors on different types of cells, resulting in enhanced cell-cell interaction and triggering a series of biochemical reactions that can lead to excessive cell proliferation and the release of inflammatory cytokines. However, although superantigens share many features, they can have very different biological effects that are potentiated by host genetic and environmental factors. This review focuses on a group of secreted pyrogenic toxins that belong to the superantigen family and highlights some of their structural-functional features and their roles in diseases such as toxic shock and autoimmunity. Deciphering the biological activities of the various superantigens and understanding their role in the pathogenesis of microbial infections and their sequelae will enable us to devise means by which we can intervene with their activity and/or manipulate them to our advantage.

Basic aspects of neuroimmunology as they relate to immunotherapeutic targets: present and future prospects

The neurological diseases with definite or putative immune pathogenesis include myasthenia gravis; Lambert-Eaton myasthenic syndrome; IgM monoclonal anti-myelin-associated glycoprotein-associated demyelinating polyneuropathy; Guillain-Barré syndrome; chronic inflammatory demyelinating polyneuropathy; multifocal motor neuropathy with or without GM1 antibodies; multiple sclerosis; inflammatory myopathies; stiff-man syndrome; autoimmune neuromyotonia; paraneoplastic neuronopathies and cerebellar degeneration; and neurological diseases associated with systemic autoimmune conditions, vasculitis, or viral infections. The events that lead to these autoimmune diseases are not clear but the following sequential steps are critical: (a) the breaking of tolerance, a process in which cytokines, molecular mimicry, or superantigens may play a role in rendering previously anergic T cells to recognize neural autoantigens; (b) antigen recognition by the T-cell receptor complex and processing of the antigen via the major histocompatibility complex class I or II; (c) costimulatory factors especially B7 and B7-binding proteins (CD28, CTLA-4) and intercellular adhesion molecule (ICAM)-1 and its leukocyte function-associated (LFA)-1 ligand; (d) traffic of the activated T cells across the blood-brain or blood-nerve barrier via a series of adhesion molecules that include selectins, leukocyte integrins (LFA-1, Mac-1, very late activating antigen [VLA]-4) and their counterreceptors (ICAM-1, vascular cell adhesion molecule [VCAM]) on the endothelial cells; and (e) tissue injury when the activated T cells, macrophages, or specific autoantibodies find their antigenic targets on glial cells, myelin, axon, calcium channels, or muscle. In designing specific immunotherapy, the main players involved in every step of the immune response need to be considered. Targets for specific therapy in neurological diseases include agents that (a) interfere or compete with antigen recognition or stimulation, (b) inhibit costimulatory signals or cytokines, (c) inhibit the traffic of the activated cells to tissues, and (d) intervene at the antigen recognition sites in the targeted organ. The various immunomodulating procedures and immunosuppressive drugs currently used for nonselective neuroimmunotherapy are discussed in the context of their interference with the above-described immune mediators.

Staphylococcal enterotoxin B increases the severity of type II collagen induced arthritis in mice

OBJECTIVE

To observe the influence of T cell subset changes on the development of experimental arthritis, by using the bacterial superantigen staphylococcal enterotoxin B (SEB) to modulate the T cell repertoire during the arthritogenic response to type II collagen (CII) in vivo.

METHODS

DBA/1 mice were injected with SEB before immunisation with CII, and assessed for the development of collagen induced arthritis (CIA) and an immune response to CII. Mice with established arthritis were also treated therapeutically with SEB. Flow cytometry was used to evaluate the effect of the therapy on T cell subsets and T cell receptor (TCR) V beta expression.

RESULTS

Mice injected with SEB developed arthritis significantly faster than saline treated control animals, and developed more severe clinical features. Mice treated with SEB after the onset of CIA were also observed to progress more rapidly to a severe arthritis than mice treated with saline alone. The level of anti-CII antibody was not affected by SEB injection. Flow cytometric analysis of TCR expression in mice 21 days after injection of CII showed decreased expression of V beta 6 and V beta 8 cells in SEB treated mice, compared with collagen immunised control mice. Injection of SEB alone caused a decrease in V beta 8, but not V beta 6 T cells compared with the values in normal DBA/1 mice. No significant variations in the T cell repertoire were detected 70 days after CII immunisation.

