Plasmacytoid monocytes migrate to inflamed lymph nodes and produce large amounts of type I interferon.

We have identified two cell subsets in human blood based on the lack of lineage markers (lin-) and the differential expression of immunoglobulin-like transcript receptor 1 (ILT1) and ILT3. One subset (lin-/ILT3+/ILT1+) is related to myeloid dendritic cells. The other subset (lin-/ILT3+/ILT1+) corresponds to 'plasmacytoid monocytes'. These cells are found in inflamed lymph nodes in and around the high endothelial venules. They express CD62L and CXCR3, and produce extremely large amounts of type I interferon after stimulation with influenza virus or CD40L. These results, with the distinct cell phenotype, indicate that plasmacytoid monocytes represent a specialized cell lineage that enters inflamed lymph nodes at high endothelial venules, where it produces type I interferon. Plasmacytoid monocytes may protect other cells from viral infections and promote survival of antigen-activated T cells.

Distinct expression pattern of IFN-alpha and TNF-alpha in juvenile idiopathic arthritis synovial tissue.

OBJECTIVES: Recent laboratory and clinical data suggest that two prototype autoimmune diseases, systemic lupus erythematosus and rheumatoid arthritis are mainly driven by distinct cytokines, interferon (IFN)-alpha and tumour necrosis factor (TNF)-alpha, respectively. We here investigated the presence and characteristics of natural type I IFN-producing cells (IPCs), as well as IFN-alpha and TNF-alpha expression at sites of inflammation in juvenile idiopathic arthritis (JIA). METHODS: Peripheral blood (PB) and synovial fluid (SF) mononuclear cells (MNCs) (n = 25 each) from JIA patients with active disease were studied. IPCs were identified as BCDA-2(+)CD123(+)HLA-DR(+)CD45RA(+) cells, and dendritic cells (DCs) as CD11c(+)CD14(-/low)lin(-) cells by flow cytometry. IPCs and DCs were analysed for Toll-like receptor-7 and -9 mRNA expression by real-time polymerase chain reaction. IFN-alpha was measured by enzyme-linked immunosorbent assay in serum, SF and in supernatants of influenza virus-infected, cultured IPCs. Synovial tissues of n = 6 additional JIA patients were analysed by immunohistochemistry using mAbs against CD123, IFN-alpha, TNF-alpha, CD3, CD19 and CD138. RESULTS: IPCs were enriched in SF MNCs compared with PB MNCs in all JIA patients. Influenza-induced, but no spontaneous IFN-alpha release was detected from SF IPCs, and serum and SF IFN-alpha levels were not elevated. Nonetheless, in synovial tissue IFN-alpha producing cells accumulated at inflammatory lymph-follicular-like structures, while TNF-alpha producing cells were mostly found at the lining and sublining layers. CONCLUSIONS: These data suggest that besides TNF-alpha-expressing cells, IFN-alpha-producing IPCs are involved in initiation, maintenance or regulation of the inflammatory response in JIA.

Specialization and complementarity in microbial molecule recognition by human myeloid and plasmacytoid dendritic cells.

Following encounter with pathogens, dendritic cells (DC) mature and migrate from peripheral tissues to the T cell areas of secondary lymphoid organs, where they produce regulatory cytokines and prime naive T lymphocytes. We investigated in two subsets of human peripheral blood DC the expression of Toll-like receptors (TLR1 through TLR9) and the regulation of chemokine receptors and cytokine production in response to different maturation stimuli. Myeloid DC express all TLR except TLR7 and TLR9, which are selectively expressed by plasmacytoid DC. Myeloid and plasmacytoid DC respond to pathogen-associated molecular patterns according to their TLR expression. In response to the appropriate stimuli both DC types up-regulate CCR7, a receptor that drives DC migration to the T cell areas. Type I IFN was produced only by plasmacytoid DC and at early time points after stimulation. Furthermore, its production was elicited by some of the maturation stimuli tested. These results reveal a remarkable specialization and complementarity in microbial molecule recognition as well as a flexibility in effector function among myeloid and plasmacytoid DC.

