Subversion of host immune responses by viral superantigens.

Recent experiments with mouse mammary tumor virus indicate that expression of a virally encoded superantigen by B cells and its subsequent recognition by T cells are essential steps for amplification of infection and virus transmission. Preliminary results suggest that superantigens may also be expressed during retroviral infection in humans.

Expression and presentation of endogenous mouse mammary tumor virus superantigens by thymic and splenic dendritic cells and B cells.

Tolerance against superantigens (SAgs) encoded by endogenous mouse mammary tumor virus (Mtv) loci involves the intrathymic deletion of SAg-reactive T cells expressing a particular TCR V beta-chain, presumably upon presentation of the SAg by specialized APC. However, although the role of dendritic cells (DC) in the induction of tolerance against conventional Ags has been demonstrated, little is known about the role played by DC in tolerance induction against Mtv SAgs. Moreover, there is conflicting evidence concerning the capacity of DC to express and present Mtv SAgs. In this report we have analyzed the expression of Mtv SAgs in highly purified thymic and splenic DC and B cells by reverse transcriptase-PCR, using primers amplifying Mtv SAg-specific spliced mRNAs. DC express Mtv SAgs at levels comparable to B cells, but display a differential expression pattern of the various Mtv loci compared with B cells. Furthermore, our results show that DC are able to induce the deletion of SAg-reactive thymocytes in an in vitro assay, indicating that Mtv SAgs are functionally expressed on the DC surface. Collectively, our data are consistent with the hypothesis that DC play a role in the induction of intrathymic tolerance to Mtv SAgs.

BAFF AND APRIL: a tutorial on B cell survival.

BAFF, a member of the TNF family, is a fundamental survival factor for transitional and mature B cells. BAFF overexpression leads to an expanded B cell compartment and autoimmunity in mice, and elevated amounts of BAFF can be found in the serum of autoimmune patients. APRIL is a related factor that shares receptors with BAFF yet appears to play a different biological role. The BAFF system provides not only potential insight into the development of autoreactive B cells but a relatively simple paradigm to begin considering the balancing act between survival, growth, and death that affects all cells.

Positive and negative regulation of T cell responses by fibroblastic reticular cells within paracortical regions of lymph nodes.

Fibroblastic reticular cells (FRC) form the structural backbone of the T cell rich zones in secondary lymphoid organs (SLO), but also actively influence the adaptive immune response. They provide a guidance path for immigrating T lymphocytes and dendritic cells (DC) and are the main local source of the cytokines CCL19, CCL21, and IL-7, all of which are thought to positively regulate T cell homeostasis and T cell interactions with DC. Recently, FRC in lymph nodes (LN) were also described to negatively regulate T cell responses in two distinct ways. During homeostasis they express and present a range of peripheral tissue antigens, thereby participating in peripheral tolerance induction of self-reactive CD8(+) T cells. During acute inflammation T cells responding to foreign antigens presented on DC very quickly release pro-inflammatory cytokines such as interferon γ. These cytokines are sensed by FRC which transiently produce nitric oxide (NO) gas dampening the proliferation of neighboring T cells in a non-cognate fashion. In summary, we propose a model in which FRC engage in a bidirectional crosstalk with both DC and T cells to increase the efficiency of the T cell response. However, during an acute response, FRC limit excessive expansion and inflammatory activity of antigen-specific T cells. This negative feedback loop may help to maintain tissue integrity and function during rapid organ growth.

Maturation of the lymphoid system. III. Induction of selective unresponsiveness by injection of a haptenated carbohydrate into neonatal mice.

Neonatal treatment of A/J mice with DNP-Ficoll reduced or eliminated indirect anti-DNP PFC normally produced in response to adult challenge with DNP-keyhole limpet hemocyanin. The remaining direct anti-DNP PFC response was of low avidity. Spleen cells from neonatal A/J mice inhibited the in vitro but not the in vivo response of adult spleen cells to DNP-Ficoll.

Frequencies of circulating MDSC correlate with clinical outcome of melanoma patients treated with ipilimumab.

