Hélène Bour-Jordan

Distinct roles of CTLA-4 and TGF-β in CD4+CD25+regulatory T cell function

Both CTLA‐4 and TGF‐β have been implicated in suppression by CD4+CD25+ regulatory T cells (Treg). In this study, the relationship between CTLA‐4 and TGF‐β in Treg function was examined. Blocking CTLA‐4 on wild‐type Treg abrogated their suppressive activity in vitro, whereas neutralizing TGF‐β had no effect, supporting a TGF‐β‐independent role for CTLA‐4 in Treg‐mediated suppression in vitro. In CTLA‐4‐deficient mice, Treg development and homeostasis was normal. Moreover, Treg from CTLA‐4‐deficient mice exhibited uncompromised suppressive activity in vitro. These CTLA‐4‐deficient Treg expressed increased levels of the suppressive cytokines IL‐10 and TGF‐β, and in vitro suppression mediated by CTLA‐4–/– Treg was markedly reduced by neutralizing TGF‐β, suggesting that CTLA‐4‐deficient Treg develop a compensatory suppressive mechanism through CTLA‐4‐independent production of TGF‐β. Together, these data suggest that CTLA‐4 regulates Treg function by two distinct mechanisms, one during functional development of Treg and the other during the effector phase, when the CTLA‐4 signaling pathway is required for suppression. These results help explain contradictions in the literature and support the existence of functionally distinct Treg.

Impairment of NK Cell Function by NKG2D Modulation in NOD Mice

Nonobese diabetic (NOD) mice, a model of insulin-dependent diabetes mellitus, have a defect in natural killer (NK) cell-mediated functions. Here we show impairment in an activating receptor, NKG2D, in NOD NK cells. While resting NK cells from C57BL/6 and NOD mice expressed equivalent levels of NKG2D, upon activation NOD NK cells but not C57BL/6 NK cells expressed NKG2D ligands, which resulted in downmodulation of the receptor. NKG2D-dependent cytotoxicity and cytokine production were decreased because of receptor modulation, accounting for the dysfunction. Modulation of NKG2D was mostly dependent on the YxxM motif of DAP10, the NKG2D-associated adaptor that activates phosphoinositide 3 kinase. These results suggest that NK cells may be desensitized by exposure to NKG2D ligands.

Intrinsic and extrinsic control of peripheral T-cell tolerance by costimulatory molecules of the CD28/B7 family

Summary:  Positive and negative costimulation by members of the CD28 family is critical for the development of productive immune responses against foreign pathogens and their proper termination to prevent inflammation‐induced tissue damage. In addition, costimulatory signals are critical for the establishment and maintenance of peripheral tolerance. This paradigm has been established in many animal models and has led to the development of immunotherapies targeting costimulation pathways for the treatment of cancer, autoimmune disease, and allograft rejection. During the last decade, the complexity of the biology of costimulatory pathways has greatly increased due to the realization that costimulation does not affect only effector T cells but also influences regulatory T cells and antigen‐presenting cells. Thus, costimulation controls T‐cell tolerance through both intrinsic and extrinsic pathways. In this review, we discuss the influence of costimulation on intrinsic and extrinsic pathways of peripheral tolerance, with emphasis on members of the CD28 family, CD28, cytotoxic T‐lymphocyte antigen‐4 (CTLA‐4), and programmed death‐1 (PD‐1), as well as the downstream cytokine interleukin‐1 (IL‐2).

