Expansion of peripheral naturally occurring T regulatory cells by Fms-like tyrosine kinase 3 ligand treatment. Blood. 2009;113:6277-6287. [PMID: 19211508 DOI: 10.1182/blood-2008-06-161026]

Flt3 ligand augments immune responses to soluble PD1-based DNA vaccine via expansion of type 1 conventional DCs

DNA vaccines are prospective for their efficient manufacturing process, but their immunogenicity is limited as they cannot efficiently induce CD8+ T cell responses. A promising approach is to induce cross-presentation by targeting antigens to DCs. Flt3L can expand the number of type 1 conventional DCs and thereby improve cross-presentation. In this study, we first constructed a DNA vaccine expressing soluble PD1 and found that the therapeutic effect of targeting DCs with only the sPD1 vaccine was limited. When combined the vaccine with Flt3L, the anti-tumor effect was significantly enhanced. Considering the complexity of tumors and that a single method may not be able to activate a large number of effective CD8+ T cells, we combined different drugs and the vaccine with Flt3L based on the characteristics of different tumors. In 4T1 model, we reduced Tregs through cyclophosphamide. In Panc02 model, we increased activated DCs by using aCD40. Both strategies triggered strong CD8+ T cell responses and significantly improved the therapeutic effect. Our study provides important support for the clinical exploration of DC-targeted DNA vaccines in combination with Flt3L.

Copper homeostasis and cuproptosis in radiation-induced injury

Radiation therapy for cancer treatment brings about a series of radiation injuries to normal tissues. In recent years, the discovery of copper-regulated cell death, cuproptosis, a novel form of programmed cell death, has attracted widespread attention and exploration in various biological functions and pathological mechanisms of copper metabolism and cuproptosis. Understanding its role in the process of radiation injury may open up new avenues and directions for exploration in radiation biology and radiation oncology, thereby improving tumor response and mitigating adverse reactions to radiotherapy. This review provides an overview of copper metabolism, the characteristics of cuproptosis, and their potential regulatory mechanisms in radiation injury.

FLT3+ DC inhibits immune rejection via interaction with Treg in liver transplantation

Fms-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase (RTK) primarily expressed in hematopoietic stem cells and dendritic cells (DCs). While FLT3 plays a critical role in the proliferation, development and maintenance of DCs, thus influencing immune responses under both normal and pathological conditions, there also exists some evidence that FLT3+DC may be involved with immune responses in liver transplantation (LT). In this study, results from single-cell sequencing data analysis revealed a clear relationship between FLT3+DCs and Regulatory T cells (Tregs) in liver tissue of LT recipients. In peripheral blood samples of LT patients, levels of FLT3+DCs were decreased post-LT-surgery, while Tregs were increased. In a LT mouse model, levels of FLT3+DCs in the liver and bone marrow exhibited an initial time-dependent decrease followed by an increase after LT surgery. Results as obtained with co-culture experiments using mature BMDCs and CD4+ T cells revealed fluctuations in Tregs in response to FLT3 inhibitors and the FLT3 ligand. These findings suggest that FLT3+DCs could emerge as a novel target for mitigating immune rejection in LT.

FLT3L-dependent dendritic cells control tumor immunity by modulating Treg and NK cell homeostasis

FLT3-L-dependent classical dendritic cells (cDCs) recruit anti-tumor and tumor-protecting lymphocytes. We evaluate cancer growth in mice with low, normal, or high levels of cDCs. Paradoxically, both low or high numbers of cDCs improve survival in mice with melanoma. In low cDC context, tumors are restrained by the adaptive immune system through influx of effector T cells and depletion of Tregs and NK cells. High cDC numbers favor the innate anti-tumor response, with massive recruitment of activated NK cells, despite high Treg infiltration. Anti CTLA-4 but not anti PD-1 therapy synergizes with FLT3-L therapy in the cDCHi but not in the cDCLo context. A combination of cDC boost and Treg depletion dramatically improves survival of tumor-bearing mice. Transcriptomic data confirm the paradoxical effect of cDC levels on survival in several human tumor types. cDCHi-TregLo state in such patients predicts best survival. Modulating cDC numbers via FLT3 signaling may have therapeutic potential in human cancer.

CAR-T cells and CAR-Tregs targeting conventional type-1 dendritic cell suppress experimental autoimmune encephalomyelitis

Conventional type 1 dendritic cells (DC1) contribute to the development of pathogenic T helper type 1 (Th1) cells in part via the production of the proinflammatory cytokine interleukin-12. Thus, depletion of DC1 has the potential to dampen autoimmune responses. Here, we developed X-C motif chemokine receptor 1 (XCR1)-specific chimeric antigen receptor (CAR)-T cells and CAR-Tregs that specifically targeted DC1. XCR1 CAR-T cells were successfully generated as CD4+ and CD8+ T cells, expressed XCR1 CAR efficiently, and induced XCR1-dependent activation, cytokine production and proliferation. XCR1 CAR-T cells selectively depleted DC1 when transferred into RAG2-/- mice with a compensatory increase in conventional type 2 DC (DC2) and plasmacytoid DC (pDC). XCR1 CAR-T cell-mediated depletion of DC1 modestly suppressed the onset of Th1-driven experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis. Diphtheria toxin-mediated DC1 depletion in XCR1-diphtheria toxin receptor mice also suppressed EAE, suggesting that DC1 depletion was responsible for EAE suppression. XCR1 CAR-Tregs were successfully generated and suppressed effector T cells in the presence of XCR1+ cells. Therapeutic treatment with XCR1 CAR-Tregs suppressed Th1-driven EAE. Therefore, we conclude that depletion of DC1 with XCR1 CAR-T cells or immune suppression with XCR1 CAR-Tregs can modestly suppress Th1-driven EAE.

Skin-derived TSLP stimulates skin migratory dendritic cells to promote the expansion of regulatory T cells

Therapeutic strategies that enhance regulatory T (Treg) cell proliferation or suppressive function hold promise for the treatment of autoimmune and inflammatory diseases. We previously reported that the topical application of the vitamin D3 analog MC903 systemically expands Treg cells by stimulating the production of thymic stromal lymphopoietin (TSLP) from the skin. Using mice lacking TSLP receptor expression by dendritic cells (DCs), we hereby show that TSLP receptor signaling in DCs is required for this Treg expansion in vivo. Topical MC903 treatment of ear skin selectively increased the number of migratory DCs in skin-draining lymph nodes (LNs) and upregulated their expression of co-stimulatory molecules. Accordingly, DCs isolated from skin-draining LNs but not mesenteric LNs or spleen of MC903-treated mice showed an enhanced ability to promote Treg proliferation, which was driven by co-stimulatory signals through CD80/CD86 and OX40 ligand. Among the DC subsets in the skin-draining LNs of MC903-treated mice, migratory XCR1- CD11b+ type 2 and XCR1- CD11b- double negative conventional DCs promoted Treg expansion. Together, these data demonstrate that vitamin D3 stimulation of skin induces TSLP expression, which stimulates skin migratory DCs to expand Treg cells. Thus, topical MC903 treatment could represent a convenient strategy to treat inflammatory disorders by engaging this pathway.

