The anatomy of fetal peripheral lymphatic vessels in the head-and-neck region: an immunohistochemical study

Using D2-40 immunohistochemistry, we assessed the distribution of peripheral lymphatic vessels (LVs) in the head-and-neck region of four midterm fetuses without nuchal edema, two of 10 weeks and two of 15 weeks' gestation. We observed abundant LVs in the subcutaneous layer, especially in and along the facial muscles. In the occipital region, only a few LVs were identified perforating the back muscles. The parotid and thyroid glands were surrounded by LVs, but the sublingual and submandibular glands were not. The numbers of submucosal LVs increased from 10 to 15 weeks' gestation in all of the nasal, oral, pharyngeal, and laryngeal cavities, but not in the palate. The laryngeal submucosa had an extremely high density of LVs. In contrast, we found few LVs along bone and cartilage except for those of the mandible as well as along the pharyngotympanic tube, middle ear, tooth germ, and the cranial nerves and ganglia. Some of these results suggested that cerebrospinal fluid outflow to the head LVs commences after 15 weeks' gestation. The subcutaneous LVs of the head appear to grow from the neck side, whereas initial submucosal LVs likely develop in situ because no communication was evident with other sites during early developmental stages. In addition, CD68-positive macrophages did not accompany the developing LVs.     

The transduction pattern of IL‐12‐encoding lentiviral vectors shapes the immunological outcome

In situ modification of antigen‐presenting cells garnered interest in cancer immunotherapy. Therefore, we developed APC‐targeted lentiviral vectors (LVs). Unexpectedly, these LVs were inferior vaccines to broad tropism LVs. Since IL‐12 is a potent mediator of antitumor immunity, we evaluated whether this proinflammatory cytokine could enhance antitumor immunity of an APC‐targeted LV‐based vaccine. Therefore, we compared subcutaneous administration of broad tropism LVs (VSV‐G‐LV) with APC‐targeted LVs (DC2.1‐LV)‐encoding enhanced GFP and ovalbumin, or IL‐12 and ovalbumin in mice. We show that codelivery of IL‐12 by VSV‐G‐LVs or DC2.1‐LVs augments CD4⁺ or CD8⁺ T‐cell proliferation, respectively. Furthermore, we demonstrate that codelivery of IL‐12 enhances the CD4⁺ T_H1 profile irrespective of its delivery mode, while an increase in cytotoxic and therapeutic CD8⁺ T cells was only induced upon VSV‐G‐LV injection. While codelivery of IL‐12 by DC2.1‐LVs did not enhance CD8⁺ T‐cell performance, it increased expression of inhibitory checkpoint markers Lag3, Tim3, and PD‐1. Finally, the discrepancy between CD4⁺ T‐cell stimulation with and without functional CD8⁺ T‐cell stimulation by VSV‐G‐ and DC2.1‐LVs is partly explained by the observation that IL‐12 relieves CD8⁺ T cells from CD4⁺ T‐cell help, implying that a T_H1 profile is of minor importance for antitumor immunotherapy if IL‐12 is exogenously delivered.     

Dendritic cell migration in inflammation and immunity

Dendritic cells (DCs) are the key link between innate immunity and adaptive immunity and play crucial roles in both the promotion of immune defense and the maintenance of immune tolerance. The trafficking of distinct DC subsets across lymphoid and nonlymphoid tissues is essential for DC-dependent activation and regulation of inflammation and immunity. DC chemotaxis and migration are triggered by interactions between chemokines and their receptors and regulated by multiple intracellular mechanisms, such as protein modification, epigenetic reprogramming, metabolic remodeling, and cytoskeletal rearrangement, in a tissue-specific manner. Dysregulation of DC migration may lead to abnormal positioning or activation of DCs, resulting in an imbalance of immune responses and even immune pathologies, including autoimmune responses, infectious diseases, allergic diseases and tumors. New strategies targeting the migration of distinct DC subsets are being explored for the treatment of inflammatory and infectious diseases and the development of novel DC-based vaccines. In this review, we will discuss the migratory routes and immunological consequences of distinct DC subsets, the molecular basis and regulatory mechanisms of migratory signaling in DCs, and the association of DC migration with the pathogenesis of autoimmune and infectious diseases.     

