Syncytium formation in aged umbilical cord blood macrophages. Attempts to demonstrate an infectious etiology.

About 50% of cultures of mononuclear cells from human umbilical cord blood kept for more than 6 weeks in tissue culture reveal syncytium formation of adherent cells involving varying percentages of cells. Syncytium formation usually occurs focally producing plaques of multinucleated giant cells followed by cell degeneration. Occasionally, plaques of giant cell formation of adherent cells were also noted in macrophage cultures obtained from healthy adult blood donors. Attempts to transfer cytopathogenic changes to other human epithelial and fibroblastic cells in tissue culture were unsuccessful. In three instances, however, fibroblast-like cells started to grow spontaneously in umbilical cord blood cultures revealing syncytium formation and were subcultured once or twice. Six to eight weeks after their appearance, these cells too degenerated spontaneously also showing syncytium formation. Nuclei of the fused cells contained prominent nucleolus-like structures. Electron microscopy failed to demonstrate viral particles within these intranuclear inclusions. Occasionally, however, virus-like particles resembling in size and some structural features that of herpesviruses, were found in degenerating cells. Nucleic acid hybridizations performed with cellular DNA derived from such cultures and 32P-labelled DNA of herpesvirus saimiri and human cytomegalovirus failed to reveal any homologies. In a series of human sera tested for reactivity against fused cells in an indirect immuno-fluorescence test, two reacted specifically at low titer against the nucleolus-like structures of syncytial cells and some additional mononuclear cells.