Flt3L therapy increases the abundance of Treg-promoting CCR7+ cDCs in preclinical cancer models

Conventional dendritic cells (cDCs) are at the forefront of activating the immune system to mount an anti-tumor immune response. Flt3L is a cytokine required for DC development that can increase DC abundance in the tumor when administered therapeutically. However, the impact of Flt3L on the phenotype of distinct cDC subsets in the tumor microenvironment is still largely undetermined. Here, using multi-omic single-cell analysis, we show that Flt3L therapy increases all cDC subsets in orthotopic E0771 and TS/A breast cancer and LLC lung cancer models, but this did not result in a reduction of tumor growth in any of the models. Interestingly, a CD81+migcDC1 population, likely developing from cDC1, was induced upon Flt3L treatment in E0771 tumors as well as in TS/A breast and LLC lung tumors. This CD81+migcDC1 subset is characterized by the expression of both canonical cDC1 markers as well as migratory cDC activation and regulatory markers and displayed a Treg-inducing potential. To shift the cDC phenotype towards a T-cell stimulatory phenotype, CD40 agonist therapy was administered to E0771 tumor-bearing mice in combination with Flt3L. However, while αCD40 reduced tumor growth, Flt3L failed to improve the therapeutic response to αCD40 therapy. Interestingly, Flt3L+αCD40 combination therapy increased the abundance of Treg-promoting CD81+migcDC1. Nonetheless, while Treg-depletion and αCD40 therapy were synergistic, the addition of Flt3L to this combination did not result in any added benefit. Overall, these results indicate that merely increasing cDCs in the tumor by Flt3L treatment cannot improve anti-tumor responses and therefore might not be beneficial for the treatment of cancer, though could still be of use to increase cDC numbers for autologous DC-therapy.

Dendritic cell Flt3 - regulation, roles and repercussions for immunotherapy

Dendritic cells (DCs) are essential for initiating immune responses. Depending on the environment, the type of DC and the way in which they interact with T cells, these immune responses can be beneficial or detrimental. DCs can be exploited as cellular vectors for vaccines against infection and cancer. The development and maintenance of DCs is dependent on the FMS-like tyrosine kinase 3 (Flt3)/Flt3 ligand (Flt3L) signaling cascade. Flt3 is also one of the most commonly mutated genes in acute myeloid leukemia and as such represents an attractive drug target. In this review, Flt3 is discussed with a particular focus on DCs. We detail the lifecycle of Flt3, from transcription to degradation, and interrogate recent studies as to how this pathway can be manipulated for immunotherapy, vaccination and treatment of autoimmune disease.

Bendamustine Conditioning Skews Murine Host DCs Toward Pre-cDC1s and Reduces GvHD Independently of Batf3

Graft-versus-host disease (GvHD) remains the second leading cause of death in allogeneic hematopoietic stem cell transplantation recipients, highlighting the need for improved preventative strategies. Our laboratory has previously demonstrated in an experimental bone marrow transplantation (BMT) model that bendamustine combined with total body irradiation (BEN+TBI) is a safer alternative to cyclophosphamide with TBI (CY+TBI). The biological mechanisms of action of BEN have not been fully elucidated and likely involve multiple cell populations. Host dendritic cells (DCs) can prime naïve donor T-cells immediately following transplantation, making host DCs critical for the initiation phase of GvHD. We hypothesized that BEN+TBI conditioning favorably alters host DC composition to reduce GvHD. We demonstrate that host DCs treated with BEN+TBI induce less allogeneic T-cell proliferation than those conditioned with CY+TBI. We further show that BEN+TBI conditioning results in greater total numbers of all host DC subsets but with a more favorable composition compared to CY+TBI with significantly larger proportions of type 1 conventional DCs (cDC1), a highly regulatory DC subset capable of suppressing GvHD. Our studies using recipient Batf3 KO mice indicate that CD8α+ cDC1s are largely dispensable for the reduced GvHD following BEN+TBI conditioning. We found a higher frequency of host pre-cDC1s with BEN+TBI conditioning in both wild-type (WT) and Batf3 KO mice, which was inversely associated with GvHD. Additionally, we observed that BEN treatment results in greater expression of Flt3 receptor (CD135) on host DCs compared to CY, potentially contributing to the skewing of host DCs toward cDC1s. Further, BEN+TBI conditioning results in host cDCs with greater expression of PIR-B, an inhibitory receptor capable of preventing lethal GvHD. We conclude that BEN+TBI is a safer alternative to CY+TBI, resulting in a greater frequency of host pre-cDC1s and limiting GvHD.

FMS-like Tyrosine Kinase 3/FLT3: From Basic Science to Clinical Implications

FMS-like tyrosine kinase 3 (FLT3) is a receptor tyrosine kinase that is expressed almost exclusively in the hematopoietic compartment. Its ligand, FLT3 ligand (FL), induces dimerization and activation of its intrinsic tyrosine kinase activity. Activation of FLT3 leads to its autophosphorylation and initiation of several signal transduction cascades. Signaling is initiated by the recruitment of signal transduction molecules to activated FLT3 through binding to specific phosphorylated tyrosine residues in the intracellular region of FLT3. Activation of FLT3 mediates cell survival, cell proliferation, and differentiation of hematopoietic progenitor cells. It acts in synergy with several other cytokines to promote its biological effects. Deregulated FLT3 activity has been implicated in several diseases, most prominently in acute myeloid leukemia where around one-third of patients carry an activating mutant of FLT3 which drives the disease and is correlated with poor prognosis. Overactivity of FLT3 has also been implicated in autoimmune diseases, such as rheumatoid arthritis. The observation that gain-of-function mutations of FLT3 can promote leukemogenesis has stimulated the development of inhibitors that target this receptor. Many of these are in clinical trials, and some have been approved for clinical use. However, problems with acquired resistance to these inhibitors are common and, furthermore, only a fraction of patients respond to these selective treatments. This review provides a summary of our current knowledge regarding structural and functional aspects of FLT3 signaling, both under normal and pathological conditions, and discusses challenges for the future regarding the use of targeted inhibition of these pathways for the treatment of patients.

Strain specific maturation of Dendritic cells and production of IL-1β controls CD40-driven colitis

Intestinal integrity is maintained by balanced numbers of CD103+ Dendritic cells (DCs), which generate peripherally induced regulatory T cells (iTregs). We have developed a mouse model where DC-specific constitutive CD40 signals caused a strong reduction of CD103+ DCs in the lamina propria (LP) and intestinal lymph nodes (LN). As a consequence, also iTregs were strongly reduced and transgenic mice on the C57Bl/6-background (B6) developed fatal colitis. Here we describe that transgenic mice on a pure Balb/c-background (B/c) do not show any pathologies, while transgenic C57Bl/6 x Balb/c (F1) mice develop weak colon inflammation, without fatal colitis. This graded pathology correlated with the effects of CD40-signalling on DCs in each background, with striking loss of CD103+ DCs in B6, but reduced in F1 and diminished in B/c background. We further show direct correlation of CD103+ DC-numbers with numbers of iTregs, the frequencies of which behave correspondingly. Striking effects on B6-DCs reflected robust loss of surface MHCII, known to be crucial for iTreg induction. Furthermore, elevated levels of IL-23 together with IL-1, found only in B6 mice, support generation of intestinal IFN-γ+IL-17+ Th17 cells and IFN-γ+ Th1 cells, responsible for onset of disease. Together, this demonstrates a novel aspect of colitis-control, depending on genetic background. Moreover, strain-specific environmental sensing might alter the CD103+ DC/iTreg-axis to tip intestinal homeostatic balance to pathology.

Absence of MHC-II expression by lymph node stromal cells results in autoimmunity

MHCII-restricted antigen presentation by lymph node stromal cells is essential for regulatory T-cell proliferation and functions, and for the regulation of autoimmunity.

How lymph node stromal cells (LNSCs) shape peripheral T-cell responses remains unclear. We have previously demonstrated that murine LNSCs, lymphatic endothelial cells (LECs), blood endothelial cells (BECs), and fibroblastic reticular cells (FRCs) use the IFN-γ–inducible promoter IV (pIV) of the MHC class II (MHCII) transactivator CIITA to express MHCII. Here, we show that aging mice (>1 yr old) in which MHCII is abrogated in LNSCs by the selective deletion of pIV exhibit a significant T-cell dysregulation in LNs, including defective Treg and increased effector CD4⁺ and CD8⁺ T-cell frequencies, resulting in enhanced peripheral organ T-cell infiltration and autoantibody production. The proliferation of LN-Tregs interacting with LECs increases following MHCII up-regulation by LECs upon aging or after exposure to IFN-γ, this effect being abolished in mice in which LECs lack MHCII. Overall, our work underpins the importance of LNSCs, particularly LECs, in supporting Tregs and T-cell tolerance.