Fetal anatomy of peripheral lymphatic vessels: a D2-40 immunohistochemical study using an 18-week human fetus (CRL 155 mm)

We demonstrated fetal peripheral lymphatic vessels (LVs) using D2-40 immunohistochemistry in a whole female fetus (18 weeks of gestation, CRL 155 mm) except for the head. There were abundant LVs in the thyroid gland, lung, stomach, small intestine, rectum and pancreas, whereas no LVs were seen in the parathyroid gland, spleen and adrenal cortex. In the liver, except for the gallbladder bed, LVs were still restricted to around hilar thick portal veins and around the hepatic vein terminals. Subcutaneous LVs were well developed throughout the body even in areas where no or few perforating LVs connected with the deep LVs. The diaphragm contained abundant, dilated LVs in the pleural half of its thickness. LVs were also seen not only along supplying arteries of muscles and cartilage but also along the epimysium and perichondrium. LVs ran in a space between the obliquus internus and transversus abdominis but not between the obliquus internus and obliquus externus. Some tight connective tissues such as the sacrotuberous ligament contained abundant LVs. The intervertebral foramen contained a lymphatic plexus. The present observations provide a better understanding of peripheral lymphatic development. The fetal lymphatic morphology seems not to represent a mini-version of the adult morphology.     

Migration of dendritic cells: physical principles,molecular mechanisms,and functional implications

Dendritic cells (DCs) constitute a complex cell population that resides in both peripheral tissues and lymphoid organs. Their major function in tissues is to patrol their environment in search of danger-associated antigens to transport to lymph nodes and present to T lymphocytes. This process constitutes the first step of the adaptive immune response and relies on specific DC properties, including a high endocytic capacity as well as efficient motility in confined three-dimensional environments. Although cell motility has been widely studied, little is known on how the geometric characteristics of the environment influence DC migration and function. In this review, we give an overview of the basic physical principles and molecular mechanisms that control DC migration under confinement and discuss how such mechanisms impact the environment-patrolling capacity of DCs.     

Complement C1q chemoattracts human dendritic cells and enhances migration of mature dendritic cells to CCL19 via activation of AKT and MAPK pathways

Dendritic cells (DC) and complement are both important effectors in innate immunity, and also potent linkers between innate immunity and adaptive immunity. As key components of innate immunity, various bioactive complement components produced at the inflammatory sites have been found to be able to regulate functions of DC. It is well known that migration of DC to the peripheral inflammatory sites benefits the recognition and uptake of invading pathogens by DC as antigen-presenting cells, and DC migration to secondary lymphatic tissues benefits the priming and activation of T cells. However, up to date, little is known about the underlying signaling mechanisms for the regulation of DC migration by the multifunctional molecule C1q, the first member of classical pathway. In this study, we show that C1q mediates the chemotaxis and transendothelial migration of immature MoDC. Additionally, C1q significantly enhances the chemotaxis of LPS-induced mature DC to CCL19 via upregulation of CCR7 expression. Activation of PI3K/AKT, ERK and JNK pathways is required for the chemotaxis of immature DC to C1q, meanwhile activation of AKT and P38 pathways is required for the C1q-mediated enhancement of mature DC chemotaxis to CCL19. Therefore, our results suggest that C1q, actively produced and accumulated at the inflammatory sites, can directly chemoattract immature DC from blood to peripheral inflammatory tissues, and promotes the migration of mature DC to secondary lymph organs via activation of AKT and MAPK pathways, thus outlining new way for favoring the link of innate immunity to adaptive immunity.     

