Divergent effects of hypoxia on dendritic cell functions. Blood. 2008;112:3723-3734. [PMID: 18694997 DOI: 10.1182/blood-2008-02-142091]

Chemokine C-C motif receptor 5 and C-C motif ligand 5 promote cancer cell migration under hypoxia

The chemokine CC motif receptor 5 (CCR5) and its ligands have been reported to be associated with cancer progression and metastasis. Although recent researches have demonstrated a fundamental role of hypoxia in cancer, the effect of hypoxia on the expression and function of CCR5 and its ligands in cancer cells is unknown. Here, we investigated the status of CCR5 and its ligands in cancer cells under hypoxic conditions. Quantitative polymerase chain reaction, western blotting and immunofluorescence staining showed that hypoxia induced a strong increase of CCR5 expression. Dual luciferase assay and mRNA stability analysis indicated that hypoxia-induced CCR5 mRNA expression relied on both transcriptional and posttranscriptional mechanisms. We detected the expression of CCR5 ligands and found that chemokine CC motif ligand 5 (CCL5) was induced under hypoxia. Recombinant human CCL5 stimulated cell migration rather than cell proliferation under hypoxia, and neutralization of CCL5 inhibited hypoxia-induced migration of cancer cells. Similarly, overexpression of CCR5 increased cell migration, and knockdown of CCR5 attenuated hypoxia-mediated cell migration. We further showed that hypoxia-inducible factor-1α (HIF-1α) was involved in CCR5 and CCL5 regulation under hypoxia. HIF-1α mRNA levels were highly correlated with CCR5 mRNA and CCL5 mRNA levels in clinical samples. CCR5 and CCL5 were highly expressed in breast cancer lymph nodes metastases. Taken together, our data suggest that CCR5-CCL5 interaction promotes cancer cell migration under hypoxia.

Coordinated regulation of myeloid cells by tumours

Myeloid cells are the most abundant nucleated haematopoietic cells in the human body and are a collection of distinct cell populations with many diverse functions. The three groups of terminally differentiated myeloid cells - macrophages, dendritic cells and granulocytes - are essential for the normal function of both the innate and adaptive immune systems. Mounting evidence indicates that the tumour microenvironment alters myeloid cells and can convert them into potent immunosuppressive cells. Here, we consider myeloid cells as an intricately connected, complex, single system and we focus on how tumours manipulate the myeloid system to evade the host immune response.

Inclusive estimation of complex antigen presentation functions of monocyte-derived dendritic cells differentiated under normoxia and hypoxia conditions

Dendritic cells (DCs) generated from monocytes under 20% O2 are now used as therapeutic tools for cancer patients. However, the O2 concentration is between 3 and 0.5% in most tissues. We evaluated these complicated functions of DCs under oxygen tensions mimicking in vivo situations. Immature DCs (imDCs) were generated from monocytes using IL-4 and GM-CSF under normoxia (20% O2; N-imDCs) or hypoxia (1% O2; H-imDCs). Mature DCs (mDCs) were induced with LPS. DCs were further exposed to normoxia (N/N-DCs) or hypoxia (N/H-DCs and H/H-DCs) conditions. Using a 2-D culture system, H-DCs were smaller in size than N-DCs, and H/H-DCs exhibited higher allo-T cell stimulation ability than N/N-DCs and N/H-DCs. On the other hand, motility and phagocytic ability of H/H-DCs were significantly lower than those of N/H-DCs and N/N-DCs. In a 3-D culture system, however, maturation of H/H-imDCs and N/H-imDCs was suppressed compared with N/N-imDCs as a result of their decreased motility and phagocytosis. Interestingly, silencing of HIF-1α by RNA interference decreased CD83 expression without affecting any antigen presentation abilities except for the ability to stimulate the allo-T cell population. Our data could help our understanding of DCs, especially therapeutic DCs, in vivo.

