Separation of dendritic cells from highly purified human monocytes by counterflow centrifugation induces tissue factor expression

Protease-activated receptor-2 signalling by tissue factor on dendritic cells suppresses antigen-specific CD4+ T-cell priming

The precise function of tissue factor (TF) expressed by dendritic cells (DC) is uncertain. As well as initiating thrombin generation it can signal through protease-activated receptor 2 (PAR-2) when complexed with factor VIIa. We investigated the expression and function of TF on mouse bone marrow (BM) -derived DC; 20% of BM-derived DC expressed TF, which did not vary after incubation with lipopolysaccharide (LPS) or dexamethasone (DEX). However, the pro-coagulant activity of DEX-treated DC in recalcified plasma was 30-fold less than LPS-treated DC. In antigen-specific and allogeneic T-cell culture experiments, the TF on DEX-treated DC provided a signal through PAR-2, which contributed to the reduced ability of these cells to stimulate CD4(+) T-cell proliferation and cytokine production. In vivo, an inhibitory anti-TF antibody and a PAR-2 antagonist enhanced antigen-specific priming in two models where antigen was given without adjuvant, with an effect approximately 50% that seen with LPS, suggesting that a similar mechanism was operational physiologically. These data suggest a novel TF and PAR-2-dependent mechanism on DEX-DC in vitro and unprimed DC in vivo that contributes to the low immunogenicity of these cells. Targeting this pathway has the potential to influence antigen-specific CD4(+) T-cell activation.

Rapid tissue factor induction by oral streptococci and monocyte-IL-1beta

The ability of pro-inflammatory cytokines to promote coagulation prompted the hypothesis that pro-inflammatory cytokines induced by oral streptococci might play a role in the pathogenesis of viridans endocarditis. We used supernatant fluids from peripheral blood mononuclear monocyte (PBMC) cultures, stimulated for just 4-6 hrs with representative streptococcal isolates, to study cytokines that promoted endothelial tissue factor (TF) activity. Neutralizing antibodies demonstrated that interleukin-1beta (IL-1beta) was a major early endothelial TF inducer, and that recombinant IL-1beta was comparable with the supernatant fluid in activity. IL-1beta-rich supernatant fluids from oral streptococci-stimulated or lipopolysaccharide-stimulated PBMC cultures up-regulated the expression of endothelial ICAM-1 and E-selectin. These molecules could help trap TF-producing monocytes or dendritic cells bearing streptococci at the site. Thus, the rapid IL-1beta-inducing capacity of oral streptococci could facilitate the early deposition of bacteria in fibrin clots and promote endocarditis.

The interface between coagulation and immunity

Coagulation proteases are involved in generating fibrin after vascular injury (hemostasis) but they also have multiple other effects, many of which are mediated independently of fibrin generation, via interactions with specific cell membrane-expressed "protease activated receptors". In inflammation, this family of proteins has a complex influence, the facets of which are still incompletely understood, though a common feature in different models appears to be amplification of innate signals that are initially generated by pathogenic elements or, in the context of transplantation, ischemia or anti-graft antibodies, for instance. There is increasing evidence that these proteases may also have specific effects on cells involved in adaptive immunity and on cells that mediate chronic inflammation and fibrosis. Understanding whether these effects are relevant in the responses generated against transplanted organs is important, as it could lead ultimately to the development of novel ways to promote long-term graft survival.

Long-term incubation with IL-4 and IL-10 oppositely modifies procoagulant activity of monocytes and modulates the surface expression of tissue factor and tissue factor pathway inhibitor

Monocytes can be induced to express both tissue factor (TF) and its inhibitor, TF pathway inhibitor-1 (TFPI-1). A short incubation (<6 h) with interleukin (IL)-4 and IL-10, two potent deactivators of monocyte functions, has been shown to modulate the synthesis and expression of TF by monocytes activated by lipopolysaccharide, but the consequences of longer incubations (up to 96 h) on both TF and TFPI-1 are unknown. The results of this study showed that adherent monocytes in culture spontaneously expressed TF and TFPI and that prolonged incubation with IL-10 induced a time- and dose-dependent decrease of monocyte TF synthesis, and an accumulation of TF/TFPI-1 complexes at the moncyte surface, suggesting a decreased clearance of these complexes. In contrast, IL-4 induced a time- and dose-dependent increase in TF synthesis, which remained intracytoplasmic, as shown by confocal microscopy. Surprisingly, TF:antigen (Ag) was decreased at the monocyte surface, but the procoagulant activity (PCA) of IL-4-treated monocytes was increased, as a result of more pronounced decrease of TFPI-1:Ag expression than that of TF. In conclusion, prolonged incubation with IL-4 and IL-10 oppositely modified PCA of cultured monocytes, and altered TF and TFPI trafficking and clearance. These data explain the respective deleterious or benefit effects of IL-4 or IL-10 in atherothrombosis.