Factors and signals that govern the migration of dendritic cells via lymphatics: recent advances

Dendritic cell (DC) migration from peripheral organs to lymph nodes plays a key role in initiating immune responses, whether migratory DCs bring antigen in tow to lymph nodes or position themselves to capture antigen that drains into the lymph node. CCR7 prominently controls DC migration into afferent lymphatic vessels and the positioning of DCs within the lymph node. Expression of CCR7 is not sufficient for function, as its function is positively regulated by a variety of other extracellular triggers. At least one of these triggers, synthesis and secretion of PGE₂, is brought on by the activation of p38 MAP kinase. The MAP kinase pathway has been well studied in DCs and exhibits a complex regulatory role in which the activation of different MAP kinase members leads to biologically distinct outcomes that are dependent upon stage of differentiation at the time of activation as well as the duration of signaling. Almost all of our knowledge of how DCs mature and ultimately mobilize to lymph nodes comes from studies in which DC migration is probed in the context of immune activation and priming. A reasonable body of evidence has gathered to suggest that many molecular events important for DC migration in this context do not affect accumulation of DCs in lymph nodes in the steady state, but mediators that interface with the signaling adaptor DAP-12 may play key roles in the steady state. It may thus become possible to devise approaches to modulate DC mobilization in the context of inflammation without affecting the traffic of DCs during more quiescent conditions. Considering the finely tuned regulation of DC maturation, migration, and cytokine production, with the realization that these phenotypes can be mutually exclusive, manipulation of DC migration in the clinic will be a challenging, albeit feasible, task.     

Interleukin-17A negatively regulates lymphangiogenesis in T helper 17 cell-mediated inflammation

During inflammation lymphatic vessels (LVs) are enlarged and their density is increased to facilitate the migration of activated immune cells and antigens. However, after antigen clearance, the expanded LVs shrink to maintain homeostasis. Here we show that interleukin (IL)-17A, secreted from T helper type 17 (T_H17) cells, is a negative regulator of lymphangiogenesis during the resolution phase of T_H17-mediated immune responses. Moreover, IL-17A suppresses the expression of major lymphatic markers in lymphatic endothelial cells and decreases in vitro LV formation. To investigate the role of IL-17A in vivo, we utilized a cholera toxin-mediated inflammation model and identified inflammation and resolution phases based on the numbers of recruited immune cells. IL-17A, markedly produced by T_H17 cells even after the peak of inflammation, was found to participate in the negative regulation of LV formation. Moreover, blockade of IL-17A resulted in not only increased density of LVs in tissues but also their enhanced function. Taken together, these findings improve the current understanding of the relationship between LVs and inflammatory cytokines in pathologic conditions.     

Dendritic Cells and Tumor Microenvironment: A Dangerous Liaison

The fact that the immune response to cancer is compromised has been convincingly demonstrated in murine tumor models as well as in cancer patients. The unresponsiveness of the host immune system is one of the major mechanisms of tumor escape as well as an important factor that limits the success of cancer immunotherapy. Inadequate function of professional antigen presenting cells dendritic cells (DC) in cancer is one of the major elements of compromised anti-tumor immune response. Despite substantial progress in recent years, the mechanism of inadequate DC function in cancer still remains unclear. The tumor microenvironment has emerged as an important component contributing to DC malfunction. In this review we will discuss the potential role of tumor microenvironment in DC dysfunction.     

Dendritic Cells and Tumor Microenvironment: A Dangerous Liaison

The fact that the immune response to cancer is compromised has been convincingly demonstrated in murine tumor models as well as in cancer patients. The unresponsiveness of the host immune system is one of the major mechanisms of tumor escape as well as an important factor that limits the success of cancer immunotherapy. Inadequate function of professional antigen presenting cells dendritic cells (DC) in cancer is one of the major elements of compromised anti-tumor immune response. Despite substantial progress in recent years, the mechanism of inadequate DC function in cancer still remains unclear. The tumor microenvironment has emerged as an important component contributing to DC malfunction. In this review we will discuss the potential role of tumor microenvironment in DC dysfunction.     