Metastatic melanoma has a poor prognosis with high resistance to chemotherapy and radiation. Recently, the anti-CTLA-4 antibody ipilimumab has demonstrated clinical efficacy, being the first agent to significantly prolong the overall survival of inoperable stage III/IV melanoma patients. A major aim of patient immune monitoring is the identification of biomarkers that predict clinical outcome. We studied circulating myeloid-derived suppressor cells (MDSC) in ipilimumab-treated patients to detect alterations in the myeloid cell compartment and possible correlations with clinical outcome. Lin(-) CD14(+) HLA-DR(-) monocytic MDSC were enriched in peripheral blood of melanoma patients compared to healthy donors (HD). Tumor resection did not significantly alter MDSC frequencies. During ipilimumab treatment, MDSC frequencies did not change significantly compared to baseline levels. We observed high inter-patient differences. MDSC frequencies in ipilimumab-treated patients were independent of baseline serum lactate dehydrogenase levels but tended to increase in patients with severe metastatic disease (M1c) compared to patients with metastases in skin or lymph nodes only (M1a), who had frequencies comparable to HD. Interestingly, clinical responders to ipilimumab therapy showed significantly less lin(-) CD14(+) HLA-DR(-) cells as compared to non-responders. The data suggest that the frequency of monocytic MDSC may be used as predictive marker of response, as low frequencies identify patients more likely benefitting from ipilimumab treatment. Prospective clinical trials assessing MDSC frequencies as potential biomarkers are warranted to validate these observations.

Engineering tolerance into transplanted beta cell lines.

In this paper we explore the possibility of improving, by genetic engineering, the resistance of insulin-secreting cells to the metabolic and inflammatory stresses that are anticipated to limit their function and survival when encapsulated and transplanted in a type 1 diabetic environment. We show that transfer of the Bcl-2 antiapoptotic gene, and of genes specifically interfering with cytokine intracellular signaling pathways, greatly improves resistance of the cells to metabolic limitations and inflammatory stresses.

In BALB/c mice, IL-4 production during the initial phase of infection with Leishmania major is necessary and sufficient to instruct Th2 cell development resulting in progressive disease.

In contrast to intact BALB/c mice, BALB/c mice rendered deficient in Vbeta4+ CD4+ T cells develop a Th1 response to infection with Leishmania major and are resistant. Vbeta4-deficient BALB/c mice are unable to generate the early IL-4 transcription occurring in Vbeta4 Valpha8 CD4+ T cells of BALB/c mice within 1 day of infection. Here we demonstrate that treatment of Vbeta4-deficient BALB/c mice with IL-4 during the first 64 h after infection instructs Th2 cell development and susceptibility to infection. The demonstrated inability of IL-4 to reverse the resistant phenotype of BALB/c mice treated with anti-CD4 mAb the day before infection suggest that these effects of IL-4 require its interaction with CD4+ T cells. In contrast to draining lymph node cells from BALB/c mice, cells from Vbeta4-deficient BALB/c mice remain responsive to IL-12 following infection. Strikingly, administration of IL-4 to Vbeta4-deficient BALB/c mice renders their lymph node cells unresponsive to IL-12 by down-regulating IL-12R beta2-chain expression. This study directly demonstrates that in BALB/c mice IL-4 is necessary and sufficient to initiate the molecular events steering Th2 cell maturation and susceptibility to L. major.

The IL-4 rapidly produced in BALB/c mice after infection with Leishmania major down-regulates IL-12 receptor beta 2-chain expression on CD4+ T cells resulting in a state of unresponsiveness to IL-12.

Within 1 day of infection with Leishmania major, susceptible BALB/c mice produce a burst of IL-4 in their draining lymph nodes, resulting in a state of unresponsiveness to IL-12 in parasite-specific CD4+ T cells within 48 h. In this report we examined the molecular mechanism underlying this IL-12 unresponsiveness. Extinction of IL-12 signaling in BALB/c mice is due to a rapid down-regulation of IL-12R beta2-chain mRNA expression in CD4+ T cells. In contrast, IL-12R beta2-chain mRNA expression was maintained on CD4+ T cells from resistant C57BL/6 mice. The down-regulation of the IL-12R beta2-chain mRNA expression in BALB/c CD4+ T cells is a consequence of the early IL-4 production. In this murine model of infection, a strict correlation is shown in vivo between expression of the IL-12R beta2-chain in CD4+ T cells and the development of a Th1 response and down-regulation of the mRNA beta2-chain expression and the maturation of a Th2 response. Treatment of BALB/c mice with IFN-gamma, even when IL-4 has been produced for 48 h, resulted in maintenance of IL-12R beta2-chain mRNA expression and IL-12 responsiveness. The data presented here support the hypothesis that the genetically determined susceptibility of BALB/c mice to infection with L. major is primarily based on an up-regulation of IL-4 production, which secondarily induces extinction of IL-12 signaling.

The parallel lives of angiogenesis and immunosuppression: cancer and other tales.

