Guillaume Oldenhove

CD4+ CD25+ Regulatory T Cells Control T Helper Cell Type 1 Responses to Foreign Antigens Induced by Mature Dendritic Cells In Vivo

Recent evidence suggests that in addition to their well known stimulatory properties, dendritic cells (DCs) may play a major role in peripheral tolerance. It is still unclear whether a distinct subtype or activation status of DC exists that promotes the differentiation of suppressor rather than effector T cells from naive precursors. In this work, we tested whether the naturally occurring CD4+ CD25+ regulatory T cells (Treg) may control immune responses induced by DCs in vivo. We characterized the immune response induced by adoptive transfer of antigen-pulsed mature DCs into mice depleted or not of CD25+ cells. We found that the development of major histocompatibility complex class I and II–restricted interferon γ–producing cells was consistently enhanced in the absence of Treg. By contrast, T helper cell (Th)2 priming was down-regulated in the same conditions. This regulation was independent of interleukin 10 production by DCs. Of note, splenic DCs incubated in vitro with Toll-like receptor ligands (lipopolysaccharide or CpG) activated immune responses that remained sensitive to Treg function. Our data further show that mature DCs induced higher cytotoxic activity in CD25-depleted recipients as compared with untreated hosts. We conclude that Treg naturally exert a negative feedback mechanism on Th1-type responses induced by mature DCs in vivo.

Endotoxin-Induced Myeloid-Derived Suppressor Cells Inhibit Alloimmune Responses via Heme Oxygenase-1

Inflammation and cancer are associated with impairment of T‐cell responses by a heterogeneous population of myeloid‐derived suppressor cells (MDSCs) coexpressing CD11b and GR‐1 antigens. MDSCs have been recently implicated in costimulation blockade‐induced transplantation tolerance in rats, which was under the control of inducible NO synthase (iNOS). Herein, we describe CD11b+GR‐1+MDSC‐compatible cells appearing after repetitive injections of lipopolysaccharide (LPS) using a unique mechanism of suppression. These cells suppressed T‐cell proliferation and Th1 and Th2 cytokine production in both mixed lymphocyte reaction and polyclonal stimulation assays. Transfer of CD11b+ cells from LPS‐treated mice in untreated recipients significantly prolonged skin allograft survival. They produced large amounts of IL‐10 and expressed heme oxygenase‐1 (HO‐1), a stress‐responsive enzyme endowed with immunoregulatory and cytoprotective properties not previously associated with MDSC activity. HO‐1 inhibition by the specific inhibitor, SnPP, completely abolished T‐cell suppression and IL‐10 production. In contrast, neither iNOS nor arginase 1 inhibition did affect suppression. Importantly, HO‐1 inhibition before CD11b+ cell transfer prevented the delay of allograft rejection revealing a new MDSC‐associated suppressor mechanism relevant for transplantation.

African Trypanosomiasis: Naturally Occurring Regulatory T Cells Favor Trypanotolerance by Limiting Pathology Associated with Sustained Type 1 Inflammation

Tolerance to African trypanosomes requires the production of IFN-γ in the early stage of infection that triggers the development of classically activated macrophages controlling parasite growth. However, once the first peak of parasitemia has been controlled, down-regulation of the type 1 immune response has been described. In this study, we have evaluated whether regulatory T cells (Tregs) contribute to the limitation of the immune response occurring during Trypanosoma congolense infection and hereby influence the outcome of the disease in trypanotolerant C57BL/6 host. Our data show that Foxp3+ Tregs originating from the naturally occurring Treg pool expanded in the spleen and the liver of infected mice. These cells produced IL-10 and limited the production of IFN-γ by CD4+ and CD8+ effector T cells. Tregs also down-regulated classical activation of macrophages resulting in reduced TNF-α production. The Treg-mediated suppression of the type 1 inflammatory immune response did not hamper parasite clearance, but was beneficial for the host survival by limiting the tissue damages, including liver injury. Collectively, these data suggest a cardinal role for naturally occurring Tregs in the development of a trypanotolerant phenotype during African trypanosomiasis.

