Journal of Experimental Medicine, Vol 142, 936-948, Copyright © 1975 by Rockefeller University Press
Continuous replication of Friend virus complex (spleen focus-forming virus-lymphatic leukemia-inducing virus) in mouse embryo fibroblasts. Retention of leukemogenicity and loss of immunosuppressive properties
RJ Eckner
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.