Continuous propagation of radiation leukemia virus on a C57BL mouse-embryo fibroblast line, with attenuation of leukemogenic activity

The radiation leukemia virus (RadLV), a murine leukemia virus derived from thymic lymphomas induced by x-irradiation in strain C57BL/Ka mice, has been successfully propagated in sustained high titer in vitro in a newly established line, BL-5, of C57BL/Ka mouse-embryo fibroblasts. In addition, the production of endogenous virus, presumed to be RadLV, has been induced and sustained through multiple serial passages after treatment of BL-5 cell cultures with 5-bromodeoxyuridine. The chronically RadLV-infected subline, designated BL-5 (RadLV), sheds virus into the supernatant culture fluids that is biologically active in vitro in the XC cell plaque assay, in interference assays for focus-formation by murine sarcoma virus, and in the intracellular induction of group-specific antigens detectable by immunofluorescence, but is apparently devoid of leukemogenic activity after intrathymic inoculation into neonatal or immunosuppressed C57BL/Ka mice. Although BL-5 cells exhibited morphological alterations suggestive of transformation in vitro and gave rise to fibrosarcomatous ascites tumors after intraperitoneal inoculation with C57BL/Ka mice, the chronically infected BL-5(RadLV) cells remained normal in morphology and failed to yield fibrosarcomas in vivo.

In vitro activation of cellular immune response to Gross virus-induced lymphoma

Spleen cells from W/Fu rats 40 days or more after immunization with a syngeneic Gross virus-induced leukemia were unreactive in direct cytotoxic assays. Incubation of these immune cells at 37 degrees C for 12 hr or longer, in the absence of antigen, resulted in the appearance of specific cytotoxic reactivity. Other lymphoid cells from the immune rats also were activated upon in vitro incubation, but to a lesser extent. Experiments were performed to define the necessary conditions and the mechanism for the in vitro incubation. Activation was temperature dependent, occurring at 37 degrees C but not at 4 degrees C. Immune serum suppressed the activation, but normal rat serum also had some inhibitory activity. Passage of immune cells through a nylon column, before preincubation, prevented activation. In contrast, exposure to nylon after preincubation did not remove cytotoxic reactivity. These findings demonstrate the reversal of a central inhibition of immune cell activity. The explanations offered for this phenomenon included change in surface characteristics of the immune cells during in vitro incubation, and the possible need for an adherent helper cell.

Replication of radiation-induced murine leukemia virus in normal and transformed mouse cells

Autonomous radiation-induced leukemia virus (RadLV) replication could be detected in mouse 3T3 cells by the development of interference with murine sarcoma virus (MSV), the appearance of covert helper activity for defective MSV, and by the induction of cytopathic effect type foci in MSV-transformed, leukemia virus-negative (S+L-) cells. A chronic infection of either 3T3 or S+L- cells with RadLV could be established. Both RadLV infectivity and helper activity were demonstrated in the same peak at a buoyant density of 1.16 g/cm(3). Additionally a soluble inhibitor of MSV focus formation was found which could be separated from infectious RadLV. Examination of cell clones derived from chronically infected 3T3 cells showed that essentially every cell was infected and produced both infectious RadLV and low levels of inhibitor. Quantitative comparisons of autonomously replicating RadLV in normal 3T3 and S+L- cells suggested that RadLV may consist of several populations of virus of varying replicative potential. Apparently 99% of RadLV can be assayed only as helper units in normal cells or as replicative units in S+L- cells. To explain the atypical results, a model for RadLV deficiency is proposed.

Quantitative studies of naturally occurring murine leukemia virus infection of AKR mice

Quantitative studies were made of the organ distribution of murine leukemia virus in AKR mice of various ages. Infectious virus first appeared shortly before or after birth and was continuously present in all mice thereafter. Highest infectivity titers were found in uterus and bone, with spleen slightly lower. Virus titers in normal thymus were relatively low, but increased significantly with the development of thymic lymphoma. The level of viremia decreased after the 1st month of life, but increased sharply in lymphomatous mice.

Effect of Friend leukemia virus and Rowson-Parr virus on immunological maturation of mice

The effect of neonatal infection with Friend virus (FV) and Rowson-Parr virus (RPV) on the maturation of the capacity to respond to sheep red cells, as measured by the numbers of hemolytic plaque-forming cells in the spleen, was investigated in BALB/c mice. Both viruses affected immunological maturation but there were significant differences between their effects. The development with age of the ability to produce plaque-forming cells in response to antigen was virtually abolished by FV and only slightly impaired by RPV. Furthermore, FV also suppressed the development of background plaque-forming cells, whereas RPV did not.