Figure 2.

Evolution of host range variants. In this model the variant of HIV-1 that is replicating in memory T cells (requiring high levels of CD4 on the surface of the cell and using CCR5 as the coreceptor, shown in green) is exposed to reduced host surveillance in the form of reduced selective pressure from antibodies. This allows the virus to evolve such that the Env protein assumes a more open conformation that allows increased binding to CD4. The open conformation may also expose a latent low level tropism for CXCR4. These changes potentiate the subsequent evolution to use CXCR4 efficiently (X4 virus—red) or to use CD4 more efficiently (macrophage tropism—blue).