Tumor-infiltrating dendritic cell precursors recruited by a beta-defensin contribute to vasculogenesis under the influence of Vegf-A

VEGFR2+PDGFRbeta+ circulating precursor cells participate in capillary restoration after hyperoxia acute lung injury (HALI)

The in vivo morphology and phenotype of circulating cells that spontaneously contribute to new vessel formation in adults remain unclear. Here, we use high-resolution imaging and flow cytometry to characterize the morphology and phenotype of a distinct population of circulating mononuclear cells contributing to spontaneous new vessel formation after hyperoxia acute lung injury (HALI). We identify a subpopulation of myeloid (CD11b/Mac1⁺) haematopoietic cells co-expressing vascular endothelial growth factor receptor 2 (VEGFR2) and platelet derived growth factor receptor beta (PDGFRβ). Moreover, we show that these CD11b⁺VEGFR2⁺PDGFRβ⁺ circulating precursor cells (CPCs) contribute structurally to the luminal surface of capillaries re-forming 2 weeks post-HALI. This indicates that these myeloid CPCs may function, at least transiently, as putative vascular precursors, and has important implications for capillary growth and repair in injury and in pathologies of the lung and other organs.

Tumor-infiltrating regulatory dendritic cells inhibit CD8+ T cell function via L-arginine metabolism

Dendritic cells (DC) have a critical effect on the outcome of adaptive immune responses against growing tumors. Whereas it is generally assumed that the presence of phenotypically mature DCs should promote protective antitumor immunity, evidence to the contrary does exist. We describe here a novel mechanism by which tumor-infiltrating dendritic cells (TIDC) actively contribute to the suppression of protective CD8(+) T-cell-based antitumor immunity. Using the BALB/NeuT model of spontaneously arising mammary carcinoma, we found that canonical MHC II(+)/CD11b(+)/CD11c(high) TIDCs act as regulatory DCs to suppress CD8(+) T-cell function, resulting in diminished T-cell-based antitumor immunity in vivo. Stimulation of naive T cells with regulatory TIDCs resulted in an altered cell fate program characterized by minimal T-cell expansion, impaired IFNgamma production, and anergy. Suppression by regulatory TIDCs overcame stimulatory signals provided by standard DCs, occurred in the absence of cognate interactions with T cells, and was mediated primarily by arginase metabolism of l-arginine. Immunosuppressive TIDCs were found in every murine tumor type examined and were phenotypically distinct from tumor-infiltrating CD11c(int-low)/CD11b(+)/Gr-1(+) myeloid-derived suppressor cells. Thus, within the tumor microenvironment, MHC II(+) TIDCs can function as potent suppressors of CD8(+) T-cell immunity.

Influence of bevacizumab, sunitinib and sorafenib as single agents or in combination on the inhibitory effects of VEGF on human dendritic cell differentiation from monocytes

Vascular endothelial growth factor (VEGF) inhibits differentiation and maturation of dendritic cells (DC), suggesting a potential immunosuppressive role for this proangiogenic factor. Bevacizumab, sorafenib and sunitinib target VEGF-mediated angiogenesis and are active against several types of cancer, but their effects on the immune system are poorly understood. In this study, VEGF and supernatants of renal carcinoma cell lines cultured under hypoxia were found to alter the differentiation of human monocytes to DC. Resulting DC showed impaired activity, as assessed by the alloreactive mixed T-lymphocyte reaction. Bevacizumab and sorafenib, but not sunitinib, reversed the inhibitory effects of VEGF, but not of those mediated by tumour supernatants. Dendritic cells matured under the influence of VEGF expressed less human leukocyte antigen-DR (HLA-DR) and CD86, and this effect was restored by bevacizumab and sorafenib. Finally, tumour-cell supernatants decreased interleukin-12 (IL-12) production by mature DC, and such inhibition was not restored by any of the tested drugs, delivered either as single agents or in combination. The deleterious effects of tumour-cell supernatants were mainly mediated by thermostable molecules distinct from VEGF. These results indicate that inhibition of the differentiation of monocytes to DC is a multifactorial effect, and that they support the development of combinations of angiogenesis inhibitors with immunological modulators.

Angiogenesis and antiangiogenic therapy in non-Hodgkin's lymphoma

Angiogenesis, the growth of new blood vessels, requires dynamic expansion, assembly and stabilization of vascular endothelial cells in response to proangiogenic stimuli. Antiangiogenic strategies have become an important therapeutic modality for solid tumors. While many aspects of postnatal pathological angiogenesis have been extensively studied in the context of nonhematopoietic neoplasms, the precise role of these processes in lymphoma pathogenesis is under active investigation. Lymphoma growth and progression is potentiated by at least two distinct angiogenic mechanisms: autocrine stimulation of tumor cells via expression of vascular endothelial growth factor (VEGF) and VEGF receptors by lymphoma cells, as well as paracrine influences of proangiogenic tumor microenvironment on both local neovascular transformation and recruitment of circulating bone marrow-derived progenitors. Lymphoma-associated infiltrating host cells including hematopoietic monocytes, T cells and mesenchymal pericytes have increasingly been associated with the pathogenesis and prognosis of lymphoma, in part providing perivascular guidance and support to neoangiogenesis. Collectively, these distinct angiogenic mechanisms appear to be important therapeutic targets in selected non-Hodgkin's lymphoma (NHL) subtypes. Understanding these pathways has led to the introduction of antiangiogenic treatment strategies into the clinic where they are currently under assessment in several ongoing studies of NHL patients.

Alarmins and antimicrobial immunity

Alarmins are endogenous mediators capable of enhancing innate and adaptive immune response through induction of concomitant recruitment and activation of antigen-presenting cells. Here we provide a brief overview of various alarmins, highlight their critical roles in innate and adaptive antimicrobial immunity, and speculate on potential usage of alarmins in combating aspergillosis.

