Analysis of the Basis for Persistence of Herpes Simplex Virus Type 1 in Undifferentiated U937 Cells

Replication of herpes simplex type 1 (HSV-1) is inhibited in the human monocyte-like cell line, U937, when the cells are in the undifferentiated state, but when the cells are stimulated to differentiate by treatment with the phorbol ester, phorbol 12-myristate 13-acetate virus is replicated. Because HSV-1 has been shown to persist in these cells and in their in vitro counterparts freshly isolated human blood monocytes, we initiated an analysis of viral persistence in undifferentiated U937 cells. No appreciable HSV-1 DNA replication was observed in undifferentiated U937 cells compared with differentiated U937 cells and with fully permissive Vero cells. However, using in situ hybridization, we established that a significant percent of the undifferentiated U937 cells contained viral DNA sequences. Interestingly, when analyzed by Southern blot hybridization, this DNA was found to have assumed a nonlinear configuration similar to that found in latently infected neurons. Analysis of viral proteins in undifferentiated U937 cells revealed a marked absence of proteins of all three kinetic classes. However, in transient transfection assays, the major viral transactivating protein ICP4, functioned normally, whereas ICP0, a promiscuous transactivator of both viral and cellular genes, was unable to transactivate viral promoters in undifferentiated U937 cells. Thus, a subtle dysfunction in the activity of ICP0 may account, at least in part, for the inability of undifferentiated U937 cells to support replication of HSV-1.