Effects of administration of progenipoietin 1, Flt-3 ligand, granulocyte colony-stimulating factor, and pegylated granulocyte-macrophage colony-stimulating factor on dendritic cell subsets in mice

A Minimal PBPK/PD Model with Expansion-Enhanced Target-Mediated Drug Disposition to Support a First-in-Human Clinical Study Design for a FLT3L-Fc Molecule

FLT3L-Fc is a half-life extended, effectorless Fc-fusion of the native human FLT3-ligand. In cynomolgus monkeys, treatment with FLT3L-Fc leads to a complex pharmacokinetic/pharmacodynamic (PK/PD) relationship, with observed nonlinear PK and expansion of different immune cell types across different dose levels. A minimal physiologically based PK/PD model with expansion-enhanced target-mediated drug disposition (TMDD) was developed to integrate the molecule's mechanism of action, as well as the complex preclinical and clinical PK/PD data, to support the preclinical-to-clinical translation of FLT3L-Fc. In addition to the preclinical PK data of FLT3L-Fc in cynomolgus monkeys, clinical PK and PD data from other FLT3-agonist molecules (GS-3583 and CDX-301) were used to inform the model and project the expansion profiles of conventional DC1s (cDC1s) and total DCs in peripheral blood. This work constitutes an essential part of our model-informed drug development (MIDD) strategy for clinical development of FLT3L-Fc by projecting PK/PD in healthy volunteers, determining the first-in-human (FIH) dose, and informing the efficacious dose in clinical settings. Model-generated results were incorporated in regulatory filings to support the rationale for the FIH dose selection.

Emerging biomaterial-based strategies for personalized therapeutic in situ cancer vaccines

Landmark successes in oncoimmunology have led to development of therapeutics boosting the host immune system to eradicate local and distant tumors with impactful tumor reduction in a subset of patients. However, current immunotherapy modalities often demonstrate limited success when involving immunologically cold tumors and solid tumors. Here, we describe the role of various biomaterials to formulate cancer vaccines as a form of cancer immunotherapy, seeking to utilize the host immune system to activate and expand tumor-specific T cells. Biomaterial-based cancer vaccines enhance the cancer-immunity cycle by harnessing cellular recruitment and activation against tumor-specific antigens. In this review, we discuss biomaterial-based vaccine strategies to induce lymphocytic responses necessary to mediate anti-tumor immunity. We focus on strategies that selectively attract dendritic cells via immunostimulatory gradients, activate them against presented tumor-specific antigens, and induce effective cross-presentation to T cells in secondary lymphoid organs, thereby generating immunity. We posit that personalized cancer vaccines are promising targets to generate long-term systemic immunity against patient- and tumor-specific antigens to ensure long-term cancer remission.

Quercetin Administration Suppresses the Cytokine Storm in Myeloid and Plasmacytoid Dendritic Cells

Dendritic cells (DCs) can be divided by lineage into myeloid dendritic cells (mDCs) and plasmacytoid dendritic cells (pDCs). They both are present in mucosal tissues and regulate the immune response by secreting chemokines and cytokines. Inflammatory bowel diseases (IBDs) are characterized by a leaky intestinal barrier and the consequent translocation of bacterial lipopolysaccharide (LPS) to the basolateral side. This results in DCs activation, but the response of pDCs is still poorly characterized. In the present study, we compared mDCs and pDCs responses to LPS administration. We present a broad panel of DCs secreted factors, including cytokines, chemokines, and growth factors. Our recent studies demonstrated the anti-inflammatory effects of quercetin administration, but to date, there is no evidence about quercetin’s effects on pDCs. The results of the present study demonstrate that pDCs can respond to LPS and that quercetin exposure modulates soluble factors release through the same molecular pathway used by mDCs (Slpi, Hmox1, and AP-1).

Single-cell analyses reveal the clonal and molecular aetiology of Flt3L-induced emergency dendritic cell development

Regulation of haematopoietic stem and progenitor cell (HSPC) fate is crucial during homeostasis and under stress conditions. Here we examine the aetiology of the Flt3 ligand (Flt3L)-mediated increase of type 1 conventional dendritic cells (cDC1s). Using cellular barcoding we demonstrate this occurs through selective clonal expansion of HSPCs that are primed to produce cDC1s and not through activation of cDC1 fate by other HSPCs. In particular, multi/oligo-potent clones selectively amplify their cDC1 output, without compromising the production of other lineages, via a process we term tuning. We then develop Divi-Seq to simultaneously profile the division history, surface phenotype and transcriptome of individual HSPCs. We discover that Flt3L-responsive HSPCs maintain a proliferative 'early progenitor'-like state, leading to the selective expansion of multiple transitional cDC1-primed progenitor stages that are marked by Irf8 expression. These findings define the mechanistic action of Flt3L through clonal tuning, which has important implications for other models of 'emergency' haematopoiesis.

Peripheral blood stem cell grafts in allogeneic hematopoietic cell transplantation: It is not all about the CD34+ cell dose

Allogeneic Hematopoietic Cell Transplantation is a curative approach in various malignant and non-malignant disorders. The majority of adult transplants in the current era are performed using mobilized stem cells, harvested from the peripheral blood by leukapheresis. Peripheral blood stem cell (PBSC) collections are designed to target a dose of stem cells that will result in safe engraftment and hematopoietic recovery; however, 99 % of the cells contained in a PBSC graft are not stem cells and a growing number of studies attempt to characterize the associations between graft composition and transplant outcomes. A better understanding of the impact of the quantity and quality of various cell types in PBSC grafts may lead to development of novel collection strategies or improved donor selection algorithms. Here we review relevant findings from recent studies in this area.

The role of dendritic cells in cancer

Cancer immunotherapy harnesses the ability of the immune system to recognize and eliminate cancer. The potent ability of dendritic cells (DCs) to initiate and regulate adaptive immune responses underpins the successful generation of anti-tumor immune responses. DCs are a heterogeneous leukocyte population comprised of distinct subsets that drive specific types of immune responses. Understanding how DCs induce tumor immune responses and the mechanisms adopted by tumors to evade DC surveillance is essential to render immunotherapies more effective. This review discusses current knowledge of the roles played by different DC subsets in human cancer and how these might be manipulated as new immunotherapeutics to improve CD8⁺ T cell-mediated immune responses, with a particular focus on the conventional type 1 DCs (cDC1).

