Dynamics of Evolution of Poliovirus Neutralizing Antigenic Sites and Other Capsid Functional Domains during a Large and Prolonged Outbreak.

We followed the dynamics of capsid amino acid replacement among 403 Nigerian outbreak isolates of type 2 circulating vaccine-derived poliovirus (cVDPV2) from 2005 through 2011. Four different functional domains were analyzed: (i) neutralizing antigenic (NAg) sites, (ii) residues binding the poliovirus receptor (PVR), (iii) VP1 residues 1 to 32, and (iv) the capsid structural core. Amino acid replacements mapped to 37 of 43 positions across all 4 NAg sites; the most variable and polymorphic residues were in NAg sites 2 and 3b. The most divergent of the 120 NAg variants had no more than 5 replacements in all NAg sites and were still neutralized at titers similar to those of Sabin 2. PVR-binding residues were less variable (25 different variants; 0 to 2 replacements per isolate; 30/44 invariant positions), with the most variable residues also forming parts of NAg sites 2 and 3a. Residues 1 to 32 of VP1 were highly variable (133 different variants; 0 to 6 replacements per isolate; 5/32 invariant positions), with residues 1 to 18 predicted to form a well-conserved amphipathic helix. Replacement events were dated by mapping them onto the branches of time-scaled phylogenies. Rates of amino acid replacement varied widely across positions and followed no simple substitution model. Replacements in the structural core were the most conservative and were fixed at an overall rate ∼20-fold lower than the rates for the NAg sites and VP1 1 to 32 and ∼5-fold lower than the rate for the PVR-binding sites. Only VP1 143-Ile, a non-NAg site surface residue and known attenuation site, appeared to be under strong negative selection. The high rate of poliovirus evolution is offset by strong selection against amino acid replacement at most positions of the capsid. Consequently, poliovirus vaccines developed from strains isolated decades ago have been used worldwide to bring wild polioviruses almost to extinction. The apparent antigenic stability of poliovirus obscures a dynamic of continuous change within the neutralizing antigenic (NAg) sites. During 7 years of a large outbreak in Nigeria, the circulating type 2 vaccine-derived polioviruses generated 120 different NAg site variants via multiple independent pathways. Nonetheless, overall antigenic evolution was constrained, as no isolate had fixed more than 5 amino acid differences from the Sabin 2 NAg sites, and the most divergent isolates were efficiently neutralized by human immune sera. Evolution elsewhere in the capsid was also constrained. Amino acids binding the poliovirus receptor were strongly conserved, and extensive variation in the VP1 amino terminus still conserved a predicted amphipathic helix.