The hypoxic environment reprograms the cytokine/chemokine expression profile of human mature dendritic cells

Myeloid dendritic cells (DCs) are professional antigen-presenting cells critical for the orchestration of immunity and maintenance of self-tolerance. DC development and functions are tightly regulated by a complex network of inhibitory and activating signals present in the tissue microenvironment, and dysregulated DC responses may result in amplification of inflammation, loss of tolerance, or establishment of immune escape mechanisms. Generation of mature (m)DCs from monocytic precursors recruited at pathological sites occurs under condition of low partial oxygen pressure (pO(2)). However, the way in which the hypoxic microenvironment modulates the functions of these cells is still not clear. We demonstrate that chronic hypoxia (4 days, 1% O(2)) promotes the onset of a highly proinflammatory gene expression profile in mDCs generated from primary human monocytes, characterized by the modulation of a significant cluster of genes coding for proinflammatory chemokines/cytokines and/or their receptors. Within the chemokine system, strong upregulation of genes encoding proteins chemotactic for neutrophils, such as CXCL2, CXCL3, CXCL5, CXCL6, and CXCL8, and for activated/memory T lymphocytes, monocytes, and immature (i) DCs, e.g. CCL20, CCL3 and CCL5, was observed, concomitant with decreased expression of genes coding for naive/resting T cells chemoattractants, CCL18 and CCL23. Other hypoxia-inducible genes coded for cytokines with a primary role in inflammation and angiogenesis, including osteopontin, vascular endothelial growth factor, and IL-1β. mRNA modulation was paralleled by protein secretion. These results suggest that conditions of reduced O(2) availability reprograms mDCs toward a proinflammatory direction by tuning the cytokine/chemokine repertoire, thus affecting their ability to regulate leukocyte trafficking and activation at pathological sites, with potential implications for the pathogenesis of chronic inflammatory diseases.

Hypoxia--a key regulator of angiogenesis and inflammation in rheumatoid arthritis

The importance of inflammation in rheumatoid arthritis (RA) is well understood. This knowledge has resulted in the development of anti-inflammatory therapies--either broadly acting (such as steroids) or more specific approaches (such as antibodies against TNF)--with biologic therapies (including TNF inhibitors) revolutionizing the treatment of RA. However, what is less well appreciated in RA are the links between inflammation, blood-vessel formation (angiogenesis) and cellular responses to changes in oxygen tension. Inadequate oxygenation, termed hypoxia, is thought to drive the increase in synovial angiogenesis that occurs in RA, through expression of hypoxia-inducible molecules, including vascular endothelial growth factor (VEGF). This process promotes further infiltration of inflammatory cells and production of inflammatory mediators, perpetuating synovitis. This Review highlights the molecular pathways activated by hypoxia, and how these pathways might interact with inflammatory signaling to promote and maintain synovitis in RA, with a particular focus on the response of macrophages to hypoxia in the context of RA. Successful treatment of RA, for example with anti-TNF antibodies, reduces levels of proangiogenic factors, including VEGF, and leads to normalization of the vasculature. These processes emphasise the close links between hypoxia, angiogenesis and inflammation in this disease and supports the concept that angiogenesis blockade could be of therapeutic benefit in RA.

When Cells Suffocate: Autophagy in Cancer and Immune Cells under Low Oxygen

Hypoxia is a signature feature of growing tumors. This cellular state creates an inhospitable condition that impedes the growth and function of all cells within the immediate and surrounding tumor microenvironment. To adapt to hypoxia, cells activate autophagy and undergo a metabolic shift increasing the cellular dependency on anaerobic metabolism. Autophagy upregulation in cancer cells liberates nutrients, decreases the buildup of reactive oxygen species, and aids in the clearance of misfolded proteins. Together, these features impart a survival advantage for cancer cells in the tumor microenvironment. This observation has led to intense research efforts focused on developing autophagy-modulating drugs for cancer patient treatment. However, other cells that infiltrate the tumor environment such as immune cells also encounter hypoxia likely resulting in hypoxia-induced autophagy. In light of the fact that autophagy is crucial for immune cell proliferation as well as their effector functions such as antigen presentation and T cell-mediated killing of tumor cells, anticancer treatment strategies based on autophagy modulation will need to consider the impact of autophagy on the immune system.