GM-CSF therapy inhibits chronic graft-versus-host disease via expansion of regulatory T cells

Regulatory T cells (Tregs) attenuate excessive immune responses, making their expansion beneficial in immune-mediated diseases, including allogeneic bone marrow transplantation associated with graft-versus-host disease (GVHD). In addition to interleukin-2, Tregs require T-cell receptor and costimulatory signals from antigen-presenting cells, such as DCs, for their optimal proliferation. Granulocyte-macrophage colony-stimulating factor (GM-CSF) increases DC number and may promote DC-dependent Treg proliferation. Here, we demonstrate that GM-CSF treatment increases CD4⁺ CD8^- DCs, which are associated with Treg expansion. In a mouse model of chronic GVHD (cGVHD), GM-CSF therapy expanded Tregs, protected against the development of skin GVHD, and regulated both Th1 and Th17 responses in the peripheral lymph nodes, resulting in an attenuation of skin cGVHD. Notably, the expanded Tregs were instrumental to GM-CSF-mediated cGVHD inhibition, which was dependent upon an increased ratio of Tregs to conventional T cells rather than augmentation of suppressive function. These data suggest that GM-CSF induces Treg proliferation by expanding CD4⁺ CD8^- DCs, which in turn regulate alloimmune responses in a cGVHD mouse model. Thus, GM-CSF could be used as a therapeutic DC modulator to induce Treg expansion and to inhibit excessive alloimmune responses in immune-related diseases.

Midostaurin reduces Regulatory T cells markers in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous hematological malignancy in which the only curative approach is allogeneic stem cell transplant (Allo-HSCT). The recognition and elimination of leukemic clones by donor T-cells contribute significantly to Allo-HSCT success. FLT3-ITD, a common mutation in AML, is associated with poor prognosis. Recently, midostaurin became the first FDA approved FLT3-inhibitor for pre-transplant patients with FLT3-ITD in combination with standard therapy. In addition to their multikinase activity which may affect T-cell signaling, FLT3-inhibitors induce apoptosis of malignant cells which may also enhance antigen presentation to activate T-cells. Considering the increased clinical use of these inhibitors in patients with AML, and the limited clinical benefit derived from their use as single agents, understanding how FLT3-inhibitors affect T cell population and function is needed to improve their clinical benefit. We examined the effect of four different FLT3 inhibitors (midostaurin, sorafenib, tandutinib, and quizartenib) on T cell populations in peripheral blood mononuclear cells (PBMC) obtained from healthy donors and from patients with AML. Midostaurin exhibited a significant decrease in CD4 + CD25 + FOXP3+ T cell population and FOXP3 mRNA expression in healthy and AML PBMCs. Similarly, samples collected from patients with AML treated with midostaurin showed a reduction in Tregs markers. Interferon-γ(IFN-γ), tumor necrosis factor-α(TNF-α), and IL-10 levels were also reduced following midostaurin treatment. Considering the FDA approval of midostaurin for use in patients with AML in the pre-transplant setting, our finding will have important clinical implication as it provides the rationale for functional investigation of the use of midostaurin in post-transplant patients.

Accumulation of Multipotent Hematopoietic Progenitors in Peripheral Lymphoid Organs of Mice Over-expressing Interleukin-7 and Flt3-Ligand

Interleukin-7 (IL-7) and Flt3-ligand (FL) are two cytokines important for the generation of B cells, as manifested by the impaired B cell development in mice deficient for either cytokine or their respective receptors and by the complete block in B cell differentiation in the absence of both cytokines. IL-7 is an important survival and proliferation factor for B cell progenitors, whereas FL acts on several early developmental stages, prior to B cell commitment. We have generated mice constitutively over-expressing both IL-7 and FL. These double transgenic mice develop splenomegaly and lymphadenopathy characterized by tremendously enlarged lymph nodes even in young animals. Lymphoid, myeloid and dendritic cell numbers are increased compared to mice over-expressing either of the two cytokines alone and the effect on their expansion is synergistic, rather than additive. B cell progenitors, early progenitors with myeloid and lymphoid potential (EPLM), common lymphoid progenitors (CLP) and lineage⁻, Sca1⁺, kit⁺ (LSK) cells are all increased not only in the bone marrow but also in peripheral blood, spleen and even lymph nodes. When transplanted into irradiated wild-type mice, lymph node cells show long-term multilineage reconstitution, further confirming the presence of functional hematopoietic progenitors therein. Our double transgenic mouse model shows that sustained and combined over-expression of IL-7 and FL leads to a massive expansion of most bone marrow hematopoietic progenitors and to their associated presence in peripheral lymphoid organs where they reside and potentially differentiate further, thus leading to the synergistic increase in mature lymphoid and myeloid cell numbers. The present study provides further in vivo evidence for the concerted action of IL-7 and FL on lymphopoiesis and suggests that extramedullary niches, including those in lymph nodes, can support the survival and maintenance of hematopoietic progenitors that under physiological conditions develop exclusively in the bone marrow.

Fms-Like Tyrosine Kinase 3-Ligand Contributes to the Development and Function of the Subpopulation of CD8α+ Plasmacytoid Precursor Dendritic Cells in CD8+ /TCR- Facilitating Cells

Facilitating cells (FC) are a CD8⁺ TCR^- bone marrow subpopulation that enhance engraftment of purified hematopoietic stem cells (HSC) and induce antigen-specific CD4⁺ CD25⁺ FoxP3⁺ regulatory T cell (Treg) in vivo. The major subpopulation in FC resembles plasmacytoid precursor dendritic cells (p-preDC) both phenotypically and functionally. Here, we report that the number of FC was significantly reduced in Fms-like tyrosine kinase 3-ligand-knockout (Flt3-L-KO) mice. Specifically, there was a selective decrease in the B220⁺ CD11c⁺ CD11b^- p-preDC FC subpopulation. The p-preDC FC subpopulation in FC total is restored after Flt3-L administration to Flt3-L-KO mice. FC from Flt3-L-KO donors exhibit impaired facilitation of allogeneic HSC engraftment in ablatively conditioned mice (B6 → NOD) as well as in mice conditioned with reduced intensity conditioning (B6 → BALB/c). In addition, the number of CD4⁺ CD25⁺ Foxp3⁺ Treg from Flt3-L-KO mice is significantly decreased. This was associated with the expression of chemokine receptor CXCR3⁺ or CCR5⁺ on Treg. Treg from the spleen of Flt3-L-KO mice showed impaired facilitation of engraftment of allogeneic HSC compared to wild-type Treg. Flt3-L treatment significantly expanded Treg, and restored their facilitating function. These results suggest that Flt3-L is an important growth factor in the development and homeostasis of p-preDC FC and in the role of FC inducing generation of Treg. Flt3-L provides potent immunoregulatory properties that may be clinically useful to improve tolerance induction and enhance the function of allogeneic cell therapies. Stem Cells 2018;36:1567-1577.

Complexity of immune responses to AAV transgene products - Example of factor IX

After two decades of research, in vivo gene transfer with adeno-associated viral (AAV) vectors has now resulted in successful treatments and even cures for several human diseases. However, the potential for immune responses against the therapeutic gene products remains one of the concerns as this approach is broadened to more patients, diverse diseases, and target organs. Immune responses following gene transfer of coagulation factor IX (FIX) for the treatment of the bleeding disorder hemophilia B has been extensively investigated in multiple animal models. Findings from these studies have not only influenced clinical trial design but have broader implications for other diseases. The impact of vector design and dose, as well as target organ/route of administration on humoral and cellular immune responses are reviewed. Furthermore, the potential for tolerance induction by hepatic gene transfer or combination with immune modulation is discussed.

