New Research Gives Hope in Respiratory Syncytial Virus Vaccine Search – Part II
This story is part two of a series about respiratory syncytial virus vaccines. Here we describe groundbreaking research that both answers questions about past vaccine failure and charts a path toward the production of effective vaccines.
Respiratory syncytial virus is one of the leading causes of infant death worldwide, and a safe vaccine has yet to be approved for use. However, the long vaccine hunt may soon be over. The new vaccine technology based on the structure behind the recent development of such vaccines is very promising. In Part I of this series, we discussed the extent of the disease and the seriousness of its consequences, and here, in Part II, we discuss the science behind recent advances in research. a vaccine against RSV.
Until recently, respiratory syncytial virus posed barriers to technology that was successful in producing functional vaccines that successfully mitigated the spread and severity of many other infectious diseases. While the search for a respiratory syncytial virus vaccine has eluded scientists and vaccine makers for decades, just as recently, there are finally promising vaccine candidates in phase 3 trials. New vaccine design technology based on the structure behind their development will have life-saving consequences, as respiratory syncytial virus is the leading cause of hospitalizations in children under five, and infants and the elderly are particularly susceptible .
Just over 55 years ago, in 1965, a small respiratory syncytial virus vaccine trial led to increased hospitalization rates and the deaths of two infants in a treatment group of just 23. The aftermath The disastrous results of the trial delayed any further development of an RSV vaccine. at the turn of the century. Recent advances in structural vaccine technology mean vaccines may soon be available, even for young children.
Figure 1. Electron microscope image of a respiratory syncytial virus particle
One path in structure-based vaccine design technology is to focus on the physical form of the pathogen, down to the individual atom. By identifying the structures to which antibodies bind, scientists can then synthesize a replica of the antigen’s structure. Formulating vaccines with the modified antigens will trigger an antibody production response in the body, preparing the immune system to react quickly to future exposures. The following is drawn largely from research published by Graham, Modjarrad and McLellan in 2015.
Figure 2. Schematic drawing of RSV particles
The fusion protein (F) has been identified as the key viral structure to target for RSV. However, we now know that the F protein has two forms, going from the pre-fusion state (pre-F) to the post-fusion state (post-F). Antibodies target both forms of F protein, but those that bind to the prefusion form are much more effective at neutralizing the virus. Additionally, the negative effects of the virus can be partly attributed to the non-neutralizing binding of antibodies to the post-fusion form. Knowing this, it became clear to scientists that to design a vaccine that produced maximum defense, they needed to focus on the structure of the prefusion antigen. The problem with this is that the prefusion F protein is very unstable. The pre-melt state is spring-loaded and easily takes the post-melt state. To incorporate the pre-F form into the vaccine to stimulate more impactful antibody production, the challenge is to stabilize the F protein in its prefusion state.
In 2013, researchers at the National Institutes of Health’s Center for Vaccine Research used X-ray crystallography to understand the precise molecular structure of the pre-F and post-F forms as they were linked to l one of the recently discovered potent antibodies. Building on previously established research that a virus could be stabilized in its prefusion form and additional structural information provided by X-ray crystallography, the researchers, Dr. Jason McLellan and Dr. Barney Graham of the Research Center on NIH vaccines, reported that they had succeeded in genetically modifying the protein to retain its prefusion state. To stabilize the pre-F conformation, McLellan and his team analyzed the pre-F structure for mutations that would maintain the structure maximizing antibody neutralization. Among more than 100 variants, three were found to retain pre-F-specific antibody binding capacity.
The first of these mutation variants, named DS, was formed by replacing amino acids Ser155 and Ser290 with pairs of cysteines that formed stable disulfide bonds. These served as covalent bridges to lock the F protein into its prefusion configuration. The next one, called Cav1, contained the S190F and V207L mutations. Change a serine to alanine phenol and a valine to atomic level cavities filled with leucine to increase hydrophobic packaging and maintain the structure of the most potent antigenic site. The third variant, called TriC, involved an F488W mutation, stabilizing the hydrophobic fusion peptide. However, after testing different combinations of the three mutations, it was clear that the DS-Cav1 combination was superior in maintaining stability under extreme conditions such as temperature, pH, osmolality, and freeze-thaw, all of which are important for the manufacture of vaccines. When tested, the DS-Cav1 model of the stabilized pre-F form induced extremely high antibody levels compared to the post-F form in an animal assay, confirming the research and paving the way to new RSV vaccine options.
Figure 3. Design of RSV F trimers stabilized at the soluble site
Fig. 4. Crystal structures of RSV F trimers, designed to preserve the Ø antigenic site
Stabilization of the prefusion form is a critical development because this pre-F form of the protein contains more effective antibody neutralization target sites. As shown in Figure 3 above, red targets, labeled Site Ø, are rated “outstanding” in value as a neutralization site. However, these are only present in the premelted state. In the post-fusion state, which occurs after the viral cell has undergone a conformational change and made physical contact with the host cell, these essential binding sites are no longer present. Prefusion F also has additional neutralization target sites which are not yet named but shown in dark orange in Figure 3. These “excellent” neutralization value sites are also only present on the prefusion form, demonstrating the importance of protein modifications that succeeded in stabilizing the protein. F for vaccine development.
Figure 5. Surface representation of RSV F glycoprotein
It has been more than half a century since the initial RSV vaccine candidate performed so poorly, leading to higher levels of hospitalizations and mortality among vaccinated children compared to the control group. With recent developments in structure-based vaccinology, we have an explanation for what went wrong. Evidence now suggests that the 1965 RSV vaccine induced high levels of antibodies binding only to the post-F state, not the pre-F state. Since only the prefusion form contains the binding sites most susceptible to neutralization, the vaccine was not effective. Scientists at the time were unaware of the different forms of the F protein, which led to an immune response of hyper-production of antibodies that bound to the virus without neutralizing it. These non-neutralizing antibodies caused young children who had not previously been naturally exposed to RSV to have an overly strong inflammatory immune response once they were exposed to the virus after vaccination. In some cases, this has resulted in severe respiratory symptoms and serious illness requiring hospitalization. This tragic first vaccine trial, coupled with recent groundbreaking structure-based research, underscores the importance of having a fundamental understanding of molecular structure. Recent advances in RSV vaccine development would not be possible without the current in-depth understanding of the RSV virus at the atomic level, as this fundamental research is essential for successful vaccine production.
In Part III of this series, we will describe the strategies behind vaccine development, as well as current vaccines against respiratory syncytial virus currently in late-stage clinical trials with the potential to save many lives.