Repeated and spontaneous switching of the chemokine co-receptors used by HIV-1 is rare in vivo

HIV-1 uses CD4 cells as its primary receptor and a chemokine co-receptor as its secondary receptor to gain entry into cells. Initial studies suggested that the dissemination of HIV-1 from lymphoid organs to non-lymphoid tissues was associated with an expansion in the number of chemokine co-receptors being used by the virus. The rapid progression to AIDS and death of patients whose viral isolates changed from R5 (non-syncytium inducing) to RSX4 (syncytium inducing) supported this conclusion. The factors that promote this switch in viral phenorype have not been established. We investigated whether repeated and spontaneous switching of the chemokine co-receptors being used by the virus is more common in vivo than has been recognized in the cross-sectional studies that have been performed.