A bovine macrophage cell line supports bovine herpesvirus-4 persistent infection

Although bovine herpesvirus-4 (BHV-4), a gammaherpesvirus lacking a clear disease association, has been demonstrated in many tissues during persistent BHV-4 infection, a likely site of virus persistence is in cells of the monocyte/macrophage lineage. To establish an in vitro model of persistent infection potentially useful for examining the molecular mechanisms of BHV-4 persistence/latency, we infected the bovine macrophage cell line BOMAC. Following extensive cell death, surviving cells were found to be persistently infected, maintaining the viral genome over many passages and producing low levels of infectious virus. Although selection was unnecessary for the maintenance of the viral genome, cells persistently infected with recombinant BHV-4 containing a neomycin-resistance gene could be selected with geneticin, thus confirming that persistent BHV-4 infection was compatible with cell survival and replication. Furthermore, persistent BHV-4 infection caused no decrease in the growth rate of BOMAC cells. Sodium butyrate, which reactivates latent gammaherpesviruses in vitro, or dexamethasone, which reactivates latent BHV-4 in vivo, increased viral DNA by 10- to 15-fold in persistently infected BOMAC cells. This suggests that reactivation of latent BHV-4 by dexamethasone in vivo might involve direct action of dexamethasone on latently infected cells.