HEMATOPOIESIS The Soluble Interleukin-6 (IL-6) Receptor/IL-6 Fusion Protein Enhances In Vitro Maintenance and Proliferation of Human CD341CD382/low Cells Capable of Repopulating Severe Combined Immunodeficiency Mice

In vitro maintenance and proliferation of human hematopoietic stem cells is crucial for many clinical applications. Early hematopoietic cells express low levels of FLT-3 and c-kit receptors, as well as the interleukin-6 (IL-6) receptor signal transducing element, gp130, but do not express IL-6 receptor itself. Therefore, we have attempted to maintain human cord blood or bone marrow CD341 cells ex vivo in serum-free cultures containing stem cell factor (SCF) and FLT-3 ligand (FL) alone or together with a new recombinant molecule of soluble IL-6 receptor fused to IL-6 (IL6RIL6 chimera). The effect of IL6RIL6 chimera on the proliferation and differentiation of CD341 cells was compared with that of each chimera component added separately. The engraftment potential of in vitro-cultured cells was determined using our recently established functional in vivo assay for primitive human severe combined immunodeficiency (SCID)-repopulating cells (SRC). We report here that IL6RIL6 chimera induced significantly higher levels of progenitors and SRC compared with SCF 1 FL alone or together with IL-6 and soluble IL-6 receptor. IL6RIL6 chimera prolonged in vitro maintenance of SRC for up to 14 days. Stimulation of CD341CD382/low enriched cells with IL6RIL6 chimera maintained the early CD341CD382/low cell subpopulation, which could be detected in vitro for up to 14 days. Moreover, IL6RIL6 chimera preferentially stimulated the growth of early CD341382/low cells, resulting in significantly higher levels of progenitors compared with more mature CD341381 cells. Taken together, these findings demonstrate the importance of IL6RIL6 chimera in stimulating the proliferation of early CD341· CD382gp1301IL-6R2 cells in vitro and extended maintenance of progenitors and SRC. r 1999 by The American Society of Hematology.