FLT3 ligand regulates thymic precursor cells and hematopoietic stem cells through interactions with CXCR4 and the marrow niche

Impaired immune reconstitution after hematopoietic stem cell transplantation (HSCT) is attributed in part to impaired thymopoiesis. Recent data suggest that precursor input may be a point of regulation for the thymus. We hypothesized that administration of FLT3 ligand (FLT3L) would enhance thymopoiesis after adoptive transfer of aged, FLT3L-treated bone marrow (BM) to aged, Lupron-treated hosts by increasing murine HSC (Lin^[minus]Sca1⁺c-Kit⁺ [LSK] cells) trafficking and survival. In murine models of aged and young hosts, we show that FLT3L enhances thymopoiesis in aged, Lupron-treated hosts through increased survival and export of LSK cells via CXCR4 regulation. In addition, we elucidate an underlying mechanism of FLT3L action on BM LSK cells-FLT3L drives LSK cells into the stromal niche using Hoescht (Ho) dye perimortem. In summary, we show that FLT3L administration leads to: (1) increased LSK cells and early thymocyte progenitor precursors that can enhance thymopoiesis after transplantation and androgen withdrawal, (2) mobilization of LSK cells through downregulation of CXCR4, (3) enhanced BM stem cell survival associated with Bcl-2 upregulation, and (4) BM stem cell enrichment through increased trafficking to the BM niche. Therefore, we show a mechanism by which FLT3L activity on hematopoeitic and thymic progenitor cells may contribute to thymic recovery. These data have potential clinical relevance to enhance thymic reconstitution after cytoreductive therapy.

The Cytokine Flt3-Ligand in Normal and Malignant Hematopoiesis

The cytokine Fms-like tyrosine kinase 3 ligand (FL) is an important regulator of hematopoiesis. Its receptor, Flt3, is expressed on myeloid, lymphoid and dendritic cell progenitors and is considered an important growth and differentiation factor for several hematopoietic lineages. Activating mutations of Flt3 are frequently found in acute myeloid leukemia (AML) patients and associated with a poor clinical prognosis. In the present review we provide an overview of our current knowledge on the role of FL in the generation of blood cell lineages. We examine recent studies on Flt3 expression by hematopoietic stem cells and its potential instructive action at early stages of hematopoiesis. In addition, we review current findings on the role of mutated FLT3 in leukemia and the development of FLT3 inhibitors for therapeutic use to treat AML. The importance of mouse models in elucidating the role of Flt3-ligand in normal and malignant hematopoiesis is discussed.

Skin-derived TSLP systemically expands regulatory T cells

Regulatory T cells (Tregs) are a subset of CD4+ T cells with suppressive function and are critical for limiting inappropriate activation of T cells. Hence, the expansion of Tregs is an attractive strategy for the treatment of autoimmune diseases. Here, we demonstrate that the skin possesses the remarkable capacity to systemically expand Treg numbers by producing thymic stromal lymphopoietin (TSLP) in response to vitamin D receptor stimulation. An ∼2-fold increase in the proportion and absolute number of Tregs was observed in mice treated topically but not systemically with the Vitamin D3 analog MC903. This expansion of Tregs was dependent on TSLP receptor signaling but not on VDR signaling in hematopoietic cells. However, TSLP receptor expression by Tregs was not required for their proliferation. Rather, skin-derived TSLP promoted Treg expansion through dendritic cells. Importantly, treatment of skin with MC903 significantly lowered the incidence of autoimmune diabetes in non-obese diabetic mice and attenuated disease score in experimental autoimmune encephalomyelitis. Together, these data demonstrate that the skin has the remarkable potential to control systemic immune responses and that Vitamin D-mediated stimulation of skin could serve as a novel strategy to therapeutically modulate the systemic immune system for the treatment of autoimmunity.

Dendritic cells as gatekeepers of tolerance

Dendritic cells (DC) are unique hematopoietic cells, linking innate and adaptive immune responses. In particular, they are considered as the most potent antigen presenting cells, governing both T cell immunity and tolerance. In view of their exceptional ability to present antigen and to interact with T cells, DC play distinct roles in shaping T cell development, differentiation and function. The outcome of the DC-T cell interaction is determined by the state of DC maturation, the type of DC subset, the cytokine microenvironment and the tissue location. Both regulatory T cells (Tregs) and DC are indispensable for maintaining central and peripheral tolerance. Over the past decade, accumulating data indicate that DC critically contribute to Treg differentiation and homeostasis.

Metabolites: deciphering the molecular language between DCs and their environment

Dendritic cells (DCs) determine the outcome of the immune response based on signals they receive from the environment. Presentation of antigen under various contexts can lead to activation and differentiation of T cells for immunity or dampening of immune responses by establishing tolerance, primarily through the priming of regulatory T cells. Infections, inflammation and normal cellular interactions shape DC responses through direct contact or via cytokine signaling. Although it is widely accepted that DCs sense microbial components through pattern recognition receptors (PRRs), increasing evidence advocates for the existence of a set of signals that can profoundly shape DC function via PRR-independent pathways. This diverse group of host- or commensal-derived metabolites represents a newly appreciated code from which DCs can interpret environmental cues. In this review, we discuss the existing information on the effect of some of the most studied metabolites on DC function, together with the implications this may have in immune-mediated diseases.

Permissive roles of cytokines interleukin-7 and Flt3 ligand in mouse B-cell lineage commitment

Hematopoietic cells are continuously generated throughout life from hematopoietic stem cells, thus making hematopoiesis a favorable system to study developmental cell lineage commitment. The main factors incorporating environmental signals to developing hematopoietic cells are cytokines, which regulate commitment of hematopoietic progenitors to the different blood lineages by acting either in an instructive or a permissive manner. Fms-like tyrosine kinase-3 (Flt3) ligand (FL) and Interleukin-7 (IL-7) are cytokines pivotal for B-cell development, as manifested by the severely compromised B-cell development in their absence. However, their precise role in regulating B-cell commitment has been the subject of debate. In the present study we assessed the rescue of B-cell commitment in mice lacking IL-7 but simultaneously overexpressing FL. Results obtained demonstrate that FL overexpression in IL-7-deficient mice rescues B-cell commitment, resulting in significant Ebf1 and Pax5 expression in Ly6D⁺CD135⁺CD127⁺CD19^- precursors and subsequent generation of normal numbers of CD19⁺ B-cell progenitors, therefore indicating that IL-7 can be dispensable for commitment to the B-cell lineage. Further analysis of Ly6D⁺CD135⁺CD127⁺CD19^- progenitors in IL-7- or FL-deficient mice overexpressing Bcl2, as well as in IL-7 transgenic mice suggests that both FL and IL-7 regulate B-cell commitment in a permissive manner: FL by inducing proliferation of Ly6D⁺CD135⁺CD127⁺CD19^- progenitors and IL-7 by providing survival signals to these progenitors.

Novel immunoregulatory role of perforin-positive dendritic cells

The recently described generation of a highly defined population of dendritic cells which express perforin and granzyme A (termed "perf-DCs") and their ability to selectively delete cognate CD8+ T cell has raised the possibility that these cells play a role in the maintenance of peripheral tolerance. Using bone marrow transplantation, we generated mice selectively lacking perforin expressing dendritic cells. These mice progressively gain weight and exhibit features resembling metabolic syndrome as well as an enhanced susceptibility to autoimmunity induction. Interestingly, these pathological phenotypes were reversed upon treatment with CD4/CD8 neutralizing antibodies. Thus, it appears that this rare subpopulation of dendritic cells (perf-DCs) displays a major regulatory role in adipose tissue inflammatory processes and in autoimmunity.

