Immature dendritic cells (iDCs) do not mature after uptake of apoptotic cells and may play a role in the induction of peripheral tolerance to self antigens derived from apoptotic material. The integrins, αvβ3, αvβ5, and the scavenger receptor, CD36, have been shown to mediate uptake of apoptotic cells by iDCs. However, it is not known whether the complement system, also takes part in this process. In this study we investigated the ability of iDCs to bind to apoptotic cells opsonized by iC3b. Monocyte-derived dendritic cells were offered apoptotic Jurkat cells opsonized by autologous iC3b and labeled with 1,1′-dioctadecyl-3,3,3′,3′-tetramethyl-indocarbocyanineperchlorate. A significant increase (P < 0.001) in the amount of cleared apoptotic cells was seen at low ratios. Despite increased efficiency of uptake, interaction between iC3b-opsonized apoptotic cells and iDCs down-regulated the expression of major histocompatibility complex class II, CD86, CC chemokine receptor (CCR)2, CCR5, and β2-integrins (P < 0.001), and up-regulated expression of CCR7 (P < 0.001). In addition, iDC maturation responses to CD40L and lipopolysaccharide were significantly inhibited. We conclude that opsonization of apoptotic cells by iC3b induces tolerant iDCs that are able to migrate to lymph nodes.

Apoptotic cells were shown to induce dendritic cell immune tolerance. We applied a proteomic approach to identify molecules that are secreted from apoptotic monocytes, and thus may mediate engulfment and immune suppression. Supernatants of monocytes undergoing apoptosis were collected and compared using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and differentially expressed proteins were identified using tandem mass spectrometry. Thrombospondin-1 (TSP-1) and its cleaved 26-kDa heparin-binding domain (HBD) were identified. We show that TSP-1 is expressed upon induction of monocyte apoptosis in a caspase-dependent pattern and the HBD is cleaved by chymotrypsinlike serine protease. We further show that CD29, CD36, CD47, CD51, and CD91 simultaneously participate in engulfment induction and generation of an immature dendritic cell (iDC) tolerogenic and phagocytic state. We conclude that apoptotic cell TSP-1, and notably its HBD, creates a signalosome in iDCs to improve engulfment and to tolerate engulfed material prior to the interaction with apoptotic cells. IntroductionIn recent years, it has become apparent that upon induction of apoptosis, apoptotic cells play an active role in their own engulfment by signaling professional phagocytes and/or antigenpresenting cells, without triggering an inflammatory or autoimmune response. [1][2][3][4][5] This process seems to play an important role in homeostasis, resolution of inflammation, and peripheral tolerance induction. 4,[6][7][8] Apoptotic cells have been shown to signal the innate immune system in a variety of ways. "Eat me" signals on apoptotic cells serve as markers for phagocytes to specifically recognize these cells and subsequently ingest them. Such signals can appear on apoptotic cell membranes. Direct signals include alteration in cell surface phospholipid composition, 9 changes in cell surface glycoprotein expression, distinct alterations in cell surface charge, 10,11 or expression of specific molecules. 12 Alternatively, certain serum or phagocyte-derived proteins can opsonize an apoptotic cell surface and signal phagocytes to engulf the opsonized cells. 4,[13][14][15][16][17] Viable cells actively express "do not eat me" signals by restriction of phosphatydilserine to the inner leaflet of their membrane, or "stay away" signals using CD31 expression. 18 Recently, attention has been given not only to apoptotic cell membrane changes and phagocyte receptors, but also to the release of a membrane-derived phospholipid, lysophosphatidylcholine, which acts as a "find me" signal that is important for phagocytic cell recruitment. 19 Most of these mechanisms suggest efficient identification and clearance of cells undergoing apoptosis, with noninflammatory and nonautoimmune consequences.We decided to further explore whether apoptotic cells can actively express and secrete molecules that have a physiological significance for their own engulfment and for the environmental immune suppression. We examined whether apoptosis-induced immune suppress...

