Interleukin-15 redirects the outcome of a tolerizing T-cell stimulus from apoptosis to anergy

Identification of IFN-γ-producing T cells as the main mediators of the side effects associated to mouse interleukin-15 sustained exposure

Interleukin-15 (IL-15) is a cell growth-factor that regulates lymphocyte function and homeostasis. Its strong immunostimulatory activity coupled with an apparent lack of toxicity makes IL-15 an exciting candidate for cancer therapy, somehow limited by its short half-life in circulation. To increase IL-15 bioavailability we constructed a recombinant adeno-associated vector expressing murine IL-15 (AAV-mIL15) in the liver. Mice injected with AAV-mIL15 showed sustained and vector dose-dependent levels of IL-15/IL-15Rα complexes in serum, production of IFN-γ and activation of CD8+ T-cells and macrophages. The antitumoral efficacy of AAV-mIL15 was tested in a mouse model of metastatic colorectal cancer established by injection of MC38 cells. AAV-mIL15 treatment slightly inhibits MC38 tumor-growth and significantly increases the survival of mice. However, mIL-15 sustained expression was associated with development of side effects like hepatosplenomegaly, liver damage and the development of haematological stress, which results in the expansion of hematopoietic precursors in the bone marrow. To elucidate the mechanism, we treated IFN-γ receptor-, RAG1-, CD1d- and µMT-deficient mice and performed adoptive transfer of bone marrow cells from WT mice to RAG1-defcient mice. We demonstrated that the side effects of murine IL-15 administration were mainly mediated by IFN-γ-producing T-cells.

CONCLUSIONS

IL-15 induces the activation and survival of effector immune cells that are necessary for its antitumoral activity; but, long-term exposure to IL-15 is associated with the development of important side effects mainly mediated by IFN-γ-producing T-cells. Strategies to modulate T-cell activation should be combined with IL-15 administration to reduce secondary adverse events while maintaining its antitumoral effect.

Expression of IL-15RA or an IL-15/IL-15RA fusion on CD8+ T cells modifies adoptively transferred T-cell function in cis

IL-15 and IL-15 receptor alpha (IL-15RA) play a significant role in multiple aspects of T-cell biology. However, given the evidence that IL-15RA can present IL-15 in trans, the functional capacity of IL-15RA expressed on CD8(+) T cells to modify IL-15 functions in cis is currently unclear. In the current study, we explore the functional consequences of IL-15RA, expression on T cells using a novel method to transfect naive CD8(+) T cells. We observed that RNA nucleofection led to highly efficient, non-toxic, and rapid manipulation of protein expression levels in unstimulated CD8(+) T cells. We found that transfection of unstimulated CD8(+) T cells with IL-15RA RNA led to enhanced viability of CD8(+) T cells in response to IL-15. Transfection with IL-15RA enhanced IL-15-mediated phosphorylation of STAT5 and also promoted IL-15-mediated proliferation in vivo of adoptively transferred naïve CD8(+) T cells. We demonstrated that IL-15RA can present IL-15 via cis-presentation on CD8(+) T cells. Finally, we showed that transfection with a chimeric construct linking IL-15 to IL-15RA cell autonomously enhances the viability and proliferation of primary CD8(+) T cells and cytotoxic potential of antigen-specific CD8(+) T cells. The clinical implications of the current study are discussed.

IL-15Ralpha is a negative regulator of TCR-activated proliferation in CD4+ T cells

Although IL-15 is known to be a T cell growth factor, the function in T cells of IL-15Ralpha, its high affinity receptor, remains unclear. We found that murine IL-15Ralpha(-/-) CD4(+) T cells hyperproliferated in response to TCR stimulation, in vitro and in vivo, and displayed a lower TCR activation threshold than wild-type CD4(+) T cells. TCR-induced activation of Zap70 and of the phospholipase C-gamma1-NFATp, Ras-ERK-c-Fos, and Rac-JNK-c-Jun pathways was all augmented in IL-15Ralpha(-/-) CD4(+) T cells. This in turn led to earlier IL-2Ralpha induction and higher IL-2 production, which most likely contribute to the hyperproliferation of IL-15Ralpha(-/-) CD4(+) T cells. Exogenous IL-15 reduced levels of TCR-activated signals, transcription factors, IL-2, and IL-2Ralpha, and division in wild-type CD4(+) T cells. These results reveal IL-15Ralpha to be a negative regulator for CD4(+) T cell activation and demonstrate a novel layer of regulation of TCR signaling by a cytokine system.

