Probing in vivo dendritic cell functions by conditional cell ablation

Neuroprotective Potential of Dendritic Cells and Sirtuins in Multiple Sclerosis

Myeloid cells, including parenchymal microglia, perivascular and meningeal macrophages, and dendritic cells (DCs), are present in the central nervous system (CNS) and establish an intricate relationship with other cells, playing a crucial role both in health and in neurological diseases. In this context, DCs are critical to orchestrating the immune response linking the innate and adaptive immune systems. Under steady-state conditions, DCs patrol the CNS, sampling their local environment and acting as sentinels. During neuroinflammation, the resulting activation of DCs is a critical step that drives the inflammatory response or the resolution of inflammation with the participation of different cell types of the immune system (macrophages, mast cells, T and B lymphocytes), resident cells of the CNS and soluble factors. Although the importance of DCs is clearly recognized, their exact function in CNS disease is still debated. In this review, we will discuss modern concepts of DC biology in steady-state and during autoimmune neuroinflammation. Here, we will also address some key aspects involving DCs in CNS patrolling, highlighting the neuroprotective nature of DCs and emphasizing their therapeutic potential for the treatment of neurological conditions. Recently, inhibition of the NAD⁺-dependent deac(et)ylase sirtuin 6 was demonstrated to delay the onset of experimental autoimmune encephalomyelitis, by dampening DC trafficking towards inflamed LNs. Thus, a special focus will be dedicated to sirtuins’ role in DCs functions.

Bone marrow dendritic cells support the survival of chronic lymphocytic leukemia cells in a CD84 dependent manner

Chronic lymphocytic leukemia (CLL) is a malignancy of mature B lymphocytes. The microenvironment of the CLL cells is a vital element in the regulation of the survival of these malignant cells. CLL cell longevity is dependent on external signals, originating from cells in their microenvironment including secreted and surface-bound factors. Dendritic cells (DCs) play an important part in tumor microenvironment, but their role in the CLL bone marrow (BM) niche has not been studied. We show here that CLL cells induce accumulation of bone marrow dendritic cells (BMDCs). Depletion of this population attenuates disease expansion. Our results show that the support of the microenvironment is partly dependent on CD84, a cell surface molecule belonging to the Signaling Lymphocyte Activating Molecule (SLAM) family of immunoreceptors. Our results suggest a novel therapeutic strategy whereby eliminating BMDCs or blocking the CD84 expressed on these cells may reduce the tumor load.

DTR-mediated conditional cell ablation-Progress and challenges

Cell ablation is a valuable complement to mutagenesis for experimentally defining specific cell functions in physiology and pathophysiology in small animal models. One of the most popular ablation strategies involves transgenic expression of a primate diphtheria toxin receptor (DTR) on murine cells that are otherwise resistant to the bacterial exotoxin. The efforts of many laboratories using the DTR approach over the years have yielded numerous valuable insights into specific cell functions. Here, we will discuss the technical aspects of the DTR approach, including the strengths, pitfalls, and future strategies to overcome the shortcomings, highlighting a recent paper published in the European Journal of Immunology [El Hachem et al. Eur. J. Immunol. 2018 https://doi.org/10.1002/eji.201747351]. A particular focus will be given to the application of DTR approach to decipher in vivo functions of the murine myeloid cell compartment.

Diversity of the CD4 T Cell Alloresponse: The Short and the Long of It

MHC alloantigen is recognized by two pathways: "directly," intact on donor cells, or "indirectly," as self-restricted allopeptide. The duration of each pathway, and its relative contribution to allograft vasculopathy, remain unclear. Using a murine model of chronic allograft rejection, we report that direct-pathway CD4 T cell alloresponses, as well as indirect-pathway responses against MHC class II alloantigen, are curtailed by rapid elimination of donor hematopoietic antigen-presenting cells. In contrast, persistent presentation of epitope resulted in continual division and less-profound contraction of the class I allopeptide-specific CD4 T cell population, with approximately 10,000-fold more cells persisting than following acute allograft rejection. This expanded population nevertheless displayed sub-optimal anamnestic responses and was unable to provide co-stimulation-independent help for generating alloantibody. Indirect-pathway CD4 T cell responses are heterogeneous. Appreciation that responses against particular alloantigens dominate at late time points will likely inform development of strategies aimed at improving transplant outcomes.

