Alternative Virus-Like Particle-Associated Prefusion F Proteins as Maternal Vaccines for Respiratory Syncytial Virus

Abstract

Maternal vaccination may be the most effective and safest approach to the protection of infants from respiratory syncytial virus (RSV) infection, a severe acute lower respiratory tract disease in infants and young children worldwide. We previously compared five different virus-like particle (VLP)-associated, mutation-stabilized prefusion F (pre-F) proteins, including the prototype DS-Cav1 F VLPs. We showed that alternative versions of prefusion F proteins have different conformations and induce different populations of anti-F protein antibodies. Two of these alternative pre-F VLPs, the UC-2 F and UC-3 F VLPs, stimulated in mice higher titers of neutralizing antibodies than DS-Cav1 F VLPs (M. L. Cullen, R. M. Schmidt, M. G. Torres, A. A. Capoferri, et al., Vaccines 7:21–41, 2019, https://doi.org/10.3390/vaccines7010021). Here we describe a comparison of these two pre-F VLPs with DS-Cav1 F VLPs as maternal vaccines in cotton rats and report that UC-3 F VLPs significantly increased the neutralizing antibody (NAb) titers in pregnant dams compared to DS-Cav1 F VLPs. The neutralizing antibody titers in the sera of the offspring of the dams immunized with UC-3 F VLPs were significantly higher than those in the sera of the offspring of dams immunized with DS-Cav1 VLPs. This increase in serum NAb titers translated to a 6- to 40-fold lower virus titer in the lungs of the RSV-challenged offspring of dams immunized with UC-3 F VLPs than in the lungs of the RSV-challenged offspring of dams immunized with DS-Cav1 F VLPs. Importantly, the offspring of UC-3 F VLP-immunized dams showed significant protection from lung pathology and from induction of inflammatory lung cytokine mRNA expression after RSV challenge. Immunization with UC-3 F VLPs also induced durable levels of high-titer neutralizing antibodies in dams. IMPORTANCE Respiratory syncytial virus (RSV) is a significant human pathogen severely impacting neonates and young children, but no vaccine exists to protect this vulnerable population. Furthermore, direct vaccination of neonates is likely ineffective due to the immaturity of their immune system, and neonate immunization is potentially unsafe. Maternal vaccination may be the best and safest approach to the protection of neonates through the passive transfer of maternal neutralizing antibodies in utero to the fetus after maternal immunization. Here we report that immunization of pregnant cotton rats, a surrogate model for human maternal immunization, with novel RSV virus-like particle (VLP) vaccine candidates containing stabilized prefusion RSV F proteins provides significant levels of protection of the offspring of immunized dams from RSV challenge. We also found that antibodies induced by VLPs containing different versions of the prefusion F protein varied by 40-fold in the extent of protection provided to the offspring of vaccinated dams upon RSV challenge.

Comparisons of Antibody Populations in Different Pre-Fusion F VLP-Immunized Cotton Rat Dams and Their Offspring

“…Animal studies have been described previously [30]. Briefly, Sigmodon hispidus cotton rats were obtained from an inbred colony maintained at Sigmovir Biosystems, Inc. (Rockville, MD USA).…”

“…We have previously described and compared immune responses in CR immunized with DS-Cav1 F VLPs and two alternative pre-fusion F VLPs [30]. CR females were RSV primed by intranasal infection, mated at 56 days after the RSV prime, and then immunized at various times of gestation with DS-Cav1 F, UC-2 F, UC-3 F, post-F VLPs, or mock-immunized.…”

“…A very important question for vaccine development is whether the different mutation-stabilized pre-fusion F proteins are indeed the same in terms of structure, antibodies induced, and protection from RSV challenge afforded by their use as immunogens. We have recently addressed these questions by comparing the reactivity to monoclonal antibodies and the immunological properties of virus-like particles (VLPs) assembled with different versions of mutation-stabilized pre-fusion F proteins [18,30]. We have reported that five different pre-fusion F proteins, in VLPs, bind differently to representative pre-fusion specific monoclonal antibodies (mAb) [18].…”

“…Furthermore, we showed that the specificities of the population of antibodies induced in mice by the five different VLP-associated pre-fusion F proteins were different as defined by differences in the serum inhibition of binding of representative monoclonal antibodies to the soluble forms of the pre-fusion F protein [18]. These combined results indicate that not all mutant stabilized pre-fusion F proteins are the same with respect to the population of antibodies they induce.Using cotton rats (CR), the preferred animal model for RSV [7,14,31,32], we reported that the immunization of pregnant animals with one of these alternative pre-fusion F VLPs increased their serum-neutralizing antibody (NAb) titers and significantly increased the protection from RSV challenge of their offspring compared to the immunization of dams with DS-Cav1 F VLPs [30]. These results raise the question of why there are differential levels of protection of offspring upon immunization of their dams with the different VLP-associated pre-fusion F proteins.…”

“…Using cotton rats (CR), the preferred animal model for RSV [7,14,31,32], we reported that the immunization of pregnant animals with one of these alternative pre-fusion F VLPs increased their serum-neutralizing antibody (NAb) titers and significantly increased the protection from RSV challenge of their offspring compared to the immunization of dams with DS-Cav1 F VLPs [30]. These results raise the question of why there are differential levels of protection of offspring upon immunization of their dams with the different VLP-associated pre-fusion F proteins.…”