A system out of breath: how hypoxia possibly contributes to the pathogenesis of systemic sclerosis

Systemic sclerosis (SSc) is an autoimmune disease characterized by vascular alterations and immunological disturbances and fibrosis, the order of which remains to be fully determined. Clinically, patients show clear signs of hypoxia in skin and internal organs. The low oxygen tension is potentially caused by a yet to be indentified circuitry involving the three features that typify SSc. In addition, once present, the hypoxia creates a vicious circle of ongoing pathology. In this paper, we provide an overview of the evidence that points towards the mechanisms causing hypoxia in SSc. In addition, data that suggest how hypoxia itself may orchestrate worsening of symptoms is presented. Altogether, it is clear that hypoxia is an important hallmark in SSc patients. By providing an overview of the mechanisms at play and the possible therapeutic avenues that have emerged, we hope to stimulate researchers to provide novel clues into the conundrum in SSc patients.

When oxygen runs short: the microenvironment drives host-pathogen interactions

Pathogens that colonize or infect the human body have to face varying oxygen concentrations within different organs. Inflammation itself promotes oxygen consumption within affected tissues and creates a low oxygen environment. As a consequence, pathogens and the host immune system have to adapt to rapid changes in oxygen availability. Here we summarize recent findings on the adaptation of pathogens, host defense mechanisms and treatment strategies against intracellular pathogens in a low oxygen environment.

Toll-like receptor activation and hypoxia use distinct signaling pathways to stabilize hypoxia-inducible factor 1α (HIF1A) and result in differential HIF1A-dependent gene expression

HIF1A is a transcription factor that plays a central role for the adaptation to tissue hypoxia and for the inflammatory response of myeloid cells, including DCs. HIF1A is stabilized by hypoxia but also by TLR ligands under normoxic conditions. The underlying signaling events leading to the accumulation of HIF1A in the presence of oxygen are still poorly understood. Here, we show that in contrast to hypoxic stabilization of HIF1A, normoxic, TLR-mediated HIF1A accumulation in DCs follows a different pathway that predominantly requires MYD88-dependent NF-κB activity. The TLR-induced HIF1A controls a subset of proinflammatory genes that are insufficiently induced following hypoxia-mediated HIF1A induction. Thus, TLR activation and hypoxia stabilize HIF1A via distinct signaling pathways, resulting in differential HIF1A-dependent gene expression.

A unique hybrid renal mononuclear phagocyte activation phenotype in murine systemic lupus erythematosus nephritis

Renal infiltration with mononuclear cells is associated with poor prognosis in systemic lupus erythematosus. A renal macrophage/dendritic cell signature is associated with the onset of nephritis in NZB/W mice, and immune-modulating therapies can reverse this signature and the associated renal damage despite ongoing immune complex deposition. In nephritic NZB/W mice, renal F4/80(hi)/CD11c(int) macrophages are located throughout the interstitium, whereas F4/80(lo)/CD11c(hi) dendritic cells accumulate in perivascular lymphoid aggregates. We show here that F4/80(hi)/CD11c(int) renal macrophages have a Gr1(lo)/Ly6C(lo)/VLA4(lo)/MHCII(hi)/CD43(lo)/CD62L(lo) phenotype different from that described for inflammatory macrophages. At nephritis onset, F4/80(hi)/CD11c(int) cells upregulate cell surface CD11b, acquire cathepsin and matrix metalloproteinase activity, and accumulate large numbers of autophagocytic vacuoles; these changes reverse after the induction of remission. Latex bead labeling of peripheral blood Gr1(lo) monocytes indicates that these are the source of F4/80(hi)/CD11c(int) macrophages. CD11c(hi)/MHCII(lo) dendritic cells are found in the kidneys only after proteinuria onset, turnover rapidly, and disappear rapidly after remission induction. Gene expression profiling of the F4/80(hi)/CD11c(int) population displays increased expression of proinflammatory, regulatory, and tissue repair/degradation-associated genes at nephritis onset that reverses with remission induction. Our findings suggest that mononuclear phagocytes with an aberrant activation profile contribute to tissue damage in lupus nephritis by mediating both local inflammation and excessive tissue remodeling.

