Abnormal expression of interleukin-3 and leukaemia

Autonomous production of a required growth factor is one mechanism by which a cell may become tumorigenic. Several leukaemias have been described which secrete growth factors which may be involved in autocrine stimulation of cell proliferation. One of these leukaemias is WEHI-3B, a myelomonocytic leukaemia that constitutively produces interleukin-3 (IL-3). Cloning of the IL-3 gene has enabled us to investigate possible genetic changes in this gene in WEHI-3B cells which may have resulted in autonomous production of growth factor. We have shown that one of the IL-3 genes in WEHI-3B has been rearranged, as a result of the insertion of a 5.1 kilobase intracisternal A-type particle genome head to head with the 5' end of the IL-3 gene, 215 bases upstream of the IL-3 TATA box. The rearranged gene, when cloned into a lambda EMBL3A vector, could readily be expressed in COS-1 monkey cells, whereas the normal gene, in the same vector, was silent. Thus the insertion of the endogenous retroviral element has resulted in abnormal expression of the IL-3 gene and is postulated to have been a key genetic change in the development of this leukaemia. In an attempt to experimentally construct IL-3 producing leukaemias, IL-3 responsive FDC-P1 and 32D cl-23 cells were transfected with a retroviral expression vector containing the IL-3 gene. This resulted in autonomous production of IL-3 and continuous proliferation of the transfected cells. As a result of transfection, the FDC-P1 and 32D cl-23 cells became leukemogenic demonstrating the oncogenic potential of abnormal expression of IL-3. The autocrine nature of the experimental leukaemias was demonstrated by blocking their proliferation with an IL-3 neutralising antiserum. Similarly treated cultures of normal bone marrow cells also produced IL-3 and could be maintained for several months after transfection but were not leukemogenic. The factor-dependent cell lines are unable to differentiate in the presence of known CSF's and presumably have undergone other genetic changes which allow them to become leukemogenic when autocrine-stimulated. In contrast, the transfected bone marrow cells could differentiate and form colonies containing mature granulocytes and macrophages. The non-tumorigenic behaviour of the transfected bone marrow cells is consistent with the concept that several other genetic changes which effectively block differentiation are required for development of tumorigenicity in these cells.