Cytoplasmic trafficking of the canine parvovirus capsid and its role in infection and nuclear transport

To begin a successful infection, viruses must first cross the host cell pla sma membrane, either by direct fusion with the membrane or by receptor-medi ated endocytosis. After release into the cytoplasm those viruses that repli cate in the nucleus must target their genome to that location. We examined the role of cytoplasmic transport of the canine parvovirus (CPV) capsid in productive infection by microinjecting two antibodies that recognize the in tact CPV capsid into the cytoplasm of cells and also by using intracellular expression of variable domains of a neutralizing antibody fused to green f luorescence protein. The two antibodies tested and the expressed scFv all e fficiently blocked virus infection, probably by binding to virus particles while they were in the cytoplasm and before entering the nucleus. The injec ted antibodies were able to block most infections even when injected 8 h af ter virus inoculation. In control studies, microinjected capsid antibodies did not interfere with CPV replication when they were coinjected with an in fectious plasmid clone of CPV. Cytoplasmically injected full and empty caps ids were able to move through the cytosol towards the nuclear membrane in a process that could be blocked by nocodazole treatment of the cells. Nuclea r transport of the capsids was slow, with significant amounts being found i n the nucleus only 3 to 6 h after injection.