Dendritic cells and tumor microenvironment: a dangerous liaison

The fact that the immune response to cancer is compromised has been convincingly demonstrated in murine tumor models as well as in cancer patients. The unresponsiveness of the host immune system is one of the major mechanisms of tumor escape as well as an important factor that limits the success of cancer immunotherapy. Inadequate function of professional antigen presenting cells dendritic cells (DC) in cancer is one of the major elements of compromised anti-tumor immune response. Despite substantial progress in recent years, the mechanism of inadequate DC function in cancer still remains unclear. The tumor microenvironment has emerged as an important component contributing to DC malfunction. In this review we will discuss the potential role of tumor microenvironment in DC dysfunction.     

B cell-driven lymphangiogenesis in inflamed lymph nodes enhances dendritic cell mobilization

Dendritic cell (DC) migration from the periphery to lymph nodes is regulated by the pattern of genes expressed by DCs themselves and by signals within the surrounding peripheral environment. Here, we report that DC mobilization can also be regulated by signals initiated within the downstream lymph nodes, particularly when lymph nodes enlarge as a consequence of immunization. Lymph node B lymphocytes orchestrate expansion of the lymphatic network within the immunized lymph node. This expanded network in turn supports increased DC migration from the periphery. These results reveal unique relationships between B cells, lymphatic vessels, and migratory DCs. Knowledge that DC migration from the periphery is augmented by B cell-dependent signals reveals new potential strategies to increase DC migration during vaccination.     

WASP deficiency leads to global defects of directed leukocyte migration in vitro and in vivo

Intact cellular migration is critically important for the induction and regulation of the immune response. The Wiskott-Aldrich syndrome protein (WASP) regulates surface receptor signaling to the actin cytoskeleton in hematopoietic cells and thus plays a pivotal role in cellular locomotion. WASP deficiency causes the Wiskott-Aldrich syndrome (WAS), characterized by immunodeficiency, thrombocytopenia, and eczema. Cell migration defects may contribute to the pathophysiology of WAS. In this study, we used a variety of in vitro and in vivo assays to comprehensively analyze migration properties of lymphocytes, dendritic cells (DC), and neutrophils from WASP-deficient mice. We provide evidence that WASP-deficient lymphocytes show a marked reduction in tethering in an in vitro flow chamber assay as well as decreased migration of T cells in response to the CC chemokine ligand 19 (CCL19). In vivo, compared with wild-type lymphocytes, WASP-deficient lymphocytes showed significantly impaired homing to Peyer's patches upon adoptive transfer into recipient mice. In addition, bone marrow-derived DC migrated less efficiently in response to CCL19. In vivo studies showed decreased migration of DC from skin to draining lymph nodes in WASP-deficient animals. Finally, we also document decreased neutrophil migration in vitro and in vivo. In summary, our studies suggest that WASP plays an important role in the locomotion of lymphocytes, DC, and granulocytes in vitro and in vivo and thus, reveal a crucial role of WASP in physiological trafficking of various hematopoietic cell lineages. These results further delineate immunological abnormalities in WASP-deficient mice, which will be useful to assess preclinical gene therapy studies.     

DC ablation in mice: promises, pitfalls, and challenges

Dendritic cells (DC) play pivotal roles in orchestrating immunity and tolerance, and, as such, they are key targets for immunotherapy. Exploiting their function depends on a precise understanding of the part that different DC subsets play in vivo, but attempts to identify definitive functions have been limited by problems depleting individual DC populations in mice. Inducible cell ablation via transgenic expression of a high-affinity diphtheria toxin receptor (DTR) is a new and powerful approach to DC research. Here, we discuss the impact of CD11c-DTR and Langerin-DTR mice on DC immunobiology, and we highlight the problems to be aware of when interpreting data from these models. The challenge now will be to refine transgenic strategies so that other DC subsets can be inducibly depleted in vivo.     

