Stimulation with specific antigen can block superantigen-mediated deletion of T cells in vivo

Impaired thymic selection and abnormal antigen-specific T cell responses in Foxn1(Δ/Δ) mutant mice

Background

Foxn1^Δ/Δ mutant mice have a specific defect in thymic development, characterized by a block in TEC differentiation at an intermediate progenitor stage, and blocks in thymocyte development at both the DN1 and DP cell stages, resulting in the production of abnormally functioning T cells that develop from an atypical progenitor population. In the current study, we tested the effects of these defects on thymic selection.

Methodology/Principal Findings

We used Foxn1^Δ/Δ; DO11 Tg and Foxn1^Δ/Δ; OT1 Tg mice as positive selection and Foxn1^Δ/Δ; MHCII I-E mice as negative selection models. We also used an in vivo system of antigen-specific reactivity to test the function of peripheral T cells. Our data show that the capacity for positive and negative selection of both CD4 and CD8 SP thymocytes was reduced in Foxn1^Δ/Δ mutants compared to Foxn1^+/Δ control mice. These defects were associated with reduction of both MHC Class I and Class II expression, although the resulting peripheral T cells have a broad TCR Vβ repertoire. In this deficient thymic environment, immature CD4 and CD8 SP thymocytes emigrate from the thymus into the periphery. These T cells had an incompletely activated profile under stimulation of the TCR signal in vitro, and were either hypersensitive or hyporesponsive to antigen-specific stimulation in vivo. These cell-autonomous defects were compounded by the hypocellular peripheral environment caused by low thymic output.

Conclusions/Significance

These data show that a primary defect in the thymic microenvironment can cause both direct defects in selection which can in turn cause indirect effects on the periphery, exacerbating functional defects in T cells.

Nedd4 augments the adaptive immune response by promoting ubiquitin-mediated degradation of Cbl-b in activated T cells

Nedd4 and Itch are E3 ubiquitin ligases that ubiquitinate similar targets in vitro and thus are thought to function similarly. T cells lacking Itch show spontaneous activation and T helper type 2 polarization. To test whether loss of Nedd4 affects T cells in the same way, we generated Nedd4(+/+) and Nedd4(-/-) fetal liver chimeras. Nedd4(-/-) T cells developed normally but proliferated less, produced less interleukin 2 and provided inadequate help to B cells. Nedd4(-/-) T cells contained more of the E3 ubiquitin ligase Cbl-b, and Nedd4 was required for polyubiquitination of Cbl-b induced by CD28 costimulation. Our data demonstrate that Nedd4 promotes the conversion of naive T cells into activated T cells. We propose that Nedd4 and Itch ubiquitinate distinct target proteins in vivo.

Ndfip1 protein promotes the function of itch ubiquitin ligase to prevent T cell activation and T helper 2 cell-mediated inflammation

Nedd4 family interacting protein-1 (Ndfip1) is a protein whose only known function is that it binds Nedd4, a HECT-type E3 ubiquitin ligase. Here we show that mice lacking Ndfip1 developed severe inflammation of the skin and lung and died prematurely. This condition was due to a defect in Ndfip1(-/-) T cells. Ndfip1(-/-) T cells were activated, and they proliferated and adopted a T helper 2 (Th2) phenotype more readily than did their Ndfip1(+/+) counterparts. This phenotype resembled that of Itchy mutant mice, suggesting that Ndfip1 might affect the function of Itch, an E3 ubiquitin ligase. We show that T cell activation promoted both Ndfip1 expression and its association with Itch. In the absence of Ndfip1, JunB half-life was prolonged after T cell activation. Thus, in the absence of Ndfip1, Itch is inactive and JunB accumulates. As a result, T cells produce Th2 cytokines and promote Th2-mediated inflammatory disease.

Calcineurin B1 Is Essential for Positive but Not Negative Selection during Thymocyte Development

During development, discrete cell fates often result from variation in the intensity of a particular signal. The mechanisms underlying these seemingly analog-to-digital switches are not understood. In developing T lymphocytes, low-intensity signals through the antigen receptor result in positive selection while more intense signals give rise to negative selection. By deleting the genetic locus encoding the regulatory B1 subunit of calcineurin specifically in thymocytes, we found an absolute requirement for calcineurin in positive selection. In contrast, calcineurin activity was dispensable in several models of negative selection. Unexpectedly, we found that removal of calcineurin activity from thymocytes results in inefficient ERK activation at the double-positive stage of thymocyte development, when selection occurs. These studies clarify the mechanism by which graded signals are converted to discrete outcomes in T cell development and further indicate that the developmental roles of calcineurin likely contribute to immunosuppression by calcineurin inhibitors.

