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Quantitation of Virus-induced (MLr) and Normal (Thy.1.2) Cell Surface Antigens in Isolated Plasma Membranes and the Extracellular Ascites Fluid of Mouse Leukemia Cells¹

Departments of Biochemistry [ W. J. V. B., P. E., D. K.], Biology [J. H., R. N.], and Electronmicroscopy [C. A. F.], The Netherlands Cancer Institute, Antoni van Leeuwenhoek-Laboratory, Sarphatistraat 108, Amsterdam, The Netherlands

Plasma membranes were isolated by two methods from mouse leukemia cells containing mammary tumor virus-induced (MLr) and normal (Thy.1.2) antigens on their surfaces. A number of chemical components, enzymic activities, and the antigenic contents were determined in subcellular fractions and found to be specifically concentrated in the plasma membrane fractions. The major part of the cellular MLr, in contrast to Thy.1.2, was present in the 105,000 x g_max supernatant of the cell homogenate. This and other results indicate an easy release of the antigen from the plasma membrane. A considerable amount of MLr was also present in the ascites fluid, partly free and partly bound, supposedly in an immune complex that allowed the isolation of three components of similar molecular weights as mammary tumor virus components. The extracellular presence of MLr may illustrate that, by shedding of antigen, the tumor may protect itself against the immunological defense of the host.

¹ This work was supported in part by Contract NOI-CP-33368 with the Virus Cancer Program of the National Cancer Institute. This paper was originally presented as part of the symposium on Immunologic Control of Virus-associated Tumors in Man: Prospects and Problems, April 7 to 9, 1975, at NIH, Bethesda, Md.

Received 3/17/75. Accepted 6/ 9/75.