Anergy and cytokine-mediated suppression as distinct superantigen-induced tolerance mechanisms in vivo

CD4(+) and CD8(+) anergic T cells induced by interleukin-10-treated human dendritic cells display antigen-specific suppressor activity

Interleukin-10 (IL-10)-treated dendritic cells (DCs) induce an alloantigen- or peptide-specific anergy in various CD4(+) and CD8(+) T-cell populations. In the present study, we analyzed whether these anergic T cells are able to regulate antigen-specific immunity. Coculture experiments revealed that alloantigen-specific anergic CD4(+) and CD8(+) T cells suppressed proliferation of syngeneic T cells in a dose-dependent manner. The same effect was observed when the hemagglutinin-specific CD4(+) T-cell clone HA1.7 or tyrosinase-specific CD8(+) T cells were cocultured with anergic T cells of the same specificity. Anergic T cells did not induce an antigen-independent bystander inhibition. Suppression was dependent on cell-to-cell contact between anergic and responder T cells, required activation by antigen-loaded DCs, and was not mediated by supernatants of anergic T cells. Furthermore, anergic T cells displayed an increased extracellular and intracellular expression of cytotoxic T-lymphocyte antigen (CTLA)-4 molecules, and blocking of the CTLA-4 pathway restored the T-cell proliferation up to 70%, indicating an important role of the CTLA-4 molecule in the suppressor activity of anergic T cells. Taken together, our experiments demonstrate that anergic T cells induced by IL-10-treated DCs are able to suppress activation and function of T cells in an antigen-specific manner. Induction of anergic T cells might be exploited therapeutically for suppression of cellular immune responses in allergic or autoimmune diseases with identified (auto) antigens.

In vitro generation of interleukin 10-producing regulatory CD4(+) T cells is induced by immunosuppressive drugs and inhibited by T helper type 1 (Th1)- and Th2-inducing cytokines

We show that a combination of the immunosuppressive drugs, vitamin D3 and Dexamethasone, induced human and mouse naive CD4(+) T cells to differentiate in vitro into regulatory T cells. In contrast to the previously described in vitro derived CD4(+) T cells, these cells produced only interleukin (IL)-10, but no IL-5 and interferon (IFN)-gamma, and furthermore retained strong proliferative capacity. The development of these IL-10-producing cells was enhanced by neutralization of the T helper type 1 (Th1)- and Th2-inducing cytokines IL-4, IL-12, and IFN-gamma. These immunosuppressive drugs also induced the development of IL-10-producing T cells in the absence of antigen-presenting cells, with IL-10 acting as a positive autocrine factor for these T cells. Furthermore, nuclear factor (NF)-kappaB and activator protein (AP)-1 activities were inhibited in the IL-10-producing cells described here as well as key transcription factors involved in Th1 and Th2 subset differentiation. The regulatory function of these in vitro generated IL-10-producing T cells was demonstrated by their ability to prevent central nervous system inflammation, when targeted to the site of inflammation, and this function was shown to be IL-10 dependent. Generating homogeneous populations of IL-10-producing T cells in vitro will thus facilitate the use of regulatory T cells in immunotherapy.

Temporal sequence and kinetics of proinflammatory and anti-inflammatory cytokine secretion induced by toxic shock syndrome toxin 1 in human peripheral blood mononuclear cells

The staphylococcal superantigen toxic shock syndrome toxin 1 (TSST-1) induces massive cytokine production, which is believed to be the key factor in the pathogenesis of TSS. The temporal sequence and kinetics of both proinflammatory and anti-inflammatory cytokines induced by TSST-1 in human peripheral blood mononuclear cells were investigated. A panel of loss-of-function single-amino-acid-substitution mutants of TSST-1, previously demonstrated to be defective in either major histocompatibility complex (MHC) class II binding (G31R) or T-cell receptor (TCR) interaction (H135A, S14N), was studied in parallel to further elucidate the mechanisms of cytokine secretion. Wild-type recombinant (WT r) TSST-1 induced a biphasic pattern of cytokine secretion: an early phase with rapid release of proinflammatory cytokines (especially gamma interferon, interleukin-2 [IL-2], and tumor necrosis factor alpha [TNF-alpha]) within 3 to 4 h poststimulation, and a later phase with more gradual production of both proinflammatory (IL-1beta, IL-12, and TNF-beta) and anti-inflammatory (IL-6, IL-10) cytokines within 16 to 72 h poststimulation. G31R, which is defective in MHC class II binding, induced a cytokine profile similar to that of WT rTSST-1, except that secretion of the early-phase proinflammatory cytokines was delayed and production of IL-1beta and IL-12 was markedly reduced. In contrast, mutant toxins defective in TCR interaction either demonstrated complete absence of any cytokine secretion during the entire observation period (H135A) or resulted in complete abolishment of IL-2 and other early-phase proinflammatory cytokines, while secretion of IL-10 appeared unaffected (S14N). Neither WT rTSST-1 nor the mutant toxins induced IL-4 or transforming growth factor beta. Our data indicate that effective TCR interaction is critical for the induction of the early-phase proinflammatory cytokine response, thus underscoring the importance of T-cell signaling in TSS.