Effects of bacterial superantigens on behavior of mice in the elevated plus maze and light-dark box

Bacterial superantigens, such as staphylococcal enteroxins A and B (SEA/SEB) stimulate T cells to produce high levels of cytokines in blood. Previously it had been shown that these toxins were capable of stimulating increased neuroendocrine activity and enhanced behavioral reactivity to novel gustatory and non-gustatory stimuli. Therefore, it was suggested that these superantigens may promote anxiety-like behavior. In the current set of experiments, BALB/cByJ and C57BL/6J male mice were challenged with either SEB (50 microg) or SEA (5 or 10 microg) and tested for behavior in the elevated plus maze (EPM). Results suggested an absence of increased anxiety-like behavior, with exploration of the open arms being enhanced by SEA or SEB treatment. In another test of anxiety, the light-dark box, SEB challenge of BALB/cByJ mice 90 min prior to testing, did not alter exit latency, activity nor time spent in the dark. However, in a second experiment, it was found that if animals were first tested for consumption, followed by testing in the light-dark box, SEB challenged animals displayed increased exit latency and reduced exploration. These studies suggest that in standard tests of rodent anxiety-like behavior, evidence for the induction of anxiety-like processes subsequent to challenge with SEA or SEB is not patently discernable. However, neurobiological events induced by immunological challenge might synergize with reactivity to psychogenic and/or gustatory stimuli, thereby resulting in increased anxiety-like behavior that could be unmasked by standard behavioral tests such as the light-dark box or EPM.

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

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

Aberrant regulation of superantigen responses during T-cell reconstitution and graft-versus-host disease in immunodeficient mice

Acute graft-versus-host disease (GVHD) after allogeneic stem cell transplantation is associated with impaired deletion and anergy of host-reactive T cells. To elucidate the immunoregulatory events that may contribute to such dysregulated T-cell responses in GVHD, we studied superantigen (SAg) responses after adoptive T-cell transfer into severe combined immunodeficient (SCID) mice. SAg responses are normally regulated by mechanisms involving deletion and anergy, with SAg-reactive T cells typically being deleted rapidly in vivo. In a SCID mouse model of GVHD, however, allogeneic host SAg-reactive T cells were not deleted rapidly, but rather persisted in increased numbers for several months. Moreover, depending on the timing of SAg stimulation and the numbers of T cells transferred, dysregulation (impaired deletion and anergy) of SAg responses could be demonstrated following the adoptive transfer of syngeneic T cells into SCID mice as well. Transgenic T-cell receptor-bearing KJ1-26.1+ T cells were then used to determine the fate of weakly reactive T cells after adoptive transfer and SAg stimulation. When transferred alone, KJ1-26.1+ T cells demonstrated impaired deletion and anergy. In the presence of more strongly staphylococcal enterotoxin B (SEB)–reactive T cells, however, KJ1-26.1+ T cells were regulated normally, in a manner that could be prevented by inhibiting the effects of more strongly SEB-reactive cells or by increasing the level of activation of the KJ1-26.1+ T cells themselves. We suggest that the control mechanisms that normally regulate strongly activated T cells in immunocompetent animals are lost following adoptive transfer into immunodeficient hosts, and that this impairment contributes to the development of GVHD.

Aberrant regulation of superantigen responses during T-cell reconstitution and graft-versus-host disease in immunodeficient mice

Acute graft-versus-host disease (GVHD) after allogeneic stem cell transplantation is associated with impaired deletion and anergy of host-reactive T cells. To elucidate the immunoregulatory events that may contribute to such dysregulated T-cell responses in GVHD, we studied superantigen (SAg) responses after adoptive T-cell transfer into severe combined immunodeficient (SCID) mice. SAg responses are normally regulated by mechanisms involving deletion and anergy, with SAg-reactive T cells typically being deleted rapidly in vivo. In a SCID mouse model of GVHD, however, allogeneic host SAg-reactive T cells were not deleted rapidly, but rather persisted in increased numbers for several months. Moreover, depending on the timing of SAg stimulation and the numbers of T cells transferred, dysregulation (impaired deletion and anergy) of SAg responses could be demonstrated following the adoptive transfer of syngeneic T cells into SCID mice as well. Transgenic T-cell receptor-bearing KJ1-26.1+ T cells were then used to determine the fate of weakly reactive T cells after adoptive transfer and SAg stimulation. When transferred alone, KJ1-26.1+ T cells demonstrated impaired deletion and anergy. In the presence of more strongly staphylococcal enterotoxin B (SEB)–reactive T cells, however, KJ1-26.1+ T cells were regulated normally, in a manner that could be prevented by inhibiting the effects of more strongly SEB-reactive cells or by increasing the level of activation of the KJ1-26.1+ T cells themselves. We suggest that the control mechanisms that normally regulate strongly activated T cells in immunocompetent animals are lost following adoptive transfer into immunodeficient hosts, and that this impairment contributes to the development of GVHD.