CD4+ CD25+ regulatory T cells control the magnitude of T-dependent humoral immune responses to exogenous antigens.

CD4+CD25+ T reg cells are critical for peripheral tolerance and prevention of autoimmunity. Here we show that CD4+CD25+ T reg also regulate the magnitude of humoral responses against a panel of T‐dependent antigens of foreign origin during both primary and secondary immune responses. Depletion of CD4+CD25+ T cells leads to increased antigen‐specific antibody production and affinity maturation but does not affect T‐independent B cell responses, suggesting that CD4+CD25+ T reg exert a feedback mechanism on non‐self antigen‐specific antibody secretion by dampening the T cell help for B cell activation. Moreover, we show that CD4+CD25+ T reg also suppress in vitro B cell immunoglobulin production by inhibiting CD4+CD25– T cell help delivery, and that blockade of TGF‐β activity abolishes this suppression.

Anti-CTLA-4 treatment induces IL-10-producing ICOS+ regulatory T cells displaying IDO-dependent anti-inflammatory properties in a mouse model of colitis

Background and aims: Cytotoxic T lymphocyte-associated antigen 4 (CTLA-4) has been shown to act as a negative regulator of T cell function and has been implicated in the regulation of T helper 1 (Th1)/Th2 development and the function of regulatory T cells. Tests were carried out to determine whether anti-CTLA-4 treatment would alter the polarisation of naive T cells in vivo. Methods: Mice were treated with anti-CTLA-4 monoclonal antibody (mAb) (UC10-4F10) at the time of immunisation or colonic instillation of trinitrobenzene sulfonic acid (TNBS). The cytokines produced by lymph node cells after in vitro antigenic stimulation and the role of indoleamine 2,3 dioxygenase (IDO) and of interleukin-10 (IL-10) were tested, and the survival of mice was monitored. Results: Injection of anti-CTLA-4 mAb in mice during priming induced the development of adaptive CD4+ regulatory T cells which expressed high levels of ICOS (inducible co-stimulator), secreted IL-4 and IL-10. This treatment inhibited Th1 memory responses in vivo and repressed experimental intestinal inflammation. The anti-CTLA-4-induced amelioration of disease correlated with IDO expression and infiltration of ICOShigh Foxp3+ T cells in the intestine, suggesting that anti-CTLA-4 acted indirectly through the development of regulatory T cells producing IL-10 and inducing IDO. Conclusions: These observations emphasise the synergy between IL-10 and IDO as anti-inflammatory agents and highlight anti-CTLA-4 treatment as a potential novel immunotherapeutic approach for inducing adaptive regulatory T cells.

Depending on their maturation state, splenic dendritic cells induce the differentiation of CD4 + T lymphocytes into memory and/or effector cells in vivo

There is increasing evidence that dendritic cells (DC) display opposite functions in the immune system, as they may induce immunity or tolerance depending on intrinsic and environmental factors. In mice, adoptive transfer of mature DC pulsed extracorporeally with antigen induces the development of antigen‐specific Th1‐ and Th2‐type CD4+ cells. In this work, we compared the adjuvant properties of immature (freshly isolated) and mature (cultured) splenic DC in vivo. Our data show that injection of either cell population induces the clonal expansion of CD4+ T cells but that only mature DC trigger their differentiation into effector cells producing IFN‐γ. In contrast, transfer of immature DC provokes the development of intermediates in the differentiation process, similar to the central memory cells. These observations, together with data in the literature, suggest that DC may induce tolerance, memory, or immunity depending on their maturationstate.

