High- and low-leukemogenic variants of the radiation leukemia virus (RadLV): immunogenic, suppressive and genetic properties in relation to leukemogenic activity

Potentiation of T cell immunity against radiation-leukemia-virus-induced lymphoma by polysaccharide K

C57BL mice inoculated with radiation leukemia virus (RadLV) develop preleukemic cells long before the onset of leukemia. These cells are potentially immunogenic but fail to elicit an immune response in the host because of the appearance of virus-specific suppressor T cells. We have studied the effect of polysaccharide K (PSK) on the generation of RadLV-specific cell-mediated immune responses in vitro. Long-term exposure to PSK in culture potentiated the ability of immunized T cells to respond to a RadLV-induced lymphoma. It also abrogated the suppressive activity of suppressor T cells and simultaneously boosted the ability of reactive T cells to respond. The dual immunostimulating activity of PSK resulted in the generation of T cytotoxic lymphocytes that could lyse lymphoma cells in vitro. The results suggest that PSK could be used as a prophylactic immune response modifier in preleukemia.

The effect of lead on chemical- and viral-induced tumor production in mice

Female Swiss mice were exposed to lead in the drinking water at concentrations ranging from 0 to 1000 ppm for 105 or 280 day periods of time. The effect of lead on urethan-induced pulmonary adenoma formation was evaluated in the 105 day study. Urethan-induced sleeping times observed following ip injection of urethan (1.5 mg/g) after 3 weeks of lead exposure were not altered by lead indicating that lead did not affect the rate of urethan elimination. Pulmonary adenoma formation was evaluated 84 days later. Lead exposure did not affect the number of tumors produced, nor did it alter the mean tumor diameter in the lead treatment groups. This suggests that the immunosuppressive activity of lead does not enhance urethan-induced adenoma formation. In the 280 day study, the incidence of spontaneous murine lymphocytic leukemia was evaluated. Leukemia was observed in all treatment groups. Mortality was greater in the lead-exposed mice. Mice exposed to 50 or 1000 ppm lead had 41.6% and 58.3% more deaths associated with the virus. The median survival time was also reduced in the lead-exposed mice. It appears that the immunosuppressive effects of lead allow for increased expression of the murine lymphocytic leukemia virus.

A highly leukemogenic radiation leukemia virus isolate is a thymotropic, immunosuppressive retrovirus with a unique RNA structure

Clones of N-, B- and NB-fibrotropic viruses were isolated from weakly (D-RadLV) and strongly (A-RadLV) leukomogenic RadLV preparations. A highly leukemogenic, thymotropic virus (TV) was isolated by ex-vivo infection of thymocytes with A-RadLV. This virus could not be isolated from D-RadLV. Two-dimensional fingerprint analysis suggested that TV recombines unique RNA sequences with RNA genomic material derived from a B-tropic endogenous virus. C57BL/6 (B6) mice injected with B- or NB-fibrotropic clones, but not with TV or N-tropic viral clones, developed reactive T lymphocytes (Tr), capable of differentiating into anti-tumor cytotoxic cells. The N-tropic virus isolates were non-immunogenic in B6 mice whereas the TV isolate induced suppressor T lymphocytes (Ts) that abrogated a potential Tr response. These results suggest that emergence of highly leukemogenic RadLV involves activation of endogenous fibrotropic virus which is immunogenic in its natural host strain (B6). This virus can further recombine with other retroviral genetic sequences, resulting in a suppressogenic and thymotropic, highly leukemogenic virus.

The effect of cadmium on chemical- and viral-induced tumor production in mice

Female Swiss mice were exposed to cadmium in the drinking water at concentrations ranging from 0 to 50 ppm for 105 or 280 day time periods. In the 105 day study, the effect of cadmium on urethan-induced pulmonary adenoma formation was evaluated. Urethan-induced sleeping times observed following i.p. injection of urethan after 3 weeks of cadmium exposure were not affected by cadmium indicating that chronic cadmium exposure did not alter the elimination of urethan. Pulmonary adenoma formation which was evaluated 84 days later was not affected by cadmium. The size and number of tumors remained unchanged. This suggests that the immunosuppressive actions of cadmium do not influence urethan-induced adenoma formation. In the 280-day study, the effects of cadmium on the incidence of spontaneous murine lymphocytic leukemia was evaluated. Mortality from the leukemia virus was greater in the cadmium-exposed mice. Mice exposed to 10 or 50 ppm cadmium experienced 33% more deaths from the virus. The average time till death was unaffected. It appears that the immunosuppressive effects of cadmium impair immunosurveillance mechanisms that control expression of the murine lymphocytic leukemia virus.