CLEC-1 Restrains Acute Inflammatory Response and Recruitment of Neutrophils following Tissue Injury

The inflammatory response is a key mechanism for the elimination of injurious agents but must be tightly controlled to prevent additional tissue damage and progression to persistent inflammation. C-type lectin receptors expressed mostly by myeloid cells play a crucial role in the regulation of inflammation by recognizing molecular patterns released by injured tissues. We recently showed that the C-type lectin receptor CLEC-1 is able to recognize necrotic cells. However, its role in the acute inflammatory response following tissue damage had not yet been investigated. We show in this study, in a mouse model of liver injury induced by acetaminophen intoxication, that Clec1a deficiency enhances the acute immune response with increased expression of Il1b, Tnfa, and Cxcl2 and higher infiltration of activated neutrophils into the injured organ. Furthermore, we demonstrate that Clec1a deficiency exacerbates tissue damage via CXCL2-dependent neutrophil infiltration. In contrast, we observed that the lack of CLEC-1 limits CCL2 expression and the accumulation, beyond the peak of injury, of monocyte-derived macrophages. Mechanistically, we found that Clec1a-deficient dendritic cells increase the expression of Il1b, Tnfa, and Cxcl2 in response to necrotic cells, but decrease the expression of Ccl2. Interestingly, treatment with an anti-human CLEC-1 antagonist mAb recapitulates the exacerbation of acute immunopathology observed by genetic loss of Clec1a in a preclinical humanized mouse model. To conclude, our results demonstrate that CLEC-1 is a death receptor limiting the acute inflammatory response following injury and represents a therapeutic target to modulate immunity.

CLEC-1 is a death sensor that limits antigen cross-presentation by dendritic cells and represents a target for cancer immunotherapy

Tumors exploit numerous immune checkpoints, including those deployed by myeloid cells to curtail antitumor immunity. Here, we show that the C-type lectin receptor CLEC-1 expressed by myeloid cells senses dead cells killed by programmed necrosis. Moreover, we identified Tripartite Motif Containing 21 (TRIM21) as an endogenous ligand overexpressed in various cancers. We observed that the combination of CLEC-1 blockade with chemotherapy prolonged mouse survival in tumor models. Loss of CLEC-1 reduced the accumulation of immunosuppressive myeloid cells in tumors and invigorated the activation state of dendritic cells (DCs), thereby increasing T cell responses. Mechanistically, we found that the absence of CLEC-1 increased the cross-presentation of dead cell-associated antigens by conventional type-1 DCs. We identified antihuman CLEC-1 antagonist antibodies able to enhance antitumor immunity in CLEC-1 humanized mice. Together, our results demonstrate that CLEC-1 acts as an immune checkpoint in myeloid cells and support CLEC-1 as a novel target for cancer immunotherapy.

230 Preclinical efficacy of CLEC-1 antagonist as novel myeloid immune checkpoint therapy for oncology

Background

C-type lectin receptors (CLRs) are powerful pattern recognition receptors shaping immune cell-mediated tissue damage by positively or negatively regulating myeloid cell functions and hence tumor elimination or evasion. We previously reported that the orphan CLR CLEC-1 expressed by dendritic cells (DCs) tempers T cell’s responses in vivo by limiting antigen cross-presentation by cDC1. Furthermore, we observed that CLEC-1 is highly expressed by myeloid cells purified from human tumor microenvironment, in particular tumor-associated macrophages.

Methods

Macrophages were generated from monocytes of healthy volunteers for phagocytosis assays. MC38 and Hepa 1.6 murine tumor cells were implanted in Clec1a KO or KI mice for immunotherapeutic treatment evaluation.

Results

Using newly developed anti-human CLEC-1 monoclonal antibodies (mAbs), we found that antagonist anti-CLEC-1 mAbs with the capacity to block CLEC-1/CLEC-1Ligand interaction, as opposed to non-antagonist CLEC-1 mAbs, increase the phagocytosis of CLEC-1Ligand-positive human tumor cells by human macrophages, in particular when opsonized by tumor-associated antigen mAbs (Rituximab, Cetuximab, Trastuzumab) or with anti-CD47 mAb (Magrolimab). In-vivo, CLEC-1 knock-out (KO) mice (n=19) display significant prolonged survival in monotherapy as compared to wild-type littermates (n=12) in an orthotopic hepatocellular carcinoma (HCC) model and anti-tumor memory responses was demonstrated by tumor rechallenge in cured mice. CLEC1 KO mice also illustrate significant eradication of MC38 colorectal tumors in combination with chemotherapy promoting CLEC-1Ligand expression by tumor cells (n=16 with Gemcitabine or n=11 with Cyclophosphamide). HCC tumor microenvironment analysis after 2 weeks of tumor implantation shows significantly higher number of CD8+ and memory CD8+ T cells with reduced PD1 expression in CLEC1 KO animals (n=16 versus n=12 for KO vs WT mice respectively). Finally, we recently generated human CLEC-1 knock-in mice expressing the extracellular human CLEC1 domain fused to the intracellular mouse CLEC1 tail and confirmed preclinical efficacy in vivo with anti-human CLEC1 antagonist mAb in monotherapy in the orthotopic HCC model.

Conclusions

These data illustrate that CLEC-1 inhibition represents a novel therapeutic target for immuno-oncology modifying T cell immune responses and tumor cell phagocytosis by macrophages.

Dendritic Cells Require TMEM176A/B Ion Channels for Optimal MHC Class II Antigen Presentation to Naive CD4+ T Cells

Intracellular ion fluxes emerge as critical actors of immunoregulation but still remain poorly explored. In this study, we investigated the role of the redundant cation channels TMEM176A and TMEM176B (TMEM176A/B) in retinoic acid-related orphan receptor γt⁺ cells and conventional dendritic cells (DCs) using germline and conditional double knockout mice. Although Tmem176a/b appeared surprisingly dispensable for the protective function of Th17 and group 3 innate lymphoid cells in the intestinal mucosa, we found that they were required in conventional DCs for optimal Ag processing and presentation to CD4⁺ T cells. Using a real-time imaging method, we show that TMEM176A/B accumulate in dynamic post-Golgi vesicles preferentially linked to the late endolysosomal system and strongly colocalize with HLA-DM. Taken together, our results suggest that TMEM176A/B ion channels play a direct role in the MHC class II compartment of DCs for the fine regulation of Ag presentation and naive CD4⁺ T cell priming.

Abstract 1636: CLEC-1 is a novel myeloid immune checkpoint for cancer immunotherapy limiting tumor cells phagocytosis and tumor antigen cross-presentation

