CCR5 and infection: In order to determine whether extracellular Tat produced by neighbor cells could affect the entry of virus particles and infection independently of its transactivating activity, a co-culture system was used in which CEMss-Tat expressing cells (which do not express CCR5) and TZM-bl cells, which are CCR5+, were co-cultured in the presence or absence of a single-cycle, Tat-independent and GFP expressing SF162 (VP/SF162) [64], as described in materials and methods.