Antimicrobial human beta-defensin-2 stimulates migration, proliferation and tube formation of human umbilical vein endothelial cells

Human beta-defensin-2 (hBD-2) is an antimicrobial peptide which is released upon microbial invasion and contributes to mucosal and epithelial defense modulating both innate and adaptive immunity. We found that hBD-2 stimulates chemotaxis of human endothelial cells with an extent similar to that exerted by the vascular endothelial growth factor (VEGF). The hBD-2-dependent chemotaxis is dose-dependent, maximal effect being reached at 500 ng/ml concentration. In the absence of any growth factor, hBD-2 favors wound healing of endothelial cells, causing an about 2-fold increase in the speed of wound closure with respect to the control. Furthermore, hBD-2 promotes endothelial cell proliferation, although at a minor extent as compared to VEGF. When plated on matrigel enriched with angiogenic factors, endothelial cells form a three-dimensional network of tubes that gives rise to capillary-like structures. Similarly to VEGF, hBD-2 promotes capillary-like tube formation of human endothelial cells. Pro-angiogenic effect promoted by hBD-2 is dose-dependent, peaks at a 500 ng/ml hBD-2 concentration and is prevented by blocking anti-alphavbeta3 monoclonal antibody. However, hBD-2-induced pro-angiogenic activity is not due to endogenously produced VEGF because it is not prevented by neutralizing anti-VEGF antibodies. Overall, our findings suggest that hBD-2 could link inflammation and the host defense through its pro-angiogenic activity.

Low levels of tumor necrosis factor alpha increase tumor growth by inducing an endothelial phenotype of monocytes recruited to the tumor site

Microenvironmental cues instruct infiltrating tumor-associated myeloid cells to drive malignant progression. A subpopulation of tumor-associated myeloid cells coexpressing endothelial and myeloid markers, although rare in peripheral blood, are primarily associated with tumors where they enhance tumor growth and angiogenesis. These biphenotypic vascular leukocytes result from the endothelial differentiation of myeloid progenitors, a process regulated by tumor necrosis factor (TNF)alpha in vitro. An in vivo increase in tumor-derived TNFalpha expression promoted tumor growth and vascularity of mouse melanoma, lung cancer, and mammary tumors. Notably, tumor growth was accompanied by a significant increase in myeloid/endothelial biphenotypic populations. TNFalpha-associated tumor growth, vascularity, and generation of tumor vascular leukocytes in mouse melanoma tumors were dependent on intact host TNFalpha receptors. Importantly, TNFalpha-expressing tumors did not exhibit increased inflammation over control tumors, suggesting a unique action related to myeloid to endothelial differentiation. Our studies suggest that TNFalpha constitutes a tumor microenvironment signal that biases recruited monocytes toward a proangiogenic/provasculogenic myeloid/endothelial phenotype.

Human defensins as cancer biomarkers and antitumour molecules

Human defensins, which are small cationic peptides produced by neutrophils and epithelial cells, form two genetically distinct alpha and beta subfamilies. They are involved in innate immunity through killing microbial pathogens or neutralizing bacterial toxins and in adaptive immunity by serving as chemoattractants and activators of immune cells. alpha-defensins are mainly packaged in neutrophil granules (HNP1, HNP2, HNP3) or secreted by intestinal Paneth cells (HD5, HD6), while beta-defensins are expressed in mucosa and epithelial cells. Using surface enhanced laser desorption/ionisation time-of-flight (SELDI-TOF) mass spectrometry (MS), alpha-defensins were found to be expressed in a variety of human tumours, either in tumour cells or at their surface. HNP1-3 peptides are also secreted and their accumulation in biological fluids was proposed as a tumour biomarker. Conversely, beta-defensin-1 (HBD-1) is down-regulated in some tumour types in which it could behave as a tumour suppressor protein. Alpha-defensins promote tumour cell growth or, at higher concentration, provoke cell death. These peptides also inhibit angiogenesis, which, in addition to immunomodulation, indicates a complex role in tumour development. This review summarizes current knowledge of defensins to discuss their role in tumour growth, tumour monitoring and cancer treatment.

Pleiotrophin produced by multiple myeloma induces transdifferentiation of monocytes into vascular endothelial cells: a novel mechanism of tumor-induced vasculogenesis

Enhanced angiogenesis is a hallmark of cancer. Pleiotrophin (PTN) is an angiogenic factor that is produced by many different human cancers and stimulates tumor blood vessel formation when it is expressed in malignant cancer cells. Recent studies show that monocytes may give rise to vascular endothelium. In these studies, we show that PTN combined with macrophage colony-stimulating factor (M-CSF) induces expression of vascular endothelial cell (VEC) genes and proteins in human monocyte cell lines and monocytes from human peripheral blood (PB). Monocytes induce VEC gene expression and develop tube-like structures when they are exposed to serum or cultured with bone marrow (BM) from patients with multiple myeloma (MM) that express PTN, effects specifically blocked with antiPTN antibodies. When coinjected with human MM cells into severe combined immunodeficient (SCID) mice, green fluorescent protein (GFP)-marked human monocytes were found incorporated into tumor blood vessels and expressed human VEC protein markers and genes that were blocked by anti-PTN antibody. Our results suggest that vasculogenesis in human MM may develop from tumoral production of PTN, which orchestrates the transdifferentiation of monocytes into VECs.

Vascular endothelium in cancer

The vascular endothelium plays an essential role during organogenesis and in tissue homeostasis. Growing evidence also supports its essential and complex role in tumour biology and cancer progression. In particular, excessive proliferation and transformation or dysfunction of endothelial cells leads to pathological (lymph)angiogenesis or vascular malfunctions, which are hallmarks of neoplastic and malignant disorders. Reciprocal interactions between endothelial cells and the local tumour microenvironment may regulate tumour progression and resistance to anti-cancer therapies in a tumour-type-specific manner.

Tumors sound the alarmin(s)

Recent evidence suggests that inflammatory molecules play critical roles in the development and progression of numerous tumors. However, one specific group of inflammatory molecules whose importance has been established in host immune responses, termed alarmins, has been largely overlooked in cancer biology. The function of several alarmins-including the defensins, LL-37, and HMGB1-in tumor development, progression, or suppression is discussed here. Taken together, these studies indicate that alarmins represent potential new targets for manipulation in a variety of tumors.

