Respiratory syncytial virus fusion protein-specific B cell repertoires induced by natural infection or vaccination

Respiratory syncytial virus (RSV) infections are the major contributor to acute lower respiratory syndrome in newborns. Infections generally result in hospitalization and sometimes in death. A vaccine is not available yet, despite decades of research. Vaccine development is hampered in consequence of a failed vaccine trial in the 1960s which entailed fatal outcomes. An alternative to direct vaccination of children is maternal vaccination for passive immunization of babies before birth. RSV infects every person repeatedly throughout life which implies that pregnant woman carry RSV-directed memory B cell (MBC) repertoires. A successful maternal vaccine would therefore elicit high titer RSV-neutralizing IgGs by reactivation of pre-existing MBCs which would protect newborns during the first months of life. RSV has two neutralizing antigens, of which the fusion protein (RSV F) is the most promising vaccine candidate. RSV F mediates the fusion process of viral and cellular membranes, wherefore it exists in a pre-fusion (pre F) or post-fusion (post F) conformation. Even though there is more interest in the pre F conformation as immunogen, the post F conformation may be equally considered as successful vaccine antigen. The pre F conformation is very metastable and readily switches to the highly stable post F conformation, which implies that a post F-based vaccine would be more cost-efficient to produce. More importantly, several neutralizing epitopes are preserved on the post F conformation and a substantial amount of RSV F-directed MBCs induced by natural infection are actually pre/post F cross-reactive. Since most of the pre/post F cross-reactive MBCs were previously shown to have higher affinities for the post F conformation, a post F-based vaccine may be ideal for reactivation and clonal expansion of MBCs which express neutralizing B cell receptors (BCRs). Every antigen, also RSV post F protein, has its own signatures within BCR repertoires because of preferential selection of BCR characteristics for B cell clonal expansions. In order to understand how RSV F protein shapes BCR repertoires, RSV F-directed BCRs were isolated from a healthy blood donor and three vaccinees who received an RSV post F vaccine. BCR repertoire analysis confirmed the assumption that the pre and post F protein have their own signatures within RSV F-directed BCR repertoires. The different characteristics indicated longer affinity maturation of pre F-reactive MBCs. Furthermore, estimation of clonal relatedness between the pre and post F-binding BCR repertoires from the healthy donor provided indications that a substantial number of the isolated BCRs are actually pre/post F cross-reactive, which confirmed a previous study. Analysis of the BCR repertoires isolated from the RSV post F-vaccinees showed that the vaccine induced a biased MBC response with preferential BCR characteristics. There were several implications that the post F-vaccine expanded primarily pre/post F cross-reactive MBCs. In contrast to the high variability of pre/post F cross-reactive BCRs induced by natural infection, the vaccine-induced MBC response indicated a skewed selection of VH4 gene family-encoded BCRs for clonal expansion and affinity maturation. More importantly, estimation of clonal relatedness revealed convergent MBC responses between the three analyzed subjects, while several MBC lineages shared stereotypic characteristics with pre F-binding BCRs or RSV-neutralizing antibodies. Some of the supposedly pre/post F cross-binding or neutralizing BCR sequences were expressed as mAbs and functionally characterized. RSV pre/post F cross-binding and neutralization activities could be demonstrated for all of the expressed mAbs. This project demonstrated the potential of ‘clonal’ grouping as novel reverse approach to screen BCR repertoires for functional antibodies.