Cytotoxic antibody fragments for eliminating undifferentiated human embryonic stem cells

Human embryonic stem cells (hESC) possess great potential for applications in regenerative medicine due to their ability to differentiate into any cell type in the body. However, it is crucial to remove residual undifferentiated hESC from the differentiated population to avoid teratoma formation in vivo. The monoclonal antibody, mAb 84, has been shown to bind and kill undifferentiated hESC and is very useful for the elimination of contaminating undifferentiated hESC prior to transplantation. As mAb 84 is an IgM, its large size may impede penetration into embryoid bodies (EB) or cell clumps. To improve penetration, four antibody fragment formats of mAb 84 were engineered and expressed in Escherichia coli: Fab 84, scFv 84, scFv 84-diabody and scFv 84-HTH. All 4 fragments bound specifically to hESC, but only scFv 84-HTH, a single chain variable fragment with a dimerizing helix–turn–helix motif, could recapitulate the cytotoxicity of mAb 84 on multiple hESC lines. The results suggest that multivalency and flexibility between the antigen-binding sites may be essential features required for killing of hESC by mAb 84 and its derivatives. Imaging of EB treated with scFv 84-HTH or mAb 84 showed an even distribution of scFv 84-HTH throughout the EB whereas mAb 84 was localized more to the periphery.