Myeloid cells represent one of the most abundant immune cell types in solid tumors that impede myeloid phagocytosis by triggering “don't eat me” and “don't find me” signals. Recent literature demonstrates that C-type lectin receptors (CLRs) are powerful pattern recognition receptors shaping immune cell-mediated tissue damage by positively or negatively regulating myeloid cell functions and hence tumor elimination or evasion. We previously reported that the orphan CLR CLEC-1 expressed by dendritic cells (DCs) and macrophages (MPs) is enhanced by TGFβ and tempers downstream T cells responses. Furthermore, we observed that CLEC-1 is highly expressed by myeloid cells purified from the human tumor microenvironment, in particular, tumor-associated MPs. We evaluated whether CLEC-1 could also be a receptor for DAMPs and influences phagocytosis. We found that CLEC-1 fusion protein binds specifically to secondary necrotic healthy or tumor cells induced by chemotherapy, radiation (UV, X-ray), or culture stress conditions suggesting that ligands of CLEC-1 are generated upon stress and programmed cell death. Importantly, further to promising results in KO CLEC-1 mice, we observed in vivo that CLEC-1 deficient mice, in contrast to wild-type littermates, eradicate MC38 colorectal tumors in combination with cytotoxic and immunogenic chemotherapy. Importantly, disruption of CLEC-1 signaling by Fc-CLEC-1 fusion protein also promotes tumor eradication. We then generated and identified different anti-human CLEC-1 antagonist monoclonal antibodies (mAbs) with the capacity to block CLEC-1/CLEC-1L interaction. We developed innovative antagonist CLEC-1 mAbs which, in contrast to non-antagonist CLEC-1 control mAb, increase the phagocytosis of CLEC-1L-positive human tumor cells by human TGFβ-polarized DCs or MPs. Indeed, TGFβ-polarized DCs phagocytose more efficiently a NSCLC cell line (A549) as well as Rituximab (anti-CD20 mAb)-opsonized Burkitt lymphoma cells (Raji) when CLEC-1 is antagonized by Abs. Similarly, macrophages significantly more efficiently engulfed human tumors in the presence of CLEC-1 antagonist Abs, in particular when tumor cells were opsonized such as Rituximab-opsonized Raji cells, Cetuximab opsonized colon carcinoma cells (DLD-1; EGFR+) or Trastuzumab opsonized mammary carcinoma cells (SK-BR-3; Her2+). Importantly, we observed both in vitro and in vivo that DCs from Clec1a deficient mice cross-present more efficiently dead cell-associated antigens to CD8+ T cells (OT-1). We generated hCLEC-1 knock-in mice and in vivo preclinical evaluation of CLEC-1 blocking mAbs is ongoing. Altogether, these data illustrate that CLEC-1 broadly notably inhibits tumor-cell phagocytosis and synergized with tumor-targeted cytotoxic monoclonal antibodies in both solid and hematological tumors, and hampers DC antigen cross-presentation.

Citation Format: Vanessa Gauttier, Sabrina Pengam, Marion Drouin, Javier Saenz, Bérangère Evrard, Kevin Biteau, Caroline Mary, Géraldine Teppaz, Ariane Desselle, Virginie Thépénier, Emmanuelle Wilhelm, Elise Chiffoleau, Nicolas Poirier. CLEC-1 is a novel myeloid immune checkpoint for cancer immunotherapy limiting tumor cells phagocytosis and tumor antigen cross-presentation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 1636.

Abstract 3423: CLEC-1 suppress dendritic cell antigen presentation and is a novel myeloid immune checkpoint target for cancer immunotherapy

Recent literature demonstrates that tumor hijacks physiological mechanisms of C-type lectin receptors (CLRs) that normally restrain immune cell-mediated tissue damage to suppress myeloid cell activation and promotes immune escape. We previously demonstrated that the orphan CLR CLEC-1 expressed by myeloid cells, is enhanced by the immunosuppressive cytokine TGFβ and tempers downstream CD4+ Th1 and Th17 responses following sterile inflammation. Interestingly, we observed a high CLEC-1 expression by human CD11b+ myeloid cells from ovarian ascites and from in-vitro generated pro-tumoral M2 macrophages. Moreover, open-source data show that Clec1a is highly expressed by XCR1-expressing mouse dendritic cells (DC). As these cDC1 are specialized in cross-presentation of dead cell-associated antigens, we evaluated whether CLEC-1 could be a receptor of damaged cells and regulate anti-tumor immunity. Interestingly, we observed that both human and mouse CLEC-1 bind specifically to secondary necrotic cells. No binding was observed with viable, primary necrotic or with early apoptotic cells suggesting that the ligand(s) of CLEC-1 correspond(s) to Damage-Associated Molecular Pattern(s) induced by the cell death process. Then, to test if CLEC-1 in DCs influences cross-presentation of dead cell-associated antigens, we immunized WT and Clec1a KO mice with OVA-loaded dead cells and co-injected OVA-specific TCR transgenic CD8+ OT-I T cells. Impressively, we observed an increased proliferation of CD8+ T cells in the absence of CLEC-1, demonstrating that CLEC-1 in DCs tempers cross-presentation of dead cell-associated antigens. Interestingly, we observed in a model of subcutaneous MC38 colon carcinoma that the combination of the absence of CLEC-1 with cytotoxic and immunogenic chemotherapy (cyclophosphamide), reduces significantly the tumor growth and cures most of the mice. Similarly, in orthotopic models of mesothelioma (AK7) or hepatocarcinoma (Hepa1.6), we observed a prolongation of survival of Clec1a KO mice. Mechanistically, this was associated with an increase of effector CD69+ and of central memory CD44hi CD62Lhi CCR7hi CD4+ and CD8+ T cells in both liver and spleen. Furthermore, we observed a decrease in myeloid-derived suppressor cells. Besides, macrophages display a more mature phenotype relative to MHC class II expression. In addition, transcriptomics analysis performed in tumor-bearing liver of Clec1a KO mice revealed a profound decrease in CSF1 expression suggesting a defect in immunosuppressive myeloid cell recruitment. Moreover, we observed an increase in CCL17 and CCL7 expression, both known to activate CD8+ T cells through DCs and to enhance anti-tumor immunity. Taken together, these results suggest that CLEC-1 in DCs by acting as sensor of cell damage tempers immune response and represents a novel pharmacological target for cancer immunotherapy.

Citation Format: Marion Drouin, Javier Saenz, Bérangère Evrard, Vanessa Gauttier, Géraldine Teppaz, Maria-Dolores Lopez-Robles, Cédric Louvet, Nicolas Poirier, Elise Chiffoleau. CLEC-1 suppress dendritic cell antigen presentation and is a novel myeloid immune checkpoint target for cancer immunotherapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3423.

C-Type Lectin-Like Receptors: Head or Tail in Cell Death Immunity

C-type lectin-like receptors (CLRs) represent a family of transmembrane pattern recognition receptors, expressed primarily by myeloid cells. They recognize not only pathogen moieties for host defense, but also modified self-antigens such as damage-associated molecular patterns released from dead cells. Upon ligation, CLR signaling leads to the production of inflammatory mediators to shape amplitude, duration and outcome of the immune response. Thus, following excessive injury, dysregulation of these receptors leads to the development of inflammatory diseases. Herein, we will focus on four CLRs of the "Dectin family," shown to decode the immunogenicity of cell death. CLEC9A on dendritic cells links F-actin exposed by dying cells to favor cross-presentation of dead-cell associated antigens to CD8⁺ T cells. Nevertheless, CLEC9A exerts also feedback mechanisms to temper neutrophil recruitment and prevent additional tissue damage. MINCLE expressed by macrophages binds nuclear SAP130 released by necrotic cells to potentiate pro-inflammatory responses. However, the consequent inflammation can exacerbate pathogenesis of inflammatory diseases. Moreover, in a tumor microenvironment, MINCLE induces macrophage-induced immune suppression and cancer progression. Similarly, triggering of LOX-1 by oxidized LDL, amplifies pro-inflammatory response but promotes tumor immune escape and metastasis. Finally, CLEC12A that recognizes monosodium urate crystals formed during cell death, inhibits activating signals to prevent detrimental inflammation. Interestingly, CLEC12A also sustains type-I IFN response to finely tune immune responses in case of viral-induced collateral damage. Therefore, CLRs acting in concert as sensors of injury, could be used in a targeted way to treat numerous diseases such as allergies, obesity, tumors, and autoimmunity.