CD8 T-cell recognition of acquired alloantigen promotes acute allograft rejection

Adaptive CD8 T-cell immunity is the principal arm of the cellular alloimmune response, but its development requires help. This can be provided by CD4 T cells that recognize alloantigen "indirectly," as self-restricted allopeptide, but this process remains unexplained, because the target epitopes for CD4 and CD8 T-cell recognition are "unlinked" on different cells (recipient and donor antigen presenting cells (APCs), respectively). Here, we test the hypothesis that the presentation of intact and processed MHC class I alloantigen by recipient dendritic cells (DCs) (the "semidirect" pathway) allows linked help to be delivered by indirect-pathway CD4 T cells for generating destructive cytotoxic CD8 T-cell alloresponses. We show that CD8 T-cell-mediated rejection of murine heart allografts that lack hematopoietic APCs requires host secondary lymphoid tissue (SLT). SLT is necessary because within it, recipient dendritic cells can acquire MHC from graft parenchymal cells and simultaneously present it as intact protein to alloreactive CD8 T cells and as processed peptide alloantigen for recognition by indirect-pathway CD4 T cells. This enables delivery of essential help for generating cytotoxic CD8 T-cell responses that cause rapid allograft rejection. In demonstrating the functional relevance of the semidirect pathway to transplant rejection, our findings provide a solution to a long-standing conundrum as to why SLT is required for CD8 T-cell allorecognition of graft parenchymal cells and suggest a mechanism by which indirect-pathway CD4 T cells provide help for generating effector cytotoxic CD8 T-cell alloresponses at late time points after transplantation.

Host-derived CD8⁺ dendritic cells protect against acute graft-versus-host disease after experimental allogeneic bone marrow transplantation

Graft-versus-host disease (GVHD) is a frequent life-threatening complication after allogeneic hematopoietic stem cell transplantation (HSCT) and induced by donor-derived T cells that become activated by host antigen-presenting cells. To address the relevance of host dendritic cell (DC) populations in this disease, we used mouse strains deficient in CD11c(+) or CD8α(+) DC populations in a model of acute GVHD where bone marrow and T cells from BALB/c donors were transplanted into C57BL/6 hosts. Surprisingly, a strong increase in GVHD-related mortality was observed in the absence of CD11c(+) cells. Likewise, Batf3-deficient (Batf3(-/-)) mice that lack CD8α(+) DCs also displayed a strongly increased GVHD-related mortality. In the absence of CD8α(+) DCs, we detected an increased activation of the remaining DC populations after HSCT, leading to an enhanced priming of allogeneic T cells. Importantly, this was associated with reduced numbers of regulatory T cells and transforming growth factor-β levels, indicating an aggravated failure of peripheral tolerance mechanisms after HSCT in the absence of CD8α(+) DCs. In summary, our results indicate a critical role of CD8α(+) DCs as important inducers of regulatory T cell-mediated tolerance to control DC activation and T cell priming in the initiation phase of GVHD.

Ovarian dendritic cells act as a double-edged pro-ovulatory and anti-inflammatory sword

Ovulation and inflammation share common attributes, including immune cell invasion into the ovary. The present study aims at deciphering the role of dendritic cells (DCs) in ovulation and corpus luteum formation. Using a CD11c-EYFP transgenic mouse model, ovarian transplantation experiments, and fluorescence-activated cell sorting analyses, we demonstrate that CD11c-positive, F4/80-negative cells, representing DCs, are recruited to the ovary under gonadotropin regulation. By conditional ablation of these cells in CD11c-DTR transgenic mice, we revealed that they are essential for expansion of the cumulus-oocyte complex, release of the ovum from the ovarian follicle, formation of a functional corpus luteum, and enhanced lymphangiogenesis. These experiments were complemented by allogeneic DC transplantation after conditional ablation of CD11c-positive cells that rescued ovulation. The pro-ovulatory effects of these cells were mediated by up-regulation of ovulation-essential genes. Interestingly, we detected a remarkable anti-inflammatory capacity of ovarian DCs, which seemingly serves to restrict the ovulatory-associated inflammation. In addition to discovering the role of DCs in ovulation, this study implies the extended capabilities of these cells, beyond their classic immunologic role, which is relevant also to other biological systems.

