Targeted deletion of regions rich in immune-evasive genes from the cytomegalovirus genome as a novel vaccine strategy

Human cytomegalovirus (CMV), a ubiquitous human pathogen, is a leading cause of congenital infections and represents a serious health risk for the immunosuppressed patient. A vaccine against CMV is currently not available. CMV is characterized by its large genome and by multiple genes modulating the immunity of the host, which cluster predominantly at genome termini. Here, we tested whether the deletion of gene blocks rich in immunomodulatory genes could be used as a novel concept in the generation of immunogenic but avirulent, herpesvirus vaccines. To generate an experimental CMV vaccine, we selectively deleted 32 genes from the mouse cytomegalovirus (MCMV) genome. The resulting mutant grew to titers similar to that of wild-type MCMV in vitro. In vivo, the mutant was 10,000-fold attenuated and well tolerated, even by highly susceptible mice deficient for B, T, and NK cells or for the interferon type I receptor. Equally relevant for safety concerns, immune suppression did not lead to the mutant's reactivation from latency. Immunization with the replication-competent mutant, but not with inactivated virus, resulted in protective immunity, which increased over time. Vaccination induced MCMV-specific antibodies and a strong T-cell response. We propose that a targeted and rational approach can improve future herpesvirus vaccines and vaccine vectors.