In order to obtain a comprehensive picture of the molecular events regulating cutaneous photodamage of intact human epidermis, suction blister roofs obtained after a single dose of in vivo ultraviolet (UV)B exposure were used for microarray profiling. We found a changed expression of 619 genes. Half of the UVB-regulated genes had returned to pre-exposure baseline levels at 72 h, underscoring the transient character of the molecular cutaneous UVB response. Of special interest was our finding that several of the central p53 target genes remained unaffected following UVB exposure in spite of p53 protein accumulation. We next compared the in vivo expression profiles of epidermal sheets to that of cultured human epidermal keratinocytes exposed to UVB in vitro. We found 1931 genes that differed in their expression profiles between the two groups. The expression profile in intact epidemis was geared mainly towards DNA repair, whereas cultured keratinocytes responded predominantly by activating genes associated with cell-cycle arrest and apoptosis. These differences in expression profiles might reflect differences between mature differentiating keratinocytes in the suprabasal epidermal layers versus exponentially proliferating keratinocytes in cell culture. Our findings show that extreme care should be taken when extrapolating from findings based on keratinocyte cultures to changes in intact epidermis.

IsraelIn recent years, it has become apparent that the removal of apoptotic cells by macrophages and DC is not only noninflammatory, but also immune-inhibitory, in most although not all circumstances. Complement may be involved in the uptake of apoptotic cells via direct binding of bridging factors in some physiological circumstances, by opsonization and engagement of the complement receptors. In the current study, we use a complementdependent system of apoptotic cell clearance by human-derived macrophages and DC. Using a luciferase reporter gene and measuring immune response to non-opsonic zymosan, we show that iC3b-apoptotic cells induce NF-jB inhibition in response to zymosan and LPS at the nuclear translocation, transcriptional and post-transcriptional levels, leading to profound inhibition of proinflammatory cytokines. In addition, interaction with iC3b-opsonized apoptotic cells is characterized by macrophage secretion of IL-10 and lack of TGF-b secretion.In conclusion, in cells with iC3b receptors, opsonized apoptotic cells mediate a distinct antiinflammatory response and transcriptional NF-jB-dependent blockage.Key words: Apoptosis . Complement . iC3b . NF-kB IntroductionCells undergoing apoptosis in the human body express cell surface changes that allow recognition by professional phagocytes and/or neighboring cells. Removal of these cells occurs rapidly and without induction of a proinflammatory milieu [1]. In recent years, it has become apparent that the removal of apoptotic cells by macrophages and DC is not only noninflammatory but also immune-inhibitory [2][3][4][5][6][7][8], in most although not all circumstances. Fadok et al. [2] showed that efferocytosis (clearance of apoptotic cells, a terminology suggested by the Henson group) inhibited the production of proinflammatory cytokines such as IL-8 and IL-1b, and induced the secretion of TGF-b, platelet-activating factor, and prostaglandin E2. They further showed and suggested that these factors inhibited a proinflammatory response to LPS and zymosan, by autocrine or paracrine mechanisms, via the secreted factors. Later, Huynh et al. [4] showed that the resolution of acute inflammation is dependent on phosphatidylserine expressed by apoptotic cells, and on TGF-b, secreted most probably by macrophages following engulfment of apoptotic cells expressing phosphatidylserine. further showed that through TGF-b, apoptotic cells simultaneously induce an anti-inflammatory milieu and suppress proinflammatory eicosanoid and NO synthesis in murine macrophages. Hence, the proposed model for à These authors contributed equally to this work. inhibition of a proinflammatory response to LPS and zymosan, as well as the resolution of acute inflammation, is based on ligation of phosphatidylserine expressed on apoptotic cells to the presumed phosphatidylserine receptor, and possibly other receptors. This ligation is expected to result in immediate preformed TGF-b secretion from macrophages, followed by de novo synthesis of TGF-b. Additional mechanisms of inflammatory res...

After Ag capture and exposure to danger stimuli, maturing dendritic cells (DCs) migrate to regional lymph nodes, where antigenic peptides are presented to T lymphocytes. To migrate from peripheral tissue such as the epidermis to regional lymph nodes, Ag-bearing epidermal Langerhans cells must move through an extracellular matrix (ECM) of various compositions. The nature of their capacity to transmigrate via ECM is not well understood, although MIP-3β and CCR7 play critical roles. We were interested in verifying whether heparanase, a heparan sulfate-degrading endo-β-d-glucuronidase that participates in ECM degradation and remodeling, is expressed and functional in monocyte-derived DCs. Using immunohistochemistry, confocal microscopy, RT-PCR, Western blot analysis, assays for heparanase activity, and Matrigel transmigration, we show that heparanase is expressed in both nuclei and cytoplasm of immature DCs, and that gene expression and synthesis take place mainly in monocytes and early immature DCs. We also found that both nuclear and cytoplasm fractions show heparanase activity, and upon LPS-induced maturation, heparanase translocates to the cell surface and degrades ECM heparan sulfate. Matrigel transmigration assays showed a MIP-3β-comparable role for heparanase. Because heparan sulfate glycosaminoglycans play a key role in the self-assembly, insolubility, and barrier properties of the ECM, the results of this study suggest that heparanase is a key enzyme in DC transmigration through the ECM.

