Human herpesvirus 6 variant A, but not variant B, infects EBV-positive B lymphoid cells, activating the latent EBV genome through a BZLF-1-dependent mechanism

Human herpesvirus 6, a predominantly T lymphotropic virus, has been recently shown to infect some EBV-positive B cell lines, and to induce in them the activation of the EBV lytic cycle. Here we have confirmed and extended such observations, showing that (1) this phenomenon is restricted to the variant A of HHV-6: in fact two isolates belonging to the HHV-6 variant B (BA92 and Z29) were neither able to infect any B cell line, independently of the EBV status, nor to induce the EBV genome expression. The only exception is represented by the P3HR1 cells, in which, however, the infection by the variant B does not determine induction of EBV antigens; (2) the presence of the EBV genome contributes to the susceptibility of the B cell lines to HHV-6 infection, increasing the binding sites and the percentage of infectable cells, as detected by immunoelectron microscopy; and (3) HHV-6 infected T cells, transfected with plasmids bearing the promoter regions of the EBV early genes BZLF1 and BMRF1, show a strong transactivation of these promoters.