Paired immunoglobulin-like receptor A is an intrinsic, self-limiting suppressor of IL-5-induced eosinophil development

Eosinophilia is a hallmark characteristic of T helper type 2 (TH2) cell-associated diseases and is critically regulated by the central eosinophil growth factor interleukin 5 (IL-5). Here we demonstrate that IL-5 activity in eosinophils was regulated by paired immunoglobulin-like receptors PIR-A and PIR-B. Upon self-recognition of β₂-microglobulin (β₂M) molecules, PIR-B served as a permissive checkpoint for IL-5-induced development of eosinophils by suppressing the proapoptotic activities of PIR-A, which were mediated by the Grb2-Erk-Bim pathway. PIR-B-deficient bone marrow eosinophils underwent compartmentalized apoptosis, resulting in decreased blood eosinophilia in naive mice and in mice challenged with IL-5. Subsequently, Pirb(-/-) mice displayed impaired aeroallergen-induced lung eosinophilia and induction of lung TH2 cell responses. Collectively, these data uncover an intrinsic, self-limiting pathway regulating IL-5-induced expansion of eosinophils, which has broad implications for eosinophil-associated diseases.

Dendritic cells and multiple sclerosis: disease, tolerance and therapy

Multiple sclerosis (MS) is a devastating neurological disease that predominantly affects young adults resulting in severe personal and economic impact. The majority of therapies for this disease were developed in, or are beneficial in experimental autoimmune encephalomyelitis (EAE), the animal model of MS. While known to target adaptive anti-CNS immune responses, they also target, the innate immune arm. This mini-review focuses on the role of dendritic cells (DCs), the professional antigen presenting cells of the innate immune system. The evidence for a role for DCs in the appropriate regulation of anti-CNS autoimmune responses and their role in MS disease susceptibility and possible therapeutic utility are discussed. Additionally, the current controversy regarding the evidence for the presence of functional DCs in the normal CNS is reviewed. Furthermore, the role of CNS DCs and potential routes of their intercourse between the CNS and cervical lymph nodes are considered. Finally, the future role that this nexus between the CNS and the cervical lymph nodes might play in site directed molecular and cellular therapy for MS is outlined.

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.

Profound depletion of host conventional dendritic cells, plasmacytoid dendritic cells, and B cells does not prevent graft-versus-host disease induction

The efficacy of allogeneic hematopoietic stem cell transplantation is limited by graft-versus-host disease (GVHD). Host hematopoietic APCs are important initiators of GVHD, making them logical targets for GVHD prevention. Conventional dendritic cells (DCs) are key APCs for T cell responses in other models of T cell immunity, and they are sufficient for GVHD induction. However, we report in this article that in two polyclonal GVHD models in which host hematopoietic APCs are essential, GVHD was not decreased when recipient conventional DCs were inducibly or constitutively deleted. Additional profound depletion of plasmacytoid DCs and B cells, with or without partial depletion of CD11b(+) cells, also did not ameliorate GVHD. These data indicate that, in contrast with pathogen models, there is a surprising redundancy as to which host cells can initiate GVHD. Alternatively, very low numbers of targeted APCs were sufficient. We hypothesize the difference in APC requirements in pathogen and GVHD models relates to the availability of target Ags. In antipathogen responses, specialized APCs are uniquely equipped to acquire and present exogenous Ags, whereas in GVHD, all host cells directly present alloantigens. These studies make it unlikely that reagent-based host APC depletion will prevent GVHD in the clinic.

Profound depletion of host conventional dendritic cells, plasmacytoid dendritic cells, and B cells does not prevent graft-versus-host disease induction

The efficacy of allogeneic hematopoietic stem cell transplantation is limited by graft-versus-host disease (GVHD). Host hematopoietic APCs are important initiators of GVHD, making them logical targets for GVHD prevention. Conventional dendritic cells (DCs) are key APCs for T cell responses in other models of T cell immunity, and they are sufficient for GVHD induction. However, we report in this article that in two polyclonal GVHD models in which host hematopoietic APCs are essential, GVHD was not decreased when recipient conventional DCs were inducibly or constitutively deleted. Additional profound depletion of plasmacytoid DCs and B cells, with or without partial depletion of CD11b(+) cells, also did not ameliorate GVHD. These data indicate that, in contrast with pathogen models, there is a surprising redundancy as to which host cells can initiate GVHD. Alternatively, very low numbers of targeted APCs were sufficient. We hypothesize the difference in APC requirements in pathogen and GVHD models relates to the availability of target Ags. In antipathogen responses, specialized APCs are uniquely equipped to acquire and present exogenous Ags, whereas in GVHD, all host cells directly present alloantigens. These studies make it unlikely that reagent-based host APC depletion will prevent GVHD in the clinic.

