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The Original Pathologische Anatomie Leiden-Endothelium Monoclonal Antibody Recognizes a Vascular Endothelial Growth Factor–Binding Site within Neuropilin-1

¹ The University of Texas M.D. Anderson Cancer Center, Houston, Texas and ² Academic Medical Center, University of Amsterdam, Amsterdam, the Netherlands

Requests for reprints: Wadih Arap or Renata Pasqualini, The University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030. Phone: 713-792-3873; Fax: 713-745-2999; E-mail: warap{at}mdanderson.org or rpasqual{at}mdanderson.org.

For two decades, the antigen recognized by the Pathologische Anatomie Leiden-Endothelium (PAL-E) monoclonal antibody, a standard vascular endothelial cell marker, has remained elusive. Here, we used a combinatorial phage display–based approach ("epitope mapping") to select peptides binding to the original PAL-E antibody. We found that a subset of the selected panel of peptides had motifs with strong homology to an exposed site within the b1 domain of human neuropilin-1 (NRP-1). We confirmed peptide binding by ELISA and by surface plasmon resonance. We also showed that the PAL-E antigen colocalizes with NRP-1 staining in endothelial cells. Crystal structure of the b1 domain in NRP-1 suggests that the PAL-E binding site overlaps with a vascular endothelial growth factor (VEGF) binding site. Taken together, these results indicate that NRP-1 is an endothelial cell antigen recognized by the true PAL-E antibody. The consistent biochemical, morphologic, and functional features between the PAL-E antigen and NRP-1 support our interpretation. Given that NRP-1 is a VEGF receptor, these results explain the attributes of the PAL-E antibody as a marker of vascular permeability and angiogenesis. [Cancer Res 2007;67(20):9623–9]