Hypoxia-Inducible Factor α Subunits Regulate Tie2-Expressing Macrophages That Influence Tumor Oxygen and Perfusion in Murine Breast Cancer

Tie2-expressing monocytes/macrophages (TEMs) are a distinct subset of proangiogenic monocytes selectively recruited to tumors in breast cancer. Because of the hypoxic nature of solid tumors, we investigated if oxygen, via hypoxia-inducible transcription factors HIF-1α and HIF-2α, regulates TEM function in the hypoxic tumor microenvironment. We orthotopically implanted PyMT breast tumor cells into the mammary fat pads of syngeneic LysMcre, HIF-1α fl/fl /LysMcre, or HIF-2α fl/fl /LysMcre mice and evaluated the tumor TEM population. There was no difference in the percentage of tumor macrophages among the mouse groups. In contrast, HIF-1α fl/fl /LysMcre mice had a significantly smaller percentage of tumor TEMs compared with control and HIF-2α fl/fl /LysMcre mice. Proangiogenic TEMs in macrophage HIF-2α-deficient tumors presented significantly more CD31+ microvessel density but exacerbated hypoxia and tissue necrosis. Reduced numbers of proangiogenic TEMs in macrophage HIF-1α-deficient tumors presented significantly less microvessel density but tumor vessels that were more functional as lectin injection revealed more perfusion, and functional electron paramagnetic resonance analysis revealed more oxygen in those tumors. Macrophage HIF-1α-deficient tumors also responded significantly to chemotherapy. These data introduce a previously undescribed and counterintuitive prohypoxia role for proangiogenic TEMs in breast cancer which is, in part, suppressed by HIF-2α.

Proangiogenic TIE2(+)/CD31 (+) macrophages are the predominant population of tumor-associated macrophages infiltrating metastatic lymph nodes

Tumor-associated macrophages (TAMs) accumulate in various cancers and promote tumor angiogenesis and metastasis, and thus may be ideal targets for the clinical diagnosis of tumor metastasis with high specificity. However, there are few specific markers to distinguish between TAMs and normal or inflammatory macrophages. Here, we show that TAMs localize in green fluorescent protein-labeled tumors of metastatic lymph nodes (MLNs) from B16F1 melanoma cells but not in necrotic tumor regions, suggesting that TAMs may promote the growth of tumor cells and the progression of tumor metastasis. Furthermore, we isolated pure populations of TAMs from MLNs and characterized their gene expression signatures compared to peritoneal macrophages (PMs), and found that TAMs significantly overexpress immunosuppressive cytokines such as IL-4, IL-10, and TGF-β as well as proangiogenic factors such as VEGF, TIE2, and CD31. Notably, immunological analysis revealed that TIE2(+)/CD31(+) macrophages constitute the predominant population of TAMs that infiltrate MLNs, distinct from tissue or inflammatory macrophages. Importantly, these TIE2(+)/CD31(+) macrophages also heavily infiltrated MLNs from human breast cancer biopsies but not reactive hyperplastic LNs. Thus, TIE2(+)/ CD31(+) macrophages may be a unique histopathological biomarker for detecting metastasis in clinical diagnosis, and a novel and promising target for TAM-specific cancer therapy.

A novel angiopoietin-2 selective fully human antibody with potent anti-tumoral and anti-angiogenic efficacy and superior side effect profile compared to Pan-Angiopoietin-1/-2 inhibitors

There is increasing experimental evidence for an important role of Angiopoietin-2 (Ang-2) in tumor angiogenesis and progression. In addition, Ang-2 is up-regulated in many cancer types and correlated with poor prognosis. To investigate the functional role of Ang-2 inhibition in tumor development and progression, we generated novel fully human antibodies that neutralize specifically the binding of Ang-2 to its receptor Tie2. The selected antibodies LC06 and LC08 recognize both rodent and human Ang-2 with high affinity, but LC06 shows a higher selectivity for Ang-2 over Ang-1 compared to LC08 which can be considered an Ang-2/Ang-1 cross-reactive antibody. Our data demonstrate that Ang-2 blockade results in potent tumor growth inhibition and pronounced tumor necrosis in subcutaneous and orthotopic tumor models. These effects are attended with a reduction of intratumoral microvessel density and tumor vessels characterized by fewer branches and increased pericyte coverage. Furthermore, anti-Ang-2 treatment strongly inhibits the dissemination of tumor cells to the lungs. Interestingly, in contrast to the Ang-2/Ang-1 cross-reactive antibody LC08 that leads to a regression of physiological vessels in the mouse trachea, the inhibition with the selective anti-Ang-2 antibody LC06 appears to be largely restricted to tumor vasculature without obvious effects on normal vasculature. Taken together, these data provide strong evidence for the selective Ang-2 antibody LC06 as promising new therapeutic agent for the treatment of various cancers.