Circulating blood dendritic cells from myeloid leukemia patients display quantitative and cytogenetic abnormalities as well as functional impairment.

Dendritic cells (DCs) are responsible for the initiation of immune responses. Two distinct subsets of blood DCs have been characterized thus far. Myeloid DCs (MDCs) and plasmacytoid monocytes (PDCs) were shown to be able to promote polarization of naive T cells. This study shows a dramatic quantitative imbalance in both circulating blood DC subsets in 37 patients with acute myeloid leukemias. Eleven patients (30%) displayed a normal quantitative profile (MDC mean, 0.37% +/- 0.21%; range, 0.01% to 0.78%; PDC mean, 0.21% +/- 0.24%; range, 0.04% to 0.62%), whereas 22 (59%) showed a tremendous expansion of MDCs (9 patients: mean, 16.76% +/- 14.03%; range, 1.36% to 41%), PDCs (4 patients: mean, 7.28% +/- 6.84%; range, 1% to 14%), or both subsets (9 patients: MDC mean, 10.86% +/- 12.36%; range, 1.02% to 37.1%; PDC mean, 4.25% +/- 3.78%; range, 1.14% to 13.04%). Finally, in 4 patients (11%), no DC subsets were detectable. Both MDC and PDC subsets exhibited the original leukemic chromosomal abnormality. Ex vivo, leukemic PDCs, but not leukemic MDCs, had impaired capacity for maturation and decreased allostimulatory activity. Also, leukemic PDCs were altered in their ability to secrete interferon-alpha. These data provide evidence that DC subsets in vivo may be affected by leukemogenesis and may contribute to leukemia escape from immune control.

The monoclonal antibody 97A6 defines a novel surface antigen expressed on human basophils and their multipotent and unipotent progenitors.

Basophils (Ba) and mast cells (MC) are important effector cells of inflammatory reactions. Both cell types derive from CD34(+) hematopoietic progenitors. However, little is known about the cell subsets that become committed to and give rise to Ba and/or MC. We have generated a monoclonal antibody (MoAb), 97A6, that specifically detects human Ba, MC (lung, skin), and their CD34(+) progenitors. Other mature hematopoietic cells (neutrophils, eosinophils, monocytes, lymphocytes, platelets) did not react with MoAb 97A6, and sorting of 97A6(+) peripheral blood (PB) and bone marrow (BM) cells resulted in an almost pure population (>98%) of Ba. Approximately 1% of CD34(+) BM and PB cells was found to be 97A6(+). Culture of sorted CD34(+)97A6(+) BM cells in semisolid medium containing phytohemagglutinin-stimulated leukocyte supernatant for 16 days (multilineage assay) resulted in the formation of pure Ba colonies (10 of 40), Ba-eosinophil colonies (7 of 40), Ba-macrophage colonies (3 of 40), and multilineage Ba-eosinophil-macrophage and/or neutrophil colonies (12 of 40). In contrast, no Ba could be cultured from CD34(+)97A6(-) cells. Liquid culture of CD34(+) PB cells in the presence of 100 ng/mL interleukin (IL)-3 (Ba progenitor assay) resulted in an increase of 97A6(+) cells, starting from 1% of day-0 cells to almost 70% (basophils) after day 7. Culture of sorted BM CD34(+)97A6(+) cells in the presence of 100 ng/mL stem cell factor (SCF) for 35 days (mast cell progenitor assay) resulted in the growth of MC (>30% on day 35). Anti-IgE-induced IgE receptor cross-linking on Ba for 15 minutes resulted in a 4-fold to 5-fold upregulation of 97A6 antigen expression. These data show that the 97A6-reactive antigen plays a role in basophil activation and is expressed on multipotent CD34(+) progenitors, MC progenitors, Ba progenitors, as well as on mature Ba and tissue MC. The lineage-specificity of MoAb 97A6 suggests that this novel marker may be a useful tool to isolate and analyze Ba/MC and their progenitors.