Ultrastructural localization of the RNA of immunodeficiency viruses using electron microscopy in situ hybridization and in vitro infected lymphocytes

Cells infected in vitro with immunodeficiency viruses have been examined by electron microscopy in situ hybridization (EM ISH) methods for localization of viral RNA. Techniques used for preparation of specimens and probes are described. Unambiguous positive results were obtained using a mixture of two or three single negative strand DNA oligonucleotides complementary to regions of the gag, env and nef genes, each 200-300 bases and labelled with dig-11-UTP. Positive strand probes were used as a negative control. Cells were fixed with a mixture of formaldehyde and glutaraldehyde, dehydrated in ethanol with progressive lowering of temperature and embedded in Lowicryl K4M or HM20 at -35 degreesC. Permeabilization or pre-treatment of sections with proteinase K was not essential. The hybridization mixture was applied for 3-4 h at 37 degreesC and probe was visualized by direct immune-staining with sheep anti-digoxigenin antibodies conjugated to 10 nm gold. This method would be suitable for future studies of the pathogenesis of retroviral infections and as a basis for further development of the EM ISH technique. EM ISH of in vitro infections of immunodeficiency viruses has shown the location of viral RNA in immature and mature viruses and its relationship to multimerized Gag protein during viral budding. The label for RNA has also been found in the cytoplasm of infected cells; it was mainly located adjacent to the plasma membrane and unassociated with visible Gag proteins. This may indicate that viral RNA migrates to the plasma membrane independently of the Gag protein and may, in some instances, arrive at the plasma membrane prior to the Gag protein. Viral RNA has also been found in the nucleus of peripheral blood mononuclear cells (PBMC) that were showing no morphological evidence of infection. The RNA was typically located in the nucleolus and in peripheral dense chromatin. These cells, which displayed morphological features of macrophage lineage, may have been the initial cell type to be infected in the PBMC. (C) 2001 Elsevier Science Ltd. All rights reserved.