Murine Immune Responses to Virus-Like Particle-Associated Pre- and Postfusion Forms of the Respiratory Syncytial Virus F Protein

Abstract

Virus IMPORTANCEThe development of vaccines for respiratory syncytial virus has been hampered by a lack of understanding of the requirements for eliciting high titers of neutralizing antibodies. The results of this study suggest that particle-associated RSV F protein containing mutations that stabilize the structure in a prefusion conformation may stimulate higher titers of protective antibodies than particles containing F protein in a wild-type or postfusion conformation. These findings indicate that the prefusion F protein assembled into VLPs has the potential to produce a successful RSV vaccine candidate.H uman respiratory syncytial virus (RSV) is the most significant cause of acute viral respiratory disease in infants and young children (1). There are from 34 to 65 million RSV infections resulting in acute lower respiratory disease requiring hospitalization and 160,000 to 199,000 deaths per year worldwide (2). Elderly populations are also at significant risk for serious RSV disease. In the United States, the virus accounts for 10,000 deaths and 14,000 to 60,000 hospitalizations per year among individuals more than 64 years of age (3-5). Indeed, RSV infection of this population is at least as significant as influenza virus infections. RSV infections result in high mortality rates in immunocompromised populations, particularly stem cell transplant recipients (6) and individuals with cardiopulmonary diseases (7). Despite the significance of RSV disease in different populations, there are no vaccines available.Failure to develop a licensed RSV vaccine is not due to lack of effort as numerous vaccine candidates have been characterized in preclinical and clinical studies spanning 5 decades (summarized in references 8 to 9). While many problems have uniquely hindered RSV vaccine development, a major hurdle has been a lack of understanding of requirements for generation of protective immunity to RSV infection. Many vaccine candidates are protective in animal models and, while stimulating antibody responses in humans, have failed to induce high levels of neutralizing antibodies and protection from virus challenge in human trials (reviewed in references 10 and 11). Although there are likely many reasons for these observations, one important issue has been a lack of clear understanding of the most effective form of the RSV antigens, particularly the F protein, for stimulating potent neutralizing antibodies.The paramyxovirus F protein is folded into a metastable conformation and upon fusion activation refolds, through a series of conformational intermediates, into the postfusion conformation, which is structurally very different from the prefusion form (12)(13)(14)(15)(16)(17)(18)(19). While it is logical to assume that the prefusion form of F protein should be more effective in stimulating optimally neutralizing antibodies, recent structural studies have shown that the postfusion form of the F protein contains at least some epitopes recognized by neutralizing monoclonal antibodies (17, 18). Thus, it has been a...

Influence of Respiratory Syncytial Virus F Glycoprotein Conformation on Induction of Protective Immune Responses

“…Although others have shown that two doses of prefusion F, either as a soluble protein (32) or incorporated into VLPs (52), can induce protection against RSV challenge in rodent models of RSV infection, our results indicate that at low antigen concentrations, individuals may remain susceptible to infection following the first vaccination and before the second dose has been administered. However, in aggregate, prefusion F is capable of inducing the highest levels of neutralizing antibodies after a single dose and conferring protection with minimal pathology at high doses of the immunogen.…”

“…The reasons for this disparity are unknown, but it is worth stressing that proteins with essentially identical primary structures may induce significantly different antibody responses depending on their conformations. A relatively similar preference for prefusion-specific antibodies was recently reported for mice inoculated with Newcastle disease virus-like particles (VLPs) that incorporated engineered prefusion hRSV F (52).…”

A live RSV vaccine with engineered thermostability is immunogenic in cotton rats despite high attenuation

“…The high capacity of pre-F to elicit neutralizing antibody titres has been demonstrated in multiple vaccine platforms, including purified proteins121314, virus-like particles15, and recombinant parainfluenza viruses16. Use of pre-F in passive immunization, either by anti-pre-F monoclonal antibody (mAb) prophylaxis or by boosting RSV neutralizing antibody (nAb) titres in pregnant mothers with pre-F protein-based vaccines, holds promise for reducing RSV disease in the youngest infants14.…”

Chimeric Pneumoviridae fusion proteins as immunogens to induce cross‐neutralizing antibody responses

“…Thus, fine tuning of changes introduced in chimeric proteins may be critical for their expression and immunogenicity. The use of other expression systems or incorporation of chimeric proteins into viral vectors (Liang et al , 2017) or virus particles (McGinnes et al , 2015) are alternatives to improve production and/or immunogenicity of chimeric F proteins. Nonetheless, the results presented here represent a proof of principle to design chimeric proteins that may be used as single immunogens to induce cross‐neutralizing and cross‐protecting immune responses against the important Pneumoviridae members, hRSV and hMPV.…”