Plasmacytoid monocytes in epithelioid cell granulomas: ultrastructural and immunoelectron microscopic study

Experimental leprosy: a model of epithelioid cell granuloma

An animal model of granulomatous hypersensitivity has been developed, which reproduces some features of the pathologies of important chronic granulomatous disorders, including tuberculosis, tuberculoid leprosy, sarcoidosis, berylliosis, Crohn's disease, and sensitivity to zirconium. The lesions consist of focal collections of epithelioid cells surrounded by lymphocytes to form tubercles. The epithelioid cell has a secretory function and is not phagocytic. Plasmacytoid dendritic cells are precursors of epithelioid cells, which are therefore part of the innate immune system. Subplasmalemmal linear densities are also present in these cells. This autoimmune model has been induced in rabbits using a non-myelin sensory peripheral antigen to reproduce the features of tuberculoid leprosy. The antigen is probably present only in human tissue. A granuloma antigen, which is tissue specific similar to that in peripheral nerves, could be present in sarcoidosis and Crohn's disease. In multiple sclerosis, mononuclear cells in the brain parenchyma are not phagocytic and are therefore similar to epithelioid cells. The induction of tolerance leading to the development of a vaccine to prevent the lesions in multiple sclerosis, sarcoidosis, and Crohn's disease is possible after purification of the granuloma antigen.

P2Y receptor signaling regulates phenotype and IFN-alpha secretion of human plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDCs) play powerful regulatory roles in innate and adaptive immune responses and are a major source of type I interferon (IFN) following viral infection. During inflammation and mechanical stress, cells release nucleotides into the extracellular space where they act as signaling molecules via G protein-coupled P2Y receptors. We have previously reported on the regulation of myeloid dendritic cell (DC) function by nucleotides. Here, we report that human PDCs express several subtypes of P2Y receptors and mobilize intracellular calcium in response to nucleotide exposure. As a functional consequence, PDCs acquire a mature phenotype that is further enhanced in the context of CD40 ligation. Strikingly, nucleotides strongly inhibit IFN-alpha secretion induced by influenza virus or CpG-A. This effect is most pronounced for the uridine nucleotides UDP and UTP and the sugar nucleotide UDP-glucose, ligands of P2Y(6), P2Y(4), and P2Y(14), respectively. Nucleotide-induced inhibition of IFN-alpha production is blocked by suramin, a P2Y receptor antagonist. Pharmacological data point toward a role of protein kinase C in the negative regulation of type I IFN. Manipulating PDC function with P2Y receptor agonists may offer novel therapeutic strategies for autoimmune diseases or cancer.

Role of adenosine receptors in regulating chemotaxis and cytokine production of plasmacytoid dendritic cells

Plasmacytoid dendritic cells (PDCs) are potent regulators of immune function and the major source of type I interferon (IFN) following viral infection. PDCs are found at sites of inflammation in allergic reactions, autoimmune disorders, and cancer, but the mechanisms leading to the recruitment of PDCs to these sites remain elusive. During inflammation, adenosine is released and functions as a signaling molecule via adenosine receptors. This study analyzes adenosine receptor expression and function in human PDCs. Adenosine was found to be a potent chemotactic stimulus for immature PDCs via an A(1) receptor-mediated mechanism. The migratory response toward adenosine was comparable to that seen with CXCL12 (stromal-derived factor-1 alpha [SDF-1 alpha), the most potent chemotactic stimulus identified thus far for immature PDCs. Upon maturation, PDCs down-regulate the A(1) receptor, resulting in a loss of migratory function. In contrast, mature PDCs up-regulate the A(2a) receptor, which is positively coupled to adenylyl cyclase and has been implicated in the down-regulation of DC cytokine-producing capacity. We show that in mature PDCs adenosine reduces interleukin-6 (IL-6), IL-12, and IFN-alpha production in response to CpG oligodeoxynucleotides (ODN). These findings indicate that adenosine may play a dual role in PDC-mediated immunity by initially recruiting immature PDCs to sites of inflammation and by subsequently limiting the extent of the inflammatory response induced by mature PDCs by inhibiting their cytokine-producing capacity.