Molecular regulation of dendritic cell development and function in homeostasis, inflammation, and cancer

Dendritic cells (DCs) are the principal antigen-presenting cells of the immune system and play key roles in controlling immune tolerance and activation. As such, DCs are chief mediators of tumor immunity. DCs can regulate tolerogenic immune responses that facilitate unchecked tumor growth. Importantly, however, DCs also mediate immune-stimulatory activity that restrains tumor progression. For instance, emerging evidence indicates the cDC1 subset has important functions in delivering tumor antigens to lymph nodes and inducing antigen-specific lymphocyte responses to tumors. Moreover, DCs control specific therapeutic responses in cancer including those resulting from immune checkpoint blockade. DC generation and function is influenced profoundly by cytokines, as well as their intracellular signaling proteins including STAT transcription factors. Regardless, our understanding of DC regulation in the cytokine-rich tumor microenvironment is still developing and must be better defined to advance cancer treatment. Here, we review literature focused on the molecular control of DCs, with a particular emphasis on cytokine- and STAT-mediated DC regulation. In addition, we highlight recent studies that delineate the importance of DCs in anti-tumor immunity and immune therapy, with the overall goal of improving knowledge of tumor-associated factors and intrinsic DC signaling cascades that influence DC function in cancer.

Dendritic Cells and Cancer: From Biology to Therapeutic Intervention

Inducing effective anti-tumor immunity has become a major therapeutic strategy against cancer. Dendritic cells (DC) are a heterogenous population of antigen presenting cells that infiltrate tumors. While DC play a critical role in the priming and maintenance of local immunity, their functions are often diminished, or suppressed, by factors encountered in the tumor microenvironment. Furthermore, DC populations with immunosuppressive activities are also recruited to tumors, limiting T cell infiltration and promoting tumor growth. Anti-cancer therapies can impact the function of tumor-associated DC and/or alter their phenotype. Therefore, the design of effective anti-cancer therapies for clinical translation should consider how best to boost tumor-associated DC function to drive anti-tumor immunity. In this review, we discuss the different subsets of tumor-infiltrating DC and their role in anti-tumor immunity. Moreover, we describe strategies to enhance DC function within tumors and harness these cells for effective tumor immunotherapy.

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.

Acutely malnourished weanling mice administered Flt3 ligand can support a cell-mediated inflammatory response

The main objective of this investigation was to determine whether, despite acute (wasting) deficits of dietary nitrogen and energy, weanling mice could respond to the dendritic cell hematopoietin, Fms-like tyrosine kinase 3 ligand (Flt3L), in terms of an index of cell-mediated inflammatory competence. Male and female C57BL/6J weanlings were used, initially 19 days of age, and malnutrition was produced using a nitrogen-deficient diet. In preliminary work ten daily subcutaneous 1.0 µg doses of murine Flt3L, comparable to a protocol effective in humans, expanded the splenic conventional dendritic cell compartment (CD11c⁺F4/80^-/low) of healthy weanlings without affecting the numbers of lymphocytes, macrophages, or recoverable mononuclear cells. Two subsequent experiments showed that, despite advancing malnutrition, exogenous Flt3L was able both to exert its classic influence on splenic conventional dendritic cell numbers and to invigorate the attenuated primary splenic cell-mediated inflammatory response to sheep erythrocytes. A final experiment showed that the cytokine intervention did not affect dendritic cell maturity according to several phenotypic indices. The findings provide new support for the proposition that dendritic cell numbers are the first limiting factor in the weak cell-mediated immune competence of acute pre-pubescent malnutrition. More substantially, intervention with Flt3L sustained an inflammatory systemic immune character despite progressive weanling malnutrition and weight loss. This outcome provides new support of fundamental character for the Tolerance Model which posits that the cell-mediated inflammatory incompetence of acute pre-pubescent protein and energy deficits is a regulated adaptive attempt, the antithesis of the classic paradigm of unregulated immunological attrition.

Autoimmune-Mediated Thymic Atrophy Is Accelerated but Reversible in RelB-Deficient Mice

Polymorphisms impacting thymic function may decrease peripheral tolerance and hasten autoimmune disease. The NF-κB transcription factor subunit, RelB, is essential for the development and differentiation of medullary thymic epithelial cells (mTECs): RelB-deficient mice have reduced thymic cellularity and markedly fewer mTECs, lacking AIRE. The precise mechanism of this mTEC reduction in the absence of RelB is unclear. To address this, we studied mTECs and dendritic cells (DCs), which critically regulate negative selection, and thymic regulatory T-cells (tTreg) in RelB-/- mice, which have spontaneous multiorgan autoimmune disease. RelB-/- thymi were organized, with medullary structures containing AIRE- mTECs, DCs, and CD4+ thymocytes, but fewer tTreg. Granulocytes infiltrated the RelB-/- thymic cortex, capsule, and medulla, producing inflammatory thymic medullary atrophy, which could be treated by granulocyte depletion or RelB+ DC immunotherapy, with concomitant recovery of mTEC and tTreg numbers. These data indicate that central tolerance defects may be accelerated by autoimmune thymic inflammation where impaired RelB signaling impairs the medullary niche, and may be reversible by therapies enhancing peripheral Treg or suppressing inflammation.

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.

Potential Pitfalls of the Mx1-Cre System: Implications for Experimental Modeling of Normal and Malignant Hematopoiesis

Conditional knockout mice are commonly used to study the function of specific genes in hematopoiesis. Different promoters that drive Cre expression have been utilized, with the interferon-inducible Mx1-Cre still being the most commonly used "deleter strain" in experimental hematology. However, different pitfalls associated with this system could lead to misinterpretation in functional studies. We present here two of these issues related to the use of Mx1-Cre: first, a high spontaneous recombination rate when applying commonly used techniques in experimental hematology, and second, undesired short-term consequences of the use of polyinosinic:polycytidylic acid, including changes in cellular phenotypes that, however, resolve within days. Our studies emphasize therefore that proper controls are crucial when modeling gene deletion using the Mx1-Cre transgene.

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.

Plasmacytoid dendritic cells in allogeneic hematopoietic cell transplantation: benefit or burden?

Plasmacytoid dendritic cells (pDCs) bridge innate and adaptive immune responses and have important roles in hematopoietic engraftment, GvHD and graft-versus-leukemia responses following allogeneic hematopoietic cell transplantation (HCT). In addition, pDCs mediate antiviral immunity, particularly as they are the body's primary cellular source of type I interferon. Given their pleiotropic roles, pDCs have emerged as cells that critically impact transplant outcomes, including overall survival. In this article, we will review the pre-clinical and clinical literature, supporting the crucial roles that pDCs assume as key immune effector cells during HCT.

