Immunomodulation by the ectopic expression of intracellular antibodies ('intrabodies') has a great potential for interfering with physiological or pathological functions in vivo in a highly specific manner. One of the major obstacles of this technology is the inability of most antibodies to properly fold and function in the reducing environment of the cytoplasm, which prevents the formation of essential disulfide bonds. We wished to assess the intracellular performance of antibodies derived from a semi-synthetic single-chain variable fragment (scFv) phage display library ('F8 library') built on a thermodynamically stable single-framework scaffold. To this purpose, we chose to modulate the infection of a pandemic plant pathogen, the cucumber mosaic virus (CMV). After in vitro 'biopanning' on immobilized virions, two scFvs were biochemically characterized, showing high affinity toward the antigen. They were transiently expressed at high yields as soluble molecules in the cytoplasm of Nicotiana benthamiana plants. Subsequently, they were expressed in the cytoplasm of transgenic tomato plants. Challenge with high viral dose showed that both scFvs were able to elicit a phenotypic effect and led to the identification of a transgenic line fully resistant to infection. In these plants, the scFv binds the virus in the inoculated leaves preventing viral long distance movement. This work represents the first demonstration that the 'F8 library' can be directly screened in vitro to rapidly isolate antigen-specific scFvs that act as effective intrabodies in vivo. These antibodies represent powerful tools to interfere with several intracellular targets, modulating pathogen infectivity and/or cellular metabolism. © Springer 2005.

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