Murine endoglin-specific single-chain Fv fragments for the analysis of vascular targeting strategies in mice.

Endoglin has been identified as a promising cell surface antigen for vascular targeting approaches in cancer therapy, e.g. employing antibody molecules as targeting moieties. However, in vivo analysis of such strategies in mouse models requires antibodies recognizing endoglin on mouse endothelial cells. Here we describe the isolation of single-chain Fv fragments (scFvs) from phage display libraries, which bind to the extracellular region of mouse endoglin. One of these clones, scFv mE12, showed strong (K(d)=11 nM) and selective binding to purified endoglin and also to the endoglin-expressing mouse endothelioma cell line eEnd.2. This antibody recognized a linear epitope located in the N-terminal region (aa 27-361) of endoglin. Cell binding was further increased by generating a bivalent scFv-Fc fusion protein composed of scFv mE12 and the human gamma1 Fc part. Moreover, scFv mE12 was endowed with an additional cysteine residue in the linker region and applied for the generation of anti-endoglin scFv immunoliposomes capable of selectively binding to endoglin-expressing cells. Thus, anti-mouse endoglin scFv mE12 should be useful to analyze vascular targeting strategies in mice.