IL-15 and cognate antigen successfully expand de novo-induced human antigen-specific regulatory CD4+ T cells that require antigen-specific activation for suppression

An important prerequisite in using regulatory T cells for immunotherapy is their ex vivo expansion without loss of suppressor function. Human anergic regulatory T cells are expandable by Ag-specific stimulation in the presence of IL-2. IL-15, like IL-2, is a T cell growth factor that, in contrast to IL-2, stimulates survival of T cells. In this study, we examined whether IL-15 could be exploited as a superior growth factor of human CD4(+) anergic regulatory T cells that were generated by costimulation blockade. Next, IL-15, as compared with IL-2, was investigated with respect to expansion and function of these regulatory T cells. Optimal expansion required cognate allogeneic stimulation in the presence of exogenous IL-15. IL-15 resulted in enhanced survival that was paralleled by an increased number of Bcl-2-expressing cells. Moreover, IL-15 induced a distinct type of anergy characterized by hyperreactivity to IL-15, resulting in improved expansion. This is likely attributed to increased propensity of these cells to up-regulate both alpha- and gamma-chains of the IL-2 and IL-15 receptor. Notably, IL-15-expanded regulatory CD4(+) T cells suppressed both naive and memory T cells in a superior way. Immunosuppression required alloantigen-specific stimulation and appeared gamma-irradiation resistant and independent of IL-10, TGFbeta, or CTLA-4 interactions. These regulatory T cells were stable suppressors, mediating bystander suppression upon TCR stimulation, but leaving recall responses unaffected in the absence of cognate Ag. Finally, human naturally occurring regulatory CD4(+)CD25(+) T cells appeared important in generating regulatory T cells by costimulation blockade. In conclusion, IL-15-expanded, de novo-induced human anergic regulatory CD4(+) T cells are of interest in Ag-specific immunotherapy.

In vitro dendritic cell-induced T cell responses to B cell chronic lymphocytic leukaemia enhanced by IL-15 and dendritic cell-B-CLL electrofusion hybrids

HLA class II-restricted proliferative and cytotoxic T cell (CTL) responses to B cell chronic lymphocytic leukaemia (B-CLL) can be generated using autologous dendritic cells (DCs) pulsed with tumour cell lysate. In this study a number of different approaches were used to optimize further the in vitro system. First, the effects of a variety of maturation agents were studied. The addition of TNF-alpha, polyriboinosinic polyribocytidylic acid (Poly(I:C)) and LPS to autologous DCs resulted in the emergence of only a small percentage of CD83+ DCs, IFN-alpha having no demonstrable effect. Only the addition of Poly(I:C) to DCs resulted in modestly increased specific cytotoxicity to B-CLL targets, IFN-alpha and LPS having no effect. Secondly, T cells were pretreated with IL-15, prior to culturing with lysate-pulsed autologous DCs. A significant increase in T cell activation (P = 0.038), IFN-gamma secretion (P = 0.030) and specific cytotoxicity to B-CLL targets (P = 0.006) was demonstrated compared to untreated T cells. Thirdly, monocyte derived DCs electrofused with B-CLL B cells were compared with lysate-pulsed DCs. T cells stimulated by fused DCs generated higher levels of specific cytotoxicity to autologous B-CLL B cell targets than those stimulated by lysate pulsed DCs (P = 0.013). Blocking studies demonstrated inhibition of this cytotoxicity by both anti-CD4 (P = 0.062) and anti-CD8 monoclonal antibodies (P = 0.018), suggesting the generation of both HLA class I- and HLA class II-restricted CTL responses. In summary, in vitro B-CLL-specific T cell responses can be enhanced further by preincubating T cells with IL-15 and using autologous fused DC-B-CLL hybrids instead of autologous lysate-pulsed DCs. These preliminary data require confirmation with larger numbers of patients. Such an approach, however, may eventually provide effective immunotherapy for treatment of B-CLL.

