Parsing the Functions of Immediate Early Proteins in HCMV Infection Outcome

Human cytomegalovirus (HCMV) is a prevalent pathogen of the herpesvirus family, infecting most of the human population worldwide. Like all herpesviruses, HCMV can establish a latent infection that persists throughout the lifetime of the host. The HCMV immediate early (IE) proteins, IE1 and IE2, are viewed as master regulators of HCMV infection and are commonly assumed to play pivotal roles in regulating the balance between latent and lytic infection, as their repression is a hallmark of latency. However, it is still unclear whether their expression can indeed determine the establishment of productive infection and what functions, either related to viral gene expression or to cellular pathways, are involved in this activity. Using THP1 monocytes, ectopically expressing the HCMV receptor, PDGFRα to boost viral entry, we show that overexpression of either IE1 or IE2 significantly enhances productive infection, illustrating their critical role in determining infection outcome. Mechanistically, we show IE2 drives expression of the viral early genes at early stages of infection, whereas IE1 acts more broadly to enhance global viral gene expression. We further show that from the many functions of IE1, its ability to promote lytic infection is mainly linked to the disruption of PML nuclear bodies. Importantly, induction of either IE1 or IE2 expression in latently infected cells enhances viral reactivation, with IE1-mediated PML representing a central mechanism. Taken together, our findings elucidate the distinct and complementary roles of IE1 and IE2 in overcoming barriers to productive infection and reactivation.