Testing mouse mammary tumor virus superantigen as adjuvant in cytotoxic T-lymphocyte responses against a melanoma tumor antigen

Cytotoxic T cells represent a powerful strategy for antitumor treatment. Depending on the route of injection, an important role for CD4 T cell-mediated help was observed in the induction of this response. For this reason, we investigated whether induction of a CTL response to the HLA-A2-restricted immunodominant peptide melanoma antigen Melan-A was improved by using rVVs expressing the CTL-defined epitope alone or in combination with an SAg. In the latter case, the few infected dendritic cells simultaneously presented an SAg and an antigen, i.e., peptide. Here, we show that the anti-Melan-A response was efficiently induced but not significantly improved by coexpression of the SAg.

Oligoclonal expansion of T cell receptor V beta 2 and 3 cells in the livers of mice with graft-versus-host disease

The nonsuppurrative destructive cholangitis lesions in the B10.D2 (donor) into BALB/c (host) mouse graft-versus-host disease (GVHD) model are dependent on CD4 T cells that use a T cell receptor-beta chain variable region (Vbeta) repertoire, which is heavily biased toward Vbeta2 and Vbeta3 usage. We hypothesized that liver Vbeta2(+) and Vbeta3(+) CD4 T cells originate from donor mice and recognize BALB/c minor histocompatibility alloantigens and BALB/c endogenous retroviral superantigen-6, respectively. To test this hypothesis, we determined the donor:host chimera status of infiltrating liver lymphocytes and the clonal states of liver Vbeta2(+) and liver Vbeta3(+) CD4 cells isolated from GVHD mice. A limited donor TCR Vbeta repertoire composed of Vbeta1(+), 2(+), 3(+), 4(+), 6(+), and 8(+) cells infiltrated the livers of GVHD mice on day 3. Consistent with a response to immunodominant host minor histocompatibility antigens, we detected oligoclonal liver Vbeta2(+) T cells in 40% of GVHD mice studied on day 3 and in 100% of GVHD mice studied on day 14. Typical of superantigen stimulation, extremely polyclonal liver Vbeta3(+) T cells were detected in 100% of GVHD mice studied on day 3 and 40% of GVHD mice studied on day 14. Yet, the liver Vbeta3(+) T cells in 60% of the day 14 GVHD mice were oligoclonal, pointing to a response to minor histocompatibility antigens.

Visualizing T cell competition for peptide/MHC complexes: a specific mechanism to minimize the effect of precursor frequency

In vivo antigenic competition of naive CD4+ TCR transgenic T cells was visualized by tracking cell division. Competition reduced both recruitment into cell division and burst size per recruited precursor cell, minimizing the effect of differences in precursorfrequency while maintaining the dose-response relationship with antigen. Competition was restricted to T cells of the same specificity, indicating that cells were competing for access to Ag-MHC complexes rather than for Ag nonspecific factors. Moreover, the qualitative distinction between the responses to i.v. peptide and s.c. peptide/CFA was unaffected by precursor frequency. These data explain the paradoxical ability of the immune system to tailor responses to the type and dose of Ag even in individuals with large differences in initial precursor frequency.

Effect of Staphylococcus enterotoxin B on the concurrent CD8(+) T cell response to influenza virus infection

Bacterial superantigens have potent in vivo effects. Respiratory viral infections are often associated with secondary bacterial infections, raising the likelihood of exposure to bacterial superantigens after the initiation of the anti-viral immune response. In this study, the general and V beta-specific effects of exposure to Staphylococcal enterotoxin B (SEB) during influenza virus infection on both the ongoing acute and the subsequent recall CD8(+) T cell responses were analyzed, using the well-characterized murine influenza model system and tetrameric MHC/peptide reagents to directly identify virus-specific T cells. The results show that although superantigen exposure during the primary viral infection caused delayed viral clearance, there was remarkably little effect of SEB on the magnitude or TCR repertoire of the ongoing cytolytic T cell response or on the recall response elicited by secondary viral infection. Thus, despite the well-characterized immunomodulatory effects of SEB, there was surprisingly little interference with concurrent anti-viral immunity.

