Welcome to the website for Ben Seon, PhD and his team. For more information, please feel free to contact us at the Department of Immunology, Roswell Park Cancer Institute, Elm and Carlton Streets, Buffalo, NY 14263.

A. Endoglin (EDG; CD105), an Angiogenesis Marker and Ancillary TGF-β Receptor  
 

a. Introduction – Angiogenesis is a fundamental process by which new blood vessels are formed from existing blood vessels and/or endothelial precursors. Pathological angiogenesis is a hallmark of cancer and various ischemic and inflammatory diseases. Tumor growth and metastasis are angiogenesis-dependent. A major research project in my laboratory is concerned with development of novel anti-angiogenesis and vascular-targeting therapies.

Fig. 1. Endoglin participation in the TGF-β-mediated signal transduction. TGF-β binds the type II TGF-β receptor (TβRII). The ligand (TGF-β)-bound TβRII recruits and transactivates the type I TGF-β receptor (TβRI) which results in the phosphorylation of a glycine serine- rich region termed the GS box (GS). TβRII and activated TβRI phosphorylate serine/threonine residues of the cytosolic domain of EDG. The phosphorylated TβRI induces activation of the downstream pathways. EDG modulates this activation and signal transduction.

EDG is a homodimeric cell membrane glycoprotein that was initially found on human leukemia cells and subsequently on endothelial cells (Haruta, Y. and Seon, B.K.,1986; Gougus, A. and Letarte, M.,1988).

EDG is a proliferation-associated antigen of endothelial cells and expressed more abundantly on the vascular endothelium of tumor tissues than that of normal tissues. In addition, EDG is essential for angiogenesis and an ancillary transforming growth factor-β (TGF-β) receptor (Fig. 1).

We have been studying EDG
 

1) as a target for therapy of cancer and other angiogenesis-associated diseases,

2) as a marker for diagnosis/prognosis of cancer and

3) as a TGF-β receptor.

A key tool in these studies is unique monoclonal antibodies (mAbs) that were generated and characterized in my laboratory. We generated 12 anti-EDG mAbs, termed SN6 series mAbs, and molecular locations of the epitopes defined by these mAbs were mapped. Antigen-binding avidity was determined for most of these mAbs. These mAbs reacted strongly with vascular endothelium of tumor tissues but less so with that of normal tissues. They did not react with tumor cells per se in the tissues. Under proper conditions, some of the mAbs selectively stained angiogenic blood vessels. Studies in vitro and in vivo showed that several of the 12 mAbs possess unique properties. For instance, some of these mAbs weakly cross-react with murine endothelial cells which allows us to use these mAbs for therapeutic studies in murine animal models. In addition, we found that soluble EDG is a useful marker for metastasis and tumor progression in breast and colorectal cancer patients.

b. Suppression of Angiogenesis, Tumor Growth and Metastasis in Mice – The selected cross-reactive anti-EDG mAbs and their immunoconjugates suppressed angiogenesis in mice. They also suppressed growth of human tumors in severe combined immunodeficient (SCID) mice. In addition, they induced regression of preformed established tumors in SCID mice and strongly suppressed metastasis in tumor-bearing immunocompetent mice. In the immunocompetent mice, CD4+ and CD8+ T cells play pivotal roles in the anti-EDG mAb-mediated tumor suppression.

c. Suppression of Age-Related Macular Degeneration (AMD) – AMD is a leading cause of irreversible blindness among old people. It is characterized by the abnormal angiogenesis under or within the eye macular. We are studying potential application of our anti-EDG mAbs for therapy of AMD in collaboration with researchers of Johns Hopkins University and a biopharmaceutical company. In a preliminary study, intravitreal injection of a small dose of an anti-EDG mAb is highly effective for suppressing excessive angiogenesis in the eye of mice.

