Figure 7. Proposed evolution of V1V2 loop size change during transmission and HIV infection.

At the time of sexual transmission, a significant genetic bottleneck occurs in which one or a small number of donor variants is transmitted to the recipient, without clear selection for loop size (represented on the y-axis). During early infection, prior to an effective host response, viral variants with a compact V1V2 loop have a competitive advantage, and V1V2 loop size remains stable or regresses. During chronic asymptomatic infection, mean V1V2 length increases in response to (humoral) immune selective pressure. As immune function wanes, V1V2 loop length gradually declines.