Active immunotherapy induces antibody responses that target tumor angiogenesis

The inhibition of VEGF signaling with antibodies or small molecules achieves clinical benefits in diverse solid malignancies. Nonetheless, therapeutic effects are usually not sustained, and most patients eventually succumb to progressive disease, indicating that antiangiogenic strategies require additional optimization. Vaccination with lethally irradiated, autologous tumor cells engineered to secrete granulocyte-macrophage colony stimulating factor (GM-CSF) and antibody blockade of cytotoxic T lymphocyte-associated antigen-4 (CTLA-4) trigger a tumor vasculopathy in some long-term responding subjects. These reactions are characterized by disrupted tumor blood vessels in association with lymphocyte and granulocyte infiltrates and zonal areas of ischemic tumor necrosis. However, the mechanisms underlying this immune-mediated destruction of the tumor vasculature remain to be clarified. Here, we show that GM-CSF-secreting tumor cell vaccines and CTLA-4 blockade elicit a functionally important humoral reaction against multiple angiogenic cytokines. Antibodies to angiopoietin-1 and angiopoietin-2 block Tie-2 binding, downstream signaling, endothelial cell tube formation, and macrophage chemotaxis. Antibodies to macrophage inhibitory factor (MIF) attenuate macrophage Tie-2 expression and matrix metalloproteinase-9 (MMP-9) production. Together, these results delineate an immunotherapy-induced host response that broadly targets the angiogenic network in the tumor microenvironment.

Tie2 expression in human embryonic tissues

Tie2 is a member of receptor tyrosine kinases family, involved in vasculogenesis and angiogenesis. Its main role is to stabilize, maintain, and facilitate the structural adaptation of the vasculature, during embryo development, and adult wound healing, or tumor development. Tissues from human embryos found in different stages of development (5 and 7-week-old), were investigated for Tie2 expression. The reaction was positive, with maximum intensity in the vascular cords, found in the mesenchymal tissue, and in the connective tissue around the primitive spinal cord. In the 7-week-old embryo, the reaction was negative in large blood vessels, and it was heterogeneous in those showing bridging phenomenon. In conclusion, during the first two months of human embryo development, we have concurrent vasculogenesis, angiogenesis, and blood vessel maturation and stabilization.

A stroma targeted therapy enhances castration effects in a transplantable rat prostate cancer model

BACKGROUND

Castration results in a major involution of the normal prostate gland. This process is initiated by effects in the prostate stroma and vasculature. Castration-induced regression of androgen sensitive prostate tumors is however less prominent and hypothetically this could be related to a limited stromal/vascular response. We therefore used animal tumor models to explore the importance of stroma and vascular effects, and if castration effects could be enhanced by a simultaneous therapy targeting the tumor stroma.

METHODS

Using rats with Dunning PAP and H tumors, stereological methods, immunohistochemistry, and Western blotting, we studied the tumor response 7 and 28 days after castration and after the addition of stroma targeted therapies.

RESULTS

In the normal ventral prostate (VP) nuclear androgen receptors (AR) were rapidly downregulated after castration. In contrast, the Dunning tumors downregulated the AR in the cancerous epithelium, but not in the surrounding stroma. Vascular regulators such as the angiopoietins, tie 2, and PDGF-Rbeta were not decreased in the stroma after castration, as observed in the VP, creating an environment that prevents vascular involution. When a tumor stroma targeted therapy inhibiting the tie 2 receptor and the PDGF-Rbeta simultaneously was added to castration it resulted in a decreased vascular density, increased tumor cell apoptosis and decreased tumor growth compared to castration alone.

CONCLUSIONS

The stroma in highly differentiated androgen sensitive Dunning tumors is apparently androgen insensitive. If this unresponsive stroma is targeted the effects of castration can be enhanced.