Emerging evidence indicates that angiogenesis and immunosuppression frequently occur simultaneously in response to diverse stimuli. Here, we describe a fundamental biological programme that involves the activation of both angiogenesis and immunosuppressive responses, often through the same cell types or soluble factors. We suggest that the initiation of these responses is part of a physiological and homeostatic tissue repair programme, which can be co-opted in pathological states, notably by tumours. This view can help to devise new cancer therapies and may have implications for aseptic tissue injury, pathogen-mediated tissue destruction, chronic inflammation and even reproduction.

Low TCR avidity and lack of tumor cell recognition in CD8(+) T cells primed with the CEA-analogue CAP1-6D peptide.

The use of "altered peptide ligands" (APL), epitopes designed for exerting increased immunogenicity as compared with native determinants, represents nowadays one of the most utilized strategies for overcoming immune tolerance to self-antigens and boosting anti-tumor T cell-mediated immune responses. However, the actual ability of APL-primed T cells to cross-recognize natural epitopes expressed by tumor cells remains a crucial concern. In the present study, we show that CAP1-6D, a superagonist analogue of a carcinoembriyonic antigen (CEA)-derived HLA-A*0201-restricted epitope widely used in clinical setting, reproducibly promotes the generation of low-affinity CD8(+) T cells lacking the ability to recognized CEA-expressing colorectal carcinoma (CRC) cells. Short-term T cell cultures, obtained by priming peripheral blood mononuclear cells from HLA-A*0201(+) healthy donors or CRC patients with CAP1-6D, were indeed found to heterogeneously cross-react with saturating concentrations of the native peptide CAP1, but to fail constantly lysing or recognizing through IFN- gamma release CEA(+)CRC cells. Characterization of anti-CAP1-6D T cell avidity, gained through peptide titration, CD8-dependency assay, and staining with mutated tetramers (D227K/T228A), revealed that anti-CAP1-6D T cells exerted a differential interaction with the two CEA epitopes, i.e., displaying high affinity/CD8-independency toward the APL and low affinity/CD8-dependency toward the native CAP1 peptide. Our data demonstrate that the efficient detection of self-antigen expressed by tumors could be a feature of high avidity CD8-independent T cells, and underline the need for extensive analysis of tumor cross-recognition prior to any clinical usage of APL as anti-cancer vaccines.

CD4+CD25+Foxp3+ regulatory T cells: from basic research to potential therapeutic use.

Regulatory T cells control immune responses to self- and foreign-antigens and play a major role in maintaining the balance between immunity and tolerance. This article reviews recent key developments in the field of CD4+CD25+Foxp3+ regulatory T (TREG) cells. It presents their characteristics and describes their range of activity and mechanisms of action. Some models of diseases triggered by the imbalance between TREG cells and effector pathogenic T cells are described and their potential therapeutic applications in humans are outlined.

Mycobacterium tuberculosis subverts innate immunity to evade specific effectors.

The macrophage is the niche of the intracellular pathogen Mycobacterium tuberculosis. Induction of macrophage apoptosis by CD4(+) or CD8(+) T cells is accompanied by reduced bacterial counts, potentially defining a host defense mechanism. We have already established that M. tuberculosis-infected primary human macrophages have a reduced susceptibility to Fas ligand (FasL)-induced apoptosis. To study the mechanisms by which M. tuberculosis prevents apoptotic signaling, we have generated a cell culture system based on PMA- and IFN-gamma-differentiated THP-1 cells recapitulating the properties of primary macrophages. In these cells, nucleotide-binding oligomerization domain 2 or TLR2 agonists and mycobacterial infection protected macrophages from apoptosis and resulted in NF-kappaB nuclear translocation associated with up-regulation of the antiapoptotic cellular FLIP. Transduction of a receptor-interacting protein-2 dominant-negative construct showed that nucleotide-binding oligomerization domain 2 is not involved in protection in the mycobacterial infection system. In contrast, both a dominant-negative construct of the MyD88 adaptor and an NF-kappaB inhibitor abrogated the protection against FasL-mediated apoptosis, showing the implication of TLR2-mediated activation of NF-kappaB in apoptosis protection in infected macrophages. The apoptosis resistance of infected macrophages might be considered as an immune escape mechanism, whereby M. tuberculosis subverts innate immunity signaling to protect its host cell against FasL(+)-specific cytotoxic lymphocytes.