d. Mechanisms by Which Naked Anti-EDG mAbs Suppress Angiogenesis and Tumors – A partially humanized human/mouse chimeric mAb, c-SN6j, showed strong antibody-dependent cell-medicated cytotoxicity (ADCC) against proliferating human endothelial cells. In addition, SN6j and other SN6 series anti-EDG mAbs showed direct suppression of proliferating endothelial cells without any effector cells. Furthermore, these mAbs synergized with TGF-β in the suppression of the proliferating endothelial cells. Our data suggest that this direct suppression is associated with apoptosis and signal transduction.

e. Preclinical Studies in Nonhuman Primates – A human/mouse chimeric mAb termed c-SN6j was generated in my laboratory. Intravenous administration of c-SN6j into female cynomolgus monkeys caused no significant toxicity in a dose escalation study after six injections for each dose. Immune response to the murine part of c-SN6j was minimal. Several pharmacokinetic parameters of c-SN6j after multiple injections of c-SN6j were determined and these parameters favor clinical application of c-SN6j. These results were confirmed by a larger scale of study in both male and female monkeys.

f. Toward Clinical Therapeutic Application – Recently, I was successful in obtaining a clinical translational research grant for the phase I and phase II clinical trials of c-SN6j for therapy of breast cancer patients. In addition, we are collaborating with a biopharmaceutical company to apply c-SN6j for therapy of cancer patients. Recently we filed an application of IND (Investigational New Drug) approval of c-SN6j to FDA. Currently we possess four US patents and two foreign patents that are directly relevant to the clinical application of c-SN6j and other anti-EDG mAbs.

B. Human B Cell Antigen Receptor

Fig. 2. Cell membrane IgM on the surface of mature B cell is associated with Igα and Igβ molecules. The resulting complex constitutes the B cell antigen receptor complex. Igα and Igβ contain ITAMs in their cytoplasmic tails that mediate signal transduction.

a. Introduction – We previously discovered a novel heterodimeric cell membrane antigen that was strongly expressed on malignant B cells (Okazaki, M. et al., 1989, 1993). This antigen was discovered by using three new mAbs termed SN8, SN8a and SN8b that were generated in my laboratory. Amino acid sequence analysis revealed that this antigen is a human homologue of the Igα/Igβ (mb-1/B29) component of the murine B cell antigen receptor complex (Vasile, S. et al., 1994) (Fig. 2).

At the 5th International Workshop and Conference on Human Leukocyte Differentiation Antigen (November, 1993), CD79a and CD79b were assigned to Igα and Igβ, respectively. At this Workshop-Conference, SN8 was determined to be the only mAb that defines an extracellular epitope of CD79. We did not submit SN8a and SN8b to this Workshop-Conference. SN8 and SN8a react with CD79b while SN8b reacts with CD79a. SN8 activates normal B cells but suppresses growth of malignant B cells. In addition, SN8 suppresses growth of B cell tumors in SCID mice. SN8 has been used for diagnosis of different B cell malignancies in several countries.

b. Therapeutic Application of SN8 – We are collaborating with a major pharmaceutical company to develop SN8 as a new therapeutic agent of patients with B cell malignancies. They showed that drug conjugates of SN8 were highly effective for eradicating B cell tumors in several animal models. We possess one US patent that is directly relevant to this clinical application.

C. Current Research Areas

1. Toward the therapeutic application of c-SN6j to patients with solid tumors. We are moving forward this project in collaboration with a biopharmaceutical company and clinicians of our institute (RPCI). Most of the relevant laboratory research has been performed in my laboratory.

2. Animal model studies to improve therapeutic efficacy of anti-EDG mAbs and understand the underlying mechanisms involving antiangiogenic and antitumor activities of these mAbs in vivo. To facilitate these studies, we have recently generated transgenic mice expressing human EDG.

3. Molecular mechanisms by which anti-EDG mAbs exert antiangiogenic and antitumor activities. We have already identified a few mechanisms but additional mechanisms remain to be clarified. These include the role of anti-EDG mAbs in the TGF-β-mediated signal transduction.

4. Toward application of anti-CD79 mAb SN8 for therapy of patients with B cell malignancies. This project has been moved forward in collaboration with a major pharmaceutical company. In my laboratory, we perform in vitro and animal model studies to understand mechanisms of the activities of SN8 and other anti-CD79 mAbs.