Genetically barcoded SIV reveals the emergence of escape mutations in multiple viral lineages during immune escape.

The rapidity of replication coupled with a high mutation rate enables HIV to evade selective pressures imposed by host immune responses. Investigating the ability of HIV to escape different selection forces has generally relied on population-level measures, such as the time to detectable escape mutations in plasma and the rate these mutations subsequently take over the virus population. Here we employed a barcoded synthetic swarm of simian immunodeficiency virus (SIV) in rhesus macaques to investigate the generation and selection of escape mutations within individual viral lineages at the Mamu-A*01-restricted Tat-SL8 epitope. We observed the persistence of more than 1,000 different barcode lineages following selection after acquiring escape mutations. Furthermore, the increased resolution into the virus population afforded by barcode analysis revealed changes in the population structure of the viral quasispecies as it adapted to immune pressure. The high frequency of emergence of escape mutations in parallel viral lineages at the Tat-SL8 epitope highlights the challenge posed by viral escape for the development of T cell-based vaccines. Importantly, the level of viral replication required for generating escape mutations in individual lineages can be directly estimated using the barcoded virus, thereby identifying the level of efficacy required for a successful vaccine to limit escape. Overall, assessing the survival of barcoded viral lineages during selection provides a direct and quantitative measure of the stringency of the underlying genetic bottleneck, making it possible to predict the ability of the virus to escape selective forces induced by host immune responses as well as during therapeutic interventions.