Rôle de la molécule CD47 sur le lymphocyte T dans la régulation de la réponse immunitaire

L’importance respective des lymphocytes T régulateurs naturels générés dans le thymus ou induits en périphérie dans la régulation immunitaire et la résolution de l’inflammation est désormais bien établie. Nous avons contribué à mettre en évidence une nouvelle voie d’induction de lymphocytes T régulateurs périphériques à partir de cellules T humaines CD4+CD25- naïves et mémoires. Nous avons montré que l’engagement de la molécule ubiquitaire transmembranaire CD47 sur la cellule T par un anticorps monoclonal ou par le peptide 4N1K (peptide dérivé du domaine carboxy-terminal de la thrombospondine-1 et spécifique du site de liaison à CD47) induisait des lymphocytes T CD4+ régulateurs exerçant une fonction suppressive sur les lymphocytes T effecteurs. Les propriétés suppressives induites par la thrombospondine-1 confortent les fonctions anti-inflammatoires de cette protéine de la matrice extracellulaire. L’inhibition exercée par les lymphocytes T régulateurs induits dépend du contact intercellulaire entre les cellules T régulatrices et leurs cibles, et est indépendante du TGF-. Nos résultats démontrent également le rôle de CD47 sur le lymphocyte T CD4+ dans la réponse immunitaire spécifique de l’antigène in vivo. En effet, les souris BALB/c déficientes pour CD47 présentent un biais de la sécrétion d’anticorps et de cytokines de type Th1, alors que les souris BALB/c sont décrites comme exprimant un profil de production de cytokines de type Th2. Nos travaux mettent en évidence le rôle de CD47 dans l’inhibition du développement d’une réponse cellulaire et humorale de type Th1 in vivo, confirmant de précédentes études in vitro réalisées avec des cellules T CD4+ humaines. Nous présentons également le rôle inhibiteur de l’engagement de CD28 in vitro sur la différenciation en cellules Th17 des lymphocytes T CD4+ naïfs isolés de souris BALB/c. Le mécanisme proposé est dépendant de la production de l’IL-2 et de l’IFN- et indépendant de la présence de lymphocytes T régulateurs. Notre étude du rôle de deux molécules transmembranaires CD47 et CD28 exprimées sur la cellule T CD4+, contribue à une meilleure connaissance des mécanismes impliqués dans la tolérance immunologique, la résolution de l’inflammation et la différenciation des cellules T "helper" CD4+.

Modulation des réactions alloimmunitaires par les cytokines maîtresses IFN-γ et TGF-β

L’injection de cellules immunologiquement compétentes à un hôte histo-incompatible amène une réaction qui peut se traduire par la maladie du greffon-contre-l’hôte (GVHD). La GVHD demeure une barrière importante à une utilisation plus répandue de la greffe allogénique de cellules hématopoïétiques (AHCT), pourtant un traitement efficace pour traiter de nombreuses maladies. Une meilleure compréhension des mécanismes qui sous-tendent cette pathologie pourrait en faciliter le traitement et la prévention. L’Interféron-gamma (IFN-γ) et le Transforming Growth Factor-béta (TGF-β) sont deux cytokines maîtresses de l’immunité impliquées dans la fonction et l’homéostasie des cellules greffées. Nous démontrons chez la souris que l’IFN-γ limite la reconstitution lympho-hématopoïétique de façon dose-dépendante en mobilisant des mécanismes d’apoptose et en inhibant la prolifération cellulaire. Le TGF-β est quant à lui généralement connu comme un immunosuppresseur qui contrôle l’immunité en utilisant plusieurs voies de signalisation. Le rôle relatif de ces voies en AHCT est inconnu. Nous avons étudié une de ces voies en greffant des cellules provenant de donneurs déficients pour le gène SMAD3 (SMAD3-KO), un médiateur central de la voie canonique du TGF-β, à des souris histo-incompatibles. Bien que l’absence de SMAD3 ne cause aucune maladie chez nos souris donneuses, l’injection de cellules SMAD3-KO amène une GVHD du colon sévère chez le receveur. Cette atteinte est caractérisée par une différenciation Th1 et une infiltration massive de granulocytes témoignant d’un rôle central de SMAD3 dans la physiologie des lymphocytes T CD4 et des cellules myéloïdes. Nous avons focalisé ensuite nos efforts sur le rôle de SMAD3 chez les lymphocytes T CD4 en sachant que SMAD3 était actif chez les lymphocytes T CD4 tolérants. Nous avons découvert que SMAD3 était rapidement inactivé après une activation des cellules T, suggérant que l’inactivation de SMAD3 était fonctionnellement importante pour briser l’état de tolérance. Des études de micro-puces d’ADNc nous ont montré que SMAD3 contrôlait en effet l’expression de nombreux transcrits de gènes connus comme étant reliés à la tolérance et/ou à des processus biologiques dont les rôles dans le maintien de la tolérance sont plausibles.