The receptor binding domain (RBD) of SARS-Coronavirus spike protein is a highly immunogenic antigen and is a promising target for both passive immunotherapy and vaccine design. For passive immunotherapy to be successful in an outbreak, antibodies have to be active against various viral isolates that may emerge. The 80R single chain Fv (Marasco et al.) has been shown to neutralize some human SARS-COV isolates, but failed to neutralize an isolate with a critical mutation at the RBD-80R interface. To examine whether increasing the affinity of 80R antibody towards RBD broadens its ability to bind and neutralize SARS-COV variants, 80R ScFv was subjected to a round of random mutagenesis and high affinity clones were isolated by a fluorescence-activated cell sorting (FACS) based bacterial display system. The RBD antigen was expressed in insect cells and used to screen the libraries via the Anchored Periplasmic Expression (APEx) system (Harvey et al.). A 60-fold increase in affinity from 10nM to 173pM was obtained in a single round of mutagenesis. These high affinity antibody fragments are currently being evaluated for neutralization of both human and zoonotic isolates of SARS-COV by in vitro studies.

Marasco, W.A. et al. (2005) Journal of Virology 79 (10):5900

Harvey, B.R. et al. (2004) Proceedings of the National Academy of Sciences of USA 101(25): 9193

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