EXPERIMENTAL MODEL AND APPROACHES TO INVESTIGATION OF THE ACQUIRED RESISTANCE TO TUMOR TRANSPLANTATION IN MICE

Abstract

Introduction. An acquired resistance to experimental tumors was detected in animals that recovered from a primary transplanted tumor due to treatment or spontaneously, and demonstrated intolerance to a renewal tumor inoculation. This phenomenon is much less frequently observed, although it is of great scientific interest and medical significance. Here, we have addressed the expression of the resistance phenomenon in a model tumor in mice – Nemeth–Kelner lymphoma (NK/Ly).
The aim of our study was to elaborate a reproducible method for induction of resistance to transplantation of lymphoma NK/Ly in mice and to investigate the mechanisms of its development.
Methods and Results. Three schemes for induction of resistance were tested. The first one included treatment of tumor-bearing mice with vinblastine and, thereafter, reconvalescent animals were checked for the development of resistance expressed as a complete suppression of tumor growth after re-inoculation of tumor cells. Mice were inoculated intraperitoneally with NK/Ly ascitic cells and then subjected to 2–4 intraperitoneal injections of vinblastine at a dose of 1mg/g of body weight. The recovered mice were re-inoculated with tumor cells and the absence of tumor growth was considered as resistance development. The disadvantage of this approach is that less than 5% of mice achieve a long lasting recovery due to the treatment. The second scheme included the immunization of mice with intraperitoneal injection of the minimal number of viable tumor cells that do not cause tumor growth, but initiate the immune response. However, this approach was not effective, since there was no reliable number of cells correspon­ding to these demands. The minimal number of 15×10³ injected cells per mouse caused a retarded but still progressive tumor growth. In the third scheme, the immunization of mice was conducted by the intraperitoneal injections of NK/Ly cells permeabilized with saponin. It should be noted that treatment with saponin leads to cell death with a minimal damage to cell morphology. The scheme of immunization with permeabilized NK/Ly cells appeared to be simple and effective. It provided a reproducible resistance to transplanted tumor and might be used as a model in studies of the mechanisms of this phenomenon. Cytological investigation of tumor and immunocompetent cells in ascites of control and of tumor-resistant mice was conducted. As revealed, the number of lymphocytes in ascites of tumor-resistant mice was about 4 times higher than such amount in the control (non-resistant) mice. A destruction of tumor cells by the adherent mono-nuclears was observed.
Conclusions. The method of induction of resistance to transplantation of experimental tumor NK/Ly by immunization of mice with tumor cells permeabilized with saponin is described. The intraperitoneal inoculation of tumor cells to the tumor-resistant mice caused the marked increase of the mononuclear leukocytes population in the peritoneal fluid, which showed a harmful effect upon tumor cells. Thus, the induction of resistance to transplantation of NK/Ly lymphoma in mice might be provided mainly via the mechanisms of cell immunity, in particular, by the appearance of cytotoxic lymphocytes specific to distinct tumor cells.