CTL hyporesponsiveness induced by 2,3,7, 8-tetrachlorodibenzo-p-dioxin: role of cytokines and apoptosis

Studies have shown that blocking B7-mediated costimulation induces T cell tolerance via anergy or apoptosis. Provision of exogenous IL-2 can reverse or prevent the induction of tolerance. We have previously shown that TCDD-induced suppression of the CTL response to allogeneic P815 tumor cells is accompanied by decreased expression of CD86 (B7-2) as well as suppressed IL-2 and IFNgamma production. In the present studies, the role of IL-2 and IFNgamma and the analysis of inappropriate deletion of CD8(+) cells was examined. Administration of IL-2 on days 7-9 relative to the injection of P815 tumor cells dose-dependently increased the CTL activity and the generation of CD8(+) CTL effector cells in TCDD-treated mice. This increased CTL response was not due to recruitment of naive CTL precursors (CTLp), suggesting that a small pool of activated CTLp in TCDD-treated mice could respond to the IL-2. A much larger pool of activated CTLp in control mice was also expanded by IL-2 treatment. In contrast, treatment with IFNgamma during the same time period did not alter CTL activity in control or TCDD-treated mice. To address the possibility that insufficient IL-2 early in the response was responsible for the reduced pool of activated CTLp in TCDD-treated mice, IL-2 was administered on days 1-3 after P815 injection. However, not only did early treatment with IL-2 fail to restore the response in TCDD-treated mice, it suppressed the CTL response of non-TCDD-treated mice. To test whether exposure to TCDD induced apoptosis of activated CD8(+) T cells, phosphatidylserine (PS) expression was measured on various days after P815 tumor challenge. Surprisingly, the percentage of apoptotic CD8(+) T cells was significantly lower in TCDD-treated mice compared to controls throughout the allograft response. Similarly, exposure to TCDD failed to enhance peripheral deletion of Vbeta3(+)CD8(+) T cells after injection of the superantigen Staphylococcal enterotoxin A (SEA). Taken together, the data indicate that TCDD induces an early defect in CTLp activation that is not due to insufficient IL-2 or deletion of CD8(+) cells and may implicate a novel mechanism by which ligands of the Ah receptor disrupt CTL precursor activation.

Sustained Activation of the Raf-MEK-ERK Pathway Elicits Cytokine Unresponsiveness in T Cells

Activation of T cells via the TCR and other costimulatory receptors triggers a number of signaling cascades. Among them, the Ras-activated Raf-mitogen-activated protein/extracellular signal-related kinase (ERK) kinase (MEK)-ERK signaling cascade has been demonstrated to be crucial for both T cell development and activation. It has previously been demonstrated that high doses of Ag or anti-CD3 mAb are able to induce in T cells a nonresponsive state to subsequent treatment with cytokines such as IL-2. The precise biochemical mechanisms underlying this effect are not fully characterized. In this study, we demonstrate that cytokine nonresponsiveness is accompanied by the induction of the cyclin-dependent kinase inhibitor p21Cip1 that is mediated, at least in part, by the activation of the Raf-MEK-ERK pathway. Furthermore, we demonstrate that selective activation of the Raf-MEK-ERK signaling pathway in T cells is sufficient to induce cytokine nonresponsiveness in both a T cell clone and naive primary T cells. In this case, nonresponsiveness is accompanied by the induction of p21Cip1 and the prevention of p27Kip1 down-regulation, leading to inhibition of cyclin E/cyclin-dependent kinase 2 activity. These data suggest that anti-CD3 mAb-induced cytokine nonresponsiveness may be a consequence of hyperactivation of the Raf-MEK-ERK pathway, leading to alterations in the expression of key cell cycle regulators. These observations may provide a novel insight into the mechanisms of induction of peripheral tolerance.

Superantigens in human disease

Superantigens have been implicated in a wide variety of human diseases. Yet, solid evidence for their role in pathogenesis is available only for Toxic Shock Syndrome and a few other conditions. This evidence is critically reviewed herein.

CD40 Activation Boosts T Cell Immunity In Vivo by Enhancing T Cell Clonal Expansion and Delaying Peripheral T Cell Deletion

In this report we show that activation of APC with an agonist anti-CD40 mAb profoundly alters the behavior of CD4 T cells in vivo. Stimulation of mice with anti-CD40 2 days before, but not 1 day after, administration of superantigen (SAg) enhanced CD4 and CD8 T cell clonal expansion by approximately threefold. Further, CD40 activation also delayed peripheral T cell deletion after activation. Dying, activated T cells were quantitated by detecting extracellular phosphatidylserine with concomitant staining for SAg-reactive T cells using a TCR Vβ-specific mAb. Upon close examination, it was shown that CD40 activation delayed the death of the activated T cells. Additionally, it was found that enhanced survival of CD4 T cells was equally dependent on APC expression of B7-1 and B7-2. This is in contrast to CD8 T cells, which did not depend as much on B7-1 as B7-2. Thus, CD40 activation indirectly promotes T cell growth and delays the death of SAg-stimulated CD4 T cells in vivo. These data suggest that one way CD40 activation promotes a more robust immune response is by indirectly increasing the production of effector T cells and by keeping them alive for longer periods of time.

