Generation of Human Single-chain Antibody to the CD99 Cell Surface Determinant Specifically Recognizing Ewing’s Sarcoma Tumor Cells

The survival of pediatric patients with cancer entities including osteosarcoma and Ewing's sarcoma (ES), remains extremely low hence novel treatment approaches are urgently needed. Therefore, based on the concept of targeted therapy, numerous potential targets for the treatment of these cancers have been evaluated pre-clinically or in some cases even clinically during the last decade. In ES the CD99 protein is an attractive target antigen. In this respect, a new entry site for therapeutic intervention may derive from specific human antibodies against CD99. Human scFvC7 was isolated from a semi-synthetic ETH-2 antibody phage library panned on the extracellular portion of recombinant human CD99 protein. The scFvC7 was genetically sequenced, tested for CD99 recognition on an array of recombinant CD99 fragments and measured for binding affinity by ELISA. Finally, it was tested for staining CD99 antigen on a large panel of tumor and normal cells and tissues by cytofluorimetric and immunohystochemical assays. The new antibody scFvC7 recognizes the CD99 extracellular domain included between residues 50 and 74 with a binding affinity of 2.4 x 10-8 M. In contrast with all other antibodies to CD99 so far isolated, scFvC7 shows a unique specificity in cancer cell recognition: It stained prevalently ES cells while no or weak reactivity was observed on the majority of the other tumor and normal cells and tissues. Thanks to its properties the new anti-CD99 antibody here described represents the first step towards the construction of new selective ES therapeutics.