Human plasmacytoid dendritic cells (pDCs), also called type 2 dendritic cell precursors or natural interferon (IFN)-producing cells, represent a cell type with distinctive phenotypic and functional features. They are present in the thymus and probably share a common precursor with T and natural killer (NK) cells. In an effort to identify genes that control pDC development we searched for genes of which the expression is restricted to human pDC using a cDNA subtraction technique with activated monocyte-derived DCs (Mo-DCs) as competitor. We identified the transcription factor Spi-B to be expressed in pDCs but not in Mo-DCs. Spi-B expression in pDCs was maintained on in vitro maturation of pDCs. Spi-B was expressed in early CD34(+)CD38(-) hematopoietic progenitors and in CD34(+)CD1a(-) thymic precursors. Spi-B expression is down-regulated when uncommitted CD34(+)CD1a(-) thymic precursors differentiate into committed CD34(+)CD1a(+) pre-T cells. Overexpression of Spi-B in hematopoietic progenitor cells resulted in inhibition of development of T cells both in vitro and in vivo. In addition, development of progenitor cells into B and NK cells in vitro was also inhibited by Spi-B overexpression. Our results indicate that Spi-B is involved in the control of pDC development by limiting the capacity of progenitor cells to develop into other lymphoid lineages.

Human plasmacytoid dendritic cells represent a rare population of leukocytes which produce high amounts of type I interferon in response to certain viruses. Although those cells were first described in 1958, there are still unsolved issues related to their origin and function. Recently, a leukemic counterpart of plasmacytoid dendritic cells was identified. Molecular approaches using either normal or leukemic plasmacytoid dendritic cells provide some new insights into the controversial lymphoid origin of those cells. The need for specific markers is still a critical aspect for the identification of plasmacytoid dendritic cells, whatever stage of differentiation, in normal as well as in pathological conditions. Hopefully, novel markers will allow delineation of the relationships between dendritic cells at different stages of differentiation/maturation along the myeloid and lymphoid lineages.

Recent studies in humans have highlighted the importance of a distinct cellular entity, the plasmacytoid dendritic cell (PDC). To identify genes for which expression is restricted to human PDCs, a cDNA subtraction technique was applied using cDNA from activated monocyte-derived DCs (MDDCs) as competitor. In the 650 sequences analyzed, 25% were for B-cell transcripts. We also found lymphoid-related genes, immunoglobulinlike transcript 7 (ILT7), granzyme B (GrB), Spi-B, and the receptor tyrosine kinase Eph-B1. Granzyme B was up-regulated on activation, and protein was detected only in PDCs. Eph-B1 protein was expressed in the cytoplasm and the nuclei of PDCs and MDDCs, respectively. Interestingly, several novel molecules have been identified that were predicted to encode for a type 2 transmembrane protein (BRI(3)), a putative cytokine (C-15, a cysteine-rich-secreted protein), and a type 1 leucine-rich repeat protein (MAPA). The identification of genes expressed in PDCs provides new insights into their function and origin.

Natural killer T (NKT) cells are a highly conserved subset of T cells that have been shown to play a critical role in suppressing T helper cell type 1–mediated autoimmune diseases and graft versus host disease in an interleukin (IL)-4–dependent manner. Thus, it is important to understand how the development of IL-4– versus interferon (IFN)-γ–producing NKT cells is regulated. Here, we show that NKT cells from adult blood and those from cord blood undergo massive expansion in cell numbers (500–70,000-fold) during a 4-wk culture with IL-2, IL-7, phytohemagglutinin, anti-CD3, and anti-CD28 mAbs. Unlike adult NKT cells that preferentially produce both IL-4 and IFN-γ, neonatal NKT cells preferentially produce IL-4 after polyclonal activation. Addition of type 2 dendritic cells (DC2) enhances the development of neonatal NKT cells into IL-4+IFN-γ− NKT2 cells, whereas addition of type 1 dendritic cells (DC1) induces polarization towards IL-4−IFN-γ+ NKT1 cells. Adult NKT cells display limited plasticity for polarization induced by DC1 or DC2. Thus, newly generated NKT cells may possess the potent ability to develop into IL-4+IFN-γ− NKT2 cells in response to appropriate stimuli and may thereafter acquire the tendency to produce both IL-4 and IFN-γ.