TCR engagement in the absence of cell cycle progression leads to T cell anergy independent of p27(Kip1)

We have proposed a model in which the prevention of anergy by costimulation is the result of IL-2-induced G1 to S phase cell cycle progression. Here we demonstrate that the reversal of anergy by exogenous IL-2 also occurs during this window of the cell cycle. Recently, it has been proposed that the cell cycle inhibitor p27(Kip1) is an anergic factor. In contrast, our data demonstrate that during the induction, maintenance and rechallenge phases of anergy, p27(Kip1) levels do not correlate with the anergic phenotype. Although p27(Kip1) levels were down-regulated by IL-2 during the G1 to S phase transition, the amount of IL-2 required to produce this effect was far lower than that required to prevent the induction of anergy. Furthermore, T cell lines from p27(Kip1) knockout mice were anergized as well as T cells from mice that were heterozygous for p27(Kip1). Interestingly, the forced overexpression of p27(Kip1) was able to decrease IL-2 promoter-induced transcription, suggesting that the cell cycle machinery may be involved in T cell activation; however, physiological levels of p27(Kip1) did not prevent IL-2 transcription. Overall, our data serve to disassociate the ability of IL-2 to down-regulate p27(Kip1) and its ability to prevent or reverse anergy.

Alloantigen-induced specific immunological unresponsiveness

When the immune system encounters alloantigen it can respond in any one of a number of different ways. The choice that is made will take into account factors such as where, when and how the contact with the alloantigen takes place, as well as the environmental conditions that prevail at the time the alloantigen is encountered. Alloantigen administration before transplantation either alone or in combination with therapeutic agents that modulate the functional activity of the responding leucocytes can be a powerful way of inducing specific unresponsiveness to alloantigens in vivo. The molecular mechanisms that influence the way the outcome of the immune response to alloantigen develops, either activation or unresponsiveness to the triggering antigen, hold the key to our ability to manipulate the immune system effectively by exposing it to donor antigen for therapeutic purposes. This review will focus on alloantigen-induced immunological unresponsiveness and how insights into the mechanisms of unresponsiveness have driven the development of novel tolerance-induction strategies that show promise for translation into the clinic in the future.

Role of CD47 in the induction of human naive T cell anergy

We recently reported that CD47 ligation inhibited IL-2 release by umbilical cord blood mononuclear cells activated in the presence of IL-12, but not IL-4, preventing the induction of IL-12Rbeta(2) expression and the acquisition of Th1, but not the Th2 phenotype. Here we show that in the absence of exogenous cytokine at priming, CD47 ligation of umbilical cord blood mononuclear cells promotes the development of hyporesponsive T cells. Naive cells were treated with CD47 mAb for 3 days, expanded in IL-2 for 9-12 days, and restimulated by CD3 and CD28 coengagement. Effector T cells generated under these conditions were considered to be anergic because they produced a reduced amount of IL-2 at the single-cell level and displayed an impaired capacity 1) to proliferate, 2) to secrete Th1/Th2 cytokines, and 3) to respond to IL-2, IL-4, or IL-12. Moreover, CD47 mAb strongly suppressed IL-2 production and IL-2Ralpha expression in primary cultures and IL-2 response of activated naive T cells. Induction of anergy by CD47 mAb was IL-10 independent, whereas inclusion of IL-2 and IL-4, but not IL-7, at priming fully restored T cell activation. Furthermore, CD28 costimulation prevented induction of anergy. Thus, CD47 may represent a potential target to induce anergy and prevent undesired Th0/Th1 responses such as graft vs host diseases, allograft rejection, or autoimmune diseases.