Fms-like tyrosine kinase 3 ligand is required for thymic dendritic cell generation from bone marrow-derived CD117⁺ hematopoietic progenitor cells

Thymic dendritic cells (TDCs) are a type of dendritic cell (DC) in the thymus, which can enhance the proliferation of thymic T lymphocytes, regulate negative selection and induce central tolerance through autoantigen presentation. However, further investigations using TDCs has been restricted due to insufficient numbers. Therefore, an effective expansion method for TDCs in vitro is urgently required to further examine their biological characteristics. In the present study, a novel system was established using fetal thymus organ culture (FTOC) and a hanging drop culture system in the presence of fms‑like tyrosine kinase 3 ligand (Flt3L), termed the Flt3L/FTOC system. TDCs were successfully generated and expanded from CD117+ bone marrow hematopoietic progenitor cells. Conventional DCs (cDCs; CD11c+B220‑ DCs) and plasmacytoid DCs (pDCs; CD11c+B220+ DCs) were found in the TDCs generated using the Flt3L/FTOC system. These cells exhibited the specific morphological features of DCs, which were confirmed using Giemsa staining. Furthermore, the cytokine and surface marker profiles were also analyzed. Higher expression levels of interferon‑α and interleukin‑12 were observed in the pDCs, compared with the cDCs, and higher expression levels of toll‑like receptor (TLR)7 and TLR9 were found in the pDCs than in the cDCs. In addition, the Flt3L/FTOC‑derived TDCs also exhibited the ability to stimulate the allogenic T cell response. In conclusion, a novel in vitro culture system of thymic cDCs and pDCs using Flt3L was established, and this may provide a methodological basis for understanding the properties of TDCs.

Innate immune regulation by STAT-mediated transcriptional mechanisms

The term innate immunity typically refers to a quick but non-specific host defense response against invading pathogens. The innate immune system comprises particular immune cell populations, epithelial barriers, and numerous secretory mediators including cytokines, chemokines, and defense peptides. Innate immune cells are also now recognized to play important contributing roles in cancer and pathological inflammatory conditions. Innate immunity relies on rapid signal transduction elicited upon pathogen recognition via pattern recognition receptors (PRRs) and cell:cell communication conducted by soluble mediators, including cytokines. A majority of cytokines involved in innate immune signaling use a molecular cascade encompassing receptor-associated Jak protein tyrosine kinases and STAT (signal transducer and activator of transcription) transcriptional regulators. Here, we focus on roles for STAT proteins in three major innate immune subsets: neutrophils, macrophages, and dendritic cells (DCs). While knowledge in this area is only now emerging, understanding the molecular regulation of these cell types is necessary for developing new approaches to treat human disorders such as inflammatory conditions, autoimmunity, and cancer.

CD1d(hi)CD5+ B cells expanded by GM-CSF in vivo suppress experimental autoimmune myasthenia gravis

IL-10-competent subset within CD1d(hi)CD5(+) B cells, also known as B10 cells, has been shown to regulate autoimmune diseases. Whether B10 cells can prevent or suppress the development of experimental autoimmune myasthenia gravis (EAMG) has not been studied. In this study, we investigated whether low-dose GM-CSF, which suppresses EAMG, can expand B10 cells in vivo, and whether adoptive transfer of CD1d(hi)CD5(+) B cells would prevent or suppress EAMG. We found that treatment of EAMG mice with low-dose GM-CSF increased the proportion of CD1d(hi)CD5(+) B cells and B10 cells. In vitro coculture studies revealed that CD1d(hi)CD5(+) B cells altered T cell cytokine profile but did not directly inhibit T cell proliferation. In contrast, CD1d(hi)CD5(+) B cells inhibited B cell proliferation and its autoantibody production in an IL-10-dependent manner. Adoptive transfer of CD1d(hi)CD5(+) B cells to mice could prevent disease, as well as suppress EAMG after disease onset. This was associated with downregulation of mature dendritic cell markers and expansion of regulatory T cells resulting in the suppression of acetylcholine receptor-specific T cell and B cell responses. Thus, our data have provided significant insight into the mechanisms underlying the tolerogenic effects of B10 cells in EAMG. These observations suggest that in vivo or in vitro expansion of CD1d(hi)CD5(+) B cells or B10 cells may represent an effective strategy in the treatment of human myasthenia gravis.

Cutting edge: Flt3 ligand mediates STAT3-independent expansion but STAT3-dependent activation of myeloid-derived suppressor cells

The Flt3-Flt3 ligand (Flt3L) pathway is critically involved in the differentiation and homeostasis of myeloid cells, including dendritic cells (DC); however, its role in the expansion and function of myeloid-derived suppressor cells (MDSC) has not been determined. In this article, we describe the ability of Flt3L to expand and activate murine MDSC capable of suppressing allograft rejection upon adoptive transfer. Although Flt3L expands and augments the stimulatory capacity of myeloid DC, MDSC expanded by Flt3L have increased suppressive activity. Although STAT3 is considered the central transcription factor for MDSC expansion, inhibition and genetic ablation of STAT3 did not block, but rather augmented, Flt3L-mediated MDSC expansion. MDSC suppressive function, preserved when STAT3 inhibition was removed, was reduced by genetic STAT3 deletion. Both STAT3 inhibition and deletion reduced Flt3L-mediated DC expansion, signifying that STAT3 had reciprocal effects on suppressive MDSC and immunostimulatory DC expansion. Together, these findings enhance our understanding of the immunomodulatory properties of Flt3L.

OX40 ligand regulates splenic CD8⁻ dendritic cell-induced Th2 responses in vivo

In mice, splenic conventional dendritic cells (cDCs) can be separated, based on their expression of CD8α into CD8(-) and CD8(+) cDCs. Although previous experiments demonstrated that injection of antigen (Ag)-pulsed CD8(-) cDCs into mice induced CD4 T cell differentiation toward Th2 cells, the mechanism involved is unclear. In the current study, we investigated whether OX40 ligand (OX40L) on CD8(-) cDCs contributes to the induction of Th2 responses by Ag-pulsed CD8(-) cDCs in vivo, because OX40-OX40L interactions may play a preferential role in Th2 cell development. When unseparated Ag-pulsed OX40L-deficient cDCs were injected into syngeneic BALB/c mice, Th2 cytokine (IL-4, IL-5, and IL-10) production in lymph node cells was significantly reduced. Splenic cDCs were separated to CD8(-) and CD8(+) cDCs. OX40L expression was not observed on freshly isolated CD8(-) cDCs, but was induced by anti-CD40 mAb stimulation for 24 h. Administration of neutralizing anti-OX40L mAb significantly inhibited IL-4, IL-5, and IL-10 production induced by Ag-pulsed CD8(-) cDC injection. Moreover, administration of anti-OX40L mAb with Ag-pulsed CD8(-) cDCs during a secondary response also significantly inhibited Th2 cytokine production. Thus, OX40L on CD8(-) cDCs physiologically contributes to the development of Th2 cells and secondary Th2 responses induced by Ag-pulsed CD8(-) cDCs in vivo.

