Original article Antibody against murine PECAM-1 inhibits tumor angiogenesis in mice Zhao Zhou 1 , Melpo Christo®dou-Solomidou 1 , Cecilia Garlanda 2 & Horace M. DeLisser 1 1 Pulmonary and Critical Care Division, Department of Medicine, University of Pennsylvania Medical Center, Philadelphia, Pennsylvania, USA; 2 Mario Negri Institute for Pharmacological Research, Milan, Italy Received 15 April 1999; accepted in revised form 30 September 1999 Key words: angiogenesis, cell adhesion, PECAM-1 Abstract Platelet endothelial cell adhesion molecule (PECAM-1/CD31), a member of the immunoglobulin superfamily expressed at high levels on endothelial cells, has been recently implicated in angiogenesis. Although antagonism of PECAM-1 inhibited neovascularization in two dierent animal models of growth factor/chemokine-induced angiogenesis, its participation in tumor angiogenesis has not been established. We therefore investigated its involvement in models of tumor angiogenesis in mice. An antibody against murine PECAM-1 that was shown to block in vitro murine endothelial tube formation inhibited the subcutaneous growth and tumor vascularity of three tumors in mice: A549 human non-small cell lung cancer in SCID mice, B16 murine melanoma in C57BL/6 mice and AB12 murine mesothelioma in Balb/c mice. These studies suggest a possible role for PECAM-1 in the complex process of tumor angiogenesis and provide additional evidence of the importance of endothelial cell adhesion molecules to the formation of new vessels. Introduction Platelet endothelial cell adhesion molecule (PECAM-1/ CD31) is a member of the immunoglobulin gene super- family found on platelets, leukocytes and on endothelial cells where it concentrates at cell±cell borders [1, 2]. It has been reported to mediate cell±cell adhesion through homophilic and heterophilic interactions and to trans- duce intracellular signals that up-regulate the function of integrins on leukocytes and platelets. The regulation of PECAM-1-mediated adhesion or signaling phenomena is still not fully understood but very likely involves activa- tional events such as phosphorylation of its cytoplasmic domain [2±5]. Data indicate that PECAM-1 may play important roles in the recruitment of leukocytes at in¯ammatory sites, cardiovascular development, matu- ration and release of bone marrow leukocytes, T-cell mediated immune responses and platelet function [1, 2]. Recently, a role for PECAM-1 has also been suggested in the process of angiogenesis [6, 7]. For example, in studies done by our group, a polyclonal antibody to human PECAM-1, which cross-reacts with rat PECAM- 1, was found to block in vitro tube formation by rat capillary endothelial cells and cytokine-induced rat cor- neal neovascularization. These ®ndings were not limited to the rat endothelium as two monoclonal antibodies against murine PECAM-1 in mice prevented vessel growth into subcutaneously implanted gels supplemented with bFGF. Although these data showed that blockade of PECAM-1 could inhibit angiogenesis in two very dier- ent animal models of growth factor/chemokine-induced angiogenesis, the role of PECAM-1 in the more complex process of tumor-induced angiogenesis [8] where ne- ovascularization arises from the interactions of multiple stimulatory factors (i.e. vascular endothelial cell growth factor [VEGF], basic FGF, IL-8, angiogenin, etc.) as well as inhibitors of angiogenesis (i.e. angiostatin, endostatin, thrombospondin, etc.) is currently not de®ned. We therefore investigated the participation of PECAM-1 in murine models of tumor angiogenesis. It was observed that an antibody against PECAM-1 that inhibited in vitro murine endothelial cell tube formation also inhibited the growth and vascularity of three tumors grown subcutaneously in mice. These data provide evidence of a role for PECAM-1 in tumor angiogenesis and further emphasize the importance of endothelial cell adhesion molecules in the formation of new vessels. Materials and methods Antibodies The following monoclonal antibodies (mAbs) were used: rat anti-mouse antibodies, Mec 13.3 [9] and 390 [10], Correspondence to: Horace M. DeLisser, MD, 806 BRB II/III, 421 Curie Blvd., Philadelphia, PA 19104-6160, USA. Tel.: +1-215-573- 9916; Fax: +1-215-573-4469; E-mail: delisser@mail.med.upenn.edu Angiogenesis 3: 181±188, 1999. 181 Ó 2000 Kluwer Academic Publishers. Printed in the Netherlands.