Human Papillomavirus 16 Capsids Mediate Nuclear Entry during Infection

Infectious human papillomavirus 16 (HPV16) L1/L2 pseudovirions were found to remain largely intact during vesicular transport to the nucleus. By electron microscopy, capsids with a diameter of 50 nm were clearly visible within small vesicles attached to mitotic chromosomes and to a lesser extent within interphase nuclei, implying nuclear disassembly. By confocal analysis, it was determined that nuclear entry of assembled L1 is dependent upon the presence of the minor capsid protein, L2, but independent of encapsidated DNA. We also demonstrate that L1 nuclear localization and mitotic chromosome association can occur in the murine cervicovaginal challenge model of HPV16 infection. These findings challenge the prevailing concepts of PV uncoating and disassembly. More generally, they document that a largely intact viral capsid can enter the nucleus within a transport vesicle, establishing a novel mechanism by which a virus accesses the nuclear cellular machinery. Papillomaviruses (PVs) comprise a large family of nonenveloped DNA viruses that include HPV16, among other oncogenic types, the causative agents of cervical cancer. Delivery of the viral DNA into the host cell nucleus is necessary for establishment of infection. This was thought to occur via a subviral complex following uncoating of the larger viral capsid. In this study, we demonstrate that little disassembly of the PV capsid occurs prior to nuclear delivery. These surprising data reveal a previously unrecognized viral strategy to access the nuclear replication machinery. Understanding viral entry mechanisms not only increases our appreciation of basic cell biological pathways but also may lead to more effective antiviral interventions.