FLT3-ligand treatment of humanized mice results in the generation of large numbers of CD141+ and CD1c+ dendritic cells in vivo

We established a humanized mouse model incorporating FLT3-ligand (FLT3-L) administration after hematopoietic cell reconstitution to investigate expansion, phenotype, and function of human dendritic cells (DC). FLT3-L increased numbers of human CD141(+) DC, CD1c(+) DC, and, to a lesser extent, plasmacytoid DC (pDC) in the blood, spleen, and bone marrow of humanized mice. CD1c(+) DC and CD141(+) DC subsets were expanded to a similar degree in blood and spleen, with a bias toward expansion of the CD1c(+) DC subset in the bone marrow. Importantly, the human DC subsets generated after FLT3-L treatment of humanized mice are phenotypically and functionally similar to their human blood counterparts. CD141(+) DC in humanized mice express C-type lectin-like receptor 9A, XCR1, CADM1, and TLR3 but lack TLR4 and TLR9. They are major producers of IFN-λ in response to polyinosinic-polycytidylic acid but are similar to CD1c(+) DC in their capacity to produce IL-12p70. Although all DC subsets in humanized mice are efficient at presenting peptide to CD8(+) T cells, CD141(+) DC are superior in their capacity to cross-present protein Ag to CD8(+) T cells following activation with polyinosinic-polycytidylic acid. CD141(+) DC can be targeted in vivo following injection of Abs against human DEC-205 or C-type lectin-like receptor 9A. This model provides a feasible and practical approach to dissect the function of human CD141(+) and CD1c(+) DC and evaluate adjuvants and DC-targeting strategies in vivo.

A novel subset of helper T cells promotes immune responses by secreting GM-CSF

Helper T cells are crucial for maintaining proper immune responses. Yet, they have an undefined relationship with one of the most potent immune stimulatory cytokines, granulocyte macrophage-colony-stimulating factor (GM-CSF). By depleting major cytokines during the differentiation of CD4(+) T cells in vitro, we derived cells that were found to produce large amounts of GM-CSF, but little of the cytokines produced by other helper T subsets. By their secretion of GM-CSF, this novel subset of helper T cells (which we have termed ThGM cells) promoted the production of cytokines by other T-cell subtypes, including type 1 helper T cell (Th1), type 2 helper T cell (Th2), type 1 cytotoxic T cell (Tc1), type 2 cytotoxic T cell (Tc2), and naive T cells, as evidenced by the fact that antibody neutralization of GM-CSF abolished this effect. ThGM cells were found to be highly prone to activation-induced cell death (AICD). Inhibitors of TRAIL or granzymes could not block AICD in ThGM cells, whereas inhibition of FasL/Fas interaction partially rescued ThGM cells from AICD. Thus, ThGM cells are a novel subpopulation of T helper cells that produce abundant GM-CSF, exhibit exquisite susceptibility to apoptosis, and therefore play a pivotal role in the regulation of the early stages of immune responses.

Diversification of dendritic cell subsets: Emerging roles for STAT proteins

The term dendritic cell (DC) refers to a population of hematopoietic cells with critical roles in immunity, including immune activation in response to pathogen-elicited danger signals and immune tolerance. Aberrant DC activity is an important contributing factor in autoimmunity, while severe DC depletion accompanies certain immunodeficiency conditions. By contrast, DCs have become attractive candidates to manipulate in immune therapy. Recent studies show that STAT transcription factors have unique roles in DCs, a feature that might be exploited in future DC-based therapies. Here, we focus on the functions of STAT1, STAT3, and STAT5 in DC generation and DC-mediated immune responses.

IRF4 transcription-factor-dependent CD103(+)CD11b(+) dendritic cells drive mucosal T helper 17 cell differentiation

CD103(+)CD11b(+) dendritic cells (DCs) represent the major migratory DC population within the small intestinal lamina propria (SI-LP), but their in vivo function remains unclear. Here we demonstrate that intestinal CD103(+)CD11b(+) DC survival was dependent on interferon regulatory factor 4 (IRF4). Mice with a DC deletion in Irf4 displayed reduced numbers of intestinal interleukin 17 (IL-17)-secreting helper T 17 (Th17) cells and failed to support Th17 cell differentiation in draining mesenteric lymph nodes (MLN) following immunization. The latter was associated with a selective reduction in CD103(+)CD11b(+) MLN DCs and DC derived IL-6. Immunized Il6(-/-) mice failed to support Th17 cell differentiation in MLN in vivo and CD103(+)CD11b(+) MLN DCs supported IL-6-dependent Th17 cell differentiation in vitro. Together, our results suggest a central role for IRF4-dependent, IL-6 producing CD103(+)CD11b(+) DCs in intestinal Th17 cell differentiation.

Identification and quantitation of MHC class II-bound peptides from mouse spleen dendritic cells by immunoprecipitation and mass spectrometry analysis

Advances in immunology and immune therapies require knowledge of antigenic peptide sequences that are presented on MHC class II and class I molecules of antigen presenting cells. The most specialized antigen presenting cells are dendritic cells (DCs). In the past, the small number of DCs that could be isolated from mouse spleen prevented direct analysis of the MHC II peptide repertoire presented by DCs. Here we describe a protocol that integrates immunological methods (in vivo enrichment of mouse spleen DCs by Flt3L treatment and immunoprecipitation of MHC II-peptide complexes), mass spectrometry analysis and peptide synthesis (LC-MS/MS and quantitation analysis for non tryptic peptides) to identify and quantitate the endogenous peptides that are bound to MHC II molecules on DCs. The described method produces quantitative data that are reproducible and reliable enough to cover a wide range of peptide copy numbers. We propose the application of this method in future studies to quantitatively investigate the MHC II repertoire on DCs presented during viral infections or different immunizations in vaccine development research.

