In Vitro Evolution of Bovine Foamy Virus Variants with Enhanced Cell-Free Virus Titers and Transmission

Virus transmission is essential for spreading viral infections and is a highly coordinated process which occurs by cell-free transmission or cell–cell contact. The transmission of Bovine Foamy Virus (BFV) is highly cell-associated, with undetectable cell-free transmission. However, BFV particle budding can be induced by overexpression of wild-type (wt) BFV Gag and Env or artificial retargeting of Gag to the plasma membrane via myristoylation membrane targeting signals, closely resembling observations in other foamy viruses. Thus, the particle release machinery of wt BFV appears to be an excellent model system to study viral adaption to cell-free transmission by in vitro selection and evolution. Using selection for BFV variants with high cell-free infectivity in bovine and non-bovine cells, infectivity dramatically increased from almost no infectious units to about 10(5)–10(6) FFU (fluorescent focus forming units)/mL in both cell types. Importantly, the selected BFV variants with high titer (HT) cell-free infectivity could still transmit via cell-cell contacts and were neutralized by serum from naturally infected cows. These selected HT–BFV variants will shed light into virus transmission and potential routes of intervention in the spread of viral infections. It will also allow the improvement or development of new promising approaches for antiretroviral therapies.