Human Tolerogenic Dendritic Cells Regulate Immune Responses through Lactate Synthesis

Cell therapy is a promising strategy for treating patients suffering from autoimmune or inflammatory diseases or receiving a transplant. Based on our preclinical studies, we have generated human autologous tolerogenic dendritic cells (ATDCs), which are being tested in a first-in-man clinical trial in kidney transplant recipients. Here, we report that ATDCs represent a unique subset of monocyte-derived cells based on phenotypic, transcriptomic, and metabolic analyses. ATDCs are characterized by their suppression of T cell proliferation and their expansion of Tregs through secreted factors. ATDCs produce high levels of lactate that shape T cell responses toward tolerance. Indeed, T cells take up ATDC-secreted lactate, leading to a decrease of their glycolysis. In vivo, ATDCs promote elevated levels of circulating lactate and delay graft-versus-host disease by reducing T cell proliferative capacity. The suppression of T cell immunity through lactate production by ATDCs is a novel mechanism that distinguishes ATDCs from other cell-based immunotherapies.

C-Type Lectin-Like Receptors As Emerging Orchestrators of Sterile Inflammation Represent Potential Therapeutic Targets

Over the last decade, C-type lectin-like receptors (CTLRs), expressed mostly by myeloid cells, have gained increasing attention for their role in the fine tuning of both innate and adaptive immunity. Not only CTLRs recognize pathogen-derived ligands to protect against infection but also endogenous ligands such as self-carbohydrates, proteins, or lipids to control homeostasis and tissue injury. Interestingly, CTLRs act as antigen-uptake receptors via their carbohydrate-recognition domain for internalization and subsequent presentation to T-cells. Furthermore, CTLRs signal through a complex intracellular network leading to the secretion of a particular set of cytokines that differently polarizes downstream effector T-cell responses according to the ligand and pattern recognition receptor co-engagement. Thus, by orchestrating the balance between inflammatory and resolution pathways, CTLRs are now considered as driving players of sterile inflammation whose dysregulation leads to the development of various pathologies such as autoimmune diseases, allergy, or cancer. For examples, the macrophage-inducible C-type lectin (MINCLE), by sensing glycolipids released during cell-damage, promotes skin allergy and the pathogenesis of experimental autoimmune uveoretinitis. Besides, recent studies described that tumors use physiological process of the CTLRs’ dendritic cell-associated C-type lectin-1 (DECTIN-1) and MINCLE to locally suppress myeloid cell activation and promote immune evasion. Therefore, we aim here to overview the current knowledge of the pivotal role of CTLRs in sterile inflammation with special attention given to the “Dectin-1” and “Dectin-2” families. Moreover, we will discuss the potential of these receptors as promising therapeutic targets to treat a wide range of acute and chronic diseases.

B cells with regulatory properties in transplantation tolerance

Induction of tolerance remains a major goal in transplantation. Indeed, despite potent immunosuppression, chronic rejection is still a real problem in transplantation. The humoral response is an important mediator of chronic rejection, and numerous strategies have been developed to target either B cells or plasma cells. However, the use of anti-CD20 therapy has highlighted the beneficial role of subpopulation of B cells, termed regulatory B cells. These cells have been characterized mainly in mice models of auto-immune diseases but emerging literature suggests their role in graft tolerance in transplantation. Regulatory B cells seem to be induced following inflammation to restrain excessive response. Different phenotypes of regulatory B cells have been described and are functional at various differentiation steps from immature to plasma cells. These cells act by multiple mechanisms such as secretion of immuno-suppressive cytokines interleukin-10 (IL-10) or IL-35, cytotoxicity, expression of inhibitory receptors or by secretion of non-inflammatory antibodies. Better characterization of the development, phenotype and mode of action of these cells seems urgent to develop novel approaches to manipulate the different B cell subsets and the response to the graft in a clinical setting.

Regulatory B Cells with a Partial Defect in CD40 Signaling and Overexpressing Granzyme B Transfer Allograft Tolerance in Rodents

Emerging knowledge regarding B cells in organ transplantation has demonstrated that these cells can no longer be taken as mere generators of deleterious Abs but can also act as beneficial players. We previously demonstrated in a rat model of cardiac allograft tolerance induced by short-term immunosuppression an accumulation in the blood of B cells overexpressing inhibitory molecules, a phenotype also observed in the blood of patients that spontaneously develop graft tolerance. In this study, we demonstrated the presence in the spleen of regulatory B cells enriched in the CD24(int)CD38(+)CD27(+)IgD(-)IgM(+/low) subpopulation, which are able to transfer donor-specific tolerance via IL-10 and TGF-β1-dependent mechanisms and to suppress in vitro TNF-α secretion. Following anti-CD40 stimulation, IgD(-)IgM(+/low) B cells were blocked in their plasma cell differentiation pathway, maintained high expression of the inhibitory molecules CD23 and Bank1, and upregulated Granzyme B and Irf4, two molecules described as highly expressed by regulatory B cells. Interestingly, these B cells recognized specifically a dominant donor Ag, suggesting restricted specificity that could lead to a particular B cell response. Regulatory B cells were not required for induction of tolerance and appeared following Foxp3(+)CD4(+)CD25(+) regulatory T cells, suggesting cooperation with regulatory T cells for their expansion. Nevertheless, following transfer to new recipients, these B cells migrated to the allograft, kept their regulatory profile, and promoted local accumulation of Foxp3(+)CD4(+)CD25(+) regulatory T cells. Mechanisms of regulatory B cells and their cell therapy potential are important to decipher in experimental models to pave the way for future developments in the clinic.

Tolerant Kidney Transplant Patients Produce B Cells with Regulatory Properties

Whereas a B cell-transcriptional profile has been recorded for operationally tolerant kidney graft patients, the role that B cells have in this tolerance has not been reported. In this study, we analyzed the role of B cells from operationally tolerant patients, healthy volunteers, and kidney transplant recipients with stable graft function on T cell suppression. Proliferation, apoptosis, and type I proinflammatory cytokine production by effector CD4(+)CD25(-) T cells were measured after anti-CD3/anti-CD28 stimulation with or without autologous B cells. We report that B cells inhibit CD4(+)CD25(-) effector T cell response in a dose-dependent manner. This effect required B cells to interact with T-cell targets and was achieved through a granzyme B (GzmB)-dependent pathway. Tolerant recipients harbored a higher number of B cells expressing GzmB and displaying a plasma cell phenotype. Finally, GzmB(+) B-cell number was dependent on IL-21 production, and B cells from tolerant recipients but not from other patients positively regulated both the number of IL-21(+) T cells and IL-21 production, suggesting a feedback loop in tolerant recipients that increases excessive B cell activation and allows regulation to take place. These data provide insights into the characterization of B cell-mediated immunoregulation in clinical tolerance and show a potential regulatory effect of B cells on effector T cells in blood from patients with operationally tolerant kidney grafts.

Fibrinogen-like protein 2/fibroleukin induces long-term allograft survival in a rat model through regulatory B cells

We previously described that in a rat model of heart transplantation tolerance was dependent on CD8+CD45RClow Tregs that over-expressed fibrinogen-like protein 2 (FGL2)/fibroleukin. Little is known on the immunoregulatory properties of FGL2. Here we analyzed the transplantation tolerance mechanisms that are present in Lewis 1A rats treated with FGL2. Over-expression of FGL2 in vivo through adenovirus associated virus -mediated gene transfer without any further treatment resulted in inhibition of cardiac allograft rejection. Adoptive cell transfer of splenocytes from FGL2-treated rats with long-term graft survival (> 80 days) in animals that were transplanted with cardiac allografts inhibited acute and chronic organ rejection in a donor-specific and transferable tolerance manner, since iterative adoptive transfer up to a sixth consecutive recipient resulted in transplantation tolerance. Adoptive cell transfer also efficiently inhibited anti-donor antibody production. Analysis of all possible cell populations among splenocytes revealed that B lymphocytes were sufficient for this adoptive cell tolerance. These B cells were also capable of inhibiting the proliferation of CD4+ T cells in response to allogeneic stimuli. Moreover, gene transfer of FGL2 in B cell deficient rats did not prolong graft survival. Thus, this is the first description of FGL2 resulting in long-term allograft survival. Furthermore, allograft tolerance was transferable and B cells were the main cells responsible for this effect.