Induction of IL-15 by TCR/CD3 aggregation depends on IFN-gamma and protects against apoptosis of immature thymocytes in vivo

TCR/CD3 aggregation by injection of anti-CD3 Ab produces T cell activation, release of cytokines such as IFN-gamma, and apoptosis in the cortical region of the thymus. We show that anti-CD3 Ab induces IL-15 mRNA in spleens of wild-type but not IFN-gamma receptor-knock-out (IFN-gammaR KO) mice. The loss of IL-15 mRNA induction in IFN-gammaR KO mice was associated with increased thymocyte apoptosis. Pretreatment of wild-type mice with neutralizing anti-IL-15 Ab increased the anti-CD3-triggered thymocyte apoptosis, thus mimicking the sensitive phenotype of IFN-gammaR KO mice. Inversely, anti-CD3-induced apoptosis in IFN-gammaR KO mice was suppressed by administration of recombinant IL-15. In IFN-gammaR KO mice and in wild-type mice that were treated with anti-IL-15, augmented apoptosis affected mainly CD4+CD8+ immature thymocytes. IL-15 as well as IL-15Ralpha mRNA expression in thymocytes was not increased by anti-CD3. These data demonstrate that systemic IL-15 exerts anti-apoptotic activity on immature T cells and establish a regulatory mechanism whereby TCR/CD3 engagement induces IL-15 expression via an IFN-gamma-dependent pathway. The self-amplifying nature of this IFN-gamma/IL-15 connection may constitute a regulatory pathway in central tolerance to self.

Inhibition of the IL-15 pathway in anti-CD25 mAb treated renal allograft recipients

Anti-CD25 mAb's are used for prophylaxis of rejection in allograft transplantation. These agents target the alpha-chain, part of the IL-2Ralphabetagamma complex. The beta- and gamma-chain are signaling components that are not specific for IL-2. The T-cell growth factors IL2, IL-7 and IL-15 utilize the gamma-chain and IL-2 and IL-15 share the beta-chain. We have studied the consequences of targeting the IL-2R alpha-chain with the anti-CD25 mAb basiliximab by measuring the IL-2R alphabetagamma expression levels and the IL-2, IL-7 and IL-15 driven proliferation. By flowcytometry and limiting dilution analysis, the IL-2R complex was analyzed in peripheral blood samples from renal allograft recipients (n = 29) who received basiliximab (20 mg IV bolus on day 0 and 4), cyclosporin and mycophenolate mofetil. In peripheral blood, after induction therapy with basiliximab, no CD25 positive T-cells were detectable for 61 days (median, range 25-93 days). When CD25 cells were covered with basiliximab, the percentage and the mean fluorescence intensity (MFI) of IL-2Rbeta positive T-cells significantly decreased, P = 0.02 and P = 0.013, respectively, whereas the expression level of the IL-2Rgamma was not affected. The inhibition of the expression of the IL-2R alpha- and beta-chain had significant consequences for the function of these cells. Both the IL-2- and the IL-15-dependent proliferation were inhibited, P = 0.007 and P = 0.01, respectively. The control, the IL-7 driven proliferation, was not influenced by basiliximab. In conclusion, therapy with anti-CD25 mAb's affect T-cell-dependent allogeneic immune responses, not only by blocking IL-2 signaling but also by impairing IL-15 signaling through downregulating the IL-2/IL-15 receptor beta-chain.

