Neuropeptides and the immune system: focus on dendritic cells

Neuropeptides are small, biologically active peptides derived from the central and peripheral nervous system. It is increasingly clear that besides their function as neurotransmitters, these peptides have an influence on almost all body functions including the immune system. Since dendritic cells are the most efficient antigen-presenting cells that stimulate naive T cells, thus promoting adaptive immunity, it is not surprising that interactions between neuropeptides and dendritic cells take place. The current review addresses several aspects of dendritic cell-related neuroimmunology and focuses on the role of neuropeptides as immunomodulators. Moreover, we present a novel concept of neuropeptide-mediated regulation of dendritic cell migration. The importance of chemokines in immunity is generally accepted. It may be that not enough attention has been paid to the possible role of nervous system-derived peptides in regulating immune reactions.     

Metastasis to sentinel lymph nodes in breast cancer is associated with maturation arrest of dendritic cells and poor co-localization of dendritic cells and CD8+ T cells

The regional immune systems of patients with breast cancer are immunosuppressed. Dendritic cells are professional antigen-presenting cells and present cancer-associated antigens to the adaptive immune system in sentinel lymph nodes. Dendritic cells may promote, or inhibit, an adaptive immune response to specific antigens. Our aim was to assess whether dendritic cells were associated with nodal metastasis in patients with breast cancer. Sentinel lymph nodes of 47 patients with breast cancer with varying degrees of nodal disease and ten controls were evaluated using immunohistochemistry for the accumulation of dendritic cells in general (CD1a⁺), mature dendritic cells (CD208⁺), and plasmacytoid dendritic cells (CD123⁺). Cytotoxic T cell and regulatory T cell accumulation were also evaluated. Sentinel lymph nodes with macrometastases demonstrated fewer mature dendritic cells than sentinel lymph nodes without metastasis (p = 0.028), but not controls. There were fewer mature dendritic cells to cytotoxic T cells in sentinel lymph nodes with metastasis than those without (p = 0.033). Also, there were more regulatory T cells to mature dendritic cells in sentinel lymph nodes with metastasis than those without (p = 0.02). In conclusion, our study suggests that sentinel lymph nodes with metastasis have arrest of maturation of dendritic cells, fewer mature dendritic cell interactions with cytotoxic T cells, and more regulatory T cells than sentinel lymph nodes without metastasis in patients with breast cancer. These findings extend our understanding of regional immunosuppression and suggest that most regional immunosuppressive changes are associated with nodal metastasis in breast cancer.     

Lung dendritic cells and the inflammatory response.

OBJECTIVE: To discuss the role of conventional and plasmacytoid dendritic cells in inducing and modulating immune responses in the lung. DATA SOURCES: The primary literature and selected review articles studying the role of dendritic cells in both rodent and human lungs as identified via a PubMed/MEDLINE search using the keywords dendritic cell, antigen-presenting cell, viral airway disease, asthma, allergy, and atopy. STUDY SELECTION: The author's knowledge of the field was used to identify studies that were relevant to the stated objective. RESULTS: Dendritic cells are well positioned in the respiratory tract and other mucosal surfaces to respond to any foreign protein. These cells are crucial to the initiation of the adaptive immune response through induction of antigen specific T-cell responses. These cells also play an important role in the regulation of developing and ongoing immune responses, an area that is currently under intense investigation. This review discusses the various subsets of human and rodent dendritic cells and the pathways involved in antigen processing and subsequent immune regulation by dendritic cells in the lung using both viral and nonviral allergenic protein exposure as examples. CONCLUSIONS: Conventional and plasmacytoid dendritic cells are uniquely situated in the immune cascade to not only initiate but also modulate immune responses. Therapeutic interventions in allergic and asthmatic diseases will likely be developed to take advantage of this exclusive position of the dendritic cell.     