Immunotherapy of tumors with protein vaccine based on chicken homologous Tie-2

PURPOSE

Tie-2 is an endothelium-specific receptor tyrosine kinase known to play a key role in tumor angiogenesis. The present study explores the feasibility of immunotherapy of tumors by using a protein vaccine based on chicken Tie-2 as a model antigen to break the immune tolerance against Tie-2 in a cross-reaction between the xenogeneic homologous and self-Tie-2.

EXPERIMENTAL DESIGN AND RESULTS

In this study, a chicken homologous Tie-2 protein vaccine (chTie-2) and a corresponding mouse Tie-2 vaccine as a control were prepared and the antitumor effect of these vaccines was tested in two tumor models (murine B16F10 melanoma and murine H22 hepatoma). Immunotherapy with chTie-2 was found effective in two tumor models. Autoantibodies against mouse Tie-2 were detected in sera of mice immunized with chTie-2 through Western blot analysis and ELISA assay. Anti-Tie-2 antibody-producing B cells were detectable by ELISPOT. Histologic examination revealed that autoantibodies were deposited on the endothelial cells of tumor tissues. Purified immunoglobulins from chTie-2-immunized mice could induce the apoptosis of human umbilical vein endothelial cells in vitro. Importantly, adoptive transfer of purified immunoglobulins led to antitumor effect in vivo; apparently, angiogenesis was significantly inhibited in these tumors. Furthermore, the antitumor activity and production of autoantibodies could be abrogated by depletion of CD4+ T lymphocytes.

CONCLUSIONS

Our findings may provide a vaccine strategy for cancer therapy and show the potential utilization of interference with Tie-2 pathway.

The use of reverse immunology to identify HLA-A2 binding epitopes in Tie-2

A potential target for a cancer vaccine would be receptors, such as Tie-2 which are over expressed on tumour endothelium. Using computer aided motif predictions for possible HLA class I epitopes, we have identified peptides from Tie-2 that should bind with a range of affinities to HLA-A*0201. No direct correlation between predicted values and actual binding affinities was observed. Although, the programs did produce a number of false positives, two epitopes were predicted that bound with relatively high affinity when compared with an influenza peptide. We have previously identified a Tie-2 epitope and shown that it was only immunogenic when we substituted preferred amino acids at key anchor residues to increase binding affinity. In this study we used a similar approach to generate modified epitopes. When HLA-A2 transgenic mice were immunised with peptides, CTL killing of the target cells was only achieved when the wild type epitope was presented at moderate levels. Moreover, the efficiency of immunisation was increased when we linked CD4 epitopes to CD8 epitopes. Caution should therefore be employed in the use of both reverse immunology and anchor modification of CTL epitopes in the identification of CTL epitopes for cancer vaccines.

Purification of hematopoietic stem cells using the side population

Hematopoietic stem cells (HSCs) primarily reside in bone marrow, are defined by their ability to maintain blood homeostasis, and replenish themselves through self-renewal. Although HSC purification schemes vary from laboratory to laboratory, the resulting cell populations are similar, if not the same. This chapter will discuss different enrichment methods for HSCs and provide a detailed protocol for staining HSC with Hoechst 33342 for the side population (SP).

[Evaluation of endothelial cells differentiated from mesenchymal stem cells of human bone marrow with Tie-2 monoclonal antibody by immunohistochemistry in vitro]

AIM

To amplify mesenchymal stem cells (MSCs) from human bone marrow (HBM) and to induce MSCs differentiated into endothelial cells (ECs) in vitro, which possibility and conditions were to be discussed.

METHODS

MSCs were separated by gradient centrifugation on Percoll (density 1.073 g/ml) from HBM, and incubated for purification and amplification in DMEM (low glucose) with 10% fetal bovine serum (FBS). MSCs, which their phenotypic characteristics were analyzed by flow cytometry, were incubated for orientation differentiated into ECs in DMEM (high glucose) with 20% FBS and VEGF (10 ng/ml) for about 14 - 21 days. Afterwards, the cells differentiated were evaluated by immunohistochemistry with Tie-2 monoclonal antibody and by transmission electron microscopy (TEM) for observation Weible-palade corpuscle in the cytoplasm.

RESULTS

The quantity of MSCs was increased from 5.0 - 10(5) in the primary culture to 8.0 x 10(12), or to increase 1.6 x 10(7) times after 15 generations incubated. The purity of MSCs was above 95% and 98% homogeneous at passages 2 and 3, respectively. The typical "cobblestone" of these cells presented from MSCs differentiation culture after 14 - 21 days was observed by light microscopy. More than 90% of the cells were positive stain for Tie-2 related antigen by immunohistochemistry assay. The Weible-palade corpuscle, which form is typical morphology of ECs, was also observed by TEM in the cytoplasm.