The equivalents of human blood and spleen dendritic cell subtypes can be generated in vitro from human CD34(+) stem cells in the presence of fms-like tyrosine kinase 3 ligand and thrombopoietin

Dendritic cells (DCs) are immune cells specialized to capture, process and present antigen to T cells in order to initiate an appropriate adaptive immune response. The study of mouse DC has revealed a heterogeneous population of cells that differ in their development, surface phenotype and function. The study of human blood and spleen has shown the presence of two subsets of conventional DC including the CD1b/c(+) and CD141(+)CLEC9A(+) conventional DC (cDC) and a plasmacytoid DC (pDC) that is CD304(+)CD123(+). Studies on these subpopulations have revealed phenotypic and functional differences that are similar to those described in the mouse. In this study, the three DC subsets have been generated in vitro from human CD34(+) precursors in the presence of fms-like tyrosine kinase 3 ligand (Flt3L) and thrombopoietin (TPO). The DC subsets so generated, including the CD1b/c(+) and CLEC9A(+) cDCs and CD123(+) pDCs, were largely similar to their blood and spleen counterparts with respect to surface phenotype, toll-like receptor and transcription factor expression, capacity to stimulate T cells, cytokine secretion and cross-presentation of antigens. This system may be utilized to study aspects of DC development and function not possible in vivo.

The signal transducers STAT5 and STAT3 control expression of Id2 and E2-2 during dendritic cell development

Cytokines and transcription factors play key roles in dendritic cell (DC) development, yet information about regulatory interactions between these signals remains limited. Here we show that the cytokines GM-CSF and Flt3L induce the transcriptional mediators Id2 and E2-2 and control DC lineage diversification by STAT-dependent pathways. We found that STAT5 is required for tissue CD103(+) DC generation and plasmacytoid DC (pDC) suppression in steady state or response to GM-CSF. STAT5 stimulates GM-CSF-dependent expression of Id2, which controls CD103(+) DC production and pDC inhibition. By contrast, pDCs, but not CD103(+) DCs, are dependent on STAT3. Consistently, STAT3 stimulates Flt3L-responsive expression of the pDC regulator Tcf4 (E2-2). These data suggest that STATs contribute to DC development by controlling transcription factors involved in lineage differentiation.

CD4 T follicular helper cell dynamics during SIV infection

CD4 T follicular helper (TFH) cells interact with and stimulate the generation of antigen-specific B cells. TFH cell interaction with B cells correlates with production of SIV-specific immunoglobulins. However, the fate of TFH cells and their participation in SIV-induced antibody production is not well understood. We investigated the phenotype, function, location, and molecular signature of TFH cells in rhesus macaques. Similar to their human counterparts, TFH cells in rhesus macaques represented a heterogeneous population with respect to cytokine function. In a highly differentiated subpopulation of TFH cells, characterized by CD150lo expression, production of Th1 cytokines was compromised while IL-4 production was augmented, and cells exhibited decreased survival, cycling, and trafficking capacity. TFH cells exhibited a distinct gene profile that was markedly altered by SIV infection. TFH cells were infected by SIV; yet, in some animals, these cells actually accumulated during chronic SIV infection. Generalized immune activation and increased IL-6 production helped drive TFH differentiation during SIV infection. Accumulation of TFH cells was associated with increased frequency of activated germinal center B cells and SIV-specific antibodies. Therefore, chronic SIV does not disturb the ability of TFH cells to help B cell maturation and production of SIV-specific immunoglobulins.

Dendritic cells: arbiters of immunity and immunological tolerance

Dendritic cells (DCs) link innate immune sensing of the environment to the initiation of adaptive immune responses. Given their supreme capacity to interact with and present antigen to T cells, DCs have been proposed as key mediators of immunological tolerance in the steady state. However, recent evidence suggests that the role of DCs in central and peripheral T-cell tolerance is neither obligate nor dominant. Instead, DCs appear to regulate multiple aspects of T-cell physiology including tonic antigen receptor signaling, priming of effector T-cell response, and the maintenance of regulatory T cells. These diverse contributions of DCs may reflect the significant heterogeneity and "division of labor" observed between and within distinct DC subsets. The emerging complex role of different DC subsets should form the conceptual basis of DC-based therapeutic approaches toward induction of tolerance or immunization.

Dendritic/tumor fusion cells as cancer vaccines

A promising cancer vaccine involves the fusion of dendritic cells (DCs) with tumor cells such that a broad array of tumor antigens are presented in the context of DC-mediated costimulation and stimulatory cytokines. In diverse animal models, vaccination with DC/tumor fusions results in protection from an otherwise lethal challenge of tumor cells and eradication of established disease. In phase I clinical studies, vaccination with DC/tumor fusions was well tolerated, and induced immunologic responses in the majority of patients and clinical responses in a subset. Vaccine efficacy may be blunted by the immunosuppressive milieu characteristic of patients with malignancy, including the increased presence of regulatory T cells, and inhibitory pathways such as the PD-1/PDL-1 pathway. A current focus of research interest lies in enhancing response to cancer vaccines, by combining vaccination with tumor cytoreduction, regulatory T-cell depletion, and blockade of critical inhibitory pathways.

Optimizing dendritic cell-based immunotherapy: tackling the complexity of different arms of the immune system

Earlier investigations have revealed a surprising complexity and variety in the range of interaction between cells of the innate and adaptive immune system. Our understanding of the specialized roles of dendritic cell (DC) subsets in innate and adaptive immune responses has been significantly advanced over the years. Because of their immunoregulatory capacities and because very small numbers of activated DC are highly efficient at generating immune responses against antigens, DCs have been vigorously used in clinical trials in order to elicit or amplify immune responses against cancer and chronic infectious diseases. A better insight in DC immunobiology and function has stimulated many new ideas regarding the potential ways forward to improve DC therapy in a more fundamental way. Here, we discuss the continuous search for optimal in vitro conditions in order to generate clinical-grade DC with a potent immunogenic potential. For this, we explore the molecular and cellular mechanisms underlying adequate immune responses and focus on most favourable DC culture regimens and activation stimuli in humans. We envisage that by combining each of the features outlined in the current paper into a unified strategy, DC-based vaccines may advance to a higher level of effectiveness.

A STATus report on DC development

DCs have a vital role in the immune system by recognizing exogenous or self-antigens and eliciting appropriate stimulatory or tolerogenic adaptive immune responses. DCs also contribute to human autoimmune disease and, when depleted, to immunodeficiency. Moreover, DCs are being explored for potential use in clinical therapies including cancer treatment. Thus, understanding the molecular mechanisms that regulate DCs is crucial to improving treatments for human immune disease and cancer. DCs constitute a heterogeneous population including plasmacytoid (pDC) and classic (cDC) subsets; however, the majority of DCs residing in lymphoid organs and peripheral tissues in steady state share common progenitor populations, originating with hematopoietic stem cells. Like other hematopoietic lineages, DCs require extracellular factors including cytokines, as well as intrinsic transcription factors, to control lineage specification, commitment, and maturation. Here, we review recent findings on the roles for cytokines and cytokine-activated STAT transcription factors in DC subset development. We also discuss how cytokines and STATs intersect with lineage-regulatory transcription factors and how insight into the molecular basis of human disease has revealed transcriptional regulators of DCs. Whereas this is an emerging area with much work remaining, we anticipate that knowledge gained by delineating cytokine and transcription factor mechanisms will enable a better understanding of DC subset diversity, and the potential to manipulate these important immune cells for human benefit.

