The biological, immunological, and physicochemical characterization of a transmissible agent capable of inducing DNA and thymine degradation in cultured human cells

A study of the mechanisms of DNA And thymine degradation in cultured human cells infected with a lipovirus

DNA and thymine degradation on cultured human, mouse, and chick cells were studied. Significant increase in DNA-degrading activity was demonstrated in human embryonic cells killed by freeze-thawing, liver cells killed with mitomycin C, mouse embryonic cells infected with encephalomyocarditis virus, and in all cells killed by the lipovirus. Twelve other viral agents, actinomycin D, and 5-fluorodeoxyuridine failed to produce a similar increase. Thymidine-2-C¹⁴-labeled cultures, either live, killed, or infected by 19 different physical-chemical and biological agents, did not release detectable quantity of C¹⁴C₂. Following infection with the lipovirus 20 to 60 per cent of the total radioactivity of thymidine-2-C¹⁴-labeled cultures was liberated as C¹⁴O₂. It was postulated that the lipovirus introduced into the host cells the missing genetic information necessary for the synthesis of one or more enzymes responsible for the reductive catabolism of thymine.

A lipogenic toxin released through the interaction of a new cytopathic agent (lipovirus) and cultured human cells

The release of a toxic product from cultured human cells infected by a new cytopathic agent (the lipovirus) was described. This toxin was dissociable from the infectious particles. It induced sudanophilia of human and mouse cells, an increase in the total fatty acid content, and a change in the major constituent fatty acids. Similar toxin was not demonstrated in cultures infected by the vaccinia herpes simplex, adeno 3, polyoma, polio 1 and 2, Coxsackie B1, parainfluenza A, and Rous sarcoma viruses. Preliminary characterization indicated that this toxin was resistant to tryptic digestion and could not be dialyzed or neutralized by human gamma globulin. It was inactivated at 58°C for 30 minutes, but stable at 37°, 4°, and –60°C. The significance of these findings is discussed.

On the nature of the "lipovirus"

Experiments designed to elucidate the nature of the "lipovirus" are described. The development of characteristic nuclear lesions in human cells in vitro depended on the presence of an ameboid cell in the inoculum. The spatial separation of the ameboid cells from the human cells by a membrane filter 150 micro in thickness was sufficient to prevent the development of nuclear lesions. Nuclear lesions appeared to be the primary change of the affected human cells. This development of nuclear lesions was partially suppressed by FUDR and the suppression was reversed by thymidine. Time-lapse microcinematography showed that a 30 min intermittent contact between an ameboid cell and a human cell resulted in the retraction of both progenies of the human cell after a lapse of about 36 hr. Other human cells not in contact with the ameboid cell remained polygonal and continued to divide. Radioautography of the ameboid cell revealed the presence in the cytoplasm of thymidine-containing DNAse-sensitive materials. The development of antigens related to the ameboid cell within the cytoplasm of the human cell is described in the accompanying report (4).

Radiation Resistance in Lipovirus-Altered Human Cells

Human cells altered by infection with the "lipovirus," a transmissible cytopathic agent, are highly resistant to ionizing radiation. Whereas about 500 roentgens will inhibit cell multiplication in normal cells, more than 500,000 roentgens are required to produce a similar effect in the altered cells. This gross change in cellular radiosensitivity may provide a useful system for studying in vitro the mechanisms of inhibition of cell division by radiation.