In vivo tumor transfection with superantigen plus cytokine genes induces tumor regression and prolongs survival in dogs with malignant melanoma

In vivo transfection of established tumors with immunostimulatory genes can elicit antitumor immunity. Therefore, we evaluated the safety and efficacy of intratumoral injections of a bacterial superantigen with a cytokine gene in dogs with malignant melanoma, a spontaneous and highly malignant canine tumor. 26 dogs with melanoma were treated with lipid-complexed plasmid DNA encoding staphylococcal enterotoxin B and either GM-CSF or IL-2. Dogs were evaluated for treatment-associated toxicity, tumor responses, immunologic responses, and survival times. The overall response rate (complete or partial remissions) for all 26 dogs was 46% (12 of 26), and was highest in patients with smaller tumors. Toxicity was minimal or absent in all dogs. Injected tumors developed marked infiltrates of CD4+ and CD8+ T cells and macrophages, and tumor regression was associated with development of high levels of antitumor cytotoxic T lymphocyte activity in peripheral blood lymphocytes. Survival times for animals with stage III melanomas treated by intratumoral gene therapy were prolonged significantly compared with animals treated with surgical tumor excision only. Thus, local tumor transfection with superantigen and cytokine genes was capable of inducing both local and systemic antitumor immunity in an outbred animal with a spontaneously developing malignant tumor.

In Vivo Effects of a Bacterial Superantigen on Macaque TCR Repertoires

A macaque model was employed to explore staphylococcal enterotoxin B (SEB) superantigen-driven T lymphocyte responses. The SEB-reactive Vβ+ cell subpopulations demonstrated a striking tri-phase response in rhesus monkeys following an SEB challenge in vivo. The hyperacute down-regulation, seen as early as 2 h through 2 days after SEB injection, was characterized by a disappearance of the reactive Vβ-restricted PBL subpopulations from the circulation and decreased expression of these cell subpopulations in lymphoid tissues. Following this, a dominant expansion of reactive Vβ-expressing CD4+ cell subpopulations occurred in lymph nodes and spleens, whereas in the peripheral blood a preferential expansion of reactive Vβ-expressing CD8+ cell subpopulations was seen. An exhaustion of this response was then seen, with a prolonged decrease in the number of the reactive Vβ+ CD4+ lymphocyte subpopulations. Interestingly, monoclonal or oligoclonal dominance was seen in the reactive Vβ+ cell subpopulations in the period of the transition from the polyclonal cellular expansion to the exhaustion of the response, suggesting that some Vβ+ cell clones may be more resistant than others to superantigen-mediated depletion. These results indicate that in vivo SEB superantigen-mediated effect on lymphocyte subpopulations in macaques is complex, suggesting that profound dynamics in the TCR repertoires may in part account for the susceptibility of higher primates to SEB-induced diseases.

Cell death mediated by Fas-FasL interaction between glial cells and MBP-reactive T cells

Apoptosis in T cells that have penetrated into the central nervous system (CNS) may be important for the physiological control of T cells with potentially dangerous reactivities to CNS antigens; such control may be dysfunctional in animals suffering from experimental autoimmune encephalomyelitis (EAE). In this study we examined the expression of Fas and FasL genes both in myelin basic protein (MBP)-reactive T cells and in glial cells and the susceptibility of these cells to death induced by Fas/FasL interaction. Both Fas and FasL gene expression is detectable in glial cells and MBP-reactive T cells. Cell death is not unidirectional: when T cells interact with glial cells death can be induced in the former or in the latter population. The ability to induce death of Fas-expressing cells varies greatly among different lines of MBP-reactive T cells, as does resistance to death induction by cells expressing FasL. Moreover, the ability of T cells both to deliver and to resist death signals is a function of their activation status: T cells freshly activated transmit a stronger apoptotic signal to Fas-positive target cells and are also more resistant to FasL-induced suicide. Soluble form of FasL provides a convenient titratable means of delivering death signals via Fas. However, comparison of the susceptibility of different targets to soluble FasL and to FasL expressed on the surface of a transfected glial line revealed differences, suggesting that signals arising from Fas/FasL interaction may be modulated by additional cell-surface molecules.

