Studies on the in vivo production of a lymphokine activity, interleukin 3 (IL-3) elaborated by lymphocytes and a myeloid leukaemic line in vitro and the fate of IL-3 dependent cell lines.

Interleukin 3 (IL-3) is produced constitutively by WEHI-3b leukaemic cells and stimulated lymphoid cell populations in vitro. We have investigated the in vivo production of IL-3 in mice rendered leukaemic with WEHI-3b cells and mice stimulated by acute graft versus host disease (GVHD). In leukaemic mice, IL-3 was not found in serum or sonicates of 18-day spleens or bone marrow, although cells from the leukaemic organs were fully competent to elaborate IL-3 in vitro. Further, elaboration of IL-3 by WEHI cells in vitro was not affected by co-culture with normal haemopoietic cells. However, intracellular IL-3 was detected in leukaemic nodules isolated from the liver. Inhibitors specific for IL-3 were not found, although liver-cell conditioned medium and leukaemic nodule sonicates contained potent non-specific inhibitors of cell growth. At 21 days, intracellular IL-3 was also present in spleens and correlated with the presence of non-specific inhibitors. In GVHD, no evidence for IL-3 elaboration in vivo was found, nor did lymphoid populations affected by GVHD spontaneously elaborate it in vitro; however, their competence to produce it was unaffected, as IL-3 was elaborated on subsequent mitogen stimulation in vitro. We also investigated the recovery and circulation of in vitro 111Indium-labelled IL-3 dependent cells after injection in vivo and the half-life of semi-purified IL-3. Dependent cells were not recovered after injection into irradiated recipients, although the cells recirculated for at least 24 hours. Inability to recover dependent cells was explicable on general cytotoxicity which masked potential recovery. The serum half-life of injected partially purified material with IL-3 activity was short (less than 30 min). We conclude that the elaboration of IL-3 by leukaemic WEHI-3b is not an in vitro artifact and these results are discussed in relationship to other growth factors and the leukaemic state, and the origin of IL-3 dependent lines.