Pathogenicity and comparative evolution in vivo of the transitional quasispecies SIVsmmPBj8.

Abstract During 14 months of infection of a pig-tailed macaque, the acutely lethal simian immunodeficiency virus SIVsmmPBj14 (SIV-PBj14) evolved from the minimally pathogenic strain SIVsmm9. The virus isolated at 8 months (SIV-PBj8) exhibited properties of both SIVsmm9 and SIV-PBj14, indicating that a phenotypic transition occurred between 6 and 10 months. To assess the influence that this new composition of biologic properties might have on pathogenicity, three pig-tailed macaques were inoculated intravenously with SIV-PBj8. Although no animals developed the severe acute disease syndrome typical of SIV-PBj14, all had high levels of viremia and died of AIDS at 4, 10.5, and 32 months. Characterization of the SIV-PBj8-derived quasispecies that evolved in these macaques showed that at 4 days after inoculation, viruses from all three animals exhibited in vitro properties different from those of the inoculum. By 4 months, the initial phenotypic profiles had changed, with the quasispecies in plasma from the animal (J90232) that died at this time most closely resembling SIV-PBj14, not SIV-PBj8. Phylogenetic trees of the gp41/Nef region of viruses in 4-month plasma from J90232 revealed three distinct populations with high bootstrap values: one group branched with SIVsmm9, one with SIV-PBj14, and one with SIV-PBj8 (ratio of clones, 5:9:5). Nucleotide sequence analysis suggested that some members of the original SIV-PBj8 quasispecies may have been evolving toward a SIV-PBj14-like genotype at the time macaque J90232 died. The use of SIV-PBj8, which was more pathogenic than SIVsmm9, but less pathogenic than SIV-PBj14, may provide the optimal genetic background on which to identify the minimal, multigenic determinants of the SIV-PBj14 phenotype. The results of our studies on SIV-PBj14 indicate that in some, but not all, cases of primate lentivirus infection more pathogenic variants evolve, selectively proliferate, and more than likely contribute to disease progression.