Curcumin improves regulatory T cells in gut-associated lymphoid tissue of colitis mice

AIM: To explore the probable pathway by which curcumin (Cur) regulates the function of Treg cells by observing the expression of costimulatory molecules of dendritic cells (DCs).

METHODS: Experimental colitis was induced by administering 2, 4, 6-trinitrobenzene sulfonic acid (TNBS)/ethanol solution. Forty male C57BL/6 mice were randomly divided into four groups: normal, TNBS + Cur, TNBS + mesalazine (Mes) and TNBS groups. The mice in the TNBS + Cur and TNBS +Mes groups were treated with Cur and Mes, respectively, while those in the TNBS group were treated with physiological saline for 7 d. After treatment, the curative effect of Cur was evaluated by colonic weight, colonic length, weight index of the colon, and histological observation and score. The levels of CD4⁺CD25+Foxp3⁺ T cells (Treg cells) and costimulatory molecules of DCs were measured by flow cytometry. Also, related cytokines were analyzed by enzyme-linked immunosorbent assay.

RESULTS: Cur alleviated inflammatory injury of the colonic mucosa, decreased colonic weigh and histological score, and restored colonic length. The number of Treg cells was increased, while the secretion of TNF-α, IL-2, IL-6, IL-12 p40, IL-17 and IL-21 and the expression of costimulatory molecules (CD205, CD54 [ICAM-1], TLR4, CD252[OX40 L], CD256 [RANK] and CD254 [RANK L]) of DCs were notably inhibited in colitis mice treated with Cur.

CONCLUSION: Cur potentially modulates activation of DCs to enhance the suppressive functions of Treg cells and promote the recovery of damaged colonic mucosa in inflammatory bowel disease.

Leukemia-associated activating mutation of Flt3 expands dendritic cells and alters T cell responses

Lau et al. show that the FLT3-ITD mutation directly affects dendritic cell development in preleukemic mice, indirectly modulating T cell homeostasis and supporting the expansion of regulatory T cells.

A common genetic alteration in acute myeloid leukemia is the internal tandem duplication (ITD) in FLT3, the receptor for cytokine FLT3 ligand (FLT3L). Constitutively active FLT3-ITD promotes the expansion of transformed progenitors, but also has pleiotropic effects on hematopoiesis. We analyzed the effect of FLT3-ITD on dendritic cells (DCs), which express FLT3 and can be expanded by FLT3L administration. Pre-leukemic mice with the Flt3^ITD knock-in allele manifested an expansion of classical DCs (cDCs) and plasmacytoid DCs. The expansion originated in DC progenitors, was cell intrinsic, and was further enhanced in Flt3^ITD/ITD mice. The mutation caused the down-regulation of Flt3 on the surface of DCs and reduced their responsiveness to Flt3L. Both canonical Batf3-dependent CD8⁺ cDCs and noncanonical CD8⁺ cDCs were expanded and showed specific alterations in their expression profiles. Flt3^ITD mice showed enhanced capacity to support T cell proliferation, including a cell-extrinsic expansion of regulatory T (T reg) cells. Accordingly, these mice restricted alloreactive T cell responses during graft-versus-host reaction, but failed to control autoimmunity without T reg cells. Thus, the FLT3-ITD mutation directly affects DC development, indirectly modulating T cell homeostasis and supporting T reg cell expansion. We hypothesize that this effect of FLT3-ITD might subvert immunosurveillance and promote leukemogenesis in a cell-extrinsic manner.

Flt3 Ligand Regulates the Development of Innate Lymphoid Cells in Fetal and Adult Mice

Flt3 ligand (Flt3L) promotes survival of lymphoid progenitors in the bone marrow and differentiation of dendritic cells (DCs), but its role in regulating innate lymphoid cells (ILCs) during fetal and adult life is not understood. By using Flt3L knockout and transgenic mice, we demonstrate that Flt3L controls ILC numbers by regulating the pool of α4β7(-) and α4β7(+) lymphoid tissue inducer cell progenitors in the fetal liver and common lymphoid progenitors in the bone marrow. Deletion of flt3l severely reduced the number of fetal liver progenitors and lymphoid tissue inducer cells in the neonatal intestine, resulting in impaired development of Peyer's patches. In the adult intestine, NK cells and group 2 and 3 ILCs were severely reduced. This effect occurred independently of DCs as ILC numbers were normal in mice in which DCs were constitutively deleted. Finally, we could show that administration of Flt3L increased the number of NKp46(-) group 3 ILCs in wild-type and even in Il7(-/-) mice, which generally have reduced numbers of ILCs. Taken together, Flt3L significantly contributes to ILC and Peyer's patches development by targeting lymphoid progenitor cells during fetal and adult life.

Role of Dendritic Cells in the Induction of Lymphocyte Tolerance

The ability of dendritic cells (DCs) to trigger tolerance or immunity is dictated by the context in which an antigen is encountered. A large body of evidence indicates that antigen presentation by steady-state DCs induces peripheral tolerance through mechanisms such as the secretion of soluble factors, the clonal deletion of autoreactive T cells, and feedback control of regulatory T cells. Moreover, recent understandings on the function of DC lineages and the advent of murine models of DC depletion have highlighted the contribution of DCs to lymphocyte tolerance. Importantly, these findings are now being applied to human research in the contexts of autoimmune diseases, allergies, and transplant rejection. Indeed, DC-based immunotherapy research has made important progress in the area of human health, particularly in regards to cancer. A better understanding of several DC-related aspects including the features of DC lineages, milieu composition, specific expression of surface molecules, the control of signaling responses, and the identification of competent stimuli able to trigger and sustain a tolerogenic outcome will contribute to the success of DC-based immunotherapy in the area of lymphocyte tolerance. This review will discuss the latest advances in the biology of DC subtypes related to the induction of regulatory T cells, in addition to presenting current ex vivo protocols for tolerogenic DC production. Particular attention will be given to the molecules and signals relevant for achieving an adequate tolerogenic response for the treatment of human pathologies.

FMS-like tyrosine kinase 3 ligand treatment does not ameliorate experimental rapidly progressive glomerulonephritis

Fms-like tyrosine kinase 3-ligand (FL) is a growth factor that may expand dendritic cell and regulatory T cell populations. We hypothesised that FL-induced regulatory T cells would protect mice from experimental rapidly progressive glomerulonephritis. To determine if FL was able to enhance regulatory T cell populations, C57BL/6 mice received 10 days of daily intraperitoneal injections of either FL or phosphate buffered saline. To induce accelerated autologous-phase anti-mouse glomerular basement membrane glomerulonephritis, mice were sensitized to sheep globulin 4 days prior to the induction of glomerulonephritis with sheep anti-mouse glomerular basement membrane globulin, and experiments ended 10 days later. FL was administered before, throughout and during the sensitization phase of this glomerulonephritis model. Renal disease and systemic immunity to the nephritogenic antigen were assessed. FL increased regulatory T cell and plasmacytoid dendritic cell proportions within spleen and lymph nodes. FL administration prior to glomerulonephritis did not protect mice from renal injury. When FL was given throughout the model, FL treated mice had reduced survival, with more interstitial neutrophils and glomerular CD11c+ cells than controls. Systemic immune responses showed increased IL-17A production from splenocytes, with more CD11c+ cells, but reduced plasmacytoid dendritic cell proportions in spleen and lymph nodes, despite increased regulatory T cell proportions. Under homeostatic conditions, FL expanded regulatory T cell and plasmacytoid dendritic cell populations, but FL enhanced systemic inflammatory responses and conventional dendritic cell populations when given during experimental glomerulonephritis, suggesting selective attempts to suppress pathogenic immunity by dendritic cell manipulation may be harmful.

