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Department of Pathology, School of Medicine, University of Connecticut Health Center, Farmington, CT 06030
Evidence is provided in a companion paper for an IL-7-associated
molecular complex that selectively stimulates the proliferation and
presumptive differentiation of pre-pro-B cells in our long-term bone
marrow culture system and "primes" them to proliferate in response
to monomeric IL-7. Here, Western immunoblot analysis reveals that this
pre-pro-B cell growth-stimulating factor (PPBSF) is a self-assembling
heterodimer of IL-7 and a cofactor with a Mr of
30,000. Thus, when developed with anti-IL-7 mAb, PPBSF migrates
electrophoretically as a covalently bound 
55-kDa molecule under
nonreducing conditions but dissociates under reducing conditions.
Furthermore, the addition of rIL-7 or native IL-7 to medium conditioned
by stromal cells from IL-7 gene-deleted (-/-) mice results in the
formation of active 45-kDa and 55-kDa molecular complexes,
respectively. Antiserum prepared in IL-7(-/-) mice against
affinity-purified PPBSF contained separable reactivities for IL-7 and
the non-IL-7 component of PPBSF. The PPBSF cofactor detected by this
antiserum migrates as an 30-kDa molecule and is able to maintain the
viability, but not the proliferation, of pre-pro-B cells. Furthermore,
the cofactor is produced constitutively by IL-7(-/-) and IL-7(+/+)
bone marrow stromal cells under pro-B- but not pre-B-type culture
conditions. Consequently, IL-7 appears to exist almost entirely as a
heterodimer (i.e., PPBSF) in pro-B-type cultures, whereas it exists
almost entirely as a monomer in pre-B-type cultures. Although the
identity of the PPBSF cofactor remains to be determined, it does not
appear to be stem cell factor, insulin-like growth factor-1, thymic
stromal-derived lymphopoietin, flt3, stromal cell-derived factor-1, or
IL-7R.
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