CONCLUSION

MSCs from HBM has the capability in differentiation into ECs in vitro, which is possible to provide the seed cells for fabrication of tissue-engineering heart valve.

Angiopoietin-like-4 is a potential angiogenic mediator in arthritis

Our previous studies of gene expression profiling during collagen-induced arthritis (CIA) indicated that the putative angiogenic factor Angptl4 was one of the most highly expressed mRNAs early in disease. To investigate the potential involvement of Angptl4 in CIA pathogenesis, Angptl4 protein levels were assessed at early stages of disease and its cellular sources were determined. In addition, the functional effects of mouse Angptl4 on endothelial cells were assessed. Angptl4 protein levels were higher in arthritic joints as compared to normal joints. In situ hybridization localized Angptl4 mRNA to stromal fibroblast-like cells within the inflamed synovium. Temporal expression of Angptl4 mRNA during CIA was similar to that of key angiogenic factors, including structurally related angiopoietin 1. Recombinant mouse Angptl4 promoted endothelial cell survival and formation of tubule-like structures. These functional effects of Angptl4, combined with very high expression at early stages of CIA, suggest a role for Angptl4 in angiogenesis in arthritis.

Phenotypic knockout of VEGF-R2 and Tie-2 with an intradiabody reduces tumor growth and angiogenesis in vivo

The endothelial cell receptor-tyrosine kinases, VEGF receptor 2 (VEGF-R2) and Tie-2, and their ligands, vascular endothelial growth factor (VEGF) and angiopoietins 1 and 2, respectively, play key roles in tumor angiogenesis. Several studies suggest that the VEGF receptor pathway and the Tie-2 pathway are independent and essential mediators of angiogenesis, leading to the hypothesis that simultaneous interference with both pathways should result in additive effects on tumor growth. In this study, a human melanoma xenograft model (M21) was used to analyze the effects of simultaneous intradiabody depletion of vascular endothelial growth receptor-R2 and Tie-2 on tumor angiogenesis and tumor xenograft growth. The intradiabodies were expressed from recombinant adenovirus delivered through subtumoral injection. Blockade of both VEGF-R2 and Tie-2 pathways simultaneously or the VEGF receptor pathway alone resulted in a significant inhibition of tumor growth and tumor angiogenesis (92.2% and 74.4%, respectively). In addition, immunohistochemical staining of intradiabody-treated tumors demonstrated a decreased number of tumor-associated blood vessels versus control treatment. Previous studies with intrabodies had demonstrated that the Tie-2 receptor pathway was essential for tumor growth. The simultaneous blockade of the VEGF and Tie-2 pathways resulted in effective inhibition of tumor growth and demonstrated the potential of simultaneous targeting of multiple pathways as a therapeutic strategy.

Targeting tumor angiogenesis with adenovirus-delivered anti-Tie-2 intrabody

Inhibition of tumor angiogenesis is a promising approach for cancer therapy. As an endothelial cell-specific receptor kinase expressed almost exclusively on the surface of vascular endothelium, Tie-2 has an important role in tumor angiogenesis. To explore the therapeutic potential of blocking Tie-2 receptor-interaction pathway, an adenoviral vector was used to deliver a recombinant single-chain antibody fragment rabbit intrabody (pAd-2S03) capable of inhibition of both mouse and human Tie-2 surface expression. pAd-2S03 was given to mice with well-established primary tumors, either a human Kaposi's sarcoma (SLK) or a human colon carcinoma (SW1222). The intrabody significantly inhibited growth of both tumors (75% and 63%, respectively) when compared with pAd-GFP control-treated tumors (P < 0.01). Histopathologic analysis of cryosections taken from mice treated with pAd-2S03 revealed a marked decrease in vessel density, which was reduced by >87% in both tumor models when compared with control-treated tumors (P < 0.01). In contrast, human Tie-2-monospecific pAd-1S05 intrabody did not affect the growth of tumors, indicating that the antitumor effect of pAd-2S03 was due to the inhibition of tumor angiogenesis in these murine models. Our results show that the Tie-2 receptor pathway is essential for both SLK sarcoma and SW1222 colon carcinoma xenograft growth. The present study shows the potential utility of antiangiogenic agents that target the endothelium-specific receptor Tie-2 for down-regulation or genetic deletion.