Dendritic cells (DCs) are professional antigen-presenting cells, capable of activating naïve T helper (TH) cells [1-3]. Differentiation of activated TH cells into IFN-γ producing effector TH1 cells or IL-4, -5 and -10 producing effector TH2 cells depends respectively on cytokines, such as IL-12 or IL-4, possibly produced by a third cell type [3-7] IL-12 produced by activated macrophages was believed to be critical for TH1 differentiation [5-7]. Because IL-4 is a TH2 prototype cytokine, the original source of IL-4 required for TH2 differentiation has been controversial [4, 5]. During early cognate DC-T cell interaction, activated T cells rapidly express T cell activation antigen, CD40ligand [8-10]. The finding that CD40-ligand and microbial product rapidly induce DCs to produce a large amount of IL-12 suggests that dendritic cells may not need a third cell type to polarize activated T cells towards TH1 effectors [11-14]. The questions are: i) are there distinct types of DCs? ii) Do distinct types of DCs induce different types of immune responses, such as TH1 versus TH2 or immunity versus tolerance? iii) how do cytokine microenvironment or innate immunity determine the functions of DCs? iv) is there an IL-4 independent mechanism for the induction of TH2 differentiation? v) what are the ideal DCs for tumor therapy or for treatment of autoimmune diseases and graft versus host diseases (GVHD)?

It is not known whether subsets of dendritic cells provide different cytokine microenvironments that determine the differentiation of either type-1 T helper (T H 1) or T H 2 cells. Human monocyte (pDC1)–derived dendritic cells (DC1) were found to induce T H 1 differentiation, whereas dendritic cells (DC2) derived from CD4 + CD3 – CD11c – plasmacytoid cells (pDC2) induced T H 2 differentiation by use of a mechanism unaffected by interleukin-4 (IL-4) or IL-12. The T H 2 cytokine IL-4 enhanced DC1 maturation and killed pDC2, an effect potentiated by IL-10 but blocked by CD40 ligand and interferon-γ. Thus, a negative feedback loop from the mature T helper cells may selectively inhibit prolonged T H 1 or T H 2 responses by regulating survival of the appropriate dendritic cell subset.

Secondary infections due to a marked immunosuppression have long been recognized as a major cause of the high morbidity and mortality rate associated with acute measles. The mechanisms underlying the inhibition of cell-mediated immunity are not clearly understood but dysfunctions of monocytes as antigen-presenting cells (APC) are implicated. In this report, we demonstrate that measles virus (MV) replicates weakly in the resting dendritic cells (DC) as in lipopolysaccharide-activated monocytes, but intensively in CD40-activated DC. The interaction of MV-infected DC with T cells not only induces syncytia formation where MV undergoes massive replication, but also leads to an impairment of DC and T cell function and cell death. CD40-activated DC decrease their capacity to produce interleukin (IL) 12, and T cells are unable to proliferate in response to MV-infected DC stimulation. A massive apoptosis of both DC and T cells is observed in the MV pulsed DC-T cell cocultures. This study suggests that DC represent a major target of MV. The enhanced MV replication during DC-T cell interaction, leading to an IL-12 production decrease and the deletion of DC and T cells, may be the essential mechanism of immunosuppression induced by MV.

