Coagulation factor X shields adenovirus type 5 from attack by natural antibodies and complement

Understanding how adenoviruses transduce cells is important for their use and development as vaccine vectors. Adenovirus type 5 (Ad5) is known to bind coagulation factor X (FX), and FX is thought to act as a bridge between the virus and its receptor on hepatocytes. Andrew Byrnes and his colleagues now report that the major role of FX binding to Ad5 is actually to protect Ad5 from neutralization by complement and natural antibodies, and in the absence of B cells, Ad5 does not require FX binding for effective liver transduction. Adenovirus type 5 (Ad5) specifically binds coagulation factor X (FX), and FX is normally essential for intravenously injected Ad5 vectors to transduce the liver. We demonstrate that the ability of FX to enhance liver transduction by Ad5 vectors is due to an unexpected ability of FX to protect Ad5 from attack by the classical complement pathway. In vitro , naive mouse serum neutralized Ad5 when FX was blocked from binding Ad5. This neutralization was mediated by natural IgM and the classical complement pathway. In vivo , FX was essential for Ad5 vectors to transduce the livers of wild-type mice, but FX was not required for liver transduction in mice that lack antibodies, C1q or C4. We conclude that Ad5 recruits FX as a defense against complement and that the sensitivity of Ad5 to inactivation by complement must be taken into account when designing vectors for systemic gene therapy.