Modulation of the antitumor immune response by complement

The involvement of complement activation products in promoting tumor growth has not yet been recognized. Here we show that generation of complement C5a in the tumor microenvironment enhanced tumor growth by suppressing the anti-tumor CD8⁺ T cell-mediated response. This suppression was associated with the recruitment of myeloid-derived suppressor cells (MDSCs) into tumors and augmentation of their T cell-directed suppressive capabilities. Amplification of MDSC suppressive capacity by C5a occurred through regulation of the production of reactive oxygen and nitrogen species. Pharmacological blockade of C5a receptor significantly impaired tumor growth to a degree comparable to the effect produced by the anti-cancer drug Taxol. Thus, this study demonstrates a therapeutic role for complement inhibition in the treatment of cancer.

Murine ovarian cancer vascular leukocytes require arginase-1 activity for T cell suppression

The predominant leukocyte population present in both human and murine peritoneal ovarian tumors is the Vascular Leukocyte (VLC). VLCs are recruited en masse to the ovarian tumor microenvironment whereupon they promote tumor progression. Importantly, the presence of VLCs is requisite for peritoneal ovarian cancer progression: selective elimination of VLCs inhibits tumor burden and ascites accumulation. Despite the critical importance of VLCs to ovarian tumors, their derivation and the mechanisms by which they facilitate tumor progression are not well understood. Here we demonstrate in vivo that the murine ID8 ovarian tumor model can usurp the host peritoneal macrophage pathway to elicit and recruit VLCs. Moreover, we demonstrate that VLCs express CD11b and Gr-1, a characteristic phenotype shared amongst heterogeneous populations of leukocytes referred to as myeloid-derived suppressor cells (MDSCs). In accord with their MDSC phenotype, both murine and human VLCs express arginase-1 (ARG1). Importantly, we demonstrate that the VLCs suppress both CD8(+) and CD4(+) T cells responses and that this immunosuppression is ARG1-dependent, since blockade of VLC ARG1 activity with nor-NOHA reversed the immunosuppression. These data further characterize the tumor-associated leukocytes in ovarian cancer and provide insights into the mechanisms by which they promote tumor growth.

Anti-angiogenic effect of high doses of ascorbic acid

Pharmaceutical doses of ascorbic acid (AA, vitamin C, or its salts) have been reported to exert anticancer activity in vitro and in vivo. One proposed mechanism involves direct cytotoxicity mediated by accumulation of ascorbic acid radicals and hydrogen peroxide in the extracellular environment of tumor cells. However, therapeutic effects have been reported at concentrations insufficient to induce direct tumor cell death. We hypothesized that AA may exert anti-angiogenic effects. To test this, we expanded endothelial progenitor cells (EPCs) from peripheral blood and assessed, whether or not high dose AA would inhibit EPC ability to migrate, change energy metabolism, and tube formation ability. We also evaluated the effects of high dose AA on angiogenic activities of HUVECs (human umbilical vein endothelial cells) and HUAECs (human umbilical arterial endothelial cells). According to our data, concentrations of AA higher than 100 mg/dl suppressed capillary-like tube formation on Matrigel for all cells tested and the effect was more pronounced for progenitor cells in comparison with mature cells. Co-culture of differentiated endothelial cells with progenitor cells showed that there was incorporation of EPCs in vessels formed by HUVECs and HUAECs. Cell migration was assessed using an in vitro wound healing model. The results of these experiments showed an inverse correlation between AA concentrations relative to both cell migration and gap filling capacity. Suppression of NO (nitric oxide) generation appeared to be one of the mechanisms by which AA mediated angiostatic effects. This study supports further investigation into non-cytotoxic antitumor activities of AA.

Circulating endothelial cells, bone marrow-derived endothelial progenitor cells and proangiogenic hematopoietic cells in cancer: From biology to therapy

Vascularization, a hallmark of tumorigenesis, is classically thought to occur exclusively through angiogenesis (i.e. endothelial sprouting). However, there is a growing body of evidence that endothelial progenitor cells (EPCs) and proangiogenic hematopoietic cells (HCs) are able to support the vascularization of tumors and may therefore play a synergistic role with angiogenesis. An additional cell type being studied in the field of tumor vascularization is the circulating endothelial cell (CEC), whose presence in elevated numbers reflects vascular injury. Levels of EPCs and CECs are reported to correlate with tumor stage and have been evaluated as biomarkers of the efficacy of anticancer/antiangiogenic treatments. Furthermore, because EPCs and subtypes of proangiogenic HCs are actively participating in capillary growth, these cells are attractive potential vehicles for delivering therapeutic molecules. The current paper provides an update on the biology of CECs, EPCs and proangiogenic HCs, and explores the utility of these cell populations for clinical oncology.

Depletion of dendritic cells delays ovarian cancer progression by boosting antitumor immunity

Dendritic cells (DC) and cytokines that expand myeloid progenitors are widely used to treat cancer. Here, we show that CD11c(+)DEC205(+) DCs coexpressing alpha-smooth muscle actin and VE-cadherin home to perivascular areas in the ovarian cancer microenvironment and are required for the maintenance of tumor vasculature. Consequently, depletion of DCs in mice bearing established ovarian cancer by targeting different specific markers significantly delays tumor growth and enhances the effect of standard chemotherapies. Tumor growth restriction was associated with vascular apoptosis after DC ablation followed by necrosis, which triggered an antitumor immunogenic boost. Our findings provide a mechanistic rationale for selectively eliminating tumor-associated leukocytes to promote antitumor immunity while impeding tumor vascularization and to develop more effective DC vaccines based on a better understanding of the tumor microenvironment.

The role of myeloid cells in the promotion of tumour angiogenesis

The use of various transgenic mouse models and analysis of human tumour biopsies has shown that bone marrow-derived myeloid cells, such as macrophages, neutrophils, eosinophils, mast cells and dendritic cells, have an important role in regulating the formation and maintenance of blood vessels in tumours. In this Review the evidence for each of these cell types driving tumour angiogenesis is outlined, along with the mechanisms regulating their recruitment and activation by the tumour microenvironment. We also discuss the therapeutic implications of recent findings that specific myeloid cell populations modulate the responses of tumours to agents such as chemotherapy and some anti-angiogenic therapies.

