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The protein pattern of a type-D retrovirus (PMFV) isolated from and propagated in human cell lines has been investigated using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). Staining with Coomassie blue and labeling with 14C-leucine/14C-lysine revealed five viral polypeptides with molecular weights of 10,000, 12,000, 15,000, 25,000, and 68,000. The 68,000 D-protein was shown to be a glycoprotein by incorporation of 3H-glucosamine and the 15,000 D-protein was identified as a phosphoprotein. By comparing PMFV with the closely related Mason-Pfizer monkey virus (MPMV) in co-electrophoresis experiments no clear difference was detected in viral 14C-leucine/14C-lysine profiles. The viruses differ, however, with respect to their glycoprotein patterns. A glycoprotein corresponding to the gp20 of MPMV has not been detected in PMFV irrespective of the cell line used for propagation of viruses.

IN an earlier communication we reported on multiple epithelial skin tumours in Syrian hamsters in which a Papova virus has been detected with great regularity, in large quantities, and in characteristic histological distribution, as evidenced by electron microscopic observation (Graffi et al., 1967). In experiments designed to transmit this disease by means of subcellular extracts from these tumours to other animals we surprisingly obtained in Syrian hamsters and, in certain circumstances, also in rats, leukoses and reticuloses. It is these that are the subject of the present report. MATERIALS AND METHODSSubcellular tumour extracts were prepared as follows: tissue from skin tumours of hamsters ( Fig. 1 and 2) was vigorously homogenized in a glass homogenizer in phosphate-buffered saline (PBS) solution and the homogenate was diluted 1: 5 to 1: 10 and centrifuged. One group of animals was treated with the supernatant of the homogenate obtained after centrifugation at 3000 r.p.m. for 20 minutes. In most experiments, however, the material injected was subjected to centrifugation at 4000 to 6000 r.p.m. twice or three times, followed by filtration through G4 glass filters twice. In some cases the filtrate was diluted 1: 1 with 0 * 25 M saccharose solution and centrifuged at 100,000 g for 40 to 60 minutes and the sediment was administered to the animals after suspension in PBS solution. By the same series of procedures, cell-free G4 filtrates were obtained from hamster leukoses induced by the injection of papilloma-derived material and from cell transplants of such leukoses. All manipulations were carried out in the cold (2°C.) as quickly as possible. The extracts were administered to (1) newborn Syrian hamsters of our own random-bred hamster colony that has a low spontaneous incidence of the type of skin tumour in question (maximum 5 per cent) and a low incidence of leukaemias mainly lymphomas (Horn and Siewert, 1968) arising in the mesenteric lymph nodes (about 3 per cent); (2) newborn Syrian hamsters of another hamster colony in which neither of the two types of tumour occur spontaneously; (3) newborn rats of a Wistar strain that has a spontaneous leukaemia incidence of about 0 5 per cent. The filtrate was in most cases injected subcutaneously, but occasionally intraperitoneally, in doses of 0 * 2 to 0 * 3 ml. per newborn hamster, or 0 -4 to 0 * 5 ml. per newborn rat.The electron microscopic investigation was performed with ultra-thin sections from tumours and from leukaemic infiltrates in liver, lymph nodes, spleen, kidney, etc. Material was fixed in glutaraldehyde and osmic acid and embedded in Epon. Uranyl and lead acetate were used to make contrast preparations.

A type-D retrovirus, derived from a human cell line and designated PMFV, which is related to, but distinct from the Mason-Pfizer monkey virus (MPMV) suppressed the in vitro mitogen response of human lymphocytes. PMFV was suppressive either as intact virus or after disruption by ether or detergents. In kinetic studies, the time course of the interaction between suppressive virus components and lymphocytes was characterized. Neither cytotoxicity nor time shifts in optimum 3H-thymidine incorporation of responding lymphocytes were observed. The suppressive activity of the disrupted virus was diminished by heating, freeze-thawing or treatment with pronase or trypsin, indicating temperature-sensitive proteins as suppressive components. By Sephacryl S-200 gel chromatography, molecular weights of approximately 70 and 15 kDa were determined for the suppressive components which were isolated from disrupted virus or virus-producing cells but not from non-infected cells.

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