CONCLUSIONS

Treatment with the bacterial enterotoxin SEB before the induction of arthritis did not suppress the immunological or arthritogenic response to CII in DBA/1 mice, despite the modulation of the V beta 8 T cell subset. Treatment of mice with established arthritis using SEB provoked a more severe disease course.

T cell receptor V beta repertoire in mice lacking endogenous mouse mammary tumor provirus

When endogenous mouse mammary tumor virus (MMTV) superantigens (SAg) are expressed in the first weeks of life an efficient thymic deletion of T cells expressing MMTV SAg-reactive T cell receptor (TcR) V beta segments is observed. As most inbred mouse strains and wild mice contain integrated MMTV DNA, knowing the precise extent of MMTV influence on T cell development is required in order to study T cell immunobiology in the mouse. In this report, backcross breeding between BALB.D2 (Mtv-6, -7, -8 and -9) and 38CH (Mtv-) mice was carried out to obtain animals either lacking endogenous MMTV or containing a single MMTV locus, i.e. Mtv-6, -7, -8 or -9. The TcR V beta chain (TcR V beta) usage in these mice was analyzed using monoclonal antibodies specific for TcR V beta 2, V beta 3, V beta 4, V beta 5, V beta 6, V beta 7, V beta 8, V beta 11, V beta 12 and V beta 14 segments. Both Mtv-8+ mice and Mtv-9+ mice deleted TcR V beta 5+ and V beta 11+ T cells. Moreover, we also observed the deletion of TcR V beta 12+ cells by Mtv-8 and Mtv-9 products. Mtv-6+ and Mtv-7+ animals deleted TcR V beta 3+ and V beta 5+ cells, and TcR V beta 6+, V beta 7+ and V beta 8.1+ cells, respectively. Unexpectedly, TcR V beta 8.2+ cells were also deleted in some backcross mice expressing Mtv-7. TcR V beta 8.2 reactivity to Mtv-7 was shown to be brought by the 38CH strain and to result from an amino acid substitution (Asn-->Asp) in position 19 on the TcR V beta 8.2 fragment. Reactivities of BALB.D2 TcR V beta 8.2 and 38CH TcR V beta 8.2 to the exogenous infectious viruses, MMTV(SW) and MMTV(SHN), were compared. Finally, the observation of increased frequencies of TcR V beta 2+, V beta 4+ and V beta 8+ CD4+ T cell subsets in Mtv-8+ and Mtv-9+ mice, and TcR V beta 4+ CD4+ T cells in Mtv-6+ and Mtv-7+ mice, when compared with the T cell repertoire of Mtv- mice, is consistent with the possibility that MMTV products contribute to positive selection of T cells.

Lipopolysaccharide interferes with the induction of peripheral T cell death

In mice injected with superantigens, T cells specific for that antigen proliferate and then die. It has been suggested that the target cells die because they encounter superantigen on the surfaces of nonprofessional presenting cells, such as B cells, which cannot deliver costimulatory signals to T cells. A number of reagents that induce costimulatory molecules on B cells were tested. Lipopolysaccharide very effectively prevented T cell death driven by superantigen. Perhaps surprisingly, the action of lipopolysaccharide was not mediated through the expected costimulatory molecule, B7. Rather, the effects of lipopolysaccharide involved the production of inflammatory cytokines, in particular TNF alpha. The rescued cells survived in vitro culture and were resistant to Fas-induced killing. These data demonstrate that LPS can block antigen-induced T cell death perhaps by interfering with Fas signaling.