Plasmacytoid dendritic cells: from the plasmacytoid T-cell to type 2 dendritic cells CD4+CD56+ malignancies

Recent identification of CD4(+)CD56(+) malignancies as pathological counterparts of the precursors of type 2 dendritic cells (DC2) has shed new light on a leukocyte lineage that long remained elusive. This review retraces how knowledge evolved, through careful examination and analysis of both normal lymphoid tissue and rare proliferative diseases, from plasmacytoid T cells to plasmacytoid dendritic cells (pDC) and then DC2. The functions of these cells and their key role at the crossroads of innate and cognitive immunity are also discussed. The major characteristics of DC2 malignancies are summarized and compared to natural killer cell (NK) lymphomas, another type of proliferative disease sharing the expression of CD56.

Activation of influenza virus-specific CD4+ and CD8+ T cells: a new role for plasmacytoid dendritic cells in adaptive immunity

Plasmacytoid dendritic cells (pDCs) contribute to innate antiviral immune responses by producing type I interferons (IFNs) upon exposure to enveloped viruses. However, their role in adaptive immune responses, such as the initiation of antiviral T-cell responses, is not known. In this study, we examined interactions between blood pDCs and influenza virus with special attention to the capacity of pDCs to activate influenza-specific T cells. pDCs were compared with CD11c(+) DCs, the most potent antigen-presenting cells (APCs), for their capacity to activate T-cell responses. We found that like CD11c(+) DCs, pDCs mature following exposure to influenza virus, express CCR7, and produce proinflammatory chemokines, but differ in that they produce type I IFN and are resistant to the cytopathic effect of the infection. After influenza virus exposure, both DC types exhibited an equivalent efficiency to expand anti-influenza virus cytotoxic T lymphocytes (CTLs) and T helper 1 (TH1) CD4(+) T cells. Our results pinpoint a new role of pDCs in the induction of antiviral T-cell responses and suggest that these DCs play a prominent role in the adaptive immune response against viruses.

'Agranular CD4+ CD56+ hematodermic neoplasm' (blastic NK-cell lymphoma) originates from a population of CD56+ precursor cells related to plasmacytoid monocytes

In 1999, we reported seven cases of an unusual hematologic malignancy with primary cutaneous presentation that appeared as a distinct clinicopathologic entity characterized by medium-sized tumor cells with a peculiar CD3- CD4+ CD56+ CD43+ HLA-DR+ cell surface phenotype. Because the origin of tumor cells was not clear and they exhibited a nonlineage-specific phenotype, we hypothesized that such tumors likely originated from hematologic-myeloid precursor cells and were tentatively assigned the designation "agranular CD4+ CD56+ hematodermic neoplasms." In the present study we report 14 cases (seven already reported and seven additional cases) of these tumors, and simultaneously we present now a rare population of cells that we have identified in the peripheral blood of healthy volunteers treated with Flt3 ligand. These cells express all the characteristic markers of CD4+ CD56+ hematodermic neoplasms. This population appears to be related to plasmacytoid monocytes because they also expressed CD68 and bright levels of CD123. To confirm the relationship between these normal cells and CD4+ CD56+ hematodermic neoplasms, we conducted an extensive comparative phenotypic study. Results show that these two cell types are indeed related because they share many phenotypic features, including the presence of CD4, CD56, CD43, CD68, and HLA-DR and the absence of other T, B, NK, or myelomonocytic markers. More importantly, we found that the bright expression of CD123 by immunohistochemistry is a distinctive characteristic of CD4+ CD56+ hematodermic neoplasms because all (n = 14) cases expressed this marker, whereas only two specimens in a control panel comprising 30 samples of related tumors expressed comparable levels of CD123. We therefore propose that oncogenic transformation of NCAM-expressing plasmacytoid monocyte-like cells may lead to "agranular CD4+ CD56+ hematodermic neoplasm."

Plasmacytoid monocytes/T cells: a dendritic cell lineage?

Plasmacytoid monocytes/T cells were first described in 1958, yet their origin and function have remained enigmatic. Recently a series of publications brought these cells to the forefront of immunological research. Indeed, plasmacytoid monocytes/T-cells contain natural type-I interferon producing cells and can differentiate in vitro into dendritic cells (DC). It has been proposed that plasmacytoid monocytes/T-cells represent a distinct lineage of cells whose fate it is to differentiate into dendritic cells. Herein we will review recent advances in our understanding of plasmacytoid monocytes/T cells and highlight arguments in favor or against this lineage hypothesis. We propose that plasmacytoid monocytes/T cells represent a composite group of both myeloid and lymphoid early-committed cells that are characterized by their ability to differentiate in vitro into DC.