Liaison between natural killer cells and dendritic cells in human gestation

A successful pregnancy relies on immunological adaptations that allow the fetus to grow and develop in the uterus, despite being recognized by maternal immune cells. Among several immunocompetent cell types present within the human maternal/fetal interface, DC-SIGN~ dendritic cells （DCs） and CD56＋ natural killer （NK） cells are of major importance for early pregnancy maintenance, not only generating maternal immunological tolerance but also regulating stromal cell differentiation. Previous reports show the presence of NK-DC cell conjugates in first trimester human decidua, suggesting that these cells may play a role in the modulation of the local immune response within the uterus. While effective immunity is necessary to protect the mother from harmful pathogens, some form of tolerance must be activated to avoid an immune response against fetal antigens. This review article discusses current evidence concerning the functions of DC and NK cells in pregnancy and their liaison in human decidua.     

B cells are resistant to immunomodulation by 'IVIg-educated' dendritic cells

Intravenous immunoglobulin (IVIg) can exert beneficial effects in autoimmune and inflammatory diseases via several mutually non-exclusive mechanisms. While, IVIg can directly modulate the functions of both innate and adaptive immune cells such as dendritic cells (DC), macrophages, B and T cells, several reports have also highlighted that the regulation of immune responses by IVIg can be indirect. In view of these results, we aimed at exploring whether indirect regulation of immune cells by 'IVIg-educated' innate cells is a universal phenomenon. We addressed this question by deciphering the modulation of B cell functions by 'IVIg-educated' DC. Our results indicate that human B cells are resistant to immunomodulation by 'IVIg-educated' DC. However, IVIg at therapeutic concentrations can directly inhibit B cell activation and proliferation. These results thus suggest that, indirect modulation of immune cells by IVIg is not a universal phenomenon.     

Emerging role of dendritic cells in respiratory viral infection

Respiratory viral infections are a major health problem, especially in the immunocompromised, young, and elderly. In order for the host to effectively clear viral infections, a productive adaptive immune response must be developed. Crucial to the initiation of the adaptive response is the dendritic cell, which induces the proliferation and activation of T cells early in an antiviral response. This review examines the role of lung dendritic cells in the immune response to respiratory viruses. The phenotypic and functional differences between conventional and plasmacytoid dendritic cells are discussed, as are the mechanisms behind homeostatic recruitment of these cells in the normal lung. Focusing on respiratory syncytial virus and influenza, the role of the two dendritic cell subsets during an antiviral response is explored. Through evolution, viruses have developed several mechanisms to interfere with the normal function of dendritic cells and prevent appropriate induction of an adaptive immune response, which are also discussed. Finally, we identify potential targets for future therapeutic strategies to ameliorate disease caused by respiratory virus infection.     

Plasmacytoid dendritic cells

Plasmacytoid dendritic cells (pDCs) have been associated with several names and functions over time, reflecting the limited availability of specific markers and their ability to produce large amounts of type I interferons, present antigens, as well as prime disparate T-cell helper responses. Yet, there is increasing evidence that pDCs are a distinct cell type in the innate immune system. This review high-lights aspects in which pDCs are unique in comparison to other antigen-presenting cells in regulating innate and adaptive immune responses.     

Against the self: dendritic cells versus cancer

The role of host defense in cancer is highly variable. Although there are cases where spontaneous cures of cancer appear to be mediated by immunologic mechanisms, malignant disease generally progresses even in patients where tumor-specific immunity can be demonstrated. It is apparent that there are complex interactions between tumor cells and dendritic cells, the dominant antigen-presenting cells of the immune system. Through their inhibitory actions upon dendritic cells, tumor cells can negatively regulate priming of tumor-specific immunity. Recent work has also shown that dendritic cells have direct cytotoxic effects upon tumor cells. These interactions may impact on the efficacy of current strategies using dendritic cell-based vaccines for tumor immunotherapy.     

Origin and migratory properties of dendritic cells in the skin

The professional antigen presenting cells of the skin, namely Langerhans cells and dermal dendritic cells, are continuously exposed to environmental stimuli. The population of antigen-carrying dendritic cells that migrates from this site to the draining lymph nodes is highly dynamic, and its density, composition and function changes as a consequence of dendritic cell recruitment and activation in tissues, as well as interactions with T lymphocytes in the specialized lymph node microenvironments. Therefore dendritic cells within the skin are the key regulators of both quantitative and qualitative aspects of immune responses to local antigenic challenge.     

