Human Cytomegalovirus UL135 and UL136 Genes Are Required for Postentry Tropism in Endothelial Cells

Abstract

Endothelial cells (ECs) are a critical target of viruses, and infection of the endothelium represents a defining point in viral pathogenesis. Human cytomegalovirus (HCMV), the prototypical betaherpesvirus, encodes proteins specialized for entry into ECs and delivery of the genome to the nuclei of ECs. Virus strains competent to enter ECs replicate with differing efficiencies, suggesting that the virus encodes genes for postentry tropism in ECs. We previously reported a specific requirement for the UL133/8 locus of HCMV for replication in ECs. The UL133/8 locus harbors four genes: UL133, UL135, UL136, and UL138. In this study, we find that while UL133 and UL138 are dispensable for replication in ECs, both UL135 and UL136 are important. These genes are not required for virus entry or the expression of viral genes. The phenotypes associated with disruption of either gene reflect phenotypes observed for the UL133/8NULL virus, which lacks the entire UL133/8 locus, but are largely distinct from one another. Viruses lacking UL135 fail to properly envelop capsids in the cytoplasm, produce fewer dense bodies (DB) than the wild-type (WT) virus, and are unable to incorporate viral products into multivesicular bodies (MVB). Viruses lacking UL136 also fail to properly envelop virions and produce larger dense bodies than the WT virus. Our results indicate roles for the UL135 and UL136 proteins in commandeering host membrane-trafficking pathways for virus maturation. UL135 and UL136 represent the first HCMV genes crucial for early- to late-stage tropism in ECs.

Importance: Human cytomegalovirus (HCMV) persists in the majority of the world's population. While typically asymptomatic in healthy hosts, HCMV can cause significant morbidity and mortality in immunocompromised or naïve individuals, particularly transplant patients and patients with congenital infections, respectively. Lifelong persistence of the virus may also contribute to age-related pathologies, such as vascular disease. One aspect of HCMV infection contributing to complex and varied pathogenesis is the diverse array of cell types that this virus infects in the host. The vascular endothelium is a particularly important target of infection, contributing to viral dissemination and likely leading to CMV complications following transplantation. In this work, we identify two viral gene products required for postentry tropism in endothelial cells. Identifying tropism factors required for replication in critical cell targets of infection is important for the development of strategies to restrict virus replication.