Subacute inflammatory polyneuropathy: two cases with plasmacytoid histiocytes in the endoneurium

Inflammatory polyneuropathies are mainly known by their acute form, Guillain-Barré syndrome, but there are also chronic cases and all are considered as having an autoimmune mechanism. In each form, peripheral nerve biopsies show scattered macrophages in the endoneurium and in certain cases macrophages invade the Schwann cell cytoplasm and destroy the myelin sheath. In rarer cases there is a primary axonal degeneration. The authors studied two chronic cases, which both exhibited mixed primitive axonal and demyelinating lesions, with peculiar histiocytes in the endoneurium. These rounded cells were characteristically well marked by KP1 but showed well-developed rough endoplasmic reticulum cysternae at ultrastructural examination. Such plasmacytoid histiocytes have mainly been studied in subacute lymphadenopathies and have been only briefly illustrated in a few cases of peripheral neuropathies due to Lyme disease or HIV infection. The two cases reported here had no associated diseases and probably correspond to a peculiar subacute autoimmune reaction.

Translocation of particle-laden alveolar macrophages and intra-alveolar granuloma formation in rats exposed to Ludox colloidal amorphous silica by inhalation

Rats were exposed to Ludox colloidal amorphous silica at concentrations of 0, 10, 50 and 150 mg/m3 for 6 h/day, 5 days/week for 4 weeks. The rats were killed after 4 weeks exposure and at 10 days or 3 months post-exposure (PE). Dose-related pulmonary lesions were observed at 50 and 150 mg/m3, but not at 10 mg/m3. Inhaled particles were mostly phagocytized by alveolar macrophages (AMs) in the alveolar duct region and a few free particles were found in Type I pneumocytes in the alveoli. Particle-laden AMs directly penetrated into the bronchiolar interstitium from alveoli and accumulated in bronchus-associated lymphoid tissue (BALT), peribronchiolar, or perivascular interstitium. The particle-laden AMs in the interstitium further migrated into the peribronchial or perivascular lymphatics and accumulated in the tracheo-bronchial lymph nodes (TBLN). Some particle-laden AMs in the BALT transmigrated directly into bronchial lumen through the epithelium. The transmigrated particle laden-AMs in the TBLN were indistinguishable from those seen in the alveoli. They were characterized by slender cytoplasmic processes, phagosomes, myelin figures (alveolar surfactant), cholesterol clefts and lipid droplets. Many migrated particle-laden AMs were necrotic and released particles in the TBLN. The released particles were phagocytized by histiocytes and formed histiocytic granulomas. Silicotic granulomas were initially formed in alveoli with particle-laden AMs and proliferating epithelioid cells. Subsequently, the granulomas were incorporated into the interstitium. However, perivascular silicotic granulomas were developed with accumulation of transmigrated particle-laden AMs and minimal collagenized fibers at 3 months PE. There was no alveolar lipo-proteinosis as seen in crystalline silica exposure.

Morphological findings on peripheral nerve biopsies in 15 patients with human immunodeficiency virus infection

A peripheral nerve biopsy was performed in 15 patients with human immunodeficiency virus (HIV) infection and polyneuropathy. Two cases [1 asymptomatic, 1 AIDS-related complex (ARC)] presented with chronic inflammatory demyelinating polyneuropathy; there was 1 case (asymptomatic) of mononeuropathy multiplex and 12 cases (1 asymptomatic, 1 ARC, 10 AIDS) with distal symmetrical polyneuropathy. Epi- or endoneurial microvasculitis was observed in 6 cases. Electron microscopy showed that nerve fiber lesions were mainly axonal. Severe segmental demyelination was also present in both cases of chronic inflammatory demyelinating polyneuropathy, with characteristic features of active demyelination in one. Numerous plasmacytoid cells were found in the endoneurium in 4 patients. Tubuloreticular inclusions were present in endothelial cells in the 10 cases with AIDS but absent in the other patients. Direct immunopathological examination with anti-immunoglobulin sera was negative in all cases. HIV was evidenced by in situ hybridization in 2 AIDS patients; no Epstein-Barr virus or cytomegalovirus was detected.