Mathematical Modeling of Ultradeep Sequencing Data Reveals that Acute CD8+ T-Lymphocyte Responses Exert Strong Selective Pressure in Simian Immunodeficiency Virus-Infected Macaques but Still Fail To Clear Founder Epitope Sequences▿ †

The prominent role of antiviral cytotoxic CD8+ T-lymphocytes (CD8-TL) in containing the acute viremia of human and simian immunodeficiency viruses (HIV-1 and SIV) has rationalized the development of T-cell-based vaccines. However, the presence of escape mutations in the acute stage of infection has raised a concern that accelerated escape from vaccine-induced CD8-TL responses might undermine vaccine efficacy. We reanalyzed previously published data of 101,822 viral genomes of three CD8-TL epitopes, Nef103-111RM9 (RM9), Tat28-35SL8 (SL8), and Gag181-189CM9 (CM9), sampled by ultradeep pyrosequencing from eight macaques. Multiple epitope variants appeared during the resolution of acute viremia, followed by the predominance of a single mutant epitope. By fitting a mathematical model, we estimated the first acute escape rate as 0.36 day−1 within escape-prone epitopes, RM9 and SL8, and the chronic escape rate as 0.014 day−1 within the CM9 epitope. Our estimate of SIV acute escape rates was found to be comparable to very early HIV-1 escape rates. The timing of the first escape was more highly correlated with the timing of the peak CD8-TL response than with the magnitude of the CD8-TL response. The transmitted epitope decayed more than 400 times faster during the acute viral decline stage than predicted by a neutral evolution model. However, the founder epitope persisted as a minor population even at the viral set point; in contrast, the majority of acute escape epitopes were completely cleared. Our results suggest that a reservoir of SIV infection is preferentially formed by virus with the transmitted epitope