Adsorbed fibrinogen regulates the behavior of human dendritic cells in a CD18-dependent manner

The involvement of fibrinogen in inflammation has been considered by many, but the roles of the protein in that process have yet to be fully elucidated. The protein readily coats surfaces and is deposited at sites of inflammation. Furthermore, adsorbed fibrinogen influences many cells of the immune system, likely a result of increased receptor recognition upon ligand immobilization. To better understand adsorbed fibrinogen's role in inflammation, we studied the effects of the protein, adsorbed to the surface of microscopic beads, on human dendritic cells. Adsorbed fibrinogen increased dendritic cell expression of IL-6, IL-8, MIP-1beta and MCP-1. In contrast, solution phase fibrinogen had no effect. Importantly, dendritic cells formed complexes with, and subsequently accumulated around, beads in fibrinogen-dependent fashion. Antibodies directed against CD18 significantly decreased cytokine/chemokine expression and bead-cell complexation. Epsilon-aminocaproic acid limited bead-cell complexation, suggesting fibrinogen degradation products modulate dendritic cell activity. In support of this proposal, fibrinogen fragment D also increased MCP-1 expression by human dendritic cells. Taken together our data indicate adsorbed fibrinogen and its degradation products directly influence human dendritic cell operation. We propose a model whereby adsorbed fibrinogen plays a distinct causatory role in inflammation through its beta(2) integrin-mediated interaction with dendritic cells.     

Murine dendritic cells pulsed in vitro with tumor antigen induce tumor resistance in vivo

The aim of this work is to induce tumor resistance to a B cell lymphoma in BALB/c mice using elements of the immune system. It has indeed been shown by us and by others that antigen-presenting cells (APC) like dendritic cells can induce efficient immune responses and can even substitute for Freund's adjuvant. Here we show that mice immunized with syngeneic dendritic cells pulsed in vitro with tumor antigen (BCL1 idiotype expressed by lymphoma cells) are protected against a subsequent tumor inoculation. The in vivo resistance can be correlated with the induction of a humoral response specific for the idiotype expressed by the tumor. No such protection can be achieved when B cells are used as APC. These data show that effector cells in tumor-bearing animals can be recruited and activated using dendritic cells, providing long-lasting immune surveillance.     

Myeloid dendritic cells.

There is considerable but as yet incomplete evidence for two developmental lineages of dendritic cells: a myeloid lineage shared with phagocytes and a lymphoid lineage shared with T cells. The two corresponding functional states, which may not require the existence of two formal lineages, are that myeloid dendritic cells capture antigens in the periphery and then migrate to the lymphoid organs to initiate immunity, whereas lymphoid dendritic cells are found in the thymic medulla and lymph node T cell areas and are responsible for tolerance. The latter may occur through immune regulation and/or deletion. Myeloid dendritic cells undergo many activities that contribute to the initiation of immunity. These are summarized here and include mobilization from progenitors and precursors in the blood and marrow, maturation from immature cells in the skin and interstitial spaces, formation of MHC-peptide complexes in MHC class II compartments or MIICs, migration to the T cell areas, and finally, mortality. The death of migratory dendritic cells seems to be accompanied by their phagocytosis and processing by other dendritic cells in the T area. We speculate that this transfer of antigens, including self-peptides captured by the uptake of apoptotic cells in peripheral tissues, is an important prelude to the regulatory function of resident or lymphoid dendritic cells in the T cell area. These features of dendritic cell biology provide targets to manipulate the immune response in vivo.     

Presentation of mycobacterial antigens by human dendritic cells: lack of transfer from infected macrophages.