Synergy between rapamycin and FLT3 ligand enhances plasmacytoid dendritic cell-dependent induction of CD4+CD25+FoxP3+ Treg

CD4(+)CD25(+)FoxP3(+) regulatory T cells (Treg) are critical elements for maintaining immune tolerance, for instance to exogenous antigens that are introduced during therapeutic interventions such as cell/organ transplant or gene/protein replacement therapy. Coadministration of antigen with rapamycin simultaneously promotes deletion of conventional CD4(+) T cells and induction of Treg. Here, we report that the cytokine FMS-like receptor tyrosine kinase ligand (Flt3L) enhances the in vivo effect of rapamycin. This occurs via selective expansion of plasmacytoid dendritic cells (pDCs), which further augments the number of Treg. Whereas in conventional DCs, rapamycin effectively blocks mammalian target of rapamycin (mTOR) 1 signaling induced by Flt3L, increased mTOR1 activity renders pDCs more resistant to inhibition by rapamycin. Consequently, Flt3L and rapamycin synergistically promote induction of antigen-specific Treg via selective expansion of pDCs. This concept is supported by the finding that Treg induction is abrogated upon pDC depletion. The combination with pDCs and rapamycin is requisite for Flt3L/antigen-induced Treg induction because Flt3L/antigen by itself fails to induce Treg. As co-administering Flt3L, rapamycin, and antigen blocked CD8(+) T-cell and antibody responses in models of gene and protein therapy, we conclude that the differential effect of rapamycin on DC subsets can be exploited for improved tolerance induction.

Phase I trial of maintenance sorafenib after allogeneic hematopoietic stem cell transplantation for fms-like tyrosine kinase 3 internal tandem duplication acute myeloid leukemia

The fms-like tyrosine kinase 3 internal tandem duplication (FLT3-ITD) mutation is associated with a high relapse rate for patients with acute myeloid leukemia (AML) even after allogeneic hematopoietic stem cell transplantation (HSCT). Sorafenib is a tyrosine kinase inhibitor, which inhibits the FLT3 tyrosine kinase and has shown encouraging activity in FLT3-ITD AML. We conducted a phase I trial of maintenance sorafenib after HSCT in patients with FLT3-ITD AML (ClinicalTrials.govNCT01398501). Patients received a variety of conditioning regimens and graft sources. A dose escalation 3 + 3 cohort design was used to define the maximum tolerated dose (MTD), with an additional 10 patients treated at the MTD. Sorafenib was initiated between days 45 and 120 after HSCT and continued for 12 28-day cycles. Twenty-two patients were enrolled (status at HSCT: first complete remission [CR1], n = 16; second complete remission [CR2], n = 3; refractory, n = 3). The MTD was established at 400 mg twice daily with 1 dose-limiting toxicity (DLT) observed (pericardial effusion). Two patients died of transplantation-related causes, both unrelated to sorafenib. Two patients stopped sorafenib after relapse and 5 stopped because of attributable toxicities after the DLT period. Median follow-up for surviving patients is 16.7 months after HSCT (range, 8.1 to 35.0). There was 1 case of grade II acute graft-versus-host disease (GVHD) after starting sorafenib and the 12-month cumulative incidence of chronic GVHD was 38% (90% confidence interval [CI], 21% to 56%). For all patients, 1-year progression-free survival (PFS) was 85% (90% CI, 66% to 94%) and 1-year overall survival (OS) was 95% (90% CI, 79% to 99%) after HSCT. For patients in CR1/CR2 before HSCT (n = 19), 1-year PFS was 95% (90% CI, 76% to 99%) and 1-year OS was 100%, with only 1 patient who relapsed. Sorafenib is safe after HSCT for FLT3-ITD AML and merits further investigation for the prevention of relapse.

Flt3/Flt3L Participates in the Process of Regulating Dendritic Cells and Regulatory T Cells in DSS-Induced Colitis

The immunoregulation between dendritic cells (DCs) and regulatory T cells (T-regs) plays an important role in the pathogenesis of ulcerative colitis (UC). Recent research showed that Fms-like tyrosine kinase 3 (Flt3) and Flt3 ligand (Flt3L) were involved in the process of DCs regulating T-regs. The DSS-induced colitis model is widely used because of its simplicity and many similarities with human UC. In this study, we observe the disease activity index (DAI) and histological scoring, detect the amounts of DCs and T-regs and expression of Flt3/Flt3L, and investigate Flt3/Flt3L participating in the process of DCs regulating T-regs in DSS-induced colitis. Our findings suggest that the reduction of Flt3 and Flt3L expression may possibly induce colonic immunoregulatory imbalance between CD103(+)MHCII(+)DCs and CD4(+)CD25(+)FoxP3(+)T-regs in DSS-induced colitis. Flt3/Flt3L participates in the process of regulating DCS and T-regs in the pathogenesis of UC, at least, in the acute stage of this disease.

IL-33 is an unconventional Alarmin that stimulates IL-2 secretion by dendritic cells to selectively expand IL-33R/ST2+ regulatory T cells

IL-33 is a recently characterized IL-1 family member that is proposed to function as an alarmin, or endogenous signal of cellular damage, as well as act as a pleiotropic cytokine. The ability of IL-33 to potentiate both Th1 and Th2 immunity supports its role in pathogen clearance and disease immunopathology. Yet, IL-33 restrains experimental colitis and transplant rejection by expanding regulatory T cells (Treg) via an undefined mechanism. We sought to determine the influence of IL-33 on hematopoietic cells that drives Treg expansion and underlies the therapeutic benefit of IL-33 administration. In this study, we identify a feedback loop in which conventional mouse CD11c(+) dendritic cells (DC) stimulated by IL-33 secrete IL-2 to selectively expand IL-33R(ST2(+))- suppressive CD4(+)Foxp3(+) Treg. Interestingly, this occurs in the absence of classical DC maturation, and DC-derived (innate) IL-2 increases ST2 expression on both DC and interacting Treg. ST2(+) Treg represent an activated subset of Foxp3(+) cells, demonstrated to be ICOS(high)CD44(high) compared with their ST2(-) counterparts. Furthermore, although studies have shown that IL-33-exposed DC promote Th2 responses, we reveal that ST2(+) DC are required for IL-33-mediated in vitro and in vivo Treg expansion. Thus, we have uncovered a relationship between IL-33 and innate IL-2 that promotes the selective expansion of ST2(+) Treg over non-Treg. These findings identify a novel regulatory pathway driven by IL-33 in immune cells that may be harnessed for therapeutic benefit or for robust expansion of Treg in vitro and in vivo.

Role and therapeutic value of dendritic cells in central nervous system autoimmunity

Dendritic cells (DCs) are professional antigen-presenting cells that control the generation of adaptive immunity. Consequently, DCs have a central role in the induction of protective immunity to pathogens and also in the pathogenic immune response responsible for the development and progression of autoimmune disorders. Thus the study of the molecular pathways that control DC development and function is likely to result in new strategies for the therapeutic manipulation of the immune response. In this review, we discuss the role and therapeutic value of DCs in autoimmune diseases, with a special focus on multiple sclerosis.

Homeostasis and function of regulatory T cells (Tregs) in vivo: lessons from TCR-transgenic Tregs

The identification of CD25 and subsequently Forkhead box protein 3 (Foxp3) as markers for regulatory T cells (Tregs) has revolutionized our ability to explore this population experimentally. In a similar vein, our understanding of antigen-specific Treg responses in vivo owes much to the fortuitous generation of T-cell receptor (TCR)-transgenic Tregs. This has permitted tracking of Tregs with a defined specificity in vivo, facilitating analysis of how encounter with cognate antigen shapes Treg homeostasis and function. Here, we review the key lessons learned from a decade of analysis of TCR-transgenic Tregs and set this in the broader context of general progress in the field. Use of TCR-transgenic Tregs has led to an appreciation that Tregs are a highly dynamic proliferative population in vivo, rather than an anergic population as they were initially portrayed. It is now clear that Treg homeostasis is positively regulated by encounter with self-antigen expressed on peripheral tissues, which is likely to be relevant to the phenomenon of peripheral repertoire reshaping that has been described for Tregs and the observation that the Treg TCR specificities vary by anatomical location. Substantial evidence has also accumulated to support the role of CD28 costimulation and interleukin-2 in Treg homeostasis. The availability of TCR-transgenic Tregs has enabled analysis of Treg populations that are sufficient or deficient in particular genes, without the comparison being confounded by repertoire alterations. This approach has yielded insights into genes required for Treg function in vivo, with particular progress being made on the role of ctla-4 in this context. As the prospect of manipulating Treg populations in the clinic becomes reality, a full appreciation of the rules governing their homeostasis will prove increasingly important.

