Genetic characterization of Sabin types 1 and 3 poliovaccine virus following serial passage in the human intestinal tract.

Poliovirus isolates types 1 and 3 were obtained from five and seven successive passages respectively, in infants who had been fed monovalent OPV in two separate clinical trials conducted in 1960. The purpose of these trials was to answer the question how much the vaccine virus would revert to its original neurovirulent phenotype following multiplication in the intestinal tract. Human passages were performed either by contact exposure or by feeding the excreted virus while the infants were maintained in isolation. Several virus isolates were obtained at each passage level. Infants participating in both studies showed no symptoms of disease. Antigenic studies (McBride, van Wezel) and protein analysis (PAGE) of the isolates, reported earlier from this laboratory, had shown that the isolates remained vaccine-like, although isolates from the later passages revealed some differences. Monkey neurovirulence test results showed that for both types 1 and 3 viruses the loss of attenuation of the vaccine strain upon passage was gradual, although the loss was faster for type 3. Examination of the oligonucleotide maps demonstrated that the oligonucleotide configuration of the isolates remained the same as for the vaccine strain but there was an increase of individual spot differences with increasing passage. The nucleotide sequence analysis of selected regions of the virus genomes revealed that there was no change from a G to A in nucleotide 480 of type 1 isolates; however, nucleotide 476 changed from a U to an A in type 1 passages 3, 4 and 5. Conversely, for type 3 the change of nucleotide 472 from a U to a C changed at the early first passage (4 days following administration of OPV), and remained a C in the six following passages; type 3 nucleotide 2034 did not change in the first passage from a U to a C, but it became a C in all further passages tested. The nucleotide changes mentioned for both virus types remained stable in successive passages. However, there was another nucleotide change for type 3 from a U to a C at position 1973 only for passages 5 and 6 which reverted to a U for passages 7L and 7LL. Study of selected human passage virus strains could further contribute to the identification of the critical nucleotides that are responsible for the attenuation of these two polio types of vaccine viruses.