Date Published: February 7, 2019
Publisher: Public Library of Science
Author(s): Qingqing Xie, Zhao Wang, Fengyun Ni, Xiaorui Chen, Jianpeng Ma, Nita Patel, Hanxin Lu, Ye Liu, Jing-Hui Tian, David Flyer, Michael J. Massare, Larry Ellingsworth, Gregory Glenn, Gale Smith, Qinghua Wang, Steven M. Varga.
Globally, human respiratory syncytial virus (RSV) is a leading cause of lower respiratory tract infections in newborns, young children, and the elderly for which there is no vaccine. The RSV fusion (F) glycoprotein is a major target for vaccine development. Here, we describe a novel monoclonal antibody (designated as R4.C6) that recognizes both pre-fusion and post-fusion RSV F, and binds with nanomole affinity to a unique neutralizing site comprised of antigenic sites II and IV on the globular head. A 3.9 Å-resolution structure of RSV F-R4.C6 Fab complex was obtained by single particle cryo-electron microscopy and 3D reconstruction. The structure unraveled detailed interactions of R4.C6 with antigenic site II on one protomer and site IV on a neighboring protomer of post-fusion RSV F protein. These findings significantly further our understanding of the antigenic complexity of the F protein and provide new insights into RSV vaccine design.
Human respiratory syncytial virus (RSV) is a leading cause of pneumonia and bronchiolitis in premature newborns, young children, and adults over 65 years old. In the United States, RSV is responsible for over 2 million outpatient visits and nearly 60,000 hospitalizations in children under 5 years of age and 11,000–17,000 deaths in adults each year [1–3]. Globally, RSV is responsible for over 30 million lower respiratory tract infections (LRTI), 3 million hospitalizations, and 50,000–75,000 deaths each year in children under 5 years with the majority of incidents in developing countries [4–6]. To date, there is no approved RSV vaccine, and palivizumab (Synagis) is the only passive prophylaxis approved for prevention of RSV infection in newborns under 24 months of age. However, the high cost and 5-dose treatment regimen prevent the use and availability of palivizumab in developing countries [7–9].
In this study, we reported the discovery of four new mAbs elicited in mice that were immunized with a near full-length RSV F NP (Table 1), followed by a series of detailed characterization of a novel mAb designated as R4.C6. Epitope binning by BLI using Octet QK384 instrument revealed that mAb R4.C6 broadly competed for binding with neutralizing humanized and human mAbs targeting antigenic site IIa (palivizumab, hRSV14N4 and R6.46) and site IIb (motavizumab and hRSV3J20) as well as non-neutralizing mAb (R6.29) binding to antigenic site VII. Unexpected, mAb R4.C6 also competed binding of neutralizing mAbs RSHZ19 and R1.42 that are directed at antigenic site IV. R4.C6 bound RSV F NP and a site II synthetic peptide with nanomole affinity (KD = 0.07 and 12.5 nM, respectively), which were 7- and 30-fold higher than palivizumab (Table 1). However, the neutralization potency of R4.C6 was slightly lower than palivizumab (IC50 = 1078 vs. 323 ng/mL). Moreover, although R4.C6 had a similar binding affinity for RSV F NP as R1.42 (KD = 0.07 vs. 0.04 nM), R4.C6 exhibited about 50-fold lower neutralization activity than R1.42 (IC50 = 1078 vs. 19.6 ng/mL). Therefore, despite a clear correlation of binding affinity and neutralization activity between palivizumab and its affinity-matured variant motavizumab (Table 1) , this correlation was not always observed in other mAbs. Probably other factors, including but not limited to, the accessibility of the target epitope(s) on the tightly packed RSV F glycoproteins on the viral surface are also in play. More structural and functional studies of RSV F-specific antibodies like the one reported here are urgently needed for a better understanding of mAb-mediated neutralization of RSV virus.