Antigen-independent selection of stable intracellular single-chain antibodies.

The intracellular expression of single-chain Fv antibody fragments (scFv) in eukaryotic cells has an enormous potential in functional genomics and therapeutics Marasco (1997) Gene Ther. 4, 11-15; Richardson and Marasco (1995) Trends Biotechnol. 13, 306-310]. However, the application of these so-called intrabodies is currently limited by their unpredictable behavior under the reducing conditions encountered inside eukaryotic cells, which can affect their stability and solubility properties W?rn et al. (2000) J. Biol. Chem. 275, 2795-2803; Biocca et al. (1995) Bio/Technology 13, 1110-1115]. We present a novel system that enables selection of stable and soluble intrabody frameworks in vivo without the requirement or knowledge of antigens. This system is based on the expression of single-chain antibodies fused to a selectable marker that can control gene expression and cell growth. Our results show that the activity of a selectable marker fused to well characterized scFvs W?rn et al. (2000) J. Biol. Chem. 275, 2795-2803] correlates with the solubility and stability of the scFv moieties. This method provides a unique tool to identify stable and soluble scFv frameworks, which subsequently serve as acceptor backbones to construct intrabody complementarity determining region libraries by randomization of hypervariable loops.