TARGETED IN-VIVO INFECTION WITH A RETROVIRAL VECTOR CARRYING THE INTERLEUKIN-3 (MULTI-CSF) GENE LEADS TO IMMORTALIZATION AND LEUKEMIC TRANSFORMATION OF PRIMITIVE HEMATOPOIETIC PROGENITOR CELLS

To measure the effect of endogenous IL-3 (Multi-CSF) expression on hem atopoietic cells in vivo, we have infected several kinds of hematopoie tic cell populations with retroviral vectors carrying the IL-3 gene (M 3MuV) in vitro and injected the virus-producing cells into mice to ''t arget'' the virus to sites of hematopoiesis. Mast cell lines (Elut cel ls) or multipotent cell lines (FDC-Pmix) were infected with MPSV-based replication defective retroviral vectors carrying either the neomycin resistance gene alone (M3neoV) or the neomycin gene plus the IL-3 gen e (M3MuV). These cell lines produced infective retroviral particles co nsisting of the replication defective vectors and helper virus constit utively produced by the target cell populations. Irradiated and non-ir radiated virus-producing Elut cells and the virus-producing FDC-Pmix c ells were transplanted into syngeneic mice to ''target'' virus infecti on to the sites of hemopoiesis. Control mice injected with M3neoV-prod ucing cells did not develop a disease up to 6 months following transpl antation, whereas mice injected with M3MuV-producing cells developed a myeloproliferative disease within 3 months. Hematopoietic cell lines were rescued from diseased and control mice. In all cases these cell l ines were of host origin. Cell lines derived from control mice were of basophil/mast cell morphology only, and required IL-3 for their conti nued proliferation (similar to cell lines derived from uninfected anim als), whereas the cell lines generated from spleen and bone marrow cel ls of host mice with myeloproliferative disease carried the M3MuV vect or, were G418 resistant and IL-3 independent. The biologic properties of M3MuV infected host derived cell lines varied considerably. Some we re multipotential and could be induced to differentiate in response to stromal cells and serum factors, others were more restricted to the g ranulocyte/macrophage lineage but were also differentiation inducible, and some were blocked in differentiation at the myeloblast/promyelocy te stage. We conclude that the injected donor cells acted as ''infecti ous centers'' to facilitate the infection of host hematopoietic cells with the M3MuV vector. Our results indicate that the ''targeted'' in v ivo infection of primitive hematopoietic cells with M3MuV can initiate the immortalization and leukaemogenesis of multipotential and lineage restricted progenitor cells.