New roles for mononuclear phagocytes in cancer biology

The primary focus in the pathogenesis and treatment of human malignancies has been the tumor cell. However, the biologic properties of a malignancy are not all intrinsically determined. Interactions between heterogeneous cell populations influence the growth and survival of both normal and malignant cells. Studies defining the origin of endothelial cells involved in tumor angiogenesis first demonstrated the contributions of normal cellular environment. Recently, the mononuclear phagocyte lineage has been found to have biologically and clinically significant tumor enhancing and tumor suppressive effects. This article reviews the multiple roles of mononuclear phagocytes in cancer biology. A companion manuscript (J Pediatr Hematol Oncol. 2008, in press) describes the targeting of these cells for therapeutic benefit. Incorporating these strategies into future childhood cancer protocols could be an innovative approach for improving patient outcome.

Circulating bone marrow-derived endothelial progenitor cells: characterization, mobilization, and therapeutic considerations in malignant disease

Until recently, tumor vascularization was thought to occur exclusively through angiogenesis. However, recent studies using different animal models of cancer suggested the importance of bone marrow-derived endothelial progenitor cells (EPCs) (i.e. postnatal vasculogenesis) in tumor vascularization and growth. EPCs are present in the peripheral blood, their levels are increased in response to certain signals/cytokines, and they home into the neovascular bed of malignant tissues. Furthermore, at the clinical level, evidence is emerging that changes in EPC levels might predict the efficacy of anticancer drug combinations that include antiangiogenic agents. On the basis of these observations, EPCs have attractive potential diagnostic and therapeutic applications for malignant diseases. In this paper, we review biological features of EPCs and speculate on the utility of these progenitor cells for medical oncology.

Chemotherapy and the tumor microenvironment: the contribution of circulating endothelial cells

Anti-angiogenic drugs, alone or in combination with chemotherapeutics, are increasingly used by medical oncologists. In many cases, however, their mechanism of action and the tailoring of optimal dosage/schedule are still elusive. Circulating endothelial cell (CEC) and progenitor (CEP) number and viability are modulated in a large series of diseases including cancer, and look promising as surrogate biomarkers for the definition of the optimal biological dose of anti-angiogenic drugs and for patients' stratification. Along with CECs and CEPs, potential EC- and CEP-related surrogate molecular markers such as VE-Cadherin and CD133 are currently under preclinical and clinical investigation.

Dendritic cell uptake of iron-based magnetic nanoparticles

We have investigated the internalization of magnetic nanoparticles (NPs) into dendritic cells (DCs) in order to assess both the final location of the particles and the viability of the cultured cells. The particles, consisting of a metallic iron core covered with carbon, showed no toxic effects on the DCs and had no effect in their viability. We found that mature DCs are able to incorporate magnetic nanoparticles in a range of size from 10 nm to ca. 200 nm, after 24 h of incubation. We describe a method to separate cells loaded with NPs, and analyze the resulting material by electron microscopy and magnetic measurements. It is found that NPs are internalized in lysosomes, providing a large magnetic signal. Our results suggest that loading DCs with properly functionalized magnetic NPs could be a promising strategy for improved vectorization in cancer diagnosis and treatment.

Immunotherapy opportunities in ovarian cancer

Ovarian cancer is responsible for the majority of gynecologic cancer deaths and despite the highest standard of multimodality therapy with surgery and cytotoxic chemotherapy, long-term survival remains low. With compelling evidence that epithelial ovarian cancer is an immunogenic tumor capable of stimulating an antitumor immune response, renewed efforts to develop immune therapies to augment the efficacy of traditional therapies are underway. Current immunotherapies focus on varied modes of antitumor vaccine development, particularly with the use of dendritic cell vaccines, effective methods for adoptive T-cell transfer and combinatorial approaches with immune modulatory therapy subverting natural tolerance mechanisms or boosting effector mechanisms. Additional combinatorial approaches include the use of cytokines and/or chemotherapy with immune therapy.

Suppression of hepatic glucose production by human neutrophil alpha-defensins through a signaling pathway distinct from insulin

In this study, we tested the hypothesis that human neutrophil alpha-defensins (HNPs) inhibit hepatic glucose production through a signaling pathway distinct from insulin. The effect of HNP-1 on fasting blood glucose levels and the expression of hepatic gluconeogenic genes was first examined. Using hyperinsulinemic-euglycemic clamps, we determined the effect of HNP-1 on endogenous glucose production, hepatic expression of key gluconeogenic genes and glucose uptake in skeletal muscle in Zucker diabetic fatty rats. In isolated primary hepatocytes, we studied the effect of HNP-1 and -2 on glucose production, expression of gluconeogenic genes, and phosphorylation of Akt, c-Src, and FoxO1. Our results show that HNP-1 reduced blood glucose levels of both normal mice and Zucker diabetic fatty rats predominantly through suppression of hepatic glucose production. HNPs inhibited glycogenolysis and gluconeogenesis in isolated hepatocytes. HNPs also suppressed expression of key gluconeogenic genes including phosphoenoylpyruvate carboxyl kinase and glucose-6-phosphatase. To investigate the mechanism, we found that HNPs stimulated phosphorylation of Akt and FoxO1 without activating IRS1. Nevertheless, HNPs activated c-Src. Blockade of c-Src activity with either a chemical inhibitor PP2 or an alternative inhibitor CSK prevented the inhibitory effect of HNPs on gluconeogenesis. Together, our results support the hypothesis that HNPs can suppress hepatic glucose production through an intracellular mechanism distinct from the classical insulin signaling pathway.

A catalytic role for proangiogenic marrow-derived cells in tumor neovascularization

Small numbers of proangiogenic bone marrow-derived cells (BMDCs) can play pivotal roles in tumor progression. In this issue of Cancer Cell, two papers, utilizing different tumor angiogenesis models, both find that activated MMP-9 delivered by BMDCs modulates neovessel remodeling, thereby promoting tumor growth. The changes in microvascular anatomy induced by MMP-9-expressing BMDCs are strikingly different between the preirradiated tumor vascular bed model employed by Ahn and Brown and the invasive glioblastoma model utilized by Du et al., likely mirroring the complexity of the real tumor microenvironment and the intricacy of roles of different BMDC populations in mediating tumor neoangiogenesis.

