Suppression of cytolytic T-cell activity by staphylococcal enterotoxin B-induced suppressor cells: role of interleukin 2

Staphylococcal enterotoxin B modulates V beta 8+ TcR-associated T-cell memory against conventional antigen

Primary in vivo challenge with the superantigen staphylococcal enterotoxin B (SEB) induces polyclonal proliferation of an unusually large proportion of circulating T-cells that bear the V beta 8-T-cell receptor (TcR) domain. Early and vigorous proliferation of V beta 8+ T-cells precedes their selective deletion, leaving the host unresponsive upon rechallenge with the native immunogen SEB. Nonetheless, this induction of anergy is incompletely understood. Recently we demonstrated that more cells than just V beta 8+ T-cells undergo clonal proliferation after challenge with SEB (Cell. Immunol. 154, 440, 1994). These findings suggested that non-V beta 8+ T-cells may have a role in the induction of superantigen-induced anergy. To further investigate this, we enumerated CD4+ and CD8+ T-cells in lymph nodes and spleens from Balb/c mice at various times after primary and secondary challenge with either a high or a low dose of SEB. Using these kinetic data we investigated whether challenge with SEB would modulate antigen-specific V beta 8-associated T-memory responses. To this end, the V beta 8+ T-cell-associated responses induced by SEB were compared with the V beta 8+ TcR-associated memory responses induced by the nominal antigen sperm whale myoglobin (SWM). Results indicated that challenge of SWM-primed mice with SEB abrogated the V beta 8-associated SWM-specific T-cell memory for an extended but transient period of time. Moreover, prechallenge with SEB blocked the establishment of de novo V beta + T-cell-mediated immunity. These findings suggest that administration of low and controlled doses of microbial superantigen could provide long-term suppression of antigen-specific cell-mediated immunity.

Staphylococcal enterotoxin B promotes deletion and functional inactivation of CD4V beta 8-positive cells in the absence of CD8 T cells

The staphylococcal enterotoxins stimulate discrete subsets of T cells depending on their expression of particular V genes. Among these, staphylococcal enterotoxin B (SEB) vigorously stimulates V beta 8+ cells. This stimulation results in proliferation of both CD4+V beta 8+ and CD8+ T cells and eventually to anergy and clonal deletion in the former subset. We have examined the possible role of CD8+ T cells in the response of CD4+ cells to SEB, by in vivo CD8+ T-cell-depletion. We found no qualitative difference in the responses of untreated and CD8+ T-cell depleted mice to SEB; however, a small quantitative difference in deletion was observed. Thus it appears that on the whole the response of CD4+V beta 8+ T cells to SEB is independent of CD8+ T-cell effector function, although the latter may play a partial role.

Staphylococcal enterotoxin B as a potent suppressant of T lymphocytes: trace levels suppress T lymphocyte proliferative responses

Staphylococcal enterotoxins have long been known to be powerful stimulators of T lymphocytes in mouse and man. In a previous study we showed that high concentrations of staphylococcal enterotoxin serotype B (SEB) failed to stimulate strong proliferative responses by Lewis rat T lymphocytes. Moreover, concentrations of SEB (10-50 micrograms/ml) that stimulated optimal mouse T lymphocyte proliferative responses suppressed a mitogen- or antigen-induced rat T lymphocytes proliferative responses. The present study shows that SEB at low concentrations (as low as 10(-3)-10(-4) micrograms/ml) and often also trace levels (about 10(-6)-10(-7) micrograms/ml) suppresses both rat and mouse T lymphocytes proliferative responses to mitogen or antigen. Furthermore, under different circumstances, SEB may have conflicting effects on the same T cells. While high concentrations (1-50 micrograms/ml) of SEB stimulate certain mouse T cell clones, low concentrations or trace levels have a potent suppressive effect on the same clones. The results indicate that the in vitro conflicting effects of SEB on the same T cells are concentration dependent and may reflect its in vivo effects on SEB-reactive T lymphocytes. The suppression of the mitogen- or antigen-induced stimulation of T cell clones by SEB was direct and did not require the agency of suppressor cells. Furthermore, the suppression by low amounts of SEB was not major histocompatibility complex restricted and affected a large proportion of both rat and mouse T lymphocyte subpopulation, regardless of their antigenic specificity. The concomitant suppressogenic and stimulatory characteristics of SEB support the conclusion that, under different conditions, SEB can be considered a "super-suppressogen" as well as a "super-antigen". Overall, the results suggest that SEB, and possibly other bacterial toxins, could be useful in immunomodulation of specific T cell responses.

