Antitumor effects of a monoclonal antibody that binds anionic phospholipids on the surface of tumor blood vessels in mice

Radiation-enhanced vascular targeting of human lung cancers in mice with a monoclonal antibody that binds anionic phospholipids

PURPOSE

New treatment strategies aimed at damaging tumor vasculature could potentially improve tumor response to radiation therapy. We recently showed that anionic phospholipids, principally phosphatidylserine, are specifically exposed on the luminal surface of tumor blood vessels. Here we tested the hypothesis that radiation therapy can increase phosphatidylserine exposure on lung tumor vasculature, thereby enhancing the antitumor properties of the anti-phosphatidylserine antibody 2aG4.

EXPERIMENTAL DESIGN

The therapeutic efficacy of radiation therapy plus 2aG4 was tested in nude mice bearing radiation-resistant A549 human lung tumors. Radiation-induced phosphatidylserine exposure on endothelial cells and A549 tumor cells was analyzed by immunofluoresence staining. The mechanism of the enhanced antitumor effect was examined by histology and antibody-dependent cell-mediated cytotoxicity experiments.

RESULTS

Focal irradiation of A549 human lung cancer xenografts increased the percentage of tumor vessels with exposed phosphatidylserine from 4% to 26%. Treatment of mice bearing A549 tumors with 2aG4 plus focal radiation therapy inhibited tumor growth by 80% and was superior to radiation therapy or 2aG4 alone (P < 0.01). Combination therapy reduced blood vessel density and enhanced monocyte infiltration into the tumor mass beyond that observed with individual treatments. In vitro, 2aG4 enhanced the ability of macrophages to kill endothelial cells with exposed phosphatidylserine in an Fc'-dependent manner.

CONCLUSION

These results suggest that 2aG4 enhances the antitumor effects of radiation therapy by increasing antibody-dependent cell-mediated cytotoxicity toward tumor vessels with externalized phosphatidylserine. Bavituximab, a chimeric version of 2aG4 in clinical trials, has the potential to enhance the therapeutic efficacy of radiation therapy in lung cancer patients.

Apoptosis assays with lymphoma cell lines: problems and pitfalls

Much attention has been focused on the manner in which tumour cells die after treatment with cytotoxic agents. The basic question is whether cells die via apoptosis or via direct damage from the toxic agent. Various assays have been used to make this distinction. However, we show herein that some of the widely used assays for apoptosis do not in fact distinguish between apoptosis and other forms of cell death. More specifically: (1) A sub-G₁ DNA content, identified by propidium iodide staining, does not distinguish between apoptotic and necrotic cells; (2) loss of mitochondrial membrane potential does not distinguish between apoptotic and necrotic cells, unless combined with an assay for an intact cell membrane; (3) subcellular fragments that arise from dead cells or from apoptotic bodies can interfere with some assays for apoptosis such as annexin V staining, as they may be close to the size of intact cells, making it difficult to decide where to set the size threshold; (4) irradiated cells display a large increase in nonspecific Ab binding. This may be partly due to an increase in cell size, but, regardless of the cause, it can lead to a mistaken conclusion that there is an increase in a particular antigen if appropriate control reagents are not tested; and (5) experiments utilising Ab crosslinking have neglected the role of cell aggregation, which can cause multiple problems including death from mechanical stress when cells are handled. Consideration of these factors will improve our ability to determine the mode of cell death.

Characterization of antibody-containing vesicles shed from B-lymphoma cell lines: exposure of annexin V binding sites

Antibodies (Abs) to CD20 or HLA-DR, after binding to the B-lymphoma cell line RL following an overnight incubation at 37 degrees C, accumulate in the form of shed vesicles, which develop in the center of the cell clusters that are spontaneously formed by this cell line. These vesicles coalesce into fairly stable large structures, which we refer to as conglomerates of shed vesicles (CSVs). In the present study, we have extended our previous investigations into the nature of this material. Electron microscopy revealed a conglomerate of heterogeneous vesicles, which looked like pinched-off cytoplasmic projections. CSVs developed similarly either with or without Ab, demonstrating that CSV production is a spontaneous process that incorporates bound Abs if they are present. Before delivery to CSVs, the Abs capped on the cell surface. CSVs had high expression of annexin V binding sites, which are phagocytic signals that are exposed on damaged cells. For CSVs that were cell bound, which are frequently observed, the annexin V binding sites were only in the CSVs, and not on the surface of the intact cell. Although all CSVs contained both Abs and annexin V binding sites, the precise distribution of these two ligands was generally different. Annexin V binding sites were present on caps as well as on CSVs, and appear as soon as caps are formed. In cells incubated with anti-HLA-DR, CD20 was delivered to the CSVs together with HLA-DR, suggesting an association between these two molecules. CSVs prepared with anti-HLA-DR, but not CSVs prepared with anti-CD20, contained considerable numbers of nuclear fragments, identified by propidium iodide staining.