Inhibition of calcineurin abrogates while inhibition of mTOR promotes regulatory T cell expansion and graft-versus-host disease protection by IL-2 in allogeneic bone marrow transplantation

Regulatory T cells (Treg)s attenuate excessive immune responses, making their expansion beneficial in immune-mediated diseases including allogeneic bone marrow transplantation (BMT)-associated graft-versus-host disease (GVHD). We have recently reported that Treg expansion does not require phospholipase Cγ activation when IL-2 is provided. As such, the combination of IL-2 and a calcineurin inhibitor (Cyclosporine A; CsA) expands Tregs while inhibiting Tconv proliferation and protects against a mouse model of multiple sclerosis. However, CsA inhibits Treg proliferation in the presence of a TCR stimulus, suggesting that CsA may negatively impact Treg proliferation when they receive strong allogeneic MHC-mediated TCR signals. In this study, we show that CsA inhibits Treg proliferation and inducible Treg generation in allogeneic but not in syngeneic BMT when IL-2 is provided. In contrast to CsA, the mTOR inhibitor (Rapamycin) almost completely suppressed IL-2-mediated Treg proliferation. However, CsA and Rapamycin inhibited Treg proliferation to a similar extent when TCR stimulation was provided. Furthermore, Rapamycin promoted Treg expansion and inducible Treg generation in allogeneic BMT recipients treated with IL-2. Consistent with these observations, CsA abrogated while Rapamycin promoted the protective effect of IL-2 on allogeneic BMT-induced GVHD. These results suggest that while CsA permits IL-2-induced Treg proliferation in the syngeneic setting (absence of strong TCR signals), CsA in combination with IL-2 may be detrimental for Treg proliferation in an allogeneic setting. Thus, in allogeneic settings, an mTOR inhibitor such as Rapamycin is a better choice for adjunct therapy with IL-2 in expansion of Tregs and protection against allogeneic BMT-induced GVHD.

Self-antigen presentation by dendritic cells in autoimmunity

The operation of both central and peripheral tolerance ensures the prevention of autoimmune diseases. The maintenance of peripheral tolerance requires self-antigen presentation by professional antigen presenting cells (APCs). Dendritic cells (DCs) are considered as major APCs involved in this process. The current review discusses the role of DCs in autoimmune diseases, the various factors involved in the induction and maintenance of tolerogenic DC phenotype, and pinpoints their therapeutic capacity as well as potential novel targets for future clinical studies.

Survivin but not Fms-like tyrosine kinase 3 ligand is up-regulated before the onset of rheumatoid arthritis: a pilot study

Introduction

Antibodies against citrullinated peptides (anti-CCP) and increased levels of cytokines precede the development of rheumatoid arthritis (RA) by several years. Recently, the proteins survivin and Fms-like tyrosine kinase 3 ligand (Flt3L) have been identified as biomarkers of RA associated with joint destruction. Our objective was to investigate the potential of survivin and Flt3L as predictors of RA in samples from patients prior to onset of symptoms.

Methods

This study included 47 individuals sampled before onset of RA (median 2.5 years (IQR 4.5) and 155 matched controls, all were donors to the Medical Biobank of Northern Sweden, and 36 RA patients. Levels of anti-CCP, survivin and Flt3L were measured using ELISAs and 29 cytokines/chemokines by multiplex detection.

Results

Levels of survivin were increased in pre-symptomatic individuals compared with controls (P = 0.003), whilst the levels of Flt3L were similar. The frequency of survivin positivity in the pre-symptomatic individuals was increased compared with the controls (36.2 vs.14.2%, P = 0.001) and predicted disease development (odds ratio (OR) =3.4 (95% confidence interval (CI) 1.6-7.2)). The frequency of survivin and Flt3L in RA patients was increased compared with the controls (both, P <0.0001, OR = 12.1 (95% CI, 5.3-27.6) and OR = 11.0 (95% CI, 3.9-30.9), respectively). Anti-CCP positive pre-symptomatic individuals and patients had significantly higher levels of survivin compared with anti-CCP2 negative individuals. In pre-symptomatic individuals, survivin correlated with IL-12, IL-1β and IL-9 whereas Flt3L correlated to a significantly broader spectrum of cytokines in RA patients.

Conclusion

Proto-oncogene survivin was increased in individuals prior to onset of symptoms of RA and was correlated to cytokines suggesting its role at pre-clinical stages of the disease.

Glucocorticoid-Induced TNF Receptor Family-Related Protein Ligand is Requisite for Optimal Functioning of Regulatory CD4(+) T Cells

Glucocorticoid-induced tumor necrosis factor receptor family-related protein (TNFRSF18, CD357) is constitutively expressed on regulatory T cells (Tregs) and is inducible on effector T cells. In this report, we examine the role of glucocorticoid-induced TNF receptor family-related protein ligand (GITR-L), which is expressed by antigen presenting cells, on the development and expansion of Tregs. We found that GITR-L is dispensable for the development of naturally occurring FoxP3⁺ Treg cells in the thymus. However, the expansion of Treg in GITR-L^−/− mice is impaired after injection of the dendritic cells (DCs) inducing factor Flt3 ligand. Furthermore, DCs from the liver of GITR-L^−/− mice were less efficient in inducing proliferation of antigen-specific Treg cells in vitro than the same cells from WT littermates. Upon gene transfer of ovalbumin into hepatocytes of GITR-L^−/−FoxP3(GFP) reporter mice using adeno-associated virus (AAV8-OVA) the number of antigen-specific Treg in liver and spleen is reduced. The reduced number of Tregs resulted in an increase in the number of ovalbumin specific CD8⁺ T effector cells. This is highly significant because proliferation of antigen-specific CD8⁺ cells itself is dependent on the presence of GITR-L, as shown by in vitro experiments and by adoptive transfers into GITR-L^−/−Rag^−/− and Rag^−/− mice that had received AAV8-OVA. Surprisingly, administering αCD3 significantly reduced the numbers of FoxP3⁺ Treg cells in the liver and spleen of GITR-L^−/− but not WT mice. Because soluble Fc-GITR-L partially rescues αCD3 induced in vitro depletion of the CD103⁺ subset of FoxP3⁺CD4⁺ Treg cells, we conclude that expression of GITR-L by antigen presenting cells is requisite for optimal Treg-mediated regulation of immune responses including those in response during gene transfer.

In vivo evidence for an instructive role of fms-like tyrosine kinase-3 (FLT3) ligand in hematopoietic development

Cytokines are essential regulators of hematopoiesis, acting in an instructive or permissive way. Fms-like tyrosine kinase 3 ligand (FLT3L) is an important cytokine for the development of several hematopoietic populations. Its receptor (FLT3) is expressed on both myeloid and lymphoid progenitors and deletion of either the receptor or its ligand leads to defective developmental potential of hematopoietic progenitors. In vivo administration of FLT3L promotes expansion of progenitors with combined myeloid and lymphoid potential. To investigate further the role of this cytokine in hematopoietic development, we generated transgenic mice expressing high levels of human FLT3L. These transgenic mice displayed a dramatic expansion of dendritic and myeloid cells, leading to splenomegaly and blood leukocytosis. Bone marrow myeloid and lymphoid progenitors were significantly increased in numbers but retained their developmental potential. Furthermore, the transgenic mice developed anemia together with a reduction in platelet numbers. FLT3L was shown to rapidly reduce the earliest erythroid progenitors when injected into wild-type mice, indicating a direct negative role of the cytokine on erythropoiesis. We conclude that FLT3L acts on multipotent progenitors in an instructive way, inducing their development into myeloid/lymphoid lineages while suppressing their megakaryocyte/erythrocyte potential.