Kinetics of vascular normalization by VEGFR2 blockade governs brain tumor response to radiation: role of oxygenation, angiopoietin-1, and matrix metalloproteinases

The recent landmark Phase III clinical trial with a VEGF-specific antibody suggests that antiangiogenic therapy must be combined with cytotoxic therapy for the treatment of solid tumors. However, there are no guidelines for optimal scheduling of these therapies. Here we show that VEGFR2 blockade creates a "normalization window"--a period during which combined radiation therapy gives the best outcome. This window is characterized by an increase in tumor oxygenation, which is known to enhance radiation response. During the normalization window, but not before or after it, VEGFR2 blockade increases pericyte coverage of brain tumor vessels via upregulation of Ang1 and degrades their pathologically thick basement membrane via MMP activation.

[Prokaryotic expression, purification and refolding of extracellular ligand binding domains of chick Tie-2 and its immunogenicity]

OBJECTIVE

To study the prokaryotic expression of extracellular ligand binding domains of chick Tie-2, the purification and refolding conditions of the recombinant protein, and to observe its immunogenicity in mouse.

METHODS

A DNA fragment encoding extracellular ligand binding domains of chick Tie-2 was obtained by PCR from a previous constructed plasmid as a template. The amplified fragment was then inserted into prokaryotic expression vector PQE30, and was expressed in E. coli XL-1 blue by adding isopropyl-beta-D-thiogalactoside(IPTG). The recombinant protein in inclusion bodies was purified by nickel-nitrilotriacetic acid (Ni-NTA) affinity chromatography under denatured conditions. Then the refolding of the purified protein was performed with gradient dialysis. The target protein was injected into mouse subcutaneously, and the antiserum of the mouse was analyzed by ELISA and Western blot analysis.

RESULTS

The recombinant protein was highly expressed in E. coli XL-1 blue, and in mouse it produced the antibody which could specifically recognize the recombinant protein.

CONCLUSION

The protein of extracellular ligand binding domains of chick Tie-2 can be highly expressed in prokaryotic expression system, and the expressed protein can induce immune response in mouse. These findings are very important for the further study of this protein in anti-angiogenesis and immunotherapy research.

Identification of an HLA-A*0201 cytotoxic T lymphocyte epitope specific to the endothelial antigen Tie-2

Tie-2 stabilises pericyte-endothelial interactions during angiogenesis and is highly expressed on endothelium during several diseases, including arthritis, age-related macular degeneration and cancer. A vaccine that targets endothelium overexpressing Tie-2 may result in vessel damage and stimulate an inflammatory cascade resulting in disease regression. We have identified a region unique to Tie-2 (amino acids 1-196) that is homologous in humans and mice. Using computer algorithms, several HLA-A*0201 epitopes that are identical in mice and humans were predicted within this region; however, binding assays showed that the majority of these epitopes were of low affinity. Modification of the anchor residues of 4 epitopes enhanced HLA binding. These epitopes were incorporated by site-directed mutagenesis into a Tie-2 DNA construct. Immunisation of HLA*0201 transgenic mice with one of the modified Tie-2 constructs stimulated CTLs that recognised both wild-type and modified peptide-pulsed target cells. In contrast, no CTLs were generated in mice immunised with wild-type Tie-2 construct, demonstrating that the modified epitope was necessary in the generation of CTLs. Moreover, CTLs from mice immunised with the modified construct killed HLA-A*0201 endothelial cells overexpressing Tie-2. Our study demonstrates that it is possible to break tolerance to the endothelial antigen Tie-2, suggesting that it may be feasible to design a vaccine to activate CTLs to kill endothelial cells overexpressing Tie-2.