Tolerance induction and autoimmune encephalomyelitis amelioration after administration of myelin basic protein-derived peptide

Experimental autoimmune encephalomyelitis (EAE), a demyelinating disease of the central nervous system, is an animal model of paralyzing human disease, multiple sclerosis. EAE is readily induced by immunization with myelin basic protein (MBP) in mice transgenic for an alphabeta T cell receptor (TCR) that is specific for MBP. Subcutaneous injection of p17 (a peptide consisting of 17 NH2-terminal aminoacids of MBP) in complete Freund's adjuvant (CFA) causes paralysis. Induction of paralysis is inhibited by prior intraperitoneal injection of the same peptide in incomplete Freund's adjuvant (IFA). In addition, ongoing paralysis is ameliorated by subsequent intraperitoneal injection of p17 in IFA. Tolerance induction is equally efficient in Fas-deficient and IL-4-deficient TCR-transgenic mice, suggesting that neither activation-induced cell death nor differentiation into Th2 type cells plays a role in the tolerance induction. Tolerance induction by p17 seems to be based on reduction in the responsiveness of anti-MBP T cells, as documented by lower overall antigen-induced lymphokine production and proliferation, as well as diminished upregulation of early activation marker CD69 by tolerized T cells. We propose that continuous encounters of MBP-specific T cells with p17 play a critical role in the induction and maintenance of tolerance.

The changing preference of T and B cells for partners as T-dependent antibody responses develop

Recirculating virgin CD4+ T cells spend their life migrating between the T zones of secondary lymphoid tissues where they screen the surface of interdigitating dendritic cells. T-cell priming starts when processed peptides or superantigen associated with class II MHC molecules are recognised. Those primed T cells that remain within the lymphoid tissue move to the outer T zone, where they interact with B cells that have taken up and processed antigen. Cognate interaction between these cells initiates immunoglobulin (Ig) class switch-recombination and proliferation of both B and T cells; much of this growth occurs outside the T zones B cells migrate to follicles, where they form germinal centres, and to extrafollicular sites of B-cell growth, where they differentiate into mainly short-lived plasma cells. T cells do not move to the extrafollicular foci, but to the follicles; there they proliferate and are subsequently involved in the selection of B cells that have mutated their Ig variable-region genes. During primary antibody responses T-cell proliferation in follicles produces many times the peak number of T cells found in that site: a substantial proportion of the CD4+ memory T-cell pool may originate from growth in follicles.

Viral superantigen drives extrafollicular and follicular B cell differentiation leading to virus-specific antibody production

Mouse mammary tumor virus (MMTV[SW]) encodes a superantigen expressed by infected B cells. It evokes an antibody response specific for viral envelope protein, indicating selective activation of antigen-specific B cells. The response to MMTV(SW) in draining lymph nodes was compared with the response to haptenated chicken gamma globulin (NP-CGG) using flow cytometry and immunohistology. T cell priming occurs in both responses, with T cells proliferating in association with interdigitating dendritic cells in the T zone. T cell proliferation continues in the presence of B cells in the outer T zone, and B blasts then undergo exponential growth and differentiation into plasma cells in the medullary cords. Germinal centers develop in both responses, but those induced by MMTV(SW) appear later and are smaller. Most T cells activated in the T zone and germinal centers in the MMTV(SW) response are superantigen specific and these persist for weeks in lymph nodes draining the site MMTV(SW) injection: this contrasts with the selective loss of superantigen-specific T cells from other secondary lymphoid tissues. The results indicate that this viral superantigen, when expressed by professional antigen-presenting cells, drives extrafollicular and follicular B cell differentiation leading to virus-specific antibody production.

Secretion of biologically active superantigens by mammalian cells

The genetic modification of tumor cells to secrete immune regulatory molecules can elicit a potent antitumor immune response. Bacterial superantigens are among the most potent T cell mitogens. Activation of tumor-sensitized T cells by bacterial superantigens can lead to immune effector cells with potent and specific in vivo antitumor activity. Retrovirus vectors encoding for the bacterial superantigens SEA and SEC2 were constructed, and recombinant retrovirus stocks were generated. SEA and SEC2 could be detected in the culture supernatant of tumor cells after a single exposure to retrovirus. Molecular analysis of the genetically modified cells revealed intact proviral DNA and abundant vector-derived superantigen RNA. Biologic activity was apparent for both superantigens. Secretion of biologically active superantigen by mammalian cells has not been reported previously, and this will enable investigating the potential for superantigen gene therapy.

B-cell superantigens: molecular and cellular implications

B cell superantigens are proteins that are capable of immunoglobulin variable region mediated binding interactions with the naive B cell repertoire at frequencies that are orders of magnitude greater than occur for conventional antigens. Within this review we discuss recent observations regarding the molecular basis of these interactions and the distribution of superantigen binding capacities in different human B cell populations. These findings and current predictions regarding the relevance of these proteins to the physiologic development of immune repertoires are also discussed.

Superantigens

Superantigens are potent modulators of the immune system. Some of their biological and immunological properties are reviewed here with special attention to their potential significance for cutaneous inflammation, specific skin immune responses and skin diseases.