The potential role of dendritic cells in the therapy of Type 1 diabetes

Type 1 diabetes (T1D) is the result of T-cell mediated autoimmune destruction of pancreatic islet β-cells. The two current treatments for T1D are based on insulin or islet-cell replacement rather than the pathogenesis of T1D and remain problematic. Islet/pancreas transplantation does not cater for the majority of sufferers due to the lack of supply of organs and the need for continuous immunosuppression regimens. The mainstay treatment is insulin replacement, but this is disruptive to lifestyle and does not protect against severe long-term complications. An early vaccination and long-term restoration of immune tolerance to self-antigens in T1D patients (reversing the immunopathogenesis of the disease) would be preferable. Dendritic cells (DCs) are potent APCs and play an important role in inducing and maintaining immune tolerance. Targeting DCs through different DC surface molecules shows effective modulation of immune responses. Their feasibility for immunotherapy to prolong transplant survival and cancer immunotherapy has been demonstrated. Therefore, DCs could potentially be used in the treatment of autoimmune diseases. This review summarizes new insights into DCs as a potential therapeutic target for the treatment of T1D.

Dendritic cells generated with Flt3L and exposed to apoptotic cells lack induction of T cell anergy and Foxp3⁺ regulatory T cell conversion in vitro

Removal of apoptotic cells, which appear during the steady state, is a pre-requisite to prevent generation of secondary necrotic cells that may lead to autoimmunity. The recognition of apoptotic material by dendritic cells (DCs) has been proposed to convert them into tolerogenic DCs equipped with specialized tolerogenic mechanisms on T cells. However, comparative studies to demonstrate functional alterations of DCs upon exposure to apoptotic cells have not been performed so far. Here we show that immature murine bone marrow-derived DCs generated with GM-CSF (GM-DCs) or Flt3L (FL-DCs) interact with live or apoptotic syngeneic thymocytes. As expected, GM-DCs phagocytose apoptotic but not live cells, FL-DCs only show trogocytosis of membrane parts. Interaction with live or apoptotic thymocytes did not lead to DC maturation. Both GM-DCs and FL-DCs present OVA as protein, peptide and membrane-associated antigens. Interestingly, only GM-DCs were able to induce T cell anergy or convert naïve T cells into FoxP3⁺ regulatory T cells (Tregs) but FL-DCs did not show either of these effects. Unexpectedly, exposure of immature GM-DCs to live or apoptotic thymocytes did not improve DC functions in both types of in vitro T cell tolerance induction assays. Together, our data suggest that these tolerogenic in vitro measures of immature BM-DCs are not further enhanced by exposure to apoptotic cells and may depend on the generating cytokine.

The role of dendritic cells in autoimmunity

Dendritic cells (DCs) initiate and shape both the innate and adaptive immune responses. Accordingly, recent evidence from clinical studies and experimental models implicates DCs in the pathogenesis of most autoimmune diseases. However, fundamental questions remain unanswered concerning the actual roles of DCs in autoimmunity, both in general and, in particular, in specific diseases. In this Review, we discuss the proposed roles of DCs in immunological tolerance, the effect of the gain or loss of DCs on autoimmunity and DC-intrinsic molecular regulators that help to prevent the development of autoimmunity. We also review the emerging roles of DCs in several autoimmune diseases, including autoimmune myocarditis, multiple sclerosis, psoriasis, type 1 diabetes and systemic lupus erythematosus.

Differential targeting of IL-2 and T cell receptor signaling pathways selectively expands regulatory T cells while inhibiting conventional T cells

Strategies to expand regulatory T cells hold therapeutic potential for ameliorating T cell-mediated autoimmunity. Recently, we reported that the requirements for T cell receptor signaling in conventional T cell and regulatory T cell proliferation are different. Using mutant mice that display defective T cell receptor-mediated phospholipase Cγ (PLCγ) activation, we hereby demonstrate that PLCγ activation is required for antigen-specific conventional T cell proliferation but not for IL-2-induced regulatory T cell proliferation. This led us to hypothesize that in conjunction with IL-2, pharmacological inhibition of T cell receptor-mediated PLCγ activation might offer a novel therapeutic strategy to expand regulatory T cells while simultaneously inhibiting conventional T cell proliferation. Indeed, using the calcineurin inhibitor Cyclosporine A to inhibit signaling downstream of PLCγ, we found that Cyclosporine A attenuated antigen-specific Tconv proliferation but permitted IL-2-induced regulatory T cell expansion in vitro and in vivo. Furthermore, the combination of Cyclosporine A and IL-2 was superior over either Cyclosporine A or IL-2 monotherapy in protection against the T cell-mediated demyelinating autoimmune disease mouse model, experimental autoimmune encephalomyelitis. Thus, a combination of TCR signaling inhibition and IL-2 might be a beneficial strategy in expanding regulatory T cells and inhibiting conventional T cell proliferation in autoimmune settings.

Dendritic cells in the periphery control antigen-specific natural and induced regulatory T cells

Dendritic cells (DCs) are specialized antigen-presenting cells that regulate both immunity and tolerance. DCs in the periphery play a key role in expanding naturally occurring Foxp3⁺ CD25⁺ CD4⁺ regulatory T cells (Natural T-regs) and inducing Foxp3 expression (Induced T-regs) in Foxp3⁻ CD4⁺ T cells. DCs are phenotypically and functionally heterogeneous, and further classified into several subsets depending on distinct marker expression and their location. Recent findings indicate the presence of specialized DC subsets that act to expand Natural T-regs or induce Foxp3⁺ T-regs from Foxp3⁻ CD4⁺ T cells. For example, two major subsets of DCs in lymphoid organs act differentially in inducing Foxp3⁺ T-regs from Foxp3⁻ cells or expanding Natural T-regs with model-antigen delivery by anti-DC subset monoclonal antibodies in vivo. Furthermore, DCs expressing CD103 in the intestine induce Foxp3⁺ T-regs from Foxp3⁻ CD4⁺ T cells with endogenous TGF-β and retinoic acid. In addition, antigen-presenting DCs have a capacity to generate Foxp3⁺ T-regs in the oral cavity where many antigens and commensals exist, similar to intestine and skin. In skin and skin-draining lymph nodes, at least six DC subsets have been identified, suggesting a complex DC-T-reg network. Here, we will review the specific activity of DCs in expanding Natural T-regs and inducing Foxp3⁺ T-regs from Foxp3⁻ precursors, and further discuss the critical function of DCs in maintaining tolerance at various locations including skin and oral cavity.

Regulatory T-cell immunotherapy for allogeneic hematopoietic stem-cell transplantation

From mouse studies to recently published clinical trials, evidence has accumulated on the potential use of regulatory T cells (Treg) in preventing and treating graft-versus-host disease following hematopoietic-cell transplantation (HCT). However, controversies remain as to the phenotype and stability of various Treg subsets and their respective roles in vivo, the requirement of antigen-specificity of Treg to reduce promiscuous suppression, and the molecular mechanisms by which Treg suppress, particularly in humans. In this review, we discuss recent findings that support a heterogeneous population of human Treg, address advances in understanding how Treg function in the context of HCT, and present data on recent clinical trials that highlight the feasibility and limitations on Treg immunotherapy for graft-versus-host disease.