Roles of I-E molecule and CD28 costimulation in induction of suppression by staphylococcal enterotoxin B in vivo

Exposure to bacterial superantigens leads to the induction of a subsequent state of immune hyporesponsiveness. Using a transwell coculture system, a previous report demonstrated that splenocytes from staphylococcal enterotoxin B (SEB)-injected BALB/c mice secreted soluble mediators to suppress the proliferative response of naive syngeneic splenocytes to SEB stimulation. We show in the present study that, in contrast to the suppressive effect induced by SEB in BALB/c (H-2(d) haplotype), MRL(+/+), and MRL-lpr/lpr (H-2(k)) mice, SEB-primed splenocytes from I-E(-) strains such as B6, B10, A. BY (H-2(b)), and A.SW (H-2(s)) mice failed to inhibit the CD25 expression and the proliferative activity of their syngeneic naive responder splenocytes. Further results revealed that the SEB-primed cells from BALB/c, but not B6, mice inhibited the CD25 expression and proliferation of naive responder cells from either BALB/c or B6 mice, indicating the critical regulatory role of the effector cells. Unlike SEB, staphylococcal enterotoxin A induced profound suppression in both BALB/c and B6 mice. Moreover, the suppressive competence of SEB-primed splenocytes was diminished in CD28-deficient BALB/c mice. Taken together, our results indicate that when SEB is used as a stimulator in vivo, both the I-E molecule and CD28 costimulation are required for the induction of regulatory cells bearing suppressive activity.

Adaptive tolerance of CD4+ T cells in vivo: multiple thresholds in response to a constant level of antigen presentation

The in vivo T cell response to persistent Ag contains a hyporesponsive phase following an initial expansion and subsequent partial deletion of the responding cells. The mechanism(s) responsible for this tolerance process is poorly understood. In this study, we describe a new paired transgenic model (TCR and Ag), which within 7-14 days produces 20-40 million hyporesponsive T cells. This state is characterized by an 85-95% reduction in all cytokine production, an impairment of re-expression of CD25 and CD69, and a desensitization of the proliferative response to Ag. TCR levels were normal, and in vivo mixing experiments showed no evidence for active suppression. The hyporesponsiveness partially dissipated without proliferation when the cells were transferred into a non-Ag-bearing host. If the second host expressed Ag, the T cells initially regained responsiveness, but then slowly entered an even deeper state of tolerance characterized by an additional 7- to 10-fold lowering of cytokine production and a greater desensitization of proliferation. Surprisingly, this readaptation took place with the same level of Ag presentation, suggesting that other parameters can influence the tolerance threshold. Both the readjustment in sensitivity and the reversal without Ag convincingly demonstrate for the first time a truly adaptive tolerance process in CD4+ T cells in vivo.

IFN-alpha and IL-10 induce the differentiation of human type 1 T regulatory cells

CD4(+) T regulatory type 1 (Tr1) cells suppress Ag-specific immune responses in vitro and in vivo. Although IL-10 is critical for the differentiation of Tr1 cells, the effects of other cytokines on differentiation of naive T cells into Tr1 cells have not been investigated. Here we demonstrate that endogenous or exogenous IL-10 in combination with IFN-alpha, but not TGF-beta, induces naive CD4(+) T cells derived from cord blood to differentiate into Tr1 cells: IL-10(+)IFN-gamma(+)IL-2(-/low)IL-4(-). Naive CD4(+) T cells derived from peripheral blood require both exogenous IL-10 and IFN-alpha for Tr1 cell differentiation. The proliferative responses of the Tr1-containing lymphocyte populations, following activation with anti-CD3 and anti-CD28 mAbs, were reduced. Similarly, cultures containing Tr1 cells displayed reduced responses to alloantigens via a mechanism that was partially mediated by IL-10 and TGF-beta. More importantly, Tr1-containing populations strongly suppressed responses of naive T cells to alloantigens. Collectively, these results show that IFN-alpha strongly enhances IL-10-induced differentiation of functional Tr1 cells, which represents a first major step in establishing specific culture conditions to generate T regulatory cells for biological and biochemical analysis, and for cellular therapy to induce peripheral tolerance in humans.

