Combination of interleukins 3 and 6 preserves stem cell function in culture and enhances retrovirus-mediated gene transfer into hematopoietic stem cells.

The effects of several hematopoietic growth factors on primitive murine bone marrow progenitor cells [colony-forming unit(s)-spleen (CFU-S)] have been investigated during culture for 2-6 days. Interleukin 3 (IL-3) was required for CFU-S survival in culture, and the combination of IL-3 and interleukin 6 (IL-6) increased the number of CFU-S in culture 10-fold over the number obtained with IL-3 alone. Stem cell function was measured by competitive repopulation; IL-3 was required, and IL-3 and IL-6 appear to act synergistically to enhance stem cell recovery from these cultures. These data appear to be relevant for retroviral-mediated gene transfer into stem and progenitor cells. Murine bone marrow cells were infected with a retrovirus containing the human beta-globin gene in the presence of various growth factors. Only 2 of 17 mice reconstituted with cells infected in the presence of IL-3 alone showed long-term expression of the human beta-globin gene (12 months), as opposed to 6 of 11 mice reconstituted with cells infected in the presence of IL-3 and IL-6. Medium conditioned by 5637 bladder carcinoma cells, a source of several hematopoietic growth factors, increased the frequency of infection of CFU-S but did not enhance stem cell infection or the repopulating potential of cultured bone marrow cells. Stem cells containing the human beta-globin provirus from these animals were shown to be capable of reconstituting secondary recipients in which the human beta-globin gene was expressed.