Transient expression of human interleukin-2 and interferon-gamma genes is regulated by interaction between distinct cell subsets

The level of transient expression of human IL-2 and IFN-gamma genes, we show, is regulated by dynamic interaction between two functionally distinct cell populations. One is able to express these genes, while the other, bearing one of several specific surface markers, actively inhibits their expression. Defined cell subsets were isolated from PBMC and tonsil cells using immunomagnetic beads coated with monoclonal antibodies directed against surface markers. Depletion of CD8, CD11a (Leu15), or Leu8 subsets led to a pronounced superinduction of IL-2 and IFN-gamma gene expression when the remaining cell population was stimulated with mitogen (PHA) or antigen (SEB). Thus, a 10-fold increase in production of IFN-gamma was observed after removal of CD11a (Leu15) cells constituting only a small percentage of the total cell population. By contrast, depletion of cells expressing CD19, a B cell marker, did not yield any superinduction. Conversely, CD8, CD11a (Leu15), or Leu8 cell subsets, but not CD19 cells, each inhibited the induction of IL-2 and IFN-gamma gene expression almost completely in depleted or total cell populations from which they were derived. Gene expression occurring within one cell subset could be effectively inhibited by cells from a second subset. Introduction of inhibitory cells (Leu8) into a population that actively expressed IL-2 and IFN-gamma mRNA resulted in an immediate cessation of gene expression. This suppression involves a soluble mediator, since the culture medium in which such cells were activated exerted a similarly effective inhibition.

Immunopathogenesis and treatment of the Guillain-Barré syndrome--Part II

In the second part of our review the role of antecedent infections in the pathogenesis of GBS is discussed. The association with Campylobacter jejuni (C. jejuni) is highlighted and the concept of molecular mimicry, i.e., sharing of epitopes between microbes and peripheral nerve, explained. Alternative mechanisms to relate an infection with the immune-mediated neuropathy are elaborated. Current therapies of the GBS include plasma exchange, high-dose intravenous immunoglobulins, and supportive treatment directed to secondary complications. Published therapeutic trials are reviewed and future approaches are outlined. Principles of general care are also summarized.

A unique tumor antigen produced by a single amino acid substitution

Mice immunized against a cancer recognize antigens unique to that cancer, but the molecular structures of such antigens are unknown. We isolated CD4+ T cell clones recognizing an antigen uniquely expressed on the UV-induced tumor 6132A; some clones inhibited the growth of tumors bearing the specific antigen. A T cell hybridoma was used to purify this antigen from nuclear extracts by RP-HPLC and SDS-PAGE using T cell immunoblot assays. A partial amino acid sequence was nearly identical to a sequence in ribosomal protein L9. The cDNA sequence of L9 from 6132A PRO cells differed from the normal sequence at one nucleotide; this mutation encoded histidine instead of leucine at position 47. A synthetic peptide containing this mutation was over 1000-fold more stimulatory of T cells than was the wild-type peptide. These results indicate that this unique tumor antigen is derived from a single amino acid substitution in a cellular protein.

Glomerulonephritis associated with MRSA infection: a possible role of bacterial superantigen

We report 10 cases of glomerulonephritis following methicillin-resistant Staphylococcus aureus (MRSA) infection. The clinical features of this syndrome were an abrupt or insidious onset of rapidly progressive glomerulonephritis (RPGN) with nephrotic syndrome and occasionally purpura, following MRSA infection. The renal histologic findings showed a variety of types of proliferative glomerulonephritis with varying degrees of crescent formation; immunofluorescence revealed of glomerular deposition of IgA, IgG, and C3. Laboratory findings showed polyclonal increases of serum IgA and IgG, with high levels of circulating immune complexes (ICs). Increased numbers of DR+CD4+, and DR+CD8+T cells were observed in the peripheral circulation, with a high frequency of T cell receptor (TCR) V beta + cells. MRSA produced enterotoxins C and A and toxic shock syndrome toxin (TSST)-1, all of which are known to act as superantigens. From the above observations, we speculate that post-MRSA glomerulonephritis may be induced by superantigens causing production of high levels of cytokines, and polyclonal activation of IgG and IgA. The formation of ICs containing IgA and IgG in the circulation result in development of glomerulonephritis and vasculitis. Accordingly, microbial superantigens may play an important role in the pathogenesis of this unique syndrome of nephritis and vasculitis.