Endocarditis-associated oral streptococci promote rapid differentiation of monocytes into mature dendritic cells

Endocarditis is frequently attributable to oral streptococci, but mechanisms of pathogenesis are not well understood, although monocytes appear to be important. High titers of interleukin-12 (IL-12) are produced by peripheral blood mononuclear cells (PBMC) after engaging Streptococcus mutans, but monocytes in developing endocardial vegetations tend to disappear rather than become macrophages. These data prompted the hypothesis that streptococcus-infected monocytes differentiate into short-lived IL-12-producing dendritic cells (DCs) rather than macrophages. PBMC from healthy subjects were stimulated with six isolates of oral streptococci, three nonstreptococcal oral bacteria, or IL-4 plus granulocyte-macrophage colony-stimulating factor, and the appearance of cells with markers typical of mature DCs (CD83(+), CD86(+), CD11c(+), and CD14(-)) was monitored. Supernatant fluids from the PBMC cultures were harvested and IL-12 p70 levels were determined. S. mutans-stimulated monocytes were analyzed for their ability to elicit allogeneic mixed-lymphocyte reactions. All streptococci examined, except one strain of Streptococcus oralis (35037), rapidly induced up-regulation of CD83 and CD86 and a loss of CD14 in the CD11c(+) monocyte population within 20 h. Induction of IL-12 was CD14 dependent and correlated with streptococcal isolates that promoted the DC phenotype. Major histocompatibility complex (MHC) class II expression was up-regulated by S. mutans, and these cells were short-lived and elicited potent allogeneic mixed-lymphocyte reactions typical of DCs. In summary, monocytes stimulated with endocarditis-associated oral streptococci rapidly exhibited the DC phenotype and functions. These data suggest that the initiation of bacterial endocarditis by oral streptococci may involve monocyte-to-DC differentiation, and this may help explain the low levels of macrophages in the site.

Emerging roles of tissue factor in viral hemorrhagic fever

Activation of coagulation by tissue factor (TF) is frequently observed in sepsis syndrome and is documented in certain viral hemorrhagic fevers. Coagulation protease complexes signal by activating the G-protein coupled, protease-activated receptors that regulate inflammation. Blockade of TF attenuates lethality in experimental models of Ebola virus infection but - similar to findings in bacterial sepsis - reduction of inflammation, rather than attenuation of coagulation, predicts survival of treated animals. Thus, targeting TF appears to aid the antiviral immune response in hemorrhagic fevers, and further studies are encouraged to define how TF-dependent signaling regulates immunity.

Porcine dendritic cells generated in vitro: morphological, phenotypic and functional properties

Despite the central role that dendritic cells (DC) play in immune regulation and antigen presentation, little is known about porcine DC. In this study, two sources of DC were employed. Bone marrow haematopoietic cell-derived DC (BM-DC) were generated using granulocyte-macrophage colony-stimulating factor (GM-CSF) in the presence or absence of tumour necrosis factor-alpha (TNF-alpha). Monocyte-derived DC (Momicron-DC) were generated with GM-CSF and interleukin-4 (IL-4). In both systems, non-adherent cells developed with dendritic morphology, expressing high levels of major histocompatibility complex (MHC) class II. The presence of TNF-alpha increased the BM-DC yield, and enhanced T-cell stimulatory capacity. Both BM-DC and Momicron-DC expressed the pan-myeloid marker SWC3, as well as CD1 and CD80/86, but were also CD14+ and CD16+. The CD16 molecule was functional, acting as a low-affinity Fc receptor. In contrast, the CD14 on DC appeared to differ functionally from monocyte CD14: attempts to block CD14, in terms of lipopolysaccharide (LPS)-induced procoagulant activity (PCA), failed. The use of TNF-alpha or LPS for DC maturation induced up-regulation of MHC class II and/or CD80/86, but also CD14. Allogeneic mixed leucocyte reactions and staphylococcal enterotoxin B antigen presentation assays demonstrated that these DC possessed potent T-cell stimulatory capacity. No T helper cell polarization was noted. Both the BM-DC and the Momicron-DC induced a strong interferon-gamma and IL-4 response. Taken together, porcine DC generated in vitro possess certain characteristics relating them to DC from other species including humans, but the continued presence of CD14 and CD16 on mature and immature porcine DC was a notable difference.