Nonplasmacytoid, high IFN-α-producing, bone marrow dendritic cells

Plasmacytoid dendritic cells (pDC) are the producers of type I IFNs in response to TLR9 ligands. However, we have found that when bone marrow is depleted of pDC, the IFN-α produced in response to TLR9 ligands is not fully removed. We assign the source of this non-pDC IFN-α as a newly described DC type. It displays the high IFN-α producing activity of pDC but to a more limited range of viruses. Unlike pDC, the novel DC display high T cell stimulation capacity. Moreover, unlike mouse pDC, they are matured with GM-CSF and are less prone to apoptosis upon activation stimuli, including viruses. We propose that these DC constitute a novel bone marrow inflammatory DC type, ideally geared to linking innate and adaptive immune responses in bone marrow via their potent IFN-α production and high T cell stimulatory capacity.

The prolyl isomerase Pin1 modulates development of CD8+ cDC in mice

Background

Pin1 has previously been described to regulate cells that participate in both innate and adaptive immunity. Thus far, however, no role for Pin1 has been described in modulating conventional dendritic cells, innate antigen presenting cells that potently activate naïve T cells, thereby bridging innate and adaptive immune responses.

Methodology/Principal Findings

When challenged with LPS, Pin1-null mice failed to accumulate spleen conventional dendritic cells (cDC). Analysis of steady-state spleen DC populations revealed that Pin1-null mice had fewer CD8+ cDC. This defect was recapitulated by culturing Pin1-null bone marrow with the DC-instructive cytokine Flt3 Ligand. Additionally, injection of Flt3 Ligand for 9 days failed to induce robust expansion of CD8+ cDC in Pin1-null mice. Upon infection with Listeria monocytogenes, Pin1-null mice were defective in stimulating proliferation of adoptively transferred WT CD8+ T cells, suggesting that decreases in Pin1 null CD8+ cDC may affect T cell responses to infection in vivo. Finally, upon analyzing expression of proteins involved in DC development, elevated expression of PU.1 was detected in Pin1-null cells, which resulted from an increase in PU.1 protein half-life.

Conclusions/Significance

We have identified a novel role for Pin1 as a modulator of CD8+ cDC development. Consistent with reduced numbers of CD8+ cDC in Pin1-null mice, we find that the absence of Pin1 impairs CD8+ T cell proliferation in response to infection with Listeria monocytogenes. These data suggest that, via regulation of CD8+ cDC production, Pin1 may serve as an important modulator of adaptive immunity.

Non-human primate dendritic cells

Non-human primates (NHP) are essential translational models for biomedical research. Dendritic cells (DC) are a group of antigen presenting cells (APC) that play pivotal roles in the immunobiology of health and disease and are attractive cells for adoptive immunotherapy to stimulate and suppress immunity. DC have been studied extensively in humans and mice but until recently, have not been well characterized in NHP. This review considers the available data about DC across a range of NHP species and summarizes the understanding of in vitro-propagated DC and in vivo-isolated DC, which is now established. It is clear that although NHP DC exist within the paradigm of human DC, there are important functional and phenotypic differences when compared with human DC subsets. These differences need to be taken into account when designing preclinical, translational studies of DC therapy using NHP models.

GM-CSF-induced regulatory T cells selectively inhibit anti-acetylcholine receptor-specific immune responses in experimental myasthenia gravis

We and others have demonstrated the ability of granulocyte-macrophage colony-stimulating factor (GM-CSF) to suppress autoimmunity by increasing the number of CD4(+)CD25(+) regulatory T cells (Tregs). In the current study, we have explored the critical role of induced antigen specific Tregs in the therapeutic effects of GM-CSF in murine experimental autoimmune myasthenia gravis (EAMG). Specifically, we show that Tregs from GM-CSF treated EAMG mice (GM-CSF/AChR-induced-Tregs) adoptively transferred into animals with EAMG suppressed clinical disease more potently than equal numbers of Tregs from either GM-CSF untreated EAMG mice or healthy mice treated with GM-CSF. In addition, GM-CSF/AChR-induced-Tregs selectively suppressed antigen specific T cell proliferation induced by AChR relative to that induced by an irrelevant self antigen, (thyroglobulin) and failed to significantly alter T cell proliferation in response to an exogenous antigen (ovalbumin). These results are consistent with the hypothesized mechanism of action of GM-CSF involving the mobilization of tolerogenic dendritic cell precursors which, upon antigen (AChR) capture, suppress the anti-AChR immune response through the induction/expansion of AChR-specific Tregs.

Mass spectrometry analysis and quantitation of peptides presented on the MHC II molecules of mouse spleen dendritic cells

Major histocompatibility complex class II (MHC II) molecules are expressed on the surface of antigen-presenting cells and display short bound peptide fragments derived from self- and nonself antigens. These peptide-MHC complexes function to maintain immunological tolerance in the case of self-antigens and initiate the CD4(+) T cell response in the case of foreign proteins. Here we report the application of LC-MS/MS analysis to identify MHC II peptides derived from endogenous proteins expressed in freshly isolated murine splenic DCs. The cell number was enriched in vivo upon treatment with Flt3L-B16 melanoma cells. In a typical experiment, starting with about 5 × 10(8) splenic DCs, we were able to reliably identify a repertoire of over 100 MHC II peptides originating from about 55 proteins localized in membrane (23%), intracellular (26%), endolysosomal (12%), nuclear (14%), and extracellular (25%) compartments. Using synthetic isotopically labeled peptides corresponding to the sequences of representative bound MHC II peptides, we quantified by LC-MS relative peptide abundance. In a single experiment, peptides were detected in a wide concentration range spanning from 2.5 fmol/μL to 12 pmol/μL or from approximately 13 to 2 × 10(5) copies per DC. These peptides were found in similar amounts on B cells where we detected about 80 peptides originating from 55 proteins distributed homogenously within the same cellular compartments as in DCs. About 90 different binding motifs predicted by the epitope prediction algorithm were found within the sequences of the identified MHC II peptides. These results set a foundation for future studies to quantitatively investigate the MHC II repertoire on DCs generated under different immunization conditions.