V beta gene repertoire in the aging mouse: a developmental perspective

To define age-associated alterations in the immune system at the molecular level, we have analyzed TCR V beta gene expression patterns at the fetal, neonatal, adult, and advanced ages of mice. In contrast to V gamma and VH genes, V beta genes rearranged without any preference related to their chromosomal organization. Endogenous superantigen-mediated clonal deletions were registered for the first time at the neonatal stage, presumably reflecting the late developmental appearance of these molecules. Such deletions, once established, were maintained throughout life with little, if any, leakage in this process. Furthermore, bone marrow transplantation and other studies indicated that an involuted thymus maintained its capacity to perform both its functions, i.e. positive and negative selection. Although overall V beta repertoires showed remarkable stability with advanced age, modifications in expression levels for some V beta, particularly those associated with the CD8 subset and presumably reflecting antigenic stimulation, were recorded. Mice with lupus and early-life thymic involution were fully capable of deleting endogenous superantigen-reactive V beta clones, and even lupus mice with a genetic defect in the apoptosis-promoting Fas gene were normal in this regard. The results indicate that, aside from some anticipated clonal expansions induced by antigenic stimulation, age-associated alterations in immune functions are not caused by any profound changes in the overall TCR repertoire.

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

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

Unbiased usage of T-cell receptor beta variable region genes in peripheral blood cells of hepatitis C patients: no correlation with superantigen effect

Hepatitis C virus (HCV) infection frequently causes chronic hepatitis and lack of virus clearance in these patients. In addition, many patients infected by HCV also present with hypergammaglobulinemia in the early stage of chronic infection. These observations raise a possible viral superantigen effect induced by HCV, because viral superantigen found in human immunodeficiency virus (HIV) or in replication of defective murine leukemia virus (MuLV) is associated with T-cell dysfunction and polyclonal activation of B cells. The possibility was investigated of whether HCV encodes any superantigen by analyzing the usage of T-cell receptor (TCR) from the peripheral blood lymphocytes (PBL) of patients with chronic hepatitis C. Two groups, one with hypergammaglobulinemia and the other without hypergammaglobulinemia, were studied for the usage of TCR beta chain by reverse transcription-polymerase chain reaction (RT-PCR) analysis. It was found that all genes of V beta variable chain were used in the PBL of these patients. Furthermore, there was no significant difference of the TCR expression pattern between these two groups, nor a complete deletion of a particular T-cell subset in either group. These results do not provide evidence for HCV superantigen, but indicate that the TCR usage in the patients was neither defective nor biased.

Subsets of HLA-DR1 molecules defined by SEB and TSST-1 binding

Superantigens bind to major histocompatibility complex class II molecules on antigen-presenting cells and stimulate T cells. Staphylococcus aureus enterotoxin B (SEB) and toxic shock syndrome toxin-1 (TSST-1) bind to the same region of human lymphocyte antigen (HLA)-DR1 but do not compete with each other, which indicates that they bind to different subsets of DR1 molecules. Here, a mutation in the peptide-binding groove disrupted the SEB and TSST-1 binding sites, which suggests that peptides can influence the interaction with bacterial toxins. In support of this, the expression of the DR1 molecule in various cell types differentially affected the binding of these toxins.

Purification and characterization of a novel superantigen produced by a clinical isolate of Yersinia pseudotuberculosis

A superantigen designated as Yersinia pseudotuberculosis-derived mitogen (YPM) was purified in an equal manner from both the culture supernatant and cell lysate of a clinical isolate (KUR-1) of Y. pseudotuberculosis serotype 4b. A significant proliferative response of human peripheral blood mononuclear cells to purified YPM was detectable even at a concentration of 1 pg/ml. The N-terminal sequence of YPM which included 23 amino acid residues was determined, by automated Edman degradation, as Thr-Asp-Tyr-Asp-Asn-Thr-Leu-Asn-Ser-Ile-Pro-Ser-Leu-Arg-Ile-Pro-Asn-Il e-Ala-Thr-Tyr-Thr-Gly-. This sequence differed from not only all the, hitherto, reported superantigens but also known proteins. While molecular weights of known bacterial superantigens are more than 22,000, electrospray ionization mass spectrometry showed that the molecular weight of YPM was 14524.4. These results indicate that YPM comprises a novel superantigen with substantial structural differences from other bacterial superantigens produced by Gram-positive cocci.

