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Summary A model system for the comparative study of human astrocytes before and after infection with the encephalomyelitic virus of sheep, visna virus, is described. The emergence of a variant of visna virus and the modifications induced in affected cultures are discussed.

Moscovici, Carlo (University of Colorado Medical Center, Denver), and E. H. Macintyre . Effect of avian myeloblastosis virus in the Japanese quail. J. Bacteriol. 92: 1141–1149. 1966.—Avian myeloblastosis virus (AMV) induced a spectrum of neoplasms in Japanese quail ( Coturnix coturnix japonica ) which was similar to that observed in the chicken, with one exception: the total absence of acute myeloblastic leukemia in quail. Studies in vivo as well as in vitro suggested that the cause for this difference may be ascribed to the heterogeneity of AMV and to the genetic makeup of the quail cell.

VISNA, a medium-sized RNA virus, is comparable in mode of maturation and a number of other properties with the RNA tumour viruses1,2. The virion also carries RNA-dependent DNA polymerase3–5. This enzyme was first discovered in RNA tumour viruses and thought perhaps unique to them6–9; but it was subsequently found in a very few other RNA viruses3,10, where tumorigenicity was not a known attribute. The question whether RNA-dependent DNA polymerase is a hallmark of malignancy therefore remains entirely open.

CHRONIC infection with measles virus is causally related to the human syndrome of subacute sclerosing panencephalitis (SSPE)1–5, and means of reproducing that condition in experimental animals are being sought6–10. Measles carrier states in vitro have been established in some human11–13 and rodent14,15 cell systems. We now report the establishment of measles carrier states in human astrocyte cultures of normal brain, and in a tumour astrocyte line, as well as the emergence of fine structural features which bear a close resemblance to findings in the brain of some patients with SSPE16.

A study was undertaken to seek fine structural differences between normal and maglinant astrocytes and normal fibroblasts in cell culture. A series of human adult normal astrocytes and malignant astrocytes (including RSV-transformed cells) whose cultural characteristics had previously been reported formed the basis of the study. Also included were matched normal human fetal fibroblasts and astrocytes (from the same fetus) and methylcholanthrene-induced murine malignant astrocytes. Pellets of cells scraped from their culture vessel and sectioned as well as in situ preparations were examined. The fibroblasts were the only cells whose external microfibrils were collagenase-sensitive and considered to be tropocollagen. Both the benign and the malignant astrocytes had intracytoplasmic microfibril bands (gliofibrils) and some had external collagenase-resistant microfibrils. Malignant astrocytes had a much more developed membrane system (used in its widest sense) than normal astrocytes, had many free ribosomes and some had morphologically abnormal mitochondria. Virus was seen only in the murine malignant astrocytes, was abundant and of the murine leukemia virus group. Areas interpreted as representing fusion in progress were seen at the surface of some RSV-transfomed giant tumor astrocytes. The fine structure of one cell type from an IgG-producing culture from human glioblastoma corresponded with that reported for immunocytes.

Human tumour astrocytes (118MG) were exposed in vitro to the Engelbreth-Holm strain of Rous sarcoma virus at a multiplicity of infection of one. Morphological transformation of the cells in 60-mm plates was complete in 9-11 weeks after viral exposure. The transformed cells (EH-118MG) grew as a monolayer, upon which nests of rounded cells developed. From these nests, rounded cells shed into the medium and in turn on seeding formed a monolayer and repeated the cycle. The transformed cells contained the viral group-specific antigen in the cytoplasm, and produced Rous sarcomas in chickens. No complete virus was demonstrated in the mammalian cells. Electron micrographs of the parent 118MG and transformed EH-118MG cells gave further details of the structure of cytoplasmic protrusions which were formed in some of the free EH-1 18MG but not in the 118MG cells. Their nature is completely unknown.

Comparison of human astrocytes (1181N1 line) and sheep choroid plexus cells (SCP) infected with visna virus (V-1181N1 and V-SCP) shows that the mode of virus assembly and the morphology and size of the free virions are identical in the 2 systems. V-SCP cells usually cytolyse soon after infection; V-1181N1 cells do not. This is considered to be due to interference with membrane assembly in the former. Syncytial giant cells from both V-1181N1 and V-SCP cultures have a much looser attachment to the growth surface than their uninfected parents. This is shown to be associated with a loss of certain microfibrillar arrays close to the plasmalemma. Uninfected SCP cultures have cilia and desmosomes and retain these after infection till cytolysis; 1181N1 cells do not have these structures, but have a well developed microtubular system, which the SCP cells lack.

Bovine fibroblasts transformed by polyoma virus have been cocultivated with syngeneic or allogeneic normal cells. The growth of the transformed cells was inhibited by the presence of normal cells. The strongest depression was obtained when a dense stationary layer of normal fibroblasts was challenged by polyoma cells. The inhibition was least pronounced if normal and transformed cells were simultaneously seeded. Polyoma-transformed cells were identified by radioautography after incorporation of tritiated thymidine. After 96 h of contact with stationary normal cells the number of grains per nucleus and the number of labelled nuclei had not changed indicating strong depression of DNA synthesis. A deficient attachment or premature detachment of polyoma cells could not account for the observed failure of growth in the normal/polyoma mixed cultures.