Anti-DNA knock-in mice serve as models for studying B cell tolerance mechanisms to a ubiquitous antigen. We have constructed six strains of double transgenic (C57BL/6xBALB/c)F1 mice, each expressing an unmutated or somatically mutated anti-DNA heavy (H) chain, combined with one of three different light (L) chains, namely V(kappa)1-J(kappa)1, V(kappa)4-J(kappa)4 and V(kappa)8-J(kappa)5. In vitro analysis of the various Ig H/L chain combinations showed that all had a similar specificity for single-stranded DNA and double-stranded DNA, but that antibodies encoded by the mutated H chain had higher affinities for the autoantigen. None of the targeted mouse strains exhibited significant levels of serum anti-DNA activity. However, while B cells from mice carrying the V(kappa)1-J(kappa)1 transgenic L chains were tolerized almost exclusively by L chain receptor editing in an affinity-independent manner, the mice expressing V(kappa)8-J(kappa)5 L chains have utilized affinity-dependent clonal anergy as their sole mechanism of B cell tolerance. V(kappa)4-J(kappa)4 targeted mice exhibited an intermediate phenotype with respect to these two mechanisms of B cell tolerance. Our results suggest that receptor editing is the preferred mechanism of B cell tolerance and that the efficiency of L chain editing is directly related to the number of available J(kappa) segments on the expressed V(kappa) allele.

Programmed cell death (PCD) is a fundamental mechanism in tissue and cell homeostasis. It was long suggested that apoptosis regulates the cell number in diverse cell populations; however no clear mechanism was shown. Neutrophils are the short-lived, first-line defense of innate immunity, with an estimated t = 1/2 of 8 hours and a high turnover rate. Here we first show that spontaneous neutrophil constitutive PCD is regulated by cell concentrations. Using a proteomic approach, we identified the S100 A8/9 complex, which constitutes roughly 40% of cytosolic protein in neutrophils, as mediating this effect. We further demonstrate that it regulates cell survival via a signaling mechanism involving MEK-ERK via TLR4 and CD11B/CD18. This mechanism is suggested to have a fine-tuning role in regulating the neutrophil number in bone marrow, peripheral blood, and inflammatory sites.

SummaryDendritic cell (DC) maturation may accelerate autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis, and may contribute to accelerated atherosclerosis seen in these patients. The immune system responds to both exogenous and endogenous 'dangerous' signals that can induce dendritic cell maturation. We have found that autologous plasma contains danger signals that induce up-regulation of major histocompatibility complex (MHC) class II and co-stimulatory molecules in immature DCs (iDCs). The objective of this study was to determine whether low-density lipoprotein (LDL) and/or oxidized LDL (oxLDL) constitute danger signals, and to assess the effect of exposure to LDL and oxLDL following monocyte differentiation into iDCs in lipoprotein-deficient serum (LPDS). IDCs were generated in the presence of autologous plasma or LPDS. Expression of maturation and migration molecules was evaluated using flow cytometry, and morphology was assessed by light microscopy. Pro-or anti-apoptotic effect was determined using annexin V and propidium iodide binding. Phagocytosis of apoptotic cells was evaluated using autologous plasma or LPDS. LDL and oxLDL were clearly able to slightly up-regulate levels of HLA-DR and co-stimulatory molecule CD86. High oxLDL concentrations (50-100 mg/ml) were associated with expression of additional maturation molecules. Moreover, iDCs that were prepared in LPDS showed partial maturation following exposure to LDL and oxLDL, and improved tolerogenic apoptotic cell uptake. This study suggests that oxLDL, and to some extent LDL, are at least partly responsible for the iDC 'danger' response induced by autologous plasma.