Myeloid cells functioning in tumor vascularization as a novel therapeutic target

Angiogenesis, the sprouting of new blood vessels to sustain growth, is an important new target in solid tumor therapy. Initial studies focused on the role of the tumor cell in promoting angiogenesis; yet more recent work has demonstrated that host cells in the tumor microenvironment also play a critical role in tumor vascularization. Additionally, vasculogenesis in which new blood vessels develop from vascular progenitor cells also contributes to tumor growth. Recent studies propose a central role for cells of the myeloid lineage in triggering vessel growth by releasing angiogenic factors and perhaps by incorporating directly into nascent blood vessels. We will review studies that support a critical role for myeloid cells in neovascularization, with a focus on cells that express various monocytic/dendritic cell markers, including vascular leukocytes (VLCs), Tie2+ monocytes, and vascular endothelial growth factor receptor 2 (VEGFR2)+ monocytes, among others. The evidence that these myeloid cells represent bona fide therapeutic targets for solid tumors will be reviewed. Finally, we will address some controversies and challenges in the field with a focus on future directions.

Cheiradone: a vascular endothelial cell growth factor receptor antagonist

BACKGROUND

Angiogenesis, the growth of new blood vessels from the pre-existing vasculature is associated with physiological (for example wound healing) and pathological conditions (tumour development). Vascular endothelial growth factor (VEGF), fibroblast growth factor-2 (FGF-2) and epidermal growth factor (EGF) are the major angiogenic regulators. We have identified a natural product (cheiradone) isolated from a Euphorbia species which inhibited in vivo and in vitro VEGF- stimulated angiogenesis but had no effect on FGF-2 or EGF activity. Two primary cultures, bovine aortic and human dermal endothelial cells were used in in vitro (proliferation, wound healing, invasion in Matrigel and tube formation) and in vivo (the chick chorioallantoic membrane) models of angiogenesis in the presence of growth factors and cheiradone. In all cases, the concentration of cheiradone which caused 50% inhibition (IC50) was determined. The effect of cheiradone on the binding of growth factors to their receptors was also investigated.

RESULTS

Cheiradone inhibited all stages of VEGF-induced angiogenesis with IC50 values in the range 5.20-7.50 microM but did not inhibit FGF-2 or EGF-induced angiogenesis. It also inhibited VEGF binding to VEGF receptor-1 and 2 with IC50 values of 2.9 and 0.61 microM respectively.

CONCLUSION

Cheiradone inhibited VEGF-induced angiogenesis by binding to VEGF receptors -1 and -2 and may be a useful investigative tool to study the specific contribution of VEGF to angiogenesis and may have therapeutic potential.

Analysis and separation of residues important for the chemoattractant and antimicrobial activities of beta-defensin 3

beta-Defensins are important in mammalian immunity displaying both antimicrobial and chemoattractant activities. Three canonical disulfide intramolecular bonds are believed to be dispensable for antimicrobial activity but essential for chemoattractant ability. However, here we show that HBD3 (human beta-defensin 3) alkylated with iodoactemide and devoid of any disulfide bonds is still a potent chemoattractant. Furthermore, when the canonical six cysteine residues are replaced with alanine, the peptide is no longer active as a chemoattractant. These findings are replicated by the murine ortholog Defb14. We restore the chemoattractant activity of Defb14 and HBD3 by introduction of a single cysteine in the fifth position (Cys V) of the beta-defensin six cysteine motif. In contrast, a peptide with a single cysteine at the first position (Cys I) is inactive. Moreover, a range of overlapping linear fragments of Defb14 do not act as chemoattractants, suggesting that the chemotactic activity of this peptide is not dependent solely on an epitope surrounding Cys V. Full-length peptides either with alkylated cysteine residues or with cysteine residues replaced with alanine are still strongly antimicrobial. Defb14 peptide fragments were also tested for antimicrobial activity, and peptides derived from the N-terminal region display potent antimicrobial activity. Thus, the chemoattractant and antimicrobial activities of beta-defensins can be separated, and both of these functions are independent of intramolecular disulfide bonds. These findings are important for further understanding of the mechanism of action of defensins and for therapeutic design.

Antimicrobial peptides in human skin disease

The skin continuously encounters microbial pathogens. To defend against this, cells of the epidermis and dermis have evolved several innate strategies to prevent infection. Antimicrobial peptides are one of the primary mechanisms used by the skin in the early stages of immune defense. In general, antimicrobial peptides have broad antibacterial activity against gram-positive and negative bacteria and also show antifungal and antiviral activity. The antimicrobial activity of most peptides occurs as a result of unique structural characteristics that enable them to disrupt the microbial membrane while leaving human cell membranes intact. However, antimicrobial peptides also act on host cells to stimulate cytokine production, cell migration, proliferation, maturation, and extracellular matrix synthesis. The production by human skin of antimicrobial peptides such as defensins and cathelicidins occurs constitutively but also greatly increases after infection, inflammation or injury. Some skin diseases show altered expression of antimicrobial peptides, partially explaining the pathophysiology of these diseases. Thus, current research suggests that understanding how antimicrobial peptides modify susceptibility to microbes, influence skin inflammation, and modify wound healing, provides greater insight into the pathophysiology of skin disorders and offers new therapeutic opportunities.

beta-Defensins chemoattract macrophages and mast cells but not lymphocytes and dendritic cells: CCR6 is not involved

beta-Defensins are natural peptide antibiotics whose immunomodulatory functions are poorly understood. In the present study, macrophages were found to migrate to human beta-defensins (HBD)-1 to -4 using Galpha(i) proteins as well as MAPK ERK, p38 and JNK as signal transducers. In addition, mast cells responded to HBD-1 to -4 with calcium fluxes as well as chemotaxis, which increased upon stimulation with IgE plus antigen or ionomycin. In contrast, human beta-defensins were unable to induce migration of memory lymphocytes and dendritic cells (DC). Similar to HBD, the murine beta-defensin (mBD)-8 mobilized macrophages and lacked the ability to recruit memory T cells. These findings were unexpected as CCR6 on memory T cells and DC has been previously observed to be a receptor for human beta-defensins. In support of our findings, however, RBL-2H3 as well as 300.19 cells stably expressing CCR6 proved to be unresponsive to HBD-2 and -3. Intriguingly, our observation of a PKC-independent homologous desensitization between HBD-1 to -4 suggests a common receptor for HBD. In summary, chemoattraction of macrophages and mast cells is evolutionary conserved within the beta-defensin family despite a considerable sequence variation and distinct antimicrobial activities. However, CCR6 is not a functional receptor for beta-defensins.

