Microevolution of Type 3 Sabin Strain of Poliovirus in Cell Cultures and Its Implications for Oral Poliovirus Vaccine Quality Control

Abstract

Screening for sequence heterogeneities in Sabin Type 3 strains of attenuated poliovirus demonstrated mutations that consistently accumulate to significant levels following 10 passages in cultures of primary African green monkey kidney (AGMK) cells or continuous cultures of Vero cells. Fourteen newly identified mutations were quantified by mutant analysis by PCR and restriction enzyme cleavage in passages and in batches of commercial vaccines made in AGMK and Vero cells from the Sabin original (SO) seed virus and from a seed virus rederived by RNA plaque purification (RSO or "Pfizer" seed). Nine of the 14 mutations were reproducibly observed in more than one series of passages. Although 5 other mutations were observed in only one set of passages each, their content gradually increased to a high percentage, suggesting that all the mutations that we found accumulated consistently, SO-derived samples accumulated more mutations than did RSO derived ones, and the number of mutations and the rates of their accumulation were higher in Vero than in AGMK cells. While the rates of accumulation of most mutations were higher when passaging was performed at 37°, a U → C transition at nucleotide 5832 occurred faster at 34°, the temperature used for vaccine production. Analysis of Type 3 oral poliovirus vaccine (OPV) monopools made by six manufacturers found only 5 of these newly identified mutations in vaccine batches (nucleotides 3956, 4935, 5357, 5786, and 5832). Some of the mutations were found in trace amounts (less than 0.1%) while others were present at up to 1.8% levels. The pattern of these mutations was characteristic for the type of seed virus and the cell substrate but demonstrated no correlation with results of the monkey neurovirulence test. Therefore the only mutation occurring in Type 3 OPV which contributed to neurovirulence in monkeys was the previously described reversion at nucleotide 472. Quantitation of reversion at nucleotide 472 can be utilized for assessment of acceptability of vaccine lots, while other mutations can be used for monitoring the consistency of vaccine production.