Exposure of NIH Swiss mouse embryo fibroblasts (MEF) to infectious Friend virus (FV) complex [containing defective spleen focus-forming virus (SFFV) and endogenous NB-tropic leukemia-inducing helper virus (LLV-F)] resulted in the productive infection of these cells by both SFFV and LLV-F. Stocks of SFFV derived after extensive growth in this Swiss MEF cell culture system are fully leukemogenic in adult mice for the induction of erythroleukemia and spleen foci. In addition, in vitro-derived LLV-F, when isolated free of SFFV, is fully leukemogenic for the induction of lymphatic leukemia when inoculated into susceptible newborn BALB/c mice. The host range of in vitro-derived FV complex (i.e., FV-TC) for focus formation in vivo is NB-tropic. Unlike in vivo-derived FV complex, FV-TC does not suppress the responsiveness of murine thymocytes to concanavalin A (Con A) in vitro. Rather, FV-TC acts as a mitogen to nonspecifically stimulate the proliferation of BALB/c thymocytes. The mitogenicity of in vitro-derived FV complex is directly associated with the presence of type-C virus particles, is a heat-labile and UV-sensitive property of the virus, and may be primarily due to LLV since equivalent amounts of LLV with or without SFFV present are equally mitogenic. One in vivo passage of FV-TC resulted in the total loss of this mitogenic property with the reappearance of full immunosuppressive properties. This result demonstrates a clear association between in vivo growth of FV and its ability to suppress mouse thymocytes, and suggests that FV complex (SFFV-LLV) is not inherently immunosuppressive for these cells. While the mechanism of this interconversion between immunostimulatory and fully suppressive virus is unknown, both virus markers appear to be dependent upon the presence of infectious FV.

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