Pulmonary dendritic cells in lungs of preterm infants: neglected participants in bronchopulmonary dysplasia?

Preterm infants are at risk for bronchopulmonary dysplasia (BPD), a chronic lung disease characterized by disrupted alveolar remodeling and microvascular dysangiogenesis. The pathogenesis of BPD is multifactorial, with contributions from antenatal and/or postnatal infection and inflammation. The potential role of dendritic cells, critical immune regulatory cells with potent angiogenic activities, remains undetermined. We studied the prevalence and topography of dendritic cells in postmortem lungs of short- and long-term ventilated preterm infants born between 23 and 29 weeks in gestation. Controls were age-matched infants who had lived less than 12 hours. Dendritic cells were identified by anti-DC-SIGN immunohistochemistry and were co-localized with endothelial and smooth muscle cells by double immunofluorescence. Lungs of early and late control infants without evidence of antenatal infection contained scattered DC-SIGN-positive dendritic cells in the peripheral lung parenchyma. Lungs of early control infants with a history of chorioamnionitis/antenatal infection and lungs of short- or long-term ventilated preterm infants showed a dramatic (more than 3-fold) increase in dendritic cells. Double labeling highlighted a close association between dendritic cells and small- or medium-sized pulmonary vessels. In conclusion, we demonstrated that dendritic cells are an integral component of normal postcanalicular lung development. Antenatal infection and ventilation/BPD are associated with significant pulmonary recruitment of dendritic cells. The recently described angiogenic effects of dendritic cells and their intimate association with the pulmonary microvasculature indicate that dendritic cells may participate in BPD-associated dysangiogenesis. Elucidation of the role of this immunovascular axis may lead to novel therapeutic approaches to BPD.

Hypoxia modulates the gene expression profile of immunoregulatory receptors in human mature dendritic cells: identification of TREM-1 as a novel hypoxic marker in vitro and in vivo

Dendritic cells (DCs) are a heterogeneous group of professional antigen-presenting cells functioning as sentinels of the immune system and playing a key role in the initiation and amplification of innate and adaptive immune responses. DC development and functions are acquired during a complex differentiation and maturation process influenced by several factors present in the local milieu. A common feature at pathologic sites is represented by hypoxia, a condition of low pO(2), which creates a unique microenvironment affecting cell phenotype and behavior. Little is known about the impact of hypoxia on the generation of mature DCs (mDCs). In this study, we identified by gene expression profiling a significant cluster of genes coding for immune-related cell surface receptors strongly up-regulated by hypoxia in monocyte-derived mDCs and characterized one of such receptors, TREM-1, as a new hypoxia-inducible gene in mDCs. TREM-1 associated with DAP12 in hypoxic mDCs, and its engagement elicited DAP12-linked signaling, resulting in ERK-1, Akt, and IκBα phosphorylation and proinflammatory cytokine and chemokine secretion. Finally, we provided the first evidence that TREM-1 is expressed on mDCs infiltrating the inflamed hypoxic joints of children affected by juvenile idiopathic arthritis, representing a new in vivo marker of hypoxic mDCs endowed with proinflammatory properties.