Chronic exposure to superantigen induces regulatory CD4(+) T cells with IL-10-mediated suppressive activity

The repeated injection of bacterial superantigens (SAg), such as staphylococcus enterotoxin (SE) A or B, has been shown in mice to induce a state of unresponsiveness characterized by the lack of secretion of Th1 lymphokines, such as IL-2 and IFN-gamma, following subsequent SAg challenge. We made the observation, in vivo as well as in vitro, that unresponsiveness to SAg could be transferred from SEA- to SEB-reactive T cells (and reversibly from SEB- to SEA-specific T cells) in C57BL/6 mice but not in BALB/c mice. Since C57BL/6 mice, unlike BALB/c mice, possess TCR V(beta)3+ and V(beta)11+ T cells able to react with both SEA and SEB, we hypothesized that SAg-unresponsive V(beta)3(+) and V(beta)11+ T cells could mediate linked suppression of other SAg-reactive T cells. To analyze further this possibility, spleen cells from BALB/c mice made unresponsive to SEB were tested for their capacity to suppress the response of normal BALB/c cells to SEB. The production of both IFN-gamma and IL-2 following SEB stimulation was greatly impaired in co-cultures containing CD4(+) T cells, but not CD8(+) T cells, isolated from unresponsive animals. In vivo, the production of both IFN-gamma and IL-2 responses to SEB was dramatically reduced in animals adoptively transferred with unresponsive spleen cells. This suppression was abrogated in recipients injected with neutralizing anti-IL-10 antibodies. Moreover, in animals made unresponsive to SEB, SAg-reactive CD4(+) T cells were found to express high levels of CTLA-4, a molecule recently described to play an essential role in the suppressive function of regulatory T cells. Taken together these results demonstrate that the repetitive injection of SAg induces the differentiation of regulatory CD4(+) T cells capable of suppressing SAg-reactive naive T cells.

GIF inhibits Th effector generation by acting on antigen-presenting B cells

Glycosylation-inhibiting factor (GIF) is a 13-kDa cytokine secreted from T cells. Administration of bioactive recombinant GIF inhibits IgG1 and IgE Ab responses in vivo. Treatment of B cells with the cytokine reduces the secretion of IgG1 and IgE induced by LPS and IL-4. To examine the effect on cognate T-B interaction, GIF was added to low-density B cells from MD4 transgenic (Tg) mice, which express B cell receptor specific for hen egg lysozyme (HEL). The B cells were subsequently pulsed with HEL-OVA conjugate and cultured with OVA-specific naive CD4 T cells from DO11.10 Tg mice. Treatment of Ag-presenting B cells with GIF reduced expansion and IL-2 secretion of naive T cells and rendered them hyporesponsive to antigenic restimulation, resulting in 50--95% reduction of IL-4 and IFN-gamma secretion upon restimulation with Ag. GIF dramatically inhibited Th effector generation when it was added to B cells before pulsing with HEL-OVA, whereas it showed little to no effect when added after B cells were pulsed with Ag. GIF was more effective when B cells from MD4 Tg mice were pulsed with HEL-OVA than when they were pulsed with OVA. This cytokine did not affect Th effector generation when B cells or irradiated splenocytes pulsed with OVA(323--339) peptide stimulated naive DO11.10 T cells. Confocal microscopy revealed that GIF inhibited internalization of HEL by B cells from MD4 Tg mice. Therefore, the cytokine may regulate early steps of Ag presentation involving B cell receptors to diminish Th effector generation from naive CD4 T cells.

The role of different subsets of T regulatory cells in controlling autoimmunity

T regulatory cells-in addition to clonal deletion and anergy-are essential for the downregulation of T cell responses to both foreign and self antigens, and for the prevention of autoimmunity. Recent progress has been made in characterising the different subsets of T regulatory cells, the factors that drive their differentiation, and their mode of action. The resolution of these mechanisms will make it possible to use T regulatory cells therapeutically in human autoimmune diseases.

Superantigen-induced CD4 T cell tolerance mediated by myeloid cells and IFN-gamma

We have previously shown that systemic staphylococcal enterotoxin A (SEA) injections cause CD4 T cells in TCR-transgenic mice to become tolerant to subsequent ex vivo restimulation. An active IFN-gamma-dependent mechanism of suppression was responsible for the apparent unresponsiveness of the CD4 T cells. In this study, we analyze the response of CD4 T cells isolated throughout the first 10 days of the in vivo response to injected SEA. We show that CD4 T cells isolated at the peak of the in vivo response undergo very little activation-induced cell death after sterile FACS sorting or restimulation in the presence of neutralizing Abs to IFN-gamma. We also show that the IFN-gamma-dependent tolerance develops soon after SEA injection in the spleens of both normal and TCR-transgenic mice. This suppression is dependent upon myeloid cells from the SEA-treated mice and is optimal when inducible NO synthase activity and reactive oxygen intermediates are both present. The data indicate that IFN-gamma, myeloid cells, and a combination of NO and reactive oxygen intermediates all contribute to a common pathway of T cell death that targets activated or responding CD4 T cells. Sorted Gr-1(+) cells from SEA-treated mice also directly suppress the response of naive CD4 T cells in mixed cultures, indicating that this tolerance mechanism may play a role in down-regulating other vigorous immune responses.