CD8α⁺ dendritic cells improve collagen-induced arthritis in CC chemokine receptor (CCR)-2 deficient mice

OBJECTIVE

Dendritic cells (DCs) have long been recognized as potential therapeutic targets of rheumatoid arthritis (RA). Increasing evidence has showed that DCs are capable of suppressing autoimmunity by expanding FoxP3⁺ regulatory T cells (T(reg)), which in turn exert immunosuppression by increasing TGFβ-1. In the SKG mice, activated DC prime autoreactive T cells causing autoantibody production and an inflammatory arthritic response. Recently, we reported that CC-chemokine receptor-2 deficient (Ccr2⁻/⁻) mice had impaired DCs migration and reduced CD8α⁺ DCs in the C57Bl/6J mice strain and that these mice were more susceptible to collagen antibody-induced arthritis (CAIA), compared to wild type mice. To examine the mechanism by which DCs contribute to the increased susceptibility of arthritis in Ccr2⁻/⁻ mice, we tested the hypothesis that CD8α⁺ DCs are protective (tolerogenic) against autoimmune arthritis by examining the role of CD8α⁺ DCs in Ccr2⁻/⁻ and SKG mice.

METHODS

To examine the mechanism by which DCs defects lead to the development of arthritis, we used two murine models of experimental arthritis: collagen-induced arthritis (CIA) in DBA1/J mice and zymosan-induced arthritis in SKG mice. DBA1/J mice received recombinant fms-like tyrosine kinase 3 ligand (Flt3L) injections to expand endogenous DCs populations or adoptive transfers of CD8α⁺ DCs.

RESULTS

Flt3L-mediated expansion of endogenous CD8α⁺ DCs resulted in heightened susceptibility of CIA. In contrast, supplementation with exogenous CD8α⁺ DCs ameliorated arthritis in Ccr2⁻/⁻ mice and enhanced TGFβ1 production by T cells. Furthermore, SKG mice with genetic inactivation of CCR2 did not affect the numbers of DCs nor improve the arthritis phenotype.

CONCLUSION

CD8α⁺ DCs were tolerogenic to the development of arthritis. CD8α⁺ DCs deficiency heightened the sensitivity to arthritis in Ccr2⁻/⁻ mice. Ccr2 deficiency did not alter the arthritic phenotype in SKG mice suggesting the arthritis in Ccr2⁻/⁻ mice was T cell-independent.

Antigen encoded by vaccine vectors derived from human adenovirus serotype 5 is preferentially presented to CD8+ T lymphocytes by the CD8α+ dendritic cell subset

Different subsets of dendritic cells (DC) elicit qualitatively different immune responses. In mice, two lymphoid tissue-resident subsets, CD8α(+) and CD8α(-), have been implicated in the induction of T helper 1 (Th1) or Th2 responses, respectively. Moreover, CD8α(+) DC appear to play a major role in priming CD8(+) T lymphocyte responses to viral antigens in the course of diverse viral infections. These considerations have been less extensively explored for vaccine vectors derived from viruses. Despite inefficient ex vivo transduction of DC, vectored vaccines derived from human adenoviruses of serotype 5 (Ad5) elicit robust immune responses, predominantly of the Th1 orientation, in humans and mice. At present it is unknown whether Ad5 interacts with DC subsets in a differential manner, thereby influencing the quality of the elicited IR. To address this issue, successive steps (attachment, transgene expression, MHC class I antigen presentation and activation of antigen-specific T lymphocytes) involved in induction of immune responses by Ad5-based vectors have been examined in CD8α(+) and CD8α(-) murine DC subsets. Although in both ex vivo and in vivo experiments CD8α(+) and CD8α(-) DC subsets captured an Ad5-based vector to a similar extent, transgene expression and subsequent MHC class I display of a transgene-encoded antigen were more efficient in CD8α(+) DC. Moreover, following in vivo and ex vivo transduction with an Ad5-based vaccine, antigen-specific CD8(+) T lymphocytes were more efficiently activated by CD8α(+) DC than by CD8α(-) DC. Thus, superior antigen expression and MHC class I display in CD8α(+) DC may contribute to preferred priming of antigen-specific CD8(+) lymphocytes by Ad5-transduced CD8α(+) DC.

Mechanisms regulating dendritic cell specification and development

Understanding the diversification of dendritic cell (DC) lineages is one of the last frontiers in mapping the developmental hierarchy of the hematopoietic system. DCs are a vital link between the innate and adaptive immune responses; thus, elucidating their developmental pathways is crucial for insight into the generation of natural immunity and for learning how to regulate DCs in clinical settings. DCs arise from hematopoietic stem cells through specialized progenitor subsets under the direction of FMS-like tyrosine kinase 3 ligand (Flt3L) and Flt3L receptor (Flt3) signaling. Recent studies have revealed important contributions from granulocyte-macrophage colony-stimulating factor (GM-CSF) and type I interferons (IFNs) in vivo. Furthermore, DC development is guided by lineage-restricted transcription factors such as IRF8, E2-2, and Batf3. A critical question centers on how cytokines and lineage-restricted transcription factors operate molecularly to direct DC diversification. Here, we review recent findings that provide new insight into the DC developmental process.

Flt3L-mobilized dendritic cells bearing H2-Kbm1 apoptotic cells do not induce cross-tolerance to CD8+ T cells across a class I MHC mismatched barrier

Tolerization of allogeneic CD8(+) T cells is still a pending issue in the field of transplantation research to achieve long-term survival. To test whether dendritic cells (DC) bearing allogeneic major histocompatibility complex (MHC) class I mismatched apoptotic cells could induce cross-tolerance to alloreactive CD8(+) T cells, the following experimental strategy was devised. Rag2/γ(c) KO B6 mice were treated with Fms-like tyrosine kinase 3 ligand (Flt3L)-transduced B16 melanoma cells to drive a rapid expansion and mobilization of DC in vivo. Of all DC populations expanded, splenic CD11c(+) CD103(+) CD8α(+) DC were selectively involved in the process of antigen clearance of X-ray irradiated apoptotic thymocytes in vivo. Considering that CD11c(+) CD103(+) CD8α(+) DC selectively take up apoptotic cells and that they are highly specialized in cross-presenting antigen to CD8(+) T cells, we investigated whether B6 mice adoptively transferred with Flt3L-derived DC loaded with donor-derived apoptotic thymocytes could induce tolerance to bm1 skin allografts. Our findings on host anti-donor alloresponse, as revealed by skin allograft survival and cytotoxic T lymphocyte assays, indicated that the administration of syngeneic DC presenting K(bm1) donor-derived allopeptides through the indirect pathway of antigen presentation was not sufficient to induce cross-tolerance to alloreactive CD8(+) T cells responding to bm1 alloantigens in a murine model of skin allograft transplantation across an MHC class I mismatched barrier.

