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.