The in vivo mechanism of action of CTLA4Ig

A single dose of CTLA4Ig, an inhibitor of CD28-mediated T cell costimulation, given 2 days after transplantation induces specific unresponsiveness to alloantigens in vivo. However, the mechanisms responsible are unknown. Using pigeon cytochrome c as a model Ag, we monitored the effect of CTLA4Ig on the fate of Ag-reactive T cells in normal mice and on pigeon cytochrome c-specific TCR transgenic cells adoptively transferred into congenic mice. CTLA4Ig significantly inhibits immunization with pigeon cytochrome c. In particular, ELISA and ELISPOT assays indicate an 80 to 90% reduction in Th1 (i.e, IL-2 and IFN-gamma) cytokine production and in the numbers of cytokine-producing cells. Interestingly, despite this profound reduction in cytokine-producing cells, Ag-reactive T cells expand in CTLA4Ig-treated animals, although the degree of expansion is reduced by 50% compared with that in control Ig-treated animals. Thus, loss of Th1 cytokine production in CTLA4Ig-treated animals is not fully explained by the decreased expansion of Ag-specific T cells. These results suggest two mechanisms of action for CTLA4Ig in vivo: inhibition of expansion of Ag-reactive cells and induction of anergy in the residual population.

Immunosuppression through blockade of CD28:B7-mediated costimulatory signals

It is now well established that T cells require two signals for activation and effector function. The first signal is provided through the T cell receptor for antigen. The best-characterized pathway which provides the second, or costimulatory, signal is through the CD28 receptor on the surface of T cells. In vitro, ligation of the T cell receptor without a second signal induces a long-lived state of anergy in T cells. CD28 has two known ligands, B7-1 and B7-2, expressed on activated antigen-presenting cells. A soluble fusion protein called CTLA4Ig has been produced which binds B7-1 and B7-2 and acts as a competitive inhibitor of CD28. In vitro and in vivo studies with CTLA4Ig demonstrate that it is an extremely effective immunosuppressive agent in models of transplantation and autoimmunity. Mechanistic studies indicate that CTLA4Ig may work by partially inhibiting the expansion of antigen-reactive cells and inducing anergy in the residual population.

Major histocompatibility complex class II-associated peptides control the presentation of bacterial superantigens to T cells

Recent studies have shown that only a subset of major histocompatibility complex (MHC) class II molecules are able to present bacterial superantigens to T cells, leading to the suggestion that class-II associated peptides may influence superantigen presentation. Here, we have assessed the potential role of peptides on superantigen presentation by (a) analyzing the ability of superantigens to block peptide-specific T cell responses and (b) analyzing the ability of individual peptides to promote superantigen presentation on I-Ab-expressing T2 cells that have a quantitative defect in antigen processing. A series of peptides is described that specifically promote either toxic shock syndrome toxin (TSST) 1 or staphylococcal enterotoxin A (SEA) presentation. Whereas some peptides promoted the presentation of TSST-1 (almost 5,000-fold in the case of one peptide), other peptides promoted the presentation of SEA. These data demonstrate that MHC class II-associated peptides differentially influence the presentation of bacterial superantigens to T cells.

The superantigen Staphylococcus enterotoxin B induces a strong and accelerated secondary T-cell response rather than anergy

The primary and secondary immune response of V beta 8+ T cells to the bacterial superantigen Staphylococcus enterotoxin B was compared in BALB/c mice. Secondary responder T cells were found to up-regulate the expression of the adhesion molecule LFA-1 faster, and to enter the cell cycle earlier than primary responder T cells. Both, primary and secondary responder T cells upregulate the expression of CD2 and CD25 and turn into blast cells with superimposable time kinetics. Secondary responder T cells terminate DNA synthesis, blast formation and the upregulation of CD25 and CD2 expression earlier than primary responder T cells and become more rapidly deleted. Two days after superantigen challenge, when primary responder T cells reach peak activity in terms of DNA synthesis and blast formation, secondary responder T cells have returned to the size of microblasts and ceased to replicate their DNA. Whereas our results are consistent with the observations leading to the concept of superantigen-induced T-cell anergy, they demonstrate, by revealing the accelerated vigorous secondary T-cell response to the superantigen, that this concept requires reconsideration.

Prevention and reversal of superantigen-induced anergy by contact allergen exposure

The superantigen Staphylococcal enterotoxin B (SEB) and the contact allergen 2,4-dinitrofluorbenzene (DNFB) both react with V beta 8+ T-cells delivering distinct signals. Pre-treatment with DNFB painted onto the same skin site where SEB was to be injected, prevented the induction of anergy in V beta + T-cells that was otherwise induced after SEB had been injected intradermally over a period of 2 weeks. Application of the irritant sodium dodecyl sulfate (SDS) instead of DNFB did not exert this effect. Application of DNFB at a site distant from the site where SEB was injected resulted in a much weaker inhibitory influence on the induction of anergy by SEB. Established anergy of V beta 8+ T-cells (proliferative non-responsiveness to SEB in vitro that could be overcome by addition of exogenous interleukin 2 (IL-2)) could be largely reversed by repeated cutaneous exposure to DNFB painted to the site where SEB had been injected before. The moderate decrease of V beta 8+ T-cells normally induced by SEB-treatment was also partially prevented by DNFB pre-treatment. The data indicate the importance of the sequence of signals delivered to T cells and the plasticity of the responsiveness of this cell type.