Transcriptional regulation of dendritic cell diversity

Dendritic cells (DCs) are specialized antigen presenting cells that are exquisitely adapted to sense pathogens and induce the development of adaptive immune responses. They form a complex network of phenotypically and functionally distinct subsets. Within this network, individual DC subsets display highly specific roles in local immunosurveillance, migration, and antigen presentation. This division of labor amongst DCs offers great potential to tune the immune response by harnessing subset-specific attributes of DCs in the clinical setting. Until recently, our understanding of DC subsets has been limited and paralleled by poor clinical translation and efficacy. We have now begun to unravel how different DC subsets develop within a complex multilayered system. These findings open up exciting possibilities for targeted manipulation of DC subsets. Furthermore, ground-breaking developments overcoming a major translational obstacle - identification of similar DC populations in mouse and man - now sets the stage for significant advances in the field. Here we explore the determinants that underpin cellular and transcriptional heterogeneity within the DC network, how these influence DC distribution and localization at steady-state, and the capacity of DCs to present antigens via direct or cross-presentation during pathogen infection.

ISCOMATRIX vaccines mediate CD8+ T-cell cross-priming by a MyD88-dependent signaling pathway

Generating a cytotoxic CD8⁺ T-cell response that can eradicate malignant cells is the primary objective of cancer vaccine strategies. In this study we have characterized the innate and adaptive immune response to the ISCOMATRIX adjuvant, and the ability of vaccine antigens formulated with this adjuvant to promote antitumor immunity. ISCOMATRIX adjuvant led to a rapid innate immune cell response at the injection site, followed by the activation of natural killer and dendritic cells (DC) in regional draining lymph nodes. Strikingly, major histocompatibility complex (MHC) class I cross-presentation by CD8α⁺ and CD8α⁻ DCs was enhanced by up to 100-fold when antigen was formulated with ISCOMATRIX adjuvant. These coordinated features enabled efficient CD8⁺ T-cell cross-priming, which exhibited prophylactic and therapeutic tumoricidal activity. The therapeutic efficacy of an ISCOMATRIX vaccine was further improved when co-administered with an anti-CD40 agonist antibody, suggesting that ISCOMATRIX-based vaccines may combine favorably with other immune modifiers in clinical development to treat cancer. Finally, we identified a requirement for the myeloid differentiation primary response gene 88 (MyD88) adapter protein for both innate and adaptive immune responses to ISCOMATRIX vaccines in vivo. Taken together, our findings support the utility of the ISCOMATRIX adjuvant for use in the development of novel vaccines, particularly those requiring strong CD8⁺ T-cell immune responses, such as therapeutic cancer vaccines.

Kidney dendritic cells induce innate immunity against bacterial pyelonephritis

Dendritic cells (DCs) are the most abundant immune cells in the kidney and form an intricate network in the tubulointerstitium, suggesting that they may play an important role in interstitial infections such as pyelonephritis. Here, we optimized a murine pyelonephritis model by instilling uropathogenic Escherichia coli two times at a 3-hour interval, which produced an infection rate of 84%. By 3 hours after the second instillation, resident kidney DCs began secreting the chemokine CXCL2, which recruits neutrophilic granulocytes. During the time studied, DCs remained responsible for most of the CXCL2 production. Neutrophils began infiltrating the kidney 3 hours after the second instillation and phagocytozed bacteria. Macrophages followed 3 hours later and contributed much less to both CXCL2 production and bacterial phagocytosis. To investigate whether DCs recruit neutrophils into the kidney for antibacterial defense, we used CD11c.DTR mice allowing conditional depletion of CD11c(+) dendritic cells. The absence of CD11c(+) DCs markedly delayed neutrophil recruitment and bacterial clearance. In conclusion, these findings suggest that the tubulointerstitial dendritic cell network serves an innate immune sentinel function against bacterial pyelonephritis.

Regulatory T-cell expansion during chronic viral infection is dependent on endogenous retroviral superantigens

Regulatory T cells (Treg) play critical roles in the modulation of immune responses to infectious agents. Further understanding of the factors that control Treg activation and expansion in response to pathogens is needed to manipulate Treg function in acute and chronic infections. Here we show that chronic, but not acute, infection of mice with lymphocytic choriomeningitis virus results in a marked expansion of Foxp3(+) Treg that is dependent on retroviral superantigen (sag) genes encoded in the mouse genome. Sag-dependent Treg expansion was MHC class II dependent, CD4 independent, and required dendritic cells. Thus, one unique mechanism by which certain infectious agents evade host immune responses may be mediated by endogenous Sag-dependent activation and expansion of Treg.