Insulin-induced remission in new-onset NOD mice is maintained by the PD-1–PD-L1 pathway

The past decade has seen a significant increase in the number of potentially tolerogenic therapies for treatment of new-onset diabetes. However, most treatments are antigen nonspecific, and the mechanism for the maintenance of long-term tolerance remains unclear. In this study, we developed an antigen-specific therapy, insulin-coupled antigen-presenting cells, to treat diabetes in nonobese diabetic mice after disease onset. Using this approach, we demonstrate disease remission, inhibition of pathogenic T cell proliferation, decreased cytokine production, and induction of anergy. Moreover, we show that robust long-term tolerance depends on the programmed death 1 (PD-1)–programmed death ligand (PD-L)1 pathway, not the distinct cytotoxic T lymphocyte–associated antigen 4 pathway. Anti–PD-1 and anti–PD-L1, but not anti–PD-L2, reversed tolerance weeks after tolerogenic therapy by promoting antigen-specific T cell proliferation and inflammatory cytokine production directly in infiltrated tissues. PD-1–PD-L1 blockade did not limit T regulatory cell activity, suggesting direct effects on pathogenic T cells. Finally, we describe a critical role for PD-1–PD-L1 in another powerful immunotherapy model using anti-CD3, suggesting that PD-1–PD-L1 interactions form part of a common pathway to selectively maintain tolerance within the target tissues.

Regulating the regulators: costimulatory signals control the homeostasis and function of regulatory T cells

Summary:  Costimulation is a concept that goes back to the early 1980s when Lafferty and others hypothesized that cell surface and soluble molecules must exist that are essential for initiating immune responses subsequent to antigen exposure. The explosion in this field of research ensued as over a dozen molecules have been identified to function as second signals following T‐cell receptor engagement. By 1994, it seemed clear that the most prominent costimulatory pathway CD28 and functionally related costimulatory molecules, such as CD154, were the major drivers of a positive immune response. Then the immunology world turned upside down. CD28 knockout mice, which were, in most cases, immunodeficient, led to increased autoimmunity when bred into the non‐obese diabetic background. Another CD28 family member, cytotoxic T‐lymphocyte‐associated protein 4, which was presumed to be a costimulatory molecule on activated T cells, turned out to be critical in downregulating immunity. These results, coupled with the vast suppressor cell literature which had been largely rebuked, suggested that the immune system was not poised for response but controlled in such a way that regulation was dominant. Over the last decade, we have learned that these costimulatory molecules play a key role in the now classical CD4+CD25+Foxp3+ regulatory T cells (Tregs) that provide critical control of unwanted autoimmune responses. In this review, we discuss the connections between costimulation and Tregs that have changed the costimulation paradigm.

Costimulation controls diabetes by altering the balance of pathogenic and regulatory T cells

The development of autoimmune diabetes in the nonobese diabetic (NOD) mouse results from a breakdown in tolerance to pancreatic islet antigens. CD28-B7 and CD40 ligand-CD40 (CD40L-CD40) costimulatory pathways affect the development of disease and are promising therapeutic targets. Indeed, it was shown previously that diabetes fails to develop in NOD-B7-2-/- and NOD-CD40L-/- mice. In this study, we examined the relative role of these 2 costimulatory pathways in the balance of autoimmunity versus regulation in NOD mice. We demonstrate that initiation but not effector function of autoreactive T cells was defective in NOD-B7-2-/- mice. Moreover, the residual proliferation of the autoreactive cells was effectively controlled by CD28-dependent CD4+CD25+ regulatory T cells (Tregs), as depletion of Tregs partially restored proliferation of autoreactive T cells and resulted in diabetes in an adoptive-transfer model. Similarly, disruption of the CD28-B7 pathway and subsequent Treg deletion restored autoimmunity in NOD-CD40L-/- mice. These results demonstrate that development of diabetes is dependent on a balance of pathogenic and regulatory T cells that is controlled by costimulatory signals. Thus, elimination of Tregs results in diabetes even in the absence of costimulation, which suggests a need for alternative strategies for immunotherapeutic approaches.