When exposed to a challenge of 10 Mycobacterium bovis BCG cells per antigen-presenting cell, most human monocytes engulf several organisms. In contrast, blood dendritic cells which are potent antigen-presenting cells for several antigens are not detectably phagocytic for mycobacteria. We investigated the possibility that infected macrophages might regurgitate antigens for presentation by populations of human blood dendritic cells. Macrophages were infected with M. bovis BCG, mixed with uninfected dendritic cells, and added to immune T cells, either bulk T cells or cloned populations from BCG vaccinees or patients recovering from tuberculosis. The macrophages were from donors who were mismatched to the T cells so that transfer of antigen to major histocompatibility complex-matched dendritic cells could be evaluated. As we describe, there was no evidence for the transfer of mycobacterial antigens from macrophages to dendritic cells in a form that was stimulatory for the T cells.     

Harnessing human dendritic cell subsets for medicine

Summary: Immunity results from a complex interplay between the antigen-non-specific innate immune system and the antigen-specific adaptive immune system. The cells and molecules of the innate system employ non-clonal recognition receptors including lectins, Toll-like receptors, NOD-like receptors, and helicases. B and T lymphocytes of the adaptive immune system employ clonal receptors recognizing antigens or their derived peptides in a highly specific manner. An essential link between innate and adaptive immunity is provided by dendritic cells (DCs). DCs can induce such contrasting states as immunity and tolerance. The recent years have brought a wealth of information on the biology of DCs revealing the complexity of this cell system. Indeed, DC plasticity and subsets are prominent determinants of the type and quality of elicited immune responses. In this article, we summarize our recent studies aimed at a better understanding of the DC system to unravel the pathophysiology of human diseases and design novel human vaccines.     

Development and maturation of Langerhans cells, spleen and bone marrow dendritic cells in TNF-alpha/lymphotoxin-alpha double-deficient mice

Dendritic cells are key regulators of immunity and tolerance. TNF-alpha has manifold effects on dendritic cells. It is an indispensable ingredient in several dendritic cell generation protocols, especially in the human, and it is included in diverse maturation stimuli for dendritic cells. Mice deficient in various components of the TNF/lymphotoxin system (TNF-alpha, lymphotoxin-alpha and -beta, TNF receptors, combinations thereof) have profound defects in mounting immune responses to infections. The dendritic cell system in these mice has been incompletely studied to date. We therefore investigated dendritic cells from the epidermis (Langerhans cells), spleen and the bone marrow of mice double-deficient in TNF-alpha and lymphotoxin-alpha. We report that dendritic cells in these mice are grossly normal. Langerhans cells, spleen and bone marrow dendritic cells can develop and mature. Their expression of MHC II and CD86 is not impaired, and their T cell-stimulatory as well as antigen-processing capacity is comparable to their normal counterparts. Thus, the described defects in these mice appear to be due the lack of lymph nodes, the disturbed architecture of the spleen, and deranged chemokine production patterns, rather than to a profoundly altered dendritic cell system.     

Murine epidermal Langerhans cells and splenic dendritic cells present tumor-associated antigens to primed T cells

We examined the ability of epidermal Langerhans cells and splenic dendritic cells to present tumor-associated antigens (Ag) to immune T cells. Methylcholanthrene (MCA)-induced subcutaneous fibrosarcomas derived from C57BL/6 mice were used as tumor models. Our data demonstrate that both murine Langerhans cells and splenic dendritic cells have the capacity to present tumor-associated Ag to primed T cells. We found that variously treated tumor preparations (irradiated viable tumor cells, irradiated frozen-stored tumor cells, mitomycin C-treated viable tumor cells, and snap freeze-thawed tumor cell lysates) can be utilized for tumor Ag-pulsing. Primed CD4⁺ T cells demonstrated in vitro specificity towards their respective tumors and did not cross-react to other syngeneic MCA-induced or non-MCA-induced tumors. The T cell proliferative response critically depended on the presence of immune CD4⁺ T cells. We discuss the implications of these findings for the adoptive immunotherapy of cancer using immune CD4⁺ T cells.     