Management of gut inflammation through the manipulation of intestinal dendritic cells and macrophages?

Inflammatory bowel diseases (IBDs) including Crohn's disease and ulcerative colitis represent a major challenge to clinicians and immunologists trying to understand why in certain individuals the peaceful coexistence of the commensal microflora and its host breaks down and results in chronic inflammation. Here we summarize the recent progress in our understanding of the organization of the intestinal mononuclear phagocytes with dendritic cells and macrophages of distinct phenotype, origin and function. Finally, we discuss potential strategies to translate the recent findings into the management of chronic inflammation in animal models of IBD.

Dendritic cells erase bad memory

DC not only activate CD4 T (Th) cell and cytotoxic CD8 T cell (CTL) responses against pathogens, but they also tolerize autoreactive T cells in order to avoid autoimmunity. Previous studies have demonstrated that steady-state DC can tolerize naïve CTL, naïve Th cells and memory CTL. A study in this issue of the European Journal of Immunology demonstrates that DC also tolerize memory Th cells. This is arguably most critical for developing therapies against autoimmune disease; first, because Th cells are the central regulators of all adaptive immune responses, and second because memory, rather than naïve T cells are the clinically relevant cells in established autoimmune diseases. This study fosters hope that DC-based specific immunotherapies for common autoimmune diseases are possible.

Endogenous IL-10 attenuates cisplatin nephrotoxicity: role of dendritic cells

Sterile inflammation is associated with tissue injury and organ failure. Recent studies indicate that certain endogenous cytokines and immune cells may limit tissue injury by reducing immune-mediated inflammatory responses. Cisplatin is a commonly used anticancer chemotherapeutic agent but causes acute kidney injury and dysfunction. In a recent study, we showed that renal dendritic cells attenuate cisplatin-induced kidney injury by reducing inflammation. In this study, we investigated the effect of endogenous IL-10 and dendritic cell IL-10 in cisplatin-mediated kidney injury. Cisplatin treatment caused increases in renal IL-10R1 expression and STAT3 phosphorylation. In response to cisplatin treatment, IL-10 knockout mice showed more rapid and greater increases in blood urea nitrogen and serum creatinine compared with wild-type mice, indicating that endogenous IL-10 ameliorates kidney injury in cisplatin nephrotoxicity. Renal infiltration of IFN-γ-producing neutrophils was markedly increased in IL-10 knockout mice compared with wild-type mice. However, IFN-γ neutralization had no impact on renal dysfunction, suggesting IFN-γ-independent mechanisms of tissue injury in cisplatin nephrotoxicity. Renal dendritic cells showed high expression of IL-10 in response to cisplatin treatment. We further investigated the effect of dendritic cell-derived IL-10 in cisplatin nephrotoxicity using a conditional cell ablation approach. Mixed bone marrow chimeric mice lacking IL-10 in dendritic cells showed moderately greater renal dysfunction than chimeric mice positive for IL-10 in dendritic cells. These data demonstrate that endogenous IL-10 reduces cisplatin nephrotoxicity and associated inflammation. Moreover, IL-10 produced by dendritic cells themselves accounts for a portion of the protective effect of dendritic cells in cisplatin nephrotoxicity.

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.

Defining in vivo dendritic cell functions using CD11c-DTR transgenic mice

The study of dendritic cell involvement in complex phenomena that rely on multi-cellular interactions, such as immune homeostasis, stimulation, and tolerization, called for the investigation of dendritic cell functions within physiological context. To this end we have developed a conditional cell ablation strategy that is based on dendritic cell-restricted expression of a Diphtheria Toxin receptor (DTR) using the CD11c/Itgax promoter. Here, we provide basic protocols that describe the use of this prototypic dendritic cell ablation model and highlight pitfalls and strengths of the approach.