Distinct Roles of Dendritic Cells and B Cells in Va14Ja18 Natural T Cell Activation In Vivo

Va14Ja18 natural T (iNKT) cells are innate, immunoregulatory lymphocytes that recognize CD1d-restricted lipid Ags such as α-galactosylceramide (αGalCer). The immunoregulatory functions of iNKT cells are dependent upon either IFN-γ or IL-4 production by these cells. We hypothesized that αGalCer presentation by different CD1d-positive cell types elicits distinct iNKT cell functions. In this study we report that dendritic cells (DC) play a critical role in αGalCer-mediated activation of iNKT cells and subsequent transactivation of NK cells. Remarkably, B lymphocytes suppress DC-mediated iNKT and NK cell activation. Nevertheless, αGalCer presentation by B cells elicits low IL-4 responses from iNKT cells. This finding is particularly interesting because we demonstrate that NOD DC are defective in eliciting iNKT cell function, but their B cells preferentially activate this T cell subset to secrete low levels of IL-4. Thus, the differential immune outcome based on the type of APC that displays glycolipid Ags in vivo has implications for the design of therapies that harness the immunoregulatory functions of iNKT cells.

CD28 function: a balance of costimulatory and regulatory signals.

Both the recognition of MHC/antigen complex by the T-cell receptor and engagement of costimulatory molecules are necessary for efficient T-cell activation. CD28 has been widely recognized as the major costimulation pathway for naive T-cell activation, and the CD28/B7 pathway plays a central role in immune responses against pathogens, autoimmune diseases, and graft rejection. In this review, we will summarize evidence that CD28 is also prominent in the regulation of immune responses and the maintenance of peripheral tolerance. Indeed, CD28 engagement increases the expression of the down-modulatory molecule CTLA-4, induces the differentiation of Th2 cells that have a protective function in autoimmunity, and has an obligatory role in the homeostasis of regulatory T cells. Therefore, CD28/B7 interactions induce a balance of costimulatory and regulatory signals that have opposite outcomes on immune responses. This new perspective on CD28 function suggests that caution should be taken in the development of immunotherapies targeting costimulatory pathways.

CTLA-4 regulates the requirement for cytokine-induced signals in T H 2 lineage commitment

The relative importance of the cytokine milieu versus cytolytic T lymphocyte-associated antigen 4 (CTLA-4) and T cell receptor signal strength on T cell differentiation remains unclear. Here we have generated mice deficient for signal transducer and activator of transcription 6 (STAT6) and CTLA-4 to determine the role of CTLA-4 in cytokine-driven T cell differentiation. CTLA-4–deficient T cells bypass the need for STAT6 in the differentiation of T helper type 2 (TH2) cells. TH2 differentiation of cells deficient for both STAT6 and CTLA-4 is accompanied by induction of GATA-3 and the migration of TH2 cells to peripheral tissues. CTLA-4 deficiency also affects the balance of the nuclear factors NFATc1 and NFATc2, and enhances activation of NF-κB. These results suggest that CTLA-4 has a critical role in T cell differentiation and that STAT6-dependent TH2 lineage commitment and stabilization can be bypassed by increasing the strength of signaling through the T cell receptor.

Distinct Effector Mechanisms in the Development of Autoimmune Neuropathy versus Diabetes in Nonobese Diabetic Mice

NOD mice deficient for the costimulatory molecule B7-2 (NOD-B7-2KO mice) are protected from autoimmune diabetes but develop a spontaneous autoimmune peripheral neuropathy that resembles human diseases Guillain-Barre syndrome and chronic inflammatory demyelinating polyradiculoneuropathy. Similar observations have now been made in conventional NOD mice. We have shown previously that this disease was mediated by autoreactive T cells inducing demyelination in the peripheral nervous system. In this study, we analyzed the molecular pathways involved in the disease. Our data showed that neuropathy developed in the absence of perforin or fas, suggesting that classic cytotoxicity pathways were dispensable for nerve damage in NOD-B7-2KO mice. In contrast, IFN-γ played an obligatory role in the development of neuropathy as demonstrated by the complete protection from disease and infiltration in the nerves in NOD-B7-2KO mice deficient for IFN-γ. This result was consistent with the inflammatory phenotype of T cells infiltrating the peripheral nerves. Importantly, the relative role of perforin, fas, and IFN-γ appears completely different in autoimmune diabetes vs neuropathy. Thus, there are sharp contrasts in the pathogenesis of autoimmune diseases targeting different tissues in the same NOD background.