Induction of nitric oxide synthase activity by toxic shock syndrome toxin 1 in a macrophage-monocyte cell line

Toxic shock syndrome toxin 1 (TSST-1) is a Mr 22,000 protein produced by Staphylococcus aureus. It is thought to be the cause of toxic shock syndrome. We investigated the hypothesis that TSST-1 induces nitric oxide (NO) synthase and that the NO formed may be involved in the pathogenesis of toxic shock syndrome. We used the murine monocyte-macrophage cell line J744.2 that responds to TSST-1 and also expresses NO synthase activity upon immunological stimulation. J774.2 macrophages stimulated with TSST-1 (10-100 nM) generated nitrite, a breakdown product of NO, and induced concentration-dependent elevations of cGMP in the pig kidney epithelial cell line (LLC-PK1). This latter effect was due to the generation of L-arginine-derived NO for it was (i) abolished by oxyhemoglobin (10 microM), a scavenger of NO, or by methylene blue (10 microM), an inhibitor of NO-activated guanylate cyclase; (ii) potentiated by superoxide dismutase (100 units/ml), which prolongs the life of NO; (iii) inhibited by NG-monomethyl-L-arginine (0.3 mM), an inhibitor of NO synthase; (iv) significantly decreased when L-arginine (0.4 mM) in the medium was replaced by D-arginine (0.4 mM). Moreover, TSST-1 (100 nM) enhanced the activity of cytosolic NO synthase in J774.2 cells. Hydrocortisone (1 microM) but not indomethacin (5 micrograms/ml) or salicylic acid (5 micrograms/ml) prevented the generation of NO2- and the increases in cGMP levels in LLC-PK1 cells induced by J774.2 cells stimulated with TSST-1. The effects of hydrocortisone were partially reversed by coincubation with RU 486 (1 microM), an antagonist of glucocorticoid receptors. Thus, TSST-1 and perhaps other exotoxins produced by Gram-positive bacteria induce NO synthase and the increased NO formation may contribute to toxic shock syndrome and possibly to changes in the immune responses that accompany infection.

Characterization of antigen-specific suppressor factors induced by staphylococcal enterotoxin B

Previous studies from this laboratory have shown that staphylococcal enterotoxin B (SEB) has the capacity to nonspecifically induce multiple T suppressor cell populations which are capable of regulating both primary and secondary in vitro antibody responses. Additional studies have revealed that the suppressive activity of these cells is mediated, at least in part, by an I-J-restricted suppressor-inducer factor. Efforts to characterize the specificity of this inducer molecule have demonstrated the presence of multiple antigen-specific suppressor factors within SEB-stimulated supernatants. Antigen-binding molecules present within these SEB-induced factor preparations were isolated and concentrated over antigen-coupled columns. The results have demonstrated that eluates from trinitrophenyl-ovalbumin and trinitrophenyl-keyhole limpet hemocyanin-coupled columns significantly suppress both primary and secondary anti-TNP plaque-forming colony responses, whereas the filtrates from these columns demonstrated little to no inhibitory activity. The filtrates, but not eluates, from these columns exhibited significant suppressive activity for anti-poly(L-Phe,L-Glu)-poly-DL-Ala-poly-L-Lys, anti-azobenzenearsonate, and anti-sheep red blood cell antibody responses. Additional studies using eluates and filtrates from a variety of antigen-coated columns have demonstrated that suppressor factors of multiple antigenic specificities are present within SEB-induced supernatants. These studies suggest that SEB polyclonally activates antigen-specific suppressor cells which produce suppressor factors which specifically regulate the antibody response.

In vivo T-cell activation by staphylococcal enterotoxin B prevents outgrowth of a malignant tumor

Treatment of T cells with staphylococcal enterotoxins in vitro is known to activate T cells in a subset restricted manner based on beta-chain variable region (V beta) gene expression. In particular, staphylococcal enterotoxin B (SEB) activates T cells bearing V beta 7 or V beta 8. We examined the ability of SEB to activate T cells in vivo. Treatment of C3H mice with doses of SEB ranging from 5 to 250 micrograms resulted in a dose-dependent activation of V beta 8+ T cells as reflected by increased interleukin 2 receptor (IL-2R) expression, proliferation to exogenous IL-2 and allogeneic cells, and production of gamma interferon. SEB also caused proliferation of the CD8+ subset of V beta 8+ cells in vivo. Thus, T-cell activation by SEB in vivo appears to be specific since V beta 2+ cells (non-SEB reactive) did not show increases in IL-2R expression similar to those seen with V beta 8+ cells nor did they proliferate. We then studied the ability of these activated cells to potentiate the immune response to a malignant progressor tumor. Treatment of C3H mice with 50 micrograms of SEB at the time of inoculation with tumor fragments resulted in a statistically significant decrease in the frequency of tumor outgrowth. These data demonstrate that treatment of C3H mice with SEB results in specific activation of V beta 8+ cells in vivo and that these activated cells are capable of preventing the outgrowth of a malignant tumor.