Renal dendritic cells ameliorate nephrotoxic acute kidney injury

Inflammation contributes to the pathogenesis of acute kidney injury. Dendritic cells (DCs) are immune sentinels with the ability to induce immunity or tolerance, but whether they mediate acute kidney injury is unknown. Here, we studied the distribution of DCs within the kidney and the role of DCs in cisplatin-induced acute kidney injury using a mouse model in which DCs express both green fluorescence protein and the diphtheria toxin receptor. DCs were present throughout the tubulointerstitium but not in glomeruli. We used diphtheria toxin to deplete DCs to study their functional significance in cisplatin nephrotoxicity. Mice depleted of DCs before or coincident with cisplatin treatment but not at later stages experienced more severe renal dysfunction, tubular injury, neutrophil infiltration and greater mortality than nondepleted mice. We used bone marrow chimeric mice to confirm that the depletion of CD11c-expressing hematopoietic cells was responsible for the enhanced renal injury. Finally, mixed bone marrow chimeras demonstrated that the worsening of cisplatin nephrotoxicity in DC-depleted mice was not a result of the dying or dead DCs themselves. After cisplatin treatment, expression of MHC class II decreased and expression of inducible co-stimulator ligand increased on renal DCs. These data demonstrate that resident DCs reduce cisplatin nephrotoxicity and its associated inflammation.

The lung vascular filter as a site of immune induction for T cell responses to large embolic antigen

The bloodstream is an important route of dissemination of invading pathogens. Most of the small bloodborne pathogens, like bacteria or viruses, are filtered by the spleen or liver sinusoids and presented to the immune system by dendritic cells (DCs) that probe these filters for the presence of foreign antigen (Ag). However, larger pathogens, like helminths or infectious emboli, that exceed 20 µm are mostly trapped in the vasculature of the lung. To determine if Ag trapped here can be presented to cells of the immune system, we used a model of venous embolism of large particulate Ag (in the form of ovalbumin [OVA]-coated Sepharose beads) in the lung vascular bed. We found that large Ags were presented and cross-presented to CD4 and CD8 T cells in the mediastinal lymph nodes (LNs) but not in the spleen or liver-draining LNs. Dividing T cells returned to the lungs, and a short-lived infiltrate consisting of T cells and DCs formed around trapped Ag. This infiltrate was increased when the Toll-like receptor 4 was stimulated and full DC maturation was induced by CD40 triggering. Under these conditions, OVA-specific cytotoxic T lymphocyte responses, as well as humoral immunity, were induced. The T cell response to embolic Ag was severely reduced in mice depleted of CD11c^hi cells or Ly6C/G⁺ cells but restored upon adoptive transfer of Ly6C^hi monocytes. We conclude that the lung vascular filter represents a largely unexplored site of immune induction that traps large bloodborne Ags for presentation by monocyte-derived DCs.

Induction of peripheral CD4+ T-cell tolerance and CD8+ T-cell cross-tolerance by dendritic cells

DC can present and cross-present self-antigens to autoreactive CD4(+) and CD8(+) T cells, respectively, and incapacitate them by inducing anergy, deletion or converting them into Treg. In this review, we summarize the recent progress in immune tolerance research, which has been achieved by employing antigen- and TCR-transgenic mice. We cover the numerous discoveries that have furthered our knowledge of the DC subsets and maturation pathways involved in tolerance; the signals, such as CD70, TGF-beta, B7-H1/PD-L1, which dictate the decision between immunity and tolerance; and the in vivo role of DC in the maintenance of CD4(+) T-cell tolerance and CD8(+) T-cell cross-tolerance.

Renal dendritic cells: an update

Discovery into the role of renal dendritic cells (rDCs) in health and disease of the kidney is rapidly accelerating. Progress in deciphering DC precursors and the heterogeneity of monocyte subsets in mice and humans is providing insight into the biology of rDCs. Recent findings have extended knowledge of the origins, anatomy and function of the rDC network at steady state and during periods of injury to the renal parenchyma. This brief review highlights these new findings and provides an update on the study of rDCs.

Lack of conventional dendritic cells is compatible with normal development and T cell homeostasis, but causes myeloid proliferative syndrome

Dendritic cells are critically involved in the promotion and regulation of T cell responses. Here, we report a mouse strain that lacks conventional CD11c(hi) dendritic cells (cDCs) because of constitutive cell-type specific expression of a suicide gene. As expected, cDC-less mice failed to mount effective T cell responses resulting in impaired viral clearance. In contrast, neither thymic negative selection nor T regulatory cell generation or T cell homeostasis were markedly affected. Unexpectedly, cDC-less mice developed a progressive myeloproliferative disorder characterized by prominent extramedullary hematopoiesis and increased serum amounts of the cytokine Flt3 ligand. Our data identify a critical role of cDCs in the control of steady-state hematopoiesis, revealing a feedback loop that links peripheral cDCs to myelogenesis through soluble growth factors, such as Flt3 ligand.