To identify genes expressed by a specific subset of dendritic cells found in vivo a polymerase chain reaction-based cDNA subtraction technique was applied to the recently described germinal center dendritic cells. A novel member of the disintegrin metalloproteinase family was cloned which comprises a not typical zinc-chelating catalytic site most similar to a bacterial metalloproteinase. Dendritic cell precursors or immature dendritic cells express no or low levels of the message. It is induced to high levels upon spontaneous or CD40-dependent maturation and in a mixed lymphocyte reaction. In situ hybridization showed distinct expression of this gene in the germinal center. This, together with the findings that certain disintegrin metalloproteinases regulate the activity of tumor necrosis factor alpha and that metalloproteinases have also been implicated in FasL processing, suggest that this novel molecule may play an important role in dendritic cell function and their interactions with germinal center T cells.

A subset of CD4+CD11c-CD3- blood cells was recently shown to develop into dendritic cells when cultured with monocyte conditioned medium. Here, we demonstrate that CD4+ CD11c-CD3- cells, isolated from tonsils, correspond to the so-called plasmacytoid T cells, an obscure cell type that has long been observed by pathologists within secondary lymphoid tissues. They express CD45RA, but not markers specific for known lymphoid- or myeloid-derived cell types. They undergo rapid apoptosis in culture, unless rescued by IL-3. Further addition of CD40-ligand results in their differentiation into dendritic cells that express low levels of myeloid antigens CD13 and CD33.

Dendritic cells (DC) are professional antigen presenting cells which are required for the initiation of immune responses.They are characterized by the expression of high levels of MHC class II products and an unusual dendritic shape.Although found in all organs, DC are present at trace level making tedious their purification and functional studies.Human DC can now be generated in vitro in large numbers by culturing CD34+ hematopoietic progenitors in presence of GM-CSF+TNFalfa during 12 days [Caux et al (1992) Nature 360 : 258].

This paper demonstrates that CD40 is expressed on rheumatoid synovial pannus and primary fibroblast cell lines established from rheumatoid and osteoarthritic synovium as well as normal skin. Among various tested cytokines, interferon-gamma (IFN-gamma) and to a lower extent, tumour necrosis factor-alpha (TNF-alpha) were found to upregulate CD40 expression on fibroblasts. Synovial and skin fibroblasts cultured over CD40 Ligand transfected L cells (L-CD40 L) demonstrate a CD40 specific increase of DNA synthesis as measured by tritiated thymidine incorporation. Cell-cycle analysis and enumeration of viable cells further show that CD40 induced fibroblast proliferation. Costimulation with L-CD40 L and IFN-gamma resulted in maximal proliferation. Engagement of fibroblasts CD40 increased the IL-1-induced production of granulocyte macrophage-colony stimulating factor and macrophage inflammatory protein-1 alpha MIP-1 alpha. CD40 L activated fibroblasts showed decreased levels of CD40, but only marginal alterations of other cell-surface antigens. Taken together, the present results indicate that fibroblasts express functional CD40 and suggest a possible role of CD40 L expressing cells, such as activated T cells and mast cells, in the development of synovium hyperplasia.

Rheumatoid synovitis is characterized by an infiltration of mononuclear cells and by the proliferation of synoviocytes. Monocytes and synoviocytes are major producers of cytokines, growth factors, and enzymes that contribute to the rheumatoid arthritis (RA) process. Since they are in close contact in vivo, we engaged in an in vitro study of the functional consequences of their interactions. Coculture of unstimulated elutriated normal blood monocytes over RA synoviocytes resulted in a synergistic increase of the production of IL-6, granulocyte-macrophage colony-stimulating factor (GM-CSF), leukemia inhibitory factor (LIF), and IL-8, when compared with their respective production in culture alone. In contrast, cytokines such as IL-10, IL-1 beta, IL-1 alpha, and TNF-alpha could not be detected. The IL-6 production in coculture was further increased by the addition of IL-1 beta, GM-CSF, IFN-gamma, or TNF-alpha, but was inhibited by the addition of IL-10, IL-4, IL-13, or IL-1Ra, an effect reverted by the addition of IL-1 beta. Moreover, an inhibition was also observed with anti-CD14 mAb and newly raised mAbs directed against RA synoviocytes. Under reducing conditions, the mAb SY12 precipitated a 150-kDa surface membrane protein, identified as amino-peptidase N (CD13/AP-N). Collectively, these results indicate that 1) monocytes and synoviocytes interact with each other to produce proinflammatory cytokines, 2) pro- and antiinflammatory cytokines have opposite effects on IL-6 production, and 3) molecules such as IL-1, CD14, and CD13 are involved.