Inhibition of the delayed-type hypersensitivity response by staphylococcal enterotoxin B-induced suppressor T cells

Staphylococcal enterotoxin B (SEB) is a member of a family of gram-positive bacterial exotoxins which act as superantigens in both mouse and man. The administration of this toxin has been shown to inhibit antibody responses in vivo. We have previously shown that SEB is a potent inducer in vitro of multiple T suppressor cell populations. The present studies show that administration of microgram quantities of this toxin result in a reduced capacity to manifest a delayed-type hypersensitivity (DTH) response. In addition, we find that the failure to generate a normal DTH response appears to be due to the generation of a T suppressor cell population following SEB administration. Adoptive transfer studies show that the suppressor cells bear the CD5+ I-J+ CD4- CD8- Thy 1+ surface phenotype. The relationship of these cells to suppressor T cell populations generated following in vitro activation by SEB is discussed.

Immunosuppressive activity of staphylococcal enterotoxin B. II. Activation of suppressor-effector cells by a staphylococcal enterotoxin B-induced suppressor factor

The capacity of staphylococcal enterotoxins to stimulate all T cells bearing certain T cell receptors has recently generated a great deal of interest. These toxins are believed to bind directly both to the TCR:CD4 complex via its V beta domains and to class II MHC molecules on accessory cells prior to T cell activation. Previous studies from this laboratory have demonstrated that staphylococcal enterotoxin B (SEB) is capable of inducing multiple T suppressor cell populations which can inhibit in vitro antibody responses. Additional studies have demonstrated that the suppressive activity of these cells is mediated, at least in part, by an I-J-restricted suppressor factor. Efforts to characterize the inhibitory activity of this factor have demonstrated that the suppressive element is capable of activating both early and late acting suppressor cell populations in vitro. Analysis by both positive and negative selection shows that cells bearing the Lyt1-2+ surface marker phenotype are active early, whereas Lyt1+2+ cells are active both early and late in the antibody response. Additional experiments using various strains of mice as sources of suppressor factor and of naive splenocyte populations have demonstrated that activation of suppressor-effector cells by this suppressor factor is restricted at the I-J, but not Igh, gene locus. These studies suggest that this SEB-induced suppressor factor alone provides the signals necessary for the induction and activation of suppressor-effector cell activity.

Immunosuppressive activity of staphylococcal enterotoxin B. I. Characterization of staphylococcal enterotoxin-B-induced suppressor cells

Staphylococcal enterotoxin B (SEB) binds specifically to major histocompatibility complex class II molecules on the surface of accessory cells and stimulates virtually all T cells bearing certain, but not all, T cell-receptor V beta alleles. We have previously shown that this superantigen is a potent inducer of multiple regulatory T cell populations. In the present report we show that SEB induces a population of suppressor T cells which inhibits the generation of alloantigen-induced cytotoxic T cell activity. Using both negative- and positive-selection analysis, we found that this suppressor population is a CD4- CD8- CD5+ IL-2R+ T cell. This cell population inhibited both syngeneic and allogeneic cytotoxic T lymphocyte activity, but the cell population which inhibited allogeneic CTL activity was radiation sensitive. In addition, allogeneic SEB-primed cells appeared to develop cytolytic activity as a result of the additional stimulation in the mixed-lymphocyte reaction culture. The relationship of the SEB-primed CD4- CD8- CD5+ T cells to related regulatory T cell populations is discussed.

Depression of prothymosin alpha production in murine thymus correlates with staphylococcal enterotoxin-B-induced immunosuppression [corrected]

Prothymosin alpha [corrected] (ProT alpha) and thymosin beta 4 [corrected] (T beta 4) were isolated from murine thymus and characterized by microsequence analysis. Murine T beta 4 has an identical sequence to bovine T beta 4, whereas murine ProT alpha is highly homologous to rat Pro T alpha. Murine Pro T alpha differs from rat Pro T alpha at two positions, Glu100 and Asp108 of the rat sequence are substituted by aspartic and glutamic acid, respectively, in murine Pro T alpha. The amount of Pro T alpha in murine thymus was found to be reduced after in vivo treatment with staphylococcal enterotoxin B [corrected] (SEB), a superantigen which stimulates T cells bearing specific V beta receptors. Results from the anti-SRBC (sheep erythrocyte) plaque-forming cell assay showed that the antibody response of the spleen cells from these animals was also suppressed. On the other hand, the amount of T beta 4 was not changed significantly. Our studies suggest that the suppression of SEB on antibody response correlates with the depression of Pro T alpha production in the thymus.

