CCR5 and infection: In order to adapt the PBMCHIV+/TZM-bl infection system to specifically quantify cell-cell transmission we made use of the fact that many CCR5 (R5) using HIV strains are only capable of efficiently infecting engineered, CCR5 and CD4 expressing target cell lines such as TZM-bl in the presence of polycations [34], [41], [42].