Dyad of CD40/CD40 ligand fosters neuroinflammation at the blood-brain barrier and is regulated via JNK signaling: implications for HIV-1 encephalitis

Human immunodeficiency virus 1 (HIV-1) infection may result in activation of peripheral monocytes followed by their infiltration into the CNS, where the release of proinflammatory mediators causes neurologic disease. Previously, we detected high levels of soluble CD40 ligand (CD40L) in CSF and plasma of HIV-infected patients with cognitive impairment. We now show that CD40, a receptor for CD40L, is highly expressed in brain endothelial cells of patients affected by HIV-1 encephalitis (HIVE), suggesting an important role for the CD40/CD40L dyad in regulating blood-brain barrier (BBB) functions. This concept was further supported by in vitro experiments. Exposure of primary human brain microvascular endothelial cells (BMVECs) to CD40L upregulated the expression of adhesion molecules intracellular adhesion molecule-1 and vascular cell adhesion molecule-1, which caused a fourfold increase in monocyte adhesion to BMVECs and stimulated migration across an in vitro BBB model. Investigations into the intracellular signaling pathways that govern these events revealed that cJUN-N-terminal kinase (JNK) is critical to CD40 activation in the BMVECs. CD40L induced activation of mixed-lineage-kinase-3 and JNK, leading to the subsequent activation of cJUN/AP-1 (activating-protein-1). JNK inhibition in the BMVECs prevented CD40L-mediated induction of adhesion molecules, monocyte adhesion, and transendothelial migration. These new findings support the concept that the CD40/CD40L dyad plays an important role in HIVE neuroinflammation.

Different tumor microenvironments contain functionally distinct subsets of macrophages derived from Ly6C(high) monocytes

Tumor-associated macrophages (TAM) form a major component of the tumor stroma. However, important concepts such as TAM heterogeneity and the nature of the monocytic TAM precursors remain speculative. Here, we show for the first time that mouse mammary tumors contained functionally distinct subsets of TAMs and provide markers for their identification. Furthermore, in search of the TAM progenitors, we show that the tumor-monocyte pool almost exclusively consisted of Ly6C(hi)CX(3)CR1(low) monocytes, which continuously seeded tumors and renewed all nonproliferating TAM subsets. Interestingly, gene and protein profiling indicated that distinct TAM populations differed at the molecular level and could be classified based on the classic (M1) versus alternative (M2) macrophage activation paradigm. Importantly, the more M2-like TAMs were enriched in hypoxic tumor areas, had a superior proangiogenic activity in vivo, and increased in numbers as tumors progressed. Finally, it was shown that the TAM subsets were poor antigen presenters, but could suppress T-cell activation, albeit by using different suppressive mechanisms. Together, our data help to unravel the complexities of the tumor-infiltrating myeloid cell compartment and provide a rationale for targeting specialized TAM subsets, thereby optimally "re-educating" the TAM compartment.

Dendritic cells/natural killer cross-talk: a novel target for human immunodeficiency virus type-1 protease inhibitors

Background

HIV-1 Protease Inhibitors, namely PIs, originally designed to inhibit HIV-1 aspartic protease, can modulate the immune response by mechanisms largely unknown, and independent from their activity on viral replication. Here, we analyzed the ability of PIs to interfere with differentiation program of monocytes toward dendritic cell (DCs) lineage, a key process in the inflammatory response.

Methodology/Principal Findings

Monocytes from healthy donors were isolated and induced to differentiate in vitro in the presence or absence of saquinavir, ritonavir, nelfinavir, indinavir or amprenavir (sqv, rtv, nlfv, idv, apv, respectively). These drugs demonstrated a differential ability to sustain the generation of immature DCs (iDCs) with an altered phenotype, including low levels of CD1a, CD86, CD36 and CD209. DCs generated in the presence of rtv also failed to acquire the typical phenotype of mature DCs (mDCs), and secreted lower amounts of IL-12 and IL-15. Accordingly, these aberrant mDCs failed to support activation of autologous Natural Killer (NK) cells, and resulted highly susceptible to NK cell-mediated cytotoxicity.