Autologous dendritic cells prolong allograft survival through Tmem176b-dependent antigen cross-presentation

The administration of autologous (recipient-derived) tolerogenic dendritic cells (ATDCs) is under clinical evaluation. However, the molecular mechanisms by which these cells prolong graft survival in a donor-specific manner is unknown. Here, we tested mouse ATDCs for their therapeutic potential in a skin transplantation model. ATDC injection in combination with anti-CD3 treatment induced the accumulation of CD8(+) CD11c(+) T cells and significantly prolonged allograft survival. TMEM176B is an intracellular protein expressed in ATDCs and initially identified in allograft tolerance. We show that Tmem176b(-/-) ATDCs completely failed to trigger both phenomena but recovered their effect when loaded with donor peptides before injection. These results strongly suggested that ATDCs require TMEM176B to cross-present antigens in a tolerogenic fashion. In agreement with this, Tmem176b(-/-) ATDCs specifically failed to cross-present male antigens or ovalbumin to CD8(+) T cells. Finally, we observed that a Tmem176b-dependent cation current controls phagosomal pH, a critical parameter in cross-presentation. Thus, ATDCs require TMEM176B to cross-present donor antigens to induce donor-specific CD8(+) CD11c(+) T cells with regulatory properties and prolong graft survival.

LIMLE, a new molecule over-expressed following activation, is involved in the stimulatory properties of dendritic cells

Dendritic cells are sentinels of the immune system distributed throughout the body, that following danger signals will migrate to secondary lymphoid organs to induce effector T cell responses. We have identified, in a rodent model of graft rejection, a new molecule expressed by dendritic cells that we have named LIMLE (RGD1310371). To characterize this new molecule, we analyzed its regulation of expression and its function. We observed that LIMLE mRNAs were rapidly and strongly up regulated in dendritic cells following inflammatory stimulation. We demonstrated that LIMLE inhibition does not alter dendritic cell maturation or cytokine production following Toll-like-receptor stimulation. However, it reduces their ability to stimulate effector T cells in a mixed leukocyte reaction or T cell receptor transgenic system. Interestingly, we observed that LIMLE protein localized with actin at some areas under the plasma membrane. Moreover, LIMLE is highly expressed in testis, trachea, lung and ciliated cells and it has been shown that cilia formation bears similarities to formation of the immunological synapse which is required for the T cell activation by dendritic cells. Taken together, these data suggest a role for LIMLE in specialized structures of the cytoskeleton that are important for dynamic cellular events such as immune synapse formation. In the future, LIMLE may represent a new target to reduce the capacity of dendritic cells to stimulate T cells and to regulate an immune response.

Cell therapy with autologous tolerogenic dendritic cells induces allograft tolerance through interferon-gamma and epstein-barr virus-induced gene 3

Innovative therapeutic strategies are needed to diminish the impact of harmful immunosuppression in transplantation. Dendritic cell (DC)-based therapy is a promising approach for induction of antigen-specific tolerance. Using a heart allograft model in rats, we analyzed the immunoregulatory mechanisms by which injection of autologous tolerogenic DCs (ATDCs) plus suboptimal immunosuppression promotes indefinite graft survival. Surprisingly, we determined that Interferon-gamma (IFNG), a cytokine expected to be propathogenic, was threefold increased in the spleen of tolerant rats. Importantly, its blockade led to allograft rejection [Mean Survival Time (MST) = 25.6 ± 4 days], showing that IFNG plays a critical role in immunoregulatory mechanisms triggered by ATDCs. IFNG was expressed by TCRαβ(+) CD3(+) CD4(-) CD8(-) NKRP1(-) cells (double negative T cells, DNT), which accumulated in the spleen of tolerant rats. Interestingly, ATDCs specifically induced IFNG production by DNT cells. ATDCs expressed the cytokinic chain Epstein-Barr virus-induced gene 3 (EBI3), an IL-12 family member. EBI3 blockade or knock-down through siRNA completely abolished IFNG expression in DNT cells. Finally, EBI3 blockade in vivo led to allograft rejection (MST = 36.8 ± 19.7 days), demonstrating for the first time a role for EBI3 in transplantation tolerance. Taken together our results have important implications in the rationalization of DC-based therapy in transplantation as well as in the patient immunomonitoring follow-up.

Long-term allograft tolerance is characterized by the accumulation of B cells exhibiting an inhibited profile

Numerous reports have highlighted the central role of regulatory T cells in long-term allograft tolerance, but few studies have investigated the B-cell aspect. We analyzed the B-cell response in a rat model of long-term cardiac allograft tolerance induced by a short-term immunosuppression. We observed that tolerated allografts are infiltrated by numerous B cells organized in germinal centers that are strongly regulated in their IgG alloantibody response. Moreover, alloantibodies from tolerant recipients exhibit a deviation toward a Th2 isotype and do not activate in vitro donor-type endothelial cells in a pro-inflammatory way but maintained expression of cytoprotective molecules. Interestingly, this inhibition of the B-cell response is characterized by the progressive accumulation in the graft and in the blood of B cells blocked at the IgM to IgG switch recombination process and overexpressing BANK-1 and the inhibitory receptor Fcgr2b. Importantly, B cells from tolerant recipients are able to transfer allograft tolerance. Taken together, these results demonstrate a strong regulation of the alloantibody response in tolerant recipients and the accumulation of B cells exhibiting an inhibited and regulatory profile. These mechanisms of regulation of the B-cell response could be instrumental to develop new strategies to promote tolerance.

Tmem176B and Tmem176A are associated with the immature state of dendritic cells

DCs play a central role in the development of innate and adaptive immunity but also in the induction and maintenance of immune tolerance. Identification of factors that govern DC activation, their maturation state, and their capacity to induce proinflammatory or tolerogeneic responses therefore represents a crucial aim of research. We previously identified a new molecule, Tmem176B (which we named TORID initially), as highly expressed in a model of allograft tolerance in the rat. We showed that its overexpression in rat DCs blocked their maturation, suggesting a role for this molecule in the maturation process. To characterize the function of Tmem176B further, we used a split-ubiquitin yeast, two-hybrid system to identify interacting partners and found that Tmem176B associated with itself but also with Tmem176A, a membrane protein similar to Tmem176B. Interestingly, these two molecules showed similar mRNA expression patterns among various murine tissues and immune cells and were both down-regulated following DC maturation. In addition, we showed that in using RNAi, these molecules are both involved in the maintenance of the immature state of the DCs. Taken together, these data suggest that Tmem176B and Tmem176A associate to form multimers and restrain DC maturation. Therefore, these two molecules may represent valid targets to regulate DC function.

Characterization of Schlafen-3 expression in effector and regulatory T cells

Members of the Slfn protein family have been implicated in the regulation of cell growth, hematopoietic cell differentiation, and T cell development/differentiation in the thymus. Ten members of this family have been described in the mouse, and they have been divided into three subgroups based on the overall sequence homology and the size of the encoded proteins. We have identified Slfn3, a member of Subgroup II, as an overexpressed gene in CD4(+) CD25(+) T cells in the periphery. Interestingly, we demonstrate that upon activation and proliferation, Slfn3 mRNA is down-regulated in CD4(+) CD25(+) Tregs and up-regulated in CD4(+) CD25(-) Teffs. Moreover, TGF-beta inhibits the expression of Slfn3 in anti-CD3/CD28-activated CD4+ T cells, and the same conditions induce FoxP3 mRNA. Our results suggest that Slfn3 could have a role in T cell differentiation and activation.

