Intermittent bulk release of human cytomegalovirus

Abstract

Human Cytomegalovirus (HCMV) can infect a variety of cell types by using virions of varying glycoprotein compositions. It is still unclear how this diversity is generated, but spatio-temporally separated envelopment and egress pathways might play a role. So far, one egress pathway has been described in which HCMV particles are individually enveloped into small vesicles and are subsequently exocytosed continuously. However, some studies have also found enveloped virus particles inside multivesicular structures but could not link them to productive egress or degradation pathways. We used a novel 3D-CLEM workflow allowing us to investigate these structures in HCMV morphogenesis and egress at high spatio-temporal resolution. We found that multiple envelopment events occurred at individual vesicles leading to multiviral bodies (MViBs), which subsequently traversed the cytoplasm to release virions as intermittent bulk pulses at the plasma membrane to form extracellular virus accumulations (EVAs). Our data support the existence of a novel bona fide HCMV egress pathway, which opens the gate to evaluate divergent egress pathways in generating virion diversity. Author summary HCMV is a clinically highly relevant virus, which causes serious disease affecting multiple organs. Despite HCMV being an important pathogen, especially for newborn children and immunocompromised patients, treatment options are still limited. Understanding how HCMV can infect a wide variety of cells is essential for developing antiviral strategies. It is well established that HCMV particles with varying glycoprotein repertoires facilitate entry into different target cells. How different glycoprotein compositions are generated at the single-particle level is still unclear. Different envelopment and egress pathways might play a role in creating this diversity. Here we present direct functional evidence that HCMV uses multiviral bodies (MViBs) for the bulk release of virus particles into extracellular viral accumulations (EVAs) as a novel, alternative HCMV egress pathway. Our data from two different HCMV strains, TB40/E and Merlin, indicate that MViBs play an important role in the production of viral particles. Our results provide a basis to illuminate how different egress pathways lead to varying virion compositions and potentially determine the tropism of HCMV progeny.