Conclusions/Significance

Our findings uncover novel functional properties of PIs within the DC-NK cell cross-talk, unveiling the heterogeneous ability of members of this class drugs to drive the generation of atypical monocyte-derived DCs (MDDCs) showing an aberrant phenotype, a failure to respond appropriately to bacterial endotoxin, a weak ability to prime autologous NK cells, and a high susceptibility to NK cell killing. These unexpected properties might contribute to limit inflammation and viral spreading in HIV-1 infected patients under PIs treatment, and open novel therapeutical perspectives for this class drugs as immunomodulators in autoimmunity and cancer.

Effects of TLR agonists on the hypoxia-regulated transcription factor HIF-1alpha and dendritic cell maturation under normoxic conditions

Dendritic cells (DC) are professional antigen presenting cells that represent an important link between innate and adaptive immunity. Danger signals such as toll-like receptor (TLR) agonists induce maturation of DC leading to a T-cell mediated adaptive immune response. In this study, we show that exogenous as well as endogenous inflammatory stimuli for TLR4 and TLR2 induce the expression of HIF-1alpha in human monocyte-derived DC under normoxic conditions. On the functional level, inhibition of HIF-1alpha using chetomin (CTM), YC-1 and digoxin lead to no consistent effect on MoDC maturation, or cytokine secretion despite having the common effect of blocking HIF-1alpha stabilization or activity through different mechanisms. Stabilization of HIF-1alpha protein by hypoxia or CoCl(2) did not result in maturation of human DC. In addition, we could show that TLR stimulation resulted in an increase of HIF-1alpha controlled VEGF secretion. These results show that stimulation of human MoDC with exogenous as well as endogenous TLR agonists induces the expression of HIF-1alpha in a time-dependent manner. Hypoxia alone does not induce maturation of DC, but is able to augment maturation after TLR ligation. Current evidence suggests that different target genes may be affected by HIF-1alpha under normoxic conditions with physiological roles that differ from those induced by hypoxia.

Hypoxic tumors and their effect on immune cells and cancer therapy

The abnormal decrease or the lack of oxygen supply to cells and tissues is called hypoxia. This condition is commonly seen in various diseases such as rheumatoid arthritis and atherosclerosis, also in solid cancers. Pre-clinical and clinical studies have shown that hypoxic cancers are extremely aggressive, resistant to standard therapies (chemotherapy and radiotherapy), and thus very difficult to eradicate. Hypoxia affects both the tumor and the immune cells via various pathways. This review summarizes the most common effects of hypoxia on immune cells that play a key role in the anti-tumor response, the limitation of current therapies, and the potential solutions that were developed for hypoxic malignancies.

Hypoxia induces T-cell apoptosis by inhibiting chemokine C receptor 7 expression: the role of adenosine receptor A(2)

Hypoxia is a major characteristic of the tumor microenvironment, and its effects on immune cells are proposed to be important factors for the process of tumor immune escape. It has been reported that hypoxia affects the function of dendritic cells and the antitumor function of T cells. Here we discuss the effects of hypoxia on T-cell survival. Our results showed that hypoxia induced apoptosis of T cells. Adenosine and adenosine receptors (AR) are important to the hypoxia-related signaling pathway. Using AR agonists and antagonists, we demonstrated that hypoxia-induced apoptosis of T cells was mediated by A(2a )and A(2b) receptors. Furthermore, we are the first, to our knowledge, to report that hypoxia significantly inhibited the expression of chemokine C receptor 7 (CCR7) of T cells via the A(2)R signal pathway, perhaps representing a mechanism of hypoxia-induced apoptosis of T cells. Collectively, our research demonstrated that hypoxia induces T-cell apoptosis by the A(2)R signaling pathway partly by suppressing CCR7. Blocking the A(2)R signaling pathway and/or activation of CCR7 can increase the anti-apoptosis function of T cells and may become a new strategy to improve antitumor potential.

