Human Herpesvirus‐6 Induces MVB Formation, and Virus Egress Occurs by an Exosomal Release Pathway

Abstract

†These authors contributed equally to this work.The final envelopment of most herpesviruses occurs at Golgi or post-Golgi compartments, such as the trans Golgi network (TGN); however, the final envelopment site of human herpesvirus 6 (HHV-6) is uncertain. In this study, we found novel pathways for HHV-6 assembly and release from T cells that differed, in part, from those of alphaherpesviruses. Electron microscopy showed that late in infection, HHV-6-infected cells were larger than uninfected cells and contained many newly formed multivesicular body (MVB)-like compartments that included small vesicles. These MVBs surrounded the Golgi apparatus. Mature virions were found in the MVBs and MVB fusion with plasma membrane, and the release of mature virions together with small vesicles was observed at the cell surface. Immunoelectron microscopy demonstrated that the MVBs contained CD63, an MVB/ late endosome marker, and HHV-6 envelope glycoproteins. The viral glycoproteins also localized to internal vesicles in the MVBs and to secreted vesicles (exosomes). Furthermore, we found virus budding at TGNassociated membranes, which expressed CD63, adaptor protein (AP-1) and TGN46, and CD63 incorporation into virions. Our findings suggest that mature HHV-6 virions are released together with internal vesicles through MVBs by the cellular exosomal pathway. This scenario has significant implications for understanding HHV-6's maturation pathway.

Rhomboid Protease PARL Mediates the Mitochondrial Membrane Potential Loss-induced Cleavage of PGAM5

“…pH 7.2, for immunoelectron microscopy (27). In the former procedure the cells were post-fixed with 1% OsO 4 and embedded in Epon 812 after dehydration.…”

“…2A), we speculated that the cleavage site was located in the N-terminal region. Therefore, we performed Edman degradation analysis of the lower band of PGAM5 and determined that its N-terminal sequence was Ala-Val-Ala-Val-Gly (amino acids [25][26][27][28][29]. This suggested that the lower band of PGAM5 was a processed form and that the cleavage site was located in the TM domain (Fig.…”

Extracellular vesicles: novel vehicles in herpesvirus infection

“…MVBs contain CD63, which is an MVB late endosome marker and envelope glycoprotein, that is localized in internal vesicles. The available evidence indicates that HHV-6 virions are released through the exosomal pathway, which demonstrates the significance of exosomes in the HHV6 maturation pathway (Mori et al, 2008). Additionally, MHC class I molecules are another rare member that have been found to be inserted into viral particles and exosomes and then released into the extracellular space via an exosomal secretary pathway, which can be blocked by the U24 protein of HHV-6 (Sullivan and Coscoy, 2010;Ota et al, 2014).…”

The Herpes Simplex Virus-1 Encoded Glycoprotein B Diverts HLA-DR into the Exosome Pathway

“…These exosomes are derived from MVBs. The route of exosomal release may also be targeted by viral proteins (21). A release of gB and of gB-DR complexes by exosomes could demonstrate a novel route for the HSV-1-encoded gB and of gB-associated DR. To elucidate some details of trafficking in the endocytic pathway, we investigated whether MJ cells secrete exosomes.…”