Proliferation is a prerequisite for bacterial superantigen-induced T cell apoptosis in vivo

Staphylococcal enterotoxin B (SEB) is a bacterial superantigen that binds to major histocompatibility complex class II molecules and selectively interacts with T cells that bear certain T cell receptor (TCR) V beta domains. Administration of SEB in adult mice results in initial proliferation of V beta 8+ T cells followed by a state of unresponsiveness resulting from a combination of clonal deletion and clonal anergy in the SEB-reactive population. At this time, it is unclear what relationship exists between the T cells that have proliferated and those that have been deleted or have become anergic. Here we show that only a fraction of the potentially reactive V beta 8+ T cells proliferate in response to SEB in vivo, and that all the cells that have proliferated eventually undergo apoptosis. Virtually no apoptosis can be detected in the nonproliferating V beta 8+ T cells. These data demonstrate a causal relationship between proliferation and apoptosis in response to SEB in vivo, and they further indicate that T cells bearing the same TCR V beta segment can respond differently to the same superantigen. The implications of this differential responsiveness in terms of activation and tolerance are discussed.

Susceptibility to tumors induced by polyoma virus is conferred by an endogenous mouse mammary tumor virus superantigen

A dominant gene carried in certain inbred mouse strains confers susceptibility to tumors induced by polyoma virus. This gene, designated Pyvs, was defined in crosses between the highly susceptible C3H/BiDa strain and the highly resistant but H-2k-identical C57BR/cdJ strain. The resistance of C57BR/cdJ mice is overcome by irradiation, indicating an immunological basis. In F1 x C57BR/cdJ backcross mice, tumor susceptibility cosegregates with Mtv-7, a mouse mammary tumor provirus carried by the C3H/BiDa strain. This suggests that Pyvs might encode the Mtv-7 superantigen (SAG) and abrogate polyoma tumor immunosurveillance through elimination of T cells bearing specific V beta domains. DNA typing of 110 backcross mice showed no evidence of recombination between Pyvs and Mtv-7. Strongly biased usage of V beta 6 by polyoma virus-specific CD8+ cytotoxic T lymphocytes in C57BR/cdJ mice implicates T cells bearing this Mtv-7 SAG-reactive V beta domain as critical anti-polyoma tumor effector cells in vivo. These results indicate identity between Pyvs and Mtv-7 sag, and demonstrate a novel mechanism of inherited susceptibility to virus-induced tumors based on effects of an endogenous superantigen on the host's T cell repertoire.

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.

Deletion of CD4+ T cells and thymocytes by apoptosis in mouse mammary tumor virus (C4)-infected V beta 2 transgenic mice

Mouse mammary tumor virus MMTV (C4) encodes a V beta 2-specific superantigen. In V beta 2 transgenic (TG2) mice more than 98% of peripheral T cells express V beta 2. Infection of Tg2 mice with MMTV (C4) at birth through their mothers' milk or at 6-8 weeks of age by intravenous injection resulted in massive deletion of peripheral CD4+ T cells and suppressed thymopoiesis. The number of peripheral CD8+ T cells was not affected in neonatally infected mice. In older mice injected with MMTV (C4), splenic CD8+ T cells were significantly elevated. Suppressed thymopoiesis was observed in both neonatally infected and older mice injected with MMTV (C4). Thymocytes which expressed high level CD3 or V beta 2 were deleted. To determine if T cells or thymocytes were deleted through apoptosis, DNA fragmentation was examined by flow cytometry and diphenylamine (DPA) binding assay. Approximately 31% of CD4+ T cells from MMTV (C4)-infected Tg2 mice as compared to 6% from normal Tg2 mice contained fragmented nuclear DNA by flow-cytometric analysis. The DPA binding assay showed significantly increased total soluble DNA in lymph node cells and thymocytes from MMTV (C4)-infected mice. The kinetics of T cell and thymocyte apoptosis correspond to their deletion, supporting apoptosis as the mechanism of T cell and thymocyte deletion. CD4+ T cell and thymocyte deletion by MMTV (C4) in Tg2 mice provides a sensitive system for the analysis of retrovirus superantigen-induced apoptosis.