On the mechanisms of T cell silencing by IL-10 DNA: direct and indirect inhibition of T cell functions

Previously we reported that mucosal IL-10 DNA administration resulted in long-term suppression of virus-induced inflammatory responses by silencing Th1-type CD4+ T cell functions. However, the mechanism by which IL-10 silences the activity of CD4+ T cells was not clear. The present report has shown that mucosal IL-10 DNA administration led to the reduction of reactivity of T cells following TCR stimulation. IL-10 DNA also downregulated APC functions to stimulate T cells but the effect was temporary. Bystander suppression, including that of IL-10 producing regulatory cells, appeared not to be directly involved in the inhibition of T cell reactivity because both anti-IL-10 and anti-IL-10R could not block the suppression of T cell functions. This silenced state could be maintained following adoptive transfer to untreated animals. The nature of the silencing appears to be a reversible anergic state since Ag stimulation in the presence of exogenous IL-2 restored T cell reactivity. Furthermore, IL-10-induced silenced T cells could be induced in vitro by culturing the T cells with rIL-10 in the presence or the absence of antigen stimulation. This state persisted in the absence of rIL-10 and persisted for at least 3 days. A more notable effect, however, was observed when the T cells were incubated with IL-10 in the presence of APC and Ag. These results indicate that IL-10 induced a long-term silenced state in T cells by direct and indirect inhibition of T cell functions.

Suppressor T cells regulate the nonanergic cell population that remains after peripheral tolerance is induced to the Mls-1 antigen in T cell receptor Vbeta 8.1 transgenic mice

We have found suppressor T cells that inhibit the proliferative response of naive CD4(+) T cells in T cell receptor (TCR) Vbeta8.1 transgenic mice rendered tolerant in vivo by inoculation of Mls-1(a)-positive cells. This suppression was mediated by CD4(+) T cells but not by CD8(+) T cells or double-negative (DN) cells, and splenic CD4(+) T cells from tolerant mice displayed a greater suppression than lymph node CD4(+) T cells. Cell contact was required for efficient suppression, and known inhibitory cytokines such as IL-4, IL-10, and transforming growth factor beta were not involved. Suppressor T cells inhibited IL-2 production by naive CD4(+) T cells, and the addition of exogenous IL-2 diminished the suppressed activity while having little activity on tolerant T cells. Suppression was abolished by the elimination of CD25(+) T cells in the tolerant CD4(+) T cell subset. CD25(+)CD4(+) T cells suppressed the proliferative response of the residual fraction of the nonanergic population, namely, 6C10(+)CD4(+) T cells still present in the tolerant mice. However, 6C10(-)CD4(+) T cells still had reduced reactivity to Mls-1(a) even after CD25(+)CD4(+) T cells were removed and exogenous IL-2 was added. Suppressor cells appear to affect only residual nonanergic cells in situ, thereby facilitating the maintenance of the unresponsive state in vivo. These data provide a framework for understanding suppressor T cells and explain the difficulties and variables in defining their activity in other systems, because suppressor T cells apparently control only a small population of nonanergic cells in the periphery and may be viewed as a homeostatic mechanism.

Inducing tolerance by intranasal administration of long peptides in naive and primed CBA/J mice

To assess the capacity of a peptide-based immunotherapy to induce systemic tolerance via the nasal route, we designed three long overlapping peptides of 44-60 aa covering the entire sequence of phospholipase A2 (PLA2), a major bee venom allergen. Both prophylactic and therapeutic intranasal administrations of long peptides to PLA2-hypersensitive CBA/J mice induced specific T cell tolerance to the native allergen. In prophylactic conditions, this tolerance was marked by a suppression of subsequent specific IgE response, whereas the therapeutic approach in presensitized mice induced a more than 60% decrease in PLA2-specific IgE. This decline was associated with a shift in the cytokine response toward a Th1 profile, as demonstrated by decreased PLA2-specific IgG1 and enhanced IgG2a levels, and by a decline in the specific IL-4/IFN-gamma ratios. T cell transfer from long peptide-tolerized mice to naive animals abrogated the expected anti-PLA2 IgE and IgG1 Ab response, as well as specific T cell proliferation, but enhanced specific IgG2a response upon sensitization with PLA2. These events were strongly suggestive of a clonal anergy affecting more profoundly Th2 than the Th1 subsets. In conclusion, these results demonstrate that allergen-derived long peptides delivered via the nasal mucosa may offer an alternative to immunotherapy with native allergens without the inherent risk of systemic anaphylactic reactions. Moreover, long peptides, in contrast to immunotherapy strategies based on short peptides, have the advantage of covering all potential T cell epitopes, and may represent novel and safe tools for the therapy of allergic diseases.