Report on the ISBTC mini-symposium on biologic effects of targeted therapeutics

The International Society for Biologic Therapy of Cancer held a mini-symposium on October 26, 2006 in Los Angeles to review current information regarding the biologic effects of both standard and targeted therapies. The purpose of the mini-symposium was to describe the existing knowledge regarding various biologic effects of current therapies, identify the most relevant issues and gaps in the knowledge base and discuss the optimal means of obtaining necessary missing information. Topics discussed included: (1) The impact of antitumor monoclonal antibody therapy on antigen presentation and adaptive immunity; (2) the effects of antiangiogenic/targeted therapy of the immune system; (3) the impact of chemotherapy on angiogenesis and immune function; (4) combination of antiangiogenic and immunotherapy at the clinical level; (5) the effects of tyrosine kinase inhibitors on TH1/TH2 response and T-regulatory cells; (6) the impact of farnesyltransferase inhibitors and other targeted agents on T-cell activation; (7) the impact of epigenetic modulators on biologic properties, and (8) the impact of the nature of cell death on the immune system. The ultimate goals of this mini-symposium were to use the above information to inform and influence basic science efforts and discussions, rationally design combination treatment regimens and optimally employ correlative studies in the context of ongoing and future clinical investigations.

Dendritic cells support angiogenesis and promote lesion growth in a murine model of endometriosis

Endometriosis affects 10-15% of women and is associated with pelvic pain and infertility. Angiogenesis plays an essential role in its pathogenesis. Dendritic cells (DCs) were recently implicated in supporting tumor angiogenesis. As both tumors and endometriosis lesions depend on angiogenesis, we investigated the possibility that DCs may also play a role in endometriosis. We induced endometriosis in 8-wk-old female C57BL/6 mice by implantation of autologous endometrium into the peritoneal cavity. We observed an abundance of CD11c(+) DCs infiltrating sites of angiogenesis in endometriosis lesions. We noticed a similar pattern of infiltrating DCs at sites of angiogenesis in the peritoneal Lewis lung carcinoma tumor model. These DCs were immature (major histocompatability complex class II(low)) and expressed vascular endothelial growth factor receptor 2. Peritoneal implanted bone marrow-derived DCs (BMDCs) incorporated into both endometriosis lesions and into B16 melanoma tumors and enhanced their growth at 8 days compared with controls (5.1+/-2.5 vs. 1.5+/-0.5 mm(2), n=4 and 4, P<0.0001 for endometriosis; 67.6+/-15.1 vs. 22.7+/-14.6 mm(2), n=5 and 7, P=0.0004 for mouse melanoma). Finally, immature BMDCs but not mature BMDCs enhanced microvascular endothelial cell migration in vitro (219+/-51 vs. 93+/-32 cells, P=0.02). Based on these findings, we suggest a novel role for DCs in supporting angiogenesis and promoting lesion growth both in endometriosis and in tumors.

Generation and characterization of an ascitogenic mesothelin-expressing tumor model

BACKGROUND

Intraperitoneal tumors expressing high amounts of mesothelin such as malignant mesothelioma and ovarian cancers tend to develop ascites and result in significant morbidity and mortality in the patient. A suitable preclinical intraperitoneal model will assist in the illustration of the mechanisms of molecular oncogenesis and facilitate in addressing issues related to early screening, diagnosis, and therapy for intraperitoneal tumors.

METHODS

In the current study, an ascitogenic malignant tumor model (WF-3) was created. The mobility and proliferation of WF-3 and its precursor cells, WF-0, were characterized using transwell and MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assays. In addition, the in vivo tumorgenicity of WF-3 and WF-0 was determined using intraperitoneal injection of the tumor cells. Microarray analysis was performed using WF-3 and WF-0. Northern blot analysis was used to characterize the expression of the mesothelin gene in WF-3 and WF-0. Furthermore, the mesothelin levels in serum and ascites were used to correlate with tumor load of WF-3 in tumor challenged mice.

RESULTS

The WF-3 tumor cells demonstrated relatively high proliferation and migration rates compared with the parental cell line, WF-0. The tumors from the WF-3 but not WF-0 were capable of forming ascites and peritoneal-based tumors after tumor challenge. The WF-3 tumor model was also capable of implanting into multiple organs including the diaphragm, intestines, and peritoneal wall. Furthermore, the WF-3 tumor expressed high levels of mesothelin, which is commonly observed in the majority of ovarian cancers, pancreatic cancer, and malignant mesothelioma. In addition, the authors found that the serum and ascites mesothelin levels correlated with tumor loads in tumor-challenged mice.

CONCLUSIONS

The data indicate that the WF-3 murine tumor model may potentially serve as a good model for understanding the molecular oncogenesis of peritoneal tumors. In addition, the preclinical model may potentially be useful for the development of diagnostic and therapeutic methods against intraperitoneal cancers.

Dendritic cell–endothelial cell cross-talk in angiogenesis

Dendritic cells (DCs) are professional antigen-presenting cells that have a pivotal role in the onset and regulation of adaptive immune responses. DCs have the ability to regulate inflammation through their capacity to release cytokines and chemokines and kill pathogens, which they share with other phagocytes. Recent observations have shown that different DC subsets produce and release various pro- and anti-angiogenic mediators depending on their activation status and cytokine milieu. In particular, alternatively activated DCs exert a potent pro-angiogenic activity that is mediated by the prototypic angiogenic growth factor vascular endothelial growth factor-A (VEGF-A). In turn, pro- and anti-angiogenic mediators can affect the biology of DCs, modulating their differentiation and maturation. Finally, DCs can trans-differentiate into endothelial-like cells, possibly contributing to vasculogenesis in the adult. Thus, DCs might exert an important impact on the neovascularization process in different physiopathological conditions.