Clearance of apoptotic and necrotic cells and its immunological consequences

The ultimate and most favorable fate of almost all dying cells is engulfment by neighboring or specialized cells. Efficient clearance of cells undergoing apoptotic death is crucial for normal tissue homeostasis and for the modulation of immune responses. Engulfment of apoptotic cells is finely regulated by a highly redundant system of receptors and bridging molecules on phagocytic cells that detect molecules specific for dying cells. Recognition of necrotic cells by phagocytes is less well understood than recognition of apoptotic cells, but an increasing number of recent studies, which are discussed here, are highlighting its importance. New observations indicate that the interaction of macrophages with dying cells initiates internalization of the apoptotic or necrotic targets, and that internalization can be preceded by "zipper"-like and macropinocytotic mechanisms, respectively. We emphasize that clearance of dying cells is an important fundamental process serving multiple functions in the regulation of normal tissue turnover and homeostasis, and is not just simple anti- or pro-inflammatory responses. Here we review recent findings on genetic pathways participating in apoptotic cell clearance, mechanisms of internalization, and molecules involved in engulfment of apoptotic versus necrotic cells, as well as their immunological consequences and relationships to disease pathogenesis.

Ocular immune privilege is circumvented by CD4+ T cells, leading to the rejection of intraocular tumors in an IFN-{gamma}-dependent manner

Although intraocular tumors reside in an immune-privileged site, they can circumvent immune privilege and undergo rejection, which typically follows one of two pathways. One pathway involves CD4(+) T cells, delayed-type hypersensitivity (DTH), and the culmination in ischemic necrosis of the tumor and phthisis (atrophy) of the eye. The second pathway is DTH-independent and does not inflict collateral injury to ocular tissues, and the eye is preserved. In this study, we used a well-characterized tumor, Ad5E1, to analyze the role of IFN-gamma in the nonphthisical form of intraocular tumor rejection. The results showed that IFN-gamma induced tumor cell apoptosis, inhibited tumor cell proliferation, and promoted rejection by inhibiting angiogenesis. Microarray analysis revealed that IFN-gamma induced up-regulation of five antiangiogenic genes and down-regulation of four proangiogenic genes in Ad5E1 tumor cells. Although IFN-gamma knockout (KO) mice have progressively growing intraocular tumors, IFN-gamma was not needed for the elimination of extraocular tumors, as all IFN-gamma KO mice rejected s.c. tumor inocula. This represents a heretofore unrecognized role for IFN-gamma in circumventing ocular immune privilege and eliminating intraocular tumors. The findings also reveal that some IFN-gamma-independent tumor rejection processes are excluded from the eye and may represent a new facet of ocular immune privilege.

Plasma protein beta-2-glycoprotein 1 mediates interaction between the anti-tumor monoclonal antibody 3G4 and anionic phospholipids on endothelial cells