The C-type lectin-like receptor CLEC-1, expressed by myeloid cells and endothelial cells, is up-regulated by immunoregulatory mediators and moderates T cell activation

C-type lectin receptors have recently been described as playing crucial roles in immunity and homeostasis since these proteins are able to recognize pathogens as well as self-Ags. We identified the C-type lectin-like receptor-1, CLEC-1, as being overexpressed in a model of rat allograft tolerance. We previously described in this model the expression of numerous cytoprotective molecules by graft endothelial cells and their interplay with regulatory CD4(+)CD25(+) T cells. In this study, we demonstrate that CLEC-1 is expressed by myeloid cells and specifically by endothelial cells in tolerated allografts and that CLEC-1 expression can be induced in endothelial cells by alloantigen-specific regulatory CD4(+)CD25(+) T cells. Analysis of CLEC-1 expression in naive rats demonstrates that CLEC-1 is highly expressed by myeloid cells and at a lower level by endothelial cells, and that its expression is down-regulated by inflammatory stimuli but increased by the immunoregulators IL-10 or TGFbeta. Interestingly, we demonstrate in vitro that inhibition of CLEC-1 expression in rat dendritic cells increases the subsequent differentiation of allogeneic Th17 T cells and decreases the regulatory Foxp3(+) T cell pool. Additionally, in chronically rejected allograft, the decreased expression of CLEC-1 is associated with a higher production of IL-17. Taken together, our data suggest that CLEC-1, expressed by myeloid cells and endothelial cells, is enhanced by regulatory mediators and moderates Th17 differentiation. Therefore, CLEC-1 may represent a new therapeutic agent to modulate the immune response in transplantation, autoimmunity, or cancer settings.

Impaired Notch4 Activity Elicits Endothelial Cell Activation and Apoptosis

Objective— Notch signaling pathway controls key functions in vascular and endothelial cells (EC). However, little is known about the role of Notch in allografted vessels during the development of transplant arteriosclerosis (TA). This study investigated regulation of the Notch pathway on cardiac allograft arteriosclerosis and further examined its implication in EC dysfunction.

Methods and Results— Here we show that, among Notch receptors, Notch2, -3, and -4 transcript levels were markedly downregulated in TA compared to tolerant and syngeneic allografts. TA correlates with high levels of tumor necrosis factor (TNF), transforming growth factor (TGF)β, and IL10, which consistently decrease Notch4 expression in transplants and cultured ECs. We found that inhibition of Notch activity, reflected by both a reduced CBF1 activity and Hes1 expression, parallels the downregulation of Notch4 expression mediated by TNF in ECs. Notch4 and Hes1 knockdown enhances vascular cell adhesion molecule-1 expression and promotes EC apoptosis. Silencing Notch4 or Hes1 also drastically inhibits repair of endothelial injury. Overall, our results suggest that Notch4 and basal Notch activity are required to maintain EC quiescence and for optimal survival and repair in response to injury.

Conclusion— Together, our findings indicate that impaired Notch4 activity in graft ECs is a key event associated with TA by triggering EC activation and apoptosis.

An immunomodulatory role for follistatin-like 1 in heart allograft transplantation

Donor-specific tolerance to heart allografts in the rat can be achieved by donor-specific blood transfusions (DST) before transplantation. We have previously reported that this tolerance is associated with strong leukocyte infiltration, and that host CD8(+) T cells and TGFbeta are required. In order to identify new molecules involved in the induction phase of tolerance, we compared tolerated and rejected heart allografts (suppressive subtractive hybridization) 5 days after transplantation. We identified overexpression of Follistatin-like 1 (FSTL1) transcript in tolerated allografts compared to rejected allografts or syngeneic grafts. We show that FSTL1 is overexpressed during both the induction and maintenance phase of tolerance, and appears to be specific to the tolerance model induced by DST. Analysis of graft-infiltrating cells revealed predominant expression of FSTL1 in CD8(+) T cells from tolerated grafts, and depletion of these cells prior to transplantation abrogated FSTL1 expression and heart allograft survival. Moreover, overexpression of FSTL1 by adenovirus gene transfer in vivo significantly prolonged allograft survival in association with inhibition of the proinflammatory cytokines, IL6, IL17 A and IFNgamma. Taken together, these results suggest that FSTL1 could be an active component of the mechanisms mediating heart allograft tolerance.

Tribbles-1 as a novel biomarker of chronic antibody-mediated rejection

Diagnosis of the specific cause of late allograft injury is necessary if more personalized and efficient immunosuppressive regimens are to be introduced. This study sought previously unrecognized biomarkers for specific histologic diagnoses of late graft scarring by comparison of gene sets from published microarray studies. Tribbles-1 (TRIB1), a human homolog of Drosophila tribbles, was identified to be a potentially informative biomarker. For testing this, mRNA expression in 76 graft biopsies, 71 blood samples, and 11 urine samples were profiled from independent cohorts of renal transplant patients with different histologic diagnoses recruited at two European centers. TRIB1 but not TRIB2 or TRIB3 was found to be a potential blood and tissue biomarker of chronic antibody-mediated rejection, an active immune-mediated form of chronic allograft failure associated with a poor prognosis. TRIB1 mRNA levels in peripheral blood mononuclear cells discriminated patients with chronic antibody-mediated rejection from those with other types of late allograft injury with high sensitivity and specificity. TRIB1 was also upregulated in a rodent model of chronic cardiac vasculopathy, suggesting that this biomarker may be useful in other solid-organ transplants and across species. It was determined that TRIB1 is expressed primarily by antigen-presenting cells and activated endothelial cells. Overall, these data support the potential use of TRIB1 as a biomarker of chronic antibody-mediated allograft failure.

Implication of matrix metalloproteinase 7 and the noncanonical wingless-type signaling pathway in a model of kidney allograft tolerance induced by the administration of anti-donor class II antibodies

In rats, tolerance to MHC-incompatible renal allografts can be induced by the administration of anti-donor class II Abs on the day of transplantation. In this study we explored the mechanisms involved in the maintenance phase of this tolerance by analyzing intragraft gene expression profiles by microarray in long-term accepted kidneys. Comparison of the gene expression patterns of tolerated to syngeneic kidneys revealed 5,954 differentially expressed genes (p < 0.05). Further analysis of this gene set revealed a key role for the wingless-type (WNT) signaling pathway, one of the pivotal pathways involved in cell regulation that has not yet been implicated in transplantation. Several genes within this pathway were significantly up-regulated in the tolerated grafts, particularly matrix metalloproteinase 7 (MMP7; fold change > 40). Analysis of several other pathway-related molecules indicated that MMP7 overexpression was the result of the noncanonical WNT signaling pathway. MMP7 expression was restricted to vascular smooth muscle cells and was specific to anti-class II Ab-induced tolerance, as it was undetectable in other models of renal and heart transplant tolerance and chronic rejection induced across the same strain combination. These results suggest a novel role for noncanonical WNT signaling in maintaining kidney transplant tolerance in this model, with MMP7 being a key target. Determining the mechanisms whereby MMP7 contributes to transplant tolerance may help in the development of new strategies to improve long-term graft outcome.

Role of IFNgamma in allograft tolerance mediated by CD4+CD25+ regulatory T cells by induction of IDO in endothelial cells

Regulatory T cells have been described to specifically accumulate at the site of regulation together with effector T cells and antigen-presenting cells, establishing a state of local immune privilege. However the mechanisms of this interplay remain to be defined. We previously demonstrated, in a fully MHC mismatched rat cardiac allograft combination, that a short-term treatment with a deoxyspergualine analogue, LF15-0195, induces long-term allograft tolerance with a specific expansion of regulatory CD4+CD25+T cells that accumulate within the graft. In this study, we show that following transfer of regulatory CD4+T cells to a secondary irradiated recipient, regulatory CD25+Foxp3+ and CD25+Foxp3(-) CD4+T cells accumulate at the graft site and induce graft endothelial cell expression of Indoleamine 2, 3-dioxygenase (IDO) by an IFNgamma-dependent mechanism. Moreover, in vivo transfer of tolerance can be abrogated by blocking IFNgamma or IDO, and anti-IFNgamma reduces the survival/expansion of alloantigen-induced regulatory Foxp3+CD4+T cells. Together, our results demonstrate interrelated mechanisms between regulatory CD4+CD25+T cells and the graft endothelial cells in this local immune privilege, and a key role for IFNgamma and IDO in this process.