T cell receptor V-gene usage in oral lichen planus; increased frequency of T cell receptors expressing V alpha 2 and V beta 3

In order to analyse the clonality of T cells in the inflammatory infiltrate of oral lichen planus (OLP), mucosal biopsies were obtained from seven patients with manifest disease. The biopsies were stained with MoAbs directed against 11 different T cell receptor (TCR) V-gene families, anti-CD4, anti-CD8 and IL-2 receptor (IL-2R). For comparison, the frequencies of the different TCR V-families were determined in biopsies from five patients with oral candidosis as well as in peripheral blood from three patients with OLP and from six healthy blood donors (HBD). The occurrence of the investigated TCR V-families varied between 0% and 7% in venous blood obtained from both HBD and OLP patients. T lymphocytes expressing the TCR V beta 3 and V alpha 2 in OLP biopsies were, however, detected in frequencies ranging between 18% and 40% of the total fraction of lymphocytes, a consistent finding for all the OLP infiltrates studied. The other nine TCR V-families examined appeared in low frequencies both in biopsies and in peripheral blood. V alpha 2+ and V beta 3+ cells were often localized adjacent to the basal membrane. In contrast, T cells in Candida-induced lesions did not express a biased TCR distribution, and most V-families studied appeared in frequencies of 0-6%. Thus, T lymphocytes in OLP lesions express a substantially higher frequency of TCR V alpha 2 and V beta 3 than expected from the distribution in blood. The clonal expansion of T cells observed in OLP suggests that a superantigen is involved in the pathogenesis of the disease. Whether this superantigen is of exogenous or endogenous origin needs to be investigated.

Genetic control of the human V beta 13.2 T cell repertoire: importance of allelic variation outside the coding regions of the TCRBV13S2 gene

In humans, the T cell repertoire is influenced by HLA, T cell receptor null alleles and antigen. Here, we describe a novel mechanism, independent of superantigen or T cell receptor structure which influences the T cell repertoire in a V beta-dependent manner. We have identified a biallelic locus, the TCRBV13S2 T cell receptor gene, where allelic differences predominate in the non-coding regions including transitions, transversions and frameshift deletions. The expressed protein is non-polymorphic at this locus. The TCRBV13S2 genotype profoundly influences the circulating levels of V beta 13.2 CD4 T cells but does not affect T cell receptor expression or function.

Early expression of cytokines in lymph nodes after treatment in vivo with Staphylococcus enterotoxin B

Excessive cytokine expression induced by superantigen may be one aspect of the pathophysiology associated with Gram positive bacteremia. We have undertaken a study of the kinetics of cytokine production in lymph nodes obtained from in vivo Staphylococcus enterotoxin B (SEB) treated animals. This study was designed to evaluate the short term cytokine profile observed using immunohistochemistry (IHC) in BALB/c mice injected intraperitoneally (i.p.). The observed immunohistochemical kinetic profiles were corroborated using reverse transcription-polymerase chain reaction (RT-PCR) RNA analysis. We report here that TNF, IL-2, and IFN-gamma are the principal cytokines which were detected within hours of SEB administration, and that other cytokines such as IL-3, IL-4, IL-5, IL-6, IL-10, GM-CSF and M-CSF were undetectable. TNF and IL-2 appeared very early following SEB priming, and were observed by 1 h. IFN-gamma which appeared later (maximally at 14 h) was produced predominantly by CD8+ cells. In contrast, the TNF and IL-2 were produced primarily by CD4+ cells. Identical results were obtained by IHC and RT-PCR; the kinetics of mRNA expression slightly preceded the appearance of protein. The TNF and IFN-gamma staining patterns observed in lymph node sections were indicative of Golgi-localized cytokine. The IL-2 staining pattern observed in lymph node sections was distinctive, covering a significant local area of cells. This local regional concentration of IL-2, which may result from cytokine attached to extracellular binding components, may be an important aspect of the activation phase of a developing immune response. Rapid induction and excessive cytokine production elicited by superantigen in vivo, may ultimately help to explain the shock and death associated with SEB.