Reoxygenation of hypoxia-differentiated dentritic cells induces Th1 and Th17 cell differentiation

Dendritic cells (DCs) are often exposed to various oxygen tensions under physiological and pathological conditions. However, the effects of various oxygen tensions on DC functions remain unclear. In this study, we showed that hypoxia-differentiated DCs expressed lower levels of MHC-II molecule, co-stimulatory molecules (CD80, CD86) and proinflammatory cytokines (IL-1beta, IL-6, and TNF-alpha), but higher levels of immunoregulatory cytokine transforming growth factor-beta (TGF-beta) than normoxia-differentiated DCs. Unexpectedly, re-exposure of hypoxia-differentiated DCs to saturated oxygen (reoxygenation) completely restored their mature phenotype and function. Specifically, the reoxygenated DCs induced naïve CD4(+) T cells to differentiate into Th1 and Th17 effector cells, but deceased the generation of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Tregs). The data indicate that hypoxic microenvironment suppresses the maturation and function of murine DCs. Reoxygenation of hypoxia-differentiated DCs however results in complete recovery of their mature phenotype and function, and has strong ability to drive immune response toward a proinflammatory direction, suggesting reoxygenated DCs may contribute to inflammation of ischemia-reperfusion injury.

An approach for understanding the inflammation and cancer relationship

It is well known that persistent inflammatory conditions can induce the cancer formation. In fact, cytokines and chemokines play a crucial role in the promoting of angiogenesis, metastasis, and subversion of adaptive immunity. These proteins are involved in cancer-related inflammation and can represent a target for innovative diagnostic and therapeutic strategies useful to clinical studies.

Dendritic cells in atherosclerotic disease

Atherosclerosis has been considered a syndrome of dysregulated lipid storage until recent evidence has emphasized the critical contribution of the immune system. Dendritic cells (DC) are positioned at the interface of the innate and adaptive immune system. Recognition of danger signals in atheromas leads to DC activation. Activated DC regulate effector T cells which can kill plaque-resident cells and damage the plaque structure. Two types of DC have been identified in atherosclerotic lesions; classical myeloid DC (mDC) which mainly recognize bacterial signatures and plasmacytoid DC (pDC) which specialize in sensing viral fragments and have the unique potential of producing large amounts of type I interferon (IFN). In human atheromas, type I IFN upregulates expression of the cytotoxic molecule TRAIL which leads to apoptosis of plaque-resident cells. This review will elucidate the role of DC in atherogenesis and particularly in plaque rupture, the underlying pathophysiologic cause of myocardial infarction.

Hypoxia or in situ normoxia: The stem cell paradigm

Although O(2) concentrations are considerably lowered in vivo, depending on the tissue and cell population in question (some cells need almost anoxic environment for their maintenance) the cell and tissue cultures are usually performed at atmospheric O(2) concentration (20-21%). As an instructive example, the relationship between stem cells and micro-environmental/culture oxygenation has been recapitulated. The basic principle of stem cell biology, "the generation-age hypothesis," and hypoxic metabolic properties of stem cells are considered in the context of the oxygen-dependent evolution of life and its transposition to ontogenesis and development. A hypothesis relating the self-renewal with the anaerobic and hypoxic metabolic properties of stem cells and the actual O(2) availability is elaborated ("oxygen stem cell paradigm"). Many examples demonstrated that the cellular response is substantially different at atmospheric O(2) concentration when compared to lower O(2) concentrations which better approximate the physiologic situation. These lower O(2) concentrations, traditionally called "hypoxia" represent, in fact, an in situ normoxia, and should be used in experimentation to get an insight of the real cell/cytokine physiology. The revision of our knowledge on cell/cytokine physiology, which has been acquired ex vivo at non physiological atmospheric (20-21%) O(2) concentrations representing a hyperoxic state for most primate cells, has thus become imperious.