CD40Ig treatment results in allograft acceptance mediated by CD8CD45RC T cells, IFN-gamma, and indoleamine 2,3-dioxygenase

Treatment with CD40Ig results in indefinite allograft survival in a complete MHC-mismatched heart allograft model in the rat. Here we show that serial second, third, and fourth adoptive transfers of total splenocytes from CD40Ig-treated recipients into secondary recipients led to indefinite donor-specific allograft acceptance. Purification of splenocyte subpopulations from CD40Ig-treated recipients demonstrated that only the adoptively transferred CD8(+)CD45RC(low) subset resulted in donor-specific long-term survival, whereas CD8(+)CD45RC(low) T cells from naive animals did not. Accepted grafts displayed increased indoleamine 2,3-dioxygenase (IDO) expression restricted in the graft to ECs. Coculture of donor ECs with CD8(+)CD45RC(low) T cells purified from CD40Ig-treated animals resulted in donor-specific IDO expression dependent on IFN-gamma. Neutralization of IFN-gamma or IDO triggered acute allograft rejection in both CD40Ig-treated and adoptively transferred recipients. This study demonstrates for what we believe to be the first time that interference in CD40-CD40 ligand (CD40-CD40L) interactions induces allospecific CD8(+) Tregs that maintain allograft survival. CD8(+)CD45RC(low) T cells act through IFN-gamma production, which in turn induces IDO expression by graft ECs. Thus, donor alloantigen-specific CD8(+) Tregs may promote local graft immune privilege through IDO expression.

New evidence for a role of allograft accommodation in long-term tolerance

BACKGROUND

Progressively better therapies have largely prevented or at least effectively treated acute allograft rejection. Consequently, the long-term survival of solid organ transplants has increasingly become limited primarily by the development of chronic allograft rejection. The mechanisms of chronic rejection remain largely unknown and the induction of specific tolerance would be the ultimate achievement in transplant immunology. We previously demonstrated, in a fully major histocompatibility complex (MHC)-mismatched rat cardiac allograft combination, that a 20-day treatment with a deoxyspergualin (DSG) analogue, LF15-0195, induces allograft tolerance with the development of potent CD4CD25 regulatory T cells. In order to better characterize the mechanisms involved in allograft tolerance, we compared long-term tolerated allografts with allografts exhibiting signs of chronic rejection induced by donor-specific blood transfusion.

METHODS

We analyzed both types of allografts for infiltration, alloantibody production and gene expression by histology, exhaustive microarray and quantitative reverse-transcriptase polymerase chain reaction.

RESULTS

Interestingly, we observed in tolerated allografts an infiltrate as dense as the one observed in chronically rejected allografts and alloantibody deposits on graft endothelial cells. Prominent gene expression of many putative proinflammatory cytokines and genes related to cell activation or cytotoxicity were observed in tolerated allografts. However, we observed a specific upregulation of cytoprotective genes such as nitric oxide synthase, BclXL, and indoleamine 2,3 dioxygenase, and a poor in situ expression of immunoglobulin chain gene.

CONCLUSIONS

This study demonstrates a state of accommodation of tolerated allografts and suggests the importance of early control of humoral immunity for the prevention of chronic rejection and the maintenance of long-term tolerance.

TLR9 ligand enhances proliferation of rat CD4+ T cell and modulates suppressive activity mediated by CD4+ CD25+ T cell

Toll-like receptors (TLRs) play a crucial role in the initiation of innate responses following microbial infection and also in adaptive immune responses by orchestrating the activation of different cell populations. TLRs are expressed at high levels in antigen-presenting cells and recent studies have demonstrated the expression and biological role of TLRs in mouse and human CD4(+) T cells. In this study, we analyzed TLR mRNA expression in rat CD4(+) T cells using stringent quantitative reverse transcription-PCR conditions enabling a direct comparison of the levels of each TLR. We show that TLR3, 5, 6 and 9 mRNAs are the highest TLRs expressed in rat CD4(+) T cells and that TLR5 mRNA is highly expressed in regulatory CD4(+) CD25(+) T cells. In addition, we show that the TLR9 ligand (TLR9L), CpG oligodeoxynucleotide, synergizes with anti-CD3 to induce proliferation of both CD4(+) CD25(-) and regulatory CD4(+) CD25(+) T cells and that TLR9L partially abrogates the suppressive activity mediated by regulatory CD4(+) CD25(+) T cells. This loss of suppression is in part due to the direct effect of TLR9L on effector T cells that are rendered more resistant to the regulation exerted by regulatory T cells. To our knowledge, this is the first study describing the expression of TLR mRNA in rat CD4(+) T cells and the capacity of TLR9L to directly regulate rat T cell responses. Thus, TLR9L may rapidly increase the host's adaptive immunity by expanding effector cells and also by attenuating the suppressive activity mediated by regulatory T cells.

TRAF6 is a T cell-intrinsic negative regulator required for the maintenance of immune homeostasis

TRAF6 has a key role in the regulation of innate immune responses by mediating signals from both TNF receptor and interleukin-1 receptor/Toll-like receptor superfamilies. Here we show that T cell-specific deletion of TRAF6 unexpectedly results in multiorgan inflammatory disease. TRAF6-deficient T cells exhibit hyperactivation of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway compared with wild-type T cells and, as a result, become resistant to suppression by CD4+ CD25+ regulatory T cells. These data identify a previously unrecognized role for TRAF6 in the maintenance of peripheral tolerance, and suggest the presence of a T cell-intrinsic control mechanism to render responder T cells susceptible to tolerizing signals.

Accumulation of T Cells with Potent Regulatory Properties and Restricted Vβ7-TCR Rearrangements in Tolerated Allografts

BACKGROUND

We have previously demonstrated that a short-course treatment with LF15-0195, a 15-deoxyspergualin analogue, induces donor-specific tolerance of cardiac allografts in rats and expansion of splenic CD4CD25 regulatory T cells.

METHODS

To further characterize long-term tolerance in this model, we have analyzed the phenotype, regulatory properties and TCR-Vbeta usage of the T cells infiltrating the tolerated allografts.

RESULTS

We demonstrate that the tolerated allografts express high levels of FoxP3 transcripts and contain a large number of CD4 T cells, half of which express CD25. Moreover, T cells from these tolerated allografts are very powerful at transferring tolerance to a subsequent allograft recipient, demonstrating the presence of potent regulatory T cells at the site of the graft. Interestingly, the T cells infiltrating the tolerated allografts systematically display restricted Vbeta7 TCR rearrangements.

CONCLUSION

These results demonstrate in this model of tolerance, a specific accumulation of T cells with potent regulatory properties and exhibiting restricted Vbeta7-TCR rearrangements at the graft site.

