IN-VITRO AND IN-VIVO STUDIES OF STROMAL NICHES

Lymphohematopoiesis occurs in the densely packed environment of the in tramedullary spaces. Primitive lymphohematopoietic stem cells exist in close apposition to a variety of supportive cells including both hemo poietic and nonhemopoietic lineages. Using an in vitro long-term Dexte r liquid culture system, we have established that a variety of cytokin es are produced constitutively by such stromal cells in culture. These cytokines include Steel factor, interleukin-6 (IL-6), and colony-stim ulating factor (CSF-1). Granulocyte-CSF and granulocyte-macrophage-CSF mRNA can be detected after refeeding of cultures, although in quiesce nt cultures message for these factors is difficult to detect. Interleu kin-3, IL-4, and IL-5 are not detectable by standard Northern blot ana lysis or bioassay of condition media. However, IL-3-detectable by reve rse-transcriptase PCR and biologic activity-was confirmed by growth of factor-dependent cells on stromal cells with IL-3 antibody blocking o f such growth. Stem cells resident on such stromal cells are mirrored by the high proliferative potential colony-forming cell assay and are responsive to a relatively large number of cytokines, with Steel facto r being of central importance, appearing to be a critical component of various synergistic combinations. Steel factor allows reduced levels of other factors in such combinations and works early in a temporal se quence.