Requirement for accessory cells in suppression of MOPC-315 IgA secretion by staphylococcal enterotoxin B-induced T-suppressor cells

Staphylococcal enterotoxin B (SEB) is a potent polyclonal activator of both human and murine T cells. We previously reported data which show that SEB-induced T cells suppress antibody secretion by various mouse plasmacytoma cell lines. This suppression of antibody secretion was found to be both idiotype and isotype nonspecific, and the suppressor cell bears the CD5-positive CD8-negative cell surface phenotype. The present studies demonstrate that accessory cells are required in the SEB-primed spleen cell (SEB-PSC) population in order for this population to mediate suppression. The suppressive activity of SEB-PSC is abrogated following accessory cell depletion by passage over Sephadex G-10 columns. B cell depletion using nylon-wool also abrogates suppression mediated by SEB-PSC. The addition of nonelicited adherent peritoneal exudate cells (PECs) restores suppressive activity to accessory cell-depleted SEB-PSC. The restoration of suppression by the PECs is not major histocompatibility complex restricted, since both syngeneic and allogeneic PECs can carry out this activity. In addition, it is not necessary for the accessory cells to be metabolically active in order to participate in the suppressive activity. This is based on results demonstrating that glutaraldehyde fixation, at levels reported to eliminate metabolic activity, does not affect the ability of PECs to restore suppression to Sephadex G-10-depleted SEB-PSC. The results are consistent with the well established requirement for accessory cells in the function of antigen-induced suppressor T cells.

Cell surface molecules involved in early events in T-cell mitogenic stimulation by staphylococcal enterotoxins

We tested the mitogenic response to staphylococcal enterotoxin (SE) type A and SE type B in spleen cells from five strains of mice and found consistent and significant differences among the strains. We chose to study the mitogenic responses of two of these strains, C58BL/6J and BALB/cJ, in greater detail. We investigated the effects of specific monoclonal antibodies to cell surface determinants on SE-induced mitogenesis. Monoclonal antibodies against Ia (class II major histocompatibility complex) determinants blocked SE-induced mitogenesis. Both I-A and I-E molecules can participate in the stimulation, and in BALB/cJ mice which express both types of class II molecules both must be blocked to prevent mitogenesis. Mitogenesis was not inhibited by monoclonal antibodies specific for class I major histocompatibility complex antigens or monoclonal antibodies specific for Mac-1, Lyt-1, or Lyt-2 cell surface proteins. Monoclonal antibodies specific for the T-cell surface antigens L3T4 and T3 also substantially inhibited SE-induced mitogenesis. This implicates participation of the T-cell antigen receptor complex in stimulation induced by the SEs. Elimination of L3T4+ helper-inducer T cells abolished the mitogenic response of spleen cells to SE. Reconstitution of L3T4-depleted spleen cells with L3T4+ T cells showed that the level of the mitogenic response was directly proportional to the number of L3T4+ cells added. Elimination of Lyt-2+ cells resulted in a 50% decrease in the response to SEs. These results indicate that L3T4+ T cells are required for the mitogenic response to SE, but both L3T4+ and Lyt 2+ T cells participate in SE-induced mitogenesis. Our results suggest that both Ia and the T-cell antigenic receptor complex are involved in SE-induced mitogenesis.

Staphylococcal enterotoxin B stimulation of BALB/c lymphocyte mitogenesis and potential relationship to the Mls response

Staphylococcal enterotoxin B (SEB) is a T cell mitogen with properties different from the plant lectin mitogens. We examined the stimulation of mitogenesis induced by SEB in BALB/c mouse spleen cells and its relationship to major histocompatibility complex (MHC) and related cell surface proteins. Based on the ability of specific monoclonal antibodies to block mitogenesis, SEB stimulation appears to be more dependent on interaction with I-E than with I-A class II MHC molecules. Additionally, anti-L3T4, and possibly other antibodies specific for proteins related to the T cell receptor complex, were inhibitory. When A20 cells were treated with SEB and used to stimulate BALB/c spleen cells which were not otherwise exposed to SEB, the treated A20 cells were capable of stimulating mitogenesis of the BALB/c spleen cells. The data support the hypothesis that SEB stimulation is mediated primarily by interactions with class II MHC proteins and possibly proteins in the T cell receptor complex. We also observed that the presence of SEB in DBA/2 (Mlsa)-stimulated BALB/c (Mlsb) spleen cell cultures enhanced the BALB/c mitogenesis three-fold over the sum of the SEB-plus Mls-stimulated mitogenesis. These results suggest that SEB may be a useful tool for further exploration of the Mls response.