Identification of a new member of the CD20/FcepsilonRIbeta family overexpressed in tolerated allografts

We identified a novel rat gene specifically overexpressed in tolerated heart allografts in a model of tolerance induced by donor-specific blood transfusion (DST). We named this gene TORID, for tolerance-related and induced transcript. We show that TORID expression can be attributed to non-T cells infiltrating tolerated grafts. Interestingly, TORID overexpression was also observed in long-term grafts from a different model of tolerance in which chronic rejection does not occur. Comparison of the predicted amino acid sequence of TORID and of its human counterpart LR8 showed an homology with the four-transmembrane CD20/FcepsilonRIbeta family proteins. We investigated TORID expression in naive rat immune cells and lymphoid tissues. TORID was found to be preferentially expressed in cells of the myeloid lineage such as macrophages and dendritic cells (DCs). Its expression dramatically decreased following activation/maturation. Similar results were obtained in human monocyte-derived DCs. Interestingly, TORID overexpression in bone marrow-derived DCs alters expression of MHC II and CD86 and production of IL12p40 following activation. These results suggest that TORID may be involved in the control of DC maturation and may, therefore, play a role in the induction or maintenance of allograft tolerance.

Induction of Fractalkine and CX3CR1 Mediated by Host CD8+ T Cells in Allograft Tolerance Induced by Donor Specific Blood Transfusion

BACKGROUND

Donor-specific tolerance to heart allografts in the rat can be achieved by donor-specific blood transfusions (DST) before transplantation. This tolerance induction requires the presence of host CD8 T cells and is characterized by the infiltration of numerous leukocytes.

METHODS

To identify new mediators involved in tolerance induction, gene searching was performed and resulted in the identification of the Fractalkine receptor, CX3CR1, as being highly expressed in tolerated allografts.

RESULTS

We showed that the high CX3CR1 mRNA accumulation found in tolerated allografts was related to the active recruitment of monocytes/macrophages. CX3CR1 transcript accumulation was preceded by an early expression of its ligand, Fractalkine, by graft endothelial cells. Interestingly, depletion of recipient CD8 cells led to a dramatic decrease in both CX3CR1 and Fractalkine mRNA levels. Moreover, in vitro, CD8 T cells from DST-primed animals were found to strongly induce Fractalkine expression in an allogeneic endothelial cell line.

CONCLUSION

This is the first report describing Fractalkine, a chemokine usually described in inflammatory processes, as being expressed in a model of allograft tolerance.

TNF receptor-associated factor 6 deficiency during hemopoiesis induces Th2-polarized inflammatory disease

Toll-like receptors (TLR) initiate rapid innate immune responses by recognizing microbial products. These events in turn lead to the development of an efficient adaptive immune response through the up-regulation of a number of costimulatory molecules, including members of the TNF/TNFR superfamily, on the surface of an APC. TNFR-associated factor 6 (TRAF6) is a common signaling adapter used by members of both the TNFR and the TLR/IL-1R superfamilies, and as such plays a critical role in the development of immune responses. As TRAF6-deficient mice die prematurely, we generated chimeras reconstituted with TRAF6-deficient fetal liver cells to analyze functions of TRAF6 in vivo in the hemopoietic compartment. We found that TRAF6-deficient chimeras develop a progressive lethal inflammatory disease associated with massive organ infiltration and activation of CD4(+) T cells in a Th2-polarized phenotype, and a defect in IL-18 responsiveness. When recombination-activating gene 2(-/-) blastocysts were complemented with TRAF6-deficient embryonic stem cells, a marked elevation of activated CD4(+) T cells and progressive inflammatory disease were also observed. Moreover, T cell activation and lethal inflammation were not reversed in mixed chimeric mice generated from normal and TRAF6-deficient fetal liver cells. These results suggest that deletion of TRAF6 induces a dominant Th2-type polarized autoimmune response. Therefore, in addition to playing a critical role in innate and adaptive immunity, TRAF6 is likely to play a previously unrecognized role in the maintenance of self-tolerance.

Bcl-XL expression in stem cells facilitates engraftment and reduces the need for host conditioning during bone marrow transplantation

The prolonged survival of donor hematopoietic stem cells is crucial to the success of bone marrow transplantation. The anti-apoptotic gene Bcl-xL has been shown to promote survival of cells of the erythroid, myeloid and lymphoid lineages. To evaluate a potential therapeutic role for Bcl-xL, we used a retroviral vector to express Bcl-xL in donor cells used for murine bone marrow transplantation. We find that Bcl-xL expression in bone marrow cells facilitates hematopoietic reconstitution (as assessed by total cellularity) without altering cell differentiation. Most importantly, cells reconstituted with Bcl-xL are able to achieve high levels of donor chimerism even in non-ablative conditioning protocols in a syngeneic model of transplantation. Thus, expression of Bcl-xL by donor cells during bone marrow transplantation may provide a means to minimize host conditioning and toxicity while still achieving therapeutic degrees of mixed chimerism.

TRAF6 is a critical factor for dendritic cell maturation and development

IL-1 receptor (IL-1R)/Toll-like receptor (TLR) family and TNF receptor (TNFR) superfamily members are critical for regulating multiple aspects of dendritic cell (DC) biology. Several signaling pathways associated with each family utilize the adapter molecule, TRAF6, but its role in DCs is unclear. By examining TRAF6-deficient mice and bone marrow (BM) chimeras reconstituted with TRAF6-deficient fetal liver cells, we show that proper DC maturation requires TRAF6. In response to either microbial components or CD40L, TRAF6-deficient DCs fail to upregulate surface expression of MHCII and B7.2, or produce inflammatory cytokines. Moreover, LPS-treated TRAF6-deficient DCs do not exhibit an enhanced capacity to stimulate naive T cells. Interestingly, a major population of splenic DCs, the CD4(+)CD8alpha(-) subset, is nearly absent in both TRAF6-deficient mice and BM chimeras. Together these results indicate that TRAF6 regulates the critical processes required for maturation, activation, and development of DCs, the primary cellular bridge between innate and adaptive immunity.

Notch signaling augments T cell responsiveness by enhancing CD25 expression

Notch receptors signal through a highly conserved pathway to influence cell fate decisions. Notch1 is required for T lineage commitment; however, a role for Notch signaling has not been clearly defined for the peripheral T cell response. Notch gene expression is induced, and Notch1 is activated in primary CD4(+) T cells following specific peptide-Ag stimulation. Notch activity contributes to the peripheral T cell response, as inhibition of endogenous Notch activation decreases the proliferation of activated T cells in a manner associated with the diminished production of IL-2 and the expression of the high affinity IL-2R (CD25). Conversely, forced expression of a constitutively active Notch1 in primary T cells results in increased surface expression of CD25, and renders these cells more sensitive to both cognate Ag and IL-2, as measured by cell division. These data suggest an important role for Notch signaling during CD4(+) T cell responses, which operates through augmenting a positive feedback loop involving IL-2 and its high affinity receptor.

Apoptosis and transplantation tolerance

Self-tolerance is maintained by several mechanisms including deletion (via apoptosis) and regulation. Acquired tolerance to allogeneic tissues and organs exploits similar strategies. One key difference between alloantigens and peptide antigens is the enormous number of T cells that are alloreactive. Accumulating evidence suggests that in the face of this large mass of potentially graft-destructive T cells, tolerance requires an initial wave of deletion. This creates a more level playing field in which a smaller number of regulatory T cells can then act to maintain an established tolerant state. Deletion of alloreactive T cells by apoptosis actively promotes immunoregulation as well, by interfering with proinflammatory maturation of antigen presenting cells. This article reviews the immune response to alloantigens, the development and use of both necrotic and apoptotic means of cell death during the evolution of the immune response, and the likely role and mechanisms by which apoptosis promotes, and may even be required for, transplantation tolerance.

