Hi Microcapfun,
Read the Celsentri briefing doc for a good backgrounder on this.
Transition from a macrophage tropism (CCR5) to T-cell (CXCR4) tropism is a natural phenomenon in the natural history of HIV, for the simple reason that the macrophages are usually among the very first cells infected after exposure to HIV because of their location. A gradual evolution to CXCR4 chemokine receptor use causes a shift in the proportion of productively infected cells toward the CD4+ T-cells.
I would assume the presence of a very small percentage of CXCR4-tropic virions in every infected individual. Otherwise T cells wouldn't be infected reasonably early after exposure, and of course they are.
Also, in reading through the aforementioned briefing doc, I noted that mutations can also arise that allow the virus to use CCR5 even in the presence of maraiviroc.
