In recent years, nuclear domain 10 (ND10), also called PML bodies, has been a topic of intense interest, especially in terms of its role in viral infection 
. Although a great deal of evidence supports the theory that ND10 components such as PML, Daxx, and SP100 are viral gene repressors and protect host cells against many viruses 
, the effects of the ND10 structure on viral infection have been not determined. The fact that several herpesviruses are able to disrupt ND10 at a very early stage of infection implies that ND10 has a defensive role in the process 
. However, several DNA viruses (such as herpesviruses) dock their input DNA, replicate DNA, and transcribe immediate-early genes at ND10, which argues that ND10 favors viral replication 
. To comparatively investigate the roles of ND10 and ND10 proteins in cross-species infections (MCMV in human cells and HCMV in mouse cells), we performed an immunofluorescence assay to detect the effects of CMV infection on ND10. We discovered that during the infection of MCMV in human cells, MCMV IE1 distributed both diffusely and as domains (), which differs from what has been found in mouse cells, where IE1 distributes only diffusely. In addition, MCMV IE1 loses the ability to disrupt the ND10 of human cells. HCMV IE1 also loses the ability to disperse mouse cell ND10.
Previously, it was found that laboratory strains of HCMV infection in mouse cells can express only IE1 and not IE2, even though that IE1 shares a promoter and its first three exons with IE2. Therefore, it was concluded that HCMV infection in mouse cells was blocked at the IE stage 
. However, it has been reported that the laboratory strains of HCMV experience profound mutations during replication in human fibroblast cells 
. The mutations lead not only to the attenuation of HCMV but also to the narrowing of cell tropisms. The mutation of HCMV cell tropism occurred at gene UL128–131 since the repaired UL128–131 caused the recovery of cell tropisms 
. Therefore, this time using both the clinical HCMV strain and the repaired HCMV, we reentered the study of HCMV infection in mouse cells. At this point, we found that HCMV could produce IE proteins and some early proteins ( and ), but failed to replicate DNA. We now conclude that HCMV infection in mouse cells was blocked before DNA replication.
Unlike what takes place in RNA viruses (in which species specificity is determined by the interaction of viral proteins and cellular receptors) 
, the species-specific restriction of CMV occurs not at the entry to cells but at the post-DNA replication stage for MCMV infection in human cells 
and at the early stage before DNA replication for HCMV infection in mouse cells. Cellular proteins, including transcription repressors (Daxx, PML, SP100), have suppressive effects on viral gene expression and represent an intrinsic, host-cell defense 
. PML is the scaffold protein and is essential for the formation of ND10 because PML knockout (k/o) cells lack ND10, and inducing exogenous PML into PML knockout cells can restore ND10 
. SUMOylation is another characteristic of PML and makes it possible for PML to interact with many other nuclear proteins. There are more than 70 different cellular proteins that have been found to be related to ND10, and the proteins that interact with PML have already been reviewed by Dr. Van Ostade1 and colleagues 
. The most frequently investigated PML-interacting proteins include Daxx and SP100. ND10 structure can be shown by indirect immunofluorescence using anti-PML, -Daxx, and -SP100 antibodies.
The inhibitory effects of ND10 proteins on viral infection have been demonstrated on PML, Daxx, and SP100. The effects of the ND10 structure on viral infection have not been determined. ND10's defensive role in the infection process can be inferred by the fact that several herpesviruses are required to disrupt it. We previously showed that IE1 is the only protein of MCMV that is capable of disrupting the ND10 of mouse cells 
. In that prior study, we reported that the IE1 of MCMV also colocalized with the ND10 of human cells; however, IE1 lost its ability to disperse ND10 in cytomegalovirus cross-species infections. This discovery supports the theory that ND10 might block the productive cross-species infection of cytomegalovirus. Consistent with this speculation, we found that human-cell ND10 proteins, such as SP100, PML, and Daxx, strongly suppress MCMV viral gene expression (), and mouse cell ND10 protein also represses HCMV gene expression ().
Interestingly, HCMV laboratory-strain infections in mouse cells can produce IE1 but not IE2, even though IE1 and IE2 share a promoter and the first three exons, all of which suggests that splicing regulation also plays a role in blocking HCMV infection in mouse cells.
In summary, we discovered that intrinsic cellular defense mechanisms participate in the blocking of CMV cross-species infection and that CMV IE1 loses its ability to disperse ND10. In HCMV-infected mouse cells, only IE1 (and not IE2) can be detected in laboratory-strain–infected mouse cells, but clinical strains and UL128–131-repaired strains can produce many more viral gene products. Future studies will focus on identifying the additional mechanisms that are involved in blocking cross-species infection.