Cutting edge: cell autonomous rather than environmental factors control bacterial superantigen-induced T cell anergy in vivo

Anergic T cells display a marked decrease in their ability to produce IL-2 and to proliferate in the presence of an appropriate antigenic signal. Two nonmutually exclusive classes of models have been proposed to explain the persistence of T cell anergy in vivo. While some reports indicate that anergic T cells have intrinsic defects in signaling pathways or transcriptional activities, other studies suggest that anergy is maintained by environmental "suppressor" factors such as cytokines or Abs. To distinguish between these conflicting hypotheses, we employed the well-characterized bacterial superantigen model system to evaluate in vivo the ability of a trace population of adoptively transferred naive or anergized T cells to proliferate in a naive vs anergic environment upon subsequent challenge. Our data clearly demonstrate that bacterial superantigen-induced T cell anergy is cell autonomous and independent of environmental factors.

T-regulatory 1 cells: a novel subset of CD4 T cells with immunoregulatory properties

Clonal deletion and clonal anergy are well established mechanisms of peripheral tolerance. A role has also been described for clonal suppression by regulatory cells in the induction of peripheral tolerance to a variety of antigens. However, it has been difficult to isolate regulatory cells and to define their mechanism of action. We have recently reported the in vitro generation and characterization of a novel subset of CD4(+) T cells that have regulatory properties and are able to suppress antigen-specific immune responses in vitro and in vivo. These T-regulatory 1 (Tr1) cells are defined by their unique profile of cytokine production and make high levels of IL-10 and TGF-beta, but no IL-4 or IL-2. The IL-10 and TGF-beta produced by these cells mediate the inhibition of primary naive T cells in vitro. There is also evidence that Tr1 cells exist in vivo, and we have documented the presence of high IL-10-producing CD4(+) T cells in patients with severe combined immunodeficiency who have received allogeneic stem-cell transplants. These findings support the notion that Tr1 cells are involved in the regulation of peripheral tolerance and that they could potentially be used as a cellular therapy to modulate immune responses in vivo.

A self-hsp60 peptide acts as a partial agonist inducing expression of B7-2 on mycobacterial hsp60-specific T cells: a possible mechanism for inhibitory T cell regulation of adjuvant arthritis?

We previously reported that resistance to the induction of adjuvant arthritis after preimmunization with mycobacterial hsp60 was mediated by T cells recognizing a conserved epitope (M256-270) of mycobacterial hsp60. These T cells were cross-reactive with the homologous rat hsp60 peptide sequence and the natural self-epitope on stressed antigen-presenting cells. Recognition of peptide M256-265, the conserved core of peptide M256-270, was shown to be essential for the generation of self-reactive T cells. The rat homologue of peptide M256-265, peptide R256-265, differs with three conservative amino acid substitutions from the mycobacterial core peptide. Thus peptide R256-265 could act as an altered peptide ligand with the potential of inducing a different functional phenotype in M256-270-specific T cells. We now show that peptide R256-265 was recognized by M256-270-specific T cells as a partial agonist, inducing TCR down-regulation and up-regulation of activation/adhesion molecules in the absence of proliferative responses. Peptide R256-265 did not induce anergy but induced B7-2 (but not B7-1) expression on M256-270-specific T cells, as opposed to the mycobacterial peptide, which preferentially induced B7-1. These effects were more pronounced at low peptide concentrations. Therefore also in vivo at the more relevant low physiological level of expression, the self-hsp could induce such phenotype. It is discussed how this selective up-regulation of B7-2 expression on (self-hsp60) autoreactive T cells might be a way by which destructive autoimmune responses are controlled.