A promising target on tumor vasculature is phosphatidylserine (PS), an anionic phospholipid that resides exclusively on the inner leaflet of the plasma membrane of resting mammalian cells. We have shown previously that PS becomes exposed on the surface of endothelial cells (EC) in solid tumors. To target PS on tumor vasculature, the murine monoclonal antibody 3G4 was developed. 3G4 localizes to tumor vasculature, inhibits tumor growth, and enhances anti-tumor chemotherapies without toxicity in mice. A chimeric version of 3G4 is in clinical trials. In this study, we investigated the basis for the interaction between 3G4 and EC with surface-exposed PS. We demonstrate that antibody binding to PS is dependent on plasma protein beta-2-glycoprotein 1 (beta2GP1). beta2GP1 is a 50-kDa glycoprotein that binds weakly to anionic phospholipids under physiological conditions. We show that 3G4 enhances binding of beta2GP1 to EC induced to expose PS. We also show that divalent 3G4-beta2GP1 complexes are required for enhanced binding, since 3G4 Fab' fragments do not bind EC with exposed PS. Finally, we demonstrate that an artificial dimeric beta2GP1 construct binds to EC with exposed PS in the absence of 3G4, confirming that antibody binding is mediated by dimerization of beta2GP1. Together, these data indicate that 3G4 targets tumor EC by increasing the avidity of beta2GP1 for anionic phospholipids through formation of multivalent 3G4-beta2GP1 complexes.

Combination of a monoclonal anti-phosphatidylserine antibody with gemcitabine strongly inhibits the growth and metastasis of orthotopic pancreatic tumors in mice

Pancreatic cancer continues to have a dismal prognosis and novel therapy is needed. In this study, we evaluate a promising new target for therapy, phosphatidylserine (PS). PS is an anionic phospholipid located normally on the inner leaflet of the plasma membrane in mammalian cells. In the tumor microenvironment, PS becomes externalized on vascular endothelium. The monoclonal antibody 3G4 binds PS and promotes an inflammatory response against tumor blood vessels, resulting in reduction of tumor growth. Mice with orthotopic pancreatic tumors were treated with 3G4, gemcitabine or a combination of both drugs. Tumor burden including pancreas weight and metastatic lesions (liver, lymph node and peritoneal) were reduced 3- to 5-fold by the combination therapy as compared with 1.5- to 2-fold with 3G4 and gemcitabine alone, respectively. Treatment of tumor-bearing animals with the combination therapy increased macrophage infiltration into the tumor mass 10-fold and reduced microvessel density in the tumor by 2.5-fold compared with tumors from untreated animals. Gemcitabine alone and 3G4 alone were less effective than the combination of the 2 agents together. The additive therapeutic effect of both agents appears to be because chemotherapy increases PS exposure on tumor vascular endothelium and amplifies the target for attack by 3G4. In conclusion, 3G4 enhanced the anti-tumor and anti-metastatic activity of gemcitabine without contributing to toxicity.

A monoclonal antibody that binds anionic phospholipids on tumor blood vessels enhances the antitumor effect of docetaxel on human breast tumors in mice

Anionic phospholipids, principally phosphatidylserine, become exposed on the external surface of viable vascular endothelial cells in tumors, providing an excellent marker for tumor vascular targeting. We recently raised an IgG monoclonal antibody, 3G4, which binds to anionic phospholipids in a beta2-glycoprotein I-dependent manner. It inhibited tumor growth in a variety of rodent tumor models by stimulating antibody-dependent cellular cytotoxicity toward tumor vessels. In the present study, we tested the hypothesis that docetaxel, which is known to have antivascular effects on tumors, might induce exposure of anionic phospholipids on tumor vasculature and, thus, enhance the antitumor activity of 3G4. Treatment of human umbilical vascular endothelial cells with subtoxic concentrations of docetaxel (20 pmol/L) in vitro caused anionic phospholipids to be externalized without inducing apoptosis. Docetaxel treatment of mice increased the percentage of tumor vessels that expose anionic phospholipids from 35% to 60%. No induction of phosphatidylserine was observed on vessels in normal tissues even after systemic treatment with docetaxel. Treatment of mice bearing orthotopic MDA-MB-435 human breast tumors with 3G4 plus docetaxel inhibited tumor growth by 93%. Treatment of mice bearing disseminated MDA-MB-435 tumors with 3G4 plus docetaxel reduced the average number of tumor colonies in the lungs by 93% and half the animals did not develop tumors. In both tumor models, the antitumor effect of the combination was statistically superior (P < 0.01) to that of docetaxel or 3G4 alone. Combination therapy reduced the tumor vessel density and plasma volume in tumors to a greater extent than did the individual drugs. The combination therapy was no more toxic to the mice than was docetaxel alone. These results indicate that, as an adjuvant therapy, 3G4 could enhance the therapeutic efficacy of docetaxel in breast cancer patients.