PML plays both inimical and beneficial roles in HSV-1 replication

After entry into the nucleus, herpes simplex virus (HSV) DNA is coated with repressive proteins and becomes the site of assembly of nuclear domain 10 (ND10) bodies. These small (0.1-1 mu M) nuclear structures contain both constant [e.g., promyelocytic leukemia protein (PML), Sp100, death-domain associated protein (Daxx), and so forth] and variable proteins, depending on the function of the cells or the stress to which they are exposed. The amounts of PML and the number of ND10 structures increase in cells exposed to IFN-beta. On initiation of HSV-1 gene expression, ICP0, a viral E3 ligase, degrades both PML and Sp100. The earlier report that IFN-beta is significantly more effective in blocking viral replication in murine PML+/+ cells than in sibling PML-/- cells, reproduced here with human cells, suggests that PML acts as an effector of antiviral effects of IFN-beta. To define more precisely the function of PML in HSV-1 replication, we constructed a PML-/- human cell line. We report that in PML-/- cells, Sp100 degradation is delayed, possibly because colocalization and merger of ICP0 with nuclear bodies containing Sp100 and Daxx is ineffective, and that HSV-1 replicates equally well in parental HEp-2 and PML-/- cells infected at 5 pfu wildtype virus per cell, but poorly in PML-/- cells exposed to 0.1 pfu per cell. Finally, ICP0 accumulation is reduced in PML-/- infected at low, but not high, multiplicities of infection. In essence, the very mechanism that serves to degrade an antiviral IFN-beta effector is exploited by HSV-1 to establish an efficient replication domain in the nucleus.