Delta-9-tetrahydrocannabinol inhibits antitumor immunity by a CB2 receptor-mediated, cytokine-dependent pathway

In this study, we show that Delta-9-tetrahydrocannabinol (THC), the major psychoactive component of marijuana, suppresses host immune reactivity against lung cancer. In two different weakly immunogenic murine lung cancer models, intermittent administration of THC (5 mg/kg, four times/wk i.p. for 4 wk) led to accelerated growth of tumor implants compared with treatment with diluent alone. In contrast to our findings in immunocompetent mice, THC did not affect tumor growth in tumor-bearing SCID mice. The immune inhibitory cytokines, IL-10 and TGF-beta, were augmented, while IFN-gamma was down-regulated at both the tumor site and in the spleens of THC-treated mice. Administration of either anti-IL-10- or anti-TGF-beta-neutralizing Abs prevented the THC-induced enhancement in tumor growth. Both APC and T cells from THC-treated mice showed limited capacities to generate alloreactivity. Furthermore, lymphocytes from THC-treated mice transferred the effect to normal mice, resulting in accelerated tumor growth similar to that seen in the THC-treated mice. THC decreased tumor immunogenicity, as indicated by the limited capacity for tumor-immunized, THC-treated mice to withstand tumor rechallenge. In vivo administration of a specific antagonist of the CB2 cannabinoid receptor also blocked the effects of THC. Our findings suggest the THC promotes tumor growth by inhibiting antitumor immunity by a CB2 receptor-mediated, cytokine-dependent pathway.

'Anergic' T cells modulate the T-cell activating capacity of antigen-presenting cells

Nowadays there is compelling evidence for immunoregulation by T cells. Recently, we showed that so-called 'anergic' T cells are not functionally inert but can act as regulatory cells by actively suppressing other T cell responses. We now show that 'anergic' T cells mediate this suppressive effect via modulation of the T-cell activating capacity of the antigen-presenting cell (APC). Upon removal of the 'anergic' T cells, the suppressive APC phenotype persisted, indicating that 'anergic' T cells conditioned the APC to become a mediator of T cell suppression. The inhibitory signal delivered by 'anergic' T cells depended on the presence of the cognate ligand for the 'anergic' T cell, and appeared to be dominant since previously activated APC were rendered inhibitory as well. These findings imply that APC upon cross-talk with T cells can adopt distinct functional phenotypes ranging from T-cell stimulatory to T-cell suppressive. The contribution of 'anergic' T cells to the functional tuning of APC offers an explanation for the maintenance of 'anergic' T cells in the repertoire, and for their role in immunoregulation.

Host resistance and immune deviation in pigeon cytochrome c T-cell receptor transgenic mice infected with Toxoplasma gondii

Resistance to Toxoplasma gondii has been shown to be mediated by gamma interferon (IFN-gamma) produced by NK, CD4(+), and CD8(+) T cells. While studies of SCID mice have implicated NK cells as the source of the cytokine in acute infection, several lines of evidence suggest that IFN-gamma production by CD4(+) T lymphocytes also plays an important role in controlling early parasite growth. To evaluate whether this function is due to nonspecific as opposed to T-cell receptor (TCR)-dependent stimulation by the parasite, we have examined the resistance to T. gondii infection of pigeon cytochrome c transgenic (PCC-Tg) Rag-2(-/-) mice in which all CD4(+) T lymphocytes are unreactive with the protozoan. When inoculated with the ME49 strain, PCC-Tg animals exhibited only temporary control of acute infection and succumbed by day 17. Intracellular cytokine staining by flow cytometry revealed that, in contrast to infected nontransgenic controls, infected PCC-Tg animals failed to develop IFN-gamma-producing CD4(+) T cells. Moreover, the CD4(+) lymphocytes from these mice showed no evidence of activation as judged by lack of upregulated expression of CD44 or CD69. Nevertheless, when acutely infected transgenic mice were primed by PCC injection, the lymphokine responses measured after in vitro antigen restimulation displayed a strong Th1 bias which was shown to be dependent on endogenous interleukin 12 (IL-12). The above findings argue that, while T. gondii-induced IL-12 cannot trigger IFN-gamma production by CD4(+) T cells in the absence of TCR ligation, the pathogen is able to nonspecifically promote Th1 responses against nonparasite antigens, an effect that may explain the immunostimulatory properties of T. gondii infection.

IL-10-producing T cells suppress immune responses in anergic tuberculosis patients

The lethality of Mycobacterium tuberculosis remains the highest among infectious organisms and is linked to inadequate immune response of the host. Containment and cure of tuberculosis requires an effective cell-mediated immune response, and the absence, during active tuberculosis infection, of delayed-type hypersensitivity (DTH) responses to mycobacterial antigens, defined as anergy, is associated with poor clinical outcome. To investigate the biochemical events associated with this anergy, we screened 206 patients with pulmonary tuberculosis and identified anergic patients by their lack of dermal reactivity to tuberculin purified protein derivative (PPD). In vitro stimulation of T cells with PPD induced production of IL-10, IFN-gamma, and proliferation in PPD(+) patients, whereas cells from anergic patients produced IL-10 but not IFN-gamma and failed to proliferate in response to this treatment. Moreover, in anergic patients IL-10-producing T cells were constitutively present, and T-cell receptor-mediated (TCR-mediated) stimulation resulted in defective phosphorylation of TCRzeta and defective activation of ZAP-70 and MAPK. These results show that T-cell anergy can be induced by antigen in vivo in the intact human host and provide new insights into mechanisms by which M. tuberculosis escapes immune surveillance.