Evidence for superantigen activity of the Bel 3 protein of the human foamy virus

The human foamy virus is a complex retrovirus that codes for several regulatory bel genes in addition to the conventional gag, pol, and env genes. The bel 3 gene is located in the 3' part of the viral genome comparable to that of the superantigen of the mouse mammary tumor virus. Superantigens bound to major histocompatibility complex (MHC) class II molecules have been shown to stimulate T cells in a V beta-specific manner. The recombinant Bel 3 protein purified to near homogeneity was assayed in vitro to determine whether or not it functions as a superantigen that stimulates human T lymphocytes expressing particular V beta T cell receptor (TCR) chains. Therefore, an analysis including all known human V beta elements was performed. The expression of different V beta chains of the TCR was analyzed by reverse transcription of the V beta RNAs and subsequent amplification of the corresponding V beta cDNA elements by polymerase chain reaction in unstimulated, phytohemaggluttinin (PHA)- and Bel 3-stimulated human T lymphocytes. In addition, eight monoclonal antibodies directed against particular V beta family members were used to determine any change in the expression of the remaining known V beta elements upon treatment with PHA and Bel 3. The comparative V beta-specific transcriptional analysis revealed that the in vitro expression of the V beta 18 chain was specifically and strongly expanded in Bel 3-stimulated human T cells, a property characteristic for a superantigen.

Toxoplasma gondii possesses a superantigen activity that selectively expands murine T cell receptor V beta 5-bearing CD8+ lymphocytes

To investigate early immune responses to the intracellular parasite Toxoplasma gondii, we examined the capacity of nonimmune splenocytes to respond in vitro to intact tachyzoites and soluble tachyzoite antigen (Ag). Both types of stimuli induced high levels of proliferation as well as interferon gamma (IFN-gamma) secretion. Based on several key criteria, the response appeared to be driven by a superantigen present in the parasite. Thus, stimulation of C57BL/6 spleen cells with T. gondii resulted in a preferential threefold expansion of a T cell population expressing the V beta 5 chain of the T cell receptor, and a survey of different inbred mouse strains revealed an inverse correlation between Ag-induced proliferation and genetic deletion of V beta 5. Moreover, proliferation was induced using irradiated Ag-pulsed and infected splenic adherent cells, and was blocked by a major histocompatibility complex class II-specific monoclonal antibody. Furthermore, paraformaldehyde-fixed IAb-, IAk-, and IEk-transfected fibroblast lines were able to specifically bind T. gondii Ag and drive proliferation of T lymphocytes, demonstrating that the response can be mediated by allogeneic class II molecules, and that it does not require cellular Ag processing. It is interesting to note that after 1 wk of culture with Ag, up to 70% of the expanded V beta 5-expressing cells were CD8+. These results provide the first description of a superantigen activity in a protozoan pathogen. In the case of T. gondii, superantigen-driven expansion of IFN-gamma-secreting CD8+ lymphocytes may play a role in the development of the dominant IFN-gamma-dependent, cell-mediated immunity characteristic of infection with this parasite.

Absence of MHC class II molecules reduces CNS demyelination, microglial/macrophage infiltration, and twitching in murine globoid cell leukodystrophy

Globoid cell leukodystrophy (GLD) is a severe genetic demyelinating disorder with an increased number of Ia (immune response antigen) positive brain microglia/macrophages. To assess the role of aberrant Ia expression in the central nervous system (CNS), twitcher mice, which represent the murine model for GLD, were mated with Ia- transgenic mice. Compared with the Ia+ controls, Ia- twitcher mice showed a profound reduction in the severity of demyelinating lesions correlated with significantly fewer microglia/macrophages. Most importantly, Ia- twitcher mice showed significantly reduced twitching compared with ia+ twitcher mice. In contrast with experimental allergic encephalomyelitis (EAE), there was no significant amount of inflammatory T cell infiltrates, implying that T cells may not play a predominant role in this disease. These findings may have broad therapeutic implications for Alzheimer's disease, Parkinson's disease, and Huntington's disease, which display enhanced Ia expression in the CNS without obvious T cell infiltrates.