MicroRNAs affect dendritic cell function and phenotype

MicroRNA (miRNA) are small, non-coding RNA molecules that have been linked with immunity through regulating/modulating gene expression. A role for these molecules in T-cell and B-cell development and function has been well established. An increasing body of literature now highlights the importance of specific miRNA in dendritic cell (DC) development as well as their maturation process, antigen presentation capacity and cytokine release. Given the unique role of DC within the immune system, linking the innate and adaptive immune responses, understanding how specific miRNA affect DC function is of importance for understanding disease. In this review we summarize recent developments in miRNA and DC research, highlighting the requirement of miRNA in DC lineage commitment from bone marrow progenitors and for the development of subsets such as plasmacytoid DC and conventional DC. In addition, we discuss how infections and tumours modulate miRNA expression and consequently DC function.     

Lymph node homing of T cells and dendritic cells via afferent lymphatics

The continuous migration of immune cells is of utmost importance for the induction of both protective immunity as well as immunological tolerance. However, relatively little is known about the molecular cues that regulate the entry of immune cells from peripheral, nonlymphoid tissues into afferent lymph vessels and, in particular, their subsequent transmigration from afferent lymphatics into the parenchyma of draining lymph nodes (LNs). Here, we review the requirements for T cells and dendritic cells (DCs) to enter initial afferent lymph vessels of the skin. We discuss how these cells subsequently gain access to the paracortex of draining lymph nodes; a location that allows for efficient interaction between both cell populations, providing the right environment for the induction of immunity as well as tolerance.     

Low molecular weight dextran sulfate as complement inhibitor and cytoprotectant in solid organ and islet transplantation

Complement is an essential part of the innate immune system and plays a crucial role in organ and islet transplantation. Its activation, triggered for example by ischemia/reperfusion (I/R), significantly influences graft survival, and blocking of complement by inhibitors has been shown to attenuate I/R injury. Another player of innate immunity are the dendritic cells (DC), which form an important link between innate and adaptive immunity. DC are relevant in the induction of an immune response as well as in the maintenance of tolerance. Modulation or inhibition of both components, complement and DC, may be crucial to improve the clinical outcome of solid organ as well as islet transplantation. Low molecular weight dextran sulfate (DXS), a well-known complement inhibitor, has been shown to prevent complement-mediated damage of the donor graft endothelium and is thus acting as an endothelial protectant. In this review we will discuss the evidence for this cytoprotective effect of DXS and also highlight recent data which show that DXS inhibits the maturation of human DC. Taken together the available data suggest that DXS may be a useful reagent to prevent the activation of innate immunity, both in solid organ and islet transplantation.     

CD137 ligand reverse signaling has multiple functions in human dendritic cells during an adaptive immune response

T cell activation via dendritic cells (DC) is an important step in the adaptive immune response, which requires DC maturation, migration to lymph nodes and presentation of antigen to T cells. CD137 receptor expressed on activated T cells is a potent costimulatory molecule. Here, we investigated the functions of CD137 ligand (CD137L) in human monocyte-derived DC during an immune response. Cross-linking of CD137L on DC leads to cell maturation in an autocrine fashion, mostly via release of TNF-alpha. Reverse signaling of CD137L also mediates migration of DC via up-regulation of the CCR7 chemokine receptor, demonstrated by an in vivo MIP-3beta-dependent SCID mouse migration model. Finally, CD137L-activated DC induce differentiation of human T cells into potent Th1 effectors. Cocultivation of autologous T cells and CD137L-activated DC in an antigen-specific reaction leads to T cell proliferation and the release of IL-12p70 and IFN-gamma. These findings deliver new insights into the multiple effects of reverse signaling of CD137L in human DC during the initiation of an adaptive immune response, including the key features of DC maturation, migration and, ultimately, antigen-specific T cell differentiation.