Tumors induced by progressor sarcoma virus (Moloney) in mice: Growth in the presence of an immune response and isolation of autonomously growing cells

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Abstract
Courses of tumors, which had been induced in adult STU mice with a regressor and with a progressor strain of Moloney sarcoma virus (MSV-M) were followed and compared. All 73 tumors induced by the regressor strain of MSV-M (R-MSV-M) regressed and 181 of 183 tumors induced by a progressor strain (P-MSV-M) grew progressively and killed their hosts between 16 and 171 days after infection. Even after inoculation of about 4 FFU of P-MSV-M tumor development may occur and lead to progressively growing tumors. Both strains of MSV-M induce strong immune responses in their host, namely cytotoxic effector cells, cytotoxic antibodies and tumor resistance. Simultaneous injection of mice on separate sites with P-MSV-M on one side and R-MSV-M producing ascitic cells on the other side did not induce mutual influences on the different tumor courses. It is therefore concluded that the immune response is obviously not decisive for the failure of P-MSV-M induced tumors to regress. From seven P-MSV-M induced primary tumors two producer and five nonproducer transformants could be isolated, maintained in culture and partly held as easily transplantable tumors in adult immunocompetent mice. Both producer transformants release sarcoma virus with the capacity to induce progressively growing tumors in adult mice. All transformants induce progressively growing tumors after transplantation independent of the applied cell dose. It is concluded that the ability of the P-MSV-M to induce stably transformed cells (in contrast to R-MSV-M,Weiland et al., Brit. J. Cancer, 1979) favours clonal development and ultimately progressive tumor course, in spite of the presence of an immune response.