Analysis of HLDA9 mAbs on plasmacytoid dendritic cells

Dendritic cells are a heterogeneous population of bone marrow derived leucocytes that play a crucial role in both pathogen recognition and the initiation of primary T cell immune responses. Plasmacytoid dendritic cells (pDCs), also known as natural interferon-producing cells, comprise one of two major human dendritic cell subsets that strongly influences immune balance. pDCs remain a poorly characterized subset. Several studies have suggested the existence of a close phenotypic and functional relationship between B-cells and pDCs. The surface reactivity of a panel of 96 monoclonal antibodies submitted to the ninth Human Leukocyte Differentiation Antigens workshop (HLDA9) B cell section was analyzed using pDCs as target cells. The results showed that eight of the mAbs reacted positively on pDCs: CD86, CD229, CD319, CD305, CD184, CD84, CD85g and Fc?RIa, confirming previously published reports. Interestingly, this study also revealed the expression of eight surface molecules not previously described on pDCs, including CD352(NTBA), CD272(BTLA), CD357(GITR), CD48, CD270(HVEM), Galectin-3, CD148, and CD361. The present report summarizes the expression of these molecules on freshly isolated pDCs. Significantly, we have identified several new molecules expressed by these intriguing cells, ones we believe will open new avenues for the study of pDC functionality and their role in human health and diseases.     

An improved method for purifying human thymic dendritic cells

Thymic dendritic cells (DC) play a prominent role in the immune response as they constitute a key element involved in the maturation of thymocytes in the thymus. Human thymic DC, like DC from other lymphoid organs, represent a minor cell population (< 2%) of the thymus. Since these cells cannot replicate in vitro, the development of efficient purification methods is an essential prerequisite for extensive functional studies. DC express high levels of HLA-DR, a cell surface marker of the MHC class II antigen which is not exclusive to DC. Since no specific human thymic DC marker has been identified so far, DC purification methods are mainly based on depletion of particular subgroups of cells. We report here an improved method for purifying human thymic dendritic cells. In contrast to prior work, CD2⁺ thymocytes were first depleted by rosetting with neuraminidase treated sheep red blood cells. The nonrosetted cells were separated in a Percoll gradient, and the low-density cells were subsequently depleted of nondendritic cells by using thymocyte and macrophage specific monoclonal antibodies and either magnetic bead depletion or cytofluorometry. Cell populations (18–55 × 10⁶ cells) obtained following magnetic bead purification were at least 80% HLA-DR⁺/CD2⁻ and exhibited ultrastructural morphological features and functional activities such as those described previously for thymic DC. This improved method was compared with different purification approaches that use various combinations of cell density-based separation techniques and cell surface specific markers antibody reactivity. The magnetic beads depletion approach provided higher yields.     

Imaging of T-cell interactions with antigen presenting cells in culture and in intact lymphoid tissue

Summary: The development of an effective immune response requires cell–cell contact between T cells and antigen‐bearing cells of several types (dendritic cells, B cells, infected tissue cells). Recent advances in light microscopy have led to intense investigation of the molecular events that accompany these cell interactions, especially the redistribution of membrane proteins into discrete organized subdomains within the zone of cell–cell contact termed the ‘immunological synapse’. Here we discuss two aspects of our own studies in this area. First, we highlight results from our in vitro analysis of the role of the cytoskeletal ezrin, radixin, moesin adapter proteins in the exclusion of CD43 from the well‐defined T cell receptor (TCR) and integrin‐rich zones of the synapse. Based on the molecular mechanism uncovered in this work, we propose a new model for how TCR‐signaled changes in cytoskeletal organization indirectly influence both protein distributions and the efficiency of signaling between T cell and presenting cell. We then discuss the development of a new method for dynamic visualization of T cell – dendritic cell interactions in intact lymphoid tissue. The remarkable longevity of monogamous lymphocyte‐presenting cell interactions is discussed, differences between our observations and those of others are laid out in detail, and prospects for future application of this technical approach to analysis of early immune responses in lymphoid organs and of effector lymphocyte function in tissues are presented.