Nature andSpecificity ofLymphokine Independence Induced bya Selectable Retroviral Vector Expressing v-src

A murine retroviral vector, LSNLsrc, hasbeenconstructed andexamined foritsability toinduce growth factor independence incells normally dependent oninterleukin 2(IL-2) orinterleukin 3(IL-3) forgrowth. The LSNLsrc vector coexpressed thev-src geneofRoussarcoma virus andtheneogenefromtransposon TnS, allowing infected cells tobeselected onthebasis ofG418resistance. Themurine cell lines CTLL-2andFD.C/1, whicharedependent forgrowth onIL-2andIL-3, respectively, werebothreadily infected withtheLSNLsrc virus. LSNLsrc-infected, G418-resistant cultures ofFD.C/1 cells wereabletogiverise toIL-3-independent progeny, butallG418-resistant CTLL-2cells retained normalIL-2dependence. Theinduction ofIL-3 independence byv-src wasnotadirect event, since limiting dilution analysis oftheLSNLsrc-infected FD.C/1 cells showedthatmostofthemwereIL-3dependent, despite expression ofv-src mRNA andactive pp60`sl kinase. However, clones selected fromthis population inthepresence ofIL-3wereable toundergo asubsequent progression event andgenerate IL-3-independent progeny. Thegeneration offactor-independent variants in theclonal cultures wasarareevent, aswitnessed bythedeath ofmostofthecells ineachclone whenIL-3was withdrawn. Together, these dataindicate that asecondary event, inaddition tov-src expression, wasrequired togenerate IL-3-independent growth. Noevidence wasfound foranautocrine mechanism oftransformation involving IL-2, IL-3, interleukin 4,orgranulocyte-macrophage colony-stimulating factor.