Small virus-like particles bud from the cell membranes of normal as well as HIV-infected human lymphoid cells.

Electron microscopy (EM) of cell sections showed cell associated virus-like particles (VLP), 50-60 nm in diameter, budding from the membrane of human lymphoid cells in culture. The particles had an envelope continuous with the cell membrane and a dense core that almost filled the particle. Particles 70-80 nm in diameter with prominent external spikes were found in the culture medium by negative staining (medium-associated VLP). Cell-associated VLP were also present in cord lymphocytes, both on initial separation and after culture with or without foetal calf serum, and therefore were considered to be endogenous to the cells and were not bovine diarrhoea virus. VLP were observed in most of the lymphoid cell lines examined. VLP were also found less frequently in established human tumour and nontumour cell lines. Both cell-associated and medium-associated VLP were also present in HIV infected cell cultures, and they could be distinguished from HIV by their characteristic morphology and smaller size. It was not determined whether the 2 types of particle represented the same entity, or whether they were defective virus particles or a cellular secretory product.