Structure-Based Design of Head-Only Fusion Glycoprotein Immunogens for Respiratory Syncytial Virus

Boyington, Jeffrey C.; Joyce, M. Gordon; Sastry, Mallika; Stewart-Jones, Guillaume B. E.; Chen, Man; Kong, Wing-Pui; Ngwuta, Joan O.; Thomas, Paul V.; Tsybovsky, Yaroslav; Yang, Yongping; Zhang, Baoshan; Chen, Lei; Druz, Aliaksandr; Georgiev, Ivelin S.; Ko, Kiyoon; Zhou, Tongqing; Mascola, John R.; Graham, Barney S.; Kwong, Peter D.

Immune-focusing of the vaccine response, through the removal of extraneous epitopes from vaccine immunogens, has been observed to elicit targeted antibody responses in several studies. With RSV, incorporation of two RSV F helices from antigenic site II into a protein scaffold resulted in immunogens that could elicit binding antibodies to site II in mice [19] and-with the addition of an intervening loop-neutralizing antibodies to site II in macaques [20] , though the macaque titers appeared to be much lower than those elicited by trimeric immunogens, such as the RSV F trimer. With influenza A virus, headless or stalk-only immunogens of the hemagglutinin (HA) glycoprotein induced significantly higher titers of protective HA stemdirected antibodies than the seasonal influenza trivalent inactivated vaccine in mice [21, 22] and ferrets [22] , and HA stem-directed antibodies that reduced symptoms of disease in macaques [21] . Furthermore, with Middle East respiratory syndrome coronavirus, mice immunized with the receptor-binding domain (RBD)-containing S1 subunit elicited more RBD-reactive neutralizing antibodies than mice immunized with DNA for full-length spike (S) protein [23] . We therefore hypothesized that a "head-only" immunogen might more ably boost antigenic site Ø-directed responses than the complete F glycoprotein. To test this hypothesis, we designed a series of RSV F immunogens comprising only the membrane-distal half (head region) of the RSV F glycoprotein, with antigenic site Ø intact and the membrane-proximal half (stalk region) completely removed. Four "head-only" RSV F immunogens were evaluated in prime-boost mouse immunogenicity studies, with two of these boosting RSV-neutralization titers to levels that were comparable to those boosted by the RSV F DS-Cav1 trimer, although increased site Ø-directed antibodies.