The host defence peptide LL-37/hCAP-18 is a growth factor for lung cancer cells

Cancer development can be viewed as dysregulated repair. Antimicrobial peptides (AMPs) are effector molecules of the innate immune system with direct antimicrobial activity. Beside this host defence function several AMPs play a role in the regulation of inflammation and tissue repair. The aim of the present study was to investigate whether the human cathelicidin AMP LL-37/hCAP-18 is involved in the biology of lung cancer. Human cancer cell lines were found to express the human cathelicidin LL-37/hCAP-18 mRNA and peptide at different levels. Immunohistochemistry of human lung cancers showed that the peptide is expressed mostly in adenocarcinoma and squamous cell carcinoma. Application of exogenous LL-37 at low concentrations of 5ng/ml to cancer cell lines increased proliferation and growth of anchorage-independent colonies. At the molecular level, LL-37 induced phosphorylation of the epidermal growth factor receptor (EGFR) and activation of downstream MAP kinase signalling pathways. Lung cancer cell lines that stably overexpress the peptide by means of a doxycycline-regulated promoter system also showed a faster growth. When these cell lines were injected subcutaneously into nude mice, cathelicidin overexpression resulted in increased tumourigenicity and the formation of significantly larger tumours. In conclusion, cathelicidin is expressed in human lung cancers. The peptide activates tumour cells resulting in increased cell growth in vitro and in an animal model. The host defence peptide cathelicidin LL-37/hCAP-18 acts as growth factor for human lung cancer.

Biologic properties of endothelial progenitor cells and their potential for cell therapy

Recent studies indicate that portions of ischemic and tumor neovasculature are derived by neovasculogenesis, whereby bone marrow (BM)-derived circulating endothelial progenitor cells (EPCs) home to sites of regenerative or malignant growth and contribute to blood vessel formation. Recent data from animal models suggest that a variety of cell types, including unfractionated BM mononuclear cells and those obtained by ex vivo expansion of human peripheral blood or enriched progenitors, can function as EPCs to promote tissue vasculogenesis, regeneration, and repair when introduced in vivo. The promising preclinical results have led to several human clinical trials using BM as a potential source of EPCs in cardiac repair as well as ongoing basic research on using EPCs in tissue engineering or as cell therapy to target tumor growth.

Scavenger receptor-A-targeted leukocyte depletion inhibits peritoneal ovarian tumor progression

Immunosuppressive leukocytes are emerging as a critical factor in facilitating tumor progression. These leukocytes are converted by the tumor microenvironment to become tolerogenic, facilitate metastasis, and to aid in neovascularization. The predominant variety of suppressive leukocytes found in human and murine ovarian cancer are called vascular leukocytes (VLC), due to sharing functions and cell surface markers of both dendritic cells and endothelial cells. Using the ID8 murine model of ovarian cancer, the aim of this study was to test the efficacy of VLC elimination as an ovarian tumor therapy. We show that carrageenan-mediated depletion of peritoneal tumor-associated leukocytes inhibits ovarian tumor progression. We then identified scavenger receptor-A (SR-A) as a cell surface receptor that is robustly and specifically expressed within human and murine ovarian tumor ascites upon VLCs. Administration of anti-SR-A immunotoxin to mice challenged with peritoneal ID8 tumors eliminated tumor-associated VLCs and, importantly, substantially inhibited peritoneal tumor burden and ascites accumulation. Moreover, the toxin required targeting to SR-A because mice that received untargeted toxin did not exhibit inhibition of tumor progression. We conclude that SR-A constitutes a novel and specific target for efficacious immunotherapeutic treatment of peritoneal ovarian cancer.

Control of human mesothelin-expressing tumors by DNA vaccines

Mesothelin has been implicated as a potential ideal target antigen for the development of antigen-specific cancer immunotherapy for the control of mesothelin-expressing cancers such as ovarian cancer, mesothelioma and pancreatic adenocarcinoma. In the current study, we utilized a DNA vaccine encoding human mesothelin (pcDNA3-Hmeso) to treat C57BL/6 mice challenged with luciferase-expressing, Hmeso-expressing ovarian cancer cell line, Defb29 Vegf-luc/Hmeso. The therapeutic effect of the tumor-challenged mice was followed by noninvasive bioluminescence imaging systems. The mechanism of the antitumor effect was characterized by depletion of subsets of lymphocytes as well as adopted transfer of serum from pcDNA3-Hmeso-vaccinated mice. We found that vaccination with pcDNA3-Hmeso DNA vaccine generates a significant antitumor effect and promotes survival in mice challenged with Defb29 Vegf-luc/Hmeso. Furthermore, we found CD4+ and CD8+ T-cell immune responses as well as the humoral immune responses are important for the observed antitumor effects in vaccinated mice. Our data indicated that vaccination with DNA vaccine targeting Hmeso could generate potent antitumor effects against mesothelin-expressing tumors through both T cell-mediated immunity as well as antibody-mediated immunity.

Pretreatment with recombinant human vascular endothelial growth factor reduces virus replication and inflammation in a perinatal lamb model of respiratory syncytial virus infection

Vascular endothelial growth factor (VEGF) is increasingly recognized as a perinatal regulator of lung maturation and surfactant protein expression. Preterm and young infants are at increased risk for pulmonary immaturity characterized by insufficient surfactant production as well as increased risk for severe manifestations of respiratory syncytial virus (RSV) infection. Innate immune components including surfactant proteins A and D, and beta-defensins have putative antimicrobial activity against pulmonary pathogens including RSV. Our hypothesis was that recombinant human VEGF (rhVEGF) pretreatment therapy would decrease RSV disease in the perinatal lamb RSV model. Newborn lambs were pretreated with rhVEGF, betamethasone, or saline and then inoculated with bovine RSV or sterile medium. Tissues were collected 5 d postinoculation, corresponding to the initiation of severe lesions and peak viral replication. In RSV-infected lambs, rhVEGF therapy increased the mean daily body temperature, decreased airway neutrophil exudate, and reduced RSV replication compared with betamethasone or saline pretreatment. Furthermore, rhVEGF therapy significantly mitigated the RSV-induced increase in surfactant protein A mRNA expression and decrease in surfactant protein D mRNA expression. In control (non-RSV-infected) lambs, pretreatment with rhVEGF increased sheep beta-defensin-1 (SBD1) mRNA expression, but no alteration in surfactant proteins A and D was detected. This novel study demonstrates that rhVEGF pretreatment mitigates RSV disease and, in addition, rhVEGF regulation of innate immune genes is dependent on RSV infection status.