Immune reconstitution after autologous peripheral blood progenitor cell transplantation: effect of interleukin-15 on T-cell survival and effector functions

OBJECTIVE

The aim of this study was to evaluate the occurrence of T-cell spontaneous apoptosis (A(spont)) and its modulation in vitro by the interleukin-2 receptor (IL-2R) gamma-chain (gammac)-signaling cytokine IL-15 in patients transplanted with autologous peripheral blood progenitor cells (PBPC) for hematologic malignancies.

MATERIALS AND METHODS

Patients were examined on days 30-60, 60-90, and 90-120 after PBPC infusion. Dissipation of mitochondrial transmembrane potential, a hallmark of T-cell apoptosis, has been detected using the fluorescent probe 3,3'-dihexyloxacarbocyanine iodide, after short-term T-cell culture in the absence or presence of exogenous cytokines. Expression of Bcl-2 family members has been studied by flow cytometry and reverse transcriptase polymerase chain reaction. T-cell proliferative responses to recall antigens have been estimated in autologous mixed leukocyte cultures.

RESULTS

A(spont) was seen in 45% +/- 6% of CD4(+) and 55% +/- 6% of CD8(+) T cells cultured in the absence of cytokines. Of interest, IL-15 and, to a lesser extent, its structural cousin IL-2 counteracted T-cell A(spont) by inhibiting the processing of caspase-3 and up-regulating Bcl-2 mRNA and protein levels. Cell division tracking confirmed that IL-15 did not rescue T cells from A(spont) by promoting proliferation but rather acted as a genuine survival factor. Addition of a gammac-blocking antibody to cytokine-conditioned cultures abrogated both apoptosis inhibition and Bcl-2 induction by IL-15, suggesting involvement of the IL-2Rgammac signal transduction pathway. Whereas cytokine-unprimed posttransplant T cells mounted inadequate responses to recall antigens, T cells conditioned with IL-15 expanded vigorously, indicating restoration of antigen-specific proliferation.

CONCLUSIONS

T cells recovering after autologous PBPC transplantation are highly susceptible to spontaneous apoptosis in vitro. This phenomenon can be counteracted by the gammac-signaling cytokine IL-15. These findings suggest that IL-15 might be a promising immunomodulating agent to improve postgrafting T-cell function.

Exogenous interleukin-2 administration corrects the cell cycle perturbation of lymphocytes from human immunodeficiency virus-infected individuals

Human immunodeficiency virus (HIV)-induced immunodeficiency is characterized by progressive loss of CD4(+) T cells associated with functional abnormalities of the surviving lymphocytes. Increased susceptibility to apoptosis and loss of proper cell cycle control can be observed in lymphocytes from HIV-infected individuals and may contribute to the lymphocyte dysfunction of AIDS patients. To better understand the relation between T-cell activation, apoptosis, and cell cycle perturbation, we studied the effect of exogenous interleukin-2 (IL-2) administration on the intracellular turnover of phase-dependent proteins. Circulating T cells from HIV-infected patients display a marked discrepancy between a metabolic profile typical of G(0) and a pattern of expression of phase-dependent proteins that indicates a more-advanced position within the cell cycle. This discrepancy is enhanced by in vitro activation with ConA and ultimately results in a marked increase of apoptotic events. Conversely, treatment of lymphocytes with IL-2 alone restores the phase-specific pattern of expression of cell cycle-dependent proteins and is associated with low levels of apoptosis. Interestingly, exogenous IL-2 administration normalizes the overall intracellular protein turnover, as measured by protein synthesis, half-life of newly synthesised proteins, and total protein ubiquitination, thus providing a possible explanation for the effect of IL-2 on the intracellular kinetics of cell cycle-dependent proteins. The beneficial effect of IL-2 administration is consistent with the possibility of defective IL-2 function in vivo, which is confirmed by the observation that lymphocytes from HIV-infected patients show abnormal endogenous IL-2 paracrine/autocrine function upon in vitro mitogen stimulation. Overall these results confirm that perturbation of cell cycle control contributes to HIV-related lymphocyte dysfunction and, by showing that IL-2 administration can revert this perturbation, suggest a new mechanism of action of IL-2 therapy in HIV-infected patients.