Induction of donor-specific allograft tolerance by short-term treatment with LF15-0195 after transplantation. Evidence for a direct effect on T-cell differentiation

A 20-day treatment with LF15-0195, a deoxyspergualine analog, induced long-term heart allograft survival in the rat without signs of chronic rejection. LF15-0195-treated recipients did not develop an anti-donor alloantibody response. Analysis of graft-infiltrating cells, IL10, TNFalpha, IFNgamma mRNA and iNOS protein expression in allografts, 5 days after transplantation, showed that they were markedly decreased in allografts from LF15-0195-treated recipients compared with allografts from untreated recipients. Surprisingly, spleen T cells from LF15-0195 recipients, 5days after grafting, were able to proliferate strongly in vitro, when stimulated with donor cells, but had reduced mRNA expression for IFNy compared with spleen T cells from untreated graft recipients. Furthermore, when T cells from naive animals were stimulated in vitro, using anti-CD3 and anti-CD28, LF15-0195 also increased T-cell proliferation in a dose-dependent fashion: however, these cells expressed less of the Th1 -related cytokines, IFNgamma and IL2, compared with untreated cells, suggesting that LF15-0195 could act on T-cell differentiation. In conclusion, we show here that a short-term treatment with LF15-0195 induced long-term allograft tolerance, decreasing the in situ anti-donor response, and we illustrate evidence for the development of regulatory mechanisms.

Role for thymic and splenic regulatory CD4+ T cells induced by donor dendritic cells in allograft tolerance by LF15-0195 treatment

A 20-day treatment with LF15-0195, a deoxyspergualine analogue, induced allograft tolerance in a fully MHC-mismatched heart allograft model in the rat. Long-term allografts displayed minimal cell infiltration with no signs of chronic rejection. CD4+ spleen T cells from tolerant LF15-0195-treated recipients were able to suppress in vitro proliferation of allogeneic CD4+ T cells and to transfer tolerance to second syngeneic recipients, demonstrating dominant suppression by regulatory cells. A significant increase in the percentage of CD4+CD25+ T cells was observed in the thymus and spleen from tolerant LF15-0195-treated recipient. In vitro direct stimulation with donor APCs demonstrated that CD4+ regulatory T cells proliferated weakly and expressed low levels of IFN-gamma, IL-10, and IL-2. CD4+CD25+ cell depletion increased IL-2 production by CD4+CD25- thymic cells, but not splenic cells. Moreover, tolerance was transferable with splenic and thymic CD4+CD25+ cells, but also in 50% of cases with splenic CD4+CD25- cells, demonstrating that CD25 can be a marker for regulatory cells in the thymus, but not in the periphery. In addition, we presented evidences that donor APCs were required to induce tolerance and to expand regulatory CD4+ T cells. This study demonstrates that LF15-0195 treatment induces donor APCs to expand powerful regulatory CD4+CD25+/- T cells present in both the central and peripheral compartments.

New insight into mechanisms of allograft transplantation in the rat by differential display: macrophage scavenger receptor-A brings to light

BACKGROUND

Donor specific tolerance to heart allografts is induced in LEW.1A rat recipient by two donor LEW.1W blood transfusions prior engraftment. Although the tolerant allograft is infiltrated by leukocytes, graft infiltrating cells are only expressing low levels of the Th1- or Th2-related cytokines suggesting that induction of tolerance is an active phenomenon in which the mechanisms remain to be elucidated.

MATERIALS AND METHODS

Differential display (DD) method was applied on RNAs extracted from graft infiltrating cells (GIC) derived from allografts either from rejecting untreated rats or donor-specific blood transfusion treated tolerant rats. Quantitative RT/PCR was performed to confirm mRNA expressions of the selected genes.

RESULTS

Among the six differentially displayed DNAs (ddDNA) overexpressed in GIC from rejected allografts, the macrophage scavenger receptor-A (A:D13265) was identified; it exhibited a stricking induction of mRNA expression from day 1 to 7 after transplantation. Among the seven ddDNAs overexpressed in GIC from tolerant allografts, the 3-hydroxy-3-methyl glutaryl coenzyme-A reductase (A:M29249) and an "unknown gene" (ddDNA EC9) were identified and both were confirmed to be up-regulated by quantitative RT/PCR.

CONCLUSIONS

The relevance of these genes in transplantation has not yet been reported and must therefore be elucidated; they represent possible targets for immunointervention. Nevertheless, our data demonstrate that the DD is a powerful tool to identify new genes involved in organ transplantation.

Anti-TCR Vbeta-specific DNA vaccination prolongs heart allograft survival in adult rats

Allospecific T cells are known to play a central role in the process of allograft rejection. Recently, it has been shown that T cell function could be specifically targeted using DNA vaccination. In our model, PCR analysis of the TCR-beta chain repertoire of T cells infiltrating rejected allografts showed specific expansions of the Vbeta13 and Vbeta2 families. In this study, we tested the effect on allograft survival of DNA vaccination against a specific TCR Vbeta, in a model of heart allograft rejection in adult rats. Our results showed that anti-TCR Vbeta13 DNA vaccination lead to a significant prolongation of allograft survival compared to vaccination against other Vbeta families or untreated recipients. The prolongation of allograft survival correlated in vitro with a decrease in anti-donor reactivity of spleen cells from Vbeta13-vaccinated rats. These results show that, in a transplantation model, DNA vaccination could be used as a method to specifically manipulate a T cell response and thus prolong allograft survival.

Anti-TCR-specific DNA vaccination demonstrates a role for a CD8+ T cell clone in the induction of allograft tolerance by donor-specific blood transfusion

Donor-specific allograft tolerance can be induced in the adult rat by pregraft donor-specific blood transfusion (DST). This tolerance appeared to be mediated by regulatory cells and to the production of the suppressive cytokine TGF-beta1. A potential immunoregulatory CD8+ clone bearing a Vbeta18-Dbeta1-Jbeta2.7 TCR gene rearrangement was previously identified in DST-treated recipients. To assess the functional role of this T cell clone in the induction of tolerance by DST, we have vaccinated DST-treated recipients with a plasmid construct encoding for the Vbeta18-Dbeta1-Jbeta2.7 TCR beta-chain. DST-induced allograft tolerance was abolished by anti-TCR Vbeta18-Dbeta1-Jbeta2.7 DNA vaccination in six of seven recipients, whereas vaccination with the vector alone, or with the construct encoding a TCR Vbeta13 beta-chain, had no effect. However, the transcript number of the Vbeta18-Dbeta1-Jbeta2.7 chain was unchanged in allografts from vaccinated DST-treated rats, suggesting that this clone was not depleted by vaccination, but rather was altered in its function. Moreover, TCR Vbeta18-Dbeta1-Jbeta2.7 DNA vaccination restored the anti-donor alloantibody production, partially restore the capacity of spleen cells from tolerized recipients to proliferate in vitro against donor cells, and decreased the inhibitory effect of TGF-beta1, seen in DST-treated recipients, in spleen cells from vaccinated DST-treated ones. This study strongly suggests that this CD8+ TCR Vbeta18-Dbeta1-Jbeta2.7 T cell clone has an effective immunoregulatory function in allograft tolerance induced by DST.

Reassessment of the role of CD8+ T cells in the induction of allograft tolerance by donor-specific blood transfusion

Donor-specific allograft tolerance can be induced in adult rats by pregraft donor-specific blood transfusion (DST). We have previously shown that DST elicits in recipients the expansion of a donor-specific CD8+ T cell clone displaying the Vbeta18-Dbeta1-Jbeta2.7 TCR rearrangement, which rapidly infiltrates allografts after transplantation, suggesting a regulatory function for this clone in DST-induced tolerance. In this study, recipients were pretreated before grafting, using an anti-CD8 monoclonal antibody to deplete CD8+ T cells. CD8 depletion before DST and transplantation abrogated allograft tolerance, and the CD8+ T cell clone was absent from allografts. These effects were not observed when CD8 depletion was performed after DST but before transplantation. We conclude that CD8+ T cells play a role in the induction of allograft tolerance by DST.