Innate immune stimuli modulate bone marrow-derived dendritic cell production in vitro by toll-like receptor-dependent and -independent mechanisms

Haematopoiesis is crucial for immunity because it results in the production of leucocytes. Bacterial and viral infections alter leucocyte production by promoting granulopoiesis or lymphopoiesis. Recent studies suggest that changes in leucocyte production may be caused by the effects of inflammatory responses on the differentiation of haematopoietic progenitors in the bone marrow. We investigated the mechanisms through which infection regulates the formation of bone marrow-derived dendritic cells (BMDCs) in vitro. We mimicked infection by stimulating developing cells with molecules associated with bacteria and viruses and with inactivated influenza viruses. We showed that toll-like receptor (TLR) ligands act as modulators of haematopoiesis, and that signalling through different TLRs results in differing effects on the production of BMDCs. We demonstrated that ligands for TLR3 and influenza viruses reduce the production of BMDCs, resulting in increased neutrophil numbers, and that ligands for TLR4 and TLR9 drive the production of plasmacytoid dendritic cells. Furthermore, there are distinct signalling mechanisms involved in these effects. Signalling pathways triggered by TLR4 and TLR9 involve MyD88 and are partially mediated by the cytokine tumour necrosis factor-α (TNF-α). Mechanisms activated by TLR3 were Tir-domain-containing adaptor-inducing interferon dependent. Haematopoietic modulation induced by inactivated influenza viruses was associated with the activation of an antiviral pathway mediated by type-1 interferons.

Mouse CD8alpha+ DCs and human BDCA3+ DCs are major producers of IFN-lambda in response to poly IC

Polyinosinic:polycytidylic acid (poly IC), a double-stranded RNA, is an effective adjuvant in vivo. IFN-λs (also termed IL-28/29) are potent immunomodulatory and antiviral cytokines. We demonstrate that poly IC injection in vivo induces large amounts of IFN-λ, which depended on hematopoietic cells and the presence of TLR3 (Toll-like receptor 3), IRF3 (IFN regulatory factor 3), IRF7, IFN-I receptor, Fms-related tyrosine kinase 3 ligand (FL), and IRF8 but not on MyD88 (myeloid differentiation factor 88), Rig-like helicases, or lymphocytes. Upon poly IC injection in vivo, the IFN-λ production by splenocytes segregated with cells phenotypically resembling CD8α(+) conventional dendritic cells (DCs [cDCs]). In vitro experiments revealed that CD8α(+) cDCs were the major producers of IFN-λ in response to poly IC, whereas both CD8α(+) cDCs and plasmacytoid DCs produced large amounts of IFN-λ in response to HSV-1 or parapoxvirus. The nature of the stimulus and the cytokine milieu determined whether CD8α(+) cDCs produced IFN-λ or IL-12p70. Human DCs expressing BDCA3 (CD141), which is considered to be the human counterpart of murine CD8α(+) DCs, also produced large amounts of IFN-λ upon poly IC stimulation. Thus, IFN-λ production in response to poly IC is a novel function of mouse CD8α(+) cDCs and their human equivalents.

Mammalian target of rapamycin controls dendritic cell development downstream of Flt3 ligand signaling

Dendritic cells (DCs) comprise distinct functional subsets including CD8⁻ and CD8(+) classical DCs (cDCs) and interferon-secreting plasmacytoid DCs (pDCs). The cytokine Flt3 ligand (Flt3L) controls the development of DCs and is particularly important for the pDC and CD8(+) cDC and their CD103(+) tissue counterparts. We report that mammalian target of rapamycin (mTOR) inhibitor rapamycin impaired Flt3L-driven DC development in vitro, with the pDCs and CD8(+)-like cDCs most profoundly affected. Conversely, deletion of the phosphoinositide 3-kinase (PI3K)-mTOR negative regulator Pten facilitated Flt3L-driven DC development in culture. DC-specific Pten targeting in vivo caused the expansion of CD8(+) and CD103(+) cDC numbers, which was reversible by rapamycin. The increased CD8(+) cDC numbers caused by Pten deletion correlated with increased susceptibility to the intracellular pathogen Listeria. Thus, PI3K-mTOR signaling downstream of Flt3L controls DC development, and its restriction by Pten ensures optimal DC pool size and subset composition.

CX3CR1+ CD8alpha+ dendritic cells are a steady-state population related to plasmacytoid dendritic cells

Lymphoid organs are characterized by a complex network of phenotypically distinct dendritic cells (DC) with potentially unique roles in pathogen recognition and immunostimulation. Classical DC (cDC) include two major subsets distinguished in the mouse by the expression of CD8alpha. Here we describe a subset of CD8alpha(+) DC in lymphoid organs of naïve mice characterized by expression of the CX(3)CR1 chemokine receptor. CX(3)CR1(+) CD8alpha(+) DC lack hallmarks of classical CD8alpha(+) DC, including IL-12 secretion, the capacity to cross-present antigen, and their developmental dependence on the transcriptional factor BatF3. Gene-expression profiling showed that CX(3)CR1(+) CD8alpha(+) DC resemble CD8alpha(-) cDC. The microarray analysis further revealed a unique plasmacytoid DC (PDC) gene signature of CX(3)CR1(+) CD8alpha(+) DC. A PDC relationship of the cells is supported further by the fact that they harbor characteristic D-J Ig gene rearrangements and that development of CX(3)CR1(+) CD8alpha(+) DC requires E2-2, the critical transcriptional regulator of PDC. Thus, CX(3)CR1(+) CD8alpha(+) DC represent a unique DC subset, related to but distinct from PDC. Collectively, the expression-profiling data of this study refine the resolution of previous DC definitions, sharpen the border of classical CD8alpha(+) and CD8alpha(-) DC, and should assist the identification of human counterparts of murine DC subsets.

Resident and monocyte-derived dendritic cells become dominant IL-12 producers under different conditions and signaling pathways

IL-12 is such a pivotal cytokine that it has been called the third signal for T cell activation, TCR engagement being the first and costimulation being the second. It has been generally viewed that the resident CD8(+) dendritic cell (DC) subset is the predominant IL-12-producing cell type. In this study, we found, although this is so under steady state conditions, under inflammatory conditions monocyte-derived DC (mDC) became a major cell type producing IL-12. Depletion of either type of DC resulted in reduced production of IL-12 in vivo. For CD8(+) DC, IL-12 production could be stimulated by various pathways viz. signaling through MyD88, Trif, or nucleotide-binding oligomerization domain (Nod)-like receptors. In contrast, for mDC, IL-12 production was mainly dependent on MyD88 signaling. Thus, conventional DCs and mDCs use different pathways to regulate IL-12 production.