Alterations in transcription factor binding at the IL-2 promoter region in anergized human CD4+ T lymphocytes

BACKGROUND

The mechanisms responsible for the induction of clonal anergy are not well understood. We have utilized an in vitro model of human T cell anergy to explore the perturbations in cell signaling at the level of interleukin (IL)-2 gene transcription and to define the contribution of other cytokines to this effect.

METHODS

An in vitro model of clonal anergy was established by using CD4+ T lymphocytes from healthy human donors. Cells were anergized by prestimulation with an anti-CD3 monoclonal antibody (mAb) followed by restimulation 72 hr later with anti-CD3 mAb with or without anti-CD28.

RESULTS

CD4+ T cells, anergized with anti-CD3 monoclonal antibody (OKT3) prestimulation, displayed a marked reduction in proliferation (P=0.0036) and IL-2 production (P<0.0001). Co-incubation with IL-10 reduced cellular proliferation in OKT3/CD28 pretreated cells by 19% (P=NS) and reduced IL-2 production by 40% (P=0.0024). Anergized T cells demonstrated a reduced binding activity of the AP-1 complex to the IL-2 promoter. Supershift experiments and Western blots confirmed that the binding of c-Fos, JunB, and JunD, but not of FosB, was reduced in anergized cells. At the sis-inducible element (SIE)-binding region of the c-Fos promoter, Stat3 binding was reduced.

CONCLUSIONS

T cell anergy, induced by prestimulation with OKT3, is characterized by reduced proliferation and a profound decrease in IL-2 production. Anergy can be prevented by co-incubation with anti-CD28 and partially re-established by IL-10. Anergy is accompanied by a reduction in AP-1 binding to the IL-2 promoter, with selective reduction in binding of c-Fos, JunB, and JunD. Defective binding for Stat3 at the c-Fos promoter suggests an involvement of the Jak-Stat pathway.

The CD7(-) subset of CD4(+) memory T cells is prone to accelerated apoptosis that is prevented by interleukin-15 (IL-15)

The CD7(-) subset of CD4(+) T cells reflects a stable differentiation state of post-thymic helper T cells with CD45R0(+)CD45RA(-) 'memory' phenotype. Here we report that CD4(+)CD7(-) T cells are prone to increased spontaneous apoptosis in vitro compared to CD4(+)CD7(+) T cells. Spontaneous apoptosis is prevented by IL-15, but not by IL-2. Moreover, IL-15 increases Bcl-2 and decreases CD95/Fas expression of CD7(-), but not of CD7(+) T cells. Because IL-15 is physiologically not secreted but expressed in a membrane-bound form, we cocultured T cells with TNF-alpha stimulated fibroblasts that expose membrane IL-15. TNF-alpha stimulated fibroblasts rescue CD4(+)CD7(-) T cells from apoptosis whereas unstimulated fibroblasts do not. Rescue from apoptosis requires cell-cell contact and is abolished by addition of neutralizing antibodies to IL-15. We conclude that membrane IL-15 prevents accelerated apoptosis of CD4(+)CD7(-) T cells. This mechanism may contribute to accumulation of CD7(-) T cells in chronic inflammatory skin lesions.

Review article: molecular signals and genetic reprogramming in peripheral T-cell differentiation

Rearrangement of gene segments occurs in T lymphocytes during thymic development as the T-cell receptor (TCR) is first expressed, allowing T cells to become central regulators of antigen specificity in the acquired immune system. However, further development of T cells occurs after population of peripheral lymphoid tissues, which can result in T-cell expansion and differentiation into effectors of various immune function, or progression to memory T cells, anergic cells or death by apoptosis. This review focuses on more recent developments concerning the choices that peripheral T cells make between first encountering antigen through TCR recognition and death. These decisions are associated with a process of genetic reprogramming that alters the behaviour of cells so that immune responses are appropriately regulated.