IDENTIFICATION OF AN IL-7-ASSOCIATED PRE-PRO-B CELL GROWTH-STIMULATING FACTOR (PPBSF) - II - PPBSF IS A COVALENTLY-LINKED HETERODIMER OF IL-7 AND A M-R 30,000 COFACTOR

Evidence is provided in a companion paper for an IL-7-associated molec ular complex that selectively stimulates the proliferation and presump tive differentiation of pre-pro-B cells in our long-term bone marrow c ulture system and ''primes'' them to proliferate in response to monome ric IL-7, Here, Western immunoblot analysis reveals that this pre-pro- B cell growth-stimulating factor (PPBSF) is a self-assembling heterodi mer of IL-7 and a cofactor with a M-r of 30,000, Thus, when developed with anti-IL-7 mAb, PPBSF migrates electrophoretically as a covalently bound similar to 55-kDa molecule under nonreducing conditions but dis sociates 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-kD a molecular complexes, respectively, Antiserum prepared in IL-7(-/-) m ice against affinity-purified PPBSF contained separable reactivities f or IL-7 and the non-IL-7 component of PPBSF, The PPBSF cofactor detect ed by this antiserum migrates as an similar to 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 e ntirely as a heterodimer (i.e., PPBSF) in pro-B-type cultures, whereas it exists almost entirely as a monomer in pre-B-type cultures, Althou gh the identity of the PPBSF cofactor remains to be determined, it doe s not appear to be stem cell factor, insulin-like growth factor-1, thy mic stromal-derived lymphopoietin, flt3, stromal cell-derived factor-1 , or IL-7R.