Background:  Simian immunodeficiency virus (SIV) has been shown to progress through a number of changes that lead to the emergence of pathogenic viral variants in macaques initially infected with a mildly cytopathic variant, SIV_MneCL8. One of these late-stage isolates, SIV_Mne170, replicates to high levels and causes a rapid disease course when re-introduced into naïve macaques, resulting in viral set-point as much as 3000-fold higher than the set-point of the parental virus SIV_MneCL8. However, both viruses replicate with similar kinetics in cell cultures with low cell turnover. We hypothesized that the late-stage variant is better adapted to rapid turnover of infected cells that occurs when cellular immunity pressures are present.

Methods:  We used a direct in vitro competition approach to measure relative fitness of early and late variants of SIV. The fitness was tested when the lifespan of infected cells was either not controlled (non-limited lifespan) or limited to 2 days (rapid-turnover system).

Results:  We found that the fitness of early variant SIV_MneCL8 was higher than fitness of late variant SIV_Mne170 in cultures with non-limited lifespan of cells. The fitness of SIV_Mne170 in a rapid-turnover system dramatically increased compared with its fitness in cultures with a non-limited lifespan of cells. The increase in fitness resulted at least partially from the rapid-turnover system that eliminates the negative influence of the cytopathic effects associated with replication of SIV_Mne170. The genetic determinant for increased fitness of SIV_Mne170 was localized to 5’ part of the genome.

Conclusions:  Late-stage virus SIV_Mne170 was better adapted to rapid turnover of infected cells in vitro. This adaptation may result from in vivo selective pressure imposed by rapid clearance of infected cells by the host immune system and may contribute to increased pathogenicity of SIV_Mne170 in virus-naïve macaques. Because the relative fitness of SIV_MneCL8 and SIV_Mne170 observed in rapid-turnover system more accurately reflects their fitness in vivo, the system represents an approach to comparing relative fitness of viruses that may better mimic viral replication in the host. Current work focuses on testing whether HIV variants isolated from late-stage patients are more fit in rapid-turnover system than early isolates.