The spontaneous production of IL-1 beta (IL-1 beta) and IL-1 receptor antagonist (IL-1Ra) by rheumatoid arthritis (RA) synovium, and the regulation of their production by IL-4 and IL-10, were studied. Supernatants from cultured synovium pieces from 19 RA patients were assayed for IL-1 beta and IL-1Ra production using ELISA and RIA, respectively. After 10 days of culture, spontaneous production of IL-1Ra was 1.42 +/- 0.43 ng/ml/100 mg of synovium whereas spontaneous production of IL-1 beta was 4.03 +/- 0.90 ng/ml/100 mg of synovium (n = 19). The addition of IL-4 reduced IL-1 beta production by 2.3-fold (p = 0.001) and increased that of IL-1Ra by 2.8-fold (p = 0.003). IL-10 had no significant effect on IL-1Ra production and suppressed IL-1 beta production (primarily in samples producing high levels of IL-1 beta). However, IL-10 was less potent than IL-4 in suppressing IL-1 beta production. IL-1Ra was mainly produced by rheumatoid synovial monocytes/macrophages. IL-4 was more potent than IL-10 in inducing IL-1Ra production by monocytes/macrophages purified from RA synovium, as well as from RA blood. Thus, RA synovium is characterized by an imbalance between IL-1Ra and IL-1 beta production, in favor of the latter. IL-4, and to a lesser extent IL-10, shift this balance in favor of an anti-inflammatory situation.

During antigen specific immune responses, antigen specific naive B cells undergo a cascade of events including activation, expansion, mutations, isotype switch, selections and differentiation into either antibody secreting plasma cells or memory B cells. These antigendependent events occur in different areas of secondary lymphoid organs, as well as other non-lymphoid organs. It requires the interaction of B cells with antigens and numerous cell types including T cells, dendritic cells (DC) and follicular dendritic cells (FDC). These cells interact with B cells through different cell surface molecules and through the release of polypeptidic mediators called cytokines.

Rheumatoid synovitis is characterized by increased activation and proliferation of synoviocytes, which are an important source of cytokines. The role of Interleukin 4 (IL-4) and IL-10 on the production of mediators of inflammation by rheumatoid synoviocytes was studied herein. While IL-4 weakly affected the spontaneous PGE2 production, it strongly inhibited its production when cells were stimulated with IL-1β and TNF-α. IL-4 decreased by 60% to 80% the spontaneous and the IL-1β or TNF-α induced synthesis of GM-CSF. In contrast, IL-4 enhanced the spontaneous (2.6-fold), and to a lower extent (1.3-1.8-fold), the cytokine stimulated production of IL-6. This induction was not due to a passive release of pre-synthesized IL-6, since IL-4 increased the level of IL-6 mRNA expression induced by IL-1β. The D50 was 5 U/ml of IL-4 for both the stimulation of IL-6 synthesis and the inhibition of GM-CSF production. Kinetic studies of the action of IL-4 revealed a rapid and sustained inhibition of GM-CSF production, and a late increase of IL-6 secretion. By contrast, IL-10 had no effect on the production of either IL-6 or GM-CSF by synoviocytes. Thus, by inhibiting synoviocyte proliferation and inhibiting their secretion of PGE2 and GM-CSF, IL-4 displays on synoviocytes a series of biological effects which complements its anti-inflammatory properties on monocytes.