In vivo staphylococcal enterotoxin B (SEB)-primed murine splenocytes secrete mediators which suppress CD25(hi) expression and cell cycle progression of naive splenocytes in response to SEB in vitro

Administration of bacterial superantigen results in clonal activation of T cells followed by a state of hyporesponsiveness to subsequent antigen stimulation. Using a coculture system, we showed that the splenocytes from staphylococcal enterotoxin B (SEB)-injected BALB/c mice suppressed the proliferative response of naive splenocytes to SEB stimulation. The suppressive effect also occurred in Fas-deficient MRL-lpr/lpr mice. When naive responder cells were separated by a semipermeable membrane from SEB-primed effector cells, the suppressive effect remained apparent. The hyporesponsiveness of responder cells did not result from excessive induction of apoptosis, but rather from prevention of entering the S and G2/M phases of the cell cycle. The IL-2 levels in culture supernatants were low with the presence of SEB-primed effector cells. However, addition of IL-2 to the cocultures only partially reversed the inhibitory effect. Further studies revealed a reduced level of the CD25(hi) subpopulation in responder cells when cultured in the transwell with the presence of SEB-primed effector cells compared to that with saline-primed controls. This inhibitory effect was not observed for SEB-induced activation of CD25(int) and CD69 expression. Taken together, using a transwell culture system, we show in this study an inhibition of CD25(hi) expression and cell cycle arrest in target cells, which may serve at least in part the mechanisms of SEB-induced hyporesponsiveness.

Induction of T cell anergy in the absence of CTLA-4/B7 interaction

Immunologic tolerance in T lymphocytes is maintained through both thymic and peripheral contributions. One peripheral tolerance mechanism is the induction of T cell anergy, a form of nonresponsiveness resulting from incomplete T cell activation, such as stimulation through the TCR in the absence of costimulation. Recent reports have suggested that engagement of the inhibitory receptor CTLA-4 by its B7 ligand is critical for the initiation of anergy. We tested the importance of CTLA-4 in anergy induction in primary T cells with an in vitro anergy system. Using both CTLA-4/B7-blocking agents and CTLA-4-deficient T cells, we found that T cell anergy can be established in the absence of CTLA-4 expression and/or function. Even in the absence of CTLA-4 signal transduction, T cells activated solely through TCR ligation lose the ability to proliferate as a result of autocrine IL-2 production upon subsequent receptor engagement. Thus, CTLA-4 signaling is not required for the development of T cell anergy.

Analysis of the requirements for the induction of CD4+ T cell alloantigen hyporesponsiveness by ex vivo anti-CD40 ligand antibody

A major goal of the transplant field is to selectively tolerize only those donor T cells recognizing host alloantigen and mediating graft-vs-host disease (GVHD). Recently, we described an ex vivo approach in which the blockade of the CD40 ligand (CD40L):CD40 costimulatory pathway in bulk MLR cultures induces donor CD4+ T cells to become specifically tolerant to MHC class II-disparate alloantigenic-bearing stimulators, resulting in a profound reduction in GVHD generation in vivo. In studies presented in this work, we investigated the ex vivo requirements for tolerance induction. We found that CD4+ T cells become profoundly more hyporesponsive to alloantigen restimulation with prolonged culture duration such that 7 to 10 but not 4 days is needed to achieve maximum alloantigen hyporesponsiveness as assessed in secondary MLR cultures and GVHD generation. By day 7, both primed and tolerized cells had substantially increased blastogenesis and CD25 expression. Primed but not tolerized cells substantially down-regulated L-selectin expression, indicating that the tolerized cells do not become fully Ag experienced. Both Th1 and Th2 cytokine production is severely impaired by CD40L:CD40 blockade. Analysis of culture supernatants and results from IL-4 and IL-10 knockout mice indicated that GVHD prevention was not mediated by a skewing toward a Th2 phenotype. The addition of IL-4 to the cultures as a survival factor precluded the induction of tolerance in the anti-CD40L-cultured cells. These data provide further impetus for the ex vivo use of anti-CD40L mAb to block GVHD generation.