Combined immunocytochemistry and non-isotopic in situ hybridization for the ultrastructural investigation of human parvovirus B19 infection.

Parvovirus B19 is a single-stranded DNA virus with a specific tropism for human erythroid precursor cells. The virus codes for two overlapping structural (capsid) proteins and one non-structural protein which is thought to perform essential functions in viral replication, transcription and packaging. The ultrastructural localization of these proteins was achieved in cultured haemopoietic cells derived from fetal liver which had been infected in vitro and subsequently embedded in LR White acrylic resin. Postembedding immunogold detection of B19 structural and non-structural proteins was combined with localization of viral nucleic acid by in situ hybridization using a digoxigenin-labelled probe and different sized gold labels. The majority of the B19 capsid protein and DNA present in cells harvested 48 hours post-infection co-localized within the centri-nuclear region of erythroid cells demonstrating characteristic chromatin margination. Relatively little DNA hybridization signal was present over paracrystalline inclusions strongly labelled with anti-capsid protein monoclonal antibody R92F6. Viral DNA and capsid protein were co-localized in apparent egress from the nucleus through nuclear pores. B19 non-structural protein was detected in association with both nuclear and cytoplasmic arrays of capsids, supporting the view that this protein plays an important role in viral packaging and remains associated with the complete viral particle until its release from the cell. Co-localization of viral nucleic acid and proteins at the ultrastructural level is a flexible, rapid and highly specific tool for examination of viral life-cycles within cells.