Cutting Edge: Hypoxia-Inducible Factor 1 Negatively Regulates Th1 Function

Tissue hypoxia can occur in physiological and pathological conditions. When O2 availability decreases, the transcription factor hypoxia-inducible factor (HIF)-1α is stabilized and regulates cellular adaptation to hypoxia. The objective of this study was to test whether HIF-1α regulates T cell fate and to define the molecular mechanisms of this control. Our data demonstrate that Th1 cells lose their capacity to produce IFN-γ when cultured under hypoxia. HIF-1α−/− Th1 cells were insensitive to hypoxia, underlining a critical role for HIF-1α. Our results point to a role for IL-10, as suggested by the increased IL-10 expression at low O2 levels and the unchanged IFN-γ production by IL-10–deficient Th1 cells stimulated in hypoxic conditions. Accordingly, STAT3 phosphorylation is increased in Th1 cells under hypoxia, leading to enhanced HIF-1α transcription, which, in turn, may inhibit suppressor of cytokine signaling 3 transcription. This positive-feedback loop reinforces STAT3 activation and downregulates Th1 responses that may cause collateral damage to the host.

Thymus-derived regulatory T cells restrain pro-inflammatory Th1 responses by downregulating CD70 on dendritic cells.

The severity and intensity of autoimmune disease in immune dysregulation, polyendocrinopathy, enteropathy, X‐linked (IPEX) patients and in scurfy mice emphasize the critical role played by thymus‐derived regulatory T cells (tTregs) in maintaining peripheral immune tolerance. However, although tTregs are critical to prevent lethal autoimmunity and excessive inflammatory responses, their suppressive mechanism remains elusive. Here, we demonstrate that tTregs selectively inhibit CD27/CD70‐dependent Th1 priming, while leaving the IL‐12‐dependent pathway unaffected. Immunized mice depleted of tTregs showed an increased response of IFN‐γ‐secreting CD4+ T cells that was strictly reliant on a functional CD27/CD70 pathway. In vitro studies revealed that tTregs downregulate CD70 from the plasma membrane of dendritic cells (DCs) in a CD27‐dependent manner. CD70 downregulation required contact between Tregs and DCs and resulted in endocytosis of CD27 and CD70 into the DC. These findings reveal a novel mechanism by which tTregs can maintain tolerance or prevent excessive, proinflammatory Th1 responses.

Hypoxia in the intestine or solid tumors: A beneficial or deleterious alarm signal?

The transcription factor hypoxia inducible factors (HIF)‐1 functions as a master regulator of oxygen homeostasis. There is increasing evidence that HIF has an essential role to prevent tissue damage in physiological and pathological situations in which cells are deprived of O2. Here, we review the effects of decreased oxygen supply on the innate and adaptive immune responses in the gut and in solid tumors in which the oxygenation profile correlates with the grade of inflammation. Data in the literature indicate that some tumors may co‐opt immune mechanisms induced by HIF‐1 to promote their survival and proliferation. By contrast, HIF‐1 stabilization would have a beneficial effect in the intestinal tract as it would dampen inflammation and promote its resolution. Therefore, stabilization of HIF‐1 in hypoxia may have opposite effects on the integrity of the host, depending on the tissue microenvironment.

Intestinal immunopathology is associated with decreased CD73-generated adenosine during lethal infection.

The ectonucleotidases CD39 and CD73 sequentially degrade the extracellular ATP pool and release immunosuppressive adenosine, thereby regulating inflammatory responses. This control is likely to be critical in the gastrointestinal tract where high levels of ATP are released in particular by commensal bacteria. The aim of this study was therefore to evaluate the involvement of the adenosinergic regulation in the intestine of mice in steady-state conditions and on acute infection with Toxoplasma gondii. We show that both conventional (Tconv) and regulatory (Treg) CD4+ T lymphocytes express CD39 and CD73 in the intestine of naive mice. CD73 expression was downregulated during acute infection with T. gondii, leading to impaired capacity to produce adenosine. Interestingly, the expression of adenosine receptors was maintained and treatment with receptor agonists limited immunopathology and dysbiosis, suggesting that the activation of adenosine receptors may constitute an efficient approach to control intestinal inflammation associated with decreased ectonucleotidase expression.