Alternative conformations of a major antigenic site on RSV F

Jones, Harrison G.; Battles, Michael B.; Lin, Chun-Chi; Bianchi, Siro; Corti, Davide; McLellan, Jason S.

D25 was the first structurally characterized antibody that specifically targets prefusion RSV F and was used to solve the structure of the prefusion RSV F conformation, facilitating the engineering of prefusion-stabilized variants that prevent conformational rearrangement to postfusion RSV F [8, 40, 41] . Recently, the structure of 5C4 bound to RSV F was determined, revealing a nearly identical conformation of prefusion RSV F as that observed in the D25-bound structure [42] . However, crystal structures of the different prefusion-stabilized variants of RSV F have revealed an alternative conformation of antigenic site Ø or weak electron density in this region, suggesting that this site is flexible. Currently, it is unclear if flexibility in this region is an inherent property of prefusion RSV F that may be important for triggering membrane fusion [41] , or if it is related to inadequate stabilization of site Ø in the engineered variants [40] . Therefore, we sought to investigate the conformational plasticity of site Ø by determining and comparing the crystal structures of prefusion RSV F in complex with AM22 and RSD5. Our results demonstrate that prefusion RSV F adopts at least three alternative conformations of site Ø and that potently neutralizing human antibodies can recognize the alternative conformations using distinct binding modes. This suggests that site Ø samples an ensemble of conformations in vivo, at least some of which can be recognized by neutralizing human antibodies. These results should influence future vaccine designs and may have implications for the mechanism of RSV F triggering.