Intradiabodies, bispecific, tetravalent antibodies for the simultaneous functional knockout of two cell surface receptors

The specific and high affinity binding properties of intracellular antibodies (intrabodies), combined with their ability to be stably expressed in defined organelles, provides powerful tools with a wide range of applications in the field of functional genomics and gene therapy. Intrabodies have been used to specifically target intracellular proteins, manipulate biological processes, and contribute to the understanding of their functions as well as for the generation of phenotypic knockouts in vivo by surface depletion of extracellular or transmembrane proteins. In order to study the biological consequences of knocking down two receptor-tyrosine kinases, we developed a novel intrabody-based strategy. Here we describe the design, engineering, and characterization of a bispecific, tetravalent endoplasmic reticulum (ER)-targeted intradiabody for simultaneous surface depletion of two endothelial transmembrane receptors, Tie-2 and vascular endothelial growth factor receptor 2 (VEGF-R2). Comparison of the ER-targeted intradiabody with the corresponding conventional ER-targeted single-chain antibody fragment (scFv) intrabodies demonstrated that the intradiabody is significantly more efficient with respect to efficiency and duration of surface depletion of Tie-2 and VEGF-R2. In vitro endothelial cell tube formation assays suggest that the bispecific intradiabody exhibits strong antiangiogenic activity, whereas the effect of the monospecific scFv intrabodies was weaker. These findings suggest that simultaneous interference with the VEGF and the Tie-2 receptor pathways results in at least additive antiangiogenic effects, which may have implications for future drug developments. In conclusion, we have identified a highly effective ER-targeted intrabody format for the simultaneous functional knockout of two cell surface receptors.

Number and adhesive properties of circulating endothelial progenitor cells in patients with in-stent restenosis

OBJECTIVE

Intact endothelialization machinery is essential to facilitate vessel healing after stent placement and to prevent restenosis. Circulating endothelial progenitor cells (EPC) have been demonstrated in the peripheral blood and shown to display endothelial functional properties, along with the ability to traffic to damaged vasculature. We reasoned that robust in-stent intimal growth could be partially related to impaired endothelialization resulting from reduced circulating EPC number or function.

METHODS AND RESULTS

Sixteen patients with angiographically-demonstrated in-stent restenosis were compared with patients with a similar clinical presentation that exhibited patent stents (n=11). Groups were similar with respect to the use of drugs that could potentially influence EPC numbers. Circulating EPC numbers were determined by the colony-forming unit assay, and their phenotype was characterized by endothelial-cell markers. Adhesiveness of EPC from both groups to extracellular matrix and to endothelial cells was also assayed. Patients with in-stent restenosis and with patent stents displayed a similar number of circulating EPC. Fibronectin-binding was compromised in patients with in-stent restenosis as compared with their controls exhibiting patent stents. Patients with diffuse in-stent restenosis exhibited reduced numbers of EPC in comparison with subjects with focal in-stent lesions.

CONCLUSIONS

Reduced numbers of circulating EPC in patients with diffuse in-stent restenosis and impaired adhesion of EPC from patients with restenosis provides a potential mechanism mediating the exuberant proliferative process. These markers, if further validated, could provide means of risk stratifying patients for likelihood of developing in-stent restenosis.

Angiopoietin/Tek interactions regulate mmp-9 expression and retinal neovascularization

The objective of the study was to determine the role of the angiopoietins in the regulation of gelatinase expression during angiogenesis, and whether inhibition of the angiopoietin/Tek interaction in vivo can suppress the extent of retinal neovascularization. Retinal microvascular endothelial cells were treated with angiopoietins and examined for the production of gelatinases. The effects of inhibiting angiopoietin binding to the Tie-2 receptor was studied in newborn mice with experimentally induced retinal neovascularization. Animals were treated with an ip injection of the Tie-2 antagonist, muTek delta Fc, while oxygen-exposed mice treated with similar concentrations of murine IgG were used as controls. The effect of muTek delta Fc on the gelatinase expression in the retina was examined by real-time RT-PCR analysis. The stimulation of cultured retinal endothelial cells with Ang-1 and -2 resulted in the increased expression of matrix metalloproteinase (MMP)-9. Ang-2 expression was up-regulated in experimental animals during the period of angiogenesis and was the greatest on Day 17 (the time of maximal angiogenic response). Histologic analysis of mice treated with the Tie-2 antagonist, muTek delta Fc, showed significant (87%; p = 0.001) inhibition of retinal neovascularization, and the response was dose-dependent. In vitro binding data support the fact that both Ang-1 and Ang-2 bind with high avidity to muTek delta Fc. The RT-PCR analysis of the retinas of the Tek-treated animals showed a similar (80%; p = 0.001) inhibition of the MMP-9 expression, which correlated with the decrease in angiogenesis. The up-regulation of gelatinases in microvascular endothelial cells by Ang-2 may be an important early response during the development of retinal neovascularization. Inhibition of the binding activity of the angiopoietins in vivo suppressed retinal neovascularization concomitant with a reduction in the expression of MMP-9.