Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract illness (LRTI) among infants and young children and contributes significantly to morbidity and mortality in this age group. RSV is classified into two subgroups, RSV/A and RSV/B, based on variation in the attachment (G) gene. Viruses from both subgroups circulate, though usually one subgroup dominates a given RSV season [1]. The G protein and fusion (F) protein are the only two surface glycoproteins capable of inducing a neutralizing antibody response [2]. However, the F protein is far more conserved than the G protein and, for this reason, has been the major antigen of focus for RSV vaccine development [3]. There is currently no licensed vaccine against RSV; however, there is a large pipeline containing candidate vaccines that are in preclinical to late stages of development [4]. Most of these vaccines are monovalent and utilize the F protein or sequence isolated in the 1960s from an RSV/A virus belonging to the GA1 genotype.
The RSV/A and RSV/B subgroups are further divided into genotypes based on variability in the distal third of the G gene, the hypervariable mucin-like domain [1,5]. During RSV season more than one genotype from the same RSV subgroup co-circulates within a community outbreak. The GA2 genotype has been the dominant genotype for RSV/A for nearly a decade. However, it is rapidly being replaced by the Ontario (ON1) genotype [6]. The Buenos Aires (BA) genotype has been the dominant genotype for RSV/B since 2005 [7]. Interestingly, both ON1 and BA have a unique duplication in the distal third of their G genes, 72 and 60 nucleotides respectively [7,8].
The F protein has been identified as having at least two dominant conformations: the pre-fusion and post-fusion F forms. The F protein’s pre-fusion conformation is metastable and readily rearranges into the stable post-fusion conformation [9]. Each of these conformations has been expressed as a protein crystal; however, modifications had to be made to stabilize the F protein, in particular, for the pre-fusion conformation. Thus it is possible that the pre-fusion protein crystallization may not represent the protein’s true form prior to virus-to-cell fusion. Both the pre-fusion and post-fusion conformation of the F protein are being explored as vaccine candidates [4,10,11]. These two conformations share some antigenic sites but also have their own antigenic sties. Two known antigenic sites (II and IV) are present in the pre- and post-fusion F [12,13]. Antigenic site II is the targeted site of the therapeutic monoclonal antibody, palivizumab. In addition, pre-fusion F has antigenic site ø, MPE-8, [alpha]2, [alpha]3, [beta]3 & [beta]4 (recognized by AM14), and p27 [9,14-16]; post-fusion F has the unique antigenic site I [17].
Although the F protein is generally thought to be well conserved, variability in some of the F domains has been observed in the signal peptide, transmembrane domain, not defined 2 site, and antigenic site ø [18]. In this report we examine the sequence variability of the F gene from a large bank of RSV sequences that span over 50 years. To better understand the impact this variability may have on vaccine development, we have focused on the antigenic sites of the pre-fusion and post-fusion F and used as our reference the F gene from a historical sequence. This reference gene belongs to the genotype GA1 which is often utilized in the development of RSV vaccines.
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Authors: Anne M. Hause, David M. Henke, Vasanthi Avadhanula, Chad A. Shaw, Lorena I. Tapia and Pedro A. Piedra
Date: Apr. 17, 2017