Differentiation of human tumour-associated dendritic cells into endothelial-like cells: an alternative pathway of tumour angiogenesis

Until recently, the only accepted mechanism of tumour vascularization was the sprouting of endothelial cells (EC) from pre-existing vessels, while recent studies suggest the contribution of stem cell-derived endothelial progenitors as well as cells from the myeloid lineage. Here, we show a new way of endothelial differentiation that involves the specific modulation of monocytes by the tumour environment. The tumour milieu is characterized by the presence of cytokines and lactate which induce the differentiation of tumour-invading monocytes into tumour-associated dendritic cells (DC). Additional incubation of tumour-associated DC with pro-angiogenic factors, such as vascular endothelial growth factor and oncostatin M, led to transdifferentiation into endothelial-like cells. The cells showed strong expression of von Willebrand factor and VE-Cadherin, both classical EC markers, while leukocytic markers were reduced. In addition, they were able to form network-like structures on matrigel, which could be blocked by the DNA-based drug Defibrotide. This finding may be of great therapeutic relevance for tumour therapy.

Defensins induce the recruitment of dendritic cells in cervical human papillomavirus-associated (pre)neoplastic lesions formed in vitro and transplanted in vivo

In addition to their direct antimicrobial activity, defensins might also influence adaptive immunity by attracting immature dendritic cells (DC). As these cells have been shown to be deficient in uterine cervix carcinogenesis, we evaluated the ability of alpha-defensin (HNP-2, human neutrophil defensin 2) and beta-defensin (HbetaD2, human beta defensin 2) to stimulate their migration in human papillomavirus (HPV)-associated (pre)cancers. We first observed, using RT-PCR and immunohistology, that HbetaD2 is absent in HPV-transformed keratinocytes and that it is weakly expressed in cervical (pre)neoplastic lesions in comparison with normal keratinocytes. We next demonstrated that defensins exert a chemotactic activity for DC in a Boyden Chamber assay and stimulate their infiltration in an in vitro-formed (pre)neoplastic epithelium (organotypic culture of HPV-transformed keratinocytes). To evaluate the ability of defensins also to recruit DC in vivo, we developed a model of immunodeficient mice grafted with organotypic cultures of HPV+ keratinocytes, which form an epithelium similar to a high-grade neoplastic lesion, with tumoral invasion and neovascularization. Intravenously injected human DC were able to infiltrate grafts of HPV+ keratinocytes after administration of HNP-2 in the transplantation chamber. Taken together, these results suggest that defensins could reverse a frequent immune alteration observed in cancer development.

Control of mesothelin-expressing ovarian cancer using adoptive transfer of mesothelin peptide-specific CD8+ T cells

Cancer immunotherapy targeting mesothelin represents a potentially plausible approach for the control of ovarian cancer as most ovarian cancers express high levels of mesothelin. In the current study, we created a mesothelin-positive luciferase-expressing ovarian cancer model, MOSEC/luc. This luciferase-expressing tumor model allowed us to quantitate tumor distribution and tumor load in tumor-challenged mice using a non-invasive bioluminescence imaging system. In addition, we identified an H-2D(b)-restricted mesothelin peptide-specific cytotoxic T-lymphocyte (CTL) epitope (amino acid (aa) 406-414) that was endogenously processed and presented by MOSEC/luc tumor cells. We showed that adoptive transfer of mesothelin peptide (aa406-414)-specific CD8(+) T cells led to the control of MOSEC/luc tumor cells. The MOSEC/luc tumor model and the newly identified H-2D(b)-restricted murine mesothelin-specific CTL epitope (aa406-414) will be very useful for the development of immunotherapy for ovarian cancer as well as for the development of quantitative CD8(+) T cell-mediated immunological assays.

Pathophysiology of tumor neovascularization

Neovascularization is essential to the process of development and differentiation of tissues in the vertebrate embryo, and is also involved in a wide variety of physiological and pathological conditions in adults, including wound repair, metabolic diseases, inflammation, cardiovascular disorders, and tumor progression. Thanks to cumulative studies on vasculature, new therapeutic approaches have been opened for us to some life-threatening diseases by controlling angiogenesis in the affected organs. In cancer therapy, for example, modulation of factors responsible for tumor angiogenesis may be beneficial in inhibiting of tumor progression. Several antiangiogenic approaches are currently under preclinical trial. However, the mechanisms of neovascularization in tumors are complicated and each tumor shows unique features in its vasculature, depending on tissue specificity, angiogenic micromilieu, grades and stages, host immunity, and so on. For better understanding and effective therapeutic approaches, it is important to clarify both the general mechanism of angiogenic events and the disease-specific mechanism of neovascularization. This review discusses the general features of angiogenesis under physiological and pathological conditions, mainly in tumor progression. In addition, recent topics such as contribution of the endothelial progenitor cells, tumor vasculogenic mimicry, markers for tumor-derived endothelial cells and pericytes, and angiogenic/angiostatic chemokines are summarized.

Intratumoral cytokines/chemokines/growth factors and tumor infiltrating dendritic cells: friends or enemies?

The tumor microenvironment consists of a variable combination of tumor cells, stromal fibroblasts, endothelial cells and infiltrating leukocytes, such as macrophages, T lymphocytes, and dendritic cells. A variety of cytokines, chemokines and growth factors are produced in the local tumor environment by different cells accounting for a complex cell interaction and regulation of differentiation, activation, function and survival of multiple cell types. The interaction between cytokines, chemokines, growth factors and their receptors forms a comprehensive network at the tumor site, which is primary responsible for overall tumor progression and spreading or induction of antitumor immune responses and tumor rejection. Although the general thought is that dendritic cells are among the first cells migrating to the tumor site and recognizing tumor cells for the induction of specific antitumor immunity, the clinical relevance of dendritic cells at the site of the tumor remains a matter of debate regarding their role in the generation of successful antitumor immune responses in human cancers. While several lines of evidence suggest that intratumoral dendritic cells play an important role in antitumor immune responses, understanding the mechanisms of dendritic cell/tumor cell interaction and modulation of activity and function of different dendritic cell subtypes at the tumor site is incomplete. This review is limited to discussing the role of intratumoral cytokine network in the understanding immunobiology of tumor-associated dendritic cells, which seems to possess different regulatory functions at the tumor site.